The salt effect on the yields of trioxane in reaction solution and in distillate

Article abstract of DOI:10.1039/c5ra02821c

Batch reaction experiments were performed to investigate the salt effect on the yield of trioxane in the reaction solution. The salts considered include NaHSO₄, Na₂SO₄, NaH₂PO₄, Na₂HPO₄, KCl, NaCl, LiCl, ZnCl₂, MgCl₂, and FeCl₃. The effects of the anionic structure and the cation charge density on the yield of trioxane in the reaction solution were elucidated and the mechanisms that govern such effects were established. It is shown that the first four salts exerted a negative effect on the yield of trioxane in the reaction solution and such an effect increased progressively from left to right. This trend is due to the formation of NaHSO₄, H₃PO₄, or (H₃PO₄ and NaH₂PO₄), which decreased the concentration of H⁺ in the solution. The latter six salts showed a positive effect on the yield of trioxane in the reaction solution. The salt effect paralleled the ability of the salt to decrease the water activity of the reaction solution and followed the order KCl < NaCl < LiCl < ZnCl₂ < MgCl₂ < FeCl₃. Continuous production experiments were performed to investigate the salt effect on the concentration of trioxane in the distillate. The salts considered were KCl, NaCl, LiCl, ZnCl₂, MgCl₂, and FeCl₃, and the salt effect increased progressively from left to right. Such an effect was shown to be determined by the ability of the salt to increase the yield of trioxane in the reaction solution and to increase the relative volatilities of trioxane and water and of trioxane and oligomers.

Full text of DOI:10.1039/c5ra02821c

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The salt effect on the yields of trioxane in reaction
solution and in distillate
Cite this: RSC Adv., 2015, 5, 37697
*
Liuyi Yin, Yufeng Hu, Xianming Zhang, Jianguang Qi and Weiting Ma
Batch reaction experiments were performed to investigate the salt effect on the yield of trioxane in the
reaction solution. The salts considered include NaHSO4, Na2SO4, NaH2PO4, Na2HPO4, KCl, NaCl, LiCl,
ZnCl2, MgCl2, and FeCl3. The effects of the anionic structure and the cation charge density on the yield
of trioxane in the reaction solution were elucidated and the mechanisms that govern such effects were
established. It is shown that the first four salts exerted a negative effect on the yield of trioxane in the
reaction solution and such an effect increased progressively from left to right. This trend is due to the
formation of NaHSO4, H3PO4, or (H3PO4 and NaH2PO4), which decreased the concentration of H+ in the
solution. The latter six salts showed a positive effect on the yield of trioxane in the reaction solution. The
salt effect paralleled the ability of the salt to decrease the water activity of the reaction solution and
followed the order KCl < NaCl < LiCl < ZnCl2 < MgCl2 < FeCl3. Continuous production experiments were
performed to investigate the salt effect on the concentration of trioxane in the distillate. The salts
considered were KCl, NaCl, LiCl, ZnCl2, MgCl2, and FeCl3, and the salt effect increased progressively
from left to right. Such an effect was shown to be determined by the ability of the salt to increase the
yield of trioxane in the reaction solution and to increase the relative volatilities of trioxane and water and
of trioxane and oligomers.
Received 13th February 2015
Accepted 9th April 2015
DOI: 10.1039/c5ra02821c
www.rsc.org/advances
the preparation of trioxane.8,9 In addition, an IL catalyst that
was successfully used in a pilot plant trial is less corrosive to
1 Introduction
instruments. However, use of this IL catalyst instead of sulfuric
acid lowers the yield of products. Furthermore, the price of this
IL catalyst is as high as US $128 000 per ton.10 Therefore, to date,
sulfuric acid is still the most generally used catalyst, as the
corresponding processing route is mature and the price of
sulfuric acid is low. However, the disadvantage of using sulfuric
acid as a catalyst for the synthesis of trioxane is that by-products
such as formic acid, methyl formate, and the like are extensively
formed. In particular, when the concentration of sulfuric acid
exceeds 8 wt% (weight percent), the formation of large amount
of by-products will decrease the yield of trioxane.11
In the last few decades, trioxane, the cyclic trimer of formal-
dehyde, has been studied with interest as a starting material for
the preparation of acetal resins, anhydrous formaldehyde,
pesticides, moulding materials, bonding materials, disinfectant
agents, and antibacterial agents.1 Due to their superior chem-
ical stability, mechanical strength, and plasticity, acetal resins
have found broad applications in areas where traditional metals
were applied.2,3 Accordingly, a rapid expansion of acetal resin
production has occurred worldwide.4,5 Such expansion requires
a more practical, more efficient, and more economic route for
the production of trioxane.
