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Article
Multi-Scale Analysis of Integrated C 1 (CH 4 and CO 2 )
Utilization Catalytic Processes: Impacts of Catalysts
Characteristics up to Industrial-Scale Process
Flowsheeting, Part I: Experimental Analysis of
Catalytic Low-Pressure CO 2 to Methanol Conversion
Hamid Reza Godini 1,2,, Mohammadali Khadivi 1 , Mohammadreza Azadi 1 , Oliver Görke 3 , Seyed Mahdi Jazayeri 1 , Lukas Thum 4 , Reinhard Schomäcker 4 , Günter Wozny 1 and Jens-Uwe Repke 1* (^1) Process Dynamics and Operation, Technische Universität Berlin, Straße des 17. Juni 135, Sekr. KWT-9, D-10623 Berlin, Germany; ali.khadivi69@gmail.com (M.K.); mra.azadi@yahoo.de (M.A.); mahdi.jazayeri@gmail.com (S.M.J.); guenter.wozny@tu-berlin.de (G.W.); j.repke@tu-berlin.de (J.-U.R.) (^2) Inorganic Membrane and Membrane Reactors, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology (TU/e), Den Dolech 2, 5612AD Eindhoven, The Netherlands (^3) Advanced Ceramic Materials, Institute of Materials Science and Technology, Technische Universität Berlin, Hardenbergstr. 40, 10623 Berlin, Germany; oliver.goerke@ceramics.tu-berlin.de (^4) Department of Chemistry, Technische Universität Berlin, Straße des 17. Juni 124, D-10623 Berlin, Germany; l.thum@mailbox.tu-berlin.de (L.T.); schomaecker@tu-berlin.de (R.S.) ***** Correspondence: h.r.godini@tue.nl; Tel.: +31 40 2476195; Fax: +49-30-
Received: 28 March 2020; Accepted: 1 May 2020; Published: 4 May 2020
Abstract: A multi-aspect analysis of low-pressure catalytic hydrogenation of CO 2 for methanol production is reported in the first part (part I) of this paper. This includes an extensive review of distinguished low-pressure catalytic CO 2 -hydrogenation systems. Specifically, the results of the conducted systematic experimental investigation on the impacts of synthesis and micro-scale characteristics of the selected Cu/ZnO/Al 2 O 3 model-catalysts on their activity and stability are discussed. The performance of the investigated Cu/ZnO/Al 2 O 3 catalysts, synthesized via different methods, were tested under a targeted range of operating conditions in this research. Specifically, the performances of these tested Cu/ZnO/Al 2 O 3 catalysts with regard to the impacts of the main operating parameters, namely H 2 /CO 2 ratio (at stoichiometric -3-, average -6- and high -9- ratios), temperature (in the range of 160–260 ◦C) and the lower and upper values of physically achievable gas hourly space velocity (GHSV) (corresponding to 200 h−^1 and 684 h−^1 , respectively), were analyzed. It was found that the catalyst prepared by the hydrolysis co-precipitation method, with a homogenously distributed copper content over its entire surface, provides a promising methanol yield of 21% at a reaction temperature of 200 ◦C, lowest tested GHSV, highest tested H 2 /CO 2 ratio (9) and operating pressure (10 bar). This is in line with other promising results so far reported for this catalytic system even in pilot-plant scale, highlighting its potential for large-scale methanol production. To analyze the findings in more details, the thermal-reaction performance of the system, specifically with regard to the impact of GHSV on the CO 2 -conversion and methanol selectivity, and yield were experimentally investigated. Moreover, the stability of the selected catalysts, as another crucial factor for potential industrial operation of this system, was tested under continual long-term operation for 150 h, the reaction-reductive shifting-atmospheres and also even after introducing oxygen to the catalyst surface followed by hydrogen reduction-reaction tests. Only the latter state was found to affect the stable performance of the screened catalysts in this research. In addition, the reported experimental reactor performances have been analyzed in the light of equilibrium-based calculated achievable performance of this reaction system. In the performed multi-scale analysis
Catalysts 2020 , 10 , 505; doi:10.3390/catal
in this research, the requirements for establishing a selective-stable catalytic performance based on the catalyst- and reactor-scale analyses have been identified. This will be combined with the techno–economic performance analysis of the industrial-scale novel integrated process, utilizing the selected catalyst in this research, in the form of an add-on catalytic system under 10 bar pressure and H 2 /CO 2 ratio (3), for efficiently reducing the overall CO 2 -emission from oxidative coupling of methane reactors, as reported in the second part (part II) of this paper.
