CO2 Hydrogenation to Ethanol over Cu@Na-Beta

•Ethanol as the only organic product obtained from high-performance CO2 hydrogenation•Key step for C–C bond is achieved between CO2 and surface methyl at Cu step sites•Current ethanol synthesis from CO2 hydrogenation should be useful in industry•Assemblies of surroundings and reactive centers are important for novel catalysts The high-performance catalyst [email protected], prepared via a unique method to embed 2∼5 nm Cu nanoparticles in crystalline particles of Na-Beta zeolite, is reported for CO2 hydrogenation to ethanol as the only organic product in a traditional fixed-bed reactor. The ethanol yield in a single pass can reach ∼14% at 300°C, ∼12,000 mL·gcat−1·h−1, and 2.1 MPa, corresponding to space-time yield of ∼398 mg·gcat−1·h−1. The key step of the reaction is the rapid bonding of CO2∗ with surface methyl species at step sites of Cu nanoparticles to CH3COO∗, which converts to ethanol in the following hydrogenation steps. The points of the catalyst seem to be that the irregular copper nanoparticles stuck in zeolitic frameworks offer a high density of step sites and that the intimate surrounding of zeolitic frameworks strongly constrains the CO2 reactions at the copper surface and blocks byproducts such as methanol, formic acid, and acetyl acid. Here, we report a high-performance catalyst [email protected], prepared via a unique method to embed 2∼5 nm Cu nanoparticles in crystalline particles of Na-Beta zeolite, for CO2 hydrogenation to ethanol as the only organic product in a traditional fixed-bed reactor. The ethanol yield in a single pass can reach ∼14% at 300°C, ∼12,000 mL·gcat−1·h−1, and 2.1 MPa, corresponding to a space-time yield of ∼398 mg·gcat−1·h−1. The key step of the reaction is considered as the rapid bonding of CO2∗ with surface methyl species at step sites of Cu nanoparticles to CH3COO∗ that converts to ethanol in following hydrogenation steps. The points of the catalyst seemed to be that the irregular copper nanoparticles stuck in zeolitic frameworks offer high density of step sites and the intimate surrounding of zeolitic frameworks strongly constrain the CO2 reactions at the copper surface and block by-products, such as methanol, formic acid, and acetyl acid. Here, we report a high-performance catalyst [email protected], prepared via a unique method to embed 2∼5 nm Cu nanoparticles in crystalline particles of Na-Beta zeolite, for CO2 hydrogenation to ethanol as the only organic product in a traditional fixed-bed reactor. The ethanol yield in a single pass can reach ∼14% at 300°C, ∼12,000 mL·gcat−1·h−1, and 2.1 MPa, corresponding to a space-time yield of ∼398 mg·gcat−1·h−1. The key step of the reaction is considered as the rapid bonding of CO2∗ with surface methyl species at step sites of Cu nanoparticles to CH3COO∗ that converts to ethanol in following hydrogenation steps. The points of the catalyst seemed to be that the irregular copper nanoparticles stuck in zeolitic frameworks offer high density of step sites and the intimate surrounding of zeolitic frameworks strongly constrain the CO2 reactions at the copper surface and block by-products, such as methanol, formic acid, and acetyl acid. The utilization of carbon dioxide has a massive impact on carbon cycling for the development of the recycling economy1Enthaler S. von Langermann J. Schmidt T. Carbon dioxide and formic acid—the couple for environmental-friendly hydrogen storage?.Energy Environ. Sci. 2010; 3: 1207-1217Crossref Scopus (478) Google Scholar,2Ghoniem A.F. Needs, resources and climate change: clean and efficient conversion technologies.Prog. Energy Combust. Sci. 2011; 37: 15-51Crossref Scopus (193) Google Scholar and has attracted much research attention worldwide. Among the products from CO2 conversion, methanol as the main product has been investigated intensively in recent years.3Moret S. Dyson P.J. Laurenczy G. Direct synthesis of formic acid from carbon dioxide by hydrogenation in acidic media.Nat. Commun. 