Catalytic hydrogenation of CO2 to isoparaffins over Fe-based

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Catalytic hydrogenation of CO2 to isoparaffins over Fe-based multifunctional catalysts Jian Wei, Ruwei Yao, Qingjie Ge, Zhiyong Wen, Xuewei Ji, Chuanyan Fang, Jixin Zhang, Hengyong Xu, and Jian Sun ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02267 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 17, 2018

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Catalytic hydrogenation of CO2 to isoparaffins over Fe-based multifunctional catalysts Jian Weia,#, Ruwei Yaoa,b,#, Qingjie Gea,*, Zhiyong Wena, Xuewei Jia,b, Chuanyan Fanga, Jixin Zhanga, Hengyong Xua & Jian Suna,* a

Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, Dalian 116023, China. b

University of Chinese Academy of Sciences, Beijing 100049, China.

#

These authors contributed equally to this work.

ABSTRACT: Direct conversion of CO2 into isoparaffins, ideal clean hydrocarbon fuel components, would be an eco-friendly way of mitigating CO2 emissions and replacing fossil fuels. Herein, we realize a one-step high-yield synthesis of isoparaffins from CO2 hydrogenation, catalyzed by a multifunctional Na– Fe3O4/HMCM-22 catalyst. A selectivity of 82% among hydrocarbons was achieved for C4+ hydrocarbons, of which isoparaffins could account for 74%, while CO selectivity was as low as 17% at a CO2 conversion of 26%. 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. 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 1

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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. KEYWORDS: CO2 hydrogenation, isoparaffins, Brønsted acid sites, coke deposition, zeolite regeneration, multifunctional catalyst

1. INTRODUCTION

The unceasingly rising concentration of CO2 in the atmosphere, mainly resulting from fossil fuel combustion, is confidently thought to have induced global warming and other climate change phenomena.1 Therefore, numerous solutions have been proposed to develop efficient CO2 capture and utilization systems in order to reduce CO2 emissions. Actually, CO2 is also a cheap (or even negative-value) and abundant carbon source to manufacture valuable fuels and commodity chemicals in view of the development of a resource-efficient and low-carbon economy.2,3 It would be a sustainable way of reducing CO2 emissions, replacing fossil fuels and facilitating the storage of renewable energy, if H2 used for converting CO2 was supplied by water electrolysis with renewable electricity.4,5 Current researches into the catalytic hydrogenation of CO2 mainly focus on the production of low-carbon chemicals,6 such as CO, methane, formic acid, methanol, ethanol and lower olefins.7-23 Due to the weak capability of carbon chain growth during CO2 hydrogenation,24,25 relatively, the selective hydrogenation of CO2 into long-chain hydrocarbons is the least studied and characterized process.26-33 2


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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 team26 and 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 3


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middle (C4–C11) isoparaffins from CO2 hydrogenation. 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. 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.

2. EXPERIMENTAL SECTION

2.1. Catalyst preparation Synthesis of Na–Fe3O4 nanoparticles. Na–Fe3O4 nanoparticles were prepared by a one-pot synthesis method. Typically, FeCl3·6H2O (7.88 g) and FeCl2·4H2O (3.14 g) were successively dissolved in a mixture of 50 ml of deionized water and 1.3 ml of 12.1 mol l−1 HCl. Then, the aqueous solution of NaOH (1.5 mol l−1) was added into the above solution dropwise with stirring, until the pH of the resulting suspension reaches 10. After aging at 60 oC for 1 h, the black precipitate generated was isolated in the magnetic field. The product was washed with deionized water (1×200 ml), and dried at 60 oC for 15 h without further calcination. The content of residual sodium in Na–Fe3O4 catalyst was 0.7 wt% derived from inductively coupled plasma optical emission spectrometer (ICP-OES, Perkin-Elmer Optima 7300DV). 4


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Preparation of composite catalysts. HMCM-22, HBeta and HZSM-5 zeolites were purchased from Nankai University Catalyst Company. The zeolites were pre-treated in air at 500 oC for 4 h before use. Na–Fe3O4 and zeolite (HMCM-22, HBeta or HZSM-5) are typically integrated by dual-bed configuration with a fixed mass ratio of the two components of 1:3. For example, Na–Fe3O4 and zeolite (HMCM-22, HBeta or HZSM-5) were pressed, crushed and sieved to 20–40 meshes (sizes, 380–830 µm), respectively. Then, the zeolites were packed below Na–Fe3O4 and separated by a thin layer of inert quartz sand.

