CO 2 hydrogenation performance

We initially prepared Na–Fe 3 O 4 nanocatalyst by a simple one-pot synthesis method and then applied it to CO 2 hydrogenation reaction. As shown in Fig. 1a, Na–Fe 3 O 4 catalyst exhibited 12% selectivity to CH 4 , 38% selectivity to C 5 –C 11 as well as a low CO selectivity (14%) at a CO 2 conversion of 34%. Notably, the hydrocarbon distribution followed a fairly linear trend for Na–Fe 3 O 4 , implying an ASF product distribution (Fig. 1c). In our quest for a compatible zeolite, a series of zeolites like HY, HBEA, HMOR, HZSM-23, HMCM-22 and HZSM-5, possessing the ability to catalyse olefin oligomerization reaction in varying degrees, were coupled with Na–Fe 3 O 4 catalyst for CO 2 hydrogenation. The description of zeolite channels and NH 3 -TPD results were listed in Supplementary Table 1 and Supplementary Fig. 1. As shown in Fig. 1a, CO 2 conversion and CO selectivity are not obviously related to zeolite type, predominantly decided by the first component of Na–Fe 3 O 4 , whereas the distribution of hydrocarbon products is evidently influenced by the zeolite pore structure on Na–Fe 3 O 4 /Zeolite catalysts for CO 2 hydrogenation. It is noteworthy that three types of zeolites with 10 member ring (MR) channels exhibit higher C 5 –C 11 selectivities in the order of HZSM-5 (3-dimensional)>HMCM-22 (2-dimensional)>HZSM-23 (1-dimensional). This result suggests zeolites with 10 MR channels can favour the oligomerization of olefins and the production of C 5 –C 11 hydrocarbons. Besides the pore structure, the acidity, which is depended on the SiO 2 /Al 2 O 3 ratio of zeolite, is another important factor affecting hydrocarbon distribution. It suggests that the stronger acidity of HZSM-5(27) could cause the over-cracking of heavy hydrocarbons to C 1 –C 4 hydrocarbons, whereas the weaker one of the HZSM-5(300) is not beneficial to the oligomerization/isomerization/aromatization of primary CO 2 -FT products, thus both disfavour the selective production of C 5 –C 11 hydrocarbons (Fig. 1a). In summary, HZSM-5(160) zeolite is suitable for C 5 –C 11 hydrocarbon synthesis due to the presence of medium/strong acid sites and 3-dimensional pore structure.

Figure 1: Catalytic performance for CO 2 hydrogenation. (a) CO 2 conversion and product selectivity over different Na–Fe 3 O 4 /Zeolite catalysts; reaction conditions: H 2 /CO 2 =3,320 °C, 3 MPa and 4,000 ml h−1g cat −1. (b) CO 2 conversion and product selectivity at different H 2 /CO 2 ratios over Na–Fe 3 O 4 /HZSM-5(160) catalyst at 320 °C, 3 MPa and 4,000 ml h−1g cat −1. (c,d) The detailed hydrocarbon product distribution obtained over Na–Fe 3 O 4 (c) and Na–Fe 3 O 4 /HZSM-5(160) (d) catalysts, an additional ASF plot and α value comparison of above two catalysts are also depicted; W n is the weight fraction of a product with n carbon atoms. Full size image

The Na–Fe 3 O 4 /HZSM-5(160) multifunctional catalyst provided a CO 2 conversion of 34% and selectivities to CH 4 , C 2 –C 4 , C 5 –C 11 and C 12+ hydrocarbons of 8, 18, 73 and 1%, respectively, under 320 °C, 3 MPa, and H 2 /CO 2 ratio of 3 (Fig. 1a). Moreover, when the H 2 /CO 2 ratio of feed gas was switched to 1, we observed an even higher selectivity to gasoline fraction (78%) and only 4% CH 4 with a CO 2 conversion of 22% over Na–Fe 3 O 4 /HZSM-5(160) catalyst (Fig. 1b). To our knowledge, this is the highest selectivity for gasoline-range hydrocarbons reported for CO 2 hydrogenation (Supplementary Table 2). A higher H 2 /CO 2 ratio benefits CO 2 conversion, which rose to 54% at H 2 /CO 2 =6, for instance, whereas it disfavours the selective formation of gasoline fraction. Selectivities varied in the range from 68 to 78% for C 5 –C 11 and 4 to 10% for CH 4 in the investigated H 2 /CO 2 ratio (1 to 6).

