J. Chem. SOC., Faraday Trans. I, 1984, 80, 1329-1340 Tin Oxide Surfaces Part 11 .-Infrared Study of the Chemisorption of Ketones on Tin(1v) Oxide BY PHILLIP G. HARRISON* AND BARRY M. MAUNDERS Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD Received 17th March, 1983 Infrared spectroscopy has been employed to study the chemisorption of a number of unsymmetrical ketones, RCOMe [R = C3H7, C,H,, (CH,),CH, (CH,),C, Ph], onto tin@) oxide. In every case, the final product was the surface carboxylate, RCO;(ads). The data suggested a mechanism involving initial coordination of the ketone to a surface tin site, followed by nucleophilic attack of a neighbouring hydroxyl group at the carbonyl carbon atom. Studies of the adsorption of ketones on metal oxides have largely been confined to acetone on alumina, rutile, magnesia, beryllia and nickel and calcium oxides.Three modes of reaction have been distinguished. Surface acetate is the most common chemisorption product,l-10 although coordinated acetone,ll* l2 mesityl oxide13-15 and surface enolatesll? l2 have also been observed. Both coordinated acetone and surface enolate have been proposed as intermediates in the formation of both mesityl oxide13 and surface a ~ e t a t e . ~ ~ lo No evidence has been found for the dissociative chemisorption of acetone on silica12 or germania,16 although hydrogen bonding to surface hydroxyl groups occurs. Other ketones which have been studied are halogenated acetones8> 1 7 9 or symmetrical ketones such as di-isopropy1ketone.ls Surface-bound carbonyl com- pounds have often been proposed as intermediates in the oxidation of hydrocarbons over tin(rv) oxide-containing catalysts. We have previouslylg briefly examined the chemisorption of acetone on tin@) oxide, where a surface acetate was also observed.In this paper, we report the chemisorption behaviour of a number of other unsymmetrical ketones towards tin(rv) oxide in order to elucidate the mechanism of oxidation of such species when bound to a tin(rv) oxide surface. EXPERIMENTAL The vacuum line and general procedure have been described previously.20 All spectra were recorded using Perkin-Elmer model 157G, 521 or 577 spectrophotometers. Tin(1v) oxide gel was prepared by precipitation from redistilled tin(rv) chloride.20 All ketone adsorbates were redistilled under nitrogen and then subjected to freeze-thaw cycles prior to use in order to remove dissolved gases.RESULTS PENTAN-2-ONE ADSORPTION Fig. 1 shows the spectra obtained from the action of pentan-2-one on an evacuated N m-2, 320 K). Exposure to pentan-2-one vapour N m-2) resulted in tin(1v) oxide disc (1.33 x (1.9 k N m-2, 320 K, 10 min) followed by evacuation (320 K, 13291330 I.R. STUDY OF KETONES ON TIN OXIDE 46- 95. 80 1800 1600 1400 1200 wavenumberlcm -' Fig. 1. Infrared spectra of pentan-2-one chemisorbed on tin(1v) oxide: (1) background of evacuated disc; (2) pentan-2-one vapour 320 K, 1.9 kN m-2; (3) exposed to pentan-2-one 320 K, 10 min, 1.9 kN m-2, then evacuated 320 K, 0.5 h, < 1.33 x N md2; subsequent evacuation (4) 320 K, 20 h, < 1.33 x N m-2; (6) N m-2; ( 5 ) 378 K, 2 h, < 1.33 x N m-2.513 K, 20 h, < 1.33 x absorption bands at 1510, 1408 and 1310 cm-l, a broad shoulder between 1770 and 1700 cm-l and a second shoulder at 1585 cm-l. Further evacuation (320 K, 20 h) removed the 1770-1700 cm-l shoulder and greatly reduced the intensity of the 1585 cm-l shoulder. Evacuation at higher temperatures (5 13 K, 20 h) greatly increased the intensity of the 1510 and 1408 cm-I bands and reduced the 1200-1000 cm-l band, because of the OH deformation, enabling two weaker bands at 1265 and 1105 cm-l to be discerned. In addition a weak shoulder at ca. 1600 cm-l was present on the high- wavenumber side of the 1510 cm-' band. The two shoulders at 1700-1770 and 1585 cm-l can be assigned to pentan-2-one hydrogen-bonded to surface hydroxyl groups and coordinated to Lewis-acidic tin sites, respectively.Shifts in carbonyl stretching frequencies of the order of 6 to 33 cm-l forP. G. HARRISON AND B. M. MAUNDERS 1331 68. 