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Concentration dependent binding of CO2 and CD4 in UiO 66(Zr)
Abstract
Porous metal-organic frameworks have emerged as promising materials for the capture of carbon dioxide (CO2) and its separation from methane (CH4) during the industrially-important “sweetening” of sour natural-gas. The excellent thermal and chemical stability of the highly-porous UiO-66(Zr) material, combined with good selectivity for CO2 over CH4, makes this material a prime candidate for such applications. Using a combination of neutron powder-diffraction and density-functional theory, we examine the details of the binding of CO2 and CH4 in UiO-66(Zr) over the industrially-relevant 3.6 to 9.0 mmol/g concentration range, corresponding to the material that is half to fully saturated with CO2. This work builds on the previously-reported preferred site for CO2 and CH4 in UiO-66(Zr), establishing further sites and determining the strength and nature of the guest-host interaction at these. We find the UiO-66(Zr)···CO2 interactions are significantly affected by the concentration of CO2 as the binding of CO2 is enhanced by interguest interactions.
... As revealed by the electrospray ionization-mass spectrometry (ESI-MS) ( Supplementary Fig. 28), H 13 COO − was the only reduction product when 13 CO 2 was used as feed gas, confirming that the formates were generated from photocatalytic CO 2 RR exclusively. Additionally, as shown in Supplementary Fig. 29, FTIR peaks (at 1497 and 1433 cm -1 ) of the surface carbonic acid species 36,37 , the intermediate species for formic acid evolution 36 , were immediately observed when humidified CO 2 was fed into the reactors containing Ir 1 /A-aUiO or A-aUiO catalysts in dark, indicating the adsorption of CO 2 by the hydroxyls on the Zr-O nodes of A-aUiO 38,39 . The reactors were then purged with Ar to remove the excess CO 2 . ...
The demand for sustainable energy has motivated the development of artificial photosynthesis. Yet the catalyst and reaction interface designs for directly fixing permanent gases (e.g. CO 2 , O 2 , N 2 ) into liquid fuels are still challenged by slow mass transfer and sluggish catalytic kinetics at the gas-liquid-solid boundary. Here, we report that gas-permeable metal-organic framework (MOF) membranes can modify the electronic structures and catalytic properties of metal single-atoms (SAs) to promote the diffusion, activation, and reduction of gas molecules (e.g. CO 2, O 2 ) and produce liquid fuels under visible light and mild conditions. With Ir SAs as active centers, the defect-engineered MOF (e.g. activated NH 2 -UiO-66) particles can reduce CO 2 to HCOOH with an apparent quantum efficiency (AQE) of 2.51% at 420 nm on the gas-liquid-solid reaction interface. With promoted gas diffusion at the porous gas-solid interfaces, the gas-permeable SA/MOF membranes can directly convert humid CO 2 gas into HCOOH with a near-unity selectivity and a significantly increased AQE of 15.76% at 420 nm. A similar strategy can be applied to the photocatalytic O 2 -to-H 2 O 2 conversions, suggesting the wide applicability of our catalyst and reaction interface designs.
... As revealed by the electrospray ionization-mass spectrometry (ESI-MS) ( Figure S25), H 13 COO − was the only product when 13 CO2 was used as feed gas, confirming that the formates were generated from photocatalytic CO2RR exclusively. Additionally, as shown in Figure S26, FTIR peaks (at 1497 and 1433 cm -1 ) of the surface carbonic acid species 27,28 , the intermediate species for formic acid evolution 27 , were immediately observed when humidified CO2 was fed into the reactors containing Ir1/d-aUiO or d-aUiO catalysts in dark, indicating the adsorption of CO2 by the hydroxyls on the Zr-O nodes of d-aUiO 29,30 . The reactors were then purged with Ar to remove the excess CO2. ...
The demand for sustainable energy has motivated the development of artificial photosynthesis. Yet the catalyst and reaction interface designs for directly fixing permanent gases into liquid fuels are still challenged by sluggish mass transfer and catalytic kinetics at the gas-liquid-solid three-phase boundary. Here, we report that breathable metal-organic framework (MOF) membranes decorated with metal single-atoms (SAs) can synergistically promote the diffusion, adsorption, and activation of gas molecules (e.g. CO 2 , O 2 ) to boost the photocatalytic conversion of them into liquid fuels. With Ir SAs as active centers, the defect-engineered MOF (e.g. NH 2 -UiO-66) matrixes can efficiently harvest visible light and sensitize the electronically tailored Ir SAs for reducing CO 2 to HCOOH with the high activity of 0.51 mmol g cat - ¹ h- ¹ at the conventional three-phase reaction interface. Furthermore, the breathable SA/MOF membranes can directly convert humid CO 2 gas into HCOOH at the high-throughput gas-solid interfaces, presenting a near-unity selectivity and an unprecedented activity of 3.38 mmol g cat - ¹ h- ¹ . Similarly, with Pd SAs as active centers, the SA/MOF membrane can catalyze the O 2 -to-H 2 O 2 conversion with an ultrahigh activity of 10.4 mmol g cat - ¹ h- ¹ under visible light, suggesting the wide applicability of our catalyst and reaction interface designs.
... The longest scans, ranging up to 24 h, are typically performed for vanadium-rod and background measurements (x3.3), very small or highly absorbing samples, and magnetic materials with a weak magnetic diffraction signal. Particular experiments which have pushed the limits of the instrument include a crystal structure study of radiation damage in pyrochlores on a sample of $50 mg (unpublished); a magnetic structure study of highly absorbing EuTiO 3 (Kennedy et al., 2014) and DyGa (Susilo et al., 2015); magnetic structure studies of S = 1/2 Cu +2 systems such as Sr 2 CuWO 6 (Vasala et al., 2014) and Sr 2 CuTeO 6 (Koga et al., 2016) with ordered magnetic moments as low as 0.5-0.7 B ; and a system containing 27% hydrogen (Chevreau et al., 2015). ...
