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An investigation of Knoevenagel condensation reaction in microreactors using a new zeolite catalyst
Abstract
New basic zeolite catalysts obtained by grafting amino groups onto NaX and CsNaX zeolites exhibit excellent catalytic activities for Knoevenagel condensation reaction between benzaldehyde and ethyl cyanoacetate (ECA), ethyl acetoacetate (EAA) and diethyl malonate (DEM). The CsNaX-NH 2 catalyst also displays higher conversion compared to aminopropylated MCM-41. Knoevenagel condensation reaction in a CsNaX zeolite microreactor performed better than the traditional packed bed reactor (PBR) with an order of magnitude higher productivity (i.e. moles ethyl 2-cyano-3 phenylacrylate produced per hour for each gram catalyst). A nearly fourfold increase in reaction conversion was obtained for the microreactor when CsNaX-NH 2 catalyst was used. A membrane microreactor was obtained by incorporating a water-selective, NaA membrane to the multi-channel microreactor. The selective removal of water byproduct during the reaction led to a 25% improvement in reaction conversion for both catalysts.
... However, in the quest to optimize this process, a wide range of catalysts has been employed, including heterogeneous catalysts as Lewis acids, zeolites, solid bases. [15,16] The nature of the catalyst, particularly its acidity or basicity, plays a critical role in the efficiency and selectivity of this reaction. Chughtai et al. [17] reported good catalytic activity for an amide functionalized MOF (LOCOM-1) as basic catalyst for the Knoevenagel condensation reaction. ...
A series of ordered mesoporous silicas (MCM‐41, SBA‐15 and KIT‐6) were successfully synthesized and modified with nickel by incipient wetness impregnation method. The supports and catalysts were characterized by N2 adsorption‐desorption, XRD, TEM, H2‐TPR, UV vis‐DR, XPS, ICP and Py FT‐IR techniques. All the materials were evaluated in the Knoevenagel condensation reaction between vanillin and malononitrile under microwave irradiation. The catalytic results show the key role that the chosen porous structure plays in the deposition of the active phase and its catalytic behaviour. Thus, by designing a suitable mesoporous catalyst it was possible to carry out such Knoevenagel condensation to obtain 2‐(4‐hydroxy‐3‐methoxybenzylidene) malononitrile through a highly efficient and environmentally friendly process, without solvents and reducing reaction times by employing microwave heating.
... In this expression, x represents the product (benzalmalononitrite) concentration, t is the reaction time, and a and b are the initial concentrations of malononitrile and benzaldehyde, respectively. The correlation coefficient of this relationship is notably high at 0.982, suggesting that the reaction follows a pseudo-secondorder kinetic model 61,62 . To further investigate the individual effects of benzaldehyde and malononitrile concentrations on the reaction, their concentrations were varied separately in a series of experiments. ...
Membrane reactors are known for their efficiency and superior operability compared to traditional batch processes, but their limited diversity poses challenges in meeting various reaction requirements. Herein, we leverage the molecular tunability of covalent organic frameworks (COFs) to broaden their applicability in membrane reactors. Our COF membrane demonstrates an exceptional ability to achieve complete conversion in just 0.63 s at room temperature—a benchmark in efficiency for Knoevenagel condensation. This performance significantly surpasses that of the corresponding homogeneous catalyst and COF powder by factors of 176 and 375 in turnover frequency, respectively. The enhanced concentration of reactants and the rapid removal of generated water within the membrane greatly accelerate the reaction, reducing the apparent activation energy. Consequently, this membrane reactor enables reactions that are unattainable using both COF powders and homogeneous catalysts. Considering the versatility, our findings highlight the substantial promise of COF-based membrane reactors in organic transformations.
... It is noticeable that the monolith reactor is more effective than other reactors such as the packed-bed reactor (as shown in Table 6) [32][33][34][35][36]. The cyanoethyl acetate conversion in our work was as high as many other reported value. ...
A green chemical approach is used to construct monolithic porous microreactors with a high catalytic performance for Knoevenagel reaction. A facile method is reported to construct monolithic microreactor with high catalytic performance for Knoevenagel reaction. The microreactor is based on hierarchically porous silica (HPS) which has interconnected macro-and mesopores. Then the HPS is surface modified by pyrogallol (PG) polymer. Al(NO 3) 3 and Mg(NO 3) 2 are loaded on the surface of HPS through coordination with-OH groups of PG. After thermal treatment, Al(NO 3) 3 and Mg(NO 3) 2 are converted Al 2 O 3 and MgO. The as-synthesized catalytic microreactor shows a high and stable performance in Knoevenagel reaction. The microreactor possess large surface area and interconnected pore structures which are beneficial for reactions. Moreover, this economic, facile and eco-friendly surface modification method can be used in loading more metal oxides for more reactions.
... Heterogeneous catalysts based on metal oxides, such as alkaline earth metal oxides, metal-doped mesoporous materials, zeolites, and metal oxides supported on solid materials have been widely proposed to overcome the demerits of homogeneous ones. [4][5][6][7][8][9][10] However, these metal oxide-based catalysts are not environmentally friendly and suffer from drawbacks, such as contamination and low catalytic activities. [11,12] Therefore, novel metal-free solid base catalysts are urgent subjects of development for Knoevenagel condensation to synthesize various types of key organic compounds. ...
Nitrogen‐rich carbon nitrides are desired materials for base‐catalysed transformations; however, their synthesis is challenging due to the volatile nature of N at high temperatures. Herein, we report on the temperature‐controlled synthesis of ordered N‐rich mesoporous carbon nitrides (MCNs) via pyrolysis of aminoguanidine by using SBA‐15 as a hard template. The properties and the nitrogen content of the materials were tuned by varying the carbonization temperature in the range of 350–500 °C. At 350 and 400 °C, higher amounts of N could be retained in the MCN framework with the predominant formation of C3N6 having a six‐membered aromatic ring with diamino‐s‐tetrazine moiety, whereas C3N5 with 1‐amino/imino‐1,2,4‐triazole moieties was produced at 450 and 500 °C. The base catalytic activity of MCNs in Knoevenagel condensation of benzaldehyde with malononitrile revealed that the MCN‐400 exhibited the highest catalytic performance by displaying a 96.4 % product yield with toluene as a solvent. The superior catalytic activity of MCN‐400 is attributed to high N content (62.6 wt%), high surface area (235 m² g⁻¹), and large pore volume (0.74 cm³ g⁻¹). The optimum temperature for obtaining the highest yield of the products is 80 °C, and the catalyst showed good cycling stability for 5 consecutive cycles.
... In addition to the common utilization to evaluate base-catalyzed reactions, [18] Knoevenagel condensation is also a key step in the production of several therapeutic drugs such as Niphendipine and Niterndipine, as well as pharmacological products such as calcium channel blockers and antihypertensives. [19] The above supported organocatalyst is also applied in the Knoevenagel condensation. [5][6][7][8]11,[20][21] For instance, high catalytic effect appears when the catalyst is supported with molecular sieves, while a high temperature of 50°C is still required. ...
Inspired by the formation of microspheres by hexachlorocyclotriphosphazene and 4, 4′‐sulfonyldiphenol, polyphosphazene‐functionalized microspheres were developed. Benefits from the supported supper basic phosphazene, the yield exceeded 99 % at room temperature in the manner of second‐order reaction kinetics toward Knoevenagel reaction and was still maintained at 99 % after 16 runs. In the experimental temperature from 0 °C to 90 °C, the yield increased from 92 % to 99 %, reflecting that the catalyst had strong applicability under mild conditions. This behavior was conducive to energy conservation. Meanwhile, simple separation and recovery further enhanced this advantage. In addition, the catalyst was also found to be insensitive to aqueous solution or organic solvents such as toluene, THF, EtOH and CH3CN. This property gave the Knoevenagel reaction a vast choice. All these features exhibit that this novel catalyst is an attractive and applicable alternative in organic synthesis.
... Functionalization of zeolites using different types of silane coupling agents is among the effective technique used to modify the zeolite properties for wide applications [5][6][7][8][9]. The most commonly used silane coupling agents are (γ-aminopropyl)-triethoxysilane (APTES), 3aminopropylmethyldiethoxysilane (APMDES) and (γ-aminopropyl)-diethoxymethylsilane (APDEMS) [10][11][12][13][14][15]. ...
