Structural Control Of SAPO-11 Zeolite And Its Hydroisomerization Performance

Diesel oil and lubricating oil contain long-chain n-alkanes which have high solidifying point and poor fluidity at low temperature,so it is necessary to improve the oil quality by the isomerism condensation process.The isomerization reaction of long-chain alkanes mainly take place on the orifice and external surface of the catalyst.Therefore,the carrier of long-chain alkane isomerization catalyst is required to have more orifices and higher external surface area to expose more active sites.Currently,SAPO-11 molecular sieve with suitable acidity are widely used in the isomerization reaction of alkanes.However,the application of SAPO-11 molecular sieve in catalytic reactions involving high-molecular-weight hydrocarbons is limited due to the microporous structure of SAPO-11 molecular sieve.To solve the above-mentioned problems,this paper controlled the structure of SAPO-11molecular sieve by reducing grain size,constructing hierarchical zeolites,synthesizing nano-sheet morphology,and designing and synthesizing Mordenite（MOR）and SAPO-11composite molecular sieve,so as to obtain hydrocarbon hydroisomerization catalysts with excellent properties.The specific research works were as follows:（1）To synthesize SAPO-11 molecular sieve with small grains by directing agent method.The effects of aging conditions and dosage of directing agent solutions,crystallization time of molecular sieve and addition amounts of template agents on the structure and properties of SAPO-11 molecular sieve with small grains synthesized by the directing agent method were studied.The synthesis mechanism of SAPO-11 molecular sieve with small grains was investigated by the UV-Raman spectrum characterization,and the hydroisomerization properties of Pt/SAPO-11 catalyst with small grains was evaluated with n-dodecane as the model compound.By adding directing agent solutions to the precursor of SAPO-11 molecular sieve,we can control the synthesis process to get SAPO-11 molecular sieve with small grains of 300-500 nm.In the synthesis process of SAPO-11 molecular sieve,the various of structural units of SAPO-11 molecular sieve contained in the directing agent solutions were self-assembled,and a large number of resulting crystal nucleus competed and grew simultaneously,causing the grain size of SAPO-11 molecular sieve reduced correspondingly.As compared with conventional SAPO-11 molecular sieve,SAPO-11 molecular sieve with small grains has higher external surface area and more medium-strength B-acid distribution,and its i-dodecane selectivity and yield also increase from 69.79%to 73.06%and from56.10%to 66.29%,respectively.（2）To construct hierarchical SAPO-11 molecular sieve by biomimetic method.We performed acid-alkali pre-treatment on natural sisal to obtain sisal cellulose,which was used as template to direct the self-assembly of grains to synthsize SAPO-11 molecular sieve with tubular hollow structure.The effect of the dosage of sisal cellulose on the construction of hierarchical SAPO-11 molecular sieve was studied.The hierarchical SAPO-11 molecular sieve synthesized by adding 4%sisal cellulose showed relatively high crystallinity.As compared with conventional microporous SAPO-11 molecular sieve,the BET specific surface area of hierarchical SAPO-11 molecular sieve is significantly improved,the number of mesoporous is increased,and the mesoporous volume is enhanced.In the hydroisomerization reaction of n-dodecane,the conversion rate of n-dodecane and the yield of i-dodecane of Pt/SAPO-11-J-4 catalyst are 90.73%and 67.48%,respectively.（3）To synthesize ordered mesoporous nano-sheet SAPO-11 molecular sieve by using uncalcined SBA-15 molecular sieve as silicon source.The effects of different silicon sources on the structure and properties of SAPO-11 molecular sieve were studied,and the growth mechanism of nano-sheet SAPO-11 molecular sieve was investigated.A electrostatic polarity region was formed between the template P123（triblock copolymer）contained in the uncalcined SBA-15 and the two-dimensional pipe of the SBA-15 molecular sieve.When Al O₂^-and PO₂⁺species diffused into the region between P123 micelle and SBA-15,the electrostatic polarity region promoted the preferential growth of SAPO-11 molecular sieve along the c axis.The nano-sheet SAPO-11 molecular sieve exposed more active sites,which improved the accessibility of the medium-strength B-acid active sites of the catalyst.In the isomerization reaction of long-chain alkanes,ordered mesoporous nano-sheet SAPO-11molecular sieve showed higher isomerization activity and selectivity.When the conversion rate of n-dodecane reached 87.96%,the yield of i-dodecane would be 68.08%,in particular the yield of multi-branched i-dodecane would be 31.65%.（4）To design and synthesize SAPO-11 and MOR composite molecular sieves by preset-seed MOR and using MOR as silicon source method,respectively.The effects of the crystallization conditions of composite molecular sieve,the addition amounts of seed and the Si/Al ratio on the synthesis of SAPO-11 and MOR composite molecular sieves by preset-seed method were studied and the crystallization process of the synthesis of composite molecular sieve by using MOR as silicon source method was investigated to explore the synthesis mechanism of the composite molecular sieve.The composite molecular sieve showed a core-shell structure with MOR as the nucleus and SAPO-11 as the shell.The chemical interaction between SAPO-11 and MOR made the composite molecular sieve present a suitable B-acid distribution.The composite molecular sieve showed better hydroisomerization properties than the pure zeolite and the mechanical mixture,and achieved higher multiple-branched i-dodecane yield.The yields of i-dodecane and multi-branched i-dodecane for Pt/MS-10 catalyst prepared by preset seed method were 69.48%and 33.42%,respectively,while the yields of i-dodecane and multi-branched i-dodecane for Pt/MOR@SAPO-11catalyst prepared by Si-Al source method were 70.27%and 33.65%,respectively.