Catalytic Conversion of Alcohols into Olefins: Spectroscopy, Kinetics and Catalyst Deactivation

Abstract

The alcohols-to-olefins (ATO) catalytic process, a technology based on oil-alternative feedstocks, has gained increasing attention due to the current high price of crude oil as well as the growing environmental concerns. Intensive academic and industrial research, mainly performed under ex-situ conditions with bulk characterization techniques as well as advanced theoretical calculations, have yielded important insights into the ATO reaction mechanism, which follows the so-called “hydrocarbon pool (HCP) mechanism”. This mechanism involves the subsequent addition of e.g. methanol onto an organic scaffold, from which olefins, such as propylene and ethylene, are formed and subsequently released. However, the nature of the HCP species under realistic reaction conditions and the catalytic chemistry are still under debate. The scope of this PhD thesis is to obtain new physicochemical insights into ATO catalytic processes over two industrially important solid acids, namely SAPO-34 and SSZ-13. Both molecular sieves possess the chabazite (CHA) framework structure, but differ in their chemical composition and hence their acid function. For this purpose, a combination of in-situ bulk and micro-spectroscopic techniques has been employed, which provides complementary information on the nature, formation mechanism, kinetic behavior and spatial distribution of HCP species during the methanol-to-olefins (MTO) and ethanol-to-olefins (ETO) catalytic processes. There are two similar types of HCP species observed during the course of both MTO and ETO. The first one is ascribed to poly-alkylated benzene (PAB) carbocations, the most active HCP species. The other one is the family of poly-aromatics (PA), the deactivating species. It was found that methylation reactions are responsible for both PAB and PA formations in the case of MTO, while for ETO olefin condensations are dominant. In addition, the HCP species, formed during the MTO reaction, possess more alkyl groups as compared to the ones generated during the ETO. The HCP species are mainly located at the outer rim of the CHA crystal in the case of MTO. This distribution will eventually block the reactant and product molecules to diffuse inwards and outwards. For ETO the distribution of HCP species occurs throughout the whole crystal at low temperatures; while at high temperatures only the outer part is working and the coke formation is concentrated in a layer in the inner parts of the crystal. Furthermore, in the case of MTO acid strength controls the rate of the methylation reaction, which is the major path for the formation of PAB and PA species. The acid site density affects the MTO process in terms of amount and distribution of HCP species. In contrast, condensation of olefins, the main path for the formation of both PAB and PA species during ETO, is not controlled by the acid strength; a higher acid site density results in a faster formation and produces a higher amount of PAB and PA species. Moreover, it was found that during MTO reactions over SAPO-34 the induction period and the lifetime of PAB carbocations gets shorter with increasing crystal size. Additionally, a larger amount of PA species is obtained with small-sized SAPO-34 crystals.