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Corresponding author.E-mail address: kakezhu@ecust.edu.cn (K. Zhu). zeolite with at least two types of porosity is called a hierarchically porouszeolite (HPZ), which normally contains both meso- (2–50 nm) andmicro-porosity in one crystalline material. The merits of using HPZswith respect to their conventional analogues include high activity, highcatalytic lifetime, higher activity towards bulky molecules, as well asimproved stability, that has recently been highlighted in a recent reviewby Egeblad et al. [35]. The product distribution can also be altered due tothe different shape or size of their controlledmass transport inside zeolit-ic architectures [26,35]. In the case of toluene methylation, however,there are controversial conclusions. For instance, Jahn et al.reportednosize effect of ZSM-5 to production distribution for toluene methylation[36], whereas Wang and co-workers [13] showed the merits of usingnanosized ZSM-5 by promoting activity (conv. 4%) in both toluenedisproportionation and methylation. Ding and co-workers [37] haveobserved a jump of 10% in conversion when ZSM-5 after steaming treat-ment was used to catalyze the methylation of toluene; however, thezeolitic size effect is not clearly shown as steaming affects both porosityand acid strength. From early studies by Christensen et al. [26], it hasbeen observed that over HPZ ZSM-5, benzene alkylation with ethyleneto ethylbenzene is enhanced in terms of benzene conversion and ethyl-benzene selectivity. The rate limiting process for the reaction over con-ventional ZSM-5 is intracrystalline diffusion of ethylbenzene, becausetheethylbenzenemoleculeislargerinsizethanotherreactants.Ontheother hand, the rate limiting process on HPZ ZSM-5 is shifted to theintrinsic reaction controlled process. These findings inspire us to findthe effect of product distribution by HPZ, but no report on crystal sizeeffect for benzene methylation has been known thus far.From the point of view of mass transport, it was once believed thatthe shape-selectivity stems from the intracrystal diffusion of productsand thereby a larger crystal size enhances selectivity [38]. However,recent transport studies show that benzene, toluene and xylene (BTX)transport depends on crystal sizes. Lercher and co-workers [39–41]found that the rate-controlling step in mass transport is switchedfrom intracrystal diffusion to the surface process as the primarysizes of ZSM-5 are reduced down to below 100 nm, as a result of smallvalues (≤10−6) for the BTX sticking probability. As small molecules(i.e.,CH3OH) access pore apertures via both sinusoidal/straight channelsand BTX entersmainly via straight channels, traffic-control can also playa role in mass transport [42]. Keeping in mind that the majority of theconventional
ZSM-5 crystal is covered by sinusoidal channels as a resultof subunit intergrowth, only traces of straight channels are exposed toexternal surfaces [43]. Moreover, the pore mismatch in boundaries ofintergrowth subunits can incur amore intricate effect tomass transportin conventional ZSM-5 [43]. From a kinetic angle of recent studies bySvelle et al. [5], it is evident that as the apparent reaction rate is first-orderwith respect to benzene partial pressure, improving BTX transportis expected to influence the concentration of BTX in the vicinity of activesites, and thereby the kinetic process. Although the reaction rate is ze-roth order with respect to CH3OH pressure, excessive CH3OH adsorbedstrongly on acidic sites can block the aperture to channels and lead toslowmass transport [5]. Changing the Si/Al ratiowill adjust the concen-tration of pre-adsorbed CH3OH. Inspiredwith the above knowledge, onecan expect that both additional porosity and Si/Al ratio can influence thecatalytic performance to benzene methylation [38].Herein, we adopt H-ZSM-5, a widely employed archetypal zeoliticcatalyst to explore the effect of hierarchical porosity and Si/Al ratioon the catalytic performance for the methylation of benzene. As themethylation of benzene is also an important step to understandmethanol to hydrocarbon (MTH) [5] or methane methylation withbenzene [7,8], these results can also be fundamentally important tothese relevant reactions. We demonstrate here that HPZ ZSM-5 en-hances both benzene methylation to toluene and its consecutivemethylation to xylene at high Si/Al ratios. HPZ ZSM-5 is thereby acatalyst that intensifies the benzene methylation selectivity to xylene,which constitutes anothermerit of using hierarchical zeolites to consec-utive reactions.2. Experimental2.1. Materials and syntheses