Reductive Catalytic Fractionation of Flax Shive over Ru/C Catalysts

Flax shive is the main waste (up to 70 wt %) in the production of flax fiber. It represents the lignified parts of the flax stem mainly in the form of small straws. Complex processing of such wastes is a significant problem due to the heterogeneity of the chemical structure of lignin. This article presents the results of reductive catalytic fractionation (RCF) of flax shive in ethanol and isopropanol at elevated temperatures (225–250 °C) in the presence of a bifunctional catalyst (Ru/C) and molecular hydrogen. This provides solvolytic depolymerization of lignin and hemicelluloses presented in flax shive. Catalytic hydrogenation effectively stabilizes the formed lignin intermediates and prevents repolymerization reactions producing the lignin fraction with a high degree of depolymerization. RCF of flax shive produces solid products with a high cellulose content and liquid products consisting mainly of monophenolic compounds. Furthermore, the effect of different characteristics (the ruthenium content, particle size, and support acidity) of the bifunctional catalysts containing ruthenium nanoparticles supported on mesoporous, graphite-like carbon material Sibunit®-4 on the yield and composition of the products of hydrogenation of flax shive in sub- and super-critical ethanol has been studied. Bifunctional catalysts Ru/C used in the RCF of flax shive increase its conversion from 44 to 56 wt % and the yield of monophenols from 1.1 to 10.2 wt % (based on the weight of lignin in the sample). Using the best Ru/C catalyst containing 3% of Ru on oxidized at 400 °C carbon support, the high degree of delignification (up to 79.0%), cellulose yield (up to 67.2 wt %), and monophenols yield (up to 9.5 wt %) have been obtained.


Introduction
The depletion of fossil resources and CO 2 emissions, negatively affecting the environment, require new methods for use of renewable plant raw materials, including agricultural waste, to ensure environmental safety and social and economic sustainability [1].
Flax shive is the nonfibrous fraction of flax straw. It has a highly lignified structure and represents an agricultural waste widespread in Europe and North America. The flax shive composition includes lignin (~25%), cellulose (50%), and hemicelluloses (20%) [2,3]. Lignin is one of the three main (20-30 wt %) polymeric components of plant biomass. It is formed by biochemical oxidation of the coniferyl, sinapic, and p-coumaric alcohols and consists mainly of the corresponding phenylpropane units [4]. The lignin of herbaceous plants contains all three structural units. herbaceous biomass has the potential of becoming the largest renewable source of bioproducts and biofuels. Lignin is conventionally extracted

Catalysts Preparation and Characterization
In early works [22][23][24], the reductive catalytic fractionation of woody biomass with powder Ru/C catalysts based on graphite-like mesoporous carbon Sibunit@-4 (S4) was considered. In this work, a number of catalyst characteristics were varied, including temperature of the support oxidation, the ruthenium content and the fractional composition of the catalyst (granular or powder).
Firstly, four carbon supports were prepared based on S4: initial powder S4, this powder oxidized at 400 and 450 • C, granular S4 oxidized at 500 • C. The supports were labeled as: S4, S400, S450, S500 g, respectively (Table 1). Varying the oxidation temperature allows us to obtain different amounts of oxidized centers on the surface of the carbon which can affect not only the acidic properties, but also the distribution of active component. The granular support was used for the estimation of diffusion limits. Secondly, Ru-containing catalysts with 3 wt % of Ru were prepared based on the obtained supports (Tables 1 and 2). Catalysts were labeled 3RSg, 3RS500g, 3RS400, 3RS450, respectively. In order to decrease the amount of the noble metal we also obtained the catalyst with 1 wt % of Ru (1RS450) based on S450 support. surface area SBET, pore volume Vpore, and average pore size dpore, probably due to partial destruction of the graphite-like structure of the Sibunit-4 CS during oxidation and blocking of some support pores with particles of the active component due to its precipitation (Table 1).
(a) (b)   1 рНpzc is the рН of the point of zero charge. 2 1 wt % Ru catalyst.
An increase in the temperature of the oxidative treatment of the CS also results in a decrease in the pHpzc value, which apparently results from an increase in the concentration of the surface acid species [25]. The deposition of ruthenium is accompanied by a slight increase in the pHpzc values (Table 1) obviously due to deactivation of the acid sites as a result of Ru particle deposition.
The statistical analysis of TEM images was performed to investigate Ru particles distribution in relation to Ru loadings (1 and 3 wt %) and catalyst support acidity ( Table 2).  Table 2. Ru particle sizes and dispersion in the prepared catalysts (results of statistical processing of the TEM data, Figure 1). 1 d min and d max -the minimum and maximum particle diameters, <d l > = Σd i /N-the mean linear particle size, <d S > = Σd i 3 / Σd i 2 -the mean volume-surface particle size, D Ru -the dispersion of Ru 2 1.0-1.6 mm fraction. 3 56-94 µm fraction. Texture properties, morphology and acid properties of the supports and catalysts were studied using N 2 adsorption (Table 1), transmission electron microscopy (TEM) (Table 2, Figure 1) and acid titration ( Table 2). An increase in the temperature of the oxidative treatment of the carbon support (CS) and the deposition of ruthenium reduced the specific surface area S BET , pore volume V pore , and average pore size d pore , probably due to partial destruction of the graphite-like structure of the Sibunit-4 CS during oxidation and blocking of some support pores with particles of the active component due to its precipitation (Table 1).

