DE2336288A1 - PROCESS FOR THE ELECTROCHEMICAL PRODUCTION OF OLEFINOXIDES - Google Patents

Description

BASf corporation

Our reference: O.Z. jjo OO3 Mu / Wil 67OO Ludwigshafen, July 12, 1973

Process for the electrochemical production of olefin oxides

Olefin oxides are important intermediates for many purposes. Particularly this applies to propylene oxide, which is used in quantities of many hundreds of thousands of tons annually for the production of polyurethane foams, unsaturated polyester resins and various other products such as surfactants, emulsion breakers, glycerin, etc. are used will.

The importance of the product is reflected in the large number of synthesis proposals. To date, however, only two of these have practical significance as large-scale technical processes. There is, on the one hand, the chlorohydrin process, in which propylene chlorohydrin is produced from propylene and chlorine and saponified to form propylene oxide; the required chlorine and caustic soda are generated by the chlor-alkali electrolysis. This process has the disadvantage of being a multistage process, of supplying large amounts of NaCl as a waste product in a form which cannot or can hardly be recycled and always supplies about 10% dichloropropane as a by-product which is difficult to utilize.

The other technical process is the so-called co-oxidation process; in the co-oxidation of olefins with i-Butane or ethylbenzene transfer the intermediate hydroperoxides one oxygen atom selectively to the olefin. Apart from that of yield problems there is an inevitable accumulation of isobutylene or styrene and there are serious separation problems, because the reactions are very inconsistent.

Direct oxidation, which is very satisfactory in the epoxidation of ethylene in the gas phase over a silver catalyst leads to only very low yields with higher olefins.

Apart from the fact that the chlorohydrin process is in principle an indirect one As an electrochemical process, there are now a number of direct electrochemical processes that make up the

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Should avoid disadvantages of the chlorohydrin process. They can be classified as follows:

I. Halohydrin process:

a) In divided cells (e.g. described in the DT-AS

1 258 856, the PR-B - 1 375 973 or from AP Tomilow, M. Yes. Fioschin, Khim. Prom 1969 , 413)

b) in undivided cells, e.g. B. U.S. Patent 3,394,059

II. Epoxidation with anodically generated oxygen:

a) Directly on the anode, which consists of Zn or Ag (see DT-OS 1 906 182)

b) indirectly via soluble complexes, e.g. B. des Ag ⁺⁺ , Tl ⁺⁺⁺ , Co ⁺⁺⁺ etc.

While the oxygen processes Ha and Hb are in practice could not be realized, the electrochemical halohydrin process gave much more favorable results achieved. They are all based on the same principle, namely the anodic formation of HOCl or HOBr, their addition to the Olefin with the formation of the halohydrin and its saponification by cathodically formed alkali. In the proceedings in divided Cells (la) here seeps through the porous halohydrin with the anolyte in the alkaline catholyte Through the diaphragm. In the undivided cell processes, the saponification step takes place in the alkaline diffusion layer on the cathode. The halohydrin must here from the anodic diffusion layer can be transported out into the alkaline cathodic diffusion layer.

While the halohydrin processes in divided cells are quite cumbersome, require a complicated cell and - at Propylene - inevitable to produce dichloropropane as a by-product in larger quantities, the processes are in undivided Cells much more attractive. The cells are structurally simple. Instead of two electrolyte circuits, you have only one. If alkali bromide is used as the electrolyte, the formation of dihalogen compounds can be drastically reduced.

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However, the energy requirement is with the previously known methods at 10 to 25 kWh (per kg of propylene oxide). In addition, are the space-time yields in all previously known direct electrochemical halohydrin processes are not large enough.

It was therefore the task of a method for direct electrochemical Production of olefin oxides via the non-isolated intermediate stage of the halohydrin in an undivided cell with indicate the lowest possible energy requirement and a high space-time yield.

It has been found that such a method is possible if an electrode spacing in the undivided cell of 0.05 to 2 mm is maintained. As the results compiled in Example 1 show, there is a relationship between the electrode spacing and the current yield of olefin oxide and the main by-product dihalo-olefin in the sense that the current yield of olefin oxide increases steadily with decreasing electrode spacing and finally reaches values of around 90 % , while at the same time the current yield for the dihalo-olefin drops steadily until - in the case of dibromopropane - to less than 1 %.

