Preparation of a solid acid derived from Japanese cedar sawdust and evaluation of its the catalytic activity

The preparation of a solid acid by chemical treatment of plant-derived biomass has been explored, along with an evaluation of its catalytic activity. The solid acid was prepared by pretreating Japanese cedar sawdust with hydrochloric acid, followed by sulfonation using sulfuric acid and fuming sulfuric acid. Surface analysis using energy-dispersive X-ray spectroscopy confirmed the presence of sulfur atoms on the surface, while X-ray photoelectron spectroscopy demonstrated the introduction of sulfo groups into lignin, forming a plant-derived biomass. A pH titration using NaOH revealed an acidity of 2.25 mmol g^–1, indicating that Japanese cedar sawdust can function as an acid following chemical treatment. Therefore, the use of solid acids as catalysts for esterification reactions, as an alternative to mineral acids, was investigated. Specifically, the reaction between acetic acid and ethanol to form ethyl acetate proceeded at room temperature, reaching chemical equilibrium within 570 min following the addition of the solid acid catalyst. The reaction rate increased with temperature, and the catalyst maintained high activity even at high temperatures. Although heterogeneous catalysts, such as solid acids have low contact efficiencies, they can still catalyze esterification reactions, similarly to mineral acids. Furthermore, solid acids exhibit greater catalytic capacity at elevated temperatures, while also being recyclable. Overall, this study presents a novel application of unused biomass in catalysis, demonstrating its potential as a sustainable catalyst.

Petroleum resources are used to synthesize chemical materials, but have an environmental impact in terms of carbon dioxide emissions and resource depletion. To reduce environmental impact, the use of plant-derived biomass has been explored. Nearly 70% of Japan's land area is forested; however, in recent years, timber imports have increased, and the domestic forestry industry has stagnated. Therefore, forested areas that should be protected and managed through thinning and undercutting are left unattended, resulting in environmental degradation of forests [1]. Furthermore, forest residues pose social problems because they flow into rivers during natural disasters such as heavy rainfall, which can cause secondary disaster events [2]. One way of mitigating this issue is to convert plant-derived biomass, specifically forest residues, into useful, chemical materials.

The main components of plant-derived biomass are cellulose, hemicellulose, and lignin, which form complex intertwined higher-order structures [3]. Cellulose has a regular chain structure and is the main component of cell walls. Hemicellulose is a branched polysaccharide with a degree of polymerization between 50 and 200, which is lower than that of cellulose, and is soluble in alkaline solutions due to its amorphous and less ordered structure [1]. Therefore, they are easily degraded and soluble in alkaline solutions. In contrast, lignin is an amorphous, highly branched aromatic polymer composed mainly of phenylpropanoid units such as coniferyl, sinapyl, and p-coumaryl alcohols. These units are polymerized via radical coupling reactions, resulting in a heterogeneous and irregular macromolecule rather than a well-defined three-dimensional network [4]. Lignin is not covalently bound to cellulose but rather associates with it through non-covalent interactions or lignin–carbohydrate complexes (LCCs), contributing to the mechanical strength of plant cell walls. Furthermore, it exhibits hydrophobicity in the cell wall, which is thought to reduce water permeability in vascular tissues. Thus, lignin contributes to the structural integrity of plants and helps reduce degradation by microbial or environmental factors [5]. However, using lignin as a chemical material is challenging because of its chemical complexity and resistance to depolymerization. Color change in lignin is not necessarily a major factor limiting its application, but it can be relevant in material processing depending on the intended use. Therefore, woods from which lignin has been removed or trees with low lignin content are typically used for chemical materials. Although lignin can be extracted and utilized as a lignophenol [6,7,8], and is used as Klason lignin from bamboo and red pine [9, 10], this study this study investigates the effective utilization of lignin-rich Japanese cedar sawdust, which is directly subjected to acid hydrolysis to concentrate the lignin component and then sulfonated under mild conditions. This approach allows the preparation of carbon-rich solid acids without isolating pure lignin, offering a simpler and more sustainable route compared to conventional lignin sulfonation methods.

