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Article

Synthesis of Silicotungstic Acid/Ni-Zr-O Composite Nanoparticle by Using Bimetallic Ni-Zr MOF for Fatty Acid Esterification

by
Qiuyun Zhang
1,2,3,*,
Mengmeng Hu
1,
Jialu Wang
2,
Yanting Lei
1,
Yaping Wu
1,
Qing Liu
1,
Yongting Zhao
1 and
Yutao Zhang
1,2,3,*
1
School of Chemistry and Chemical Engineering, Anshun University, Anshun 561000, China
2
College Rural Revitalization Research Center of Guizhou, Anshun 561000, China
3
Engineering Technology Center of Control and Remediation of Soil Contamination of Guizhou Science and Technology Department, Anshun University, Anshun 561000, China
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(1), 40; https://doi.org/10.3390/catal13010040
Submission received: 10 November 2022 / Revised: 16 December 2022 / Accepted: 22 December 2022 / Published: 25 December 2022
(This article belongs to the Special Issue Green Catalysis in Biodiesel and Biomass Valorisation)
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Abstract

:
In this study, the bimetallic Ni-Zr MOF-derived nickel-zirconium oxide (Ni-Zr-O)-impregnated silicotungstic acid (HSiW) nanocomposite catalyst (HSiW@Ni-Zr-O) was prepared via a hydrothermal procedure followed by a pyrolysis treatment, and its structural, morphological, and surface components and oxidation states were characterized by using XRD, FTIR, TPD-NH3, SEM, TEM, N2 physisorption, and XPS analyses. Then, the nanocomposite catalysts were successfully applied to the esterification of oleic acid (OA) with methanol. According to its characteristics, the obtained HSiW@Ni-Zr-O-1 catalyst would generate larger pores, a higher acidity, and active interfaces at the calcining temperature of 300 °C. Therefore, HSiW@Ni-Zr-O-1 exhibits an excellent catalytic activity of 95.2% under optimal reaction conditions. Additionally, the catalyst can be reused with a good catalytic activity after nine cycles. This study highlights the opportunity of using bimetallic MOFs as precursors to the synthesis of highly nanoporous metal oxide, which supports the larger-industrial scale production of biofuels.

1. Introduction

Due to the energy crisis, non-renewable fossil fuels, and environmental concerns, especially sizeable greenhouse gas emissions, there is an urgent need to develop renewable biofuels to meet the energy and environmental demand [1,2]. Compared with fossil-based fuels, biodiesel is known as a renewable, biodegradable, nontoxic, and ecofriendly energy source. Its emits a relatively small amount of carbon dioxide, and possesses an appropriate cetane number and high flash point [3,4]. In general, biodiesel refers to fatty acid methyl ester obtained through the catalytic esterification or transesterification of fatty acids (e.g., oleic acid, palmitic acid, lauric acid, etc.) [5,6], vegetable oils [7], animal oils [8], and microbial oils [9] with methanol, where the liquid catalysts of HCl, H2SO4, KOH, and NaOH are mostly employed, owing to liquid catalysts’ high conversion rate [10,11]. However, the liquid catalysts used in the production of biodiesel are limited due to some drawbacks of these homogeneous reaction approaches, such as soap formation, vast wastewater generation, and difficulty in reusing the catalysts [12].
Heterogeneous catalytic approaches have distinct advantages over homogeneous catalytic approaches because the heterogeneous catalysts with excellent reusability can be easily separated from the reaction mixture [13]. Currently, various types of heterogeneous catalysts have been reported for biodiesel production, such as zeolite [14], Amberlyst [15], acidic ionic liquids [16], and heteropoly acid [17,18]. However, in some heterogeneous catalysts, the leaching of active components is found in the reaction process, leading to a decrease in stability [19]. To address this challenge, it is essential to further develop large surface area and porous carrier impregnated active components for the production of biodiesel.
Quite recently, metal–organic frameworks (MOFs) with a high porosity, large surface area, outstanding stability, and crystalline open structure have been used as ideal templates to prepare metal oxides impregnated with active components through pyrolyzation [20,21,22,23]. Here, this work describes impregnating the skeleton structure of bimetallic Ni-Zr MOF-derived nickel-zirconium oxide (Ni-Zr-O) with silicotungstic acid (HSiW) to prepare a nanocomposite catalyst via a hydrothermal and pyrolysis method. The chemical and physical properties of the as-prepared nanocomposites were studied. Moreover, the effect of the pyrolysis temperature on the catalytic performance in the esterification of oleic acid (OA) with methanol was evaluated. Furthermore, the reused properties and thermal filtration test of the as-prepared nanocomposites in catalyzation were also investigated in detail. This study supplies insight into the devising and synthesis of bimetallic MOF-derived hybrid nanomaterials for biodiesel production.

