Shape-selective alkylation of naphthalene and methylnaphthalene with methanol over H-ZSM-5 zeolite catalysts

Abstract

Gas-phase alkylation of naphthalene and 2-methylnaphthalene (2-MN) can be carried out efficiently over H-mordenite (HM), HY and H-ZSM-5. A high β-selectivity is observed with various kinds of H-ZSM-5 but not with HM and HY. At 350 °C, with naphthalene:methanol:mesitylene, 1:1:3 (wt) feed and space velocity, WHSV ~0.5 h −1, 15% naphthalene conversion is achievable with 76% selectivity to 2-MN and 2,6/2,7-dimethylnaphthalene (2,6/2,7-DMN). At 400 °C, 2-MN can be methylated using a 2-MN:methanol:mesitylene, 1:0.25:3.0 (wt) feed to give 60% 2,6/2,7-DMN at 19% conversion. This shape-selectivity is explained in terms of size differences between relevant molecules and zeolite pores. Nonselective alkylation of naphthalene and 2-MN on HM and HY is suggested to occur in the intracrystalline voids of these large-port zeolites, which easily accommodate both reactant and product molecules and allow their rapid diffusion. On the other hand, it is postulated that the β-selective alkylation over H-ZSM-5 occurs in external surface sites which perfectly suit small-size naphthalenics; these catalytic sites are too large, however, to allow any para-selectivity in the alkylation of toluene with methanol.