A long-life catalyst for glycerol dehydration to acrolein

Article abstract of DOI:10.1039/c0gc00254b

While the initial catalytic performances of silica-supported silicotungstic acid are high in the glycerol dehydration reaction, they rapidly decrease with time on stream and the acrolein yield quickly decreases. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0gc00254b

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www.rsc.org/greenchem | Green Chemistry
A long-life catalyst for glycerol dehydration to acrolein†
Benjamin Katryniok,a,b,c Se´bastien Paul,a,b,c Mickae¨l Capron,a,b Christine Lancelot,a,b
Virginie Bellie`re-Baca,d Patrick Reye and Franck Dumeignil*a,b
Received 23rd June 2010, Accepted 24th August 2010
DOI: 10.1039/c0gc00254b
While the initial catalytic performances of silica-supported
silicotungstic acid are high in the glycerol dehydration
reaction, they rapidly decrease with time on stream and the
acrolein yield quickly decreases.
which results in their rapid deactivation.6–8 For instance, Chai
et al. reported a decrease in catalytic activity by a factor two after
only 10 h on stream for HZSM-5 catalysts or phosphotungstic
acid supported on alumina,4 which is obviously an important
drawback when targeting industrial applications.
Among the possible formulations, supported inorganic acids
have been extensively studied by several research groups, who
have investigated either common inorganic acids like phosphoric
acid or more complex acids like Keggin-type heteropolyacids
(HPA). Apparently, the use of an HPA as an active phase
offers the possibility of easily controlling the acidity, and
thereby the catalytic performance, through fine tuning of the
HPA composition. HPA-based catalysts have been tested using
several supports, such as silica,7 alumina,8 zirconia9 and active
carbon.10 Among them, silica and zirconia seem to be especially
interesting:
—Silica offers the advantage of facile control of the pore
size distribution, which has been identified as an important
parameter for obtaining good catalytic performance. Tsukuda
et al. have shown that the pore diameter (PD) of the silica should
be chosen to be around 10 nm to obtain the best performance.7
Additionally, silica does not alter the acidic properties of the
supported HPAs, contrarily to alumina.8
In the biodiesel production process, vegetable oils and fats—
most usually from canola, soy or corn—are reacted with a mono-
alcohol (usually methanol) to cleave the fatty acids from their
glycerol backbone. The resulting fatty acid esters are directly
used as biodiesel, whereas glycerol remains as a rather valueless
by-product. As a consequence, various reactions have been
investigated for valorizing glycerol, such as reforming, oxidation,
hydrogenolysis, etherification and dehydration.1 When consid-
ering a commercial target, the dehydration of crude glycerol to
yield acrolein (Scheme 1) is one of the most promising options
due to the important role of acrolein as a precursor for the
synthesis of DL-methionine and acrylic acid.2
—As claimed by Chai et al., Zirconia offers an increased
long-term stability of catalysts. Nevertheless, over pure zirconia
supports, the acrolein yield does not exceed 54% after 10 h on
stream.9
Scheme 1 The principle of acrolein production using the glycerol by-
product of biodiesel synthesis.
In this paper, we report for the first time the combination of
the advantages of these two components by using ZrO2-grafted
silica as a support for an HPA active phase. SBA-15, prepared by
the method of Roggenbuck et al.11,† was chosen as the preferred
silica host support. It offers the feature of a controlled PD,
adjusted here to 8 nm, which is consistent with the optimal
pore size postulated by Tsukuda et al.7 A commercial silica
The dehydration of glycerol to acrolein requires acid catalysts
with active sites of a suitable strength in order to efficiently
promote the reaction while limiting coke formation. Solid acid
catalysts with a Hammett acidity between -3 and -8, such
as zeolites, supported inorganic acids and metal oxides, have
previously shown good performances with yields of up to
80%.3–5 Unfortunately, the rather strong acidic properties of
these catalysts are also responsible for the formation of coke,
R
(CARiACTꢀ-Q10, Fuji Silysia) was also chosen for checking
the influence of its larger PD of 15 nm. An SBA-15 support
with a PD of 6 nm† was also used as a comparative sample. The
silica host support was grafted using zirconium n-propoxide as
a ZrO2 precursor in order to yield a ZrO2 loading from 10 to
40 wt%.† The obtained supports were then impregnated with
10 to 30 wt% silicotungstic acid (STA). Impregnated catalysts
based on non-grafted silica were also prepared as comparative
samples.
