Reduction of benzaldehyde on metal oxides

Article abstract of DOI:10.1006/jcat.1997.1854

The study of benzaldehyde reduction over simple metal oxides was carried out at 300°C in a helium or dihydrogen atmosphere. The results obtained show that benzyl alcohol is produced by the Cannizzaro reaction under helium and by direct hydrogenation under dihydrogen on reducible oxides such as ZnO and Cr₂O₃. Over slightly reducible oxides such as ZrO₂, both reactions occur under H₂: the Cannizzaro reaction involves the most basic OH groups and is followed by the reduction of benzoate species by H₂ to benzaldehyde, regenerating the active hydroxyl groups. Toluene is formed under He on the most acidic oxides (Al₂O₃, TiO₂) from a reaction involving benzyl alcoholate species, and under H₂ on reducible oxides (Fe₂O₃) by means of consecutive reduction reactions. The present study shows that the benzaldehyde transformation via the Cannizzaro reaction is a good test for determining the mobility of surface hydroxyl groups on metal oxides, whereas its direct reduction under H₂ characterizes the metal oxide redox properties.

Full text of DOI:10.1006/jcat.1997.1854

JOURNAL OF CATALYSIS 172, 85–92 (1997)
ARTICLE NO. CA971854
Reduction of Benzaldehyde on Metal Oxides
D. Haffad, U. Kameswari,† M. M. Bettahar, A. Chambellan, ,1 and J. C. Lavalley
UMR 6506, Catalyse et Spectrochimie, ISMRa-Universite´, 14050 Caen, France; and †Indian Institute of Technology, Madras 600 036, India
Received February 10, 1997; revised May 13, 1997; accepted July 16, 1997
considered to be basic, amphoteric, and weakly acidic, re-
spectively.
The study of benzaldehyde reduction over simple metal oxides
was carried out at 300 C in a helium or dihydrogen atmosphere. The
results obtained show that benzyl alcohol isproduced by the Canniz-
zaro reaction under helium and by direct hydrogenation under di-
hydrogen on reducible oxides such as ZnO and Cr2O3. Over slightly
reducible oxides such as ZrO2, both reactions occur under H2: the
Cannizzaro reaction involves the most basic OH groups and is fol-
lowed by the reduction of benzoate species by H2 to benzaldehyde,
regenerating the active hydroxyl groups. Toluene is formed under
He on the most acidic oxides (Al2O3, TiO2) from a reaction involving
benzyl alcoholate species, and under H2 on reducible oxides (Fe2O3)
by means of consecutive reduction reactions. The present study
shows that the benzaldehyde transformation via the Cannizzaro re-
action is a good test for determining the mobility of surface hydroxyl
groups on metal oxides, whereas its direct reduction under H2 char-
c
Transition metal oxides are more difficult to characterise.
The properties depend a priori on their positions in the pe-
riodic table and the oxidation state of the ion. For a given
oxidation state, reducing ability and basicity of these ox-
ides increase in a given column from top to bottom and in
a period from right to left. Numerous analysis techniques
allow these tendencies to be confirmed: titration (5), mi-
crocalorimetry (6), or use of IR with probe molecules (7).
In particular: pyridine and carbon monoxide adsorption
demonstrate the presence of Lewis acidic sites on all sam-
ples tested; Bro¨nsted acidity is also detected, especially on
V2O5 (8); CO2 adsorption reveals no notable basicity on
V2O5 and SiO2 (8, 9); and the presence of strongly basic
hydroxydes has only been described for a few samples such
as MgO, ZrO2, and CeO2 (4).
acterizes the metal oxide redox properties.
1997 Academic Press
INTRODUCTION
EXPERIMENTAL
The transformation of benzaldehyde into reduced com-
The metal oxides used are reported in Table 1. Some
pounds such as benzyl alcohol or toluene can occur on metal are commercial, the others were prepared by hydrolysis of
oxides (1–3). Two main processes seem to be involved de- the corresponding isopropylate, followed by calcination at
pending on whether or not the formation of the reduced 550 C (10).
compounds is accompanied by that of oxygenated products
Before the reaction, a current of either helium (denoted
such as benzoate. In the former case (Cannizzaro reaction) as N, like the neutral pretreatment) or dihydrogen (denoted
the active sites are mobile hydroxyl groups, whereas in the as R, like the reducing pretreatment) was passed over the
latter case, which requires the presence of hydrogen, re- oxides (100 mg) for 2 h at 350 C.
duced sites are certainly involved (4).