2.2. Catalyst characterization The chemical composition of zeolites was determined by X-ray fluorescence (XRF) using a PANAlytical Epsilon 5 spectrometer. The morphology of zeolites was characterized by scanning electron microscopy (SEM) on a JSM-7800F microscope operated at 1.0 kV in the GB display mode. Powder X-ray diffraction (XRD) spectra of the catalysts were recorded with a PANalytical X’Pert Pro diffractometer. Cu-Kα irradiation (40 kV, 40 mA) was used as the X-ray source. The BET surface area and pore volume of zeolites were determined by N2 adsorption-desorption measurements at −196 oC on a Quantachrome instrument.

27

Al Magic Angles Spinning Nuclear

Magnetic Resonance (MAS NMR) spectra were measured at room temperature on a Bruker DRX-400 spectrometer equipped with a magic angle spin probe operating at 104.3 MHz. The 27Al chemical shift was referenced using an aqueous AlCl3 solution. Temperature-programmed desorption of NH3 (NH3-TPD) was performed on a home-made apparatus to study the acidity of zeolites. Prior to adsorption experiments, 5


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100 mg of samples were pretreated in a quartz tube in He stream at 600 oC for 30 min, and then were cooled down to 100 oC. Then, NH3 was introduced into the sample for saturated adsorption. It was switched to He flow for 30 min in order to remove reversibly and physically bound NH3 from the surface. Finally, NH3 desorption was carried out from 100 to 700 oC at a heating rate of 10 oC min−1 in a stream of He (30 ml min−1). The acidity of zeolites was also studied by pyridine-adsorbed Fourier transform infrared spectroscopy (Py-IR) on a Bruker IF113V FT-IR spectrometer. The zeolites were compressed into a self-supported wafer (~16 mg and 13 mm of diameter) and placed in an in-situ IR cell with CaF2 windows. Prior to measurements, the fresh zeolites were pretreated under vacuum (10−2 Pa) at 400 oC for 1 h and then cooled down to room temperature. Spectra of degassed samples were collected as background. Then, pyridine was introduced into the sample and saturated for 5 min at room temperature. Then, the IR spectra were collected after evacuation at 200 oC (or 350 oC) for 30 min. The difference spectra were obtained by subtracting the background spectrum previously obtained. Coke deposition of spent zeolites after catalytic testing was probed by temperature-programmed oxidation (TPO) in a home-built setup equipped with a Pfeiffer OmniStar mass spectrometer. Typically, a 40 mg sample was first pretreated in Ar stream at 200 oC for 1 h and then cooled down to room temperature. Finally, the samples were heated to 900 oC with a heating rate of 10 oC min−1 under a flow of 5% O2/95% Ar. MS signals, including O2 consumption (m/z = 32), production of H2O 6


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(m/z = 18) and CO2 (m/z = 44), were monitored as a function of temperature. Coke content was also measured by heating spent zeolites from ambient temperature to 1,000 oC at 10 oC min−1 under air flow (40 ml min−1) in a thermogravimetric (TG) analyser (Perkin-Elmer Diamond TGS-2). The composition of coke deposits was determined according to the method reported by Guisnet and Magnoux40. In this technique, a 40 mg sample of spent zeolite was treated with 1 ml of HF acid (40%) in order to dissolve the zeolite and liberate the coke. The soluble components of coke extracted by CH2Cl2 were analysed by an Agilent GC–MS. The ‘‘insoluble coke’’ refers to the part of coke which is not dissolved in CH2Cl2. The 57Fe Mössbauer spectra of catalysts were collected in transmission mode by a constant-acceleration spectrometer (Topologic 500A). The radioactive source was 57

Co (in Rh matrix), and all samples were investigated at room temperature over a

velocity range of ±10 mm/s. Data analyses were carried out assuming a Lorentzian lineshape for computer folding and fitting.

2.3. Catalytic tests CO2 conversion was conducted in a continuous-flow fixed-bed reactor. The reactant gas with a H2/CO2 ratio of 2:1 was employed in this work unless otherwise noted. N2 with a v(N2)/v(CO2) of 1/6 contained in the feed gas was used as an internal standard to calculate the conversion of CO2. Before reaction, the composite catalyst was in-situ reduced in H2 at 350 oC for 8 h. The reaction conditions were typically at 320 oC, 3 MPa and 4,000 ml h−1 gcat−1. The products were analysed by two online GC. 7


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One GC system (VARIAN 3800) was equipped with a thermal conductivity detector (TCD) and a TDX-01 packed column to analyse N2, CO, CH4 and CO2. Another GC system (Agilent 7890B) was equipped with a flame ionization detector (FID) and a PONA capillary column to detect the hydrocarbons. CO2 conversion, CO selectivity and the hydrocarbon selectivity were calculated on a molar carbon basis, i.e. % = where CO2

in

and CO2

out

× 100%

denote the moles of CO2 at the inlet and outlet,

respectively. % =

× 100%

where CO out represents the moles of CO at the outlet. The selectivity of individual hydrocarbon (Ci) in total hydrocarbons was given according to: )*+, *- ./01*2314*5×

