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Tuning Zeolite Properties towards CO2 Methanation: AnOverviewM. Carmen Bacariza,*[a] Inês Graça,[b] José M. Lopes,[a] and Carlos Henriques[a]

MinireviewsDOI: 10.1002/cctc.201900229

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This work provides a short review of the literature concerningthe utilization of zeolites as supports for CO2 methanationcatalysts. Indeed, the application of these materials in carbondioxide hydrogenation into methane has led to considerablyrelevant performances according to the literature, being zeolite-based catalysts more active and selective than commercialmaterials reported. Considering the well-known zeolite features

and the possibility to finely modulate its properties, inducingrelevant changes on the adsorbed metal species and in the CO2

activation ability of the catalyst, potentially interesting catalyticproperties can be generated also towards this transformation.Possible correlations between the characteristics of thesematerials, found as relevant for CO2 methanation, and thecatalytic properties will be then presented and discussed.

1. Introduction

In the last years, the relevant production of renewable energyhas motivated an increased interest in electric energy storage(EES) systems.[1] In line with this fact, several reviews have beenpublished dealing with energy storage methods and theperspectives for the future.[2,3] It has been reported that asuitable EES system could definitely deal with the intermittencyof the renewable sources derived energy and the unpredict-ability of their output. The produced surplus could be in factstored during periods when intermittent generation exceedsthe demand, and then be used to cover periods when the loadis greater than the generation.[3] Consequently, a definitiveextension of the renewable sources’ utilisation will be allowed.Among all, fuels production as energy vectors has attractedattention in the last years.

The utilization of CO2 as feedstock has also been focus ofhundreds of works in the last years,[4–8] with energy vectorsproduction from carbon dioxide and surplus renewable electricenergy being considered as a promising route for the achieve-ment of a more efficient electric energy network.[2,3]

In this way, Blanco et al.[9] published a detailed reviewregarding power-to-x (x=heat, liquids, chemicals, fuels andmobility) technologies, where the production of H2 throughelectrolysis and its subsequent conversion into methane withCO2 from different sources (e.g. carbon capture, biogas, air) wasincluded. Additionally, Schaaf et al.[10] reported the suitability ofCO2 methanation as an energy storage strategy, since itpresents a high storage capacity combined with high charge/discharge periods. Thus, among all the power-to-x alternatives,CO2 methanation (power-to-methane) represents an interestingstrategy. Indeed, being CH4 the main component of natural gas(NG), the existence of an established and solid network of NGallows the incorporation of the synthetic natural gas (SNG)produced from CO2 in the existing NG distribution grid.[11]

Furthermore, if CO2 methanation is implemented in industrieswith large CO2 emissions (e.g. cement industry or power plants),which can use natural gas for combustion processes, the

produced methane could be re-injected into the plants, savingmoney and decreasing CO2 emissions at the same time.

The efficiency of power-to-gas plants depends on the CO2

methanation catalysts. Indeed, the rise of active, selective andstable catalysts belongs to the core of the methanationprocess.[12] CO2 methanation has been focus of hundreds ofresearch studies in the last years.[4–6,12–20] Ni and Ru-basedmaterials have been commonly used for this reaction while, interms of supports, SiO2, Al2O3, Ce and Zr oxides, mesoporousmaterials, carbons, hydrotalcite-derived materials and zeoliteshave been reported.[4–6,12–20]

The development of suitable zeolite catalysts, through thepreparation of well-defined samples, with controlled type ofactive sites (metals, metal oxides and acidity), could not onlyallow taking advantage from zeolite ability to stabilize differentmetal species, but also from zeolite confinement effects (zeolitecages, channels and channels intersections really act likenanoreactors, boosting catalysts activity).[21–24] Different zeolitestructures could be used as supports and their basicity/aciditycould be modified by cationic-exchange with alkaline metalsand by post-synthesis treatments (e.g. dealumination).[22,25]

Moreover, zeolites structure hydrothermal stability could beimproved by steaming treatments, becoming more resistant tothe water formed during CO2 hydrogenation.[22,25] Among themain reported methods for metallic supported catalystspreparation,[26–29] the introduction of metal species in zeolitescould be mainly performed by ion-exchange or impregnationmethods.[30] The metallic phase dispersion could be thusinfluenced by the preparation method, as well as by activationconditions (e.g. calcination temperature). All these features ofzeolite-based catalysts, which made them suitable materials formany type of catalysis applications,[31] could be indeed tuned, inorder to satisfy the characteristics found as favourable for theachievement of active, selective and stable materials for CO2

methanation.In the present work, a review of peer-reviewed publications

using zeolite materials for CO2 methanation will be attempted.In this way, the main conclusions of the published works will besummarized, and the performances of the best reportedsamples compared, taking into account the limitations due tothe dependence of the results on the experimental conditions.Finally, the main characteristics found as favourable in theliterature for the achievement of interesting catalysts for carbondioxide conversion into methane will be presented and possiblecorrelations with the catalytic properties will be discussed.

[a] Dr. M. C. Bacariza, Prof. Dr. J. M. Lopes, Prof. Dr. C. HenriquesDepartment of Chemical EngineeringCentro de Química EstruturalInstituto Superior Técnico, Universidade de LisboaAv. Rovisco Pais, 1049-001 Lisboa (Portugal)E-mail: maria.rey@tecnico.ulisboa.pt

[b] Dr. I. GraçaDepartment of Chemical EngineeringImperial College LondonLondon SW7 2AZ (UK)

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2. CO2 Methanation over Zeolite-basedCatalysts under Thermal Conditions

Several works have been reported in the last years dealing withthe utilization of zeolite-based materials as catalysts for CO2

methanation. The main results found in the literature will besummarized in this chapter both in terms of monometallic andbimetallic catalysts applied under conventional or cyclicsorption enhanced methanation conditions. Additionally, thereported findings regarding the mechanism will be alsopresented.

