Production of green diesel from karanja oil (Pongamia pinnata) using mesoporous NiMo-alumina composite catalysts

Bioresource Technology Reports 7 (2019) 100288

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Production of green diesel from karanja oil (Pongamia pinnata) using mesoporous NiMo-alumina composite catalysts

T

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Sudhakara Reddy Yenumala1, Pankaj Kumar, Sunil K. Maity , Debaprasad Shee Department of Chemical Engineering, Indian Institute of Technology Hyderabad, Kandi, Sangareddy 502285, Telangana, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Green diesel Hydrodeoxygenation Karanja Oil NiMo-alumina Mesoporous catalysts

NiMo-alumina catalysts with a small quantity of Mo showed ordered mesoporous structure. For 4.3 mmol total metals content, mesoporous structure, however, became disorder for 1.7 mmol and higher Mo content. The C18 alkane was the leading product among hydrocarbons (C15–C22) formed during hydrodeoxygenation of karanja oil. The catalytically active NiMo complex and Mo oxide species with lower oxidation states were substantial in NiMo catalysts with the modest quantity of both Mo and Ni and relatively small for the other two extremes. The catalytic activity and selectivity to C18 alkane were thus enhanced with the rise in Mo content up to 3.4 mmol. The catalytic activity was also improved with growing total metals content and temperature. The optimum catalyst (0.9 mmol Ni and 3.4 mmol Mo) showed the complete conversion of oxygenates with 13 wt% < C18, 75 wt% C18, and 12 wt% > C18 alkanes at 340 °C and 4 h reaction.

1. Introduction

of 15–20% for usage in unmodified combustion engines. The presence of oxygen in the structure of these biofuels also causes lower fuel mileage than petroleum-based liquid transportation fuels. The availability of hydrocarbon biofuels from biomass is thus crucial to avoid the erection of capital-intensive new infrastructures. Triglycerides are composed of the long and linear hydrocarbon backbone with a lesser quantity of oxygen compared to sugar, starch, and cellulosic biomass. It is, therefore, considered as the attractive biomass for manufacturing hydrocarbon biofuels. Pyrolysis, catalytic cracking, and hydrodeoxygenation (HDO) are possible routes for manufacturing diesel-range hydrocarbons from triglycerides, commonly known as green diesel. Among these processes, HDO is the widely accepted route owing to the high yield of green diesel. Additionally, this route is associated with minimal loss of fatty acid's carbons as volatile hydrocarbons. This route further offers the possibility of co-processing of triglycerides with crude oil fractions in the existing hydrotreatment unit. In general, HDO of triglyceride can be carried out through two distinct ways: (i) direct HDO, where green diesel is produced by HDO of neat triglycerides with propane as the co-product and (ii) two-step HDO, where mixed fatty acids are first produced by hydrolysis of vegetable oils with glycerol as the co-product (Mailaram and Maity, 2019). The HDO of these mixed fatty acids is then carried out to produce green diesel. Recently, Mailaram and Maity (2019) evaluated the

Fossil fuels are the primary energy sources all over the world. The consumption of fossil fuels is increasing gradually to meet the energy demands of the growing population in the world. The gradual diminution of fossil fuels, rise in crude oil price, and emission of greenhouse gases are the primary motivations for developing energy technologies from sustainable resources. On the other hand, transportation fuels play an important role in today's society. The transportation fuels sector alone consumes about 28% of global energy. At present, transportation fuels are mainly produced from limited petroleum. The production of transportation fuels from the renewable sources of carbon such as biomass is thus indispensable to minimize the import of crude oil and hence improve the economics of the country (Maity, 2015). Biodiesel and bioethanol are accepted as the potential renewable transportation fuels. Biodiesel is currently manufactured from vegetable oils, microalgal oils, waste cooking oil, and animal fats by transesterification reaction with methanol. The transesterification reaction is generally carried out in the presence of an alkali catalyst under mild reaction temperature (around 50–80 °C). On the other hand, bioethanol is traditionally produced by yeast fermentation of biomass-derived sugars. The sugars are generally obtained from either sugar or starchy biomass. Owing to the poor fuels properties, biodiesel and bioethanol are typically blended with diesel and petrol, respectively, to the extent ⁎

Corresponding author. E-mail address: [email protected] (S.K. Maity). 1 Present address: Biomass Conversion Area (BCA), Materials Resource Efficiency Division (MRED), CSIR-Indian Institute of Petroleum (IIP), Dehradun 248005, India. https://doi.org/10.1016/j.biteb.2019.100288 Received 22 March 2019; Received in revised form 15 July 2019; Accepted 15 July 2019 Available online 17 July 2019 2589-014X/ © 2019 Elsevier Ltd. All rights reserved.

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Bhaumik, 2016). A few studies are, however, reported on HDO of vegetable oils using mesoporous alumina supported catalysts. For example, the NiMo/mesoporous alumina catalyst demonstrated higher catalytic activity compared to NiMo/alumina (Priecel et al., 2011). The NiMo/mesoporous alumina catalyst also displayed the low tendency of cracking and high selectivity to diesel-range fuel with a high cetane number for HDO of soya oil (Tiwari et al., 2011). The NiMo/mesoporous alumina also showed higher selectivity to HDO products compared to NiMo/SAPO-11 (Chen et al., 2015). The NiMo-S/mesoporous alumina catalyst further showed higher selectivity to diesel-range hydrocarbons compared to CoMo-S/mesoporous alumina (Taromi and Kaliaguine, 2018). The ordered mesoporous carbon was reported to facilitate the diffusion and found to be less prone to deactivation than the activated carbon (Kim et al., 2017). HDO of karanja oil using NiMo-alumina composite catalysts was, however, not reported so far. This work thus provides a comprehensive study on HDO of karanja oil using the novel mesoporous NiMo-alumina composite catalyst synthesized by one-pot evaporation-induced selfassembly (EISA) method. On the other hand, the structure, reducibility, and the formation of catalytically active species strongly depend on the relative quantity of Mo and Ni in NiMo catalysts. This work further provides a novel approach to elucidate the formation of various catalytically active species for different Ni/Mo (mole) and total metals loading. The catalytic activity and selectivity to various products were then correlated qualitatively with these active species for HDO of karanja oil.