Support
An increase in the temperature of the oxidative treatment of the CS also results in a decrease in the pH pzc value, which apparently results from an increase in the concentration of the surface acid species [25]. The deposition of ruthenium is accompanied by a slight increase in the pH pzc values (Table 1) obviously due to deactivation of the acid sites as a result of Ru particle deposition.
The statistical analysis of TEM images was performed to investigate Ru particles distribution in relation to Ru loadings (1 and 3 wt %) and catalyst support acidity ( Table 2). All the catalysts based on the oxidized supports have a smaller ruthenium particle size and a narrower pore size distribution as compared with the initial support (Tables 1 and 2, Figure 1). The average ruthenium particle diameter decreases in the series 3RSg >3RS500g >3RS400 >3RS450 >1RS450 (Table 2). This can be explained by the interaction of negatively charged surface oxidized centers, the number of which increases with temperature of the support oxidative treatment, with ruthenium ions. The distribution of ruthenium particles on granular supports is less uniform.
Ru loading as low as 1 wt % gives a better Ru distribution in comparison to 3 wt % catalysts providing smaller ruthenium particle size (Table 2, Figure 1). Ru dispersion in the 1RS450 catalyst, appeared to be higher than 1 (Table 2). Taking into account the error in the mean particle diameter measurements, a conclusion may be made, that for 1RS450 catalyst all ruthenium atoms are located on the surface and fully available for the reaction.
The X-ray photoelectron spectroscopy (XPS) was used for a quantitative and qualitative analysis of the obtained catalysts (Table 3, Figure 2). The XPS shows that for all the samples, the most intense peaks are observed in the range of 284-285 eV, which is typical of graphite. In the range of 285-287 eV, there are peaks attributed to carbonyl, hydroxyl, and ether groups, as well as fragments of carboxyl groups. The Ru 3d 5/2 peak observed near 281 eV is typical for oxidized ruthenium RuO 2 . Metallic ruthenium is characterized by a peak near 280 eV; however, this shift toward higher binding energies can also be explained by the relaxation shift observed for very fine metallic particles [26,27].

Catalytic hydrogenation of Flax Shive in Ethanol
The choice of the process temperature, duration and hydrogen pressure is based on the analysis of literature [11,19,24,28]. The results of experiments on hydrogenation of flax shive in ethanol without and with the different Ru/C catalysts are given in Table 4. During hydrogenation of flax shive in subcritical ethanol, the catalysts significantly intensify the biomass conversion (from 44 to 56 wt %) and increase the yield of liquids (from 38 to 44 wt %) and gases (from 5.6 to 16 wt %). While using the catalysts, the yield of monophenols grows significantly (from 1.5 to 3.5-5.6 wt %) ( Table 4). These characteristics are improved due to the depolymerization of native lignin in the presence of the catalysts, which accelerates the cleavage of the C-O and C-C bonds [29].
The maximum yield of monophenols (5.6 wt % per shive and 10.2 wt % per lignin), which are the most valuable products, is obtained using the catalyst oxidized at 450 • C and containing 3 wt % of Ru. The reductive catalytic fractionation of flax shive with the use of the carbon support (Sib-4) yields 2.24 wt % of monophenols. When using the granular catalysts (runs 4 and 6), the monophenol yield does not exceed 4.4 wt %. It should be noted that the use of the granular 3% Ru/C catalyst on CS oxidized at 500 • C enhances the gas yield up to 16.5 wt %.
Thus, the highest efficiency of the catalytic process is attained by the 3RS450 characterized by the maximum specific surface area and porosity (Table 1) and the minimum ruthenium particle size (Table 2). The catalysts change the CO 2 yield of the process insignificantly, but fundamentally increase the CO and CH 4 yields, from trace amounts to 2-7 wt % per shive weight ( Table 5). The catalysts on the support oxidized at 450 • C is most active in the shive conversion (Table 5) and gives the highest CO and CH 4 yields. The growth of the CO yield is caused by the intensification of decarbonylation of the Gibbert ketones [30][31][32][33] formed on the catalyst metal sites. Methane is formed mainly at hydrocracking of the aliphatic structural fragments of lignin [34,35]. Increase the CO and CO 2 yields in the presence of catalyst indicates the catalytic deoxygenation of the lignin structural fragments.  Table 6 shows the data on the composition of the solid residue of the flax shive hydrogenation process. The highest (79.5 wt %) cellulose and the lowest (15.5 wt %) lignin content in the solid product were obtained with the most active 3RS450 catalyst. The comparison of the data given in Tables 3-5 shows that 3RS450 catalyst both destroys the ether bonds of the lignin structure and partially prevents destruction of the cellulose polymer chain. It should be noted that addition of oxidized Sibunit without ruthenium reduces the hemicellulose yield, whereas ruthenium reached catalysts increase it ( Table 6). The decomposition of hemicelluloses into soluble products can be caused by the acidic properties of the carbon support, and deposition of ruthenium onto Sibunit reduces its acidity (Table 1).
It should be emphasized that the use of the 3RS400 catalyst leads to the formation of a solid product with the highest (67.2 wt %) cellulose yield at high degree of delignification (79.0%). Increasing the process temperature up to 250 • C leads to a higher degree of delignification (up to 93%) and stronger depolymerization of cellulose, the yield of which drops to 40 wt %.
