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Keywords:

  • Catalyst synthesis;
  • Heterogeneous catalysis;
  • Methanol synthesis;
  • Copper;
  • Zinc

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

In this research report we summarize recent progress that has been made in the field of Cu/ZnO catalyst synthesis. We briefly introduce the fields of application of this catalyst: methanol synthesis, the water gas shift reaction, and methanol steam reforming. The review is focused on the well-documented industrial synthesis protocol and on the early stages of catalyst synthesis. The setting of the most critical synthesis parameters during co-precipitation and ageing, like pH and temperature, is discussed in detail. We show how these parameters effect the phase formation and identify zincian malachite, (Cu, Zn)2(OH)2CO3, as the relevant precursor phase for high-performance catalysts. A special emphasis is placed on the solid state chemistry of this precursor phase, in particular on the structural effects of Cu, Zn substitution. Based on the structural analysis, it is shown that the industrial synthesis recipe was empirically optimized to maximize the zinc incorporation into zincian malachite. From this insight a simple and generic geometric concept for the synthesis of nanostructured composite catalysts based on de-mixing of solid solution precursors is derived. With this concept, the complex multi-step industrial synthesis can be rationalized and the so-called “chemical memory” of this catalyst synthesis can be understood. We also demonstrate how application of this concept can lead to new interesting catalytic materials, which help to address fundamental questions of this catalyst system like to role of the Al2O3 promoter or the so-called Cu-Zn synergy.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

Heterogeneous catalysis is a field that involves many sub-disciplines of chemistry. The essential step of the synthesis of solid catalysts is in itself a heterogeneous area of research that combines methods from solid state chemistry, surface science, colloidal approaches, inorganic coordination chemistry, metal-organic synthesis, chemical engineering and others. Additional critical aspects include the shaping of catalyst powders and high throughput catalyst synthesis. A powerful and common approach in model catalysis is to decouple the material's surface from its preparation history to limit the complexity and to generate fundamental insight into catalytic reactions at surfaces, which usually cannot be directly observed by other approaches.

However, applied solid catalysts usually are complex composite powders with nanostructured components. Thus, there is a gap between traditional model studies conducted on well-defined and simplified clean surfaces at low pressures and the real industrial processes. The possibilities of model metal-support systems prepared by physical vapor deposition in combination with scanning probe characterization techniques have helped greatly to reduce this gap.1 However, understanding the role of irregularities built into the catalyst components during their chemical formation is, however, still a substantial challenge in studying of bulk heterogeneous catalysts and their mode of operation.

One reason for the still substantial dimension of the gap between model and performance catalysis is the dynamical nature of high performance catalysts. Through the advent and consequent utilization of in-situ functional analysis, it was found that many systems undergo massive restructuring2 during contact with their reagents. In many cases, this restructuring creates the active sites that do hence not exist in the pre-catalyst isolated from its reaction environment. These so-called pressure and materials gaps often prevent straightforward extrapolation of model studies to real applications. Hence, a true holistic approach to heterogeneous catalysis must complement the model approach by studies of a catalytic material that takes its preparation history as well as the chemical potentials of the final application into account.

The retrospective investigation of a known industrially applied catalyst synthesis is an ideal starting point for reconstructing the chemistry of arriving at a functional catalyst. In this situation, one can be sure that the critical details of all unit operations have been tested for relevance and are directly imperative for the best final result. A performance catalyst with high activity, selectivity, and stability is a solid material deviating substantially from the thermodynamic equilibrium system given by its composition.

Hence, its synthesis is entirely controlled by kinetic parameters that are rarely identified and even more rarely systematically studied.

The development of industrially applied catalyst syntheses is usually a continuous process that to a large extent is based on the experience of the manufacturer. The accumulated knowledge from the combination of empirical trial-and-error experimentation often leads to complex recipes. This complexity is sometimes generalized as the “black magic” of catalyst preparation. Further optimization is usually guided by elaboration of relationships between synthesis parameter and catalytic function within the boundary conditions of a feasible and scalable synthesis. Such phenomenological correlations are useful within narrow ranges of parameters, but suffer from two problems. They are not based upon physical and causal structure function relations and they are often even obscured by influences of transport kinetic phenomena that unfortunately are only seldom probed in studies aiming at catalyst optimization. Thus, their predictive value is rather limited and leads rarely to more than evolutionary improvements in performance.

The goal of such studies thus must be to upgrade empirical synthesis parameter-function relationships to synthesis parameter-structure-function relationships that allows for an understanding and more rational optimization of the catalyst. In the case of bulk catalysts, i.e. materials not prepared by deposition of a minority active component on a pre-assembled support, the key often lies in the solid state chemistry of its synthesis. Herein, we demonstrate the pivotal role of solid state chemistry in catalyst synthesis for the example of Cu/ZnO-based systems,3 which in a promoted form are industrially applied for the synthesis of methanol.4 We review the recent progress that has been made in analyzing and understanding the well-documented industrial synthesis of this important catalyst and show examples of how this insight can be used to make better methanol synthesis catalysts. Moreover, the chemical concept behind the “black magic” of this particular synthesis can be generalized and transferred to other catalyst systems. The actual catalytic reaction is not part of this research report, but we note that also the progress in the mechanistic understanding industrial methanol synthesis by new structure-function relationships is always based on the controlled synthesis of the catalysts and must be coupled to the understanding of the materials chemistry.

We start by briefly introducing the methanol synthesis process and other important C1 reactions that are catalyzed by Cu/ZnO catalysts. Subsequently we will discuss the microstructure and synthesis recipe of the industrial catalyst. Next, the genesis of a Cu/ZnO catalyst will be traced through the early synthesis steps with an emphasis on the solid state chemistry of the catalyst precursor compound. In the last sections, the concept behind the industrial recipe and its application to synthesize new catalytic materials will be discussed.

Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

In 2009 worldwide production of methanol was around 40 million metric tons. Chemical intermediates dominate methanol consumption and formaldehyde, a platform molecule for the synthesis of polymer resins, is responsible for nearly half of the total demand. Acetic acid, MTBE, and methyl methacrylate (MMA) constitute another 25 %.45 Direct fuel and additive usage accounts for 15 % of demand, but is expected to rise.

