Enhancing catalytic isomerization ability of SAPO-11 by typical acid modification in preparation of green diesel by one-step hydrotreatment of FAME

SAPO-11 molecular sieve is a promising candidate for preparation of hydrocarbon-based biofuels. Herein, two typical acids including citric acid (CA) and nitric acid (NA) are used to modify SAPO-11. The results show that the acid concentration was within the range of 0.05–1.0 mol L−1, SAPO-11 maintained a complete crystal framework structure, and more (002) crystal planes were exposed. The specific surface area, pore diameter and acid content of molecular sieve by acid modification are significantly higher than those of the fresh SAPO-11. The pore size increased from 7 nm to 17 and 18 nm, an increase of the total acid from 104 μmol g−1 to 174 and 162 μmol g−1, respectively, when SAPO-11 was treated by CA with 0.5 mol L−1 and NA with 0.3 mol L−1. Pt–Sn/SAPO-11-CA-0.5 showed the best catalytic activity with selectivity of C15–C18 alkanes of 95.8% and iso-alkanes of 33.1%. Additionally, the deoxygenation ability of Pt–Sn/SAPO-11-CA-0.5 decreased 30% after running 1200 h. Using propionic acid (PA) as the model probe molecule, the DFT reveals steps of the hydrodecarbonylation pathway of PA absorbed on the Pt (111) plane as below: CH3CH2COOH*→CH3CH2CO*→CH3CHCO*→CH3CCO*→CH3C*+CO*, and the rate-limiting step is CH3CH2COOH*→CH3CH2CO*+OH*.