Despite the obviously higher hydrogen donating activity of isopropanol as compared to ethanol, the use of this secondary alcohol as a solvent reduces the hydrogenation efficiency: The degree of delignification decreases to 55% from 83% for ethanol and the cellulose yield increases to 74% from 52% in ethanol. This can be caused by several factors, for example, the isopropanol amount used in the experiment (0.8 mol) is less than the ethanol amount (1.05 mol). This leads to the weaker extraction of lignin from the flax shive and, therefore, retards its depolymerization.
Thus, the maximum degree of delignification (up to 83%) is obtained with the 3RS450 catalyst used in the reductive catalytic fractionation of flax shive in the subcritical ethanol medium. however, the use of the 3RS400 catalyst ensures the higher (up to 67 wt %) cellulose yield at the comparable degree of delignification (79 wt %).

Characterystics of the Liquid Products of Reductive Catalytic Fractionation of Flax Shive
GC-MS analysis of the liquid product composition of the flax shive hydrogenation showed that the products of lignin conversion are mainly guaiacyl derivatives and, to a lesser extent, syringyl derivatives, since flax shive have a high content of guaiacyl-type fragments [36]. The main products are guaiacylpropanol (1), guaiacylpropene (2), and guaiacylpropane (3) (Tables 7-9). fragments [36]. The main products are guaiacylpropanol (1), guaiacylpropene (2), and guaiacylpropane (3) (Tables 7-9). In addition, other aromatic compounds with the carbonyl and ester groups were identified. Their total yield ranged from 0.5 to 2 wt %, depending on the process conditions. Along with the methoxyphenol derivatives, the liquid products contained compounds formed from biomass polysaccharides: furfural derivatives (furfuryl alcohol and difurfuryl ether), methyl esters of the oxy-and hydroxy acids, ethyl levulinate, alcohols, and ketones (1-hydroxy-2-propanone and 3-hydroxy-2-butanone) with yields of 0. 6 fragments [36]. The main products are guaiacylpropanol (1), guaiacylpropene (2), and guaiacylpropane (3) (Tables 7-9). In addition, other aromatic compounds with the carbonyl and ester groups were identified. Their total yield ranged from 0.5 to 2 wt %, depending on the process conditions. Along with the methoxyphenol derivatives, the liquid products contained compounds formed from biomass polysaccharides: furfural derivatives (furfuryl alcohol and difurfuryl ether), methyl esters of the oxy-and hydroxy acids, ethyl levulinate, alcohols, and ketones (1-hydroxy-2-propanone and 3-hydroxy-2-butanone) with yields of 0.6-2.3  In addition, other aromatic compounds with the carbonyl and ester groups were identified. Their total yield ranged from 0.5 to 2 wt %, depending on the process conditions. Along with the methoxyphenol derivatives, the liquid products contained compounds formed from biomass polysaccharides: furfural derivatives (furfuryl alcohol and difurfuryl ether), methyl esters of the oxy-and hydroxy acids, ethyl levulinate, alcohols, and ketones (1-hydroxy-2-propanone and 3-hydroxy-2-butanone) with yields of 0. 6 fragments [36]. The main products are guaiacylpropanol (1), guaiacylpropene (2), and guaiacylpropane (3) (Tables 7-9). In addition, other aromatic compounds with the carbonyl and ester groups were identified. Their total yield ranged from 0.5 to 2 wt %, depending on the process conditions. Along with the methoxyphenol derivatives, the liquid products contained compounds formed from biomass polysaccharides: furfural derivatives (furfuryl alcohol and difurfuryl ether), methyl esters of the oxy-and hydroxy acids, ethyl levulinate, alcohols, and ketones (1-hydroxy-2-propanone and 3-hydroxy-2-butanone) with yields of 0.6-2.3 fragments [36]. The main products are guaiacylpropanol (1), guaiacylpropene (2), and guaiacylpropane (3) (Tables 7-9). In addition, other aromatic compounds with the carbonyl and ester groups were identified. Their total yield ranged from 0.5 to 2 wt %, depending on the process conditions. Along with the methoxyphenol derivatives, the liquid products contained compounds formed from biomass polysaccharides: furfural derivatives (furfuryl alcohol and difurfuryl ether), methyl esters of the oxy-and hydroxy acids, ethyl levulinate, alcohols, and ketones (1-hydroxy-2-propanone and 3-hydroxy-2-butanone) with yields of 0. 6 fragments [36]. The main products are guaiacylpropanol (1), guaiacylpropene (2), and guaiacylpropane (3) (Tables 7-9). In addition, other aromatic compounds with the carbonyl and ester groups were identified. Their total yield ranged from 0.5 to 2 wt %, depending on the process conditions. Along with the methoxyphenol derivatives, the liquid products contained compounds formed from biomass polysaccharides: furfural derivatives (furfuryl alcohol and difurfuryl ether), methyl esters of the oxy-and hydroxy acids, ethyl levulinate, alcohols, and ketones (1-hydroxy-2-propanone and 3-hydroxy-2-butanone) with yields of 0. 