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Methylation of benzene is an alternative low-cost route to produce xylenes, but selectivity to xylene remains lowover conventional zeolitic catalysts. In this work, a combined dry-gel-conversion and steam-assisted-crystallization method is used to synthesize hierarchically porous zeolite ZSM-5 with varied Si/Al malar ratios.X-ray diffraction (XRD), N2 physisorption, NH3-temperature programmed desorption (TPD), scanning electronicmicroscopic (SEM)measurement and Fourier transforminfrared (FT-IR) are employed to characterize the struc-ture and acidity of both hierarchically porous zeolites and their conventional counterparts. The method is foundto be applicable to ZSM-5 with molar ratios of Si/Al from 20 to 180. The ZSM-5 zeolites are used as catalysts forbenzene methylation at 460 °C to investigate the effect of additional porosity and Si/Al ratios. At low Si/Al ratios,the benzene conversions over conventional and hierarchical ZSM-5 are close, and selectivity to toluene is highover hierarchical ZSM-5. It is found that hierarchical porositymarkedly enhances the utility of zeolite and the se-lectivity towards xylenes via improved mass transport at higher Si/Al ratios. Under an optimized hierarchicalZSM-5 catalyst, xylene selectivity reaches 34.9% at a Si/Al ratio of 180.© 2014 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved. 1. IntroductionXylenes are important feedstock for the industrial production ofterephthalic acid, isophthalic acid, dimethyl terephthalate, polyesterresins, and the synthesis of vitamins or other pharmaceutical products.35697
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Nowadays,most xylenes aremass produced in industrial scale via eithernaphtha catalytic reforming or toluene disproportionation [1].Separa-tion of xylene isomers and the ethylbenzene mixture co-product isexpensive due to their close boiling points. In the established toluenedisproportionation process, a large amount of benzene is producedsimultaneously, which was previously added to gasoline as co-fuels.Nonetheless, governments are imposing stringent guidelines to reducebenzene content in gasoline, with the awareness of its toxicity to publichealth and our ecological system. An alternative route to producexylenes is through the methylation of toluene or benzene, and zeolitesare the preferred solid acidic catalyst for their shape-selectivity [1–9].For toluene methylation, since the pioneering work by Yashima et al.[2] over various cation modified Y-zeolite (FAU) catalysts in the 1970s,Kaeding and co-workers have optimized the modification methods ofZSM-5 (MFI structure) with a wide range of modifiers(Mg,P,Si, etc.)tomaximize the p-xylene yields [3,4,11–16]. Later on, other zeolites, suchas
ZSM-11 (MEL),MCM-22 (MWW) [17],ITQ-2(MWW) [17],Mordenite(MOR) [18,19], β-(BEA) [20],SAPO-34(CHA) [18],SAPO-11/MnAPO-11(AEL) [6,18], and SAPO-5/MnAPO-5 (AFI) [8,18,21] have been extensivelyexplored for catalyzing methylation of toluene. In most cases, valuablep-xylene is the major product because p-xylene diffusivity in ZSM-5channels is 3 orders ofmagnitude faster than m-or o-xylene, which en-dows desirable product selectivity [3,22]. On the other hand, there areonly sporadic reports on benzene methylation using methanol com-pared with that of toluene; a complicated product spectrum with highselectivity to toluene and low selectivity towards xylenes is normallyobserved [5–10,23,24]. It is demanding to design a single intensifiedprocess to convert low-cost benzene and methanol through consecu-tivemethylation to value added xylenes, for which the current selectiv-ity to xylene remains low (≤20%) [5,23].As the crystal size ofmost zeolites is in the range ofmicrometers, andmolecular transport withinmicroporous channels is solely governed byconfigurational diffusion [25]. One side effect of using zeolitic catalystsis the restriction of diffusion that limits the utility of zeolites or accessi-bility to and from the internal active centers. In the past decade or so, ithas been recognized that creating additional porosity to zeolitic crystalsor decreasing crystal size can alleviate the diffusion limitations and there-by promote the catalytic performance of zeolitic catalysts [25–34].The☆ Supported by theNational Natural Science Foundation of China (21006024), the CNPCInnovation Foundation (2011D-5006-0507), the Shanghai Pujiang Program (11PJ1402600),the New Century Excellent Talents in University (NCET-11-0644), and the FundamentalResearch Funds for the Central Universities (WB1213004-1).⁎