83 n 83 0 s 4-3 c ._ 89 E 2 4-3 75 1800 1600 1400 1200 wavenum ber/cm-' Fig. 2. Infrared spectra of butyric acid chemisorbed on tin(1v) oxide: (1) background of evacuated disc, (2) exposed to butyric acid vapour, 320 K, 0.5 h, < 0.1 kN m-2; subsequent evacuation (3) 320 K, 1.5 h, < 1.33 x N m-2; ( 5 ) N m-2, (4) 378 K, 20 h, < 1.33 x N mP2. 453 K, 3.5 h, < 1.33 x ketones interacting with hydroxyl groups of molecules have been ascribed by Denisov21 to the formation of a hydrogen bond of the type OH. - .O=C.Con- siderably greater shifts are observed when ketones coordinate to strong aprotic electron acceptors; for example, Filimonov22 reports that the v(C=O) stretching band of acetone shifts to 1625-1635 cm-l upon coordination with AlBr,. The 1510 and 1408 cm-l bands can be assigned to the v,,(COO) and v,(COO) stretching modes, respectively, of a surface butyrate formed by oxidative chemisorption of pentan-2-one, while the 1310 cm-l band can be attributed to a C-H deformation mode of the butyrate and/or coordinated hydrogen-bonded pentan-2-one. The former is most likely since an absorption band occurs in this position on a tin(rv) oxide disc exposed to butyric acid vapour and its intensity is unaltered after prolonged evacuation.1332 .R. STUDY OF KETONES ON TIN OXIDE 1 1 1 1 1 1 1 1 1 1 1 1800 1600 1400 1200 wavenumber/cm -I Fig.3. Infrared spectra of butan-2-one chemisorbed on tin(1v) oxide : (1) background of evacuated disc; (2) exposed to butan-2-one vapour, 320 K, 0.5 h, 4.3 kN m-2, then evacuated, 320 K, 1 h, c 1.33 x N m-2; subsequent evacuation (3) 393 K, 20 h, < 1.33 x N m-2. The weak band at 1265 cm-I and shoulder at ca. 1600 cm-l can tentatively be assigned to the (COH) deformation and (C=O) stretching modes, respectively, of free acid, formed by protonation of the carboxylate, coordinated through the carbonyl oxygen to surface Lewis acidic tin sites. The 1105 cm-l band can possibly be assigned to a (C-C) stretching mode. The assignment of the chemisorption product to a surface butyrate was confirmed by the adsorption of butyric acid vapour (< 0.1 kN m-2, 320 K, 0.5 h) on a similarly treated tin(1v) oxide disc (fig.2), where, after evacuation, absorption bands were observed at 1610, 1512, 1410 and 1310 cm-l. The 1610 cm-' band was reduced to a shoulder after evacuation at 453 K, at the same time two bands at 1265 and 1105 cm-l were observed. The 1512, 1410, 13 10 and 1105 cm-I bands are due to the butyrate, while the 1610 and 1265 cm-l bands are due to the v(C=O) stretching and (COH) deformation modes, respectively, of coordinated butyric acid. The increases in band intensity observed on evacuation at 5 13 K after pentan-2-one adsorption can be attributed either to oxidation of any remaining coordinated pentan-2-one or to pentan-2-one, adsorbed on the silica walls at lower temperatures, being desorbed then oxidised on the tin@) oxide surface.BUTAN-2-ONE ADSORPTION Exposure of an evacuated tin(1v) oxide disc (1.33 x N m+, 320 K) to butan-2-one vapour (320 K, 0.5 h, 4.3 k N m-2), then subsequently re-evacuated, exhibited strong infrared absorption bands at 1512 and 1410 cm-l, together withP. G. HARRISON AND B. M. MAUNDERS 1333 76 - 95 - 87- * 94- E a) E: +-I Y .