Bifunctional metal/base catalysts (Pt/X) for the direct synthesis of MIBK from acetone

Article abstract of DOI:10.1006/jcat.2002.3625

The synthesis of MIBK was studied over Pt supported on NaX and CsX zeolites. The activity increased as both the temperature and the H₂/Ac ratio were increased. Temperature had also a beneficial effect on the selectivity to MIBK but the H₂/Ac ratio had an opposite effect. Activities and selectivities were always higher for Pt/NaX than for Pt/CsX. For Pt/NaX, increasing the reduction temperature increased the activity but decreased the selectivity to MIBK. Both catalysts were quite stable and selectivities to MIBK of 70% were obtained over PtNaX at 613 K and H₂/Ac = 0.5. The absence of both strong acidic and strong basic sites and a proper balance between metallic and basic sites are responsible for the behavior observed.

Full text of DOI:10.1006/jcat.2002.3625

Journal of Catalysis 209, 166–176 (2002)
doi:10.1006/jcat.2002.3625
Bifunctional Metal/Base Catalysts (Pt/X) for the Direct
Synthesis of MIBK from Acetone
∗,2
Lisiane V. Mattos,∗,1 Fa´bio B. Noronha,† and Jose´ Luiz F. Monteiro
∗
NUCAT-COPPE-UFRJ, Universidade Federal do Rio de Janeiro, Ilha do Funda˜o, C.P. 68502, CEP 21945-970 Rio de Janeiro, Brazil; and
†Instituto Nacional de Tecnologia (INT), Av. Venezuela, 82, CEP 20081-310, Rio de Janeiro, Brazil
Received December 4, 2001; revised March 19, 2002; accepted March 19, 2002
Those drawbacks strongly induced the search for a one-step
process using a catalyst that may perform condensation,
dehydration, and reduction simultaneously. Irreversibly hy-
drogenating MO to MIBK would favorably shift the equi-
librium of the other two previous steps. Furthermore, rapid
removal of mesityl oxide from the product mixture before
it can enter into further condensations prevents the forma-
tion of high-boiling products (8).
Several patents can be found on the use of bifunctional
catalysts for the one-step synthesis of MIBK in liquid
phase using palladium as the active metal (9–16). Various
acidic and basic supports have been used, such as KOH–
Al2O3 (9), MgO–SiO2 (10), CaO–MgO–SrO–Al2O3 (11),
Nb2O5 (12, 17), Zr3(PO4)2 (1, 5), ZrO(OH)2–C (13), Ce,
Hf, and/or Ta oxides–C (14), resin (15), C (+ resins) (16),
and Nb2O5 + SiO2 (18).
The synthesis of MIBK was studied over Pt supported on NaX
and CsX zeolites. The activity increased as both the temperature
and the H2/Ac ratio were increased. Temperature had also a ben-
eficial effect on the selectivity to MIBK but the H2/Ac ratio had
an opposite effect. Activities and selectivities were always higher
for Pt/NaX than for Pt/CsX. For Pt/NaX, increasing the reduction
temperature increased the activity but decreased the selectivity to
MIBK. Both catalysts were quite stable and selectivities to MIBK
of 70% were obtained over PtNaX at 613 K and H2/Ac = 0.5. The
absence of both strong acidic and strong basic sites and a proper
balance between metallic and basic sites are responsible for the be-
c
havior observed.
ꢀ 2002 Elsevier Science (USA)
Key Words: acetone; methyl isobutyl ketone; Pt/X zeolites, one-
step process.
INTRODUCTION
When the reaction is carried out in a batch, the rate of
reaction decreases very rapidly due to water buildup in
the system (17, 18). This limitation can be overcome in a
continuous system. In such a case, process conditions often
comprise temperatures in the range 393–433 K, pressures
between 20–50 bar, and hydrogen/acetone molar ratios of
0.15–0.4, and very stable operation can be achieved (1, 17,
18). Although very high selectivities to MIBK (>90%) at
acetone conversions of about 30–40% have been reported,
the high operating pressures are a limitation of the liquid-
phase single-step process (19).
So, there has recently been a trend for gas-phase single-
step processes operating at atmospheric pressure. Reports
can be found on the use of Pd supported on SAPO-34
(20), SAPO-11 (6), AlPO4-11 (6), calcined hydrotalcites
(21), ZSM-5 (22–25), and Na–MgO (4); of Pt supported on
ZSM-5 (7, 25–27); of Ni supported on MgO (19), ALPON
(28), Al2O3 (29), calcined hydrotalcites (20), and ZSM-5
(30); and of Cu supported on MgO (3). The results show
that selectivities to MIBK in gas phase are usually below
80% (although Narayanan and Unnikrishnan (29) have re-
ported values as high as 95%, they did not measure gaseous
products), especially due to the parallel hydrogenation of
acetone leading ultimately to propene. However, the ma-
jor limitation seems to be the catalyst stability. Whenever
Methyl isobutyl ketone (MIBK) is the most important
product obtained from acetone (Ac). It is used mainly as
a solvent for paints, inks, lacquers, and protective coating
systems (1, 2), but it is also used in the extraction of inor-
ganic salts (3) and as a reagent in dewaxing mineral oils (4).
