Characterization of basic catalysts by the use of nitromethane as NMR probe molecule and reactant

Article abstract of DOI:10.1016/j.jcat.2003.12.017

Characterization of the basicity of mixed oxides obtained from hydrotalcite-like precursors by ¹³C CP/MAS NMR spectroscopy using nitromethane-reactive adsorption is described. On Lewis bases, featuring acid-base pairs, the nature of adsorbed sp

Full text of DOI:10.1016/j.jcat.2003.12.017

Journal of Catalysis 223 (2004) 28–35
www.elsevier.com/locate/jcat
Characterization of basic catalysts by the use of nitromethane
as NMR probe molecule and reactant
Enrique Lima, Monique Laspéras, Louis-Charles de Ménorval, Didier Tichit,∗
and François Fajula
Laboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS ENSCM, 8, rue de l’École Normale,
34296 Montpellier cedex 5, France
Received 16 October 2003; revised 22 December 2003; accepted 23 December 2003
Abstract
13
Characterization of the basicity of mixed oxides obtained from hydrotalcite-like precursors by C CP/MAS NMR spectroscopy us-
ing nitromethane-reactive adsorption is described. On Lewis bases, featuring acid–base pairs, the nature of adsorbed species namely
nitromethane, aci-anion nitromethane, and methazonate salt analogue depends on the relative strength of the basic and acid sites on the
pairs. None of these species is obtained using Brønsted bases such as layered double hydroxide (LDH) of the meixnerite type. In that case
a formate analogue is proposed to be formed. Heterogeneous base catalysis of the Michael addition of nitromethane to cyclohex-2-en-1-one
13
is studied. A relationship is established between the activity of the catalyst and the C CP/MAS NMR chemical shift of the aci-anion
nitromethane which is a measure of the basic strength. Discussion of NMR and catalytic results takes into account the respective roles of
basicity and of conjugate Lewis acidity. Basicity is needed to form a reactive anion comparable to aci-anion nitromethane, but the higher the
stabilization of aci-anion nitromethane by Lewis acidity, the lower the activity. Increasing the basic strength by decreasing the Lewis acidity
13
of the acid–base pairs leads to an increase of activity concomitant with the formation of methazonate as shown by C MAS NMR. The
efficiency of meixnerite which is the most active and selective catalyst is related not only to its Brønsted basicity but also to its weak Lewis
acidity.
2004 Elsevier Inc. All rights reserved.
Keywords: Base catalysis; Nitromethane; NMR; Michael addition
1. Introduction
aci-anion nitromethane were only observed and the role of
acid–base pairs of the surface was not really established.
On the other hand, Iglesia et al. [6] claimed that bi-
functional pathways are mainly involved for heterogeneous
acid or base catalysis. In the domain of carbon–carbon
bond formation, Michael addition of nitromethane to α, β-
unsaturated carbonyl compounds is known to be catalyzed
by liquid bases such as NaOH, KOH, or EtONa [7]. Results
are also described in heterogeneous catalysis using KF/basic
alumina [8], KF/oxides [9], basic alumina [10,11], Am-
berlyst A-21 [12], and Amberlyst A-27 [13]. KF/alumina or
KOH/alumina exhibited high activities whatever the unsatu-
rated compound [14]. However, these catalysts lose activity
upon recycling [8]. More recently, Choudary et al. reported
good yields for Michael addition catalyzed by rehydrated
Mg–Al mixed oxides [15].
The surface of inorganic oxides, mixed oxides, or zeolites
features acid–base pairs whose strength can be measured
using probe molecules [1–3]. Among the various probes ni-
tromethane has been little studied. We showed that heating
nitromethane adsorbed on various basic surfaces led to ad-
sorbed isocyanate species. Their formation was followed by
FT-IR spectroscopy and their frequencies were correlated
to the heats of carbon dioxide adsorption which is a mea-
sure of the basic strength [4]. Reactive chemisorption of
nitromethane as aci-anion nitromethane was investigated by
Kheir and Haw [5] to probe MgO and CaO basicity, using
cross-polarization magic-angle spinning (CP/MAS) solid-
state 13C NMR spectroscopy. Physisorbed nitromethane and
*
The adsorption of a molecule either as a probe to analyze
the surface by spectroscopy or as a reactant in a catalytic
Corresponding author.
