SBA-15 and carbon supported nickel, heteropoly acid catalysts for the hydrodeoxygenation of lignin derived trans-anethole to sustainable aviation fuel

Catalyst is the key for the hydrodeoxygenation (HDO) of lignin derivatives to produce alkyl aromatics and alkyl cycloalkanes, key components of 100% sustainable aviation fuels (SAF). Mono and bi-functional catalysts of nickel (Ni) and heterophosphotungstic acid (HPW) supported on SBA-15 and carbons were prepared, characterized and evaluated for the HDO of lignin derived trans-anethole. The SBA-15 structure was intact but its surface area and pore volume decreased with the incorporation of Ni and HPW. UV–Visible DRS and XPS confirmed the presence of Nio and Ni2+ in the bi-functional catalysts. FTIR spectra of pyridine adsorbed Ni(10)-HPW(10)/SBA-15 catalyst showed the presence of both Brønsted and Lewis acid (Ni2+) sites and hence its acidity was higher than mono functional HPW/SBA-15 catalyst as indicated by the ammonia TPD studies. The synergy between them might be the possible reason for the higher and stable conversion, higher HDO selectivity, and more importantly higher selectivity for propyl benzene and propyl cyclohexane over bi-functional catalysts. Among the bi-functional catalysts, the highest conversion and HDO selectivity were observed over Ni(10)-HPW(10)/SBA-15. Its highest selectivity for propyl cyclohexane was due to its mean Ni particle size of 31.02 nm, large enough for the co-adsorption of hydrogen and benzene ring of propyl benzene, thus led to its saturation. Optimum temperature for the maximum conversion and HDO selectivity was found to be 400 °C for the bi-functional catalysts. Comparing carbon and SBA-15 supported Ni-HPW catalysts, later have shown higher HDO selectivity owing to strong metal-support interaction.


