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Development of tandem catalysts for CO hydrogenation to olefins Zhiqiang Ma, and Marc D Porosoff ACS Catal., Just Accepted Manuscript • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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ACS Catalysis

Development of tandem catalysts for CO2 hydrogenation to olefins Zhiqiang Ma and Marc D. Porosoff*

Department of Chemical Engineering University of Rochester Rochester, NY 14627, USA *Corresponding author: Prof. Marc D. Porosoff, 4305 Wegmans Hall, Phone: (+1) 585-2767401, Email: [email protected]

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Abstract The utilization of CO2 as a carbon source for synthesis of value-added chemicals and fuels, particularly light olefins, is one of the most attractive routes to convert CO2 as part of a large-scale process. Designing active, selective and stable catalysts for olefin production is challenging because of the difficulty characterizing structure-property relationships for the highly complex CO2 hydrogenation reaction network. To understand the challenges and opportunities in converting CO2 directly to olefins over a single tandem catalyst, this perspective reviews the following three routes: (1) Direct hydrogenation of CO2 to olefins over promoted catalysts; (2) Methanol synthesis followed by methanol-to-olefins (MeOH-mediated route); (3) CO production via the reverse-water-gas-shift reaction, followed by Fischer-Tropsch synthesis (CO-mediated route). Future research directions are proposed on the critical research areas of elucidating reaction mechanisms by combining in situ characterization techniques with density functional theory calculations, identifying structure-property relationships for the zeolite support, strategizing methods to increase catalyst lifetime and developing advanced synthesis techniques for depositing a metal-based active phase within a zeolite for highly active, selective and stable tandem catalysts.

Keywords: CO2 hydrogenation, tandem catalysts, heterogeneous catalysts, catalysis, FischerTropsch synthesis

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1. Introduction To reduce the negative effects associated with the increasing atmospheric concentration of CO2, including climate change and ocean acidification, efforts must be put forth to decrease emissions, permanently sequester CO2 and convert CO2 into valuable products.1-2 Figure 1 compares the net global consumption of olefins, gasoline and methanol in 2017 with the overall CO2 emissions on a basis of gigatons carbon equivalent. As clearly shown by the figure, there is a significant gap between net consumption of the carbon-based products that are commonly targeted in CO2 hydrogenation research and the overall CO2 emissions. Therefore, in addition to converting CO2 into chemicals and fuels, which can sequester up to 42% of global emissions,3-4 CO2 conversion technologies must be used in concert with energy conservation, emissions reductions and carbon sequestration to close the carbon gap and reduce the atmospheric concentration of CO2.

Figure 1. Comparison of global net consumption of natural gas (methane), olefins (ethylene and propylene), methanol and gasoline compared with CO2 emissions on a basis of gigatons (Gt) carbon equivalent in 2017. Sources: IHS Chemicals,5 IGP Energy,6 and the U.S. Energy Information Administration (EIA).7 The large gap can be attributed to other CO2 emission sources, such as coal-fired powerplants, deforestation and concrete production. On the commercial level, conversion of CO2 into chemicals and fuels is a rapidly growing field because CO2 is an abundant and low-cost C1 feedstock that can be used as a platform chemical for storing renewable energy. For example, corporations in Iceland and Canada have already 1 ACS Paragon Plus Environment

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deployed pilot scale projects for both CO2 conversion and sequestration. In Iceland, Carbon Recycling International has developed a process that is driven by geothermal energy to reduce CO2 into methanol with a capacity of 4000 tons per year.8 In Canada, Carbon Engineering is developing a combined process for direct air capture of CO2, followed by conversion of the captured CO2 into synthetic fuels.8-9 Furthermore, Carbon Engineering has recently reported that the cost for sequestering CO2 has been reduced by a factor of six to $100 per ton during scale up of their direct air capture process.9 Although these pilot scale projects are encouraging for the field of CO2 utilization, future research efforts must balance process feasibility with the value of the target product to maximize the economic incentive for capturing and converting CO2. Perhaps the most promising route for CO2 conversion is selective hydrogenation to lower olefins, e.g. ethylene and propylene, the top two produced petrochemicals worldwide.10 In 2017, annual global ethylene and propylene consumption were approximately 150 million and 100 million metric tons, respectively.4 This high demand for lower olefins is due to their usage as building blocks in the chemical industry for production of plastics, polymers, solvents and cosmetics.11 Furthermore, olefins can be oligomerized into long-chain hydrocarbons for fuel, making them a desirable low-molecular weight target product with high potential for utilizing up to 23% of CO2 emissions (including gasoline), as shown in Figure 1. Currently, research into hydrogenation of CO2 to olefins focuses on three synthetic routes: (1) Direct hydrogenation of CO2 to olefins over multi-functional, promoted catalysts;12-15 (2) The MeOH-mediated pathway via MeOH synthesis followed by methanol-to-olefins (MTO);16-25 and (3) The CO-mediated pathway via the reverse water-gas shift reaction (RWGS) followed by Fischer-Tropsch synthesis to olefins (FTO).26-44 Controlling the selectivity of CO2 hydrogenation to the desired olefin product requires a fundamental understanding of the thermodynamics and kinetics of the above three pathways. RWGS (CO2 + H2  CO + H2O, H0298= 42.1 kJ mol-1) is endothermic and is thus favored at higher temperatures,45 while FTO (CO + 2H2  -(CH2)- + H2O, H0298= -152.0 kJ mol-1), MeOH synthesis (CO2 + 3H2  CH3OH + H2O, H0298= -45.9 kJ mol1)

and CO2 methanation (CO2 + 4H2  CH4 + 2H2O, H0298= -165.0 kJ mol-1) are all exothermic

processes.46-47 The thermodynamic data indicates that lower temperatures favor FTO, MeOH and methane synthesis, while higher temperatures are needed to activate CO2 and achieve fast reaction rates, thus complicating the identification of an ideal reaction temperature for targeting olefins.

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Clearly, the above thermodynamics suggest that design and synthesis of catalysts for selective production of olefins is challenging. In identifying catalysts that can directly hydrogenate CO2 to olefins with high selectivity (Route 1), one important area of consideration are Fe and Cobased FTO catalysts, which use synthesis gas (CO + H2) as the reactant and show high potential to be extended to olefin production from CO2 and H2.48 However, these FTO catalysts are still under development, requiring additional studies to understand the effect of promoters before they can be adapted to CO2 hydrogenation with the desired activity, selectivity and stability.49 As an alternative, instead of directly hydrogenating CO2 to olefins over a single active phase, it is possible to achieve more control over the reaction pathway, and therefore, higher selectivity to olefins, by coupling two sequential reactions over a tandem catalyst. For example, in the MeOH-mediated pathway (Route 2), a two-reactor system typically performs the MeOH synthesis step at ~250 °C, while the second MTO step is at temperatures of ~450 °C. By selecting an appropriate temperature to couple these two pathways over a single tandem catalyst, there is potential to reduce the cost of the process, but achieving high selectivity is challenging, especially while avoiding the undesired RWGS reaction, which is endothermic and favored at the higher MTO temperatures. For the CO-mediated pathway (Route 3), temperatures of ~300 °C are necessary to achieve fast rates of reaction and activate the inert CO2 molecule to drive the endothermic RWGS reaction forward, which is equilibrium limited to 23% conversion at 300 °C with a 3:1 H2:CO2 ratio. If FTO is coupled with RWGS via the CO-mediated pathway over a tandem catalyst, this route must utilize (1) A fast FTO component to drive the reaction to the right via Le’Chatelier’s Principle and (2) Recover heat from the FTO reaction step to minimize the heat input for the overall process.47 Another significant challenge of the CO-mediated reaction pathway is that CO2 methanation via the Sabatier reaction is undesired and difficult to avoid because the FTO component of tandem catalysts is typically active for CO2 methanation.50 In this perspective, the recent progress, challenges, opportunities and future research directions for controlling the selectivity of CO2 hydrogenation to light olefins will be discussed. After reviewing the primary routes for olefin production from CO2, the challenges and primary strategies will be outlined for controlling the selectivity in future research studies. Designing novel catalysts that are active, selective and stable under the desired reaction conditions requires a


