Preparation Of Modified MCM-22 Zeolites And Its Performance In Synthesis Of Polyoxymethylene Dimethyl Ethers

Polyoxymethylene dimethyl ethers is a general designation for a series of substances,and the structural formula is CH₃O?CH₂O?_nCH₃?n?1?.Due to its high oxygen content?45% to 49%?and cetane number?>63?,PODE_n is considered as an excellent green diesel additive,and it can effectively reduce NOx and soot generated during combustion.At present,most raws of the synthesis of polymethoxy dimethyl ether are methylal and paraformaldehyde.Catalysts are mainly ionic liquids,but they are more expensive others and are not easy to separate with products.In this paper,methanol and paraformaldehyde are used as raw materials to prepare catalysts with good catalystic performances to improve the conversion of raw materials and the selectivity of PODE_n.In this paper,anion-modified HMCM-22?X? series catalysts were prepared,and the effects of catalyst structure and acid properties on the synthesis reaction were investigated.A series of super acid and sulfate supported catalysts were prepared by using HMCM-22 as carrier.The preparation process of the catalyst was investigated.The effect of catalyst acid properties on the synthesis reaction was investigated.The reaction conditions using methanol and paraformaldehyde as raw materials was optimized.The HMCM-22?X? catalyst was prepared by introduceing additional anions(SO₄^2-,NO₃^-and ClO₄^-)into the gels to change the position of Al by dynamic hydrothermal synthesis method,and they were evaluated for catalystic performances in an autoclave.The structure of the catalyst was characterized by XRD,FT-IR,SEM,BET and ²⁷Al-NMR.There were little influences on the structure of the catalyst by anions,but the morphology,specific surface area and pores of the catalyst were changed.At the same time,more Al was exposed on the outer surface and the super cage.The acid properties of the catalyst were investigated by NH₃-TPD,and the results showd there are different effects in acidity and acid strength of the catalyst by different anions.The relationship between the synthesis reaction and the acid center position and the acid properties of the catalyst were investigated by combining the acidic position and acid properties with the distribution of the reaction products.It was found that the conversion of the raw materials and the generation of DMM and by-product MF are not affected by the acidic position and the medium strong acid on the outer surface of the catalyst was benefical for the chain extension reaction.Three kinds of super acid molecular sieves and eight kinds of sulfate supported catalysts were prepared by impregnation method with HMCM-22 molecular sieve as carrier.The structure and acid properties of the catalyst were analyzed by correlation characterization methods and the catalytic performances was examined under the same conditions.It is found that the super acid molecular sieves which there were excessive amount of strong acid resulte in the formation of a large amount of by-product methyl formate in the reaction,and the selectivity of the target product was decreased.The supported sulfate catalysts reduce the acid strength and increases the amount of the weak acid,the conversion of methanol and the selectivity of the target product were effectively increased.Finally,NiSO₄ was selected as the optimal load by screening.The preparation conditions of NiSO₄/HMCM-22 catalyst is investigated,and the optimal preparation conditions were obtained:the concentration of the immersion liquid was 1 mol/L,and the calcination temperature was 400?.In this condition,the catalyst showd the best catalystic performances,the conversion of methanol and the selectivity PODE_2?8 and PODE_3?8was 69.1%,62.2% and 30.1%,respectively.NiSO₄/HMCM-22 prepared under optimal conditions was used as catalyst to optimize the reaction conditions of the synthesis of PODE_n.The optimal reaction conditions were as follows:the catalyst dosage was 1%?wt.?,n_MeOH:n_PF=1.5:1,and the reaction temperature was 140?,The reaction time was 4 h.In this condition,the conversion of methanol and the selectivity of PODE_2?8 and PODE_3?8 were 79.1%,72.0% and 46.5%,respectively.