Vapour-phase reaction of acetophenone with methanol or dimethyl carbonate on magnesium oxide and magnesium phosphates

Article abstract of DOI:10.1006/jcat.1998.2360

The vapour-phase reaction of acetophenone with methanol on magnesium oxide, various magnesium phosphates, and combinations of the two types of catalysts was studied. The process was found to involve the Meerwein-Ponndorf-Verley reaction, aldol condensatio

Full text of DOI:10.1006/jcat.1998.2360

Journal of Catalysis 183, 119–127 (1999)
Article ID jcat.1998.2360, available online at http://www.idealibrary.com on
Vapour-Phase Reaction of Acetophenone with Methanol or Dimethyl
Carbonate on Magnesium Oxide and Magnesium Phosphates
Mar´ıa Angeles Aramend´ıa, Victoriano Borau, Ce´sar Jime´nez,1 Jose´ Mar´ıa Marinas,
and Francisco Jose´ Romero
Department of Organic Chemistry, Faculty of Sciences, University of Co´rdoba, Avenida San Alberto Magno s/n, E-14004 Co´rdoba, Spain
Received September 23, 1998; revised November 20, 1998; accepted November 20, 1998
This process involves dehydrations, dehydrogenations,
and cross-coupling reactions.
The vapour-phase reaction of acetophenone with methanol on
magnesium oxide, various magnesium phosphates, and combina-
tions of the two types of catalysts was studied. The process was
found to involve the Meerwein–Ponndorf–Verley reaction, aldol
condensations, dehydrations, and hydrogenations. The presence of
basic sites is indispensable for the reaction to develop; however, acid
sites also play an active role. The selectivity for each reaction prod-
uct depends on the particular catalyst used. The total conversion
is maximal with the catalysts containing the largest populations of
acid and basic sites. Also, catalysts with large numbers of acid sites
exhibit an increased selectivity towards the corresponding alkenes.
The use of dimethyl carbonate instead of methanol alters the re-
action selectivity to an extent dependent on the particular catalyst
and operating conditions. However, this also results in markedly
c
Active catalysts for this process include MgO doped with
2–15% by weight of a transition metal such as Al, Fe, Cr,
Ni, or Cu (4). On these catalysts, acetonitrile reacts with
methanol to give acrylonitrile and, by subsequent hydro-
genation, propionitrile,
2 CH3CN + 2 CH3OH
==
→ CH2 CHCN + CH3–CH2CN + 2 H2O + H2,
which reacts with methanol to yield methacrylonitrile as a
major product and small amounts of isobutyronitrile and
crotonitrile (6). Chromium(III)-doped MgO exhibits high
activity and selectivity in both reactions.
Acetone reacts with methanol to form methyl vinyl ke-
tone as a major product and methyl ethyl ketone, C5 ke-
tones, and isopropyl alcohol as minor products (7). MgO
doped with Cr(III), Cu(II), or, especially, Fe(III) provides
high activity and selectivity.
decreased total conversion in some instances.
1999 Academic Press
Key Words: acetophenone; methanol; condensation; MPV reac-
tion; magnesium phosphates; magnesium oxide; zinc oxide.
INTRODUCTION
Propionic ester also reacts with methanol. In addition to
methyl methacrylate, the reaction produces methyl 2-meth-
ylpropionate and C5–C6 ketones. The highest selectivity for
the methacrylate, 65% , is provided by Mn(II)-doped MgO.
The reaction of methanol with acetone has also been de-
veloped on H-ZSM-5 (9). Based on the results, the reaction
yields propene and isobutene as major products, in addi-
tion to small amounts of dimethyl ether and various aro-
matics. Experiments involving Cs-impregnated X zeolites
(10) have revealed that most of these products result from
aldol condensation of acetone and that small amounts of
methyl vinyl ketone are formed in the process.
, -Unsaturated compounds are very important for the
chemical industry [particularly in the manufacturing of
polymers] (11). Thus, vinyl ketones are gaining increasing
significance (12) as polymer photosensitizers and electro-
conducting polymers.
The use of C1 sources in applied catalysis research has
grown enormously in the past few years (1). The fact that
methanol can be conveniently obtained from methane or
coal has promoted research into the reactions involved [e.g.,
the methanol to gasoline conversion based on ZSM-5 (2)].
The acid-catalysed coupling of olefins to form ethers is also
a major reaction in this context and is used, among oth-
ers, to synthesize octane-enhancing methyl tert-butyl ether
(MTBE) from methanol and isobutene (3). Ueda et al. (4–8)
found methanol to react with activated C–H containing
compounds such as nitriles, ketones, and ethers to form
,
-unsaturated compounds. The reaction takes place on
catalysts possessing both acid and basic functions and can
be formulated as follows,
==
RCH2 Z + CH3OH → CH2 CRZ + H2O + H2,
with Z = –CN, –CO R0, –COO R00and R = –H or –CH3.
In recent years, various magnesium phosphates have
been used as catalysts for organic processes. Thus, Sokolo-
vskiietal. (13) have examined the oxidative transformations
1 To whom correspondence should be addressed.
119
0021-9517/99 $30.00
c
Copyright
1999 by Academic Press
All rights of reproduction in any form reserved.

´
ARAMENDIA ET AL.
