Direct Production of Lower Olefins from CO2 Conversion via

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Direct production of lower olefins from CO2 conversion via bifunctional catalysis Peng Gao, Shanshan Dang, Shenggang Li, Xianni Bu, Ziyu Liu, Minghuang Qiu, Chengguang Yang, Hui Wang, Liangshu Zhong, Yong Han, Qiang Liu, Wei Wei, and Yuhan Sun ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02649 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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Direct production of lower olefins from CO2 conversion via bifunctional catalysis Peng Gaoa, Shanshan Danga,b, Shenggang Lia,c, Xianni Bua, Ziyu Liua, Minghuang Qiua, Chengguang Yanga, Hui Wanga, Liangshu Zhonga*, Yong Hanc,d, Qiang Liuc,d, Wei Weia,c, Yuhan Suna.c* a

CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201203, PR China b

University of the Chinese Academy of Sciences, Beijing 100049, PR China

c

School of Physical Science and Technology, ShanghaiTech University, Shanghai 201203, PR China

d

State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, PR China ABSTRACT: Direct conversion of carbon dioxide (CO2) into lower olefins (C2=–C4=) – generally referring to ethylene, propylene and butylene is highly attractive as a sustainable production route for its great significance in greenhouse gas control and fossil fuel substitution, but such a route always tends to be low in selectivity towards olefins. Here we present a bifunctional catalysis process that offers C2=–C4= selectivity as high as 80% and C2–C4 selectivity around 93% at more than 35% CO2 conversion. This is achieved by a bifunctional catalyst composed of indium-zirconium composite oxide and SAPO–34 zeolite, which is responsible for CO2 activation and selective C–C coupling, respectively. We demonstrate that both the precise control of oxygen vacancies on the oxide surface and the integration manner of the components are crucial in the direct production of lower olefins from CO2 hydrogenation. No obvious deactivation is observed over 150 h, indicating a promising potential for industrial application.

KEYWORDS: CO2 hydrogenation, Lower olefins, Bifunctional catalysts, C–C coupling, Heterogeneous catalysis.

1.

INTRODUCTION

Carbon dioxide (CO2) is an easily available renewable carbon resource with the advantages of being economical, nontoxic, and abundant.1 The utilization of CO2 as a feedstock for producing various chemicals not only contributes to alleviating global climate changes caused by increasing CO2 emissions, but also offers a sustainable solution to replacing dwindling fossil fuel reserves.2-5 One promising route is the selective conversion of CO2 into value-added chemicals, such as lower olefins (C2=–C4=, generally referring to ethylene, propylene and butylene), which are the key building blocks of the chemical industry and traditionally produced by thermal cracking of naphtha.6-8 However, few studies focused on the selective hydrogenation of CO2 to the product containing two or more carbon atoms (C2+) due to the extreme inertness of CO2 and a high C–C coupling barrier.9-11 Generally, the direct CO2 hydrogenation to hydrocarbons proceeds via a modified Fischer-Tropsch synthesis (FTS) process, which consists of two main consecutive reactions: reverse water gas shift (RWGS) reaction to pro-

duce CO followed by the further conversion of CO to hydrocarbons via Fischer-Tropsch reaction.12-15 For FTS, the produced hydrocarbons usually follow the AndersonSchulz-Flory (ASF) distribution, which is characterized by a maximum of C2 to C4 hydrocarbon (C2–C4) fraction including olefins (C2=–C4=) and paraffins (C2o–C4o) of about 56.7% and an undesired methane (CH4) fraction of about 29.2%.16-18 Moreover, the low CO2 adsorption rate on the surface due to the thermodynamical and chemical stability of CO2 molecules leads to a high H/C ratio on the catalyst surface during CO2 hydrogenation.12,19 This favors the hydrogenation of surface-adsorbed intermediates, resulting in the ready formation of methane with a decrease in chain growth. Iron-based catalyst, conventionally used for commercial FTS, was also extensively studied for the production of olefins from CO2 hydrogenation, and a C2=–C4= selectivity of 35–50% was achieved with CH4 selectivity above 16%.20-22 Thus, it remains a grand challenge to simultaneously achieve high selectivity for lower olefins and low selectivity for CH4. The indirect route of production of lower olefins from CO2 includes conversion of CO2 into methanol (CH3OH) and olefins production from CH3OH in a separate stage.13


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Different from industrial methanol synthesis from syngas, the formation of water vapor is inevitable for CO2 hydrogenation to methanol, which inhibits the reaction strongly and leads to serious catalyst deactivation,23,24 and thus an efficient catalyst is in need to improve catalytic stability. For conversion of methanol to olefins (MTO), the SAPO-34 zeolite is recognized as the best catalysts owing to its unique topology, while it undergoes quick deactivation as a result of coke deposition.25-27 Therefore, the selective formation of olefins from CH3OH with high stability is still challenging. Moreover, as compared with the indirect route, the direct conversion of CO2 into olefins would be more economic and energy-efficient. Here, we report an alternative process for the direct production of lower olefins from CO2 hydrogenation via bifunctional catalysis. The bifunctional catalysis has resulted in a significant breakthrough in the synthesis of gasoline fuels from CO2 hydrogenation28 and selective conversion of syngas to lower olefins7,29. We design a high efficient bifunctional catalysts composed of indiumzirconium composite oxides (In–Zr oxide) and SAPO–34 zeolites to convert CO2 directly into lower olefins. As a result, a C2=–C4= selectivity reached up to 80% in hydrocarbons with only about 4% methane. The product distribution was completely different from that obtained via FTS and deviated greatly from the classical ASF distribution. In addition, there was no obvious deactivation over 150 h, suggesting the promising potential for industrial applications. 2.

