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Sustainable production of light olefins from greenhouse gas CO2 over SAPO-34 supported modified cerium oxide
Microporous and Mesoporous Materials 297 (2020) 110029
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Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Sustainable production of light olefins from greenhouse gas CO2 over SAPO-34 supported modified cerium oxide Mostafa Ghasemi a, Majid Mohammadi b, Mehdi Sedighi c, d, * a
Chemical Engineering Section, Sohar University, Sohar, 311, Oman Department of Energy Engineering, Faculty of Science, Qom University of Technology, Qom, Iran c Division of Energy Systems, Department of Chemical Engineering, University of Qom, Qom, Iran d Center of Environmental Research, University of Qom, Qom, Iran b
A R T I C L E I N F O
A B S T R A C T
Keywords: CO2 hydrogenation NiCu/CeO2-SAPO-34 catalyst Light olefins Temperature Space velocity Ratio of H2/CO2
The direct supply of light olefins from CO2 hydrogenation has led to a tremendous interest in its important roles in reducing CO2 emissions. We study here a significantly effective, reliable and multifunctional catalyst, NiCu/ CeO2-SAPO-34, capable of directly converting CO2 to light olefins with selectivity up to 76.6% (C2H4 ¼ 22.7%, C3H6 ¼ 35.5%, and C4H8 ¼ 18.4%), while only 2.1% CH4 with CO2 conversion of 15.3% at H2/CO2 of 3, 12 L. gcat1 h 1, 375 � C and 20 bar. Under optimum reaction conditions, the CO selectivity is lower than 65%. Physi ochemical characterization of the catalyst was performed using BET, NH3-TPD, H2-TPR, XRD, TEM, and SEM techniques. Compared to the XRD patterns of SAPO-34 and NiCu/CeO2, the composite showed all characteristic XRD peaks of both samples. In addition, our designed hybrid catalyst also has beneficial catalytic stability, which can operate for 90 h without loss of apparent activity.
1. Introduction Carbon dioxide is the important pollutant that is recently causing an alarming level of environmental degradation. In particular, CO2 is not just a greenhouse gas, but can certainly be utilized as a carbon source to produce a range of high quality products such as light olefins [1–7]. Lower olefins (C2H4 and C3H6) are the chief petrochemical building blocks used to manufacture a host of chemicals [8–13]. However, carbon dioxide is definitely stable molecule caused by the double bonds be tween carbon as well as oxygen, making it chemically inert. Accord ingly, the activation of CO2 is apparently a significant advance in the use of carbon dioxide. Regarding the mechanisms for the CO2 hydrogenation to light olefins, two reaction pathways are generally recognized: MTO (methanol-mediated reaction) and modified Fischer-Tropsch synthesis (CO-mediated reaction). In the modified Fischer-Tropsch synthesis method, CO2 is first converted to carbon monoxide by a reverse water –gas shift reaction (RWGS), and then hydrogenated to lower olefins by a modified F-T synthesis method [14–19]. In the process using methanol, CO2 and H2 are taken up on a hybrid catalyst to synthesize methanol and then converted to light olefins [20–25]. The olefin selectivity of on the modified FT catalysts is mostly less than 60%, as the Anderson-Schulz-Flory distribution is limited [26]. We expect the
methanol-mediated process (MTO) to overcome such a limitation with a bifunctional catalyst. In InZr-SAPO-34, olefins selectivity reached 74.5% with CO2 conversion of 26.2% at space velocity of 9 L g 1h 1, 380 � C and 20 bar [20]. Using ZnZr-SAPO, the CO2 conversion of 13% with the selectivity of 80% was achieved at temperature of 380 � C as well as 20 bar [27]. The introduction of metals such as nickel-coupled copper is another way of synthesizing catalysts for the CH3OH synthesis from CO2 [28,29]. Because of their unique properties, transition metal are often applied in heterogeneous catalytic process compared to pure transition metals [30]. Ni–Cu alloys are presently of notable interest for several chemical process, such as CH4/C2H5OH/C2H6O steam reforming, CH4 hydroge nation and decomposition processes using carbon monoxide, carbon dioxide and C4H6O4, etc [29,31–34]. In the meantime, it has been documented that using metal oxides (Al2O3, CeO2, MnO2 and La2O3) as supports enhances catalytic selectivity as well as activity [30,35–38]. For example, CeO2 supported Ni catalyst indicated high selectivity (99%) due to the modified electronic property of Ni with CeO2 through the strong interaction between metal and support [36]. It is known that the formation of C–C bonds, such as lower olefins, can take place on a zeolitic catalyst during the conversion of methanol [39–42]. Zeolites are already widely utilized as definitely beneficial
* Corresponding author. Division of Energy Systems, Department of Chemical Engineering, University of Qom, Qom, Iran. E-mail addresses: [email protected], [email protected] (M. Sedighi). https://doi.org/10.1016/j.micromeso.2020.110029 Received 21 September 2019; Received in revised form 3 January 2020; Accepted 13 January 2020 Available online 16 January 2020 1387-1811/© 2020 Elsevier Inc. All rights reserved.
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adsorbents, catalysts and ion exchangers in various industries [43–51]. Among these zeolite, SAPO-34 has attracted excellent catalytic perfor mance due to its special framework with a CHA cage (9.4 Å) [52], as well as its mild acidity [43,46,53], which allows it to attract excellent cata lytic performance [39,40,54]. In this work, we design for the first time a highly efficient hybrid catalyst of NiCu/CeO2 and SAPO–34 zeolite, which converts CO2 directly into lower olefins. In this bifunctional catalyst, the methanol produced on the surface of NiCu/CeO2 can be converted to lower olefins in situ on the porous SAPO-34 zeolite. The influence of operating con ditions, consisting of gas space velocity (SV), reaction temperature and H2/CO2 ratio for conversion of CO2 into lower olefins has been exten sively examined. Furthermore, the stability of hybrid catalysts was measured under the best situations.