Keywords: low-pressure methanol synthesis; catalytic CO 2 -hydrogenation; catalyst synthesis; equilibrium-based calculation; systematic performance analysis; add-on CO 2 -utilization process
1. Introduction Efficient conversion of generated carbon dioxide in industries to valuable fuels or chemicals is evolving from being a promising alternative to becoming a necessity due to the ever growing CO 2 emission rate as well as the cost and limitations of CO 2 storage [ 1 – 3 ]. Less undesired byproduct CO 2 will be generated in the first place if less energy and fuels are utilized [ 4 ], or if the selective performance of the catalytic and non-catalytic base-process is improved. Having considered the potential of supplying hydrogen either using renewable sources or via the reforming of hydrocarbons, CO 2 -hydrogenation to methanol becomes one of the promising CO 2 -utilization concepts as extensively investigated elsewhere [ 5 , 6 ]. On the other hand, methanol is an important product and an equally important intermediate chemical for producing more valuable chemicals such as olefins, dimethyl-ether (DME), fuels and solvents in general. Industrial-scale methanol production is mainly based on medium- to high-pressure catalytic conversion technologies of syngas (CO/H 2 with and without CO 2 ) even utilizing waste resources [7]. Guiding the conceptual design of any retrofitted CO 2 hydrogenation process from an industrial-operating point of view, it should be taken into consideration that the carbon dioxide containing gas streams often have a low delivery pressure, being either the product stream of CO 2 -removal-stripping section or the purged and flue gas streams. Therefore, CO 2 hydrogenation to methanol even under atmospheric pressure has been investigated, but the observed levels of CO 2 conversion and the methanol yield in that case are not high enough for the perspective of possible industrial-application [ 8 , 9 ]. Moreover, industrial-scale catalytic reactors usually do not operate below 5–10 bar pressure, which is needed to secure feed flow along the reactor and downstream units. Increasing the operating reaction pressure up to medium and high levels (more than 50 bar) and processing the pressurized carbon dioxide and hydrogen with the conventional methanol production technologies are usually very expensive and operationally challenging because their CO 2 content is low. This has been demonstrated to be a crucial conceptual design aspect also for combining the syngas production and methanol synthesis [ 10 ]. Compressing the carbon oxides and unreacted hydrogen separated in the downstream units and recycling them back in to the medium and high level pressure methanol reactor is also very costly. Specifically, some condensable corrosive components present in the feed streams make the required compression and the required equipment very expensive. Therefore, the relatively low pressure (up to 20 bar) CO 2 -catalytic conversion processes are especially attractive from a techno–economic point of view. In the current study, catalytic hydrogenation of carbon dioxide to methanol specifically around 10 bar pressure, which can be easily integrated with many industrial processes producing CO 2 , was investigated. Specifically, minimizing the CO 2 generation in the upstream methane activation process [ 11 , 12 ] and the efficient conversion of CO 2 to methanol in the downstream add-on CO 2 –hydrogenation process will be secured for efficient utilization of methane and carbon dioxide (C 1 : CH 4 and CO 2 ) via integrated oxidative coupling of methane (OCM) processes to be discussed in detail in part II of this paper.
Cu/ZnO/Al 2 O 3 , Cu/ZnO/ZrO 2 , Cu/ZrO 2 , Cu/ZnO/Ga 2 O 3 catalysts and other multi-component-oxides supports and promotors have been widely investigated for this application [ 18 , 19 ]. Among them, Al 2 O 3 supported Cu-ZnO catalyst has shown a promising performance in terms of selectivity towards methanol for the practically relevant high range of carbon dioxide conversion so that the methanol yield of more than 20% can be secured [ 16 – 18 ]. All the studies reporting more than 20% methanol yield have been highlighted in Table S1. Continue following the reported studies listed in Table S1, after reviewing the reported performances of the Cu/ZnO catalysts under low–medium pressure (listed reports in rows10–19), the performance of the alumina supported catalysts (listed reports in rows 20–42) show the highest methanol yield as expected to be achieved at the highest operating pressure of 110 bar. Here, the impacts of operating pressure, H 2 /CO 2 ratio, synthesis method as well as the catalyst composition (even some commercial catalysts) can be tracked. Similarly, the listed reports 43–65 (all having alumina in their supports) and 66–109 in Table S1 can be compared, through which the impacts of mixed supports and promotors can be highlighted. Performances of the catalysts containing Cu, ZnO or their combinations over different supports can be analyzed by reviewing the reported data listed in rows 110–177 in Table S1. Similarly the performances of other catalysts’ active components and supports under wide range of conditions can be compared by analyzing the rest of reported data in this table. Even though that the main focus in the current research study is on the low-pressure CO 2 hydrogenation