COMMUNICATION
The hydrothermal synthesis of tetragonal tungsten bronze-based
catalysts for the selective oxidation of hydrocarbons{
Pablo Botella,a Benjam´ın Solsona,a Ester Garc´ıa-Gonza´lez,b Jose´ M. Gonza´lez-Calbetb and
Jose´ M. Lo´pez Nieto*a
Received (in Cambridge, UK) 23rd July 2007, Accepted 7th September 2007
First published as an Advance Article on the web 21st September 2007
DOI: 10.1039/b711228a
completely modified when trivalent and tetravalent elements of the
V and VI groups are present in the hydrothermal process. In
addition, it will be shown that the catalytic performance of this
crystalline phase can be tailored by changing the composition of
both the heteropolyacid precursor and the synthesis gel.
Mixed metal oxides with tetragonal tungsten bronze (TTB)
structure, showing high activity and selectivity for the gas phase
partial oxidation of olefins, have been prepared by hydro-
thermal synthesis from Keggin-type heteropolyacids.
MoVTe(Sb)NbO oxidic bronze catalysts initially reported by
Mitsubishi1 are promising catalysts in the (amm)oxidation of
propane1,2 and in the oxidative dehydrogenation of ethane.3
Generally speaking, they mainly constitute two crystal phases:1–5
the orthorhombic Te2M20O57 (M = Mo, V, Nb; the so-called M1)
and the pseudohexagonal bronze Te0.33MO3.33 (M = Mo, V, Nb;
the so-called M2). The M1 phase is active and selective in the
partial oxidation of propane or ethane, while M2 is only active and
selective in the oxidation of propene to acrolein and/or acrylic
acid.2–6 Moreover, the synthesis procedure strongly influences both
physicochemical and catalytic performance.1–7 It has been
suggested that these transition metal composite oxides could be
prepared by self-organization of polyoxometalates,2b these being
MoVTe(Sb)NbO oxides obtained from slurries prepared with
Anderson heteropolyacids.2b,3–6 Anderson heteropolyacids have
also been employed in the hydrothermal synthesis of these
materials, although orthorhombic3,5,6 or trigonal7 phases were
obtained depending on specific synthesis conditions. It has been
suggested that the Mo-intermediate formed during the hydro-
thermal synthesis7 could be similar to those proposed by Muller
et al.8 in the preparation of giant polyoxomolybdates.
Mo- and/or W-containing TTB-type phases were prepared by
hydrothermal synthesis using a polyoxometalate, niobium oxalate,
vanadyl sulfate, and a X-metal oxide (X = Te, Sb or Bi) with a
(Mo,W)/Nb/V/X atomic ratio of 1/0.17/0.20/x (x = 0 to 0.08) and a
(Mo,W)/H2O molar ratio of 1/87. The gels were loaded in Teflon-
lined stainless-steel autoclaves and heated at 175 uC for 48 h. The
resulting solids were dried at 80 uC for 16 h, and calcined in
the 700–800 uC temperature range during 2 h in a N2-stream. The
synthesized catalysts were tested in a fixed bed quartz reactor, at
reaction temperatures ranging from 300 to 400 uC and atmospheric
pressure. The reaction feed consisted of a mixture of propene–
oxygen–steam–helium with a molar ratio of 1.5/6/15/77.5. The
catalytic tests refer to steady-state conditions; no deactivation was
observed over about 100 h. A blank run showed that under our
reaction conditions the homogeneous process could be neglected.
Fig. 1 shows some characterization results of sample H1-T
(Table 1). The powder X-ray diffraction pattern of calcined sample
shows the formation of a high purity TTB-related crystal phase
(Fig. 1a). This has been observed in all prepared catalysts
independently of the heteropolyacid precursor and/or the chemical
composition.12 Fig. 1b corresponds to the electron diffraction
pattern of the same sample. Diffuse scattering around the main
TTB reflections is indicative of a short range order situation which
is clear from the observation of the corresponding electron
micrograph in Fig. 1c. Moreover, the structural characterization
suggests that the microstructure of these Mo- or W-containing
TTB-bronzes depends on the final niobium content.13,14
During the last few years several groups have been working on
the synthesis of new similar materials. Among these new materials,
we can highlight the heat-treated pyridine and niobium molybdo-
vanadophosphoric acid based catalysts, active and selective in the
partial oxidation of light alkanes,9 or new Cs-containing M1-phase
oxides, active and selective in the partial oxidation of C3–C4-
olefins.10 Recently, we reported that mixed metal oxides prepared
from Keggin-type heteropolyacids could be used as catalysts in the
selective oxidation of hydrocarbons.11 The present work shows
how the hydrothermal synthesis from Keggin-type heteropolyacids
can facilitate the preparation of Mo(W)VNbO mixed oxides with a
tetragonal tungsten bronze (TTB) structure, and how the catalytic
performance in the partial oxidation of olefins of these solids is
The final calcination temperature required in the formation of
crystalline phases depends on the catalyst composition (Table 1).
Thus, it has been observed that W-based catalysts need heat-
treatment temperatures higher than the corresponding Mo-based
catalysts. On the other hand, no significant differences in their
XRD patterns were observed in solids prepared in the presence or
absence of elements of the V and VI groups in the X/(Mo + W)
atomic ratio of 0 to 0.10. However, it is important to mention that
the presence of these elements in the W-based catalysts favours a
decrease in the calcination temperature needed to obtain the
tetragonal bronze.
aInstituto de Tecnolog´ıa Qu´ımica, UPV-CSIC, Avda. Los Naranjos s/n,
46022 Valencia, Spain. E-mail: jmlopez@itq.upv.es;
Fax: + 34 963877809; Tel: + 34 963877808
bDpto. Qu´ımica Inorga´nica, Facultad de Qu´ımicas, Universidad
Complutense, Madrid 28040, Spain
We must indicate that no TTB bronzes but a mixture of M5O14-
type phases and single oxides (basically MoO3) were obtained
when phosphoric acid and ammonium heptamolybdate (or
{ Electronic supplementary information (ESI) available: XRD patterns of
different Keggin-type heteropolyacids based materials and additional
TEM information. See DOI: 10.1039/b711228a
5040 | Chem. Commun., 2007, 5040–5042
This journal is ß The Royal Society of Chemistry 2007
Botella, Pablo
