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Article

Visible-Light-Active Zn–Fe Layered Double Hydroxide (LDH) for the Photocatalytic Conversion of Rice Husk Extract to Value-Added Products

by
Muhammad Saeed
1,†,
Muhammad Zafar
2,†,
Abdul Razzaq
1,
Shenawar Ali Khan
3,
Zakir Khan
1,* and
Woo Young Kim
3,*
1
Department of Chemical Engineering, COMSATS University Islamabad, Lahore 54000, Pakistan
2
Institute of Energy and Environmental Engineering, University of the Punjab, Lahore 54590, Pakistan
3
Department of Electronic Engineering, Faculty of Applied Energy System, Jeju National University, Jeju-si 63243, Jeju Special Self-Governing Province, Korea
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Appl. Sci. 2022, 12(5), 2313; https://doi.org/10.3390/app12052313
Submission received: 17 January 2022 / Revised: 15 February 2022 / Accepted: 21 February 2022 / Published: 23 February 2022
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Versions Notes

Abstract

:
One of the major causes of excess CO2 in the atmosphere is the direct burning of biomass waste, which can be obviated by the photocatalytic biomass conversion to useful/valuable chemicals/fuels, a sustainable and renewable approach. The present research work is focused on the development of a novel Zn–Fe LDH by a simple co-precipitation method and its utilization for the photocatalytic conversion of a rice husk extract (extracted from rice husk by means of pyrolysis) to value-added products. The synthesized, pure Zn–Fe LDH was characterized by various analytical techniques such as XRD, SEM, FTIR, and UV–Visible DRS spectroscopy. The rice husk extract was converted in a photocatalytic reactor under irradiation with 75 W white light, and the valued-added chemicals were analyzed by gas chromatography–mass spectrometry (GC–MS). It was found that the compounds in the rice husk extract before the photocatalytic reaction were mainly carboxylic acids, phenols, alcohols, alkanes (in a small amount), aldehydes, ketones, and amines. After the photocatalytic reaction, all the carboxylic acids and phenols were completely converted into alkanes by complex reactions. Hence, photocatalytic biomass conversion of a rice husk extract was successfully carried out in the present experimental work, opening new avenues for the development of related research domains, with a great potential for obtaining an alternate fuel and overcoming environmental pollution.

1. Introduction

Biomass was essential to mankind for a long time to produce heat energy before the rise of fossil fuels [1]. Today, modern technologies are relying on fossil fuels to produce of energy. The burning of either fossil fuels or biomass produces an enormous amount of carbon dioxide (CO2), which causes global warming [2]. CO2 is a major greenhouse gas that is considered to contribute a large portion of overall greenhouse gas (GHG) emissions throughout the world. Recently, the rate of greenhouse gases emission has been reaching the value of 6% per annum, and a major portion of greenhouse gases consists of carbon dioxide [3].
Biomass is a major renewable carbon-based energy resource that is readily available to fulfill the essential requirements of human beings; for carbon-based chemical production, it is the best alternative to fossil fuels. Until now, biomass has been utilized to produce heat energy or power generation in industries, but it can also be utilized to obtain valuable fuels, chemicals, materials, and other products, and this will also contribute to alleviating the emissions of carbon dioxide [4]. For the production of numerous materials and feedstocks, many biomass-derived chemicals to facilitate the green processing of biomass are readily available [5].
Different types of biomass are produced in a country, such as harvests waste, food processing trash, municipal garbage, animal waste, etc. [6].
Presently, certain technologies can convert biomass to valuable products. Liquid, gaseous, and solid fuels can be produced by the thermochemical conversion of biomass through different methods such pyrolysis [7], gasification [8], combustion, and liquification [9]. The main products produced by the thermochemical conversion of biomass are bio-oil, methanol, aerosols, syngas, methane gas, hydrogen fuel, etc. Biomass can also be biologically converted into liquid fuels such as ethanol, dimethyl ether, and biogas by digestion and fermentation [10]. The conversion of biomass through thermochemical (combustion, pyrolysis, gasification, and liquification) and biological (digestion and fermentation) methods also produces a large amount of carbon dioxide [11]. Amongst various approaches, recently, photocatalysis has demonstrated to be an alluring approach for the conversion of biomass into useful and valuable chemicals/fuels, without polluting the environment. Photocatalysis is a process in which photons (solar irradiations) re used to accelerate chemical reactions in the presence of a suitable photocatalyst [12].
Photocatalysis is portrayed as a unique process to deal with the environmental crisis and energy rectification to preserve a good quality of life and avoid acute and chronic damages caused by CO2 emission. Photocatalysis is the most versatile and effective of emerging processes for the conversion of biomass [12]. The objective of the present experimental work was to catalytically convert biomass into useful/valuable products and avoid the direct combustion of biomass—so to keep the atmosphere as clean as possible—using a visible-light-active photocatalyst. For the conversion of biomass, TiO2 is most commonly used as a photocatalyst due to its valuable properties. In fact, TiO2 is abundant, has a favorable surface area, is nontoxic, is chemically and biologically inert and cost-effective, and has a high stability under sunlight irradiation [13,14]. However, despite certain benefits, TiO2 possesses a major limitation consisting in its wide band gap pf 3.2–3.4 eV, not favorable for efficient catalysis, because of the absorption of ultraviolet radiations comprising only 4% of the solar spectrum [14]. Such specific drawback has led to its replacement by visible-light-active photocatalysts capturing a major portion of light and hence promoting reaction efficiency. Until today, various visible-light-active photocatalysts have been developed with the aim of better performance. Amongst them, recently layered double hydroxides (LDHs) have attracted considerable attention due to their particular properties and vast potential applications, such as superb adsorption, enhanced capacity of light absorption, greater stability than ordinary photocatalyst, and improved activity due to their layered morphology which may provide favorable porosity and surface area for the attachment of biomass, as compared to other ordinary photocatalysts [15]. The zinc–iron layered doubled-hydroxide (Zn–Fe LDH) photocatalyst synthesized by a simple co-precipitation method was employed for value addition to biomass husk extract under while light irradiation. The results obtained are preliminary but interesting and provide a new direction for the application of photocatalysts in biomass value addition, which is the ultimate target of the present research work.