2014; 5: 4017Crossref PubMed Scopus (363) Google Scholar, 4Larmier K. Liao W.C. Tada S. Lam E. Verel R. Bansode A. Urakawa A. Comas-Vives A. Copéret C. CO2-to-methanol hydrogenation on zirconia-supported copper nanoparticles: reaction intermediates and the role of the metal-support interface.Angew. Chem. Int. Ed. 2017; 56: 2318-2323Crossref PubMed Scopus (241) Google Scholar, 5An B. Zhang J. Cheng K. Ji P. Wang C. Lin W. Confinement of ultrasmall Cu/ZnOx nanoparticles in metal–organic frameworks for selective methanol synthesis from catalytic hydrogenation of CO2.J. Am. Chem. Soc. 2017; 139: 3834-3840Crossref PubMed Scopus (258) Google Scholar Comparatively, few results have been reported about the synthesis of ethanol or C2+OH from CO2 hydrogenation, even though ethanol is not only a nontoxic but also a more valuable product that can be easily converted to high-value chemicals such as ethylene. Considering the synthetic routes of ethanol from feed gases of CO2+3H2 or CO+2H2, the relevant reactions are shown as Equations 1, 2, and 3. This means that the hydrogenation of CO2 to ethanol highly relates to the syngas conversion and the difference is just the water-gas shift reaction, a well-established industrial reaction.6Spivey J.J. Egbebi A. Heterogeneous catalytic synthesis of ethanol from biomass-derived syngas.Chem. Soc. Rev. 2007; 36: 1514-1528Crossref PubMed Scopus (508) Google Scholar As to the utilization of chemical energy included in the feed gases, one C2H5OH (g) reserves ∼94% or ∼89% chemical energy included in 6H2 or 2CO+4H2, respectively, reflecting to some extent the advantages of CO2 hydrogenation to ethanol over the syngas conversion.2CO2 + 6H2 → C2H5OH (g) + 3H2O (g) △H= −173.7 kJ/mol(Equation 1) 2CO + 4H2 → C2H5OH (g) + H2O (g) △H= −255.9 kJ/mol(Equation 2) CO + H2O (g) → H2 + CO2 △H= −41.1 kJ/mol(Equation 3) For the practical use of CO2 hydrogenation to ethanol, a significant promotion on catalytic efficiency and ethanol selectivity is highly desired.7Kusama H. Okabe K. Sayama K. Arakawa H. Ethanol synthesis by catalytic hydrogenation of CO2 over Rh-FeSiO2 catalysts.Energy. 1997; 22: 343-348Crossref Scopus (59) Google Scholar, 8Nieskens D.L.S. Ferrari D. Liu Y. Kolonko R. The conversion of carbon dioxide and hydrogen into methanol and higher alcohols.Catal. Commun. 2011; 14: 111-113Crossref Scopus (43) Google Scholar, 9Li S. Guo H. Luo C. Zhang H. Xiong L. Chen X. Ma L. Effect of iron promoter on structure and performance of K/Cu–Zn catalyst for higher alcohols synthesis from CO2 hydrogenation.Catal. Lett. 2013; 143: 345-355Crossref Scopus (53) Google Scholar, 10Prieto G. Beijer S. Smith M.L. He M. Au Y. Wang Z. Bruce D.A. de Jong K.P. Spivey J.J. de Jongh P.E. Design and synthesis of copper-cobalt catalysts for the selective conversion of synthesis gas to ethanol and higher alcohols.Angew. Chem. Int. Ed. 2014; 53: 6397-6401Crossref PubMed Scopus (173) Google Scholar, 11He Z. Qian Q. Ma J. Meng Q. Zhou H. Song J. Liu Z. Han B. Water-enhanced synthesis of higher alcohols from CO2 hydrogenation over a Pt/Co3O4 catalyst under milder conditions.Angew. Chem. Int. Ed. 2016; 55: 737-741Crossref PubMed Scopus (121) Google Scholar, 12Chen Y. Choi S. Thompson L.T. Low temperature CO2 hydrogenation to alcohols and hydrocarbons over Mo2C supported metal catalysts.J. Catal. 