MgO nanoparticle-based multifunctional catalysts in the cascade reaction allows the green synthesis of anti-inflammatory agents

Article abstract of DOI:10.1016/j.jcat.2007.02.003

Up to now, the anti-inflammatory agent nabumetone has been synthesized by a two-step process involving either a Heck reaction between 6-bromo-2-methoxynaphthalene and methyl vinyl ketone or a condensation between 6-methoxy-2 naphthaldehyde and acetone to give an intermediate that is separated, purified, and hydrogenated in a second and separate process to give the final product, while producing a large amount of waste products. Here we report a residue-free catalytic process for the production of nabumetone with ≥ 98 % yield and 100% selectivity achieved through a cascade reaction system involving a multifunctional base/acid/hydrogenation catalyst based on nanocrystalline (～3 nm) MgO.

Full text of DOI:10.1016/j.jcat.2007.02.003

Journal of Catalysis 247 (2007) 223–230
www.elsevier.com/locate/jcat
MgO nanoparticle-based multifunctional catalysts in the cascade reaction
allows the green synthesis of anti-inflammatory agents
Maria Jose Climent, Avelino Corma ∗, Sara Iborra, Maria Mifsud
Instituto de Tecnología Quimica, UPV-CSIC, Universidad Politécnica de Valencia, Avda. de los Naranjos s/n, 46022 Valencia, Spain
Received 15 January 2007; revised 1 February 2007; accepted 2 February 2007
Available online 8 March 2007
Abstract
Up to now, the anti-inflammatory agent nabumetone has been synthesized by a two-step process involving either a Heck reaction between
6-bromo-2-methoxynaphthalene and methyl vinyl ketone or a condensation between 6-methoxy-2 naphthaldehyde and acetone to give an inter-
mediate that is separated, purified, and hydrogenated in a second and separate process to give the final product, while producing a large amount of
waste products. Here we report a residue-free catalytic process for the production of nabumetone with ꢀ98% yield and 100% selectivity achieved
through a cascade reaction system involving a multifunctional base/acid/hydrogenation catalyst based on nanocrystalline (∼3 nm) MgO.
© 2007 Elsevier Inc. All rights reserved.
Keywords: Aldol condensation; Anti-inflammatory; Cascade reaction; MgO; Multifunctional catalyst
1. Introduction
acts 6-bromo-2-methoxynaphthalene and methyl vinyl ketone
(Heck reaction) in dimethylformamide as a solvent, using a ho-
mogeneous palladium (II) catalyst at 405 K. The Pd catalyst
generally results from a Pd(II) salt and a phosphine ligand, such
as triphenylphosphine (PPh3). Hydrogenation of the unsatu-
rated product formed during the Heck reaction with a palladium
on carbon prewetted (Pd–C) catalyst in a second reaction step
yields 87.4% of nabumetone [3] (2) (see Scheme 1) and leaves
bromide salts as byproducts.
Nabumetone, 4-(6-methoxy-2-naphthyl)-2-butanone, like
α-aryl propionic acids (e.g., ibuprofen, naproxen, ketoprofen)
have nonsteroidal anti-inflammatory activity (NSAI). Besides
being an effective anti-inflammatory and analgesic in the treat-
ment of various rheumatic and arthritic diseases, nabumetone
has fewer side effects and lower toxicity to the gastrointestinal
tract than other anti-inflammatory agents [1].
Various industrial processes for the preparation of nabume-
tone have been proposed that involve the preparation of an
analogue compound with a double bond in the aliphatic chain
4-(6-methoxy-2-naphthyl)-3-buten-2-one (1), followed by hy-
drogenation of the double bond. It is interesting that although
the unsaturated intermediary presents the same analgesic and/or
anti-inflammatory activity as the final hydrogenated compound,
it produces a certain degree of estrogenicity; for this reason,
the final hydrogenated product is medically recommended. Two
main standard methods for carbon–carbon formation have been
used for the preparation of intermediate 1: the Heck reaction [2]
and aldol condensation. Thus, the Hoechst Celanese process re-
Reaction byproducts formed in this process are generated
from the subsequent Heck C–C coupling of 1 to give prod-
uct A and the corresponding hydrogenated (C), as well as
the formation of 4-(6-methoxy-2-naphthyl)-2-butanol (B) (see
Scheme 1).
Rhodia Chimie [4] synthesized 1 by reacting in an au-
toclave 1,6-dibromo-2-methoxynaphthalene with methyl vinyl
ketone (MVK) in toluene anhydrous in the presence of Et3N,
Pd(OAc)2, PPh3, and H2 at 423 K and 40 bar (yield of 1 ∼58%,
and only a 5% of 2 was detected). Alternatives to MVK for the
synthesis of nabumetone have been sought by industry due to
its limited stability (due to competing side reactions, such as
oligomerization) and toxicologic concerns associated with its
use. An alternative method of obtaining 2 involves 2-bromo-6-
methoxynaphthalene, 3-buten-2-ol, palladium acetate, PPh3, in
*
Corresponding author. Fax: +34 (96) 3877809.
E-mail address: acorma@itq.upv.es (A. Corma).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.02.003

