Acidity & BasicityAcidity & Basicity

Dr.K.R.Krishnamurthy

NCCR,IITM, Chennai

Acidity/Basicity- PrarametersType/Nature of sites Number/population of sites Strength/distribution of acid sits

Bronsted-Lewis acid interconversionBronsted-Lewis acid interconversion

Substitution of Si4+ by Al3+

Excess electron balanced by protonattached with Al-O-Si bridgeSurface hydroxyls- Bronsted sitesOn de-hydroxylation form Lewis sites

Acids & BasesAcids & Bases• Definitions in solution phase

Acid Base pH < 7. 0 pH >7.0

Donates H+ ion Accepts proton/generates (OH)-

• Solution Vs Solids – Homogeneous/heterogeneous

Acids -TypesAcids -Types

Arhenius acidsAn acid when dissolved in water gives hydronium ion as per the equilibrium

2H2O(l) ↔ H3O+(aq) + OH-

(aq)

Proton, H+, is stable in solution phase, only in hydrated form.

Bronsted (-Lowry) acidAcids can transfer protons - Donation of proton to water in solution by acetic acid

ie., produces an hydronium ion

In reaction with ammonia it

does not produce hydronium

Ion; but donates a proton

to ammonia forming

ammonium ion

Acids & BasesAcids & Bases

Lewis acidsA Lewis acid accepts a pair of electrons from other species

Bronsted acids transfer protonswhile Lewis acids accept electrons

A Lewis base transfers a pair of electrons to other species BF3- Lewis acid; Ammonia- Lewis base

Proton transfer reactions occur w/o hydronium ion H3O+(aq) + Cl-(aq) + NH3 → Cl-(aq) + NH4+(aq) HCl(benzene) + NH3(benzene) → NH4Cl(s)

HCl(g) + NH3(g) → NH4Cl(s)

Bronsted-Lewis acid inter conversionBronsted-Lewis acid inter conversion

Acidity by IR SpectroscopyAcidity by IR Spectroscopy

On heating ammoniated form hydroxyl groups are formed which display IR bands at 3742,3643 &3540 cm-1 as shown in structures I & II- Bronsted acid sitesBeyond 450ºC, de-hydroxylation takes place resulting in Structure III leading to Lewis acid sites, tri-co-ordinated Al - Lewis acid site

Acid dissociationAcid dissociation

HA ↔ H+ + A-

Ka = [H+] [A-] Higher Ka stronger the acid/ ability to loose proton

[HA]

pKa = -log10(Ka) Lower pKa stronger the acid

pKa = -2 to 12 Weak acid – Extent of dissociation small

pKa < -2 Strong acid – Nearly complete dissociation

Formula Name pKa[1]

HF hydrofluoric acid 3.17

H2O water 15.7

NH3 ammonia 38

CH4 methane 48

Mono protic acidHA(aq) + H2O(l)    H3O

+(aq) + A−(aq)         Ka

Di-protic acidH2A(aq) + H2O(l)    H3O

+(aq) + HA−(aq)       Ka1

HA−(aq) + H2O(l)    H3O+(aq) + A2−(aq)       Ka2

Strength of an acidStrength of an acidDefined as the ability of a solid acid to convert an adsorbed neutral base to

its conjugate acid

B + H+ BH+

aB aH+

Acid dissociation constant K BH+ = aBH

+ = aH+ [B] γB

[BH+] γBH+

log KBH+ = log aH

+ γB + log [B]

γBH+ [BH+]

pKaBH+ = H0 - log [B] H0 = - log aH

+ γB

[BH+] γBH+

H0 = pKBH+ + log [B] H0 – Hammet acidity function

[BH+]

Similar to Henderson-Hasselbalch equation for pH

At equivalence point [B] = [BH+] , pKBH+ = H0

γB & γBH+ - Activity coefficients

Henderson- Hasselbalch equation- For solutionsHenderson- Hasselbalch equation- For solutions

