CHEMM003 Homogenous and Heterogenous Catalysts PDF

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Summary

CHEMM003 GS Homogeneous vs heterogeneous catalysis Homogeneous industrially problematic Difficult to recover and recycle trace amounts may remain in product Advantages: reaction rates and product selectivities generally higher than heterogeneous systems Heterogeneous Uncontaminated product isolation...

Description

CHEMM003 GS

Homogeneous vs heterogeneous catalysis 

Homogeneous - industrially problematic  

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Difficult to recover and recycle catalyst; trace amounts may remain in product Advantages: reaction rates and product selectivities generally higher than

heterogeneous systems Heterogeneous  Uncontaminated product isolation and catalyst reuse 

Reaction rates and selectivities low

Heterogeneous catalysis - gas-phase reaction 1.

Adsorption of reactants

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Surface diffusion of reactants

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Surface reaction

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Surface diffusion of products

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Desorption of products

Terminologies    

Activity - amount of reactant converted to products Turnover number (TON) - activity per site Turnover frequency (TOF) - TON per unit time Selectivity - amount/percentage of a specific product formed compared to other products

Catalyst Characterisation techniques 

X-ray diffraction (XRD)  Provides complete structure solution - if material crystalline  

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Phase identification - phase purity of material Particle size determination

X-ray absorption spectroscopy (XANES, EXAFS)  X-ray absorption near edge structure (XANES) - determine oxidation state and local 

coordination geometry Extended X-ray absorption fine structure (EXAFS) - determine precise local

structural geometry, in particular bond distances, element specific Can be done during reaction Raman spectroscopy 

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Study vibrational, rotational, and other low-frequency modes in system Vibrational information is specific to chemical bonds and symmetry of molecules Determine local structure of metal ions present in system 1

CHEMM003 GS

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Infra red spectroscopy

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Molecules absorb specific frequencies that are characteristic of their structure NMR  Liquid samples - mainly for organic samples 

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Solid-state - used extensively to characterise certain solids - eg silica, alumina,

aluminophosphate, aluminosilicate Electron microscopy  Morphology Structure at atomic level Surface area  higher surface area, higher reactivity 

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Operates on adsorption property of a molecule on the surface of a solid

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Expressed in m2/gm - amount of available surface area over a solid particle for a gram of solid Molecules can adsorb on surface: 1) physisorption - weak van der Waals

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interaction; 2) chemisorption - forming chemical bond Typically nitrogen is used as it is inert and unlikely to strongly adsorb on the

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surface Temperature programmed reduction (TPR)  Provides info on nature of reduction of catalysts at various temperature by

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monitoring hydrogen uptake Temperature programmed desorption (TPD)  Provides info on nature of acid sites in catalyst - acidity measurement

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Thermo gravimetric analysis (TGA)  Change in weight of sample with temperature

Product characterisation 

Gas chromatography  Separate and analyse compounds that can be vaporised without decomposition  Typical use: separate different components of a mixture 

Carrier gas (inert gas eg He/N) used to carry other molecules Stationary phase located inside a piece of glass/metal tubing - column

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Gaseous compounds analysed interact with walls of column which is coated with

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specific material Each compound elute at different time - retention time

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Comparison of evolution of gases at various times - estimate amount of each

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product present in a mixture Mass spectrometry 2

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Measure molecular mass of a sample

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Mass spectrometer divided to 3 fundamental parts: ionisation source, analyser, detector

Types of catalytic materials 

Nano porous materials - micro and mesoporous catalysts Zeolites, aluminophosphates Solid-state materials, dense framework, bulk oxide 

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Bismuth molybdates, vanadium phosphates, iron phosphates, titania, perovskites,

nitrides, sulphides Supported catalysts  Metal clusters/nano particles supported on high surface area carriers

Nano porous materials Acid catalysis - many chemical compounds can act as sources for protons to be transferred eg sulphuric acid - used in petroleum refining; to overcome issues with use of sulphuric acid, solid acid catalysts widely used and also in high temperature applications Zeolite as acid catalyst Advantages: relatively cheap, reusable/recyclable, reduce waste production, easily handled and stored, can be tailored to suit the chemistry in which they are used Catalysis through ion-exchanged systems Shape - reactant and product selective Properties required for zeolite as catalysts  Stability - integrity of atomic arrangement  Pore size - stability ensures retention of pore dimensions for shape selective catalysis  

