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SBA-15 catalytic performance of CO2 hydrogenation to methanol
JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 47, Issue 10, October 2019 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2019, 47(10), 12141225
RESEARCH PAPER
Effect of additive on CuO-ZnO/SBA-15 catalytic performance of CO2 hydrogenation to methanol LIN Min1,2, NA Wei1,3,*, YE Hai-chuan1,3, HUO Hai-hui1,3, GAO Wen-gui1,3 1
State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming
650093, China; 2
Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650093, China;
3
Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China
Abstract: Three kinds of porous catalysts CuO-ZnO/SBA-15 (CZ/SBA-15), CuO-ZnO-MnO2/SBA-15 (CZM/SBA-15) and CuO-ZnO-ZrO2/SBA-15 (CZZ/SBA-15) were synthesized by impregnation method with a siliceous framework mesoporous molecular sieve SBA-15. The performance of all catalysts for catalytic hydrogenation of CO 2 to methanol was evaluated on a fixed bed reactor, combined with N2 adsorption-desorption (BET), X-ray diffraction (XRD), H2 temperature-programmed reduction (H2-TPR), temperature programmed desorption (H2-TPD, CO2-TPD), N2O titration, X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM) . The results show that the introduction of metal oxide in the catalyst changes the pore size and specific surface area of the SBA-15 molecular sieve support. The CuO-ZnO-MnO2/SBA-15 and CuO-ZnO-ZrO2/SBA-15 have high copper dispersion (DCu), large specific surface area (ACu), small surface CuO particle size, and easy to be reduced. Compared with the Mn–O cluster, the Zr–O cluster enhances the basic site and improves the methanol selectivity. In addition, CuO-ZnO-ZrO2/SBA-15 has the highest oxygen vacancy concentration and better catalytic activity among three catalysts. The methanol selectivity of CuO-ZnO-ZrO2/SBA-15 is 25.02%, which is 28% and 136.9% higher than those of CuO-ZnO/SBA-15 and CuO-ZnO-MnO2/SBA-15, respectively. Key words:
molecular sieve SBA-15; oxygen vacancy; alkaline sites; methanol
Methanol is an important chemical raw material[1]. There are two main ways to produce methanol: one is to oxidize methane directly to methanol[2], the other is to use CO2 hydrogenation to produce methanol. Nowadays, the increase of carbon dioxide concentration has caused possibly irreversible changes to climate and environment[3]. Catalytic conversion of CO2 to methanol is a promising route that may offer a solution to energy and environment[4,5]. The excellent catalysts have been a key research to improve the catalytic performance. The main factors affecting the performance of catalysts are active components, supports and catalytic promoters [6,7]. At present, most researchers believe that copper-based catalysts are the most effective catalysts in the methanol production process[8–10]. Although changing preparation methods or adding additives can improve the activity of copper-based catalysts, it also has some problems, such as uneven
dispersion of active components and easy sintering of catalysts. Therefore, the conventional copper-based catalysts have been widely modified with different supports such as Al2O3 and ZrO2, but the hydrophilicity of alumina impairs the stability of the catalyst. Thus, there is an urgent need to study alternative carrier materials[11,12]. Koh et al[13–16] synthesized a series of copper catalysts Cu-ZnO-MnO (CZM) supported on porous siliceous SBA-15 with different morphologies to study the effect of support morphology on the catalyst. The results showed that the catalytic activity of mesoporous molecular sieve SBA-15 was significantly improved. Phongamwong et al[17] concluded in a recent study that adding SiO2 to Cu/ZnO/ZrO2 could improve the methanol synthesis activity by increasing the surface area of active site copper and the basicity of catalyst. In addition, Toyir et al[18] further discovered that the performance of Cu-Zn-Ga supported on hydrophobic silica (SiO2) was enhanced during methanol
Received: 25-Jun-2019; Revised: 05-Sep-2019. Foundation items: Supported by the Natural Science Foundation of China (51404122, 51404099) and the Natural Key Technologies R&D Program of China (2011BAC01B03). *Corresponding author. E-mail: [email protected]. Copyright 2019, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
synthesis process. Chen et al[19] prepared small particles of Cu/SBA-15 catalyst with modified SBA-15 as support, which improved the catalytic activity of water vapor reaction. More studies have shown that SBA-15 has good application prospects as catalyst carrier material because of its flexible pore structure and large specific surface area[20–23]. The porous SBA-15 also has a great advantage in gas phase adsorption[24]. In view of this, SBA-15, a porous siliceous material, was used as a carrier in this study to improve the sintering resistance of the catalyst and promote the dispersion of active components on the surface of the carrier. Early studies show that Cu/SiO2 catalyst has good activity performance in hydrogenation reaction[25,26]. However, single SiO2 carrier can’t effectively stabilize the active components, which results in the growth and inactivation of copper particles in high temperature and high space velocity reduction atmosphere. How to improve the stability of active components in the reaction process has always been a scientific problem to be solved urgently in related fields. Many studies have proved that the addition of some metal oxides can greatly promote the synthesis of methanol. Li et al[27] prepared Cu-Zn-Zr/SBA-15 with high specific surface area to study the effects of different surface structures on the catalytic activity, and it was proved that the surface structure of the catalyst played a key role in the catalytic activity. In an earlier study, Słoczyński et al[28] found that adding metal oxides B, Ga, In, Gd, Y, Mn and Mg to Cu/ZnO/ZrO 2 could change the structure or properties of the catalyst. In their other studies, it was proved that the addition of promoters Zn and Mn improved the dispersion of reduced copper and the size of copper microcrystals, which was beneficial to methanol synthesis[29,30]. However, the amount of additives is also important. Zhu et al[31] investigated the effect of different zinc content on the dispersion of Cu species on Al2O3 support. They found that proper zinc addition could improve the dispersion of Cu species on Al2O3 support, and excessive Zn led to the formation of a large number of CuO crystals and the decrease of catalyst activity. In the study of the effect of additives on the size and distribution of CuO grains, Yin et al[32] found that adding appropriate amount of manganese promoter could significantly improve the activity and thermal stability of the catalyst, and manganese played a role in preventing the growth of CuO grains and promoted the dispersion of CuO. Furthermore, Hao et al[33] investigated the effects of three additives (Zr, Ba and Mn) on the catalytic performance of Cu/ZnO/Al2O3 (CZA). The results showed that the introduction of Zr reduced the maximum active temperature of CZA catalyst and enhanced the catalytic stability. Moreover, the mechanism of the effect of additives on active sites has also been studied. Wang et al[34] found that the introduction of zirconium, aluminium and manganese
facilitated the formation of different size copper microcrystalline, which resulted different amounts of active sites. Witoon et al[35] studied the effect of ZrO2 on the activity of copper catalyst. The results showed that amorphous ZrO 2 could increase the alkaline site concentration on the surface of the catalyst, enhance the interaction between active components and improve the performance of the catalyst. And then, Atakan et al[36–39] studied the effect of Zr-doped mesoporous silica (Zr-SBA-15) with symbiotic copper nanoparticles on the hydrogenation of carbon dioxide. It was found that the loading method of copper affected the selectivity of products, and the addition of Zr promoted the activity of catalysts. The results of Liu et al[40] showed that Al and Mn could also significantly affect the specific surface area and basicity of the catalyst. In summary, the addition of additives would affect the performance of catalysts to some degree. However, the effects of different promoters on the performance of catalysts need be further studied. For the above reasons, mesoporous molecular sieve SBA-15 was used as support to improve the dispersion of active components of the catalyst in this study. We synthesized mesoporous catalysts CZ/SBA-15, CZM/SBA-15 and CZZ/SBA-15 with different additives by traditional impregnation method, and compared the physical, and chemical properties of these catalysts, finally, studied the effect of adding Zr and Mn on the catalytic performance of CO2 hydrogenation to methanol.
