Direct selective hydrogenation of fatty acids and Jatropha oil to fatty

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Biofuels and Biomass

Direct selective hydrogenation of fatty acids and Jatropha oil to fatty alcohols over cobalt-based catalysts in water Wenda Jia, Guangyue Xu, Xiaohao Liu, Feng Zhou, Huixia Ma, Ying Zhang, and Yao Fu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01114 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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Direct selective hydrogenation of fatty acids and Jatropha oil to fatty alcohols over cobalt-based catalysts in water Wenda Jiaa, Guangyue Xua, Xiaohao Liua, Feng Zhoub, Huixia Mab, Ying Zhang a,*, Yao Fua a

iChEM, CAS Key Laboratory of Urban Pollutant Conversion, Anhui Province Key

Laboratory for Biomass Clean Energy and Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China. b

Fushun Research Institute of Petroleum and Petrochemicals, SINOPEC, Fushun 113001, Liaoning, China

ABSTRACT: Inedible natural oils are desired resources for renewable fuel and chemical production. Herein, a non-precious metal cobalt catalytic system was developed for selectively hydrogenating fatty acids and natural oil into fatty alcohols or long-chain alkanes. The cobalt-based catalysts were prepared by wet-impregnation method with a series of supports including HZSM-5, CeO2, ZrO2, SiO2, Al2O3, TiO2 and hydroxyapatite (HAP) for hydrogenating stearic acid. Among these catalysts, Co/HAP exhibited the highest activity and 97.1% yield of 1-octadecanol was obtained at 190 °C and 4 MPa H2 in water. Additionally, the Co/HAP was capable of directly hydrogenating the natural oil, Jatropha oil, to fatty alcohols without any preprocessing and 83.1 wt% yield of alcohols could be achieved at 190 °C and 4 MPa H2 in water. Co/HAP could also catalyze the complete conversion of stearic acid and Jatropha oil to

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long-chain alkanes when dodecane was used as solvent. TEM, XRD, H2-TPR and NH3TPD were carried out, and the high catalytic activity of Co/HAP could be due to its desired acidity, cobalt particle dispersion, and stronger metal-support interaction. The FT-IR results indicated the high efficiency of Co/HAP could also be due to the absorption of fatty acid on the surface of catalyst which thus promoted the hydrogenation process over Co species. The possible reaction pathway was also proposed according to the conversion process tracking of stearic acid.

KEYWORDS: biomass, hydrogenation, fatty acids, fatty alcohols, cobalt, catalysis

INTRODUCTION Inedible renewable oils in the form of triglycerides are one of the promising types of bio-based feedstock for producing biodiesel and high-valued chemicals.1,2 The conversion process of fatty acids and inedible natural oils to first and second generation of biodiesel has been widely studied. However, under current low petroleum price, it is hard to compete with petrochemical process. To achieve the profit margin, development of a flexible process which can selectively convert natural oils and fatty acids to higher fatty alcohols or long-chain alkanes, major component of diesel fuel, should be a good choice. Higher fatty alcohols are important economic products in natural oils conversion process which could be as intermediates to produce lubricants, health care products, and cosmetics.3,4 Meanwhile, high fatty alcohols can also be further converted into long-chain alkanes via hydrodeoxygenation process.5-7 Recently, the world market value of fatty alcohols has reached about $1.9 billion and with an estimated growth rate


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of 7.3%/a.8 With the increasing demand and expanding market for global green chemicals, the fatty alcohols market is expected to experience significant growth in the near future.9,10

In the earlier researches, the copper-chromite salts were mainly used as catalysts for preparing fatty alcohols from natural fatty acids or esters,11-13 which exhibited good selectivity (>90%) toward alcohols at temperatures of 200-400 °C and hydrogen pressures of 20-30 MPa. Nevertheless, environmental risk due to the use of chromite and drastic reaction conditions make it crucial to develop eco-friendly catalysts for the production of fatty alcohols under mild conditions.

Over the past two decades, both homogeneous and heterogeneous systems have been developed devoting to direct hydrogenation of fatty acids to synthesize fatty alcohols. Homogeneous

catalysts

included

Co(BF4)2·6H2O,14

Ru(acac)3/Triphos,15

[Ru(Triphos)(TMM)],16 Cu(OTf)2/TMDS ,17 and so on. However, certain deficiencies in the application process (e.g. catalysts/products separation, cost of ligand, etc.) make the development of heterogeneous catalysts particularly important. Various precious metal catalysts have been applied in the hydrogenation of fatty acids to fatty alcohols, such as Ru,18-20 Pd,21-24 Pt,25 and Re.26-29 Especially, Pt/TiO2 catalyst realized acid to alcohol with 82% conversion and 93% selectivity at remarkable mild reaction conditions (130 °C and 2 MPa H2).25 In comparison with precious metal catalysts, only a few researches on heterogeneous non-precious metal catalysts in directly hydrogenating fatty acids into alcohols have been reported due to generally the low activity of non-precious metal on the carboxyl group conversion.30,31 Slowing et al.32


