Catalytic activity of highly ordered mesoporous VMCM-48

Mesoporous VMCM-48 molecular sieves with Si/V (molar) ratios of 25-200 were synthesized hydrothermally and systematically characterized by various analytical and spectroscopic techniques, viz., XRD, N-2 sorption, TG-DTA, TEM, EPR, V-51 MAS-NMR, FT-IR, and DRUV-VIS. XRD studies suggest that the substitution of vanadium occurs in the silicate framework structure of MCM-48. TEM and ED investigations confirm the highly ordered cubic structure of VMCM-48. EPR and V-51 NMR studies indicate the presence of pentavalent vanadium ions in tetrahedral framework positions, while DRUV-VIS spectra show their existence in two different environments. The catalytic activity of these well-characterized (both in the calcined and the washed forms) materials was evaluated for cyclohexane oxidation under mild reaction conditions. All these catalysts gave high substrate conversion and excellent product (cyclohexanol) selectivity. Furthermore, unlike many other vanadium-based heterogeneous catalysts reported, the mesoporous VMCM-48 catalysts show minimal leaching of the active vanadium species. This was confirmed by washing, recycling and quenching experiments where only a small amount of vanadium ions leaches out in the case of calcined samples, while vanadium was not detected for the washed ones. Finally, the catalytic activity of VMCM-48 was also compared with mesoporous VMCM-41 as well as microporous VS-1 catalyst. The results indicate that the former showed superior activity to those of the latter two, such superiority could, however, be directly related to the amount of vanadium incorporated in MCM-48 being larger than the amounts in MCM-41 and MF1structures. (C) 2004 rights reserved

Mesoporous H-AlMCM-48: highly efficient solid acid catalyst

Mesoporous cubic Na-AlMCM-48 molecular sieve catalyst with a Si/Al (molar) ratio of 60 was synthesized hydrothermally and characterized by various analytical and spectroscopic techniques. Al-27 MAS-NNIR results indicate that the presence of aluminum in tetrahedral framework (Bronsted acid site) in both as-synthesized and calcined samples. However, in the case of the latter, a small amount of aluminum was found to be in octahedral coordination. This observation is well supported by NH3-TPD studies over protonated catalyst (H-AlMCM-48), where desorption profile at higher temperature shows features characteristic of Lewis acid sites. Further, these studies also indicate that the presence of high concentration of moderate-to-strong Bronsted acid sites at lower temperature, which are more suitable for the para-selective tertiary-butylation reaction of phenol. Hence, in the present investigation, the reaction was carried out H-AIMCM-48, which however showed much higher activity as compared to the hexagonal H-AlMCM-41. In addition, it was also found that the former does not get deactivated owing to three-dimensional pore system while the latter is susceptible to deactivation on account of one-dimensional pore system. (C) 200

Tertiary butylation of phenol over mesoporous MeMCM-48 and MeMCM-41 (Me = Ga, Fe, Al or B) solid acid catalysts

Mesoporous Na-GaMCM-48 molecular sieves having a silicon-to-gallium (molar) ratio of 30-90 were synthesized hydrothermally, and the structure; coordination geometry of the gallium, acidic properties, and catalytic activity were systematically investigated using a number of analytical and spectroscopic techniques. XRD, TEM and N-2 sorption investigations indicate an MCM-48 structure with highly ordered mesoporosity. Ga-17 MAS-NMR studies reveal that gallium substitutes isomorphously in the mesoporous silicate framework of MCM-48. The NH3-TPD profiles suggest the presence of high concentrations of moderate-to-strong Bronsted acid sites in H-GaMCM-48. The catalytic performance of this protonated catalyst was evaluated for the t-butylation of phenol. The results indicate that the H-GaMCM-48 catalysts are highly active for the chosen reaction, and show much higher substrate conversion than many other catalyst systems. However, compared to the analogous H-GaMCM-41, the H-GaMCM-48 shows a slight decrease in p-t-butyl phenol selectivity owing to the formation of 2,4-di-t-butyl phenol. On the other hand, the deactivation is very minimal on account of the three-dimensional pore system of the MCM-48 structure compared to the one-dimensional pore opening of the MCM-41 structure. (C) 2004 Elsevier B.V. All rights reserved

