Acid-catalyzed Synthesis of Trioxane in Aprotic Media

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Acid-catalyzed Synthesis of Trioxane in Aprotic Media Wei-Ting Ma, Yu-Feng Hu, Jianguang Qi, Lihu Wei, Xian-Ming Zhang, Zhenyu Yang, and Siqi Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 29 May 2017 Downloaded from http://pubs.acs.org on June 3, 2017

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Industrial & Engineering Chemistry Research

Acid-catalyzed Synthesis of Trioxane in Aprotic Media Weiting Ma,† Yufeng Hu,*,† Jianguang Qi,† Lihu Wei,† Xianming Zhang,† Zhenyu Yang,† Siqi Jiang† †

State Key Laboratory of Heavy Oil Processing and High Pressure Fluid Phase

Behavior & Property Research Laboratory, China University of Petroleum, Beijing 102249, China. *Corresponding author. Email: [email protected] ABSREACT: Effects of solvent, acid specificity, acid concentration, added salt, and reaction temperature on the yields of trioxane and formic acid (by-product) in [paraformaldehyde + acid (or acid + salt) + aprotic solvent] were investigated. The mechanism that underlay the yield of paraformaldehyde and the selectivity of trioxane was determined. A highly practical and efficient synthesis of trioxane by salt-mediated and acid-catalyzed yield of paraformaldehyde in sulfolane media was developed. The method increased the yield of paraformaldehyde by more than 5 times and decreased the formic acid concentration by 10.0 times compared to the commercial synthesis of trioxane in aqueous reaction system (formaldehyde + H2SO4 + H2O). Keyword: Trioxane, Aprotic Solvent, Acid Catalyst, Salt Effect, Mechanism

1

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1. INTRODUTION Trioxane is the cyclic trimer of formaldehyde [(CH2O)3] and has been extensively used as polymerizing monomer to produce high-performance plastics such as polyoxymethylene (POM) polymers, which account for 80% POMs products all over the world1−3 and to produce poly(oxymethylene) dimethyl ethers (DMMn) that serve as components of tailored diesel fuel.4 Trioxane has also been widely used in preparation of bulk and fine chemicals.3 The biggest problem for POMs and DMMn products arises from trioxane production.3,5 Trioxane is produced from aqueous formaldehyde solutions by homogeneous catalysis with sulfuric acid at temperatures up to 373.15 K.2,,3,5 However, in aqueous solutions formaldehyde is almost entirely chemically distributed into methylene glycol HO(CH2O)H (MG1) and poly(oxymethylene) glycols HO(CH2O)nH (MGn) (n>1) by:6−8

CH 2 O ( Formaldehyde ) + H 2O ↔ HO ( CH 2 O ) H

(1)

HO(CH 2 O) n −1H + CH 2O ↔ HO(CH 2 O) n H

(2)

But only HO(CH2O)3H is real reactant that produces trioxane at the presence of sulfuric acid at about 373 K:8 HO(CH 2 O)3H ↔ (CH 2 O)3 (Trioxane) + H 2O

(3)

The equilibrium concentration of HO(CH2O)3H is a function of the overall concentration of formaldehyde. Therefore, the production of trioxane always starts with concentrating by distillation an aqueous formaldehyde solution from 37 wt% (weight percent) to 50−60 wt%. Even so, the equilibrium concentration of HO(CH2O)3H, and thereby the concentration of trioxane formed in aqueous formaldehyde solutions at 373 K, is still very low.3 Furthermore, under the reaction conditions several side reactions, in particular, the disproportionation of formaldehyde 2


