Recycling of CO2 by Hydrogenation of Carbonate Derivatives to Methanol: Tuning Copper–Oxide Promotion Effects in Supported Catalysts

Abstract The selective hydrogenation of organic carbonates to methanol is a relevant transformation to realize flexible processes for the recycling of waste CO2 with renewable H2 mediated by condensed carbon dioxide surrogates. Oxide‐supported copper nanoparticles are promising solid catalysts for this selective hydrogenation. However, essential for their optimization is to rationalize the prominent impact of the oxide support on performance. Herein, the role of Lewis acid centers, exposed on the oxide support at the periphery of the Cu nanoparticles, was systematically assessed. For the hydrogenation of propylene carbonate, as a model cyclic carbonate, the conversion rate, the apparent activation energy, and the selectivity to methanol correlate with the Lewis acidity of the coordinatively unsaturated cationic sites exposed on the oxide support. Lewis sites of markedly low and high electron‐withdrawing character promote unselective decarbonylation and decarboxylation reaction pathways, respectively. Supports exposing Lewis sites of intermediate acidity maximize the selectivity to methanol while inhibiting acid‐catalyzed secondary reactions of the propanediol product, and thus enable its recovery in cyclic processes of CO2 hydrogenation mediated by condensed carbonate derivatives. These findings help rationalize metal–support promotion effects that determine the performance of supported metal nanoparticles in this and other selective hydrogenation reactions of significance in the context of sustainable chemistry.


Introduction
In supported metal catalysts, the role of the oxide support often goes beyond that of ah igh-surface-areac arriert hat increasest he metal dispersion and mechanical stability. Oxide speciesa re knownt oe xert promotion effects that profoundly modify the catalytic performance and stabilityo ft he metal speciesw ith whicht hey are in contact. [1] Metal/oxide promotion effectsa re particularly significant for reactions that require the activation of polarized bonds (CÀO, NÀO, SÀO, etc.) on "oxophilic" oxide surfaces to operate in conjunction with classical metal-catalyzed elementary steps, fore xample, H 2 dissociation. [2] Identifying experimentally accessible physicochemical descriptors as am eans to reach aq uantitative, ideally predictive, description of such metal/oxide promotion effects is currently am ajor goal. Such insights are expected to set af irmer basis for ad eeper understanding of existingc atalysts ystems and am ore rational development of innovative catalysts and processes.
Catalysis by nanoparticles of coinage metals,t hat is, Au, Ag, and Cu, is particularly dominated by metal/oxide promotion effects. This is in part associated with the relativelyl ow oxophilicity and remarkablyl ow carbophilicity of these metals, which often require the concerted action of oxide species to activate reactants with polar bonds. For example, copper/oxide interfaces have been proposed to play ap ivotal role in the very rich, but also strongly support-dependent, reactivity of copper nanoparticles dispersed on oxide carriers. This includes reactions of particular significance for am ore sustainablec hemical industry,s uch as selective hydrogenation of (biogenic) oxygenated compounds, [3] alcohol synthesisf rom syngas [4] and CO 2 , [5] lignin depolymerization, [6] and (bio)esterhydrogenolysis. [7] The hydrogenation reactions of organic derivatives of CO 2 , that is, carbonates,c arbamates, and ureas to methanol are of significant interesti nt he context of chemical CO 2 recycling, and are archetypal catalytic reactions in which the activation of highly polarized bonds is central to the overall selectivity.I n certain scenarios, the hydrogenation of CO 2 to methanol with H 2 of renewableo rigin is considered af easible route to chemically recycle waste CO 2 streams into versatile platform chemicals and to store renewable energy in al iquid vector. [8] By surmounting some of the limitations envisagedf or direct CO 2 hydrogenation processes, the aforementioned organicd erivatives could playasubstantial role as high-density,a nd therefore cost-effectively transportable, intermediate CO 2 couriers and thus facilitatet he bridging of remote point sources of waste The selectiveh ydrogenation of organic carbonates to methanol is ar elevant transformation to realize flexible processes for the recycling of waste CO 2 with renewable H 2 mediated by condensed carbon dioxide surrogates. Oxide-supported copper nanoparticles are promising solid catalysts for this selective hydrogenation. However,e ssential for their optimization is to rationalize the prominenti mpacto ft he oxide support on performance. Herein, the role of Lewis acid centers,e xposed on the oxide support at the periphery of the Cu nanoparticles, was systematicallya ssessed.F or the hydrogenation of propylene carbonate, as am odel cyclic carbonate,t he conversion rate, the apparent activation energy,a nd the selectivity to methanol correlate with the Lewis acidity of the coordinatively unsaturated cationic sites exposed on the oxide support. Lewis sites of markedly low and high electron-withdrawing character promote unselective decarbonylation and decarboxylation reaction pathways, respectively.S upports exposing Lewis sites of intermediate acidity maximize the selectivity to methanol while inhibiting acid-catalyzed secondary reactions of the propanediolp roduct, and thus enable its recovery in cyclic processes of CO 2 hydrogenation mediatedb yc ondensed carbonate derivatives. These findings help rationalize metal-support promotion effects that determine the performance of supported metal nanoparticles in this and other selectiveh ydrogenation reactions of significance in the context of sustainable chemistry. CO 2 and renewable H 2 ,a sw ell as buffering the transientfluctuations that are inherent to both supplies (Scheme S1 in the Supporting Information). [9] These compounds can be obtained with high selectivity by reactiono fC O 2 with different organic compounds, such as alcohols, glycols, epoxides, and amines. [10] In as eparate catalytic step, organic CO 2 derivatives can be hydrogenatively converted in the presence of H 2 as coreactant. If the latter transformation is achieved with high selectivity,t he CO 2 -derived carbon may act as the precursor for methanoli na reactiont hat, unlike direct CO 2 hydrogenation, is not bound by thermodynamic limitations to the methanol yield, while the organic residues can be recovered andr ecycled for furtherC O 2 capture, and ac yclic overall process consisting of an et conversion of CO 2 and H 2 to methanol is enabled.
