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Article

Improved Microwave-Assisted Ethyl Levulinate Production Using Rice Husk-Derived Biobased Mesoporous Silica as Catalyst

by
Susana O. Ribeiro
1,†,
Inês Marques
1,†,
Aleksandr Bamburov
2,
Aleksey A. Yaremchenko
2,
Andreia F. Peixoto
1 and
Andreia Leite
1,*
1
LAQV/REQUIMTE, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal
2
CICECO-Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, University of Aveiro, 3810-193 Aveiro, Portugal
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2024, 14(8), 482; https://doi.org/10.3390/catal14080482
Submission received: 17 June 2024 / Revised: 14 July 2024 / Accepted: 24 July 2024 / Published: 27 July 2024
(This article belongs to the Special Issue Cutting-Edge Catalytic Biomass Conversion in Europe)
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Abstract

:
This study presents the synthesis and characterization of mesoporous silica using biobased silica recovered from rice husks (RHs) as an excellent example of the circular economy. Distinct hydrothermal methods were used, namely, the autoclave hydrothermal method and microwave irradiation. Furthermore, the microwave-synthesized SBA-15 material was subjected to post-functionalization with –SO3H groups using the organosilane 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane (CSPTMS). The structural and chemical properties of the prepared materials were rigorously characterized through several techniques, thereby confirming the successful preparation of this functionalized material. Subsequently, the functionalized SBA-15 (CSPTMS@SBA-15) was employed as a catalyst in the synthesis of ethyl levulinate (EL) from 5-hydroxymethylfurfural (5-HMF) using different methodologies: typical high-pressure batch reactor, conventional heating, and microwave irradiation. This investigation aimed to elucidate the influence of microwave and non-microwave heating methods on the efficient conversion of 5-HMF into EL. The findings revealed that the microwave reactor exhibited superior conversion rates and selectivity when compared to the non-microwave heating methods. The study also explored the effects of temperature and utilization of various alcohols as both solvents and reagents. The results demonstrated that higher temperatures favored the production of alkyl levulinate and that complete conversion of 5-HMF was attainable for all the alcohols employed. Specifically, for methanol and ethanol a 100% yield of alkyl levulinates was achieved, while for 1-propanol and butanol a reduction in the yield of alkyl levulinates was observed. These outcomes underscore the feasibility of achieving significant yields of various alkyl levulinates through the utilization of CSPTMS@SBA-15 as a catalyst.

1. Introduction

Global energy demand is on the rise, and the population is expanding rapidly. This, in turn, leads to the depletion of fossil fuel sources on a daily basis [1]. Furthermore, fossil fuels are major contributors to several environmental concerns, including the release of toxic pollutants, acid rain, smog formation, and global warming. Consequently, the lack of a viable replacement for fossil fuels is a continuous cause for alarm, prompting researchers worldwide to seek alternative energy sources for the future. In this context, biomass has been explored for producing carbon-based fuels or chemicals for a more sustainable future [2]. The production of alkyl levulinates has recently gained significant interest due to their unique physicochemical properties making them suitable for applications in the chemical and petrochemical sectors [3]. Furthermore, because of their low sulfur content, excellent lubricity, improved flow characteristics, and flash point, alkyl levulinates have shown substantial potential for use in gasoline blends and diesel engines. In this regard, ethyl levulinate (EL) has been suggested as one of the most promising fuel additives because it is soluble in diesel fuels, even with a higher aromatic content. It is also less toxic, promotes cleaner combustion, and may not require changes to existing engine designs [4,5]. EL is typically produced from levulinic acid (LA), which is considered one of the top 12 building-block chemicals. To achieve this, an esterification step is required in the presence of ethanol, using catalysts such as heteropolyacids [6,7], phosphotungstic acid, zeolites [8], mesoporous metal oxides [9], MOFs [10,11], and sulfonic acid silicas [12,13,14]. However, due to the propensity to produce polymeric humins that are challenging to separate, the synthesis of high-purity LA is a labor-intensive and expensive process [15]. Recently, 5-hydroxymethylfurfural (5-HMF), a biobased platform compound, has garnered significant attention. By converting 5-HMF, a variety of high-value derivatives, including EL, can be obtained. Consequently, the conversion of 5-HMF into EL has been proposed as an alternative route due to the ease and cost effectiveness of EL production from cellulose and hemicellulose resources. The reaction pathway begins with an acid-driven etherification with ethanol to yield 5-ethoxymethylfurfural (EMF), which is then converted into EL through ethanolysis. However, the conversion of 5-HMF into EL typically results in low product yields in the presence of catalysts with complex preparation, such as MOFS, or involves extended reaction times, making it a limiting step in EL production [16]. To enhance the efficiency of the process for converting 5-HMF into EL, more active and selective solid catalysts are needed.
In this context, rice husk (RH), an agricultural waste product from rice production, is a low-cost, environmentally friendly, and readily available material rich in silica. RH comprises approximately 50% cellulose, 30% lignin, and 20% silica. Through a calcination process, all the organic matter is removed, resulting in RH ash (RHA) with a microporous structure containing around 97% silica. RHA can be used to prepare sodium silicate solutions for the synthesis of ordered mesoporous silicas [17,18]. This approach offers a more sustainable alternative to traditional synthesis methods, as it replaces toxic and expensive tetraethoxysilanes with sodium silicate solutions obtained from RHA. Additionally, ordered mesoporous silicas are highly attractive for catalyst support due to their outstanding properties, including easy surface functionalization, high surface area, catalytically active surface, and thermal and chemical stability. Particular attention has been given to SBA-15, which has a large and tunable pore size distribution with uniform hexagonal channels, allowing for enhanced access to active sites [19]. The synthesis of SBA-15 is commonly achieved through a hydrothermal process using autoclave hydrothermal heating, as reported by Zhao et al. [20]. In their study, the researchers successfully synthesized SBA-15 by treating it at 100 °C for 24 h in an autoclave. This experiment confirmed that the formation of silica SBA-15 requires high temperatures and extended reaction times. To reduce the energy consumption associated with this process, researchers are actively exploring alternative approaches to shorten reaction times and improve yields. One such approach involves the use of microwave irradiation, which enhances the reaction, reduces the reaction time, provides a greater uniformity of pores, and generates materials with a higher concentration of silanol groups [21]. In this work, SBA-15 was synthesized using RHA as a silicon source under microwave hydrothermal conditions. Although SBA-15 may lack sufficient acidic strength for the conversion of 5-HMF into EL, functionalization can be performed to enhance its acid character using simple methods [22]. Sulfonic groups are a typical class of Brønsted acids that have been reported as a key factor in the production of EL from 5-HMF. Furthermore, –SO3H groups are particularly interesting for this purpose due to their low cost, high stability, and high activity [23,24,25]. In the present work, SBA-15 functionalized with CSPTMS was prepared from RH. This material was investigated as a catalyst for the one-pot conversion of 5-HMF into EL using three different heating methods: microwave irradiation, a high-pressure batch stirred reactor, and conventional heating. The effects of temperature and solvents were evaluated on the activity and selectivity of this catalytic process using SBA-15 functionalized with CSPTMS, as well as its recyclability. Overall, this work contributes to advancements in the production of alkyl levulinates using efficient, low-cost, and sustainable materials, contributing to the concepts of circular economy and biorefinery.