ℎ" # $ %– '%( = ∑

78 )*+, *- ./01*2314*5×

× 100%

3. RESULTS AND DISCUSSIONS

3.1. Structure and acidity of various zeolites The Na–Fe3O4/Zeolite multifunctional catalyst consists of three types of active sites.26 During CO2 hydrogenation, the multiple active sites in this catalyst worked cooperatively and enabled a tandem of reactions, including the initial reduction of CO2 to CO on Fe3O4 sites, the subsequent hydrogenation of CO to olefins on Fe5C2 sites, and hydrocarbon oligomerization, isomerization, aromatization on zeolite acid sites.26 Therefore, the structure and acidity of zeolite play an important role in 8


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controlling the selectivity of hydrocarbons. XRF analysis showed that the SiO2/Al2O3 ratios of three zeolites are very similar (28 for HMCM-22, 24 for HBeta and 25 for HZSM-5). SEM images and XRD patterns of three zeolites confirmed that they are highly crystalline materials of the expected phases (Figure S1). The N2 adsorption-desorption isotherms and BET results of zeolites are displayed in Figure S2 and Table S1. The micropore volumes of three zeolites are similar, while the mesopore volumes of HMCM-22 and HBeta are much larger than that of HZSM-5. HMCM-22 possesses the largest pore volume because of its typical thin platelet morphology and the presence of supercages in the MWW framework.39 Furthermore, the pore-size distributions of both HMCM-22 and HBeta are widely ranged from 3 to 50 nm, which is characteristic of amorphous pores resulting from the interparticle space. For the acid-catalyzed reactions, the acid density and strength of zeolites have a significant impact on the catalytic performance. Figure 1 and Table S1 give the detailed acidity data of zeolites measured by NH3-TPD and Py-IR techniques. Both techniques showed that the total acid site densities of zeolites have an obvious distinction, in the same order of HMCM-22 < HBeta < HZSM-5, despite similar Al contents three zeolites possess. This can be explained by the fact that not all the Al atoms distributes in the tetrahedral sites of zeolite framework.41 As indicated by the 27

Al MAS NMR spectra of three zeolites (Figure 1c), the percentage of tetrahedral Al

(framework Al) in the total Al increased in the order HMCM-22 (76%) < HBeta (79%) < HZSM-5 (86%), in agreement with the NH3-TPD and Py-IR results. Consequently, 9


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the zeolites have an obvious difference in the acid density, acid type and acid strength. As shown in Figure 1a, the temperature of second NH3 desorption peak for HZSM-5 (427 oC) is much higher than those of HMCM-22 (338 oC) and HBeta (329 oC), which has a strong correlation with the acid strength and the pore structure. Further study by Py-IR (Figure 1b) displayed that all three zeolites have Brønsted and Lewis acid sites. The difference is the acid sites of HZSM-5 are mainly Brønsted acid sites, with a high Brønsted/Lewis (B/L) ratio of 7.13. While for HMCM-22 and HBeta, the amounts of Brønsted and Lewis acid sites are close, with a low B/L ratio of 1.37 and 1.23, respectively.

10


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Figure 1. Acidity of different zeolites. a, NH3-TPD profiles of various zeolites. The NH3-TPD profiles were fitted by two Gaussian functions associated with the desorption peaks at low and high temperatures, respectively (dashed line).42 A desorption peak around 100–300 oC attributes to weak acid sites and the peak above 300 oC to strong acid sites. b, Py-IR spectra of different zeolites. The Py-IR spectra were recorded after weakly adsorbed pyridine was removed by evacuation at 200 oC. The band at 1546 cm−1 corresponds to Brønsted acidity and the band at 1455 cm−1 to Lewis acidity. c, 27Al MAS NMR spectra of different zeolites. All the spectra of three zeolites showed two peaks, one at 54 ppm, attributed to tetrahedrally-coordinated Al (framework Al) and one at 0 ppm, assigned to octahedrally-coordinated Al (extraframework Al).

