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Adsorption properties of HKUST-1 toward hydrogen and other small molecules monitored by IR
Abstract
Among microporous systems metal organic frameworks are considered promising materials for molecular adsorption. In this contribution infrared spectroscopy is successfully applied to highlight the positive role played by coordinatively unsaturated Cu2+ ions in HKUST-1, acting as specific interaction sites. A properly activated material, obtained after solvent removal, is characterized by a high fraction of coordinatively unsaturated Cu2+ ions acting as preferential adsorption sites that show specific activities towards some of the most common gaseous species (NO, CO2, CO, N2 and H2). From a temperature dependent IR study, it has been estimated that the H2 adsorption energy is as high as 10 kJ mol(-1). A very complex spectral evolution has been observed upon lowering the temperature. A further peculiarity of this material is the fact that it promotes ortho-para conversion of the adsorbed H2 species.
... In particular, HKUST-1, also referred to as Cu-BTC or Basolite ® C300, is one of the few commercially available frameworks and is among the most temperature/water-resistant MOFs. Since it was first documented by Chui et al. in 1999 [22], this material has been proposed in numerous applications such as gas storage [3,23,24], adsorbent for the separation of gas mixtures [25,26], molecular sensing [27], and applications as a catalyst [24,28,29]. Cu-BTC presents a three-dimensional porous framework formed by the coordination of copper cations (Cu 2+ ) and benzene-1,3,5-tricarboxylate (BTC) linker molecules which form the dimeric copper paddle-wheel structural building blocks (see Figure 1, left side) [30]. ...
... In the context of Cu-BTC MOF, different works have been performed to elucidate the interactions of guest species with the framework. It is noteworthy to note the spectroscopic study regarding the adsorbate-adsorbent interaction in HKUST-1 performed by Bordiga et al. [23,40]. In their first work regarding this topic [40], the authors investigated the dehydration process (activation) by means of the XRD, UV−Vis, EXAFS, XANES, and Raman spectroscopies. ...
... In the dehydrated state, they observed the formation of labile Cu 2+ ···CO and Cu 2+ ···H2 adducts, detecting for the first time the signal of Cu(II) carbonyl and dihydrogen complexes formed inside a crystalline microporous hosting matrix. In a follow-up study, Bordiga et al. [23] improved the preparation method for the Cu-BTC synthesis and performed IR spectroscopic measurements in transmission mode using a properly designed cryogenic cell, assessing the interaction of HKUST-1 sites with several adsorbates, such as NO, CO2, CO, N2, and H2. Interestingly, the interaction of CO2, CO, and N2 allowed to distinguish between a first type of Cu 2+ sites located at the external faces of the crystals and a second type of Cu 2+ sites regularly contained within the cages of the framework. ...
This contribution aims at providing a critical overview of experimental results for the sorption of low molecular weight compounds in the Cu-BTC Metal–Organic Framework (MOF) and of their interpretation using available and new, specifically developed, theoretical approaches. First, a literature review of experimental results for the sorption of gases and vapors is presented, with particular focus on the results obtained from vibrational spectroscopy techniques. Then, an overview of theoretical models available in the literature is presented starting from semiempirical theoretical approaches suitable to interpret the adsorption thermodynamics of gases and vapors in Cu-BTC. A more detailed description is provided of a recently proposed Lattice Fluid approach, the Rigid Adsorbent Lattice Fluid (RALF) model. In addition, to deal with the cases where specific self- and cross-interactions (e.g., H-bonding, Lewis acid/Lewis base interactions) play a role, a modification of the RALF model, i.e., the RALFHB model, is introduced here for the first time. An extension of both RALF and RALFHB is also presented to cope with the cases in which the heterogeneity of the rigid adsorbent displaying a different kind of adsorbent cages is of relevance, as it occurs for the adsorption of some low molecular weight substances in Cu-BTC MOF.
... MIL-101(Cr) [40], MIL-101(Fe) [41], ZIF-8(Zn) [42], HKUST-1 [43] and MIL-68(Al) [44] were synthesized according to the method described in the literature. ...
... In this study, various MOFs, including MIL-101(Cr), MIL-101(Fe), ZIF-8(Zn), HKUST-1(Cu), and MIL-68(Al), were synthesized and then subjected to analyses by scanning electron microscopy (SEM, Hitachi SU-8020, Japan), and X-ray diffraction (XRD, Rigaku Ultima IV, Japan). The XRD patterns (Fig. 2) of the synthesized MOFs were in good agreement with those reported previously [19,42,43,45,46], and their SEM images also confirmed their proper morphology. The prepared MIL-68(Al) material consisted of needle-like crystals with different lengths and were disorderly arranged (Fig. 2e). ...
In this study, metal–organic framework (MOF)-functionalized sponge columns were prepared using a simple approach by embedding melamine sponge in a homogeneous MOF solution and first applied in the removal of methyl orange dye from water samples. Various MOF materials (MIL-101(Cr), MIL-101(Fe), ZIF-8(Zn), HKUST-1(Cu), and MIL-68(Al)) with different structures were synthesized and used as coating materials for a blank sponge. This method not only can be used for a wide variety of MOF materials, but also can retain their unique properties. The results show that the sponge as an adsorbed base material can effectively simplify the separation procedures of adsorption. Under the optimal conditions, the prepared MOF/PVDF-sponge column had an excellent adsorption capacity (229.4 mg g⁻¹) toward methyl orange and reached the equilibrium within 1 h. With simple one-step infiltration, excellent adsorption properties and simple operation, the MOF/PVDF-sponge column presents a promising commercial value. Moreover, this study provides a new approach for exploring functional porous sponges for adsorption of dyes from environmental water samples.
... Given the low coverages (red curves), it is observed that the main peak at 2337 cm À1 sharpens as time goes by and has a high frequency shoulder at 2356 cm À1 . These components, due to the ν 3 mode of CO 2 (asymmetric O¼C¼O stretching), are red-shifted relative to CO 2 in the gas phase (2349 cm À1 ), as previously reported in case of HKUST-1 with open metal site [52]. Indeed, the ν 3 mode associated with M 2þ (Ni 2þ and Co 2þ )⋅⋅⋅ O¼C¼O complexes formed on MOF-74 framework with open metal site, corroborating an strong interaction between CO 2 and M 2þ [52,53]. ...
... These components, due to the ν 3 mode of CO 2 (asymmetric O¼C¼O stretching), are red-shifted relative to CO 2 in the gas phase (2349 cm À1 ), as previously reported in case of HKUST-1 with open metal site [52]. Indeed, the ν 3 mode associated with M 2þ (Ni 2þ and Co 2þ )⋅⋅⋅ O¼C¼O complexes formed on MOF-74 framework with open metal site, corroborating an strong interaction between CO 2 and M 2þ [52,53]. In order to investigate the CO 2 capture profile with temperature on the Ni 1 Co 1 -MOF-74, in situ CO 2 adsorption experiment was implemented at different temperatures (25,50,100, and 150 C), and their spectra were collected. ...
... Finally, the band observed at 2180 cm -1 is assigned to the formation of Cu(II)-CO adducts, as in this case the interaction is dominated by polarization of the probe molecule interacting with the cation. 43,45 The low intensities of both the components associated to the formation of Cu(I)-CO and Cu(II)-CO adducts suggest that in the adopted conditions the amount of accessible copper sites with a coordinative vacancies are very limited. Table S10). ...
Metal-organic frameworks (MOFs) featuring zirconium-based clusters are widely used for the development of functionalized materials due to their exceptional stability. In this study, we report the synthesis of a novel...
... Each BTC ligand is consist of three dimeric Cu paddle wheels, resulting in face-centered cubic symmetry of a microporous open framework [44]. The 3D unit cell crystal structure is made up of huge core cavities (dia 9.0 o A) surrounded by tiny pockets (dia 5.0 o A) [45]. These lateral channels are linked to the central cage via a 3.5 o A triangular window. ...
In this study, copper based metal organic frame work (Cu-MOF) was synthesized by using different solvents i. e. H2O, Dimethylformamide (DMF) and DMF + C2H5OH + H2O solutions at various temperatures. Cu-MOF synthesized in different solvents exhibited a large number of properties and advantages including easy synthesis procedure, high porosity, good crystallinity, high surface area, massive pore volume, controllable pore surface properties (zeta potential), rigid and chemical stability. The as-synthesized Cu- MOF was analysed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area, zeta potential, electrostatic attraction and Thermal gravimetric analysis (TGA). A higher BET surface area of 121.1 m² g⁻¹ was obtained for Cu-MOF synthesized in DMF + C2H5OH + H2O compared with H2O (109.3 m² g⁻¹) and DMF (115.1 m² g⁻¹) solvents. The as-obtained Cu-MOF was used to investigate adsorption kinetic performances of methylene blue (MB) and methyl orange (MO) dye molecules. Compared with other samples, Cu-MOF synthesized in (DMF + C2H5OH + H2O) has a significantly enhanced adsorption ability for MB but a lower for MO dyes. After 4 min, the adsorption co-efficient (k) values were calculated to be 0.506, 0.846 and 1.1915 min⁻¹ for MB and 0.029, 0.024 and 0.005 min⁻¹ for MO dyes for Cu-MOF (H2O), Cu-MOF (DMF) and Cu-MOF (DMF + C2H5OH + H2O), respectively. It was found that Cu-MOF has a higher adsorption for MB dye but a poor adsorption capacity for MO, which was attributed to the electrostatic attraction/repulsion phenomena. Moreover, compared with Cu-MOF (DMF) and Cu-MOF (H2O), Cu-MOF (DMF + C2H5OH + H2O) shows an improved stability at 370, 417 and 423 °C temperatures. The result obtained through experiment suggested that porous Cu-MOF microstructures synthesized in DMF + C2H5OH + H2O have potential application for the treatment wastewaters having MB and MO dyes.
... Studied MOF-hydrate synergies include Al metals (MIL-53), Fe based (MIL-100), Cr (MIL-101) and Cu metals (HKUST-1), and Cr-based MOF-1 and Y-shaped MOF-5 are used. MIL-53 and HKUST-1 are widely studied MOFs for gas adsorption due to their higher hydrothermal stability [67,68]. The water adsorption isotherm suggests that MIL-101 and MIL-100 (Fe) are the most stable MOFs in water as hydrophilic mesoporous compounds and compared with HKUST-1, and zeolitic imidazolate frameworks (ZIF-8 and ZIF-67) are characterized as hydrophobic MOFs [66]. ...
Recent research on the role of nanomaterials in gas hydrate science and a few review papers have highlighted the positive synergies between gas hydrates and metal–organic frameworks (MOFs) for gas separation and storage. Metal–organic frameworks consist of metal nodes and organic linkers connected by coordination bonds to form programmable modular structures that are symmetric and have tunable properties. Metal–organic frameworks, also known as microporous or nanoporous materials, provide a large pore volume and surface area suitable for capturing, separating and storing gases through physisorption mechanisms. However, water and water interactions within the nanopores, open metal sites, coordination bonds and surface make metal–organic framework usage in water-based technologies an exciting research topic. Water-based gas hydrate technology could be potential technology that can take advantage of MOF tunable properties, such as a large surface area and a high pore volume, to improve its efficiency and formation mechanism. For the authors of this review, the synergy of MOFs and gas hydrates resembles a Pandora’s box of unanswered questions and revelations. Therefore, this review examines the current state of the art, including present research on gas storage and separation using gas hydrates in the presence of a MOF. In addition, critical technical aspects, such as the water stability of MOFs, the nano confinement effect and water properties in the nanopores, are presented to stimulate critical thinking among scientists in hydrate research to fully exploit the synergies between MOFs and hydrates. This review ends with the authors’ opinion on potential research areas, unanswered questions and practical implications and prospects.
... The introduction of the analyte molecules also induces distortion of the mechanical structure, as well as changes in the electronic band structure that lead to changes in the conductivity of the MOF. In a pure nitrogen ambient, the N 2 adsorbs on the metal sites [53], but in the presence of Lewis base molecules, such as aliphatic alcohols, the analyte is expected to displace pre-adsorbed N 2 molecules to form adducts from the open metal sites. ...