Fortunately, a positive salt effect has been found in the pal-
ladium(II) catalyzed isomerization of alkylidene cyclopropyl
ketones.12 The salt effect has also been identied in the reaction
of the in-mediated allylation of N-tert-butanesulnyl imines.13
More recently, the remarkable role of halide salt additives in the
Negishi reaction involving aryl zinc reagents has been repor-
ted.14 Therefore, the present efforts explored the effect of salts
on the formation of trioxane. The structure–activity relationship
for the salt effect on the synthesis of trioxane was elucidated for
the rst time. In addition, the batch reaction and continuous
production experiments were performed simultaneously for the
rst time to uncover new mechanisms that govern the salt effect
on the yield of trioxane in the reaction solution and in the
distillate. The results thus obtained are important for us to use
The usual method for the production of trioxane consists of
heating aqueous formaldehyde in the presence of an acid
catalyst such as sulfuric acid, acidic ion exchange resin, heter-
opolyacid, and acidic ionic liquid.1,4,5 Using either acidic ion
exchange resin or heteropolyacids as the catalyst yields rela-
tively good results. However, these methods require a high
concentration of formaldehyde, a large amount of catalyst or
high reaction conditions (e.g., temperature, pressure and
material purity).6,7 Recently, acidic ionic liquids (ILs) such as
imidazole- and pyridine-based ILs have been used as catalysts in
State Key Laboratory of Heavy Oil Processing and High Pressure Fluid Phase Behavior
& Property Research Laboratory, China University of Petroleum, Beijing 102249,
China. E-mail: huyf3581@sina.com; Tel: +86-10-89733846
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the (sulfuric acid–salt) system in place of sulfuric acid as a When the formaldehyde solution feed was nished, the distil-
catalyst for trioxane synthesis to decrease the concentration of late was analyzed by gas chromatography with a thermal
sulfuric acid and to achieve greater yields of trioxane in the conductivity detector (TCD).
reaction solution and in the distillate.
3 Typical reactions
2 Experimental
Aqueous solutions of formaldehyde are inherently reactive
complex multicomponent mixtures in which formaldehyde is
predominately bound in different oligomers:15
2.1 Materials and reagents
50 wt% aqueous formaldehyde solution was prepared by
concentrating ꢀ37 wt% aqueous formaldehyde solution (Sino-
pharm Chemical Reagent Co., Ltd) using vacuum distillation at
343.15 K. Analytical grade inorganic salts and sulfuric acid were
supplied by Aladdin Industrial Corporation (Shanghai, China)
and they were used without further purication.
CH2O + H2O # HOCH2OH
(1)
HO(CH2O)nꢂ1H + HOCH2OH # HO(CH2O)nH + H2O (2)
The most probable mechanism by which trioxane is formed
can be described as:15
2.2 Batch reaction
Hþ
ƒƒƒƒ!
A known amount of solution containing 50 wt% formaldehyde
and an inorganic salt was added to a 250 mL spherical reactor
equipped with a sampling device. A Pt/100 temperature sensor
monitored by a CSOOB digital thermometer was inserted into
the reactor to control the temperature with an uncertainty of
ꢁ0.1 K. The reactor was pressurized to 1.0 MPa with nitrogen
gas via a conduit system to prevent the solution from boiling.