2. Results and Discussion

2.1. Material Characterization

The pristine SBA-15 and the functionalized catalyst were characterized regarding their physicochemical and textural properties by several techniques.
The obtained N2 adsorption isotherms (Figure 1) are consistent with type IV classification with H1 hysteresis loops, typical of these mesoporous materials, and indicate the uniform distribution of mesopores [26,27]. SBA-15 obtained using the autoclave hydrothermal method presented higher surface area (SBET) and pore volume (Vp) when compared with the MW-irradiated materials; however, the time needed for its preparation is 4 times higher (24 h) compared to the time needed to prepare the material using MW irradiation (6 h). Comparing the MW-prepared SBA-15 materials (SBA-15 MW 2 h, 4 h, and 6 h) it is possible to notice that as the reaction time increases there is an increase in both SBET and Vp and the obtained N2 adsorption isotherm patterns become more similar to the one obtained for SBA-15 synthetized by the autoclave hydrothermal methodology (Figure 1 and Table 1). The pore size distributions for all the materials can be found in Figure S1 in the Supporting Information (SI).
Figure 2 shows the low-angle XRD patterns of SBA-15 synthetized by the two different heating methods in the 2θ range of 0.5–10°.
For SBA-15 synthesized by microwave irradiation only the material irradiated for 6 h presents the set of peaks characteristic of the hexagonal and well-ordered arrangement of the SBA-15 channels. Furthermore, the XRD patterns of both SBA-15 synthetized by the autoclave hydrothermal method and SBA-15 MW 6 h display the characteristic peaks of SBA-15 materials in the low-angle area corresponding to the (100), (110), and (200) reflections of the hexagonal symmetry lattice P6mm [28,29].
The morphologies of the SBA-15 materials synthetized by the two methods were observed by SEM analysis and are depicted in Figure 3. It is possible to observe the formation of rope-like particles for all samples, although for reaction times of 2 h and 4 h the presence of spherical particles can also be seen (Figure 3A,B).
These results may explain the values obtained for SBET of SBA-15 MW 2 h; the presence of a high number of spherical particles (Figure 3A) could be responsible for the higher SBET compared to the SBET of SBA-15 MW 4 h.
For the 6 h reaction time only the elongated particles typical of SBA-15 can be observed, which indicates that only in these conditions is the formation of SBA-15 with uniform morphology and size observed (Figure 3C).
Taking in to consideration the results, SBA-15 MW 6 h (referred to from now on as SBA-15) was further used as the support for functionalization with CSPTMS. After that, several techniques were employed to characterize the physicochemical and textural properties of the resultant material.
The chemical composition obtained from EA and XPS revealed an increase in the carbon content from the parent and the functionalized sulfonated SBA-15-based catalyst (CSPTMS@SBA-15), Figure 4 left. Furthermore, the presence of sulfur on CSPTMS@SBA-15 was detected by EA with 3.2% (bulk content) and by XPS with 2.1% (surface content), which was assigned to the presence of –SO3H groups, as observed in the high-resolution spectra of CSPTMS@SBA-15 in the S 2p region at ~167 eV, generally attributed to sulfur in its oxidized form. The presence of a small peak at ~165 eV was also observed and assigned to the presence of some non-oxidized S (Figure 4 right) [30].
The effective functionalization of SBA-15 with CSPTMS was also investigated through FTIR spectroscopy.
Figure 5A displays the FTIR spectra of CSPTMS@SBA-15 in comparison to the unfunctionalized SBA-15 sample. Both samples show the characteristic bands around 1220 and 1089 cm−1 assigned to the Si–O–Si asymmetric stretching, 808 cm−1 assigned to the Si–O–Si symmetric stretching, and 464 cm−1 assigned to bending vibrations of a condensed silica network [31].
A peak at 970 cm−1 can also be observed in both spectra corresponding to the Si–O in-plane stretching of Si–OH and Si–O– groups [31].
The significant difference between SBA-15 and CSPTMS@SBA-15 is the presence of aromatic –SO3H groups. The CSPTMS@SBA-15 material presents two peaks at 669 and 698 cm−1 due to the stretching vibration of S–O and a peak at 576 cm−1 assigned to the out-of-plane bending of the para-disubstituted aromatic rings, Figure 5B. The broad band around 3440 cm−1 is due to the stretching vibrations of adsorbed H2O [24]. Bands corresponding to S=O stretching vibrations are normally found in the range of 1000–1400 cm−1. To assess the presence of these bands the subtraction of the SBA-15 spectrum from CSPTMS@SBA-15 spectrum was performed; Figure 6.
In this figure two peaks are shown at 1170 and 1126 cm−1, assigned to the asymmetric stretching vibration of O=S=O, and a peak at 1010 cm−1, assigned to the symmetric stretching vibration of O=S=O [23].
Less intense peaks are observed in the region of 1300 to 1600 cm−1. According to Joseph et al. [32] stretching phenyl modes are expected in this region for para-substituted benzene rings. The bands observed at 1600, 1406, and 1359 cm−1 in the spectrum could be due to the phenyl ring stretching modes.
These findings confirm the successful incorporation of organo-sulfonic acid groups into the pristine SBA-15.
N2 adsorption–desorption isotherms were measured for both SBA-15 and SBA-15@CSPTMS (Figure 7). The sulfonic acid-functionalized SBA-15 retains the same shape of the isotherms of bare SBA-15. In addition, there is also a decrease in the surface area and pore volume (Table 2) of SBA-15@CSPTMS. These results are consistent with the successful functionalization with sulfonic acid groups anchored in the surface wall of SBA-15 that might partially block the access of nitrogen molecules to micropores [25,33,34]. The pore size distributions for both SBA-15 and SBA-15@CSPTMS can be found in Figure S2 in the Supporting Information.
Figure 8 shows the low-angle XRD patterns of SBA-15 and SBA-15@CSPTMS in the 2θ range of 0.5–10°.
Comparing the XRD of these materials it can be seen that both display the characteristic peaks of SBA-15 materials in the low-angle area corresponding to the (100), (110), and (200) reflections of the hexagonal symmetry lattice p6mm [28,29].
The morphologies of the prepared materials were observed by SEM analysis and are depicted in Figure 9. The images obtained for SBA-15 and CSPTMS@SBA-15 reveal the characteristic morphology of the SBA-15 materials with hexagonal and rope-like particles indicating that the morphology of the silica support was maintained after SBA-15 functionalization [34,35]. The presence of sulfonic acid groups in the functionalized material could also be confirmed by the presence of sulfur in the EDS spectra and the increase in the relative intensity of the Si and C elements related to O, which is indicative of the success of the functionalization of the organosilane on the SBA-15 surface.
STEM studies of SBA-15 samples revealed an onset of formation of an ordered structure after 2 h of microwave treatment (Figure 10A). Semi-amorphous particles comprised short-order nanodomains with a comparatively small interplanar spacing of 2.0–2.2 nm. The formation of the desired mesoporous structure was promoted by increasing the microwave treatment time (Figure 10B). Finally, the samples processed for 6 h showed agglomerated granules or rope-like particles (Figure 10C) with a well-defined mesoporous nanostructure characteristic of SBA-15, in agreement with the XRD data.
Figure 11 depicts representative examples of STEM images of SBA-15 prepared by microwave irradiation, without (A) and with (B) functionalization. The images confirm the formation of a well-organized long-range ordered hexagonal array of one-dimensional mesopores. The mesoporous nanostructure is retained after functionalization. In both cases, the average pore diameter and the wall thickness estimated by image analysis corresponded to approximately 6 nm and 3.5–4.0 nm, respectively. The STEM results prove that the microwave-assisted synthesis enables the preparation of SBA-15 mesoporous silica in a shorter time and with characteristics very similar to the samples prepared by the autoclave hydrothermal method (Figure 11C), which had a slightly smaller pore size of ~5.5 nm and wall thickness of ~3.0–3.3 nm.
The TGA curve of the pristine and grafted SBA-15 catalyst is shown in Figure 12.
TGA analysis depicted a greater mass loss for the SBA-15 catalyst compared with the SBA-15 matrix, suggesting that sulfonation reduces the thermal stability of SBA-15. At 850 °C, the mass loss percentage of SBA-15 was around 7.4%, while for the sulfonated SBA-15 it was 23.8%. For both unfunctionalized and functionalized materials the first weight loss occurred up to 150 °C and is attributed to the water adsorbed in SBA-15’s surface. This loss of water can be ascribed to the moisture content, which varied from 6.3% for the SBA-15 sample to 5.3% for the catalyst. The TG curve of the CSPTMS@SBA-15 showed a weight loss of 11.5% in the temperature range of 300–600 °C, which can be attributed to the thermal degradation of the arylsulfonic acid groups [36,37].
The TGA data also showed that the thermal stability of the sulfonic acid groups in the prepared SBA-15-based catalysts was maintained up to around 300 °C, meaning that degradation did not occur in the range of 130–160 °C, where 5-HMF etherification and ethanolysis reactions were conducted.
Bearing in mind the application of the synthetized catalyst in the transformation of 5-HMF into EL and the fact that this transformation is associated with the acidity of the catalyst, the Brønsted acid density of the sulfonated SBA-15 catalyst was characterized by potentiometric acid–base titration. The data revealed that the concentration of Brønsted acidity in the catalyst was 1.27 mmol H+·g−1, while for SBA-15 no significant acidity was found, as previously reported [38], indicating that there is a correlation between the acid density and the bulk sulfur content.

