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ESR spectra of perylene and tetracyanoethylene adsorbed on alumlna were measured. The spectra of perylenium ions on sulfate-containing aluminas,.
Reaction Kinetics and Catalysis Letters, Vol 2, No. 4, 355-362 (19~5) ELECTRON-ACCEPTOR AND ELECTRON-DONOR PROPERTIES OF SULFATE-CONTAINING ALUMINAS R. F1edorow and A. Wieckowski Institute of Chemistry, A. Mickiewicz University, Poznan, Poland, and Laboratory of P,adiospectroscopy, Institute of Physics, Polish Academy of Sciences, Poznan, Poland Received January 20, 1975

ESR spectra of perylene and tetracyanoethylene adsorbed on alumlna were measured. The spectra of perylenium ions on sulfate-containing aluminas, in contrast to those on pure alumina samples, show a well-resolved hyperfine structure in the absence of oxygen, which persists even after 75 hrs. The electron-donor properties of sulfate-containing aluminas are not different from those of pure alumina. B~

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In our previous p a p e r s / 1 - 4 / , special attention was directed to changes in the surface properties of alumina caused by the admixture of water-imoinble basic ainminium sulfates introduced during the preparation of the catalyst. These sulfates cause a marked increase in the acidic and oxidizL.~ properties of alumina but do nor affect the basic and reducing properties/5/. Using a modified sample prepa355

H~ORDW. WIECKOWSKI:SULFATE-COI~rAININGALUMINAS

ration technique/2, 3/, evidence for the different electron-acceptor properties of sulfate-containing aluminas and pure alumina has been obtained from the ESR spectra.

EXPERIMENTA L Gamlysts used were precipitated at 20 ~

from an aluminium sulfate solu-

tion by the addition of ammonium hydroxide to a pH of 7.7 for catalyst S-600 and a pH of 9.5 for catalyst A-9.5/VIII. After washing to the disappearance of sulfate ions in the filtrate, the precipitates were dried at 110 ~ then calcined at 600 ~

for 18 hrs, and

(catalyst S-600) or 800 ~ (catalyst A-9.5/VIII) for 6 hrs.

Sulfate contents (expressed as SO3 content) in these catalysts were found to be 9.8 and 2.2 wt.O~o, respectively. Surface areas of the above catalysts were 154 and 134 m 2 / ~

respectively. Moreover, for comparison purposes, we used a c o m -

mercial silica-alumina catalyst of Soviet origin, containing 13~ alumina. Its surface area after calcination at 550 ~ was 250 m2/g. The electron-acceptor and electron-donor properties of the catalyst surfaces were characterized by changes in the ESR spectra of the pesylenium cation radical (Pc+) and tetraeyanoethylene (TCNE') anion radical, resulting from the interaction between perylene or tetracyanoethylene and the catalyst surface. ESR measurements were carried out at room temperature with a Polish spectrometer SE-25/71 A, at a ldystron frequency of 9.6 GHz and a magnetic field modulation of 100 kHz. An ultramarine sample having a spin concentration of 1.1 x 1017 was used as a standard for estimating the TCNE" radical concenlxation. The spectroscopic splitting factor, g, was determined by comparison with the position of the DPPH line. The samples for the ESRstudy were prepared as follows: the sample of alumina was 356

FIEDOROW. WIECKOWSKI:SULFATE'CONTAINING ALUMINAS

placed in a quartz tube and evacuated at 400 ~

and 10 -5 Tort for 3 hrs. Then a

solution of 0. 005 Mperylene (Fluka, Switzerland) in dry benzene or a solution of 0.1M tetracyanoethylene (Eastman=Kodak, U.S.A.) also in dry benzene, was intro= duced to the catalyst sample by breaking a thin glass seal. Both of these solutiom were previously deaesated by several freeze-evacuate-thaw cycles. ESR spectra were recorded 75 hrs after the solutionc a m e into contact with the solid.

RESULTS AND DISCUSSION Spectra of perylenium ions adsorbed on both sulfate-containing alumina samples (Fig. 1) show a well-resolved hyperflne structure (HFS) with 9 lines cziginating from the interaction of the unpaired electron with 8 hydrogen nuclei. The intensities of these lines are expressed by the relation I : 8 : 2 8 : 5 6 : 7 0 : 5 6 : 2 8 : 8 : 1 . The hypesflne coupling constant, A, is 0.35 mT. The spectra of perylenium radical ions adsorbed on our alumina samples, shown in Figs. la and b, are analogous to those of Pe+ on silica-alumina observed by Hodgson and P4tley /6/, Fiockhart et al./7/ and ourselves(Fig. lc). It has been stated by Flocldmrt et al. /7/that a we11-resolved HFS in the spectrum of peryleniumiom on alumina can be observed at room temperature oniy for a few minutes following the addition of perylene to the alumina sample and the presence of oxygen is necemary. If perylene adsorption occurs at a temperature of - 4 5 ~

a

well-resolved HFS is still observable after 9 hrs upon maintaining the sample at this temperature. On the other hand, the perylenium ion on silica-alumina gave a spectrum with 9 HF lines in the absence of oxygen and the spectrum did not change over long periods of time. 357

