Highly efficient conversion of fatty acids into fatty alcohols with a Zn over Ni catalyst in water

Abstract

A new route to convert fatty acids into fatty alcohols under hydrothermal conditions with a Zn reductant over an Ni catalyst is presented. The highest yield of fatty alcohols from fatty acids (81.4%) was achieved at 300 °C for 2 h with a water filling of 40%. The Zn and Ni used in this research were both commercially available powders, and thus this process provides a highly efficient and simple method for reducing fatty acids into fatty alcohols.

---

With the increasing demand for limited petroleum resources, it is imperative to develop economical and energy-efficient processes for the sustainable production of chemicals and fuels.¹ Over the past several decades, biomass has attracted more and more attention to be used for the production of high value-added chemicals due to its excellent properties, such as abundance, renewability, and less pollution.²

Natural oils and fats, derived from biomass (e.g., plant and animal oils), are complex mixtures of triglycerides, which consist of a glycerol backbone with three fatty acid moieties. These natural oils and fats can be easily converted into fatty acids through hydrolysis with carbon numbers ranging from about 8 to 24, in which the C₁₂, C₁₆, and C₁₈ fatty acids are usually the most abundant.³ These natural materials can serve as feedstock for the production of renewable liquid transportation fuels and high value-added chemicals such as fatty alcohols.¹

Fatty alcohols derived from natural fats and oils usually have a high molecular weight and straight chain with carbon numbers ranging from as few as 4–6 to as many as 22–26.⁴ Fatty alcohols usually have an even number of carbon atoms and a single alcohol group (–OH) attaching to the terminal carbon. Some are unsaturated and some are branched. Fatty alcohols are widely used in the production of lubricants, resins, perfumes, cosmetics, shampoos and conditioners.^5,6 Recently, the potentials of fatty alcohols uses in medicine, health supplements and biofuel production have been discovered.⁷

In the past several decades, many methods of fatty alcohol production have been proposed. However, most of the previous work^5,6,8–10 on fatty acid hydrogenation were done in organic reaction media, such as dodecane and toluene, which were adverse to the environment. These routes usually involve expensive or complicatedly prepared hydrotreatment catalysts, such as Pt/TiO₂, ReO_x–Pd/SiO₂, Cu(OTf)₂ and Co(BF₄)₂·6H₂O (Scheme 1).^5,9,10,23 Moreover, the high hydrogen consumption associated with these processes is another major drawback. Currently, hydrogen is not available in large quantities from renewable resources and not easy to obtain at a reduced energy cost. On the other hand, it is difficult and unsafe to transport and store high-purity and high-pressure gaseous hydrogen due to its instability and flammability.¹¹ Therefore, developing new methods for the hydrogenation of fatty acids into fatty alcohols with alternative hydrogen sources and simple catalysts is urgently required.

Scheme 1 Previous and present proposed processes for the conversion of fatty acids into fatty alcohols.

A series of researches on the conversion of biomass into value-added chemicals using high-temperature water have been reported.^12–14 In these processes, water acts not only as an excellent and environmentally benign reaction medium but also as an in situ hydrogen donor through the reaction of water with cheap metal reductants.¹⁵ Recently, some research groups have reported that ZnO could be converted into Zn through a ZnO/Zn redox process using solar energy.^16,17 These findings inspired us to study the potential of the conversion of natural oils and fats derived from biomass into fatty alcohols with water as a hydrogen source in the presence of metals. In this paper, the reaction of palmitic acid, which was used as a model compound of fatty acids with high molecular weights, into hexadecanol proceeded effectively with a high yield and a high selectivity in high-temperature water in the presence of Ni as the catalyst.¹⁷ The detailed results are reported herein.

An experiment of palmitic acid, as a model compound of fatty acids, with Zn as a reductant and Ni as a catalyst at 300 °C for 1 hour was conducted to investigate the possibility of the hydrothermal conversion of fatty acids into their corresponding alcohols. The reaction proceeded efficiently and produced the desired hexadecanol with a 51.5% yield, which was determined via GC-FID and GC-MS (Table 1, entry 4). The remaining solid residue after the reaction was collected and analyzed via XRD. The results showed that Ni remained unchanged, while Zn was oxidized into ZnO after the reaction (see Fig. SI-4†). Therefore, Zn acted as a reductant, and Ni performed as a catalyst in the conversion of palmitic acid into hexadecanol.