2.2. Catalytic Results

2.2.1. Microwave Irradiation vs. Non-MW Irradiation

Different methodologies, including a typical high-pressure batch reactor and conventional approach, were used and compared with microwave-assisted technology in order to assess the effect of microwave heating on the efficiency of the one-pot conversion of 5-HMF into EL. Figure 13 depicts the catalytic activity of CSPTMS@SBA-15 for the one-pot conversion of 5-HMF into EL using the three different methods and the results demonstrate the influence of the heating method on the overall catalytic performance. In fact, the MW-assisted method shows higher 5-HMF conversion and EL selectivity when compared with non-MW heating methods, under the same experimental conditions. The main advantage of MW-assisted heating resides in the extraordinary acceleration of the reaction rate. Opposite to conventional heat transfer processes of conduction and convection, microwave irradiation-induced heating is an energy conversion process, in which the electromagnetic energy can be converted directly to internal thermal energy in the microwave-absorbing system, reducing the side reactions [39,40].
The results obtained from the studied systems can truly reflect the advantages of microwave-assisted conversion. Over the CSPTMS@SBA-15 catalyst, under the conventional approach, 5-HMF was converted (83%) into EL with a yield of 52% after 6 h. Using a high-pressure reactor with control of temperature and autogenous pressure we observed a significant increase in conversion (94%) and selectivity (81%). Comparatively, under microwave irradiation, EL was produced with a high yield (98%) with almost 100% 5-HMF conversion.
Regarding the values obtained for the catalysis promoted by the conventional approach and high-pressure reactor, the conversion increased by about 17% and 6%, and the EL yield increased significantly, by 46% and 17%, respectively, due to the use of microwave irradiation.
The findings show that in comparison to conventional heating methods (via conduction and convection), microwave irradiation efficiently promotes the one-pot catalytic conversion of 5-HMF into EL in the presence of a CSPTMS@SBA-15 catalyst, suggesting that this catalyst possesses microwave-response characteristics, enhancing the catalytic conversion under microwave irradiation [41].

2.2.2. Effect of Temperature

In order to evaluate the microwave-responsive performance of CSPTMS@SBA-15 catalyst in the 5-HMF to EL conversion, three different temperatures were evaluated. Figure 14 illustrates that the CSPTMS@SBA-15 catalyst exhibited different catalytic behavior at different reaction temperatures. The conversion of 5-HMF increased from 85% to 100% when the temperature rose from 130 °C to 150–160 °C. Furthermore, at higher reaction temperatures the selectivity for EL increased significantly. Notably, at 160 °C, both the 5-HMF conversion and the EL selectivity reached 100%. These results reveal the remarkable effect of temperature in microwave irradiation, enhancing both the conversion of 5-HMF and the selectivity towards EL, with the optimal conditions being achieved at 160 °C, where all 5-HMF is converted into EL, indicating the high activity and suitability of this biogenic silica-based catalyst for future catalytic applications. A similar trend was observed by Wang Z. [42]. The author reported that higher temperatures favored the conversion of 5-HMF into EL.

2.2.3. Effect of Reaction Time

The influence of reaction time was also studied at 160 °C and the results are depicted in Figure 15. The EL selectivity reaches its maximum after 3 h, with all the 5-HMF being converted into EL. After this reaction time, both 5-HMF conversion and EL selectivity remain constant. These results highlight the highly microwave-responsive characteristics of CSPTMS@SBA-15 to promote the one-pot conversion of 5-HMF to EL under microwave irradiation.

2.2.4. Effect of the Solvent

To demonstrate the versatility of the CSPTMS@SBA-15 in promoting the production of other alkyl levulinates and to gain a deeper understanding of the solvent’s role in microwave-assisted technology, a series of reactions were conducted using low-carbon-chain alkyl alcohols, including methanol, 1-propanol, and butanol, as reactants.
The results are depicted in Figure 16, and the analysis shows that complete conversion of 5-HMF was achieved for all the alcohols. Methyl levulinate and ethyl levulinate, were obtained with 100% selectivity and 100% 5-HMF conversion, while 1-propanol and butanol showed slightly lower reactivity, resulting in a 76% selectivity of propyl levulinate and 45% of butyl levulinate. These differences could potentially be explained by considering the reaction mechanism. The conversion of 5-HMF with different alcohols follows several steps: firstly, an acid-driven etherification with the alcohol, followed by alcohol-assisted ring-opening of the furan that leads to the formation of levulinate compounds. The nucleophilic attack of bulkier alcohols, such as propanol, is more hindered compared to methanol and ethanol due to steric limitations, which might limit the formation of propyl and butyl levulinates [12]. This is notable when using butanol, where a significantly poorer catalytic performance is evident. Nevertheless, these results underscore the feasibility of achieving various alkyl levulinates when employing CSPTMS@SBA-15 as catalyst with MW irradiation.
The conversion and selectivity results for EL production under different conditions (heating method, temperature, and solvent), can be found in Tables S1 and S2 in the Supporting Information.