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multifaceted approach that combines density functional theory (DFT) calculations and in situ reactor studies with novel catalyst synthesis methodologies. 2. Catalytic hydrogenation of CO2 to olefins 2.1. Inspiration for direct CO2-FTO catalysts from FTO catalysts Direct hydrogenation of CO2 to olefins is an attractive route for CO2 hydrogenation because of the simplicity of conducting the process over a single catalyst without the energy-intensive, post-reaction separation steps. The challenge, however, is achieving high selectivity towards the desired light olefin products, while suppressing the undesired C1 side products (CO and/or CH4).51 A large fraction of research has focused on using Fischer-Tropsch (FT) catalysts composed of Fe, Co and/or Ru, with minor modifications, as evidenced by Table 1, but these catalysts are typically limited by low selectivity to C2-C4 olefins, as characterized by the Anderson-Shulz-Flory (ASF) distribution.52 Dorner et al. studied a Fe-Mn/γ-Al2O3 catalyst for CO2 hydrogenation at T = 290 °C, P = 1.4 MPa, weight hourly space velocity (WHSV) = 5.4 L h-1 g-1 and a H2:CO2 ratio of 3:1.53 Their results show a moderate CO2 conversion of 37.7% with a C2-C5+ selectivity of 55.3%. The chain growth probability, represented by the quantity, α, is obtained from the slope of plotting ln(WN/N) versus N, where WN is the weight fraction of hydrocarbon products with hydrocarbon chain length, N. For the unmodified Fe-Mn catalyst and Fe-Mn promoted with 10 wt% Ce, both have α values of approximately 0.47, indicating a wide distribution of hydrocarbon products. Furthermore, upon addition of Ce, the olefin to paraffin (O:P) ratio is virtually constant, decreasing from 1.16 to 1.14.53 Clearly, these Fe-based catalysts are unselective towards the desired products, requiring further study of catalyst promoters and structure-property relationships to improve the olefin selectivity. A difficulty of improving the selectivity of CO2 to olefins (CO2-FTO) catalysts is that FTO catalysts are not fully mature, thus limiting the availability of model catalysts that could logically be extended from FTO to CO2-FTO. Recently, however, the exploitation of natural gas reserves has inspired research into novel routes for utilizing synthesis gas produced via steam reforming of natural gas. Because light olefins are the top produced petrochemicals worldwide, synthesizing these commodities through FTO is highly desirable, and a novel research direction in this area 4 ACS Paragon Plus Environment

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could provide guidelines for designing CO2 hydrogenation catalysts that are selective for olefin production. One potential approach is modifying Fe and Co-based FTS catalysts with appropriate promoters to adjust the electronic and structure properties of the catalysts to increase the selectivity towards light olefins.49 For FTS over Fe promoted with varying amounts of K supported on reduced graphene oxide, the highest selectivity to lower olefins is achieved at the highest K content, where the catalytic activity is also the lowest (reaction conditions: T= 340 °C, P = 2 MPa, WHSV = 36 L h-1 g-1, H2:CO ratio = 1).54 Using Mössbauer spectroscopy, the authors theorize that the addition of K to the Fe-based catalyst favors the formation of Hägg carbide (Fe5C2), the active phase for olefin production, but an excess of K results in increasing size of the carbidic particles and blocking of the active phase via carbon deposition.54 Mn has also been widely-studied as a promoter for FTS, which attenuates the electronic and structure properties of the catalytically active phase, resulting in increased olefin selectivity and catalyst stability.55-56 For example, over Mn-promoted Co2C synthesized by a co-precipitation method (reaction conditions: T = 250 °C, P = 0.1 MPa, WHSV = 2 L h-1 g-1, H2:CO ratio = 2), Zhong et al. report a CO conversion of 31.8% with a C2–C4 olefin selectivity of 60.8% and a very low CH4 selectivity of less than 5%.56 From high resolution transmission electron microscopy (HRTEM) imaging at different reaction times, the authors suggest that Co2C nanocrystals form and then gradually evolve into a parallelepiped shape during the reaction. Upon the addition of Mn, the Co2C restructures into a nanoprism geometry with exposed (101) and (020) crystal planes, which are theorized to be the active facets for syngas conversion to olefins.56 This has been recently confirmed by Li et al. who clearly show that the Co-Mn composite oxide also forms nanoprisms with exposed (101) and (020) facets that are selective towards C2-C4 olefins, while the Co-Mn catalyst prepared by traditional impregnation is spherical and unselective towards light olefins.57 Another promising result has been achieved by Galvis et al., who studied Fe-based catalysts supported on α-Al2O3, prepared with an ammonium iron citrate precursor.58 The iron-based precursor contains trace amounts of sodium and sulfur promoters, which have a positive effect on selectivity towards light olefins. The authors postulate that the presence of sulfur in the catalyst formulation favors β-hydride abstraction of the adsorbed alkyl intermediates, resulting in formation of olefins instead of chain growth termination via hydrogenation to form the undesired methane and paraffins.58 5 ACS Paragon Plus Environment

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Follow-up studies systematically introduced Na and S into the Fe-based catalyst supported on carbon nanofibers (CNF) to determine catalyst stability and the extent of carbon deposition and carburization of the Fe active phase. Figure 2a shows that the promoted catalyst exhibits high initial catalytic activity and selectivity towards light olefins, but the catalysts deactivate after 100 h on stream.59 This behavior is contrasted with the unpromoted Fe-based catalysts, where catalytic activity increases with time on stream.59 The rapid deactivation observed over the promoted catalysts is a result of carbon deposition, as indicated by in situ tapered element oscillating microbalance (TEOM) measurements. Additionally, Mössbauer spectroscopy reveals a greater extent of Fe carburization over the promoted catalyst, which is theorized to agglomerate under reaction conditions, as shown in Figure 2b.59 Previous Mössbauer spectroscopy studies by the same research group indicate a stronger Fe-CO bond and increased chain growth probability upon the addition of Na, while S weakens the Fe-CO bond, and reduces the surface coverage of hydrogen, resulting in lower selectivity towards the undesirable paraffins and methane.60

Figure 2. Effect of Na and S promoters on C2–C4 olefin selectivity over Fe/CNF supported catalysts. a) Olefin yield versus time-on-stream; b) Illustration of agglomeration of the promoted Fe-based nanoparticles. Reaction conditions: T = 340 °C, P = 2 MPa, gas hourly space velocity (GHSV) = 54,000 h−1 and H2:CO ratio = 1. Reprint with permission from ACS.59 Although electronic (Na, K and S) and structural (K, Mn) promoters are promising for improvement of light olefin selectivity during FTO, the specific effect of promoters on the catalytic 6 ACS Paragon Plus Environment

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properties and mechanism is not well-characterized. The result is an observed limitation on the efficacy of promoters as they substantially improve the selectivity towards light olefins, but unwanted side-reactions and catalytic deactivation are still observed. Clearly, additional research on the electronic and structural effects of promoters is necessary, but in practice, the characterization is challenging because the intrinsically low concentration of promoters (as low as 0.1 wt%) requires reproducible synthesis methods and advanced characterization techniques, such as Mössbauer spectroscopy,59 X-ray adsorption fine structure (XAFS)61 and X-ray photoelectron spectroscopy (XPS)62 to elucidate the environment of the promoting species. Nevertheless, researchers have still extended the use of promoters from FTO to CO2 hydrogenation because of their clear benefit in improving olefin selectivity.