Molecular catalysts based on Ru and Ir have been designed to achieve high reaction rates and product selectivities for the selectiveh ydrogenation of organic CO 2 derivatives in solution. [11] However,s olid catalysts are highly preferredb ecause they enablee ffective product recovery and catalyst recycling. Catalystsb ased on supportedc opper nanoparticles have been proposed for the heterogeneously catalyzed hydrogenation of organic carbonates to methanol. [12] Stark differences in activity and selectivityh ave been reported as af unctiono ft he nature of the oxide support.H owever,i ns pite of the significance of these oxide-support effects for the rational development of advancedcatalysts, they remain poorly understood at present.
Herein, we address the promotion effects of peripheral Lewis oxide centers on the Cu-catalyzed hydrogenation of propylene carbonate,a samodel organic carbonate,t om ethanol. For as eries of model Cu catalysts, the surface Lewis acidic (electron-accepting) charactero ft he peripheral oxide support was systematicallym odified and quantitativelya ssessed in a broad study space.T his physicochemical feature is shown to be ar ather general and quantitative descriptor for both activity and selectivity,a nd thus as uitable design parameter for optimizedcopper-based catalysts.

Results and Discussion
Characterization of oxide support materials To synthesize as eries of oxide support materials showings imilar textural properties (specific surface area,p ore volume, and diameter) but different surfaceL ewis acidities in ab road study space, the surface of am esoporous g-Al 2 O 3 support was overlaid with different transition-metal and lanthanide oxides (MO x ; M = Sm, Y, Sc, Zr,T a) of markedlyd ifferenti ntrinsic Lewis acidity.I na ll cases, the surfacec overage of Ma toms (M at )w as set to approximately 4.5-5.0 M at nm À2 ,w hich has been previously identified to correspond to the monolayer content of different oxideso ng-Al 2 O 3 . [5c, 13] The resulting series of support oxidesi s hereafter denoted MO x @Al 2 O 3 ,i nw hich MO x is the overlay oxide.X RD did not detectc rystallites of the MO x species (Figure S1 in the Supporting Information). All materials, including the neat g-Al 2 O 3 substrate, showed type IV N 2 physisorption isotherms characteristic of mesoporousm aterials ( Figure S2 in the Supporting Information), with essentially identicalA l 2 O 3normalized specific surfacea reas (277 AE 21 m 2 g À1 )a nd mesopore volumes (0.70 AE 0.05 cm 3 g À1 ), as listed in Table 1. These resultsp rove the absence of any significant mesopore plugging following the depositiono fM O x ,w hich would have blocked part of the porosity of the alumina carriert oN 2 uptake. In addition, the MO x @Al 2 O 3 oxidess howeda na verage mesopore size of 9.2 AE 0.6 nm, that is, approximately 2nmn arrower than that of the pristine g-Al 2 O 3 carrier. This reduction in mesopore size agreesr easonably well with that expected on deposition of am onolayer of MO x on the inner wall of the Al 2 O 3 mesopores.I ti st herefore inferred that the MO x species are in all cases deposited as an amorphous, essentially 2D overlay on the Al 2 O 3 surface. Indeed,s pherical aberration (C s )corrected high-angle annular dark-field scanning-transmission electron microscopy (HAADF-STEM) could provide direct visual proof for the existence of a( few-)atom-thick oxide overlay envelopingt he g-Al 2 O 3 nanocrystals for selected materials, for example,T aO x @Al 2 O 3 and SmO x @Al 2 O 3 ,f or which the Z contrast Table 1. Chemical composition and texturala nd surfacee lectronicp roperties of MO x @Al 2 O 3 oxide supports and metal dispersion in the corresponding copper-based catalysts.

