Aminothermal synthesis of CHA-type SAPO molecular sieves and their catalytic performance in methanol to olefins (MTO) reaction

Aminothermal synthesis of SAPO molecular sieves, in which organic amines are used as both solvent and template, is explored based on a variety of amines. Di-iso-propylamine (DIPA) and N,N,N',N'-tetramethylethylenediamine (TMEDA) are found to lead to the rapid crystallization of SAPO-34 with high solid yield. A solid yield of 96.2% could be acquired using the TMEDA system (200 degrees C, 12 h), which is the highest value ever reported for SAPO molecular sieves. SAPO-44 is obtained for the first time using the hexamethyleneimine (HMI) template. Detailed synthetic investigation shows that the silicon content in the initial gel has an important effect on the crystalline nature of the final products, and higher Si concentration favours the synthesis of pure SAPO-34 and SAPO-44. In addition, it is shown that the Si coordination environment in the samples is closely related to the choice of template. Among the three samples investigated, SAPO-34-DIPA has the lowest threshold of Si content for the formation of Si islands in the framework due to the smallest charge compensation centers occluded in its CHA cage. The catalytic performance of the synthesized samples is tested by the MTO reaction and a high olefin selectivity of 85.8% is obtained on SAPO-34 templated by DIPA.