Possibilities of Liquefied Spruce (Picea abies) and Oak (Quercus robur) Biomass as an Environmentally Friendly Additive in Conventional Phenol–Formaldehyde Resin Wood Adhesives

Abstract

Considerable efforts have been made to replace formaldehyde-containing adhesives in the manufacturing of wood products, particularly particleboard, with natural alternatives. One promising solution is the liquefaction of lignocellulosic materials such as wood using glycerol (C₃H₈O₃) under sulfuric acid catalysis (H₂SO₄). The aim of this study was to investigate the chemical composition and properties of spruce and oak biomass after liquefaction and to evaluate its potential as a formaldehyde-free adhesive substitute. All samples were liquefied at 150 °C for 120 min in five different wood–glycerol ratios (1:1 to 1:5). The liquefaction percentage, the insoluble residue, the dry matter and the hydroxyl (OH) number were determined as characteristic values for the polymer properties of the liquefied samples. The results showed the liquefaction percentage was up to 90% for spruce and oak. The insoluble residue ranged from 10 to 29% for spruce and from 10 to 22% for oak, the dry matter ranged from 54 to 70% for spruce and from 51 to 62% for oak, while the highest xydroxyl number was 570 mg KOH/g for spruce and 839 mg KOH/g for oak. Based on these results, liquefied wood was shown to be an effective natural alternative to synthetic resins in particleboard adhesives and a way to reduce formaldehyde emissions. This research not only supports environmentally sustainable practices but also paves the way for various bioproducts derived from liquefied biomass and points to future avenues for innovation and development in this area.

1. Introduction

Lignocellulosic biomass from forests is a widespread and economically accessible raw material source from forestry (e.g., trees, bark, branches, roots, leaves and energy crops) or the wood processing industry (including wood dust, wood shavings and sawdust) [1]. It serves as a promising substitute for fossil fuels and complements other energy sources such as wind and solar power as a renewable energy source [2]. Wood, which is predominantly used for energy production in many countries with a strong wood processing industry and extensive forest areas, is used in private households for heating and cooking as well as in the commercial and industrial sectors for the production of heat and hot water [3,4].

Lignocellulosic biomass consists mainly of cellulose, lignin, hemicellulose and small amounts of ash, proteins and pectin (Figure 1). The distribution and structure of these components vary depending on the type and degree of maturity of the plant cell wall [5]. Cellulose, a linear polymer of ß-d-glucopyranose units linked by ß-1,4-glycosidic bonds, forms insoluble microfibrils surrounded by hemicellulose and lignin in plant cell walls due to hydrogen and van der Waals bonds [6,7,8,9]. Lignin, a complex three-dimensional polymer with aromatic structures and multiple chemical bonds, acts as an “adhesive” in wood, providing strength and reinforcement [10]. Hemicellulose, an amorphous, branched heteropolymer of pentose and hexose sugars and uronic acids, is the second most abundant after cellulose and is valuable for biobased products after it has been removed during the pretreatment of lignocellulosic biomass [9,11,12].

The composition of lignocellulosic biomass varies depending on the plant species and part and is influenced by age and growth conditions. In conifers (45–50%) and hardwoods (40–55%), cellulose predominates, while in wheat straw (50%) and grasses (35–50%), hemicellulose predominates. Softwoods and hardwoods also contain a lot of lignin (25–35% and 18–25%, respectively), while the lignin content in grasses is lower (10–30%) [9]. These polymers bind together in the cell walls and make them resistant to chemical and microbial degradation [13,14].

Figure 1. Schematic representation of the chemical structure of cellulose, lignin and hemicellulose (modified from [15]).

Figure 1. Schematic representation of the chemical structure of cellulose, lignin and hemicellulose (modified from [15]).

Biochemical and thermochemical processes are frequently used to convert lignocellulosic biomass into various bioproducts [16]. Cellulose and hemicellulose can be converted into fermentable sugars and chemicals, while lignin is a potential source of high-value products such as epoxy resins, phenolic resins, adhesives and polyolefins [9,17]. Phenolic resins derived from formaldehyde and phenol are central to the adhesives industry due to their mechanical properties, flame retardancy and chemical resistance, although there are concerns about formaldehyde emissions [18]. Efforts to replace phenol and formaldehyde in resin synthesis are aimed at improving health and performance properties [19].

Liquefaction of lignocellulosic biomass, including wood, involves preparation of the biomass (wood residues, sawdust or other lignocellulosic materials are chopped to increase the reaction surface area), the addition of a solvent (the biomass is mixed with a solvent, which may be water, glycerol, phenol or a mixture of these substances), the addition of a catalyst (an acidic or basic catalyst is often added to accelerate the liquefaction reaction), a thermochemical reaction (a mixture of biomass, solvent and catalyst is heated to a high temperature, usually 150 to 300 °C, which breaks down the lignin and cellulose in the biomass into smaller molecules and produces a liquid product), product separation (after the reaction, the liquid product is separated from solid residues and impurities. Depending on the desired application, the liquid product can be further purified or chemically modified) [20,21,22].