Material
Composition Textural properties [a] cus Lewis acidity [b]  of the Ma toms (M = Ta,S m) relative to the Al 2 O 3 matrix was the highest among the series of materials ( Figure 1). Notable uniformity in textural properties was achieved in the series of MO x @Al 2 O 3 support materials, which would otherwise not be possible should bulk oxides be employed. Moreover,t he incorporation of the different oxide overlays on the commonA l 2 O 3 substrate resulted in as eries of materials spanning ab road range of surface Lewis acidity. Lewis acidity in oxides stems from coordinatively unsaturated metal sites (cus) exposed on their outer surface. The relative Lewis acidityo f the cus on the surfaceo ft he oxide support materials was quantified by means of UV/Vis spectroscopyw ith 1,2dihydroxyanthraquinone (alizarin) as as urfacep robe molecule, as describede lsewhere. [14] The lowesti ntramolecular charge transfer (IMCT) from the catechol subunit to the polycyclic system of the probe molecule appearsa saband around 505 nm, in the visible range of the spectra (FigureS3i nt he Supporting Information). This energy is hereafter denoted the spectroscopic parameter h,a nd it is plotted in Figure 2v ersus the theoretical Lewis acidity of the corresponding bulk-type MO x oxide for the entire series of MO x @Al 2 O 3 materials. [14] The linear correlation observedd emonstrates that the ranking of Lewis acidities established experimentally for the cus exposed on the surface of the Al 2 O 3 -supported oxide overlays matches well with that expected for the correspondingb ulk-type oxides, and ab road range of acidity is covered.

Characterization of copper-based catalysts
Copper was incorporated in the series of MO x @Al 2 O 3 support materials by incipient wetness impregnation. The nominal sur-face copperc ontent was generally set to 1.5 Cu at nm À2 .O ns elected supports, catalysts with highers urface Cu contents of 3.0 and 4.5 Cu at nm À2 were also synthesized.T he thermal treatment following impregnationw as designed to ensuref ull dehydration of the copper nitrate precursor at mild temperature (343 K) in af low of N 2 ,p rior to its decomposition at higher temperatures. Previous studies have shown that this route leads to highly dispersed Cu II specieso nt he surface of the oxide support. [15] XRD of the as-calcined catalysts showedn o signs of CuO crystalsf or catalysts with aC uc ontent of 1.5 Cu at nm À2 (2.5-4 wt %), indicative of the very highd egree of copper dispersion achieved ( Figure S4 in the Supporting Information). Hydrogen temperature-programmed reduction (H 2 -TPR) experiments showedH 2 consumptions, corresponding to the reduction of Cu II species to their metallic state, peaking in the temperature range of 425-510 K. On the basis of this reducibility study,atreatment at 523 Ku nder flow of 20 %H 2 /N 2 was selected to activate the catalysts prior to catalysis (Figure S5 in the Supporting Information).
Following in situ catalystr eduction, X-ray photoemission spectroscopy (XPS) showed Cu 2p 3/2 binding energies (BEs) of 932.2 AE 0.4eVf or the entire series of materials (Table S1 in the Supporting Information), with Cu L 3 M 4,5 M 4,5 at ak inetic energy of approximately9 19 eV.J ointly,t heseo bservations point to full reduction of coppers peciest ot he metallic state, in line with what could be inferred from the H 2 -TPR profiles. The overlay MO x oxides, however, showedn oa ppreciable reduction followingt he H 2 activation step at 523 K( Ta ble S1 in the Supporting Information). Only in the case of Ta O x could ac ontribution from as ub-oxide speciesb ed etected. However,t his contribution was determined to account for less than 15 %ofthe tantalum specieso nt he catalyst surface. These results testify to the low reducibility of the oxides deposited as an overlay on the surfaceoft he g-Al 2 O 3 support.  Comparison between the surface acidity determined experimentally for the overlay MO x oxidess upported on g-Al 2 O 3 with the theoretical (Lewis)a cidity of the corresponding bulk oxides,a sd efined by Jeong et al., [14] thatis, represented by N M À2 d M ,inwhich N M is the formalo xidation state and d M the Sanderson partial chargeo ft he cations in the bulk oxides: Sm 2 O 3 ,Y 2 O 3 ,Sc 2 O 3 ,ZrO 2 ,and Ta 2 O 5 . ChemSusChem 2020ChemSusChem , 13,2043ChemSusChem -2052 www.chemsuschem.org 2020 The Authors. Publishedb yWiley-VCH Verlag GmbH &Co. KGaA, Weinheim C s -corrected HAADF-STEM was coupled with energy dispersive X-ray (EDX) spectroscopy to investigate the dispersion of Cu and MO x species on the surface of the reduced catalysts. Figure 3s hows representativeS TEM images and compositional EDX mappings, collected at both the mesoscopic and nanometric length scales, respectively,f or selected catalysts after reduction. Ar emarkably uniform mesospatial distribution of the MO x overlay oxidesc ould be ascertained in all cases,w ithout any appreciable zoning or agglomeration. Owing to the relatively high Z contrastc ontributed by the MO x @Al 2 O 3 support oxides, the presence of Cu nanoparticles (with sizes of % 6-15 nm) could only be ascertained in few catalysts, with the assistanceo fE DX compositionalm aps to identify copperenrichedn anoscale regions. For other materials, however, direct visualization of the Cu nanoparticles proved challenging owing to limited Z-contrast differences and smaller Cu nanoparticles izes (Figure 3c and Figure S6 in the Supporting Information).