over Cu-based catalysts, some selected comparable catalytic performances on higher pressures (covered range of up to 110 bar) have been also listed in Table S1 to highlight the impact of operating pressure. Having reviewed these reported performances, it can be concluded that targeting 20–30% methanol yield via low-pressure CO 2 -hydrogenation would present this technology as an attractive alternative, even as a competing technology for medium-to-high pressure syngas to methanol processes. Inexpensive hydrogen supply, for instance from renewable resources, is a key aspect here. In addition, reviewing the representative mechanisms and the kinetic data for this catalytic system hints how selective reactions paths can be intensified [ 16 ]. Especially the quantitative and qualitative contributions of the Cu on the catalytic activity have been extensively investigated. One should however be aware of the difficulties and limitations of comparing the reported results with each other while reviewing the observed trends and the impacts of operating conditions in previous studies. Therefore, one of the objectives of current study is to select a set of catalysts and test their performances under comparable conditions. This also enables the determining of the targeted set of conditions under which, this catalytic system can be utilized in the form of industrial-scale low-pressure CO 2 hydrogenation as added/integrated part of the upstream CO 2 -generating process. Concluding this review, it should be emphasized that the selected catalysts to be the subject of further experimental studies in this research are not necessarily the most effective catalysts known for this system. Nevertheless, these selected catalysts represent different synthesis recipes of Cu/ZnO catalysts and analyzing their performances will provide valuable information to better understand and efficiently utilize the catalytic CO 2 -hydrogenation as add-on low-pressure (e.g., 10 bar) methanol production process. The main challenges to be addressed in this context are the low CO 2 -conversion, methanol selectivity and the stability of the catalyst. In order to address these challenges and screen the catalysts and their performances with regard to the effects of the catalysts characteristics and operating conditions, the selected catalysts were characterized and tested in a standard fixed-bed reactor under the targeted range of operating conditions explained in the next sections. As a result of the performed literature review summarized in Table S1, beside the main targeted catalyst synthesized via hydrolysis co-precipitation method [ 17 ], the conventional-, carbonate-, and gel-coprecipitation methods reported in references [ 20 , 21 ] were selected to be applied in this research for synthesizing the Cu/ZnO catalyst. In addition, the citrate impregnation method as reported in references [ 22 , 23 ] was also applied to synthesize the catalyst and test it along with the above mentioned
synthesized catalysts under comparable testing conditions. Some of the benchmark commercial catalysts have been synthesized similarly [20]. On the other hand, a comprehensively-analyzed catalyst with known performance trajectory should be also chosen and tested as a reference catalyst along with these selected catalysts in a comparative study. Therefore, a benchmark research catalyst from Fritz-Haber Institute, which is the results of several years of research and optimization [ 24 – 26 ], was synthesized and supplied by Fritz-Haber Institute and tested in this research. The current study therefore aims to systematically review and complete the previously reported catalyst studies, and to consolidate the possible conclusions made based on analyzing the impacts of the catalysts’ characteristics as well as the operating conditions on their activity, selectivity and stability. This further improves our understanding of the parameters shaping the performance of the CO 2 -hydrogenation catalysts. The generated added-value information thereby supports efficient utilization of these catalysts in the targeted range of operating conditions in this research in the context of multi-scale analysis of the integrated catalytic process.
3. Results and Discussion In this section, first the results of catalyst characterizations for catalysts MET1-MET7 defined in Section 4) are presented and discussed.
3.1. Catalyst Characterizations The BET results show that except for MET6 and MET7, the nitrogen adsorption-desorption isotherms for all other catalysts (as typically observed for MET2), are type IV with the hysteresis loop, which usually is an indication for a mesoporous structure with slit-shaped pores. Detailed BET results of these samples have not been reported here for the sake of shortening the paper, but they have been observed to represent relatively flat isotherms, which is an indication for the porosity of the samples caused by a dense agglomeration of metal oxides. On the other side, type II isotherms were identified for the samples MET6 and MET7, which generally is an indication for their macro porous characteristic. The specific surface areas and the pore size distributions of all samples, which have been calculated by (Barrett–Joyner–Halenda) BJH method, are shown in Table 1. These should be considered while analyzing the performance of the catalytic samples.
