# One flow through hydrolysis and hydrogenation of semi-industrial xylan from birch (betula pendula) in a continuous reactor-Kinetics and modelling

Xiaojia Lu, Paula Junghans, Stephanie Weckesser, Johan Wärnå, Gerd

Hilpmann, Rüdiger Lange, Heather Trajano, Kari Eränen, Lionel Estel, Sébastien Leveneur, et al.

► To cite this version:

Xiaojia Lu, Paula Junghans, Stephanie Weckesser, Johan Wärnå, Gerd Hilpmann, et al. One flow through hydrolysis and hydrogenation of semi-industrial xylan from birch (betula pendula) in a continuous reactor-Kinetics and modelling. Chemical Engineering and Processing: Process Intensification, 2021, 169, pp.108614. 10.1016/j.cep.2021.108614. hal-03498066

HAL Id: hal-03498066

https://hal.science/hal-03498066v1

Submitted on 15 May 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# One flow through hydrolysis and hydrogenation of semi-industrial xylan from birch (betula pendula) in a continuous reactor—Kinetics and modelling

Xiaojia Lu<sup>a,b</sup>, Paula Junghans<sup>c</sup>, Stephanie Weckesser<sup>c</sup>, Johan Wärnå<sup>a</sup>, Gerd Hilpmann<sup>c</sup>, Rüdiger Lange<sup>c</sup>, Heather Trajano<sup>d</sup>, Kari Eränen<sup>a</sup>, Lionel Estel<sup>b</sup>, Sebastien Leveneur<sup>a,b</sup>, Henrik Grénman<sup>a,*</sup>

<figure><img src="image_3.png" /></figure>

Check for
updates

## ARTICLE INFO

### Keywords:

Xylan

Combined hydrolysis and hydrogenation

Continuous reactor

Kinetics

Increased selectivity

Process intensification

## ABSTRACT

Xylan obtained from birch (betula pendula) by a novel semi-industrial scale aqueous based method was used for studying the hydrolysis and consecutive hydrolysis-hydrogenation processes in continuous reactors. Dowex 50WX2-100 was chosen as the hydrolysis catalyst based on the results of catalyst screening performed previously in batch reactor. It was also observed to perform well in continuous reactor converting xylan to xylose in high yield under the studied reaction conditions. The influence of several reaction parameters were investigated for optimization. Similar experimental conditions used in the hydrolysis were then applied for studying one flow through hydrolysis and hydrogenation of the semi-industrial xylan. A consecutive catalyst bed consisting of ruthenium on carbon was introduced into the continuous reactor downstream from the hydrolysis bed to hydrogenate monosaccharides to xylitol. Hydrogen was co-fed into the reactor with the xylan solution. Reaction parameters, including temperature, residence time and hydrogen pressure, were varied to maximize the xylitol yield. The developed continuous process was demonstrated to be highly selective and efficient for the valorization of the semi-industrial xylan by yielding over 90% xylitol under optimal experimental conditions. It was noticed, that co-feeding hydrogenation decreased the degradation of monosaccharides during hydrolysis, thus improving the selectivity towards the target product and enabling remarkable process intensification. Moreover, mathematical modelling was performed for the hydrolysis and one flow through hydrolysis and hydrogenation processes. The models take into account the consecutive reaction pathways and the influence of the experimental conditions. Good fits of the model to the experimental data were obtained. The conversion of this novel, well characterized wood-based xylan to produce xylose or xylitol in continuous reactors has not been studied previously. The current work contributes significantly to understanding the processing of real feedstock in one flow through employing consecutive reactions and provides necessary data for process intensification.

## 1. Introduction

Platform chemicals, such as sugars, sugar alcohols and sugar acids can be obtained from hemicelluloses deriving from biomass. Hemicellulose is one of the main components in lignocellulosic biomass, which constitutes 25-35% of the total mass [1]. Large quantities of hemicellulose are produced as side product in industry every year, however, it remains a relatively underutilized resource [2,3]. The most important hardwood hemicellulose is xylan, especially O-acetyl-4-O-methylglucuronoxylan, amounting to about 80-90% of the hardwood

hemicelluloses [4]. Xylan is a heterogeneous linear heteropolysaccharide that mainly consists of xylose in addition to other C5 and C6 sugar units. The xylose units are connected by β-(1-4)-D-xylopyranose bonds. The side groups are glucuronic acids and some other sugars such as galactose, rhamnose, and mannose. Xylose is the main monosaccharide obtainable from xylan via hydrolysis and it can be utilized as an important platform molecule, which can be valorised to e.g. xylitol via hydrogenation [5,6], to furfural via dehydration [6–9] or to xylonic acid through oxidation [10–12]. These compounds can be utilized to a wide variety of bio-chemicals to be used in e.g. paper, alimentary, cosmetic and pharmaceutical industry [13–17].

<footnote><sup>a</sup> Laboratory of Industrial Chemistry and Reaction Engineering, Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Biskopsgatan 8, 20500 Turku, Finland</footnote>

<footnote><sup>b</sup> Normandie University, INSA Rouen, UNIROUEN, LSPC, EA4704, 76000 Rouen, France</footnote>

<footnote><sup>c</sup> Technische Universität Dresden, Institut für Verfahrens- und Umwelttechnik, Professur für Chemische Verfahrens- und Anlagentechnik, Dresden, Germany</footnote>

<footnote><sup>d</sup> The University of British Columbia, Chemical and Biological Engineering, Vancouver Campus, 2360 East Mall, Vancouver, BC Canada V6T 1Z3</footnote>