Given the limitations associated with gas chemisorption methods to reliably determine Cu dispersionf or as eries of catalysts supported on aw ide range of oxides, [16] we resorted to quantitative analysis of the XP spectra of the in-situ-reduced catalysts as am eans to evaluateC ud ispersion and average particle size in the series of Cu/MO x @Al 2 O 3 catalysts. This bulk-sampling methodp rovides a quantitative assessment that is complementary to the local electron-microscopy analysis. As summarized in Table 1a nd Figure S7 in the Supporting Information, the average Cu nanoparticle sizes determined for the series of catalysts synthesized with a copper content of 1.5 Cu at nm À2 were in the range of 4-16 nm. At this constant Cu surfacec ontent, the average Cu nanoparticle size increased for catalysts supported on increasingly Lewis basic oxides, notably YO x @Al 2 O 3 and SmO x @Al 2 O 3 .T his trend may be associated with partial hydrolysis of the coppern itrate precursor during impregnation and drying, which is known to result in ag reater extento fm etal agglomeration [15c] andi se xpected to be facilitated on oxide supports of basic character by local pH increases of the mesoporeconfined impregnation solution during catalysts ynthesis. Catalysts with similar average Cu nanoparticle sizes could also be synthesized on the mostL ewis acidic Ta O x @Al 2 O 3 support by increasing the surfacespecific Cu loading ( Figure S7 in the Supporting Information).

Hydrogenationo fp ropylene carbonate
The catalytic performance of the set of Cu/ MO x @Al 2 O 3 model catalysts was evaluated in the hydrogenation of propylenec arbonate in the liquid phase. Methanol, propane(di)ols, CO, and CO 2 were the major reactionp roducts. Light hydrocarbons (C 1 -C 3 )w ere detected under certain reaction conditions, albeit only in small amounts( < 5%). Under identical reaction conditions, blank experiments with the Cu-free MO x @Al 2 O 3 oxide support materials showedi nsignificant propylenec arbonate conversion (< 5% after 24 h), that is, the presence of Cu is essential for catalysis.
Remarkably,a ss hown in Figure 4a,t he initial propylene carbonatec onversion rate showeds trong dependence on the Lewisacidity of the cus on the oxide support. The highest reaction rates (0.29-0.31 mol g Cu À1 L À1 min À1 )w ere registered for Cu nanoparticles deposited on oxide supports exposing surface cus of the lowest Lewisa cidity,t hat is, SmO x @Al 2 O 3 and YO x @Al 2 O 3 .W ith progressively increasing surfaceL ewis acidity of the support, the reactionr ate decreased progressively by a factor of approximately three over the entire study space.
To assess whether as econdary reactionp athway involving hydrogenation of CO 2 contributed to the observed methanol formation rates, as et of controle xperiments was conducted with CO 2 and H 2 as reactants. In theset ests, the CO 2 partial pressurei nt he gas feed was set to 10 bar,t hat is, simulating the case in which propylene carbonate had initially been fully decarboxylated under the reaction conditions. However,t he methanolf ormationr ates registered from CO 2 were approximately one to two orders of magnitude lower than those ob- served from propylene carbonate (see Table S2 and the accompanying discussion in the Supporting Information). These results confirmed the prevalence of ad irect carbonatehydrogenation pathway as the major methanol production route.
The differences observed in the overall carbonate conversion rates among the series of catalysts might arise from active sites of different intrinsic reactivity and/ord ifferences in the relative abundance of identicala ctive sites. However, an analysis of the initial carbonate conversion rates in the temperature range of 413-473K ( Figure S8 in the SupportingI nformation) revealed as tark dependence of the apparent activation energy E a forc arbonate conversion on the oxide cus Lewis acidity.A s shown in Figure 4b, E a increased rather linearly with the spec-troscopicp arameter h,f rom 28 kJ mol À1 for Cu/SmO x @Al 2 O 3 to 53 kJ mol À1 for Cu/TaO x @Al 2 O 3 .I ti sh ence inferred that the differencesi nr eactivity are indeedc onnected to reaction pathways with different overall energetic barriers operating on differentc atalysts, rather than simply to differences in the surface density of active sites.
To assess the influence of the support Lewis acidityo np roduct selectivity,t he reaction product pattern was examined as a functiono fc arbonate conversion for all catalysts at ar eaction temperature of 453 K. Relatively steady product selectivity patterns were obtained in all cases after ap ropylene carbonate conversion of 70 %h ad been reached( Figure S9 in the Supporting Information). Figure 5a depictst he evolutiono ft he methanols electivity,a tac onstant propylene carbonate con-  version of 80 %, as af unction of h. Av olcano dependence is observed, according to whicht he selectivity to methanol is maximized (> 60 %) for Cu nanoparticles deposited on oxides exposing cus of intermediate Lewis acidity, namely ZrO x @Al 2 O 3 , and decreases remarkably as the cus sites exposed on the surface of the support becomee ither more electron-donating or more electron-accepting.
