Catalytic Upgrading of Rice Straw Bio-Oil via Esterification in Supercritical Ethanol over Bimetallic Catalyst Supported on Rice Straw Biochar

Abstract

This research explores the enhancement of bio-oil quality through upgrading with the magnetic bimetallic oxide (CuO-Fe₃O₄) catalysts supported on activated rice straw biochar (AcB). These catalysts were employed in a supercritical ethanol-based upgrading process. Various characterization techniques, including elemental analysis, Fourier transform infrared (FTIR), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), were utilized to characterize the prepared catalysts. This study revealed significant improvements in the physical characteristics and chemical composition of the bio-oil, with an increase in the heating value (HHV) from 21.3 to 32.1 MJ/kg. Esterification and transesterification were identified as key reactions contributing to this improvement. Notably, the pH of bio-oil increased from 4.3 (raw bio-oil) to 5.63 (after upgrading), signifying reduced acidity. The analysis of the bio-oil’s chemical composition highlighted a decrease in oxygen content and an increase in carbon and hydrogen content. At the optimum conditions, the application of supercritical ethanol proved to be an efficient method for enhancing the bio-oil’s properties. A crucial transformation occurred during the upgrading process and more than 90% of carboxylic acids were converted into esters, primarily ethyl acetate at the optimal conditions. This study has demonstrated the effective enhancement of raw bio-oil from rice straw through the utilization of carbon-based bimetallic oxide catalysts in a supercritical upgrading procedure.

1. Introduction

The rapid depletion of fossil fuels, coupled with the environmental threat it poses as a result of greenhouse gas emissions, has triggered the search for an alternate and sustainable source of fuels from lignocellulosic biomass [1]. Lignocellulosic biomass fuels have enormous advantages over current fossil fuels due to their negligible sulfur content, renewability, and 80–90% lower greenhouse gas emissions [2]. By 2022, the United States estimated 360 billion gallons of the world’s total fuel consumption would be made up of renewable resources. Wood and agricultural residues are the most common sources of renewable resources [3]. Agricultural residues, including rice straw, are some of the most abundant and least costly lignocellulosic biomasses, accounting for about 283 million tons worldwide annually [4]. Consequently, their effective utilization can significantly lead to a major reduction in greenhouse gas emissions. At the same time, their utilization will represent an additional source of renewable biofuels [5]. Rice straw biomass from one of the world’s most consumed foods is gaining more attention for bio-oil production through the fast pyrolysis technique under both catalytic and non-catalytic conditions [6].

Fast pyrolysis is a thermochemical process which can produce a liquid fuel that may be used to replace fossil fuels in any static heating or electrical generation applications [7]. The fast pyrolysis process thermally degrades the biomass into liquid bio-oil, a mixture of gases, and solid biochar in the absence of air at a temperature of 400 to 450 °C and pressure of 0.1 to 0.5 MPa. Over the years, there has been substantial work in developing pyrolysis processes for creating high bio-oil yield from lignocellulosic biomass [8]. The major challenges with fast pyrolysis bio-oil, irrespective of the biomass source, are the bio-oil chemical composition. Bio-oil typically consists of a complex mixture of organic compounds such as acids, alcohols, aldehydes, esters, ketones, sugars, guaiacols, syringols, furans, lignin-derived phenols, levoglucosans, and other compounds [9]. The presence of a high water content and the high percentage of oxygenated compounds (35–40 wt.%) are the main reasons for bio-oil’s negative properties, such as low heating value (13–20 MJ/kg) and immiscibility with fossil fuels [10]. Additionally, bio-oil has a strong tendency to polymerize during storage and transit and contains substantial quantities of carboxylic acids, such as formic, propanoic, and acetic acids, which results in low pH values (2–3) and makes bio-oil more corrosive [11]. Therefore, the development of suitable upgrading techniques for decreasing the acid value and oxygen-containing species, increasing the calorific value, and stabilizing bio-oil is necessary for the utilization of fast pyrolysis bio-oil [12].

Several upgrading techniques, such as hydrogenation, hydrodeoxygenation (HDO), catalytic cracking, emulsification, and esterification in supercritical fluids have been developed to improve the physicochemical properties of fast pyrolysis bio-oil [13]. The esterification process to transform the enormous quantities of carboxylic acids that exist in the bio-oil, such as formic acid, acetic acid, and propionic acid can be achieved by using alcohols, which reduces the acidic value, thus decreasing the corrosiveness of the bio-oil [14]. The esterified bio-oil represents a greener approach to ester production which can be essentially used as biodiesel [15]. The esterification process can be performed in the presence of a liquid homogeneous acid catalyst or a solid acid catalyst. The esterification process with a heterogenous solid catalyst alongside the HDO is seen as the best approach for upgrading bio-oil [14]. It is also important to note that esterification carried in the presence of a solid heterogeneous catalyst does not only catalyze the esterification reaction but also aids the deoxygenation of the esterified bio-oil, thereby further enhancing the calorific value of the upgraded fuel [16]. It is therefore not surprising that there has been increasing interest in the development in the enhancement of fast pyrolysis bio-oils through supercritical alcohol treatment, presenting an alternative to traditional techniques like HDO and catalytic cracking [17,18].

