ReviewSustainable production of formic acid from biomass and carbon dioxide
Graphical abstract
Introduction
Among the 100 most important chemical compounds, formic acid (FA) is the simplest yet strongest organic acid with prominent advantages. It is more eco-friendly, noncorrosive, easily biodegradable, etc. than other organic and inorganic acids, and is broadly applied in agricultures, rubbery, pharmaceuticals, animal feeds, leather and textiles industries [1]. Moreover, it is a frequently-used reductant, green solvent as well as a building block in various chemical syntheses. In 2019, the approximate annual global market for FA is about US$ 620 million which would maintain a constant increase in the future. Not only being an important chemical commodity, FA is also a key energy carrier/medium which may furnish solutions to the energy crisis and contribute to establishing renewable energy structures [[2], [3], [4]]. It can directly produce or be upgraded into a variety of high-quality fuels including hydrogen (H2), carbon monoxide (CO), methanol, bio-oils, etc. Especially, FA is regarded as one of the most promising H2 storage materials with a remarkable volumetric capacity of ∼53.4 g/L, equivalent to 4.4 wt% of H2, which is close to the set value of 5.5 wt% by the US Department of Energy for efficient H2 storage substances. FA as the energy carrier is crucially beneficial for gaseous fuels to ease the storage and transportation in practical uses, and the dehydrogenation of FA into H2 has been extensively investigated which can be achieved in a simple, mild and easily-controlled manner. By manipulating the reaction parameters, FA can be alternatively decomposed into CO and therefore it is also a potential CO storage material. As a result, with its prevalent applications and outstanding virtues, FA is of principal significance to both the modern chemical societies and the energy industries (see Fig. 1).
FA naturally occurs in most ants, some species of bees, and in the atmosphere because of forest emissions. In fact, the name “formic” originates from Latin language formica which means ants because it was initially isolated by the distillation of ants. The current industrial production of FA involves a fossil-based, two-step method by first reacting methanol with CO to generate methyl formate which is then hydrolyzed to form FA (see Fig. 2, top). However, large-scale FA production from renewable resources are more preferable to mitigate carbon emission and fight against global warming, as part of a more sustainable human society [5]. So far, persistent endeavors have been made to obtain FA from renewable resources primarily the biomass or CO2 feedstock [[6], [7], [8], [9]]. Biomass grows based on photosynthesis utilizing solar energy with CO2 and water as the starting materials. Upon decomposition, the carbon emits back into the atmosphere to close the carbon cycle, in which no extra carbon is released. Biomass represents the largest carbon resource around the world and is considerably decent for chemical syntheses due to the inherently rich functionalities [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]]. Among the diverse types of biomass, cellulosic biomass with ample hydroxyl groups is inexpensive, readily available and the most abundant, which is therefore the most studied feedstock to produce FA by exploiting different transformation strategies.
Rather than treating it as a bothersome gas, CO2 itself can be regarded as a C1 building block chemical for the generation of a series of value-added chemicals and fuels such as FA [[23], [24], [25]]. This CO2-based refinery will help mitigate the carbon emission and simultaneously create economic values. Since CO2 is in the most oxidized state and relatively stable, highly active species or high energy inputs are usually necessary to reduce it, but the conditions can be modified by the rational design and development of catalysts. In fact, the reduction of CO2 into FA is relatively easier than to other products (methane, etc.) via hydrogenation, and the commonly used reducing agents include H2, water, etc. Notably, CO2 reduction into FA can be realized by chemocatalytic, photocatalytic and electrochemical methods. The reduction efficiency and selectivity are closely relevant to the surface chemistry, nanostructures, electronic states, etc. of the employed catalytic systems. In this review, renewable production of FA using biomass and CO2 as the feedstock will be the major focus (an overview is provided in Fig. 2, bottom). First, conversion of biomass (mainly cellulosic biomass) into FA by different transformation routes such as hydrolysis, wet oxidation, etc. is illustrated in detail. Next, recent advances in CO2 reduction into FA by chemical, photochemical and electrochemical methods will be emphasized along with the catalyst optimization tactics. Then, the applications of FA as an energy carrier to produce H2 will be briefly depicted to showcase its essential role in future hydrogen energy economy. Lastly, the challenges and prospects of renewable FA production will be discussed and summarized.
Section snippets
Acid hydrolysis of biomass
The conversion of glucose and cellulose under the catalysis of acids would lead to the simultaneous production of the value-added products levulinic acid (LeA) and FA [26]. It is generally recognized that in the presence of an acid, cellulose first undergoes hydrolysis to give oligosaccharides and glucose which are subsequently dehydrated into 5-hydroxymethylfurfural (5-HMF) via glucose isomerization, and then rehydration of 5-HMF happens to afford LeA and FA (see Fig. 3). Hence, in this
CO2 reduction into FA
The C1 chemistry based on CO2 as the feedstock has been increasingly attractive since it offers an appealing solution to both the carbon mitigation and the sustainable development of the society [[69], [70], [71]]. CO2 is a relatively stable compound and a reducing agent is indispensable for its utilization into fuels or chemicals [70,[72], [73], [74], [75], [76], [77]] which normally involve multiple electron-transfer processes. According to the calculated Gibbs energy, CO2 reduction to FA is
The dehydrogenation of FA to produce H2 fuel
A salient advantage and potential application of FA is its role as a H2 energy carrier to solve the storage issue for gas fuels, ease the transportation and improve the safety of using H2 as a fuel. The dehydrogenation of FA is a reverse reaction of CO2 hydrogenation, and many of the catalysts that catalyzed the CO2 hydrogenation into FA are also able to promote FA dehydrogenation. A common side reaction is the FA dehydration into water and CO which should be avoided to favor H2 fuel
Challenges and outlook
In the past decade, the conversion of biomass into FA has remarkably developed with prominent outcomes. The efficiency of the transformation is not any more a major challenge, and many of the catalytic systems can result in highly selective conversion of glucose into FA. Some works exploited powerful catalysts can even directly transform raw woody biomass such as waste paper into FA with relatively satisfactory yields. In particular, a room-temperature transformation strategy has been put
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 manuscript was supported by the Young Scientists Fund of the National Natural Science Foundation of China (No. 21908145), the Shanghai Sailing Program (19YF1422100) and the SJTU Global Strategic Partnership Fund (WF610561702).
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