Catalytic upgradation of bio-oil over metal supported activated carbon catalysts in sub-supercritical ethanol
Graphical Abstract
Introduction
Worldwide demand for energy resources is on the rise due to the rapid development of society. Renewable energy sources are becoming more important due to the issues arising from burning of fossil fuels [1], [2]. Another attractive approach for fuel production is the transformation of waste materials to bio-oil through thermochemical processes which is widely considered as a replacement for conventional fossil fuel. However, bio-oils possess several undesired properties i.e. the presence of unsaturated and oxygenated compounds, high oxygen, water content, acid value and high reactivity limits its utilization for fuel applications [3], [4]. The presence of oxygenated compounds i.e. aldehydes and phenols increases the viscosity of crude bio-oil which is disadvantageous for flow properties of fuel, lowers the heating value and produces relatively thermally unstable and extremely corrosive oil [5]. With the apparent shortcomings, bio-oil upgradation is thus necessary to exploit its true potential as a renewable energy source. Solvents at supercritical fluid conditions that enhance the heat and mass transfer rates have been widely used to upgrade bio-oil [4], [5], [6]. Supercritical fluids may work in the same way as liquids, with high dissolving ability, and in the same way as gas, with high diffusivity, liquid like density which increases dissolution power and in the formation of a homogenous reaction system [6]. In general, supercritical solvents acquired to upgrade bio-oil mostly contains methanol and ethanol [7], [8]. Recent literature related to supercritical solvent provides widely studied factors that influence the cycle of bio-oil upgrading process [9], [10], [11], and the performance of different catalysts [10], [11]. The solvent also has substantial impact on the upgrading process of bio-oil. Ethanol can be obtained from certain materials having lignocelluloses and it is also known as an eco-friendly solvent [12]. Sub-supercritical ethanol offers a single-phase reaction environment as it is a super solvent (243.1 °C, 6.3 MPa). Moreover, ethanol having sub-supercritical properties is also used as a hydrogen donor to produce significant and higher concentrations of hydrogen during hydrogenation process and to prevent the formation of coke during upgradation process [13].
Different studies having been conducted to obtain hydrogen-rich fuel [14], [15]. Zhang et al. [14] studied the utilization of Ru/α-Al2O3 catalyst for the conversion of heavy bio-oil in to a rich hydrocarbon fuel (23.15%), through fast pyrolysis of rice husk with catalytic depolymerization and hydrodeoxygenation in ethanol. Zhang et al. used Ni/MgO catalyst in an organic solvent to upgrade bio-oil obtained from rice husk and reported an increase in HHV (24.9 MJ/kg) with optimized catalytic upgrading process [16]. Zhang (2015) used multi-functional catalyst Ni/SiO2-ZrO2 in ethanol solvent by employing esterification, hydrogenation, hydrodeoxygenation, and depolymerization processes. The findings indicated alcohols, phenols, esters, and cyclic ketones as the main volatile components present in the upgraded bio-oil. Furthermore, the aldehydes were fully disappeared due to catalytic hydrogenation and the organic acids were converted into esters [13]. Ardiyanti et al. used mono and bimetallic catalysts to upgrade wood-based pyrolysis oil and reported maximum bio-oil yield (37–47 wt%) in the product. Whereas, the oxygen content remarkably decreased from 40.1 wt% to 7–11 wt% [17]. Thermal stability experiments on chemical modifications in bio-oil pyrolysis performed at high temperatures (up to 90 °C) for a longer period of time (up to one week) have been reported [18], [19], [20]. However, there are no studies available for upgrading of bio-oil using metal embedded activated carbon catalysts in sub-supercritical conditions.
The present study was carried out to generate high-quality and improved bio-oil in sub-supercritical ethanol with Pt/AC and Pd/AC catalysts for subsequent applications in a single pot process. The objectives of this work are (1) to elucidate the role of ethanol as a solvent in sub-supercritical conditions; (2) to describe the solvent-mediated reactions between raw bio-oil and upgraded bio-oil by analyzing the modifications in chemical compounds to investigate different reaction pathways; (3) to outline the role of metal embedded carbon catalysts.
Section snippets
Materials
The raw bio-oil used in this study for upgrading was derived from the co-liquefaction of polyethylene and sugarcane bagasse and reported in our previous study [21]. Platinum on activated carbon (10 wt% Pt/AC) and palladium on activated carbon (10 wt% Pd/AC) catalysts were ordered from Sigma-Aldrich Pty Ltd., were used without any pre-treatment.
Experimental setup and upgrading procedure
Bio-oil upgrading was performed in 600 mL Parr stirred reactor at sub/supercritical conditions i.e. 200 °C and 250 °C. 30 g of bio-oil, 300 mL ethanol
XRD analysis of catalysts
The XRD diagrams of six different catalysts such as fresh Pt/AC catalyst, fresh Pd/AC catalyst, Pd/AC and Pt/AC catalysts used in subcritical (200 °C) and supercritical (250 °C) conditions during upgrading process are shown in Fig. 2. The diffraction at 2θ < 30° refers to support of carbon black [23]. As seen in Fig. 2, a weak Pt and Pd crystalline peaks were observed at around 2θ = 43° in XRD diagrams of fresh Pd/AC and Pt/AC catalyst, demonstrating a well dispersion of Pd and Pt on carbon
Conclusion
Bio-oil upgrading was investigated in this study in subcritical and supercritical ethanol with C-supported catalysts (Pt/AC and Pd/AC). Upgrading process in subcritical and supercritical ethanol with C-supported catalyst not only improved the physical properties of the bio-oil but also changes composition of organic compounds of bio-oil. The HHV of bio-oil increased from 29.55 to 36.4 MJ/kg which is close to conventional bio-diesel fuel. Esterification, transesterification, and hydrogenation
CRediT authorship contribution statement
Humair Ahmed Baloch: Conceptualization, Lab experiments, Data analysis, Methodology, Writing - original draft. Sabzoi Nizamuddin: Conceptualization, Methodology, Writing - review & editing. Mohammad Tahir Hussain Siddiqui: Data analysis, Methodology. Sajid Riaz: Writing - review & editing. Mubarak Mujawar Data analysis, Review. Kristina Konstas. BET experiments and analysis. Srinivasan Madapusi: Supervision, Conceptualization, Methodology, Writing - review & editing. Gregory Griffin:
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.
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