Infrared study of acetophenone and 4-methoxyacetophenone adsorbed on titania

Spectrochimica Acta Part A 54 (1998) 319–326

Infrared study of acetophenone and 4-methoxyacetophenoneadsorbed on titania

Imtiaz Ahmad, James A. Anderson, Trevor J. Dines, Colin H. Rochester *Chemistry Department, Dundee Uni6ersity, Dundee DD1 4HN, UK

Received 16 June 1997; received in revised form 8 September 1997; accepted 19 September 1997

Abstract

Infrared spectra are reported for acetophenone and its 4-methoxy derivative adsorbed on titania. Adsorptioninvolved ligation via carbonyl oxygen atoms to Lewis acidic Ti4+ sites followed by slow enolisation and subsequentcondensation to, for acetophenone, dypnone. Surface carboxylate species were formed at 353 K. Acetophenones areunsatisfactory indicators for testing the Brønsted acid character of oxide surfaces. © 1998 Elsevier Science B.V. Allrights reserved.

Keywords: Acetophenone; Adsorption; Titania; Surface acidity

1. Introduction

The weak basicity of acetophenone derivativesin aqueous solution [1] suggests their use as indi-cators for detecting strong Brønsted acidic siteson oxide surfaces. Study of the electronic spec-trum of adsorbed benzalacetophenone has beenreported in this context [2]. Ketones act as Lewisbases and undergo ligation to metal cations [3]and therefore also constitute probe molecules forthe detection of Lewis acidic sites on oxides [4].Ketones may also form hydrogen bonds withsurface hydroxy groups, a mode of adsorptionwhich has been explored for acetophenones onsilica [5].

One problem of using visible spectra to monitor

indicator adsorption is that the observed colourchanges may not necessarily relate to the particu-lar mode of adsorptive interaction which it isdesired to study [2,6,7]. Undesirable side reactionsmay also occur. For example, acetone on rutileundergoes enolisation and dimerisation to formadsorbed mesityl oxide [8]. FTIR spectroscopyprovides a good method of unambiguously char-acterising the adsorption of molecules on oxides.The present study was aimed at characterisingacetophenone adsorption on titania which ex-hibits strong Lewis acidity [9–12] but no Brønstedacidity in the presence of weak bases.

2. Experimental

Titanium dioxide (Degusa P25) had a surfacearea of 59 m2 g−1 and was shown by powder

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I. Ahmad et al. / Spectrochimica Acta Part A 54 (1998) 319–326320

XRD to be ca. 80% anatase and ca. 20% rutile.Before exposure to adsorbate vapour at 295 Kin an infrared cell with fluorite windows presseddiscs of the oxide were heated at 673 K first ina flow of dry CO2-free air (100 cm3 min−1) for1 h and then for 0.5 h in vacuum. Spectra wererecorded using a Perkin Elmer 1720 FTIRspectrometer at 4 cm−1 resolution.

3. Results

Series of spectra of acetophenone on titaniashowed that at least three modes of adsorptionoccurred. Fig. 1 shows the three spectra recordedduring exposure of a disc to acetophenone va-pour. The low vapour pressure of the liquid atroom temperature led to the adsorption rate beingslow and therefore the build up of adsorbed spe-cies took several hours. The most significant dif-ferences between the spectra of adsorbed andliquid [13] or solution phase [14,15] acetophenonewas in the n(CO) spectral region. At low surfacecoverages (Fig. 1a) the band at 1685 cm−1 forliquid acetophenone had shifted to 1674 cm−1 onadsorption. Furthermore, a band at 1268 cm−1

attributable to an aromatic �C–H in-plane defor-mation vibration [16] for the liquid was at 1280cm−1 for acetophenone on titania. These twobands have previously been reported to be partic-ularly sensitive to the ligation of acetophenone totransition metal cations [3]. The existence ofstrong Lewis acidic sites on titania [9–12] leads tothe conclusion that the first mode of adsorptionof acetophenone on titania involved O-ligation toTi4+ surface sites. Acetone behaves similarly onrutile and gives a perturbed n(CO) band at 1680cm−1 [8].

