| Due to the global watming and the uncertainty of non-renewable resources,more and more scholars have focused their attention on biomass conversion,hoping to efficiently convert biomass into fuel or other high value-added platform compounds.Biomass-based 2,5-furandicarboxylic acid(2,5-FDCA)is an important platform compound,which can polymerize with ethylene glycol to obtain poly(ethylene furandicarboxylate)(PEF).PEF is an excellent polyester which can be used in food packaging,film and other industries.2,5-FDCA is mainly derived from the oxidation of 5-hydroxymethylfurfural(5-HMF).5-HMF has high reactivity,instability,difficulty in separation,and high cost,it is not suitable as a raw material for large-scale industrial production.2,5-FDCA can also be prepared by cyclodehydration of hexaric acid,which is very stable and easy to separate.Hexaric acid includes galactaric acid,glucaric acid,and mannaric acid.Now,there are problems such as low yield,inaccurate qualification of by-products,and lack of kinetic data for the preparation of 2,5-FDCA by cyclodehydration of hexaric acid.In view of these problems,we selected galactaric acid,and calcium saccharate as raw materials,a systematic study has been carried out on the preparation of 2,5-FDCA by cyclodehydration of hexaric acid.The specific research contents are as follows:First of all,in this paper,galactaric acid was used as a raw material,and the catalyst and solvent were screened on the basis of qualification of by-products.The results showed that sulfone is the best solvent and sulfuric acid is the best catalyst.Then,the process was optimized on the basis of catalyst and solvent screening,and the relatively good condition were obtained when galactaric acid was used as the raw material:sulfone was used as the solvent,and the reaction was performed at 130? for 16 h under 15 wt%sulfuric acid,the yield of 2,5-FDCA was 49.1%.Because the preparation of 2,5-FDCA by hexaric acid is a dehydration reaction,the generated water is likely to inhibit the forward reaction or cause some other degradation reactions,the effect of water content on the reaction was investigated.The results showed that with the increase of water content,the conversion rate of galactaric acid and the yield of 2,5-FDCA both showed a downward trend,and the yield of furoic acid showed an upward trend,the yield of 4-hydroxy-2-oxopyran-6-carboxylic acid(HOPC)increases first and then decreases,and the higher the water content,the more stable 2,5-FDCA.Galactaric acid is poorly soluble and has low solubility.In this paper,the effect of substrate concentration and stirring on the reaction was investigated using galactaric acid as the raw material.The results showed that the initial concentration has a great effect on the reaction without stirring,the yield of 2,5-FDCA decreased from 39.6%to 17.5%when the concentration of the substance increased from 0.02 g/mL to 0.06 g/mL,presumably because the galactaric acid dissolves too slowly and fails to participate in the ring-forming reaction in a timely manner,and dehydrated to humins.The effect of stirring on the reaction was examined,and the results showed that stirring could slow down the declining rate of 2,5-FDCA yield,but increasing the stirring speed had little effect on the reaction,from 300 r/min to 700 r/min,The yield of 2,5-FDCA does not change much.Secondly,due to the greater reserves of glucose in nature,the preparation of 2,5-FDCA using glucaric acid as a raw material has more research value.In this paper,under the same conditions,the four raw materials of galactaric acid,potassium bisaccharate,calcium saccharate,and saccharic acid 1,4-lactone were compared,the yields of 2,5-FDCA were 45.9%,51.6%,53.6%,and 56.3%respectively.Overall,the effect of glucaric acid derivatives is better than that of galactaric acid,presumably due to the low solubility and slow dissolution of galactaric acid.Calcium saccharate is relatively cheap and easy to obtain,and the yield of 2,5-FDCA is high.In this paper,the catalyst and solvent were screened using calcium saccharate as the raw material.The results showed that sulfone is the best solvent and sulfuric acid is the best catalyst.Then.the process was optimized on the basis of catalyst and solvent screening,and the best process conditions were obtained when calcium saccharate was used as the raw material:sulfone was used as the solvent,and the reaction was carried out at 120?with for 20 h under 15 wt%sulfuric acid,the yield of 2,5-FDCA is 56.3%.Finally,using the first-order reaction kinetics fitting,the activation energy of galactaric acid reaction catalyzed by 15 wt%sulfuric acid is 90.0 kJ/mol,2,5-FDCA formation activation energy is 87.4 kJ/mol,and other side reaction activation energy is 92.7 kJ/mol,activation energy of further degradation reaction of 2,5-FDCA is 130.6 kJ/mol;using first-order reaction kinetics fitting,the activation energy of calcium saccharate reaction catalyzed by 15 wt%sulfuric acid is 91.7 kJ/mol,2,5-FDCA formation activation energy is 87.7 kJ/mol,other side reaction activation energy is 98.2 kJ/mol,2,5-FDCA further degradation reaction activation energy is 130.6 kJ/mol.Since the mechanism for the preparation of 2,5-FDCA by hexaric acid is not clear,this article speculated on the possible reaction mechanism:first,the hexaric acid removes a molecule of water from the 3-position hydroxyl group,then keto-enol tautomerism to form an intermediate 5-dehydro-4-deoxyhexaric acid,then 5-dehydro-4-deoxyhexaric acid is condensed to form a ring,finally dehydration to produce 2,5-FDCA;furoic acid is formed after 5-dehydro-4-deoxyhexaric acid condensed to a ring,then decarboxylated and dehydrated;HOPC is obtained by forming 1,5-lactone from hexaric acid and then dehydrated. |
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