Preparation Of Me/SAPO-31 Bifunctional Catalyst And Its Catalytic Reaction Performance

The design,preparation and performance study of n-alkane hydrogenation isomerization reaction for efficient bifunctional catalysts have important research value and application prospect on developing production technology of clean fuel oil which is composed of iso-paraffin.In this study,based on the traditional hydrothermal synthesis microporous SAPO-31 molecular sieves with different morphology and acidity were synthesized by one-step or two-step crystallization with DBA as the template.Additionally,in the ethanediol-water system,the SAPO-31 samples with small particle size（S31-yEG）was synthesized by changing the ratio of ethanediol/water,so that the diffusion properties of the isomer products were improved due to the abundant intergranular mesopores.And Pd/SAPO-31 bifunctional catalysts with 0.5wt.%Pd loading were prepared by incipient wetness impregnation method.The characteristics of SAPO-31 samples and Pd/SAPO-31 bifunctional catalysts,including topology,composition,morphology,pore structure,acidity and metal properties were characterized by XRD,XRF,SEM,N₂ physical adsorption,Py-IR and H₂ chemisorption measurements.In addition,the catalytic performance of bifunctional catalysts was tested by using the hydroisomerization of n-hexadecane as the probe reaction.The test results indicated that the 0.5Pd/SAPO-31 catalyst synthesized by one-step crystallization showed the highest catalytic activity of n-hexadecane and selectivity to iso-hexadecane.SAPO-31 acicular aggregates（S31-yNH₄F）or nanosheets（S31-xHF）were also synthesized by adding different amounts of HF or NH₄F to the initial gel,and the morphology and acidity of SAPO-31 molecular sieves were modified as a result.The bifunctional catalyst with 0.1 wt.%Pd loading were prepared with SAPO-31molecular sieve nanosheets as the acid supports（S31-xHF）.Among all 0.1Pd/S31-x HF catalysts,the 0.1Pd/S31-0.2HF catalyst exhibited the highest iso-hexadecane selectivity of 93%at an n-hexadecane conversion of 80.4%.Compared with the traditional bifunctional catalysts with microporous SAPO-31supports,the higher selectivity to multy-branched isomers was obtained over the0.5Pd/S31-25EG catalyst.By using polyhexamethylene biguanide（PHMB）as the mesopore template,the hierarchical pore SAPO-31 samples with micro-mesopore structure were synthesized.The influence of the amount of PHMB to the structure,acidity and catalytic performance of corresponding bifunctional catalysts were investigated.The results demonstrated that the introduction of mesopore structure enhanced the dispersion of Pd in Pd/SAPO-31 catalysts,promoted the generation and diffusion of the branched isomers,and also improved the selectivity to iso-hexadecane.The highest iso-hexadecane selectivity of 94.5%was obtained over the0.5Pd/S31-0.02PHMB catalyst at an n-hexadecane conversion of 89.1%.What’s more,the selectivity to bibranched isomers over the 0.5Pd/S31-0.02PHMB catalyst（41.0%）was almost two folds compared with the conventional bifunctional catalysts with microporous SAPO-31 supports（23.5%）.The bifunctional catalysts with different distance between metal and acid sites were also prepared by changing the methods of Pd loading,and the effects of calcination parameters were discussed.By using Ni₂P as the metal site,Ni₂P/S31bifunctional catalysts were prepared through the temperature programmed reduction method,and the catalytic performance was then investigated.The results showed that,for bifunctional catalysts with only Pd metal sites,the catalysts calcined at 600℃in air had the most excellent catalytic performance for the isomerization of n-hexadecane,due tothe favorable distance between metal and acid sites.For bifunctional catalysts with Ni₂P metal sites,the 4Ni₂P/S31 catalyst with 4%Ni₂P loading showed the highest catalytic activity and isomerization selectivity.The highest iso-hexadecane yield of68.3%was obtained at an n-hexadecane conversion of 80.5%.In this paper,the physicochemical properties of Me/S31 bimetallic catalysts with Pd-Pt and Pd-Ni₂P bimetallic sites and the catalytic performance of n-hexadecane hydroisomerization were investigated.The results showed that 0.5Pd-Pt（1:1）/S31catalyst prepared by co-impregnation method has the suitable C_Me/C_H+ratio,so that the synergistic catalysis was achieved between two active sites.In addition,the0.05Pd-4Ni₂P/S31 bimetallic bifunctional catalyst prepared by loading 0.05 wt%Pd on the 4Ni₂P/S31 could significantly improve the catalytic performance,and the highest iso-hexadecane yield of 75.0%was obtained at an n-hexadecane conversion of 83.1%.