2016; 343: 147-156Crossref Scopus (63) Google Scholar, 13Bai S. Shao Q. Wang P. Dai Q. Wang X. Huang X. Highly active and selective hydrogenation of CO2 to ethanol by ordered Pd–Cu nanoparticles.J. Am. Chem. Soc. 2017; 139: 6827-6830Crossref PubMed Scopus (191) Google Scholar, 14Wang L. Wang L. Zhang J. Liu X. Wang H. Zhang W. Yang Q. Ma J. Dong X. Yoo S.J. et al.Selective hydrogenation of CO2 to ethanol over cobalt catalysts.Angew. Chem. Int. Ed. 2018; 57: 6104-6108Crossref PubMed Scopus (106) Google Scholar, 15Song Y. Chen W. Zhao C. Li S. Wei W. Sun Y. Metal-free nitrogen-doped mesoporous carbon for electroreduction of CO2 to ethanol.Angew. Chem. Int. Ed. 2017; 56: 10840-10844Crossref PubMed Scopus (174) Google Scholar At the present time, it is a great challenge to obtain high or ideally exclusive selectivity to ethanol by using traditional heterogeneous catalysts at valuably high CO2 conversion. New ideas for the catalyst design are called to achieve high ethanol selectivity as well as a high activity for CO2 conversion. Copper-based catalysts have been widely applied to catalyze the hydrogenation of COx and the production of organic oxygenates from syngas,16Liao F. Huang Y. Ge J. Zheng W. Tedsree K. Collier P. Hong X. Tsang S.C. Morphology-dependent interactions of ZnO with Cu nanoparticles at the materials’ interface in selective hydrogenation of CO2 to CH3OH.Angew. Chem. Int. Ed. 2011; 50: 2162-2165Crossref PubMed Scopus (281) Google Scholar, 17Su J. Zhang Z. Fu D. Liu D. Xu X.-C. Shi B. Wang X. Si R. Jiang Z. Xu J. Han Y.-F. Higher alcohols synthesis from syngas over CoCu/SiO2 catalysts: dynamic structure and the role of Cu.J. Catal. 2016; 336: 94-106Crossref Scopus (61) Google Scholar, 18Kattel S. Ramírez P.J. Chen J.G. Rodriguez J.A. Liu P. Active sites for CO2 hydrogenation to methanol on Cu/ZnO catalysts.Science. 2017; 355: 1296-1299Crossref PubMed Scopus (626) Google Scholar of which the activity and selectivity strongly depend on the support and/or promoters. Especially, Liao et al. have revealed the importance of morphology-dependent interactions of ZnO with Cu nanoparticles at the interface in between in selective hydrogenation of CO2 to CH3OH.16Liao F. Huang Y. Ge J. Zheng W. Tedsree K. Collier P. Hong X. Tsang S.C. Morphology-dependent interactions of ZnO with Cu nanoparticles at the materials’ interface in selective hydrogenation of CO2 to CH3OH.Angew. Chem. Int. Ed. 2011; 50: 2162-2165Crossref PubMed Scopus (281) Google Scholar On the other hand, zeolites with well-organized 3D porous structure have strong confining or modulating effects on entering molecules and the clusters enclosed, as reported in the recent years.19Gu J. Zhang Z. Hu P. Ding L. Xue N. Peng L. Guo X. Lin M. Ding W. Platinum nanoparticles encapsulated in MFI zeolite crystals by a two-step dry gel conversion method as a highly selective hydrogenation catalyst.ACS Catal. 2015; 5: 6893-6901Crossref Scopus (95) Google Scholar, 20Li L. Lv J. Shen Y. Guo X. Peng L. Xie Z. Ding W. Hexadecylphosphate-Functionalized iron oxide nanoparticles: mild oxidation of benzyl C–H bonds exclusive to carbonyls by molecular oxygen.ACS Catal. 2014; 4: 2746-2752Crossref Scopus (16) Google Scholar, 21Liu L. Díaz U. Arenal R. Agostini G. Concepción P. Corma A. Generation of subnanometric platinum with high stability during transformation of a 2D zeolite into 3D.Nat. Mater. 2017; 16: 132-138Crossref PubMed Scopus (302) Google Scholar, 22Zhang Z. Li Y. Gu J. Ding L. Xue N. Peng L. Guo X. Zhu Y. Ma J. Ding W. The effect of electrostatic field on the catalytic properties of platinum clusters confined in zeolite for hydrogenation.Catal. Sci. Technol. 