120
of methane on magnesium–phosphorus catalysts. Based on on a Siemens D 500 diffractometer using CuK radiation.
their results, increasing the catalyst acidity by raising its Scans were performed over the 2 range from 2 to 80.
phosphorus content shifts the direction of the methane The catalysts were also characterized by thermogravimetric
transformation from dimerization to selective oxidation. analysis (TA) and diffuse reflectance infrared spectroscopy
Sugiyama et al. (14, 15) used a commercially available (DRIFT).
magnesium phosphate in the oxidative dehydrogenation of
The composition of the catalysts was determined by en-
ethane and the oxidative coupling of methane. Our group ergy dispersive X-ray analysis (EDAX) on a Jeol JSM-
(16) has studied the influence of the procedure used to syn- 5400 instrument equipped with a Link ISI analyser and a
thesize various magnesium phosphates on their structure, Pentafet detector (Oxford), as well as from XPS measure-
composition, and surface properties, as well as its effect on ments made on a Leybold–Heraeus LHS-10 spectrometer
the gas-phase conversion of alcohols.
using a constant pass energy of 50 eV and MgK radiation
This paper reports the results obtained in the vapour- for excitation (h = 1253.6 eV).
phase reaction of acetophenone with methanol or dimethyl
carbonate on various magnesium phosphates, magnesium
Chemical and Textural Properties of the Catalysts
oxide, and combinations of both types of solid. These cata-
lysts possess differential structure, composition, and surface
properties that influence the selectivity of the process.
The specific surface area of the solids was determined
by using the BET method on a Micromeritics ASAP 2000
analyser.
Acid and basic sites were quantified from the reten-
tion isotherms for two different titrants (cyclohexylamine
and phenol, respectively, dissolved in cyclohexane). The
amount of titrant retained by each solid was measured spec-
EXPERIMENTAL
Catalyst Synthesis
Catalysts MgPC (Aldrich, ref. 34, 470-2), MgPPC
(Aldrich, ref. 34, 075-8), MgOC (Probus, ref. 3225), and
ZnOC (Probus ref. 50286) were commercially available
solids of formula Mg3(PO4)2 xH2O, MgHPO4 3H2O,
Mg(OH)2, and Zn(OH)2, respectively. The procedure used
to synthesize solid NaMgP has been described elsewhere
(16).
trophotometrically (
226 and 271.6 nm for cyclohexy-
max
lamine and phenol, respectively). By using the Langmuir
equation, the amount of titrant adsorbed in monolayer
form, Xm, was obtained as a measure of the concentration
of acid and basic sites (17).
Reactor
Catalyst MgP(Cl,H,w) was prepared as follows. A solution
containing 40 g of MgCl2 6H2O and 8.9 ml of 85% H3PO4
in 125 ml of distilled water was placed in an ice bath and sup-
plied with 3 N NaOH dropwise up to pH 9 under continuous
stirring. The solid thus obtained was filtered, suspended in
distilled water (20 ml/g solid), and stirred for 30 min, af-
ter which it was allowed to stand for 24 h, filtered, and air-
dried.
The procedure used to synthesize catalyst NaMgP–O in-
volved placing 12.3 g of Mg(OH)2 and 20 g of Na2HPO4
in a flask and suspending the mixture in 200 ml of distilled
water. Following refluxing for 24 h, the suspended solid was
filtered while hot and washed with distilled water and iso-
propyl alcohol prior to air-drying.
Catalyst NaMgP–P was synthesized by refluxing a mix-
ture of 24.5 g of MgHPO4 3H2O and 6.27 g of NaOH in
200 ml of distilled water for 5 days. The suspended solid was
filtered while hot and air-dried.
All solids were subjected to the following stepwise calci-
nation programme: 1 h at 473 K, 1 h at 573 K, 1 h at 673 K,
and 3 h at 773 K. All were sifted through 200–250 mesh.
Reactions were carried out in a glass tubular reactor of
6 mm ID that was fed at the top with the reaction mixture
by means of a SAGE propulsion pump, the flowrate of
which was controlled via a nitrogen flowmeter. The reactor
was loaded with 0.175 g of catalyst, over which 5 g of
glass beads acting as a vaporizing layer were placed. The
temperature was controlled via an externally wrapped
heating wire that covered the height of both the catalytic
bed and the vaporizer, and was connected to a temperature
control. Evolved gases from the reactor were passed
through a condenser and onto a collector that allowed
fluids to be withdrawn at different times.
No diffusion control mechanism was detected (18), nor
were any of the reactor elements found to contribute to
the catalytic effect under the reactor’s operating conditions
(viz. feed rate 1–10 ml h 1, T 575–773 K, N2 flowrate 30 ml
min 1, amount of catalyst 0.1–0.5 g) used in preliminary
blank runs.
Collected samples were analysed by gas chromatography
using both an SPB-5 60 m 0.25 mm ID phenylsilicone
capillary column and raising the temperature from 343 to
553 K at 10 K min 1, and a 2 m 1/8 in ID column packed
Catalyst Characterization
The solids were structurally characterized from their with Carbowax over Chromosorb P-10% CW 20 M that was
1
X-ray diffraction (XRD) patterns, which were recorded heated linearly from 333 to 423 K at 30 K min
.

