1. Introduction
The worsening pollution induced by the increasing consumption of conventional fossil fuels and the ever-increasing dependency on imported oil make researchers search for eco-friendly energy which is sustainable and renewable [
1,
2]. Biomass is regarded as the most prospective alternative energy source, as it is the only renewable resource which can be directly translated into liquid carbon fuels, and it is carbon dioxide neutral due to photosynthesis. Generally, bio-oil is the liquid produced from lignocellulosic biomass, such as rice husk, miscanthus, pinewood, cellulose, and lignin, through thermochemical conversion [
3,
4]; it has some similarities to the chemical composition of petroleum crude and is recognized as the most promising alternative to petroleum, which could improve the nation’s energy independence as well as reduce greenhouse gas emissions [
5]. However, compared to petroleum, the bio-oil contains a significant quantity of oxygen (up to 35–40 wt %), which results in its lower pH and calorific value, chemical and thermal instability, and poor storage stability [
6,
7,
8,
9], and these properties make it difficult to directly use bio-oil as the liquid fuel for transportation. So, upgrading of the bio-oil method is needed.
Long-carbon-chain (C
16–C
22) fatty acids are normally selected as a model compound of bio-oil, especially algae-based bio-oil, to investigate bio-oil upgrading via deoxygenation reactions [
10]. After upgrading, long-carbon-chain hydrocarbons are obtained as final products, which can be a substitution for traditional diesel oil. Recently, there have been three deoxygenation approaches for fatty acids as follows: decarboxylation (DCX:
R–COOH →
R–H + CO
2), hydrodecarbonylation (HDC:
R–COOH + H
2 →
R–H + CO + H
2O) and hydrodeoxygenation (HDO:
R–COOH + 3H
2 →
R–CH
3 + 2H
2O) [
11,
12,
13,
14,
15]; further, three main catalyst systems have been applied to the deoxygenation of fatty acids: (1) NiMo or CoMo sulfide/supported catalysts [
16,
17]; (2) noble metal–supported catalysts, mainly including Pt- or Pd-based catalysts [
1,
18,
19,
20]; (3) supported non-noble transition metal catalysts (e.g., Fe, Co, Ni, Mo et al.) [
21,
22,
23,
24]. Sulfide/supported catalysts may contaminate the products as a result of sulfur leaching, and the noble metals are high in price, and these deficiencies restrict the practical usage for the former two catalyst systems. Meanwhile, the supported non-noble transition metal catalysts have the most promising prospect for their high catalytic activity and low cost and are feasible for application in the fatty acid deoxygenation process. In the published literatures, Chao et al. [
25] deoxygenated palmitic acid with the presence of water at 300 °C and 0.69 MPa of H
2 pressure over Ni/ZrO
2 and concluded that decarbonylation was the major route to generate 30.2% of C
15, 26.0% of C
8–C
14 and 18.6% of CH
4 as the main products. Imane et al. [
26] studied nickel supported on SiO
2 and γ-Al
2O
3 performed in HDO of stearic acid in a 3 MPa hydrogen atmosphere at 300 °C, obtaining both 100% acid conversion and 96.3% and 96.2% of n-heptadecane selectivity, respectively. Hui et al. [
27] used activated carbon-supported nickel phosphide catalyst in the deoxidation of palmitic acid at 350 °C and 5% H
2/Ar with the rate of 30 mL/min, and found that the coexistence of Ni
2P and Ni
12P
5 resulted in the optimum C
15 selectivity of 74.9%. Ning and his colleagues [
25,
28] examined different metal acetate salts for the catalytic deoxygenation of stearic acid, and found Ni(OAc)
2 exhibited the highest activity with 62% yield achieved at 350 °C for 4.5 h with only 1 mol % of the catalyst in the absence of H
2 and solvent; furthermore, they stated the diversified applications of nickel-based bimetallic catalysts for energy and the environment since Ni has a high alloying efficiency with all noble metals as well as many transition metals in different mass ratios.
Our group also explored the hydroconversion of octanoic acid with Ni/ZrO
2 at 320 °C and 3 MPa of H
2, obtaining 69.9% yield of C
7 as the main product through HDC of ocatnoic acid [
29]. These papers revealed Ni active particles as promising catalytically active components for fatty acid deoxygenation, but the reaction conditions were too demanding for the reaction temperatures all greater than or equal to 300 °C. It is well known that a relatively higher reaction temperature results in high energy consumption, and can induce undesirable side reactions such as coking or polymerization, which sequentially leads to catalyst deactivation [
11]. So, it is meaningful to investigate the deoxygenation of fatty acids under milder conditions. In our other former research, carbon nanotubes (CNTs)-supported MoO
2, Co-MoO
2 or β-Mo
2C catalysts were applied to the conversion of palmitic acid and stearic acid at 220 °C or 180 °C with 4 MPa of H
2, and these catalysts all had sufficient deoxygenation activity to obtain 100% acid conversion with around a 90% yield of alkanes without carbon loss [
30,
31,
32]. This phenomenon demonstrates that the CNTs are a promising catalyst support since the CNTs have high thermal conductivity, accessibility of the active phase, good chemical stability in aggressive media and a high surface area [
33,
34]; the unique electronic structure of CNTs exhibits a large charge transfer and deficient electron, as well as gaps, which decrease the diffusion resistance kinetically [
35,
36]. In addition, the active metal atoms have a stronger interaction with outside CNTs over the supported catalysts, which can also lead to structure defects of CNTs that affect the physical properties and chemical behavior of the catalysts, and further affect the hydrodeoxygenation of the reactant and the transfer of products inside or outside the CNTs [
37]. Furthermore, the CNTs possess a hydrophobic surface with relatively few or no functional groups, which decreases the probability of side reactions [
38] and promotes water desorption that favors the HDO reaction [
39,
40]. Although the reaction temperature has been lowered, the high H
2 pressure of 4 MPa is unfavorable to the safety and expense of hydrogenation equipment, so it is of significance to further research fatty acid conversion to alkanes at much milder reaction conditions.
In this paper, CNTs-supported Ni catalyst (5% Ni loading) was synthesized and applied in the deoxygenation of palmitic acid, and the 5% Ni/CNTs catalyst was characterized by XRD (X-ray powder diffraction), TEM (Transmission electron microscope) and nitrogen sorption. This work aims to design a new style of catalyst, displaying high efficiency for palmitic acid hydroconversion at a milder reaction condition, and hopes to further disclose the deoxygenation mechanism for fatty acids, sequentially contributing to the upgrading of bio-oil.