Novel synthesis of a vanadium-titanium aluminophosphate molecular sieve of MFI structure (VTAPO-5) and catalytic activity for the partial oxidation of methanol

Article abstract of DOI:10.1039/a903876k

A new vanadium-titanium substituted aluminophosphate molecular sieve of MFI structure (VTAPO-5) was synthesized hydrothermally and found to be active and selective in partial oxidation of methanol; catalytic activity depends on the vanadium sites; however, the presence of titanium influences the reactivity of vanadium species during reaction.

Full text of DOI:10.1039/a903876k

Novel synthesis of a vanadium–titanium aluminophosphate molecular sieve of
MFI structure (VTAPO-5) and catalytic activity for the partial oxidation of
methanol
M. P. Kapoor* and Anuj Rajb
a
a
Osaka National Research Institute, 1-8-31 Midorigaoka, Ikeda, Osaka-563, Japan. E-mail: kapoor@onri.go.jp
National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki-305, Japan
b
Received (in Cambridge, UK) 14th May 1999, Accepted 15th June 1999
A new vanadium–titanium substituted aluminophosphate
molecular sieve of MFI structure (VTAPO-5) was synthe-
sized hydrothermally and found to be active and selective in
partial oxidation of methanol; catalytic activity depends on
the vanadium sites; however, the presence of titanium
influences the reactivity of vanadium species during reac-
tion.
phate-5 (VTAPO-5), which is found to be an active and
selective catalyst for the partial oxidation of methanol.
VTAPO-5 was synthesized using a gel composition 0.30
NPr
3
+0.70 TPAOH+0.02 TiO
2
+0.047 V
2
O5 : P
2
O
5
2
+42 H O. In
a typical synthesis, the calculated amounts of phosphoric acid
(85 wt%) and pseudoboehmite, as P and Al sources re-
spectively, were mixed and dissolved in distilled water. Then
vanadium and titanium as hydrated oxovanadium(iv) sulfate
and titanium isopropoxide, respectively, were introduced drop-
wise to the homogenous gel under vigorous stirring. A mixture
Aluminophosphate (AlPO) molecular sieves classified as a new
class of microporous crystalline inorganic solids are contempo-
rary to the zeolites. A variety of AlPOs have been reported with
a rich diversity of structures.1 They are framework oxides with
Al and P in tetrahedral coordination with oxygen. In spite of
different structural properties from zeolites, AlPOs also appar-
ently obey Loewenstein’s rule with an avoidance of Al–O–Al
bonds and thus include framework topologies analogous to
zeolites. Although titanium and vanadium substitution is well
3
of tripropylamine (NPr ) and TPAOH (an organic template as
,2
structure directing agent), was used to direct the MFI structure
type in the VTAPO-5 molecular sieve and was slowly
introduced into the gel. Finally the entire gel was further stirred
for at least 1 h and transferred into a Teflon-coated stainless-
steel autoclave for hydrothermal crystallization. Based on a
series of systematic syntheses, pure and highly crystalline
VTAPO-5 was synthesized aging the gel directly at 458 K for
200 h. After heating, the autoclave was cooled and solid product
was separated by centrifugation and repeatedly washed with
distilled water followed by drying at 358 K for 6 h followed by
calcination in air at 813 K for 8 h. It was interesting that the
3–7
documented in various topologies, very little information on
isomorphously substituted Ti and V AlPOs have been reported.
Excluding the original patent on synthesis of some titanium
containing AlPOs, only the synthesis and characterization of
8
TAPSO-5, and more recently TAPO-5, 211, 231 and –36
have been published.9 Many important issues encountered in
the synthesis and application of substituted AlPOs are still un-
addressed. Very few reports are available in the literature
regarding the catalytic behavior of substituted AlPOs such as
TAPOs and VAPOs.
–11
systems involving only NPr
molecular sieve while pure VTAPO was obtained from the gel
with a mixture of NPr and TPAOH. Substituted TAPO-5 and
VAPO-5 were also synthesized by essentially following a
3
or TPAOH gave no VTAPO
3
reported procedure,1
0.04 TiO +P +40 H
respectively.
0,14
using the gel composition TPAOH :
O+NPr +0.047 V +P +40 H
8
Ulagappan and Krishnasamy found that TAPO-5 and -11
2
2
O
5
2
3
O
2 5
2
O
5
2
O
were active in phenol hydroxylation. Also some activity was
reported for VAPO-5 and Ti-VPI-5 in the catalytic oxidation of
cyclohexane to cyclohexanol using tert-butyl hydroperoxide
The samples were characterized by X-ray diffraction (XRD),
diffuse reflectance UV-VIS spectroscopy (DRS), Raman spec-
troscopy and 27Al and P MAS NMR. Pore volumes were
31
(TBHP). Recently, it has shown that TAPOs of structure types
AFI, AEL, ATO and ATS (TAPO-5, -11, -31 and -36
respectively) have comparable activities to titanium silicates
2
measured by n-butane adsorption and N adsorption methods
(BET). Bulk chemical analysis was carried out using in-
ductively coupled plasma atomic emission spectroscopy (ICP).
Chemical and physical properties of all samples are given in
Table 1.
(
TS-1) in a series of oxidation reactions using dilute H
2
O
2
(30
10,11
wt%) as the oxidant.
Vanadium–titanium catalysts, (either
supported or impregnated) are widely used in industry for the
partial oxidation of hydrocarbons and for the selective catalytic
reduction of NO with NH . In the literature there are some
3
The XRD patterns of TAPO-5 and VAPO-5 were similar to
those reported earlier and confirm the purity of the crystallized
product with the XRD pattern of VTAPO-5 being closer to that
of pure TAPO-5. Intercalation of vanadium moieties in the
framework can be discounted since no change in the basal
spacing is observed. Pore plugging as well as evidence for an
extraneous phase were checked by n-butane and nitrogen
adsorption. The measured BET surface area for VTAPO-5 was
interesting reports12 concerning the structural characteristics of
coprecipitated VTiO systems and their activity in n-butene
oxidation for the selective production of acetaldehyde and
acetic acid. In a very recent report13 the nature of vanadium sites
in a V/a-Ti phosphate catalyst for the oxidative dehydrogena-
tion of ethane were investigated. Here, we report for the first
time, the novel synthesis of vanadium–titanium aluminophos-
2
21
21
304 m g and the void volume was 0.121 ml g . The amount
Table 1 Physico-chemical properties of substituted AlPOs.
Chemical analysis
2
N adsorption
n-Butane adsorption
V /ml g
p
2
21
/ml g21
21
Material
Ti
V
Al
P
S
BET/m g
V
p
VTAPO-5
TAPO-5
VAPO-5
0.015
0.028
—
0.022
—
0.024
0.484
0.489
0.492
0.491
0.481
0.483
304
311
298
0.121
0.117
0.123
0.114
0.110
0.118
Chem. Commun., 1999, 1409–1410
1409