EXPERIMENTAL SECTION

2.1 Catalyst Preparation Various oxides were prepared by a co-precipitation method. Typically, 3.25 g In(NO3)3 • 4.5H2O, 14.63 g Zr(NO3)3 • 5H2O were dissolved in a mixture of 48 mL deionized water and 140 mL ethanol, followed by the addition of a mixture of 36 mL NH4OH (28–30 wt.% in H2O) and 108 mL ethanol. The product was aged at 80oC for 10 min and then filtered and washed with deionized water. The filter cakes were dried over night at 60 oC and calcined in air at 500 oC for 5 h. Other oxides were synthesized by the similar procedures. SAPO-34 zeolites were prepared by a hydrothermal route. Parent SAPO-34 crystals were prepared by a hydrothermal route. The synthesis gel recipe in molar composition is 1 Al2O3:0.44 SiO2: 1.1 P2O5:2.25 triethyl-amine: 35 H2O, and SAPO-34 crystal seeds with a mass ratio of 1:500 to the gel were mixed in a closed autoclave. Then the mixture was heated from room temperature to 165 °C in 7 h and kept at 165 oC for 33 h before cooling down. The solid product was filtered, washed and dried, followed by calcination at 600 oC for 5 h. For the preparation of the In2O3/SAPO-34 and In– Zr/SAPO-34 composite catalysts, the In2O3 or In–Zr oxides and the SAPO-34 were pressed, crushed and sieved to granules in the range of 40–60 mesh (granule sizes of

250–400 μm), respectively. Then, the granules of the two samples were mixed together by shaking in a vessel. The samples prepared by this method were denoted as In2O3/SAPO-34-G and In–Zr/SAPO-34-G. For comparison, the In2O3 or In–Zr oxides and the SAPO-34 were mixed in an agate mortar for 10 min. Then, the mixed sample were pressed, crushed and sieved to particles in the range of 40–60 mesh. The obtained catalysts were denoted as In2O3/SAPO-34-M and In–Zr/SAPO-34-M. 2.2 Catalyst Characterization The crystalline and phase of the samples were investigated by powder X–ray diffraction (XRD) performed on a Rigatku Ultima 4 X–ray diffractometer with Cu Ka radiation (40 kV, 40 mA). The intensity data were collected over a 2θ range of 5o–90o and scanning step length of 0.0167o. For in situ XRD measurement, the sample firstly remained in pure Ar at a flow rate of 60 mL min–1. Temperature-ramping programs were exhibited from 20 to 400 oC at a heating rate of 10 oC min–1, and maintained at 400 oC for 1 h. Then the gas flow was switched to the reactant gas H2/CO2/N2 (73/24/3) mixture and the temperature was maintained at 400 oC. The textural properties such as surface area (BET), micropore area (t-plot method), pore volume (BJH and HK) and pore size distribution (BJH) of the samples were derived from N2 adsorption–desorption measurements carried out at 77 k using a TriStar II 3020 instrument. Prior to the measurements, the samples were treated in vacuum at 473 K for 10 h. The chemical composition of the zeolites was determined with the use of an X-ray fluorescence (XRF) spectroscopy (Rigaku ZSX Primus Ⅱ, Japan) by SQX calculation. The morphology of the samples was observed by a SUPRRATM 55 Scanning electron microscopy (SEM) with an accelerating voltage of 2.0 kV. The nanostructure of catalysts was investigated by a Tecnai G2 20 S-Twin highresolution transmission electron microscope (HRTEM) and the TEM operated at 200 kV. The energy X-ray dispersive spectroscopic (EDX) analyses have been performed by using a Li–Si EDS detector with an energy resolution of 0.05 eV. In situ near ambient pressure X-ray photoelectron spectroscopy (NAP–XPS) was performed on a system which is manufactured by SPECS Surface Nano Analysis GmbH. The facility is composed of two chambers, an analysis chamber and a quick ample load-lock chamber. The analysis chamber is equipped with a PHOIBOS NAP hemispherical electron energy analyzer, a microfocus monochromatized Al Kα X-ray source with beam size of 300 μm, a SPECS IQE-11A ion gun, and an infrared laser heater. CO2 temperature-programmed desorption (CO2–TPD) experiments were carried out with a Micromeritics Chemi Sorb 2920. Firstly, the catalyst (0.1g) was reduced at 400 o C for 60 min in a flow of pure Ar of 60 mL min–1, and then cooled down to 50 oC. After that, the catalyst was


