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Highly selective conversion of CO2 to hydrocarbons over composite catalysts of ZnO-ZrO2 and SAPO-34
Microporous and Mesoporous Materials 284 (2019) 133–140
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Highly selective conversion of CO2 to hydrocarbons over composite catalysts of ZnO-ZrO2 and SAPO-34
T
Guanchao Wanga,b, Liying Zenga,b, Jianxin Caoa,b,c, Fei Liua,b,c,∗, Qian Lina,b,c, Yun Yia,b,c, Hongyan Pana,b,c a
School of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou, 550025, PR China Key Laboratory of Green Chemical and Clean Energy Technology, Guiyang, Guizhou, 550025, PR China c Key Laboratory of Efficient Utilization of Mineral and Green Chemical Technology, Guiyang, Guizhou, 550025, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Lower olefins Hydrogenation of CO2 Hydrothermal coating Physical blending Hydrogen reduced ZnO-ZrO2
Direct conversion hydrogenation of CO2 to hydrocarbons is a highly attractive but challenging method for obtaining value-added lower olefins with low CO selectivity. In this work, composites of SAPO-34 and ZnO-ZrO2 were successfully fabricated via hydrothermal coating and physical blending methods, respectively. Characterization results indicated that the composite phase together with coexisting acidic and basic sites and distinct micro-mesoporous structure was obtained without overly tight contact between the two active compounds in the physically blended composite. As a result the selectivity for lower olefins was 70% among all hydrocarbon products while CO selectivity was around 41%. Then, H2 reduction on ZnO-ZrO2 precursor was performed to explore the electronic properties of the physically blended composite. Interestingly, the CO byproduct was significantly suppressed from 41% to 27% by the alterations of the acid and basic sites resulting from electronic property tuning of Zn and Zr sites during H2 reduction at 500 °C. The synergetic effect of composite phase, suitable electronic property tuning, large number of acidic-basic sites and distinct micromesoporous structure all made essential contributions to the enhanced catalytic activity for direct CO2 conversion, the superior selectivity for lower olefins and less amount of CO by-product over ZnO-ZrO2/SAPO-34 composite.
1. Introduction The environmental and ecological problems caused by a large amount of greenhouse gas CO2 emissions are becoming increasingly prominent [1]. It is generally accepted that the CO2 molecule is relatively stable, and its activation requires surmounting high energy barriers [1]. The utilization of higher energy hydrogen molecules is a promising strategy for activation of CO2. The direct synthesis of lower (C2 to C4) olefins, key building-block chemicals, from CO2 and H2 is highly attractive. Hydrogenation of CO2 to lower olefins not only contributes to alleviating global climate changes caused by increasing CO2 emissions but also offers a sustainable solution to reducing the consumption of non-renewable resources such as petroleum and coal in the production of lower olefins [2]. There are two main reaction routes for the CO2 hydrogenation to lower olefins. The first route involves a two-step process, with initial reduction of CO2 to CO via the reverse water gas shift (RWGS) reaction followed by the conversion of CO to lower olefins via a Fischer-Tropsch
∗
synthesis (FTS). However, for FTS, the hydrocarbons produced usually follow the Anderson−Schulz−Flory (ASF) distribution, in which the maximum amount of C2-C4 hydrocarbon product is limited to 56.7% with undesired methane product of about 29.2%. Thus, it remains a major challenge to achieve high selectivity for lower olefins and low selectivity for CH4 simultaneously [1,3]. In the second route, lower olefins are produced from CO2 and H2 by two consecutive processes, namely methanol synthesis and methanol to olefins (MTO) reaction. This reaction path via methanol as an intermediate product raises considerable concern because the product distribution is completely different from that obtained via FTS and greatly deviates from the classical ASF distribution [1]. Since the 1980s, the target products of CO2 hydrogenation via methanol route mainly include hydrocarbons such as low-carbon hydrocarbons, dimethyl ether, aromatic hydrocarbons and gasoline [4–7]. To achieve both molecular activation of CO2 and H2 and C-C bond formation, Cu-ZnO based catalysts are used for methanol synthesis at low temperature while acidic zeolites are employed for methanol
Corresponding author. School of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou, 550025, PR China. E-mail address: [email protected] (F. Liu).
https://doi.org/10.1016/j.micromeso.2019.04.023 Received 10 October 2018; Received in revised form 20 March 2019; Accepted 12 April 2019 Available online 15 April 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
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procedure, an aqueous solution of ammonium carbonate (1 mol L−1) was added drop-wise into a mixture of zirconium and zinc nitrates (0.7 mol L−1) at 70 °C. The pH value of the mixture was maintained at 7.0 by adjusting the addition rate of ammonium carbonate. The precipitate was aged for 2 h. After filtration and washing, the precipitated gel was then dried at 105 °C for 6 h and calcined at 500 °C for 3 h. The obtained ZnO-ZrO2 was then crushed and passed through 200 mesh sieve. SAPO-34 molecular sieve was fabricated via a hydrothermal synthesis route. The gel was synthesized using the raw materials of quasi-boehmite, phosphoric acid, tetraethyl orthosilicate, morpholine and deionized water in the molar composition of 1.0 Al2O3:0.8 P2O5:0.6 SiO2:2.5 MOR: 80H2O. The gel was aged for 24 h. Then, the resultant gel was transferred to a homogeneous reactor and heated at 200 °C for 48 h. After filtration and washing, the precursor was dried at 105 °C for 6 h and calcined at 550 °C for 6 h. The obtained SAPO-34 was then crushed and sieved to 200 mesh size powder.
conversion at high temperature (ZSM-5, SAPO-34, HY, etc.). However, hydrogenation of CO2 to lower olefins on the traditional composite catalyst Cu-ZnO-Al2O3/SAPO-34 produces undesired alkanes and CO [8,9]. More importantly, the catalyst is unstable and has a short life, showing poor potential for industrial applications. The process of CO2 hydrogenation to methanol is usually accompanied by the reverse water gas shift reaction (Eq. (1) and (2)). Generally, the optimal temperature for methanol synthesis is 220–270 °C [10–15], because lower temperature could suppress the reverse water gas shift reaction on most methanol synthesis catalysts. However, compared with the methanol synthesis, the MTO reaction thermodynamically requires higher reaction temperature. It is known that the optimal reaction temperatures for ZSM-5 and SAPO-34 are above 400 °C [16–20]. As the temperature decreases from 400 °C, the catalytic performance of SAPO-34 significantly decreases with an exponential relationship. In addition, the abundant generation of adamantane hydrocarbons with large molecular size causes the deactivation of SAPO34 at 300–350 °C [21]. Thus, it can be seen that significantly different temperatures are required for the two-step reaction of CO2 hydrogenation to methanol and methanol to olefins. CO2+3H2→CH3OH + H2O; ΔrHo(500K) = −62 kJ mol−1 CO2+H2→CO + H2O; ΔrH (500K) = +40 kJ mol o
−1
2.1.2. Preparation of SAPO-34/ZnO-ZrO2 composite sample SAPO-34/ZnO-ZrO2 was fabricated via liquid phase precipitation coating process. The SAPO-34 molecular sieve with 200 mesh size was added into a mixed solution of zirconium and zinc nitrates (0.7 mol L−1) under magnetic stirring with the mass ratio of SAPO34:ZnO-ZrO2 = 1:3. Then, an aqueous solution of ammonium carbonate (1 mol L−1) was added into the above suspension. The pH value of the mixture was kept at 7.0 and the gel was aged for 2 h. After filtration and washing, the precipitated gel was dried at 105 °C for 6 h and calcined at 500 °C for 3 h. The obtained SAPO-34/ZnO-ZrO2 was then crushed and sieved to 20–40 mesh size particles, named as SZ-LPC.