Until the commercialization of the first heterogeneous catalytic process for methanol synthesis by BASF in the 1920's, methanol was produced exclusively from the dry distillation of wood. The catalyzed synthesis of methanol corresponds to the conversion of synthesis gas (or syngas), a mixture of CO2, CO, and H2, according to the following three reactions:

  • CO2 + 3H2 [lrarr2] CH3OH + H2O; ΔH0 = –49.8 kJ·mol–1 ((1))
  • CO + 2H2 [lrarr2] CH3OH; ΔH0 = –91.0 kJ·mol–1 ((2))
  • CO + H2O [lrarr2] CO2 + H2; ΔH0 = –41.2 kJ·mol–1 ((3))

Methanol synthesis from CO2 [Equation (1)] and CO [Equation (2)] as well as the water gas shift (WGS) reaction [Equation (3)] are mildly exothermic and the former two result in volumetric contraction. According to Le Chatelier's principle, high pressures and low temperatures favor methanol synthesis. It should be noted that any two of the three reactions are linearly independent and therefore sufficient in describing the compositions of equilibrated mixtures.

The BASF process utilized sulfur-containing coal or coke derived synthesis gas and ZnO/Cr2O3 catalyst operating at 300–450 °C.6 High pressures (100–300 bar) were required to counteract these thermodynamically unfavorable temperatures. Although the superior activity of Cu-based methanol synthesis catalysts was reported shortly thereafter,7 only the advent of natural-gas derived sulfur-free synthesis gas allowed for feasible industrial application. The commercialization of more active Cu/ZnO/Al2O3 based catalysts (see below) by ICI in the 1960's lead to the application of milder reaction conditions of 240–260 °C and 50–100 bar in a “low pressure” process.8 Since its inception, this process has been optimized to yield methanol with a >99 % selectivity and 75 % energy efficiency, and has thus become the exclusive means of methanol production.9 The low pressure methanol synthesis relies almost exclusively on promoted Cu/ZnO catalysts. Radio-tracer experiments conducted in the 1980's proved conclusively that CO2 was the primary methanol source in the industrial process according to reaction (1).10

In addition to the steam reforming of natural gas or the gasification of coal, also mixtures of anthropogenic CO2 with hydrogen from water electrolysis are considered as a syngas source for methanol synthesis. This latter option has led to a renewed interest in methanol synthesis in the last years as the so-called “methanol economy” provides a sustainable energy scenario, which is based on the usage of methanol as a fuel and a recycle of the emitted CO2 in methanol synthesis.9 The switch from the industrial process that relies on fossil sources of syngas to a hydrogenation process of anthropogenic CO2 is, however, related to several challenges, such as the large-scale production of “renewable” hydrogen. Another one is the unfavorable thermodynamic of the CO2 hydrogenation reaction, which requires improved methanol synthesis catalysts with a better low-temperature performance. For instance, the thermodynamically expected methanol yield in a COx/H2 (1:3) mixture at 50 bar is approximately 60 % at 500 K for COx = CO, but only 20 % for COx = CO2. Yields around 60 % for CO2 hydrogenation require temperatures below 400 K, at which the kinetics are far too slow with the currently available catalysts.

Cu/ZnO-based catalysts are also applied in a promoted form for the low-temperature water gas shift reaction (3), which is used in industry to reduce the CO content in syngas feeds. Also the formal reverse reaction of CO2 hydrogenation (1), the methanol stream reforming (MSR) reaction is catalyzed by Cu/ZnO catalysts. In areas, where steam reforming of natural gas is not an option, MSR is applied in a stationary mode to produce hydrogen in relatively small-sized units, but most current research activity focuses on the use of MSR for on-board hydrogen generation in combination with downstream PEM fuel cells.11

The Cu/ZnO Catalyst

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

Promoted Cu/ZnO catalysts for methanol synthesis are commercially available and contain 50–70 atomic % CuO, 20–50 % ZnO and 5–20 % of the Al2O3 promoter. Instead of alumina, also chromium oxide, magnesium oxide, and rare earth oxides have also been used. The oxide catalysts are activated with dilute hydrogen at 190–230 °C, by which copper oxide is completely reduced to metallic crystallites interspersed by ZnO/Al2O3.

Commercial Cu/ZnO/Al2O3 methanol synthesis catalysts are often mistaken as supported systems, but neither ZnO nor the low amounts of Al2O3 used represent classical porous oxide supports. This is apparent, when considering the typical composition of modern Cu/ZnO/(Al2O3) catalysts, which is characterized by a molar Cu:Zn ratio close to 70:30, while the amount of Al2O3 typically is significantly lower than that of ZnO. This Cu-rich composition manifests itself in a peculiar microstructure of the industrial Cu/ZnO/Al2O3 catalyst,12 which is composed of spherical copper nanoparticles of a size of 5–15 nm (Figure 1) and often even smaller ZnO nanoparticles arranged in an alternating fashion. From a microstructural perspective, one important role of ZnO is to act as spacer and stabilizer avoiding direct contact of the copper particles and preventing them from sintering13 (see below for other roles of ZnO). Thus, porous aggregates are formed, in which the oxide particles act as spacers between Cu particles (Figure 1a). The presence of inter-particle pores as seen in the central upper half of the HRTEM image (Figure 1b) allows some access to the “inner surface” of the larger Cu/ZnO aggregates.

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Figure 1. (High resolution) TEM images of a Cu/ZnO/Al2O3 methanol synthesis catalyst consisting of porous aggregates (a)15 of metallic Cu and ZnO nanoparticles (b)12.

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This unique microstructure with a proper balance of Cu dispersion and loading enables a reasonably high dispersion of Cu and exposure of many Cu-ZnO interfaces at a high total Cu content. The specific Cu surface area (SACu) of methanol catalysts can be estimated by reactive N2O titration,14 which causes surface oxidation of the Cu particles and allows calculation of SACu from the amount of evolved N2. The SACu of state-of-the-art methanol synthesis catalysts, measured by this method, amounts to 25–35 m2·g–1.

The SACu has been observed to scale linearly with the activity for sample families with a similar preparation history.16 However, between these families considerably different intrinsic activities, i.e. activities normalized by SACu, can be found.17 Thus, in agreement with the structure sensitivity of methanol synthesis over Cu,18 different “qualities” of Cu surfaces can be prepared, which vary in the activity of their active sites and/or in the concentration of these sites. Differences in intrinsic activity of the exposed SACu can be related to defects and disorder in the Cu nanoparticles19 and to the “synergetic” role of ZnO,4,19a,20 which was observed in many studies to exceed the function of a mere physical stabilizer. The nature of this synergy and the active site of methanol synthesis are vividly debated. However, this discussion is beyond the scope of this research report, which is limited to aspects of catalyst synthesis.

