Hierarchical SAPO-11 molecular sieve-based catalysts for enhancing the double-branched hydroisomerization of alkanes

Abstract

To convert linear alkanes into double-branched isomers, a novel catalyst was prepared on the basis of hierarchical silicoaluminophosphate-11 (SAPO-11) molecular sieves with the large external surface area (ESA) and the high amount of medium and strong Brønsted (MSB) acid sites. The hierarchical SAPO-11 molecular sieves were synthesized in water with low-temperature crystallization using supercritical carbon dioxide (ScCO2) and the triblock copolymer polyoxyethylene-polyoxypropylene-polyoxyethylene (F127, EO106PO70EO106) as combined mesoporous templates. During the crystallization of hierarchical SAPO-11, the addition of F127 promotes more ScCO2 to be dissolved in water, and the micelles produced by F127 are dilated by ScCO2 to match the SAPO-11 fragments in size. In addition, ScCO2 makes silica species well dispersed in the aluminum phosphate (AlPO4) framework of the as-synthesized SAPO-11, which forms small silica islands (SIs); the small SIs produce a high relative concentration of Si (nAl) (n = 1–3), which increases the number of MSB acid sites. As a result, the as-synthesized SAPO-11 exhibits the ESA of 207 m2/g and the amount of MSB acid sites of 59.5 μmol pyridine/g, which are more than those of the conventional microporous SAPO-11 (87 m2/g and 38.7 μmol pyridine/g) and the counterpart synthesized in ScCO2-water (159 m2/g and 48.6 μmol pyridine/g) or F127-water (117 m2/g and 39.2 μmol pyridine/g). The large ESA and the high amount of MSB acid sites over the as-synthesized SAPO-11 provide accessible active sites for the double-branched hydroisomerization of alkanes. The as-synthesized SAPO-11-based catalyst displays the selectivity of 33.5% to double-branched isomers and the cracking selectivity of 8.0%.