Zeolites, metal oxides, sulfates, phosphates, and cation-exchange resins are examples of solid acids [11, 12]. These materials are used in the chemical industry as catalysts for hydration, dehydration, alkylation, and esterification reactions [13]. In particular, cation exchange resins donate protons and therefore have catalytic activity as Brønsted acids [14]. Recently, sulfonated carbon materials, especially those derived from lignin or lignosulfonate, have been widely studied as solid acid catalysts owing to their strong Brønsted acidity and thermal stability [15, 16]. Inspried by these studies, we prepared and evaluated a lignin-derived solid acid containing sulfo groups, obtained from Japanese cedar sawdust using concentrated and fuming sulfuric acid.

Solid acids are classified as heterogeneous catalysts and are increasingly replacing liquid homogeneous catalysts; however, their reduced contact efficiency with reactants remains a disadvantage [17]. Chemical treatment of plant-derived biomass increases the specific surface area and improves contact efficiency, rendering it suitable for use as a catalyst. Furthermore, solid acids can be separated from the reactants and products (i.e. gases and liquids) and can be used repeatedly, whereas mineral acids (e.g. sulfuric acid, hydrochloric acid, and nitric acid), which are liquids, are difficult to separate from products following reactivity and require considerable energy for disposal. Therefore, the use of solid acids as acid catalysts instead of mineral acids can reduce the environmental impact of catalyst use.

In this study, solid acids derived from Japanese cedar sawdust were prepared, which is one of the most lignin-rich types of plant-derived biomass. We report that the resulting solid acid exhibits appreciable acidity and demonstrates satisfactory catalytic activity.

Chemicals and apparatus

Japanese cedar sawdust (60 mesh) was gifted by Toyo Jushi Co., Ltd. Sulfuric acid was purchased from Yoneyama Chemical Industry and fuming sulfuric acid was obtained from Kanto Chemical. Analytical grade acetic acid (Wako Pure Chemical Industries) was used. All other chemicals were commercially available and of extra pure grade.

The surface of the solid acid derived from Japanese cedar sawdust was analyzed using scanning electron microscopy (SEM, JSM-IT700HR, JEOL Ltd.). Surface roughness was measured using a laser microscopy (VK-X3000, Keyence Corporation). Particle size distribution was laser diffraction particle size distribution analyzer (SALD-2300, Shimadzu Corporation). X-ray photoelectron spectroscopy (XPS, Versa Probe CU, ULVAC-PHI, Inc.) was used to analyze surface functional groups. pH was measured using a pH meter (HM-30 V, DKK-TOA Corporation). Ester formation was confirmed by gas chromatography (GC-14B, Shimadzu Corporation) using a capillary column (DB-WAX, Agilent Technologies, Ltd.).

Solid acid prepares

Concentrated hydrochloric acid (75 mL) was gradually added to 15 g of Japanese cedar sawdust. The mixture was heated to approximately 100 °C for 2 h with stirring. After heating, the reaction was quenched by cooling the mixture to room temperature. The solid was then collected by filtration and thoroughly washed with 500 mL of distilled water followed by 200 mL of acetone to remove soluble residues and reaction byproducts. After the mixture was dried, pretreated Japanese cedar sawdust was obtained [18]. The pretreated Japanese cedar sawdust (5 g) was immersed in 50 mL of concentrated sulfuric acid (98%) at room temperature (approximately 25 °C) for 20 min. Subsequently, 5 mL of fuming sulfuric acid (20% SO3) was slowly added dropwise over 5 min under continuous stirring, and the reaction was continued at room temperature for an additional 40 min. The mixture was then washed with water and dried at 100 °C, and the resulting solid acid on Japanese cedar was obtained (crude yield: 4 g) [19]. To evaluate the composition of the product, the holocellulose content was determined using the sodium sulfite method. The holocellulose content decreased from 57% in the raw Japanese cedar sawdust to 26% after sulfuric acid treatment, indicating that the resulting material is highly enriched in lignin. Therefore, we refer to this material as "Cedar-lignin-SO3H" in this study.

True density measurement of Cedar-lignin-SO₃H

The true density of the Cedar-lignin-SO3H sample was measured using a pycnometer (25 mL) with distilled water as the reference liquid. Approximately 1.0 g of the powdered sample was used for each measurement. The sample was gently added to the pycnometer, filled with water, and the mass difference before and after sample addition was used to calculate the volume displaced. The measurements were performed at 25 °C and repeated three times to ensure accuracy.

