Process and arrangement for producing ethylene glycol by paired electrolysis

Abstract

The present invention relates to a process and an arrangement for producing ethylene glycol by paired electrolysis from formaldehyde and methanol. In the process an electrolyte composition comprising aqueous formaldehyde, methanol, at least one salt, and (2,2,6,6-tetramethylpiperidin-1-yl)oxyl is subjected to electrolysis in an undivided electrolytic cell having at least one carbon-based anode and at least one carbon-based cathode. The ethylene glycol is formed by reduction of formaldehyde at said at least one carbon-based cathode. At least part of the formaldehyde is formed by oxidation of methanol at said at least one carbon-based anode.

Description

[0001] PROCESS^ AND^ ARRANGEMENT^ FOR^ PRODUCING^ ETHYLENE^ GLYCOL^ BY^ PAIRED^ELECTROLYSIS^ FIELD OF THE INVENTION The present invention relates to a process for producing ethylene glycol by paired electrolysis from formaldehyde and methanol. The present invention also relates to an arrangement for producing eth-ylene glycol by paired electrolysis from formaldehyde and methanol. The present invention further relates to use of 2,2,6,6-tetramethylpi-peridin-1-yl)oxyl (TEMPO) in production of ethylene glycol by paired electrolysis from formaldehyde and methanol. BACKGROUND OF THE INVENTION Ethylene glycol (EG) is a heavy hitter commodity chemical that is cur- rently produced mainly from fossil-based sources. Several bio-based routes havebeen studied as well but there is a growing concern towards using biomass as achemical feedstock. At elevated scale, this can lead to degradation of ecosystemswhich can contribute, for example, to anthropogenic extinction. In the past, sub-stantial effort has been directed towards the direct dehydrodimerization of formal-dehyde into ethylene glycol with organic electrosynthesis. As formaldehyde can besynthesized from carbon dioxide or from carbon dioxide derived methanol, theelectrosynthesis route could allow CO2-based synthesis of ethylene glycol. As eth-ylene glycol (EG) is a high-volume commodity chemical, the synthesis must be asefficient as possible. Typically, the focus in electrosynthesis is either at the cathode or anode and the other electrode process is neglected. However, if both electrodes contrib- ute to the synthesis meaningfully, a substantial economic savings can be made. Compared to methanol, formaldehyde is more expensive starting material, thus, paired electrolysis where formaldehyde is produced at anode would decrease the amount of needed formaldehyde and increase cost-effectiveness of ethylene glycol synthesis from formaldehyde. Noteworthy, a direct synthesis of EG using methanol as starting material seems unrealistic as the generation of two formaldehyde mol-ecules requires four electrons and the dehydrodimerization of two formaldehydesto EG two electrons. Therefore, there will always be a need to use formaldehyde in the reaction mixture.Existing solutions to the synthesis of ethylene glycol are many. For ex-ample, the current industrial approach proceeds from ethene via oxirane to eth- ylene glycol. Bio-based alternatives could encompass either producing ethene from ethanol or by hydrogenolysis of sugars. In publication US4478694A is disclosed methods for electrosynthesis ofpolyols. Example IX in US4478694A describes paired electrosynthesis where thecathode is used to produce ethylene glycol from formaldehyde and the anode isused to produce formaldehyde from methanol utilizing a divided cell with a Pro-totech PWB-3 gas diffusion electrode (methanol is introduced as a gas) with plati-num impregnated anode to produce formaldehyde. The formaldehyde is not di-rectly used in the synthesis of ethylene glycol and the products formed at the cath-ode and anode sides are collected separately. The formaldehyde that is producedat the anode is not consumed in the synthesis of ethylene glycol. A similar strategy as described above has been used with a membraneassembled electrolyzer in the article Xia, R., Wang, R., Hasa, B. et^al. Electrosynthe-sis of ethylene glycol from C1 feedstocks in a flow electrolyzer. Nat^Commun 14,4570 (2023). BRIEF DESCRIPTION OF THE INVENTION The objects of the invention are achieved by a process, an arrangementand a use which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims. As defined in independent claim 1, one aspect of the invention is a pro- cess for producing ethylene glycol by paired electrolysis from formaldehyde and methanol, wherein an electrolyte composition comprising aqueous formaldehyde, methanol, at least one salt, and (2,2,6,6-tetramethylpiperidin-1-yl)oxyl is subjected to electrolysis in an undivided electrolytic cell having at least one carbon-based an- ode and at least one carbon-based cathode, wherein ethylene glycol is formed by reduction of formaldehyde at said at least one carbon-based cathode, and at least part of the formaldehyde is formed by oxidation of methanol at said at least one carbon-based anode. In one preferred embodiment of the process according to the invention,the at least one carbon-based cathode is a graphite cathode, which preferably is treated with sulfuric acid or H2O2. According to an alternative preferred embodiment of the process, the atleast one