Synthesis of the Industrial Catalyst or the “Chemical Memory”

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

In the technical catalyst, three requirements for high catalytic activity20c – large SACu, defective Cu nanoparticles and many reactive interfaces to ZnO – are elegantly realized by the nanoparticulate and porous Cu/ZnO arrangement shown in Figure 1. Preparation of this microstructure requires a homogeneous and maximized intermixing of the Cu and Zn species in order to stabilize the alternating arrangement of small Cu and ZnO nanoparticles. Thus, the main goal of catalyst synthesis is to carry over and maintain the perfectly homogeneous cation distribution in the starting mixed solutions to a maximum extent to the final catalyst.21 Different methods of Cu/ZnO catalyst preparation can be found in literature,3 but the aforementioned microstructure is most successfully achieved by co-precipitation, which is by far the most important and technically applied technique.

Figure 2 gives a schematic overview of the multistep synthesis route of Cu/ZnO catalysts introduced by ICI in the 1960s.13,22 It comprises co-precipitation21 and ageing19c,23 of a mixed Cu, Zn,(Al) hydroxy-carbonate precursor material, thermal decomposition yielding an intimate mixture of the oxides, and finally activation of the catalyst by reduction of the Cu component.24 The synthesis parameters of this route have been studied in many academic and industrial groups and a high degree of optimization could be achieved over the last decades by mostly empirical fine-tuning of the conditions.21,25 Preferred Cu:Zn ratios for ternary Cu, Zn, Al and binary Cu, Zn catalysts are near 70:3025a or 2:1.22 It was reported that the best catalysts can be obtained by constant pH co-precipitation with Na2CO3 solution at pH 6 or 7 and at elevated temperatures around 333–343 K.25b,25c Ageing of the initial precipitate is crucial19c,21,25c and takes from around 30 min to several hours. Calcination is typically performed at relatively mild temperatures around 600–700 K.

The delicate nanoparticulate and porous microstructure of the industrial methanol synthesis catalyst can only be obtained if the optimized parameters are strictly obeyed during synthesis. Especially the synthesis conditions during the early co-precipitation and ageing steps turned out to be crucial for the catalytic properties of the resulting methanol synthesis catalyst. This phenomenon, sometimes termed the “chemical memory” of the Cu/ZnO system,26 indicates the critical role of the preparation history of this catalyst system.19b,25b,27 Baltes et al.25b elaborated a quantitative basis of the chemical memory in a systematic study. They incrementally varied the synthesis conditions in a wide parameter field and tested their resulting materials against a commercial reference catalyst. The authors reported dramatic difference in SACu and catalytic activity for Cu/ZnO/Al2O3 catalysts of the same composition as pH or temperature of the co-precipitation step was varied (Figure 3). Variations in only fractions of a pH unit or a few degrees in temperature can have a huge effect on the resulting meta-stable composite material. In light of these results, it needs to be considered that variation of numerous other “non-chemical” parameters like batch size, stirring speed, or mixing geometry28 will also affect the outcome and the reproducibility of the catalyst synthesis. Minimization of so-called batch-effects on the material's properties is thus one major first challenge for a reliable linking of synthesis parameters to structural properties of the products in this system.

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Figure 3. Chemical memory: Dependence of catalytic activity in methanol synthesis (relative to an industrial standard) on the conditions of the co-precipitation and ageing steps. Reprinted from Ref. 25b with permission from Elsevier.

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The type of Cu/ZnO/(Al2O3) catalyst that is represented by the maximum in Figure 3 is very suitable as a reference point for new catalysts to monitor e.g. the progress in catalyst development for methanol synthesis or to compare different catalyst classes in methanol steam reforming. If self-made Cu/ZnO catalysts are used for this purpose, it is thus very important to carefully prepare the reference catalyst according to the above described recipe as it is much easier to synthesize a poor Cu/ZnO catalyst than to hit exactly and reproducibly the above mentioned sharp maximum. Furthermore, understanding and controlling the catalyst synthesis is the basis for an understanding and rational manipulation of the material's chemistry of the catalyst that can lead to new insights into the catalytic process itself. Such insight is often built on structure-performance relationships, which require a carefully prepared and comprehensively characterized series of samples.

The Role of the Precursor Compound

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

Much of the complexity of the industrial synthesis recipe can be explained with the solid state and material's chemistry of the co-precipitated precursor as shown in the following sections. The careful analysis of the co-precipitation, crystallization, and substitution chemistry of mixed Cu, Zn basic carbonates turned out to be the key to the understanding of this particular catalyst synthesis. A generalization of the results will be discussed later. The sometimes misunderstood term “chemical memory” finds its explanation in the details of the synthesis kinetics controlling the real structure of the final material; it will be shown that the precursor chemistry controls morphology and defects within the final Cu/ZnO system through determining the treatment temperatures of oxidation and reduction of the oxide intermediate.

“Co”-Precipitation

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

The term co-precipitation is often used for all precipitations involving mixed salt solutions, but in a stricter sense it means the simultaneous solidification of different species by adsorption or formation of binary precipitates at conditions where it would not occur from pure solutions with only a single species present. Precipitation titration is an elegant way to study the hydrolysis of Cu2+ and Zn2+ upon pH elevation under conditions relevant for catalyst preparation.21 Such experimental results are shown in Figure 4.

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Figure 4. Precipitation titration curves relevant for the co-precipitation of Cu/ZnO catalyst precursors at 338 K using aq. Na2CO3 as precipitation agent (adopted from Ref. 21).

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The hydrolysis of the cations is characterized by an underlying neutralization of the acidic starting solution with the basic precipitating agent. In case of Cu2+, a plateau of precipitation near pH 3 interrupts the S-shaped neutralization curve (Figure 4a). A qualitatively similar picture emerges for the Zn2+ solution (Figure 4b), but with the important difference that the Zn-precipitate is formed at pH 5 instead at pH 3. Such differences in hydrolysis behavior are of course not at all uncommon and are the basis for the traditional wet chemical ion separation techniques used for qualitative cation analysis. Interestingly, the precipitation titration curve of the binary system (Figure 4c) is a superposition of the single systems. This fact indicates that there is no formation of a mixed binary precipitate under these conditions, but that Cu2+ is first completely precipitated at pH 3, which can be also seen from the vanishing blue color of the mother solution at pH 4, while Zn2+ is precipitated “on top” later at pH 5.