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saturated in flowing CO2 for 1 h with 30 mL min–1 and followed by flushing in Ar for 3 h to remove any physisorbed molecules. The CO2–TPD measurement was then carried out at 50-750 oC and heating rate of 10 oC min–1 under continuous flow of Ar with 40 mL min–1. In situ diffuse reflectance infrared fourier transform spectroscopy (in situ DRIFTS) measurements were performed in quartz cell having a cylindrical cavity (5 mm in diameter and 5 mm vertical length) for the sample placement. Approximately, 20 mg of catalyst powder was placed in the cell and reduced at 400 oC for 1 h under continuous flow of He with 40 mL min–10), and then cooled down to 30 oC. After that, the catalyst was saturated in flowing CO2 for 2 h with 30 mL min–1 and followed by flushing in He for 30 min to remove any physisorbed molecules. After that, temperature-ramping programs were exhibited from 30 to 100, 150, 200, 250, 300, 350 and 400 o C with the He gas flow of 30 mL min–1. The spectra were collected on a Fourier-transform infrared spectroscopy (Thermo Scientific, Nicolet 6700). NH3 temperature-programmed desorption (NH3–TPD) experiments were performed in a similar manner as CO2– TPD. After pretreatment of 0.2 g catalyst in Ar at 400 oC for 1 h, the sample was cooled to 100 oC and brought to saturation with ammonia using NH3 flow at 30 mL min−1. Following the ammonia saturation, the system was purged with Ar flow at 100 oC for 30 min to remove any gas phase ammonia in the system and unadsorbed ammonia trapped in the catalyst bed. For desorption analysis, the catalyst bed temperature was raised from 100 oC to 600 oC at 10 oC min–1. 29

Si, 27Al and 31P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AVANCE III 400 WB spectrometer equipped with a 4 mm standard bore CP MAS probehead whose X channel was tuned to 79.50, 104.27 and 162 MHz for 29Si, 27Al and 31P, respectively, using a magnetic field of 9.39T at 297 K. The dried and finely powdered samples were packed in the ZrO2 rotor closed with Kel-F cap which were spun at 8 or 12 kHz rate. All 29Si, 27Al and 31P MAS NMR chemical shifts are referenced to the resonances of 3(trimethylsilyl)-1-propanesulfonic acid sodium salt (DSS), [Al(H2O)6]3+ and monoammonium phosphate (NH4H2PO4) standards (d = 0.0), respectively. 2.3 Catalytic Evaluation Activity measurements in the hydrogenation of CO2 were carried out in a continuous-flow, high-pressure, fixed-bed reactor. Typically, 1.0 g of composite catalyst (40–60 mesh) was placed in a stainless steel tube reactor (inner diameter, 12 mm). Prior to reaction, the catalyst was pretreated at 400 ºC for 1 h in pure Ar (150 mL min−1). Then reactant gas mixture with a H2/CO2/N2 ratio of 73/24/3 and a pressure of 3.0 MPa was introduced into the reactor. The catalytic reaction for methanol conversion was performed in the same reactor. After the 1.0 g of zeolite (40–60 mesh) was pretreated in the reactor in pure Ar (150 mL min−1) at 400 oC for 1 h.

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Methanol was then pumped into the reactor with an Ar or H2 (150 mL min−1) under 0.1–3.0 MPa and 400 oC. The space velocity of liquid methanol was set to 0.18 mL gzeo–1 −1 lite h , equivalent to the yield of methanol from CO 2 hydrogenation over the composite catalyst with In– Zr/SAPO-34 mass ratio of 2/1. The effluents were quantitatively analyzed online with a Shimadzu GC-2010C gas chromatograph equipped with thermal conductivity and flame ionization detectors. The catalytic performance after 48 h of reaction was typically used for discussion. CO2 conversion was calculated on a carbon atom basis according to the following equation: 𝐶𝑂2 𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 =

𝐶𝑂2 𝑖𝑛𝑙𝑒𝑡 −𝐶𝑂2 𝑜𝑢𝑡𝑙𝑒𝑡 𝐶𝑂2 𝑖𝑛𝑙𝑒𝑡

× 100%

where CO inlet and CO outlet represent moles of CO at the inlet and outlet, respectively. CO was formed by the reverse water gas shift reaction and CO selectivity was calculated according to: CO 𝑆𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 =

𝐶𝑂 𝑜𝑢𝑡𝑙𝑒𝑡 𝐶𝑂2 𝑖𝑛𝑙𝑒𝑡 −𝐶𝑂2 𝑜𝑢𝑡𝑙𝑒𝑡

× 100%

where CO2 outlet denotes moles of CO2 at the outlet. The selectivity of individual hydrocarbon product CnHm based on CO-free was obtained according to: 𝐶𝑛 𝐻𝑚 𝑆𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 =

𝑛𝐶𝑛 𝐻𝑚 𝑜𝑢𝑡𝑙𝑒𝑡 𝐶𝑂2 𝑖𝑛𝑙𝑒𝑡 −𝐶𝑂2 𝑜𝑢𝑡𝑙𝑒𝑡 −𝐶𝑂𝑜𝑢𝑡𝑙𝑒𝑡

× 100%

where CnHm outlet represents moles C of individual hydrocarbon product at the outlet. 2.4 DFT calculations Periodic density functional theory (DFT) calculations were carried out with the Vienna ab initio simulation package (VASP) using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional and projector augmented wave (PAW) potentials. 3

