Catalytic In Situ Hydrogenation of Fatty Acids into Fatty Alcohols over

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Catalytic In-situ Hydrogenation of Fatty Acids into Fatty Alcohols over Cu-based Catalysts with Methanol in Hydrothermal Media Zihao Zhang, Feng Zhou, Kequan Chen, Jie Fu, Xiuyang Lu, and Pingkai Ouyang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01621 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on October 2, 2017

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Catalytic In-situ Hydrogenation of Fatty Acids into Fatty Alcohols over Cu-based Catalysts with Methanol in Hydrothermal Media Zihao Zhanga, Feng Zhoub, Kequan Chenc, Jie Fua*, Xiuyang Lua, Pingkai Ouyanga,c a

Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of

Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China b

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

China c

State Key Laboratory of Materials-Oriented Chemical Engineering, College of

Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing 211816, China * Corresponding author Jie Fu, Tel: +86 571 87951065 E-mail address: [email protected]

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Abstract: The catalytic hydrogenation of fatty acids has witnessed rapid development in recent years. However, the conventional hydrogenation process often requires high pressure hydrogen. This paper describes a novel protocol to produce fatty alcohols via an in situ hydrogenation of fatty acids in water and methanol using Cu-based catalysts. Cu/ZrO2, Cu/MgO and Cu/Al2O3 were prepared by the co-precipitation method. All Cu-based catalysts exhibited excellent activity for the in-situ hydrogenation of fatty acids, and the stability of Cu/ZrO2 was the best. The structures and properties of Cu-based catalysts are demonstrated by TEM, XRD and H2-TPR, N2 adsorption-desorption, CO-TPD, and CO2-TPD. The stability of Cu/ZrO2 is caused by the good hydrothermal stability and tetragonal phase formation of ZrO2, which strongly binds to the active Cu. The better activity over Cu/Al2O3 is caused by the larger surface area, higher Cu dispersion, smaller Cu particle size and stronger basicity of the Cu/Al2O3. Furthermore, the eﬀects of reaction time, catalyst loading, methanol loading, carbon number and types of hydrogen donor on the in-situ hydrogenation of the fatty acids were investigated to demonstrate the reaction behaviors. Keywords: Copper-based catalysts; Fatty acids; Fatty alcohols; Methanol; Hydrothermal hydrogenation

1. Introduction Fatty alcohols, particularly C12 and higher alcohols in the detergent range, have 2


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become an important basic material for a host of derivatives and applications, such as emulsifiers, emollients and thickeners in the food and cosmetic industries.1-2 The importance of fatty alcohols is reflected in the increase in global production from 2.5 million tons in 2005 to an estimated 3.1 million tons in 2015 with an estimated global demand growth (2012–2017) of 3.2% per year.

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Currently, large-scale fatty alcohol

production is performed mainly by the petrochemical industry using non-renewable resources.3 Oils and fats are regarded as an ideal substitute feedstock for fatty alcohol production because they consist of large amounts of free fatty acids, are relatively inexpensive and represent an inexhaustible renewable resource.3 Homogeneous catalysts,4-5 have been studied for the production of fatty alcohols from fatty acid or esters, but the separation and recovery of the catalysts are difficult. For heterogeneous catalysts, Cu-Cr-based catalysts have good selectivity for alcohol production from fatty acids or esters, but the addition of Cr would introduce environmental pollution.2,3,6 Recently, more research has focused on the development of new catalysts without Cr for the efficient hydrogenation of fatty acids. Noble Ru-based catalysts are reported to be active for the selective hydrogenation of fatty acids, such as bimetallic Ru-Sn supported on Al2O3, SiO2, ZrO2, or TiO2.6-8 Additionally, Pt and Re have good catalytic performances for the hydrogenation of carboxylic acids.9-10 In order to reduce the cost of 3


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catalyst, non-noble Cu as well as bimetallic Cu-Fe or Cu-Zn were also studied to catalyze the hydrogenation of esters and carboxylic acids to alcohols.11 However, all of the above studies on the hydrogenation of fatty acids or esters inevitably require high-pressure hydrogen. From an industrial point of view, it is important to achieve high activity and selectivity at lower pressures and/or lower H2/oil ratios since the major drawback of the current processes is the high hydrogen compression cost.3 Hydrogen storage and transportation remain a big challenge, and the large hydrogen consumption negatively impacts the safety and efficiency of hydrogenation process. 12-14 As a result, an in-situ release of the hydrogen from stable liquid substance, which can offer one way of ensuring its safe storage and transportation, is highly desirable.15,16 Aqueous-phase-reforming of methanol (APRM) has been considered as an ideal method for the in-situ hydrogen production, since methanol is inexpensive and water as solvent is well suitable for high-moisture oil and fat feedbacks.12, 16, 17 What’s more, APRM can release hydrogen with a high gravimetric density.18 Herein, we, for the first time, demonstrate the in-situ hydrogenation of fatty acids to produce fatty alcohols over Cu-based catalysts by coupling the following two steps: in-situ hydrogen production from APRM and in-situ hydrogenation of fatty acid. Cu/ZrO2, Cu/MgO and Cu/Al2O3 were prepared by the co-precipitation method and their activities 4


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and stabilities were evaluated. Their structures and properties are demonstrated by TEM, XRD and H2-TPR, N2 adsorption-desorption, CO-TPD, and CO2-TPD. The eﬀects of reaction time, catalyst loading, methanol loading, carbon number and type of hydrogen donor on the in-situ hydrogenation of the fatty acids were investigated. This study provides a new strategy for the production of fatty alcohols by replacing high-pressure hydrogen with the mixture of methanol and water.

