Synthesis of polyoxymethylene dimethyl ethers from methylal and trioxane catalyzed by Brønsted acid ionic liquids with different alkyl groups

Abstract

Brønsted acid ionic liquids with different alkyl group carbon chain lengths and an alkane sulfonic acid group were synthesized through bromoalkane, imidazole and 1,4-butane sultone as raw materials. The structures and properties of the ionic liquids were experimentally characterized. Catalytic reaction of methylal (DMM) with trioxane (TOX) for preparation of polyoxymethylene dimethyl ethers (PODME_n, CH₃O(CH₂O)_nCH₃, where n > 1) was investigated in various Brønsted acid ionic liquids with different carbon chain length of alkyl groups. The carbon chain length of alkyl groups and activity correlation for the ionic liquids was studied. It was found that the structures of ionic liquids were consistent with the designed structure and their purities were high. They possessed high thermal stability and wide liquid range. The hydrophobicity of ionic liquids became stronger with the increase of carbon chain length. With increasing the carbon chain length of ionic liquids, the selectivity of PODME_3–8 is increased at first and then decreased. Among all the ionic liquids, [C₆ImBS][HSO₄] shows the best catalytic performance and the selectivity of PODME_3–8 is 57.85%.

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Introduction

Diesel engines have revolutionized transportation due to their high thermal efficiency. Environmental and linked regulatory issues have driven the development of low emission diesel engines. Still, even in modern high performance diesel engines, the formation of soot during the combustion remains a problem. Oxygenated compounds (molecules which contain oxygen in their structure) are known to reduce soot formation during the combustion, when added to diesel fuels.^1,2 Polyoxymethylene dimethyl ethers (PODME_n) are novel oxygenated compounds with the formula CH₃O(CH₂O)_nCH₃, where n ≥ 1. The features of PODME_n are high oxygen content (42–51%) and high cetane number, which can improve the combustibility of diesel oil, enhance the efficiency of combustion, and reduce pollutants.^3–7 Thus, it is believed to have wide application prospect as a diesel additive.⁴ However, considering the influences of vapor pressure, boiling point, and solubility of PODME_n, the suitable n value is 3–8, and the average cetane number of PODME_3–8 is up to 76.⁸ It was reported that the emissions of particulate pollutants and NO_x can be reduced up to 80–90% and 50%, respectively, by adding 20% PODME_3–8 to diesel oil.⁵

PODME_n is usually produced by the end-group (–CH₃) provider and the chain-group (–CH₂O–) provider with the existence of catalysts. Generally, the former contains methanol, methylal or dimethyl ether, the latter contains trioxane, formaldehyde or paraformaldehyde.^5–7 Patrini et al. used CF₃SO₃H as the catalyst to prepare PODME_n, with methylal and paraformaldehyde as starting materials.⁹ However, homogeneous liquid catalysts, such as sulfuric acid,¹⁰ CF₃SO₃H, are corrosive and are not easily recovered, leading to the release of environmentally unfriendly effluents, which inevitably leads to a series of environmental problems. Arvidson et al. prepared PODME_n by the reaction of trioxane, methylal and paraformaldehyde using cation exchange resin as catalyst.¹¹ Although heterogeneous acid catalysts are environmentally friendly,¹² they also have some drawbacks, such as low catalytic activity and easy deactivation of the catalyst. Therefore, the development of environmental and efficient catalysts for synthesis of PODME_n is extremely urgent.

In recent years, room temperature ionic liquids, as environmentally benign and “designer” solvents or catalysts, have attracted much attention due to numerous advantages, such as wide liquid range, high catalytic activity, ignorable vapor pressure, designed structure and easy separation from the reaction system.^13–20 Chen et al. prepared PODME_n from methanol and trioxane by employing ionic liquid as the catalyst.²¹ But the yield of PODME_3–8 was low for the reaction of methanol and trioxane. In our previous work, several Brønsted acid ionic liquids with different nitrogen groups and different alkane sulfonic acid groups were synthesized. Catalytic reaction of methylal with trioxane for preparation of polyoxymethylene dimethyl ethers was investigated in these ionic liquids. The Brønsted acidity–activity correlation for the ionic liquids was studied.²²

In this work, Brønsted acid ionic liquids with different carbon chain length of alkyl groups were synthesized. They were characterized by infrared spectrum (IR), nuclear magnetic resonance (NMR), electrospray ionization mass spectrum (ESI-MS), solubility analysis and differential thermal analysis (TG-DTA). Their catalytic performances for the synthesis of PODME_n from methylal (DMM) and trioxane (TOX) were investigated in various Brønsted acid ionic liquids with different carbon chain length of alkyl groups. The carbon chain length of alkyl groups and activity correlation for the ionic liquids was studied.

