Pore-mouth catalysis boosting the formation of iso-paraffins from syngas over bifunctional catalysts

Recent studies confirm that the emerging bifunctional catalysts consisting of metal oxide and zeolites can directly convert syngas into high-quality gasoline, however, the formation mechanism of iso-paraffins and the difference with the conventional FT/zeolite catalyst have not been investigated. Herein, three one-dimensional SAPO zeolites with diverse micropore sizes were synthesized and assembled with ZnAlOx with spinel structure. It was found that ZnAlOx/SAPO-41 and ZnAlOx/SAPO-11 with medium micropore sizes favored the formation of C5–C11 hydrocarbons with a high content of iso-paraffins. The characterizations pointed out that the formation of iso-paraffins over SAPO-11 followed a pore-mouth catalysis mechanism, which means the isomerization of linear hydrocarbons can only take place near the pore mouth region of zeolites. This mechanism only allows the formation of mono-branched iso-paraffins in the C5–C11 range, which are less prone to be cracked than their di-branched isomers. A careful comparative analysis between ZnAlOx/SAPO-11 and Co/H-meso-ZSM-5 was also made in terms of product distribution, activity, and stability.