Catalytic In Situ Hydrogenation of Fatty Acids into

0 downloads 0 Views 5MB Size Report
Sep 29, 2017 - fatty acid or esters, but the separation and recovery of the catalysts are ... hydrogenation of esters and carboxylic acids to alcohols.11. However ...
See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/320114275

Catalytic In Situ Hydrogenation of Fatty Acids into Fatty Alcohols over CuBased Catalysts with Methanol in Hydrothermal Media Article  in  Energy & Fuels · September 2017 DOI: 10.1021/acs.energyfuels.7b01621

CITATIONS

READS

2

67

6 authors, including: Zihao Zhang

Jie Fu

Zhejiang University

Zhejiang University

11 PUBLICATIONS   25 CITATIONS   

59 PUBLICATIONS   571 CITATIONS   

SEE PROFILE

Lu Xiuyang Beijing Jiaotong University 82 PUBLICATIONS   1,557 CITATIONS    SEE PROFILE

All content following this page was uploaded by Zihao Zhang on 05 June 2018.

The user has requested enhancement of the downloaded file.

SEE PROFILE

Article pubs.acs.org/EF

Cite This: Energy Fuels 2017, 31, 12624-12632

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†,§ †

Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China ‡ Fushun Research Institute of Petroleum and Petrochemicals, SINOPEC, Fushun, Liaoning 113001, People’s Republic of China § State Key Laboratory of Materials-Oriented Chemical Engineering, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing, Jiangsu 211816, People’s Republic of China S Supporting Information *

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 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 transmission electron microscopy, X-ray diffraction, H2 temperature-programmed reduction, N2 adsorption− desorption, CO temperature-programmed desorption, and CO2 temperature-programmed desorption. The stability of Cu/ZrO2 is caused by the good hydrothermal stability and tetragonal phase formation of ZrO2, which strongly binds to 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 Cu/Al2O3. Furthermore, the effects of the reaction time, catalyst loading, methanol loading, carbon number, and types of hydrogen donor on in situ hydrogenation of the fatty acids were investigated to demonstrate the reaction behaviors. 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 because 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 the hydrogenation process.12−14 As a result, an in situ release of hydrogen from a 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 in situ hydrogen production, because methanol is inexpensive and water as a solvent is well-suitable for high-moisture oil and fat feedbacks.12,16,17 In addition, APRM can release hydrogen with a high gravimetric density.18 Herein, we, for the first time, demonstrate 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 coprecipitation method, and their activities and stabilities were evaluated. Their structures and properties are demonstrated by transmission electron microscopy (TEM), X-ray diffraction

1. INTRODUCTION Fatty alcohols, particularly C12 and higher alcohols in the detergent range, have 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.3 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 catalysts4,5 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−Crbased 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 To reduce the cost of catalyst, non-noble Cu as well as bimetallic Cu−Fe or Cu−Zn were also studied to catalyze the © 2017 American Chemical Society

Received: June 6, 2017 Revised: September 29, 2017 Published: September 29, 2017 12624

DOI: 10.1021/acs.energyfuels.7b01621 Energy Fuels 2017, 31, 12624−12632

Article

Energy & Fuels

of the crystalline phase of elemental copper. For Cu/ZrO2, the reflections at 2θ = 30.1° (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 Figure 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 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

(XRD), H2 temperature-programmed reduction (H2-TPR), N2 adsorption−desorption, CO temperature-programmed desorption (CO-TPD), and CO2 temperature-programmed desorption (CO2-TPD). The effects of the reaction time, catalyst loading, methanol loading, carbon number, and type of hydrogen donor on 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 in. tube and two 3/8 in. caps purchased from Swagelok, Solon, OH, U.S.A. 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. 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 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 %.

Table 1. Physical Properties of Different Cu-Based Catalysts catalyst

Cu crystallite sizea (nm)

BET surface areab (m2 g−1)

pore volumec (cm3 g−1)

Cu/ZrO2 Cu/Al2O3 Cu/MgO Cu/Al2O3 (reuse) Cu/ZrO2 (reuse)

31.2 10.9 10.1d 16.9 18.8

13.4 151.5 69.5

0.07 0.56 0.38

a

Average particle sizes of the Cu species calculated using the Scherrer equation at 2θ ∼ 43.3°. bBrunauer−Emmett−Teller (BET) surface area. cBarrett−Joyner−Halenda (BJH) adsorption cumulative volume of the pores between 1.7000 and 300.000 nm in diameter. dThis result was calculated at 2θ for both Cu2O (42.4) and Cu (43.3).

