Electron-transfer complex formationand oxidation of naphthalene in zeolites

H.K. Beyer, H.G. Karge, I. Kiricsi and J.B. Nagy (Eds.)

Catalysisby MicroporousMaterials

Studies in Surface Science and Catalysis, Vol. 94 9 1995 Elsevier Science B.V. All rights reserved.

606

ELECTRON-TRANSFER

COMPLEX FORMATION

A N D O X I D A T I O N OF N A P H T H A L E N E IN Z E O L I T E S

A.M.Eremenko, V.M.Ogenko, A.A.Chuiko

Institute of Surface Chemistry, National Ukrainian Academy of Sciences Prospect Nauki 31, Kiev 252022, Ukraine

Low-temperature luminescence spectra of naphthalene (Nph) adsorbed in the alkali and alkaline-earth forms of the X zeolites indicate molecules physically adsorbed and bound in donor-acceptor complexes. Using spectral methods the Nph oxidation reaction has been found to be promoted by water vapor. The oxygen chemisorbed in the cages is considered as an oxidizer ofNph. Nph forms photostable dimer associates on the surface of amorphous aerosil.

INTRODUCTION

Photophysical and photochemical behavior of the polyacene molecules such as pyrene, anthracene, perylene adsorbed in the cages of X and Y zeolites depend on the cation and hydration degree of zeolites [ 1-3 ]. According to [4] the polyacene molecules with an ionization potential less than 7 eV can be ionized in cages of the X and Y zeolites. The excited molecules are quenched by transition metal ions such as Cu 2+ , T1+, the active sphere radius of which ranges from 10 to 18 A [3,4 and references therein]. The zeolite lattice participates in both the donation and acceptation of electrons in regard to various substrates in many processes of importance. In regard to polyacenes the dehydrated zeolite lattice manifests, as a rule, acceptor-like properties. D. Oelkrug with co-authors [5] found the formation of the Nph donor-acceptor

607 charge transfer complex (CTC) on the surface of the catalytic active alumina-silicates. For the MgO surface one marks luminescence of adsorbed Nph associate with a peak at 400 nm [5]. In [6,7] no additional band indicative of a donor-acceptor compound formation has been found in the spectra of naphthalene adsorbed on pure silica. Luminescence in the 400 nm region for Nph on the silica gel surface with a coverage 36 % of a monolayer is assigned to formation of a true excimer. Polyacenes on the amorphous silica surface form associates similar to physical dimers [7-10]. In the present paper we compare the photophysical properties of naphthalene being chemically and photochemicallythe most stable, on the dehydrated X zeolite surface of various ionexchanged forms and on the surface of the pyrogeneous non-porous silica aerosil.

EXPERIMENTAL

We used faujazites type NaX zeolite with the common formula: 0.94Na20*A1203 92.77SIO2.6.12H20 - - (Si/AI = 1.38).The cation forms of the zeolites were prepared by ionic exchange using 0.025 M solution of the Ca, Mg, Ba, Rb and Cs salts. The degree of ion exchange in all cases was 40 to 48 %. The thermal activation conditions of the samples: 550 ~ in air and at~erwards - - 400 ~ in vacuum cuvettes. The Ao300 aerosil (Ssp =260 m2/g) was activated under conditions of dehydroxylation at 800 ~ in air and ax~

at 400 ~ in high

vacuum before carrying out Nph adsorption. The naphthalene, which was spectrally pure, was purified by the zone melting method. Hexane was dried under zeolites and then distillated.We used vacuum cuvettes made from optical quartz or uviol glass. The adsorption of Nph was carried out from its solutions in hexane. The samples were pumped down to 1.3.10 .2 Pa during 4 hours and sealed in vacuum. Stationary luminescence and excitation spectra were recorded using SDL-1 spectrograph with excitation from a xenon lamp at 77 K and room temperature. The measurement accuracy for line intervals in line spectra was 0.08 rim. The kinetics of luminescence decay was recorded with the pulse nanosecond fluorimeter PRA-1000. Instant spectra were recorded with a laser fluorimeter with excitation from a pulse nitrogen laser (~oxc = 337.1 rim; pulse duration = 10 ns).