REACTION OF ACETOPHENONE AND METHANOL
121
TABLE 2
Reaction products were identified by comparison with
standards and by gas chromatography–mass spectrometry
on a Hewlett–Packard 5971A analyser.
Chemical Textural Properties and Crystal Phases Present
in the Catalysts Studied
SBET
(m2 g
Acidity
mol g
Basicity
mol g
RESULTS AND DISCUSSION
1
1
a
1
b
Catalyst
)
(
)
(
)
Phases
c
Catalyst Characterization
MgPC
MgPPC
7
11
31
8
30
241
108
2
24
51
83
12
52
123
88
—
6
9
44
5
25
254
77
4
Mg3(PO4)2
-Mg2P2O7
Based on its XRD patterns, uncalcined solid MgPC con-
sists of Mg3(PO4)2 8H2O. Its crystallization water can be
removed by heating at 442 K. Above 923 K, the solid con-
sists of Mg3(PO4)2 (crystalline farringtonite phase). At the
operating temperature, 773 K, this solid consists of amor-
phous Mg3(PO4)2. Uncalcined MgOC consists of Mg(OH)2
that becomes periclase MgO at 773 K. Similarly, solid ZnOC
is Zn(OH)2 and becomes zincite ZnO upon calcination
at 773 K. On the other hand, uncalcined MgPPC is
MgHPO4 3H2O, loses its crystallization water at ca. 440 K,
and becomes -Mg2P2O7 at 773 K. Its structure and thermal
behaviour were examined elsewhere (19).
Solid NaMgP, which was studied in previous work (16,
20), consists largely of NaMgPO4, accompanied by resid-
ual amounts of MgO, NaCl, and Na2CO3. Based on its
XRD patterns, solid NaMgP–P calcined at 773 K consists
of NaMgPO4 and Mg2P2O7 but is poorly crystalline. Finally,
solid NaMgP–O calcined at 773 K consists of NaMgPO4 and
MgO.
c
MgP(Cl,H,w)
NaMgP
NaMgP–P
MgOC
NaMgP–O
ZnOC
Mg3(PO4)2
NaMgPO4
NaMgPO4, Mg2P2O7
MgO
MgO, NaMgPO4
ZnO
a Vs cyclohexylamine.
b Vs phenol.
c Crystalline only above 923 K.
as large as those of MgPC, MgPPC, and NaMgP. Solid ZnOC
is the one with the smallest surface area.
The catalysts also differ markedly in acidity and basicity.
The two oxides, MgOC and ZnOC, are essentially basic but
possess a markedly different number of sites. MgOC is an-
other solid containing a fairly large population of acid sites.
Magnesium oxide is considered a strongly basic compound
by virtue of the presence of O2 surface sites that can act as
proton acceptors; however, Mg2+ ions are considered to be
weak Lewis acids (21). Catalyst NaMgP–O contains a sim-
ilar number of acid and basic sites. The other solids possess
more acid sites than basic sites (particularly MgPPC, which
is essentially acidic).
Table 1 shows the percentage atomic composition of each
catalyst.
Textural and Surface Acid–Base Properties of the Catalysts
Table 2 summarizes the textural and acid–base properties
of the catalysts studied. It also shows the above-described
crystal phases as identified from the XRD patterns.
Catalytic Activity
Reaction between acetophenone and methanol. This re-
action can yield a wide variety of products depending on
the particular catalyst used. As a rule, the formation of
the main reaction products can be accounted for on the
basis of Scheme 1. A series of dehydrogenation, hydrogen-
transfer, dehydration, isomerization, and condensation re-
actions result in a wide range of products, the identification
and analysis of which entail the use of gas chromatography
in combination with mass spectrometry.
As can be seen, the specific surface area varies markedly
among the solids. Thus, the areas of the catalysts containing
MgO as main phase are much higher than those of the solids
consisting largely of a magnesium phosphate. The latter
also exhibit some differences among individual solids. Thus,
catalysts MgP(Cl,H,w) and NaMgP–P have areas three times
TABLE 1
The main products of the reaction include styrene, 1-
phenylethanol, phenyl vinyl ketone, and propiophenone.
Alkene homologues include C9 olefins such as 1- and
2-propenylbenzene, and C10 olefins such as 1-methyl-1-
propenylbenzene and 2-methyl-1-methylpropenylbenzene,
the former product being obtained in a lower proportion.
These compounds are grouped together because they can
Percentage Atomic Composition of the Catalysts as Determined
by EDAX and XPS
Catalyst
Mg
P
O
Na
Cl
MgPC
MgPPC
MgP(Cl,H,w)
NaMgP
NaMgP–P
MgOC
22.3
16.4
22.6
14.1
15.0
33.2
20.6
13.6
14.2
11.2
7.4
9.3
—
64.1
69.3
65.1
58.1
63.5
66.8
63.5
—
—
1.0
8.6
9.3
—
—
—
0.1
2.3 occur in various isomeric forms in addition to those de-
—
—
—
scribed above. Similarly, C10 ketones are also dealt with as
a whole since they occur in various structures of molecular
weight 146 and 148.
NaMgP–O
6.6
9.3

´
122
ARAMENDIA ET AL.
SCHEME 1. Selected products formed in the reaction between acetophenone and methanol.