Over the years, functionalization of zeolite is gaining popularity among researchers to further modify the properties of the zeolite for wide applications. The procedure of functionalization is crucial to ensure that the framework and structure of the zeolite would not be destroyed by the functionalization process. In this work, zeolite AlPO-18 was synthesized via hydrothermal synthesis method and functionalized by (3-Aminopropyl) triethoxysilane (APTES). The effect of the APTES functionalization on zeolite AlPO-18 was investigated in this work. Both unfunctionalized and silane-functionalized zeolite AlPO-18 were characterized using Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and Thermogravimetric analysis (TGA) for their properties. The morphology and the composition of the elements present in zeolite AlPO-18 and zeolite NH 2 -AlPO-18 were examined using Field Emission Scanning Electron Microscopy (FESEM) and Energy-Dispersive spectroscopy (EDX) respectively. The XRD pattern of NH 2 -AlPO-18 was similar to that of zeolite AlPO-18, however, the intensity of the peaks was lower compared to zeolite AlPO-18. Based on the FTIR spectra, the presence of N-H stretching and bending vibration band of aminosilane were observed in the NH 2 -AlPO-18 sample. According to FESEM images, the morphology of NH 2 -AlPO-18 was comparable to that of zeolite AlPO-18 even after functionalization, proving that functionalization of aminosilane on zeolite does not affect on the zeolite structure. Besides that, EDX proves the presence of 3.02 % of element N in the NH 2 -AlPO-18 sample which is absent in the zeolite AlPO-18 sample. All of the characterizations evinced the presence of aminosilane, APTES in the NH 2 -AlPO-18 sample.
... For the catalysts based on zeolites as supports and gold NPs as the main active centers it is relatively easy to determine the structure and type of active sites (e.g., Brønsted acid sites (BAS) in zeolites). Moreover, zeolites can be easily modified by isomorphous substitution in their skeleton (e.g., by incorporation of boron instead of aluminum in the aluminosilica framework [10,11]) or by post-synthesis modification (e.g., by functionalization of the zeolite surface with amino-organosilanes [12,13]). Changes in the zeolite composition imply changes in the surface (acid-base) properties and thus modify the interaction between the support and gold NPs. ...
Activity of gold supported catalysts strongly depends on the type and composition of support, which determine the size of Au nanoparticles (Au NPs), gold-support interaction influencing gold properties, interaction with the reactants and, in this way, the reaction pathway. The aim of this study was to use two types of zeolites: the three dimensional HBeta and the layered two-dimensional MCM-36 as supports for gold, and modification of their properties towards the achievement of different properties in oxidation of glucose to gluconic acid with molecular oxygen and hydrogen peroxide. Such an approach allowed establishment of relationships between the activity of gold catalysts and different parameters such as Au NPs size, electronic properties of gold, structure and acidity of the supports. The zeolites were modified with (3-aminopropyl)-trimethoxysilane (APMS), which affected the support features and Au NPs properties. Moreover, the modification of the zeolite lattice with boron was applied to change the strength of the zeolite acidity. All modifications resulted in changes in glucose conversion, while maintaining high selectivity to gluconic acid. The most important findings include the differences in the reaction steps limiting the reaction rate depending on the nature of the oxidant applied (oxygen vs. H2O2), the important role of porosity of the zeolite supports, and accumulation of negative charge on Au NPs in catalytic oxidation of glucose.
... Functionalized zeolites, such as FAU and zeolite X, were reported as efficient base catalysts in Knoevenagel reactions [9,10]. However, with a possible mass-transfer-limitation of zeolites due to their small pore systems (less than 2 nm), efforts have been directed to use other supports with a wider pores such as mesoporous materials [11,12] and metal organic framework (MOF) [13]. ...
This research, for the first time, reports the design and development of a heterogeneous nano-catalyst based on sodium ions (Na+) incorporation in Technical University of Delft (TUD-1) mesoporous silica for Knoevenagel condensation reaction. Facile one-step fabrication of Na-TUD-1 nano-catalysts (varying Si/Na ratio as 100–5) was demonstrated using the sol–gel route. The catalytic performance of Na-TUD-1 was evaluated as a base heterogeneous catalyst in Knoevenagel condensation reaction, which took place under conventional and microwave irradiations conditions using ethanol as a solvent. Na-TUD-1 exhibited superior catalytic activity in comparison to available homogeneous base catalysts such as sodium ethoxide. The Na-TUD-1 nano-catalyst demonstrated identical performance till the fourth run along with high stability and negligible leaching of Na. Moreover, the use of microwave heating reduced the reaction time from 240 to 20 min only with a TOF of 0.58 min−1. Such excellent performance of Na-TUD-1 heterogeneous nano-catalysts will certainly increase its industrial acceptability to achieve affordable and efficient waste-effluent treatments.
... In connection with the second group, porous as well as layered structures were probed as catalysts in Knoevenagel reaction. Zeolites [16,17], montmorillonite [18], layered double hydroxides [19,20] and their modified versions [21,22] displayed advantageous features from increasing the selectivity to decreasing the reaction time. However, either the reusability or the general applicability of the catalysts often caused serious problems. ...
Both Bi- and Ce-based CeBi mixed oxides were prepared by a modified sol-gel process from their precursor salts. The mixed as well as the parent oxides were characterized by X-ray diffractometry, Raman, XPS, UV–DRS and X-ray photoelectron spectroscopies, ICP–OES method, scanning and transmission electron microscopies as well as BET surface area, CO2- and NH3-temperature-programmed desorption measurements. After characterizing the morphology, the acid-base properties, the oxidation states of the cationic components and the porosity of these structures, their catalytic activities were probed in the Koevenagel condensation of benzaldehyde and diethyl malonate and the toluene oxidation to benzaldehyde reactions. Based on the catalytic activities of the oxides in the individual reactions, a catalyst mixture from the Bi- and Ce-based mixed oxides was used successfully in the toluene to benzaldehyde oxidation and benzaldehyde to benzylidene malonate Knoevenagel condensation domino reaction under environmentally benign conditions.
... Systematic studies of oily wastewater filtration using SiC ceramic membranes are rare in the published literature. 5,6,21 The permeability, TDS, COD, and oil rejection results of the membrane prepared in the present work are comparable to the reported membrane prepared from α-Al 2 O 3 , clay, mullite, etc. [47][48][49][50] Performance of the used membrane after cleaning with n-hexane followed by acid, alkali cleaning was found to remain same. The result suggests that ceramic membranes used for oily-wastewater filtration can be recycled and reused. ...
Porous SiC ceramic is considered as a suitable material for hot gas filtration, microfiltration, and many others industrial applications. However, full utilizations of porous SiC ceramics have been limited by high‐processing costs. In this study, mullite‐bonded porous SiC ceramics membranes were prepared using commercial SiC powder, alumina, clay, and different sacrificial pore formers. The effect of different pore formers on the microstructure, mechanical strength, porosity and pore size distribution, air, and water permeability of porous SiC ceramics were investigated. The average pore diameter, porosities, and flexural strength of the final ceramics varied in the range 3.7‐6.5 µm, 38‐50 vol. %, and 28‐38 MPa, respectively, depending on the characteristics of pore former. The Darcian (k1) and non‐Darcian (k2) permeability evaluated from air permeation behavior at room temperature was found to vary from 1.48 × 10⁻¹³ to 4.64 × 10⁻¹³ m² and 1.46 × 10⁻⁸ to 6.51 × 10⁻⁸ m, respectively. All membranes showed high oil rejection rate (89%‐93%) from feed wastewater with oil concentration of 1557 mg/L. The membrane with porosity ~48 vol% and mechanical strength 31.5 MPa showed and highest pure water permeability of 13 298 Lm⁻²h⁻¹bar⁻¹.
... Many heterogeneous catalysts, such as Ni-SiO 2 -supported catalysts [10], nitrogen-doped carbon materials [11], MCM-41 [12], zeolites [13], enzymes [14], amino groups immobilized on silica gel [15], Lewis acidic/basic bifunctional organobismuth complex in ionic liquids [16], and other catalysts based on alumina [17], silica sulfuric acid [18,19], zinc and magnesium oxides [20], microporous polyurethane [21], zeolite membrane microreactor [22][23][24] have been exploited as catalysts for the Knoevenagel condensation. There has been a long standing desire in preparative chemistry to avoid waste, to substitute benign chemicals and solvents for toxic ones and to use solvent-less reactions. ...