This novel approach of utilizing supercritical alcohol for upgrading via esterification using a heterogenous catalyst offers a promising solution to tackle the common issues of hydrogen availability and coke formation encountered in conventional bio-oil upgrading methods [19,20]. The demand for hydrogen in the other upgrading technique, such as the HDO process, can be fulfilled by a hydrogen transfer from the supercritical alcohol with the help of the catalyst [19,21]. The distinctive characteristics of supercritical alcohols as solvents create a unified reaction environment by dissolving gases, such as hydrogen, after dissociation [18]. This setup facilitates the HDO reaction by eliminating mass transfer barriers and curbs the formation of coke by stabilizing the precursors that lead to coking [22]. Ethanol, among various alcohols, has gained significant attention as an optimal solvent for supercritical upgrading because alcohols are cheaper, safer, and more appropriate to use as a hydrogen donor than molecular H₂ [22,23]. This preference is attributed to its exceptional capability to dissolve organic compounds within the bio-oil, along with its origin from biomass. Furthermore, the application of supercritical ethanol aids in minimizing waste production during the upgrading procedure [24]. Moreover, the alcohols employed in this process exhibit heightened efficiency in esterification reactions, effectively stabilizing the acidic components present in the bio-oil [25,26].

The HDO of bio-oils has extensively been studied using conventional petroleum hydrotreating catalysts, such as noble metals Rh, Pd, and Pt [27]. Although noble metals have demonstrated notable catalytic efficacy in HDO, their substantial production expenses impose restrictions on their practical application [28]. In contrast, transition metals like Ni, Cu, Fe, Co, and Mo, while more cost-effective, tend to exhibit comparatively reduced catalytic activity in HDO [29,30]. Other combinations of bimetallic catalysts such as CoMo and NiMo supported on alumina were also studied [31,32]. The bimetallic NiCo catalysts were also used in the deoxygenation of palm oil to methyl ester [33]. Of all the above-discussed transition metals, Cu and Fe are not only cheaper and abundantly available, but also comparably the least studied.

In addition to the activities of metal loading in catalysts, the type of support also plays a critical role in bio-oil upgrading. Several supports, including biochar, metal oxides, and zeolites, had reportedly been used as catalyst supports for chemical biomass conversion to bio-oil [34,35,36]. Additionally, it was found that acidic catalysts like zeolites and silica-alumina work effectively in upgrading bio-oil even under atmospheric pressure without needing hydrogen [37]. Although all these catalyst supports have been proven to be effective in bio-oil upgrading, biochar-based support catalysts are more cost-effective, readily available, and environmentally benign [38]. The homologous biochar-supported inlaid Cu–Ni catalysts were used for lignin depolymerization to monophenols and bio-oil without the use of external hydrogen [39]. In a few instances, trimetallic catalysts, such as MgNiMo supported on activated charcoal, were used successfully for bio-oil esterification under supercritical ethanol conditions [14]. It is worth noting that most of these biochar-based catalysts were prepared using commercially purchased activated carbon, some of which had been utilized for the purpose of upgrading bio-oil derived from fast pyrolysis in supercritical ethanol [14,40]. It is therefore not surprising that recently biochar has become a more popular catalyst support, similar to zeolite, alumina, oxides etc. On the other hand, activated biochar is a reliable resource for organic carbon-based supports which is not only sustainable and environmentally friendly, but also a cheaper alternative to zeolite, alumina, and other commercial options, etc. [41,42].

Rice straw biochar in the form of activated biochar was used as a support due to its unique chemical composition, such as higher silica content and morphological characteristics, allowing for successful loading of metal oxides such as Fe₃O₄ and CuO on its acid sites compared to biochar form other biomass sources [43,44]. The choice of bimetallic catalysts, Fe₃O₄ and CuO, for the modification of rice straw biochar, is substantiated by various research studies. Ma and Dongdong showed that the metal-modified catalysts were effective in improving both the yield and selectivity of bio-oil in the hydro-liquefaction process of rice straw [45,46]. Furthermore, another study highlighted the potential of modified biochar to catalyze the production of value-added chemicals from biomass [47]. Therefore, this study will focus primarily on the synthesis and characterization of the magnetic bimetallic oxide catalysts supported on the activated rice straw biochar (CuO-Fe₃O₄/AcB) and using it for the catalytic upgrading of rice straw bio-oil under supercritical ethanol conditions. The prepared catalyst was characterized using X-ray diffraction (XRD), BET Surface area, and pore size distribution by BJH, elemental analyzer, thermogravimetric analysis (TGA), and scanning electron microscope (SEM). The esterified upgraded bio-oil was characterized by the common physical and chemical characterization methods.

2. Materials and Methods

2.1. Materials

Rice Straw feedstock was collected from the Delta Research and Extension Center (DREC) at Stoneville, MS, USA. The metal precursors, such as Copper (II) nitrate trihydrate Cu (NO₃)₂·3H₂O and iron (III) nitrate nonahydrate Fe₂(NO₃)₃·9H₂O, were purchased from Fisher Scientific (Waltham, MA, USA) with a percent purity ≥ 99%. Sodium hydroxide (NaOH) and potassium bicarbonate (K₂CO₃) were obtained from Millipore-Sigma (St. Louis, MO, USA). Methanol, ethanol, and acetone with a percent purity ≥ 99% were purchased from Fisher Scientific and used without further purification.

2.2. Pyrolysis of Rice Straw

A schematic diagram of the overall pyrolysis and upgrading processes is presented in Figure 1. The pyrolysis of rice straw was conducted at a feed rate of approximately 7 kg/h within a stainless-steel auger reactor, as described in our previous publication [48]. The operation of the auger reactor excluded the use of a carrier gas or additional heat carrier, relying instead on nitrogen to eliminate oxygen from the system. Pyrolysis was achieved by maintaining a pyrolysis temperature of 425 °C, with a calculated gas residence time within the range of 1 to 2 s. Multiple heaters, situated along the reactor pipe, were responsible for supplying the required heat for the pyrolysis reactions. During the experimental process, the solid feed material took approximately 30 s to traverse the 450 °C pyrolysis zone, with a total transit time of approximately 50 s to reach the char exit point. The resulting pyrolysis vapors were subsequently conveyed through the pipe into a condenser train, where they underwent condensation to yield liquid bio-oil. The condenser system was designed to recover multiple liquid fractions from the pyrolysis vapors. The non-condensable gases generated within this process were collected at the end of the condenser train using a gas sampling bag and a gas sampler kit (GAV-200 MK 2, SGE Inc., Austin, TX, USA). Liquid condensate resulting from the condensation process was collected from the exits of four condensers and subsequently assembled to form a composite bio-oil sample. The biochar, a solid byproduct of pyrolysis, was collected within a sealed vessel positioned at the end of the reactor tube, and the weights of the biochar samples were recorded.

