Study of trioxane production process with super- or subcritical fluid as solvent and extractant

Article abstract of DOI:10.1080/00397910600631908

A super- or subcritical fluid was used as a reaction solvent for nonaqueous trioxane synthesis instead of common organic solvents. The generation of trioxane from paraformaldehyde was observed in the presence of the catalyst when carbon dioxide reached a supercritical region, suggesting that the supercritical carbon dioxide acted as the reaction solvent. In the case of Freon 12, the trioxane was effectively produced even in a subcritical state. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910600631908

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Study of Trioxane Production Process with
Super‐ or Subcritical Fluid as Solvent and
Extractant
Michio Tanaka a & Kenji Ogino b
a Resarch and Development Planning and Coordination Division , Mitsui
Chemicals , Tokyo, Japan
b Graduate School of Bio‐applications and Systems Engineering , Tokyo
University of Agriculture and Technology , Tokyo, Japan
Published online: 20 Aug 2006.
To cite this article: Michio Tanaka & Kenji Ogino (2006) Study of Trioxane Production Process with Super‐ or
Subcritical Fluid as Solvent and Extractant, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 36:14, 1927-1932, DOI: 10.1080/00397910600631908
To link to this article: http://dx.doi.org/10.1080/00397910600631908
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Synthetic Communicationsw, 36: 1927–1932, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600631908
Study of Trioxane Production Process
with Super- or Subcritical Fluid
as Solvent and Extractant
Michio Tanaka
Resarch and Development Planning and Coordination Division,
Mitsui Chemicals, Tokyo, Japan
Kenji Ogino
Graduate School of Bio-applications and Systems Engineering,
Tokyo University of Agriculture and Technology, Tokyo, Japan
Abstract: A super- or subcritical fluid was used as a reaction solvent for nonaqueous
trioxane synthesis instead of common organic solvents. The generation of trioxane
from paraformaldehyde was observed in the presence of the catalyst when carbon
dioxide reached a supercritical region, suggesting that the supercritical carbon
dioxide acted as the reaction solvent. In the case of Freon 12, the trioxane was effec-
tively produced even in a subcritical state.
Keywords: Carbon dioxide, Freon 12, subcritical fluid, supercritical fluid, trioxane
synthesis
INTRODUCTION
Trioxane is the cyclic trimer of formaldehyde and the starting material for
high-performance polyacetal. The formation of trioxane is an equilibrium
reaction. The equilibrium constant is so small that only 3–4 wt% of trioxane
can be generated even if more than 60 wt% of higher concentration of
formalin, an aqueous solution of formaldehyde, is used as the raw
Received in Japan October 24, 2005
Address correspondence to Michio Tanaka, Resarch and Development Planning
and Coordination Division, Mitsui Chemicals, Inc., 1-5-2 Higashi-Shimbashi,
Minato-ku, Tokyo 105-7117, Japan. E-mail: michio.tanaka@mitsui-chem.co.jp
1927