RESULTS and DISCUSSIONS

3.1 Catalytic functionality of oxides and zeolites The bifunctional catalyst contained an In–Zr oxide with an In/Zr molar ratio of 1:4 (Table S1 in the Supporting Information) and a SAPO-34 zeolite with Si/(Si+Al+P) molar ratio of 0.062 and a pore size around 0.4 nm (Table S2 and Figure S1 in the Supporting Information). It was confirmed that the In–Zr oxide mainly contained In1– xZrxOy and ZrO2 nanoparticles from XRD and TEM characterizations (Figure S2 in the Supporting Information and Figure 1a, b). Firstly, the catalytic performance for the sole In–Zr oxide or In2O3 was investigated. Both In–Zr and In2O3 samples exhibited much high activity for CO2 hydrogenation, and CO2 conversion was more than 30% at the reaction temperature of 400 ºC, whereas usage of ZrO2 alone showed only 7.3% of CO2 conversion (Table S3 in the Supporting Information). In addition, the In–Zr oxide displayed an In-based reaction rate (i.e. the number of


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Olattice IV: CO2@400 oC Odefect OH

Odefect

Intensity (a.u.)

IV: 15.2% III: H2@400 oC

III: 21.7% II: CO2@400 oC

II: 11.4%

I: Ar@400 oC

CH4

C2=-C4=

C20-C40

C5+

r

Hydrocarbon distribution (%)

c

100 90 80 70 60 50 40 30 20 10 0

◨

moles of CO2 converted per gram of indium per second) approximately five times that of the bulk In2O3, suggesting that the incorporation of Zr enhanced the catalytic activity significantly (Figure 2).

I: 22.3%

) ) /1) r /1) 4/1) 1/2 1/1 In 2O 3 -34(2 In-Z -34( -34( -34(2 -34( PO APO APO APO PO S SA S S /SA Zr/ n-Zr/ n-Zr/ In-Zr/ In 2O 3 InI I

1.0x10-4 8.0x10-5

r (molCO2 gIn1 s1)

6.0x10-5 4.0x10-5 2.0x10-5 0.0

Figure 2. Hydrocarbon distribution and reaction rate (r) over In2O3 and In–Zr oxides and bifunctional catalysts composed of metal oxides and SAPO-34 zeolites with different mass ratios as shown in the parentheses. Reaction conditions: 400 ºC, 3.0 MPa, 9000 mL gcat−1 h−1, H2/CO2/N2 = 73/24/3. CO2 conversions and the formation of CO by the RWGS reaction are reported in Table S3 in the Supporting Information.

535 534 533 532 531 530 529 528 527

Binding energy (eV)

d CO2 desorption (a.u.)

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ACS Catalysis

123

478

690

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108 170

ZrO2

510

In2O3

100

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400 500 600 o Temperature ( C)

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Figure 1. Characterizations of oxides used in the bifunctional catalyst. HRTEM images of (a) In–Zr and (b) In2O3 samples. (c) In situ NAP–XPS O 1s spectra of In–Zr oxide sequentially exposed to 50 Pa Ar, CO2, H2, and again to CO2 atmosphere under different conditions. In situ NAPXPS O 1s spectra can be de-convoluted into three distinct peaks: the main oxide peak at 530.2 eV (‘Olattice’) and two additional peaks at 531.4 eV and 532.4 eV, which were assigned to O atoms next to a defect (‘Odefect’) and surface hydroxyls (‘OH’), respectively. (d) CO2−TPD spectra for the reduced In2O3, ZrO2 and In–Zr oxides (thermal treatment in Ar at 400 ºC for 1 h).

Density functional theory (DFT) calculations revealed that In2O3 was a unique catalyst in CO2 activation and hydrogenation to methanol with its surface oxygen vacancy and that the reaction followed a mechanism comprising the cyclic creation and annihilation of oxygen vacancies.30,31 Surface Zr dopants were predicted to prefer to substitute the In4a and In4b atoms (Figure S3a–d in the Supporting Information). Additionally, based on previous DFT studies, oxygen vacancy at the O3a (D3) or O4a (D4) site may be able to catalyze the CO2 hydrogenation reaction.28,30 CO2 chemisorption energies at the different surface oxygen vacancy sites (D3 and D4) on the pure In2O3 and Zr-doped In1–xZrxOy surfaces are shown in Figure S3e,f in the Supporting Information and Figure 3a, b, respectively. For the defect surface, CO2 chemisorption at the D3 and D4 sites resulted in the formation of a metalcarbon bond. In addition, for the In1–xZrxOy surface, CO2 chemisorption at the oxygen vacancy site near the Zr dopant was much stronger than that at the oxygen vacancy site of pure In2O3, for example, the presence of the Zr dopant leads to stronger CO2 adsorption by ~1.1 eV at the D4 site (Figure 3a, b). As shown in Figure 3c, other intermediates were also significantly stabilized during CO2 hydrogenation, especially the methoxy (CH3O*) species adsorbed at the defect-free surface (P), which was the least stable among the various intermediates for the reaction on the pure In2O3 surface. Therefore, the incorporation of Zr benefitted the formation of methanol from CO2 hydrogenation.


