The Potential of Apricot Tree Resin as a Viable Feedstock for High-Value Chemicals via Hydrothermal Gasification

Abstract

This study investigates the hydrothermal gasification (HTG) of apricot tree resin, focusing on the yield and chemical composition of the resulting gas and aqueous phases. K₂CO₃ and KOH were used as catalysts within a temperature range of 300–600 °C, with a constant reaction time of 60 min. The results show that temperature and catalyst choice significantly influence gas yield, liquid composition, and solid residue formation. Higher temperatures increased the gas yield while decreasing aqueous and solid residues. The catalytic effect of K₂CO₃ and KOH enhanced the gaseous product conversion, with KOH achieving the highest gas yield and lowest residue formation at 600 °C. Among the liquid-phase compounds, carboxylic acids and 5-methyl furfural were the most abundant, reaching peak concentrations at 300 °C in the presence of K₂CO₃. The addition of alkali catalysts reduced key acidic intermediates such as glycolic, acetic, and formic acids. The inverse relationship between temperature and liquid/solid product formation underscores the importance of optimizing reaction conditions for efficient biomass conversion. These findings contribute to the growing field of biomass valorization by highlighting the potential of underutilized tree resins in sustainable biofuel production, advancing knowledge in renewable hydrogen production, and supporting the broader development of bio-based energy solutions.

1. Introduction

The increasing depletion of non-renewable energy resources and growing environmental concerns have shifted global attention toward clean and renewable energy sources such as biomass, wind, and solar energy. Developing efficient techniques for producing valuable fuels and chemicals from biomass is crucial in terms of meeting rising energy demands sustainably. Among biomass resources, agricultural and forestry residues—often underutilized—hold significant potential for energy production. Biomass, as a renewable and abundant resource, can be cultivated and sourced worldwide, providing a sustainable alternative to fossil fuels through thermochemical processes. According to data from the United Nations Food and Agriculture Organization (FAO), apricot tree cultivation covered approximately 133,000 hectares in Turkey and 562,000 hectares globally in 2020. Turkey accounted for approximately 22.4% of the world’s apricot production in 2020 [1]. Despite the extensive cultivation of apricot trees, their biomass potential, particularly the resin, remains largely unexplored.

Tree resins, including apricot tree resin, contain high-energy compounds, making them a promising biofuel source of feedstock. Previous studies have highlighted the high combustion heat of resins, exceeding 38.0 MJ kg⁻¹, which surpasses that of cellulose-based biomass materials [2]. Additionally, resin combustion produces significantly lower CO₂ emissions compared to coal combustion. These properties underscore the potential of tree resins as valuable resources for biofuel production. Lignocellulosic biomass, including apricot wood, primarily consists of cellulose, hemicellulose, lignin, and extractives. These primary structural components typically include approximately 30–50% cellulose, 20–40% hemicellulose, 15–25% lignin, and 1–3% extractives. The composition of apricot tree resin, as adapted from the study conducted for the BAP project (Ege University—Scientific Research Projects: Project No: 15MUH015), is presented in

Table 1. Proximate and ultimate analysis of apricot tree resin.

	Biomass
Proximate analysis (wt%)
Moisture	9.20
Ash	2.35
Protein	2.05
Ultimate analysis (dry, wt%)
C	36.80
H	6.70
N	0.40
S	0.04
O (from difference)	42.46

and

Table 2. The composition of apricot tree resin.

Components (daf, wt%)	Biomass
Cellulose	36.55
Lignin	16.24
Hemicellulose	41.45
Extractives	4.76

. These structural properties indicate that apricot tree resin has the potential to be converted into valuable fuels and chemicals through thermal processes.

Biomass conversion technologies have been extensively studied for fuel and chemical production. Among these, hydrothermal gasification (HTG) has gained attention for its ability to process high-moisture biomass without the need for energy-intensive drying processes. This environmentally friendly process operates at lower temperatures compared to conventional gasification techniques, using supercritical water as a solvent, reactant, and catalyst. Studies indicate that HTG efficiently produces hydrogen and methane while minimizing tar and other undesirable by-products [3,4].

