J.C.S. Faraday I, 1980,76, 19-29Adsorption and Decomposition of Isopropyl Alcoholover Zinc OxideInfraed and Kinetic StudyBY OSAMU KOGA, TAKAHARU ONISHI* AND ~ N Z I TAMARUDepartment of Chemistry, Faculty of Science,University of Tokyo, Bunkyo-ku, Tokyo 113, JapanReceived 19th December, 1977The adsorption of isopropyl alcohol and acetone on zinc oxide was studied by an infraredtechnique which revealed that isopropyl alcohol is dissociatively adsorbed at room temperature toform zinc alcoholate and hydroxyl group on the surface, while the adsorption of acetone takes placein its enolic form. When adsorbed isopropyl alcohol was heated to 363 K, the zinc alcoholatespecies changed gradually to acetone adsorbed in its enolic form, which further desorbed at highertemperatures as acetone, being replaced by the attacking isopropyl alcohol.The behaviour of the adsorbed species during decomposition of isopropyl alcohol on zinc oxidewas studied in more detail, leading to the overall reaction mechanism described by eqn (V).The dehydrogenation of isopropyl alcohol over metal oxides is both technicallyimportant and is a basic catalytic reaction in view of its selectivity and catalyticactivity.Krylov studied a scheme for dehydrogenation of isopropyl alcohol overzinc oxide, a typical dehydrogenating catalyst. He considered the relation betweenthe activity of various catalysts and their properties and suggested an adsorbedalcohol " dissociated at a-hydrogen " and " enol- or keto-type of acetone " asreaction intermediate, and that the dehydrogenation step from dissociated alcoholto adsorbed acetone was rate-determining.Teichner and coworkers measuredthe pressure increase under various reaction conditions and concluded that the ratedetermining step is the dehydrogenation of adsorbed alcohol to form acetone, amechanism similar to that of Krylov. Both, however, obtained no direct informationabout either the adsorbed species or the reaction sites and assumed some partialequilibrium in their reaction sequences. Desorption of isopropyl alcohol from zincoxide was studied by Kemball and coworkers who reported that even at tempera-tures as high as 573 K some species are not removed from the catalyst surface; inparticular less acetone was desorbed in cornpaxison with hydrogen.In this report the adsorption of isopropyl alcohol and acetone on zinc oxide wasstudied by infrared spectroscopy, and the mechanism of decomposition of isopropylalcohol to produce acetone is elucidated by measuring not only its rate but also theadsorption and kinetic behaviour of adsorbed species during the reaction.EXPERIMENTALZinc oxide (Kadox 25 from New Jersey Zinc Co., surface area 10 m2 g-l) was pressedinto a self-supporting disc ( M 0.2 g), which was placed in an infrared cell and connected toa conventional closed circulating system, as reported previou~ly.~ The catalyst disc wasevacuated N m-2) at 690 K for 2 h and further treated for 2 h at 730 K in 13 kN m-2oxygen being circulated through a trap cooled by liquid nitrogen.120 ADSORPTION OF ISOPROPYL ALCOHOLIsopropyl alcohol and acetone were used after purification by repeated distillation.[2H8]Isopropyl alcohol and [2H6]acetone were obtained from Merck.The infrared spectra of the adsorbed species were measured in the region 4000-900 cm-l .If necessary, the 900-600 cm-I range was also measured by putting almost the same amountof catalyst in the reference beam.The procedure and apparatus employed in measuring the adsorption were similar tothose employed previo~sly.