ACS Catalysis

Figure 3. CO2 adsorption energy at the O4a surface oxygen vacancy (D4) site on (a) the pure In2O3 and (b) Zr doped In1–xZrxOy surfaces. (c) Energy profiles of CO2 hydrogenation to form CH3OH on the In2O3 and In1–xZrxOy surfaces shown in black and red, respectively. D and P stand for defective and perfect surfaces with and without the oxygen vacancy, respectively. DFT calculations revealed that oxygen-defective In2O3 can be created through direct thermal desorption or exposure to reducing agents.30,31 However, the treatment of fresh In2O3 in H2 at 300 oC resulted in a significant decrease in surface area.24 Therefore, In2O3 pretreated in Ar can yield more active sites than In2O3 activated in H2. In situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) of In–Zr oxide surface suggested an increase in the amount of surface oxygen defects detected from oxygen atoms adjacent to the defects (Odefect, 22.3%, Table S2 in the Supporting Information; see also Figure 1c, 531.4 eV) upon exposure to Ar at 400 ºC. The Odefect concentration of In–Zr oxide was higher than the sum of the individual contributions from pure ZrO2 and In2O3 (Table S2 in the Supporting Information). Odefect concentration dropped remarkably in CO2 at 400 ºC, while H2 benefited the regeneration of the oxygen vacancies via hydrogenation, thereby, sustain the catalytic cycle (Figure 1c). The CO2 temperature programmed desorption (CO2−TPD) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurement also confirmed that CO2 could be activated over such an In2O3 surface with oxygen vacancies. Several signals in the temperature range of 50−250 and 400−600 ºC were observed from CO2−TPD profiles and a higher temperature peak around 690 ºC only appeared for the In−Zr sample (Figure 1d). The peaks at 400−600 ºC originated from desorption of CO2 that interacted strongly with oxygen vacancies and the peak at approximately 690 oC for In–Zr oxide was attributed to the surface oxygen vacancy site near the Zr dopant based on our DFT calculations. In addition, the area of the high temperature peak was larger than the sum of ZrO2 and In2O3, similar to the trend of Odefect signal (Table S2 in the Supporting Information). These re-

sults indicated that the incorporation of Zr into In2O3 created new kinds of vacancies with high concentration, consistent with our DFT calculations, which progressively enhanced the reaction rate. As shown in Figure S4 in the Supporting Information, several bands at 900−1100 cm–1 were visible in the DRIFT spectrum collected upon CO2 adsorption over both In2O3 and In–Zr oxides activated in Ar at 400 ºC, which were assigned to the adsorbed CO2 bridging two In atoms around the oxygen vacancy sites and was blue-shift with the incorporation of Zr due to stronger CO2 adsorption.24,32 In addition, the region between 1200 and 1700 cm–1 contained a large variety of different bands corresponding to carbonate species.33,34 These findings are in agreement with our DFT results. Moreover, the adsorbed species are relatively stable and cannot be removed completely by He treatment even up to 350 ºC (Figure S4b in the Supporting Information), while the intensities decreased significantly at 200 ºC in the presence of H2 due to the formation of hydrogenated species (Figure S4c in the Supporting Information). CH4

100


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=

=

C2 -C4

o

o

C2 -C4

C5+

90 80 70 60 50 40 30 20 10 0

1 -0. Ar

a a a a a a MP MP MP MP MP MP 0 1 0 . . .0 . .0 1 0 3 3 2 H2 H2 H2 H2 Ar

Figure 4. Influence of the atmosphere and H2 pressure on the product selectivity for the conversion of methanol over the SAPO-34 catalyst. Reaction conditions: catalyst, 1.0 g; Ar or H2, 0.1−3.0 MPa; 150 mL min−1; liquid CH3OH, 0.003 mL min−1; time on stream, 15 h. As far as the product selectivity was concerned, only about 1% of CH3OH and 1% of hydrocarbons were obtained with either In–Zr oxide or bulk In2O3 as the sole catalyst for CO2 hydrogenation at 400 ºC, and the main product was CO with much high CH4 selectivity in the hydrocarbon distribution. It is worth noting that methanol formation from CO2 hydrogenation is restrained at high temperature due to its exothermic character,35,36 and the equilibrium selectivity to CH3OH is only 0.5% at 400 ºC (Figure S5 in the Supporting Information). When the metal oxides were combined with SAPO-34 zeolites, CO selectivity decreased from 97% to 85% and the reaction rate per gram of In and C2=−C4= selectivity increased remarkably because of the thermodynamic driving force. The obtained hydrocarbons distribution from CO2 hydrogenation over the oxide/zeolite catalyst was similar to


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that derived from MTO over the sole SAPO-34 zeolite used in the bifunctional catalyst (Figure 4). Moreover, the CO2 conversion can be tuned by varying the mass ratio of In−Zr oxides to SAPO-34 zeolites (Figure 2). Therefore, it can be concluded that the oxide/zeolite composite catalyst separates CO2 activation and C−C coupling onto different sites with complementary and compatible properties. During CO2 hydrogenation, CO2 is activated on oxide surface with oxygen vacancies and hydrogenated to


a

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 5. Catalytic performance for CO2 hydrogenation. CO2 conversion and hydrocarbon distribution at different (a) H2/CO2 ratios and (b) space velocities. (c) Stability test of composite In–Zr/SAPO-34 catalyst. Reaction conditions: 400 ºC, 3.0 MPa, 9000 mL gcat−1 h−1, H2/CO2/N2 = 73/24/3 and oxide/zeolite mass ratio = 2. The formation of CO by the RWGS reaction is reported in Table S4 in the Supporting Information.