(1) (2)
In summary, high temperature is favorable for the formation of C-C bonds, CO and CH4 by-products, but not conducive for the selectivity of methanol. In addition, it is well-documented that water vapor is inevitably produced at high temperature, which strongly restrains the reaction and results in severe deactivation of Cu metal nanoparticles [22–24]. Therefore, the traditional composites of Cu-based catalyst and SAPO-34 molecular sieve have the problem that different temperatures are required for the two-step reaction, leading to shorter catalyst life and lower C2=-C4= selectivity (≤60%). Due to the thermodynamic and dynamic properties of the CO2 hydrogenation to methanol synthesis, suitable reaction temperatures are generally in the range of 220–270 °C for most methanol synthesis catalysts, such as Cu-based [10–15](Cu/ZnO/Al2O3, Cu/ZrO2, Cu/ZnO, Cu/ZnO/ZrO2), and noble metal based (GaPd2/SiO2, PtCo/CeO2, Pt3Co, Pd2Ga) catalysts [25,26]. Recently, Martin et al., in 2016 [24] and Wang et al., in 2017 [27] reported that metal-oxide solid solutions such as In2O3-ZrO2 and ZnO-ZrO2 exhibit high methanol selectivity at higher reaction temperatures (360–400 °C). Then, Gao et al. [22] and Li et al. [28] developed In2O3-ZrO2/SAPO-34 and ZnO-ZrO2/SAPO-34 composite catalysts by physical blending. At the reaction temperature of 380–400 °C, by optimizing the close interplay between two active compounds, the selectivity for C2=-C4= hydrocarbon products was improved significantly (≥70%). As can be seen, the previous research revealed that the close interplay between two active compounds in composites had a significant effect on the hydrogenation of CO2 via methanol as an intermediate product to lower olefins. Therefore, in order to further explore the influence of interfacial characteristics of composites on the catalytic performance, ZnO-ZrO2/SAPO-34 composite was fabricated via various routes in this work. Then, we comparatively investigated the effects of physical blending, liquid phase precipitation coating and hydrothermal coating methods on the structural characteristics and catalytic performance of the composite catalysts, to establish the underlying structurereactivity relationship of various composites.
2.1.3. Preparation of ZnO-ZrO2/SAPO-34 composite sample The ZnO-ZrO2/SAPO-34 composite sample was fabricated via hydrothermal coating method. The obtained ZnO-ZrO2 with 200 mesh size was added into the precursor solution of SAPO-34 and the mass ratio of SAPO-34:ZnO-ZrO2 was 1:3. Then, the resultant gel was transferred to a homogeneous reactor and heated at 200 °C for 48 h. After filtration and washing, the precursor was dried at 105 °C for 6 h and calcined at 550 °C for 6 h. The obtained ZnO-ZrO2/SAPO-34 composite sample was then crushed and sieved to 20–40 mesh size, denoted as ZS-HC. 2.1.4. Preparation of ZnO-ZrO2/SAPO-34-M ZnO-ZrO2/SAPO-34-M composite sample was fabricated via physical blending with the mass ratio of SAPO-34: ZnO-ZrO2 = 1:3. The obtained ZnO-ZrO2/SAPO-34-M composite sample was then crushed and sieved to 20–40 mesh size, named as ZS-PB. 2.1.5. H2 reduction treatment of ZnO-ZrO2/SAPO-34-M Hydrogen-reduced ZnO-ZrO2 was fabricated via the aforementioned co-precipitation method. The precipitated gel was then dried at 105 °C for 6 h and hydrogen reduction was conducted at various temperatures for 3 h. The obtained ZnO-ZrO2 was then crushed and sieved to 200 mesh size. Hydrogen-reduced ZnO-ZrO2/SAPO-34-M was then fabricated via physical blending with the mass ratio of SAPO-34: Hydrogen-reduced ZnO-ZrO2 = 1:3. The obtained composite sample was crushed and sieved to 20–40 mesh size, labelled as ZS-HPB. 2.2. Characterization of catalysts
2. Experimental
X-ray diffraction (XRD) patterns of the samples were recorded on an X'Pert Powder diffractometer using Cu Kα radiation, operating at 40 KV and 40 mA, ranging from 5 to 90° with a scanning speed of 5°/min. Scanning electron microscopy (SEM) images were taken with a ZEISS ΣIGMA microscope at an acceleration voltage of 5.0 kV. Transmission electron microscopy (TEM) measurements and EDX analysis were performed on a Phillips Analytical FEI Tecnai 80 electron
2.1. Preparation of catalysts 2.1.1. Synthesis of ZnO-ZrO2 and SAPO-34 ZnO-ZrO2 was fabricated via the co-precipitation method and the mass ratio of Zn(NO3)2·6H2O:Zr(NO3)4·5H2O was 1:2. In a typical 134
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microscope operated at an acceleration voltage of 200 kV. Textural properties were determined by N2 adsorption at −196 °C using an Auto Chem 2020 surface area & pore size analyzer (Micromeritics). Prior to the adsorption-desorption measurements, all samples were degassed at 200 °C for 4 h. The surface area was calculated using the BET equation. The micropore and mesopore size distributions were determined according to Horvath–Kawazoe (HK) and Barrett–Joyner–Halenda (BJH) methods, respectively. NH3-temperature programmed desorption (NH3-TPD) and CO2temperature programmed desorption (CO2-TPD) measurements were carried out using a chemisorption analyzer (Micromeritics, Auto Chem Ⅱ 2920). The procedure for NH3-TPD was as follows: 0.06 g sample was placed in a quartz tube and heated at 300 °C under N2 flow (30 mL/min) for 1 h. The sample was cooled to 45 °C and then NH3 adsorption was performed by passing NH3-He mixture (2.5 vol% NH3, 30 mL/min) over the sample for 1.5 h. The sample was flushed with 30 mL/min N2 flow to remove physically adsorbed NH3 on the catalyst surface for 1.5 h. The procedure for CO2-TPD was similar to that for NH3-TPD. Briefly, the sample was cooled to 45 °C and then CO2 adsorption was performed by passing CO2-He mixture (10 vol% CO2, 30 mL/min) over the sample for 1.5 h. NH3-TPD and CO2-TPD analyses were carried out in 30 mL/ min N2 flow and from 45 to 700 °C, at the heating rate of 10 °C/min. X-ray photoelectron spectroscopy (XPS) was performed with a KALPHA+ spectrometer under the Al Kα irradiation (hν = 1486.6 eV). X-ray fluorescence (XRF) analysis was performed with Malvern Panalytical Zetium spectrometer.
