FT-IR evidence of the interaction of benzothiophene with

the hydroxyl groups of H-MFI and H-MOR zeolites

Aıda Gutierrez-Alejandre a,b,*, Maria Angeles Larrubia a,c,Jorge Ramirez b,d, Guido Busca a,*

a Laboratorio di Catalisi, Dipartimento di Ingegneria Chimica e di Processo, Universita, Genova, P.le J.F. Kennedy, I-16129 Genova, Italyb UNICAT, Depto. de Ingenierıa Quımica, Facultad de Quımica, UNAM, Cd. Universitaria, 04510 Mexico D.F., Mexico

c Departamento di Ingenierıa Quımica, Facultad de Quımica, Universidad de Malaga, 29071 Malaga, Spaind Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas No. 152, 07730 Mexico D.F., Mexico

Received 10 October 2005; received in revised form 9 December 2005; accepted 21 December 2005

Available online 2 February 2006

Abstract

The interaction of benzothiophene (BT) with H-MFI and H-MOR has been investigated by FT-IR spectroscopy. The results from BT adsorption

on acid H-MFI and H-MOR coupled with the study of adsorbed naphthalene show that two types of interactions occur between BT and the strongly

acidic protonic sites of the zeolites: one interaction through the sulfur lone electron pair and another one, similar to that observed with naphthalene,

involving the p-type electron cloud. With the external silanols, only a weak hydrogen bonding type interaction with the p-type electron cloud of the

aromatic ring of BT was clearly observed.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Benzothiophene; FT-IR; H-MFI; H-MOR zeolites IR spectroscopy; H-bonding; Adsorption

www.elsevier.com/locate/vibspec

Vibrational Spectroscopy 41 (2006) 42–47

1. Introduction

The development of new catalysts [1,2] and of new

adsorbants [3] is of great importance for the elimination of

thiophenic sulfur compounds from transport fuels, in order to

fulfill the strict sulfur content regulations expected for the near

future (15 ppm or less). Materials containing protonic zeolites

are among candidates in both fields.

Several studies have been reported in recent years

concerning the interaction of thiophene with protonic zeolites.

Garcia and Lercher [4,5], Geobaldo et al. [6] and Yu et al. [7]

reported spectroscopic studies of thiophene interaction with H-

MFI and H-Y. Soscun et al. [8], Saintigny et al. [9] and

Rozanska et al. [10] reported theoretical studies on the

adsorption and reactions of thiophene over protonic zeolites.

Garcia and Lercher [4,5] concludes that on H-MFI thiophene is

weakly hydrogen bonded to the SiOH groups and that initially,

* Corresponding author. Tel.: +39 010 3536024; fax: +39 010 3536028.

E-mail addresses: aidag@servidor.unam.mx (A. Gutierrez-Alejandre), Gui-

do.Busca@unige.it (G. Busca).

0924-2031/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.vibspec.2005.12.008

thiophene hydrogen bonds to the Si–OH–Al groups. This

interaction is followed by ring opening and oligomerization

reactions promoted by the strong acidity of the H-MFI zeolite.

These results are consistent with those reported in [6].

According to Yu et al. [11] and Li et al. [12] thiophene

converts into benzothiophene and likely higher heteroaromatics

plus H2S on H-MFI zeolite and this reaction is greatly enhanced

by the presence of hydrogen or alkanes. The same group found

that thiophene oligomers form, during adsorption and that their

size, depends on spatial constraints within zeolite channels. The

data reflect specific interactions of thiophene with Brønsted

acid sites [13,14].

Much less is known about the interaction of benzothio-

phene (BT) with solids. To our knowledge, the only

spectroscopic study of benzothiophene adsorbed on solids

has been performed by our group and concerns about the

interactions with Al, Mg and Zr oxides [15]. The molecular

adsorption of benzothiophene on protonic zeolites has been

the object of few recent studies [16,17]. A theoretical study

has been performed on the isomerization of alkyl–benzothio-

phenes on protonic mordenite [18]. Clearly, better knowledge

of the interactions of benzothiophenes and aromatics with

A. Gutierrez-Alejandre et al. / Vibrational Spectroscopy 41 (2006) 42–47 43

Fig. 1. FT-IR spectra of H-MFI zeolite upon adsorption of benzothiophene

(BT). (a) Activated at 773 K; (b–f) in contact with increasing pressures of BT up

to 0.75 Torr at r.t., (g) after outgassing at r.t., (h) after outgassing at T = 373 K,

(i) after outgassing at T = 473 K.

zeolites is needed in order to design better catalysts and

adsorbents for the sulfur and aromatic compounds present in

the different petroleum fractions. Here, we report a FT-IR

study of the interaction of benzothiophene over protonic

zeolites (H-MFI and H-MOR) to identify interactions relevant

to HDT and the ring opening mechanism of heterocyclic

aromatic compounds.

