Applications of supercritical technologies to CO2 reduction: Catalyst development and process intensification

1 Applications of supercritical technologies to CO2 capture and utilization Ángel Martín*, Alexander Navarrete, María Dolores Bermejo High Pressure Processes Group, Department of Chemical Engineering and Environmental Technology, University of Valladolid C/Dr. Mergelina, s/n, 47011 Valladolid, Spain Tel:+34 983184077. e-mail: [email protected] (Á. Martín) Abstract Keywords: Carbon dioxide capture and utilization, supercritical water, photocatalysis, nanomaterial, aerogel, process intensification 1. Introduction There is an increasing global concern about the negative effects of climate change, which are related to the increasing concentration of greenhouse gases in the atmosphere, and in particular CO2, due to the use of fossil fuels. The recent Paris Agreement, set in 2015, has as its main objective the reduction of CO2 emissions in order to avoid a global temperature increase higher than 2ºC. To attain this objective, it is necessary to gradually replace fossil fuels by renewable energies with zero CO2 emissions, but it can be expected that fossil fuels will remain as the main energy source for many years, particularly for production of electricity and as fuels for vehicles. Therefore, other solutions such as carbon capture and storage (CCS) technologies must be considered, because they would enable a significant reduction of the CO2 emissions of thermal power plants or chemical industries such as the industries of production of ammonia, hydrogen and cement [1]. Different CO2 capture technologies have been researched. Among them, the most developed technology at industrial level is the absorption using aqueous solutions of alkanolamines, a method that has been used since the 30s to remove CO2 and other acid gases from natural gas [2]. Alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA) amd 2 methyldiethanolamine (MDEA) selectively and reversibly react with CO2 forming carbamates, and can capture 0.5-0.7 mol CO2/mol amine [3,4]. However, the heat of absorption is high, implying that the amine regeneration step, which is carried out by desorption at temperatures of 100 – 150ºC, has high energy consumption. Alternative absorption processes for CO2 capture processes have been developed, such as the “Chilled Ammonia” process presented by the French company Alstrom [5]. This method uses an aqueous ammonia solution at low temperature (2-10ºC), where CO2 is absorbed as ammonia carbonate, bicarbonate and carbamate. Typically, 0.33-0.67 molCO2/molNH3 can be absorbed. As in the case of amines, regeneration is carried out by desorption at temperatures of 100-150ºC and pressures of 2-136 bar. Compared to the case of amines, the heat of desorption from the ammonia solution is much lower, which implies that the energy costs of the regeneration also are lower. The disadvantage of this method is the need of big amounts of refrigeration water. For this reason, in general this method is in only considered for facilities located near the sea. Besides these absorption methods, other CO2 capture technologies are being developed, such as adsorption of membrane separation [6]. In general, all these processes are costly, because in addition to the cost of the CO2 separation process, the cost of compressing and transporting CO2 to the storage site is also high. In fact, it has been estimated that even employing favorable technologies for CO2 separation such as oxy-fuel combustion, the incorporation of a CO2 capture and storage unit in a thermal power plant implies a significant penalty on the global efficiency of the plant, which is estimated as 2 – 3 % of the global efficiency of the power plant [7], a penalty that is assumed estimating that the benefits of reduced CO2 emissions outweigh it. Therefore, it can be concluded that CO2 capture is a complex and costly process, which justifies the general appreciation of CO2 as a problematic residue that causes considerable economic problems and environmental harms. In contrast, looking into nature, carbon dioxide is the basic chemical resource used by plants. By transforming it into chemical fuels, they store 3 surplus solar energy for periods of need. By converting it into complex organic compounds, they obtain the building blocks that they need to grow and live. A similar approach can be followed in the industry, converting CO2 in valuable compounds that can generate a profit instead of simply storing it [8]. Furthermore, it must be considered that fossil fuels not only provide most of the energy we use, but also are the main raw material used by the chemical industry. Therefore, it is necessary to find an alternative of base compounds for the chemical industry that can gradually replace the compounds obtained from fossil fuels as they run out. CO2 utilization technologies can also contribute to this purpose, because they can produce important platform chemicals such as formic acid or methanol, and CO2 is a widespread resource that can contribute to the creation of a novel decentralized industrial structure that produces “just enough” [9]. Following this approach, different Carbon Capture and Utilization (CCU) technologies are being researched [10]. Among other options, four key CO2 conversion technologies are being intensively studied: (1) catalytic copolymerization of CO2, particularly with highly reactive epoxides for the synthesis of polycarbonates [11]; (2) thermo-catalytic CO2 conversion processes [12, 13]; (3) CO2 fixation in photo-bioreactors [14]; and (4) photocatalytic CO2 conversion processes, also known as “artificial photosynthesis” methods [15]. The application of supercritical fluids can contribute to the development of these technologies in several aspects such as materials design, process intensification and reaction engineering. The objective of this article is to present an overview of the recent progress and the perspectives in the application of supercritical fluids in carbon dioxide utilization technologies. Since the copolymerization of CO2 is discussed in another article of this special issue, this application will not be considered in this article. 