Fig. 1. XRD patterns of samples (a) the synthesized composites ZS-HC, SZ-LPC, SAPO-34, ZnO-ZrO2 and physically blended ZS-PB; (b) magnified view of 2 theta from 8° to 52° for composite ZS-PB and ZS-HC.
diffraction peaks at 2θ = 9.5°, 12.8°, 20.6°, and 30.7° for SAPO-34 and at 30.3°, 35.2°, and 50.6° for ZnO-ZrO2 of ZS-HC composite sample slightly shifted toward low 2θ relative to the corresponding peaks of physically blended ZS-PB (shown in Fig. 1b). According to the Bragg's Equation: 2dsinθ = λ [29], the diffraction angle θ decreases, resulting in an increase in the plane spacing d of the ZS-HC crystal. It demonstrates that the larger lattice of composite phase existed in the ZS-HC composite sample. Consequently, to a certain extent the tight contact for composite phase might result in altered physicochemical nature of the composite sample. Conversely, compared with sole SAPO-34 and ZnO-ZrO2, the characteristic diffraction peaks of zeotype SAPO-34 and ZnO-ZrO2 phase in ZS-PB composite sample showed little shift toward low or high 2θ, indicating that there was a relatively wide distance between the two compounds in the composite catalyst. In order to further reveal the interface characteristics of the composite phase obtained via the hydrothermal coating and the physical blending methods, the binding energies of Zn and Zr elements in the composite samples were determined by XPS technique. The corresponding binding energies of Zn and Zr elements in the synthesized composite sample ZS-HC and physically blended ZS-PB are presented in Fig. 2. The binding energy of Zn2p in ZS-HC composite sample was evidently decreased, whereas the binding energy of Zr3d in ZS-HC composite sample was significantly increased compared to that of ZS-PB composite sample. This result was due to the electronic property alteration of the Zn and Zr sites by the interaction of Zn and Zr in ZnO-ZrO2 crystal phase with the framework elements such as Al, Si and P of neighboring SAPO-34 crystal phase in ZS-HC composite sample. The alteration of binding energy for Zn2p and Zr3d was closely related to the distance between two compounds, demonstrating that a tight contact between the two phases existed in ZS-HC composite sample but a relatively wide distance existed between two compounds in the ZS-PB composite sample, in agreement with the results of XRD analysis.
2.3. Catalytic test The catalytic reaction was performed using 1.0 g composite sample placed in a fixed-bed stainless steel reactor at 3.0 MPa. Typically, the total flow rate of the gas mixture (H2/CO2 = 3) was 60 mL/min, and the reaction temperature was set to 380 °C. Prior to the reaction, the composite sample was first pretreated under N2 flow (30 mL/min) at 380 °C for 1 h. The CO2 conversion and selectivity of CO by-product were analyzed by an online GC-9560 gas chromatograph with a TCD detector and a Porapak-Q column. A GC-9560 gas chromatograph with a FID detector and a TDX-01 column was used to determine the selectivity of hydrocarbons online. All the data of catalytic performances was collected after 6 h of reaction with no deactivation for CO2 hydrogenation reaction. 3. Results and discussion 3.1. XRD and XPS analyses The XRD patterns for a series of synthesized composites ZS-HC, SZLPC, SAPO-34, ZnO-ZrO2 and physically blended ZS-PB are presented in Fig. 1a. No diffraction peak of ZnO-ZrO2 phase was observed in the diffractogram, and the characteristic diffraction peak of SAPO-34 crystal phase was obviously weakened, indicating that no composite phase of SAPO-34 and ZnO-ZrO2 existed in the SZ-LPC composite sample fabricated via liquid phase precipitation coating process. The result suggested that the added SAPO-34 powder inhibited the transition of zinc-zirconium precursor to ZnO-ZrO2 crystal phase during liquid phase precipitation, and also the coated SAPO-34 phase was significantly eroded by the matrix solution. This unexpected phenomenon was mainly due to the pH variation in the matrix solution caused by the continuous dropping of alkaline precipitant. In addition, the typical peaks (2θ = 9.5°, 12.8°, 20.6°, 25.9°, 30.7°) were the characteristic diffraction peaks of zeotype SAPO-34 (PDF # 47–0630), and the peaks at 30.3°, 35.2°, 50.6° and 60.2° were attributed to the ZnO-ZrO2 phase (PDF # 50–1089). These results show that ZS-HC and ZS-PB composite samples were successfully fabricated via hydrothermal coating and physical blending methods, respectively. It must be noted that the
3.2. XRF analysis The elemental analysis results of synthesized composite ZS-HC and physically blended ZS-PB are shown in Table 1. It can be seen that the contents of Zn, Zr, Si, Al and P in the synthesized composite ZS-HC were almost the same with those in the physically blended ZS-PB. 3.3. SEM, TEM and EDX analyses SEM and TEM images of the as-prepared ZS-PB and ZS-HC samples 135
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Fig. 2. Zn2p and Zr3d XPS spectra of synthesized composite sample ZS-HC and physically blended ZS-PB.
for investigating the surface acidic and basic natures. CO2-TPD and NH3-TPD profiles over synthesized composite sample ZS-HC and physically blended ZS-PB are illustrated in Fig. 6 and Fig. 7, respectively. As shown in Fig. 6, the differences in the positions of the desorption peaks were very small for all the samples, but the amount of weak base in ZSPB sample was significantly higher than that in ZS-HC sample. This indicates that there was obvious change in the amount of weak base with the different distances between the two compounds in composite samples. In Fig. 7, the desorption step at 108 °C was observed for the ZS-HC sample, indicating that weak acid sites simultaneously appeared. Compared with ZS-HC sample, the ZS-PB composite sample presented three peaks with maxima at the temperatures of 112 °C, 354 °C and 656 °C, clearly suggesting that weak, medium-strong and strong acid sites simultaneously existed in ZS-PB samples. The results also demonstrated that the close interaction between the two compounds had a significant impact on the acid properties of the composite sample. As can be seen, the considerable chemical strength between two phases induced by overly tight contact between composite phases was unfavorable for the formation of a large number of active sites in ZnOZrO2/SAPO-34 composite sample.
Table 1 Elemental analysis of synthesized composite ZS-HC and physically blended ZSPB. Samples
ZS-HC ZS-PB
Element content (W %) Zn
Zr
Si
Al
P
19.48 19.49
59.71 59.75
2.74 2.71
8.48 8.44
8.46 8.48
are shown in Fig. 3 and Fig. 4, respectively. In addition, the local elemental composition of ZS-HC composite sample was analyzed by energy-dispersive X-ray spectrometry, and the results are shown in Fig. 5. As shown in Figs. 3b, 4c and 5, the ZS-HC composite sample exhibited typical encapsulated structure. The results of XRD, TEM and EDX analyses confirmed that SAPO-34 molecular sieve was completely coated with ZnO-ZrO2 crystal phase, indicating that the ZnO-ZrO2 particles were tightly attached to SAPO-34 particles (Fig. 4(c)). In the case of ZS-PB composite sample, the cubic SAPO-34 crystal phase and the nanoparticle ZnO-ZrO2 crystal phase were independent of each other, as seen from Figs. 3a and 4a. Only a small portion of ZnO-ZrO2 crystals were adhered to the surface of SAPO-34 grains. This result also demonstrated that a relatively wide distance existed between the two compounds in ZS-PB composite sample fabricated via physical blending.
3.5. N2 isothermal adsorption-desorption analysis Fig. 8 displays N2 adsorption-desorption isotherms of synthesized composite ZS-HC and physically blended ZS-PB. The isotherm presented the H4 hysteresis loop and the characteristic trend of type-I isotherm, indicating that the ZS-HC composite sample could be classified as microporous material with uniform pore structure. In comparison with ZSHC, a typical shape of H3 hysteresis loop was observed in ZS-PB composite sample, indicating that various pore distributions existed in ZSPB composite. The data of BET specific surface area and pore volume of
3.4. CO2-TPD and NH3-TPD analyses Generally, the basicity of metal oxide has a great influence on the adsorption and activation of acidic CO2. However, the acidity of molecular sieves exerts a significant impact on the catalytic behavior for MTO reaction. Hence, NH3-TPD and CO2-TPD are efficient techniques
Fig. 3. SEM images of synthesized composite ZS-HC and physically blended ZS-PB. (a) ZS-PB; (b) ZS-HC. 136
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Fig. 4. HRTEM images of synthesized composite ZS-HC and physically blended ZS-PB. (a) and (b) HRTEM images of ZS-PB; (c) and (d) HRTEM images of ZS-HC.