techno-economics of these two routes assuming 0.5 USD/kg as the market price of karanja oil. The latter route displayed the higher utility cost; whereas hydrogen and electricity consumption was much more in the former route. The co-product credit was, however, considerably higher in the latter route. The green diesel manufacturing cost was, therefore, somewhat lesser in two-step HDO (0.798 USD/kg) than direct HDO (0.84 USD/kg) for the optimum plant capacity of 0.12 million metric tons per annum of karanja oil. The capital investment was, however, somewhat higher in two-step HDO compared to direct HDO. The minimum green diesel selling price was thus slightly lesser in direct HDO (1.186 USD/kg) than two-step HDO (1.21 USD/kg) for 8.5% return on investment per annum and the payback period of five years. Following the former approach, this study is focused on the HDO of karanja oil. The HDO of vegetable oils has been investigated over various supported metal catalysts. Alumina-supported Ni, Co, NiMo, CoMo, and NiW are generally used as the catalysts in the hydrotreatment of various crude oil fractions. These inexpensive supported metal catalysts displayed decent catalytic activity and extended time-on-stream stability in comparison with noble metal catalysts. Therefore, these catalysts have been employed extensively for HDO of vegetable oils. The active metal and co-metal generally play a vital role in the reaction pathways and catalytic activity. The C18 and heavier hydrocarbons were mainly observed over Pt and Pd catalysts in the HDO of triolein, tristearin, and soybean oil, while lighter than C18 hydrocarbons were observed over the Ni catalyst (Morgan et al., 2010). The reaction ensues the decarbonylation route predominantly over monometallic catalysts such as Ni and Co, while the reaction follows HDO route over bimetallic catalysts such as NiMo and CoMo (Hachemi et al., 2017; Kumar et al., 2014). The bimetallic catalyst showed superior catalytic activity than monometallic catalysts (Kumar et al., 2019a). For example, NiMo/ alumina exhibited higher catalytic activity and better selectivity to C18 alkane compared to CoMo/alumina and NiW/SiO2-Al2O3 (Taromi and Kaliaguine, 2018). The NiMo catalyst also displayed higher selectivity to paraffin than CoMo catalysts (Taromi and Kaliaguine, 2018). On the other hand, B2O3-Al2O3 supported NiMo catalyst showed higher isomerization activity (Giraldo and Centeno, 2008). The ratio of C17/C18 alkane was improved at the elevated reaction temperature with the simultaneous drop in hydrocarbons yield for HDO of sunflower oil (Kovács et al., 2011). The sulfided NiMo catalysts, however, exhibited a lesser green diesel yield from waste cooking oil and sunflower oil than non-sulfided catalyst (Kordouli et al., 2017). The catalyst supports also play a crucial role in the reaction. For example, Ni/γ-Al2O3 showed slightly higher catalytic activity compared to Ni/SiO2 for HDO of stearic acid (Kumar et al., 2014). It was due to the strong Ni-alumina interaction with enhanced metal dispersion over γ-Al2O3 compared to SiO2. The strongly acidic catalyst such as Ni/ZSM5 showed higher catalytic activity for HDO of stearic acid compared to weakly acidic (Ni/γ-Al2O3) or neutral catalysts (Ni/SiO2) (Kumar et al., 2014). The highly acidic catalyst such as Co/HZSM-22 also favors hydro-isomerization reaction resulting in high selectivity to branched alkanes (Cao et al., 2018). The catalytic cracking is also inevitable over the acidic catalyst (Ni/HZSM-5) (Chen et al., 2019). The supports with optimum acidity such as γ-Al2O3 was found to be promising for high catalytic activity with minimal catalytic cracking compared to neutral (SiO2) and acidic (HZSM-5 and HY) supports (Zuo et al., 2012). The alumina was thus considered as support for this work. Generally, mesoporous materials show a high surface area that facilitates the dispersion of metals, thereby improving the catalytic activity. The large pore diameter of the mesoporous materials further reduces the diffusional resistance inside the pores. On the other hand, mesoporous composite catalysts offer high surface area, large pore diameter, and strong metal-support interaction. The strong metal-support interaction additionally facilitates the formation of various catalytically active species. The mesoporous materials have thus been considered actively as the supports for HDO reaction (Bhanja and

