Dual behavior of ZSM-5 as bronsted acid and electron acceptor in the adsorption of N,N'-diphenylhydrazine

Article abstract of DOI:10.1002/(SICI)1099-0690(200002)2000:3<473::AID-EJOC473>3.0.CO;2-N

Incorporation of N,N'-diphenylhydrazine into HZSM-5 led to the formation of azobenzene and aniline (70-80 mol-% mass balance). The formation of azobenzene can be followed by the appearance in diffuse reflectance UV/Vis spectroscopy of a characteristic band (λmax = 420 nm). IR spectra of the extracted solids indicate that some aniline is retained in the zeolite, probably due to acid-base interaction with active sites. The products are rationalized by two competing mechanisms: the formation of azobenzene by electron transfer to an oxidizing site, the generation of aniline by proton transfer from a Bronsted acid site. A long-wavelength diffuse-reflectance band (λmax = 800 nm) in the initial stages of the reaction supports the intermediacy of a radical cation. These results show that acid zeolites can exhibit a dual behavior against basic substrates with low oxidation potential, serving simultaneously as electron acceptor and Bronsted acid.

Full text of DOI:10.1002/(SICI)1099-0690(200002)2000:3<473::AID-EJOC473>3.0.CO;2-N

FULL PAPER
Dual Behavior of ZSM؊5 as Brønsted Acid and Electron Acceptor in the
Adsorption of N,NЈ-Diphenylhydrazine
[
a]
[a]
[a]
[b]
Vicente Mart ´ı , Vicente Forn e´ s, Hermenegildo Garc ´ı a* and Heinz Dieter Roth*
Keywords: Zeolites / Electron transfer / Proton transfer / Radical cation
Incorporation of N,NЈ-diphenylhydrazine into HZSM-5 led to
the formation of azobenzene and aniline (70Ϫ80 mol-% mass
balance). The formation of azobenzene can be followed by
the appearance in diffuse reflectance UV/Vis spectroscopy of
a characteristic band (λmax = 420 nm). IR spectra of the
extracted solids indicate that some aniline is retained in the
zeolite, probably due to acid-base interaction with active
sites. The products are rationalized by two competing
mechanisms: the formation of azobenzene by electron
transfer to an oxidizing site, the generation of aniline by
proton transfer from a Brønsted acid site. A long-wavelength
diffuse-reflectance band (λmax = 800 nm) in the initial stages
of the reaction supports the intermediacy of a radical cation.
These results show that acid zeolites can exhibit a dual
behavior against basic substrates with low oxidation
potential, serving simultaneously as electron acceptor and
Brønsted acid.
Introduction
their decay in solution; accordingly, they have increased life-
times and can be studied by conventional spectroscopic
techniques. Furthermore, the limiting geometry of the zeo-
lite pores may manifest itself in two ways: a) it may restrict
the geometry of the sequestered intermediates; and b) it
may selectively incorporate substrates with suitable geo-
metries. Thus, radiolysis of n-hexane and n-octane in penta-
The structures and catalytic properties of zeolites have
been extensively investigated as prototypes of acidic indus-
trial catalysts. Zeolites were first introduced as catalysts for
large-scale gas-phase reactions in petrochemistry. Com-
pared to conventional liquid acids these materials showed
significant advantages for processes such as cracking, aro-
matic isomerization, or disproportionation because of easy
separation of the reaction mixture, reactor design, control
of the reaction outcome, shape selectivity, and the possibil-
ity to regenerate the catalyst after deactivation.[ The re-
sults of these gas-phase reactions triggered intense research
aimed at developing the potential of zeolites for general
acid-catalyzed organic reactions in the liquid phase under
batch conditions. During the past two decades an ever-in-
creasing volume of research has been carried out, promot-
ing the use of zeolites as solid catalysts for organic reactions
in the liquid phase, e.g. in the production of fine chemicals
under environmentally benign conditions. As a result of
[8]
sil zeolite only generated the extended radical cations.
Likewise, diaryl disulfides or diselenides gave rise selectively
to “extended” sulfur- or selenium-centered radical cat-
[9][10]
ions.