Fig. 5. TEM–EDX characterization of synthesized composite sample ZS-HC. Fig. 6. CO2-TPD spectra of synthesized composite sample ZS-HC and physically blended ZS-PB, ZS-HPB-300 °C, ZS-HPB-400 °C, ZS-HPB-500 °C, ZS-HPB-600 °C and ZS-HPB-700 °C.
synthesized composite ZS-HC and physically blended ZS-PB are shown in Table 2. The ZS-HC composite mainly exhibited a large amount of micropores and less amount of mesopores. However, the ZS-PB composite had significantly increased surface area of mesopores (121 m2 g−1) and mesopore volume (0.24 cm3 g−1) compared with the ZS-HC composite. From the above discussion, it is evident that the ZSPB composite fabricated via physical blending retained the initial pore structures of the two phases of SAPO-34 and ZnO-ZrO2, while the ZS-HC composite fabricated via hydrothermal coating showed mainly
microporous structure. This could be attributed to the chemical interaction of ZnO-ZrO2 with SAPO-34 during the hydrothermal coating process. 4. Catalytic performances The effects of the preparation methods on the CO2 conversion and 137
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Fig. 9. Catalytic performance in CO2 hydrogenation of synthesized composite sample ZS-HC and physically blended ZS-PB.
Fig. 7. NH3-TPD spectra of synthesized composite sample ZS-HC and physically blended ZS-PB, ZS-HPB-300 °C, ZS-HPB-400 °C, ZS-HPB-500 °C, ZS-HPB-600 °C and ZS-HPB-700 °C.
affinity of the system for CO2. In addition, the selectivity for primary olefin products and methanol conversion are strongly dependent on the Brønsted acidic sites of SAPO-34, while the Lewis acidic sites on ZnOZrO2 are beneficial to obtain B-L synergistic acidic sites in the composite samples [20,30]. Hence, the existence of the overall Lewis basic and B-L synergistic acidic sites on composite samples is a significant advantage for the CO2 conversion and formation of lower olefins. It is worth mentioning that the mesopores can act as channels for the enhanced transfer of reaction products from the pores to the bulk gas phase [31]. Thus, the existence of the structure with micro-mesopores is advantageous for the hydrogenation of CO2 to hydrocarbons. As a result, extremely tight contact of two-phase structure of the composite might increase the probability of unfavorable lower olefins formation in target hydrocarbons produced from SAPO-34 nearby ZnOZrO2. Conversely, the existence of discrete states between these two phases was beneficial to the catalytic performance, implying that only suitable distance of two-phase structure could result in the enhanced catalytic performance. In order to further promote the catalytic performance of ZS-PB composite sample fabricated via physical blending method in the hydrogenation reaction of CO2 to hydrocarbons, the ZnO-ZrO2 precursor was subjected to H2 reduction treatment at different temperatures. The catalytic performances of the ZS-HPB composite samples after H2 reductions at various temperatures are shown in Fig. 10. In comparison with ZS-PB, it was found that the performance of ZS-HPB composite sample was significantly affected by H2 reduction temperature. As the H2 reduction temperature increased, the selectivity of CO was lowered from 41% to 27%, and a minimum of 27% was observed over the composite sample after H2 reduction treatment at 500 °C. However, higher H2 reduction temperature led to the increase in CO selectivity. In addition, compared with ZS-PB, the selectivity for CO on H2 reduced composite decreased by approximately 14%. On the other hand, the selectivity for all hydrocarbon products showed a similar trend and the highest selectivity for hydrocarbons was reached 73% with the 70% proportion of lower olefins over the sample of 500 °C H2 reduction treatment.
Fig. 8. N2 adsorption desorption isotherms of synthesized composite sample ZSHC and physically blended ZS-PB.
product selectivity are shown in Fig. 9. As seen from Fig. 9, the CO2 hydrogenation on the ZS-HC composite sample displayed 21% selectivity for C2=- C4=, 19% for C20- C40, 51% for CH4, and 9% for C5+ among all hydrocarbon products, at the CO2 conversion of 13% under reaction conditions of 3 MPa, 3500 mL g−1 h−1 and 380 °C. Also, the CO selectivity was as high as 51%. Conversely, ZS-PB composite sample exhibited much higher C2=- C4= selectivity (70%) and lower CH4 selectivity (3%) compared with ZS-HC composite sample. It can be inferred that the better catalytic performance of ZS-PB composite was directly related to the synergetic effects of higher activity and distinct micro-mesoporous structure. As is known, zirconia possesses surface Lewis basic and acidic sites, and more basic sites would enhance the
Table 2 Specific surface area, pore volume and average pore diameter data of synthesized composite sample ZS-HC and physically blended ZS-PB. Samples
ZS-PB ZS-HC
BET surface area (m2·g−1)
Pore volume (cm3·g−1)
Pore diameter (nm) D
SBET
Smic
Smes
Vt
Vmic
Vmes
229 178
108 145
121 33
0.39 0.21
0.15 0.09
0.24 0.12
138
6.8 4.5
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the intensities of medium-strong acid sites became stronger and some weak base sites transferred to medium-strong base sites in ZS-HPB composite samples (Figs. 6 and 7). These alterations of the acid and basic sites roused by H2 reduction might be the primary reason for lower amount of CO by-product over ZS-HPB composite samples. The above result could be further confirmed from the XPS data (Fig. 12). Significant changes in the peak strengths and positions of Zn2p and Zr3d were observed in ZS-HPB-500 °C sample, which was possibly because the electronic property of Zn and Zr sites was altered by H2 reduction. The electronic property tuning of Zn and Zr sites result in alterations of the acid and basic sites, significantly suppressing the formation of CO by-product.
5. Conclusions In summary, composites of SAPO-34 and ZnO-ZrO2 were successfully fabricated via hydrothermal coating and physical blending methods, respectively. The obtained composites exhibited completely different physicochemical properties caused by the interaction between the two active compounds. Characterization results indicated that the composite phase was obtained along with a large number of coexisting acidic and basic sites and distinct micro-mesoporous structure, and without tight contact between the two active compounds, in the physically blended composite. Catalytic test results showed that physically blended composite exhibited higher catalytic performance with 70% selectivity for lower olefins among all hydrocarbon products, but the CO selectivity was around 41%. Furthermore, H2 reduction treatment was performed on ZnO-ZrO2 precursor to explore the electronic properties of the physically blended composite. The results showed that the alterations of the acid and basic sites resulting from electronic property tuning of Zn and Zr sites significantly suppressed the formation of CO by-product. The CO selectivity significantly decreased from 41% to 27%. The excellent catalytic performance for physically blended composite after H2 reduction could be attributed to the synergetic effect of composite phase with suitably tuned electronic property, large number of acidic-basic sites and distinct micro-mesoporous structure. Therefore, this study demonstrated that enhanced catalytic activity for CO2 conversion, superior shape selectivity for lower olefins and less amount of CO by-product could be achieved by using hydrogen-reduced ZnOZrO2/SAPO-34 composite catalyst.
Fig. 10. Catalytic performance of composite sample after H2 reduction treatment.
Fig. 11. XRD patterns of synthesized composites SAPO-34 and ZnO-ZrO2 and physically blended ZS-PB and ZS-HPB-500 °C.
In comparison with the composite ZS-PB, a trace amount of ZnO phase was present in the composite reduced at 500 °C from XRD data (2θ = 32°, 36°, 48°, 57°, Fig. 11), but it was interestingly observed that
Notes The authors declare no competing financial interest.
