Vapor-phase dehydration of glycerol into hydroxyacetone over silver catalyst

Article abstract of DOI:10.1246/cl.2012.965

Silica-supported silver exhibited high catalytic activity in the dehydration of glycerol: glycerol was dehydrated into hydroxyacetone with the selectivity higher than 86% at 91% conversion over Ag/SiO₂ in H ₂ flow at 240°C. Silver metal provides an active site and showed stable catalytic activity for the glycerol dehydration in H₂ atmosphere, while the dehydration activity decreased in N₂ atmosphere. The hydrogenation of hydroxyacetone into 1,2-propanediol and the decomposition to ethylene glycol did not proceed over silver.

Full text of DOI:10.1246/cl.2012.965

doi:10.1246/cl.2012.965
Published on the web September 8, 2012
965
Vapor-phase Dehydration of Glycerol into Hydroxyacetone over Silver Catalyst
Satoshi Sato,* Daisuke Sakai, Fumiya Sato, and Yasuhiro Yamada
Graduate School of Engineering, Chiba University, Yayoi, Inage-ku, Chiba 263-8522
(Received June 26, 2012; CL-120684; E-mail: satoshi@faculty.chiba-u.jp)
Silica-supported silver exhibited high catalytic activity in the
O
−H2O
OH
dehydration of glycerol: glycerol was dehydrated into hydroxy-
acetone with the selectivity higher than 86% at 91% conversion
over Ag/SiO2 in H2 flow at 240 °C. Silver metal provides an
active site and showed stable catalytic activity for the glycerol
dehydration in H2 atmosphere, while the dehydration activity
decreased in N2 atmosphere. The hydrogenation of hydroxyace-
tone into 1,2-propanediol and the decomposition to ethylene
glycol did not proceed over silver.
HO
OH
OH
Ag in H2
Figure 1. Dehydration of glycerol into hydroxyacetone (HA).
Table 1. Synthesis of HA over metal catalysts of Group 11a
Selectivity/mol %b
Catalyst
Conversionb
/mol %
(metal content/wt %)
HA 12-PDO EG
Au/Al2O3
Au/SiO2
(5)
(5)
13.7
1.5
57.0
26.3
3.9
0
2.5
0
Catalytic conversion of carbon-neutral biomass to produce
valuable chemicals is expected to reduce damage to the environ-
ment.1,2 This is one of the answers to the problem of global
warming. Renewable biomass fuels, such as biodiesel fuel that is
fatty acid methyl esters, are produced from vegetable oil i.e.,
triglyceride. In the manufacture of biodiesel, glycerol is produced
as a by-product, and the supply of biodiesel is increasing year by
year. Glycerol is one of such promising biomass resources.
Recently, research papers on the reaction of glycerol have
increased drastically, and a large amount of research has been
introduced in recent reviews,3,4 as we also introduced in a
previous paper.5 In the vapor-phase dehydration of glycerol,
glycerol can be dehydrated into acrolein over acidic catalysts3,6
and is also dehydrated into hydroxyacetone (1-hydroxy-2-
propanone) often called acetol, hereafter abbreviated as HA, in
an inert atmosphere over supported copper7-9 and sodium-doped
base10 catalysts.
In the liquid-phase reaction glycerol under compressed
hydrogen conditions, pioneering work using Ru catalyst has been
reported by Montassier et al.,11 and numerous research papers
have been reported since 2004.4 In the liquid phase, side reactions
reduce the selectivity to 1,2- and 1,3-propanediols along with the
formation of various by-products such as propanol, propanoic
acid, lactic acid, acetaldehyde, and ethylene glycol. In the vapor