Clearly, such an increasing pH precipitation processes is not a true co-precipitation and cannot yield a well intermixed precipitate that is needed for an intimate mixture of the species constituting the final catalyst. The solution to this problem is the application of the constant pH co-precipitation technique,29 meaning that the acidic metal solution and the precipitating agent are dosed simultaneously in such a way that the average pH in the reaction vessel is maintained constant. Using this mode of precipitation, it is possible to precipitate Cu and Zn very close in space and time. However, although direct analytical insight is difficult, this method probably also does not correspond to a true (simultaneous) co-precipitation, but can be envisaged as a much faster sequential precipitation, which happens for each single droplet instead of for the whole batch as in the titration experiments. Thus, the cation distribution in the precipitate obtained by constant pH co-precipitation is much more homogeneous.21 Such a “pseudo-homogeneous” blue precipitate can undergo homogenization reactions still within the mother liquor and thus lead indirectly to a meta-stable true co-precipitate.

Also a first idea of a suitable pH for the precursor preparation can be deduced from the titration curves shown in Figure 4. It should not be lower than pH 5 to guarantee complete precipitation of Zn2+ (and Al3+), which otherwise would remain at least partially in solution. On the other hand, the pH should stay below pH 9, because in a very basic solution de-mixing of the Cu, Zn precipitate by oxolation of basic copper precipitates into stable tenorite, CuO, occurs.21 This oxolation can be seen as a dip in the titration curves at high pH. It is noted that the position of the precipitation plateaus are also a function of temperature. A decrease in temperature leads to a shift of the titration curves to higher pH values. As a consequence, the Zn content of the mixed solution will not be fully precipitated if the pH is too low at a given temperature, but also if the temperature is too low at a given pH confirming that the proper selection of pH and temperature is crucial to guarantee the rapid and complete solidification of all components. Thus, pH 6–7 can only be regarded as optimal within a certain temperature window of 333–343 K.21,27,30

Two features of the complex dependency of catalyst performance on the synthesis parameters (Figure 3) can be easily understood based on the results of the titration experiments: The sharp breakdown from the maximum at pH 6.5 and T = 65 °C towards lower pH and lower temperature is due to a lack of Zn in the catalyst (and its stabilizing function on the microstructure, see above) as has been confirmed by elemental analysis by Baltes et al.25b

The Chemistry of Precipitate Ageing

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

Titration experiments have shown that no binary precipitate can be directly obtained, but that the constant-pH method needs to be used to form a pseudo-homogeneous precipitate with the best possible distribution of Cu- and Zn-domains. This initial precipitate is X-ray amorphous and can lower its free energy by crystallization during ageing in the mother liquor. Ageing critically affects the micro-structural and consequently the catalytic properties of the resulting Cu/ZnO catalyst.31 The crystallization is associated with a change in color from blue to bluish green32 and, as expected, a change in particle size and morphology.33 The phase composition of the ageing product is mostly determined by the Cu:Zn ratio25a,34 but also by the mode of precipitation,35 and the speed of addition of the precipitating agent.34 Typical phases obtained when going from Cu-rich to Zn-rich compositions are malachite Cu2(OH)2CO3, zincian malachite (Cu, Zn)2(OH)2CO3 (sometimes called rosasite, see Ref. 36), aurichalcite (Cu, Zn)5(OH)6(CO3)2, hydrozincite Zn5(OH)6(CO3)2, and mixtures thereof. At the industrially relevant Cu:Zn ratio, only zincian malachite and minor amounts of aurichalcite have been found. In ternary Cu, Zn, Al systems, also layered double hydroxides (LDHs) can be observed (see below).

The phase-formation occurs rather step-like than gradually and is accompanied by a transient minimum in pH as shown in Figure 5. The peak in temperature indicates the exothermicity of the crystallization process. Particle size is decreasing while the turbidity of the suspension is increasing. XRD investigations before and after the pH minimum confirm that the precipitate has transformed from the amorphous to the crystalline state.

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Figure 5. Evolution of pH, reactor temperature and turbidity during co-precipitation (constant pH 7, tageing < 0 min) and ageing (tageing > 0 min) at 338 K (adopted from Ref. 33).

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The chemistry of ageing has been studied with in-situ energy-dispersive XRD (EDXRD),30 a technique well suitable to directly observe the crystalline reaction intermediates of hydrothermal reactions.37 Decoupled precipitation and ageing experiments were used to study the influence of temperature and pH on the catalyst precursor crystallization (Figure 6).

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Figure 6. EDXRD patterns (converted to °2θ values of Cu-Kα radiation) during ageing of the amorphous Cu, Zn precursor at pH 7 and 323 K after 2 (a), 26 (b) and 98 min (c). At the bottom PDF 72-75 (dark grey bars) and PDF 1-457 (light grey bars) are shown as references for zincian malachite (Cu, Zn)2(CO3)(OH)2 and for sodium zinc carbonate Na2Zn3(CO3)4·3H2O, respectively. The position of the 201equation image peak of zincian malachite is shifted compared to the pure malachite reference because of zinc incorporation (see text).

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While the effect of increasing temperature was found to accelerate the crystallization at a given pH, the acidity had a crucial influence on the reaction mechanism. At a pH higher than 6.5 transient crystallization of a Na2Zn3(CO3)4·3H2O intermediate was observed before the desired (see below) zincian malachite precursor phase was formed. It was found that the occurrence of this transient phase had a negative effect on the Zn incorporation into the zincian malachite phase, which on a first sight can be easily explained with the intermediate segregation of the sodium zinc carbonate. Such segregation is undesired and contrasts the efforts of achieving a homogeneous distribution of Cu and Zn during co-precipitation. Further investigations with potassium containing artificial mother liquors however pointed rather at a direct negative effect of a basic pH on the phase formation independent of the transient presence of the sodium zinc carbonate phase.30

The EDXRD results clearly show how the exact setting of the ageing conditions for a co-precipitate of given composition can affect the phase formation and composition of the solid products. The negative effect of a too high pH on the zinc incorporation into zincian malachite offers an explanation for the fast decay of catalytic activity with an increase in precipitation pH into the neutral or even basic regime as shown in Figure 3. The effect of Cu, Zn-substitution in zincian malachite will be discussed in the next section in detail.

Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

Due to the miscibility ranges of the mixed phases, a comprehensive characterization of the precursor material is often difficult if a phase mixture is present. Zincian malachite was suggested to be the relevant precursor phase for industrial catalysts,25c which is also confirmed by the industrially applied Cu-rich composition near Cu:Zn = 70:30, falling into the regime, in which zincian malachite is the main product. This view is confirmed by a positive correlation of the Zn-content in zincian malachite and the SACu of the resulting catalyst.33 The zinc fraction can be estimated from the angular position of the characteristic 201equation image XRD peak of the zincian malchite phase near 32° in 2θ using Cu-Kα radiation.36 This particular lattice plane distance strongly contracts as Zn2+ is incorporated into zincian malachite and the corresponding peak is shifted to higher angles. This can be explained with the average lowering of Jahn-Teller distortions of the copper-containing MO6 building units in zincian malachite, whose elongated axes are aligned nearly perpendicular to this orientation (Figure 7a).33,35,36

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Figure 7. (a) Unit cell of malachite, slightly tilted view towards (501equation image). Only the Jahn-Teller elongated bonds of the CuO6 units are shown. They are oriented either perpendicular to (201equation image) (shown in dark grey) or to (211equation image) (shown in light grey). Arrows indicate the directions of the strongest unit cell contraction upon Cu-Zn substitution. Carbonate groups are omitted for clarity. (b) Changes in the powder XRD pattern of zincian malachite samples as a function of zinc content x for the six most intensive reflections (adopted from Ref. 36).

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It is thus possible to conveniently measure the Zn-content of this phase by conventional XRD despite the similar ionic radii and scattering factors of Cu and Zn. It is noted that due to compensating effects during the anisotropic contraction of the monoclinic unit cell (space group P21/a, No. 14) the other strong XRD peaks at lower angles, which are often employed for phase identification, are only hardly affected by Cu, Zn substitution and do not give much diagnostic insight (Figure 7b).36 The full pattern refinements results indicate linear dependencies of the individual lattice parameters on the composition as expected with a contraction of the a and c axes and an expanding b axis. The monoclinic angle β is decreasing with increasing zinc incorporation. Analysis of the distortion of the coordination environment of the two crystallographically independent metal position in the malachite crystal structure reveals that incoming zinc is preferably incorporation on the M2 site, which changes to a more regular coordination, whereas the M1 site remains almost unaffected at low substitution levels.36

The decrease of the (201equation image) lattice plane distance upon incorporation of zinc into malachite was measured for catalyst precursor phases obtained after ageing of precipitates with different nominal Cu:Zn ratio (Figure 7b).36 It was found that, under the conditions applied in this study, the limit of Zn incorporation is found near 28 %, i.e. at a composition of zincian malachite close to (Cu0.72Zn0.28)2(OH)2CO3. For higher nominal zinc content no further shift of the (201equation image) reflection and crystallization of the Zn-richer aurichalcite phase as a side-product were observed.

Thus, certain amounts of copper in malachite can be substituted by zinc, but a critical composition exists, where no more Zn can be incorporated into zincian malachite and formation of a phase mixture of zincian malachite and aurichalcite is energetically favorable. The reason is most probably the increasing concentration of regular MO6 octahedra, which destabilizes the aligned arrangement of the Jahn-Teller distortions present in malachite (Figure 7a). It is understandable from what was discussed in the previous sections that this critical value strongly depends on the exact setting of the synthesis conditions like pH, temperature, and ageing time, but also on the general mode of co-precipitation. For instance, if the decreasing pH technique is used at otherwise the same conditions, the critical composition cannot exceed 11 %.35 It is thus important for that catalyst synthesis to know not only the nominal Cu:Zn ratio, but in particular the effective Cu:Zn ratio of the zincian malachite precursor phase, which will differ at unfavorable synthesis conditions due to leaching or formation of Zn-rich by-phases.

The reason, why the zinc incorporation into malachite is critical for the success of the catalyst synthesis can be seen in Figure 8: The largest SACu of the resulting catalyst is observed for materials prepared from precursors near the critical composition. This observation strongly suggests that the desired porous microstructure of Cu/ZnO catalysts (Figure 1) is formed from highly substituted zincian malachite precursors, and that the applied Cu:Zn ration near 70:30 is beneficial, because it lies near the incorporation limit of Zn into the malachite phase.

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Figure 8. Lattice contraction in the direction perpendicular to the (201equation image) planes of zincian malachite (a measure of zinc incorporation) in the precursor phase and SACu of the final Cu/ZnO catalyst as a function Cu content (Phase composition: M: malachite; zM: Zincian malachite; A: Aurichalcite); adopted from Ref. 33.

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This insight leads to a simple geometrical model for the compositional optimization of industrial Cu/ZnO catalysts comprising subsequent meso- and nano-structuring of the material33 (Figure 9). In a first microstructure directing step (meso-structuring) the homogeneous Cu, Zn precipitate obtained by constant-pH co-precipitation crystallizes as thin needles of zincian malachite. Thin and interwoven needles are desired, because the porosity of the final catalyst is already pre-determined at this step. In a second step, the individual needles are decomposed into CuO and ZnO and pseudo-morphs of the precursor needles can be still observed after mild calcinations.25b Because both oxides are only poorly miscible, a de-mixing cannot be avoided at this stage and nanoparticles of CuO and ZnO are formed. The effectiveness of this nanostructuring step depends critically on the Zn-content of the precursor. The closer a hypothetical 1:1 ratio of Cu2+ and Zn2+ in the zincian malachite phase is achieved, the smaller the newly formed oxide particles will be and the higher is the dispersion of the Cu phase.33 A 1:1 ratio of Cu and Zn in synthetic zincian malachite, however, seems to be inaccessible by conventional co-precipitation and ageing and due to solid state chemical constrains the limit is near 70:30.36 If we leave aside the synergetic effects of Cu and ZnO in the elementary steps of the catalytic reaction sequence, the general benefit of using Zn2+ for the preparation of highly dispersed Cu-based catalysts is a simple geometric effect due to the chemical similarity of Cu2+ and Zn2+ concerning cation charge and size in the precursor state. This similarity enables a common solid-state chemistry of Cu2+ and Zn2+ in a single mixed precursor phase. Thus, highly intermixed precursors can be prepared in the Cu-Zn system, leading to highly dispersed Cu and ZnO particles after de-mixing upon thermal decomposition and reduction.

