Studies On The Simultaneous Catalytic Conversion Of C5 And C6 Sugars To Methyl Lactate/Methyl Levulinate In Near-critical Methanol

Due to the increasingly diminishing fossil resources and emission of greenhouse gases,the utilization of green renewable resources is imperative.Biomass is abundant and renewable.Converting biomass to fuels and value-added chemicals is a promising way to reduce the dependence on fossil resources.The greenhouse gas CO2 is recycled by the plants via photosynthesis.Therefore,the utilization and regeneration of biomass will not increase the emission of net greenhouse gases so as to alleviate the greenhouse effect.Lignocellulose consists of cellulose,hemicellulose and lignin.Hemicellulose and cellulose make up 2/3 of lignocellulose.The hydrolysis products of hemicellulose and cellulose are C5 and C6 sugars.Most researchers studied the conversion of C5 and C6 sugars separately,resulting in the low utilization rate of raw materials,the low product yield and the high separation cost.In order to solve these problems,the simultaneous catalytic conversion of C5 and C6 sugars to one value-added product in near-critical methanol was studied in this work.Methyl lactate and methyl levulinate are the two main target products.The work is summarized as follows:Firstly,a high methyl lactate yield could be detected via the homogeneous catalytic conversion of biomass such as some metal chlorides.However,there is no clear explanation to demonstrate the trends of the catalytic conversions of C6 and C5 sugars with different metal chlorides.Hence,the effect of 15 different metal chlorides on the simultaneous catalytic conversion of C5 and C6 sugars to methyl lactate in near-critical methanol solutions was studied.The results showed that C5 and C6 sugars reacted independently and did not affect each other.Moreover,the trends of the conversions of C5 and C6 sugars were similar as different metal chlorides.The product yield of methyl lactate initially increased then decreased when increasing the pKa value of metal ions.This indicated that the medium Lewis acidity was efficient for the production of methyl lactate.The highest methyl lactate yield was detected with InCl3(pKa value=4.4).On the catalytic conversion of glucose,fructose,xylose,and the mixture of glucose and xylose,the methyl lactate yields were 45.9%,57.4%,32.3%,and 35.8%,respectively.A possible reaction pathway for the simultaneous catalytic conversion of C5 and C6 sugars to methyl lactate was proposed.Secondly,even though high methyl lactate yield could be obtained via the homogeneous catalytic conversion of biomass,the recovery of homogenous catalyst is often difficult and unsatisfactory.In contrast,heterogeneous catalysts are more attractive.Some researches detected that NiO was efficient for the production of methyl lactate from biomass.Moreover,NiO is relatively cheap and it possesses interesting magnetic properties so it can potentially be recovered.However,the catalytic mechanism of NiO has not been studied.In this chapter,the catalytic mechanism on the conversion of fructose to methyl lactate over NiO was clarified.The NiOs were calcined at 300℃ 400℃ 500℃ and 600℃ respectively.NiOOH was found on the surface of NiO.The amount of NiOOH and the yield of methyl lactate yield decreased with the increase of the calcination temperature of NiO.So,we speculated that NiOOH promoted the catalytic conversion of fructose to methyl lactation in this reaction condition.On the other hand,NiOOH also influenced the catalytic conversion of intermediate products,1,3 dihydroxy acetone and methylglyoxal.The influence of NiOOH on the catalytic conversion of reactant is in the order of fructose>1,3 dihydroxyacetone>methylglyoxal.However,in this reaction condition,NiO showed inferior reusability since some carbonaceous species occupied the surface of NiO blocking the access of reactant to the active sites of catalyst.Then,currently,research is mainly focused on the catalytic conversion of model compounds to methyl lactate.If the raw biomass could be directly converted to methyl lactate,the cost would be decreased significantly.However,the raw biomass comprises a certain amount of water,which would result in the hydrolysis of methyl lactate.So,the effect of water content on the catalytic conversion of sugars to methyl lactate in the near-critical methanol solutions was studied.The model compounds glucose,xylose,fructose,sucrose,maltose and lactose were chosen as the reactants.NiO was chosen as the catalyst.For different sugars,the product yield of methyl lactate initially increased and then decreased with the increase of the water content.Because some side products such as glycolaldehyde dimethyl acetal(GADMA)were converted to methyl lactate in the presence of water.GADMA was hydrolyzed to glycolaldehyde with water.Then glycolaldehyde could be converted to C6 sugar via the aldol condensation reaction.And the C6 sugar was converted to methyl lactate.With a certain amount of water,the production rate is higher than the hydrolysis rate of methyl lactate.Therefore,a higher methyl lactate yield was obtained under optimized water dosage.The optimal water content for achieving the maximum methyl lactate yield varied for the conversion of different sugars.The effect of water content on the conversion of GADMA and the stability of methyl lactate were detected.NiO could be regenerated by calcination at 250℃,and water showed almost no adverse effect on the activity of NiO catalyst.The hydrous high fructose corn syrup was directly converted to methyl lactate with NiO in the near-critical methanol solution.We detected the effect of reactant loading on the product yield of methyl lactate.A 42.4%methyl lactate product yield was obtained.Finally,compare with methyl lactate,it’s more difficult to simultaneous catalytic conversion of C5 and C6 sugars to methyl levulinate.Because the hydrogenation reaction and multisteps were required.However,a one-pot,hydrogen-free conversion of C5 and C6 sugars into methyl levulinate over zeolites at near-critical methanol conditions with the assistance of 1,3,5-trioxane was reported in this study.The effect of 1,3,5-trioxane loading on the conversion of xylose was studied.The hydroxymethylation of furfural/furfural derivative and formaldehyde to 5-hydroxymethylfurfural is the key step.The effect on conversion of xylose over different zeolites(Hβ,MCM-22,MCM-41,Mordenite and SAPO-11)was studied.Hβzeolite with both high Lewis and Br?nsted acid sites density contributed to the highest methyl levulinate yield.By optimization of reaction conditions,the highest methyl levulinate yield could reach 47.4%at 160℃ for 18 h.The effects of 1,3,5-trioxane on different sugars were studied.For the conversion of C6 sugars and disaccharide,1,3,5-trioxane was not required,further proving 1,3,5-trioxane was only favorable for the hydroxymethylation reaction.The mixture of glucose and xylose afforded high methyl levulinate yield of 50.4%at 160℃ for 18 h.The possible reaction pathway of the conversion of C5 and C6 sugars to methyl levulinate with the assistance of 1,3,5-trioxane was proposed.To further confirm the proposed pathways,the conversion of furfural and 5-hydroxymethylfurfural were assessed.Hβ zeolite can be reused for at least five times without obvious deactivation.