2.3. Preparation of Biochar-Based Catalysts

The activated rice straw biochar (AcB) was prepared according to the following procedure [49]. Briefly, 100 g biochar was impregnated in 300 mL of 0.6 M K₂CO₃ solution for 24 h. The mixture was stirred for 48 h using a magnetic stirrer. The impregnated biochar was separated using a centrifuge and dried at 80 °C in the oven and then pyrolyzed in a furnace tube (OTF-1200X) with a heating rate of 10 °C/min under inert conditions using nitrogen gas. The heating temperature was set to 600 °C for 2 h. The obtained activated biochar was ground to pass through a 0.15 mm sieve. Then, it was treated with 1 M HCl to remove ash and washed with deionized water until the conductivity of the washed filtrate was less than 10 μS/cm. Finally, the activated biochar was vacuum-dried at 80 °C and kept covered.

2.4. Preparation of CuO-Fe₃O₄/AcB

Five different combinations of the (CuO-Fe₃O₄/AcB) biochar-based catalysts (1, 2, 3, 4, and 5) were prepared using the precipitation–deposition method with slight modifications [50]. The activated biochar was added to 250 mL of deionized water and then sonicated for 30 min at room temperature to obtain a good dispersion. The required weight of (Cu) in the form of Cu (NO₃)₂·3H₂O and (Fe) in the form of Fe₂(NO₃)₃·9H₂O were weighed and dispersed in 250 mL water and sonicated for 10 min for complete dissolution. Then the metal-nitrate mixtures were added into the activated biochar dispersion drop-wise with constant stirring using a mechanical stirrer. Subsequently, both metals were precipitated by using 1.0 M NaOH at 0 °C using ice, keeping the suspension pH fixed at 9.5 by monitoring with a pH meter. The suspension was then stirred at room temperature for 1 h, filtered using a centrifuge, and washed with deionized water continuously until the detection of a neutral pH of the filtrate. Afterward, the residue was put into a vacuum oven to dry overnight at 80 °C. The precipitate was then calcined in a furnace tube (OTF-1200X) at 550 °C for 5 h in the presence of air. Finally, the obtained catalyst was reduced at 400 °C under 5% H₂ in an N₂ flow (1 L/min) atmosphere for 5 h with a heating rate of 10 °C/min in a continuous flow in a furnace tube (OTF-1200X).

Table 1. Conditions of the different prepared catalysts.

EXP.	Catalyst	AcB (wt.%)	Cu and Fe (wt.%)	Cu (wt.%)	Fe (wt.%)	Cu:Fe Ratio
1	AcB support	100	0	0	0	-
2	1	80	20	10	10	1:1
3	2	80	20	13.33	6.66	2:1
4	3	80	20	15	5	3:1
5	4	90	10	7.5	2.5	3:1
6	5	70	30	22.5	7.5	3:1

summarizes the different catalyst preparation conditions including the ratios and contents of AcB, Cu, and Fe. The total weight percentages of bimetals (Cu:Fe) were 10%, 20%, and 30% for each prepared catalyst. The selection of these ratios was based on a strategic consideration of several factors, including economic value and environmental friendliness. The AcB support was chosen as 100% in Exp. 1 to serve as the reference point, while varying ratios of Cu and Fe (1:1; 2:1; and 3:1) were introduced to observe the impact on catalytic activity and stability.

2.5. Catalyst Characterization

Characterization of the catalyst was performed by using different analytical techniques, such as TGA, XRD, SEM-EDX, and Brunauer-Emmet-Teller (BET) analysis. The TGA (thermogravimetric analysis) of the AcB and CuO-Fe₃O₄/AcB catalyst was performed using the SDT Q600 TA instrument (Eden Prairie, MN, USA). All measurements were made between 20 °C and 800 °C at a heating rate of 10 °C/min under N₂ atmosphere. X-ray Diffraction (XRD) patterns for the prepared catalysts were determined using a RINT Ultima III XRD (Rigaku Corp., Akishima-shi, Japan) operating with CuKα1 radiation (λ = 1.54 Å) at 40 KV 44 mA in the range of 3 to 90 (2θ) at 40 kV voltage, and peak intensities were recorded every 0.03° at a sweep rate of 1.0° (2θ/min). A scanning electron microscope (SEM) equipped with an Energy Dispersive X-ray spectroscopy (SEM-EDX) was used to obtain the image of the morphology as well as the elemental composition (Cu, Fe, Si, O, and C) of the catalyst, and the two samples were sputter-coated with 15 nm platinum and imaged at a 5 keV accelerating voltage. The Fourier Transform Infrared (FTIR) spectroscopy on the CuO-Fe₃O₄/AcB catalyst in the range of 500 to 4000 cm⁻¹ was performed using a Thermo Scientific (Waltham, MA, USA) Nicolet iS50 FTIR spectrometer. The surface area (BET), pore volume, and size distribution (BJH) of AcB and CuO-Fe₃O₄/AcB were determined by adsorption–desorption isotherms of nitrogen at −196 °C using a Quantachrome Autosorb iQ gas sorption analyzer (Quantachrome, Boynton Beach, FL, USA).