E-mail address: tichit@cit.enscm.fr (D. Tichit).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2003.12.017
2004 Elsevier Inc. All rights reserved.

E. Lima et al. / Journal of Catalysis 223 (2004) 28–35
29
Table 1
Chemical compositions of the LDH precursors
Sample Mg/
Chemical formula
III
M
−
2−
2−
2−
2−
Mg(Al)O 3.0 [Mg
Mg(Al)O 2.5 [Mg
Mg(Al)O 2.0 [Mg
KW 2200 2.3 [Mg
Mg(Ga)O 5.0 [Mg
Mg(Ga)O 3.0 [Mg
Mg(Ga)O 2.0 [Mg
Al
(OH) ](CO
2
(OH) ](CO
2
(OH) ](CO
2
(OH) ](CO
2
(OH) ](CO
2
(OH) ](CO
2
(OH) ](CO
2
)
)
)
)
(NO
(NO
(NO
)
)
)
· 0.41H O
0.75 0.25
3
3
3
3
0.102
0.123
0.164
0.045
0.058
0.032
2
3
−
Al
· 0.63H O
0.71 0.29
2
3
−
Al
· 0.49H O
0.66 0.34
2
3
)
−
Al
(NO
· 0.4H O
0.69 0.31
0.14
0.04
2
3
−
2−
2−
2−
Ga
)
)
)
(NO
(NO
(NO
)
)
)
· 0.52H O
0.79 0.21
3
3
3
0.098
0.108
0.151
0.053
0.055
0.035
2
3
−
Ga
· 0.59H O
0.73 0.27
2
3
−
Ga
0.68 0.32
· 0.47H O
2
3
reaction provides a valuable tool for a better understand-
ing of the surface and its reactivity. In a previous work [16]
we showed that in the Michael addition of nitromethane on
cyclohex-2-en-1-one, low reaction rates were observed for
catalysts which only physisorbed nitromethane (γ -Al2O3,
NaX, CsX). Intermediate reaction rates were observed for
catalysts such as Mg(Al)O mixed oxides which led to aci-
anion nitromethane. The most efficient catalysts were MgO
and CsX 9Cs zeolite which showed the formation of metha-
zonate salt analogue.
The aim of this work was to use nitromethane as probe
and reactant to correlate the acido–basicity and the catalytic
properties of Mg(Al)O and Mg(Ga)O mixed oxides obtained
by calcinations of LDH precursors, hydrated LDH (meixner-
ite), and MgO samples obtained from magnesium carbonate
or magnesium hydroxide precursors.
Scheme 1. Nitromethane reaction with sodium hydroxide in homogeneous
medium.
The rehydration of the Mg(Al)O mixed oxide (Mg/Al =
3.0) calcined at 623 K to yield the meixnerite-like type com-
pound was carried out in liquid water at 298 K for 3 days
under a N2 atmosphere.
CsX and the strongly basic overexchanged CsX 9Cs zeo-
lites were obtained from commercially available NaX sam-
ple (Aldrich) as described elsewhere [17].
MgOc was obtained by calcination of MgCO3 (Aldrich)
at 873 K under dry synthetic air. Activation in situ before
catalytic reaction of Mg(OH)2 (Stremm) at 623 K for 6 h led
to MgOh.
γ -Al2O3 was prepared by calcination of boehmite at
873 K.
2. Experimental
2.2. Synthesis of the salts of reaction products
2.1. Material preparation
The addition at 273 K of a solution of NaOH (0.1 mol)
in 50 mL of methanol to a solution of nitromethane in
equilibrium with the aci-anion nitromethane (hereafter 1,
Scheme 1) led to the sodium salt of aci-anion nitromethane
(hereafter 2, Scheme 1). CH3OH was then removed by distil-
lation under vacuum, and the solid dried in an oven at 353 K.