224
M.J. Climent et al. / Journal of Catalysis 247 (2007) 223–230
Scheme 1. Reaction steps involved in the Hoechst Celanese process.
1-methyl-2-pyrrolidone as solvent at 413 K for 5 h to give a
yield of 60% [5].
tion A. Both solutions were co-added at a rate of 1 mL/min
under vigorous mechanical stirring at room temperature. The
gel was aged under autogenous pressure conditions at 333 K
for 12 h. Then the hydrotalcite was filtered and washed un-
til pH 7 was reached, at which point the solid was dried. The
Mg–Al mixed-oxide (HTc) base catalyst was obtained by cal-
cination of Mg–Al-LDH at 723 K for 6 h in a dry nitrogen
flow.
Synthetic approaches to 2 via aldol condensation also have
been reported in the literature [6]. Aldol condensation between
6-methoxy-2-naphthaldehyde and acetone was performed in
the presence of an aqueous alkali metal hydroxide solution
(NaOH and/or KOH) as catalyst, followed by acidification to
give 1 (yield 95%). Then hydrogenation of 1 was carried out
in a second step using Pd–C as catalysts, yielding the corre-
sponding saturated ketone. This two-step process yields >90%
of product 2 but generates high amounts of waste salts and
effluents. To avoid this handicap, a two-step process using
a solid catalyst (basic alumina) for the condensation step of
6-methoxy-2-naphthaldehyde and acetone has been described
in the patent literature [7]. However, this procedure achieves
only a 78% yield in the first reaction after 22 h, and the
intermediate 1 must be hydrogenated in a second, separate
process.
A MgO sample with a surface area of 120 m2/g was pur-
chased from Aldrich. A MgO sample with a surface area of
320 m2/g was prepared starting from magnesium oxalate fol-
lowing the method described by Putanov et al. [8]. A MgO
sample with a surface area of 670 m2/g was purchased from
NanoScale Materials.
Pd catalysts with different Pd loadings (1, 1.5, and 2 wt%
of palladium) were obtained by contacting the MgO or Mg–Al-
LDH samples with an anhydrous toluene solution with different
amounts of palladium acetylacetonate [Pd(acac)2] for 12 h. Af-
ter evaporation of toluene at reduced pressure, the solid was
dried overnight at 353 K in vacuum and then calcined in ni-
trogen flow at 823 K (ramp rate, 5 ◦/min) for 3.5 h. In the
case of Pd-HTr, the calcined sample (Pd-HTc) was hydrated at
room temperature by the direct addition of decarbonated water
(MilliQ) (36 wt%) just before their use as catalysts.
The present work demonstrates that a multifunctional acid–
base–metal catalyst based on nanocrystallite (∼3 nm) MgO is
able to catalyze the synthesis of nabumetone with yields ꢀ98
and 100% selectivity through a one-pot cascade reaction sys-
tem. The catalyst can be regenerated and recycled several times,
resulting in high turnover numbers (TONs).
Samples of Pd-MgO and Pd-HTc were activated before re-
action by heating the solid at 723 K under air atmosphere for
5 h and then for 5 h under nitrogen. Metal reduction was per-
formed by heating the solid at 723 K in a flow of H2/N2 (90/10)
for 2 h.
2. Experimental
2.1. Catalysts
The magnesium–aluminum layered double hydroxide sam-
ple (Mg–Al-LDH) was prepared from a gel produced by
mixing two solutions. Solution A contained (3 − x) mol of
Mg(NO3)2·6H2O and x mol of Al(NO3)3·9H2O in the (Al +
Mg) concentration of 1.5 mol/L, for a Al/Al + Mg molar ra-
tio of 0.25. Solution B was formed by (6 + x) mol of NaOH
and 2 mol of Na2CO3 dissolved in the same volume as solu-
The catalysts thus prepared were characterized by XRD,
TEM, and nitrogen adsorption. N2 adsorption/desorption iso-
therms were performed at 77 K in a Micromeritics ASAP 2010
apparatus after the samples were pretreated under vacuum at
673 K overnight, and the BET surface areas were obtained
using the BET methodology. The main characteristics of the
catalysts are given in Table 1.