Introduction
Robust growth of global civil aviation sector has seen a surge in number of air passengers and revenue passenger kilometers to 4.5 billion and 8.69 trillion respectively in 2019 [1]. Hence global jet fuel consumption reached 106 billion gallons in 2019 [2], which accounted for 8.1% of the total petroleum products consumed [3]. Civil aviation sector contributed 2% of the total anthropogenic greenhouse gas (GHG) emitted in 2019 [4] and hence International Civil Aviation Organization (ICAO) aimed at carbon neutral growth of civil aviation from 2020 onwards [5]. The best feasible option to achieve this is the development and use of biomass derived sustainable aviation fuel (SAF), considering the huge inventory of jet fuel based aircrafts currently in use and difficulty in their electrification [6]. Seven SAF production processes have been certified by ASTM International so far, all of them lack required quantity of cycloalkanes and aromatics thus necessitating their blending with minimum of 50% petroleum derived jet fuel [4]. Presence of 8% of alkyl aromatics and 20% alkyl cycloalkanes in the SAF help to meet the jet fuel specifications such as energy density, freezing point, flash point and more importantly swelling of the sealant to prevent leakage in the aircraft engine [7,8].
Lignocellulose is the most abundant, non-food biomass containing 15 to 30% lignin which is a polymer of phenols [9]. Lignin is a by-product in pulp and 2G ethanol industries and only around 2% of it is valorized, thus readily available for SAF production [8]. Lignin has the highest potential to produce alkyl aromatics and alkyl cycloalkanes in the jet fuel range by its fast pyrolysis or hydrothermal liquefaction to bio-oil followed by catalytic hydrodeoxygenation (HDO) [10], to comply with ASTM D7566 and ASTM D1655 specifications [11]. Catalytic HDO of lignin derived bio-oil is aimed at removing the oxygen, adding hydrogen while preserving the C-C bond to get hydrocarbons in the jet fuel range. The HDO catalysts need to be cost effective with high deoxygenation activity under less severe reaction conditions to slow down its deactivation with time on stream. Wide range of catalyst systems viz. transition metals, sulfides, phosphides, nitrides, carbides and oxides of metals on different supports have been evaluated for HDO of lignin derived bio-oil and model compounds to hydrocarbon fuels under high H 2 pressure [8]. Most of the HDO catalysts reported were bi-functional catalysts containing an active metal viz. Ni, Mo, Co, Pt, Pd, Ru and a solid acid, catalyzing hydrogenation and deoxygenation respectively [12]. Cheaper non-noble metal catalysts demanded harsher reaction conditions such as high temperature and H 2 pressure, leading to its quick deactivation [13], necessitating the development of highly active and stable catalysts under milder conditions. Hence polymethylhydrosiloxane (PMHS), a safe H-donor as the substitute for high pressure H 2 was explored for the HDO of acetophenone, a bio-oil model compound under mild conditions. Pd/HZSM-5, fluorine enhanced Pd/HZSM-5 and Pd-Zr/HZSM-5 catalysts, studied for the same at 35 to 65 •C for 3 h in hydrocarbon solvents, reportedly yielded more than 95% of ethyl benzene [14][15][16]. Recently HDO of vanillin in aqueous phase using Ni/Nb 2 O 5 catalysts was reported to be efficient [17] and the mechanistic aspects of HDO of biomass in water-oil biphasic systems to biofuels was critically reviewed [18].
Hydrogen has become an energy carrier or storage for green electricity and its availability and affordability will be on the increase due to the ever increasing transition from fossils based to renewables based electricity such as solar and wind [19]. Low volumetric energy density of H 2 and potential fire hazards in its transportation, necessitated the development of safe and energy dense chemical hydrogen carriers [20]. Bio jet fuels produced by the HDO of bio-oil in large scale, using hydrogen instead of hydrogen donor is a biomass valorization as well as a hydrogen carrier, thus having the advantage. Zhang et al. studied the HDO of mixture of six lignin derived phenolic compounds consisted of 10 Wt% trans-anethole in a batch reactor containing 1.5 g of Ni(10Wt%)/SiO 2 -ZrO 2 catalyst at 300 °C, 5 MPa of H 2 for 8 h, using dodecane solvent [21]. Conversion of 100% and a selectivity of 98.9% for propyl cyclohexane were reported. They have also studied the MoO 3 catalyst for the HDO of trans-anethole in octane solvent at 340 ℃, 0.5 MPa H 2 , for 6 h and reported a conversion of 93.2% with the selectivity of 75.8% for propenyl benzene [22], whereas olefins in aviation fuel need to be restricted to 5% [8]. Heteropoly acid was reported to be the green catalyst with high acidity and thus gained importance [23]. Cheng et al. studied the bifunctional catalyst of Ni(10Wt%)-HPW/Hierarchical zeolite Y containing varying amount of heterophosphotungstic acid (HPW) and identified the optimum loading of HPW for the maximum selectivity of bio jet fuel, from the HDO of microalgae derived biodiesel [24].
In the present study, trans-anethole is selected as a model lignin compound as it has three competing sites for hydrogen insertion viz. olefinic double bond, benzene ring and methoxy group. Hence it will bring out the ability of the catalyst to preferentially hydrogenate which of them and thus the selectivity for HDO, alkyl benzene and alkyl cyclohexane. Further SBA-15 is chosen as a support, being mesoporous with good thermal stability. Bi-functional catalysts of Ni-HPW(10)/SBA-15 of varying amount of Ni viz. 5, 10 and 15Wt%, and mono functional catalysts viz. Ni(10)/SBA-15 and HPW(10)/SBA-15 for comparison were synthesized, characterized by XRD, BET, TPD, FT-IR, FT-IR(Pyridine adsorbed), HR-TEM with EDAX, XPS, and evaluated for the HDO of trans-anethole. Carbon supports namely commercially available activated carbon (AC), synthesized Biomass Derived Activated Carbon (BDAC) and Graffiti Carbon Sphere (GCS) were also studied as support for Ni(10)-HPW(10) in lieu of SBA-15 for comparison. The catalyst characteristics were correlated with the catalytic activity to identify the optimum Ni loading and the support for maximum selectivity to HDO and SAF components.