Suitable electrolysis cells for carrying out the method according to the invention are undivided and known per se. For example, suitable constructions are described in DT-OS 1 8o4 809 and 2 039 590. The electrodes are bipolar in series and are vertical plates arranged in parallel in a trough cell, vertical plates in special plate and frame cells, or preferably a stack of horizontally arranged circular plates with inner bores and radially arranged spacers. The last-mentioned cell is preferably used because it largely eliminates sealing problems and the required planarity of the electrodes is easiest to achieve. Cells of the type mentioned preferably have an electrode spacing between 0.3 and 0.8 mm. _t

The electrolyte consists of a soluble chloride or bromide, the cation of which cannot be deposited cathodically, preferably NaCl, KCl, NaBr and KBr. The use of bromides has

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over the chlorides the advantage that the formation of the dihalogen compound is pushed back significantly. The concentration of the supporting electrolyte is kept as small as possible in order to achieve the Formation of the hypohalic acid in the vicinity of the anode

Br ₂ + H ₂ O ^ H ⁺ + Br "+ HOBr

to favor. When using the above-mentioned capillary gap cells, favorable current voltage conditions are set in spite of the small electrolyte concentrations. The concentration of the conductive salts (NaBr, NaCl) is 0.01 to 10% by weight , preferably 0.5 to 4% by weight. It is possible to improve the conductivity by adding an inert foreign electrolyte such as NapSOi or NaClOh.

Water is used as the solvent. The addition of organic auxiliary solvents such as glacial acetic acid or dimethylformamide, which are sufficiently resistant to bromination, is optionally suitable for increasing the solubility of the olefin.

The anode can be made of graphite, graphite-filled plastic, magnetite, platinum or other platinum metals or the oxides the platinum metals exist. The metal electrodes are preferably designed as composite electrodes. A thin film from platinum metal z. B. applied to a base made of graphite or titanium with the help of a graphite-filled hot melt adhesive. Thin platinum metal layers can also be applied to suitable base metals using galvanic or ceramic methods will. A preferred embodiment of an anode for the method according to the invention consists of a titanium / TiOg / RuOg composite electrode, which has been produced by known ceramic processes (cf. BE-PS 710 551 and DT-OS 1 814 567).

In addition to graphite, metals such as steel, chrome-nickel steel, Copper, iron, titanium and lead. The material of the cathode is of minor importance for the process.

The current density is in the range from 0.1 to 100 A / dm, preferably

1 to 30 A / dm. You can easily adjust the halogen concentration be adjusted. Surprisingly, even at

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very low bromide concentrations, relatively high current densities can be set (cf. Example 2).

The temperature for the process according to the invention has a relatively small influence on the result and can, for. B. between about 0 and 100 ⁰ C, preferably 35 to 50 ⁰ C, lie.

The pH value in the electrolyte is not critical. It can be set to mean values in the range from 3 to 12, preferably 6 to 10 will. Acids such as HBr, HCl or HgSOn or bases such as NaOH are added to regulate the pH value. Extremely acidic or alkaline pH values are to be avoided in order to prevent the saponification of the olefin oxide dissolved in the electrolyte to form the glycol.

Sufficient convection is expediently established by pumping the electrolyte around. The ones about the radius and the electrode gap mean linear velocity of the electrolyte is for example 1 to 1000 cm / sec, preferably 10 to 100 cm / sec.

The olefin, which generally only slightly dissolves in the aqueous electrolyte, is suspended as finely as possible in the liquid phase. A suspension of the (possibly gaseous) olefin in the electrolyte is generally pumped through the capillary gap, the electrolyte being sufficiently soluble for the olefin. The ratio of the volume flows of a gaseous olefin to the electrolyte at the prevailing pressure is adjusted to 1: 100 to 1: 1, preferably 1:25 to 1: 5. Specifying an even higher gas load on the liquid than about 1: 1 (50 vol. Of gas) is generally not expedient, since otherwise the cell voltage rises drastically. The olefin metered in and finely dispersed in the electrolyte is specified in a ratio to the prevailing current strength of 1 to 100 equivalents. With a ratio of 1, the current would have to convert the fed-in olefin quantitatively, with a quantitative current yield; with a ratio of 100, the theoretical current conversion would be 1 %. A ratio of 2 to 10 is preferably set. It has surprisingly been found that even with the addition of

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theoretical amount of olefin (1 equivalent based on the Faraday's see amount of electricity) the olefin oxide with high current efficiency is formed. The olefin concentration in the solution can be increased by increasing the pressure. There will be prints of z. B. 1 to 100 atmospheres set, which can be realized in the undivided cells without difficulty. The excess Olefin, which leaves the cell with the hydrogen formed, is separated off by condensation and again the Capillary gap fed.