2. Results and Discussion

2.1. Catalyst Characterization

XRD diffractograms of HSiW, Ni-Zr MOF, HSiW@Ni-Zr MOF, HSiW@Ni-Zr-O-1, HSiW@Ni-Zr-O-2, and HSiW@Ni-Zr-O-3 samples are shown in Figure 1a,b. From Figure 1a, it can be seen that the XRD patterns of the Ni-Zr MOF and HSiW@Ni-Zr MOF samples are identical, and the characteristic peaks of HSiW cannot be found in the HSiW@Ni-Zr MOF. It is implied that HSiW molecules were encapsulated in the Ni-Zr MOF framework, and the decoration of HSiW has no effect on the structure of the Ni-Zr MOF; the results also agree well with a previous report [24]. After pyrolysis treatment, for HSiW@Ni-Zr-O-1 and HSiW@Ni-Zr-O-2, the sharp peak at around 8.3° and the weak broad peaks at 15–35° were observed, but the other Ni-Zr MOF characteristic peaks almost disappeared. This can possibly be attributed to the partial decomposition of the Ni-Zr MOF to Ni-Zr-O via the calcination process. However, the XRD patterns of the HSiW@Ni-Zr-O-3 sample only exhibit a weak broad peak at 15–35°, indicating that the high calcination temperature would cause a deeper breaking of the Ni-Zr MOF framework. It can be speculated that the HSiW@Ni-Zr-O hybrids were successfully synthesized.
Figure 1c depicts the FTIR spectrum of the HSiW, Ni-Zr MOF, HSiW@Ni-Zr MOF, HSiW@Ni-Zr-O-1, HSiW@Ni-Zr-O-2, and HSiW@Ni-Zr-O-3. For the HSiW and HSiW@Ni-Zr MOF, the characteristic peaks centered at 978, 927, 884, and 804 cm−1 imply the presence of a Keggin structure [25]. For the Ni-Zr MOF and HSiW@Ni-Zr MOF, the absorption bands in the range of 1000–1600 cm−1 were a result of the stretching and bending of H2-BDC [26], and the FTIR absorption bands in the range of 450–800 cm−1 may be ascribed to the vibration of Ni-O and Zr-O [27,28]. Based on this, the FTIR spectra of the HSiW@Ni-Zr MOF contained not only absorption bands corresponding to the Ni-Zr MOF, but also peaks belonging to HSiW. Thus, this suggests that HSiW was successfully embedded into the Ni-Zr MOF matrix. Interestingly, the FTIR spectra of HSiW@Ni-Zr-O-1, HSiW@Ni-Zr-O-2, and HSiW@Ni-Zr-O-3 are similar to that of the HSiW@Ni-Zr MOF sample. Noticeably, with an increasing pyrolysis temperature, the characteristic peak intensity of the HSiW@Ni-Zr MOF decreased, which is mainly responsible for the interaction between Ni-Zr-O and HSiW; this finding is similar to our research results from our previous studies [29]. Surprisingly, when the pyrolysis temperature is 400 °C, the characteristic peaks of the HSiW@Ni-Zr-O-3 sample are almost unobservable; this may possibly be due to the partial decomposition of the Keggin structure and the collapse of the skeletal structure, which was additionally confirmed via XRD analysis. That is to say, the mild calcination temperature could successfully synthesize HSiW@Ni-Zr-O hybrids.
Moreover, a TPD-NH3 analysis was also performed to determine the acidity of the HSiW@Ni-Zr MOF pyrolyzed at 300 °C, 350 °C, and 400 °C. As shown in Figure 1d, TPD-NH3 curves of the HSiW@Ni-Zr-O-1, HSiW@Ni-Zr-O-2, and HSiW@Ni-Zr-O-3 catalysts display two desorption peaks; a broad desorption peak observed at 100~300 °C is assigned to the weak acidic sites, which are derived from the center of Ni2+ and Zr4+ (Lewis acid sites). Additionally, a sharp desorption peak at 500~650 °C can be classified as the strong acidic sites, which could be attributed to the HSiW group (Brønsted acid sites). Interestingly, a maximum desorption peak of the HSiW@Ni-Zr-O-2 and HSiW@Ni-Zr-O-3 catalysts at 563 °C was slightly higher than those of HSiW@Ni-Zr-O-1. However, the amount of total acidic sites for HSiW@Ni-Zr-O-1, HSiW@Ni-Zr-O-2, and HSiW@Ni-Zr-O-3 was 8.08, 6.14, and 3.27 mmol/g, respectively, indicating that the acid strength of HSiW@Ni-Zr-O-1 is larger than that of HSiW@Ni-Zr-O-2 and HSiW@Ni-Zr-O-3, and implying that there are more acid sites in HSiW@Ni-Zr-O-1 due to a mild pyrolysis treatment temperature. Therefore, the results reveal the significant impact of the pyrolysis treatment temperature on the amount of total acidic sites, which is the crucial factor for esterification.