laboratory scale [60]. The catalyst performance was analyzed in a fixed-bed reactor (L ¼ 45 cm, I.D. ¼ 1.51 cm). Typically, 1.0 g of catalyst was put into the reactor. The catalyst was pretreated for 3 h at 400 � C in pure Ar. The temperature was then increased to the desired temperature to begin the reaction. A gas mixture including CO2, H2 and N2 at a specific stoichiometric ratio was fed into the reactor. The products were analyzed using Varian Chrompack CP3800 gas chromatograph equipped with a TCD and a FID detector. The activity tests were performed at different temperatures between 350 and 450 � C, gas space velocity varied from 2 to 12 Lgcat1 h 1, the H2/CO2 ratio was in the range of 1–5, the pressure 20 bar and the reaction time ¼ 10 h. The conversion as well as selectivity are defined as follows: CO2 inlet CO2outlet � 100% CO2inlet
(1)
CO outlet � 100% CO2 inlet CO2outlet
(2)
CO2 Conversion ¼
2. Experiment
CO Selectivity ¼
2.1. Catalyst synthesis
Cn Hm Selectivity ¼
2.1.1. NiCu/CeO2 synthesis In a special preparation of NiCu/CeO2, the catalyst is synthesized by the co-precipitation process. First, 12 mmol Ce(NO3)3.6H2O was dis solved in distilled water (DW). Then, 15% KOH (w/w) and 0.05 M Ni (NO3)2 and Cu(NO3)2 solutions were applied simultaneously at 85 � C with steady stirring. The solution pH was kept at 11. The obtained precipitate was digested for 36 h with stirring at 80 � C. The obtained solid product was washed properly with DW repeatedly to remove the potassium contamination, then air-dried at 110 � C for 10 h and then calcined at 600 � C for 5 h.
CO2inlet
nCn Hmoutlet � 100% CO2outlet CO outlet
(3)
3. Results and discussion 3.1. Catalysts characterizations Fig. 2 illustrates the X-ray diffraction pattern of various samples after calcination at 550 � C and measurement at room temperature. The diffraction peaks at 28.5, 33.2, 46.8, as well as 56.4 indicate the exis tence of the fluorite type structure of ceria in all samples [61–63]. In addition, in the NiCu/CeO2 oxide sample, CuO (2 theta ¼ 35.3 and 39.1) and NiO (2 theta ¼ 37.2 and 43.1) phase characteristics were also detected. This indicates the presence of bulk oxides of both CuO as well as NiO in the sample synthesized by the co-precipitation method. The XRD patterns of the as-synthesized SAPO-34 verified the chabazite structure [52]. It can be seen that the structure has the peak positions at 2 theta ¼ 9.6, 13.1, 16.1, 20.5, 24.9, 25.9, 30.7, as well 31.2� . Compared to the XRD patterns of SAPO-34 and NiCu/CeO2, the composite showed all characteristic XRD peaks of both samples. It shows that the binary structure of NiCu–CeO2/SAPO-34 composite could be synthesized for physical blending synthetic method. Table 1 shows the crystallinity of all catalysts. The relative crystallinity of the samples was measured via the sum of the intensity of the more important peaks based on the reference intensities. The crystallinity of the modified SAPO-34 (87%) is lower than that of as-synthesized SAPO-34 (91%), which can be caused by a reduction in surface smoothness and surface area (Table 1). The specific surface area as well as the pore volume of CeO2 and NiCu/CeO2 are 57 (m2g 1), 0.14 (cm3g 1) and 38 (m2g 1), 0.09 (cm3g 1), respec tively. The BET surface area of SAPO-34 (427 m2g-1) was decreased by NiCu/CeO2 blending, which may be due to the clogging of some chan nels and pores of the unmodified catalyst. Fig. 3a shows the distribution and dispersion of the NiCu/CeO2 powder. The average particle size of the NiCu/CeO2 approximated by TEM was about 20 nm with a narrow distribution. Fig. 3b shows that the SAPO-34 sample has characteristic cubic crystals with a uniform size of about 2.5 μm. The synthesized SAPO-34 shows the typical morphology of the cubic-like rhombohedra, which is nearly the same as natural chabacite. The SEM images of NiCu/CeO2-SAPO 34 (Fig. 3c) shows a morphology other than SAPO-34, in which irregular crystals are observed on their surface. This suggests that the addition of NiCu/CeO2 significantly alters the regular morphology of SAPO-34, however, the structure of modified SAPO-34 is nearly identical to that of SAPO-34. In Fig. 4a, two broad H2-TPR peaks realized at 388 and 758 � C for CeO2 can be related to the reduction of surface capping oxygen as well as bulk oxygen, respectively [64,65]. In addition, a TPR profile of NiCu/ CeO2 sample synthesized by co-precipitation method is presented. The
2.1.2. Synthesis of SAPO-34 The crystalline SAPO-34 was hydrothermally produced with mor pholine and TEAOH as templates. The sources of Si, Al and P were Silicic acid (SiO2, Merck), aluminum isopropoxide (AIP, Merck), and phos phoric acid (85 wt% H3PO4, Merck), respectively. The composition of gel was 1Al2O3:1P2O5:0.6SiO2:0.5TEAOH, 0.5 MOR, 60H2O (molar basis). The full explanation of SAPO-34 preparation is in our earlier work [39–41,55–59]. As-synthesized product was then dried and calcined at 550 � C for 6 h. 2.1.3. Hybrid NiCu/CeO2-SAPO-34 The hybrid composite was prepared by physical mixing with the mass ratio of NiCu/CeO2: SAPO-34 ¼ 1:2. The resulting sample was then crushed and screened to a size of 20–40 mesh and assigned as NiCu/ CeO2-SAPO-34. 2.2. Catalyst characterization techniques XRD Patterns were recorded with Cu Kα radiation (λ ¼ 1.5406 � A) on a Bruker D8 Advance instrument. The particle size of catalysts were determined by TEM on a JEOL, JEM-2200FS electron microscope. The microscopic morphology of the sample was monitored by SEM, AIS2100 (Seron Technology, South Korea). The specific surface area was studied according to the Brunauer-Emmett-Teller (BET) equation using the multi-point method (Quantachrome ChemBET-3000). H2-TPR mea surements were conducted on a Micromeritics AutoChem II 2920 in strument in a H2–Ar mixture. Before the reduction, the catalyst was pretreated in an air stream at 400 � C for 2 h and then cooled to 50 � C. The H2-TPR experiment was then started by raising the temperature to 800 � C at a rate of 10 � C/min. The surface acidity of the catalysts was determined on a Micromeritics 2000 adsorption equipment using NH3TPD. 2.3. Catalytic performance Fig. 1 provides a schematic representation of the layout on a 2
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Fig. 1. Schematic flow diagram of the experimental setup for CO2 hydrogenation to light olefins [60].