2. Catalysts for Artificial Photosynthesis The origin of “artificial photosynthesis” systems is usually tracked back to the seminal work of of Fujishima and Honda, who in 1972 demonstrated the photoelectrochemical splitting of 4 water using a single-crystal TiO2 photoanode and a Pt cathode with an external electrical bias [16]. Following this work, a considerable work has been carried out in the development of photoelectrochemical and photocatalytic CO2 conversion systems, and particularly in the development of suitable photocatalysts, which are an essential element of these systems. The photocatalysts employed are materials that are able to absorb light energy, generating free electrons (e-) and holes (h’), as shown in Figure 1 [17]. Some of the most employed materials are TiO2 and other semiconductors. The photo-induced electron-hole pairs produced in these materials can initiate the redox reactions required to convert CO2. Therefore, a fundamental aspect determining the photocatalytic activity of the material is the efficiency of stabilization of the charge separation induced by light excitation. Charge separation can be enhanced adding metal co-catalysts to the material, such as Pt, that actuate as sinks for the free electrons produced. Such semiconductor-cocatalyst systems are some of the most promising photocatalytic materials for water splitting and CO2 reduction available today [18]. Charge separation can also be stabilized by geometrical factors, producing catalysts with appropriate and controlled (nano) sizes and shapes [19]. Following this approach, different (and, in some cases, patented) designs describe nano-structured materials with a high aspect ratio (e.g. fibers or rods), including: TiO2 nanofibers [20], TiO2-coated ZnO nanorods deposited over a silicon substrate [21], or a double-layer structure, formed by Si nanowires vertically disposed over the two sides of a PEM membrane [22] (Figure 2). Moreover, the range of light wavelengths in which the photocatalytic material is active is an extremely important property. Ideally, the material should have a maximum activity in the visible light range, but commonly used semiconductor photocatalysts, such as commercial TiO2, show their maximum activity in the UV range. The development of photocatalysts suitable for light absorption in a wide wavelength range is extremely challenging, because light absorption depends on many factors, including the chemical composition, the crystalline structure and type and density of defects, the particle size, or the interactions at the interface between the photocatalyst and the co- 5 catalyst. It is therefore necessary to develop a method for production of the catalyst that can control all these aspects. Considering the possibilities of supercritical fluid technologies for the development of such materials with tailored properties, several researchers have studied the production of photocatalysts in supercritical media. Figure 1: Schematic representation of the reaction mechanism of the photocatalytic CO2 reduction over a semiconductor particle. Reproduced from [17] with permission. As previously described, semiconductor-cocatalyst photocatalytic systems are frequently used as particles of appropriate size, generally in the nanometer range. Different supercritical fluid processes can be used to produce such metal nanoparticles with controlled and narrow particle size distribution [23]. A first alternative is the synthesis in supercritical CO2 media by thermolysis of an organometallic precursor. In order to apply this method, a suitable precursor with a high solubility in supercritical CO2 must be available. The first step of the process is the dissolution of this precursor in CO2. If the deposition of the produced particles over a scaffold is desired, the supercritical dissolution is put into contact with the scaffold during this step, with the objective of adding the precursor to the scaffold by adsorption or physical deposition. After this, the supercritical dissolution is heated up, causing the decomposition of the organic part of the organometallic precursor. Metal particles are thus produced, generally as metal oxides. Employing this method, Alonso et al. [24] researched the synthesis of TiO2 nanoparticles in supercritical CO2 and evaluated the influence of synthesis conditions on the photocatalytic activity of the material. In this work, TiO2 anatase nanoparticles were obtained 6 by thermohydrolysis of DIPBAT (diisopropoxititanium bis acetylacetonate) in supercritical carbon dioxide with different alcohols (ethanol and isopropylalcohol). Moreover, the photocatalytic activity of the material was tested considering the degradation of aqueous solutions of methyl orange as model reaction. It was found that the crystallinity was a fundamental parameter on the photocatalytic activity of the material. The crystallinity could be tailored modifying the operation temperature. The operation range was 200ºC – 300ºC, and higher crystallinity was obtained at higher temperatures, leading to a higher photocatalytic activity. In contrast, other operating parameters such as the operating pressure did not have a significant effect on the photocatalytic activity. The activity