2. Experimental

The data presented here refer to ZSM5 (SiO2/Al2O3 molar

ratio = 40) and mordenite sample (SiO2/Al2O3 molar ratio =

20) from zeolyst.

The IR spectra were recorded on a Nicolet Magna 750 Fourier

transform instrument, using pressed disks of pure zeolite

powders (15 mg for 2 cm diameter), activated by outgassing

at 773 K for 1 h into the IR cell. A conventional gas

manipulation/outgassing system connected to the IR cell was

used. The adsorption procedure involves contact of the activated

sample disk with BT or naphthalene vapors at increasing

pressures (1 Torr) and outgassing in steps at r.t. or increasing

temperatures. Benzothiophene and naphthalene were pure

products from Aldrich and were vaporized in vacuum at 373 K.

3. Results

Detailed studies of complete fundamental, overtone and

combination band assignments of solid and gaseous ben-

zothiophene can be found in the literature [19,20]. The n15 ‘‘in

plane’’ mode, which is perhaps the most sensitive to the state

of the sulfur atom is unavailable when BT is adsorbed on

zeolites due to the superimposition of the Si–O–Si (Al)

asymmetric stretching modes of zeolites that cut-off the

radiation. Therefore, the analysis of the interaction of BT with

zeolites has to relay mostly on the changes observed in the OH

region.

3.1. Adsorption of benzothiophene on H-ZSM5

Fig. 1 reports the spectra relative to the interaction of

benzothiophene with H-MFI. As it is well known [21,22], OH

stretching bands in the protonic zeolites spectra are generally

assigned to bridging Si–(OH)–Al groups internal in the cavities

(bands in the 3650–3500 cm�1 region), OHs on extra-frame-

work species (bands in the region 3720–3650 cm�1) and

terminal silanols at the external crystal surface (ca. 3745 cm�1).

According to the relatively low Al content in the zeolite sample

(molar SiO2/Al2O3 = 40), the spectrum of the pure zeolite

(Fig. 1a) shows that the OH stretching band due to external

silanol groups, sharp at 3745 cm�1, is stronger than the band

due to internal bridging Si–(OH)–Al groups, detected at

3609 cm�1. No bands at about 3680 cm�1 are evident showing

that OHs on extra framework material are not present.

The band associated to the internal bridging OHs

(3609 cm�1) decreases steadily in intensity when BT is dosed

to the sample (Fig. 1b and c), showing that benzothiophene first

penetrates the zeolite channels. In contrast, the first doses of BT

(Fig. 1b and c) cause little change in the intensity and position

of the band due to terminal silanols (3745 cm�1). After further

doses of BT the silanol band intensity decreases considerably.

This decrease in the intensity of the external silanols band is

accompanied by the appearance and increase of a broader band

centered at 3607 cm�1. This band is associated to the same

silanol groups but shifted down, Dn = 138 cm�1, due to their

interaction with BT. The fact that with the first BT doses the

band related to internal OHs decreases while that associated to

external silanols remains practically constant indicates that BT

interacts first with the strongest acid sites located in the zeolite

channels. Simultaneously to the progressive decrease of the

band due to internal OH groups, a new broad band centered at

3220 cm�1 and a weaker one near 2450 cm�1 are clearly

formed. The former absorption is likely composed of two

features, both broad, one near the maximum at 3220 cm�1 and

the other near 3000 cm�1, to which the sharp CH stretchings are

superimposed. The maximum at 3220 cm�1 is due to the OH

stretching of the bridging OHs perturbed by a medium–strong

interaction with the adsorbate (Dn = 390 cm�1). The compo-

nents at 3000 and 2450 cm�1 are likely the A and B

components of a so called ABC spectrum, due to a strong

H-bonding involving bridging OHs and the adsorbate. This

spectrum is due to the Fermi Resonance of the OH stretching

mode (strongly shifted downwards) with the first overtones of

the in-plane and out-of-plane bending modes of the bridging

OHs [23], respectively.