As olefins are, for. B. ethylene, propylene, 1-butene, 2-butene, allyl chloride or styrene. Of particular interest is the synthesis of propylene oxide with the inventive Process can be carried out with very favorable yields. Propylene can be used in pure form, but also in Mixture with inert gases or as a technical C J5 cut (minor components Propane and others).

The olefin oxide generally dissolves in the electrolyte. A part is also discharged from the cell together with the excess olefin and the inert gases and condensed. Concentrations of 0.01 to 20 % of olefin oxide in the electrolyte can be set.

For work-up, the electrolyte and condensate are fractionally distilled. With small concentrations of olefin oxide in the aqueous For the electrolyte, an extraction or an extractive distillation is expedient.

The electrosynthesis can be carried out either discontinuously or continuously. in the In the former case, the olefin oxide accumulates in the electrolyte and in the condensate and is worked up after the electrolysis has ended.

Switching to continuous operation is easy possible. After a steady concentration of olefin oxide has been reached in the electrolyte, the electrolyte becomes continuous withdrawn from the cell, removed from it the olefin oxide and the by-products and the remaining halide solution,

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if necessary after replacement of used halide, returned to the cell. The one with the excess olefin the inert gases, the cathodically generated hydrogen and optionally with other volatile by-products discharged olefin oxide is continuously condensed out of the cycle gas or washed out.

example 1

This example uses a discontinuous process for the electrosynthesis of propylene oxide (PO) at low current densities and propylene oxide concentrations at different electrode spacings.

The capillary gap cell (1) used (see FIG. 1) has a

ρ
Electrode area F of 1 dm. The capillary gap with a width of d = 0.5 mm is formed by a horizontal, circular, overhead anode and a cathode arranged at the bottom. The anode consists of a 1 mm thick, made with TIOP / RuOp by a ceramic process and activated at 500 ⁰ C titanium sheet, which is adhered to a V2A base plate by means of a graphite-filled epoxy adhesive. The sides of the base plate are covered with polyethylene. The cathode is made of V2A steel. The electrolyte flows from the inside of the cell through the capillary gap radially outwards, is cooled with water at the heat exchanger (2) and then pumped back into the cell with the centrifugal pump (3) via a liquid rotameter (4) and a gas supply vessel (5) will. The pH value in the electrolyte is measured with the glass electrode (6) and regulated by metering in 5 η HgSO ^ via the inlet (7). The building material for the electrolyte circuit is glass on an experimental scale. The leaving the cell gases are first washed in two successively sailed included cold traps (8a) and (8b) with n-propanol at O ⁰ C and -10 ⁰ C. In a third cold trap (9b) propylene is condensed at -78 ⁰ C. After reaching a certain liquid level excess propylene is siphoned into the evaporator (11), where it is evaporated at 20 ⁰ C. Together with the non-condensable gases reaches residual propylene of (9a) to (9b), where it is condensed in n-propanol at -78 ⁰ C. there

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there are also traces of propylene oxide. Hydrogen, which is formed cathodically, leaves the apparatus together with some oxygen and nitrogen via the exhaust pipe (10). The vaporized propylene is finally pumped back into the cell with a gas membrane pump (12) via the gas rotameter (13). A glass frit in the gas supply vessel (5) ensures that the gas is finely dispersed in the electrolyte. Propylene to be supplemented is introduced discontinuously into the system via a line (14) with an overpressure regulator (15). The cell is supplied with direct current via the power supply unit (16). The oxygen analyzer (17) based on an electrochemical cell with a silver cathode detects the oxygen content in the exhaust gas. In the steady state, an oxygen value is found that corresponds to less than 1% current efficiency.