M.J. Climent et al. / Journal of Catalysis 247 (2007) 223–230
225
Table 1
by GC–MS (Varian Saturn II, using the same column as for
GC) and H NMR spectroscopy (Varian Geminis, 300 MHz,
with CDCl3 as a solvent).
1
Main characteristic of the catalyst
a
Catalyst
Surface area Total pore
Average pore Crystal size
2
−1
)
(m g
volume (cc/g)
diameter (Å)
(nm)
Pd-HTc-(1) 264
0.553
–
0.871
0.563
0.644
84
<50
–
13
5
2.3. Catalyst regeneration
Pd-HTr-(1)
MgO-120
MgO-320
MgO-670
18
120
320
670
–
292.41
70.80
39.55
Pd-HTc(1) and Pd-MgO(1)-670 samples were regenerated
after each use by calcination of the solids at 723 K under air
atmosphere for 5 h and then under nitrogen for 5 h. Reduction
of the Pd was performed by heating the solid at 723 K in a flow
of H2/N2 (90/10) for 2 h.
3
a
Crystallite size measured by X-ray diffraction spectrum.
2.2. Reaction procedure
3. Results and discussion
For a typical experiment, a solution of 6-methoxy-2-naphth-
aldehyde (500 mg, 2.6 mmol) dissolved in an excess of acetone
(26 mmol) was placed in a batch slurry reactor that was then
immersed in a thermostat silicone oil bath at 333 K and mag-
netically stirred. When the desired temperature was reached,
a 50-mg catalyst sample (10 wt% with respect to naphthalde-
hyde) was added, and hydrogen at a constant hydrogen pressure
of 5 bar was applied. When Pd-HTr was used, the Pd-HTc pre-
cursor catalyst was previously hydrated at room temperature
with decarbonated water (36 wt%). The course of the reaction
was followed by gas chromatography (GC) using a Hewlett
Packard GC 5988 A with a flame ionization detector and a
HP-5 capillary column of 5% phenylmethylsilicone. In all of
the experiments, dodecane was used as an internal standard. At
the end of the reaction, after cooling, the reaction mixture was
filtered to remove the catalyst. The solid was extracted thor-
oughly with dichloromethane, and the extract was considered
in the mass balance. The reaction mixture was concentrated un-
der reduced pressure, and the reaction products were analyzed
When we first approached the synthesis of 1 through the
aldol condensation between 6-methoxy-2-naphthaldehyde and
acetone, the solid base catalyst of choice was a calcined Mg/Al
hydrotalcite. Calcined hydrotalcites are base compounds that
have been successfully used in catalyzing condensation reac-
tions for the production of chemicals and pharmaceuticals [9].
Furthermore, a series of excellent reports has shown that
metal-supported hydrotalcites can be used to produce methyl
isobutyl ketone [10], 2-methyl-3-phenyl propanal [11], and
α-alkylated nitriles [12] in a single-stage condensation hydro-
genation process. Therefore, and because three reaction steps
are required for the synthesis of nabumetone, involving basic,
acid, and hydrogenation sites (see Scheme 2), we used Pd on a
calcined hydrotalcite (Pd-HTc-(1), with 1% wt/wt of Pd load-
ing) as a catalyst, in which Lewis basic sites are associated with
framework oxygens, Lewis acid sites are related to Al3+ and
Mg2+ cations, and finally metal Pd as the hydrogenating func-
Scheme 2. Reaction steps involved in the formation of nabumetone.

