• International Journal of Technology (IJTech)
  • Vol 14, No 2 (2023)

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

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

Title: Nickel Supported on MIL-96(Al) as an Efficient Catalyst for Biodiesel and Green Diesel Production from Crude Palm Oil
Afifah Nur Aisyah, Dwi Ni’maturrohmah, Riandy Putra, Syaiful Ichsan, Grandprix T M Kadja, Witri Wahyu Lestari

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Cite this article as:
Aisyah, A.N., Ni’maturrohmah, D., Putra, R., Ichsan, S., Kadja, G.T.M, Lestari, W.W., 2023. Nickel Supported on MIL-96(Al) as an Efficient Catalyst for Biodiesel and Green Diesel Production from Crude Palm Oil . International Journal of Technology. Volume 14(2), pp. 276-289

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Afifah Nur Aisyah Chemistry Department, Faculty of Mathematics and Natural Sciences, Universitas Sebelas Maret, Jl. Ir. Sutami No.36A, Kentingan, Jebres-Surakarta, Central Java, 57126, Indonesia
Dwi Ni’maturrohmah 1. Chemistry Department, Faculty of Mathematics and Natural Sciences, Universitas Sebelas Maret, Jl. Ir. Sutami No.36A, Kentingan, Jebres-Surakarta, Central Java, 57126, Indonesia, 2. Research Center
Riandy Putra 1. Chemistry Department, Faculty of Mathematics and Natural Sciences, Universitas Sebelas Maret, Jl. Ir. Sutami No.36A, Kentingan, Jebres-Surakarta, Central Java, 57126, Indonesia, 2. Department of C
Syaiful Ichsan Chemistry Department, Faculty of Mathematics and Natural Sciences, Universitas Sebelas Maret, Jl. Ir. Sutami No.36A, Kentingan, Jebres-Surakarta, Central Java, 57126, Indonesia
Grandprix T M Kadja 1. Division of Inorganic and Physical Chemistry, Faculty of Mathematics and Natural Sciences, Institut Teknologi Bandung, Jalan Ganesha No. 10, Bandung 40132, Indonesia, 2. Center for Catalysis and R
Witri Wahyu Lestari Chemistry Department, Faculty of Mathematics and Natural Sciences, Universitas Sebelas Maret, Jl. Ir. Sutami No.36A, Kentingan, Jebres-Surakarta, Central Java, 57126, Indonesia
Email to Corresponding Author

Abstract
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

Introduction

    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 2nd 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.

Several MOFs were tested as heterogeneous catalysts in esterification and trans- esterification reactions such as HSO3-MIL-101(Cr) and HSO3-(Zr) MOF (Juan-Alcañiz et al., 2013), ZIF-8 (Chizallet et al., 2010) and [HPW/Cu3(BTC)2] (Wee et al., 2011). Peng et al. (2012) conducted metal impregnation and found that Ni metal loaded into zirconium oxide increases catalytic activity in the selective reduction of microalgae oil into alkanes resulting in high selectivity for C17 (up to 96%). Nickel in the form of metal nanoparticles has also been incorporated into MOF-5 (Zhao, Chou, and Song, 2012) and Zn-MOCP (Zhao Chou, and Song, 2011) through wet impregnation. These materials have been used for hydrogenation reactions of crotonaldehyde, as well as for improving catalytic activity and stability in CO2 methanation (Zhen et al., 2015). Furthermore, Ni metal exposure in MIL-120(Al) also has shown activity prominently converting 100% of benzene to cyclohexane in hydrogenation reaction as reported by Wan et al. (2013).
Considering biofuel sustainability, Fang et al. (2020) reviewed the use of metal-organic frameworks (MOFs) and derivatives as catalysts for biomass conversion, including lignocellulosic feedstocks. Previous research conducted by Celic et al. (2015) reported that 3wt.% Ni/MIL-77 catalyst showed high hydrogenolysis activity during hydrotreatment of biomass material (lignocellulose) and successfully converted biomass oil into polar and non-polar phases. Noteworthily, MOF-derived catalyst on the valorization of lignocellulosic demonstrated the synergism between active sites and functionalized ligands for enhancing the catalytic activity (Liao, Matsagar, and Wu 2018). Hence, remarkable attention has been devoted to the development of innovative MOF catalysts for value-added biochemical production (Khemthong et al., 2021). MIL-96(Al) and its post-synthesis with Ni metal, which could perform as a heterogeneous catalyst for transesterification processes and hydrogenation, have never been studied before. This research will present an innovative synthetic method of MIL-96(Al) in the presence of DMF as an assisting agent. Furthermore, the material was then embedded with nickel in various weight percent (3, 5, and 10 wt.%) and tested as a catalyst in esterification-transesterification and hydrogenation as well as HDO reaction of CPO forming biodiesel and green diesel.

Experimental Methods

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 af 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):


    Where C(TG) represents the triglyceride concentration (%) in the oil product as evaluated by GC, Yi represents the yield of C16 hydrocarbons (%) as determined by GC analysis, and C represents the crude palm oil conversion (%) as determined by the Equation (2).

