Acidity & Basicity
Dr.K.R.Krishnamurthy
NCCR,IITM, Chennai
Acidity/Basicity- Prarameters
Type/Nature of sites
 Number/population of sites
 Strength/distribution of acid sits
Bronsted-Lewis acid interconversion
Substitution of Si4+ by Al3+
Excess electron balanced by proton
attached with Al-O-Si bridge
Surface hydroxyls- Bronsted sites
On de-hydroxylation form Lewis sites
Acids & Bases
• Definitions in solution phase
Acid
pH < 7. 0
Donates H+ ion
•
Base
pH >7.0
Accepts proton/generates (OH)-
Solution Vs Solids – Homogeneous/heterogeneous
Acids -Types
Arhenius acids
An 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) acid
Acids 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 & Bases
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)
Lewis acids
A Lewis acid accepts a pair of
electrons from other species
Bronsted acids transfer protons
while Lewis acids accept electrons
A Lewis base transfers a pair of
electrons to other species
BF3- Lewis acid; Ammonia- Lewis base
Bronsted-Lewis acid inter conversion
Acidity 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 sites
Beyond 450ºC, de-hydroxylation takes place resulting in Structure III leading to
Lewis acid sites, tri-co-ordinated Al - Lewis acid site
Acid dissociation
HA ↔ H+ + A-
Ka = [H+] [A-]
[HA]
pKa = -log10(Ka)
pKa = -2 to 12
pKa < -2
Lower pKa stronger the acid
Weak acid – Extent of dissociation small
Strong acid – Nearly complete dissociation
Mono protic acid
HA(aq) + H2O(l)
H3O+(aq) + A−(aq)
Higher Ka stronger the acid/ ability to loose proton
Ka
Di-protic acid
H2A(aq) + H2O(l)
HA−(aq) + H2O(l)
H3O+(aq) + HA−(aq)
H3O+(aq) + A2−(aq)
Ka1
Ka2
Formula
Name
pKa[1]
HF
hydrofluoric acid
3.17
H2O
water
15.7
NH3
ammonia
38
CH4
methane
48
Strength of an acid
Defined 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]
[BH+]
γB & γBH+ - Activity coefficients
H0 = - log aH+ γB
γBH+
+ log [B]
H0 – Hammet acidity function
[BH+]
Similar to Henderson-Hasselbalch equation for pH
At equivalence point [B] = [BH+] , pKBH+ = H0
H0
= pKBH+
Henderson- Hasselbalch equation- For solutions
Equation can be used to calculate pH of buffer solutions
Typical Hammett
acidity (Ho)
of some strong acids
used in catalysis
a
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
: Denotes the strength of the strongest acid sites in solid acids
Acids- Ranking as per the strength
Measurement of acidity
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•
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•
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 measurements
Acidity 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
HR indicators
Mostly aromatic alcohols
Highly specific for protonic acids
R-OH + H+  R+ + H2O
HR = -log AH+γROH γR+ - log AH2O
2. Adsorption – desorption of bases (TPD)
a) Adsorption of bases
Heat of ads. of NH3
on two acid catalysts
Difficult to relate
reaction requirement
to heat of adsorption
Temperature Programmed Desorption methods
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.