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 Figure 5. 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 Figure 2. In Figure 2a, 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 binds to active Cu and is proven by the XRD, highresolution transmission electron microscopy (HRTEM), H2TPR, and energy-dispersive spectroscopy (EDS) mapping results.19 In addition, the stability results in the superior reuse performance of the Cu/ZrO2 catalyst. However, as shown in Figure 2b, 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) [Joint Committee on Powder Diffraction Standards (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 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.

3. RESULTS AND DISCUSSION 3.1. Structure and Properties of the Different CuBased Catalysts. The XRD patterns of all of the Cu-based catalysts are shown in Figure 1. The appearance of the indexed diffraction angles at 2θ = 43.3° (111), 50.5° (200), and 74.2° (220) [Powder Diffraction File (PDF) 04-0836] in the Cu/ ZrO2, Cu/MgO, and Cu/Al2O3 catalysts indicate the presence

Figure 1. XRD results for the Cu/ZrO2, Cu/MgO, and Cu/Al2O3 catalysts. 12625

DOI: 10.1021/acs.energyfuels.7b01621 Energy Fuels 2017, 31, 12624−12632

Article

Energy & Fuels

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

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 200−320 °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 Cu and ZrO2, which contributes to its greater stability under the reaction conditions.19 The stability of 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. Figure S1 of the Supporting Information shows that only one broad peak was observed in all of the CO-TPD profiles, and it can be attributed to desorption of CO from the surface of 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. Figure S2 of the Supporting Information shows the CO2TPD results that were 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. The TEM images of the Cu/Al2O3, Cu/ZrO2, and Cu/MgO catalysts are shown in Figure 4. As shown in Figure 4a, 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 Figure 4b, 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. Panels c and d of Figure 4 show the low- and

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

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 m2 g−1) was 10 times greater than that of Cu/ZrO2 (13.4 m2 g−1). In addition, the pore volume of Cu/Al2O3 (0.56 cm3 g−1) was also much larger than that of Cu/ZrO2 (0.07 cm3 g−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 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 Figure 3, all of 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 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 12626

DOI: 10.1021/acs.energyfuels.7b01621 Energy Fuels 2017, 31, 12624−12632

Article

Energy & Fuels

Figure 4. (a) TEM image of Cu/Al2O3, (b) Cu particle distribution, (c) low-resolution TEM image of Cu/MgO, (d) high-resolution TEM image of Cu/MgO, (e) TEM image of Cu/ZrO2, (f) HRTEM image of Cu/ZrO2, and (g and h) elemental mapping (Cu and Zr) of Cu/ZrO2. Green stands for Cu, and red stands for Zr.

high-resolution images of the Cu/MgO catalyst. As shown in panels c and d of Figure 4, 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 H2-TPR. Although very large Cu particles (>50 nm) were observed in the Cu/MgO catalyst, the average Cu particle size obtained from XRD was similar to that of the Cu/ 12627

DOI: 10.1021/acs.energyfuels.7b01621 Energy Fuels 2017, 31, 12624−12632

Article

Energy & Fuels

acid and methanol. As the yield of lauryl alcohol increased, the yield of the byproduct, 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 COTPD, 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. Figure 6 shows the conversion of lauric acid and yields of lauryl alcohol and methyl laurate for the reduction reaction of

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 Figure 4e, 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 Cu and ZrO2. The results are shown in panels f−h of Figure 4. We identified Cu and ZrO2 by calculating the interplanar spacing of the particles. As shown in Figure 4f, Cu and ZrO2 are extremely similar in their morphology and size. In addition, Cu and ZrO2 are mixed together 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 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 panels g and h of Figure 4, and Cu and ZrO2 have a strong correlation and confirmed the homogeneous elemental dispersion. There is a strong interaction between Cu and ZrO2. In addition, the H2TPR 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 Figure 5, the

Figure 6. 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; and reaction time, 1−10 h.