also showed some variation when the alcohol employed in the thermohydrolysis was changed, with higher activity in the case of particles obtained with isopropanol. According to authors, this result could be related to the density of surface hydroxyl groups in the particles. Figure 2 presents a SEM image of the TiO2 particles produced. Figure 2: TiO2 microparticles synthetized in supercritical CO2. Adapted from [24] with permission. Camarillo et al. [25] applied TiO2 nanoparticles synthetized in supercritical CO2 with a similar procedure to the photocatalytic reduction of CO2, comparing them with commercial Degussa 7 P-25 TiO2 particles (Evonik). They characterized the light absorption properties of the TiO2 particles, finding that particles synthetized from supercritical CO2 exhibited a higher light absorption in the visible range than the commercial particles. This is a major improvement of the properties of the catalyst, since TiO2 catalysts are generally limited to operation in the UV range and show poor light absorption in the visible range. In connection with this result, particles synthetized in supercritical media also showed lower band gap energy than commercial catalysts, a result that also indicates a more efficient light absorption. Authors suggested that this result could be due to the existence of surface oxygen vacancies in the materials synthetized in supercritical fluids, which effectively harvest visible light and create a color center [26]. Indeed, as presented in Figure 3, it could be observed by a simple visual inspection that while commercial particles showed a bright white color, particles synthetized in supercritical fluids presented darker grey or green colors, a clear indication of light absorption in the visible range. Camarillo et al. [25] also assessed the stabilization of the charge separation attained with the different materials. X-ray diffraction (XRD) assays showed that the particles produced in supercritical fluid had smaller crystallite sizes than commercial particles, a property that is favorable for the stabilization of charge separation, because the crystalline fractures and the contact surfaces between crystallites slow down the recombination of free electrons and holes. However, while commercial particles consisted of a mixture of anatase and rutile crystalline phases, CO2-syntetized particles consisted exclusively of anatase, as also observed by Alonso et al. [24], which in this case is an unfavorable property because the presence of different crystalline phases can also slow down charge recombination. As a result of the combination of these properties, particles synthetized in supercritical CO2 showed a higher photocatalytic activity in the reduction of CO2, enabling the production of CH4 and CO with reaction rates that were 22 and 1.7 times higher, respectively, than the rates obtained with commercial TiO2 particles. 8 Figure 3: TiO2 particles synthetized by Camarillo et al. [25] after photocatalytic experiments, prepared from different Ti precursors and solvents: (a) TTIP-isopropyl alcohol; (b) TTIP-ethanol; (c) DIPBAT-isopropyl alcohol; (d) DIPBAT-ethanol. Reproduced from [25] with permission The synthesis procedure in supercritical CO2 can also be used to obtain composite TiO2 – metal cocatalyst particles. In further works, S. Tostón et al. [27] and Camarillo et al. [28] investigated the application of TiO2 particles doped with Cu or Pt. TiO2/Pt catalysts, obtained by simultaneous thermohydrolysis of TiO2 and Pt precursors (titanium tetraisopropoxide or diisopropoxititanium bis acetylacetonate, and palladium acetyl acetonate, respectively). As in previous cases, XRD assays showed a high crystallinity of the TiO2 particles produced, composed exclusively of anatase phase, but in this case diffraction peaks that could correspond to PdO phases could also be observed, indicating that a fraction of Pd was not well dispersed in the TiO2 matrix. As in the previous work, a shift in the light absorption range to the visible range was observed in UV-VIS diffuse reflectance assays, which is favorable for the application of the catalysts with natural sunlight. CO2 reduction experiments showed the production of CO, methane and ethane, and the rates of formation were 3-22 times higher 9 than the rates observed with commercial, undoped TiO2 particles. Comparing with the results of their previous works, authors observed that the addition of Pd enhanced the selectivity toward methane. The best results were obtained with the catalysts with the highest Pd load (3 wt%), indicating that the presence of segregated PdO phases observed in XRD assays did not have a detrimental effect on the photocatalytic activity. Another method for the synthesis of metal and semiconductor nanoparticles is the hydrothermal synthesis under supercritical conditions [29]. The process consists of mixing an aqueous solution of a metal precursor with a stream of water at near-critical or supercritical conditions, which causes the degradation of the organic part of the precursor and the formation of metal oxide nanoparticles. The mixing kinetics between the precursor solution and the supercritical water are a crucial parameter for the successful synthesis of the nanoparticles. With an appropriate design of this mixer, the process can be carried out continuously. Chowdhury et al. [30] tested the photocatalytic activity of TiO2 particles synthetized with this method in the visible light range. The synthesis was carried out using water-ethanol solvent mixtures, at operating temperatures ranging from 200ºC to 400ºC. Particles with anatase crystalline structure were obtained, with sizes that ranged from 10 nm at 250ºC to 100 nm at 400ºC. The