amorphous CuO, weakly interacting with CeO2 support and to the reduction of CuO, strongly interacting with the support. However, the shoulder peak above 200 � C was related to the greatly dispersed NiO reduction [31,53,66–68]. Fig. 4b shows the acidity of the catalyst using NH3-TPD. The strong acid sites are generally directly related to olefin production [33–35]. The desorption curve shows two typical peaks corresponding to weak (at 120–260 � C) and strong acid sites (at 320–480 � C). The modified SAPO-34 has a low acidity compared to SAPO-34. 3.2. Catalytic activity The concept of multifunctional catalysis was established by Weisz [69]. The synthesized hybrid catalysts, which combine two separate types of active sites for the single step carbon dioxide to light olefins, was generally prepared by the simple powder blending method and used for this reaction. NiCu/CeO2 catalyst was used to synthesize methanol and SAPO-34 zeolite for MTO. Typically, this process demands two in dependent reactors in which the CH3OH is achieved in the first reactor, and the light olefins can be produced from CH3OH on zeolite in the second reactor. Alternatively, the methanol can be simultaneously synthesized in the single reactor and further dehydrated to lower olefins [20,22,27,70–72]. This process offers a more thermodynamic and economical benefit than a two-step process, since the thermodynamic limit for methanol product is weakened, which can change the equi librium to higher CO2 conversion.
Fig. 2. XRD patterns of the samples CeO2, NiCu/CeO2, SAPO-34 and NiCu/ CeO2-SAPO-34. Table 1 Physicochemical properties of all samples. Sample
SBET (m2g 1)
Vpore (cm3g 1)
Average pore diameter (nm)
Crystallinity (%)
CeO2 NiCu/CeO2 SAPO-34 NiCu/CeO2SAPO-34
57 38 427 308
0.14 0.09 0.36 0.25
9.45 6.73 0.47 4.92
94 89 91 84
reduction temperatures of NiCu/CeO2 samples are lower than that of pure CeO2. The NiCu/CeO2 sample shows reduction peaks in the range of 120–390 � C with complete reductions of CuO as well as NiO to metallic phase. The peak below 200 � C was attributed to the reduction of
CO2 þ 3H2 →CH3 OH þ H2 O ðCO2 to methanolÞ
(4)
nCH3 OH → Cn H2n þ nH2 O ðMTOÞ
(5)
Here we examine the various factors, including the reaction tempera ture, space velocity as well as H2/CO2 ratio, which affected catalytic 3
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Fig. 3. (a) TEM image of NiCu/CeO2, (b) SEM image of SAPO-34, and (c) SEM image of modified SAPO-34.
Fig. 4. (a) H2-TPR profile and (b) NH3-TPD curve.
(C2–C04) and C5þ fractions are also promoted by raising the temperature from 375 to 450 � C. Due to the CO2 conversion, selectivity of olefins and CH4, it is believed that 375 � C is the appropriate temperature for the synthesis of light olefins from carbon dioxide on a NiCu/CeO2-SAPO-34 catalyst.
activity. 3.2.1. Effect of reaction temperature It has been found that temperature is a more important factor affecting the product with lower olefin selectivity. The NiCu/CeO2SAPO-34 catalyst has been studied at several reaction temperatures of 350, 375, 400, 425 as well as 450 � C at pressure of 20 bar; space velocity of 12 Lgcat1h 1; H2/CO2 molar ratio of 3 and process time is set to 10 h. The conversion of CO2 seems to increase significantly with reaction temperature. The CO selectivity as the function of the reaction tem perature accompanies the identical pattern for the conversion of carbon dioxide. As the temperature decreased from 450 to 375 � C, the CO selectivity decreased significantly from 85.87% to 64.27% (Fig. 5a), but the C2–C¼ 4 selectivity increased from 60.14% (C2H4 ¼ 18.6%, C3H6 ¼ 26.4%, and C4H8 ¼ 15.1%) to 76.66% (C2H4 ¼ 22.7%, C3H6 ¼ 35.5%, and C4H8 ¼ 18.4%) (Fig. 5b) and the selectivity for CH4 decreased slightly from 5.2% to 2.1% (Fig. 5b). The formation of light paraffins
3.2.2. Effect of SV The effect of space velocity on the hybrid catalyst of NiCu/CeO2 -SAPO-34 is explained in Fig. 6. The temperature of 375 � C; pressure of 20 bar; H2/CO2 molar ratio of 3 and the reaction time is set to 10 h. At SV of 2 Lg-1 h 1, the conversion of CO2 is 18.10% (Fig. 6a) and the olefin selectivity is 64.79% (C2H4 ¼ 19.2%, C3H6 ¼ 30%, and C4H8 ¼ 15.6%) (Fig. 6b). By increasing the SV from 2 to 5 Lg-1 h 1, the CO2 conversion is reduced to 17.33% and the olefin selectivity is relatively improved to 69.36%. Further increasing the SV from 8 to 12 Lg-1 h 1 significantly reduces the CO2 conversion to 15.27% and the selectivity to olefin (C2–C¼ 4 ) is slightly higher, 76.66% (C2H4 ¼ 22.7%, C3H6 ¼ 35.5%, and 4
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0 Fig. 5. Effect of different temperature on (a) CO2 conversion and CO selectivity, (b) olefins (C2–C¼ 4 ), alkanes (C2–C4), CH4 and C5þ selectivity; P ¼ 20 bar, SV ¼ 12 Lgcat1 h 1, H2/CO2 ¼ 3:1 and reaction time ¼ 10 h.