carbon-based anode is a glassy carbon anode, a reticulated vitreous carbon (RVC), a boron-doped diamond (BDD) anode or a graphite anode, said graphite anode being preferably treated with sulfuric acid. According to a further preferred embodiment of the process, current isfed to the undivided electrolytic cell with at a current density in the range of 200 to 400 mA / cm2, preferably 300 to 400 mA / cm2, more preferably 300 to 380 mA / cm2,most preferably 300 to 350 mA / cm2. The undivided electrolytic cell may be eithera batch or flow-cell. In a still further preferred embodiment of the process the at least one salt comprises sodium chloride, a quaternary salt, or a tertiary salt, or a combina-tion of at least two of these. The quaternary salt is preferably selected from quater-nary ammonium and phosphonium salts, and the tertiary salt is preferably selected from tertiary sulfonium salts. In a particularly preferred embodiment, the quaternary ammonium salt is a quaternary alkyl ammonium salt, such as a quaternary alkyl ammonium chlo- ride, preferably NMe4Cl, NMeEt3Cl, NBu4Cl, NBu3MeCl or NEt4Cl, most preferably NBu3MeCl. Most preferably, the at least one salt comprises sodium chloride and NBu3MeCl, preferably consists of sodium chloride and NBu3MeCl. It is further preferable in the process that the temperature of the elec-trolyte composition during electrolysis is set in the range of 50 to 85 °C, more pref- erably 60 to 80 °C. In a further preferred embodiment of the process of the invention(2,2,6,6-tetramethyl-piperidin-1-yl)oxyl is present in the electrolyte composition at an initial concentration in the range from 0.020 M to 0.1 M. In the process it is further preferable that sodium chloride is preferablypresent in the electrolyte composition at an initial concentration in the range from0.62 to 1.03 M. The tributylmethylammonium chloride is preferably present in theelectrolyte composition at an initial concentration in the range from 0.076 to 0.13 M. According to an alternative embodiment of the process according to theinvention, additional formaldehyde is added to the electrolyte composition during electrolysis. As defined in independent claim 15, another aspect of the invention isan arrangement for producing ethylene glycol by paired electrolysis from formal- dehyde and methanol, the arrangement comprising an undivided electrolytic cell having at least one carbon-based anode and at least one carbon-based cathode, and an electrolyte composition comprising aqueous formaldehyde, methanol, at least one salt, and (2,2,6,6-tetramethylpiperidin-1-yl)oxyl. In one preferred embodiment of the arrangement according to the in-vention the at least one carbon-based cathode is a graphite cathode, which is pref-erably treated with sulfuric acid or H2O2. According to an alternative preferred embodiment of the arrangement the at least one carbon-based anode is a glassy carbon anode, a boron-doped dia- mond (BDD) anode or a graphite anode, preferably a sulfuric acid treated graphite anode. According to a further preferred embodiment of the arrangement itcomprises means for feeding current to the cell at a current density in the range of200 to 400 mA / cm2, more preferably 300 to 380 mA / cm2, most preferably 300 to 350 mA / cm2. The undivided cell of the arrangement is preferably either a batch orflow-cell. In a still further preferred embodiment of the arrangement the at least one salt comprises sodium chloride, a quaternary salt, or a tertiary salt, or a com-bination of at least two of these. The quaternary salt is preferably selected fromquaternary ammonium and phosphonium salts, and the tertiary salt is preferably selected from tertiary sulfonium salts. In a particularly preferred embodiment, the quaternary ammonium salt is a quaternary alkyl ammonium salt, such as a quaternary alkyl ammonium chlo- ride, preferably NMe4Cl, NMeEt3Cl, NBu4Cl, NBu3MeCl or NEt4Cl, most preferably NBu3MeCl. Most preferably, the at least one salt comprises sodium chloride andNBu3MeCl, preferably consists of sodium chloride and NBu3MeCl. It is further preferable in the arrangement that it comprises means forheating the electrolyte during electrolysis to a temperature in the range of 50 to 85 °C, preferably 60 to 80 °C. In a further preferred embodiment of the arrangement of the invention (2,2,6,6-tetra-methylpiperidin-1-yl)oxyl is present in the electrolyte composition at an initial concentration in the range from 0.020 M to 0.1 M. It is further preferable in the arrangement that sodium chloride is pre-sent in the electrolyte composition at an initial concentration in the range from 0.62 to 1.03 M. It is yet further preferred that the tributylmethylammonium chloride is present in the electrolyte composition at an initial concentration in the range from 0.076 to 0.13 M. According to a further preferred embodiment of the arrangement of theinvention, it comprises means for addition of additional formaldehyde into theelectrolyte composition in the undivided electrolytic cell during electrolysis. As defined in independent claim 29 the invention also concerns use of 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) in production of ethylene glycol by paired electrolysis from formaldehyde and methanol. The present invention has several advantages