To further elucidate the function of HZSM-5, a detailed product distribution has been done on Na–Fe 3 O 4 /HZSM-5(160) catalyst (Fig. 1d). Compared with Na–Fe 3 O 4 catalyst (Fig. 1c), the use of HZSM-5 as the second component significantly decreased the selectivities to CH 4 and C 2 –C 4 , and altered the product distribution towards gasoline-range isoparaffins and aromatics. Moreover, oxygenates formation is inhibited at the presence of zeolite (Supplementary Table 3). An additional ASF plot and the probability of chain growth (α) value comparison of above two catalysts are also given in Fig. 1c,d. Relatively, Na–Fe 3 O 4 /HZSM-5 catalyst exhibited an α value of 0.70, higher than that of 0.59 for Na–Fe 3 O 4 catalyst, confirming that the production of long-chain hydrocarbons was promoted on the multifunctional catalyst. The product distribution on the multifunctional catalyst deviated greatly from the typical ASF distribution, which could be attributed to the secondary reactions, such as oligomerization, isomerization and aromatization, occurring on zeolite acid sites.

Further, a tunable isoparaffin/aromatic ratio in gasoline-range hydrocarbons is achieved by simply altering zeolite type (Supplementary Fig. 2). Under the same conditions, HZSM-5(27), HZSM-5(160) and HZSM-5(300) with MFI topology produced higher amount of aromatics (up to 61% of aromatics in gasoline fraction) while HMCM-22 with MWW topology produced mainly isoparaffins (46% of isoparaffins in gasoline fraction). This phenomenon has a close correlation with the topology of different zeolites. HMCM-22 zeolite with 10 MR pore openings has a unique lamellar structure consisting of two independent pore systems, which leads to HMCM-22 with potential catalytic properties in isomerization, alkylation and disproportionation25. In addition, the major aromatics produced over Na–Fe 3 O 4 /HZSM-5 catalyst, were identified to be toluene, xylene, ethyltoluene, trimethylbenzene and dimethyl ethylbenzene, while less benzene and durene formed (both <1% in gasoline) (Supplementary Table 4). Such aromatic product distribution is evidently different from that derived from MTG process. It will not need an extra separation process usually applied in MTG process due to the higher content of durene in gasoline.

Structural characterization

To reveal the nature of active sites that favors the formation of gasoline-range hydrocarbons, we resorted to multiple characterization techniques to investigate the structure of multifunctional catalyst. Na–Fe 3 O 4 catalyst was composed of nanosized Fe 3 O 4 with an average size of 13.1 nm, and the residual Na (0.7 wt%, determined by inductively coupled plasma (ICP)) was well distributed on the surface of Fe 3 O 4 nanoparticles, with no obvious segregation (Fig. 2a,b,e; Supplementary Figs 3 and 4e). HZSM-5(160) was highly crystalline and appeared to be cuboid crystals ranged from 200 to 500 nm (Supplementary Fig. 4). Characterization of high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and Mössbauer spectra showed that two different types of iron phase were discerned in the spent Na–Fe 3 O 4 catalyst, with 32.4% of Fe 3 O 4 and 67.6% of χ-Fe 5 C 2 phase (Fig. 2c–f; Supplementary Table 5). Metallic iron is formed when Na–Fe 3 O 4 is reduced in H 2 prior to reaction (Supplementary Fig. 4c). Upon exposure of the catalyst to the reaction atmosphere, Fe 5 C 2 and Fe 3 O 4 are formed as a result of the interaction of metallic iron with carbon and oxygen species from the dissociated carbon oxides26. Appropriate proportion and arrangement of Fe 3 O 4 (active sites for RWGS) and Fe 5 C 2 (active sites for FTS)26, we speculated, is responsible for low CO selectivity (lower than 20%) with relatively high CO 2 conversion during CO 2 hydrogenation.