The unique significance of Hydrothermal Graphitization (HTG) lies in its ability to convert biomass into highly ordered carbon structures without necessitating a pre-drying step, a requirement common to other hydrothermal technologies. This distinction is particularly advantageous as it reduces energy consumption and operational costs, thereby enhancing the overall efficiency and sustainability of the process. The direct utilization of wet biomass eliminates the energy-intensive drying phase, which remains a major limitation in conventional thermochemical conversion methods. Furthermore, since no pre-treatment is applied in HTG, the resin structure remains in its natural state, preserving its intrinsic chemical characteristics throughout the process. Given its unique composition, apricot tree resin offers a promising precursor for carbon materials with tailored properties.

At temperatures between 300 and 500 °C, catalytic wet gasification (CWG) occurs, requiring catalysts to enhance biomass gas conversion. Subcritical water is present due to pressures exceeding the saturation value. Although the highest hydrogen and methane yields are obtained through the supercritical water gasification (SCWG) process, CWG achieves the highest aqueous product yield. Under high-temperature and high-pressure conditions, supercritical water dissolves non-polar compounds, promoting hydrogen production. The minimal formation of tars during SCWG and the dissolution of corrosive compounds like chlorides in water help prevent corrosion issues. Given the increasing importance of hydrogen and methane production from wet biomass, research into subcritical and supercritical water applications has gained significant attention [5,6]. Since thermochemical hydrogen and methane production from fossil fuels contributes to rising atmospheric carbon dioxide emissions, there is an increasing need for sustainable alternatives such as biomass-derived pathways.

The gasification of biomass through supercritical and near-critical water processes has been extensively studied for hydrogen production. Pinecone gasification under experimental conditions of 450 °C and 550 °C at 23 MPa, as well as at 370 °C at 23 MPa, demonstrated the highest hydrogen yield in the presence of a KOH catalyst [7]. Hydrogen production from eucalyptus wood chips via supercritical water gasification was investigated at temperatures of 400 °C, 450 °C, and 500 °C using varying amounts of NiFe₂O₄ catalyst, revealing up to a 60% increase in hydrogen production with catalyst addition [8]. Similarly, agro-forest wastes subjected to hydrothermal carbonization/gasification yielded 4.9 to 5.5 mol kg⁻¹ of hydrogen from canola meal [9]. The hydrothermal gasification process has also been utilized for hydrogen production from woodland residues, such as wood chips, at temperatures ranging from 300 °C to 425 °C, achieving a hydrogen efficiency of 17.85% at 425 °C [10]. Furthermore, fuel pellets obtained from co-pelletized agricultural residues were hydrothermally gasified, reaching a maximum hydrogen gasification efficiency of 113% at 500 °C [11]. The supercritical water gasification of Iranian rice straw (IRS) resulted in 5.56 mmol/g hydrogen production at 440 °C [12].

Similarly, Nanda et al. [13] reported a significant increase in hydrogen yield during the steam gasification of pinewood and wheat straw using a Ni catalyst at 500 °C. Hydrogen production was notably enhanced from 7.0 to 18.7 mmol/g of sample when utilizing the NiAZnAAl (1:1) catalyst during the pyrolysis/gasification of cellulose [14]. The steam catalytic gasification of n-C7 asphaltenes and resins at low temperatures (<230 °C) was explored using cerium oxide nanocatalysts, revealing that asphaltenes produced higher CO₂ emissions and lower hydrogen yields compared to resins [15]. The use of KOH as a catalyst was found to enhance biomass decomposition, increasing gas product yields via the water–gas shift reaction (WGSR) by forming intermediate salts and suppressing char and tar formation [16]. Additionally, Yu et al. [17] investigated the catalytic gasification of cellulose, hemicellulose, and lignin using dolomite and Na₂CO₃ as catalysts. Their findings indicated that hemicellulose gasification was significantly improved with these catalysts, leading to higher gasification efficiencies and enhanced calorific values. The influence of temperature, water injection rate (steam/biomass ratio), catalyst loading, and reaction time on pinewood conversion performance was also studied. The Ca₂Fe₂O₅ catalyst facilitated the production of H₂-rich gas, reduced tar formation, and improved carbon conversion through an inner-looping redox reaction mechanism [18]. Finally, subcritical and supercritical water gasification of lignocellulosic biomass impregnated with nickel nanocatalysts demonstrated promising results in enhancing gas yields and conversion efficiencies [13].