0 � Fig. 6. Effect of different space velocity on (a) CO2 conversion and CO selectivity, (b) olefins (C2–C¼ 4 ), alkanes (C2–C4), CH4 and C5þ selectivity; T ¼ 375 C, P ¼ 20 bar, H2/CO2 ¼ 3:1 and reaction time ¼ 10 h.
C4H8 ¼ 18.4%). When the SV increases from 2 to 12 Lg-1 h 1, the CO selectivity decreases slightly from 68.94% to 64.27% (Fig. 6a). The higher SV results in a shorter reaction time of the CO2 and H2 molecules on the catalyst surface. Therefore, the CO2 conversion is low; however, the light olefin formation is somewhat high. This may be caused by the production of CH3OH immediately reacted with SAPO-34 zeolite,
resulting in more olefin formation. Consequently, the high olefin selectivity was achieved at higher SV of 12 Lg-1 h 1. 3.2.3. Effect of ratio of H2/CO2 To study this effect, the temperature of 375 � C; reaction pressure of 20 bar; space velocity of 12 L g 1 h 1 and reaction time is set to 10 h. At
0 � Fig. 7. Effect of different H2/CO2 ratio on (a) conversion of CO2 as well as selectivity of CO, (b) olefins (C2–C¼ 4 ), alkanes (C2–C4), CH4 and C5þ selectivity; T ¼ 375 C, SV ¼ 12 Lgcat1 h 1, P ¼ 20 bar and reaction time ¼ 10 h.
5
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the H2/CO2 ¼ 1, the conversion of CO2 is only 6.55% (Fig. 7a) and olefin selectivity is 79.85% (Fig. 7b). As the H2/CO2 ratio increases from 2 to 3, the CO2 conversion increases to 15.27% and the selectivity to olefins is reduced slightly to 76.66% (C2H4 ¼ 22.7%, C3H6 ¼ 35.5%, and C4H8 ¼ 18.4%). As the H2/CO2 ratio is further increased from 3 to 5, the CO2 conversion increases to 23.53%, but the selectivity to olefins is reduced to 69.87%. In addition, the selectivity of methane and paraffin increase to 4.40% and 23.12%, respectively. According to Eqs. (4) and (6), we conclude that as the H2/CO2 ratio decreases from 3 to 1, the reaction Eq. (6) proceeds further, which increases CO selectivity: CO2 þ H2 →CO þ H2 O
(6)
As the H2/CO2 ratio increases from 3 to 5, CH4 selectivity also in creases. Since the methanol produced reacts with additional amounts of H2, it produces CH4, thus reducing the selectivity of olefins: CH3 OH þ H2 ↔ CH4 þ H2 O
(7)
The above results have shown that the H2/CO2 ratio of 3 is suitable. Therefore, it can be stated that the optimum reaction conditions are T ¼ 375 � C, SV ¼ 12 L g 1 h 1, H2/CO2 ¼ 3 and P ¼ 20 bar.
Fig. 8. Stability test at 375 � C and 20 bar with SV of 12 L gcat1 h 1.
the work reported in this paper.
3.2.4. Catalyst stability Fig. 8 indicates the long-term test of 100 h on the NiCu/CeO2-SAPO34 hybrid catalyst at the optimum reaction conditions (T ¼ 375 � C, P ¼ 20 bar, H2/CO2 ¼ 3, SV ¼ 12 L g 1 h 1). It is noteworthy that the catalyst has good stability over 90 h on stream without obvious deac tivation. The conversion of CO2 was maintained at about 15% during 90 h of continuous reaction. In addition, methane and olefin selectivities remained stable by 2.6% and 76%, respectively. Also, it was found that the NiCu/CeO2-SAPO-34 deactivated slowly after 90 h, for example, the carbon dioxide conversion was decreased from 15% to 10% and the olefin selectivity decreased from 76% to 70%. These data show that the developed catalysts are particularly stable and selective for hydroge nation of CO2 to produce lower olefins.
CRediT authorship contribution statement Mostafa Ghasemi: Conceptualization, Data curation, Writing original draft. Majid Mohammadi: Data curation, Investigation, Methodology. Mehdi Sedighi: Methodology, Supervision, Writing - re view & editing. Acknowledgements The authors wish to thank the laboratory unit of the University of Alberta, Canada and the Central Laboratory of Amirkabir University of Technology, Iran for characterization techniques. Appendix A. Supplementary data
4. Conclusion
Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2020.110029.
For the direct formation of lower olefins from CO2 hydrogenation, we synthesized a hybrid catalyst composed of NiCu/CeO2 and SAPO–34 zeolites to convert CO2 directly into light olefins. Various factors influ encing the catalytic activity, including the gas space velocity (SV), the reaction temperature, the H2/CO2 ratio and the stability test were investigated in detail. It was found that the optimum reaction conditions were T ¼ 375 � C, P ¼ 20 bar, H2/CO2 ¼ 3, SV ¼ 12 L g 1 h 1. Under optimal conditions, the selectivity for C2–C¼ 4 olefins was up to 76.6% (C2H4 ¼ 22.7%, C3H6 ¼ 35.5%, and C4H8 ¼ 18.4%) and the selectivity for paraffin (C2–C04) and methane was 18.3% and 2.1%, respectively, with less than 65% carbon monoxide at the conversion of 15.3% CO2. By raising the reaction temperature, the CO2 conversion is improved; however, the light olefin selectivity is reduced. Raising the gas space velocity reduces the CO2 conversion, and the light olefin selectivity is increased. Increasing the H2/CO2 ratio from 1 to 3, the conversion of CO2 increases to 15.27% and the selectivity to olefins decreases slightly to 76.66% (C2H4 ¼ 22.7%, C3H6 ¼ 35.5%, and C4H8 ¼ 18.4%). As the H2/CO2 ratio increases from 3 to 5, the conversion of CO2 increases to 23.53%, but the lower olefin selectivity decreases to 69.87%. In addi tion, the selectivity of methane and paraffin increase to 4.40% and 23.12%, respectively. Furthermore, our developed hybrid catalyst also provides beneficial catalytic stability, which can operate 90 h continu ously without apparent loss of activity; showing the potential industrial catalyst for CO2 utilization to light olefins.
