Chapter 3: Hydrolysis and Condensation II - Silicates

Chapter 3: Hydrolysis and Condensation II - Silicates

Outline • Aqueous Silicates • Hydrolysis and Condensation of Silicon Alkoxides • General Trends • Precursor Molecules • Hydrolysis • Condensation • Sol Gel Kinetics • Structural Evolution • Summary • Multicomponent Silicates

Aqueous Silicates Coordination of silicon: Generally silicon have 4 coordination number Coordination expansion does not spontaneously occur Kinetics of reaction is considerably slower than transition metal Electronegativity of silicon Silicon is generally less electropositive Less susceptive to nucleophilic attack δ(Si) in Si(OEt)4 = +0.32 δ(Ti) in Ti(OEt)4= +0.63 δ(Zr) in Zr(OEt)4= +0.65 δ(M): partial positive charge Coordination number and reaction kinetics Condensation by nucleophilic substitution Condensation by nucleophilic addition Z=2 Z=1

Hydrolysis of silicon Silicon have small ionic radius (0.42Å) → Polarizing power of ions are large Easily hydrolyzed Below pH 7 : Si(OH)4 is predominant Above pH 7 : SiO(OH)3-is predominant SiO2 (OH)2- is observed in appreciable quantities only above pH 12

3-stage polymerization of silicates Polymerization of monomer to form particles Growth of particles Linking of particles into chains, network formation and gelation First stage: particle formation Formation of dimer, trimer, tetramer, pentamer, hexamer, ….

Particle growth Large solubility difference between large & small particles Particle growth by Ostwald ripening Network formation Solubility difference between large & small Particles under few ppm Network formation by Particle aggregation

Particle condensation Tends to maximize the number of Si-O-Si bonds minimize the number of terminal hydroxyl groups Particles condense to the most compact state leaving OH groups on the outside

pH-Dependence pH 2 : Isoelectric point pH 7 : At or above pH 7, silica particles are ionized and its solubility is maximized

Polymerization pH 2-7 Between pH 2 ~ pH 6 generally, Increasing pH → Increasing number of [OH-] → Shorter gelation time When basic OR and OH are replaced with OSi, electron density on Si is reduced → Acidity of the protons on the remaining silanols are increased More acidic silanols are more likely to be deprotonated by (4) Rate of dimerization is low but once dimers form they react preferentially Cyclic trimers are much less stable in this pH range because of reduced Si-O-Si bond angles

Polymerization above pH 7 Particles are more likely to be ionized and repulsive Growth by: Addition of monomers > Particle aggregation Above pH 12, most of silanols are deprotonated Planar, cyclic configuration permits the greatest separation of charge between deprotonated sites O- O- O- O- Si Si O O O O O- O- O- O- O O Si Si Si Si O- O- O- O-

Salt concentration Salt addition → Increased electrolyte concentration → Decreased double layer thickness → Repulsion force between particles decreased → Reduced gel times

Polymerization below pH 2 Polymerization rate is quite low and is proportional to [H+] Iler and others propose mechanism involves a siliconium ion intermediate (≡Si+) In fact, on alkoxide polymerization condensation process is more likely proceeds via an ≡Si(OH)R(OH2)+ intermediate (discussed in the following section) Solubility of silica below pH 2 is quite low Growth by: Ostwald ripening < Particle aggregation Ripening contribute little to growth after the particles exceed 2nm in diameter → Gel networks are composed of small primary particles Addtion of F- decrease the gel times (discussed in the following section)

Hydrolysis and Condensation of Silicon Alkoxides General Trends Three reactions are generally used to Describe the sol-gel process Water and alkoxysilanes are immiscible Alcohol is normally used as a homogenizing agent Without added alcohol, gels can be prepared with produced alcohol by-product

Dependence of H2O:Si molar ratio (r) Theoretically r value of 2 is sufficient for complete reaction nSi(OR)4 + 2nH2O → nSiO2 + 4nROH In reality, reaction does not complete even is r >> 2 Instead, a spectrum of intermediate species are generated ([SiOx(OH)y(OR)z]n; where 2x+y+z=4) 1<r<2 with acid catalyst Viscous, spinnable sol (capable of being drawn into a fiber) Weakly branched “polymeric” sols are produced r>2 or with base catalyst Produced solutions that were not spinnable at equivalent viscosities Highly condensed “particulate” sols are produced

Precursor Molecules TEOS and TMOS is most common Tetraalkoxysilanes used in sol-gel process To reduce the functionality of the precursor, organotrialkoxysilane or diorganodialkoxysilane precursor can be used (R’Si(OR3) or R’2Si(OR)2) R’ : nonhydrolyzable organic substituent

Oligomer Precursors Oligomeric precursors can be used to increase the silicate content of a solution or to make tailor-made substructure Steric and inductive characteristics of precursor largely determine reaction kinetics Organically modified silicates Hybrid systems in which several precursor types are combined If organofunctional group is polymerizable, it is possible to form an organic network in addition to the inorganic one