Important process parameters are temperature (the key factor influencing the reaction rate and the final composition of the liquid product), reaction time (the duration of heating influences the degree of decomposition of the biomass), the ratio of solvent to biomass (this ratio influences the efficiency of the reaction and the quality of the liquid product) and the type of catalyst (the choice in catalyst influences the rate and selectivity of the reaction) [20,21,22,23].

This process produces a viscous liquid mixture rich in phenolic units when phenol is used as a solvent, or polyhydric alcohols lead to a different product profile [24,25,26].

Liquefied wood is used as an additive in formaldehyde-based resins used for particleboard, epoxy resins and polyurethane foams, with ongoing research focused on optimizing these applications [27]. The properties of liquefied wood depend on factors such as the solvent type, catalyst, temperature, duration, wood species and chemical composition and influence the properties of the final product [28,29]. Two types of wood were used in this study, softwood spruce and hardwood oak.

The European or Norwegian spruce (Picea abies L.) is an ecologically and economically highly valued conifer in Europe. This tall, evergreen species is indispensable for its essential oils and is mainly used for its wood. It accumulates volatile substances throughout its structure, with the needles having the highest concentration. Interestingly, however, the needles are underutilized in the wood industry compared to other parts of the tree [30,31]. Geographically, the natural habitat of the spruce covers a large area in Europe and Siberia. Its distribution area extends over about 31 degrees of latitude and reaches from the Balkan Peninsula in the south to the Chatanga River in Siberia in the north. Longitudinally, the distribution area extends from the French Alps in the west to the Sea of Okhotsk in eastern Siberia in the east. This wide distribution range underlines the adaptability and resilience of the European spruce to the different climates and environments in its range [32].

The oak (Quercus robur L.) is a widespread tree species found throughout Europe, the Caucasus and certain parts of Asia. This deciduous tree thrives on deep, loamy or sandy soils that are fertile and predominantly moist, often with high groundwater levels. It forms both pure forests and mixed stands with hornbeam, ash and other species [33]. Oak wood is known for its strength, resistance and durability. It contains both organic compounds (extractives) and inorganic compounds (salts and oxides). These organic extractives, which can be extracted with various solvents, influence the natural resistance and color of the wood, but can sometimes also affect the surface treatment [34]. Oak wood is particularly hard and durable and is therefore preferred worldwide to produce parquet flooring, furniture, joinery, railroad beams, ships, bridges and much more. The oak root system is also very extensive, with the roots penetrating several meters deep into the ground and spreading widely [35].

This study investigates how the chemical composition and five different ratios of solvent to biomass (from 1:1 to 1:5) with an acidic catalyst at a constant time and temperature (120 min and 150 °C) during liquefaction affect properties such as liquefaction efficiency, the percentage of insoluble residues, dry matter content and the hydroxyl number. By examining multiple ratios, this research aims to provide comparative insights and establish correlations useful for optimizing the use of liquefied wood in formaldehyde-based adhesives and other wood bioproducts.

2. Materials and Methods

2.1. Wood Samples

Norway spruce (Picea abies L., Figure 2a) and oak (Quercus robur L., Figure 2b) samples came from the Spačva forest area in Vukovar-Srijem County and were collected in Bjelin d.o.o., a wood processing plant (45°09′01.8″ N, 18°55′11.6″ E).

2.2. Sample Preparation

Test method T257 [36] of the Technical Association of the Pulp and Paper Industry (TAPPI) was used for sampling. The samples were taken immediately after sawing the tree at a height of 2 m, making sure that the sawn rings were between 5 and 15 cm thick. The samples were then air-dried at room temperature for two weeks. The bark was then separated from the sapwood and heartwood, and a mixture of sapwood and heartwood was used for this study.

The air-dried samples were ground with a Retsch mill SM400 (RetschGmbH, Haan, Germany) (Figure 3a) using a Retsch sieve with round openings of 10.0 mm (HRN EN ISO 14780:2017) [37]. Further grinding was carried out with a rotating hammer mill SR300 Retsch (RetschGmbH, Haan, Germany) (Figure 3b) using a sieve with trapezoidal openings of 1.0 mm according to Retsch (HRN EN ISO 14780:2017) [37]. After grinding, the samples were sieved using a vibrating Retsch sieve shaker AS200 BASIC (RetschGmbH, Haan, Germany) (Figure 3c) (HRN EN ISO 17827-2:2024) [38] with different Retsch mesh sizes (I-SO3310-1:2016) [39].

After grinding and sieving, particles with a size between 0.50 and 1.00 mm were selected for further analysis, as previous studies had recommended this size range as ideal for isolating the chemical composition of interest. Five smaller samples were taken from the ground and sieved samples. All chemical analyses and liquefaction procedures were performed on these five samples, and the results were reported using their mean values.

This detailed methodological approach ensures consistency and accuracy in the characterization and analysis of samples, which is essential for scientific research and field trials.