~ Zinc oxide catalyst ( m 3.0 g) for the kinetic study was usedin the form of a disc and treated under the same conditions as for the infrared study.Theadsorption and the rate of decomposition of isopropyl alcohol were measured in a circulatingsystem at 363 K.The amounts of the adsorbed alcohol and acetone were determined by the peak heightof the infrared spectra using the reference cell to compensate gas phase absorptions. Thecalibration curves for adsorbed isopropyl alcohol and acetone shown in fig.1 were deter-mined by measuring the 1129 and 1500 cm-l bands when known amounts of isopropylalcohol and acetone were separately adsorbed.The reactant and product gases were analysed by gas chromatography using a 1 mdioctylsebasate column at room temperature.1.61 . 41 . 21.00 80.60.40.220 40 60amount/pmol g1FIG. 1.-Calibration curve of adsorbed species at room temperature (shaded symbols) and 363 K(open symbols). 0, isopropyl alcohol (IPO), 1129 cm-I; A, A, acetone (En), 1500 cm-l.RESULTS AND DISCUSSIONINFRARED STUDYADSORPTION OF ISOPROPYL ALCOHOLThe infrared spectra of isopropyl alcohol adsorbed on zinc oxide at room tem-perature are shown in fig.2. The amount of saturated adsorption was nearl0. KOGA, T. ONISHI AND K . TAMARU 2160 pmol (g cat)-’. The sharp bands at 3616 and 3665 cm-l, due to the hydroxylgroup on zinc oxide, decreased on alcohol adsorption and a new broad band around3300 em-1 increased.wavenumber /cm-’FIG. 2.-Infrared spectra of adsorbed isopropyl alcohol (IPO). - - -, ZnO background ; -,adsorbed alcohol (IPOH); --- , adsorbed alcohol after exchange with D2 (IPOD) at roomtemperature.TABLE UP O INFRARED BANDS OF ISOPROPYL ALCOHOL, ITS ZINC ALCOHOLATE AND ITS ADSORBEDSPECIES ON ZnOadsor bedalcohol zinc species(liquid) alcoholate (this work) assignment/cm-l /cm-l /crn-l1470 vs1412 m1382 vs1372 vs1342 s1311 vs1162 m1130 vs1450 vs 1458 vs WH3)&CH)1366 vs 1380 vs WH3)1356 vs 1366 vs 6033)1330 s 1338 s wm&OH)1160 m 1159 m skeletal1133 vs 1129 vs skelet a1The hydrogen exchange reaction between adsorbed alcohol and deuterium gas(13 kN m-2) at room temperature was also studied and revealed that only hydrogenin the hydroxyl group of alcohol was exchanged.The broad band of hydroxylgroup around 3300 cm-l was shifted to 2500 cm-l by the exchange reaction, all theother bands staying unchanged (fig. 2).The spectrum in fig. 2 was compared with those of liquid alcohol and zincalcoholate in table 1.The table shows that the band positions are reasonably simila22 ADSORPTION OF ISOPROPYL ALCOHOLto those of alcoholate, which suggests that alcohol is adsorbed dissociatively to formzinc alcoholate and hydroxyl group on the surface, as follows,H H(1)I II IICH3-C-CH3- CH,-C-CH, + HI-0- .ZnO0-Zn-OHADSORPTION OF ACETONE ON ZnOTypical infrared spectra of small amounts of acetone [ZO pmol (g cat)-l] adsorbedon zinc oxide at room temperature are shown in fig. 3. There was no indicationthat the adsorbed acetone desorbed during evacuation at 383 K for a few hours.3600 2800 2000 1700 1500 1300 1100 900 700wavenumber /cm-FIG. 3.-Infrared spectra of adsorbed acetone (En). - - -, ZnO background, nearly the same amountof ZnO in the reference beam ; (a) adsorbed E2Ho]acetone, (b) adsorbed [2Hs]acetone.The spectrum of adsorbed acetone lacks the absorption band characteristic ofC=O stretching of acetone vapour at 1745 cm-l.In addition the following newbands appeaxed in the spectrum of adsorbed acetone; (a) a sharp band for OHgroup at 3360 cm-l with weak absorption of the H-C- group at 3040 cm-l and(b) a strong skeletal vibration band at 1500 cm-5To collect more information on the behaviour of hydrogen in the adsorbedstate, a hydrogen exchange reaction was carried out at room temperature betweendeuterium (13 kN m-2) and acetone [20 pmol (g cat)-l] adsorbed on zinc oxide.The results of the exchange demonstrated that the hydrogen atoms both in thehydroxyl and in the hydrocarbon group were