Study On The Drying And Conversion Of Xylan To Sugar Alcohols

Lignocellulosic biomass contributes the large proportion to the biomass resource,which is mainly composed of polymers(cellulose and hemicellulose),and aromatic macromolecules(lignin).Hemicellulose is second only to cellulose in content and is the second most abundant renewable biomass in nature.Xylan is the main component of hemicellulose,which is made of xylose units linked by ?-1,4 linkages.Xylan can be used to prepare high value-added xylooligosaccharides(XOS)by bio-enzymolysis and xylitol by chemical catalysis,which is an important biomass-based platform molecule for the production of various liquid fuels,fuel additives,spices,pharmaceutical intermediates and other downstream chemicalsDifferent polymerization degree is the main reason for various properties and functions of xylan.Xylooligosaccharides(XOS)composed of 2-9 xylose molecules are promising prebiotics and have attracted tremendous attention.XOS have the functions of non-cariogenicity,promoting the growth of colonic bifidobacteria,and calcium absorption.It has important applications not only in food and medicine,but also widely used in agriculture and feed formulations.XOS can be obtained from rich,inexpensive and renewable agricultural and forestry wastes by stages such as pretreatment,enzymatic hydrolysis,concentration and refining,and drying.However,the hygroscopicity,agglomeration,and even surface liquefaction which are likely to occur during drying and storage of XOS,leading to an inferior quality,a lower drying yield,and packaging and utilization difficulties.Therefore,it is extremely important to explore the key factors affecting the quality change of XOS during drying and storage,and to improve the quality of XOS.Besides,xylitol is one of the 12 most important target chemicals that selected by the US Energy Department's Energy Response Center.At present,xylose is mainly synthesized by a conventional high-pollution,high-cost two-step process:hydrolysis of xylan-rich hemicellulose to xylose through inorganic acid,followed by conversion of xylose to xylitol by noble metal catalysts.Therefore,in order to solve the quality deterioration during drying and storage of XOS as well as high environmental pollution and high-cost during preparation of xylose,a series of studies were carried out,as follows:First,the powder quality and microstructure of prebiotic XOS were evaluated,when carried by maltodextrin(MD).The relationships between glass transition temperature(Tg)and microencapsulating efficiency,moisture content as well as drying yield under different inlet air temperatures and MD concentrations were investigated.Antioxidant activity retention,hygroscopicity,color attributes,X-ray diffraction(XRD),scanning electron microscopy(SEM)and Fourier transform infrared spectroscopy(FTIR)of the spray-dried XOS product were also assessed.The results indicated that an increase in inlet air temperature decreased the moisture content,hence increased the Tg value.Higher MD concentration increased the Tg value,and was attributed to the molecular weight increase.The wall deposition was a tendency to occur when the temperature beyond the Tg,reduced the drying yield.Higher temperature and wall deposition promoted the decomposition of XOS into monosaccharide.The antioxidant activity retention of XOS exhibited a concentration-dependent behavior.The hygroscopicity of the microparticles could be reduced by both of the low inlet air temperature and high MD concentration.According to the results of color attributes,XRD and SEM,the particles were colorless and amorphous,and tended to become more spherical and scattered with the addition of MD.The FTIR analysis confirmed that no chemical reaction occurred between XOS and agent,which indicated that MD is a promising carrier for producing bioactive XOS powder.Second,using gum Arabic(GA)as a carrier,the rheological properties of the fluids,as well as the antioxidant activity,physicochemical and morphological characterization of spray-dried XOS were studied.The antioxidant effects of the products were evaluated by 2,2-Diphenyl-1-picry lhydrazyl(DPPH),2,2'-Azinobis(3-ethylbenzothiazoline-6-sulfonic acid ammonium salt)(ABTS+),ferric reducing antioxidant power(FRAP),and oxygen radical absorbance capacity(ORAC).The intuitive perturbation plot was applied to analyse the effects of the independent variables on the responses.A polynomial equation and the Gordon-Taylor equation were used to model the experimental data,and the fitted equations revealed the powder quality and storage stability well.The molecular weight of the carbohydrate affected the rheological properties and Tg.Increasing the solid content of the initial solution tended to increase the apparent viscosity and median diameter.The EC50(the antioxidant concentration providing 50%reduction of DPPH radicals)values of representative products were 1126.00 and 651.69 ?g/mL by DPPH and ABTS+assays,and antioxidant activity values of representative products were 49.68 and 64.95 ?mol TE/g by FRAP and ORAC assays,which shown the remarkable antioxidant effects.The drying yield,hygroscopicity,Tg,microencapsulating efficiency,and colour attributes(L*,a*,and b*)of representative high GA concentration(GAC)products were 76.10%,13.00 g H2O/100 g dry weight,71.10? 