The contact with BT (Fig. 1b–f) causes the appearance of a

sharp CH band at 3066 cm�1 due to adsorbed BT, which

increases with subsequent doses of BT. Components are also

evident at 3090, 3077, 3030 and 2968 cm�1. Outgassing causes

the new features associated to BT adsorption to disappear and

the original spectrum to be restored (Fig. 1g–i). The bands

related to internal and external OHs follow the inverse trend

to the one shown during adsorption; the weakest acidic sites

(external silanols) are restored before the strong internal

A. Gutierrez-Alejandre et al. / Vibrational Spectroscopy 41 (2006) 42–4744

Fig. 2. FT-IR spectra of BT adsorbed on H-MFI. In-plane and out-of-plane

vibrational modes region. From bottom to top: BT liquid, the adsorbed species

in contact with increasing pressures of BT up to 0.75 Torr at r.t., after outgassing

at r.t., after outgassing at T = 373 K, after outgassing at T = 673 K.

Fig. 3. FT-IR spectra of H-MFI zeolite upon adsorption of naphthalene. (a)

Activated at 773 K, (b) naphtalene at r.t., (c) outgassed at r.t., (d) outgassed at

373 K and (e) outgassed at 423 K.

Scheme 1. H-bonding interactions of the naphthalene aromatic electron cloud

with (a) the silanol groups and (b) interaction of naphthalene with the bridging

OHs.

bridging OHs. Almost complete BT desorption is only obtained

by outgassing at 473 K (Fig. 1i).

The spectra in the lower frequency region (Fig. 2) show,

after adsorption and outgassing at r.t., the main BT bands, all

sharp, at 1498 (weak), 1458, 1423 (both very strong), 1346

and 1323 cm�1. In the windows between the zeolite’s

skeletal vibrations (not shown), bands of adsorbed BT at

866, 851, 762, 734 and 688 cm�1 were evident. The bands in

the region above 1300 cm�1 correspond to in-plane vibra-

tional modes of BT. The position of the bands, with respect

to the values reported for liquid BT are shifted few cm�1

except for n13, which is involved in Fermi resonances. The

lower frequency detected bands, in particular those at 851,

734 and 688 cm�1, which correspond to out-of-plane

vibrations (n31, n33 and n34), are also shifted few cm�1.

Upon outgassing from room temperature up to 473 K no

evidence of the formation of reaction products different from

BT itself was found.

The interaction of BT with the zeolite OH groups can take

place in two different modes: through the p electron cloud of

the aromatic ring, or through the sulfur lone electron pair. Each

of these interactions will present a characteristic shift of the

different OH bands perturbed by BT adsorption. To analyze if

the OH band shifts observed upon BT adsorption are purely due

to interactions with the p electron cloud of the aromatic ring,

experiments were made with naphthalene, a molecule with

similar steric hindrance, molecular shape, and aromatic

electronic structure than BT.

3.2. Adsorption of naphthalene on H-MFI

Naphthalene adsorption over H-MFI (Fig. 3) is associated to

the formation of two broad features: one with two unresolved

components near 3620 and 3550 cm�1 and another one which

seems also split with components near 3250 and 3150 cm�1.

According to our previous data concerning IR studies on the

adsorption of hydrocarbons on H-MFI [24], the former feature

is associated to H-bonding interactions of the naphthalene

aromatic electron cloud with the silanol groups, while the latter

is due to the interaction of naphthalene with the bridging OHs.

The different observed shifts, Dn OH 125–200 cm�1 for the

silanol, and 360–460 cm�1 for bridging OHs, agree with the

different acidities of the two OH groups. The splitting for both

bands can be either due to the existence of two families of

silanol groups and two families of bridging OHs with a

remarkably different acidity, or a different steric hindrance, or

(more likely) to the possibility of formation of two types of

adsorption complexes with naphthalene for both kinds of OHs

(e.g. an interaction with fully delocalized p orbitals and an

interaction localized on one C–C bond). In any case, the

interaction of aromatic hydrocarbons with OH groups is

interpreted as evidence of a medium–strong hydrogen bonding

involving the p electron cloud (see Scheme 1) and both

bridging and terminal OHs.

