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

doi:10.21203/rs.3.rs-1880168/v1

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

https://doi.org/10.21203/rs.3.rs-1880168/v1

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Catalytic hydrodeoxygenation (HDO) of lignin derivatives is the important means of producing alkyl aromatics and 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 Ni^o and Ni²⁺ in the bi-functional catalysts. FTIR spectra of pyridine adsorbed Ni(10)-HPW(10)/SBA-15 catalyst has shown 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 over bi-functional catalysts besides higher HDO selectivity, and more importantly selectivity for propyl benzene and propyl cyclohexane, the desired components of 100% SAF, compared to mono functional catalysts. Among the bi-functional catalysts, best catalytic performance was observed over Ni(10)-HPW(10)/SBA-15 due to its mean Ni particle size of 31.02 nm which is large enough for co-adsorption of hydrogen and benzene ring of propyl benzene may possibly be the reason for its highest selectivity to propyl cyclohexane by following the hydrogenation mechanical pathway. Optimum temperature for maximum conversion and, HDO selectivity was found to be 400^oC for the bi-functional catalysts. Carbon supported Ni-HPW catalysts have shown lower selectivity for HDO, thus mesoporous SBA-15 was found to be advantageous.

Hydrodeoxygenation

sustainable aviation fuels

trans-Anethole

SBA-15

bio jet fuel

Robust growth of global civil aviation sector in the last decade has seen a surge in number of air passengers and revenue passenger kilometers from 2.5 billion and 3.08 trillion in 2010[1] respectively to 4.5 billion and 8.69 trillion in 2019[2], leaving the exceptional year of 2020 which witnessed a sharp decline of 60% due to air travel restriction necessitated by COVID – 19 pandemic. Hence the global jet fuel consumption in 2019 reached 106 billion gallons[3] accounted for 8.1% of the petroleum products consumed[4] and hence resulted in 2% of the total anthropogenic greenhouse gas (GHG) emitted. Number of air passengers and jet fuel consumption are projected to reach 16 billion and 230 billion gallons respectively in 2050[5] and aviation sector is expected to account for 5% of the total anthropogenic GHG emitted in 2035[6]. International Civil Aviation Organization (ICAO) aimed at carbon neutral growth of civil aviation from 2020 onwards while IATA seek to bring down the GHG emission to 50% of 2005 level by 2050 and Airliners for America committed to net-zero emission by 2050. On the other hand, oil the source of jet fuel who’s global reserve and annual consumption reported by BP’s statistical review of world energy 2022[7] suggested that it may last for 59 years. To address the twin issues of long term availability of jet fuel and GHG emission, the best feasible option is the development and use of biomass derived sustainable aviation fuel (SAF), considering the huge inventory of current fleet of aircrafts and difficulty in their electrification[8]. Seven SAF production processes have been certified by ASTM international so far, and all of them lack required quantity of cycloalkanes and aromatics thus necessitating their blending with minimum of 50% petroleum derived jet fuel[9]. Presence of 8% of C₉ aromatics and 20% cycloalkanes in the jet fuel helps to meet the jet fuel specifications such as energy density, freezing point, flash point and more importantly to prevent leakage in engine by swelling the sealant[10, 11].

Lignocellulose is the most abundant, non-food biomass containing 15 to 30% lignin, a polymer of phenols has the highest potential to produce alkyl aromatics and alkyl cycloalkanes in the jet fuel range by fast pyrolysis followed by catalytic hydrodeoxygenation (HDO)[12], thus to produce 100% SAF[13] that comply with ASTM D7566 and ASTM D1655. Lignin is a by-product in pulp and 2G ethanol industry and only around 2% of it is valorized, thus readily available for SAF production[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, and to meet the other jet fuel’s specifications. 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[11]. Zhang et al[14] studied the MoO₃ catalyst for the HDO of several phenolic compounds including trans- anethole and reported a conversion of 93.2% with propenyl benzene as the predominant product, not desirable to be present in SAF, being an unsaturated compound. 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[15]. Cheaper non-noble metal catalysts demanded harsher reaction conditions leading to its quick deactivation[16] necessitating the development of highly active catalysts that work under mild conditions with longer stability. Heteropoly acid was reported to be green catalyst with high acidity and thus gained importance[17]. Heterophosphotungstic acid (HPW), H₃PW₁₂O₄₀ is the strongest among heteropoly acids having cage like structure known as Keggin structure which easily fits within the pores of mesoporous support[18]. HPW loaded on Ni-based hierarchical mesoporous zeolite Y catalyst[18] was reported to increase the hydrodeoxygenation of microalgae biodiesel to quality SAF. Ni-HPW/nano-hydroxyapatite[19] and Ni-HPW/Al₂O₃ catalyst[20] were reported to have high activity and selectivity for the hydroprocessing of Jatropha oil to green diesel.

