Multiphase Hydrodechlorination of 1,3,5-Trichlorobenzene on Palladium Catalysts Supported on Alumina: Effect of the Support Properties and Modification by Heteropoly Acid Based on Silicon and Tungstenстатья
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Дата последнего поиска статьи во внешних источниках: 17 июля 2019 г.
Аннотация:Catalytic systems 2 wt % Pd/Al2O3 were prepared using noncalcined boehmite (NC) and two types
of alumina support: one was prepared by the calcination of boehmite at 600°C (C) and the other produced by
Engelhard (E). To prepare 2 wt % Pd/HPC–Al2O3 samples, these supports were modified by impregnation
by a heteropoly compound (HPC) (20 wt % Н8[Si(W2O7)6] ⋅ 6Н2О). The effect of the Al2O3 structure and its
modification by the heteropoly compound on the physicochemical properties, activity, selectivity and stability
of catalysts in the reaction of multiphase hydrodechlorination of 1,3,5-trichlorobenzene (TCB) was studied.
All catalysts showed activity in the considered reaction with the predominant formation of benzene but
were deactivated in the reaction medium. Modification by the heteropoly compound resulted in increased
stability and was especially effective for catalyst supported on Al2O3(E). The method of scanning electron
microscopy (SEM) was used to determine the morphological differences of supports. According to the data
of transmission electron microscopy, all catalysts contained palladium in the form of particles less than 20 nm
in size. The particle size and width of the size distribution increases in the series Pd/Al2O3(NC) <
Pd/Al2O3(C) < Pd/Al2O3(E). Modification by the heteropoly compound was favorable for the decrease in the
size of palladium particles. The method of temperature-programmed reduction with hydrogen (TPR-H2)
showed that all catalysts included in their composition palladium hydride along with more strongly surfacebound
metal forms that are reduced at elevated temperatures, and their content decreases after modification
by the heteropoly compound and increases after catalytic tests. According to diffuse reflectance infrared Fourier
transform spectroscopy (DRIFTS), the deposition of a heteropoly compound leads to a change in the
type of Lewis acid sites on the alumina surface and in the electronic state of palladium. According to the
results of infrared spectroscopic studies of adsorbed CO, the relatively large particles of Pd0 are the main form
on the surface of nonmodified catalysts. The catalysts modified by the heteropoly compound contain single
Pd+ and Pd2+ cations, and the fraction of Pd0 is substantially smaller. The specific features of the Lewis acidity
of the catalyst surface determine the possibility of 1,3,5-trichlorobenzene adsorption and activation on the
support and the spillover of hydrogen from Pd0. An increase in the catalyst stability as a result of support modification
by the heteropoly compound can be explained by the appearance of new active sites in the interaction
of palladium with the heteropoly compound or its thermal decomposition products.