Ionization and protonation of aromatic diamines by sorption in zeolites

Ionization and protonation of aromatic diamines by sorption

in zeolites

S. Marquisa, A. Moissettea,*, C. Bremarda, H. Vezinb

aLaboratoire de Spectrochimie Infrarouge et Raman UMR-CNRS 8516, Bat. C5 Universite des Sciences et Technologies de Lille,

59655 Villeneuve d’Ascq cedex, FrancebLaboratoire de Chimie Organique et Macromoleculaire ESA-CNRS 8009, Bat. C3 Universite des Sciences et Technologies de Lille,

59655 Villeneuve d’Ascq cedex, France

Received 2 September 2002; accepted 23 September 2002

Abstract

In situ diffuse reflectance UV visible, Raman scattering and EPR spectroscopies were used to monitor spontaneous ionization

of N,N,N0,N0-tetramethyl-p-phenylenediamine (TMPD) by direct exposure to dehydrated MnZSM-5 zeolite (M ¼ Naþ, Hþ;

n ¼ 0,3,6). The TMPD†þ radical cation is found to be generated in low yield in purely siliceous silicalite-1 whereas TMPD†þ is

generated in high yield in aluminated NanZSM-5 and HnZSM-5. The ejected electron is also characterized though electronic

absorption spectra. The charge separation was found to be persistent over several months. The tight fit between the shape of

TMPD and the pore size of straight channels of zeolites is considered to be the main factor responsible for the stabilization of

the TMPD radical ion by preventing rapid electron back transfer. Within NanZSM-5, the amounts of TMPD†þ and trapped

electron were found to decrease over long time and to recombine to molecular TMPD. In contrast, in acidic HnZSM-5 zeolite

the charge recombination generates diprotonated TMPDH22þ occluded species. Furthermore, within HnZSM-5, protonation

appears competitive to the ionization efficiency of zeolite.

q 2003 Elsevier Science B.V. All rights reserved.

Keywords: Zeolite; Aromatic amine; Radical cation; Ionization; Protonation

1. Introduction

A variety of organic radical cations can be

generated spontaneously by inclusion of their

precursors in the void space of porous materials

such as zeolites [1–7]. The tight fit between the

shape of the occluded species and the pore size of

zeolites is considered to be an important factor

responsible for the stabilization of radical cations

[3,6]. The strong electron donor aromatic amines are

known to have low ionization potential in the gas

phase and to exhibit proton affinity. The rod-shaped

N,N,N0,N0-tetramethyl-p-phenylenediamine (TMPD)

molecules can penetrate within the channels of

ZSM-5 type of zeolites [7]. The fully N-methylated

derivative was chosen to avoid further intrazeolite

conversion. The spectroscopy and behavior of

TMPD and protonated species as well as radical

cation in solution are well documented [8–12].

0022-2860/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved.

PII: S0 02 2 -2 86 0 (0 2) 00 6 46 -4

Journal of Molecular Structure 651–653 (2003) 305–314

www.elsevier.com/locate/molstruc

* Corresponding author. Fax: þ33-3-20436755.

E-mail address: [email protected] (A. Moissette).

http://www.elsevier.com/locate/molstruc

After dehydration, ZSM-5 zeolites are microporous

crystalline aluminosilicates which contain two types

of intersecting channels with openings of 5.5 mm in

size. The silicalite-1 (Si/Al , 1000) is the purely

siliceous analogue of the ZSM-5 zeolite. The

incorporation of aluminum (III) in the framework

induces the presence of counterbalancing extrafra-

mework cation in MnZSM-5 zeolites (M ¼ Naþ,

Hþ; n ¼ 3, 6).

In this paper, we report results obtained upon

incorporating TMPD in MnZSM-5 zeolites with the

chemical composition Mn(AlO2)n(SiO2)962n. The

resulting samples were probed by the application of

diffuse reflectance UV-visible absorption (DRUVv),

electron paramagnetic resonance (EPR) and Raman

scattering spectroscopies.

