Guerbet condensation of methanol with n-propanol to isobutyl alcohol over heterogeneous copper chromite/Mg-Al mixed oxides catalysts

Article abstract of DOI:10.1016/j.molcata.2004.05.034

The synthesis of isobutyl alcohol (ⁱBuOH) from methanol (MeOH) and n-propanol (PrOH) through the Guerbet condensation was studied at 200°C and under inert atmosphere (3 MPa of N₂, using a two-component heterogeneous catalytic system based on pre-activated copper chromite and Mg-Al mixed oxides deriving from hydrotalcite-type precursors with different Mg/Al ratios. The catalyst productivity increased by reducing the Mg/Al ratio in the heterogeneous base, according to the increase of the fraction of medium-strong and strong basic sites, which favor the aldol condensation between the aldehydes derived from MeOH and PrOH. The heterogeneous base with the lowest Mg/Al atomic ratio exhibited the maximum of productivity in ⁱBuOH when employed in the lowest amount, thus opening interesting application perspectives.

Full text of DOI:10.1016/j.molcata.2004.05.034

Journal of Molecular Catalysis A: Chemical 220 (2004) 215–220
Guerbet condensation of methanol with n-propanol to isobutyl alcohol
over heterogeneous copper chromite/Mg–Al mixed oxides catalysts
Carlo Carlinia,∗, Cristina Flegob, Mario Marchionnab, Marilena Novielloa,c,
Anna Maria Raspolli Gallettia, Glauco Sbranaa, Francesco Basiled, Angelo Vaccarid
a
Dipartimento di Chimica e Chimica Industriale, University of Pisa, Via Risorgimento 35, 56126 Pisa, Italy
b
EniTecnologie S.p.A., Via Maritano 26, 20097 San Donato Milanese MI, Italy
c
Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
d
Dipartimento di Chimica Industriale e dei Materiali, ALMA MATER STUDIORUM, University of Bologna,
Viale del Risorgimento 4, 40136 Bologna, Italy
Received 13 April 2004; accepted 24 May 2004
Available online 15 July 2004
Abstract
The synthesis of isobutyl alcohol (iBuOH) from methanol (MeOH) and n-propanol (PrOH) through the Guerbet condensation has been
studied at 200 ◦C and under inert atmosphere (3.0 MPa of N2), using a two-component heterogeneous catalytic system based on pre-activated
copper chromite and Mg–Al mixed oxides deriving from hydrotalcite-type (HT) precursors with different Mg/Al ratios. All the investigated
catalysts displayed a significant activity, with an almost complete selectivity to iBuOH. Unlike the copper chromite/soluble sodium methoxide
system, the catalysts were tolerant of the co-produced water and did not display any appreciable deactivation during the course of the reaction.
The catalyst productivity was found to increase by reducing the Mg/Al ratio in the heterogeneous base, according to the increase of the fraction
of medium–strong and strong basic sites which favour the aldol condensation between the aldehydes derived from MeOH and PrOH.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Guerbet reaction; Hydrotalcite; Copper chromite; Mg/Al mixed oxides; Isobutyl alcohol; Methanol; n-Propanol; Aldol condensation
1. Introduction
high-temperature-methanol catalysts [4–6]. However, the
latter step has several drawbacks, particularly in terms
of selectivity to BuOH and retroconversion of MeOH to
i
The selective synthesis of isobutyl alcohol (iBuOH)
has recently gained an increasing interest because this
substrate is a potential precursor of gasoline additives
such as methyl-tertbutyl-ether or isooctane. iBuOH
may be directly synthesized from syngas through the
higher-molecular-weight alcohols synthesis (HAS) carried
out at high temperature and pressure over heterogeneous
catalysts. However, this process is characterized by rather
low selectivity and productivity [1]. An alternative route
would be represented by a two-step process [2,3] where in
the first stage methanol (MeOH) and higher alcohols are
obtained at low temperature from syngas adopting modi-
fied low-temperature-methanol catalysts and in the second
syngas. In this context, taking into account that from the
first step of the above mentioned process a MeOH/ethanol
(EtOH)/n-propanol (PrOH) mixture with a large excess of
MeOH was generally obtained [2], it appeared convenient
to transform this mixture in an iBuOH-rich product through
the Guerbet reaction. Indeed, the Guerbet reaction consists
of a condensation between alcohols, promoted by a bifunc-
tional catalytic system based on a basic component and a
metal species with dehydrogenating/hydrogenating proper-
ties. Moreover, it has to be underlined that iBuOH does not
yield subsequent condensation products, probably due to its
steric hindrance and to the presence of only one hydrogen
on the carbon atom in ␣-position to the methylol group [7].