Acidity - good acid properties for acid catalysed reactions Redox - change in oxidation property for oxidation reactions

Aluminosilicates (zeolites) Acid catalysts: Al3+, Ga3+, Fe3+ Redox catalyst: Ti4+, V4+ Synthesis: hydrothermal method  Silica powder, aluminium hydroxide mixed in water along an organic template (amines 

or quaternary ammonium compounds) to form an uniform gel Heated in an sealed autoclave between 100-200 °C for several hours 3

CHEMM003 GS

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Once cooled, product is filtered, washed and dried XRD measurement to ensure product is crystalline and phase pure

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Sample calcined in air to remove organic template molecules to obtain open framework

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system = empty channels Convert zeolites to acid catalysts Lower valent metal ions substituted in the framework of silicate introduces charge imbalance, when compensated with protons they act as solid acid catalyst (Bronsted acid site) Al3+ substituted in place of Si4+ Higher Al content - higher acidity Acid strength depends on Si/Al ratio Lewis acid and Bronsted acid sites  Lewis acid sites - coordinatively unsaturated sites around substituted lower valent cations 

Bronsted acid sites - bridging hydroxyl joining Al3+ and Si4+ ions  Created by substituting a lower valent metal ion in the tetrahedral sites

Determination of acidity and strength  Proton NMR 

FTIR - distinguish terminal and bridging hydroxyls  

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Bridging hydroxyls (Bronsted acid sites) give a unique band at 3600 cm-1 Base molecules eg pyridine/ammonia allowed to absorbed into acid zeolite - shift

in N-H frequency used to determine types and strength of acid sites TPD - determine type and strength of acid sites using ammonia/other base molecules 

Adsorption of bases from gas phase with inert nitrogen as carrier gas at RT Sample heated subsequently in reactor

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Connected to MS/GC to measure amount of out coming/desorbed gas - recorded

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as a function of temperature Base interacting with weak acid sites (Lewis acid) desorbed at lower temperature Strong acid sites (Bronsted acid) retain base longer - desorb at higher temperature Results obtained at different temperatures give measure of acid strength distribution of catalyst

Methanol to olefins/gasoline  Mechanism involves C-C bond formation from C1 fragments 

Olefins - intermediates in conversion of methanol to hydrocarbons - lead to aromatic 4

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hydrocarbons - inhibit conversion to aromatics to enhance yield of light olefins Even if larger molecules formed inside zeolite cage - could not diffuse through pores hence undergo subsequent cracking to smaller molecules Catalyst - zeolite ZSM-5 - straight channels Product selectivity depends on temperature and pore size

Aluminophosphate - SAPO (silicoaluminophosphates) more stable Synthesis: hydrothermal method Phosphoric acid, aluminium hydroxide (and metal sources to be substituted) mixed in water Convert to acid catalysts Divalent metal ions substituted in the Al3+ sites eg Mn2+, Co2+, Fe2+ Tetravalent metal ions substituted in the P5+ sites eg Si4+, Ti4+ Use of zeolites (acid catalysis)  Methanol - converted to gasoline (and diesel)  Dimethylether (DME) - another important product of gas conversion - produced directly 

from syngas or indirectly via methanol dehydration Catalytic cracking - obtain alkane and alkene

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Alkylation, isomerisation

Transition metal ions substituted zeolites Effective shape-selective and recyclable catalysts for several oxidation by using environmentally friendly oxidants Redox chemistry associated with TM critical Titano silicate/zeolite silicalite/Ti-silicalite 

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To avoid formation of TiO2 (octahedrally coordinated Ti centres) small amounts of Ti4+ substituted in silicate matrix Requirements: stable zeolite structure, synthesised in pure silicate form, synthesis to be perfected to avoid any clustering of Ti ions TS-1 - for selective and partial oxidation of organic substrates using H2O2 as oxidant Advantage of using H2O2 - final product has water as major side product

Synthesis: hydrothermal method Silica source (silica/tetraethyl orthosilicate/colloidal silica) and Ti source (TiO2, tetraethyl orthotitanate/titanium isopropoxide) mixed in water along an organic template (tetrapropyl ammonium hydroxide) 5

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Characterisation  XRD - unit cell volume increases linearly with Ti content 