The actual quantities of deposited ZrO2 were determined by
elemental analysis. The measured contents were only slightly
lower than the theoretical ones,† with a maximal relative error
for the 10 wt% ZrO2 sample, which contained 7.2 wt% ZrO2
(Table S1†). The textural properties of the prepared supports
aUniv. Lille Nord de France, F-59000, Lille, France.
E-mail: franck.dumeignil@univ-lille1.fr; Fax: +33 (0)3.20.43.65.61;
Tel: +33 (0)3.20.43.45.38
bCNRS UMR8181, Unite´ de Catalyse et Chimie du Solide, UCCS,
F-59655, Villeneuve d’Ascq, France
cECLille, F-59655, Villeneuve d’Ascq, France
dRhodia France, 52 Rue de la Haie Coq, 93308, Aubervilliers, France
eAdisseo France SAS, Antony Parc 2-10, 92160, Antony, France
† Electronic Supplementary Information (ESI) available: Experimental
and analytical methods. See DOI: 10.1039/c0gc00254b
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Table 1 The catalytic performances of the catalysts supported on SBA-
15 with a mean pore diameter of 8 nm
and catalysts were determined using nitrogen physisorption
(Table S2†). Grafting with ZrO2 led to a decrease in both
the specific surface area (SSA) and the pore volume (PV). A
comparison between the theoretical and the experimental SSAs
and PVs suggests that plugging of the micropores occurred with
low amounts of ZrO2 (10 wt%). On the other hand, samples
containing 20 and 40 wt% ZrO2 showed higher experimental
values than those theoretically predicted. This can be attributed
to the porous character of the deposited zirconia, which develops
its own surface and PV. Meanwhile, neither the impregnation
with 10–30 wt% STA (Fig. S1†) nor the grafting with quantities
of ZrO2 to 20 wt% affected the PD (7.2 0.4 nm; Fig. 1; Table
S2†). Only in the case of 40 wt% ZrO2 was a decrease in the
PD observed (5.9 nm), which was attributed to the formation of
large zirconia crystal domains, as evidenced by XRD (detection
of tetragonal zirconia; Fig. S2†) and HRTEM (Fig. S3-b1†).
ZrO2 amount (wt%):
STA amount (wt%):
0
10 20
40 20 20 20
20 10 30
20 20 20
0
TOS 0–5 h
C (%)
84 87 96
90 34 87 89
65 19 49 75
13 10 13 10
SAC (%) 83 77 74
SHA (%)
Y (%)
CB (%) 91 89 96
C (%) 41 62 78
SAC (%) 57 69 88
SHA (%)
Y (%)
CB (%) 85 90 100 75
5
12 12
71 67 71
59
6
42 67
80 75 62 86
TOS 24–25 h
60
44
13
26
—
—
—
—
—
70 66
44 78
14 12
31 52
63 83
5
13 12
24 43 69
TOS = time on stream, C = glycerol conversion, SAC = selectivity for
acrolein, SHA = selectivity for hydroxyacetone, Y = yield of acrolein,
CB = carbon balance.
SBA-15 and on bare SBA-15 were compared using ammonia
temperature programmed desorption (TPD). The initial SBA-
15 host support showed almost no acidity (0.004 mmol g-1),
whereas the grafting of zirconia led to a significant increase
in the number of acid sites (+0.174 mmol g-1; Fig. 2), which
are supposedly of Lewis-type due to the nature of the support.
