THERMAL-BEHAVIOR AND EFFECT OF COMPACTION ON PHASE-CHANGES AND CATALYTIC PROPERTIES OF SOME SOLID ACID CATALYSTS

The thermal dehydration of AI(OH)(3), Mg(OH)(2) and the mixed hydroxid e (1:1) were investigated, both powder and compact forms using DTA and TGA techniques. Kinetic parameters, such as activation energy, pre-ex ponential factor and activation entropy were calculated from TG curves . The results obtained showed that the dehydration process inside the pores of AI(OH)(3) appears to be the largest complexity compared to th at inside the pores of all other samples (Delta S = -190.3 JK(-1)mol( -1)). In mixed compacted samples the migration of water molecules from the pores of Mg(OH)(2) to those of Al(OH)(3) takes place. The catalyt ic activities for conversion of methanol to ether and methane were fou nd to decrease sharply by compaction up to 8 tons inch(-2) beyond whic h it remains nearly constant up to 20 tons inch(-2). This indicates th at compaction hinders the accessible diffusion of the methanol molecul es to active sites. The water molecules collected by compaction blocke d the active centres participating in the reaction. These sites retain their original activity by evacuation. The most active catalyst was g amma-Al2O3 which gives the lowest negative Delta S values (-252.5, -1 43.1 JK(-1)mol(-1)) for ether and methane formation respectively. The activities of gamma-Al2O3-MgO before and after compaction were differe nt, which can be attributed to the difference in the number of acid si tes of their catalysts but not in their strengths Delta S values are not significantly different).