Table 1. Surface area (BET) and pore diameter of all catalyst samples. Catalyst (^) SBET (m^2 g −^1 ) Mean Pore Size (nm) MET1 79 7 MET2 45 6 MET3 19 5. MET4 22 5. MET5 52 5 MET6 11 6. MET7 28 6
All the peaks indicated in this figure are related to CuO and ZnO. All the samples have very clear CuO (111) reflection based on which the size of copper oxide crystallites could be calculated. The broadened peaks of MET1 and MET5 indicate a rather small crystallite size in the nanometer range. This is also indicated through their relatively larger specific surface area as reported in Table 1. The relation between the catalyst selectivity and the size of the Cu particles is known and has been fully explained elsewhere [ 22 ]. More details on the implementation of different methods to calculate the copper oxide particle sizes and their impacts on the performance of the catalysts have been also already discussed extensively and can be found elsewhere [ 21 , 23 ]. However, it should be mentioned that by using the XRD data for analyzing the surface characteristics of the samples, usually only the large particle-sizes are observed, while the impacts of the small particle sizes needs to be also
Figure 2. Scanning electron microscope-energy dispersive x-ray spectroscopy (SEM-EDX) mapping of ( a ) MET1, ( b ) MET2, ( c ) MET6 (top left: SEM picture; top right: EDX visualization of copper distribution; bottom left: EDX visualization of zinc distribution; bottom right: EDX visualization of aluminum distribution).
The EDX pictures for MET1 and MET2 show a homogenous distribution of copper over the entire surface of these catalyst samples established by the implemented synthesis methods. On the contrary, the dispersion of copper over the surface of MET6 and MET7 is not homogenous. It is known that the better distribution of the metal species over the catalyst surface results in relatively lower local metal loading and increases the portion of the strong basic sites, which positively contribute to selective methanol formation [ 27 , 28 ]. Having generally reviewed all these and without going into details, the main focus in this step is to explain the observed different selectivity of the investigated catalysts. For instance as will be discussed in next section, the observed relatively higher methanol selectivity of MET1 and MET2 (synthesized via co-precipitation method) can be mainly attributed to the proven relatively homogenous distribution of the Cu and Zn species over the surface of these catalysts.
3.2. Catalytic Performance Testing the catalytic performance of the samples in this research was designed to be conducted in four steps in order to step-by-step screen the catalysts’ activity, selectivity, and stability with the view on their possible industrial-scale operation: Step one: comparative performance analysis of all selected catalysts under the operating range recommended in the original references. Step two: comparative sensitivity analysis of the short-listed catalysts under the preferred range of operating conditions (GHSV and temperature) determined after the first step (step one). Step three: searching for the best catalytic performance through a well-designed full-factorial experimentation, represented in Tables 2 and 3, to analyze the impacts of the operating temperature and H 2 /CO 2 ratios on the performance of the final selected catalysts. Results are evaluated also in reference to the equilibrium-based calculated achievable performance of the CO 2 -hydrogenation under these conditions. Step four: stability tests under (a) long-term hydrogenation reaction, (b) sequence of reducing-reaction atmospheres and (c) before and after exposing the catalysts to oxygen. In the first step (step one) of the experimentation, all identified Cu/ZnO catalysts (MET1–MET7) were tested close to the range of operating conditions recommended for each one in the original references, under which their best observed performance have been observed. In this manner and in order to compare the performance of these catalysts, the GHSV was set to 684 (h−^1 ) while they were tested in operating temperature range of 200–260 ◦C at H 2 /CO 2 ratios of 3, 6 and 9. All experiments were repeated at least three times and the standard deviations were calculated to be below 5%. For all tested catalysts, methanol and CO were observed to be the main products and only a trace of CH 4 was detected via the GC. The observed results of this screening step are reported in Table 2. Most of the reported results in Table 2 follow the expected trends as also previously recorded for these catalysts [ 16 , 17 ], confirming that an increase in the operating temperature causes an increase in the CO 2 conversion and CO selectivity and thereby a decrease on the methanol selectivity. This could be explained based on the impact of temperature on the rates of competing CO 2 hydrogenation reaction to methanol and the reverse water gas shift (RWGS) reaction. These reactions are represented as followings: CO 2 + 3H 2 CH 3 OH + H 2 O ∆H 298 = −49.
kJ.mol−^1
(R-1)
CO 2 + H 2 CO + H 2 O ∆H 298 = 41.
kJ.mol−^1
(R-2)
Increasing the temperature is more favorable for the RWGS reaction due to its endothermic thermal characteristic. Therefore, for the next step of the experimentation, narrower lower range of temperature (200–230 ◦C) was applied.