2. Materials and Methods

2.1. Chemicals and Materials

The materials and chemicals for the synthesis of the Zn–Fe LDH photocatalyst included iron nitrate nonahydrate (Fe(NO3)3·9H2O, 99.99%), zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98.0%), and sodium hydroxide (NaOH, 97.0%). All chemicals were purchased from Merck, Sigma-Aldrich and, used as such. Highly pure de-ionized water (6.22 pH, 2.40 mg/L TDS, 5.84 µS/cm electrical conductivity) was produced by Aqua Flow® (Reverse Osmosis water purifier system, Taiwan). Absolute ethanol (99.5% pure) was also purchased from Sigma-Aldrich. The rice husk extract (100 mL) was provided by Energy Engineering Research and Development Center, University of the Punjab, Lahore, produced from a pyrolysis reactor system. Temperature is a key promotor in pyrolysis and significantly affects the mass yield of the pyrolysis products; thus, the optimum temperature should be selected to attain the optimum yield [16]. Rice husk extract was obtained by subjecting dried rice husk to a pyrolysis reaction (without oxygen) in a pyrolysis reactor at a temperature of about 400 °C, with a 20 °C/min heating rate.

2.2. Synthesis of Pure Zn–Fe LDH

The Zn–Fe LDH photocatalyst was synthesized by a simple co-precipitation method [15,17]. Briefly, 3.57 g of zinc nitrate hexahydrate (0.3 M, 297.5 g/mol) was dissolved into 40 mL of deionized water, and 1.616 g of iron nitrate nonahydrate (0.1 M, 404 g/mol) was dissolved in a separate beaker containing 40 mL of deionized water and magnetically stirred for proper dissolving [18]. The two solutions were then mixed in a 250 mL beaker under continuous stirring. Sodium hydroxide (2 M, 40 g/mol), used as a precipitating agent, was then added dropwise into the mixed solution under vigorous magnetic stirring until a dark orange-colored colloidal solution was obtained. The resulting orange-colored colloidal solution was filtered using vacuum filtration, and the filtered cake was washed twice to eliminate the impurities. The washed filter cake was dried in an air oven at 60 °C overnight, followed by grinding to finally obtain a fine powder of the Zn–Fe LDH photocatalyst. The concentration of Zn and Fe was measured by flame atomic absorption spectroscopy (AAS, Agilent 280 FS AA) and was found negligible, i.e., Zn concentration was 0.0128 µg/L, and Fe concentration was 0.0095 µg/L. For Zn detection, the wavelength was 307.6 nm, whereas for Fe detection, the wavelength was 372 nm. The flame consisted of air/acetylene gas.

