Photo-Induced Preparation of Ag@MOF-801 Composite Based Heterogeneous Nanocatalyst for the Production of Biodiesel

Abstract

Hybrid materials based on metal-organic frameworks (MOFs) and nanoparticles (NPs) have gained considerable popularity in a variety of applications. Particularly, these types of materials have demonstrated excellent efficiency in heterogeneous catalysis due to the synergistic effect between the components. Herein, we report a simple, eco-friendly, photocatalytic method for the fabrication of Zr containing MOF-801 and a silver (Ag) NPs-based hybrid (Ag@MOF-801). In this method, the photocatalytic property of the central metal ion (Zr) of MOF was exploited to promote the formation and deposition of Ag NPs on the surface of the MOF-801 under the irradiation of visible light. The successful incorporation of Ag NPs was ascertained by powder X-ray diffraction (XRD) and UV-Vis analysis, while the morphology and surface area of the sample was determined by N₂ adsorption–desorption and scanning electron microscopy (SEM), respectively. The resulting Ag@MOF-801 hybrid served as a highly efficient catalyst for the transesterification of used vegetable oil (UVO) for the production of biodiesel. The Ag@MOF-801 catalyst exhibited superior catalytic activity compared to its pristine MOF-801 counterpart due to the enhanced surface area of the material.

1. Introduction

MOFs are metal ions and multifunctional organic ligands based on crystalline, porous materials, which have gained significant attention from researchers worldwide [1,2]. They possess a unique 3D structure formed by the self-assembly of metal ions with multi-dentate organic ligands via coordination bonds. Owing to their novel properties, these materials are extensively used in a variety of applications such as gas separation, chemical sensing, biomedical and catalytic applications, etc. [3,4,5]. Particularly, due to their easy customization by the variation of metal ions and linkers and further modifications via post-synthesis processes, MOFs offer extraordinary structure flexibility [6,7]. In addition, due to their highly porous structure, MOF-based materials offer an ultra-high surface area (10,000 m²g⁻¹), which is considerably higher than carbonaceous materials and zeolites; hence, MOFs are also considered excellent catalyst templates [8,9]. The central metal ions in MOFs offer coordinatively unsaturated metal sites which act as superb Lewis acid catalysts either by themselves or due to the synergistic effect with inserted materials such as metallic nanoparticles (NPs), etc. [10,11].

Furthermore, additional catalytically active metal sites can be introduced in MOFs by the metalation of organic ligands either during the synthesis of MOFs or by the post-synthesis modifications [12,13]. Currently, MOFs are already established as efficient catalytic materials due to their variety of topologies and the possibility of combining them with other guest materials such as metal, metal oxide NPs, carbon materials, etc. [14,15,16,17]. The deposition of NPs in MOFs during post-synthesis modifications facilitates the growth of ultra-fine nanomaterials, and the functional groups of organic ligands prevent the leaching of catalytically active material, which significantly enhances the catalytic activity of MOF-based nanocatalysts [18,19]. Furthermore, the insolubility of MOFs in organic solvents makes them ideal catalysts in heterogeneous catalysis, and the uniform pore sizes and shapes in MOFs can be exploited in size-selective catalysis [20,21].

Due to this, the trend of combining MOFs with other materials, especially metal and metal oxide NPs, has gained considerable attention to enhance the intrinsic properties of MOFs, such as surface area, thermal, electrical, and electronic properties and so on [21]. MOFs are efficient in supporting materials when compared to other porous materials due to their novel 3D pore matrices and organic ligands, which facilitate the stabilization of NPs during their growth [22]. Often, the central metal ion and organic linkers act in synergy with the guest NPs resulting in the enhanced activity of the composite [23]. So far, significant efforts were devoted to producing high-quality MOF-NP-based nanocomposites using a variety of methods, including the liquid impregnation method, solid grinding, chemical vapor deposition (CVD), etc. [24,25]. Among these methods, the impregnation method is most commonly applied, which involves the reduction of a metal precursor using a reducing agent to produce NPs.