The copper particles ize is another parameter that may potentiallya ffect the catalytic performance. Structure-sensitivity phenomena, that is, particle-size-dependent turnover frequencies, have been reported for hydrogenationr eactions with supported copper nanoparticles in the sub-10 nm size regime. [17] However,a nalysis of the catalytic performance for catalysts with different average Cu nanoparticle sizes in the range of 4.3-15.2nms upported on ac ommon oxide carrier, that is, Ta O x @Al 2 O 3 ,s howedv ery similara pparent activation energies (Figure 4a)a nd product selectivities at ac onstant carbonate conversion of 80 %( Figure 5). These results further confirm that the Lewis acidity of the oxide support is ad ominant factor for reactivity.I na ddition, these findings suggest that cus on the surface of the oxide species, at the periphery of the coppern anoparticles, are involved in ak inetically relevant reaction step.
Not only the overall selectivityt oc arbon oxides buta lso the composition of these gas-phase byproducts dependedm arkedly on the Lewis acidic character of the peripheral cus. As shown in Figure 5b the CO/CO 2 molar ratio in the products at ac arbonate conversion of 80 %d ecreased linearly with increasing h. Different side-reaction pathways might be responsible for the production of CO and CO 2 under the reaction conditions applied for propylenec arbonate hydrogenation. Figure 6a summarizes thesep ossible reactionp athways (ii)-(iv), together with the desired selective hydrogenation to methanol and propanediol [pathway (i)].C arbon monoxide formation might be the result of decarbonylation of the carbonate sub-strate [pathway (ii)].I ti st hereforei nferred from all these catalytic results that oxide supports exposing surface cus with increasingly stronger Lewis basic character promote fast propylene carbonate decarbonylation. This reactionp athway is by and large responsible for both the higher carbonate conversion rates and the highers electivity to CO observed as the spectroscopic parameter h decreases (Figures 4a nd 5, respectively).
Carbon dioxide may be formed by direct decarboxylation of propylene carbonate [pathway (iii)i nF igure 6a]o rb yc arbonate hydrolysis in the presence of H 2 O[ pathway (iv)].I nt urn, water may be formed in situ in the reaction medium as the product of secondary intermoleculard ehydration reactions (etherification with methanol and/or oligomerization) of the 1,2-propanediol primary product, as summarized schematically in Figure 6b.B oth routes undesirably contribute to depletion of the glycol product and thus disable its recycling as CO 2 carrier in an overall process of CO 2 utilization. These secondary reactions are known to be acid-catalyzed. Indeed, ad etailed analysiso ft he reactionp roducts arising from the propylene moiety in the carbonate reactant revealed that the extent to which secondary dehydration reactions take place is greater for catalysts supported on oxides exposing cus of markedly Lewis acidic nature. As shown in Figure7a, the lumped selectivity to dehydration products, including products of methanol cross-etherification with propane(di)ols and propylene glycol oligomers, increased on increasing h beyond2 .48 eV.
In view of these results, ah igher water concentration in the reactionm edium, and thusastronger driving force for propylene carbonate hydrolysis, is inferred for catalysts supportedo n oxides bearingstrongly Lewis acidic cus. However,the selectivity to 1-propanol, the product of the direct carbonate decarboxylation route [pathway (iii)i nF igure 6a], also increased remarkably on increasing h beyond2 .45 eV and dominated over dehydration side products for the most Lewis acidic catalyst, that is, Cu/TaO x @Al 2 O 3 .
To assess the effect of water on the conversion of propylene carbonate, separatee xperimentsw ere conducted on the most Lewis acidic catalystC u/TaO x @Al 2 O 3 in which different amounts of water were intentionally added at the beginning of the reaction. As showni nF igure 7b,t he addition of increasing amountso fe xogenous water resulted, as expected,i nasteep decreasei nm ethanols electivity,f rom approximately 30 to less than 5% for H 2 O/carbonate initial molar ratios ! 0.1, in favor of CO 2 as the product of carbonate hydrolysis. In parallel, as ignificant increase in the initial carbonate consumption rate by a factor of approximately four was also observed. These results evidencet hat reaction conditions favoringp ropylenec arbonate hydrolysis lead to notably higherc arbonate conversion rates alongside remarkably lower methanols electivityt han those registeredu nder standard reaction conditions, that is, without the addition of exogenous water to the reaction medium. It wast herefore deduced that carbonate hydrolysis pathways driven by the presence of endogenousw ater are not the major reactionp athway underlying the productiono fC O 2 under the conditions of propylene carbonate hydrogenation. Rather, our results suggestt hat the direct carbonate decarbox- (ii)direct decarbonylationpathway; (iii)direct decarboxylation pathway;and (iv) hydrolysis to CO 2 and 1,2-propanediol. b) Illustrative acid-catalyzed (i)esterification with methanol and (ii)oligomerization secondaryr eactions for the 1,2-propanediol primary product. ylation pathway dominates and is by and large responsible for the formation of CO 2 on Cu