soid state synthesis

8/16/2019 Soid State Synthesis

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Solid State Chemistry: Synthetictechniques

Lectures

2 and 3

4th year undergraduate course

Leigh F. Jones

School of Chemistry

NUI Galway

Rm. 133

Lecture 2: Outcomes

Conventional Solid State Reaction (SSR) Routes

Solid state Reaction rates (SSRs)

Shake and bake methods

Equipment used in SSRs

Problems encountered + ways around them

Advantages and Disadvantages of SSR routes


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Synthetic techniques towards Inorganic Materials

• Inorganic materials are characterised by extended lattices, notdiscrete molecules

• Inorganic materials are mainly prepared in the form of powders andsingle crystals

NOTE: It is important to prepare single phase compounds (no secondarycompounds), as the products cannot be purified.

SYNTHESIS ROUTES

Conventional Routes Non-conventional Routes

Synthesis & Reactions of 3D Inorganics

Rules of thumb…

•Making, breaking, modifying multiple bonds (3D) = generally need lots of E= high Temps

• Very different to making molecules!

Major problems:

1) Purity2) Control of stoichiometry3) High Temp = expensive!

Factors to consider when choosing reactants:1) availability & cost2) purity3) Avoidance of problems!


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Conventional Synthetic Routes

Solid State Reactions (SSR)

• Oldest, simplest and most widely used method is to mixtogether powdered reactants (sometimes they are pressed into

pellets) and heat for prolonged periods of time.

• Some call it ‘shake and bake’

• It is very effective….almost all the new superconductors werefirst prepared via SSR methods

Solid state reaction: the direct reaction of a mixture of starting reagents (usuallypowders) at high temperature (700 - 1600°C)

High temperature provides the necessary energy for the reaction to occur

Products from solid state reactions are alwaysthermodynamically stable compounds

General route = mix components together and heat for extended periods

Solid-solidSolid-liquidSolid-gasLiquid-gas

Gas-gas

“ Shake and Bake”

2 general reaction types:

BaO(s) + TiO2(s) BaTiO3(s)900 °C

“addition” e.g.

“exchange” e.g. ZnS(s) + CdO(s) ZnO(s) + CdS(s)900 °C

In general, they are all EXOTHERMIC.The driving force being the difference between the free energies of

formation ( ∆Gfor ) of the products and reactants.

Conventional synthetic routes…


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MgAl2O4 spinel

• The Spinels are any class of compound of general formula XY2O4

• Crystallise in the cubic crystal system

• Oxide anions exhibit CCP arrangement in the lattice

• X and Y occupy some or all of the tetrahedral and octahedral holes

• X and Y can be divalent, trivalent or quadrivalent:

• Zn, Mg, Fe, Mn, Al, Cr, Ti, Si

•MFe2O4 (M = Fe, Ni, Zn) are used as materials for magnetic recording

(Fe 3+)(Fe2+,Fe 3+)O4 = Magnetit e !

• O2- anions form CCP (FCC) lattice (similar to NaCl).• The metals ( A and B) occupy 1/8th of the tetrahedral sites and ½ (which

alternate) the octahedral sites.•Large cubic cells (~8 Å for MgAl2O4).


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NaCl (Rock-Salt)

Polyhedral representation‘Balls and sticks ’ representation

COMPOUND WITH CCP LATTICES

Cl¯ Na+

For the ions to migrate = ENERGY to overcome lattice E = high T

T = 400ºC-2000ºC.

NB: only true below MPt = “SINTERING”

e.g.

“Exchange” reactions – much more complicated and not understood!

T = diffusion rate = reaction rate

MgO + Al2O3 MgAl2O41400ºC

2 days (AB2O4 = “spinel” )

Making materials from powdersby heating at below their melting

points until the particles adhere toeach other. Common in making

ceramics.


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Decomposing precursors

Use precursors that decompose at low T to gen. metal oxide reactants:

I.E: metal carbonates, CO32-; metal hydroxides, OH ¯ ; metal nitrates, NO3

¯

BaCO3heat

-CO2BaO

heat

+TiO2BaTiO3

Advantage: precursors air-stablevery fine (


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Solid state reaction - Reaction rates

Solid state reactions are usually slow (from 8 hours to several days)

• large amount of structural reorganization

• bonds break and ions migrate through a solid

• Unlike gas phase and solution reactions, the limiting factor in solid-solidreactions is usually diffusion.

• The rate controlling step in a solid state reaction is the diffusion of thecations through the product layer

Why???

1 kx

dt

dxor 2

1't k x

',k k

t

x thickness of product layer

time

rate constantsRate law governing diffusion througha planar layer

A solid-state reaction will not occur until the temperature reaches at least 2/3of the melting point of one of the reactants.