3.2. CO2 hydrogenation performance of composite catalysts The proximity of the two components in Na–Fe3O4/Zeolite multifunctional catalysts has been reported to exert significant influence on catalytic activity and product distribution during CO2 hydrogenation26. The distance between the two components has an influence on the hydrogen concentration and the transport of the reaction intermediates in the gas phase, leading to varying degrees of secondary reactions over zeolites. It is inclined to produce more aromatics under the manner of granule mixing of Na–Fe3O4 and zeolites, probably because the hydrogen consumption steps (RWGS and FTS) over Na–Fe3O4 bring about a lower hydrogen partial pressure over zeolites, which favors for the dehydro-aromatization rather than hydro-isomerization. While more isoparaffins are produced under the dual-bed configuration (zeolites packed below Na–Fe3O4). To obtain a higher isoparaffin selectivity, Na–Fe3O4 and zeolites are combined by dual-bed configuration in this

11


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work. The CO2 hydrogenation performance and a detailed hydrocarbon product distribution over the sole Na–Fe3O4 catalyst and different Na–Fe3O4/Zeolite catalysts are listed in Table 1 and Figure 2. There were no substantial differences in CO2 conversion and CO selectivity between the base and the composite catalysts (Table 1), indicating that these catalysts exhibited comparative RWGS reaction rates. With Na– Fe3O4 as the sole catalyst, the major products of CO2 conversion were hydrocarbons (80% in all products) containing 66% of C4+ hydrocarbons, while only 3% of oxygenates (C2H5OH) and 17% of CO were detected. Unfortunately, most of these hydrocarbon products were olefins and normal paraffins, and isoparaffin selectivity was only 6%. After combining Na–Fe3O4 with zeolites, the total selectivity of C1–C3 hydrocarbons could be successfully suppressed to less than 20%, and the olefin to paraffin ratio (Cole/Cp) of hydrocarbons decreased greatly from 5.1 to less than 0.1 (Table 1). By contrast, the selectivity of isoparaffins increased dramatically from 6% to more than 34%, and the selectivity of aromatics increased from 1% to more than 8%. It can be deduced that olefins such as ethylene and propylene are important intermediate products during the process of CO2 hydrogenation to long-chain hydrocarbons. The isoparaffins and aromatics are formed through the oligomerization, isomerization and aromatization reactions at the expense of olefins on acidic zeolites. The results showed that the selectivities of isoparaffins were different among four catalysts in the order: Na–Fe3O4/HBeta (59%) > Na–Fe3O4/HMCM-22 (57%) > Na– Fe3O4/HZSM-5 (34%) >> Na–Fe3O4 (6%). Unprecedented space time yields (STY) of 12


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isoparaffins of 102 and 105 mgiso gcat-1 h-1, which are far higher than the isoparaffin yields reported on In2O3/HZSM-5 (63 mgiso gcat-1 h-1)

27

and Na–Fe3O4/HZSM-5 (60

mgiso gcat-1 h-1) catalysts, were achieved over Na–Fe3O4/HMCM-22 and Na– Fe3O4/HBeta catalysts, respectively. This was owing to the well matching between RWGS and FTS, as well as the effective suppression of aromatization reaction. Over Na–Fe3O4/Zeolite catalysts, the selectivities of C4+ hydrocarbons are similar at around 81% while the compositions of C4+ hydrocarbons are totally different. Over Na–Fe3O4/HMCM-22 and Na–Fe3O4/HBeta catalysts, 70% and 74%, respectively, of C4+ hydrocarbons are isoparaffins (Figure 3a). Nevertheless, the selectivity of C4 hydrocarbon (mainly isobutane) reached up to 43% in all hydrocarbons over Na– Fe3O4/HBeta, far beyond the other catalysts (Figure 2b). Unlike Na–Fe3O4/HMCM-22 and Na–Fe3O4/HBeta, the Na–Fe3O4/HZSM-5 produced higher amount of aromatics, and isoparaffins make up only 44% of C4+ fractions (Figure 3a). This phenomenon has a close correlation with the topology of various zeolites (Table S2). Zeolite HMCM-22, an unusual two-dimensional layered zeolite with MWW structure and 10-membered ring (MR) channel, has a unique lamellar structure consisting of two independent, non-intersecting pore systems. HBeta is a large pore zeolite with BEA topology structure, possessing a three dimensional, intersected and 12-MR channel system. These give rise to HMCM-22 and HBeta with potential catalytic properties in isomerization, alkylation and disproportionation.39,43 However, the well-known HZSM-5 zeolite with MFI structure composes of smaller 10-MR channels (5.3×5.6 Å and 5.1×5.5 Å), which favor for the formation of space-saving aromatics rather than 13


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long-chain isoparaffins. Besides the pore structure, zeolite acidity also has a significant effect on the product distribution. It is known that isomerization and aromatization reactions mainly occurred on the Brønsted acid sites of zeolite.34,36,44,45 Although HZSM-5 beared the highest Brønsted acid sites density and strength (Table S1), it exhibited the lowest selectivity to isoparaffins among three catalysts (Table 1). Based on the above results, we speculated that the appropriate density and strength of Brønsted acid sites of zeolites are the key point of improving isoparaffin selectivity. This can be explained by the fact that hydrocarbon isomerization and aromatization reactions are competing reactions during CO2 hydrogenation. As shown in Figure 1, HZSM-5 has a much stronger Brønsted acid sites and far higher B/L ratio than HMCM-22 and HBeta. As a result, HZSM-5 with the excessive levels of Brønsted acid sites would be inclined to catalyse the aromatization reaction, and favor the formation of aromatics instead of isoparaffins.

