Efficient hydrodeoxygenation of biomass-derived ketones over bifunctional Pt-polyoxometalate catalyst

Article abstract of DOI:10.1039/c2cc33189f

Acidic heteropoly salt Cs_2.5H_0.5PW₁₂O ₄₀ doped with Pt nanoparticles is a highly active and selective catalyst for one-step hydrogenation of methyl isobutyl and diisobutyl ketones to the corresponding alkanes in the gas phase at 100 °C with 97-99% yield via metal-acid bifunctional catalysis.

Full text of DOI:10.1039/c2cc33189f

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COMMUNICATION
Efficient hydrodeoxygenation of biomass-derived ketones over
bifunctional Pt-polyoxometalate catalystw
Mshari A. Alotaibi, Elena F. Kozhevnikova and Ivan V. Kozhevnikov*
Received 3rd May 2012, Accepted 22nd May 2012
DOI: 10.1039/c2cc33189f
Acidic heteropoly salt Cs2.5H0.5PW12O40 doped with Pt nano-
particles is a highly active and selective catalyst for one-step
hydrogenation of methyl isobutyl and diisobutyl ketones to the
corresponding alkanes in the gas phase at 100 8C with 97–99%
yield via metal-acid bifunctional catalysis.
Table 1 Hydrogenation of MIBKa
Selectivity [%]
MP
MP-ol
Other
Catalyst
Conversion [%]
0.5%Pt/SiO2
10%Pt/C
64
95
52
3
95
84
96
o1
92
14
6
86
93
0
1
4
0
1
1
3
0
0
10%Pt/Cb
0.5%Pd/SiO2
10%Pd/C
87
0
6
9
100
93
Efficient utilization of biomass resources is a major goal for
academia and industry.1 It is expected that biomass-based
n-butanol will play a major role in the next generation of
biofuels together with ethanol and biodiesel.2 Biobutanol can be
produced by acetone-butanol fermentation of carbohydrates
and cellulosic raw materials using Clostridium acetobutylicum
strain which yields butanol, acetone and ethanol in a weight
ratio of 6 : 3 : 1.3 Therefore, finding new outlets for acetone,
preferably in the transportation fuel sector, would greatly
improve the economy of biobutanol production. Acetone can
be transformed into C6, C9 and larger organic molecules by
aldol condensation. This is employed for the industrial synthesis
of methyl isobutyl ketone (MIBK) and diisobutyl ketone
(DIBK) (Scheme 1) that are used as solvents in paints. The
synthesis can be carried as one-pot process using a bifunctional
metal-acid catalyst, e.g., Pd on an acidic support.4 MIBK and
DIBK could be hydrogenated to alkanes, 2-methylpentane
(MP) and 2,6-dimethylheptane (DMH), which could be blended
with gasoline and used through the existing fuel infrastructure.
However, to the best of our knowledge, there is little data
available on catalytic hydrogenation of MIBK and DIBK.
Here we report a very efficient gas-phase hydrogenation
(hydrodeoxygenation) of MIBK and DIBK to MP and DMH,
using heterogeneous bifunctional metal-acid catalysis.
10%Pd/Cb
5%Ru/C
5%Cu/SiO2
94
2
5
95
0
0
100
100
c
2CuOÁCr2O3
a
At 100 1C, 0.20 g catalyst pretreated in H2 at reaction temperature
for 1 h, 3.6% MIBK in H2 flow, 20 ml minÀ1 flow rate, 4 h time on
b
c
stream. At 300 1C. Pre-reduced in H2 at 400 1C for 2 h; exhibited
XRD pattern of Cu metal (Supplementary Information).
First, we tested a number of conventional supported metal
catalysts comprising Pd, Pt, Ru and Cu supported on silica
and active carbon for MIBK hydrogenation in a continuous flow
fixed bed reactor (9 mm internal diameter) in the temperature
range 100–400 1C (for experimental details, see Supplementary
Informationw). Some representative results are shown in Table 1.
