Ir Spectroscopic Identification of Adsorbed Surface Species on Oxidation Catalysts Exposed To Propene and Ethene in Air

G. Centi and F. Trifiro' (Editors),New Developments in Selective Oxidation 1990 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

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IR SPECTROSCOPIC IDENTIFICATION OF ADSORBED SURFACE SPECIES ON OXIDATION CATALYSTS EXPOSED TO PROPENE AND ETHENE IN AIR

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M. J PIRES'

, N. T .DO2 ,

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M. BAERNS2 and M.F PORTELA1

Grupo de Estudos de Cathlise Heteroggnea, Centro de Processos QuL micos (INIC), Universidade Tgcnica de Lisboa, Instituto Superior Tgcnico, Aven. Rovisco Pais, 1096 Lisboa Codex (Portugal)

' Lehrstuhl

fur Technische Chemie, Ruhr-Universitst Bochum, Postfach 102148, D-4630 Bochum (West Germany)

SUMMARY IR spectroscopic studies have shown different surface complexes on three catalysts, i.e. Bi20 MOO Y-Al2O3 supported thallium silver when exposed under wnoxide and if-A1 0 supported m&all?i tinuous flow tz mixture of C H or C H and air. Spectra recordThese results can ed after desorption in air w e d h s o be related to the different catalytic behaviours observed in previous studies.

d

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INTRODUCTION Selectivity in hydrocarbon oxidation is associated with the structure and the energy differences between the surface intermediates formed by the hydrocarbon and oxygen and the catalyst. Infrared spectroscopy is a powerful technique for the study of adsorbed species and a helping tool for the understanding of the mechanisms in complex catalytic reactions. For these reactions we may assume that they involve several chemisorption forms at the catalyst surface, leading to different products. The aim of this work is the elucidation of the nature of adsorbed propene species on the surfaces of silver, bismuth-molybdate and thallium based catalysts, for which previous studies (refs.1, 2) have shown different activities (epoxidation, allylic and total oxidations) EXPERIMENTAL Catalysts The low temperature Bi203.Mo03 pure phase was prepared by a reproducible coprecipitation technique (ref.3). X-ray diffraction and Raman SpeCtrOSCOpy did not show traces of impurities. The IR spectrum recorded with the catalyst in air at 303Kwas identical

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to the ones reported in literature [refs. 4 , 5 1 . XPS confirmed the expected Bi/Mo atomic ratio on the surface. The BET surface area was 2.1 m 2 g-1 Thallium(~II1) oxide catalyst was prepared from thallium(II1) nitrate by addition of nitric acid (pH = 1.5) and ammonium hydroxide (pH = 8). The precipitate was dried 2 h at 353 K and activated under air at 573 K ( 4 h). The silver supported in I(-alumina catalyst was prepared by dry impregnation technique, with silver nitrate, followed by drying and further reduction with formaldehyde (8 h at 393 K).

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IR equipment and method of investigation For measuring infrared spectra a double beam IR spectrophotometer (Perkin-Elmer, model 580A) attached to a minicomputer (Dietz, model 621 x 2/Mulheim) was used. Two cells built after Gallei and Stolz (ref. 6) and described by Ramstetter (ref. 7) were incorporated into the spectrometer. Adsorbate spectra were obtained by compensating the overall spectrum by subtraction of the spectra of the gasphase and the clean catalyst from it. A detailed description of the method was given by Baerns and Ramstetter (refs. 8,9). Experimental conditions of irs measurements To obtain disks of good quality all catalysts except Bi203,Mo03 were mixed with bl-A1203 as support before pressing the disk. BET surface areas after this pretreatment were 140 and 156 m 2 g-l for silver and thallium oxide catalysts. Each catalyst was left overnight prior to the experiment under air stream at temperatures lower than 720 K and was tested in continuous flow adsorption runs with C3H6-air mixture followed by desorption runs in air at the same temperature. The gases were dried and purified before contact with the catalyst. Adsorbate formation lasted for several hours. Spectra corresponding to different adsorbed amounts were recorded as function of time. Experimental conditions were empirically established by previous tests, including checking of the inertness of alumina for the used experimental conditions. With Bi203.Mo03 catalyst no adsorption was observed at temperatures above 473 K; thallium oxide and silver required higher olefin concentration in order to observe any adsorbates. RESULTS Olefin species adsorbed on the catalysts were identified

by

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their ir spectra after adsorption and after subsequent desorption. Bi203.Mo03 A ) Adsorption and desorption of C3H6 in the presence of air