<footnote>* Corresponding author.</footnote>
<table><thead><tr><th>Notation</th><th></th></tr></thead><tbody><tr><td>k</td><td>Conversion dependent pre-exponential factor (min<sup>-1</sup>)</td></tr><tr><td>k<sub>0</sub></td><td>Pre-exponential factor (min<sup>-1</sup>)</td></tr><tr><td>c<sub>xylan</sub></td><td>Concentrations of xylan (g/L)</td></tr><tr><td>c<sub>xylose</sub></td><td>Concentrations of xylose (g/L)</td></tr><tr><td>c<sub>degr.</sub></td><td>Concentrations of xylose degradation products (g/L)</td></tr><tr><td>c<sub>xylitol</sub></td><td>Concentrations of xylitol (g/L)</td></tr><tr><td>c<sub>H+</sub></td><td>Concentration of H<sup>+</sup> (mol/L)</td></tr><tr><td>P<sub>H2</sub></td><td>Pressure of hydrogen (Pa)</td></tr><tr><td>k<sub>01/02</sub></td><td>Pre-exponential factors (L mol<sup>-1</sup> min<sup>-1</sup>)</td></tr><tr><td>E<sub>a</sub></td><td>Apparent activation energy (kJ/mol)</td></tr><tr><td>R</td><td>Gas constant (J mol<sup>-1</sup> K<sup>-1</sup>)</td></tr><tr><td>θ</td><td>Modified temperature</td></tr><tr><td>T</td><td>Temperature (K)</td></tr><tr><td>T<sub>ref</sub></td><td>Reference temperature (K)</td></tr><tr><td>Q</td><td>Sum of residual squares</td></tr></tbody></table><table><tbody><tr><td>c<sub>exp</sub></td><td>Concentration measured experimentally (g/L)</td></tr><tr><td>c<sub>est</sub></td><td>Concentration estimated by modelling (g/L)</td></tr><tr><td>R<sup>2</sup></td><td>Degree of explanation</td></tr><tr><td>V</td><td>Volume (mL)</td></tr><tr><td>V̇</td><td>Volumetric flow rate (mL/min)</td></tr><tr><td>t<sub>c 1/2</sub></td><td>Half life</td></tr><tr><td>RTD</td><td>Residence time distribution</td></tr><tr><td>Pe</td><td>Peclet number</td></tr><tr><td>u</td><td>Superficial velocity (m/s)</td></tr><tr><td>L</td><td>Length of the reactor (m)</td></tr><tr><td>D<sub>z</sub></td><td>Axial dispersion coefficient (m<sup>2</sup>/s)</td></tr><tr><td>Q</td><td>Volumetric flow rate (m<sup>3</sup>/s)</td></tr><tr><td>a</td><td>Cross-sectional area (m<sup>2</sup>)</td></tr><tr><td>ε<sub>v</sub></td><td>Void fraction</td></tr><tr><td>r</td><td>Reaction rate (mol L<sup>-1</sup> min<sup>-1</sup>)</td></tr><tr><td>τ</td><td>Residence time (min)</td></tr></tbody></table>

The kinetics and mechanism of hydrolysis of the semi-industrial xylan was previously investigated in a batch reactor utilizing selected commercial heterogeneous solid catalysts [18]. The aim of the current work is to utilize the information gained in the batch reactor experiments and enable the hydrolysis of the hemicelluloses in a continuous reactor. A continuous packed bed reactor offers many advantages compared with homogeneous batch systems: no need for further separation, continuous operation, low corrosion and efficient temperature control. The aim was to achieve high yield of xylose and to study, if the severe degradation i.e. dehydration of the monosaccharides can be at least partly avoided by utilizing a continuous reactor and optimizing the residence time.

A second aim was to study the feasibility of producing xylitol from the semi-industrial xylan in a continuous reactor setup by a single pass through. The major challenge for such operations is typically; that the two different steps and catalysts require different reaction conditions e. g. temperature and that the kinetics of the reactions differ so much that single pass through is not possible, especially if further consecutive degradation reactions are likely. Based on our previous research on xylan hydrolysis [18] and xylose hydrogenation reactions in batch reactors, the operating conditions and stability of the catalysts and the reaction kinetics could allow single pass through operation, by introducing a second consecutive catalyst bed for the hydrogenation after the hydrolysis step and by co-feeding hydrogen to the reactor already from the inlet. A hypothesis was also that the reductive conditions could help to prevent the undesired dehydration of the sugars, as the presence of oxygen has been observed in previous studies to promote degradation [19].

In a hydrolysis process, the glycosidic bond is cleaved by protonation and water addition [20]. The polymer is stepwise cleaved into smaller molecules, resulting in a mixture of oligomers with varying degree of polymerization (DP). The oligomers can be further transformed into monomers via either thermal, physio-thermal, micro-biological or chemical cleavage. In addition, a further dehydration to furfural is possible under harsher conditions [7-9]. The formation of the covalent bond between the oxygen atom of the water and the carbon of the polysaccharide is the rate-determining step in the reaction, which can be accelerated with a suitable catalyst. The hydrolysis can be carried out using homogeneous [21-24] or heterogeneous [25] catalysts. High yields of sugar monomers can be achieved with homogeneous catalysts such as mineral acids, however, it usually brings safety risk to both environment and workers, and it is challenging and expensive to separate the products [26]. Heterogeneous catalysts can be utilized to overcome such challenges as they are easy to recycle, which greatly reduces costs and makes them more attractive for industrial processes.

However, heterogeneous catalysts do not typically provide equally fast kinetics as e.g. mass transfer limitations may be present, which affects the obtained overall reaction rates and results in a less efficient hydrolysis. Therefore, the selection of catalyst is essential as their characteristics such as acidity, surface area, particle size, porosity and degree of crosslinking vary greatly, which influences their performance.

In a typical chemical catalytic hydrogenation of xylose, a heterogeneous catalyst is usually employed with the hydrogen gas, avoiding the use of stoichiometric reducing agents and the subsequent salts forming [27]. The key to the process often lies in selecting the catalyst to obtain high activity and selectivity towards xylitol while leading to less deactivation by the fouling species. A variety of base and noble metal-based catalysts was investigated. Raney nickel [28] has been a popular choice for a while due to its lower price, ease of use, good activity and selectivity. However, fast deactivation can always be observed, which consequently leads to slower reaction kinetics and weaker selectivity towards xylitol production. Numerous catalytic systems based on noble metals, such as Pt, Pd and Ru, was then tested, among which ruthenium based catalysts have shown much less deactivation, being considered an ideal alternative to Ni based catalysts for hydrogenation processes in recent years [29,30].

Hemicellulose extracted from Finnish silver birch (betula pendula) was selected as the raw material in the current work, as birch is one of the major tree species in Finland and other boreal areas, it contains high amounts of xylan and it does not compete with food production. The current work focuses on utilizing a novel, modified aqueous extraction based xylan from silver birch, which is currently being produced in pilot scale (500 kg/d) [31,32]. The novel semi-industrial process fractionates efficiently in an almost closed loop system the hemicellulose and the lignin fraction from cellulose fibers, making it possible to utilize the whole biomass for further valorization. This is consistent with the requirements of modern sustainable biorefineries and complies with the principles of green chemistry and engineering. The extracted xylan has advantages such as being sulfur free, having a low lignin content and relatively high molar mass and low content of degradation products. These characteristics all contribute to the processing of xylan to platform chemicals and high-value products. The xylan was used as obtained from the semi-industrial process, without further purification or separation. It is crucial to study also real feedstock instead of highly purified typically very deacetylated hemicelluloses model compounds.