At 100–200 1C, these catalysts were active for the hydrogenation
of MIBK to 4-methyl-2-pentanol (MP-ol). Best results
showed 10%Pt/C, 10%Pd/C, 5%Ru/C and copper chromite,
with 93–100% MP-ol selectivity at 92–96% MIBK conversion
at 100 1C. The selectivity to MP-ol decreased with increasing
temperature due to formation of MP and hydrogenolysis of
C–C bonds giving C1–C5 hydrocarbons. The highest MP
selectivities 87–94% were observed for 10%Pt/C and 10%Pd/C
catalysts at 52–84% conversion, but only at a temperature as
high as 300 1C (Table 1), which indicates the difficulty of direct
hydrogenation of MP-ol to MP.
Next, we tested bifunctional metal-polyoxometalate catalysts
comprising Pt, Pd, Ru, or Cu supported on Keggin heteropoly salt
Cs2.5H0.5PW12O40 (CsPW). This salt is widely used as a solid acid
catalyst. It is a strong Brønsted acid, almost as strong as the parent
heteropoly acid H3PW12O40, with the important advantage over
the latter of having much larger surface area (111 m2 gÀ1) and a
higher thermal stability (B500 1C decomposition temperature),5
hence a longer lifetime.6 Our idea was to explore bifunctional
metal-acid catalysed pathway to achieve MIBK-to-MP hydro-
genation in one step on a single catalyst bed. This pathway
Scheme 1 MIBK and DIBK production from acetone.
Department of Chemistry, University of Liverpool, Liverpool,
L69 7ZD, United Kingdom. E-mail: kozhev@liverpool.ac.uk
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc33189f
c
7194 Chem. Commun., 2012, 48, 7194–7196
This journal is The Royal Society of Chemistry 2012

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Scheme 2 Hydrodeoxygenation of MIBK via bifunctional metal-acid
catalysis.
Table 2 Hydrogenation of MIBK over bifunctional metal-acid
catalystsa
Fig. 1 Time course for MIBK hydrogenation over 0.5%Pt/CsPW
(0.05 g) at 100 1C (3.6% MIBK in H2 flow, 100 ml minÀ1 flow rate, the
catalyst diluted with 0.15 g SiO2).
Selectivity [%]
MP
MP-ol
Other
Catalyst
Conversion [%]
0.5%Pt/CsPW
0.5%Pt/CsPWb
0.5%Pt/HZSM-5c
0.5%Pt/HZSM-5b,c
0.5%Pd/CsPW
0.5%Ru/CsPW
5%Ru/CsPW
99
95
94
100
7
100
88
65
83
0
0
34
0
0
0
0
12
1
17
66
0
34
5
100
100
100
99
o1
0
0
0
0
5%Cu/CsPW
a
At 100 1C, 0.20 g catalyst pretreated in H2 at reaction temperature
for 1 h, 3.6% MIBK in H2 flow, 20 ml minÀ1 flow rate, 4 h time on
b
c
stream. At 200 1C. Atomic ratio Si/Al = 12.
Fig. 2 Effect of MIBK concentration on the rate of MIBK hydro-
genation over 0.5%Pt/CsPW (0.025 g) at 100 1C (100 ml minÀ1 H2
flow rate, the catalyst diluted with 0.175 g SiO2).
would involve MIBK hydrogenation to MP-ol on metal sites
followed by MP-ol dehydration on acid sites to form olefin
and finally olefin hydrogenation to MP on metal sites
(Scheme 2). Since dehydration of secondary alcohols on
heteropoly acid catalysts is fast7 and so is hydrogenation of
CQC bond on platinum metals, the bifunctional pathway was
expected to be more efficient that the hydrogenation only
pathway. Our study proved that this was indeed the case.