The benzaldehyde transformation was performed in he-
Therefore comparison of benzaldehyde conversion on lium atmosphere (N process) or in dihydrogen atmosphere
metal oxides, in the presence or absence of hydrogen, would (R process) in a quartz microflow reactor at atmospheric
allowthe classification ofthe metaloxidesaccordingto their pressure and 300 C. The flow rate was 40 ml/min, and the
1
hydroxyl mobility (basicity) and reducibility. This paper space velocity was W/F = 0.21 kg mol 1 h
.
deals with the conversion of benzaldehyde under helium
or under hydrogen over a large number of metal oxides:
The reaction mixture was analysed on line by gas chro-
matography equipped with a FID detector, the first analysis
being performed after a reaction time of 21 min.
MgO, ZnO, Al2O3, V2O3, Cr2O3, Fe2O3,
SiO2, TiO2, ZrO2, CeO2, V2O5.
REDUCTION OF BENZALDEHYDE IN A
HELIUM ATMOSPHERE
1. Results
These oxides possess very distinctive acid/base and redox
properties. MgO, Al2O3, and SiO2 are fixed valence oxides
Influence of the Nature of Metal Oxides
In helium atmosphere, the reactivity decreased with time
1 Corresponding author. E-mail: Spectrocat@ISMRA. UNICAEN. Fr. on flow (Fig. 1), generally tending to zero or to a very low
85
0021-9517/97 $25.00
c
Copyright
1997 by Academic Press
All rights of reproduction in any form reserved.

86
HAFFAD ET AL.
TABLE 1
Origin and Surface Area of the Different Metal Oxides Used
Oxide
Origin
Surface area (m2/g)
MgO
ZnO
Merck
54
40
179
24
42
12
200
70
50
120
8
Asturienne des mines
ex-isopropylate
Merck
Rhoˆne–Poulenc
Rhoˆne–Poulenc
Degussa
ex-isopropylate
ex-isopropylate
Rhoˆne–Poulenc
Merck
Al2O3
V2O3
Cr2O3
Fe2O3
SiO2
TiO2
ZrO2
CeO2
V2O5
residual value. After three on flow, all oxides were inactive
with the exception of TiO2 (0.1% T).
The benzaldehyde conversions f after 21 min of reaction
are reported in Fig. 2. Reaction products were benzyl alco-
hol (A), toluene (T), and benzene (B).
Activities and selectivities depend on the metal oxide
used. The reactivity of benzaldehyde decreases in the fol-
lowing order:
FIG. 2. Benzaldehyde conversion on metal oxides in a helium atmo-
sphere at 300 C after 21 min under flow: B, benzene; T, toluene; A, benzyl
alcohol.
the conversion of benzaldehyde decreased from 2.2 to 0% ,
whereas ZnO remained inactive. No change was observed
in the selectivities.
Influence of the R Pretreatment in a
Dihydrogen Atmosphere
MgO
TiO2 ZrO2 > Al2O3 > Cr2O3 > CeO2
V2O3 Fe2O3 > ZnO = SiO2 = V2O5 = 0.
The same oxides (MgO, ZrO2, and ZnO) were pretreated
by molecular hydrogen at 350 C. The MgO activity was not
affected (12.9% A); ZrO2 became totally inactive, and no
change was observed in the case of ZnO which remained
inactive.
Influence of the N Pretreatment Temperature
Previous work showed some influence of the pretreat-
ment temperature on benzaldehyde reduction on the ba-
sic oxides MgO, ZrO2, and ZnO (4). Thus, for MgO, af-
ter 21 min of reaction, the conversion of benzaldehyde
decreased from 12.7 to 3.5% when the pretreatment tem-
perature increased from 350 to 450 C. Similary, on ZrO2,
Influence of Water Pretreatment
After the R pretreatment at 350 C, a mixture of water
(P = 10 Torr) in helium was passed over the oxide sam-
ples for 10 min at 300 C (RW pretreatment); thereafter the
benzaldehyde–helium mixture was passed over the samples
(RWN process).
In relation to the NN process (pretreatment and reac-
tion in a helium atmosphere), the influence of water de-
pended on the metal oxide used. The results show that the
zirconia activity was not only re-established but, moreover,
enhanced, since the conversion of aldehyde to benzyl al-
cohol increased from 2.2 to 6.1% . A positive effect was
also observed for cerium, vanadium (V2O3) and iron ox-
ides which became slightly active ( 0.3–0.5% conversion).