Cross-linked polystyrene copolymer beads with the average particle size in the range of (50–80 mesh size) were prepared by a new method, characterized and functionalized with titanium tetrachloride to afford the corresponding polystyrene‑titanium tetrachloride complex in one step reaction and characterized by FT-IR, UV, TGA, DSC, XRD, SEM, BET. This polymer metal complex (PS/TiCl 4 ) was used as a heterogeneous, recoverable, reusable Lewis acid for solvent-free Knoevenagel condensations of 1,3-diketones with aromatic aldehydes under green and mild conditions. The rate of reactions was found to decrease with an increasing percentage of crosslinking and the mesh size of the copolymer beads. This complex showed good stability and catalytic activity in the Knoevenagel reactions.
... Other applications apart from the single gas permeation tested here include gas-liquid contactors, where the 3D structure presents an advantage due to the higher surface-to-volume ratio compared to 2D membranes, and also applications requiring high temperatures and gas reactant distribution or product removal, such as micromembrane microreactors [15]. ...
The novel concept of a microfluidic chip with an integrated three-dimensional fractal geometry with nanopores, acting as a gas transport membrane, is presented. The method of engineering the 3D fractal structure is based on a combination of anisotropic etching of silicon and corner lithography. The permeation of oxygen and carbon dioxide through the fractal membrane is measured and validated theoretically. The results show high permeation flux due to low resistance to mass transfer because of the hierarchical branched structure of the fractals, and the high number of the apertures. This approach offers an advantage of high surface to volume ratio and pores in the range of nanometers. The obtained results show that the gas permeation through the nanonozzles in the form of fractal geometry is remarkably enhanced in comparison to the commonly-used polydimethylsiloxane (PDMS) dense membrane. The developed chip is envisioned as an interesting alternative for gas-liquid contactors that require harsh conditions, such as microreactors or microdevices, for energy applications.
The synthesis of ethyl (hetero)arylidene cyanoacetates, valuable intermediates in organic chemistry, has been investigated using several commercially available and synthetic nitrogen-based organocatalysts. 4,4′-Trimethylenedipiperidine (TMDP) was chosen as an efficient organocatalyst regarding short reaction times, excellent yield with a conversion of 100%, and high selectivity. The pure products could be isolated and devoid of a costly workup. The residue could be directly reused for the next catalytic run. Encouragingly, TMDP exhibited chemical stability and high recyclability in five subsequent runs without a significant loss in catalytic activity. Scale-up experiments also demonstrate the current strategy’s promising industrial application. Mechanistic studies were conducted to provide insights into the reaction pathways and possible interaction/reaction between organocatalyst and each reactant through performing control experiments and studying FTIR in neat state and NMR spectra in aprotic and protic deuterated solvents. The developed metal-free and halogen-free catalytic strategy offers cost-effectiveness, low toxicity, and enhanced sustainability.
Graphical Abstract
Non-hygroscopic polystyrene-supported chloroaluminate ionic liquid was prepared from the reaction of Merrifield resin with pyridine followed by reaction with aluminium chloride. This Lewis acidic ionic liquid is an environmentally friendly heterogeneous catalyst for the Knoevenagel condensation of aromatic and aliphatic aldehydes with ethyl cyanoacetate. The catalyst is stable (as a bench top catalyst) and can be easily recovered and reused without appreciable change in its efficiency.
This research work presents the use of base-modified Tunisian Smectite as catalysts for the condensation of Knoevenagel. The prepared clay-catalysts were characterized via FTIR and XRD studies. A series of condensation reactions based on ultrasonic irradiation of various types of active methylene compounds and a variety of aryl aldehydes have shown the high efficacy of prepared catalysts. The stability and catalytic performance of the clay-catalysts were excellent since they could be reused four times without considerable loss in catalytic activity.
To achieve the aims of green chemistry, persimmon tannin-propane-melamine-Zirconium (PT-Pr-Me-Zr) was synthesized as a bio-based bifunctional heterogeneous catalyst polymer. Persimmon tannin was used as a biopolymer for the synthesis of bio-organic catalyst, and the properties of PT-Pr-Me-Zr were characterized using X-ray powder diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, Energy-dispersive X-ray spectroscopy, and field emission scanning electron microscopy. The as-synthesized composite was applied for the Knoevenagel condensation between aromatic aldehyde derivatives and malononitrile, then the conditions of reaction such as solvent and catalyst loading amount were optimized. The bifunctional property of PT-Pr-Me-Zr catalyst was improved the yield and rate of Knoevenagel reaction. The PT-Pr-Me-Zr can be recovered for five times, which was illustrated the reusability of catalyst. Based on the findings, the PT-Pr-Me-Zr can be applied as an inexpensive and ecofriendly catalyst in organic reaction such as Knoevenagel condensation.
1-Butyl-3-vinylimidazolium chloride was synthesized and polymerized with acrylamide to furnish an ionic liquid-containing polymer, which was then used for the formation of a composite with iron-based metal–organic framework. The resultant composite was characterized with XRD, TGA, FE-SEM, FTIR, EDS and elemental mapping analyses and its catalytic activity was appraised for ultrasonic-assisted Knoevenagel condensation. The results confirmed that the prepared composite could promote the reaction efficiently to furnish the corresponding products in high yields in very short reaction times. Moreover, the composite exhibited high recyclability up to six runs. It was also established that the activity of the composite was higher compared to pristine metal–organic framework or polymer.
A simple one‐pot procedure under mild conditions has been developed with the aim of preparing a set of hybrid organic‐inorganic multifunctional materials. This procedure is based on the use of catechol and KIO4 as oxidizing agent in conjunction with 3‐aminopropyl‐, substituted 3‐aminopropyl‐ and 3‐methylimidazolium‐1‐trimethoxypropylsilane mimicking polydopamine‐like chemistry. Reactions were carried out in water at room temperature and 70 °C to give fifteen materials that were characterised by using several techniques (nitrogen physisorption, TGA, XPS, ¹³C and ²⁹Si CP‐MAS NMR, IR, pHPZC). Knoevenagel reaction was chosen as a good probe to investigate the availability of functional groups on the surface of the materials. The most active catalytic materials were tested in recycling procedures. Such simple procedure is of broad interest for the scientific community and it open the doors to the development of new hybrid organic‐inorganic multifunctional materials for several purposes.
A detailed investigation on the crystallization of offretite (OFF) zeolite templated by non-toxic cetyltrimethylammonium bromide (CTABr) is reported. Various hydrothermal synthesis conditions are prepared by systematically adjusting the synthesis variables, namely crystallization time, heating temperature and amount of reactants (SiO2, Al2O3, K2O, M2O (M = Na⁺, Cs⁺), CTABr and H2O) to investigate their effects on the crystal growth of OFF zeolite. It is shown that each variable has its profound influences on the crystallization process, which further affect the properties of final zeolite products (e.g. phase purity, morphology, crystal size). Unlike classical synthesis pathway using tetramethylammonium (TMA⁺) as organic template, the CTABr-templating approach successfully produces large OFF single crystals (framework Si/Al ratio = 4.11) with hexagonal prism shape (ca. 9.4 × 1.8 μm²) and smooth surface. Besides that, the OFF zeolite (81.9% conversion) also shows comparable or even higher catalytic performance than other zeolites reported in this work and other works in microwave assisted-Knoevenagel reaction of benzaldehyde and ethyl cyanoacetate, offering a sustainable and facile pathway for preparing zeolitic materials for advanced applications.
Knoevenagel condensation is a C–C bond formation reaction widely used in fine chemistry, which can be catalyzed by bases, such as Na-zeolites and supported amines. Due to the high size of the molecules that are used, its application is highly influenced by the accessibility of the catalytic sites that promote the reaction. In this context, this work studied the effect of the crystallization time of aluminosilicate precursors of LTA zeolite on the propylamine anchoring capacity. The materials were compared in terms of the amount of grafted propylamine as well as the accessibility of the sites in the Knoevenagel condensation reaction. Functionalized solids prepared with shorter times in hydrothermal synthesis were more active in the reaction due to the greater propylamine anchoring capacity. Also, the amorphous precursors of LTA zeolite were more active, due to their greater accessibility of the catalytic sites before the microporous structure was formed.