6 fragments [36]. The main products are guaiacylpropanol (1), guaiacylpropene (2), and guaiacylpropane (3) (Tables 7-9). In addition, other aromatic compounds with the carbonyl and ester groups were identified. Their total yield ranged from 0.5 to 2 wt %, depending on the process conditions. Along with the methoxyphenol derivatives, the liquid products contained compounds formed from biomass polysaccharides: furfural derivatives (furfuryl alcohol and difurfuryl ether), methyl esters of the oxy-and hydroxy acids, ethyl levulinate, alcohols, and ketones (1-hydroxy-2-propanone and 3-hydroxy-2-butanone) with yields of 0.6-2.3 wt %. The liquid products contained also ethyl esters of fatty acids (palmitic, stearic, and fragments [36]. The main products are guaiacylpropanol (1), guaiacylpropene (2), and guaiacylpropane (3) (Tables 7-9). In addition, other aromatic compounds with the carbonyl and ester groups were identified. Their total yield ranged from 0.5 to 2 wt %, depending on the process conditions. Along with the methoxyphenol derivatives, the liquid products contained compounds formed from biomass polysaccharides: furfural derivatives (furfuryl alcohol and difurfuryl ether), methyl esters of the oxy-and hydroxy acids, ethyl levulinate, alcohols, and ketones (1-hydroxy-2-propanone and 3-hydroxy-2-butanone) with yields of 0.6-2.3 wt %. The liquid products contained also ethyl esters of fatty acids (palmitic, stearic, and  fragments [36]. The main products are guaiacylpropanol (1), guaiacylpropene (2), and guaiacylpropane (3) (Tables 7-9). In addition, other aromatic compounds with the carbonyl and ester groups were identified. Their total yield ranged from 0.5 to 2 wt %, depending on the process conditions. Along with the methoxyphenol derivatives, the liquid products contained compounds formed from biomass polysaccharides: furfural derivatives (furfuryl alcohol and difurfuryl ether), methyl esters of the oxy-and hydroxy acids, ethyl levulinate, alcohols, and ketones (1-hydroxy-2-propanone and 3-hydroxy-2-butanone) with yields of 0.6-2.3 wt %. The liquid products contained also ethyl esters of fatty acids (palmitic, stearic, and fragments [36]. The main products are guaiacylpropanol (1), guaiacylpropene (2), and guaiacylpropane (3) (Tables 7-9). In addition, other aromatic compounds with the carbonyl and ester groups were identified. Their total yield ranged from 0.5 to 2 wt %, depending on the process conditions. Along with the methoxyphenol derivatives, the liquid products contained compounds formed from biomass polysaccharides: furfural derivatives (furfuryl alcohol and difurfuryl ether), methyl esters of the oxy-and hydroxy acids, ethyl levulinate, alcohols, and ketones (1-hydroxy-2-propanone and 3-hydroxy-2-butanone) with yields of 0. 6 Figure 3. Yields of the methoxyphenols (1), (2), (3) for different catalysts used.  Figure 3. Yields of the methoxyphenols (1), (2), (3) for different catalysts used.  Figure 3. Yields of the methoxyphenols (1), (2), (3) for different catalysts used.  Figure 3. Yields of the methoxyphenols (1), (2), (3) for different catalysts used.  Figure 3. Yields of the methoxyphenols (1), (2), (3) for different catalysts used.  Figure 3. Yields of the methoxyphenols (1), (2), (3) for different catalysts used. The catalysts differ not only in the ruthenium particle size, but also in acidity of the support. Liu et al. showed [37] that, in the process occurring with the Ni@ZIF-8 catalyst at 260 °C for 8 h, monolignols are hydrogenated first along the C=C bond with the formation of substituted propanols, acid-catalyzed dehydration of the latter, and hydrogenation of the resulting propenes to phenylpropanes (Figure 4). The presence of Lewis acid sites, e.g., Zn and Fe, on the catalyst surface is shown to shift the selectivity of product formation from guaiacylpropanol to guaiacylpropane [31,38]. The results presented in Figure 3 allow us comparing the two catalysts, 3RS400 and 3RS450, with different oxidation temperatures and, correspondingly, different acidities. The maximum yield of compound (1) (2.9 wt %) is obtained on the less acidic 3RS400 catalyst, and the maximum yield of compound (3) (4.9 wt %) is done on the more acidic 3RS450 catalyst. This is probably due to the acidcatalyzed dehydration of guaiacylpropanol to guaiacylpropene; Hence, the similar regularities of these transformations are observed on the Bronsted (Figure 3) and Lewis [35,36] sites.  The catalysts differ not only in the ruthenium particle size, but also in acidity of the support. Liu et al. showed [37] that, in the process occurring with the Ni@ZIF-8 catalyst at 260 °C for 8 h, monolignols are hydrogenated first along the C=C bond with the formation of substituted propanols, acid-catalyzed dehydration of the latter, and hydrogenation of the resulting propenes to phenylpropanes (Figure 4). The presence of Lewis acid sites, e.g., Zn and Fe, on the catalyst surface is shown to shift the selectivity of product formation from guaiacylpropanol to guaiacylpropane [31,38]. The results presented in Figure 3 allow us comparing the two catalysts, 3RS400 and 3RS450, with different oxidation temperatures and, correspondingly, different acidities. The maximum yield of compound (1) (2.9 wt %) is obtained on the less acidic 3RS400 catalyst, and the maximum yield of compound (3) (4.9 wt %) is done on the more acidic 3RS450 catalyst. This is probably due to the acidcatalyzed dehydration of guaiacylpropanol to guaiacylpropene; Hence, the similar regularities of these transformations are observed on the Bronsted (Figure 3) and Lewis [35,36] sites.  The catalysts differ not only in the ruthenium particle size, but also in acidity of the support. Liu et al. showed [37] that, in the process occurring with the Ni@ZIF-8 catalyst at 260 °C for 8 h, monolignols are hydrogenated first along the C=C bond with the formation of substituted propanols, acid-catalyzed dehydration of the latter, and hydrogenation of the resulting propenes to phenylpropanes (Figure 4). The presence of Lewis acid sites, e.g., Zn and Fe, on the catalyst surface is shown to shift the selectivity of product formation from guaiacylpropanol to guaiacylpropane [31,38]. The results presented in Figure 3 allow us comparing the two catalysts, 3RS400 and 3RS450, with different oxidation temperatures and, correspondingly, different acidities. The maximum yield of compound (1) (2.9 wt %) is obtained on the less acidic 3RS400 catalyst, and the maximum yield of compound (3) (4.9 wt %) is done on the more acidic 3RS450 catalyst. This is probably due to the acidcatalyzed dehydration of guaiacylpropanol to guaiacylpropene; Hence, the similar regularities of these transformations are observed on the Bronsted (Figure 3) and Lewis [35,36] sites.   