Effect of metal–support interaction on the selective hydrodeoxygenation of anisole to aromatics over Ni-based catalysts
Graphical abstract
Introduction
Lignocellulose can be transformed by fast or flash pyrolysis, yielding up to 70% of a liquid fraction known as bio-oil, which can be used for biofuel production. However, the high oxygen content of this product leads to chemical instability, immiscibility with fossil fuels and corrosion, preventing the direct application of this liquid in the transportation sector [1]. Therefore, upgrading bio-oils to liquid hydrocarbons is an important milestone in the development of a feasible route for obtaining sustainable fuels from lignocellulose biomass [2]. Zeolite upgrading [3], [4], [5], [6], [7] and hydrotreating [8], [9], [10], [11] are the two main approaches for bio-oil conversion, and accordingly these processes are being intensively studied. Though zeolites are effective in conversion of small oxygenates (such as aldehydes and ketones) through acid catalyzed reactions, their capability for deoxygenation of phenolic compounds derived from lignocellulose is limited due to extensive formation of coke [12]. Hydrodeoxygenation (HDO) at high pressure (30–50 bars) is an effective method to yield aliphatic hydrocarbons, but full hydrogenation of the aromatic ring of phenols and other aromatic chemicals over metal sites is inevitable under these conditions [8]. As a consequence the hydrogen consumption is high during these processes. Minimizing both carbon losses and also hydrogen demand (i.e. improving hydrogen efficiency) are important parameters for reducing the cost of biomass-derived liquids [13]. In this context, production of aromatic hydrocarbons such as toluene or benzene from the lignin fraction of biomass arises as an economically favorable process. Furthermore, arenes are important building blocks in petrochemical industry and replacing the feedstock from fossil to renewable resources in a biorefinery scheme is certainly advantageous.
The aim of this work is to gain better understanding of the factors influencing the performance of the catalytic systems for the selective production of aromatic chemicals by bio-oil upgrading. In the present study anisole has been chosen as a molecule representative of those present in bio-oil obtained from feedstocks with high lignin content, because it contains a methoxy-phenyl moiety, which is characteristic of some of the main components of these liquids [14]. Furthermore, the relatively simple product distribution should facilitate the interpretation of the activity of the assayed catalysts. Previous studies on anisole hydrodeoxygenation were performed using catalysts based on Pt [15], Pt–Sn [16], metallic phosphides [17] or HZSM-5 and HY zeolites [18]. More recently, the use of non-sulfided Ni–Cu catalysts has been proposed as interesting alternative for hydrotreating bio-oils [19], while copper–chromite catalysts have been successfully applied to the HDO of a number of lignin model molecules [20]. In this line, the present work explores the capacity of the metal–support interaction to modulate the performance of Ni-based hydrotreating catalysts. With this aim, supports with properties as diverse as microporous carbon, γ-Al2O3, SBA-15, Al-SBA-15, TiO2 and CeO2 were selected for this study. These carrier materials are widely used to disperse active metals and they provide a variety of morphological and chemical characteristics, which can aid to determine the influence of textural properties, composition and acidity on modulating the characteristics of the Ni centers for the selective hydrodesoxygenation of anisole toward aromatics. In this respect, high surface carriers are expected to enhance metal dispersion at high loading, leading to an improved hydrogenation capacity. However, the metal distribution on the support will also depend on the specific surface interactions established during impregnation, such as those determined by the point of zero charge (PZC) of the solid. On the other hand, acid centers crucially determine catalytic behavior, and they have been reported to facilitate the hydrogenolysis of anisole and transmethylation reactions [18], [20]. In addition, a moderate acidity of the support may promote the hydrogenation capacity of Ni [21]. Similarly, redox oxides such as TiO2 may also contribute to promote HDO activity, as observed in bimetallic Ni–Cu catalysts [19].
Section snippets
Catalysts preparation
For the synthesis of SBA-15, the structure-directing agent, Pluronic 123 (MW 5800, Aldrich), was dissolved in HCl solution at room temperature, and then heated to 40 °C before adding tetraethyl orthosilicate (TEOS; Aldrich), used as silica source. This mixture was stirred for 20 h at 40 °C, and aged at 100 °C for 24 h under autogeneous pressure. Thereafter, the solid product was recovered by filtration and dried overnight. The surfactant was removed by calcination at 550 °C for 5 h. Al-SBA-15
Physicochemical characterization of the catalysts
In the present work a series of Ni-containing (20% loading) catalysts based on SBA-15, Al-SBA-15, γ-Al2O3, microporous carbon, TiO2 and CeO2 supports were prepared to determine the influence of metal–support interactions on the hydrotreating performance. In order to stabilize the activated form of the catalysts for characterization, they were previously reduced and passivated according to the procedure mentioned above. Textural properties of both the supports and the corresponding Ni-supported
Conclusions
In this work, a series of different Ni-based catalysts with a broad range of physicochemical properties were prepared and tested for the hydrodeoxygenation of anisole. All these catalysts show high activity and excellent selectivity toward benzene production under relatively low hydrogen pressure (3 bars) and moderate temperatures (290–310 °C). Regarding aromatic production, better results are obtained at higher temperature and lower space velocity. Significant differences of the product
Acknowledgements
This study has received financial support of the RESTOENE program funded by Consejería de Educación of Comunidad de Madrid and LIGNOCATUP from the Spanish Secretary of State for Science and Innovation (ENE2011-29643-C02-01). YXY and VPO thank the financial support of AMAROUT (European Commission) and “Ramón y Cajal” (MINECO) programs, respectively.
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