Structure–property relationships of some amorphous and crystalline aluminophosphates

Abstract

Precipitation of aluminophosphates was carried out over a wide range of alkalinity (pH 4.5–8.5) and P : Al ratios (0.17–1.02). X-Ray diffraction (XRD) showed that for P : Al < 1 the materials were completely amorphous whereas at P : Al = 1.02 they were crystalline, although the crystallinity decreased markedly with increasing pH. A large increase in porosity occurred as P : Al increased above 0.17 accompanied by major changes in ²⁷Al and ³¹P magic-angle spinning (MAS) NMR spectra. A strong correlation exists between the ³¹P chemical shift and the pore-size distribution. Catalytic activity was probed via, dehydrogenation and dehydration of n-butanol. Dehydrogenation selectivity is maximal at P : Al = 1.02 whereas the overall catalytic activity correlates with total pore volume and is maximal at P : Al = 0.66. NMR, XRD and studies of catalytic selectivity confirm the absence of separate alumina and aluminium phosphate phases in these aluminophosphates.