On the other hand, trans-1,2-diphenylcyclopro-
1]
pane was incorporated readily into ZSM-5 (and oxidized
within its pores) whereas the cis isomer failed to be incor-
[11]
porated.
The majority of studies in this area are focused on the
spectroscopic characterization of the persistent radical cat-
ion species; the application of zeolites to trigger electron-
transfer (ET) mediated reactions has received comparatively
little attention, in contrast to the interest that other chemi-
cal and photochemical ET promoters have attracted. For
these studies, heterogeneous catalysis of organic reactions
[
12]
by zeolites is a mature field.[2Ϫ6]
example, the ET reactions of cyanoaromatics,
tri-
[13]
phenylpyrylium ion,
or tris(4-bromophenyl)ammonium
One of the most intriguing properties of acid zeolites is
their ability to generate spontaneously organic radical cat-
ions upon adsorption of electron-rich organic molecules.
Spontaneous generation of organic radical cations upon ad-
sorption of organic electron-donors into acid zeolites is well
documented. Within the pores of the zeolite, the radical cat-
ions are protected from reagents that typically would cause
ion[ have been studied in detail.
14]
[7]
In one of the few preparative studies on zeolites, Bauld
and co-workers found that 1,3-cyclohexadiene undergoes
dimerization to the corresponding DielsϪAlder adducts in
[15][16]
the presence of NaX on a preparative scale.
1,3-
Cyclohexadiene is an excellent probe molecule to differen-
tiate ET processes since the product distribution changes
[
[
[
a]
Instituto de Tecnolog ´ı a Qu ´ı mica CSIC-UPV, Universidad Poli-
t e´ cnica de Valencia,
[17][18]
with the operating reaction mechanism;
the formation
of DielsϪAlder (endo) adducts is typical for ET-induced di-
merization of this substrate. Another recent example of an
ET-induced conversion on a zeolite is the dehydrogenation
of 4-propylanisole upon adsorption on pentasil zeolite
which generates propenylanisole (anethole) radical cation,
a process that formally involves the loss of three electrons
Apartado 22012, E-46071 Valencia, Spain
Fax: (internat.) ϩ 34-96/387-7809
E-mail: hgarcia@qim.upv.es
Department of Chemistry, Wright-Rieman Laboratories, Rut-
gers University,
b]
6
10 Taylor Road, New Brunswick, NJ 08854-8087, USA
Fax: (internat.) ϩ 1-732/445-5312
E-mail: roth@rutchem.rutgers.edu
Profesor Visitante IBERDROLA de Ciencia et Tecnolog ´ı a.
°]
[19]
and two protons.
Eur. J. Org. Chem. 2000, 473Ϫ477

WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ193X/00/0203Ϫ0473 $ 17.50ϩ.50/0
473

V. Mart ´ı , V. Forn e´ s, H. Garc ´ı a, H. D. Roth
FULL PAPER
The electron acceptor ability of zeolites has been attri- clusion procedure, the solid mixture of the reagents showed
buted to the presence of acid sites, although a controversy an additional DR band, λmax ϭ 800 nm, at an intermediate
seems to persist regarding the (Lewis or Brønsted) nature stage, which disappeared upon further heating (Figure 1).
of the active site. Brønsted acid sites are associated with
bridged ϵSi(OH)Alϵ hydroxy groups, whereas Lewis sites
are due to octahedral oligomeric and polymeric nonframe-
work positive aluminate clusters of the general composition
[
3nϪxϪ2y]ϩ [20]
Al (OH) O
.
These are generated by migration
n
x
y
of tetrahedral framework Al out of the crystal lattice.
Therefore, substrates having electron lone pairs as well as a
low oxidation potential may have two alternative pathways
available, initiated by electron or proton transfer. However,
examples showing the simultaneous operation of these two
pathways are scarce.[
21][22]
In this paper we report the conversion of N,NЈ-diphenyl-
hydrazine (DPH) to azobenzene (AB) upon adsorption
onto acid H-ZSM-5; herein, the dehydrogenation product
is accompanied by a reduced bond-cleavage product, aniline
Figure 1. Diffuse reflectance spectra (plotted as the inverse of re-
(
AN). Diffuse reflectance optical spectroscopic results lead flectance, 1/R) of HZSM-5 zeolite after incorporating N,NЈ-diphe-
•
ϩ
nylhydrazine (b) and in the early phase of heating the two solids
us to suggest that DPH is an intermediate in the forma-
tion of AB.