1H NMR (CDCl3, ref TMS): δ = 5.7 ppm (s), H2C;
13C NMR (CD3OD), δ = 101 ppm, 13C CP/MAS NMR,
δ = 99 ppm, H2C.
The synthesis of LDH precursors of mixed oxides (Ta-
ble 1) was performed by coprecipitation of a gel at con-
stant pH. An aqueous solution of Mg(NO3)2 · 6H2O and
Al(NO3)3 · 6H2O or Ga(NO3)3 · 9H2O with the desired
Mg/MIII ratio was delivered into a polypropylene reac-
tor by a chromatography-type pump at a constant flow of
1 cm3 min−1. A second aqueous solution of 1 M NaOH +
2 × 10−2 M Na2CO3 was simultaneously fed by a pH stat
apparatus (718 Stat Titrino Metrohm). The flow was con-
trolled in order to maintain the pH in the reactor at a constant
value of 9 0.2. After completion of the precipitation the
gels were refluxed at 353 5 K for 15 h. The solids were
separated by centrifugation, washed thoroughly with deion-
ized water until nitrate elimination, and dried overnight in an
oven at 353 K.
Synthesis of sodium salt of methazonic acid (hereafter 3,
Scheme 1) was performed by using 0.1 mol of nitromethane
added to 0.2 mol of NaOH in 50 mL of CH3OH at 323 K for
2 h. CH3OH was then removed by distillation under vacuum,
and the solid dried in an oven at 353 K.
1H NMR (CDCl3, ref TMS), δ = 6.82 ppm (d, J =
9.0 Hz) and 7.23 ppm (d, J = 8.8 Hz), H C=NOH; 7.66 ppm
(d, J = 8.8 Hz) and 8.20 ppm (d, J = 9.0 Hz), H C=NO2−
Na+. As shown by irradiation, signals at 6.82 and 8.20 ppm
The Mg(Al)O and Mg(Ga)O mixed oxides were obtained
by calcination of Mg/Al and Mg/Ga LDH at 723 K in a
dry synthetic air flow (25 cm3 min−1, ramp: 1 K min−1, 6 h
plateau at 723 K).
belong to the same isomer.
13C NMR (CD3OD), δ = 108 and 114 ppm, C=NO2
–
−
Mg(Al)O mixed oxide KW 2200 (Mg/Al = 2.3) was pur-
Na+; 142 and 145 ppm, C=NOH; 13C CP/MAS NMR: δ =
chased from Kyowa.
106, 111, 140, and 143 ppm.

30
E. Lima et al. / Journal of Catalysis 223 (2004) 28–35
Moreover 13C CP NMR spectra of the previous solid
show a peak at δ = 167.2 ppm corresponding to the same
shift as the sodium formate (δ = 167.4 ppm) or formic acid
1
(δ = 166.7 ppm). H NMR corroborates this assignment:
H C δ (CD3OD) = 8.6 ppm (s).
The same products are identified when reaction is per-
formed in water. No reaction is observed without addition of
base.
2.3. Characterizations
Chemical analyses were performed at the Service Central
d’Analyse du CNRS (Solaize, France) by ICP-MS.
XRD powder patterns were collected on a CGR Theta 60
Scheme 2. Model Michael addition of nitromethane to cyclo-
hex-2-en-1-one.
instrument using monochromatized Cu-Kα radiation (λ =
1
0.15401 nm, 40 kV, and 50 mA).
BET specific surface areas were determined by N2 ad-
sorption at 77 K with a Micromeritics ASAP 2000 apparatus
on samples outgassed at 523 K and 10−4 Pa.
Scheme 3. Formation of the aci-anion sodium salt analogue on acid–base
pairs.
For 13C CP/MAS NMR studies, 99% 13C-enriched ni-
tromethane (Aldrich) was distilled under vacuum before ad-
mission onto the solids. Solids were activated in situ under
vacuum at 673 K for 8 h before nitromethane adsorption.