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Figure 9. Simplified preparation scheme of Cu/ZnO catalyst and electron microscopy images of the different stages of preparation. The co-precipitate (a) crystallized upon ageing in the mother liquor yielding zincian malachite (zM, b). Under optimized conditions zM is obtained in form of thin needles with large inter-particle pores. During calcination the individual needles of zM undergoe thermal decomposition yielding an intimate mixture of CuO and ZnO (c). Inter-dispersion of these two phases is higher the more zinc was incorporated into zM to dilute the Cu ions. Finally, the CuO component is reduced in hydrogen yielding nanoparticulate Cu/ZnO with a unique microstructure exhibiting high porosity and high Cu dispersion (adopted from Ref. 38).

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The insights described above allow for deduction of a guide for a rational predictive optimization of the industrial catalyst synthesis: Following the synthesis concept shown in Figure 9, conditions of the precursor synthesis need to be found that allow a more efficient meso-structuring by formation of thinner zincian malachite needles and a more efficient nano-structuring by shifting the effective Cu:Zn ratio of zincian malachite beyond the critical limit of 30 %.

Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

In this section two examples are discussed, in which the understanding of the applied synthesis as explained above has contributed to new insights that are directly relevant for the industrial methanol synthesis catalyst. The first example concerns the aforementioned Cu-Zn synergy. One part of this phenomenon was already explained in the previous section: The similar chemistry of Cu2+ and Zn2+ at the precursor stage enables exploitation of the mixed lattices in the precursor compound and results in an exceptionally high Cu dispersion after precursor decomposition. However, this function can in principle also be fulfilled by other elements. Substitution of Zn2+ by Mg2+ in the catalyst synthesis (with adjustment of the co-precipitation pH to 9) resulted indeed in a magnesium-substituted malachite with a clear shift of the 201equation image peak indicative of successful non-Jahn-Teller ion incorporation at a Cu:Mg ratio of 80:2020c (Figure 10a). The Mg-malachite precursor was decomposed to a Cu/MgO catalyst with a microstructure that was similar to that shown in Figure 1 with the only difference that the spacer particles between the Cu nanoparticles were MgO and not ZnO. The SACu of this catalyst was higher than a Cu/ZnO catalyst of the same copper content. Thus, the geometrical part of the Cu-ZnO synergy is not unique to ZnO and other refractory oxides can also be used for the structural function of forming and stabilizing a porous composite microstructure. Interestingly, the unique role of ZnO was strikingly reflected in the catalytic properties as the Cu/MgO unlike Cu/ZnO and despite the high SACu was unable to synthesize methanol from CO2/CO/H2 at high rates (Figure 10b). This inability is likely related to a lack of strong metal support interaction and was easily overcome by addition of a small amount of ZnO by subsequent impregnation, which switched on the CO2 hydrogenation activity of the resulting highly disperse Cu/MgO/ZnO catalyst.

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Figure 10. (a) XRD patterns of the substituted malachite precursors obtained from co-precipitation of Cu, Zn (CZ, light grey) and Cu, Mg (CM, dark grey) with 80 % Cu. The reference pattern is malachite (black bar graph; PDF 72-75). (b) Methanol synthesis activity (gray) of the catalysts prepared from these precursors. The dashed column is the CO formation activity. The catalyst CMZ was prepared by addition of 5 wt-% ZnO to CM by impregnation (adopted from Ref. 20c).

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With this experiment it was shown that only a small amount of ZnO is needed on an already highly dispersed copper catalyst to generate the synergetic effect necessary for methanol synthesis. This view is in agreement with an involvement of small amounts of Zn species in the Cu-based active site for the methanol synthesis reaction.19a The fact that the CO formation rates are affected to a much lesser extent (Figure 10b) suggests that the reverse WGS occurs on different sites, which are not much affected by the presence or absence of zinc.

The second example addresses the role of the Al2O3 promoter that is used in industrial formulations to increase catalyst lifetime and activity. Using the methodology described above for ternary Cu, Zn, Al mixed solutions (Cu:Zn = 70:30) revealed a very strong contraction of d(201) in a narrow window of Al concentrations around 3–4 %.38 The resulting catalysts were much more active than the binary Cu/ZnO catalysts or those with higher promoter content. The presence of the promoter affected the phase formation of the precursor during ageing in a beneficial way that leads to an increase in SACu as a result of a structural promotion effect. An additional electronic promotion effect was proposed, which was related to the formation of doped ZnO:Al particles upon calcination. Interestingly, among the differently promoted samples containing various amounts of Al, Ga, or Cr a simple linear correlation was observed between d(201) of the precursor and the absolute catalytic activity of the resulting catalysts (Figure 11).

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Figure 11. Linear correlation of the catalytic performance of the final catalyst and the d-spacing of the 201equation image planes in the zincian malachite precursor for the promoter series with Al, Ga, and Cr (from Ref. 38).

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Alternative Precursor Materials

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

The concept that the solid state chemistry of the zincian malachite precursor determines the success of the catalyst synthesis and the catalytic properties of the resulting material (Figure 9) can be used to explore other mixed Cu, Zn precursor compounds. For example, the principles shown in Figure 9 were transferred to a mixed Cu, Zn basic formate system, (Cu1–xZnx)2(OH)3HCO2.39 After determining the right co-precipitation conditions by means of titration experiments, a series of solid solution with varying x was prepared and structurally characterized. The system showed interesting parallels with the zincian malachite route. Similar to the malachite case, an anisotropic change of the unit cell was observed as a function of Zn content. Also a critical composition was found and the (Cu1–xZnx)2(OH)3HCO2 target phase started to disappear for x > 0.21. Even the crystal morphology was similar and also needle-like particles have been obtained. Figure 12 shows how the phase-pure precursor needles with a maximal zinc incorporation, (Cu0.78Zn0.22)2(OH)3HCO2, can be thermally decomposed to a porous oxide pre-catalyst and finally to a nanostructured Cu/ZnO composite. The generic nature of the above described concept for catalyst synthesis was finally confirmed by the higher catalytic activity compared to a zincian malachite derived catalyst of the same composition, while the promoted and optimized industrial catalyst was still substantially better.39

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Figure 12. SEM micrographs of the (Cu1–xZnx)2(OH)3HCO2 needles (Cu:Zn = 78:22) before (a) and after (b) calcination in O2 at 200 °C.