Cerium oxide produced some benzyl alcohol (selectivity:
33% A + 67% B), in contrast to vanadium oxide (100% T)
or iron oxide (67% T + 33% B). No effect was observed in
the case of ZnO, Cr2O3, and TiO2.
Conversely, the conversion of benzaldehyde to benzyl
alcohol decreased from 12.7 to 4.5% A on magnesia. The
same feature was observed for alumina, since the toluene
production decreased from 1.2 to 0.1% T.
FIG. 1. Benzaldehyde conversion versus time on flow on metal oxides
in a helium atmosphere at 300 C.

REDUCTION OF BENZALDEHYDE
87
2. Discussion
most active oxide: MgO, from the extrapolation to t → 0 of
the curve “activity versus time.” However, Fig. 1 shows that
the determination of the initial activity is imprecise; it was
estimated tentatively as 20 mol/m2h. Taking into account
the amount of benzyl alcohol formed with time, we can
deduce that the number of hydroxyl groups n(OH), which
were consumed after two of reactions, are approximately
Under helium atmosphere, benzyl alcohol is selectively
formed on MgO, ZrO2, and CeO2, whereas toluene is ob-
tained on Al2O3, V2O3, TiO2, and Fe2O3. Small amounts of
benzene were also found in the case of Cr2O3 and TiO2.
Benzyl Alcohol Formation
Analogous to results obtained in homogeneous medium
(11), benzyl alcohol should be produced by the Cannizzaro
reaction, formally:
n(OH) = 20 mol/m2 = 12 OH/nm2.
This OH number may be different from the total number
of hydroxyl groups present on the surface.
(ii) In infrared studies of partially dehydroxylated metal
oxides, several types of the OH group were detected. The
most basic, referred to as type I, is a terminal OH coor-
2 PhCHO + OH → PhCOO + PhCH2OH. [1]
The mechanism on oxide surfaces can be written as follows
(4):
–
dinated to one metal cation (M OH). It corresponds to
the highest (OH) wave number in the IR spectrum. Such
species give rise to a very strong absorption band on MgO,
a weaker band on ZrO2, and the weakest band on ZnO (4).
In the case of CeO2, which was not very active in the con-
sidered reaction, it has been shown that type-I OH groups
are thermally unstable, explaining the poor activity of this
oxide (13). The presence of basic OH groups was proven
by the formation of hydrogen carbonate species with the
adsorption CO2 (14) or by CS2 and COS hydrolysis (15).
The latter test confirms the low basicity of the ZnO surface
OH groups and, on the other hand, the efficiency of those
on ZrO2 (16).
(iii) The pretreatment in a H2 atmosphere is expected to
reduce the oxide surfaces and to affect the hydroxyls. The
activity of the irreducible magnesia was not modified by the
RN process; on the contrary, in the case of the slightly re-
ducible zirconia (17), the H2 pretreatment drastically inhib-
ited the Cannizzaro reaction; no conversion was observed.
One infers that this activation mode provokes the modifi-
cation of hydroxyl groups and, as a result, the total inhi-
bition of the dismutation reaction. The mechanism of de-
hydroxylation is probably complex. At 350 C, dihydrogen
produced some oxygen vacancies by moderate reduction
of the surface and altered the surface structure (monoclinic
The first step is the nucleophilic addition of the hydroxyl
group to the first aldehyde molecule. The second step is the
nucleophilic addition of the intermediate hydride species
on the second aldehyde molecule. Such a scheme involves
the disappearance of surface basic hydroxyls and the forma-
tion of adsorbed benzoate species. Stoichiometrically, the
formation of one benzyl alcohol molecule consumes one
surface hydroxyl group, thus explaining the decrease in re-
activity with time.
The observed activities MgO (12.7% ) ZrO2 (2.2% )
CeO2 (0.05% ) > ZnO (0% ) mean that the number of active
hydroxyls varies in the same way. This order is surprising
and not in agreement with the bulk basicity (PIES) in the
Parks scale (5):
MgO (12.4) > ZnO (8.9) > CeO2 (6.8) ZrO2 (6.7).
However, the strength of basic sites, estimated from the to quadratic) (18). No change was observed on ZnO, be-
adsorption of Hammett indicators is different (12): H0, MAX cause it does not possess active hydroxyl groups and, thus,
(ZrO2) = 9.5 H0, MAX (ZnO) = 6.4. Thus the observed ac- remains inactive after pre-reduction.
tivity would only depend on the number of strong basic
hydroxyls.