2.1. Catalysts

The most remarkable findings in terms of catalyst composition,method used for the incorporation of the metallic phase, pre-reduction temperature (Tred), H2 :CO2 ratio, gas hourly spatialvelocity (GHSV) and best catalytic performances (CO2 conver-sion and CH4 selectivity at a specific reaction temperature) canbe found in Table 1.

Starting with the monometallic catalysts, Kitamura Bandoet al.[32] prepared Rh-exchanged Y zeolite (with Na+ as compen-sating cation and a Si/Al of 2.4) and performed catalytic tests at30 atm. Rh metal particles (~3 nm) were found to be locatedoutside the zeolite cages whatever the Rh loading (from 1 to

6 wt.%Rh). The authors verified that the performances of theRh� Y catalysts were higher than those of a conventional Rh/SiO2 sample. They speculated that the activity of the Rh� Ycatalyst was due to the zeolite cages, which played animportant role in condensing CO2 molecules and supplyingthem to the Rh sites located outside the cage for the promotionof the reaction. The authors also verified an intense deactivationafter 100 min under reaction conditions due to the accumu-lation of the produced water inside the zeolite cages, with aconsequent suppression of the CO2 reservoir inside the cage.Aziz et al.[33] prepared a HY zeolite (without any information interms of Si/Al ratio) with 5 wt.% Ni by impregnation with theaim of comparing it with other Si-based materials. Their zeolite-based catalyst presented lower performances than the corre-sponding mesostructured silica nanoparticles and MCM-41based catalysts, while the Ni/SiO2 and Ni/γ-Al2O3 samples wereless active than the Ni/HY zeolite (Figure 1).

Ocampo[34] also prepared a 5%Ni-HY zeolite by ion-exchange, in order to compare it with other Ni� Ce-Zr materials.The author did not report relevant performances for thiscatalyst in terms of CO2 methanation, being reverse water gasshift (RWGS) promoted by the acid support. Additionally, Graçaet al.[35] studied the effect of the preparation method (ion-exchange or incipient wetness impregnation) and the Nicontent in the performances of Ni/HNaUSY zeolites. The authorsverified that only impregnation could lead to significant activitytowards CO2 methanation due to the higher temperatures

M. Carmen Bacariza was born in Caldas deReis (Galiza, Spain) in 1989 and studiedChemical Engineering in Escola Técnica Supe-rior de Enxeñería (ETSE, Universidade deSantiago de Compostela) from 2007 to 2013.She completed her PhD thesis in ChemicalEngineering, entitled “CO2 conversion to CH4

using metallic catalysts supported on zeolites”,under the supervision of Prof. José ManuelLopes and Prof. Carlos Henriques in May 2018,at Instituto Superior Técnico (IST, Universidadede Lisboa, Portugal, CATSUS-FCT PhD Pro-gram). Currently, she works as a postdoctoralresearcher at Instituto Superior Técnico, CATH-PRO/CQE, in the area of NOx reduction fromstationary sources by selective catalytic reduc-tion using VOCs and zeolite-based catalysts.

Inês Graça was born in Lisbon, Portugal, andobtained her master’s degree in ChemicalEngineering from Instituto Superior Técnico,University of Lisbon, in 2007. She received herPhD in Chemical Engineering with distinctionfrom both Instituto Superior Técnico, Univer-sity of Lisbon, and the University of Poitiers,France, in 2010. After the PhD, she worked asa postdoctoral researcher at Instituto SuperiorTécnico, University of Lisbon, from 2011 to2014. She received the Young ResearchersUTL/Delloite award from the Rector of theUniversity of Lisbon in 2011. In 2014, shejoined the Chemical Engineering Departmentof Imperial College London, where she is

currently a postdoctoral researcher. Her mainresearch interests focus on the design andapplication of zeolite-based catalysts andmixed oxide catalysts to the catalytic valor-isation of lignocellulosic biomass, carbondioxide transformation and selective oxidationof alkanes.

José M. Lopes received his PhD in ChemicalEngineering from Instituto Superior Técnico(IST), Universidade de Lisboa, Portugal (1993).His teaching activity started in 1986, at IST, inthe Chemical Engineering Department, wherehe works nowadays as Associate Professor, inthe field of reaction engineering and hetero-geneous catalysis. His research studies focuson hydrocarbon transformations promoted byzeolites, namely in the field of catalyticcracking and on the catalytic chemical trans-formation of CO2.

Carlos Henriques is Associated Professor (En-gineering Sciences) in the Chemical Engineer-ing Department of Instituto Superior Técnico(IST), University of Lisbon. He holds the chairsof Catalysis and Catalytic Processes (CPC) andHeterogeneous Catalysis. His research studiesare performed in the CATHPRO/Centro deQuímica Estrutural (CQE) group at IST. Hismain research interests are related to catalysisby zeolites, environmental catalysis (DeNOxand VOC elimination) and catalysis and en-ergy, based on CO2 conversion.

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required for the reduction of Ni2+ species ion-exchanged in theUSY zeolite, which could indeed explain the low performancesverified by Ocampo.[34] Moreover, higher metal loadings led tobetter catalytic performances, which the authors attributed tothe increased number of Ni0 active sites available for thedissociation and/or activation of H2 and CO2.