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Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Sustainable production of light olefins from greenhouse gas CO2 over SAPO-34 supported modified cerium oxide Mostafa Ghasemi a, Majid Mohammadi b, Mehdi Sedighi c, d, * a
Chemical Engineering Section, Sohar University, Sohar, 311, Oman Department of Energy Engineering, Faculty of Science, Qom University of Technology, Qom, Iran c Division of Energy Systems, Department of Chemical Engineering, University of Qom, Qom, Iran d Center of Environmental Research, University of Qom, Qom, Iran b
A R T I C L E I N F O
A B S T R A C T
Keywords: CO2 hydrogenation NiCu/CeO2-SAPO-34 catalyst Light olefins Temperature Space velocity Ratio of H2/CO2
The direct supply of light olefins from CO2 hydrogenation has led to a tremendous interest in its important roles in reducing CO2 emissions. We study here a significantly effective, reliable and multifunctional catalyst, NiCu/ CeO2-SAPO-34, capable of directly converting CO2 to light olefins with selectivity up to 76.6% (C2H4 ¼ 22.7%, C3H6 ¼ 35.5%, and C4H8 ¼ 18.4%), while only 2.1% CH4 with CO2 conversion of 15.3% at H2/CO2 of 3, 12 L. gcat1 h 1, 375 � C and 20 bar. Under optimum reaction conditions, the CO selectivity is lower than 65%. Physi ochemical characterization of the catalyst was performed using BET, NH3-TPD, H2-TPR, XRD, TEM, and SEM techniques. Compared to the XRD patterns of SAPO-34 and NiCu/CeO2, the composite showed all characteristic XRD peaks of both samples. In addition, our designed hybrid catalyst also has beneficial catalytic stability, which can operate for 90 h without loss of apparent activity.
1. Introduction Carbon dioxide is the important pollutant that is recently causing an alarming level of environmental degradation. In particular, CO2 is not just a greenhouse gas, but can certainly be utilized as a carbon source to produce a range of high quality products such as light olefins [1–7]. Lower olefins (C2H4 and C3H6) are the chief petrochemical building blocks used to manufacture a host of chemicals [8–13]. However, carbon dioxide is definitely stable molecule caused by the double bonds be tween carbon as well as oxygen, making it chemically inert. Accord ingly, the activation of CO2 is apparently a significant advance in the use of carbon dioxide. Regarding the mechanisms for the CO2 hydrogenation to light olefins, two reaction pathways are generally recognized: MTO (methanol-mediated reaction) and modified Fischer-Tropsch synthesis (CO-mediated reaction). In the modified Fischer-Tropsch synthesis method, CO2 is first converted to carbon monoxide by a reverse water –gas shift reaction (RWGS), and then hydrogenated to lower olefins by a modified F-T synthesis method [14–19]. In the process using methanol, CO2 and H2 are taken up on a hybrid catalyst to synthesize methanol and then converted to light olefins [20–25]. The olefin selectivity of on the modified FT catalysts is mostly less than 60%, as the Anderson-Schulz-Flory distribution is limited [26]. We expect the
methanol-mediated process (MTO) to overcome such a limitation with a bifunctional catalyst. In InZr-SAPO-34, olefins selectivity reached 74.5% with CO2 conversion of 26.2% at space velocity of 9 L g 1h 1, 380 � C and 20 bar [20]. Using ZnZr-SAPO, the CO2 conversion of 13% with the selectivity of 80% was achieved at temperature of 380 � C as well as 20 bar [27]. The introduction of metals such as nickel-coupled copper is another way of synthesizing catalysts for the CH3OH synthesis from CO2 [28,29]. Because of their unique properties, transition metal are often applied in heterogeneous catalytic process compared to pure transition metals [30]. Ni–Cu alloys are presently of notable interest for several chemical process, such as CH4/C2H5OH/C2H6O steam reforming, CH4 hydroge nation and decomposition processes using carbon monoxide, carbon dioxide and C4H6O4, etc [29,31–34]. In the meantime, it has been documented that using metal oxides (Al2O3, CeO2, MnO2 and La2O3) as supports enhances catalytic selectivity as well as activity [30,35–38]. For example, CeO2 supported Ni catalyst indicated high selectivity (99%) due to the modified electronic property of Ni with CeO2 through the strong interaction between metal and support [36]. It is known that the formation of C–C bonds, such as lower olefins, can take place on a zeolitic catalyst during the conversion of methanol [39–42]. Zeolites are already widely utilized as definitely beneficial
* Corresponding author. Division of Energy Systems, Department of Chemical Engineering, University of Qom, Qom, Iran. E-mail addresses: [email protected], [email protected] (M. Sedighi). https://doi.org/10.1016/j.micromeso.2020.110029 Received 21 September 2019; Received in revised form 3 January 2020; Accepted 13 January 2020 Available online 16 January 2020 1387-1811/© 2020 Elsevier Inc. All rights reserved.
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adsorbents, catalysts and ion exchangers in various industries [43–51]. Among these zeolite, SAPO-34 has attracted excellent catalytic perfor mance due to its special framework with a CHA cage (9.4 Å) [52], as well as its mild acidity [43,46,53], which allows it to attract excellent cata lytic performance [39,40,54]. In this work, we design for the first time a highly efficient hybrid catalyst of NiCu/CeO2 and SAPO–34 zeolite, which converts CO2 directly into lower olefins. In this bifunctional catalyst, the methanol produced on the surface of NiCu/CeO2 can be converted to lower olefins in situ on the porous SAPO-34 zeolite. The influence of operating con ditions, consisting of gas space velocity (SV), reaction temperature and H2/CO2 ratio for conversion of CO2 into lower olefins has been exten sively examined. Furthermore, the stability of hybrid catalysts was measured under the best situations.