2.3. Analysis of the Physicochemical Composition

The proximate analysis of the samples (ash, moisture content, volatile matter and fixed carbon) and structural analysis of the lignocellulose (extractives, cellulose, lignin and hemicellulose) (Figure 4) were performed according to standard methods. The structural analysis included the determination of the proportion of extractives (TAPPI T 204) [40] by the solvent reaction of methanol (CH₃OH) and benzene (C₆H₆) in a volume ratio of 1:1 using a Soxhlet R108S BEHRotest extractor (Labor-Technik-GmbH, Dusseldorf, Germany) (Figure 3d), cellulose by reaction of the sample with a solution of nitric acid (HNO₃) and ethanol (C₂H₅OH) in a volume ratio of 1:4 by boiling in a water bath Hydro H9V Lauda (Lauda GmbH, Lauda-Königshofen, Germany) (Figure 3e), lignin by reaction of the sample with 72.0% sulfuric acid (H₂SO₄) (TAPPI T 222) (TAPPI, Peachtree Corners, GA, USA) [41] by boiling on a magnetic stirrer IKA C-MAG HS7 (IKA^®-Werke GmbH & Co.KG, Staufen, Germany) (Figure 3f), while the amount of hemicellulose was determined according to the following Equation (1):

$$\mathrm{H}\mathrm{C}=100-\left(\mathrm{a}\mathrm{s}\mathrm{h}\%+\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{s}\%+\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{l}\mathrm{u}\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{e}\%+\mathrm{l}\mathrm{i}\mathrm{g}\mathrm{n}\mathrm{i}\mathrm{n}\%\right) \%$$

(1)

All chemicals used were of high purity (p.a.) and were purchased commercially. In the physicochemical analysis, the moisture content (HRN EN ISO 18134-2:2024) [42] was determined using a UF160 Memmert laboratory dryer (Memmert GmbH + Co.KG, Schwabach, Germany) (Figure 3g). The ash content (HRN EN ISO 18122:2022) [43] was measured in a Nabertherm L9/11/B170 muffle furnace (Nabertherm GmbH, Lilienthal, Germany) (Figure 3h), while the volatile matter content was calculated based on HRN EN ISO 18123:2023 [44]. The calorific value (HRN EN ISO 18125:2017) [45] was determined using an IKA C6000 calorimeter (IKA^®-Werke GmbH & Co. KG, Staufen, Germany) (Figure 3i). Each analysis was performed in five replicates to ensure the reliability of the results.

2.4. Liquefaction of the Wood Biomass Samples

As already mentioned, wood liquefaction is a process of thermochemical conversion of wood or agricultural biomass into a liquid form with the aim of producing liquid products that can be used in various industrial applications [24].

Glycerol (C₃H₈O₃), a type of polyhydric alcohol, was used as the liquefying agent, with 3% sulfuric acid (98% H₂SO₄) serving as the acid catalyst. Liquefaction was carried out in a 500 mL round bottom flask with a condenser, using a magnetic stirrer for heating, controlled by a heater and a thermostat.

The liquefaction process began with the addition of 50 g glycerol and 1.5 g sulfuric acid to the flask. The mixture was stirred at 500 rpm until it reached a temperature of 150 °C. Once the solvent had reached this temperature, a certain amount of the wood sample was added to the flask. The amount of sample used varied depending on the ratio of the sample mass to the solvent mixture (1:1, 1:2, 1:3, 1:4 and 1:5) and was between 10 and 50 g. The reaction mixture was then liquefied at 150 °C for 120 min (Figure 5), while the temperature of the solvent was continuously monitored. To better understand why an almost black product is formed from a transparent solvent and yellowish samples, a blank test was carried out using only glycerol and sulfuric acid before the liquefaction of the biomass samples began. As can be seen in Figure 6, the solvent takes on a reddish color as the temperature increases, while it takes on an almost black color when the liquefaction temperature of around 150 °C is reached. The change in the color of glycerol was also confirmed by Shahbazi et al. (2020) [46], where the color of the clear solution changed to yellow, and as the temperature increased, the color of the solution gradually turned black. One of the main reasons for the black color of the liquefied samples is also that after adding a sample of the wood biomass to the solvent, which consists of glycerol and sulfuric acid heated to 150 °C, chemical decomposition of the lignin takes place.

When lignin undergoes decomposition, particularly through chemical processes like oxidation or thermal degradation, it breaks down into a variety of smaller, more complex molecules. These molecules include phenolic compounds, quinones and other aromatic structures. The dark coloration observed in the liquid is primarily due to the formation of these complex aromatic compounds, many of which are chromophores—molecules that absorb light in the visible spectrum, particularly in the blue and green wavelengths. As a result, they reflect red and yellow wavelengths, which combine to give the liquid a brown or black appearance [20,21,22,23,24,26].

2.5. Analysis of the Liquefied Samples

After completion of the liquefaction process, the liquefied samples (Figure 7) were characterized as shown in Figure 8.

2.5.1. Insoluble Residue

The insoluble residue of liquefied wood refers to the biomass fraction that is not dissolved during the liquefaction process and remains as a solid residue. The liquefied spruce and oak samples were dissolved with a solvent mixture of 1,4-dioxane and water (8/2), which was selected due to its universality in the liquefaction of biomass or lignocellulose. This liquefied wood solution was thoroughly mixed on a magnetic stirrer for 60 min and then filtered through a B2 glass fiber filter. The filtered solution was washed several times until a colorless filtrate was obtained. The resulting dioxane-insoluble product was then dried in a laboratory dryer at 105 °C until a constant mass was achieved. The mass of the insoluble residue was then measured to determine its percentage in relation to the initial mass of the liquefied sample [22,23].

