An efficient route for electrooxidation of methanol to dimethoxymethane using ionic liquid as electrolyte

An ionic liquid 1-ethyl-3-methyl imidazolium tetrafloroborate (EmimBF4) was found to be highly active for one-pot synthesis of dimethoxymethane (DMM) by electrooxidation of methanol on platinum electrode, exhibiting 34.7% conversion, 96.9% selectivity to DMM, high current efficiency (99.2%) as well. The electrode reaction mechanism was proposed according to the experimental results and reported literature.

The selective oxidation of methanol to several important organic chemicals, ranging from formaldehyde, dimethoxymethane (DMM or methylal) and methyl formate has attracted more and more attention.Among these products, DMM is known to be an attractive candidate for fuels or fuel additive due to its ability to reduce soot formation during the combustion in diesel engines with a high chemical stability. 1oreover, this molecule represents an important building block and can thus be applied as a source for the synthesis of polyoxymethylene dimethyl ether (POMM), which could be used as a safe embalming agent in substitution of the currently used formaldehyde, 2 a wellknown human carcinogen.It is also applied as reagent in organic synthesis and as an excellent solvent in pharmaceutical and perfume industries because of low toxicity. 3Usually, DMM is produced by the condensation of formaldehyde with methanol over acidic catalysts. 4owever, this approach is intricate and costly owing to the high reaction temperature and equipment corrosion caused by the acidic catalysts.Many efforts have been dedicated to developing one step direct oxidation of methanol into DMM and a number of selective oxidation catalysts such as rhenium oxide, 5 SbRe 2 O 6 , 6 heteropoly acids, 7 Cu-ZSM-5, 8 V 2 O 5 /TiO 2 9 and VO x /TS-1 10 etc. have been reported, where three methanol molecules are incorporated into one DMM molecule via oxidation of methanol to formaldehyde by O 2 gas and the condensation of formaldehyde with methanol.Most of these systems use bifunctional heterogeneous catalysts to control the sequential in situ formation of formaldehyde and its subsequent condensation with methanol in gas-phase fixed-bed reactor.Recently, Li et al. 11 described a successful liquid-phase oxidation of methanol for the synthesis of DMM using RuCl 3 as a catalyst.
However, the most attractive attribute of the electrochemical synthesis of chemicals is that one can control the reaction at the electrode by controlling the potential or current and thus controlling the reactions by means of electricity is a simple and attractive prospect for chemists and engineers devoting themselves to the catalytic synthesis of organic chemicals.Recently, the efforts toward electrooxidation of methanol have been mostly made for methanol fuel cells.In these processes, the Pt based electrode materials including Pt/Ru, 12,13 Pt/Sn 14,15 and Pt/Se 16 etc.have been used.The studies on the base anode materials showed that Pt electrode is the site of methanol binding and dehydrogenation. 17The investigations on electrooxidation of methanol to products DMM and HCOOH has been reported yet by Matare et al. 18 and Anthony et al., 19 but these studies only act as a part of methanol fuel cells with a very low yield of DMM.However, the solvent dichloroethane and catalysts Ru/Sn, Ru/Pt, Ru/Pd and Ru/Au bimetallic complexes were used in these systems.To circumvent the application of organic solvents and bimetallic complexes that can z E-mail: caiqinghai@yahoo.com;lupin630411@163.comcause inconvenient separation of the catalysts from the reaction mixture, a direct approach to generate DMM from electrochemical oxidation of methanol on Pt electrode and using ionic liquid as supporting electrolyte or/and catalyst would provide a attractive alternative route.Herein, we describe the electrocatalytic process to achieve this goal, opening the posibility for controllable conversion of methanol into DMM on the electrode under solvent-free condition.

Experimental
The ionic liquids such as 1-benzyl-3-methyl imidazolium chloride (BzmimCl), 1-butyl-3-methylimidazolium chloride (BmimCl), 1-butyl-3-methylimidazolium hydroxide (BmimOH), 1-butyl-3-methylimidazolium bromide (BmimBr), 1-butyl-3methylimidazolium tetrafloroborate (BmimBF 4 ) and 1-ethyl-3methyl imidazolium tetrafloroborate (EmimBF 4 ) were synthesized according to previously reported methods. 20,21yclic voltammetry was carried out with a CHI620B electrochemical analyzer.Before the measurement, Pt electrodes with rectangle and surface area 0.2826 cm 2 (Pt-213, Weiye instrument factory in Shanghai) were burnished with metallographic abrasive paper [W20(02)-500 # , Shanghai grinding wheels plant], followed by washing with acetone and distilled water in turn.The rectangle Pt electrodes were used as working and counter electrodes, respectively, and saturated calomel as reference one at scan rate of 0.05 V/s.The electrolysis of anhydrous methanol using bulk electrolysis with coulometry was performed in an undivided cell with rectangle Pt piece as a cathode and an anode.In a typical procedure, 20 mL (0.49 mol) methanol and 24 mmol ionic liquid were charged into the cell.The electrolysis was carried out at 3.0 V potential for 24 h.After the electrolysis, the reaction mixture was distilled to separate the methanol and products from the ionic liquids.The liquid products collected from the outlet were directly analyzed by GC (Agilent 7820) and GC-MS (Agilent GC 7890-MS 5975c).The conversion was calculated by the following formulae: con.= m c /m o × 100%; m c is the converted mass and m o is the initial mass of methanol added in the reaction.The current efficiecy was calculated basis on two electrons transfer.