Representative results are shown in Table 2. It can be seen
that amongst the bifunctional catalysts studied 0.5%Pt/CsPW
clearly stands out yielding 99% MP at 100 1C, with 100% MP
selectivity at Z 99% MIBK conversion (top entry). Impor-
tantly, no MP isomerisation was observed at this temperature,
thus allowing synthetically viable complete transformation of
ketone to alkane without carbon backbone alteration. At 200 1C,
about 12% of MP isomerised to form n-hexane, 3-methylpentane
and 2,3-dimethylbutane, in agreement with the literature.8
0.5%Ru/CsPW matched the Pt catalyst in MP selectivity, but
had much lower activity per unit metal weight. About 5% Ru
loading was required to match the activity of the 0.5% Pt catalyst.
0.5%Pd/CsPW was inferior to the Pt catalyst regarding both
catalytic activity and MP selectivity, giving a large amount of
heavy condensation products. Previously, Pd/CsPW has been
found to be active bifunctional catalyst for the one-pot hydro-
condensation of acetone to MIBK and further to DIBK.4a
Finally, 5%Cu/CsPW practically did not show catalytic activity
at 100 1C.
not change after reaction, thus demonstrating high stability
of Pt nanoparticles under reaction conditions.
The reaction rate practically did not change with MIBK
concentration in the gas feed (Fig. 2), which means that the
order in MIBK was close to zero. Therefore, MIBK conversion
is equivalent to the rate of MIBK consumption in our system.
The order in the catalyst was close to one, as shown in Fig. 3.
The apparent activation energy was found to be 50 kJ molÀ1 in
the temperature range 60–100 1C, which indicates that the
reaction occurred under kinetic control. From these results,
the turnover frequency (TOF) was estimated to be 160 hÀ1 per
exposed Pt atom at 100 1C.
Further, to prove the bifunctional mechanism (Scheme 2),
we tested the dehydration of MP-ol on 0.5%Pt/CsPW catalyst.
This was carried out at 100 1C under the conditions applied for
MIBK hydrogenation (Table 2), except with N2 (20 ml minÀ1
)
instead of H2. As expected, the reaction was very fast, yielding
two olefins, 4-methylpentene-1 and 4-methylpentene-2, in about
1 : 1 ratio at 100% conversion (eqn (1)). The same reaction
under H2 gave MP in a 100% yield (eqn (2)). These results,
The Pt catalyst showed excellent performance stability. It
reached steady state in about 1 h and operated without
deactivation for at least 14 h (Fig. 1). It should be noted that
to set in this run the conversion at about 80%, the amount of
catalyst had to be reduced fourfold and the flow rate increased
fivefold as compared to those in Table 2. Less than 1% of coke
was found in the catalyst after reaction. Pt dispersion, i.e.,
the fraction of exposed Pt atoms, which was 0.36 in the
fresh 0.5%Pt/CsPW catalyst (from H2 chemisorption), did
Fig. 3 Effect of catalyst weight on conversion for MIBK hydrogenation
over 0.5%Pt/CsPW at 100 1C (3.6% MIBK in H2 flow, 100 ml minÀ1
flow rate, 0.5%Pt/CsPW+SiO2 = 0.20 g).
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 7194–7196 7195

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Table 3 Effect of Pt loading in Pt/CsPW on MIBK hydrogenationa
Table 4 Hydrogenation of DIBKa
Pt loading [wt%]
Conversion [%]
MP selectivity [%]
Selectivity [%]
DMH DMH-ol Otherb
Catalyst
T/1C Conversion [%]
0.25
0.50
1.0
30
78
100
93
98
100
0.5%Pt/CsPW 100
300
100
96
97
92
0
0
3
8
c
2CuOÁCr2O3
a
Catalyst mixture 0.05 g Pt/CsPW+0.15 g SiO2, 100 1C, 3.6% MIBK
0.20 g catalyst, 1.7% DIBK in H2 flow, 20 ml minÀ1 flow rate, 4 h
a
in H2 flow, 100 ml minÀ1 flow rate, 4 h time on stream.
b
time on stream. Other products: C9 alkane isomers. Pre-reduced in
c
H2 at 400 1C for 2 h; exhibited XRD pattern of Cu metal.
therefore, support the bifunctional mechanism for MIBK hydro-
genation over 0.5%Pt/CsPW catalyst.
