Applied chemical process - vscht.czpaidarm/ACHP/erasmus/5eng... · 2017-05-10 · 10.5.2017 1 Applied chemical process Heterogeneous catalysed reactions Catalysis •Homogeneous catalysis

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Applied chemical process

Heterogeneous catalysed reactions

Catalysis

• Homogeneous catalysis – the catalyst is in the same form as reactants

• Heterogeneous catalysis – the catalyst is in other form than reactants

• Biocatalysis – enzyms (biotechnologic reactions)• Environmental catalysis – catal. processes are

connected with environment(green catalytic processes)

Principle of catalysis• The catalyst accelerates

the chemical reaction• The activation energy for

the catalytic reaction is lower than the non-catalyzed reaction

• Decreased amount of activation energy cause higher probability of reactants reaction

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Early history of catalysis

• 6000 bc brewing of beer from malt

• 3000 bc preparation of vine from fruit

• 2000 bc fermentation of different hydrocarbons to alcohol

• 800 bc chees preparation by casein hydrolysis with extract from calf stomach

History of catalysis

• Von Marum dehydrogenation of alcohol by metals 1796

• Davy oxidation of methane on platinum 1817

• Faraday hydrogen ignition in air on Pt 1825

• Berzelius formulation of catalyst definition 1836

• Von Hoffmann Ag as catal. for oxidation CH3OH to HCHO 1869

• Messel try of industrial oxidation of SO2 to SO3 on Pt 1875

• Winkler discovery of contact process of H2SO4 production 1879

• BASF industrial synthesis of H2SO4 on Pt 1889-1901

History of catalysis in 20th cent.

• Ostwald 2 NH3 + 5/2 O2 → 2 NO + 3 H2O on Pt 1901

• Sabatier Hydrogenation of alkens on Ni ( Nobel p.1912) 1902

• Haber Ammonia preparation from elements (NP 1918) 1904

• Langmuir Formulation of adsorption theory (Nobel p. 1932) 1915

• Chem.Construction Industrial manufacturing of HNO3 1917

• Taylor Theory of catalysis 1925

• …

Lot of industrial applications of heterogeneous catalysis during WWII.Mostly dehydrogenations on Pt (alumina carrier), oxidations (V2O5) etc.

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Principle of catalysis• A suitable catalyst can enhance the rate of a

thermodynamically feasible reaction but cannot change the position of the thermodynamic equilibrium

• Catalysts don´t change termodynamic of reactions• Heterogeneous catalysis involves systems in which catalyst

and reactants form separate physical phases. Typical heterogeneous catalysts are inorganic solids such as metals, oxides, sulfides, and metal salts, but they may also be organic materials such as organic hydroperoxides, ion exchangers, and enzymes

Principle of catalysis

1)Mass transfer of thereactants from the bulk ofthe fluid to the externalsurface of the particle(external diffusion)

2)Transport of the reactants through the pores into the

particle by diffusion(internal diffusion)

Principle of catalysis

eqLeqA

eqALA cc

cK

A

ALLAA Kccckr

eqBeqAL

eqCL

ccc

K

A

L L L L

A

B

C

C

Kccckr CL

BAL

eqLeqC

eqCLC cc

cK

LC

C

CLC ccKckr

3)Adsorption of reactants on

the solid surface

4) 5)Chemical conversion Desorption of in the adsorbed state products

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Principle of catalysis

7) Transport of products from the

external surface to the bulk

6)Transport of products through the

pores to the external surface of the particle

Adsorption

• Adsorption isotherms– The dependence of the equilibrium amount adsorbed

substance on the composition of the mixture• Freundlich isotherm• Lanqmuir isotherm

– number of occupied active sites– monolayer

• BET isotherm(Brunauer, Emmett and Teller)– multilayer adsorption

AA

AA

pKpK

1

npkmx 1

000

0

11PP

PPC

PP

PPC

VVmono

total

Langmuir adsorption isotherm- surface coverages are correlated with partial pressures or concentrations in the fluid phase

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For bimolecular, heterogeneously catalyzed reactions, two mechanisms can be

distinguished:

A) Eley – Rideal MechanismB) Langmuir – Hinshelwood Mechanism

Catalyst• A catalyst is a substance increasing the rate of a

chemical transformation without modifying the yield and it is found intact in the final reaction product.