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Another alternative class of precursor compounds that has attracted considerable attention, is that of layered double hydroxides (LDHs) or hydrotalcite-like compounds. These hydroxy-carbonate precursors can be derived from the naturally occurring Mg-Al salt hydrotalcite, Mg1–xAlx(OH)2(CO3)2/x·mH2O (0.25 < x < 0.40). Mg and Al form layers of edge-sharing (Mg, Al)(OH)6 octahedra. The hydrogen atoms point to the interlayer space, where also the carbonate anions are located. Carbonate ions (or other anions) are needed in the structure to compensate for the extra positive charge introduced by the trivalent Al3+ ions. LDHs are well-established precursor compounds for synthesis of various catalysts.40 They are especially interesting due to their ability to isomorphously substitute Mg2+ as well as Al3+ by other bi- or trivalent cations, in particular those from the first row of transition metals. Thus, they are very attractive precursors for Cu/ZnO/Al2O3 catalysts as they can provide a perfect atomic distribution of all metal species in one single-phase precursor compound and should yield structurally uniform catalysts of high Cu dispersion and enhanced interaction between Cu metal and the Zn, Al oxide phase.

LDH is often observed as a by-phase in the synthesis of ternary Al2O3-promoted Cu/ZnO catalyst. Cu-rich phase-pure LDH precursors (up to 49 at-% copper) can be prepared by a modified direct co-precipitation and yield Cu/ZnAl2O4-type catalysts.41 The major differences of the syntheses are: (i) the higher Al content of 30–40 % to obtain phase pure LDH compared to typically 10–20 %, (ii) the use of a mixture of NaOH and Na2CO3 as precipitating agent instead of pure Na2CO3 to avoid formation of carbonate-richer phases (like malachite), (iii) an increase of the precipitation pH from ca. 7 to 8 to favor formation of the hydroxide-rich LDH phase, (iv) a lower reaction temperature to avoid oxolation of copper hydroxide species to CuO at this higher pH.

The resulting material exhibits a different microstructure than that of the industrial Cu/ZnO/Al2O3 catalyst shown in Figure 1.41 Despite a smaller average Cu particle size observed in the ex-LDH material, which is a result of the perfect cation distribution in the precursor at a lower total Cu content, the accessible Cu surface area is considerable lower, around 5 m2·g–1. This is a result of the much stronger embedment of the small metal particles in the ZnAl2O4 matrix. After calcination at 603 K and reduction, the interface-to-surface ratio was determined to be 89 % compared to ca. 35 % for the industrial system. The major challenge in the preparation of such ex-LDH Cu/ZnAl2O4 catalyst can thus be seen to optimize the “nuts-in-chocolate”-like morphology by adjusting the LDH particle size, e.g. the precursor platelet thickness, to affect the degree of embedment in order to find the proper compromise between Cu metal-oxide interactions and Cu dispersion. A microemulsion approach for the precipitation of the precursor to “nano-cast” the platelet morphology of the LDH phase has been shown to increase SACu by 75 % compared to a conventionally co-precipitated ex-LDH Cu/ZnAl2O4 catalyst (Cu:Zn:Al = 50:17:33).42 Tang et al.43 recently reported a very high SACu of 39 m2·g–1 for a LDH-derived Cu/ZnO/Al2O3 catalyst (Cu:Zn:Al = 37:15:48) after calcination of the precursor at elevated temperature of 873 K associated with the crystallization of spinel-type oxides. Ex-LDH Cu/ZnO/Al2O3 catalysts have been reported for MSR44 as well as for CO2 hydrogenation.45 Also the synthesis of mixed Zn1–xCuxM2O4 (M = Al, Ga) spinels by direct co-precipitation46 or subsequent microwave-assisted hydrothermal synthesis47 has been shown to lead to active catalysts.

Calcination and Activation

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

No matter, which mixed Cu, Zn precursor was used to prepare a Cu/ZnO catalyst, the actual nanostructuring, i.e. the step from a solid solution to a segregated composite should happen during the thermal treatment of the precursor and not at the stage of the co-precipitated precursor. Usually it is the calcination, which leads to decomposition of the anionic sub-lattice and de-mixing of the cationic sub-lattice. In case of the mixed spinel precursor, the nanostructuring will happen during reduction. The previous sections have emphasized the role of the precursor preparation and explained why it is absolutely critical in case of the industrial catalyst synthesis. However, also the calcination and activation procedure are important and will have an effect on the finally resulting catalyst. Still, while is it possible to destroy a well-prepared precursor by wrong thermal treatment, it will not be possible to turn a bad precursor into a good catalyst only by proper setting of the calcination and activation conditions. A suitable tool to study these last synthesis steps is thermal analysis.48 There are many research reports on the thermal activation of CuO/ZnO that combine temperature-programmed methods with structural analytics and fewer on the thermal decomposition during calcination. This research report is focused on the chemistry of the co-precipitation and ageing steps and a detailed review of the thermal treatment of the catalyst precursors will be given in a forthcoming paper.

Summary

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

Many secrets of the industrial synthesis of Cu/ZnO-based catalysts for methanol synthesis were revealed by studying the solid-state chemistry of the precursor compounds. The structural effects and limitations of the Cu-Zn substitution in zincian malachite are directly relevant for the catalytic properties of the finally resulting nano-structured composite material. This is because the morphology and in particular the effective Cu:Zn ratio of this precursor phase determines the accessible SACu in the final catalyst according to a simple geometric model. The target of the early preparation steps is to maximize the Zn incorporation into the malachite phase and all synthesis parameters have been (partly in an unintended manner) optimized to incorporate all Zn in zincian malachite. The target phase is zincian malachite with an effective cation composition equal to the nominal Cu:Zn ratio of the co-precipitation. The d-spacing of the (201equation image) lattice planes is thus a good lead for the catalyst synthesis. From these considerations, it is obvious that the setting of synthesis parameters including the nominal Cu:Zn ratio is directly related to the zincian malachite precursor phase and not general to all types of Cu/ZnO catalysts. In turn, if other precursor phases are employed, all parameters need to be newly adjusted. The analytics of the industrial synthesis recipe provides a guide to a general scheme, which should be applicable also to other bulk catalysts than Cu/ZnO, and which is briefly recapitulated in the following.