In particular, it is well established that the Guerbet reac-
tion may be split into the following three steps: (1) dehy-
drogenation of alcohols to the corresponding aldehydes, (2)
aldol condensation of the resulting aldehydes, and (3) hydro-
i
stage they are converted to an BuOH-rich product using
∗
Corresponding author. Tel.: +39 050 2219222;
fax: +39 050 2219260.
E-mail address: carlini@dcci.unipi.it (C. Carlini).
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.05.034

216
C. Carlini et al. / Journal of Molecular Catalysis A: Chemical 220 (2004) 215–220
Scheme 1.
genation of the unsaturated condensation products to give
the higher alcohols [8,9]. In Scheme 1 the reaction pathway
is reported for the MeOH/PrOH condensation.
which can also be easily and cheaply synthesized [20–22].
The properties may be easily tailored by varying the amount
of the cations (or their mixtures) and/or the anions, giving
rise to a wide range of HT compounds having the general
formula [(M1−x2+Mx3+(OH)2)x+(Ax/mm−)·nH2O], where
Recently, the Guerbet reaction of MeOH with PrOH
[7] or with an EtOH/PrOH mixture [10] has been in-
vestigated by some of us adopting copper-based/sodium
methoxide (MeONa) catalytic systems. In particular, when
MeOH/PrOH mixtures were reacted in the presence of
copper chromite/MeONa, an almost complete selectivity to
iBuOH was observed, the yield increasing with a tempera-
ture enhancement from 180 to 220 ◦C [7]. The productivity
was also found to improve on increasing the relative amount
of MeONa with respect to the copper component. When
the reaction was carried out under N2 atmosphere, better
catalytic performances were obtained than under H2 atmo-
sphere, thus indirectly confirming that the dehydrogenation
of the alcohols to the corresponding aldehydes is the rate
limiting step of the reaction. In addition, the recycle tests
of both the solid copper component and the liquid reaction
mixture allowed to conclude that copper chromite essen-
tially works in heterogeneous phase, unlike that observed
for Pd-, Rh- or Ru-based catalysts [11,12].
m−
M2+ and M3+ are cations and A
is an interlayer anion
[20–22]. When HT precursors are calcined at temperatures
lower than ca. 650 ◦C, the resulting mixed oxides display
(i) high surface area, (ii) basic properties, (iii) memory ef-
fect, since they may recover the original HT structure when
in contact with water solution containing various anions
[20–22].
Therefore, HT precursors with various Mg/Al ratios were
prepared, calcined and tested in combination with copper
chromite in the Guerbet condensation of MeOH with PrOH
i
to obtain BuOH with high yield and selectivity in a com-
pletely heterogeneous process. In this particular case, the
heterogeneous bases derived from HT precursors appeared
very promising, since the basic properties are not only re-
tained, but very probably enhanced in the presence of the
co-produced water [23,24].
However, the main drawback of the process was the use
of MeONa as homogeneous basic component, due to its
progressive hydrolysis to MeOH and inactive NaOH by the
H2O co-produced in the condensation step [7–10]. There-
fore, in the present work for the first time a fully heteroge-
neous system was adopted, i.e. combining a solid base with
the copper chromite component. The replacement of a sol-
uble base with an insoluble one would facilitate the separa-
tion and recovery of the catalyst from reaction products and
would strongly decrease corrosion phenomena and environ-
mental constraints [13–15]. Moreover, the use of solid base
catalytic components in the reaction of MeOH with PrOH
could allow an easier integration of the condensation step
with the subsequent hydrogenation reaction [16], whenever
needed.