2.6. Bio-Oil Esterification Process

The bio-oil esterification process was carried out under a supercritical ethanol condition by first mixing bio-oil with ethanol at a ratio of 1:1 w/w, and the bio-oil ethanol mixture was then used for the upgrading process. Briefly, 100 g of the bio-oil/ethanol mixture was taken for the catalytic upgrading process in a 350 mL Parr reactor equipped with a glass temperature controller, stirrer, and heating mantle. A total of 5 g of the catalyst was loaded in the reactor with the ethanol/bio-oil mixture. Then, the reactor was purged with N₂ 5 times to remove the air inside. Subsequently, the reactor was heated to 300 °C at a heating rate of 5.0 °C/min. After 3 h, the heater was turned off and the reactor was cooled to room temperature using an ice-water bath. The catalyst particles were separated from the liquid products in centrifuge vials via centrifugation at 4000 rpm for 30 min. The coke products, along with the used catalysts, were then recovered using vacuum filtration. The recovered solids were rinsed several times with acetone and dried at 105 °C for 12 h and weighed. The amount of coke formed during the HDO process was calculated by subtracting the initial weight of the catalyst from the weight of the total solids, assuming no catalyst loss. The aqueous fraction (AF) and oil fraction (OF) were unable to be separated since they were soluble in ethanol solvent. Thus, the OF was separated using a rotary evaporator to evaporate all the solvent and water at 65 °C under reduced pressure.

The yields (Y) of the upgraded products (liquids, solids, and gas) were then determined according to Equations (1)–(3) and presented in

Table 2. Textural properties of the prepared catalysts.

EXP. No.	Catalyst	BET Surface Area (m2/g)	Pore Volume (cc/g)	Average Pore Diameter (nm)
1	AcB support	203.75	0.389	3.71
2	1	58.32	0.253	14.75
3	2	56.41	0.234	15.39
4	3	53.19	0.213	16.03
5	4	74.81	0.299	10.64
6	5	23.89	0.176	36.78

.

$$\mathrm{Y}\mathrm{l}\mathrm{i}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{d}=\frac{\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}\left(\mathrm{l}\mathrm{i}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{d}\right)}{\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}(\mathrm{b}\mathrm{i}\mathrm{o}-\mathrm{o}\mathrm{i}\mathrm{l}+\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{l})}\times 100\mathrm{\%}$$

(1)

$$\mathrm{Y}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}=\frac{\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}\left(\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}\right)}{\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}(\mathrm{b}\mathrm{i}\mathrm{o}-\mathrm{o}\mathrm{i}\mathrm{l}+\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{l})}\times 100\mathrm{\%}$$

(2)

$$\mathrm{Y}\mathrm{g}\mathrm{a}\mathrm{s}=1-\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}(\mathrm{L}\mathrm{i}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{d}+\mathrm{S}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d})\times 100\mathrm{\%}$$

(3)

2.7. Physical and Chemical Characterization of Raw and Esterified Bio-Oil

The physical and chemical properties of rice straw bio-oil, such as pH, water content, viscosity, acid value, and heating value, were determined according to the appropriate standard methods. Water content was determined by the Karl Fisher titration method using a Cole-Parmer Model C-25800–10 titration apparatus (Thermo Fisher Scientific Inc., Waltham, MA, USA) via the ASTM D5291 method. Viscosities were determined using a Stabinger Viscometer TM SVM 3000 (Anton Parr, Graz, Austria) at 40 °C, according to the ASTM D7042 method. Total acid number (TAN) was obtained by dissolving a 1 g sample in 50 mL of 35:65 volume ratios of isopropanol to distilled water mixture and titrating the mixture to obtain a final pH of 8.5 with 0.1 N KOH solution via the ASTM D664 method. Fourier transform infrared spectroscopy analysis was performed using a Perkin Elmer FTIR spectrometer. FTIR spectra were recorded in transmittance mode in the range of 4000 to 500 cm⁻¹ using the standard potassium bromide disk technique. Carbon, hydrogen, nitrogen, and oxygen (by difference) content was measured using a CE-440 Elemental Analyzer (Exeter Analytical, Chelmsford, MA, USA) according to the ASTM D3291 method, with a standard of acetanilide (C = 71.09 wt.%, H = 6.71 wt.%, N = 10.36 wt.% and O = 11.84 wt.%). The higher heating value (HHV) was calculated using Equation (4) [14]. The volatile and semi-volatile components of each specimen were analyzed using a Perkin Elmer Clarus 500 Gas Chromatograph/Mass Spectrometer (GC/MS) system. The gas chromatograph was equipped with a DB-5MS capillary column of 30 m × 0.32 mm ID × 1 μm film thickness. Samples were injected in the split-less mode and the injector temperature was set to 270 °C. The initial oven temperature of the GC was 40 °C for 4 min and then programmed at a rate of 5 °C/min to 280 °C, with a total run time of 60 min. The mass spectrometer detector was an electron impact ionization device operating at 70 eV with a source temperature of 210 °C and interface temperature of 225 °C. The chemical component data obtained from GC/MS were analyzed using a chemical integration program together with the NIST mass spectral search library. The peak area percentage was calculated for each identified compound. However, compounds that were associated with some peaks were not identified.

$$\mathrm{H}\mathrm{H}\mathrm{V}\left(\mathrm{M}\mathrm{J}/\mathrm{k}\mathrm{g}\right)=\frac{\left(34\times \mathrm{C}+124.3\times \mathrm{H}+6.3\times \mathrm{N}+19.3\times \mathrm{S}-9.8\times \mathrm{O}\right)}{100}$$

(4)

(C, H, N, S, O was the elemental composition of the respective element)

2.8. Catalyst’s Stability

The potential use of the catalytic activity is fully controlled by the stability of the catalyst. The catalyst’s stability was measured via a rerun of the optimum experiment (Exp. 5) using the spent catalyst for three more runs as follows: The spent CuO-Fe₃O₄/AcB catalyst was activated in the furnace tube (OTF-1200X) of MTI Corporation (Richmond, CA, USA) at 550 °C under continuous N₂ flow (1 L/min) for 5 h before each cycle. After each run, the catalyst was washed, filtered, dried, and activated for the next cycles.