Green and efficient routes for a sustainable and renewable strategy in chemicals and energy delivery are highly demanded in the current status. Developing production routes from bio-alcohols is of immense interest. To achieve such a goal, an enhanced membrane-assisted dehydration reactor for the conversion of bio-alcohols (mainly made up of methanol and ethanol) is suggested. The system can eliminate the innate and produced water through the dehydration reaction, representing an intensified process that favors a conversion enhancement without needing a water purification unit. This study includes a rigorous feasibility study utilizing a validated mathematical model. The impacts of input variables such as feed flowrate, feed temperature, feed composition, and feed pressure, as well as the sweep gas flowrate and temperature, are examined. The sensitivity analysis is conducted in terms of feed conversion, temperature profiles, pressure drop, and desired product distributions. Then, an optimization strategy was developed to obtain the optimum operating conditions of each system leading to maximized conversion and product yields. According to the achievements of this study, the enhanced membrane-assisted dehydration reactor offers a great capacity for converting bio-alcohols and producing considerable quantities of DME from bio-methanol as well as DEE, ethylene, and butylene from bio-ethanol.
Electron-donating groups in the robust MOF motif are able to provide an excellent catalytic platform, therefore obtaining site-isolated metal sites. In this study, the terminal alkyne is firstly introduced into UiO-66-type metal-organic frameworks (UiO-66-alkyne). Herein, to further study the application potential of this material, a covalently bonded nickel catalyst based on the alkynyl-tagged UiO-66-alkyne has been prepared and afforded us unprecedented highly dispersed and highly efficient catalytic active species. Meanwhile, this nickel-contained catalyst method for joining metals provided an alternative pathway regarding catalyst designing. With UiO-66-alkyne-Ni, the heterogeneous transformation of homogeneous catalysts is realized. Using benzaldehyde and malononitrile as starting materials, we were able to catalyze the Knoevenagel condensation within 45 min under room temperature with yield (> 99%). Moreover, the recovery rate of the UiO-66-alkyne-Ni also outperformed previous MOFs in both small-scale and gram-level reactions, which shows UiO-66-alkyne-Ni is a potential contributor to the subsequent industrialization.
Alanine functionalized mesoporous SBA-15 (SBA-15-Alanine) as an active and selective catalyst for Knoevenagel condensation reaction of benzaldehyde and ethyl cyanoacetate is reported. The catalyst is prepared by 2-step grafting approach using (3-chloropropyl)triethoxysilane (CPTES) followed by dl-alanine. The XRD, TEM and N2 adsorption results reveal that the mesoporous characteristic of SBA-15-Alanine is retained upon modification. Furthermore, the IR, TG/DTG, CHN and TPD analyses indicate the successful functionalization of alanine, leading to the formation of bifunctional mesoporous catalyst bearing both acidic (-COOH) and basic (-NH) sites. This catalyst exhibits acid-base cooperative effect and good catalytic performance in the Knoevenagel condensation between benzaldehyde with ethyl cyanoacetate (96.7% conversion) with 100% selective to ethyl α-cyanocinnamate under novel non-microwave instant heating conditions. The SBA-15-Alanine also shows stable catalytic performance with high reusability where insignificant decline in catalytic reactivity is observed after five successive cycles, thus suggesting a promising replacement of base catalysts in condensation reactions.
Bifunctional acid‐base catalysts have been developed by fabricating core‐shell faujasite@aqueous miscible organic‐layered double hydroxides (FAU@AMO‐LDHs) composites via the growth of AMO‐LDHs on zeolite surfaces. Interestingly, by adjusting the acid/base properties of the composites, the one‐pot synthesis of ethyl trans‐α‐cyanocinnamate via deacetalization‐Knoevenagel reaction can be achieved up to 90% yield. In contrast, almost no desired products can be observed in cases of isolated zeolites and AMO‐LDHs, demonstrating the synergistic effect of zeolites and AMO‐LDHs as acid‐base catalysts. These findings open up new perspectives of the development of tunable acid‐base zeolite@LDHs composites in an acid‐base tandem reaction, which can selectively produce ethyl trans‐α‐cyanocinnamate up to 90% yield. The tunable acid/base FAU@AMO‐LDHs composites promote an excellent catalytic performance in one‐pot tandem reaction.
A novel biochar-based graphitic carbon nitride was prepared through calcination of Zinnia grandiflora petals and urea. To provide acidic and ionic-liquid functionalities on the prepared carbon, the resultant biochar-based graphitic carbon nitride was vinyl functionalized and polymerized with 2-acrylamido-2-methyl-1-propanesulfonic acid, acrylic acid and the as-prepared 1-vinyl-3-butylimidazolium chloride. The final catalytic system that benefits from both acidic (–COOH and –SO3H) and ionic-liquid functionalities was applied as a versatile, metal-free catalyst for promoting some model acid catalyzed reactions such as Knoevenagel condensation and Biginelli reaction in aqueous media under a very mild reaction condition. The results confirmed high activity of the catalyst. Broad substrate scope and recyclability and stability of the catalyst were other merits of the developed protocols. Comparative experiments also indicated that both acidic and ionic-liquid functionalities on the catalyst participated in the catalysis.
Three amino‐functionalized alkaline earth Metal‐Organic Frameworks (MOFs), including Mg‐ABDC [Mg(ABDC)(DMF)], Ca‐ABDC [Ca(ABDC)(DMF)] and Sr‐ABDC [Sr(ABDC)(DMF)]( ABDC=2‐aminobenzene‐1,4‐dicarboxylate anion), were synthesized under solvothermal conditions. The incorporation of alkaline earth metals and N‐containing ligands is an effective way to increase the basicity of MOFs. To explore catalytic activity of three alkaline earth MOFs, they were applied to catalyze the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate to produce ethyl (E)‐α‐cyanocinnamate. All three alkaline earth MOFs had high catalytic performance, among which Mg‐ABDC showed the best performance. The activity of catalyst was expressed by the conversion rate and selectivity. The conversation rate was up to 94.7 % and the selectivity was nearly 100 % by adding Mg‐ABDC. Mg‐ABDC was proved to be an efficient, stable and heterogeneous catalyst through hot filtration and recycling tests.
A novel application is described for utilizing hydrogel dots as organocatalyst carriers inside microfluidic reactors. Tertiary amines were covalently immobilized in the hydrogel dots. Due to the diffusion of reactants within the swollen hydrogel dots, the accessible amount of catalysts inside a microfluidic reactor chamber can be increased compared to the accessible amount of surface‐bound catalysts. To perform fast Knoevenagel reactions, important flow parameters had to be validated to optimize the reactor performance while keeping the dimensions of the reactor chamber constant; e.g. the height of the hydrogel dots had to be adjusted to the invariable dimensions of the reactor chamber, or an adjustment of organocatalysts in the hydrogel dots had to be validated to achieve the highest conversion rate during a certain residence time. To characterize the conversion, nuclear magnetic resonance (NMR) and UV/Vis‐spectroscopy were utilized as an offline and online method, respectively. With suitable hydrogel dots, the influence of different flow parameters (e.g., operating flow rate and reactant concentration) on the selected model reactions in the microfluidic reactor was investigated. Finally, a variety of reactants were screened with the optimized flow parameters. With these results, the turnover frequency was determined for the Knoevenagel reactions in a microfluidic reactor, and the results were compared with published data that were determined by other synthetic approaches.
Solid Lewis-base catalysis is important in the production of fine chemicals. A Lewis-base Ga4B2O9 was synthesized by high temperature solid state reactions. It exhibited a high yield (90 %) and a high stability in Knoevenagel condensation reactions, where several aldehydes were combined with malononitrile to form α, β-unsaturated compounds through nucleophilic addition reactions. Reaction kinetics analyses indicated Knoevenagel condensation reactions over Ga4B2O9 catalyst obeyed a second-order characteristics and the calculated activation energy was ∼61.6 kJ/mol, suggesting that Langmuir-Hinshelwood absorption pathway was probably employed. The structural evolution from Ga4B2O9→GaBO3→β-Ga2O3 evidenced that the structure-induced basicity of Ga4B2O9 attributed to the special μ3-O atoms linked exclusively to 3 five-coordinated Ga³⁺. This investigation proves a convincible structure-property correlation in Ga-based solid base materials and helps developing an alternative avenue towards the design of new intrinsic solid base catalysts.