2018; 8: 6384-6395Crossref Google Scholar Zeolite beta is a disordered intergrown hybrid of tetragonal and clinorhombic system, which are two distinct but closely related structures and possesses a three-dimensional pore system containing 12-membered ring apertures. The inner field potential of Beta zeolite has fascinating characteristics, different from the surrounding effects of the ZSM-5 and X zeolites, which we have reported earlier.19Gu J. Zhang Z. Hu P. Ding L. Xue N. Peng L. Guo X. Lin M. Ding W. Platinum nanoparticles encapsulated in MFI zeolite crystals by a two-step dry gel conversion method as a highly selective hydrogenation catalyst.ACS Catal. 2015; 5: 6893-6901Crossref Scopus (95) Google Scholar,22Zhang Z. Li Y. Gu J. Ding L. Xue N. Peng L. Guo X. Zhu Y. Ma J. Ding W. The effect of electrostatic field on the catalytic properties of platinum clusters confined in zeolite for hydrogenation.Catal. Sci. Technol. 2018; 8: 6384-6395Crossref Google Scholar In current work, we combined the Beta zeolite and the active Cu nanoparticles to prepare a highly efficient catalyst, in an intimately cooperated structure with copper enclosed in crystalline particles of zeolite Beta, which showed the superior performance for CO2 hydrogenation to ethanol. The CuO nanoparticles were embedded in crystal particles of Na-Beta zeolite (abbreviated as [email protected]) by a two-step preparation procedure using dry gel conversion and crystallization methods (Preparation of Catalysts), similar to what we previously described.19Gu J. Zhang Z. Hu P. Ding L. Xue N. Peng L. Guo X. Lin M. Ding W. Platinum nanoparticles encapsulated in MFI zeolite crystals by a two-step dry gel conversion method as a highly selective hydrogenation catalyst.ACS Catal. 2015; 5: 6893-6901Crossref Scopus (95) Google Scholar Then the sample was reduced by hydrogen to [email protected] before the catalytic reaction. The obtained material is considered as ∼2–5 nm Cu nanoparticles embedded in the zeolite crystal particles, as characterized by various techniques (see Experimental Procedures). The equilibrium conversions of CO2 with the mixture of CO2+3H2 as reactants at different temperatures and pressures were calculated according to thermodynamic parameters of the related compounds (Supplemental Information; Figure S1), implying the reaction conditions for ethanol synthesis from CO2 hydrogenation can be quite mild. Figures 1A–1D show the catalytic performance of the catalyst, ∼8.24 wt % [email protected] (the Cu content was measured by X-ray fluorescence [XRF] spectroscopy) for CO2 hydrogenation. With the reaction temperature increasing from 200°C to 350°C, the conversion of CO2 increases from 0.85% to 12.2% at a rather high space velocity 12,000 mL·gcat−1·h−1 and a pressure of mere 1.3 MPa. When the temperature reaches 250°C, CO appears as a by-product and the selectivity of CO increases to ∼ 45.2% as the temperature increases to 350°C (Figure 1A), while the ethanol is the only organic product. The pressure has a great effect on the performance of the catalyst. When the pressure changes from 0.5 to 2.1 MPa (300°C), the conversion of CO2 increases significantly from ∼2.0% to ∼18% and the selectivity of CO decreases from 94.6% to 21% (Figure 1B), and ethanol is still the only organic product observed in the tests of the reaction. When the space velocity increases from 6,000 to 18,000 mL·gcat−1·h−1, the CO2 conversion decreases from 13.5% to 3.7%, and the CO selectivity increases from ∼26.9% to ∼35.9% (Figure 1C). The ethanol yield of ∼14 % is obtained at 300°C, 2.1 Mpa, and 12,000 mL·gcat−1·h−1. More importantly, the catalytic performance of the catalyst [email protected] remains stable during the 100-h reaction on stream (Figure 1D), and ethanol is the only organic product formed under the reaction conditions (H2/CO2 = 3/1, 300°C, 1.3 MPa, 12,000 mL·gcat−1·h−1). The products of the first 24-h