608 R E S U L T S AND D I S C U S S I O N

The N p h molecules adsorbed from a solution in hexane on the NaX zeolite surface exhibite a strong emission consisting of structured fluorescence and phosphorescence and an additional structureless band with a peak at 400 nm (Fig.l). The vibrational analysis of the structural spectra is presented in Table 1. Table 1 Band positions and electron vibrational analysis of the luminescence spectrum o f N p h adsorbed in the NaX zeolite Phosphorescence

Fluorescence Assignment

Band frequence, cm

-1

00

31900

01

31370

01

Vibration frequence, cm

Assignment

Band frequence, cm 1

-1

Vibration frequence, cm

-1

00

21300

530

01

20790

512

30930

970

01

19900

1380

01

30480

1420

02

19410

512 + 1380

02

30015

530 + 1360

02

18520

2 • 1380

02

29560

970 + 1380

02

29070

2 x 1410

28120

2 x 530 + 2 x 1360

Their position in regard to the energy scale and intensity distribution place these spectra in the same series with the spectra of the molecules in neutral solvents (nparaffins) and are indicative of physical adsorption in zeolite cages. The spectral band broadening seems to refer to several spatial positions of Nph in the zeolite. The spectral position of an additional band corresponds to that of the Nph excimer i.e. excited bimolecular associates [11,12]. It is known that the polyacene excimers in solutions are formed by molecule coming close together by diffusion and that they have a dissociative ground state [13]. Recording of the adsorbate spectrum at low temperatures helps to understand the mechanism of formation of new

609 interaction products, since under these conditions the diffusion of the molecules is retarded and excited dimers cannot appear in the spectrum. Fig.1 gives evidence of formation of a stable compound . Associates which are stable in the ground state are discussed in [7,8] for Nph adsorption on silica gel. The excitation spectrum is slightly shifted with regard to that the Nph monomer. For zeolites, the excitation spectrum with a peak at 400 nm is shifted in regard to the Nph molecular form spectrum by 40 nm (Fig. 1). The spectra of Nph in the zeolite lattice and on the amorphous cartier aerosil (Fig.2) are of different kind.

Ill i

o ,),, ~"

I

"i"l,tl,

C

b )" ",,, /

-:c I-

0

--1

260

I

it1

,

,

~

w

m

!

,

,

360

,

,',

!

,

,

,

!

,

,

460

,

i

,'f',

'l

,"!

560

o

,

,

,

,

,

,

250

X, nm

,

,

,

!

,

,

,

350

,

,

,

,

w

,

i

,

i

4.50

nm

Figure 1. Excitation (a) and luminescence (b) spectra of Nph on NaX zeolite, a =

Figure 2. Excitation (a) and luminescence (b, c) spectra of Nph on aerosile, a =

1.7.10 .4 mole/g, 77 K.

1.10 6 mole/g. ~,oxc= 254 nm (b) and 320 (a-

nm (c). adsorbed amount of Nph)

The spectrum of Nph form on aerosil is not resolved. The wide-band fluorescence contribution relative to the molecular emission is large, afterglow is not observed. The wideband excitation spectrum at 400 nm is shifted relatively to that of a molecular form by 10 nm. For zeolites this is mainly CTC and for aerosil a bimolecular associate of Nph. When adsorbing from a vapor phase, the emission spectrum of Nph in a zeolite consists of a continuous structureless band which is a superposition of CTC and dimers adsorbed at the outer surface (Fig. 3a).In the case of co-adsorption of water vapor or hexane the spectrum transforms with the appearance of structured fluorescence and phosphorescence components (Fig.3b). The coadsorbate seems to promote breaking up of dimers and diffusion of molecules in zeolite cages.