the hydrogen content in the feed significantly increases the CO 2 conversion and methanol yield and decreases the CO selectivity. This has been explained also elsewhere via quantitative analyzing of the relative impact of the partial pressure of CO 2 and hydrogen on the formation rate of methanol [ 16 ]. Interaction of the adsorbed species with the Cu and the oxide components, and in general the contribution of the Cu particle size and Cu surface area on the activity and selectivity of these catalysts have been also discussed there [ 16 ] as well as in many other references [ 22 ], suggesting that for the given Cu content and oxide supports, the smaller size Cu particles cause higher Cu surface area and thus a higher yield towards methanol. This was observed for the catalyst samples MET3, MET and MET5. After analyzing the experimental results of the first step, in order to highlight the main practical theme of this research, the catalysts showing a methanol yield of higher than 4% using H 2 /CO 2 = 3 and temperature of 200 ◦C were selected to be further investigated in the next steps. It was found that MET6 and MET7 catalysts synthesized by the impregnation method were not active enough, as their conversion was relatively low. The relatively lower Cu content (only 15%), heterogeneous dispersion of the Cu components on the surface of these catalysts, confirmed via several SEM-EDX images of different parts of the catalysts samples, and their relatively less mesoporous structure are believed to be the main reasons for relatively low activity–selectivity of these catalysts. Therefore these catalysts (MET6 and MET7) were not further investigated and the analysis continued with the remaining catalysts. Having fixed the operating pressure and temperature as well as the size of catalytic bed, less feed and therefore lower GHSV could be applied in order to improve the conversion of carbon dioxide in the next step of the experimentation. Therefore, comparative performance tests of the remaining catalysts (MET1–MET5) under the lowest range of feed flow were conducted. The targeted feed flow was therefore determined, based on the targeted feed composition and by considering the lowest range of flow in the mass flow controllers, which ultimately led to establishing the actual GHSV of 200 h−^1 inside the catalytic bed. The results of the performed comparative tests under this low GHSV (200 h−^1 ), enabled analyzing the impacts of varying temperature and H 2 /CO 2 ratios on the CO 2 conversion and methanol selectivity of the investigated catalysts under such intense reaction environment as reported in Table 3. Mainly due to the longer contact time established in this set of experiments, the CO 2 conversion was increased. It should be taken into consideration however, that decreasing the gas hourly space velocity may increase the intensity of reaction, resulting in higher reaction temperature along the catalytic bed, and also because of that it may affect the CO 2 conversion and reaction performance in general. Having considered such interactive effects, reducing the GHSV does not always favor methanol formation and the RWGS reaction can be intensified in this way and thereby more CO can be produced. This indicates that not only the activity, but also the selectivity of the reaction system will be affected by varying the GHSV or the contact time. Therefore, beside considering the involved catalytic kinetic and mechanism aspects, thermal effects of the reactor design should be also taken into analysis for describing the performance of this reaction system. Having analyzed the performance of all catalysts in these series of experiments, MET1 and MET catalysts showed the highest methanol yield under H 2 /CO 2 ratio of 3, which represents the lowest stoichiometric ratio of expensive-to-inexpensive educts and therefore the most practically relevant conditions for this catalytic reaction system in its industrial-scale operating perspective. Among the synthesized catalysts, these two catalysts also have shown relatively the most homogenous distribution of the surface–active components, as earlier discussed. In fact, the best recorded methanol yield for low-pressure CO 2 -hydrogenation has been previously reported for catalyst MET2 [ 17 ]. As expected, the research-benchmark MET1 catalyst has also performed very well. Therefore, the observed results for these catalysts in current research reconfirm their promising potential.
3.3. Equilibrium-Limited Achievable Performance The finally selected catalysts MET1 and MET2 were tested in wide range of operating temperatures (160–260 ◦C) to track the impact of temperature on the observed methanol selectivity and CO 2 conversion, while comparing these values with their ultimate achievable values calculated based on the thermodynamic equilibrium limitations and the Gibbs free energy minimization of products’ formation. These values were calculated using Aspen Plus simulator 8.8 and SR-Polar equation of state for a given set of reaction conditions (T, P, H 2 /CO 2 ratio). In such calculations, all involved reactions including the equilibrium CO 2 and CO hydrogenations and water gas shift reaction have been taken into consideration. As shown in Figure 3, by tracking the trends showing the effect of operating temperature on the CO 2 -conversion and methanol selectivity, it can be observed that for the GHSV of 200 h−^1 , a pressure of 9 bar and H 2 /CO 2 ratio 3, the highest methanol yield can be secured by operating at temperature of 200 ◦C.
Figure 3. The impacts of reaction temperature on the CO 2 conversion (top) and MeOH selectivity (bottom) for the ultimate selected catalysts and comparing their performances with the thermodynamic-equilibrium limited achievable performance; Reaction conditions: H 2 /CO 2 = 3, T= 160–260 ◦C, P = 9 bar g, GHSV = 200 h−^1.
Although achieving higher methanol selectivity and yield is theoretically predicted at lower temperatures from the sole equilibrium point of view, the temperatures lower than 200 ◦C cannot activate the catalyst enough to secure a significant CO 2 -conversion and thereby high methanol yield. The reaction temperatures of higher than 200 ◦C, thermodynamically favor unselective conversion. Therefore, at higher temperatures, the results show a decrease in methanol yield mainly because of the poor selectivity. This is the case for both the investigated catalysts, MET1 and MET2.