2. Materials and methods 2.1. Catalysts preparation Following the procedure outlined by Morris et al. (2008), the mesoporous NiMo-alumina composite catalysts (hereafter denoted as NiMo–alumina) were synthesized by the one-pot EISA method using pluronic P123 as the structure directing agent. The catalysts were designated as xNyM throughout the article (x mmol Ni and y mmol Mo per g of alumina). The catalysts were calcined at 700 °C for 6 h with 1 °C/ min heating rate and represented as the cal catalyst. Cal catalysts were further reduced using 5 vol% H2-N2 at 700 °C and represented as the red catalyst. 2.2. Catalyst characterization The physicochemical properties of the catalysts were evaluated by BET (Micromeritics, ASAP 2020), temperature programmed reduction by hydrogen (H2-TPR) (Micrometrics, AutoChem II 2920), powder XRD (X-PERT Pro PAN analytical), small angle XRD (SAXess, Anton-Parr), Raman spectroscopy (Bruker Senterra II, Germany), and UV–vis-NIR spectroscopy (PerkinElmer, Lamda-1050, USA). The images of a few representative samples were acquired using the transmission electron microscope (TEM, JOEL JEM 2100, Japan). The BET surface area was estimated from multipoint N2 adsorption isotherm in the relative pressure (P/P0) range of 0.05 to 0.3. The volume of N2 adsorbed at P/P0 of approx.1.0 was considered as the pore volume of the catalyst. The pore size was calculated using the BJH method. The H2-TPR studies of cal catalysts were performed in the temperature range of 100–900 °C with 5 °C/min as the heating rate using 10 vol% H2 in Ar. The powder XRD pattern of both cal and red catalysts was acquired in the 2θ range of 10–100 0 with a scan rate of 0.016°/s using CuKα Xray source (λ = 1.5418 Å) at 45 kV, 30 mA current, and Ni filter. The small angle XRD pattern of red catalysts was acquired in the 2θ range of 0.01–100 using CuKα X-ray source (λ = 1.5418 Å). The Raman spectra was recorded using a 532 nm laser (20 mW) at a resolution of 5 cm−1 using Raman spectrometer equipped with a confocal microscope and 1200 grooves/mm grate. The UV–vis spectra of cal catalysts were acquired using UV–vis-NIR spectrophotometer equipped with diffuse 2

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reflectance accessories (Harrick scientific, USA) under the ambient condition. The spectra were acquired in the 200–800 nm wavelength range using BaSO4 as a reference. For TEM analysis, the finely ground sample was first dispersed in anhydrous ethanol by sonication. A drop of dispersed sample solution was then placed in lacey carbon coated (200 mesh) copper grid and the sample specimen was dried and stored in a vacuum desiccator before analysis. The TEM images of the sample were then collected at an accelerated voltage of 200 kV. The energy dispersed spectra was also collected for the selected area.

Table 1 Physicochemical properties of mesoporous alumina and NiMo-alumina catalysts. BET Catalysts

H2-TPR

SAa

PV

cal Alumina 4.3N 3.4N0.9M 2.6N1.7M 1.7N2.6M 0.9N3.4M 0.4N3.9M 4.3M 0.7N2.7M 0.6N2.2M 0.5N1.8M

2.3. Catalytic activity HDO of karanja oil (Pongamia pinnata) (Maruti Agro Ltd.) was carried out in a 300 ml magnetically-driven high-pressure batch reactor. An electrically-heated furnace was used for heating the reactor. The reactor temperature was controlled within ± 1 °C using a PID temperature controller. For a typical HDO reaction, known quantities of karanja oil, n-dodecane (≥99%, Sigma-Aldrich), and red catalyst were added to the reactor. Hydrogen was purged initially through the reactor for around 10 min and the reactor pressure was then increased to 30 bar H2. The reactor was first heated to 200 °C and remained there for an hour for hydrogenation of the unsaturated bonds present in the triglycerides. The temperature of the reactor was further increased to the required reaction temperature. The first reaction sample was collected after reaching the reaction temperature. In this study, the conversion of oxygenated compounds was calculated with reference to this sample. Additional samples were acquired at different time intervals. The compounds present in the samples were identified by a gas chromatography–mass spectrometry (GC–MS, Shimadzu GCMSQP2010 ultra) and quantified by a GC-flame ionization detector (FID, Shimadzu, GC2014, ZB-5HT capillary column: 30 m × 0.32 mm × 0.10 μm) using helium as the carrier gas. After the reaction, the gas samples were collected and analyzed using GC-thermal conductivity detector (TCD) using Carbosieve-S2 (Chromatopak, 1/8 in × 3 m) packed column and argon as the carrier gas. For reusability study, the catalyst was filtered from the reaction mixture, washed with ethanol, and dried at 100 °C for 12 h. The dried spent catalyst was regenerated by calcination followed by the reduction at 700 °C and then tested for HDO reaction. The selectivity to a particular product was calculated based on the wt% of that product in the liquid sample. HDO of karanja oil was performed at different speed of revolution of the stirrer to reduce the thickness of the mass transfer boundary layer and to enhance the diffusion rate of reactants from bulk fluid to the catalyst surface. The conversion of oxygenated compounds was, however, not increased much above 1000 rpm. This study was thus conducted above 1000 rpm to eradicate the external mass transfer resistance. The standard deviation calculated from the repeated experiments showed that the reaction results are highly reproducible (Tables 1 and 2).

red

cal

210 138 89 69 64 43 54 63 66 73

0.56 0.40 0.43 0.28 0.43 0.37 0.19 0.45 0.35 0.32

230 233 154 85 61 53 43 24 59 59 72

dav red

cal

0.53 0.43 0.47 0.31 0.51 0.3 0.38 0.47 0.41 0.34

8.9 8.2 10.3 10.7 12.4 13.2 12.9 12.3 11.7 10.2

0.49

MMH red

cal

8.5 10.6 9.2 10.5 11.1 9.2 8.9 8.9 8.8 8.9 10.6

1.68 3.53 4.19 3.70 2.58 2.33 1.95 2.17 1.89 1.77

cal = calcined catalysts; red = reduced catalysts; SA = surface area, m2/g; PV = pore volume, cm3/g; dav = average pore size, nm; MMH = H2 consumption, mmol/g. a Maximum deviation: ± 1.0 m2/g.