1 1.1
Experimental Experimental materials
Cu(NO3)2·3H2O, Zn(NO3)2·6H2O, Zr(NO3)2·5H2O, Mn(NO3)2, TEOS, HCl and citric acid all purchased in Guoyao Group Company Limited. P123 was purchased from Sigma Aldrich. The standard mixture of H2 and CO2/H2 was purchased from Kunming Messel Gas Company Limited. 1.2
Catalyst preparation
All catalysts were synthesized by impregnation method. The SBA-15 was prepared by hydrothermal method[41,42]. Carriers account for 70% and active components account for 30% (by weight) in all catalysts. The active component was composed of a certain amount of copper and zinc and another metal element with the mass ratio of 5:4:1 among them. Firstly, a certain amount of Cu(NO3)2·3H2O, Zn(NO3)2·6H2O, Zr(NO3)2·5H2O and Mn(NO3)2 were dissolved in deionized water, and then we added the carrier SBA-15 and citric acid with corresponding mass ratio to make the solutes disperse uniformly by ultrasound about 10 min. The mixture was dried
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
in a water bath at 90°C, and then we transferred it into an oven at 60°C for 12 h until drying to obtain the catalyst precursor. A certain amount of the catalyst precursor was calcined in a muffle furnace at a heating rate of 1°C /min to 350°C for 4 h to obtain the catalyst CZX/SBA-15, where X represents an equivalent amount of Zn, Mn and Zr. 1.3
Catalyst characterization
XRD (D/MAX-2400 X diffractometer, Japan) was performed with a Cu Kα radiation at 40 kV and tube current at 100 mA form 0°–10° (the scan rate: 2(°)/min) and 10°–90° (the scan rate: 5(°)/min). The specific surface areas and pore volume were determined from N2 adsorption-desorption (Autosorb-iQ-C, Quantachrome, America). The catalyst samples 150 mg were degassed at 300°C under vacuum for 3 h, and then N2 adsorption-desorption analysis was carried out under liquid nitrogen (–196°C). The N2O titration characterization was performed on a Chemisorbent (Chem BET Pulsar & TPR/TPD, Quantachrome, America). References method is used to calculate specific surface area and dispersion of copper in catalysts[43]. The whole characterization process was completed by twice TPR program. The first consumption of H2 is recorded as n1, the second TPR program will be executed after cooling to room temperature, and the amount of H2 consumed is recorded as n2. The dispersion (D) and specific surface area (A) of copper are calculated as follows: D = (2×n2/n1)×100% (1) A = 2×n2×Nav/(n1×MCu×1.4×1019) = 1353×n2/n1 (m2(Cu)/g(Cu)) (2) In the formula, Nav denotes Avogadro constant, MCu denotes the relative atomic mass of copper (63.456 g/mol), 1.4×1019 is the relative density of copper atoms on the surface. The crystallite size of copper in the reduced catalyst was calculated based on the Debye-Scherrer formula: d = K/(cos) (3) Where d is grain size, is the full width at half maximum (FWHM), is a diffraction angle, K represents a constant (K = 0.89) and is the wavelength of Cu K X-rays ( = 0.154 nm)[44]. H2-TPR, H2-TPD and CO2-TPD were completed on the Chemisorbent (Chem BET Pulsar & TPR/TPD, Quantachrome, America). H2-TPR was carried out using 50 mg catalyst in 10% H2/90% He gas flow with a flow rate of 25 mL/min and a heating rate as 10°C/min. In characterization of CO2-TPD, the He was used as carrier gas, catalyst (50 mg) was first reduced in H2 gas stream at 280°C for 1 h, and then switched to He blowing for 30 min, when the temperature was lowered to 40°C, the adsorption of CO2 was switched to saturation and He was purged for 60 min. At last, the CO 2 desorption was
completed by temperature programmed desorption to 900°C using a heating rate of 10°C/min. The characterization method of H2-TPD was similar to that of CO2-TPD. The differences were 100 mg catalyst used and H2 adsorption to saturation after He purging and cooling. The desorption was completed by purging with He for 60 min and then heating up to 900°C at 10°C /min. XPS results (PHI5000 Versaprobe-II) were obtained using Al K X-ray at 15 eV (hν=1486.6 eV). All binding energies were calibrated by the C 1s (284.8 eV) standard line. The morphology of sample was probed by TEM (JEM-2001, Japan). Firstly, the powder sample was dispersed in absolute ethanol by ultrasound, and then the suspended liquid droplets from the upper layer were added to the nickel mesh, and the nickel mesh was placed on the electron microscope instrument for observation. Elements of materials were probed by EDX (APOLLO, Idax, America). 1.4
Catalytic activity measurements
The activity evaluation of catalytic hydrogenation of CO2 to methanol was carried out in a self-made high temperature and high pressure tubular micro reactor. 1.0 g (20–40 mesh) catalyst and 1.5 g quartz sand were mixed and evenly packed into tubular reactor. Catalyst was reduced first by H2 for 2 h at 280°C and natural cooled below 160°C after completion. The catalyst activity was tested under the condition of VH2:VCO2=3:1, 3.0 MPa, 250°C, GHSV = 6000 h –1. The final product was separated by condensation and analyzed by gas chromatography (Agilent6890) with capillary columns (HP-MOLESIEVE (30 m×0.535 mm×50.00 m), HP-PLOT/Q (30 m×0.535 mm×40.00 m)). The content of each target component was compared with the standard signal values previously and calibrated with a certain concentration of CO, CO2, CH4 and CH3OH. The content of CO and CO2 were measured by TCD, and CH4 and CH3OH were measured by FID. Finally, the conversion of CO2, selectivity and yield of methanol as the target product were calculated by referring to the detection signal and the standard signal value.
2 2.1
Results and discussion X-ray diffraction (XRD)
Small-angle X-ray diffraction patterns of SBA-15 are illustrated in Figure 1(a). The diffraction peaks at 0.8, 1.4 and 1.6 correspond to the characteristic diffraction peaks of (100), (110) and (200) plane of SBA-15, respectively. It shows that the prepared carrier SBA-15 has hexagonal orderly mesoporous structure. Figure 1(b) shows the small angle XRD spectrum of the catalyst.
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
Fig. 1
XRD patterns of the catalyst
(a): low-angle XRD pattern of SBA-15; (b): wide-angle XRD patterns of SBA-15; (c): wide -angle XRD patterns of the catalysts; (d): wide-angle XRD patterns of the catalysts after reduction
Table 1
a
Grain size of Cu in the catalyst after reduction
Sample
C/SBA-15
CZ/SBA-15
CZM/SBA-15
CZZ/SBA-15
daCu/nm
19.8
18.9
17.8
15.4
: the crystallite size of Cu NPs was calculated by Scherrer's equation, where 2θ = 43.3°, 50.5°, 74.2°.
All catalysts exhibit a small angle XRD diffraction peak consistent with SBA-15, indicating that the mesoporous structure of SBA-15 is not destroyed after metal (Cu, Zn, Mn, Zr) loaded. At the same time, with the addition of Zr and Mn, the positions of the diffraction peaks of the catalysts shift to a low angle, indicating that the lattice constants of the catalysts are changed and the ordering of the pore structure of the supports is decreased[45]. The results of small angle XRD show that the mesoporous structure of support SBA-15 is not destroyed after the adding of metal oxides (Cu, Zn, Mn, Zr). Results of wide-angle X-ray diffraction patterns of samples are illustrated in Figure 1(c). As shown in Figure 1(c), all samples exhibit a wide diffraction peak at 24, which belong to amorphous SiO2. The corresponding diffraction peaks of CuO (110), (–111), (111), (–202), (–113) crystal plane (PDF#74-1021) and ZnO (111), (200), (311) crystal plane (PDF#77-0191) in the catalyst have been marked at corresponding positions, respectively. Results show that metal oxides are successfully introduced into the catalyst. It is worth noting that there are not the diffraction peaks corresponding to
the oxides of Mn and Zr. This is because the addition of these two elements is too small to be detected by XRD. In addition, in order to further illustrate the effect of additives on copper crystal size, a single copper component (C/SBA-15) is designed, reduced by H2 and characterized by wide-angle XRD. The results are shown in Figure 1(d) and Table 1. The results show that the grain sizes of the reduced Cu are in the order of C/SBA-15 ˃ CZ/SBA-15 ˃ CZM/SBA-15 ˃ CZZ/SBA-15. Wide-angle XRD results show that the grain size of Cu decreased after the introduction of Zn. Interestingly, the particle size of Cu further decreased with the addition of Zr and Mn. This is because the addition of metal oxides increases the dispersion of copper particles and decreases the grain size of copper. In addition, the BET results of each sample show that the specific surface area (ABET), pore volume (vpore) and pore size (dpore) of the catalyst are lower than those of SBA-15 (Table 2). It indicates that the metal particles enter the pore structure of SBA-15 or adhere to the surface of the support, which reduces the pore size and specific surface area, but the catalyst still has a good pore structure.
