CO2 hydrogenation to long-chain hydrocarbons can be catalyzed through modified Fischer-Tropsch synthesis (FTS) route or methanol-mediated route to promote the hydrocarbon chain growth.5 Very recently, our team26and Gao et al.27 have both reported the direct conversion of CO2 into gasoline-range (C5–C11) hydrocarbons with high selectivity and a very low CH4 production over composite catalysts. However, the yields to isoparaffins are not high due to poor matching among reverse water-gas shift (RWGS), C-C coupling and isomerization. To reduce the emission of pollutants in the automobile exhausted gas, the olefin and aromatic contents in gasoline fraction were strictly restricted by law. By contrast, environmentally benign and high-octane isoparaffins are ideal gasoline components.34-36 Meanwhile, isobutane is clean hydrocarbon fuel components of liquefied petroleum gas (LPG), and also used to synthesize high-octane isooctane via alkylation. Despite the great importance of selective synthesis of isoparaffins via CO2 hydrogenation, developing a high-efficient catalyst is still challenging.
The zeolite component in a multifunctional catalyst is a key factor to precisely control hydrocarbon selectivity for above reactions.37 Except HZSM-5, other types of zeolites possessing different framework topology, such as HMCM-22 and HBeta, have shown unusual catalytic characteristics in the skeletal isomerization reactions of hydrocarbons, while they undergo quick deactivation as a result of coke deposition.38,39 To date, however, there is little known about the catalytic properties and deactivation behaviors of HMCM-22 and HBeta in CO2 hydrogenation reaction.
Herein, we develop a high-efficient multifunctional catalyst system for synthesizing middle (C4–C11) isoparaffins from CO2 hydrogenation (Table 1). The effects of zeolite pore structure and acidity on the product distribution and coke formation were investigated over Na–Fe3O4/Zeolite composite catalysts containing different topology but similar SiO2/Al2O3 ratios of zeolites: HMCM-22, HBeta and HZSM-5. The product distributions over HMCM-22 and HBeta were strikingly different from that obtained over HZSM-5 and displayed very high initial selectivities to isoparaffins (Figure 1). In addition, the zeolite deactivation behavior and nature of coke formation have been studied in detail, which provide important information to take into account for the regeneration of zeolites in this study.
Studies on the reaction schemes of CO2 hydrogenation to isoparaffins over Na-Fe3O4/HMCM-22 catalysts show that CO2 and H2 are activated on Na–Fe3O4 to generate olefins as intermediates, and their oligomerization and isomerization are preferentially performed on HMCM-22 or HBeta. The high yield to isoparaffins was derived owing to well matching of three tandem reactions comprising reverse water-gas shift, C-C coupling and isomerization. Unique pore structure and appropriate Brønsted acid properties of HMCM-22 effectively suppressed aromatization, whilst promoting isomerization (Figure 2). Coke deposition inside the micropores of HMCM-22 causes the decline of isoparaffin yield without changing the total yield of heavy hydrocarbons. Both the physico-chemical properties and catalytic performances of the catalysts could still keep their original levels after several reaction-regeneration cycles, indicating a promising potential application in the future commercialization process of CO2 hydrogenation.
Table 1. Reaction performance for CO2 hydrogenation. a
Catalysts |
CO2 conv. (%) |
CO sel. (%) |
Hydrocarbon distribution (C-mol %) |
Cole/Cp c |
STY d (mgiso gcat-1 h-1) |
|||
C1 |
C2-3 |
N-C4+b |
Isoparaffins |
|||||
Na–Fe3O4/HMCM-22 |
25.9 |
17.1 |
8 |
10 |
25 |
57 |
0.08 |
102 |
Na–Fe3O4/HBeta |
25.8 |
17.4 |
9 |
11 |
21 |
59 |
0.02 |
105 |
Na–Fe3O4/HZSM-5 |
25.6 |
17.3 |
6 |
16 |
44 |
34 |
0.02 |
60 |
Na–Fe3O4 |
25.1 |
17.7 |
8 |
26 |
60 |
6 |
5.11 |
12 |
a Reaction conditions: H2/CO2 = 2, 320 oC, 3 MPa, 4,000 ml h−1, time on stream of 90 min.
b N-C4+: C4+ products except for isoparaffins. c Cole/Cp is the molar ratio of all olefins to all paraffins with n > 1. d STY: Space time yield of isoparaffins.
Figure 1. The detailed hydrocarbon product distribution obtained over different catalysts.
a, Na–Fe3O4/HMCM-22. b, Na–Fe3O4/HBeta. c, Na–Fe3O4/HZSM-5. d, Na–Fe3O4. Reaction conditions as in Table 1.
Figure 2. Reaction scheme of isoparaffin synthesis and coke formation during CO2 hydrogenation over Na–Fe3O4/HMCM-22 catalyst.
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