that makes it more eco-nomically feasible than the prior art solutions. Noteworthy, in contrast to prior artsolutions, the formaldehyde generated at the anode is directly consumed in thesynthesis of ethylene glycol at the cathode and therefore contributes towards syn-thesis of ethylene glycol. Further, the present invention utilizes an undivided elec-trolytic cell, which makes the electrochemical system easier to scale-up and it canalso lower cell resistance, which reduces the amount of electricity consumed.Further, the TEMPO redox-mediator, which is added into the reactionmixture in the electrolytic cell, enables oxidation of methanol into formaldehydewhich elevates the current efficiency (c.e.) of the reaction. Most strikingly, thiseffect is seen at industrially relevant high current densities between 200 to 400mA / cm2. The addition of TEMPO (2,2,6,6-tetramethylpiperidin-1-yl)oxyl) intothe reaction mixture of the process also converts the anodic counter-reaction intomethanol oxidation to formaldehyde. This way formaldehyde concentration stays higher during electrolysis than it would stay without TEMPO. This enables bettercurrent efficiency (up to 94%) or yield (up to 55%) than without TEMPO.The effect of TEMPO is demonstrated further with control experimentswithout TEMPO at various current densities between 200 to 400 mA / cm2: Highercurrent efficiency is always achieved with TEMPO.DETAILED DESCRIPTION OF THE INVENTION In the following, the present invention is explained in more detail byway of specific examples in the form of laboratory experiments made. Experimental 1.^General^Information All chemicals and solvents were purchased and used without any puri-fication unless otherwise stated. Analytical grade formaldehyde was supplied as a37% water solution, stabilized with 10% MeOH. Graphite electrodes (Cgr, Sigrafine V2100) were wetted before use. Graphite surfaces were cleaned after reactions with a synthetic fiber polishing pad after which they were sonicated for around 10minutes in a distilled water bath, washed with distilled water and acetone and pol-ished with paper towel and acetone. Prior to use the electrodes were placed in 5%H2SO4solution for 10 min and subsequently rinsed with water and acetone. The glassy carbon (GC, Sigradur G) surface was washed with acetone and water and wiped with a paper towel prior the use. Boron-doped diamond electrodes (BDD) were conditioned by electrolyzing 20% H2SO4with BDD as an anode and Cgr-plate as a cathode (10 C / cm², 10 mA / cm²) with subsequent rinsing with water and ace-tone. Electrode dimensions were 2x6 cm for all the electrodes while the interelec-trode gap was 0.5 cm. All the electrolysis experiments were carried out in custom-manufac-tured 25 mL glass cells (diameter 3 cm, height 7 cm) and the cell was powered bya constant current power supply. NMR spectroscopy was used to monitor the con-version, yield and selectivity of the reactions. All the spectra were acquired on D2O and referenced to methyl sulfone (DMSO2, 3.60 ppm). Calculation of current efficiency (c.e.):^. ^. =^(^^) ×100% =^(^^)2^(^^) × ^^ × ^(^^)1 1^× 100% = × 100%× ^^2^ × ^2 (^)2^(^^) × ^^where ^(^^) is charge of an electron, ^^ is Avogardo’s constant, ^(^^)is the molar amount of produced ethylene glycol, ^ is electrolysis current in am-peres, and ^ is electrolysis time in seconds.2.^Standard^operation^conditions Sodium chloride, TEMPO and quaternary ammonium salt wereweighted into the glass cell equipped with a magnetic stirring bar.15 mL 37% for- maldehyde solution was then measured to the cell using a measuring burette, fol-lowed by 7 mL MeOH and optionally with 3 mL H2O (for clarity, throughout thisapplication mL means milliliter, which also may be abbreviated as ml). A condenser with a water circulation and electrodes in a PTFE holder were then attached to the cell. The cell was then placed in an oil bath and preheated for 15 minutes at thedesired temperature. The reaction temperature refers to the solution of the oilbath. After preheating, a constant current was passed through the electrodes for apredetermined charge amount. After completion of the electrolysis, the mixturewas removed from the oil bath and diluted to 25.0 mL with water and analyzedwith1H NMR spectroscopy. 3.^Screening^of^reaction^conditions 3.1.^Preliminary^screening Preliminary screenings (Table 1, Entries 1-5) were conducted in solu-tions consisting of 15 mL formaldehyde solution and 7 mL MeOH with NaCl (1.2 g),TEMPO (0, 100 or 200 mg) and tributylmethylammonium chloride (600 mg). Se- lected examples of these studies are presented below in Table 1. In preliminary screenings, we found that TEMPO increases both ethylene glycol yields and currentefficiency (Entries 1 – 3) and we decided to include TEMPO in the further optimi-zation. High current density (300 mA cm-2) and temperature (70 °C) were alsofound favourable (Entries 4 – 5), and they were therefore chosen as starting valuesfor further optimization. Increasing current density and temperature to 350 mAcm-2 and 80 °C while slightly increasing amount of TEMPO to 120 mg, improvedcurrent efficiency to 94% (Entry 6). Noteworthy, the completion degree of electrol-ysis was 30% and amounts of NaCl and tributylmethylammonium chloride were900 mg and 750 mg, respectively.

