Modification Of SAPO-11 Molecular Sieves For Catalytic Alkylation Of 2-Methylnaphthalene

2,6-Dimethylnaphthalene（2,6-DMN） is an important intermediate for the synthesis of new high-end polyester of polyethylene naphthalate（PEN）,but the industrial production process of 2,6-DMN is cumbersome,resulting in higher production costs.In recent years,the one-step synthesis of 2,6-DMN from 2-methylnaphthalene（2-MN） and methanol by molecular sieves has many advantages such as short process route,cheap raw materials and high selectivity of target products,which has attracted the interest of many scholars.In this paper,SAPO-11 molecular sieves were modified by different methods to obtain catalysts with high 2,6-DMN selectivity,high 2,6-/2,7-DMN ratio and good stability.By means of XRD,SEM,N₂adsorption-desorption,FT-IR,ICP,NH₃-TPD and TG,the effects of the structure and acidity of modified SAPO-11 molecular sieves were investigated,and the catalytic performance of modified SAPO-11 molecular sieves for alkylation of 2-MN and methanol were investigated in a fixed bed reactor.（1）The hydrothermally synthesized SAPO-11 molecular sieves were used as a carrier,which were modified with Ni（NO₃）₂·6H₂O,Co（NO₃）₂·6H₂O,tetraethylorthosilicate（TEOS）and Mg（NO₃）₂·6H₂O,respectively.In the molecular sieves modified by Ni（NO₃）₂·6H₂O and Co（NO₃）₂·6H₂O,the metal oxides are not only deposited on the outer surface of molecular sieves,but also deposited in the channels;TEOS is mainly deposited on the outer surface of the molecular sieves,which has a little effect on the channels.The total pore volume of SAPO-11 molecular sieves increased after Mg²⁺ions exchange.The acid content of SAPO-11 molecular sieves revised by the four modifiers was reduced,and the acid strength was weakened to a certain extent.The modified molecular sieves were applied in the alkylation of 2-MN reaction,and the catalytic performance of the modified SAPO-11 molecular sieves was improved.After 7 h of reaction under the proper reaction conditions,the conversion of2-MN was 10.4%,the selectivity of 2,6-DMN was 46.4%and the ratio of 2,6-/2,7-DMN was 2.04 over 2% Ni/SAPO-11 molecular sieves;However,a 2-MN conversion of 8.7%,a 2,6-DMN selectivity of 36.0%and a 2,6-/2,7-DMN ratio of 1.72 were obtained over SAPO-11 molecular sieves.This may be related to the reduction of acid sites of the modified molecular sieves,which inhibits the isomerization of raw materials,inter-product isomerization and deep alkylation,reduces the formation of carbon deposits,and improves the selectivity of 2,6-DMN.（2）The SAPO-11 molecular sieves were dealuminated with citric acid and oxalic acid,respectively,and the optimized SAPO-11 molecular sieves were characterized and analyzed.The results show that the effects of two kinds of organic acid dealumination modification on the chemical composition,pore structure,acidity and catalytic performance of SAPO-11 molecular sieve are very different.The modification by citric acid can effectively remove the non-framework aluminum and framework aluminum of the molecular sieves without destroying the original crystal phase structure,morphology and microporous structure of the molecular sieves,which improves the alkylation performance of the catalyst;However,the morphology and microporous structure of SAPO-11 molecular sieves were destroyed after oxalic acid modification,and the catalytic performance was also poor,which may be related to the stronger acidity of oxalic acid and the deeper degree of dealumination.The molecular sieves treated with 0.5 M citric acid showed good catalytic performance.After 6 h of reaction,the conversion of 2-MN was 15.5%,the selectivity of 2,6-DMN could reach 41.7%,and the ratio of 2,6-/2,7-DMN could reach 1.88.