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 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 of the Cu-based catalysts performed well for in situ hydrogenation of lauric acid to produce lauryl alcohol. 3.2.2. Effect of the Catalyst Loading. Figure 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 Figure 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 from 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

Figure 5. 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; and water loading, 0.5 mL.

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 lauric acid and the yields of lauryl alcohol over Cu/Al2O3 and Cu/MgO were much higher than those 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 byproduct and the yield was 5.9%. The side reaction was caused by the esterification between lauric 12628

DOI: 10.1021/acs.energyfuels.7b01621 Energy Fuels 2017, 31, 12624−12632

Article

Energy & Fuels

hydrogen, obtained from APRM, is an 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 influenced the hydrogenation behavior of 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 lauric acid was hydrogenated into lauryl alcohol, irrespective of how much alcohol was added. The addition of methanol significantly improves the hydrogenation activity but 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 media. The cleavage of C−H and O−H bonds in methanol were reported to form CO and H2 (eq 2.1), followed by a water-gas shift (eq 2.2) to form H2 and CO2. The overall aqueous steam reforming reaction for producing H2 is shown in eq 2.23 The hydrogenation of fatty acid to produce fatty alcohol using H2 as the hydrogen source has been widely studied.6−11 The reaction equation of the hydrogenation of lauric acid is shown in eq 3. In our APRM reaction system, 250 mg of methanol, 2.5 mL of water, and 75 mg of Cu/Al2O3 were loaded into a 8 mL batch reactor. Before reaction at 350 °C for 3 h, the air in the reactor was replaced by N2 3 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 fractions were 71.4, 14.2, and 14.3%, respectively. The ratio of H2/CO and CO2 was about 2.5, and the mole ratio of CO/CO2 was about 1. If CO was converted completely to CO2 (eq 2.2), the mole ratio of H2/CO and CO2 would be 3 from eq 2. It indicated that H was produced first from the cleavage of C−H and O−H in methanol and then obtained from the water-gas shift reaction of CO. Therefore, we deduced that eq 4 was the total equation for the hydrogen production from APRM in our reaction system. H in fatty alcohol should be obtained from both methanol and water, and in situ hydrogen obtained from methanol was much more than that from water. The total equation for in situ hydrogenation of lauric acid is shown in eq 1. In addition, the possible reaction pathway for in situ hydrogenation of fatty acids with methanol in hydrothermal media is shown in Figure S3 of the Supporting Information.

Figure 7. 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; and reaction time, 3 h.

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 Cubased catalyst and esterification can occur without the catalyst. 3.2.3. Effect of Methanol and Water. Figure 8 shows the different methanol loading experiments over the Cu/Al2O3

Figure 8. Conversion of lauric acid and 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, 0−70 mg; catalyst loading, 15 mg; water loading, 0.5 mL; and reaction time, 1 h.

5C11H 23COOH + 2CH3OH + H 2O

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 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 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 experimental results are shown in Table S1 of the Supporting Information. The intermolecular reforming of methanol can produce hydrogen;18 therefore, the hydrogenated product (lauryl alcohol) was detected without the addition of water. However, the deoxygenation reaction to produce byproduct alkanes and esterification of fatty acids were promoted. In addition, the isomerization reaction was also discovered without the addition of water. Therefore, in situ

↔ 5C12H 25OH + CO + CO2

(1)

CH3OH + H 2O ↔ CO2 + 3H 2

(2)

CH3OH ↔ CO + 2H 2

(2.1)

CO + H 2O ↔ CO2 + H 2

(2.2)

C11H 23COOH + H 2 ↔ C12H 25OH

(3)

2CH3OH + H 2O ↔ CO + CO2 + 5H 2

(4)

3.2.4. Hydrogenation of Fatty Acids with Various Carbon Numbers. The hydrogenation of different fatty acids (lauric, myristic, palmitic, stearic, and arachidic acids) over the Cu/ Al2O3 catalyst with the same reactant (50 mg), methanol (30 12629

DOI: 10.1021/acs.energyfuels.7b01621 Energy Fuels 2017, 31, 12624−12632

Article

Energy & Fuels mg), catalyst (15 mg), and water (0.5 mL) was investigated at 330 °C for 1 h, and the results are shown in Figure 9. All of the

Figure 9. 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; and reaction time, 1 h.