reaction during the prolonged test have been collected with a condenser cooled by liquid nitrogen. The weight of the products obtained is ∼0.71 g, in the composition of ∼43.5 wt % ethanol, coincidently according to the stoichiometry of the reaction 2CO2 + 6H2 = C2H5OH + 3H2O, as analyzed by gas chromatography (GC) using acetone as inner standard. Referring to the collection result, the isolated yield of ethanol is calculated as ∼5.1%, which is in accordance with the analysis results by online GC. The NMR spectrum of the liquid product collected is also listed in Figure S2 and only the signals from ethanol and water are observed. It is very interesting that the catalyst gives totally different results with syngas CO+2H2 used as the reactants, of which products similar to Fischer-Tropsch synthesis are obtained, even though some methanol and ethanol are also produced, as shown in Figure 1E. If the mixture of CO, CO2, and H2 was used as the feed gas, the ethanol became the only organic product with a ratio of CO2/(CO + CO2) in the feed gas larger than 0.65, indicating the CO2 is important for the formation of ethanol (Figure 1F). For insight into the catalytic mechanism, three control samples were prepared with the same copper content. Two of them were prepared by traditionally impregnated copper over Na-Beta zeolite (Cu/Na-Beta) and silica (Cu/SiO2). The third control sample was [email protected], which was prepared using the same method as [email protected] with Na-ZSM-5 zeolite. The typical catalytic performances of the catalysts for the title reaction are listed in Table 1. Under the same reaction conditions, the catalyst Cu/SiO2 shows poor activity for CO2 conversion, and no ethanol is detected in products and the results seem consistent with that reported by Wang et al.,23Wang Z. Xu Z.N. Peng S.Y. Zhang M.J. Lu G. Chen Q.S. Chen Y. Guo G.C. High-performance and long-lived Cu/SiO2 nanocatalyst for CO2 hydrogenation.ACS Catal. 2015; 5: 4255-4259Crossref Scopus (118) Google Scholar in which the Cu/SiO2 needed higher pressure to promote its activity. Comparatively, the Cu/Na-Beta is active for CO2 conversion and ethanol formation but much inferior to the [email protected], and the yield of ethanol over the former is only ∼16% of the latter. The catalyst [email protected] shows fairly good catalytic performance but the selectivity to ethanol is not exclusive and methanol and methane are detected. The results of the papers published on conversion of CO2 to ethanol are listed in Table 1. Among traditional catalysts, the results reported by this work are much more excellent for the selectivity of ethanol. Almost 100% selectivity to ethanol can be achieved by electroreduction over nitrogen-doped mesocarbon; however, the conversion of CO2 is low. These experimental results suggest that the zeolites and the cooperation manner between the zeolites and copper are important to obtain the excellent catalytic performance and the enclosure of copper nanoparticles in the crystal particles of Beta zeolite is the key factor. The intimate cooperation or synergism between them, however, appears different from previously reported remarkable results on syngas conversion using Ox-Zel catalysts, in which the bifunctional catalytic mechanism is established, i.e., CO hydrogenation on oxides to intermediates that are further converted to hydrocarbons on acidic SAPO-34 or H-ZSM-5 at distance, as reported by Bao et al. and Wang et al.24Jiao F. Li J. Pan X. Xiao J. Li H. Ma H. Wei M. Pan Y. Zhou Z. Li M. et al.Selective conversion of syngas to light olefins.Science. 