The ECHIDNA high-resolution neutron powder diffractometer at the 20 MW OPAL research reactor in Australia produces high-quality data for a broad spectrum of crystal and magnetic structural studies. The paper presents an overview of the current status of the hardware, latest developments in data-reduction software, statistics on instrument usage and the user programme, and instrument limitations.
... It is particularly interesting to note that in the lowconcentration regime, the Zn(hba) unit-cell behaviour is essentially indistinguishable for both CO 2 and CD 4 at equal loadings. Although the binding enthalpy determined for CO 2 in Zn(hba) was 8 kJ mol −1 higher than that for CH 4 (White et al., 2015), as is typical in MOFs because of the quadrupole moment of CO 2 (D' Alessandro et al., 2010;Chevreau et al., 2015), the Zn(hba) lattice expansion appears to be independent of guest interaction strength and dependent only on the total concentration of guest molecules. It can be speculated that the lower observed uptake of CH 4 by Zn(hba) results from a steric inability of the bulkier CH 4 molecule to access the proposed secondary binding site occupied by CO 2 at higher loadings. ...
Analysis of in situ neutron powder diffraction data collected for the porous framework material Zn(hba) during gas adsorption reveals a two-stage response of the host lattice to increasing CO 2 guest concentration, suggesting progressive occupation of multiple CO 2 adsorption sites with different binding strengths. The response of the lattice to moderate CH 4 guest concentrations is virtually indistinguishable from the response to CO 2 , demonstrating that the influence of host–guest interactions on the Zn(hba) framework is defined more strongly by the concentration than by the identity of the guests.
Zirconium terephthalate UiO-66 has aroused great interest in catalysis since it exhibits significant flexibility and compatibility for accommodating a high number of defects as well as exceptional thermal and chemical stability. Until now, many works have focused on the modulations of the Zr6-oxo clusters in UiO-66 in terms of diverse synthesis, advanced characterizations, and their catalytic applications. To achieve high catalytic efficiency, it is still highly desired for rationally constructing and modulating the Zr6-oxo clusters with exposed catalytic sites and diverse microenvironments for advanced catalysis. In this review, we provide a comprehensive summary of recent progress on the synthesis of defective UiO-66, qualitative and quantitative characterizations, as well as a logical overview of heterogeneous catalytic applications over the past few years. Finally, the outlooks for the research paradigm of defective UiO-66 are discussed.
UiO-66 is one of the most valuable metal-organic frameworks because of its excellent adsorption capability for gas molecules and its high stability towards water. Herein we investigated adsorption of carbon dioxide (CO2), acetone, and methanol to infinite UiO-66 using DFT calculations on an infinite system under periodic-boundary conditions and post-Hartree-Fock (SCS-MP2 and MP2.5) calculations on cluster models. Three to four molecules are adsorbed at each of four μ-OH groups bridging three Zr atoms in one unit cell (named Site I). Six molecules are adsorbed around three pillar ligands, where the molecule is loosely surrounded by three terephthalate ligands (named Site II). Also, six molecules are adsorbed around the pillar ligand in a different manner from that at Site II, where the molecule is surrounded by three terephthalate ligands (named Site III). Totally fifteen to sixteen CO2 molecules are adsorbed into one unit cell of UiO-66. The binding energy (BE) decreases in the order Site I > Site III > Site II for all three molecules studied here and in the order acetone > methanol ≫ CO2 in the three adsorption sites. At the site I, the protonic H atom of the μ-OH group interacts strongly with the negatively charged O atom of CO2, acetone and methanol, which is the origin of the largest BE value at this site. Although the DFT calculations present these decreasing orders of BE values correctly, the correction by post-Hartree-Fock calculations is not negligibly small and must be added for obtaining better BE values. We explored NMR spectra of UiO-66 with adsorbed CO2 molecules and found that the isotropic shielding constants of the 1H atom significantly differ among no CO2, one CO2 (at Sites I, II, or III), and fifteen CO2 adsorption cases (Sites I to III) but the isotropic 17O and 13C shielding constants change moderately by adsorption of fifteen CO2 molecules. Thus, 1H NMR measurement is a useful experiment for investigating CO2 adsorption.
In this work, we report that a simple post-synthetic treatment of the metal-organic framework (MOF) UiO-66 through an exposure to ultraviolet light (UV-C) allows a marked increase in its CO2 uptake capacity. It is shown that this is due to the generation of cluster defects at nanoregions of the MOF while its overall architecture and crystallinity are maintained. UiO-66 modified in this way has a greater structural complexity with a more open framework and new interaction sites that lead to the increased capture of CO2. The results presented here are the first report of an improvement in the CO2 uptake of a Metal-organic framework using this procedure. Comparing with existing methods, the treatment is a promising alternative to increase the CO2 uptake of UiO-66 preserving its structural integrity and robustness through a simple, short and sustainable procedure that avoids the use of chemicals.