Mesoporous H-GaMCM-48: A remarkable solid acid catalyst for tertiary butylation of phenol

A mesoporous gallosificate (GaMCM-48) molecular sieve having a silicon-to-gallium (molar) ratio of 60 was synthesized hydrothermally, and the structure, coordination geometry of gallium, acidic properties, and catalytic activity were investigated systematically employing various analytical and spectroscopic techniques. XRD, TEM, and N-2 sorption investigations show a MCM-48 structure with highly ordered (meso)porosity. Ga-71 MAS-NMR studies reveal that gallium substitutes isomorphously in the mesoporous silicate framework of MCM-48. Further, NH3-TPD profiles suggest the presence of a high concentration of moderate-to-strong Bronsted acid sites in H-GaMCM-48. The catalytic performance of this protonated catalyst was evaluated for the t-butylation of phenol reaction. The results indicate that the H-GaMCM-48 catalyst is highly active for the chosen reaction and shows a much higher substrate conversion than many other catalyst systems. However, compared to the analogous H-GaMCM-41. the H-GaMCM-48 shows only a slight decrease in p-t-butyl phenol selectivity owing to the formation of 2,4-di-t-butyl phenol. On the other hand, the deactivation is very minimal on account of the three-dimensional pore system of MCM-48 structure compared to the one-dimensional pore opening of the MCM-41 structure. (C) 200

The effect of vanadium sources on the synthesis and catalytic activity of VMCM-41

Mesoporous VMCM-41 was synthesized hydrothermally using various vanadium sources, viz, tetravalent vanadium such as vanadyl sulfate and vanadyl acetylacetonate, as well as pentavalent vanadium like sodium vanadate and ammonium vanadate. The influence of different vanadium sources on the framework substitution of vanadium, as well as their catalytic activity for the oxidation of cyclohexane, was investigated. Among the different vanadium stocks, the tetravalent vanadium sources showed maximum vanadium incorporation in the silicate framework of MCM-41. As it consequence, these catalysts gave much higher substrate conversion and excellent product selectivity. On the other hand, the catalysts prepared from pentavalent vanadium sources showed lower activity owing to the smaller amounts of vanadium in the matrix. Although the activity of the catalyst slightly decreased after first recycle as due to leaching of small amounts of active vanadium species, it however remained nearly the same even after several recycles. This was further confirmed by washing experiments wherein non-framework vanadium ions were removed upon ammonium acetate treatment; the washed catalysts showed a similar activity to those of the recycled catalysts. Thus, recycled/washed VMCM-41 behaves truly as heterogeneous catalyst. Furthermore, the influence of pore size of the catalyst was tested for the oxidation of bulkier substrate, viz, cyclododecane. (C) 200

Mesoporous (Cr)MCM-41 and (Cr)MCM-48 molecular sieves: promising heterogeneous catalysts for liquid phase oxidation reactions

Chromium-containing mesoporous MCM-41 and MCM-48 silicate molecular sieves were synthesized and characterized. Unlike the chromium-containing microporous materials, the mesoporous analogues show significant activity for the chosen (ethylbenzene oxidation) reaction even after several recycling or washing treatments

Nanosized metal oxides in the mesopores of MCM-41 and MCM-48 silicates

The present work describes the preparation and characterization of nanosized metal oxide particles (Fe2O3, ZnO and PbO) inside the mesopore channels of MCM-41 and MCM-48 silicate molecular sieves. The encapsulation of the metal oxides was carried out at room temperature by the incipient wetness method. Diffuse reflectance ultraviolet-visible spectroscopic studies showed a significant shift in the absorption band for the entrapped metal oxides as compared to the corresponding bulk oxides. Thus, confirming the quantum confinement of the incorporated nanoparticles in MCM-41 and MCM-48. (C) 2001 Elsevier Science B.V. All rights reserved

