Preparation of catalysts.Chapter 1: Methods of synthesis of

catalysts

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Universidad de Antioquia

University of Antioquia

Environmental catalysis Group

Present by: Manuel J Cano

Professors: PhD Aída L Villa & PhD Lina M Gonzalez

February 11th, 2015

Introduction. Brief history of catalysis

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1836

Jöns Jacob Berzelius a Swedish chemist coined the word “catalysis”. He wrote in Edinburg New Philosophical Journal:

“The substances that cause the decomposition of H2O2 do not achieve this goal by being incorporated into the new compounds (H2O and O2); in each case they remain unchanged and hence act by means of an inherent force whose nature is still unknown... So long as the nature of the new force remains hidden, it will help our researches and discussions about it if we have a special name for it. I hence will name it the catalytic force of the substances, and I will name decomposition by this force catalysis. The catalytic force is reflected in the capacity that some substances have, by their mere presence and not by their own reactivity, to awaken activities that are slumbering in molecules at a given temperature”.

1875

Large-scale production of sulfuric acid on platinum catalyst. This catalyst was developed by Pregrin Philips (British Patent No 6096, 1831).

1900 - 1950

1903: Ammonia oxidation over Pt leading to the production of HNO3

1913: Ammonia synthesis over iron and subsequent development of ammonia synthesis process by Bosch and Haber.

1920-1940: Catalytic processes for hydrogenation of CO to methanol or liquid hydrocarbons. . Germany used this technology during World War II when petroleum supplies had been cut off.

1936-1942: Catalytic cracking over SiO2 – Al2O3, this was the first significant use of solid catalysts in petroleum industry. Very important developments allowed refiners to increase gasoline yield and the petroleum crude more efficiently.

1960

1960: Weisz and Frilette coined the word “shape-selective catalysis” to describe the unique selectivity properties of crystalline molecular sieves or zeolites in cracking n-alkanes to exclusively straight chain products.

The “shape-selectivity” of zeolites is based on their unique ability to selectively admit or reject molecules of characteristic size and/or shape at the entrance to molecular-size pores containing active sites.

1970 - 1990 Nowadays

1964-1968: zeolites found application to catalytic cracking and hydrocracking of petroleum feedstocks.

Later several and numerous applications to shape-selective petroleum processing and production of chemicals and fuels: xylene synthesis and isomerization, ethylbenzen synthesis and gasoline production from methanol.

Noble metal catalysts were developed for control de CO, HC and NO emissions from automobiles. Increased emphasis on environmental control in the United States and Europe during the 1980s and 1990s led to the development of vanadium titania and zeolite catalysts for selective reduction of nitrogen oxides with ammonia. Catalysts for removal volatile organic hydrocarbons (VOCs) and hazardous organics such as chlorohydrocarbons were also developed.

? ? ?CATALYSIS in Modern World.mp4

Stationary

sources

Classification of catalyst technologies.

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Catalyst technologies

Petroleum refining Chemicals manufacturing

Environmental

Clean-up

Technologies for control of emissions

Mobile sources

Reaction type

- Hydrogenations.

- Oxidations.

- Synthesis.

- Polymerizations.

- Enzyme reactions.

Definition: What is a catalysts?

• Increase reaction rate (kinetics).

• It does not change thermodynamic equilibrium of the reaction.

• Remains unalterable during the reaction.

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• Catalyst is a material that converts reactants into products, through a series of elementary steps, in which the catalyst participates while being regenerates to its original form at the end of each cycle during its lifetime.

• A catalyst changes the kinetics of the reaction, but does not change the thermodynamics.

Example: Three Way Catalyst forAutomobile emission control.

• The internal combustion engine in automobiles combusts a gasoline/air mixture to generate heat that is converted to mechanical work in the engine.

• The combustion process is not 100% efficient, there are some undesired by products: CO, HC (unburned hydrocarbons), NOx (NO, NO2, N2O), CO2, H2O.