2. Experimental

2.1. Materials

The sodium and NH4þ-exchanged ZSM-5 samples

(Si/Al ¼ 13, 27, particle size ,1 mm) were obtained

from VAW Aluminum (Schwandorf, Germany).

Crystals of silicalite-1 (Si/Al . 1000) were syn-

thesised in high purity according to the fluoride

medium procedure and was a gift of Dr J. Patarin

(Laboratoire des Materiaux Mineraux, ESA 7016,

CNRS- ENSC Mulhouse). All the zeolite samples

were used after a calcination procedure up to 500 8C.

The unit cell compositions of the calcined and

dehydrated MnZSM-5 (M ¼ Naþ, Hþ, n ¼ 0, 3.4,

6.6) were found to be Si96O192, Na3.4(AlO2)3.4

(SiO2)92.6, Na6.6(AlO2)6.6(SiO2)89.4, H3.4(AlO2)3.4

(SiO2)92.6, and H6.6(AlO2)6.6(SiO2)89.4 from elemental

analysis. In the following text, we use the notations

Na3ZSM-5, Na6ZSM-5, H3ZSM-5, H6ZSM-5 for

convenience. The crystallinity of the samples was

checked by XRD and was not reduced by the thermal

treatment. TMPD molecules (C10H16N2) were syn-

thesized and purified as described previously [11].

Pure and dry Ar and O2 gas were used.

2.2. Sorption of TMPD

Weighted amounts (,1.4 g) of freshly calcined

zeolite were held under vacuum and then under dry

argon. Weighted amount of TMPD, corresponding to

one mole per unit cell (UC), was introduced into the

cell under dry argon and the powder mixture was

shaken. The sample was transferred under dry argon

in a Suprasil quartz glass cuvette for DRUVv and FT-

Raman experiments and in a sealed quartz tube for

EPR measurements.

2.3. Instrumentation

The UV-visible absorption spectra of the sample

were recorded between 200 and 800 nm using a Cary

3 spectrometer. The instrument was equipped with an

integrating sphere to study the powdered zeolite

samples through diffuse reflectance; the correspond-

ing bare zeolite was used as the reference. The

DRUVv spectra were plotted as the Kubelka–Munk

function: FðRÞ ¼ ð1 2 RÞ2=2R ¼ K=S; where R rep-

resents the ratio of the diffuse reflectance of the loaded

zeolite to that of the dehydrated neat zeolite, K

designates an absorption coefficient proportional to

the concentration of the chromophore and S the

scattering coefficient of the powder.

Raman scattering spectra were collected on a

LabRAM spectrometer (Instruments S.A) equipped

with a Peltier-cooled Charge Coupled Device. The

excitation wavelength used was 632.8 nm with low

laser power to avoid irreversible laser effects for the

sample. The laser lines were supplied by ionized

argon laser and Helium–Neon laser. The spec-

trometer calibration was verified using the Raman

lines of silicon. This resulted in an accuracy of less

than ^1 cm21.

A BRUKER IFS 88 instrument was used as a FT-

Raman spectrometer with a cw Nd:YAG laser at

1064 nm as excitation source. A laser power of 100–

200 mW was used. The spectra (3500–150 cm21)

were recorded with a resolution of 2 cm21 using 400

scans. The Opus BRUKER software was used for

spectral acquisition, data treatment and plotting.

The (X-band) EPR spectra were recorded at room

temperature on a Bruker ESP-300 spectrometer. The

EPR signals were double integrated using Bruker

software and the spin concentration was determined

relatively to a reference standard.

The structural modeling calculations were per-

formed on a Silicon Graphics workstation using

Cerius2 (version 3.8) package from Molecular

S. Marquis et al. / Journal of Molecular Structure 651–653 (2003) 305–314306

Simulations. The zeolite structural parameters, force

field and calculation details used to carry out the

minimization procedure of the non-bonding energy

were as described previously [13].