nanoparticles supported on the most Lewis acidic oxides. Facilitation of the carbonate decarboxylation pathway on oxides of pronouncedL ewis acidic character is in agreement with previousp redictions based on quantum-mechanicalcalculations. [18] The fact that the catalytic performance correlates with the Lewis acidity of the cus exposed on the oxide surface at the periphery of the Cu nanoparticles suggestss trongly the involvement of these Lewis centers in the activation of the carbonater eactant. Highly electron-withdrawing (acidic) Lewis centers promote cleavage of CÀOb onds between the carbonate functionalg roup and the propane backbone of the substrate molecule, and hence its unselective decarboxylation. In contrast, centers with weaker electron-withdrawing character (more Lewis basic) seem to facilitatea ctivation of the electrophilic carbonate functional group in the reactant molecule, which results in significantly reduced overall activatione nergies and notably higher conversion rates. However,t he en-hanced reactivity is in this case largely owing to the promotion of CÀOb ond cleavage in the carbonate group, leading to undesired decarbonylation of the substrate. Oxides bearing cus Lewis centers of intermediate acidity,s uch as those exposed on ZrO x ,a re essential to favor the selectiveh ydrogenation of the carbonate group in the reactant to form methanol.M oreover,m ild Lewis acidityo ft he oxide support effectively suppresses undesired secondary reactions of the glycol side product, such as dehydration and oligomerization, which are catalyzed by stronger Lewis acid sites. Such preservation of the propanediol backboneo ft he carbonate molecule enables its recycling as CO 2 carrieri na no verall cyclic process of CO 2 hydrogenation to methanol. These findings underscore the central role of the oxide support at the periphery of Cu nanoparticles in determining the catalytic performance. Moreover,t hey serve to clearlyi dentify the relative Lewis acidity (electronwithdrawing character) of coordinatively unsaturated cationic sites on the oxide surface as ac entral physicochemical parameter to rationalize and optimize such metal-oxide promotion effects, which play ap ivotalr ole in an umber of catalytic reactions of environmental significance, such as hydrogenation of CO 2 to alcohols [19] and CO [20] chemical vectors, or the selective conversion of biogenic oxygenates. [3b,d]

Conclusions
The deposition of monolayer-content overlays of varioustransition-metal and lanthanide oxideso nahigh-surface-area g-Al 2 O 3 carrierresulted in oxide support materials with essentially identicalp orosity,w hile exposing coordinatively unsaturated metal sites (cus)w ith aw ide range of surface Lewis acidity.T he dispersion of copper nanoparticles on these oxide supports enabled as ystematic assessment of metal-support promotion effects in the selectiveh ydrogenation of propylene carbonate to methanol. Under relevant reaction conditions, the overall (apparent) activation energy for the reaction scales with the relative electron-withdrawing character of the cus on the oxide at the periphery of the Cu nanoparticles, as quantified by UV/Vis spectroscopy with 1,2-dihydroxyanthraquinone as probe molecule, owing to the involvement of oxide Lewis centers in the activation of the carbonater eactant. Moreover,t he selectivity of the reaction also correlates with the Lewis acidity of the cus on the oxide support. Lewis sites of lower acidity,t hat is, lower electron-withdrawing power,p romotel ow-activation-energy, and hence faster albeit unselective, decarbonylation of the carbonater eactant. In contrast, Cu nanoparticles interfaced with oxides bearing strongly Lewis acidic sites favor ad ecarboxylation route. Oxide supports exposings ites of intermediate Lewis aciditya re optimal for hydrogenation of the carbonate functional group and maximize the selectivity to methanol. In addition, mild Lewis acidity of the oxide carrier is also essential to inhibit secondary,a cid-catalyzed reactions of the propanediol primary product, to enable its effective recovery and reuse, as required to realize ac yclic process of hydrogenation of waste CO 2 to methanol mediated by the condensed carbonate derivative.The insights achieved with this set of model catalysts are significant for the design of optimized copperc ata- Figure 7. Contribution of dehydration secondary reactions and water byproducts to the catalytic performance. a) Evolution of the selectivity to npropanol, 1,2-propanediol, and soluble products of 1,2-propanediol dehydration reactionsa safunction of the Lewis acidity of the cus on the oxide support at ap ropylene carbonate conversion of 80 %. b) Influenceoft he addition of increasing amounts of exogenous water on the initial reaction rate (*)a nd methanol selectivity (~)for the hydrogenation of propylene carbonate catalyzedbyC u/TaO x @g-Al 2 O 3 .Reaction conditions: P H2 = 40 bar,i nitial propylene carbonate concentration = 0.25 m in 1,4-dioxane, catalystconcentration = 0.15 mm Cu. Lines in b) were added as guide to the eye. lysts for the selective hydrogenation of CO 2 -derived carbonates to methanol. More broadly,t hey contributet owards au nifying and quantitative description of copper-oxidep romotion effects, which play ac rucial role in al arge number of selective hydrogenation reactions of significance for sustainable chemistry.