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reported Cu and Fe oxides supported on mesoporous silica nanoparticles (MSN) for hydrogenating stearic acid into 1-octadecanol. Under moderate conditions (180 °C，3 MPa H2), the stearic acid conversion was up to 100% with 95% yield of 1-octadecanol, but the preparation of catalyst is relatively complex. Afterwards, a Cu/SiO2 catalyst was developed by Zhao et al.33 for converting fatty esters (acids) and coconut oil to fatty alcohols at 240 °C in methanol without extraneous hydrogen, and 85% of conversion and 100% of selectivity were achieved, respectively. Recently, Li et al.34 reported a partially reduced cobalt oxides catalyst for hydrogenation of carboxylic acids to corresponding alcohols in alkane with good substrate adaptability. Currently, most of the processes of fatty acids hydrogenation to alcohols were performed in organic solvents derived from fossil resources, which require product/solvent separation and may cause the environmental impacts. Therefore, the development of simple, green and efficient non-precious catalyst systems in water is desirable for the selective hydrogenation of fatty acid to corresponding alcohol.

Cobalt is earth abundant and has moderate hydrogenation activity which results in high performance in selective catalytic hydrogenation of biomass derivatives.35-39 Therefore, we synthesized different cobalt-based catalysts by a simple wet impregnation method to investigate the selective hydrogenation process of stearic acid under mild conditions. The catalysts were well characterized by different techniques of X-ray power diffraction (XRD), transmission electron microscopy (TEM), H2 temperature-programmed

reduction

(H2-TPR),

NH3

temperature-programmed

desorption (NH3-TPD), Fourier transform infrared spectroscopy (FT-IR), and


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inductively coupled plasma-optical emission spectrometer (ICP-OES). The effect of reaction parameters (temperature and H2 pressure) was investigated and a possible reaction pathway for stearic acid to 1-octadecanol was also investigated via time-course experiments. Additionally, the direct one step hydrogenation of Jatropha oil, an inedible plant oil, to fatty alcohols or long-chain alkanes was tested. The stability of the catalyst was also studied.

EXPERIMENT SECTION

Materials Stearic acid (>99.0%, AR), C12-C18 alkanes (≥99.5%, AR), 1-octanol (>99.0%, AR), 1-hexadecanol (>99.0%, AR), 1-octadecanol (>99.0%, AR), hydroxylapatite (HAP, ≥ 97.0%)，SiO2 (Aerogel) and Al2O3 were provided by Shanghai Aladdin Bio-Chem Co., Ltd. Co(NO3)2·6H2O (AR), ethyl acetate (AR), hexane (AR), acetone (AR), ZrO2 and CeO2 were provided by Sinopharm Chemical Reagent Co., Ltd. TiO2 (Rutile) was provided by Sigma-Aldrich Co., Ltd. HZSM-5 (Si/Al=25) was provided by Tianjin Nanhua Catalyst Co., Ltd. Crude Jatropha oil was obtained from Yunnan Shenyu New Energy Co., Ltd. N2, H2, Ar, NH3 and H2/Ar mixture gas were provided by Jiangsu Shangyuan Special Gas Co., Ltd. The above materials obtained from commercial purchase were used directly without further pretreatment.

Catalyst Preparation

The different cobalt-based catalysts were prepared by the same wet-impregnation method and the content of cobalt metal on different supports was about 10 wt%. For


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preparing Co/HAP catalyst, 1.00g of HAP was put in a 250 mL round-bottom flask containing 110 mL acetone with vigorous magnetic stirring maintaining at 45 °C. 54.8 mg of Co(NO3)2·6H2O was put in a 25 mL beaker using 10 mL acetone for dissolution and then the Co(NO3)2/acetone solution was dropwise added to the above 250 mL round-bottom flask within 0.5 h, keeping vigorously stirring for 24 h. After impregnation, the acetone in the flask was evaporated by rotary evaporation with a vacuum pump and the residue was placed in an oven at 105 °C for overnight drying. Thereafter, the dried residue was calcined in a still air of muffle furnace at 600 °C for 2 h with 5 °C·min-1 of heating rate to obtain the as-calcined catalyst. The as-calcined catalyst was reduced in a tube furnace by high-purity H2 (100 mL·min-1) at 600 °C for 2 h with 1 °C·min-1 of heating rate to obtain the as-reduced catalyst, and then the asreduced catalyst was sealed in solvent for test.

Catalyst Characterization

The transmission electron microscopy (TEM) images of as-reduced cobalt-based catalysts were collected by a JEOL-2010 electron microscope. The catalysts were sonicated in ethanol for a better dispersion before the sampling.

The X-ray power diffraction (XRD) patterns of as-reduced cobalt-based catalysts were recorded by an X-ray (TTR-III) diffractometer with a CuKα radiation. The recording areas of 2θ were set from 10°to 70°.

