Structure and acidity of Mo/H-MCM-22 catalysts studied by NMR spectroscopy

A comprehensive study on physical and chemical properties of Mo/MCM-22 bifunctional catalysts has been made by using combined analytic and spectroscopic techniques, such as adsorption, elemental analysis, and Xe-129 and P-31 NMR of adsorbed trialkylphosphine oxide probe molecules. Samples prepared by the impregnation method with Mo loadings ranging from 2-10 wt.% have been examined and the results are compared with that obtained from samples prepared by mechanical mixing using MoO3 or Mo2C as agents. Sample calcination treatment is essential in achieving a well-dispersed metal species in Mo/MCM-22. It was found that, upon initial incorporation, the Mo species tend to inactivate both Bronsted and Lewis sites locate predominantly in the supercages rather than the 10-membered ring channels of MCM-22. However, as the Mo loading exceeds 6 wt.%, the excessive Mo species tend to migrate toward extracrystalline surfaces of the catalyst. A consistent decrease in concentrations of acid sites with increasing Mo loading < 6 wt.% was found, especially for those with higher acid strengths. Upon loading of Mo > 6 wt.%, further decreases in both Bronsted and Lewis acidities were observed. These results provide crucial supports for interpreting the peculiar behaviors previously observed during the conversion of methane to benzene over Mo/MCM-22 catalyst under non-oxidative conditions, in which an optimal performance was achieved with a Mo loading of 6 wt.%. The effects of Mo incorporation on porosity and acidity features of the catalyst are discussed. (C) 2004 Published by Elsevier B.V.

The effect of acidity on olefin aromatization over potassium modified ZSM-5 catalysts

The transformation of olefin to aromatics over ZSM-5 catalysts with different K-loadings has been investigated both in a continuous flow fixed-bed reactor and in a pulse microreactor. Investigation of variation of olefin aromatization activity with K-loadings shows that strong acid sites are indispensable for the converting of olefin to aromatics. As intermediates of olefin aromatization process, butadiene and cyclopentene not only show much higher aromatization activity than mono-olefins, but also can be transformed into aromatics over relatively weak acid sites of K/ZSM-5. A proposal is put forward, stating that among all the steps experienced in olefins aromatization, the formation of diene or cycloolfin from mono-olefins through hydrogen transfer is the key step and can be catalyzed by strong acid sites.

Effect of acidity in TS-1 zeolites on product distribution of the styrene oxidation reaction

The role of Bronsted acidity of titanium silicalite zeolite (with different ratios of Si/Ti) in oxidation reactions of styrene has been investigated and discussed. For zeolites with Si/Ti > 42, most of the titanium is in the zeolite framework. These framework titanium species, which act both as the isolated titanium centers and as Bronsted acidity centers (together with the Bronsted acidity produced by the tetrahedral aluminum impurity introduced during synthesis), can catalyze both the epoxidation and the succeeding rearrangement reactions, thus promoting the formation of phenylacetaldehyde. With an increase in the titanium content of the zeolite, titanium will tend to stay outside the zeolite lattice, except for the TiOx nanophases which can be occluded in the zeolite channels or on the external surface. These non-framework titanium species are favorable for the carbon-carbon bond scission, leading to the production of additional benzaldehyde. The catalytic performances of these zeolites with different Si/Ti ratios are correlated here with their structural information by using solid-state NMR and UV-Vis methods. (C) 2003 Elsevier B.V. All rights reserved.

Molecular imprinting of nitrophenol and hydroxybenzoic acid isomers: effect of molecular structure and acidity on imprinting

Three nitrophenol isomer-imprinted polymers were prepared under the same conditions using 4-vinylpyridine as a functional monomer. Different recognition capacities for template molecules were observed for the three polymers. Another imprinting system with stronger acidity than nitrophenol isomers, 2-hydroxybenzoic acid (salicylic acid) and 4-hydroxybenzoic acid, was imprinted using 4-vinylpyridine or acrylamide as functional monomer respectively. Both 4-hydroxybenzoic acid-imprinted polymers using the two monomers showed recognition ability for the template molecule. However, when acrylamide was chosen as functional monomer, the salicylic acid-imprinted polymer showed very weak recognition for the template molecule, whereas strong recognition ability of the resultant polymer for salicylic acid was observed with 4-vinylpyridine as functional monomer. It seems that the structure and acidity of template molecules is responsible for the difference in recognition, by influencing the formation and strength of interaction between template molecule and functional monomer during the imprinting process. An understanding of the mechanism of molecular imprinting and molecular recognition of MIPs will help to predict the selectivity of MIPs on the basis of template molecule properties. Copyright (C) 2003 John Wiley Sons, Ltd.

Determination of the interfacial acidity of the functional alkanethiols self-assembly membrane by capacitance measurement

The acid-base equilibrium of self-assembly monolayer(SAM) of mercaptopropionic acid on gold electrode was studied by capacitance titration. The change of capacitance was due to protonation and deprotonation of the modified electrode surface. This method can be used to measure pH value in solution and capacitance of solid/liquid on SAMs surface synchronously. The shift of pK(a) was observed during the experiment as previous reports. The factors that affect the capacitance measurement were discussed in detail.

NMR study on the acidity of TS-1 zeolite

The acidic properties of TS-1 and Silicalite-I zeolites have been investigated by the solid-state MAS NMR technique capable of in situ sample pretreatment. As shown by a combination of the P-31 MAS NMR and H-1 MAS NMR techniques with trimethylphosphine, not only Bronsted acid sites but also Lewis acid sites exist in the TS-1 zeolites. Moreover, TS-1 zeolite is more acidic compared with Silicalite-1. The H-1, Si-29 MAS NMR spectra and the resonance related to Bronsted acid species in the P-31 MAS NMR spectra demonstrate clearly that the presence of titanium in the framework results in the formation of a new hydroxy group, titanols, which is more acidic than silanols of Silicalite-1. The P-31 MAS NMR measurements also illustrate convincingly the existence of at least two different Lewis acid species on the TS-1 zeolites. The conversion of propylene oxide into methoxypropanol catalyzed by TS-1 or Silicalite-I zeolite in methanol solution as a test reaction has also been described. With the increase of titanium in zeolite, TS-1 appears to have a higher activity during the reaction of propylene oxide to methoxypropanol.