A Pickering emulsion catalytic system was developed using amphiphilic amine-functionalized graphene oxide (GO-NH2) for a Knoevenagel condensation in aqueous medium instead of organic solvent. The properties of GO-NH2 and the Pickering emulsion stabilized by GO-NH2 were characterized by Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS), organic elemental analysis (OEA), contact angle (CA), Brunauer–Emmett–Teller surface area, and optical microscopy. The results indicated that the properties of the as-prepared Pickering emulsion system, such as droplet density, size, and distribution, were greatly affected by the amount of GO-NH2 and the oil–water ratio, which were closely related to the catalytic performance for Knoevenagel condensation reactions. Furthermore, this Pickering emulsion system for the Knoevenagel condensation in an aqueous medium exhibited a much higher catalytic activity than those of GO-NH2 in organic solvents under the same reaction conditions. The reaction rate constant (k) in the Pickering emulsion system was calculated to be 0.043 min⁻¹ at 50 °C. The enhanced activity of the Pickering emulsion was mainly attributed to large interfaces between GO-NH2 and reactants in the emulsion, and the clear confinement effect. Furthermore, GO-NH2 remained stable for preparing Pickering emulsion systems after recycling in six runs, exhibiting its good reusability.
Graphic Abstract
A novel Pickering emulsion catalytic system was developed using amphiphilic amine functionalized graphene oxide (GO-NH2) for Knoevenagel condensation in aqueous medium instead of organic solvents. It exhibited excellent catalytic performance in the reaction. Open image in new window
A novel metal-free solid acid catalyst, PHSA, has been designed and synthesized through facile and environmentally benign procedure via using halloysite nanoclay as a natural and biocompatible support. The synthetic protocol included co-polymerization of 2-acrylamido-2-methylpropane sulfonic acid and acrylic acid in the presence of vinyl functionalized halloysite. The metal-free solid catalyst that contained both carboxylic acid and sulfonic acid functionality exhibited high catalytic activity for catalyzing both Knoevenagel condensation and Xanthenes synthesis under mild reaction conditions in aqueous media. The presence of halloysite in the structure of PHSA rendered the catalyst heterogeneous and facilitated its recovery and recyclability compared to the halloysite-free homogeneous counterpart. High catalytic activity and recyclability of the catalyst, simplicity of the preparation procedure, using available and natural clay as a support are the merits of this new metal-free solid acid catalyst.
A novel metal -free triple composite that benefits from the chemistry of chitosan, β-cyclodextrin and ionic liquid has been prepared through polymerization of vinyl functionalized chitosan and cyclodextrin decorated ionic liquid. The resulting compound, CS-CD-IL, was characterized and successfully applied as a heterogeneous catalyst for promoting Knoevenagel condensation reaction under mild reaction condition in aqueous media. It was believed that the presence of cyclodextrin in the structure of the catalyst could facilitate the reaction in aqueous media through transferring the hydrophobic substrate in the vicinity of the catalytic sites. Chitosan and ionic liquid, on the other hand, could activate the substrates via two pathways. The effects of the reaction variables as well as the recyclability of the catalyst were also investigated. The results confirmed high recyclability of the catalyst up to six reaction runs. Notably, the developed catalyst was versatile and could promote similar condensation reactions to furnish more complex chemicals such as Xanthane.
This work presented a microfluidic processing of zeolite film in a capillary microreactor and its potential application for biomass conversion. Uniform and continuous HZSM-5 zeolite film was fabricated under a continuous flow hydrothermal system in which the film thickness could be precisely tuned by flow rate and synthesis time. The catalytic performance of this acidic HZSM-5 film capillary microreactor was evaluated by continuous flow acetalisation reaction of glycerol with acetone, affording ~62.61% solketal yield under mild and solvent free conditions. No deactivation was observed even after 100 h, indicating the excellent stability of this HZSM-5 film capillary microreactor. This microfluidic processing strategy provided an extended applicability in the fabrication of zeolite films-based capillary microchannel with high length-to-diameter ratio or other chip-based microchannel devices.
In the present work, a new MOF material from UiO-family called Zr-UiO-66-NH-CH2-Py (1) has been obtained by solvothermal technique and successfully characterized. The MOF structure is contains 2-((pyridin-4-ylmethyl) amino) terephthalic acid (H2BDC-NH-CH2-Py) linker. The activated form of 1 (called 1′) exhibits considerable thermal as well as chemical stability. Compound 1′ showed very rapid and selective response for the fluorometric sensing of superoxide (O2•−) in aqueous medium even in presence of the potentially competitive reactive oxygen species (ROS). The limit of detection value for O2•− sensing is 0.21 μM, which is comparable with the reported O2•− sensors. This is the first MOF based fluorescent sensor for the detection of O2•−. The response time of this MOF sensor for O2•− is very short (60 s). On the other hand, 1′ was employed as a solid heterogeneous catalyst for Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate at 80 °C in ethanol by providing a very high yield of the desired product. The effects of the free linker (H2BDC-NH-CH2-Py) and corresponding metal salt (ZrCl4) on this catalytic reaction were examined separately. We have also scrutinized the substrate scope elaborately for the catalytic reaction by catalyst 1′.
Catalytically active membranes can realize chemical reaction and membrane separation simultaneously and have been recognized as an effective strategy for process intensification to offer smaller footprint, lower energy consumption, and enhanced process efficiency. Reaction-separation coupling involving high temperature reactions has traditionally evolved around inorganic membranes. However, great advancements have also been achieved on development of more versatile and abundant polymeric catalytically active membranes (pCAM) for a range of niche applications. This review is the first attempt to provide an overview of reaction-separation coupling by pCAM. Liquid phase reactions often occur at mild temperatures and the majority of studies dealing with pCAM are focused on these reactions. Research progress of reaction-separation coupling using pCAM for liquid phase reactions including fine chemical synthetic, photocatalytic and biocatalytic reactions is first examined. The limited studies on the use of pCAM for gas phase reactions are also summarized. Perspectives on the future development of pCAM and their potential applications are also highlighted.
Layered zeolites of MWW family, MCM-22 and pillared MCM-36, were used as supports for base modifiers: cesium species (introduced via cation exchange or impregnation) and 3-aminopropyl trimethoxysilane (AP). The obtained materials were characterized by different methods (ICP-OES, N 2 adsorption, XRD, XPS, TG/DTA, FTIR combined with pyridine adsorption, 2-propanol decomposition) for evaluation of chemical, structural and surface properties. All materials obtained were subjected to the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate and ethyl acetoacetate. The effect of the zeolite structure, the stability of the catalysts as well as acid-base properties of zeolites on the activity in Knoevenagel condensation were considered. Of particular interest was the role of Brønsted acid sites (BAS). The nature of basic sites and BAS played different roles depending on the methylene compound used in the Knoevenagel condensation. AP-modified zeolites were the most active in the condensation between benzaldehyde and ethyl cyanoacetate, in which, in the first step of the reaction AP abstracted hydrogen from methylene carbon in ethyl cyanoacetate. A different reaction pathway was postulated for the condensation with ethyl acetoacetate on the basis of the highest activity of unmodified zeolites and the relationship between benzaldehyde conversion and the number of BAS. For this reaction protonation of benzaldehyde was postulated as the initial step of the reaction.
Two new fluorinated metal−organic frameworks [Zn2(hfipbb)2(4-bpdh)].0.5DMF (TMU-55) and [Zn2(hfipbb)2(4-bpdb)].2DMF (HTMU-55) have been solvothermally synthesized by the reaction of two linear N-containing ligands as pillars (4-bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene, 4-bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4- hexadiene), and a flexible V-shaped fluorinated linker, H2hfipbb = 4,4'-(hexafluoroisopropylidene) bis(benzoic acid)). The two compounds were characterized by different techniques such as X-ray crystallography, powder X-ray diffraction (PXRD), infrared spectroscopy (IR), contact angle (CA), thermo gravimetry (TGA), field emission scanning electron microscope (FE-SEM), inductively coupled plasma (ICP), and Brunauer–Emmett–Teller (BET) surface area analysis. Both compounds are structurally similar and exhibit three dimensional (3D) coordination frameworks with hydrophobic properties. The catalytic activity of these isoreticular F-MOFs as efficient base heterogeneous catalysts toward Knoevenagel condensation reaction was tested and compared to each other. TMU-55 and HTMU-55 exhibited good catalytic activity (with a minimum amount of catalyst, 2.5 mg (0.5%mol), and the shortest time, 5 min) in water media as a green solvent with excellent conversions. Also, the results were considerably higher in comparison with the most given values in the literature for this reaction at ambient temperature. The remarkable catalytic performance of TMU-55 and HTMU-55 in water can be attributed to the presence of hydrophobic fluoro groups near the basic reaction center in the F-MOFs. These catalysts maintain their crystalline frameworks after the reaction and are easily recovered and reused at least for three cycles without significant loss in their catalytic activity activity. Furthermore, the conversion of benzaldehyde can be kept over 95% while the selectivity of the Knoevenagel reaction is kept at 100%.