2.3. Materials Characterization

The crystallinity of the prepared photocatalysts was measured using X-ray diffraction (XRD) spectroscopy, using a PANalytical X-pert Powder diffractometer (model: Pro DY38059) and employing Cu Kα radiations of 0.15406 nm. The scan range was 2θ = 10°–80°. The surface morphology of pure Zn–Fe LDH was investigated by scanning electron microscopy (SEM, TESCAN Vega LMU), and images were taken at 15 kV. The functional groups analysis for Zn–Fe LDH were identified by Fourier-transform infrared (FTIR) spectrometry using a PerkinElmer spectrometer in ATR mode within the range from 4000 cm−1 to 700 cm−1. The absorption of the prepared Zn–Fe LDH was investigated by UV–visible diffuse reflectance spectroscopy (DRS, Jasco V-770), employing an integrated sphere kit. Rice husk extract, before and after the photocatalytic reaction, was analyzed by gas chromatography–mass spectrometer (GC–MS) using an Agilent Technologies/7890 GC system. The injection of the sample was done manually using a glass syringe, and the injection volume was 1 µL in the presence of helium (He, 99.999% pure) as a carrier gas. The flow rate of He was 1.2 mL/min with a split ratio of 1:20. An HP-5MS capillary column (30 m × 0.25 mm × 0.25 µm) was used, and the column oven was held at 40 °C initially for 3 min. The oven temperature was then raised to 190 °C (ramp 5 °C/min) and held for 20 min. The injection temperature was 270 °C.
A three-electrode arrangement was employed to measure the photocurrent density. In brief, an FTO substrate coated with Zn–Fe LDH (exposed are 1.0 cm2) acted as the working electrode, a platinum (Pt) wire as the counter electrode, and Ag/AgCl as the reference electrode. All electrodes were immersed in a beaker containing 0.1 M Na2SO4 solution, and the working electrode was illuminated with a 75 W LED light during photocurrent measurements. The lamp was switched on and off for 20 s.

2.4. Photocatalytic Conversion of Biomass

For the evaluation of the photocatalytic performance, the rice husk extract was subjected to a photocatalytic reaction with the prepared Zn–Fe photocatalyst. For the photocatalytic reaction, 0.750 g of rice husk extract was put in a beaker containing 100 mL ethanol and 0.15 g of the synthesized Zn–Fe LDH powder [19]. The mixture of photocatalysts and rice husk extract was stirred for 30 min in the dark until reaching the adsorption–desorption equilibrium (if any) and a good dispersion of the photocatalyst. The dispersed solution was then transferred into a quartz photocatalytic reactor equipped with a white LED light (75 W) lamp. The photocatalytic reactor was placed on a digital magnetic stirrer. The photocatalytic reaction was started when the light was turned on. After one hour, 1 mL of sample was taken from the photocatalytic reactor, properly centrifuged, filtered, and injected into the GC-MS for analysis. The schematic portraying the experimentation is shown in Figure 1.

3. Results and Discussion

3.1. Crystallinity

The crystallinity of the prepared Zn–Fe LDH was analyzed using XRD, as shown in Figure 2. It is evident that Zn–Fe LDH exhibited 2θ peaks appearing at 31.91°, 34.51°, 36.41°, 47.77°, 56.74°, 62.9°, 67.97°, and 69.17°, corresponding to the basal reflections (inter-planner spaces) of (003), (006), (009), (012), (015), (118), (110), and (113), respectively. The largest peak corresponding to (009) demonstrated that the prepared compound was a Zn–Fe LDH with high crystallinity, as previously reported [20,21].
The peaks appearing at the lower angles (2θ) of 31.91°, 34.51°, and 36.41° were ascribed to maximum inter-planner spacing, representing the respective planes of (003), (006), and (009) [21]. The intensity of the peaks also varied due to the position of the atoms attached inside the LDH unit cell, with body-centered or face-centered arrangement [22]. In the XRD pattern, there was one large peak at 36.41°, which was due to constructive interference indicating that the atoms attached within the crystallite were in a body-centered position. Similarly, there were two mediums and five small peaks at 31.91°, 34.51°, 47.77°, 56.74°, 62.9°, 67.97°, and 69.17° because of destructive interference due to the body-centered arrangement of atoms [23]. For each peak, the size of the crystallite varied corresponding to different widths of the peaks, because the synthesized Zn–Fe LDH was not completely homogenized. A big crystallite showed all peaks diffracted at a single angle, but a small crystallite exhibited a mild variation of the angle from a single plane [22].