The hydrogen temperature-programmed reduction (H2-TPR) analyses of as-calcined cobalt-based catalysts were performed on a self-made device coupled to a gas


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chromatograph (GC) with thermal conductivity detector (TCD).The as-calcined catalysts were tested for H2-TPR after preprocessing in a stream of argon at 500 °C for 1 h. The temperature-programmed process under 5% H2/Ar was conducted from 40 °C to 800 °C (10 °C/min). The cold trap was employed for removing the moisture.

The ammonia temperature-programmed desorption (NH3-TPD) analyses of asreduced cobalt-based catalysts were also performed on the above self-made device with the same preprocessing. The adsorption of catalysts in ammonia stream and the removal of ammonia adsorbed physically on the catalysts surface in argon stream were conducted at 40 °C for 1 h, respectively. The temperature-programmed process under argon stream was conducted from 40 °C to 600 °C (10 °C/min).

Fourier transform infrared (FT-IR) spectra was obtained using a Nicolet 8700 spectrograph. The as-reduced Co/HAP was added in a certain concentration of stearic acid/hexane solution with 24 h of vigorous stirring at room temperature (RT). The treated Co/HAP catalyst was tested after centrifugal separation with at least 20 washes in hexane and drying for 24 h in a stream of nitrogen at RT.

The elemental composition of catalysts were measured using an inductively coupled plasma-optical emission spectrometer (ICP-OES) of Optima 7300 DV.

Catalyst Test

The hydrogenation of stearic acid and Jatropha oil was performed in a 25 mL batch autoclave with a magnetic stirrer and a temperature controller. Typically, 0.2 mmol of stearic acid (or 60 mg of Jatropha oil), 10 mL of water (or dodecane), 50 mg of 10 wt%


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Co/HAP and stirring magneton were added into the reactor. The reactor was aerated to the set H2 pressure after gas replacement by H2 at least 6 times and then was heated to the set temperature for reaction. After reaction finished, cool the reactor with ice water to RT. The organics were fully extracted from the reaction mixture by ethyl acetate. The qualitative and quantitative analyses were performed using a gas chromatographmass spectrometer combination (GC-MS, Agilent 5975C, HP-5 column) and a gas chromatograph (GC, kexiao1690, HP-5 column), respectively. The internal standard method was used for quantification and 1-octanol was used as internal standard. The quantitative analyses of reactants and products were based on the established standard curve. The detecting conditions were as follows: high-purity N2 was used to carrier gas; both the injection port and detector (FID) temperature were set at 320 °C; the temperature of column oven was raised from 120 °C to 250 °C (30 °C/min). The gas collected after reaction was identified using gas chromatograph (GC, lunanSP6890) of thermal conductivity detector (TCD) and flame ionization detector (FID), respectively.

Each of experiment was repeated for at least three times. The data obtained were the average of repeated experiments and the errors were less than 2%. The data in tables and figures were expressed as the mean ±standard deviation.

Using stearic acid as initial reactant, the conversion of stearic acid and the yield of products were calculated by mol%.

 n(stearic acid after reaction)  Conv. (%)= 1 100%  n(stearic acid in initial reactant)  n(each product) Yield (%)= 100% n(stearic acid in initial reactant)


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Using Jatropha oil as initial reactant, the yield of products were calculated by wt%.

Yield (wt%)=

m(each product) 100% m(Jatropha oil in the initial reactant)

RESULTS AND DISCUSSION

Hydrogenation of stearic acid to 1-octadecanol

The hydrogenation of stearic acid over cobalt-based catalysts with a series of supports (HZSM-5, CeO2, ZrO2, SiO2, Al2O3, TiO2 and HAP) were carried out at 200 °C with 4 MPa H2 in water to explore the catalysts activities. As shown in Table 1, cobalt-based catalysts showed different catalytic activities. The hydrogenated products were 1-octadecanol, heptadecane, octadecane and stearyl stearate and 1-octadecanol was the dominated product for all the catalysts. The activity of Co/HZSM-5 was very low and the yield of 1-octadecanol was only 3.3%, together with a main by-product, 1.1% of stearyl stearate. The Co/HZSM-5 showed poor hydrothermal stability and the division of metal and support could be observed after reaction. When CeO2 was employed as the support, the catalyst exhibited relatively low activity. Only 19.0% of stearic acid was converted with 18.3% of 1-octadecanol yield. Co/ZrO2, Co/SiO2, Co/Al2O3 and Co/TiO2 showed moderate catalytic activity. The yields of 1-octadecanol for these catalysts were from 51.8% to 66.1%, with trace amount C17 and C18 alkanes and stearyl stearate. Among thess catalysts, Co/HAP showed the best catalytic performance. The conversion of stearic acid was 100% and the yield of 1-octadecanol reached to 95.2% at 200 °C. The unreduced Co/HAP was tested for hydrogenation of stearic acid under the same conditions. The conversion of stearic acid and the yield of


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1-octadecanol were 12.2% and 12.1%, respectively. HAP barely showed catalytic activity. Since Li’s group reported that the partially reduced Co oxides could catalyze stearic acid hydrogenation,34 the unreduced Co/HAP catalyst could be partially reduced under the liquid phase hydrogenation condition.