100 200

5

1 0

1 5

2 0

time (hours)

x 2

1 0 6 ( c m 2 )

1500°C

1400°C

1300°C

MgO + Al2O3 MgAl2O4

x = thickness of the product l ayer

Solid state reaction - Temperature dependance

The reaction occurs much more quickly with increasing temperature

….again the synthesis of MgAl2O4 spinel


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Problems encountered using SSRs

• Some starting reagents are chemically reactive and / or contain ions thatdiffuse easily.

• Other problems may then arise such as:

1) Evaporation of a reactant due to high T (e.g. Alkali oxides, Tl2O, PbO, Bi2O3,HgO).

2) Reaction of a reagent with the container (i.e. Transition metal containingvessels).

However, SSRs can be (and has been) used to synthesis thousands of solids.With due care and attention to detail the above problems can be avoided.

Reagents and Equipment

Another example of solid-solid addition: Reagents (synthesis of BaTiO3)

Alkaline earth oxides are moisture sensitive and therefore not used as starting reactants

Hydroxides, nitrates, oxalates and carbonates are often used as starting reactantsinstead of oxides.

Pestle and mortar

• Starting reagents are mixed indesired stoichiometric

amounts.

a newer version is the ball mill:

BaCO3 + TiO2 BaTiO3 (1200°C)

BaO + CO2(g)

BaO + TiO2 BaTiO3

Volatile by

product


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Furnaces (Ovens)

• Starting materials (reagents) are

placed inside crucible and put into hightemperature furnace.

• Furnaces provide the temperatureto initiate and carry on the reaction.

Reaction vessels (crucibles)

Crucibles are made of refractory materials(Al2O3, ZrO2, Pt) for 2 reasons:

1. Chemically inert to the reactants

2. High melting point

Refractory material: retains strengthat high temperatures (> 540 ºC)

Reactions in controlled atmospheres

Compounds containing metals in unstable oxidation state cannot be prepared inair. A reducing or oxidising atmosphere is required.

Experimentally: a gas (H2/N2,O2) is passed overthe reaction mixture in a tubular furnace

H2 extracts O2via formation of

H2O

Reaction in a reducing atmosphere (V5+ V2+)

sVOsOV N / H 2252

0.5/2 O2 is added to thechemical formula

Reaction in an oxidising atmosphere (Ni2+ Ni3+)

s LaNiOs LaNiO O. 352 2


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Advantages:

• Starting materials readily available

• Compounds can be prepared in large amounts

• Easy to carry out the synthetic procedure

• Long duration (can take ages!!)

• Poor chemical homogeneity (sometimes secondary phases are present, asBaTi2O5 in the preparation of BaTiO3)

• Large grain size (often not mono-disperse)

• Use of high temperature (always dangerous and tricky to handle safely)

• Lack of control of products formed

Disadvantages:

Solid State Reactions

Lecture 3: Outcomes

Non conventional syntheticroutes

• Soft chemistry methods

• Precipitation method

• Sol-gel processes

• Inter- and de-intercalation (topotactic process)

• Hydrothermal synthesis

• Zeolites

PRECURSOR ROUTES


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Non-conventional synthetic routes

• Products from solid state reactions are always thermodynamically stablecompounds

Non-conventional synthetic routes can be complementary or an alternative to SSR

Complementary to SSR: Aims to improve

homogeneity and particlesize in thermodynamically

stable compounds

Alt ernative to SSR:The synthesis

of metastable compoundswhich cannot be prepared

by SSR

Co-precipitation Soft chemistry routes

Sol-gel methods

Co-precipitation

Sol-gel Precursor routes

Soft chemistry routes are used to prepare non-thermodynamically stablecompounds (called metastable)

Soft Chemistry Routes (chimie douche)

Soft Chemistry reactions are carried out under moderate conditions(typically T


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The use of low temperature does not provide enough energy to ‘build’new structures (conventional SSR), but useful for modification or tuning

of existing structures.

Soft chemistry reactions are topotactic, i.e. only specific parts of the

starting materials (parent compounds) are targeted.

Advantages

• Design of new compounds with targeted structural (physical) properties

• Synthesis of metastable compounds

Disadvantages • Difficulty in finding appropriate precursors

• Metastable compounds decompose at high temperatures

Products retains the main structural features of the parent compounds

therefore:

Intercalation and deintercalation

DEFINTIONS:Intercalation: The insertion of ions into a 3-D structure.

Deintercalation: The removal of ions from 3-D structure.(commonly involve H+, Li+, Na+ and O2-)

AIMS:

These are carried out to improve on the materials physical properties(i.e. to increase conductivity (ion mobility).

• These processes are topotactic reactions (thus a form of soft chemistry /chimie douche)

• An elegant way to synthesise new materials with extremely similarstructures as their precursors with improved physical properties.