Commercial heterogeneous catalysts were chosen based on the previous studies and references, and different reaction conditions were tested to achieve selective and sufficient conversion of this special xylan material. The goal was to achieve high yields of desired products, i.e., xylose and xylitol from the two developed processes, respectively, and as
<figure><img src="image_4.png" /><figcaption>Fig. 1. Reactor set-up for xylan hydrolysis.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 2. Schematic diagram of continuous reactor for consecutive hydrolysis-hydrogenation of xylan.</figcaption></figure>

few by-products as possible. Moreover, mathematical modelling were also developed to quantify the influence of reaction parameters on the reaction rates.

## 2. Experimental section

### 2.1. Materials and chemicals

Semi-industrial xylan, originating from Finnish silver birch was used in the current work. The xylan was extracted by a spinoff company with a process developed at Åbo Akademi University. First, the polysaccharides were isolated from the fibrous wood structure with a heated aqueous phase at 150 °C and a pressure below 8 bars. Then, nearly all of the oxygen present in the reactor was removed to minimize oxidation and degradation of hemicelluloses. The extraction was performed under continuous recirculation to increase the concentration of the hemicelluloses in the liquid phase.

The same batch of xylan with a dry content of 38.8% and an average molar mass of 4.649 kg/mol was used in all experiments. The degree of depolymerization was estimated to be around 31 assuming an average monomer molar mass of 150.13 g/mol and 100 % xylose in the substrate. The sugar composition was analysed by GC and xylose was the main component of the substrate, making up 79.3 % of the total mass.

Dowex 50WX2-100 was used as the acidic hydrolysis catalyst. It is a styrene-divinylbenzene (gel), with a 78% moisture content when supplied, a 50-100 mesh particle size, maximum operating temperature of 150 °C and 4.8 meq/g of protons. It is very stable under the investigated conditions while only 2.7% of sulfonic groups were observed to be leached from the catalyst [18].

0.7% Ru/C purchased from Engelhard (Italiana S.p.A., Italy) was employed for the hydrogenation of xylose to produce xylitol. The particle size of the extruded catalyst was between 355-500 µm.

### 2.2. Reactor setup

#### 2.2.1. Hydrolysis experiments

A laboratory scale tubular stainless steel reactor (Fig. 1) was used with up-stream flow to guarantee a good liquid distribution inside the column. The prepared xylan solution (S) was pumped into the reactor (R1) with an Eldex dualpiston pump (P1) at certain flow rates to obtain desired residence times. A three-way valve (V1) was mounted at the bottom of the reactor to keep the liquid inside in case of disuse or catalyst replacement. Two external heaters and an aluminium jacket were utilized to distribute the heat evenly in the reactor. The temperature was measured inside the reactor at its top (T101) and bottom (T102). To keep the temperature constant, a temperature sensor (TIC01) was placed inside the reactor to control the electronic heaters. After leaving the reactor, the solution was cooled down with water in a heat exchanger (H1) and dispensed into a 200 mL container (C1). A three-way valve (V2) was installed in between to take samples. The pressure was kept constant with an external valve (V3) while taking samples.

The total reactor volume ($V_{reactor}$) was about 24 mL and the free volume in the catalyst bed ($V_{liquid}$) after filling was determined to be 12 mL. The reactor was filled with quartz and catalyst bed, which consisted of a mixture of quartz and catalyst in order to avoid too low local pH and to obtain an even distribution of liquid flow. The glass wool were placed at the ends of the reactor. The dilution of the catalyst bed was tested in order to see if efficient hydrolysis could be obtained while still avoiding severe dehydration as the local pH was higher in the diluted bed even though the catalyst amount stayed the same. The ratio between quartz and catalyst was determined by the required pH and the free liquid volume of the reactor. In some experiments, the catalyst amount was kept constant, but the degree of dilution was varied. Therefore, the length of the catalyst bed varied according to the desired pH. Two different local pH (0.3 and 0.5) were tested in the current work. The catalyst bed lengths to the corresponding pH were 8.1 cm and 13.0 cm.

A substrate concentration of 2 g/L was mainly utilized in the experiments. A few experiments were also conducted with 20 g/L in order to investigate the influence of higher concentration on the catalyst performance.
<table><caption>**Table 1**
Experimental matrix.</caption><thead><tr><th>Temp. [°C]</th><th>pH</th><th>Conc. Xylan [g/L]</th><th>Conc. Xylose [g/L]</th><th>Residence times [min]</th></tr></thead><tbody><tr><td>145</td><td>0.5</td><td>1.939</td><td>1.538</td><td>10, 15, 20, 25</td></tr><tr><td>145</td><td>0.75</td><td>1.939</td><td>1.538</td><td>10, 20, 30, 40, 50</td></tr><tr><td>137</td><td>0.2</td><td>1.939</td><td>1.538</td><td>20, 30, 40</td></tr><tr><td>137</td><td>0.5</td><td>1.939</td><td>1.538</td><td>10, 15, 25, 35</td></tr><tr><td>137</td><td>0.75</td><td>1.939</td><td>1.538</td><td>10, 15, 20, 30, 40, 50</td></tr><tr><td>130</td><td>0.2</td><td>1.939</td><td>1.538</td><td>20, 30, 40, 50, 60</td></tr><tr><td>130</td><td>0.5</td><td>1.939</td><td>1.538</td><td>20</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** The xylose concentration as a function of residence time for experiments performed at 137 °C and different pH.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** The xylose concentration as a function of residence time for experiments performed at 145 °C and different pH.</figcaption></figure>

While heating the reactor, water was pumped through it. The water was replaced by the prepared xylan solution upon reaching the desired temperature. The first sample was taken after four times the residence time and the outlet was flushed before sampling to eliminate the influence of previous samples. The samples were filtered with 0.45 µm PVDF filters before analysis. Additional samples were taken to check whether steady state was achieved and the sampling interval was set according to the reaction conditions. During the whole experiment, the temperature was kept constant and monitored. Each experiment lasted for at least 6 times the residence time. After the experiment, the reactor was flushed with water for several hours while being cooled down. The samples were quantitatively analysed by liquid and gas chromatography.

### 2.2.2. Consecutive hydrolysis-hydrogenation of xylan

A tubular stainless steel reactor with smaller diameter was applied to the consecutive hydrolysis-hydrogenation study in order to lower the amount of catalyst needed in one experiment. The reactants, i.e. xylan solution and hydrogen were both fed from the top of the reactor resulting in top-down concurrent operation. The liquid and gas passed first through the acidic hydrolysis catalyst bed (Dowex 50WX2-100) followed by the hydrogenation catalyst bed (0.7% Ru/C) to the sampling at the end of the reactor. A schematic representation of the reactor configuration is depicted in Fig. 2. The pressure was kept constant during the experiments. The samples were filtered with 0.45 µm PVDF filters prior to analysis.