Recently, metal–organic frameworks (MOFs) have been extensively investigated as fluorescence chemsensors due to their tunable porosity, framework structure and photoluminescence properties. In this paper, a well-known Zr(IV)-based MOF, UiO-66-NH2 was demonstrated to have capability for detection of L-lysine (Lys) and L-arginine (Arg) selectively from common essential amino acids in aqueous media via a fluorescence turn-on mechanism. Further investigation reveals its high sensitivity and strong anti-interference properties. Moreover, the possible mechanism for sensing Lys and Arg was explored by FT-IR and ¹H-NMR, and the results indicate that the enhancement of the fluorescence could be ascribed to the adsorption of Lys/Arg and the hydrogen bonding interactions between Lys/Arg and the amino group of UiO-66-NH2. The difference of the sensing capacity and sensitivity between UiO-66 and UiO-66-NH2 revealed that the amino group plays an essential role in the sensing performance. This work presents a unique example of the functional group dependent sensing properties of MOFs.
Stabilizing metal nanoparticles (MNPs) with metal−organic frameworks (MOFs) has become a favorable pathway for developing efficient hybrid photocatalysts, while the effective charge transfer within the hybrid catalysts is largely elusive. In this work, we employed a semiconductor-like Zr-MOF, NH2-UiO-68 with amino-functionalized linker and permanent porosity enabling to confine guest species for the first time. Pt NPs have been embedded inside (referred as Pt@MOF), outside (referred as Pt/MOF) or both inside and outside (referred as Pt-MOF) of the flowerlike NH2-UiO-68 by varied synthetic methods and Pt NPs amounts. In these different hybrids, the sample 2 wt% Pt@NH2-UiO-68 presents the highest photocatalytic CO2 reduction activity under visible light irradiation, far superior to the samples 2 wt% Pt/NH2-UiO-68, 1 wt% Pt@NH2-UiO-68, and 4 wt% Pt-NH2-UiO-68. The appropriate location and content of Pt NPs in the catalyst would facilitate the contact between Pt NPs and NH2-UiO-68, which is beneficial to form Pt-MOF heterojunctions, thereby acquiring efficient charge transfer in the photocatalytic process. More importantly, effective charge transfer from NH2-UiO-68 to the excited Pt NPs via Pt-MOF Schottky junctions can accordingly inhibit the recombination of photogenerated charge carriers, ultimately enhancing the photocatalytic activity.
Metal-organic frameworks (MOFs) have been one of the most intensely studied family of hybrid porous crystalline materials for decades, whose potential applications cover almost all the concerned technological fields ranging from environment, medical, and biology to energy and electronics. However, the low stability of MOFs has been the most crucial weakness of this type of material, hindering its development in many applications not only as pure MOFs but also in the grand field of porous nanomaterials. Fortunately, UiO-66, a zirconium terephthalate–based MOF, is found to possess remarkably uniform and tunable pore sizes, exceptionally high surface areas as well as active Zr-O clusters. It also holds incredible hydrothermal stability even higher than 350°C because of its strong Zr-O bond with high coordination number of Zr(IV), which further offers high resistance to a wide range of solvents. These outstanding qualities have granted UiO-66 and its derived UiOs extraordinarily promising future potentials. Since the very applications of UiO-66 itself are of high values and, more essentially, the high comprehensive properties of UiO-66 can lead researchers to explore and accomplish achievements that no other MOFs can do, it is of vital importance to systematically categorize and analyze the properties and applications of UiO-66 as it is an ideal representative to study the modification and applicable properties of general UiOs, MOFs, and even porous materials. However, no work has been done to summarize MOFs through the perspective of one classic Zr-MOF, namely, UiO-66. Moreover, there is hardly no work on the study of MOFs via the angle from one single branch such as UiOs or ZIFs. Therefore, in this organized review, details of the synthesis, modulation, and functionalization of a series of UiOs, especially those of UiO-66, are presented.
Carbon dioxide (CO2) capture and separation in sorbent materials relies on the interactions that take place between guest molecules and topological or chemical features of the sorbent. Although researchers have developed an immense library of metal-organic framework (MOF) designs, which range in shape, size, and chemical functionality, a molecular-level understanding of how MOF structure and functionality affect capture and transport has yet to be fully developed. In this work, in situ infrared spectroscopic techniques, coupled with density functional theory, were used to provide insight to the binding locations and energetics of CO2 on the zirconium-based MOF, UiO-66. Two unique CO2 binding motifs in UiO-66 were identified through differing vibrational frequencies of the asymmetric OCO vibrational mode. One configuration is established through hydrogen bonding with μ3-OH groups on the MOF nodes and a second binding mode existis where CO2 is stabilized through dispersive interactions. Variable temperature infrared spectroscopy (VTIR) revealed adsorption enthalpies of −38.0 ± 1.5 kJ/mol and −30.2 ± 1.3 kJ/mol for the hydrogen-bonded and dispersion-stabilized complexes, respectively. The techniques and results from this study can be extended to other gas–MOF systems to help reveal how topological features affect gas sorption and to provide insight into next generation sorbent materials design.
Control of the thermomechanical properties of functional materials is of great fundamental and technological significance, with the achievement of zero or negative thermal expansion behavior being a key goal for various applications. A dynamic, reversible mode of control is demonstrated for the first time in two Prussian blue derivative frameworks whose coefficients of thermal expansion are tuned continuously from negative to positive values by varying the concentration of adsorbed CO2. A simple empirical model that captures site-specific guest contributions to the framework expansion is derived, and displays excellent agreement with the observed lattice behaviour.
We report a record-high SO2 adsorption capacity of 12.3 mmol g-1 in a robust porous material, MFM-601, at 298 K and 1.0 bar. SO2 adsorption in MFM-601 is fully reversible and highly selective over CO2 and N2. The binding domains for adsorbed SO2 and CO2 molecules in MFM-601 have been determined by in situ synchrotron X-ray diffraction experiments, giving insights at the molecular level to the basis of the observed high selectivity.