We leveraged chemical-induced changes to microwave signal propagation characteristics (i.e., S-parameters) to characterize the detection of aliphatic alcohol (methanol, ethanol, and 2-propanol) vapors using TCNQ-doped HKUST-1 metal-organic-framework films as the sensing material, at temperatures under 100 °C. We show that the sensitivity of aliphatic alcohol detection depends on the oxidation potential of the analyte, and the impedance of the detection setup depends on the analyte-loading of the sensing medium. The microwaves-based detection technique can also afford new mechanistic insights into VOC detection, with surface-anchored metal-organic frameworks (SURMOFs), which is inaccessible with the traditional coulometric (i.e., resistance-based) measurements.
... This induces an impedance increase in the device under test (DUT) due to distortion of the mechanical structure, along with changes in the electronic band structure that lead to changes in the conductivity of the MOF. In a pure nitrogen ambient, the N 2 adsorbs on the metal sites [68], but in the presence of Lewis base molecules, such as aliphatic alcohols, the analyte is expected to displace pre-adsorbed N 2 molecules from the open metal sites. (2) The aliphatic alcohol probably coordinates to the open metal center via the hydroxyloxygen atom. ...
The most common gas sensors are based on chemically induced changes in electrical resistivity and necessarily involve making imperfect electrical contacts to the sensing materials, which introduce errors into the measurements. We leverage thermal- and chemical-induced changes in microwave propagation characteristics (i.e., S-parameters) to compare ZnO and surface-anchored metal–organic-framework (HKUST-1 MOF) thin films as sensing materials for detecting ethanol vapor, a typical volatile organic compound (VOC), at low temperatures. We show that the microwave propagation technique can detect ethanol at relatively low temperatures (<100 °C), and afford new mechanistic insights that are inaccessible with the traditional dc-resistance-based measurements. In addition, the metrological technique avoids the inimical measurand distortions due to parasitic electrical effects inherent in the conductometric volatile organic compound detection.
... Among the often reported MOFs, copper benzene-1,3,5-tricarboxylate (Cu-BTC) is a very representative compound for versatile applications, including high-efficiency adsorptive removal of aqueous pollutants, due to its easy preparation and low cost [1,[13][14][15][16]. Specifically, Cu-BTC has a unique double connected network with main and secondary channel diameters of 0.9 and 0.5 nm [17][18][19], respectively. The cavity formed along the channel connection may function as useful space for the capture and storage of guests (e.g., dyes, metals and gas). ...
Metal–organic frameworks (MOFs) featuring porous structures and large specific surface areas have shown great potential in removing organic pollutants from wastewater via adsorption processes. Although the particle size of MOFs determines the adsorption performance (something known as the size-dependent effect), engineering it into desirable dimensions for enhancing the adsorption performance is a great challenge. Here, we develop a practical and facile approach to regulate the particle size of copper benzene-1,3,5-tricarboxylate (Cu-BTC) adsorbents with high tunability by screening the functional modulator of various surfactants adding in hydrothermal synthesis procedure. The effect of surfactant type and concentration on the particle size of Cu-BTC was systematically investigated. The results show that the nonionic surfactant polyvinylpyrrolidone (PVP) demonstrated the greatest ability to control the particle size of Cu-BTC among other counterparts (e.g., N, N, N-trimethyl-1-dodecanaminium bromide (DTAB), polyethylene glycol (PEG1000), sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS) and hexadecyl trimethyl ammonium bromide (CTAB)). By increasing the PVP concentration to 0.14 mmol L−1, the average particle size of Cu-BTC could be correspondingly reduced by more than ten times, reaching to a comparative smaller value of 2.4 μm as compared with the reported counterparts. In addition, the PVP allowed a large increase of the surface area of Cu-BTC according to porosity analysis, resulting in a great enhancement of methylene blue (MB) adsorption. The PVP-modulated Cu-BTC showed fast adsorption kinetics for MB removal accompanied with a maximum adsorption capacity of 169.2 mg g−1, which was considerably competitive with most of the analogs reported. Therefore, our study may inspire concepts for engineering the particle size of Cu-BTCs with improved properties for more practical applications.
... s-polarised light only contributes to the fractions of the evanescent wave in the y-direction (compare Figure 1b). In addtion, IR spectroscopy is a rapid technique that is well suited for quantitative real-time analysis of adsorbing guest molecules, which has been largly used to investigate the adsorption sites in MOFs using probe molecules, for example, CO, NO, or CO 2 , [28][29][30][31][32][33] or changes in the MOF connectivity. [34][35][36] Thus far, analytical techniques have only permitted seperate investigation of each fundamental process. ...
The degree of pore filling is an important parameter for defining guest@MOF properties in applications including electronics, optics, and gas separation. However, the interplay of key aspects of host–guest interactions, such as a quantitative description of the guest alignment or the structural integrity of the host as function of pore filling are yet to be determined. Polarisation‐dependent infrared spectroscopy in attenuated total reflection configuration combined with gas sorption allowed to simultaneously study the orientation of the guest molecule and structural changes of the MOF framework during the pore filling process. Thereby we found, that initially randomly oriented guest molecules align with increasing pore filling during adsorption from the gas phase. At the same time, the framework itself undergoes a reversible, guest molecule‐dependent rotation of the aromatic linker and a linker detachment process, which induce defects.
... The absorption band is blue-shifted and its absorption peak becomes higher. This is because of the temperature ramping and consequent formation of the O@C@O adduct over Ni 2+ sites on MOF-74 (Ni), which excites it to a higher vibration state [37][38][39]. ...
Though the highest CO2 capture capacity belongs to liquid amine-solutions, solid matters capable of CO2 capture are also highly sought, providing that, they offer at least analogous CO2 adsorption capacity and CO2/N2 selectivity. Herein, a surprisingly high-performance Ni-based metal-organic framework for CO2 adsorption, namely MOF-74(Ni), was synthesized by a facile condensation reflux approach. It was found that the structure and CO2 adsorption isosteric heat of MOF-74(Ni) could tune upon varying the synthesis duration under various temperatures. The optimized MOF-74(Ni)-24-140 (synthesized at 140 °C for 24 h) displays outstanding CO2 adsorption capacity of 8.29/6.61 mmol/g at 273/298 K under normal pressure of 1.0 bar, several times higher than previously reported MOF-74-Ni (2.0/2.1 times), UTSA-16 (1.5/1.6 times), and DA-CMP-1 (3.6/4.9 times) under similar conditions. The excellent CO2 capture capacity is associated to the abundant adsorption sites (mainly arising from the cationic Ni²⁺ ions) and narrow micropore channels (mainly arising from the cage structure of Ni²⁺ ions coordinated with organic linkers). Offering a high CO2 selectivity (CO2/N2 = 49) and a well-tuned isosteric heat of CO2 adsorption (27-52 kJ/mol) besides its decent CO2 capture capacity, MOF-74(Ni) strongly stands out as an efficient and strong CO2 capturing material with industrial scale applicability.
... Crystallographic defects play vital roles as far as the visiblerange optical properties of wide-bandgap MOFs are concerned. Types of defects in HKUST-1 reported thus far include plane dislocations with free COOH groups, 53,54 dislocation growth spirals, 55,56 fractures propagating in the crystal interior, 53 monovalent copper, 54,57,58 metal vacancy, 59 linker vacancy, 60 defective linkers, [61][62][63] and temporary defects as Brønsted sites. 64 Some defects can be caused post synthesis by exposure to moisture. ...
Synthesis of crystalline materials is elemental in the field of coordination chemistry towards optical applications. In the present work, coordination between copper and benzene-1,3,5-tricarboxylic acid (BTC) is controlled by adjusting the pH scale of the reaction mixture at room temperature to synthesize two crystalline structures: metal-organic framework HKUST-1 and coordination polymer Cu(BTC)·3H2O. The post-synthesis transformation of HKUST-1 into Cu(BTC)·3H2O is further demonstrated. Single crystals of both structures are studied by multi-laser Raman and luminescence spectroscopy. It is found that both crystals exhibit photoluminescence in the range of 700-900 cm-1 within the optical gap of the bulk materials, which can be associated with crystallographic defects. This work gives impetus for the synthesis of large metal-organic crystals based on which optical properties can be studied in depth.
... MOFs whilst retaining their structural integrity, which is comprehensively described by Mondloch et al.. 95 The conventional method remains the use of a vacuum with heating, 96 however, this technique is not applicable to all MOFsespecially those with larger pore volumes-due to a balance of capillary force strength and surface tension against the coordination bond strength. ...
Metal–organic frameworks (MOFs) are a highly topical class of three-dimensional porous materials proposed for applications such as gas storage, separations, and catalysis. Typically, MOFs are synthesised as microcrystalline powders of nanometer- to millimetre-sized particles ill-suited to industrial settings without prior processing. However, recent research has revealed solid-liquid transitions within the family, which is used here to create a class of functional, stable and porous composite materials. Described herein is the design, synthesis and characterisation of MOF crystal–glass composites formed by dispersing crystalline MOFs within a MOF glass matrix. The first of these novel materials incorporates MIL 53 within a ZIF 62 glass matrix where the crystalline phase’s coordinative bonding and chemical structure are preserved. Whilst the phases are separated, the interfacial interactions between the proximate microdomains improve the mechanical properties of the glass composite. More significantly, the high-temperature, open-pore phase of MIL 53, which spontaneously transforms to a narrow pore phase upon cooling in the presence of water, is stabilised at room temperature in the crystal–glass composite. This leads to a significant improvement in CO2 adsorption capacity. This enhancement is further explored and maximised by synthesising a compositional series of composites. The distribution and integrity of the crystalline component in this series were determined, and these findings were used to identify the maximum crystalline loading and maximum CO2 adsorption capacity. In addition to the study of MIL 53, other MOF crystal-glass composite (MOF CGC) systems were explored, and the thermal stability considerations in the formation of MOF CGCs are highlighted. Resultantly, two separate MOFs were identified, MIL 118 and UL MOF 1, with which MOF CGCs were successfully synthesised. These new materials, alongside the prototypical MOF CGC, formed using MIL 53, were studied using scanning electron microscopy, powder X-ray diffraction, and gas sorption techniques to reveal an approximate kinetic diameter limitation of gases that may permeate through the glass matrix. Furthermore, the thermal expansion behaviour of these three MOF CGCs was investigated. Specifically, variable-temperature powder X-ray diffraction data and thermomechanical analysis show the suppression of thermal expansivity in each of these three crystalline MOFs when suspended within a ZIF 62 glass matrix. In particular, for the two flexible frameworks, the average volumetric thermal expansion (αV) was found to be near-zero in the crystal–glass composite.
... The presence of partially reduced Cu + /Cu 2+ dimeric sites has been observed by CO-probed FTIR and XPS, 29−33 and a broad discussion has been established in the literature on the origin of the Cu + species in HKUST-1. In particular, two different hypotheses have been made: the former suggests that the Cu + ions originate from amorphous Cu 2 O impurities that are formed upon heating at high temperature (e.g., 350°C), 29 and the latter suggests Cu + / Cu 2+ dimeric sites originate from the Cu 2+ ions in the MOF framework either by reduction of defective clusters or by reduction of Cu 2+ cations in perfectly coordinated paddlewheels. 32,33 Consequently, in our experimental conditions, it appears reasonable to hypothesize that the temperature treatment provokes both the dehydration of the pristine Cu 2+ /Cu 2+ paddlewheel units and the formation of defective Cu + /Cu 2+ sites on the surface of HKUST-1. ...
Rapid industrialization and urban development around the world have significantly increased carbon dioxide emissions, adversely affecting climate and ecosystems. Therefore, carbon capture and storage emerged as a promising route to reduce environmental CO 2 concentration. Among various CO 2 capture technologies, adsorption through carbon‐based porous materials has attracted particularly strong attention. This is primarily due to their high specific surface area, selective CO 2 adsorption, moderate heat of adsorption, tunable morphology, and reduced degradation in moisture. This review critically examines carbon‐based CO 2 sorbents derived from diverse sources. The key factors controlling adsorption performance, including the impact of structural and functional properties are discussed. The future research directions in this rapidly emerging field, contributing to the decarbonization of the global economy and society, are highlighted.