- A 2 * 90. 8 3 . - l o o / o - 1 1 1 1 1 1 1 1 1 1 1 1800 1600 1400 1200 w avenum berlcm-' Fig. 4. Infrared spectra of 3-methylbutanone chemisorbed on tin(rv) oxide : (1) background of evacuated disc; (2) exposed to 3-methylbutanone, 320 K, 10 min, 1.5 kN m-2, (3) 3- methylbutanone vapour, 320 K, 1.5 kN mP2; subsequent evacuation (4) 320 K, 1.5 h, < 1.33 x Nm-2; (5) 373 K, 2 h, < 1.33 x lop4 Nm-2; (6) 453 K, 20 h, < 1.33 x N mp2.weaker bands at 1378 and 1300 cm-l and a broad shoulder centred at ca. 1610 cm-1 (fig. 3). Evacuation (393 K, 20 h) increased the intensity of the 1512 and 1410 cm-1 bands, the 1512 cm-l band being unsymmetrical and broader on the high-wavenumber side. The 1512 and 1410 cm-l bands can be assigned to the (COO) stretching modes, of a surface propionate, with the 1378 and 1300 cm-l bands being attributed to the symmetric methyl deformation and a C-H deformation mode, respectively. The shoulder at 1610 cm-l is due to the v(C-0) stretching mode of coordinated butan-2-one. The broad, unsymmetrical nature of the 15 12 cm-l band after evacuation at 393 K suggests that some coordinated ketone is still present.1334 I.R.STUDY OF KETONES ON TIN OXIDE 3-METHYLBUTANONE ADSORPTION Fig. 4 shows the spectra obtained when an evacuated tin(1v) oxide disc (1.33 x lo-* N m+, 320 K) was exposed to 3-methylbutanone vapour (320 K, 10 min, 1.5 kN m-2). Absorption bands were observed at 1680, 1610, 1530, 1460, 1412 and 1362 cm-l, along with a shoulder at ca. 17 10 cm-l. Subsequent evacuation removed the 1710 cm-l shoulder, reduced the 1680 cm-l band to a shoulder on the broad, strong 161 5 cm-l band, shifted from 16 10 cm-l. The 1530 cm-l band was shifted to 1515 cm-l and, like the 1412 cm-l band, became more distinct. The 1460 cm-l band, shifted to 1470 cm-l, and the 1362 cm-l band, shifted to 1370 cm-l, were both reduced in intensity. In addition weak bands were now apparent at 1290 and 1252 cm-l.Further evacuation at 373 and then 453 K reduced further the intensities of the 1680 cm-l shoulder and the 1610, 1470 and 1370 cm-l bands, while the 151 5 , 1412 and 1290 cm-l bands all increased in intensity, with the 1290 and 1252 cm-l bands shifting to 1300 and 1265 cm-l, respectively. Absorption bands also became apparent at 11 80 and 1100 cm-l. The 1710 cm-l shoulder can be assigned to the v(C=O) mode of the vapour-phase ketone, or possibly the same species weakly hydrogen-bonded to the surface. The 1680 cm-l band is in the position expected for the v(C=O) stretching mode of the ketone coordinated, through the carbonyl oxygen, to Lewis-acidic tin sites. The 1610 cm-l band could be ascribed to ketone coordinated to a second, stronger, Lewis-acid site, or possibly to a carbon-carbon double-bond stretch of an enolate structure, a third, and perhaps the most probable, assignment could be due to coordinated isobutyric acid, formed by oxidation of the ketone to the isobutyrate followed by protonation to give the free acid, since a band occurs in this position upon isobutyric acid absorption.The bands at 1515 (shifted from 1530 cm-l upon evacuation) and 1412 cm-1 are due to the v(C00) stretching modes, respectively, of a surface isobutyrate, while the 1460 and 1362 cm-l bands can be assigned to the S,,(CH,) and S,(CH,) deformation modes, respectively, of the ketone and possibly isobutyrate. The decrease in intensity, upon evacuation, of the 1680, 1460 and 1362 cm-l bands is evidence for the removal of the majority of the coordinated ketone.The shift of the latter two bands to slightly higher wavenumbers, 1470 and 1370 cm-l, reflects the smaller surface coverage by the ketone and hence less