Equilibration was performed under 30 Torr of 13CH3NO2
at room temperature for 30 min, and then the samples were
evacuated at 323 K for 20 min. Spectra were recorded at
50.32 MHz with a contact time of 5 ms, a 4 KHz spin-
ning rate, and 90◦ pulses of 5 us [16,18] using a ASX-
200 Bruker spectrometer with a home-built NMR probe-
head [19]. Chemical shifts were referenced to solid CH2
adamantane shift at 38.2 ppm relative to TMS.
cyclohex-2-ene-1-one 4 conversion (mol%),
ꢀ
ꢁꢂ
C = 100 × [4]0 − [4]t [4]0;
selectivity of mono-adduct (mol%),
ꢃ
ꢂ
ꢀ ꢄ
,
S = 100 × 1 (IHa + IHb
)
12 (IHa + IHb) + IHa
2
where IHa and IHb are the integrated intensities of the signals
ꢀ
corresponding to 2 protons of the mono-adduct 5, and IHa
to that of a proton of the bis-adduct 6.
Nitromethane (98% purity, Lancaster) and cyclohex-2-
en-1-one 4 (Fluka) were used without purification. DMSO
purchased from SDS was kept on molecular sieve A under
N2 atmosphere.
2.4. Catalytic tests
Before reaction, catalysts were activated in situ in a glass
reactor at 623 K for 6 h under a dry synthetic air flow
(ramp: 2 K min−1). Meixnerite was activated at 323 K un-
der vacuum. After adjusting the temperature to reaction
temperature under N2 flow, stoichiometric amounts of re-
actants (0.01 mol) in 20 mL of DMSO were introduced
on the activated catalyst (0.4 g) in the batch reactor with
a syringe, under magnetic stirring. The progress of the re-
action was monitored by periodically withdrawing samples
which were analyzed by gas chromatography on a HP-
5890 with capillary column HP-1. Calibration of 3 was per-
formed using decane (Aldrich) as a reference. Retention
times of nitromethane, cyclohex-2-ene-1-one (hereafter 4,
Scheme 2), decane, mono-adduct (hereafter 5, Scheme 2),
bis-adduct (hereafter 6, Scheme 3) were 0.9, 5.5, 7.1, 10.9,
and 14.8 min, respectively.
3. Results
Nitromethane and its tautomeric form, aci-nitromethane
1 lead in the presence of sodium hydroxide to the sodium
salt 2 [20]. A typical reaction of nitromethane also con-
cerns the formation of the sodium salt of methazonic acid
3 [21] by condensation with an equivalent of the aci-anion
salt (Scheme 1).
Sodium aci-anion salt 2 and sodium methazonate 3
were obtained by reaction of nitromethane with NaOH in
methanol and characterized by 13C NMR (Fig. 1). The sin-
glet observed in 13C CP/MAS NMR at 99 ppm (Fig. 1a) for
the methylene group of 2 gives a multiplet in CD3OD cen-
tered at 101 ppm (Fig. 1c) explained by H/D exchange [22].
Two isomers of sodium methazonate are clearly observed
in CDCl3 (Fig. 1c) (108, 114, 142, 145 ppm) which cor-
respond to Z and E isomers around the C=N–OH double
The concentration of reaction products was determined
from 1H NMR spectra performed with a Bruker Avance
DRX—400 MHz spectrometer, after solvent evaporation
and dissolution of the residual fraction in CD3Cl. The fol-
lowing parameters were determined to evaluate the catalytic
properties:
1
bond (Scheme 1) [23]. Their H NMR spectra agree with
the formula 3. Another signal (167 ppm) is observed when

E. Lima et al. / Journal of Catalysis 223 (2004) 28–35
31
13
Fig. 1. C CP NMR spectra of solid sodium aci-anion salt (a), solid
sodium methazonate (b), sodium methazonate in CD OD (c), and products
3
in D O (d).
2
reaction is performed in water (Fig. 1d) which is assigned to
formic acid [23] or sodium formate by comparison with an
authentic sample.
13
Fig. 2. C CP/MAS NMR spectra of species formed from nitromethane
adsorption on MgO (a), Mg(Al)O mixed oxides with Mg/Al = 3.0 (b),
c
3.1. Reactive adsorption of nitromethane on oxides and
mixed oxides followed by 13C CP/MAS NMR
2.5 (c), 2.0 (d), and Al O (e).
