Liquid-phase oxidative dehydrogenation of 2,3,5-trimethyl-1,4-hydroquinone in the presence of titanium dioxide hydrogel was studied by a kinetic method. Associative interactions between the substrate, oxidant, and gel were detected by voltammetry and ESR and IR spectroscopy.Russian Foundation for Basic Researc

Buldakova, L. Y.

Kharchuk, V. G.

Koryakova, O. V.

Kovaleva, E. G.

Kuznetsova, O. V.

Molochnikov, L. S.

Petrov, L. A.

Shishmakov, A. B.

Yanchenko, M. Y.

Russian Journal of General Chemistry

English

Buldakova, L. Yu.

Electronic archive of the Ural State Forest Engineering University

1070-3632/04/7401-0101 2004 MAIK Nauka/InterperiodicaRussian Journal of General Chemistry, Vol. 74, No. 1, 2004, pp. 101 104. Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 1, 2004,pp. 110  113.Original Russian Text Copyright   2004 by Kharchuk, Buldakova, Shishmakov, Kuznetsova, Kovaleva, Koryakova, Molochnikov, Yanchenko,Petrov.                                                                                    Oxidative Dehydrogenation of 2,3,5-Trimethyl-1,4-hydroquinonein the Presence of Titanium Dioxide HydrogelV. G. Kharchuk, L. Yu. Buldakova, A. B. Shishmakov, O. V. Kuznetsova, E. G. Kovaleva,O. V. Koryakova, L. S. Molochnikov, M. Yu. Yanchenko, and L. A. PetrovInstitute of Solid-State Chemistry, Ural Division, Russian Academy of Sciences, Yekaterinburg, RussiaInstitute of Organic Synthesis, Ural Division, Russian Academy of Sciences, Yekaterinburg, RussiaUral State Academy of Forestry Engineering, Yekaterinburg, RussiaReceived April 24, 2001Abstract Liquid-p[hase oxidative dehydrogenation of 2,3,5-trimethyl-1,4-hydroquinone in the presence oftitanium dioxide hydrogel was studied by a kinetic method. Associative interactions between the substrate,oxidant, and gel were detected by voltammetry and ESR and IR spectroscopy.We showed in our previous papers how activepolydisperse highly ordered oxide systems can affectliquid-phase oxidation processes as catalytically activestructuring ingredients [1, 2]. Here we consider oxi-dative dehydrogenation of 2,3,5-trimethyl-1,4-hydro-quinone I (a model hydroxyarene) in the presence oftitanium dioxide gel II.Electrophilic liquid-phase oxidation of I with at-mospheric oxygen in a neutral medium occurs selec-tively by the concerted molecular mechanism andyields 2,3,5-trimethyl-1,4-benzoquinone III [3]: H3COHCH3CH3OH [O]H3C CH3CH3OOI IIIThe reaction was performed in aqueous methanol at50 C in air. Its progress was monitored by GLC [4].Hydrogel II, when present in the sphere of reactionof I with atmospheric oxygen, decreases the initialreaction rate v0 to 0.10 mol l1 h1 (v0 without gel IIis 0.26 mol l1 h1). This result suggests the occur-rence of competing interparticle interactions involvinggel II and reactants. Indeed, an IR study of substrate Isorbed on gel II reveals significant changes in theabsorption bands at 1344 and 1412 cm1, which aresuperpositions of the CO stretching bands and vibra-tion bands of the aromatic ring [5]. In going from freeto sorbed I, the band maximum at 1344 cm1 shifts to1324 cm1, and the ratio of the intensity of this bandto that of the reference (CH3) band at 1464 cm1 (A)decreases from 0.932 to 0.772; the band at 1412 cm1changes in the shape and intensity, with its A decreas-ing from 0.969 to 0.597. Furthermore, the bendingvibration band of the OH group of I ( 1216 cm1)transforms into a doublet at 1232 and 1220 cm1. Thehydroxonium absorption band at 17301680 cm1,characteristic of gel II [6], disappears upon sorptionof I. The maxima of the stretching ( 3400 cm1) andbending ( 1620 cm1) bands of hydration water in IIshift to 3280 and 1640 cm1, respectively. WeakTiOH bending bands of II ( 1192, 1168, 1090 cm1)are not observed in the spectrum of the sample withsorbed I.These changes in the IR spectra are apparently dueto binding of the hydroxonium proton and hydrogenatoms of water molecules localized on the surface