Xiaoqi Gong,Zongwei Zhang,Yonghao Ni,2,*,Xinhua Ouyang,Lihui Chen,Liulian Huang,Huichao Hu,*

1.College of Material Engineering,Fujian Agriculture and Forestry University,Fuzhou,Fujian Province,350002,China

2.Department of Chemical Engineering,University of New Brunswick,Fredericton,New Brunswick,E3B 5A3,Canada

Abstract:Hemicellulose has a wide range of applications,including that as an emulsifier for the food industry and raw material for the synthesis of bioethanol/biochemicals and biodegradable films.Hemicellulose is usually present as a spent liquor,such as the prehydrolysis liquor of the prehydrolysis kraft dissolving pulp production process and the alkali extraction liquor of the cold caustic extraction of pulp fibers.Due to its dilute nature,hemicellulose needs to be dried for practical utilization,and this is challenging.In this study,cellulose and hemicellulose in a bleached bamboo kraft pulp were separated using an alkali extraction process.Hemicellulose obtained from the extraction liquor was dried by an ammonium carbonate-assisted drying process.The effects of drying time and drying temperature were determined.Structure of the hemicellulose obtained by the ammonium carbonate-assisted drying process was similar to that of original hemicellulose,as revealed by detailed Fourier transform infrared and X-ray diffraction analyses.The novel drying method was more energy efficient and required a shorter drying time than the conventional freeze drying method,and the excellent solubility in alkaline solutions favored the chemical modification of hemicellulose.The dried hemicellulose can be used as a renewable raw material for the preparation of hydrogels and other substances such as bioethanol/biochemicals and biodegradable films.

Keywords:bamboo kraft pulp;hemicellulose;alkali extraction;ammonium carbonate;drying process;dissolving pulp

1 Introduction

Hemicellulose is one of the most abundant natural polymers.Hemicellulose,together with cellulose and lignin,is a major constituent of the cell wall of lignocellulosic plants[1-2].It is a low molecular weight branched polymer composed of sugar units including xylose,glucose,arabinose,galactose,mannose,glucuronic acid,and galacturonic acid[3-5].Hemicellulose has potential applications as strength additives and sizing agents for the pulp and paper industry,emulsifiers for the food industry,and raw materials for the preparation of chemicals and pharmaceuticals,hydrogels,bioethanol/biochemicals,and biodegradable films[6-8].

Bleached chemical pulp consists almost exclusively of cellulose and hemicellulose,with negligible amounts of other impurities such as lignin and wood extractives.The cold alkali extraction process is an effective method to separate cellulose and hemicellulose;in fact,it is used in the industries for dissolving pulp production from bleached paper-grade pulp[9-11].Dissolving pulp can be converted to various valueadded green cellulose products, including nanocellulose,cellulose ethers,cellulose esters,synthetic fibers,and other cellulose derivatives[12-15].The alkali extraction liquor contains hemicellulose,and there are other sources of hemicellulose in industrial spent liquors.For example,the prehydrolysis liquor of the prehydrolysis kraft dissolving pulp production process is rich in hemicellulose[16].Another example is the filtrate from the extraction filtrate during the conversion of paper-grade pulp to dissolving pulp using ionic liquid(IL)[17-18]andγ-valerolactone(GVL)[19].A significant challenge impeding their large-scale industrial utilization is the drying of hemicellulose in these liquors/filtrates,as transportation of dilute hemicellulose-containing liquors from pulp mills to processing plants can be expensive.In addition,dried hemicelluloses can be stored for a long time,while the dilute liquor is difficult to store due to bacterial and fungal growth in it.