• How_Car_Exhaust_System_Works.mp4

• Three-way_catalytic_converter-Basf.mp4

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Catalysts and Activation Energy.

• Rate or reaction increases exponentially with decreasing the activation energy, then reduction of this increases dramatically the reaction rate. 6

• The catalyst reduce the energy barrier or activation energy necessary for electron exchange between the reactants and products.

• The catalyst lowers the activation barrier by providing a surface or site for adsorption and dissociation of the reactants, in which they are more readily transformed to products.

• Heat of the reaction does not change.

Heterogeneous catalysis

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• Heterogeneous catalysis is the processes whereby reactants absorb onto the surface of a solid catalyst, are activated by chemical interaction with the catalyst surface and selectively transformed to adsorbed products, which desorb from the catalytic surface.

• Numerous industrial applications: chemical, food, pharmaceutical, etc. It has been estimated that 90% of all chemical processes use heterogeneous catalysts.

• Principles of heterogeneous catalysts are complex because is a highly interdisciplinary field and require the cooperation between chemist and physicists, between surface scientists and reaction engineers, between theorists and experimentalists, between spectroscopists and kineticists and between materials scientists involved with catalyst synthesis and characterizations.

Heterogeneous Catalysts

• Main advantage: the catalyst being a solid material, it is easy to separate from the gas and/or liquid reactants and product of the overall catalytic reaction.

• Heterogeneous catalyst involves:- Active sites (or active centers)

- The surface of the solid, typically a high-surface area material (10 – 1000 m2/g)

- It is desirable maximize the number of active sites per reactor volume.

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Heterogeneous Catalysts

• Reaction intermediates: They are transitory and highly reactive species that are chemically identifiable. Examples: atoms, radicals, ions and molecules in excited states.

• Identify the reaction intermediates – and hence the mechanism – for a heterogeneous catalytic reaction is often difficult, because many of these intermediates are difficult to detect using conventional methods (e.g., gas chromatography or mass spectrometry) because they do not desorb as significant rates from the surface of the catalyst.

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Steps in a Heterogeneous Catalytic Reaction.

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Steps:1) Bulk diffusion.

2) Intraparticle Diffusion.

3) Adsorption

4) Surface reaction

5) Desorption

6) Intraparticle diffusion

7) Bulk diffusion.

Re

actants

Pro

du

cts

Steps:1) Bulk diffusion: Diffusion

of the reactants through the stagnant gas film or boundary layer surrounding the catalyst particle to the external catalyst surface.

Steps:2) Intraparticle Diffusion: Diffusion of reactants through the porous network of the catalyst to the catalytic surface (active sites).

Steps:3) Adsorption: Adsorption of the reactants onto the catalyst surface (Active sites).

Steps:4) Surface reaction: involving formation or conversion of various adsorbed intermediates.

Steps:5) Desorption: of products from the catalysts sites.

Steps:6) Intraparticle diffusion: Intraparticle diffusion of the products through the catalyst pores.

Steps:7) Bulk diffusion (film mass transfer of product): bulk diffusion of the products across the boundary layer surrounding the catalyst particle.

Catalyst Characteristics

• The suitability of a catalyst for an industrial process depends mainly on the following three properties:

- Activity: is a measure of how fast one or more reactions proceed in the presence of the catalyst. Activity can be defined in terms of kinetics (reaction rate), turnover number (which originates from the field of enzymatic catalysis) and turnover frequency to quantify the specific activity of a catalytic center for a special reaction under defined reaction conditions.

- Selectivity: is the fraction of the starting material that is converted to the desired product P.

- Stability (deactivation behavior): Chemical, thermal and mechanical stability of a catalyst determines its lifetime in industrial reactors.

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Reaction rate (r).

• The specific reaction rate for a stoichiometric catalytic reaction is formally defined as:

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Volume, weight or surface area or the catalyst.

Stoichiometric coefficient in the stoichiometric reaction.

Number of moles of each species.