Hydrolysis: Effect of catalysts Hydrolysis is most rapid and complete when catalysts are employed Generally mineral acids are more effective catalyst than equivalent concentration of base In case of γ–glycidoxypropyl-trialkoxysilane, Hydrolysis appears to be both H3O+ and OH- catalyzed In case of tris(2-methoxy-ethoxy)phenylsilane, similar behavior was observed Above pH 10, pH independent hydrolysis rate constant was observed Investigation of TEOS hydrolysis (Aelion et al.,1950): not reliable Reaction rate was most influenced by the strength and concentration of catalysts Secondary, temperature and solvent are important Solvent effect was more significant with basic catalyst

Hydrolysis: Effect of catalysts F- is about the same size as OH- and has ability to increase the coordination of silicon Hydrolysis is catalyzed by 2-step reaction First step: formation of a pentavalent intermediate Second step: Necleophilic attack of water on weaken Si-OR bonding site Proton transfer and elimination of ROH

Hydrolysis: Steric and inductive effects Steric effects Increased length or branch of alkoxy groups → lowered hydrolysis rate Inductive effects Alkyl substitution on alkoxy group → increase electron density on silicon Hydrolysis(OH→OR) or condensation(OH,OR→OSi) → decrease electron density on silicon High rate with basic condition High rate with acidic condition

Hydrolysis: H2O:Si ratio, r Increased r → Acceleration of hydrolysis reaction Low hydrolysis rate Reduced silicate concentration reduced hydrolysis and condensation rate

Hydrolysis: Solvent effects Polarity Determines solubility of silicate More polar solvents are used to solvate polar, tetrafunctional silicate Less polar solvents are used in alkyl-substituted systems Dipole moment Determines the length over which charge on one species can be “felt” by surrounding species Availability of labile protons Protic solvents hydrogen bond to hydroxyl ions or hydronium ions → Reduce the catalytic activity Aproticsolvents do not hydrogen bond to hydroxyl ions or hydronium ions Hydrogen bonding may also influence the hydrolysis reaction or reverse reaction example)

Hydrolysis: Mechanisms Acid-catalyzed hydrolysis By bimolecular nucleophilic displacement reactions (SN2-Si reactions): First, alkoxide is protonated under acidic conditions and electron density is withdrawn from silicon. Silicon is more electrophilic and more susceptible to attack by water Displace of alcohol acompanied by inversion of the silicon tetrahedon Consistent with this mechanism, hydrolysis rate is increased with less steric crowding around silicon Electron-providing substituent help stabilize developing positive charges → Increased hydrolysis rate

Acid-catalyzed hydrolysis without inversion Inversion is influenced by specific bonding configurations In case of cubic octagomer, inversion cannot take place Flank-side attack without inversion

Base-catalyzed hydrolysis Under basic conditions nucleophilic hydroxyl anions attack the silicon atom SN2-Si mechanism with inversion of silicon tetrahedron: In this case, steric factors are more important SN2**-Si or SN2**-Si mechanism involving a stable 5-coordinated intermediate In this case, inductive factors are more important (stabilization of transition state)

Hydrolysis: Transesterification, Reesterification, and hydrolysis Acid-catalyzed reesterification reaction: Acid-catalyzed reesterification reaction involves the protonation of a silanol group Base-catalyzed reesterification reaction involves the deprotonation of alcohol For normal conditions, protonation of silanol groups is much easier than that of alcohols In this reason reesterification proceeds much further under acidic conditions Transesterification Alcohol displaces an alkoxide group Steric and inductive characteristics of the exchanged alkoxide influence reaction kinetics

Condensation: Effects of catalysts Minimum at ~pH 2 corresponds to the isoelectric point of silica pH < 2 : protonation of silanols are involved in condensation pH > 2 : deprotonationof silanols are involved in condensation Under more basic conditions, condensation proceed but gelation does not occur → Paticles grow only until critical size and become stable (Stöber silica particles)

HF catalyst Condensation mechanism may involve a bimolecular intermediate F- increases coordination of silicon to five or six and increases reaction rate Other mechanism proposed: F- displaces an OH-, causing localized attractions to other silanol species → Increasing condensation rate Alternative argument: F-is more electron-withdrawing than OH-, F- substitution reduces the electron density on Si → Making Si more susceptible to nucleophilic attack

Condensation: Steric and inductive effects Generally, increased steric crowding reduces condensation rate Increased silanols on the silicon atom(increasing silanol acidity) → condensation rate increases Acidity of silanol determines isoelectric point ex) reduced acidity by alkyl substituent → IEP toward higher pH value Significantly influencing the pH-dependence of the condensation mechanism (acid- or base catalyzed condensation) Generally, base-catalyzed condensation occurs above pH 2 Inductive effects are predominated by steric effects and less important