In this work, we thoroughly assess the CO2 adsorption behaviour of a recently reported, pillared-layered Cuᴵᴵ ethylenediphosphonate of formula Cu2(H2O)1.7(O3P-C2H4-PO3)·1.5H2O (Cu-EtP) that features narrow channel-like pores (diameter < 5 Å)....
This study introduces an innovative approach for synthesizing metal-organic frameworks (MOFs) on 3D-structured hierarchically porous nanofibrous aerogels (NFAs). The NFA was initially fabricated by solid-templating cellulose diacetate (CDA)-silica electrospun nanofibers...
Using metal–organic frameworks (MOFs) technology, this chapter looks at the most recent improvements in removing pollutants from industrial products and organic dyes. MOFs offer a revolutionary technique for the adsorption of toxic dye and removing harmful contaminants that are often found in wastewater. MOF sponges possessed outstanding ability and excellent dynamic adsorption capacity planning as the commercial division sponge. Access to potable water is essential for the proper development of all living things. Getting rid of toxins from aquatic systems should be one of the main goals of research if we want to restore ecological balance and ensure the future is more sustainable. Because of their interesting structures as well as unique physical properties, MOFs are great platforms for removing dangerous species from water. In this chapter, we tried to give a broad overview of the different synthetic methods for cleaning up water and focusing on how easily MOFs can be changed. We have put these two types of pollution into separate subfamilies based on the chemicals they contain or how often they are used. Last, we have discussed some possible trends and problems that need to be fixed to make MOFs more valuable and make significant steps toward sustainable development. Herein, recent advancements of MOFs and materials-based MOFs are use in environmental applications like primarily treatment water as well as separation. Lastly, the future difficulties and innovations of MOFs or materials-based MOFs are considered and investigated.
Gaseous thallium (Tl) pollution events, primarily caused by non-ferrous mineral refineries and fossil fuel combustion, have increased over the past few decades. To prevent gaseous Tl distribution from flue gas, MnO2/CeO2@HKUST-1 (MCH) was synthesized and found to achieve a gaseous Tl(I) removal level of up to 90% at 423 K, a weight hourly space velocity (WHSV) of 2000 h-1/mL with an Mn dose of 10%, maintained over 10 h. The best Mn/Ce ratio was found to be 9:1. To further investigate surface kinetic behavior, four commonly used kinetic models were applied, including the Eley-Rideal (ER) model, Langmuir-Hinshelwood (LH) model, Mars-van Krevelen (MVK) model, and pseudo-first-order (PFO) model. While the ER and LH models had the slightest deviation, the MVK model was the most reliable. The CatMAP software was also used to match the simulation deviation. This work demonstrated the Tl removal mechanism and provided insights into the accuracy of kinetic models on minor-radius heavy metal. Thus, this research may help promote the design of reactors, heavy metal removal rates, and flue gas purification technology selection.
The first part of this overview on functional materials for gas storage is mainly focused on environmental applications, which include the separation and capture of carbon dioxide and air purification where adsorbent materials play a decisive role to remove a large diversity of toxic compounds. The exploitation of fossil fuels has generated huge amounts of gaseous pollutants, namely, CO 2 , NO x , and countless volatile organic compounds ( VOC s), among others, which urge capture and reuse whenever possible. In many industrial processes, emission of harmful gases and vapors are common, which require their removal for personnel protection and odor control. This part of overview includes a brief discussion on the fundamentals of gases and vapors' adsorption. During the past decades, tens of thousands of porous solids have been prepared and evaluated as absorbents. To rationalize that huge volume of information, the available data are critically reviewed according to the nature of the guest–host interactions that determine the adsorption forces and the related adsorption heats. Carbon dioxide can be captured and stored by clathrate formation, which is also briefly discussed.
Increased gas adsorption in a series of post-synthetically modified metal-organic frameworks (MOFs) of the type HKUST-1 was achieved by the partial cation exchange process. Manipulation of post-synthetic conditions demonstrates high tunability in the site substitution and gas adsorption properties during the dynamic equilibrium process. In this work, post-synthetic modification of Cu3(BTC)2 is carried on by exposure to TM2+ solutions (TM = Mn, Fe, Co, Ni) at different time intervals. The crystal structure, composition, and morphology were studied by powder X-ray diffraction, Fourier-transform infrared spectroscopy, inductively coupled plasma optical emission spectroscopy, and scanning electron microscopy. Structural analysis supports the retention of the crystal structure and partial substitution of the Cu metal nodes within the framework. A linear increase in the transmetalation process is observed with Fe and Co with a maximum percentage of 39 and 18%, respectively. Conversely, relatively low cation exchange is observed with Mn having a maximum percentage of 2.40% and Ni with only 2.02%. Gas adsorption measurements and surface area analysis were determined for each species. Interestingly, (Cu/Mn)3(BTC)2 revealed the highest CO2 adsorption capacity of 5.47 mmol/g, compared to 3.08 mmol/g for Cu3(BTC)2. The overall increased gas adsorption can be attributed to the formation of defects in the crystal structure during the cation exchange process. These results demonstrate the outstanding potential of post-synthetic ion exchange as a general approach to fine-tuning the physical properties of existing MOF architectures.
Materials that simultaneously exhibit permanent porosity and high-temperature magnetic order could lead to advances in fundamental physics and numerous emerging technologies. Herein, we show that the archetypal molecule-based magnet and magnonic material V(TCNE)2 (TCNE = tetracyanoethylene) can be desolvated to generate a room-temperature microporous magnet. The solution-phase reaction of V(CO)6 with TCNE yields V(TCNE)2·0.95CH2Cl2, for which a characteristic temperature of T* = 646 K is estimated from a Bloch fit to variable-temperature magnetization data. Removal of the solvent under reduced pressure affords the activated compound V(TCNE)2, which exhibits a T* value of 590 K and permanent microporosity (Langmuir surface area of 850 m2/g). The porous structure of V(TCNE)2 is accessible to the small gas molecules H2, N2, O2, CO2, ethane, and ethylene. While V(TCNE)2 exhibits thermally activated electron transfer with O2, all the other studied gases engage in physisorption. The T* value of V(TCNE)2 is slightly modulated upon adsorption of H2 (T* = 583 K) or CO2 (T* = 596 K), while it decreases more significantly upon ethylene insertion (T* = 459 K). These results provide an initial demonstration of microporosity in a room-temperature magnet and highlight the possibility of further incorporation of small-molecule guests, potentially even molecular qubits, toward future applications.
In situ diffuse reflectance infrared fourier transform (DRIFT) spectroscopy was firstly developed to observe the hydrogen isotope separaion behaviors at active CuI sites within CuI-MFU-4l, and clear evidences of the...
Herein, novel carbons that, owing to a high density of micropores (up to 79%) and N‐content (up to 14.9%), offering exciting potential for post‐combustion CO2 capture are reported. Given that little is known about how starting materials impact the structure and performance of carbons, three different microporous materials are pyrolyzed. These include a Co‐(metal‐organic framework) (MOF), a Co‐MOF‐polymer composite, and a coordination polymer derived from the same monomer and cobalt ions. Notably, the cobalt, which is required to drive the polymerization, is subsequently leached from the carbons via acid for its reuse in MOF synthesis. Next, various metrics including CO2 capacity, selectivity, isosteric heat of adsorption, breakthrough time and cyclability are assessed. The acid treated carbons adsorb 0.21, 0.99, and 1.11 mmol CO2 g⁻¹, respectively, (313 K, 0.15 bar) with CO2/N2 selectivity ranging from 37 to 52. Due to superior capacity, the polymer‐derived carbons also reveal impressive breakthrough times in simulated flue gas mixtures (15% CO2/85% N2, 80% RH, 313 K) ranging from 33 to 40 min g⁻¹. Similar performance is also observed under dry conditions and after pre‐saturation with water for 1.5 h. Remarkably, no loss in working capacity is observed after 100 CO2 TSA cycles (313 K/393 K).
Catalysis has always been part of the development of mankind; from the fermentation of alcoholic drinks, through the development of fertilisers in the agricultural revolution and production of bulk chemicals in the 20th Century. Today, society demands improved production routes with greater product output and energy efficiency; the ultimate goal to achieving this would be having all catalytic reactions in concert, effectively functioning like a biological cell.
Metal organic frameworks (MOFs) are a relatively new type of hybrid material. Their crystalline porous structure, built up from organic and inorganic building blocks, presents a vast array of composition, porosity and functionality offering enormous potential in catalytic systems.
This book examines the latest research and discovery in the use of MOFs in catalysis, highlighting the extent to which these materials have been embraced by the community. Beyond presenting a digest of recent research by major players in the field, the book presents the strategies behind recent developments, providing a lasting reference for seasoned researchers and newcomers to the field.
Catalysis has always been part of the development of mankind; from the fermentation of alcoholic drinks, through the development of fertilisers in the agricultural revolution and production of bulk chemicals in the 20th Century. Today, society demands improved production routes with greater product output and energy efficiency; the ultimate goal to achieving this would be having all catalytic reactions in concert, effectively functioning like a biological cell.
Metal organic frameworks (MOFs) are a relatively new type of hybrid material. Their crystalline porous structure, built up from organic and inorganic building blocks, presents a vast array of composition, porosity and functionality offering enormous potential in catalytic systems.
This book examines the latest research and discovery in the use of MOFs in catalysis, highlighting the extent to which these materials have been embraced by the community. Beyond presenting a digest of recent research by major players in the field, the book presents the strategies behind recent developments, providing a lasting reference for seasoned researchers and newcomers to the field.
Catalysts for the oxidation of CH4 commonly suffer from poisoning by sulfur-containing molecules, such as H2S and SO2. This is a huge problem in real energy applications. As a possible solution, a material capable of selectively capturing the poisoning SO2 is necessary. In this work, water-stable MOF-303 shows high performance for SO2 capture at low pressures (6.21 mmol·g–1 at 298 K and 0.1 bar), ideal characteristics for the adsorption of trace amounts of SO2. Furthermore, MOF-303 showed high chemical stability toward dry and humid SO2 and a remarkable cycling performance with facile regeneration. In addition, MOF-303 displayed high selectivity toward SO2 over CH4 and CO2. Finally, fluorescence experiments proved that the SO2 detection properties of this MOF material are highly promising, even in the presence of CH4 and CO2.
Spectroscopy on metal–organic framework (MOF) films and the molecular phenomena associated to them is mostly limited to grazing incidence methods. To allow for transmission‐based characterization and highlight the applicability of MOFs on transparent substrates, UiO‐67, UU‐1, and ZIF‐8 MOFs are synthesized on CaF2 windows. It is revealed that the growth of the UiO‐67 MOF follows a Volmer–Weber mechanism using scanning electron microscopy (SEM). This growth is assisted by growing seeds in solution, which anchor on the film and become part of the intergrown film itself. UU‐1, a copper‐based MOF, is formed after spin coating the Cu‐BTC precursors, showing its characteristic fiber‐like morphology and resulting inter‐fiber macroporosity. ZIF‐8 is formed using a “flash”‐synthesis, and it is shown that this approach resembles a Volmer–Weber growth mode as well. Last, CO probe molecule adsorption FT‐IR spectroscopy is utilized to study the effect of methanol exposure. UiO‐67 becomes inaccessible toward CO, due to the formation of methoxy species, whereas UU‐1 undergoes a topotactic transformation to HKUST‐1. ZIF‐8 is the most stable as methanol only removes impurities from the framework. This approach opens the venue for other film materials to be studied in situ synthesis, sorption, or catalysis using transmission‐based spectroscopy. Metal–organic framework (MOF) films are a challenge to study in situ using vibrational spectroscopy. Here, films of three MOFs are grown on transparent CaF2 windows. The growth modes are studied, and their transparency allows for FT‐IR spectroscopy during CO and methanol exposure. This reveals methoxy species in UiO‐67, a topotactic transformation for UU‐1 and a cleaning effect on ZIF‐8.
