Compaction versus Surface Parameters of Certain Solid Catalysts

Abstract

The effect of compaction at 0.31–3.1 t/cm2 on Al(OH)3, Mg(OH)2 and their mixed hydroxide was studied by nitrogen adsorption. With Al(OH)3, compaction gave no significant changes up to 1.24 t/cm2, but above 1.55 t/cm2 a considerable decrease in SBET and an increase in pore radius occurred with small changes in pore volume. This could be ascribed to the presence of free water between the structural layers in the material which normally prevents their contact and a consequent destruction of the pore structure. With Mg(OH)2, compaction at low pressures decreased SBET and increased the pore dimensions as a result of adhesion between neighbouring particles, leading to a blocking of that fraction of the micropore structure originally accessible to nitrogen molecules. Increasing compaction led to a marked increase in the adsorption capacity as a consequence of plastic deformation associated with the breakage of fragile primary particles and the creation of new surfaces. Compaction of the mixed hydroxide led at first to an increase in both the SBET and Vp values (due to fragmentation of the particles), followed by a loss of SBET due to the presence of a mixture of particles in the system which increase the compression ability of the latter. Complete pore structure analysis showed that samples of Al(OH)3 powder when compacted at 0.31 and 1.55 t/cm2 were microporous. All other samples contained mainly mesopores.