2.2.5. Recyclability of the Catalyst

An important characteristic is the catalyst’s reusability under the selected reaction conditions. In this study, the recyclability of CSPTMS@SBA-15 was evaluated at 130 °C for three consecutive cycles. The choice of a mild temperature was to avoid catalyst deactivation under higher temperatures, as observed previously by the authors. After each cycle, the catalyst was recovered, washed with ethanol, dried, and reused, maintaining the same experimental conditions. The catalytic data in Figure 17 show that 5-HMF conversion remains constant throughout the cycles. In contrast, EL selectivity decreases abruptly from 78% to 37% in the second cycle, and it is almost negligible in the third cycle, indicating a low stability of the catalyst under microwave irradiation. In order to assess what contributes to this decrease in catalyst activity, the used catalyst was characterized.
The percentage of carbon and sulfur in the CSPTMS@SBA-15 catalyst after catalysis was determined through elemental analysis. This showed a significant increase in carbon content (from 10.22 to 25.89%) and a considerable decrease in sulfur content (from 3.21 to 1.74%), which are indicative of some organic contaminants adsorbed to the catalyst surface. TG analysis was also performed on the catalyst after reaction. As can be seen in Figure S3 in the Supporting Information, an increase in the weight loss is observed in the region 300–500 °C for the used catalyst, this may be due to adsorbed organic species, namely, polymeric compounds.
Consequently, the leaching of sulfonic groups and carbon deposition may be the primary causes of catalyst deactivation. This is in line with findings from previous work by Velasco et al. [43], which observed carbon deposition on the surface of the SBA-15 catalyst, and they suggested that this deposition could be linked to humin formation [43]. The author reported that the formation of humins might reduce the selectivity toward the desired product due to their deposition on the catalyst surface, leading to the deactivation of catalytic sites. Furthermore, Peixoto et al. [44] claimed that leaching of sulfonic groups decreases the active sites, and consequently, reduces the activity of the catalyst [44].
To avoid catalyst deactivation, more hydrophobic surface catalysts should be preferred, as observed by Yang and co-authors [45] when preparing a series of solid acid silica materials with different hydrophobic alkylorganosilanes and evaluating the catalytic activity on the conversion of fructose to 5-HMF. The most hydrophobic catalyst prevented the leaching of sulfonic groups and led to the highest yield and selectivity.
Organic deposition on active sites of the catalyst results in the covering of pores and reduces the surface area, leading to poorer catalytic performance of the catalyst. In this case, we can subject the materials to solvent extraction in order to carry away the organic contaminants [46].

3. Experimental

3.1. Materials, Reagents, and Methods

The biomass material used in this work was rice husks from Oriza sativa L., kindly supplied by NOVARROZ-PRODUTOS ALIMENTARES, S.A. (Aveiro, Portugal) and the post-treatment was performed as described in the experimental section. All reagents and solvents were used as received without further purification. 2-(4-Chlorosulfonylphenyl) ethyltrimethoxysilane (CSPTMS, 50 wt% in methylene chloride) was acquired from ABCR GmbH, 5-(hydroxymethyl) furfural (≥99%) and tetradecane (≥99%) were purchased from Sigma-Aldrich (Saint Louis, MO, USA), and ethanol (99.8%) was purchased from Fisher Chemicals (Pittsburgh, PA, USA).
The microwave-assisted rice husk chemical leaching and SBA-15 preparation was carried out in an MARS 6 microwave from CEM Corporation (Matthews, NC, USA).
The elemental analyses (EA) for C, H, S, and N elements were performed in a Leco instrument (CHNS-932 model) at the University of Santiago de Compostela, Spain.
X-ray photoelectron spectroscopy (XPS) was performed at “Centro de Materiais da Universidade do Porto” (CEMUP), Portugal, using a Kratos (San Diego, CA, USA) AXIS Ultra HSA, with the VISION software for data acquisition. The analysis was carried out with a monochromatic Al Kα X-ray source (1486.7 eV), operating at 15 kV (90 W), in FAT mode (fixed analyzer transmission), with a pass energy of 40 eV for ROI regions and 80 eV for survey. The powdered samples were pressed into pellets prior to the XPS studies. Data acquisition was performed with a pressure lower than 1 × 10−6 Pa, and a charge neutralization system was used. To correct possible deviations caused by the electric charge of the samples, the calibration of the binding energies (BEs) was performed using the C 1 s peak at 284.6 eV (C–H/C–C) as internal reference. High-resolution C 1s, O 1s, Si 2p, and S 2p core-level spectra were analyzed with CasaXPS (Casa Software 2.3.26). For curve fitting of the core-level spectra, a Gaussian/Lorentzian product function peak shape model (G/L = 30) was used in combination with a Shirley background.
The BET surface area (SBET) was calculated by using the relative pressure data in the 0.05–0.3 range. The total pore volume (Vp) was evaluated based on the amount adsorbed at a relative pressure of about 0.95. The pore size distributions were obtained from the adsorption branches of the isotherms, applying the BJH method with the modified Kelvin equation and a correction for the statistical film thickness of the pore walls.
Powder X-ray diffraction (XRD) patterns were obtained at room temperature in Bragg–Brentano para-focusing geometry using a Rigaku Smartlab (Cambridge, MA, USA) diffractometer, equipped with a D/tex Ultra 250 detector and using Cu K-α radiation (K α1 wavelength 1.54059 Å), 45 kV, 200 mA in continuous mode, step 0.01°, speed 0.6°/min in the 0.3 ≤ 2θ ≤ 10°.
Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) studies were performed at “Centro de Materiais da Universidade do Porto” (CEMUP, Porto, Portugal) using a high-resolution (Schottky) scanning electron microscope with X-ray microanalysis and electron backscattered diffraction analysis Quanta (Fremont, CA, USA) 400 FEG ESEM/EDAX Genesis X4 M. The samples were studied as powders and were coated with an Au/Pd thin film by sputtering using the SPI Module Sputter Coater equipment.
Scanning transmission electron microscopy (STEM) studies were performed using a Hitachi (Tokyo, Japan) HD-2700 microscope operating at an acceleration voltage of 200 kV. The samples for analysis were prepared by dispersing a powdered sample in ethanol followed by immersion of a 400-mesh copper grid covered with a holey carbon film into the prepared suspension. The acquired STEM images were analyzed using the Gatan DigitalMicrograph 3.53 software.
Thermogravimetric analysis (TGA) was performed at FCUP|DQB-Lab&Services, Porto, Portugal, using a Hitachi (STA7200RV) thermal analyzer with a ramp of 5 °C/min under nitrogen (200 mL/min) from 30 to 850 °C.
Potentiometric titration was performed in an HACH TitraLab (Vésenaz, Switzerland) AT 1000 Series using a PHC705 electrode to determine the Brønsted acid density of the functionalized SBA-15 materials (mmol H+·g−1). Prior to the analysis, a solution of 20 mg of solid catalyst and 20 mL of 2 M NaCl was kept at room temperature under vigorous stirring for 18 h. Once the suspension was filtered, the final solution was titrated with 0.02 M NaOH aqueous solution.
Microwave-assisted catalytic reactions were carried out in a CEM Discovery Labmate circular single-mode cavity instrument (300 W max magnetron power output) from CEM Corporation, (Matthews, NC, USA). The performance of the prepared silica catalyst was also studied in a high-pressure 70 mL batch stirred reactor (Berghof) equipped with a BTC-3000 control interface and a digital stirring hot plate (IKA RH) and using conventional heating using an oil bath.
Product identification and quantification were carried out using a Thermo Scientific (Waltham, MA, USA) TRACE 1300 GC (FID detector) equipped with a TG-5MS capillary column (30 m, 0.25 mm i.d., 0.25 μm) and using standard reference compounds like 5-HMF (Sigma-Aldrich, Saint Louis, MO, USA; >99%) and EL (Aldrich; >98%). Identifications of reaction products were carried out by GC–MS (Thermo scientific Trace 1300 with an ISQ Single Quadrupole Mass Spectrometer, Porto, Portugal) equipped with a TG5-MS capillary column (60 m, 0.25 mm i.d., 0.25 μm film).
FTIR spectra were obtained on a Jasco (Tokyo, Japan) 460 Plus spectrometer using KBr pellets, in the range 4000–400 cm−1, with 64 scans.