The percentage of insoluble residue (Equation (2)):

$$\mathrm{I}\mathrm{n}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{b}\mathrm{l}\mathrm{e} \mathrm{r}\mathrm{e}\mathrm{s}\mathrm{i}\mathrm{d}\mathrm{u}\mathrm{e}\left(\mathrm{I}\mathrm{R}\right)={\displaystyle \frac{[\mathrm{m}\left(\mathrm{f}\mathrm{p}+\mathrm{d}\mathrm{s}\right)-\mathrm{m}(\mathrm{f}\mathrm{p}\left)\right]}{\mathrm{m}\left(\mathrm{s}\right)}}\times 100\left(\%\right)$$

(2)

m(fp + ds)—the mass of the filter paper together with the dried sample (g);

m(fp)—the mass of the filter paper;

m(s)—the mass of the sample.

2.5.2. Liquefaction Percentage

The liquefaction percentage (LP) is a measure that describes the amount of biomass that has been converted into a liquid form during the liquefaction process.

A total of 1 g of the liquefied biomass sample was weighed into a dry beaker with an accuracy of ±0.1 mg, then 100 mL of distilled water and a magnet were added to the beaker. The beaker was placed on a magnetic stirrer with ten positions and stirred for 30 min (Figure 9).

While the samples were being mixed, the mass of the filter paper previously dried and cooled in the desiccator was weighed. The samples, mixed for 30 min, were then filtered through a filter paper placed in a glass funnel placed on an Erlenmeyer flask. The undissolved part remained on the filter paper, while the liquid part was drained into the Erlenmeyer flask. The filter paper was then dried for 24 h at a temperature of 80 ± 2 °C, i.e., until a constant mass was achieved.

The percentage of liquefaction was then calculated according to the following Equation (3) [23]:

$$\mathrm{L}\mathrm{i}\mathrm{q}\mathrm{u}\mathrm{e}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{p}\mathrm{e}\mathrm{r}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{a}\mathrm{g}\mathrm{e}\left(\mathrm{L}\mathrm{P}\right)=100-\mathrm{I}\mathrm{n}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{b}\mathrm{l}\mathrm{e} \mathrm{r}\mathrm{e}\mathrm{s}\mathrm{i}\mathrm{d}\mathrm{u}\mathrm{e} \left(\mathrm{I}\mathrm{R}\right)\left(\%\right)$$

(3)

2.5.3. Dry Matter

The dry matter of liquefied wood refers to the amount of solid substance present in the liquid product after the thermochemical process of liquefying wood biomass. The measurement of dry matter is important because it provides information on the amount of non-volatile matter remaining after the removal of water and other volatile components [20,22,23]. The dry matter content (Figure 10) was determined by weighing the empty watch glass and 1 g of the liquefied sample. These samples were dried in an oven at 150 ± 2 °C for 24 h, cooled in a desiccator and then weighed again together with the watch glass.

The percentage of dry matter was then calculated using the following Equation (4) [23]:

$$\mathrm{D}\mathrm{r}\mathrm{y} \mathrm{M}\mathrm{a}\mathrm{t}\mathrm{t}\mathrm{e}\mathrm{r}={\displaystyle \frac{[\mathrm{m}\left(\mathrm{w}\mathrm{g}+\mathrm{d}\mathrm{s}\right)-\mathrm{m}(\mathrm{w}\mathrm{g}\left)\right]}{\mathrm{m}\left(\mathrm{s}\right)}}\times 100\left(\%\right)$$

(4)

m(wg + ds)—mass of the watch glass and the dried sample (g);

m(wg)—mass of an empty watch glass (g);

m(s)—mass of the sample (g).

With these methods, the content of insoluble residues and dry matter in the liquefied biomass could be precisely determined, which is crucial for evaluating the effectiveness of the liquefaction process.

2.5.4. Hydroxyl (OH) Number

The hydroxyl number (OH number) is defined as the amount of potassium hydroxide (KOH) in milligrams required to neutralize the hydroxyl groups in one gram of the sample and is a measure of the hydroxyl group (–OH) content of a chemical compound. It is expressed in mg KOH/g. The hydroxyl number (OH) is an important indicator of the concentration of hydroxyl groups present in a polyol. The amount of reactive hydroxyl groups in the polyol has a direct effect on the formation of urethane bonds, which significantly influence the physical properties of the final product. Monitoring and controlling this parameter during polyol production is crucial. Polyols with a higher concentration of reactive hydroxyl groups lead to a higher degree of cross-linking, resulting in a stiffer and stronger end product [20,21,22,23].

