| Conventional ZSM-5 zeolite shows a poor activity in many reaction processes because of poor accessibility and diffusion limitation resulting from their microporous networks. To overcome the diffusion limitations and enhance the availability of active sites in ZSM-5 zeolite, hierarchical ZSM-5 zeolite possessing structured microporosity and secondary mesoporosity within zeolite crystals has being synthesized. The presence of secondary mesopores effectively lowers the diffusion and steric limitation and improves the accessibity of acid sites. Compared to the conventional zeolites, therefore, hierarchical ZSM-5 zeolite has exhibited excellent catalytic activity in many reactions involving large molecules.The distribution, nature, number and strength of acid sites in the hierarchical zeolites can be changed as a result of the introduction of mesopores, which are critical factors determining catalytic behaviour of zeolite catalysts. So, there is a strong incentive to study comprehensively the effect of the acidity and porosity of hierarchical zeolites on their catalytic performance, which can provide a theoretical basis for the synthesis of efficient catalysts.Microspherical MFI mesoporous zeolite (M-ZSM-5) aggregated from nanosized zeolite crystals with intra- and inter-crystalline mesoporous structure was prepared by bond-blocking principle. Organic functioned fumed silica is used as silica source and tetrapropylammonium bromine as microporous structure-directing agent. According to the results of XRD, nitrogen adsorption-desorption isotherms, SEM and TEM, M-ZSM-5 exhibits typical MFI crystalline structure. Meanwhile, obviously higher BET surface area of 544 m2/g, external surface area of 327 m2/g and mesoporous volume of 0.30cm3/g are observed in M-ZSM-5 because of the presence of secondary mesopores and nanometer-sized crystals. In comparison, BET surface area, external surface area and mesoporous volume of conventional ZSM-5 are 427 m2/g,37 m2/g and 0.03 cm3/g, respectively. Moreover, a broad pore distribution with pore diameter of 2-20 nm is observed for M-ZSM-5 zeolites, and most of pore sizes are comprised in the range 3-7 nm.Their acidic properties were characterized via NH3-TPD and FT-IR spectra in combination with probe molecules of different sizes. It is revealed that surface acidity of M-ZSM-5 zeolites is not significantly sacrificed due to the introduction of mesopores and change of morphology. It is obvious that the conventional ZSM-5 zeolite possess higher concentration of Bronsted sites than M-ZSM-5 zeolites. On the contrary, higher concentration of Lewis acid sites exist in M-ZSM-5 zeolites compared to the conventional ZSM-5 zeolite due to increasing non-framework octahedrally coordinated aluminum confirmed by 27 Al MAS NMR, and these Lewis sites are quite strong. Meanwhile, the introduction of mesopores made M-ZSM-5 zeolites more accessible to bulky molecules but weaken their acid strength.2,6-dimethylpyridine (DMPy) and 2,6-di-tert-butylpyridine (DTBPy) can probe 97% and 89% of Bronsted acid sites in M-ZSM-5 zeolite with external surface areas of 327 m2/g. However, only 1.2% of Bronsted acid sites are located on the external surface of ZSM-5 zeolite.Mass transfer ability of zeolites is demonstrated via the diffusion experiments of adsorbed benzene.The effective diffusivities of benzene increase with the increasing external surface area and mesoporous volume. The effective diffusivities of benzene for M-ZSM-5-100-1, M-ZSM-5-100-2 and M-ZSM-5-100-3 are 6.61×10-20m2/s,8.58×10-20m2/s and 11.44×10-20m2/s at 308 K and 0.2 mbar, respectively.The benzylation of aromatics is a very important Friedel-Crafts type reaction in organic chemistry.These reactions are catalyzed by strong homogeneous acid catalysts, which present several problems, such as corrosion, high toxicity, difficulty in separation and recovery of catalysts, and post-handling, etc. Hence, the development of environmentally more friendly solid acid catalysts is of great practical importance. In the benzylation of aromatics (viz. benzene, toluene, xylene, mesitylene, anisole and naphthalene) with benzyl chloride (BC), conventional ZSM-5 zeolite display a poor catalytic activity, conversion of BC is less than 2% at 373 K after a reaction time of 8 h. In contrast, M-ZSM-5 zeolite possesses the enhanced catalytic activity and reusability. Conversion of BC reachs above 94% for mesitylene and anisole at 373K for 2h. Meanwhile, Similar distribution between the isomers of monobenzylated aromatics for M-ZSM-5 and conventional ZSM-5 zeolites reflects that these reactions completely occur at the outer surface of the catalysts, in which case no steric effect is expected for these reactions over M-ZSM-5 and conventional ZSM-5catalysts. In addition, the reactivities of different aromatics with BC are in the following order:anisole> mesitylene> o-xylene> toluene> m-xylene> p-xylene>benzene. This result is totally in accordance with the classical mechanism of Friedel-Crafts benzylation reaction, where the benzylation of aromatics is easier owing to the introduction of one or more electron donating groups in aromatic rings. It is concluded that electron donating groups in aromatic rings are mainly contributed to the enhanced catalytic activity for the benzylation of different aromatics with BC over the zeolites. Meanwhile, electron donating groups in aromatic rings will hinder the interaction between electrophilic carbocation and benzene ring, which has some effect on the catalytic activity.The catalytic behavior of the zeolites in these reactions is affected not only by their porosity but also by their acidity. M-ZSM-5 zeolites with additional mesoporosity and external surface areas display an improved catalytic activity because of their better mass transfer ability and enhanced accessibility of acid sites. Generally, the liquid-phase benzylation of aromatics with benzyl chloride has been reported to be catalysed by Lewis acid sites. More Lewis acid sites will result in accelerated reaction rate. Bronsted acid sites would also promote the process of the reaction, but the role of Bronsted acid sites for the benzylation of different aromatics is different. Catalytic results of M-ZSM-5 zeolites are related to the number and strength of external acid sites (Bronsted and Lewis acid sites). In the benzylation of benzene, toluene, m-xylene and p-xylene, the catalytic activity of M-ZSM-5 zeolites is increased with the increase in the amount of external acid sites. There is a good linear correlation between the apparent rate constant ka and the amount of strong external acid sites. For the benzylation of o-xylene, anisole and mesitylene, however, a higher concentration of Bronsted acid sites could cause a low catalytic activity owing to the different nature of aromatic subatrates resulting in serious poison on the catalyst surface. |
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