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
Fig. 2
N2 adsorption-desorption isotherm of the catalyst
In a word, the addition of metal oxides facilitate the dispersion of Cu in the catalyst, resulting in the improvement of the catalytic performance. 2.2
N2 adsorption-desorption isotherms
The N2 adsorption-desorption isotherms and pore size distributions of samples are illustrated in Figure 2. As shown in Figure 2, the mesoporous molecular sieve SBA-15 exhibits a typical Langmuir IV adsorption isotherm. According to the classification of IUPAC, H1-type adsorption hysteresis loops appear in the middle section, and the hysteresis loops are steep. It shows that SBA-15 is a cylindrical hole with uniform diameter distribution and high orderliness. Moreover, Table 2
a
2
CZ/SBA-15, CZM/SBA-15 and CZZ/SBA-15 all exhibit the same typical Langmuir IV adsorption isotherms, indicating that all catalysts have the similar pore structure as SBA-15. The hysteresis loops of the catalysts are relatively flat, indicating that the introduction of metal oxides has a certain effect on the pore structure of SBA-15. According to the literature report[46], the inflection point of N2 adsorption-desorption isotherm can reflect the pore size of mesoporous materials. Compared with SBA-15, the adsorption isotherms of all catalysts are flat and move towards lower pressure, which indicate that the pore size, specific surface area and pore volume of mesoporous molecular sieve SBA-15 are reduced after loading metal oxides (shown in Table 2). 2.3
N2O titration
In order to further study the distribution of CuO particles in the catalyst, N2O titration of the catalyst were carried out. The dispersion and specific surface area of copper are calculated according to the formula. The related parameters are shown in Table 2. The results show that the dispersion (DCu) and specific surface area (ACu) of copper on CZM/SBA-15 and CZZ/SBA-15 are larger than those on CZ/SBA-15 catalysts. Among them, CZZ/SBA-15 has the largest DCu and ACu, 19.6% and 133.2 m2/g respectively.
Physical property parameter table of the catalyst
–1
Sample
ABET/(m g )
vapore/(cm3g–1)
dpore/nm
DCu/%
ACu/(m2·g–1)
SBA-15
863.25
1.17
7.41
–
–
CZ/SBA-15
382.96
0.60
6.27
7.80
52.6
CZM/SBA-15
412.87
0.65
6.24
11.4
77.3
CZZ/SBA-15
387.60
0.54
5.57
19.6
133.2
: total pore volume obtained from p/p0 = 0.99
It is attributed to the formation of a more complex Cu-Zn-Mn and Cu-Zn-Zr composite oxide layer than that of pure Cu-Zn due to the addition of a small amount of Mn and Zr metal oxides in the catalyst[27]. It has been reported that the addition of metals during the formation of catalysts enhances the interaction between metal oxides, resulting in copper oxides adhering to other metal oxides and evenly distributing on the surface of catalysts[47], and then the dispersion of copper (DCu) and specific surface area (ACu) on the catalyst surface are improved. 2.4 Fig. 3
H2-TPR patterns of the catalyst
H2-TPR
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
Fig. 4
H2-TPD (a) and CO2-TPD (b) patterns of the catalyst
Table 3 Sample CZ/SBA-15 CZM/SBA-15 CZZ/SBA-15
Catalysts adsorption performance parameters
H2-TPD area
maximum desorption temperature t/℃
479
6415
149
183
470
1736
476 480
The characterization of catalysts by H2-TPR is used to explore the difficulty of hydrogen reduction of different catalysts. As shown in Figure 3, three catalysts show obvious reduction peaks at 250–400°C, which corresponded to the reduction of CuO, since ZnO, ZrO2 and MnO2 couldn’t be reduced in this temperature range. In addition, the reduction temperatures of all catalysts are different. The order of reduction temperature from high to low is as follows: CZ/SBA-15 (354°C) ˃ CZM/SBA-15 (334°C) ˃ CZZ/SBA-15 (322°C). The results indicate that with the addition of Zr and Mn the reduction temperature of catalyst decreases, and the CuO in the catalyst is easier to be reduced. According to the XRD characterization of the catalyst, the oxides of zirconium and manganese inhibit the growth of CuO particles, increase the dispersion of CuO particles, lead to the reduction of CuO species more easily, and decrease the reduction temperature of the catalyst. 2.5
CO2-TPD
maximum desorption temperature t/℃
Adsorption properties of catalysts (H2-TPD, CO2-TPD)
Figure 4 (a) is the H2-TPD curve of the catalyst. A wide range H2 desorption curves (200–700°C) are appeared with different strength. It is clear that the whole desorption process can be divided into two strong stages. It is worth noting that the mesoporous SBA-15 only has desorption peak at high temperatures, which indicates that the pore structure of the catalyst is helpful to the adsorption of H 2.
7485 7262
area
138
118
473
1988
141
159
488
2253
According to the relevant literature [48], only the weak desorption peaks at low temperature (200–300°C) belong to the desorption of hydrogen atoms on the Cu site, while the high temperature section indicates the desorption of hydrogen strongly adsorbed by copper or other metal oxides (Cu+–O–Zn2+), and the hydrogen adsorption performance is mainly attributed to the interaction of copper and zinc [49,50]. In order to know more clearly about the hydrogen adsorption capacity of the catalysts, the maximum desorption temperature and peak area of each catalyst are shown in Table 3. As shown in Table 3, the H 2 desorption capacity of the three catalysts is in the order: CZM/SBA-15 > CZZ/SBA-15 > CZ/SBA-15, suggesting the addition of Mn and Zr oxides facilitates the hydrogen adsorption of the catalyst. Relevant studies have shown that [51], the strong interaction between ZnO and Cu enhances H 2 spillover and improves the adsorption of H 2 on ZnO metal oxides, and the addition of metal oxides (Mn, Zr) further enhances the adsorption of H2 on catalysts[13]. Figure 4(b) is the CO2-TPD curve of the catalyst. From Figure 4(b), it can be seen that the desorption peaks of carbon dioxide appear in all catalysts and SBA-15 support in different temperature ranges, indicating that mesoporous material SBA-15 is helpful to the adsorption of CO2 on catalysts. However, as far as the system is concerned, the adsorption of CO2 on the catalyst at low temperature is the main factor affecting the activity of the catalyst.
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
Fig. 5
XPS spectra of the catalyst
(a): full spectra; (b): O 1s; (c): Cu 2p; (d): Zn 2p; (e): Mn 2p; (f): Zr 3d
Table 4 Sample
XPS peak data of each element in the catalyst
E/eV
E/eV
Cu 2p3/2
Cu 2p1/2
Mn 2p3/2
E/eV
O 1s/%
Mn 2p1/2
Zr 2p5/2
Zr 2p3/2
OI
OII
OIII/ OIII
(OI+OII)
CZ/SBA-15
933.1
953.1
–
–
–
–
22.8
42.2
35
0.53
CZM/SBA-15
933.1
953.1
641.6
654.1
–
–
22.4
47.4
30.2
0.43
CZZ/SBA-15
933.1
953.1
–
–
182.4
184.8
20
42.4
37.6
0.60
Compared with SBA-15, the desorption temperature of all catalysts shifts to low temperature in varying degrees. This phenomenon is related to oxygen atoms in low coordination M–O clusters (M denotes different metals such as Zr, Mn, and Zn)[48], which demonstrates that the desorption peaks in the low temperature range of the catalyst are related not only to the Si–OH clusters in the catalyst[52], but also to the interaction of metal Cu , Zn oxides. The parameters of desorption peaks
in catalysts are listed in Table 3. The results show that the CO2 adsorption peak of catalyst CZM/SBA-15 is small at low temperature, while the CO2 adsorption capacities of other catalysts are large at low temperature, and the alkaline site provided by the oxide of Mn is below 500°C[53], indicating the basic sites provided by Mn–O clusters are not as many as those provided by Zn–O and Zr–O clusters in low temperature.