The procedure for the determination of the hydroxyl number was carried out in accordance with previous studies [20,23] and began with the addition of 0.51 to 0.56 g of the sample to a beaker, into which 25 mL of the esterification reagent was then added. The beaker with the sample was then placed over a water bath and heated for 5 to 10 min, i.e., until the sample detached from the bottom of the beaker. The sample was then placed in a water bath at a temperature of 98 ± 2 °C for 15 min, with the water level high enough to submerge the part of the beaker containing the sample. After the water bath, the beakers containing the samples were removed and cooled. Then, 50 mL of pyridine and 10 mL of hot distilled water were pipetted into a beaker containing a sample, and a magnet was attached. The beaker was placed on a magnetic stirrer on which the sample was mixed with the aid of a magnet, and a pH meter was placed in the beaker. For this reason, beakers with lids were used instead of flasks. The samples prepared in this way were titrated to the equivalence point with a previously standardized 0.5 M potassium hydroxide solution (Figure 11).

The OH number in mg KOH/g of the liquefied biomass sample was calculated according to the following Equation (5) [23]:

$$\mathrm{O}\mathrm{H} \mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}={\displaystyle \frac{\left(\mathrm{B}-\mathrm{A}\right)\times \mathrm{N}\times 56.1}{\mathrm{m}}}\times 100\left(\%\right)$$

(5)

B—consumption of KOH (mL);

A—consumption of KOH for the blank test (mL);

N—normality of the solution;

m—mass of the sample (g).

2.6. Statistics

The statistical analysis was carried out using the statistical program TIBCO STATISTICA 14.0.0. Results are presented as means with standard deviations calculated using the Tukey post-hoc HSD test to detect differences between the observed samples.

3. Results

Chemical Composition of the Biomass of Spruce and Oak

The investigated chemical composition of spruce and oak biomass is shown in

Table 1. Proximate analysis of spruce and oak biomass (%).

Wood Species	Chemical Composition
	MC	Ash	FC	VM
Spruce	8.60 ± 0.20 b	0.39 ± 0.07 b	17.63 ± 0.45 b	80.2 ± 1.11 a
Oak	8.18 ± 0.07 a	0.25 ± 0.04 a	12.99 ± 0.09 a	79.87 ± 0.19 a

MC—moisture content; FC—fixed carbon; VM—volatile matter. Different letters in the exponent represent the difference in means according to the Tukey post-hoc HSD test (p < 0.05).

and includes moisture content, ash, fixed carbon and volatile matter.

Table 2. Heating value of spruce and oak biomass (MJ kg⁻¹).

Wood Species	Chemical Composition
	HHV	LHV
Spruce	19.20 ± 0.36 b	18.20 ± 0.36 b
Oak	18.43 ± 0.47 a	17.47 ± 0.42 a

HHV—higher heating value; LHV—lower heating value. Different letters in the exponent represent the difference in means according to the Tukey post-hoc HSD test (p < 0.05).

shows the higher (HHV) and lower (LHV) heating values. The particle size of the analyzed samples ranged from 0.50 to 1.00 mm.

In the evaluation of biomass for energy production, the ash content serves as an indicator of the mineral components present in the biomass that do not contribute to heat production. Table 1 shows that spruce has a higher ash content compared to oak, which can have a negative impact on fuel quality and the liquefaction process.

The fixed carbon, a solid residue after combustion, increases with a higher ash content and lower volatile matter content. This phenomenon leads to a higher total fixed carbon content in biomass burned at higher temperatures, as the carbon loss from volatile compounds is minimal [47]. The volatiles, on the other hand, represent the combustible fraction, which is crucial for fuel ignition and combustion stability. Biomass with a higher content of volatile components, such as spruce and oak with around 80%, ignites more easily and requires less energy for ignition. However, fuels with a higher volatile matter content release less heat, as a large proportion of the volatile matter escapes into the atmosphere [48].

These properties must be carefully considered when selecting wood species as a raw material for energy production.

As mentioned above, the heating values provide a basic insight into the energy yield that can be obtained from a given biomass sample. The data in Table 2 show that spruce has higher HHV and LHV values compared to oak, indicating a greater energy potential per unit mass. Consequently, spruce appears to be a more efficient raw material for energy production, as it offers a higher energy yield per unit compared to oak.

However, it should not be forgotten that the selection of raw materials for energy production depends on several other factors, such as availability, cost, environmental impact and logistical feasibility. While the heating value is an important indicator of energy potential, a comprehensive assessment of these additional factors is essential to make informed decisions about the selection of biomass for energy purposes.

The results presented in

Table 3. Lignocellulosic composition of spruce and oak biomass (%).

Wood Species	Lignocellulosic Components
	C	L	HC	E
Spruce	49.77 ± 0.29 a	26.38 ± 0.54 a	21.72 ± 0.2 a	2.44 ± 0.21 b
Oak	49.81 ± 0.58 a	25.97 ± 0.15 a	21.43 ± 0.4 a	1.70 ± 0.02 a

C—cellulose; L—lignin; HC—hemicellulose; E—extractives. Different letters in the exponent represent the difference in means according to the Tukey post-hoc HSD test (p < 0.05).

show that the cellulose content was almost 50% in both samples tested, with almost equal proportions of lignin (26%) and hemicellulose (22%). The main difference between the samples was in the extractives, which were about 1% higher in the spruce sample. These differences in the lignocellulosic composition of the wood could potentially affect properties such as mechanical and chemical resistance, color, density and other related characteristics.