[35,36,50]

Bacariza et al. also prepared 15 wt.% Ni-based USY zeolitesand studied the effect of the zeolite composition in terms ofcompensating cation[51] and Si/Al ratio.[37] They verified that

larger monovalent cations slightly enhanced the reducibility ofNiO species in the calcined samples, while Ni0 average particlesizes in the reduced catalysts were not significantly affected bythe cation nature.[51] CH4 yields were promoted by larger cations(Cs+>Na+>Li+>H+) due to the increasing basicity of theframework oxygens and the positive effect on the CO2 affinity.NiO species reducibility was again favoured for larger divalentcations (Ba2+>Ca2+>Mg2+) but the verified differences interms of metallic dispersion in these samples hindered a properevaluation of the effects in the catalytic performances, beingMg2+ sample the most promising.[51] The effect of the cationnature above described[51] could indeed justify the low perform-ances verified by Aziz et al.[33] and Ocampo[34] since theseauthors chose acidic zeolites (HY) as supports.

Regarding the effect of the Si/Al ratio, Bacariza et al.[37] alsoverified that, whatever the compensating cation, the catalyticperformances were importantly favoured by increasing the Si/Alratio of USY zeolites (Figure 2). Indeed, higher ratios led tomore hydrophobic samples, whereas zeolites basicity wasconsiderably higher for lower Si/Al ratios.[37] As a result, samples’hydrophobicity was found as the key parameter explaining theperformance enhancements, since H2O plays an inhibitory rolein the reaction.

The same authors[38,39] prepared Ni samples supported overUSY, BEA, ZSM-5 and MOR zeolites with similar structurecompositions and the same compensating cations in order toassess the effect of the framework type on the methanationperformances. According to their findings, USY was the mostadvantageous structure due to its weaker interaction with H2O.In second position, the improvement of the dispersionpromoted by BEA explained the results obtained for this

Table 1. Most performant catalysts based on zeolites reported in the literature.

Catalyst composition Zeolite composition Metal(s) incorporation method Tred[b]

[°C]H2 :CO2 GHSV

[h� 1]Best catalytic performances achieved

Ref.CC[a] Si/Al T[°C]

CO2 conversion[%]

CH4 selectivity[%]

6%Rh-Y Na+ 2.4 ion exchange 450 3 :1 n.a. 150 6 100 [32]

5%Ni/Y H+ n.a. impregnation 500 4 :1 50000 n.a. 50 95 [33]

5%Ni/Y H+ n.a. ion exchange 400 4 :1 43000 450 10 20 [34]

14%Ni/USY Na+ 3 impregnation 470 4 :1 43000 400 65 94 [35,36]

15%Ni/USY Na+/Cs+ 38 impregnation 470 4 :1 43000 400 73 97 [37]

15%Ni/BEA Na+/Cs+ 38 impregnation 470 4 :1 43000 400 70 96 [38,39]

15%Ni/BEA Na+ 243 impregnation 470 4 :1 43000 400 71 97 [38,39]

15%Ni/MOR Na+/Cs+ 47 impregnation 470 4 :1 43000 400 66 95 [38,39]

15%Ni/ZSM-5 Na+/Cs+ 40 impregnation 470 4 :1 43000 400 65 95 [38,39]

5%Ni/d-S1[c] n.a. 1 impregnation 500 4 :1 60000 450 57 91 [40]

10%Ni/ZSM-5 n.a. n.a. impregnation 500 4 :1 2400 400 76 75 [41]

2%Ru/ZSM-5 H+ 15 impregnation 500 10 :1 n.a. 350 100 100 [42]

10%Ni/BEA Na+ 12 impregnation 500 4 :1 10000 350 33 88 [43]

5%Ni/5 A[d] Ca2+,Na+ n.a. impregnation 500 4.05 :1 92 300 85 100 [44]

5%Ni/13X[d] n.a. 1.5 impregnation 500 4.05 :1 92 300 85 100 [44]

6%Ni-5 A[d] Ca2+,Na+ n.a. ion exchange 650 8 :1 1000 300 100 100 [45]

Ni-Catalystcom + 4 A[d] n.a. n.a. mechanical mixture 250 4 :1 42 270 70 96 [46]

10%Mg/13%Ni/USY Na+ 3 impregnation 700 4 :1 43000 400 63 93 [47]

7%Ce/14%Ni/USY Na+ 3 impregnation 700 4 :1 43000 400 68 95 [35]

15%Ni-20%Ce/USY Cs+ 38 co-impregnation 470 4 :1 43000 305 78 99 [48]

0.5%Pt-2%Co–MOR Na+ 5 ion exchange 350 4 :1 1091 350 41 15 [49]

10%Ni/10%La/BEA Na+ 12 impregnation 500 4 :1 10000 350 65 99 [43]

[a] Compensating cation; [b] Pre-reduction temperature; [c] Desilicated Silicalite-1 material; [d] Catalysts applied in sorption enhanced methanationconditions.

Figure 1. Catalytic performances reported by Aziz et al.[33] for different typesof supported Ni catalysts: (A) Effect of H2 :CO2 mass ratio and GHSV at 300 °C.(B) Effect of the type of support. Reproduced with permission from Ref. [33]Copyright 2013 Elsevier.

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sample, slightly lower than those of USY catalyst. Finally, MORand ZSM-5 presented comparatively lower performances, whichwas mainly attributed to the lower metallic dispersion.[38,52]

Goodarzi et al.[40] prepared 5 wt.% Ni catalysts using silica-lite-1 (S1) and desilicated silicalite-1 (d-S1) materials as supports.The authors performed a post-synthesis desilication treatmentcreating intra-particle voids and mesopores in order to promotemetallic dispersion. Indeed, the encapsulation effect led to aremarkable effect in the Ni0 average size (from ~14 nm in the5%Ni/S1 catalyst to ~6 nm in the 5%Ni/d-S1 sample) and,

consequently, in the catalytic activity (CO2 conversion and CH4

selectivity from 42 and 40% to 57 and 91% in the 5%Ni/S1 and5%Ni/d-S1, respectively).