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to formic acid and methanol (Cannizzaro reaction) and their esterification reaction, occur.1,2,9 The formed formic acid enters the azeotrope of trioxane and water, thereby causing serious corrosiveness to following equipments.2,9 The formic acid present in trioxane also decreases the velocity of polymerization of trioxane into POMs. The POMs thus obtained have a low degree of polymerization and therefore cannot have good physical properties. POMs are widely used in a wide variety of industries such as car manufacture, machinery, electricity, etc. The POMs market is continuously growing, and a rapid expansion of the facilities for production of trioxane has occurred worldwide.2,3 In recent years we have severe over capacity in methanol production,10 and air pollution levels in many countries have hit dangerous levels. Fortunately, using DMMn as components of diesel oils can reduce the amounts of powdered pollutants and NOx released upon combustion by 80–90% and 50%, respectively.4 The scale yield of methanol through formaldehyde and trioxane to DMMn as an additive of diesel oils not only can absorb the methanol production capacity by 50%,11 but also can bring enormous benefits with respect to economics and environmental protection. Therefore, new facilities for production of trioxane will be constructed in the near future.3 For new plants, it is important to overcome the disadvantages of sulfuric acid method.3,12 Our results (Fig. 1) confirm the finding1,13 that the synthesis of trioxane in sulfolane medium can overcome the fundamental problems arising from the consumption of formaldehyde by formed MGn (n≥1) and from the energy consumption due to the great latent heat of vaporization of H2O and the huge amount of water involved in the synthesis of trioxane (production of 1 tons of trioxane consumes 10−12 tons of steam),3 thereby achieving a higher reactivity and selectivity. However,

our

preliminary

experiments

using

common

3


acids

such

as


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trifluoromethanesulfonic

acid

(CF3SO3H),

perchloric

acid

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(HClO4)

and

phosphoricacid (H3PO4) show that the reactivity and selectivity obtained in aprotic solvents depend strongly on the property of the solvents, the specificity of the acids, the acid concentration, and the reaction temperature. More importantly, the mechanism that governs the reactivity and selectivity in aprotic solvents has not been uncovered so far, leaving the choice of the acids at random. Therefore, in this study the activity and selectivity of typical acids is investigated. An acid catalyst and a (acid + salt) catalyst system are developed to drastically increase the yield of formaldehyde to trioxane and to decrease the formation of formic acid. The mechanism that underlies the performance of the acid catalysts is reported. 2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. CF3SO3H, camphor sulfonic acid (C10H17OSO3H), trioxane, sulfolane, dimethyl sulfoxide (DMSO), 1,3-bis(hydroxymethyl)-2-imidazolidinone (DMI), and dimethylsulfone were purchased from Shanghai (China) Aladdin biological technology co., LTD. CH3SO3H, benzenesulfonic acid (C6H5SO3H), p-methylbenzenesulfonic acid (p-CH3C6H4SO3H), H2SO4, HClO4, 1-chlorine naphthalene and sodium methanesulfonate (CH3SO3Na) were obtained from Tianjin (China) Guangfu fine chemical co., LTD. Sodium camphor sulphonate (C10H17OSO3Na) was purchased from Tianjin Heowns biochemical technology co., LTD. Paraformaldehyde (HO(CH2O)nH) was supplied by Shandong (China) Chenxin new energy co., LTD. All materials were of analytical grade and were used without further purification. 2.2. Synthesis of Trioxane and measurements of Hammett acidity function H0 and decomposition rate of trioxane. The reactor that has been used in our previous study for solid−liquid state reactions was used in this study.13 A known amount of solution that contained paraformaldehyde, sulfolane, an acid or an acid and a salt was added in the reactor. 4


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Then, the reactor was heated in an oil bath with continuous stir. The cyclotrimerization reactions were typically allowed to proceed for 1 h. During each experiment, traces of samples of the reaction solution were sampled and analyzed at a certain interval to monitor the change with reaction time in the composition of reaction solution. The batch reactions were also carried out on [formaldehyde (60 wt%) + H2SO4 (0.2 mol⋅(kg H2O)−1) + H2O] using our previous procedure.3 The measurements of the Hammett acidity function H0 and the decomposition of trioxane were carried out using the our procedure.13 Samples were analyzed as follows:3 For trioxane concentration by gas chromatography, with the relative error in mass fraction being smaller than 1%;3 For acid value of solution by acid−base titration using potentiometric titrimeter (Leici ZDJ−5), with the relative errors being 1%.3 3. RESULTS AND DISCUSSION Figure 1 shows the batch-reaction results that were made on (paraformaldehyde + MgCl2·6H2O + sulfolane) and (formaldehyde + H2SO4 + H2O). A1

A

Water

Crystalliferous water (MgCl2·6H2O)

Methanol Dimethoxy methane

Formaldehyde

Trioxane

Ethanol (internal standard compound)

Trioxane

Ethanol (internal standard compound)

MGn of Fig. B1 B1

B

MG2

MG1

Sulfolane MG3 MG j ≥ 4

Dimethyl sulfoxide (internal standard compound)