Then, the reactor was heated in an electric heating jacket with
continuous stirring. When the temperature reached the pre-set
value (ꢀ371.15 K), a given amount of sulfuric acid was intro-
duced into the reaction solution to initiate the reaction and the
reaction time was recorded. The cyclotrimerization reactions
were typically allowed to proceed for 30 min. During the reac-
tion, a known amount of reaction solution was sampled and
analyzed at a certain interval (e.g., 3 min.) to monitor the change
with reaction time in the composition of the reaction solution.
At the same time, another known amount of reaction solution
was injected into a conical ask to determine the acid value of
the reaction solution by acid–base titration using a potentio-
metric titrimeter (Leici ZDJ-5).
HOðCH OÞ H
ðCH OÞ þ H O
(3)
ƒƒƒƒ
2
2
2
3
3
However, information on the true species distribution is not
available under different reaction conditions.15 HO(CH2O)3H is
usually not individually observable in experiments that involve
a high concentration of acid.15 Accordingly, it must use the
calculated concentration of HO(CH2O)3H. However, models
that can calculate the concentration of HO(CH2O)3H in the
reaction solution (formaldehyde–H2SO4) are not available to our
knowledge. Therefore, the formation of trioxane is usually
described in the literature as:15–17
Hþ
ƒƒƒƒ!
3CH O
2
ðCH OÞ þ H O
(4)
ƒƒƒƒ
2
2
3
and quantitative results are presented using overall concentra-
tions (neglecting the oligomer formation).15,16,18
4 Results
Fig. 1 compares the results of the two independent batch
experiments performed at 371.15 K.
2.3 Continuous production
It is clear that the reproducibility of the measurements is
highly satisfactory. Therefore, a series of batch experiments was
For the continuous production of trioxane, a 250 mL glass
reactor with a formaldehyde solution feeder and a packed tower
was used. The packed tower was lled by stainless steel raschig
rings. A reux condenser, which was added to the top cover of
the packed tower, was heated with warm water at approximately
323.15 K so that the vapor phase condensed on the inner face
of reux condenser without the precipitation of para-
formaldehyde. A Pt/100 temperature sensor monitored by a
CSOOB digital thermometer was inserted into the reactor to
control the temperature of reaction with an uncertainty of ꢁ0.1
K. The reaction mixture (100 mL) comprising 50 wt% formal-
dehyde solution, the acid catalyst, and the inorganic salt was
heated in an electric heating jacket. The reaction time was
recorded when the temperature reached ꢀ375.15 K. Aer 1 h,
the 50 wt% formaldehyde solution (100 mL) was continuously
fed to the reactor from the feeder, and the feed rate of the
formaldehyde solution was adjusted to ensure a constant level
Fig. 1 The results of the two independent batch reactions in [(50 wt%)
of the reaction solution. A reux ratio of 2 : 1 was maintained. formaldehyde–(0.4 mol Lꢂ1) H2SO4] performed at T ¼ 371.15 K.
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carried out to explore the effects of salts on the yield of trioxane
The experiments were made to uncover the mechanisms that
and on the formation of by-product(s) in the reaction solution. govern the effect of the addition of Clꢂ-based salts on the
The results thus obtained by adding various sodium salts are formation of trioxane in the reaction solution. The results are
shown in Fig. 2 and 3.
shown in Fig. 4 and show several features of interest. The
Fig. 2 shows that, in comparison with the results obtained concentration of trioxane in the reaction solution increased in
for (formaldehyde–H2SO4), the addition of Na2SO4, NaH2PO4, or the order KCl < NaCl < LiCl < ZnCl2 < MgCl2 < FeCl3, which is
Na2HPO4 considerably decreased the concentration of trioxane exactly the order for the decreasing radius of the constituent
in the reaction solution (formaldehyde–H2SO4–salt) over the cations of the salts considered. However, the mechanism that
entire experimental reaction time. The effect of NaHSO4 on the fundamentally controls the effect of the Clꢂ-based salts
yield of trioxane in the reaction solution was also negative. considered on the yield of trioxane in the reaction solution is
However, this effect was considerably smaller than that still poorly understood to date. Note that the yield of trioxane in
exerted by the above-mentioned salts. On the contrary, the [(50 wt%) formaldehyde–(0.4 mol Lꢂ1) H2SO4–(1 mol Lꢂ1) LiCl/
presence of NaCl increased the concentration of trioxane in ZnCl2/MgCl2/FeCl3] is greater than the yield in [(50 wt%) form-
the reaction solution over the entire experimental reaction aldehyde–(1.4 mol Lꢂ1) H2SO4]. The effect of these salts on the
time. The order observed in Fig. 3 for the increase in acid change in the acid strength of the reaction solution with reac-
strength of the reaction solution with the reaction time is tion time was small {see Fig. 3 for the rapidity of the change
Na2SO4 < NaH2PO4 < Na2HPO4.