2. Experimental Section 2.1 Experimental procedure All experiments were performed in a microbatch reactor (1.67 mL) that was assembled from a 3/8-inch tube and two 3/8-inch caps purchased from Swagelok, USA. The microbatch reactor was charged with 50 mg of the reactant, 0~15 mg of the catalyst, 0.5 mL of deionized water and methanol (0~70 mg). The sealed reactor was heated in a fluidized sand bath (Techne SBL-2) up to the reaction temperature. After the reaction, the reactor was soaked in water to quench the reaction. Then, the reaction mixture in the reactor was centrifuged to recover the solid catalyst, and the liquid phase was rinsed and diluted in a 10 mL volumetric flask with acetone for analysis. 2.2 Catalyst preparation A series of carrier-supported Cu catalysts were prepared via a co-precipitation method. 5


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Briefly, the calculated amounts of Cu(NO3)2•3H2O, ZrO(NO3)3•xH2O, Al(NO3)3•9H2O and Mg(NO3)2•3H2O were dissolved in 400 mL of deionized water to form a transparent solution, which is referred to as solution A. Solution B was a mixture of NaOH and Na2CO3 with concentrations of 0.8 and 0.25 mol/L, respectively. Solutions A and B were simultaneously added dropwise to a three-neck flask under vigorous stirring at 30 °C and a pH value of 9.5. After aging for 7 h at 30 °C, the precipitate was separated via filtration and washed thoroughly with deionized water until the pH was approximately 7. The precipitate was further dried in a forced air oven at 80 °C for 12 h and then calcined at 600 °C for 4 h. Prior to the reaction and testing, the catalysts were activated in a tube furnace using flowing 10% H2/Ar (a flow rate of 80 mL/min) at 550 °C for 1 h at a heating rate of 10 °C/min. In addition, the catalysts were cooled under a N2 flow. The theoretical metal loading of the self-designed catalysts was 20 wt%.

3. Results and Discussion 3.1 The structure and properties of the different Cu-based catalysts The XRD patterns of all the Cu-based catalysts are shown in Fig. 1. The appearance of the indexed diffraction angles at 2θ=43.3° (111), 50.5° (200) and 74.2° (220) (PDF#04-0836) in the Cu/ZrO2, Cu/MgO and Cu/Al2O3 catalysts indicate the presence of the crystalline phase of elemental copper. For Cu/ZrO2, the reflections at 2θ = 30.1° 6


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(111), 35.0° (200), 50.2° (220), and 59.7° (311) (PDF#65-3288) are attributed to the tetragonal ZrO2 phase: t-ZrO2 (PDF#49-1642). For Cu/MgO, a new diffraction peak at 2θ=38.7° (PDF#45-0937) indicated the formation of CuO (111), as shown in Fig. 1. The MgO phase at 2θ = 36.9° (111), 42.9° (200), 62.3° (220) and 74.7° (311) was observed. For Cu/Al2O3, the Al2O3 phase was also observed. The diffraction peak intensity of the Al2O3 support is much lower than that of the Cu. The XRD results showed that the main active site phase of the Cu-based catalysts is metallic copper. The scherrer equation at 2θ of 43.3° was applied to calculate the Cu crystallite size, which is shown in Table 1. The data showed that the Cu crystallite size of the Cu/ZrO2 catalyst are much larger than that in the Cu/Al2O3 and Cu/MgO catalysts. In addition, the Cu/Al2O3 and Cu/MgO catalysts showed better hydrogenation activity than the Cu/ZrO2 catalyst under the same reaction conditions, which can be seen in Fig. 1. This suggests that a small Cu crystallite size is required to achieve a higher catalytic hydrogenation activity. The XRD patterns of the fresh and used Cu/ZrO2 and Cu/Al2O3 catalysts are shown in Fig. 2. In Fig. 2(a), the diffraction peak did not change, and only the Cu and ZrO2 phases are observed. In addition, the Cu crystallite size of 18.8 nm obtained from the Scherrer equation decreased after use, as seen in Table 1. The stability of the Cu/ZrO2 catalyst may be caused by the stable tetragonal phase formation of ZrO2, which strongly 7


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binds to the active Cu and is proven by the XRD, HRTEM, H2-TPR and EDS mapping results.19 In addition, the stability results in the superior reuse performance of the Cu/ZrO2 catalyst However, as shown in Fig. 2 (b), new diffraction peaks appear at 14.5° (020), 28.2° (020), 38.3° (031), 49.2° (200), 55.2° (151), 60.6° (080), 64.0° (231), 65.0° (002) and 71.9° (251), and they belong to boehmite (AlOOH) (JCPDS #21-1307). Therefore, the formation of AlOOH is responsible for the poor reuse performance of Cu/Al2O3. Additionally, the Cu crystallite size increased from 10.9 nm to 18.8 nm after use and is another reason for the poor reuse performance of Cu/Al2O3. The N2 adsorption-desorption results were used to determine the total surface area and pore volume of the reduced Cu-based catalysts. These data suggested that the surface area and pore volume of the Cu-based catalysts decreased in the order of Cu/Al2O3 > Cu/MgO > Cu/ZrO2. The most active catalyst (Cu/Al2O3) had the highest BET surface area and the largest pore volume. In contrast, Cu/ZrO2 had the lowest catalytic activity and the smallest surface area and pore volume. The surface area of Cu/Al2O3 (151.5 m2g-1) was ten times greater than that of Cu/ZrO2 (13.4 m2g-1). In addition, the pore volume of Cu/Al2O3 (0.56 cm3g-1) was also much larger than that of Cu/ZrO2 (0.07 cm3g-1). The above results show that the surface area and pore volume are very important for the activity of the catalysts for the hydrothermal hydrogenation of lauric acid with methanol as a hydrogen donor. A high surface area and large pore volume should lead to a high dispersion of the Cu particles on 8