Table 1 Effects of catalysts on the yields of hexadecanol from palmitic acid^a

Entry	Reductant	Catalyst	Yield^b (%)
a Reaction conditions: palmitic acid: 0.2 mmol, Zn: 5.0 mmol, catalyst: 1.0 mmol, water filling: 40%, temp.: 300 °C, time: 1 h.b The hexadecanol yield is calculated as the percentage of hexadecanol to the initial palmitic acid based on carbon.
1	—	—	0
2	Zn	—	11.2
3	—	Ni	0
4	Zn	Ni	51.5
5	Zn	Cu	33.4
6	Zn	CuO	1.2
7	Zn	Cu2O	14.2
8	Zn	Mo	0
9	Zn	TiO2	0
10	Zn	Fe2O3	3.3
11	Zn	CuFe2O4	0
12	Zn	AgSiO2	0
13	Zn	MnO2	0
14	Zn	WC	2.2
15	Zn	Pd/C	8.4
16	Zn	Ru/C	7.2

---

The effects of various catalysts other than Ni on the conversion of fatty acids into fatty alcohols were also investigated. The results are summarised in Table 1. There was no hexadecanol detected in the absence of both the reductant and the catalyst or in the presence of the catalyst only (entries 1 and 3). However, in the reaction using 6 mmol of Zn, hexadecanol was generated with a yield of 11.2% even without the addition of any catalyst (entry 2), which suggested that the reductant was indispensable in this reaction. It can be seen from Table 1 that no formation of hexadecanol was observed when the catalysts of Mo, TiO₂, CuFe₂O₄, AgSiO₂, MnO₂ were used (entries 8–9 and 11–13), and low yields of hexadecanol were obtained when using CuO, Cu₂O, Fe₂O₃, WC, Pd/C and Ru/C as the catalysts in the presence of Zn (entries 6, 7, 10 and 14–16). Among all of the catalysts we investigated, only Ni and Cu exhibited good catalytic activities in promoting the yield of hexadecanol from palmitic acid (entries 4 and 5), and Ni performed better than Cu.

The effects of different reductants on the conversion of palmitic acid into hexadecanol were investigated by using various metals, such as Fe and Mn. Among all of the tested metals, Zn was the most effective in converting palmitic acid into hexadecanol with the best yield of 51.5% compared with Fe and Mn (see Fig. SI-5†). Thus, Zn was chosen as the reductant for the following experiments.

The effect of the reaction time on the yields of hexadecanol was investigated by changing the reaction time from 0.5 h to 6.0 h at 300 °C. Fig. 1 shows that the yield of hexadecanol increased drastically when the reaction time was less than 2 hours. However, a gradual decrease in the hexadecanol yield with time was observed after 2 hours. On the other hand, production of pentadecane was also detected, the yield of which increased significantly as the yield of hexadecanol decreased, suggesting that the produced hexadecanol was further reduced into pentadecane as the reaction was prolonged. The GC spectra of the liquid products after these reactions can be found in Fig. SI-7 and 8.† The maximum yield of hexadecanol with a value of 81.4% was obtained at the reaction time of 2 hours. And an almost 100% total yields of hexadecanol plus pentadecane from palmitic acid can be achieved at reaction time ranging from 2 to 6 h (Scheme 2).

Fig. 1 (a) Effect of the reaction temperatures on the yield of hexadecanol (2 h) and the effect of the reaction time on the yield of (b) hexadecanol and (c) pentadecane (300 °C, palmitic acid: 0.2 mmol, Zn: 5.0 mmol, Ni: 1.0 mmol, water filling: 40%).

Scheme 2 Conversion of palmitic acid into hexadecanol and pentadecane.

The effect of reaction temperatures from 250 to 325 °C on the yield of hexadecanol was also studied with the results shown in Fig. 1. The yield of hexadecanol improved remarkably as the reaction temperature increased from 250 °C to 300 °C. However, a slight decrease in the yield of hexadecanol was observed as the temperature further increased from 300 to 325 °C. These results indicated that side reactions of palmitic acid might occur at a higher temperature, which resulted in the slight decline in the yield of hexadecanol to 78.4% at 325 °C. The maximum hexadecanol yield of 81.4% was obtained at 300 °C. Therefore, the temperature of 300 °C was used as the optimal reaction temperature in the following experiments.