laboratory scale [60]. The catalyst performance was analyzed in a fixed-bed reactor (L ¼ 45 cm, I.D. ¼ 1.51 cm). Typically, 1.0 g of catalyst was put into the reactor. The catalyst was pretreated for 3 h at 400 � C in pure Ar. The temperature was then increased to the desired temperature to begin the reaction. A gas mixture including CO2, H2 and N2 at a specific stoichiometric ratio was fed into the reactor. The products were analyzed using Varian Chrompack CP3800 gas chromatograph equipped with a TCD and a FID detector. The activity tests were performed at different temperatures between 350 and 450 � C, gas space velocity varied from 2 to 12 Lgcat1 h 1, the H2/CO2 ratio was in the range of 1–5, the pressure 20 bar and the reaction time ¼ 10 h. The conversion as well as selectivity are defined as follows: CO2 inlet CO2outlet � 100% CO2inlet
(1)
CO outlet � 100% CO2 inlet CO2outlet
(2)
CO2 Conversion ¼
2. Experiment
CO Selectivity ¼
2.1. Catalyst synthesis
Cn Hm Selectivity ¼
2.1.1. NiCu/CeO2 synthesis In a special preparation of NiCu/CeO2, the catalyst is synthesized by the co-precipitation process. First, 12 mmol Ce(NO3)3.6H2O was dis solved in distilled water (DW). Then, 15% KOH (w/w) and 0.05 M Ni (NO3)2 and Cu(NO3)2 solutions were applied simultaneously at 85 � C with steady stirring. The solution pH was kept at 11. The obtained precipitate was digested for 36 h with stirring at 80 � C. The obtained solid product was washed properly with DW repeatedly to remove the potassium contamination, then air-dried at 110 � C for 10 h and then calcined at 600 � C for 5 h.
CO2inlet
nCn Hmoutlet � 100% CO2outlet CO outlet
(3)
3. Results and discussion 3.1. Catalysts characterizations Fig. 2 illustrates the X-ray diffraction pattern of various samples after calcination at 550 � C and measurement at room temperature. The diffraction peaks at 28.5, 33.2, 46.8, as well as 56.4 indicate the exis tence of the fluorite type structure of ceria in all samples [61–63]. In addition, in the NiCu/CeO2 oxide sample, CuO (2 theta ¼ 35.3 and 39.1) and NiO (2 theta ¼ 37.2 and 43.1) phase characteristics were also detected. This indicates the presence of bulk oxides of both CuO as well as NiO in the sample synthesized by the co-precipitation method. The XRD patterns of the as-synthesized SAPO-34 verified the chabazite structure [52]. It can be seen that the structure has the peak positions at 2 theta ¼ 9.6, 13.1, 16.1, 20.5, 24.9, 25.9, 30.7, as well 31.2� . Compared to the XRD patterns of SAPO-34 and NiCu/CeO2, the composite showed all characteristic XRD peaks of both samples. It shows that the binary structure of NiCu–CeO2/SAPO-34 composite could be synthesized for physical blending synthetic method. Table 1 shows the crystallinity of all catalysts. The relative crystallinity of the samples was measured via the sum of the intensity of the more important peaks based on the reference intensities. The crystallinity of the modified SAPO-34 (87%) is lower than that of as-synthesized SAPO-34 (91%), which can be caused by a reduction in surface smoothness and surface area (Table 1). The specific surface area as well as the pore volume of CeO2 and NiCu/CeO2 are 57 (m2g 1), 0.14 (cm3g 1) and 38 (m2g 1), 0.09 (cm3g 1), respec tively. The BET surface area of SAPO-34 (427 m2g-1) was decreased by NiCu/CeO2 blending, which may be due to the clogging of some chan nels and pores of the unmodified catalyst. Fig. 3a shows the distribution and dispersion of the NiCu/CeO2 powder. The average particle size of the NiCu/CeO2 approximated by TEM was about 20 nm with a narrow distribution. Fig. 3b shows that the SAPO-34 sample has characteristic cubic crystals with a uniform size of about 2.5 μm. The synthesized SAPO-34 shows the typical morphology of the cubic-like rhombohedra, which is nearly the same as natural chabacite. The SEM images of NiCu/CeO2-SAPO 34 (Fig. 3c) shows a morphology other than SAPO-34, in which irregular crystals are observed on their surface. This suggests that the addition of NiCu/CeO2 significantly alters the regular morphology of SAPO-34, however, the structure of modified SAPO-34 is nearly identical to that of SAPO-34. In Fig. 4a, two broad H2-TPR peaks realized at 388 and 758 � C for CeO2 can be related to the reduction of surface capping oxygen as well as bulk oxygen, respectively [64,65]. In addition, a TPR profile of NiCu/ CeO2 sample synthesized by co-precipitation method is presented. The
2.1.2. Synthesis of SAPO-34 The crystalline SAPO-34 was hydrothermally produced with mor pholine and TEAOH as templates. The sources of Si, Al and P were Silicic acid (SiO2, Merck), aluminum isopropoxide (AIP, Merck), and phos phoric acid (85 wt% H3PO4, Merck), respectively. The composition of gel was 1Al2O3:1P2O5:0.6SiO2:0.5TEAOH, 0.5 MOR, 60H2O (molar basis). The full explanation of SAPO-34 preparation is in our earlier work [39–41,55–59]. As-synthesized product was then dried and calcined at 550 � C for 6 h. 2.1.3. Hybrid NiCu/CeO2-SAPO-34 The hybrid composite was prepared by physical mixing with the mass ratio of NiCu/CeO2: SAPO-34 ¼ 1:2. The resulting sample was then crushed and screened to a size of 20–40 mesh and assigned as NiCu/ CeO2-SAPO-34. 2.2. Catalyst characterization techniques XRD Patterns were recorded with Cu Kα radiation (λ ¼ 1.5406 � A) on a Bruker D8 Advance instrument. The particle size of catalysts were determined by TEM on a JEOL, JEM-2200FS electron microscope. The microscopic morphology of the sample was monitored by SEM, AIS2100 (Seron Technology, South Korea). The specific surface area was studied according to the Brunauer-Emmett-Teller (BET) equation using the multi-point method (Quantachrome ChemBET-3000). H2-TPR mea surements were conducted on a Micromeritics AutoChem II 2920 in strument in a H2–Ar mixture. Before the reduction, the catalyst was pretreated in an air stream at 400 � C for 2 h and then cooled to 50 � C. The H2-TPR experiment was then started by raising the temperature to 800 � C at a rate of 10 � C/min. The surface acidity of the catalysts was determined on a Micromeritics 2000 adsorption equipment using NH3TPD. 2.3. Catalytic performance Fig. 1 provides a schematic representation of the layout on a 2
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Fig. 1. Schematic flow diagram of the experimental setup for CO2 hydrogenation to light olefins [60].
