Synthesis And Modification Of Low Silicon-Aluminum Ratio ZSM-48 Molecular Sieve And Hydroisomerization Performance Of N-dodecane

The rapid economic and social development has caused a large amount of energy consumption,and the energy crisis has become increasingly prominent.In this background,increasing the effective use of energy and reducing energy consumption have become the consistent goals of today's global society.Coupled with the “green revolution” currently underway around the world,it poses a challenge to the clean use of energy.At the same time,the environmental negative impact of various types of oil used in automobiles cannot be underestimated.To reduce pressure of resources and environment,needing to improve the production process of existing automotive oil products.The catalytic reaction of n-alkane hydroisomerization converts n-alkane to i-alkane with the same carbon number and branch chain,which improves the low-temperature fluidity of lubricating base oil.The n-paraffin hydroisomerization catalytic reaction technology also has the advantages of greenness and environmental protection,which is the main method of producing high-quality lubricants at present.Moreover,the core of this technology is the preparation of n-alkane hydroisomerization catalytic reaction catalyst with high catalytic activity and good selectivity of isomer products.ZSM-48 is a kind of one-dimensional,ten-membered ring,non-interpenetrating molecular sieve.Due to its unique channel structure's “shape selection effect”,it can effectively improve the selectivity of isomer products.At the same time,the appropriate acid amount of ZSM-48 molecular sieve can also effectively improve the yield of isomer products.However,as a high-silicon molecular sieve,its low acid content and weak acidity have limited its wide application.Therefore,based on previous work,this paper synthesized a series of ZSM-48 molecular sieve with lower silicon-aluminum ratio.The bifunctional catalyst was prepared by incipient wetness impregnation metal Pt or Ni.Select n-dodecane as the model reactant,the effects of lower silicon-aluminum ratio ZSM-48 molecular sieve on the catalytic activity and product distribution of n-dodecane hydroisomerization catalytic reaction were investigated.The specific research content is as follows:(1)Preparation of ZSM-48 molecular sieves with different silicon-aluminum ratios and study its catalytic performance on hydroisomerizationThe static hydrothermal synthesis method was used to synthesize ZSM-48 molecular sieve with a series feeding silicon-aluminum ratio of 50,70,90,200.ZSM-48 molecular sieve with an feeding silicon-aluminum ratio of 50 has an actual silicon-aluminum ratio of 38.6 characteried by ICP-AES.The 0.5 wt.%Pt/HZSM-48-70 catalyst with a feeding silicon-aluminum ratio of 70 has reached the best balance of acid site and metal site and has the best catalytic performance.The single branch isomer main products and the multil branch isomer main products are 5-Methylundecane and 2,6-Dimethyldecane,respectively,which are both located in the middle positions of carbon chain,which conform to the “Key-Lock” shape-selecty catalytic mechanism.(2)Alkali treatment of ZSM-48 molecular sieve and study its hydroisomerization catalytic performanceThe ZSM-48 molecular sieve with silicon-aluminum ratio of 50 was treated with Na OH alkali solution with a concentration of 0.1 M,0.3 M,and 0.5 M respectively,to obtain hierarchical molecular sieve carrier.Hierarchical molecular sieve decreased diffusion resistance of product,besides it's beneficial to reduce cracking reaction so the selectivity of iso-dodecane was improved.(3)Synthesis Fe-ZSM-48 molecular sieve and study its hydroisomerization catalytic performanceSince the acidity of Si-OH-Fe bridge hydroxyl groups is lower than that of Si-OH-Al bridge hydroxyl groups,the method of doping heteroatoms Fe into the ZSM-48 molecular sieve framework partially replaces the Al atoms in the framework to adjust the acidity of the molecular sieve carrier.The UV-vis characterization showed that Fe atoms entered the molecular sieve framework and existed in the tetracoordinated form in the ZSM-48 molecular sieve framework.Due to the reduction in acid content and the weakening of acidity,the selectivity of the isomerized product of Fe-ZSM-48 catalyst remains above 74.00% at a reaction temperature of 360 °C.(4)Preparation of Ni/HZSM-48 bifunctional catalyst and study its hydroisomerization catalytic performance2 wt.%Ni,4 wt.%Ni,6 wt.%Ni,8 wt.%Ni,and 10 wt.%Ni were loaded on ZSM-48 molecular sieve which treated with 0.2 M Na OH alkaline solution using incipient wetness impregnation method to prepare Ni-based bifunctional catalyst.When the Ni loading of 8 wt.% is catalyted at 320 °C,conversion is 73.16%,the selectivity of isomer products is 91.59%,and the yield of isomer products is 67.00%.Its catalytic performance is better than catalyst of ZSM-48 molecular sieve loaded with 0.5 wt.%Pt.