Table 3 gives the results obtained in the reaction be-
As can be seen, all the catalysts except MgPPC (with
tween acetophenone and methanol on the different cata- XT < 2% ) exhibit acceptable selectivity in the reaction be-
lysts tested, at 773 K, under the conditions stated under tween acetophenone and methanol. This suggests that the
Experimental. XT denotes the percentage overall conver- reaction requires the presence of basic sites to develop—
sion to acetophenone derivatives and Si the selectivity in fact, solid MgPPC possesses a large population of acid
towards compound i (100 conversion to i/XT), which sites and a very small one of basic sites. Its typically acidic
includes styrene (styr.), C9 and C10 olefins (alkenes), 1- nature was demonstrated elsewhere (19). Also promi-
phenylethanol (1-ph.e.), phenyl vinyl ketone (ph.v.ket.), nent among the catalysts is solid NaMgP, which contains
propiophenone (propioph.), and C10 ketones (C10 ket.).
few acid and basic sites. Although its acid sites are in
greater numbers, the solid has been found to be pre-
dominantly basic, as it only yields the dehydrogenation
product in the transformation of cyclohexanol (16). Con-
sistent with this behaviour, the solid exhibits a high selec-
tivity for 1-phenylethanol, which results from a Meerwein–
Ponndorf–Verley (MPV) reaction between acetophe-
none and methanol. The absence of dehydrating activity
prevents the subsequent formation of alkenes. Also, the
presence of basic sites facilitates the formation of the con-
densation products (mainly phenyl vinyl ketone and pro-
piophenone). Figure 1 shows the variation of the total con-
version and the selectivity towards each product during the
reaction. As can be seen, the total conversion decreases
slightly as the reaction develops; in contrast, the selectiv-
ity for each product remains virtually constant through-
out.
TABLE 3
Total Conversion and Selectivity in the Reaction between
Acetophenone and Methanol on the Catalysts Studied
Catalyst
MgPC
MgP(Cl,H,w) 61.5 18.1
NaMgP 27.7
NaMgP–P 32.4 13.3
MgOC
NaMgP–O 61.7
ZnOC 10.7
XT Sstyr. Salkenes S1-ph.e. Sph.v.ket. Spropioph. SC10 ket.
24.3 32.1
7.9
25.6
—
—
—
28.2
12.6
24.0
25.3
7.8
21.0
21.9
22.9
30.1
22.7
37.6
22.0
6.8
19.8
5.6
—
34.3
9.3
5.5
—
12.6
14.6
4.5
7.4
78.2
8.8
5.5
4.9
17.1
25.1
5.0
20.4
63.1
4.9
—
Note. Reaction conditions: N2 flowrate 30 ml min 1, amount of catalyst
1
0.175 g, acetophenone/methanol molar ratio 1 : 5, feed rate 3.88 ml h
reac 773 K. Data obtained at treac = 80 min.
,
T

REACTION OF ACETOPHENONE AND METHANOL
123
and selectivitytowardsdifferent productsachieved with this
catalyst.
Using the same reactor feed rate but an acetophe-
none/methanol molar ratio of 1 under the operating con-
ditions of Table 3, we obtained the following results: XT =
26.5, Sstyr. = 28.5, Salkenes = 11.7, Sph.v.ket. = 21.9, Spropioph.
=
25.0, and SC10 ket. = 7.6. As expected, the total conversion to
acetophenone was lower than in the previous experiments.
Also worth noting is the decreased selectivity for C9–C10
alkenes and C10 ketones, as well as the increased selectiv-
ity towards styrene and phenyl vinyl ketone (the first two
products obtained according to Scheme 1).
Unlike MgPPC, catalyst NaMgP–P, which contains
Mg2P2O7 and NaMgPO4 phases, is also active in this re-
action. Obviously, the presence of basic sites, provided by
the NaMgPO4 phase, is indispensable for the solid to have
catalytic activity. This solid is especially selective towards
propiophenone and phenyl vinyl ketone. Its total conver-
FIG. 1. Total conversion (᭹) and selectivity towards styrene (5), C9–
C10 alkenes (4), 1-phenylethanol (ᮀ), phenyl vinyl ketone (᭢), propio- sion, however, is similar to those of MgPC and NaMgP.
phenone (᭡), and C10 ketones (᭜) in the reaction between acetophenone
and methanol on catalyst NaMgP at 773 K.
Catalyst MgOC exhibits outstanding activity in this reac-
tion. Figure 3 shows the total conversion and the selectivity
towards the different reaction products obtained with it.
Catalyst MgPC exhibits a differential behaviour. Al- As can be seen, the solid preserves its activity throughout
though it possesses acid and basic sites, it is predominantly the reaction. Worth a special note is its low selectivity for
acidic. Accordingly, it is the solid exhibiting the highest phenyl vinyl ketone, a result of its ability to promote fur-
selectivity for styrene, which is accompanied by phenyl ther reactions. Although this is an essentially basic solid,
vinyl ketone and propiophenone, in addition to smaller the presence of acid sites endows it with dehydrating activ-
amounts of other alkenes and C10 ketones. The magnesium ity that leads to the production of alkenes (mainly C9 and
orthophosphate MgP(Cl,H,w) possesses higher specific sur-
C10 olefins). In addition, the catalyst gives a number of con-
face area, acidity, and basicity than its commercially avail- densation products of high molecular weights that are all
able counterpart. It exhibits a high conversion and yields a obtained in low proportions but, as a whole, decrease the
wide range of products. Figure 2 shows the total conversion selectivity towards the previous ones.
