Laws Of Chloride - Ions Oxidation On Various Electrodes and Green Electrochemical Method of Higher α-olefins Processing

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1 / The lectrochemical Society Laws Of Chloride - Ions Oxidation On Various lectrodes and Green lectrochemical Method of Higher α-olefins Processing Yu. H. Budnikova, S. A. Krasnov, I. M. Magdeev, O. G. Sinyashin Institution of Russian Academy of Sciences Institute of Organic and Physical Chemistry named after A..Arbuzov of Kazan Scientific Centre of Russian Academy of Sciences, 8 Acad. Arbuzov st., Kazan , Russian Federation Fax: (843) mail: yulia@iopc.knc.ru Oxidation of chloride-ions and chlorine gas on different electrodes was studied. The Cl - oxidation potential increases as follows: RuO2/TiO2 < Pt < GC. The results obtained supplement present concepts on electrochemical behaviour of chloride-ions and reveal substantial differences in proceeding electrochlorination processes on electrodes of different nature. lectrochlorination of higher α- olefins on platinum (Pt) and RuO 2 /TiO 2 (DSA) anodes was shown to proceed as substitutive chlorination to give chlororaffins containing up to 50% of chlorine, while on glassy carbon (GC) electrodes an additive chlorination (up to 25% of chlorine) occurs. Introduction Reaction of chlorine gas evolution during electrochemical oxidation of chlorideions has been of researchers s interest over a long period of time due to its extreme importance for electrochemical technology (3). Application of DSA electrodes covered with metal oxides of platinum group came as technological advance in chlorine and alkali production. According to S.Trasatti, the technology run ahead basic research(3). Though a great deal of studies on electrooxidation of chloride-ions exists, there is still not unambiguously proposed general mechanism of oxidation and explanation of differences resulted from different electrodes used, as well as how oxide electrodes produce much better results. Many attempts of retrospective literature analysis were made, but the scope of results and conclusions is too wide (2). Taking into account up-to-date trends to change over to environmentally friendly technologies and full-scale waste processing, our aim was to devise environmentally friendly method for production of haloraffins and for recovery of large quantities of higher α-olefins resulted from the processing of raw hydrocarbons. An effective, lowwasted and ecologically friendly electrochemical method for production of chlororaffins has been thus proposed (4, 5) as alternative to industrial one that uses chlorine gas. lectrosynthesis is based on anode oxidation of initial chlorinating agents - chlorides (hydrochloric acid and sodium chloride). For synthesis rameters to be optimized and the most useful and cost-effective electrodes to be chosen, there is however a need to understand the mechanism of electrochemical reactions proceeding on electrodes and induced electrochemically in solution. 7

2 chlororaffins H 2 Cl 2H α -olefines Cl 0 This work is aimed to reveal differences in voltammetric behaviour of chlorideions on electrodes of different nature (platinum, glassy carbon, DSA) and to comre voltammetric and prerative data of α-olefins electrochlorination on different electrodes, as well as to specify mechanisms of these processes. Results and Discussion lectrochemical chlorination of higher olefins was studied by using platinum, glassy carbon, and DSA electrodes. Hydrochloric acid was used as chlorinating agent. lectrosyntheses performed showed that higher α-olefins are completely chlorinated (double bonds disappear, ca. 100% product yield) in the presence of electrochemically generated chlorinating agent to give chlororaffins (Table I) in strongacid media, but process effectiveness depends dramatically on anode nature (Fig.1). As the content of hydrochloric acid decreases down to 2 mol/l with other conditions being equal, not only addition of chlorine but also hydroxyl groups to olefin occurs. Table I. Chlororaffin С 16 -С 18 characteristics vs. quantity of electricity ssed (Pt anode). lectric quantity, F/mole 20 n D %, Cl 1 1,458 13,1 2 1,464 17,20 3 1,465 18,20 4,5 1,469 21, ,483 32,00 11,5 1,484 33, ,491 38, ,499 45,00 Fig. 1. The plot of chlorine content in chlororaffin С 16 -С 18 product vs. quantity of electricity ssed through different anodes. 8