steric restraints upon the deformation modes. The increase in intensity, with evacuation, of the 1515, 1412 and 1290 cm-l bands and the appearance of the 1180 and 1100 cm-l bands confirm that these are all due to the same species. The assignment of the species to the iso- butyrate was confirmed by adsorption of isobutyric acid vapour. The observation of a band at 1375 cm-l after adsorption indicates that the 1370 cm-l band observed on 3-methylbutanone adsorption must be partially due to the isobutyrate. The weak band at 1625 cm-l could be due to a (COH) deformation mode associated with the free acid, for which the 1610 cm-l band is assigned to the v(C=O) stretching mode.The spectra from both 3-methylbutanone and isobutyric acid both show the presence of a shoulder at ca. 1610 cm-l on the 15 15 cm-l band together with the weak 1265 cm-l band. The 1180 and 1100 cm-l bands can tentatively be assigned to carbon-carbon skeletal vibrations of the isopropyl unit [(CH3)2C],23 while the 1290-1 300 cm-l band is probably due to a (C-H) deformation mode. 2,2-DIMETHYLBUTAN-3-ONE ADSORPTION Fig. 5 shows the spectra obtained when an evacuated tin(1v) oxide disc (1.33 x N mP2, 320 K) was exposed to 2,2-dimethylbutan-3-one vapour (320 K, 10 min, 2.0 kN m-2). A broad, strong absorption band was observed with itsP.G. HARRISON AND B. M. MAUNDERS 1335 1800 1600 1400 wavenumberlcm-' Fig. 5. Infrared spectra of 2,2-dimethylbutan-3-one chemisorbed on tin(1v) oxide; (1) background of evacuated disc, (2) 2,2-dimethylbutan-3-one vapour, 320 K, 2.0 kN m-2; (3) exposed to 2,2-dimethylbutan-3-one, 320 K, 10 min, 2.0 kN m-2; subsequent evacuation (4) 320 K, 1 h, < 1.33 x N m-2; ( 5 ) 388 K, 48 h, < 1.33 x lop4 N m-2; (6) 493 K, 2 h, N mp2; (7) 623 K, 2 h, < 1.33 x < 1.33 x N m-2. maximum at 1680 crn-l, together with absorption bands at 1470,1360 and 1 140 cm-l. Subsequent evacuation (320K) left bands at 1630 (broad and strong), 1480 and 1370 cm-l, along with a slight shoulder at 1530 cm-l and a weak band between 1430 and 1385 cm-l. Prolonged evacuation (388 K, 60 h) produced a change in the spectrum with two broad, overlapping bands appearing at 1555 and 1520 cm-l, along with other bands at 1480, 1410, 1370, 1220 and 1170 cm-l.A broad band, with its maximum at 1515 cm-l, remained after evacuation at 493 K for 10 min, along with a shoulder on the low-wavenumber side at 1480 cm-l. The absorption bands at 1410 (now shifted to 1415), 1370, 1220 and 1170 cm-l still remained. From a comparison with the infrared spectrum of the vapour-phase ketone, which exhibited strong absorption bands at 1728, 1485, 1370, 1360 and 1140 cm-l, the absorption bands at 1630, 1480 and 1370 cm-l, remaining after evacuation at 320 K, can be assigned to the v(C=O) stretching, S,,(CH,) and S,(CH,) deformation modes,1336 I.R. STUDY OF KETONES ON TIN OXIDE 60 78 I I I I I I I I I I I 1800 1600 1400 1200 wavenumber/cm -' Fig.6. Infrared spectra of acetophenone chemisorbed on tin@) oxide: (1) background of evacuated disc; (2) exposed to acetophenone vapour, 320 K, 10 min, c 0.1 kN m-2; subsequent evacuation (3) 320 K, 20 h, < 1.33 x N m-2; (4) 393 K, 20 h, c 1.33 x N m-2. respectively, of coordinated 2,2-dimethylbutan-3-one, where the ketone is coordinated via the carbonyl oxygen to Lewis sites on the oxide surface. The very broad band at 1680 cm-l observed in the presence of the vapour is due to overlapping of the free v(C=O) stretching, hydrogen-bonded and coordinated v(C=O) stretching vibrations of the ketone. At 320 K there is very little evidence for any other surface species, only the very weak bands at 1530 and 1430-1385 cm-l.However, at