If dodecane was selected as reaction solvent, stearic acid was mostly converted to alkanes instead of fatty alcohols over Co/HAP and the 1-octadecanol should be the key intermediate for alkanes. The details could be seen in Table S1. Compared with water, dodecane can promote the 1-octadecanol conversion to alkanes. It could be related to the different adsorption strength between the catalyst and 1-octadecanol caused by solvent effect.

Characterization of the catalysts The physical and chemical properties of the catalysts were characterized by different techniques. The TEM were employed to characterize the morphology of the cobalt catalysts (Figure 1, a-g). The size and distribution of cobalt particles were different because of the different natures of the supports. Except for CeO2 and ZrO2, cobalt particles showed better dispersion on the other supports, especially for SiO2 and Al2O3 which could be restricted by the smaller size of the supports.

The XRD patterns of the as-reduced cobalt catalysts were shown in Figure 2. For Co/Al2O3, except for the diffraction peaks of Al2O3, no diffraction peaks of cobalt were observed, indicating the Co particles could be well dispersed on the support, which was consistent with the TEM image. In addition to Co/Al2O3, the small diffraction peaks


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around 44.2°were observed for all the other cobalt catalysts, which could be assigned to (111) plane of Co0 (JCPDS database). Other diffraction peaks of these catalysts in Figure 2 came from the corresponding supports and no diffraction peak of cobalt oxides was found. If there were any Co oxides, they should be well dispersed on the catalysts surface.

The H2 temperature programmed reduction experiments were carried out to study the reduction behaviors and metal-support interactions of different as-calcined cobalt catalysts. Conventionally, the H2-TPR profiles of Co catalysts could show two reduction peaks, which were attributed to the two-step reduction process of Co3O4 (Co3O4 to CoO and then CoO to Co0).40 As shown in Figure 3, different supported Co catalysts exhibited different reducibility due to the different interaction between cobalt phase and supports. Based on the H2-TPR measurements, HZSM-5, CeO2, ZrO2 and SiO2 supported catalysts showed two reduction peaks in the similar reduction temperature ranged from 260 °C to 500 °C, and the first reduction peak appeared at 320-360 °C with similar intensity. The main reduction peak of Co/Al2O3 appeared above 500 °C. The emerging reduction peak at above 650 °C might result from the reduction of the cobalt aluminate species.41 The two obvious peaks from Co/TiO2 were observed in the temperature range of 350 °C to 600 °C with the first reduction peak at 420 °C. For Co/HAP, the first peak at 330 °C related to the reduction of Co3O4 to CoO, while the reduction peak at the wide range of temperature between 400 °C and 700 °C could be the overlap of the reduction of CoO on the HAP surface and the reduction of Co2+ in the HAP structure by ion exchange.42 Since higher reduction temperature


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indicated stronger interaction between metal and support, the interaction between metal and support of Co/Al2O3, Co/TiO2 and Co/HAP was stronger than that of other cobaltbased catalysts.

The surface acidic properties of as-reduced cobalt catalysts were determined by NH3TPD and the desorption profiles were shown in Figure 4. From the NH3-TPD curves, all of these catalysts exhibit two sets of broad desorption peaks. Desorption peak in the temperature range of 40-200 °C was attributed to the weak acid sites and that above 350 °C was associated with the strong acid sites. The Co/HZSM-5 exhibited the largest surface adsorption peaks, as well as adsorption bonding strength. The CeO2 and ZrO2 had similar surface weak acid sites while the strong acid sites of Co/ZrO2 was much less than those of Co/CeO2. For the Co/SiO2, Co/Al2O3, Co/TiO2 and Co/HAP, the acid sites were gradually decreased.

All the catalysts had a certain catalytic effects in the reaction but the catalytic activity was different, which could be influenced by many factors. The low stability of Co/HZSM-5 in hydrothermal reaction could cause the lowest activity. From the TEM images, the cobalt particles on Co/CeO2 and Co/ZrO2 showed poor dispersion, and the catalytic activity of them was lower than that of other catalysts. The size and distribution of metal particles of catalysts could affect the activity of catalysts. Furthermore, the interaction between metal and support has influence on the catalytic activity, and the stronger metal-support interaction could enhance the catalytic activity of the catalysts in selective hydrogenation reaction.19,43,44 From the results of H2-TPR and the catalytic activity of cobalt catalysts, the catalysts which had the stronger metal-


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support interaction showed better catalytic activity. This result was consistent with previous reports, indicating stronger metal-support interaction may also contribute to the hydrogenation activity of catalysts. Based on the NH3-TPD profiles and the catalytic activity of cobalt-based catalysts, with decrease of the surface acidity, the catalytic performance of cobalt-based catalysts was improved. The acidity of these catalysts could also affect the activity of catalysts. Among these catalysts, Co/HAP exhibited best catalytic activity, which could be due to its desired acidity, cobalt particles dispersion, and stronger metal-support interaction.