MG j≥5 Trioxane

MG4 MG3

Trioxane

(paraformaldehyde + MgCl2·6H2O + sulfolane) [formaldehyde (50 wt%) + H2SO4 (4 wt%) + H2O] Figure 1. Chromatograms (A and A1) and NMR (B and B1) spectra for reaction 5



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solutions (paraformaldehyde + MgCl2·6H2O + sulfolane) at 383.15 K and [formaldehyde (50 wt%) + H2SO4 (4 wt%) + H2O] at 373.15 K. The aqueous reaction solution contains H2O, trioxane, a series of MGn (n≥1), HCOOH, and catalyst. The sulfolane reaction solution comprises of sulfolane, trioxane, HCOOH, and catalyst, confirming that the synthesis of trioxane in sulfolane medium can overcome the above-mentioned fundamental problems.

Table

1.

Yields

of

trioxane

and

formic

acid

in

(paraformaldehyde + CH3SO3H + solvent)a at 423.15 K Solvent

Yield (%)b

HCOOH (ppm)

1-Chlorine naphthalene

37.63

4135

DMSO

56.43

1908

DMI

31.47

5436

Dimethylsulfone

28.78

2258

Sulfolane

67.28

1033

a

The weight ratio of paraformaldehyde and sulfolane is 0.15. The

concentration of CH3SO3H is 0.1 mol⋅(kg solvent)−1. Yield (%) = 3wtrioxane/(Fwparaformaldehyde), where wtrioxane and wparaformaldehyde are weights of trioxane and paraformaldehyde, and F is weight percent of formaldehyde in paraformaldehyde.

Table 1 shows solvent effect on yields of trioxane and formic acid in (paraformaldehyde + CH3SO3H + solvent). The solvents considered include sulfolane, DMSO, DMI, 1-chlorine naphthalene, and dimethylsulfone. The best result is obtained in sulfolane solution. Figure 2 shows effect of reaction time on yields of 6


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trioxane and formic acid in (paraformaldehyde + catalyst + sulfolane) with (CH3SO3H + CH3SO3Na) or HClO4 as the catalyst. In both cases trioxane concentration reaches its maximum at 1 h and remains approximately constant within 0.8 to 2 h. However, the most striking thing about Figure 2 is the pronounced specificity in the acid effects on formic acid concentration, which increases with reaction time very mildly in the reactions catalyzed by (CH3SO3H + CH3SO3Na) but raises significantly in the reactions catalyzed by HClO4 (Table S1 of Supporting Information shows that increasing HClO4 concentration does not yield better results). On the basis of these results, the cyclotrimerization reactions in (paraformaldehyde + catalyst system + sulfolane) were typically allowed to proceed for 1 h. The results are shown in Tables 2 and 3, Tables S1 and S2 (Supporting Information) and Figures 3 and 4.

A

B

100

CH3SO3H + CH3SO3Na

CH3SO3H + CH3SO3Na HClO4

HClO4

60 40 20 0

6000

HCOOH /ppm

80

Yield/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5

1.0

1.5

2.0

4000

2000

0

2.5

Time/h

0.5

1.0

1.5

2.0

Time /h

Figure 2. Yields of trioxane and formic acid in (paraformaldehyde + catalyst system + sulfolane) at 383.15 K (the weight ratio of paraformaldehyde and sulfolane is 0.09. The concentrations of CH3SO3H, CH3SO3Na, and HClO4 are 0.33, 0.15, and 0.01 mol⋅(kg sulfolane)−1).

Table 2 and Table S1 (Supporting Information) show that the concentrations of trioxane and formic acid produced in (paraformaldehyde + acid + sulfolane) depend strongly on the particular acid used, the acid concentration, and the reaction 7