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the carrier surface and smaller Cu particles, which eventually contribute to a higher catalytic activity. A H2-TPR analysis was performed on the Cu/ZrO2, Cu/MgO and Cu/Al2O3 catalysts calcined at 600 °C before reduction. As shown in Fig. 3, all the Cu-based catalysts consumed H2 because of the reduction of copper oxide under the 5% H2/Ar atmosphere. For the Cu/MgO catalyst, the H2 consumption profile showed two peaks at 220 °C and 276 °C, which indicated that the copper oxide species have different redox behaviors. The peak at 220 °C can be attributed to the well-dispersed CuO phase, and the high-temperature peak at 276 °C is attributed to the reduction of the larger CuO particles.20 For the Cu/Al2O3 catalyst, the only reduction band at 244 °C is ascribed to the reduction of the highly dispersed CuO phase. The particle size in the Cu/Al2O3 catalyst should be larger than the particle size at 220 °C in the Cu/MgO catalyst and smaller than that at 276 °C in the Cu/MgO catalyst. As a result, the reduced Cu/MgO and Cu/Al2O3 catalysts have similar Cu particle sizes, which is proven by the XRD data in Table 1. For Cu/ZrO2, a broad band of H2 consumption in the range of 200 °C to 320 °C was discovered. The shape of the H2 consumption peak should include a complex overlapping of several elemental reduction processes, such as the sequential reduction of CuO to Cu0 via Cu2O.19 The first reduction peak at 256 °C was also ascribed to the reduction of the highly dispersed, larger CuO phase, and the highest reduction peak at 292 °C confirmed the strong interaction of 9


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Cu and ZrO2, which contributes to its greater stability under the reaction conditions.19 The stability of the Cu/ZrO2 was proven by testing the catalytic maintenance of Cu/ZrO2. The copper particle sizes (Cu/ZrO2 > Cu/MgO > Cu/Al2O3) observed via H2-TPR were also in good agreement with the XRD results. Fig. S1 shows only one broad peak was observed in all the CO-TPD profiles, and it can be attributed to desorption of CO from the surface of the copper. Cu/Al2O3 showed the largest amount of CO desorption, and Cu/ZrO2 had the lowest amount of CO desorption. This suggested that the dispersity of the copper nanoparticles in Cu/ZrO2 is worse than that in Cu/Al2O3. In addition, this is also the reason why Cu/Al2O3 had better in situ catalytic hydrogenation activity than Cu/ZrO2 under the same reaction conditions. Fig. S2 shows the CO2-TPD results that was used to evaluate the strength of the basic sites for various copper catalysts. The CO2-TPD results revealed the presence of basic sites among the catalysts, and the amount of CO2 desorption increased in the order of Cu/ZrO2 < Cu/MgO < Cu/Al2O3. This was mainly attributed to the presence of inherent Lewis basic sites on the supports, ZrO2, MgO and Al2O3, which have a strong affinity for CO2.21 Although the basicity of Cu/Al2O3 is not strong, the relatively stronger basicity of Cu/Al2O3 may be another reason for its efficient hydrogenation activity for fatty acids. Because fatty acids are more easily adsorbed on the surface of Cu/Al2O3 owing to its relatively stronger basicity compared to Cu/ZrO2. 10


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The TEM images of the Cu/Al2O3, Cu/ZrO2 and Cu/MgO catalysts are shown in Fig. 4. As shown in Fig. 4 (a), the copper particles (black spots) were highly dispersed on the Al2O3 support. The copper particle distribution was calculated by determining the number of particles of a specific size, as shown in Fig. 4 (b), and the average particle size was determined by Gauss Fit. The average size of Cu is 10.0 nm, which is very close to the 10.9 nm value obtained from the XRD results. Fig. 4 (c) and 6 (d) show the low and high-resolution images of the Cu/MgO catalyst. As shown in Fig. 4 (c) and 6 (d), Cu with very small and large particle sizes was dispersed on MgO, making it difficult to obtain an accurate particle size. The relatively larger Cu particles in the Cu/MgO catalyst can also be explained by the reduction peak at 276 °C in the H2-TPR. Although very large Cu particles (>50 nm) were observed in the Cu/MgO catalyst, the average Cu particle size obtained from the XRD was similar to that of the Cu/Al2O3 catalyst owing to the presence of smaller copper particles, which makes the catalytic activity of Cu/MgO equal to that of Cu/Al2O3. The TEM image of Cu/ZrO2 is shown in Fig. 4 (e), and it is very difficult to clearly distinguish ZrO2 with Cu. Therefore, the HRTEM image and the elemental mapping of the Cu/ZrO2 sample were used to determine the existing form of the Cu and ZrO2. The results are shown in Fig. 4 (f), (g) and (h). We identified Cu and ZrO2 by calculating the inter-planar spacing of the particles. As shown in Fig. 4 (f), Cu and ZrO2 are extremely similar in their morphology and size. In addition, Cu and ZrO2 are mixed together 11


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without a clear boundary. Therefore, it is difficult to determine the precise average Cu particle size. Instead, we measured a Cu particle in the HRTEM image, and the size was 15.88 nm. The copper size is larger than that in the Cu/MgO and Cu/Al2O3 catalysts, but it is still smaller than the size (31.2 nm) in the Cu/ZrO2 catalyst obtained using XRD. The results are reasonable because we only measured one Cu particle. The EDS-mapping results are shown in Fig. 4 (g) and (h), and Cu and ZrO2 have a strong correlation and confirmed the homogenous elemental dispersion. There is a strong interaction between Cu and ZrO2. In addition, the H2-TPR result also indicated a strong interaction between Cu and ZrO2. 3.2 Catalytic hydrothermal hydrogenation performance of different Cu-based catalysts