[0002] Table^1. Examples from preliminary screening of ethylene glycol electrosynthesis,using graphite electrodes as cathode and anode.j (mA^ T^(°C)yield^ c.e.^(%) (%) cm-2) (mg) (%) 168 300 70 0 33 492 71 300 70 100 41 583 66 300 70 200 43 664 68 30 70 100 15 225 66 300 40 100 9.9 156[a] 30 350 80 120 28 94 [a] Performed with 750 mg of NBu3MeCl and 900 mg of NaCl in solution of 15mL 37% aqueous formaldehyde, 7 mL MeOH and 3 mL H2O. Both graphite elec-trodes were pretreated with sulfuric acid. 3.2.^Effect^of^sulfuric^acid^pretreatment^of^graphite^electrodes To study the effect of pretreatment, three replicate electrolysis were performed first with untreated graphite electrodes, followed by another three rep-licates with acid-treated graphite electrodes. For these experiments a solution of15 mL formaldehyde, 7 mL MeOH and 3 mL H2O, containing NaCl (1200 mg) andtributylmethylammonium chloride (600 mg) and TEMPO (100 mg) was electro-lysed at 70 °C with a current density of 300 mA cm-2 until 41% completion. For acidtreatment, the electrodes were placed in a 5% H2SO4solution for 10 min after which they were rinsed with water and acetone. The results of the study on the effect of sulfuric acid treatment for reproducibility, yield and current efficiency(c.e.) across three replicates are presented in Table 2.Table^2. Effect of sulfuric acid treatment for reproducibility, yield and current effi- ciency across three replicates. untreated^graphite sulfuric^acid^treated^graphiteyield^(%) c.e. (%) yield^(%) c.e. (%)25 62 38 8624 60 38 8623 58 35 85 3.3.^Screening^of^anode^and^cathode^materials Previous studies have indicated that carbon-based cathodes are supe- rior to metal-based cathodes in the cathodic coupling of formaldehyde to ethylene glycol (EG). Therefore, we decided to screen different carbon-based cathodes usingthe optimal conditions from Table 1, Entry 6, and Table 3, Entry 1, using sulfuricacid treated graphite electrodes as anodes. The alternative cathodes (i.e., graphitefelt, glassy carbon, and boron-doped diamond (BDD)) gave only very poor yieldsand c.e. for ethylene glycol (Table 3, Entries 2–4). In addition, intense gas formation was observed with all alternative cathodes, indicating towards more pronounced parasitic hydrogen formation compared to the graphite cathode. Next, we in- spected the suitability of alternative carbon anodes (Table 3, Entries 6–7). Glassy carbon and BDD gave slightly lower current efficiencies.Table^3. Effect of anode and cathode for paired ethylene glycol formation Entry Anode Cathode EG yieldConversion c.e. (%) (%)[a](%)[a]1Graphite[b] 28[c] 44[c] 94[c]2 Graphite[b] Graphite felt 2 44 73 Graphite[b] Glassy car- 2 39 5bon 4Graphite[b] BDD <1 51 16 Glassy car- Graphite[b] 24 49 79bon [a] Yields and conversions determined by1H NMR using dimethyl sulfone (DMSO2) as an internal standard. [b] Graphite cathodes were treated for ten minutes in 5% H2SO4prior the reaction [c] Average of three experiments.3.4.^Effect^of^different^supporting^electrolyte^cations and^TEMPO mediatorDifferent supporting electrolyte cations (NMe4, NEt4, NBu4, NEt3Me)were then screened, using the optimal conditions from Table 1, Entry 6, aiming atimproving the current efficiency by further attenuating the parasitic hydrogen for-mation at the graphite cathode (Scheme 1). These experiments show thatNBu3MeCl gave the best c.e. for ethylene glycol (EG). The second-best performingadditive was NEt4Cl. Other tetra-alkyl ammonium cations gave lower, but still sat-isfactory c.e. ranging from 77 to 88%. Conversely, the effect of counter-anion seemssomewhat critical, as NBu4BF4and NBu4PF6gave 23 and 55% lower c.e. thanNBu4Cl. Surprisingly, replacing NaCl and NBu4Cl with NaBF4 and NBu4BF4 leads toa further dramatic decrease of c.e. to 15%. Thereafter we investigated the effect of redox mediators. In Table 1, En-try 6 the use of TEMPO mediator provided 94% c.e. compared