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

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

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Acidity & Basicity Dr.K.R.Krishnamurthy NCCR,IITM, Chennai Slide 2 Acidity/Basicity- Prarameters Type/Nature of sites Number/population of sites Strength/distribution of acid sits Slide 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 Slide 4 Acids & Bases Definitions in solution phase Acid Base pH 7.0 Donates H + ion Accepts proton/generates (OH) - Solution Vs Solids Homogeneous/heterogeneous Slide 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 Slide 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) Slide 7 Slide 8 Bronsted-Lewis acid inter conversion Slide 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 450C, de-hydroxylation takes place resulting in Structure III leading to Lewis acid sites, tri-co-ordinated Al - Lewis acid site Slide 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 Slide 11 Slide 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] B [BH + ] BH + log K BH + = log a H + B + log [B] BH + [BH + ] pK aBH + = H 0 - log [B] H 0 = - log a H + B [BH + ] 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 B & BH+ - Activity coefficients Slide 13 Henderson- Hasselbalch equation- For solutions Equation can be used to calculate pH of buffer solutions Slide 14 Acid HoaHoa Conc. H 2 SO 4 ~ -12 Anhydrous HF~ -10 SiO 2 -Al 2 O 3 - 8.2 - 10 SiO 2 -MgO< + 1.5 SbF 5 - Al 2 O 3 < -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 (H o ) of some strong acids used in catalysis Slide 15 Acids- Ranking as per the strength Slide 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 Slide 17 Indicators used for acidity measurements Slide 18 Slide 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 Slide 20 H R indicators Mostly aromatic alcohols Highly specific for protonic acids R-OH + H + R + + H 2 O H R = -log A H+ ROH R + - log A H2O Slide 21 Slide 22 Slide 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 Slide 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 Slide 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> 600C ammonia may decompose Quantitative analysis to be carried out with caution Slide 26 Pulse chemisorption set up Helium Ammonia Slide 27 Laboratory reactors Pulse micro reactor Small amount of catalyst (mg) / reactants (l) 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 Slide 28 Chemisorptive titration Pt adsorbs H 2 & O 2 reversibly at RT Titration cycles are possible Pt + H Pt.H Pt.H + O 2 PtO +H PtO +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 Slide 29 Typical Ammonia TPD pattern Plots are deconvoluted to derive WEAK and STRONG acidity Slide 30 Acidity & acid strength distribution SampleSi/Al Weak acidity (meq/g) Strong acidity (meq/g) H-Beta150.550.66 H-Deal 1340.210.30 H-Deal 2460.090.30 H-Deal 3175-- Slide 31 Ammonia TPD- Finger prints for Zeolites Type of Zeolite Effect of SAR Slide 32 Ammonia TPD- Effect of metals on acidity Al-MFI Slide 33 Ammonia TPD-Effect of heating rate Two different types of sites Slide 34 Acidity by ammonia TPD- RE HY Samples Ref.GI.Kapustin et.al,Appl.Catal. 42,239,1988 Slide 35 Acidity by ammonia TPD- REHY samples 3.3 8,3 12.8 17.3 A.Corma et.al, Zeolites, 7,561,1987 Slide 36 Ref.GI.Kapustin et.al,Appl.Catal. 42,239,1988 Slide 37 Heat of asdorption of ammonia Ref.GI.Kapustin et.al,Appl.Catal. 42,239,1988 Slide 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 Slide 39 Slide 40 Slide 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 Slide 42 Slide 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 450C, 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 Slide 44 Surface hydroxyls by IR Spectroscopy JW.Ward, J.Catalysis, 9,225,1967 Slide 45 Surface hydroxyls- Effect of temperature JW.Ward, J.Catalysis, 9,225,1967 Slide 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 Slide 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) Slide 48 Slide 49 Slide 50 Slide 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 Slide 52 Slide 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 C pK w = 13.9964 ~ 14 pK b = 14 - pK a Slide 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 Slide 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 YellowOrange17.2 4-NitroanilineYellowOrange18.4 4.ChloroanilineColorlessPink26.5 * pK a of indicator Slide 56 Basicity & activity RJ.Davis, Res.Chem.Intermed.26,21,2000 Slide 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 Slide 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 indicatorbenzene carboxylic acid titration was used Slide 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 Slide 60 Solid Super bases H.Gorzawski & W.F.Hoelderich, J.Mol.Catal. 144, 181,1999 Slide 61 Shape selective base catalysts J Zhu et.al, Catal.Today, 51,103,1999 Slide 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 Slide 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