Experimental

Materials

Sodium, bromomethane, bromoethane, 1-bromobutane, 1-bromohexane and 1-bromooctane were purchased from Sionpharm Chemical Reagent Co., Ltd. 1-Bromodecane, 1-bromododecane, 1-bromotetradecane, 1-bromohexadecane, methylal and trioxane were obtained from Alfa Aesa. 1,4-Butane sultone were purchased from WuHan Fengfan Chemical Factory. Imidazole was obtained from Linhai Kaile Chemical Factory. Ethanol, acetone, dichloromethane, ether, ethyl acetate and sulfuric acid were received from Beijing Chemical Co., Ltd. All reagents and starting materials were commercially available and used without any further purification unless, otherwise, noted.

Methods

The structures of the ionic liquids and their intermediates were analyzed by IR spectroscopy, NMR and ESI-MS. IR spectra were recorded on a Perkin-Elmer Spectrum One FTIR spectrometer using KBr windows suitable for Fourier transform infrared (FTIR) transmittance technology to form a liquid film. ¹H NMR spectra were obtained on a Varian mercury-plus 400 MHz nuclear magnetic resonance spectrometer. Chemical shifts were reported in parts per million (ppm, δ). ESI-MS spectra were obtained on Varian 500-MS instrument. The thermal decomposition temperature was determined by Perkin-Elmer TGA7 TG-DTA analysis, with a heating rate of 10 °C min⁻¹ from room temperature to 700 °C. The melting points of the ionic liquids with different carbon chain of alkyl groups were measured by TA DSC Q2000. The viscosity of the ionic liquids with different carbon chain of alkyl groups was measured by capillary viscosimetry at 60 °C.

Preparation of Brønsted acid ionic liquids with different alkyl groups

The synthetic procedure of Brønsted acid ionic liquids with different alkyl groups is illustrated in Scheme 1.

Scheme 1 Synthetic procedure of Brønsted acid ionic liquids with different alkyl groups.

(1) Preparation of alkylimidazole with different carbon chain (RIm). The sodium was slowly added to ethanol. The produced sodium ethoxide solution, which was colorless transparent liquid, was purified through filtration.

The imidazole was added to sodium ethoxide solution, and the mixture was stirred in the oil bath pan for eight hours, resulting in the formation of the sodium imidazole. The sodium imidazole was purified through filtration.

A stoichiometric amount of RBr was added to the ethanol solution of sodium imidazole, and the mixture was stirred in the oil bath pan for two days, resulting in the formation of the alkylimidazole (RIm). The RIm was washed repeatedly with ethanol, acetone and dichloromethane, leached and distilled accordingly. Finally, the RIm was purified by column chromatography on silica gel.

(2) Preparation of acidic ionic liquids ([RImBs][HSO₄]). A stoichiometric amount of 1,4-butane sultone was added dropwise to the RIm at 0 °C and the mixture was stirred at 50 °C for 2–3 days. The zwitterions BsImR produced, all of which were white solids, were washed three times with ethyl acetate and ether to remove unreacted material and dried for 12 hours in a vacuum.

A stoichiometric amount of concentrated sulfuric acid was added dropwise to the zwitterions at 0 °C, and the mixture was stirred at 50 °C for 1–2 days, resulting in the formation of the [RImBs][HSO₄] ionic liquids. The ionic liquids were washed repeatedly with ethyl acetate and ether to remove unreacted material and dried under vacuum.²³

Solubility measurement of the [RImBs][HSO₄] ionic liquids

The method for the solubility measurement of ionic liquids in selected solvents was the same as that described in the literature.²⁴ The equilibrium cell was a sealed 120 mL glass measuring flask. The flask was immersed in a constant-temperature water bath. A magnetic stirrer was utilized for solution preparation. The precision of the analytical balance was 0.1 mg.

Synthesis of polyoxymethylene dimethyl ethers (PODME_n) catalyzed by the [RImBs][HSO₄] ionic liquid

The reaction of methylal with trioxane was carried out in a 50 mL Teflon-lined stainless-steel autoclave, equipped with thermostat and mechanical stirring. Methylal, trioxane, and [RImBs][HSO₄] ionic liquid catalyst with different molar rations were quantitatively introduced into the reactor successively. The reaction was allowed to proceed for 10 h and kept at the desired temperature. After the reaction completed, the mixture was cooled and kept at 0 °C for 0.5 h and two phases were formed. The upper phase consisted of the produced PODME_n and the unreacted materials, and the lower phase contained the ionic liquid catalyst. The lower phase was simply separated from the upper phase by decantation.