3. Results

3.1. Catalyst Characterization

The textual properties of the synthesized (CuO-Fe₃O₄/AcB) bimetallic oxide catalyst and AcB support used in this study were determined through N₂ physisorption using BET and BJH methods. The results, as presented in Table 2, demonstrate the impact of metal loadings on the AcB support. The data underscore the noticeable impact of metal loadings on the AcB support, resulting in a decrease in both surface area and pore volume. Importantly, the average pore diameter exhibited an inverse proportionality to the BET surface area. This observation suggests that the filling and blocking of AcB pores during the loading of metal oxides play a role in shaping the catalyst’s textual properties.

The prepared catalyst number 4 (10% bimetals with a Cu:Fe ratio of 3:1) was chosen for further detailed characterization because it demonstrated superior catalytic activity compared to the other formulations. The N₂ adsorption/desorption isotherm of catalyst (4) is depicted in Figure 2. The isotherm exhibits a type IV isotherm pattern with an H3 hysteresis loop, characteristic of meso- and macro-porous materials with a plate-like layered structure [51]. Notably, the catalyst showed a decrease in BET surface area compared to the AcB support, indicative of bimetallic oxide deposition on the AcB surface [52]. These findings are in line with previous studies on supported bimetallic catalysts, emphasizing the influence of metal loading on structural characteristics and catalytic performance. The integration of these N₂ physisorption results provides a comprehensive understanding of the evolution of the catalyst’s properties during synthesis.

The phase and the crystallinity of the prepared AcB support and CuO-Fe₃O₄/AcB catalyst were determined using XRD as well as the average crystal size. As shown in Figure 3, the characterization spectrum of the AcB support was wide with a broad peak with no crystallinity, and was thus amorphous, as expected. The XRD spectrum of the CuO-Fe₃O₄/AcB catalyst, on the other hand, shows crystallinity with a group of diffraction peaks at 2θ of 43.52°, 57.4°, and 74.04° consistent with Fe₃O₄ assigned to the cubic spinel unit cell that matches the standard magnetite structural data(JCPDS file no. 19-0629) [53]. These diffraction peaks of Fe₃O₄ 400, 511, and 522 lattice planes confirm the existence of Fe₃O₄. Also, a single cubic phase of Cu₂O, consistent with a standard cuprite structure with the diffraction peaks 111, 200, and 311 lattice planes, were found at 2θ of 38.5°, 42.4°, and 50.2° (JCPDS file no. 05-0667) [54]. Furthermore, diffraction peaks at 2θ of 38.5°, 47.5°, and 52.3° were consistent with CuO phase crystalline with monoclinic (tenorite) structure [54,55]. Hence, the XRD has confirmed the presence of both metal oxides of CuO and Fe₃O₄ crystalline phases present in the synthesized CuO-Fe₃O₄/AcB catalyst.

Thermogravimetric analysis (TGA) was conducted on catalyst 4 under the presence of N₂ gas. This analysis aimed to determine the thermal stability and degradation within the catalysts. The outcomes of this analysis are presented in Figure 4. The TGA results revealed a weight loss of 2.4% from ambient temperature to 250 °C, primarily attributed to the removal of water. Notably, catalyst 4 exhibited enhanced thermal stability and reduced degradation tendencies in the temperature range between 250 and 600 °C. These are consistent with the findings of other two studies [56,57] that emphasize the importance of catalyst stability in biomass conversion processes. This is a significant finding, indicating that the catalyst is expected to remain stable throughout the reaction, especially considering that our bio-oil upgrading process occurs predominantly at a temperature of 300 °C. The stability observed in the crucial temperature range aligns with the desired conditions for the esterification of rice straw bio-oil. This thermal stability is a key attribute, ensuring that the catalyst remains active and effective during the bio-oil upgrading process. Beyond the activation temperature (600 °C), a slight weight loss of approximately 2.5% was noted. This minor degradation is likely associated with the breakdown of AcB compounds. While this weight loss is observed, it is within an acceptable range and may not significantly impact the overall performance of the catalyst. Thus, the TGA analysis of catalyst 4 provides valuable insights into its thermal behavior, affirming its suitability for the bio-oil upgrading process. The observed thermal stability in the relevant temperature range supports the catalyst’s resilience under the anticipated reaction conditions.

SEM equipped with an EDX was used to study the morphology as well as the elemental composition (Cu, Fe, Si, O, and C) of the AcB support and fresh and used catalyst, as shown in Figure 5. The SEM images of the AcB support (a) appear as a sponge-like micropore structure while the SEM image of the fresh catalyst (b) shows the deposition of the metallic oxide crystal shape on the surface. The SEM-EDX elemental composition of the fresh catalyst (c) spectra confirm the presence of bimetallic oxide loading of CuO-Fe₃O₄ and the peak of Si and O confirms the abundance of silica (Si-O-Si) in rice straw biochar [41]. The mapping of the fresh catalyst (d–h) demonstrates that all metal oxides on the AcB support in the synthesized CuO-Fe₃O₄/AcB catalyst were well dispersed. The SEM image of the used catalyst (i) shows that the surface became less porous compared with the fresh catalyst. The SEM-EDX elemental composition of the used catalyst (j) spectra confirm the abundance of silica (Si-O-Si) in the used catalyst. The mapping of the used catalyst (k–o) also confirms that the metals are well dispersed on the surface of the used catalyst.