2016; 351: 1065-1068Crossref PubMed Scopus (640) Google Scholar,25Cheng K. Gu B. Liu X. Kang J. Zhang Q. Wang Y. Direct and highly selective conversion of synthesis gas into lower olefins: design of a bifunctional catalyst combining methanol synthesis and carbon–carbon coupling.Angew. Chem. Int. Ed. 2016; 55: 4725-4728Crossref PubMed Scopus (301) Google Scholar For current [email protected] catalyst, the surrounding Na-Beta framework constrains the copper nanoparticles at special shapes and electronic status suitable for the CO2 hydrogenation to ethanol occurr at the nanoparticles, and the enveloping of zeolitic frameworks on the nanoparticle surface is also highly influential, which restricts side reactions for the production of methanol, formic acid, acetic acid, etc.Table 1Catalytic Performance of Some Related Catalysts for CO2 HydrogenationCatalystP (MPa)T (°C)Conv. (%)Sel. (%)bSpace-time yield of ethanol (mg·gcat−1·h−1). Data in parentheses are turnover frequencies (TOF in h−1).STYc0.1 g Cat; GHSV: 12,000 mL·gcat−1·h−1.RefCH4CH3OHC2H5OHCOCu/SiO2aCO is not included to calculate the organic product selectivity.1.33000.47N/D∼100N/D41.60 (0)This workCu/Na-BetaaCO is not included to calculate the organic product selectivity.3.15.630.164.366.446 (3.0)[email protected]aCO is not included to calculate the organic product selectivity.1.86.88.285.025.459 (3.9)[email protected]aCO is not included to calculate the organic product selectivity.7.9N/DN/D∼10030.5258 (17.1)Ru-Fe/SiO25.026026.734.729.41619.7−Kusama et al.7Kusama H. Okabe K. Sayama K. Arakawa H. Ethanol synthesis by catalytic hydrogenation of CO2 over Rh-FeSiO2 catalysts.Energy. 1997; 22: 343-348Crossref Scopus (59) Google ScholarCoMoS10.33403236.747.112.957.5−Nieskens et al.8Nieskens D.L.S. Ferrari D. Liu Y. Kolonko R. The conversion of carbon dioxide and hydrogen into methanol and higher alcohols.Catal. Commun. 2011; 14: 111-113Crossref Scopus (43) Google ScholarCuZnFe0.5K0.156.030042.356.44.732 (C2+)6.9148(C2+)Li et al.9Li S. Guo H. Luo C. Zhang H. Xiong L. Chen X. Ma L. Effect of iron promoter on structure and performance of K/Cu–Zn catalyst for higher alcohols synthesis from CO2 hydrogenation.Catal. Lett. 2013; 143: 345-355Crossref Scopus (53) Google ScholarK-CuCo/MoOxdCO hydrogenation to ethanol.4.0270−−−−−27 (C2+)Prieto et al.10Prieto G. Beijer S. Smith M.L. He M. Au Y. Wang Z. Bruce D.A. de Jong K.P. Spivey J.J. de Jongh P.E. Design and synthesis of copper-cobalt catalysts for the selective conversion of synthesis gas to ethanol and higher alcohols.Angew. Chem. Int. Ed. 2014; 53: 6397-6401Crossref PubMed Scopus (173) Google ScholarPt/Co3O48.0200−−12.557−0.29He et al.11He Z. Qian Q. Ma J. Meng Q. Zhou H. Song J. Liu Z. Han B. Water-enhanced synthesis of higher alcohols from CO2 hydrogenation over a Pt/Co3O4 catalyst under milder conditions.Angew. Chem. Int. Ed. 2016; 55: 737-741Crossref PubMed Scopus (121) Google ScholarCo/Mo2C4.0200∼109.546259.5−Chen et al.12Chen Y. Choi S. Thompson L.T. Low temperature CO2 hydrogenation to alcohols and hydrocarbons over Mo2C supported metal catalysts.J. Catal. 2016; 343: 147-156Crossref Scopus (63) Google ScholarPd2Cu/P253.2200−−−92−41.5Bai et al.13Bai S. Shao Q. Wang P. Dai Q. Wang X. Huang X. Highly active and selective hydrogenation of CO2 to ethanol by ordered Pd–Cu nanoparticles.J. Am. Chem. Soc. 2017; 139: 6827-6830Crossref PubMed Scopus (191) Google ScholarCoAlOx4.0140−−92.1−0.444Wang et al.14Wang L. Wang L. Zhang J. Liu X. Wang H. Zhang W. Yang Q. Ma J. Dong X. Yoo S.J. et al.Selective hydrogenation of CO2 to ethanol over cobalt catalysts.Angew. Chem. Int. Ed. 2018; 57: 6104-6108Crossref PubMed Scopus (106) Google ScholarMeso CarboneElectroreduction of CO2 to ethanol.