This research presents the modification of MOF-199 and ZIF-8 using furfuryl alcohol (FA) as a carbon source to subsequently fix lipase from Pseudomonas cepacia and use these biocatalysts in the transesterification of African palm oil (APO). The need to overcome the disadvantages of free lipases in the biodiesel production process led to the use of metal organic framework (MOF)-type supports because they provide greater thermal stability and separation of the catalytic phase, thus improving the activity and efficiency in relation to the use of free lipase, disadvantages that could not be overcome with the use of other types of catalysts used in transesterification/esterification reactions for the production of biodiesel. The modification of MOFs ZIF-8 and MOF-199 with FA increases the pore volume which allows better immobilization of Pseudomonas cepacia lipase (PCL). The results show that these biocatalysts undergo transesterification with biodiesel yields above 90%. Additionally, studies were carried out on the effect of (1) enzyme loading, 2) enzyme immobilization time, (3) enzyme immobilization temperature, and (4) pH on the % immobilization of the enzyme and the specific activity. The results show that the highest immobilization efficiency for the FA@ZIF-8 support has a value of 91.2% when the load of this support was 3.5 mg/mg and has a specific activity of 142.5 U/g protein. The FA@MOF-199 support presented 80.3% enzyme immobilization and 125% U/g specific activity protein. We established that the specific activity increases in the period from 0.5 to 5.0 h for the systems under investigation. After this time, both the specific activity and the % efficiency of enzyme immobilization decrease. Therefore, 5.0 h (immobilization efficiency of 95 and 85% for FA@MOF-199, respectively) was chosen as the most appropriate time for PCL immobilization. Methods of adding methanol, with three and four steps, were tested, where biodiesel yields greater than 90% were obtained for the biocatalysts synthesized in this work (FA@ZIF-8-PCL and FA@MOF-199-PCL) and above 70% for free PCL, and the maximum yield was reached at a molar ratio between methanol and APO of 4:1 when using the one-step method under the same reaction conditions (as mentioned above). Only the results of FA@ZIF-8-PCL are presented here; however, it should be noted that the results for biocatalyst FA@MOF-199-PCL and lipase-free PCL presented the same behavior. The order of biocatalyst performance was FA@ZIF-8-PCL > FA@MOF-199-PCL > PCL-Free, which demonstrates that the use of FA as a modifier is a novel aspect in the conversion of palm oil into biodiesel components.
The CuxRh3-x(BTC)2catalyst (abbreviated CuRhBTC, BTC³⁻= benzene tricarboxylate) provides excellent dispersion of active metal sites coupled with well-defined, robust structures for propylene hydrogenation reactions. This material therefore serves as a unique prototype for understanding catalytic activity in metal organic frameworks (MOFs). The mechanism of gas-phase hydrogenation at the bimetallic metal nodes of a MOF has been investigated in detail for the first time using in situ spectroscopy and diffraction experiments combined with density functional theory (DFT) calculations. The reaction occurs via a cooperative process in which the metal and linker sites play complementary roles; specifically, H2is dissociated at a Rh²⁺site with a missing Rh-O bond, while protonation of the decoordinated carboxylate linker stabilizes the active sites and promotes H2dissociation. In situ X-ray diffraction experiments show that the crystalline structure of the MOF is retained under reaction conditions at 20-100 °C. In situ Raman spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments demonstrate that propylene adsorbs at both Rh²⁺and Cu²⁺sites via πbonding. Cu²⁺is catalytically inactive, but at Rh²⁺sites, a propyl intermediate is observed when H2is introduced into the propylene feed. Furthermore, the appearance of the O-H stretch of COOH at ∼3690 cm⁻¹in the DRIFT spectra is characteristic of defects consisting of missing Rh-O bonds. These experimental results are in general agreement with a reaction mechanism proposed by DFT, in which the decoordinated carboxylate linker is protonated, and the active Rh²⁺site remains available for readsorption of reactants in the subsequent catalytic cycle.
Effective separation of hydrogen isotopes still remains one of the extremely challenging tasks in industry. Compared to the present methods that are energy- and cost-intensive, quantum sieving technology based on nanostructured materials offers a more efficient alternative approach, where metal-organic frameworks (MOFs) featuring open metal sites (OMS) can serve as an ideal platform. Herein, a combination of periodic density functional theory (DFT) with dispersive correction and high-throughput molecular simulation was employed from thermodynamic viewpoints to explore the D2/H2 separation properties of 929 experimental MOFs bearing a copper-paddlewheel unit. The DFT calculations showed that there is a negligible rotational energy barrier for the molecule adsorbed at the OMS, and the movement of the Cu atoms along the Cu-Cu axis vector almost has no influence on the interaction energy. On the basis of the DFT results, a new force field with a proposed cutoff scheme was developed to accurately describe the strong isotope-OMS interaction. Under practical conditions (40 K and 1.0 bar), large-scale computational material screening demonstrated that the OMS interaction plays a more important role in highly selective materials and ignoring such interactions can lead to completely wrong identification of the most promising materials. Using the adsorption selectivity and adsorbent performance score as evaluation metrics, this work demonstrated that the materials with sql topology notably outperform many benchmark adsorbents reported so far.
“We shape clay into a pot, but it is the emptiness inside that holds whatever we want.” This old Chinese proverb underlines the importance of porous materials, that is, creating materials that possess empty spaces within. Naturally occurring porous clay materials “Zeolite” have been known to us for centuries. Both natural zeolites and man-made aluminosilicate (often called “zeotype”) find applications in petrochemical industry as heterogeneous catalyst in the last century. Apart from this, zeolites are often used in gas separation, desiccant, detergent builder, etc. With large internal surface area, hydrothermal stability zeolites have channels, channel intersections, and/or cages that attracted chemists for utilization in industry. Examples are faujasite, mordenite, offretite, ferrierite, chabazite, etc. All these and related synthetic microporous materials containing titanium, vanadium, cobalt, etc. are of considerable interest in catalysis and routinely dealt with at the International Zeolite Association.
The degree of pore filling is an important parameter for defining guest@MOF properties in applications including electronics, optics, and gas separation. However, the interplay of key aspects of host‐guest interactions, such as a quantitative description of the guest alignment or the structural integrity of the host as function of pore filling are yet to be determined. Polarisation‐dependent infrared spectroscopy in attenuated total reflection configuration combined with gas sorption allowed to simultaneously study the orientation of the guest molecule and structural changes of the MOF framework during the pore filling process. Thereby we found, that initially randomly oriented guest molecules align with increasing pore filling during adsorption from the gas phase. At the same time, the framework itself undergoes a reversible, guest molecule‐dependent rotation of the aromatic linker and a linker detachment process, which induce defects.
We experimentally and theoretically investigate the thermal conductivity and mechanical properties of polycrystalline HKUST-1 metal-organic frameworks (MOFs) infiltrated with three guest molecules: tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), and (cyclohexane-1,4-diylidene)dimalononitrile (H4-TCNQ). This allows for modification of the interaction strength between the guest and host, presenting an opportunity to study the fundamental atomic scale mechanisms of how guest molecules impact the thermal conductivity of large unit cell porous crystals. The thermal conductivities of the guest@MOF systems decrease significantly, by on average a factor of 4, for all infiltrated samples as compared to the uninfiltrated, pristine HKUST-1. This reduction in thermal conductivity goes in tandem with an increase in density of 38% and corresponding increase in heat capacity of ∼48%, defying conventional effective medium scaling of thermal properties of porous materials. We explore the origin of this reduction by experimentally investigating the guest molecules' effects on the mechanical properties of the MOF and performing atomistic simulations to elucidate the roles of the mass and bonding environments on thermal conductivity. The reduction in thermal conductivity can be ascribed to an increase in vibrational scattering introduced by extrinsic guest-MOF collisions as well as guest molecule-induced modifications to the intrinsic vibrational structure of the MOF in the form of hybridization of low frequency modes that is concomitant with an enhanced population of localized modes. The concentration of localized modes and resulting reduction in thermal conductivity do not seem to be significantly affected by the mass or bonding strength of the guest species.
Nitrogen oxide (NOx) is a family of poisonous and highly reactive gases formed when fuel is burned at high temperatures during anthropogenic behavior. It is a strong oxidizing agent that significantly contributes to the ozone and smog in the atmosphere. Thus, NOx removal is important for the ecological environment upon which the civilization depends. In recent decades, metal–organic frameworks (MOFs) have been regarded as ideal candidates to address these issues because they form a reticular structure between proper inorganic and organic constituents with ultrahigh porosity and high internal surface area. These characteristics render them chemically adaptable for NOx adsorption, separation, sensing, and catalysis. In additional, MOFs enable potential nitric oxide (NO) delivery for the signaling of molecular NO in the human body. Herein, the different advantages of MOFs for coping with current environmental burdens and improving the habitable environment of humans on the basis of NOx adsorption are reviewed.
Natural gas is major resource in UAE, constitute about 90% methane and as compared
to other fossil fuels, it is more environmentally friendly. Energy demand from natural
gas can be projected to exceed two hundred exajoules per year in 2040. In the UAE,
many natural gas filling stations are already built for utilizing natural gas as a vehicle
transportation fuel where these materials have potential applications to store and
deliver this fuel. This research aims to study various kinds of Metal-Organic
Frameworks, and to investigate adsorption properties for the storage of natural gas and
its delivery. MOFs possess porous material that exhibits a high deliverable capacity of
gases. These are synthesized using strategies such as crystal engineering with varying
organic groups such as linker length and hydrophilicity, pore shape, and phase
changes. The main challenges in designing MOFs for methane storage are
understanding the mechanical properties, developing thermal management solutions,
and the effect of impurities on the working capacity as well as the manufacturing costs
of MOFs. This thesis gives pathway to tackle such problems. Overall, HKUST-1
showed promising results for the various MOFs tested. Synthesis and characterization
were done by scanning electron microscope, thermogravimetric analysis, X-ray
diffraction, and nitrogen adsorption. Adsorption process, reaction heat, and total heat
involved in the process were studied using Tian-Calvet calorimeter and gas
chromatography. A significant part of this research was dedicated to designing and
setting up the calorimeter used in obtaining the heat of adsorption. Moreover, the
adsorption properties and separation of the gaseous mixture are also studied using the
gas chromatography with some equipment modifications. Designing of MOFs, a class
of adsorbents, is described considering the thermodynamics of adsorption of these
porous materials for natural gas and methane storage. The thermodynamics of
adsorption governs the adsorption isotherm and, therefore the deliverable capacity of
stored natural gas and methane. Calorimetric and gas chromatography studies
indicated that HKUST-1 has the best adsorbent among the tested MOFs
The continuous carbon dioxide (CO2) gas emissions associated with fossil fuel production, valorization, and utilization are serious challenges to the global environment. Therefore, several developments of CO2 capture, separation, transportation, storage, and valorization have been explored. Consequently, we documented a comprehensive review of the most advanced strategies adopted in metal‐organic frameworks (MOFs) for CO2 capture and separation. The enhancements in CO2 capture and separation are generally achieved due to the chemistry of MOFs by controlling pore window, pore size, open‐metal sites, acidity, chemical doping, post or pre‐synthetic modifications. The chemistry of defects engineering, breathing in MOFs, functionalization in MOFs, hydrophobicity, and topology are the salient advanced strategies, recently reported in MOFs for CO2 capture and separation. Therefore, this review summarizes MOF materials′ advancement explaining different strategies and their role in the CO2 mitigations. The study also provided useful insights into key areas for further investigations. In this review, an extensive literature survey has been covered to comprehensively summarize the strategies adopted in MOFs for CO2 capture and separation.