exchanged, resulting finally in thecomplete exchange of all the hydrogen atoms in acetone?As the exchange reaction proceeded, the band around 1500 cm-l shifted graduall0.KOGA, T. ONISHI AND K. TAMARU 23to 1465 cm-l. The amounts of hydrogen and/or deuterium in the hydroxyl groupof the adsorbed species was estimated from their intensity. In fig. 4 the deuteriumcontent in the hydroxyl group of the adsorbed molecule (OD/OH+OD) is plottedagainst the peak position of the adsorbed molecule near 1500cm-l. The skeletalC=C bond shifts linearly with the deuterium content of the hydroxyl group, support-ing the idea that the OH group is directly attached to the C-C bond.0 0.2 0.4 0.6 0.8 1 .OOD/OH+ODFIG. 4.-Band shift of peak at nearly 1500cm-l for En under partial deuteration.OH (OD)represents OH (OD) concentration of En from the infrared absorbance at 3360 cm-l (2510 cm-l).Roginsky and coworkers observed an OH band in the region 3550-3750 c dand two strong bands at 1575 and 1515 cm-l in the range characteristic of the C=Ostretching vibration in the spectra of acetone adsorbed on NiO. They proposed anenolate type adsorption species as follows :CH3\1 CH3COCH3 --+ C=O - - - NiO.NiO".CH H/ \\/IC CC M-C\ /A M--- 11 T-/ \. H H H/ \H(1) (2) (3)which is a hybridized structure of (2) and (3).124 ADSORPTION OF ISOPROPYL ALCOHOLIn table 2 the characteristic infrared bands of acetone adsorbed on zinc oxideare compared with the enol type molecules.This comparison leads to the conclusionthat the adsorbed species on ZnO is an enol type as follows :CH3ICHO H\ / c=cNagai and Miyahara l5 studied the infrared spectra of acetone adsorbed on ZnOand found that the adsorbed acetone species is an enolate.When large amounts of acetone [ > 40 pmol (g cat)-l] were introduced onto zincoxide, the spectrum differed from that of fig. 2, as shown in fig. 5. This spectrum,which has a new peak around 1600 cm-l, is similar to that reported by Nagai et al.It is known that polymerization of acetone takes place on zinc oxide. Accordingly,the spectrum in fig. 5 can be assigned to a polymerized species of acetone such asacetylacxtone.1700 1500 1300 1100wavenumber/cm-lFIG. 5.-Infrared spectra of adsorbed acetone in excess amounts (52 pmol g-l)0.KOGA, T . ONISHI AND K . TAMARU 25TABLE 2.-cHARACTERISTIC BANDS OF ENOL TYPE SPECIES AND ADSORBED ACETONEadsorbed species(RCHZ- (this work)CMeOH)+Br- l4 C3HsC120-Pt l2 CH3COCH3 CD3COCD3 assignment1515 s3300 vs 3360 w 2510 m3020 w 3040 w2960 w 2220 w2930 w2880 w1545 s 1500 vs 1465 vs1435 m 1046 w1362 m 1350 s1301 s1080 w --__-__--.,1050 w ---------.,'::940 w930 w w o w765 vs 793 w752 w-OH(-OD)H-C==-CH3(-CD,)-CH3-CH3 c=c-CH3(-CDs) c-050 100 150 200 250When adsorbed isopropyl alcohol was heated to 363 K, the spectrum of theadsorbed species changed from that in fig. 2 to curve (a) in fig. 3 ; isopropyl alcoholatespecies on ZnO changed to an enol type acetone species at this temperature, indicatingFIG.6.-Reaction between En(a) and hydrogen (18 kNme2) to produce IPO(a) on ZnO at 363 K.reaction time/mi26 ADSORPTION OF ISOPROPYL ALCOHOLthat the first step in the decomposition of isopropyl alcohol is the dissociative adsorp-tion to alcoholate and hydroxyl groups on the surface which react further to enol(a)and hydrogen.When adsorbed acetone, En(a), was heated at 363 K in the presence of 18 kN m-2hydrogen, it was converted into isopropyl alcoholate (IPO) as shown in fig. 6, furthersupporting the reversible process H2 + En(a) + IPO(a) + H(a).(2) (3) (4)reaction timelminFIG. 7.-Decomposition of isopropyl alcohol on ZnO at 363 K. (1) Gas phase was removed by aliquid nitrogen cold trap just after introducing alcohol to the reaction cell.