FIG. 2. Total conversion (᭹) and selectivity towards styrene (5), C9–
C10 alkenes (4), phenyl vinyl ketone (᭢), propiophenone (᭡), and C10
ketones (᭜) in the reaction between acetophenone and methanol on cata-
lyst MgP(Cl,H,w) at 773 K.
FIG. 3. Total conversion (᭹) and selectivity towards styrene (5), C9–
C10 alkenes (4), 1-phenylethanol (ᮀ), phenyl vinyl ketone (᭢), propio-
phenone (᭡), and C10 ketones (᭜) in the reaction between acetophenone
and methanol on catalyst MgOC at 773 K.

´
124
ARAMENDIA ET AL.
SCHEME 2. Mechanism for the reaction between acetophenone and methanol.
Solid NaMgP–O is also highly active (see Table 3). In ad- face basic site reacts with adsorbed methanol to yield the
dition, it exhibits a low selectivity towards alkenes and an product. Consequently, the activated state of methanol on
acceptable one for C9 and C10 ketones. It produces virtu- the oxide must play a central role in the process. By anal-
ally no other product, which suggests that its basic sites are ogy with the aldol condensation and the reported mecha-
weaker than those of MgOC.
nism for the alkylation of toluene on basic solids to form
Table 3 also shows the results provided by catalyst ZnOC. styrene and ethylbenzene (25–27), the reaction probably
It exhibits a high selectivity for phenyl vinyl ketone but takes place via a formaldehyde intermediate resulting from
also a low total conversion; accordingly, its main reaction the dehydrogenation of methanol, followed by condensa-
products must be those obtained in the early reaction steps tion with the carbanion. The mechanism on our catalysts
(see Scheme 1). The reaction also produces alkenes as the must be similar to that put forward by Ueda and co-workers
likely result of the presence of Lewis acid sites. Although (23, 24) for the reaction between acetonitrile and methanol.
this catalyst is considerably less active than MgOC in this Scheme 2 shows the mechanism for that between acetophe-
reaction, it clearly surpasses it in the dehydrogenation of al- none and methanol. Steps 1 and 2 are in equilibrium. Both
cohols (22); consequently, the reaction between acetophe- involve the dissociation of a proton that is adsorbed at
none and methanol requires the presence of other types of a basic site on the catalyst surface. The methoxide anion
active sites.
or the carbanion formed from acetophenone must be ad-
Some of the above-mentioned products can only result sorbed on a surface Lewis acid site. Step 3 must be the
from dehydration, dehydrogenation, or cross-coupling re- rate-determining one. This, however, depends strongly on
actions. Both acid and basic sites are likely to take part in the particular catalyst, substrate, and operating conditions.
the process. Basic sites must play a prominent role since Thus, if the reactant pKa is very high (e.g., that for toluene is
the reaction rate is highly correlated to the acidity of the 37), then the second step is the rate-determining one. Step 4
reactants. Thus, Ueda et al. (8) determined the reaction involves the aldol condensation between the acetophenone
rate for reactants of variable pKa for their proton. The carbanion and adsorbed acetaldehyde.
pKa is associated to the dissociation of the H atom that is
Similar steps are also possible. Thus, any ketone possess-
abstracted in the cross-coupling reaction. As a result, the ing strong enough acid protons could be dissociatively
more readily the H atom can dissociate as H+, the higher adsorbed on the catalyst and give successive condensa-
the reaction rate is. The pKa for the proton in acetophe- tion reactions that would yield, for example, C10 ketones
none is 19. At a later stage, Ueda et al. (23, 24) studied as shown in Scheme 1.
in greater depth the conversion and selectivity of MgO
Based on the previous results, the dehydrogenation of
doped with transition metal ions, the influence of their sur- methanol on the catalyst surface under the operating con-
face basic properties on the reaction, and, especially, the ditions used in this work must proceed via an MPV reaction
mechanism for the process, which they investigated by us- between acetophenone and methanol. As a rule, the MPV
ing deuterium-labelled methanol-d1(CH3OD), methanol- reaction is concerted and takes place at a lower tempera-
d4(CD3OD), and acetonitrile-d3(CD3CN).
ture than the dehydrogenation of the alcohol, which acts as
In the base-catalysed reaction ofmethanol, the C–H bond a donor; this suggests that adsorbed ketone in the vicinity
in the methyl or methylene group of the other reactant of the alcohol favours its dehydrogenation and lowers the
must be activated by an electron-withdrawing group such temperature at which the hydrogen is lost from it. Based
as carbonyl, cyano, or phenyl. The intermediate carban- on the mechanism of Kibby and Hall (28), the alcohol and
ion formed after the uptake of the acid hydrogen by a sur- the ketone are both adsorbed at acid sites but require the

REACTION OF ACETOPHENONE AND METHANOL
125
sistent with previous results, as the reaction between ace-
tone and methanol takes place at a higher temperature, it
causes the dehydration of the diacetonalcohol and its even-
tual decomposition to isobutene. This reaction is effectively
hindered by a low partial pressure of acetone, an increased
basic strength, and a decreased acid strength. The methyla-
tion of acetone is also believed to be dependent on Lewis
acidity and basicity. The increased basicity and decreased
acidity of X zeolites exchanged with alkali ions result in the
second and third reaction developing to a dramatically de-
creased extent and in a consequent increase in selectivity
towards methyl vinyl ketone. However, the methylation of
methyl vinyl ketone under the operating conditions used
in this work prevents its dehydrogenation and leads to the
formation of C5 ketones (mainly 3-pentanone).