Equation can be used to calculate pH of buffer solutions

Acid Hoa

Conc. H2SO4 ~ -12

Anhydrous HF ~ -10

SiO2-Al2O3 - 8.2 - 10

SiO2-MgO < + 1.5

SbF5- Al2O3 < -13.2

Zeolite, H-ZSM-5 -8.2 - 13

Zeolite, RE-H-Y -8.2 - 13

a : Denotes the strength of the strongest acid sites in solid acids

Typical Hammett acidity (Ho)

of some strong acidsused in catalysis

Acids- Ranking as per the strengthAcids- Ranking as per the strength

Measurement of acidityMeasurement of acidity

• In heterogeneous catalysts acid sites of different strengths exist

• By titrating a catalyst with a series of indicators with different pKa values one can obtain an acid strength distribution in terms of H0

• Known quantity of catalyst is dried and covered with inert solvent ( Benzene,Iso-octane)

• Few drops of an indicator is added, that gives specific colour

• Followed by titration with n- butyl amine, allowing sufficient time for equilibration after every addition

• End point is indicated by the indicator colour change

• Quantity of amine taken up indicates total acidity and the pKa value of the indicator gives the strength of the sites

Indicators used for acidity measurementsIndicators used for acidity measurements

Acidity using indicatorsAcidity using indicators

• Activity coefficients are seldom equivalent to unity

• Colour changes in some indicators are not associated with protonic acidity

• Coloured samples could not be used

• Presence of moisture interferes with measurement-competes with indicator

• End point detection is visual

HHRR indicators indicators

Mostly aromatic alcohols Highly specific for protonic acids

R-OH + H+ R+ + H2O

HR = -log AH+γROH γR+ - log AH2O

a) Adsorption of bases

Heat of ads. of NH3 on two acid catalysts

2. Adsorption – desorption of bases (TPD)

Difficult to relatereaction requirement to heat of adsorption

Determining the quantity and strength of the acid sites on catalysts like silica-alumina, zeolites, mixed oxides is crucial to understand and predict performance.

For some of acid catalyzed reactions, the rate of reaction linearly related to acid sites.

There are three types of probe molecules for TPD: NH3, non- reactive vapors and reactive vapors.

Advantages and disadvantages of NH3 as a probe

Its molecular size facilitates access into all pores in a solid. It is highly basic, hence titrates even weak acid sites. Strongly polar adsorbed NH3 also capable of adsorbing additional NH3 from gas phase.

Temperature Programmed Desorption methodsTemperature Programmed Desorption methods

Probe molecules- Ammonia, Amines – For acidity Acids ( Acetic/Benzoic),CO2 For basicity

TPD of ammonia & aminesTPD of ammonia & amines

Large non-reactive amines such as pyridine and t-butyl amine are alternative to NH3.

They titrate only the strong and moderate acid sites.

Though pyridine chemisorption studies by IR spectroscopy is most appropriate, lack of extinction coefficient data complicates.

Most commonly used are propyl amines.

It reacts and decompose to propylene and ammonia over B-acid sites.

CH3-CH2-CH2-NH2 CH3-CH2= CH2 + NH3

Amines are known to decompose to higher temperature; hence may not desorb as amines; This aspect to be kept in mind in analysis of TPD patterns of amines

Even in the case of ammonia at T> 600ºC ammonia may decompose

Quantitative analysis to be carried out with caution

Pulse chemisorption set upPulse chemisorption set up

Helium

Ammonia

Laboratory reactorsLaboratory reactors

Pulse micro reactor• Small amount of catalyst (mg) / reactants (µl)• Reactants are injected as liquid/gas pulses• Carrier gas (CG) takes the reactant vapors to

the catalyst bed • Reactor effluent directly enters GC for

analysis • Direct comparison of reactant concentration

-before & after the reaction• The reaction takes place under non- steady

state conditions• Useful for fast screening of catalysts

CG

GC

R

Preliminary screening of catalysts

GSVLiquid

Chemisorptive titrationChemisorptive titration

• Pt adsorbs H2 & O2 reversibly at RT

• Titration cycles are possible

Pt + H Pt….H

Pt….H + O2 Pt…O +H

Pt…O +3H Pt….H + H2O O2 & H2 cycles to be repeated up to saturation

H2 consumed in titration is 3 times higher than that in chemisorption

Typical Ammonia TPD patternTypical Ammonia TPD pattern

100 200 300 400 500 6000

2

4

6 H-Beta

Des

orpt

ion

Temperature(o C)

Plots are deconvoluted to derive WEAK and STRONG acidity

Acidity & acid strength distributionAcidity & acid strength distribution

100 200 300 400 500 600

108

110

112

114

116

118 H-Beta D-Beta 34 D-Beta 46 D-Beta 175

Deso

rptio

n

Temperature(oC)