• These are effectively solid-state redox processes


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Intercalation and deintercalation

Insertion of ions = intercalationremoval of ions = deintercalation

• Reduction = cations insertion of the host• Oxidation (anion insertion) of the host

(usually in layered structures)

TiS2 + nBuLi LixTiS2

Intercalation

deintercalation

Li xVS2 VS2

+ Li++ Li+

- Li+

Example of a remarkable physical property transformation via intercalation

(Anatase polymorph)Insulating solid

TiO2 xn-BuLi inhexane

Lithium Titanate superconductor Tc = 13 K

• The Li+ and electrons intercalate inside theanatase structure

• The anatase structure has open 1-D channelssimilar to rutile (right).

Lithiated anatase(no structural change occurred, still an

insulating white solid)

Li xTiO2

Li xTiO2

• Causes structural reorganization.• Now has a spinel structure (like

MgAl2O4 earlier)

• Now a superconductor!!

+ x/2 octane

500 ºC


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Co-precipitation - synthesis of BaTiO3

Precursor: A solution of BaCl2 is added to an aqueous solution of TiO(CO2)2

)]())(([)()()( 2222 sCOOTiO BaaqCOOTiOaq Ba

Barium titanyl oxalate precursor containing Ba2+ and Ti4+

in the correct ratio to form BaTiO3

)(2)(2)()]())(([ 23920

22 gCOgCOs BaTiOsCOOTiO Ba K

(BaTiO3 is an important ferroelectric material used in capacitors)

Decomposes on heating

Precursor Routes (co-precipitation and sol-gel processes)

-a wet technique forming the fabrication of a material: (i.e. a metal oxide)Relatively cheap technique requiring reasonably low T

Sol-Gel synthesis

Colloid: a material consisting of so lid particles (size range:1nm to 1 µm) dispersed in a solution

*The last stage compri ses high T heating o f the gel (firing) to:

• Remove volatile components• Remove any side bonded organics (explained later)

Reagent(s) Sol-gel process

Homogenous amorphoussolid

3-D integrated network

Product

HomogenousSolution(s) or

colloid

Dried(heated)

gradually)

*

Fired at

high TViscous solturning to

gel


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Typical precursors (starting materials / reagents):Metal alkoxides and Metal Chlorides

1) TEOS:

Tetraethylorthosilicate

2) Ti(OiPr)4:

Si(OCH2CH3)4

• Starting reagent for the sol-gel synthesis of SiO2 (quartz)(described in Lecture 1)

Titanium isopropoxide

• Precursor to the sol -gelsynthesis of TiO2

Uses:

• Paint / pigment• Sunscreen

• Food colouring

Extremely high refractive index

Sol-gel synthesis: Mechanisms / processes

Note:

Al (OCH2CH2CH2CH3)3

Si(OCH2CH3)4

Al l covalent liquidsTi(OCH(CH3)2)4

• Covalent liquids used in appropriate ratios / stoichiometries

• Alcohol often added → Promotes miscibility of H2O with Alkoxide

• Water is vital!!.............H2O hydrolyses alkoxides

• This process can be acid and base catalysed (described later)

M(OR) M(OH)


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2) Condensation polymerisation to eliminate H2O

• Hydrolysis occurs in 2 STEPS:

1) Replacement of ¯OR with ¯OH

i.e.

Si(OCH2CH3)4 Si(OCH2CH3)3(OH) + Si(OCH2CH3)2(OH)2 etc

(RO)3-Si-OH + HO-Si(OR)3 (RO)3-Si-O-Si(OR)3

i.e.

ETC

-H2O

Composite + structure + viscosity depends GREATLY on:DEGREE OF HYDROLYSIS + CONDENSATION = need synthetic control

Cross condensation:

-M-OH + HO-M’-

-M-O-M’-

H2O

Consider synthesising a material comprisingtwo (or more cations) M and M'


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ACID CATALYSED: Electrophillic attack (H+ i.e using HCl)

BASE CATALYSED: Nucleophillic substitut ion (¯OH)

Si-(OR)4 + ¯OH Si(OR)3(OH) + ¯OR

Si(OR)4 + H+ + Cl¯ Si(OR)3(OH) + RCl

Calcination (‘firing’ as described earlier in notes):

• Heating (calcine) the gel at relatively high temperatures (200-450°C)

BURNS OFF VOLATILE ORGANICS TO GIVE METAL OXIDE PRODUCT aftercondensation of Si(OH)x species (i.e. to eventually give SiO2 in this instance)

Sol-gel hydrolysis step:May be acid or base catalysed….

Sol-Gel: Alternative synthesis o f MgAl2O4 (spinel)

Mg(OCH3)2 + Al(OCH2CH2CH2CH3)3

(i) Mixing(ii) Hydrolysis(iii) Condensation(iv) Drying

Amorphous GEL

FINAL ‘FIRING’ at 250°C (calcination)leads to thermal decomposition to very

fine particles of SPINEL MgAl2O4 Advantages: Energetically favourable compared to other preparation SSR route

(250° vs. heating at 1400 °C for days).