Guo et al.[41] compared the performances of Ni-supportedmicro- and mesoporous materials using ZSM-5 zeolite, SiO2,SBA-15, MCM-41 and Al2O3 as supports. All catalysts wereprepared by impregnation, containing 10 wt.% Ni. The authorsverified an effect of the support on the metallic Ni dispersion,the H2 adsorption capacity and the basicity, being the ZSM-5zeolite the most promising support (Figure 3), leading to thesmallest Ni0 particles and the higher basicity in terms of weakand medium strength sites (found as the most beneficial for themethanation reaction[53]).

Furthermore, Scirè et al.[42] studied the effect of the supporton the performances of Ru supported catalysts by performingFTIR studies. The authors impregnated 2 wt.%Ru in two silicamaterials and also over a H-ZSM-5 zeolite. They suggested thatCO2 methanation is a consecutive reaction that proceedsthrough the dissociation of CO2 to adsorbed CO, followed byCO hydrogenation. They reported better results for the zeolite-based catalyst and attributed this to the more favourablemetal-support interactions established between Ru and theZSM-5 material, with a consequent effect on the selectivity toCH4.

Finally, recent studies reported by Quindimil et al.,[43] inwhich the authors used Y and BEA zeolites with Si/Al ratios of~2.6 and ~12 respectively, confirmed the beneficial effect ofusing alkali exchanged zeolites rather than their acidic forms,owing to the enhancement of the CO2 activation and animprovement of the Ni dispersion, as already reported in theliterature.[51] The authors also compared the results obtained forthe two types of zeolites impregnated with ~10 wt.%Ni. Whencomparing Ni/H-zeolites, BEA led to the best results, since itallowed the presence of more reducible Ni particles, thusincreasing the available metallic surface area during thereaction. However, similar performances were reported for bothNi/Na-zeolites. Another possible effect, not analysed by theauthors, could arise from the difference in the Si/Al of thesamples chosen for this study. Indeed, according theliterature,[37] higher Si/Al ratios are responsible for a reduction inthe inhibitory role of water in the CO2 methanation reaction.

Figure 2. Effect of the Si/Al ratio (3, 15 and 38) in the performances of 15%Ni/USY zeolites reported by Bacariza et al.[37] Reproduced with permissionfrom Ref. [37] Copyright 2018 Elsevier.

Figure 3. Catalytic performances reported by Guo et al..[41] Reproduced with permission from Ref. [41] Copyright 2018 American Chemical Society.

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Consequently, the better results found for the BEA samplescould be also related to the higher Si/Al ratio of the parentzeolite used (~12, while the Y samples presented ratios of~2.6).

In terms of application of zeolite based materials to sorptionenhanced methanation, Delmelle et al.[44] prepared 5 wt.% Nibased 13X and 5A zeolites with an enhanced water uptakecapacity by impregnation. Both catalysts yielded comparableCO2 conversion and selectivity to CH4. In addition, Borgschulteet al.[45] (Figure 4) also prepared Ni-5A catalysts by ion-exchangewith Ni contents below 6 wt.% and observed that theconversion of CO2 into CH4 was obtained with a CH4 yield of~100%, using a H2 :CO2=8 :1, doubling the stoichiometric ratio.Furthermore, Walspurger et al.[46] reported the beneficial effectsof mixing a Ni-based commercial catalyst with an hydrophilic4A zeolite (commercial catalyst:zeolite=1 :5) on the catalyticperformances, when using an inlet flow composed by 2.5%CO2, 9.9% H2, 81.6% CH4 and 6.0% N2. In these three works,

[44–46]

the interesting results obtained were attributed to the ability ofthe used zeolites to trap water, shifting the reaction towardsCH4 formation. However, two remarks have to be taken intoaccount in both studies: (1) the authors did not performsystematic studies using zeolites with different hydrophilic/hydrophobic properties and (2) they performed the reaction ofCO2 methanation followed by steps of drying treatments underair and/or H2 flows, thus eliminating the water adsorbed in thezeolite. Consequently, their findings cannot be directly com-pared to the results found in the literature and reported underconventional CO2 methanation conditions.

Regarding bimetallic catalysts, Bacariza et al.[47] studied theeffect of Mg incorporation to Ni/USY zeolites and found outthat 1–3 wt.%Mg impregnation over a 5%Ni/USY zeolitepromotes Ni0 dispersion, CO2 adsorption and, consequently, CH4

yields. However, Mg loadings higher than 6 wt.% weredisadvantageous for CO2 methanation since NiO-MgO solidsolutions were formed, damaging zeolite structure and hinder-ing Ni species reduction. The authors also studied samples withhigher metals content, being verified a relevant enhancementof the performances by increasing the temperature used in thepre-reduction treatment.

Furthermore, Graça et al.[35] analysed the effect of Ceincorporation to Ni/USY. They verified that the incorporation ofincreasing amounts of Ce over the 5%Ni/USY zeolite favouredNi dispersion and CO2 activation over CeO2 species.[35,54]

Consequently, the performances towards CO2 methanationwere better for the promoted samples. Regarding a 15%Ni/USYsample promoted with 7 wt.%Ce, the interactions between NiOand CeO2 species responsible for the remarkable improvementof Ni species reducibility in the 5%Ni/USY based sample werenot as significant, probably due to the lower Ce contentrespectively to Ni. However, Ce induced again an enhancementof the CO2 conversion, especially at lower reaction temper-atures.

