Catalytic Production Of P-xylene From 2,5-Dimethylfuran/2,5-hexanedione And Ethylene Over Solid Acid

Para-xylene(PX)is one of the most important bulk chemicals,mainly used as a raw material for the production of polyester.Since entering the 21st century,bio-based PX has attracted more attention and research due to the attractiveness of its renewable raw materials.The production of bio-based PX from 2,5dimethylfuran(DMF)and ethylene has the potential and application prospects to replace petrochemical PX.Under the catalysis of solid acid,DMF and ethylene undergo Diels-Alder cycloaddition reaction followed by dehydration to produce PX.This research starts from the study of the requirements for the pore structure and acid property of the catalyst,and constructs highly selective solid acid catalysts.Then we further study the hydrolysis and condensation sidereactions,and find out the mechanism of the acid strength during 2,5hexanedione(HDO,a side-product of DMF hydrolysis)conversion.And the study on the highly selective production of PX from HDO and ethylene by solid acid has been carried out,and a new two-step process for the preparation of PX from carbohydrate biomass via HDO is constructed.Firstly,the influence of the pore structure and acid properties of catalyst on the selectivity for PX from DMF and ethylene was explored.Using SiO2 with different structures as carriers and sulfonic acid group as the active sites,it is found out that the active sites directly expose on the outer surface of the catalyst have much higher activity than those in the tenuous pores,that is to say,the external active sites can weaken the inhibition of internal diffusion.The activity of mesoporous catalysts is better than microporous catalysts.WO3/SBA-15 catalyst is used to explore the effect of different acid properties on PX selectivity.By adjusting the calcination temperature,the ratio of L acid(Lewis acid)to B acid(Br?nsted acid)of the catalyst is adjustable from 5.5 to 13,4.Catalyst with a higher ratio of L acid to B acid achieves a higher PX selectivity.It indicates that L acid has weaker catalytic ability for hydrolysis and HDO condensation raction and L acid sites are easier to achieve higher PX selectivity than B acid sites.Compared with the super acid WO3/ZrO2,it is found out that the weaker acid WO3/SBA-15 has weaker ability catalyzing hydrolysis and HDO condensation reactions.Higher PX selectivity of 74.5%is achieved on WO3/SBA-15 than 36.9%on WO3/ZrO2,indicating that the catalysts with high PX selectivity should not have strong acid sites.Based on the features of the above highly selective catalysts,mesoporous solid acid tin phosphate(SnPO)is constructed as a highly effective catalyst with application prospects for PX production from DMF and ethylene.With the increase in the ratio of P/Sn element content from 1.00 to 2.00 during the synthesis process,the structures of SnPO catalysts change from the accumulation of nanoparticles to a layered structure.The P-OH content on the surface of the catalyst gradually decreases,and the acid properties of catalysts change from B acid to L acid.The active sites change from P-OH to tetrahedrally coordinated Sn(Ⅳ).SnPO-1.50 and SnPO-1.75(mainly in L acid property,tetrahedrally coordinated Sn(Ⅳ)as active sites)exhibite high PX selectivity of 90.4%and 93.3%,respectively,20%higher than SnPO-1.00 and SnPO-1.25(mainly in B acid property,P-OH as active sites).And they also exhibit higher PX productivity of 155 molpx·molacid sites-1·h-1 and 173 molPX·molacid sites-1·h-1,significantly higher than SnPO-1.00(117 molPX·molacid sites-1·h-1)and SnPO-1.25(121 molPX·molacid sites-1·h-1).It is concluded that in SnPO catalysts the tetrahedrally coordinated Sn(Ⅳ)active sites is more effective than P-OH active site for PX production.DFT calculations reveale that tetrahedrally coordinated Sn(Ⅳ)is more effective than P-OH in catalyzing the Diels-Alder reaction between DMF and ethylene and the cleavage of the C-O bond in the initial step of dehydration of the cycloaddition intermediate.The catalytic activity of the SnPO catalysts are compared with the zeolite catalysts H-Beta,Sn-Beta and P-Beta.PX productivity of SnPO-1.50 and SnPO-1.75 are 1.5 times than that of H-Beta(108 molPX·molacid sites-1·h-1).In the meanwhile,they have a lower activity to the side reaction of hydrolysis and sequent condensation reaction.And SnPO catalysts have less carbon deposition.Cyclic experiments show that the SnPO catalyst has good regeneration performance and has the prospect and potential of being applied to the production of renewable PX.It is found out that the acid strength of SnPO,AlPO,TiPO,and ZrPO gradually increases in the further research on phosphate catalysts.The varied acid strength of phosphate catalysts provides a good platform for studying the mechanism of acid strength in the hydrolysis and condensation side-reactions and the relationship between acid strength and PX selectivity.Through the study of the reaction conversion path,isotope labeling has discovered the reversible reaction between DMF and HDO that occurs simultaneously under the reaction conditions.It is found out that the hydrolysis of DMF to HDO is not the intrinsic cause of the decrease in PX selectivity.Moreover,it is found out that the production rate of HDO condensation products has a strong positive correlation with the concentration of strong acid sites and the R2 value reachs 0.998.It indicates that the strong acid sites catalyze the condensation of HDO and destroy the balance between the reversible reaction of DMF and HDO,resulting in irreversible carbon loss and being the intrinsic cause of the decrease in PX selectivity.Under the catalysis of phosphates with different acid strengths,HDO used as a raw material instead of DMF reacts with ethylene to prepare PX.As the acid strength of SnPO to ZrPO increases,the PX selectivity gradually decreases,again verifying the important influence of acid strength on PX selectivity.It provides a basis and guidance for the design of highly selective catalysts to avoid strong acids.Based on the discovered principle that HDO can replace DMF to react with ethylene to prepare PX,a new path for the production of PX through two steps from carbohydrate biomass via HDO is proposed for the first time:the first step is the hydrodeoxygenation of sugar in an acidic system to obtain HDO;the second step is HDO reacts with ethylene to prepare PX.In this thesis,the process of preparing HDO from sugar in the CH2Cl2/concentrated HCl twophase system is preliminarily explored.Under Pd/C catalysis,the yield of HDO reachs 41.4%when cellulose is used as raw material.The reaction conditions and various solid acid catalysts for the conversion of HDO and ethylene to PX are emphatically studied.P-Beta and SnPO-1.75 exhibit PX selectivity of 86.7%and 89.2%,respectively,in PX production from HDO and ethylene.Moreover,it is found out that HDO can be rapidly converted to DMF,and the rate-limiting step is the reaction of DMF and ethylene to prepare PX.Finally,a new two-step process for preparing PX from carbohydrate biomass(such as cellulose)via HDO is established.