(
a) ground together in a mortar
A complete mass balance is vital to determine the true
Results
product distribution of a chemical reaction in a hetero-
geneous medium; products retained in the solid (and not
accounted for) can distort or vail the true outcome of the
reaction. Determining mass balances in zeolites has in-
herent inaccuracies. However, in the case of DPH, the frac-
tion of material not recovered by solid-liquid extraction far
exceeds any experimental error (Table 1). A significant
amount of organic material is retained in the solid. IR spec-
Inclusion of DPH into H-ZSM-5 was carried out by ad-
sorption from organic solutions or by heating a mixture of
solid DPH and hydrated H-ZSM-5 progressively to 150°C.
With both procedures, the zeolite turned yellow, indicating
that a chemical reaction was taking place. The inclusion of
DPH from solution was essentially complete, judging by the
depletion of DPH in the supernatant solution. This con-
clusion was confirmed by combustion analyses of the dried,
filtered, and degassed zeolite: The C/N ratio corresponded
to that of DPH. Exhaustive solid-liquid extraction of the
yellow solids yielded a reaction mixture consisting of AB
and AN accompanied by minor amounts of unchanged
starting material (70Ϫ80 mol-% recovery; Table 1).
tra of the solid after degassing (Figure 2, b) showed bands
Ϫ1
(
1638, 1618, 1583 cm ) characteristic for AN (Figure 2, c).
The presence of DPH (spectrum not shown) or benzidine
Ϫ1 [21]
(
bands at 1608, 1537 cm
)
are clearly ruled out. Appar-
ently, extraction of the zeolite recovers AB almost com-
pletely whereas a significant portion of AN is retained by
the acid zeolite sites. The preferential retention of AN re-
flects its greater basicity compared to the azo compound.
Table 1. Products obtained by liquid extraction of HZSM-5 after
incorporation of DPH
Mass balance[
[%]
a]
Product distribution
[b]
Method of
loading
DPH
AB
AN
Solution
Solid phase
70
75
Ϫ
13
30
59
70
23
[
a]
Σ[DPH ϩ AB ϩ (AN/2)]/DPH
0
, where DPH, AB and AN are
is
the molar quantities of each product in the extract and DPH
0
the initial quantity of DPH. The product distribution was corrected
to take into account the initial amount of 12% AB in the commer-
[b]
cial sample of DPH used in this work. Ϫ Product yield ϭ mass
balance ϫ percentage in the product distribution.
Diffuse reflectance UV/Vis spectroscopy (DR) of the yel-
low loaded zeolite samples showed a new band, λmax
20 nm (Figure 1) which coincides with that of AB in di-
chloromethane; apparently, AB is formed within the zeolite.
ϭ
Figure 2. IR spectra of HZSM-5 samples after adsorption of N,NЈ-
diphenylhydrazine on HZSM-5 (a), after exhaustive solid-liquid ex-
traction (b), and of pure aniline (c), respectively, adsorbed on
HZSM-5; the most characteristic band corresponding to AB (spec-
4
While this band is ultimately observed regardless of the in- trum not shown) is indicated with an asterisk
474 Eur. J. Org. Chem. 2000, 473Ϫ477

ZSMϪ5 as Brønsted Acid and Electron Acceptor in the Adsorption of N,NЈ-Diphenylhydrazine
FULL PAPER
Discussion
near-IR is typical for radical cations as a consequence of
[25]
their open shell configuration.
DPH , can be deprotonated and the resulting free radical
generate AB by loss of an additional hydrogen atom
The radical cation,
The fate of DPH upon incorporation into the zeolite may
at first appear surprising because strong acids are known to
cause rearrangement of DPH to benzidine,
•ϩ
[
23]
and HZSM-
(
Scheme 3). There is ample evidence that acid sites in
5
is a solid acid whose acid strength at room temperature
HZSM-5 can function as single electron ac-
[20]
is comparable to 75% aqueous solution of H SO .