Catalyst synthesis
SBA-15 was synthesized using P123 tri-block copolymer (EO 20 PO 70 EO 20 ) as the structure directing agent, by following the reported procedure [25]. With the addition of 4 g of P123 to 30 mL of deionized water in polypropylene bottle, a clear solution was obtained after stirring for 4 h. About 120 mL of 2 M hydrochloric acid was added and stirred for 2 h. Then 9 g of TEOS was added to it and the resulting mixture was stirred for 24 h at 40 °C. The contents were transferred to stainless steel autoclave of 250 ml capacity and then kept in an oven for 48 h at 100 °C. The autoclave was removed from the oven, cooled and the product was filtered, washed several times with distilled water and dried at 100 °C overnight. Then it was calcined at 550 °C for 6 h to remove the occluded template. SBA-15 obtained was impregnated with Ni and HPW separately as well as together by following the wet impregnation method. 1 g of SBA-15 was transferred to a 50 mL round bottom (RB) flask to which 30 mL of deionized water was added and stirred for 1 h. The required weight of nickel (II) nitrate hexahydrate was added to the mixture and the stirring continued for 6 h. The water was evaporated and the residue left was dried at 100 °C for 12 h and then calcined at 550 °C for 6 h in air at atmospheric pressure giving mono functional Ni(10)/ SBA-15 catalyst, containing 10 weight% of Ni. To 1 g of SBA-15 taken in the 50 ml RB flask, 30 mL of deionized water was added followed by the addition of 0.1 g of HPW.

Characterization of catalyst
The XRD pattern of catalysts were recorded on Bruker D8 diffractometer with Cu-Kα radiation and nickel filter. With JCPDS (Joint Committee on Powder Diffraction Standards) data files, the XRD peaks of the catalysts were matched to arrive the crystalline phase. Nitrogen adsorption was carried out in Quanta chrome 2010-09 at liquid nitrogen temperature (− 196 °C) after degassing the catalyst samples for 3 h at 350 °C. The surface area was calculated by Brunauer-Emmet-Teller (BET) method while pore diameter and pore volume of the catalysts were obtained using Barret-Joyner-Halenda (BJH) method, from the nitrogen adsorption measurements. All the catalyst samples were heated at 300 °C for 2 h to remove the moisture and then FTIR spectra was recorded with Agilent Cary 630 in the range of 4000-400 cm −1 , using pellet obtained by mixing 20 mg of dried sample with 80 mg of KBr powder. Catalyst samples were adsorbed with pyridine and then FTIR spectra were recorded. Temperature programmed desorption (TPD) of ammonia was carried out in Micromeritics ChemiSorb 2750 to determine the acidity of the catalysts. UV-visible diffuse reflectance spectra (UV-Visible DRS) were recorded with Shimadzu UV-Visible spectrometer (UV-2450) using BaSO 4 as reference in the range 200-800 nm to understand the co-ordination nature of the active catalytic species. The surface morphology of the catalysts were characterized using JEOL JEM 2100 high resolution transmission electron microscope (HRTEM) by applying the acetone suspended catalyst over the carbon coated copper grids. TEM microscope attached with EDX detector was used for identifying the elements present. The binding energy and oxidation state of the elements present in the sample were identified by X-ray photoelectron spectroscopy (XPS) with Omicron XM1000 mono using monochromatic Al Kα radiation of 1483 eV operated at 300 W. Amount of Ni and HPW loading of the catalysts were analyzed using Agilent 5900 SVDV ICP-OES system.

Catalytic studies on HDO of trans-anethole
Catalytic HDO of trans-anethole was studied in a fixed bed down flow reactor at atmospheric pressure in vapor phase. The quartz tubular reactor of 2 cm diameter and 40 cm length was used for the reaction. The catalyst (0.1 g) was placed on silica beads supported on glass wool. It was activated at 500 °C for 3 h with hydrogen flow of 50 mL/min. Then HDO of trans-anethole was carried out over the catalyst at 300, 350 and 450 °C with the trans-anethole flow rate of 1 mL/h and hydrogen flow rate of 50 mL/min for 12 h of time on stream (TOS). Bottom of the reactor was connected to a coiled water cooled condenser to condense and collect the product after every 2 h. The products were analyzed by GC-17A Shimadzu gas chromatograph with Rtx-5 column (30.0 m × 0.25 mm, 0.25 µm film thicknesses) with flame ionization detector (FID). GC-MS (Perkin-Elmer Clarus 500) was used for further confirmation of the products. The conversion, product selectivity, selectivity for HDO (HDO%), specific reaction rate (r) and turn-over frequency (TOF) were calculated using the Eqs. (1), (2), (3), (4) and (5) respectively [27]. (1)

