Supercritical Methanolysis Of Sweet Sorghum Stalk And Its Catalytic Conversion Over Solid Bases

Biomass,as one of the most abundant and renewable resources,is being considered as a potential alternative for fossil resources.Sweet sorghum,as a new industrial high energy crop due to its high photosynthetic efficiency,high biomass yield per hectare,short growth period,highly resistant to drought and salinity,and well adapted to temperate climates,has attracted many countries' attention.Many researchers focused on producing bioethanol from sweet sorghum with fermentation.However,few attempts have been made on other technology for sweet sorghum transformation.In this study,methanol soluble portion?MSP?and organic compounds were obtained from sweet sorghum stalk?SSS?supercritical methanolysis under different conditions including temperature,holding time,and SSS-to-methanol ratio.The optimal reaction temperature and time were determined to be 300 °C and 30 min,respectively,based on the MSP yield and the maximum yield of MSP is 40.5 wt% with higher heating value?HHV?of 25.1 kJ.g-1.Low SSS-to-methanol ratio,i.e.,either more methanol volume or less SSS dose,benefits the MSP yield.In addition,the effect of pretreatment on SSS methanolysis and the structure and stability of the three major components of biomass was also examined.The results show that the pretreatment could markedly improve the yield and HHV of MSP and decrease the optimal temperature for obtaining maximum MSP yield.The pretreatment could be helpful to partly remove lignin and hemicellulose and disrupt the crystalline structure of cellulose,resulting in the destruction of tightly association of the three polymers.The changes of physical and chemical structures should be responsible for the higher MSP yield and lower optimal temperature.The key to improving the MSP yield and obtaining value-added chemicals is preparing appropriate catalysts which can promote the breakage of covalent bonds?especially –C–O– bonds?during SSS methanolysis.Three common solid bases?Na₂CO₃,NaOH,and CaO?together with two supported solid bases?MMCB and ATB?were used to catalyze SSS methanolysis.The results exhibit that yield of each soluble portion from SSS methanolysis over the 5 types of catalysts at 280 ? exceeds 50 wt%,which increases ca.10 wt% comparing with the soluble portion from non-catalytic methanolysis.The yield of soluble portion from the three common solid bases catalytic methanolysis increased in the order: Na₂CO₃ < NaOH < CaO.Comparing with Na₂CO₃,NaOH,and CaO,higher yield of soluble portion was obtained from MMCB and ATB catalytic SSS methanolysis with less dosage.The molecular composition of MSP was identified by various advanced analytical techniques and detailedly understanding the MSP composition is necessary for upgrading the MSP to biofuels or using the MSP as a base for chemicals.According to GC/MS analysis,the compounds detected in the MSP can be grouped into hydrocarbons,alcohols,phenolic compounds,ethers,ketones,and esters.Among them,phenolic compounds and esters are the most abundant.The origin of group components was discussed.Phenolic compounds mainly are from lignin,while esters and ketones primarily come from cellulose and hemicellulose.The distribution of compounds significantly changed with reaction condition.The total yield of ketones almost trended contrary to that of esters under the reaction conditions,and the same tendency was exhibited between the total yield of long-chain methyl esters and short-chain methyl esters,indicating that the competition may exist in the formation process of these compounds.Thousands of oxygen-containing species were identified with ESI FTICRMS,which are attributed to On?n = 1–10?class species with double bond equivalent?DBE?values of 1–14 and carbon atom numbers?CANs?of 5–35.They could be assigned to acidic species,such as aliphatic acids,monomers,dimers and trimmers of lignin based on DBE and CAN.GC x GC/TOFMS and FTICRMS analyses also reveal nitrogen-containing compounds,which are difficult to be detected with GC/MS.The results display that N1On?n = 0–10?class species are the main neutral nitrogen compounds and N2On?n = 0–6?class species are the main basic nitrogen compounds.Nitrogen atoms in neutral N1 On class species mainly exist in pyrrole and indole ring,while those in basic N2 On class species predominantly present in amine,imidazole,and pyrazine,providing important information on organic nitrogen compounds in MSP.To realize efficient utilization of MSP,a series of group components were enriched from MSP by extraction and column chromatography.The MSP from solid bases catalytic methanolysis were sequentially extracted with petroleum ether,carbon disulfide,and ethoxyethane.As s result,a series of phenolic compounds,alcohols,benzoic acids and fatty acids were enriched.In addition,a series of alkanes,arenes,methyl alkanoates,methyl alkenoates,alkylphenols,and guaiacols were enriched to large extents with medium pressure liquid chromatograph using mixture of petroleum ether/carbon disulfide as the elution solvent.