The data processing of the UV-visible absorption

spectra recorded during the course of the sorption

was performed using the SIMPLISMA (SIMPle-to-

use Interactive Self-modeling Mixture Analysis)

approach [14]. This method was applied to extract

the characteristic spectra of species generated

through sorption from many spectral data, which

resolves spectrum of mixture into pure component

spectra without any prior information. In addition

the program calculates the relative concentration of

the species. The accuracy of the calculation is

given by residuals, which represent the difference

between the reconstructed, and the original

data. This coefficient can be seen as a standard

deviation and was defined previously [15]. The

values are between 0 and 1. The algorithm for

SIMPLISMA calculation was described in detail

elsewhere [15].

3. Results and discussion

3.1. Sorption of TMPD into silicalite-1 (Si/Al ¼ 1000)

The mere exposure of solid TMPD to dehydrated

silicalite-1 powder generates weak purple coloration.

The EPR experiments provide evidence of ioniz-

ation. The formation of radical cation is detected by

the characteristic absorption features (611, 562,

518 nm) exhibited in the DRUVv spectra recorded

after the mixing of powders [7]. The sorption of

TMPD molecules within the void space of zeolite

was monitored through the increase of the absorp-

tion bands at 276 and 325 nm. The sorption course

of TMPD at loading corresponding to one mole per

unit cell (1 TMPD/UC) was monitored as a function

of time by FT Raman spectrometry. The progressive

disappearance of solid TMPD is observed through

the decrease of the Raman bands at 777 and

1620 cm21 whereas the occupancy of the void

space is observed through the appearance of

Raman bands at 783 and 1630 cm21. The ionization

yield was found to be too weak to be detected by

Raman spectrometry with off resonance conditions.

However, after a long exposure, TMPD†þ was

detected in low yield through the band at

1632 cm21.

Energy minimization procedures of the nonbond-

ing interactions between rigid planar molecule and

fixed zeolite framework provide a reasonable

structural picture of the ability of TMPD to pass

through the pore openings of silicalite-1. The

molecules were found to penetrate into the internal

void space of the zeolite and the preferred

locations lay in straight channels in the

vicinity of the intersection with zigzag channel

with the C–N bonds running along the b direction

of the cell, Fig. 1.

3.2. Sorption of TMPD into NanZSM5 (Si/Al ¼ 27, 13)

Na3ZSM-5 (Si/Al ¼ 27)—The exposure of solid

TMPD to dehydrated Na3ZSM-5 generates rapidly

a deep purple colour. The EPR experiments

indicate an intense signal in agreement with

ionization of TMPD in high yield upon sorption.

DRUVv spectra recorded as a function of time

after the mixing of the powders with 1 TMPD/UC

loading exhibit new absorption bands in the 500–

650 nm region characteristic of TMPD radical

cation, TMPD†þ (Fig. 2). The SIMPLISMA

approach carried out on the spectra set provides

evidence of three extracted spectra (Fig. 3). The

first extracted spectrum is assigned to molecular

TMPD. The second extracted spectrum displays the

spectral features at 611, 562, 518 and 325 nm of

TMPD†þ. The spectrum with broad band observed

in the 350–600 nm region is assigned with respect

to previous works [6] to the electron transferred to

the zeolite framework after the spontaneous TMPD

ionization. The broad band observed is explained

by different environments and so different

local fields for electrons during the sorption course,

Fig. 3(c) [16]. Over one year, DRUVv spectra

provide evidence of progressive decrease of radical

cation and electron signatures. These decreases are

correlated with a significant increase of the 270 nm

band intensity characteristic of molecular TMPD.

This feature shows that recombination of radical

cation occurs through the capture of an electron

and formation of neutral TMPD occluded in the

zeolite channels.

S. Marquis et al. / Journal of Molecular Structure 651–653 (2003) 305–314 307

The sorption process of TMPD at 1 TMPD/UC

loading was monitored as a function of time by FT-

Raman spectrometry. Over one month, the Raman

features of solid TMPD disappear nearly totally

whereas the Raman bands of TMPD†þ at 1633, 1508,

1418 cm21 are found to increase significantly. After

more than 3 months, the characteristic bands of neutral

occluded TMPD centered at 777 and 1620 cm21

increase at the expense of TMPD†þ consistent with

the recombination of radical cation as occluded TMPD.