Fig. 12. Zn2p and Zr3d XPS spectra of physically blended ZS-PB and ZS-HPB-500 °C. 139
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Acknowledgments
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Highly selective conversion of CO2 to hydrocarbons over composite catalysts of ZnO-ZrO2 and SAPO-34
T
Guanchao Wanga,b, Liying Zenga,b, Jianxin Caoa,b,c, Fei Liua,b,c,∗, Qian Lina,b,c, Yun Yia,b,c, Hongyan Pana,b,c a
School of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou, 550025, PR China Key Laboratory of Green Chemical and Clean Energy Technology, Guiyang, Guizhou, 550025, PR China c Key Laboratory of Efficient Utilization of Mineral and Green Chemical Technology, Guiyang, Guizhou, 550025, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Lower olefins Hydrogenation of CO2 Hydrothermal coating Physical blending Hydrogen reduced ZnO-ZrO2
Direct conversion hydrogenation of CO2 to hydrocarbons is a highly attractive but challenging method for obtaining value-added lower olefins with low CO selectivity. In this work, composites of SAPO-34 and ZnO-ZrO2 were successfully fabricated via hydrothermal coating and physical blending methods, respectively. Characterization results indicated that the composite phase together with coexisting acidic and basic sites and distinct micro-mesoporous structure was obtained without overly tight contact between the two active compounds in the physically blended composite. As a result the selectivity for lower olefins was 70% among all hydrocarbon products while CO selectivity was around 41%. Then, H2 reduction on ZnO-ZrO2 precursor was performed to explore the electronic properties of the physically blended composite. Interestingly, the CO byproduct was significantly suppressed from 41% to 27% by the alterations of the acid and basic sites resulting from electronic property tuning of Zn and Zr sites during H2 reduction at 500 °C. The synergetic effect of composite phase, suitable electronic property tuning, large number of acidic-basic sites and distinct micromesoporous structure all made essential contributions to the enhanced catalytic activity for direct CO2 conversion, the superior selectivity for lower olefins and less amount of CO by-product over ZnO-ZrO2/SAPO-34 composite.
1. Introduction The environmental and ecological problems caused by a large amount of greenhouse gas CO2 emissions are becoming increasingly prominent [1]. It is generally accepted that the CO2 molecule is relatively stable, and its activation requires surmounting high energy barriers [1]. The utilization of higher energy hydrogen molecules is a promising strategy for activation of CO2. The direct synthesis of lower (C2 to C4) olefins, key building-block chemicals, from CO2 and H2 is highly attractive. Hydrogenation of CO2 to lower olefins not only contributes to alleviating global climate changes caused by increasing CO2 emissions but also offers a sustainable solution to reducing the consumption of non-renewable resources such as petroleum and coal in the production of lower olefins [2]. There are two main reaction routes for the CO2 hydrogenation to lower olefins. The first route involves a two-step process, with initial reduction of CO2 to CO via the reverse water gas shift (RWGS) reaction followed by the conversion of CO to lower olefins via a Fischer-Tropsch
∗
synthesis (FTS). However, for FTS, the hydrocarbons produced usually follow the Anderson−Schulz−Flory (ASF) distribution, in which the maximum amount of C2-C4 hydrocarbon product is limited to 56.7% with undesired methane product of about 29.2%. Thus, it remains a major challenge to achieve high selectivity for lower olefins and low selectivity for CH4 simultaneously [1,3]. In the second route, lower olefins are produced from CO2 and H2 by two consecutive processes, namely methanol synthesis and methanol to olefins (MTO) reaction. This reaction path via methanol as an intermediate product raises considerable concern because the product distribution is completely different from that obtained via FTS and greatly deviates from the classical ASF distribution [1]. Since the 1980s, the target products of CO2 hydrogenation via methanol route mainly include hydrocarbons such as low-carbon hydrocarbons, dimethyl ether, aromatic hydrocarbons and gasoline [4–7]. To achieve both molecular activation of CO2 and H2 and C-C bond formation, Cu-ZnO based catalysts are used for methanol synthesis at low temperature while acidic zeolites are employed for methanol
Corresponding author. School of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou, 550025, PR China. E-mail address: [email protected] (F. Liu).
https://doi.org/10.1016/j.micromeso.2019.04.023 Received 10 October 2018; Received in revised form 20 March 2019; Accepted 12 April 2019 Available online 15 April 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
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procedure, an aqueous solution of ammonium carbonate (1 mol L−1) was added drop-wise into a mixture of zirconium and zinc nitrates (0.7 mol L−1) at 70 °C. The pH value of the mixture was maintained at 7.0 by adjusting the addition rate of ammonium carbonate. The precipitate was aged for 2 h. After filtration and washing, the precipitated gel was then dried at 105 °C for 6 h and calcined at 500 °C for 3 h. The obtained ZnO-ZrO2 was then crushed and passed through 200 mesh sieve. SAPO-34 molecular sieve was fabricated via a hydrothermal synthesis route. The gel was synthesized using the raw materials of quasi-boehmite, phosphoric acid, tetraethyl orthosilicate, morpholine and deionized water in the molar composition of 1.0 Al2O3:0.8 P2O5:0.6 SiO2:2.5 MOR: 80H2O. The gel was aged for 24 h. Then, the resultant gel was transferred to a homogeneous reactor and heated at 200 °C for 48 h. After filtration and washing, the precursor was dried at 105 °C for 6 h and calcined at 550 °C for 6 h. The obtained SAPO-34 was then crushed and sieved to 200 mesh size powder.
conversion at high temperature (ZSM-5, SAPO-34, HY, etc.). However, hydrogenation of CO2 to lower olefins on the traditional composite catalyst Cu-ZnO-Al2O3/SAPO-34 produces undesired alkanes and CO [8,9]. More importantly, the catalyst is unstable and has a short life, showing poor potential for industrial applications. The process of CO2 hydrogenation to methanol is usually accompanied by the reverse water gas shift reaction (Eq. (1) and (2)). Generally, the optimal temperature for methanol synthesis is 220–270 °C [10–15], because lower temperature could suppress the reverse water gas shift reaction on most methanol synthesis catalysts. However, compared with the methanol synthesis, the MTO reaction thermodynamically requires higher reaction temperature. It is known that the optimal reaction temperatures for ZSM-5 and SAPO-34 are above 400 °C [16–20]. As the temperature decreases from 400 °C, the catalytic performance of SAPO-34 significantly decreases with an exponential relationship. In addition, the abundant generation of adamantane hydrocarbons with large molecular size causes the deactivation of SAPO34 at 300–350 °C [21]. Thus, it can be seen that significantly different temperatures are required for the two-step reaction of CO2 hydrogenation to methanol and methanol to olefins. CO2+3H2→CH3OH + H2O; ΔrHo(500K) = −62 kJ mol−1 CO2+H2→CO + H2O; ΔrH (500K) = +40 kJ mol o
−1
2.1.2. Preparation of SAPO-34/ZnO-ZrO2 composite sample SAPO-34/ZnO-ZrO2 was fabricated via liquid phase precipitation coating process. The SAPO-34 molecular sieve with 200 mesh size was added into a mixed solution of zirconium and zinc nitrates (0.7 mol L−1) under magnetic stirring with the mass ratio of SAPO34:ZnO-ZrO2 = 1:3. Then, an aqueous solution of ammonium carbonate (1 mol L−1) was added into the above suspension. The pH value of the mixture was kept at 7.0 and the gel was aged for 2 h. After filtration and washing, the precipitated gel was dried at 105 °C for 6 h and calcined at 500 °C for 3 h. The obtained SAPO-34/ZnO-ZrO2 was then crushed and sieved to 20–40 mesh size particles, named as SZ-LPC.