A series of nitrogen-containing ordered mesoporous carbon (NOMC) materials have been prepared by a convenient soft-templating method. The physicochemical properties of the NOMC samples were characterized by N2 adsorption and desorption, small-angle XRD, FT-IR, XPS, and TEM. As solid base catalysts, NOMCs exhibited remarkable catalytic activities for Knoevenagel condensation reactions under mild reaction conditions, depending on their synthetic calcination temperature. Among them, NOMC-500 showed the highest catalytic activity. For Knoevenagel condensation of benzaldehyde and malononitrile, the conversion of aldehyde was higher than 85% under 50 °C. Moreover, the catalysts had good catalytic applicability for wide substrates. The catalytic activity obtained on NOMC catalyst is compared with other nitrogen-containing carbon-based materials.
Graphical abstract
Nitrogen-containing ordered mesoporous carbon materials, prepared by a soft-templating method under aqueous solution and as solid base catalysts, demonstrated high catalytic activity in Knoevenagel condensation reactions under mild reaction conditions. Open image in new window
The use of strongly basic anion exchange resins as catalysts for Knoevenagel condensations under continuous‐flow conditions was investigated. It was found that Amberlite FPA60 was an efficient solid base catalyst for the flow synthesis of 1,2‐disubstituted alkenyl nitriles from arylmethyl nitriles and aldehydes. On the other hand, the Knoevenagel reaction involving acetonitrile required elevated reaction temperatures, which necessitated a change to a thermally robust ammonium hydroxide polymer catalyst. The potential applications of these strongly basic anion exchange resins for multi‐step continuous‐flow synthesis were also demonstrated.
The Knoevenagel condensation with chlorine substituted benzaldehyde is difficult because of deactivation of aldehyde group. In this work we have shown that the activity of mesoporous cellular foams (MCF and Nb/MCF), modified with 3-aminopropyl-trimethoxysilane (AP) as a source of basicity, can be enhanced for this reaction by the anchoring of gold species. Gold species were loaded on amine modified materials towards Au/AP-MCF and Au/AP-Nb/MCF. Moreover, copper was used as a dopant for the latter sample giving rise to AuCu/AP/MCF. All these materials were characterized in details by different complementary techniques (N2 adsorption isotherms, TEM, UV–vis, TG/DTA, XPS, DFT calculations) which allowed the evaluation of states of all components of the catalysts. The interaction between niobium species and AP was evidenced to increase the stability of the basic modifier. This interaction was weakened by the introduction of gold and copper. The presence of both, negatively charged gold nanoparticles (NPs) and gold cations was crucial for the activation of 2,4-dichlorobenzaldehyde via the interaction with chlorine substituent atoms and aldehyde oxygen atoms as well as breaking of the “pseudo hydrogen bond” between aldehyde hydrogen atom and chlorine substituent, which destabilized the structure making it more active. Gold dopants did not influence significantly the condensation of ethyl cyanoacetate with benzaldehyde.
Environmental imperatives to conserve natural resources and to divert waste streams have stimulated significant interest in mineral recycling. This study illustrates that waste green container glass cullet is a suitable feedstock material for the facile synthesis of basic metallosilicate minerals that have potential as heterogeneous catalysts for industrially significant organic reactions. The target product phases, tobermorite (Ca5Si6O16(OH)2·4H2O), lithium metasilicate (Li2SiO3) and hydroxycancrinite (Na6Ca(AlSiO4)6(OH)2·2H2O) were synthesised by hydrothermal treatment of waste green cullet in alkaline media at 125 °C for 14 days. The reaction products were characterised by powder X-ray diffraction analysis, Fourier transform infra-red spectroscopy and scanning electron microscopy. Phase-pure tobermorite (TB) was prepared from a mixture of glass and lime in sodium hydroxide solution. Impure lithium metasilicate (LS) containing minor proportions of portlandite and calcite was synthesised from the glass in lithium hydroxide solution. A mixed product of hydroxycancrinite (HC), with minor proportions of hydroxysodalite, tobermorite and hydrogarnet, was produced from the glass in a solution of aluminium and sodium hydroxides. All three glass-derived metallosilicate products were found to be effective catalysts for the Knoevenagel synthesis of ethyl (2E)-2-cyano-3-phenylacrylate from ethyl cyanoacetate and benzaldehyde. The order of catalytic efficacy followed the trend in basicity of the metallosilicate products, LS > TB > HC.
Synthetic attempts towards submicron macropore and skeleton dimensions of hierarchical silica monoliths have so far been plagued by compromised structural homogeneity and/or mechanical stability. We overcome these problems by introducing urea as an agent to control macropore size and skeleton thickness (in addition to mesopore size) as well as a low-cost, straightforward method to achieve a seamless stainless-steel cladding of the monolithic silica rods tolerating pressures of >100 bar. Increasing the urea content of the starting sol comprising a sulfuric acid solution of tetraethoxysilane, poly(ethylene oxide), and urea from 3 to 24 wt % decreases the macropore size from 2.3 to 0.6 µm, the skeleton thickness from 2.0 to 0.4 µm, and increases the mesopore size from 10 to 26 nm. We assume that with increasing urea content of the starting sol, phase separation and gelation are retarded as well as shifted closer together, so that the formed monolithic structures represent a less evolved state of spinodal decomposition, preserving smaller macropores and a thinner skeleton. After cladding, the surface functionalization with aminopropyl groups yields a continuous-flow microreactor used for heterogeneous catalysis of the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate. The catalytic testing and kinetic studies with an on-line coupled reaction–analysis system reveal macroscopic plug-flow conditions in the microreactor and the elimination of diffusive transport limitations demonstrating the overall success of the preparation. The proposed scheme enables academic laboratories to prepare hierarchical silica monoliths with desirable morphological properties (addressing particularly submicron macropore size and skeleton thickness) and versatile surface functionalization for demanding applications in adsorption, separation, organic synthesis, and catalysis.
Formamidine sulfonic acid (FSA) stabilized on silica-coated Fe3O4 magnetic nanoparticles as an efficient and hydrogen bonding catalyst was employed for Knoevenagel condensation and synthesis of pyrano[2, 3-d]pyrimidinone derivatives by the one-pot three-component condensation reaction in water at room temperature. The catalyst can be separated from the mixture reaction which is in close agreement with green chemistry disciplines.
A zeolite-based microengineered reactor was fabricated and tested for 1-pentene epoxidation over titanium silicalite-1 (TS-1) catalyst, which has been selectively incorporated within the microreactor channel using a new synthesis procedure.
Zeolite micromembranes' application is not limited to gas, gas-liquid and liquid-liquid separations. They also find uses as selective barrier for sensors and ion-conducting membrane for electrochemical systems (e.g., microfuel cell). Miniaturization benefits membrane separation by improving mass and heat transfer rates (Franz et al., 2000; Losey et al., 2000). It also allows larger membrane area to be packed in a smaller volume enabling the design of a more efficient and compact separation unit. This work reports the fabrication of HZSM-5 zeolite micromembrane and its performance for hydrogen permeation and proton transport. Although there are numerous works that discuss the fabrication of zeolite micromembranes, to our knowledge this is the first successful demonstration of gas permeation and proton transport in the zeolite micromembrane.
Half-title pageAlso of interestTitle pageCopyright pageDedicationPrefaceContents
Basic zeolites were prepared by in situ formation of caesium oxide by calcination of the parent acetate loaded in an increasing amount up to 26 caesium atoms per unit cell. X-ray diffraction and BET studies are consistent with good crystallinity and site accessibility retainings. C02 TPD results show homogeneous location of the basic species inside the pores with one caesium oxide per supercage. The results are fairly correlated with the initial rates of the Knoevenagel reaction of benzaldehyde and ethylcyanoacetate. These basic solids provide well-adapted selective microporous catalysts for condensation reaction.
