Test of the nonane method for micropore evaluation. Use of nitrogen, n-hexane and carbon tetrachloride as adsorptives with ammonium phosphomolybdate, phosphotungstate and silicomolybdate as absorbents

Abstract

The adsorption isotherms of nitrogen, n-hexane and carbon tetrachloride on each of three microporous solids have been determined gravimetrically both before and after pre-adsorption of n-nonane. The solids were ammonium phosphomolybdate (APM), ammonium phosphotungstate (APT) and ammonium silicomolybdate (ASM), and a second series of experiments was carried out on compacts of the solids prepared at a pressure of 930 MN m^–2.

The nonane method for micropore evaluation was found to function satisfactorily when nitrogen was used for the determination of the isotherms, ASM being shown to be almost wholly microporous, APM to possess a small surface area (∼4 m² g^–1) and APT a considerable one (∼59 m² g^–1). With both APM and APT (ASM was not tested) the majority of the micropore volume survived compaction and was presumably therefore structural in nature, i.e., contained within the lattice. The results obtained with hexane and carbon tetrachloride before pre-adsorption of nonane are explicable in terms of molecular sieve effects; but after pre-adsorption the uptake of both adsorptives was higher than that of nitrogen, probably because of their diffusion through the “stoppers” of nonane blocking the entrances to the micropores.

It is concluded that the applicability of the nonane method for micropore evaluation is restricted to adsorptives such as nitrogen which are used at temperatures much below ambient and which have low solubility in n-nonane.