−0.56 V−−100−−Song et al.15Song Y. Chen W. Zhao C. Li S. Wei W. Sun Y. Metal-free nitrogen-doped mesoporous carbon for electroreduction of CO2 to ethanol.Angew. Chem. Int. Ed. 2017; 56: 10840-10844Crossref PubMed Scopus (174) Google Scholara CO is not included to calculate the organic product selectivity.b Space-time yield of ethanol (mg·gcat−1·h−1). Data in parentheses are turnover frequencies (TOF in h−1).c 0.1 g Cat; GHSV: 12,000 mL·gcat−1·h−1.d CO hydrogenation to ethanol.e Electroreduction of CO2 to ethanol. Open table in a new tab Figure 2 shows the characterization results of the related samples. The alkali-treatment on raw Na-Beta zeolite led to the formation of mesopores in their crystal particles, and the sizes of the mesopores were measured by N2 sorption as ∼4 nm (Figure 2A and inset). CuO nanoparticles were introduced into the mesopores and then enclosed in the interior of the zeolite crystal particles after the two-step dry gel conversion (DGC) synthesis. After a reduction in 5% H2/N2 at 350°C, the [email protected] was reduced to [email protected] and the copper species remain as highly dispersed nanoparticles enclosed in the Beta zeolite crystal particles, as shown in Figure 2B. HRTEM of irregular copper nanoparticles in [email protected] is shown in Figure 2C. The irregular shape of Cu nanoparticles was embedded in Na-Beta zeolites. The high-resolution image of a copper particle in a gourd shape enclosed in the zeolite is shown in Figures 2D and S3A. By the difficultly obtained image, the copper nanoparticle enclosed in the zeolite is constructed as accurately as possible and shown in Figure S3B. It appears that step sites show up on the particle surface, due to the constraint of the irregular mesopores of the zeolite etched by alkali during the preparation. After a long-term reaction of 100 h, the copper nanoparticles retain their original sizes, as revealed by the TEM image of spent [email protected] (Figure 2E), in agreement with its stable catalytic performance, as shown in Figure 1D. The XRD (X-ray diffraction) patterns of related samples are shown in Figure S4, indicating the reduction of CuO to metallic copper during the treatment in 5% H2/N2 at 350°C. The crystallinities of the zeolite in [email protected], [email protected], and spent [email protected] appear to be similar to the original Beta zeolite. The stability of Na-Beta framework surrounding the copper nanoparticles ensures the stable activity of the catalyst. For the control catalyst Cu/Na-Beta, however, Cu nanoparticles significantly grow up after reaction (Figure S5) and, accordingly, the catalyst, though with poor activity, deactivates in a much shorter time on stream. It should be pointed out that the copper species loaded on or enclosed in the zeolite Beta are significantly different, as shown by the measurement of H2 temperature-programed reduction. The copper species enclosed are reduced at temperatures about 100°C higher than those loaded on the zeolite (Figure 2F). The results show that, in [email protected], Cu particles are tightly surrounded by a molecular sieve and interact with each other. Operando X-ray Afine structure (XAFS) measurements were performed to determine the status of copper species during the reaction under 250°C and 0.5 MPa pressure and the results are shown in Figures 2G and 2H. The XANES spectra in Figure 2G show that as soon as reaction gas is introduced, the intensity of the peak at 8,997 eV starts to decrease, indicating the reduction of CuO. Simultaneously, the peak at 8,981.3 eV due to the Cu species appears and increases with time on stream. By the edge fitting, CuO species are reduced to Cu after 40 min on stream, indicating that metallic copper is mainly the active species for the reaction. The radial distribution functions of the copper obtained by Fourier transform of the EXAFS are shown in Figure 2H and those of the control catalysts, i.e., Cu/Na-Beta and Cu/SiO2, are also listed. The coordination parameters of Cu in the three samples are given in Table 2. The Cu-Cu shell coordination number and distances in the three samples are given in Table 2, which shows the copper nanoparticles in [email protected] and Cu/Na-Beta are similar in size of around ∼3 nm26Bersani M. Gupta K. Mishra A.K. Lanza R. Taylor S.F.R. Islam H.