ofgel II with the -electron system and hydroxyl oxygenatoms of I. Formation of hydrogen-bonded complexesdecreases the electron density on the oxygen atoms ofthe hydroxy groups in I [7, 8], which is responsiblefor the decreased reactivity of I in electrophilic oxi-dation.Addition of gel II to the reaction system in oxida-tion of I with Cu2+ ions in the presence of oxygenaccelerates the reaction: v0 becomes as high as1.05 mol l1 h1, against 0.54 mol l1 h1 in the ab-sence of gel II. In this system, substrate I is involvedin two parallel reactions, one of which is inhibited bygel II (see above). Intensification of the reaction in-volving Cu2+ ions is apparently due to their associa-tive interaction with fragments of the added hydro-gel II.RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 1 2004102 KHARCHUK et al.40dI/d,A s1040801201 0E, V1234Fig. 1. Oxidation of substrate I on carbon paste electrode:(1) no additions, (2) in the presence of 5  103 M Cu2+solution, (3) with introduction of TiO2Cu2+ads into theelectrode, and (4) with introduction of TiO2 into theelectrode in the presence of 5  103 M Cu2+ solution.Oxidation of hydroquinone I in the presence ofCu2+ ions was also studied electrochemically with acarbon paste electrode into which air-dry gel II wasintroduced. Upon addition of Cu2+ ions into solution,or upon preliminary adsorption of Cu2+ on gel IIfollowed by introduction of the gel into the carbonpaste electrode, the course of oxidation of I changescompared to the process performed on the carbonpaste electrode without introduced gel II (Fig. 1).The current of the oxidation peak of I increases, andthe shift of the peak potential reaches 0.150.16 V,which is indicative of increased oxidation rate; also,an additional oxidation peak appears at a lower poten-tial (0.350.40 V).Study of the electrochemical behavior of Cu2+ ionsadsorbed on gel II shows that reduction of Cu2+ toCu+ and of Cu+ to Cu0 is facilitated (Fig. 2, curves 1,2). Whereas the reduction of Cu2+ from solution on acarbon paste electrode is characterized by a commontwo-electron wave, reduction of Cu2+ ions adsorbedon gel II occurs in several steps. In adsorption ongel II, the waves of copper oxidation shift, and theprocess of copper oxidation/reduction becomes morereversible (the difference between the peaks ofCu2+/Cu0 reduction/oxidation is 1.27 and 0.86 V,respectively). These data suggest that, upon adsorptionof Cu2+, its reduction to Cu+ and reverse oxidation toCu2+ are appreciably facilitated, and the Cu2+ and Cu+ions can form with gel II two strongly bound and oneweakly bound complexes. Formation of the latter issuggested by broadening of the peak of copper anodicdI/d,A s140040801 0 1 E, V321Fig. 2. Influence of adsorption on the surface of TiO2gel on the electrochemical behavior of Cu2+ ions:(1) solution of Cu2+ ions and carbon paste electrode,(2) Cu2+ ions adsorbed on the surface of TiO2 gelintroduced into carbon p[aste electrode, and (3) carbonpaste electrode with introduced TiO2 (background).oxidation (at +0.5 V) and by an ill-defined oxidationpeak at +0.8 V (Fig. 2, curve 2).An ESR study of Cu2+-containing hydrogels IIbased on TiO2 showed that Cu2+ ions, depending ontheir content in the sorbent phase (Cu2+), form mono-nuclear complexes IV, associates of mononuclearcomplexes V with increased local concentration ofmetal ions, and Cu2+ compounds VI giving no ESRsignal at the given frequency and temperature. Suchstructures were also revealed on the surface of nano-structured films and powders based on TiO2 [9]. Webelieve that a separate phase of insoluble Cu(OH)2can play the role of compound VI at relatively lowcontent of sorbed Cu2+ ions, and at their high contentESR-inactive bridged polynuclear Cu2+ compounds(clusters with strong exchange coupling or polynu-clear structures with bridging OH groups) can alsoform, by analogy with the related structures formedin ion exchangers [10, 11].By varying Cu2+, we