Regulation of product distribution in CO2 hydrogenation to light olefins

Conversion of CO2 into specific light olefins with high selectivity is a challenge. In this issue of Chem, Fan and co-workers propose an ethanol-intermediate route with the design of a new catalyst: Cr2O3(SG)/H-SAPO-34. The C2=–C4= selectivity can reach 95.7%, and ethene accounts for 74% of light olefins in CO2 hydrogenation. Conversion of CO2 into specific light olefins with high selectivity is a challenge. In this issue of Chem, Fan and co-workers propose an ethanol-intermediate route with the design of a new catalyst: Cr2O3(SG)/H-SAPO-34. The C2=–C4= selectivity can reach 95.7%, and ethene accounts for 74% of light olefins in CO2 hydrogenation. The massive emission of carbon dioxide (CO2) leads to a serious greenhouse effect, which is a major constraint on economic and social development. On the other hand, CO2 is also a clean, non-toxic, and abundant carbon-containing resource. Using CO2 as a raw material can not only reduce the amount of CO2 in the atmosphere and slow down the greenhouse effect but also open up a new way to prepare clean fuels. CO2 can be catalyzed into a variety of chemicals, among which light olefins are important building blocks in modern chemical industry.1Centi G. Quadrelli E.A. Perathoner S. Catalysis for CO2 conversion: A key technology for rapid introduction of renewable energy in the value chain of chemical industries.Energy Environ. Sci. 2013; 6: 1711-1731https://doi.org/10.1039/c3ee00056gCrossref Scopus (957) Google Scholar Nowadays, direct hydrogenation of CO2 into light olefins can be achieved by Fischer-Tropsch (FT) synthesis and methanol-intermediate routes. In FT synthesis, CO2 is generally transformed into CO via the reverse water-gas shift (RWGS) reaction and then into light olefins via the coupling of CHx intermediates.2Torres Galvis H.M. de Jong K.P. Catalysts for production of lower olefins from synthesis gas: A review.ACS Catal. 2013; 3: 2130-2149https://doi.org/10.1021/cs4003436Crossref Scopus (750) Google Scholar Because the distribution of hydrocarbon products follows the ASF rule, the light-olefin selectivity in FT synthesis is no higher than 61%, whereas that of methane reaches 15%–25%. The recently proposed methanol-intermediate route gave a different manner for light-olefin formation in CO/CO2 hydrogenation. Methanol intermediate is generated on metal oxides and rapidly diffuses onto the acid sites of zeolite to produce light olefins via dehydration and C–C coupling.3Jiao F. Li J.J. Pan X.L. Xiao J.P. Li H.B. Ma H. Wei M.M. Pan Y. Zhou Z.Y. Li M.R. et al.Selective conversion of syngas to light olefins.Science. 2016; 351: 1065-1068https://doi.org/10.1126/science.aaf1835Crossref PubMed Scopus (999) Google Scholar,4Cheng K. Gu B. Liu X.L. Kang J.C. Zhang Q.H. 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: 4725-4728https://doi.org/10.1002/anie.201601208Crossref PubMed Scopus (462) Google Scholar Compared with the FT synthesis route, the methanol-intermediate route can effectively break the limitation of the ASF rule and increase the selectivity of light olefins in hydrocarbons to 70%–87%.5Bao J. Yang G.H. Yoneyama Y. Tsubaki N. Significant advances in C1 catalysis: Highly efficient catalysts and catalytic reactions.ACS Catal. 2019; 9: 3026-3053https://doi.org/10.1021/acscatal.8b03924Crossref Scopus (205) Google Scholar Nevertheless, further elevation of light-olefin selectivity beyond 90% is still quite difficult. Moreover, the composition of light olefins is rather fixed at ∼25%–30% ethene, ∼35%–40% propene, and ∼10%–15% butene. Although some research has indicated that a change in zeolite structure and acidic properties can modulate the distribution of light olefins,6Wang M.H. Kang J.C. Xiong X.W. Zhang F.Y. Cheng K. Zhang Q.H. Wang Y. Effect of zeolite topology on the hydrocarbon distribution over bifunctional ZnAlO/SAPO catalysts in syngas conversion.Catal. Today. 