3 Comrison of the efficiency of different electrodes when chlorinating С 16 -С 18 olefin shows that a substitutive chlorination occurs on DSA electrode, with a high halogen content (47%) in the product being achieved as early as 15F of electricity has ssed. High current densities can also be used without anodic corrosion. Platinum anode allows to achieve 45% of chlorination at most but with greater quantity of electricity ssing (less current yield), and current density has to be limited because of electrode corrosion in hydrochloric solutions. A substitutive chlorination on glassy carbon electrode failed. In the last case an additive chlorination may be possible (25% yield) by olefin double bond. Glassy carbon electrodes can also suffer slight corrosion, and pitting corrosion is observed especially when high current densities (more than 900 A/m 2 ) are used. When chlorine gas is used as chlorinating agent (with other synthesis conditions being equal) an additive chlorination by double bond was only possible, with chemical chlorination efficiency being less than under electrolysis conditions. When reproducing industrial conditions the rate of chlorination with chlorine gas by double bond was found to be times less than that in electrochemical chlorination. A substitutive chlorination with use of chlorine gas but no radical-inducing additives does not occur. It can be seen from Fig.2 (a, b) that curve slopes of ΔЕ vs. current density on working electrode (anode) or anode potential Е а vs. current density differ depending on anode nature. Fig. 2. a. The plot of anode potential vs. current density on electrode. b. The plot of cell voltage vs. anode current density. Strong HCl (35%) was used as electrolyte. Analysis of DSA surface performed with scanning probe microscope showed (Fig.3) that catalyst grains do not generally exceed nm in size, i.e. electrode surface may be considered to be nanomaterial. Nanodimension of electrode layer should increase its catalytic activity. Unlike DSA electrode, glassy carbon and platinum electrodes have smooth surface (Fig. 3). 9

a b с Fig. 3. lectrode surfaces obtained with scanning probe microscope; DSA electrode rticles size distribution histogram.

$Thus, under fractional differentiation conditions it can be possible to see the reversibility of corresponding stages in cases when peak current values are small in current potential coordinates.$ CV analysis of Cl - and Cl 2 on platinum electrode lectrochemical oxidation of 10-2 М hydrochloric acid in MeCN on stationary platinum electrode was found to be characterized by two peaks ( = 1493

CV analysis of Cl - and Cl 2 on platinum electrode lectrochemical oxidation of 10-2 М hydrochloric acid in MeCN on stationary platinum electrode was found to be characterized by two peaks ( = 1493

4 a b с Fig. 3. lectrode surfaces obtained with scanning probe microscope; DSA electrode rticles size distribution histogram. To find the reasons why electrochlorination processes proceed so differently the voltammetric behaviour of chloride-ions on electrodes chosen was studied by cyclic voltammetry (CV). To obtain additional information and improve resolution of complex curves, the curves were processed as relationships of current semi-derivative vs. potential. Thus, under fractional differentiation conditions it can be possible to see the reversibility of corresponding stages in cases when peak current values are small in current potential coordinates. CV analysis of Cl - and Cl 2 on platinum electrode lectrochemical oxidation of 10-2 М hydrochloric acid in MeCN on stationary platinum electrode was found to be characterized by two peaks ( = 1493 mv, 2 = 2378 mv), peak currents have close values ( i 1 = A, i = A) (рис. 4), the second peak is irreversible. 1 Fig. 4. Oxidation of aqueous M HCl solution in MeCN, 0.1M Bu 4 NBF 4 on Pt electrode a) in current potential coordinates, b) first peak in current semi-derivative potential coordinates. Complex ttern of the first peak can be seen more clearly in current semiderivative potential coordinates (Fig. 4b). 10