phase, however, copper metal works as a catalyst in the formation
of 1,2-propanediol with selectivity higher than 90 mol % from
glycerol.5,12-15 In the catalytic conversion of glycerol into 1,2-
propanediol, the developed processes can control the thermody-
namic equilibrium of the second-step hydrogenation of HA.5
It is known that copper catalyzes the dehydrogenation of
1,2-diols.16 However, it should be noticed that copper metal
works as an effective catalyst for the dehydration of triol i.e.,
glycerol.7-9,17,18 SiO2-supported copper catalyst shows 98.8%
Ag/SiO2
Ag/Al2O3
Ag/ZrO2
Ag/CeO2
Ag/TiO2
(10)
(10)
(10)
(10)
(10)
46.2
84.7
37.1
10.2
12.4
30.8d
91.1
59.8
77.3
64.7
64.1
84.6
3.4
9.1
9.7
0.6
0
1.6
2.4
3.1
6.3
3.4
0
Ag powderc (100)
4.0
Cu/SiO2
Cu/Al2O3
(10)
33.0
84.9
66.8
44.5
1.7
32.9
14.1
10.9
c
aReaction temperature, 240 °C; carrier gas, H2, 210 cm3 min¹1; catalyst
weight, 0.5 g. Average conversion in the initial 5 h; 12-PDO: average
selectivity to 1,2-propanediol; EG: ethylene glycol. cCommercially
b
d
available sample. Catalyst weight, 4.0 g.
Details of experimental procedures of catalytic reaction are
described in the Supporting Information.19 Procedures are briefly
explained as follows: all the supported metal catalysts were
prepared by incipient wetness impregnation using a solution with
a prescribed amount of metal nitrate or chloroauric acid dissolved
in distilled water. The catalyst sample was calcined at 500 °C for
3 h. Hereafter, unless otherwise specified, the Ag/SiO2 catalyst is
silver supported on a silica (CARiACT Q-10, 310 m2 g¹1), which
was supplied by Fuji Silycia Chemical Ltd.
The catalytic reaction of glycerol was performed in a fixed-
bed down-flow glass reactor at atmospheric pressure and temper-
atures between 200 and 300 °C. After the temperature of catalyst
bed had been maintained at 200 °C in H2 flow for 1 h, an aqueous
solution of glycerol at 30 wt % was fed into the reactor at a feed
rate of 1.8 cm3 h¹1, which corresponds to 5.9 mmol of glycerol
per hour, in either H2 or N2 flow. The liquid products recovered at
¹78 °C every hour were analyzed on a gas chromatograph
connected to a hydrogen flame ionization detector (FID-GC,
Shimadzu GC-8A) using a 60-m capillary column of TC-WAX
(GL Science).
glycerol conversion with HA selectivity of 84.6% under the
¹1
conditions of 220 °C, weight hourly space velocity of 0.08 h
,
and ambient N2 pressure with 5% H2.17 Raney Cu is also active in
the vapor-phase dehydration of glycerol into HA.14 We prelimi-
narily found that silver metal catalyzed the dehydration of
glycerol to produce HA in H2 flow (Figure 1). In this paper, we
report that HA is produced from glycerol with high selectivity
over supported silver catalysts in H2 flow. We also compare the
catalytic activity of Group 11 elements in the reaction of glycerol.
In our preliminary screening of metal catalysts, gold, cobalt,
and platinum were tested on several supports (Table S1).19 Pt/
Al2O3 and Co/Al2O3 decomposed the C-C bond of glycerol to
produce ethylene glycol. Among the tested samples, Au/Al2O3
was found to be selective to HA whereas the conversion of
glycerol was low (Table 1). Ag/SiO2 showed the highest
Chem. Lett. 2012, 41, 965-966
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

