Permanent blockade of in situ-generated acid Brønsted sites of vanadyl
pyrophosphate catalysts by pyridine during the partial oxidation of toluene
Andreas Martin,* Ursula Bentrup, Bernhard Lücke and Angelika Brückner
Institut für Angewandte Chemie Berlin-Adlershof e.V., Rudower Chaussee 5 D-12484 Berlin, Germany.
E-mail: a.martin@aca-berlin.de
Received (in Cambridge, UK) 6th April 1999, Accepted 20th May 1999
The permanent blockade of in situ-formed Brønsted-acid
OH groups and an effective lowering of the catalyst acidity
during the partial oxidation of toluene to benzaldehyde is
demonstrated by an efficient method using a continuous
dosing of pyridine to the feed that leads to drastically
increased aldehyde selectivities.
with water vapour as mentioned above (d). Normally, the
benzaldehyde adsorption is characterised by the appearance of
two vibration bands at ca. 1720 and 1705 cm21 assigned as
carbonyl stretching vibrations. In the spectrum of the water
vapour pretreated sample a further carbonyl stretching band at
1685 cm21 has been observed. The shift of the carbonyl band to
lower wavenumbers points to a weakening of the CNO bond.
This effect can be explained by a strengthening of the
adsorption of the aldehyde7 by interaction of neighbouring OH
groups with the carbonyl groups probably via hydrogen
bonding. This can also be seen in the case of the toluene–air
adsorption under working conditions [Fig. 1, spectra (a) and
(b)]: The carbonyl band is obviously shifted to lower wav-
enumbers, independent of the catalyst pretreatment, i.e. the
water produced during the catalytic reaction causes the same
effect as has been found for the water vapour pretreatment of the
catalyst. This behaviour explains the findings of the recent in
situ-EPR measurements4 and provides a reason for the strong
chemisorption of aldehyde intermediates and therefore, their
subsequent oxidation in consecutive reactions.
Scheme 1 illustrates these ideas. The VPP catalyst surface
consists of vanadyl dioctahedra units linked via V–O–P bridges
with the phosphate tetrahedra of the pyrophosphate chains. A
toluene molecule is chemisorbed on the Lewis sites via its p-
electron system. An electrophilic attack on the methyl group
leads to a methylene-like species, water is probably eliminated
by the collaboration of bulk oxygen, which is in turn replaced by
gas-phase oxygen. Thus, benzaldehyde is formed that is able to
desorb, but simultaneously, the liberated water molecule can
attack the neighbouring V–O–P bond, generating OH groups
(V–OH and P–OH) which interact with benzaldehyde via
hydrogen bonding and its fast desorption is markedly hin-
dered.
Vanadium phosphates (VPO) are well known as catalysts for
selective O- and N-insertion reactions of aliphatics and
methylaromatics.1,2 Recently, we have studied the ammoxida-
tion of toluene on vanadyl pyrophosphate [(VO)2P2O7] in detail
and obtained knowledge on the formation of benzaldehyde,
acting as a reaction intermediate.3 Therefore, the idea arose to
test these catalysts for the partial oxidation of toluene. However,
the catalytic performance of these solids is rather poor:
benzaldehyde selectivities > 40% could only be reached at
toluene conversions < 5% and mainly carbon oxides were
identified as by-products in the effluent. A number of further
oxidised intermediates (cyclic anhydrides are of prime im-
portance) that remain chemisorbed on the catalyst surface were
detected by FTIR spectroscopy.4 The reason for the rather low
aldehyde selectivity can be seen in a very strong adsorption of
toluene and/or intermediates on the rather acidic surface of the
VPO catalyst as confirmed by in situ-EPR measurements
carried out recently.4 Furthermore, it is also supposed that the
existence of M–OH sites could be involved in the oxidation of
the aromatic nucleus, leading to total oxidation.5
Here, we present investigations on the partial oxidation of
toluene to benzaldehyde with vanadyl pyrophosphate used as
catalyst, especially on the effect of Brønsted-acid OH groups of
the catalyst surface using in situ-FTIR spectroscopy. A
procedure is proposed to block such in situ-generated acid OH
groups by pyridine. Additionally, an effective increase of the
basicity of the catalyst surface can be reached that might
enhance the desorption rate of the desired aldehyde.
(VO)2P2O7 (VPP) was generated by the usual dehydration of
VOHPO4·0.5H2O used as precursor compound (723 K, 4 h, 10
l h21 N2). The catalytic properties were determined during the
oxidation of toluene to benzaldehyde, using a fixed bed quartz-
glass reactor. The catalyst (0.5 g) was applied as sieve fraction
(1–1.25 mm) and mixed with an equal portion of quartz glass (1
mm) to avoid local overheating. The effluent was analysed by
GC and the formation of CO/CO2 was permanently followed by
non-dispersive IR photometry. The in situ-FTIR investigations
were carried out on a Bruker IFS 66 FTIR spectrometer using
self-supporting discs with a diameter of 20 mm and a weight of
50 mg, mounted in an heated IR cell.
Fig. 1 depicts FTIR spectra of adsorbates of the partial
oxidation of toluene (air– toluene = 100+1, 573 K) on VPP [(a)
fresh VPP, (b) VPP pretreated with water vapour (4.2 mmol h21
H2O in air, 30 min) at 573 K prior to feed exposure). Beside the
bands of the aromatic ring vibration (1605, 1500 cm21) and that
of the adsorbed benzaldehyde (1678 cm21), the bands of cyclic
anhydrides (1858, 1783 cm21) always appeared. These cyclic
anhydrides can be considered as total oxidation precursors.6
Additionally, Fig. 1 also shows FTIR spectra of air–benzalde-
hyde mixtures (air–benzaldehyde = 91+1) adsorbed at 573 K
on the parent VPP sample (c) and on a VPP specimen pretreated
Fig. 1 FTIR spectra of adsorbates on a (VO)2P2O7 catalyst at 673 K: (a) air–
toluene flow (molar ratio = 100+1, 4.2 mmol toluene h21, 60 min), (b) the
same air–toluene flow after water vapour pretreatment (4.2 mmol H2O h21
in air, 30 min), (c) air–benzaldehyde flow (molar ratio = 91+1, 2.3 mmol
benzaldehyde h21, 60 min) and (d) the same air-benzaldehyde flow after
water vapour pretreatment (4.2 mmol H2O h21 in air, 30 min).
Chem. Commun., 1999, 1169–1170
1169
Martin, Andreas
