Operating Conditions and Composition Effect on the Hydrogenation of Carbon Dioxide Performed over CuO/ZnO/Al2O3 Catalysts

A series of catalysts constituted of mixed copper and zinc oxides supported on alumina were prepared by co-precipitation method. The cooper content was in the 10-90 wt.% range. Their catalytic behavior in the hydrogenation of carbon dioxide to methanol was investigated at high pressure (up to 75 bars). The catalysts were characterized by elemental analysis, N2-adsorption, N2O-chemisorptions, and X-ray diffraction (XRD). The catalysts showed a clear activity in the hydrogenation reaction that could be correlated to the surface area of the metallic copper and to the reaction pressure. The CuO/ZnO/Al2O3 catalyst with a Cu/Zn/Al weight ratio of 60/30/10, exhibits the highest carbon dioxide conversion and methanol selectivity. Finally, a mechanism pathway has been proposed on copper active sites of (Cu0/CuI) oxidation state. Copyright © 2019 BCREC Group. All rights reserved


Introduction
Methanol is an important intermediate in the petrochemical industry used in the production of a variety of products, including gasoline and alternative raw materials for the production of olefin such as ethylene and propylene [1]. The increasing demand for methanol has drawn considerable attention in enhancing its production. Currently, methanol is industrially produced starting from CO/CO2/H2 mixture over CuO/ZnO/Al2O3 catalysts operating at 50-100 bars, and 220-300 °C [2]. If the CuO/ZnO/Al2O3 catalyst exhibits high activity in the methanol tain Cu-Zn oxides as main constituents, and others components can be added in order to modify the catalytic properties.
The aim of the present research is to study of the influence of CuO/ZnO weight ratios, surface property, and reaction pressure on the activity of CuO/ZnO/Al2O3 catalyst in the synthesis of methanol starting from CO2/H2. The listed parameters play indeed an important role in driving the catalytic performance of such mixed oxides catalyst.

Catalysts Preparation
All the chemicals were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used without further purification. A series of CuO/ZnO/Al2O3 catalysts with 10 wt.% of Al2O3 and different CuO/ZnO weight ratios were prepared by co-precipitation method on 10 wt% alumina [41]. The copper and zinc nitrate precursors Cu(NO3)2.2.5H2O (98% purity) and Zn(NO3)2.6H2O (99% purity) were dissolved in deionized water and sodium carbonate Na2CO3 (99.5% purity) was used as precipitating agent. Prior to precipitation, the estimated 10 wt.% of alumina, Al2O3 (98% purity) was added to the solution and the slurry was stirred and kept at 85 °C. The addition of sodium carbonate increased the pH to 7, allowing the coprecipitation of Cu and Zn hydroxides on the alumina surface. Then, the obtained solid material was aged during 2 h under intensive stirring. In the next step, sodium and nitrate ions were thoroughly removed by washing the solid with redistilled water (6 times) until the total elimination of Na + and NO3ˉ ions. Analysis of the filtrate by electrical conductivity confirmed this result.
To avoid agglomeration of the CuO and ZnO particles in the CuO-ZnO solid solution, the dried precipitate was calcined in air at 350 °C for 12 h. Several studies have shown that when the calcination temperature exceeded 350 °C, specific area and catalytic activity decreased. After calcination, all catalysts were characterized by IR spectroscopy (Table 1), only O-H stretching and metallic carbonates vibration band were observed.

Catalysts Characterization
X-ray patterns were collected on PANATI-CAL MPD X'Pert Pro diffractometer operating with Cu-K radiation (K = 0.15418 nm) and equipped with an X' Accelerator. The real-time multiple strip pattern was collected at 295 K in the 5°-70° 2θ range with a step of 0.017° 2θ and a time /step of 220 sec; the total collecting time was about 2h.
The copper and zinc content were determined using an AAS 6800 spectrometer (Shimadzu). The specific surface areas of the alumina support and the obtained catalyst were measured by nitrogen adsorption at -196 °C by applying the BET method and using a Quantachrome apparatus. The metallic copper surface area was measured by the decomposition of N2O at 90 °C [42][43][44] on the surface of metallic copper by the following reaction: The pulse titration technique was employed and a thermal conductivity detector (TCD) was used to detect the amount of N2O consumed.

Catalytic Tests
The catalytic tests have been performed in a continuous tubular flow fixed-bed microreactor at different reaction pressures and at constant temperature of 230 °C for all catalysts. For the most active catalyst (C4) further investigations at different temperature (in the 170-270 °C range) have been performed. Prior to reaction, the catalyst (0.5 g) was reduced in H2 flow (1.3 L.h -1 ). The reduction program consisted in an increment of the temperature from room temperature to 350 °C at 10 °C.min -1 and of a plateau to 350 °C for 8 h. After the reduction step, the reactor was fed with the reaction mixture: CO2/H2 = 1/3. The total flow rate was in the 0.3-3.6 L.h -1 range and the tested operating pressures were: 1, 20, 35, 50, and 75 bar, respectively. The reaction mixture and products were analyzed on line by using gas chromatographs equipped with TCD and FID detectors and Carbosieve and Porapak Q columns, respectively. The products, detected in the stream flow exiting form the reactor during CO2 hydrogenation on CuO/ZnO/Al2O3 catalysts, were almost exclusively constituted of methanol and carbon monoxide. Only traces of methane could be observed. Methanol and carbon monoxide selectivity and CO2 conversion were calculated using the following equations:

N2 adsorption-desorption
The BET surface area of the CuO/ZnO/Al2O3 catalysts with different Cu/Zn theoretical and experimental weight ratios are summarized in Table 2. The surface area significantly decreases when increasing the Cu/Zn ratio. The increase of Cu/Zn ratio should be responsible for the decrease of surface area. The growth of crystal grain or agglomeration of particles with the increase of Cu/Zn ratio should be responsible for the decrease of surface area. At the opposite, the metallic copper surface area increased with the Cu/Zn ratio, suggesting that the Cu-containing phase is mainly distributed on the surface of the composite.

XRD analysis
The XRD patterns of the calcined CuO-ZnO/Al2O3 samples with different Cu/Zn weight ratios are constituted of mixtures of CuO, ZnO and Al2O3 (Figure 1 The results of the CO2 hydrogenation over the various CuO/ZnO/Al2O3 catalysts are given in Table 2. The selectivity to methanol increased with the metallic copper surface area for the samples containing 10-60% of copper, while it decreased when the copper content exceeded 60%, although the metallic copper surface area was even higher, a result that is consistent with other reports [45,48,49]. This can be explained by the positive synergetic effect obtained by the contact between copper and zinc oxides. On the other hand, the conversion of CO2 continuously increased by increasing the Cu/Zn weight ratios, while the selectivity to CO decreased with the Cu/Zn weight ratios augmentation, for the catalysts containing from 10 to 60% of copper content, but increased when the copper content exceeded 60% [48]. The effect of metallic cooper surface area on the CO2 conversion at 230 °C is shown in Figure 2. With increasing the Cu/Zn weight ratios, the metallic cooper surface area increased and the CO2 conversion was linearly enhanced. This indicated that the CO2 conversion is directly proportional to the surface area of metallic copper. These results confirm that there must be some other factors affecting the catalytic performance during the synthesis of methanol from CO2/H2. Apparently, Cu 0 atoms are the active sites in the dissociation of CO2 to CO and Cu-O-Cu species [50][51][52][53]. The dissociation reaction is the follows: The  Table 3, illustrates the effects of temperature on CO2 hydrogenation reaction at atmospheric pressure and stoichiometric feed ratio (H2/CO2 = 3).The product stream mostly contains CO and methanol at reaction temperature in the range between 170 and 270 ºC. Moreover, by increasing the temperature, the CO2 conversion and carbon monoxide selectivity was enhanced, while the methanol selectivity decreased simultaneously. At constant reaction pressure, a lower temperature leads to   higher methanol selectivity. This result suggests that the formate, which is the main route for methanol synthesis is unstable at high temperature and decompose to CO and H2O through the reverse water gas shift (RWGS) reaction.

Temperature effect
The effect of flow-rate variation was also tested at 0.6, 1, 1.4, 2, and 3.6 L/h, and 230 °C. The data for the selectivity ratios during 10 hours on-stream with the C4 catalyst are reported in Figure 3. By increasing the flow-rate, the CO2 conversion and CO selectivity decreased, while the methanol selectivity considerably increased. A high flow-rate is favorable to methanol formation, whereas the CO formation is enhanced at low flow-rate.
The effects of the reaction pressure, on the activity and selectivity has been studied over the most active catalyst C4 (CuO/ZnO/Al2O3: 60/30/10) at the reaction temperature of 230 °C ( Table 4). The conversion of carbon dioxide and methanol selectivity increased with the total pressure, while the carbon monoxide selectivity decreased by increasing the total pressures. At 75 bar, the methanol selectivity reached the maximum value. Such a behavior may be due to the decomposition of the formate species that is much lower at high pressure. Consequently, CO formation can be minimized increasing the pressure. The CO/CH3OH selectivity ratios expressed as function of the pressure show the same trend that observed as a function of the flow-rate, Figure 4. Finally, high pressure and high flow-rate are favorable to methanol production, while they present the opposite effect on the CO formation.

The activation energy
Overall, apparent activation energy can be determined from the effect of temperature on the rate for the CO2 hydrogenation reaction, with constant composition and pressure.