It is produced commercially by a three-step process in liq-
uid phase: (i) aldol condensation of acetone to diacetone
alcohol (DAA) over basic (Ba(OH)2, NaOH, KOH, and
Ca(OH)2) or acidic catalysts; (ii) acid (H3PO4, H2SO4)-
catalyzed DAA dehydration to mesityl oxide (MO); and
(iii) selective hydrogenation of MO to MIBK over nickel
or copper chromite or over noble metal catalysts (1, 3, 5–7).
The two first steps are conventional homogeneous pro-
cesses and are usually equilibrium limited and the third
one may yield a relatively large amount of less useful
methyl isobutyl carbinol (MIBC). Furthermore, these pro-
cesses require many separation and neutralization steps,
present corrosion problems, and represent a significant risk
to the environment, and the operational costs are high.
1 Present Address: Instituto Nacional de Tecnologia (INT), Av.
Venezuela, 82, CEP 20081-310, Rio de Janeiro, Brazil.
2 To whom correspondence should be addressed. Fax: (55-21) 25628300.
E-mail: monteiro@peq.coppe.ufrj.br.
166
0021-9517/02 $35.00
c
ꢀ 2002 Elsevier Science (USA)
All rights reserved.

SYNTHESIS OF MIBK OVER Pt/X
167
reported, it is always very poor (7, 24–26, 29) over acidic Catalyst Characterization
supports, since on these supports coke formation leads to
A detailed description of the characterization of the
present catalysts is beyond the scope of this work and will be
given in another paper, under preparation. The information
given here is that necessary for the proper interpretation of
the catalytic results.
fast deactivation of the catalyst. On mildly acidic or basic
supports, reported catalyst stabilities (4, 6, 21) were some-
what better, but still limited. In all of those gas-phase stud-
ies, the temperature of reaction was kept below 473 K so
as to minimize further condensation reactions leading to
the formation of high-boiling point products and catalyst
deactivation. The only exception seems to be the work of
Chika´n et al. (3), who reported a reasonable constant MIBK
yield over the course of 24 h TOS (conversion decrease was
counterbalanced by increasing MIBK selectivity along the
run) at 553 K on Cu/MgO catalyst. Trying to suppress the
deactivation by increasing the hydrogen/acetone molar ra-
tio increases the hydrogenation of acetone to isopropanol,
thus reducing the yield of methyl isobutyl ketone. On the
other hand, a too-low hydrogen/acetone ratio not only con-
tributes to catalyst deactivation, it also limits acetone con-
version. The production of MIBK requires 1 mol of hydro-
gen for every 2 mol of acetone (not taking into account the
direct hydrogenation of acetone), so that hydrogen is the
limiting reactant for hydrogen/acetone ratios below 0.5.
We have previously reported the formation of MIBK
over CsX zeolites at 563 K and hydrogen/acetone =
6.2 (molar ratio) (31). Since no data could be found on
the one-step synthesis of MIBK from acetone over Pt on
mildly basic supports in gas phase, the goal of this work
was to study the effect of hydrogen/acetone ratio, reaction
temperature, and metal particle size on this reaction over
platinum (∼1 wt%) supported on X zeolites with Na and
Cs as compensating cations. Expectedly, the use of a mildly
basic support could overcome the stability limitations men-
tioned before.
The chemical composition was determined by atomic ab-
sorption (Na, Pt, Si, and Al) and atomic emission (Cs)
spectrometry on a Perkin–Elmer AAS 1100B spectropho-
tometer. 29Si-, 27Al-, and 133Cs-MAS–NMR spectra were
recorded on a Bruker DRX-300 spectrometer with a 7-mm
zirconia rotor spun at 5 kHz for Si and Al and at 14 kHz for
Cs. Spectra were taken at 59.6 MHz for Si, 78.2 MHz for
Al, and 39.4 MHz for Cs. BET specific surface areas and
micro- (t-plot) and mesopore (BJH) volumes were deter-
mined by N2 adsorption at 77 K on a Micromeritics ASAP
2000. The crystallinity of the samples was determined by
XRD using a Rigaku Dmax Ultima+ diffractometer with
CuKα radiation, 40 kV and 40 mA.