Table 4. Liquefaction properties of the liquefied spruce.

Liquefaction Ratio	Liquefaction Properties
	LP (%)	IR (%)	DM (%)	OH (mg KOH/g)
1:1	71.34 ± 1.21 a	28.66 ± 1.22 d	69.74 ± 1.60 d	338.36 ± 0.48 a
1:2	73.94 ± 0.23 a	26.06 ± 0.44 c	63.63 ± 1.05 c	352.21 ± 0.15 a
1:3	85.09 ± 0.42 b	14.91 ± 0.11 b	60.08 ± 2.97 b	415.88 ± 0.51 b
1:4	87.74 ± 0.13 c	12.26 ± 0.23 a	55.64 ± 1.83 a	499.01 ± 0.22 c
1:5	89.54 ± 0.10 c	10.46 ± 0.47 a	54.70 ± 1.17 a	569.74 ± 0.33 d

LP—liquefaction percentage; IR—insoluble residue; DM—dry matter; OH—hydroxyl number. Different letters in the exponent represent the difference in means according to the Tukey post-hoc HSD test (p < 0.05).

shows that the highest percentage of liquefaction of spruce wood occurred at a liquefaction ratio of 1:5. Conversely, the highest percentage of insoluble residues was observed at a ratio of 1:1. This indicates that a larger proportion of the biomass remained insoluble at this ratio, which is probably due to incomplete liquefaction in the solvent. The highest percentage of dry matter was obtained at a ratio of 1:1, indicating that at this ratio, the presence of moisture or other foreign matter in the tested sample was minimized. In addition, the highest value of the OH number was obtained at a liquefaction ratio of 1:5.

The liquefaction results for oak (

Table 5. Liquefaction properties of the liquefied oak.

Liquefaction Ratio	Liquefaction Properties
	LP (%)	IR (%)	DM (%)	OH (mg KOH/g)
1:1	77.85 ± 0.40 a	22.15 ± 0.50 c	62.37 ± 0.48 c	492.12 ± 0.05 a
1:2	79.93 ± 3.910 a	20.07 ± 1.24 b	55.53 ± 2.54 b	513.42 ± 0.11 a
1:3	87.24 ± 0.38 b	12.76 ± 1.01 a	57.68 ± 0.81 b	645.08 ± 0.29 b
1:4	89.69 ± 0.38 c	10.31 ± 0.19 a	51.35 ± 2.13 a	751.23 ± 0.20 c
1:5	87.84 ± 0.44 b	12.16 ± 0.59 a	52.25 ± 1.14 a	839.01 ± 0.18 d

LP—liquefaction percentage; IR—insoluble residue; DM—dry matter; OH—xydroxyl number. Different letters in the exponent represent the difference in means according to the Tukey post-hoc HSD test (p < 0.05).

) are consistent with those for spruce. For oak, the highest liquefaction percentage was obtained at a ratio of 1:4, while the highest percentages of insoluble residue and dry matter were obtained at a ratio of 1:1. As with spruce, the highest value of hydroxyl number was also obtained at a liquefaction ratio of 1:5. For both species, the 1:4 and 1:5 ratios were the most effective, indicating optimal leaching of the material from the samples. With an increasing sample mass in relation to the solvent, the proportion of liquefaction decreased, while the proportion of solvent remained constant. Table 4 and Table 5 illustrate the importance of choosing the right biomass-to-solvent ratio for efficient liquefaction processes. In addition, the type of wood has a significant influence on the results of the process.

4. Discussion

Efforts to replace a certain percentage of formaldehyde and phenolic resins in wood adhesives are a central focus of the circular bioeconomy. One promising approach is wood liquefaction, where the liquefied material can be used in adhesives to manufacture wood products. These products can in turn be liquefied again at the end of their use, closing the cycle.

This study is novel in that it compares the liquefaction properties of two different types of wood at five different ratios of liquefaction, unlike other studies which usually refer to a single ratio of liquefaction. Based on these properties and previous studies, manufacturers of products such as particleboard can gain new insights into the use of liquefied biomass as a catalyst in commercial adhesives.

During liquefaction, it was found that softwoods liquefy faster than hardwoods but also undergo recondensation reactions more frequently. This is due to the fact that softwoods contain large amounts of guaiacylpropane units, which are more reactive than the syringylpropane units found in hardwoods. In addition to these units, other physicochemical properties of the starting material also significantly influence the process [49].

The air temperature influences the moisture content of the biomass, which has a negative effect on the fuel properties. Although a high moisture content affects the liquefaction process less than other thermochemical conversion processes, it still has an impact on the biomass utilization and product yield during liquefaction [50]. For example, the moisture content of the spruce sample was 8.60%, while the moisture content of the oak sample was 8.18%.

Similar to the moisture content, the ash content also reduces the fuel quality, so a lower percentage is desirable [49]. The ash content of the spruce samples in this study was 0.39%, which is consistent with the literature values: Ulusal et al. (2021) [51] reported 0.50% for spruce, and Čajová Kantová et al. (2022) [52] reported 0.42%. In the oak samples, the ash content was 0.24%, which is significantly lower than the 0.73% reported by Öcal and Yüksel (2023) [53] and the 1.39% reported by Antonović et al. (2019) [20]. Both spruce and oak have a lower ash content than some agricultural biomass, as found in other studies [54].

