The conversion of chloromethane to light olefins over SAPO-34: The influence of dichloromethane addition

Abstract

The effect of CH 2Cl 2 impurities on the conversion of CH 3Cl to olefins over SAPO-34 has been investigated. CH 3Cl was co-reacted with CH 2Cl 2 in a 10:1 ratio at 400 °C, WHSV = 8 h −1, p(CH 3Cl) = 620 mbar and p(CH 2Cl 2) = 65 mbar, over a typical SAPO-34 catalyst with (Al + P)/Si = 11. It was observed that CH 2Cl 2 promotes the formation of aromatic compounds and leads to rapid deactivation of the catalyst for the CH 3Cl to olefins reaction. The induction period, which is typically observed as an increase in activity before maximum conversion is reached is, however, significantly shortened by the addition of CH 2Cl 2. Feeding a mixture of 12CH 3Cl and 13CH 2Cl 2 in a 10:1 ratio led to formation of olefins with about 5% 13C, which was indistinguishable from the 13C content in the aromatic hydrocarbons (reaction intermediates and coke) retained inside the SAPO-34 crystals in the same experiment. This shows that CH 2Cl 2 takes part in the reactions leading to both products, including the formation of aromatic reaction intermediates, and deactivation. Hypothesized reaction steps for the incorporation of carbon atoms from CH 2Cl 2 into both gas phase products and coke were found to be plausible by studying the reactions of xylene/CH 2Cl 2 mixtures and of 2,5-dimethylbenzyl chloride alone. The intimate mechanistic links between the desired conversion of CH 3Cl to olefins and the detrimental effects of CH 2Cl 2 addition imposes restrictions on the purity of the CH 3Cl feedstock in a commercial SAPO-34 based CH 3Cl to olefins application.