(2) Release of condensedgases from the trap. (3) Reintroduction of isopropyl alcohol. (4) Evacuation of gas phase.-0-, IPA, isopropyl alcohol in gas phase ; -@-, ACT, acetone (g) ; - x -, PP, propylene (g) ;--A-, Hz (8); -0-, IPO, adsorbed isopropyl alcohol (alcoholate); -El-, En, adsorbedacetone (enol).KINETIC STUDY OF DECOMPOSITION OF ISOPROPYL ALCOHOLIsopropyl alcohol decomposes over zinc oxide at 363 K to form mainly acetoneand hydrogen ( M 90 % selectivity) and small amounts of propylene and water (10 %).A typical result of the dynamic study at 363 K is given in fig. 7, where the changes inpartial pressure of the reactants and products and the amounts of each adsorbedspecies are shown. Soon after contact of alcohol with the catalyst, the gas phas0 .KOGA, T. ONISHI AND K. TAMARU 27components other than hydrogen were removed by circulating the reacting gas andcondensing in a liquid nitrogen trap (period 1) ; after 2 h the condensed gases werereleased by removing the liquid nitrogen (period 2).During period 1 , the amounts of species adsorbed during reaction were estimatedfrom i.r. intensities, and by measuring the pressure and composition of the ambientgas and the amount of condensate (determined by gas chromatography at suitabletime intervals) in the trap. The amount of adsorbed alcohol decreased, whilehydrogen in the gas phase and the adsorbed acetone (enol species) increased withtime by almost the same amount. The adsorbed acetone desorbed negligibly asKemball et aL3 observed in the absence of alcohol in the gas phase.- P1 2log ([IPOIIWOl g-'1FIG.8.-Dependence of the initial rate of En formation upon IPO coverages.At the start of period 2, the adsorbed alcohol increased, whereas the adsorbedacetone decreased desorbing into the gas phase by amounts corresponding to therelease of condensed vapours from the trap. The decomposition of alcohol con-tinued subsequently, while adsorbed alcohol and acetone remained almost constant.Thus the following scheme for the overall reaction can be proposed to explain theseresults :IPOH(g) e IPO(a) + H(a)+ H(a) + En(a) -I- H,(g)(1)(2)(3) En(a) + IPOH(g) + IPO(a) + ACT(g)where IPOH(g), IPO(a), En(a) and ACT(g) show gaseous isopropyl alcohol, isopropylalcolate (adsorbed), enol type adsorbate and gaseous acetone, respectively.Step (3)represents the expulsion by alcohol vapour of the enol type acetone adsorbed specieswhich is adsorbed instead on the catalyst surface28 ADSORPTION OF ISOPROPYL ALCOHOLTo discover the role of adsorbed alcohol and acetone in dehydrogenation, therate of reaction (2) was examined at various alcohol and acetone coverages. Theresults using fresh catalysts are shown in fig. 8.1 . 2 c-bDH 5 1.0 .50 100 150reaction time/&FIG. 9.-Formation of En at various coverages of IPO and En. IPO coverage in pmol (g cat)-'.Vacant site (S) : 17.9 pmol (g cat)-I (see text). (a) IPO = 9.81, (6) IPO = 19.7, (c) IPO = 32.7, (d)IPO = 36.6, (e) IPO = 50.8.The initial rate of formation of enol type adsorbed species was second order withrespect to IPO(a) coverage in the initial stage of the reaction, which suggests thatinitially IPO(a) can react with dissociatively adsorbed hydrogen H(a) to form enolspecies.The preadsorption of acetone (En) inhibited this reaction, demonstratingthat the initial rate of En formation is proportional to the vacant site (S) for Enadsorption (fig. 9). Summarizing these data the following equation was obtainedd[En(a)]/dt = k[IPO(a)][H(a)][S - En(a)].An overall mechanism for the reaction is thus proposed :CH3ICCH3 CH3 CH3 CH3HzW I \\ /CHOH 0 1 Zn\ /CH ZnO l - 1 + H + HZC".. .. .:.' OHI 1Zn 0CH3 CH3 CH3 CH3 (V)\ /C\ /CHI + H + II 0 I 0I 0cH3\c y3HO ' '*___3Z0. KOGA, T. ONISHI AND K . TAMARU 290. V . 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