This chapter presents an updated review of the advancement achieved in the field of MOFs as CO2 sorbents. The different functionalization strategies are comprehensively reviewed along with their impacts on CO2 capacity, selectivity, and enthalpies of adsorption and desorption. The potential application of amines and ionic liquid‐modified MOFs and MOF composite materials for CO2 separation from flue gas is also thoroughly reviewed and discussed in this chapter. This review also sheds light on the issue of stability of MOFs under humid flue gas conditions and the different strategies employed to address this challenge of MOFs as CO2 separation adsorbents in order for their mass integration into existing power plants. The future of MOFs in CO2 separation is very promising, and with the sharply growing research in this field, the commercial application is inevitable.
UiO-66/TiO2 composites were fabricated via self-assembly using a solvothermal method. The composites exhibits high efficiency and stability for removing rhodamine B and methyl blue under strongly acidic conditions via a simple regeneration process. The participation of the UiO-66 structure in TiO2 was found to reduce the electron–hole recombination but did not change its adsorption ability on dyes, which leads to its enhanced photocatalytic properties under visible light radiation. In addition, the introduced TiO2 in MOFs was firstly found to increase the number of active sites, which is beneficial for CO2 capture. Therefore, it can be suggested that porous MOFs combined with photoactive semiconductors may introduce a new class of dual-functional materials for use in water treatment and CO2 capture.
The CO2 adsorption process in the family of porous metal-organic framework materials CPO 27 M (M = Mg, Mn, Co, Ni, Cu and Zn) has been studied by variable temperature powder synchrotron X-ray diffraction under isobaric conditions. Rietveld analysis of the data provided a time-lapse view of the adsorption process with an unprecedented level of structural detail. It confirms the temperature dependent order of occupation of the up to three adsorption sites found in the pores of the CPO-27-M materials. In CPO-27-M (M = Mg, Mn, Co, Ni and Zn), the adsorption sites are occupied in sequential order, which is primarily due to the high affinity of CO2 towards the open metal site. CPO-27-Cu deviates from this stepwise mechanism in that the adsorption site at the metal cation and the second site are occupied in parallel. The temperature dependence of the site occupancy of the individual CO2 adsorption sites derived from the diffraction data is reflected in the shape of the volumetric sorption isotherms. The fast kinetics and high reversibility observed in these experiments confirm the suitability of the materials for use in temperature or pressure swing processes for carbon capture.
Porous Interpenetrated Zirconium-Organic Frameworks (PIZOFs) with various functional groups are explored for CO2 capture using molecular simulation and experiment. Functionalisation enhances the CO2 uptake and selectivity over other gases, but small cavities play an even more important role. PIZOF-2 outperforms the other PIZOF structures for CO2 separation from methane and nitrogen (related to raw natural gas and post-combustion of coal mixtures) due to optimal formation of small cavities around 6.5 Ȧ in diameter which are also laced with methoxy groups. The small cavities within the interpenetrated structures are crucial for achieving high selectivities, especially for cavities surrounded by a combination of 6 benzene rings, 2 metal clusters and 4 methoxy groups that offer a tight overlapping potential energy field, ideal for “catching” CO2.
A system for positioning powder samples in top-loading cryofurnaces during neutron scattering experiments, while facilitating the successive delivery of gas doses at set temperatures to the sample, has been designed and tested. The positioning system is compatible with a Hiden Isochema IMI instrument as a gas-dosing platform, enabling gases to be delivered to the sample through a centrally located and thermally stabilized capillary line and valve. The positioning system separates into an upper and a lower section, with the lower section enabling the sample to be isolated and inserted into a glove box. This work describes the system using example neutron powder diffraction results obtained with this system in closed-cycle cryofurnaces.
Metal–organic frameworks (MOFs) are promising solid sorbents, showing gas selectivity and uptake capacities relevant to many important applications, notably in the energy sector. To improve and tailor the sorption properties of these materials for such applications, it is necessary to gain an understanding of their working mechanisms at the atomic and molecular scale. Specifically, it is important to understand how features such as framework porosity, topology, chemical functionality and flexibility underpin sorbent behaviour and performance. Such information is obtained through interrogation of structure–function relationships, with neutron powder diffraction (NPD) being a particularly powerful characterization tool. The combination of NPD with first-principles density functional theory (DFT) calculations enables a deep understanding of the sorption mechanisms, and the resulting insights can direct the future development of MOF sorbents. In this paper, experimental approaches and investigations of two example MOFs are summarized, which demonstrate the type of information and the understanding into their functional mechanisms that can be gained. Such information is critical to the strategic design of new materials with targeted gas-sorption properties.
This book distills the knowledge gained from research into atoms in molecules over the last 10 years into a unique, handy reference. Throughout, the authors address a wide audience, such that this volume may equally be used as a textbook without compromising its research-oriented character. Clearly structured, the text begins with advances in theory before moving on to theoretical studies of chemical bonding and reactivity. There follow separate sections on solid state and surfaces as well as experimental electron densities, before finishing with applications in biological sciences and drug-design. The result is a must-have for physicochemists, chemists, physicists, spectroscopists and materials scientists.
In this paper we introduce AIM-UC, a free application that allows to create graphs related to Quantum Theory of Atoms in Molecules (QTAIM). The input data are files that contain 3D grid of electron density, in some known formats: CUBE from gaussian, grd from DMol and CHGCAR from VASP. Also it supports wave function files from AIMPAC. The application calculates 2D regular grids (electron densities and its laplacians) in a plane positioned by the user. By using bicubic interpolation and the Newton-Raphson method, the critical points at the plane can be calculated, and the gradient field and molecular graph can also be generated using the Cash-Karp Runge-Kutta method. It also permits plotting the electron density and its negative laplacian on the selected plane. Moreover, for both 2D grids, it is possible to calculate the contour maps using the "Marching Squares" algorithm. This application also permits finding the critical points in the 3D space. The output consist of plain text files and images in PNG, BMP and EPS (encapsulated post script) formats. AIM-UC was programmed in C++, using the GUI toolkit FLTK (fast light toolkit) and OpenGL (open graphic library).