Glycerolysis of triolein and rapeseed oil were carried out with base catalysts such as Cs-MCM-41, Cs-Sepiolite, MgO, and calcined hydrotalcites with different Al/Al+Mg ratios. A wide range of basicities were covered in order to extract active hydrogens with pKa values from 9 to 16. MgO and hydrotalcites with low Al content, basic enough to extract protons from aliphatic alcohols such as glycerol, are active and selective catalysts for glycerolysis. By optimizing the main process variables, such as temperature and the glycerol/oil ratio it is possible to obtain a high yield of monoglycerides as well as to meet the quality requirements established by the European Union.
The catalytic activity of alkali ion-exchanged zeolites X, Y and Beta as well as of non- zeolitic porous solids impregnated with cesium hydroxide or cesium acetate has been studied in the Knoevenagel condensation of benzaldehyde and malononitrile. Additional information concerning the basic surface sites of the catalysts was obtained by FTIR spectroscopic investigations of adsorbed deuterochloroform. Comparison of the results revealed a good agreement of the spectroscopic and catalytic characterization for the ion-exchanged zeolites. In the case of more strongly basic cesium-impregnated catalysts, the application of deutero- chloroform as a suitable probe molecule may be limited.
A novel rearrangement of the ω-phenylalkanals phenylacetaldehyde, 3-phenylpropionaldehyde and 4-phenylbutyraldehyde towards
phenyl alkyl ketones is reported. The liquid-phase isomerizations are catalyzed by cesium oxide or binary cesium-lanthanum
oxide supported on the mesoporous aluminosilicate molecular sieves MCM-41 and HMS and on amorphous supports like silica-alumina
and γ-alumina. Beside the title reaction the aldol condensation is observed as a side-reaction. A mechanism is proposed in
which the aromatic ring is involved in the rearrangement process. Factors governing the activities and product selectivities
are presented.
The presence of basic centers in some oxides has been recognized for a long time as being important in catalysis [1-4]. Usually both basic and acid sites exist simultaneously. The two centers may work independently or in a concerted way. For instance, in alcohol transformation, dehydration is favored on acidic sites and dehydrogenation on basic centers [3,5]. A large variety of materials are cited as having basic character. They include single-metal oxides (MgO, CaO, ZnO), supported alkali metals (Na/MgO, K/K2CO3), mixed-metal oxides (MgO-A12O3, ZnO-SiO2, MgO-TiO2), zeolites (X and Y saturated with alkaline cations of low electronegativity), hydrotalcite-type anionic clays, asbestoslike materials, carbon-supported basic catalysts, and basic organic resins.* Present address: 16 rue François Gillet, 69003 Lyon, France.
Ordered mesoporous silica adsorbents were prepared by grafting amino- and carboxylic-containing functional groups onto MCM-41 for the removal of Acid blue 25 and Methylene blue dyes from wastewater. The amino-containing OMS−NH2 adsorbent has a large adsorption capacity and a strong affinity for the Acid blue 25. It can selectively remove Acid blue 25 from a mixture of dyes (i.e., Acid blue 25 and Methylene blue). The OMS−COOH is a good adsorbent for Methylene blue displaying excellent adsorption capacity and selectivity for the dye. The better selectivity of the OMS-based adsorbents means longer operating life and less maintenance. Furthermore, these adsorbents can be regenerated by simple washing with alkaline or acid solution to recover both the adsorbents and the adsorbed dyes.
The arrangements of residual water and alkali-metal cations in zeolites Y and X have been determined by Rietveld refinement in combination with Fourier analysis of X-ray powder patterns. The sodium forms NaY and NaX as well as partially cation exchanged forms MNaY and MNaX (M = K, Rb, Cs) were studied. It was shown that only sodium participates in the interaction with the water molecules, therewith causing the formation of distinctive sodium−water structures in the large cavity system of the hosts. Heavier alkali-metal ions that are also present affect the formation of these structures in an indirect way by blocking cation positions for sodium. The results, gleaned from X-ray diffraction, were put into relation to temperature-programmed desorption (TPD) measurements, obtained from previous investigation of these zeolites.
Heterogeneous acid catalysis attracted much attention primarily because heterogeneous acidic catalysts act as catalysts in petroleum refinery and are known as a main catalyst in the cracking process which is the largest process among the industrial chemical processes. In contrast to these extensive studies of heterogeneous acidic catalysts, fewer efforts have been given to the study of heterogeneous basic catalysts. The types of heterogeneous basic catalysts are listed in Table 1. Except for non-oxide catalysts, the basic sites are believed to be surface O atoms. The studies of heterogeneous catalysis have been continuous and progressed steadily. They have never been reviewed in the chemical Reviews before. It is more useful and informative to describe the studies of heterogeneous basic catalysis performed for a long period. In the present article, therefore, the cited papers are not restricted to those published recently, but include those published for the last 25 years. The paper first describes the generation of basic sites before describing methods used in the characterization of basic surfaces. These are indicator methods, temperature programmed desorption (TPD) of COâ, UV absorption and luminescence spectroscopies, TPD of Hâ, XPS, IR of COâ, IR of pyrrole, and oxygen exchange between COâ and the surface. The paper then discusses studies on the catalysis by heterogeneous basic catalysts. Some of these reactions are dehydration, dehydrogenation, hydrogenation, amination, alkylation, ring transformation, and reactions of organosilanes. Catalysts discussed are single component metal oxides, zeolites, non-oxide types, and superbasic catalysts. 141 refs.
Mesoporous MCM-41 containing intraporous cesium oxide particles have been prepared by impregnation of aqueous cesium acetate and subsequent calcination, and characterized by 133Cs MAS NMR, CO2 temperature programmed desorption and nitrogen physisorption. The amount of framework aluminum of the MCM-41 support appeared to be very important regarding the strength of the basic sites. The Michael addition of diethyl malonate to neopentyl glycol diacrylate catalyzed by MCM-41 supported cesium oxide showed a high regioselectivity, in contrast to bulk cesium oxide.
Isomerization of glucose into fructose was performed in a batch mode in the presence of a series of alkaline solid catalysts such as cation-exchanged A, X, Y zeolites and hydrotalcites.Under optimized operating conditions, water as the solvent, 95°C, up to 250 g/l of initial glucose concentration, up to 20 wt.% of catalyst , 700–1200 rpm and 8 bar of nitrogen pressure to avoid oxidation reactions, the reaction is not controlled by external or internal diffusional limitations. Among the different catalysts used, Li-, Na-, K-, Cs-, Ca- and Ba-exchanged A, X and Y zeolites, and hydrotalcites, Ca- and Ba-exchanged A, X and Y zeolites were found less selective, whereas those with a moderate basicity such as NaX and KX were found to achieve isomerization of glucose into fructose with a selectivity to fructose of about 90%, but at low glucose conversion, around 10–20%. However, the high selectivity to fructose is only obtained at glucose conversions lower than 25%. Furthermore, it was found that a significant amount of the cation passed into water (around 15% in the case of monovalent cation-exchanged X zeolites). This leaching phenomenon is no longer observed after a second run. The conversion of glucose is stabilized at about 10% without loss of selectivity to fructose, so that a continuous process may be considered.
A TS-1 coated, microchannel reactor was fabricated onto silicon substrate using selective seeding and regrowth. The preparation technique's ability to engineer the zeolite film microstructure and chemistry was used to prepare TS-1 coatings with different titanium-contents and crystal sizes. The zeolite microreactor was utilized for 1-pentene epoxidation to 1,2-epoxypentane with hydrogen peroxide. This exothermic reaction can benefit by good heat transfer and containment of the oxidizing agent. A computational model was employed to simulate the reaction in the microreactor using kinetic data from batch reactor experiments. The influence of reactor geometry, catalyst properties and reaction conditions was evaluated and the results compared with experimental data.