Using the 632 nm excitation line, the resonance

Raman spectra for TMPD occluded within Na3ZSM-5

displays exclusively the spectral features of radical

cation.

Na6ZSM-5 (Si/Al ¼ 13)—The ionization process

is found to be more rapid for TMPD adsorbed into

Na6ZSM-5 compared to TMPD within Na3ZSM-5 but

the process and maximum yield of TMPD†þ are

found to be analogous before the charge

recombination.

3.3. Sorption of TMPD into acidic HnZSM-5

(Si/Al ¼ 13, 27)

H3ZSM-5 (Si/Al ¼ 27)—The mixing of TMPD

and dehydrated zeolite induces nearly instantaneous

deep purple coloration. The EPR experiments

indicate an intense signal in agreement with

ionization of TMPD in high yield upon sorption.

DRUVv spectra recorded as a function of time from

the mixing of the powders exhibit new absorption

bands in the 500–650 nm spectral range character-

istic of TMPD radical cation. The maximum

intensity of TMPDþ† band is reached within 3

days. Concomitantly to the TMPDþ† bands in the

visible region, we can also observe the significant

increase of the 325 nm and 260 nm bands, Fig. 4(a).

These two bands are assigned to contribution of

both TMPD†þ and TMPD. The 325 nm band is

mainly due to TMPD†þ whereas the 260 nm one

comes essentially from TMPD. Within 7 days,

Fig. 1. Modeling of the structure of TMPD occluded in the straight channel of silicalite-1. Black and shaded sticks represent the O and Si atoms

of the (SiO2)96 framework, respectively. The white, dark grey and black cylinders represent the H, C and N atoms of the C10H16N2 molecule,

respectively.


the 260 nm band is no more visible in agreement

with nearly total ionization of TMPD. However, it

should be noted that a new weaker band is observed

at 255 nm. This band is assigned to TMPDH22þ, the

diprotonated form of TMPD by comparison with the

spectrum of diprotonated TMPD obtained in con-

centrated chloride acid solution (12 M), (not shown).

Over longer period of time, the TMPD†þ contri-

bution slowly decreases relatively to the weak band

of protonated TMPD. One year after the mixture of

the powders, the DRUVv spectrum displays only

weak contribution of TMPD†þ, Fig. 4(b). It should

be noted that the extinction coefficients of

TMPDH22þ are markedly weaker than the radical

Fig. 2. DRUV spectra recorded at room temperature during the course of adsorption of TMPD into Na3ZSM-5 [Na3.4(AlO2)3.4(SiO2)92.6] zeolite

by mixing of powders. (a) 5 min to 48 h (b) 48 h to 4 months.


cation ones [8]. Thus, assumption is made that

TMPD†þ recombination within acid zeolite chan-

nels occurs exclusively through diprotonation of

TMPD and that a mixture of TMPD†; and

TMPDH22þ is observed one year after the beginning

of the reaction.

Data processing using the SIMPLISMA program

of numerous diffuse reflectance UV-visible spectra

recorded over several months during the course of

TMPD sorption into calcined H3(AlO2)3(SiO2)93

provides evidence of four independent spectra

(Fig. 5). The first extracted spectrum is assigned to

solid TMPD. The second spectrum exhibits the

characteristic features of TMPD†þ. The spectrum

with broad band observed in the 350–600 nm region

is assigned to the ejected electron transferred to the

zeolite framework after the spontaneous TMPD

ionization [6]. The shape of the spectral signature of

the electron differs slightly from that observed for

electron within NaZSM-5. The fourth spectrum

corresponds to TMPDH22þ.

Protonation is only a minor phenomenon compared

to ionization during the first steps of the sorption

process. However, the increase of concentration of

TMPDH22þ corresponds to the TMPD†þ decrease.

Thus, it is obvious that TMPDH22þ essentially

comes from TMPD†þ recombination with electron.

The neutral molecule TMPD contribution is found to

fall to zero in agreement with total ionization and

protonation.