2.2. Direct CO2 hydrogenation to olefins with promoters There have been an extensive number of studies attempting to extend the work on promoters for FTO to CO2 hydrogenation to olefins, as shown in Table 1. For example, alkali metal promoters have been found to increase the binding energy of CO2, and decrease the hydrogen binding energy,63 thereby suppressing the formation of undesirable methane and light alkanes.64 For Fe-Co-Zr polymetallic ﬁbers, the addition of potassium increases the selectivity to C2–C4 olefins from 90%). The lack of dual-functionality of the In-Zr oxide prevents further conversion of the methoxy intermediates into desirable olefins.89 Therefore, adding a MTO catalyst, e.g. SAPO-34, to the catalyst formulation can result in coupling of the methoxy intermediates to light olefins with high selectivity (>80%).89 The findings are further confirmed by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and similar observations have been reported over a comparable ZnZr/SAPO-34 system.90 11 ACS Paragon Plus Environment

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A study of the specific reaction mechanism for the formation of CH4 and C2H4 over Cu-Fe bimetallic surfaces, shown in Figure 3,91 suggests that CH* is a critical adsorbed intermediate, at which the pathways for methane and ethylene production diverge. For methane production, the adsorbed CH* undergoes three successive hydrogenation steps to form CH4*, which then desorbs from the surface as CH4. Conversely, if two of these CH* species are first coupled to form C2H2*, and are subsequently hydrogenated to C2H4*, the resultant product upon desorption is the desired C2H4.91 The authors find that over the bimetallic Cu-Fe(100) surface with 4/9 monolayer of Cu, CH4 formation is suppressed and C2H4 production is enhanced relative to monometallic Fe(100), due to a higher hydrogenation barrier of adsorbed CH* versus C−C coupling to form C2H2*.91 However, translating these theoretical calculations to a real catalytic system does not always have the anticipated result as a recent study shows a decrease in selectivity towards C2-C4 olefins upon addition of Cu to a K-Fe/Al2O3 catalyst.92

Figure 3. Proposed reaction mechanism of CO2 hydrogenation to CH4 and C2H4 over Cu-Fe bimetallic surfaces by DFT calculations. Reprint with permission from ACS.91 To adequately predict and understand a model catalyst system, DFT calculations must be combined with experimental techniques to validate their applicability on a real system. As an example, ambient pressure X-ray photoelectron spectroscopy (AP-XPS), DFT and kinetic Monte Carlo simulations have been utilized together to identify the active site for CO2 hydrogenation to methanol over a Zn-Cu catalyst.93 As shown in Figure 4a, deposition of ZnO onto Cu(111) results in increasing methanol production rates with increasing of ZnO coverage, until reaching a maximum rate at ~0.2 monolayer (ML). The increased catalytic activity of Cu(111) with low coverages of ZnO can be attributed to the larger number interfacial sites created by the welldispersed ZnO particles.93 Energy barrier calculations for the CO2 hydrogenation pathway in Figure 4b show that the reaction proceeds mainly via *HCOO, *HCOOH, and *H2COOH 12 ACS Paragon Plus Environment

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intermediates and the formation of the critical formate (*HCOO) intermediate is stabilized at the ZnO-Cu(111) interface, resulting in a significant decrease in activation barrier and higher methanol production rate.93

Figure 4. a) Reaction rate of methanol synthesis over ZnO/Cu(111); and b) Proposed reaction mechanism of CO2 hydrogenation to methanol over Zn-Cu bimetallic surfaces by DFT calculations. Insert in Figure 4b: *HCOO on ZnO/Cu(111). Reaction conditions: T = 525, 550, 575 K, P = 0.5 MPa, H2:CO2 ratio = 9. Reprint with permission from AAAS.93

Kattel et al. further examined Cu-based catalysts with DFT calculations, specifically the binding energies of key intermediates at the Cu-TiO2 and Cu-ZrO2 interfaces during CO2 hydrogenation to methanol.94 The DFT calculations suggest that carboxyl intermediates are stabilized at the Cu-Zr interfaces, resulting in increased methanol production, while formate intermediates are likely spectators.94 The ZrO2 support results in higher activity relative to TiO2 because the binding energies of key reaction intermediates, *CO2, *CO, *HCO and *H2CO, are slightly increased in the presence of reduced Zr3+ at the Cu-ZrO2 interface.94 On the other hand, *HCOO intermediates bind strongly on both catalysts, resulting in poisoning.94 The DFT calculations are in excellent agreement with in situ DRIFTS experiments,94 demonstrating the importance of the metal-oxide interface for CO2 hydrogenation to methanol and subsequently, olefins. Although DFT calculations are widely applied to better understand complex problems in catalysis, there are still limitations on the capabilities of DFT for design of novel catalysts.95-96 DFT has been very successful for relatively simple approximations under ideal conditions, such as 13 ACS Paragon Plus Environment

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a single crystal surface with only a few molecules at ultra-low temperature and pressure, but over heterogeneous catalysts under real reaction conditions, bridging the pressure and materials gap is challenging, which limits the accuracy and utility of DFT calculations.97 The two major errors within DFT that result in decreased accuracy are the delocalization error and the static correlation error of approximating functionals.96 Understanding the fundamental cause of these errors is essential for development of new functionals, thus improving the accuracy of DFT predictions.96 Even by combining DFT with advanced analytical techniques such as, Mössbauer spectroscopy,59 XAFS,61 in situ Fourier transform infrared (FTIR) spectoscopy,89 AP-XPS93 and environmental TEM,98 bridging the pressure and materials gap is challenging and will depend highly on the complexity of the catalyst and the specific reaction conditions.99 Without experimental data to support the calculations, the findings might not necessarily be useful, as has been shown in the above Cu-Fe example. Nevertheless, there are also examples in the above section that clearly illustrate how combining DFT calculations with in situ characterization and reactor studies can help identify important details regarding reaction mechanisms and active sites. Proper utilization of DFT with experiments will help guide the development of novel catalysts, which is an important part of the approach for the production of light olefins from both CO and CO2 over tandem catalysts.