intermediate species (methanol), while C−C coupling is controlled within the pores of SAPO-34. 3.2 Catalytic performance over bifunctional catalysts In−Zr/SAPO-34 catalyst exhibited C2=−C4= selectivity of 76.4% in hydrocarbons and CH4 selectivity was only 4.3% with CO2 conversion of 35.5% at 400 °C. The effects of the H2/CO2 ratio and reaction pressure were explored. As shown in Figure 5a and Figure S6a in the Supporting Information, CO2 conversion rose significantly with increasing H2/CO2 ratio (from 22.1% at H2/CO2 of 1.0 to 44.0% H2/CO2 of 5.0) and pressure (from 29.5% at 1.0 MPa to 37.9% at 5.0 MPa). CO selectivity was above 80% under different these conditions. However, higher H2/CO2 ratio and pressure decreased C2=−C4= selectivity and increased selectivities to CH4 and C2o−C4o gradually, indicating that the high H2 partial pressure was not favorable for the formation of C2=−C4=. In order to further reveal the influence of H2 partial pressure, the catalytic performance of MTO was investigated in the presence of H2 with different H2 pressures. Under the Ar atmosphere typically used for the MTO reaction, the C2=−C4= selectivity reached around 80% over SAPO-34 at 0.1 MPa, and it decreased slightly to 76% with Ar pressure rose to 3.0 MPa (Figure 4). The C2−C4 olefin selectivity maintained 80% when the reaction atmosphere switched from Ar to H2 at 0.1 MPa, whereas it decreased significantly and C2−C4 paraffins selectivity increased sharply with increasing H2 pressure due to the second hydrogenation of the olefins over the acid sites of SAPO-34 (Figure 4).37 Consequently, decreasing the H2/CO2 ratio and pressure favors the formation of lower olefin. In addition, the selectivity of

lower olefin can be further increased from 68% to 84% when the space velocity was changed from 4500 to 15750 mL gcat−1 h−1, and the selectivities of CH4 and C5+ were largely suppressed (Figure 5b). The C–C coupling from methanol is thermodynamically more favorable at high temperature, and 400−450 ºC is optimal for methanol to C2−C4 olefins over SAPO-34.27,29,38 However, methanol synthesis is thermodynamically restrained at such high temperature (Figure S5 in the Supporting Information). Two parallel reactions, methanol synthesis and RWGS, occur simultaneously during the CO2 hydrogenation process. The RWGS reaction is favored at higher temperature, since it is an endothermic reaction.12,39,40 Meanwhile, it is kinetically favored under methanol synthesis conditions for CO2 hydrogenation. The reaction temperature is usually relative low (200−300 º C) over methanol synthesis catalysts. Although the coupling with the MTO reaction can derived the CO2 conversion, the zeolite is not active for the C–C coupling reaction at such low temperature. The mismatch of their reaction temperatures results in the production of a large amount of CO. Therefore, one of the key challenges in the selective formation of lower olefins from CO2 hydrogenation is how to suppress CO formation. Theoretical equilibrium selectivity of CO expected on the basis of thermodynamic analysis of CO2 hydrogena-


ACS Catalysis tion to methanol is as high as 98.5−99.7% with CO2 conversion of 31.7−39.9% at 360−420 ºC (Figure S5 in the Supporting Information). Thus, it is necessary to investigate the effect of reaction conditions on CO selectivity.

With the reaction temperature decreasing from 420 to o 360 C, CO selectivity decreased sharply from 91% to 51% and these values are far from the equilibrium values, and the selectivity of C2−C4 olefins increased from 68% to 86% (Figure 6a). In addition, higher H2/CO2 ratio and pressure also led to significantly reduced CO selectivity (Table S4 and Figure S6 in the Supporting Information). Furthermore, the CO selectivity can be greatly suppressed by adding CO into the feed gas (Figure 6b). With increasing CO concentration to 14.4%, CO selectivity was only 20.5%, indicating that a larger fraction of the available carbon resource in the CO2/H2 mixture has been converted into hydrocarbons. The activity for CO2 hydrogenation to lower olefins and the selectivity of C2−C4 olefins remained almost unchanged with the increasing CO concentration in the feed gas.