2
3
Species formed by reactive adsorption of nitromethane on
the surface of various solids were identified by comparison
of the resulting 13C CP/MAS NMR spectra with those of the
compounds obtained in homogeneous conditions.
Their nature depends on the acid–basic properties of the
surface. Only physisorbed nitromethane (62 ppm) is ob-
served for zeolites NaX and CsX [16]. Reactive adsorption
of nitromethane on Al2O3 (Fig. 2e) led to physisorbed ni-
tromethane (δ = 62 ppm) and aci-anion analogue 2 (δ =
112 ppm). The same species are observed by adsorption
of nitromethane on Mg(Al)O mixed oxides (Fig. 2b–d).
On MgOc (Fig. 2a) physisorbed nitromethane and aci-anion
2 are also obtained. However, the presence of a peak at
143 ppm and of a signal at 107 ppm, which could be de-
composed into 2 peaks at 110 and 102 ppm, show that met-
hazonate 3 analogue (δ = 110, 143 ppm) and aci-anion 2
(δ = 102 ppm) are also formed on this surface after decom-
position.
13
Fig. 3. C CP/MAS NMR spectra of species formed from nitromethane
adsorption on Mg(Ga)O mixed oxides with Mg/Ga = 5.0 (a), 3.0 (b), and
2.0 (c). Decomposition of the peak at 109 ppm in (a) leads to two peaks in
(d) at 104 and 112 ppm.
Therefore, incorporation of Al in the MgO structure leads
to the disappearance of the signals of the methazonate anion
analogue 3. The decrease of the Mg/Al ratio from 3.0 to
2.0 by increasing the Al content (from Figs. 2b to d) has
no effect on the species formed (physisorbed nitromethane
and aci-anion 2). However, on the one hand, the CH2 signal
of 2 shifts upfield and, on the other hand, as shown by the
signal to noise ratio obtained under the same conditions, the
amount of 2 decreases with increase of the Al content.
Adsorption of nitromethane onto Mg(Ga)O mixed oxides
leads to three species (Fig. 3) as for MgOc. Whatever the
Mg/Ga ratio between 2.0 and 5.0 methazonate 3, aci-anion
2, and physisorbed nitromethane are obtained.
Species formed upon nitromethane adsorption allow con-
sideration of 3 distinct families of solids, i.e., zeolites NaX
and CsX leading to physisorbed nitromethane; Al2O3 and
Mg(Al)O mixed oxides leading to physisorbed nitromethane
and aci-anion nitromethane 2; and Mg(Ga)O mixed oxides,
MgOc, and modified CsX 9Cs zeolite leading to physisorbed
nitromethane, aci-anion nitromethane 2, and methazonate
anion analogue 3.

32
E. Lima et al. / Journal of Catalysis 223 (2004) 28–35
1
Fig. 5. H NMR spectra of the mono-adduct 5 (•) and bis-adduct 6 (∗)
implied in the calculation of the mono-adduct selectivity.
Table 2
13
Fig. 4. C CP/MAS NMR spectra of species formed from nitromethane ad-
sorption on the meixnerite activated at 323 K (a), 353 K (b), and 473 K (c).
Effect of the order of addition of reactants using stoichiometric concentra-
tions and MgO as catalyst (m catalyst = 0.4 g)
h
Method
r
Conversion (t = 4 h)
5 Selectivity
0
−1
(mol min
)
(%)
(%)
3.2. Example of reactive adsorption of nitromethane on
LDH followed by 13C CP/MAS NMR
a
a
0.18
1.10
0.36
56
97
73
54
35
70
a
b
b
c
By adsorption of nitromethane under the same conditions
on meixnerite activated at 323 K a single signal assigned to
a formate analogue species is observed at 167 ppm (Fig. 4).