FTIR - compare titanosilicalite with pure silicalite: band at 960 cm-1 very evident

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Raman - compare with pure silicalite - band at 960 cm-1 very evident, slightly shifted when Ti present in sample UV-VIS - sharp tetrahedral Ti peak for when no amorphous titania, octahedral peak

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appears when amorphous titania present 

Ti K-edge XANES and EXAFS show presence of tetrahedral Ti4+ centres

Comparison of amorphous and crystalline titanosilicate materials  Crystalline TS-1 - hydrophobic, high selectivity, produce more epoxide, epoxide gets 

hydrolysed to diol with time Amorphous co-gel - hydrophilic, low selectivity, epoxide selectivity very low, main product is diol

Oxidation of hydrocarbons with zeolites Epoxidation of alkenes (eg cyclohexene)

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Calcined titanium silicate (TS-1) catalyst - Ti4+ in framework surrounded by Si4+ linked through oxygen in crystalline solid Batch reaction, temperature: 333K, atmospheric pressure, liquid phase reaction

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Solvent: acetonitrile, necessary to mix H2O2/water and cyclohexene

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Catalytic cycle - formation of η1 and η2 Ti-peroxo species - both intermediates promote reaction

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Iron silicalite Synthesis: hydrothermal method Silica source and iron source (iron acetate) mixed in water along an organic template (tetrapropyl ammonium hydroxide) Characterisation  XRD - white crystalline solid formed under hydrothermal conditions  

Fe K-edge XANES and EXAFS show presence of tetrahedral Fe3+ centres Coordinatively unsaturated (Lewis site) site of Fe is the active site

Oxidation of benzene to phenol  Industrially, phenol produced from benzene via Cumene process 

Multistep - low phenol yield, highly energy consuming, by-products eg acetone 6

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Poor ecology, explosive intermediate

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Iron silicalite (Fe-ZSM5) catalyst - Fe3+ in tetrahedral coordination, coordinatively unsaturated sites Direct oxidation using nitrous oxide N2O as oxidant - much better selectivity as

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compared to oxidation by dioxygen No highly reactive intermediate, no acetone byproduct, low expenses

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Gas-phase used (He carrier gas), temperature: 653K, atmospheric pressure,

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benzene/N2O = 1/4   

N2O decomposes to N2 and atomic oxygen at Fe3+ sites at around 350 °C Reactive oxygen readily reacts with benzene to form phenol At high temperatures molecular oxygen produced and oxidise benzene to CO2

Iron substituted aluminophosphate - FeAlPO4-5 Improved/modified approach Advantage - lower temperature for activation and direct generation of coordinatively unsaturated Fe3+ sites Synthesis: hydrothermal method Phosphoric acid, aluminium hydroxide and iron acetate mixed in water Selective oxidation of alkanes  Oxidation of organic molecules eg cyclohexane, n-hexane, n-octane, n-decane  Selective/partial oxidation most difficult under environmentally conditions  

Eg adipic acid for nylon production - catalyst cobalt acetate Transition metal ions containing aluminophosphate (Mn, Co or Fe) good for this  TM undergo efficient oxidation-reduction process 

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Synthesis: hydrothermal method - phosphoric acid, aluminium hydroxide and

Fe/Mn/Co acetate mixed in water along an organic template (triethylamine/tetraethyl ammonium hydroxide/cyclohexylamin) Catalytic conditions

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Calcined catalyst - remove organic template, also convert M2+ ions to M3+ Liquid alkane - cyclohexane = liquid phase under pressure Placed in high-pressure autoclave vessel, pressurise and heat

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Liquid products analysed using GC

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Bulk oxides Widely used in industry as heterogeneous catalysts for selective oxidation reactions  Methanol oxidation to formaldehyde 7

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Propylene oxidation/ammoxidation to acrolein/actylonitrile n-butane oxidation to maleic anhydride

Complexity of bulk mixed oxide catalyst powders  Variable oxidation states  Variable coordination for each oxidation state   

Chemical nature of surface sites - redox, basic or acidic Participation of surface and bulk lattice oxygen atoms in oxidation reactions Possess several different oxygen sites in their crystallographic bulk structure

Principles for selective oxidation  Lattic oxygen - labile/reactive  Metal-oxygen bond strength - some should be weak to provide source for oxidation 

