Synthesis of Fe-substituted Pt/H-ZSM-12 catalysts and their performances in n-dodecane hydroisomerization
The unique pore structure of H-ZSM-12 zeolite is conducive to the formation of multi-branched isomers in long chain alkane hydroisomerization,but its strong acidity exacerbates the occurrence of cracking reactions.Substitution of Fe for Al in the framework of zeolites could effectively reduce the acid strength.In order to improve the selectivity of i-alkanes,Z12-Al,Z12-Al-Fe and Z12-Fe zeolites with Fe-substitution degrees(calculated as n(Fe):n(Al + Fe))of 0%,50%and 100%,synthesized by hydrothermal method.The characterization results of ultraviolet-visible absorption spectroscopy(UV-Vis)indicate that the majority of Fe introduced in Z12-Al-Fe and Z12-Fe exist in the molecular sieve framework in the form of[FeO4].The characterization results of infrared absorption spectroscopy(FT-IR)and X-ray photoelectron spectroscopy(XPS)indicate that Fe in the H-Z12-Al-Fe and H-Z12-Fe frameworks combines with Si—OH to form Si—OH—Fe bonds.The temperature programmed desorption of ammonia(NH3-TPD)and pyridine infrared(Py-IR)characterization results indicate that with the increase of Fe content in the zeolite,the acid strength of its strong acid site gradually decreases,and the total amount of Brønsted acid significantly decreases.After loading sufficient metal Pt on the zeolite support,the hydroisomerization performance of the bifunctional catalyst was evaluated using n-dodecane(n-C12)as a model compound(reaction pressure is 2.0 MPa,n(H2):n(n-C12)is 6.0).The reaction results show that the decrease in Brønsted acid strength and acid sites quantity of Pt/H-ZSM-12-Fe results in lower catalytic activity than Pt/H-ZSM-12-Al.However,the inhibition of weak Brønsted acids on cracking reaction results in the highest total isomers selectivity(85.7%)and multi-branched isomers selectivity(53.8%)at a n-C12 conversion rate of about 90%,which are 18.6%and 9.7%higher than Pt/H-Z12-Al,respectively.In addition,Pt/H-Z12-Fe exhibited excellent stability in a continuous reaction of 120 h.
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