226
M.J. Climent et al. / Journal of Catalysis 247 (2007) 223–230
Scheme 3. Reaction steps involved in the formation of compound 2a.
Table 2
Table 3
Synthesis of nabumetone using Pd-supported hydrotalcites at different reaction
temperatures
Results of reuses of the Pd-HTc(1) catalyst
Catalyst
Cycles
1
Conversion (%)
Yield of 2 (%)
a
b
Catalyst
T
r
0
r
2
Conversion Yield of
0
Pd-HTc(1)
Pd-HTc(1)
Pd-HTc(1)
81
66
62
81
66
62
−1 −1
−1 −1
g ) (%)
(K) (mol min
g
) (mol min
2 (%)
a
2
a
4
4
10
10
3
c
c
d
d
c
Pd-HTc(1) 333 4.5
Pd-HTr(1) 333 5.2
Pd-HTc(1) 348 5.8
Pd-HTc(1) 373 7.8
2.4
2.8
3.2
4.8
15
32
85
98
15
Note. Reaction conditions: molar ratio acetone/aldehyde = 10, 10 wt% of cat-
alyst with respect to the aldehyde, at 1 h reaction time, at 373 K, working at
constant pressure of hydrogen (5 bar).
c,e
29
d
85
d
a
98
Reuse after calcination and reduction of the catalyst.
Note. Reaction conditions: molar ratio acetone/aldehyde = 10, 10 wt%, of cat-
alyst with respect to the aldehyde and working at constant pressure of hydrogen
(5 bar). In brackets the amount of palladium content (wt%) of the catalyst.
Table 4
Results of the use and reuses of Pd-HTr(1) catalyst
a
Initial reaction rates for the condensation step.
Initital reaction rates of formation of nabumetone.
At 1 h reaction.
At 2 h reaction time.
b
Catalyst
Cycles
1
Conversion
Yield of 2 (%)
Yield of 2a (%)
c
Pd-HTr(1)
Pd-HTr(1)
Pd-HTr(1)
100
18
44
89
17
35
11
1
9
d
a
2
b
e
3% of compound 2a was detected.
2
Note. Reaction conditions: relation molar acetone/aldehyde = 10, 40 wt% of
catalyst with respect to the aldehyde, 75 min, at 348 K, working at constant
pressure of hydrogen (5 bar).
tion. The results, given in Table 2, show that this catalyst was
not very active at 333 K, contrary to our expectations.
a
Reuse after washed and reduction.
Reuse after calcinations, reduction and rehydration of the catalyst. In brack-
To increase the catalyst activity, the calcined hydrotalcite
containing palladium metal was rehydrated [Pd-HTr-(1)] (see
Section 2), because rehydration restores the layered structure in
which the CO23− compensating anions in the interlayer space
are replaced by OH−, resulting in strong Brønsted base cat-
alysts that effectively catalyze a greater number of reactions
[13–19].
The results in Table 2 indicate that the catalytic activity
increased on rehydration, but a secondary product (2a) was
formed by condensation of product 1 with a second acetone
molecule, followed by hydrogenation (Scheme 3). The reaction
temperature was increased to improve the yield to nabume-
tone; as shown in Table 2, a 98% yield of nabumetone could
be achieved in a one-pot reaction after 2 h of reaction time
using Pd-HTc(1) and working at 373 K. This yield is much
higher than the values reported in the patent literature using
a basic alumina for the condensation step, followed by hydro-
genation of the product thus formed [7]. Unfortunately, the cat-
alyst became deactivated, and regeneration was not achieved.
Indeed, when the reaction was completed and the catalyst was
b
ets the amount of palladium content (wt%) of the catalyst.
filtered, washed with acetone, and reused, no activity was ob-
served. In principle, this can be attributed to the presence of
organic products remaining adsorbed on the catalyst. To elimi-
nate these products, regeneration was attempted by calcination
at 623 K for 5 h in the presence of air, followed by 5 h in a
stream of N2. After this regeneration procedure, the sample was
reused in a second cycle and then in a third cycle. The results,
given in Table 3, indicate an irreversible deactivation of the cat-
alyst.
With the rehydrated hydrotalcite, improved yields of nabu-
metone also were obtained in shorter reaction times by increas-
ing the temperature and the amount of catalyst (Table 4), but
selectivity was lower than in the case of calcined hydrotalcite,
due to formation of larger amounts of compound 2a. Unfor-
tunately, also in this case, catalyst deactivation occurred very
rapidly, and regeneration of the solid by either washing or cal-
cination and subsequent rehydration was not successful. Owing