Fig. 2 shows the effect of the Zn amount on the yield of hexadecanol. There was no hexadecanol production observed without the addition of Zn. The yield of hexadecanol improved gradually as the amount of Zn increased from 0 to 5.0 mmol. Since Zn was oxidized to ZnO to produce H₂, which has been well studied in our previous studies,²⁴ more hydrogen was produced with a higher amount of Zn. Then, the conversion of palmitic acid into hexadecanol was facilitated. When the amount of Zn increased to 6.0 mmol, the yield of hexadecanol had no significant difference with that obtained with 5.0 mmol of Zn. Therefore, the optimal amount of Zn was 5.0 mmol, yielding hexadecanol with a maximum value of 81.4%.

Fig. 2 Effects of the amounts of Zn and Ni on the yields of hexadecanol (palmitic acid: 0.2 mmol, water filling: 40%, temperature: 300 °C, time: 2 h).

The effect of Ni amount was then investigated at 300 °C for 2 hours, and the results are also shown in Fig. 2. The yield of hexadecanol firstly ascended significantly when the amount of Ni increased from 0 to 1.0 mmol. However, no further increase in the hexadecanol was observed when more Ni was added. The best hexadecanol yield of 81.4% was achieved with 1.0 mmol Ni.

Furthermore, we investigated the effects of water filling with a fixed reaction concentration at 300 °C for 2 hours. The results showed that water filling had a significant effect on the hexadecanol yield (see Fig. SI-6†). An increase in water filling from 20% to 40% led to an increase in the yield of hexadecanol from 35.4% to 81.4%, and a decrease when water filling exceeded 40%. Under our experimental conditions, H₂ is generated during the reaction. When increasing water filling, the spare space in the reactor should decrease, which causes an increase in the partial pressure of hydrogen. The increase in the partial pressure of hydrogen would be favorable for the reduction/hydrogenation of organics. Thus, the observed results that increasing water filling led to the increase in the yield of hexadecanol can be explained as the increase in the partial pressure of hydrogen, and results that the decrease in alcohols with further increase in water filling may be because a higher partial pressure of hydrogen leads to the reduction hexadecanol into pentadecane.

Finally, an experiment using lauric acid as another model compound of fatty acids to replace palmitic acid was conducted. After reacting 0.2 mmol of lauric acid, 5.0 mmol of Zn and 1.0 mmol of Ni with a water filling of 40% at 300 °C for 2 hours, the desired dodecanol with a yield of 53.2% was generated, indicating that fatty acids can be converted into the corresponding alcohols.

The reduction pathway of carbonic acids into alcohols, which involves the carbonic acid first reducing into an aldehyde and then to an alcohol, has been well established and widely accepted.^25,26 Therefore, our proposed reaction mechanism based on the literature was reasonable. However, in order to further support our proposed reaction mechanisms under hydrothermal conditions, experiments with dodecanoic acid and dodecanal instead of palmitic acid were conducted at 300 °C. Dodecanal was detected via GC-mass after the reaction of dodecanoic acid. After 2 h, a total of 95% of dodecanal was changed to dodecanol. So it is reasonable to deduce that dodecanal was an intermediate (see Fig. SI-9†). The probable reaction pathway of the conversion of fatty acids into their corresponding alcohols over Ni catalyst in high temperature water (HTW) can be proposed in Scheme 3. The new experimental data are now added in the manuscript.

Scheme 3 Proposed mechanism of the reduction of fatty acids into fatty alcohols with the Zn over Ni catalyst in HTW.

The reaction probable pathway of the conversion of fatty acids into their corresponding alcohols over the Ni catalyst in high temperature water (HTW) can be proposed in Scheme 3.^17–22 First, hydrogen is formed via the reaction of Zn with HTW.²¹ The in situ formed hydrogen is adsorbed onto the surface of Ni + ZnO through the hydrogen bonds between hydrogen and Ni. Subsequently, fatty acids are adsorbed onto the surface of Ni + ZnO with carbonylic C and O atoms.²² When hydrogen reacts with the fatty acid molecules, an intermediate hexadecanal is obtained. Then, H is added to C=O and an intermediate linked by α-bond is formed. The intermediate acquires one more H from the surface of Ni and the desired fatty alcohols are obtained. When Zn was substituted by gaseous hydrogen in the reduction of palmitic acid with Ni as a catalyst in HTW, there was no hexadecanol generated, which suggested that the in situ formed hydrogen showed a higher activity than ordinary hydrogen. Therefore, the in situ formed hydrogen is key to promote the reduction of fatty acids into fatty alcohols.