The surface topographies of samples are depicted in Figure 2. Figure 2a shows the big block structure of the pure HSiW sample. The SEM image of the HSiW@Ni-Zr MOF illustrates the accumulated small, irregular nanoparticles (Figure 2b). After pyrolysis treatment, as observed for the HSiW@Ni-Zr-O-1 nanocatalyst (Figure 2c), the particle size was significantly decreased with a hexagonal-like shape morphology, implying the existence of interactions between the two components during pyrolysis treatment. Furthermore, the HSiW@Ni-Zr-O-1 catalyst shared some microstructures with the aggregate nanoparticles. From Figure 2d, it can be seen that the small shape of the HSiW@Ni-Zr-O-2 sample was similar to that of HSiW@Ni-Zr-O-1, but the large bulk structure was also observed in the HSiW@Ni-Zr-O-2 sample. Moreover, HSiW@Ni-Zr-O-3 (Figure 2e) exhibits near-spherical particles with a nonuniform size, and the change in morphology was probably due to an unsuitable calcination process. This SEM analysis was consistent with the results of the XRD and FTIR analysis. Furthermore, Figure 2f,g shows the TEM image, where it is clearly seen that the particle structure of HSiW@Ni-Zr-O-1 is in the shape of a hexagon with a size of about 50–100 nm, and also shows some micropores with an agglomeration of the nanoparticles, which is consistent with the SEM analysis. These suitable textural and morphological properties of HSiW@Ni-Zr-O-1 are conductive and can improve the diffusion transfer and increase catalytic conversion.
As shown in Figure 3a, the N2 physisorption isotherms of the HSiW@Ni-Zr MOF sample display the type-IV isotherm, indicating that there were a large number of mesoporous structures in the materials. After 300 °C calcination of the HSiW@Ni-Zr MOF, the HSiW@Ni-Zr-O-1 generated a typical type-I isotherm, which reveals that the sample is a microporous material. In addition, the BET surface area, average N2-adsorption pore diameters, and pore volumes of the HSiW@Ni-Zr MOF without calcination were found to be 313.2 m2/g, 2.29 nm, and 0.179 m3/g, whereas those of the as-prepared HSiW@Ni-Zr-O-1 were 49.8 m2/g, 3.04 nm, and 0.038 m3/g, respectively. This is probably due to the excessive shrinkage of the MOF precursor and the generation of the microporosity through the pyrolysis process, leading to the decrease in the surface area and pore volume [30,31,32]. The BJH pore-size distribution curves in Figure 3b confirm the existence of a micropore as a main pore type in the HSiW@Ni-Zr-O-1 sample. However, we observed that the pore diameter of the HSiW@Ni-Zr-O-1 sample was substantially increased during pyrolysis. Thus, the larger pore diameters and relatively large specific surface areas are beneficial for the easy and fast transport of the substrate on the active interface, which is expected to achieve a much better synergistic catalytic performance.
The XPS of the HSiW@Ni-Zr-O-1 composite was measured to identify elemental surface components and oxidation states (Figure 4). The C 1s peak of the HSiW@Ni-Zr-O-1 sample (Figure 4a) could be deconvoluted into two fitted peaks at 284.8 eV and 288.7 eV, which are assigned to C=C/C-C and C=O from terephthalate groups [33]. Figure 4b shows the two peaks of Zr 3d at 182.6 eV and 184.9 eV that corresponded to Zr 3d5/2 and Zr 3d3/2, determining the presence of Zr4+ in HSiW@Ni-Zr-O-1 [34]. The Ni 2p spectrum of HSiW@Ni-Zr-O-1 (Figure 4c) could be deconvoluted into four peaks at 880.8 eV, 874.1 eV, 861.5 eV, and 856.3 eV, which are due to Ni 2p1/2 and Ni 2p3/2, and the satellite peaks at 861.5 eV and 880.8 eV are associated with Ni2+, indicating the presence of Ni2+ in the composite [35]. Most importantly, the XPS spectrum of W 4f (Figure 4d), which shows two strong peaks at 35.9 eV and 38.0 eV, is associated with the species of W 4f7/2 and W 4f5/2, respectively, which are in good agreement with the existence of W6+ [36]. Interestingly, an extra lower-energy peak at 30–32 eV was found, which is ascribed to W atoms with the original W6+ in HSiW@Ni-Zr-O-1 and has been partly reduced to a lower chemical valence (W0) in the preparation process [37]. The results confirm that the synergistic effect occurred between HSiW and Ni-Zr-O through the transferring of electrons, and also imply that the HSiW–Ni-Zr-O active interfaces may be generated, resulting in the improved catalytic activity of HSiW@Ni-Zr-O-1 in the esterification system. The above analysis shows that HSiW species exist in the Ni-Zr-O frame structure.