Increasing adsorption gave two additionaln(CO) bands at 1685 and 1659 cm−1 (Fig. 1) thelatter appearing first at moderate coverages butthe former becoming dominant as more acetophe-none was adsorbed. The growth of the band at1685 cm−1 was accompanied by concomitantgrowth of a maximum at 1268 cm−1, both bandsbeing in identical positions to those for liquidadsorbate. This suggests the build up of weakly

physically adsorbed acetophenone molecules onthe titania surfaces at high coverages. However, aswell as the slow adsorption of acetophenone therewas also a slow chemical reaction taking place onthe titania surface. Fig. 2 shows the spectralchanges which occurred after the adsorption ofacetophenone and subsequent standing in vacuumfor 16 h at 295 K. The strong bands due tophysically adsorbed acetophenone were consider-ably weakened in intensity but significant in-creases in band intensities occurred at 1659, 1280and 1216 cm−1. Comparison of Fig. 1b and 2d

Fig. 1. Spectra of acetophenone on titania at (a–c) increasingsurface coverages.

I. Ahmad et al. / Spectrochimica Acta Part A 54 (1998) 319–326 321

Fig. 2. (c) as Fig. 1c, high coverage, and (d) after subsequentstanding at 295 K for 16 h.

same sites. The corresponding band positions formesityl oxide and acetone were 1660 and 1680cm−1, respectively [8]. The band at 1280 cm−1

for dypnone is, by analogy to the same band foracetophenone, ascribed to an aromatic in plane�C–H deformation vibration [16] of �CHCOC6H5

groups ligated to Ti4+ ions. The band at 1216cm−1, for which there was no equivalent in thespectrum of liquid acetophenone, is assigned to asimilar vibration of C6H5(CH3)C� groups in lig-ated dypnone. A similar band exists in spectra ofboth styrene and benzalacetophenone.

Despite the existence of weak bands at 3719,3673 and 3500 (vw) cm−1 in spectra of titaniaafter heat treatment at 673 K [7] there was noevidence in the spectra for hydrogen bondinginteractions between hydroxy groups and ad-sorbed acetophenone molecules. Acetone on rutilegave a similar result [8].

Acetophenone-methyl-d3 was adsorbed in orderto test for H/D isotope exchange between surfacehydroxy groups and CD3 groups which would bean expected outcome of enolisation, or for thegeneration of surface deuteroxy groups whichwould result from the condensation reaction giv-ing dypnone. Spectra during adsorption are inFig. 3. Aromatic ring vibrations gave similarbands at 1599, 1578, 1494 and 1448 cm−1 for theCH3 and CD3 compounds in the adsorbed state.The absence of the bands at 1358 and 1428 (sh)cm−1 from spectra of the CD3 compound confi-rms their assignment to ds(CH3) and da(CH3)vibrations [13]. The n(CO) vibrations of acetophe-none-d3 and dypnone-d4 ligated to Ti4+ sites wereat 1671 and 1661 cm−1, respectively, and a bandat 1280 cm−1 for both compounds was due to anin plane aromatic �C–H deformation vibration.Adsorption was not carried on sufficiently to gen-erate physically adsorbed acetophenone-d3 on thetitania surface. However, three bands at 2650,2554 and 2487 cm−1 on a broad band envelopegrew with increasing surface coverage. Thesebands were due to OD-stretching vibrations ofdeuteroxy species (OD or D2O) [11,17] thus prov-ing that C–D bonds had been broken in theadsorption reactions.

Evacuation of titania after exposure to ace-tophenone led to slow desorption of acetophe-

shows that the band at 1674 cm−1 due to ligatedacetophenone had also been weakened.