amorphous CuO, weakly interacting with CeO2 support and to the reduction of CuO, strongly interacting with the support. However, the shoulder peak above 200 � C was related to the greatly dispersed NiO reduction [31,53,66–68]. Fig. 4b shows the acidity of the catalyst using NH3-TPD. The strong acid sites are generally directly related to olefin production [33–35]. The desorption curve shows two typical peaks corresponding to weak (at 120–260 � C) and strong acid sites (at 320–480 � C). The modified SAPO-34 has a low acidity compared to SAPO-34. 3.2. Catalytic activity The concept of multifunctional catalysis was established by Weisz [69]. The synthesized hybrid catalysts, which combine two separate types of active sites for the single step carbon dioxide to light olefins, was generally prepared by the simple powder blending method and used for this reaction. NiCu/CeO2 catalyst was used to synthesize methanol and SAPO-34 zeolite for MTO. Typically, this process demands two in dependent reactors in which the CH3OH is achieved in the first reactor, and the light olefins can be produced from CH3OH on zeolite in the second reactor. Alternatively, the methanol can be simultaneously synthesized in the single reactor and further dehydrated to lower olefins [20,22,27,70–72]. This process offers a more thermodynamic and economical benefit than a two-step process, since the thermodynamic limit for methanol product is weakened, which can change the equi librium to higher CO2 conversion.
Fig. 2. XRD patterns of the samples CeO2, NiCu/CeO2, SAPO-34 and NiCu/ CeO2-SAPO-34. Table 1 Physicochemical properties of all samples. Sample
SBET (m2g 1)
Vpore (cm3g 1)
Average pore diameter (nm)
Crystallinity (%)
CeO2 NiCu/CeO2 SAPO-34 NiCu/CeO2SAPO-34
57 38 427 308
0.14 0.09 0.36 0.25
9.45 6.73 0.47 4.92
94 89 91 84
reduction temperatures of NiCu/CeO2 samples are lower than that of pure CeO2. The NiCu/CeO2 sample shows reduction peaks in the range of 120–390 � C with complete reductions of CuO as well as NiO to metallic phase. The peak below 200 � C was attributed to the reduction of
CO2 þ 3H2 →CH3 OH þ H2 O ðCO2 to methanolÞ
(4)
nCH3 OH → Cn H2n þ nH2 O ðMTOÞ
(5)
Here we examine the various factors, including the reaction tempera ture, space velocity as well as H2/CO2 ratio, which affected catalytic 3
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Fig. 3. (a) TEM image of NiCu/CeO2, (b) SEM image of SAPO-34, and (c) SEM image of modified SAPO-34.
Fig. 4. (a) H2-TPR profile and (b) NH3-TPD curve.
(C2–C04) and C5þ fractions are also promoted by raising the temperature from 375 to 450 � C. Due to the CO2 conversion, selectivity of olefins and CH4, it is believed that 375 � C is the appropriate temperature for the synthesis of light olefins from carbon dioxide on a NiCu/CeO2-SAPO-34 catalyst.
activity. 3.2.1. Effect of reaction temperature It has been found that temperature is a more important factor affecting the product with lower olefin selectivity. The NiCu/CeO2SAPO-34 catalyst has been studied at several reaction temperatures of 350, 375, 400, 425 as well as 450 � C at pressure of 20 bar; space velocity of 12 Lgcat1h 1; H2/CO2 molar ratio of 3 and process time is set to 10 h. The conversion of CO2 seems to increase significantly with reaction temperature. The CO selectivity as the function of the reaction tem perature accompanies the identical pattern for the conversion of carbon dioxide. As the temperature decreased from 450 to 375 � C, the CO selectivity decreased significantly from 85.87% to 64.27% (Fig. 5a), but the C2–C¼ 4 selectivity increased from 60.14% (C2H4 ¼ 18.6%, C3H6 ¼ 26.4%, and C4H8 ¼ 15.1%) to 76.66% (C2H4 ¼ 22.7%, C3H6 ¼ 35.5%, and C4H8 ¼ 18.4%) (Fig. 5b) and the selectivity for CH4 decreased slightly from 5.2% to 2.1% (Fig. 5b). The formation of light paraffins
3.2.2. Effect of SV The effect of space velocity on the hybrid catalyst of NiCu/CeO2 -SAPO-34 is explained in Fig. 6. The temperature of 375 � C; pressure of 20 bar; H2/CO2 molar ratio of 3 and the reaction time is set to 10 h. At SV of 2 Lg-1 h 1, the conversion of CO2 is 18.10% (Fig. 6a) and the olefin selectivity is 64.79% (C2H4 ¼ 19.2%, C3H6 ¼ 30%, and C4H8 ¼ 15.6%) (Fig. 6b). By increasing the SV from 2 to 5 Lg-1 h 1, the CO2 conversion is reduced to 17.33% and the olefin selectivity is relatively improved to 69.36%. Further increasing the SV from 8 to 12 Lg-1 h 1 significantly reduces the CO2 conversion to 15.27% and the selectivity to olefin (C2–C¼ 4 ) is slightly higher, 76.66% (C2H4 ¼ 22.7%, C3H6 ¼ 35.5%, and 4
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0 Fig. 5. Effect of different temperature on (a) CO2 conversion and CO selectivity, (b) olefins (C2–C¼ 4 ), alkanes (C2–C4), CH4 and C5þ selectivity; P ¼ 20 bar, SV ¼ 12 Lgcat1 h 1, H2/CO2 ¼ 3:1 and reaction time ¼ 10 h.