-U. Hollingsworth N. Hardacre C. de Leeuw N.H. Darr J.A. Combined EXAFS, XRD, DRIFTS, and DFT study of Nano copper-based catalysts for CO2 hydrogenation.ACS Catal. 2016; 6: 5823-5833Crossref Scopus (29) Google Scholar,27Márquez F. Palomares A. EXFAS electron spectroscopy as a new tool of local characterisation of copper in Cu-Beta zeolite.Solid State Sci. 2001; 3: 637-640Crossref Scopus (1) Google Scholar in accordance with the TEM results and the copper nanoparticles in Cu/SiO2 are slightly larger in size. This means that the crystal structure of the copper nanoparticles in the three samples are in fact similar, which cannot explain their significant differences in the catalytic property for CO2 hydrogenation. We think the true cause lay in the different shapes of the copper nanoparticles in the samples and the intimate interaction between the copper and the support. The [email protected] has irregular copper nanoparticles and the most intimate interaction between copper and zeolite. The intimacy of copper and zeolite for Cu/Na-Beta is inferior to that of [email protected] and only the bottoms of the copper particles contact with the zeolite and it was prepared with traditional impregnation method and without encapsulation (Figure S5). A small amount of ethanol observed on the Cu/Na-Beta may form at the boundary of copper and Na-Beta zeolite. The results of X-ray photoelectron spectroscopy (XPS) and Cu LMM Auger spectra (Figure S6) recorded with the samples reduced in the instrument show that a small amount of electropositive copper species is detected in the [email protected] but not detected in other two samples, which implies copper particles, or the surface copper atoms in the particles, interact strongly with the framework oxygen of the Na-Beta surrounding, and, a relatively more intimate cooperation between Cu site and the zeolite of [email protected] can be deduced.Table 2Cu k-edge EXAFS Data for the Cu-Based CatalystsSampleScatteredR(Å)aBond distancesCN (Cu-Cu)bCoordination numberσ2(Å2)cDebye-Waller factor[email protected]Cu2.578.59 ± 0.860.0138Cu/Na-BetaCu2.547.74 ± 0.870.0136Cu/SiO2Cu2.5510.29 ± 0.630.0135All the parameters are obtained with the catalysts in reaction of CO2+3H2 in the XAS cell.a Bond distancesb Coordination numberc Debye-Waller factor Open table in a new tab All the parameters are obtained with the catalysts in reaction of CO2+3H2 in the XAS cell. As to the formation mechanism of CH3CH2OH from CO2 hydrogenation on [email protected], it was explored by a transient measurement between the switching of H2/12CO2 mixture (3/1) to H2/13CO2 mixture (3/1). Under steady-state reaction conditions (300°C and 0.5 MPa), the stream of H2/12CO2 mixture (3/1) was switched to H2/13CO2 mixture (3/1) and then switched back to the original H2/12CO2 mixture (3/1) gases. The products at the exit of the reactor were stored using a multiposition micro-electric valve actuators with 12 sample loops of 500 μl and analyzed afterward by a GC-MS (Figure S7), and the results are shown in Figure 3. The products of 12CH312CH2OH, 12CH313CH2OH, 13CH312CH2OH, and 13CH313CH2OH are detected by their characteristic m/z peaks listed in Table S1. The changes in the relative concentration of 13CO2 and 12CO2 respond to the transient switching ac