None of the previous species are identified for activation
temperatures of 323 or 353 K. Physisorbed nitromethane
(62 ppm) and aci-anion nitromethane (111 ppm) appear after
activation of meixnerite at 473 K (Fig. 4c).
a
Nitromethane or enone 4 was added as the first reactant, and the second
reactant was added after 5 min at 353 K.
b
The two reactants were added together.
of two diastereoisomers (RR, SS) and (RS, SR) which give
ꢀ
ꢀ
two doublets (J Ha Hc = 7.1 Hz). Selectivities were deter-
mined from 1H NMR spectra of the crude media (Fig. 5).
3.3. Michael addition of nitromethane on
cyclohex-2-en-1-one 4
3.4. Effect of the order of addition of reactants
Table 2 shows results obtained with MgOh using stoichio-
metric amounts of reactants (0.5 mol L−1).
The model reaction (Scheme 2) was performed using sto-
ichiometric amounts of nitromethane and 4 (0.5 mol L−1) in
DMSO.
The order of addition of the reactants on the catalyst
presents a major effect either on the initial rate or on the
mono-adduct selectivity. The initial rate is six times higher
when enone 4 (method b) instead of nitromethane is first
added (method a). Comparison of methods a and b shows
that selectivity decreases when conversion decreases as ex-
pected for consecutive reactions. Therefore the bis-adduct 6
is the major product by method b. Higher conversion and
selectivity are reached by method c, when reactants are si-
multaneously added, than by method a. Method c will be
used throughout the rest of the study.
This reaction was usually performed in protic solvents,
but dimethyl sulfoxide (DMSO) rather than ethanol was
likely used taking into account our previous results show-
ing that it was a better solvent in the Knoevenagel reaction
catalyzed by basic zeolites [24]. Initial rates were calculated
from cyclohexenone consumption with time. Two products
were identified: (i) the mono-adduct 5 resulting from mono
addition of nitromethane on cyclohex-2-en-1-one 4, (ii) the
bis-adduct 6 formed by the consecutive reaction of 5 with 4
1
(Scheme 2). 5 and 6 were well characterized by H NMR
(Fig. 5). Two quartets are observed for Ha and Hb of 5 and
ꢀ
3.5. Effect of the weight of catalyst on the reaction rate and
selectivity
two doublets for Ha of 6.
The two diastereotopic protons Ha and Hb of the prochiral
methylene group of the CH2NO2 moiety in 5 present an AB
spectra (J HaHb = 12 Hz), each proton being coupled with
the vicinal HC (J HaHC = 6 Hz). The formation of the bis-
adduct 6 with two asymmetric centers leads to the obtention
The reaction does not proceed without catalyst. The re-
action rate increases with the weight of catalyst (Table 3)
and does not vary with the stirring rate (not shown). Thus

E. Lima et al. / Journal of Catalysis 223 (2004) 28–35
33
Table 3
CsX 9Cs (entry 10), the selectivity remaining similar. Ac-
tivity of CsX 9Cs zeolite is higher than that of MgOc with
similar selectivity. Accordingly physisorbed nitromethane
is identified for NaX, CsX, and γ -Al2O3, whereas metha-
zonate 3 is also obtained for CsX 9Cs zeolite.
Finally, the better activities and selectivities in mono-
adduct are obtained with MgOh and meixnerite (Table 4,
entries 11, 12). The increase of the initial rate when going
from MgOc to MgOh does not accounted for an increase of
the surface areas. Formation of formate upon nitromethane
adsorption shows the different behavior of the meixnerite
and the other catalysts.
Effect of the weight of MgO catalyst using stoichiometric concentrations
of 4 and nitromethane
h
m
r
Conversion (t = 4 h)
5 Selectivity
0
−1
(g)
(mol min
)
(%)
(%)
0.0
0.0
0.28
0.40
0.50
0.24
0.36
0.46
62
73
85
76
70
52
the reaction rate is not controlled by external diffusional
constraints. The mono-adduct selectivity, as previously de-
scribed, decreases with conversion.
3.6. Catalytic results in the model reaction
4. Discussion
For a given catalyst the selectivity in mono-adduct de-
creases when the conversion goes on, as expected when con-
secutive reactions are taking place (Table 3). The reaction
rates vary within broad limits depending on the nature of the
catalyst (Table 4).