The results for acetophenone are explained byanalogy to acetone adsorption on rutile [8]. Liga-tion to Lewis acidic sites was followed by slowenolisation and condensation reactions, which foracetophenone lead to adsorbed dypnone [C6H5

(CH3)C�CHCOC6H5]. The latter is itself ligatedto Lewis acidic Ti4+ sites and is responsible forinfrared bands at 1659, 1280 and 1216 cm−1. Theslow conversion of adsorbed monomer to thecondensation product leads to increased occu-pancy of Ti4+ sites by dypnone with n(CO) at1659 cm and therefore the displacement of ace-tophenone with n(CO) at 1674 cm−1 from the


Fig. 3. Spectra of (a) liquid acetophenone-methyl-d3 and (b–f) increasing coverages of acetophenone-methyl-d3 on titania.

none molecules ligated to Ti4+ sites. Results forthe CD3 compound are in Fig. 4; the band at 1671cm−1 due to ligated C�O groups had disappearedafter 24 h at 295 K leaving a sharp maximum at1661 cm−1 due to ligated dypnone which wasmore difficult to desorb. Raising the temperatureto 353 K promoted desorption of dypnone which,however, was incomplete after 7 h. At the highertemperature, bands at 1414 and ca. 1500 cm−1

grew in intensity and are ascribed to surface car-boxylate species [8,18]. A band due to surfacedeuteroxy groups remained at 2630 cm−1 theother bands due to OD groups having disap-peared.

On silica, 4-methoxyacetophenone was ad-sorbed via hydrogen bonding interactions involv-

ing both carbonyl and methoxy groups [5].Adsorption on titania was studied to test whetherthe molecule was ligated to the surface via its C�Oor OMe group or both. Spectra in the range1620–1150 cm−1 and in the n(CH) region resem-bled, with slight band shifts or relative intensitychanges, spectra for adsorption on silica [5] or ofthe solid compound. However, the shape of theband envelope due to n(CO) vibrations was sig-nificantly affected by the titania surface. Then(CO) band is at 1684 cm−1 for the methoxycompound in CCl4 [14], 1668 cm−1 for the solid,and 1667 cm−1 for molecules hydrogen bonded tosurface silanol groups on silica [5]. For adsorptionon titania (Fig. 5) a sharp maximum at 1663cm−1 was accompanied by a shoulder at ca. 1650


Fig. 4. Spectra of titania at 295 K after adsorption of acetophenone-methyl-d3 and evacuation for (a) 5 min at 295 K, (b) 24 h at295 K, and (c) 7 h at 353 K.

cm−1. The form of the band envelope closelyresembled that for acetophenone at low coveragessuggesting that similar adsorption behaviour wasresponsible for the results. The adsorption of themethoxy compound could not be studied athigher coverages because of the low vapour pres-sure of the solid. The band at 1663 cm−1 isassigned to the n(CO) vibration of 4-methoxyace-tophenone ligated to Ti4+ sites. Conversion to theproduct of condensation was small but the shoul-der at 1650 cm−1 is ascribed to n(CO) for theligated 4,4%-dimethoxy derivative of dypnone. Theabsence of a band due to unperturbed carbonyl

groups shows that none of the adsorbed moleculeswere ligated to Ti4+ sites via methoxy groupsalone.

The adsorption of both acetophenone and its4-methoxy derivative on titania led to the initiallywhite samples turning pale yellow.

4. Discussion

The dominant surface reactions of acetophe-none on titania are represented by the followingequations:


Surface complex (I) involving ligation of car-bonyl groups to Lewis acidic Ti4+ sites [9–12] gavethe n(CO) band at 1674 cm−1. Surface oxide orhydroxide ions on titania are strongly basic andenable slow proton abstraction from CH3 groupsgiving the adsorbed enolate complex (II) whichreacts with a further acetophenone molecule to givethe ligated dypnone complex (III) with a n(CO)band at 1659 cm−1. The enolate complex was notdetected but must have constituted an intermediatein the overall condensation reaction. The experi-ments with acetophenone-d3 confirmed that ODgroups, and from the broad underlying band envel-ope at ca. 2500 cm−1, D2O resulted from theadsorption reactions. Similar spectra in the ODregion resulted from acetone-d6 adsorption onundeuterated rutile surfaces [8].

The n(CO) band positions of 1674 and 1663cm−1 for complex (I) involving acetophenone andits methoxy derivative, respectively, compare withbands at 1691 cm−1 [15] and 1684 cm−1 [14] forthe molecules in CCl4 solution. The correspondingDn(CO) band shifts of 17 and 21 cm−1 are as wouldbe expected from the electron donating effect of the4-methoxy group [19] added to the acetophenonering. However, these shifts are less than thosereported for a series of transition metal complexesinvolving acetophenone as a ligand [3] suggestingthat the ligation in (I) was comparatively weak.This is consistent with the ease of desorption ofacetophenone from the titania surface.

