Synthesis of trioxane using heteropolyacids as catalyst

Article abstract of DOI:10.1002/1521-3773(20000616)39:12<2102::AID-ANIE2102>3.0.CO;2-E

A high formaldehyde feed concentration and a low reaction temperature are among the advantages of using heteropolyacids (phosphotungstic acid and silicotungstic acid) as catalysts in the formation of trioxane; this cyclic trimer of formaldehyde (see scheme) is used in the industrial production of acetal resins. Compared to reactions with a conventional catalyst, like sulfuric acid, trioxane can be produced in higher yield and with higher selectivity.

Full text of DOI:10.1002/1521-3773(20000616)39:12<2102::AID-ANIE2102>3.0.CO;2-E

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Soc. 1998, 120, 11279 ± 11284; f) P. A. Evans, V. S. Murthy, Tetrahe-
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homopolymer utilizing the worldꢁs third type of polyacetal
technology.[3] Asahi Chemical also industrialized the acetal
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resins in the world is about 400000 t per year.
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phenylsiloxy)methyl]tetrahydrofurane-2-carbaldehyde (U. Koert, M.
Stein, H. Wagner, Liebigs Ann. 1995, 1415 ± 1426) by side-chain
elongation with a Wittig reaction.
The biggest problem in the case of acetal resins is the
energy consumption during its production. The main aspect is
the energy requirement in the monomer process. For example,
it requires a great deal of energy to get the monomeric
trioxane that is needed for the acetal copolymer from aqueous
formaldehyde. In the commercial process, trioxane is obtained
by heating aqueous formaldehyde in the presence of an acid
catalyst like sulfuric acid [Eq. (1)].
[12] L. Syper, K. Kloc, J. Mlochowski, Z. Szula, Synthesis 1979, 521 ± 522.
[13] 14 was prepared by routes described for the synthesis of squamocin D
see reference [8].
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Kuwabara, H. Iwamura, T. Yagi, Biochemistry 1998, 37, 6436 ± 6445;
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Presley, G. P. McCabe, J. L. McLaughlin, Nat. Toxins 1996, 181 ± 188.
[15] A. L. Smith, Methods Enzymol. 1967, 10, 81 ± 86.
[16] P. C. Hinkle, M. L. Yu, J. Biol. Chem. 1979, 254, 2450 ± 2455.
Even though the equilibrium concentration of trioxane is
low in the reaction mixture, in the commercial production
process trioxane is removed as the distillate from the reaction
mixture in the distillation tower.[5] The vapor± liquid equili-
brium between trioxane and aqueous formaldehyde is such
that when the trioxane concentration in the liquid phase is
low, trioxane shows a very high volatility compared to
formaldehyde and water in the vapor phase. Thus, almost all
the trioxane contained in the vapor phase from the reaction
mixture can be concentrated into the distillate in the
distillation tower under the proper refluxing conditions. Since
the heat of vaporization of the water± formaldehyde mixture
is much higher than that of trioxane,[6] most of the energy for
trioxane synthesis is consumed in the vaporization of water
and formaldehyde.[7] From the viewpoint of the energy
requirement, the key point of the trioxane synthesis is the
high yield and high selectivity in a one-pass vaporization.
At Asahi Chemical, we developed the tert-butyl alcohol
process, that is the selective hydration of isobutene using a
highly concentrated heteropolyacid as the catalyst.[8] We also
developed a new process for producing polyoxytetramethy-
lene glycol with a narrow molecular weight distribution using
a heteropolyacid as a catalyst for the polymerization of
tetrahydrofuran.[9, 10] With this knowledge, we investigated the
catalytic activity of heteropolyacids for the synthesis of
trioxane. We observed some interesting phenomena and a
superior advantage of heteropolyacids over conventional
catalysts like sulfuric acid.
Synthesis of Trioxane Using Heteropolyacids as
Catalyst
Junzo Masamoto,* Katsuhiko Hamanaka,
Kohichi Yoshida, Hajime Nagahara, Kenji Kagawa,
Toshiyuki Iwaisako, and Hajime Komaki
Acetal resin is a term used to describe the high molecular
weight polymers and the copolymers of formaldehyde. First
commercialized as a homopolymer in 1960 by DuPont, acetal
resins are engineering thermoplastics which have found broad
use in areas where traditionally metals were applied.[1] Shortly
thereafter, researchers at Celanese developed an acetal resin
based on the copolymerization of trioxane and cyclic ethers,
such as ethylene oxide.[2] In 1962 a commercial plant began
producing this acetal copolymer. Since then, a rapid expan-
sion of acetal resin production has occurred worldwide.