The FTIR spectra (Figure 6) of CuO-Fe₃O₄/AcB bimetallic catalyst nanoparticles exhibit two faint absorption bands at 2981 cm⁻¹ and 1381, corresponding to the stretching and bending vibrations, respectively. There were small O-H groups in the adsorbed water on the silica surface. A broad and highly intense band at 1060 cm⁻¹ is associated with the asymmetric stretching vibrations of Si–O–Si bonds thus further confirming the abundance of silica as was observed in the SEM elemental mapping. The band observed at 801 cm⁻¹ is designated for the symmetric stretching vibration of Si–O–Si in the rocking mode [58].

3.2. Product Yields, Elemental Analysis, and HHV

The yield of liquid products in the absence of a catalyst (Exp. 1) was relatively higher (56.3%) than the yield of liquid products (36.0–38.4%) in experiments performed in the presence of the catalyst (Exps. 2–6). Also, the yield of solids was relatively lower in the absence of a catalyst (4.3%) than in the presence of a catalyst (17.3–19.6%). The above results clearly indicate that the presence of a catalyst had a great influence on the product yields of liquid, solid, and gas. The elemental composition of raw and esterified bio-oils with different catalysts was also investigated (

Table 3. Product yield and elemental analysis results of the raw and upgraded bio-oil.

EXP. No.	Y (Liquid)	Y (Solid)	Y (Gas)	Elemental Analysis (wt.%)				HHV (MJ/kg)
				C	H	N	O
Raw bio-oil	-	-	-	54.2	7.7	1.8	36.3	21.3
1	56.3	4.3	39.4	60.7	7.2	2.1	28.0	24.3
2	37.4	17.6	45.0	62.3	7.6	2.2	27.9	27.8
3	38.4	17.3	44.3	62.9	6.6	3.1	27.4	28.9
4	36.0	19.1	44.9	65.4	9.1	2.8	22.7	30.9
5	37.4	19.3	43.3	66.9	9.1	2.2	21.8	32.1
6	36.9	19.6	43.5	67.0	8.8	3.3	20.9	31.9

(Upgrading Temp. = 300 °C, time = 3 h, and bio-oil/ethanol mixture = (1:1 w/w)).

.) As per the upgrading procedure, there has been an increase in the carbon (C) content and a decrease in the oxygen (O) content. Due to the increased (C) content and decreased (O) concentration, the esterified bio-oils exhibited Higher Heating Values (HHVs) than the raw bio-oil. In Exp. 1, when the bio-oil was upgraded, even in the absence of the bimetallic oxide catalyst, the HHV increased to 24.3 (MJ/kg) compared to the raw bio-oil at 21.3 (MJ/kg), indicating the ability of the silica-rich biochar (AcB) alone to act as a catalyst due to the presence of silica [42]. Replacing 20% AcB with metals (Exps. 2 and 3) led to a pronounced increase in both carbon content (62.3 and 62.9%) and heating value (27.8 and 28.9 MJ/kg). The increase in the HHV was more pronounced in Exp. 3 when the ratio of Cu:Fe loading was 2:1. Increasing the ratio of Cu:Fe to (3:1) in the catalyst (Exps. 4–6) led to greater improvement in both the carbon content and HHV. The maximum HHV of 32.1 MJ/kg, which represented about a 49.8% increase in the HHV of the raw bio-oil, was achieved in Exp. 5.

3.3. Physical Chractarization of Raw and Esterified Upgraded Bio-Oils

Rice straw bio-oil, like other bio-oils, is composed of a complex mixture of oxygenated compounds, with the majority being carboxylic acids. The other oxygenated compounds primarily consist of alcohols, aldehydes, phenols, and ketones, among others [59]. The upgrading of rice straw bio-oil in supercritical ethanol primarily leads to the formation of esters through esterification reactions between ethanol and carboxylic acids.

Table 4. Physical properties of the raw and upgraded bio-oils.

EXP. No.	Water Content	Viscosity at 40 °C Cst	Density at 40 °C (g/cm3)	pH	TAN (mg KOH/g)
Raw bio-oil	38.5	1.88	0.92	4.3	83.9
1	35.2	1.78	0.87	4.6	75.7
2	34.4	1.68	0.84	4.8	74.5
3	34.0	1.75	0.84	4.8	72.6
4	33.9	1.79	0.85	4.9	70.8
5	32.1	1.63	0.78	5.6	60.3
6	32.7	1.77	0.86	5.1	71.1

shows the physical properties of both the raw and the upgraded bio-oil. It is not surprising that desirable biofuel properties, such as the lowest water content, least viscosity, and density, were observed in Exp. 5. Notably, Exp. 5 had the highest pH value of 5.6, rendering it the least acidic in comparison to raw bio-oil (Exp. 4 and Exp. 6), which had pH values of 4.3, 4.9, and 4.7, respectively. This result has a strong correlation with the total acid number (TAN) values. The TAN value in Exp. 5 significantly decreased by 28.1% in comparison to the raw bio-oil. This decrease in acid number was mainly related to the esterification of the carboxylic acid compounds in the raw bio-oil. In general, all upgraded bio-oil experiments showed lower TAN values, viscosity, water content, and density compared to the raw bio-oil, suggesting a significant improvement in the physical properties of the esterified bio-oil.

3.4. Chemical Chractarization of Raw and Esterified Upgraded Bio-Oils

In this study, the chemical composition of bio-oil from each experimental run was analyzed using Gas Chromatography-Mass Spectrometry (GC/MS). The compounds with the most prominent peak areas were selected, integrated, and quantitatively determined based on their area percentages. The analysis revealed that the chemical compositions of the esterified bio-oils produced in each experiment (Exp. 1, Exp. 2, Exp. 3, Exp. 4, Exp. 5, and Exp.6) using the same experimental conditions were similar, though with differing peak intensities.

Table 5. Compounds in the raw and esterified bio-oil detected and identified by GC-MS.