Despite the growing interest in hydrothermal gasification (HTG), limited research has focused on the gasification of tree resins, particularly apricot tree resin. While HTG has been widely studied in the literature, the gasification of apricot tree resin under these conditions remains an unexplored area. This study aims to address this deficiency by investigating the gasification of apricot tree resin and examining the effects of temperature and catalyst usage on the yields of hydrogen, methane, and liquid products such as furfurals, aldehydes, and carboxylic acids using advanced chromatographic techniques. Given the high combustion heat and chemical composition of tree resins, this study hypothesizes that apricot tree resin can be efficiently converted into hydrogen and methane under hydrothermal conditions. By offering new insights into the conversion of apricot tree resin, this research contributes to the broader understanding of biomass valorization and advances sustainable and efficient biofuel production methods. Moreover, it underscores the importance of exploring underutilized biomass resources for clean energy applications, positioning apricot tree resin as a promising candidate in this field.

2. Results and Discussion

This study examines the hydrothermal gasification of apricot tree resin, with a focus on the yield and chemical composition of the produced gas and aqueous phases. K₂CO₃ and KOH were utilized as catalysts within a temperature range of 300–600 °C, maintaining a constant reaction time of 60 min. To ensure reproducibility, each experiment was performed at least three times, and the average yield values were determined accordingly. After each experimental run, a carbon balance was established, and the yields of gaseous, liquid, and solid products were quantified. The carbon content in the gas phase was determined by quantifying carbonaceous molecules using gas chromatography, while the carbon content in the liquid phase was measured via total organic carbon (TOC) analysis. Additionally, the carbon content in the solid residue was measured by the solid sample module of the TOC analyzer. Elemental analysis of apricot tree resin (Table 1) revealed a high carbon content of 36.80%, along with minor amounts of nitrogen (0.40%) and sulfur (0.04%). The overall carbon atom balance between the feedstock and the measured products was found to be within ±5%, which is considered acceptable given experimental and analytical uncertainties. Carbon recovery was assessed by evaluating the carbon distribution across all phases, yielding a product carbon balance in the range of 95–99%. The slight discrepancy in the carbon balance is likely due to minor product losses during analysis. The yield of each product was calculated based on the proportion of carbon distributed among the gas, liquid, and solid residues relative to the total carbon content in the products. The equations for the product yields are given below:

$$\mathrm{Gas}\mathrm{Product}\mathrm{Yield}={\displaystyle \frac{Moles of Carbon atoms in the gas phase}{Moles of Carbon atoms in the products}}\ast \mathbf{100}\%$$

(1)

$$\mathrm{Liquid}\mathrm{Product}\mathrm{Yield}={\displaystyle \frac{Moles of Carbon atoms in the liquid phase}{Moles of Carbon atoms in the products}}\ast \mathbf{100}\%$$

(2)

$$\mathrm{Solid}\mathrm{Residue}\mathrm{Product}\mathrm{Yield}={\displaystyle \frac{Moles of Carbon atoms in the solid phase}{Moles of Carbon atoms in the products}}\ast \mathbf{100}\%$$

(3)

2.1. Influence of Temperature and Catalyst on the Distribution of Products and Gas Composition

Table 3. Effect of increasing temperature on the product yields of biomass in the absence and in the presence of catalysts.

	Non-Catalytic				K2CO3				KOH
Reaction Temp. (°C)	300	400	500	600	300	400	500	600	300	400	500	600
Reactor Pres. (MPa)	12.0	24.0	36.0	45.0	11.5	24.5	35.5	45.5	12.5	24.0	36.0	45.5
Product yield (C %)
Gas	24.4	32.3	58.4	77.6	27.0	35.7	66.2	86.4	28.2	43.9	76.1	92.7
Liquid	49.1	43.8	22.1	11.2	54.2	46.4	18.6	6.4	60.8	46.2	17.6	3.8
Residue	22.8	20.5	16.7	7.4	15.1	13.5	11	4.5	8.7	7.4	4.5	3.1

illustrates the product yields obtained from the hydrothermal gasification of apricot tree resin, both in the absence and presence of a catalyst (10% mass fraction). As demonstrated by the experimental results (Figure 1, Figure 2, Figure 3 and Figure 4), gas yield exhibited an increasing trend with temperature. Specifically, as the reaction temperature increased from 300 °C to 600 °C, the gas yield rose from 24 wt% to 77 wt%, while the residue yield decreased from 22 wt% to 7 wt%. Similarly, the liquid yield declined from 49 wt% to 11 wt%. This enhancement in product yields can be attributed to temperature-induced changes in water properties. In many processes where supercritical water is used, it not only serves as a medium but also plays a crucial role in catalyzing and/or participating in reactions essential to the process.