3.2. Synthesis of SBA-15 Materials

The preparation of the silica materials involved several steps. Initially, rice husks (RHs) were washed with water to remove soil and dust, and then, dried at 100 °C for 2 h. Rice husk ash (RHA) was obtained by calcination of the RH at 600 °C for 4 h with a heating rate of 5 °C per minute. Treated rice husk ash (TRHA) was produced by subjecting RHA to chemical leaching using a 5% citric acid solution under microwave irradiation at 100 W and 95 °C for 30 min, followed by heat treatment at 600 °C for 4 h with a heating rate of 5 °C per minute.
Next, a sodium silicate solution was prepared by refluxing under MW irradiation 1 g of TRHA in 40 mL of 1 moldm−3 NaOH in H2O at 100 °C and 100 W for 15 min, followed by filtration.
The synthesis of Santa Barbara Amorphous-15 (SBA-15) was carried out through a hydrothermal process based on previously reported procedures [20,21]. To begin, 1.0 g of Pluronic P123 was dissolved in 38 mL of aqueous HCl (1.6 moldm−3) under stirring at 40 °C, and 40 mL of the sodium silicate solution obtained from the rice husks was added dropwise. The mixture was then stirred at 40 °C for 24 h, when using the autoclave hydrothermal procedure, or for 2 h, 4 h, or 6 h when using microwave irradiation. Subsequently, independently of the procedure, the temperature was raised to 100 °C for an additional 2 h under microwave irradiation. The resulting product was filtered, dried, and the organic template was removed through calcination at 550 °C for 5 h with a heating rate of 1 °C per minute.

3.3. SBA-15 Functionalization

SBA-15 functionalization was performed by adding 1 g of SBA-15 to 40 mL of anhydrous dichloromethane under stirring. To the resulting mixture, 1 mL of CSPTMS was added and the solution was stirred over night at 30 °C, in an inert atmosphere. The solid product was filtered, washed with dichloromethane, and left to dry at 80 °C, overnight [31,47].

3.4. Catalytic Reactions

The performance of the as-prepared sulfonic acid-functionalized SBA-15 catalyst was studied using different techniques: a microwave-assisted reactor (air, 700 W, close vessel), a high-pressure batch reactor (air, autogenous pressure 5 bar) with mechanical stirring, and a glass reactor in oil bath conventional heating (air, close vessel), referred to as the conventional approach. The reactions were performed using 5 mL of ethanol, 50 mg of 5-HMF, 25 mg of catalyst, and 20 μL of tetradecane (as internal standard). After reaction, the reactors were cooled to room temperature and the suspension filtered, collected, and stored in sealed vials for analysis. The analysis was performed by gas chromatography [31,44]. The 5-HMF conversion into EL was calculated based on the 5-HMF/EL ratio, with the presence of the internal standard. The catalyst was recovered from the reaction solution by centrifugation, washed with ethanol, left to dry in a glass vacuum drying pistol, and then, subjected to the next catalytic run to assess the catalyst recyclability regarding its conversion and selectivity.
A control experiment was performed using the pristine SBA-15 as catalyst and the results confirmed that SBA-15 alone does not exhibit any catalytic activity; Figure S3 in the Supporting Information.