In the present study, trans-anethole is selected as a model compound representing the lignin derived phenolic compounds and is a good source to synthesize propyl cyclohexane and propyl benzene desirable to be present in the 100% SAF. Further SBA-15 is chosen as a support, being mesoporous with good thermal stability. Bi-functional catalysts viz. Ni(5)-HPW(10)/SBA-15, Ni(10)-HPW(10)/SBA-15, Ni(15)-HPW(10)/SBA-15 along with 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, 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.

2.1 Materials

Tri-block copolymer (Pluronic P123, EO₂₀ PO₇₀ EO₂₀; M.W. 5800), tetraethyl orthosilicate (TEOS), trans-anethole, Nickel (II) nitrate hexahydrate and HPW were all Aldrich (98% pure), activated carbon (SRL) and hydrochloric acid (35%) was AR grade of Merck were used.

2.2 Catalyst synthesis

SBA-15 was synthesized using P123 tri-block copolymer (EO₂₀PO₇₀EO₂₀) as the structure directing agent, by following the reported procedure [21]. 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 4h. About 120 mL of 2M 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 24h at 40°C. The contents were transferred to stainless steel autoclave of 250 ml capacity and then kept in an oven for 48h at 100°C. The autoclave was removed from 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 6h to remove the occluded template. SBA-15 obtained was impregnated with Ni, HPW and both 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 1h. The required weight of nickel (II) nitrate hexahydrate was added to the mixture and the stirring continued for 6h. 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. To 1g 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. The content was stirred for 3h at 80°C, dried in the air-oven at 100°C for 6h, followed by calcination at 500°C for 3h to obtain the mono functional catalyst HPW(10)/SBA-15. Bi-functional catalysts viz. Ni(5)-HPW(10)/SBA-15, Ni(10)-HPW(10)/SBA-15, and Ni(15)-HPW(10)/SBA-15 were obtained by combining the Ni and HPW impregnation steps. Biomass Derived Activated Carbon (BDAC) support was synthesized by heating the husk of the seeds of Sterculia foetida at 800°C in an inert atmosphere using CVD for three hours. Graffiti Carbon Sphere (GCS) was synthesized by the reported procedure[22]. Activated carbon (AC) of SRL make was used as such. These three forms of carbon were impregnated with 10% of Ni and HPW each to get Ni(10)-HPW(10)/AC, Ni(10)-HPW(10)/BDAC and Ni(10)-HPW(10)/GCS.

2.3 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 3h 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 2h to remove the moisture and then FTIR spectra was recorded with Agilent Cary 630 in the range 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 BaSO₄ 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.

2.4 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 of length was used for the reaction. The catalyst (0.1 g) was placed on silica beads supported on cotton glass wool. It was activated at 500°C for 3h with hydrogen flow of 50 mL min^-1. Then HDO of trans-anethole was carried out over the catalyst at 300, 350 and 450°C with the flow rate of 1mL h^-1 and 50mL min^-1 of trans-anethole and hydrogen respectively for 12 h of time on stream (TOS). Bottom of the reactor was connected to a coiled water 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 x 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 equations (1), (2), (3), (4) and (5) respectively[23].

Conversion of trans-anethole (%) =\(\frac{ {\left(m\right({\alpha }-\text{t}\text{r}\text{a}\text{n}\text{s} \text{a}\text{n}\text{e}\text{t}\text{h}\text{o}\text{l}\text{e})}_{in}-{m({\alpha }-\text{t}\text{r}\text{a}\text{n}\text{s} \text{a}\text{n}\text{e}\text{t}\text{h}\text{o}\text{l}\text{e})}_{out}}{ {\left(m\right({\alpha }-\text{t}\text{r}\text{a}\text{n}\text{s} \text{a}\text{n}\text{e}\text{t}\text{h}\text{o}\text{l}\text{e})}_{in}}\)× 100\(\)(1)

Product selectivity % = \(\frac{Moles of specific product}{ Moles of all tℎe product }\)× 100\(\) (2)

HDO% = ∑ Product selectivity % of all deoxygenated products (3)

Specific Reaction Rate (r) = \(\frac{F \times conversion of trans-anetℎole}{W}\) (4)

Turn over frequency (TOF) = \(\frac{r}{Quantity of sites}\) (5)

Where F = Molar inflow rate of trans-anethole in mol s^− 1

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) to understand the extent of deactivation by coking with Perkin Elmer Diamond.

