Shape-Selective Hydroisomerization Of Long-Chain N-Alkanes Over Multi-Functional Catalysts

Hydroisomerization of long-chain n-alkanes plays an important role in the process of producing lubricating base oil with high qualities, which is usually carried out over metal-acid bifunctional catalysts with shape-selectivity. In this work, we investigated the catalytic properties of Pt/SAPO-11 and Pt/MeAPO-11 catalysts with different acidities, and elucidated the important role of the balance between the acid function and the (de)hydrogenation function in the hydroisomerization of long-chain n-alkanes.The H2O-alcohol biphase system can effectively improve the Si content, the amount of the acid sites, and thus the activity of SAPO-11 for isomerization. However, due to the acidity of SAPO-11 is too high to balance with the (de)hydrogenation function of the supported Pt, the isomerization selectivity of Pt/SAPO-11 catalyst was decreased.Different metal substituted AlPO-11 molecular sieves were characterized by various techniques, and their catalytic performance was tested in the hydroisomerization of n-dodecane. It was found that the activity of Pt/MeAPO-11 catalysts was primarily determined by the acid strength of MeAPO-11 molecular sieves, and decreased in the order of Pt/MgAPO-11 > Pt/CoAPO-11 > Pt/ZnAPO-11 > Pt/MnAPO-11. Nevertheless, the isomerization selectivity of Pt/MnAPO-11 and Pt/CoAPO-11 was remarkably decreased, because the strong interaction of Mn and Co species with Pt significantly weakened the metallicproperties of Pt.The influence of the synthesis conditions on the crystalline phase, the Mg content, the acidity and the catalytic performance of MgAPO-11 molecular sieves for hydroisomerization of n-dodecane was systematically studied. It was found that the acidity of MgAPO-11 molecular sieves was increased with the increase of the Mg content, whereas the metallic properties of Pt were weakened by the interaction between the Pt and the strong acid sites of Me-OH-P generated by the substitution of Me2+ ions into the framework. Once the Mg content exceeded a certain value, the (de)hydrogenation function of Pt was incapable to balance with the acidity of MgAPO-11 molecular sieves. So the activity of Pt/MgAPO-11 catalysts was increased until the molar ratio of MgO/Al2O3 in the gels was 0.03, and then decreased with the further increase of the Mg content.The above results indicate that in the hydroisomerization of long-chain n-alkanes both the metallic properties and the acidity have effect on the activity and selectivity of the catalysts. Only when the metallic function and the acid function are in proper balance, the catalytic performance of the catalysts is optimum.