3.2.1 Catalytic activity over different Cu-based catalysts

The catalytic hydrothermal hydrogenation experiments over the Cu/ZrO2, Cu/Al2O3 and Cu/MgO catalysts were conducted with 50 mg of lauric acid, 50 mg of methanol, 15 mg of catalyst, and 0.5 mL of deionized water at 330 °C for 3 h. As shown in Fig. 5, the conversion percentages for the hydrogenation of lauric acid over the Cu/ZrO2, Cu/Al2O3 and Cu/MgO catalysts were 53.9%, 98.9% and 96.9%, respectively. The corresponding yields of lauryl alcohol were 45.8%, 99.2% and 97.6%, respectively. The conversions of

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lauric acid and the yields of lauryl alcohol over Cu/Al2O3 and Cu/MgO were much higher than that over the Cu/ZrO2 catalyst. The results suggest that Al2O3 and MgO are better supports than ZrO2 for the reduction of fatty acids. For the low-conversion reactions over the Cu/ZrO2 catalyst, methyl laurate was the main by-product, and the yields were 5.9%. The side reaction was caused by the esterification between lauric acid and methanol. As the yield of lauryl alcohol increased, the yield of the by-product, methyl laurate, decreased to 0. This result indicated that if the hydrogenation activity is high enough (such as over the Cu/Al2O3 and Cu/MgO catalysts), the fatty acids are hydrogenated rather than esterified. The higher dispersion, smaller Cu particle size and stronger basicity of the Cu/Al2O3 and Cu/MgO catalysts, determined using CO-TPD, XRD and CO2-TPD, respectively, are the main reasons why the Cu/Al2O3 and Cu/MgO catalysts show better catalytic activity than the Cu/ZrO2 catalyst under the same reaction conditions. Fig. 6 shows the conversion of lauric acid and yields of lauryl alcohol and methyl laurate for the reduction reaction of fatty acids over Cu/ZrO2 at different reaction times. The experiments were performed at 330 °C with 50 mg of lauric acid, 50 mg of methanol, 15 mg of catalyst, and 0.5 mL of deionized water and reaction times from 1 to 10 h. The conversion of lauric acid and the yield of lauryl alcohol increased simultaneously as the reaction time increased from 1 to 7 h, and the yield of methyl laurate decreased to 0.23% at 7 h. The highest conversion of lauric acid and yield of lauryl alcohol were 94.5% and 13


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92.9%, respectively, and the selectivity to lauryl alcohol is up to 98.3% at 7 h. The complete conversion of lauric acid and yield of 97.8% of lauryl alcohol were achieved at 10 h, and the yield of methyl laurate decreased to 0. Although the hydrogenation catalytic activity of Cu/ZrO2 is inferior to that of Cu/MgO and Cu/Al2O3, lauric acid and methyl laurate can be further converted to lauryl alcohol by prolonging the reaction time. Therefore, all the Cu-based catalysts performed well for the in situ hydrogenation of lauric acid to produce lauryl alcohol.

3.2.2 Effect of catalyst loading

Fig. 7 shows the different catalyst loading experiments over the Cu/Al2O3 catalyst at 330 °C for 3 h with 50 mg of lauric acid, 50 mg of methanol, 0.5 mL of deionized water and 0~15 mg of catalyst. As shown in Fig. 7, lauric acid was only converted to methyl laurate, and lauryl alcohol was not detected with a catalyst loading of 0. The conversion of lauric acid and the yield of lauryl alcohol increased continuously from 15.6% to 98.9% and 0 to 99.2%, respectively, as the catalyst loading increased from 0 to 15 mg. At the same time, the yield of methyl laurate slightly decreased as the catalyst loading increased from 0 to 2 mg, decreased remarkably from 16.1% to 0.6% as the catalyst loading increased from 2 to 10 mg, and reached 0 with a catalyst loading of 15 mg. These results suggest that the hydrogenation activity comes from the Cu-based catalyst, and

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esterification can occur without the catalyst.

3.2.3 The effect of methanol and water

Fig. 8 shows the different methanol loading experiments over the Cu/Al2O3 catalyst at 330 °C for 1 h with 50 mg of lauric acid, 0.5 mL of deionized water, 15 mg of catalyst and 0~70 mg of methanol. Without the added methanol, lauric acid was barely converted in the hydrothermal reaction system. This indicated that methanol is necessary for the hydrogenation of lauric acid and is responsible for producing the in-situ hydrogen. The experiments with 50 mg of methanol and no deionized water were performed to determine the role of water in the hydrogenation of lauric acid. The experiment results are shown in Table S1. The intermolecular reforming of methanol can produce hydrogen, [18] therefore, the hydrogenated product (lauryl alcohol) were detected without the addition of water. However, the deoxygenation reaction to produce by-product alkanes and esterification of fatty acids were promoted. In addition, isomerization reaction was also discovered without the addition of water. Therefore, in-situ hydrogen, obtained from APRM, is excellent hydrogen donor for the high selective hydrogenation of fatty acids to produce corresponding fatty alcohols. Owing to the superior hydrogenation activity of the Cu/Al2O3 catalyst, the methyl laurate yield was maintained at a relatively low level with a methanol loading of 5~70 mg. However, the addition of methanol significantly

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influenced the hydrogenation behavior of the lauric acid. As the methanol loading increased from 0 to 70 mg, the conversion of lauric acid increased remarkably from 1.3% to 91.0%, and at the same time, the lauryl alcohol yield increased from 0 to 91.4%. These results show that almost all the lauric acid was hydrogenated into lauryl alcohol irrespective of how much alcohol was added. The addition of methanol significantly improves the hydrogenation activity, but it does not lead to the occurrence of the side reaction. According to our experimental results, lauric acid could be efficiently hydrogenated to lauryl alcohol in water and methanol medium. It was reported that the cleavage of C-H and O-H bonds in methanol to form CO and H2 (Eq. 2.1), followed by water-gas shift (Eq. 2.2) to form H2 and CO2. The overall aqueous steam reforming reaction for producing H2 is shown in Eq. 2.