Freeze drying and spray drying are the two common drying processes and can also be applied for drying hemicellulose.Freeze drying is based on the principle of ice crystal sublimation.After drying,the material retains the original chemical composition and physical properties;however,the operational cost is high,thus limiting its wide application at an industrial scale.Spray drying is a simple and fast process—the hemicellulose solution is atomized into fine droplets in a large chamber filled with hot air.The resultant dry particles are then collected with a cyclone.Although spray drying is less expensive than freeze drying,it is an energy-intensive process;in addition,it requires complex equipment,and the associated capital investment is high.Thus,it is of practical interest to develop an alternative green and effective process for drying hemicellulose.In this study,the applicability of an ammonium carbonate-assisted process for drying hemicellulose were investigated.In this process,CO2and NH3are generated by thermal decomposition,which facilitates the drying of hemicellulose.

In this work,caustic extraction was conducted to extract hemicellulose from a bleached bamboo kraft pulp.Hemicellulose present in the extraction filtrate was then collected and subjected to drying under different processes,namely,①ammonium carbonateassisted drying,②freeze drying,and③air drying.This work focuses on the ammonium carbonateassisted drying of hemicellulose.Detailed extraction conditions,such as drying time,temperature,and the mass ratio of ammonium carbonate to hemicellulose,were investigated.Moreover,the hemicellulose samples were characterized by X-ray diffraction(XRD), Fourier transform infrared (FT-IR)spectroscopy,Brunauer-Emmett-Teller(BET)isotherm studies,and their re-dissolution was investigated.Furthermore,the solid residues(alkali-extracted pulp fibers)were qualified as dissolving pulp and were further characterized by scanning electron microscopy(SEM),XRD,and FT-IR spectroscopy.

2 Experimental

2.1 Materials

Bleached bamboo kraft pulp(BKP)sample containing 10.8% of hemicellulose was generously provided by a pulp mill in Sichuan Province.All analytical grade chemicals used in this study,including NaOH,H2SO4,and ammonium carbonate((NH4)2CO3,99.99%),were purchased from Sigma-Aldrich.All chemicals were directly used without further purification.

2.2 Methods

2.2.1 Alkali extraction of BKP

Cold alkali extraction of BKP was performed using a previously reported procedure[20].Briefly,the BKP(50 g)was mixed with NaOH(2.5 mol/L),and the alkali extraction was performed with 5% pulp consistency at 25℃for 60 min.Subsequently,the treated pulp slurry was filtered.The alkali filtrate contained hemicellulose,which was subsequently neutralized by adding H2SO4under continuous agitation until the pH value was 2.5.The mixture was kept overnight in a refrigerator to allow the precipitation of hemicellulose.The hemicellulose precipitate was separated from the filtrate by centrifugation and washed repeatedly with deionized water until the pH value of the slurries was~5.Hemicellulose slurries in water at a concentration of 10% were prepared for the next drying experiments.

The solid residue(alkali-extracted pulp fibers,purified cellulose)was obtained as the filtration cake and was thoroughly washed and collected for further analyses.Yield of hemicellulose obtained using this cold caustic extraction(CCE)method was 72.2%.The hemicellulose was composed of 91.6% xylose,2.1%arabinose,and 6.3%glucose.

2.2.2 Drying of hemicellulose samples

Well-mixed hemicellulose slurries containing about 0.5 g hemicellulose were transferred to glass petri dishes and dried using three different procedures as described below.

2.2.2.1 Ammonium carbonate-assisted drying

Typically,hemicellulose slurries with about 0.5 g hemicellulose(10% solid content)was transferred to a glass petri dish.Subsequently,9.6 g ammonium carbonate was added with continuous stirring.After mixing well,the glass petri dish with the sample was left in a drying oven until a constant weight was reached.For the optimization experiments,the ammonium carbonate-assisted hemicellulose drying was studied under different mass ratios of ammonium carbonate to hemicellulose(AC∶HC=0.8∶1-1.2∶1),different temperatures(60-100℃),and different drying times(0-20 h).