In addition, other aromatic compounds with the carbonyl and ester groups were identified. Their total yield ranged from 0.5 to 2 wt %, depending on the process conditions. Along with the methoxyphenol derivatives, the liquid products contained compounds formed from biomass polysaccharides: furfural derivatives (furfuryl alcohol and difurfuryl ether), methyl esters of the oxy-and hydroxy acids, ethyl levulinate, alcohols, and ketones (1-hydroxy-2-propanone and 3-hydroxy-2-butanone) with yields of 0.6-2.3 wt %. The liquid products contained also ethyl esters of fatty acids (palmitic, stearic, and linoleic). Most of them were obtained without catalysts or using only the support as a catalyst, while in the case of using the ruthenium-containing catalysts, these compounds were found in trace amounts or not found at all.
The use of the ruthenium catalysts in the hydrogenolysis process enhanced the total yield of methoxyphenols in the liquid products by a factor of 1.6-2.3 (Tables 7 and 8). The maximum total yield (9.48 and 10.21 wt %) of methoxyphenols was observed for the 3RS400 and 3RS450 catalysts. When the granular catalysts are used, the monomer yields are twice as low. This is probably due to the slow diffusion of reagents to the inner surface of a catalyst grain in the liquid-phase processes.
Comparing the results of Figure 3 and Tables 2 and 3 shows that the total yield of monomeric methoxyphenols increases with increasing the ruthenium dispersion and Comparing the results of Figure 3 and Tables 2 and 3 shows that the total yield of monomeric methoxyphenols increases with increasing the ruthenium dispersion and availability: 1RS450 catalyst possess highest dispersion and almost the same surface ruthenium concentration compared to 3RS450. A significant increase (by a factor of more than 20) is noted in the guaiacylpropane (3) yield while growing the total monomeric methoxyphenols yield by a factor of 3 due to the increase of the catalysts activity ( Figure 3). This is probably due to the high stability of the guaiacylpropane (3), the final product of hydrogenation, as compared to guaiacylpropene (2) and guaiacylpropanol (1).   A significant increase (by a factor of more than 20) is noted in the guaiacylpropane (3) yield while growing the total monomeric methoxyphenols yield by a factor of 3 due to the increase of the catalysts activity ( Figure 3). This is probably due to the high stability of the guaiacylpropane (3), the final product of hydrogenation, as compared to guaiacylpropene (2) and guaiacylpropanol (1).
The catalysts differ not only in the ruthenium particle size, but also in acidity of the support. Liu et al. showed [37] that, in the process occurring with the Ni@ZIF-8 catalyst at 260 • C for 8 h, monolignols are hydrogenated first along the C=C bond with the formation of substituted propanols, acid-catalyzed dehydration of the latter, and hydrogenation of the resulting propenes to phenylpropanes (Figure 4). The presence of Lewis acid sites, e.g., Zn and Fe, on the catalyst surface is shown to shift the selectivity of product formation from guaiacylpropanol to guaiacylpropane [31,38]. The results presented in Figure 3 allow us comparing the two catalysts, 3RS400 and 3RS450, with different oxidation temperatures and, correspondingly, different acidities. The maximum yield of compound (1) (2.9 wt %) is obtained on the less acidic 3RS400 catalyst, and the maximum yield of compound (3) (4.9 wt %) is done on the more acidic 3RS450 catalyst. This is probably due to the acid-catalyzed dehydration of guaiacylpropanol to guaiacylpropene; hence, the similar regularities of these transformations are observed on the Bronsted (Figure 3) and Lewis [35,36]  The catalysts differ not only in the ruthenium particle size, but also in acidity of the support. Liu et al. showed [37] that, in the process occurring with the Ni@ZIF-8 catalyst at 260 °C for 8 h, monolignols are hydrogenated first along the C=C bond with the formation of substituted propanols, acid-catalyzed dehydration of the latter, and hydrogenation of the resulting propenes to phenylpropanes (Figure 4). The presence of Lewis acid sites, e.g., Zn and Fe, on the catalyst surface is shown to shift the selectivity of product formation from guaiacylpropanol to guaiacylpropane [31,38]. The results presented in Figure 3 allow us comparing the two catalysts, 3RS400 and 3RS450, with different oxidation temperatures and, correspondingly, different acidities. The maximum yield of compound (1) (2.9 wt %) is obtained on the less acidic 3RS400 catalyst, and the maximum yield of compound (3) (4.9 wt %) is done on the more acidic 3RS450 catalyst. This is probably due to the acidcatalyzed dehydration of guaiacylpropanol to guaiacylpropene; Hence, the similar regularities of these transformations are observed on the Bronsted (Figure 3) and Lewis [35,36] sites. In addition, the use of a ruthenium-containing catalyst leads to the formation of mainly propyl-substituted phenolic compounds, while with the use of the Pd/C catalyst, propanol-substituted compounds are the major ones [28,39,40].