## 2.3. Analysis

### 2.3.1. HPLC

Two different HPLC methods were employed for the quantitative analysis of samples from xylan valorization experiments. (A) A VWR Hitachi Chromaster HPLC equipped with a 5450 refractive index detector (RI). A Bio-Rad Aminex HPX-87C column with a diameter of 300 mm and a length of 7.8 mm was used. The eluent (1.2 mmol/L CaSO₄ solution) was pumped into the column at a flow rate of 0.5 mL/min and a constant temperature of 80 °C. (B) A Hewlett Packard series 1100 HPLC was also employed equipped with a Bio-Rad Aminex HPX-87H column and a RI detector. The analysis was performed at 45 °C under isocratic conditions and a flow rate of 0.6 mL/min. The mobile phase consisted of 5 mmol/L H₂SO₄ solution. Samples were filtered with 0.45 µm PVDF filters before injection. The calibration curves were determined by preparing 5 calibration samples in a concentration range of 0.31-5 g/L with pure compounds.

### 2.3.2. GC-FID

An Agilent J&W GC, model PeriknEmler Clarus 500, equipped with a 7.2 m × 0.530 mm column was employed to analyse the hydrolysis samples. The carrier gas was hydrogen with a flow rate of 7 mL/min. The injection volume was 0.5 µl. The injector had an initial temperature of 80 °C. After 0.1 min of injection, the temperature was increased by 50 °C/min up to 110 °C and afterwards 15 °C/min to 330 °C. This temperature was stable for 7 min. The oven was heated at 100 °C for 0.5 min and the temperature was then increased at 12 °C/min up to 340 °C for 5 min. An FID detector was used with 45 mL/min hydrogen and 450 mL/min air at 340 °C. The samples were freeze dried overnight and diluted to 1 g/L with distilled water. Cholesterol was added as the internal standard. The solutions were then mixedt with the silylation reagent (Pyridine:N,O-Bis(trimethylsilyl)trifluoroacetamide:Trimethylsilyl chloride = 1:4:1) prior to injection.

## 3. Hydrolysis in continuous reactor

The influence of experimental conditions on the reaction rates was studied by employing different temperatures and pH in the continuous reactor while keeping the substrate concentration mainly constant. The substrate concentration was increased in a few experiments in order to see the influence of catalyst concentration of the conversion, but results showed, that with concentrations up to 20 g/L, very small differences were observed. This is most probably due to the fact that the catalyst influences the local pH in close vicinity of the catalyst and, thus, act as a “pseudo-homogeneous” catalyst. Rather low substrate concentrations were mainly used due to experimental and analytical convenience. Moreover, the mean residence time was varied. The temperature and pH range as well as the range of residence times were planned based on previous data obtained in batch reactor results [18]. The experimental matrix is presented in Table 1.
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** The xylose concentration as a function of residence time for experiments performed at pH 0.75 and different temperature.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** The xylose concentration as a function of residence time for experiments performed at pH 0.5 and different temperature.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Influence of temperature on the mass distribution of products of experiments performed with 2 g/L solution at pH 0.3 and a residence time of 12.3 min.</figcaption></figure>

### 3.1. Influence of pH

The influence of pH on the reaction rate at 137 °C and 145 °C is displayed in Figs. 3 and 4. It is demonstrated that pH has a clear influence on the reaction kinetics. The results show that at lower residence times, lower pH results in higher xylose yields as the hydrolysis rate is increased. However, at longer residence times, the yield of xylose decreases with increasing rate when the pH is lower, as the consecutive dehydration of the monosaccharides increases. At a higher pH, however, the concentration of xylose gradually increases with the prolongation of

retention time while little degradation is observed. It seems logical that a close to linear dependence of the xylose concentration (yield) as a function of residence time was obtained under investigated conditions before the severe degradation started to take place. This is consistent with the batch reactor results [18], even though the dehydration at higher residence times did not lead to equally advanced dehydration as in the batch reactor at corresponding experimental time. Moreover, by simply comparing the experimental data shown in the two figures, a preliminary conclusion can be drawn that higher temperature promotes the reaction kinetics, whereas the kinetics of side reaction is also accelerated. The degradation of sugar products can be clearly observed at shorter retention times, already at 20 min, when 145 °C was employed.

### 3.2. Influence of temperature

The influence of temperature on the reaction kinetics at pH 0.75 and 0.5 is displayed in Figs. 5 and 6, respectively. The results in Fig. 5 show that the concentration of xylose increases when prolonging the residence time while keeping the temperature constant at 137 °C, whereas at higher temperature i.e. 145 °C, the concentration is observed to increase with residence time in the beginning, followed by a significant decrease when the residence time exceeds 20 min. Consequently, at a pH of 0.75 at 137 °C and a residence time of 50 min, 0.92 g/L of xylose was obtained, while at 145 °C a lower xylose yield of 0.74 g/L was reached. A similar trend can be observed in the experimental result obtained at pH 0.5 (Fig. 6) as at 145 °C with lower flow rates i.e. higher mean residence times, dehydration of the monosaccharides was more profound and higher yields were obtained at lower residence times. This is also in compliance with the previously obtained batch reactor results [18], but again the sugar degradation was not as profound at corresponding residence time compared to batch reactor experiments at the same experimental time. Moreover, a significant difference in the reaction rate was observed when increasing the temperature from 137 to 145 °C, which clearly demonstrates a rather high activation energy for the hydrolysis reaction. Given this, it is evident that optimization of the reaction temperature is very crucial in the acidic hydrolysis process. A general conclusion can be drawn that high yields of monosaccharides can be obtained at high temperature and low pH, however, the residence time must be kept short in order to avoid degradation of desired sugar products.

Fig. 7 showed the mass distribution of compounds determined in the products at three different temperatures at a low pH of 0.3. The results show that when the temperature exceeded 140 °C, the amount of degradation products as well as some condensation products that are detected as xylo-oligomers increased significantly. At 145 °C, the degradation of xylose is already severe.

### 3.3. Influence of local pH

The influence of local pH was investigated using 3.6 g of catalyst and diluting it into different degree (pH 0.3 and 0.5) and performing experiments at 135-145 °C. The results are displayed in Fig. 8. The xylan conversion were generally higher at pH 0.3 whereas the xylose yield was greater at pH 0.5 in all experiments. This indicates that pH 0.3 was too harsh and severe dehydration of the sugars occurred. The highest xylose yield of around 91 % was achieved at a temperature of 140 °C, a residence time of 20 min and a pH of 0.5.