2.4. Direct carbon oxide hydrogenation to olefins over tandem catalysts 2.4.1. FTO catalysts as inspiration for CO2 hydrogenation Tandem catalysts, which couple multiple reactions over a single catalyst, have recently become a promising area of investigation for controlling the selectivity of carbon oxide hydrogenation to olefins. A landmark study occurred in 2016 when Jiao et al. reported a novel process for CO hydrogenation to light olefins over a ZnCrOx catalyst combined with a mesoporous SAPO zeolite (MSAPO).100 Over this bifunctional catalyst, the ZnCrOx component activates CO to a ketene intermediate, and then controlled C–C coupling occurs within the confined zeolite pores of MSAPO, resulting in an exceptionally high C2–C4 olefin selectivity of 80%, on a hydrocarbon basis.100


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As shown in Figure 5a, the authors measure the effect of MSAPO on catalytic performance by adjusting the configuration of the catalyst bed, which shows that ZnCrOx alone primarily produces CH4 with greater than 50% selectivity (Mode 1). Upon addition of MSAPO (Mode 2), CH4 production is significantly suppressed, leading to high selectivity of the desirable C2–C4 olefins. The selectivity towards C2–C4 olefins can be further improved by packing the ZnCrOx and MSAPO in the catalyst bed in an alternating sequence (Mode 3) or in a well-mixed composite catalyst (Mode 4). This suggests that reducing the distance between the ZnCrOx and MSAPO results in rapid transport of the active ketene intermediate from the metal oxide to the zeolite, increasing selectivity towards C2–C4 olefins, while suppressing CH4 formation.100 Furthermore, Figure 5b shows the high-stability and minimal CH4 production of this catalyst after 110 h on stream, but the ~45% selectivity to CO2 via the water-gas shift side reaction is difficult to avoid over the highly active Zn-based catalyst.

Figure 5. Catalytic performance of ZnCrOx/MSAPO for syngas to light olefins. a) Reaction performance over the catalysts packed in different modes as a function of space velocity. Mode 1: ZnCrOx only; Mode 2: MSAPO packed below ZnCrOx; Mode 3: MSAPO and ZnCrOx packed in an alternating layers; Mode 4: MSAPO and ZnCrOx well-mixed. b) Stability test of a composite catalyst with ZnCrOx:MSAPO ratio = 0.9. Reaction conditions: T = 400 °C, P = 2.5 MPa, WHSV = 6.8 L h-1 g-1, H2:CO2 ratio = 2.5. Reprint with permission from AAAS.100

It is clear from the work of Jiao et al., that the zeolite component of the catalyst is necessary to achieve high olefin selectivity. The effect of other zeolites, including ZSM-5, mordenite and zeolite beta, on CO hydrogenation selectivity have been studied over a Fe-Cu catalyst promoted 15 ACS Paragon Plus Environment

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with K.101 Of the three zeolites, ZSM-5 was found to exhibit the highest activity and olefin selectivity of ~30% due to the moderate acidity of the ZSM-5. The acidity is a critical feature of a highly selective olefin synthesis catalyst, which promotes the cracking of paraffins into olefins, without further olefin transformations, including C-C bond scission and oligomerization into higher order hydrocarbons.101 SAPO-34 is another zeolite which has shown superior selectivity for olefins, as reported by Cheng et al.102 In their work, a C2–C4 olefin selectivity of 90% was achieved over a ZrO2/SAPO-34 catalyst. However, the activity was low, only reaching a CO conversion of 1%, because of the weak ability of ZrO2 to dissociate H2.102 As these zeolite-based tandem catalysts have shown promise for high olefin selectivity via FTO, researchers have recently put forth efforts to extend these principles to CO2-FTO.

2.4.2. Tandem catalysts for CO2 hydrogenation to olefins The high olefin selectivity achieved by FTO tandem catalysts has inspired the natural extension of these catalysts to CO2 hydrogenation. As shown in Table 2, tandem catalysts for CO2FTO contain a variety of active phases, including Fe or Co,65, 73-74 bimetallics (Co-Fe and Co-FeZr)65, 74 and noble metals (Pt and Pt-Co).103 Metal oxides have also been studied, which include Fe3O4104 and In2O3.105-106 The second component of a highly-selective CO2 hydrogenation to olefins catalyst is a zeolite, which is used for two primary functions: (1) To control C-C bond formation and hydrocarbon chain growth via MTO and (2) To crack larger hydrocarbon fragments into olefins over acid sites. As an example of controlled C-C bond formation, supporting a mixed Zr-In oxide on a SAPO-34 zeolite results in high selectivity towards ethylene and propylene of up to 90% during CO2 hydrogenation.105 Zeolites other than SAPO have also been studied as supports to further control the hydrocarbon selectivity during CO2 hydrogenation, including ZSM-5, Y and beta.104107

These zeolites have shown high selectivity to olefins due to their unique pore structure and

strong acidity. Furthermore, a core-shell system consisting of a mixed metallic Fe-Zn-Zr core catalyst with a H-ZSM-5 shell can achieve C2–C4 selectivity to as high as 76% upon addition of a second zeolite, H-beta to the shell (reaction conditions: T = 340 °C, P = 5.0 MPa, WHSV = 3 L h-1 g-1 and H2:CO2 ratio = 3).107 The excellent performance of this core-shell catalyst is due to the synergistic effect of the H-ZSM-5 and H-beta zeolites. The authors have further shown that by 16 ACS Paragon Plus Environment

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varying the ratios of these zeolites in the shell, the selectivity towards light hydrocarbons can be tightly controlled.107 For example, the selectivity to C2–C4 hydrocarbons increases from 61.7% to 71.3% by increasing the H-ZSM-5 to H-beta ratio from 2:1 to 1:2, respectively. Similarly to the Zr-In oxide catalyst outlined above, other groups have coupled methanol synthesis with MTO over tandem catalysts, which typically use SAPO-34 zeolites. SAPO-34 is a desirable MTO catalyst because high activity (~100% MeOH conversion) and selectivity (~80% C2–C3 olefins) are achievable at relatively mild temperatures of 450 °C.108 Although SAPO-34 is a promising zeolite for MTO, these catalysts suffer from rapid deactivation.108-115 For example, Dai et al. studied MTO over a H‑SAPO-34 zeolite at 325 °C, and found the conversion increased during a 15 minute induction period from 20 to 80%, and then gradually decreased to 25% after 60 minutes on stream.116 Wang et al. studied MTO at 470 °C over a SAPO-34 zeolite that achieved ~100% conversion and ~81% selectivity to C2–C3 olefins, however, the zeolite deactivated after 155 minutes on stream, falling below 10% conversion after less than 5 h.117 Although SAPO-34 zeolites have a tendency to deactivate, Li et al. discovered that a ZnZrOx/SAPO-34 tandem catalyst for CO2 hydrogenation achieves a selectivity for light olefins as high as 80% among hydrocarbon products, as shown in Figure 6a.90 The reaction kinetics and the surface intermediates over the tandem catalyst are determined by in situ FTIR spectroscopy coupled with a mass spectrometer. The authors propose that the tandem reaction proceeds as follows: (1) Generation of CHxO adsorbed species via CO2 hydrogenation on ZnZrOx, followed by (2) Diffusion of the adsorbed CHxO onto SAPO zeolite for C-C bond formation, resulting in light olefins, as shown in Figure 6b.90


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Figure 6. a) Catalytic conversion of carbon dioxide to lower olefins over a tandem ZnZrOx/SAPO-34 catalyst; b) Schematic for proposed reaction mechanism. Reaction conditions: T = 380 °C, P = 2 MPa, WHSV = 3.6 L h-1 g-1, H2:CO2 ratio = 3. Reprint with permission from ACS.90