(1) (2)
In summary, high temperature is favorable for the formation of C-C bonds, CO and CH4 by-products, but not conducive for the selectivity of methanol. In addition, it is well-documented that water vapor is inevitably produced at high temperature, which strongly restrains the reaction and results in severe deactivation of Cu metal nanoparticles [22–24]. Therefore, the traditional composites of Cu-based catalyst and SAPO-34 molecular sieve have the problem that different temperatures are required for the two-step reaction, leading to shorter catalyst life and lower C2=-C4= selectivity (≤60%). Due to the thermodynamic and dynamic properties of the CO2 hydrogenation to methanol synthesis, suitable reaction temperatures are generally in the range of 220–270 °C for most methanol synthesis catalysts, such as Cu-based [10–15](Cu/ZnO/Al2O3, Cu/ZrO2, Cu/ZnO, Cu/ZnO/ZrO2), and noble metal based (GaPd2/SiO2, PtCo/CeO2, Pt3Co, Pd2Ga) catalysts [25,26]. Recently, Martin et al., in 2016 [24] and Wang et al., in 2017 [27] reported that metal-oxide solid solutions such as In2O3-ZrO2 and ZnO-ZrO2 exhibit high methanol selectivity at higher reaction temperatures (360–400 °C). Then, Gao et al. [22] and Li et al. [28] developed In2O3-ZrO2/SAPO-34 and ZnO-ZrO2/SAPO-34 composite catalysts by physical blending. At the reaction temperature of 380–400 °C, by optimizing the close interplay between two active compounds, the selectivity for C2=-C4= hydrocarbon products was improved significantly (≥70%). As can be seen, the previous research revealed that the close interplay between two active compounds in composites had a significant effect on the hydrogenation of CO2 via methanol as an intermediate product to lower olefins. Therefore, in order to further explore the influence of interfacial characteristics of composites on the catalytic performance, ZnO-ZrO2/SAPO-34 composite was fabricated via various routes in this work. Then, we comparatively investigated the effects of physical blending, liquid phase precipitation coating and hydrothermal coating methods on the structural characteristics and catalytic performance of the composite catalysts, to establish the underlying structurereactivity relationship of various composites.
2.1.3. Preparation of ZnO-ZrO2/SAPO-34 composite sample The ZnO-ZrO2/SAPO-34 composite sample was fabricated via hydrothermal coating method. The obtained ZnO-ZrO2 with 200 mesh size was added into the precursor solution of SAPO-34 and the mass ratio of SAPO-34:ZnO-ZrO2 was 1:3. Then, the resultant gel was transferred to a homogeneous reactor and heated at 200 °C for 48 h. After filtration and washing, the precursor was dried at 105 °C for 6 h and calcined at 550 °C for 6 h. The obtained ZnO-ZrO2/SAPO-34 composite sample was then crushed and sieved to 20–40 mesh size, denoted as ZS-HC. 2.1.4. Preparation of ZnO-ZrO2/SAPO-34-M ZnO-ZrO2/SAPO-34-M composite sample was fabricated via physical blending with the mass ratio of SAPO-34: ZnO-ZrO2 = 1:3. The obtained ZnO-ZrO2/SAPO-34-M composite sample was then crushed and sieved to 20–40 mesh size, named as ZS-PB. 2.1.5. H2 reduction treatment of ZnO-ZrO2/SAPO-34-M Hydrogen-reduced ZnO-ZrO2 was fabricated via the aforementioned co-precipitation method. The precipitated gel was then dried at 105 °C for 6 h and hydrogen reduction was conducted at various temperatures for 3 h. The obtained ZnO-ZrO2 was then crushed and sieved to 200 mesh size. Hydrogen-reduced ZnO-ZrO2/SAPO-34-M was then fabricated via physical blending with the mass ratio of SAPO-34: Hydrogen-reduced ZnO-ZrO2 = 1:3. The obtained composite sample was crushed and sieved to 20–40 mesh size, labelled as ZS-HPB. 2.2. Characterization of catalysts
2. Experimental
X-ray diffraction (XRD) patterns of the samples were recorded on an X'Pert Powder diffractometer using Cu Kα radiation, operating at 40 KV and 40 mA, ranging from 5 to 90° with a scanning speed of 5°/min. Scanning electron microscopy (SEM) images were taken with a ZEISS ΣIGMA microscope at an acceleration voltage of 5.0 kV. Transmission electron microscopy (TEM) measurements and EDX analysis were performed on a Phillips Analytical FEI Tecnai 80 electron
2.1. Preparation of catalysts 2.1.1. Synthesis of ZnO-ZrO2 and SAPO-34 ZnO-ZrO2 was fabricated via the co-precipitation method and the mass ratio of Zn(NO3)2·6H2O:Zr(NO3)4·5H2O was 1:2. In a typical 134
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microscope operated at an acceleration voltage of 200 kV. Textural properties were determined by N2 adsorption at −196 °C using an Auto Chem 2020 surface area & pore size analyzer (Micromeritics). Prior to the adsorption-desorption measurements, all samples were degassed at 200 °C for 4 h. The surface area was calculated using the BET equation. The micropore and mesopore size distributions were determined according to Horvath–Kawazoe (HK) and Barrett–Joyner–Halenda (BJH) methods, respectively. NH3-temperature programmed desorption (NH3-TPD) and CO2temperature programmed desorption (CO2-TPD) measurements were carried out using a chemisorption analyzer (Micromeritics, Auto Chem Ⅱ 2920). The procedure for NH3-TPD was as follows: 0.06 g sample was placed in a quartz tube and heated at 300 °C under N2 flow (30 mL/min) for 1 h. The sample was cooled to 45 °C and then NH3 adsorption was performed by passing NH3-He mixture (2.5 vol% NH3, 30 mL/min) over the sample for 1.5 h. The sample was flushed with 30 mL/min N2 flow to remove physically adsorbed NH3 on the catalyst surface for 1.5 h. The procedure for CO2-TPD was similar to that for NH3-TPD. Briefly, the sample was cooled to 45 °C and then CO2 adsorption was performed by passing CO2-He mixture (10 vol% CO2, 30 mL/min) over the sample for 1.5 h. NH3-TPD and CO2-TPD analyses were carried out in 30 mL/ min N2 flow and from 45 to 700 °C, at the heating rate of 10 °C/min. X-ray photoelectron spectroscopy (XPS) was performed with a KALPHA+ spectrometer under the Al Kα irradiation (hν = 1486.6 eV). X-ray fluorescence (XRF) analysis was performed with Malvern Panalytical Zetium spectrometer.