Up to 1971 DuPont and Celanese (alone or in joint ventures
with other companies) were the sole producers of acetal
[*] Prof. J. Masamoto[‡]
Department of Polymer Science and Engineering
Kyoto Institute of Technology
Sakyo-ku, Kyoto 606-8585 (Japan)
E-mail: masamoto@ccmails.fukui-ut.ac.jp
The results for the reaction at atmospheric pressure and
1008C are shown in Table 1. The best features of using the
heteropolyacid, instead of sulfuric acid, as a catalyst for
trioxane synthesis were the higher conversion and selectivity.
For example, for the same selectivity of 97%, the conversion
by sulfuric acid was 20%, while the conversion by the
heteropolyacid was 27% (drawn from entries 10 and 6
respectively in Table 1). Heteropolyacids provided a 35%
higher yield (yield ˆ conversion  selectivity) than sulfuric
acid. A similar result was also obtained for the hydration of
isobutene.[8] In this case, high selectivity of heteropolyacids
was reported to be related to the big size of their anions.
K. Hamanaka, K. Yoshida, H. Nagahara, K. Kagawa, T. Iwaisako,
H. Komaki
Technical Research Laboratory
Asahi Chemical Industry Co., Ltd.
Fuji-shi 416- 8501 (Japan)
‡
[ ] New address:
Department of Management Science
Fukui University of Technology
Gakuen, Fukui-shi 910-8505 (Japan)
Fax : (‡81)776-29-7891
2102
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Table 1. Results from the synthesis of trioxane with heteropolyacids and
with sulfuric acid as the catalyst.[a]
Entry
Catalyst[b]
t [min][c]
Conv. [%][d, e]
S [%][e, f]
1
2
3
4
5
6
7
8[g]
9
10
11
12
13
14
PW (30)
39
34
50
23
42
27
41
19
3814.5
52
31
12
1822.3
27
18.3
24.6
25.5
22.0
22.7
26.7
27.5
28.0
98
17.4
17.9
21.0
94.5
23.6
98.5
98.3
98.1
99.3
99.2
97.1
95.4
97.1
PW (100)
PW (100)
SiW (30)
SiW (30)
SiW (100)
SiW (100)
SiW (100)
H2SO4 (2)
H2SO4 (5)
H2SO4 (10)
H2SO4 (30)
H2SO4 (30)
H2SO4 (30)
.5
97.5
97.4
96.5
Figure 1. Plot of the lowest temperature T for which upon cooling no
whitening (paraformaldehyde formation) occurs as a function of the
amount m of heteropolyacid per 100 g of formalin. The catalyst was
phosphotungstic acid; 55 wt% formalin was used; contact time 12 ± 30 min.
93.5
[a] The reaction was carried out at 1008C under atmospheric pressure;
unless noted otherwise, 55 wt% formalin was used. [b] PWˆ phospho-
tungstic acid, SiWˆ silicotungstic acid; the number in parentheses indi-
cates the amount of catalyst (in g) per initial 100 g of formalin. [c] The
contact time t is defined here for simplicity as follows: t [min] ˆ volume of
the reaction mixture [mL]/rate of feed formalin [mLminÀ1], assuming the
of heteropolyacid, ever lower temperatures could be reached
without whitening. In the case of a conventional catalyst like
sulfuric acid, with feeding of 55 wt% formalin, the critical
temperature was around 1008C. However, through the use of
heteropolyacids, we could work at lower reaction temper-
atures without whitening by increasing the amount of the
heteropolyacids in the reaction mixtures. In Figure 1, we show
the example of using phosphotungstic acid as catalyst. Similar
results were obtained in the case of silicotungstic acid. These
facts suggested that the heteropolyacids have a high solvation
power for formalin.
density of the feed formalin is 1.0 gmLÀ1
.
[d] Conversion [%] ˆ
{(3[trioxane] ‡ 2[methyl formate] ‡ [methylal])/[feed formalin]}  100.
Almost all the by-product is methyl formate, and only a small amount of
methylal was observed. The amount of formic acid was negligibly small.