evaluated the quantitativerelationships between mononuclear complexes IV,their associates V, and Cu2+ compounds VI. As Cu2+is increased, the content of mononuclear complexesIV (nIV) decreases, and that of compounds VI (nVI)grows. The content of associates V (nV) passesthrough a maximum at Cu2+ 0.37 mmol Cu2+ pergram of gel II. In this case, the three types of Cu2+species IVVI are present in the phase of gel II inapproximately equal amounts. At Cu2+ 0.57 mmolCu2+ per gram of gel II, the ratio nIV : nV : nVI was0.06 : 0.20 : 0.74. The reactivities of species IVVIRUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 1 2004OXIDATIVE DEHYDROGENATION OF 2,3,5-TRIMETHYL-1,4-HYDROQUINONE 103were judged from the initial reaction rates v0(IV VI),estimated at 0.35, 1.00, and 1.20 mol l1 h1, respec-tively. That is, the Cu2+ ions bound in magnetic asso-ciates V and polynuclear compounds VI are morereactive than those in mononuclear complexes IV.This is apparently due to the mobility of electrons insystems V and VI, in which the Cu2+ ions are linkedto each other and the electron density transfer is facili-tated. Previous measurements of the microwave con-ductivity [12, 13] revealed a correlation between theelectron mobility and catalytic activity of Cu2+ ions.Acceleration of the dehydrogenation is due to the factthat oxidation of substrate I and reduction of oxygencan be in this case separated in space.Thus, we can conclude from our results that theoxidative transformation of I in the presence of gel IIoccurs in the sphere of homo- and heteroassociativeinterparticle interactions involving the gel, substrate,and oxidant.EXPERIMENTALTitanium dioxide gel II with the specific surfacearea Ssp 26013 m2 g1 (as determined by adsorptionof an inert gas) was prepared by hydrolysis of analcoholic solution of tetrabutoxytitanium with waterat room temperature with vigorous stirring.Experiments were performed in a temperature-controlled reactor equipped with a reflux condenserand a bubbler for air supply. The mixture was stirredat a rate of 2 s1; the air flow rate was 6.2 l h1. Theseconditions ensure kinetic control of the reaction. Thereaction was performed in aqueous methanol (water :methanol = 1 : 1 by volume) at 500.2 C. Oxygenand copper dichloride dihydrate in the presence ofatmospheric oxygen were used as oxidants. The sub-strate was dissolved in aqueous methanol to obtain a6.6  102 M solution, and the gel (0.13 mol l1)and CuCl2 2H2O (0.59  102 M) were added. Thisamount of CuCl2 2H2O was completely sorbed on thegel within 45 s (content of sorbed Cu2+ ions on thegel Cu2+ 0.57  103 mol g1 gel).Kinetic measurements were performed by takingaliquot samples and determining the content of thestarting compound. The functions obtained wereapproximated by polynomials. The initial reactionrates v0 were determined by numerical differentiationand interpolation; their error did not exceed 10%.The initial reaction rates v0(IV VI) characterizingthe individual reactivities of Cu2+ species IVIV inthe phase of gel II were calculated by solving a sys-tem of equations of the formv0i = v0IVnIVi + v0VnVi + v0VInVIi,where v0i is the initial reaction rate at a Cu2+ contentin the gel phase of i; v0IV, initial reaction rate in thepresence of mononuclear Cu2+ complexes IV; v0V,initial reaction rate in the presence of associates V ofmononuclear complexes; v0VI, initial reaction rate inthe presence of Cu2+ hydroxide VI; n(IV VI)i, relativecontents of species IVVI at a total Cu2+ concentra-tion in the gel phase of i. The quantities v0(IV VI)were determined with an error of 15%, which in-cludes the error of determining n(IV VI) by ESR.Quantitative determination of I was performed byGLC on a Chrom-4 chromatograph similarly to [4].The IR spectra were recorded on a Specord M-80spectrometer in the range 4000400 cm1 in KBr andmineral oil. The spectra were taken in the optical den-sity scale; the error of indication of the optical