According to previous research work, HAP has adsorption abilities for stearic acid.45,46 The high activity of the Co/HAP catalyst in the conversion of stearic acid to 1-octadecanol could also be influenced by the important mutual effect between the catalyst and substrate. To investigate the interaction between the Co/HAP and the stearic acid, the Co/HAP modified with stearic acid was analyzed by FT-IR and the results were presented in Figure 5. The characteristic peaks in black FT-IR spectrum clearly showed the vibrational bands of stearic acid. The peak at 2955 cm-1 was assigned to the C-H stretching vibration of -CH3. The peaks at 2920 cm-1 and 2850 cm-1 were assigned to C-H stretching vibration of -CH2. The peak at 1470 cm-1 was belonged the deformation vibration of -CH2 group and the peak at 719 cm-1 was belonged to the C-C skeletal vibration in alkyl chain. The peaks at 1705 and 941 cm-1 were corresponded respectively to the characteristic vibration of -COOH and its dimer.47,48 For the Co/HAP, the characteristic peaks of the hydroxyl and the phosphate species in HAP phase were observed at 3571, 1095, 1034, 963, 631, 604, 566 and 474 cm-1. The peaks at 3571 and


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631 cm-1 were belonged to the stretching vibration and librational vibration of -OH, respectively. The peaks at 963 cm-1 (ν1), 474 cm-1 (ν2), 1095 cm-1 (ν3), 1034 cm-1 (ν3) and 604 cm-1 (ν4), 566 cm-1 (ν4) were ascribed to the vibration of PO43-.49-51 After treating the surface of Co/HAP with stearic acid, some new absorption bands were observed at the roughly same position of the alkyl chain in stearic acid, but the peaks of carboxyl group disappeared. It could be ruled out as a physical adsorption of stearic acid. It was noteworthy that a new peak appeared at 1552 cm-1. It corresponded to the two equivalent carbon-oxygen bond stretching vibration of bidentate carboxylate coordinated on the Co/HAP surface, which maybe originated from the interaction between the Ca2+ ions from HAP structure and the carboxyl groups from stearic acid. However, this carboxylate species was not the form of calcium stearate. Therefore, the above evidences revealed the chemisorption of stearic acid interacted with the Co/HAP surface by means of the carboxylate formation, which was consistent with the adsorption of stearic acid on pure HAP in our previous study.52 On the other hand, water as the reaction medium probably facilitated the ionization of carboxyl group and then would result in the electrostatic interaction between carboxylate anions and calcium cations of the surface of Co/HAP. The possible catalytic mechanism was proposed, which could be seen in Figure 6. In this aqueous phase hydrogenation reaction, stearic acid could be adsorbed on the surface of Co/HAP at the beginning. Then, as the hydrogenation active centers, the Co species of the catalyst surface could activate the hydrogen and subsequently hydrogenate the adsorbed stearic acid to 1-octadecanal,


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which could be hydrogenated to 1-octadecanol immediately. It seems to be a synergistic effect between metal and support.

Meanwhile, HAP can help disperse the active species and has appropriate acid-base properties.53-55 Abundant hydroxyl groups on the surface of HAP enable it to better dispersion in aqueous phase, and nonporous structure of HAP makes it easier for mass transfer in the reaction.52,56 All of the factors could contribute jointly to the high catalytic performance of Co/HAP for the selective production of 1-octadecanol from stearic acid. Therefore, Co/HAP was selected to further explore the influence of various reaction parameters on the selective hydrogenation of stearic acid to 1-octadecanol.

Effect of reaction temperature

Temperature is a key parameter in the liquid-phase hydrogenation process of stearic acid. To further investigate the influence of reaction temperature over Co/HAP, the reaction at different temperature was conducted and the results were shown in Table 2. At 140 °C, almost no stearic acid was converted. At 160 °C, the yield of 1-octadecanol was 22.5% and a small amount of the alkanes and ester was formed. With the temperature rising to 180 °C, the yield of 1-octadecanol rose to 46.7%, meanwhile, the amount of ester decreased to a negligible level. At 200 °C, the conversion of stearic acid was 100% and the yield of 1-octadecanol greatly increased to 95.2%. Moreover, the yield of alkanes reached to 4.8% with no ester product. Further increasing the reaction temperature to 220 °C led to an increase of the decarbonylation product with 11% yield of total alkanes and 1-octadecanol yield decreased to 89%. The above results


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revealed that appropriate reaction temperature could effectively accelerate the hydrogenation process of stearic acid and prevent the main product 1-octadecanol excessively converting to alkanes.

Effect of hydrogen pressure

Except for the reaction temperature, another major factor affecting the yield of 1octadecanol was the hydrogen pressure. The conversion of stearic acid was proceeded under different hydrogen pressures to better understand the reaction process, and the results were shown in Figure 7.

