Catalyst

INORGANIC AND BIO-INORGANIC CHEMISTRY – Vol.II - Homogeneous and Heterogeneous Catalysis - Erica Farnetti, Roberta Di Monte and Jan Kašpar HOMOGENEOUS AND HETEROGENEOUS CATALYSIS Erica Farnetti, Roberta Di Monte and Jan Kašpar University of Trieste, Trieste, Italy Keywords: Homogeneous, Heterogeneous catalysis, Hydrogenation, Hydroformylation, Carbonylation, Oxidation, Metathesis, Homo and copolymerization, Oligomerization, Acid-base catalysts, Redox catalysts, Polyfunctional catalysts Contents U SA N M ES PL C E O– C E H O AP L TE SS R S 1. Introduction 1.1. Catalyzed vs Non-Catalyzed Reaction 1.2. Homogeneous vs. Heterogenized and Heterogeneous Catalysts 1.3. Catalysis for Sustainability of Chemical Processes 2. Homogeneous Catalysis 2.1. General Concepts 2.2. Current Applications 3. Heterogeneous Catalysis 3.1. Synthesis Methodologies and Catalyst Formulation 3.2. Metal Support Interactions 3.3. Acid-base Catalysts 3.4. Redox Catalysts 3.5. Shape Selectivity 3.6. Polyfunctional Catalysts 4. Heterogenized Catalysts Acknowledgements Glossary Bibliography Biographical Sketches 1. Introduction The term “catalysis” was first employed by Berzelius in 1836 to identify a new entity capable of promoting the occurrence of a chemical reaction by a “catalytic contact”. In his view, the catalyst was seen as something that is added to the reaction to speed up the rate of the reaction (catalytic force) without being consumed or produced in the process. It is important to recognize that the catalysis can be traced back to ancient terms if we consider for example the fermentation processes, which are examples of biocatalysis. As for the industrial catalytic processes, catalytic production of sulfuric acid, i.e. lead chamber process, where oxidation of sulfur dioxide ( SO 2 ) to sulfur trioxide ( SO3 ) was performed in the presence of a mixture of nitric oxides ( NO/NO 2 ) as catalyst, dates back to eighteenth century. However, catalysis started to play a major impact on the chemical industry starting from the beginning of the twentieth century, nowadays more than 95% of chemicals being produced via a process that includes at least one catalytic step. The volcano-like shape reported in Figure (data from ref. (Kieboom et al. 1999)), which illustrates the introduction of major industrial catalytic processes as a function of ©Encyclopedia of Life Support Systems (EOLSS) D represent respectively the reagents and the products and the ©Encyclopedia of Life Support Systems (EOLSS) . The line is only an eye-guide.INORGANIC AND BIO-INORGANIC CHEMISTRY – Vol. an overview of the catalytic technology that has been introduced on the US market in the 1990s.1.Erica Farnetti. Figure 1: Summary of the historical development of the major industrial catalytic processes number of major commercialized industrial processes per decade (in the 20th century) ). world catalysts sales accounted for some $ 7. catalysis nowadays plays a key role in the production of chemicals and materials. reported over 130 examples of new catalysts or catalyst improvements for operating processes (both in some advanced stage of scale up or commercialized) (Armor 2001). II . B and C . could suggest that catalysis could be considered to be a mature technology. indicating the vital role of the catalytic technology to many industrial processes. However. Catalyzed vs Non-Catalyzed Reaction From a chemical point of view.4 billion in 1997. Consistently. To explain these concepts. After (Kieboom et al. In summary. 1999) 1. Roberta Di Monte and Jan Kašpar U SA N M ES PL C E O– C E H O AP L TE SS R S time. the concept of catalyst is intimately linked to the mechanism of the chemical reaction. where major developments have already been achieved.Homogeneous and Heterogeneous Catalysis . let us consider a general chemical reaction which is described by the following equation: r1 aA + bB ⎯ ⎯→ cC + dD (1) where A . The catalyst is capable of accelerating the reaction rate or to change the selectivity of the reaction towards different products with respect to the situation when the reaction occurs in the absence of the catalyst. molecules must overcome the energy barrier represented by the activation energy. Clearly. ( k1 (mol L) − m−n+1s −1 ) is the kinetic constant for the reaction (1). the activation energy for the reaction considered is defined as the energy barrier which must be overcome by the system so that products are formed in the reaction. For such a reaction we can define the reaction rate as: 1 d [ A] 1 d [ B] 1 d [C ] 1 d [ D] r=− × =− × = × = × a dt b dt c dt d dt U SA N M ES PL C E O– C E H O AP L TE SS R S where the use of square brackets is conventionally employed to indicate the concentration of the different species in mol L-1. and c. b.32 J mol-1 K-1). as dictated by Eq.Erica Farnetti. when we consider a hypothetical reaction pathway leading from reagents to products. moving from the reagents to products.. Eq. is schematically illustrated in Figure (dotted line). T reaction temperature ( K ) and ΔE a is the activation energy (J mol-1) for the reaction. resulting