After STA impregnation, the total number of acid sites increased
(+0.335 mmol g-1 over SBA-15 and +0.274 mmol g-1 over
ZrO2-grafted SBA-15). While the catalyst prepared from bare
SBA-15 had slightly fewer acid sites than its ZrO2-grafted
homologue (0.339 mmol g-1 vs. 0.442 mmol g-1, respectively),
it was rather stronger (DT = 25 K; Fig. 2). This effect is
attributed to a difference in the electronic interaction between
the HPA and the support. Similar to what was proposed for
Al2O3-supported HPAs by Atia et al.,8 we suggest hereafter that
STA has a stronger interaction with ZrO2 than with SiO2. In
contact with water, the hydroxyl groups at the surface of MOx
supports (M = Al, Si or Zr) are protonated, which yields M–
Fig. 1 Pore size distribution of SBA-15 grafted with (a) 0 wt%, (b)
10 wt%, (c) 20 wt% and (d) 40 wt% ZrO2.
+
Catalytic performances were determined at 548 K according
to the procedure described in the ESI.† First, the influence of
the amount of ZrO2 was investigated at an iso-HPA loading
(20 wt%). The catalytic performances during the first 5 h on
stream were similar for samples containing up to 20 wt% ZrO2,
with an acrolein yield of ca. 70% (Table 1). The yield over the
sample grafted with 40 wt% ZrO2 was lower (59%) due to the
low selectivity for acrolein (65%). Note that the selectivity for
hydroxyacetone doubled by introducing ZrO2 (12–13% vs. 5%).
After 24 h on stream, the beneficial effect of the zirconia grafting
was revealed, whereas the conversion of the catalyst without
ZrO2 was divided by a factor of two (41% after 24 h vs. 84%
during the first 5 h). The catalysts containing ZrO2 continued to
exhibit rather high glycerol conversions of 60% (40 wt% ZrO2),
62% (10 wt% ZrO2) and even 78% (20 wt% ZrO2). However,
the selectivity for acrolein of the sample grafted with 40 wt%
zirconia remained low (44%).
OH2 species (Scheme S1†). These species strongly interact via
electronic effects with the negatively-charged heteropolyanion,
as postulated by Wu et al.13 Depending on the acidic character
of the support, this interaction can be either strong (over ZrO2
and Al2O3) or weak (over SiO2), and its strength has an influence
on the Brønsted acidity and thermal stability of the supported
The above results show the beneficial effect of zirconia in
preventing catalytic deactivation. The best catalytic performance
was obtained over the 20 wt% ZrO2 catalyst, which showed a
quite stable acrolein yield of 69% after 24 h vs. 71% during
the first 5 h. To understand the effect of the zirconia on
the catalytic performance of the supported STA, the acidity
of the catalysts containing 20 wt% STA on ZrO2-grafted
Fig. 2 Temperature programmed NH3 desorption of the (a) host SBA-
15, (b) 20 wt% ZrO2-grafted SBA-15, (c) STA on SBA-15 catalyst and
(d) STA on 20 wt% ZrO2-grafted SBA-15 catalyst.
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HPA.8 A strong interaction leads to distortion of the Keggin
structure, which induces a decrease in the acid strength. This
slight diminution of acid strength leads to less formation of coke,
with a higher carbon balance (96% vs. 91% for 0 and 20 wt%
ZrO2, respectively). As a second effect, the strong interaction
between zirconia and STA stabilizes the Keggin structure, which
results in a higher thermal stability of the HPA. This was
confirmed by thermogravimetric analysis (Fig. S4†).
From this last series of experiments, two different factors
affecting the catalytic performance were identified: (i) the surface
density of acid sites and (ii) the nature of the acid sites. A low
amount of STA (i.e., 10 wt%) resulted in partial coverage (Table
S3†) of the ZrO2-grafted support. Accordingly, the Lewis acid
character of the uncovered zirconia became dominant, which
led to a low selectivity for acrolein (also observed at a high ZrO2
loading (40 wt%)). On the other hand, a high amount of STA
(i.e., 30 wt%) led to the deposition of a larger quantity of active
phase over bare silica, thereby the acid strength increased again.
As a consequence, coke formation became more dominant (CB
of 86% vs. 96%), which resulted in accelerated deactivation after
24 h on stream.6–8
From the above results, we conclude that the Brønsted acid
site strength of the STA is decreased over zirconia, which has
a positive impact on long-term catalytic performance due to
reduced carbon deposits and therefore a slowed deactivation
of the catalyst. On the other hand, the presence of bare
zirconia adds Lewis acidic character to the catalyst, which
favors side reactions and leads to a decrease in the selectivity
for acrolein.12–13 Therefore, these two effects must be properly
balanced.