with the reaction time in the acid strength of the reaction
solution [(50 wt%) formaldehyde–(0.4 mol Lꢂ1
H2SO4]}.
Therefore, the corresponding results are not shown.
)
The values of the rate constants (k) were determined by tting
the experimental data for the concentration of trioxane in the
reaction solution (cp) as a function of reaction time to eqn (5):19
3
dcp/dt ¼ k1cA ꢂ k2cp
(5)
where cA is the concentration of formaldehyde. k1 and k2 are the
rate constants for the forward reaction and the reverse reaction,
respectively. Therefore, the inuence of the Clꢂ-based salts
considered on the formation of trioxane is quantitatively rep-
resented by the ratio ksalt/k, where ksalt and k are the rate
constants for the salt-containing solution and for the salt free
solution, respectively. The results are shown in Table 1.
The results of the continuous production experiments are
summarized in Table 2. It is clear that the addition of the salts
considered simultaneously increased the reaction conversion
and time/space yield. In addition, such positive effects increase
Fig. 2 The effect of the addition of a sodium salt on the yield of tri-
oxane in the reaction solution [(50 wt%) formaldehyde–(0.4 mol Lꢂ1
)
H2SO4–(1 mol Lꢂ1) salt]. ( ): Na2HPO4; ( ): NaH2PO4; ( ): Na2SO4; ( ):
NaHSO4; ( ): NaCl. For the sake of comparison, the results obtained
for the reaction solution [(50 wt%) formaldehyde–(0.4 mol Lꢂ1) H2SO4]
are also shown and are symbolized by ( ).
Fig. 3 The effect of the addition of a sodium salt on the acid strength Fig. 4 The effect of the addition of a Clꢂ-based salt on the yield of
of the reaction solution [(50 wt%) formaldehyde–(0.4 mol Lꢂ1) H2SO4– trioxane in the reaction solution [(50 wt%) formaldehyde–(0.4 mol Lꢂ1
)
(1 mol Lꢂ1) salt]. ( ): Na2HPO4; ( ): NaH2PO4; ( ): Na2SO4; ( ): H2SO4–(1 mol Lꢂ1) salt]. ( ): KCl; ( ): NaCl; ( ): LiCl; ( ): ZnCl2; ( ):
NaHSO4; ( ): NaCl. For the sake of comparison, the results obtained MgCl2; ( ): FeCl3. For the sake of comparison, the results obtained for
for the reaction solution [(50 wt%) formaldehyde–(0.4 mol Lꢂ1) H2SO4] the reaction solution [(50 wt%) formaldehyde–(1.4 mol Lꢂ1) H2SO4] are
are also shown and are symbolized by ( ).
also shown and are symbolized by ( ).
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Table 1 The influence of the salts considered on the sulfuric acid
catalyzed formation of trioxane at ꢀ371.15 K
addition of Na2SO4, NaH2PO4, or Na2HPO4 signicantly
decreases the yield of trioxane in the reaction solution and that
the negative effect increases with decreasing Kqa value of the
corresponding acid in the order Na2SO4 < NaH2PO4 < Na2HPO4.
These results indicate that the effect of decreased concentration
of H+ in the reaction solution overrides the water-structuring
effect.