The data reported in [37,41] showed that compound (3) can be formed as a result of the catalytic hydrogenation of the C=C bond of compound (2) formed due to the dehydration of compound (1), which is the result of hydrogenation of coniferyl alcohol (Figure 4). Indeed, the most yields of guaiacylpropene (2) were observed for Sibunit-4 and 1RS450 In addition, the use of a ruthenium-containing catalyst leads to the formation of mainly propyl-substituted phenolic compounds, while with the use of the Pd/C catalyst, propanol-substituted compounds are the major ones [28,39,40].
The data reported in [37,41] showed that compound (3) can be formed as a result of the catalytic hydrogenation of the C=C bond of compound (2) formed due to the dehydration of compound (1), which is the result of hydrogenation of coniferyl alcohol (Figure 4). Indeed, the most yields of guaiacylpropene (2) were observed for Sibunit-4 and 1RS450 catalyst (1.9-3.2 wt %), the acid systems with minimum hydrogenating activity.
The liquid products of fractionation of herbaceous plants contain some specific compounds, including ferulic and p-coumaric acid esters [28,40,42]. In the products of flax shive hydrogenation these compounds were not found, and this is apparently due to their almost complete lack or decarboxylation to guaiacylethane, with its maximum content of 0.73 wt % in the presence of the 3RS500g catalyst.

Molecular Weight Distribution of the Liquid Products
The studied ruthenium catalysts significantly affect the molecular weight distribution of the liquid products of flax shive hydrogenation in ethanol ( Figure 5, Table 9). In the absence of a catalyst, the liquids are mainly the products of incomplete depolymerization with the fractions of dimers and small amounts of monomeric compounds. The Ru/C catalysts significantly change the molecular weight distribution profile and, consequently, the qualitative composition of the liquid products. In particular, the region of oligomers with a molecular weight above 1 kDa lacks in the presence of a catalyst; a significant signal intensity shift toward lower molecular weights is observed, which suggests a large amount of monomer fraction in the liquid products. A slight change in the region of dimeric products of catalytic hydrogenation confirms stabilization of the obtained liquids and absence of repolymerization reactions. The maximum depth of lignin depolymerization was obtained with 3RS450 and 3RS400 catalysts.

Influence of Solvent
Hydrogenation of flax shive in isopropyl alcohol in the presence of the 3RS450 catalyst results in the lower yields of monomeric methoxyphenols as compared to the sub-and supercritical ethanol used (Table 10). These results responds to the data [44]: The use of isopropanol also resulted in a lower monomer yields in comparison with such solvents as ethanol, methanol, and ethylene glycol.
quently, the qualitative composition of the liquid products. In particular, the region of oligomers with a molecular weight above 1 kDa lacks in the presence of a catalyst; a significant signal intensity shift toward lower molecular weights is observed, which suggests a large amount of monomer fraction in the liquid products. A slight change in the region of dimeric products of catalytic hydrogenation confirms stabilization of the obtained liquids and absence of repolymerization reactions. The maximum depth of lignin depolymerization was obtained with 3RS450 and 3RS400 catalysts.

Influence of Solvent
Hydrogenation of flax shive in isopropyl alcohol in the presence of the 3RS450 catalyst results in the lower yields of monomeric methoxyphenols as compared to the suband supercritical ethanol used (Table 10). These results responds to the data [44]: The use of isopropanol also resulted in a lower monomer yields in comparison with such solvents as ethanol, methanol, and ethylene glycol. The yields and composition of the major monomers during hydrogenation of flax shive in supercritical ethanol (250 °C, 9.7 wt %) are close to the results obtained in subcritical ethanol (225 °C, 10.2 wt %), except for the higher 4-guaiacylethane yield and almost complete lack of guaiacylpropene (2). A higher total yield (27 wt %) of monomeric compounds was obtained in RCF of miscanthus in methanol with the 5% Ru/C catalyst at 250 °C [28]. These differences can be caused by the nature of the hydrogenating substrates (flax and miscanthus), solvent, and also by the higher ruthenium content in the catalyst [28].
Based on the results obtained, several conclusions may be made. First, temperature is the most powerful parameter that determines conversion of flax shive and gas yield. Second, the catalysts acidity increases the carbohydrates conversion. Third, the best yields of monophenols and guaiacylpropane are obtained on the most active catalysts, 3RS450 and 3RS400, possessing highest ruthenium activity and acidity. The yields and composition of the major monomers during hydrogenation of flax shive in supercritical ethanol (250 °C, 9.7 wt %) are close to the results obtained in subcritical ethanol (225 °C, 10.2 wt %), except for the higher 4-guaiacylethane yield and almost complete lack of guaiacylpropene (2). A higher total yield (27 wt %) of monomeric compounds was obtained in RCF of miscanthus in methanol with the 5% Ru/C catalyst at 250 °C [28]. These differences can be caused by the nature of the hydrogenating substrates (flax and miscanthus), solvent, and also by the higher ruthenium content in the catalyst [28].