In addition, Bacariza et al.[48] optimized the impregnationstrategy for the synthesis of NiCe/USY zeolites (Figure 5).Authors found out that co-impregnation favours Ni and Cespecies interaction, enhancing the metallic dispersion and theCO2 adsorption/activation. Indeed, the optimized sample re-ported by the authors in this work[48] presented higher perform-ances than a commercial Ni-based alumina catalyst.[38]

Additionally, Boix et al.[49] prepared Pt� Co based mordenite(MOR) catalysts with Pt contents of 0.5, 1 and 5 wt.% and3 wt.% of Co. The bimetallic samples were found to containCoPt3 clusters and presented better activities than the obtainedfor Pt/MOR and Co/MOR samples, the best Co/Pt ratio being0.6. However, the highest selectivity was found for the Pt/MORsample. The promoting effect of Pt on the activity wassuggested to result from the enhancement of the H2 activationand the formation of PtCoxOy active species.

Finally, Quindimil et al.[43] identified La2O3 (5, 10 and15 wt.%) as a promoter for Ni-based Na-BEA zeolites. Indeed,they prepared samples containing 10 wt.%Ni and verified thatLa incorporation favoured the metallic dispersion (Ni particlesof ~20 and ~10 nm for 10%Ni/Na-BEA and 10%Ni/10%La/Na-BEA samples, respectively), the CO2 activation and also thereducibility of the Ni species. The best sample contained10 wt.%Ni and 10 wt.%La, since higher La contents wereresponsible for a reduction of the reactant’s accessibility to theactive sites.

Figure 4. Sorption enhanced methanation model developed by Borgschulte et al.[45] . Reproduced with permission from Ref.[45] Published by the PCCP OwnerSocieties.

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2.2. Mechanism Proposals

In terms of mechanistic studies, Scirè et al.[42] first suggestedthat CO2 methanation could be a consecutive reaction proceed-ing through the dissociation of CO2 to adsorbed CO and thenfollowing the same reaction pathway as CO hydrogenation,under Ru-based zeolite catalysts. In addition, Westermannet al.[50] proposed some pathways over Ni/Na-USY zeolitesprepared by impregnation (Figure 6). In their findings, authorssuggested that CO2 could be adsorbed onto compensatingcations (e.g. Na+) as linear complexes or in Ni0 particles andextra-framework Al species (EFAL), as monodentate carbonatespecies. Then, these species were proposed to react with Hatoms supplied by Ni0 active sites, giving rise to formatespecies. Up to 300 °C, formates were mainly dissociated tocarbonyls, since CH4 formation was found to be low. Above350 °C, carbonyl and formyl species were further hydrogenatedonto Ni0 particles to form probably formaldehydes, methoxyspecies and, finally, methane. The mechanism proposed byWestermann et al.[50] is indeed in accordance with somesuggested pathways also found in the literature,[55,56] whereboth formates and CO could act as intermediate species for CO2

methanation reaction, and contrary to those which consideredthat only CO (mainly over monometallic catalysts)[42,57–63] orformate species (mainly over bimetallic catalysts)[64–71] could bethe true intermediates in this reaction. Additionally, West-ermann et al.[54] also proposed that by adding Ce to Ni/zeolites(Ce/Ni/USY) the bifunctional mechanism suggested in theliterature without CO as intermediate species[64–71] is favoured.Actually, in this mechanism proposal, and also according to themain findings observed for other multifunctional catalysts,[64–71]

H2 molecules are activated and dissociated into H on Ni0 sites,while CO2 is mainly adsorbed on the promoted support formingmonodentate carbonates. These monodentate carbonates arethen sequentially hydrogenated producing hydrogen-carbo-nates, formates, formaldehydes, methoxy species and, finally,methane.

Figure 5. Performances reported by Bacariza et al.[48] for sequentially (Ce/Ni) and simultaneously (Ni� Ce) impregnated NiCe/Zeolites. Reproduced withpermission from Ref. [48] Copyright 2018 Wiley.

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3. CO2 Methanation over Zeolite-basedCatalysts under Plasma Conditions

Zeolites were also reported as active and selective catalysts forcarbon dioxide methanation under non thermal plasma-assistedconditions.[72–75] Indeed, Jwa et al.[73] proposed that plasma couldenhance the dissociation of the methanation reaction inter-mediates adsorbed over a Ni/BEA sample. Later, Azzolina-Juryand Thibault-Starzyk[74] proposed a mechanism for plasmaassisted methanation over a Ni/USY catalyst (Figure 7) andverified that methane could only be formed in systemconfigurations where the catalyst could directly interact withplasma discharges leading to the widely reported synergeticeffects.[72]

Finally, Bacariza et al.[75] studied the effect of Si/Al ratio andCe addition on Ni-based USY zeolites under non-thermal DBDplasma conditions. In this way, they found out that zeolites withlower Al content led to better results, which they attributed tothe lower interaction of water with the catalyst surface. Addi-tionally, they observed a remarkable effect derived from cerium

incorporation, being the performances again better than theones obtained for a commercial catalyst. These effects hadalready been reported under thermal assisted methanation bythe same authors[37,48] but they are even more remarkable underDBD plasma conditions.[75]

4. Relevant Properties of Zeolites for CO2

Methanation

Several parameters have been identified in the literature asfavourable for CO2 methanation reaction over metal basedcatalysts,[6,12,18] mainly the following: metallic phase highlydispersed, basic support and/or high affinity to CO2, additionalactive sites for CO2 activation (e.g. oxygen vacancies), hydro-phobicity/hydrophilicity character and resistance to water.Zeolites can be suitable and promising materials for CO2

methanation catalysts, since these parameters can be regulatedin order to achieve a proper selection of the zeolite structure,composition and preparation method, as it will be discussedbelow.

4.1 Metallic Phase Dispersion

The most active monometallic based zeolites reported in theliterature for CO2 methanation present typically metal particleswith average sizes of ~20 nm, mainly located on their externalsurface. In spite of the relatively large size and the location ofthe active metals on the outer surface of the zeolites, favourablemetal-support interactions were established in these materials,turning them into more active and selective catalysts thancommercial unsupported NiO.[35] Consequently, the improve-ment of the metallic dispersion is mandatory for furtherenhancing the performances of the zeolite-based catalysts, thusallowing a reduction of the required metal loadings.