### 3.4. Residence time distribution

In order to evaluate the residence time distribution (RTD) and thus investigate the flow regime in the reactor with the current packing, step experiments were performed with sodium chloride as the tracer. The response at the outlet of the reactor was analysed by measuring the conductivity of the solution. The reactor was first flushed with distilled
<figure><img src="image_11.png" /><figcaption>Fig. 8. Influence of pH on the conversion of xylan (left) and yield of xylose (right) of experiments performed with 2 g/L solution at different temperatures and residence times.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 9. Conductivity as a function of time on stream in a step response experiment. The flow rate corresponded to a mean residence time of 20 min. The pumping the different liquid was started at zero min.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 10. Measured data compared to calculated data by modelling.</figcaption></figure>

water and then the input liquid was changed to a sodium chloride water mixture. Three different concentration were tested, 1 g/L, 2 g/L and 4 g/L. The conductivity was measured directly after the reactor, at least every 10 s. All the experiments were carried out with a flow rate corresponding to a mean residence time of 20 min. The step response is displayed in Fig. 9.

The results showed (Fig. 9), that about one and a half hour was

required until the reactor was filled with the salt solution and the output concentration stabilized. However, this does not represent only the reactor volume, but also the piping before the reactor inlet. Assuming laminar flow, the volume of the reactor and piping from the substrate container to the outlet of the reactor could be estimated taking into account the time point, when half of the concentration change had occurred (mean value for the three experiments was 4023 s) and the flow rate 0.594 mL/min.
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Hydrogenation of 25 g/L xylose to xylitol with Ru/C catalyst. Reaction conditions: 120 °C, 20 bar H₂ (total pressure), 1400 rpm.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Influence of temperature on the xylose and xylitol yields in experiments performed at a flow rate of 0.1 mL/min under 20 bar H₂.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 13.** Influence of flow rate on the xylose and xylitol yields in experiments performed at 140 °C under 20 bar H₂.</figcaption></figure>

$$V_{\text{reactor system}} = t_c^{1/2} * \dot{V} \qquad (1)$$

$$V_{\text{whole}} = \frac{0.594 \text{ mL}}{60 \text{ s}} * 4023 \text{ s} = 39.83 \text{ mL} \qquad (2)$$

It can be concluded, that the volume of the entire reactor system before the outlet was about 40 mL. Since the outlet responses were very symmetrical, it was assumed that no dead zones were present in the reactor. In order to gain a better understanding of the flow inside the reactor, the Peclet number was calculated according to the formula:

<figure><img src="image_17.png" /><figcaption>**Fig. 14.** Influence of hydrogen pressure on the xylose and xylitol yields in experiments performed at 140 °C and a flow rate of 0.1 mL/min.</figcaption></figure>

<table><caption>**Table 2**

Parameters after the final modelling approach.</caption><thead><tr><th>Parameter</th><th>Value</th><th></th><th>Est. relative standard error %</th></tr></thead><tbody><tr><td>k<sub>1</sub></td><td>0.138</td><td>L/(g•min)</td><td>4.3</td></tr><tr><td>k<sub>2</sub></td><td>0.0382</td><td>L/(g•min)</td><td>8.1</td></tr><tr><td>E<sub>a1</sub></td><td>205</td><td>kJ/mol</td><td>6.0</td></tr><tr><td>E<sub>a2</sub></td><td>97.1</td><td>kJ/mol</td><td>19.6</td></tr></tbody></table>

<figure><img src="image_18.png" /><figcaption>**Fig. 15.** Fit of the model (y-axis) to the experimental data points (x-axis) of the concentration of xylose.</figcaption></figure>

$$Pe = \frac{uL}{D_z} \qquad (3)$$

where *u* was the superficial velocity, *L* the length of the reactor and *D*<sub>z</sub> the axial dispersion coefficient. The superficial velocity was calculated based on

$$u = \frac{Q}{a\varepsilon_v} \qquad (4)$$

where *Q* was defined as flowrate, *a* was the cross-sectional area and *ε*<sub>v</sub> was the void fraction with the following formula.

$$\varepsilon_v = \frac{V_{liq}}{V_{total}} \qquad (5)$$

The axial dispersion coefficient (*D*<sub>z</sub>) was calculated based on the step response function (6)
<figure><img src="image_19.png" /><figcaption>Fig. 16. Contour plots of the correlation of the parameters.</figcaption></figure>

<table><caption>**Table 3**

Parameters after the final modelling approach.</caption><thead><tr><th>Parameter</th><th>Value</th><th></th><th>Est. relative standard error %</th></tr></thead><tbody><tr><td>k<sub>1</sub></td><td>0.101</td><td>L/(mol·min)</td><td>13.1</td></tr><tr><td>k<sub>3</sub></td><td>0.0157</td><td>1/(min·Pa)</td><td>10.9</td></tr><tr><td>E<sub>a1</sub></td><td>68.3</td><td>kJ/mol</td><td>27.5</td></tr><tr><td>E<sub>a3</sub></td><td>43.7</td><td>kJ/mol</td><td>71.2</td></tr></tbody></table>

<figure><img src="image_20.png" /><figcaption>Fig. 17. Fit of the model (y-axis) to the experimental data points (x-axis) of the concentrations of xylose and xylitol.</figcaption></figure>

$$F(t) = \frac{1}{2} \left( 1 - \text{erf} \left( \frac{L - ut}{(4D_{\text{ct}})^{\frac{1}{2}}} \right) \right) \qquad (6)$$

Since, F(t) can only obtain values between 0 and 1, the measured conductivities were divided by the maximal conductivity in the relative experiment and an average response was calculated based on the three measurements. The volume of the empty reactor was 24 mL and displacement measurements revealed that after packing, the liquid volume was 12 mL. Taking into account the volume of the entire reactor system including the piping as well as the liquid volume inside the reactor, it was possible to determine the volume of the piping before the reactor inlet. The time required for the tracer liquid to pass through the piping was subtracted from the tracer experiment, in order to estimate

the residence time distribution in the reactor. The F(t) function (Eq. 6) was fitted to the experimental data by adjusting the value of the axial dispersion coefficient. The fit is displayed in Fig. 10.

The minimal possible $\sum (\Delta F(t))^2$ with adjusting were 12.257 with a $D_z$ of $1.059*10^{-6}$.

Consequently, the Peclet number could be calculated

$$Pe = \frac{1.06753*10^{-4m_s*0.12 m}}{1.059*10^{-6m_s^2}} = 12.1 \qquad (7)$$

A Peclet number of 12 shows, that significant turbulence was present inside the reactor despite the rather low flow rate, caused by the packing.

## 4. Consecutive hydrolysis-hydrogenation of semi-industrial hemicellulose to sugar alcohols

Previous studies on the hydrolysis of the semi-industrial xylan [18] and hydrogenation (Fig. 11) of xylose indicated the possibility of performing a consecutive hydrolysis and hydrogenation in one flow through in a continuous reactor to obtain xylitol directly from hemicellulose, as the optimal operation temperatures and kinetics of the two reactions were very similar. Therefore, the feasibility of the process was examined by performing a set of experiments using Dowex 50WX2-100 as the hydrolysing catalyst in the first catalyst bed and ruthenium (0.7%) on activated carbon for the hydrogenation in the second catalyst bed. Based on the previous measurement of the continuous reactor used for the hydrolysis of xylan, the free liquid volume accounted for about 50% of the entire reactor after being filled with the catalyst and quartz. This value was also utilized to calculate the retention times at different liquid flow rates when performing the consecutive hydrolysis-hydrogenation experiments.