Probe molecules- Ammonia, Amines – For acidity
Acids ( Acetic/Benzoic),CO2 For basicity
TPD 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 up
Ammonia
Helium
Laboratory reactors
Pulse micro reactor
•
•
•
•
•
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•
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

Liquid
GSV
R
GC
Preliminary screening of catalysts
Chemisorptive titration
•
•
Pt adsorbs H2 & O2 reversibly at RT
Titration cycles are possible
Pt + H
Pt….H + O2
Pt….H
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 pattern
Plots are deconvoluted to derive
WEAK and STRONG acidity
H -B eta
6
D esorp tion
4
2
0
1 00
2 00
3 00
4 00
o
T em p eratu re( C )
5 00
6 00
Acidity & acid strength distribution
H-Beta
D-Beta 34
D-Beta 46
D-Beta 175
118
Desorption
116
114
112
110
108
100
200
300
400
Temperature(oC)
500
600
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
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-
Ammonia TPD- Finger prints for Zeolites
Type of Zeolite
Effect of SAR
Ammonia TPD- Effect of metals on acidity
Al-MFI
Ammonia TPD-Effect of heating rate
Two different types of sites
Acidity by ammonia TPD- RE HY Samples
Ref.GI.Kapustin et.al,Appl.Catal. 42,239,1988
Acidity by ammonia TPD- REHY samples
3.3
8,3
12.8
17.3
A.Corma et.al, Zeolites, 7,561,1987
Ref.GI.Kapustin et.al,Appl.Catal. 42,239,1988
Heat of asdorption of ammonia
Ref.GI.Kapustin et.al,Appl.Catal. 42,239,1988
Heats of adsorption –TPD & Microcalorimetry
Eqn-3
Eqn-4
Calculation of F* & V* based on TPD patterns; F Flow rate, Vs- Sample volume
Ref.GI.Kapustin et.al,Appl.Catal. 42,239,1988
Ref.GI.Kapustin et.al,Appl.Catal. 42,239,1988
H+
H
OH OH
-Heat
-O-Al-O-Al-O
O-
O
+
-O-Al-O-Al-O
-O-Al-O-Al-O
+H2O
-H2O
Lewis
Acid site
O-
Bronsted
acid site
Basic site
Basic site
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 Spectroscopy
Mol. Seives, as synthesized- in Na form
H- Protonic form has maximum acidity
Generation of H-form- NaY  NH4Y  H-Y
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 sites
Beyond 450ºC, de-hydroxylation takes place resulting in Structure III leading to
Lewis acid sites, tri-co-ordinated Al - Lewis acid site
Surface hydroxyls by IR Spectroscopy
JW.Ward, J.Catalysis, 9,225,1967
Surface hydroxyls- Effect of temperature
JW.Ward, J.Catalysis, 9,225,1967
IR 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
Pyridinium ion
N
H+
Coordinately bound Pyridine
N
..↓
Effect of calcination of NH4Y- Bronsted & Lewis acid
sites evolution- IR spectra of adsorbed Pyridine
Decrease in intensity of 1545 cm-1 peak (Bronsted acid sites) &
Appearance of peak at 1451cm-1( Lewis acid sites)
JW.Ward, J.Catalysis, 9,225,1967
In-situ- IR cell for reaction/adsorption
Size
o.d. 4", height 3.75"
Operating Pressures
10-5 torr - 15 atm
Material
Stainless Steel
Windows
CaF2 or any other
standard IR
transparent material
Catalyst Sample Size
2 cm o.d., typically
80 mg of solid
One minithermocouple for
Temperature
reactor body temp
Control/Measurement control and one for
sample surface
measurement
Flow Pattern:
Gases are flown
parallel on both sides
of the wafer
Gaskets
Viton O-rings
Basicity
Base- Ability to form (OH)- ion
2H2O  H3O+ + OHB + H2O  BH+ + OHKw = [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
Basicity
Basic 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 measurement
Indicators
pKa*
Colour
Acid form
Basic form
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-2nitroaniline
Yellow
Orange
17.2
4-Nitroaniline
Yellow
Orange
18.4
4.Chloroaniline
Colorless
Pink
26.5
* pKa of indicator
Basicity & activity
RJ.Davis, Res.Chem.Intermed.26,21,2000
Basicity 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-chloroaniline-(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 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–benzene
carboxylic acid titration was used
Solid 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 bases
H.Gorzawski & W.F.Hoelderich, J.Mol.Catal. 144, 181,1999
Shape selective base catalysts
J Zhu et.al, Catal.Today, 51,103,1999
Acidity & Basicity of ZrO2
Addition of B2O3 increases
acidity
Acidity by Ammonia TPD &
basicity by Acetic acid TPD
J.Fung & I.Wang, Appl.Catal.
A166,327,1998
Acidity & Basicity of ZrO2
Addition of K2O increases
Basicity
Acidity by Ammonia TPD &
basicity by Acetic acid TPD
J.Fung & I.Wang, Appl.Catal.
A166,327,1998