3.2. Morphological Analysis

The surface morphology of the prepared Zn–Fe LDH was investigated by SEM, as shown in in Figure 3. It can be clearly observed that the Zn–Fe LDH exhibited a sort of agglomerated laminar plates located one on the other. The surface morphology was smooth and homogenous in phase. We also observed large pores compacted within the layers, with several pores in the layers, indicating a somewhat porous nature of the LDH.

3.3. Fourier-Transform Infrared Spectroscopy (FTIR) Analysis

The obtained FT-IR spectrum for the Zn–Fe LDH is shown in Figure 4, with some peaks corresponding to functional groups. The broad band spectrum centered at the wavenumber of 3402 cm−1 was attributed to the stretching vibration of the hydroxyl group (–OH) and was due to interlayer water molecules [24] present between the layers of the LDH and interacting the means of hydrogen bonds [25]. A weak band around 1608 cm−1 could be assigned to the bending mode of interlayer water molecules [26] associated with anions by means of H-bonds [27,28]. The peak at the wavenumber of 1384 cm−1 was ascribed to the stretching vibration of nitrate (NO−3) groups attached to the LDH [25]. The peaks appearing at 901 cm−1 and 826 cm−1 were assigned to the lattice vibrations of the metal oxide groups M–O–H and O–M–O (M = Zn2+ and Fe3+), respectively [27,29].

3.4. Optical Absorbance and Photocurrent Measurement

The light absorbance for the prepared Zn–Fe LDH is shown by the UV–visible DRS spectrum presented in Figure 5a. It is obvious form the absorption that Zn–Fe LDH absorbs a good portion of visible light, up to the wavelength of 550 nm, with the absorption peak appearing at the wavelength of 490 nm. As established for photocatalytic materials, the photogeneration of electrons and holes in Zn–Fe LDH is dependent on light photons, which are very important to initiate the photocatalytic reaction and provide an active surface for the reaction [26]. The DRS spectrum thus obtained clearly demonstrates that the surface of Zn–Fe LDH presented greater electrons and holes produced by the absorption of visible light due to a narrow band gap [30]. The band gap estimated from the absorption data was found to be 2.53 eV for Zn–Fe. The band gap was estimated by the band edge equation which represents the relationship between band gap energy and wavelength maxima [31,32] of an absorbing material, Ebg (eV) = 1240/λm.
Figure 5b shows the photocurrent density of Zn–Fe LDH with respect to the illumination time (20 s illumination interval). It is obvious that the photocurrent density of the Zn–Fe LDH was approximately around 0.10 mA cm−2 compared to a sample in the dark, thus supporting the role of Zn–Fe LDH in the photocatalytic conversion of biomass to valuable products.

3.5. Photocatalytic Activity Evaluation

Rice husk extract was analyzed by GC–MS to demonstrate the photocatalytic performance of rice husk biomass conversion into the useful/value-added chemicals existing in it. The analysis by GC–MS indicated that the rice husk extract was a combination of complex hydrocarbons, with a long and branched chain structure. For the recognition of compounds present in the rice husk extract, the NIST (National Institute of Standards and Technology) mass spectra database was used.