• Unsupported (Bulk) Catalysts• Supported catalysts• Hybrid catalysts• Polymerization catalysts

Characterization of Solid Catalystsunder Working Conditions

• Inorganic technology– synthesis gas, hydrogen, ammonia, methanol, sulphuric

acid• Organic technology

– hydrogenation, dehydrogenation etc.• Polymerization reaction• Environmental Catalysis

– car exhaust catalyst• Biotechnology

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Classification of Solid Catalysts• (1) unsupported (bulk) catalysts• (2) supported catalysts• (3) confined catalysts (ship-in-a-bottle catalysts)• (4) hybrid catalyst• (5) polymerization catalysts• (6) others

Unsupported (Bulk) CatalystsMetal Oxides

• Oxides of metals are usually solids• Metal oxides have widely varying electronic properties

and include insulators (e.g.,SiO2, Al2O3), semiconductors (e.g.,TiO2, NiO, ZnO), metallic conductors (typically reduced transition metal oxides such as TiO, NbO, and tungsten bronzes), superconductors (e.g., BaPb1-xBixO3)

• They have acid, basic and redox properties • Metal oxides can have simple composition, like binary

oxides, but many technologically important oxide catalysts are complex multicomponent materials

Unsupported (Bulk) CatalystsSimple Binary Oxides

• Simple binary metal oxides may behave as acids or bases or amphoteric materials

• Amphoteric oxides ( Al2O3, ZnO) form cations in acidic and anions in basic conditions.

• Acidic oxides (SiO2) dissolve with formation of acids or anions. Transition metal oxides in their highest oxidation state (V2O5, CrO3) behave similarly

• Basic oxides (MgO) form hydroxides or dissolve byforming bases or cations.

• Silica, alumina and magnesia are commonly usedcatalysts and catalyst supports representative for a wide range of surface acid – base properties.

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Unsupported (Bulk) CatalystsAluminas

• amphoteric oxides, which form a variety of different phases depending on the nature of the hydroxide or oxide hydroxide precursor and the conditions of their thermal decomposition

• Bayerite, nordstrandite, boehmite and gibbsite can be used as starting materials

• Catalyst for elimination reactions, alkene isomerization,Claus process

• Besides their intrinsic catalytic properties they are often used as catalyst support

• The surface area and particle size of aluminas can be controlled by the preparation conditions, thermal stability is given by the surface active phases

Unsupported (Bulk) CatalystsAluminas preparation

Unsupported (Bulk) CatalystsSilicas

• weakly Bronsted acidic oxide which occur in a variety of structures such as quartz, tridymite and cristobalite.

• The most commonly used silica in catalysis is amorphous silica.

• The siloxane bridges are essentially unreactive. For this reason, silicas are not used as active catalysts, but they play an important role as oxide supports and for the synthesis of functionalized oxide supports.

• Thus, surface area, particle size andmorphology, porosity and mechanical stability can be varied by modification of the synthesis parameters.

• special forms of silica like zeolites and other mesoporous structures can be prepared by special hydrothermal synthesis

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Unsupported (Bulk) Catalysts

Magnesium oxide

• Solid • It has the simple structure, with octahedral coordination of

magnesium and oxygen.• MgO is used as a supportfor transition metal ions.

Unsupported (Bulk) CatalystsTitania

• anatase and rutile (both tetragonal structure)• Anatase is more frequently used modification since it

develops a larger surface area• Rutile is thermodynamically more stable phase• Vanadium impurities lower temperature of rutilization (820 K)• Other impurities such as surface sulfate and phosphate

stabilize the anatase phase• Titania is a semiconductor with a wide band gap and as such

is an important material for photocatalysis

600-850 oCAnatase Rutile

Unsupported (Bulk) CatalystsZirconia

• has attracted significant interest in the recent past as a catalyst support and as a base material for the preparation of strong solid acids by surface modification with sulfate or tungstate groups.

• The most important crystallographic phases of ZrO2 for catalytic applications are tetragonal and monoclinic.

• Monoclinic is the thermodynamically stable phase. Highersurface areas, however, are developed by the metastable tetragonal phase, which is stabilized at low temperatures by sulfate impurities or intentional addition of sulfate or tungstate.

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Unsupported (Bulk) CatalystsZeolites

• Hydrothermal synthesis can be used for preparation of a large family of crystalline aluminosilicates, known as zeolites.

• Zeolites are microporous solids with pore sizes ranging from 0,3 to 70 nm.

• Characteristic properties of these structurally well-defined solids are selective sorption of small molecules (molecular sieves), ion exchange and large surface areas.