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It has been widely studied that the hydrogenation of fatty acid to

produce fatty alcohol using H2 as the hydrogen source.6-11 The reaction equation of the hydrogenation of lauric acid is shown in Eq. 3. In our APRM reaction system, 250 mg methanol, 2.5 mL water and 75 mg Cu/Al2O3 were loaded into 8 mL batch reactor. Before reaction at 350 °C for 3 h, the air in the reactor was replaced by N2 for three times, and the reactor kept 1 bar N2 eventually. The results showed that methanol was completely converted. H2, CO and CO2 were detected in the gaseous products and their mole fraction were 71.4%, 14.2% and 14.3%, respectively. The ratio of H2 to CO and CO2 was about 16


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2.5, and the mole ratio of CO to CO2 was about 1. If CO was converted completely to CO2 (Eq. 2.2), the mole ratio of H2 to CO and CO2 would be 3 from Eq. 2. It indicated that H was produced firstly from the cleavage of C-H and O-H in methanol, and then obtained from water gas shift reaction of CO. Therefore, we deduced that Eq. 4 was the total equation for the hydrogen production from aqueous-phase reforming of methanol in our reaction system. H in fatty alcohol should be obtained from both methanol and water, and the in-situ hydrogen obtained from methanol was much more than that from water. The total equation for the in-situ hydrogenation of lauric acid is shown in Eq. 1. In addition, the possible reaction pathway for the in-situ hydrogenation of fatty acids with methanol in hydrothermal media is shown in Fig. S3. 3C11H23COOH+ 2CH3OH + H2O ↔ 3C12H25OH+ CO+ CO2+5 H2 CH3OH + H2O ↔ CO2 + 3H2

(1) (2)

CH3OH ↔ CO + 2H2

(2.1)

CO + H2O ↔ CO2 + H2

(2.2)

C11H23COOH+ H2 ↔ C12H25OH

(3)

2 CH3OH + H2O↔ CO+ CO2+5 H2

(4)

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3.2.4 Hydrogenation of fatty acids with various carbon numbers

The hydrogenation of different fatty acids (lauric acid, myristic acid, palmitic acid, stearic acid and arachidic acid) over the Cu/Al2O3 catalyst with the same reactant (50 mg), methanol (30 mg), catalyst (15 mg) and water (0.5 mL) was investigated at 330 °C for 1 h, and the results are shown in Fig. 9. All the fatty acids (C14~C20) tested reached the same conversion and yield of fatty alcohols as the experiment with lauric acid. However, the conversion of stearic acid and arachidic acid were slightly higher than that of lauric acid. The results seem reasonable as Toba and co-workers12, have reported that the conversion of an acid increased with an increase in the carbon number. Therefore, the Cu-based catalysts showed good hydrothermal hydrogenation performance for fatty acids with various carbon numbers using methanol as a hydrogen donor. These results suggested that Cu-based catalysts are capable for the in-situ hydrothermal hydrogenation of different fatty acids in lipids using methanol as hydrogen donor.

3.2.5 The catalytic maintenance of Cu/Al2O3 and Cu/ZrO2

The activity maintenance of Cu/Al2O3 and Cu/ZrO2 was evaluated at 330 °C with 15 mg of the fresh or used catalyst, 50 mg of lauric acid, 15 mg of methanol, and 0.5 mL of deionized water. The reaction time was 1 h for Cu/Al2O3 and 4 h for Cu/ZrO2, and the results are shown in Fig. 10. For the repeated experiment with Cu/Al2O3 in Fig. 10(a), the 18


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conversion of lauric acid and the lauryl alcohol yield over the fresh (1st), used once (2nd), and used twice (3rd) catalyst decreased remarkably. The lauryl alcohol yield decreased from 72.4% (fresh Cu/Al2O3) to 3.0% as the recycle time increased to 3rd, and the methyl laurate yield increased. This result suggested that the hydrogenation activity of the Cu/Al2O3 catalyst decreased significantly after use, which led to a decline in the lauryl alcohol yield and an increase in the methyl laurate yield. For the repeated experiments with Cu/ZrO2 in Fig. 10(b), both the conversion of lauric acid and yield of lauryl alcohol were maintained over the fresh (1st), used once (2nd), and used twice (3rd) catalyst. Even the yield of lauryl alcohol over the Cu/ZrO2 catalyst used twice (3rd) was approximately 62.3%, which is almost the same as that (61.3%) over fresh Cu/ZrO2. The superior stability of Cu/ZrO2 may be caused by good hydrothermal stability of ZrO2 and the strong interaction between Cu and ZrO2, which was proven by the HRTEM, H2-TPR and EDS-mapping results. Lu et.al has reported an interesting phenomena that pores formed as a result of structural changes in the amorphous Al2O3 by dehydration of AlOOH.22 So it’s not surprised that pore disappeared after the formation of AlOOH. For Cu/Al2O3, the formation of AlOOH, leading to the disappearance of pores, is responsible for the poor reuse performance of Cu/Al2O3. Additionally, the Cu particle size increased from 10.9 nm to 18.8 nm after use, which is another reason for the poor reuse performance of Cu/Al2O3. In addition, the ICP-OES results indicated that the concentration of cooper ion in solution 19


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after reaction over Cu/ZrO2 and Cu-Al2O3 were very low.