Neutron powder diffraction measurements were carried out on the evacuated and CO2-loaded Prussian blue analogue, Fe3[Co(CN)6]2, identifying two distinct CO2 adsorption sites: site A, in which CO2 uniquely bridges between two bare-metal sites, and site B, in which it interacts in a face capping motif. The saturation of site A at low loadings of CO2 demonstrates the favourable nature of the interaction of CO2 with bare-metal sites within the material.
A series of porous Zr oxoclusters-based MOFs was computationally explored for their gas storage/capture performances. The highly porous UiO-67(Zr) and UiO-68(Zr) solids show exceptionally high CH(4) and CO(2) adsorption capacities under operating conditions that make these thermal, water and mechanical resistant materials very promising for physisorption-based processes.
A description and justification of the
EXPGUI
program is presented. This program implements a graphical user interface and shell for the
GSAS
single-crystal and Rietveld package. Use of the Tcl/Tk scripting language allows
EXPGUI
to be platform independent. Also included is a synopsis of how the program is implemented.
The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blöchl's projector augmented wave (PAW) method is derived. It is shown that the total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addition, critical tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed core all electron methods. These tests include small molecules (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
The ligand functionalization effect on the CO(2)/CH(4) separation performance of the MOF type UiO-66(Zr) was explored computationally. The -SO(3)H and -CO(2)H functionalized forms show the highest selectivity, good working capacity and medium ranged CO(2) adsorption enthalpy that make these materials very promising for physisorption-based processes.
An unusually large expansion upon solvent adsorption occurs without apparent bond breaking in the network of a series of isoreticular chromium(III) or iron(III) diarboxylates labeled MIL-88A to D [dicarbox = fumarate (88A); terephthalate (1,4-BDC) (88B); 2,6-naphthalenedicarboxylate (2,6-NDC) (88C); and 4-4'-biphenyldicarboxylate (4-4'-BPDC) (88D)]. This reversible "breathing" motion was analyzed in terms of cell dimensions (extent of breathing), movements within the framework (mechanism of transformation), and the interactions between the guests and the skeleton. In situ techniques show that these flexible solids are highly selective absorbents and that this selectivity is strongly dependent on the nature of the organic linker.
This article focuses on high valence 3p and transition metal based metal organic frameworks. In the first part we will discuss the complex solution chemistry of these metals which makes this sub-class of MOFs more of a challenge than the traditional low valence metal based MOFs. This is followed by a short review of the different classes of solids based on phosphonates, carboxylates and other linkers. Finally, we report some of the most relevant properties of these solids such as their chemical or thermal stability as well as their catalytic, redox- and photo-activities.
Two new MOF materials, i.e. USO-2-Ni MOF and UiO-67/MCM-41 hybrid, are evaluated as regards their potential for pre-combustion CO2 capture within an IGCC power plant. Both materials have been formulated as particles, thus allowing for a meaningful characterization in view of process scale-up. This is done by first conducting fixed bed experiments, where the adsorbent material is packed into a column and exposed to three different CO2/H2 mixtures at a given temperature (25 °C) and in a broad range of pressures (1–25 bar) in order to determine the corresponding transfer parameters. Second, complete PSA simulations are conducted. More precisely, a multi-objective analysis is carried out having the CO2 purity and recovery as objectives. A fixed process configuration is considered and the optimization is repeated for different process conditions. The results of this analysis are then compared to a benchmark which has activated carbon as adsorbent. This comparison shows that the USO-2-Ni MOF indeed increases both the separation performance and the productivity of the PSA process with respect to activated carbon. UiO-67/MCM-41 hybrid exhibits a slightly worse separation performance, but a better specific adsorbent productivity than activated carbon. Furthermore, it is shown that the particle formulation is crucial, as it has a big impact on the packing characteristics, which in turn have a big impact on the process performance.
UiO-66 is a highly important prototypical zirconium metal-organic framework (MOF) compound because of its excellent stabilities not typically found in common porous MOFs. In its perfect crystal structure, each Zr- metal center is fully coordinated by 12 organic linkers to form a highly connected framework. Using high-resolution neutron power diffraction technique, we found the first direct structural evidence showing that real UiO-66 material contains significant amount of missing-linker defects, an unusual phenomenon for MOFs. The concentration of the missing-linker defects is surprisingly high, ~10% in our sample, effectively reducing the framework connection from 12 to ~11. We show that by varying the concentration of acetic acid modulator and the synthesis time, the linker vacancies can be tuned systematically, leading to dramatically enhanced porosity. We obtained samples with pore volumes ranging from 0.44 cc/g to 1.0 cc/g and BET surface areas ranging from 1000 m2/g to 1600 m2/g, the largest values of which are ~150% and ~60% higher than the theoretical values of defect-free UiO-66 crystal, respectively. The linker vacancies also have profound effects on the gas adsorption behaviors of UiO-66, in particular CO2. Finally, comparing the gas adsorption of hydroxylated and dehydroxylated UiO-66, we found that the former performs systematically better than the latter (particularly for CO2) suggesting the beneficial effect of the -OH groups. This finding is of great importance because hydroxylated UiO-66 is the practically more relevant, non-air sensitive form of this MOF. The preferred gas adsorption on the metal center was confirmed by neutron diffraction measurements and the gas binding strength enhancement by the -OH group was further supported by our first-principles calculations.