Not only ketones [8] but also ethers [20] ligate toLewis acidic Ti4+ sites in titania surfaces. 4-Methoxyacetophenone could ligate to Ti4+ cationsthrough either OMe or COCH3 groups. However,the absence of unperturbed carbonyl groups inadsorbed molecules shows that adsorption musthave involved ligation of carbonyl groups. Theequivalent of structure (I) for the methoxy com-pound fulfils this requirement. However, a conclu-sion that substituted benzene derivatives lie flat onsilica and alumina surfaces [21] could, if applicableto titania, imply that both the OMe and COCH3

groups in each adsorbed molecule might ligate totwo separate Ti4+ sites in a bridging configuration.Ab initio Hartree-Fock calculations at the SCFlevel suggest for a 4-methoxyacetophenone that,apart from H atoms in CH3 groups, all the atomsare coplanar. A bridging configuration becomesunlikely unless the coplanarity of the COCH3

group with the aromatic ring is destroyed by anenergetically more favourable adsorption complex.Under these conditions the electronic strengtheningeffect of the OMe group on the CH3CO···Ti4+

interaction would be weakened not only because ofthe distortion from planarity but also because ofthe separate interaction between OMe groups andTi4+ sites. The Dn(CO) data show that theCH3CO···Ti4+ ligand is strengthened by themethoxy group. The shifts for acetophenone andthe 4-methoxy compound on TiO2 were 17 and 21cm−1 which compare with shifts of 13 and 17 cm−1


Fig. 5. Spectra of (a–d) increasing coverages of 4-methoxyace-tophenone on titania.

contrary to the experimental result for 4-methoxy-acetophenone on titania (Fig. 5) for which themaximum at 1601 cm−1 was much more intenserelative to other bands in the spectrum than for4-methoxyacetophenone on silica. A similar effectwas not observed for acetophenone on titania.The result is explicable if ligation via the acetylgroup alone causes enhanced electron transferfrom the electron donating methoxy group thusenhancing both the dipolar character of themolecule and the 1601 cm−1 band intensity. Thebalance of the argument therefore points to struc-ture (I) as representing the mode of adsorption of4-methoxyacetophenone on titania. The bridgingconfiguration appears less likely but is not com-pletely disproved by the present data.

The formation of carboxylate species on titaniaon thermal treatment of adsorbed acetophenone isnot surprising as both acetone [8] and hex-afluoroacetone [22] behave similarly. Both O2−

and OH− ions on titania act as bases and nucle-ophiles attacking the carbonyl carbon of ketonesleading to carboxylate species in a chelatingbidentate, rather than bridging or unidentate,configuration [18].

Titania does not exhibit Brønsted acidity in thepresence of ammonia [9,11], pyridine [10,11] ortriethylamine [23] and therefore it is impossiblethat the much more weakly basic acetophenones[1] would undergo protonation. Despite this therewas a colour change to pale yellow induced by theadsorption of both compounds. The neutral ad-sorbates are colourless although benzalacetophe-none, of which dypnone is a methyl derivative, ispale yellow. A methoxy substituent moves theelectronic band maximum nearer to the visibleregion [1]. The most likely source of the presentcolour was therefore the product of the surfacecondensation reaction. The colour for acetophe-none appeared slowly in parallel to the generationof dypnone. Less condensed product appeared for4-methoxyacetophenone but this was balanced bythe higher extinction coefficients of band en-velopes extending into the visible region of thespectrum. Once again, therefore, [2,6,7] colourchanges induced by adsorption of acid-base indi-cators may not necessarily be the result of protontransfer equilibria. The problem for acetophe-

for the molecules forming hydrogen bonds withsilanol groups on silica [5]. Exposed Ti4+ ionsacting as Lewis sites are spaced ca. 0.3 nm apartin rows on some exposed planes of rutile andanatase. A single 4-methoxyacetophenonemolecule could bridge next-door-but-one neigh-bours with the middle Ti4+ interacting with thearomatic ring in a configuration parallel with thesurface plane. The required distortion of the ad-sorbed molecules from planarity would lead to anoverall decrease in the lengthwise polarity of themolecule which in turn should decrease the inten-sity of the ca. 1600 cm−1 component of the splitE2g band for the adsorbed molecules. This is


