H-ZSM-5 Zeolites Deactivation Mechanisms in Catalytic Transformation of Methanol to Hydrocarbons

Abstract

Zeolite deactivation during the methanol transformation into hydrocarbons is a complex chemical process that includes reversible and irreversible degradation of active sites. The irreversible deactivation of the catalyst is associated with the degradation of active sites during long-term functioning of the zeolite under high-temperature exposure conditions of to water vapor as one of the main reaction products. The carbon deposits formation on the catalyst surface is the main cause of reversible deactivation. The formation of carbon can occur both in the cavities of the zeolites channels, which usually leads to a change in the selectivity of the process for light hydrocarbons, and at the junctions of the channels, which leads to pores blockage and a decrease in the activity of the catalyst. In addition, carbon deposition can occur directly on the active site of the zeolite, which also reduces the activity of the catalyst. The study of the synthesized catalytic systems deactivation rate to process time correlation was carried out in a tubular reactor installation of continuous operation, consisting of a reactor for producing dimethyl ether and a reactor for transformation of dimethyl ether into hydrocarbons. Determination of the kinetic regularities of the deactivation process of zeolite H-ZSM-5 makes it possible to adequately describe the actual course of the catalytic transformation of methanol into hydrocarbons. As a result of solving the differential equations of catalyst deactivation by numerical methods, the values of the preexponential factors and activation energies were obtained. Base on the values of the preexponential factors and activation energies, the methanol into hydrocarbons conditions range optimal for the catalytic transformation was determined, providing the minimum rate of catalyst deactivation, Ptotal = 1 Bar, W (methanol) = 2.3 kg (methanol) / (kg (cat) × h), t = 330–360 °C, which correlates with the results of the literature data on the transformation of methanol into zeolite of the H-ZSM-5 type.