Figure 4. Stability and reducibility of ( a ) MET1, ( b ) MET2; reaction condition: H 2 /CO 2 = 3, T = 200–245 ◦C, P = 9 bar g, GHSV = 200 h−^1.
4. Material and Methods
4.1. Selected Catalysts and Synthesis Methods The recipes and the chemicals used for synthesizing these selected types of Cu/ZnO catalysts are reported in this section and the list of utilized chemicals is provided in Table 4.
Table 4. List of chemicals used for the catalysts’ synthesis. Used Basic Material CAS-Number Supplier Copper II nitrate trihydrate 10031-43-3 Sigma-Aldrich (Darmstadt, Germany) Zinc nitrate hexahydrate 10196-18-6 Sigma-Aldrich(Taufkirchen, Germany) Citric acid anhydrous 77-92-9 Sigma-Aldrich (Taufkirchen, Germany) Oxalic acid 144-62-7 Sigma-Aldrich (Taufkirchen, Germany) Aluminium nitrate 7784-27-2 Sigma-Aldrich (Darmstadt, Germany) Sodium carbonate 497-19-8 Sigma-Aldrich (Taufkirchen, Germany)
The details of the implemented synthesis approaches including all required information allowing to reproduce these catalysts, are reported in this section and can be also found in more details in the provided original references in each case.
4.1.1. Co-Precipitation Method for Preparation of Cu/ZnO/Al 2 O 3 Catalyst This type of Cu/ZnO catalyst was prepared by the coprecipitation method described by Behrens et al. [26]. Molar aqueous solutions of Cu(NO 3 ) 2 ·3H 2 O, Zn(NO 3 ) 2 ·6H 2 O and Al(NO 3 ) 3 were prepared. They were then mixed together to result in an overall metal composition of 68:29: (Cu:Zn:Al). An aqueous solution of one molar sodium carbonate was added to the mixture under stirring at 65 ◦C to reach the pH of 6.5. In this manner, the resulted 2000 cc metal nitrate solution was acidified with 15 cc concentrated HNO 3 and the carbonate solution as a basic precipitating agent, dosed in 1200 cc deionized water. The precipitation was established at the same temperature for 30 min and the color of the mixture was changed from green to bluish green. The precipitate was centrifuged and washed with deionized water and dried overnight at 110 ◦C and then calcined at 330 ◦C for 4 h with the heating ramp of 2 ◦C per minute. Here, the resulted catalyst is referred to as MET1.
4.1.2. Hydrolysis Method for Preparation of Cu/ZnO/Al 2 O 3 Catalyst The selected recipe for preparing Cu/ZnO/Al 2 O 3 catalyst using hydrolysis method has been reported by Xu et al. [ 17 ]. Cu(OH) 2 -Zn(OH) 2 precipitate was prepared by adding one molar Na 2 CO 3 to the premixed solution of Cu(NO 3 ) 2 and Zn(NO 3 ) 2 , prepared by mixing a liter of one molar solution of each of them, to reach the pH of 8 under stirring and ambient temperature. The precipitate was then washed with deionized water. In parallel, a proportional amount of one molar solution of Na 2 CO 3 was added to the prepared 254 cc one molar solution of Al(NO 3 ) 3 to reach the pH of 8 under stirring and ambient temperature. The precipitate Al(OH) 3 (gel form) was also washed with deionized water. Precipitated Cu(OH) 2 -Zn(OH) 2 and Al(OH) 3 were then mixed in a mortar and stirred to get a homogenous mixture to obtain the hydroxide mixture containing Cu(OH) 2 , Zn(OH) 2 and Al(OH) 3 in a molar ratio of 47, 47 and 6, respectively. The mixture was then dried at 120 ◦C for 12 h. The dried product was calcined at 350 ◦C for 3 h followed by further calcination at 500 ◦C for 1 h with the heating ramp of 3 ◦C per minute. Here, the prepared catalyst is referred to as MET2.