Table 2 Partial oxidation of methanol over substituted AlPOs
Methanol
Conversion
(%)
Product selectivity (mol%)
Material
HCHO
CO
(CH
3
)
2
O
CO
2
3
CH CHO
MF
FA
a
TAPO-5
VAPO-5
0.01
nd
nd
nd
nd
nd
nd
nd
nd
8.1
9.3
nd
nd
nd
nd
29.5
33.4
35.3
nd
nd
nd
nd
1.2
1.6
1.7
a
5.73
79.2
74.8
69.2
31.4
22.9
13.8
12.6
14.4
16.3
20.7
22.2
24.4
5.8
7.2
9.4
6.0
7.3
9.6
3.4
3.6
5.1
3.1
3.3
4.5
b
7
1
.28
c
0.16
a
VTAPO-5
5.91
b
7
1
.87
1.09c
10.7
Reaction conditions: Feed = 20% methanol in Ar; 10% O
0.64 s); catalyst = 0.2 g; MF = methyl formate; FA = formic acid; nd = not detected.
2
in Ar, reaction temp. = 200 °C; F/W (contact time) = a 21400 (0.29 s), b 15600 (0.40 s), c 9800
(
of n-butane adsorbed at STP was 0.114 ml g21. The DRS UV–
VIS spectrum of VTAPO-5 shows a single peak with lmax
2CH3OH
CH3OH
CH3OH
HCHO
(CH3)2O
2H+ 2e–
HCHO + +
+
H2O
(1)
(2)
(3)
(4)
(5)
(6)
(7)
+
+
{O2}
{O2}
232–238 nm and no evident shoulder around 300 nm. This
CO
CO
CO2
+
+
4H+
+
+
4e–
2e–
suggested that VTAPO-5 is free from anatase or extraframe-
work species such as titanium or vanadium oxide, in line with
the BET and n-butane adsorption results. Moreover, in all
samples studied, no oxide species were detected by Raman
spectroscopy. 31P and Al MAS NMR spectra of calcined
TAPO-5, VAPO-5 and VTAPO-5 indicate that in all samples
the aluminium was present in a tetrahedral environment and
2H+
6H+ 6e–
+
CH3OH
CH3OH
+
+
H2O
H2O
+
27
HCO2H
+
4H+
+
4e–
15
HCHO
+
CH3OH
[CH3OCH2OH]
HCO2CH3
octahedral Al was absent as found in pseudoboehmite. The
27
Al MAS NMR spectra of TAPO-5 and VAPO-5 show a main
peak at d 32.4 while VTAPO-5 showed a slight shift to d 34.1.
Scheme 1
31
Similar results were obtained from P MAS NMR spectra; a
single 31P band assigned to framework tetrahedral phosphorus
was observed at d 229.8 for VAPO-5 and TAPO-5 samples
whereas VTAPO-5 showed a shift to d 227.2. The overall
characterization results indicates that the novel VTAPO and its
analogs TAPO-5 and VAPO-5 are pure and well crystallized.
Previous work indicates that the incorporated metal environ-
ment is essentially the same for all substituted AlPO analogs
and is quite different from that in metal silicates. However,
VTAPO-5 is found to be an active catalyst for the partial
oxidation of methanol. The catalytic measurements were carried
out in a conventional continuous flow differential reactor
system. The typical reaction products were methyl formate,
formaldehyde, acetaldehyde dimethyl ether, formic acid, CO
acetaldehyde, methyl formate, formaldehyde, dimethyl ether
and carbon oxides. While titanium itself is not an active site for
the partial oxidation of methanol, VTAPO-5 selectively gen-
erates methyl formate and acetaldehyde which are more
complex than products generated from pure VAPO-5. Thus
titanium plays an important role in catalysis possibly due to
strong V–Ti interactions. Verification of the precise reaction
mechanism is linked to the measurement of the quantity and
nature of the active sites. The fact that titanium modifies the
active reaction sites would explain why VAPO-5 does not
produce acetaldehyde or methyl formate. Further investigation
of the catalytic behavior of VTAPO-5 and analogous VAPO-5
catalysts in various hydrocarbon conversion reactions such as
oxidative dehydrogenation and epoxidation are in progress.