Conclusions

A highly efficient method of reduction of fatty acids with a high molecular weight into their corresponding fatty alcohols over a commercial available Ni powder catalyst and Zn powder reductant in high temperature water (HTW) was, for the first time, reported in this work. An excellent fatty alcohol yield of approximately 81.4% was obtained. The recycled Ni catalyst displayed stable catalytic activity. This study provides a significant process for the conversion of fatty acids into fatty alcohols with high selectivity and high yield.

Acknowledgements

The authors thank the financial support of the National Natural Science Foundation of China (No. 21277091), the State Key Program of National Natural Science Foundation of China (No. 21436007), Key Basic Research Projects of Science and Technology Commission of Shanghai (No. 14JC1403100) and the Postdoctoral Research Fund (No. AF0500041). We gratefully acknowledge the financial support from the ENN Institute.

Notes and references

1. G. W. Huber, S. Iborra and A. Corma, Chem. Rev., 2006, 106, 4044–4098.
2. F. Jin and H. Enomoto, Energy Environ. Sci., 2011, 4, 382–397.
3. J. Fu, X. Lu and P. E. Savage, ChemSusChem, 2011, 4, 481–486.
4. K. Noweck and W. Grafahrend, Ullmann’s encyclopedia of industrial chemistry, 2006.
5. H. G. Manyar, C. Paun, R. Pilus, D. W. Rooney, J. M. Thompson and C. Hardacre, Chem. Commun., 2010, 46, 6279–6281.
6. B. Peng, X. Yuan, C. Zhao and J. A. Lercher, J. Am. Chem. Soc., 2012, 134, 9400–9405.
7. J. L. Hargrove, P. Greenspan and D. K. Hartle, Exp. Biol. Med., 2004, 229, 215–226.
8. E. Brenna, F. Cannavale, M. Crotti, F. Parmeggiani, A. Romagnolo, F. Spina and G. C. Varese, J. Mol. Catal. B: Enzym., 2015, 116, 83–88.
9. Y. Takeda, Y. Nakagawa and K. Tomishige, Catal. Sci. Technol., 2012, 2, 2221–2223.
10. Y.-J. Zhang, W. Dayoub, G.-R. Chen and M. Lemaire, Tetrahedron, 2012, 68, 7400–7407.
11. Z. Huo, L. Xu, X. Zeng, G. Yao and F. Jin, in Application of Hydrothermal Reactions to Biomass Conversion, Springer, 2014, pp. 139–152.
12. G. W. Huber, J. N. Chheda, C. J. Barrett and J. A. Dumesic, Science, 2005, 308, 1446–1450.
13. G. W. Huber, R. D. Cortright and J. A. Dumesic, Angew. Chem., Int. Ed., 2004, 43, 1549–1551.
14. G. W. Huber, J. Shabaker and J. Dumesic, Science, 2003, 300, 2075–2077.
15. A. Kruse and E. Dinjus, J. Supercrit. Fluids, 2007, 39, 362–380.
16. A. Steinfeld, Solar Energy, 2005, 78, 603–615.
17. L. Xu, Z. Huo, J. Fu and F. Jin, Chem. Commun., 2014, 50, 6009–6012.
18. F. Chen, G. Yao, Z. Huo and F. Jin, RSC Adv., 2015, 5, 11257–11260.
19. A. A. Peterson, F. Vogel, R. P. Lachance, M. Fröling, M. J. Antal Jr and J. W. Tester, Energy Environ. Sci., 2008, 1, 32–65.
20. J. N. Chheda, G. W. Huber and J. A. Dumesic, Angew. Chem., Int. Ed., 2007, 46, 7164–7183.
21. F. L. Resende and P. E. Savage, Ind. Eng. Chem. Res., 2010, 49, 2694–2700.
22. Z.-p. Yan, L. Lin and S. Liu, Energy Fuels, 2009, 23, 3853–3858.
23. T. J. Korstanje, J. I. van der Vlugt, C. J. Elsevier and B. de Bruin, Science, 2015, 350, 298–302.
24. F. Jin, X. Zeng, J. Liu, Y. Jin, L. Wang, H. Zhong, G. Yao and Z. Huo, Sci. Rep., 2014, 4, 4503.
25. D.-H. He, N. Wakasa and T. Fuchikami, Tetrahedron Lett., 1995, 36(7), 1059–1062.
26. X. Cui, Y. Li, C. Topf, K. Junge and M. Beller, Angew. Chem., 2015, 127, 10742–10745.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental details, characterization data. See DOI: 10.1039/c6ra01150k

---