0 � Fig. 6. Effect of different space velocity on (a) CO2 conversion and CO selectivity, (b) olefins (C2–C¼ 4 ), alkanes (C2–C4), CH4 and C5þ selectivity; T ¼ 375 C, P ¼ 20 bar, H2/CO2 ¼ 3:1 and reaction time ¼ 10 h.
C4H8 ¼ 18.4%). When the SV increases from 2 to 12 Lg-1 h 1, the CO selectivity decreases slightly from 68.94% to 64.27% (Fig. 6a). The higher SV results in a shorter reaction time of the CO2 and H2 molecules on the catalyst surface. Therefore, the CO2 conversion is low; however, the light olefin formation is somewhat high. This may be caused by the production of CH3OH immediately reacted with SAPO-34 zeolite,
resulting in more olefin formation. Consequently, the high olefin selectivity was achieved at higher SV of 12 Lg-1 h 1. 3.2.3. Effect of ratio of H2/CO2 To study this effect, the temperature of 375 � C; reaction pressure of 20 bar; space velocity of 12 L g 1 h 1 and reaction time is set to 10 h. At
0 � Fig. 7. Effect of different H2/CO2 ratio on (a) conversion of CO2 as well as selectivity of CO, (b) olefins (C2–C¼ 4 ), alkanes (C2–C4), CH4 and C5þ selectivity; T ¼ 375 C, SV ¼ 12 Lgcat1 h 1, P ¼ 20 bar and reaction time ¼ 10 h.
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the H2/CO2 ¼ 1, the conversion of CO2 is only 6.55% (Fig. 7a) and olefin selectivity is 79.85% (Fig. 7b). As the H2/CO2 ratio increases from 2 to 3, the CO2 conversion increases to 15.27% and the selectivity to olefins is reduced slightly to 76.66% (C2H4 ¼ 22.7%, C3H6 ¼ 35.5%, and C4H8 ¼ 18.4%). As the H2/CO2 ratio is further increased from 3 to 5, the CO2 conversion increases to 23.53%, but the selectivity to olefins is reduced to 69.87%. In addition, the selectivity of methane and paraffin increase to 4.40% and 23.12%, respectively. According to Eqs. (4) and (6), we conclude that as the H2/CO2 ratio decreases from 3 to 1, the reaction Eq. (6) proceeds further, which increases CO selectivity: CO2 þ H2 →CO þ H2 O
(6)
As the H2/CO2 ratio increases from 3 to 5, CH4 selectivity also in creases. Since the methanol produced reacts with additional amounts of H2, it produces CH4, thus reducing the selectivity of olefins: CH3 OH þ H2 ↔ CH4 þ H2 O
(7)
The above results have shown that the H2/CO2 ratio of 3 is suitable. Therefore, it can be stated that the optimum reaction conditions are T ¼ 375 � C, SV ¼ 12 L g 1 h 1, H2/CO2 ¼ 3 and P ¼ 20 bar.
Fig. 8. Stability test at 375 � C and 20 bar with SV of 12 L gcat1 h 1.
the work reported in this paper.
3.2.4. Catalyst stability Fig. 8 indicates the long-term test of 100 h on the NiCu/CeO2-SAPO34 hybrid catalyst at the optimum reaction conditions (T ¼ 375 � C, P ¼ 20 bar, H2/CO2 ¼ 3, SV ¼ 12 L g 1 h 1). It is noteworthy that the catalyst has good stability over 90 h on stream without obvious deac tivation. The conversion of CO2 was maintained at about 15% during 90 h of continuous reaction. In addition, methane and olefin selectivities remained stable by 2.6% and 76%, respectively. Also, it was found that the NiCu/CeO2-SAPO-34 deactivated slowly after 90 h, for example, the carbon dioxide conversion was decreased from 15% to 10% and the olefin selectivity decreased from 76% to 70%. These data show that the developed catalysts are particularly stable and selective for hydroge nation of CO2 to produce lower olefins.
CRediT authorship contribution statement Mostafa Ghasemi: Conceptualization, Data curation, Writing original draft. Majid Mohammadi: Data curation, Investigation, Methodology. Mehdi Sedighi: Methodology, Supervision, Writing - re view & editing. Acknowledgements The authors wish to thank the laboratory unit of the University of Alberta, Canada and the Central Laboratory of Amirkabir University of Technology, Iran for characterization techniques. Appendix A. Supplementary data
4. Conclusion
Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2020.110029.
For the direct formation of lower olefins from CO2 hydrogenation, we synthesized a hybrid catalyst composed of NiCu/CeO2 and SAPO–34 zeolites to convert CO2 directly into light olefins. Various factors influ encing the catalytic activity, including the gas space velocity (SV), the reaction temperature, the H2/CO2 ratio and the stability test were investigated in detail. It was found that the optimum reaction conditions were T ¼ 375 � C, P ¼ 20 bar, H2/CO2 ¼ 3, SV ¼ 12 L g 1 h 1. Under optimal conditions, the selectivity for C2–C¼ 4 olefins was up to 76.6% (C2H4 ¼ 22.7%, C3H6 ¼ 35.5%, and C4H8 ¼ 18.4%) and the selectivity for paraffin (C2–C04) and methane was 18.3% and 2.1%, respectively, with less than 65% carbon monoxide at the conversion of 15.3% CO2. By raising the reaction temperature, the CO2 conversion is improved; however, the light olefin selectivity is reduced. Raising the gas space velocity reduces the CO2 conversion, and the light olefin selectivity is increased. Increasing the H2/CO2 ratio from 1 to 3, the conversion of CO2 increases to 15.27% and the selectivity to olefins decreases slightly to 76.66% (C2H4 ¼ 22.7%, C3H6 ¼ 35.5%, and C4H8 ¼ 18.4%). As the H2/CO2 ratio increases from 3 to 5, the conversion of CO2 increases to 23.53%, but the lower olefin selectivity decreases to 69.87%. In addi tion, the selectivity of methane and paraffin increase to 4.40% and 23.12%, respectively. Furthermore, our developed hybrid catalyst also provides beneficial catalytic stability, which can operate 90 h continu ously without apparent loss of activity; showing the potential industrial catalyst for CO2 utilization to light olefins.
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