A novel vapor-phase catalytic synthetic approach for industrial production of 1,1,1,3,3,3-hexafluoroisopropyl methylether
Introduction
Owing to the implementation of the Montreal Protocol, the chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), which show high ozone depletion potentials (ODP) and high global warming potentials (GWP), were banned from using [1]. Thereafter, hydrofluoroethers (HFEs) were considered as potential substitutes for CFCs and HCFCs due to their zero ODP and relatively short atmospheric lifetimes [[2], [3], [4]]. Among the various HFEs, 1,1,1,3,3,3-hexafluoroisopropylmethyl ether (HFE-356mmz) has attracted particular attention because it exhibits an ODP of zero, an atmospheric lifetime of 62 days, a GWP100 value of 6, weak flammability, high chemical stability, and low toxicity [[3], [4], [5], [6]]. These advantageous properties enable huge application potentials of HFE-356mmz in coolants, solvents, cleaners, and the like. As a result, mass production of HFE-356mmz with excellent synthetic efficiency is highly desired.
Up to now, a number of synthetic strategies for HFE-356mmz from different starting materials have been reported (Scheme 1). One of the most popular methods is the liquid-phase O-methylation of 1,1,1,3,3,3-hexafluoroisopropanol (HFiP) using (CH3)2SO4 [7], dimethyl carbonate (DMC) [8], or CH3X (X = Cl, Br, I) [9] as the methylating reagent. However, (CH3)2SO4 is highly toxic and the reaction using DMC needs to be carried out in an autoclave under a harsh condition. In addition, strong bases are required as catalysts in all these reactions, leading to cumbersome post-processing problems. All the other methods employ high-cost starting materials such as 1,1,3,3,3-pentafluoro-2-meth-oxyprop-1-ene [10], methyl-3,3,3trifluoro-2-hydroxy-2-(trifluoromethyl) propanoate [10], 2-methoxymalo-nonitrile [11], 1,1,1,3,3,3-hexafluoro-2-(trichlorometh-oxy) propane [12], and sevoflurane [13]. Moreover, most of these methods are based on liquid-phase reactions which are of intermittent operation and high pollution due to the production of a large amount of unrecyclable solvents and base.
Therefore, there is still no suitable method for mass production of HFE-356mmz so far.
To develop a green, pollution-free, and continuous process for the synthesis of HFE-356mmz, we came up with the idea of using a Lewis acid catalyst, namely, metal fluoride, because Lewis acid catalysts (eg. FeCl3, AlCl3, and zeolites) [14,15] and Lewis base catalysts (eg, K2CO3, CsF/α-Al2O3, and MgO) [16,17] were reported to be constructive in the methylation reaction. The metal fluoride was selected because it can resist the corrosive HF, which might be produced from the decomposition of HFiP at high temperatures. In addition, metal fluorides, in many cases, are powerful catalysts to help achieve green gas-phase reactions such as dehydrohalogenation reaction [18], Cl/F exchange reaction [19], and isomerisation reaction [20]. To our best knowledge, the metal fluoride has not been disclosed in any open literature as a catalyst in the methylation reaction. Herein, we achieved a green vapor-phase synthetic approach for HFE-356mmz for the first time by utilizing metal fluoride as the catalyst. As shown in Scheme 2, the methylation of HFiP using DMC gave rise to HFE-356mmz and a series of volatile by-products. To achieve high synthetic efficiency, a number of metal fluorides with an order of decreasing Lewis acidity (AlF3, MgF2, CaF2, SrF2, and BaF2) were prepared and tested. MgF2 with moderate Lewis acidity and moderate Lewis basicity exhibited the highest activity. Through systematic investigation of all these metal fluorides, their catalytic activity was found to depend on the combination of the surface acidity and basicity, as well as the total amount of surface Lewis acid sites. In addition, the MgF2 showed high stability over 1000 h and HFE-356mmz in 50 kg scale could be prepared, suggesting great potential of the synthetic route for industrial production of HFE-356mmz. Accordingly, a rational mechanism of the vapor-phase methylation was proposed at the end. Our study not only paves the way for industrial production of HFE-356mmz, but also expands the application of metal fluorides.
Section snippets
Preparation of the catalysts
Metal fluorides (AlF3, MgF2, CaF2, SrF2, and BaF2) were prepared by fluorination of the corresponding metal oxides (Al2O3, MgO, CaO, SrO, and BaO) [21,22], in which MgO and Al2O3 were obtained by a precipitation method. The typical process for synthesizing MgF2 was as follows: Mg(NO3)2 (148 g, 1.0 mol) was dissolved in distilled water (400 ml) and stirred for 1 h, then ammonium hydroxide (5 %) was slowly added to the solution at room temperature until pH = 8.5 was reached. The precipitate
Synthesis of HFE-356mmz
The HFE-356mmz was synthesized through methylation reaction of HFiP using DMC as the methylation reagent over different metal fluorides (AlF3, MgF2, CaF2, SrF2, or BaF2) at 250 °C under 101.1 kPa. According to the GC analysis results of the outlet reaction mixture, methanol (CH3OH) and methyl ether (CH3OCH3) were produced in addition to the product HFE-356mmz and the unreacted raw materials (DMC and HFiP). CH3OH was probably formed by the methylation reaction of DMC while CH3OCH3 might be
Conclusions
A novel and efficient vapor-phase synthetic approach to prepare HFE-356mmz was demonstrated by utilizing metal fluorides as the catalysts in the methylation reaction of HFiPfor the first time. The catalysts enabled 100 % selectivity of HFE-356mmz since no by-product from HFiPwas produced. All the other by-products from the methylation reagent (DMC) are volatile, making the reaction solvent-free. Through systematic investigation of a series of metal fluorides (AlF3, MgF2, CaF2, SrF2, and BaF2),
Credit author statement
X.Z. is the principal investigator. W.L. and X.Z designed the work. W.L. and G.Y. performed the research. W.L. and F.L. analyzed the data and wrote the manuscript. All authors discussed the results and commented on the manuscript.
Declaration of Competing Interest
The authors declare no competing financial interest.
CRediT authorship contribution statement
Wei Li: Conceptualization, Project administration. Gang Yang: Conceptualization, Project administration. Fengniu Lu: Conceptualization, Project administration. Xiaoling Zhang: .
Acknowledgments
The authors are grateful to the financial support by Sinochem, Lantian, P. R. China (grant no. ZHLT2015AC20).
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Vapor-phase catalytic methylation of 1,1,1,3,3,3-hexafluoroisopropanol for the mass production of 1,1,1,3,3,3-hexafluoroisopropyl methyl ether
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