4.1.3. Coprecipitation Method for Preparation of Cu/ZnO/Al 2 O 3 Catalyst The third type of Cu/ZnO/Al 2 O 3 catalyst was prepared by the coprecipitation method described by Jingfa et al. [ 21 ]. Aqueous solutions of Cu(NO 3 ) 2 ·3H 2 O, Zn(NO 3 ) 2 ·6H 2 O and Al(NO 3 ) 3 each in 0. concentration were prepared and mixed together to reach the metal composition of 45Cu:45Zn:10Al in the precipitate phase. An aqueous solution of 1 molar oxalic acid was added rapidly to the prepared
It should be mentioned that most of the investigated catalysts in this research have been also characterized in previous studies. For instance, general characterization techniques such as XRD, TEM and thermal analysis (TG/DTG) have been previously [ 21 ] applied for the most of the selected catalysts in this research. In order to conduct a comparative study, the characteristics of the selected catalyst samples, using XRD, BET and SEM characterizations, have been analysed and reported and conceptually compared with each other in the current research. These results can be also directly compared with the reported characteristics of similar catalytic systems in literature. This can be generalized for the other measured/reported characteristics, except for the few parameters such as pore dimeters of the MET3 and MET4 which have been measured to be 50% smaller than their original reported values of 10–14 nm [ 21 ]. The results of these basic characterization techniques (i.e., XRD, BET, SEM) can be also used for studying the impacts of the structure and the morphology of the catalysts as well as their involved crystalline phases on their catalytic performances. Moreover, the results of the physical–chemical characterization methods which are sensitive enough to identify the characteristics of the external surface, for instance for detecting the copper species before and after the reaction using XPS analysis, and the ones determining the reactivity of the solids (chemisorption, IR) for some of these catalysts are available elsewhere [17,23]. Calcined catalysts samples were characterized by applying X-ray powder diffraction (XRD) analysis technique in the 10 ≤ 2 θ ≤ 90 range on a Brucker D8 advance diffractometer via Co-Ko radiation (using 1.541 Å, 40 kV, 35 mA, Berlin, Germany) at a scanning rate of 2K/min to identify the crystalline phases. The BET surface area and the pore size distribution of all catalysts were calculated based on the N 2 adsorption-desorption measurements at − 196 ◦C. The proper amount of catalyst samples (corresponding to 10–100 m^2 surface) were degassed under vacuum atmosphere. During the pre-treatment period, each sample was heated under a step-wise rising temperature profile starting with 80 ◦C, followed by being heated under 120 and 180 ◦C each for one hour and finally at 220 ◦C for 4 more hours with the heating rate of 3 ◦C per minute in order to clean the surface and pores of the sample. After degassing the BELprep-VACII preparation equipment (Berlin, Germany), the sample was weighed again to quantify the dried mass of the materials. The cell was then placed in the BELSORP-mini II measuring equipment (Berlin, Germany) and then into a Dewar vessel of liquid nitrogen. N 2 was stepwise introduced to the samples until reaching the ambient pressure and then was step-wise vacuumed in order to measure the N 2 adsorption and desorption behaviour of the samples. In order to study the surface morphology and the homogeneity of the samples, a Zeiss Gemini Leo 1530 Field Emission Scanning Electron Microscope (FESEM, Berlin, Germany) was used, which is equipped with energy-dispersive X-ray spectroscopy (EDX).
5. Experimentation for Catalyst Testing The specification of the experimental setup and the design of experiments as well as the procedure of testing the catalysts are reported in this section.
Experimental Setup The schematic of the experimental setup utilized for testing the catalyst is depicted in Figure 5. A picture of the setup and the visualization interface of the control system can also be seen there. In this setup, the operating temperature, pressure and feed composition are respectively controlled via an electrical tube furnace, back pressure regulator and several mass flow controllers. The set-points and the measured values of the feed flow rates, operating pressures and the applied temperature of the three electrical elements/zones in the furnace are set and monitored online through the user-interface PCS-ILS control system as shown in Figure 5.
Figure 5. Experimental setup: ( a: left ) picture of the reactor, ( b: middle ) schematic flow diagram, ( c: right-top ) monitor-control system.
The temperature could be set either as the temperature of the furnace (local-mode) or the temperature inside the catalytic bed (cascade-mode), which was the case for most of the experiments in this research. The thermocouple located inside the tube was in touch with the top of the catalytic bed inside the vertical reactor, where feed gas enters from the top. The volumetric feed flow rates of nitrogen, carbon dioxide and hydrogen were controlled using Bronkhorst F-series mass flow controllers (MFC) with a 0.8% precision range around the measured values. The operating pressure inside the reactors was controlled using an electronic back-pressure controller. For each experiment, a 20 cm long catalytic bed, made of 200–400 micron sieved powder of each catalyst, was located inside a stainless steel fixed-bed reactor with an inner diameter of 8 mm. The bed was positioned in the height of the middle electrical heating element/zone. In order to fix the location of the catalytic bed, the bottom part of the reactor was filled with inert packing and quartz wool. For calculating the required amount of inert packing and the catalyst for each run and in order to ensure proper filling the reactor, the locations of the heating zones as well as the values of inner diameter of the reactor and the density of catalysts were taken into calculation. After placing the reactor inside the furnace and fixing the connections, the feed-mixture composed of the desired portions of nitrogen (or air solely used for specific type of stability tests), hydrogen and CO 2 were tuned for establishing the targeted reducing, purging and reaction atmospheres. The required overall gas feed flow rate to be introduced to the reactor for representing the targeted actual gas hourly space velocity (GHSV) inside the catalytic bed is calculated using Equation (1):
GHSV =
60 × Q × (273.15 + Tcb) Vcb × P × (273.15 + Ta)
values in this manuscript. This means that for instance, the actual value of CH 3 OH-Yield lies in the range of 0.9 × CH 3 OH-Yieldreported^ < CH 3 OH-Yieldactual^ < 1.1 × CH 3 OH-Yieldreported.