Table 1. Reaction performance for CO2 hydrogenation. a CO2 conv. (%)

CO sel. (%)

Na–Fe3O4/HMCM-22

25.9

Na–Fe3O4/HBeta

Catalysts

Hydrocarbon distribution (C-mol %)

Cole/Cp c

STY d (mgiso gcat-1 h-1)

C1

C2-3 N-C4+ b Isoparaffins

17.1

8

10

25

57

0.08

102

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 b

Reaction conditions: H2/CO2 = 2, 320 oC, 3 MPa, 4,000 ml h−1, time on stream of 90 min. 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. 14


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Figure 2. 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 3. Catalytic performance of composite catalysts. a, The composition of C4+ hydrocarbons over different Na–Fe3O4/Zeolite catalysts. Reaction conditions as in 15


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Table 1. b, CO2 conversion and product selectivity at different H2/CO2 ratios over Na– Fe3O4/HMCM-22 catalyst at 320 oC, 3 MPa, and 4,000 ml gcat−1 h−1. c, CO2 conversion, C4+ and isoparaffin selectivity over different Na–Fe3O4/Zeolite catalysts as a function of TOS. HCs: Hydrocarbons. Reaction conditions: H2/CO2=2, 320 oC, 3 MPa, 4,000 ml gcat−1 h−1. CO2 conversion and C4+ selectivity are indicated by solid lines and isoparaffin selectivity by a dashed line. Then, the effect of H2/CO2 ratio of the feed gas on CO2 hydrogenation performance was investigated over Na–Fe3O4/HMCM-22 catalyst (Figure 3b). It can be seen that higher H2/CO2 ratios in the reaction system, were beneficial to the processes of both CO2 reduced to CO (RWGS) and CO hydrogenation to hydrocarbons (FTS), consequently increasing the utilization ratio of carbon. With increasing H2/CO2 ratios from 1 to 5, CO2 conversion increased from 17 to 44%, and CO selectivity decreased from 26 to 10%. Nevertheless, the increased CO2 conversion accompanied the enhanced methane formation and secondary hydrogenation of olefin intermediates at the expense of heavier hydrocarbons. Upon increasing H2/CO2 ratios from 1 to 5, both CH4 (from 6 to 13%) and C2–C3 selectivities (from 9 to 12%) increased, whereas total C4+ (from 85 to 75%) and isoparaffin selectivities (from 57 to 53%) decreased substantially. These results indicate that our multifunctional catalyst is applicable to the conversion of CO2 with different H2/CO2 ratios and thus could allow the utilization of different feed gas resources. The time-on-stream (TOS) evolutions of CO2 conversion and hydrocarbon selectivity over different Na–Fe3O4/Zeolite catalysts are provided in Figure 3c. It can be seen that CO2 conversions stayed around 26% and the selectivities of C4+ 16


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hydrocarbons stably maintained at ~81% during the whole 12 h of TOS studied. It indicated that all three multifunctional catalysts are suitable for the direct conversion of CO2 to long-chain hydrocarbons. In detail, the changes in the selectivities of isoparaffins in hydrocarbons have been plotted as a function of TOS in Figure 3c for each of the multifunctional catalysts for a better comparison of the deactivation behavior. As shown in Figure 3c, both Na–Fe3O4/HMCM-22 and Na–Fe3O4/HBeta exhibit higher isoparaffin selectivities at the initial stage of reactions. Then, rapid declines in the isoparaffin selectivities are observed during the first 6 h on stream, after which the selectivities drop slowly. A fast deactivation in FTS reaction was also observed in previous work by Martínezet et al.46,47 for hybrid catalysts comprising large-pore zeolites, such as USY, Beta, and mordenite. Relatively, Na–Fe3O4/HZSM-5 displayed a lower but more stable selectivity to isoparaffins. In order to account for the differences in the deactivation trends observed, the amount and nature of the coke deposits in spent zeolites have been determined by various characterization techniques. The results obtained are presented and discussed in the following section.