The upper solution was analyzed by a Shimadzu GC-2014 equipped with a FID detector. A capillary column (DB-5MS, 30 m × 0.25 μm, id × 0.25 μm) was used. The quantitative analysis was carried out by internal standard method. The internal standard compound was methanol. The calculation of trioxane conversion and PODME selectivity was based on carbon balance. Analytical conditions were as follows: injection port temperature, 290 °C; FID temperature, 300 °C; oven temperature program, initially the temperature was held at 40 °C for 5 min, then ramped to 280 °C at a rate of 5 °C min⁻¹ and held for 5 min; carrier gas, nitrogen of 0.4 Mpa; reagent gases, air of 0.4 Mpa, hydrogen of 0.3 Mpa.

Results and discussion

IR analysis of the intermediate [RIm] and [RImBs][HSO₄]

Fig. 1 show the IR spectra of C₈H₁₇Im and C₈H₁₇Br. For the C₈H₁₇Im, the bands at 3130 cm⁻¹ and 1511 cm⁻¹ were assigned to the C–H and C–N ring stretching vibration of imidazole. In addition, the 550 cm⁻¹ band peak disappeared or weakened. It was shown that the imidazole group had substituted Bromine group and the C₈H₁₇Im had been synthesized successfully. Similarly, the C_nH_2n+1Im had been synthesized successfully, respectively.

Fig. 1 IR spectra of C₈H₁₇Im and C₈H₁₇Br.

Fig. 2 display the infrared spectra of the R₈Im, R₈ImBS and [R₈ImBS][HSO₄] ionic liquids. As can be seen from Fig. 2, for the BsImR₈, the bands at 1245 and 1167 cm⁻¹ were assigned to the O=S=O stretching vibrations. The 1040 cm⁻¹ band was assigned to the C–S–O stretching vibrations. These results showed the existence of sulfonic acid group. For the [R₈ImBS][HSO₄] ILs, the peak intensity of sulfonic acid group increased obviously compared with the R₈ImBS. The 596 cm⁻¹ band was assigned to the –SO₄H group vibration further indicative of the existence of main group. In addition, the wide peak at 3300–3500 cm⁻¹ was assigned to the O–H group of water because the [R₈ImBS][HSO₄] is super water-absorbable. The similar results were obtained for the other [R_nImBS][HSO₄] ionic liquids. Therefore, the IR data are consistent with the structures for the ionic liquids shown in Scheme 1.

Fig. 2 IR spectra of R₈Im, R₈ImBS and [R₈ImBS][HSO₄].

NMR analysis of the [RImBS][HSO₄] ionic liquids

The ionic liquids were analyzed by ¹H NMR spectroscopy. The NMR spectral data of the ionic liquids are as follows.

C₁-IL([C₁ImBS][HSO₄]): ¹H NMR (400 MHz, DMSO-d₆): δ 1.540 (m, 2H), 1.877 (m, 2H), 2.496 (t, 2H), 3.852 (s, 3H), 4.183 (t, 2H), 7.707 (s, 1H), 7.772 (s, 1H), 9.137 (s, 1H).

C₂-IL([C₂ImBS][HSO₄]): ¹H NMR (400 MHz, DMSO-d₆): δ 1.420 (m, 3H), 1.547 (m, 2H), 1.871 (m, 2H), 2.510 (t, 2H), 4.201 (m, 4H), 7.807 (s, 1H), 7.833 (s, 1H), 9.250 (s, 1H).

C₄-IL([C₄ImBS][HSO₄]): ¹H NMR (400 MHz, DMSO-d₆): δ 0.9160 (m, 3H), 1.276 (m, 2H), 1.536 (m, 2H), 1.776 (m, 2H), 1.886 (m, 2H), 2.506 (t, 2H), 4.187 (m, 4H), 7.815 (s, 2H), 9.237 (s, 1H).

C₆-IL([C₆ImBS][HSO₄]): ¹H NMR (400 MHz, DMSO-d₆): δ 0.8020 (m, 3H), 1.210 (m, 6H), 1.510 (m, 2H), 1.738 (m, 2H), 1.848 (m, 2H), 2.529 (t, 2H), 4.139 (m, 4H), 7.775 (s, 2H), 9.228 (s, 1H).