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
Fig. 6
TEM images of the catalyst
(a): C/SBA-15; (b): CZ/SBA-15; (c): CZM/SBA-15; (d): CZZ/SBA-15; (e): energy spectrum of CZM/SBA-15; (f): energy spectrum of CZZ/SBA-15
Table 5
Catalytic performance evaluation of catalysts
Catalyst
xCO2/%
sCH3OH/%
sCO/%
wCH3OH/(mmolg–1h–1)
CZ/SBA-15
8.7
19.54
80.46
0.42
CZM/SBA-15
8.2
10.56
89.40
0.25
CZZ/SBA-15
8.1
25.02
74.97
0.99
reaction condition: t=250°C, p=3 MPa, H2/CO2(volume ratio)=3:1, and SV=6000 mL/(g·h)
In addition, the adsorption data at high temperature show that the addition of Zr and Mn increase the desorption area of CO2. The desorption peak of this temperature segment is attributed to the adsorption of CO2 on the catalyst surface O2–[13,54]. In conclusion, the results of CO2-TPD show that the basic sites on the surface of the catalyst are mainly attributed to the interaction of metal Zr, Zn, Cu and O. Zr–O and Zn–O clusters increase the basic sites of catalysts, especially CZZ/SBA-15. It’s attributed to the formation of Zr4+–O2– acyl
pairs[50]. However, the effect provided by Mn–O clusters at relatively low temperatures is weak, resulting in poor catalytic performance. 2.6
XPS spectra
Figure 5(a) is the XPS full spectrum of the catalyst. The binding energies of each element are shown in the figure. Peaks of Cu 2p are observed at 932–934 eV and 952–954 eV
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
for all catalysts (Figure 5(c)), corresponding to Cu 2p3/2 and Cu 2p1/2, respectively. It is noteworthy that there are satellite peaks between 936–946 eV. These satellite characteristic peaks are due to charge transfer between copper 3d orbital and oxygen 2p orbital, indicating that copper in the catalyst exists in the form of Cu2+[55]. In addition, all catalysts exhibit characteristic peaks of Zn 2p (Zn 2p3/2 and Zn 2p1/2) at 1022–1045 eV (Figure 5(d)), corresponding to Zn2+. In order to better analyze the influence of metal Mn and Zr on the catalyst, the peak-fit processing are carried out. The corresponding results are list in Table 4. The peak values at 642.7 and 654.4 eV belong to Mn4+ in Figure 5(e), and there is not satellite peak at near 647.8 eV, indicating that there is not Mn2+ in the catalyst[56,57]. In Figure 5(f), the characteristic peaks of catalyst CZZ/SBA-15 at 181–185 eV belong to Zr 3d, where 182.4 eV belongs to Zr 3d5/2 and 184.8 eV belongs to Zr 3d3/2, indicating that zirconium in the catalyst exists in the form of Zr4+[58–60]. It has been shown that the oxygen vacancy in the catalyst has an important influence on the formation of intermediate products in methanol synthesis[61,62]. In order to further explore the distribution of oxygen species in catalysts, peak fitting of O 1s is carried out[63], in Figure 5(b). The lowest peak of binding energy is the lattice oxygen of the catalyst itself (OI), corresponding to M–O bond (M denotes Cu, Zn, etc.). The highest peak of binding energy corresponds to the surface oxygen (OIII) of the catalyst. The peak at the middle position corresponds to the oxygen adsorbed on the catalyst surface (OII), which belongs to the Si–O–Si bond. According to the literature[64], lattice oxygen can provide effective catalytic area and ratio of surface oxygen to lattice oxygen indicates the oxygen vacancy concentration of catalyst [65]. The catalyst with higher oxygen vacancy concentration has better activity[66]. The distribution of different types of oxygen content and oxygen vacancy concentration are listed in Table 4. The results show that CZZ/SBA-15 has the largest oxygen vacancy concentration among all catalysts. The addition of metal Zr promotes the interaction between metal oxides and supports, increases the number of oxygen vacancies on the surface of catalysts, and facilitates the formation of intermediate products in methanol synthesis. 2.7
TEM photographs
Figure 6 is the high resolution image of the catalyst under transmission electron microscopy. As can be seen from Figure 6, the pore structure of support SBA-15 in each catalyst is well arranged, indicating that the addition of metal oxides does not damage the pore structure of support. It can be found that with the addition of metal oxides additives, the dispersion of CuO on SBA-15 becomes better, and the agglomeration of
metal oxides in catalysts decreases significantly. Especially for CZZ/SBA-15 with component Zr, the metal oxides are uniformly dispersed on the surface of SBA-15, which indirectly promotes the dispersion of CuO in the catalyst to a large extent. These results are consistent with the previous characterization. In addition, The energy spectrum analysis results show that Mn and Zr are found on the surface of CZM/SBA-15 and CZZ/SBA-15, indicating that Zr and Mn are successfully added to the catalyst. 2.8
Catalytic activity
Table 5 shows the activity evaluation of the catalysts for the hydrogenation of carbon dioxide to methanol under the same conditions. It shows that the conversion rate of carbon dioxide for CZ/SBA-15 catalyst is 8.7%, the selectivity and yield of methanol are 19.54% and 0.42 mmol/(g·h) respectively. For catalyst CZZ/SBA-15, the carbon dioxide conversion decreases, but the methanol selectivity and yield are improved to 25.02% and 0.99 mmol/(g·h) respectively. Compared with CZ/SBA-15 and CZM/SBA-15, the methanol selectivity and yield of CZZ/SBA-15 increased by 28%, 135.7% and 136.9%, 296%, respectively. Combining with XRD and N2O characterization, the radius of Cu particles is the smallest in CZZ/SBA-15. In addition, XPS characterization shows that more oxygen vacancies are formed on the surface of CZZ/SBA-15, which promote methanol synthesis reaction on its surface and enhanced methanol selectivity. On the contrary, the addition of MnO2 does not show a good performance.
3
Conclusions
In this study, mesoporous molecular sieve material SBA-15 was used as catalyst carrier and the catalytic performance of different metal oxides supported SBA-15 was carried out by CO2 hydrogenation to methanol. The results show that the pore structure of mesoporous molecular sieve SBA-15 doesn’t affected by the addition of a small amount of metal oxides, but the pore size and specific surface area are declined at some degree. With the addition of various metal oxides, a more complex multi-oxide layer is formed on the surface of the catalyst, which reduces the size of CuO grains, promotes the dispersion of CuO grains on SBA-15 surface and decreases the reduction temperature of the catalyst. Besides, the addition of Zr enhances the basic site of the catalyst, improves the interaction between metal and oxygen (M–O) in the catalyst, and leads to the higher oxygen vacancy concentration and better catalytic activity. However, the alkaline sites obtained by Mn–O are relatively weak and the catalytic activity is poor, indicating that the particle size and distribution of the active component aren’t the only factors determining the
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
performance of the catalyst. The highest methanol selectivity is obtained by CZZ/SBA-15 catalyst, which increases by 28% and 136.9% respectively compared with the other two catalysts.