The sorption process of TMPD (1 TMPD/UC) was

monitored as a function of time by FT-Raman

spectrometry. Over several days, the Raman features

evolved from the characteristic spectra of solid TMPD

[11,12] to spectra characteristic of TMPD†þ (Fig. 6).

In these off resonance conditions, the prominent line

of neutral TMPD centered at 777 cm21 disappears

after 5 days. The observation of only radical cation

features with high signal/background ratio provides

evidence of nearly complete ionization of TMPD

molecule. After about 20 days, the Raman spectra

display supplementary weak lines at 797, 900, 995,

1132, 1467 cm21 and shoulder at ca. 1610 –

1620 cm21. These lines are assigned to TMPDH22þ,

by comparison with Raman spectrum recorded in

hydrochloride concentrated solution, Fig. 6 [11]. One

year and a half after the mixing of the powders, the

Raman spectrum is still representative of a mixture of

TMPD†þ and TMPDH22þ. No Raman line of molecu-

lar TMPD was found even for very long time after the

mixture.

Resonance Raman scattering was carried out using

the 632.5 nm exciting laser line within the electronic

absorption of TMPD†þ. The resonance Raman

Fig. 3. Pure DRUV spectra extracted using the SIMPLISMA method of the DRUV data recorded during the sorption of TMPD into

Na3.4(AlO2)3.4(SiO2)92.6: (a) TMPD; (b) radical cation TMPD†þ; (c) ejected electron.


spectra obtained several hours to one year after the

mixing of the solids display exclusively the spectral

features of TMPD†þ.

The DRUVv intensities measured at 611 nm as a

function of time for 0.25, 0.5 and 1 TMPD/UC

loadings indicate the TMPD†þ yields. The maximum

of generated TMPD†þ yield is found to be approxi-

mately proportional to the loading value. However,

the ionization yield value deduced from double

integrated EPR signals does not correspond to

Fig. 4. DRUV spectra recorded at room temperature during the course of adsorption of Anth in H3ZSM-5 [H3.4(AlO2)3.4(SiO2)92.6] zeolite by

mixing of powders. (a) 5 min to 48 h (b) 48 h to 6 months.


proportional behaviour (Fig. 7). The increase of

loading generates probably ejected electrons in high

yield. The close proximity of trapped electrons can

lead to antiferromagnetic coupling and induce a

decrease of the spin density.

H6ZSM-5 (Si/Al ¼ 13)—The ionization and pro-

tonation processes which occur upon sorption of

TMPD in H6ZSM-5 were found to be analogous to

those observed for H3ZSM-5. However, the main

difference is the weaker ionization yield as deduced

from UV-visible, EPR and Raman experiments.

3.4. Discussion

EPR, UV-visible and Raman spectra recorded

during the sorption of TMPD in MnZSM-5

(M ¼ Naþ, Hþ; n ¼ 0, 3, 6) zeolites provide evidence

of ionization in all the zeolites under study.

TMPD†þ is produced in low yield in purely

siliceous silicalite-1 whereas TMPD†þ is generated

in high yield in aluminated NanZSM-5. The ejected

electron is also characterized though electronic

absorption spectra. These paramagnetic

species are stabilised in high yield over more

than one month at room temperature. After long

times of exposure, the amounts of TMPD†þ and

trapped electron decrease and indicate charge

recombination to generate occluded molecular

TMPD.

The sorption of TMPD in acidic HnZSM-5

zeolite induces durable charge separation in high

yield for several weeks. After long times of

exposure the amounts of TMPD†þ and trapped

electron decrease and indicate charge recombina-

tion. However, in contrast to non acidic zeolites,

charge recombination generates diprotonated

TMPDH22þ occluded species. The protonation

appears competitive to the ionization efficiency of

HnZSM-5 zeolite. The ionization maximum yield of

TMPD appears somewhat weaker in H6ZSM-5 than

in H3ZSM-5 whereas the protonation was found to

be more rapid.

