Tiangui Wang,Chuang Li,Mengmeng Song,Rongrong Fan

College of Chemistry,Chemical and Environmental Engineering,Henan University of Technology,Zhengzhou 450001,China

Keywords:

Xylo-oligosaccharides

Hemicelluloses

Hydrolysis

Catalysis

ABSTRACT

The preparation of xylo-oligosaccharides(XOSs)through hydrolysis of hemicelluloses was studied.The hemicelluloses were isolated from the press lye discharged in the production of viscose,which contained about 30%xylan.Then,a factorial experimental design was applied to compare the influences of several factors including the concentrations of sulphuric acid and hemicelluloses,the duration and temperature of the hydrolysis,on the conversion of xylan,and the selectivity for the product-XOSs.The results showed that the hydrolysis duration affects the yield of XOSs to the greatest extent.It is difficult to obtain a high yield of XOSs with sulphuric acid as the hydrolysis catalyst.

1.Introduction

Viscose fibre derives from natural cellulose.The pulp for viscose preparation is made of cellulose-rich raw materials,such as wood,cotton lint,or bamboo.The pulp consists of cellulose,some hemicelluloses,and other impurities.Generally,pulp is steeped in a hot aqueous solution of sodium hydroxide to remove the non-cellulose constituents and to obtain sodium cellulose.During the steeping process,non-cellulose impurities and some degraded cellulose,which are generally called hemicellulose(HC)[1],dissolve in the solution.After press-filtration,the filtrate,called pressing lye(PL)can be recycled and reused only if its HC content reaches the required specification.Otherwise,the PL must be discharged.If the HC in the PL is removed,PL can be recycled and reused.Some approaches to the regeneration of PL have been developed and nano-filtration has been extensively adopted.The HC in the PL is concentrated in the retained material.To recover sodium hydroxide from this retained material,an alcohol can be used to precipitate the HC.After precipitation and separation of HC,the solution containing sodium hydroxide and the alcohol is rectified to obtain,respectively,the aqueous solutions of sodium hydroxide and the alcohol which are recycled and reused.This article focuses on the value-added application of the HC separated from the retained material,and more specifically on the transformation of the xylan in the HC.Xylan is one of the main constituents of natural hemicelluloses in plants and agricultural residues.Traditionally,xylan is used to produce xylose,furfural,and their derivatives.In recent years,producing xylo-oligosaccharides(XOSs)with xylan has aroused interest among scholars[2-5]and shows good market potential.XOSs are prebiotics and can be added to feed,food products,and beverages.XOSs seem to impart nutritional benefits in various animal species and humans and have many functions such as:immunomodulatory activity,anti-cancerous activity,growth regulatory activity,anti-microbial activity,etc.[6,7].The methods used for preparation of XOSs can be classified into three groups: autohydrolysis [8-12],acidhydrolysis [13-16],and enzymatic hydrolysis [17-20].There are merits and demerits for each method.Autohydrolysis needs not any catalysts and seems to be simple,but it must conduct at a higher temperature(150-200°C)and pressure so that obtains an adequate hydrolysis rate,which incurs side reactions such as pyrolysis or degradation of the hemicelluloses.Furthermore,it is difficult to exactly control the duration and temperature of autohydrolysis in laboratory at elevated temperature and pressure.Enzymatic hydrolysis has the highest selectivity of XOSs,mild reaction conditions,and is the only known commercial production process of XOSs,but the process is complex and the hydrolysis rate is slower.Acid-hydrolysis rate is faster,then the efficiency is higher,but there may be substantial side reactions,especially xylose production.These three processes were all investigated in our research.Here only the results of sulphuric acid catalytic hydrolysis are presented.The purpose is to recognize the major factors affecting the yield of XOSs by means of experimental design methodology.Other contents will be published later.

2.Materials and methods

2.1.HC preparation

HC was isolated from PL(Xinxiang Chemical Fibre Co.,Ltd.,China)by means of ethanol precipitation.The PL had been concentrated by membrane process,and contained 50-60 g/L hemicelluloses(as determined by dichromate oxidation method).The pentosan content of the isolated hemicelluloses was about 30%(dry weight basis),as determined by distillation[21],hydrolysis[22],colorimetric methods[23],and HPLC,which is consistent with the results of Mozdyniewicz et al.[1].Before used,HC were neutralised,washed,dried,ground,and analysed.

2.2.HC hydrolysis

Professional software Design-Expert was adopted to plan the experiments.Sulphuric acid was chosen as the catalyst of HC hydrolysis.The concentrations of sulphuric acid(Cs)and HC(CL),temperature,and duration of hydrolysis were chosen as influential factors.The values of the factors are listed in Table 1.The two-level factorial experimental design is shown in Table 2.Experiments were conducted,at random,according to the plan.A certain amount of HC was put into a 50-mL conical glass bottle,to

Table 1 Influential factors and their levels.

which 25-mL sulphuric acid solution prepared was also added according to the required concentration.Then,the conical glass bottle was sealed and fixed on the shaker preheated to the set temperature and shaken at a suitable velocity.After a specific time,the conical glass bottle was taken out and a solution of sodium hydroxide was added to neutralise the acid.Afterwards,the conical glass bottle was cooled to room temperature,followed by centrifugation.The supernatant was extracted and then diluted to a certain volume before analysis of the contents of xylose and XOSs therein.The hydrolytic residues were washed and dried to analyse the content of xylan therein.On this basis,the conversions(XX)of xylan and selectivity for XOSs(SXOS)were calculated in accordance with the following Eqs.(1)and(2).

