Opportunities and prospects in the chemical recycling of carbon dioxide to fuels

Abstract

This review analyses the opportunities and prospects in the chemical recycling of carbon dioxide to fuels, as a complementary technology to carbon sequestration and storage (CSS). It is remarked that the requisites for this objective are (i) minimize as much as possible the consumption of hydrogen (or hydrogen sources), (ii) produce fuels that can be easily stored and transported, and (iii) use renewable energy sources. From this perspective, the preferable option is to produce alcohols (preferably ≥C2) using solar energy to produce the protons and electrons necessary for the reaction of CO₂ reduction. It is evidenced, however, that this is still a long-term objective, even if already some good advances in this direction exist. The different topics discussed in the review include CO₂ (i) reverse water–gas shift and (ii) hydrogenation to hydrocarbons, alcohols, dimethyl ether and formic acid, (iii) reaction with hydrocarbons to syngas, (iv) photo- and electrochemical/catalytic conversion, and (v) thermochemical conversion. Other relevant options, such as the use of micro-algae or other bio-catalysis based processes, or the use of microwave and plasma processes are instead not addressed. Therefore, the area of carbon dioxide conversion to fuels and chemicals is a very active R&D sector, and it is anticipated that it represents a challenging possibility for companies to develop complementary strategies to CSS to reduce greenhouse gas emissions.

Introduction

Carbon dioxide is turning image recently and there are increasing attempts to consider it a resource and a business opportunity rather than a waste with a cost of disposal [1], [2], [3]. Increasing amounts of low-cost and relatively pure CO₂ will be soon available from current and planned plants for carbon sequestration and storage (CSS). Therefore, CO₂ will be a feedstock of nearly zero (or even negative) cost for conversion to fuels and chemicals, in addition to the many benefits in terms of positive image for companies, which will adopt politics of reduction of CO₂ emissions. The other factor stimulating the interest in CO₂ chemical recycling is the presence of many emissions for which the CSS option is unsuitable: distance from safe sequestration sites, diluted concentration of CO₂ in the emitting gas, small-medium size sources, and uncertain on the impact on environment. It can be roughly estimated that about 5–10% of the total CO₂ emissions (about 30 Gt worldwide in 2008 [4]) could be suited for production of fuels and chemicals [1], e.g. about one order of magnitude higher than actual use of CO₂ in industry.

The use of CO₂ as a building block in organic syntheses to obtain valuable chemicals and materials has been discussed in many reports and review articles [5], [6], [7], [8], [9], [10], [11], [12], [13]. Currently, the utilization of CO₂ as chemical feedstock is limited to a few processes: synthesis of urea (for nitrogen fertilizers and plastics), salicylic acid (a pharmaceutical ingredient), and polycarbonates (for plastics). However, the actual use corresponds to a few percentage of the potential CO₂ suitable to be converted to chemicals. Therefore, a chemical recycling of CO₂ may significantly contribute to a reduction of its emissions only when the target products are components for the fuel pool, which worldwide consumption is two order of magnitude higher than that of chemicals. The main products of CO₂ conversion must be fuels to reduce CO₂ emissions significantly and to create great economic value, although some of them (methanol, ethanol, etc.) could be considered in the double role of fuel and chemical. In addition, valorisation of carbon dioxide emissions could be one important part of the general strategy for reducing CO₂ emissions and push chemical and energy companies towards a more sustainable use of the resources [14], [15].

There are different options to convert CO₂. Hydrogenation of CO₂ to form oxygenates and/or hydrocarbons are the most intensively investigated area of CO₂ conversion. Methanol synthesis from CO₂ and H₂ has been investigated at the pilot plant stage with promising results. The alternative possibility is the production of DME, a clean-burning fuel that is a potential diesel substitute. Ethanol formation, either directly or via methanol homologation, or the conversion of CO₂ to formic acid are also potentially interesting routes. Methanol, ethanol, and formic acid may be used as feedstocks in fuel cells, providing a route to store energy from CO₂ and then produce electricity.