Conversion of trans-anethole
Product selectivity % = Moles of specific product Moles of all the product × 100 where F = Molar inflow rate of trans-anethole in mol/s; W = Weight of catalyst in g. Quantity of sites were calculated by dividing the weight of Ni and HPW present in the catalyst (determined by ICP-OES) with their atomic and molecular weight respectively. The spent catalysts left after 12 h of TOS were studied by thermogravimetric (TG) analyzer to understand the extent of deactivation by coking with Perkin Elmer Diamond.

XRD characterization of catalysts
The low angle XRD patterns of calcined SBA-15 and Ni(5)-HPW(10)/SBA-15 are shown in Fig. 1a. The reflections at 2θ of 0.5°, 1.2° and 1.4° corresponding to (1 0 0), (1 1 0) and (2 0 0) respectively were observed for both the samples. This indicated that the ordered hexagonal mesoporous SBA-15 was formed and the impregnation of Ni and HPW to SBA-15 have not altered its long range order, similar to the observation reported for Ni/SBA-15 [25]. From the wide angle XRD patterns of the samples (Fig. 1b), the broad peak observed for all the samples in the 2θ range of 15-30° is attributed to amorphous silica of the SBA-15 [28]. Three peaks observed Specific Reaction Rate (r) = F × Conversion of trans-anethole (%) W (5) Turn over frequency (TOF) = r Quantity of sites at 2θ positions of 37°, 43° and 63° were found to be matching with the standard NiO peaks of JCPDS 04-0835 and intensity of these three peaks increased with the increase in Ni loading from 5 to 15%. The reflection at 2θ of 34.1°, characteristic of HPW [29] was not found in the XRD pattern of Ni(5,10,15)HPW(10)/SBA-15 catalysts, due to the thorough dispersion of HPW in SBA-15.

Nitrogen adsorption isotherms of catalysts
The physico-chemical and textural characteristics of the SBA-15 support and supported catalysts are presented in Table 1. All the SBA-15 supported catalysts were found to have BET surface area, pore volume and pore diameter less than the support SBA-15, suggested that incorporation of either Ni or HPW or both partially blocked the mesopores of SBA-15. Further, with the increase in Ni loading of HPW(10)/SBA-15 from 0 to 15 Wt%, BET surface area, pore volume and pore diameter decreased. N 2 adsorption isotherms (Fig. 2a) of, all the samples showed type IV(a) adsorption isotherm with the hysteresis loop evidencing the characteristic of cage-like (Im3m) pore structure [30]. Absence of the sharp inflection at the relative pressure (p/p 0) range 0.22 to 0.26 indicated the absence of micropores. This was evidenced further from the pore size distribution, much of which were ranging from 2 to 8 nm (Fig. 2b), falling in the range of mesopores, characteristic of SBA-15.

FT-IR spectra of the support and catalysts
The

UV-visible diffuse reflectance spectra of the catalysts
The UV-Visible diffuse reflectance spectra (UV-Visible DRS) of the catalysts are shown in Fig. 5. SBA-15 did not show any peak in 200 to 800 nm whereas HPW(10)/SBA-15 showed a broader peak between 200 and 300 nm, attributed to the oxygen to metal charge transfer in tungstophosphate anion [34]. Ni(10)/SBA-15 showed a sharp peak around 250 nm, attributed to the charge transfer transition of NiO from O 2− to Ni 2+ ions [35] and absorption increased with the increase in Ni loading from 5 to 15% for the bi-functional Ni(5,10,15), HPW(10)/SBA-15 catalysts. Absence of plasma band for the Ni particles proved them to be of nano size.