Fig. 1. XRD patterns of samples (a) the synthesized composites ZS-HC, SZ-LPC, SAPO-34, ZnO-ZrO2 and physically blended ZS-PB; (b) magnified view of 2 theta from 8° to 52° for composite ZS-PB and ZS-HC.
diffraction peaks at 2θ = 9.5°, 12.8°, 20.6°, and 30.7° for SAPO-34 and at 30.3°, 35.2°, and 50.6° for ZnO-ZrO2 of ZS-HC composite sample slightly shifted toward low 2θ relative to the corresponding peaks of physically blended ZS-PB (shown in Fig. 1b). According to the Bragg's Equation: 2dsinθ = λ [29], the diffraction angle θ decreases, resulting in an increase in the plane spacing d of the ZS-HC crystal. It demonstrates that the larger lattice of composite phase existed in the ZS-HC composite sample. Consequently, to a certain extent the tight contact for composite phase might result in altered physicochemical nature of the composite sample. Conversely, compared with sole SAPO-34 and ZnO-ZrO2, the characteristic diffraction peaks of zeotype SAPO-34 and ZnO-ZrO2 phase in ZS-PB composite sample showed little shift toward low or high 2θ, indicating that there was a relatively wide distance between the two compounds in the composite catalyst. In order to further reveal the interface characteristics of the composite phase obtained via the hydrothermal coating and the physical blending methods, the binding energies of Zn and Zr elements in the composite samples were determined by XPS technique. The corresponding binding energies of Zn and Zr elements in the synthesized composite sample ZS-HC and physically blended ZS-PB are presented in Fig. 2. The binding energy of Zn2p in ZS-HC composite sample was evidently decreased, whereas the binding energy of Zr3d in ZS-HC composite sample was significantly increased compared to that of ZS-PB composite sample. This result was due to the electronic property alteration of the Zn and Zr sites by the interaction of Zn and Zr in ZnO-ZrO2 crystal phase with the framework elements such as Al, Si and P of neighboring SAPO-34 crystal phase in ZS-HC composite sample. The alteration of binding energy for Zn2p and Zr3d was closely related to the distance between two compounds, demonstrating that a tight contact between the two phases existed in ZS-HC composite sample but a relatively wide distance existed between two compounds in the ZS-PB composite sample, in agreement with the results of XRD analysis.
2.3. Catalytic test The catalytic reaction was performed using 1.0 g composite sample placed in a fixed-bed stainless steel reactor at 3.0 MPa. Typically, the total flow rate of the gas mixture (H2/CO2 = 3) was 60 mL/min, and the reaction temperature was set to 380 °C. Prior to the reaction, the composite sample was first pretreated under N2 flow (30 mL/min) at 380 °C for 1 h. The CO2 conversion and selectivity of CO by-product were analyzed by an online GC-9560 gas chromatograph with a TCD detector and a Porapak-Q column. A GC-9560 gas chromatograph with a FID detector and a TDX-01 column was used to determine the selectivity of hydrocarbons online. All the data of catalytic performances was collected after 6 h of reaction with no deactivation for CO2 hydrogenation reaction. 3. Results and discussion 3.1. XRD and XPS analyses The XRD patterns for a series of synthesized composites ZS-HC, SZLPC, SAPO-34, ZnO-ZrO2 and physically blended ZS-PB are presented in Fig. 1a. No diffraction peak of ZnO-ZrO2 phase was observed in the diffractogram, and the characteristic diffraction peak of SAPO-34 crystal phase was obviously weakened, indicating that no composite phase of SAPO-34 and ZnO-ZrO2 existed in the SZ-LPC composite sample fabricated via liquid phase precipitation coating process. The result suggested that the added SAPO-34 powder inhibited the transition of zinc-zirconium precursor to ZnO-ZrO2 crystal phase during liquid phase precipitation, and also the coated SAPO-34 phase was significantly eroded by the matrix solution. This unexpected phenomenon was mainly due to the pH variation in the matrix solution caused by the continuous dropping of alkaline precipitant. In addition, the typical peaks (2θ = 9.5°, 12.8°, 20.6°, 25.9°, 30.7°) were the characteristic diffraction peaks of zeotype SAPO-34 (PDF # 47–0630), and the peaks at 30.3°, 35.2°, 50.6° and 60.2° were attributed to the ZnO-ZrO2 phase (PDF # 50–1089). These results show that ZS-HC and ZS-PB composite samples were successfully fabricated via hydrothermal coating and physical blending methods, respectively. It must be noted that the
3.2. XRF analysis The elemental analysis results of synthesized composite ZS-HC and physically blended ZS-PB are shown in Table 1. It can be seen that the contents of Zn, Zr, Si, Al and P in the synthesized composite ZS-HC were almost the same with those in the physically blended ZS-PB. 3.3. SEM, TEM and EDX analyses SEM and TEM images of the as-prepared ZS-PB and ZS-HC samples 135
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Fig. 2. Zn2p and Zr3d XPS spectra of synthesized composite sample ZS-HC and physically blended ZS-PB.
for investigating the surface acidic and basic natures. CO2-TPD and NH3-TPD profiles over synthesized composite sample ZS-HC and physically blended ZS-PB are illustrated in Fig. 6 and Fig. 7, respectively. As shown in Fig. 6, the differences in the positions of the desorption peaks were very small for all the samples, but the amount of weak base in ZSPB sample was significantly higher than that in ZS-HC sample. This indicates that there was obvious change in the amount of weak base with the different distances between the two compounds in composite samples. In Fig. 7, the desorption step at 108 °C was observed for the ZS-HC sample, indicating that weak acid sites simultaneously appeared. Compared with ZS-HC sample, the ZS-PB composite sample presented three peaks with maxima at the temperatures of 112 °C, 354 °C and 656 °C, clearly suggesting that weak, medium-strong and strong acid sites simultaneously existed in ZS-PB samples. The results also demonstrated that the close interaction between the two compounds had a significant impact on the acid properties of the composite sample. As can be seen, the considerable chemical strength between two phases induced by overly tight contact between composite phases was unfavorable for the formation of a large number of active sites in ZnOZrO2/SAPO-34 composite sample.
Table 1 Elemental analysis of synthesized composite ZS-HC and physically blended ZSPB. Samples
ZS-HC ZS-PB
Element content (W %) Zn
Zr
Si
Al
P
19.48 19.49
59.71 59.75
2.74 2.71
8.48 8.44
8.46 8.48
are shown in Fig. 3 and Fig. 4, respectively. In addition, the local elemental composition of ZS-HC composite sample was analyzed by energy-dispersive X-ray spectrometry, and the results are shown in Fig. 5. As shown in Figs. 3b, 4c and 5, the ZS-HC composite sample exhibited typical encapsulated structure. The results of XRD, TEM and EDX analyses confirmed that SAPO-34 molecular sieve was completely coated with ZnO-ZrO2 crystal phase, indicating that the ZnO-ZrO2 particles were tightly attached to SAPO-34 particles (Fig. 4(c)). In the case of ZS-PB composite sample, the cubic SAPO-34 crystal phase and the nanoparticle ZnO-ZrO2 crystal phase were independent of each other, as seen from Figs. 3a and 4a. Only a small portion of ZnO-ZrO2 crystals were adhered to the surface of SAPO-34 grains. This result also demonstrated that a relatively wide distance existed between the two compounds in ZS-PB composite sample fabricated via physical blending.
3.5. N2 isothermal adsorption-desorption analysis Fig. 8 displays N2 adsorption-desorption isotherms of synthesized composite ZS-HC and physically blended ZS-PB. The isotherm presented the H4 hysteresis loop and the characteristic trend of type-I isotherm, indicating that the ZS-HC composite sample could be classified as microporous material with uniform pore structure. In comparison with ZSHC, a typical shape of H3 hysteresis loop was observed in ZS-PB composite sample, indicating that various pore distributions existed in ZSPB composite. The data of BET specific surface area and pore volume of
3.4. CO2-TPD and NH3-TPD analyses Generally, the basicity of metal oxide has a great influence on the adsorption and activation of acidic CO2. However, the acidity of molecular sieves exerts a significant impact on the catalytic behavior for MTO reaction. Hence, NH3-TPD and CO2-TPD are efficient techniques
Fig. 3. SEM images of synthesized composite ZS-HC and physically blended ZS-PB. (a) ZS-PB; (b) ZS-HC. 136
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Fig. 4. HRTEM images of synthesized composite ZS-HC and physically blended ZS-PB. (a) and (b) HRTEM images of ZS-PB; (c) and (d) HRTEM images of ZS-HC.