(iv) Conversely, the hydration of the reduced samples
(RWN process) is expected to re-establish or even increase
The effects of different pretreatments as well as com- the number of surface hydroxyls and, consequently, the
plementary results corroborate such a conclusion. The pre- Cannizzaro reaction. This is notably the case on zirconia
treatment of the oxides modifies their surface properties and, to a lesser extent, on ceria. No detectable effect was
and, especially for a given oxide, the strength and number observed on ZnO, whereas a negative effect was noted on
of the surface hydroxyl groups:
magnesia. Water molecules should inhibit some active hy-
droxyl groups on magnesia by means of a nondissociative
(coordinative) adsorption (19).
(i) The increase in the MgO activation temperature leads
to the decrease in the number of the hydroxyl surface
species and, consequently, in benzaldehyde conversion. The
In summary, benzaldehyde reduction to benzyl alco-
number of active sites can be estimated tentatively for the hol on oxides in helium atmosphere is the result of the

88
HAFFAD ET AL.
Cannizzaro dismutation. The reaction requires basic hy-
To differentiate mechanisms [3] and [4], small amounts of
droxyl groups. The observed conversions depend only on benzyl alcohol were added to the benzaldehyde flow during
the initial number and strength of these entities.
the transformation. Alcohol addition leads to an increase
in toluene formation and has little influence on the quan-
tity of benzaldehyde detected which is slightly diminished.
These results suggest that mechanism [4] prevails. Thus, it
seems to us that reaction [4] better reflects the experimen-
tal conditions, especially the presence of benzaldehyde in
the gas phase and the participation of Bro¨nsted acid sites.
It appears therefore that, whatever the reaction products
formed (benzyl alcohol or toluene), the surface-active sites
(basic hydroxyls) involved in the first step (formation of
benzyl alcohol or alkoxy species) are the same.
Reduction to Toluene
Toluene is produced selectively on amphoteric or acidic
oxides (mainly TiO2 and Al2O3 and, to a lesser extent, on
V2O3 and Fe2O3). Formally, this reaction requires two hy-
drogen atoms to produce toluene from benzaldehyde and
four hydrogen atoms if water appears. In a helium atmo-
sphere, if no coking occurs, hydrogen comes from surface
hydroxyl or aldehydic groups.
Sreekumar and Pillai (20) showed that benzyl alcohol un-
dergoes disproportionation to toluene, benzaldehyde, and
water over alumina at 300 C:
The benzaldehyde conversion to toluene decreases in the
order:
TiO2 > Al2O3
V2O3 Fe2O3 > Cr2O3
2 C6H5CH2OH → C6H5CH3 + C6H5CHO + H2O.
= SiO2 = V2O5 = 0.
This result suggests that toluene may arise from succes-
sive reactions: benzaldehyde dismutation to benzyl alcohol
(Cannizzaro reaction) and then benzyl alcohol transforma-
tion to toluene (Pillai reaction). However, on the metal
oxides which produce toluene, no trace of benzyl alcohol
was detected in the reaction mixture, suggesting that alkoxy
species do not desorb (reaction [2]) and react directly by a
new surface step to toluene:
This should reflect the relative abundance of the basic hy-
droxyls on these metal oxides.
Titania appearsto be nearlyasactive aszirconia (2.2% A)
after 21 min on flow, but its selectivity is quite different.
We observed that both oxides are active in COS hydrolysis
which also involves basic hydroxyl groups (21). In addition,
on TiO2, a slight residual activity ( 0.1% T) persists even
after a very long time on flow. This formation, considered
to be nearly catalytic, requires the regeneration of some
basic hydroxyls and the continuous disappearance of sur-
face benzoate species, explaining the presence of benzene
traces in the products formed. However, the weak activity
and the use of a FID detector in GC analysis do not allow
us to prove the concomitant formation of CO2, as would be
from [5], reported below.
2 PhCHO + OHads → PhCOOads + PhCH2Oads + Hads [2]
The mechanism could involve either two nearby alkoxy
species according to Sreekumars and Pillai’s suggestion (re-
action [3]):
2 PhCH2Oads + 2 Hads → PhCH3 + PhCHO + H2O [3]
The hydration of the samples after pretreatment under
dihydrogen (RWN process) induces various effects. As in
the case of magnesia, the activity on the irreducible alumina
clearly decreases, water inhibiting the reaction through a
possible nondissociative adsorption. No detectable effect
appears on titania, whereas the reactivity of benzaldehyde
is enhanced on V2O3 and Fe2O3 as observed above in the
case of ceria. Note that the reduction of such transition
metal oxides could increase their surface basicity (22).