Based on the results obtained, several conclusions may be made. First, temperature is the most powerful parameter that determines conversion of flax shive and gas yield. Second, the catalysts acidity increases the carbohydrates conversion. Third, the best yields of monophenols and guaiacylpropane are obtained on the most active catalysts, 3RS450 and 3RS400, possessing highest ruthenium activity and acidity. The yields and composition of the major monomers during hydrogenation of flax shive in supercritical ethanol (250 °C, 9.7 wt %) are close to the results obtained in subcritical ethanol (225 °C, 10.2 wt %), except for the higher 4-guaiacylethane yield and almost complete lack of guaiacylpropene (2). A higher total yield (27 wt %) of monomeric compounds was obtained in RCF of miscanthus in methanol with the 5% Ru/C catalyst at 250 °C [28]. These differences can be caused by the nature of the hydrogenating substrates (flax and miscanthus), solvent, and also by the higher ruthenium content in the catalyst [28].
Based on the results obtained, several conclusions may be made. First, temperature is the most powerful parameter that determines conversion of flax shive and gas yield. Second, the catalysts acidity increases the carbohydrates conversion. Third, the best yields of monophenols and guaiacylpropane are obtained on the most active catalysts, 3RS450 and 3RS400, possessing highest ruthenium activity and acidity. The yields and composition of the major monomers during hydrogenation of flax shive in supercritical ethanol (250 °C, 9.7 wt %) are close to the results obtained in subcritical ethanol (225 °C, 10.2 wt %), except for the higher 4-guaiacylethane yield and almost complete lack of guaiacylpropene (2). A higher total yield (27 wt %) of monomeric compounds was obtained in RCF of miscanthus in methanol with the 5% Ru/C catalyst at 250 °C [28]. These differences can be caused by the nature of the hydrogenating substrates (flax and miscanthus), solvent, and also by the higher ruthenium content in the catalyst [28].
Based on the results obtained, several conclusions may be made. First, temperature is the most powerful parameter that determines conversion of flax shive and gas yield. Second, the catalysts acidity increases the carbohydrates conversion. Third, the best yields of monophenols and guaiacylpropane are obtained on the most active catalysts, 3RS450 and 3RS400, possessing highest ruthenium activity and acidity. The yields and composition of the major monomers during hydrogenation of flax shive in supercritical ethanol (250 °C, 9.7 wt %) are close to the results obtained in subcritical ethanol (225 °C, 10.2 wt %), except for the higher 4-guaiacylethane yield and almost complete lack of guaiacylpropene (2). A higher total yield (27 wt %) of monomeric compounds was obtained in RCF of miscanthus in methanol with the 5% Ru/C catalyst at 250 °C [28]. These differences can be caused by the nature of the hydrogenating substrates (flax and miscanthus), solvent, and also by the higher ruthenium content in the catalyst [28].
Based on the results obtained, several conclusions may be made. First, temperature is the most powerful parameter that determines conversion of flax shive and gas yield. Second, the catalysts acidity increases the carbohydrates conversion. Third, the best yields of monophenols and guaiacylpropane are obtained on the most active catalysts, 3RS450 and 3RS400, possessing highest ruthenium activity and acidity. The yields and composition of the major monomers during hydrogenation of flax shive in supercritical ethanol (250 °C, 9.7 wt %) are close to the results obtained in subcritical ethanol (225 °C, 10.2 wt %), except for the higher 4-guaiacylethane yield and almost complete lack of guaiacylpropene (2). A higher total yield (27 wt %) of monomeric compounds was obtained in RCF of miscanthus in methanol with the 5% Ru/C catalyst at 250 °C [28]. These differences can be caused by the nature of the hydrogenating substrates (flax and miscanthus), solvent, and also by the higher ruthenium content in the catalyst [28].
Based on the results obtained, several conclusions may be made. First, temperature is the most powerful parameter that determines conversion of flax shive and gas yield. Second, the catalysts acidity increases the carbohydrates conversion. Third, the best yields of monophenols and guaiacylpropane are obtained on the most active catalysts, 3RS450 and 3RS400, possessing highest ruthenium activity and acidity. The yields and composition of the major monomers during hydrogenation of flax shive in supercritical ethanol (250 °C, 9.7 wt %) are close to the results obtained in subcritical ethanol (225 °C, 10.2 wt %), except for the higher 4-guaiacylethane yield and almost complete lack of guaiacylpropene (2). A higher total yield (27 wt %) of monomeric compounds was obtained in RCF of miscanthus in methanol with the 5% Ru/C catalyst at 250 °C [28]. These differences can be caused by the nature of the hydrogenating substrates (flax and miscanthus), solvent, and also by the higher ruthenium content in the catalyst [28].
Based on the results obtained, several conclusions may be made. First, temperature is the most powerful parameter that determines conversion of flax shive and gas yield. Second, the catalysts acidity increases the carbohydrates conversion. Third, the best yields of monophenols and guaiacylpropane are obtained on the most active catalysts, 3RS450 and 3RS400, possessing highest ruthenium activity and acidity. The yields and composition of the major monomers during hydrogenation of flax shive in supercritical ethanol (250 °C, 9.7 wt %) are close to the results obtained in subcritical ethanol (225 °C, 10.2 wt %), except for the higher 4-guaiacylethane yield and almost complete lack of guaiacylpropene (2). A higher total yield (27 wt %) of monomeric compounds was obtained in RCF of miscanthus in methanol with the 5% Ru/C catalyst at 250 °C [28]. These differences can be caused by the nature of the hydrogenating substrates (flax and miscanthus), solvent, and also by the higher ruthenium content in the catalyst [28].