H2 chemisorption (Micromeritics ASAP 2900 C) and
transmission electronic microscopy (JEOL 2010 micro-
scope) were used to determine metal particle size and dis-
persion. For TEM analyses, sections with thickness around
80–90 nm were obtained with a microtome. The particle
size as given by TEM was taken from various micrographs.
About 700 particles were examined for each sample and
the average diameter, assuming spherical particles, was de-
termined from (32)
ꢀ
ni × di3
d = ꢀ
,
[1]
ni × di2
where ni is the number of particles with diameter di .
Dispersions and particle sizes were related by (33)
EXPERIMENTAL
1
D
d =
.
[2]
Catalyst Preparation
The parent sample was a NaX zeolite (Si/Al = 1.2) ob-
tained from IPT (Instituto de Pesquisas Tecnolo´gicas, Sa˜o
Paulo, Brazil). It was ion exchanged (Cs+/Na+ = 1.5) with a
0.85MCsClsolution(sampleCsX)and/orwitha5 × 10−3 M
Pt(NH3)4Cl2 solution to give samples PtCsX and PtNaX,
respectively. The solutions were added dropwise under stir-
ring for 6 h to an aqueous suspension with 4 wt% solids at
353 K and kept at room temperature for 20 h. After fil-
tration and washing until no chloride was detected in the
filtrate, the samples were dried overnight at 393 K. Next,
they were calcined under pure O2 (1.0 L/g/min) at 1 K/min
up to 633 K and kept at the final temperature for 2 h. Then,
IR spectra were recorded with a Perkin–Elmer FTIR
2000 spectrophotometer at room temperature. Self-
supported wafers were used to investigate surface hydrox-
yls after drying at 633 K for 2 h under He and evacuation
at 10−4 Torr for 1 h at this same temperature. Acid sites
were probed by pyridine adsorption at 423 K followed by
evacuation at 10−4 Torr for 30 min at this temperature. All
spectra were taken at room temperature.
Reaction Conditions
The gas-phase reaction was carried out in a fixed-bed re-
the calcined samples were reduced under H2 at 5 K/min up actor at atmospheric pressure. Various independent runs
to 633 or 773 K. Three samples were obtained: (i) Pt6/X6 at different temperatures, space velocities, and H2/Ac mo-
(PtNaX calcined and reduced at 633 K), (ii) Pt7/X6 (PtNaX lar ratios were performed. The reactants were fed to the
calcined at 633 K and reduced at 773 K), and (iii) Pt6/CsX6 reactor by bubbling hydrogen through a saturator held at
(PtCsX calcined and reduced at 633 K).
the desired temperature. The exit stream was analyzed by

168
MATTOS, NORONHA, AND MONTEIRO
TABLE 1
online gas chromatography with a Carbowax 20 M column
and a flame ionization detector.
The conversion of acetone (XA) and the selectivity (S) to
the various products were defined on the basis of converted
acetone (5, 8).
BET Areas and Micropore Volumes as Determined
by N2 Adsorption
BET area
(m2/g)
Micropore volume
(cm3/g)
Sample
yM + 2 × yD + 3 × yT
NaX
CsX
Pt6/X6
Pt7/X6
Pt6/CsX6
656
490
660
650
450
0.30
0.22
0.30
0.31
0.20
XA
=
,
[3]
[4]
yA + yM + 2 × yD + 3 × yT
yM, 2 × yD or 3 × yT
S =
,
yM + 2 × yD + 3 × yT
where yA, yM, yD, and yT are, respectively, the mole frac-
tions of acetone, monomers (IPA, C3), dimers (DAA, IMO,
MO, MIBK, MIBC), and trimers (DIBK, DMHA) in the
product stream.
Accordingly, the activities were also defined on the basis
of converted acetone (mol Ac/h/gcat), as follows.
XRD analyses confirmed that the crystallinity of the sam-
ples was preserved along the various treatments.
The results obtained from H2 chemisorption and TEM
are shown in Table 2. The particle size distribution for sam-
ples Pt6/X6 and Pt7/X6 are presented in Fig. 1. The metal
particle size was not significantly affected by the nature of
the compensating cation but the particles became larger
when the temperature of reduction was increased from 633
to 773 K.
Most of the particles were located within the zeolite crys-
tal, most probably causing a local disruption of the frame-
work, small enough not to be detected by the textural anal-
ysis. Although only a relatively small fraction of the metal
particles (the largest ones, between 3.0 and 5.0 nm) were
on the external surface, particularly for the sample reduced
at the lower temperature, they contributed heavily to the
overall dispersion and average particle size.