Sample Si/Al Weak acidity (meq/g)

Strong acidity (meq/g)

H-Beta 15 0.55 0.66

H-Deal 1 34 0.21 0.30

H-Deal 2 46 0.09 0.30

H-Deal 3 175 - -

Ammonia TPD- Finger prints for ZeolitesAmmonia TPD- Finger prints for Zeolites

Type of Zeolite Effect of SAR

Ammonia TPD- Effect of metals on acidityAmmonia TPD- Effect of metals on acidity

Al-MFI

Ammonia TPD-Effect of heating rateAmmonia TPD-Effect of heating rate

Two different types of sites

Acidity by ammonia TPD- RE HY SamplesAcidity by ammonia TPD- RE HY Samples

Ref.GI.Kapustin et.al,Appl.Catal. 42,239,1988

Acidity by ammonia TPD- REHY samplesAcidity by ammonia TPD- REHY samples

3.38,312.817.3

A.Corma et.al, Zeolites, 7,561,1987

Ref.GI.Kapustin et.al,Appl.Catal. 42,239,1988

Heat of asdorption of ammoniaHeat of asdorption of ammonia

Ref.GI.Kapustin et.al,Appl.Catal. 42,239,1988

Heats of adsorption –TPD & MicrocalorimetryHeats of adsorption –TPD & Microcalorimetry

Eqn-3Eqn-4

Calculation of F* & V* based on TPD patterns; F Flow rate, Vs- Sample volumeRef.GI.Kapustin et.al,Appl.Catal. 42,239,1988

Ref.GI.Kapustin et.al,Appl.Catal. 42,239,1988

-O-Al-O-Al-O

OO-

+H2O

-Heat-O-Al-O-Al-O

+OH

LewisAcid site

Basic site

Bronsted acid site

Basic site-H2O

-O-Al-O-Al-O

O-OH

H H+

Acidic and basic sites in alumina Surface hydroxyl groups can have different environ ments ie., OH groups surrounded by 4 , 3, 2 ,1,0 -oxide ions as neighbors Accordingly net charge on O- in OH group varies Basicity/acidity varies accordingly Alumina displays 5 different surface hydroxyl groups characterized by IR absorption bands, at 3800, 3780,3744,3733,3700 cm-1

These bands can be observed by in-situ IR spectroscopy of alumina after proper activation – heating in vacuum at > 300C

Acidity by IR SpectroscopyAcidity by IR Spectroscopy

On heating ammoniated form hydroxyl groups are formed which display IR bands at 3742,3643 &3540 cm-1 as shown in structures I & II- Bronsted acid sitesBeyond 450ºC, de-hydroxylation takes place resulting in Structure III leading to Lewis acid sites, tri-co-ordinated Al - Lewis acid site

Mol. Seives, as synthesized- in Na formH- Protonic form has maximum acidityGeneration of H-form- NaY NH4Y H-Y

Surface hydroxyls by IR SpectroscopySurface hydroxyls by IR Spectroscopy

JW.Ward, J.Catalysis, 9,225,1967

Surface hydroxyls- Effect of temperatureSurface hydroxyls- Effect of temperature

JW.Ward, J.Catalysis, 9,225,1967

IR data on Pyridine adsorbed on acid sitesIR data on Pyridine adsorbed on acid sites

On Bronsted acid sites, Pyridine gets adsorbed as Pyridinium ion with very

strong IR absorption band at 1545 cm-1

On Lewis acid sites, Pyridine gets adsorbed coordinately through the lone pair on N, forming very strong IR absorption band at 1451 cm-1

N

N

H+

Pyridinium ion

..↓

Coordinately bound Pyridine

Effect of calcination of NHEffect of calcination of NH44Y- Bronsted & Lewis acid Y- Bronsted & Lewis acid

sites evolution- IR spectra of adsorbed Pyridinesites evolution- IR spectra of adsorbed Pyridine

JW.Ward, J.Catalysis, 9,225,1967

Decrease in intensity of 1545 cm-1 peak (Bronsted acid sites) &Appearance of peak at 1451cm-1( Lewis acid sites)

Size o.d. 4", height 3.75"