2.2. Exploration of the Optimal Esterification Conditions

Figure 5a depicts the OA conversion catalyzed by using synthesized HSiW@Ni-Zr MOF and HSiW@Ni-Zr-O-1 catalysts at a temperature of 140 °C, with the catalyst being 0.15 g and the methanol to OA molar ratio being 20. From Figure 5a, it can be seen that HSiW@Ni-Zr-O-1 with a 98.5% conversion was remarkably higher than the HSiW@Ni-Zr MOF (41.7% conversion) over 4 h, indicating that the calcined HSiW@Ni-Zr-O-1 catalyst was effective at catalyzing the esterification of OA to biodiesel, which is possibly attributable to the increases in pore size, the high acidity of the catalyst, and the bare HSiW–Ni-Zr-O active interface after calcination. To compare, HSiW@Ni-Zr MOFs at different calcination temperatures in the preparation of the HSiW@Ni-Zr-O catalyst were also examined after a 4 h activity test. From Figure 5b, it can be seen that the highest OA conversion of 98.5% was obtained using HSiW@Ni-Zr-O-1 after 300°C calcination at a reaction temperature of 140 °C for a 4 h reaction, and the conversion was 43.6% and 40.1% for HSiW@Ni-Zr-O-2 and HSiW@Ni-Zr-O-3, respectively, under the same conditions above. Next, based on the XRD, SEM, and TPD-NH3 results, a moderate calcination temperature of 300 °C was selected; consequently, the bare HSiW–Ni-Zr-O interfaces may be formed and the acidic property of the HSiW@Ni-Zr-O-1 catalyst was increased. Importantly, it is noteworthy to mention that the high calcination temperature may lead to the decomposition of HSiW from the HSiW@Ni-Zr MOF, which would decrease the catalytic activity.

2.3. Optimum Esterification Conditions of OA and Methanol for HSiW@Ni-Zr-O-1 Catalyst

2.3.1. Influence of Reaction Temperature and Time

Due to the esterification of OA and methanol being an endothermic reaction, the high temperature can accelerate the reaction rate [38,39]. The effect of the reaction temperature on the OA conversion was studied, and the results are shown in Figure 6. It turns out that a temperature of 140 °C has a clear effect on the conversion of OA compared to other temperatures. The OA conversion at 140 °C was 95.2%, whereas in the cases of 120 °C and 130 °C, the reaction achieved a conversion of 54.6% and 84.3% after 3 h. This phenomenon can be attributed to the fact that an increase in temperature increases the collisions between reactant molecules and enhances the mass transfer of the reactants into the active site of the catalyst [40]. When the reaction time is further prolonged to 4 h, the conversion of OA slight increases at 130 °C and 140 °C. Hence, 140 °C and 3 h for HSiW@Ni-Zr-O-1 were considered to be the best reaction temperature and time.