At 1 MPa of the reaction hydrogen pressure, only a small amount of stearic acid (9.0%) was converted to 1-octadecanol (8.9%) and alkanes (0.1%), which might be due to the relatively lower water solubility of hydrogen. The conversion of stearic acid and the yields of products were improved sharply when the initial pressure raised from 1 MPa to 2 MPa. As the hydrogen pressure further increased, both the conversion of stearic acid and the yield of 1-octadecanol increased. The alkanes yield also increased slightly. Under 4 MPa hydrogen, the stearic acid was completely transformed and the 1-octadecanol yield reached up to 95.2%. The total yield of alkanes was 4.8%. When the reaction was carried out at higher hydrogen pressure (5 MPa), the result showed no significant changes compared with 4 MPa. Interestingly, the amount of ester product was much small and could be ignored when the reaction was conducted under 1-3 MPa. The appropriate reaction hydrogen pressure could also accelerate the hydrogenation process of stearic acid. According to the effects of temperature and pressure, the optimal


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reaction conditions were further screened and when the reaction was carried out at 190 °C with 4 MPa H2, 97.1% of 1-octadecanol yield was obtained after 16 h. (Table 2, Entry 6).

Plausible pathway investigation

To investigate the effect of the reaction time and obtain the preliminary insight into the reaction pathway, the hydrogenation of stearic acid was carried out at 200 °C and 4 MPa H2 and the products distribution was traced and analyzed at different reaction time (Figure 8). At 0 h, the reaction temperature was just heated to 200 °C and then quenched rapidly. The conversion of stearic reached 10.9% with 9.7% of 1-octadecanol yield. Small amounts of ester (1.0%), the dehydration product of fatty acid and alcohol, and alkanes (0.2%) were produced at the same time. Afterwards, the conversion of stearic acid continuously increased as the reaction progressed with a subsequent increase of the 1-octadecanol yield. Meanwhile, the yield of alkanes increased along with the reaction time and the yield of ester decreased slowly afterward. When the reaction maintained at 200 °C for 10 h, the conversion of stearic acid reached to 100% and the yield of 1-octadecanol increased up to 95.2%. The yield of alkanes was 4.8% and the ester was converted completely to 1-octadecanol. 1-Octadecanol was the main product of the hydrogenation of stearic acid. Generally, the hydrogenation of stearic acid would first go through the key reaction intermediate 1-octadecanal. However, 1-octadecanal was far from stable and would soon be converted to more stable products, 1-octadecanol or heptadecane, so 1-octadecanal was not detected during the conversion process. Among the end-products, heptadecane and octadecane were the two by-products. It


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indicated that the dehydration of 1-octadecanol to octadecane occurred. Heptadecane with one carbon atom less could be from the decarboxylation of stearic acid or decarbonylation of 1-octadecanal. To verify the existence of decarboxylation process, the reaction was conducted under the same condition with nitrogen atmosphere, because the decarboxylation does not require the participation of hydrogen. After 10h, only a trace amount of heptadecane was detected, indicating almost all the heptadecane was derived from the decarbonylation of 1-octadecanal. A separate experiment with 1octadecanol as reaction substrate was also performed at 200 °C and 4 MPa H 2 for 6 h. After reaction, only 5.3% of 1-octadecanol was transformed into 4.5% of heptadecane and 0.8% of octadecane, which indicated that the dehydration-hydrogenation process and the dehydrogenation-decarbonylation of 1-octadecanol occurred. Thus, the possible conversion pathway of stearic acid over Co/HAP in water was proposed according to the above results (Scheme 1).

Stearic acid was first hydrogenated into 1-octadecanal, and then the 1-octadecanal was mainly hydrogenated 1-octadecanol. 1-Octadecanal from the hydrogenation of fatty acids and the dehydrogenation of 1-octadecanol was decarbonylated into the linear alkane with one carbon atom less. Although, the decarboxylation of fatty acids occurred, it could be almost neglected. 1-Octadecanol was dehydrated and then hydrogenated into the linear alkane with the same number of carbon atoms. The generated 1-octadecanol and the unreacted stearic acid occurred the dehydration reaction to form the ester, which could be further decomposed to 1-octadecanol or stearic acid via the hydrogenolysis or hydrolysis process.