[13] Koh M K, Khavarian M, Chai S P, Mohamed A R. The morphological
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copper-catalysts for selective direct CO2 hydrogenation to methanol. Int J Hydrogen Energy, 2018, 43(19): 9334–9342. [14] Koizumi N, Jiang X, Kugai J, Song C. Effects of mesoporous
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Chollacoop
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Chen S,
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Synthesis
RESEARCH PAPER
Effect of additive on CuO-ZnO/SBA-15 catalytic performance of CO2 hydrogenation to methanol LIN Min1,2, NA Wei1,3,*, YE Hai-chuan1,3, HUO Hai-hui1,3, GAO Wen-gui1,3 1
State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming
650093, China; 2
Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650093, China;
3
Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China
Abstract: Three kinds of porous catalysts CuO-ZnO/SBA-15 (CZ/SBA-15), CuO-ZnO-MnO2/SBA-15 (CZM/SBA-15) and CuO-ZnO-ZrO2/SBA-15 (CZZ/SBA-15) were synthesized by impregnation method with a siliceous framework mesoporous molecular sieve SBA-15. The performance of all catalysts for catalytic hydrogenation of CO 2 to methanol was evaluated on a fixed bed reactor, combined with N2 adsorption-desorption (BET), X-ray diffraction (XRD), H2 temperature-programmed reduction (H2-TPR), temperature programmed desorption (H2-TPD, CO2-TPD), N2O titration, X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM) . The results show that the introduction of metal oxide in the catalyst changes the pore size and specific surface area of the SBA-15 molecular sieve support. The CuO-ZnO-MnO2/SBA-15 and CuO-ZnO-ZrO2/SBA-15 have high copper dispersion (DCu), large specific surface area (ACu), small surface CuO particle size, and easy to be reduced. Compared with the Mn–O cluster, the Zr–O cluster enhances the basic site and improves the methanol selectivity. In addition, CuO-ZnO-ZrO2/SBA-15 has the highest oxygen vacancy concentration and better catalytic activity among three catalysts. The methanol selectivity of CuO-ZnO-ZrO2/SBA-15 is 25.02%, which is 28% and 136.9% higher than those of CuO-ZnO/SBA-15 and CuO-ZnO-MnO2/SBA-15, respectively. Key words:
molecular sieve SBA-15; oxygen vacancy; alkaline sites; methanol
Methanol is an important chemical raw material[1]. There are two main ways to produce methanol: one is to oxidize methane directly to methanol[2], the other is to use CO2 hydrogenation to produce methanol. Nowadays, the increase of carbon dioxide concentration has caused possibly irreversible changes to climate and environment[3]. Catalytic conversion of CO2 to methanol is a promising route that may offer a solution to energy and environment[4,5]. The excellent catalysts have been a key research to improve the catalytic performance. The main factors affecting the performance of catalysts are active components, supports and catalytic promoters [6,7]. At present, most researchers believe that copper-based catalysts are the most effective catalysts in the methanol production process[8–10]. Although changing preparation methods or adding additives can improve the activity of copper-based catalysts, it also has some problems, such as uneven
dispersion of active components and easy sintering of catalysts. Therefore, the conventional copper-based catalysts have been widely modified with different supports such as Al2O3 and ZrO2, but the hydrophilicity of alumina impairs the stability of the catalyst. Thus, there is an urgent need to study alternative carrier materials[11,12]. Koh et al[13–16] synthesized a series of copper catalysts Cu-ZnO-MnO (CZM) supported on porous siliceous SBA-15 with different morphologies to study the effect of support morphology on the catalyst. The results showed that the catalytic activity of mesoporous molecular sieve SBA-15 was significantly improved. Phongamwong et al[17] concluded in a recent study that adding SiO2 to Cu/ZnO/ZrO2 could improve the methanol synthesis activity by increasing the surface area of active site copper and the basicity of catalyst. In addition, Toyir et al[18] further discovered that the performance of Cu-Zn-Ga supported on hydrophobic silica (SiO2) was enhanced during methanol
Received: 25-Jun-2019; Revised: 05-Sep-2019. Foundation items: Supported by the Natural Science Foundation of China (51404122, 51404099) and the Natural Key Technologies R&D Program of China (2011BAC01B03). *Corresponding author. E-mail: [email protected]. Copyright 2019, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
synthesis process. Chen et al[19] prepared small particles of Cu/SBA-15 catalyst with modified SBA-15 as support, which improved the catalytic activity of water vapor reaction. More studies have shown that SBA-15 has good application prospects as catalyst carrier material because of its flexible pore structure and large specific surface area[20–23]. The porous SBA-15 also has a great advantage in gas phase adsorption[24]. In view of this, SBA-15, a porous siliceous material, was used as a carrier in this study to improve the sintering resistance of the catalyst and promote the dispersion of active components on the surface of the carrier. Early studies show that Cu/SiO2 catalyst has good activity performance in hydrogenation reaction[25,26]. However, single SiO2 carrier can’t effectively stabilize the active components, which results in the growth and inactivation of copper particles in high temperature and high space velocity reduction atmosphere. How to improve the stability of active components in the reaction process has always been a scientific problem to be solved urgently in related fields. Many studies have proved that the addition of some metal oxides can greatly promote the synthesis of methanol. Li et al[27] prepared Cu-Zn-Zr/SBA-15 with high specific surface area to study the effects of different surface structures on the catalytic activity, and it was proved that the surface structure of the catalyst played a key role in the catalytic activity. In an earlier study, Słoczyński et al[28] found that adding metal oxides B, Ga, In, Gd, Y, Mn and Mg to Cu/ZnO/ZrO 2 could change the structure or properties of the catalyst. In their other studies, it was proved that the addition of promoters Zn and Mn improved the dispersion of reduced copper and the size of copper microcrystals, which was beneficial to methanol synthesis[29,30]. However, the amount of additives is also important. Zhu et al[31] investigated the effect of different zinc content on the dispersion of Cu species on Al2O3 support. They found that proper zinc addition could improve the dispersion of Cu species on Al2O3 support, and excessive Zn led to the formation of a large number of CuO crystals and the decrease of catalyst activity. In the study of the effect of additives on the size and distribution of CuO grains, Yin et al[32] found that adding appropriate amount of manganese promoter could significantly improve the activity and thermal stability of the catalyst, and manganese played a role in preventing the growth of CuO grains and promoted the dispersion of CuO. Furthermore, Hao et al[33] investigated the effects of three additives (Zr, Ba and Mn) on the catalytic performance of Cu/ZnO/Al2O3 (CZA). The results showed that the introduction of Zr reduced the maximum active temperature of CZA catalyst and enhanced the catalytic stability. Moreover, the mechanism of the effect of additives on active sites has also been studied. Wang et al[34] found that the introduction of zirconium, aluminium and manganese
facilitated the formation of different size copper microcrystalline, which resulted different amounts of active sites. Witoon et al[35] studied the effect of ZrO2 on the activity of copper catalyst. The results showed that amorphous ZrO 2 could increase the alkaline site concentration on the surface of the catalyst, enhance the interaction between active components and improve the performance of the catalyst. And then, Atakan et al[36–39] studied the effect of Zr-doped mesoporous silica (Zr-SBA-15) with symbiotic copper nanoparticles on the hydrogenation of carbon dioxide. It was found that the loading method of copper affected the selectivity of products, and the addition of Zr promoted the activity of catalysts. The results of Liu et al[40] showed that Al and Mn could also significantly affect the specific surface area and basicity of the catalyst. In summary, the addition of additives would affect the performance of catalysts to some degree. However, the effects of different promoters on the performance of catalysts need be further studied. For the above reasons, mesoporous molecular sieve SBA-15 was used as support to improve the dispersion of active components of the catalyst in this study. We synthesized mesoporous catalysts CZ/SBA-15, CZM/SBA-15 and CZZ/SBA-15 with different additives by traditional impregnation method, and compared the physical, and chemical properties of these catalysts, finally, studied the effect of adding Zr and Mn on the catalytic performance of CO2 hydrogenation to methanol.