Silicon-based nanosheets (SNS) were synthesised via a mild (60 °C) and time-saving (8 h) modified topochemical method. Then, Cu3(BTC)2 and [email protected]3(BTC)2 were successfully synthesised by microwave irradiation, and their characteristics and hydrogen storage performance were analysed by multiple techniques. The accordion-like SNS exhibited void spaces, a unique low buckled structure, and ultrathin, almost transparent, loosely stacked layers with a high specific surface area (362 m²/g). After in-situ synthesis with Cu3(BTC)2, the SNS compound achieved a high specific surface area (1526 m²/g), outstanding hydrogen storage performance (5.6 wt%), and a desirable hydrogen diffusion coefficient (10⁻⁷). Thus, SNS doping improved the hydrogen storage performance of Cu3(BTC)2 by 64% through electron transfer reactions with Cu enabled by the unique composite nanostructure of [email protected]3(BTC)2. This study presents a promising method of synthesising SNS and porous composite materials for hydrogen storage.
“Defect” in materials is an exciting concept that can be also appeared in metal‐organic frameworks. This property can be produced inherently or intentionally and it can provide new opportunities in materials for different applications. Different kinds of defects can produce heterogeneous structures with various degrees of complexity and chemical‐physical properties. So, in this chapter, we discuss the generated heterogeneity from structural disorders and defects in MOFs and its effects. It should be said that defects do not necessarily have adverse effects.
An important problem when studying the interaction between a CO probe molecule and a Na4Ca4A type zeolite is the estimation of the central repulsive coefficients versus the internuclear distance of CO. In particular, this dependence cannot be estimated in the case of the unstable linear “framework oxygen–CO molecule” pair due to the electrostatic repulsive interaction. Hence, we discuss the application of two approximate forms of this dependence either allowing or disregarding the repulsive contribution in the interval wherein the vibrational CO probability distribution cannot be neglected. The consequences of these approximations are compared through calculation of the interaction energy and band shift of CO adsorbed inside Na4Ca4A. The CO spatial parameters (semi-axes) are estimated by fitting both the band shift, corresponding to two different positions of CO relative to the zeolite, and the
interaction energy values to the experimental data obtained at small coverage.
The physisorption energy of molecular hydrogen (H-2) on flat carbon nanoparticles (graphitic platelets) and polycyclic aromatic hydrocarbons (PAHs) is determined to be attractive between 3.5 and 7.2 kJ mol(-1), depending on the orientation of H-2 and on the particle size. Entropy, estimated from experimental data, reduces the interaction energy by 3.4 kJ mol(-1) at room temperature. Therefore, nanostructured graphitic platelets might be suitable for hydrogen storage. Computations have been carried out for PAHs from benzene to coronene using second order Moller-Plesset (MP2) theory at the basis set limit, and the results are extrapolated to graphene layers.
The interaction energy and band shift values of the fundamental vibrational modes of CO and N2 adsorbed within the NaY and NaRbY zeolite forms are calculated using a pair-wise addition scheme. A clear difference between the most stable positions of CO and N2 in NaY and NaRbY is observed. While, for NaY, both diatomic probes are close to the alkali-metal ion, this is not the case for NaRbY. For CO, the isolated Rb+–CO pair-wise interaction cannot describe the system, because the Rb–CO and O–CO distances are comparable whereas for N2 within NaRbY, the molecule is closer to the framework oxygens than to the Rb. The influence of both molecular and zeolite framework parameters (ionic charges, polarizabilities, and radius) on the interaction energy and band shift values is definitively proven.
M+···CO adducts (M = Li, Na, K, Rb, Cs) formed upon CO adsorption on alkali-metal exchanged zeolites show IR spectra characterized by a main (cation specific) IR absorption band which appears in the range 2150-2180 cm-1, and which is due to the fundamental C-O stretching mode (v(CO)). However, a weaker band is often observed some 90-140 cm-1 higher than the main band. This weaker band can be assigned to the combination mode v(CO) + v(MC) where v(MC) represents the frequency of the cation-carbon bond vibration. This assignment is supported by direct observation of the corresponding band (at 139 cm-1) for CO adsorbed on Na-Y, which was detected by using far IR radiation from a synchrotron source. Observation of the combination mode is relevant to zeolite characterization by IR spectroscopy, since the characteristic cation-carbon stretching vibration is very sensitive to the specific cation present in the zeolite.
Grand canonical Monte Carlo simulations in conjunction with high-resolution low-pressure argon adsorption experiments were employed to study adsorption mechanisms on the copper(II) benzene-1,3,5-tricarboxylate metal-organic framework (Cu−BTC). We constructed a molecular structural model of Cu−BTC. The pore network of Cu−BTC has a simple cubic symmetry. It consists of main channels of a square cross-section of ca. 0.9 nm diameter and tetrahedral side pockets of ca. 0.5 nm, which are connected to the main channels by triangular windows of ca. 0.35 nm diameter. Using a parameterized united-atom force field, we have determined the preferential adsorption sites and the sequence of adsorption mechanisms from a gradual filling of the side pockets to a stepwise adsorption and condensation in the main channels. The simulation results agree quantitatively with the experimental isotherm of argon up to almost complete filling of the pore network.
The unique properties of the neutron, which is a charge neutral subatomic particle, make it a nearly ideal particle for scattering investigations of materials. The enormous utility of the neutron in studies of dynamical phenomena stems from the ability to measure the momentum and energy transferred during the neutron scattering event simultaneously, thereby providing the experimentalist with information in both the spatial and time domains, respectively. Thus not only are neutron inelastic scattering techniques sensitive to the frequencies associated with a fluctuating object, they are also sensitive to the structure or geometry of the excitation. Advances in neutron scattering instrumentation along with new sources are providing exciting new opportunities to study increasingly complex issues in materials research. Here we provide a basic introduction into the properties of the neutron that make it a nearly ideal probe of materials. We then describe recent examples that demonstrate how these properties have been exploited to study the dynamics of hydrogen and water in a variety of materials.
This review describes methods of preparing hybrid inorganic–organic mesoporous silicates with uniform channel structures, as well as some of their applications. Both reactive and passive organic groups can be incorporated in the porous solids by grafting methods or by co-condensation under surfactant control. Functional groups have been placed selectively on the internal or external pore surfaces or even within the walls of the mesoporous solids. Organic functionalization of these solids permits tuning of the surface properties (hydrophilicity, hydrophobicity, binding to guest molecules), alteration of the surface reactivity, protection of the surface from attack, and modification of the bulk properties (e.g., mechanical or optical properties) of the material. Recent applications of hybrid mesoporous silicates are highlighted, including catalysis, sorption of metals, anions, and organics, reactors for polymerization, fixation of biologically active species, and optical applications.
The adsorption of hydrogen on H–Y and Pd(0)–H–Y zeolites was investigated by IR (in the 300–20 K temperature range) and EXAFS spectroscopies and the energy barriers for diffusion of hydrogen in the internal voids of the faujasite structure evaluated by ab initio calculations. On H–Y, H2 is adsorbed at 20 K inside the super-cages giving rise to 1:1 –OH⋯H2 hydrogen bonded complexes with the Brønsted acid groups, as well as less specific interaction with the cage walls. Due to the high energy barrier for crossing the framework hexagonal apertures, H2 cannot enter the sodalite-cages. On Pd(0) containing zeolites, characterized as far the dispersion of the metal is concerned by TEM, EXAFS and IR spectroscopy of adsorbed CO, hydrogen is adsorbed in atomic form to give a metal-hydride phase; subsequent spill-over effects allow confinement of hydrogen also in the small sodalite-cages, as demonstrated by H/D isotopic substitution experiments.
The pure rotational lines and the Q branch lines of the 1–0 vibrational bands of H2, HD, and D2 have been photographed with a 21 ft. grating spectrograph. From these spectra, a complete set of constants for the ν = 0 and 1 levels of all three molecules have been determined. When these constants are combined with Herzberg's results of the forbidden infrared spectra of H2 and HD they lead to improved values of the electronic ground state constants of H2 and HD. The leading terms in the Dunham power series expansion of the potential are calculated for H2 and HD and are found to be mutually consistent. The isotopic relations are obeyed within experimental accuracy, and the small constants De, Be, and He, are in agreement with values given by theoretical formulae.
Extraframework cation sites in the sodium forms of the zeolites ZSM-5, mordenite, Linde-4A and faujasite-type X and Y have been investigated by using low-temperature adsorption of dihydrogen and carbon monoxide as IR spectroscopic probes. The extent of H—H and C—O bond polarization was found to be dependent not only on the cation electrostatic field, but also on the neighbouring oxygen atoms of the zeolite framework. The influence of these oxygen atoms is most keenly felt by adsorbed molecular hydrogen, but they also affect the IR frequency shift of the stretching vibration of adsorbed carbon monoxide. The Si : Al ratio of the zeolite framework modulates the basic strength of the oxygen atoms, and this was found to be reflected in the IR stretching frequency of both adsorbed molecules, H2 and CO.
Cationic forms of different zeolites have been studied by diffuse reflectance infrared spectroscopy using low-temperature adsorption of molecular hydrogen as a probe. The frequency of the stretching vibrations of the H—H bond was found to be strongly dependent on the charge of the cations. For alkaline-earth-metal forms of high-silica zeolites the frequency shifts correlated with the polarizing ability (e/r) of the cations. The influence of different factors (hydrolysis of cations and their localization) on hydrogen polarization is also discussed.
The localization sites for univalent cations in cationic forms of zeolites (A, X, Y, ZSM-5 and mordenite) have been investigated using the low-temperature adsorption of molecular hydrogen as a spectroscopic test. The extent of H—H bond polarization, estimated from the low-frequency shift of the fundamental stretching vibration of perturbed H2 molecules, was found to be strongly dependent on the basicity of the neighbouring oxygen atoms of the framework.
Correlations between the adsorption heats of carbon monoxide on aprotic acidic centers and the Δv
CO values in IR spectra have been studied. In the case of aprotic centers formed by cations of non-transition metals, Δv
CO is shown to be linearly dependent on the adsorption heat.
A crystalline and thermally stable metal-organic framework (MOF) with PtII and GdIII sites incorporated in the structure has been recently reported. This material has been synthesized with the aim to develop a heterogeneous PtII counterpart to homogeneous metal-organic complexes having C−H activating properties. The first account focused on the MOF synthesis and on structural and stability characterization of the material. In the present work, a multitechnique approach has been adopted to investigate the effect of solvent removal and the reversibility of this process. Structural features have been investigated by means of powder X-ray diffraction and extended X-ray absorption fine structure spectroscopy at both Pt and Gd L3-edges. Electronic properties have been studied with diffuse reflectance surface UV−vis and X-ray absorption near-edge spectroscopies. Finally, IR spectroscopy has been used to determine the vibration properties. Thermogravimetric methods have been used to quantify the water loss. X-ray absorption spectroscopy has been used to compare the Pt environment in the periodic MOF structure, in related molecular complexes, and in the linkers. Our results demonstrate that the environment around Pt is more or less unaffected by the incorporation of the Pt centers in the molecular complexes into the 3D extended framework of the Pt−Gd MOF. The principle of using known homogeneous complexes as building blocks for the construction of single-site heterogeneous catalysts therefore seems applicable in the present case. Removal of solvent water molecules from the internal voids of the as-prepared MOF presents an opportunity to attain a porous material with accessible PtII sites. We observe that the structure undergoes a reversible loss of long-range order upon dehydration at ambient temperature. The environment of Gd is somewhat perturbed in the dehydration/hydration process, while that of Pt is almost unaffected. When a total dehydration is achieved, the original structure is only partially recovered upon rehydration.
Carbon monoxide adsorption at 77 K on zeolites NaX, NaY, NaZSM-5 and HZSM-5 was studied by Fourier transform IR spectrometry. All samples showed strong absorption in the 2160–2180 cm−1 range, which is ascribed to the stretching mode of the C-O bond in carbon monoxide molecules, polarized by Na+ or H+ ions inside the zeolite supercage (faujasites) or channels (ZSM-5). Further, a complex band was observed at 2138 cm−1 and was assigned to liquid-like carbon monoxide. A computer band-fitting method was applied to analyse these bands. Some other minor IR absorptions were also observed and assigned to different adsorbed species.
