Mild-Temperature hydrodeoxygenation of vanillin a typical bio-oil model compound to Creosol a potential future biofuel
Graphical abstract
Introduction
Fossil-derived fuels such as petroleum, coal and natural gas have long been the major source of energy to the world [1]. However, the capacity of light crude oil reserves responsible for producing most hydrocarbon fuels have been declining for many years. In addition, consumption of fossil derived fuels adversely affects the environment. As a result of these factors, attention has shifted toward alternative sources of energy. Preferably sources that are renewable and sustainable should be exploited [[2], [3], [4], [5]]. Plant biomass represents a promising source of energy because of the renewability and reduced carbon footprint [6]. Liquid fuel known as bio-oil is usually produced from plant biomass via fast pyrolysis (FP), this process occurs rapidly (typically 1–2 s) at temperatures around 773 K in the absence of air [[7], [8], [9], [10], [11]]. As a result, produced bio-oil usually contains a significant amount of thermodynamically unstable oxygenated compounds [[12], [13], [14], [15]]. Up to 400 different oxygenated compounds have been reported in analytical studies on bio-oil [[16], [17], [18]]. In contrast, the oxygen content in conventional fuels is less than 1 wt %. This high oxygen content in bio-oils (20−30 wt %) leads to problems such as blocked filters, excessive corrosion, pump breakdown etc. during initial attempts to substitute conventional fuels with bio-oils in operation of furnaces, gas turbines, boilers and space heaters [[19], [20], [21], [22], [23]]. Hence, upgrading is necessary for bio-oils to fulfil their potential as substitute fuels. Hydrodeoxygenation (HDO) is the leading technology for upgrading them. It involves rejection of oxygen from the bio-oil with hydrogen as the reducing agent, and usually involves temperatures of 423–573 K and 7.5–30 MPa hydrogen gas pressure. However, the complexity of bio-oil and HDO reaction network compelled many laboratory scale studies to single or mixtures of model compounds present in bio-oil [[24], [25], [26], [27], [28]]. Common model compounds used in past studies on HDO include cresol, guaiacol, and anisole. These compounds were chosen because they contain multiple functional groups and are member of the guaiacyl species [29]. These species are favoured as model of oxygenates in bio-oil because they represent the primary structure of lignin fraction used to produce bio-oil. Vanillin (VL), an aromatic aldehyde compound cited in different analytical studies on bio-oil, is of interest in this work. This compound is selected because it contains a carbonyl group which is largely responsible for the thermodynamic instability of real bio-oil [[30], [31], [32], [33], [34]]. Moreover, the three functional groups aldehyde, ether, and hydroxyl present in the structure of vanillin makes it a good representative model of oxygenates present in bio-oil. Finally, it belongs to the same group as cresol, guaiacol, and anisole. Catalysts used in past studies on real bio-oil and model compound HDO reactions include transition sulfided metals (CoMoS, and NiMoS), noble or precious metals (Pd, Pt, Ru and Rh) and non-noble metals (Ni, Cu, Fe, and their alloys) [28,35]. Looking at the performance of these catalysts in past HDO studies, undoubtedly noble metals demonstrated superior activity. However, the exorbitant cost in procuring these metals and ease at which they are poisoned raises concerns about the economics of the process [[36], [37], [38]]. Fortunately, resistance of these metals to poison can be increased by combining them with another metal. In addition, higher selectivity toward deoxygenated products was achieved in past HDO studies that utilised bimetallic catalysts [[39], [40], [41]]. Hence, a novel bimetallic catalyst comprising of palladium and rhodium on alumina support was synthesised, characterised and tested in this work. Furthermore, the catalytic performance of noble metals (Pd, Rh and Pt) on inert (c) and conventional supports (SiO2 and Al2O3) were compared to the synthesised bimetallic catalyst. The role of solvent and processing parameters in VL HDO reaction was probed using different solvents and conditions. Finally, Taguchi analysis was used to derive the best mild condition for transforming vanillin to creosol which is a potential future biofuel.
Section snippets
Synthesis of PdRh/Al2O3 catalyst
PdCl2 solution (10.0 g, 5 wt % in 10 wt % HCl, Sigma – Aldrich) was added dropwise to methanol (30 mL, Fischer Scientific, 99 %) under vigorous stirring and then 5 wt % Rh/Al2O3 (10.0 g, Johnson Matthey) was introduced. The resultant mixture was stirred at room temperature for 4 h and dried overnight at 298 K. Further drying in an oven set at 353 K was carried out for 2 h prior to calcination at 773 K for 4 h.
Characterisation of the catalysts
Nitrogen adsorption – desorption isotherms of the catalyst used in this study were
Results and discussion
The range of conditions investigated ensures liquid phase only reaction with insignificant coke and gas formation. Also, negligible changes in the weight of solvents used were observed, which suggests the solvents are not converted into gases. Consequently, only two products vanillyl alcohol and creosol were recorded, meaning the solvents were not involved in the reaction. This section is organized as follows; characteristics of the catalysts used are presented and discussed in section 3.1,
Conclusion
In this work, vanillin (VL) a model compound of bio-oil was used to investigate the effect of solvent, catalyst active metal, supports and processing parameters on hydrodeoxygenation (HDO) reaction. It was observed that non-catalytic and catalytic support influences on the reaction are not significant when compared to the catalytic active metals (Pd, Pt, Rh, and PdRh). Product distribution and conversion changed significantly with the reaction solvents. This was correlated to observed
CRediT authorship contribution statement
Elias Aliu: Methodology, Investigation, Writing - original draft, Visualization. Abarasi Hart: Formal analysis, Writing - review & editing, Supervision. Joseph Wood: Conceptualization, Resources, Supervision, Project administration, Funding acquisition.
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.
Acknowledgements
The financial support for this work was provided by School of Chemical Engineering, University of Birmingham.
References (53)
- et al.
Appl. Catal. A Gen.
(2016) - et al.
Renew. Sustain. Energy Rev.
(2016) - et al.
Bioresour. Technol.
(2012) Catal. Today
(1996)Appl. Catal. A Gen.
(1994)- et al.
Biomass Bioenergy
(2000) Biomass Bioenergy
(2012)- et al.
Fuel
(2014) - et al.
Catal. Today
(2014) - et al.
J. Catal.
(2011)
Fuel
Appl. Catal. A Gen.
Appl. Catal. A Gen.
Renew. Sustain. Energy Rev.
Appl. Catal. A Gen.
J. Anal. Appl. Pyrolysis
J. Anal. Appl. Pyrolysis
Fuel Process Technol.
J. Anal. Appl. Pyrolysis
Curr. Opin. Chem. Eng.
Appl. Catal. A Gen.
J. Catal.
J. Catal.
J. Catal.
J. Catal.
J. Mol. Catal. A Chem.
Cited by (18)
Review on the production of renewable biofuel: Solvent-free deoxygenation
2024, Renewable and Sustainable Energy ReviewsHydrodeoxygenation of vanillin to 2-methoxy-4-methylphenol over carbon dots supported Pd catalyst
2024, Molecular CatalysisCatalytic hydrodeoxygenation of bio-oil and model compounds - Choice of catalysts, and mechanisms
2023, Renewable and Sustainable Energy Reviews