99.69%,96.93,1.13,and 3.66,respectively,which shown better quality properties than low GAC products.According to the XRD and SEM analysis,the microparticles were amorphous,and well-separated under an appropriate GAC.The FTIR spectra results shown that the XOS and GA preserved their structural integrity during the spray drying process.Third,four drying methods such as freeze drying,vacuum drying,hot-air drying,and spray drying,with or without carriers(MD and GA)were applied to evaluate their influence on the physicochemical properties,microstructures,and storage stability of the resulting XOS products.Rheological properties of the fluids,Tg,freezable water content,water activity,water sorption isotherms,microstructure(characterized by XRD,FTIR,and SEM),chemical quality,and shelf life were mainly studied.The results showed that freezable water content was negatively correlated with Tg.Tg and monolayer water content(Xm)greatly affect the processability and stability of the XOS product,which had a further impact on the microstructure of the product during the storage.For comprehensive consideration,the freeze-dried product with 32-50%MD and 6-18%GA has higher Tg and lower Xm,and storage stability are relatively good,which can be used as the best conditions for drying the crude XOS liquid.Then,an inexpensive Ni6.66Fe1Al1.55 catalyst was synthesized by co-precipitation method for xylose hydrogenation experiments.A maximum xylose conversion and xylitol yield of 100%and 99.78%was achieved in Ni6.66Fe1Al1.55 at 120? in 3 h.During the experiment,we also detected that the highest yield of arabitol was 20.73%.The catalyst exhibited excellent catalytic stability in the catalyst recycle experiment,and its catalytic activity remained relatively stable in the subsequent 5 repeated experiments.The catalyst has superparamagnetism and the saturation magnetization reaches 20.52 emu/g,which can rapidly separate the catalyst and the reaction solution under an external magnetic field.According to a series,of characterization and activity determinations,we proposed the formation process of Al2Ni1-xO4-x solid solution and the mechanism of xylose hydrogenation reaction.First,in the coprecipitation process,the catalyst precursor can form a well-crystallized form of Ni6.66Fe1Al1.55(OH)n(CO3)m hydrotalcite.During the 500? reduction process,the Ni ions in the hydrotalcite interact strongly with the Al ions,and the Ni2+ions are embedded in the alumina lattice to form NiAl2O4.During the reduction process,a part of NiAl2O4 is reduced,and the reduced Ni metal interacts strongly with the unreduced NiAl2O4 to form a solid solution and enlarges the lattice of Ni.The lattice of Ni is kept compatible with the distance between the carbonyl groups of the xylose molecule,promoting hydrogenation activity.We also used this catalyst in the glucose hydrogenation experiment.Under the reaction conditions of 130? and 5 h,the highest glucose conversion and sorbitol yield were 99.00%and 98.89%,respectively.Finally,a series of different ratio of Ni/Fe catalysts were prepared by using inexpensive silica gel as carrier to efficiently hydrolyze xylan to sugar alcohols in one pot.Among them,Ni8.98Fe1@1.54SiO2 has the highest magnetization,and the yield of xylitol and arabitol is as high as 88.16%and 20.55%after reacting for 4 h at 180?.The catalyst has excellent recycle performance,and good separation of the reaction liquid under an external magnetic field.Through a series of characterization and reaction experiments,it is shown that the catalyst with Bronsted acidic site can accelerate the catalytic hydrolysis of xylan to xylose at high temperature,and H2 is adsorbed on the acidic site of Bronsted supported by highly active Ni catalyst,producing activated H on the surface of catalyst.Finally,xylose reacts with the activated H on the surface of catalyst,which is an irreversible reaction,and then the product desorbs from the catalyst and diffuses into the liquid phase.