In this context, among the different types of heterogeneous
basic catalysts reported in the literature Mg/Al mixed ox-
ides prepared by controlled calcination of hydrotalcite-type
(HT) precursors have shown the best basic features and, in
the recent years, these materials have been applied as cat-
alysts for many reactions [17–19]. Hydrotalcite is a lay-
ered mineral having the formula Mg6Al2(OH)16CO3·4H2O,
2. Experimental
2.1. Materials
MeOH (Prolabo) and PrOH (Carlo Erba) were dried by
distillation under dry argon after refluxing for 6 h on magne-
sium methoxide, according to the Lund and Bjerrum method
[25].
MeONa (Aldrich) was used as received and stored under
dry argon.
Copper chromite catalyst (Cu 1955-P, Engelhard), with
the following composition: Cu (36.0 wt.%), Cr (33.0 wt.%),
Mn (3.0 wt.%), was dried under vacuum at room temperature
and then activated before the use in a mechanically stirred
Parr reactor in the presence of methanol at 180 ◦C, for 4 h
and under 8.0 MPa of H2.
HT precursors with different Mg/Al atomic ratios were
prepared by co-precipitation of magnesium and aluminium
nitrates by Na2CO3, maintaining the pH = 10.0
0.1
by dropwise addition of a concentrated solution of NaOH
[20,22,26]. The main characteristics of the prepared Mg/Al

C. Carlini et al. / Journal of Molecular Catalysis A: Chemical 220 (2004) 215–220
217
Table 1
of pure compounds of known composition, with benzene as
internal standard.
Composition and surface area of the heterogeneous basic catalysts obtained
by calcination at 500 ◦C for 5 h of the HT precursors
In all the experiments an almost total selectivity to
iBuOH was observed. Indeed, only traces (<0.5 mol%)
of methyl and propyl formate were detected by GC/MS
analysis carried out employing a HP5973 Mass Selective
Detector and a HP6890 Series GC System, equipped with
a HP5-MS crosslinked 5% phenylmethylsiloxane column
(30 m × 0.25 mm × 0.25 ␮m).
Catalyst
Mg/Al (atomic
ratio)a
Weight
composition (%)
BET surface
area (m2/g)
MgO
Al2O3
Cat1
Cat2
Cat3
80/20
71/29
66/34
76.0
65.9
60.5
24.0
34.5
39.5
208
232
214
a
Ratio between aluminium and magnesium nitrates in the starting
solution used for the preparation of the HT precursors.
2.1.3. Catalysts characterization
The adsorption and temperature programmed desorption
(TPD) of CO2, as probe-molecule for determining the ba-
sic properties of the Mg/Al mixed oxides, was carried out
on a Pulse Chemisorb 2705 Micromeritics instrument. Each
sample (almost 0.5 g) was pre-treated under He at 500 ◦C for
2 h in order to eliminate water and gas impurities eventually
present. After cooling the sample, the adsorption of CO2 at
21 ◦C was carried out until surface saturation was reached;
the presence of physically adsorbed CO2 was avoided by
performing the adsorption under He flow. The temperature
was then increased with a linear rate from room temper-
ature up to 500 ◦C and the CO2 evolution was monitored
by mass spectrometry. The basic strength distribution of the
sites was evaluated from the capability of the material to
retain the probe molecule during the desorption. The TPD
profiles were analyzed by a deconvolution program (Galac-
tic Peaksolve) in order to determine the signal number and
position. The quality of the elaboration obtained by the
Levenberg–Marquadt algorithm was evaluated on the basis
of the correlation coefficient (R2 > 0.97).
samples, after calcination at 500 ◦C for 5 h, crushing and
sieving to 125–250 ␮m particle size and finally storage
under argon, are reported in Table 1.