Mesoporous VMCM-41: highly efficient and remarkable catalyst for selective oxidation of cyclohexane to cyclohexanol

Liquid phase oxidation of cyclohexane was carried out under milder reaction conditions over mesoporous VMCM-41 molecular sieve catalysts using aqueous hydrogen peroxide as oxidant, acetic acid as solvent, and methyl ethyl ketone as initiator. The catalysts showed high substrate conversion and excellent product (cyclohexanol) selectivity. Although the activity of the catalyst slightly decreased after first recycle, owing to leaching of small amount of non-framework vanadium ions, it, however, remained nearly same thereafter. This observation was further confirmed by washing experiments where the non-framework vanadium ions were removed upon ammonium acetate treatment. Further, the washed catalyst also showed an activity similar to that of the recycled catalyst. Thus, the recycled or washed VMCM-41 behaves truly as heterogeneous catalyst. This observation was complemented and confirmed by both filtrate and quenching studies. The effects of reaction time, temperature, Si/V molar ratio, and catalyst concentration on the catalyst performance were examined in order to optimize the conversion of cyclohexane and selectivity of cyclohexanol. However, the use of strong oxidizing agent, e.g., tertiary butyl hydroperoxide, resulted in the formation of cyclohexanone as the major product. In addition, the use of solvents like methanol, dioxan and acetone showed lower activity. (C) 2004 Elsevier B.V. All rights reserved

Allylic oxidation of cyclohexene over chromium containing mesoporous molecular sieves

Allylic oxidation of cyclohexene was carried out over mesoporous (Cr)MCM-41 and (Cr)MCM-48 molecular sieve catalysts. In both the cases, 2-cyclohexen-1-one was obtained as the major product with small amounts of cyclohexene oxide and 1.2-cyclohexandiol. (Cr)MCM-48 showed higher activity than (Cr)MCM-41 owing to the high chromium content in the former. The use of polar solvents such as acetonitrile and methanol decrease the 2-cyclohexen-1-one selectivity; however, such a procedure produces double bond oxidized product, viz. cyclohexene oxide. Furthermore, unlike many other chromium-based solid catalysts, the activity over recycled as well as washed (Cr)MCM-41 and (Cr)MCM-48 remains nearly the same, indicating that the mesoporous chromosilicate materials behave truly as heterogeneous catalysts. In other words, after the initial loss of non-framework chromium ions for the first time, no leaching was noticed for the chromium-based systems. (C) 2003

Novel mesoporous (Cr)MCM-48 molecular sieves: Promising heterogeneous catalysts for selective oxidation reactions

A series of chromium-containing mesoporous MCM-48 molecular sieves, (Cr)MCM-48, with varying Si/Cr (25 - proportional to) ratios were synthesized hydrothermally, and characterized systematically by various analytical and spectroscopic techniques. The catalytic performance of the materials was evaluated for the oxidation reactions of certain industrially important organics such as ethylbenzene, cyclohexene and cyclohexane. For all the chosen reactions, the catalyst with a Si/Cr ratio of 50 showed very good conversion and excellent selectivity of the products. Furthermore, (Cr)MCM-48 exhibits much higher activity than the one-dimensional mesoporous (Cr)MCM-41 due to the incorporation of chromium ions in large concentrations as well as to the three-dimensional pore system of the former. Unlike many other chromium-based heterogeneous catalysts, the (Cr)MCM-48 showed only a slight decrease in catalytic activity during first recycling (or second run) experiments, and thereafter it remains nearly the same. Further, the quenching experiments as well as washing studies clearly support the above conclusion. In addition, it can also be deduced from several recycling, washing, filtrate and quenching studies that the leaching of the active chromium ions from the mesoporous matrix is minimal compared to almost all the chromium-based heterogeneous catalysts reported so far. The catalytic activity of (Cr)MCM-48 was also compared with various chromium-containing microporous silicalite-1 and beta, i.e., (Cr)S-1 and (Cr)beta, molecular sieves