First synthesis of SAPO molecular sieve with LTL-type structure by hydrothermal conversion of SAPO-37 with FAU-type structure

Abstract

An interzeolite conversion method was applied to the synthesis of silicoaluminophosphate (SAPO) molecular sieves. In the interzeolite conversion process, a structural similarity between the starting zeolite and the final crystallized zeolite was the crucial factor for zeolite crystal growth. We succeeded in the first synthesis of a SAPO with LTL structure from SAPO-37 with FAU structure in the presence of KOH. The d6r unit was the common building unit between both SAPOs. The crystallization of the LTL-type SAPO was accomplished at 170°C and in only one day when tetramethylammonium hydroxide (TMAOH) was added to the starting materials. As the LTL-type SAPO could not be obtained from the amorphous hydrogel prepared from fumed silica, pseudoboehmite, and H3PO4, even with prolonged reaction time, it was found that the SAPO formation process via such interzeolite conversion was more efficient, with an enhanced crystallization rate. The hydrothermal conversion of SAPO-37 into LTL-type SAPO was also found to depend significantly upon the synthesis parameters of H2O/T (T=Si+Al+P), KOH/T, and TMAOH/T, as well as the synthesis temperature.