### 4.1. Influence of temperature

The influence of temperature on the yields of main products (xylose and xylitol) was investigated. The hydrogen pressure was kept constant at 20 bar during the whole process and a liquid flow rate of 0.1 mL/min was employed to ensure sufficient contact time between the reactants and the catalyst beds. At this flow rate, the total residence time in the reactor was 36.3 min, of which the residence times in the hydrolysis and hydrogenation catalyst beds were calculated to be 11.2 min and 5.5 min, respectively.

As shown in Fig. 12, the yield of xylitol increased from 77% to 90%
<figure><img src="image_21.png" /><figcaption>Fig. 18. Contour plots of the correlation of the parameters.</figcaption></figure>

by increasing the temperature from 130 °C to 140 °C, which indicated that elevated temperatures promoted the hydrolysis and hydrogenation significantly. Moreover, less monosaccharide degradation i.e. dehydration was observed compared to the previously presented hydrolysis experiments where oxygen was present in the reaction mixture. It seems plausible, that the reductive atmosphere and rapid hydrogenation of the sugars after hydrolysis influenced the selectivity of the process and decreased the amount of unwanted dehydration. Almost all of the xylose was hydrogenated as only very low concentrations, a few per cent, of xylose was detected in the liquid product The high yield of xylitol also confirmed that the substrate was effectively hydrolysed to xylose under the selected experimental conditions.

## 4.2. Influence of flow rate

The influence of flow rate on the xylose and xylitol yields was studied in order to evaluate how the mean residence time influences the hydrolysis and hydrogenation kinetics. The results are shown in Fig. 13. A significant decrease in the xylitol yield was observed, only 31% xylitol was detected in the final product when the highest flow rate of 0.4 mL/min, which corresponded to a total residence time of 9.075 min (2.8 min for hydrolysis and 1.375 min for hydrogenation), was employed. The xylan conversion also decreased as the flow rate increased fourfold, leaving 36% of the substrate unreacted. It can be concluded, that the residence time greatly affected the kinetics of both hydrolysis of xylan and hydrogenation of xylose. On the other hand, more xylose was produced by increasing the flow rate, indicating that the hydrogenation step was more sensitive to the change in flow rate.

## 4.3. Influence of hydrogen pressure

Higher hydrogen pressures were also tested in order to see if the yield of xylose could be further enhanced by increasing the hydrogen concentration in liquid phase. A flow rate of 0.2 mL/min, corresponding to a total residence time of 18.15 min, of which 5.6 min were rationed for hydrolysis and 2.75 min hydrogenation, was applied to lower the yield and make the influence more visible. As shown in Fig. 14, no gain could be obtained by increasing the hydrogen pressure, which demonstrated that the catalyst surface was already saturated with hydrogen at a pressure of 20 bars.

# 5. Mathematical modelling

## 5.1. Hydrolysis

The mathematical model used for the kinetics of acid-catalysed hydrolysis of xylan was based on the data previously obtained in batch reactor experiments [18]. The Peclet number in the current experiments was surprisingly high (~12), which indicates rather turbulent conditions due to the mixing effect of the reactor packing, despite the low flow rates. Moreover, the diffusion rate of the bulky xylan molecules is slower than that of the salt used as a tracer, which leads to the hemicellulose solution having in practise an even higher Peclet number than the obtained 12. Even though the flow cannot be considered as complete plug flow, based on the rather high Peclet number and for the sake of simplicity, the mean residence time was used in the mathematical modelling. The model is based on the monomers formed, and thus, the formation of oligomers was neglected, even though dimers, trimers and tetramers were formed as intermediates based on previous GC analysis. Moreover, the dehydration step is included in the model.

The overall reaction can be described by a two-step consecutive reaction pathway

$$Xylan + \frac{1}{2}H_2O \xrightarrow[\text{H}^+]{r_1} C_5H_{10}O_5 \xrightarrow[\text{H}^+]{r_2} C_5H_4O_2 + 3H_2O$$

The reactions were considered irreversible and first order with respect to the reactants and the proton concentration according to the reaction mechanism and the concentration of water was assumed to be abundant and constant, so it was excluded from the model. Hence the reaction rates could be described as follows:

$$r_1 = k_1 c_{H^+} c_{xylan} \qquad (8)$$

$$r_2 = k_2 c_{H^+} c_{xylose} \qquad (9)$$

The mass balance for each compound in the reaction liquid was then described as:

$$\frac{dc_{xylan}}{d\tau} = -r_1 = -k_1 c_{H^+} c_{xylan} \qquad (10)$$

$$\frac{dc_{xylose}}{d\tau} = r_1 - r_2 = k_1 c_{H^+} c_{xylan} - k_2 c_{H^+} c_{xylose} \qquad (11)$$

$$\frac{dc_{degr.}}{d\tau} = r_2 = k_2 c_{H^+} c_{xylose} \qquad (12)$$
where $c_{H^+}$ is the concentration of the protons that can be calculated based on the number of sulfonic acid sites in the catalyst.

A modified form of the Arrhenius equation was used to decouple the correlation of the pre-exponential factor and the activation energy.

$$k_0 = k_{01}\exp\left(-\frac{E_a}{R\theta}\right). \qquad (13)$$

The parameter $k_{01}$ is the pre-exponential factor, $E_a$ the activation energy and R the universal gas constant. The temperature was transformed to:

$$\frac{1}{\theta} = \frac{1}{T} - \frac{1}{T_{\text{ref}}}. \qquad (14)$$

where the reference temperature $T_{\text{ref}}$ was 137 °C.

The overall mass balance was then expressed as:

$$\frac{dc_{xylan}}{d\tau} = -k_{01}\exp\left(-\frac{E_{a1}}{R\theta}\right)c_{H^+}c_{xylan} \qquad (15)$$

$$\frac{dc_{xylose}}{d\tau} = k_{01}\exp\left(-\frac{E_{a1}}{R\theta}\right)c_{H^+}c_{xylan} - k_{02}\exp\left(-\frac{E_{a2}}{R\theta}\right)c_{H^+}c_{xylose}. \qquad (16)$$

$$\frac{dc_{degr.}}{d\tau} = k_{02}\exp\left(-\frac{E_{a2}}{R\theta}\right)c_{H^+}c_{xylose} \qquad (17)$$

For the kinetic parameter estimation, a non-linear regression analysis that included parts of the experimental data was utilized. The model consisting of a system of ordinary differential equations (ODEs) was solved with the backward difference method implemented in MODEST software.