2.2.2.2 Freeze drying

Vacuum freeze drying is an effective technique in which the material is frozen to the eutectic point temperature,so that the water in the material is frozen to ice;the ice directly sublimes into water vapor under vacuum.The hemicellulose samples were frozen overnight in a freezer.They were then transferred to a freeze drier (VFD-2000, Beijing Boyikang Experimental Instrument Co.,Ltd.)and left there for several hours until dry.

2.2.2.3 Air drying

For air drying,the hemicellulose sample was left in a constant temperature/constant humidity room until a constant weight was reached.

2.2.3 Re-dissolution of dried hemicellulose

Dried hemicellulose samples(2.0 g)obtained by ammonium carbonate-assisted drying,freeze drying,and air drying were re-dissolved respectively in different solvent systems—water,N-methylmorpholine-N-oxide(NMMO)/water(1∶1,mass ratio),and NaOH(2.5 mol/L),and then stirred for 1 min for subsequent analyses.

2.2.4 Characterization

2.2.4.1 SEM

Morphologies of the cellulose and hemicellulose samples before and after alkali extraction were examined with field-emission scanning electron microscopy(FESEM,HITACHI UHR FE-SEM SU8010).

2.2.4.2 FT-IR spectroscopy

FT-IR spectroscopy(Nicolet 5700 Fourier transform infrared spectrometer)was used to investigate the potential changes in the components and structures during the drying process,the spectra were recorded in the range of 400-4000 cm−1.

2.2.4.3 XRD

XRD patterns were recorded on a Bruker D8 advance diffractometer in the reflection mode in the 2θrange 5°-50° at steps of 0.02°.The Cu KR radiation generated at 40 kV and 40 mA was monochromatized using a 20μm Ni filter.The scanning speed was 0.0385°/s[14].The cellulose crystallinity index(CrI)was determined following a literature method[20-21]and was calculated based on Eq.(1):whereI200is the maximum intensity of crystalline scatter at the 200 reflection(2θ=22.5°)andIamis the intensity of diffraction at 2θ=18.0°.

2.2.4.4 BET analysis

The specific surface area(SSA)and pore size were estimated from the BET analysis of the low-temperature nitrogen adsorption-desorption isotherms obtained using a BelsorpMax volumetric gas adsorption instrument(Bel Japan,Inc.,Osaka,Japan).The data were subsequently analyzed using the BET equation following the procedure reported in literature[22].

3 Results and discussion

Fig.1 shows the schematic of the alkali extraction of hemicellulose from BKP and the ammonium carbonateassisted drying of the extracted hemicellulose.We added ammonium carbonate to the hemicellulose to be dried.Upon the addition of ammonium carbonate,water molecules quickly combine with it to form crystal water;during drying,the crystal water volatilizes rapidly with NH3and CO2which are formed from the decomposition of ammonium carbonate at a high temperature.Water can evaporate in this novel drying method while minimizing the hydrogen bond formation with the resultant dried hemicellulose due to thein-situevolution of NH3and CO2gases in the drying process.

3.1 Effects of drying time and temperature

To systematically investigate the effects of the drying time and temperature on the solid hemicellulose content,the hemicellulose dryness(xylan content)was determined as a function of time.Fig.2(a)shows that the drying time decreases with increasing drying temperature(ammonium carbonate to hemicellulose mass ratio was 1:1).Specifically,it was found that the drying time of hemicellulose significantly decreases at temperatures higher than 80℃.However,as the drying time increases,the color of the dried sample gradually changes from white to light yellow(Fig.2(c)),which is probably caused by hemicellulose oxidation at extremely high temperatures.Considering this,we decided that the optimal drying temperature was 80℃.Fig.2(b)shows the hemicellulose dryness as a function of the drying time under different ammonium carbonate to hemicellulose mass ratios at a fixed temperature(80℃).It is clear that the rate of purification of hemicellulose decreases with increasing salthemicellulose mass ratio.However,as the proportion of the added salt decreases,the dried samples gradually turn yellow and degrade.Therefore,from the perspective of energy conservation,the optimal ammonium carbonate to hemicellulose mass ratio is 1:1.