In conclusion, benzaldehyde reduction to toluene on
metal oxides is the result of two consecutive reactions, the
Cannizzaro dismutation leading to adsorbed alkoxy species
which form toluene by hydrogen transfer. This transforma-
tion requires amphoteric oxides with basic hydroxyl groups
to trigger the first step and with acidic sites to avoid benzyl
alcohol desorption.
or an alkoxy species and a benzaldehyde molecule (reac-
tion [4]):
PhCH2Oads + PhCHO → PhCH3 + PhCOOads [4]
Benzene Formation
Some benzene appears in the case of chromia and tita-
nia: Cr2O3 (0.1% ) > TiO2 (0.05% ). Benzene is usually con-
sidered to be the result of the decomposition of surface

REDUCTION OF BENZALDEHYDE
89
benzoate species with carbon dioxide formation (23):
C6H5COOads + Hads → C6H6 + CO2.
[5]
Over TiO2, benzoate and alkoxy species are formed via
the Cannizzaro reaction. Over Cr2O3, the high selectivity
(100% ) and the low activity to benzene suggests a reduction
in the Cr2O3 surface by the benzaldehyde molecule. Let us
recall that the Cr2O3 surface is readily reduced (24).
All results strongly suggest that benzoate surface species
always appear during the formation of benzyl alcohol,
toluene, or benzene. A possible intermediate is a hemiac-
–
etal specie Ph CH(OH)Oads (25). It may be regarded as a
hydride producer for reductions (via alkoxy species) or a
benzene precursor. The final products should be dependent
only on the acidic or redox surface properties.
FIG. 4. Influence of the dihydrogen pressure on benzaldehyde con-
version at 300 C.
HYDROGENATION IN A DIHYDROGEN ATMOSPHERE
helium: benzyl alcohol, toluene, benzene, and traces of con-
densed products (not analysed). Of course, the activities
and selectivities are different.
1. Results
Influence of the Metal Oxide Nature
The irreducible oxides (magnesia, alumina, and silica)
remained almost inactive after dihydrogen introduction. In
contrast, all transition metal oxides exhibited various activ-
ities:
The benzaldehyde reduction was carried out under a di-
hydrogen flow after the total disappearance of strong basic
hydroxyls by pretreatment and reaction in a helium atmo-
sphere: (NNR process). Dihydrogen replaced the helium
flow 5 h after the test was started. Under such conditions,
the oxide surface was partially covered with carboxylate
species and all metal oxides were inactive before introduc-
ing H2 (with the exception of TiO2 0.1% T under He).
The activity and selectivity of the various metal oxides
used are reported in Fig. 3. The reported data describe the
stationaryconditions, no deactivation beingobserved in this
case.
(i) ZnO gave rise to large amounts of benzyl alcohol
and some toluene.
(ii) The metal oxides of the fourth period, with three
as the oxidation number, became active, and the benzalde-
hyde conversion increased with the atomicnumber (V2O3 <
Cr2O3 < Fe2O3). The selectivities differed considerably
from one oxide to the other (100% toluene on V2O3 and
Fe2O3; 88% alcohol and 12% toluene on chromia).
(iii) As for the metal oxides of the fourth column, zir-
conia displayed the same activity and selectivity as at the
beginning of the reaction under He (Cannizzaro reaction),
whereas the activity of titania was weaker. Ceria became
slightly active ( 0.5% ) under hydrogen flow, giving rise
mainly to benzyl alcohol and to very small amounts of
toluene and benzene.
The reaction is catalytic and the products obtained are
the same as those obtained for the transformation under
Influence of the Dihydrogen Partial Pressure
The effects of the partial pressure of dihydrogen on ben-
zaldehyde hydrogenation are reported in Fig. 4.
During the test, the carrier gas was a helium–hydrogen
mixture, the dihydrogen pressure being increased in 25%
steps from 0 to 100 kPa. It is shown that the conversion
increased with dihydrogen pressure. If we consider the plug
flow reactor to be differential, the apparent reaction order
in the dihydrogen pressure, , can be determined;it depends
on the metal oxide used and is not clearly defined on Fe2O3:
(Cr2O3) 2.5,
(ZrO2) 2,
(TiO2) 0.5,
FIG. 3. Benzaldehyde conversion on metal oxides in a dihydrogen
atmosphere at 300 C after 3 h under flow.