3.3. Nature of coke formation SEM images of spent zeolites HMCM-22, HBeta and HZSM-5, showed that there was no evident change to the shape and size of the crystal after reaction (Figure S3). XRD patterns of spent zeolites displayed that the crystallinity of zeolites decreased obviously as compared to fresh zeolites (Figure S3d). The N2 physical adsorption data of spent zeolites are listed in Table S3. Obviously, the coke deposition generated in zeolites influenced their BET surface areas and pore volumes. Larger falls in the 17


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micropore surface areas and micropore volumes were observed for spent zeolites, when compared with the external surface areas and mesopore volumes. This indicated that the coke was formed predominantly inside the zeolite micropores, blocked up their micropore opening, and led to their deactivation. Moreover, the surface areas and pore volumes of HMCM-22 and HBeta decreased more strikingly than HZSM-5, indicating that HMCM-22 and HBeta suffered from coke more severely. The coke amounts of spent zeolites have been studied by means of TPO and TG analysis. As shown in Figure 4a, obvious CO2 (m/z = 44) and H2O (m/z = 18) signals were detected by the mass spectrometer (MS) over three spent zeolites. Among these zeolites studied, HZSM-5 was recorded as having the lowest content of coke formed on its acid sites, consistent with the good stability of Na–Fe3O4/HZSM-5 catalyst during CO2 hydrogenation reaction (Figure 3c). From TG analysis, two weightlessness steps were detected in the heating process at air atmosphere (Figure 4b). The former step before 300 oC corresponds to the removal of adsorption water in zeolite channels. However, the latter step after 300 oC is related to the burning of zeolite coke. It was also observed that the total amount of coke deposited on HZSM-5 after 12 h reaction was 3%, which was much lower than those on spent HMCM-22 of 14% and spent HBeta of 12% (Figure 4b). It is noteworthy that the carbon mass balances over different Na–Fe3O4/Zeolite catalysts were calculated according to the product distribution analysis, which were 98% for Na–Fe3O4/HZSM-5, 82% for Na– Fe3O4/HMCM-22, and 83% for Na–Fe3O4/HBeta. The carbon mass balances corresponded well with the total amounts of coke detected by TG analysis over 18


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various zeolites. These results, in line with those of N2 adsorption-desorption and TPO measurements, indicated that HZSM-5 showed superior coke resistance. Next, to reveal the nature of coke retained on spent zeolites, the soluble components of coke were determined by gas chromatographs-mass spectrometer (GC-MS) according to the method reported by Guisnet and Magnoux40. The insoluble coke was not dissolved in CH2Cl2 and its chemical composition is hard to be determined. GC-MS profiles (Figure 4c) showed that the carbonaceous compounds retained in HMCM-22, HBeta and HZSM-5 are indeed similar, despite the large differences in concentrations of various compounds. Table S4 presents the molecular formula and molecular weights of the components detected in the soluble coke. Figure S4 gives the mass spectra of the soluble coke species in spent zeolites. The coke extracted from spent zeolites constituted mainly single-ring aromatic compounds (such as toluene (C7H8), p-xylene (C8H10) and mesitylene (C9H12)), as well as oxygenated

aromatic

compounds

(2,4-dimethyl-benzaldehyde

(C9H10O)

and

2,4-di-tert-butylphenol (C14H22O)). Besides, minor amounts of adamantane (C10H16) and 1,6-dimethyl-naphthalene (C12H22) were also identified by GC-MS. In conclusion, the shape selectivity of zeolite controls not only product distribution but also the amount and nature of coke deposition.

19


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Figure 4. Coke analysis of spent zeolites. TPO (a) and TG (b) analysis of spent zeolites. (c) GC-MS analysis of the soluble coke in spent zeolites. The zeolites went through reaction tests of 12 h in Fig. 3c. The coke formation process of zeolite is a complex shape-selective catalytic reaction occurring on acid sites.48-50 The pore structure plays a significant role in coke formation. The characterization results indicate that, during CO2 hydrogenation, coke is originated and developed inside the micropore or on the external surface. As the coke content increased, the specific surface area and total pore volume of zeolites decreased, and the micropore surface area and micropore volume of zeolites decreased first. At last, the coke blocked the access of the reactant to the active sites, 20