1 1.1
Experimental Experimental materials
Cu(NO3)2·3H2O, Zn(NO3)2·6H2O, Zr(NO3)2·5H2O, Mn(NO3)2, TEOS, HCl and citric acid all purchased in Guoyao Group Company Limited. P123 was purchased from Sigma Aldrich. The standard mixture of H2 and CO2/H2 was purchased from Kunming Messel Gas Company Limited. 1.2
Catalyst preparation
All catalysts were synthesized by impregnation method. The SBA-15 was prepared by hydrothermal method[41,42]. Carriers account for 70% and active components account for 30% (by weight) in all catalysts. The active component was composed of a certain amount of copper and zinc and another metal element with the mass ratio of 5:4:1 among them. Firstly, a certain amount of Cu(NO3)2·3H2O, Zn(NO3)2·6H2O, Zr(NO3)2·5H2O and Mn(NO3)2 were dissolved in deionized water, and then we added the carrier SBA-15 and citric acid with corresponding mass ratio to make the solutes disperse uniformly by ultrasound about 10 min. The mixture was dried
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
in a water bath at 90°C, and then we transferred it into an oven at 60°C for 12 h until drying to obtain the catalyst precursor. A certain amount of the catalyst precursor was calcined in a muffle furnace at a heating rate of 1°C /min to 350°C for 4 h to obtain the catalyst CZX/SBA-15, where X represents an equivalent amount of Zn, Mn and Zr. 1.3
Catalyst characterization
XRD (D/MAX-2400 X diffractometer, Japan) was performed with a Cu Kα radiation at 40 kV and tube current at 100 mA form 0°–10° (the scan rate: 2(°)/min) and 10°–90° (the scan rate: 5(°)/min). The specific surface areas and pore volume were determined from N2 adsorption-desorption (Autosorb-iQ-C, Quantachrome, America). The catalyst samples 150 mg were degassed at 300°C under vacuum for 3 h, and then N2 adsorption-desorption analysis was carried out under liquid nitrogen (–196°C). The N2O titration characterization was performed on a Chemisorbent (Chem BET Pulsar & TPR/TPD, Quantachrome, America). References method is used to calculate specific surface area and dispersion of copper in catalysts[43]. The whole characterization process was completed by twice TPR program. The first consumption of H2 is recorded as n1, the second TPR program will be executed after cooling to room temperature, and the amount of H2 consumed is recorded as n2. The dispersion (D) and specific surface area (A) of copper are calculated as follows: D = (2×n2/n1)×100% (1) A = 2×n2×Nav/(n1×MCu×1.4×1019) = 1353×n2/n1 (m2(Cu)/g(Cu)) (2) In the formula, Nav denotes Avogadro constant, MCu denotes the relative atomic mass of copper (63.456 g/mol), 1.4×1019 is the relative density of copper atoms on the surface. The crystallite size of copper in the reduced catalyst was calculated based on the Debye-Scherrer formula: d = K/(cos) (3) Where d is grain size, is the full width at half maximum (FWHM), is a diffraction angle, K represents a constant (K = 0.89) and is the wavelength of Cu K X-rays ( = 0.154 nm)[44]. H2-TPR, H2-TPD and CO2-TPD were completed on the Chemisorbent (Chem BET Pulsar & TPR/TPD, Quantachrome, America). H2-TPR was carried out using 50 mg catalyst in 10% H2/90% He gas flow with a flow rate of 25 mL/min and a heating rate as 10°C/min. In characterization of CO2-TPD, the He was used as carrier gas, catalyst (50 mg) was first reduced in H2 gas stream at 280°C for 1 h, and then switched to He blowing for 30 min, when the temperature was lowered to 40°C, the adsorption of CO2 was switched to saturation and He was purged for 60 min. At last, the CO 2 desorption was
completed by temperature programmed desorption to 900°C using a heating rate of 10°C/min. The characterization method of H2-TPD was similar to that of CO2-TPD. The differences were 100 mg catalyst used and H2 adsorption to saturation after He purging and cooling. The desorption was completed by purging with He for 60 min and then heating up to 900°C at 10°C /min. XPS results (PHI5000 Versaprobe-II) were obtained using Al K X-ray at 15 eV (hν=1486.6 eV). All binding energies were calibrated by the C 1s (284.8 eV) standard line. The morphology of sample was probed by TEM (JEM-2001, Japan). Firstly, the powder sample was dispersed in absolute ethanol by ultrasound, and then the suspended liquid droplets from the upper layer were added to the nickel mesh, and the nickel mesh was placed on the electron microscope instrument for observation. Elements of materials were probed by EDX (APOLLO, Idax, America). 1.4
Catalytic activity measurements
The activity evaluation of catalytic hydrogenation of CO2 to methanol was carried out in a self-made high temperature and high pressure tubular micro reactor. 1.0 g (20–40 mesh) catalyst and 1.5 g quartz sand were mixed and evenly packed into tubular reactor. Catalyst was reduced first by H2 for 2 h at 280°C and natural cooled below 160°C after completion. The catalyst activity was tested under the condition of VH2:VCO2=3:1, 3.0 MPa, 250°C, GHSV = 6000 h –1. The final product was separated by condensation and analyzed by gas chromatography (Agilent6890) with capillary columns (HP-MOLESIEVE (30 m×0.535 mm×50.00 m), HP-PLOT/Q (30 m×0.535 mm×40.00 m)). The content of each target component was compared with the standard signal values previously and calibrated with a certain concentration of CO, CO2, CH4 and CH3OH. The content of CO and CO2 were measured by TCD, and CH4 and CH3OH were measured by FID. Finally, the conversion of CO2, selectivity and yield of methanol as the target product were calculated by referring to the detection signal and the standard signal value.
2 2.1
Results and discussion X-ray diffraction (XRD)
Small-angle X-ray diffraction patterns of SBA-15 are illustrated in Figure 1(a). The diffraction peaks at 0.8, 1.4 and 1.6 correspond to the characteristic diffraction peaks of (100), (110) and (200) plane of SBA-15, respectively. It shows that the prepared carrier SBA-15 has hexagonal orderly mesoporous structure. Figure 1(b) shows the small angle XRD spectrum of the catalyst.
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
Fig. 1
XRD patterns of the catalyst
(a): low-angle XRD pattern of SBA-15; (b): wide-angle XRD patterns of SBA-15; (c): wide -angle XRD patterns of the catalysts; (d): wide-angle XRD patterns of the catalysts after reduction
Table 1
a
Grain size of Cu in the catalyst after reduction
Sample
C/SBA-15
CZ/SBA-15
CZM/SBA-15
CZZ/SBA-15
daCu/nm
19.8
18.9
17.8
15.4
: the crystallite size of Cu NPs was calculated by Scherrer's equation, where 2θ = 43.3°, 50.5°, 74.2°.
All catalysts exhibit a small angle XRD diffraction peak consistent with SBA-15, indicating that the mesoporous structure of SBA-15 is not destroyed after metal (Cu, Zn, Mn, Zr) loaded. At the same time, with the addition of Zr and Mn, the positions of the diffraction peaks of the catalysts shift to a low angle, indicating that the lattice constants of the catalysts are changed and the ordering of the pore structure of the supports is decreased[45]. The results of small angle XRD show that the mesoporous structure of support SBA-15 is not destroyed after the adding of metal oxides (Cu, Zn, Mn, Zr). Results of wide-angle X-ray diffraction patterns of samples are illustrated in Figure 1(c). As shown in Figure 1(c), all samples exhibit a wide diffraction peak at 24, which belong to amorphous SiO2. The corresponding diffraction peaks of CuO (110), (–111), (111), (–202), (–113) crystal plane (PDF#74-1021) and ZnO (111), (200), (311) crystal plane (PDF#77-0191) in the catalyst have been marked at corresponding positions, respectively. Results show that metal oxides are successfully introduced into the catalyst. It is worth noting that there are not the diffraction peaks corresponding to
the oxides of Mn and Zr. This is because the addition of these two elements is too small to be detected by XRD. In addition, in order to further illustrate the effect of additives on copper crystal size, a single copper component (C/SBA-15) is designed, reduced by H2 and characterized by wide-angle XRD. The results are shown in Figure 1(d) and Table 1. The results show that the grain sizes of the reduced Cu are in the order of C/SBA-15 ˃ CZ/SBA-15 ˃ CZM/SBA-15 ˃ CZZ/SBA-15. Wide-angle XRD results show that the grain size of Cu decreased after the introduction of Zn. Interestingly, the particle size of Cu further decreased with the addition of Zr and Mn. This is because the addition of metal oxides increases the dispersion of copper particles and decreases the grain size of copper. In addition, the BET results of each sample show that the specific surface area (ABET), pore volume (vpore) and pore size (dpore) of the catalyst are lower than those of SBA-15 (Table 2). It indicates that the metal particles enter the pore structure of SBA-15 or adhere to the surface of the support, which reduces the pore size and specific surface area, but the catalyst still has a good pore structure.
