Nickel Supported on MIL-96(Al) as an Efficient Catalyst for Biodiesel and Green Diesel Production from Crude Palm Oil

In this study, a new class of heterogeneous catalyst in the form of metal-organic frameworks (MOFs), namely Material of Institute Lavoisier-96(Al), which is called MIL-96(Al), was employed for the production of biodiesel and green diesel. The synthesis of MIL-96(Al) was conducted via a hydrothermal method at 210 °C for 4 hours with dimethylformamide (DMF) as an assisting agent. The Ni was loaded into MIL-96(Al) via incipient wetness impregnation method with variations 3, 5, and 10 wt.% to form Ni/MIL-96(Al). Based on X-Ray diffraction (XRD) analysis, the obtained material has good crystallinity with characteristic peaks observed at 2? = 5.8°; 7.8°, and 9.1°. Fourier Transform Infrared (FTIR) analysis demonstrated an essential shift from 1715 cm^-1 to 1666 cm^-1, indicating the coordination of the carboxylate group with Al³⁺ metal ions. Moreover, MIL-96(Al) is stable up to 390 °C according to the thermogravimetric analysis (TGA). Based on structural and morphological analysis (using XRD, FTIR, and Scanning Electron Microscope (SEM)), the loading of Ni into MIL-96(Al) does not change the basic structure of MIL-96(Al). However, the pore diameter of MIL-96(Al) decreased from 5.7 nm to 1.4 nm after the Ni was embedded in the structure. The largest surface area was obtained from 10% Ni/MIL-96(Al) (up to 595.5 m²/g). The catalytic test exhibits that 3% Ni/MIL-96(Al) could attain an optimum yield of up to 85.24% of biodiesel, while in the case of hydrodeoxygenation (HDO) reaction, the optimum catalyst shown by 10% Ni/MIL-96(Al) with conversion and selectivity of C₁₆ up to 90.70% and 55.22%, respectively.

Biodiesel; Green Diesel; Hydrodeoxygenation; Ni/MIL-96(Al); Trans-esterification

    The highlighted impact of the population and industrial sector growth has been significantly increased due to the global energy demand. Based on the United States Energy Information Administration (U.S. EIA) International Energy Outlook 2017, the total energy requirement is predicted to increase up to 28% from 2015 to 2040 (Capuano, 2020). Meanwhile, reserves and the production of fossil fuels are threatened to be limited. Moreover, the use of fossil fuels as the main energy source causes scarcity and air pollution. Therefore, changing fossil-based energy to environmentally friendly renewable energy is necessary. Biomass feedstocks (either lignocellulose or triglyceride) have been proposed and widely studied as sustainable resources for generating of renewable fuels. Several review articles have discussed comprehensive information on this matter (Hoang et al., 2021a-c; Zhao et al., 2017). One of the potential alternative energies to be developed in Indonesia is palm oil-based biofuels, like biodiesel and green diesel.
    Biodiesel can be produced through a trans-esterification reaction, while green diesel (the 2^nd generation of diesel) is manufactured through an HDO reaction (Prihadiyono et al., 2022; Muharam and Adinda, 2018; Putra et al., 2018; Susanto et al., 2016). Hoang and Li (2019) thoroughly examined biodiesel as a fuel in diesel engines, focusing on engine performance, deposit formation, combustion, and emissions characteristics. In addition, biodiesel manufacturing employing various processes and catalysts (e.g., heterogeneous catalysts) has also been extensively described in multiple review articles (Cong et al., 2021; Hoang et al., 2021). Heterogeneous catalysts have been studied and significantly impacted the reaction stage. In addition, heterogeneous catalysts are greener due to the ease of handling and separation from the product and reusability. One relatively new class of promising materials for the heterogeneous catalyst is Metal-Organic Frameworks (MOF) (Yap, Fow, and Chen, 2017; Xamena and Gascon, 2013).
    MOFs are organic-inorganic hybrid nanoporous materials consisting of metal cations or metal oxide clusters as nodes and bidentate or poli-dentate ligands as organic linkers to form infinite networks. MOFs combine the advantages of homogeneous and heterogeneous catalysts due to their unique features such as nanoporosity, thermal and chemical stability, insolubility in water and organic solvents, and the presence of various catalytic active sites. The catalytic activity of MOFs depends on their active site, which can be derived from unsaturated metal centers, functionalized organic linkers, as well as additional metal sites inserted via post-synthesis modification (Huang et al., 2017). As a catalyst, MOFs should have a stable porosity that can be accessed by the substrates.
    Material of Institut Lavoisier-96(Al) [MIL-96(Al)] represents the first synthesized MOFs using Al³⁺ metal ions as nodes and trimesic acid [benzene-1,3,5-tricarboxylic] or H₃BTC as ligand by hydrothermal method resulting [Al₁₂O(OH)₁₈(H₂O)₃(Al₂(OH)₄)[BTC]₆ 4H₂O (Loiseau et al., 2006). MIL-96(Al) is stable up to 500 °C and has a high BET surface area (ca. 687 m²/g) as well as an average pore radius and total pore volume of 0.97 nm and 0.335 cc/g, respectively (Abid et al., 2016). The use of Al³⁺ as a trivalent metal ion node causes higher thermal and chemical stability than divalent metal ions (Chughtai et al., 2015). MIL-96(Al) was first synthesized via the hydrothermal method employed by Loiseau et al. (2006) for 24 hours at 210 °C. Yang et al. (2016) innovated to use of assisting agents such as methanol, ethanol, diethylformamide (DEF) or dimethylformamide (DMF) in the synthesis of MIL-100(Al) with a shorter time of only for 4 hours at 210 °C. Moreover, the advantage of using DMF as an assisting agent is that it could produce material with better crystallinity.