XA × WHSV
Overall activity, Ao =
.
[5]
[6]
58
Activity for acetone hydrogenation, Ah = Ao × SM.
Activity for acetone condensation, Ac = Ao × (SD + ST).
[7]
Conversion, selectivity, and activity values reported for
each run refer to those taken at steady conditions, typically
at 180 min TOS.
RESULTS
Figure 2 shows the results obtained from infrared anal-
yses in the hydroxyl region. For both samples NaX and
PtNaX after calcination only a band around 3690 cm−1 was
observed, which is attributed to the interaction of water
with the sodium cation (35). This band was not observed for
sample CsX, since the interaction of this cation with water is
much weaker (36). No bands associated to hydroxyl groups
could be seen either before or after the ion-exchange and
calcination steps, confirming the absence of structural de-
fects and of bridged hydroxyls.
Characteristics of the Samples
The unit cells of the calcined samples were Pt0.62Na83.8
Al85Si107O384 (0.90 wt% Pt) and Pt0.91Cs42.2Na41Al85Si107
O384 (0.98 wt% Pt). 29Si-MAS–NMR gave SAR (silica/
alumina ratio) values of 2.3–2.4 for all the calcined and re-
duced samples used in this work, a value very close to that
of the bulk composition (2.4).
Absence of extraframework aluminum species was con-
firmed by 27Al-MAS–NMR. A signal at 31 ppm in the 133Cs-
MAS–NMR spectrum of the cesium-containing sample af-
ter calcination is due to Cs cations in the zeolite cavities
(34).
The IR results for sample Pt6/X6 obtained after pyri-
dine adsorption are presented in Fig. 3. The band related to
Brønsted acid sites could hardly be seen at 1540 cm−1, in-
dicating that even the reduced samples had a very small
VariousTPD–, TPR–, andTPO–MSexperimentsshowed
no signals that could be attributed to chlorine species. The
same applies to XPS and XRF analyses.
TABLE 2
All samples exhibited type I isotherms, characteristic of
microporous materials. No mesopores were observed. The
BET areas and the micropore volumes were not affected
by the various treatments (Table 1). For the sodium sample
those values were within the range 650–660 m2/g and 0.30–
0.31 cm3/g, respectively. For the cesium samples the corre-
sponding values were 450–490 m2/g and 0.20–0.22 cm3/g,
since the bulky cesium cations limit the space available
within the cavities.
Dispersion (D) and Average Particle Size (d) as Given by H2
Chemisorption and TEM
H2 chemisorption
D (%) d (nm)
TEM
Sample
d (nm)
D (%)
Pt6/X6
Pt7/X6
Pt6/CsX6
35
25
30
2.8
4.0
3.3
3.1
3.7
2.5
32
27
40

SYNTHESIS OF MIBK OVER Pt/X
169
FIG. 1. Particle size distribution as determined from TEM micrographs for samples calcined at 633 K and reduced at (a) 633 K (sample Pt6/X6)
and (b) 773 K (sample Pt7/X6).
Brønsted acidity. Only bands due to the interaction of 2,6-dimethyl-2-hepten-4-ol (DMHA). Formation of these
pyridine with the cations could be detected at 1439 and products can be explained by means of the following re-
1484 cm−1
.
action scheme (Fig. 4). DAA formed by the aldol conden-
sation of acetone on basic sites is dehydrated to mesityl
oxide (MO) and its isomer (IMO), which are hydrogenated
to MIBK on metallic sites. MIBK may go through addi-
tional condensation and hydrogenation steps, ultimately
leading to DIBK and DMHA, or it may be hydrogenated
to MIBC. Along a parallel reaction, acetone can also be
directly hydrogenated to IPA, which is dehydrated and hy-
drogenated to propane. Therefore, acetone transformation
occurred by two main routes: (i) hydrogenation of acetone,
Reaction Products
The products resulting from acetone (Ac) transforma-
tion over all the samples were propane (C3), isopropyl
alcohol (IPA), methyl isobutyl ketone (MIBK), methyl
isobutyl carbinol (MIBC), diisobutyl ketone (DIBK), and
FIG. 2. Infrared spectra for samples NaX, PtNaX (after calcination),
FIG. 3. Infrared spectrum of adsorbed pyridine for the sample cal-
and CsX in the hydroxyl region.
cined and reduced at 633 K (sample Pt6/X6).