3.1 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 observation reported for Ni/SBA-15[24]. 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^o is attributed to amorphous silica of the SBA-15[25]. Three peaks observed at 2θ positions of 37, 43 and 63^o 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^o, characteristic of HPW[26] 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.

3.2 Nitrogen adsorption isotherms of catalysts

The physico-chemical and textural characteristics of the catalyst are presented in Table 1. All the SBA-15 supported

Table 1

Textural and physico-chemical characteristics of the SBA-15 support and catalysts
Sample	^a_BET [m²/g]	^bV_p [cc/g]	^cd_p [nm]	Ni & HPW Loading^d [wt %]		Metal crystallite size^e [nm]	Total acidity^f (mmol g − 1)	^ga₀ _(nm)	^hP_W _(nm)
Sample	^a_BET [m²/g]	^bV_p [cc/g]	^cd_p [nm]	Ni	HPW	Metal crystallite size^e [nm]	Total acidity^f (mmol g − 1)	^ga₀ _(nm)	^hP_W _(nm)
SBA-15	871.782	0.81	3.4	–	–	-	0.01	14.7	11.3
Ni(10)/SBA-15	768.002	0.73	3.4	9.90	–	4.82	-	11.9	8.5
HPW(10)/SBA-15	720.116	0.62	3.2	–	9.87	-	0.09	12.4	9.2
Ni(5)-HPW(10)/SBA-15	680.433	0.58	3.0	4.80	9.91	3.44	-	10.5	7.5
Ni(10)-HPW(10)/SBA-15	650.536	0.54	2.7	9.85	9.70	2.49	0.14	10.3	7.6
Ni(15)-HPW(10)/SBA-15	612.417	0.51	2.5	14.70	9.89	3.43	-	9.8	7.3

^aBET surface area calculated from the N₂ adsorption isotherm.

^btotal pore volume calculated from the N₂ adsorption branch using the BJH method

^caverage pore diameter calculated from the N₂ adsorption branch using the BJH method

^dactual Ni and HPW loading from ICP-OES analysis

^ecrystallite size of Nickel calculated from the wide angle XRD using Debey- Scherrer equation

^fvalue obtained from NH₃-TPD results

^gunit cell parameter obtained from XRD results

^hwall thickness of the sample estimated by the expression: P_w= a₀− d_p where (a₀ = 2d₁₀₀/√3)

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. From the N₂ adsorption isotherms (Fig. 2a), all the samples showed type IV(a) adsorption isotherm with hysteresis loop evidencing the characteristic of cage-like (Im3m) pore structure[27]. Absence of the sharp inflection at the relative pressure (p/p₀₎ 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.

3.3 NH₃‑TPD and FT-IR spectra of pyridine adsorbed catalysts

The NH₃-TPD profiles of SBA-15, HPW(10)/SBA-15, Ni(10)-HPW(10)/SBA-15 are shown in Fig. 3a. Two distinct peaks, first one up to 300^oC and second one from 350 to 650^oC were observed, indicated the presence of two distinct strength of acid sites of medium and strong respectively. TPD of ammonia increased in the order of SBA-15 < HPW(10)/SBA-15 < Ni(10)-HPW(10)/SBA-15, possibly due to the presence of both Brønsted acid sites of HPW and Lewis acid sites of Ni²⁺ in Ni(10)-HPW(10)/SBA-15. This was further evidenced from the FTIR spectra of pyridine adsorbed Ni(10)-HPW(10)/SBA-15 catalyst (Fig. 3b) which showed bands at 1590 and 1450 cm^-1, attributed to Brønsted and Lewis acid sites respectively[28].