Host structure - fairly stable host structure, should not collapse/change during reaction Redox characteristics - some metal ions can under change in oxidation state without

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destroying structure Multi-functionality of active sites - each of the sites should be able to promote part of

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the conversion process Site isolation - same metal ions should not be clustered together, should be isolated to

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avoid clustering or sintering which may form an inactive species Phase cooperation

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Simple binary oxides  Eg CeO2 - oxygen storage/release capacity  Used in automobile three way catalyst  Stores oxygen in bulk of catalyst under oxidising conditions and uses oxide ion stored 

under reducing conditions Reduce emission of NOx, CO and hydrocarbons

Ternary oxides  ABO3 - perovskite type catalyst  M-Mo-O - M = Bi, Fe, Co etc  Vanadium phosphate - VPO 

AB2O4 - spinel type catalysts

Different methods of preparing dense framework solids  Solid state method - low surface area, heating temperature critical 

Stoichiometric amounts of respective oxides missed together and heated to a 8

CHEMM003 GS

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samples regrind and reheat until phase pure material formed Co-precipitation method - higher surface area, heating temperature critical 

Stoichiometric amounts of water soluble salts of 2 oxides (nitrate, chloride, acetate

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salts) dissolved in water and mixed Solution reacted with small amounts of base eg ammonium hydroxide/sodium

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hydroxide to obtain a precipitate Precipitate filtered and dried, then heated to specific temperature to obtain the

crystalline product Hydrothermal method - higher surface area, heating not necessary 

Stoichiometric amounts of water soluble salts of 2 oxides dissolved in water and

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mixed Base added to obtain a precipitate Liquid/gel mixture with precipitate loaded in sealed autoclave and heated to 100-

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specific temperature in air Once cooled XRD collected - if required phase not formed or impurities present,

200 °C for several hours  Once cooled, washed and dried to obtain crystalline product Soft chemical routes: co-precipitation and hydrothermal provide better catalytic system

Reaction pathway with bulk oxide catalyst  Bulk oxides catalysts fairly dense; unlike zeolites - do not have pores/voids  Majority of reactions occur on surface of catalyst  Key steps involved  

Removal of hydrogen attached to C and addition of oxygen to form epoxide Reduction of molybdenum from +6 state

Oxidation of molybdenum back to active +6 state by reacting with molecular O2 Mars van Kravelen mechanism 

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Alkene approaches surface of catalyst Oxidation of alkene with lattice oxygen Replenish of surface with oxygen from bulk = oxygen migrate to fill hole on surface Replenish of bulk with oxygen from gas phase - fill hole in lattice Rate of oxidation depends on temperature - oxygen mobility; at higher

temperature, oxygen too labile - other oxidation processes can take over Lattice oxygen main source for oxidation of hydrocarbon Molecular oxygen provide source for recreating oxidic phase

Bismuth molybdate  Propylene oxidation to acrolein, ammoxidation to actylonitrile 9

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3 catalytically dominant phases: α-Bi2Mo3O12, β-Bi2Mo2O9, γ-Bi2MoO6

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Bi: +3 oxidation state, Mo: +6 oxidation state Preparation: mix oxides Bi2O3 + xMoO3; salt ammonium hepta molybdate + bismuth

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nitrate + NH4OH Role of Bi and Mo  Bi2O3 - highly active but not selective; MoO3 - highly selective but not active Bismuth molybdates - active and selective Proceed through Mars van Krevelen mechanism 

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Cyclic reduction-reoxidation of catalyst Catalyst contain binary/multi component metal oxides

Iron molybdate  Oxidation of methanol to formaldehyde 

Formaldehyde - in nature formed by incomplete combustion/photochemical processes  High chemical reactivity, irreplaceable C1 building block

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2 forms:  ferric form Fe2(MoO4)3 - effective for methanol to formaldehyde oxidation  ferrous form FeMoO4 - formed during redox catalytic process

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Preparation (ferric form): mix oxides Fe2O3 + 3 MoO3; salt ammonium hepta molybdate + iron acetate + NH4OH  Usually based on co-precipitation in aqueous phase  Variables eg Mo and Fe precursors, conc. of initial solutions, order of addition,

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temp, pH, stirring affect physical-chemical characteristics of catalysts and their catalytic behaviour Calcination conditions - major importance