RT (min)	Compound Name	(Area %)
		Raw Bio-Oil	Exp. 1	Exp. 2	Exp. 3	Exp. 4	Exp. 5	Exp. 6
	Acids
1.57	Formic acid	2.14	0.00	0.00	0.00	0.00	0.00	0.00
2.68	Acetic acid	32.32	11.02	6.62	10.29	6.27	2.08	6.27
12.66	4-hydroxybutanoic acid	2.06	0.00	3.03	0.00	2.44	2.03	2.42
36.61	Palmitic acid	3.59	1.86	0.00	1.56	0.00	0.00	0.00
39.48	Oleic acid	2.57	0.00	0.00	0.00	0.00	0.00	0.00
	Total	42.68	12.88	9.65	11.85	8.71	4.11	8.69
	Ketones
1.49	Acetone	0.00	1.90	0.00	1.87	1.98	4.12	2.01
3.87	1-hydroxypropan-2-one	11.65	0.00	0.00	0.00	0.00	0.00	0.00
7.50	1-hydroxy-2-butanone	1.82	0.00	0.00	0.00	0.00	0.00	0.00
9.93	Cyclopent-2-en-1-one	2.29	0.00	0.00	0.00	0.00	0.00	0.00
12.50	2-methylcyclopent-2-en-1-one	0.00	3.68	3.18	3.01	2.84	2.80	3.19
14.41	3-methylcyclopent-2-en-1-one	1.99	3.45	2.31	2.48	2.21	2.15	2.26
16.30	3-methylcyclopentane-1,2-dione	5.31	0.00	0.00	0.00	0.00	0.00	0.00
16.60	2,3-dimethylcyclopent-2-en-1-one	0.00	2.06	3.46	3.70	3.52	3.35	3.84
17.29	3,4,4-trimethylcyclopent-2-en-1-one	0.00	2.49	2.11	1.81	1.36	1.49	0.00
	Total	23.06	13.58	11.06	12.87	11.91	13.91	11.3
	Phenolics
14.79	Phenol	3.30	5.97	4.19	4.52	4.64	4.73	4.54
17.03	o-cresol	2.76	3.52	2.71	2.79	2.90	2.48	5.57
17.63	p-Cresol	2.23	2.81	2.27	2.41	3.02	2.53	0.00
18.04	2-methoxyphenol	4.07	8.17	5.86	5.08	5.24	4.75	5.17
20.11	4-ethylphenol	2.49	5.43	4.27	3.89	4.49	3.84	4.04
20.82	3-ethoxyphenol	5.02	5.44	5.03	3.68	3.17	2.90	3.13
22.96	4-ethyl-2-methoxyphenol	2.50	4.48	3.52	2.62	2.99	2.13	2.68
24.64	2,6-dimethoxyphenol	2.70	0.00	2.04	0.00	0.00	0.00	0.00
25.32	4-Ethylcatechol	2.25	2.53	2.41	1.71	0.00	2.04	1.50
	Total	27.32	38.35	32.3	26.7	26.45	25.4	26.63
	Alcohols
18.33	Cyclopropylmethanol	2.81	0.00	0.00	0.00	0.00	0.00	0.00
	Total	2.81	0.00	0.00	0.00	0.00	0.00	0.00
	Esters
2.71	Ethyl acetate	0.00	16.67	24.83	27.69	33.89	36.55	32.55
5.41	Ethyl propionate	0.00	2.76	5.98	7.04	6.99	7.59	7.46
8.86	Ethyl butyrate	0.00	0.00	1.02	1.20	1.49	1.52	1.40
37.13	Ethyl palmitate	0.00	4.24	3.34	3.19	3.22	3.04	3.15
39.92	Ethyl oleate	0.00	2.48	1.62	1.82	1.75	1.39	2.01
	Total	0.00	26.15	36.79	40.94	47.34	48.57	46.57
	Ethers
1.54	Ethoxyethane	2.22	1.77	2.10	3.25	1.67	3.91	2.09
	Total	2.22	1.77	2.10	3.25	1.67	3.91	2.09
	Others
21.44	2,3-dihydrobenzofuran	1.91	2.19	0.00	0.00	0.00	0.00	0.00
17.76	Isobutyric anhydride	0.00	5.08	8.10	4.39	3.92	4.10	4.72
	Total	1.91	7.27	8.1	4.39	3.92	4.1	4.72

outlines the major compounds detected in the GC/MS analysis of both raw and esterified upgraded bio-oils employing five different catalysts and the AcB support, all processed at 300 °C. The table distinctly demonstrates that all five bimetallic oxide catalysts, supported on activated rice straw biochar (CuO-Fe₃O₄/AcB) in Exp. 2, Exp. 3, Exp. 4, Exp. 5, and Exp. 6, induced significant changes in the chemical composition of the esterified bio-oil compared to the raw bio-oil. Conversely, the chemical compounds found in the raw bio-oil were predominantly oxygenated compounds resulting from the thermal conversion of rice straw biomass to bio-oil. These identified chemical products closely resemble those reported in the literature concerning fast pyrolysis of raw bio-oil, such as acids, aldehydes, ketones, esters, furans, and phenols [60]. Notably, the primary chemical compounds identified in this upgrading experiment were esters appearing at specific retention times (2.71, 5.41, 8.86, 37.1, and 39.92 min), including ethyl acetate, ethyl propionate, ethyl butyrate, ethyl palmitate, and ethyl oleate, respectively (as detailed in Table 4), confirmed by the chromatogram in Figure 7. Moreover, a substantial decrease in the levels of acid compounds at specific retention times (1.57, 2.68, 12.66, 36.61, and 39.48 min) was observed, corresponding to formic acid, acetic acid, 4-hydroxybutanoic acid, palmitic acid, and oleic acid, respectively. The absence of esterified compounds in raw rice straw bio-oils and their presence in the upgraded products (Exp. 2, Exp. 3, Exp. 4, Exp. 5, and Exp. 6) indicate the occurrence of esterification reactions between the carboxylic acid groups and supercritical ethanol in the presence of acid catalyst or support. Additionally, in all upgrading experiments, formic acid and oleic acid were completely absent at specific retention times (1.57 and 39.48, respectively) (Figure 7). Figure 8 shows the non-existence of esters in the raw rice straw bio-oil and their presence at varied concentrations in the experiments. It is important to note that in Exp. 4, Exp. 5, and Exp. 6 with a Cu:Fe ratio of 3:1, the ester yields were notably higher at 47.34%, 48.57%, and 46.57%, respectively, in terms of chemical composition compared to the other experiments, as presented in Table 5. Conversely, phenolic compounds experienced a slight reduction in these three experiments compared to the other experiments. Exp. 5 displayed the highest conversion of carboxylic acids into esters during the upgrading process, representing a substantial 90.4% conversion, leading to reduced acidity in the esterified oil. This esterification also resulted in an increase in esters from 0.0% to 48.57%, along with a minor decrease in phenolic compounds from 27.32% to 25.54% in comparison with the raw bio-oil, with selectivity towards ethyl acetate.