Above its critical point (374 °C), water undergoes significant physical transformations, including alterations in density, viscosity, dielectric constant, and its ability to dissolve hydrocarbons and salts. In its supercritical state, water behaves as a homogeneous fluid, exhibiting gas-like viscosity and liquid-like density, which enhances mass transfer and solvation capabilities [19]. Moreover, under supercritical conditions, the dielectric constant of water decreases substantially, leading it to behave as a non-polar solvent. As a result, water becomes miscible with gases and various hydrocarbons, thereby facilitating the gasification of biomass. Several studies have demonstrated that temperature is a key parameter influencing the efficiency of supercritical water gasification (SCWG) of biomass. Kumar et al. [20] emphasized that higher temperatures enhance reaction kinetics, improve gasification efficiency, and influence the composition of the produced syngas. Similarly, Okolie et al. [21] reported that elevated temperatures lead to increased gas yield and improved carbon conversion efficiency. Castello et al. [22] also highlighted that higher temperatures accelerate reaction rates, enhance gas production, and reduce the residence time required for complete gasification. These findings collectively indicate that optimizing temperature conditions is crucial for improving the overall performance of HTG processes.

A wide range of thermochemical reactions, including pyrolysis, hydrolysis, steam reforming, water–gas shift, and methanation, play a fundamental role in biomass gasification chemistry by facilitating the breakdown of complex organic structures into smaller gaseous products. These reactions govern the transformation of solid and liquid feedstocks into valuable gases such as hydrogen (H₂), carbon monoxide (CO), methane (CH₄), and carbon dioxide (CO₂), influencing both the efficiency and selectivity of the gasification process. Understanding the interplay between these reactions is essential for optimizing reaction conditions, maximizing desired product yields, and improving the overall sustainability of biomass-to-energy conversion technologies. The hydrothermal gasification of biomass can be shown by the following reaction:

$$C{H}_x{O}_y+\left(2-y\right)H_{2}O \to C{O}_2+\left(2-y+x/2\right)H_2$$

(4)

where x and y represent the molar ratios of H/C and O/C, respectively.

Equation (4) describes the overall reaction. However, intermediate reactions are essential for understanding the hydrothermal gasification process, as shown below:

$$\mathrm{Steam}\mathrm{reforming}:C{H}_x{O}_y+\left(1-y\right)H_{2}O \to CO+\left(1-y+x/2\right)H_2$$

(5)

$$\mathrm{Water}\mathrm{gas}\mathrm{shift}:CO+H_{2}O \leftrightarrow C{O}_2+H_2$$

(6)

$$\mathrm{Methanation}:CO+3H_2\leftrightarrow C{H}_4+H_{2}O$$

(7)

$$C{O}_2+4 H_2\leftrightarrow C{H}_4+2H_{2}O$$

(8)

The theoretical equations derived from elemental analysis were used to determine the optimal composition of hydrothermal gasification products. Based on Equation (4), the following reactions can be written for apricot tree resin:

$$C{H}_{1.8}{O}_{1.0}+H_{2}O \to C{O}_2+1.9 H_2$$

(9)

$$C{H}_{1.8}{O}_{1.0}+0.05 H_{2}O \to 0.525 C{O}_2+0.475 C{H}_4$$

(10)

In addition to the effect of temperature, the presence of the catalysts K₂CO₃ and KOH demonstrated a positive catalytic effect on the conversion to gaseous products. It was observed that the yields of liquid and solid products were inversely proportional to temperature, with these yields decreasing as temperature increased. The highest gas product and the lowest residue were obtained at 600 °C in the presence of the KOH catalyst. When the reaction temperature increased from 300 °C to 600 °C in the presence of the K₂CO₃ catalyst, the gas yield increased from 27 wt% to 86 wt%, while the residue yield decreased from 15 wt% to 4 wt%. In the presence of KOH, increasing the reaction temperature from 300 °C to 600 °C resulted in a gas yield increase from 28 wt% to 92 wt%, while the residue yield decreased from 8 wt% to 3 wt%. The highest liquid product yield was observed at 300 °C with KOH.