higher evacuation temperatures absorption bands of a new surface species appear as those due to the coordinated ketone disappear. The new bands at 1515-1520 and 1415 cm-1 can be assigned to the v,,(COO) and v,(COO) stretching modes, respectively, of a surface 2,2-dimethylpropionate, while the 1480 (shoulder) and 1370 cm-l bands can be assigned to 6,,(CH,) and 6,(CH,) deformation modes, respectively. Although adsorp- tion of the corresponding acid was not carried out, the two bands at 1220 and 1170 cm-l lend weight to the proposed structure, since they occur in the expected position for the skeletal vibrations of the 2,2-dimethyl group.23P. G. HARRISON AND B. M. MAUNDERS 1337 ACETOPHENONE ADSORPTION Fig. 6 shows the spectra obtained when an evacuated tin(1v) oxide disc (1.33 x lop4 N m-2, 320 K) was exposed to acetophenone vapour (320 K, 10 min, < 0.1 kN mP2).Strong absorption bands were observed at 1655, 1640, 1590, 1565 (shoulder), 1265 and 12 15 cm-l, with weaker bands at 1490,1445, 1390 (broad), 1360, 1180 and 1160 cm-l. The 1640 cm-l bands can be attributed to the v(C=O) stretching mode of the ketone coordinated or hydrogen bonded to the surface [cf. v(C=O) of the liquid of 1690 cm-’1. The 1590 and 1445 cm-l bands are due to the aromatic-ring vibrations, while the 1360 cm-l band can be ascribed to the d(CH,) deformation. After evacuation at 320 K strong absorption bands remained at 1590, 1490 and 1390 cm-l with weaker bands at 1560, 1445, 1220, 1180 and 1160 cm-l, and a shoulder on the high-wavenumber side of the 1590 cm-l band. The large decrease in intensity of the 1640 cm-l band indicates the removal of the majority of the coordinated ketone.Other aromatic-ring stretches are still quite strong, indicating that the aromatic portion of the molecule is still present, but the methyl deformation band at 1360 cm-l has been removed. Two previously weak bands, at 1490 and 1390 cm-l, have increased in intensity, thus it can be concluded that the acetophenone has been chemisorbed on the surface with the loss of the methyl group. The two bands at 1490 and 1390 cm-l can be ascribed to the va,(COO) and v,(COO) stretching modes, respectively, of a surface benzoate group. Further evacuation for 20 h at 393 K completely removed the shoulder due to coordinated ketone, leaving strong, sharp, absorption bands at 1590 and 1445 cm-l due to aromatic-ring stretching vibrations, at 1495 and 1395 cm-l due to the two (COO) stretching modes and at 1180 and 1160 cm-l due to the (C-H) out-of-plane deformations of the aromatic ring.Absorption bands for the carboxylates observed from adsorption of all ketones are summarised in table 1. DISCUSSION The tin(rv) oxide surface can be considered as consisting of Lewis-acidic and -basic sites, the Lewis-acidic sites being the exposed Sn ions and the basic sites the surface hydroxyl groups or oxide ions. Weak Brsnsted-acid sites can also be detected from ammonia adsorption and from the observation of absorption bands attributable to butyric and isobutyric acid after penton-2-one and 3-methylbutanone adsorption, respectively.The mechanism of surface carboxylate formation’ from adsorption of the ketones is uncertain, though it is likely that chemisorption proceeds via the nucleophilic attack of a reactive surface hydroxyl group at the carbonyl carbon of the ketone, following coordination to an unsaturated surface tin ion (scheme 1). R R’ R f?’ R R‘ ‘-0 L II JI 0 .. 0 /sn. ,Sn, 0 0 0 Scheme 9, 2”. 