4. Conclusions

In this work, SBA-15 mesoporous silica was synthetized using biobased silica obtained from rice husks and under two hydrothermal methods: autoclave and MW irradiation. MW-synthesized SBA-15 was then post-functionalized with –SO3H groups using 2-(4-Chlorosulfonylphenyl) ethyltrimethoxysilane (CSPTMS). The structural and chemical compositions of the prepared materials were confirmed by several techniques, confirming the successful preparation of the materials.
Functionalized SBA-15 (CSPTMS@SBA-15) was applied as a catalyst in the synthesis of ethyl levulinate (EL) from 5-HMF using different heating methodologies, namely, MW-assisted, high-pressure batch reactor, and the conventional approach. The effects of the MW and non-microwave heating methods on the efficient conversion of 5-HMF into EL were assessed. The results showed that MW technology allowed the best conversion and selectivity when compared with the non-MW heating methods. The effects of temperature and use of different alcohols, simultaneously as solvent and reagent, were also evaluated and the results showed that higher temperatures favor the production of EL and that a complete conversion of 5-HMF was obtained for all the used alcohols. For methanol and ethanol, 100% selectivity of alkyl levulinates was obtained, while for 1-propanol and butanol a decrease in the amount of alkyl levulinates could be observed. These results demonstrate the feasibility of achieving different alkyl levulinates with a good selectivity by using CSPTMS@SBA-15 as a catalyst.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal14080482/s1. Figure S1: BJH pore size distribution obtained for the materials; Figure S2: BJH pore size distribution obtained for SBA-15 and SBA-15@CSPTMS; Table S1: Catalytic performance of the SBA-15@CSPTMS catalyst in the conversion of 5-HMF to EL, under different conditions; Table S2: Catalytic performance of the SBA-15@CSPTMS catalyst in the conversion of 5-HMF into alkyl levulinates, using different solvents; Figure S3: Thermogravimetric profile for SBA-15@CSPTMS before (red line) and after catalysis (black line); Figure S4: GC chromatogram for the catalytic reaction of conversion of 5-HMF into EL by SBA-15 catalyst at (A) 0 h and (B) 6 h.

Author Contributions

Conceptualization, S.O.R., A.L. and A.F.P.; methodology, S.O.R., I.M., A.B. and A.A.Y.; validation, A.L. and A.F.P.; formal analysis, A.L., I.M., S.O.R. and A.A.Y.; data curation, A.L., I.M., S.O.R., A.B. and A.A.Y.; writing—original draft preparation, A.L., I.M. and A.A.Y.; writing—review and editing, A.L. and A.F.P.; supervision, A.L.; funding acquisition: A.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work received financial support from FCT/MCTES: UIDB/50006/2020 (https://doi.org/10.54499/UIDB/50006/2020) through national funds.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Acknowledgments