2.1.1. Catalytic experiments for the condensation of
methanol with n-propanol
A 300 ml Parr reactor, equipped with a mechanical stir-
rer, a heating system and a control device assisted by a J
thermocouple inserted into the reactor, a sampling valve for
liquids, an inlet valve for gas introduction and an outlet
sampling valve for gaseous products, was used in the cat-
alytic batch experiments. The copper chromite catalyst was
introduced in the reactor, then this latter was evacuated and
MeOH was added through the sampling valve. The reactor
was subsequently pressurized with H2 up to 8.0 MPa and
heated at 180 ◦C for 4 h. After the activation step, the reactor
was cooled to room temperature and degassed. The reactor
was evacuated and MeOH removed under vacuum, then it
was opened and the alcohols mixture and the heterogeneous
basic catalyst were introduced under argon atmosphere. Fi-
nally, the reactor was pressurized with N2. The reaction was
followed by collecting at different times portions of the re-
action mixture, quickly cooled to 0 ◦C, through the sampling
valve. At the end of each test, the reactor was rapidly cooled
to room temperature, slowly degassed through a trap main-
tained at −30 ◦C, in order to condense any liquid products
present in the gas phase, and finally the liquid reaction mix-
ture was analyzed by gas-chromatography (GC) after the
addition of a known amount of benzene as internal standard.
BET surface area values were obtained using a single
point ThermoQuest Surface Area Analizer Qsurf S1.
3. Results and discussion
A preliminary test (entry 1, Table 2) was carried out
using copper chromite as dehydrogenating/hydrogenating
catalyst and a solid basic component having an interme-
diate Mg/Al atomic ratio (71/29) as well as a BET sur-
face area equal to 232 g/m2 (Cat2), to check if a fully het-
erogeneous two-component catalyst may be active in the
Guerbet condensation of MeOH with PrOH to give iBuOH.
The reaction was performed at 200 ◦C and under N2 atmo-
sphere (3.0 MPa) as these conditions were previously proved
to give the best catalytic performances when the copper
chromite/MeONa system was employed [7,10]; moreover
copper chromite was pre-activated (see Section 2), this pro-
cedure allowing to significantly improve the activity of the
metal-containing component. By comparison, a test under
the same conditions (entry 2, Table 2) was performed using
a pre-activated copper chromite/MeONa system. The ob-
tained results clearly indicate that the Cat2/copper chromite
system was active in the Guerbet reaction, the iBuOH yield
being 26.3 mol% after 12 h. These data suggest that the ac-
2.1.2. Analytical procedures
The analysis of the reaction products was performed
by GC using a Perkin-Elmer Sigma 3B chromatograph
equipped with a thermal conductivity detector, a CE Instru-
ments DP 700 integrator and a (3 mm × 2 m) Poropak PS
packed column with a stationary phase based on ethylvinyl-
benzene/divinylbenzene resin. Helium was used as carrier
gas with a 25 ml/min flow rate. The following temperature
program was adopted for the oven: 80 ◦C for 5 min, then
the temperature was increased by a 8 ◦C/min heating until
210 ◦C was reached, maintaining this value constant for
further 15 min. For the quantitative determination of the
reaction mixtures, the chromatographic response factor for
each individual component was determined using mixtures

218
C. Carlini et al. / Journal of Molecular Catalysis A: Chemical 220 (2004) 215–220
Table 2
i
Synthesis of BuOH by condensation of MeOH with PrOH using the Cat2/pre-activated copper chromite catalytic systema
Entry
Cu-chromite (mmol)
Basic component
iBuOH yield (mol%)b
TNc (h−1
)
Type
Mg/Al (atomic ratio)
MgO (mmol)
1 h
6 h
12 h
1
2
3
4
5
6
7
2.5
2.5
2.5
2.5
5.4
Cat2
71/29
–
20
–
6.8
18.3
20.8
17.0
18.4
24.3
14.6
15.9
26.3
24.2
25.6
30.5
31.5
18.5
–
1.22
1.33
1.13
1.23
0.75
0.22
0.24
MeONad
Cat2
13.3
5.2
4.3
7.8
5.1
5.0
71/29
71/29
71/29
71/29
71/29
45
90
20
45
90
Cat2
Cat2
Cat2
Cat2
11.0
11.0
a
Reaction conditions: MeOH = 1250 mmol; PrOH = 100 mmol; PN = 3.0 MPa; T = 200 ◦C.