Note:XOSs represent xylan with the degree of polymerization from two to nine.

2.3.XOSs quantification

The supernatants resulting from hydrolysis of HC were analysed by high-performance liquid chromatography to detect the yield of XOSs released.Separation was performed using Shimadzu instrument equipped with an Xbridg Amide analytical column (Waters,3.5 μm,4.6×250 mm)coupled to an Evaporative Light-Scattering Detector.The temperature of the column was 40 °C,and the detector temperature was 60 °C.Air was used as carrying gas flowing at 1.5 L/min.The mobile phase consisted of 0.2% triethylamine-acetonitrile (A)/0.2% triethylaminewater(B),and the gradient elution mode was as follows:26%-43.5%B at 0-25 min,43.5% B at 25.0-25.5 min,and 26% B maintained at 25.5-41.0 min.The flow rate was 1.0 mL/min.The concentrations of each saccharide were quantified based on the construction of calibration curve with a mixture of xylose and XOSs standards.The selectivity(SXOS)for XOSs was calculated by dividing the total concentration of XOSs by the total concentration of all saccharides.

3.Results and discussion

The main effects of the four factors were analysed taking XXand SXOSas the target responses,respectively.The rates of hydrolysis of xylan and the selectivity for XOSs were list in Table 2.

We performed half-normal probability plot for XXanalysis,and the variance analysis was conducted.

Fig.1 shows the resulting half-normal probability plot for XXanalysis,in which there are influence and error columns of the selection.On this basis,the variance analysis was conducted,with the results shown in Table 3.Table 3 indicates that P-values of all the factors were ＜0.05 which means that the changes in all of the factors had significant effects on the conversion of xylan.

The effect plots are displayed in Fig.2: a higher concentration of sulphuric acid(Fig.2A),a longer duration(Fig.2B),a higher temperature of hydrolysis(Fig.2C),and a lower HC content(Fig.2D)helped to improve the conversion of xylan,which was consistent with our expectations and similar to findings of other research.

Fig.3 shows the resulting half-normal probability plot for SXOSanalysis,in which there are influence and error columns of the selection.On this basis,variance analysis was conducted,with the results shown in Table 4.Table 4 indicates that P-values of B,AB,AC,and AD were ＜0.05 which means that the change in hydrolysis duration had a significant effect on the selectivity of XOSs.Moreover,the interactions of sulphuric acid with the duration,temperature,and HC content also had a significant effect on the selectivity of XOSs.The duration effect plot is displayed in Fig.4:the selectivity of XOSs decreased over time,which countered the duration effect on the conversion of xylan.The interaction effect plots are displayed in Fig.5.

Based on above analysis,in the range of values of each factor,hydrolysis duration had the most significant effect on the yield of XOSs(yield is defined as conversion multiplied by selectivity).The conversion of xylan increased while the selectivity of XOSs decreased with increasing hydrolysis duration.There was optimal hydrolysis duration,in accord with the principle of chemical reaction kinetics.HC hydrolysis is a sequential reaction and the XOSs produced from HC hydrolysis will further hydrolyse to xylose.In the beginning,HC hydrolysis only produced XOSs.Over time,XOSs hydrolyse to xylose and xylose would hydrolyse to furfural at a higher temperature.Therefore,during a certain period of the hydrolysis,because the rate of disappearance of XOSs is less than the rate at which they are produced,and vice versa,then,the total concentration of XOSs decreased.The time at which the total concentration of the XOSs peaked was also related to the aforementioned interaction.The largest yield of XOSs among the 16 runs was 20.6%from Run 15(if we multiply the last two columns in Table 2)which indicated that it is difficult to obtain an economically viable yield by HC hydrolysis when using sulphuric acid as the catalyst.

Table 2 Experimental factorial design and the results.

Fig.1.Half-normal probability plot for XX analysis.

4.Conclusion

The research investigated the preparation of XOSs through HC hydrolysis using sulphuric acid as the catalyst.The hydrolysis conditions were compared by using the factorial design method.A higher concentration of sulphuric acid,a longer duration and a higher temperature of hydrolysis,and a lower HC content were conducive to conversion of xylan.The selectivity of XOSs decreased with increasing duration of hydrolysis.It was difficult to obtain a high yield of XOSs when using sulphuric acid as the hydrolysis catalyst.

Table 3 Analysis of variance for XX.

Table 4 Analysis of variance for SXOS analysis.

Fig.2.One-factor effect plot for XX analysis.

Fig.3.Half-normal probability plot for SXOS analysis.

Fig.4.One-factor effect plot for SXOS analysis.

Fig.5.Interaction plot for SXOS analysis.

Declaration of competing interest

All the authors confidently declare that there is no conflict of interest.

Acknowledgements

This article was finically supported by Science and Education Integration Program of Henan University of Technology.