The magnitude of the OH band shifts observed upon

naphthalene adsorption on bridging OHs (Dn OH 360–

460 cm�1) is in the same range to that observed for BT

adsorption (Dn OH 390 cm�1), suggesting that interactions

A. Gutierrez-Alejandre et al. / Vibrational Spectroscopy 41 (2006) 42–47 45

Fig. 4. FT-IR spectra of H-MOR zeolite upon adsorption of BT. (a) Activated at

773 K, (b) BT at r.t., (c) outgassed at r.t., (d) outgassed at 373 K and (e)

outgassed at 423 K.

through the p-type orbitals occur also for benzothiophene. The

simultaneous additional formation of the ABC components for

BTadsorption only, suggests that, for BT, an additional stronger

H-bonding interaction occurs. In fact, according to theoretical

studies, the bridging OHs can interact with the sulfur atom lone

electron pair [18] (see Scheme 2). So, it appears that BT

interacts with the zeolite OHs through both the p electron cloud

of the aromatic ring and the sulfur atom lone electron pair

(Scheme 2).

3.3. Adsorption of benzothiophene on H-MOR

Fig. 4a shows the spectrum of H-MOR after activation. The

high intensity of the band associated to internal hydroxyls

(3605 cm�1) with respect to that related to the external silanol

groups agrees with the large amount of Al in the framework.

Adsorption of BT (Fig. 4c and d) causes the complete

disappearance of the weak band associated to external silanol

groups (3747 cm�1) while thevery strong band related of internal

bridging OHs (3605 cm�1) decreases only partially suggesting

that a great part of the bridging hydroxyls is located in positions

inaccessible to BT, such as those located within the side pockets

and at the intersections between side pockets and main channels

[25,26]. The feature (shoulder) appearing near 3615 cm�1 as a

tail in the higher frequency side of the main bridging OH band

(Fig. 4b), is assigned, in agreement with the above data obtained

on H-MFI, to the shifted band (Dn = 132 cm�1) of silanol groups

perturbed by the adsorption of BT.

The small fraction of bridging hydroxyl groups perturbed by

BT adsorption gives raise to a broad band at 3250 cm�1 (main

band shift of around Dn = 355 cm�1) with additional compo-

nents near 2900 and 2400 cm�1. As for H-MFI, the last two

Scheme 2. H-bonding interactions of benzothiophene with the OH groups, (a) thro

features are likely due to an ABC type interaction, showing that

some of the H-MOR OHs interact strongly with BT, likely

through the sulfur atom. The band at 3250 cm�1 arising from

an additional interaction, similar to that observed with

naphthalene on H-MFI, i.e. very likely involving H-bonded

complexes of bridging OHs with BT through the p-type

electron cloud, gives rise to a weaker perturbation of the OHs.

However, surprisingly, this band resists outgassing at r.t. while

the ABC type interaction disappears. It appears then that, in

spite of the lower strength of the hydrogen bonding, the

interaction through the p-type electron cloud gives rise to

complexes of higher stability. This can be rationalized

suggesting that stronger additional van der Waals interactions

ugh the sulfur atom lone electron pair and (b) involving the p electron cloud.

A. Gutierrez-Alejandre et al. / Vibrational Spectroscopy 41 (2006) 42–4746

occur in this case and stabilize these weaker hydrogen bonding

complexes.

Previous IR studies using hindered nitriles as probe

molecules [25,26] allowed distinguishing three families of

bridging OHs in H-MOR. The hydroxyl groups absorbing at

3605 cm�1 are thought to stand near the center of the main

channels. Those responsible for the shoulder at 3588 cm�1 are

likely located inside the side-pockets. Others, absorbing at

3609 cm�1, are likely at the intersection between side pockets

and main channels. The spectra relative to BT adsorbed on H-

MOR indicate that only the external silanols and the bridging

OHs located well in the middle of the H-MOR’s main channels

are actually able to interact with BT. The bridging OHs located

not only in the side pockets (3588 cm�1) but also those at the

intersection of the main channels with the side pockets

(unresolved in the main OH band, near 3609 cm�1), do not

appear to be perturbed. This has been already shown in a

previous study adsorbing highly hindered and rigid molecules

such as pivalonitrile and orthotoluonitrile [25,26]. From these

data, it is evident that only the OHs standing near the middle of

the main channels are able to interact with such large

molecules. This occurs also with BT whose critical radius is

near 6 A and allows its entrance in the main channels of

mordenite even better than in the channels of HMFI.