Ideally, by combining MeOH synthesis and MTO over one multifunctional catalyst, olefins can be directly produced from CO2, as illustrated by Li et al. in the previous paragraph.90 However, as shown in the supporting information of their publication, CO comprises about 50% of all products.90 This is because MeOH synthesis is a slightly exothermic process, which typically operates at a relatively low temperature range of 200–300 °C under elevated pressures to achieve the highest MeOH yields.118 In contrast, MTO over SAPO-34 typically takes place above 400 °C to obtain high conversion and selectivity to light olefins.119-121 When MTO is conducted at typical CO2 hydrogenation temperatures of 300 °C over SAPO-34, conversion is below 10%.122 To achieve high conversion and selectivity for CO2 hydrogenation to light olefins, Li et al. chose an operating temperature between the ideal values for methanol synthesis and MTO (350 °C), thus the result is a catalyst with very high selectivity toward light olefins, but as shown in the supporting information of the report by Li et al., also under conditions where ZnZrOx is highly selective towards the undesired CO byproduct via the RWGS reaction.90 Therefore, the high CO selectivity obtained over this MeOH-mediated, CO2 hydrogenation catalyst is likely unavoidable until hightemperature, highly selective MeOH synthesis catalysts are developed. The ZnZrOx/SAPO-34 catalyst marks one of the first examples of a true two-phase tandem catalyst for CO2 hydrogenation. Prior to this example, most researchers physically mixed the active phase and zeolite to illustrate that the proximity between the two phases can affect product selectivity.123-126 For example, in Figure 7, Gao et al. show a clear proximity effect for CO2 hydrogenation by using different particle sizes and reactor configurations over In2O3/H-ZSM-5.125 In case of a dual-bed, selectivity to C2-C4 hydrocarbons is below 10% if the zeolite is packed above In2O3 (Figure 7a), while CH4 and MeOH selectivity are approximately 65% and 30%, respectively. However, if In2O3 is packed above the zeolite (Figure 7b), C2-C4 selectivity increases to 25%, while the MeOH selectivity drops below 5%, suggesting MeOH produced over In2O3 is rapidly converted to C2-C4 and C5+ hydrocarbons with minimal CH4 production (< 5%). Further decreasing the distance between the In2O3 phase and zeolite via granule stacking enhances the production of 18 ACS Paragon Plus Environment

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C5+ hydrocarbons, while suppressing CO and CH4 production (Figure 7c and d). If the distance between the In2O3 and zeolite is further reduced by mortar mixing to achieve intimate contact (Figure 7e), CH4 selectivity is greater than 94%.125 Clearly, the proximity of the active phase and zeolite influences the hydrocarbon selectivity, but it is difficult to control and study the specific location of each component in a physically mixed catalyst.

Figure 7. The effect of proximity of the active component and zeolite on catalytic conversion of carbon dioxide over a In2O3/H-ZSM-5. (Reaction conditions: T = 340 °C, P = 3 MPa, WHSV = 6.75 L h-1 g-1, H2:CO2 ratio = 3, In2O3/H-ZSM-5 mass ratio = 2:1). a and b) Dual-bed configuration; c) Granule stacking of In2O3, H-ZSM-5 and quartz sand with particle sizes of 250–380 µm; d) Well-mixed In2O3 and H-ZSM-5 particles without quartz sand; e) In2O3 and HZSM-5 mixed with a mortar. Reprint with permission from Springer Nature.125

To better control the location of each active component, and therefore, the reaction cascade, Xie et al. have taken a different approach for CO2 hydrogenation to olefins, by using the COmediated route over a core-shell tandem catalyst with a well-defined nanostructure.127 The CeO2Pt@mSiO2-Co catalyst consists of two metal-oxide interfaces: a core CeO2-Pt catalyst and a shell 19 ACS Paragon Plus Environment

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mSiO2-Co catalyst (mSiO2: mesoporous silica), as shown in Figure 8a.127 The interfaces catalyze two sequential reactions: (1) CeO2/Pt first converts CO2 and H2 into CO via the RWGS reaction, and (2) Co/mSiO2 subsequently hydrogenates CO to C2–C4 hydrocarbons via FTS, resulting in a C2–C4 selectivity of up to 60% (Figure 8b). Interestingly, although the selectivity to C2–C4 hydrocarbons is not as high as reported over other tandem catalysts, for instance 78% C2–C4 olefin selectivity over Fe3O4/H-ZSM-5, there is no CO detected, which is in stark contrast to the above MeOH-mediated example.104 The low CO selectivity is a result of CO forming solely over the core Pt/CeO2 catalyst, because the shell Co/mSiO2 catalyst is inactive for the RWGS reaction, as displayed by the near 100% CH4 selectivity in the catalyst without Pt in Figure 8b. As illustrated by the figure, the CO formed over the Pt/CeO2 core is subsequently hydrogenated to C2–C4 hydrocarbons over the shell Co/mSiO2 catalyst at a faster rate than diffusion of CO out of the shell, resulting in very low CO selectivities. However, CH4 selectivity over this unique core-shell catalyst remains at a moderate level of ~40% because of the high CO2 methanation activity of the Co/mSiO2 shell catalyst, illustrating the limitation of the CO-mediated route, in that catalysts active for FTS are also typically active for CO2 methanation.127

Figure 8. a) Schematic of CeO2-Pt@mSiO2-Co tandem catalyst for direct CO2 hydrogenation to C2-C4 hydrocarbons; b) Catalytic performance under reaction conditions, T = 250 °C, P = 0.6 MPa, WHSV= 50.4 L h-1 g-1 and H2:CO2 ratio = 3. Reprint with permission from ACS.127


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Overall, the authors attribute the increased C2–C4 selectivity to the unique spatial arrangement of two metal-oxide interfaces, which creates local environments conducive for a twostep tandem reaction that is unachievable over a physical mixture of the two catalyst components, Figure 8b.127 Perhaps if the SiO2 shell catalyst were replaced with an acidic zeolite, such as ZSM-5, the olefin selectivity could be increased by cracking of the C2-C4 paraffins into olefins over the Brønsted acid sites of ZSM-5, however, synthesis of such a tandem catalyst with high encapsulation efficiency and selectivity to target products, is still challenging. Furthermore, the chemical effect of binding an active phase on a zeolite surface is further convoluted by the proximity effect illustrated in the above In2O3/ZSM-5 example.127 To address the difficulties of encapsulating metallic particles within zeolites, Cho et al. recently developed a novel approach to synthesize a tandem catalyst consisting of Pt nanoparticles encapsulated within zeolites.128 The authors directly introduced anionic Pt precursors (PtCl62-) into the zeolite synthesis media along with a cationic polydiallydimethylammonium chloride (PDDA) polymer (Figure 9a), resulting in self-assembly of Pt nanoparticles within the H-ZSM-5 crystals. After calcination to remove the PDDA template and reduction to obtain metallic Pt particles, the resultant tandem catalysts contain ~2 nm Pt nanoparticles encapsulated within H-ZSM-5 (Pt@HZSM-5) and pure silica (Pt@Si-MFI).128 Most importantly, it is challenging to determine the encapsulation efficiency of nanoparticles within a zeolite using an imaging technique, such as TEM, because of the 2D projection of the 3D catalyst. Creative size-exclusion reactions must be used to accurately determine the encapsulation degree (Z value), which are commonly used in literature.129 In this example of Pt@H-ZSM-5, the authors determined the Z value over the tandem catalyst (Pt@HZSM-5) and a control catalyst prepared by incipient wetness impregnation (Pt/H-ZSM-5, Figure 9b) via selective oxidation of cyclohexanol and cyclododecanol. The smaller kinetic diameter of cyclohexanol (0.60 nm) permits access to the Pt nanoparticles both inside and outside of the zeolite crystals, while the larger cyclododecanol (0.84 nm) can only access the Pt nanoparticles located on the zeolite surface. Therefore, it is expected that the oxidation of cyclohexanol could proceed over both catalysts, while the larger cyclododecanol would only be oxidized over the Pt/H-ZSM5 control catalyst. From the reactor experiments, their results indicate Z values greater than 0.9 for Pt@Si-MFI and Pt@H-ZSM-5, and 0.02 for Pt/H-ZSM-5, respectively, suggesting more than 90% of the Pt nanoparticles are encapsulated within the Pt@H-ZSM-5 tandem catalyst, and only ~ 2% 21 ACS Paragon Plus Environment