3.5.1. Composition of the Rice Husk Extract before the Photocatalytic Reaction

Figure 6 displays the total ion chromatogram (TIC) of the organic compounds detected in rice husk extracted before the photocatalytic reaction. Typically, a TIC shows peaks corresponding to compounds present in the analyzed sample; the graph represents the relationship between retention time in minutes and abundance of the compound in the sample. The GC–MS results of the rice husk extract before the photocatalytic reaction indicated that it contained a large fraction of carboxylic acid (41.33%), phenolic compounds (30.32%), and alcoholic compounds (10.12%). It also contained a smaller quantity of long-chain alkanes (5.06%), aldehyde compounds (3.2%), amine groups (0.41%), and some other compounds. Table 1 presents the GC–MS results, showing a list of identified compounds from rice husk extract before the photocatalytic reaction.
Figure 7 presents the compounds identified in rice husk extract according to their parental compound. The major fraction in rice husk consisted of carboxylic acids, phenols, alcohols and alkanes, aldehyde/ketones, and other compounds [33].
Carboxylic acids in the rice husk extract: Carboxylic acids derive from cellulosic and hemi-cellulosic components, when these components undergo hydration or dehydration during pyrolysis [34]. The largest peaks that appeared after 24 and 21.84 min with 18.42% and 12.23% area, represent carboxylic acids (9-octadecanoic acid and n-hexadecanoic acid, respectively), which were highly abundant in the rice husk extract, as shown in Table 1.
Phenols in the rice husk extract: The presence of phenolic compounds was indicated by their corresponding peaks, as illustrated in Table 1. Phenolic compounds originated by the degradation of rice husk [35] that contained a large amount of lignin [36]. Phenyl propane group present in lignin is the best source of phenolic compounds, as reported by wood chemistry studies. Phenols are key compounds when checking quality, because the presence of phenols may cause aging during long-term storing in a container e [37,38].
Alcohols in the rice husk extract:Table 1 reports the presence of alcoholic compounds before the photocatalytic reaction. Alcoholic compounds are the most important for the conversion to long-chain alkanes and other important products obtained from rice husk. Rice husk contains sugar molecules, and when it is converted by means of pyrolysis, these sugar molecules are degraded forming different acids, alcohols, and ketones [39].
Aldehydes in the rice husk extract: Aldehyde compounds were present in a very small amount in the rice husk extract after pyrolysis. Table 1 reports the presence of compounds containing aldehydes in the rice husk extract: at 6.91 min, furfural was detected in a small amount, and at 9.18 min, 5-methyl 2-furancarboxaldehyde was detected, with 0.43% area coverage, as also reported by [40].
Ketones in the rice husk extract:Table 1 reports the presence of ketone-containing compounds in the rice husk extract before the photocatalytic reaction. The peak at the retention time of 5.68 min corresponds to 1-hydroxy 2-butanone, that at 6.41 min to dihydro 2-methyl 3(2H)-furanone, that at 8.24 min peak to 2-methyl 2-cyclopenten-1-one, that at 8.31 min to1-(2-furanyl) ethanone, that at 10.18 min peak to 3-methyl 1,2-cyclopentanedione, and that at 21.59 min to 3-carbethoxy-2-piperidone [40].
Alkanes in the rice husk extract: A peak near 3.53 min in Table 1 indicated the presence of cyclohexane, and a short peak at 4.90 min corresponds to 1,1-diethoxy ethane.
Other compounds in smaller quantity: Some other compounds were also detected by GC–MS in the rice husk extract but in minute quantity, as reported in Table 1. They included amines with a retention time of 3.73 min corresponding to 1,1-bis(2,2-dimethylpropoxy) N,N-dimethyl methanamine and of 7.54 min corresponding diethyl (3-decyn-1-yl) amine, with percentage areas of 0.20 and 0.21, respectively. At 6.54 min, a smaller peak with % area of 0.44 represents butyl ester acetic acid. An organic aromatic compound containing methyl and hydroxyl groups, named cresol, was also found in the rice husk at 12.50 min, with 0.45 of % area. Palmitate compounds also originated from the rice husk extract with, nearly 25 min of retention time. At 27.73 min and 27.83 min, the peaks with % area of 1.66% and 0.83% indicated the presence of glycidyl palmitate, whereas the peak appearing at 27.94 min, corresponded to glycerol 1-palmitate, with 1.14% area.