Fig.1 presents the characteristic bands observed in the range 4000- 800 cm-l. In the region near 3000 crn-’, corresponding to C-H stretching vibrations, bands above 3000 cm-l suggest that the hydrocarbon fragment is olefinic, i.e., propene is absorbed on the surface without breaking the double bond; this is confirmed by the presence of ir bands due to out-of-plane deformation vibrations C-H at 990 and 910 crn-l. The band at 3450 cm-l developped after 15 h at 303 K can be reasonably assigned to an OH frequency; hence it appears that d i s s ~ r ciation accompanies propene adsorption. At 373 K a different spectrum was recorded in this region: bands at 3450, 3080 and 2860 cm-l disappear while the intensities of other bands at 3100, 2960, 2935 and 2885 cm-l are reduced by broadening. In the range of 1600-1200 cm-l the bands 1475 and 1445 cm-l can be assigned to C-H deformation vibrations, but there are other unusual bands: 1665, 1655, 1640, 1560, 1545, 1510 and 1340 cm-’.Considering the double-bond stretching vibration band at 1652 cm-l for gaseous propene one could interpret the values around 1545 cm-l as a band shifted due to the interaction of the double bond with the surface. The presence of the 1655 cm-l band at the same time would indicate a partial interaction of T-bonding. The same effect of broadening is shown in the spectrum at 373 K for the 1650, 1530, 1400 and 1340 cm’l bands. During desorption of C3H6 from a Bi/Mo oxide surface at 303 K the following observations were made: 1. Olefinic C-H band 3080 cm-’, 990 and 910 cm-l bands disappeared. 2. Bands at 1665-1640 cm-l were also removed after 3 h in air. These bands are possibly due to a more weakly bound form of propylene. A longer desorption time did not essentially change the surface except that a band developped at 1465 crn-l and that the two other bands at 1540 and 1560 cm-’ disappeared. The effect of cleaning of the surface by desorption at 373 K during 3 h in air was also observed but a new thin band was detected at 1510 cm”.

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I 3100

I 1400

I 1100

I 1400

I 1000

Frsqusncv (em-’)

Fig.1. Compensated IR spectra of Bi 03.Mo0 after adsorption of 15% C H6 in air (a1303 K (15 h), (b) 37?K (20 and desorption in air ($1 303 K (14 h) , (d) 373 K (3 h); after adsorption of 40% C2H4 in air, (el 303 X (10 h) and desorption in air, (f) 303 K (15 min).

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B) Adsorption and desorption of C2H4 in the presence of air

The same disk of Bi203.Mo03, after being heated at 673 K during 1.5 h under air, was tested in an adsorption run with ethene at 303 K, followed by desorption at the same temperature. The adsorption spectrum after 1 h showed nothing. But after 1 0 h there was a development of bands, resulting in a rather complex spectrum. By comparing these results with the characteristic bands of gaseous ethene ( s e e Fig.1) we verify the presence of all these bands and other little ones at 1660, 1625, 1565 and 1510 cm-l already detected during propene adsorption. Fifteen minutes under desorption conditions were enough to change the catalyst surface which presented mostly a development of the bands of adsorbed water at 3450 cm-I and 1640

ern-'.

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Thallium oxide/t-Al 203 The results obtained over the supported thallium oxide are in -1 Fig.2. The spectra were recorded in the range 4000-600 cm but below 1000 cm-l the catalyst was not transparent. Adsorption and desorption spectra of C3H6 on the supported thallium oxide show no bands in the frequency range 4000-2000 cm-l.

Fig.2. Compensated IR spectra of thallium oxide/&-alumina after adsorption of 30% C H6 in air (a) 303 K (24 h), (b) 373 K (12 h ) , (c) 573 K (8.5 h) aad desorption in air, (d) 303 K (4 h), (e) 373 K (4 h), (f) 573 K (8). Adsorption spectra at 303 K show bands at 1985, 1845 and 1825 cm-l which decrease with temperature and disappear at 573 K. The double bond stretch vibration band (1670-1640 cm-’) was always observed. But some changes occur in the region near 1600 cm-1 : 1. A broad band arises around 1595 cm-l at 373 K. 2 . Adsorption at 573 K during 1.5 h leads to a better definition of the broad band at 1595 cm-l into 1580, 1575 and 1565 cm-’ and to a new band at 1335 cm-l. Spectra recorded after desorption runs show a cleaner surface at 303 and 373 K. But at 573 K after 8 h in air the spectrum is

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not very different from the one recorded after adsorption run at the same temperature except the relative intensities of bands around 1655 and 1575 cm-l. Bands under desorption conditions become better defined and seem to increase with time. Silver/b/-Al203 This catalyst was studied in the range 4000-800 cm-'. After 1.5 h and 4 h at 303 K under a mixture of 15% C3H6 in air the catalyst did not present measurable adsorption. In Fig.3 spectra show only a few adsorbed species under 30% C3H6 in air and indicate fast and complete desorption at both temperatures. ~~

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3000

2000

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Fraqumncv (cm.')