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led to the deactivation of zeolites.51-53 Moreover, there is a great difference in the zeolite coking rates, which is given as the weight percentage of coke (determined by TG analysis) deposited in spent zeolite divided by the reaction time of 12 h. HZSM-5 cokes very slowly, whereas on the contrary HMCM-22 and HBeta cokes rapidly. There is a strong correlation between the pore structure and coking rate. When the space available near the acid sites is large, the coking rate is usually faster.40 The pore network of HZSM-5, which is constituted of interconnecting channels without cavities, exerts steric constraints on the formation of the bulky intermediates of coke, and can be responsible for this low coking rate. The large cavity of HBeta (10 Å diameter) and the supercage of HMCM-22 (with inner diameter 7.1 Å, delimited by 12-MR) provide a spacious room for the following chain growth reactions to form higher hydrocarbons like polymethylbenzene, which consequently develop to coke and lead to zeolite deactivation. It is generally believed that coke formation starts from oligomerization of lower olefins, followed by cyclization of the oligomers, transformation through hydrogen transfer into monoaromatics, alkylation of these monoaromatics, and then undergoes hydrogen transfer and cyclization to form polyaromatics species.40,47,48 Coke precursors (with aromatic nature) tend to grow, up to a point where they have steric hindrance, so the growth involves the formation of aliphatic chains. The growth in larger-pore zeolites is mainly aromatic, giving rise to polyaromatic structures that remain in the cages of the pore network. During CO2 hydrogenation, due to the 21


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existence of oxygen species in the reaction system, monoaromatics may occur through zeolite catalytic oxidation reaction to form oxygenated aromatic compounds, such as methyl benzaldehyde and methyl phenol, etc. The study of coke species would help us understand the reaction mechanism more deeply, thereby providing more information for further research on catalyst improvement and regeneration.

3.4. Reaction scheme of isoparaffin synthesis XRD and Mössbauer spectra of Fe-based catalysts displayed that the main iron phase in Na–Fe3O4 transformed from Fe3O4 to the coexistence of χ-Fe5C2 (87%) and Fe3O4 (13%) during CO2 hydrogenation (Figure 5 and Table S5). Based above discussion, we propose a reaction scheme of isoparaffin synthesis and coke formation during CO2 hydrogenation over Na–Fe3O4/HMCM-22 catalyst (Figure 6). In particular, CO2 and H2 were activated on the Fe3O4 sites to form CO, and then CO was hydrogenated to olefins on the Fe5C2 sites. The lower olefins formed on Na– Fe3O4 then diffused to the zeolite Brønsted acid sites, on which higher olefins generated through the oligomerization of lower olefins. The hydrocarbon isomerization and aromatization reactions are competing reactions on the zeolite acid sites. Over HMCM-22 and HBeta with particular pore structure and appropriate acid properties, the isomerization of olefins would proceed preferentially with respect to aromatization reaction. Olefins undergo a series of reactions such as skeletal isomerization, dimerization, cracking, hydrogen transfer to form isoparaffins.54,55 As a result, a high selectivity to isoparaffins was obtained. To some extent, there will always be some aromatics formed. On account of the limitation of diffusion of 22


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aromatics, part of aromatics will continue to undergo hydrogen transfer, cyclization and alkylation to form polyaromatics species, finally form coke, deposit on the acid sites, and lead to zeolite deactivation.

Figure 5. Characterization of Fe-based catalysts. a, XRD patterns of fresh and spent Na–Fe3O4 catalyst. b, 57Fe Mössbauer spectra of spent Na–Fe3O4 catalyst in the Na–Fe3O4/HMCM-22 composite catalyst after reaction tests of 12 h in Fig. 3c. The detailed Mössbauer parameters are listed in Table S5.

The reaction behavior of C2H4 conversion was investigated to study the priority of isomerization or aromatization over various zeolites. As can be seen from Table 2, olefin oligomerization occurs on all three zeolites, resulting in C4+ hydrocarbons with selectivities of >91%. But the isoparaffin selectivities over HMCM-22 and HBeta reached up to 62.3% and 56.9%, respectively, while which of HZSM-5 was only 26.3%. This proved that olefins would prefer to isomerization rather than cyclization or aromatization over HMCM-22 and HBeta, which is well consistent with the proposed reaction scheme.

23


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Figure 6. Reaction scheme of isoparaffin synthesis and coke formation during CO2 hydrogenation over Na–Fe3O4/HMCM-22 catalyst.

Table 2. Catalytic performance for ethylene conversion over different zeolites. Product selectivity (%)

Catalysts

C2H4 conv. (%)

C1

C2

C3

N-C4+ a

Isoparaffins

HMCM-22

97.5

0.1

0.4

4.6

32.6

62.3

HBeta

80.1

0.1

1.3

6.5

35.2

56.9

HZSM-5

90.4

0.2

1.3

7.1

65.1

26.3

Reaction conditions: 0.75 g of catalyst, 320

o

C, 3 MPa, 4,000 ml h−1,

C2H4/N2/He=8/4/88. a

N-C4+: C4+ products except for isoparaffins.