A first series (Table 4, entries 1–5) of catalysts includes
γ -Al2O3, Mg(Al)O, and MgOc. The initial reaction rate in-
creases when going from γ -Al2O3 to MgOc. The same trend
is observed when the surface area of the catalyst is taken
into account. This is consistent with the concurrent increase
in the amount of aci-anion nitromethane already reported
(Fig. 2).
Nitro and aci tautomeric forms of nitroalkanes (Scheme 1)
with hydrogen atoms on the α-carbon position exhibit acidic
properties. pKa’s of nitromethane and of its aci-form 1
in water at 25 ◦C were evaluated to 10.2 and 3.2, respec-
tively [20]. The equilibrium proportion of the aci-form 1
under these conditions represents 0.000011%. Reaction with
sodium hydroxide leads to the sodium salt of the aci-anion 2.
However, nitromethane has a peculiar behavior among ni-
troalkanes because the sodium salt of methazonic acid 3 can
be readily obtained [21] (Scheme 1).
So, in order to generate the reactive aci-anion analogue
a basic site is needed. Taking into account the pKa’s of
nitromethane and aci-nitromethane 1, 10.2 and 3.2, respec-
tively, a higher basic strength is required for the abstraction
of a proton from adsorbed nitromethane leading either to 1
and then to 2 or directly to 2 (Scheme 1).
Oxides, mixed oxides, and zeolites bear acid–base pairs
which can be differentiated by their overall basicity or acid-
ity. By adsorption of nitromethane on acid–base pairs an
analogue of aci-nitromethane 2 is obtained (Scheme 3) when
basic strength is able to abstract a proton. The Lewis acid site
allows the stabilization of the aci-anion.
The mono-adduct selectivity is lower for Mg(Al)O (en-
try 4) than for MgOc (entry 5) at the same conversion. This
selectivity slightly increases when the conversion decreases
for the various Mg(Al)O mixed oxides.
Mg(Ga)O mixed oxides (Table 4, entries 6, 7) lead to ini-
tial reaction rates and selectivities markedly higher than with
the previous catalysts. Higher conversions and mono-adduct
selectivities (Table 4) are achieved. Moreover formation of
methazonate 3 and aci-anion nitromethane 2 is also observed
(Fig. 3).
The activity of exchanged NaX and CsX (Table 4, en-
tries 8, 9) is low but increases markedly with the basicity for
Table 4
Catalytic results using a same weight of activated catalyst
III
2
4
Entry
Catalyst
Mg/M
Surface area
r × 10
r × 10
Conversion
(t = 8 h) (%)
5 Selectivity
0
0
−1
2
−1
−1
−1 −2
(mol mol
)
(m g
)
(mol min
)
(mol min
m
)
(%)
1
2
3
4
5
6
7
8
9
γ -Al O
172
140
180
240
160
176
140
666
471
423
200
–
0.2
4.8
7.2
0.12
3.4
4.0
3.7
8.2
22.7
18.6
0.03
0.21
5.7
18.0
15
25
39
45
45
65
60
10
15
55
73
95
–
2
3
Mg(Al)O
KW 2200
Mg(Al)O
2.0
2.3
3.0
40
42
34
59
57
60
–
60
58
70
80
8.8
MgO
13.2
40.0
26.0
0.2
c
Mg(Ga)O
Mg(Ga)O
NaX
CsX
CsX 9Cs
5.0
2.0
1.0
10
11
12
24.0
36.0
92.0
MgO
h
Meixnerite
3.0
–

34
E. Lima et al. / Journal of Catalysis 223 (2004) 28–35
Scheme 4. Reaction mechanism proposed for acid–base catalysis of the
Michael addition.
LDH such as meixnerite are characterized by a Brønsted
basicity provided by OH-compensating anions and by a very
weak Lewis acidity due to their hydration [25].
The adsorption of nitromethane on this solid (Fig. 4)
probably leads to the formation of formate. Such a com-
pound has been evidenced in the presence of water under
basic conditions (Fig. 1d).
