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

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Presentation on theme: "Acidity & Basicity Dr.K.R.Krishnamurthy NCCR,IITM, Chennai."— Presentation transcript:

1 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 Si 4+ by Al 3+ 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 7.0 Donates H + ion Accepts proton/generates (OH) - Solution Vs Solids – Homogeneous/heterogeneous

5 Acids -Types Arhenius acids An acid when dissolved in water gives hydronium ion as per the equilibrium 2H 2 O (l) ↔ H 3 O + (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 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 BF 3 - 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)

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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 - K a = [H + ] [A - ] Higher K a stronger the acid/ ability to loose proton [HA] pK a = -log 10 (K a ) Lower pK a stronger the acid pK a = -2 to 12 Weak acid – Extent of dissociation small pK a < -2 Strong acid – Nearly complete dissociation FormulaNamepKa[1] HFhydrofluoric acid3.17 H2OH2Owater15.7 NH 3 ammonia38 CH 4 methane48 Mono protic acid HA(aq) + H 2 O(l) H 3 O + (aq) + A − (aq) K a Di-protic acid H 2 A(aq) + H 2 O(l) H 3 O + (aq) + HA − (aq) K a1 HA − (aq) + H 2 O(l) H 3 O + (aq) + A 2− (aq) K a2

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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 + a B a H + Acid dissociation constant K BH + = a BH + = a H + [B] γ B [BH + ] γ BH + log K BH + = log a H + γ B + log [B] γ BH + [BH + ] pK aBH + = H 0 - log [B] H 0 = - log a H + γ B [BH + ] γ BH + H 0 = pK BH + + log [B] H 0 – Hammet acidity function [BH + ] Similar to Henderson-Hasselbalch equation for pH At equivalence point [B] = [BH + ], pK BH + = H 0 γ B & γ BH+ - Activity coefficients

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

14 Acid HoaHoa Conc. H 2 SO 4 ~ -12 Anhydrous HF~ -10 SiO 2 -Al 2 O SiO 2 -MgO< SbF 5 - Al 2 O 3 < Zeolite, H-ZSM Zeolite, RE-H-Y a : Denotes the strength of the strongest acid sites in solid acids Typical Hammett acidity (H o ) of some strong acids used in catalysis

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 pK a values one can obtain an acid strength distribution in terms of H 0 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 pK a value of the indicator gives the strength of the sites

17 Indicators used for acidity measurements

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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 H R indicators Mostly aromatic alcohols Highly specific for protonic acids R-OH + H +  R + + H 2 O H R = -log A H+ γ ROH γ R + - log A H2O

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23 a) Adsorption of bases Heat of ads. of NH 3 on two acid catalysts 2. Adsorption – desorption of bases (TPD) Difficult to relate reaction requirement to heat of adsorption

24 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: NH 3, non- reactive vapors and reactive vapors. Advantages and disadvantages of NH 3 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 NH 3 also capable of adsorbing additional NH 3 from gas phase. Temperature Programmed Desorption methods Probe molecules- Ammonia, Amines – For acidity Acids ( Acetic/Benzoic),CO 2 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. CH 3 -CH 2 -CH 2 -NH 2 CH 3 -CH 2 = CH 2 + NH 3 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 Helium Ammonia

27 Laboratory 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 GSV  Liquid

28 Chemisorptive titration Pt adsorbs H 2 & O 2 reversibly at RT Titration cycles are possible Pt + H Pt….H Pt….H + O 2 Pt…O +H Pt…O +3H Pt….H + H 2 O O 2 & H 2 cycles to be repeated up to saturation H 2 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 SampleSi/Al Weak acidity (meq/g) Strong acidity (meq/g) H-Beta H-Deal H-Deal H-Deal

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

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, 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, V s - Sample volume Ref.GI.Kapustin et.al,Appl.Catal. 42,239,1988

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41 -O-Al-O-Al-O OO-O- +H 2 O -Heat -O-Al-O-Al-O + OH Lewis Acid site Basic site Bronsted acid site Basic site -H 2 O -O-Al-O-Al-O O-O- OH H 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

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43 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 Mol. Seives, as synthesized- in Na form H- Protonic form has maximum acidity Generation of H-form- NaY  NH 4 Y  H-Y

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 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+H+ Pyridinium ion..↓ Coordinately bound Pyridine

47 Effect of calcination of NH 4 Y- Bronsted & Lewis acid sites 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)

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51 Sizeo.d. 4", height 3.75" Operating Pressures10 -5 torr - 15 atm MaterialStainless Steel Windows CaF 2 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 GasketsViton O-rings In-situ- IR cell for reaction/adsorption

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53 Basicity Base- Ability to form (OH) - ion 2H 2 O  H 3 O + + OH - B + H 2 O  BH + + OH - K w = [H 3 O + ] [OH - ] K b = [BH + ] [OH - ] [H 2 O] 2 [B] Since water concn. is constant K w = [H + ] [OH - ] & [OH - ] = K w / [H + ] -logK w = -log[H + ] –log[OH - ] K b = [BH + ] K w = K w pK w = pH + pOH [B] [H + ] K a pK b = pK w - pK a At 25 ºC pK w = ~ 14 pK b = 14 - pK a

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 - = pK BH + log [B - ] ; When B - = BH, H - = pK BH [BH] Basic strength H - is the equivalent term for acid strength H 0 Approx. value of basic strength is given by the pK a 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 Indicators Colour Acid form Basic form pK a * Bromothymol blueYellowGreen7.2 PhenolphthaleinColorlessRed9.3 2,4,6,TrinitroanilineYellowReddish orange12.2 2,4,DinitroanilineYellowViolet15.0 4Chloro-2- nitroaniline YellowOrange NitroanilineYellowOrange ChloroanilineColorlessPink26.5 * pK a 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 ZrO 2 Addition of B 2 O 3 increases acidity Acidity by Ammonia TPD & basicity by Acetic acid TPD J.Fung & I.Wang, Appl.Catal. A166,327,1998

63 Acidity & Basicity of ZrO 2 Addition of K 2 O increases Basicity Acidity by Ammonia TPD & basicity by Acetic acid TPD J.Fung & I.Wang, Appl.Catal. A166,327,1998


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