SCHEME 3. Mechanism for the Meerwein–Ponndorf–Verleyreaction
between acetophenone and methanol.
presence of a basic site in their vicinity. The transfer must
take place as depicted in Scheme 3.
The adjacent basic site favours ionization of the adsorbed
alcohol and facilitates the transfer of a hydride ion to the
ketone. In binding to a basic site, the proton weakens the
Reaction between acetophenone and dimethyl carbonate.
Dimethyl carbonate has proven to be an efficient methylat-
ing agent in, for instance, the selective N-monomethylation
of aniline (35, 36). Thus, the reaction between aniline and
dimethyl carbonate yields N-methylaniline in addition to
methanol and carbon dioxide. Accordingly, in the reaction
with acetophenone, dimethyl carbonate could be directly
involved, leading to methylation of carbanions. In addition,
we should expect reactions similar to those described above
resulting from the presence of methanol produced in the re-
action.
Table 4 shows the results obtained in the reaction be-
tween acetophenone and dimethyl carbonate on some of
the catalysts studied. Although the total conversion and
the selectivity for each product depend on the particular
catalyst, the use of dimethyl carbonate instead of methanol
as alkylating reagent can be assumed to decrease the se-
lectivity towards all alkenes. For catalyst MgPC, the total
conversion is slightly lower with dimethyl carbonate; also,
the selectivity towards propiophenone is very high (71.4% ).
Catalyst MgP(Cl,H,w) exhibits very similar selectivity with
methanol and dimethyl carbonate. The latter results in in-
creased selectivity towards C9 and C10 ketones.
==
O–H bond and facilitates the formation of the C O bond
and the transfer of the hydride ion. In the last step of the
process, the proton adsorbed at the basic site attacks the
alkoxide formed from the ketone.
The MPV reaction takes place preferentially on basic
catalysts and the activity of MgO–SiO2 in it has been un-
equivocally correlated to its basicity (29–31). The produc-
tion of 1-phenylethanol in our case can be ascribed to this
type of reaction. Similarly, the production of styrene and
other alkenes of lower molecular weight can be attributed
to the dehydration subsequently undergone by the alco-
hols at the surface acid sites. Thus, a catalyst such as NaMgP,
which has no dehydrating ability, is highly selective towards
1-phenylethanol. On the other hand, more acidic catalysts
such as MgPC and MgP(Cl,H,w) exhibit a high selectivity for
alkenes.
The overall reaction scheme also includes hydrogenation
reactions that yield products such as propiophenone.
Recently, Huang et al. (32) carried out the vapour-phase
reaction (623 K) between methanol and acetone on A, X,
and Y zeolites exchanged with alkali metals. They identified
three types of reaction, namely: (a) the methylation of ace-
tone followed by dehydrogenation to methyl vinyl ketone;
(b) the MPV reaction between acetone and methanol, cou-
pled to additional dehydration to propene and formalde-
hyde; and (c) the condensation of acetone followed by de-
composition to isobutene. The latter two types of reaction
predominate on LiX and NaX zeolites, which exhibit a rel-
atively strong Lewis acidity. When the Lewis basicity is in-
creased and the Lewis acidity decreased by exchange with
K, Rb, and Cs, both reactions develop to a dramatically de-
creased extent and the selectivity to methyl vinyl ketone
rises markedly as a result. The aldol condensation of ace-
tone at 453 K on zeolites exchanged with alkali metals was
previously studied (33, 34) and found to be related to Lewis
acidity (cations) and basicity (lattice oxygens), the NaX ze-
olite being the most active in the process. Although, con-
Also worth special note among the data of Table 4 is
the low conversion obtained with catalyst MgOC relative to
the use of methanol as alkylating agent (Table 3). Figure 4
TABLE 4
Total Conversion and Selectivity in the Reaction between Aceto-
phenone and Dimethyl Carbonate (DMC) on Selected Catalysts
Catalyst
XT
Sstyr.
Salkenes
Sph.v.ket
Spropioph.
SC10 ket.
MgPC
MgP(Cl,H,w)
MgOC
17.0
61.0
29.6
3.0
8.8
5.0
1.1
17.6
11.2
8.8
14.0
36.3
71.4
28.0
33.7
7.2
26.1
8.6
Note. Reaction conditions: N2 flowrate 30 ml min 1, amount of catalyst
0.175 g, acetophenone/DMC molar ratio 1 : 2.5, feed rate 3.88 ml h 1, Treac
773 K. Data obtained at treac = 80 min.

´
ARAMENDIA ET AL.
126
TABLE 6
Total Conversion and Selectivity towards Different Products
in the Reaction between Acetophenone and Dimethyl Carbonate
(1 : 2.5) on Catalyst MgP(Cl,H,w) at Two Different Temperatures
T(K)
XT
Sstyr.