Results and Discussion
Cyclic voltammetry in ionic liquids.-Cyclicvoltammetry (CV) of pure ionic liquids EmimBF 4, BzmimCl and BmimBr showed that almost no redox peaks were observed in the sweeping region from −2.0 to + 2.0 V, exhibiting > 4.0 V electrochemical window of EmimBF 4, BzmimCl and BmimBr, respectively (Figure 1).This phenomenon proved their electrochemical stability of these anions BF 4 − , Cl − and Br − on the Pt electrode.After addition of anhydrous methanol to the ionic liquid EmimBF 4 , oxidized peaks of methanol were found at the potentials (E) of 1.09 and 1.45 V, corresponding to oxidation of methanol into CH 2 OH and HCHO, respectively (Figure 2c) via the electrode reactions of methanol carried out on the Pt surface.Firstly, the adsorbed methanol loses a electron and a hydrogen cation on the surface of the anode to form CH 2 OH, and then the adsorbed CH 2 OH continued to lose another electron and hydrogen cation to produce formaldehyde HCHO. 22,23The reduction peaks at −1.03 and 0.46 V were ascribed to reduction of the oxidized intermediates such as CH 2 OH and HCHO due to electrochemical inertness of ionic liquid EmimBF 4 .The influence of scan rate on the CV responses of methanol electrooxidation-reduction was recorded in the same conditions with various sweep rates, as shown in Figures 2b-2e.When the scan rate was increased from 0.01 to 0.09 V/s, the oxidation peaks shifted to more positive direction and the reduction ones shifted to the contrary.Simultaneously, the peak current increased, respectively.A good linear relationship was constructed between the peak currents and the root of scan rate, as shown in Figure 3, which implied that the electrode process was controlled by diffusion. 24The oxidation and reduction reactions taking place on the Pt electrode established the basis of methanol conversion on the electrode.
Electrosynthesis in various electrolytes.-Basedon the above cyclic voltammogram, electrochemical conversion of methanol into DMM was conducted in various ionic liquids as supporting elec- trolytes.The results were summarized in Table I.As shown in the  (3.64 mS/cm) in methanol.These findings suggested that the relevant properties of ionic liquids for solute interaction and their effect on catalytic reaction are significantly determined by the nature of the anion, which has been reported in previous literatures yet. 25,26oreover, the effect of cations on the electrochemical reaction of methanol on the Pt electrode was also remarkable since BzmimCl and EmimBF 4 possessed higher activity than BmimCl and BmimBF 4 did, respectively, in our cases.When the electrolysis voltages increased from 2 to 3 V, the conversion of methanol with EmimBF 4 as electrolyte was promoted, exhibiting 21.6% conversion with 97.3% selectivity (entry 7).Continuously raising the voltage to 4.0 V unexpectedly led to almost unchanged activity (entry 8).However, the amount of EmimBF 4 used in the system also had an influence on the electrolysis.The conversion increased from 21.6 to 33.8 and 34.7% when the molar ratio of EmimBF 4 to methanol was increased from 0.049 (0.024/0.49) to 0.062 (0.024/0.39) and 0.069 (0.024/0.35) due to enhancing the electric conductance of EmimBF 4 -methanol solution (entries 9 and 10).The curves of electrolysis currents vs. time at various molar ratios of EmimBF 4 to methanol indicated that the curents were almost constant after the electrolysis carried out about 50 min.According to the current-time dependence, 64.3%-99.2%current efficiencies were obtained for conversion of methanol when using EmimBF 4 as electrolyte, as shown in Table I.Besides, the effect of electrolysis time on the conversion was also remarkable.11.9% and 29.9% conversions were obtained when the electrolysis proceeded 12 and 36 h, respectively (entries 11 and 12).It was also seen from entries 7-12 of Table I that the current density had a great influence on the electrolysis process.When the electrolysis time was suficient to long, the conversion of methanol was increased with the current density increasing.For example, the conversion increased from 18.2% to 34.7 due to the current density increasing from 0.34 to 0.52 A/cm 2 .In addition, when inorganic salts NaCl and NH 4 BF 4 were substituted for ionic liquids as supporting electrolyte, no product was found for NaCl-methanol system, and trace of DMM was detected in NH 4 BF 4 -methanol These findings further elucidated the synergistic effect of organic cations and counter anions composed of ionic liquids in the electrochemical synthesis of DMM.It was noteworthy that the selectivity to DMM under various conditions is all more than 95.0% when using EmimBF 4 as electrolyte, which is highly selective for DMM synthesis as compared with oxidation of methanol catalyzed by bifunctional catalyst. 10The main by-products in this process were determined to be dimethyl carbonate (DMC), dimethoxyethane (DMO), methoxyethanol (MOE), ethanediol (EDO), methyl formate (MF) and a little bit of trioxane (Tr) by GC-MS with their selectivities of 1.5-4.7% in these cases.In order to identify if the gas products emitted from the electrolysis cell, the outlet of the cell was imported into soda-lime solution.Unexpectedly, no any precipitation or turbidity was found in the soda-lime solution, indicating that over-oxidation of the methanol into CO 2 could not take place and the species formed by methanol oxidation on the electrode all enter in the main product DMM, as well as other byproducts DMC, DMO, MOE, EDO, MF and a little bit of Tr.The existence of these byproducts in the electrolysis systems was probably beneficial to deduce reaction mechanism on the electrode.
According to Goodenough and Hamnett, 27  to form formic acid, the formic acid further reacts with methanol to produce MF.Evidently, a trace of MF and Tr in the mixture products supports the above deduction.This finding also suggested that the condensation of HCHO with CH 3 OH to form DMM is faster step; the electrooxidation of methanol into CH 2 OH is rate-determined one.Consequently, the electrolysis of methanol on the surface of Pt electrode and the condensation of produced HCHO on the electrode with methanol in the solution are both enhanced by the ionic liquid EmimBF 4 as electrolyte and catalyst.
Li et al. 28 and Wang et al. 29 revealed by means of the ab initio and DFT approaches, as well as experiments, that the catalytic activity on the surface of Pt metal to oxidation of methanol was attributed to formation of a stronger bond between Pt and hydrogen atoms in methanol molecule.As a result, CH 3 O • (or CH 3 O − ) was inevitably present on the surface of the electrode.Consequently, the two intermediate CH 3 O • or CH 3 O − could attack the species 3 to generate DMC (Scheme 2b).On the same way, the interaction between the two species 5 proceeded to produce EDO.Sequentially, EDO reacted with methanol to form MOE and DMO, respectively.These steps are pathway for the above byproducts.