However, there are several issues that limit the applications of these methods [26,27]. For example, the CVD method requires volatile metal precursors and complicated operating conditions, whereas the reduction methods involve the use of high temperatures, harsh reducing agents, additional stabilizing ligands, etc. [23]. Recently, to overcome these issues, MOF-NP-based composites were fabricated by the simultaneous irradiation of UV and/or visible light on the metal precursors and photoactive MOFs [28]. Using these methods, NPs can be specifically deposited in the targeted area of MOFs [29]. Upon irradiation, electrons in the valence band of the central metal ion of semiconducting MOFs are excited to the conduction band, which facilitates the reduction of the metal precursor leading to the deposition of NPs in the resulting composite [30,31]. Using this method, different types of NPs, including Pt, Au and Pd, etc., are deposited on semiconducting MOFs such as TiO₂-based MIL-125, which has demonstrated enhanced photocatalytic activity due to the incorporation of NPs [32,33]. Apart from this, various MOFs consisting of other photoactive central metal ions, such as Zn, Fe, Zr, etc., are also exploited for the photocatalytic deposition of NPs [24].

Among these photocatalytically active MOFs, Zr-based MOFs have gained decent attention due to their higher stability [34]. Recently, we published the preparation and catalytic properties of Zr-fumarate-MOF, which is also referred to as MOF-801 [35]. In order to exploit the decent photocatalytic properties of Zr-based MOFs, in the present research, we report a simple and eco-friendly photocatalytic method to produce MOF-801 and Ag hybrids (Ag@MOF-801). To the best of our knowledge, the MOF-801 has never been used to produce the reported hybrid using the photocatalytic method. In this method, the central Zr ion of MOF-801 facilitated the reduction of AgNO₃ upon irradiation with visible light. In addition, the as-prepared hybrid effectively catalyzed the transesterification reaction of used vegetable oil (UVO) for the production of biodiesel (Scheme 1). Compared to the parent MOF-801, the as-prepared hybrid MOF-801@Ag demonstrated a decent improvement in the catalytic behavior due to the enhancement of the surface area of the material.

2. Results and Discussions

2.1. X-ray Powder Diffraction (XRD) Analysis

The Ag NPs were deposited on MOF-801 (freshly prepared according to our previously published study [35]), using AgNO₃ as a precursor. In this eco-friendly approach, the visible light source and photocatalytic properties of the central metal ion (Zr) of MOF were exploited to promote the successful reduction of metal precursors to produce the Ag@MOF-801 hybrid. Initially, the formation of MOF-801 was confirmed by matching the XRD pattern of as-prepared MOF (cf. Figure 1) with the published data. When compared, as shown in Figure 1, the XRD pattern of Ag@MOF-801 consists of several additional reflections apart from the characteristic reflections of MOF-801, i.e., the reflections at ~10°, 13.9°, 19.9°, and 21.7°, which are assigned to the (200), (222), (420), and (440) planes. The additional reflections which appeared at ~37.5°, 44.1°, 64.3°, 79.3°, and 81.2°, are the characteristic reflections of Ag NPs belonging to the (111), (200), (220), (311), and (222) Miller indices (Khan et al., 2013). This confirms the formation of a highly crystalline, face-centered cubic (fcc) structure of the Ag NPs on MOF-801.

2.2. SEM and EDX Analysis of Ag@MOF-801

The structural details of the Ag@MOF-801 hybrid was ascertained by FESEM. As shown in Figure 2a, the SEM image of the hybrid depicts the presence of an octahedral shape material belonging to the MOF-801. On the other hand, the surface of the octahedra is covered by well-distributed spherical shape particles, which indicates the presence of Ag NPs on the surface of MOF. The presence of Ag in the sample is further confirmed by the elemental analysis using EDX. The EDX spectrum in Figure 2b exhibited a characteristic Ag peak at ~3 keV, which confirmed the presence of silver in the sample. Apart from this, the spectrum also showed the presence of Zr, C, and O, which are represented by the intense peak of Zr at 2.1 keV and relatively smaller peaks at ~0.28 and 0.55 keV for C and O, respectively. These peaks pointed toward the presence of MOF-801 in the sample.