HRTEM characterization of catalysts
The HRTEM image of the catalysts are shown in Fig. 6. The image of SBA -15 showed the hierarchical mesoporous structure and the same was retained after the incorporation of Ni and HPW. The particle size of Ni in Ni(10)/SBA-15 catalyst varied between 15 to 45 nm and the mean size was found to be 30.57 nm. The particle size distribution of HPW in HPW(10)/SBA-15 was found to be between 10 to 70 nm and the mean size was 32.08 nm. For the bi-functional Ni(10)-HPW(10)/SBA-15 catalyst, particle size distribution was between 10 to 70 nm and the mean particle size of was found to be 31.02, lying between Ni (10)-SBA-15 and HPW (10)-SBA-15. This might be possibly due to the strong interaction between the silica wall of SBA-15, Ni and HPW. The EDAX spectrum

X-ray photoelectron spectra (XPS) of the catalysts
XPS spectrum of nickel 2p, silicon, tungsten 4f for Ni(10)-HPW(10)/SBA-15 catalyst are given in Fig. 7. The Ni 2p signal is deconvoluted into four peaks in the range of

Products and reaction pathways of catalytic HDO of trans-anethole
Products of HDO of trans-anethole were identified by GC-MS. Products identified over the bi-functional catalysts [Ni(5,10,15)-HPW(10)/SBA-15] were 4-propylanisole and 4-propylphenol, 1-propenylbenzene, propylbenzene, propylcyclohexane, benzene and cyclohexane. The reaction pathways leading to all the identified products is given in Fig. 8. Trans-anethole (1) got hydrogenated to 4-propyl anisole (2) which subsequently demethylated to 4-propyl Phenol (3), thus accounted for the two oxygen containing products. Both of them have undergone hydrodeoxygenation to yield propylbenzene (4) which either hydrogenated further to propylcyclohexane (5) or hydrodealkylated to benzene (6). The propylcyclohexane has undergone hydrodealkylation to cyclohexane (7). Formation of propenylbenzene (8) might be due to the direct hydrodeoxygenation of trans-anethole, which would have got hydrogenated to yield additional propylbenzene. Absence of C 9 alkanes or isoalkanes in the product indicated that the hydrogenation did not proceed to the extent

Effect of time on stream on the HDO of trans-anethole
The effect of time on stream (TOS) on the conversion of trans-anethole and HDO selectivity (HDO%) for different catalysts are presented in Fig. 9a, b respectively. Among all the catalysts studied, Ni(10)-HPW(10)/SBA-15 has shown the highest conversion at TOS of 2 h and remained almost the same except for the TOS of 10 h. For all other catalysts conversion declined with TOS and the extent of decline varied according to catalyst composition. From the thermogravimetric analysis of spent catalysts (Fig. 10), weight loss was found to be minimum for Ni(10)-HPW(10)/SBA-15,  Thus it could be concluded that the bi-functional catalysts have stable HDO selectivity compared to mono-functional catalysts. Effect of TOS on the selectivity for different products obtained over different catalysts are presented in Fig. 11. At the TOS of 2 h, Ni(10)/SBA-15 has shown higher selectivity for benzene but zero selectivity for cyclohexane, which was in accordance with the result reported for the HDO of anisole over Ni(7)/Al-SBA-15 [39], both involved a flow reactor with H 2 pressure of 1 atm. and residence time of 0.1 h. But in contrast to the higher cyclohexane selectivity reported for directly precipitated Ni on SBA-15 [40] and Ni/ IM-5 catalysts [41], attributed to the strong hydrogenation activity of Ni. However both were studied in a batch reactor with a residence time 2 to 6 h and 50 bar of H 2 , which might be the actual reason for higher selectivity for cyclohexane. A flow reactor with low residence time and moderate H 2 pressure in which the reaction would be in kinetically controlled regime be better to ascertain the real activity of the catalysts. Whereas a batch reactor with long residence time and high H 2 pressure, the reaction would be in thermodynamically controlled regime would overestimate the activity of the catalyst. The other HDO product found in the present study was propenylbenzene of low selectivity. As TOS was increased from 2 to 12 h, product selectivity changed but no specific trend was observed. HPW(10)/SBA-15 was the only catalyst gave 4-propylphenol as the lone oxygen containing product, while all the other four catalysts gave both 4-propylphenol and 4-propylanisole. This confirmed that the Brønsted acidic sites of HPW was mainly responsible for the demethylation of trans-anethole by cleaving the C-O bond of the methoxy group. It gave benzene and cyclohexane as HDO products. Selectivity for cyclohexane was found to be higher than benzene up to the TOS of 2 h, indicated that the acidic HPW(10)/SBA-15 catalyst was found to have higher hydrogenation activity accounting for its higher cyclohexane selectivity compared to metallic Ni(10)/SBA-15 catalyst. With further increase in TOS, selectivity for both cyclohexane and benzene decreased, but the selectivity for benzene became higher than the cyclohexane. This suggested that the active centers of HPW responsible for the hydrogenation of benzene to cyclohexane got selectively deactivated with TOS. Benzene and cyclohexane formed were shorter in carbon length to be the part of SAF, but valuable petrochemicals. Propenylbenzene another HDO product was also not a preferred component of SAF. Hence both the mono functional catalysts could be concluded as less suitable for SAF production, but useful for the sustainable production of petrochemicals. All the five HDO products and two oxygen containing products were obtained over the three bi-functional catalysts Ni(5, 10, 15)-HPW(10)/SBA-15. The selectivity for propylcyclohexane and propylbenzene were found to increase with TOS over Ni(10)-HPW(10)/ SBA-15 and Ni(15)-HPW(10)/SBA-15 catalysts and were always higher over the former at all TOS. Selectivity for these two compounds were only 1 and 3% respectively over Ni(5)-HPW(10)/SBA-15 at TOS of 2 h, decreased with TOS and ceased to be in the product after 6 and 10 h respectively. Thus Ni(10)-HPW(10)/SBA-15 could be concluded as the best catalyst for the HDO of trans-anethole due to its higher and stable conversion, selectivity for HDO and SAF components.