The sum of residual squares Q was calculated according to

$$Q = \| c_{exp} - c_{est} \|^2 \qquad (18)$$

The objective function Q was then minimized by the Simplex-Levenberg-Marquard method, where $c_{exp}$ represents the concentration obtained in the experiment and $c_{est}$ the concentration calculated based on the model.

The parameters obtained in the modelling are presented in Table 2. As the degradation of xylose occurs simultaneously with hydrolysis of xylan, it makes the accurate determination of the activation energy of the degradation more difficult, which most probably explains the higher estimated relative standard error of $E_{a2}$.

The fit of the model to the experimental data can be seen in Fig. 15.

Fig. 16 shows the Monte Carlo plots of the objective function. An almost circular shape with a clearly defined minimum value in the middle of the circular plot was obtained. The circular shape indicated a low or even negligible correlation between the parameters. The clear minima shows that the parameters are well defined with the developed model.

## 5.2. Hydrolysis followed by hydrogenation

In the modelling approach chosen for the combined hydrolysis and hydrogenation of xylose to xylitol, the hydrolysis step was calculated according to equation presented in the previous chapter, but excluding the degradation of the monosaccharides, as the results showed that the reductive conditions prevented efficiently the dehydration. Concentrations were calculated as mol/L to take into account the stoichiometry in the consecutive reactions. When considering irreversible hydrolysis and hydrogenation steps, the overall reaction scheme becomes, where the hydrogenation is denoted as $r_3$, as $r_2$ was used previously for the dehydration.

$$\text{Xylan} + \frac{1}{2}\text{H}_2\text{O} \underset{H^+}{\overset{r_1}{\rightleftharpoons}} \text{C}_5\text{H}_{10}\text{O}_5 + \text{H}_2 \underset{H^+}{\overset{r_3}{\rightleftharpoons}} \text{C}_5\text{H}_{12}\text{O}_5$$

The xylose concentration from the hydrolysis catalyst bed, the

catalyst mass, and pressure of hydrogen were incorporated in the hydrogenation step. The adsorption of xylose and hydrogen were also taken into account in the model. The overall rate equations used in the model are

$$\frac{dc_{xylose}}{d\tau} = r_1 - r_3 = k_1c_{H^+}c_{xylan} - \frac{k_3c_{xylose}p_{H2}}{1 + K_{xylose}c_{xylose} + K_{H2}p_{H2}}. \qquad (19)$$

$$\frac{dc_{xylitol}}{d\tau} = r_3 = \frac{k_3c_{xylose}p_{H2}}{1 + K_{xylose}c_{xylose} + K_{H2}p_{H2}} \qquad (20)$$

The inclusion of adsorption to the model did not improve the model fit nor provide any statistical advantage, so the model could be further simplified coefficients did not result in any statistical relevance in the

$$\frac{dc_{xylitol}}{d\tau} = r_3 = k_3c_{xylose}p_{H2} \qquad (21)$$

The parameters obtained in the modelling are presented in Table 3.

The fit of the model (R²=98.7%) to the experimental data can be seen in Fig. 17.

The contour plots (Fig. 18) shows that the correlation of the parameters was low and that they are well defined.

The average degree of depolymerisation was estimated based on the kinetics via the stoichiometry observed in the analysis. The estimated average DP became 24, which is somewhat lower than the one obtained by SEC-MALS analysis. The standard error of the estimated DP was, however, very low being only 4.3%.

## 6. Conclusions

A continuous process was first developed for the acidic hydrolysis of semi-industrial xylan. The catalyst and preliminary reaction conditions were chosen based on results obtained in batch reactor and optimized to find a compromise between the reaction kinetics of hydrolysis and unwanted dehydration, to obtain the highest possible xylose yield. The highest yield of over 90% was achieved at 140 °C, pH 0.5 and a residence time of 20 min. A one flow through hydrolysis and hydrogenation process was then developed for producing xylitol from the semi-industrial xylan by installing a Ru/C catalyst bed downstream from the hydrolysis bed. An elevated temperature and a low liquid flow rate was observed to be beneficial for obtaining high yield and selectivity. The highest yield of over 90% was achieved at 140 °C with 20 bar H₂. The results demonstrated that the reductive conditions and rapid hydrogenation of the monosaccharides prevented undesired dehydration of the sugars. The one flow through reactor system enabled significant process intensification, as only one reactor was required and higher selectivity and yield towards xylitol was obtained compared to performing the two reaction steps separately. Moreover, mathematical modelling was successfully performed on the reaction kinetics for both hydrolysis and the consecutive hydrolysis-hydrogenation processes.

### CRediT authorship contribution statement

Xiaojia Lu: Investigation, Methodology, Writing – original draft.

Paula Junghans: Investigation. Stephanie Weckesser: Investigation.

Johan Wärnå: Software, Formal analysis. Gerd Hilpmann: Methodology. Rüdiger Lange: Methodology. Heather Trajano: Formal analysis.

Kari Eränen: Investigation. Lionel Estel: Methodology, Supervision.

Sebastien Leveneur: Conceptualization, Methodology, Writing – review & editing, Supervision, Project administration, Funding acquisition.

Henrik Grènman: Conceptualization, Methodology, Writing – review & editing, Supervision, Project administration, Funding acquisition.

### Declaration of Competing Interest

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.

Acknowledgement

The current study has been performed in the framework of the AMED
project financed with the support from the European Union within the
European Regional Development Fund (ERDF) and from the Regional
Council of Normandie. The China Scholarship Council: Cooperation
Program with the UTs (Universités de technologie) and INSAs (Institut
National des Sciences Appliquées) (France) is gratefully acknowledged
for financial support as is Åbo Akademi University and its strategic
profiling area Technologies for a Sustainable Future. Additionally, the
authors are grateful for CH-Bioforce for supplying the hemicellulose.
Support from Academy of Finland (310652) is gratefully acknowledged.

References

[1] X. Kang, Y. Wang, S. Wang, X. Song, Xylan and xylose decomposition during hot water pre-extraction: A pH-regulated hydrolysis, Carbohydr. Polym. 255 (2021), 117391.

[2] J. Krogell, K. Eränen, K. Granholm, A. Pranovich, S. Willför, High-temperature pH measuring during hot-extraction of hemicelluloses from wood, Ind. Crops Prod. 61 (2014) 9–15.

[3] W. Farhat, R. Venditti, A. Quick, M. Taha, N. Mignard, F. Becquart, A. Ayoub, Hemicellulose extraction and characterization for applications in paper coatings and adhesives, Ind. Crops Prod. 107 (2017) 370–377.