only. The mesoporous structure, however, became a disorder for NiMoalumina catalysts with 1.7 mmol and higher Mo content. This result shows that the ordered mesoporous structure depends on the Mo content in NiMo-alumina catalysts. The TEM image of pure alumina showed the hexagonal ordered mesoporous structure. The TEM images of NiMo-alumina catalysts also showed the mesoporous structure. The appearance of bright-intensity rings in the selected area diffraction pattern of NiMo-alumina catalysts suggests the polycrystalline nature of the materials. The energy dispersive spectra further confirm the homogeneous distribution of Ni, Mo, and Al. 3.1.2. BET surface area and pore volume The mesoporous alumina exhibited 220 m2/g specific surface area and 0.49 cm3/g pore volume (Table 1). The surface area, as well as the pore volume of mesoporous Ni-alumina catalyst, was quite close to that of mesoporous alumina. It is the most attractive feature of the EISA method for catalyst preparation. For fixed total metals (4.3 mmol/g) content, the surface area and pore volume of both cal and red NiMoalumina catalysts were, however, reduced with the increase in the quantity of Mo in the catalyst. The surface area and pore volume of NiMo-alumina catalysts were further reduced with the increase in total metal content for a fixed Ni/Mo (mole) of 1:4. It might be due to the (i) coverage of surfaces and the blockage of pores by metals/metal oxides, (ii) high molecular weight of Mo, or (iii) the formation of the metalsupport complex. The red NiMo-alumina catalysts, however, showed a slightly higher surface area and pore volume than the corresponding cal catalysts. The additional surface area may be attributed to the Al2(MoO4)3 reduction to MoO2 or MoO4 at high temperature.

3. Results and discussion 3.1. Catalyst characterization

3.1.3. Powder XRD pattern of cal catalysts Cal 4.3N and 3.4N0.9M showed the NiO diffraction peaks at 2θ of 37.3° and 43.3° (PCPDF#780643) and NiAl2O4 peaks at 2θ of 45.6° and 65.5° (Fig. 1a) (Wang et al., 2008). The formation of NiAl2O4 was also reported in the previous studies (Al-Ubaid and Wolf, 1988; Kordouli et al., 2018). The high metal loading and high calcination temperature (750 °C and above) was reported to enhance the formation of NiAl2O4 (Al-Ubaid and Wolf, 1988). The NiO peaks were, however, weak in cal NiMo-alumina catalysts and disappeared for Ni content below 3.4 mmol. It might be attributed to the small NiO crystallite size (owing to good dispersion) for detection by powder XRD. However, NiAl2O4 could not be distinguished from MoO3 diffraction peaks in NiMo-alumina catalysts with the small quantity of Ni. Cal NiMo-alumina catalysts showed the diffraction peaks of MoO2 at

3.1.1. Nitrogen adsorption–desorption isotherm, small angle XRD (SAXS), and TEM For fixed total metals (4.3 mmol/g) content, the isotherms were type IV in nature with H1 hysteresis loop and pore size distribution was unimodal for NiMo-alumina catalysts with Mo content up to 2.6 mmol. The pore size distribution, however, became bimodal above this Mo content. The pore size at around 35 nm may be attributed to the interparticle void space present in the catalyst. In this study, the pore size was thus reported based on the first peak only (Table 1). The small angle XRD pattern (SAXS) of the red NiMo-alumina catalysts showed two characteristics diffraction peaks for the ordered mesoporous structure at 2θ of 0.13° and 0.512° (Morris et al., 2008). The peak at 2θ of 0.512° was, however, observed in the alumina, 4.3N, and 3.4N0.9M 3

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Table 2 Effect of temperature on selectivity to products, wt% and reusability and regeneration ability of the catalyst. Temperature, °C

X

C15

C16

C17

C18

> C18

C16OH

C17CHO

C18OH

C15COOH

C17COOH

MG

EST

280 300 320 340 360

10 55

0.0 0.0 0.7 1.1 2.0

1.1 1.0 5.0 4.5 4.2

0.6 0.5 5.7 8.6 11.9

5.3 5.2 39.7 35.6 30.8

3.0 3.3 3.9 5.2 6.2

3.7 3.1 2.7 2.5 1.7

1.5 0.9 1.0 1.5 1.4

38.6 45.2 32.1 29.6 27.6

2.8 2.1 0.9 0.3 0.8

32.8 28.7 0.0 3.4 6.0

2.8 2.8 0.0 0.2 0.2

7.8 7.2 8.2 7.3 7.2

Standard deviation

88b 93b 90b 2.5

1.0 0.1 0.5 0.5

3.8 2.9 3.5 0.5

7.5 7.0 7.2 0.3

23.0 24.1 23.7 0.6

4.7 5.9 5.3 0.6

2.8 3.8 3.6 0.5

1.9 2.1 2.5 0.3

39.8 37.0 38.2 1.4

0.6 0.0 0.0 0.3

5.8 6.3 6.1 0.3

0.4 1.1 0.5 0.4

8.7 9.7 8.9 0.5

Reusabilitya 340

27b

1.6

4.0

10.2

20.8

4.2

2.6

1.4

23.9

1.3

8.6

0.0

21.4

Regeneration ability of the catalysta 340 87b 0.6

3.2

7.4

23.6

5.5

3.4

2.2

38.5

0.2

6.2

1.0

8.2

Repeatabilitya 340

X = conversion of oxygenated compounds, %, MG = monoglycerides, EST = fatty esters. Conditions: 5 (w/v)% karanja oil, 100 ml n-dodecane, 30 bars H2, 20 (w/ w)% 0.9N3.4M. a Selectivity to products at 40% conversion of oxygenated compounds, wt%. b Conversion of oxygenated compounds at 3 h of reaction time, %.