Salt
Cationic radius
(pm)
(1 mol Lꢂ1
)
k1,salt/k1
k2,salt/k2
KCl
NaCl
LiCl
ZnCl2
MgCl2
FeCl3
138
102
76
74
72
1.08
1.22
1.50
1.59
1.70
2.24
1.04
1.16
1.33
1.37
1.43
1.83
The presence of Clꢂ-based salts in the reaction solution may
inuence the yield of trioxane in several ways. First, according
to the model proposed by Debye et al.22 and Gross et al.,23 the
molecules of water tend to congregate in the vicinity of the salt
ions, in effect forcing the reactant molecules into the portions
of solution remote from the ion elds and therefore raising the
true concentration of the reactant in the latter regions. Second,
the addition of a salt to the reaction solution decreases water
activity of the solution and therefore favors the forward reaction
of eqn (3). The water activity of the binary solution (salt–H2O)
with the molality of the salt being 1 mol kgꢂ1 at 298.15 K is
0.9682, 0.9669, 0.9640, 0.9572, 0.9419, and 0.9268 for KCl, NaCl,
LiCl, ZnCl2, MgCl2, and FeCl3, respectively.24 This indicates that
the ability of these salts to decrease the water activity of the
reaction solution (formaldehyde–H2SO4–salt) increases in the
order KCl < NaCl < LiCl < ZnCl2 < MgCl2 < FeCl3,25 which is
exactly the order for the value of (k1,salt/k1)/(k2,salt/k2) (¼ 1.04,
1.05, 1.13, 1.16, 1.19, 1.22 from le to right, see Table 1) and the
order for the effect of these salts on the yield of trioxane in the
reaction solution (Fig. 4). In other words, the effect of these salts
on the yield of trioxane relies primarily on their ability to
decrease the water activity of the reaction solution, as is
demonstrated in Fig. 5, which shows that the rapid increase in
the value of (k1,salt/k1)/(k2,salt/k2) corresponds to the rapid
decrease in water activity (i.e., the rapid increase in the ability of
the salt to decrease the water activity of the reaction solution).
The Lewis acid strength of the cations of the salts considered
has been characterized by a D value related to the diagonal
elements and an O value related to the off diagonal elements of
the secular determinant.26 The values of D and O increase in the
order K+ < Na+ < Li+ < Mg2+ < Zn2+.26 This order means that the
effect of the present salts on the yield of trioxane in the reaction
65
Table 2 Effect of the added salt on the conversion and time/space
yield for trioxane production
Conversiona Time/space yieldb
Acid and salt
(%)
(g hꢂ1 Lꢂ1
)
0.4 mol Lꢂ1 H2SO4
18.07
19.08
47.51
49.42
55.17
63.28
64.56
67.89
86.89
58.20
0.4 mol Lꢂ1 H2SO4 + 1 mol Lꢂ1 KCl
0.4 mol Lꢂ1 H2SO4 + 1 mol Lꢂ1 NaCl 22.07
0.4 mol Lꢂ1 H2SO4 + 1 mol Lꢂ1 LiCl 27.20
0.4 mol Lꢂ1 H2SO4 + 1 mol Lꢂ1 ZnCl2 28.79
0.4 mol Lꢂ1 H2SO4 + 1 mol Lꢂ1 MgCl2 30.63
0.4 mol Lꢂ1 H2SO4 + 1 mol Lꢂ1 FeCl3 38.47
1 mol Lꢂ1 H2SO4
23.06
a
b
The percent of trioxane converted. The grams of trioxane formed
within one hour in one liter of solution.
in the order KCl < NaCl < LiCl < ZnCl2 < MgCl2 < FeCl3. Notably,
it can be deduced from Table 2 that the reaction conversion and
time/space yields in [(50 wt%) formaldehyde–(0.4 mol Lꢂ1
)
H2SO4–(1 mol Lꢂ1) LiCl/ZnCl2/MgCl2/FeCl3] were greater than in
[(50 wt%) formaldehyde–(1.4 mol Lꢂ1) H2SO4]. This suggests
that the salt effects on the reaction conversion and time/space
yield cannot be understood solely by the Lewis acid strength
of the salt added.