3.2 Characterization

3.2.1 Characterization of hemicellulose samples

Hemicellulose samples used to characterize,respectively,were prepared by air-drying,freezedrying,and ammonium carbonate-assisted drying(80℃,ammonium carbonate to hemicellulose mass ratio of 1:1).

3.2.1.1 FT-IR spectroscopy

FT-IR spectroscopy was used to investigate the potential changes in hemicellulose structure as a result of the different drying processes.The absorption peaks at 3442,2923,1630,1256,1044,and 901 cm−1are characteristic peaks of hemicellulose(Fig.3).The strong absorption peak at 3392 cm−1corresponds to the O—H stretching vibration and is less intense in freezedried samples.The absorption peaks at 2926,1653,and 1254 cm−1can be indexed to the—CH2stretching vibration,carbonyl group vibration,and vibration of the methyl group outside the C—H plane,respectively.The strong absorption peak at 1043 cm−1corresponds to the C—O stretching vibration,indicative of the presence of ether bonds in hemicellulose.The absorption peak at 890 cm−1appears due to the glycosidic bond between the sugar units.Furthermore,all the spectra are similar in pattern,with only slight differences in the relative intensities.This indicates that the different drying processes do not lead to any significant changes in the component and structure of hemicellulose.

3.2.1.2 XRD

The fractions of the three dried hemicellulose samples were also analyzed by XRD to investigate the crystal structure.No distinct crystalline peaks(Fig.4)were observed for the sample obtained by the ammonium carbonate-assisted drying method,while the air-dried and freeze-dried samples showed a sharp distinct peak at 2θ≈20°.These results indicate that the hemicellulose sample obtained by ammonium carbonate-assisted drying has a less ordered structure,which results from its loose structure.Thus,it is reasonable to assume that hemicellulose samples obtained by ammonium carbonate-assisted drying would be easier to redissolve than the air-dried and freeze-dried samples.

3.2.1.3 BET analysis of dried hemicellulose samples

Fig.5(a)shows that the pore size distribution of the hemicellulose sample obtained by ammonium carbonate-assisted drying is relatively narrow(0-20 nm),and its average pore size is significantly smaller than those of the other two samples(air-dried:20-40 nm,freeze-dried:0-40 nm).Pore structure parameters of the hemicellulose samples obtained by the different drying methods are shown in Fig.5(b)and Fig.5(c).It is clear that the hemicellulose sample obtained by ammonium carbonate-assisted drying has a larger BET surface area and pore volume and a smaller average pore diameter.This observation was consistent with the pore size distributions shown in Fig.5(a).Based on the above discussion,it is reasonable that the hemicellulose sample obtained by ammonium carbonate-assisted drying has a less compact structure than the air-dried and freeze-dried samples,thereby facilitating its re-dissolution.

3.2.2 Characterization of alkali-extracted BKP(purified cellulose)samples

3.2.2.1 SEM

In addition to the hemicellulose samples,the original sample and the alkali-extracted fiber(purified cellulose)samples were characterized by SEM,FT-IR,and XRD analyses.Compared to the original sample(Fig.6(a)),the morphology of the alkali-extracted fibers indicates extensive fiber destruction—the fibers were shortened.Fragmentation of fibers and wrinkles and cracks on the fiber surface occur after the removal of hemicellulose(Fig.6(b)).

3.2.2.2 FT-IR spectroscopy

FT-IR spectra of the original sample and alkaliextracted fibers are shown in Fig.6(c).The original sample and alkali-extracted fibers exhibit similar FT-IR spectra,with major functional groups matching with those of the cellulose molecule.The peaks arise due to—OH stretching vibrations(broad bands at 3400 cm−1)and—CH stretching vibrations(bands at 2900 cm−1).This indicates that the cellulose structure remains intact after the alkali extraction process,in agreement with some of the previous studies[23-25].

