Investigation of the acidic nature of MCM-68 zeolite based on the adsorption of CO and bulky probe molecules

Abstract

The aluminosilicate MCM-68 has an MSE topology, a unique pore system, and solid acid characteristics. For these reasons, it has potential applications as a highly efficient catalyst for paraffin cracking, methanol-to-olefin/dimethyl ether-to-olefin reactions, and the alkylation of aromatic compounds. In this work, the acidic properties of both the parent MCM-68 and dealuminated versions were evaluated using infrared spectroscopy in conjunction with the adsorption of CO and/or 2,4,6-trimethylpyridine (collidine). Notably, the relatively bulky collidine molecules could not enter the 12-ring pore-mouths of the MCM-68 at room temperature but were able to do so at 403 K, due to the excitation of pore-mouth breathing vibrations that allowed increased diffusion at higher temperatures. During CO adsorption study, two types of Lewis acid sites and one type of Brønsted acid sites were identified on the parent MCM-68. After dealumination by acid treatment, the Brønsted and stronger Lewis acid sites, especially those on the external surfaces of the MCM-68 particles, were preferentially removed.