Disadvantages: Starting materials are very expensive.


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Sol-Gel: Synthesis of ScMnO3

Sc2O3 and MnCO3 separately dissolved in heated aqueous solutions of formic acidto form the formate salts:

Sc(HCOO)3 and Mn(COOH)2·2H2O are added to melted citric acid monohydrateand this results in the formation of a (Sc,Mn) citrate polymer.

Heat to 180°C = Removal of excess water and organics

Heat to 450°C = Formation of an amorphous oxide product

Heat to 690°C = Formation of crystalline ScMnO3

The solid state reaction at 700° C gives a a mixture of binaryoxides:

2Sc(HCOO)3 + 2Mn(COOH)2·2H2O Sc2O3 + Mn2O3 + 5CO2 + 2H2O + H2

O H ) HCOO(Sc)l( HCOOH )s(OSc 2332 326

)l(CO H )s(O H )COOH ( MnO H )l( HCOOH )s( MnCO 322223 222

Hydrothermal Synthesis (used in 2nd step of zeolite production)

Extended to heating the water soln: HYDROTHERMAL

Sealed reaction vessel Advantages:• Controlled heating / cooling rates / pressure• Heat above BPt (super-heating)• Excellent method for obtaining crystalline material (cool slowly)• Con. Oven / microwave (rel.cheap)

ReactionMixture in

H2O

Teflonpot

steelpot


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Hydrothermal Synthesis Excellent technique for crystal growth due tovery slow cooling rates:

• NaOH acts as a “mineraliser”

• SiO2 only sparingly soluble in H2O

• Solubility increases with presence of NaOH

• Solubility also increases with higher Temperature

• NaOH/H2O soln. increases crystal growth rate cf. pure H2O

(NOTE: Solvothermal = carriedout in another solvent (not H2O)

SiO2(solid)

NaOH in H2O SiO2(single crystals)Hydrothermal

bomb

SiO2(solid)

SiO2(single crystals)Hydrothermal

bomb

Solvothermal Oven @ School of Chemistry


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ZEOLITES “ Zein” Greek for “to boil”“ Lithos” Greek for “a stone”

“ A stone that boi ls ”

• A large class of extremely porous Aluminosilicate 3-D extended framework

•Crystalline compounds made both naturally and may be synthesised in the laboratory

• 175 Zeolites known (to date), 80 are naturally occurring.

• Comprise of SiO4 and AlO4 tetrahedral building blocks (think of molecular lego)

• Discovered in 1756 by Alex Cronstedt

AlO4SiO4

=

ZEOLITES: structure SiO4 and AlO4 building blocks:

General formula: My+(SiO2)x(AlO2

¯ )y.nH2O

• Some AlO4 ¯ tetahedra have substituted SiO4 units (neutrally charged).

• Negative charge of AlO4 ¯ is accounted for by the presence of M+ cations.

• These are located within the channels and cavities.


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Synthetic routes to ZEOLITES

1.) Sol-gel technique

A typical reaction is:

NaAl(OH)4 (aq) + Na2SiO3 (aq) + NaOH

GEL

Naw AlxSiyOz zeolite

• The specific zeolite made also depends on thetemplate used: usually an Alkylammonium cation.

• The templates are then calcined or burned off byheating between 300-400°C.

• Particular zeolite formed depends on startingcompostion, temperature and pressure conditions.

(the 3-D frameworkcrystallises around the

template)

Leaving desired inorganic framework (zeolite)

25-175°C in H2O (hydrothermalconditions: heat in high pressureH2O)

1st step: 25°C

• Microporous solids possess largeinterconnected VOIDS and CHANNELS

ZEOLITES: their uses (1)

Large VOIDS and CHANNELS permit:

Very regular pore (channel / void) size of molecular dimensions

→ means that they ideal for separating (or sorting) molecules depending on their size

MOLECULAR SIEVES

•Industrial Catalytic Applications

• Ion exchange (water softening and purifying)

• Gas separation and removal


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ZSM-5

1) HETEROGENOUS CATALYSTS(shape selective)

• ZSM-5 is a high Si / low Al (conc.) zeolite madeby Mobil Oil.

• ZSM-5 is highly acidic (H+ rife in channels) andtherefore used by hydrocarbon interconversion by

petrochemical Industry.

i.e. Xylene isomerisation (carbocation isomerisation)

meta

para


Ion exchange for H2Oseparation and purification

Sodium Zeolite A

• Water hardness stems from presence of Ca2+ and Mg2+ ions in waterwhich do not decompose on boiling the water.

• The loosely bound Na+ in the cavities / channels are readily replaced byCa2+ and Mg2+ ions in aqueous conditions. This process is water softening.

• This is why you will find zeolites in commercial washing powders


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