Based on the results obtained, several conclusions may be made. First, temperature is the most powerful parameter that determines conversion of flax shive and gas yield. Second, the catalysts acidity increases the carbohydrates conversion. Third, the best yields of monophenols and guaiacylpropane are obtained on the most active catalysts, 3RS450 and 3RS400, possessing highest ruthenium activity and acidity. The yields and composition of the major monomers during hydrogenation of flax shive in supercritical ethanol (250 °C, 9.7 wt %) are close to the results obtained in subcritical ethanol (225 °C, 10.2 wt %), except for the higher 4-guaiacylethane yield and almost complete lack of guaiacylpropene (2). A higher total yield (27 wt %) of monomeric compounds was obtained in RCF of miscanthus in methanol with the 5% Ru/C catalyst at 250 °C [28]. These differences can be caused by the nature of the hydrogenating substrates (flax and miscanthus), solvent, and also by the higher ruthenium content in the catalyst [28].
Based on the results obtained, several conclusions may be made. First, temperature is the most powerful parameter that determines conversion of flax shive and gas yield. Second, the catalysts acidity increases the carbohydrates conversion. Third, the best yields of monophenols and guaiacylpropane are obtained on the most active catalysts, 3RS450 and 3RS400, possessing highest ruthenium activity and acidity.  Table 9.
The yields and composition of the major monomers during hydrogenation of flax shive in supercritical ethanol (250 • C, 9.7 wt %) are close to the results obtained in subcritical ethanol (225 • C, 10.2 wt %), except for the higher 4-guaiacylethane yield and almost complete lack of guaiacylpropene (2). A higher total yield (27 wt %) of monomeric compounds was obtained in RCF of miscanthus in methanol with the 5% Ru/C catalyst at 250 • C [28]. These differences can be caused by the nature of the hydrogenating substrates (flax and miscanthus), solvent, and also by the higher ruthenium content in the catalyst [28].
Based on the results obtained, several conclusions may be made. First, temperature is the most powerful parameter that determines conversion of flax shive and gas yield. Second, the catalysts acidity increases the carbohydrates conversion. Third, the best yields of monophenols and guaiacylpropane are obtained on the most active catalysts, 3RS450 and 3RS400, possessing highest ruthenium activity and acidity. dimethoxyphenol (Sigma-Aldrich, St. Louis, Missouri, USA), ethyl palmitate (Tokyo Chem. Ind., Tokyo, Japan) were used. Phenanthrene was used as an internal standard.
Molecular-weight distribution of the liquid samples were determined by gel permeation chromatography (GPC) on Agilent 1260 Infinity II Multi-Detector GPC/SEC System (Agilent, Santa Clara, CA, USA, 2018). The separation was made by a PLgel Mixed-E column with tetrahydrofuran stabilized with 250 ppm 2,6-di-tert-butyl-4-methylphenol as an eluent. The columns were calibrated with polystyrene calibration kit (Agilent, Santa Clara, CA, USA). The eluent flow was set at 1mL/min and the volume of the injection loop was 100 µL. All samples were dissolved in eluent with concentration 1 mg/mL and left to dissolve completely overnight. After dissolution all samples were filtered through a 0.22-µm Agilent PTFE membrane filter.

Conclusions
Flax shive is a low-value waste residue of the flax fiber production. In this paper, a reductive catalytic fractionation (RCF) of flax shive was studied. The bifunctional ruthenium catalysts based on oxidized Sibunit demonstrated the high efficiency in the reductive fractionation of flax shive at 225 • C for 3 h. In the experiments without a catalyst under these process conditions, the conversion was found to be no more than 44.1 wt % and the content of phenolic monomers in the liquid fractionation products was 1.1 wt %. The use of the Ru/C catalysts result in a significant increase in the monophenol yield (up to 10.2 wt % per lignin weight in the sample)) and enhanced the conversion of flax shive to 56 wt %.
The effect of different characteristics (the ruthenium content, particle size, and substrate oxidation temperature) of the bifunctional Ru/C catalysts on the reductive catalytic fractionation of flax shive was examined. The highest degree of delignification (up to 79.0%) and cellulose yield (up to 67.2 wt %) were obtained with the Ru/C catalyst containing 3 wt % of Ru on carbon support oxidized at 400 • C. In addition, the use of this catalyst ensured the high (up to 9.5 wt %) monophenol yield. The main characteristics of the catalysts that determine the high conversion of lignin to guaiacylpropane are the ruthenium dispersion and surface area. Bronsted acid sites plays an important role in the guaiacylpropane formation due to catalyzing the dehydration of guaiacylpropanol to guaiacylpropene.
The granular ruthenium-containing catalysts showed the low monomer yields and degrees of delignification in comparison with the powdered catalysts due to the low diffusion that limits the transfer of reagents to the inner surface of catalyst grains in the liquid phase.
To conclude, the catalytic conversion of flax shive provides a solution to the problem of its waste utilization. The most promising individual compounds obtained as products of the hydrogenation processing of the flax shive lignin can be guaiacylpropane.  Data Availability Statement: Data on the methodology of the experimental part is available in [22][23][24].