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One-step conversion of Jatropha Oil into Fatty Alcohols The direct hydrogenation of the natural fats and oils is a particularly elegant method for the production of fatty alcohols, which affords advantages by reducing the outlay on equipment and improving profitability because the starting materials do not have to be highly refined.57 Hence, we also attempted to apply Co/HAP catalyst to direct hydrogenation of crude Jatropha oil (the detailed composition information as shown in Table S2) to desired fatty alcohols. The experiments on the transformation of crude Jatropha oil were conducted at 190 °C and 4 MPa H2 in aqueous phase. The product distribution varied with time was shown in Figure 9 (The detailed experimental data were shown in Table S3). At 0 h, 1.2 wt% of fatty alcohols, 5.8 wt% of saturated fatty acids, a small amount of alkanes and esters were detected, and at this time the yield of saturated fatty acids was up to the maximum. In the following conversion process, the yield of fatty alcohols increased continuously and the yield of alkanes increased with a slow rate. The yield of fatty acids decreased to 0 as transformation continued while the ester products increased at the beginning and then transformed gradually. After 18 h, 83.1 wt% of the desired fatty alcohols were obtained. However, the glycerol was further transformed into gas products mainly including CH4 and C2H4 identified by GC. It could be seen that the Co/HAP was efficient to convert Jatropha oil directly to fatty alcohols in water without any pretreatment. 84.1 wt% of alkanes could also be achieved from the Jatropha oil over Co/HAP at 200°C and 4 MPa H2 in dodecane (Table S4).

Stability of the catalyst


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We also investigated the stability of Co/HAP by cyclic experiment in hydrogenation of stearic acid to 1-octadecanol under 200 °C, 4 MPa H2 and 10 h. The Co/HAP catalyst could be collected by an external magnet from the liquid due to the magnetism of the Co metal. After two runs, the yield of 1-octadecanol slightly decreased to 88.6%. However, after three runs, the catalytic activity of Co/HAP declined greatly. The yield of 1-octadecanol decreased to 61.5%. Thus, some characterizations were carried out to investigate the reason for deactivation of the catalyst. From the TEM images (Figure S2), severe aggregation of the cobalt particles on the catalyst surface occurred after three runs. The XRD (Figure S3) analyses showed no obvious differences between the fresh and used catalyst. The leaching of the cobalt metal was detected by ICP-OES analyses of the used catalyst and the cobalt content decreased from 9.7 wt% to 9.3 wt%. Based on the above results, the major reasons of deactivation for Co/HAP could be the particle aggregation and metal leaching of cobalt. The exploration for improving the catalyst stability is currently underway.

CONCLUSION

A new efficient non-precious metal catalytic system was established for the selective hydrogenation of fatty acids and nature oil into fatty alcohols or long-chain alkanes over Co/HAP. At 190 °C, 4 MPa H2 in water, 97.1% yield of 1-octadecanol from stearic acid was obtained and 83.1 wt% yield of fatty alcohols from Jatropha oil without any preprocessing was obtained. When changing the solvent to dodecane, the stearic acid and Jatropha oil could also be converted to alkanes over Co/HAP. The high catalytic activity of Co/HAP could be due to its desired acidity, cobalt particle


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dispersion, and stronger metal-support interaction. The absorption of stearic acid on the surface of catalyst which thus promote the hydrogenation process over the Co species could also lead to a high yield of 1-octadecanol. Further research is being carried out to improve the stability of the catalyst. This green, efficient and inexpensive non-precious metal catalytic system has the potential for the production of fatty alcohols or longchain alkanes from bio-renewable feedstocks. ASSOCIATED CONTENT Supporting Information

XPS of reduced Co/HAP catalyst; TEM image of Co/HAP after three runs; XRD patterns of Co/HAP before and after three runs; Conversion of stearic acid over Co/HAP in dodecane; Composition of Jatropha oil; The detailed experimental data of Jatropha oil conversion over Co/HAP catalyst in water; The conversion of Jatropha oil over Co/HAP in dodecane; The hydrogenation of stearic acid over Cu/HAP, Ni/HAP and Fe/HAP in water. AUTHOR INFORMATION Corresponding Author：

* Email: [email protected] (Y. Zhang), Tel: +86-551-63603463, Fax: +86-55163606689.

ORCID Ying Zhang: 0000-0003-2519-7359 Yao Fu: 0000-0003-2282-4839


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Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS

We sincerely acknowledge the financial supports from the NSFC (21572213), the Program for Changjiang Scholars and Innovative Research Team in University of the Ministry of Education of China and the Fundamental Research Funds for the Central Universities (wk 2060190040).

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Figures and scheme Captions Figure 1. TEM images of as-reduced cobalt-based catalysts with a series of supports. Figure 2. XRD patterns of the as-reduced cobalt-based catalysts. Figure 3. H2-TPR profiles for the as-calcined cobalt-based catalysts. Figure 4. NH3-TPD profiles for the as-reduced cobalt-based catalysts. Figure 5. FT-IR spectra of as-reduced Co/HAP with and without stearic acid adsorption. Figure 6. Possible main catalytic mechanism in water. Figure 7. The hydrogenation of stearic acid under different reaction hydrogen pressures. Figure 8. The time-course of stearic acid conversion. Figure 9. The conversion of Jatropha oil. Scheme 1. Possible reaction pathways of stearic acid conversion over Co/HAP in water.


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Figure 1. TEM images of as-reduced cobalt-based catalysts with a series of supports. a) Co/HZSM-5, b) Co/CeO2, c) Co/ZrO2, d) Co/SiO2, e) Co/Al2O3, f) Co/TiO2, g) Co/HAP.