5 CV analysis of Cl - and Cl 2 on glassy carbon electrode The Cl - oxidation on glassy carbon electrode is characterized by three oxidation peaks at concentration of М (Fig. 5). Fig. 5. Oxidation of aqueous M HCl solution in MeCN on glassy carbon electrode: a) three peaks of the Cl - 1 oxidation ( = 1320 mv, 2 = 1670 mv, 3 = 2463 mv), b) CV-curve of the first two waves, в) CV-curve of the first two waves in current semi-derivative potential coordinates. As concentration increases up to М the first two waves of chloride-ions oxidation disappear (merge) (Fig. 6). Fig. 6. Oxidation of aqueous M HCl solution in MeCN on glassy carbon electrode: a) two peaks of the Cl oxidation ( = 1772 mv, = 2417 mv), b) CVcurve of the first peak in current semi-derivative potential coordinates. CV analysis of Cl - and Cl 2 on DSA electrode Oxidation of hydrochloric acid on DSA electrode was found to be characterized by a single slightly diffused oxidation peak (Fig.7). 11

6 Fig. 7. НCl oxidation on DSA electrode: a) at various concentrations - a) М, b) 10-2 М, c) М, b) CV-curve of oxidation at concentration of М in current semi-derivative potential coordinates. Oxidation currents are very low at low concentrations of chloride-ions, as if an accumulation period takes place, and only at concentration of М the wave gets distinct. The current semi-derivative potential relationship allows to see a complex ttern of the Cl - oxidation wave (at concentration of up to М) qualitatively similar to that on smooth electrodes (Fig. 7b). CV-curves of Cl 2 oxidation on DSA electrode recorded under the same conditions as on Pt and glassy carbon electrodes show no oxidation peak. Diffused trace peaks seem to be recorded due to oxidation of chloride-ions formed in solution when left to stand or reducing elementary chlorine at the starting potentials of scanning (Fig. 8). Fig. 8. CV-curves of Cl 2 oxidation on DSA electrode at concentration of М. a) oxidation, б) reduction. 12

7 Table II. Characteristics of CV-peaks when using different electrodes at concentration of М. Saturated calomel electrode (SC) was used as reference electrode. Substrate Parameter lectrode material Glassy carbon Pt DSA Cl -, mv , mv Cl , mv Basing on the results obtained and some literature sources the following can be resumed: 1. The Cl - oxidation potential increases as follows: RuO2/TiO2 < Pt < GC (Table II); 2. The order of a reaction by chloride-ion is second with smooth electrode (Pt or glassy carbon nature was not taken into account in calculation) and first with DSA (6-8). Rate-determining stage is the Cl + + Cl - Cl 2 reaction with Pt or glassy carbon electrode (8) and the Cl + e Cl + reaction with DSA occurring at active sites of electrode via renburg mechanism (9). 3. The Cl - to Cl 2 oxidation peak on smooth electrodes consists of two singleelectron peaks, i.e. the processes Cl - - е Cl and Cl - e Cl + proceed at different potentials and merge into a single wave only at high Cl - concentrations, that has never been taken into account before in calculations. Under voltammetric conditions the Cl 2 is only detected on the second stage (peak) of these two - the first one is irreversible and has no cathode curve during inverse scanning (recombination of Cl to Cl 2 does not proceed with detectable rate that shows no peak referred to Cl 2 reduction). With Pt and glassy carbon electrodes, the Cl 2 oxidation peak is observed at substantial anode potentials with its potential being comrable to the peak of chlorine gas which was specially run through electrolyte free of chloride-ions. With DSA, a single but complex oxidation wave is only observed being detected at only high Cl - concentrations (as if there is accumulation period at concentration of up to ca М when oxidation current is almost zero). There is no Cl 2 oxidation wave (in comrison with smooth electrodes) that may be caused by the Cl and Cl + formation on a complex wave, and a second order stage of the Cl 2 formation is complicated. This is the explanation for disappearance of gas bubbles effect on DSA (3). Thus, chlorine radicals exist in DSA pores and provide successful olefin chlorination via radical mechanism (preferred) to give the product containing up to 50% of chlorine. This process occurs with high rate at low anode potentials of 2V and at A/m 2. ffluent gases contain practically no chlorine gas. The Cl 2 formation should mainly proceed on glassy carbon electrode (the third oxidation wave with higher current corresponds to Cl 2 ; the Cl 2 reduction wave with higher current corresponds the wave detected in reverse scanning after the first complex anode wave). An additive olefin chlorination by double bond (with maximum 25% of chlorine being achieved) only occurs on glassy carbon electrode. In prerative synthesis, chlorine gas is liberated vigorously on glassy carbon electrode, with its major fraction 13