2.3.2. Influence of Catalyst Amount and Substrate Molar Ratio

Figure 7a shows the effect of the catalyst amount (0.03–0.27 g) on the OA esterification. As shown in Figure 7a, the conversion of OA increased with the increase in the amount of the HSiW@Ni-Zr-O-1 catalyst from 0.03 g to 0.15 g, and the conversion reached 95.2% when the amount of catalyst was 0.15 g. Once the catalyst amount exceeds 0.15 g, the OA conversion slightly increases as compared to that of 0.15 g. Meanwhile, exceeding this catalyst amount can lead to poor integration between the HSiW@Ni-Zr-O-1 catalyst and reactants [41]. Thus, 0.15 g of the HSiW@Ni-Zr-O-1 catalyst was established as the best catalyst amount. In addition, the influence of the OA/methanol molar ratio in the range from 1:5 to 1:30 on the conversion of OA was explored. As depicted in Figure 7b, the conversion of OA remarkably increased, and a plateau of 95.2% was reached as the OA/methanol molar ratio increased from 1:5 to 1:20. However, further increasing the methanol amount did not have a positive effect on esterification, and the conversion of OA percentage began to decrease. This is due to the positive esterification reaction direction probably being inhibited when the methanol dosage went up, as similar observations were reported by Guo et al. [42]. Consequently, in this work, the optimal molar ratio between OA and methanol was 1:20.

2.4. Reusability and Thermal Filtration Test of the HSiW@Ni-Zr-O-1 Catalyst

To evaluate the stability of the prepared HSiW@Ni-Zr-O-1, recycling of the used catalyst should be performed. The used HSiW@Ni-Zr-O-1 catalyst was segregated from the reaction mixture via centrifugation, and was directly used for the subsequent nine cycles of catalysis under the same conditions (0.15 g catalyst, 140 °C, 3 h, and an OA/methanol molar ratio of 1:20) as the first cycle. The results are shown in Figure 8a. It was found that the OA conversion slightly decreased after each cycle, and at the ninth cycle, an OA conversion of 79.1% was observed. Furthermore, the XRD and FTIR spectra of fresh HSiW@Ni-Zr-O-1 and the HSiW@Ni-Zr-O-1 used after the ninth cycle are displayed in Figure 1b,c; the analysis exhibits that the XRD and FTIR spectra of both catalysts from the first and ninth cycles possess nearly similar signals. It is confirmed that the hybrid HSiW@Ni-Zr-O-1 could retain its original frame structure without noticeable changes during the reaction process. Subsequently, the thermal filtration test further confirmed the stability of HSiW@Ni-Zr-O-1 (Figure 8b). When the reaction time reached 1 h, the HSiW@Ni-Zr-O-1 catalyst was removed, and the conversion of OA in the filtrate slightly increased (16.8%) in the subsequent esterification reaction at 3 h, which further shows that the strong interaction between HSiW and Ni-Zr-O effectively enhances the stability. However, a possible explanation for the slight decrease in the conversion would be the loss of part of the catalyst during the recycling process and the leaching of a smaller amount of active species, causing a reduction in the catalytic active sites [43]. According to the results, HSiW@Ni-Zr-O-1 has powerful operational and chemical stability.

2.5. Comparative Study on the Catalytic Activities

An overview of various reports on the esterification of fatty acid with alcohol is presented in Table 1. From the results reported in Table 1, it is apparent that the HSiW@Ni-Zr-O-1 performance is comparable with other heterogenous catalysts that use raw fatty acid types and various reaction conditions. Moreover, this also shows that the optimum conditions in this study obtained a higher fatty acid conversion at a moderate reaction temperature and shorter time. Hence, the esterification process could be more economical using an HSiW@Ni-Zr-O-1 catalyst.

3. Materials and Methods

3.1. Chemicals Used

Zirconium (IV) chloride (ZrCl4, AR), silicotungstic acid (HSiW, H4SiW12O40·nH2O, AR), and benzene-1,4-dicarboxylic acid (H2-BDC, AR) were obtained from Shanghai Aladdin Industrial Inc. Nickel nitrate hexahydrate (Ni(NO3)2·6H2O, AR), oleic acid (OA, AR), anhydrous methanol (AR), and N,N-dimethylformamide (DMF, AR) were provided by Sinopharm Chemical Regent Co., Ltd (Shanghai, China). All chemical reagents are commercially available without further treatment. Deionized water was used for all of the experiments.