Figure 2: Structural characterization of Na–Fe 3 O 4 catalyst. (a,c) TEM images of fresh (a) and spent (c) Na–Fe 3 O 4 catalyst. Scale bar, 100 nm. (b,d) HRTEM images of fresh (b) and spent (d) Na–Fe 3 O 4 catalyst. Scale bar, 10 nm. (e) XRD patterns of fresh and spent Na–Fe 3 O 4 catalyst. (f) Mössbauer spectra of spent Na–Fe 3 O 4 catalyst. Full size image

Reaction scheme for CO 2 hydrogenation

In the basis of the results above, we propose a reaction scheme of CO 2 hydrogenation to hydrocarbons over Na–Fe 3 O 4 /Zeolite multifunctional catalyst as illustrated in Fig. 3. This scheme indicates that the multifunctional catalyst, with three types of active sites, exhibits complementary and compatible properties. During CO 2 hydrogenation, CO 2 is initially reduced to CO by H 2 via RWGS on Fe 3 O 4 sites, followed by a subsequent hydrogenation of CO to α-olefins via FTS on Fe 5 C 2 sites. The olefin intermediates generated on the iron-based catalyst then diffuse to zeolite acid sites, on which they undergo acid-catalysed reactions (oligomerization, isomerization and aromatization), as a consequence, the gasoline-range isoparaffins and aromatics are selectively formed and finally diffuse out of zeolite pores. Besides, CO 2 conversion and product selectivity could be modulated by varying the mass ratio of Na–Fe 3 O 4 relative to zeolite (Supplementary Fig. 5), which provides further support to the above hypothesis that Na–Fe 3 O 4 /Zeolite catalyst is multifunctional and the reaction involves intermediate migration among different active sites.

Figure 3: Reaction scheme for CO 2 hydrogenation to gasoline-range hydrocarbons. The CO 2 hydrogenation reaction over Na–Fe 3 O 4 /Zeolite multifunctional catalyst takes place in three steps: (1) an initially reduced to CO intermediate via RWGS, (2) a subsequent hydrogenation of CO to α-olefins intermediate via FTS and (3) the formation of gasoline-range hydrocarbons via the acid-catalysed oligomerization, isomerization and aromatization reactions. Full size image

Proximity effect in multifunctional catalysts

The proximity of the two components in multifunctional catalysts has been reported to exert significant influence on catalytic activity (refs 27, 28, 29). In our case, we found that it is also vital for selective conversion of CO 2 to hydrocarbons (Fig. 4a). When Na–Fe 3 O 4 and HZSM-5 were integrated by powder mixing, the closest proximity between iron-based sites and zeolite acid sites turned out to be detrimental, exhibiting a very low CO 2 conversion (13%) and high undesired CH 4 selectivity up to 60%. The reason, we speculated, is that the zeolite acid sites poison the Na-induced alkali sites on the Fe 3 O 4 surface, leading to a decrease in the surface basicity and carburization degree of Fe 3 O 4 catalyst. Likewise, another 2%Na–10%Fe/HZSM-5 catalyst with a close intimacy we prepared by an incipient wetness impregnation method as a comparison also presented a poor performance on CO 2 hydrogenation (Supplementary Table 3). When Na–Fe 3 O 4 and HZSM-5 were combined by granule mixing, the distance between iron-based and zeolite acid sites was enlarged, and the olefin intermediates formed on iron-based sites diffused through wide pores to zeolite, where they immediately underwent oligomerization, isomerization and aromatization reactions, giving rise to the highest C 5 –C 11 selectivity (73%) at a CO 2 conversion of 34%. It demonstrated an appropriate distance between iron-based and acid sites is critical for achieving excellent performance. With regard to dual-bed configuration, where HZSM-5 was packed below Na–Fe 3 O 4 and separated by a thin layer of inert quartz sand, the distance between iron-based and acid sites got larger. It exhibited a slightly lower C 5 –C 11 selectivity (67%) and the same CO 2 conversion as the manner of granule mixing.

Figure 4: CO 2 hydrogenation performance over the multifunctional catalysts with different proximity. (a) CO 2 conversion and product selectivity over different combinations of Na–Fe 3 O 4 and HZSM-5 catalysts conducted at the same reaction conditions as Fig. 1a; HCs: hydrocarbons. (b) The composition of gasoline-range hydrocarbons on different Na–Fe 3 O 4 /HZSM-5(160) composite catalysts. (c) The stability of the Na–Fe 3 O 4 /HZSM-5 catalyst with dual-bed configuration under the same reaction conditions as Fig. 1a. The hydrocarbon selectivities are normalized with the exception of CO. Full size image