3.5.2. Composition of the Rice Husk Extract after the Photocatalytic Reaction

Figure 8 displays the TIC obtained for the rice husk extract sample after the photocatalytic reaction. A major portion of the organic compounds consisted mainly of long-chain alkanes molecules and some alcohols. Hence, the photocatalytic reaction promoted biomass conversion and value addition to the rice husk extract by transforming organic compounds present before the reaction, i.e., carboxylic acids, phenols, amines, aldehydes, and ketones, to long-chain alkanes and some alcohols. Figure 8b presents the enlarge view of TIC.
Table 2 presents the percentage areas for organic compounds formed after the photocatalytic reaction (1 h) of the rice husk extract with Zn–Fe LDH.
Figure 9 presents the bar chart of the organic compounds in the rice husk extract after the photocatalytic reaction. These compounds are indicated according to their parental compound. Hence, GC–MS showed that, after the photocatalytic reaction, the rice husk contained 82.62% of long-chain alkanes, 7.83% of alcohols, 0.38% of ketones, 9.17% of other compounds. These compounds were the outcome of many interconnected reactions that took place during the photocatalytic conversion.
Alkanes after the photocatalytic reaction: Table 2 displays the data relevant to the peaks of alkane-containing compounds after the photocatalytic reaction. The amount of aliphatic compounds was high as compared to that of other compounds detected in the sample after the photocatalytic reaction. Aromatic compounds present in the rice husk extract before the photocatalytic reaction were converted in aliphatic compounds due to the high accessibility of hydrogen radicals [H+] present there [41].
The reactions involved in the photocatalytic conversion are condensation, hydration, dehydration [33], absorption, desorption, recombination, oxidation, reduction, polymerization of compounds, and repolymerization of compounds, as reported by [42]. Here, we observed that, due to the complex reactions in the photocatalytic process within the rice husk extract, it was quite challenging to infer a reaction mechanism that clearly explained all the conversion reactions occurring in photocatalytic reaction of the rice husk extract. However, based on the reported literature [4,5,43,44,45], it can be presumed herein that a series of reactions involving Brønsted and Lewis acids occurred, resulting in a variety of key products. A proposed mechanism is shown in Figure 10, in which major compounds present in the rice husk extract, are associated with photocatalytic conversion mechanisms in the presence of Zn–Fe LDH.
Alcohols after the photocatalytic reaction: Table 2 reports the presence of alcoholic compounds in the sample after the photocatalytic reaction, in a smaller amount, compared to alkanes. The peak at the retention time of 3.06 min represents 2-methyl 1-propanol, that at 5.08 min indicates 2-methyl 1-butanol, and that at 4.89 min peak represents of tri-methyl silyl methanol.
Other compounds in smaller quantity after the photocatalytic reaction: Some other compounds were also detected by GC–MS after the photocatalytic reaction. Table 2 reports the detected compounds, i.e., amines, corresponding to the peak appearing at retention time of 3.73 min and assigned to 1,1-bis(2,2-dimethylpropoxy) N,N-dimethyl methanamine and the peak at 7.54 min indicating the presence of diethyl (3-decyn-1-yl) amine, with % areas of 0.20 and 0.21, respectively. A smaller amount of acetic acid was also found as shown by the small peak at 6.54 min retention time, with area percentage of 0.44, representing butyl ester acetic acid.
Based on GC–MS analysis, it is obvious that the rice husk extract obtained by pyrolysis from rice husk contained many products, some of which are desirable while others are undesirable. The desirable products are those allowing the use of rice husk extract as a bio-oil for heating purposes or for the production of many compounds, such as hydro-carbons and alcohols. The formation of undesirable products should be inhibited to make bio-oil the best alternative fuel [46]. These undesirable products are oxygenated compounds (such as carboxylic acids, phenols, aldehyde, ketones, esters, etc.) and nitrogenized compounds (nitromethane, etc.). These oxygenated and nitrogenized compounds are undesirable for the following main reasons [47]: (i) they promote corrosiveness when bio-oil is stored for some time, (ii) they promote the instability of components of bio-oil, (iii) they may reduce the heating value of bio-oil. Therefore, it is necessary to decrease or eliminate these undesirable oxygenated and nitrogenized compounds to make biomass such as bio-oil the best alternative fuel.

