Trace water amounts can increase benzene H/D exchange rates in an acidic zeolite

Reactions between benzene and the zeolite HZSM-5, carried out at low pressure and room temperature, revealed that controlled addition of sub-stoichiometric amounts of water increases the rate of isotopic H-1/H-2 exchange between benzene-d(6) and the surface acid site by almost an order of magnitude relative to the control case in which no water is added. Single-ring aromatics like benzene have been identified as an active form of the hydrocarbon pool in methanol-to-hydrocarbon chemistries, but their role as reactive centers for alkylation and dealkylation chemistry is general to all of solid-acid catalysis. Maximum reaction rates were observed for water loadings of ca. 0.1 equivalents for catalysts with Si/Al equal to 15. No beneficial water effect was observed in benzene/zeolite reactions using a much lower acid-density catalyst HZSM-5 with Si/Al equal to 40. In-situ experiments strongly suggest that the origin of the water enhancement effect for the high-acid density catalyst arises from an increase in the "vehicle hopping" proton-transfer mechanism, which cannot occur when acid sites are sufficiently far apart as in the Si/Al equal to 40 catalyst. When the water loading is too high, water's competitive adsorption and excluded volume reduces the benzene adsorption probability due to the increased proton affinity of a water cluster. (C) 2017 Elsevier Inc. All rights reserved.