with a Si/Al ratio of 70. The effect of acidic strength canbe negligible because the acid strength for all HPZ ZSM-5-HTS samplesis close or slightly weaker than conventional ZSM-5 (NH3-TPD), where-as they overperform in the catalytic tests. The overall carbon balance isestimated to be above 96%. It is important to note that the catalyst iscurrently running in an industrial side-track experiment, and remainsstable for up to a test of 2000 h. The high conversion of benzene andthe high selectivity to xylene over HPZ ZSM-5 can be explained by thepositive effects of additional porosity.To understand the effect of additional porosity to the product distri-bution, it is important to take into consideration the role of mass The effectiveness factor denotes the utility of active sites in a porouscatalyticmaterial such as zeolite. At Si/Al=20, the primary particulatesthat build up the aggregates of conventional or HPZ ZSM-5 crystalsare close in size, i.e.,withsimilar R values. The effect of zeolite util-ity in terms of η is not so obvious, and the benzene conversions for con-ventional and HPZ ZSM-5 are close. Besides, the low Si/Al ratio alsoincreases the Al content and associated acid density in micropores, andmore CH3OH is adsorbed on acidic sites which can aggravatemass trans-port even in HPZ ZSM-5. Both factors contribute to the close benzeneconversion when Si/Al (molar ratio) = 20. As the Si/Al ratio increases,R for HPZ remains small and for conventional ZSM-5 it appears larger,and the effectiveness factor governs the conversion and selectivity ofbenzene in conventional ZSM-5. From the above speculations, one canexpect that it is the length of the diffusion path in zeolitic microporesthat determines the conversion/selectivity. For HPZ ZSM-5, theintracrystal diffusion is no longer the rate-limiting process of the masstransport process. According to recent mass transport studies, benzene,toluene or xylene transport in HPZ or nanosized ZSM-5 is dictated by asurface barrier that is associatedwith sticking probability,whereas in con-ventional micron-sized ZSM-5 it is restricted by intracrystalline diffusion[39,40]. As sticking probability of benzene, toluene, and xylene followsthe order: B N T N X; it is likely that reactants such as benzene and tolueneare more kinetically favorable to enter HPZ ZSM-5 than xylenes [39,40].This surface barrier controlled mass transport, in turn, means that morebenzene/toluene can be converted on HPZ than xylene totrimethylbenzenes. Interestingly, this effectiveness factor also reflects anincrease in primary product toluenemethylation to xylene, and the corre-sponding increase in xylene selectivity is detected simultaneously. Aquantified description of the observed selectivity is still lacking thus far,due to the unknown basic parameters to describe the reaction kineticdata. But the trend is obviously manifested that using HPZ zeolite notonly enhances themethylation of benzene to toluene, but also the tolueneto xylenemethylation. The acceleration of two consecutive reactions con-stitutes a merit to use HPZ zeolite for process intensification purposes. 4. ConclusionsA series of MFI structured ZSM-5 with varied Si/Al ratios have beensuccessfully prepared by extrapolating a combined dry-gel conversionand steam-assisted crystallization method.When employed as catalystfor themethylation of benzene, at a Si/Al ratio of 20, the primary partic-ulates of conventional ZSM-5 and HPZ ZSM-5 are both small after dry-gel conversion, and only toluene selectivity is improved. As Si/Al ratioincreases, it is found that selectivity towards toluene increases with in-creasing Si/Al ratios in conventional ZSM-5, and xylene selectivity doesnot exhibit remarkable change.On HPZ
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