2.1. Materials

    All chemicals used in this research are in analytical grade and used as received without further purification. Aluminum nitrate nonahydrate, Al(NO₃)₃·9H₂O (98%), and benzene-1,3,5-tricarboxylic acid (H₃BTC) (95%) were purchased from Sigma Aldrich, Germany. Nickel(II) nitrate hexahydrate, Ni(NO₃)₂·6H₂O (98%), nitric acid (HNO₃), N-N' dimethyl formamide (DMF, 99.8%), ethanol (96%), and methanol (96%) were commercially obtained from Merck, Germany. Crude Palm Oil (CPO) was obtained from PT. Salim Ivomas Pratama Tbk., nitrogen, and hydrogen gas (UHP, 99,9%) was supplied by PT. Samator Indonesia. Other materials, such as pH paper, filter paper, acetone, and aquadest, were obtained from Bratachem Indonesia.

2.2. Synthesis of MIL-96(Al)

    The synthesis of MIL-96(Al) was performed via a hydrothermal method based on modified and combined previous procedures reported by Loiseau et al. (2006) and Yang et al. (2016). Al(NO₃)_3·9H₂O (0.75 g, 2.40 mmol) and H₃BTC (0.35 g, 1.67 mmol) were dissolved in 10 mL of aquadest. Subsequently, 0.2 mL DMF was added slowly then stirred for 10 minutes to form a homogeneous solution. HNO₃ (4M) was added dropwise to the solution until pH 1. The solution was then placed into Teflon vessels covered with stainless-steel autoclaves and heated at 210°C for 4 hours. The final product was filtered and washed with ethanol to remove the residual acid. The yellow precipitate was dried at room temperature and activated at 200 °C for 2 hours.

2.3. Loading Ni into MIL-96(Al)

      Loading of nickel into MIL-96(Al) was conducted through the incipient wetness impregnation method refers to previous procedures reported by Peng et al. (2012). Ni(NO₃)₂·6H₂O metal salts as Ni source was loaded into MIL-96(Al) with variation 3, 5, and 10 wt.% of 2 g of MIL-96(Al). The mixture of Ni(NO₃)₂·6H₂O and MIL-96(Al) was stirred in 10 mL of aquadest at room temperature for 24 hours, then filtered, dried and calcined, and reduced at 200 °C for 2 h under nitrogen and hydrogen atmosphere (with a 15 mL/min flow rate) (Figure 1).