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of Pt nanoparticles are located within the Pt/H-ZSM-5 synthesized using conventional synthesis methods.128 To illustrate the importance of encapsulating the Pt particles within the acidic ZSM-5, the catalytic performance is evaluated by tandem aldol condensation and hydrogenation of furfural and acetone (Reaction conditions: 1 mmol of furfural, 20 mmol of acetone, 2 mL isopropanol in a batch reactor; P = 4.1 MPa of H2, T = 180 °C for 3 h).128 The results indicate that the Pt@H-ZSM-5 catalyst produces >80% yield of the target aldol products (4-(2-furyl)-butan-2-one and 4-(2furyl)butan-2-ol), while large amounts of 2-methylfuran (41%) and cyclopentanone (13%) are produced over the traditional Pt/H-ZSM-5. The observed difference in selectivity can be explained by comparing the reaction pathways over the two catalysts. On Pt@H-ZSM-5, furfural and acetone first react to form aldols (4-(2-furyl)-buten-2-one) over the H-ZSM-5 acid sites before hydrogenation over the Pt nanoparticles. However, in the case of Pt/H-ZSM-5, the furfural is directly converted to undesired products via decarbonylation or hydrodeoxygenation over the Pt nanoparticles on the surface of the zeolite, which can occur before aldol condensation, thus significantly decreasing the selectivity to the target products.128 This example is analogous to the previously discussed core-shell CO2 hydrogenation example in Figure 8, suggesting that with an appropriate synthetic procedure, a tandem catalyst with the desired structure can be synthesized to study the effect of nanoparticle encapsulation within a zeolite for CO2 hydrogenation.

Figure 9. Schematic of zeolite-encapsulated Pt nanoparticles within H-ZSM-5 and Si-MFI. a) Schematic of the synthesis procedure; b) Encapsulation degree (Z value) of Pt nanoparticles determined by selective oxidation of cyclohexanol and cyclododecanol. Reaction conditions: 0.1


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mmol reactant; 0.3 mmol of tert-butyl hydroperoxide (5.0-6.0 M in decane) and 2 mL acetonitrile, T = 60 °C for 2 h. Reprint with permission from ACS.128

Table 2. Summary of performance of selected tandem catalysts for CO and CO2 hydrogenation to olefins. Catalysts are listed in order of reactant with CO hydrogenation first, followed by CO2 hydrogenation to olefins.

Catalyst

Reactant

H2:C ratio

Temperature (°C)

Pressure (MPa)

Conversion (%)

Target Product

Selectivity (%)

Space Time Yield (L h-1 g-1)d

Zn-CrOx /MSAPO100

CO

2.5

400

2.5

17

C2–C4=

80a

0.50

K-FeCu/ZSM5101

CO

2

300

1

81.1

C2–C4=

30.3a

0.32

KFeCu/Zeolite Beta101

CO

2

300

1

63.9

C2–C4=

29.2a

0.36

ZrO2/SAPO34102

CO

2

400

1

1

C2–C4=

90a

0.04

MnO/SAPO34130

CO

2.5

400

2.5

8.5

C2–C4=

79.2a

0.18

ZnCrOx/Morde nite131

CO

2.5

360

2.5

26

C2H4

73a

n/a

Co/Y-zeolite132

CO

2

260

1

75

Olefins, unspecified

18.3b

0.07

ZrIn2O3/SAPO3489

CO2

3

400

3

35

C2–C4=

80c

0.47

ZnZrO2/SAPO3490

CO2

3

380

2

12.6

C2–C4=

85c

0.22

Na-Fe3O4/HZSM-5104

CO2

3

320

3

34

C2–C4=

18c

0.22

In2O3ZrO2/SAPO34105

CO2

3

400

1.5

20

C2 + C3=

80-90c

0.22-0.25


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In2O3 /SAPO34106

CO2

3

380

3

15.3

C2–C4=

82c

0.36

Fe-ZnZr@zeolites107

CO2

3

340

5

14.3

C2–C4

76c

0.18

In2O3/H-ZSM5125

CO2

3

340

3

15

C2–C4

25c

0.18

CeO2Pt@mSiO2Co127

CO2

3

250

0.6

26

C2–C4

40c

1.30

Cu-ZnAl/HBeta zeolite133

CO2

3

300

1

27.6

C2–C4

43.2c

0.18

C2-C4= denotes C2-C4 olefins, while C2-C4 denotes total C2-C4 hydrocarbon species. a) Molar percentage of olefins in hydrocarbon products (without CO2 and oxygenates); b) Molar percentage of olefins in all products (includes hydrocarbon, CO, and oxygenates); c) Molar percentage of olefins in hydrocarbon products (without CO and oxygenates); d) Calculation based on the reported conversion, selectivity (in all products), catalyst weight, gas hourly space velocity (or flowrate) in the reference; n/a: not available.


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3. Outlook and future directions Although much effort has been devoted to catalyst development for CO2 hydrogenation to olefins, further improvements in activity, selectivity and stability are required. To design improved catalysts for CO2 hydrogenation to olefins, future research should focus on the following directions: Elucidating Reaction Mechanisms It is important that future studies reveal the key reaction intermediates and mechanisms for production of light olefins during CO2 hydrogenation. DFT calculations offer extensive insight into model systems, leading to the discovery of catalysts that can activate CO2 with a minimal activation barrier. However, the information gained from DFT calculations can be limited because the systems studied are under idealized conditions of ultra-high vacuum or on well-defined, single crystal surfaces, which do not always directly translate to a heterogeneous catalyst in a packed bed reactor under high temperature and pressure.134 As computing power continues to increase, more complex and realistic catalytic systems can be modeled, which offer insight into the mechanism of CO2 hydrogenation to olefins. For example, a recent report has studied the effect of oxygen coverage on CO2 activation for a potassium-promoted Mo2C-based catalyst. The authors find that when the surface is covered with up to 0.5 ML of oxygen, a situation that approaches the actual state of the catalyst under reaction conditions,135 CO2 can still adsorb and dissociate on the catalyst surface.136 As illustrated by the examples outlined in this perspective, producing light olefins via CO2 hydrogenation is generally difficult and unselective. Part of the challenge in designing highly selective catalysts for CO2-FTO is that clear structure-property relationships are unavailable, which are necessary to identify the critical features that govern selectivity towards olefins. In CO2 hydrogenation, the dozens to hundreds of intermediates, associated elementary steps and reaction pathways for this complex reaction network makes it extremely difficult to infer the rate determining step (RDS), and in turn, design a catalyst with high olefin selectivity. An emerging area of research that will likely be extremely important for further understanding of CO2-FTO is coupling DFT calculations and experiments with machine learning to gain a better understanding of complex reaction networks, and infer the most important elementary step(s) for production of desired products.137-138 25 ACS Paragon Plus Environment