8 leaving electrolyzer together with effluent gases. The results obtained supplement current concepts on electrochemical behaviour of chloride-ions and reveal substantial differences in proceeding electrochlorination processes on electrodes of different nature. lectrochemical measurements (CV) xperimental lectrochemical measurements were carried out with potentiostat Е2Р psilon (BASi, USA) consisting of detector, personal computer Dell Optiplex 320 with software psilon С-USB-V200, and electrochemical cell С3 having three-electrode scheme. A linear potential scan rate was 100 mv/s. Stationary glassy carbon electrode (d = 3.0 mm), platinum electrode (d = 1.5 mm), and DSA electrode (d = 1.0 mm) were used as working electrodes. Calomel system Hg/Hg 2 Cl 2 /KCl was used as reference electrode in voltammetric measurements. Platinum wire (d = 0.5 mm) was used as auxiliary electrode. Tetrabutylammonium tetrafluorborate with concentration 10-1 M was used as a background salt in voltammetric studies. Hydrochloric solution for CV studies was prered from corresponding volume of 35% HCl and 0.1M Bu 4 NBF 4 in MeCN, and then diluted to 5 ml. Prerative electrolyses Prerative electrolyses were performed with constant-current source B5-71 in thermostatically-controlled diaphragm-free cylindrical electrolyzer (three-electrode cell) with cacity of 200 cm 3. DSA electrode with surface area of 52 cm 2 (a), glassy carbon electrode with surface area of 50 cm 2 (b), and platinum electrode with surface area of 44.8 cm 2 (c) were used as anodes. Titanium plate with surface area of 22 cm 2 was used as cathode. Saturated calomel electrode (Hg/Hg 2 Cl 2 ) was applied as reference electrode. To stir electrolyte during electrolysis a magnetic stirrer was used. Acknowledgments The study was fulfilled due to financial support of Russian Foundation of Basic Research (grants , ), Branch of General and Technical Chemistry of Russian Academy of Sciences. References 1. S.Trasatti, lectrochim.acta, 32, 369 (1987). 2..J.Kelly, D..Heatherly, C..Vallet, C.W.White, J.lectrochem.Soc., 134, 1167 (1987). 3. S.Trasatti, lectrochim.acta, 45, 2377, (2000). 4. Budnikova Yu.H., Magdeev I.M., Reznik V.S., Sinyashin O.G., Tazeev D.I., Jakushev I.A., Yarullin R.S. Patent RU , priority Yu.G. Budnikova, T. V. Gryaznova, S. A. Krasnov, I. M. Magdeev, and O. G. Sinyashin, lektrokhimiya, 43, 1291 ( 2007). 14

9 6. A.Cornell, lectrochimica Acta, 48, 473, (2003). 7. M.Thomassen, B.Borresen, G.Hagen, R.Tunold, lectrochim.acta, 50, 2909, (2005). 8. M.H.P.Santana, L.A.De Faria, lectrochim. Acta, 51, 3578, (2006). 9. R.G.renburg, L.I.Krishtalik, I.P.Yaroshevskaya, lektrokhimiya, 11, 1236, (1975). 15

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