2θ of 26°, 36.9°, and 53.7° (PCPDF#860135), MoO3 at 2θ of 34.6° and 45.6° (PCPDF#891554), and Al2(MoO4)3 at 2θ of 15.67°, 20.8°, 22.1°, 23.2°, 23.5°, 25.5°, 26.2°, 27.8°, and 30.8° (PCPDF#852286). The intensity of the MoO3 and Al2(MoO4)3 peaks were amplified with the increase in the quantity of Mo in the catalyst. Cal NiMo-alumina catalysts exhibited NiMoO4 diffraction peaks at 2θ of 14.5°, 23.4°, 25.4°, 26.7°, 27.3°, 28.8°, 32.7°, 33.9°, 43.8°, and 47.5° (Liu et al., 2014; Wang et al., 2008). The NiMoO4 diffraction peak at 2θ of 26.7° was, however, weak in NiMo-alumina catalysts with a small quantity of both Ni and Mo and disappeared in 0.4N3.9M and 3.4N0.9M. 3.1.4. Powder XRD pattern of red catalysts The Ni diffraction peaks were observed only in red 4.3N and 3.4N0.9M (Fig. 1b). These peaks were observed at 2θ of 44.4°, 51.8°, and 76.4° (PCPDF#701849). The Mo diffraction peak was observed at 2θ of 40.6° (PCPDF#895023). The Mo diffraction peak was intense in the NiMo-alumina catalyst with 2.6 mmol Mo (1.7 mmol Ni) and faded for both above and below this Mo content. The Mo diffraction peak was, however, not observed in 4.3M. This result demonstrates the formation of Mo by the reduction of NiMoO4. Rodriguez et al. (2002) also reported the formation of Ni, Mo, and NiMo complex by the reduction of NiMoO4 at 600 °C and above. The red NiMo-alumina catalysts and 4.3M additionally showed the diffraction peaks of MoO2 at 2θ of 26°, 36.9°, and 53.7° and MoO3 at 2θ of 45.6° (Dhanala et al., 2015). The presence of these peaks demonstrates the incomplete reduction of Mo oxides. It may be due to the poor reducibility of Mo oxides or formation of Mo oxides from Al2(MoO4)3 during the reduction. However, the diffraction peaks corresponding to Al2(MoO4)3, NiMoO4, and NiAl2O4 were not detected in red NiMoalumina catalysts. On the contrary, red NiMo-alumina catalysts showed the diffraction peaks corresponding to the NiMo complex at 2θ of 43.0° (MoNi3) (PCPDF#652587) and 43.7° (MoNi4) (PCPDF#651533) (Sene and Motta, 2013). The diffraction peaks of NiMo complex were intense in the red NiMo-alumina catalyst with 2.6 mmol Mo (1.7 mmol Ni), faded for both above and below this Mo content, and practically disappeared for 0.9 mmol Mo. It may be due to the thermodynamically unfavorable metals composition for the formation of NiMo complex. The phase diagram suggests the formation of NiMo complex in the Ni mole fraction range of 0.5–0.8 at 700 °C (Press, 1990; Rocha and Guirardello, 2009).

Fig. 1. Powder XRD pattern of (a) cal and (b) red NiMo-alumina catalyst.

3.1.5. UV–Vis DRS spectra The bands at 280–320 nm for cal 4.3N were attributed to the 4

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Fig. 2. Raman analysis of cal NiMo-alumina catalysts. Fig. 3. H2-TPR profile of cal NiMo-alumina catalysts.

O2− → Ni2+ charge transfer. The absorption bands at 380 and 646 nm for 4.3N, 3.4N0.9M, and 2.6N1.7M were due to NiAl2O4 (Palla et al., 2014). These absorption bands were, however, poor in NiMo-alumina catalysts with low Ni content. The absorption bands at 230–260, 270–285, and 320–330 nm in NiMo-alumina catalysts and 4.3M were due to ligand-to-metal charge transfer (O2− → Mo6+). The bands at 230–260 nm were due to tetrahedral and octahedral coordinated Mo in MoO3 species (Williams et al., 1991). Only one absorption band was, however, observed for NiMo-alumina catalysts with low Mo content (3.4N0.9M and 2.6N1.7M). It may be due to the variation of the extent of tetrahedral and octahedral coordinated Mo. The absorption bands at 270–285 nm and 320–330 nm were, however, due to the charge transfer of O2− → Mo6+ for the isolated MoO4 units associated with Al2(MoO4)3 (Tian et al., 2005). These absorption bands were, however, weak in NiMo-alumina catalysts with low Mo content (3.4N0.9M and 2.6N1.7M). The powder XRD also revealed similar findings.

Eswaramoorthi et al., 2008; Kumar et al., 2019a). The peak at 786 °C, however, corresponds to the reduction of tetrahedral coordinated Mo in Al2(MoO4)3 (Briand et al., 2000; Ferdous et al., 2004; Kumar et al., 2019a; Park et al., 1997). The Al2(MoO4)3 was also observed in Raman spectroscopy of 4.3M (Fig. 2). Cal 4.3N also showed three reduction peaks at 460 °C, 639 °C, and 734 °C. The peak at 460 °C represents the reduction of dispersed NiO (Kumar et al., 2014). The peaks at 639 °C and 734 °C correspond to the reduction of tetrahedral and octahedral coordinated NiAl2O4, respectively (Priecel et al., 2011). The powder XRD pattern of 4.3N also showed the formation of NiAl2O4 (Al-Ubaid and Wolf, 1988). The cal NiMo–alumina catalyst showed four distinct reduction peaks. The peaks at 387–399 °C correspond to the combined reduction of NiMoO4 and MoO3. For NiMo-alumina catalysts with a small quantity of Mo such as 3.4N0.9M, the NiMoO4 species was, however, not observed in powder XRD and Raman spectroscopy. The peak appeared at 360 °C for 3.4N0.9M was thus deliberated for the reduction of MoO3. A similar reduction peak appeared as a shoulder at around 374 °C for 2.6N1.7M and can be considered for the reduction of MoO3. The 2nd peak at 486–513 °C corresponds to the MoO2 reduction to Mo. Powder XRD and Raman spectroscopy showed an insignificant quantity of Al2(MoO4)3 in 2.6N1.7M and 3.4N0.9M. The reduction peak at about 712–722 °C for these catalysts was intense and mainly due to the reduction of NiAl2O4. For 1.7N2.6M, 0.9N3.4M, and 0.4N3.9M, the peak at 677–787 °C was broad and owing to the concurrent reduction of NiAl2O4 and Al2(MoO4)3 (Chen et al., 2015; Wang et al., 2008).