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temperature. At 383.15 K the maximum concentration of trioxane is 5.60 mol⋅(kg sulfolane)−1 and the corresponding concentration of formic acid is about 567 ppm, both of which were obtained in [paraformaldehyde + CH3SO3H (0.33 mol⋅kg−1) + sulfolane). However, at 423.15 K the maximum concentration of trioxane is 8.35 mol⋅(kg sulfolane)−1 and the second minimum concentration of formic acid is about 1033 ppm, both were obtained in [paraformaldehyde + CH3SO3H (0.10 mol⋅kg−1) + sulfolane). Table 2 and Table S1 show that the concentrations of trioxane and formic acid produced in reaction solution depend drastically on the acid strength of the Brönsted and the Lewis acid catalysts. CF3SO3H is the strongest of all acids in organic solvents.14 However, CF3SO3H always provides significantly much greater concentrations of formic acid in the reaction solutions than CH3SO3H, indicating that the acidity of the acid catalyst cannot be too strong. On the contrary, the acidity of camphor sulfonic acid is weaker than that of CH3SO3H (the measured H0 increases from CH3SO3H to camphor sulfonic acid), but the yield of trioxane by camphor sulfonic acid is also lower than that by CH3SO3H. The results obtained with C6H5SO3H and p-CH3C6H4SO3H as the catalyst also confirm that it is difficult to find a catalyst with proper acid strength to achieve as high as possible the yield and the selectivity of trioxane. Table 2 also shows that, even for CH3SO3H, the optimum acid concentration that gave rise to the maximum yield and selectivity of trioxane depends strongly on the reaction temperature. Table 2 and Figure 3 show that increasing reaction temperature increases the yields of both trioxane and formic acid in (paraformaldehyde + acid + sulfolane) but decreases the yield of paraformaldehyde to trioxane.

8


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Table 2. Yields of trioxane and formic acid in (paraformaldehyde + catalyst + sulfolane) and in (formaldehyde + H2SO4 + H2O) and decomposition of trioxane in (trioxane + catalyst + sulfolane) at different temperatures Catalyst

Yield

Decomposition

mol⋅(kg sulfolane)−1

Yield (%)a HCOOH

Trioxane HCOOH

(ppm)

(%)

H0

(ppm)

(Paraformaldehyde + catalyst + sulfolane) at 383.15b K CF3SO3H (0.01)

64.46 (5.08)

2377

48.53 d

3936

CF3SO3H (0.33)

0 (0)

22365

100.0

21898

p-CH3C6H4SO3H (0.33)

50.51(3.79)

1428

58.76

1634

1.1361

C6H5SO3H (0.33)

45.95 (3.59)

2565

70.50

2356

0.9908

C10H17OSO3H (0.33)

70.87 (5.25)

1018

43.08

631

1.5791

CH3SO3H (0.05)

61.94 (4.89)

228

35.18

155

2.3038

CH3SO3H (0.10)

64.89 (5.10)

480

37.95

310

1.9187

CH3SO3H (0.33)

73.08 (5.60)

567

41.90

482

1.5249

CH3SO3H (0.40)

66.02 (5.06)

711

44.22

800

1.3932

(Paraformaldehyde + catalyst + sulfolane) at 423.15e K C10H17OSO3H (0.33)

54.40 (6.91)

5229

46.01

1852

1.5791

CH3SO3H (0.05)

61.52 (7.67)

774

35.98

264

2.3038

CH3SO3H (0.10)

67.28 (8.35)

880

38.40

401

1.9187

CH3SO3H (0.20)

62.26 (7.66)

2648

39.95

1720

1.6890

CH3SO3H (0.33)

59.89 (7.29)

3322

45.72

3559

1.5249

(Formaldehyde (60 wt%) + H2SO4 (0.2 mol⋅(kg H2O)−1) + H2O) at 373.15 K H2SO4 (0.2) a

13 (3.18)

10024

Yield (%) = 3wtrioxane/(Fwparaformaldehyde), where wtrioxane and wparaformaldehyde are

weights of trioxane and paraformaldehyde, and F is weight percent of 9



formaldehyde in paraformaldehyde. bThe weight ratio of paraformaldehyde and sulfolane is 0.09. cThe weight percent of trioxane in reaction solution. dThe weight percent of the decomposed trioxane (the initial weight ratio of trioxane and sulfolane is 0.15). eThe weight ratio of paraformaldehyde and sulfolane is 0.15.

5000

Trioxane (CH3SO3H+CH3SO3Na)

70

Trioxane (CH3SO3H) HCOOH (CH3CSO3H+CH3SO3Na)

Yield/%

4000

HCOOH (CH3SO3H)

60

3000

50

2000

40

1000

30 20

380

390

400

410

420

HCOOH/ppm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 430

T/K Figure 3. Yields of trioxane and formic acid in (paraformaldehyde + catalytic system + sulfolane) at different temperatures (the weight ratio of paraformaldehyde

and sulfolane is 0.15; the concentrations of CH3SO3H and CH3SO3Na are 0.33 and 0.15 mol⋅(kg sulfolane)−1).