In order for any material to be considered in a post-combustion carbon capture technology, it must have high working capacities of CO2 from flue gas and be regenerable using as little energy as possible. Shown here is an easy to use method to calculate both working capacities and regeneration energies and thereby predict optimal desorption conditions for any material.
Recent extensions of the DMol3 local orbital density functional method for band structure calculations of insulating and metallic solids are described. Furthermore the method for calculating semilocal pseudopotential matrix elements and basis functions are detailed together with other unpublished parts of the methodology pertaining to gradient functionals and local orbital basis sets. The method is applied to calculations of the enthalpy of formation of a set of molecules and solids. We find that the present numerical localized basis sets yield improved results as compared to previous results for the same functionals. Enthalpies for the formation of H, N, O, F, Cl, and C, Si, S atoms from the thermodynamic reference states are calculated at the same level of theory. It is found that the performance in predicting molecular enthalpies of formation is markedly improved for the Perdew–Burke–Ernzerhof [Phys. Rev. Lett. 77, 3865 (1996)] functional. © 2000 American Institute of Physics.
Zirconium-metal organic frameworks (Zr-MOFs) were synthesized with or without ammonium hydroxide as an additive in the synthesis process. It was found that addition of ammonium hydroxide would change the textural structure of Zr-MOF. The BET surface area, pore volume, and crystal size of Zr-MOF were reduced after addition of ammonium hydroxide. However, the crystalline structure and thermal stability were maintained and no functional groups were formed. Adsorption tests showed that Zr-MOF presented much higher CO2 adsorption than CH4. Zr-MOF exhibited CO2 and CH4 adsorption of 8.1 and 3.6 mmol/g, respectively, at 273 K, 988 kPa. The addition of ammonium hydroxide resulted in the Zr-MOF with a slight lower adsorption of CO2 and CH4, however, the selectivity of CO2/CH4 is significantly enhanced.Graphical abstractZr-metal organic frameworks (Zr-MOF) exhibit high CH4 and CO2 adsorption and additive of ammonium hydroxide produces Zr-MOF with higher selectivity of CO2/CH4.View high quality image (95K)Highlights► Zr-MOFs were prepared with addition of ammonia as additive. ► Ammonia additive affects textural structure of Zr-MOF. ► Nanosize Zr-MOF presents high adsorption of CH4 and CO2. ► Ammonia modified Zr-MOF exhibits high selectivity of CO2/CH4.
The quantum mechanics of proper open systems yields the physics that governs the local behavior of the electron density, ρ(r). The Ehrenfest force F(r) acting on an element of ρ(r) and the virial field ν(r) that determine its potential energy are obtained from equations of motion for the electronic momentum and virial operators, respectively. Each is represented by a “dressed” density, a distribution in real space that results from replacing the property in question for a single electron with a corresponding density that describes its average interaction with all of the remaining particles in the system. All bond paths, lines of maximum density linking neighboring nuclei in a system in stable electrostatic equilibrium, have a common physical origin in terms of F(r) and ν(r), regardless of the nature of the interaction. Each is homeomorphically mirrored by a virial path, a line of maximally negative potential energy density linking the same nuclei. The presence of a bond path and its associated virial path provide a universal indicator of bonding between the atoms so linked. There is no net force acting on an element of ρ(r) or on an atom in a molecule in a stationary state, and ν(r) is attractive everywhere. Thus, contrary to what has appeared in the literature, no repulsive forces act on atoms linked by a bond path, nor on their nuclei. All atomic interactions, including those described as nonbonded and responsible for binding in condensed states of matter, result from a local pairing of the densities of opposite spin electrons. This local pairing, which varies throughout space and with the strength of the interaction, should be distinguished from the notion of an electron pair, as embodied in the Lewis model.
A combination of experimental (gravimetry, microcalorimetry, and quasi-elastic neutron scattering) measurements and molecular modeling was employed to understand the coadsorption of CO2 and CH4 in the zirconium terephthalate UiO-66(Zr) material from both the thermodynamic and kinetic points of view. It was shown that each type of molecules adsorb preferentially in two different porosities of the material, that is, while CO2 occupy the tetrahedral cages, CH4 are pushed to the octahedral cages. Further, a very unusual dynamic behavior was also pointed out with the slower molecule, that is, CO2, enhancing the mobility of the fast one, that is, CH4, that contrasts with those usually observed so far for the CO2/CH4 mixture in narrow window zeolites where the molecules are most commonly diffusing independently or slowing-down the partner species. Such behavior was interpreted in light of molecular simulations that evidenced a jump type mechanism involving a tetrahedral cages–octahedral cages–tetrahedral cages sequence that occurs more frequently for CH4 when in presence of CO2. The consequences in terms of CO2/CH4 selectivity and the possible use of this MOF-type material in a PSA process are then discussed. It is thus clearly emphasized that this MOF material combines several favorable features including a good selectivity, high working capacity, and potential easy regenerability that make it as a good alternative candidate of the conventional NaX Faujasite used in pressure swing adsorption.
Different measures of H-bond strength based on X−H proton donating bond properties and on parameters of H···Y distance (Y−proton acceptor within X−H···Y H bridges) are investigated. Correlations between such measures and H-bond energy are studied. The parameters of H-bonds are taken from geometry of simple complexes optimized within HF/6-311++G** and MP2/6-311++G** levels of theory. The Bader theory of atoms in molecules is also applied for an estimation of electronic densities at bond critical points and Laplacians of these densities, these topological parameters are also used to define H-bond strength measures. Apart from the conventional statistical analysis, the factor analysis is applied to study the properties of H bridges. The results show that the set of geometrical, energetic, and topological variables describing the H bridge may be replaced by one new variable, one factor. It is also shown that the geometrical and topological parameters of the proton donating bond better correlate with the H-bond energy and with the new factor than the parameters of H···Y contact.