Figure 6. CO2 methanation mechanism proposed for Westermann et al.under conventional thermal conditions over Ni/USY zeolite catalysts.Reproduced with permission from Ref. [50] Copyright 2015 Elsevier.

Figure 7. Carbon dioxide methanation mechanism proposed by Azzolina-Jury and Thibault-Starzyk under plasma conditions.[74] Reproduced withpermission from Ref. [74] Copyright 2017 Springer Nature.

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As previously referred, metals can be mainly incorporatedby ion-exchange or impregnation methods on zeolites. Themetallic dispersion is known to be affected by the metalloading, but also by the metal neighbourhood. Consequently,the preparation conditions, especially in terms of calcinationand pre-reduction temperatures, as well as the incorporation ofpromoters, could be key aspects for guaranteeing the presenceof dispersed metallic species over metal-based zeolites.

Calcination Temperature

Calcination is typically the last step in the preparation ofcatalysts, where the precursors are decomposed, and presentsan unquestionable relevance.[65,76–80] In the specific case ofzeolites, this treatment is crucial since parameters such as thetype of species, particle size or even the position of the metalspecies in the framework can be affected by the calcinationconditions.[30,81–83] Indeed, calcining at high temperatures couldinduce a displacement of the particles towards more internalcavities,[30,83,84] hindering its reducibility. Moreover, high calcina-tion temperatures promote agglomeration of particles,[81–83]

which can affect the zeolite textural properties[84] Consequently,by adjusting the calcination temperature the metallic dispersionof zeolite supported catalysts for CO2 methanation could beenhanced favouring the performances, as reported in theliterature.[83] A reduction of the calcination temperature from500 to 300 °C allowed a decrease in the Ni0 particle size of~7 nm (from ~19 to ~12 nm) over 5 wt.%Ni/USY samples.[83]

Pre-Reduction Temperature

Pre-reduction treatments are typically performed prior tocatalytic tests and its relevance cannot be ignored.[37,55,57,83] Thepre-reduction conditions (namely, the temperature) are knownto affect metal particle sizes[30,81] and, as expected, the fractionof species effectively reduced.[85–87] Thus, by optimizing thisparameter the metallic dispersion and metal-support interac-tions can also be enhanced. Indeed, it was reported that higherpre-reduction temperatures could partially increase themethane yields obtained for 5%Ni/USY zeolites,[83] despite theslight increase in the average Ni0 particle size from ~19 to~22 nm after reduction at 470 and 700 °C, respectively.However, in the case of 15%Ni/USY samples,[37,83] the pre-reduction temperature did not lead to relevant impacts on theactivity and selectivity, since two opposed effects were verified.On one hand, the degree of reduced Ni species increases withthis parameter and, on the other hand, the dispersion of themetallic Ni species decreases due to sintering (Ni0 average sizesincreasing 3–6 nm after reducing at 700 °C instead of 470 °C).However, when studying Mg-promoted Ni/zeolites, the effect ofthe pre-reduction temperature can be positive. Indeed, it wasreported[47] that increasing the pre-reduction treatment of abimetallic MgNi/USY sample from 470 to 700 °C led to animportant enhancement of the CH4 yield. This enhancementwas attributed to the anti-sintering properties of Mg (able to

prevent the agglomeration of Ni0 sites), as well as to thepossible reduction of the Ni� Mg mixed oxides present in theMgNi/USY sample, so that only the positive effect of increasingthe amount of reduced Ni species with the increased temper-ature was verified.[47]

Incorporation of Promoters

While CO2 methanation catalysts based on noble metals such asRh or Ru do not normally present sintering problems, Ni, Cu orCo based samples, among others, are more susceptible to metalparticles agglomeration and aggregation, leading to significanteffects on the dispersion. In this context, several works reportedthe beneficial effect of adding transition metals, noble metals,lanthanides or even alkali earth metals in partially suppressingNi particles sintering.[35,36,43,47,48,54,66,78,88–100] Among all, Ce hasbeen the most commonly used,[35,36,48,54,66,78,89–91,99] mainly due toits effects, not only on the dispersion of the metallic species(from ~19 to ~3 nm Ni0 particles in 5%Ni/USY and 5%Ni-3%Ce/USY catalysts, respectively), but also due to the favouredactivation of CO2 over Ce species, as it will be further discussedlater. The incorporation of La over 10 wt.%Ni/BEA was alsofound to lead to a decrease in the metallic Ni particles size from~20 in the Ni/BEA catalyst to ~12, ~9 and ~7 nm in thebimetallic samples containing 10 wt.%Ni and 5, 10 and15 wt.%La.[43]

Framework Type Effect

Additionally, the zeolite framework type could also play a rolein tuning the metallic dispersion, since the metal supportinteractions could be influenced by the spatial arrangementand bond angles present in zeolites. Indeed, it was reportedthat BEA zeolite, when comparing to USY, MOR and ZSM-5 withsimilar structure composition, leads to the most favourable Nidispersion[38,52] (average Ni0 sizes of ~18 nm in Ni/Na-BEA and~24 nm in Ni/Na-USY, Ni/Na-MOR and Ni/Na-ZSM-5).

4.2 Basicity and/or Affinity to CO2

Several works reported that the basicity of the catalysts,especially from weak and medium basic sites,[53] could beresponsible for an important improvement of theperformances.[33,43,47,51,53,58,98,101,102] Zeolites basicity arises from thecharge and number of framework oxygens in the zeoliteslattice, which depend on the compensating cations nature andSi/Al ratios.[103] Additionally, activity promoters can be alsoadded, giving additional sites for CO2 adsorption and activation.