170
MATTOS, NORONHA, AND MONTEIRO
TABLE 3
Effect of H2/Ac Ratio on the Overall Activity (Ao), on the Activ-
ity for Acetone Hydrogenation (Ah), and on the Activity for Aldol
Condensation of Acetone (Ac) (T = 563 K)
H2/Ac
Ao
(molar WHSV XA (mol/h/
Ah
(mol/h/g
Ac
(mol/h/g
Sample
ratio) (h−1
)
(%)
g
cat
)
)
)
cat
cat
Pt6/X6
Pt6/X6
Pt6/CsX6
Pt6/CsX6
0.5
6.2
0.5
6.2
106
295
27
6.7
9.3
7.4
7.8
0.12
0.47
0.035
0.070
0.045
0.415
0.017
0.059
0.076
0.055
0.018
0.011
50
and DMHA. On the other hand, MIBC production was not
favored. Moreover, the results (Fig. 5) show that increasing
the H2/Ac ratio decreased the C3/IPA ratio, and that the
selectivity to both of them increased.
FIG. 4. Main reactions observed during acetone transformation.
producing IPA and C3, and (ii) aldol condensation of ace-
tone, forming MIBK and other condensation products.
DAA, MO, IMO, propene, isobutene, and heavier com-
pounds (phorone, isophorone, isoxylitones, etc.) were not
observed among the products.
Effect of Reaction Temperature
Taking into account the results reported in the previous
section, the influence of temperature was studied at H2/
Ac = 0.5 for the samples reduced at 633 K.
Now, for the sample in the sodium form, increasing the
reaction temperature from 563 to 613 K at a conversion of
about 10% increased both the overall activity and the activ-
Effect of H2/Ac Ratio
For both samples reduced at 633 K, increasing the H2/Ac ity for aldol condensation of acetone, but decreased its rate
ratio at nearly isoconversion at 563 K had a positive effect of hydrogenation (Table 4). For the sample in the cesium
on the overall activity (Table 3), since the direct hydrogena- form, this behavior held for the overall and for the con-
tionofacetoneincreasedsignificantly. However, theactivity densation activity as the temperature was raised, but now
for the aldol condensation reaction slightly decreased. As also the hydrogenation activity showed a slight increasing
a consequence, the selectivity to MIBK (Fig. 5) was signif- trend. For this sample, very limited activity for condensa-
icantly reduced, particularly for the sample in the sodium tion was observed below 473 K, a range of temperatures
form. The same was observed for the selectivities to DIBK usually employed when acid catalysts are used.
FIG. 5. Effect of H2/Ac molar ratio on the selectivity at 563 K: (a) Pt6/X6 and (b) Pt6/CsX6.

SYNTHESIS OF MIBK OVER Pt/X
171
TABLE 4
TABLE 5
Effect of Reduction Temperature (Tred) on the Overall Activity
Effect of Reaction Temperature on the Overall Activity (Ao), on
the Activity for Acetone Hydrogenation (Ah), and on the Activity (Ao), on the Activity for Acetone Hydrogenation (Ah), and on the
for Aldol Condensation of Acetone (Ac) (H2/Ac = 0.5)
Activity for Aldol Condensation of Acetone (Ac) for Sample PtNaX
Calcined at 633 K (H2/Ac = 0.5)
Ao Ah
Ac
WHSV
XA
(%)
(mol/h/ (mol/h/
(mol/h/
Ao
(mol/h/
Ah
(mol/h/
Ac
(mol/h/
Sample
T (K)
(h−1
)
g
cat
)
g
)
g
cat
)
Tred
(K)
T
(K)
WHSV
XA
(%)
cat
Sample
(h−1
)
g
)
g
)
g
)
cat
cat
cat
Pt6/X6
Pt6/X6
Pt6/CsX6
Pt6/CsX6
Pt6/CsX6
Pt6/CsX6
563
613
413
473
563
613
106
154
6
13.5
27
6.7
7.2
12.6
13.0
7.4
0.120
0.190
0.013
0.030
0.035
0.076
0.045
0.029
0.013
0.025
0.017
0.025
0.076
0.161
Pt6/X6
Pt7/X6
Pt6/X6
Pt7/X6
633
773
633
773
563
563
613
613
106
159
181
242
6.7
6.7
5.6
5.0
0.12
0.18
0.17
0.21
0.045
0.108
0.023
0.042
0.076
0.072
0.147
0.168
2.26 × 10−4
0.005
0.018
0.051
54
8.3
Also, for both samples the increase in the reaction tem-
perature favored the dehydration of IPA (Fig. 6). This had
a positive effect on C3 selectivity for sample Pt6/CsX6 but
not for sample Pt6/X6, since for the latter the enhanced de-
hydration of IPA did not overcome the reduction of the ac-
tivity along the hydrogenation route when the temperature
was increased. Only trace amounts of C3 were observed at
or below 473 K.