An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way. The method allows high-quality first-principles molecular-dynamics calculations to be performed using the original fictitious Lagrangian approach of Car and Parrinello. Like the LAPW method it can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function. The augmentation procedure is generalized in that partial-wave expansions are not determined by the value and the derivative of the envelope function at some muffin-tin radius, but rather by the overlap with localized projector functions. The pseudopotential approach based on generalized separable pseudopotentials can be regained by a simple approximation.
The uptake and adsorption enthalpy of carbon dioxide at 0.2 bar have been studied in three different topical porous MOF samples, HKUST-1, UiO-66(Zr), and MIL-100(Fe), after having been pre-equilibrated under different relative humidities (3, 10, 20, 40%) of water vapor. If in the case of microporous UiO-66, CO(2) uptake remained similar whatever the relative humidity, and correlations were difficult for microporous HKUST-1 due to its relative instability toward water vapor. In the case of MIL-100(Fe), a remarkable 5-fold increase in CO(2) uptake was observed with increasing RH, up to 105 mg g(-1) CO(2) at 40% RH, in parallel with a large decrease in enthalpy measured. Cycling measurements show slight differences for the initial three cycles and complete reversibility with further cycles. These results suggest an enhanced solubility of CO(2) in the water-filled mesopores of MIL-100(Fe).
We present a detailed description and comparison of algorithms for performing ab-initio quantum-mechanical calculations using pseudopotentials and a plane-wave basis set. We will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temperature density-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N-atoms(2) scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge density including a new special 'preconditioning' optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. We have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio molecular-dynamics package), The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
In addition to its high thermal stability, repetitive hydration/dehydration tests have revealed that the porous zirconium terephthalate UiO-66 switches reversibly between its dehydroxylated and hydroxylated versions. The structure of its dehydroxylated form has thus been elucidated by coupling molecular simulations and X-ray powder diffraction data. Infrared measurements have shown that relatively weak acid sites are available while microcalorimetry combined with Monte Carlo simulations emphasize moderate interactions between the UiO-66 surface and a wide range of guest molecules including CH(4), CO, and CO(2). These properties, in conjunction with its significant adsorption capacity, make UiO-66 of interest for its further evaluation for CO(2) recovery in industrial applications. This global approach suggests a strategy for the evaluation of metal-organic frameworks for gas-based applications.
Quasi-elastic neutron scattering (QENS) measurements combined with molecular dynamics (MD) simulations were conducted to deeply understand the concentration dependence of the self- and transport diffusivities of CH(4) and CO(2) , respectively, in the humidity-resistant metal-organic framework UiO-66(Zr). The QENS measurements show that the self-diffusivity profile for CH(4) exhibits a maximum, while the transport diffusivity for CO(2) increases continuously at the loadings explored in this study. Our MD simulations can reproduce fairly well both the magnitude and the concentration dependence of each measured diffusivity. The flexibility of the framework implemented by deriving a new forcefield for UiO-66(Zr) has a significant impact on the diffusivity of the two species. Methane diffuses faster than CO(2) over a broad range of loading, and this is in contrast to zeolites with narrow windows, for which opposite trends were observed. Further analysis of the MD trajectories indicates that the global microscopic diffusion mechanism involves a combination of intracage motions and jump sequences between tetrahedral and octahedral cages.
The escalating level of atmospheric carbon dioxide is one of the most pressing environmental concerns of our age. Carbon capture and storage (CCS) from large point sources such as power plants is one option for reducing anthropogenic CO(2) emissions; however, currently the capture alone will increase the energy requirements of a plant by 25-40%. This Review highlights the challenges for capture technologies which have the greatest likelihood of reducing CO(2) emissions to the atmosphere, namely postcombustion (predominantly CO(2)/N(2) separation), precombustion (CO(2)/H(2)) capture, and natural gas sweetening (CO(2)/CH(4)). The key factor which underlies significant advancements lies in improved materials that perform the separations. In this regard, the most recent developments and emerging concepts in CO(2) separations by solvent absorption, chemical and physical adsorption, and membranes, amongst others, will be discussed, with particular attention on progress in the burgeoning field of metal-organic frameworks.
Ask the locals Dynamic deformation of the dicopper tetracarboxylate paddlewheel unit within a metal organic framework from square prismatic left to distorted right occurs at very low energies. This deformation, which contributes strongly to the negative thermal expansion of this system, is a local vibration induced by a redistribution of electron density at the Cu O junctions
Hydrothermal stability is a pertinent issue to address for many industrial applications where percent levels of water can be present at temperatures ranging from subambient to several hundred degrees. Our objective is to understand relative stabilities of MOF materials through experimental testing combined with molecular modeling. This will enable the ultimate design of materials with improved hydrothermal stability, while maintaining the properties of interest. The tools that we have employed for these studies include quantum mechanical calculations based upon cluster models and combinatorial steaming methods whereby a steam stability map was formulated according to the relative stability of different materials. The experimental steaming method allows for high throughput screening of materials stability over a broad range of steam levels as well as in-depth investigation of structural transformations under more highly resolved conditions, while the cluster model presented here yields the correct trends in hydrothermal stability. Good agreement was observed between predicted relative stabilities of materials by molecular modeling and experimental results. Fundamental information from these studies has provided insight into how metal composition and coordination, chemical functionality of organic linker, framework dimensionality, and interpenetration affect the relative stabilities of PCP materials. This work suggests that the strength of the bond between the metal oxide cluster and the bridging linker is important in determining the hydrothermal stability of the PCP. Although the flexibility of the framework plays a role, it is not as important as the metal-linker bond strength. This demonstration of alignment between experimental and calculated observations has proven the validity of the method, and the insight derived herein insight facilitates direction in designing ideal MOF materials with improved hydrothermal stability for desired applications.