At the beginning of the electrolysis, the cell is charged with 2 kg of a 0.5% solution of sodium bromide (technical grade) in distilled water. The electrolyte is pumped in a circle through the capillary gap cell with a volume flow of 2.56 l / min. After flushing with nitrogen, flushing is carried out with propylene until there is only a little nitrogen in the system. It is then switched to circuit operation, the propylene being pumped around at a rate ψ _n “ of 0.11 l / min

^C 3 ^H 6
will. The ratio of the two volume flows to propylene

The electrolyte is 1:23

The electrolysis is carried out at a current strength of 1 A, corresponding to a current density j of 1 A / dm. Based on this current is the volume flow of Propylene ie in 15-fold excess (at 30 ⁰ C). The temperature T is 45 °; the pH value of 9 is maintained by adding 5n HpSO ^ (specific consumption 1.6 mVal / Ah). The experiment is terminated after 4 hours, i.e. after Q = 4 Ah has passed. The electrolyte is colorless and clear. (Glycolic column polypropylene 6m, T = 100 ⁰ C, flame ionisation, T = 150 ⁰ C) by gas chromatography for the content of propylene oxide to methyl ethyl ketone is determined as the internal standard. The following are found:

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in the electrolyte
in the O ⁰ C pallet
in the -78 ⁰ C case

o.z. 30 003 2336288

2.23 s po

0.64 g PO

0.25 g po

a total of 3.12 g PO

The product was thus formed with a current efficiency of 72.5 %. Dibromopropane was found in detectable amounts in the electrolyte and in the O ⁰ C pallet, corresponding to a current efficiency of 2.5 fo 5 % would have corresponded. Apart from propylene oxide and dibromopropane, no reaction products of propylene were found. The cell voltage was 3.0 volts. This results in an energy requirement of 3.9 kWh per kg PO.

Maintaining the parameters mentioned, but adapting them the volume flow of the electrolyte to achieve a constant linear velocity at a certain point of the Capillary gap, the electrode spacing of the capillary gap cell was varied systematically. In Table 1 are the obtained Results compiled. They show that a minimized electrode spacing leads to an optimal result in terms of current yield PO, current efficiency of the by-product dibromopropane and energy consumption leads.

Table 1

Capillary gap cell with 1 capillary gap (P = 1 dm ² ), T =

pH 9, 0.5 fo NaBr, j = 1 A / dm ² ,

= 0.11 l / min, Q, =

45 ⁰ C, 4 Ah

Electrodes VEIy 2 * 7 Current yield {%) C ₃ H ₆ Br ₂ Energy requirement distance / mm / / l / min 7 2.85 C ₃ HgO 1.5 / kWh / kg PO / 0.15 0.75 2.9 87 2.0 2.9 0.25 1.25 2.95 82 2.0 3.3 0.35 1.75 3.0 76.5 2.5 3.6 0.50 2.5 3.1 72.5 3.0 3.9 0.75 3.75 3.15 71 4.0 4.0 0.9 4.5 3.2 70 4.5 4.1 1.2 6.0 3.4 69 5.0 4.3 1.8 9 68 4.7

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Example 2

In the apparatus described in Example 1 were discontinuous Experiments carried out with variation of the current density. The amount of electricity Q sent through was kept constant at 4 Ah. The sodium bromide concentration was kept as low as possible, d. H. compared to Example 1, it was not increased proportionally to the current density. In Table 2 are the results compiled. It can be seen that the current efficiency for PO changes only slightly over a wide current density range. Only when the electrolyte concentrations are too high is a significant decrease in the PO current yield observed (cf.

corresponding tests at 1, 10 and 40 A / dm). But even at the highest current densities - despite the moderate electrolyte concentrations - the cell voltage at favorable values. at

At the highest current density (40 A / dm) the propylene inflow is only 150 % of the theoretically required minimum amount; nevertheless, the product is still formed with a good current yield. As a result of the short duration of the experiment, in this case almost all of the PO that has been formed is contained in the electrolyte, as the following figures show (j = 40 A / dm ² , 4 % NaBr). P0 quantities found

in the electrolyte 2.73 g

in the 0 "C case Tabel 0, 2 OY G in the -78 ⁰ C case 0, 01 G all in all 2, 81 G

Capillary gap cell with 1 capillary gap, F = I dm ² , T = 45 ⁰ C, pH 9, 0.5 to 8 % NaBr, j = 1 to 40 A / dm ² , \ / _Ely = 2.56 l / min,