Experimental Section
Catalyst synthesis Ah igh-surface-area g-Al 2 O 3 carrier was synthesized from an anosized pseudo-boehmite precursor (Disperal-P2, Sasol Materials). A synthesis gel containing the alumina precursor,t he polyethylene glycol ether nonionic surfactant Tergitol 15-S-7 (Sigma-Aldrich) as porogen agent, and water,w ith am olar composition of Al/EO/H 2 O of 1:1.51:43 (EO represents the ethylene oxide building units in the polymer) was hydrothermally treated at 383 Ki na no ven for 48 h. Then, the gel was dried at 353 Kf or 48 h, at 393 Kf or 5h,a nd finally air-calcined in am uffle oven at 873 K( 3Kmin À1 )a nd sieved in the 100-200 mms ize range for further catalyst synthesis steps. As eries of oxide support materials was synthesized by deposition of monolayer amounts of different transition-metal and lanthanide oxides (MO x ;M = Sm, Y, Sc, Zr,T a) on the g-Al 2 O 3 support. Metal precursors were selected on the basis of previous studies [13a,b] to achieve strong interaction with the g-Al 2 O 3 surface. Sm(NO 3 ) 3 ·6H 2 O (99.9 %, Sigma-Aldrich), Y(NO 3 ) 3 ·6H 2 O( 99.8 %, Sigma-Aldrich), Sc(NO 3 ) 3 ·x H 2 O( 99.9 %, Sigma-Aldrich), Zr(OC 3 H 7 ) 4 (70 wt %i n1propanol, Sigma-Aldrich), and Ta (OC 2 H 5 ) 5 (99.98 %, Sigma-Aldrich) were used as received. Stock solutions were prepared by dissolving nitrate precursors in Milli-Q water,Z r(OC 3 H 7 ) 4 in dry 1-propanol, and Ta (OC 2 H 5 ) 5 in dry ethanol. The g-Al 2 O 3 support was dried at 523 Kf or 3hunder dynamic vacuum (3 mbar). Then, metal/lanthanide incorporation was achieved by incipient wetness impregnation. In all cases, the metal content was adjusted to achieve as urface-specific metal content corresponding to the experimentally determined monolayer,t hat is, 4.5-5.0 M at nm À2 . [13a] The as-impregnated solid was transferred into aq uartz packed-bed reactor,d ried at 343 Kf or 10 h, and calcined at 773 Kf or 4hunder aflow of synthetic air (heating rates of 3Kmin À1 ). The calcined solid was transferred to ag lovebox under exclusion of air.T he series of thus-synthesized support materials was denoted MO x @Al 2 O 3 ,i nw hich MO x stands for the overlay oxide. Copper was incorporated on the surface of the series of MO x @Al 2 O 3 support oxides by incipient wetness impregnation of Cu(NO 3 ) 3 ·3H 2 O( 99 %, Sigma-Aldrich) dissolved in 0.25 m HNO 3 (aq). The Cu concentration of the stock solution was adjusted to attain preset surface-specific Cu contents of 1.5, 3.0, or 4.5 Cu at nm À2 .T he as-impregnated solids were transferred to aq uartz packed-bed reactor,d ried at 343 Kf or 10 h, and calcined at 623 K( 3Kmin À1 )f or 4h under N 2 flow.T hen, the catalysts were transferred to aUshaped packed-bed glass reactor and activated by reduction at 523 K( 1Kmin À1 )i nf low of 10 vol %H 2 /N 2 .L astly,t he reactor was allowed to cool to RT,a nd the solid was recovered, transferred under exclusion of air to ag lovebox (O 2 < 0.1 ppm, H 2 O < 0.1 ppm), and ground into afine powder.

Characterizationm ethods
Nitrogen physisorption isotherms were recorded with aM icromeritics ASAP instrument (3Flex) unit after degassing the sample ( % 100 mg) at 423 Ku nder vacuum for 5h.S pecific surface areas were derived by using the BET method in the relative pressure (P/ P 0 )r egime of 0.05-0.30. To tal mesopore volumes were obtained from the amount of N 2 taken up at P/P 0 = 0.95. Pore size distributions and average mesopore diameters were determined by applying the BJH method to the desorption branch of the isotherms.
Powder XRD patterns were collected with aS TOE Theta/Theta diffractometer by using graphite-monochromatized CuK a radiation (l = 1.5406 )w ith as tep size of 0.028 and ad well time of 3sstep À1 .
UV/Vis spectroscopy with alizarin (1,2-dihydroxyanthraquinone) as surface probe molecule was applied to assess the Lewis acidity of the cus exposed on the surface of the oxide supports, as reported previously. [14] The solids were dried and soaked in as tock solution of the probe in dry ethanol (0.15 mm)w ith exclusion of air.A fter solid recovery by filtration, excess (unbound) alizarin was washed off with dry ethanol and the solid was dried at RT under vacuum (3 mbar). Diffuse-reflectance UV/Vis spectra were recorded with a PerkinElmer Lamda 365 spectrometer by using BaSO 4 as reflectance standard and converted to absorption by using the Kubelka-Munk formalism. The peak energy for the IMCT band E IMCT of the adsorbed probe was determined after subtraction of the spectra for the support material prior to alizarin uptake.