The
[9Ϫ11,19,26Ϫ28]
2
4
ceptors;
has precedent.
dehydrogenation in zeolites likewise
At the low loading levels typical for
benzidine rearrangement may have been expected also be-
cause benzidine was obtained previously as a product of
AB in an acid zeolite:[ Irradiation of cis-AB sequestered
in HY-100 faujasite pores resulted in cyclization/dehydro-
genation to benzocinnoline and reduction/rearrangement to
[9,11,19,28]
these experiments, the transfer of the electron, proton, or
the hydrogen atom to the ZSM-5 matrix does not cause any
noticeable changes in the zeolite. The electron will deacti-
21]
vate the Lewis sites by reducing the nonframework alumin-
[
21]
benzidine.
On the other hand, the limited size of the
[3nϪxϪ2y]ϩ
ate clusters, Al (OH) O
which are characterized
n
x
y
˚
2
pentasil zeolite channels (straight elliptical, 5.2 ϫ 5.7 A ,
only indirectly (vide infra).
or sinusoidal channels with nearly circular diameter, ca. 5.5
˚
2 [24]
ϫ 5.5 A )
may prevent the “folding” of protonated
DPH, which is a prerequisite of the rearrangement. In fact,
irradiation of cis-AB in the more limiting HZSM-5 did not
generate benzidine.[ In view of these considerations it is
not surprising that the reaction of DPH on HZSM-5 took
an alternative course, generating AB (revealed by the
21]
4
20 nm DR band and product analysis) and AN (supported
by product analysis and IR spectroscopy). Benzidine clearly
can be eliminated as a product retained in the zeolite by
Scheme 3. Mechanism proposed to rationalize formation of azo-
benzene from N,NЈ-diphenylhydrazine in HZSM-5
comparison with the IR spectrum of an authentic sample
of benzidine adsorbed on HZSM-5.[21]
The competing pathway leading to AN may proceed by
proton transfer from the Brønsted acid sites of the zeolite
(
ϵSi(OH)Alϵ) to DPH. The resulting hydrazonium ion
may undergo NϪN bond cleavage. Formation of a dication,
the key intermediate of the benzidine rearrangement in
solution, is less likely. The low density of Brønsted acid sites
within the zeolite (Si/Al ratio 34) does not appear to favor
double protonation. Moreover, even if the dication were
formed, the restrictive environment of ZSM-5 would likely
frustrate the “folding” required for the benzidine rearrange-
ment (vide supra). These considerations suggest that bond
cleavage may be preferred.
Scheme 1. Products obtained upon irradiation of cis-azobenzene in
HY-100 faujasite
Similar to the irradiation of cis-AB in HY-100 faujasite
the products isolated from incorporation of DPH into
HZSM-5 formally correspond to a disproportionation reac-
tion. However, the recovered product distribution (Table 1)
does not support the required stoichiometry; the molar ra-
tio, AN/AB, falls short of the required value of 2, even
when taking into account the incomplete material recovery
and assuming that AN is the only product retained in the
zeolite. To account for the excess AB (Table 1) a competing
oxidation mechanism is required, at least to some extent.
Scheme 4. Mechanism proposed to rationalize formation of aniline
from N,NЈ-diphenylhydrazine in HZSM-5
The mechanism proposed here, competing mechanisms
with two different primary intermediates generated at two
types of active sites, readily account for all observed fea-
tures. In media other than zeolites, there is ample precedent
Scheme 2. Products obtained upon incorporation of N,NЈ-diphe-
nylhydrazine into HZSM-5
Alternatively, the two products, AB and AN, could be for the quantitative oxidation of DPH to AB using various
explained by two competing primary reactions. For the for- mild oxidizing agents, including atmospheric oxygen;[ the
mation of AB we envisage electron transfer from DPH to converse reductive NϪN cleavage is less common.