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

Results and Discussion

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)

        Corresponding to the FTIR spectra of MIL-96(Al) (Figure 4), the absorption band at 1715 cm-1 is defined as the C=O stretching vibration of the carboxylate group in H3BTC. It shifts significantly to 1665.6 cm-1 after C=O coordinated with Al3+ in the MIL-96(Al) formation. Moreover, the absorption at 3000-3100 cm-1 and 1461 cm-1 reveals stretching vibration of C–H and C=C in aromatic groups of BTC parts. Al–O vibration is indicated by the peak at 768-734 cm-1, as reported in a previous study (Liu et al., 2015). Furthermore, the absorption peak shift from 3300-2500 cm-1 to 3416 cm-1 shows the existence of water coordination on Al3+-BTC and formed hydrogen bonding. Due to the same profile of FTIR spectra before and after loading with nickel (Figure 4, right) confirms that the main functional groups constructing the structure of MIL-96(Al) remained unchanged after Ni loading.

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 (left) shows type II N2 adsorption-desorption isotherm indicating the mixture of micro and mesoporosity in the material. The loading of Ni at 3, 5, and 10 wt.% into MIL-96(Al) results in an increase in surface area and a decrease in pore volume, as shown in Table 1. Therefore, it may be assumed that Ni impregnation successfully filled the pores of MIL-96(Al). Furthermore, the Brunner-Emmett-Teller (BJH) pore distribution revealed that the pore volume distribution of the materials changed to have an inhomogeneous size after being loaded with Ni (Figure 5, right). However, the materials still fall within the range of micro- and mesoporous materials.

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)

Material

Surface Area (m2/g)

Pore size (nm)

MIL-96(Al)

52.93

5.70

3% Ni/MIL-96(Al)

191.3

4.43

5% Ni/MIL-96(Al)

432.3

1.84

10% Ni/MIL-96(Al)

595.5

1.43

       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

Material

Elemental composition (wt.%)

Ni

C

O

Al

MIL-96(Al)

-

37.73

40.73

21.54

3% Ni/MIL-96(Al)

1.09

41.06

40.18

17.68

5% Ni/MIL-96(Al)

1.66

40.09

41.5

16.75

10% Ni/MIL-96(Al)

6.55

38.85

33.72

20.89

    The thermal stability of MIL-96(Al) (presented in Figure 7, left) was studied by thermogravimetric analysis. It can be explained by two different regions of mass loss. The first weight loss of 26.95% occurred between 34°C and 200°C indicating the removal of 42 molecules of H2O as the solvent, which is estimated 39 molecules of H2O are on the surface and trapped in pores and 3 other H2O molecules coordinate with metal ions Al3+on MIL-96(Al). A sudden weight loss of 52.53% at the second period from 391 to 598°C is the degradation of MIL-96(Al) structure, as confirmed in the previous literature studies (Liu et al., 2015; Loiseau et al., 2006). After that, the thermogram did not show any change up to 644°C, and the weight remained at 20.52%, which is expected as the Al2O3 phase as reported by Loiseau et al. (2006) Ni loaded into MIL-96(Al) did not change the thermal stability of MIL-96(Al) as shown in Figure 7 (right).

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

       As a preliminary study, a catalytic activity test of MIL-96(Al) and Ni/MIL-96(Al) was performed to convert CPO into biodiesel through a trans-esterification reaction. Figure 8 shows catalytic activity increased after Ni metal was loaded into MIL-96(Al). Loading of Ni 3 wt.% resulted in a significant change of up to 85.2% of yield compared to unloaded MIL-96(Al), which converted only 51.8%. However, loading 5 and 10 wt.% Ni only achieved the conversion up to 55.4% and 61.4%, respectively. Therefore, 3% Ni/MIL-96(Al) was selected as the optimum catalyst in this reaction due to the occupancy of suitable pore size between the host catalytic material and the substrate size. In our earlier investigations, Larasati et al. (2017) discovered that employing Zr4+-BTC based MOF with 0.6 wt.% catalyst results in a maximum conversion of biodiesel of 69.2%. Furthermore, Ni/MIL-96(Al) serves as a bimetallic catalyst with a stronger active site and higher Lewis acid catalyst properties than MIL-96(Al). Finally, it leads to better catalytic performance and greater product conversion.

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).

        MIL-96(Al) reached 5.83% of yield C15 alkane, while Ni 5 wt.% increased this product up to 7.01% compared to pristine MIL-96(Al) (Figure 9). This reaction is supposed to take place via the decarbonylation/decarboxylation mechanism of CPO (consisting of palmitic acid) with CO/CO2 as the by-products as proposed by Zuo et al. (2012). Research conducted by Hachemi et al. (2017) has proven that the metallic Ni strongly promotes decarbonylation and/or decarboxylation in the ester hydrogenation reaction. In addition, the proportion of loaded Ni determines the level of catalytic performance.

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.

       Interestingly, HDO, with the loading of Ni 3 wt.% produced C13 alkanes with a yield up to 44.43% over CPO (palmitic acid and oleic acid). This phenomenon might be due to the strong adsorption of 3 wt.% Ni and give impact in the weakening of the “C–C” bond, which can cause strong catalytic cracking and generate propane molecules as side-products (Chen et al., 2018). Hence, it seemed that the best catalyst was 10% Ni/MIL-96(Al) under the HDO conditions from oxygen-free green diesel compared to 3% Ni/MIL-96(Al) as the optimum catalyst to produce biodiesel.