4. Conclusions

A pure Zn–Fe LDH having extraordinary characteristics of light absorption was successfully synthesized by a simple co-precipitation method. The synthesized material was then characterized by XRD, SEM, FTIR, and UV–Visible DRS spectroscopy for the investigation of crystallinity, surface morphology, surface functionality, and optical absorbance, respectively. Since Zn–Fe LDH possesses a narrow bandgap (2.53 eV), it is presumed that photogenerated charges on the surface of the photocatalyst will improve photocatalytic reactions. The Zn–Fe LDH photocatalyst was employed successfully for biomass conversion/value addition using a rice husk extract. The results were confirmed by qualitative and quantitative analysis of the obtained products via GC–MS. Before the photocatalytic reaction, the rice husk extract contained 41.33% of carboxylic acids, 30.32% of phenols, 10.12% of alcohols, 5.06% of long-chain alkanes, 3.2% of aldehydes, 1.93% of ketones, and 4.04% of other compounds. In contrast, after the photocatalytic reaction, GC–MS detected 7.83% of alcohols, 82.62% of long-chain alkanes, and 9.17% of some other compounds in the rice husk extract. The reaction chemistry seems to be complicated, as many reactions such as condensation, polymerization, oxidation, and reduction may have occurred during the photocatalytic process. Because of the inexpensive methodology utilized, this work delivers an alluring approach to effectively utilize rice husk and its extract for the production of useful chemicals and value-added products, as an alternative to burning it for energy purposes, a process that produces CO2 gas.

Author Contributions

Conceptualization, M.S., S.A.K. and A.R.; Investigation, M.S., A.R. and Z.K.; Data curation, M.Z.; Visualization, M.S. and S.A.K.; Writing—original draft preparation, M.S., M.Z. and A.R.; Writing—review and editing, Z.K. and W.Y.K.; Supervision, A.R. and Z.K.; Methodology; A.R., M.Z. and W.Y.K.; Funding acquisition, A.R. and W.Y.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by a National University Development Project funded by the Ministry of Education (Korea) and the National Research Foundation of Korea (2021).