densitywas 0.0001. Samples for measurements were preparedby sorption of hydroquinone I from solution on gel II,followed by drying in an inert gas flow. The bandassignment was based on [5].The ESR spectra of Cu2+ in the gel phase wererecorded on a PS-100Kh ESR radiospectrometer in the3 cm wavelength range at room temperature in thin-walled round quartz ampules (d 4 mm). The spectrawere processed with special programs from the spec-trometer software. The integral intensioties of ESRsignals were analyzed using a reference sample andsoftware for spectrum processing, following the pro-cedures suggested in [1416]. Samples for studieswere prepared by sorption of Cu2+ ions on gel II fromCuCl2 2H2O solutions, followed by drying at roomtemperature. The concentration of Cu2+ ions in the gelphase was determined by atomic absorption spectros-copy on a PerkinElmer-403 spectrometer.The procedure of voltammetric experiments wasdescribed in [17]. The supporting electrolyte was0.025 M HCl in aqueous methanol (1 : 1 by volume);the Cu2+ concentration in solution was 5  103 M,and the concentration of I in solution, 102 M. Thepotential sweeping rate was 20 mV s1.ACKNOWLEDGMENTSThe study was financially supported by the RussianFoundation for Basic Research (project no. 01-03-96 521).REFERENCES1. Petrov, L.A., Kharchuk, V.G., Shishmakov, A.B., andTokarev, E.A., Zh. Org. Khim., 1998, vol. 34, no. 3,p. 376.2. Kharchuk, V.G., Shishmakov, A.B., Tokarev, E.A.,RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 1 2004104 KHARCHUK et al.and Petrov, L.A., Abstracts of Papers, XVI Mendele-evskii s ezd po obshchei i prikladnoi khimii (XVIMendeleev Congr. on General and Applied Chem-istry), Moscow, 1998, vol. 2, p. 517.3. Gekhman, A.E., Guseva, V.K., and Moiseev, I.I.,Kinet. Katal., 1989, vol. 30, no. 2, p. 362.4. Kharchuk, V.G., Kolenko, I.P., Petrov, L.A., andGus’kova, L.M., Zh. Org. Khim., 1986, vol. 22, no. 11,p. 2306.5. Bellamy, L.J., The Infra-Red Spectra of ComplexMolecules, New York: Wiley, 1957.6. Koryakova, O.V., Zhuravlev, N.A., Kovyanina, S.A.,Vovkogrud, E.G., Maksimova, L.G., Kharchuk, V.G.,Shishmakov, A.B., Kuznetsova, O.V., and Mikushi-na, Yu.V., Abstracts of Papers, Seminar SO RANUrO RAN po termodinamike i neorganicheskim ma-terialam (Workshop of the Siberian and Ural Divi-sions of the Russian Acad. Sci. on Thermodynamicsand Inorganic Materials), Novosibirsk, 2001, p. 94.7. Andrews, L.J. and Keefer, R.M., Molecular Com-plexes in Organic Chemistry, San Francisco: Holden-Day, 1964.8. Aleksanyan, V.T., Vibrational SpectroscopyModernTrends, Barnes, A.J. and Orville-Thomas, W.J., Eds.,Amsterdam: Elsevier, 1977.9. Poznyak, S.K., Pergustov, V.I., Kokorin, A.I., Ku-lak, A.I., and Schlapefer, C.W., J. Phys. Chem. (B),1999, vol. 103, no. 8, p. 1308.10. Frolova, E.N., Cand. Sci. (Phys.-Math.) Dissertation,Kazan, 1994.11. Nikitaev, A.T., Pomogailo, A.D., Golubeva, N.D., andIvleva, I.N., Kinet. Katal., 1986, vol. 27, no. 3, p. 709.12. Shaikhutdinov, Sh.K., Trukhan, E.M., Astanina, A.M.,Molochnikov, L.S., Frumkina, E.L., and Kopylo-va, V.D., Zh. Fiz. Khim., 1986, vol. 60, no. 4, p. 997.13. Min’ Tyau Van, Tuen Nguen Van, and Rudenko, A.P.,Vestn. Mosk. Gos. Univ., Ser. 2: Khim., 1995, vol. 36,no. 3, p. 278.14. Poole Ch.P., Jr., Electron Spin Resonance. A Com-prehensive Treatise on Experimental Techniques, NewYork: Interscience, 1967.15. Te Tse-Chang, Magn. Reson. Rev., 1984, vol. 9.nos. 13, p. 65.16. Molin, Yu.N., Chibirkin, I.N., Shabalkina, V.A., andShuvalov, V.F., Zavod. Lab., 1966, vol. 32, no. 8,p. 933.17. Volkov, V.L. and Buldakova, L.Yu., Elektrokhimiya,1994, vol. 30, no. 7, p. 892.

Oxidative dehydrogenation of 2,3,5-trimethyl-1,4-hydroquinone in the presence of titanium dioxide hydrogel

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Oxidative dehydrogenation of 2,3,5-trimethyl-1,4-hydroquinone in the presence of titanium dioxide hydrogel

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