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As a simplified example, if machine learning were applied to the RWGS reaction, a component of the CO-mediated pathway, an important question that could be answered is if the pathway for methane formation exhibits a common intermediate with RWGS (likely adsorbed CO), or if there is a separate, but parallel reaction pathway that can be suppressed to avoid methane formation.139 It could be further elucidated if olefin production is more favorable if the RWGS reaction proceeds via (1) CO2 dissociation (CO2* → CO* + O*) or (2) Carboxyl-mediation, where CO2 is hydrogenated to a carboxyl, which is then dissociated to CO and OH (CO2* + H* → COOH* → CO* + OH*).140 However, as discussed in the DFT section, it will always be necessary to combine these theoretical calculations with advanced in situ analytical techniques to characterize the catalysts under reaction conditions and determine the utility of the theoretical models. Without support of real experimental data, it is impossible to discern whether or not the calculated reaction intermediates, mechanisms and rate determining steps are valuable for guiding future catalyst development. Zeolite Selection As discussed previously, zeolites are potentially a key element of the catalyst formulation to promote high selectivity towards the desirable light olefins. Different zeolites, such as mordenite,131 beta,133 Y,132 H-SAPO-34,141 H-ZSM-5,142 H-ZSM-22,142 DNL-6143 and H-RUB50,144 have been tested extensively for FTS, FTO and CO2 hydrogenation. However, to date, a systematic study relating zeolite properties to CO2 hydrogenation activity and selectivity has not been conducted. As evidenced by Table 2, there are promising zeolites available, but ultimately the structure-property relationships, and therefore, guidelines to select a zeolite for a CO2-FTO tandem catalyst are ultimately unknown. Part of the difficulty in studying these relationships is that when synthesizing zeolites, there are several variables and conditions that can yield distinctly different results including synthesis template, crystallization temperatures, acidity (Si/Al ratio), pore system (1D, 2D and 3D), pore size, extra-framework cations and hierarchical structures.145 As it currently stands, commonly used zeolites are H-ZSM-5 for controllable acidity and SAPO34 for high MTO activity, but this is likely because these zeolites are well-characterized and easily synthesized, not because they are the ideal zeolites for CO2 conversion to olefins. An attempt has been made by Wei et al. to study the effect of zeolites with varying acidity and pore structures for CO2 hydrogenation over Na-Fe catalysts (reaction conditions: T = 320 °C, 26 ACS Paragon Plus Environment

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P = 3 MPa, WHSV = 4 L h-1 g-1 and H2:CO2 ratio = 3).104 The results suggest that the 10 member ring zeolite with moderate acidity (Si/Al = 160) and 3D pore structure (H-ZSM-5) show higher selectivity to C5-C11 hydrocarbons than those over 2D (H-MCM-22) and 1D (H-ZSM-23) zeolites. Unfortunately, a direct comparison between the zeolite topology and olefin selectivity is difficult because of the large amount of aromatic products obtained over the tandem catalysts studied in this example. Furthermore, for several of the catalysts studied, olefin selectivity is not reported, but within the supporting information of Wei et al., a clear trend shows that the C5-C11 olefin selectivity increases with increasing Si/Al ratio, thus decreasing acid site density. However, the acid strength may vary with the acid site density, even at the same Si/Al ratio, depending on the specific configuration of the Al and H+ atoms.146-147 This suggests that at the moderate temperatures for CO2 hydrogenation (~300 °C), the overall number and strength of acid sites are both important descriptors for controlling olefin selectivity. The study of tandem catalysts for CO2 hydrogenation to olefins is in the very early stages, and with additional research and detailed studies, the important characteristics for a highly active and selective zeolite will be better characterized. When combined with an active metal, the system becomes increasingly more complex, and a slight property change, such as the extra-framework cation or Si/Al ratio, could have important ramifications for the overall catalytic performance. Additionally, the method of impregnation is extremely important when designing a tandem catalyst, because a core-shell structure is likely much more favorable than impregnation of metallic particles onto the surface of a zeolite crystal, and the distance between active components also plays an important role. Additionally, studying advanced synthesis methods can offer strategies for isolating individual nanoparticles within the zeolitic framework, helping to address another difficulty of tandem catalysts for CO2-FTO, which is preventing particle sintering and catalyst deactivation. Improving Catalyst Lifetime Catalytic deactivation is mainly due to the carbonaceous deposits on the catalyst surface which block the active sites and micropores in the porous catalysts that are required for high olefin selectivity, e.g. SAPO-34 for MTO.148 In addition, promoters can cause phase transformations, particle growth and agglomeration, especially for FTS catalysts, resulting in drifting selectivity over time and a decrease in catalytic activity, as shown in the previous sections.59, 27 ACS Paragon Plus Environment

65

An

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understanding of how the particle growth and agglomeration occurs, as well as how to prevent it, must be the subject of a detailed CO2-FTO study to preserve catalysts that display promising initial performance, such as what was observed over the Fe-based catalysts promoted with Na and S.59-60 Moreover, RWGS, FTS and MTO all produce large amounts of water as a byproduct, which can have both positive and negative effects for these reactions.149 For a Co-based FTS catalyst, when a small amount of water is added to the reactor, or produced during the reaction, the hydrogen partial pressure decreases and it is hypothesized that the selectivity to olefins will increase as a result of less hydrogen adsorbed on the catalyst surface. However, further increases to the water content enhance CO dissociation, and in turn, increase the concentration of monomeric carbon species on the catalyst surface, resulting in higher selectivity to long-chain hydrocarbons at the expense of olefins.149-150 The effect of water is further convoluted over zeolites at high temperature, which can cause structural changes and loss of crystallinity, thus deactivating the catalyst.151-153 Therefore, catalysts with improved stability that are resistant to coke formation and water should be developed. A few strategies to achieve improved coke and water resistance are by varying the zeolite acidity,154 attenuating the extra-framework cations,122 actively removing water via a membrane technology149 or by burning off the coke simultaneously during the CO2-FTO reaction, similar to technology used in a fluid catalytic cracking unit.108 A potential method to increase the lifetime of SAPO for MTO is to synthesize nano or hierarchical zeolites to reduce the diffusion limitation of the catalyst.155-156 A top-down route via post milling and recrystallization has been developed to prepare SAPO-34 nanocrystals.155 The obtained nano-SAPO-34 shows considerably improved catalytic performance, which is due to a combination of shortened diffusion paths, decreased acidity and reduced Si on the crystal surface. Hierarchical SAPO-34 has also been synthesized using sacrificial organic templates, which show a significant increase in catalyst lifetime because of larger pore openings.119-120,

141

Another

interesting approach to prevent deactivation of the tandem catalysts is co-feeding other reactants, such as dimethyl ether and acetaldehyde, which greatly enhance the lifetime of the SAPO zeolite, while maintaining high selectivity to olefins.110, 121, 157 Novel Synthesis Methods for Tandem Catalysts Ideally, a multifunctional tandem catalyst should combine several features: (1) Activate CO2 at relatively mild conditions; (2) Produce CO or MeOH; (3) Control C-C coupling to yield 28 ACS Paragon Plus Environment