Figure 1 Schematic illustration showing the preparation of Ni/MIL-96(Al) catalyst

2.4. Materials Characterization

     X-ray diffraction (XRD) measurements were carried out on a Rigaku Miniflex 600 Benchtop  in  range of 5-50°. SEM type: inspect S50-FEI equipped with EDX analysis was used to analyze the morphology of materials and its elemental composition. FTIR spectroscopy type Shimadzu IR Prestige-21 in KBr pellets was used to analyze the change in the functional group of the material and recorded in the wavenumber range of 400-4000 cm^-1. A Quadrasorp evo (Quantachrome instruments) was used to measure nitrogen sorption isotherm at 77 K. Thermal stability of the materials was investigated using STA Linseis PT-1600 from 30 until 700 °C under nitrogen flow with a heating rate of 20 °C/min.

2.5. Catalytic test of MIL-96(Al) and Ni/MIL-96(Al)

     The catalytic activity in the transesterification reaction of CPO into biodiesel was carried out in a batch system by mixing the catalyst material, CPO:methanol (1:4 of volume), and heating at 60 °C for 5 h. The final product was extracted using a separation funnel until it formed two phases, biodiesel, and glycerol. The upper phase (biodiesel) was analyzed using GC-MS. The HDO reaction of CPO was carried in an autoclave hydrotreating batch reactor with a feed ratio of 50 mL:0.5 g (CPO:catalyst), equipped with a stirrer, condenser and pump, as shown in Figure 2. The HDO reactions were performed at 200 °C with 10 bar of H₂ pressure for 2 hours. The bottom product was distilled using a Koehler Model K 45090 with the American Society for Testing and Materials (ASTM)-D86 method. A gas chromatography-mass spectroscopy (GC-MS) (Agilent Technologies 7890A gas chromatograph (HP-INNOWax with capillary column 30 m × 0.25 mm × 0.20  film thickness) with an autosampler and a 5975C mass selective detector) was used to analyze the HDO products. The measurements started at 40 °C for 0.5 minutes, increased at 8 °C/min to 195 °C in 0 minutes, and finally increased at 1 °C/min to 225 °C and held for 22 minutes. Helium carrier gas (1.8614 mL/min) was used, and an injection volume of 1 µl was used to identify the HDO product components. The formula defined by Equation (1) was used to calculate the biodiesel conversion (Carmo et al., 2009):

Where  is the initial fatty acid of the mixture, and a_f is the fatty acid in the final mixture. The conversion and selectivity of HDO products were defined using Equation (2) and (3), according to Putra et al. (2018):

Figure 2 Experiment setup and apparatus of HDO reactor (Putra et al., 2018)

3.1. Materials Characterization

        XRD analysis (Figure 2, left) shows that MIL-96(Al) was successfully synthesized. Three prominent peaks at 5.8° (002), 7.8° (101), and 9.1° (102) are confirmed through similarity with the standard pattern of MIL-96(Al) generated from CCDC No. 622598 (Loiseau et al., 2006). DMF utilization as an assisting agent produces MIL-96(Al) in a shorter time with high crystallinity (Yang et al., 2016). However, the peaks identified at around 11°-27° are observed to be the remaining H₃BTC ligand. Further diffractogram shown in Figure 3 (right) compares MIL-96(Al) and loaded MIL-96(Al) with Ni 3, 5, and 10 wt.%. The peaks found at : 20.38°, 23.51° and 41.09° indicate the presence of Ni according to JCPDS No. 01-1266. In addition, Ni loading did not alter the basic structure of MIL-96(Al) based on the remaining three main peaks, as discussed before.

Figure 3 X-Ray Diffractogram of synthesized MIL-96(Al) compared to the simulated pattern (CCDC 622598) (left) and Ni/MIL-96(Al) in comparison with MIL-96(Al) and Ni standard (right)

Figure 4 FTIR spectra of MIL-96(Al) in comparison with H₃BTC (left) and MIL-96(Al) compared to Ni/MIL-96(Al) (right)

Figure 5 Nitrogen adsorption-desorption isotherm (left) and BJH pore size distribution (right) of MIL-96(Al) and Ni/MIL-96(Al) (right)

Table 1 Textural properties of MIL-96(Al) and various Ni/MIL-96(Al)

       SEM images (Figure 6) describe that the morphology of MIL-96(Al) is hexagonal with an average particle size in the range of 0.5-1.5  in accordance with previous studies (Liu et al., 2015; Peng et al., 2012). In addition, this result suggests that Ni metal exposure does not damage the morphology of MIL-96(Al) and is well distributed on the surface and pore of the host materials. SEM-EDX analysis (Table 2) showed that Ni nanoparticle was successfully embedded into MIL-96(Al). However, the weight of Ni particles was not exactly detected based on theoretical calculation. The low amount of Ni metals is caused by several factors, such as a long process prior to analysis, then Ni metals might be partially lost during the filtration stage, drainage, or reduction process.