• Properties of zeolites can be easily changed.• Zeolites possess a framework structure of corner-linked SiO4

4-

and AlO45- tetrahedrawith two-coordinate oxygen atoms that

bridge two tetrahedral centers (so-called T atoms).

Zeolites – common types

Zeolites - shape-selective effects

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Unsupported (Bulk) CatalystsMixed metal oxides

• are multimetal multiphase oxides which typically contain one or more transition metal oxide and exhibit significant chemical and structuralcomplexity

Application• selective oxidation• oxydehydrogenation• ammoxidation• other redox reactions

Unsupported (Bulk) CatalystsPerovskite

• A mineral CaTiO3, • The structure also tolerates a wide variety of compositions• Applications of perovskite-type oxides in catalysis are in fuel

cells, as catalysts for combustion and for DeNOx reactions.Hydrotalcit

• Hydrotalcite is a claymineral. It is a hydroxycarbonate of Mg and Al of general formula Mg6Al2(OH)16]CO3 . 4 H2O.

• Hydrotalcites develop large surface areas and basic properties. They have consequently been applied as solid catalysts for basecatalyzed reactions for fine-chemicals synthesis, polymerization of alkene oxides…

Unsupported (Bulk) CatalystsMetals and Metal Alloys

• Metals and metal alloys are used as bulk, unsupportedcatalysts in only a few cases.

• Metal gauzes or grids are used as bulk catalysts in strongly exothermic reactions which require catalyst beds of small height. Typical examples are platinum – rhodiumgrids used for ammonia oxidation in the nitric acid process and silver grids for the dehydrogenation of methane to formaldehyde.

• Skeletal (Raney-type) catalysts, particularly skeletal nickel catalysts, are technologically important materials which are specifically applied in hydrogenation reactions

• Skeletal catalysts are prepared by the selective removal of aluminum from Ni – Al alloy particles by leaching with aqueous sodium hydroxide (surface areas between 30 do 100 m2/g)

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Unsupported (Bulk) CatalystsCarbon

• Although carbons are frequently used as catalystsupports, they may also be used as catalysts in their own right

• Carbons exist in a variety of thermodynamic phases and metastable structures, which are often ill defined

• There are two functions of the carbon surface act simultaneously during a catalytic reaction

- the reactants are chemisorbed selectively on the carbon surface by ion exchange via oxygen functional groups ordirectly by dispersion forces involving the graphite valence-electron system.

- production of atomic oxygen occurring on the graphene basal faces of all sp2 carbon materials

Unsupported (Bulk) CatalystsCarbon

• Carbon can already be catalytically active under ambient conditions and in aqueous media.

• Catalytic applications of carbons include the oxidation of sulfurous to sulfuric acid, the selective oxidation of hydrogen sulfide to sulfur with oxygen in the gas phase at ca. 400 K, the reaction between phosgene and formaldehyde and the selective oxidation of creatinine by air in physiological environments.

• carbon nanotubes and nanofibers - catalysts and catalyst supports

Supported catalysts• Supported catalysts play a significant role in many industrial

processes. The support provides high surface area and stabilizes the dispersion of the active component

• Active phase – support interactions,which are dictated by the surface chemistry of the support for a given active phase, are responsible for the dispersionandthe chemical state of the latter.

• Although supports are often considered to be inert, this is not generally the case.

• Typicalexamples fortheactiveinterplaybetween support and active phase are bifunctional catalysts such as highly dispersed noble metals supported on the surface of an acidic carrier.

• Supports are typically porous materials having high thermostability.• For application in industrial processes they must also be stable

towards the feed and they must have a sufficient mechanicalstrength.

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Supported catalystsSupports

• The most frequently used supports are binary oxides including transitional Al2O3, SiO2, TiO2, ZrO2, MgO a ternární oxidy typu Al2O3 - SiO2 and zeolites.

• Luminophosphates, mullite, kieselguhr, bauxite, and calcium aluminate.

• Carbons in various forms (charcoal, activatedcarbon)

• Monolithic supports

Supported catalystsMonolithic supports

• The channel walls are nonporous or contain macropores. For the above application the monoliths must have high mechanical strength and low thermal expansion coefficients to give sufficient thermal shock resistance.

• The preferred materials of monolith structures are ceramics (cordierite) or highquality corrosion-resistant steel– Cordierite is a natural aluminosilicate (2MgO.2Al2O3.5SiO2)– surface area 50 - 300 m2/g

Supported Metal Oxide Catalysts• Are consisted of at least one active metal oxide

component dispersed on the surface of an oxide support

• The active oxides are often transition metal oxides, while the support oxides typically include transitional aluminas (preferentially g-Al2O3), SiO2, TiO2 (anatase), ZrO2 (tetragonal), and carbons.