3.1.6. Raman spectra No Raman bands were observed in alumina, 4.3N, and 3.4N0.9M (Fig. 2). The Raman bands appeared in NiMo-alumina catalysts with higher Mo content at 1024, 1003, 998, 962, 950, 824, 434, and 378 cm−1. The Raman bands at 1024, 1003, and around 993 cm−1 represent the Mo=O bond vibration of the MoO4 units associated with Al2(MoO4)3. Tian et al. (2005) also observed these Raman bands for bulk Al2(MoO4)3. The characteristic bands observed at 378 and 434 cm−1 were correspond to symmetric and asymmetric bending modes of the isolated MoO4 unit. The Raman band at 824 cm−1 denotes the asymmetric stretching of MoO4 units. The Raman band at 555 cm−1 (Mo-O-Mo symmetric stretching of Al2(MoO4)3) was, however, not observed. The Raman bands at 950 and 905 cm−1 represent the symmetric and asymmetric stretching of the terminal Mo=]O bond of octahedral coordinated MoO3 species (Kim et al., 1992). The Raman band at 894 cm−1 represents the tetrahedral coordinated isolated MoO42− (Hu et al., 1995). The Raman band at 962 attributed to NiMoO4 species present in NiMo catalyst (Ozkan and Schrader, 1985). This band was intense in NiMo-alumina catalysts with 1.7, 2.6, and 3.4 mmol of Mo and weak for both below and above these Mo content. However, Raman bands of bulk NiO and NiAl2O4 were not detected in 4.3N and NiMo-alumina catalysts.

3.2. Reaction mechanism Karanja oil (Pongamia pinnata) was composed of 10.3, 7.4, 53.5, 15.8, 3.7, 1.7, 1.2, and 6.4 wt% C16:0, C18:0, C18:1, C18:2, C20:0, C20:1, C20:3, and C22:0 fatty acids, respectively (Yenumala et al., 2016). The C15–C22 range alkanes were observed as the hydrocarbon products over NiMo-alumina catalysts with C18 alkane being the major one. Several oxygenated intermediate compounds were also observed as the product during the HDO of karanja oil such as stearic acid (C17COOH), palmitic acid (C15COOH), octadecanol (C18OH), hexadecanol (C16OH), octadecanal (C17CHO), monoglycerides (MG), and fatty ester of palmitic acid and stearic acid (EST). About 35 wt% fatty acids and around 2 wt% monoglycerides were observed at 1.0 h reaction time. The high concentration of fatty acids and the low concentration of MG represent the rapid decomposition of karanja oil to corresponding fatty acids and propane (Scheme 1).

3.1.7. H2-TPR profiles Cal 4.3M showed three distinct reduction peaks at 401 °C, 494 °C, and 786 °C (Fig. 3). First two peaks correspond to the reduction of MoO3 to MoO2 and MoO2 to Mo, respectively (Dhanala et al., 2015; 5

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O decomposition unsaturated hydrogenation saturated 3 CnH2n+1 C OH triglyceride triglyceride +3H2 C3H8 +3H2 fatty acid O reduction RR- I dehydration hydrogenation CnH2n+1 C OH CnH2n+1CH2OH Cn-1H2n-1CH=CH2 CnH2n+1CH3 fatty acid H2 H2O H2 H2O RR- III esterification reduction CnH2n+1COOCn+1H2n+3 CnH2n+1CHO CnH2n+1COOH H2O fatty aldehyde Water-gas shift reaction H2 H2O RR- II CO+H2O CO2+H2 decarbonylation Methanation reaction CnH2n+2 CO+3H2 CH4+H2O CO Scheme 1. Reaction mechanism of HDO of karanja oil.

3.3. Effect of Ni/Mo mole ratio

Propane was also detected in the gas sample. These fatty acids were later converted to alkanes via various oxygenated intermediate compounds. In this reaction, the fatty acids are first hydrogenated to fatty aldehydes over metallic centers of the catalyst. However, only small quantities of fatty aldehydes were detected in this study. It may be due to the rapid conversion of fatty aldehydes. The alkanes are then formed from fatty aldehydes via two distinct reaction routes (RR-I to RR-II) (Scheme 1). Following RR-I, fatty aldehydes are hydrogenated to the equivalent fatty alcohols over the active metal sites present in the catalyst. The fatty alcohols are then transformed to the alkanes via dehydration followed by the hydrogenation reaction. In this route, the number of carbon atoms present in the alkane is same as in the fatty acids. The coordination of Ni with Mo in NiMo-complex decreases the binding energy of oxygen with Mo (Kumar et al., 2019b). Therefore, the electron deficient NiMo-complex is speculated as active sites for removal of oxygen from fatty alcohol. Palla et al. (2014) studied the HDO of 1octanol over supported nickel catalysts and observed octanal as one of the products. This result showed that the formation of fatty alcohol from fatty aldehyde is a reversible reaction. The concentration of fatty alcohols was increased with the progress of the reaction, reached maxima (about 50 wt% at 2 h reaction time), and then decreased with further increase in reaction time. The conversion of fatty alcohols to the alkane can thus be considered as the slowest reaction step in the RR-I. Karanja oil was composed of about 75 wt% C18 fatty acids. The C18 alkane was witnessed as the main product during the HDO of karanja oil. Therefore, RR-I is the dominating reaction pathway over mesoporous NiMo-alumina catalysts. Following RR-II, fatty aldehydes are converted to alkanes via decarbonylation reaction. In this route, there is a loss of carbon atom of the fatty acids in the form of CO. In this study, reasonable amounts of fatty esters were also observed over NiMo-alumina catalysts. These fatty esters were formed by esterification reaction (RR-III). The concentration of fatty esters was increased with increasing reaction time, reached maxima, and then decreased with further increase in the reaction time. It is due to the reversible nature of the esterification reaction. The analysis of gas samples further showed the peaks of C1–C5 hydrocarbons, CO2, and CO. The CO2 might be originated from CO by the thermodynamically favorable water-gas-shift reaction. The methanation of CO/CO2 might be responsible for the formation of methane. A small quantity of ethane and C4–C5 hydrocarbons might be formed by cracking of hydrocarbons or oxygenated intermediate compounds.