0 0 0 1.CI w w 00 Table 1. Vibrations of surface carboxylates derived from adsorbed ketones 2,2-dimethyl- ketone: pentan-2-one butan-2-one 3-methylbutanone butan-3-one acetophenone corresponding adsorbed 2,2-dime thyl- carboxylate : butyrate propionate isobutyrate propionate benzoate assignments ; 1600(sh) 1590 ring vibration 3 1512 1515 1515 1490 - va,(COO) % 1510 1445 ring vibration 1408 1410 1412 1415 1390 - v,(COO) 0 1378 1375 1370-1 - 375 - 3 v(c=o)a 5 - ! - - 1610 - das(CH& 1480-1485 - - - - - - 6s(CH,) 6(COH)a 2 d(C-H) skeletal vibration of 2,2-dimethyl group - 1290-1300 1250 - 1310 1300 - - - - 1170 - \ 0 1220-1 225 - 1265 - - - - 1 180 6(C-H) out-of-plane of 1160 ) aromatic ring - - ?$ - U m - - - skeletal vibration - - - - - ) of isopropyl group - 1180 1100 - - v(C-C) - - - - 1105 - - a These bands arise from acid formed by protonation of the carboxylates, see text.P.G . HARRISON AND B. M. MAUNDERS 1339 In our previous studieP of the adsorption of [2H,]acetone on tin(1v) oxide we observed the formation of a surface acetate and suggested that an enolate intermediate may be involved due to the presence of absorption bands at ca.1600 and 1240- 1250 cm-l which were assigned to the v(C=C) stretching and 6(COH) deformation modes, respectively, of the enolate species. In this investigation absorption bands were observed in similar positions with pentan-2-one and 3-methylbutanone adsorptions ; however, as similar absorption bands were observed with adsorption of the corre- sponding acids to the observed carboxylates, butyric and isobutyric acids, respect- ively, the assignment of these bands to coordinated protonated acid seems more reasonable in this case, formed by the abstraction of hydrogen from an adjacent sur- face hydroxyl group (scheme 2). R I R 0 - H ‘ r ’ Scheme 2. All the ketones employed in the present study were unsymmetrical ketones of the type RCOMe.However, in every case, the surface carboxylate formed was that derived from the group R and in no case was a surface acetate observed. Thus, pen tan- 2-one, butan- 2-one, 3 -met h ylbu tanone, 3,3 -dime t h ylbu tan-2-one and ace to- phenone gave a surface butyrate, propionate, 2-methylpropionate7 2,2-dimethyl- propionate and benzoate, respectively. The underlying reason for this observation may be thermodynamic or kinetic in origin. A comparison of the relative heats of formation of the two possible sets of products [i.e. RCO;(ads) + CH, and MeCO;(ads) + RH (assuming that the tin-oxygen bond remains the same in each case, and using data for the free acid, which should parallel those of the carboxylates)] shows that, where data are a~ailable,~, the marginally favoured thermodynamic products would be expected to be MeCO;(ads) + RH by ca.3-33 kJ mol-l. Similarly, simple bond dissociation energy estimates2, of the two bonds, R-COMe and Me-COR, shows that the bond required to be broken to give formation of surface acetate, i.e. R-COMe, is consistently the weaker of the two by ca. 2-2 1 kJ mol-l. Thus, contrary to observation, thermodynamic arguments favour the formation of surface acetate and methane. Hence, it may be concluded that the reactions are kinetically controlled, which is consistent with the nucleophilic process shown in scheme 1. Although in this study the constitution of the gas phase was not investigated, met- hane was confirmed to be a constituent of the gas phase in the adsorption of acetone.19 M.L. Hair and I. D. Chapman, J . Phys. Chem., 1965,69, 3949. H. Knozinger, Adv. Catal., 1976, 25, 184. H. Knozinger, H. Krietenbrink, H. D. Muller and W. Schulz, Proc. 6th. In?. Congr. Catalysis (The Chemical Society, London, 1976), p. 183. P. Fink, Rev. Roum. Chim., 1969, 14, 81 1 . W. Schulz and H. Knozinger, J . Phys. 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