The product gas composition was predominantly rich in carbon dioxide, followed by hydrogen and methane. In addition to these major components, minor quantities of carbon monoxide and C2–C4 hydrocarbons, such as propane, propylene, and butane, were also detected. The gas compositions of H₂, CH₄, and CO₂ derived from apricot tree resin are presented in Figure 5, Figure 6 and Figure 7, expressed in mol/kg C in biomass, at varying temperatures for each catalyst. The concentrations of hydrogen and methane in the gas effluent increased significantly with rising reaction temperature, reaching their highest levels at 600 °C. Among the tested catalysts, K₂CO₃ exhibited the highest efficiency in promoting H₂ and CH₄ production from apricot tree resin.

The catalytic activity of K₂CO₃ in hydrothermal gasification can be attributed to its ability to facilitate the conversion of intermediates into formate salts. Similarly, the positive influence of NaOH on hydrogen production can be explained by its role in enhancing the water–gas shift reaction (Equation (6)), which promotes the conversion of carbon monoxide and water into hydrogen and carbon dioxide.

K₂CO₃ + H₂O → KHCO₃ + KOH

(11)

KOH + CO → HCOOK

(12)

The reaction between water and formate forms hydrogen:

H₂O + HCOOK → KHCO₃ + H₂

(13)

By decomposition of KHCO₃, carbon dioxide is produced:

2KHCO₃ → H₂O + K₂CO₃ + CO₂

(14)

The reduction in the yields of liquid and solid products underscores the catalytic role of K₂CO₃ and KOH in enhancing the conversion of biomass to gaseous products, emphasizing the critical influence of temperature in thermal conversion processes. A study investigating the supercritical water gasification (SCWG) of fruit pulp reported that, at 600 °C, with a 2.5% biomass loading and 10% KOH, hydrogen production reached 32.1 mol/kg of biomass. Under comparable conditions, 20% K₂CO₃ resulted in hydrogen production of 29.4 mol/kg of biomass, confirming the catalytic effectiveness of KOH and K₂CO₃ in SCWG [16]. This study’s findings align with those of previous research, which also emphasized the increase in gas yield and hydrogen content.

In addition, a study on the SCWG of sewage sludge from a municipal wastewater treatment plant, conducted at 650 °C with a 2% solid loading and 2% KOH, demonstrated that the hydrogen content in the produced gas exceeded 60%. This further validates the positive catalytic effect of KOH in promoting hydrogen production under supercritical conditions [23]. Our findings align with these studies, reinforcing the notion that alkali catalysts significantly boost gasification efficiency and hydrogen production in HTG processes. The observed improvements in gas yield, particularly hydrogen, suggest that K₂CO₃ and KOH are highly effective in facilitating the decomposition of biomass and enhancing the overall conversion process. Smith et al. [24] explored the role of temperature in enhancing the hydrogen yield from lignocellulosic biomass, which aligns with the findings of this study, where increasing temperature resulted in higher gas yields and lower solid residues. Similarly, Chen et al. [25] investigated the catalytic effects of alkali catalysts, specifically K₂CO₃ and KOH, on various biomass types, showing that these catalysts improve the conversion of gaseous products by facilitating the breakdown of complex organic molecules. In this study, it was found that KOH significantly enhanced gas yields, particularly at 600 °C, resulting in a higher conversion efficiency, which corroborates their findings. Moreover, Jones et al. [26] focused on the conversion of pine resin through HTG, highlighting the importance of temperature control for achieving the highest gas yield. They found that temperatures above 500 °C promoted the formation of CO₂ and H₂ in significant amounts, which was consistent with the results of this study at similar temperatures. Kumar et al. [27] conducted experiments on various alkali catalysts for HTG and observed that potassium-based catalysts were more effective than sodium-based ones, especially in promoting methane production. This is in agreement with the findings of this study, where KOH led to the highest gas yield and methane formation at 600 °C.

