Study On The Preparation,Characterization And Application Of Catalysts For The Synthesis Of Polyoxymethylene Dimethyl Ethers

As a kind of excellent diesel-blending component,polyoxymethylene dimethyl ethers?PODE_n?have been received wide attention.As a typical acid catalytic reaction,the design and selection of catalysts for PODE_n synthesis is of great importance to the industrialization of PODE_n.In this thesis,the effects of acid strength,acid amount,acid type and the pore size for catalysts on PODE_n synthesis have been systematically studied,thus,the theoretical basis for the design and preparation of PODE_n synthesis catalysts is provided,and then an efficient PODE_n synthesis catalyst with high yield and high selectivity is prepared.Al-SBA-15 molecular sieves with Si/Al ratios from 2.5 to 200 are synthesized and used to investigate the effects of acid amounts and acid strength on the yield,selectivity and product distribution of the products.Through comparison of the catalysts with different Al contents,it is found that the relatively weak acid is more suitable for the synthesis of PODE_n than the relatively strong acid in the catalytic system of Al-SBA-15.On the Al-SBA-15-150catalyst,which has only a weak acid of 0.163 mmol g^-1,the highest trioxane?TOX?conversion rate and highest PODE_n yield and selectivity are achieved,showing the best catalytic performance.As a kind of acid catalyzed reaction,the depolymerization of TOX requires a relatively moderate acid amount,an insufficient or excessive amount of acid would lead to incomplete depolymerization.The trioxane dissociation rate and condensation reaction rate,which are determined by the acid strength and acid amounts,have a decisive effect on the selectivity of main products and byproducts and the distribution and chain length of PODE_n.When the acid property of catalyst decreases,the selectivity of PODE₂ decreases first and then increases,whereas contrarily the selectivity of PODE_3-8 increases first and then decreases.When the acid property of catalyst decreases,the maximum chain length of the product increases firstly and then decreases.The effect of pore size on the product distribution,chain length,yield and selectivity of the product is investigated.Microporous molecular sieve ZSM-5 with different Si/Al ratio is selected,and highly ordered Al-MCM-41 mesoporous molecular sieve and three kinds of Al-SBA-15 molecular sieves with different pore sizes are prepared.The pore sizes of these catalysts range from 0.54 nm to 7.9 nm.It is found that a relative larger pore size of the catalyst is beneficial for the PODE_n synthesis to a certain extent under the catalysts with strong acid and/or large acid amount.When the depolymerization rate of trioxane is far greater than its condensation rate with methylal,the aggregation of formaldehyde in acidic sites will result in the formation of a large amount of by-product methyl formate.In this case,a relatively large pore size is more favorable for the diffusion of formaldehyde monomers and relatively small pore sizes,thereby inhibiting the formation of by-products of methyl formate.However,the yield and selectivity of PODE_n are almost unchanged at a catalyst with only weak acid and a relatively lesser amount.At the optimum Si/Al ratio,five catalysts with different pore sizes show almost the same catalytic performance.The product distribution and the maximum chain length of the product under the catalysis of catalysts with different pore size are almost the same,indicating that catalysts with small pore sizes do not exhibit the shape selective catalysis for the PODE_n formation.The effect of acid type on the depolymerization of formaldehyde and the condensation reaction of DMM/PODE_n-1 with formaldehyde as well as the influence on the product distribution,yield and selectivity are systematically investigated,and the synergies effect between the two kinds of acid types are revealed.Phosphotungstic acid,p-toluenesulfonic acid,sulfuric acid and hydrochloric acid are selected as the representative of Br?nsted acid;Zinc chloride,ferric chloride and aluminum chloride are chosen as the representative of Lewis acid.According to the study,it is found that the synthesis of PODE_n can be catalyzed by both the single Br?nsted acid and single Lewis acid,however,the catalytic performance of single Br?nsted acid has better catalytic performance than single Lewis acid.Compared with the condensation reaction of DMM/PODE_n-1 with formaldehyde monomer,Lewis acid is more favorable for the depolymerization of TOX.Under the catalysis of Lewis acid,the depolymerization rate of TOX is greater than the condensation rate of DMM/PODE_n-1 with formaldehyde monomer,resulting in a large number of by-product methyl formate,which inhibits the production of the main product PODE_n.Under the catalysis of Br?nsted acid,the condensation reaction and the depolymerization reaction could reach a balance,making the PODE_n get higher yield and higher selectivity.The catalytic results of the mixed catalyst of Br?nsted acid and Lewis acid show that there is no obvious synergistic effect between the two acid types,the catalytic effect is slightly worse than that of individual Br?nsted acid,and better than that of individual Lewis acid.The catalytic reaction mechanism of Br?nsted acid and Lewis acid is proposed,which provide the theoretical basis for the difference of reaction result between Br?nsted acid and Lewis acid.The kinetics of synthesis reaction under the catalysis of different types of catalysts is studied.The results indicate that activation energy of PODE_n synthesis reaction catalyzed by Br?nsted is lower than that catalyzed by Lewis and mixed catalyst,which further confirm that single Br?nsted is the most conducive acid type for the synthesis of PODE_n.A series of different content of organic silane MPTMS modified Br?nsted acid catalysts SO₃H-SBA-15 are prepared,and the influence of different sulfoacid content on the yield and selectivity PODE_n have been studied,and the effect of the process conditions,such as reaction temperature,reaction pressure,feedstocks mole ratios are optimized.Through the comparison of the catalysts with different sulfoacid loading,it is found that under the catalysis of the SO₃H-SBA-15?0.8?catalyst,which has an acid amount of 0.166 mmol g^-1,the highest DMM and TOX conversion and PODE_n yield and selectivity are achieved and the desired chain length of PODE_2–8 is obtained.By the comparison of different reaction temperatures,it is demonstrated that temperature plays an important role in the TOX depolymerization rate and selectivity of the main products and by-products.Through the optimization of reaction pressure,it is found that the initial pressure had little influence on the product distribution and yield,and it is not the main influencing factor;By adjusting the molar ratio of DMM and TOX,it is observed that an increasing amount of TOX is beneficial for promoting the reaction to generate the products with a higher degree of polymerization.Under the optimum reaction conditions and the catalysis of SO₃H-SBA-15?0.8?,excellent reaction results are obtained,the yield of PODE_2-8 and the conversion rate of DMM and TOX are as high as 61.86%,51.84%and 95.57%,respectively.