To prepare a nanostructured bulk metal/oxide catalyst by co-precipitation, it is recommended to start with precipitation titration experiments of the single species and mixed solutions. These will give information on the suitable pH of the experiment and also reveal if a direct true co-precipitation into a multinary compound is possible. If this is not the case, an ageing period might be required to initiate crystallization. The target of the co-precipitation and ageing procedures should be to synthesize a mixed cationic lattice of all components of the catalyst in a well-defined precursor compound with thermo-labile anions like hydroxide, carbonate, formate, oxalate, formate, acetate, etc. The crystalline nature of the precursor will allow distributing the different species in an atomic manner within a solid solution. Other advantages of preparing catalysts from phase-pure and crystalline precursors are an improved synthetic reproducibility and control thereof due to the well-defined intermediate and an expected uniform microstructure of the resulting catalyst being suitable for the establishment of structure-function relationships. At this point, the synthesis should be further guided by the solid-state chemistry of this solid solution. The aim is to find a composition allowing for optimized dilution of the active species within a refractory matrix species. As a rule of thumb a 1:1 ratio should be approached to achieve high inter-dispersion upon de-mixing. This ratio – like in the case of zincian malachite – may actually not be compatible with the miscibility range of the target precursor phase. An additional parameter to consider is the mesostructure of the precursor compound. Thin needles or platelets are very suitable to generate porous aggregates with large accessible specific surface areas. Finally, the de-mixing or nanostructuring step occurs by segregation either due to immiscibility of the oxide formed upon calcinations or due to selective reduction of the active metal. All thermal processing steps should be conducted at the lowest possible temperatures in order to preserve large interfaces between the final phase and its matrix. To this end a careful optimization of the process conditions and redox potentials, in which the calcinations and reduction is strongly recommended.

Outlook

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

Improved methanol synthesis catalysts are urgently needed not only to make the current industrial process more economic, but in particular to enable a large-scale conversion of CO2 into sustainable fuels or chemicals.49 At this time Cu/ZnO catalysts are by far the most powerful materials for CO2 conversion to methanol and among them the technical catalysts are superior. The deciphering of their empirically found synthesis recipe is a first step that hopefully paves the way for a more rational optimization of the potential of Cu/ZnO catalysts. New modifications of the zincian malachite route, e.g. attempts to increase the Zn incorporation by better selection of the synthesis conditions, will be helpful in this respect. Also alternative precursor systems are very promising. The future work on these new systems can draw inspiration from the increasing understanding of the conventional system. Last but not least, the conceptual approach taken to from reconstruction the empirical recipe can also be transferred to other catalyst systems, where nanostructured composite materials are desired.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

This research report has summarized recent progress that was made in the field of Cu/ZnO catalyst synthesis with a focus on the well-documented industrial synthesis protocol and on the early stages of catalyst synthesis. Zincian malachite, (Cu, Zn)2(OH)2CO3, was identified as the relevant precursor phase for high-performance catalysts and its solid state chemistry as the key to understand the established synthesis conditions. Based on structural analysis, it was shown that the industrial synthesis recipe was optimized to maximize the Zn incorporation into zincian malachite. From this insight a simple and generic geometrical concept for the synthesis of nanostructured composite catalysts was derived, which is based on de-mixing of solid solution. This concept can lead to new interesting catalytic materials, which help to address fundamental questions of this catalyst system.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information

We thank all Department Members of the Inorganic Chemistry at FHI for supporting this work in the last years. The work of the Nanostructure Group members is especially appreciated. Martin Muhler and Olaf Hinrichsen and their groups are acknowledged for fruitful discussions and collaboration. Süd-Chemie AG, now Clariant Produkte GmbH, accompanied our research as industrial partner. Financial support was given by the BMBF (FKZ 01RI0529), the BayStaMinWi (NW-0810–0002) and the DFG (BE 4767/1–1). Parts of this work have been previously published.50

Biographical Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information
Thumbnail image of

Malte Behrens studied Chemistry at the Christian-Albrechts-Universität in Kiel and worked in the group of Wolfgang Bensch on the reactivity of transition metal chalcogenides. He received his Ph.D. in Chemistry in 2006. He then joined the Department of Inorganic Chemistry at the Fritz-Haber-Institut der Max-Planck-Gesellschaft in Berlin and did his Habilitation with Robert Schlögl on the materials chemistry of Cu/ZnO catalysts in 2013 at the Technische Universität Berlin. His research interest is the development of nanochemically-optimized catalytic materials for energy storage applications.

Biographical Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Industrial Methanol Synthesis and Other Reactions Catalyzed by Cu/ZnO
  5. The Cu/ZnO Catalyst
  6. Synthesis of the Industrial Catalyst or the “Chemical Memory”
  7. The Role of the Precursor Compound
  8. “Co”-Precipitation
  9. The Chemistry of Precipitate Ageing
  10. Substitution Chemistry of Malachite or the Effect of the Cu:Zn Ratio
  11. Insights from Precursor Chemistry into the Functionality of the Industrial Catalyst
  12. Alternative Precursor Materials
  13. Calcination and Activation
  14. Summary
  15. Outlook
  16. Conclusions
  17. Acknowledgements
  18. Biographical Information
  19. Biographical Information
Thumbnail image of

Robert Schlögl received his Ph.D. in Chemistry at Ludwig Maximilians University in Munich in 1982. He did his Habilitation with Gerhardt Ertl at the Fritz-Haber-Institut der Max-Planck-Gesellschaft (FHI) in Berlin and went to Frankfurt University for a Full Professorship. Since 1994 he is back in Berlin as the Director of the Department of Inorganic Chemistry at the FHI and since 2011 he is also the Founding Director of the Max Planck Institute for Chemical Energy Conversion in Mülheim. Robert Schlögl's research focuses primarily on the investigation of heterogeneous catalysts, with the aim to combine scientific and technical applicability.