3.2. Catalyst Preparation

HSiW@Ni-Zr-O was prepared through a hydrothermal procedure followed by a pyrolysis treatment. Specifically, 0.5 mmol of Ni(NO3)2·6H2O and 0.5 mmol of ZrCl4 were added into 18 mL of DMF under robust stirring for 0.5 h. Afterward, 0.4 g of HSiW and 1.5 mmol of H2-BDC were added to the above-mentioned solution with stirring for 1 h. Then, the resulting mixture was kept in a 25 mL Teflon autoclave reactor at 150 °C for 6 h under static conditions. After naturally cooling down to an ambient temperature, the solid product was collected by centrifugation, washed with DMF and distilled water, and dried to obtain the precursor, namely, HSiW@Ni-Zr MOF. Lastly, the HsiW@Ni-Zr MOF precursor was calcined at 300 °C, 350 °C, and 400 °C for 3 h in an air atmosphere with a heating ramp of 5 °C/min to obtain samples of HSiW@Ni-Zr-O with different calcination temperatures, and the obtained samples were denoted as HSiW@Ni-Zr-O-1, HSiW@Ni-Zr-O-2, and HSiW@Ni-Zr-O-3. Further, a Ni-Zr MOF sample without HSiW was also synthesized via a similar procedure.

3.3. Characterization Techniques

Fourier transform infrared spectra (FT-IR) of the prepared hybrids were recorded on a PerkinElmer device (spectrum100); the powder X-ray diffraction (XRD) patterns were recorded on the Bruker D8 ADVANCE (Bruker, Berlin, Germany) using a CuKa radiation source (0.15406 nm); the surface morphological analysis of the prepared samples was performed using field-emission scanning electron microscopy (SEM, Hitachi/S-4800, Tokyo, Japan) and transmission electron microscopy (TEM, FEI Tecnai G2 20, FEI Co., Hillsboro, OR, USA); the pore size and surface area characteristics of the catalysts were characterized using a Quantachrome Nova 3200 S apparatus (Boynton Beach, FL, USA); the temperature-programmed desorption was carried out with ammonia as the probe gas (TPD-NH3, Auto TP-5080B, Tianjin, China); and the X-ray photoelectron spectroscopy (XPS) images were acquired with the Thermo Fisher ESCALAB 250XI (Waltham, MA, USA).

3.4. Catalytic Activity Test

To study the catalytic activity of the HSiW@Ni-Zr-O nanocatalyst, the esterification of OA using methanol was performed in a closed, high-pressure autoclave, and the system was heated to 120–140 °C in an oil bath under magnetic stirring. The esterification process lasted for 0.5–5 h. After the reaction was completed, the HSiW@Ni-Zr-O nanocatalyst was simply separated from the product mixture via a centrifugal process, and the excess methanol and water produced were removed from the product by vacuum distillation. The acid value of OA and the liquid product were analyzed with the ISO 660-2020 standard, and the catalytic activity of the HSiW@Ni-Zr-O nanocatalyst was estimated via the acid-value reduction efficiency.

4. Conclusions

In summary, hydrothermal and pyrolysis syntheses were used to prepare Ni-Zr-O from bimetallic Ni-Zr MOF-supported HSiW nanohybrids as promising, highly active acid catalysts for the esterification of OA with methanol. It was concluded that the HSiW@Ni-Zr-O catalyst calcinated at 300 °C showed a larger pore diameter, a higher total acidity (8.08 mmol/g), and a presence of active interfaces due to the synergistic effect between the catalyst components, leading to a significantly enhanced catalytic performance. The obtained results revealed that the highest OA conversion of 95.2% was attained with 0.15 g of the HSiW@Ni-Zr-O-1 catalyst at 140 °C and by using a molar ratio of OA to methanol of 1:20 after 3 h. Furthermore, after nine cycles, HSiW@Ni-Zr-O-1 still achieves a 79.1% conversion. Compared to other recent studies, the presented HSiW@Ni-Zr-O-1 catalyst in this study allows for the esterification process to occur with high conversion. The results and findings provide a simple and effective pyrolysis synthesis strategy for developing highly active and stable nanoporous catalysts with excellent acid catalytic activity and show great promise for applications in the biorefinery field.