Fig.3. IR spectra of silver/l-alumina in air (a) 303 K, (b) 623 K; after adsorption of 30% C3H6 in air (c) 303 K (1.5 h) , (d) 623 K (9 h) and desorption in air (el 303 K (3 h), (f) 623 K (15 min)

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DISCUSSION AND CONCLUSIONS According to literature there are two adsorbed forms of C H 3 6 at the surface of catalytic oxides. Davydov and Budneva (ref.10) refered a reversible weakly bounded form, precursor of n-allylic and 6-allylic complexes and a irreversible one which undergoes disso-

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ciation on desorption. The latter will be responsible of oxidated complexes, carbonate and carboxylate type andn-complexes. Gerei et al. (ref.11) observed on the total oxidation catalysts an adsorbed propylene mainly in an irreversible form with evidence for double bond scission. In the mild oxidation catalysts the double bond is preserved though somewhat perturbed: the result being aT-complex, weakly bound and precursor of ther-allylic complex. Both complexes are also mentioned by Dent and Kokes (ref.12). Force and Bell (refs. 13,141 refer the formation of carbonated species over metallic silver; the band 2180 cm-l can be assigned to a carbon-metallic structure Ag-CO. Table 1 presents the characteristic bands of the different adsorbed species, possible intermediates in total and mild oxidation of C3H6, according to literature. The comparison with the bands observed in our work leads to the following conclusions: 1. The Bi203.Mo03 catalyst seems to presentr-allylic complexes as well as precursors of total oxidation even at low temperatures. This result agrees with our previous studies (ref.1). The structures of irreversible form leading to the total oxidation could be TABLE 1 IR bands of C3H6 adsorbed species according to literature Davydov and Budneva Gerei et al. Dent and Kokes (ref.10) (ref.11) (ref.12) a-allylic complex 1440 no ll-allylic complex 1545 1350 Reversible r-allylic complex 1600 Form 1580 weakly bound 1626 T- complex 1430 1620 1364 strongly bound 1510 <-complex 1410 Formate 1560 1575 1370 1390 Irreversible Form Acetate 1560 1575 1450 1440 1410 1730 Carbonate 1620-1660 1320 1654 1610 1550 explained by the desorption spectra: bands at 1540-1560, 1465 and 1385 cm-l can be assigned to a formate (bands at 1560 and 1370

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cm-’) as well as an acetate structure (bands at 1540-1560 and 1450 cm-l). The CH stretching vibration of both complexes absorb at 2870 and 2950 cm-l. 2. The surface complexes on Ag0/Y-Al2O3 seem to be carbonated species, precursors of total oxidation in agreement with the low epoxidation selectivities of this catalyst. Bands of adsorbed propene on Ago at 303 K at 1840, 1820, 1655 and 1440 cm-l correspond to those of unadsorbed propene (bands at 1830, 1810, 1630-1660 and 1444-1476 cm-’). Physisorbed propene desorbs by purging with air at 303 K and was not observed by adsorption at 623 K. The bands at 1650 and 2180 cm-l observed at 623 K can be assigned to a carbonate and a Ag-CO structure. 3. &-A1203 supported thallium oxide also shows precursors of total oxidation, whose presence becames more evident at higher temperature. Propene adsorbs on thallium oxide at l o w temperature (303 K) as on Ag. The bands at 1565-1580 and 1335 cm-l observed at high temperature (573 K) can be assigned to a formate structure. 4. Ag0/8-Al2O3 catalyst presents a clean surface 15 minutes after desorption in air, unlike Bi203.Mo03. This fact may indicate different binding energies for the surface complexes on both catalysts. Thallium oxide presents a different behaviour under desorption conditions at 573 K characterized by a better definition and development of the bands. The assignment of the bands around 1650 cm-l is rather difficult. They are present in the adsorbate spectra of the three catalysts. This is the region of the double bond stretch vibration but it is also assigned to some carbonate structures. AKNOWLEDGEMENT The experimental work has been supported by a grant of the Volkswagen Foundation. REFERENCES

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