A.L. acknowledges financial support within the projects PTDC/QUI-QIN/28142/2017 (https://doi.org/10.54499/PTDC/QUI-QIN/28142/2017), LA/P/0008/2020 (https://doi.org/10.54499/LA/P/0008/2020), UIDP/50006/2020 (https://doi.org/10.54499/UIDP/50006/2020), and UIDB/50006/2020 (https://doi.org/10.54499/UIDB/50006/2020) through national funds. A.B. and A.A.Y. acknowledge financial support within the projects CICECO-Aveiro Institute of Materials, UIDB/50011/2020 (https://doi.org/10.54499/UIDB/50011/2020), UIDP/50011/2020 (https://doi.org/10.54499/UIDP/50011/2020), and LA/P/0006/2020 (https://doi.org/10.54499/LA/P/0006/2020), financed by national funds through the FCT/MCTES (PIDDAC). A. Leite acknowledges FCT for funding through the program DL 57/2016—Norma Transitória. We also acknowledge Novarroz—Produtos Alimentares, S.A. for the rice husk samples.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. N2 adsorption isotherms of SBA-15 (black line) and SBA-15 obtained by microwave irradiation at 2 h (green line), 4 h (blue line), and 6 h (red line).
Figure 1. N2 adsorption isotherms of SBA-15 (black line) and SBA-15 obtained by microwave irradiation at 2 h (green line), 4 h (blue line), and 6 h (red line).
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Catalysts 14 00482 g002
Figure 2. XRD patterns of SBA-15 synthetized by autoclave hydrothermal method (black line) and MW at different times: 2 h (green line), 4 h (blue line), and 6 h (red line).
Figure 2. XRD patterns of SBA-15 synthetized by autoclave hydrothermal method (black line) and MW at different times: 2 h (green line), 4 h (blue line), and 6 h (red line).
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Catalysts 14 00482 g003
Figure 3. SEM images of (A) SBA-15 MW 2 h; (B) SBA-15 MW 4 h; (C) SBA-15 MW 6 h; (D) SBA-15.
Figure 3. SEM images of (A) SBA-15 MW 2 h; (B) SBA-15 MW 4 h; (C) SBA-15 MW 6 h; (D) SBA-15.
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Catalysts 14 00482 g004
Figure 4. C, H, N, and S contents for SBA-15 and SBA-15@CSPTMS (left) and deconvoluted XPS high-resolution spectra of CSPTMS@SBA-15 in the S 2p region (right). The overall simulated spectrum is represented by the red line.
Figure 4. C, H, N, and S contents for SBA-15 and SBA-15@CSPTMS (left) and deconvoluted XPS high-resolution spectra of CSPTMS@SBA-15 in the S 2p region (right). The overall simulated spectrum is represented by the red line.
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Catalysts 14 00482 g005
Figure 5. FTIR spectra (A) of SBA-15 (black line) and SBA-15 functionalized with CSPTMS (red line) and (B) superposition of the 1050–500 cm−1 region.
Figure 5. FTIR spectra (A) of SBA-15 (black line) and SBA-15 functionalized with CSPTMS (red line) and (B) superposition of the 1050–500 cm−1 region.
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Catalysts 14 00482 g006
Figure 6. FTIR spectra (A) of SBA-15 (black line) and CSPTMS@SBA-15 minus SBA-15 (red line).
Figure 6. FTIR spectra (A) of SBA-15 (black line) and CSPTMS@SBA-15 minus SBA-15 (red line).
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Catalysts 14 00482 g007
Figure 7. N2 adsorption isotherms of SBA-15 (black line) and SBA-15@CSPTMS (red line).
Figure 7. N2 adsorption isotherms of SBA-15 (black line) and SBA-15@CSPTMS (red line).
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Catalysts 14 00482 g008
Figure 8. XRD patterns of SBA-15 (black line) and CSPTMS@SBA-15 (red line).
Figure 8. XRD patterns of SBA-15 (black line) and CSPTMS@SBA-15 (red line).
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Catalysts 14 00482 g009
Figure 9. SEM images of SBA-15 and SBA-15@CSPTMS and EDS results of SBA-15, top, and SBA-15@CSPTMS, bottom.
Figure 9. SEM images of SBA-15 and SBA-15@CSPTMS and EDS results of SBA-15, top, and SBA-15@CSPTMS, bottom.
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Catalysts 14 00482 g010
Figure 10. STEM images of SBA-15 particles after microwave irradiation for (A) 2 h, (B) 4 h, and (C) 6 h.
Figure 10. STEM images of SBA-15 particles after microwave irradiation for (A) 2 h, (B) 4 h, and (C) 6 h.
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Catalysts 14 00482 g011
Figure 11. STEM images of mesoporous silica parallel (top) and perpendicular (bottom) to the mesopores: (A) SBA-15 prepared by microwave irradiation for 6 h, (B) functionalized SBA-15, and (C) SBA-15 prepared by autoclave hydrothermal procedure.