Furthermore, the low selectivity for acrolein over the catalyst
containing 40 wt% ZrO2 can be explained by the decreased pore
diameter (5.9 nm) and the presence of the large zirconia domains,
as detected by XRD and HRTEM. Tsukuda et al. and Atia et al.
have demonstrated that the selectivity for acrolein decreases
with smaller pores.7–8 On the other hand, the aforementioned
zirconia domains, formed due to Zr overloading, increase the
Lewis acid character of the catalyst. The Lewis acid sites are
known to favor the formation of acetol (Scheme 2).12–13 In fact,
Brønsted acid sites directly protonate the secondary hydroxyl
group of glycerol, which finally leads to the formation of acrolein
(Scheme 2a). On the other hand, the Lewis acid sites lead
to activation of the terminal OH-group of glycerol, whereby
acetol is formed (Scheme 2b). The Lewis acid site is thereby
transformed into a pseudo-Brønsted acid site, which can then
either catalyse the dehydration of glycerol in the same way as
‘normal’ Brønsted acid catalysts or regenerate the initial Lewis
site by the loss of water. As a consequence, the selectivity
for acetol is increased over Lewis-acid catalysts. This effect
was observed for all ZrO2-containing catalysts, which—without
exception—showed significantly increased selectivity for acetol
compared to the ZrO2-free examples.
The impact of the amount of deposited STA on the catalytic
performance was further investigated for samples containing the
optimal ZrO2 content of 20 wt%. The catalytic performances
during the first 5 h on stream were rather similar for the samples
containing 20 and 30 wt% STA, with an acrolein yield of 71 and
67%, respectively. In contrast, the acrolein yield over the catalyst
containing a low amount of active phase (10 wt%) was only 42%
due to a low selectivity for acrolein of 49% (Table 1). After 24 h,
the catalyst containing 30 wt% STA showed a significantly lower
conversion (66%) than that containing 20 wt% STA (78%), while
the performance of the 10 wt% STA catalyst was lower still, again
due to its low selectivity for acrolein (44%).
As the best results were obtained for 20 wt% STA, this loading
was chosen for the last study, where the PD of the silica support
was varied using an SBA-15 with a PD of 6 nm and a commercial
R
silica with a PD of 15 nm (CARiACTꢀ-Q10). For these supports,
ZrO2 grafting also led to enhanced long-time performances com-
pared to those over catalysts based on bare silica. Nevertheless,
the positive impact of zirconia on the long-term performance
was less important than in the case of SBA-15 with an 8 nm PD
(Table S4†). This is consistent with the results of Tsukuda et al.,
who suggested an optimal pore size close to 10 nm.7
Conclusion
The increased long-time performance in the reaction of glycerol
dehydration to acrolein of a catalyst based on silicotungstic
acid supported on an SBA-15 modified by zirconia grafting is
interpreted in terms of a modified electronic interaction between
the support and the HPA, which results in a slightly lower
Brønsted acidity.
Scheme 2 The possible reaction mechanism over (a) Brønsted acid and (b) Lewis acid catalysts, as proposed by Alhanash et al.12
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The important parameters for optimizing the performance
are: (i) the amount of zirconia, (ii) the amount of active
phase and (iii) the pore size of the support. The best catalyst
was obtained by using an 8 nm pore size SBA-15 grafted
with a 20 wt% zirconia support that was impregnated with
20 wt% silicotungstic acid after an intermediate calcination
step at 923 K. The high catalytic performance of this cat-
alyst (71% acrolein yield after 5 h on stream) was stable
(69% acrolein yield after 24 h on stream) and is particularly
remarkable.
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Acknowledgements
The authors thank O. Gardoll and G. Cambien for their
technical help and ADISSEO for its financial support.
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Notes and references
1 M. Pagliaro, M. Rossi, The Future of Glycerol: New Uses of a Versatile
Raw Material, RSC Green Chemistry Book Series, 2008.
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