H 2 -TPR experiments were performed with aM icromeritics Autochem 2910 device. First, the sample was flushed with Ar at 393 Kf or 2h (10 Kmin À1 ). Materials containing SmO x and YO x overlays, of markedly basic character,w ere additionally calcined in situ at 773 K (10 Kmin À1 )f or 4h under af low of 5% O 2 /Ar to decompose surface carbonate species that might have developed through uptake of atmospheric CO 2 during specimen manipulation and experimental setup. After cooling to RT,t he temperature was ramped to 1073 K( 5Kmin À1 )u nder af low of 10 %H 2 /Ar.E volved water was removed from the outlet stream by ac old trap (195 K), and the H 2 consumption profiles were recorded with at hermal conductivity detector (TCD).
HAADF-STEM and EDX studies were performed with aC s -corrected dedicated STEM microscope (Hitachi HD-2700) equipped with a cold-field emission gun and two EDAX Octane TU ltra WE DX detectors and operated at 200 kV.P rior to observation, the reduced catalysts were embedded in al ow-viscosity resin (Spurr,S igma-Aldrich) in ag lovebox. The resin was then cured in an oven at 333 Ko vernight. Specimen cross sections with an ominal thickness of 50 nm were obtained with aD IATOME diamond knife mounted on aR eichert Ultracut ultramicrotome and collected on an ickel TEM grid (400 mesh) coated with aLacey carbon film (PLANO).
XP spectra were collected with ac ustomized spectrometer equipped with ah emispherical SPECS PHOIBOS 100 analyzer in fixed-transmission mode at 20 eV pass energy.S pectra were acquired with an onmonochromatic dual X-ray source (MgK or AlK radiation) with an anode current of 20 mA and ap otential acceleration of 12 kV.A s-calcined samples were pressed into small disks and evacuated in ap re-chamber at 423 Ka nd < 10 À7 mbar.T hen, catalyst reduction was performed in ah igh-temperature SPECS HPC-20 reaction cell with IR heating. In this case, the samples were treated in af low of 20 vol %H 2 /Ar,b yh eating from RT to 543 K (3 Kmin À1 )a nd holding the final temperature for 2hat 1bar.A fter cooling to RT,t he samples were evacuated at < 10 À7 mbar and transferred to the chamber of the spectrometer.G iven the low surface carbon content after the in situ reduction treatment, BEs were referred to the Al 2p signal from g-Al 2 O 3 at 74.10 eV.T od erive surface relative atomic ratios, peak intensities were determined after nonlinear Shirley-type background subtraction and corrected by sensitivity factors (Scofield). AverageC up article sizes were derived from the experimental Cu/Al surface ratios by using the Kerkhof-Moulijn model [21] for high-surface-area supported catalysts, modified to consider am onolayer of the corresponding MO x oxide on the surface of the Al 2 O 3 carrier.

Catalytic experiments
Catalytic experiments were performed in ap olytetrafluoroethylene (PTFE)-lined 36 mL autoclave reactor.T he pre-reduced catalyst powder (< 20 mm) was loaded into the reactor under exclusion of air in ag lovebox, in which the Ar overpressure was kept constant at 3mbar.I natypical experiment, anhydrous propylene carbonate (0.210 g, 2mmol, 99.7 %, Sigma-Aldrich), anhydrous 1,4-dioxane as solvent (8 mL, 99.8 %, Alfa-Aesar), and an amount of catalyst corresponding to 75 mmol of Cu were added into the PTFE liner of the autoclave. The reactor was then sealed, taken out of the glovebox, and dosed with H 2 (Air Liquide, 99.999 %) up to at otal pressure of 40 bar at RT.A ll catalytic tests were performed with vigorous magnetic stirring (1000 rpm). The temperature in the reactor was increased to the preset reaction temperature by using an aluminum heating jacket coupled to aheating plate (8 Kmin À1 ). After selected reaction times, the reaction was quenched by immersing the autoclave in an ice bath. The gas phase was then recovered in ag as bag and analyzed with an Agilent 7890B GC equipped with two sampling loops, the first of which was connected to aRestek RTX-1 capillary column (60 m) and af lame ionization detector (FID), and the second to two consecutive packed-bed columns (Agilent HS-Q 80/120, 1a nd 3m,r espectively), a1 3X molecular sieve column, and two TCD detectors for the analysis of permanent gases. Argon was used as internal standard for quantification of gaseous products. Liquid-phase samples were collected and analyzed with an Agilent 6890 GC equipped with aD B-WAXetr capillary column (15 m) and aT CD detector by using 1,3-propanediol and 2pentanol as standards. Initial reaction rates were determined by linear regression of analyses taken at reaction times in the range from 0t o6 0min. Product selectivities are reported on am olar basis at preset propylene carbonate conversions, and independently for products arising from either the OÀCÀO( methanol, dimethylether,a nd carbon oxides) or the propane-backbone (C 3 alcohols, oligomers thereof, and hydrocarbons) "synthons" in the propylene carbonate reactant.