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olefins with high selectivity; and (4) Suppress side-reactions that lead to the formation of undesirable CO and CH4. Some efforts have been devoted to synthesis of tandem catalysts that display all of these properties, for example, the Pt-Co core-shell catalyst with very low CO selectivity127 and the ZnZrOx/SAPO-34 bifunctional catalyst with minimal CH4 formation.100, 104 However, these catalysts are still limited by significant production of undesirable C1 byproducts (CH4 and CO, respectively). An interesting approach to increase the selectivity towards the desired olefins is to encapsulate the metal-based active phase within a zeolite.158-165 By encapsulating the above active phases within a hollow zeolite, a multifunctional catalyst can be obtained as shown in Figure 10, similar to the above CeO2-Pt@mSiO2-Co tandem catalyst. However, the development of this type of catalyst with a zeolite shell requires a novel synthesis methodology, because introduction of metals exclusively within a hollow zeolite cavity generally results in a significant fraction of the impregnated metal depositing on outer surface of the zeolite, preventing precise control over the reaction cascade, and therefore, the selectivity.165-169 The TEM images in Figure 10a clearly show that the Ni nanoparticles in the as-synthesized catalysts are located both inside and outside of the silicalite-1 (MFI structure). After post-treatment with citric acid, most of the Ni nanoparticles on the outer surface of silicalite-1 are removed (Figure 10b and c), but further analysis of Ni/Silicalite-1 catalysts indicates that the citric acid washing step is neither selective nor complete. Careful inspection of the TEM images indicates a fraction of the smaller Ni nanoparticles located within the silicalite-1 are removed,161 while reactor data suggests that surface Ni nanoparticles still remain. Clearly, this example illustrates that both encapsulating and evaluating the presence of metallic nanoparticles within zeolites is a difficult aspect of synthesizing and characterizing tandem catalysts.170


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Figure 10. TEM images of Ni encapsulated in silicalite-1 a) before; and b and c) after citric acid post-treatment. The yellow circle highlights the Ni particles on the outer silicalite-1 surface. Reprint with permission from Wiley-VCH.170 Li et al. have developed a similar strategy to encapsulate transition-metal nanoparticles in hollow ZSM-5 and silicalite-1 zeolites using a post-impregnation NaOH or TPAOH washing step to synthesize bifunctional and size-selective hydrogenation catalysts.166 Co, Ni and Cu were all successfully introduced with uniform particle sizes, and these catalysts have shown excellent activity and shape selectivity.166 Other metals, including Fe, Au, Ag, Pt and Pd have also been successfully encapsulated in hollow SAPO-34, Y, ZSM-5 and beta zeolites using more advanced and time consuming techniques.171-173 These studies typically utilize templates, in the form of mercaptosilanes or cationic polymers, such as PDDA, to efficiently encapsulate Pt, Pd, Ir, Rh, and Ag clusters within zeolites.172 These novel synthesis methods could likely be extended to other active metals and zeolites for synthesis of tandem catalysts for CO2-FTO, however the synthesis conditions need to be further developed and optimized.


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By synthesizing a core-shell tandem catalyst with the appropriate active phases, it could be possible to exercise precise control over reaction selectivity. Several active phases have shown promise in CO2 hydrogenation, including metal carbides, which have properties similar to noble metals and high selectivity for the RWGS reaction,174-176 while Cu-Zn-based catalysts are preferred for MeOH synthesis.118, 134 Fe and Co are highly active FTS catalysts, which have been shown to be selective to olefins with and without promoters, but are also highly active for the undesirable Sabatier reaction.50 Clearly, the above active phases must be carefully engineered into a metalzeolite tandem catalyst to achieve precise control over the CO2 hydrogenation reaction. As discussed within the perspective, controlling both the proximity and chemical interactions between the active metal phase and zeolite is extremely difficult, but necessary, to achieve high activity, olefin selectivity and minimal deactivation.177 Economic Considerations The development of a commercial process for CO2 hydrogenation to olefins requires further development of an active and selective catalyst. Although different approaches are currently being studied, the cost of the raw materials and synthesis will determine the applicability for scale-up and industrial applications. Novel tandem catalysts are likely more selective towards olefins, but the complex synthetic routes will make industrial adaptations difficult. For example, the advanced synthesis approach for a core-shell catalyst127-128, 172 is time consuming and requires expensive (e.g. PDDA) and/or harsh conditions and chemicals.171 In contrast, conventional wetimpregnation159, 164, 166 is low-cost and achievable on a large scale, but the location of the metal particles cannot be precisely controlled, thus resulting in a decrease in selectivity towards the desired products, requiring a post-reaction separation step. A detailed cost/benefit analysis comparing the expensive, but highly selective tandem catalysts, with the low-cost, but unselective traditional catalysts would be required before scale-up of any CO2 hydrogenation technology. However, as demonstrated in this perspective, tandem catalysts are clearly promising for CO2 hydrogenation and efforts must be put forth to understand CO2 hydrogenation chemistry over these novel catalysts. If tandem catalysts are shown to be highly active, selective and stable for CO2 hydrogenation, perhaps low-cost synthesis methods will be later developed and adapted by industry.


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4. Conclusions Converting CO2 into light olefins requires increased efforts to develop novel catalysts with high activity, selectivity and stability. Several routes have been evaluated for hydrogenating CO2 into olefins, which are important platform chemicals for the production of polymers, drugs, plastics, chemicals and fuels. Direct conversion of CO2 to olefins over modified FTO catalysts and olefin production via a MeOH or CO intermediate over tandem catalysts are all promising approaches that have shown moderate selectivity to light olefins. The challenges are avoiding the undesirable CO or CH4 byproducts, while increasing the lifetime of the CO2-FTO catalysts. Understanding how to improve the selectivity toward light olefins requires a combination of DFT calculations and in situ studies, as well as advanced synthesis techniques to understand the metal phase-zeolite interactions and efficiently encapsulate the metal-based active phase within the appropriate zeolite, which has not necessarily been identified. To help prevent the negative effects associated with the increasing atmospheric concentration of CO2, including ocean acidification and climate change, CO2 hydrogenation to light olefins offers great potential for reduction of CO2 emissions by generating an economically attractive product that can be produced from the abundant CO2 feedstock. Acknowledgements We acknowledge support from the U.S. Department of Defense, Office of Naval Research, under Award No. N00173-18-P-1439. References (1) Posada-Pérez, S.; Ramírez, P. J.; Evans, J.; Viñes, F.; Liu, P.; Illas, F.; Rodriguez, J. A. Highly active Au/δ-MoC and Cu/δ-MoC catalysts for the conversion of CO2: The metal/C ratio as a key factor defining activity, selectivity, and stability. J. Am. Chem. Soc. 2016, 138, 82698278. (2) Ji, G.; Yang, Z.; Zhang, H.; Zhao, Y.; Yu, B.; Ma, Z.; Liu, Z. Hierarchically mesoporous o-hydroxyazobenzene polymers: Synthesis and their applications in CO2 capture and conversion. Angew. Chem. Int. Ed. 2016, 55, 9685-9689. (3) Onay, O.; Kockar, O. M. Slow, fast and flash pyrolysis of rapeseed. Renew. Energ. 2003, 28, 2417-2433. (4) Vitolo, S.; Bresci, B.; Seggiani, M.; Gallo, M. G. Catalytic upgrading of pyrolytic oils over HZSM-5 zeolite: Behaviour of the catalyst when used in repeated upgrading-regenerating cycles. Fuel 2001, 80, 17-26. 32 ACS Paragon Plus Environment

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