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press the undesired RWGS reaction. Our DFT calculations suggested that the intermediates involved in methanol synthesis were more stable on the oxygen vacancy site near the Zr dopant than those on the oxygen vacancy site of pure In2O3, which strongly suppressed CO formation. As shown in Table S5 and Figure S6c in the Supporting Information, CO selectivity for both CO2 hydrogenations to methanol and lower olefins declined with the incorporation of Zr, which agrees well with our DFT calculations. Furthermore, CO selectivity for the bifunctional catalyst can be significantly suppressed from 84% to 70% at 400 o C, and 64% to 47% at 380 oC when the In–Zr oxide was further modified by the Zn promoter (Figure S6c in the Supporting Information). It is noteworthy that the In–Zr/SAPO-34 sample exhibited an excellent catalytic stability and no obvious decline in activity (CO2 conversion ~35%) was observed after a time-on-stream of 152 h (Figure 5c). In addition, CH4 and C2=−C4= selectivities remained stable around 4.5% and 77%, respectively. However, In2O3/SAPO-34 encountered severe deactivation after 152 h, for example, the CO2 conversion decreased from 34.6% to 27.7% and CH4 selectivity increased from 2.0% to 6.0% (Figure S6b in the Supporting Information). As mentioned above, the partially reduced oxide and SAPO-34 zeolite were responsible for the activation of CO2 to methanol and the selective C−C coupling, respectively. The metal oxide nanoparticles are susceptible to agglomeration during reaction at high temperature, which is the primary factor for severe deactivation and low stability.24,41-43 In situ XRD characterization revealed that the crystal size of sole In2O3 increased sharply from 14.8 to 22.5 nm after the exposure to the H2 and CO2 mixture at 400 ºC during the initial stage (8 h), and rose gradually in the following reaction time (Figure S7a in the Supporting Information). According to the TEM results of In2O3 /SAPO-34 catalysts in Figure S8 in the Supporting Information, much larger In2O3 nanoparticles with an average diameter up to 34.6 nm were present over the spent In2O3/SAPO-34 after 48 h of reaction compared with the fresh sample (12.2 nm), consistent well with the XRD results. Nevertheless, the mean particle size grew moderately for the In−Zr sample with increasing reaction time (Figure S7b in the Supporting Information), indicating that ZrO2 played a crucial role in preventing the sintering of the active nanoparticles during reaction process.

0 0.0

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3.3 The effect of integration manner

Figure 6. (a) CO2 conversion, CO selectivity and hydrocarbon distribution over the In–Zr/SAPO-34 catalyst at

various reaction temperatures; (b) Catalytic performance over In–Zr/SAPO-34 as a function of CO concentration in the feed at 380 oC. Reaction conditions: 3.0 MPa, 9000 mL gcat−1 h−1, H2/CO2/N2 = 73/24/3 and In–Zr/ SAPO34 mass ratio = 0.5. Aside from the influence of the reaction conditions, the modification of the defective oxide surface can also sup-

We further investigated the influence of integration manner of the active components on catalytic performance for CO2 hydrogenation to lower olefins. When SAPO-34 was packed below the oxide and separated by a layer of inert quartz sand (dual-bed configuration), the CH4 selectivity was above 55% and small amounts of C2=−C4= with the selectivity around 30% were formed due to the catalytic function of SAPO-34 in the downstream (Figure 7a). Then, we mixed the granules of oxides and zeolites together by shaking in a vessel to increase the


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proximity of the two components. The CH4 selectivity decreased significantly (< 5%) and C2=−C4= selectivity increased to above 75% (Figure 7b). Combined with the results that C2=−C4= selectivity increased gradually with increasing space velocities (Figure S6c in the Supporting Information), it follows that fast transport of reaction intermediates in the gas phase benefits the selective formation of olefins from CO2 hydrogenation. For granulestacking configuration, micrometer-sized particles were stacked together (Figure S9a−d in the Supporting Information).


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ACS Catalysis

0

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In2O3 InZr In2O3 InZr In2O3 InZr Dual-bed Granule-stacking Mortar-mixing

a

b

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Figure 7. Effect of the integration manner of the active components on catalytic performances. (a) Dual-bed configuration with SAPO-34 packed below In2O3 or In–Zr oxides, separated by a layer of quartz sand. (b) Stacking of granules with the In2O3, In–Zr oxides and SAPO-34 particle sizes of 250–380 μm. (c) Mixing of the two compounds in an agate mortar. All catalysts with oxide/zeolite mass ratio = 2 were evaluated under the same conditions shown in Figure 5. In order to further shorten the distance among the two components, the grinding powders of oxides and zeolites were mixed in an agate mortar. As shown in Figures S8, S9 in the Supporting Information, smaller oxide particles with a mean size of ~10 nm were observed to be attached closely on much bigger SAPO-34 particles of 2 μm in size. However, CO2 was converted mainly to CH4 with much lower CO2 conversion and CH3OH was detected, indicating remarkable deactivation of SAPO-34. Especially for In2O3/SAPO-34, the CH4 and C2=−C4= selectivities in hydrocarbons distribution were 98.2% and 0.5%, respectively, and CO2 conversion was only 18.2% (Figure 7c). Moreover, In–Zr/SAPO-34 exhibited much higher C2=−C4= selectivity (44.7%) and CO2 conversion (26.7%) compared with In2O3/SAPO-34, though overly close contact of bifunctional active sites decreased its performance for CO2 hydrogenation to lower olefins. The effect of integration manner on physiochemical properties for the bifunctional catalyst was investigated in details. It was found that the distance among the two components had no significant effect on textural and structural properties as well as surface acidity for fresh