Ion-exchange of ZSM-5-type zeolites with copper ion was carried out by three different methods to obtain information on the states and roles of the effective sites for NO decomposition and N2 adsorption activity of copper-ion-exchanged ZSM-5-type zeolites (CuZSM-5). The first method of preparation is chemical vapour deposition (CVD) using bis(1,1,1,5,5,5-hexafluoroacetylacetonato)copper(II), [Cu(hfac)2], as a volatile complex, and the second one is to utilize CuCl as a vaporizing source. These were compared with the third method, that is, the ordinary ion exchange method using an aqueous solution of CuCl2. It is apparent that in the first method the Cu2+ species deposited on ZSM-5 as [Cu(hfac)2]
(i.e., via the hydrogen bonding between a Brønsted acid site and a pseudo-aromatic ring of ligands) is reduced to the monovalent species (Cu+) by evacuation at 573 K, and also that the reducibility of Cu2+ is superior to that in the sample prepared by the conventional ion-exchange in an aqueous solution. As for the sample prepared by using CuCl, Cu+ deposited as CuCl was exchanged with H+ on a Brønsted acid site in the HZSM-5 sample through treatment at temperatures above 573 K with a release of HCl. The CuZSM-5 sample prepared by the CVD method gave a single IR band at 2159 cm−1 due to the adsorbed CO species, while the sample prepared by evaporation of CuCl at 573 K and also the sample ion-exchanged in an aqueous solution gave a broad band at around 2155 cm−1
(composed of two bands at 2159 and 2151 cm−1) for the adsorbed CO species, indicating the existence of at least two dominant types of exchangeable sites in these CuZSM-5 samples. Each sample reveals different features for NO decomposition reactivity and N2 adsorption, and such behaviours are explained by the difference in ratio of the respective sites occupied by copper ions. As a result, it was clearly demonstrated that the simultaneous existence of two types of sites lying close together has important implications for the catalytic activity for NO decomposition by CuZSM-5, and that the site giving the 2151 cm−1 band is the effective site for N2 adsorption.
Dihydrogen, dinitrogen, carbon monoxide and nitric oxide have been adsorbed, at nominally liquid nitrogen temperature, on Na+- and K+-exchanged ETS-10. IR spectroscopy shows formation of M+···(H2), M+···(N2)n, M+···(CO)n and M+···(NO) (n=1, 2, . . . ; M+=Na, K) adducts prevalently involving alkali-metal cations located in the 12-membered channels. These adducts give main IR absorption bands in the range 4050–4150 cm-1 for H2, 2331–2333 cm-1 for N2, 2148–2176 cm-1 for CO, and 1820–1900 cm-1 for NO, which are assigned to the fundamental stretching mode of the diatomic molecules polarized by the electric field created by the metal ions. On Na-exchanged samples, the Na+···(N2) and Na+···(CO) species, formed at lowest dosage, evolve into Na+···(N2)n and Na+···(CO)n (n=2, 3) species upon increasing the gas phase pressure. This reversible "‘solvation’' process is not observed for K-exchanged samples. For adsorbed CO, the high intensity of the IR spectra allowed us to observe and assign overtone bands and combination modes. This result does not find a comparable precedent for CO adsorbed on other zeolites. From bands corresponding to the combination of metal–carbon with carbon–oxygen fundamental stretching modes, the wavenumber values corresponding to metal–carbon stretching vibrations in Na+···CO and K+···CO were deduced: 122 and 107 cm-1, respectively. Due to the strong tendency of NO towards dimerization, IR spectroscopy of adsorbed NO is complicated by the manifestations of cis and trans dimers interacting with alkali cations, and Na+···(NO) and K+···(NO) species could be distinctly observed only at the very initial stages of the adsorption process. Finally, as far as the H2/ETS-10 system is concerned, the most relevant result is that the ortho and para forms are clearly detected in the Na+-ETS-10 sample.
CO adsorption at ambient temperature leads to formation of the well-known Cu+–CO species (2158 cm−1) and Cu+(CO)2 dicarbonyls (2177 and 2150 cm−1). In addition, Cu2+–CO complexes (2179 cm−1) appear under equilibrium CO pressure and are very easily destroyed by evacuation. The CO stretching frequency almost coincides in position with νs of the Cu+(CO)2 species. Lowering the temperature in the presence of gas-phase CO leads to formation of Cu2+(CO)2 geminal dicarbonyls (2170 cm−1) and a small amount of Cu+(CO)3 tricarbonyls (evidenced by a band at 2191 cm−1). The results demonstrate the high coordinative unsaturation of both Cu+ and Cu2+ ions in Cu-ZSM-5.
IR spectroscopy of adsorbed probe molecules is one of the most powerful characterization techniques for the investigation of surface active sites on high surface area materials like oxides and zeolites. In the last 20 years the use of specific IR cells allowing the in situ sample activation, gas dosage and sample cooling down to liquid nitrogen temperature has remarkably improved the number and the quality of the information on the surface structure with respect to the first experiments carried out at room temperature. Commercial cryostats able to reach liquid helium temperatures are available since decades, but the incompatibility of the materials used to reach and confine very low temperatures with the high temperatures usually needed to activate the surfaces of catalysts has prevented for long time the breaking down of the 77K frontier in IR experiments of species adsorbed on active surface sites. In our group we have very recently designed, realized and tested a new experimental set-up able to perform IR experiments in the 15–300K interval on samples previously activated under vacuum conditions (P
After a brief overview of the reasons why, in spite of the high fraction of framework Ti(IV) atoms, Engelhard titanosilicate (ETS-10) cannot be used as competitive catalyst in partial oxidation reactions, we draw the attention on the fact that the high cation density of ETS-10 can be the key property for potential new catalytic applications of this recent material. Among all, cation exchange with Cu2+ can yield to Cu-ETS-10, a promising material for environmental catalysis. We so present a detailed characterization of this material using N2, CO and NO as probe molecules. In spite of the rather high complexity of the obtained spectra, a comparison with similar experiments (described in the literature or ad hoc performed for this work) on other Cu-exchanged zeolites and on Cu2O dispersed on silica and on MCM-41, allows a full interpretation of the spectroscopic properties. It is shown that copper is present both as counterion and in the form of Cu2O nanoclusters dispersed in the ETS-10 channels and in the external surface. Finally, IR spectroscopy has been used to demonstrate that Cu-ETS-10 is active in the decomposition of NO.
A comparative FTIR spectroscopic study of CO adsorption on Cu2O and on silica-supported Cu2O microcrystals has been carried out in order to investigate the relations between the IR spectrum of adsorbed CO and the morphology of the particles (as obtained by SEM and TEM analyses). Modelling of the crystal morphology was also performed by means of atomistic simulations, and the results were compared with the electron micrographs. On both samples, CO adsorption occurs on Cu(I) ions located on the predominant and unpolar (111) faces. The simultaneous presence in the IR spectra of adsorbed CO of two components at 2158 and 2162cm−1 (weak and fully reversible) and at 2127 and 2132cm−1 (strong and quite irreversible), which shift downward of about Δν̄≅−8–10cm−1 and Δν̄≅−12–14cm−1, respectively, as a consequence of the progressive building up of lateral interactions within the bidimensional phase, is explained in terms of two alternative models. The first model explains the presence of the two IR bands in terms of CO adsorption on the two different Cu(I) sites (mono and bicoordinated) present on the (111) face of Cu2O. Following the second model, the observed IR doublet is on the contrary associated with the formation of Cu(I)(CO)2 species on monocoordinated Cu(I) sites. On the basis of the calculated electric field at the two surface sites of the relaxed and unrelaxed (111) faces, the second model seems to be more appropriate.
A comparison of the DRIFT spectra of conventional hydrogen (with a ratio of to equal to 3) and those of the o-H 2 p-H 2 para-enriched hydrogen adsorbed at 77 K on LiX and CsX zeolites, con—rms the earlier assignment of the weak high-frequency bands to the vibrationrotation transitions. The results obtained indicate almost free rotation of the hydrogen adsorbed on CsX, but substantially hindered rotation for hydrogen adsorption on the LiX form. However, the adsorption isotherms for LiX, NaX and CsX zeolites are almost identical indicating close lying values of heats of adsorption. To explain this discrepancy, it is suggested that the interaction of the adsorbed hydrogen with the cations provides only a part of the adsorption energy, to which the interaction with the basic oxygen of the zeolite framework also substantially contributes. The frequency of the oscillations of the adsorbed hydrogen relative to the adsorption sites, as obtained from the positions of the satellites in the DRIFT spectra, is used to discuss the motion of the hydrogen inside the zeolite micropores.
By means of variable-temperature FTIR spectroscopy, the standard adsorption enthalpy of hydrogen on the zeolite (Mg,Na)-Y was found to be ΔH°
=
−17.5 kJ mol−1, which suggests that magnesium-containing porous materials are good candidates in the search for suitable adsorbents for reversible hydrogen storage.
Direct ion-exchange of monovalent copper ions into a ZSM-5-type zeolite was carried out using an aqueous solution of diammine–copper(I) ions, [Cu(NH3)2]+, to prepare a copper ion-exchanged ZSM-5 zeolite including only monovalent copper ions, and the effect of monovalent copper ions on the zeolite's adsorption properties for dinitrogen (N2) was examined. Strangely enough, the reoxidation of monovalent copper-ion exchanged in ZSM-5 took place in the evacuation process at around 473 K. The changes in valence and structure of the exchanged copper-ions during the evacuation process and the interaction with N2 molecules at room temperature have been investigated by using spectroscopic techniques such as X-ray absorption fine structure (XAFS), IR and photoemission spectroscopy, as well as by measurements of adsorption isotherms and adsorption heats. On the basis of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses, it has become apparent that the Cu+ species exchanged in ZSM-5 zeolite is oxidized by water to form a divalent species having a CuO-like structure through heat-treatment in vacuo at 473 K. Further heat-treatment at temperatures above 673 K caused a reduction of the divalent species to a monovalent one that exhibits a pronounced adsorption feature for N2 even at room temperature. XANES and photoemission data clearly indicated that the Cu+ species in an 873 K-treated CuZSM-5 sample has a three-coordinate structure with lattice oxygen atoms and interacts strongly with an N2 molecule at room temperature. The strong interaction with N2 was also verified through the adsorption heat and IR data: an initial adsorption energy of 85 kJ mol−1 and an absorption band at 2295 cm−1. A prominent feature of this system is that some of the adsorbed species survives after evacuation at 300 K, indicative of a strong interaction between N2 and the three-coordinate copper ion.
This review aims to highlight the most important recent advances in the area of anion-templated syntheses in supramolecular and coordination chemistry. We published a comprehensive review on this area in 2003 and hence examples prior to this date will only be discussed when essential for clarity of presentation. The current review has been divided into three main sections: (a) anion-templated synthesis of systems with well-defined molecular weights; this includes macrocycles and cages, interlocked species (such as catenanes and rotaxanes), helical assemblies and other selected examples. (b) Anions as templates in polymeric systems; this includes metal-organic frameworks, molecularly imprinted polymers and other selected examples, such as liquid crystalline materials. (c) Anion templates in dynamic combinatorial libraries.
With this review on metal ion containing coordination polymer networks, we wish to highlight the current research in the field by giving a short overview on the concept of coordination polymers networks, how and why they are made. Several reviews on different aspects of coordination polymer compounds will be grouped and shortly presented in the introduction. Recent typical examples for different dimensionalities of the networks will be presented as well as representative compounds for specific applications. One focus will also be the problematic of polymorphism, respectively, supramolecular isomerism and pseudo-polymorphism or solvates. With a view to the large number of polymer coordination compounds that can be found in the literature, we will limit the review to the more recent results in the field and restrain the examples to the more common O-donor and mainly N-donor ligands. As our own interests lie in the field of group 11 elements, some focuses are more related with this field.The expression “Coordination Polymer” was first used by J.C. Bailar in 1967, when he compared organic polymers with inorganic compounds which can be considered as polymeric species. In comparison he established rules for the building and the required properties of new species involving metal ions and organic ligands [J.C. Bailar Jr., Prep. Inorg. React. 1 (1964) 1]. During the last 15 years the number of publications concerning coordination polymers has dramatically increased from 100 articles per year to 1000 in 2004. What really are coordination polymers? Why do these huge developments happen?