1928
M. Tanaka and K. Ogino
material.[1] In addition, the trioxane and water system makes an azeotropic
mixture.[2] That means a great amount of energy is necessary to separate
pure trioxane from the water–formaldehyde binary system.[3] Therefore, the
development of a low-energy-consumption process for trioxane production
is very important from a practical viewpoint. Several sophisticated methods
have been proposed to improve energy efficiency for the trioxane synthesis.[4,5]
Previously we found that trioxane can be concentrated up to 20 to 65 wt%
by weight from the trioxane–HCHO–H2O system by extraction with super-
critical fluids[6] and proposed the reactive extraction process for trioxane
synthesis utilizing supercritical fluid.[7,8] In the process, it was found that
carbon dioxide is the best extractant, and a trioxane solution at a concentration
of more than 40% was obtained in one pot with supercritical fluid extraction
from aqueous formaldehyde and a catalyst.
To reduce the production cost, a nonaqueous process is one of the candi-
dates, because the process of separation of trioxane from the azeotropic
mixture with water can be omitted. Because nonaqueous formaldehyde has
been industrially produced from the aqueous source until now, no attention
has been paid to nonaqueous process. If a reasonable process like the dehydro-
generation of methanol[9] for the production of nonaqueous formaldehyde is
developed, a nonaqueous process will be of importance.
In this report, a super- or subcritical fluid of carbon dioxide and Freon 12
was used as a reaction solvent for trioxane synthesis in the absence of water.
Compared with conventional organic solvents, this type of fluid is more easily
vaporized under an atmospheric pressure. This leads to the easy separation of
product and makes the reuse of the fluid as a reaction solvent facile.
EXPERIMENTAL
In an autoclave of 50 ml equipped with a stirrer, 1.0 to 1.5 g of 98% parafor-
maldehyde, 0.3 g of the catalyst, and a predetermined quantity of dry ice as
carbon dioxide source (Tc ¼ 304.1 K, Pc ¼ 7.2 MPa) or Freon 12 (CCl2F2)
(Tc ¼ 385.0 K, Pc ¼ 4.1 MPa) were placed. The autoclave was heated to
reaction and extraction temperature, and the temperature was kept constant
for 6 to 11 h under shaking and stirring. After that, a super- or subcritical
phase was introduced to dry-ice traps, keeping the autoclave at the extraction
temperature. The extract was dissolved in 1-propanol and analyzed by gas
chromatography with thermal conductivity detector detection, helium gas
carrier, using a glass column of 2 m  3 mm, packed with 20% PEG-20 M
on Chromosorb WAW DMCS. Injector and detector temperatures were
473 K, and column temperature was 413 K. Formaldehyde concentration
was analyzed by aqueous sodium sulfite titration method directed by JIS
K1502, and water content was measured by the Karl Fisher titration method.
Residual materials in the autoclave were also analyzed by gas chromato-
graphy and titration methods mentioned previously as a 1-propanol solution.

Trioxane Synthesis
1929
The supported catalyst, 10 wt% phosphoric acid on silica gel, was
prepared as follows: sieved silica gel (20 mesh pass/48 mesh on) was
dipped into aqueous phosphoric acid. The weight fraction of phosphoric
acid was adjusted at 10 wt%. The mixture was dried overnight at 393 K and
was calcinated successively at 493 K for 5.5 h. The acid strength (H0) by an
indicator method was 23.0.
RESULTS AND DISCUSSION
We examined whether a supercritical fluid can be utilized as a reaction solvent
as well as whether produced trioxane can be efficiently extracted and/or
separated by the supercritical fluid under a practically nonaqueous system.
Supported catalysts were used for the convenience of separating trioxane.
Moreover, inhomogeneous catalysts were convenient for keeping the
generated trioxane away from the influence of a strong acid because
trioxane is easily decomposed to formaldehyde in the presence of a strong
acid. Here we chose 10 wt% phosphoric acid on silica gel as the supported
catalyst, of which acid strength was 23.0, and reactions were performed at
the temperature of 373 K. Higher temperature is desirable for the solvation
or plasticization of paraformaldehyde. However, at the higher temperature,
we found that the side reactions were not negligible and the decomposition
of trioxane occurred.[10] Taking the reaction rate into consideration, the
reaction temperature was optimized at 373 K.
The reaction and extraction/separation were carried out by placing the
paraformaldehyde and catalyst, which were basic materials, the supercritical
fluid as a reaction solvent, and the extraction/separation media in an
autoclave. The experimental apparatus is illustrated schematically in Fig. 1.
In Fig. 2, a trioxane yield is plotted against an operating pressure when
carbon dioxide was used as a reaction and extraction solvent. Here, the
yield is defined as the total yield both for vapor phase (extracted) and liquid
phase (residual fraction).
Figure 2 shows that when carbon dioxide reached a supercritical region, a
quantity of produced trioxane abruptly increased, suggesting that the super-
critical carbon dioxide acted as the effective reaction solvent. We also
Figure 1. Schematic representation of reaction process.