2021; 371: 85-92https://doi.org/10.1016/j.cattod.2020.07.076Crossref Scopus (28) Google Scholar,7Wang S. Zhang L. Zhang W.Y. Wang P.F. Qin Z.F. Yan W.J. Dong M. Li J.F. Wang J.G. He L. et al.Selective conversion of CO2 into propene and butene.Chem. 2020; 6: 3344-3363https://doi.org/10.1016/j.chempr.2020.09.025Abstract Full Text Full Text PDF Scopus (48) Google Scholar the highly selective generation of a single specific olefin remains a challenge. In this issue of Chem, Fan and co-workers propose the new concept of an ethanol-intermediate route for direct conversion of CO2 into light olefins abundant in ethene.8Wang S. Zhang L. Wang P. Liu X. Chen Y. Qin Z. Dong M. Wang J. He L. Olsbye U. Fan W. Highly effective conversion of CO2 into light olefins abundant in ethene..Chem. 2022; 8: 1376-1394https://doi.org/10.1016/j.chempr.2022.01.004Abstract Full Text Full Text PDF Scopus (20) Google Scholar They first fabricated a composite catalyst consisting of Cr2O3 prepared by the sol-gel method and H-SAPO-34 zeolite prepared by the hydrothermal synthesis method. In situ X-ray diffraction, in situ X-ray photoelectron spectroscopy, transmission electron microscopy (TEM), high-resolution TEM, and selected-area electron diffraction results indicated that Cr2O3(SG) oxide has a hexagonal structure with uniform dispersion of Cr and O atoms. Pre-treatment with hydrogen (H2) resulted in the reduction of Cr6+ to Cr3+ and the formation of more surface oxygen vacancies benefitting the adsorption and activation of CO2. H-SAPO-34 showed a cubic morphology with high crystallinity. Powder mixing of Cr2O3(SG) oxide and H-SAPO-34 zeolite did not significantly affect their crystal structure. In CO2 hydrogenation, the Cr2O3(SG)/H-SAPO-34 bifunctional catalyst gave a C2=–C4= selectivity in hydrocarbons and an olefin-to-paraffin (O/P) ratio as high as 95.8% and 41.5, respectively, at 370°C and 0.5 MPa (Figure 1A). Of note, the ethene selectivity accounted for ∼70% of the light olefins with an ethene-to-propene (E/P) ratio higher than 2.6. These results are superior to those on ZnZrOx/H-SAPO-34, ZnAl2O4/H-SAPO-34, ZnGa2O4/HSAPO-34, InZrOx/H-SAPO-34, and ZnCrOx/H-SAPO-34 in CO2 hydrogenation under the same conditions (Figure 1B). The authors further investigated the influence of reaction temperature, pressure, space velocity, the synthesis method of metal oxide, and the integration manner between metal oxide and zeolite on the catalytic activity and product distribution. Cr2O3(SG)/H-SAPO-34 also exhibited good catalytic stability and regenerability. After 600 h of reaction, the selectivity of ethene and C2=–C4= in hydrocarbons could be maintained at 62.5% and 91.0%, respectively. The used catalyst was subsequently regenerated by H2 or 2% O2/Ar at 500°C–550°C. After ten recycles (above 1,000 h), it still gave ethene and C2=–C4= selectivities of 61% and 92%, respectively, with an E/P ratio of 2.3. The catalytic performance of pure Cr2O3(SG) in CO2 hydrogenation was then evaluated, and some ethanol (in addition to methanol) was detected at the initial reaction stage. This implies that the high ethene selectivity of Cr2O3(SG)/H-SAPO-34 in CO2 hydrogenation could originate from the rapid conversion of generated ethanol and methanol intermediates on Cr2O3 to ethene and light olefins on the acid sites of H-SAPO-34. It was verified that feeding a methanol-ethanol mixture on H-SAPO-34 can significantly improve the ethene selectivity, whereas for the typical methanol conversion process, the ethene selectivity is no higher than 40%, as previously reported.9Hereijgers B.P.C. Bleken F. Nilsen M.H. Svelle S. Lillerud K.P. Bjørgen M. Weckhuysen B.M. Olsbye U. Product shape selectivity dominates the methanol-to-olefins (MTO) reaction over HSAPO-34 catalysts.J. Catal. 