Post-synthetic modification of basic CsNaX and CsNaY zeolites was performed by impregnation with cesium acetate at varoous loadings followed by thermal decomposition of the cesium acetate into oxide. A comparative study of the nature and basic character of intrazeolitic species in CsNaX and CsNaY zeolites is reported. Crystallinities of modified X zeolites are largely retained after activation at 550°C for six hours. Under the same activation conditions modified Y zeoolites are less stable as evidenced by XRD, N2 sorption, 27Al and 29Si MAS NMR and stepwise thermal desorption of CO2 (TPD). The modified CsNaY zeolite crystallinities were largely maintained when the activation temperature was lowered to 400°C. The TPD of CO2 below 500°C allows the differentiation of the structures of guest cesium species occluded in the host CsNaX or CsNaY zeolites. A shift of the desorption peak maximum from 250 to 150°C accounts for a higher basicity of the species within the pores of the CsNaX host than in the CsNaY one. Linear correlations between the amount of desorbed CO2 and the cesium loading suggest a homogeneously dispersed loading up to 16 and 24 cesium atoms per unit cell for the modified X and Y zeolites, respectively. The formation of oxide (Cs2O) inside the cages of the CsNaX zeolite is proposed. In the case of the modification of the CsNaY zeolite various structures are discussed involving either the formation of local lattice cesium silicate or aluminate defects or the encapsulation of cesium oxide.
MCM-41 type silica surfaces were grafted with primary amino groups through silanation process. Tertiary amino groups covalently bound to the surface were prepared through displacement of previously anchored halogen atom by piperidine. The catalytic activities of the immobilized amino groups were studied in the Knoevenagel condensation of benzaldehyde with ethylcyanoacetate. The mechanism of the catalyst action is discussed
Using a new fabrication method based on microelectronic fabrication and zeolite thin film technologies, MFI-type zeolites with engineered structures were incorporated as catalyst, membrane and structural material within the design architecture of a microreactor, membrane microseparator and microeletrochemical cell. Complex microchannel geometry and network (<5 μm), as well as zeolite arrays (<10 μm) were successfully fabricated onto highly orientated supported zeolite films. The zeolite micropatterns were stable even after repeated thermal cycling between 303 and 873 K for prolonged period of time. Blueprints for zeolite-based microchemical systems were presented, and test units were fabricated and the structural details of the microdevices’ architecture were analysed.
This paper demonstrates that zeolites (i.e., Sil-1, ZSM-5 and TS-1) can be employed as catalysts, membrane or structural materials in miniature chemical devices. Traditional semiconductor fabrication technology was employed in micromachining the device architecture. The fabricated miniature zeolite-based structures find applications as catalytic microreactors and membrane microseparators. Four strategies for the manufacture of zeolite catalytic microreactors were discussed: zeolite powder coating, uniform zeolite film growth, localized zeolite growth, and etching of zeolite–silicon composite film. These zeolites were deposited either as film or discrete islands with controlled particle size, crystal morphology, layer thickness (3–16 μm) and film orientation (e.g., highly oriented (1 0 1) and (2 0 0) films). Crystal intergrowth was also manipulated through the use of growth inhibitors. Silicalite-1 was also prepared as free-standing membrane for zeolite membrane microseparators.
The condensations of benzaldehyde with ethyl cyanoacetate, ethyl malonate, and ethyl acetoacetate were carried out with high activity and selectivity on lithium-, sodium-, potassium- and caesium-exchanged X and Y zeolites. The activity of the zeolites increased with decrease in the framework silicon-to-aluminium ratio of the zeolite and increase in the radius of the counter cation. Under reaction conditions, it was found that most of the basic sites in alkaline X and Y zeolites have pKb⩽10.3, and sites with pKb⩽13 were present only in the CsX sample. This catalyst is more active than pyridine, and less active but more selective than piperidine. It is shown that, on these catalysts, in the condensation reactions studied, the controlling step is not proton abstraction but attack of the carbonyl group by the carbanion.
During the past decade, new classes of porous solid bases such as organic bases grafted onto the walls of ordered mesoporous materials and nitrided zeolites have been studied as catalysts in a wide variety of base-catalyzed reactions. Important examples are the mercaptane oxidation in petroleum refining, well known as sweetening of gasoline and kerosene, carbon–carbon bond forming reactions in fine chemistry such as aldol additions and condensations, and (trans-)esterification reactions interesting for food, pharmaceutical and cosmetic industries. The present work gives a survey on these new solid bases, their applications in heterogeneous catalysis and on new spectroscopic techniques allowing a more detailed investigation of base sites and adsorbates on working catalysts.
The condensation of benzaldehyde, 2-nitrobenzaldehyde, and 2-trifluoromethylbenzaldehyde with ethyl cyanoacetate and ethyl malonate to obtain intermediates for the production of dihydropyridine derivatives has been carried out on NaX-type zeolites. The substitution of Si by Ge during the zeolite synthesis results in a strong increase in activity, which has been related to changes in basicity. High activities and selectivities are obtained with the NaGeX zeolite when operating in a batch reactor at moderate temperatures. This zeolite is more active than pyridine and less active than piperidine.
An elegant way to prepare catalytically active microreactors is by applying a coating of zeolite crystals onto a metal microchannel structure. In this study the hydrothermal formation of ZSM-5 zeolitic coatings on AISI 316 stainless steel plates with a microchannel structure has been investigated at different synthesis mixture compositions. The procedures of coating and thermal treatment have also been optimized. Obtaining a uniform thickness of the coating within 0.5 mm wide microchannels requires a careful control of various synthesis variables. The role of these factors and the problems in the synthesis of these zeolitic coatings are discussed. In general, the synthesis is most sensitive to the H2O/Si ratio as well as to the orientation of the plates with respect to the gravity vector. Ratios of H2O/Si=130 and Si/template=13 were found to be optimal for the formation of a zeolitic film with a thickness of one crystal at a temperature of 130°C and a synthesis time of about 35 h. At such conditions, ZSM-5 crystals were formed with a typical size of 1.5 μm×1.5 μm×1.0 μm and a very narrow (within 0.2 μm) crystal size distribution. The prepared samples proved to be active in the selective catalytic reduction (SCR) of NO with ammonia. The activity tests have been carried out in a plate-type microreactor. The microreactor shows no mass transfer limitations and a larger SCR reaction rate is observed in comparison with pelletized Ce-ZSM-5 catalysts.
Different strategies leading to the preparation of acid and base catalysts derived from ordered mesoporous silicas are reviewed. These include aluminum incorporation into the silicate network, entrapping of heteropolyacids, deposition of oxide precursors and direct or post-synthesis of anchoring of organic moieties. Their use in a variety of reactions related to fine chemicals synthesis is critically discussed.
Solid base catalysts such as zeolites, sepiolites and hydrotalcites have been used to prepare prepolymers of malononitrile with cyclohexanone, benzophenone, andp-amino acetophenone which are of interest for preparing living polymers. The reactivity depends on both the catalyst and the nature of the ketone. Among the catalysts studied, hydrotalcites are the most active, followed by cesium-exchanged sepiolites. High selectivities and good yields have been obtained with these two solid base catalysts. It is proposed that the controlling step for these condensation reactions is not the formation of the carbanion, but the addition of the carbanion to the carbonyl group.
Microengineered reactors are a new type of reactor. Their novelty dictates a new approach in design, which requires familiarization with the various manufacturing techniques and materials, and with differences in fluid behaviour and dominant phenomena in the microscale. New approaches for energy management, catalyst incorporation and integration of functions and unit operations can be employed. Microengineered reactors have some unique characteristics which create the potential for high performance chemicals and information processing. These include efficient mass and heat transfer and precise control of the hydrodynamic environment. They can provide significant advantages in information generation in high throughput experimentation and process development, and from difficult to obtain operating regimes. In terms of chemicals manufacture, they allow distributed, mobile and intensified processing. This technology, still in its infancy has the potential to change the chemical engineering landscape.
Using a new fabrication strategy, novel, self-enclosed microporous silicalite-1 zeolite microtunnel and microchunnel architectures were successfully prepared.
A zeolite-based microengineered reactor was fabricated and tested for 1-pentene epoxidation over titanium silicalite-1 (TS-1) catalyst, which has been selectively incorporated within the microreactor channel using a new synthesis procedure.
A multi-channel membrane microreactor was fabricated and tested for Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate using Cs-exchanged faujasite NaX as the catalyst; the membrane microreactor achieves supra-eqiulibrium conversion at higher product purity.
Free-standing silicalite-1 (Sil-1) zeolite micromembranes have been successfully fabricated onto silicon substrate. Gas permeation test using permanent gases (i.e., helium, hydrogen, argon and nitrogen) and hydrocarbons (i.e., methane and n-butane) indicates that the micromembranes have excellent permeance flux and high permselectivity.
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