Methyl formate is generated by the Cannizzaro reaction from two
molecules of formaldehyde. Methylal is formed by the reaction of two
molecules of methanol and one molecule of formaldehyde. [e] For the
calculation of conversion and selectivity S concentrations in molgÀ1
were used. [f] S [%] ˆ {[trioxane]/([trioxane] ‡ 2[methyl formate] ‡
[methylal])} Â 100. [g] 61 wt% formalin was used.
The effect of temperature on the trioxane synthesis is
shown in Table 2. An increase in temperature results in a
decrease in conversion and selectivity of trioxane formation.
Thus, high selectivity and high conversion were attained using
highly concentrated heteropolyacids under reduced pressure
(to achieve reaction temperatures of 81 ± 848C; entries 7 ± 10).
These phenomena are thought to be characteristic for
heteropolyacids and could not be achieved using a conven-
tional catalyst like sulfuric acid, considering the fact that in
the sulfuric acid catalyst system the critical temperature was
1008C for the 55 wt% formalin feed.
With continuous feeding of 55 wt% formalin to the
reaction mixture, no scale formation was observed for the
heteropolyacids; that is, no paraformaldehyde was formed.
However, for sulfuric acid, some whitening of the reaction
mixtureÐthat is, formation of paraformaldehydeÐwas ob-
served. Thus the upper limit of the feed formaldehyde
concentration using sulfuric acid as catalyst was considered
to be 55 wt%. On the other hand, for the heteropolyacids, by
increasing the amount of the heteropolyacid in the reaction
mixture from 30 to 100 g (per initial 100 g of formalin),
continuous feeding of 61 wt% formalin to the reaction
mixture was possible without any problems of scale forma-
tion. This phenomenon meant an increase in the solubility of
formalin in the presence of heteropolyacids, which might
result from the interaction of the heteropolyacids with an
ether linkage of the oligooxymethylene glycol.
In Figure 2, the relationships between the yield and
selectivity obtained under various conditions are shown. With
Table 2. Effects of temperature T on conversion and selectivity S in the
synthesis of trioxane.[a]
Increasing the formaldehyde concentration from 55 to
61 wt% with the same conversion and same selectivity means
an increase of 10% in the yield of trioxane per energy for
trioxane synthesis.[7] Thus, considering the increase in con-
version and the increase in feed formalin concentration, an
increase of 50% for the yield of trioxane per energy for
trioxane synthesis may be possible.
We investigated the solubility of formalin in the hetero-
polyacids. Figure 1 shows that increasing the weight percent of
heteropolyacid enhanced the solubility of formalin. The
reaction temperature was changed by altering the vaporizing
pressure. We thus determined the whitening temperature for
the reaction mixtures. With an increase in the weight percent
Entry
Catalyst[b]
T [8C]
t [min][c]
Conv. [%][d, e]
S [%][e, f]
1[g]
2[g]
3[g]
4[h]
5[h]
6[h]
7[i]
PW (30)
PW (30)
PW (30)
PW (30)
PW (100)
PW (100)
PW (300)
PW (400)
PW (400)
PW (600)
110
110
110
100
100
100
84
83
83
81
15
20
4822.9
39
34
50
25
15
37
17.7
18.3
97.3
18.3
24.6
25.5
30.1
27.5
28.9
31.2
98.6
98.1
98.5
98.3
98.1
98.4
99.2
98.6
99.2
8[j]
9[j]
10[k]
36
[a] 55 wt% formalin was used. [b] ± [f] See footnotes [b] ± [f] of Table 1.
[g] p ˆ 2.65 atm. [h] p ˆ 1 atm. [i] p ˆ 0.44 atm. [j] p ˆ 0.42 atm. [k] p ˆ
0.33 atm.
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condenser, and the condensate (water/formaldehyde/trioxane) was ana-
lyzed by gas chromatography. The level of the reaction mixture was
controlled by a photo level controller, and the feed rate of formalin was
adjusted to obtain a constant level of the reaction mixture. The contact time
was regulated by changing the vaporizing speed, which was adjusted by
changing the oil bath temperature. The measurement for the reaction was
carried out after the stationary state was confirmed; the carbon balance
(wt%) in feed formalin and distillate was equal.
For the trioxane synthesis above 1008C (that is, under pressure, 1108C), a
stainless steel reactor was used, feeding 55 wt% formalin and drawing out
the vapor phase into a condenser at low pressure until atmospheric pressure
was reached. For the trioxane synthesis below 1008C (that is, the reaction
under reduced pressure), the same system was used as for reactions at
atmospheric pressure.
