Silicoaluminophosphate Molecular Sieves STA-7 and STA-14 and Their Structure-Dependent Catalytic Performance in the Conversion of Methanol to Olefins

Abstract

Details of the synthesis of the small-pore silicoaluminophosphate (SAPO) molecular sieves STA-7 (SAV) and STA-14 (KFI) prepared via a co-templating approach are described. STA-7 includes Si in the framework with Si/(Si+Al+P) ratios varying from 0.04 to 0.17, whereas STA-14 crystallizes from gels with a narrower compositional range (Si/(Si+Al+P) = 0.2−0.3). The main route to Si incorporation is by substitution for P, but in the presence of fluoride, aluminosilicate regions are formed in STA-7. 27Al 3Q MAS NMR studies enable resolution of tetrahedral Al in Al(OP)4 and Al(OP3,OSi) environments in both as-prepared and calcined materials. For STA-7 with low Si content the presence of five- and six-fold Al coordinated with water molecules or fluoride or hydroxide ions has also been confirmed. Upon calcination, all Al adopts tetrahedral coordination. High pressure CO2, CO, and CH4 adsorption at 100 °C indicates that the proton forms of the SAPO D6R zeotypes present effective polarities intermediate between cationic zeolites and pure silica chabazite. The methanol-to-olefin (MTO) performance of the SAPOs H-SAPO-34, H-STA-7, and H-STA-14 was found to depend on both crystallite size and cage connectivity and topology. H-STA-7 with a crystal size of 2−3 μm has MTO stability comparable to that observed for H-SAPO-34, whereas the same materials with larger crystal sizes (≥10 μm) deactivate rapidly. Ex situ GC-MS analyses of the SAPO catalysts after MTO reaction demonstrate that the uniformity in cage shape and size in cage-based, small-pore molecular sieves is a critical factor governing the type of the accumulated aromatic hydrocarbons and, hence, their MTO activity and deactivation behavior.