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Supported Metal Catalysts• Metals typically have high surface free energies

and therefore a pronounced tendency to reducetheir surface areas by particle growth.

• Therefore, for applications as catalysts they aregenerally dispersed on high surface areasupports, preferentially oxides such astransitional aluminas, with the aim of stabilizingsmall, nanosized particles under reactionconditions.

Characterization of Solid CatalystsPhysical Properties

• Surface area (BET)• Distribution of pores (BJH)• Porosity (Hg-porosimetry)• Particle size and distribution• Structure and Morphology

– X-ray powder diffraction– Electron Microscopy and Diffraction– Vibrational Spectroscopy (IR, Raman)– Neutron techniques

• Local Environment of Elements– Solid State Nuclear Magnetic

Resonance– Moossbauer spectroscopy

Chemical Properties• Surface Chemical Composition

– Electron Spectroscopy (AES, XPS)– Ion-scattering Spectroscopies– Secondary Particles

• Valence States and RedoxProperties

• Acidity and Basicity• Activity• Selectivity

Mechanical PropertiesAbrasion resistance and Stability

Important properties of catalysts

• activity• selectivity• stability• regeneration• shape• porosity• environment

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Preparation of catalysts

without support• mechanical treatment• melting• precipitation• sol-gel process• hydrolysis• thermal decomposition• hydrothermal synthesis

with support• mechanical treatment• impregnation• precipitation on support• coating of nonporous

support• reductive coating• thermal decomposition• adsorption• ion exchange

Mechanical treatment• Mixing, grinding, kneading of active material or ots precursors

with promotors, structural stabilizators or pore-formingsubstances (template)

• low production of waste water

Examples of unsupported catalysts prepared by mechanical treatment:

Examples of supported catalysts prep. by mechanical treatment:

Unsupported catalyst ApplicationFe2O3 (K, Cr, Ce, Mo) Dehydrogenationof ethylbenzene

Fe2O3 (K) Fischer-Tropschsynthesis

ZnO – Cr2O3 hydrogenation of carbonyl substances

Supportedcatalyst ApplicationNi - diatomaceousearth Hydrogenation of glucose to sorbitol

MoO3 - alumina Transesterificationof sunflower oil

V2O5 – Alumina/TiO2 Selective oxidation of hydrocarbons

• Only for few unsupported catalysts• It is possible to prepare alloy of two elements unmixable

in solution or at solid state• High energy costs

Examples:• magnetite Fe2O3 with K, Al, Ca and Mg oxides for

ammonia synthesis• V2O5 with K2S2O7 (with Cs as promoter) for SO2 oxidation• Pt/Rh meshes for oxidation of ammonia

Melting

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Precipitation (coagulation)• large ammount of waste solutions which contain salts

after precipitation and washing• reactants are (soluable) salts of metals (sulfates,

chlorides, nitrides, acetates, formates..)• coagulators are hydroxides, carbonates, and

hydrocarbonates od pottasium sodium and ammonia• mixed photocatalysts can be prepared:Cu(OH)NH4CrO4 or Ni6Al2(OH)16CO3 which form afterthermal annealing binary oxides CuO-Cr2O3 or NiO-Al2O3.• Crucial parameters are:

– pH, time, temperature, concentrations, mixing, purity• High concentrations, low t, short time – small particles

(but complicated filtration)– vice versa

Precipitationsteps in industry:

• preparation of reactants• precipitations• ageing of precipitate• washing (decantate)• filtration• washing of precipitate• drying• thermal annealing• forming/shaping• activation

Precipitationcatalyst examples

Catalyst Precursor ApplicationAlumina Na[Al(OH4)], HNO3 support, Claus processSilica Water glass, H2SO4 supportFe2O3 Fe(NO3)3, NH4OH Dehydrogenation of

ethylbenzeneTiO2 FeTiO3, TiOSO4, NaOH support, Claus process,

reduction NOx

CuO – ZnO – (Al2O3) Ni(NO3)2, Al(NO3)3, Cu(NO3)2,Na2CO3

Syntésis of methanol

Fe2(MoO4)3 Fe(NO3)3, (NH4)2MoO4, NH4OH

Oxidation of methanol to formaldehyde

NiO – Al2O3 Ni(NO3)2, Al(NO3)3, Na2CO3

Hydrogenation of arens

NiO – SiO2 Ni(NO3)2, silicate Na, Na2CO3

Hydrogenation of arens

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Sol-gel process• preparation of sol followed by forming of gel• sol (liquid suspension of solid particles smaller than 1 mm) is

prepared by hydrolysis and partial condensation of inorganicsalts and metal alcoxides

