- 上一篇：钻削技术英文文献和中文翻译
- 下一篇：甲苯甲基化工艺生产对二甲苯英文文献和中文翻译
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
ZSM-5, selectivity to toluene de-creaseswith increasing Si/Al ratios, and a simultaneous increase in ben-zene conversion and xylene selectivity is observed. The presence ofadditional porosity not only accelerates benzene methylation, but alsocontributes to the faster sequentialmethylation of primary product tol-uene to xylenes. The selectivity tomulti-methylated benzene ormethyl-ated ethylbenzene is not notably affected by the presence of hierarchicalpores. A high selectivity of 34.9% to xylene by benzene methylation isachieved by a HPZ with a Si/Al ratio of 180, in contrast to 25.6% for con-ventional ZSM-5.References[1] O. Xu, H. Su, J. Ji, X. Jin, J. Chu, Kineticmodel and simulation analysis for toluene dis-proportionation and C9-aromatics transalkylation, Chin. J. Chem. Eng. 15 (2007)326–332.[2] T. Yashima, K. Yamazaki, H. Ahmad, M. Katsuta, N. Hara, Alkylation on syntheticzeolites: II. Selectivity p-xylene formation, J. Catal. 17 (1970) 151–156.[3] N.Y. Chen, W.W. Kaeding, F.G. Dwyer, Para-directed aromatic reactions over shape-selective molecular sieve zeolite catalysts, J. Am. Chem. Soc. 101 (1979) 6783–6784.[4] W.W. Kaeding, C. Chu, L.B. Young, B. Weinstein, S.A. Butter, Selective alkylation oftoluene with methanol to produce para-Xylene, J. Catal. 67 (1981) 159–174.[5] J. Van der Mynsbrugge, M. Visur, U. Olsbye, P. Beato, M. Bjørgen, V. Van Speybroeck,S. Svelle, Methylation of benzene by methanol: single-site kinetics over H-ZSM-5and H-beta zeolite catalysts, J. Catal. 292 (2012) 201–212.[6] K.J.A. Raj, E.J.P. Malar, V.R. Vijayaraghavan, Shape-selective reactions with AEL andAFI typemolecular sieves alkylation of benzene, toluene and ethylbenzenewith eth-anol, 2-propanol,methanol and t-butanol, J. Mol. Catal. A Chem. 243 (2006) 99–105.[7] M.O. Adebajo, M.A. Long, The contribution of the methanol-to-aromatics reaction tobenzene methylation over ZSM-5 catalysts, Catal. Commun. 4 (2003) 71–76.[8] M.O. Adebajo, R.F. Howe,M.A. Long,Methylation of benzenewithmethanol over ze-olite catalysts in a low pressure flow reactor, Catal. Today 63 (2000) 471–478.[9] S.J.X. He, M.A. Long, M.A.Wilson, M.L. Gorbaty, P.S. Maa, Methylation of benzene bymethane-13C over zeolitic catalysts at 400 °C, Energy Fuel 9(1995)616–619.[10] E.M. Kennedy, F. Lonyi, T.H. Ballinger, M.P. Rosynek, J.H. Lunsford, Conversion ofbenzene to substituted aromatic products over zeolite catalysts at elevated pres-sures, Energy Fuel 8(1994)846–850.[11] J.P. Breen, R. Burch, M. Kulkarni, D. McLaughlin, P.J. Collier, S.E. Golunski, Improvedselectivity in the toluene alkylation reaction through understanding and optimisingthe process variables, Appl. Catal. A Gen. 316 (2007) 53–60.[12] G. Mirth, J.A. Lercher, In situ IR spectroscopic study of the surface species duringmethylation of toluene over HZSM-5, J. Catal. 132 (1991) 244–252.[13] K.Wang, X.Wang, Comparison of catalytic performances on nanoscale HZSM-5 andmicroscale HZSM-5, Microporous Mesoporous Mater. 112 (2008) 187–192.[14] D. Van Vu, M. Miyamoto, N. Nishiyama, Y. Egashira, K. Ueyama, Selective formationof para-xylene over H-ZSM-5 coated with polycrystalline silicalite crystals, J. Catal.243 (2006) 389–394.[15] F.J. Llopis, G. Sastre, A. Corma, Xylene isomerization and aromatic alkylation inzeolites NU-87, SSZ-33, beta, and ZSM-5: molecular dynamics and catalytic studies,J. Catal. 227 (2004) 227–241.[16] P.G. Smirniotis, E. Ruckenstein, Alkylation of benzene or toluene with MeOH or C2H4over ZSM-5 or beta zeolite: effect of the zeolite pore openings and of the hydrocarbonsinvolved on the mechanism of alkylation, Ind. Eng. Chem. Res. 34 (1995) 1517–1528.[17] S. Inagaki, K. Kamino, E. Kikuchi,M.Matsukata, Shape selectivity ofMWW-type alu-minosilicate zeolites in the alkylation of toluene with methanol, Appl. Catal. A Gen.318 (2007) 22–27.[18] Z. Zhu, Q. Chen, Z. Xie,W. Yang, C. Li, The roles of acidity and structure of zeolite forcatalyzing toluene alkylation with methanol to xylene, Microporous MesoporousMater. 88 (2006) 16–21.[19] A.M. Vos, X. Rozanska, R.A. Schoonheydt, R.A. van Santen, F. Hutschka, J. Hafner, Atheoretical study of the alkylation reaction of toluene with methanol catalyzed byacidic mordenite, J. Am. Chem. Soc. 123 (2001) 2799–2809.[20] P. Ratnasamy, R.N. Bhat, S.K. Pokhriyal, S.G. Hegde, R. Kumar, Reactions of aromatichydrocarbons over zeolite beta, J. Catal. 119 (1989) 65–70.[21] Z.K. Xie, Z.C. Liu, Y.D.Wang, Q.H. Yang, L.Y. Xu,W.P. Ding, An overview of recent de-velopment in composite catalysts from porous materials for various reactions andprocesses, Int. J. Mol. Sci. 11 (2010) 2152–2187.[22] J. Wei, A mathematical theory of enhanced para-xylene selectivity in molecularsieve catalysts, J. Catal. 76 (1982) 433–439.[23] T. Odedairo, S. Al-Khattaf,