Fixed carbon (FC) refers to the energy released when biomass is burned, with higher percentages indicating a higher calorific value [55]. As shown in Table 1, the FC content of spruce was almost 5% higher than that of oak. In this study, the FC content of spruce was 17.63%, higher than the 14.68% reported by Wang et al. (2022) [56]. In oak, the FC content was 12.99%, which is consistent with Ulusal et al. (2021) [51].

The average content of volatile matter (VM) in raw biomass (wood, forestry residues and straw) is usually between 75 and 85%, with higher contents reducing the energy value of the fuel [57]. Čajová Kantová et al. (2022) [52] reported a VM content of 82.13% for spruce wood. Ulusal et al. (2021) [51] reported a VM content of 78.52% for oak, while Öcal and Yüksel (2023) [53] determined a significantly higher VM content of 99.27%. The results of the present study are consistent with the literature and show a VM content of 81.4% for spruce and 79.96% for oak.

The calorific value represents the chemically bound energy in the biomass that is converted into thermal energy by combustion [58]. It is a decisive property for the numerical simulation and design calculation of biomass heat conversion systems. The heating value is divided into the higher heating value (HHV) and the lower heating value (LHV). The HHV indicates the total amount of energy released during the combustion of a mass of solid fuel, including the condensation of the water produced during combustion. In contrast, the LHV represents the energy produced during combustion under constant volume conditions, with all the water remaining in the form of steam [59].

Lignocellulosic biomass, which has a higher lignin content, is particularly suitable for conversion into solid biofuels. These biofuels are often used by direct combustion to generate electricity or thermal energy. Studies have shown that there is a significant correlation between the HHV and LHV of biomass fuels and their lignin content [60].

Forest biomass, including hardwood and softwood, has an average calorific value of 19.5 MJ kg⁻¹, while freshly harvested wood can have a calorific value of only 5.9 MJ kg⁻¹ [61]. The calorific value of spruce is between 18.2 MJ kg⁻¹ and 19.2 MJ kg⁻¹ and thus corresponds exactly to the value of 19.16 MJ kg⁻¹ given by Gandek et al. (2023) [62]. The calorific value of oak was between 17.47 MJ kg⁻¹ and 18.43 MJ kg⁻¹, which corresponds to the range of 17.42 MJ kg⁻¹ to 18.68 MJ kg⁻¹ determined by Jasinskas et al. (2020) [63]. These values are comparable to those of other biomass types such as sawdust, straw and agricultural residues [47,64].

The chemical composition of biomass, whether lignocellulosic or herbaceous, is determined by several main components: cellulose, lignin, hemicellulose and extractive substances. These components give plants structure and strength [65]. Cellulose, hemicellulose and lignin together make up more than 90% of lignocellulosic biomass and 80% of herbaceous biomass [66]. Lignin poses a major challenge in the production of liquid biofuels; therefore, biomass with a higher lignin content is more suitable for conversion into solid fuels, as the calorific value of lignocellulosic fuels increases with the lignin content [67]. Esteves et al. (2023) [68] give the calorific value of lignin as 23.26 to 25.58 MJ kg⁻¹, while polysaccharides have a slightly lower higher heating value (HHV) of 18.6 MJ kg⁻¹.

The lignocellulosic composition of spruce wood (Table 3) agrees quite well with the literature values. Zhang et al. (2022) [5] report 38.0 to 47.0% cellulose and 18.0 to 22.0% hemicellulose, Borovkova et al. (2022) [69] report 30.6% lignin and 1.8% extractives, while Dhyani and Bhaskar (2018) [70] report 50.8% cellulose, 27.5% lignin and 21.2% hemicellulose. There are also no major differences in the composition of oak. Antonović et al. (2019) [20] report 47.23% cellulose and 21.82% lignin, Ghavidel et al. (2020) [71] report 24.48% hemicellulose, Öcal and Yüksel (2023) [53] report 41.0% cellulose, 28.3% lignin, 23.7% hemicellulose and 7.0% extractives, while Ulusal et al. (2021) [51] report 39.25% cellulose, 24.38% lignin and 26.7% hemicellulose.

The choice of solvent for the liquefaction process is a crucial parameter that influences the degree of depolymerization and the structure of the final product. Solvents with higher polarity usually led to a higher liquefaction efficiency. Glycerol is a commonly used solvent, as it can slow down the recondensation reactions to a certain extent. The ratio of the liquefaction reaction medium is also critical, with a common solvent ratio being between 3:1 and 5:1 of biomass [26].

The data presented in Table 4 and Table 5 show that the liquefaction performance increases with a higher solvent-to-sample ratio, while the insoluble residue decreases proportionally. It can also be seen that the proportion of dry matter decreases as the liquefaction percentage increases.