in higher reaction rates under comparable reaction conditions. (4). II . As a matter of fact. Notice that in this simple description we do not consider the contribution of the reverse reaction to the overall reaction rate. d . we will consider the reaction rate for a process occurring in a single homogeneous phase. For the sake of simplicity. featuring lower activation energy compared to non-catalyzed system.Homogeneous and Heterogeneous Catalysis . The role of the catalyst is therefore to modify this energy profiles so that alternative reaction pathways. in order to transform the reagents into products. It is important to realize that the presence of a catalyst can modify also the product distribution as illustrated in Figure for a hypothetical catalyst offering a new reaction pathway leading to products B. i.INORGANIC AND BIO-INORGANIC CHEMISTRY – Vol. which is measured experimentally and represents the relationship between the reaction rate and the reactant concentration: r1 = k1 [ A] m [ B] n (2) where m and n represent the reaction order with respect to reagents A and B . (2) to obtain the net reaction rate. (3) must obviously be subtracted from Eq. R the universal gas constant (8. Roberta Di Monte and Jan Kašpar terms a.e. represent the stoichiometric coefficients of the reaction. Such products may not be reachable in the ©Encyclopedia of Life Support Systems (EOLSS) . This case can be described by a kinetic law. can be offered. The dependency of the reaction rate on the temperature is described by the Arrhenius law: k1 = A × e − ΔE a RT (4) where A is the geometric factor. The energy profile for the reaction as a function of the reaction coordinate. Under conditions where the contribution of the reverse process to the overall reaction rate cannot be neglected. This contribution would be of the type: r−1 = k −1 [C ] p [ D] q (3) where the sign minus is conventionally used to indicate the reverse process. when carried out under kinetic control. i. This distinction is linked to the fact that the catalyst operates respectively in the same phase where the reaction occurs (homogeneous catalysts) or in a different phase (heterogeneous or heterogenized ©Encyclopedia of Life Support Systems (EOLSS) .2. Roberta Di Monte and Jan Kašpar U SA N M ES PL C E O– C E H O AP L TE SS R S absence of the catalyst due to high activation energy for their formation. In summary. the reaction is stopped before the equilibrium conditions are attained. Homogeneous vs. catalytic reactions occur under milder conditions and. the product distribution at equilibrium. solid thick line: catalyzed reaction leading to the same products. Heterogenized and Heterogeneous Catalysts Traditionally catalysts were distinguished into homogeneous and heterogeneous. As a result of this.Homogeneous and Heterogeneous Catalysis . solid thin line: catalyzed reaction leading to different products (see text for details).e. Figure 2: Energy diagram for a generic reaction and the effects of a catalyst on the reaction profile: dotted line: uncatalyzed reaction. i. The differences in the energies indicated by the arrows represent the activation energy barrier ( ΔEa ) for the forward reaction and enthalpy of the reaction ( ΔH ) . heterogenized catalysts were also introduced. the presence of the catalyst affects the kinetic aspects of a reaction by lowering the activation energy. with higher or different selectivity compared to non catalytic processes.Erica Farnetti. without affecting its thermodynamics.e. 1. However. II .INORGANIC AND BIO-INORGANIC CHEMISTRY – Vol. subsequently. it should be observed that high yields of products B can be obtained in the presence of the catalyst only when the reaction is run under kinetic control. artificial enzymes. not only the nature of the catalyst. organometallic complexes. in fact. which occurs in the gaseous phase. U SA N M ES PL C E O– C E H O AP L TE SS R S A great variety of homogeneous catalysts are known. most of the processes using homogeneous catalysts occur in a liquid phase whereas for the heterogeneous catalysts. metal particle. metals ions. organic molecules up to biocatalysts (enzymes. chemical composition. Property Catalyst recovery Thermal stability Selectivity Homogeneous difficult and expensive poor excellent/good – single active site Heterogeneous easy and cheap good good/poor . This example highlights the paramount importance of strict control of the preparation of the heterogeneous catalysts in order to ensure reproducible and well-defined systems. homogeneous catalysts are summarized in Table 1. II . a common feature of the heterogeneous catalysts. As exemplified in Figure 3 for a cubooctahedron f. as a matter of fact the fist industrial catalyzed reaction (1750) was the oxidation of SO 2 to SO3 using NO as a homogeneous catalyst. metal complexes. etc). On the other hand. and the reaction occurs either in the liquid or gaseous phase. This makes homogeneous catalysts intrinsically more active and selective compared to traditional heterogeneous catalysts such as oxides or supported metal particles. pioneered