Effect of Ni loading into HPW(10)/SBA-15 on the HDO of trans-anethole
Effect of Ni loading into HPW(10)/SBA-15 on the average conversion% and HDO% at 400° C are presented in Fig. 12a. Conversion increased with increase in Ni loading from 0 to 10% and then decreased with further increase to 15%. HDO% did not change much with the increase in Ni% from 0 to 5%, but increased sharply with further increase to 10% and then decreased with further increase to 15%. HPW(10)/ SBA-15, having 0% Ni, has not yielded C 9 compounds of SAF range (Fig. 12b). With the addition of 5% Ni i.e. Ni(5)-HPW(10)/SBA-15, a bi-functional catalyst yielded C 9 compounds of SAF range viz. 6.6% propenylbenzene, 2.8% propylbenzene, 0.5% propylcyclohexane. Increase in Ni from 5 to 10%, increased the selectivity for propenylbenzene, propylbenzene and propylcyclohexane by 6.3, 7.8 and 10% respectively. Further increase in Ni to 15%, decreased the selectivity for propenylbenzene, propylbenzene and propylcyclohexane by 2.3, 5.1 and 7.5 respectively. Thus it might be concluded that Ni(10)-HPW(10)/SBA-15 catalyst containing 10% Ni loading was the best catalyst among the five SBA-15 supported catalysts studied for the HDO of transanethole to SAF. Further it appeared that equal weight% of Ni and HPW on the SBA-15 would give the maximum selectivity for the SAF components and the same proposition of other percentages need to studied in the future to confirm the same.