2
b
c
d
Referred to the amount of PrOH feed.
i
Productivity expressed as mole of BuOH/(mol Cu h) and calculated at 6 h of reaction.
20 mmol were used.
tivity of this system is of the same order of magnitude as that
To investigate the effect of the nature of the heteroge-
neous basic component on the catalytic performances, het-
erogeneous bases having different Mg/Al atomic ratios with
respect to Cat2 (Cat1 and Cat3) were employed in combina-
tion with pre-activated copper chromite. Indeed, the above
samples, having substantially the same specific surface area,
would be characterized by different number and strength of
basic sites [27–29].
When the entries 1 and 3 of Table 2 were repeated using
Cat1, characterized by a higher Mg/Al atomic ratio (80/20)
(entries 8 and 9 in Table 3, respectively) a significant reduc-
tion in both yield and productivity to iBuOH was observed.
In the case of Cat1, the increase of the relative amount of
the basic component, still retaining constant the other re-
action parameters, lowered the catalytic productivity (com-
pare entries 8–10 in Table 3). When Cat3 (Mg/Al atomic
ratio = 66/34), was used in the lowest amount (entry 11,
Table 3) the best performance was obtained in compari-
son with the analogous tests carried out using Cat1 (en-
try 8, Table 3) and Cat2 (entry 1, Table 2). An increase in
the amount of the basic component with respect to copper
chromite from 20 to 60 mmol (entries 12 and 13, Table 3)
of the copper chromite/MeONa system, for which a con-
version up to 24.2 mol% was obtained. However, the trend
of the conversion with the reaction time in the two exper-
iments (Fig. 1) clearly indicates that the former system is
much less sensitive to the effect of co-produced water, since
i
the BuOH yield increased monotonically with the reaction
time for the copper chromite/Cat2 system, whereas it sub-
stantially reached a plateau after few hours of reaction in
the case of the copper chromite/MeONa system, showing a
subsequent substantial deactivation.
On progressively increasing the amount of the basic com-
ponent Cat2 (from 20 up to 90 mmol), still retaining constant
that of the copper chromite (entries 3 and 4, Table 2), the
iBuOH yield did not significantly change, unlike what was
previously observed for the copper chromite/MeONa sys-
tem [7,10]. Moreover, the increase of the relative amount of
the copper chromite, still maintaining constant the amount
of the basic component, gave rise to a decrease in the cat-
alytic performances in terms of productivity (TN) to iBuOH
(compare entries 1, 3 and 4 with entries 5, 6 and 7 of Table 2,
respectively).
i
Fig. 1. Conversion to BuOH as a function of the reaction time for the Guerbet condensation of MeOH with PrOH in the presence of: (᭡) pre-activated
copper chromite/Cat2 or (᭹) pre-activated copper chromite/MeONa systems.

C. Carlini et al. / Journal of Molecular Catalysis A: Chemical 220 (2004) 215–220
219
Table 3
Synthesis of iBuOH by condensation of MeOH with PrOH using pre-activated copper chromite in combination with different heterogeneous basic
components (Cat1 and Cat3)a
Entry
Cu-chromite (mmol)
Basic component
iBuOH yield (mol%)b
TNc (h−1
)
Type
Mg/Al (atomic ratio)
MgO (mmol)
1 h
6 h
9.5
8.3
2.6
19.3
8.9
7.0
12 h
8
9
10
11
12
13
2.5
2.5
2.5
2.5
2.5
2.5
Cat1
Cat1
Cat1
Cat3
Cat3
Cat3
80/20
80/20
80/20
66/34
66/34
66/34
20
45
60
20
45
60
1.2
1.6
0.2
3.4
2.3
1.7
13.5
15.2
4.6
29.5
14.2
13.7
0.63
0.55
0.17
1.29
0.59
0.47
a
Reaction conditions: MeOH = 1250 mmol; PrOH = 100 mmol; PN = 3.0 MPa; T = 200 ◦C.