Whatever the reaction temperature (T ), the overall rate
of reaction increased when the metal was reduced at a
highertemperature(Table5). AcloserlookatTable5shows
that indeed at each reaction temperature the activity for al-
dol condensation was hardly affected. On the other hand,
the rate of acetone hydrogenation over the sample reduced
at 773 K is about twice as large as that over the sample
reduced at 633 K, for both temperatures of reaction.
The selectivities to the various products was affected
in an analogous manner. The selectivity to those formed
along the route of acetone hydrogenation (IPA and C3) was
larger for the sample reduced at 773 K, while those origi-
nating from acetone condensation (MIBK, MIBC, DIBK,
DMHA) were selectively obtained over sample Pt6/X6
(Fig. 7).
The selectivity to MIBK was also favored by higher tem-
peratures. As a consequence, large selectivities to MIBK
∼
( 70%) were obtained over Pt6/X6 at 613 K and H /Ac =
=
2
0.5, since at this temperature the hydrogenation of ace-
tone to IPA was largely inhibited. However, further trans-
formation of MIBK also took place, thus reducing the
MIBK/(MIBC + DIBK + DMHA) ratio from 6 to 4 for
Pt6/X6 and from 5 to 3 for Pt6/CsX6.
Effect of the Compensating Cation
Effect of Reduction Temperature
For all reaction conditions, the introduction of cesium
decreased the overall activity, as can be seen from the data
in Tables 3 and 4. The activity for acetone hydrogenation
The influence of metal dispersion was explored for the
sample in the sodium form at H2/Ac = 0.5.
FIG. 6. Effect of reaction temperature on the selectivity for H2/Ac = 0.5: (a) Pt6/X6 and (b) Pt6/CsX6.

172
MATTOS, NORONHA, AND MONTEIRO
FIG. 7. Effect of reduction temperature on the selectivity over Pt6/X6 and Pt7/X6 for H2/Ac = 0.5: (a) reaction temperature = 563 K and
(b) reaction temperature = 613 K.
of the Cs-containing sample was less sensitive to hydrogen Stability
partial pressure than that of the sample in the sodium form
The stability of sample Pt6/X6 was evaluated at two dif-
(Table 3), and it was less affected by increasing reaction
ferent conditions and the catalyst was rather stable in both
temperatures (Table 4). It was also observed that the de-
conditions after a small initial transient period. Figure 8
hydration reactions were more significant over Pt6/X6 than
shows the results obtained. When the reaction was run at
over Pt6/CsX6 (Fig. 5); as a consequence, C3/IPA ratios
563 K and H2/Ac = 6.2, the overall activity increased along
were larger over the former (Fig. 5). Moreover, selectivities
the first 120 min on stream, essentially due to an increase
to MIBK were always smaller over Pt6/CsX6 compared to
in the hydrogenation activity, as can be observed from the
Pt6/X6.
selectivity results in Fig. 9. At 613 K and H2/Ac = 0.5, a
slight decrease in the activity was observed in the begin-
ning of the run, but the catalyst became quite stable after
about 180 min time on stream (TOS). On the other hand the
FIG. 9. Selectivity as a function of time on stream (H2/Ac = 6.2, re-
FIG. 8. Activity as a function of time on stream for sample Pt6/X6.
action temperature = 563 K).

SYNTHESIS OF MIBK OVER Pt/X
173
Contrary to what is observed on acid catalysts (7, 26),
the hydrogenation of acetone was not the controlling step
along route (i), since both IPA and C3 were detected in
most cases (Figs. 5–7). Similar results have been observed
over other basic supports (3, 4, 21). The results in Fig. 5
confirm that the dehydration of IPA was slower than the
hydrogenation of acetone since the C3/IPA ratio decreased
as the H2/Ac ratio was increased, at 563 K. IPA dehydration
was rapid enough only at 613 K over Pt6/X6 (Figs. 6 and 7).
This reaction was always more important over Pt6/X6 than
over Pt6/CsX6. This is probably due to the fact that Na+
cations are stronger Lewis acidic sites than are Cs+ cations.
Although the protons derived from the reduction of Pt2+
cations may have contributed to this dehydration, the role
of the compensating cation and the absence of C3 at or
below 473 K suggests that this effect, if present, was not
significant.
FIG. 10. Selectivity as a function of time on stream (H2/Ac = 0.5,
reaction temperature = 613 K).