by Haruta and co-workers (Haruta 1997. which will be treated in Section 2.c. every single catalytic entity can act as a single active site.c. ©Encyclopedia of Life Support Systems (EOLSS) . Haruta & Date 2001). As a general picture. i. whereas gold particles with an optimum size of about 2-3 nm showed extremely high catalytic activity and were able to fully oxidize CO even at -100°C. However.Erica Farnetti. there is no limitation on the phase to be considered. the main difference is the fact that in the case of homogeneous catalysts.Homogeneous and Heterogeneous Catalysis . but also the dimension and the form of the particle plays a role in determining the relative number and type of exposed surface atoms (Anderson 1975). plays a role in defining the properties of the heterogeneous catalysts.e.INORGANIC AND BIO-INORGANIC CHEMISTRY – Vol.multiple active sites Table 1: Comparison of main advantages/disadvantages of homogeneous vs. The major advantages/disadvantages of heterogeneous vs. The heterogeneity of the surface sites is. The fact that the catalysts is in a distinct phase with respect to the reaction medium. nowadays the term homogeneous catalyst is often referred to organometallic or coordination complexes. the catalyst is usually in a solid form. In principle. ranging from Brønsted and Lewis acids widely used in organic synthesis. is a nice example of such importance: gold particles larger than 5 nm were essentially catalytically inactive for CO oxidation. Catalytic oxidation of carbon monoxide at low temperatures over gold-containing catalysts. accounts for the major advantage of the heterogeneous catalysts over the homogenous as it makes the separation e reutilization of heterogeneous catalysts simple and cheap compared to the homogenous catalysts. heterogeneous catalysts. Roberta Di Monte and Jan Kašpar catalysts). c. Notice the cubo-octahedron-like form of the Rhodium particle. Roberta Di Monte and Jan Kašpar U SA N M ES PL C E O– C E H O AP L TE SS R S Figure 3: High resolution transmission electron micrograph and digital diffraction patterns of a Rhodium metallic particles deposited on a Ce0.Homogeneous and Heterogeneous Catalysis .Erica Farnetti. relative particle dimension defined as ratio of a radius of a sphere containing N atoms to the radius of the volume occupied by an atom in the unit cell.32O 2 mixed oxide as support. Rhodium crystallizes in the f. After (Anderson 1975).68 Zr0. II . Figure 4: Relative ratio ( Fi ) of surface atoms with different CNs and dispersion ( Ds ) vs.INORGANIC AND BIO-INORGANIC CHEMISTRY – Vol.c. structure and has a CN=12 for bulk metal atoms. homogenous catalysts. which exhibits different surfaces (indicated) and Rhodium atomic sites with different coordination number (CN) as evidenced in the right part. In contrast to this surface heterogeneity. due to their molecular ©Encyclopedia of Life Support Systems (EOLSS) . diffusion within the pellet. 7) external diffusion.e.1). e. Several routes to anchor or encapsulate the homogenous. however. 5) desorption of products. diffusion towards the catalyst body. 6) internal diffusion. The recognition of these inherent limitations of the homogeneous catalyst led to a flourishing activity attempting to heterogenize homogeneous catalysts. below discussed. However.1: Hydroformylation of 1-octene to linear and branched aldehyde. The aim is that of combining the advantages of the homogenous catalyst (activity and selectivity) with the facility of the heterogeneous systems to recycle the catalyst. i. Cobalt carbonyl complexes are therefore used industrially. typically featuring a single catalytic site. II . typically 1-octene as depicted in (Scheme 1. elimination of the precipitating counter-ion. e. one must always take into account that this process consists of different steps which.g. the rhodium phosphine complex decomposes due to the higher distillation temperature in this case. there is no internal diffusion. are typically needed in order to re-utilize homogeneous catalysts. are well defined systems.Erica Farnetti. U SA N M ES PL C E O– C E H O AP L TE SS R S A major drawback of the homogeneous catalysts is the difficulty of their recovery from the reaction medium.INORGANIC AND BIO-INORGANIC CHEMISTRY – Vol.g. were suggested. The important aspect is that four of the seven steps are concerned with the diffusion and hence with the physical form of the catalyst bed within the reactor. 4) surface reaction. 2) internal diffusion. Such operation may often deactivate the catalyst. may include: 1) external diffusion. despite the fact that rhodium complexes operate under much milder reaction conditions and achieve higher selectivity to the desired linear aldehyde compared to the cobalt catalyst (Cole-Hamilton 2003). ©Encyclopedia of Life Support Systems (EOLSS) .e.e.Homogeneous and Heterogeneous Catalysis . 