M.J. Climent et al. / Journal of Catalysis 247 (2007) 223–230
227
had to be increased. To do this, we considered the MgO struc-
tural model of Coluccia et al. [21] who proposed a highly de-
fective surface structure for MgO, with step edges, corners, and
kinks, which provide O2− sites of low coordination numbers
2−
(see Fig. 1). These low-coordinated O2− sites (O on faces,
5c
2−
2−
O
on edges, and O on corners) are expected to be respon-
4c
3c
sible for the presence of basic sites with different strengths. The
basic strength of surface O2−2+sites increases as the coordina-
2−
tion number drops, with Mg3c
our purposes.
O
sites the most reactive for
3c
Taking the foregoing findings into account, we believed that
the number of sites at the corners of the crystals should increase
with decreasing crystallite size of MgO, and consequently the
catalyst activity should increase accordingly. To evaluate this,
we studied two new MgO catalysts with 320 and 670 m2 g−1
,
Fig. 1. Structure of MgO taken from Ref. [21].
and MgO crystallite sizes of 5 and 3 nm, respectively, as de-
termined from the Debye–Scherrer equation (see Table 1 and
Fig. 2). Thus, plotting the activity (initial reaction rates) for the
first reaction step (i.e., the base-catalyzed condensation reac-
tion (Scheme 2)) versus the crystallite size of the three MgO
samples demonstrates that the initial reaction rate increases
not linearly, but rather exponentially with decreasing crystal-
lite size of the MgO (Fig. 3). The result indicates that this
reaction is sensitive to the structure, following the nomencla-
ture of Boudart [22]; in other words, the basic sites associated
with corners and edges (which are the most basic) are also the
most active for the condensation reaction. At this point, select-
ing the MgO sample with a nanocrystallite size of 3 nm (see
Fig. 4) and depositing 1 wt% of Pd on the surface should yield
a highly active and selective multifunctional catalyst. Indeed,
carrying out a one-pot reaction at 333 K with the Pd-MgO(1)-
670 sample produced a 98% yield of nabumetone with 100%
selectivity after 60 min of reaction time; this yield was 87% for
Pd-MgO(1)-120 after 150 min and 88% for Pd-MgO(1)-320 af-
ter 120 min (see Fig. 5).
to this, we have searched for another type of multifunctional
acid–base metal catalyst.
Alkaline earth oxides, and more specifically MgO, have
shown to be active for aldol condensation reactions [20]. In
this case, the basic sites able to abstract protons from a reac-
tant molecule are those associated with M2+O2− ion pairs, in
which the O2− acts as a Lewis basic site and the adjacent metal
cation acts as a weak Lewis acid that catalyzes the dehydra-
tion of the intermediate benzylic alcohol to the corresponding
enone, because this would be a facile reaction. Nevertheless, the
benzylic alcohol intermediate also may undergo deoxygenation
directly by C–O bond hydrogenolysis. We observed that MgO
with 120 m2 g−1 and 13 nm nanocrystalline size with 1 wt% Pd
[Pd-MgO(1)-120] (see Section 2) had twice the activity for syn-
thesis of nabumetone than rehydrated hydrotalcite [Pd-HTr(1)],
whereas the former was 100% selective.
Based on this result, MgO appeared to be a promising candi-
date for performing the aldol condensation between 6-methoxy-
2-naphthaldehyde and acetone, although the catalytic activity
Fig. 2. XDR spectra of the MgO samples.

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M.J. Climent et al. / Journal of Catalysis 247 (2007) 223–230
Fig. 3. Initial reaction rates for the condensation step using MgO of different crystal size. Reaction conditions: molar ratio acetone/aldehyde = 10, 10 wt%, of
catalyst with respect to the aldehyde, at 333 K.
(a)
(b)
Fig. 4. (a) Scanning electron microscopy image and (b) transmission electron microscopy image of MgO-670 sample kindly supplied by NanoScale Materials Inc.
These results clearly confirm that using nanocrystalline
MgO, it is possible to synthesize nabumetone with very high
yield and selectivity through a cascade or domino reaction in
which the condensed product formed on the basic sites of the
catalysts is rapidly dehydrated on an acid site and the result-
ing double bond is hydrogenated on the Pd. It should be noted
that under our reaction conditions, increasing the amount of the
hydrogenating function from 1 to 1.5 or 2 wt% Pd increased
the rate of nabumetone formation, demonstrating that under
these reaction conditions, hydrogenation is the controlling step.
However, 1 and 1.5 wt% Pd have similar final conversion and
selectivity.
Finally, the used catalyst was subjected to three cycles of
reaction–regeneration (calcination)–reaction (see Section 2 for
regeneration conditions). The catalyst maintained its activity
(Fig. 6).
4. Conclusion
This study has demonstrated that using a multifunctional cat-
alyst based on nanocrystalline MgO, it is possible to develop a
new green process for the production of nabumetone in a one-
pot reaction. This new process allows a yield ꢀ98% with 100%
selectivity.

M.J. Climent et al. / Journal of Catalysis 247 (2007) 223–230
229
Fig. 5. Results of one pot synthesis of nabumetone using: (") Pd-MgO(1)-120, (1) Pd-MgO(1)-320, (×) Pd-MgO(1)-670, as catalysts. Reaction conditions: molar
ratio acetone/aldehyde = 10, 10 wt%, of catalyst with respect to the aldehyde, at 333 K and at constant pressure of hydrogen (5 bar).
Fig. 6. Results of the regeneration of Pd-MgO(1)-670 catalyst: 1st ("), 2nd (1) and 3rd (E) cycle.
Acknowledgment
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Financial support by CICyT MAT2003-07769-C02-01 is ac-
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[5] M. Aslam, V. Elango, K.G. Davenport, Synthesis (1989) 869.
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