Recently, the remarkable role of salt additives in the Negishi reaction involving aryl zinc reagents has been reported.15 As can be seen from Table 3 and Table S2 (Supporting Information), the addition of a suitable salt also appreciably increases the trioxane concentration but decreases the formic acid concentration. In particular, Figure 3 shows that the trioxane concentration is greater, but the formic acid concentration is remarkably smaller, in [paraformaldehyde + (CH3SO3H + CH3SO3Na) + sulfolane] than in (paraformaldehyde + CH3SO3H + sulfolane).

10


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Table 3. Yields of trioxane and formic acid in (paraformaldehyde + catalyst system + sulfolane)a and decomposition of trioxane in (trioxane + catalyst system + sulfolane) at 423.15 K

Catalyst system

Yield

Decomposition

H0

Yield (%)b HCOOH Trioxane HCOOH

mol⋅(kg sulfolane)−1

(ppm)

(%)

(ppm)

4312

44.40d

1289

1.6435

C10H17OSO3H (0.33) +

58.44b

C10H17OSO3Na (0.075)

(7.13)c

C10H17OSO3H (0.33) +

55.06 (6.72)

5085

46.82

1809

1.6213

68.97 (8.52)

880

32.24

481

2.0145

65.06 (8.01)

1206

34.49

1021

1.7146

66.08 (8.01)

1401

39.35

1885

1.5425

CH3SO3Na (0.15) CH3SO3H (0.10) + CH3SO3Na (0.05) CH3SO3H (0.20) + CH3SO3Na (0.10) CH3SO3H (0.33) + CH3SO3Na (0.15) a

The weight ratio of paraformaldehyde and sulfolane is 0.15.

b

Yield (%) =

3wtrioxane/(Fwparaformaldehyde), where wtrioxane and wparaformaldehyde are the weights of trioxane and paraformaldehyde. F is the weigh percent of formaldehyde in paraformaldehyde. cThe weight percent of trioxane in the reaction solution. dThe weight percent of the decomposed trioxane.

Figure 4 shows that the yields of trioxane and formic acid in [paraformaldehyde + (CH3SO3H + CH3SO3Na) + sulfolane] depend on the mole ratio of acid and salt. Comparisons of the results of Tables 2 and 3 and Tables S1 and S2 (Supporting Information) reveal that the yield of paraformaldehyde to trioxane (68.97%) and the concentration of formic acid (∼1000 ppm) in [paraformaldehyde + CH3SO3H (0.10 mol⋅kg−1) + CH3SO3Na (0.05 mol⋅kg−1) + sulfolane] at 423.15 K are much more 11



favorable than those in [formaldehyde (60 wt%) + H2SO4 ((0.2 mol⋅(kg H2O)−1) + H2O] at 373.15 K (13% and ∼10000 ppm). Note that the concentrations of trioxane and formic acid in (formaldehyde + H2SO4 + H2O) increase with increasing the concentrations of formaldehyde and H2SO4. However, further increasing the concentrations of formaldehyde and H2SO4 may induce formation and precipitation of paraformaldehyde. The precipitated paraformaldehyde consumes reactant and is very difficult to be removed from reaction mixtures by depolymerization at usual reaction temperatures.

70

Trioxane HCOOH

4000

65

Yield/%

3000 60

2000

55

50

1000

0.00

0.05

0.10

0.15

0.20

0.25

0.30

HCOOH/ppm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0

-1

c(CH3SO3Na)/mol.(kg sulfolane)

Figure 4. Yields of trioxane and formic acid in [paraformaldehyde + (CH3SO3H + CH3SO3Na) + sulfolane] at different concentrations of CH3SO3Na and 423.15 K

(the weight ratio of paraformaldehyde and sulfolane is 0.15. The concentration of CH3SO3H is 0.33 mol⋅(kg sulfolane)−1). The decomposition of trioxane in (trioxane + catalyst system + sulfolane) and the indicator acidity function H0 of Hammett and Deyrup16 were investigated to uncover how the catalyst and the salt additive modulated reactivity and selectivity. The results are shown in Tables 2 and 3. It can be seen that in most cases the concentration of trioxane produced in (paraformaldehyde + acid + sulfolane) or [paraformaldehyde + 12