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
Fig. 2
N2 adsorption-desorption isotherm of the catalyst
In a word, the addition of metal oxides facilitate the dispersion of Cu in the catalyst, resulting in the improvement of the catalytic performance. 2.2
N2 adsorption-desorption isotherms
The N2 adsorption-desorption isotherms and pore size distributions of samples are illustrated in Figure 2. As shown in Figure 2, the mesoporous molecular sieve SBA-15 exhibits a typical Langmuir IV adsorption isotherm. According to the classification of IUPAC, H1-type adsorption hysteresis loops appear in the middle section, and the hysteresis loops are steep. It shows that SBA-15 is a cylindrical hole with uniform diameter distribution and high orderliness. Moreover, Table 2
a
2
CZ/SBA-15, CZM/SBA-15 and CZZ/SBA-15 all exhibit the same typical Langmuir IV adsorption isotherms, indicating that all catalysts have the similar pore structure as SBA-15. The hysteresis loops of the catalysts are relatively flat, indicating that the introduction of metal oxides has a certain effect on the pore structure of SBA-15. According to the literature report[46], the inflection point of N2 adsorption-desorption isotherm can reflect the pore size of mesoporous materials. Compared with SBA-15, the adsorption isotherms of all catalysts are flat and move towards lower pressure, which indicate that the pore size, specific surface area and pore volume of mesoporous molecular sieve SBA-15 are reduced after loading metal oxides (shown in Table 2). 2.3
N2O titration
In order to further study the distribution of CuO particles in the catalyst, N2O titration of the catalyst were carried out. The dispersion and specific surface area of copper are calculated according to the formula. The related parameters are shown in Table 2. The results show that the dispersion (DCu) and specific surface area (ACu) of copper on CZM/SBA-15 and CZZ/SBA-15 are larger than those on CZ/SBA-15 catalysts. Among them, CZZ/SBA-15 has the largest DCu and ACu, 19.6% and 133.2 m2/g respectively.
Physical property parameter table of the catalyst
–1
Sample
ABET/(m g )
vapore/(cm3g–1)
dpore/nm
DCu/%
ACu/(m2·g–1)
SBA-15
863.25
1.17
7.41
–
–
CZ/SBA-15
382.96
0.60
6.27
7.80
52.6
CZM/SBA-15
412.87
0.65
6.24
11.4
77.3
CZZ/SBA-15
387.60
0.54
5.57
19.6
133.2
: total pore volume obtained from p/p0 = 0.99
It is attributed to the formation of a more complex Cu-Zn-Mn and Cu-Zn-Zr composite oxide layer than that of pure Cu-Zn due to the addition of a small amount of Mn and Zr metal oxides in the catalyst[27]. It has been reported that the addition of metals during the formation of catalysts enhances the interaction between metal oxides, resulting in copper oxides adhering to other metal oxides and evenly distributing on the surface of catalysts[47], and then the dispersion of copper (DCu) and specific surface area (ACu) on the catalyst surface are improved. 2.4 Fig. 3
H2-TPR patterns of the catalyst
H2-TPR
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
Fig. 4
H2-TPD (a) and CO2-TPD (b) patterns of the catalyst
Table 3 Sample CZ/SBA-15 CZM/SBA-15 CZZ/SBA-15
Catalysts adsorption performance parameters
H2-TPD area
maximum desorption temperature t/℃
479
6415
149
183
470
1736
476 480
The characterization of catalysts by H2-TPR is used to explore the difficulty of hydrogen reduction of different catalysts. As shown in Figure 3, three catalysts show obvious reduction peaks at 250–400°C, which corresponded to the reduction of CuO, since ZnO, ZrO2 and MnO2 couldn’t be reduced in this temperature range. In addition, the reduction temperatures of all catalysts are different. The order of reduction temperature from high to low is as follows: CZ/SBA-15 (354°C) ˃ CZM/SBA-15 (334°C) ˃ CZZ/SBA-15 (322°C). The results indicate that with the addition of Zr and Mn the reduction temperature of catalyst decreases, and the CuO in the catalyst is easier to be reduced. According to the XRD characterization of the catalyst, the oxides of zirconium and manganese inhibit the growth of CuO particles, increase the dispersion of CuO particles, lead to the reduction of CuO species more easily, and decrease the reduction temperature of the catalyst. 2.5
CO2-TPD
maximum desorption temperature t/℃
Adsorption properties of catalysts (H2-TPD, CO2-TPD)
Figure 4 (a) is the H2-TPD curve of the catalyst. A wide range H2 desorption curves (200–700°C) are appeared with different strength. It is clear that the whole desorption process can be divided into two strong stages. It is worth noting that the mesoporous SBA-15 only has desorption peak at high temperatures, which indicates that the pore structure of the catalyst is helpful to the adsorption of H 2.
7485 7262
area
138
118
473
1988
141
159
488
2253
According to the relevant literature [48], only the weak desorption peaks at low temperature (200–300°C) belong to the desorption of hydrogen atoms on the Cu site, while the high temperature section indicates the desorption of hydrogen strongly adsorbed by copper or other metal oxides (Cu+–O–Zn2+), and the hydrogen adsorption performance is mainly attributed to the interaction of copper and zinc [49,50]. In order to know more clearly about the hydrogen adsorption capacity of the catalysts, the maximum desorption temperature and peak area of each catalyst are shown in Table 3. As shown in Table 3, the H 2 desorption capacity of the three catalysts is in the order: CZM/SBA-15 > CZZ/SBA-15 > CZ/SBA-15, suggesting the addition of Mn and Zr oxides facilitates the hydrogen adsorption of the catalyst. Relevant studies have shown that [51], the strong interaction between ZnO and Cu enhances H 2 spillover and improves the adsorption of H 2 on ZnO metal oxides, and the addition of metal oxides (Mn, Zr) further enhances the adsorption of H2 on catalysts[13]. Figure 4(b) is the CO2-TPD curve of the catalyst. From Figure 4(b), it can be seen that the desorption peaks of carbon dioxide appear in all catalysts and SBA-15 support in different temperature ranges, indicating that mesoporous material SBA-15 is helpful to the adsorption of CO2 on catalysts. However, as far as the system is concerned, the adsorption of CO2 on the catalyst at low temperature is the main factor affecting the activity of the catalyst.