to the 74% c.e. (notpresented in Table 1) obtained with the same conditions without a mediator. Scheme^1. Screening of different quaternary ammonium additives (with TEMPO) 3.5^Effect^of^applied^charge^for^paired^ethylene^glycol^formation The effect of the applied charge for paired ethylene glycol formationwas studied with respect of yield of ethylene glycol (EG), conversion degree (%)and current efficiency (c.e.) in a series of tests with different applied chargesranging from 0.6 F to 2 F presented as Entries 1–6 in Table 4 below. Apart fromthe charge, which was altered between 0.6 F to 2 F, all the other parameters werekept constant and their values taken from Table 1, Entry 6. As can be seen fromTable 4, the preferred charge is 1.4F–1.8F for optimizing EG yield.Table^4.^ Effect of applied charge for paired ethylene glycol formation. yield (%)[a] Conversion (%)[a] c.e.(%)[a]10.6 F / 30% 28 41 942 1.2F / 60% 44 76 733 1.4F / 70% 47 80 674 1.6F / 80% 52 92 665 1.8F / 90% 49 91 546 2F / 100% 44 98 44The completion degree, referred to as “completion” in Table 4, is usedto describe theoretical completion of electrolysis and it is determined by dividing the applied electric charge by the theoretical amount of charge that would be needed to convert all the formaldehyde loaded to the electrolytic cell in the begin-ning. Charge is expressed as F, where F is the theoretical amount of electrons inmoles that is needed to produce one mole of the product. Two electrons are needed to produce ethylene glycol from formaldehyde. Thus, the theoretical charge needed for 100% completion of electrolysis is 2 F.EG yields were then optimized further by varying temperature, theamount of TEMPO, and current density with 1.6F of applied charge. Gratifyingly,we obtained 55% yield and 69% c.e. with 15 mL 37% aqueous formaldehyde, 7mL MeOH, 3 mL H2O, 900 mg NaCl, 750 mg tributylmethylammonium chloride,120 mg TEMPO, 75 °C, 300 mA cm-2. Furthermore, the beneficial effect of TEMPOwas observable also at high conversions as the reaction without TEMPO additive produced 41% yield with 52% c.e. 3.6.^Effect^of^redox-mediator^on^current^efficiency^as^a^function^of^current^ density Finally, the benefit of redox-mediator was further demonstrated bystudying differences in c.e. with 120 mg of TEMPO and in its absence by electro-lysing 15 mL 37% aqueous formaldehyde, 7 mL MeOH, 3 mL H2O, 900 mg NaCland 750 mg tributylmethylammonium chloride at 75 °C with charge of 1.6 F at dif-ferent current densities (Table 5). At 200 mA cm-2 current density, the reactionworks as well with and without TEMPO. However, at current densities over 300 mA cm-2, the use of TEMPO provides significantly higher c.e. The maximum cur- rent efficiency is obtained at 350 mA cm-2. Furthermore, the current density can be doubled from 200 mA cm-2to 400 mA cm-2in the presence of TEMPO.Table^5. Current efficiency as a function of current density.Current density (mA Current efficiency (%) Current efficiency (%) cm-2) with TEMPO w / o TEMPO 200 76.9 76.5250 80.2 76.2300 81.7 77.7350 94.1 76.2400 81.0 60.43.7.^Flow-electrolysis The scalability of the presented invention was demonstrated in a flow electrolyser (10 cm2electrode area, 2 mm electrode gap). In these experiments,preheated electrolyte solution (11.2 mL MeOH and 4.8 mL H2O for every 320 mmolof formaldehyde) was pumped at rate of 20 mL / min to a heated flow cell whichwas operated at 75 °C and 300 mA / cm2. The solution was recirculated in the electrolyser until the desired charge of 1.6 F was passed, providing ethylene glycolin 47 and 46% yields when 320 and 640 mmol formaldehyde were used, respec-tively. These results are presented in Table 6.Table^6. Flow-electrolysis Entry Formaldehyde (mmol) EG yield (%) Conversion (%) c.e. 1320 47 81 592 640 46 85 58It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The inven- tion and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

