EFFECT OF THE SYNTHESIS CONDITIONS ON THE CRYSTALLINITY AND SURFACE-ACIDITY OF SAPO-11

A series of AlPO4-11 molecular sieves with Si content ranging from zero to 0.24 Si molar fraction were crystallized from reaction gels prepared using different sources of Al and Si. It was found that the lower the crystallinity of the starting alumina the higher its reactivity and the higher the crystallinity and purity of the resulting molecular sieves. Factors affecting the initial pH of the departing gel strongly influence both the crystallinity and purity of the resulting solid. Thus, starting from the same source of alumina, it can be observed that the higher the pH within the acid region the higher the crystallinity and purity of the product. There seems to be an upper limit for the amount of Si incorporated in the AlPO4-11 framework, above which isolated silica patches (silica domain) are formed and subsequently dislodged from the structure when calcination is carried out at 773 K, yielding alpha-quartz. At this limiting value the population of both Lewis and Bronsted acid sites passes through a maximum. Unreacted pseudoboehmite coexisting with SAPO-11, was transformed upon calcination at 773 K into gamma-alumina. These species seem to act as Lewis centres which would interact with Bronsted sites to increase surface acidity.