610

O

.

rO

a

.0 E3C'4

6o m

0

i

540

,

,

360

380

' 400

' 420

X,

!

" 4z~O

'

9

460

nm

5

3o6

,

9

,

i

,

i

,

466

9

,

,

)~,

w

9

v

56c~

"',

9

,

,

9

,

,

66o

lqlqq

@ :::3 o_

_6 ~ k._ oo m

O "~",

310"

""~3'20 . . . . . 5 ' 3 0 "

X.,

w

w

,,

34b'. . . . .

w

~,',~0

w',

,

~

,"~

'~,'60

w

i

,

w

I

~

'~

I

370

rim

Figure .3. Luminescence spectra for Nph on zeolite NaX when adsorbed from the vapor phase(a); after water vapor admission under vacuum conditions (b); appearance of the Nph oxidation product: o~-naphthoquinon (c).

611 The N p h monomer form decay has a polyexponential run both in the zeolite and on aerosil. The fluorescence lifetime computed in the two-exponent approximation according to equation If(t)=A exp(-t/~l)+A exp(-t/'c2) varies within 7 to 40 ns depending on the N p h concentration. The emission decay duration for the 400 nm band is within 100 to 170 ns. The relaxation processes indicative of changing into a mutual orientation and of different intermolecular distances for the aerosil adsorption centers occur in the excited associates of Nph adsorbed by the aerosil surface. This fact is confirmed by shi~s from 400 to 440 nm in the

instant spectra with delayed recording [9]. Space limits in the zeolite cages prevent the molecules from easy movement. When the filling is more than one molecule per cage, favorable conditions for dimer stabilization arise. The conditions in our experiments eliminated such cage filling in the case of adsorption from solutions. Contacts of the evacuated zeolite samples containing alkali and alkaline-earth cations with water vapor promote a chemical reaction leading to the appearance of some new luminescent products (Fig. 3b, c) . An vibrational analysis of the spectrum and the relatively high intensity of the purely electronic transition are indicative of the correspondence of this spectrum to one of the low-symmetry Nph derivatives. This spectrum is identical with that of o~-naphthoquinon in the Nph crystal (Table 2). Table 2 Band positions and vibrational analysis of the luminescence spectrum for the Nph oxidation product in the NaX zeolite Assign- Band frequence, ment cm 1 00 31060

Vibration frequence,

Assignment 01

Band frequence, cm ~ 30104

Vibration frequence, cm "~ 954

01

30780

280

01

30026

1034

01

30660

400

01

29668

1392

01

30550

510

01

29586

1474

01

30303

757

02

29158

510 + 1382

c m "1

An oxidizer seems to be on the zeolite surface in form of chemisorbed oxygen which has not removed under the usual conditions of activation. Water vapor adsorption seems to decrease

612 the strength of chemisorbed oxygen to the zeolite surface. In early papers [14,15] oxidizing properties of the decationized zeolites were assigned to the Lewis-type aprotonic acid centers with oxygen chemisorbed on them. The results obtained indicate that oxidizing properties are observed also for cationized zeolites which seem to contain parts of a structure similar to decationized zeolites and alumosilicates. For examination of this idea the sequence of treatment of zeolite NaX was as follows: 1) evacuation of zeolite at 550 ~

2) adsorption of water vapor in the zeolite and consequent

evacuation at 200 ~ (the chemisorbed oxygen is removed from zeolite lattice at this stage); 3) adsorption of

Nph. In this case no Nph oxidation product was observed. This experiment is

indicative of the fact that the sodium zeolite surface contains strongly bound oxygen which is removed during the first cycle of treatment by water vapor and which is responsible for oxidizing properties of this surface. At the same time, the oxygen, which can be considered as an acceptor center during the process of CTC formation, is not the single acceptor. The CTC spectrum remains even after removing this chemisorbed oxygen (Fig.3b). It hould be noticed that acceptor centers in regard to the positions of the

