Conversion of Lower Alcohols into C2-C4 Olefins over Acid-Base Catalysts

摘要

The conversion reactions of methanol,ethanol,and their ethers in the presence of acid-base catalysts based on mesoporous ZSM-5 and ZSM-11 zeolites,microporous SAPO zeolites,heteropoly acids,and perfluorinated sulfonated cation exchangers are discussed.The set of reactions reflects the formation of olefins beginning from ethylene to butenes.The heats of reaction were estimated from the calculated values of the equilibrium constants and enthalpies at 700 K.It was assumed that the available experimental data for all three types of catalysts should be described by a complex mechanism of transformations of alcohols and ethers.The reaction sequence includes fast dehydration of alcohols into ethers.It is likely that,at temperatures of 512-573 K,the initial reactions have a common initiation mechanism for all three types of catalysts,which involves the formation of the protonated forms of substrate molecules that trigger the olefin synthesis reactions.For thermally stable catalysts(zeolites),the mechanism with participation of free radicals and olefins generated from the alkoxylated hydroxyl groups of zeolites is possible above 573 K.As the degree of conversion of ethers increases,carbenium and arenonium ions begin to play a progressively increasing role as active intermediates instead of the alkoxy groups.This crossover may be responsible for changes in both the kinetic parameters of the substrate transformation and the direction of the reaction over zeolites at 573-623 K.The schemes proposed in some published works for the conversion of oxygen-containing organic compounds and olefins with participation of superacid centers of the catalysts in question should be considered speculations in light of the effect of water leveling the strength of Bronsted sites.