Author Contributions

Writing—original draft preparation, review and editing, and project administration, Q.Z.; methodology and investigation, M.H.; conceptualization, J.W.; methodology and investigation, Y.L.; methodology and investigation, Y.W.; writing—review and editing, Q.L.; writing—review and editing, Y.Z. (Yongting Zhao); supervision and funding acquisition, Y.Z. (Yutao Zhang). All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (22262001), the 2018 Thousand Level Innovative Talents Training Program of Guizhou Province, the Guizhou Science and Technology Foundation ([2020]1Y054), the Guizhou Province Key Laboratory of Ecological Protection and Restoration of Typical Plateau Wetlands ([2020]2002), the Academician Workstation of Guizhou Science and Technology Plan (Guizhou S&T Cooperation Platform Talents [2016]5602), and the innovative entrepreneurship training program for undergraduates of the Guizhou education department (202210667006, S202210667167).

Data Availability Statement

The data implemented to support the results of the study are included within the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 00040 g001a 550Catalysts 13 00040 g001b 550
Figure 1. (a) XRD patterns of HSiW, Ni-Zr MOF, and HSiW@Ni-Zr MOF; (b) XRD patterns of HSiW@Ni-Zr-O-1, HSiW@Ni-Zr-O-2, HSiW@Ni-Zr-O-3, and the reused HSiW@Ni-Zr-O-1; (c) FTIR spectra of HSiW, Ni-Zr MOF, HSiW@Ni-Zr MOF, HSiW@Ni-Zr-O-1, HSiW@Ni-Zr-O-2, HSiW@Ni-Zr-O-3, and the reused HSiW@Ni-Zr-O-1; (d) TPD-NH3 of HSiW@Ni-Zr-O-1, HSiW@Ni-Zr-O-2, and HSiW@Ni-Zr-O-3.
Figure 1. (a) XRD patterns of HSiW, Ni-Zr MOF, and HSiW@Ni-Zr MOF; (b) XRD patterns of HSiW@Ni-Zr-O-1, HSiW@Ni-Zr-O-2, HSiW@Ni-Zr-O-3, and the reused HSiW@Ni-Zr-O-1; (c) FTIR spectra of HSiW, Ni-Zr MOF, HSiW@Ni-Zr MOF, HSiW@Ni-Zr-O-1, HSiW@Ni-Zr-O-2, HSiW@Ni-Zr-O-3, and the reused HSiW@Ni-Zr-O-1; (d) TPD-NH3 of HSiW@Ni-Zr-O-1, HSiW@Ni-Zr-O-2, and HSiW@Ni-Zr-O-3.
Catalysts 13 00040 g001aCatalysts 13 00040 g001b
Catalysts 13 00040 g002 550
Figure 2. SEM images of (a) HSiW, (b) HSiW@Ni-Zr MOF, (c) HSiW@Ni-Zr-O-1, (d) HSiW@Ni-Zr-O-2, and (e) HSiW@Ni-Zr-O-3; TEM images of (f,g) HSiW@Ni-Zr-O-1.
Figure 2. SEM images of (a) HSiW, (b) HSiW@Ni-Zr MOF, (c) HSiW@Ni-Zr-O-1, (d) HSiW@Ni-Zr-O-2, and (e) HSiW@Ni-Zr-O-3; TEM images of (f,g) HSiW@Ni-Zr-O-1.
Catalysts 13 00040 g002
Catalysts 13 00040 g003 550
Figure 3. (a) N2 physisorption isotherms and (b) BJH pore-size distribution of HSiW@Ni-Zr MOF and HSiW@Ni-Zr-O-1.
Figure 3. (a) N2 physisorption isotherms and (b) BJH pore-size distribution of HSiW@Ni-Zr MOF and HSiW@Ni-Zr-O-1.
Catalysts 13 00040 g003
Catalysts 13 00040 g004 550
Figure 4. XPS spectra for (a) C 1s, (b) Zr 3d, (c) Ni 2p, and (d) W4f of HSiW@Ni-Zr-O-1 composite.
Figure 4. XPS spectra for (a) C 1s, (b) Zr 3d, (c) Ni 2p, and (d) W4f of HSiW@Ni-Zr-O-1 composite.
Catalysts 13 00040 g004
Catalysts 13 00040 g005 550
Figure 5. (a) OA esterification catalyzed by HSiW@Ni-Zr MOF and HSiW@Ni-Zr-O-1; (b) OA esterification catalyzed by HSiW@Ni-Zr-O-1, HSiW@Ni-Zr-O-2, and HSiW@Ni-Zr-O-3.
Figure 5. (a) OA esterification catalyzed by HSiW@Ni-Zr MOF and HSiW@Ni-Zr-O-1; (b) OA esterification catalyzed by HSiW@Ni-Zr-O-1, HSiW@Ni-Zr-O-2, and HSiW@Ni-Zr-O-3.
Catalysts 13 00040 g005