(Fe2O3) = unspecified (nearly 2).

90
HAFFAD ET AL.
Influence of the Pretreatment Conditions
ducible oxide, but it presents n-type semiconductivity due
to its tendency to include some interstitial Zn atoms or
Zn+ ions in the lattice. Therefore, IR studies on such an
oxide have shown heterolytic and homolytic dissociation
of dihydrogen (28). Therefore, hydrogen surface species
may be considered as active intermediates for the direct
hydrogenation of benzaldehyde to benzyl alcohol. Subse-
quently, the reaction can continue to toluene and possibly
benzene:
A seriesofexperimentswasperformed on the same metal
oxides pretreated at 350 C for 2 h in dihydrogen and then
water. After the benzaldehyde reaction under helium, the
latter was replaced by hydrogen (RWNR process). Very
similar activities were observed, as after the NNR process,
with the exception of iron and zirconium oxides. The con-
version of benzaldehyde was very high on the former (99%
T + 1% B), whereas on the latter it doubled from 2.2 to
4.3% A.
H2
PhCHO → PhCH2OH
H2
H2
2. Discussion
→ PhCH3 + H2O → C6H6 + CH4
. [6]
The replacement of helium by dihydrogen as the carrier
gas can provoke not only the direct hydrogenation of ben-
zaldehyde but it can also modify the surface properties: the
basicity may increase due to surface reduction and regener-
ation ofsurface hydroxylsmayoccur. Under the experimen-
tal conditions, the hydrogen molecules could be activated
by the transition metal oxides but not by the irreducible
oxides. However, the observed activities were not related
directly to the reducibility of the metal oxides. The order
of activity over the set of oxides (ZnO Fe2O3 ZrO2 >
The first step may take place as follows:
Cr2O3 > TiO2 CeO2 > V2O3
ferent from the order of the standard potentials redox E0
(Ox/Red) (26):
MgO = SiO2 = 0) is dif-
It is worth noting that ZnO, which is considered to be
strongly basic, mainly produces benzyl alcohol (91% ), a
small amount of toluene (9% ), and no benzene.
(ii) ZrO2 is a nearly irreducible oxide, although partial
surface reduction has been reported. Recent conductiv-
ity measurements showed that zirconia can show a weak
n-type semiconductivity (29). However, neither EPR (30)
nor IR studies (31) clearly confirm the occurrence of Zr3+
cations when reducing zirconia. The assignment of an IR
E0(CeO2/Ce2O3) > E0(Fe2O3/Fe3O4) > E0(Fe2O3/FeO)
> 0 > E0(Cr2O3/CrO) E0(ZnO/Zn) E0(TiO2/Ti2O5)
E0(V2O3/V2O2) > 0.6 > E0(Si O2/Si)
E0(ZrO2/Zr) E0(Al2 O3/Al) > E0(MgO/Mg).
1
(CO) wave number near 2120 cm when adsorbing CO
Nevertheless, not all E0 values are known, in particular is nuclear, since CO adsorption on basic sites, giving rise to
for partly reduced or nonstabilised forms such as Zr2O3 carbonite and ketene species, seems more likely than CO
or nonstoichiometric phases. The lack of data makes com- adsorption on Zr3+ sites. On the other hand, stabilization
parison incompete. However, all transition oxides (includ- of the superoxide radical ion on H2 treated zirconia may
ing ZrO2) present semiconducting properties. By partial provide indirect evidence of its reducive properties (32).
reduction under dihydrogen, some oxygens of the lattice Hydrogen molecule dissociation was also found on zirco-
will be eliminated, while some cations pass to a lower ox- nia, after activation under conditions different from those
idation state. Thus, some reactions, particularly hydrogen used in the present study (33).
or carbon monoxide addition, are best explained by rad-
Zirconium oxide was active under Cannizzaro reaction
icals such as neutral intermediate species on metals and
conditions. Therefore, carboxylate species covered the ox-
semiconducting oxides (27). Moreover, the occurrence of
ide surface when dihydrogen was introduced. The conver-
oxygen vacancies or mobile oxygen over the surface might
sion in the steady state observed under H2 (2.2% A) is of
favor deoxygenating–oxygenation reactions.
the same magnitude as that found in a helium atmosphere
Taking these data into account, the most striking results
(2.3% A) after 21 min of reaction (Figs. 2 and 3). Thus, hy-
are observed with zinc and zirconium oxides, which show
droxyl groups may play an important role in benzaldehyde
substantial activity despite their low reducibility.