most interesting result of the work was the nano-twinned structure of the particles, which is a favorable property for the photocatalytic activity of the material because the changes of orientation of the crystal at twin crystal surfaces enhance charge separation. In this work, the photocatalytic activity of the material was tested considering the degradation of methylene blue as model reaction. A high photocatalytic activity was observed even working under visible light, which is a promising result for the application of this material in CO2 reduction reactions. The results of these works show that TiO2 micro/nano particles produced with supercritical fluids show promising photocatalytic properties. However, the handling of micro/nano particles in real applications can be problematic, as in flow reactors they are prone to 16 chemical compounds and fuels [44]. The conversion of several biomass or organic residues in hydrothermal media has already been demonstrated, by hydrolysis with or without catalysts [45, 46] or by gasification [47]. These processes have even achieved an incipient commercialization. Renmatix [48] has commercialized the production of sugars in hydrothermal conditions, while Hydromethan AG [49] applies the catalytic gasification of organic residues in supercritical water to the production of methane. Following these results, the reduction of CO2 in hydrothermal media has raised a considerable interest because it can overcome one of the main difficulties of other CO2 conversion methods: the production of H2 by a cheap and a simple method. Indeed, it has been proven that in hydrothermal reactors, hydrogen can be produced in-situ by oxidation of zero-valent metals. Therefore, in this method water acts simultaneously as environmentally friendly solvent and as hydrogen source. Moreover, the in-situ production of hydrogen also enhances the rate of the reduction reaction of CO2, probably due to the participation of radical intermediates or of adsorbed species in the reaction [50]. Figure 7 presents a schematic representation of the reactions involved in the hydrothermal conversion of CO2 with metal reductants. Figure 7: Schematic diagram of the mechanism of conversion of CO2 by hydrothermal reduction with metal reductants CO 2 +H 2 -H 2 HCOOH CO+H 2 O +H 2 -H 2 O +H 2 O -H 2 CH 2 O +H 2 -H 2 CH 3 OH +H 2 -H 2 O +H 2 O -H 2 CH 4 reduction CH 4 +H 2 Ni Me(0)+ H 2 O+CO 2 oxidation Me x O y + H 2 +CO 2 (Basic media) (High T) MeCO 3 + H 2 Me x O y + R-OH R=O+ H 2 O+ M(0) Reacciones de reducción de CO 2 Reacciones de oxidación de metales para generar H 2 Reacciones de reducción de los óxidos metálicos con alcoholes CO 2 reduction reactions Reduction of metal oxides with alcohols Oxidation of metals to generate H 2 17 The reduction of CO2 in hydrothermal media has been proved employing different metals as reductants and/or catalysts, forming formic acid. The most frequently studied combination is Fe as reductant, with Ni [50] or Cu [51] as catalyst. With these materials, yields of conversion to formic acid higher than 75% have been achieved operating at temperatures of 200ºC – 350 ºC with residence times of 1 – 2 h. Other authors have proved that the reduction can be carried out employing only Fe, without catalysts, but in this case reaction times increased up to 75 h [51, 52, 53]. Besides Fe, other metals have been used as reductants, including Mg, Mn, Zn and Al. The most promising results have been obtanied using Zn or Al, obtaining yields up to 60% without using another metal as catalyst [50]. In the case of Zn, conversion to formic acid with a yield of 60% has been achieved operating at 325ºC with a residence time of only 10 min, while at longer operating times methane traces are formed [54]. In this process, Zn acts as reductant, and the ZnO thus formed acts as catalyst. In further studies of the same authors it has been reported that employing Cu as catalyst and Zn as reductant, the formic acid initially obtained by hydrothermal conversion of CO2 is further converted to methanol [55]. Usually, formic acid is the main product obtained with this process [50, 51], although some authors have obtained phenol [53], methane [52] or methanol [55] by a similar process. Although the precise mechanism of the reaction is unknown, the results so far obtained suggest that formic acid is the first product of CO2 reduction, which is then converted to formaldehyde, methanol and finally methane [43]. In fact, these products are observed when reaction times are increased [52, 53, 55]. Experimental results indicate that the selectivity toward formic acid is favored by operation in basic pH conditions, which stabilize formiate as a cation in aqueous solution. For this reason, experiments are usually carried out using bicarbonate as CO2 source, or adding NaOH to the medium if CO2 is supplied as a gas in order to dissolve it as bicarbonate [56]. Moreover, changes in the catalyst employed or its concentration can lead to the production of different compounds. For example, when Ni is 18 used as catalyst, methanol production is favored at high Ni concentrations, while the use of Fe favors the formation of formic acid, and the use of Fe3O4 increases the formation of methanol [56]. As it can be observed, a drawback of this method is that the metal used as reductant is consumed during the process, forming a metal oxide. In order to recover the metal, the photocatalytic reduction of the metal has been proposed [54]. Another alternative is adding another reductant to the hydrothermal medium suitable for reducing the metal oxide, thus enabling the operation in a closed cycle with respect to the metal. 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