Upon outgassing at 423 K, increasing BT desorption occurs

and the above discussed OH stretching bands disappear.

However, inspection of the 1600–1300 cm�1 region shows that

at the same time, new bands appear (Fig. 5) at 1590, 1490 and

1365 cm�1. Fig. 5 shows that the spectra of adsorbed and solid

BT are similar with small shifts and intensity modifications.

Therefore, the significant differences observed in the spectrum

of adsorbed BT when the outgassing temperature is raised from

r.t. up to 423 K, are interpreted as evidence of the occurrence of

a BT chemical transformation. The resilient persistence of the

CH stretching band near 3075 cm�1 suggests that the resulting

species is still aromatic. The spectrum of this new species is

similar to that observed after interaction with some metal

oxides and is likely due to a species where the thiophenic ring

Fig. 5. FT-IR spectra of BT adsorbed on H-MOR. In-plane and out-of-plane

vibrational modes region. From bottom to top: BT solid, BT at r.t., outgassed at

r.t., outgassed at 373 K, outgassed at 423 K.

is opened while the aromatic ring is still intact, as suggested

previously [15].

The above results show that under our experimental

conditions (r.t., exposure times of less than 30 min), H-MOR

is active towards BT transformation while H-MFI is not.

4. Discussion

Comparison of the data arising from BT adsorption on the

acid H-MFI and H-MOR zeolites coupled with the study of

adsorbed naphthalene allow concluding that with the external

silanols, only a weak hydrogen bonding type interaction is

observed whose extent is similar to that occurring through the

p-type electron cloud of the aromatic ring of naphthalene. On

the contrary, two types of interactions occur between BT and

the strongly acidic protonic sites of the zeolites studied here.

The interaction of BT with the acidic OHs through the sulfur

lone electron pair is likely responsible for the formation of the

ABC type spectrum typical of strong H-bonding (Figs. 1 and 4).

An additional interaction similar to that observed with naph-

thalene, i.e. involving the p-type electron cloud, also observed,

gives rise to a weaker perturbation of the OHs. This latter

interaction leads to complexes of greater stability than the

former one, in spite of the different strength of the hydrogen

bonding, possibly because in this case additional stronger van

der Waals interactions occur and tend to stabilize the weaker

hydrogen bonding complexes formed with the aromatic ring.

The formation of species different from BT after its

adsorption at r.t. and outgassing at higher temperatures was

observed on H-MOR while on H-MFI was not. It seems likely

that these species are formed by reaction of BT with the zeolite

acid sites, hence, the difference between H-MOR and H-MFI

could be due to differences in the acid strength, in agreement

with the acidity scale for the zeolites studied here, reported

to be H-MOR > H-MFI [27,28]. A transition state shape

selectivity effect (i.e. to produce the transition state needed to

perform this reaction a little large cavity is needed) or

diffusional limitations due to the difference in channel size

between the two zeolites could also play a role in the absence of

reaction products over H-MFI. For H-MOR, the identification

of the resulting product adsorbed species cannot be ascertained

with our experimental data only, nevertheless, the presence of

the typical C–H stretching band at 3075 cm�1 and the aromatic

ring stretching bands at 1600 and 1490 cm�1, after BT

adsorption and outgassing at 423 K, suggests the formation

of an aromatic compound. So, opening of the BT sulfur-

containing ring very likely occurred [15]. Further studies are in

progress to ascertain this suggestion.

In any case, the interaction of acidic protons with the aromatic

electron cloud is considered in basic organic chemistry to be

precursor for electrophilic aromatic substitution. This kind of

reactivity would lead to different reaction products than those

expected from activation of the molecule at the sulfur atom.

These two different reaction paths may give rise to carbon sulfur

bond breaking or, alternatively, to carbon–carbon bond breaking,

making them interesting to hydrodesulfurization and aromatic

ring-opening reactions.

A. Gutierrez-Alejandre et al. / Vibrational Spectroscopy 41 (2006) 42–47 47

Acknowledgments

Authors acknowledge Ministero degli Esteri (Italy) and

CONACyT-SRE (Mexico), PAPIIT IN112802–DGAPA and

IMP-FIES programs for financial support.

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