Synthesis and Characterization of Silicoaluminophosphate CIT-16P and Its Transformation to SAPO-17.

A silicoaluminophosphate molecular sieve, CIT-16P, is synthesized using butane-1,4-bis(quinuclidinium) [(C 7 H 13 N)-(CH 2 ) 4 -(NC 7 H 13 )] 2+ dihydroxide (DiQ-C 4 -(OH) 2 ) as an organic structure-directing agent (OSDA). Upon the removal of the OSDA, either by thermal treatment in air at temperatures exceeding 490 °C or by extended ozone treatment at 150 °C, CIT-16P transforms to SAPO-17 (ERI topology). The structure solution of CIT-16P in its as-synthesized form is obtained using a Rietveld refinement of the powder X-ray diffraction pattern. The primary composite building units (CBUs) of CIT-16P are highly distorted cancrinite ( can ) CBUs that transform into stable can units of the ERI-type framework as a result of the OSDA removal. The distortion of can CBU is maintained without transformation by the presence of tightly bound DiQ-C 4 dications in the as-synthesized form of CIT-16P. The transformed material is characterized and evaluated as a catalyst in the methanol-to-olefins (MTO) reaction. The catalytic behavior of the formed SAPO-17 (Si/T-atom = 0.022) (T = Si + Al + P) at 400 °C and WHSV of 1.3 h -1 produces elevated C 3+ olefin products ( i.e. , propylene, butenes, and pentenes) in early stages of the reaction. However, as the reaction proceeds, the C 3+ fraction decreases in favor of more ethylene.