Figure 11. STEM images of mesoporous silica parallel (top) and perpendicular (bottom) to the mesopores: (A) SBA-15 prepared by microwave irradiation for 6 h, (B) functionalized SBA-15, and (C) SBA-15 prepared by autoclave hydrothermal procedure.
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Catalysts 14 00482 g012
Figure 12. TG curve of SBA-15 (black line) and SBA-15 functionalized with CSPTMS (red line).
Figure 12. TG curve of SBA-15 (black line) and SBA-15 functionalized with CSPTMS (red line).
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Catalysts 14 00482 g013
Figure 13. One-pot conversion of 5-HMF to EL using CSPTMS@SBA-15 as catalyst and different MW-assisted and non-microwave-assisted technologies at 150 °C after 6 h.
Figure 13. One-pot conversion of 5-HMF to EL using CSPTMS@SBA-15 as catalyst and different MW-assisted and non-microwave-assisted technologies at 150 °C after 6 h.
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Catalysts 14 00482 g014
Figure 14. Conversion of 5-HMF and selectivity towards EL at different temperatures after 6 h under MW irradiation.
Figure 14. Conversion of 5-HMF and selectivity towards EL at different temperatures after 6 h under MW irradiation.
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Catalysts 14 00482 g015
Figure 15. Conversion of 5-HMF and EL selectivity during 6 h over CSPTMS@SBA-15.
Figure 15. Conversion of 5-HMF and EL selectivity during 6 h over CSPTMS@SBA-15.
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Catalysts 14 00482 g016
Figure 16. Conversion of 5-HMF and selectivity towards alkyl levulinates using different solvents at 160 °C after 6 h under MW irradiation.
Figure 16. Conversion of 5-HMF and selectivity towards alkyl levulinates using different solvents at 160 °C after 6 h under MW irradiation.
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Catalysts 14 00482 g017
Figure 17. Conversion of 5-HMF and selectivity towards EL at 130 °C after 6 h, under MW irradiation for five consecutive cycles.
Figure 17. Conversion of 5-HMF and selectivity towards EL at 130 °C after 6 h, under MW irradiation for five consecutive cycles.
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Table 1. Textural properties of SBA-15 materials prepared under MW and by hydrothermal heating.
Table 1. Textural properties of SBA-15 materials prepared under MW and by hydrothermal heating.
CatalystTextural Properties (a)
SBET
(m2/g)		Vp
(cm3/g)
SBA-15 MW 2 h355.660.206
SBA-15 MW 4 h318.210.382
SBA-15 MW 6 h449.050.571
SBA-15574.400.823
(a) BET specific surface area (SBET) and pore volume at P/P0 = 0.95 (Vp) calculated from N2 adsorption isotherms at −196 °C.
Table 2. Textural properties of SBA-15 and SBA-15@CSPTMS.
Table 2. Textural properties of SBA-15 and SBA-15@CSPTMS.
CatalystTextural Properties (a)
SBET
(m2/g)		Vp
(cm3/g)
SBA-15449.050.571
SBA-15@CSPTMS334.630.421
(a) BET specific surface area (SBET) and pore volume at P/P0 = 0.95 (Vp) calculated from N2 adsorption isotherms at −196 °C.
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Ribeiro, S.O.; Marques, I.; Bamburov, A.; Yaremchenko, A.A.; Peixoto, A.F.; Leite, A. Improved Microwave-Assisted Ethyl Levulinate Production Using Rice Husk-Derived Biobased Mesoporous Silica as Catalyst. Catalysts 2024, 14, 482. https://doi.org/10.3390/catal14080482

AMA Style

Ribeiro SO, Marques I, Bamburov A, Yaremchenko AA, Peixoto AF, Leite A. Improved Microwave-Assisted Ethyl Levulinate Production Using Rice Husk-Derived Biobased Mesoporous Silica as Catalyst. Catalysts. 2024; 14(8):482. https://doi.org/10.3390/catal14080482

Chicago/Turabian Style

Ribeiro, Susana O., Inês Marques, Aleksandr Bamburov, Aleksey A. Yaremchenko, Andreia F. Peixoto, and Andreia Leite. 2024. "Improved Microwave-Assisted Ethyl Levulinate Production Using Rice Husk-Derived Biobased Mesoporous Silica as Catalyst" Catalysts 14, no. 8: 482. https://doi.org/10.3390/catal14080482

APA Style

Ribeiro, S. O., Marques, I., Bamburov, A., Yaremchenko, A. A., Peixoto, A. F., & Leite, A. (2024). Improved Microwave-Assisted Ethyl Levulinate Production Using Rice Husk-Derived Biobased Mesoporous Silica as Catalyst. Catalysts, 14(8), 482. https://doi.org/10.3390/catal14080482

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