The effect of temperature on the activity for the two
routes (Table 4) clearly indicates that the activation energy
of the condensation reaction is higher than that of acetone
hydrogenation, as also observed by Yang and Wu (6) and
Chen et al. (21). The results from Table 4 show that reduc-
ing the metal at a higher temperature increased the hydro-
genating activity but not the activity for aldol condensation,
thus indicating that the condensation of two molecules of
acetone over basic sites is the limiting step of MIBK pro-
duction along route (ii). This suggests that the amount of
platinum used (1.0 wt%) may be reduced, since mesityl
oxide hydrogenation proceeded readily over this catalyst.
Other metals, like Pd, which has a relatively low activity for
selectivitiestothevariousproductsremainedquiteconstant
along the run, as can be seen from Fig. 10.
DISCUSSION
Bifunctional metal/acid catalysts are used in a variety of
commercialprocesses, especiallyinrefiningandpetrochem-
icals production, such as reforming, hydrocracking, hydro-
isomerization of parafins, and isomerization of aromatics.
Bifunctional catalysts usually work under milder condi-
tions than monofunctional ones, their deactivation is much
slower and they can perform several steps simultaneously
(37). On the other hand, metal catalysts on nonacidic sup-
ports have recently attracted much attention, particularly
since Pt/KL zeolite catalysts have been shown to be very
active and selective for the aromatization of n-hexane (38).
Bifunctional catalysts have also a large potential of appli-
cation in the field of intermediates and fine chemicals, since
in these cases a number of successive steps are usually in-
volved. Such is the case for MIBK production, as illustrated
in Fig. 4. Actually, many other side reactions and reversible
steps could have been drawn (6, 7, 26, 31); their relative
contributions to the overall product distribution depend
on the catalyst and operational conditions.
==
C O hydrogenation, and other basic zeolites should also
be tested. The facile hydrogenation of the double bond of
the mesityl oxide molecule has been observed over various
metals on different supports (18, 26, 30).
The aldol condensation of two molecules of acetone over
a solid base can be represented by the following steps
(39, 40).
Step A: H+ abstraction from acetone by the base, form-
ing an enolate anion.
O
O
–ꢀ –
– ꢀ–
−
+
ꢁ
ꢀ
CH3 C CH + B
CH C CH2 + H B
3
3
The absence of DAA in the present work showed the
facility of dehydration of this tertiary alcohol. On the other
hand, the absence of mesityl oxides and propene and the
presence of MIBC, DIBK, and DMHA is evidence of the
hydrogenating capacity of the catalysts studied, particularly
–
CH3 C CH2
O
−
Step B: Nucleophilic addition of the enolate ion to the
carbonyl group of another acetone molecule forming a new
C–C bond and an alkoxide ion.
==
for the C C double bond. The absence of heavier products,
contrary to what has been reported by other authors who
used basic supports (3, 4, 21), may be related to both a
proper balance between the metallic and the basic functions
of our catalysts and the steric restraints imposed by the
channel system of the zeolite.
O
O
CH3
CH3
ꢀ
ꢀ
−
−
ꢁ
ꢀ
CH –C–CH + C O
CH –C–CH –C–O
3
3
2
2
CH3
CH3

174
MATTOS, NORONHA, AND MONTEIRO
Step C: Reaction of the alkoxide ion with H+ to give A too-basic support induces additional condensation reac-
diacetone alcohol (DAA).
tions, creating the heavier products responsible for catalyst
deactivation (3, 4). An additional benefit of mildly basic
supports is the possibility of using higher temperatures of
reaction, thus favoring the desorption of undesirable inter-
mediates like trimers, eventually formed in small amounts,
and thus avoiding their further condensation. The desorp-
tion of products is also favored from the large pores of
O
O
CH3
CH3
ꢀ
ꢀ
−
+
ꢁ
ꢀ
CH –C–CH –C–O + H B CH –C–CH –C–OH + B
3
2
3
2
CH3
CH3
In the case of zeolites the basic sites are the oxygen atoms zeolite X.
of the framework.
When acidic zeolites are chosen, neither large pore struc-
Since all of those steps are reversible, as is the dehydra- tures nor high temperatures can be used, since coke forma-
tion of DAA to MO (8), good yields of MIBK can only be tion is very rapid. Although HMFI has been suggested (25)
attained if hydrogenation of MO proceeds at a rate suffi- and extensively used for this reaction, its stability is also
cient enough to shift the equilibrium of the previous steps. very poor, even when its acidity is reduced (27).