Figure 2. Relationships between yield Y and selectivity S under various
conditions (see Tables 1 and 2). Unless noted otherwise, 55 wt% formalin
Received: December 29, 1999 [Z14485]
&
was used. The catalyst and reaction temperature are as follows: sulfuric
~
*
acid at 1008C, phosphotungstic acid at 1108C, phosphotungstic acid at
!
^
1008C,
phosphotungstic acid at 83 ± 848C,
phosphotungstic acid at
[1] a) C. E. Schweitzer, R. N. MacDonald, J. O. Punderson, J. Appl.
Polym. Sci. 1959, 1, 158; b) T. A. Koch, P. E. Lindvig, J. Appl. Polym.
Sci. 1959, 1, 164; c) W. H. Linton, H. H. Goodman, J. Appl. Polym. Sci.
1959, 1, 179 ± 184.
~
&
818C, silicotungstic acid at 1008C, silicotungstic acid at 1008C using
61 wt% formalin.
[2] C. Walling, F. Brown, K. Bartz (Celanese), US-A 3027352, 1962.
[3] a) Y. Kobayashi, I. Suzuki, S. Ishida, Hydrocarbon Process. 1972,
51(11), 111; b) K. Matsuzaki, J. Masamoto, Ind. Eng. Chem. Res. 1998,
37, 1729.
[4] J. Masamoto, T. Iwaisako, K. Yoshida, K. Matsuzaki, K. Kagawa, H.
Nagahara, Makromol. Chem. Macromol. Symp. 1991, 42/43, 409.
[5] J. S. T. Wall (Celanese), JP-A 73-11357, 1973.
an increase in yield, a decrease in selectivity was observed.
However, a decrease in reaction temperature was accompa-
nied by an increase in selectivity. Comparing the results
obtained using heteropolyacids with those obtained using
sulfuric acid, it is apparent that the heteropolyacid provides a
higher yield and higher selectivity than sulfuric acid. Espe-
cially, by lowering the reaction temperature, a higher yield
and higher selectivity can be attained.
In conclusion, considering the increase in conversion and
selectivity as well as the increase in formalin solubility upon
the use of heteropolyacids as a catalyst for trioxane synthesis,
it might be possible to increase the yield of trioxane by over
50% yield of trioxane per energy for trioxane synthesis.[7]
Therefore, heteropolyacids are worth developing as a catalyst
for the commercial production of trioxane. This would open a
new aspect for the industrial application of heteropolyacids as
a catalyst in addition to the synthesis of tert-butyl alcohol
(selective hydration of isobutene)[8] and polymerization of
tetrahydrofuran to polyoxytetramethylene glycol.[9]
[6] Heat of vaporization for an aqueous formaldehyde ± trioxane mixture:
water: 2.4, formaldehyde: 2.0, trioxane: 0.46 kJgÀ1
.
[7] If the trioxane concentration in the distillate of one pass vaporization
is 10 wt%, the energy requirement of trioxane synthesis is about 10 g
of steam per 1 g of trioxane, using steam as the heat source.
[8] A. Aoshima, Shokubai 1987, 29, 378.
[9] A. Aoshima, S. Tonomura, S. Yamamatsu, Polym. Adv. Technol. 1990,
2, 127.
[10] A. Aoshima, Shokubai 1991, 33, 34.
Experimental Section
We mainly used a 55 wt% aqueous formaldehyde solution, which was
prepared by dissolving the 84 wt% paraformaldehyde flake (Sumitomo
Chemical) in water at 1008C. For the heteropolyacids, we used the
commercial chemical grade of silicotungstic acid (H4SiW12O40 ´ 26H2O) and
phosphotungstic acid (H3PW12O40 ´ 29H2O), supplied by Nippon Inorganic
Color and Chemical, without further purification.
For the reactions, a 200-mL glass reactor was used. A thermal jacket, which
was added to the top cover of the reactor, was heated with steam so that the
vapor phase in the reactor did not condense on the inner face of the top
cover. Thus, the reaction gas mixture was correctly carried (without change
in composition) to the condenser without inner refluxing. (If partial
condensation occurred on the inside surface of the top cover, the relative
amount of trioxane was higher and thus the data measured gave the wrong
results.) The reaction mixture (100 mL) consisting of the catalyst and
55 wt% formalin feed was heated in an oil bath. Formalin was continuously
fed to the reactor, and the reaction product continuously vaporized from
the homogenous liquid reaction mixture. The vapor phase condensed in the
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