Thermal behavior and effect of compaction on phase changes and catalytic properties of some solid acid catalysts

Abstract

The thermal dehydration of Al(OH)3 , Mg(OH)2 and the mixed hydroxide (1:1) were investigated , both powder and compact forms using DTA and TGA techniques. Kinetic parameters, such as activation energy, pre-exponential factor and activation entropy were calculated from TG curves. The results obtained showed that the dehydration process inside the pores of Al(OH)3 appears to be the largest complexity compared to that inside the pres of all other samples . In mixed compacted samples the migration of water molecules from pores of Mg(OH)2 to those of Al(OH)3 takes place. The catalytic activities for the conversion of methanol to ether and methane were found to decrease sharply by compaction up to 8 tons /inch2 beyond which it remains nearly constant up to 20 tons/inch2. This indicates that compaction hinders the accessible diffusion of the methanol molecules to active sites. The water molecules collected by compaction blocked the active centers participating in the reaction. These sites retain their original activity by evacuation. The most active catalyst was gamma-alumina which gives the lowest negative entropy values for ether and methane formation respectively. The activities of gamma-Al2O3-MgO before and after compaction were different, which can be attributed to the difference in the number of acid sites of their catalysts but not in their strengths (the values are not significantly different).