13
Fig. 6. Correlation between the initial reaction rate and the C chemical
shift of the methylene group of the aci-anion nitromethane 2.
fore weak reaction rates are observed with poorly basic NaX
and CsX catalysts leading to nitromethane physisorption, or
Al2O3 leading to a low amount of 2. As previously suggested
from theoretical calculations [26], nitromethane physisorp-
tion likely takes place between the nitro group and a Lewis
acid site as shown by 133Cs CP/MAS NMR for CsX zeo-
lite [16].
Then, the initial reaction rate (Table 4, entries 2–5) in-
creases with the amount of 2 (Scheme 2). Moreover it is
well correlated with the 13C chemical shift of the methylene
group of aci-anion nitromethane 2 (Fig. 6). The higher the
initial rate, the lower the chemical shift.
This correlation is in line with previous results showing
that 13C chemical shifts of the aci-anion nitromethane 2 are
correlated with the heat of carbon dioxide adsorption [18].
Taking into account the species involved in the adsorption of
CO2 on the oxide surface [27], the same acid–base pairs are
concerned with the various processes.
When it is formed, aci-anion nitromethane 2 is stabilized
by interaction with a Lewis acid site. Therefore, it is highly
stabilized on Lewis acid sites of high strength as previously
suggested by Allouche for MgO and CaO [26], and its reac-
tivity decreases (Scheme 4). The withdrawing effect of the
Lewis acid site lowers the anionic character of the methylene
group of 2. This feature accounted for both the decrease of
the initial rate and the deshielding in 13C CP/MAS NMR in
the series MgOc, Mg(Al)O, Al2O3.
In the base-catalyzed Michael addition of nitromethane
on α, β-unsaturated carbonyl compounds, nitromethane acts
as the donor, the nitro group being particularly effective in
stabilizing a negative charge on an adjacent carbon [7], and
cyclohex-2-en-1-one 4 as the acceptor (Scheme 4). So, the
formation and the reactivity of 2 may be considered as the
key factor for the catalytic activity or the reactive adsorption
of nitromethane.
The formation of the two products 5 and 6 obtained by
the Michael addition of cyclohexenone 4 and nitromethane
(Scheme 2) is well explained by consecutive reactions. Ab-
straction of a proton from nitromethane or mono-adduct 5
leads to competitive addition of the anions on the C=C
double bond of cyclohexenone 4. Selectivity to 5 decreases
with conversion as observed for a same catalyst, MgOh (Ta-
ble 3). Reaction rate is not controlled by external diffusion as
checked by varying catalyst weight:reactant ratio for MgOh
(Table 3) and the order of activity of the various solids varies
only slightly if the surface area of the catalysts is taken into
account (Table 4).
The influence of the order of addition of the reactants
on reaction rates and selectivities accounted for their com-
petitive adsorption on the surface (Table 2). Higher enone
activation occurring by adsorption on a Lewis acid site
(Scheme 4), and in contrast lower stabilization of aci-anion
nitromethane, leads to an enhanced initial reaction rate
(method b). In this case, mono-adduct 5 reacts easily with
chemisorbed enone to give the bi-adduct 6. Chemisorption
of nitromethane limits chemisorption of enone 4 (method a).
Thus, a good compromise is achieved by method c involv-
ing simultaneous addition of the reactants. It is noteworthy
that a higher selectivity in the mono-adduct 5 is obtained at
a high conversion level, indicating the role of chemisorbed
enone.
Considering the acid–base pairs the strength of the basic
sites decreases as that of the conjugated acid site increases.
Therefore, reaction rate is well correlated with the basic
strength of the catalyst.
When 2 is weakly stabilized on Lewis acid sites forma-
tion of methazonate 3 occurs. Concomitantly, for Mg(Ga)O
mixed oxides (Table 4, entries 6, 7) and CsX 9Cs (entry 10)
the higher basic strength leads to an enhancement of initial
reaction rates when the decrease of Lewis acid strength ac-
counted for the increase of selectivities.
The first step (Scheme 3) of the base-catalyzed mecha-
nism (Scheme 4) implies the abstraction of a proton. There-

E. Lima et al. / Journal of Catalysis 223 (2004) 28–35
35
One can note the different behavior of MgOc obtained
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5. Conclusions
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