3.4 FT-IR spectra of the support and catalysts

The FT-IR spectra of the support SBA-15 and Ni/SBA-15 and Ni,HPW/SBA-15 catalysts are presented in Fig. 4. A broad band at 3343 cm^− 1 is due to the defective Si-OH stretching vibration of SBA-15. The intense broad band observed over 1300 to 1000 cm^− 1 is due to the asymmetric and symmetric vibrations of Si–O–Si. Absorption bands at 810 and 436 cm^− 1 are due to the bending vibrations of Si–O–Si while the shoulder around 966 cm^− 1 is due to the stretching vibration of the defective Si–OH. Bulk HPW with a Keggin phase generally shows four strong peaks at 1084 cm^− 1 (P–O), 983 cm^− 1 (W–O), 895 cm^− 1, and 801 cm^− 1 (W–O–W) and a weak band at 524 cm^− 1 (W–O–P), were found to be absent for Ni (5,10,15%)-HPW(10)/SBA-15 samples, indicated the absence of Keggin phase thus reinforced the similar conclusion from the XRD results.

3.5 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 from 200 to 300 nm, attributed to oxygen to metal charge transfer in tungstophosphate anion[29]. Ni(10)/SBA-15 showed a sharp peak around 250 nm, attributed to charge transfer transition of NiO from O^2- to Ni²⁺ ions[30] and absorption increased with 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.

3.6 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, which may be possibly due to the interaction between silica wall of SBA-15, Ni and HPW. The EDAX spectrum (Fig. 6e) confirmed the presence of Ni, W, P, and Si, in the bi-functional catalyst Ni(10)-HPW(15)/SBA-15.

3.7 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 850–890 eV. The peaks centered at 853.31 and 858.9 eV were attributed to Ni^o and Ni²⁺of Ni 2p_3/2 respectively, while the peaks at 870.50 and 878.2 eV were attributed to Ni^o and Ni²⁺of Ni 2p_1/2 respectively, and the proposition of Ni⁰ and Ni²⁺ in the sample were similar[31]. The Si 2p has shown a characteristic peak at 104.1 eV, attributed to SiO₂ of SBA-15 support[32]. W 4f XPS spectrum showed a doublet at 33.57 and 35.80 eV assigned to W 4f_7/2, characteristic of the W⁶⁺ ion interacting and non-interacting with the support respectively[33].

3.8 Products and reaction pathways of catalytic HDO of trans-anethole

Products of HDO of trans-anethole were identified by GC-MS. Oxygenated products identified were 4-propyl anisole formed by the hydrogenation of propenyl group and, 4-propyl phenol formed by the hydrogenation of propenyl group and demethylation of methoxy group of the trans-anethole. Propyl benzene, propyl cyclohexane, 1-propenyl benzene, benzene and cyclohexane were the deoxygenated products identified. Two reaction pathways namely direct HDO and hydrogenation were reported for the HDO of lignin derived model compounds[11]. Direct HDO reportedly involved hydrogenolysis of C-O bond, yielding aromatics while hydrogenation involved hydrogenation of benzene ring followed by hydrogenolysis of C-O bond yielding cycloalkanes. Thus formation of propyl benzene is accounted by the direct HDO of C-O bond of the methoxy group and hydrogenation of propenyl group of trans-anethole while 1-propenyl benzene is due to the occurrence of the first step and the absence of second step. Propyl cyclohexane formation is by hydrogenation pathway involving the hydrogenation of propenyl group and benzene ring followed by hydrogenolysis of C-O bond of the methoxy group of trans-anethole. Absence of C₉ alkanes or isoalkanes in the product indicated that the hydrogenation did not proceed to the extent of opening up of the cyclohexane ring, unlike reported for HDO of eugenol over Ni supported on HZSM-5 and combined Al-SBA-15 and HZSM-5 supports[34]. This indicated the efficacy of the present catalyst system in producing and preserving the propyl cyclohexane desired to be present in the 100%SAF. Benzene and cyclohexane observed in the product are due to the consecutive dealkylation of propyl benzene and propyl cyclohexane respectively.

