Formation of small pore SAPO-44 type molecular sieve

The hydrothermal transformation of silico–aluminophosphate gel with cyclohexylamine to SAPO-44 has been examined. The hydrothermal crystallisation products of the SAPO have been investigated by X-ray diffraction, FTIR, nitrogen and water adsorption, thermogravimetric analysis, surface analysis and ²⁷Al, ³¹P, and ²⁹Si MAS NMR. Structural changes were observed in the silico–aluminophosphate gel with and without organic template and during the hydrothermal crystallisation. The silico–aluminophosphate gel converted to pure SAPO-CHA phase in 168 h at 473 K. The surface of SAPO-44 was silicon rich as compared with that of SAPO-34 and SAPO-18. The ²⁷Al MAS NMR signal of tetrahedrally coordinated Al observed in the silico–aluminophosphate gel without the organic template was changed to octa-, penta- and tetrahedrally coordinated aluminium upon the addition of the cyclohexylamine template to the SAPO gel. After 3 h of hydrothermal treatment at 473 K however, the ²⁷Al MAS NMR signals of the octahedral and pentacoordinated aluminium were removed. This was also confirmed by ³¹P and ²⁹Si MAS NMR. The tetrahedrally coordinated P and Si were detected within 3 h at 473 K. The sorption capacity and adsorption–desorption trends of the SAPO gels and the crystallisation products were found to be different. ²⁹Si MAS NMR results indicated that the percentage of Si (4Al) and its distribution were significantly affected by the crystallization period. SAPO-44 was thermally stable up to 973 K with phase change observed over the calcination temperature of 1193 K. The SAPO gels and the crystallisation products have also been investigated for their catalytic behaviour in n-hexane and ethanol conversion reactions.