C₈-IL([C₈ImBS][HSO₄]): ¹H NMR (400 MHz, DMSO-d₆): δ 0.9120 (m, 3H), 1.250 (m, 10H), 1.521 (m, 2H), 1.752 (m, 2H), 1.872 (m, 2H), 2.509 (t, 2H), 4.187 (m, 4H), 7.812 (s, 2H), 9.227 (s, 1H).

C₁₀-IL([C₁₀ImBS][HSO₄]): ¹H NMR (400 MHz, DMSO-d₆): δ 0.8610 (m, 3H), 1.282 (m, 14H), 1.532 (m, 2H), 1.781 (m, 2H), 1.852 (m, 2H), 2.521 (t, 2H), 4.334 (m, 4H), 7.781 (s, 2H), 9.208 (s, 1H).

C₁₂-IL([C₁₂ImBS][HSO₄]): ¹H NMR (400 MHz, DMSO-d₆): δ 0.8112 (m, 3H), 1.253 (m, 10H), 1.521–1.853 (m, 10H), 2.532 (t, 4H),4.139 (m, 6H), 7.775 (s, 2H), 9.205 (s, 1H).

C₁₄-IL([C₁₄ImBS][HSO₄]): ¹H NMR (400 MHz, DMSO-d₆): δ 0.8921 (m, 3H), 1.117 (m, 20H), 1.511–1.832 (m, 6H), 2.532 (t, 4H), 4.176 (m, 4H), 7.871 (s, 2H), 9.204 (s, 1H).

C₁₆-IL([C₁₆ImBS][HSO₄]): ¹H NMR (400 MHz, DMSO-d₆): δ 0.9612 (m, 3H), 1.031 (m, 22H), 1.511–1.801 (m, 6H), 2.521 (t, 6H), 4.210 (m, 4H), 7.775 (s, 2H), 9.119 (s, 1H).

The NMR spectral data of the ionic liquids agreed with their designed structures (Scheme 1). As can be seen from the spectrum of these ionic liquids, there was no impurity peak in the ¹H NMR spectrum. This demonstrated that the purity of the ionic liquids was high. Therefore, the synthesis and purification methods for the ionic liquids were reliable.

ESI-MS analysis of the [RImBS][HSO₄] ionic liquids

The positive and negative ion electrospray ionization mass spectra of the [C₈ImBS][HSO₄] ionic liquid are shown in Fig. 3 (the upper spectra are the positive ion spectra and the lower spectra are the negative ion spectra). As seen from Fig. 3, large peak in the positive ion mode occurred at m/z 317.1, corresponding to the positive ion of [C₈ImBS][HSO₄]. In addition, the negative ion mode showed the presence of peak at m/z 96.9, corresponding to the HSO₄⁻ ion. The peak in the negative ion mode at m/z 315.2 may be assigned as a negative ion being produced by the loss of two H⁺ from the positive ion of ionic liquid. The similar results were obtained for the other [C_nImBS][HSO₄] ionic liquids. Large peaks in the positive ion mode occurred at m/z 219, 233, 261, 289, 317, 345, 373, 401 and 429, corresponding to the positive ion of ionic liquids (n = 1–14) respectively. All negative ion modes showed the presence of peak at m/z 97, corresponding to the HSO₄⁻ ion. Therefore, the ESI-MS data are consistent with the structures for the ionic liquids shown in Scheme 1.

Fig. 3 Electrospray ionization mass spectra (ESI-MS) of the [C₈ImBS]HSO₄.

TG-DTA analysis of the [RImBS][HSO₄] ionic liquids

Thermal decomposition temperature was determined by TG-DTA analysis. The thermal decomposition temperatures of ionic liquids C₁-IL, C₂-IL, C₄-IL, C₆-IL, C₈-IL, C₁₀-IL, C₁₂-IL, C₁₄-IL and C₁₆-IL were 311, 301, 288, 274, 273, 280, 296, 265 and 253 °C, respectively. It was shown that these ionic liquids possessed high thermal stability and wide liquid range, which are more than 250 °C.

Melting points of the ionic liquids

The melting points of the ionic liquids with different carbon chain of alkyl groups were measured by TA DSC Q2000. The melting points of ionic liquids C₁-IL, C₂-IL, C₄-IL, C₆-IL, C₈-IL, C₁₀-IL, C₁₂-IL, C₁₄-IL and C₁₆-IL were <−80, <−80, <−80, −68, −51, −41, −5, 55 and 63 °C, respectively. It was shown that these ionic liquids possessed wide liquid range.