2.3. UV-Visible and Fourier-Transform Infrared Spectroscopy Analysis

The formation of Ag NPs can also be confirmed by the UV analysis; typically, Ag displays a distinct peak between 380–470 nm depending on the size and shape of NPs, due to the localized surface plasmon resonance effect [36]. In this case, the UV spectrum of Ag@MOF-801 shown in Figure 3a also exhibits an absorption peak at ~450 nm, which points toward the formation of Ag NPs in the sample. Furthermore, FT-IR analysis was performed to investigate the chemical interactions between deposited Ag NPs with the chemical functionalities of MOF-801. As shown in Figure 3b, the IR spectrum of Ag@MOF-801 exhibited prominent peaks at the characteristic positions which are almost identical to the peaks of pristine MOF-801 [35]. For instance, a broad OH peak at ~3400 cm⁻¹, C=O stretching peak at 1650 cm⁻¹, O–C–O asymmetric stretching peak of the carboxylic group at 1578 cm⁻¹, the symmetrical and asymmetrical stretching peak of C–H between 3100–2800 cm⁻¹, etc. Indeed, the characteristic sharp vibration peaks of Zr₆(OH)₄O₄, and asymmetric stretching of the Zr-(OC) group also appeared at the same positions, i.e., at 655 and 491 cm⁻¹. This clearly indicated that the Ag NPs are physically adsorbed on the surface of pristine MOF-801 and do not interact chemically with the ligands and the central ion of pristine MOF. For catalytic applications, the surface area of the material plays a critical role in enhancing the catalytic properties. N₂ adsorption-desorption isotherm analyses were conducted to identify the Brunauer-Emmett-Teller (BET) specific surface area and Barrett-Joyner-Halenda (BJH) pore size distribution of Ag@MOF-801. While the pristine MOF-801 itself demonstrated a high BET surface area of 750.11 m²/g, its metallic NPs modified hybrid counterpart (Ag@MOF-801) showed a more enhanced BET surface area of 827.70 m²/g (cite our crystal paper here). This can be attributed to the presence of Ag NPs on the surface of MOF, as the size of the micropores remained unaffected with the pores’ distribution mostly microporous (Figure 4a), which is similar to the pore size of pure MOF-801 as shown in Figure 4b.

2.4. Catalytic Evaluation of Ag@MOF-801

To ascertain the catalytic properties of Ag@MOF-801, the hybrid was applied as a catalyst in the transesterification of used vegetable oil (UVO). In our previously published study, the MOF-801 was used to catalyze a similar reaction, where the progress of the reaction was tracked via ¹H NMR spectroscopy. In this case, the yield of the product (glycerol) is analyzed by NMR, ultimately indicating the amount of conversion of UVO. To begin with, several transesterification reactions were performed by varying the amount of Ag@MOF-801catalyst. Reactions were carried out at 180 °C for 8 h using a mixture of oil (as reactant) and methanol (50 wt%). Throughout these reactions, the conditions remained the same, except for the amount of catalyst; further details are provided in Table 1. Results of the NMR analysis confirming the conversion of UVO are provided in the Supplementary File Figure S1. Similar to the results obtained in the case of pristine MOF-801, in this case, 10 wt% of the catalyst with respect to the amount of oil offered maximum conversion (~70.1%). However, the Ag@MOF-801 hybrid catalyst demonstrated higher catalytic activity compared to its pristine counterpart, which rendered only ~60% conversion when the same amount of catalyst was used (cite our crystal paper here). The enhanced catalytic property of the Ag@MOF-801 hybrid can be attributed to the increased surface area of the catalyst. Notably, upon adjusting the pH of the reaction with the addition of HCl (10% v/v), the yield of the reaction further increased to more than 73.1%; this can be ascribed to the inherent catalytic property of HCl towards the esterification reaction [37]. Expectedly, the variation of the quantity of the catalyst between 10–20 wt.% has little effect on the amount of conversion, as the yield of the product slightly decreased up to ~3% (Figure 5a). On the other hand, using a smaller amount of catalyst, lower than 10 wt% (5 wt%), significantly decreased the quantity of the product and rendered only ~43% of yield (cf. Table 1). Furthermore, several experiments were carried out to optimize the time and temperature of the reactions. As appeared in the case of pristine MOF-801, herein also 8 h of reaction time rendered maximum conversion (~70.1%), while less reaction time produced a lower reaction yield (cf. Figure 5b). Likewise, a temperature of 180 °C produced maximum conversion, while below this temperature, less amount of product was obtained (cf. Figure 6a). Furthermore, the reusability tests of the Ag@MOF-801 performed under optimized reaction conditions revealed the decent stability of the catalyst up to three cycles, during which the yield of products slightly decreased (only up to ~8%) (cf. Figure 6b).

