Elsevier

Renewable Energy

Volume 153, June 2020, Pages 1439-1454
Renewable Energy

Synergistic effects between Cu and Ni species in NiCu/γ-Al2O3 catalysts for hydrodeoxygenation of methyl laurate

https://doi.org/10.1016/j.renene.2020.02.099Get rights and content

Highlights

  • The deoxygenation pathway of methyl laurate can be affected by the content of Ni and Cu active components.

  • Formed NiCu alloy can effectively promote the electronic effect between Ni and Cu.

  • Developed mesoporous structure and highly dispersed Ni, Cu, and NiCu alloy are beneficial for hydrodeoxygenation.

  • NixCuy/γ-Al2O3 catalysts present superior methyl laurate hydrodeoxygenation performance and good stability.

Abstract

Cu was introduced into Ni/γ-Al2O3 to prepare mesoporous NixCuy/γ-Al2O3 catalysts with different Ni and Cu contents. H2-TPR, XRD, BET, H2-TPD, and in-situ XPS were used to study the physicochemical properties of the prepared NixCuy/γ-Al2O3 catalysts. The catalytic performances of NixCuy/γ-Al2O3 catalysts were evaluated by methyl laurate catalytic hydrodeoxygenation (HDO) reaction. The NixCuyO/γ-Al2O3 precursors can be reduced to Ni0, Cu0, and NiCu alloy active species by H2 at 420°C. Formed NiCu alloy can effectively promote the electronic effect between Ni and Cu, and enhance the adsorption and activation abilities of the corresponding catalyst for the reactant molecules. The Ni active sites preferentially catalyzes the decarbonylation/carboxyl (DCO) reaction in the deoxygenation of methyl laurate, while the HDO pathway is predominant on the Cu active sites. The deoxygenation pathway obviously changes from DCO to HDO at the mole ratio of Ni/Cu lower than 3/7, and the main deoxygenation products change from C11 to C12 alkane. At the H2/Oil ratio of 500N, the space velocity (SV) of 1.5 h−1, H2 pressure(P) of 2 MPa, and the reaction temperature of 380°C, the Ni3Cu7/γ-Al2O3 catalyst shows the best methyl laurate DCO properties. And methyl laurate conversion and the main deoxygenation products C11 alkane selectivity can reach 98.3 and 87.4%, respectively. Moreover, Ni3Cu7/γ-Al2O3 catalyst also exhibits good stability.

Graphical abstract

The introduction of Cu into the Ni based catalyst can enormously improve the dispersion of Ni0, Cu0, and NiCu alloy active components. Meanwhile, formed NiCu alloy can also effectively promote the electronic effect between Ni and Cu. The main deoxygenation pathway of methyl laurate and the product selectivity can be directly affected by the content of Ni and Cu active components in the prepared catalysts. The prepared NixCuy/γ-Al2O3 catalysts present superior catalytic hydrodeoxygenation performance and good use stability.

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Introduction

In recent years, the contradiction of the rapid development of society and energy supply, and environmental problems caused by the harmful gases releasing from fossil fuels are becoming more and more serious. It is urgent to find a substitute products of renewable energy, the good environmental friendly and renewability of biofuel (such as the aviation fuels and biodiesel) has aroused widespread concern [1]. Oils with different chain lengths can be hydrodeoxygenation (HDO) to produce alkanes with different chain lengths.

In the HDO reaction of oils, how to realize the efficiently conversion of reactants into target products, the design and development of catalyst with high performance is still the key to the current research. Unfortunately, the high costs limit the industrial application of noble metal catalysts. While NiMoS/γ-Al2O3 and CoMoS/γ-Al2O3 catalysts, which have been extensively used into the oils HDO reaction, easily cause the corresponding catalyst poisoning in the process of the oil HDO. At the same time, the obtaining diesel-range hydrocarbons also contain sulfide and their performance can be seriously affected. To solve these issues, researchers have paid more attention to non noble metal catalysts without sulfide. At present, non noble metal catalysts without sulfide used for oils catalytic HDO study is mainly focused on the Ni based catalysts. CHEN et al. [2] prepared Ni/KIT-6 catalyst to study the ester HDO reaction. The results show that the reactants can be completely converted, the selectivity of deoxygenation products can reach 98% at atmospheric pressure and 300 °C, thus the good ester catalytic HDO performance is obtained. In general, most oil HDO reactions are conducted under high temperature and pressure. From the point of kinetics, high temperature and pressure are more conducive to HDO reaction of oils. However, high temperature and pressure are easy to cause coking or carbon deposition on catalyst surface, and then affect catalytic activity. And formed carbon deposition is difficult to eliminate [3]. Confirmed by research, when Cu component is introduced into the Ni based catalyst, the dispersion of Ni active components on the carrier surface is greatly improved and metal Ni particle size decreases. The retention time of oxygen containing compounds on the Ni active sites is also shortened, which can decrease the formation rate of carbon deposition, and then prevent the burning of the active components [4]. When 1 wt% Cu is added into Ni–Al2O3 catalyst, the anti-carbon deposite ability of Ni–Al2O3 catalyst in the methane reforming reaction can be enhanced, and thus the activity and stability of the corresponding Ni-based catalyst is improved [5]. The outer electron configuration of Ni and Cu are 3d84s2 and 3d104s1, respectively [6]. Metal Ni has more vacant d orbit. Therefore, metal Ni has stronger adsorption ability for hydrogen species and bonding ability than metal Cu. While metal Cu can preferentially occupy the active sites of the support surface and form active sites, which is because that Cu has lower surface free energy than metal Ni [7]. H2 molecules are preferential to be adsorbed on the Ni active sites and then dissociated. The formed dissociated hydrogen species can easily move to Cu active sites, which leads to the formation of vacancies on the Ni active sites, and the formed vacancies on Ni active sites can adsorb other H2 molecules again. The hydrogen species migrated to Cu active sites are easily form adsorbed hydrogen species. These adsorbed hydrogen species can be desorbed from the catalyst surface at appropriate temperature, and thus provide reactive hydrogen species for the catalytic hydrogenation reaction [3].