3.2.6 Effect of different hydrogen donors over Cu/ZrO2

The effect of different alcohols as hydrogen donors on the hydrogenation of lauric acid was evaluated over Cu/ZrO2 at 330 °C with 50 mg of lauric acid, 15 mg of catalyst, 1.56 mmol of alcohol and 0.5 mL of deionized water and a reaction time of 3 h, as shown in Fig. 11. The lauryl alcohol yields with methanol, ethanol, propanol and isopropanol under the same reaction conditions were 45.8%, 13.5%, 10.8% and 18.3%, respectively. Methanol showed the best hydrothermal hydrogenation activity of all the alcohols. It’s generally accepted that alcohols such as methanol, ethanol and propanol, where the C-H bond of each carbon atom is activated by adjacent OH groups, might be converted to H2.23 For ethanol and propanol, a part of C-H bond in the non-activated methyl and ethyl group respectively is relatively difficult to be activated compared to the C-H bond in methanol. Therefore, methanol performed better than ethanol and propanol as a hydrogen donor. In addition, the secondary alcohol (isopropanol) performed better than the primary alcohol (propanol) with the same carbon number. It might be caused by that the C-H bond in isopropanol is susceptible by adjacent OH groups, contributing to the cleavage of C-H bond.

20


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4. Conclusions Cu-based catalysts were synthesized by the co-precipitation method and exhibited excellent activity for the hydrogenation of lauric acid. The almost complete conversion of lauric acid was achieved over Cu/Al2O3 and Cu/ZrO2, more than 98% selectivity for lauryl alcohol, respectively, by optimizing the reaction conditions. Cu/ZrO2 showed the best reuse performance. The superior stability of Cu/ZrO2 is caused by the good hydrothermal stability of ZrO2 and strong interaction between Cu and ZrO2, which can be proven using the HRTEM, H2-TPR and EDS-mapping results. For Cu/Al2O3, the formation of AlOOH is responsible for its poor reuse performance. Additionally, the Cu crystallite size increased from 10.9 nm to 18.8 nm after use, which is another reason. Furthermore, the addition of water inhibited the deoxygenation reaction to produce alkanes and promoted the hydrolysis of fatty esters, which decreased the fatty ester yield. Isomerization reaction was also not detected under the hydrothermal reaction conditions. The Cu-based catalysts showed good hydrothermal hydrogenation performances for fatty acids with various carbon numbers. Methanol showed the best hydrothermal hydrogenation activity compared with ethanol, propanol and isopropanol. In conclusion, we demonstrated the hydrogen-free hydrogenation of fatty acids to produce fatty alcohols in water with methanol as the hydrogen donor in the presence of Cu-based catalysts.

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Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21436007, 21676243) and Zhejiang Provincial Natural Science Foundation of China (No. LR17B060002, LZ14B060002).

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References 1.

Kreutzer, U. R. Manufacture of fatty alcohols based on natural fats and oils. J. Am. Oil

Chem. Soc. 1984, 61, 343-348. 2.

Rozmysłowicz, B.; Kirilin, A.; Aho, A.; Manyar, H.; Hardacre, C.; Wärnå, J.; Salmi,

T.; Murzin, D. Y. Selective hydrogenation of fatty acids to alcohols over highly dispersed ReOx/TiO2 catalyst. J. Catal. 2015, 328, 197-207. 3.

Sánchez, M. A.; Torres, G. C.; Mazzieri, V. A.; Pieck, C. L. Selective hydrogenation

of fatty acids and methyl esters of fatty acids to obtain fatty alcohols-a review. J. Chem. Technol. Biot. 2017, 92, (1), 27-42. 4.

Tan, X.; Wang, Y.; Liu, Y.; Wang, F.; Shi, L.; Lee, K.-H.; Lin, Z.; Lv, H.; Zhang, X.

Highly efficient tetradentate ruthenium catalyst for ester reduction: especially for hydrogenation of fatty acid esters. Org. Lett. 2015, 17, (3), 454-457. 5.

Fairweather, N. T.; Gibson, M. S.; Guan, H. Homogeneous hydrogenation of fatty acid

methyl esters and natural oils under neat conditions. Organometallics 2015, 34, (1), 335-339. 6.

Miyake, T.; Makino, T.; Taniguchi, S.-i.; Watanuki, H.; Niki, T.; Shimizu, S.; Kojima,

Y.; Sano, M. Alcohol synthesis by hydrogenation of fatty acid methyl esters on supported Ru–Sn and Rh–Sn catalysts. Appl. Catal., A 2009, 364, (1-2), 108-112. 7.

Toba, M.; Tanaka, S.; Niwa, S.; Mizukami, F.; Koppány, Z.; Guczi, L.; Cheah, K. Y.; 23


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Tang, T. S. Synthesis of alcohols and diols by hydrogenation of carboxylic acids and esters over Ru-Sn-Al2O3 catalysts. Appl. Catal., A 1999, 189, (2), 243-250. 8.

Mendes, M. J.; Santos, O. A. A.; Jordão, E.; Silva, A. M. Hydrogenation of oleic acid

over ruthenium catalysts. Appl. Catal., A 2001, 217, 253-262. 9.

Manyar, H. G.; Paun, C.; Pilus, R.; Rooney, D. W.; Thompson, J. M.; Hardacre, C.