3.5. Catalyst regeneration test Based on the above analysis on coke deposition, the regeneration of HMCM-22 zeolite was conducted by calcination at 500 oC for 3 h in air atmosphere. Most of the coke on spent zeolite could be burned off at this temperature as indicated in TG and TPO analysis. From the appearance, the color of HMCM-22 zeolite changes from black back to white after regeneration (Figure S5). XRD patterns displayed that the crystallinity of regenerated zeolite remarkably increased as compared to spent zeolite (Figure S6). BET results showed that the specific surface area and total pore volume 24


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grew substantially in comparison with spent zeolite (Table 3). It was seen from Table 3 that the acid amount of zeolite declined sharply during reaction due to the accumulation of carbon deposits. Py-IR analysis of regenerated zeolite revealed that the total acid amount can be recovered to the comparative level as that of fresh zeolite (Table 3).

Table 3. Properties of fresh, spent and regenerated HMCM-22 zeolites. Zeolites

Fresh

Spent

Regenerated

SBET (m2/g) a

489.8

149.1

497.1

Smicro (m2/g)

361.1

90.0

349.2

Sexternal (m2/g)

128.7

59.1

147.9

0.49

0.28

0.68

Vmicro (cm /g)

0.14

0.04

0.14

Vmeso (cm3/g)

0.35

0.24

0.54

Brønsted acidity

0.116

0.014

0.089

Lewis acidity

0.085

0.018

0.071

total

0.200

0.032

0.160

B/L ratio

1.37

0.81

1.25

3

Vtotal (cm /g)

b

3

Acidity

a

(mmolpyridine gcat−1) c

SBET: specific surface area calculated by the BET method; Smicro: micropore volume

determined by t-plot method.

b

Vtotal: total pore volume; Vmicro: micropore volume

determined by t-plot method; Vmeso: mesopore volume determined by Vtotal–Vmicro. c

Determined from Py-IR. Figure 7 shows the CO2 hydrogenation performance at the same reaction

conditions over the fresh and regenerated Na–Fe3O4/HMCM-22 catalysts. It can be seen that the catalytic performance of spent catalysts were both thoroughly recovered after the first and second regenerations. The initial isoparaffin selectivities of regenerated catalysts were almost the same as that of fresh catalyst. Thus, it can be 25


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seen that both the physico-chemical properties and the catalytic performances of HMCM-22 zeolite can be back up to its original level after regeneration. Considering that Na–Fe3O4 and zeolites are integrated by dual-bed configuration and separated by a thin layer of inert quartz sand, the regeneration of spent zeolite in the presence of Fe-based catalyst is not a big issue. The multifunctional catalysts have a promising potential to be applied in the future commercialization process of CO2 hydrogenation.

Figure 7. CO2 hydrogenation performance over the fresh and regenerated Na– Fe3O4/HMCM-22 catalysts. Reaction conditions: H2/CO2 = 2, 320 oC, 3 MPa, 4,000 ml gcat−1 h−1.

4. CONCLUSIONS

In summary, we have designed high-efficient multifunctional catalysts for the direct synthesis of isoparaffins from CO2 hydrogenation. 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 unique pore 26


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network, Brønsted acid density and strength of above zeolites are responsible for their high yields to isoparaffins. Nevertheless, the coke is formed and trapped in the cavities and channels of HMCM-22 and HBeta, and blocks the access of the reactants to the acid sites, finally leads to the deactivation of zeolites. The soluble coke extracted from spent zeolites constituted mainly aromatic compounds as well as oxygenated aromatic compounds. Further studies indicated that the zeolites could be regenerated easily by burning off the coke trapped in zeolites. This study enriches the design of highly selective catalysts for CO2 hydrogenation into high-value chemicals.

ASSOCIATED CONTENT

Supporting Information Figures S1−S6 and Tables S1−S5 as described in the text. (PDF)

AUTHOR INFORMATION

Corresponding authors * Email for Q.G.: [email protected] * Email for J.S.: [email protected]

Author contributions J.W., Q.G. and J.S. conceived and designed this work, and wrote the paper. J.W. and R.Y. synthesized, characterized and evaluated the catalysts. Z.W., X.J., C.F, and J.Z. did part of the catalytic reaction testing. All the authors participated in the analysis and discussion of the experimental data. J.S., H.X. and Q.G. supervised the whole 27


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project.

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS

This work has been supported by the National Natural Science Foundation of China (21773234, 21802138, 91745107 and 21503215), the "Transformational Technologies for Clean Energy and Demonstration", Strategic Priority Research Program of the Chinese Academy of Sciences (XDA 21090203), and the Youth Innovation Promotion Association of Chinese Academy of Sciences (2018214). We thank Shoufu Hou, Xiuying Gao and Zhimin Li, for the helps in catalyst evaluation and characterization.

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Catalytic hydrogenation of CO2 to isoparaffins over Fe-based

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