Evolution of coke structures during electrochemical upgrading of bio-oil
Graphical abstract
Introduction
With the continued depletion of fossil resources and excessive emissions of greenhouse gases, fossil-free energy consumption strategies have become a global trend [1]. Biomass is an abundant carbon-containing renewable resource that exhibits enormous potential as an alternative to fossil resources for producing fuels and chemicals [2]. Fast pyrolysis is a relatively mature technology for converting biomass into liquid bio-oil, solid biochar, and non-condensable biogas products [3]. The latter two can be used as fertilizer and gas fuel, respectively. Raw bio-oil, however, is difficult to be used directly due to its intrinsic corrosiveness, high viscosity, chemical instability, and low heating-value. These properties constitute a bottleneck to the industrialization of the fast pyrolysis technology [4]. Thus, it is essential to upgrade the bio-oil to improve its physical and chemical properties prior to production of value-added fuels and chemicals. Typical methods for upgrading the bio-oil include hydrocracking, hydrogenation, deoxygenation and stem reforming based on thermochemical principles [[5], [6], [7]]. These methods face operational difficulties and high costs to achieve the severe high-temperature, −pressure, and hydrogen consumption conditions [8]. To avoid the negative effects of those severe conditions, non-thermal methods for upgrading bio-oil have been explored. In particular, the electrochemical method has been introduced in bio-oil upgrading and drawn increasing attention as a means of producing sustainable fuels and chemicals because of its mild reaction conditions, convenient control, and adaptable integration with renewable energy sources [[9], [10], [11]].
There has been increasing research on the electrochemical upgrading of bio-oil in the recent years as the research community explores different methodologies to store the intermittent energy harnessed from different renewable sources [12]. Most studies on the electrochemical upgrading of bio-oil have focused on the biomass derived model compounds, such as aldehydes, ketones, acids, furans, phenols and sugars [13]. Various electrocatalysts, electrolytes, electrolytic reactors and potentials have been examined in these studies to improve the upgrading effect and efficiency. The unsaturated low molecular compounds in the bio-oil have been successfully hydrogenated, and occasionally deoxygenated via the electrochemical method [[14], [15], [16], [17], [18], [19]]. The abovementioned studies have demonstrated the feasibility and advantages of the electrochemical upgrading of biomass-derived model compounds. Electrochemical upgrading can be performed at room temperature and under ambient pressure without the need to engage any pressurized hydrogen environment. The process can be easily controlled by regulating the potential difference. Integrating electrochemical upgrading with renewable energy results in a carbon-neutral process [20]. Based on previous attempts at the electrochemical upgrading of bio-oil model compounds, several studies have involved the whole bio-oil and its fractions, in which the acidity of the bio-oil is reduced and the chemical stability is improved [[21], [22], [23], [24]]. The application of the electrochemical method for upgrading bio-oil has demonstrated great potential.
Bio-oil contains hundreds of highly reactive organic compounds. The combination of a high abundance of carbonyl compound and high acidity gives the bio-oil a high polymerization tendency [25]. Polymerization even occurs during the storage of bio-oil at room temperature, leading to an increase in viscosity. It is easy to form coke when the bio-oil is heated in a thermochemical process [26]. Model compound studies on the thermochemical upgrading have shown that sugars, acids, phenols and furans can involve in the polymerization process. Similarly, in the electrochemical process, where the activation energy is supplied by electricity, the problem of solid carbonaceous materials (coke) formation may also occur at room temperature. Li et al. found that a small amount of precipitate appeared in the electrochemical hydrogenation of water-soluble bio-oil. Further investigation suggested that carbohydrate and phenolic oligomers could form by polymerization, even under the mild conditions in the electrochemical process [21]. Our previous study also indicated that the content of the condensed aromatic components increased after electrochemical treatment of the bio-oil, while carbonaceous solids were observed on the electrode surface [24].
Once the coke is formed, it covers the catalyst active sites and the electrodes, hindering the transfer of electrons, which will eventually reduce the energy efficiency or shutdown of the electrochemical process. Moreover, the formation of coke can reduce the amount of biomass carbon in the resulting fuel product, which in turn compromises the role of biomass in the renewable fuel sector. Surprisingly, little attention has been devoted to coke formation in the electrochemical upgrading of bio-oil. The reason may lie in the small amounts of coke that form under low current densities in the laboratory. However, the issue could severely cripple the application of electrochemical upgrading in the large-scale industrial setting. For fuel production purposes, it is essential to understand its formation mechanism and thereby inhibit its formation. From another perspective, bio-oil has the potential to produce carbonaceous materials due to its high polymerization tendency. The valuable carbonaceous materials produced through the thermal treatment of bio-oil can be applied in electrodes, high-strength materials and energy storage [[27], [28], [29]]. Inspired by these studies, it seems likely that the electrochemical method (electrochemical polymerization) may be introduced in the oil-to-material strategy. Therefore, in terms of both inhibiting coke formation in the electrochemical upgrading of bio-oil and producing carbonaceous materials from bio-oil feedstock using electrochemical polymerization, it is essential to understand the coke formation mechanism during the electrochemical treatment of bio-oil.
The present study investigates the evolution of coke structures during electrochemical processing of bio-oil. The effects of current density and reaction time on coke formation are investigated in terms of the quantity and property of the coke. The reaction pathways involved in coke formation are proposed through quantitative and qualitative analysis of solid products. The potential applications of the coke formed from the bio-oil by electrochemical polymerization are determined based on its physical morphology and chemical structure.
Section snippets
Electrochemical experiments
The original bio-oil used in this study was produced from the fast pyrolysis of rice husk at 500 °C in a fluidized-bed reactor. The main compounds of the bio-oil are organic acids, furans, phenols and levoglucosan, as shown in Table S1. The bio-oil was dissolved in a mixture of CH3OH and CH2Cl2 (4:1 by weight) solvent to obtain the whole bio-oil sample with the concentration of 5 wt%. The sample was stored in a freezer for experimental use. The electrochemical treatment of the bio-oil was
Coke yields and its impact on operating voltage
Fig. 1 shows the coke yields from the electrochemical treatment of bio-oil as a function of current density of under different reaction time. It should be noted that the coke yields are expressed on the basis of “per g of bio-oil”. The results show that coke forms at low current densities with short reaction times (50 mAcm−2, 1 h). The coke is a direct result of the electric current, since a control experiment found that no coke formed in the absence of a potential bias, even when the bio-oil
Conclusions
This study has demonstrated the evolution of coke structures during the electrochemical processing of bio-oil. The coke formation is the direct result of the applied potential,and comes primarily from the polymerization of aromatic components. The coke is distributed on the anode surface and at the bottom of the electrolytic cell. The anodic oxidation can either cause coke formation on the anode directly, or generate reactive intermedia products inducing polymerization in the bio-oil bulk
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
This work was supported by the National Natural Science Foundation of China (NSFC) (No. 51976074, 52122608) and the Graduates' Innovation Fund of Huazhong University of Science and Technology (No. 2020yjsCXCY025). The authors would also acknowledge the Analytical and Testing Center of Huazhong University of Science and Technology for the help on the experiments.
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