This hydrogenation activity should also be selective for the
C C double bond, since the hydrogenation of the carbonyl time as mildly basic materials (41). Our samples had no
group of either acetone or MIBK is undesirable. significant concentration either of strong basic sites (as fur-
Alkali-exchanged X zeolites have been known for a long
==
Our results (Table 5) show that acetone hydrogena- ther supported by the absence of products derived from
tion was more intense over the sample reduced at 773 K Michael condensation reactions such as isophorone) or of
(Pt7/X6).
strong acidic sites (as shown by IR, by their limited activ-
Characterization of samples Pt6/X6 and Pt7/X6 by H2 ity for IPA dehydration, and by the absence of isobutene
chemisorption and TEM (Table 2) indicated that the metal among the reaction products).
particles were larger for the sample reduced at the higher
In this work, very good selectivities to MIBK (∼70%)
temperature. For this sample, most particles had a size of were obtained over the catalyst calcined and reduced at
about 1.6–4.0 nm and many of them were on the outer 633 K, when the reaction was carried out at 613 K and
surface, thushavingalimitedinteractionwiththebasicsites, H2/Ac = 0.5 (molar ratio). Since selectivities were defined
mostly located within the channels. The reverse was true on the basis of molecules of acetone consumed and the main
for sample Pt6/X6, for which most particles had diameters by-product (C3) is gaseous, the corresponding molar frac-
between 0.0 and 2.4 nm and were located in the porous tion of MIBK in the liquid product is about 0.85. Further-
structure, close to the basic sites.
more, this high selectivity was obtained under conditions
Yang and Wu (6) and Gand´ıa et al. (28) suggested that sig- for which the catalyst activity was about 0.19 mol Ac/h/gcat
,
nificant yields of MIBK depend on an adequate balance be- which gives a yield of MIBK of 66 × 10−3 mol/h/gcat
.
tween basic and metallic sites. In the present work, acetone
When this reaction is performed in one step in liquid
hydrogenation was favored when the number of metallic phase over Pd supported on basic carriers, the yield of
particles on the outer surface was large (sample reduced at MIBK is impractical (5). When Pd supported on solid acids
773 K) and the ratio between metallic and basic sites was was used (1, 5, 17, 18), stable continuous operation in liquid
also large. On the other hand, a proper balance between phase has been reported with MIBK selectivities above
those two types of sites led to a larger selectivity to MIBK 90%, but the pressures used were rather high. The reported
when the sample was reduced at 633 K and most metallic activities ranged between about 4 × 10−3 (17) and 60 × 10−3
particles remained in the zeolite porous system.
(1) mol Ac/h/gcat. On the other hand, when the reaction
When samples Pt6/X6 and Pt6/CsX6 are compared was carried out in gas phase over acidic supports, either
(Tables 3 and 4), both the hydrogenation activity and the the activities were in the lower end of the range mentioned
condensation activity are lower for the Cs-containing sam- before or the stability was very poor. Most often, both
ple. Since this sample is more basic, the decreased conden- effects were observed. Over basic supports in gas phase
sation activity can be related to the geometric constraints (3, 4, 6, 19, 21, 28), activities and stabilities are usually supe-
imposed by the bulky cesium cations. As the size and lo- rior to those over acidic supports. Chen et al. (21) reported
cation of the metal particles were similar in both samples, surprisingly high activities over Pd supported on calcined
this same reasoning can be used to explain the decreased hydrotalcites at 413 K (3.2 mol Ac/h/gcat), but the simulta-
hydrogenation activity of Pt6/CsX6. However, in this case, neous formation of heavier products led to limited stability.
an electronic effect may also have played a role, since the Chika´n et al. (3) reported quite stable MIBK yields for 24 h
particles supported on the more basic cesium-containing over Cu/MgO catalysts at 553 K, but the activity of their
sample should be more electron rich.
catalyst was relatively low. Although they did not disclose
Finally, the good stability of the samples used in this work appropriately their experimental conditions, apparently
can be related to the proper basicity of the supports used. both results derive from the use of diluted (with He) feeds.

SYNTHESIS OF MIBK OVER Pt/X
175
Lin and Ko (4) observed an activity of about 0.10 mol
ACKNOWLEDGMENTS
Ac/h/gcat and reasonably stable MIBK yield for 8 h TOS
over Pd supported on magnesium oxide doped with sodium.
The Ni/MgO catalyst of Gand´ıa and Montes (19) showed
very little stability while no data are reported on the deac-
We are grateful to Dr. Joa˜o Poc¸o (IPT—Instituto de Pesquisas Tec-
nolo´gicas, Sa˜o Paulo, Brazil) for the NaX zeolite supplied and to IRC
(Institut de Recherches sur la Catalyse/France) for TEM analyses. One
of the authors (Lisiane V. Mattos) acknowledges the scholarship received
tivation of the Ni/ALPON used by Gand´ıa et al. (28). All from CNPq.
of those basic supports are very strong and this often re-
sulted in good activity but poor stability. To the best of our
knowledge, the present work is the first one to use Pt over a
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