2009; 264: 77-87https://doi.org/10.1016/j.jcat.2009.03.009Crossref Scopus (346) Google Scholar The authors systemically investigated the reaction mechanism for CO2 activation and the formation and evolution of various intermediates (including formate, methoxy, acetate and ethoxy species) by combining in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), isotope-labeled in situ DRIFTS, gas chromatography-mass spectrometry (GC-MS), and theoretical calculation. In CO2 hydrogenation on Cr2O3(SG), the characteristic peaks of formate at 1,592, 1,562, 1,354, and 1,307 cm−1 and of methoxy at 1,025–1,090 cm−1 were clearly identified, and their intensities gradually increased with the reaction time. In addition, the bands ascribed to acetate (1,541 and 1,620 cm−1), acetaldehyde (1,575 and 1,715 cm−1), and ethoxy (1,012 cm−1) were visible at the initial stage. In the effluents, some ethanol was detectable even though their amounts were much lower than that of methanol (Figures 1C and 1D). As a comparison, only the signal peaks of formate and methoxy species were observed in the in situ DRIFTS of ZnZrOx in CO2 hydrogenation, and no ethanol could be detected in the effluents. Therefore, upon coupling with H-SAPO-34, ZnZrOx/H-SAPO-34 exhibited an ethene selectivity of only 38.6%, such that the light-olefin distribution was similar to the typical methanol-to-olefin process. The reaction kinetics of various elemental steps for methanol and ethanol formation were investigated by density functional theory (DFT) calculation and molecular dynamics (MD) simulation. The CO2 hydrogenation to HCOO∗, H3CO∗, and CH3OH∗ needs to overcome free-energy barriers of 0.62–1.45 eV (Figure 1E). The reaction of H3CO∗ with CO2 and H∗ to form CH3COO∗ and OH∗ requires a higher free-energy barrier of 1.74 eV even though hydrogenation of CH3COO∗ to CH3CH2OH∗ requires lower free-energy barriers of 1.25–1.44 eV (Figure 1F). This result indicates that methanol formation is easier than ethanol on Cr2O3. It is worth noting that introducing CO into the feedstocks leads to the formation of more methane and C20–C40 at the expense of ethene and light olefins. This is because CO can be directly dissociated and hydrogenated to form alkanes on Cr2O3. Indeed, DFT calculation suggests that the free-energy barrier of the rate-determining step for methane formation via CO dissociation and hydrogenation is comparable to that of methanol and ethanol formation. Therefore, the intensity of peaks characteristic of formate, acetate, methoxy, and ethoxy in the in situ DRIFTS is much weaker in CO hydrogenation than in CO2 hydrogenation. In summary, effectively regulating the light-olefin distribution and selectively promoting the formation of a single specific olefin are highly desirable. In this issue of Chem, Fan and co-workers propose a new strategy to improve ethene selectivity by forming an ethanol intermediate on Cr2O3(SG) oxides and then rapidly converting the generated ethanol into ethene on H-SAPO-34 (Figure 1G). The prepared Cr2O3(SG)/H-SAPO-34 composite catalyst shows high ethene and light-olefin selectivity in CO2 hydrogenation, and no significant deactivation was observed even after 600 h of reaction. The results of in situ spectroscopy, DFT calculation, and MD simulation indicate that ethanol is generated from the hydrogenation of CH3COO∗ intermediate, which is formed through the reaction of H3CO∗ with CO2 and H∗. Financial support from the National Natural Science Foundation of China (22002157, 21991092, and 21991090) is acknowledged. The authors declare no competing interests. Highly effective conversion of CO2 into light olefins abundant in etheneWang et al.ChemFebruary 4, 2022In BriefEfficient conversion of CO2 into light olefins is a potential route for realizing carbon neutrality. The challenge is to prepare a highly selective catalyst. This work finds that Cr2O3(SG)/H-SAPO-34 shows a light olefin selectivity of 95.7% but a CO selectivity of 36% at a CO2 conversion of 13%. Particularly, it provides a strategy for controlling light olefin distribution by regulating alcoholic intermediates. The formation of ethanol through the reaction of H3CO∗ with CO2 and H∗ on Cr2O3(SG) makes ethene account for 74.2% of light olefins. Full-Text PDF Open Archive