Figure 2. XRD patterns of the as-reduced cobalt-based catalysts.


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Figure 3. H2-TPR profiles for the as-calcined cobalt-based catalysts.

Figure 4. NH3-TPD profiles for the as-reduced cobalt-based catalysts.


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Figure 5. FT-IR spectra of as-reduced Co/HAP with and without stearic acid adsorption. The red dotted line represents the untreated Co/HAP, while the blue solid line represents the treated Co/HAP with stearic acid.

Figure 6. Possible main catalytic mechanism in water.


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Figure 7. The hydrogenation of stearic acid under different reaction hydrogen pressures. Reaction condition: 0.2 mmol of stearic acid, 50 mg of 10% Co/HAP, 200 °C for 10 h, 10 mL of water. C18OH: 1-octadecanol; Alkanes: heptadecane + octadecane.


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Figure 8. The time-course of stearic acid conversion. Reaction conditions: 0.2 mmol stearic acid, 50 mg Co/HAP, 200 °C, 4 MPa H2, 10 mL water. C18OH: 1-octadecanol, Alkanes: heptadecane + octadecane, Ester: stearyl stearate. The time -0.5 h denotes the heating initial time. The time 0 h denotes the temperature was just heated to 200°C and then cooled to room temperature.


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Figure 9. The conversion of Jatropha oil. Reaction conditions: 60 mg Jatropha oil, 50 mg Co/HAP, 190 °C, 4 MPa H2, 10 mL water. The time -0.5 h denotes the heating initial time. The time 0 h denotes the temperature was just heated to 190 °C and then cooled to room temperature.

Scheme 1. Possible reaction pathways of stearic acid conversion over Co/HAP in water.


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Tables Captions Table 1. The hydrogenation of stearic acid over various cobalt-based catalysts. Table 2. The hydrogenation of stearic acid under different reaction temperatures.


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Energy & Fuels

Table 1. The hydrogenation of stearic acid over various cobalt-based catalysts.a

Yield/mol%

Entry

Catalyst

Conv./%

C18OH

C17

C18

Ester

1

Co/HZSM-5

4.9 ± 0.40

3.3 ±0.29

0.1 ±0.02

0.4 ±0.03

1.1 ±0.21

2

Co/CeO2

19.0 ±0.71

18.3 ±0.66

0.4 ±0.05

0.1 ±0.04

0.2 ±0.05

3

Co/ZrO2

52.4 ±0.63

51.8 ±0.54

0.3 ±0.04

0.2 ±0.01

trace

4

Co/SiO2

55.2 ±1.29

54.8 ±1.07

0.2 ±0.06

0.1 ±0.03

trace

5

Co/Al2O3

58.3 ±0.65

56.9 ±0.49

0.7 ±0.08

0.1 ±0.05

0.6 ±0.06

6

Co/TiO2

66.6 ±0.21

66.1 ±0.19

0.3 ±0.05

0.1 ±0.04

0.1 ±0.02

7

Co/HAP

100

95.2 ±0.39

4.0 ±0.28

0.8 ±0.02

-

8b

Co/HAP

12.2 ±0.08

12.1 ±0.04

0.1 ±0.03

trace

trace

9

HAP

0.2 ±0.05

-

0.2 ±0.03

-

-

a

Reaction conditions: 0.2 mmol stearic acid, 10 mL water, 50 mg catalyst, 200 °C, 4

MPa H2, 10 h. C18OH: 1-octadecanol, C17: heptadecane, C18: octadecane, Ester: stearyl stearate. bUnreduced Co/HAP. -: Not detected.


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Table 2. The hydrogenation of stearic acid under different reaction temperatures.a

Yield/mol%

a

Entry

T/°C

Conv./%

C18OH

C17

C18

Ester

1

140

0.9 ±0.11

0.8 ±0.07

-

-

0.1 ±0.03

2

160

24.8 ±0.95

22.5 ±0.54

1.0 ±0.22

0.1 ±0.03

1.2 ±0.19

3

180

48.3 ±0.48

46.7 ±0.33

1.3 ±0.16

0.2 ±0.07

trace

4

200

100

95.2 ±0.39

4.0 ±0.28

0.8 ±0.02

-

5

220

100

89.0 ±0.42

9.6 ±0.24

1.4 ±0.06

-

6b

190

100

97.1 ±0.30

2.3 ±0.05

0.6 ±0.09

-

Reaction conditions: 0.2 mmol stearic acid, 10 mL water, 50 mg 10% Co/HAP, 4 MPa

H2, 10 h. bThe reaction time was 16 h. C18OH: 1-octadecanol, C17: heptadecane, C18: octadecane, Ester: stearyl stearate. -: Not detected.


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Energy & Fuels

TOC/ Synopsis

Synopsis: Co/hydroxyapatite catalyst displayed high efficiency for hydrogenation of fatty acids and Jatropha oil to fatty alcohols in water.


Direct selective hydrogenation of fatty acids and Jatropha oil to fatty

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