*1.1. General Aspects*

While carbon-rich fossil fuels like coal, oil, and natural gas have powered human civilization, the massive emission of CO2 as a greenhouse gas has caused severe and harmful effects on the ecological environment [1]. For example, the rise of sea levels is accelerating, the number of large hurricanes and wildfires is growing, and dangerous heat waves and more severe droughts are occurring in many areas. The CO2 concentration in the atmosphere had climbed to 415 ppm by 2020 (Figure 1), an increase of more than 40% relative to the pre-industrial era [2]. The atmospheric CO2 concentration will continue to rise to ~570 ppm by the end of the 21st century if no alleviation measures are taken [3]. Therefore, there is an urgent need to control CO2 emissions in order to mitigate their negative impact on the environment. In recent years, capture and storage technologies for the CO2 released from the burning of fossil fuels have emerged and developed in potential commercial scale applications [4–7]. In order to close the carbon gap, transforming the

**Citation:** Weber, D.; He, T.; Wong, M.; Moon, C.; Zhang, A.; Foley, N.; Ramer, N.J.; Zhang, C. Recent Advances in the Mitigation of the Catalyst Deactivation of CO2 Hydrogenation to Light Olefins. *Catalysts* **2021**, *11*, 1447. https:// doi.org/10.3390/catal11121447

Academic Editors: Javier Ereña and Ainara Ateka

Received: 10 November 2021 Accepted: 25 November 2021 Published: 28 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

captured gas into value-added fuels and chemicals has become an urgent task for CO2 remediation [8,9].

**Figure 1.** Trends in the atmospheric CO2 concentration (ppm) [2].

The catalytic conversion of CO2 is a favorable approach to the mitigation of CO2 emissions by producing chemicals and fuels [8,10–17]. Light olefins such as ethylene, propylene and butylene (C2 <sup>=</sup>−C4 =), which are currently among the top petrochemicals, are the building blocks for the production of a wide variety of polymers, plastics, solvents, and cosmetics [8,13,18–21]. Moreover, light olefins can be oligomerized into long-chain hydrocarbons which can be used as fuels, making them a desirable product with high potential for the utilization—and therefore elimination—of up to 23% of CO2 emissions [8]. A highly promising route is selective CO2 hydrogenation to produce light olefins [10]. The huge market demand for the lower olefins offers a great opportunity for the target technology to profoundly impact the scale of CO2 utilization once it is developed with renewable hydrogen. The current chemical industry relies heavily on petroleum (the steam cracking of naphtha) for the production of light olefins [22]. The depletion or movement away from the refining of petroleum and the gap between the supply and demand of light olefins call for a new strategy to synthesize light olefins from alternative carbon sources [18–21,23,24]. A one-step process for the conversion of CO2 to light olefins is a highly desirable tactic to address the "3Rs" (reduce, reuse, and recycle) associated with ever-increasing CO2 levels, and to solve the paradox between the supply and demand of light olefins [25].

Currently, there are two primary pathways, as shown in Scheme 1, to produce light olefins from CO2 reduction by hydrogen (H2) in a one-step process: (1) the CO2 Fischer-Tropsch synthesis (CO2–FTS) route consists of two consecutive processes, the reverse water–gas shift (RWGS) reaction (Equation (1)) and subsequent Fischer–Tropsch synthesis (FTS) (Equation (2)); (2) the methanol (MeOH) mediated route consists of two consecutive processes, i.e., CO2-to-MeOH (Equation (3)) and a subsequent MeOH-to-olefins process (MTO) (Equation (4)). The complex reaction network in Scheme 2 indicates the competing reactions (i.e., Equation (5)) with the formation of light olefins. The control of the selectivity of the CO2 hydrogenation to the desired olefin product requires the design of catalysts for reaction pathways that are compatible with favorable thermodynamics and a good understanding of the reaction kinetics [26]. The thermodynamic values in the equations (Equations (1)–(5)) indicate that lower temperatures favor FTS (Equation (2)), methanol (Equation (3)), and methane synthesis (Equation (5)), while higher temperatures are needed to activate CO2 (Equation (1)) for rapid reaction rates [27]. The complex reaction network in Scheme 2 and thermodynamics suggest that the design and synthesis of catalysts for a one-step process to selectively produce olefins are challenging.

**Scheme 1.** Reaction route for CO2 hydrogenation to light olefins.

**Scheme 2.** Complex reaction network for CO2 conversion to chemicals through hydrogenation.

CO2–FTS reaction pathway:

Reverse water-gas shift reaction (RWGS):

$$\text{CO}\_2 + \text{H}\_2 \rightarrow \text{CO} + \text{H}\_2\text{O} \qquad \triangle \text{H}\_0^{298} = 41.1 \text{ kJ mol}^{-1} \tag{1}$$

Fischer-Tropsch synthesis to olefins (FTS):

$$\text{nCO} + 2\text{nH}\_2 \rightarrow \text{(CH}\_2\text{)}\_\text{n} + \text{nH}\_2\text{O} \qquad \triangle \text{H}\_0^{298} = -210.2 \text{ kJ mol}^{-1} \text{ (}n=2\text{)}\tag{2}$$

Methanol mediated reaction pathway:

Methanol synthesis:

$$\text{CaCO}\_2 + 3\text{H}\_2 \rightarrow \text{CH}\_3\text{OH} + \text{H}\_2\text{O} \qquad \triangle \text{H}\_0 \\ \overset{298}{\rightleftharpoons} -49.3 \text{ kJ mol}^{-1} \tag{3}$$

Methanol to olefins (MTO):

$$\text{nCH}\_3\text{OH} \rightarrow \text{(CH}\_2\text{)}\_\text{n} + \text{H}\_2\text{O} \qquad \triangle \text{H}\_0^{298} = -29.3 \text{ kJ mol}^{-1} \text{ (n=2)}\tag{4}$$

CO2 methanation:

$$\text{CO}\_2 + 4\text{H}\_2 \to \text{CH}\_4 + 2\text{H}\_2\text{O} \qquad \triangle \text{H}\_0^{298} = -165.0 \text{ kJ mol}^{-1} \tag{5}$$

*1.2. Mechanistic Insights for CO2 Conversion to Light Olefins*

In reviewing the mechanistic details of the light olefin formation, it is clear that controlling the active H to C ratio is of primary importance. The presence of too much H\* on the surface will result in excessive hydrogenation, and therefore methanation, while too little H\* on the surface will restrict the hydrogenation ability of the catalyst and

therefore reduce the CO2 conversion activity. At its most fundamental, the pivotal steps of CO2 conversion to light olefins are the cleavage of the C–O bonds and the formation of C–C bonds [25].

Iron-based catalysts have been extensively studied for use in the CO2–FTS route due to their relatively high utility and activity for both the RWGS and FTS component reactions. When using Fe-based catalysts for CO2–FTS, the initial Fe2O3 phase is reduced by hydrogen to Fe3O4 or a mixture of Fe3O4 and FeO. The resulting Fe3O4 is the active component for the RWGS reaction, and can be further reduced to form metallic Fe [27]. The reaction mechanism for the CO2–FTS pathway is suggested as shown in Scheme 3a. CO2 is first adsorbed and activated on the RWGS active phases (e.g., Fe3O4) to form a carboxylate (\*CO2, \* representing the adsorption state). The \*CO2 can then be hydrogenated by adsorbed H to form an \*HOCO intermediate. The intermediate then dissociates into \*OH and \*CO. The \*OH is then hydrogenated into \*H2O. Then, \*CO either desorbs as CO gas or reacts further via successive FTS. In order to form hydrocarbons, the \*CO is first partially hydrogenated into \*HCO and then undergoes complete hydrogenation, dissociation, and finally dehydration to form \*CH*x* species. The \*CH*x* species are precursors for the formation of olefins. In an alternative mechanism, \*CO can dissociate into \*C and \*O. Some \*C can diffuse into the Fe-metal lattice to form metal carbides as χ-Fe5C2, the active component for the FTS reaction [27]. The C\* on the χ-Fe5C2 surface can then be hydrogenated to CH*x*\* species. C\* + CH*x*\* and CH*x*\* + CH*x*\* were the most likely coupling pathways [25].

**Scheme 3.** (**a**–**c**) Reaction mechanism for CO2 hydrogenation to light olefins (modified and adapted with permission from ref. [27]. Copyright 2021 Elsevier).

As indicated above, the \*C from the dissociation of \*CO during the FTS reaction may diffuse into the α-Fe metal lattice, resulting in the formation of Fe7C3, χ-Fe5C2, θ-Fe3C, ε -Fe2.2C, and ε-Fe2C phases, depending on reaction conditions [27]. Iron carbides play an essential role in CO hydrogenation/dissociation and C–C coupling. Some researchers have proposed that χ-Fe5C2 is the active phase, while θ-Fe3C is less active and can cause catalyst deactivation due to production of graphite, which has increased stability under typical FTS reaction conditions and may block the production of other active phases [27,28].

Alternatively, the reaction mechanism for the MeOH pathway is suggested as shown in Scheme 3b,c. The synthesis of MeOH can proceed via two pathways: (1) CO-mediated, in which the \*CO intermediate, which was produced from the RWGS reaction via the

dissociation of the carboxyl (\*HOCO) species, is hydrogenated to methanol via \*HCO and \*COH, and (2) formate-mediated, in which the formate (\*HCOO) species results from the hydrogenation of the carboxylate intermediate (\*CO2), which is then reacted further to \*H2COOH, \*H2CO, \*H2COH, and \*H3CO. Through dehydration coupling, the methanol forms \*CH2CH, and then forms olefins via subsequent hydrogenation [27].

The factors that may affect the CO2 conversion and light olefin selectivity are the catalyst composition (metals, supports, promotors, etc.), functionality (i.e., metal/zeolite bifunctionality), structure (i.e., layered metal oxide, core–shell, etc.), preparation methods (e.g., impregnation, hydrothermal, sol-gel, etc.) and testing conditions (e.g., temperature, pressure, CO2/H2 molar ratio, gas hourly space velocity, etc.). The focus of this review will be on the catalyst composition, functionality and structure. Other factors of catalyst preparation methods and testing conditions for CO2 conversion to light olefins can be found elsewhere [15,20,27,29,30].
