A mechanism for the catalytic cracking of hydrocarbons in the presence of typical catalysts of the silica-alumina type is proposed, based on the acceptance of protons from the hydrocarbon by adsorbed water and on the donation of protons to the hydrocarbon by the catalyst. Water-deficient catalysts have low cracking activity, and deuterium oxide adsorbed on cracking catalysts readily exchanges deuterium for hydrogen in adsorbed hydrocarbons at temperatures far below those required for cracking. The high concentration of protons on the surface of silicaalumina as compared to that on pure silica gel further substantiates the proposed mechanism.
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