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We construct a generalized gradient approximation (GGA) for the density {ital n}{sub xc}({ital r},{ital r}+{ital u}) at position {ital r}+{ital u} of the exchange-correlation hole surrounding an electron at {ital r}, or more precisely for its system and spherical average {l_angle}{ital n}{sub xc}({ital u}){r_angle}=(4{pi}){sup {minus}1}{integral}{ital d}{Omega}{sub {ital u}}{ital N}{sup {minus}1}{integral}{ital d}{sup 3}{ital r} {ital n}({ital r}){ital n}{sub xc}({ital r},{ital r}+{ital u}). Starting from the second-order density gradient expansion, which involves the local spin densities {ital n}{sub {up_arrow}}({ital r}),{ital n}{sub {down_arrow}}({ital r}) and their gradients {nabla}{ital n}{sub {up_arrow}}({ital r}),{nabla}{ital n}{sub {down_arrow}}({ital r}), we cut off the spurious large-{ital u} contributions to restore those exact conditions on the hole that the local spin density (LSD) approximation respects. Our GGA hole recovers the Perdew-Wang 1991 and Perdew-Burke-Ernzerhof GGA{close_quote}s for the exchange-correlation energy, which therefore respect the same powerful hole constraints as LSD. When applied to real systems, our hole model provides a more detailed test of these energy functionals, and also predicts the observable electron-electron structure factor. {copyright} {ital 1996 The American Physical Society.}
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
The homochiral and heterochiral hydrogen-bonded (HB) dimers of a set of small model molecules (alpha-amino alcohols) have been studied by means of ab initio methods. The gas-phase calculations have been carried out with the hybrid HF/DFT B3LYP method and the 6-311++G** basis set. The electron density of the complexes has been analyzed using the atoms in molecules (AIM) methodology, which allows characterization of the HB interactions and additional intermolecular contacts. To take into account the water solvation effect, the polarized continuum model (PCM) method has been used to evaluate the Delta G(solv). The gas-phase results show that the heterochiral dimers are the most stable ones for each case studied, while in solution for several cases, the relative stability is reversed and the homochiral dimers become more stable. The AIM analysis shows the typical bond critical points characteristic of the HB and additional bond critical points denoting, in this case, destabilization of intermolecular interaction as CF(3)...F(3)C and CH(3)...H(3)C contacts.
A large H2 sorption capacity and high surface area are properties of the depicted metal-organic porous material, which is easily synthesized on a large scale from readily available chemicals. The rigid framework of [Zn2(1,4-bdc)2(dabco)] (1,4-H2bdc = 1,4-benzenedicarboxylic acid; dabco = diazabicyclo[2.2.2]octane) is also flexible enough to exhibit unusual guest-dependent dynamic behavior, as shown (DMF = N,N-dimethylformamide).
It is shown that the electron density at the hydrogen bond critical point increases approximately linearly with increasing stabilization energy in going from weak hydrogen bonds to moderate and strong hydrogen bonds, thus serving as an indicator of the nature and gradual change of strength of the hydrogen bond for a large number of test intermolecular complexes.
The second-order Møller-Plesset (MP2) and density functional theory (DFT) calculations have been carried out to investigate the structures and stabilities of hydrogen (H-) bonded 2-hydroxypyridine (2HP)/2-pyridone (2PY) dimeric forms as well as 2HP-2PY complexes. The results on single-point counterpoise (CP) correction of these complexes were compared against CP-optimized correction. The nature of the intermolecular contacts in the sense of normal H-bond or blue-shifting H-bond was determined on the basis of harmonic vibrational, atom-in-molecule (AIM), and natural bond orbital (NBO) analysis. A blue-shifting C-H...N H-bond was found and NBO analysis revealed a slight decrease in the population of the contacting sigmaC-H* antibonding orbital as the primary reason of the C-H contraction. Good correlations have been established between the interaction energies and the H-bond distances versus other characteristic H-bond parameters.
An interdigitated porous coordination polymer with hydrophobic pore surface shows size and affinity dependent selective gas sorption properties accompanying the reversible structure transformation.
Von Dreele General Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR 86-748, 2000. (24) Toby, B. Expgui, a Graphical User Interface for GSAS
- A C Larson
A. C. Larson; R. B. Von Dreele General Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR 86-748, 2000. (24) Toby, B. Expgui, a Graphical User Interface for GSAS. J. Appl. Crystallogr. 2001, 34, 210-213.
The Quantum Theory of Atoms in Molecules: From Solid State to DNA and Drug Design An Application for QTAIM Analysis
- R J Boyd
Boyd, R. J. The Quantum Theory of Atoms in Molecules: From Solid State to DNA and Drug Design.; Wiley-VCH Verlag GmbH & Co. KGaA, 2007. (35) Vega, D.; Almeida, D. AIM-UC: An Application for QTAIM Analysis. J. Comput. Methods Sci. Eng. 2014, 14, 131-136.
Structural Methods in Molecular Inorganic Chemistry
- C Morrison
Morrison, C. Structural Methods in Molecular Inorganic Chemistry; John Wiley & Sons, Hoboken, 2013.
- Long J. R.