The ability of acid zeolites to act as single

electron acceptors and to generate spontaneously

significant amounts of radical cation upon sorption

of organic compounds is well documented. So far,

ionization is known to occur through abstraction of

electron by Lewis acid sites in acid zeolites [6].

The electron acceptor site is a prerequisite for the

spontaneous ionization. For instance, it was

reported previously that the yield of ionization of

biphenyl increases gradually with the temperature

Fig. 5. Pure DRUV spectra extracted using the SIMPLISMA method of the DRUV data recorded during the sorption of TMPD into

H3.4(AlO2)3.4(SiO2)92.6: (a) TMPD; (b) radical cation TMPD†þ; (c) ejected electron; (d) diprotonated TMPDH22þ.


of thermal treatment under oxygen from 200

to 600 8C and is maximum at high aluminum

content [17].

In the case of aromatic amines with low ionization

potential (6.6 eV) the presence of strong electron

acceptor sites such as Lewis acid sites is not

necessary. It should be noted that Lewis acid sites

were obtained by calcination of acid zeolites under

oxygen. After gentle dehydration of non acidic

NanZSM-5, ionization of TMPD occurs in high

yield in the present work. The strong electrostatic

field in the porous void as well as efficient trapping

sites of radical cation and ejected electron can be

invoked to explain this behavior.

Moreover, TMPD is known to undergo readily

and spontaneously ionization in polar solvents such

as acetonitrile, alcohols and water [8]. Therefore,

there is no doubt that the highly polar environment

constituted by the zeolite framework provides a

suitable environment capable to ionize spon-

taneously in high yield the TMPD molecule.

Analogous behaviour was previously reported [16]

for Na sorption in aluminated zeolite. Upon

sorption, Na atom is ionized producing Naþ ion

and an electron that is trapped in the framework.

This electron absorbs visible light and is respon-

sible for the change of colour. The presence of

Brønsted and Lewis acid sites in HnZSM-5 does

not provide supplementary ionization efficiency but

induces protonation of the amine function. These

protonation properties appear competitive with the

ionization ability.

The tight fit between the rod shape TMPD†þ

radical cation and the pore size of ZSM-5 zeolite is an

important factor responsible for the stabilization

of radical cation. It appears that the presence of

aluminium in the siliceous framework is also a

requirement to prevent rapid electron back transfer.

At this stage of the study, it is possible to give a

reasonable scheme of the sorption of TMPD in ZSM-5

zeolite. The MnZSM-5 induces the uptake of TMPD

molecule at the openings of pores. Simultaneous rapid

ionization occurs through abstraction of electron by

the framework before any protonation by Brønsted

acid sites in acidic zeolites. The sorption goes to

completion by slow diffusion of radical cations and

electron migration within the framework. During the

slow diffusion of radical cation, the uptake of an

electron of the framework induces charge recombina-

tion of molecular sorbate and generates unstable

electron-hole pairs before the rapid electron-hole

annihilation. In the case of acidic HnZSM-5,

the charge recombination is concerted with

the protonation of aromatic amines to provide

quaternary salts of amines.

Acknowledgements

The authors are very grateful to Dr B. Sombret for

assistance and advice while using FT-Raman spec-

trometry. The Centre d’Etudes et de Recherches

Lasers et Applications (CERLA) is supported by the

Ministere charge de la recherche, the region Nord/Pas

Fig. 6. FT-Raman spectra (1064 nm excitation laser line) recorded

during the sorption of TMPD in H3.4ZSM-5 [H3.4(AlO2)3.4

(SiO2)92.6] zeolite by mixing of powders. (a) TMPD at the solid

state (b) 48 h after the mixing of powders; (c) 20 days; (d) 120 days;

(e) 1.5 year; (f) TMPDH22þ recorded in 12 M hydrochloride

solution.


de Calais, and the Fonds Europeen de Developpement

Economique des Regions.

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Fig. 7. Double integrated EPR signal as a function of time for several loadings: 0.25 (solid line), 0.5 (dashed and dotted line), 1 (dashed line) and

2 (dotted line) TMPD molecules per unit cell.


Ionization and protonation of aromatic diamines by sorption in zeolites

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