2.5. Mechanism of Biodiesel Production Using Ag@MOF-801

The components of biodiesel involve a series of fatty acid alkyl esters (FAAE) which are formed by the transesterification of triglycerides typically present in the vegetable oils (UVO) and animal fats, etc. The high contents of free fatty acid (FFA) present in UVO is typically removed via esterification reaction, where FFAs react with alcohol to produce FAAE and water [38]. In other words, esterification reduces the acid content of feedstock oil for transesterification. For both esterification and transesterification reactions, catalysts are required (homogenous and/or heterogeneous catalysts) to achieve effective conversion. Heterogeneous catalysts are further divided into two categories: acidic and basic, and the mechanism of the transesterification reactions varies according to the variety of catalysts [39]. In this study, Ag@MOF-801 is applied as a heterogeneous catalyst; the MOF-based catalysts are categorized based on the types of organic ligands, which usually involve carboxylic acid-based ligands, nitrogen heterocyclic rings based ligands, and other ligands (Figure 7) [40,41]. The Ag@MOF-801 consists of dicarboxylic acid (fumaric acid) as an organic ligand which makes it an acidic heterogeneous catalyst. In the case of an acid catalyst, the mechanism of transesterification initially involves the protonation of carbonyl of the carboxylic acid. Subsequently, the positive charge of carbonyl carbon facilitates the nucleophilic reaction of alcohol with the formation of intermediates. Thereafter, the proton is transferred with the removal of water, which is finally eliminated with the formation of ester. In the case of Ag@MOF-801, the reaction may occur in the following steps, (i) CH₃OH coordinates with the central Zr atoms of the MOF-801, (ii) triglyceride of UVO attach to the Zr atom of the MOF-801, (iii) the nucleophilic oxygen atom from methanol then attacks the electrophilic carbon at the triglyceride ester group which facilitates transesterification, this nucleophilic attack produces glycerol by-product, (iv) finally, FAAE (biodiesel) detach from the MOF through electron delocalization at the oxygen atoms to produce biodiesel [42].

3. Materials and Methods

3.1. Materials

Zirconium (IV) oxychloride octahydrate (ZrOCl₂·8H₂O), silver nitrate (AgNO₃), fumaric acid, dimethylformamide (DMF), formic acid, ethanol, methanol, and hexane were purchased from Sigma-Aldrich, St. Louis, MO 68178, USA. Used vegetable oil was obtained from a fast-food center in Riyadh, Saudi Arabia.

3.2. Sample Preparation (Ag@MOF-801)

All reagents and solvents were procured from commercial sources and used as received without any further purification. MOF-801 was produced according to our previously published study [35]. For the preparation of Ag@MOF-801 hybrid (50 wt% of silver), an aqueous solution of AgNO₃ (100 mg dissolved in 2.5 mL water) was first prepared. Separately, 200 mg of freshly prepared MOF-801 were dispersed in 25 mL ethanol via gentle stirring at room temperature for 30 min. Thereafter, the aqueous solution of AgNO₃ was added to the ethanol dispersion, and the mixture was gradually heated (increased the heat slowly, 10 °C every 10 min) to 80 °C. Once the temperature reached 80 °C, the mixture was irradiated with a 24 W Philips light-emitting diode (LED) lamp (λ > 400 nm). The lamp was placed at a distance of ~10 cm from the reaction flask; meanwhile, the mixture was continuously stirred until the color of the mixture changed to light brown (~30 min). After this, the reaction was stopped, and the product was isolated via centrifugation; a light brown powder was obtained, which was further washed three times with a mixture of water and ethanol (1:1) to remove the unreacted Ag precursor. Finally, a brown powder was obtained, which was then dried at 70 °C in an oven overnight to derive the Ag@MOF-801 hybrid. The experimental details of the conversion of UVO to esters and the details about the confirmation of the formation of the product via ¹H NMR are provided in the Supplementary information. Furthermore, the Supplementary File also includes all the details about sample preparation for characterization and other technical information about the instruments used during this study.