Effect of temperature on the HDO of trans-anethole
Effect of temperature on the average conversion of transanethole and selectivity for HDO (HDO%) of the five SBA-15 supported catalysts are presented in the Fig. 13. For the three bi-functional catalysts viz. Ni(5)-HPW(10)/SBA-15, Ni(10)-HPW(10)/SBA-15, Ni(15)-HPW(10)/SBA-15, average conversion% increased with increase in temperature from 350 to 400 and then decreased with further increase to 450 °C (Fig. 13a). While for the mono-functional catalyst HPW(10)/SBA-15 average conversion% increased continuously with increase in temperature, but the lowest among all the catalysts. For the other mono-functional catalyst Ni(10)/ SBA-15, average conversion% was the highest among all the catalysts at 350 °C, decreased with an increase in temperature to 400 °C and then increased with further increase to 450 °C. The selectivity for HDO (HDO%) increased from 350 to 400 °C and decreased with further increase to 450 °C for all the catalysts, except Ni(5)-HPW(10)/SBA-15 whose average HDO% was not much affected by temperature (Fig. 13b). Increase in temperature markedly affected the selectivity of both the oxygen containing and HDO products which varied with the nature of catalysts (Fig. 14). For Ni(10)/SBA-15, selectivity for propyl phenol increased with increase in temperature from 350 to 450 °C, selectivity for propyl anisole was higher at all the temperatures. At 350 and 400 °C propenyl benzene and benzene were obtained as HDO products and only at 450 °C propyl benzene and cyclohexane were also obtained, indicated that the activation energy for hydrogenation was higher for mono-functional Ni(10)/SBA-15 catalyst. Other mono-functional catalyst HPW(10)/SBA-15 has shown a complimentary behavior. It has shown an exclusive selectivity for propyl phenol among the oxygen containing products at 350 and 400 °C and only at 450 °C propyl anisole was also found, indicated its stronger demethylation ability. Benzene and cyclohexane were the exclusive HDO products at 350 and 400 °C and the selectivity for the cyclohexane increased with temperature. Among the bi-functional catalysts, for Ni(5)-HPW(10)/SBA-15, average selectivity for cyclohexane decreased with increase in Fig. 13 Effect of temperature on a average conversion and b average selectivity for HDO of trans-anethole over different catalysts temperature while the selectivity for propenyl and propylbenzene increased from 350 to 400 and then decreased with further increase to 450 °C. For Ni(10)-HPW(10)/SBA-15, average selectivity for all the HDO products including SAF components increased to the highest with increase in temperature from 350 to 400 °C, and then decreased with further increase to 450 °C. Similar trend was shown by the Ni(15)-HPW(10)/SBA-15 catalyst and hence it could be concluded that 400 °C was the optimum temperature.

Effect of carbon support on the HDO of trans-anethole
As Ni(10)-HPW(10)/SBA-15 has shown the highest conversion, selectivity and stability for the HDO of trans-anethole, effect of different carbon support were studied in lieu of SBA-15. Activated carbon (AC), biomass derived activated carbon (BDAC), and graphitic carbon sphere (GCS) were the supports studied for the HDO of trans-anethole. For the various catalysts, specific reaction rate (SRR), turnover frequency  Table 2. Among all the catalysts studied Ni(10)-HPW(10)/BDAC gave the highest conversion of 97.94% and SRR of 1.79 × 10 -5 mol g −1 s −1 , but its HDO% was only 15.9%. Ni(10)-HPW(10)/AC has shown the lowest HDO% of 6.57%. On the other hand Ni(10)-HPW(10)/SBA-15 has shown the highest HDO% of 79.5 among all the catalysts studied with comparable SRR of 1.64 × 10 -5 , suggested that SBA-15 is a better support for HDO of trans-anethole. This might be due to the strong metal-support interaction (SMSI) as its prevalence was established from the HRTEM results.

Conclusions
Incorporation of Ni and HPW on SBA-15 did not alter its structure, but the surface area and pore volume decreased marginally. Acidity of bi-functional catalyst Ni(10)-HPW(10)/SBA-15 was found to be higher than the mono functional catalyst HPW/SBA-15 due to the presence of both Bronsted and Lewis acid sites of Ni 2+ whose presence was confirmed by both UV-Visible DRS and XPS. HRTEM result indicated the interaction between the silica wall of SBA-15, Ni and HPW for Ni(10)-HPW(10)/SBA-15 catalyst. Bi-functional catalysts were found to have stable conversion and HDO selectivity for trans-anethole and among them, Ni(10)-HPW(10)/SBA-15 catalyst containing Ni and HPW 10% each, was found to have the highest and the most stable HDO selectivity, more importantly for the desirable SAF components viz. propylbenzene, propylcyclohexane. Optimum temperature for the bi-functional catalysts was found to be 400 °C for the maximum conversion, HDO selectivity and SAF components. Among the carbon supports viz. activated carbon, biomass derived activated carbon, and graphitic carbon sphere impregnated with Ni and HPW 10% each studied, biomass derived activated carbon showed the conversion of 97.94% for the HDO of trans-anethole higher than over SBA-15 support, but the HDO selectivity was lower. The strong metal-support interaction (SMSI) could be the reason for the higher HDO selectivity of SBA-15 support, thus proven to be the better support for HDO of trans-anethole.