Operating Pressures 10-5 torr - 15 atm

Material Stainless Steel

WindowsCaF2 or any other standard IR transparent material

Catalyst Sample Size2 cm o.d., typically 80 mg of solid

Temperature Control/Measurement

One mini-thermocouple for reactor body temp control and one for sample surface measurement

Flow Pattern:Gases are flown parallel on both sides of the wafer

Gaskets Viton O-rings

In-situ- IR cell for reaction/adsorption

BasicityBasicity

Base- Ability to form (OH)- ion 2H2O H3O+ + OH-

B + H2O BH+ + OH- Kw = [H3O+] [OH-]

Kb = [BH+] [OH-] [H2O]2

[B] Since water concn. is constant

Kw = [H+] [OH-] & [OH-] = Kw/ [H+]

-logKw = -log[H+] –log[OH-]

Kb = [BH+] Kw = Kw pKw = pH + pOH

[B] [H+] Ka

pKb = pKw- pKa

At 25 ºC pKw= 13.9964 ~ 14

pKb = 14 - pKa

BasicityBasicityBasic strength of a solid surface is defined as its ability to convert an

adsorbed electrically neutral acid to its conjugate base

This signifies the ability of the surface to donate an electron pair to the

adsorbed acid

For the reaction of an acid indicator BH with a solid base B

BH + B B- + BH+

Basic strength H- = pKBH + log [B-] ; When B- = BH, H- = pKBH

[BH]

Basic strength H- is the equivalent term for acid strength H0

Approx. value of basic strength is given by the pKa value of the indicator at which color changes

Amount of basic sites can be measured by titrating a suspension of the solid

base in Benzene/iso-Octane containing an indicator (in its conjugate basic form) with benzoic acid in benzene

Basicity is expressed in terms of mmolg-1 or mmolm-2 of benzoic acid

Indicators for basicity measurementIndicators for basicity measurement

Indicators Colour

Acid form Basic form

pKa*

Bromothymol blue Yellow Green 7.2

Phenolphthalein Colorless Red 9.3

2,4,6,Trinitroaniline Yellow Reddish orange 12.2

2,4,Dinitroaniline Yellow Violet 15.0

4Chloro-2-nitroaniline

Yellow Orange 17.2

4-Nitroaniline Yellow Orange 18.4

4.Chloroaniline Colorless Pink 26.5

* pKa of indicator

Basicity & activityBasicity & activity

RJ.Davis, Res.Chem.Intermed.26,21,2000

Basicity Vs Transesterification for BiodieselBasicity Vs Transesterification for Biodiesel

Basic strength, H- measured using Hammett indicators; dimethylaminoazobenzene(H_=3.3), phenolphthalien (H_=8.2), 2,4-dinitroaniline, (H_=15), nitroaniline (H_=18.4) and 4-chloro-aniline-(H_=26.5).Basicity measured by titration of dryvmethanolic slurry of catalyst against carboxylic acid

W.Xie & X.Huang, Catal.Lett., 107,53, 2006

Basicity & catalytic acivityBasicity & catalytic acivity

Hammett indicators: Dimethylaminoazobenzene (H =3.3), Phenolphthalein (H =8.2), 2,4-dinitroaniline (H =15), and nitroaniline (H =18.4). For basicity of the catalysts, the method of Hammett indicator–benzenecarboxylic acid titration was used

Solid Super basesSolid Super bases

Basic strength measured using Hammett indicators and basicity by benzoic acid titration

H.Gorzawski & W.F.Hoelderich, J.Mol.Catal. 144, 181,1999

Solid Super basesSolid Super bases

H.Gorzawski & W.F.Hoelderich, J.Mol.Catal. 144, 181,1999

Shape selective base catalystsShape selective base catalysts

J Zhu et.al, Catal.Today, 51,103,1999

Acidity & Basicity of ZrOAcidity & Basicity of ZrO22

Addition of B2O3 increases acidityAcidity by Ammonia TPD & basicity by Acetic acid TPDJ.Fung & I.Wang, Appl.Catal.A166,327,1998

Acidity & Basicity of ZrOAcidity & Basicity of ZrO22

Addition of K2O increases BasicityAcidity by Ammonia TPD & basicity by Acetic acid TPDJ.Fung & I.Wang, Appl.Catal.A166,327,1998