28]
the Lewis sites of HZSM-5. The intermediacy of the re-
In the context of competing mechanisms in zeolites, the
•
ϩ
sulting radical cation, DPH , in the oxidation/dehydrogen- photoreaction of cis-AB was discussed as a formal dispro-
ation reaction is supported by a transient DR band at portionation.[ This reaction generated benzocinnoline, a
21]
8
00 nm in the initial stages of the reaction (Figure 1, a); the (cyclized) dehydrogenated product, along with benzidine, a
appearance of long-wavelength absorptions in the visible or hydrogenated one. On the other hand, the zeolite-induced
Eur. J. Org. Chem. 2000, 473Ϫ477
475

V. Mart ´ı , V. Forn e´ s, H. Garc ´ı a, H. D. Roth
FULL PAPER
conversion of 1,1-diphenylethylene was ascribed to dual
pathways initiated at different sites.[
22]
The conclusion that the divergent products are generated
by different ZSM-5 sites suggests the interesting experiment
to determine the product distribution obtained with a series
of ZSM-5 samples containing exclusively Brønsted or ex-
clusively Lewis sites. However, this intriguing experiment
has not been possible to date. Although ZSM-5 is a syn-
thetic zeolite and considerable progress has been achieved
in the control of its synthesis, all zeolite batches charac-
terized to date contain both Brønsted and Lewis sites.
Conclusion
Figure 3. FT-IR spectra of pyridine retained after vapor-phase ad-
sorption at room temperature onto dehydrated H-ZSM-5 and sub-
sequent desorption by heating successively at 250, 350, and 400°C
Adsorption of DPH onto HZSM-5 generates AB and
AN. Although these products are formally compatible with under 10Ϫ1 Pa for 1-h periods; the bands characteristic for
disproportionation, they are ascribed to two competing
processes, involving electron transfer from DPH to the oxi-
Brønsted and Lewis sites are designated by letters B and L, respec-
tively
dizing sites of HZSM-5 and proton transfer from Brønsted
acid sites to DPH. DR spectroscopy provides sound evi-
dence for the initial ET. Dehydrogenation with formation
was removed under reduced pressure, the residue weighed, and the
product distribution quantified by integration of the corresponding
1
3
H-NMR signals (Varian Gemini 300 MHz, CDCl ; TMS as in-
of CϭC bonds had been observed previously upon adsorp-
ternal standard). DR spectra of the solids were recorded with a
Cary 5G spectrophotometer using a praying mantis attachment. IR
spectra of self-supported wafers (10 mg, prepared by compressing
[19]
tion of p-(n-propylanisole) in acid zeolites.
Ϫ2
zeolite powders at 1 Ton ϫ cm ) were recorded with a Nicolet
Experimental Section
710 FTIR spectrophotometer using a greaseless cell with CaF win-
dows. The samples were degassed under 10 Pa for successive 1-
h periods at 25, 100, 200 and 300°C before recording the IR spectra
at ambient temperature.
2
Ϫ2
General Remarks: A commercial sample of DPH (Aldrich) was
used as received; this sample contained an initial amount of 12%
of AB. H-ZSM-5 was obtained by deep-bed calcination (600°C,
air stream) of an as-synthesized tetra-n-propylammonium pentasil
zeolite (nPr
4
NϪZSM-5) prepared according to the patent litera-
[
30]
Acknowledgments
ture.
Degradation of the quaternary ammonium ion readily af-
fords the protonated form of ZSM-5 (Brønsted acid sites); however,
this type of thermal treatment also generates Lewis acid sites aris-
ing from partial framework dealumination. The H-ZSM-5 batch
used in the present work was characterized by chemical analysis
and by the pyridine adsorption-desorption method. The chemical
analysis showed a Si/Al ratio of 34:1; the IR-spectroscopic analysis
of pyridine-loaded zeolite samples may identify Brønsted as well
as Lewis acid sites by the presence of aromatic bands distinctive of
The authors acknowledge financial support from the Spanish
DGICYT (Grant MAT97Ϫ1016-CO2) and the US National Sci-
ence Foundation (Grant NSF CHEϪ9714850). V. M. thanks the
Generalidad Valenciana for a postgraduate scholarship; H. D. R.
gratefully acknowledges the 1997 IBERDROLA Award and thanks
the Generalidad Valenciana for a fellowship.
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ZSMϪ5 as Brønsted Acid and Electron Acceptor in the Adsorption of N,NЈ-Diphenylhydrazine
FULL PAPER
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