Determination of acidity for Cedar-lignin-SO₃H

Cedar-lignin-SO₃H (1 g) was placed in a 200 mL beaker, to which 40 mL of pure water was added. The mixture was stirred for 30 min. Then, the mixture was titrated with a 0.05 mol L^–1 NaOH solution, and a pH titration curve was plotted. The acidity was determined using Eq. (1):

Esterification reaction using Cedar-lignin-SO₃H

Cedar-lignin-SO₃H (1 g) was added into a 50 mL flat bottom flask 2 mL of containing acetic acid and 20 mL of ethanol, and the effect of changing the REACTION temperature was evaluated. Aliquots of the reaction mixture were removed and analyzed, and the formation of ethyl acetate was confirmed by GC. Equilibrium was defined as the time point at which the yield of ethyl acetate remained constant (± 1%) over 30 min, indicating that no further conversion was occurring.

Evaluation of the physical properties for Cedar-lignin-SO₃H

The SEM images of Japanese cedar sawdust, pretreated Japanese cedar sawdust, and Cedar- lignin-SO₃H are shown in Fig. 1.

The surface of the Japanese cedar sawdust was initially smooth (Ra; 1.56 ± 0.22, Rz; 19.14 ± 6.05); however, following pretreatment and chemical treatment with sulfuric acid and fuming sulfuric acid, it became rough (Ra; 6.48 ± 1.23, Rz; 44.22 ± 9.74). The particle size distribution results are shown in Fig. 2.

For the Japanese cedar sawdust, mode size was 419.6 μm and median size was 389.8 μm, while for Cedar- lignin-SO₃H, mode size was 100.6 μm and median size was 92.8 μm. These changes resulted in an increase in specific surface area and an improvement in contact efficiency. Therefore, the disadvantages of heterogeneous solid acids were overcome. Furthermore, the apparent density for Japanese cedar sawdust, pretreated Japanese cedar sawdust, and Cedar- lignin-SO₃H was 0.236, 1.614, and 1.839 g cm^–3, respectively; while Japanese cedar sawdust has a density less than 1 and floats on water, Cedar- lignin-SO₃H has an apparent density greater than 1 and is therefore suitable for solid–liquid reactions.

Next, the elemental distribution on the Cedar- lignin-SO₃H surface was investigated using energy-dispersive X-ray spectroscopy (EDS, Fig. 3).

The pretreated Japanese cedar sawdust contained several carbon-containing chemical environments on its surface, whereas Cedar- lignin-SO₃H contained sulfur derived from sulfo groups. The S 2p XPS spectrum of the sulfur atoms on the Cedar- lignin-SO₃H surface is shown in Fig. 4; S1 (S–C: 164.1 eV) is assigned to the bond between carbon of the Japanese cedar sawdust and the sulfo group. S2 (S–O: 164.9 eV) and S3 (SO₃2–, SO₂, –SO₂–: 165.5–166.9 eV) indicate the presence of surface sulfo groups.

These spectra are consistent with those reported previously [20,21,22]. In addition, a peak at 170.0 eV associated with a –C–O–SO₃H group was not observed. Therefore, the bond between Japanese cedar sawdust and the sulfo group is best described as –C–SO₃H and not –C–O–SO₃H. Furthermore, the sulfo groups were likely introduced into aromatic domains presumed to be lignin-derived in the Japanese cedar sawdust, based on indirect evidence such as the observed decrease in holocellulose content and the presence of aromatic signals in the XPS spectrum. In addition, a combustion analysis revealed that 2.06 mmol g⁻¹ of sulfo groups were introduced.

The acidity of Cedar-lignin-SO₃H was determined by constructing a pH titration curve (Fig. 5).

An equivalence point was located near pH 4.5, giving rise to an acidity of 2.25 mmol g^–1 on the basis of hydronium ion concentration calculated from the amount of NaOH required to reach the equivalence point of the titration.

Evaluation of the catalytic activity of Cedar-lignin-SO₃H in esterification reactions

The results of esterification using Cedar- lignin-SO₃H are shown in the Fig. 6.

