H/D exchange of molecular hydrogen with Brønsted acid sites of Zn- and Ga-modified zeolite BEA

Abstract

Kinetics of hydrogen H/D exchange between Brønsted acid sites of pure acid-form and Zn- or Ga-modified zeolites beta (BEA) and deuterated hydrogen (D₂) has been studied by ¹H MAS NMR spectroscopy in situ within the temperature range of 383–548 K. A remarkable increase of the rate of the H/D exchange has been found for Zn- and Ga-modified zeolites compared to the pure acid-form zeolite. The rate of exchange for Zn-modified zeolite is one order of magnitude higher compared to the rate for Ga-modified zeolite and two orders of magnitude larger compared to the pure acid-form zeolite. This promoting effect of metal on the rate of H/D exchange was rationalized by a preliminary dissociative adsorption of molecular hydrogen on metal oxide species or metal cations. The adsorbed hydrogen is further involved in the exchange with the acid OH groups located in vicinity of metal species. The role of different metal species in the possible mechanisms of the exchange with involvement of zeolite Brønsted acid sites and metal species is discussed.