Hydrogenation of carbon dioxide to hydrocarbons consumes much more hydrogen (per unit of product) than formation of oxygenates. Therefore, this route is valuable in principle only when hydrogen is made mainly from renewable or non-fossil resources, but other thermodynamic aspects have to be also considered. Dry reforming of methane with CO₂ is a known technology that is available on a nearly industrial scale, although the positive impact on CO₂ emissions is questionable, i.e. whether CO₂ emissions due to energy consumption are larger than the amount of CO₂ consumed in the reaction. An improvement in the positive direction is the tri-reforming, which operates autothermically and does not require a pure CO₂ feed stream, but large-scale demonstration is necessary.

This review will introduce these different options and then focus the discussion on the processes that are at an early stage, but with relevant potential in the future. In the long term, the thermal, photocatalytic, and photoelectrocatalytic reduction of CO₂ under solar irradiation could greatly increase carbon recycling and reduce fossil fuel consumption. Biological conversion of CO₂, and in particular the use of micro-algae for producing chemicals or fuels [16], is another attractive route, but it still needs much development to become economically feasible.

In a recent report, the US Department of Energy (DoE) [17] has identified the challenges in catalysis for sustainable energy and priority areas in this field. In particular, one of the three priority research directions for advanced catalysis science for energy applications is the development of advanced catalysts for the photo- and electro-driven conversion of carbon dioxide and water.

The conversion of CO₂ at r.t. and atmospheric pressure using solar light represents a highly challenging approach to close the CO₂ cycle and develop photosynthesis mimic approaches. An interesting solution to realize this objective is a novel photoelectrochemical (PEC) reactor operating in the gas phase and using nanoconfined electrodes [18], [19], [20], [21], differently from the conventional PEC systems. On one side of the proposed device, water is converted to O₂, electron and protons on a nanostructured TiO₂ thin film. This photocatalyst is growth over a porous titanium layer, which acts as electron collector and is in contact with the proton membrane to transport the protons on the other side of the cell. A wire connects the Ti substrate with the electrocatalyst on the other side of the cell. On the other side of the cell, an electrocatalyst based on metal nanoparticles deposited on conductive nanostructured carbon (carbon nanotubes or other similar C-based materials) and assembled to form a GDM (gas diffusion membrane) layer is present. On this electrocatalyst, CO₂ reacts with the electrons and protons (generated on the other side of the cell) to form hydrocarbons and alcohols.

These results evidence the possibility to develop “artificial trees” [21] able to capture the CO₂ and convert it to liquid fuels (hydrocarbons, alcohols). Therefore, the implementation of this concept will allow to reduce the levels of CO₂ in the atmosphere and at the same time capture a renewable source of energy (solar radiation) transforming it in a form (liquid fuels) which can be stored, used and transformed, preserving thus the large investments made on fossil fuels. The liquid hydrocarbons and alcohols can be alternatively used also as chemical feedstocks.

In summary, carbon dioxide conversion to fuels and chemicals is a very active R&D area that presents great challenges and opportunities for industry, along with great benefits to society. Further advances in catalysts, reactors, separations, processes, unconventional energy sources, and combinations of processes will be needed. Some of these aspects will be discussed here, with specific focus on catalysis, although it is not possible an exhaustive review of the topic. Further details can be found in Ref. [1]. To be mentioned also the recent reviews on the electrocatalytic and homogeneous reduction of carbon dioxide to liquid fuels [22] and on the advances in CO₂ capture, storage, fixation, and utilization [23]. The latter, in addition to carbon capture and storage, overviews some aspects of the chemical fixation of CO₂, in particular of the conversion to fuels (methanol, formic acid, di-Me carbonate, Me formate, higher hydrocarbons, photoreduction of CO₂. The problem of solar photocatalysis for hydrogen production and CO₂ conversion has been reviewed by Minero et al. [24], while the conversion of carbon dioxide to methanol and dimethyl ether by Olah et al. [25].