5 Discussion
The mechanisms that govern the effect of the sodium salts
considered on the formation of trioxane in the reaction solu-
tions are very complex. The value of the acid dissociation
constant Kqa decreases in the order H2SO4 (ꢀ102) > HSO4ꢂ (1.0 ꢃ
ꢂ
10ꢂ2) > H3PO4 (6.7 ꢃ 10ꢂ3) > H2PO4 (6.2 ꢃ 10ꢂ8).20 Accord-
ingly, the addition of Na2SO4, NaH2PO4, or Na2HPO4 to the
reaction solution (formaldehyde–H2SO4) must have induced the
formation of NaHSO4, H3PO4, or (H3PO4 and NaH2PO4),
respectively. Such formation inevitably decreases the concen-
tration of H+ that serves as the catalyst for trioxane synthesis.
On the other hand, NaH2PO4 and Na2HPO4 are kosmotropic
salts (water-structuring salts).21 The addition of these salts to
the reaction solution may induce the formation of an oligomer-
rich phase and a salt-rich phase (both of which are aqueous).21
Such formation will increase the true concentrations of the
oligomers including the reactant. Therefore, their effect on the
yield of trioxane in the reaction solution will be a result of the
Fig. 5 The changes with cation radius of the Clꢂ-based salts in (k1,salt
/
competition between these two factors. Fig. 2 shows that the k1)/(k2,salt/k2) and water activity.
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solution is not dominated by the Lewis acid strength of their NaH2PO4), which inevitably decreases the concentration of H+
cations. In addition, an unexpected decrease in the yield of in the solution. The effect of KCl, NaCl, LiCl, MgCl2, ZnCl2, and
trioxane has been observed by the addition of NaHSO4 to the FeCl3 on the yield of trioxane in the reaction solution relies
reaction solution (Fig. 2). These comparisons reveal that the salt primarily on their ability to decrease the water activity of the
effect on the yield of trioxane in the reaction solution does not reaction solution.
rely solely on the Lewis acid strength of the salt.
Continuous production experiments were performed to
It is notable that salt effects on the palladium(II)-catalyzed investigate the salt effects on the concentration of trioxane in
regioselective isomerization of methylenecyclopropyl ketone the distillate. The results showed that the addition of KCl, NaCl,
apparently increase from LiBr to NaI and from Bu4NBr to Bu4NI LiCl, MgCl2, ZnCl2, and FeCl3 to the reaction solution can
(Table 1 of ref. 12), indicating that salt effects increase with considerably increase the concentration of trioxane in the
increasing radius of the constituent cation or/and anion of the distillate. The salt effect increased progressively from le to
salts. On the contrary, salt effects on the yield of trioxane in the right. Such an effect is a manifestation of the synthetic perfor-
reaction solution decrease with increasing radius of the cations mance of the salt to increase the yield of trioxane in the reaction
of the salts. The salt effect on asymmetric synthesis of chiral solution and to increase the relative volatility of trioxane and
homoallylic amines by the in-mediated allylation of chiral N- water and of trioxane and oligomers.
tert-butanesulnyl imines in aqueous media at room tempera-
ture follow the order LiBr ¼ KBr, NaCl < NaI < NaBr (Table 1 of
Acknowledgements
ref. 13). Therefore, no clear connection between the salt effect
and the radius of the constituent cation or/and anion of the salt We acknowledge the National Natural Science Foundation of
is observed.
China (21076224 and 21276271) and Science Foundation of
During the continuous production of trioxane, it is removed China University of Petroleum, Beijing (qzdx-2011-01) for
as a distillate from the reaction solution in the distillation nancial support.
tower. Therefore, the effect of salts on the conversion and
time/space yield will emerge as manifestations of much
more inuential factors. In addition to their effect on the
References
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and water and of trioxane and HO(CH2O)nH, as the specic
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salt are considerably smaller than those between water
(or HO(CH2O)nH) and these ions due to the lack of the –OH
group(s) in the molecular structure of trioxane. The heat of
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