3.9 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. 8a and 8b 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, while 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. 9), it was found that weight loss was minimum for Ni(10)-HPW(10)/SBA-15, indicated less coke deposition and hence less deactivation accounted for stable conversion over the TOS of 12 h. On the other hand Ni(5)-HPW(10)/SBA-15 has shown the highest weight loss due to higher coke deposition and thus accounted for its faster deactivation. Selectivity for the hydrodeoxygenated products (HDO%) was found to be the highest for HPW(10)/SBA-15 at TOS of 2 h and declined sharply with the increase in TOS and reached the lowest at 8 h, among the five catalysts studied. Another mono-functional catalyst Ni(10)/SBA-15 has also shown a sharp decline in HDO% from 2 to 4 h of TOS beyond which, fluctuating trend was observed. In contrast, for the bi-functional catalysts viz. Ni(5)-HPW(10)/SBA-15, Ni(10)-HPW(10)/SBA-15, Ni(15)-HPW(10)/SBA-15, HDO% increased with the increase in TOS from 2 to 4 h and further increase in TOS has not resulted any significant change in HDO%, except for Ni(5%)-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. 10. HPW (10)/SBA-15 has given exclusively benzene and cyclohexane as HDO products, because of the dealkylation of propyl benzene and propyl cyclohexane. Selectivity for cyclohexane was found to be higher than benzene up to the TOS of 2 h and with further increase up to 12 h, selectivity for benzene became higher than the cyclohexane. This suggested that there may be two distinct active centers of HPW, one facilitated hydrogenation and other direct HDO pathway, of which the former get selectively deactivated with TOS. The only oxygenated product obtained was propyl phenol indicated the strong demethylation activity of HPW. The other mono-functional catalyst Ni(10)/SBA-15 yielded both the propyl anisole and propyl phenol as oxygenated products and the selectivity of former was higher than later, thus Ni based catalyst has less demethylation activity compared to HPW based catalyst. Benzene and cyclohexane alone obtained as HDO products by Ni(10%)/SBA-15 catalyst as well, but the selectivity for cyclohexane was much lower than benzene and did not change much with TOS. This suggested that the hydrogenation mechanistic pathway was less prevalent compared to HPW(10)/SBA-15, but at the same time, the active centers facilitated the hydrogenation were not selectively deactivated with TOS unlike over HPW(10)/SBA-15 catalyst. Exclusive formation of benzene and cyclohexane by both the mono-functional catalysts which are shorter in carbon length to be part of SAF, but valuable petrochemicals indicated that mono-functional catalysts are not useful for SAF production, but useful for the sustainable production of petrochemicals. All the five HDO products and two oxygenated products were obtained over the three bi-functional catalysts Ni(5, 10, 15)-HPW(10)/SBA-15. Selectivity for C₉ compounds including propyl cyclohexane, the most desired HDO product because of being naphthenic and in the SAF range, did not change much with TOS for Ni(5)-HPW(10)/SBA-15 and Ni(10)-HPW(10)/SBA-15 whereas significantly changed for Ni(15)-HPW(10)/SBA-15 and the selectivity is the highest for Ni(10)-HPW(10)/SBA-15 at all TOS. 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, HDO selectivity and SAF components.

3.10 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^o C are presented in Fig. 11a. Increase in Ni% from 0 to 10% increased the conversion and then decreased with further increase to 15%. HDO% did not change much from 0 to 5% of Ni, but increased sharply with an increase to 10% and then decreased with further increase to 15% and hence concluded that 10% is the optimum Ni loading for HDO. HPW(10)/SBA-15, a purely acidic catalyst having 0% Ni, yielded benzene and cyclohexane exclusively as HDO products and propyl phenol as the only oxygenated product (Fig. 11b). With the addition of 5% Ni i.e. Ni(5)-HPW(10)/SBA-15, a bi-functional catalyst yielded C₉ compounds of SAF range viz. propenyl benzene, propyl benzene and propyl cyclohexane as HDO products and both propyl anisole and propyl phenol as oxygenated products. Increase of Ni from 5 to 10% increased the selectivity for propyl cyclohexane from 1 to 11%, selectivity for C₉ compounds from 11 to 35% and further increase to 15% decreased the selectivity for propyl cyclohexane and C₉ compounds to 3 and 21% respectively. Thus it may be concluded that Ni(10)-HPW(10)/SBA-15 is the best catalyst among the five SBA-15 supported catalysts studied.

3.5 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 viz. activated carbon, biomass derived activated carbon, and graphitic carbon sphere on the HDO of trans-anethole were studied in lieu of SBA-15. For the various catalysts, specific reaction rate (SRR), turnover frequency (TOF), conversion% and selectivity for HDO (HDO%) are presented in Table 2.

