GEOMETRIC FACTOR IN HETEROGENEOUSLY CATALYZED DEHYDRATION

Geometric conditions for the dehydration of alcohols on γ-Al2O3 and ThO2 have been studied with the aid of a computer. Computed data rendered it possible to explain some experimental data which have as yet been explicable only with difficulty (the anti-elimination mechanism and high cis/trans isomer ratio in the dehydration on γ-Al2O3 and the selective formation of 1-alkenes in the dehydration on ThO2). It has been found that the reaction may proceed on the catalyst surface not only as the syn-elimination but also as the energetically more favorable anti-elimination. Steric demands with respect to the product formation increase in the order: 1-alkene, cis-2-alkene, and trans-2-alkene. Based on a Monte-Carlo model of coverage of the (100) surface of γ-Al2O3 by hydroxyl groups, the activity of the catalyst, the selectivity of the stereochemical course of the reaction, and the selectivity with respect to the products are discussed in relation to the degree of coverage. On low-index surfaces of ThO2, i.e., (100), (111), and (110), only such adsorption complexes have been identified as lead to preferential formation of 1-alkenes. © 1979.