CLAIMS 1. Process for producing ethylene glycol by paired electrolysis from for- maldehyde and methanol, c h a r a c t e r i z e d in that an electrolyte composition comprising aqueous formaldehyde, methanol, at least one salt, and (2,2,6,6-tetra- methylpiperidin-1-yl)oxyl is subjected to electrolysis in an undivided electrolytic cell having at least one carbon-based anode and at least one carbon-based cathode, wherein ethylene glycol is formed by reduction of formaldehyde at said at least one carbon-based cathode, and at least part of the formaldehyde is formed by oxidation of methanol at said at least one carbon-based anode.

2. Process according to claim 1, c h a r a c t e r i z e d in that the at least one carbon-based cathode is a graphite cathode, preferably treated with sulfuric acid or H2O2.

3. Process according to claim 1 or 2, c h a r a c t e r i z e d in that the at least one carbon-based anode is a glassy carbon anode, a reticulated vitreous car- bon (RVC), a boron-doped diamond (BDD) anode or a graphite anode, said graphite anode being preferably treated with sulfuric acid.

4. Process according to any one of claims 1 to 3, c h a r a c t e r i z e d in that current is fed to the undivided electrolytic cell with at a current density in the range of 200 to 400 mA / cm2, preferably 300 to 400 mA / cm2, more preferably 300 to 380 mA / cm2, most preferably 300 to 350 mA / cm2.

5. Process according to any one of claims 1 to 4, c h a r a c t e r i z e d in that the undivided cell is either a batch or flow-cell.

6. Process according to any one of claims 1 to 5, c h a r a c t e r i z e d in that the at least one salt comprises sodium chloride, a quaternary salt, or a tertiary salt, or a combination of at least two of these.

7. Process according to claim 6, c h a r a c t e r i z e d in that the quater- nary salt is selected from quaternary ammonium and phosphonium salts, and the tertiary salt is selected from tertiary sulfonium salts.

8. Process according to claim 7, c h a r a c t e r i z e d in that the quater- nary ammonium salt is a quaternary alkyl ammonium salt, such as a quaternary alkyl ammonium chloride, preferably NMe4Cl, NMeEt3Cl, NBu4Cl, NBu3MeCl or NEt4Cl, most preferably NBu3MeCl.

9. Process according to any one of claims 1 to 8, c h a r a c t e r i z e d in that the at least one salt comprises sodium chloride and NBu3MeCl, preferably con- sists of sodium chloride and NBu3MeCl.