Figure 6 SEM images of (a) MIL-96(Al); (b) 3% Ni/MIL-96(Al); (c) 5% Ni/MIL-96(Al) and (d) 10% Ni/MIL-96(Al) with magnification 20kx

Table 2 Elemental Composition of MIL-96(Al) and Ni/MIL-96(Al) determined by EDX

Figure 7 TGA curve of MIL-96(Al) (left) and Ni/MIL-96(Al) in comparison with MIL-96(Al) (right)

3.2. Catalytic Test

Figure 8 The graph of the conversion result of CPO into biodiesel with MIL-96(Al) and Ni/MIL-96(Al) as catalyst and without catalyst

     As an innovation, the HDO reaction of CPO has also been investigated over the prepared MIL-96(Al) and Ni embedded into MIL-96(Al) with different Ni contents at 200 °C under pressure 10 bar H₂ for 2 hours. Figure 9 and Table 3 show the catalytic test results and liquid phase product distribution of HDO reaction over CPO using MIL-96(Al) and its variation with Ni metals as the catalyst. The observed main product is hydrocarbon diesel fraction generally consisting of a straight chain of n-paraffin (C₁₀-C₁₇), with low amounts of by-products such as light alkanes and kerosene which involve the oxygenates, alkenes and aromatics as the intermediates. Meanwhile, as reported previously, CO₂, CO, CH₄, and propane could also serve as gas products (Putra et al., 2022; Muharam and Sudarsono, 2020; Putra et al., 2018; Peng et al., 2012).

Figure 9 Catalytic test results of MIL-96(Al) and Ni/MIL-96(Al) in HDO reaction of CPO

       Loading of 10 wt.% Ni achieved HDO product from CPO up to 50.09% of yield C₁₆ alkane (Table 3). The production of C₁₆ hydrocarbons with the selectivity 55.22% promoted hydrogenolysis of C–O by the interaction of Lewis acid from metallic Ni and oxygen atom (CPO) produced low oxygenated with H₂O as a side product. In comparison to our previously published MOF-based catalysts by Lestari et al. (2021), palladium supported MIL-100(Fe) as a selective catalyst in the hydrogenation of citronellal showed a selectivity of 22.2% of citronellol products. Thus, in view of their remarkable designability, replacing palladium metals in heterogeneous hydrogenation catalysts with low-cost metals (e.g. nickel) for the creation of Ni–H bonds (nickel hydride bonds) has driven scientists as a desirable novel class of catalytic materials.

       The presence of Ni catalyst promotes the adsorption of substrates (reactants) more effectively, thus facilitating the electron transfer from Ni’s d-orbital to bond the hydrogen’s orbital (Chen et al., 2018). Some unfilled energy levels in the d-orbital of nickel will form lone pair electrons and can form chemical bonding with the adsorbed substrates. Hydrogen molecules are then activated and broken down from “H–H” bonds to generate "Ni-H" through a spillover mechanism, generating highly reactive dissociative hydrogen species that are essential for catalytic hydrogenation reactions (Figure 10). Palmitic acid is the most important component of CPO in this study (40.77 wt.%). The presence of Ni predominantly speeds up the hydrodeoxygenation reaction in this research, which agrees with another group's study (Chen et al., 2018; Zuo et al., 2012). Direct C–O bonds scission with the release H₂O as a by-product (direct HDO) is the suggested mechanism for carboxylic acid hydrogenation, which results in n-C₁₆ alkanes (without carbon loss) (Figure 10). The C=O double bond is initially cleaved during direct HDO, leading to the production of alcohol. In addition, the C–O bond is hydrogenated, resulting in alkane formation. Nickel has been shown to be an active component in the C=O and C–O breakings in palmitic acid hydrodeoxygenation. Generally, the distribution of desirable hydrocarbon on the product relies on the reaction route and catalyst type.