Carbon dioxide hydrogenation mechanism
The CuO/ZnO/Al2O3 catalytic system have been widely used in industry in the synthesis of methanol from syngas and then widely studied in the hydrogenation reaction of pure CO2 to methanol. In the literature the debate remains still open on the methanol synthesis mechanism and on the nature of catalytic active sites involved. Two types of reaction pathways have been identified in the literature for the hydrogenation of CO2 to methanol. The first consists in the direct hydrogenation of CO2 to methanol. The second pathway suggests that CO2 is firstly converted to CO (through the RWGS reaction), then to methanol. Several active sites configurations have been proposed, some authors, proposed that Cu + is the site active in methanol synthesis, for others the active species are constituted of Cu 0 or of a mixture of Cu + and Cu 0 . It is often reported that methanol formation occurs preferentially on Cu + centers [35,36], however, it seems that methanol formation is activated only in presence of Cu 0 [37,40].
Generally, methanol synthesis by CO2 hydrogenation over CuO/ZnO based catalysts implicates three competitive reactions. The first reaction is the direct synthesis of methanol from CO2: The second one is the inter-conversion between carbon dioxide and carbon monoxide (RWGS reaction): CO2 (g) + H2 → CO (g) + H2O The third one is the synthesis of methanol from CO: By subtracting the reaction in Equation (7) from the reaction in Equation (6) results in: The [CO]/[CH3OH] selectivity ratio is inversely proportional to the total pressure as displayed in Figure 4, for the reaction performed at 230 °C. It is clear that at low pressure, the carbon monoxide is the main product, while, at high pressure, CO is transformed to methanol, demonstrating that carbon monoxide and methanol are produced from CO2 by parallel reactions. This mechanism is also supported by the fact that methanol forms very fast if the pressure is high. Moreover, the increasing of the flow-rate also enhances methanol formation. When the CO2/H2 mixture was feed over the CuO/ZnO/Al2O3 catalysts, CH3OH was produced together with CO and H2O. The methanol synthesis reaction, CO2 + 3H2  CH3OH + H2O, takes place in parallel to the reverse water gas shift reaction, CO2 + H2  CO + H2O. The impact of pressure, flow-rate and temperature on the products formation suggests that CH3OH and CO are produced through parallel pathways. By increasing the pressure, the CH3OH selectivity increased, while the CO selectivity decreased. The same trend was observed by varying the feedstock flow-rate. At the contrary, by increasing the reaction temperature, the methanol selectivity decreased and the carbon monoxide selectivity increased. These observations suggest that methanol and carbon monoxide are directly formed starting from the surface formate (O-CH=O) species that is formed via hydrogenation of CO2.
The proposed mechanism for carbon dioxide hydrogenation at high pressure suggests that the reaction between formate (HCOO) and hydrogen brings to the formation of dioxomethylene (H2COO). The dioxomethylene formation reaction may strongly compete with the decomposition reaction of formate to CO. Hence, high pressures favor the methanol selectivity, as confirmed by the results reported in Table 3. Pressure has a very strong influence on the production of methanol, probably due to the increasing of moles that characterize the involved reactions and that consequently shifts the equilibrium towards the condensation reaction. Previous works suggested that over copper based catalysts, the hydrogenation of carbon dioxide to methanol proceeds via the formation of the formate surface intermediate [53,[56][57][58]. This reaction is usually considered the ratedetermining step. The idea was previously reported by Fujita and co-workers [59] whose used IR spectroscopy to study the chemisorption of CO2/H2 on a CuO/ZnO-based catalyst. The reaction intermediates observed on the catalyst surface were carbonate (CO3 2-), formate (HCOOˉ), dioxomethylene (H2COO), formaldehyde (CH2O), methoxy (CH3Oˉ) species and the final product, CH3OH. The adsorbed species detected on the catalyst (carbonates, formates and methoxy) are also in agreement with a similar study reported by Bailey et al. [60].
Chinchen et al. [61] proposed that the surface atomic oxygen O* (Cu-O-Cu) (a) plays an important role during the methanol synthesis by promoting the adsorption of CO2, and by participating in the hydrogenation step. Based on the observations just reported, a tentative mechanism is schematized in Figure 6, for the methanol synthesis from CO2/H2 over a CuO/ZnO containing catalyst. At the first, the carbonate adsorption species (b) are produced by exposing the CuO/ZnO catalyst surface to CO2/H2. Then the hydrogenation to formate (HCOOˉ) (c), followed by the formation of dioxomethylene (H2COO) (f) and of methoxy (CH3Oˉ) species (g) take place to bring to the final product, methanol (CH3OH) (Sequence B). The CO is produced by decomposition of the monodentate and bidentate formate. In the same way than that of the formate decomposition, the decomposition of surface hydroxyl species leads to water formation (sequence A).

Conclusions
Methanol synthesis over CuO/ZnO/Al2O3 catalysts occurs via CO2 hydrogenation on the partially oxidized copper surface (Cu 0 /Cu I ). CO2 conversion and methanol selectivity strongly depend on the catalyst composition, pressure, reaction temperature and flow rate. High pressure and high flow-rate enhance the methanol formation. Besides a high reaction temperature favors the carbon monoxide production. The present results show the great influence of the catalyst composition and operating pressure on the kinetic and catalytic performances of CuO/ZnO/Al2O3 catalysts in the CO2/H2 reaction. Methanol is directly produced from CO2 whatever the pressure, while carbon monoxide can be produce by the decomposition of the formate at low pressure or directly from CO2 reduction.