At a reaction temperature of 25 °C, the reaction proceeded slowly and reached equilibrium after 570 min. However, the rate of the reaction increased with increasing temperature; the reaction reached equilibrium within 60 min at a temperature of 100 °C. Overall, the prepared Cedar- lignin-SO₃H catalyst exhibited good reactivity at elevated temperatures.

The catalytic activity of Cedar- lignin-SO₃H was compared with those of common mineral acids, such as H₂SO₄, HCl, and HNO₃, and a commercially available strongly acidic cation exchange resin, SK-1B; the results are summarized in Table 1. Instead of Cedar- lignin-SO₃H, 1 mL of commercially available mineral acids and 1.0 g of SK-1B were added, respectively.

Although the catalytic activity of Cedar-lignin-SO3H was somewhat lower than that of commercial mineral acids and SK-1B resin in terms of time to equilibrium, it exhibited superior reaction rate compared to nitric acid and SK-1B at 100 °C. Notably, unlike conventional lignin sulfonation methods that require lignin isolation or chemical pulping processes, our one-pot method directly converts Japanese cedar sawdust into a functional solid acid under mild conditions. This approach eliminates the need for lignin extraction and complex purification steps, offering a simpler, more sustainable, and practical alternative for preparing lignin-derived Brønsted acid catalysts. The catalyst demonstrated sufficient activity for esterification reactions and holds promise for green catalytic applications.

Next, the recyclability of Cedar- lignin-SO₃H in esterification reactions was investigated. When Cedar- lignin-SO₃H was used at a reaction temperature of 150 °C, 1 mL of acetic acid was completely esterified. Upon completion of the reaction, Cedar- lignin-SO₃H was recovered by filtration, washed with hydrochloric acid, and reused in the esterification reaction. The reaction proceeded at 100% until the sixth, but the reaction rate was about 75% after the seventh. Future efforts will focus on investigating the reaction conditions for catalyst reusability and improving the reaction rate.

To demonstrate utilization and application of forest residue, Japanese cedar sawdust was chemically treated to prepare a solid acid that contains sulfo groups. The sulfo groups were introduced into lignin, forming Cedar- lignin-SO₃H, as confirmed by energy-dispersive X-ray spectroscopy (EDS) and XPS. The acidity was determined by titration and found to be 2.25 mmol g^–1. The prepared Cedar- lignin-SO₃H, when used as a catalyst for esterification reactions, showed significant catalytic activity even at room temperature. The rate of reaction increased with temperature, showing stable catalytic activity even at high temperatures. Although the solid is a heterogeneous catalyst and has a lower contact efficiency than homogeneous liquid catalysts, Cedar- lignin-SO₃H was utilized as an efficient esterification catalyst in the same manner as a mineral acid. In addition, Cedar- lignin-SO₃H may be recovered after the reaction, making it reusable and reducing its environmental impact.

Not applicable.

LCCs:

Lignin-carbohydrate complexes

GC:

Gas Chromatography

SEM:

Scanning Electron Microscopy

XPS:

X-ray Photoelectron Spectroscopy

EDS:

Energy-dispersive X-ray spectroscopy

This research was supported by the Hibi Science Foundation.

Authors and Affiliations

1. Department of Applied Chemistry, Graduate School of Engineering, Chubu University, 1200 Matsumoto, Kasugai, Aichi, 487-8501, Japan

   Toshiyuki Miyauchi, Yui Igaki, Ryoya Murata, Sumire Tanabe, Masaki Tsujimoto & Yuko Shikami

2. Department of Applied Chemistry, College of Engineering, Chubu University, 1200 Matsumoto, Kasugai, Aichi, 487-8501, Japan

   Toshiyuki Miyauchi

Contributions

Miyauchi T. conceived and designed the research and drafted the manuscript. Igaki Y. conducted the synthesis of the Japanese cedar -based chemical materials. Murata R. performed the XPS and SEM analyses for physical characterization. Tanabe S. carried out the esterification experiments and related evaluations. Tsujimoto M. contributed to data analysis and interpretation. Shikami Y. supervised the overall study and critically revised the manuscript for intellectual content. All authors read and approved the final manuscript.

Corresponding author

Correspondence to Toshiyuki Miyauchi.

Competing interests

The authors declare that they have no competing interests.

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