Experimental And Theoretical Studies On The Trioxane Synthesis In Non-aqueous System

The development of the C1 chemical industry chain with methanol as the leading product is a major strategic demand for energy in China,but there is huge overcapacity in the methanol and its traditional downstream products industries.Therefore,methanol can be sequentially converted into formaldehyde,paraformaldehyde,and then produce a series of bulk chemicals and fine chemicals,polyoxymethylene engineering plastics and polymethoxy dimethyl ether.However,the production cost of trioxane is very high,which includes production energy consumption and equipment investment.This seriously restricts the development of the above-mentioned industries.The reasons for the high energy consumption are as follws.First,formaldehyde has undergone hydration and oligomerization reactions in the industrial reaction system（formaldehyde+H₂SO₄+H₂O）,which results in a very low conversion rate of formaldehyde.Second,the azeotrope is formed between the product trioxane and water,which makes it impossible to separate the trioxane by distillation technology.Third,the latent heat of vaporization of water is very high.One of the reasons for the high equipment investment is due to the increased investment in separation equipment because of the low selectivity of the catalyst,which generates a large amount of by-product formic acid.In this paper efforts will be made to develope a new reaction system to eliminate the hydration reaction and oligomerization reactions,to eliminate the azeotrope of trioxane and solvent and to reduce the latent heat of vaporization of the solvent.Efforts will also be made to develope a new catalytic system,of which the catalytic selectivity is significantly improved.The experimental data of vapor-liquid and liquid-liquid phase equilibria will be measured and the theoretical model of phase equilibrium will be established,which will lay a foundation for the establishment of a new technology for separation of trioxane.At the same time,a new technology that uses ionic liquids to store and release formaldehyde will be developed that will provide new technology reserves for extending the C1 chemical industry chain.The achievements of this paper mainly include the following four aspects.（1）In order to develop a new reaction system,the effects of reaction time,reaction temperature and solvent type（including sulfolane,dimethyl sulfoxide,1,3-dimethyl-2-imidazolidinone,1-chloronaphthalene and dimethyl sulfone）on the yield of trioxane and the amount of by-product formic acid produced in the system（organic solvent+paraformaldehyde+CH₃SO₃H）were studied,and the optimum organic solvent（sulfolane）and the suitable reaction time and temperature were determined.The catalytic performances of N-methylpyrrolidium ionic liquids and1-sulfopropyl-3-methylimidazolium ionic liquids in the trioxane synthesis using the system（sulfolane+paraformaldehyde+ionic liquid catalyst）were studied.It was found that the catalytic performances of the ionic liquids of the same cation considered follow the order[MSA]^-≈[p–TSA]^->[p–Cl BSA]^->[HSO₄]^->[BSA]^->[H₂PO₄]^->[TFO]^-.At the same time,the catalytic performances of the two kinds of ionic liquids having the same anion are similar.The catalytic performance of Zn Cl₂ was better than that of MgCl₂·6H₂O in the trioxane synthesis using the sysytem（sulfolane+paraformaldehyde+lewis acids/Br（?）nsted acids）,whihe the catalytic performance of Br（?）nsted acids followed the order CH₃SO₃H>C₁₀H₁₇OSO₃H>p-CH₃C₆H₄SO₃H>C₆H₅SO₃H>H₃PO₄>CF₃SO₃H≈HCl O_4.Furthermore,the performances of the acid catalysts for the decomposition of trioxane in the system（sulfolane+paraformaldehyde+Lewis acid/Br（?）nsted acid）were studied.And it was found that the decomposition rate of trioxane and the formation rate of formic acid followed the following order:MgCl₂·6H₂O>Zn Cl₂ and CF₃SO₃H>HCl O₄>C₆H₅SO₃H>p-CH₃C₆H₄SO₃H>CH₃SO₃H>C₁₀H₁₇OSO₃H>H₃PO₄.The catalytic performance of C₁₀H₁₇OSO₃Na,CH₃SO₃Na,C₃SMIM（1-sulfopropyl-3-methylimidazole）and C₃SNHP（1-sulfopropyl-2-pyrrolidone）in the trioxane synthesis in the system[sulfolane+paraformaldehyde+(CH₃SO₃H/C₁₀H₁₇OSO₃H+salt)]were studied.And the catalytic performances of the salts followed the order CH₃SO₃Na>C₃SMIM>C₃SNHP>C₁₀H₁₇OSO₃Na.It was also found that the compound ratio of acid and salt has a significant effect on the catalytic performance,and the higher the acid concentration,the more remarkable the cocatalysis effect of salt is.Moreover,the cocatalysis effect of the salt in the trioxane decomposition in the system[sulfolane+paraformaldehyde+(CH₃SO₃H/C₁₀H₁₇OSO₃H+salt)]was studied.The results showed that the decomposition rate of trioxane and the formation rate of formic acid followed the order C₁₀H₁₇OSO₃Na>C₃SNHP>C₃SMIM>CH₃SO₃Na.On the basis of the above results,new reaction systems[sulfolane+paraformaldehyde+（CH₃SO₃H+CH₃SO₃Na）]and[sulfolane+paraformaldehyde+[HNMP][MSA]（N-methylpyrrolidone methylsulfonate）]and a new catalytic system（CH₃SO₃H+CH₃SO₃Na）were