(1)
(2)
To determine the rate-limiting step in this mechanism, we
looked at the effect of Pt loading in Pt/CsPW catalyst on
MIBK hydrogenation (Table 3). As can be seen, MIBK
conversion increases from 30 to 100% with increasing Pt
loading from 0.25 to 1%. In contrast, the MP selectivity
practically does not change with Pt loading, being in the range
of 93–100%. This clearly shows that the reaction is limited by
the first step–hydrogenation of MIBK to MP-ol on Pt sites
(Scheme 2). Subsequent dehydration and hydrogenation steps
are fast under the chosen conditions. The fast dehydration of
MP-ol over Pt/CsPW can be explained by the strong acidity of
CsPW. Its acid strength corresponds to 164 kJ molÀ1 in terms
of the differential heat of NH3 adsorption.7b In this respect it
was interesting to test a Pt catalyst with weaker acid sites. As such
we tested 0.5%Pt/HZSM-5 with zeolite HZSM-5 as an acidic
support, possessing acid sites with a heat of NH3 adsorption of
145–150 kJ molÀ1.9 This catalyst at 100 1C gave MIBK conversion
close to that of 0.5%Pt/CsPW, but its MP selectivity (65%) was
lower than for Pt/CsPW due to incomplete dehydration of MP-ol
on the weaker acid HZSM-5 (Table 2). This shows that the strong
acidity of CsPW is essential for the high efficiency of the Pt/CsPW
catalyst in hydrodeoxygenation of MIBK. At 200 1C, Pt/HZSM-5
matched the performance of Pt/CsPW, but at this temperature
considerable isomerisation of MP took place (Table 2).
Fig. 4 Time course for DIBK hydrogenation over 0.5%Pt/CsPW
(0.05 g) at 100 1C (1.7% DIBK in H2 flow, 80 ml minÀ1 flow rate, the
catalyst diluted with 0.15 g SiO2).
very good durability, operating without deactivation for at
least 14 h. As determined with Pt/CsPW catalyst, the reactivity
of DIBK was 8 times lower than that of MIBK at 100 1C. This
can be explained by greater steric hindrance in the case of
DIBK hydrogenation.
In conclusion, we developed a very efficient catalyst Pt/CsPW
for selective one-step hydrodeoxygenation of biomass-derived
aliphatic ketones MIBK and DIBK under mild conditions without
isomerisation of carbon backbone via metal-acid bifunctional
mechanism.
Support from Salman Bin Abdulaziz University, Saudi Arabia
(PhD scholarship for M. Alotaibi) is gratefully acknowledged.
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Finally, we tested 0.5%Pt/CsPW catalyst for DIBK hydro-
genation (Table 4). For comparison, copper chromite was also
tested. The Pt catalyst again showed excellent performance
under the conditions similar to those applied for MIBK
hydrogenation. At 100 1C, it gave 97% selectivity to DMH at
100% conversion, with 3% of C9 alkane isomers also formed. No
2,6-dimethyl-4-heptanol (DMH-ol) was found amongst the reaction
products. Copper chromite, possessing weak acidity, performed
differently like in the case of MIBK reaction. At 100–200 1C, it gave
DMH-ol as the main product (90–92% selectivity at 74–80%
conversion). At 300 1C, it effected complete hydrogenation
to give DMH with 92% selectivity at 96% conversion, with 8%
selectivity to other C9 alkane isomers. Both catalysts, Pt/CsPW
at 100 1C (Fig. 4) and copper chromite at 300 1C, exhibited
c
7196 Chem. Commun., 2012, 48, 7194–7196
This journal is The Royal Society of Chemistry 2012