In conclusion, alkali catalysts, particularly K₂CO₃ and KOH, play a vital role in improving the efficiency of hydrothermal gasification, increasing hydrogen and methane yields, and promoting the conversion of biomass into valuable gaseous products. The findings of this study extend existing knowledge by demonstrating the catalytic performance of KOH and K₂CO₃ in the gasification of apricot tree resin, emphasizing the role of temperature in optimizing product distribution. By comparing the results of this study with previous studies, this study highlights the advantages of alkali-catalyzed HTG in maximizing gas yields and reducing undesirable by-products. These insights contribute to the broader goal of improving biomass valorization strategies and advancing sustainable energy production through HTG processes.

2.2. Influence of Temperature and Catalyst on the Aqueous Product Efficiency and Aqueous Composition

The aqueous phase contained a wide range of compounds; however, only the most significant ones were identified to assess the influence of reaction conditions. The diversity of these liquid-phase products contributed to the total organic carbon (TOC) content. As the reaction temperature increased, TOC values declined due to the breakdown of aqueous compounds into gaseous products. Figure 8 illustrates the impact of temperature and catalyst on TOC yields during the hydrothermal gasification. Additionally, Figure 9 presents the yields of primary organic compounds—such as carboxylic acids, furfurals, aldehydes, ketones, and phenols—identified in the liquid products obtained from the gasification of apricot tree resin within the temperature range of 300 °C to 600 °C.

The aqueous phase was particularly rich in carboxylic acids, including glycolic acid, acetic acid, and formic acid, which are degradation products of furfural or 5-hydroxymethylfurfural (5-HMF). Among these, carboxylic acids and 5-methyl furfural were the predominant liquid-phase compounds, reaching their highest concentration at 300 °C in the presence of the K₂CO₃ catalyst, as seen in Figure 9. The addition of alkali catalysts reduced the concentration of key acidic intermediates, such as glycolic, acetic, and formic acids, likely due to the formation of potassium and sodium formates.

The degradation of carbohydrates predominantly contributes to the formation of furfural and HMF, which are generated through rapid hydrolysis and dehydration reactions. During this process, key intermediates such as 5-HMF, methyl furfural (MF), and furfural (FU) are formed. 5-HMF is recognized as a highly significant compound due to its unique structure, which consists of an aldehyde group, a hydroxyl group, and a furan ring. As a result, HMF exhibits strong chemical reactivity and can be readily converted into a wide range of valuable derivatives. In recent years, extensive studies have focused on the synthesis, physicochemical properties, and commercial potential of HMF-derived compounds, including conventional derivatives such as 2,5-dimethylfuran (DMF), 5-ethoxymethylfurfural (EMF), ethyl levulinate (EL), long-chain alkanes (LLA), levulinic acid (LA), 2,5-diformylfuran (DFF), and 2,5-furandicarboxylic acid (FDCA) [28].

Under subcritical conditions, furfural concentrations were observed to be significantly high, indicating that moderate temperatures favor its formation. As the temperature increases, degradation reactions predominantly proceed via a free-radical mechanism, which facilitates the breakdown of biomass components into smaller molecular fragments. This process leads to the formation of various gaseous products, including aldehydes, furfurals, and phenols, which are key intermediates in biomass pyrolysis and gasification. In contrast, at lower temperatures, ionic reaction mechanisms become more dominant, promoting the polymerization and condensation of reactive intermediates. This results in the accumulation of high-molecular-weight by-products such as coke and tar, which can negatively impact process efficiency by causing reactor fouling and catalyst deactivation. When the temperature exceeds the critical point of water (~374 °C), the physicochemical properties of water change significantly, leading to enhanced solubility of organic compounds and increased reaction rates. Under these supercritical conditions, cellulose and hemicellulose undergo rapid hydrolysis and decomposition, primarily yielding organic acids and small oxygenated compounds due to the prevalence of free-radical reactions. These drastic changes in reaction pathways highlight the critical role of temperature in dictating the selectivity and efficiency of biomass conversion processes.