10. Process according to any one of the claims 1 to 9,c h a r a c t e r i z e d in that the temperature of the electrolyte composi- tion during electrolysis is set in the range of 50 to 85 °C, preferably 60 to 80 °C.

11. Process according to any one of the preceding claims, ch a r a c t e r i z e d in that (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl ispresent in the electrolyte composition at an initial concentration in the range from 0.020 M to 0.1 M.

12. Process according to any one of claims 6 to 11, c h a r a c t e r i z e d in that sodium chloride is present in the electrolyte composition at an initial con- centration in the range from 0.62 to 1.03 M.

13. Process according to any one of claims 6 to 12, c h a r a c t e r i z e d in that tributylmethylammonium chloride is present in the electrolyte composition at an initial concentration in the range from 0.076 to 0.13 M.

14. Process according to any one of the preceding claims, c h a r a c t e r i z e d in that additional formaldehyde is added to the electrolyte composition during electrolysis.

15. Arrangement for producing ethylene glycol by paired electrolysis from formaldehyde and methanol, c h a r a c t e r i z e d by the arrangement com- prising an undivided electrolytic cell having at least one carbon-based anode andat least one carbon-based cathode, and an electrolyte composition comprisingaqueous formaldehyde, methanol, at least one salt, and (2,2,6,6-tetra¬methylpiper- idin-1-yl)oxyl.

16. Arrangement according to claim 15, c h a r a c t e r i z e d in that the at least one carbon-based cathode is a graphite cathode, which is preferably treated with sulfuric acid or H2O2.

17. Arrangement according to claim 15 or 16, c h a r a c t e r i z e d in that the at least one carbon-based anode is a glassy carbon anode, a boron-doped diamond (BDD) anode or a graphite anode, preferably a sulfuric acid treated graph- ite anode.

18. Arrangement according to any one of the claims 15 to 17, c h a r a c t e r i z e d by comprising means for feeding current to the cell at a current density in the range of 200 to 400 mA / cm2, more preferably 300 to 380 mA / cm2, most preferably 300 to 350 mA / cm2.

19. Arrangement according to any one of the claims 15 to 18, c h a r a c t e r i z e d in that the undivided cell is either a batch or flow- cell.

20. Arrangement according to any one of the claims 15 to 19,c h a r a c t e r i z e d in that the at least one salt comprises sodium chlo- ride, a quaternary salt, or a tertiary salt, or a combination of at least two of these.

21. Arrangement according to claim 20, c h a r a c t e r i z e d in that quaternary salt is selected from quaternary ammonium and phosphonium salts, and the tertiary salt is selected from tertiary sulfonium salts.

22. Arrangement according to claim 21, c h a r a c t e r i z e d in that the quaternary ammonium salt is a quaternary alkyl ammonium salt, such as a quater- nary alkyl ammonium chloride, preferably NMe4Cl, NMeEt3Cl, NBu4Cl, NBu3MeCl or NEt4Cl, most preferably NBu3MeCl.

23. Arrangement according to any one of claims 15 to 22, c h a r a c t e r i z e d in that the at least one salt comprises sodium chlo- ride and NBu3MeCl, preferably consists of sodium chloride and NBu3MeCl.

24. Arrangement according to any one of claims 15 to 23, c h a r a c t e r i z e d in that the arrangement comprises means for heat- ing the electrolyte during electrolysis to a temperature in the range of 50 to 85 °C, preferably 60 to 80 °C.

25. Arrangement according to any one of claims 15 to 24, c h a r a c t e r i z e d in that (2,2,6,6-tetra-methylpiperidin-1-yl)oxyl is present in the electrolyte composition at an initial concentration in the range from 0.020 M to 0.1 M.

26. Arrangement according to any one of claims 15 to 25, c h a r a c t e r i z e d in that sodium chloride is present in the electrolyte composition at an initial concentration in the range from 0.62 to 1.03 M.

27. Arrangement according to any one of claims 15 to 26, c h a r a c t e r i z e d in that tributylmethylammonium chloride is present in the electrolyte composition at an initial concentration in the range from 0.076 to 0.13 M.

28. Arrangement according to any one of claims 15 to 27, c h a r a c t e r i z e d by comprising means for addition of additional formaldehyde into the electrolyte composition in the undivided electrolytic cell during electrolysis.

29. Use of 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) in produc- tion of ethylene glycol by paired electrolysis from formaldehyde and methanol.

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