• The number of moles of species I at any point in time is defined by:

• Conversion:

Reaction rate dependence of temperature and reactant

concentrations. • The rate equation is generally approximated as a product of a function

of temperature and concentration:

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• k(T) is the rate constant described by the Arrhenius law as:

• Frequency factor is proportional to the number of collisions, which might lead to reaction and in the case of a catalytic reaction to the concentration of catalytic sites.

Fraction of collisions that results in reaction. A catalyst increases the rate and hence the rate constant by increasing A (providing catalytic sites) and/or decreasing the activation energy (hence increasing the fraction of collisions resulting in reaction.

Definition: Turnover Frequency (TOF)

• TOF is a specific reaction rate based on number of active sites. Represents the frequency at which molecules react on an active site under specific reaction conditions.

• It is the number of molecular reactions (catalytic cycles) occurring at the center per unit time.

• TOF must be defined at specified conditions of temperature, concentration of reactants, and conversion. It must be measured in the absence of heat and mass transport limitations and pore diffusional restrictions.

• Industrial applications TOF range from 10-2 – 102 s.

• In principle, the TOF is a constant for a given metal, metal oxide, or metal sulfide in a given reaction at specified reaction conditions.

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Turnover Number (TON)

• TON specifies the maximum use that can be made of a catalyst for a special reaction under defined conditions by a number of molecular reactions or reactions cycles occurring at the reactive center up to the decay of activity.

• For industrial applications the TON is in the range of 106 – 107.

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Selectivity

• The selectivity of a reaction is the fraction of the starting material that is converted to the desired product P. It is expressed by the ratio of the amount of desired product to the reacted quantity of a reaction partner A and therefore gives information about the course of the reaction. In addition to the desired reaction, parallel and sequential reaction can also occur.

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Stability

• The chemical, thermal and mechanical stability of a catalyst determines its lifetime in industrial reactors. The catalysts deactivation can be followed by measuring activity or selectivity as a functions of time.

• Causes for catalyst deactivation includes: - Decomposition. - Coking.- Poisoning. - Sintering.- Phase changes.

• Catalyst that lose activity during a process can often be regenerated before they ultimately have to be replaced. The total catalyst lifetime is of crucial importance for the economics of a process.

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Key attributes of a good catalyst

• Good selectivity for production of the desired products and minimal production of undesirable byproducts.

• It should achieve adequate rates of reaction at the desired reaction conditions of the process. (Achieve good selectivity is usually more important than archiving high catalytic activity)

• Stability: the catalyst should show stable performance at reaction conditions for long period of time, or it should be possible to regenerate good catalyst performance by appropriate treatment of the desactivated catalyst after short periods.

• It should have good accessibility of reactants and products to the active sites such that high rates can be achieved per reactor volume.

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Catalyst Design

• Level 1 synthesis of materials: All studies of heterogeneous catalysis begin at the Materials Level. High-surface area catalytic materials must be synthetized with specific structures and textures. Characterizations studies are needed for determine the structures, compositions and textures of the materials that have been prepared.

• Level 2 Quantification of catalyst performance: These studies can be carried out over a wide range of catalytic materials (e.g. high-throughput studies) to identify promising catalysts and reaction conditions for further studies. It is necessary determine catalytic activity, selectivity and stability with respect time-on-stream for various reaction conditions.

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• Level 3 Elucidation: The aim is to identify the fundamental building blocks of knowledge which can be assembled to build a molecular-level understanding of catalyst performance in order to guide further investigations to improve catalyst performance. At this level is needed to determine the surface composition and nature of the surface sites on the catalyst.

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Catalyst Design

Combinatorial AssaysThe High-Throughput Approach

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Combinatorial Chemistry The High-Throughput Approach

• When new active solids are developed empirically, by trial-and-error procedure is highly speculative and leads to a very slow rate of discovery for the industry in question. This research strategy based on exhaustive studies and complete understanding is also very time-consuming.