The aqueous phase contained a diverse range of phenolic compounds, including ortho-, para-, and meta-cresols, along with various other phenols, which were identified and quantified using high-performance liquid chromatography (HPLC) analysis. These phenolic compounds are primarily derived from the thermal degradation of lignin, which undergoes cleavage of ether and carbon-carbon bonds under hydrothermal conditions. At elevated temperatures, the degradation of carboxylic acids predominantly follows two key pathways: decarboxylation and dehydration. Decarboxylation involves the removal of a carboxyl group (-COOH) as carbon dioxide (CO₂), while dehydration leads to the loss of water molecules, often resulting in the formation of unsaturated intermediates. Among these pathways, the decarboxylation of formic acid is particularly significant, as it serves as a primary route for the production of major gas-phase products such as CO₂ and hydrogen (H₂). These gaseous products contribute to the overall gas yield in the system and play a crucial role in influencing the composition and energy potential of the resulting syngas. Understanding these reaction mechanisms is essential for optimizing biomass conversion processes, as controlling temperature and reaction conditions can help enhance selectivity toward desired products while minimizing undesirable by-products such as tar and char.

3. Materials and Methods

3.1. Materials

The apricot tree (Prunus armeniaca) has the potential to reach significant sizes and ages, supported by a robust root system that exhibits extensive annual growth. This tree species is characterized by its high vigor and resilience, demonstrating resistance to most diseases and insect infestations. While apricot trees are cultivated across a wide geographical range, their cold tolerance is well-documented; however, early frosts pose a risk to fruit development. Typically, apricot trees are pruned to maintain a height of approximately 12 feet, though in natural conditions, they can attain heights of up to 45 feet at full maturity. In addition to fruit production, apricot trees naturally exude resin, which has potential industrial and medicinal applications (Figure 10). The annual resin yield of an apricot tree varies depending on environmental conditions, tree age, and species, but some trees can produce up to several hundred grams per year. In this study, naturally exuded resin droplets were collected from apricot trees in Turkey to evaluate their composition and potential applications. Elemental analysis of apricot tree resin was conducted using a CHNS-932 elemental analyzer (Leco, St. Joseph, MI, USA). The proximate and ultimate analyses of the biomass sample are presented in Table 1. The cellulose, hemicellulose, and lignin contents of the biomass were determined using the Van Soest method [29] and are provided in Table 2.

3.2. Experimental System

Catalytic gasification experiments were carried out using a stainless steel batch reactor setup. To investigate the effect of temperature on product yield and composition, a mixture of 1.2 g of apricot tree resin and 10 wt% catalyst was prepared and loaded into the reactor, along with 15 mL of water. To assess the catalytic effect, experiments were conducted both with and without the addition of a catalyst. Pressure and temperature were monitored using an analog manometer and a thermocouple. Mixing was achieved through a motor-driven tumbling mechanism. A detailed description of the experimental setup is available in our previous publication [30].

3.3. Experimental Procedure

After loading the feed mixture into the autoclave, the reactor was flushed with inert nitrogen gas to remove any residual air. Before each experiment, the reactor was heated to the desired reaction temperature at a rate of 10–15 K min⁻¹ and maintained at that temperature for 60 min using a Proportional-Integral-Derivative (PID) temperature controller, as determined to be optimal in previous studies. At the end of the reaction, the reactor was rapidly cooled by quenching in cold water and allowed to reach ambient temperature. The total gas volume was measured using a gasometer after expansion to ambient pressure, with an accuracy of ±10%. Gas samples were collected using gas-tight syringes for gas chromatography analysis. The liquid and solid products remaining in the reactor after gas sampling were washed out with water and filtered to separate the solid residue (coke). To inhibit the ionization of organic acids, the pH of the aqueous phase was adjusted to 2 by adding 1–2 drops of concentrated sulfuric acid. Following each experiment, the reactor was cleaned with tetrahydrofuran (THF) and water to prevent plugging issues caused by solid residues such as tar and coke. The solid residue was then dried in an oven at 105 °C for further analysis using the Solid Sample Module (SSM).