Conclusions
In conclusion, we demonstrated a new route for electrochemical oxidation of methanol to dimethoxymethane (DMM) using Pt and ionic liquid EmimBF 4 as electrocatalyst.The reaction was carried out under solvent-free condition.The rate of the product formation and the selectivity to DMM could be expediently controlled by the applied voltage between the Pt electrodes.Besides, we provided a reasonable explanation for the formation of the electrolysis products according to the electrochemical reaction mechanism.

Figure 1 .
Figure 1.Cyclic voltammograms of Pt electrode in the ionic liquids.

Table ,
BzmimCl showed better activity for the electrochemical reaction with 4.2% conversion of methanol and 89.8% selectivity to DMM (entry 1), while BmimCl exhibited relatively lower activity and selectivity for the reaction (entry 2), implying that benzyl group was beneficial to the conversion of methanol into DMM as compared with butyl one.Contrarily, substitution of OH − and Br − for Cl −to form BmimOH and BmimBr in the systems can improve the activity (entries 3 and 4).Surprisingly, BmimBF 4 proved highly active for the electrochemical reaction, exhibiting 15.8% conversion and 95.1% selectivity (entry 5).EmimBF 4 exhibited slightly higher activity than BmimBF 4 did (entry 6), giving 17.1% conversion and 96.1% selectivity, probably caused by the smaller cation in EmimBF 4 that led to higher electric conductivity of EmimBF 4 (6.53 mS/cm) than that of BmimBF 4

Table I . Synthesis of DMM in various electrolytes.
entry 13 and 14).The NaCl-methanol and NH 4 BF 4 -methanol system are all supersaturated solutions of NaCl and NH 4 BF 4 in methanol. ( four bound species are present on Pt electrode during the electro-oxidation of methanol, ) unless CC License in place (see abstract).ecsdl.org/site/terms_useaddress.Redistribution subject to ECS terms of use (see 128.184.188.10Downloaded on 2018-07-12 to IP