966
Table 2. Synthesis of HA over Ag on silica supportsa
Table 3. Effect of carrier gas on the synthesis of HA over 10 wt % Ag/
SiO2 catalyst at 240 °Ca
Selectivity/mol %c
Support
(Ag/wt %) /m2 g
SAb
Conversionc
/mol %
Selectivity/mol %b
HA 12-PDO EG
Gus, Flow rate
Conversionb
/mol %
¹1
HA 12-PDO EG
¹1
/cm3 min
Q-10 (5)
257
245
180
125
9.9
46.2
82.3
39.3
8.6
83.3
91.1
83.1
90.9
71.6
0
3.4
10.3
3.1
0
0
1.6
1.9
1.6
0.6
N2, 210
H2, 5 + N2, 55
H2, 30 + N2, 30
7.4
50.4
78.9
62.3
85.6
89.7
0
0.1
0
0
0
3.1
Q-10 (10)
Q-10 (20)
Q-10 (30)
Q-10 (50)
9.6
H2, 30
H2, 60
H2, 120
H2, 210
H2, 360
90.5
91.5
94.7
96.9
86.9
86.7
86.0
83.1
80.8
80.6
4.3
4.4
7.8
10.6
11.1
2.4
2.3
2.5
2.6
2.9
Q-3 (10)
Q-6 (10)
Q-15 (10)
218
261
153
34.2
43.0
48.8
86.3
86.5
88.5
6.4
4.9
2.9
1.4
2.2
1.0
aReaction temperature, 240 °C; carrier gas, H2, 210 cm3 min¹1; catalyst
aCatalyst weight, 1.0 g. bAverage conversion and selectivity in the
initial 5 h.
c
weight, 0.5 g. bSA: specific surface area. Average conversion in the
initial 5 h. 12-PDO: 1,2-propanediol; EG: ethylene glycol.
selectivity to HA, and the selectivity depends on support
materials: the other supports such as Al2O3 and ZrO2 decreased
the selectivity. Even silver powder with the specific surface area
most selective to produce HA in H2 flow because of its low
hydrogenation ability (Table S419). Table 3 shows effect of
carrier gas on the conversion of glycerol over Ag/SiO2. In N2
flow, Ag/SiO2 was not active for the dehydration of glycerol into
HA. Addition of H2 into the N2 flow drastically increases the
glycerol conversion as well as the HA selectivity. It was
confirmed that carbon deposition was inhibited during the
reaction in H2 using thermogravimetric analysis of the samples
used in the reaction (Figures S6 and S719).
The vapor-phase reaction of glycerol was performed over
silver metal catalysts at ambient H2 pressure. Glycerol was
efficiently dehydrated into HA over Ag/SiO2 catalyst. Glycerol
was dehydrated into HA with the selectivity higher than 86% at
91% conversion over Ag/SiO2 in H2 flow at 240 °C. It should be
noted that metallic silver catalyzes the dehydration of glycerol
into HA and that the further hydrogenation to 1,2-propanediol
was suppressed over the Ag/SiO2 even in H2 flow.
¹1
lower than 1 m2 g showed a sufficient HA selectivity higher
than 80%. Therefore, silver metal surface provides active sites for
the vapor-phase dehydration of glycerol to HA. The catalytic
activity of Ag/SiO2 was gradually decreased with time on stream
in H2 flow, as shown in detail in the Supporting Information
(Figure S119). The deactivation is probably caused by carbon
deposition, as discussed below.
Effect of calcination temperature was examined between 400
and 700 °C (Table S2, Figures S2 and S3).19 Ag in the samples
heated at 500 °C was reduced and formed Ag metals, whereas Ag
in the sample heated at 400 °C was Ag2O. After reduction in H2
at 240 °C, Ag2O in all the samples was reduced to Ag metal. The
size of Ag on 20 wt % Ag/SiO2 was estimated to be 28 nm by an
FWHM of the XRD peak at 2ª = 37.9°, which agrees with the
size observed in the TEM photograph (Figure S419).
Table 2 shows effect of Ag content in Ag/SiO2 catalysts on
the synthesis of HA. The HA selectivity exceeded 90% at Ag
content of 10 wt %. The conversion is strongly dependent on Ag
content: the maximum conversion was attained at Ag content of
20 wt %. Table 2 also shows the catalytic performance of silver
supported on different silicas. The number in the name of silica
indicates mesopore size in nm;6 i.e., Q-10 has mesopores with
mean pore diameter of 10 nm. The glycerol conversion seems to
be dependent on pore structures: silica with large pore and small
specific surface area would increase the conversion. Degradation
behavior seems to depend on mesopore size of support:
small pore-size catalyst such as Q-3 was readily deactivated
(Figure S519). A similar deactivation has been observed in the
dehydration of glycerol: a catalyst with a small pore support was
seriously deactivated by coke formation.6
At different concentrations of aqueous glycerol solution,
there was no effect of water content on the catalytic performance
of Ag/SiO2: both the glycerol conversion and the selectivity to
HA become similar to those at high glycerol concentrations of
90 wt % (Table S319).
We have previously reported that copper supported on
alumina shows the dehydration ability to produce HA in an N2
atmosphere.8 In H2 flow, the Cu/Al2O3 has hydrogenation ability
to produce 1,2-propanediol.5 However, copper supported on
silica was not selective to HA because it is capable of C-C bond
scission (Table 1). Among the tested catalysts, Ag/SiO2 was the
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Chem. Lett. 2012, 41, 965-966
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/