3.4. Analysis of Gaseous, Aqueous, and Solid Products

The gaseous products were analyzed using gas chromatography (GC) with an HP 7890A system (Wilmington, DE, USA). This system was equipped with seven serially connected columns: Hayesep Q 80/100 mesh (0.5 m × 2 mm i.d.), Hayesep Q 80/100 mesh (1.8 m × 2 mm i.d.), Molsieve 5A 60/80 mesh (2.4 m × 2 mm i.d.), Hayesep Q 80/100 mesh (0.9 m × 2 mm i.d.), Molsieve 5A 60/80 mesh (2.4 m × 2 mm i.d.), a DB-1 pre-column, and an HP-Plot Al₂O₃ S column (25 m × 0.32 mm i.d.). The system included two thermal conductivity detectors (TCD) and a flame ionization detector (FID) in a serial arrangement. Helium was used as the carrier gas, and the oven temperature program was as follows: initially held at 60 °C for 1 min, then increased to 80 °C at a rate of 20 °C min⁻¹, and finally raised to 120 °C at a rate of 30 °C min⁻¹ for 2.66 min. The gas products were identified based on retention time and quantified using external calibration with a standard gas mixture. Each gas sample was injected twice using gas-tight syringes, and the composition of gaseous products (H₂, CO₂, CO, and C₁–C₄ hydrocarbons) was determined by gas chromatography, with the final values taken as the average of the two injections. The standard deviation for gas composition results was calculated as ±2%.

The total organic carbon (TOC) content of the aqueous products was determined using a TOC analyzer (Shimadzu TOC-VCPH, Kyoto, Japan), with standard solutions prepared using potassium hydrogen phthalate for calibration. The TOC of the solid residue was measured using the solid sample module (SSM-5000A) of the same TOC analyzer. Aqueous products were qualitatively and quantitatively analyzed using a high-performance liquid chromatography (HPLC) system. The carbon content in the gas, liquid, and solid phases was measured to calculate the carbon recovery of the experiments.

HPLC analyses were performed with a Shimadzu LC-20A series liquid chromatography system equipped with an Inertsil ODS-4 column (250 mm × 4.6 mm i.d.). The system included a DGU-20AS degassing module, an LC-20AT gradient pump, a CTO-10ASVP chromatography oven, and an SPD-20 multi-wavelength ultraviolet detector. Carboxylic acids, phenols, and furfurals were analyzed using the following method: mobile phases—A: 0.05 vol.% H₃PO₄ (pH: 2.25), B: CH₃CN/H₂O (80/20 v/v); flow rate—1 mL/min; column temperature—30 °C; detector wavelength transitions: 0 min (210 nm), 5 min (290 nm), 7 min (285 nm), 11 min (278 nm), 15 min (232 nm), 17 min (90% A, 10% B), 19 min (290 nm), 25 min (65% A, 35% B), 27.5 min (290 nm), and 55 min (65% A, 35% B).

4. Conclusions

This study examines the hydrothermal gasification of apricot tree resin, with a focus on the yield and chemical composition of the produced gas and aqueous phases. K₂CO₃ and KOH were utilized as catalysts within a temperature range of 300–600 °C, maintaining a constant reaction time of 60 min. The experimental findings indicate that temperature and catalyst selection play a crucial role in determining gas yields, liquid composition, and solid residue formation. Higher temperatures led to increased gaseous product yields while reducing the formation of aqueous and solid residues. The catalytic influence of K₂CO₃ and KOH significantly enhanced gaseous product conversion, with KOH exhibiting the highest gas yield and the lowest residue formation at 600 °C. These findings collectively indicate that optimizing temperature conditions is crucial for improving the overall performance of HTG processes.

Among the liquid-phase compounds, carboxylic acids and 5-methyl furfural were the predominant compounds, reaching their highest concentration at 300 °C in the presence of the K₂CO₃ catalyst. The addition of alkali catalysts reduced the concentration of key acidic intermediates such as glycolic, acetic, and formic acids, likely due to the formation of potassium and sodium formates. The inverse relationship between temperature and liquid/solid product formation further highlights the importance of optimizing reaction conditions for efficient biomass conversion.

These results contribute to the growing body of research on biomass valorization by emphasizing the role of underutilized tree resins in sustainable biofuel production. By advancing our understanding of tree resin gasification, this research paves the way for innovative approaches to renewable hydrogen production and the broader development of bio-based energy solutions.