6. Conclusions A comprehensive review analysis on the low-pressure performances of the selected CO 2 -hydrogenation catalysts has been provided in this paper. The performed experimentation and testing of the selected CO 2 -hydrogenation catalysts showed that some receipts of Cu/ZnO/Al 2 O 3 catalysts exhibit a stable active catalytic performance promising for industrial-scale operation. It was observed that the coprecipitation catalyst synthesis approach, which is known for its potential of establishing a homogenous distribution of active components over the whole catalytic body, provided the highest methanol selectivity and yield. All tested catalysts have shown their best performance, in term of methanol yield, for the highest applied H 2 /CO 2 ratio and lowest GHSV. A methanol yield of 21% using the H 2 /CO 2 ratio of 9 and the temperature of 200 ◦C under 10 bar pressure was the best catalytic performance achieved. After testing the stability of the selected catalysts under long-term operation as well as the reaction-reducing shifting-atmospheres and even after introducing oxygen to the catalyst surface followed by reducing-reaction tests, it was found that exposing the catalyst with oxygen can significantly affect the stability of some of the catalysts. Moreover, it was found that the performance of some of the selected catalytic systems, especially in the average range of temperatures, can come very close to the ultimate achievable performance predicted by considering the thermodynamic-equilibrium limitations. Thermal characteristics of the reactor operation were shown to also be important in determining the ultimate achievable yield of methanol in this system. The experimental analysis conducted in this research (part I) will support the novel efficient implementation strategy for industrial-scale utilization of the CO 2 -hydrogenation catalytic system (represented by the selected catalyst MET2) as an add-on process to be integrated with the case-study process oxidative coupling of methane (OCM). This will be analyzed in part II of this manuscript.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/ 10 / 5 / 505 /s1, Table S1: A selective overview of the reported performances for catalytic hydrogenation of CO 2 to methanol (Specially the studies performed at low-medium pressure).
Author Contributions: Conceptualization, H.R.G. and S.M.J.; Formal analysis, L.T. and O.G.; Investigation, H.R.G., M.K., M.A. and O.G.; Methodology, H.R.G., M.K. and S.M.J.; Resources, R.S., G.W. and J.-U.R.; Software, M.A.; Supervision, H.R.G., O.G., S.M.J., L.T., R.S., G.W. and J.-U.R.; Visualization, O.G.; Writing—original draft, H.R.G. and M.A.; Writing—review & editing, H.R.G., O.G., G.W. and J.-U.R. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding.
Acknowledgments: Catalyst sample preparation by Elias Freie (Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Berlin, Germany) is sincerely acknowledged. The authors acknowledge the financial support from the Cluster of Excellence UniCat “Unifying Concepts in Catalysis” coordinated by the Technische Universität Berlin and funded by the German Research Foundation—Deutsche Forschungsgemeinschaft. Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
BET Measuring the specific surface based onBrunauer–Emmett–Teller theory BJH Barrett–Joyner–Halenda Cat Catalyst CCSU Carbon capture separation utilization Dilu. Dilution DME Dimethyl ether (Methoxymethane) EDX Energy-dispersive X-ray spectroscopy FESEM Field emission scanning electron microscopy Gas Gas phase GC Gas chromatography In Inlet stream MeOH (CH3OH) Methanol MET Methanol catalysts prepared with different methods Out Outlet stream PC-ILS Process control system—integrated lab solution RWGS Reverse water gas shift UniCat “Unifying Concepts in Catalysis” (a research group in Berlin) XRD X-ray diffraction
Nomenclature A Ambient - Cb catalytic bed - D Diameter or equivalent diameter nm F Molar flow rate mol/min GHSV Gas hourly space velocity L/h P Pressure bar Q Total flow rate Nml/min S (Selectivity)
Portion of the whole consumed carbon dioxide which appears in the (desired) products
T Temperature ◦C V Volume ml X (CO2 Conversion)
Portion of the inlet carbon dioxide converted to the desired and undesired products
X Mole fraction -
Y (Yield)
Amount of the converted carbon dioxide appears in each product per whole total amount of the inlet carbon dioxide
∆HR Reaction enthalpy kJ/mol P Density kg/m^3
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