2
and CO . Measurement of formic acid was performed by acid–
base volumetric analysis while other products were analyzed by
gas chromatography. The methanol oxidation reaction results
Notes and references
(
at steady state) and reaction conditions are summarized in
1
W. M. Meier, D. H. Olson and Ch. Baerlocher, Atlas of Zeolite structure
Types, Elsevier, Amsterdam–New York, 1996.
Table 2. The catalytic tests were conducted at 200 °C and
measured at different contact times (space velocities) of the feed
under differential conditions. TAPO-5 was found to be totally
inactive for methanol oxidation at the reaction condition studied
except for very low activity observed at higher temperature ( >
2 S. T. Wilson, B. M. Lok, C. A. Messina, T. R. Cannan and E. M.
Flanigen, J. Am. Chem. Soc, 1982, 104, 1146.
3
4
5
N. Taramasoo, G. Perego and B. Notari, US Pat. 4410501, 1983.
J. S. Reddy, R. Kumar and P. Ratnasamy, Appl. Catal., 1990, 58, L1.
M. A. Camblor, A. Corma, A. Martinez and J. Perez-Pariente, J. Chem.
Soc., Chem. Commun., 1992, 589.
2
80 °C). VAPO-5 showed a significant activity. The major
products obtained were formaldehyde and CO along with small
amount of CO and dimethyl ether. On VTAPO a considerable
quantity of methyl formate and acetaldehyde were obtained as
major products in addition to CO, formaldehyde, CO and
6
J. Kornatovski, M. Sychev, S. Kuzenkov, K. Strnadova, W. Pilz, D.
Kassner, G. Pierper and W. H. Baur, J. Chem. Soc., Faraday Trans.,
2
1
995, 91, 2217.
2
7 H. E. B. Lampers and R. A. Sheldon, Stud. Surf. Sci. Catal. 1997, 105,
1061.
8 N. Ulagappan and V. Krishnasamy, J. Chem. Soc., Chem. Commun.,
dimethyl ether. For methanol oxidation the product formation is
proposed to be accompanied by several known complex
equlibrium reactions (1)–(7) (Scheme 1).
1
995, 589.
M. H. Z. Niaki, P. N. Joshi and S. Kaliaguine, Chem. Commun., 1996,
7.
0 M. H. Z. Niaki, M. P. Kapoor and S. Kaliaguine, J. Catal., 1998, 177,
31.
9
With regard to the influence of contact time of the feed on the
activity and selectivity, the system clearly showed that
increased contact time improves methanol conversion and
selectivities to methyl formate, acetaldehyde and CO with a
decrease in the selectivity to formaldehyde. Thus, it can be
proposed that formaldehyde is the initial product of methanol
partial oxidation and is an intermediate species in the produc-
tion of methyl formate, acetaldehyde and CO while the
4
1
1
2
1 M. H. Z. Niaki, M. P. Kapoor and S. Kaliaguine, Proc. 12th IZC,
Baltimore, USA, 1998, p. 1221.
12 W. E. Slinkard and P. B. DeGroot, J. Catal, 1981, 68, 423.
13 J. S. Gonzalez, M. Martinez-Lara, M. A. Ba n˜ ares, M. V. M. Huerta, E.
R. Castell o´ n, J. L. G. Fierro and A. J. Lopez, J. Catal, 1999, 181,
2
80.
4 M. J. Haannepen and J. H. C. van Hooff, Appl. Catal., 1997, 152, 183;
03.
2
formation of dimethyl ether and CO are independent of the
production of the major products. According to these results, the
methanol chemisorbs on vanadium species as a surface methoxy
1
1
2
5 C. S. Blackwell and R. I. Patten, J. Phys. Chem., 1988, 92, 3965.
3
(CH O), and depending on the nature of Ti sites present on the
surfaces of the catalyst, can react via various pathways to form
Communication 9/03876K
1410
Chem. Commun., 1999, 1409–1410