Catalytic Performance Of Hierarchical ZSM-5 Zeolites For Benzylation Of Aromatic Hydrocarbon

The benzylation of aromatics with benzyl chloride is a very important class of reactions to produce benzylated aromatics, which are key intermediates in the phar-maceuticals, fragrances, dyes, and agrochemicals. In recent years, zeolites drew some researchers’s attention, especially hierarchical zeolite which changes the pore structure and the acidity of the zeolte catalysts.At the same time, hierarchical zeolite not only retains the microporous properties of zeolite, but also introduces the supermicropores, mesopores or macropores in the materials, which largely shorten the diffusion path, reduce the diffusion residence, speed up the diffusion of molecular, enhance the accessibility and availability of active sites. The appearance of hierarchical zeolite widely broadens the applied scope of zeolite.In this paper, hierarchical ZSM-5 zeolites were synthesized using three different silica source organic functioned by different organosilane (Y5669,Z-6032,TPOAC) and tetrapropylammonium bromine(TPABr) as microporous structure-directing agent. The resulting hierarchical zeolite materials can be categorized into three distinctly different types of materials. The bulky molecule benzyl chloride(BC) as the alkylating agent, and the catalytic performance for the benzylation reaction of substituted aromatics such as benzene,toluene, p-xylene,anisole and mesitylene with benzyl chloride over different hierarchical ZSM-5 samples and the conventional ZSM-5 was performed in detail in order to demonstrate find the relationship between acidity, pore structure and catalytic performance.The obtained samples were characterized by X-ray diffraction, N2 adsorption-desorption methods, scanning electron microscopy and transmission electron microscope. The results comfirmed that the presence of secondary mesopores and high external surface in the hierarchical ZSM-5. Acidities of zeolites were characterized by Fourier transform infrared spectroscopy after pyridine adsorption and 2,6-di-ter-butylpyridine (DTBPy). The results showed that a large number of strong Lewis acid sites existed in the hierarchical ZSM-5 samples. Meanwhile, the concentration of external Bronsted acid sites,especially the strong external Bronsted acid sites, increased obviously with increasing the external surface.These facts were suggested to be mainly responsible for the enhanced catalytic activity in the benzylation of aromatic hydrocarbon with benzyl chloride. Hierarchical ZSM-5 samples showed higher activity than conventional ZSM-5 under the same reaction conditions, which may be interpreted as a direct result of the presence of secondary mesopores,providing a significant amount of external surface area and removing the diffusion limitation and the steric effect for large molecules. At the same time, many accessible Bronsted acid sites and a large amount of strong Lewis acid sites appear in the hierarchical ZSM-5samples, which can contribute to the enhanced conversion of BC. Hence,the catalytic behavior of zeolite in the benzylation of aromatic hydrocarbon with BC is affected not only by their porosity but also by their acidity. The all samples afford complete selectivity to the monobenzylated aromatic, and the distribution between the isomers (o/m/p) is similar, in which case no shape selectivity is expected and reactions occur at the outer surface of the samples.Hierarchical ZSM-5 zeolites without the diffusion restriction and the steric effect exhibited the increasing BC conversion from the benzylation of benzene to that of mesitylene, which was mainly due to ring activation by electron donating effects (-(CH3)3>-OCH3>-CH3>-H). The conversion of BC in the benzylation of different aromatic compounds over zeilte samples is in the following order:mesitylene> anisole> toluene> p-xylene> benzene. However,there is a intresting experimental phenomena that the catalytic performance of all hierarchical ZSM-5 samples for the benzylation of p-xylene with BC is lower than those of toluene.The hierarchical ZSM-5 zeolites can be reused for two cycles without a significant change in the activity and selectivity,which can be regarded as promising heterogeneous acid catalyst in aromatic benzylation and has potential application for industry in the reactions involving larger molecules.