3) adsorption of reagents. pellet. Roberta Di Monte and Jan Kašpar nature. used as starting materials for plasticizers. linear H C +CO/H2 O O CH branched Scheme1. diffusion towards the exterior of the pellet. homogeneous Rhodium phosphine catalysts are used on a scale of 4 − 5 × 1012 g/year to catalyze hydroformylation of propene to volatile butanal where the product is continuously distilled off the reaction vessel. when the hydroformylation of long chain alkenes. i. Precipitation with subsequent recovery. When a catalytic reaction is considered. which is an energy intensive process. is attempted. it must be underlined that a major drawback of the heterogenized catalyst is the leaching of the catalyst during its use and recycle. Since the homogeneous catalysts are molecularly dispersed within the reaction medium. soaps and detergents. in the case of heterogeneous catalytic reaction. For example. or distillation of the reaction products. i. leading to deactivation. A (1997). Lefebvre. Some recent results on metal/support interaction effects in NM/CeO2 (NM : noble metal) catalysts. Anastas. (The Royal Society of Chemistry. F. A. (1999).. [A review paper on industrial processe catalyzed in homogeneous phase] Corker. D J (2003). Applied Catalysis A General. [A specific example of heterogenised catalyst reactivity] Corma. Roberta Di Monte and Jan Kašpar - - TO ACCESS ALL THE 37 PAGES OF THIS CHAPTER. [A review on inorganic acids and bases] Corma. Choplin. Science..M.M. (2001). 50. 51. 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Roberta Di Monte obtained her PhD in Materials Engineering in 1995 at the University of Trieste. Jan Kaspar is a full professor of Inorganic Chemistry at the University of Trieste. (WileyVCH. UK) in professor's C. (Elsevier Science Publishers B. Current interests include the transition metal speciation in the preparation of supported catalysts. leading automotive and catalyst industries. R.. Venanzi.. New solid acids and bases.G. Misono.J.. In the same year she went to Zűrich as a postdoc at ETH with professor L. P.M.Y. and Averill. Amsterdam) pp. 100. Leeuwen. van Santen. she went back to Trieste as a graduated student. revised and enlarged edition. W. working on the synthesis and characterization of homogeneous reduction catalysts. Strong metal-support interactions. 170.. J.P.H.A. Stirling's group.D.INORGANIC AND BIO-INORGANIC CHEMISTRY – Vol. Recent fields of research regard the oligo and polymerization reactions of alkynes homogeneously catalyzed by late transition metals.V. (1978).. Moulijn.. C. where her research topic was concerned with the synthesis and reactivity of heterometallic multinuclear hydrides. in co-operation with leading industries such as Magneti Marelli (FIAT Holding.. and on the other hand the study of organometallic compounds by means of electronic photoemission techniques using synchrotron radiation. II . Weinheim.. Group 8 noble metals supported on titanium dioxide. S. and Averill..W. [Reference textbook on heterogeneous catalysis] van Santen. He is a co-author of over 115 publications and five patents in the fields of homogeneous. Italy) and MEL Chemicals (UK).B.. (1989). which was awarded the Bracco-Salata price.A... A. A.1-574.M.. [Reference textbook on Catalysis] U SA N M ES PL C E O– C E H O AP L TE SS R S Bibliographic Sketches Erica Farnetti studied chemistry at the University of Trieste – her birthplace – where she got her degree in 1983 with a thesis in homogeneous catalysis. M. He is a member of the executive board of the Interuniversity Consortium Chemistry for the Environment . heterogenized and heterogeneous catalysis. He has been a member of the editorial board of Catalysis Today and Journal of Rare Earths.M.Erica Farnetti. L. and Thomas. more than 20 industrial reports and three patents. and Garten. and Hattori. (1999). After a 12month period at the University of North Wales (Bangor.V. Journal of the American Chemical Society. A.M. In 1990 she became assistant professor of inorganic chemistry at the University of Trieste where she is presently working. B. Roberta Di Monte and Jan Kašpar Tanabe.J. Her main research interests are focussed on the study of organo-transition metal compounds and of their catalytic properties. Leeuwen.. Principles and Practice of Heterogeneous Catalysis. (Elsevier Science B. she got her Ph. Currently.J. ©Encyclopedia of Life Support Systems (EOLSS) . J.1-365..K. [A reference text on solid acids and bases] Tauster. in Chemical Sciences at the university of Trieste in 1989. Second.M. [Tauster’s paper on SMSI] Thomas. N.J... Germany).N.. has primarily been focused on advanced catalytic materials based on nanostructured CeO2-ZrO2 mixed oxides as oxygen storage promoters for the automotive three-way catalysts. S. Moulijn. she is a researcher at the Department of Chemistry Sciences of the University of Trieste (Italy). Her research interests focus the synthesis and characterization of the nanostructured CeO2-ZrO2 and CeO2-ZrO2-Al2O3 materials. His research. Catalysis: An integrated approach. Amsterdam) pp. R. (1996).Homogeneous and Heterogeneous Catalysis .W. She is a co-author of over 35 papers. Ono.A.INCA which joins researchers from over 30 Italian Universities. which are widely used as critical ingredients of automotive catalytic converters.