The 4.3N showed 92% conversion of oxygenated compounds at 4 h of reaction time (Fig. 4). The addition of a small quantity of Mo in Nialumina catalyst, however, resulted in a slight decline in the conversion of oxygenated compounds. The conversion of oxygenated compounds was, however, increased with the further addition of Mo up to 3.4 mmol and decreased again beyond this Mo content. 4.3M, however, showed no catalytic activity. The catalytic activity of NiMo-alumina catalysts is governed by the concentration of various catalytically active surface species (Ni2+, NiMoO4, and Mo oxides with lower oxidation states) depending on the total metals content and the relative amount of Mo and Ni in the catalyst. As observed from H2-TPR, the reduction peaks of MoO3, NiMoO4, and Al2(MoO4)3 were moved slowly towards lower reduction temperatures with the increase in the quantity of Mo in NiMo-alumina catalysts (Fig. 3). This result shows that reducibility of the Mo oxide species improves with increasing the quantity of Mo in catalysts. As a result, the catalyst was enriched with Mo oxide species with lower oxidation states with the increase in the quantity of Mo. On the other hand, the hydrogen consumption in H2-TPR studies showed a slightly declining trend with increasing the quantity of Mo in the catalyst (Table 1). These results further confirmed the increasing trends of concentration of Mo oxide species with lower oxidation states with the increase in the quantity of Mo in the catalyst. On the other hand, the formation of NiMo complex depends strongly on the appropriate quantity of Ni and Mo in catalysts. The powder XRD pattern of the red NiMo-alumina catalysts exhibited the absence of NiMo complex for the low quantity of Mo (3.4N0.9M) (Fig. 1). The NiMo complex was, however, enriched with the increase in the amount of Mo up to 2.6 mmol and declined for further increase in Mo content (Fig. 1). The NiMo complex was formed by the interaction between Ni and Mo and originated by the reduction of NiMoO4 (Rodriguez et al., 2002). The Raman band corresponding to NiMoO4 was absent in cal NiMo-alumina catalysts with a low Mo content (3.4N0.9M) and weak for high Mo content (0.4N3.9M) (Fig. 2). The intensity of this Raman band was, however, amplified with increasing the quantity of Mo up to 0.9N3.4M (Fig. 2). The reduction peak corresponding to NiMoO4 species was also observed in H2-TPR profile of NiMo-alumina catalysts (Fig. 3). From the above discussion, it can be concluded that the catalytically active NiMo complex (NiMoO4−, MoNi3, and MoNi4) and Mo oxides with lower oxidation states were insignificant for low Mo content, increased with increasing the quantity of Mo up to 3.4 mmol, and 6

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Fig. 5. Effect of total metals content on the (a) conversion of oxygenated compounds and (b) selectivity to products at 60% conversion of oxygenated compounds. Reaction conditions: 5 (w/v)% karanja oil, 100 ml n-dodecane, 340 °C, 30 bars H2, and 20 (w/w)% catalyst. Fig. 4. Effect of Ni/Mo (mole) on the (a) conversion of oxygenated compounds and (b) selectivity to products at 60% conversion of oxygenated compounds. Reaction conditions: 5 (w/v)% karanja oil, 100 ml n-dodecane, 340 °C, 30 bars H2, and 20 (w/w)% catalyst.

selectivity to C18 alkane was enhanced with the increase in the quantity of Mo in the catalysts up to 3.4 mmol. It was due to the increase in the NiMo complex and Mo oxides with lower oxidation states in the catalyst with the increase in the quantity of Mo. The fatty acids were observed as the major oxygenated intermediate compounds for the 4.3N catalyst. It was due to the slow rate of reduction of fatty acids to the equivalent fatty aldehyde followed by the rapid transformation of the fatty aldehyde to alkanes over the Ni-alumina catalyst. On the contrary, the fatty alcohols were observed as the major oxygenated intermediate compounds over NiMo-alumina catalysts. Furthermore, the selectivity to fatty alcohols was increased with the increase in the quantity of Mo in the NiMo-alumina catalysts. It was due to the rapid reduction of fatty acids to fatty alcohols over NiMoalumina catalysts followed by the sluggish reaction of fatty alcohols to alkanes. Based on these results, the Ni/Mo (mole) of 1:4 (0.9N3.4M) was considered as the optimum.

