1. Dr.K.R.Krishnamurthy NCCR,IITM, Chennai
Acidity & Basicity
Dr.K.R.Krishnamurthy
NCCR,IITM, Chennai

2. Acidity/Basicity- Prarameters
Type/Nature of sites
Number/population of sites
Strength/distribution of acid sits

3. 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

4. Acids & Bases Definitions in solution phase Acid Base
pH < pH >7.0
Donates H+ ion Accepts proton/generates (OH)-
Solution Vs Solids – Homogeneous/heterogeneous

5. Acids -Types Arhenius acids Bronsted (-Lowry) acid
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

6. Acids & Bases Lewis acids
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

8. Bronsted-Lewis acid inter conversion

9. 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

10. Acid 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 Weak acid – Extent of dissociation small
pKa < Strong acid – Nearly complete dissociation
Mono protic acid
HA(aq) + H2O(l)    H3O+(aq) + A−(aq)         Ka
Formula
Name
pKa[1] HF
hydrofluoric acid
3.17
H2O
water
15.7
NH3
ammonia 38
CH4
methane 48
Di-protic acid
H2A(aq) + H2O(l)    H3O+(aq) + HA−(aq)       Ka1
HA−(aq) + H2O(l)    H3O+(aq) + A2−(aq)       Ka2

12. 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+ = H log [B] H0 = - log aH+ γB
[BH+] γBH+
H = 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

13. Henderson- Hasselbalch equation- For solutions
Equation can be used to calculate pH of buffer solutions

14. Typical Hammett acidity (Ho) of some strong acids used in catalysis
Hoa
Conc. H2SO4
~ -12
Anhydrous HF
~ -10
SiO2-Al2O3
SiO2-MgO
< + 1.5
SbF5- Al2O3
< -13.2
Zeolite, H-ZSM-5
Zeolite, RE-H-Y
Typical Hammett
acidity (Ho)
of some strong acids
used in catalysis
a : Denotes the strength of the strongest acid sites in solid acids

15. Acids- Ranking as per the strength

16. Measurement 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

17. Indicators used for acidity measurements

19. 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

20. HR indicators Mostly aromatic alcohols
Highly specific for protonic acids
R-OH + H+  R+ + H2O
HR = -log AH+γROH γR+ - log AH2O

23. 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

24. 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

25. 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-NH CH3-CH2= CH 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

26. Pulse chemisorption set up
Ammonia
Helium

27. Laboratory reactors Preliminary screening of catalysts
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 
Liquid
GSV R GC
Preliminary screening of catalysts

28. Chemisorptive titration
Pt adsorbs H2 & O2 reversibly at RT
Titration cycles are possible
Pt + H Pt….H
Pt….H + O 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

29. Typical Ammonia TPD pattern
Plots are deconvoluted to derive WEAK and STRONG acidity

30. Acidity & acid strength distribution
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
H-Deal 3
175 -

31. Ammonia TPD- Finger prints for Zeolites
Effect of SAR
Type of Zeolite

32. Ammonia TPD- Effect of metals on acidity
Al-MFI

33. Ammonia TPD-Effect of heating rate
Two different types of sites

34. Acidity by ammonia TPD- RE HY Samples
Ref.GI.Kapustin et.al,Appl.Catal. 42,239,1988

35. Acidity by ammonia TPD- REHY samples
3.3
8,3
12.8
17.3
A.Corma et.al, Zeolites, 7,561,1987

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

37. Heat of asdorption of ammonia
Ref.GI.Kapustin et.al,Appl.Catal. 42,239,1988

38. 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

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

41. Acidic and basic sites in aluminaH+ H OH OH O- O O-
-Heat +
-O-Al-O-Al-O
-O-Al-O-Al-O
-O-Al-O-Al-O
+H2O
Bronsted acid site
Basic site
-H2O
Lewis
Acid 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

43. 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

44. Surface hydroxyls by IR Spectroscopy
JW.Ward, J.Catalysis, 9,225,1967

45. Surface hydroxyls- Effect of temperature
JW.Ward, J.Catalysis, 9,225,1967

46. 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 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
..↓

47. 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

51. 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
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

53. Basicity 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= ~ 14
pKb = 14 - pKa

54. 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

55. Indicators for basicity measurement
Colour
Acid form Basic form
pKa*
Bromothymol blue
Yellow
Green
7.2
Phenolphthalein
Colorless
Red
9.3
2,4,6,Trinitroaniline
Reddish orange
12.2
2,4,Dinitroaniline
Violet
15.0
4Chloro-2-nitroaniline
Orange
17.2
4-Nitroaniline
18.4
4.Chloroaniline
Pink
26.5
* pKa of indicator

56. Basicity & activity
RJ.Davis, Res.Chem.Intermed.26,21,2000

57. 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-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

58. 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

59. 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

60. Solid Super bases
H.Gorzawski & W.F.Hoelderich, J.Mol.Catal. 144, 181,1999

61. Shape selective base catalysts
J Zhu et.al, Catal.Today, 51,103,1999

62. 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

63. 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