Condensation: Effects of solvents Under base-catalyzed condensation conditions, aproticdioxane is unable to hydrogen bond to SiO- And nonpolardioxane does not tend to stabilize reactants → Significantly enhances condensation rate Polar aprotic DMF and acetonitrile, stabilize anionic reactants → Slowing down the reaction Protic solvents methanol and formamide hydrogen bond to SiO- → Slowing down the reaction Formamide forms stronger hydrogend bond but partial hydrolysis of formamide produces ammonia + formic acid → Ammonia increases condensation rate with increased [OH-]

Condensation: Mechanisms Base-catalyzed condensation Attack of a nucleophilicdeprotonatedsilanol on a neutral silicate This mechanism favors reactions between larger, more highly condensed species (More acidic and easier to be deprotonated) Base-catalyzed condensation mechanism involves penta- or hexacoordinated silicon intermediates Proposed mechanisms: These mechanisms can explain pressure-accelerated reaction (In transition state or intermediate state, activation volume is reduced)

Acid-catalyzed condensation Gel times are observed to decrease below the isoelectric point of silica (more protonation) Most basic silanol species (monomers) are most likely to be protonated Condensation occur preferentially between monomers For both base- and acid catalyzed condensation involve pentacoordinate intermediate, condensation reaction kinetics will be influenced by both steric and inductive factors Less steric crowding Electron-withdrawing group for base-catalyzed Electron-providinggroup for base-catalyzed → enhances condensation

Condensation: Effects of reverse reactions Dissolution and alcoholysis of siloxane bonds occurs under basic conditions Unhydrolyzed monomers commonly observed past gel point in base-catalyzed gelation Dissolution of amorphous silica above pH 2 is catalyzed by OH- ions (OH-ions increase the coordination of silicon and weaken surrounding siloxane bonds)

Sol-gel kinetics Again, three reactions (and three reaction rates) 15 distinguishable local chemical environments and 165 rate coefficients (10kh+ 55kcw + 100kca, forward reaction only) Considering next-nearest functional group 1365 distinct local silicon environment 199,290 rate coefficients

Statistical model of Kay and Assink Assumptions: Hydrolysis- and condensation-rate depend only on the functional group reactivity Rate coefficient for a particular species undergoing one of the three reactions is simply the pruduct of a statistical factor and the appropriate functional group rate coefficient ex) kh of (400) = 4x kh of (130) kcw of (040)-(040) = 4x 4xkcw of (310)-(310) → Only 3 rate coefficient required (kh, kcw, kca) Limits : Ignores steric and inductive effects and reverse reactions In case of TMOS and early stage → good agreement In case of TEOS → Inconsistent Statistical model of Kay and Assink is only available model

Structural evolution Acidic conditions In silicates, condensation-rate constant is sufficiently small → Growth is assumed to occur under reaction-limited conditions Condensation occurs by reaction-limited cluster-cluster aggregation Condensation is irreversible after monomer depletion (Dissolution rate is low in acidic environment) → Unable to restructure and fill in voids For r<4, condensation with incompletely hydrolyzed species Alcohol-producing condensation is slower than water-producing Reduced functionality → weakly branched “extended” structure

Basic conditions Dissolution reactions occurs preferentially at weakly-branched Q1 sites → Provide a continual source of monomers Condensation occurs preferentially between monomers (weakly acidic) and clusters (strongly acidic) → Reaction-controlled monomer-cluster growth Understoichiometric additions of water (r<4) cause unhydrolyzed sites in the clusters → Probability of condensation decreases

Multicomponent Silicates Two general approaches: hydrolysis of mixed-alkoxide or metal organic precursors Sequential addition of alkoxides to partially hydrolyzed precursors – more homocondensation Heterocondensation : condense with another species Homocondensation : condense with themselves Al2O3-SiO2 system Coordination of aluminum and the relative amounts of hetero- vs. homocondensation are influenced by both the choice of precursors and the processing conditions (BuO)2-Al-O-Si-(OEt)3 : 62% tetrahedral Al and 38% octahedral Al (BuO)2-Al-O-Si-(OEt)3with no acid : Pentacoordinated aluminum additionally Al-O-Al bonding is predominant Al(OBu) 3 and TEOS : Si-O-Al bonding formation

Borosilicate systems Trigonal boron is very electrophilic compared to silicon and preferentially hydrolyze Water promotes borosiloxane bond hydrolysis (reverse of Eq. 69) Methanol promotes borosiloxane bond alcoholysis(reverse of Eq. 68)

TiO2-SiO2 systems Homocondensation of the silicates is slower than heterocondensation reaction: But Ti(OR)4 catalyzes silanol condensation, promoting homocondensation of the silicate Silanol containing monomer and dimerfastly removed with Ti(OR)4 To ensure homogeneity, mixed-metal precursor is necessary

Chapter 3: Hydrolysis and Condensation II - Silicates