nones rests in the availability of an enolisable Hatom at a-carbon which allows a condensationreaction to take place. The use of acetophenonesas indicators of Brønsted acidity on oxides issuspect especially if detection involves colourime-try or visible spectroscopy. Ketones without a-hy-drogen atoms may be more useful but evenhexafluoroacetone undergoes irreversiblechemisorption reactions on titania [22]. Benzophe-nones could prove to be useful Brønsted indica-tors [24] but were too involatile to be adsorbed atthe solid/gas interface. The use of ketones todetect Lewis acidic sites on oxides is also notalways straightforward. Diisopropyl ketone andhexamethylacetone are well behaved on alumina,but acetone and hexachloroacetone underwentchemical reactions and proved to be unsuitableprobes for Lewis acidic surface sites [4].

Acknowledgements

We thank the High Commission of Pakistan fora research studentship.

References

[1] R. Stewart, K. Yates, J. Am. Chem. Soc. 80 (1958) 6355.[2] B.S. Umansky, W.K. Hall, J. Catalysis 124 (1990) 97.[3] W.L. Driessen, W.L. Groeneveld, Recl. Trav. Chim. 90

(1971) 258.

[4] W. Schulz, H. Knozinger, J. Phys. Chem. 80 (1976) 1502.[5] I. Ahmad, J.A. Anderson, T.J. Dines, C.H. Rochester, J.

Colloid Int. Sci. (1998) in press.[6] C.H. Rochester, Discuss. Faraday Soc. 52 (1971) 285.[7] I. Ahmad, T.J. Dines, C.H. Rochester, J.A. Anderson, J.

Chem. Soc. Faraday Trans. 92 (1996) 3225.[8] D.M. Griffiths, C.H. Rochester, J. Chem Soc. Faraday

Trans. 1 (74) (1978) 403.[9] G.D. Parfitt, J. Ramsbotham, C.H. Rochester, Trans.

Faraday Soc. 67 (1971) 841.[10] G.D. Parfitt, J. Ramsbotham, C.H. Rochester, Trans.

Faraday Soc. 67 (1971) 1500.[11] N.D. Parkyns, in: P. Hepple (Ed.), Chemisorption and

Catalysis, Institute of Petroleum, London, 1970, p. 150.[12] C. Morterra, J. Chem. Soc. Faraday Trans. 1 (84) (1988)

1617.[13] H.-W. Chen, C.-S. Chen, S.-J. Harn, J. Phys. Chem. 99

(1995) 10557.[14] N. Fuson, M.-L. Josien, E.M. Shelton, J. Am. Chem. Soc.

76 (1954) 2526.[15] R.N. Jones, W.F. Forbes, W.A. Mueller, Can. J. Chem.

35 (1957) 504.[16] G. Socrates, Infrared Characteristic Group Frequencies,

Ch. 11, Wiley, Chichester, 1980.[17] D.M. Griffiths, C.H. Rochester, J. Chem. Soc. Faraday

Trans. 1 (73) (1977) 1510.[18] D.M. Griffiths, C.H. Rochester, J. Chem. Soc. Faraday

Trans. 1 (73) (1977) 1988.[19] G.B. Barlin, D.D. Perrin, Quart. Revs. 20 (1966) 75.[20] J. Graham, R. Rudham, C.H. Rochester, J. Chem. Soc.

Faraday Trans. 1 (80) (1984) 895.[21] L.R. Snyder, J. Phys. Chem. 72 (1968) 489.[22] D.M. Griffiths, C.H. Rochester, J. Chem. Soc. Faraday

Trans. 1 (73) (1977) 1913.[23] J. Graham, R. Rudham, C.H. Rochester, J. Chem. Soc.

Faraday Trans. 1 (79) (1983) 2991.[24] T.G. Bonner, J. Phillips, J. Chem. Soc. B (1966) 650.

.

Infrared study of acetophenone and 4-methoxyacetophenone adsorbed on titania

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