Highly selective and efficient hydrogenation of carboxylic acids to alcohols using titania supported Pt catalysts. Chem. Commun. 2010, 46, (34), 6279-81. 10. Yoshino, K.; Kajiwara, Y.; Takaishi, N.; Inamoto, Y.; Tsuji, J. Hydrogenation of carboxylic acids by rhenium-osmium bimetallic catalyst. J. Am. Oil Chem. Soc. 1990, 67, 21-24. 11. Kandel, K.; Chaudhary, U.; Nelson, N. C.; Slowing, I. I. Synergistic Interaction between Oxides of Copper and Iron for Production of Fatty Alcohols from Fatty Acids. ACS Catal. 2015, 5, (11), 6719-6723. 12. Vardon, D. R.; Sharma, B. K.; Jaramillo, H.; Kim, D.; Choe, J. K.; Ciesielski, P. N.; Strathmann, T. J. Hydrothermal catalytic processing of saturated and unsaturated fatty acids to hydrocarbons with glycerol for in situ hydrogen production. Green Chem. 2014, 16, (3), 1507. 13. Yu, L.; Du, X. L.; Yuan, J.; Liu, Y. M.; Cao, Y.; He, H. Y.; Fan, K. N. A versatile aqueous reduction of bio-based carboxylic acids using syngas as a hydrogen source. 24


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ChemSusChem 2013, 6, (1), 42-6. 14. Sharma, S.; Ghoshal, S. K., Hydrogen the future transportation fuel: From production to applications. Renew. Sust. Energ. Rev. 2015, 43, 1151-1158. 15. Yuan, J.; Li, S.-S.; Yu, L.; Liu, Y.-M.; Cao, Y.; He, H.-Y.; Fan, K.-N. Copper-based catalysts for the efficient conversion of carbohydrate biomass into γ-valerolactone in the absence of externally added hydrogen. Energy Environ. Sci. 2013, 6, 3308. 16. Lee, J. K.; Ko, J. B.; Kim, D. H. Methanol steam reforming over Cu/ZnO/Al2O3 catalyst: kinetics and effectiveness factor. Appl. Catal., A 2004, 278, 25-35. 17. Sá, S.; Silva, H.; Brandão, L.; Sousa, J. M.; Mendes, A., Catalysts for methanol steam reforming—A review. Appl. Catal. B-Environ. 2010, 99, 43-57. 18. Lin, L.; Zhou, W.; Gao, R.; Yao, S.; Zhang, X.; Xu, W.; Zheng, S.; Jiang, Z.; Yu, Q.; Li, Y. W.; Shi, C.; Wen, X. D.; Ma, D. Low-temperature hydrogen production from water and methanol using Pt/alpha-MoC catalysts. Nature 2017, 544, (7648), 80-83. 19. Hengne, A. M.; Rode, C. V. Cu–ZrO2 nanocomposite catalyst for selective hydrogenation of levulinic acid and its ester to γ-valerolactone. Green Chem. 2012, 14, (4), 1064. 20. Xu, S.; Huang, C.; Zhang, J.; Chen, B. Catalytic activity of Cu/MgO in liquid phase oxidation of cumene. Korean J. Chem. Eng. 2010, 26, (6), 1568-1573. 21. Song, F.; Tan, Y.; Xie, H.; Zhang, Q.; Han, Y. Direct synthesis of dimethyl ether from 25


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biomass-derived syngas over Cu–ZnO–Al2O3–ZrO2(x)/γ-Al2O3 bifunctional catalysts: Effect of Zr-loading. Fuel Process. Technol. 2014, 126, 88-94. 22. Lu, J.; Fu, B.; Kung, M. C.; Xiao, G.; Elam, J. W.; Kung, H. H.; Stair, P. C. Coking- and sintering-resistant palladium catalysts achieved through atomic layer deposition. Science 2012, 335, (6073), 1205-8. 23. Nozawa, T.; Mizukoshi, Y.; Yoshida A.; Naito, S. Appl. Catal. B-Environ., 2014, 146, 221-226.

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Page 27 of 38

Cu-ZrO2 Cu-MgO Cu-Al2O3

43.3

• Cu

•

♦ ZrO2 ⊕ Al2O3

∇ MgO ⊗ CuO

36.9∇

42.9

∇

38.7

Intensity /a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

50.5

•

62.3

74.2 • 74.7

∇

⊗

∇

50.2

♦•

30.1

♦

59.7

♦

35.0

♦

⊕ 20

30

⊕ 40

50

60

2θ

Fig. 1 XRD results for the Cu/ZrO2, Cu/MgO and Cu/Al2O3 catalysts

27


70

80

Energy & Fuels

30.1 ♦

50.5 • 43.3 • 35.0 ♦

59.7 ♦

Intensity / a.u.

50.2 ♦

• Cu ♦ ZrO2

74.2 •

Cu-ZrO2 used Cu-ZrO2 fresh

20

30

40

50 2θ

60

70

80

(a) 28.2⊗

14.5 ⊗

• Cu ⊕ Al2O3 ⊗ AlOOH

49.2 38.3 ⊗ ⊗ 43.3 •

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 38

71.9 50.555.2 64.0 ⊗ 65.0 ⊗ • ⊗ 74.2 60.6 ⊗ • ⊗ ⊕ ⊕ Cu-Al2O3 fresh Cu-Al2O3 used 10

20

30

40

50

60

70

80

2θ

(b) Fig. 2 XRD results for the (a) fresh and used Cu/ZrO2 and (b) fresh and used Cu/Al2O3 catalysts.