samples. However, the amounts of strongly acidic sites for both spent In2O3/SAPO-34 and In–Zr/SAPO-34 catalysts prepared by mixing of two compounds in an agate mortar decreased remarkably after reaction for 48 h, especially for In2O3/SAPO-34 (Figure S10 in the Supporting Information). Solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy was employed to investigate the chemical environment of framework atoms (Si, Al and P) of fresh and spent SAPO-34 used in the bifunctional catalyst. Nearly no changes were observed for the 29Si, 27Al and 31P MAS NMR spectra of spent In2O3/SAPO-34 and In–Zr/SAPO-34 catalysts prepared by mortar-mixing configuration compared with the fresh sample, indicating that the reaction process had little or no effect on the silicoaluminophosphate frameworks of SAPO-34 (Figure S11 in the Supporting Information). In addition, the local elemental compositions of fresh and spent In2O3/SAPO-34 and In–Zr/SAPO-34 catalysts were analyzed by transmission electron microscopy and energy-dispersive X–ray spectrometry (Figures S12, S13 in the Supporting Information). In the SAPO-34 zeolite regions, the signal of the In element for the spent catalysts prepared by mortar-mixing configuration was much higher than that for the fresh samples, especially for In2O3/SAPO-34, indicating the presence of In within the zeolite crystal or in the close proximity to zeolite crystal after the reaction. Nevertheless, traces of In were detected in zeolite regions for spent In2O3/SAPO-34 and In– Zr/SAPO-34 catalysts prepared by granule-stacking configuration. Consequently, overly tight contact of bifunctional active sites resulted in migration of indium during the reaction followed by ion exchange of indium ions with zeolite protons, which decreased the number of strongly acidic sites significantly, leading to severe deactivation with very low C2=−C4= selectivity. 4 CONCLUSIONS In summary, we have discovered that a bifunctional catalyst contained In–Zr oxide and SAPO-34, which was responsible for the CO2 activation and the selective C–C coupling, respectively, could realize the direct production of lower olefins from CO2 hydrogenation with excellent selectivity and high activity. The selectivity of C2−C4 olefin reached up to around 80% with much low CH4 selectivity and CO2 conversion was above 35%. We demonstrated that the incorporation of zirconium significantly enhanced the activity by creating new kinds of vacancies, and remarkably improved the catalytic stability by preventing the sintering of the oxide nanoparticles. The precise control of the proximity of the two active sites also played an important role in the direct conversion of CO 2 to lower olefins. Additionally, the product distribution depended on the nature of the oxygen vacancies. The defective In2O3 surface could be modified to further increase the stability of the key intermediates involved in methanol formation, significantly suppressing the undesired RWGS reaction.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is

available free of charge on the ACS Publications website at DOI: xxxxxx. Details of the experiments and Tables S1-S5 and Figures S1-S13 as described in the text (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail for Liangshu Zhong: [email protected]; *E-mail for Yuhan Sun: [email protected] Author Contributions P.G., L.Z. and Y.S. conceived the project, analyzed the data and wrote the paper. P.G., S.D., S.L. and W.W. drafted the manuscript. S.D. and Z.L. prepared the samples. S.L. performed DFT calculations. S.D., X.B., M.Q. and C.Y. performed the catalytic evaluation. S.D., P.G., Y.H. and Q.L. characterized the samples. All authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work has been supported by the Ministry of Science and Technology of China (2016YFA0202802, 2017YFB0602202), the National Natural Science Foundation of China (21773286, 21503260, 91545112, 11227902), Shanghai Municipal Science and Technology Commission, China (16DZ1206900, 15DZ1170500), the Chinese Academy of Sciences (QYZDB-SSWSLH035), and Science and Technology Innovation Fund of Shanghai Advanced Research Institute, CAS (172001). REFERENCES (1) Olah, G. A.; Goeppert, A.; Prakash, G. K. S. J. Org. Chem. 2009, 74, 487-498. (2) Olah, G. A.; Prakash, G. K. S.; Goeppert, A. J. Am. Chem. Soc. 2011, 133, 12881-12898. (3) Banerjee, A.; Dick, G. R.; Yoshino, T.; Kanan, M. W. Nature 2016, 531, 215-219. (4) Studt, F.; Sharafutdinov, I.; Abild-Pedersen, F.; Elkjaer, C. F.; Hummelshoj, J. S.; Dahl, S.; Chorkendorff, I.; Norskov, J. K. Nat. Chem. 2014, 6, 320-324. (5) Moret, S.; Dyson, P. J.; Laurenczy, G. Nat. Commun. 2014, 5, 4017. (6) Zhong, L. S.; Yu, F.; An, Y. L.; Zhao, Y. H.; Sun, Y. H.; Li, Z. J.; Lin, T. J.; Lin, Y. J.; Qi, X. Z.; Dai, Y. Y.; Gu, L.; Hu, J. S.; Jin, S. F.; Shen, Q.; Wang, H. Nature 2016, 538, 84-87. (7) Jiao, 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.; Miao, S.; Li, J.; Zhu, Y. F.; Xiao, D.; He, T.; Yang, J. H.; Qi, F.; Fu, Q.; Bao, X. H. Science 2016, 351, 1065-1068. (8) Galvis, H. M. T.; Bitter, J. H.; Khare, C. B.; Ruitenbeek, M.; Dugulan, A. I.; de Jong, K. P. Science 2012, 335, 835-838.

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Table of Contents artwork


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Direct Production of Lower Olefins from CO2 Conversion via

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