• condensation of sol particles forming 3D structure – gel• Alumina a silica can be prepared from sodium aluminate/silicate

and nitric/sulphuric acid• Spherical silica or silica alumina can be formed by dripping of

sol to the oil. (drying and thermal• High purity materials can be formed by gol-gel process such

alumina, TiO2, ZrO2. Basic materials are proper metal alcoxides(Ti(n-C4H9O)4

Flame hydrolysis

• Mixture of catalyst or its precursor and hydrogen and airis introduced into flame (continual reactors)

• Precursors (mainly chlorides AlCl3, SiCl4, TiCl4 or SnCl4)are hydrolyzed by water vapor( formed by oxidation ofhydrogen).

• Corresponding oxides are formed

• Several hundrets tons of silica, alumina and TiO2 areproduced by this process.

Thermal decomposition

• mixture of Cu- and Zn(NH3)4 (HCO3)2 is decomposing at 370K and form binary compound Cu-Zn carbonate, which isdecomposing by thermal annealing to corresponding oxides(catalyst for low temp. conversion of CO to CO2)• Cu-Cr oxides from basic ammonium-copper chromate[CuNH4(OH)CrO4] at 620-670 K. (for hydrogenation of organicsubstances)• High active Ni catalyst for oil and fat hydrogenation is formedfrom nickel formate at 390-420K. (It is done in directly fat toprevent surface oxidation of Ni by air)

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Thermal decomposition

Catalyst type Precursor method usagealumina (high purity)

Aluminiumalkoxide

Sol-gel Noble metal carrier

alumina (acidic)

AlCl3 Flame hydrolysis Carrier or additive

silica SiCl4 Flame hydrolysis Carrier or additive

TiO2 Ti(n-C4H9O)4 Sol-gel carrierTiO2(low density)

TiCl4 Flame hydrolysis Carrier or additive

CuO – ZnO Cu, Zn (NH3)4HCO3

Thermaldecomposition

Methanoltransformation

CuO – Cr2O3 Cu(NH4)OHCrO4 Thermaldecomposition

Hydrogenation ofcarbonylsubstances

Ni (diatomite) Ni formate Thermaldecomposition

Oil hydrogenation

Hydrothermal synthesis• important methot for preparation of zeolites a other

molecular meshes.– catalyst or support at petrochemical reactions

• preparation of pure chemicals• preparation of zeolites - mix of substances of

silicone, aluminium, alcaline metals, water and organic compounds is transformed by hydrothermal synthesis to mikroporous, crystalic alumino-silicate.

Catalyst Deactivation

The main reasons :• Poisoning• Fouling• Thermal degradation• Volatilization of active components

• reversible x irreversible

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Catalyst Poisoning• The blocking of active sites by certain elements or

compounds accompanied by chemisorption or formation of surface complexes are the main causes of catalyst deactivation.

• 1. Group 15 and 16 elements such as As, P, S,• Se, and Te• 2. Metals and ions, e.g., Pb, Hg, Sb, Cd• 3. Molecules with free electron pairs that are• strongly chemisorbed, e.g., CO, HCN, NO• 4. NH3, H2O and organic bases, e.g., aliphatic• or aromatic amines, pyridine, and quinoline• 5. Various compounds which can react with different• active sites, e.g., NO, SO2, SO3, CO2..

Catalyst Fouling• Because most catalysts and supports are

porous, blockage of pores, especially of micropores, by polymeric compounds is a frequent cause of catalyst deactivation

• At elevated temperatures (> 770 K) such polymers are transformed to blackcarbonaceous materials generally called coke

• Catalysts possessing acidic or hydrogenating – dehydrogenating functions are especially sensitive to these fouling (coking)

Thermal Degradation

• One type of thermal degradation is the agglomeration of small metal crystallites below the melting point, called sintering

• The rate of sintering increases with increasing temperature.

• The presence of steam in the flow can accelerate the sintering of metal crystallites.

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Volatization of Active Components

• Some catalytic systems containing P2O5, MoO3, Bi2O3, etc. lose their activity on heating close to the sublimation point

• Cu, Ni, Fe and noble metals can escape from catalysts after conversion to volatile chlorides if traces of chlorine are presentin the reaction mixture.