Viscosity analysis of the ionic liquids

The viscosity of the ionic liquids with different carbon chain of alkyl groups was measured by capillary viscosimetry. Fig. 4 displays the viscosity of the different ionic liquids. As could be seen from Fig. 4, when the carbon chain length of ionic liquids is less than 6, the viscosity of ionic liquid has a slight decrease. However, when the carbon chain length of ionic liquids increases from 6 to 10, the viscosity of ionic liquid sharply increases. It was shown that the carbon chain length of ionic liquids has important effect on the viscosity of the ionic liquids.

Fig. 4 The viscosity of the ionic liquids with different carbon chain of alkyl groups.

Solubility analysis of the [RImBS][HSO₄] ionic liquids

The solubility of ionic liquids in methanol was studied at different temperature (see Table 1). As shown in Table 1, the [C_1–8ImBS][HSO₄] ionic liquids were miscible in methanol. For the [C_10–16ImBS][HSO₄] ionic liquids, the solubility of ionic liquids decreased with the increase of carbon chain length at each selected temperature and increased with the increase of temperature. It was shown that the hydrophobicity of ionic liquids became stronger with the increase of carbon chain length.

Table 1 Solubility (mass fraction) of the ionic liquids in methanol at different temperatures

Ionic liquids	5 °C	20 °C	30 °C	40 °C
[C1ImBS]HSO4	Miscible	Miscible	Miscible	Miscible
[C2ImBS]HSO4	Miscible	Miscible	Miscible	Miscible
[C4ImBS]HSO4	Miscible	Miscible	Miscible	Miscible
[C6ImBS]HSO4	Miscible	Miscible	Miscible	Miscible
[C8ImBS]HSO4	Miscible	Miscible	Miscible	Miscible
[C10ImBS]HSO4	63.74	82.56	Miscible	Miscible
[C12ImBS]HSO4	58.35	73.75	85.73	Miscible
[C14ImBS]HSO4	41.84	52.35	64.68	86.75
[C16ImBS]HSO4	31.36	48.94	57.90	67.94

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The solubility of ionic liquids in methylal was studied at different temperature (see Table 2). As shown in Table 2, the ionic liquids were slightly soluble in methylal.

Table 2 Solubility (mass fraction) of the ionic liquids in methylal at different temperatures

Ionic liquids	10 °C	20 °C	40 °C
[C1ImBS][HSO4]	0.25	0.21	0.26
[C4ImBS][HSO4]	0.11	0.29	0.31
[C6ImBS][HSO4]	0.19	0.17	0.25
[C8ImBS][HSO4]	0.23	0.14	0.19
[C10ImBS][HSO4]	0.19	0.13	0.25
[C12ImBS][HSO4]	0.25	0.23	0.30

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It was seen that the ionic liquid catalyst can be easily separated from methylal via simple decantation and reused.

Reaction of methylal with trioxane for preparation of PODME_n

The effect of ionic liquids with different carbon chain of alkyl groups on the catalytic reaction was studied (see Fig. 5). Fig. 5(a) displays that the conversions of trioxane (X_TOX) are all above 90% for the different carbon chain of ionic liquids, but the selectivity of the PODME_3–8 are distinctly different. As the carbon chain length of ionic liquids increased, the conversion of trioxane, which exceeds 90%, is basically unchanged. When the carbon chain length of ionic liquids is less than 6, the selectivity of PODME_3–8 has a slight increase. However, when the carbon chain length of ionic liquids increases from 6 to 16, the selectivity of PODME_3–8 sharply decreases. The carbon chain length of ionic liquid has a significant effect on its catalytic activity. Among all the ionic liquids, [C₆ImBS][HSO₄] shows the best catalytic performance and the selectivity of PODME_3–8 is 57.85%. On the basis of the above discussion of the hydrophobicity of the ionic liquids (see the section of solubility analysis of the [RImBS][HSO₄] ionic liquids), the catalytic activity of the ionic liquid may be related to the hydrophobicity. A proper hydrophobicity contributes to the catalytic reaction of methylal with trioxane for preparation of PODME_n. However, when the carbon chain length of ionic liquids is greater than 6, the effect of steric hindrance on reaction reduces catalytic activity. In addition, the catalytic activity of the ionic liquids is related to the viscosity of the ionic liquids. When the viscosity of the ionic liquids is high, the collision rate between the ionic liquid catalyst and reactants becomes small. So, the reaction rate is slow. On the contrary, when the viscosity of the ionic liquids is low, the reaction rate is fast. On the basis of the above discussion of the viscosity of the ionic liquids (see the section of viscosity analysis of the ionic liquids), the carbon chain length of ionic liquids has important effect on the viscosity of the ionic liquids. When the carbon chain length of ionic liquids is less than 6, the viscosity of ionic liquid has a slight decrease. However, when the carbon chain length of ionic liquids increases from 6 to 10, the viscosity of ionic liquid sharply increases. The viscosity of [R₆ImBS][HSO₄] ionic liquids is the lowest among all the ionic liquids. Therefore, the carbon chain length of ionic liquids have effect on the catalytic activity of the ionic liquids. Among all the ionic liquids, [C₆ImBS][HSO₄] shows the best catalytic performance and the selectivity of PODME_3–8 is 57.85%. With increasing the carbon chain length of ionic liquids, the variation trend of viscosity is contrary to that of catalytic activity.