For spruce wood, the highest liquefaction percentage, ranging from 85.09% to 89.54%, was observed at a solvent-to-sample ratio of 1:3 to 1:5. Conversely, the lowest liquefaction percentage of 71.34% was observed at a 1:1 ratio. The insoluble residue was highest at a ratio of 1:1 at 28.66% and lowest at a ratio of 1:5 with a value of 10.46%. The lowest percentage of dry matter was observed at a ratio of 1:5 (54.70%), while percentages of over 60.0% were observed at ratios between 1:1 and 1:3.

A comparison of these results with the literature data shows that the liquefaction percentage of spruce in this study was slightly lower than the 94.94% reported by Antonović et al. (2019) [20], while the insoluble residue was higher than the 5.06% reported by the same authors [20]. A higher liquefaction percentage was found for the energy crop biomass Miscanthus x giganteus at a ratio of 1:5.62, with values of 98.75% and 99.36% and an insoluble residue of 0.64% and 1.25%, depending on the particle size [72].

In the oak samples, the highest liquefaction percentage of 89.69% was found at a ratio of 1:4, and at ratios of 1:3 and 1:4, it was almost 88%. The lowest liquefaction percentage was 77.85% at a ratio of 1:1. Accordingly, the highest percentage of insoluble residue was also found at a ratio of 1:1 at 22.15%, while the lowest percentage of insoluble residue was measured at a ratio of 1:4 (10.31%). The percentage of dry matter was lowest at a ratio of 1:4 (51.35%), and only at a ratio of 1:1 was it above 60.0%.

Comparing these results with previous studies, the liquefaction percentage of the oak samples in this study is within the range of 59.0 to 98.0% reported in previous studies [73]. It is noteworthy that the liquefaction percentage was below 91.98% for oak and below 91.0% for black poplar, as noted by Antonović et al. (2019) [20]. The same authors also reported a lower percentage of insoluble residues of 8.02% for oak and 6.20% for fir compared to the 10.31% found in this study. For further comparison, the liquefaction of lignocellulosic biomass from corn stover ranges from 69.79% to 82.62% [74], while for twelve tropical hardwood species, it ranges from 68.89% to 93.02%, with liquefaction ratios between 1:1 and 1:3 [75].

The highest value of the hydroxyl number in the liquefied spruce samples was measured at a liquefaction ratio of 1:5 and amounted to almost 570 mg KOH/g, while the lowest value of slightly more than 338 mg KOH/g was measured at a ratio of 1:1. In the liquefied oak samples, the lowest hydroxyl number was found at ratios of 1:1 and 1:2 (about 500 mg KOH/g), while the highest value of almost 840 mg KOH/g was measured at a ratio of 1:5.

The comparison of the determined values with the literature shows that the hydroxyl number for both spruce and oak is within the range of the values given in the literature. For example, Antonović et al. (2019) [20] give a value of 575 mg KOH/g for spruce, 744 mg KOH/g for oak and 798 mg KOH/g for beech, while Gosz et al. (2021) [73] give values between 352 and 813 mg KOH/g for oak. For further comparison, Đurović et al. (2024) [76] show values between 429.41 and 612.00 mg KOH/g for three liquefaction ratios (1:3–1:5) for soybeans and between 569.76 and 835.38 mg KOH/g for hemp.

5. Conclusions

The use of liquefied wood as an additive to commercially available formaldehyde or phenol-based wood adhesives depends on various factors and conditions. The properties of the liquefied product are primarily influenced by the type of wood (softwood or hardwood), its physicochemical properties, the composition of the lignocellulose, the type of solvent, the ratio of liquefaction, the temperature and the duration of liquefaction. The novelty of this study is that the liquefaction process was carried out in five different ratios, as rarely more than one ratio is investigated in the literature.

Concerning the physicochemical properties and the lignocellulosic composition of the starting material, there were no significant differences that could influence the liquefaction process. Therefore, the optimal liquefaction properties (liquefaction percentage, insoluble residue, dry matter and hydroxyl number) were determined for both forest types using glycerol as the solvent and sulfuric acid as the catalyst, with the liquefaction ratio of the solvent to sample mass ranging from 1:1 to 1:5 under constant conditions.

The results show that the choice of the correct liquefaction ratio is crucial for both economic and technical reasons. Ratios where the solvent and the sample mass are almost the same result in a lower degree of liquefaction, which makes the material unusable and leads to unnecessary energy consumption and financial losses. Therefore, it is important to find an optimal balance between the expected utilization of the liquefied biomass and the associated costs.

The hydroxyl number is the most important property for liquefaction in the context of various bioproduct applications. It takes precedence over factors such as undissolved residue and liquefaction percentage. In this respect, hardwoods have better properties for further applications compared to softwoods.

This research shows that liquefied spruce and oak wood can serve as environmentally friendly catalysts, partially replacing commercially available chemical catalysts such as ammonium chloride or ammonium sulfate in particleboard production. The results of this study confirm previous studies that measured the mechanical properties and formaldehyde release of particleboard and confirmed that it meets the European quality standard [77] when between 20% and 50% liquefied wood was added as a substitute in phenol–formaldehyde resin wood adhesives.

These results pave the way for further scientific studies in which different solvents, ratios, temperatures or liquefaction times are researched to develop different bioproducts based on liquefied wood biomass as a sustainable raw material.