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(acid + salt) + sulfolane] at a given reaction temperature is inversely related to the weight percent of trioxane that was decomposed in (trioxane + acid + sulfolane) or [trioxane + (acid + salt) + sulfolane]. By contrary, the formic acid concentrations in both [paraformaldehyde + acid (or acid + salt) + sulfolane] and [trioxane + acid (or acid + salt) + sulfolane] are strongly related to the weight percent of trioxane that was decomposed in [trioxane + acid (or acid + salt) + sulfolane]. On the basis of these results, the trioxane formation can be described as:12,17,18 kTrioxane 3CH2 O ( Formaldehyde ) ←→ (CH 2O)3 ( Trioxane ) kd

(4)

where the reverse reaction of reaction (4) (i.e., the decomposition reaction of trioxane) can be described as: 12,17,18 +

Equilibrium Slow (CH2O)3 + H+ ←→ → 2CH2O + HOCH+2 [(CH2O)3H]  kd

HOCH +2 ↔ CH2 O + H +

(5) (6)

The formation of formic acid can be described as:

CH2O + HOCH2+ + H2O → HCOOH + CH3OH + H+

(7)

Table 2 suggests that the rate of the decomposition of trioxane in (trioxane + acid + sulfolane) by reactions (5) and (6) at the same molality of acid increases with acid strength in the order CH3SO3H < CF3SO3H. In addition, the observed rate of the decomposition of trioxane increases with acid strength (i.e., with decreasing the H0 value). These results are consistent with the results reported in ref. 17, confirming once more that the decomposition of trioxane in (trioxane + acid + sulfolane) can indeed be described by reaction (5).17 These results indicate that the overall performance of a catalyst is manifested in its activity to accelerate the forward reaction and to delay the reversion reaction of reaction (4) and that the catalyst activity and selectivity to trioxane are both strongly related to its ability to retard the decomposition reaction of trioxane 13



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itself. Tables 2 and 3 show that added salts increased the H0 value (decreased acid strength), thereby decreasing the rate of the decomposition of trioxane. Therefore, the concentration of trioxane produced in [paraformaldehyde + (acid + salt) + sulfolane] was greater than in (paraformaldehyde + acid + sulfolane) whilst the formic acid concentration in the salt-containing reaction solution was smaller than that in the salt-free reaction solution.

4. CONCLUSIONS

Both the yield of formaldehyde and the selectivity of trioxane depend strongly on the decomposition rate of trioxane. Therefore, specifity of solvent, acid and salt, concentrations of acid and salt, and reaction temperature should be selected to accelerate as much as possible the forward reaction and to retard as much as possible the reverse reaction, both of reaction (4). Under the present conditions, the reaction in [paraformaldehyde + CH3SO3H (0.10 mol⋅kg−1) + CH3SO3Na (0.05 mol⋅kg−1) + sulfolane] at 423.15 K provides the best results, with the yield of paraformaldehyde to trioxane being 68.97% and the concentration of formic acid being ∼1000 ppm. The presence of CH3SO3Na increases the H0 of reaction medium and decreases the decomposition rate of trioxane, therefore increases both the yield and the selectivity. ASSOCIATED CONTENT

Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: The procedures of systhnsis of zwitterionic-type salts (C3SMIM and C3SNHP) and of measurements of Hammett acidity function H0, Table S1 for yields of trioxane and formic acid in (paraformaldehyde + catalyst + sulfolane) at 383.15 K, and Table S2 for yields of trioxane and formic acid in (paraformaldehyde + catalyst system + 14


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sulfolane) at 423.15 K. AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] ORCID Yufeng Hu: 0000-0002-9777-3805 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (grant numbers 21576285, 21276271), Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resource (Qinghai Institute of Salt Lake, Chinese Academy of Sciences) (grant numberHX20150636) and Innovation Foundation of China University of Petroleum, Beijing (grant numberZX20160004). REFERENCES

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trioxane catalyzed by Brønsted acid ionic liquids with different alkyl groups. RSC Adv. 2015, 5, 57968. 5.

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Table of Contents

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Acid-catalyzed Synthesis of Trioxane in Aprotic Media

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