Therefore, Exp.5 serves as the optimal condition for effectively esterifying rice straw bio-oil in supercritical ethanol. These findings indicate that ester formation might be the primary cause of the reduction in the acidity of the bio-oil, evident in both a pH increase from 23.2% and a reduction of 28.1% in TAN values. The reduction in oxygenated compounds, acid content, and increase in esters indicate the catalyst’s excellent activity, consequently reducing viscosity and, thereby, the hydrophilicity of the final product [19]. In contrast to acids, esters display greater promise in fuel composition, owing to their reduced corrosive impact on the engine surface. Ester formation is potentially a result of an esterification reaction between bio-oil acids and alcohol [61]. These changes not only boost stability but also improve the potential for blending hydrocarbons into the upgraded bio-oil. The enhanced levels of ethers and esters in the upgraded bio-oil can strengthen it due to a substantial drop in acids and aldehydes. Finally, the properties of the upgraded bio-oil were significantly enhanced [19].

3.5. Reaction Mechanism

To demonstrate the reaction mechanism, the catalyst was divided into three parts, including the biochar support that was rich in silica, copper oxide (CuO), and iron oxide (Fe₃O₄). The silica in the biochar had enough Brønsted acid sites to catalyze deoxygenation reactions, such as dehydration (esterification), decarbonylation, aromatization, and decarboxylation, which allowed the elimination of the oxygen in bio-oil in the form of CO and CO₂ [62]. In addition, both copper oxide and iron oxide served as Lewis acids to deoxygenate the bio-oil by facilitating the Meerwein-Ponndorf-Verley transfer hydrogenation (MPV) reduction of bio-oil components, including acids, ketones, aldehydes, phenols, esters, sugars using the hydrogen that was donated from the supercritical ethanol. Also, the presence of metal oxides helped to improve the catalytic activity and stability due to the synergetic effects between metals. In addition, the introduction of iron oxide in the catalyst can produce cost-effective, highly abundant, superior magnetic material which makes it easy to be recovered from the chemical reaction by applying an external magnet [55,63].

3.6. Catalyst’s Stability

The catalyst exhibited good stability after testing it for four consecutive cycles of the optimum experiment (Exp. 5), as shown in Figure 9. After the second run, the yield of the esters slightly decreased (from 48.57% to 47.05%), with a slight increase in acid yield (from 4.11% to 5.24%). The yield of esters decreased by 16% after the fourth cycle and the amount of the remaining acids increased to 10.08%, which could be attributed to a minimal leaching of the metal oxides on the surface of the catalyst. This study confirms that the catalyst retained its catalytic activity toward the esterification of bio-oil and may be effectively reused for up to four regeneration cycles. These results indicate that although a small amount of metal oxide leaching occurred after four runs of upgrading of rice straw bio-oil, the catalyst still has a unique catalytic reactivity for the upgrading of bio-oil in supercritical ethanol.

4. Conclusions

This study focused on enhancing bio-oil using upgrading methods involving supercritical ethanol. Bimetallic oxide catalysts supported on activated rice straw biochar (CuO-Fe₃O₄/AcB) were prepared with different Cu:Fe ratios. The application of these catalysts during supercritical ethanol-based upgrading resulted in significant enhancements in both the physical and chemical properties of the bio-oil. Esterification, transesterification, and hydrogenation were identified as the key reactions contributing to this improvement. The elemental analysis results revealed an increase in carbon and hydrogen content due to the substantial hydrocarbon content in ethanol after blending, while the oxygen content in the bio-oil decreased. Analysis of the bio-oil’s physical properties indicated the removal of acids during upgrading, leading to a pH increase because of the esterification capacity of supercritical ethanol, facilitated by the presence of newly formed esters. Also, all upgraded bio-oil experiments showed lower TAN values, viscosity, water content, and density compared to raw bio-oil. The higher heating value (HHV) of the bio-oil reached 32.1 MJ/kg, approaching the value of conventional biodiesel fuel (36.4 MJ/kg). GC/MS chemical analysis showed that the bio-oil upgrading did not only improve bio-oil quality by decreasing oxygen-containing compounds and increasing calorific value, but also transformed the abundant carboxylic acids into esters, primarily ethyl acetate. The production of esters from the significant carboxylic acid reservoir addresses its corrosive nature and further enhances bio-oil’s utility. Ethyl acetate, a prominent ester, offers several key advantages, including its favorable physicochemical properties, making it a valuable target for sustainable biofuel production. This research not only showcases innovative upgrading techniques, but also underscores their potential to revolutionize agricultural residue management, fostering cleaner energy production.