Salkenes
Sph.v.ket.
Spropioph.
SC10 ket.
623
698
15.4
53.3
7.8
12.5
12.6
10.3
11.5
10.3
57.1
38.1
10.8
21.9
Note. Reaction conditions: N2 flowrate 30 ml min 1, amount of catalyst
0.175 g, feed rate 3.88 ml h 1. Data obtained at treac = 80 min.
In addition to the products shown in Table 5, the reaction
yields 1-phenylethanol and 1-phenyl-2-propen-1-ol, with
selectivities of 10.4 and 8.3% , respectively. Unlike at 773 K,
the use of dimethyl carbonate instead of methanol boosts
the selectivity for the alkenes. In any case, the selectivity to-
wards phenyl vinyl ketone and propiophenone is very high.
The effect of temperature on the reaction between ace-
tophenone and dimethyl carbonate was studied by using
catalyst MgP(Cl,H,w). Table 6 gives the conversion and se-
lectivity results obtained at 623 and 698 K (see Table 4 for
the data at 773 K). Increasing temperatures increase the
total conversion; however, the change is especially marked
between 623 and 698 K. As regards selectivity, that for pro-
piophenone decreases, whereas those for C10 ketones and
C9–C10 alkenes increase as the temperature is raised.
FIG. 4. Total conversion (᭹) and selectivity towards styrene (5), C9–
C10 alkenes (4), phenyl vinyl ketone (᭢), propiophenone (᭡), and C10
ketones (᭜) in the reaction between acetophenone and dimethyl carbon-
ate on catalyst MgOC at 773 K.
shows the total conversion and selectivity for the different
products obtained with this catalyst and dimethyl carbonate
instead of methanol. As can be seen, the catalyst exhibits
a high initial conversion but gradually loses activity. This
reveals that the presence of basic sites on the surface of
these catalysts is essential for the reaction to develop since,
unlike methanol, dimethyl carbonate releases CO2, which
is known to be adsorbed at surface basic sites. In addition,
there is decreased selectivity for C9 and C10 alkenes, and C10
ketones, the formation of which calls for the relative strong
basic sites required by typical condensation reactions. Rel-
ative to methanol, dimethyl carbonate also decreases the
selectivity towards all the alkenes.
Table 5 shows the results obtained in the reaction be-
tween acetophenone and methanol or dimethyl carbon-
ate on catalyst MgOC at 623 K. The conversion is time-
independent in both cases and higher with methanol than
with dimethyl carbonate. The selectivity towards propio-
phenone and, especially, phenyl vinyl ketone is very high.
CONCLUSIONS
The vapour-phase reaction of acetophenone and metha-
nol on various magnesium phosphates [MgPC, MgP(Cl,H,w)
,
NaMgP, and NaMgP–P], magnesium oxide [MgOC], zinc
oxide [ZnOC], and a magnesium oxide–sodium magnesium
orthophosphate mixed system [NaMgP–O] was studied
and found to yield a wide range of products that are
formed via a series of MPV reactions, aldol condensations,
dehydrations, and hydrogenations. The result is a series
of products including 1-phenylethanol, styrene, phenyl
vinyl ketone, propiophenone, C9–C10 alkenes, and C10
ketones primarily. This reaction requires the presence of
acid and basic sites in the catalyst. As a result, magnesium
pyrophosphate [MgPPC] is inactive in the process. Also, the
TABLE 5
catalysts that exhibit the highest conversions [MgP(Cl,H,w)
,
Total Conversion and Selectivity towards Different Products in
the Reaction between Acetophenone (ACP) and Methanol (MeOH)
or Dimethyl Carbonate (DMC) on Catalyst MgOC at 623 K
MgOC, and NaMgP–O] are those possessing the largest
acid and basic site populations. MgP(Cl,H,w), where acid sites
predominate, exhibits the highest selectivity for alkenes.
NaMgP, which consists chiefly of sodium magnesium or-
thophosphate, gives no alkenes and yields 1-phenylethanol
as the main product. ZnOC is the least active of the
catalysts studied. The use of dimethyl carbonate instead of
methanol as alkylating agent introduces significant changes
in selectivity. Thus, it markedly increases the selectivity of
solid MgPC for propiophenone. In contrast, it decreases
Molar
Reactants
ratio
XT Sstyr. Salkenes Sph.v.ket. Spropioph. SC10 ket.
ACP : MeOH 1 : 5
ACP : DMC 1 : 2.5
15.3 3.0
9.7 8.3
—
14.0
51.8
30.1
23.1
42.9
3.0
4.7
Note. Reaction conditions: N2 flowrate 30 ml min 1, amount of catalyst
0.175 g, feed rate 3.88 ml h 1. Data obtained at treac = 80 min.

REACTION OF ACETOPHENONE AND METHANOL
127
the selectivity towards all alkenes, to an extent dependent 14. Sugiyama, S., Satomi, K., Kondo, N., Shigemoto, N., Hayashi, H., and
Moffat, J. B., J. Mol. Catal. 93, 53 (1994).
15. Sugiyama, S., Kondo, N., Satomi, K., Hayashi, H., and Moffat, J. B.,
J. Mol. Catal. A 95, 35 (1995).
16. Aramend´ıa, M. A., Borau, V., Jime´nez, C., Marinas, J. M., Romero,
on the particular reaction conditions. However, the use of
dimethyl carbonate with catalyst MgOC leads to its rapid
deactivation by CO2 released during the reaction.