1930
M. Tanaka and K. Ogino
Figure 2. Trioxane synthesis in carbon dioxide. Initial charge: 98% paraformalde-
hyde, 1.5 g; catalyst: 10 wt% phosphoric acid on silica gel, 0.3 g; temperature:
373 K; and reaction time: 6 h.
examined subcritical fluid using Freon 12 as a reaction solvent. Although
supercritical carbon dioxide has a solvating power as a reaction media of
trioxane synthesis as mentioned previously, more efficiency is expected
using a more polar medium, because liquefied gas or subcritical fluid can be
thought to have more solvating power. From the viewpoint of a reaction
solvent, common organic solvents such as 1,2-dichloroethane have strong
solvating power, but considering the isolation process of trioxane from the
reaction mixture, the super- or subcritical fluid, which easily vaporizes
under an atmospheric pressure, can be considered to be more preferable
because of the energy savings.
Freon 12 was chosen as a subcritical fluid because of the following
reasons. The critical temperature of Freon 12 (Tc ¼ 385.0 K) is higher than
the reaction temperature (373 K). A small difference between both tempera-
tures makes it possible for the reaction to be carried out in the subcritical
region, and its boiling point is so low (229.88C) that a facile and low-cost
purification process is possible. Another reason is the chemical stability of
Freon 12. Because the formaldehyde is a highly reactive substance, a nonreac-
tive medium is necessary for the reaction solvent under the reaction condition.
Paraformaldehyde and catalyst were placed in the autoclave, as well as
Freon 12 as a solvent. The reaction was performed at 373 K, which did not
reach the critical temperature of Freon 12. The pressure was released while
retaining the temperature at 373 K, and a distillate accompanied by subcritical
fluid and a residue inside the autoclave were analyzed.
Figure 3 shows that trioxane was also produced even in the liquefied gas
or subcritical fluid as a solvent. The higher the pressure the more the yield was
improved, and the total yield of trioxane was nearly three times higher than

Trioxane Synthesis
1931
Figure 3. Trioxane synthesis in Freon 12: V Trioxane/gas (mg/1); B Trioxane total
yield (%); initial charge: 98% paraformaldehyde, 1.0 g; catalyst: 10 wt% phosphoric
acid on silica gel, 0.3 g; temperature: 373 K; and reaction time: 6 h.
that of the supercritical carbon dioxide. It can be said that the liquefied gas or
subcritical fluid is more preferable as a reaction solvent than supercritical
carbon dioxide. Recently Abbott et al. reported that the rates of Friedel–
Crafts alkylation reaction in supercritical difluoromethane were higher than
in carbon dioxide at comparable conditions.[11]
Taking the separation of trioxane from the reaction mixture into account,
supercritical fluid is more preferable, because the solubility of trioxane can be
tuned just by changing its density with a small temperature or pressure change,
keeping the state supercritical. On the other hand, in the case of the utilization
of subcritical fluid as a reaction solvent, it is necessary to supply energy to
change its high-density state to the lower-density one, usually the vapor
phase, for the drastic solubility change. This process corresponds to the
normal separation by flash distillation. It needs vaporization heat. Final separ-
ation cost depends on the balance of trioxane yield with solvent recovery
energy. There remains the possibility to use super- or subcritical fluid as a
solvent instead of organic solvent.
CONCLUSION
Effective synthesis and separation methods for trioxane production were
studied using super- or subcritical fluids. A quantity of produced trioxane
abruptly increased when carbon dioxide reached a supercritical region,
which shows that the supercritical carbon dioxide has enough solvating
power to allow the reaction to proceed.
Not only supercritical fluid but also liquefied gas or subcritical gas were
studied as a reaction solvent to examine an effective synthesis and separation
system.

1932
M. Tanaka and K. Ogino
The results show that liquefied gas or subcritical gas was more effective as
a solvent for the formation of trioxane than supercritical carbon dioxide,
which yielded about three times more trioxane.
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