The adsorption of carbon dioxide on sodium- and magnesium-exchanged ETS-10 molecular sieves was investigated by FTIR spectroscopy. CO2 was found to adsorb reversibly onto the charge-balancing cations with formation of linear end-on adducts of the type M...OCO (with M = Na+ or Mg2+). Such adducts are characterised by intense nu(3) bands at 2353 and 2365 cm(-1), for CO2 interacting with Na+ and Mg2+, respectively. More interestingly, carbon dioxide was also found to form different carbonate-like species upon adsorption on the Na-ETS-10 (and, to a minor extent, also on Mg-ETS-10), which are the origin of the activity of ETS-10 in heterogeneous base-catalysed processes.
that is as yet unobserved in the well established chemistry of porous zeolitic oxides. The versatility of organic functional groups and the diversity of metal co-ordination geometries coupled with the ability to exploit weak inter- molecular forces such as p-p interactions and hydrogen bond- ing have allowed access to porous co-ordination solids that are capable of size-, shape- and electronic-selective binding. Specif- ically, this has been illustrated through the systematic study of the principles governing the assembly of metal(ii) ions and 1,3,5-benzenetricarboxylate (BTC), a, into one-, two- and three-dimensional porous networks. 3 In our continuing efforts to uncover the parameters regulating the supramolecular assembly of co-ordination porous solids, we chose to focus on cis,cis-1,3,5-cyclohexanetricarboxylate (CTC), b, as a building block. The specific choice of b was motivated by its structural similarity to BTC, its virtually unexplored assembly reactions into extended co-ordination networks, and the utility of deriv- atives in designing molecular recognition host systems. 4 This
Although zeolites and related materials combine nanoporosity with high thermal stability, they are difficult to modify or
derivatize in a systematic way. A highly porous metal coordination polymer [Cu3(TMA)2(H2O)3]n(where TMA is benzene-1,3,5-tricarboxylate) was formed in 80 percent yield. It has interconnected [Cu2(O2CR)4] units (where R is an aromatic ring), which create a three-dimensional system of channels with a pore size of 1 nanometer
and an accessible porosity of about 40 percent in the solid. Unlike zeolites, the channel linings can be chemically functionalized;
for example, the aqua ligands can be replaced by pyridines. Thermal gravimetric analysis and high-temperature single-crystal
diffractometry indicate that the framework is stable up to 240°C.
The C–O stretching frequencies, obtained by a 77 K Fourier transform infrared study of the interaction of CO with the extraframework cations in alkali–metal exchanged M‐ZSM‐5 and M‐mordenites (M=Na,K,Rb,Cs) have been used to evaluate the local electric field strength tested by the probe molecule. For all the investigated zeolites, the dipolar CO molecule was found to interact specifically with extra‐framework metal ions, and its stretching frequency was found to increase continuously from Cs+ to Na+ samples. The observed frequency shifts, with respect to that of the free CO molecule (νCO=2143 cm−1), were used to evaluate the electrostatic field created by the cation sites. Electric fields in the range of 2 to 6 V nm−1 were obtained; experimental values are inferior to those expected if generated in vacuo, at the distance of the probe molecule, from a point charge of +‖e‖. The observed fields can be reproduced by the sum of a positive contribution of the cation itself, and by two negative contributions, the first due to the negatively charged oxygen atoms surrounding the cation and the second due to the polarized zeolite framework. The last contribution has been found to increase from ZSM‐5 to Mordenite, in agreement with the decreasing Si/Al ratio. © 1995 American Institute of Physics.
A thorough vibrational characterization of CO2 molecules adsorbed at room temperature on alkali metal exchanged M–ZSM-5 zeolites (M = Li+, Na+, K+, and Cs+) has been obtained: All three fundamental modes (ν1, ν2, and ν3) have been measured in the mid-IR, together with combination bands of the ν1 and ν3 modes with modes at 18, 45, and 60 cm−1. The nature of these low-lying modes is discussed, and it is proposed that these are framework vibrations. Ab initio calculations on molecular models mimicking the building blocks of zeolites revealed the systematic presence of such low-lying modes, and far-IR measurements using synchrotron radiation showed a couple of bands at 55–45 cm−1 in the spectrum of the bare samples, which undergo a slight perturbation upon CO2 adsorption. The presence of combination modes between molecular and framework vibrations lends support to the concept already advanced [Bonelli et al., J. Phys. Chem. B 104, 10978 (2000)] that the CO2 molecule has a primary interaction with the cation and a secondary one with an adjacent oxygen anion; the latter not strong enough to lead to a carbonate species. The occurrence of similar combination bands for CO2 molecularly adsorbed on other systems is also discussed. © 2002 American Institute of Physics.
The infrared analysis of the induced bands of molecular hydrogen isotopes, adsorbed in NaA zeolite, shows several features; (i) when the amount of adsorbed molecules increases, the bands become complex, showing three components, which can be related to different energetic situation of the molecules in the cavity; (ii) the band frequency is lower than the gas frequency; for each species the shifts are proportional to the inverse square root of the mass (same interaction with the crystal); (iii) the absolute intensity increases when the temperature decreases, this is due to the fact that the molecule remains longer in the vicinity of the adsorption site where the electric field is the highest. Its amplitude is deduced from the absolute intensities measured at the lowest temperature ( ∼ 1010 V/m, in agreement with other results). © 1998 American Institute of Physics.
The Cu(I)↔Cu(II) redox chemistry in a Cu-ZSM-5 was investigated by means of XPS and XANES spectroscopies. It has been found that all copper sites are virtually able to perform the complete Cu(I)→Cu(II)→Cu(I) redox cycle. This implies that all Cu sites are potentially active sites in the de-NOx reactions. CO and NH3 were used as probe molecules in order to assess the coordinative unsaturation of the Cu(I) cations. The quantitative and energetic aspects of the formation of carbonyl-like and amino-complexes at the metallic sites were studied by means of adsorption microcalorimetry while the spectroscopic aspects were investigated by IR and XANES.
The long-standing challenge of designing and constructing new crystalline solid-state materials from molecular building blocks is just beginning to be addressed with success. A conceptual approach that requires the use of secondary building units to direct the assembly of ordered frameworks epitomizes this process: we call this approach reticular synthesis. This chemistry has yielded materials designed to have predetermined structures, compositions and properties. In particular, highly porous frameworks held together by strong metal–oxygen–carbon bonds and with exceptionally large surface area and capacity for gas storage have been prepared and their pore metrics systematically varied and functionalized.
Fourier-transform IR spectroscopy was used to study the interaction of CO with the acidic hydroxyl groups in H-mordenite and with extraframework cations in alkali-metal exchanged X-mordenites (X = Li, Na, K, Rb, Cs). The dipolar CO molecule was found to interact specifically with extraframework metal ions located at two different sites, the main channels and the side pockets. For cations inside the main channels of the zeolite, the C-O stretching frequencies increase continuously from 2155 cm(-1) for Cs+ up to 2188 cm(-1) for Li+. Cations located at the bottom of the side pockets were found to induce a smaller shift from the frequency corresponding to the free CO molecule (<(nu)over tilde>(CO) = 2143 cm(-1)). The observed frequency shifts were used to evaluate the electrostatic field (inside the main channel) created by the extraframework cations. Values in the range of 2-8 V nm(-1) were obtained. Nonspecifically adsorbed (physisorbed) CO was also observed, which gave rise to a complex IR absorption band at about 2138 cm(-1).
The redox behavior of Cu/ZSM-5 zeolites prepared by ion exchange from Cu2+ aqueous solutions has been followed by a variety of spectroscopic techniques to provide a thorough picture of the so-called “self-reduction” of cupric ions, which occurs upon dehydration of the hydrated system at various temperatures, and of the reverse process of reoxidation as well. Conflicting hypotheses on both these processes are, in fact, present in the literature. The experimental techniques employed in this work are electron paramagnetic resonance (EPR), IR, and optical spectroscopies, extended X-ray absorption fine structure, and X-ray absorption near-edge structure. The early stages of dehydration (from room temperature to about 470 K) involve cupric ion migration and formation of EPR silent moieties but no reduction to Cu+. The onset of this latter phenomenon starts at 470 K and, in the range 470−670 K, involves the majority of copper ions present in the system. Rehydration of Cu+-containing samples does not cause direct Cu+ oxidation to Cu2+ but favor this latter process when O2 is used as oxidant. Oxidation of Cu+ to Cu2+ by molecular oxygen, in fact, does not take place at room temperature if O2 is contacted with the dehydrated material but easily occurs when oxygen is adsorbed on rehydrated samples at the same temperature.
The structural and spectroscopic similarities between homogeneous and heterogeneous [CuI(CO)n]+A- (n = 1−3; A- = [AsF6]- or zeolite anion Z-) carbonyls are evidenced and discussed. While [CuI(CO)2]+ and [CuI(CO)3]+ have linear and planar structures in [CuI(CO)n]+[AsF6]- solid compounds, they are bent in the zeolite channels. The [AsF6]- anion has a base strength lower than that of the zeolite anion; consequently, the [CuI(CO)n]+ moieties have a more positive character in the solid compounds than in the intrazeolitic ones. The CuI−framework distance is influenced by the formation of the [CuI(CO)n]+ complexes: this is demonstrated by both EXAFS and IR results concerning the effect of CO complexation on the CuI−framework distance and on the CuI-sensitive skeletal modes, respectively. The role of basic ligands in increasing the π-character of the Cu−CO bond (with simultaneous decrease of the electrostatic and σ contributions) has been studied on [CuI(CO)n(NH3)]+Z- (n = 1, 2) and [CuI(CO)(H2O)n]+Z- (n = 1, 2) complexes synthesized in situ in the zeolite channels.
XAFS, IR, and UV−vis (diffuse reflectance and photoluminescence) spectroscopies have been employed to investigate the local environment of CuI in CuI-ZSM-5 prepared by gas phase reaction of H-ZSM-5 (SiO2/Al2O3 = 90−50) with CuCl. The measurements confirm that the absolutely predominant copper species formed with this procedure are CuI:Cu species in other oxidation states are less than 1%. Direct proof of the absence of both unreacted CuCl microaggregates and of copper clusters in the zeolite channels is also given. CuI-ZSM-5 prepared in this way represents a model solid for investigating the structure of isolated CuI sites in zeolites. EXAFS analysis reveal that, in vacuum conditions, Cu+ cations are surrounded by 2.5 ± 0.3 oxygens at 2.00 ± 0.02 Å. The combined use of EXAFS, IR, and photoluminescence UV−vis spectroscopies and computer graphic simulations allows to infer that Cu+ ions in CuI-ZSM-5 are located in two families of sites, indicated as I and II, (site II having the highest coordinative unsaturation) which are nearly equipopulated at RT. The formation of Cu+−(CO)n (n = 1, 2) adducts at RT is observed by IR and supported by XAFS spectroscopies. At 110−120 K, the CuI of family II adsorbs up to three CO molecules with formation of CuI−(CO)3 cis-carbonyl complexes; copper(I) in sites I are less unsaturated and can form only cis-CuI−(CO)2. Both sites form, at 110−120 K, stable cis-CuI−(NO)2 species. IR spectroscopy gives evidence of the N2 interaction with both families of sites at 110−120 K: the formation of stable CuI(N2) adducts even at RT.