Table 2

Catalytic activity for HDO of trans anethole over different catalysts at 400°C
Catalysts	SRR (mol g-1 s-1)	TOF (s-1)	Conversion%	HDO%
Ni(5)-HPW(10)/SBA-15	0.951x10^− 5	0.117x10^− 1	51.45	35
Ni(10)-HPW(10)/SBA-15	1.64x10^− 5	8.90x10^− 4	81.86	79.5
Ni(15)-HPW(10)/SBA-15	1.347x10^− 5	5.27x10^− 4	72.7	58.5
Ni(10)/SBA-15	0.95x10^− 5	5.59x10^− 3	51.39	37.7
HPW(10)/SBA-15	0.592x10^− 5	0.17x10⁶	32.14	38.7
Ni(10%)-HPW(10%)/AC	14.5x10^− 6	8.52x10^− 3	78.76	6.57
Ni(10%)-HPW(10%)/ BDAC	17.9x10^− 6	10.52x10^− 3	97.94	15.88
Ni(10%)-HPW(10%)/ GCS	12.4x10^− 6	7.29x10^− 3	67.46	12.17

Among all the catalysts studied Ni(10)-HPW(10)/BDAC gave the highest conversion of 97.94% and SRR of 1.79 x 10^− 5 mol g^− 1 s^− 1, but its HDO% was only 15.88%. Ni(10)-HPW(10)/AC has shown the HDO% of 6.57%, the lowest among all the catalysts studied and for Ni(10)-HPW(10)/GCS, HDO% was 12.17%. 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 x 10^− 5, suggested that SBA-15 is a better support for HDO of trans-anethole compared to all the three carbon supports. This reinforces the conclusion of HRTEM that there is a synergetic interaction between silica wall of SBA-15, Ni and HPW which may be the reason for higher HDO%.

Though there are no studies reported on HDO of trans anethole using fixed bed plug flow reactor, HDO of lignin derived phenolic compounds in octane solvent was studied in a batch reactor over MoO₃ catalyst at 340^o C and 3.5 MPa (3 MPa N₂ + 0.5 MPa H₂) for 6 h and reported 93.6% conversion for trans anethole with selectivity of 75.8, 8.3 and 1.2% for the propenyl benzene, propyl benzene, and propyl cyclohexane respectively while 12.9 and 1.7% for propenyl phenol and propyl phenol respectively[14]. Zhang et al[35] reported the HDO of mixture of lignin derived phenolic compounds in dodecane solvent over Ni/SiO₂–ZrO₂ catalysts at 300^oC and H₂ pressure of 5 MPa in a batch reactor for 8 h and got 100% conversion of trans anethole with 98.9% selectivity for propyl cyclohexane. Whereas in the present study Ni(10)-HPW(10)/SBA-15 catalyst which has given 81.86% conversion with 79.5% selectivity for HDO at 400^oC, 0.1 MPa of H₂ and residence time of just 0.1012 h and hence it may be concluded that Ni(10)-HPW(10)/SBA-15 is a much better catalyst for HDO of trans anethole.

3.6 Effect of Temperature on the HDO of trans anethole

Effect of temperature on the average conversion of trans-anethole and selectivity for HDO (HDO%) for the five SBA-15 supported catalysts are presented in the Fig. 12. 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^oC. 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^oC, decreased with an increase in temperature to 400^oC and then increased with further increase in temperature to 450^oC. The selectivity for HDO (HDO%) increased from 350 to 400^oC and decreased with further increase to 450^oC for all the catalysts, except Ni(5)-HPW(10)/SBA-15 whose average HDO% was not much affected by temperature.

Increase in temperature markedly affected the selectivity of both the oxygenated and HDO products which varied with the nature of catalysts (Fig. 13). For Ni(10)/SBA-15, propyl anisole was the major oxygenated product at all temperatures though the selectivity for propyl phenol increased with increase in temperature from 350 to 450^oC. At 350 and 400^oC propenyl benzene and benzene were obtained as HDO products and only at 450^oC additionally propyl benzene and cyclohexane were obtained indicated that the activation energy for hydrogenation is 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 exclusive selectivity for propyl phenol among the oxygenated product at 350 and 400^oC and only at 450^oC propyl anisole was also found as the oxygenated product, indicated its stronger demethylation ability. Benzene and cyclohexane are the exclusive HDO products at 350 and 400^oC and the selectivity for the cyclohexane increased with temperature indicated that surprisingly it has better hydrogenation ability than Ni(10)/SBA-15. Among the bi-functional catalysts, for Ni(5)-HPW(10)/SBA-15, average selectivity for cyclohexane decreased with increase in temperature while the selectivity for propenyl and propyl benzene increased from 350 to 400 and then decreased with further increase to 450^oC. 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^oC, and then decreased with further increase to 450^oC. Similar trend was shown by the Ni(15)-HPW(10)/SBA-15 catalyst and hence it could be concluded that 400^oC was the optimum temperature.