Nph molecule are similar by their nature since the

Nph emission and excitation spectra within the CTC band are identical for the

cationized zeolites of alkali and alkaline-earth forms. At the same time, the photonics of the

Nph molecular adsorbate form depend strongly on the cation exchange character. In the case of presence of such heavy ions like Rb, Cs and Ba one marks a sharp increase of the phosphorescence yield in consequence of an increase of the singlet-triplet interconversion. The

Nph oxidation reaction changes its direction in presence of alkaline-earth cations. Along with the oc-naphthoquinon (v00 =31060 spectrum (v00 =29912

cm1) the 13-naphthol molecules with a characteristic line

cm~) are formed in cages of the Ca, Mg, Ba forms. The product

identification is made according to the spectrum of ]3-naphthol in the quenching the

Nph crystal. Efficient

Nph fluorescence occurs in presence of the transition metals in zeolite lattices.

Quenching seems to include total electron transfer into vacant orbitals of the Co, Ni, Cu ions in static complexes.

613 CONCLUSIONS

The photonics of Nph molecules on the surfaces of the zeolite X as well as on aerosil has

Nph in zeolite voids is present as physically adsorbed molecules and bound in donor-acceptor complexes (CTC). Water vapor admission to an adsorbed Nph causes

been studied.

oxidation under formation of luminescent products of a-naphthoquinon on the Na-forms and its mixture with b-naphthol in presence of the alkaline-earth cations. Assumption about participation of oxygen chemisorbed on acid centers in an oxidation reaction is substantiated. In presence of transition metal ions the

Nph luminescence is totally quenched. The Nph

molecules adsorbed on the dehydroxylated aerosil surface form bimolecular excimer-like associates with a stable ground state and resistance to UV irradiation.

REFERENCES 1. J.K. Thomas, Chem. Rev.,93 (1993) 301. 2. K.B. Yoon, Chem.Rev.,93 (1993) 321. 3. X. Liu, K-K. Iu and J.K. Thomas, J.Phys.Chem., 95 (1989) 4120, Langmuir, 6 (1990) 471, J.Phys.Chem., 95 (1991) 506. 4. X. Liu and J.K. Thomas, Langmuir, 9 (1993) 727. 5. D. Oelkrug, M. Radgapour and H. Erbse, Z. Phys. Chem.N.F., 88 (1974) 23. 6. K. Hara, P. de Mayo, W.R. Ware and K.C. Wu, J.Phys. Chem., 86 (1982) 3781. 7. R.K. Bauer, P. de Mayo, W.R. Ware and K.C. Wu, J. Phys. Chem., 86 (1982) 3781. 8. P. de Mayo, L.V. Natarajan and W.R. Ware, Symp. 18th Meet. Am. Chem. Soc. Washington D.C. (1985) 1. 9. A.M. Eremenko, Photonics of Adsorption Complexes of Aromatic Compounds on Silica. Thesis Dr. of Sciences, Moscow, Karpov Phys. Chem. Institute, 1989. 10. A.M. Eremenko, L.A. Bykovskaja and A.A. Chuiko, J.Molec. Struct., 218 (1990) 345. 11. N. Mataga, T. Okada and K. Ezumi, Mol.Phys., 9 (1966) 367. 12. F.J. Smith, A.T. Armstrong and S.P. McGlynn, J.Chem.Phys., 44 (1966) 442. 13. Th. Forster, Excimeres and Exciplexes, M. Gordon and W.R. Ware (Eds), New York, Acad.press. 1975, 1. 14. F.R. Porter and W.K. Hall, J.Catal.,5 (1966) 366. 15. F.R. Dollish and W.K. Hall, J.Phys. Chem., 71 (1967) 1005. 16. Y. Mao and J.K. Thomas, Langmuir, 8 (1992) 2501.