Fig. 5. TEM–EDX characterization of synthesized composite sample ZS-HC. Fig. 6. CO2-TPD spectra of synthesized composite sample ZS-HC and physically blended ZS-PB, ZS-HPB-300 °C, ZS-HPB-400 °C, ZS-HPB-500 °C, ZS-HPB-600 °C and ZS-HPB-700 °C.
synthesized composite ZS-HC and physically blended ZS-PB are shown in Table 2. The ZS-HC composite mainly exhibited a large amount of micropores and less amount of mesopores. However, the ZS-PB composite had significantly increased surface area of mesopores (121 m2 g−1) and mesopore volume (0.24 cm3 g−1) compared with the ZS-HC composite. From the above discussion, it is evident that the ZSPB composite fabricated via physical blending retained the initial pore structures of the two phases of SAPO-34 and ZnO-ZrO2, while the ZS-HC composite fabricated via hydrothermal coating showed mainly
microporous structure. This could be attributed to the chemical interaction of ZnO-ZrO2 with SAPO-34 during the hydrothermal coating process. 4. Catalytic performances The effects of the preparation methods on the CO2 conversion and 137
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Fig. 9. Catalytic performance in CO2 hydrogenation of synthesized composite sample ZS-HC and physically blended ZS-PB.
Fig. 7. NH3-TPD spectra of synthesized composite sample ZS-HC and physically blended ZS-PB, ZS-HPB-300 °C, ZS-HPB-400 °C, ZS-HPB-500 °C, ZS-HPB-600 °C and ZS-HPB-700 °C.
affinity of the system for CO2. In addition, the selectivity for primary olefin products and methanol conversion are strongly dependent on the Brønsted acidic sites of SAPO-34, while the Lewis acidic sites on ZnOZrO2 are beneficial to obtain B-L synergistic acidic sites in the composite samples [20,30]. Hence, the existence of the overall Lewis basic and B-L synergistic acidic sites on composite samples is a significant advantage for the CO2 conversion and formation of lower olefins. It is worth mentioning that the mesopores can act as channels for the enhanced transfer of reaction products from the pores to the bulk gas phase [31]. Thus, the existence of the structure with micro-mesopores is advantageous for the hydrogenation of CO2 to hydrocarbons. As a result, extremely tight contact of two-phase structure of the composite might increase the probability of unfavorable lower olefins formation in target hydrocarbons produced from SAPO-34 nearby ZnOZrO2. Conversely, the existence of discrete states between these two phases was beneficial to the catalytic performance, implying that only suitable distance of two-phase structure could result in the enhanced catalytic performance. In order to further promote the catalytic performance of ZS-PB composite sample fabricated via physical blending method in the hydrogenation reaction of CO2 to hydrocarbons, the ZnO-ZrO2 precursor was subjected to H2 reduction treatment at different temperatures. The catalytic performances of the ZS-HPB composite samples after H2 reductions at various temperatures are shown in Fig. 10. In comparison with ZS-PB, it was found that the performance of ZS-HPB composite sample was significantly affected by H2 reduction temperature. As the H2 reduction temperature increased, the selectivity of CO was lowered from 41% to 27%, and a minimum of 27% was observed over the composite sample after H2 reduction treatment at 500 °C. However, higher H2 reduction temperature led to the increase in CO selectivity. In addition, compared with ZS-PB, the selectivity for CO on H2 reduced composite decreased by approximately 14%. On the other hand, the selectivity for all hydrocarbon products showed a similar trend and the highest selectivity for hydrocarbons was reached 73% with the 70% proportion of lower olefins over the sample of 500 °C H2 reduction treatment.
Fig. 8. N2 adsorption desorption isotherms of synthesized composite sample ZSHC and physically blended ZS-PB.
product selectivity are shown in Fig. 9. As seen from Fig. 9, the CO2 hydrogenation on the ZS-HC composite sample displayed 21% selectivity for C2=- C4=, 19% for C20- C40, 51% for CH4, and 9% for C5+ among all hydrocarbon products, at the CO2 conversion of 13% under reaction conditions of 3 MPa, 3500 mL g−1 h−1 and 380 °C. Also, the CO selectivity was as high as 51%. Conversely, ZS-PB composite sample exhibited much higher C2=- C4= selectivity (70%) and lower CH4 selectivity (3%) compared with ZS-HC composite sample. It can be inferred that the better catalytic performance of ZS-PB composite was directly related to the synergetic effects of higher activity and distinct micro-mesoporous structure. As is known, zirconia possesses surface Lewis basic and acidic sites, and more basic sites would enhance the
Table 2 Specific surface area, pore volume and average pore diameter data of synthesized composite sample ZS-HC and physically blended ZS-PB. Samples
ZS-PB ZS-HC
BET surface area (m2·g−1)
Pore volume (cm3·g−1)
Pore diameter (nm) D
SBET
Smic
Smes
Vt
Vmic
Vmes
229 178
108 145
121 33
0.39 0.21
0.15 0.09
0.24 0.12
138
6.8 4.5
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the intensities of medium-strong acid sites became stronger and some weak base sites transferred to medium-strong base sites in ZS-HPB composite samples (Figs. 6 and 7). These alterations of the acid and basic sites roused by H2 reduction might be the primary reason for lower amount of CO by-product over ZS-HPB composite samples. The above result could be further confirmed from the XPS data (Fig. 12). Significant changes in the peak strengths and positions of Zn2p and Zr3d were observed in ZS-HPB-500 °C sample, which was possibly because the electronic property of Zn and Zr sites was altered by H2 reduction. The electronic property tuning of Zn and Zr sites result in alterations of the acid and basic sites, significantly suppressing the formation of CO by-product.
5. Conclusions In summary, composites of SAPO-34 and ZnO-ZrO2 were successfully fabricated via hydrothermal coating and physical blending methods, respectively. The obtained composites exhibited completely different physicochemical properties caused by the interaction between the two active compounds. Characterization results indicated that the composite phase was obtained along with a large number of coexisting acidic and basic sites and distinct micro-mesoporous structure, and without tight contact between the two active compounds, in the physically blended composite. Catalytic test results showed that physically blended composite exhibited higher catalytic performance with 70% selectivity for lower olefins among all hydrocarbon products, but the CO selectivity was around 41%. Furthermore, H2 reduction treatment was performed on ZnO-ZrO2 precursor to explore the electronic properties of the physically blended composite. The results showed that the alterations of the acid and basic sites resulting from electronic property tuning of Zn and Zr sites significantly suppressed the formation of CO by-product. The CO selectivity significantly decreased from 41% to 27%. The excellent catalytic performance for physically blended composite after H2 reduction could be attributed to the synergetic effect of composite phase with suitably tuned electronic property, large number of acidic-basic sites and distinct micro-mesoporous structure. Therefore, this study demonstrated that enhanced catalytic activity for CO2 conversion, superior shape selectivity for lower olefins and less amount of CO by-product could be achieved by using hydrogen-reduced ZnOZrO2/SAPO-34 composite catalyst.
Fig. 10. Catalytic performance of composite sample after H2 reduction treatment.
Fig. 11. XRD patterns of synthesized composites SAPO-34 and ZnO-ZrO2 and physically blended ZS-PB and ZS-HPB-500 °C.
In comparison with the composite ZS-PB, a trace amount of ZnO phase was present in the composite reduced at 500 °C from XRD data (2θ = 32°, 36°, 48°, 57°, Fig. 11), but it was interestingly observed that
Notes The authors declare no competing financial interest.
Fig. 12. Zn2p and Zr3d XPS spectra of physically blended ZS-PB and ZS-HPB-500 °C. 139
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Acknowledgments
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