[4] byand I. Spiridon, V.I. Popa, M.N Belgacem, A. Gandhini, Hemicelluloses?: Major Sources, Properties and Application. Monomers, Polymers and Composites from Renewable Resources, Elsevier, Amsterdam, 2008, pp. 289–304. byand.

[5] K.K. Cheng, J.A. Zhang, E. Chavez, J.P. Li, Integrated production of xylitol and ethanol using corncob, Appl. Microbiol. Biotechnol. 87 (2010) 411–417.

[6] V.A. Sifontes, D. Rivero, J. Wärnå, J.P. Mikkola, T. Salmi, Sugar hydrogenation over supported Ru/C—kinetics and physical properties, Top. Catal. 53 (2010) 1278–1281.

[7] M. Cuevas, J. García, G. Hodaifa, S. Sánchez, Oligosaccharides and sugars production from olvie stones by autohydrolysis and enzymatic hydrolysis, Ind. Crops Prod. 70 (2015) 100–106.

[8] H. Li, J. Ren, L. Zhong, R. Sun, L. Liang, Production of furfural from xylose, water-insoluble hemicelluloses and water-soluble fraction of corncob via a tin-loaded montmorillonite solid acid catalyst, Bioresour. Technol. 176 (2015) 242–248.

[9] F. Delbecq, Y. Wang, A. Muralidhara, K. El Ouardi, G. Marlair, C. Len, Hydrolysis of hemicellulose and derivatives—a review of recent advances in the production of furfural, Front. Chem. 6 (2018) 146–174.

[10] H. Zhang, X. Han, C. Wei, J. Bao, Oxidative production of xylonic acid using xylose in distillation stillage of cellulosic ethanol fermentation broth by Gluconobacter oxydans, Bioresour. Technol. 224 (2017) 573–580.

[11] E.Y. Li, M.S. Feather, The conversion of D-Xylose to Xylitol and D-Xylonic acid by a bovine lens preparation, J. Carbohydr. Chem. 13 (1994) 499–505.

[12] T. Suzuki, H. Onishi, Oxidation and reduction of D-Xylose by cell-free extract of pichia quercuum, Appl. Microbiol. 25 (1973) 850–852.

[13] O. Akpinar, K. Erdogan, S. Bostanci, Production of xylooligosaccharides by controlled acid hydrolysis of lignocellulosic materials, Carbohydr. Res. 344 (2009) 660–666.

[14] byand R. Katzen, D.J. Schell, B. Kamm, P.R. Gruber, M. Kamm, Lignocellulosic feedstock biorefinery: history and plant development for biomass hydrolysis, Biorefineries-Industrial Processes and Products, Wiley-VCH Verlag GmbH & Co., Weinheim, 2006, pp. 129–138. byand.

[15] J.P. Lange, E.V.D. Heide, J.V. Buijtenen, R. Price, Furfural-A promising platform for pignocellulosic biofuels, ChemSusChem 5 (2012) 150–166.

[16] J.J. Bozell, G.R. Petersen, Technology development for the production of biobased products from biorefinery carbohydrates—the US Department of Energy’s “Top 10” revisited, Green Chem. 12 (2010) 539–554.

[17] M. Li, X. Meng, E. Diao, F. Du, Xylitol production by Candida tropicalis from corn cob hemicellulose hydrolysate in a two-stage fed-batch fermentation process, J. Chem. Technol. Biotechnol. 87 (2012) 387–392.

[18] X. Lu, P. Junghans, J. Wärnå, G. Hilpmann, R. Lange, H. Trajano, K. Eränen, L. Estel, S. Leveneur, H. Grénman, Hydrolysis of semi-industrial xylan from birch (*betula pendula*) employing commercial catalysts – kinetics and modelling, J. Chem. Technol. Biotechnol. (under review) (2021).

[19] A.S. Schmidt, A.B. Thomsen, Optimization of wet oxidation pretreatment of wheat straw, Bioresour. Technol. 64 (1998) 139–151.

[20] A. Sharples, The hydrolysis of cellulose and its relation to structure, Trans. Faraday Soc. 53 (1957) 1003–1013.

[21] A.P. Nebreda, V. Russo, M. Di Serio, T. Salmi, H. Grenman, Modelling of homogeneously catalyzed hemicelluloses hydrolysis in a laminar-flow reactor, Chem. Eng. Sci. 195 (2019) 758–766.

[22] A.P. Nebreda, H. Grénman, P. Mäki-Arvela, K. Eränen, J. Hemming, S. Willför, D. Y. Murzin, T. Salmi, Acid hydrolysis of O-acetyl-galactoglucomannan in a continuous tube reactor: a new approach to sugar monomer production, Holzforschung 70 (2016) 187–194.

[23] R. Zhang, X. Lu, Y. Sun, X. Wang, S. Zhang, Modelling and optimization of dilute nitric acid hydrolysis on corn stover, J. Chem. Technol. Biotechnol. 86 (2011) 306–314.

[24] M. Li, X. Yang, T. Lu, L. Zhou, Selective hydrolysis of hemicellulose component of wheat straw in high-pressure CO₂ and water with low concentration of acetic acid, J. Chem. Technol. Biotechnol. 95 (2020) 2237–2242.

[25] L. Vilcocq, P.C. Castilho, F. Carvalheiro, L.C. Duarte, Hydrolysis of oligosaccharides over solid acid catalysts: a review, ChemSusChem 7 (2014) 1010–1019.

[26] R. Ormsby, J.R. Kastner, J. Miller, Hemicellulose hydrolysis using solid acid catalysts generated from biochar, Catal. Today 190 (2012) 89–97.

[27] A.P. Tathod, P.L. Dhepe, Towards efficient synthesis of sugar alcohols from mono- and poly-saccharides: role of metals, supports & promoters, Green Chem. 16 (2014) 4944–4954.

[28] M. Herskowitz, Modelling of a trickle-bed reactor—the hydrogenation of xylose to xylitol, Chem. Eng. Sci. 40 (1985) 1309–1311.

[29] H. Ling, K. Cheng, J. Ge, W. Ping, Statistical optimization of xylitol production from corncob hemicellulose hydrolysate by *Candida tropicalis* HDY-02, Nat. Biotechnol. 28 (2011) 673–678.

[30] W.Y. Jeon, B.H. Yoon, W.Y. Shim, J.H. Kim, Reduction of arabitol in xylitol production by changed substrate preference of xylose reductase in *Candida tropicalis*, J. Biotechnol. 150 (2010) 345–346.

[31] S. Von Schoultz, Method for extracting biomass, WO2014009604A1 (2014).

[32] S. Von Schoultz, Method for extracting lignin, WO2015104460A1 (2015).