Received: November 24, 2013 | Accepted: December 5, 2013 | Web Released: December 11, 2013
CL-131098
Direct Oxidative Transformation of Glycerol into Acrylic Acid
over Phosphoric Acid-added W-V-Nb Complex Metal Oxide Catalysts
Kaori Omata, Keeko Matsumoto, Toru Murayama, and Wataru Ueda*
Catalysis Research Center, Hokkaido University, N-21, W-10, Sapporo, Hokkaido 001-0021
(E-mail: ueda@cat.hokudai.ac.jp)
The addition of phosphoric acid to W-Nb-O catalyst active
for glycerol transformation to acrolein and to W-V-Nb-O
catalyst active for direct transformation of glycerol to acrylic
acid appreciably improved their catalytic performance. The
phosphoric acid-added W-Nb-O catalyst gave acrolein yield of
81.8%, and the phosphoric acid-added W-V-Nb-O catalyst gave
acrylic acid yield of 59.2% in the direct glycerol transformation.
The improvement of the catalytic performance seems due to the
increases of the acid amount and the Brønsted acidity.
Phosphoric acid-added catalysts were prepared by impreg-
nation of uncalcined W-Nb-O or W-V-Nb-O with an aqueous
solution of phosphoric acid, followed by calcination at 673 K in
air. Phosphoric acid-added W-Nb-O and W-V-Nb-O were
denoted as H3PO4/WNb and H3PO4/WVNb, respectively. The
content of P was set to be 2.5 wt % of the supports after
optimization of P content. XRD analysis confirmed that the
layered structure of the W-Nb-O and W-V-Nb-O catalysts was
maintained after the phosphoric acid treatment (Figure S1).7
Surface area of the catalysts was estimated by BET method
where nitrogen physisorption amount was measured at 77 K with
a BELSORP max (BEL Japan Inc.). Prior to the measurement,
the samples were evacuated at 473 K for 2 h. Powder X-ray
diffraction (XRD) pattern of the catalysts was recorded on a
RINT2200 (Rigaku) with Cu Kα radiation (tube voltage: 40 kV,
tube current: 20 mA). The acid amount of catalysts was
measured with NH3-TPD with a TPD apparatus (BEL Japan
Inc.). Prior to the measurement, the samples were pretreated
under He flow at 673 K for 2 h. NH3 was adsorbed on the
catalysts at 473 K. Acidity of catalysts was measured by FT-IR
spectroscopy (PARAGON 1000, Perkin-Elmer) of adsorbed
pyridine with an evacuable furnace cell with CaF2 windows,
containing a self-supporting disk of sample. Pyridine was
adsorbed at 373 K, and after evacuation at 523 K for 1 h the
adsorption spectrum was recorded. The spectrum of adsorbed
pyridine on sample in the presence of water vapor (4.6 Torr) was
also recorded.
Glycerol is a main by-product in biodiesel production by
transesterification of plant oils or animal fat with methanol and
has been produced heavily at a relatively low price.1 Because of
this situation, transformation of glycerol into other desirable
chemicals by various catalytic reactions has been attempted by
many researchers.2 Dehydration of glycerol to acrolein is one of
the most valuable reactions, since acrolein is an important
intermediate for chemical and agricultural industries. Various
solid acid catalysts have been reported for the dehydration of
glycerol.3 We have reported that layer-structured W-Nb-O
catalysts synthesized via hydrothermal method gave acrolein in
high yield (75%) in gas-phase glycerol dehydration.4 Direct
oxidative transformation of glycerol to acrylic acid is also a very
important reaction.5 Achievement of this reaction is, however,
challenging because not only improvement of selectivity for
both dehydration of glycerol and selective oxidation of acrolein
but also tuning of optimum catalytic functions for each reaction
are required to achieve higher acrylic acid yield. Nevertheless,
we have recently found that modification of the W-Nb-O
catalyst with vanadium turned out to be an efficient catalyst for
the direct transformation of glycerol to acrylic acid6 but the yield
of acrylic acid was as low as 45%.
Very recently we found that the addition of phosphoric acid
to the W-Nb-O catalyst and the W-V-Nb-O catalysts had
pronounced effects on the transformation of glycerol to acrolein
and the direct transformation to acrylic acid, respectively. The
achieved single pass yield of acrylic acid was about 60% and is
currently the highest amongst the reported results. This paper
reports the effects of phosphoric acid addition to the W-Nb-O
catalyst and the W-V-Nb-O catalysts on the selectivity in the
glycerol transformation.
Transformation of glycerol was carried out in a vertical
fixed-bed reactor. The molar percent composition of reaction gas
was glycerol/O2/N2/H2O = 5/14/56/25 (mol %). Reaction
products and unconverted glycerol in both gas and liquid phases
were collected hourly and analyzed with GC. Oxidation of
acrolein was carried out in the same reactor as the transformation
of glycerol.
The catalytic performance of the W-Nb-O and H3PO4/
WNb catalysts in the glycerol transformation was first examined,
and the results are shown in Table 1. Both the glycerol
conversion and the acrolein yield were clearly increased by
the phosphoric acid addition, and the H3PO4/WNb catalyst
¹3
gave the acrolein in yield of 81.8% at W/F = 2.5 © 10
gcat min mL¹1. The improvement can be explained by changes
in the surface acidity because of the following results. As shown
in Table 2, the number of acid sites per gram largely increased
by the addition of phosphoric acid. At the same time, the
addition increased the ratio of Brønsted to Lewis acidity.
Moreover, it was observed in the FT-IR study that water
substantially decreased the intensity of the IR-band ascribed to
the adsorption of pyridine on Lewis acid sites and on the other
hand increased the intensity of IR-band ascribed to that on
Brønsted acid sites. This result indicates that Lewis acid sites are
hydrated and change into Brønsted acid sites in the presence of
The complex metal oxide catalysts of W, V, and Nb (W-V-
Nb-O) were prepared by hydrothermal synthesis. (NH4)6-
[H2W12O40]¢nH2O (W: 2.7 mmol), VOSO4¢nH2O (V: 0.6 mmol),
and Nb2O5¢nH2O (Nb: 2.0 mmol) were added to 45 mL of ion-
exchanged water under stirring. This mixed suspension was put
in a stainless steel autoclave with a Teflon liner and heated at
448 K for 72 h. The formed solid was filtered, washed with ion-
exchanged water, dried at 353 K, and then calcined at 673 K for
4 h in air. W-Nb-O (W/Nb = 1.35) was similarly prepared.
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