Q = 4 Ah, i.e. = 0.5 mm Vc ₃ H ₆ ^U Z Current yield (%) C ₃ H ₆ Br ₂ energy Current density
/ Ä / dmf / ⁰ NaBr / I / min / DJ C ₃ H ₆ O 2.5 requirement
/ kWh / kg P07 1 /Weight J7 0.11 3.0 72.5 2.5 3.9 . 1 0.5 0.11 2.9 66 3.5 4.1 4th 1.0 0.48 3.0 68 2.0 4.1 10 2.0 0.48 3.3 66 4.6 2.0

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Current density ⁰ NaBr ^ C ₅ H ₆ ^U Z Current yield {%) U ₃ Ii ₆ Br ₂ energy 2A / dm £ 7 / I / min 7 ra C ₃ H ₆ O 3.5 requirement
/ kWh / kg P07 10 3.5 0.48 3.1 63 4.5 4.5 20th 4.0 0.48 3.8 69 2.5 5.1 30th 4.0 0.48 4.3 66 3.0 6.0 40 4.0 0.48 5.2 65 2.5 7.4 40 8.0 0.48 4.8 48 9.3

Example 3

In the apparatus described in Example 1, discontinuous tests were carried out with variation of the test duration _s, that is to say the amount of electricity sent through. The final concentration of propylene oxide was increased in this way up to 3.2 fo in the electrolyte. Table 3 shows the results.

Table 3

Capillary gap cell with 1 capillary gap (F = 1 dm ² ), T pH 9, 2 % NaBr, j = 10 A / dm ² ,
min, d = 0.5 mm

^C 3 ^H 6

= 0.48 l / min,

45 ° C,

= 2.56 1 /

Q / Uh7 ^Z Current yield ($) C ₃ H ₆ Br ₂ Energy requirements 3.3 C ₃ H ₆ O 2.5 / kWh / kg P07 4th 3.2 66 4.5 4.6 8th 3.3 61 5.5 4.4 16 3.4 66 4.5 4.6 32 3.4 61 5.5 4.7 64 56 5.6

As a result of the higher current density, the higher gas load and the longer running time, the product is compared to the in Examples 1 and 2 mentioned cases were discharged much more strongly from the cell and transferred to the cold traps. Examples for the PO quantities found:

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-Vc-

In the electrolyte (g) in the O ⁰ C case (g) in the -78 ⁰ C case (g)

o.z. 30 003 2336288 32 Ah attempt 64 Ah attempt 17.08 23.05 5.75 14.35 0.33 1.25

total (g)

23.16

38.65

Example 4

In the apparatus described in Example 1 are discontinuous Tests carried out on a graphite anode (type EXN from Conradty in Nuremberg). The amount of Strotn is varied or the electrolysis time as in Example 3 · The cathode exists made of V2A steel. Table 4 shows the results obtained.

Table 4

Capillary gap cell with 1 capillary gap (F = 1 dm ² ), T = 45 ⁰ C, pH = 9, 2 % NaBr, j = 10 A / dm ² , V _{c E} = 0.48 l / min, V _Ely = 2, 56 l / min, d = 0.5 mm ^{5 6}

QZ £ h7 ^ Z YOU Current yield {%) C ₅ H ₆ Br ₂ Energy requirements C ₃ H ₆ O 3.1 / Bfti / kg PC7 4th 3.4 75 5.9 4.2 8th 3.5 73 6.8 4.0 16 3.5 69 6.0 4.7 32 3.2 62 5.5 4.9 64 3.4 53 6.0

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Claims (4)

O.Z. 23 .Z. 30 QP 233628 Claims Process for the direct electrochemical production of olefin oxides via the non-isolated intermediate stage of halohydrin in an undivided cell, characterized in that an electrode spacing of 0.05 to 2 mm is maintained in the undivided cell. 2. The method according to claim 1, characterized in that a soluble alkali bromide used as a supporting electrolyte in a concentration of 0.01 to 10 wt.%. 3. The method according to claim 1, characterized in that there is used as anode graphite or titanium activated with ruthenium dioxide. 4. The method according to claim 1, characterized in that the olefin used is propylene which is finely dispersed in the electrolyte, the ratio of the amount of propylene added to the amount of propylene theoretically consumed in the cell is 1 to 100. Sign. BASF Aktiengesellschaft 509808/1122 Blank page