hydrogenation over ZrO2. As shown previously (4), some
(i) Zinc oxide was the only inactive transition metal surface benzoate species on ZrO2 react with hydrogen at
oxide in the Cannizzaro reaction. Thus, a basic hydroxyl 300 C, leading back to benzaldehyde. This result suggests
regeneration under dihydrogen is not the cause of its ac- the regeneration of active species such as basic hydroxyls
tivity under H2. ZnO is not considered to be an easily re- (OHads) or intermediate species (C6H5CH(OH)Oads) and

REDUCTION OF BENZALDEHYDE
91
the reactions:
H2 + C6H5COOads → C6H5CHO + OHads
C6H5CHO + OHads → C6H5CH(OH)Oads
C6H5CH(OH)Oads + C6H5CHO
[7]
→ C6H5COOads + C6H5CH2OH
H2 + C6H5CHO → C6H5CH2OH
or, when Cannizzaro steps (full line) are introduced near
the regenerative steps (dotted line):
FIG. 5. Benzaldehyde conversion on ZrO2 under dihydrogen at
300 C. The influence of the dihydrogen pressure.
increases its activity in benzaldehyde hydrogenolysis to
toluene and benzene. A similar result was also found for the
hydrogenolysis of methyl benzoate and acetic acid. In the
To check such a mechanism, we investigated the effect of latter case, iron oxide was prereduced at 450 C, a temper-
the dihydrogen pressure, at a given pressure of benzalde- ature sufficiently high to give rise to metal iron detectable
hyde, on the reaction rate. Figure 4 shows that the reaction by XRD (34).
increases with p(H2) but that the variation is not linear. To
As Fe2O3, V2O3, and Cr2O3 are only slightly active un-
explain the surprising second-order (ZrO2) 2, we as- der helium atmosphere, therefore, mechanism [6] probably
sume that the hydrogenation of benzoate species [7] is the most often occurs, although these oxides are less reducible
rate determining step: r = k[C6H5COOact] p(H2) in which than Fe2O3. However, the observed selectivities, 88% of
[C6H5COOact.] is the surface concentration of active ben- benzyl alcohol on chromia and 100% of toluene on vana-
zoate intermediate species in the benzylalcohol formation. dium oxide, are quite different. The latter result may be due
According to mechanism [8] these intermediate species are to the occurrence of a partially oxided phase over the vana-
reduced by H2 and the apparent second order can be ex- dium sesquioxide used. V2O4, detected by X-ray analysis,
plained by considering their number to be proportional is more acidic than V2O3 which could favor toluene over
to p(H2): r = k p(H2)2. In addition, benzyl alcohol produc- benzaldehyde formation.
tion is directly related to his number of active sites: p(A) ∝
Titanium, zirconium, and cerium are three elements of
[C6H5COOact.] and r = k00 p(A) p(H2). Figure 5 shows that the fourth column in the periodic table. Contrary to TiO2
the product p(A) p(H2) is indeed linear in conversion. Strict and ZrO2, ceria appears to be almost inactive or slightly
proportionality is obtained at high p(H2), confirming the active in benzaldehyde conversion both in a helium and in
previous assumption. Conversely, when p(H2) is low, the a dihydrogen atmosphere, although it is partially reduced
rate-determining step may be different, implying a change by dihydrogen at 300 C. High selectivities in benzene or
in the oxide surface according to the dihydrogen pressure. light products were observed at 300 C (15% ) and 350 C
It appears, therefore, that the behavior of ZnO and ZrO2 (97% ); these results are probably related to the occurrence
is quite different, both producing mainly benzyl alcohol of highly stable benzoate species due to the high basicity of
from benzaldehyde reduction but in different ways, depend- ceria or coking which could hinder the benzaldehyde con-
ing on the metal oxide reducibility and/or the occurrence of version. TiO2-expropylate is active in the Cannizzaro reac-
active surface basic OH groups. The behavior of the other tion, although it is generally classified as a weakly acidic
oxides appears to be even more complicated, but is consid- metal oxide. Titania can also generate a reducing property
ered to imply both mechanisms [6] and [8]. They can be re- (an electron donating property): according to ESR studies,
garded as limiting cases; their relative occurrence depends titanium oxide is reduced to form Ti3+ (35), and this cation
on the oxide properties.
can go back to Ti4+ and an electron which may lead to an
Therefore, the reducible, rather acidic Fe2O3 oxide may anion radical with an electrophilic molecule such as ben-
produce toluene mainly via the direct consecutive hydro- zaldehyde (36). The latter is a possible intermediate for the
genation route (mechanism [6]). An interesting feature hydrogen transfer reaction and may participate in mech-
is the pre-reduction effect of Fe2O3, which dramatically anism [6]. Under H2, the rehydroxylation via mechanism

92
HAFFAD ET AL.