In the catalytic HDO reaction of oils, the carrier is also a key factor affecting the catalytic performances of the catalyst. The large specific surface area and developed pore structure of the carrier materials can effectively promote the well dispersion of active components on the support surface, and enhance the interaction between the active components. The developed pore structure of the catalyst is beneficial for the diffusion and migration of large reactant and product molecules in the pore structure of the catalyst to increase the contact probability with the active sites, thus the catalytic performances of the corresponding catalyst can be improved. In the oils catalytic HDO reaction, C, metal oxides (such as Al2O3, TiO2, SiO2, CeO2, and ZrO2, etc.), and mesoporous silicon (such as SBA-15, MCM-41, Al-SBA-15, and Al-MCM-41, etc.) are commonly used as support for preparing supported catalysts [8]. The acidic property of supports also play an important role in the HDO reaction of oils, acidic carriers (for example Al2O3 and zeolites) are contribute to improve the hydrogenolysis activity of C–O bonds and the selectivity of HDO products [9,10]. However, the excessively high acidity of carrier material can not only promote aggregation of product moleculars, but also increase cracking of the alkane products, which lead to the increase of selectivity to low carbon alkanes or even forming carbon deposite. This phenomenon is not conducive to obtain the target products with high selectivity [[11], [12], [13]]. Porous carrier material γ-Al2O3 has large specific surface area and developed pore structure, and contains rich weak acid sites on the surface, which can effectively promote the dispersion of active components on the support surface. The interaction between the active components and the carrier can be enhanced, and provide additional active center for the catalytic hydrogenation reaction. At the same time, it can also improve the use stability of HDO reaction, prolong the service life, prevent carbon deposition and sintering for the corresponding catalyst [14]. Besides, porous γ-Al2O3 has achieved large-scale industrial production, and it is easy to obtain in low cost [15]. Therefore, porous γ-Al2O3 is an ideal support material for oils catalytic HDO to produce alkanes. At present, there are few studies on the application of NiCu/γ-Al2O3 catalyst into the HDO of oils.

In this study, metal Ni and porous γ-Al2O3 are used as active component and carrier, respectively. Metal Cu was used as promoter to prepare porous NixCuy/γ-Al2O3 catalysts with different Ni and Cu contents. The effect of Cu dosage on the performances of NixCuy/γ-Al2O3 catalyst in methyl laurate HDO reaction was studied. The physicochemical properties of the catalyst were studied by H2-TPR, H2-TPD, XRD, BET, and in-situ XPS, and the use stability of the NixCuy/γ-Al2O3 catalyst was also investigated.

Section snippets

Catalyst preparation

The support γ-Al2O3 (Zibo Alum Yifeng New Material Limited company, China) was appended in an aqueous nickelous nitrate solution and copper nitrate solution, which was prepared by dissolving a specified amount of Ni(NO3)2⋅6H2O (Chengdu Kelong Chemical Reagent Factory, China) and Cu(NO3)2⋅3H2O (Chengdu Kelong Chemical Reagent Factory, China) in deionized water. The mixtures were then evaporated at a fixed temperature, dried at 100 °C for 12.0 h, and calcinated in air at 500 °C for 4.0 h to

H2-TPR studies

To investigate the reducibility of NxCyOA samples, H2-TPR studies were performed on the prepared N10C0OA, N7C3OA, N5C5OA, N3C7OA, N1C9OA, and N0C10OA samples. The H2-TPR profiles and quantitative analysis of H2 consumption are shown in Fig. 1 and Table 1.

In Fig. 1, the H2 consumption peaks appeared at detected temperature can be devided into two regions named α and β, respectively. Except for N10C0OA and N0C10OA samples, the H2 consumption peaks α and β gradually become stronger and shaper with

Conclusion

NixCuy/γ-Al2O3 catalyst prepared by impregnation method exhibited superior catalytic HDO performances. The physicochemical properties and catalytic HDO performance of NixCuy/γ-Al2O3 catalyst were obviously influenced by Cu content. NixCuyO/γ-Al2O3 precursors can be effectively reduced to NixCuy/γ-Al2O3 catalysts by H2 at 420 °C. The prepared NixCuy/γ-Al2O3 catalysts maintain the large specific surface area and the developed mesoporous structure of the carrier γ-Al2O3, which is beneficial for

CRediT authorship contribution statement

Caixia Miao, Guilin Zhou, and Xianming Zhang conceived and designed the experiments. Guilin Zhou and Xianming Zhang supervised the study. Caixia Miao and Shuang Chen carried out materials syntheses. Caixia Miao, Hongmei Xie, and Shuang Chen performed all activity tests and related measurements and interpretation of results. Caixia Miao, Guilin Zhou, and Hongmei Xie performed the original draft. All authors discussed the results and edited the manuscript. All authors reviewed and approved the

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledge

This research is funded by Chongqing Technological Innovation and Application Demonstration (general social and livelihood) projects (cstc2018jscx-msybX0341); Research Foundation of Chongqing Technology and Business University (No. 1752001); Scientific and Technological Key Program of Chongqing Municipal Education Commission (KJZD-K201800801).

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