Fig. 5 Effect of ionic liquids with different carbon chain of alkyl groups on the catalytic reaction. Reaction conditions: n (methylal) : n (trioxane) : n (ILs) = 180 : 60 : 1, 150 °C, 10 h.

In addition, as shown in Fig. 5(b), with increasing the carbon chain length of ionic liquids, the selectivity of PODME₂, PODME₃, and PODME_n=6–10 are increased at first and then decreased and the selectivity of PODME_4,5 is slightly increased. The selectivity of the PODME_n decreases with the increase of polymerization degrees. The selectivity of PODME_n is in the sequence of n = 2 > n = 3 > n = 4 > n = 5 > n = 6 > n = 7 > n = 8. Therefore, the reaction mechanism is reliable in Fig. 6. First trioxane is converted to formaldehyde over the acidic catalyst. Formaldehyde represents the monomer of the PODME oligomers and reacts with methylal (DMM) to PODME_n=2. In general PODME_n may react with formaldehyde to obtain PODME_n+1.²

Fig. 6 Reaction mechanism of PODME_n.

The reuse of ionic liquid

In order to investigate the potential reusability of the ionic liquids in the synthesis of PODME_n, a series of recycle experiments were conducted with a methylal to trioxane molar ratio of 3 and methylal to ionic liquid molar ratio of 180 at 150 °C for 10 h. After each recycle reaction, the ionic liquid was simply separated from the upper phase by decantation and was dried for 12 h in vacuum. As shown in Fig. 7, the [C₆ImBS][HSO₄] was used five times and its catalytic activity changed very little. The recovery rate of the ionic liquid catalyst was 91% after used five times. Therefore, the [C₆ImBS][HSO₄] as the catalyst for the synthesis of PODME_n is recyclable and stable.

Fig. 7 The reuse of ionic liquid.

Conclusions

Brønsted acid ionic liquids with different carbon chain length of alkyl groups and an alkane sulfonic acid group were synthesized through bromoalkane, imidazole and 1,4-butane sultone as raw materials. They were characterized by infrared spectrum (IR), nuclear magnetic resonance (NMR), ESI-MS, solubility analysis and differential thermal analysis (TG/DTA). Catalytic reaction of methylal (DMM) with trioxane (TOX) for preparation of polyoxymethylene dimethyl ethers (PODME_n, CH₃O(CH₂O)_nCH₃, where n > 1) was investigated in various Brønsted acid ionic liquids with different carbon chain length of alkyl groups. It was found that their structures were consistent with the designed structure and their purity was high. The thermal decomposition temperatures of ionic liquids were more than 250 °C by analyzing TG/DTA. The hydrophobicity of ionic liquids became stronger with the increase of carbon chain length. These acidic ionic liquids showed excellent catalytic performance. The catalytic activity of each ILs is dependent on the carbon chain length of alkyl groups. The selectivity of PODME_3–8 are increased at first and then decreased with lengthening carbon chain of ionic liquids. The ionic liquid – [C₆ImBS][HSO₄] showed the best catalytic performance.

Acknowledgements

This work is supported by the National Nature Science Foundation of China (20706006), the Excellent Young Scholars Research Fund of Beijing Institute of Technology (2007Y0509) and the Fundamental Research Foundation of Beijing Institute of Technology (20121042007) and the Foundation of Beijing Key Laboratory for Chemical Power Source and Green Catalysis (2013CX02031, 2014CX02026).

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