Compensating Cations Effect

The influence of zeolite compensating cations nature in theinteraction with CO2 was widely reported in the

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literature.[51,104–113] In this way, larger cations, due to theirpolarizing properties, induce higher basicity on the frameworkoxygens.[51,104–113] In addition, carbon dioxide molecules inter-action with zeolites, which can involve both oxygens andcations, was clearly affected by the cation size and even itsspecific location in the structure.[104,113] Several authors reportedthat CO2 presents higher affinity towards zeolites exchangedwith cations such as K+ or Cs+ rather than Li+ or Na+,[104,105]

which implies that tuning this parameter could lead toeventually relevant effects on the methanation activity andselectivity. Indeed, it was reported for Ni-based Y and BEAzeolites[43,51] that larger cations are more suitable for thisreaction, since they enhance nickel reducibility and promoteCO2 activation through the formation of carbonate-like speciesinvolving basic framework oxygens and acting as reactionintermediates.

Si/Al Ratio Effect

The interaction of CO2 molecules with the zeolite samples isalso modified by the Al content, since catalysts with higher Si/Al ratios (↓Al) present less negative charges in the structure(↓basicity). As a result, the type/nature of interaction betweenthe zeolite adsorption sites and the reactant molecules will beaffected.[21,25,114–119] Zeolites with lower Si/Al ratios can adsorb agreater number of carbon dioxide molecules since, as alreadyreferred, CO2 interacts with basic framework oxygens and/orcompensating cations, whose number will increase directly withthe Al content in the structure.[117,118] Additionally, the strengthof the interactions established between zeolites and CO2,typically as carbonate-like species, could be also affected by theSi/Al ratio.[119] Indeed, systematic studies performed in theliterature for evaluating the influence of this parameter[37]

confirmed that the affinity and interactions of zeolite samplesto CO2 is favoured for lower Si/Al ratios, even though theobserved catalytic activity decreases.

Framework Type Effect

The type of zeolite framework used could also present aninfluence in the interaction with CO2, being reported in theliterature that MOR and ZSM-5 promote the interaction andaffinity to CO2 more than USY and BEA zeolites.[38,52]

Incorporation of Promoters

As previously mentioned, in the case of the metallic phasedispersion, the addition of promoters can prevent sinteringprocesses, especially for Ni catalysts.[35,36,43,47,48,54,66,78,88–100] Addi-tionally, these promoters can also be responsible for theenhancement of the sample’s basicity and/or the supplying ofadditional sites for CO2 molecules adsorption and activation.Among all, Mg and Ce have been commonly reported. In thecase of Mg, the positive effect in the affinity to CO2 comes not

only from the intrinsic basicity of MgO, but also from thepresence of defective surfaces which could play a role in CO2

activation.[47,67,68,88] In the case of Ce, its beneficial effect derivesfrom the presence of oxygen vacancies in cerium oxidespecies.[35,36,48,54,66,78,89–91,99] Consequently, O atoms from CO2 canoccupy the oxygen vacancies, so that the stability of themolecules can be affected, favouring its activation. Finally,recent studies pointed out the positive effect of La2O3

incorporation in the performances of Ni-based BEA zeolites dueto the favoured CO2 activation, metallic dispersion and Nispecies reducibility.[43]

4.3 Hydrophobicity/Hydrophilicity

Several authors reported the effect of water in CO2 methanationreaction.[34,37,44–46,120–123] Indeed, since carbon dioxide methana-tion is a reversible reaction (CO2+4 H2 !CH4+2 H2O), thepresence of the produced water in the reaction medium wouldincrease the reversal reaction rate.[34,120–122] Additionally, watermolecules could play another eventual relevant inhibitory rolein the reaction rate, as they occupy active sites that wouldparticipate in the CO2 transformation mechanism, as alreadyreported in a recent study where it was proved that H2Omolecules are preferentially adsorbed on CO2 adsorption sitesover NaX and BaX zeolites.[124] In the case of zeolites, thehydrophobicity/hydrophilicity properties can be tuned bychanging the Si/Al ratio.[125–127] Indeed, H2O molecules will bepreferably adsorbed in regions of the zeolite samples wherecharges separation occurs, as well as in CO2 adsorption sites,according to the literature.[124] Consequently, samples present-ing lower Si/Al ratios are known to interact more strongly andwith a greater amount of H2O molecules, since the presence ofa higher number of negatively charged oxygens in the frame-work and compensating cations will lead to more regions withcharge separation.[126] Regarding the published works dealingwith the use of hydrophilic/hydrophobic zeolite-based catalystsfor carbon dioxide methanation, Delmelle et al.,[44] Borgschulteet al.[45] and Walspurger et al.[46] found out that the use ofhydrophilic zeolites (4A, 5A, 13X) under cyclic sorptionenhanced methanation could displace the reaction equilibriumtowards products. Nevertheless, no comparison with othermore hydrophobic zeolites was provided in these studies,namely concerning the catalytic activity and deactivation. Onthe other hand, Bacariza et al.[37] reported, under conventionalmethanation conditions, that the use of hydrophobic USYzeolites (high Si/Al ratios) was in the origin of a remarkableimprovement of the methane yields due to the loweradsorption/interaction of water with the catalyst surface, with-out the need for cyclic water desorption steps, and keeping theactivity for, at least, 10 h. Thus, zeolites are considerablytuneable materials also regarding their affinity to water. Finally,the type of zeolite framework (USY, BEA, MOR, ZSM-5) was alsofound as relevant for the water interaction properties beingUSY the one presenting the lowest affinity with water and,consequently, the best results.[38,52]

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4.4 Resistance to the presence of water

In the literature, damages to the catalysts structure due to thewater produced during the reaction have been also reported.[120]

