Table 1 The secondary structures and the catalytic activities of MTBE synthesis over H
3
PW12O40 and H
6 2 18 62
P W O
Secondary structure
Catalytic activitya
Primary
structure
Pretreatment
temperature/K
Before
reaction
After
reaction
Initial
Stationary
H
3
PW12
O
40
423
523
423
Cubic
Amorphous Cubic
Cubic
2
20
> 0
< 2
(Keggin type)
H
6
P
2
W
18
O
62
®
©
Amorphous Amorphous 17
18
(Dawson type) 523
a
MTBE yields (%) at the initial (10–15 min) and stationary (after 2 h) stages.
(
a)
(b)
easily for the Dawson-type at low temperatures, both hetero-
polyacids were nearly anhydrous after treatment at 523 K.
Moreover, the differences in the water content between the
(
i)
(i)
pretreatments at 423 and 523 K were smaller for H
3
PW12
O
40
(
0.3 H O/heteropolyanion) than for H 62 (ca. 1 H
2
6
P
2
W
18
O
2
O/
heteropolyanion). This is in contrast to the fact that the catalytic
behaviour differed more for the Keggin-type [Fig. 1(a)] than the
Dawson-type [Fig. 1(b)] between the 423 and 523 K treatments.
Hence, the water content is not the main factor controlling these
activities.
(ii)
(ii)
3
We previously indicated that the high catalytic activity of
6 2 18
H P W O62 is brought about by a high-activity state of the
pseudo-liquid phase in which moderate amounts of molecules
are absorbed and the absorption–desorption is rapid, while the
(
iii)
(iii)
(iv)
3
pseudo-liquid phase of H PW12O40 is in a low-activity state
where the absorption is excessive and slow. The transformation
between active and less active pseudo-liquid phases with partial
pressure was reported previously.10 If this conclusion is
combined with the present results summarized in Table 1, an
interesting correlation is deduced between the nature of the
pseudo-liquid phase (secondary structure) and the polyanion
structure (primary structure). That is, the elliptical shape of the
Dawson anion is not suitable for the formation of a stable
crystalline structure and leads to an amorphous and flexible
structure, the absorption–desorption being easier and catalytic
activity high, while the spherical shape of the Keggin anion
favors a crystalline cubic structure, where absorption–desorp-
tion is slow and catalytic activity is low.
(iv)
20
30
40 20
q / °
30
40
2
Fig. 2 Powder XRD patterns of H
3
PW12
O
6 2 18
40 (a) and of H P W O62 (b),
before and after MTBE synthesis reaction. (i) After pretreatment at 423 K,
ii) after reaction at 323 K of the sample (i), (iii) after pretreatment at 523
(
K, and (iv) after reaction of the sample (iii). (5) Peaks of polyethylene film,
(
*) peaks due to cubic structure and (8) unknown structures.
In contrast, for the Dawson-type heteropolyacid, the XRD
A part of the present study was presented at the Symposium
lines were weak and broad before and after the reaction for both
treatments [Fig. 2(b)(i)–(iv)], indicating that the secondary
structure was always mostly amorphous. IR spectra showed that
the primary structure (heteropolyanion) was retained. Although
there remain unidentified structures, it is evident that the
samples of Fig. 2(a)(i), (ii) and (iv) are mostly crystalline and
those of Fig. 2(b)(i)–(iv) and Fig. 2(a)(iii) are mostly amor-
phous.
of Catalysis Society of Japan.11
Notes and References
† E-mail: tmisono@hongo.ecc.u-tokyo.ac.jp
1
2
A. Igarashi, T. Matsuda and Y. Ogino, J. Jpn. Petrol. Inst., 1979, 22,
31.
S. Shikata, T. Okuhara and M. Misono, Sekiyu Gakkaishi, 1994, 37,
32.
3
These structural data are compared with the results of
catalytic activity in Table 1. A close correspondence was noted
in Table 1 between the secondary structures and the reaction
rates observed for the eight cases; two heteropolyacids with two
different pretreatments for the initial and stationary states. A
high rate was always observed when the secondary structure
was amorphous, and a low rate was observed for crystalline
6
3 S. Shikata, T. Okuhara and M. Misono, J. Mol. Catal., 1995, 100, 49.
4 M. Misono, Catal. Rev., -Sci. Eng., 1987, 29, 269; K. Y. Lee, T. Arai,
S. Nakata, S. Asaoka, T. Okuhara and M. Misono, J. Am. Chem. Soc.,
1
992, 114, 2836.
5
6
7
T. Okuhara, T. Hashimoto, M. Misono, Y. Yoneda, H. Niiyama,
Y. Saito and E. Echigoya, Chem. Lett., 1983, 573.
S. Shikata, S. Nakata, T. Okuhara and M. Misono, J. Catal., 1997, 166,
3
cubic structures. For example, H PW12O40 treated at 523 K
2
63.
showed high activity at first, but the activity declined to a very
low level at the steady state, accompanying the change of the
secondary structure from amorphous to crystalline (cubic),
G. M. Brown, M.-R. Noe-Spirlet, W. R. Busing and H. A. Levy, Acta
Crystallogr., Sect. B, 1977, 33, 1038.
8 K. Na, T. Okuhara and M. Misono, J. Chem. Soc., Faraday. Trans.,
1995, 91, 367.
9 F. Lefebvre, F. X. Liu-Cai and A. Auroux, J. Mater. Chem., 1994, 4,
6 2 18
while H P W O62 always showed a high activity and amor-
phous XRD pattern. It was confirmed experimentally that
surface areas and pore volumes of the two heteropolyacids little
changed with the change of the pretreatment temperature (423
or 523 K). The acid strength also remains constant. According
to the DTA–TG analyses, although the water desorbs more
1
25.
1
1
0 K. Takahashi, T. Okuhara and M. Misono, Chem. Lett., 1985, 841.
1 S. Shikata and M. Misono, Shokubai, 1997, 39, 174.
9
Received in Cambridge, UK, 18th March 1998; 8/02138D
1294
Chem. Commun., 1998