2
b
c
Referred to the amount of PrOH feed.
i
Productivity expressed as mol of BuOH/(mol Cu h) and calculated at 6 h of reaction.
Table 4
Distribution of the basic sites evaluated by CO2 adsorption in the TPD experiments
Basic catalyst (Mg/Al
atomic ratio, %)
Overall amount of
desorbed CO2 (␮mol/g)
Weak basic sites
(␮mol/g)
Medium basic sites
(␮mol/g)
Medium–strong basic
sites (␮mol/g)
Strong basic sites
(␮mol/g)
Cat1 (80/20)
Cat2 (71/29)
Cat3 (66/34)
282
249
270
36
17
39
112
59
10
108
135
117
25
38
118
caused a detrimental effect on the yield and productivity to
iBuOH.
4. Conclusions
Thus, an excess of the heterogeneous base must be
avoided considering both catalytic activity and application
perspectives.
On the basis of the obtained results the following con-
cluding remarks can be drawn:
In order to have a deeper insight into the relations be-
tween activity and number and strength of basic sites in the
catalysts, TPD of CO2 was used. The results obtained are
reported in Table 4.
The low temperature CO2 evolution peak corresponds to
species adsorbed on weakly basic OH− groups, whereas the
medium strength basic sites, related to intermediate temper-
ature peaks, are connected with the oxygen in Mg2+–O2−
site pairs and, finally, the strong basic sites, corresponding
to the high temperature peaks, are the isolated O2− anions
[30].
Although the overall number of basic sites is not appre-
ciably influenced by the Mg/Al atomic ratio value, the frac-
tion of medium–strong and strong basic sites however in-
creases when this parameter decreases, accordingly to what
was previously reported by Corma et al. which also evi-
denced that the aldol condensation reaction requires strong
basic sites [27]. Analogous results were obtained by Tichit
et al. in their study about the effect of the Mg/Al ratio on the
initial condensation rate of benzaldehyde with acetone [31].
Therefore, we have to conclude that the increase of catalyst
activity observed when moving from Cat1 to Cat3, i.e. de-
creasing the Mg/Al atomic ratio, has to be addressed to the
enhancement of the fraction of medium–strong and strong
basic sites which favour the aldol condensation of the alde-
hydes formed by dehydrogenation of PrOH and MeOH on
pre-activated copper chromite.
(1) For the first time fully heterogeneous two-component
catalysts, based on pre-activated copper chromite and
Mg/Al mixed oxides derived from HT precursors, have
been proved to be active in the synthesis of iBuOH start-
ing from MeOH/PrOH mixtures through the Guerbet
condensation. In all cases an almost complete selectiv-
ity to iBuOH was found to occur.
(2) The activity of these systems was not affected by the
co-produced water, unlike that detected for the corre-
sponding catalysts containing sodium methoxide as ba-
sic component, without any evidence of inhibition dur-
ing the course of the reaction.
(3) The activity was found to be influenced by the Mg/Al
atomic ratio in the HT precursor, according to the
variation of the relative amount of medium–strong
and strong basic sites. The heterogeneous base with
the lowest Mg/Al atomic ratio showed the maxi-
i
mum of productivity in BuOH when employed in the
lowest amount, thus opening interesting application
perspectives.
Work is in progress to prepare and check bifunctional
heterogeneous catalysts, i.e. characterized by both dehydro-
genating/hydrogenating and basic sites present in the same
matrix, to simplify the catalytic system and exploit potential
synergistic effects in the Guerbet condensation.

220
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