decreased with further increase in the amount of Mo. The HDO activity of the NiMo-alumina catalyst was thus boosted with the increase in the quantity of Mo in the catalysts up to 3.4 mmol. Mo oxides with lower oxidation states were also reported to be responsible for the enhancement in catalytic activity for HDO of triglycerides (Chen et al., 2015). NiMoO4− and Mo+6 were reported as responsible for decarboxylation reaction (Chen et al., 2015). Karanja oil was comprised of about 75 wt% and 10 wt% C18 and C16 fatty acids, respectively. The alkanes containing the odd number of carbon atoms (C15 and C17) were observed as the dominating products over the 4.3N catalyst (Fig. 4). The RR-II was thus the dominating reaction route over Ni species present in the Ni-alumina catalyst. On the other hand, the alkanes having the even number of carbon atoms (C16 and C18) were noticed as the major products over NiMo-alumina catalysts (Fig. 4). This result shows that the reaction followed the HDO route (RR-I) over the NiMo complex and Mo oxides with lower oxidation states present in NiMo-alumina catalysts. Furthermore, the

3.4. Effect of total metals content For fixed Ni/Mo (mole) of 1:4, the conversion of oxygenated compounds was enhanced with increasing total metals content up to 0.7N2.7M (Fig. 5). The conversion of oxygenated compounds was, 7

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karanja oil. 3.6. Practical applications and future research perspectives HDO of karanja oil produces green diesel in the range of C15–C22 alkanes analogous to petroleum-derived diesel. It can be used directly in the existing diesel engines. In this process, the catalyst plays a crucial role. The fundamental understanding of catalyst characteristics such as pore structure and formation of active surface species and their role on the reaction mechanism and catalytic activity is essential for the future process development. Further research is also needed in this area to develop the highly active and stable catalysts with the low tendency of cracking and optimize the process conditions before commercial process development. On the other hand, the use of edible oils should be minimized in this process to avoid the food crisis. The study can also be extended to other inedible oils and waste cooking oil. Nowadays, highly-productive microalgae are considered as a promising source of triglycerides. The research on the HDO of microalgal oil is also needed to overcome the shortage of feedstock. 4. Conclusions

Fig. 6. Effect of reaction temperature on the conversion of oxygenated compounds. Reaction conditions: 5 (w/v)% karanja oil, 100 ml n-dodecane, 30 bars H2, 20 (w/w)% 0.9N3.4M.

The catalytic activity of mesoporous NiMo-alumina catalysts was increased with increasing the concentration of catalytically active NiMo complex and Mo+ species up to 3.4 mmol Mo. The conversion of oxygenated compounds was enhanced with growing total metals content and reaction temperature. The reaction mainly followed the HDO route over these species with C18 alkane as the main product. The selectivity to C18 alkane was further enhanced with the increasing concentration of these species with the concurrent decrease in selectivity to C17 alkane. The catalysts with 0.9 mmol Ni and 3.4 mmol Mo was the optimum for HDO of karanja oil.

however, not increased much beyond this total metals content. The enhancement in the conversion of oxygenated compounds was due to the increase in the number of active sites (Ni2+, NiMo complex, and Mo oxides with lower oxidation states) in the catalyst. The selectivity to C18 alkane was enhanced with the increase in total metals content up to 0.7N2.7M with a concurrent decrease in C17 alkane selectivity. Similarly, selectivity to fatty alcohols was increased with increasing total metals content up to 0.7N2.7M with the simultaneous decrease in the selectivity to fatty acids and fatty esters. The number of active Mo oxide species with lower oxidation state and NiMo complex were increased with increasing total metals content. RR-I was thus favored leading to the increasing trends of selectivity to C18 alkane and fatty alcohols with increasing total metals content. Based on the above results, the total metals content of 4.3 mmol (0.9N3.4M) was deliberated as the optimum. The reusability of this optimum catalyst was performed using the dried spent catalyst. The dried spent catalyst showed significantly lower HDO activity than the virgin catalyst (Table 2). It may be due to the blockage of active sites by alkanes or oxygenated compounds. Regenerated spent catalysts, however, showed the catalytic performance identical to the virgin catalyst (Table 2) (Kumar et al., 2019a).

Nomenclature Cn CnCHO CnOH CnCOOH EISA HDO xNyM MG EST

alkane with n number of carbon atoms fatty aldehyde with n + 1 number of carbon atoms fatty alcohol with n number of carbon atoms fatty acid with n + 1 number of carbon atoms evaporation-induced self-assembly hydrodeoxygenation NiMo-alumina catalysts with x mmol Ni and y mmol Mo monoglycerides fatty esters

Declaration of Competing Interest 3.5. Effect of temperature No conflict of interest. The conversion of oxygenated compounds was enhanced with the rise in reaction temperature, as shown in Fig. 6. The conversion of oxygenated compounds was about 13% at 280 °C in 5 h of reaction time and almost 100% at 360 °C in 2 h of reaction time. At 280, 300, 320, 340, and 360 °C, the initial reaction rate was 0.776 × 10−4, 1.79 × 10−4, 4.74 × 10−4, and 9.31 × 10−4 kgm−3 s−1, respectively. The initial rate was enhanced by a factor of about 12 for increasing the reaction temperature from 280 °C to 360 °C. The enhancement in the reaction rate was due to the greater hydrogen solubility in the solvent as well as an increase in rate constant following Arrhenius law at the elevated reaction temperature (Safamirzaei and Modarress, 2011). Arora et al. (2019) reported 3.63 × 10−5 kmolm−3 s−1 as the initial reaction rate for HDO of stearic acid over the sulfided NiMo/Al2O3 catalyst at 325 °C and 50 bar. The selectivity to alkanes containing the odd number of carbon atoms (C15 and C17) was increased with the rise in the reaction temperature (Table 2). It was due to the enhancement in the decarbonylation route (RR-II) with increasing the reaction temperature. The apparent activation energy was 127.1 kJ/mol for HDO of

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