Short communicationPromoting selective hydrodeoxygenation of guaiacol over amorphous nanoporous NiMnO2
Graphical abstract
Introduction
Lignocellulosic biomass is considered to be the cheapest, readily available, and abundant renewable resource that has the potential for replacing fossil energy in the near future [1]. Converting biomass to bio-oil by fast pyrolysis is one of the most simple and cost-effective approaches [2]. However, the oxygen content in bio-oil is as high as 50 wt%, thereby making it unusable as a transportation fuel [3]. Moreover, bio-oil has the disadvantages of low calorific value, low stability, and high corrosivity [4]. Thus, hydrodeoxygenation (HDO) of bio-oil seems to be an effective approach to decrease its oxygen content. Guaiacol is an ideal model compound as representative products from lignin-derived bio-oil; thus, HDO of guaiacol is extensively employed to evaluate catalysts for their potential industrial use [5].
Guaiacol contains aryl ethers (Ar-OCH3) and phenolic hydroxyl groups (Ar-OH), including CO bonds, in three different chemical environments. Its highly selective HDO is generally considered to be challenging because the HDO process mainly involves CO bond cleavage and H-addition. Lee et al. found that Rh/SiO2-Al2O3 and Ru/SiO2-Al2O3 exhibited the highest cyclohexane yield at 250 °C, and the acidic supporter was essential for the deoxygenation of oxygenates [6]. Zheng et al. studied the hydrogenolysis catalytic capability of Ru/HZSM-5 with diverse morphologies [7]. Cross-shaped Ru/HZSM-5 showed excellent activity due to its preferred adsorption to guaiacol. Zhao et al. revealed that commercial Pd/Al2O3 had better catalytic activity than metal phosphide catalyst [8]. Although noble metal catalysts are active for HDO reaction, the cost will inevitably be increased and limit their broad industrial use.
Metallic Ni, especially supported Ni catalysts, are widely used for HDO reaction [9]. For example, Ni/TiO2 − ZrO2 was used for HDO of guaiacol, and the conversion was 100% with 86.4% selectivity of cyclohexane at 300 °C and 4 MPa H2 [10]. A recent study showed that the selectivity of phenolic compounds was significantly promoted due to the strong interaction of Ni with TiO2 [11]. The abovementioned supported catalysts tend to be performed under relatively higher reaction temperatures probably because of the higher energy barrier of activating CO bond.
The nanoporous metals consist of nanopores and ligaments and a large number of active sites are interconnected in the interior of the catalyst. They have exhibited excellent catalytic performance under mild conditions in hydrogenation and dehydration reactions [12]. Nanoporous Ni (NP-Ni) has attracted much attention due to its low price, high activity, and easy separation [13]. Wang et al. demonstrated that HDO of phenolic and aromatic biorefinery feeds over RANEY® Ni were realized by hydrogen transfer [14]. However, the catalytic performance of HDO will be limited by a large amount of water in the bio-oil [15]. In addition, because water is the main component of bio-oil, hydrodeoxygenation of guaiacol as representative products from lignin-derived bio-oil has been widely studied in the water phase.
In general, nanoporous catalysts have relatively lower specific activity over supported catalysts, wherein the use of support would allow the active component to have a larger exposed surface. Thus, NP-Ni was doped by the modified components with the mechanical alloying method to promote its catalytic performance in this work. The platform and step sites of NP-Ni were designed to be further dissociated into smaller one in the form of amorphous state, which might be favorable for promoting the catalytic performance of nanoporous-type catalysts. Under optimized conditions, a highly selective HDO of guaiacol to cyclohexanol was achieved with 100% conversion at 150 °C and 0.5 MPa H2 over NP-NiMnO2 catalyst.
Section snippets
Catalyst preparation
Mechanical alloying of metal powders was carried out in the planetary ball mill (QM-3SP04, Nanjing Nanda Instrument Co., Ltd.). A certain mass ratio of 4 g Ni, Al, and MnO2 metal powders (high purity and superfine, Nangong Xinshi Alloy Welding Material Spraying Co., Ltd.) and 0.4 mL ethanol (analytical purity, Tianjin Fuyu Fine Chemical Co., Ltd.) were added into the ball mill pot. The grinding balls were made of zirconia materials with a diameter of 3 mm. The weight ratio of ball to powder was
Catalyst characterization
Fig. 1 shows the XRD characterization results of the nanoporous catalysts. Two samples had similar diffraction peaks (2θ) at 44.5°, 51.9°, 76.4°, 92.9° and 98.4°, which correspond to Ni (111), (200), (220), (311) and (222) (JCPDS Card No. 04–0850), respectively. At the same time, intermetallic compounds AlNi (JCPDS Card No. 20–0019) and AlNi3 (JCPDS Card No. 50–1265) remained in the catalysts. However, NP-NiMnO2 showed a sharp and intense diffraction peak only at 44.5°, and the other peaks were
Conclusions
In conclusion, HDO of guaiacol was evaluated over nonprecious nanoporous metal catalysts at relatively lower reaction temperatures. The study demonstrated that NP-NiMnO2 is preferred to better catalytic activity than NP-Ni for the conversion of guaiacol to cyclohexanol under the same conditions because of its unique amorphous structure, although the reaction mechanism was similar for two kinds of catalysts. Guaiacol is mainly converted into cyclohexanol through demethoxylation and hydrogenation
Declaration of Competing Interest
The authors declare that they have no conflict of interest in this work.
Acknowledgements
This work was financially supported by the Fundamental Research Funds for the Central Universities (DUT19LK29, DUT19TD28, and DUT2019TA06) and the National Key Research and Development Program of China (SQ2019YFC180016).
References (26)
- et al.
Influence of preparation method and doping of zirconium oxide onto the material characteristics and catalytic activity for the HDO reaction in nickel on zirconium oxide catalysts
J. Catal.
(2018) - et al.
Catalytic roles of metals and supports on hydrodeoxygenation of lignin monomer guaiacol
Catal. Commun.
(2012) - et al.
Hydrodeoxygenation of guaiacol as model compound for pyrolysis oil on transition metal phosphide hydroprocessing catalysts
Appl. Catal. A Gen.
(2011) - et al.
High-temperature stability of nano-grained B2-structured RuAl-based intermetallics by mechanical alloying
Intermetallics
(2005) - et al.
Selective cyclohexanol production from the renewable lignin derived phenolic chemicals catalyzed by Ni/MgO
Energy Convers. Manag.
(2015) - et al.
Demethoxylation of guaiacol and methoxybenzenes over carbon-supported Ru–Mn catalyst
Appl. Catal. B Environ.
(2016) Catalytic hydrodeoxygenation
Appl. Catal. A Gen.
(2000)- et al.
Mechanistic details of C O bond activation in and H-addition to guaiacol at water-Ru cluster interfaces
J. Catal.
(2019) - et al.
Catalytic transformation of lignin for the production of chemicals and fuels
Chem. Rev.
(2015) - et al.
Heterogeneous catalytic transfer hydrogenation as an effective pathway in biomass upgrading
ACS Catal.
(2016)