Indeed, zeolites can also suffer dealumination (removal of Alatoms from the framework) processes, which can partiallydestroy or damage their structure, when submitted to water athigh temperatures.[22,25,30] To avoid these problems, commercialUSY zeolites (commonly used in FCC industrial processes) andprepared by ultra-stabilization treatments consisting of a deal-umination by steaming at controlled conditions could be usedas supports for CO2 methanation catalysts due to theirimproved hydrothermal stability. Indeed, the use of USY zeoliteas support for CO2 methanation catalysts has beenreported,[35,37,50,51,54,83] without any structural damage observed inthe samples after the conventional and deactivation tests dueto the presence of water in the reaction.[35,37]

5. Roadmap for Zeolites Application in CO2

Methanation: A Safe Bet

In previous chapters, the state of the art regarding theapplication of zeolites application in the carbon dioxidemethanation was summarized and the most advantageousproperties to guarantee in a methanation catalyst identified. Inaddition, strategies on how to tune zeolite properties werereported, highlighting the works which have already proved thesuitability of these strategies. The encouraging results reportedfor zeolite-based catalysts (more active than commercial ones)both under thermal and plasma-assisted conditions confirm thesuitability of these materials for the methanation of CO2, despitethe large metal particles typically formed when using this typeof supports. Thus, some indications and/or recommendationsare proposed for researchers interested in using these materialsfor the reaction under study (Figure 8).

Furthermore, in terms of future perspectives in this researcharea and taking into account the results already reported in theliterature for this type of supports, enhancing metallic dis-persion (for example by tuning other preparation conditionsrather than calcination and pre-reduction temperatures oradding new promoters) seems to be one of the mostchallenging tasks for the future. Furthermore, studying theinfluence of other compounds typically present in CO2 effluents(H2O, SO2, O2) on the properties and performances of zeolitecatalysts is mandatory. In this way, supplementary investigationmust be carried out for restraining Ni0 species reoxidationprocesses induced by the presence of oxygen in the reactorfeed.

6. Conclusions

Catalytic systems for the CO2 methanation reaction could be ofextreme complexity due to the multiplicity of functionalitiesrequired (hydrogenating, acid-basic, redox). Therefore, several

combinations of metals and supports can be considered.However, not all of them will satisfy the catalytic intricacies ofthis reaction. This is the reason why it is of fundamentalimportance to understand the insightful simplicity of a singlecatalytic function to move forward to the successful complexityof a final catalyst formulation, with potential for industrialapplication.

Early formulations for CO2 methanation catalysts were basedon the materials developed for the methane production fromsyngas dating from more than 100 years ago. However, withthe current environmental pressures for renewable energystorage and CO2 abatement, the search for catalysts on-purposedesigned for the CO2 methanation has gained more attention.Therefore, new supports, apart from the traditional high surfacearea oxides, such as for instance Al2O3 and SiO2, have beentested.

Thus, this overview shows the suitability of zeolites assupports for the CO2 methanation from a fundamental point ofview. It has been demonstrated that zeolites can be promisingsupports, as long as their properties are adjusted to meet therequirements of the CO2 methanation. The most relevantproperty that zeolites need to fulfil for the CO2 methanation ishydrophobicity, as inhibition from water can be severe duringthe reaction. This can be achieved through an increase of theSi/Al ratio of the zeolite framework. However, this meansdecreasing the zeolite intrinsic basicity, and so interaction withthe CO2 molecules. This can be overcome through the use ofmore basic compensating cations, such as Cs+, and addition ofpromoters. Mg, Ce and La have been the most commonlyreported, being able not only to enhance the zeolites basicity,but also to provide additional sites for the CO2 moleculesadsorption and activation. However, many other promotorsmight possibly be used. Good dispersion of the metallichydrogenating phase is other important characteristic thatthese catalytic systems need to have. This can be primarilycontrolled by the proper selection of the preparation con-

Figure 8. Main properties to guarantee in a methanation catalyst andproposals for achieving them when using zeolite-based materials.

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ditions, but addition of promotors or certain compensatingcations can also be beneficial. However, further research mustbe carried out on finding more effective techniques forimproving metallic dispersion in this type of materials, as theuncontrolled growth/agglomeration of metal particles on theexternal surface of the zeolites seems to be one of the maindrawbacks for their application as methanation supports, dueto the high metal loadings required.

Although a lot of research has been carried out to under-stand the impact of the zeolite properties on their catalyticperformances for CO2 methanation, few studies have dealt withthe mechanistic and kinetic aspects of this reaction over zeolite-based catalysts. Therefore, more attention needs to be focusedon this direction, in a perspective of process development.Moreover, the use of zeolite based-catalysts for CO2 methana-tion does not seem to be only limited to the conventionalthermal catalysis, as zeolites are also able to activate the CO2

molecule under plasma conditions.Hence, this review has shown the potential of zeolites as

support for the CO2 methanation catalysts, so that a widewindow of future research possibilities is now opened to fullyexplore the benefits of the use of zeolite-based catalysts for CO2

valorisation into fuels.

Acknowledgements

M.C. Bacariza thanks to Fundação para a Ciência e Tecnologia forher PhD grant (SFRK/BD/52369/2013) and for the financial supportof the research group (UID/QUI/00100/2013). Authors thank alsoto CEOPS Project (CO2-loop for Energy storage and conversion toOrganic chemistry through advanced catalytic Processes Systems),which has received funds from the European Union’s SeventhFramework Programme for research, technological developmentand demonstration under grant agreement number [309984].

Conflict of Interest

The authors declare no conflict of interest.

Keywords: CO2 conversion · CO2 methanation · Zeolites ·Tuning properties · Structure-reactivity

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Manuscript received: February 7, 2019Revised manuscript received: March 21, 2019Version of record online: April 11, 2019

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