developed.Compared with industrial reaction systems（formaldehyde+H₂SO₄+H₂O）,the new reaction system eliminates the hydration and oligomerization reactions of formaldehyde and the azeotrope formed between the solvent and trioxane and,therefore,the yield of trioxane is considerably improved and the amount of formic acid produced is greatly reduced.The catalytic activity and selectivity of the new catalytic system（CH₃SO₃H+CH₃SO₃Na）are significantly greater than those of the literature catalysts.The chemical reaction and the rate control step for the formation of by-product formic acid during the trioxane synthesis in the new reaction system[sulfolane+paraformaldehyde+（catalyst+salt promoter）]were determined.Moreover,the chemical reaction which determines the yield of trioxane and the formation rate of formic acid was also determined.The factor that determines the activity and selectivity of the catalyst and the salt-cocatalyst was uncovered,and such factor is the appropriate acid strength.The catalytic mechanisms were established.（2）In order to develop an extractive distillation technology for separationof trioxane from its mixtures with sulfolane,the experimental data for the vapor-liquid equilibrium of the（sulfolane+trioxane+extractive distillation agent）system were measured and the thermodynamic consistency test was made.The extractive distillation agents considered include benzene and toluene.And then the measured data were used to determine the binary interaction parameters of the NRTL,Wilson and UNIQUAC models and the group interaction parameters of the UNIFAC model.The effects of benzene and toluene on the vapor-liquid phase equilibrium were compared.The results showed that both benzene and toluene can increase the relative volatility of trioxane and sulfolane and that benzene has a stronger ability to increase that relative volatility than toluene.（3）In order to develop an extraction technology for separation of trioxane from its mixtures with sulfolane,the experimental data for the liquid-liquid equilibrium of the（sulfolane+trioxane+extractant）systems were measured.The Othmer-Tobias equation and the Hand formula as well as the NRTL model were used to correlate the measured data for the ternary systems（sulfolane+trioxane+cyclohexane）,（sulfolane+trioxane+n-hexane）and（sulfolane+trioxane+cyclopentane）.And the UNIFAC model was used to predict the liquid-liquid equilibrium data for these ternary systems.The results indicated that the above systems conform to the Othmer-Tobias correlation and Hand formula and that the description of the NRTL model for the liquid-liquid equilibrium data of these systems achieved good results and the UNIFAC model also predicted the LLE of the system very well.Comparisons of the selectivity coefficient and partition coefficient of the above systems were made,and the results showed that the extraction efficiency of cyclopentane is greatest among the three extractants.（4）In order to develop a new technology for storage of anhydrous formaldehyde,systematic studies were made to determine the capacity of 1-methylimidazole hydrogen sulfate[MIM][HSO₄]and imidazole hydrogen sulfate[IM][HSO₄]for storage of formaldehyde.The influence of the molecular weight distribution of paraformaldehyde,the reaction time and the reaction temperature on the capacity of[MIM][HSO₄]and[IM][HSO₄]for storage of formaldehyde were determined.Their maximum capacity for storage of formaldehyde were also determined.The results are 31.07 and 34.60（g formaldehyde）·（mol ionic liquid）^-1,indicating that[IM][HSO₄]has a greater storage capacity than[MIM][HSO₄].The structure of the reaction product HCHO@[MIM][HSO₄]was determined by NMR spectroscopic measurements.In order to study the renewability of formaldehyde,the rates for the decomposition of the reaction products HCHO@[MIM][HSO₄]and HCHO@[IM][HSO₄]as a function of decomposition time and decomposition temperature were measured.The results showed that both the proportion of the reation produc that has been decomposed and the maximum decomposition rate gradually increased with increasing temperature.Formaldehyde stored in HCHO@[MIM][HSO₄]and HCHO@[IM][HSO₄]can be completely released and regenerated.Furthermore,the kinetics equations for the formation and decomposition of the products HCHO@[MIM][HSO₄]and HCHO@[IM][HSO₄]were developed.The activation energy E,pre-finger factor A and decomposition rate constant of the decomposition reaction,and the rate constant of the reaction between paraformaldehyde and ionic liquids were determined.Comprehensive analyses were made.The results showed that[MIM][HSO₄]and[IM][HSO₄]can efficiently store formaldehyde,and[IM][HSO₄]has a larger molar storage capacity than[MIM][HSO₄].However,the activation energy for decomposition of HCHO@[IM][HSO₄]is greater than that for decomposition of HCHO@[MIM][HSO₄],and the decomposition rate of HCHO@[IM][HSO₄]is smaller that that of HCHO@[MIM][HSO₄].These results provide a new method for storing and releasing anhydrous formaldehyde.