• Because of this, new research strategies have to be developed in order to produce advances and revitalize the field of chemical research.

• Concepts of combinatorial chemistry were originally developed for the synthesis of small organic molecules. If a family of molecules has a common core structure, normally termed “scaffold”, bearing different functional groups, which can varied independently of each other, then a large number of different molecules can be generated based on this common scaffold, following combinatorial principles.

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• HT experimentation in drug discovery and medical chemistry is widely applied and mature approach from which combinatorial catalysis has taken and adapted most of its experimental and software tools.

• However the proper descriptions of solid catalysts and the quantum chemical properties are very complex and are still in their infancy, especially when compared with the available chemoinformatics software for virtual screening of drug candidate molecules.

• Molecular descriptors used in the pharmaceutical chemistry are variables that represent the physiochemical properties of a class for compounds and they are commonly classified into four types: Constitutional, topological, geometrical and quantum chemical properties.

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Combinatorial Chemistry The High-Throughput Approach

Combinatorial Chemistry The High-Throughput Approach

• However, for solids, the situation is much more difficult than for molecules. While for well-defined reaction steps, the product of an organic synthetic sequence is generally also well defined; this is not necessarily true for solids.

• The particle sizes may differ, the crystallinity can be dependent on treatment temperature, certain phases may need different temperature to form at differen levels of composition, different precursors will have different reactivities, and so on.

• A simple combinatorial synthesis of complex oxides is therefore not straightforward, and the situation is made even more complicated by the fact that synthesis planning for solids synthesis is only in its infancy.

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• It is a pragmatic alternative.

• It is based on the fast and systematic screening of libraries of diverse samples.

• The main idea is screening techniques used in HT methods, dozens or even hundred of experiments involving many variables can be performed at once. Two questions arise from this:

- Which experiments are the most relevant to carry out?

- What is the most efficient screening strategy?

• Quantitative structure-property relationship (QSPR) models.

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Combinatorial Chemistry The High-Throughput Approach

• The use of fundamental descriptors is very limited, since heterogenous catalysts are complex systems, with heterogeneous multicomponent active sites and unpredictable metastable structures.

• Tabulate information corresponding to tabulated attributes of the starting elements/oxides making up the catalyst is an useful tool.

• A good selection of the tabulated attributes that could have an impact in the performance and a good predictive model using neural networks and classification trees may achieved good results.

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Combinatorial Chemistry The High-Throughput Approach

Quantitative structure-property relationship (QSPR) models.

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• If a direct relationship is found between the catalysts (Space A), the molecular descriptors (space B), and the figures of performance (space C), it becomes theoretically possible to backtrack from space C to space A.

• The identification of a good catalyst according to the appropriate positions in spaces B and C should enable the selection of structures in space A.

Space A Space B Space C

Quantitative structure-property relationship for predictive models in

catalysis.

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• Spectral data can be used together with synthesis and theoretical data as input descriptors for catalytic QSPR modeling. Spectral descriptors can be obtained automatically by processing of the raw characterization data.

• The QSPR model is obtained by different data-mining techniques can be used i) as a predictive model and ii) for extraction of rules and relationships between variables, gaining knowledge about catalysis.

References

1. R. Farrauto, C. Bartholomew. Fundamentals of industrial catalyticprocesses. 1a ed. London: Blackie, 1997

2. G. Ertl, H. Knozinger, F. Schuth, J. Weitkamp. Handbook ofHeterogeneous Catalysis 2008. Wiley-VCH Verlag GmbH & Co. KGaA.

3. C. Barbero, R. Furlán y E. Mata. Química combinatoria. Volumen 21,número 124 (2011) 39-45.

4. J. Hagen, Industrial Catalysis, 2 nd. 2006.

5. Schuth , F., & Schunk, S. (2006). Combinatorial Approaches to DesignComplex Metal Oxides. En J. Fierro, Metal Oxides. Chemistry andApplications (págs. 391-408)

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