Mono and bi-functional catalysts of Ni and HPW of varying composition, supported on SBA-15 were prepared, characterized and evaluated for the HDO trans-anethole. Structure of SBA-15 was not altered, but the surface area and pore volume decreased marginally by the incorporation of Ni and HPW. 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 Ni²⁺ whose presence was confirmed by both UV-Visible DRS and XPS. Bi-functional catalysts were found to have stable 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 SAF the components viz. propyl benzene, propyl cyclohexane. Optimum temperature for the bi-functional catalysts was found to be 400^oC for the maximum conversion and HDO selectivity. Among the carbon supports viz. activated carbon, biomass derived activated carbon, and graphitic carbon sphere impregnated with Ni and HPW 10% each, biomass derived activated carbon showed the highest conversion of 97.94% but all of them showed lower HDO selectivity. Hence interaction between the silica wall of SBA-15, Ni and HPW was concluded to be the reason for its higher HDO selectivity, thus proven to be the better support for HDO of trans-anethole.

I declare that the authors have no competing or financial interests that might be perceived to influence the results and discussion reported in this paper.

Acknowledgements

One of the authors, Siva Kannan, thanks Centre for Research, Anna University, Chennai, India for providing Anna Centenary Research Fellowship. The facilities provided by IIT Madras and CLRI are gratefully acknowledged.

Author contributions Siva kannan carried out the work as Junior Research Fellow and wrote the first draft Gopalakrishnan Govindasamy helped in correcting and editing the paper Pandurangan Arumugam contributed to the research idea, guidance during the experimental work, performed analysis and wrote the paper.

Conflict of interest The authors declare that they have no conflict of interest.

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You are reading this latest preprint version

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

Status:

Version 1

Abstract

Figures

1 Introduction

2 Experimental

2.1 Materials

2.2 Catalyst synthesis

2.3 Characterization of Catalyst

2.4 Catalytic studies on HDO of trans-anethole

3 Results And Discussion

3.1 XRD characterization of catalysts

3.2 Nitrogen adsorption isotherms of catalysts

3.3 NH₃‑TPD and FT-IR spectra of pyridine adsorbed catalysts

3.4 FT-IR spectra of the support and catalysts

3.5 UV-visible diffuse reflectance spectra of the catalysts

3.6 HRTEM characterization of catalysts

3.7 X-ray photoelectron spectra (XPS) of the catalysts

3.8 Products and reaction pathways of catalytic HDO of trans-anethole

3.9 Effect of Time on Stream on the HDO of trans-anethole

3.10 Effect of Ni loading into HPW(10)/SBA-15 on the HDO of trans-anethole

3.5 Effect of carbon support on the HDO of trans anethole

3.6 Effect of Temperature on the HDO of trans anethole

4. Conclusions

Declarations

References

Additional Declarations

Status:

Version 1

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

Status:

Version 1

Abstract

Figures

1 Introduction

2 Experimental

2.1 Materials

2.2 Catalyst synthesis

2.3 Characterization of Catalyst

2.4 Catalytic studies on HDO of trans-anethole

3 Results And Discussion

3.1 XRD characterization of catalysts

3.2 Nitrogen adsorption isotherms of catalysts

3.3 NH3‑TPD and FT-IR spectra of pyridine adsorbed catalysts

3.4 FT-IR spectra of the support and catalysts

3.5 UV-visible diffuse reflectance spectra of the catalysts

3.6 HRTEM characterization of catalysts

3.7 X-ray photoelectron spectra (XPS) of the catalysts

3.8 Products and reaction pathways of catalytic HDO of trans-anethole

3.9 Effect of Time on Stream on the HDO of trans-anethole

3.10 Effect of Ni loading into HPW(10)/SBA-15 on the HDO of trans-anethole

3.5 Effect of carbon support on the HDO of trans anethole

3.6 Effect of Temperature on the HDO of trans anethole

4. Conclusions

Declarations

References

Additional Declarations

Status:

Version 1

3.3 NH₃‑TPD and FT-IR spectra of pyridine adsorbed catalysts