Catalysts 13 00040 g006 550
Figure 6. Influence of reaction temperature on OA conversion of the HSiW@Ni-Zr-O-1 catalyst (reaction conditions: 0.15 g catalyst; OA/methanol molar ratio 1:20).
Figure 6. Influence of reaction temperature on OA conversion of the HSiW@Ni-Zr-O-1 catalyst (reaction conditions: 0.15 g catalyst; OA/methanol molar ratio 1:20).
Catalysts 13 00040 g006
Catalysts 13 00040 g007 550
Figure 7. Influence of different esterification conditions of (a) catalyst amount; (b) molar ratio of OA to methanol on OA conversion (reaction conditions: 140 °C; 3 h).
Figure 7. Influence of different esterification conditions of (a) catalyst amount; (b) molar ratio of OA to methanol on OA conversion (reaction conditions: 140 °C; 3 h).
Catalysts 13 00040 g007
Catalysts 13 00040 g008 550
Figure 8. (a) The repetitive experiment of the designed HSiW@Ni-Zr-O-1 catalyst on OA conversion (reaction conditions: 0.15 g catalyst, 140 °C, 3 h, and OA/methanol molar ratio 1:20); (b) the thermal filtration test for HSiW@Ni-Zr-O-1 catalyst (reaction conditions: 0.15 g catalyst, 140 °C, and OA/methanol molar ratio 1:20).
Figure 8. (a) The repetitive experiment of the designed HSiW@Ni-Zr-O-1 catalyst on OA conversion (reaction conditions: 0.15 g catalyst, 140 °C, 3 h, and OA/methanol molar ratio 1:20); (b) the thermal filtration test for HSiW@Ni-Zr-O-1 catalyst (reaction conditions: 0.15 g catalyst, 140 °C, and OA/methanol molar ratio 1:20).
Catalysts 13 00040 g008
Table
Table 1. Comparison of various catalysts’ performance for esterification of fatty acid with alcohol.
Table 1. Comparison of various catalysts’ performance for esterification of fatty acid with alcohol.
Catalyst TypeFeedstockReaction ConditionsActivity (%)Ref.
Temp. (°C)Alcohol to Fatty Acid Molar RatioCatalyst Loading (wt%)Time (h)
SO42−/TiO2-SiO2Palm fatty acid distillate150Methanol (1:5.85)2.973.1293.3[44]
MnO2@Mn(btc)Oleic acid100Ethanol (1:12)31298[45]
ZrO2/SiO2Stearic acid120Ethanol (1:120)10369.2[46]
MoO3/B-ZSM-5Free fatty acid160Methanol (1:20)3698[47]
Ni/Mn/Na2SiO3Palm fatty acid distillate120Methanol (1:16)2496[48]
SO3-MIL-53(Al)Oleic acid150Methanol (1:60)3297.2[49]
Cu3(MoO4)2(OH)2Oleic acid140Methanol (1:5)5598.38[50]
HSiW@Ni-Zr-O-1Oleic acid140Methanol (1:20)5395.2Present
work
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Zhang, Q.; Hu, M.; Wang, J.; Lei, Y.; Wu, Y.; Liu, Q.; Zhao, Y.; Zhang, Y. Synthesis of Silicotungstic Acid/Ni-Zr-O Composite Nanoparticle by Using Bimetallic Ni-Zr MOF for Fatty Acid Esterification. Catalysts 2023, 13, 40. https://doi.org/10.3390/catal13010040

AMA Style

Zhang Q, Hu M, Wang J, Lei Y, Wu Y, Liu Q, Zhao Y, Zhang Y. Synthesis of Silicotungstic Acid/Ni-Zr-O Composite Nanoparticle by Using Bimetallic Ni-Zr MOF for Fatty Acid Esterification. Catalysts. 2023; 13(1):40. https://doi.org/10.3390/catal13010040

Chicago/Turabian Style

Zhang, Qiuyun, Mengmeng Hu, Jialu Wang, Yanting Lei, Yaping Wu, Qing Liu, Yongting Zhao, and Yutao Zhang. 2023. "Synthesis of Silicotungstic Acid/Ni-Zr-O Composite Nanoparticle by Using Bimetallic Ni-Zr MOF for Fatty Acid Esterification" Catalysts 13, no. 1: 40. https://doi.org/10.3390/catal13010040

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