fatty acids (C14−C20) tested reached the same conversion and yield of fatty alcohols as the experiment with lauric acid. However, the conversions of stearic and arachidic acids were slightly higher than that of lauric acid. The results seem reasonable because 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 in situ hydrothermal hydrogenation of different fatty acids in lipids using methanol as a hydrogen donor. 3.2.5. 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 Figure 10. For the repeated experiment with Cu/Al2O3 in Figure 10a, the conversion of lauric acid and the lauryl alcohol yield over the fresh (first), used once (second), and used twice (third) catalyst decreased remarkably. The lauryl alcohol yield decreased from 72.4% (fresh Cu/Al2O3) to 3.0% as the recycle time increased to third, 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 Figure 10b, both the conversion of lauric acid and yield of lauryl alcohol were maintained over the fresh (first), used once (second), and used twice (third) catalyst. Even the yield of lauryl alcohol over the Cu/ZrO2 catalyst used twice (third) 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, H2TPR, and EDS mapping results. Lu et al. have reported an interesting phenomena that pores formed as a result of structural changes in amorphous Al2O3 by dehydration of AlOOH.22 Therefore, it is not surprised that pores disappeared

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

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 to 18.8 nm after use, which is another reason for the poor reuse performance of Cu/Al2O3. In addition, the inductively coupled plasma optical emission spectrometry (ICP−OES) results indicated that the concentration of cooper ion in solution after reaction over Cu/ZrO2 and Cu/Al2O3 was 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 Figure 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 of the alcohols. It is 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 the C−H bond in the non-activated methyl and ethyl groups, respectively, is relatively difficult to be activated in comparison to the C−H bond in methanol. 12630

DOI: 10.1021/acs.energyfuels.7b01621 Energy Fuels 2017, 31, 12624−12632

Article

Energy & Fuels



possible reaction pathway for in situ hydrogenation of fatty acids with methanol in hydrothermal media (Figure S3), and selectivities of the different products on the conversion of lauric acid with and without water added (Table S1) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-571-87951065. E-mail: [email protected]. ORCID

Jie Fu: 0000-0002-3652-7715 Notes

Figure 11. 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; and alcohol added, 1.56 mmol (50 mg of methanol, 72 mg of ethanol, and 94 mg of propanol or isopropanol).

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21436007 and 21676243) and the Zhejiang Provincial Natural Science Foundation of China (LR17B060002 and LZ14B060002).

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 the C−H bond in isopropanol being susceptible by adjacent OH groups, contributing to the cleavage of the C− H bond.



4. CONCLUSION 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, 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, H2TPR, 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 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. The 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 to ethanol, propanol, and isopropanol. In conclusion, we demonstrated the hydrogenfree hydrogenation of fatty acids to produce fatty alcohols in water with methanol as the hydrogen donor in the presence of Cu-based catalysts.



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. Biotechnol. 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.; 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), 6279−6281. (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.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b01621. Materials, characterization, analysis method, CO-TPD profiles of the Cu/ZrO2, Cu/MgO, and Cu/Al2O3 catalysts (Figure S1), CO2-TPD profiles of the Cu/ ZrO2, Cu/MgO, and Cu/Al2O3 catalysts (Figure S2), 12631

DOI: 10.1021/acs.energyfuels.7b01621 Energy Fuels 2017, 31, 12624−12632

Article

Energy & Fuels (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. ChemSusChem 2013, 6 (1), 42−46. (14) Sharma, S.; Ghoshal, S. K. Hydrogen the future transportation fuel: From production to applications. Renewable Sustainable Energy 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 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. Lowtemperature hydrogen production from water and methanol using Pt/ α-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 γvalerolactone. 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. 2009, 26 (6), 1568−1573. (21) Song, F.; Tan, Y.; Xie, H.; Zhang, Q.; Han, Y. Direct synthesis of dimethyl ether from 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− 1208. (23) Nozawa, T.; Mizukoshi, Y.; Yoshida, A.; Naito, S. Aqueous phase reforming of ethanol and acetic acid over TiO2 supported Ru catalysts. Appl. Catal., B 2014, 146, 221−226.

12632

View publication stats

DOI: 10.1021/acs.energyfuels.7b01621 Energy Fuels 2017, 31, 12624−12632