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 12 02313 g001 550
Figure 1. Schematic view of the photocatalytic reactor setup and experimentation of the photocatalytic conversion of rice husk extract.
Figure 1. Schematic view of the photocatalytic reactor setup and experimentation of the photocatalytic conversion of rice husk extract.
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Figure 2. X-Ray diffraction (XRD) pattern of Zn–Fe LDH.
Figure 2. X-Ray diffraction (XRD) pattern of Zn–Fe LDH.
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Figure 3. SEM image of the Zn–Fe LDH synthesized by a co-precipitation method.
Figure 3. SEM image of the Zn–Fe LDH synthesized by a co-precipitation method.
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Figure 4. FT-IR spectrum of the Zn–Fe LDH synthesized by a co-precipitation method.
Figure 4. FT-IR spectrum of the Zn–Fe LDH synthesized by a co-precipitation method.
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Figure 5. (a) UV–Visible Diffuse Reflectance and (b) photocurrent density of Zn–Fe LDH.
Figure 5. (a) UV–Visible Diffuse Reflectance and (b) photocurrent density of Zn–Fe LDH.
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Figure 6. Total ion chromatogram of the rice husk extract before the photocatalytic reaction.
Figure 6. Total ion chromatogram of the rice husk extract before the photocatalytic reaction.
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Figure 7. Organic compounds in rice husk extract before the photocatalytic reaction.
Figure 7. Organic compounds in rice husk extract before the photocatalytic reaction.
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Figure 8. (a) Total ion chromatogram of the rice husk extract after the photocatalytic reaction and (b) enlarged view of (a).
Figure 8. (a) Total ion chromatogram of the rice husk extract after the photocatalytic reaction and (b) enlarged view of (a).
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Figure 9. Compounds in the rice husk extract after the photocatalytic reaction.
Figure 9. Compounds in the rice husk extract after the photocatalytic reaction.
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Figure 10. Proposed photocatalytic reaction mechanisms for the conversion of the rice husk extract to value-added chemicals.
Figure 10. Proposed photocatalytic reaction mechanisms for the conversion of the rice husk extract to value-added chemicals.
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Table
Table 1. Major compounds present in the rice husk extract before the photocatalytic reaction.
Table 1. Major compounds present in the rice husk extract before the photocatalytic reaction.
No.RT (min)Name of the Compound Found in the Rice Husk Extract before the Photocatalytic ReactionArea (%)
Carboxylic Acids in Rice Husk Extract
112.03Benzoic Acid1.88
217.00Monomethyl Ester, Nonanedioic Acid0.39
321.84n-Hexadecanoic Acid12.23
423.55Methyl Ester, Octadecenoic Acid0.45
524.009-Octadecenoic Acid18.42
627.62Oxiranylmethyl Ester, 9-Octadecenoic Acid3.49
729.762,3-Dihydroxypropyl Ester, 9-Octadecenoic Acid1.22
Phenols in Rice Husk Extract
89.42Phenol1.25
910.823-Methyl Phenol0.66
1011.12-Methoxy Phenol1.69
1112.034-Ethyl Phenol0.3
1228.45Pentamethoxyflavone10.26
1334.95Heptamethoxyflavone3.3
1435.27Hexamethoxyflavone7.07
Alcohols in Rice Husk Extract
154.993-methyl 1-Butanol3.94
165.072-methyl 1-Butanol1.08
177.332-Furanmethanol0.82
189.84Acetate Tetrahydro 2-Furanmethanol0.35
1928.132-(6-Ethoxy-4-Methyl-Quinazolin-2-Ylamino)-5-Methyl-Pyrimidine-4,6-Diol3.93
Aldehydes in rice husk extract
206.91Furfural2.77
219.185-methyl 2-Furancarboxaldehyde0.43
Ketones in Rice Husk Extract
225.681-Hydroxy 2-Butanone0.29
236.41Dihydro 2-Methyl 3(2H)-Furanone0.30
248.242-Methyl 2-Cyclopenten-1-One0.38
258.311-(2-Furanyl) Ethanone0.26
2610.183-Methyl 1,2-Cyclopentanedione0.27
2721.593-Carbethoxy-2-Piperidone0.43
Alkanes in Rice Husk Extract
283.53Cyclohexane2.30
294.901,1-Diethoxy Ethane2.76
Other Compounds in Smaller Quantity
303.731,1-Bis(2,2-Dimethylpropoxy) N,N-Dimethyl Methanamine0.20
317.54Diethyl (3-decyn-1-yl) Amine0.21
326.54Butyl Ester Acetic Acid0.44
3312.50Creosol0.45
3425.73Glycidyl Palmitate1.66
3527.83Glycidyl Palmitate0.83
3627.94Glycerol 1-Palmitate1.14
3732.89Hexadecamethyl Octasiloxane2.30
3836.35N-Methyl-1-Adamantaneacetamide0.78
Table
Table 2. Compounds in the rice husk extract after the photocatalytic reaction.
Table 2. Compounds in the rice husk extract after the photocatalytic reaction.
No.RT (min)Name of Compounds Found after Its Photocatalytic ReactionArea (%)
Alkanes after Photocatalytic Reaction
12.182-methyl Pentane12.27
22.323-methyl Pentane15.50
32.51n-Hexane30.92
42.94Methyl Cyclopentane15.69
53.53Cyclohexane3.37
65.023,3-Dimethyl-1,2-epoxybutane4.87
Alcohols after Photocatalytic Reaction
73.062-methyl 1-Propanol2.96
84.89Tri methyl silyl methanol3.49
95.082-methyl 1-Butanol1.38
Other Compounds in Smaller Quantity after Photocatalytic Reaction
102.27Allyl fluoride6.43
112.77Ethyl Acetate2.19
126.42Dihydro-2-methyl 3(2H)-Furanone0.38
136.55Acetic acid, butyl ester0.55
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Saeed, M.; Zafar, M.; Razzaq, A.; Khan, S.A.; Khan, Z.; Kim, W.Y. Visible-Light-Active Zn–Fe Layered Double Hydroxide (LDH) for the Photocatalytic Conversion of Rice Husk Extract to Value-Added Products. Appl. Sci. 2022, 12, 2313. https://doi.org/10.3390/app12052313

AMA Style

Saeed M, Zafar M, Razzaq A, Khan SA, Khan Z, Kim WY. Visible-Light-Active Zn–Fe Layered Double Hydroxide (LDH) for the Photocatalytic Conversion of Rice Husk Extract to Value-Added Products. Applied Sciences. 2022; 12(5):2313. https://doi.org/10.3390/app12052313

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Saeed, Muhammad, Muhammad Zafar, Abdul Razzaq, Shenawar Ali Khan, Zakir Khan, and Woo Young Kim. 2022. "Visible-Light-Active Zn–Fe Layered Double Hydroxide (LDH) for the Photocatalytic Conversion of Rice Husk Extract to Value-Added Products" Applied Sciences 12, no. 5: 2313. https://doi.org/10.3390/app12052313

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