F. J., Nav´ıo, J. A., and Barrios, J., J. Catal. 157, 97 (1995).
17. Aramendı´a, M. A., Borau, V., Jime´nez, C., Marinas, J. M., and Rodero,
F., Colloids Surf. 12, 227 (1984).
ACKNOWLEDGMENTS
The authors express their gratitude to Spain’s DGES of Ministerio de
Educacio´n y Cultura for funding this research in the framework of Project
PB97-0446 and to the Consejer´ıa de Educacio´n y Ciencia of the Junta de
Andaluc´ıa for additional support.
18. Koros, R. M., and Novak, E. J., Chem. Eng. Sci. 22, 470 (1967).
19. Aramend´ıa, M. A., Borau, V., Jime´nez, C., Marinas, J. M., Romero,
F. J., and Ruiz, J. R., J. Colloid Interface Sci. 202, 456 (1998).
20. Aramendı´a, M. A., Borau, V., Jime´nez, C., Marinas, J. M., and Romero,
F. J., J. Colloid Interface Sci. 179, 290 (1996).
REFERENCES
21. Morrow, B. A., Stud. Surf. Sci. Catal. 57A, A202 (1990).
22. Aramendı´a, M. A., Borau, V., Jime´nez, C., Marinas, J. M., and Romero,
F. J., J. Catal. 151, 44 (1995).
1. Catal. Today 23, special issue (1995).
2. Chang, C. D., and Silvestri, A. J., ChemTech 7, 624 (1987).
3. Chase, J. D., in “Catalytic Conversions of Synthesis Gas and Alcohols
to Chemicals” (R. G. Herman, Ed.), p. 307. Plenum, New York, 1984.
4. Ueda, W., Yokoyama, T., Moro-oka, Y., and Iwaka, T., Ind. Eng. Chem.
Prod. Res. Dev. 24, 340 (1985).
23. Kurokawa, H., Kato, T., Ueda, W., Morikawa, Y., Moro-Oka, Y., and
Ikawa, T., J. Catal. 126, 199 (1990).
24. Kurokawa, H., Kato, T., Kuwabara, T., Ueda, W., Morikawa, Y., Moro-
Oka, Y., and Ikawa, T., J. Catal. 126, 208 (1990).
25. Itoh, H., Miyamoto, A., and Murakami, Y., J. Catal. 64, 284 (1980).
5. Ueda, W., Yokoyama, T., Moro-oka, Y., and Iwaka, T., J. Chem. Soc. 26. Itoh, H., Hattori, T., Suzuki, K., Miyamoto, A., and Murakami, Y.,
Chem. Commun., 39 (1984). J. Catal. 72, 170 (1981).
6. Ueda, W., Kurokawa, H., Moro-oka, Y., and Iwaka, T., Chem. Lett., 27. Itoh, H., Hattori, T., Suzuki, K., and Murakami, Y., J. Catal. 79, 21
819 (1985). (1983).
7. Ueda, W., Yokoyama, T., Kurokawa, H., Moro-oka, Y., and Iwaka, T., 28. Kibby, C. L., and Hall, W. K., J. Catal. 31, 65 (1973).
J. Petrol. Inst. 29, 72 (1986). 29. Niyama, H., and Echigoya, E., Bull. Chem. Soc. Jpn. 45, 938 (1972).
8. Kurokawa, H., Ueda, W. Morikawa, Y., and Moro-oka, Y., in “Acid– 30. Tanabe, K., Misono, M., Ono, Y., and Hattori, H., Stud. Surf. Sci. Catal.
Base Catalysis” (K. Tanabe, T. Yamaguchi, and T. Tanaka, Eds.), p. 93.
Kodansha–VCH, Tokyo/New York/Weinhelm, 1989.
9. Novakova, J., Bosacek, V., Doleisek, Z., and Kubelkova, L., J. Mol.
Catal. 78, 43 (1993).
10. Hathaway, P. E., and Davis, M. E., J. Catal. 119, 497 (1989).
11. Moser, R. E., and Cassidy, H. G., J. Polym. Sci. B 2, 545 (1964).
12. Ogawa, T., Cedeno, R., and Inoue, M., Polym. Bull. 2, 275 (1980).
51, 319 (1989).
31. Ivanov, V. A., Bachelier, J., Audry, F., and Lavalley, J. C., J. Mol. Catal.
91, 45 (1994).
32. Huang, M., Zielinski, P. A., Moulod, J., and Kaliaguine, S., Appl. Catal.
118, 33 (1994).
33. Huang, M., and Kaliaguine, S., Catal. Lett. 18, 373 (1993).
34. Huang, M., and Kaliaguine, S., Stud. Surf. Sci. Catal. 78, 559 (1993).
13. Sokolovskii, V. D., Osipova, Z. G., Davydov, A. A., and Budneva, 35. Fu, Z., and Ono, Y., Catal. Lett. 22, 277 (1993).
A. A., Appl. Catal. A 101, 15 (1993). 36. Trotta, F., Tundo, P., and Moraglio, G., J. Org. Chem. 52, 1300 (1987).