3.3. Transesterification of Used Vegetables Oil (UVO) Using Ag@MOF-801

The transesterification of used vegetables oil (UVO) using the Ag@MOF-801 procedure is provided in the Supplementary File.

3.4. Characterization

Characterization details are provided in the Supplementary File.

4. Conclusions

In summary, we presented a highly efficient, eco-friendly photocatalytic method for the preparation of MOF-801 and a Ag NP-based Ag@MOF-801 hybrid. The hybrid was prepared by utilizing the efficient photocatalytic properties of the central Zr ion of MOF-801. The central ion facilitated the reduction of the metal precursor under visible light irradiation leading to the successful deposition of Ag NPs on the surface of MOF. The presence of Ag NPs enhanced the surface area of the hybrid catalyst, due to which the resulting nanocatalyst demonstrated superior catalytic properties when compared to its parent material, towards the esterification of UVO for the production of biodiesel. Therefore, the protocol demonstrated here for the preparation of Ag@MOF-801 can be further extended to prepare other MOFs-NPs composites by utilizing the photocatalytic properties of semiconducting central ions-based MOFs.

Author Contributions

O.M.A.: conceptualization, visualization, resources, supervision, writing—review and editing, funding acquisition; M.R.S.: investigation, formal analysis, writing—original draft preparation, data curation; S.F.A.: investigation, visualization, validation, methodology, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Acknowledgments

The authors extend their appreciation to the Deputyship for Research& Innovation, Ministry of Education in Saudi Arabia for funding this research work through the project number (DRI-KSU-572).

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures, Scheme and Table

Scheme 1. Graphical representation of transesterification of UVO to biodiesel utilizing the synthesized MOF-801 and silver (Ag) NPs based hybrid (Ag@MOF-801) catalyst.

Figure 1. XRD analysis of MOF-801 and Ag@MOF-801.

Figure 2. (a) SEM and (b) EDX analysis of Ag@MOF-801.

Figure 3. (a) UV-Visible and (b) FT-IR analysis of Ag@MOF-801.

Figure 4. (a) N2 adsorption-desorption isotherm and (b) pore size distribution of Ag@MOF-801.

Figure 5. Conversion of UVO in the presence of Ag@MOF-801 as catalysts using different reaction parameters (a) at varying catalyst loading (green line conversion based on NMR and red line conversion based on the yield of glycerol) and (b) at a different time interval.

Figure 6. Transesterification of UVO over Ag@MOF-801, (a) at different temperatures and (b) reusability study of the Ag@MOF-801.

Figure 7. Tentative mechanism of transesterification reaction of UVO for the production of biodiesel using Ag@MOF-801 catalyst; (Zr-Zirconium; FA-Fumaric acid).

Table 1

The transesterification reactions of UVO in methanol using different amounts of oil.

Catalyst	Oil (g)	Amount of Catalyst		Product Yield (g)		Conversion (%)
		Wt. % to Oil	Amount (g)	Biodiesel	Glycerol	1H-NMR	Yield of Glycerol
Ag@MOF-801	1.5	5	0.075	1.334	0.058	42.5	37.4
Ag@MOF-801		10	0.150	1.423	0.107	70.1	69.0
Ag@MOF-801		15	0.225	1.419	0.105	69.5	67.7
Ag@MOF-801		20	0.300	1.416	0.101	67.5	65.2
Ag@MOF-801/HCl		10	0.150	1.491	0.124	73.1	72.9

Reaction conditions: Temperature—180 °C, Methanol to oil (50 wt%), and time 8 h.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12050533/s1. Figure S1: SEM analysis of MOF-801; Figure S2: FT-IR analysis of MOF-801.

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