3.2.2.3 XRD

XRD patterns of the original sample and alkaliextracted fibers are shown in Fig.6(d).TheCrIof the obtained solid sample(65%)is lower than that of the original sample(68%).This could be due to the swelling of cellulose and the destruction of the cellulose crystal structure.Although this phenomenon is not obvious,it occurs due to the relatively low concentration of NaOH(2.5 mol/L).This observation was consistent with the conclusions from the SEM images shown in Fig.6(a)and Fig.6(b).

3.3 Re-dissolution of dried hemicellulose samples

To explore the influence of the different drying methods on the solubility of the dried hemicellulose samples,the samples obtained by the different drying processes were re-dissolved in different solvent systems:water,NMMO/water(1∶1,mass ratio),and NaOH solution(2.5 mol/L).It was observed that all the dried hemicellulose samples could not be re-dissolved in water;however,they were soluble in NMMO/water(Table 1).These observations support the conclusion that the solubility of dried hemicellulose is not affected by the drying method.Interestingly,while the hemicellulose samples obtained by air drying and freeze drying were not soluble in 2.5 mol/L NaOH solution,that obtained by ammonium carbonateassisted drying was soluble in it(Fig.7).This is probably due to the loosely packed hemicellulose structure formed by the NH3and CO2generatedin-situin the ammonium carbonate-assisted drying process.

Table 1 Re-dissolution of hemicellulose samples obtained by different drying methods in different solvents

3.4 Proposed industrial application

The BKP was subjected to alkaline purification using 2.5 mol/L NaOH solution.Hemicellulose was selectively hydrolyzed in this process.The alkaliextracted fibers,which are largely free of hemicellulose,are ready to be used as dissolving pulp[18,22].

The NaOH extraction of hemicellulose from BKP,with the objectives of producing both dissolving pulp and hemicellulose,and the ammonium carbonateassisted drying of the extracted hemicellulose are proposed as an industrial process(Fig.8).

The alkaline filtrate,which contains the extracted hemicellulose,is pre-evaporated to about 10% pulp consistency and then mixed with ammonium carbonate.The mixture is then dried by the ammonium carbonateassisted method under vacuum.The resultant dried hemicellulose(xylan)will be either sold in the market or converted to other valued-added products such as furfural.NH3and CO2generated will be recombined to form ammonium carbonate,which can be recycled/reused for the process.

The proposed process is straightforward and has several advantages as mentioned below:

(1)The proposed method involves lower equipment investment compared to the traditional dissolving pulp process.

(2)There is minimal or no effluent discharge.

(3)Both dissolving pulp and hemicellulose will be marketable products,and the overall process concept fits into the aspect of biorefinery.

4 Conclusions

Bamboo kraft pulp was subjected to an alkali extraction process,with the objective of producing dissolving pulp and hemicellulose.The resultant hemicellulose was dried using an ammonium carbonate-assisted drying process,with the optimal conditions being 80℃and 1∶1(mass ratio)of ammonium carbonate and hemicellulose.An industrial process concept was proposed,in which the NH3and CO2generated during the drying process could be recycled and reused.The structure of the dried hemicellulose obtained by the ammonium carbonate-assisted process is similar to that of the original hemicellulose,as revealed by detailed Fourier transform infrared(FT-IR)and X-ray diffraction(XRD)analyses.The dried hemicellulose can be used as a renewable raw material for the preparation of hydrogels in different applications including tissue engineering as well as for the synthesis of other substances such as bioethanol/biochemicals and biodegradable films.The alkali-extracted pulp fibers can be used as dissolving pulp.

Acknowledgments

The authors are grateful to the National Key Research and Development Program of China(2017YFB0307900,2019YFC1905903),the National Natural Science Foundation of China(31700507),and the Major Project on the Integration of Industry-Education-Research of Fujian Provincial Department of Science and Technology(2018H6003).