[8], would be different for the reduced surfaces and, thus, 10. Lahousse, C., Aboulayt, A., Mauge´, F., Bachelier, J., and Lavalley, J. C.,
J. Mol. Catal. 84, 283 (1993).
11. March, J., in “Advanced Organic Chemistry,” Internat. Stud. Ed.,
mechanism [6] would be predominant on reducible TiO2.
McGraw–Hill, Tokyo, 1997.
12. Tanabe, K., Misono, M., Ono, Y., and Hattori, H., in “New Solid Acids
CONCLUSION
and Bases” (B. Delmon and J. T. Yates, Eds.), Vol. 51, p. 21. Elsevier,
Amsterdam, 1989.
This study shows that benzaldehyde reduction at 300 C
is a sensitive test reaction for determining the surface prop- 13. Badri, A., Binet, C., and Lavalley, J. C., J. Chem. Soc. Faraday Trans.
92, 4669 (1996).
erties of metal oxides. When transformation occurs un-
der helium, the active sites are basic hydroxyl groups. On
irreducible basic oxides such as MgO, benzyl alcohol is
14. Morterra, C., and Orio, L., Mater. Chem. Phys. 24, 247 (1990).
15. Hoggan, P. E., Aboulayt, A., Pieplu, A., Nortier, P., and Lavalley, J. C.,
J. Catal. 149, 300 (1994).
obtained selectively, whereas on irreducible amphoteric ox-
ides (Al2O3) toluene is usually formed. Under such condi-
tions, the conversion decreases with time on-stream up to
the final consumption of the active hydroxyls.
Under H2, a stationary state is reached and most of the
active metal oxides are ZnO, Cr2O3, and Fe2O3, which
are known to be reducible, whereas Al2O3 and MgO are
inactive.
16. Aboulayt, A., Mauge´, F., Hoggan, P. E., and Lavalley, J. C., Catal. Lett.
39, 213 (1996).
17. Morterra, C., Giamello, E., Orio, L., and Volante, M., J. Phys. Chem.
94, 3111 (1990).
18. Trunschke, A., Huang, D. L., and Lieske, H., J. Chem. Soc. Faraday
Trans. 91, 4441 (1995).
19. Balint, I., and Aika, K., J. Chem. Soc. Faraday Trans. 91, 1805
(1995).
20. Sreekumar, R., and Pillai, C. N., Catal. Lett. 19, 281 (1993).
21. Bachelier, J., Aboulayt, A., Lavalley, J. C., Legendre, O., and Luck, F.,
Catal. Today 17, 55 (1993).
22. Noguera, C., in “Physique et chimie des surfaces d’oxydes,” p. 173.
Eyrolles, Paris, 1995.
A special case is constituted by ZrO2 and, to lesser extent,
by TiO2, both being active whatever the nature (He or H2)
of the flow gas used. This is explained by an auto-catalytic
mechanism involving the Cannizzaro reaction in the first 23. King, S. T., and Strojny, E. J., J. Catal. 76, 274 (1982).
24. Curry-Hyde, H. E., Musch, H., Baiker, A., Schramal-Marth, M., and
Wokaun, A., J. Catal. 133, 397 (1992).
25. Busca, G., Lamotte, J., Lavalley, J. C., and Lorenzelli, V., J. Amer.
stage, the benzoate species thus formed being reduced to
benzaldehyde, leading to the regeneration of the active hy-
droxyl sites. Those metal oxides have both properties: they
Chem. Soc. 209, 5197 (1987).
are slightly (TiO2) or moderately (ZrO2) basic and slightly
(ZrO2) or moderately (TiO2) reducible.
26. Pourbaix, M., in “Atlas des Equilibres Electroniques.” Gauthiers-
Villars, Paris, 1963.
27. Germain, J. E., in “Catalytic Conversion of Hydrocarbons,” Academic
Press, New York, 1969.
28. Saussey, J., Lavalley, J. C., Lamotte, J., and Rai, T., J. Chem. Soc. Chem.
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