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292 256 220 276

H2 consumption / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Cu-ZrO2 Cu-MgO Cu-Al2O3 244

100

200

300

400

500

600

700

800

Temperature / °C

Fig. 3 H2-TPR profiles of the Cu/ZrO2, Cu/MgO and Cu/Al2O3 catalysts.

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15

Average size=10.0 nm obtaied by Gauss Fit

12 Frequency / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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9 6 3 0 6

8

10

12

14

16

18

20

22

24

Particle size / nm

(a)

(b)

(c)

(d)

(e)

(f)

Cu Zr (g)

(h)

Fig. 4 (a) TEM image of Cu/Al2O3; (b) Cu particle distribution; (c) low-resolution TEM image of 30


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Cu/MgO; (d) high-resolution TEM image of Cu/MgO; (e) TEM image of Cu/ZrO2; (f) HRTEM image of Cu/ZrO2; (g) and (h) elemental mapping (Cu, Zr) of Cu/ZrO2, Green stands for Cu and Red stands for Zr. 100

Lauric acid Lauryl alcohol Methyl laurate

80 Conversion or yield / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

60

40

20

0

Cu-ZrO2

Cu-Al2O3

Cu-MgO

Fig. 5 The conversion of lauric acid and the yield of lauryl alcohol and methyl laurate for lauric acid hydrogenation over the Cu/ZrO2, Cu/Al2O3 and Cu/MgO catalysts. Reaction conditions: T=330 °C, time=3 h, lauric acid loading=50 mg, methanol loading=50 mg, catalyst loading=15 mg, water loading=0.5 mL.

31


Energy & Fuels

100


80 Conversion or Yield / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 40 20 0 1h

2h

3h

4h

5h

7h

10 h

Reaction time

Fig. 6 The conversion of lauric acid and yield of lauryl alcohol and methyl laurate for lauric acid hydrogenation over the Cu/ZrO2 catalyst. Reaction conditions: T=330 °C, lauric acid loading=50 mg, methanol loading=50 mg, catalyst loading=15 mg, water loading=0.5 mL, reaction time=1~10 h.

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100

Conversion or yield / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels


80 60 40 20 0 0 mg

2 mg

5 mg

10 mg

15 mg

Catalyst loading

Fig. 7 The conversion of lauric acid and the yield of lauryl alcohol and methyl laurate for lauric acid hydrogenation over the Cu/Al2O3 catalyst. Reaction conditions: T=330 °C, lauric acid loading=50 mg, methanol loading=50 mg, catalyst loading=0~15 mg, water loading=0.5 mL, reaction time=3 h.

33


Energy & Fuels

100


80 Conversion or Yield / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

40

20

0 Non

5 mg

10 mg 30 mg

50 mg

70 mg

Methanol loading

Fig. 8 The conversion of lauric acid and yields of lauryl alcohol and methyl laurate for lauric acid hydrogenation over the Cu/Al2O3 catalyst. Reaction conditions: T=330 °C, lauric acid loading=50 mg, methanol loading=0~70 mg, catalyst loading=15 mg, water loading=0.5 mL, reaction time=1 h.

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80 70 Conversion or yield / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

60 50

Fatty acid Fatty alcohol Fatty ester

40 30 20 10 0 C12

C14

C16

C18

C20

Fatty acid with different carbon number

Fig. 9 The conversion of fatty acids and yields of fatty alcohols and fatty esters for different fatty acid hydrogenations over the Cu/Al2O3 catalyst. Reaction conditions: T=330 °C, fatty acid loading=50 mg, methanol loading=30 mg, catalyst loading=15 mg, water loading=0.5 mL, reaction time=1 h.

35


Energy & Fuels


Conversion or yield / %

80

60

40

20

0 1st

2nd

3rd

2nd

3rd

(a) 100

80 Conversion or yield / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 38


60

40

20

0 1st

(b) Fig. 10 The conversion of lauric acid and yields of lauryl alcohol and methyl laurate for lauric acid hydrogenation over Cu/Al2O3 (a) and Cu/ZrO2 (b) after the first, second and third uses. Reaction conditions: T=330 °C, llauric acid loading=50 mg, catalyst loading=15 mg, water loading=0.5 mL, reaction time=1 h and 4 h respectively, methanol loading=50 mg.

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50 Yield of lauryl alcohol (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

45.8

40 30

18.3

20

13.5

10.8

10 0 Methanol

Ethanol

Propanol

Isopropanol

Fig. 11 The lauryl alcohol yields for lauric acid hydrogenation over Cu/ZrO2. Reaction conditions: T=330 °C, fatty acid loading=50 mg, catalyst loading=15 mg, water loading=0.5 mL, reaction time=3 h, alcohol added=1.56 mmol (50 mg of methanol, 72 mg of ethanol and 94 mg of propanol or isopropanol).

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Table 1 Physical properties of different Cu-based catalysts

Cu crystallite

BET surface area b

Pore volume c

size a/nm

/m2g-1

/cm3g-1

Cu/ZrO2

31.2

13.4

0.07

Cu/Al2O3

10.9

151.5

0.56

Cu/MgO

10.1d

69.5

0.38

Cu/Al2O3 (reuse)

16.9

~

~

Cu/ZrO2(reuse)

18.8

~

~

Catalyst

a

Average particle sizes of the Cu species calculated using the Scherrer equation at 2θ ~ 43.3°

b

BET surface area

c

BJH adsorption cumulative volume of the pores between 1.7000 nm and 300.000 nm in diameter

d

This result was calculated at 2θ for both Cu2O (42.4) and Cu (43.3).

38


Catalytic In Situ Hydrogenation of Fatty Acids into Fatty Alcohols over

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