FIEDOROW, WIECKOWSKI:SULFATE-CONTAINING ALUMINAS

2/

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Fig. 1. ESR spectra of perylene adsorbed on: a) alumina S-600; b) alumina A-O.5/VIII; c) silica-alumina

358

FIEDORDW, WIECKOWSKI: SULFATE-CONTAINING ALUMINAS

In this study special care was taken to exclude adsorbed oxygen from the alumina samples and to maintain the samples at room temperature for long periods of time (75 hrs). Well-resolved HFS, similar to those repceted in the above papers/6, 7 / f o r perylenium ions on silica-alumina, were observed for Pe+ on the sulfate-containing alumina samples. This indicates a difference in the interaction of perylene with the sulfate-containing aluminas relative to pure alumina as described by F1ockhart et al. / 7 / . As stated in our previous papers/I, 3, 0/, sulfatecontaining alumina samples show only a quite imignificant proton acidity, determined in the presence of triphenylmethanol. For this reason, the participation of Br6nsted acids in perylene oxidation (e. g. in the sense proposed by Hirschler and Hudson/10/ for silica-alumina) should be excluded. We suggest that Lewis acid sites are the centers on which the conversion of perylene into perylenium ion occurs on sulfate-containing aluminas. The spectra of paramagnetic tetracyanoethylene complexes formed on the surface of sulfate-containing aluminas are similar to those formed on the surface of silica-alumina (Fig. 2). The line-shapes are approximately Lorentzian, with nearly the same line-widths for all samples (Table 1). Although the spectra have Table 1 Concentration of tetracyanoethylene radicals on the surface of the catalysts studied

Catalyst L S-600 A-9.5/VIII Silica-alumina

spin x 10 "17 g j

x 10 -4 2 ~ BIS (mT)

factor

8

52

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FIEDOROW, WIECKOWSKI: SULFATE-CONTAINING ALUMINAS

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2. ~ R spectra of tetracyanoethylene adsorbed on: a) aiurmna 5-o00; b) alumina A-9.5/VIII; c) silica-alumina

FIEDOROW, WIECKOWSKI:SULFATE-CONTAININGALUMINAS

a similar slope, the concentration of TCNE" radicals per unit catalyst weight is by about one order of rr~gnitude higher on sulfate-containing aluminas than on the silica-alumina catalyst. The density of electron-donor centers per unit surface of sulfate-containing aluminas is also much higher than that for the silica-alumina surface (Table 1). From this point of view, sulfate-containing aluminas are not different from pure alumina. The spectra shown in Hg. 2 resemble the spectrum of TCNE" on the high-temperature pure alumina sample in the paper by Flockb a r t e t al. /8/, i . e . they do not have the HFS observed in the case of alumina calcined at lower temperatures and rich in surface hydroxyl groups. As the t e m perature of calcination of our alumina samples was high, the surfaces were extensively dehydroxylated and thus the formation of TCNE" radicals occurred on surface defects involving oxide ions. Such defects, according to the Peri model of the alumina surface /11/,

can play the role of surface Lewis bases. The relation-

ship between the ability of an alumina surface to form TCNE" radicals and the basictty of the alumina sample will be discussed in a forthcoming p a p e r / 5 / .

REFERENCES 1. tL Fledorow, W. Kanla, W. Kuczymki, A. Wieckowski: Proc. 5th Internatl. Congr. Catalysis, 2_. 14,54 (1973). 2. IL Fiedorow: Bull. Acad. Pol. Sci., Set. Sei. Chim., 22__, 325 (1974)' 3. W. Kuczynski, IL Fiedorow, A. Wfeckowski: Rocz. Chem., 2221 (1970). 4. 1L Ffedorow, W. Kuczynski: Quatrl~me Colloque Franeo-Polonais sur la Catalyse, Lyon-Villeurbanne 1973, p. 18-20. 5. 1L Fiedorow et al. : Rocz. Chem. (to be published). s

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FIEDOROW, WIECKOWSKhSULFATE-CONTAININGALUMINAS

6. I L L . Hodgson, L H. l~ley: I. Caral., 4_ 6 (1965). 7. B. D. FIockhart, 3. A. N. Scott, R. C. Pink: Tram. Faraday Soc., ~2._.., 730 (1966). 8. B. D. Flockhart, I. IL Leith, 11. C. Pink: Trans. Faraday Soc., 65, 542 (1969), 9. P~ Fiedorow: surface Chemistry of Some Alumina Modifications. UAM, Poznan 1972. 10. A. E. Hirschler, I. O. Hudson: I. Caral., 3_ 239 (1964). 11. J. B. Peri: J Phys. Chem., 65_., 220 (1965).

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