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
Fig. 5
XPS spectra of the catalyst
(a): full spectra; (b): O 1s; (c): Cu 2p; (d): Zn 2p; (e): Mn 2p; (f): Zr 3d
Table 4 Sample
XPS peak data of each element in the catalyst
E/eV
E/eV
Cu 2p3/2
Cu 2p1/2
Mn 2p3/2
E/eV
O 1s/%
Mn 2p1/2
Zr 2p5/2
Zr 2p3/2
OI
OII
OIII/ OIII
(OI+OII)
CZ/SBA-15
933.1
953.1
–
–
–
–
22.8
42.2
35
0.53
CZM/SBA-15
933.1
953.1
641.6
654.1
–
–
22.4
47.4
30.2
0.43
CZZ/SBA-15
933.1
953.1
–
–
182.4
184.8
20
42.4
37.6
0.60
Compared with SBA-15, the desorption temperature of all catalysts shifts to low temperature in varying degrees. This phenomenon is related to oxygen atoms in low coordination M–O clusters (M denotes different metals such as Zr, Mn, and Zn)[48], which demonstrates that the desorption peaks in the low temperature range of the catalyst are related not only to the Si–OH clusters in the catalyst[52], but also to the interaction of metal Cu , Zn oxides. The parameters of desorption peaks
in catalysts are listed in Table 3. The results show that the CO2 adsorption peak of catalyst CZM/SBA-15 is small at low temperature, while the CO2 adsorption capacities of other catalysts are large at low temperature, and the alkaline site provided by the oxide of Mn is below 500°C[53], indicating the basic sites provided by Mn–O clusters are not as many as those provided by Zn–O and Zr–O clusters in low temperature.
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
Fig. 6
TEM images of the catalyst
(a): C/SBA-15; (b): CZ/SBA-15; (c): CZM/SBA-15; (d): CZZ/SBA-15; (e): energy spectrum of CZM/SBA-15; (f): energy spectrum of CZZ/SBA-15
Table 5
Catalytic performance evaluation of catalysts
Catalyst
xCO2/%
sCH3OH/%
sCO/%
wCH3OH/(mmolg–1h–1)
CZ/SBA-15
8.7
19.54
80.46
0.42
CZM/SBA-15
8.2
10.56
89.40
0.25
CZZ/SBA-15
8.1
25.02
74.97
0.99
reaction condition: t=250°C, p=3 MPa, H2/CO2(volume ratio)=3:1, and SV=6000 mL/(g·h)
In addition, the adsorption data at high temperature show that the addition of Zr and Mn increase the desorption area of CO2. The desorption peak of this temperature segment is attributed to the adsorption of CO2 on the catalyst surface O2–[13,54]. In conclusion, the results of CO2-TPD show that the basic sites on the surface of the catalyst are mainly attributed to the interaction of metal Zr, Zn, Cu and O. Zr–O and Zn–O clusters increase the basic sites of catalysts, especially CZZ/SBA-15. It’s attributed to the formation of Zr4+–O2– acyl
pairs[50]. However, the effect provided by Mn–O clusters at relatively low temperatures is weak, resulting in poor catalytic performance. 2.6
XPS spectra
Figure 5(a) is the XPS full spectrum of the catalyst. The binding energies of each element are shown in the figure. Peaks of Cu 2p are observed at 932–934 eV and 952–954 eV
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
for all catalysts (Figure 5(c)), corresponding to Cu 2p3/2 and Cu 2p1/2, respectively. It is noteworthy that there are satellite peaks between 936–946 eV. These satellite characteristic peaks are due to charge transfer between copper 3d orbital and oxygen 2p orbital, indicating that copper in the catalyst exists in the form of Cu2+[55]. In addition, all catalysts exhibit characteristic peaks of Zn 2p (Zn 2p3/2 and Zn 2p1/2) at 1022–1045 eV (Figure 5(d)), corresponding to Zn2+. In order to better analyze the influence of metal Mn and Zr on the catalyst, the peak-fit processing are carried out. The corresponding results are list in Table 4. The peak values at 642.7 and 654.4 eV belong to Mn4+ in Figure 5(e), and there is not satellite peak at near 647.8 eV, indicating that there is not Mn2+ in the catalyst[56,57]. In Figure 5(f), the characteristic peaks of catalyst CZZ/SBA-15 at 181–185 eV belong to Zr 3d, where 182.4 eV belongs to Zr 3d5/2 and 184.8 eV belongs to Zr 3d3/2, indicating that zirconium in the catalyst exists in the form of Zr4+[58–60]. It has been shown that the oxygen vacancy in the catalyst has an important influence on the formation of intermediate products in methanol synthesis[61,62]. In order to further explore the distribution of oxygen species in catalysts, peak fitting of O 1s is carried out[63], in Figure 5(b). The lowest peak of binding energy is the lattice oxygen of the catalyst itself (OI), corresponding to M–O bond (M denotes Cu, Zn, etc.). The highest peak of binding energy corresponds to the surface oxygen (OIII) of the catalyst. The peak at the middle position corresponds to the oxygen adsorbed on the catalyst surface (OII), which belongs to the Si–O–Si bond. According to the literature[64], lattice oxygen can provide effective catalytic area and ratio of surface oxygen to lattice oxygen indicates the oxygen vacancy concentration of catalyst [65]. The catalyst with higher oxygen vacancy concentration has better activity[66]. The distribution of different types of oxygen content and oxygen vacancy concentration are listed in Table 4. The results show that CZZ/SBA-15 has the largest oxygen vacancy concentration among all catalysts. The addition of metal Zr promotes the interaction between metal oxides and supports, increases the number of oxygen vacancies on the surface of catalysts, and facilitates the formation of intermediate products in methanol synthesis. 2.7
TEM photographs
Figure 6 is the high resolution image of the catalyst under transmission electron microscopy. As can be seen from Figure 6, the pore structure of support SBA-15 in each catalyst is well arranged, indicating that the addition of metal oxides does not damage the pore structure of support. It can be found that with the addition of metal oxides additives, the dispersion of CuO on SBA-15 becomes better, and the agglomeration of
metal oxides in catalysts decreases significantly. Especially for CZZ/SBA-15 with component Zr, the metal oxides are uniformly dispersed on the surface of SBA-15, which indirectly promotes the dispersion of CuO in the catalyst to a large extent. These results are consistent with the previous characterization. In addition, The energy spectrum analysis results show that Mn and Zr are found on the surface of CZM/SBA-15 and CZZ/SBA-15, indicating that Zr and Mn are successfully added to the catalyst. 2.8
Catalytic activity
Table 5 shows the activity evaluation of the catalysts for the hydrogenation of carbon dioxide to methanol under the same conditions. It shows that the conversion rate of carbon dioxide for CZ/SBA-15 catalyst is 8.7%, the selectivity and yield of methanol are 19.54% and 0.42 mmol/(g·h) respectively. For catalyst CZZ/SBA-15, the carbon dioxide conversion decreases, but the methanol selectivity and yield are improved to 25.02% and 0.99 mmol/(g·h) respectively. Compared with CZ/SBA-15 and CZM/SBA-15, the methanol selectivity and yield of CZZ/SBA-15 increased by 28%, 135.7% and 136.9%, 296%, respectively. Combining with XRD and N2O characterization, the radius of Cu particles is the smallest in CZZ/SBA-15. In addition, XPS characterization shows that more oxygen vacancies are formed on the surface of CZZ/SBA-15, which promote methanol synthesis reaction on its surface and enhanced methanol selectivity. On the contrary, the addition of MnO2 does not show a good performance.
3
Conclusions
In this study, mesoporous molecular sieve material SBA-15 was used as catalyst carrier and the catalytic performance of different metal oxides supported SBA-15 was carried out by CO2 hydrogenation to methanol. The results show that the pore structure of mesoporous molecular sieve SBA-15 doesn’t affected by the addition of a small amount of metal oxides, but the pore size and specific surface area are declined at some degree. With the addition of various metal oxides, a more complex multi-oxide layer is formed on the surface of the catalyst, which reduces the size of CuO grains, promotes the dispersion of CuO grains on SBA-15 surface and decreases the reduction temperature of the catalyst. Besides, the addition of Zr enhances the basic site of the catalyst, improves the interaction between metal and oxygen (M–O) in the catalyst, and leads to the higher oxygen vacancy concentration and better catalytic activity. However, the alkaline sites obtained by Mn–O are relatively weak and the catalytic activity is poor, indicating that the particle size and distribution of the active component aren’t the only factors determining the
LIN Min et al / Journal of Fuel Chemistry and Technology, 2019, 47(10): 12141225
performance of the catalyst. The highest methanol selectivity is obtained by CZZ/SBA-15 catalyst, which increases by 28% and 136.9% respectively compared with the other two catalysts.
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