The adsorption of H2 on high surface area, sintered and smoke MgO samples fully characterized by HRTEM and AFM microscopies has been investigated in the 300−20 K temperature interval by FTIR spectroscopy On high surface area MgO, dissociative adsorption of H2 has been observed with formation of reversible (absorbing at 3454 and 1325 cm-1) and irreversible (absorbing at 3712 and 1125 cm-1) OH and MgH species already reported in previous studies at 300 K. Cooling the MgO/H2 system down to 20 K results in the irreversible formation at about 200 K of new OH (absorbing at 3576−3547 cm-1) and MgH (absorbing at 1430−1418 cm-1) surface groups never observed before. The spectra recorded at 20 K in H2 atmosphere also show absorptions in the 4800−4000 cm-1 frequency interval undoubtedly due to molecularly adsorbed species. Decreasing the MgO surface area results in the disappearance of all of the spectroscopic manifestations due to the hydride and hydroxyl groups formed upon dissociative adsorption of hydrogen, whereas those due to H2 adsorbed in molecular form are maintained (although with much reduced intensity). This behavior is the consequence of the reduction, revealed by HRTEM and AFM, of the concentration of surface defects (cationic and anionic sites located on edges, corners, steps, inverse edges and inverse corners). On the basis of the morphological characterization and of the IR spectroscopic studies, it is concluded that the sites responsible for the H2 dissociative adsorption are mainly inverse steps “coupled” with edges and corners, whereas more usual “isolated” defects (edges, steps, and corners) adsorb hydrogen only in molecular form. The specific adsorption energy for the formation of molecular MgnC2+···H2 adducts on Mg3C2+ (corners; 7.5 kJ/mol), Mg4C2+ (edges; 4.6 kJ/mol), and Mg5C2+ (on (100) planes; 3.6 kJ/mol) coordinatively unsaturated sites has been also calculated from the temperature dependence of the intensity of the related IR bands (ν(HH) mode).
Second-order Møller−Plesset (MP2) calculations (using the approximate resolution of the identity, RI-MP2) in the TZVPP basis are performed to study the interaction of molecular hydrogen with the aromatic systems C6H5X (X = H, F, OH, NH2, CH3, and CN), C10H8 (naphthalene and azulene), C14H10 (anthracene), C24H12 (coronene), p-C6H4(COOH)2 (terephthalic acid), and p-C6H4(COOLi)2 (dilithium terephthalate). Various adsorption positions are studied for C6H5F. The most favorable configuration places H2 above the aromatic plane with its axis pointing toward the middle of the ring. The electronic (van der Waals) interaction energy for the differently substituted benzenes correlates with the ability of the substituents to enrich the aromatic system electronically. The largest interaction energy (among the singly substituted benzenes) is found for aniline (4.5 kJ mol-1). Enlarging the aromatic system increases the interaction energy; the value for coronene amounts to 5.4 kJ mol-1. Extending the basis set and including terms linear in the interelectronic distances increases the interaction energy by about 1 kJ mol-1 relative to that of the TZVPP basis, whereas the inclusion of higher excitations by coupled-cluster calculations (including all single and double excitations with a perturbative estimate of triples, CCSD(T)) decreases the interaction energy by about the same amount.
We present a multitechnique (EPR, XANES, EXAFS, and IR of adsorbed NO) study of the coordination and oxidation chemistry of copper species hosted in mordenite (MOR) zeolite under different conditions. Starting from a 100% Cu2+−MOR, the progressive thermal activation causes first the loss of water molecules from the Cu2+ coordination sphere, accompanied by a partial aggregation in Cu2+−O−Cu2+ complexes, and then the Cu2+ → Cu+ reduction with oxygen elimination. The presence of EPR inactive cupric pairs, witnessed by EXAFS, explains the systematic underestimation of the fraction of Cu2+ species evaluated by EPR, with respect to that obtained from XANES. The data discussed here confirm the interpretation of the so-called “self-reduction” phenomenon of cupric ions emerging in a previous study performed on Cu−ZSM-5 [J. Phys. Chem. B 2000, 104, 4064]. The reoxidation of the so obtained Cu+−MOR by O2 is dramatically favored by the presence of water. This fact explains the poisoning effect of water in the deNOx activity of Cu-exchanged zeolites. The coordination of NO molecules on the Cu+−MOR system was studied in situ at liquid nitrogen temperature. The deNOx chemistry was then switched on by allowing the system to reach room temperature in the NO atmosphere. In all stages of this study, comparison is made with a Cu+−ZSM-5 model system. The differences observed between these two systems are explained in terms of the different structural (cation concentration and environment) characteristics.
Siting of copper ions in CuI−Y zeolite, prepared by gas-phase exchange of NH4−Y with CuCl, has been investigated employing XRPD, XAS and IR spectroscopies. An X-ray powder diffraction study of the zeolite in a vacuum shows that 23.4(2) cuprous ions are located at site I*, 6.1(3) at site II, and 11.5(3) at site II* (sites I* and II* are at the center of the plane of the six-membered ring connecting the hexagonal prism with the sodalite and the sodalite with the supercage, respectively). Addition of CO induces a relevant migration of copper ions from sites II and II* to a more exposed type II. EXAFS analysis shows that CuI ions in the outgassed zeolite are surrounded by 2.8(3) oxygen atoms of the zeolite framework, the average CuI−O distance being 1.99(2) Å. Both X-ray measurements and FTIR spectroscopy show that CO is adsorbed on the zeolite at room temperature with formation of carbonyl adducts. At liquid-nitrogen temperature and low CO pressure, two types of monocarbonyl species are observed, corresponding to CO adsorbed on copper ions located at sites II and II*. On increase of the CO pressure and subsequent formation of polycarbonylic species, cations at site II* move to the more exposed position II, and a single kind of tricarbonyl adducts is observed. IR spectroscopy also provides evidence for the interaction of NO with copper ions located at sites II and II*, which are the first sites able to adsorb up to two molecules of NO, whereas cations at site II*, because of their lower coordinative unsaturation, can only form CuI(NO) adducts. NO proves to be a sensitive probe not only for cuprous but also for cupric ions.
The adsorption of carbon dioxide onto M-ZSM-5 zeolites (M = Li, Na, K, Cs) was studied by means of FTIR spectroscopy and adsorption microcalorimetry. Quantum chemical calculations, at the B3-LYP level, on the interaction of CO2 with the bare alkali-metal cations were performed to assist interpretation of the experimental results. With the likely exception of Li+, CO2 was found to undergo a two-step interaction with the metal ions. At a low equilibrium pressure linear 1:1 adducts of the type M+···OCO (M = Na+, K+, Cs+) are formed; upon increasing the CO2 equilibrium pressure, the metal cation coordinates a second CO2 molecule, forming a 2:1 adduct. Calculated (ab initio) bond lengths for the 1:1 adduct are given, as well as corresponding values of the binding energy and enthalpic term. Experimentally derived values of the main thermodynamic functions (ΔH°, ΔG°, and ΔS°) are discussed and correlated with detailed results from IR spectroscopy. The interaction cation/CO2 alone cannot account for the body of evidence, and the contribution of nearby O2- anions has to be invoked.
Diffusion of triethylamine into an ethanol solution containing 1,3,5-benzenetricarboxylic acid (H3BTC) and zinc(II) nitrate hexahydrate yields crystalline Zn2(BTC)(NO3)·(H2O)(C2H5OH)5, which was formulated by elemental microanalysis, solid-state NMR, and single-crystal X-ray diffraction [cubic, P213, a = 14.728(2) Å, V = 3195(2) Å3, Z = 4]. This compound possesses a 3-D structure with nearly 44% of the framework represented by an extended channel system having a 14 Å cross-section, where highly mobile ethanol and water guest molecules reside. The multidentate functionality of BTC imparts rigidity to the structure, consequently allowing the guests to be removed or exchanged without destruction of the porous framework. X-ray powder diffraction, solid-state NMR (13C CP MAS and static), gas chromatography, and thermogravimetry analyses reveal that this material is highly selective to alcohols because of the coordination environment adopted by Zn(II) within its framework. Other molecular inclusions such as tetrahydrofuran, methyl ethyl ketone, acetonitrile, and acetone are not permitted into the channels, due to the specific electronic demands of the Zn(II) center and its ability to direct the inclusion process even in cases where incoming molecules have the appropriate shape and size for inclusion.
Fourier transform IR spectra of carbon monoxide adsorbed at liquid nitrogen temperature on a series of alkalimetal exchanged ZSM-5 zeolites [X-ZSM-5 (X = Li, Na, K, Rb, Ca)] show that the dipolar:CO molecule specifically interacts with the zeolite extraframework metal ions. This interaction modifies the C-O stretching frequency, which increases continuously from 2157 cm(-1) for Cs-ZSM-5 upto 2188 cm(-1) for Li-ZSM-5. It is shown that the observed frequency shift (from nu(CO) = 2143 cm(-1)) can be used to evaluate the electrostatic field created by the metal cations. Nonspecifically adsorbed (physisorbed) CO was also observed as well as some minor IR absorption bands which were tentatively assigned.
N-2 interacts with positive M(+) centers (M(+) = H+, Li+ Na+, K+, Rb+, and Cs+) present in M(+)-mordenites, with activation in the IR of the N=N stretching vibration. From the analysis of the IR spectra and from the examination of the available space present in the main channels and in the lateral pockets, it is concluded that N-2 can interact with smallest ions located in both positions and that only when the biggest ions are involved (Rb+, Cs+), the penetration of N-2 into the lateral pockets is not possible. The ($) over bar v(N=N) frequency is found to depend linearly upon the 1/(R(x) + R(NN))(2) (R(x) = cation radius): this is considered a clear proof of the predominant role of the electrostatic forces. When the perturbed ($) over bar v(N=N) stretching frequency is compared with the ($) over bar v(N=N) of N-2 perturbed by dispersion forces only (as for N-2 in the cages of rare gas matrices), it is concluded that the effect of the perturbation induced by the positive ions is always to shift the frequency toward higher values (as found for CO in M(+)-mordenite and M(+)-ZSM5): this result strongly suggests an end-on interaction of N-2 with all alkali-metal extraframework cations, Indications that N-2 behaves as an hindered rotator (especially in Cs+-mordenite) were also obtained and discussed.
We propose a new mechanism for paramagnetically catalyzed ortho-para conversion in molecular hydrogen. In this mechanism, a 3Σu+ excited-state character of H2 is mixed into the ground state of a collisional complex, inducing spin-uncoupling and removal of antisymmetrization of the parts of the wave function localized to the hydrogen nuclei. This induces an effective Fermi contact hyperfine interaction between the ortho and para states. Using the collision complex H2 + O2 as model system, we show that the proposed mechanism is more than 2 orders of magnitude stronger than the traditional inhomogeneous magnetic field model.
XRD, UV−Vis, EXAFS, XANES, and Raman techniques have been used to study the removal of water molecules coordinated to the Cu(II) framework atoms of the novel HKUST-1 metal-organic framework. The dehydration process preserves the crystalline nature of the material, just causing a reduction of the cell volume due to the shrinking of the [Cu2C4O8] cage. The removal of adsorbed H2O molecule makes the framework Cu(II) sites available for interaction with other probe molecules. In situ IR spectroscopy has evidenced the formation at liquid nitrogen temperature of labile Cu(II)···CO adducts characterized by a ν̃(C−O) = 2178 cm-1 and at 15 K of Cu(II)···H2 adducts characterized by a ν̃(H−H) = 4100 cm-1. To the best of our knowledge, we have observed for the first time a clear signal of Cu(II) carbonyl and dihydrogen complexes formed inside a crystalline microporous hosting matrix. The sinking of the oxygens of the carboxyl units, undergone by the Cu(II) framework ions in the dehydration process, is responsible for the rather low coordinative unsaturation of Cu(II). The important shielding effect of the four oxygen framework atoms is testified by the low polarization factor of the Cu(II) site probed by both CO and H2 molecules.
This short review describes the evolution of the nature of microporous solids and the related concepts that were at the origin of this evolution. The paper begins with the different families of classical organically templated inorganic porous solids, their parameters of synthesis, the mechanisms of formation, their consequences, and their limits. From the latter, the concept of hybrid organic−inorganic frameworks is introduced as well as their tentative classification according to the decrease of the dimensionality of the inorganic subnetwork. The last part of the paper is devoted to the new trends in the field: the creation of very large pores, their design from topological considerations, and the introduction of computational methods.
The quantitative assessment of the acid–base properties of the surface of heterogeneous catalysts by i.r. spectral studies of
adsorbed molecules is discussed. The quantities through which the protonic (Brønsted), aprotic (Lewis), acidic and basic
centres of the surface can be most easily characterised are analysed. Reported methods of determining the number and
strength of the surface centres are described. 135 references.