DK141277B - Process for Electrochemical Preparation of Silver Catalysts. - Google Patents

Description

(11) PUBLICATION 141277 DENMARK (51) int.ci.3b 01 j 23/60 § (21) Application No. 37 ^ 8/72 (22) Filed on 28 Jul. 1972 (23) Race day 28 Jul. 1972 (44) The application presented and

the disclosure order published it in O. 16D. 1 yOU

DIRECTORATE OF

PATENT AND TRADEMARK (3 °) Priority requested from 31 Jul. 1971, 27022/71, IT (71) SNAM EROGEETI S.P.A., Corso Venezia 16, Milan, IT.

(72) Inventor: Lulgi Rivola, Via Soreslna 1B, San Donato Milanese, IT: Vitto = rlo Mormlno, Via Gramscl 18, San Donato Milanese, IT: Bruno Notari,

Via Tiadena 6, San Donato Milanese, IT.

(74) Proxy during the proceedings:

International Patent Office.

(54) Process for the electrochemical preparation of silver catalysts.

The present invention relates to a process for the electrochemical preparation of silver catalysts whose particles are less than 1500 Å.

These silver catalysts can be used in the catalytic production of ethylene oxide.

Many processes are known for preparing silver catalysts which can be used in the production of ethylene oxide. However, it is also known that the silver structure and operating conditions during the silver manufacture are important for achieving high activity and selectivity. The chemical methods heretofore proposed essentially result in the decomposition of the silver salt or silver compound to provide comminuted silver. The main disadvantage of these methods is the poor reproducibility and the inevitable loss of silver that can occur during treatment, and therefore expensive extraction processes are required.

It should be possible to produce silver in high yields and in a reducible way using the electrochemical separation method of the metal. However, it is also known that by electrolysis of silver solutions the metal is produced with a compact structure or at least a large particle size, due to which such activity and selectivity cannot be obtained that the metal can be used in an industrial process. for the preparation of ethylene oxide by the oxidation of ethylene.

It has now been found that it is possible to electrochemically prepare silver catalysts whose particles are less than 1500 A, preferably between 300 and 1500 Å, and as used for the production of ethylene oxide by the oxidation of ethylene give high activity and selectivity.

Process according to the invention for the preparation of silver catalysts is characterized by electrolysing solutions of silver salts in a pulsating manner, ie. with intermittent interruption of power supply. The silver salt solution is advantageously added to a complexing agent.

Said power cut may in some cases be followed by a change of direction.

The power supply periods can e.g. be between 3 and 10 seconds and are followed by power cut periods of between 3 and 60 seconds. Optionally, after 10 to 15 cycles of power supply and power failure, the current can be reversed, preferably for a period of between 1 and 60 seconds.

The silver salt solutions used according to the invention may advantageously consist of silver nitrate, silver chloride, silver sulfate, silver acetate or silver oxalate, complexed with ammonia.

The concentrations of the silver salt solutions are preferably between 0.1 and 10 g of silver / 1 solution.

Preferably, between 3 and 50 moles of complexing agent / gram atom used silver is used.

It is also preferred to use a buffer solution to keep the pH of the electrolytic solution constant.

Appropriate buffer solutions should keep the pH between 9 and 12.5. Examples of such buffer solutions are: glycine + sodium hydroxide, disodium phosphate 4 sodium hydroxide and the like. Very good results have been obtained with the borax + sodium hydroxide mixture.

The electrolysis is advantageously carried out at a temperature between 0 and 80 ° C and preferably between 10 and 40 ° C.

The potential can be maintained between -500 and -1500 mV measured relative to the saturated calorel electrode. The apparent current density may be between 0.1 and 0.5 and preferably between 0.2 and 0.3 amp / cm.

The anode of the electrolytic cell may be graphite, platinum, platinum rhodium, titanium or, in general, any good conductor which cannot be attacked by an alkaline material, and the cathode may be silver, stainless steel, graphite or usually the the same materials used as an anode.

During the electrolytic separation, the silver salt solution is preferably kept under vigorous stirring. The powdered silver formed after washing can be used directly as is or, preferably, carried by a ceramic material as a catalyst in the production of ethyl pnoxide.

The method according to the invention is illustrated in more detail below with reference to the drawing, in which FIG. 1 shows an electrolytic cell for silver secretion according to the invention; FIG. 2 is a plan of an industrial scale unit for producing silver; FIG. 3 and 4, respectively, are a sectional and plan view of the embodiment of FIG. 2, and FIG. 5 is a power supply diagram of the embodiment of FIG. 3 and 4.

In FIG. 1 is 1 a circulation pump for the electrolyte; 2 is an anode, e.g. of graphite, 3 is a cathode, e.g. a silver grid, 4 is a cell in which the electrolysis takes place and the bottom of which consists of the silver grid, 5 is an electrolyte contained in a reservoir 11, 6 is a voltmeter for measuring the DC voltage applied to the cell, 7 is a time relay for the exchange of the current direction in the cell, 8 is a direct current generator, 9 is an amperometer for measuring the direct current delivered to the cell, and 10 is a time relay for the pulsating electrical separation.

The cell operates as follows: the electrolyte 5 produced in a container, not shown, is stored in the reservoir 11 and pumped into the cell 4 from above by means of the pump 1. The electrolyte 5 is periodically renewed when its silver content has dropped below a predetermined value.

The cell 4 is supplied with DC by means of the generator 8 through the time relay 10, which is intended to send current pulses to the cell. The string direction is inverted periodically by means of the time relay 7.

The electro-separated silver detached from the electrode 3 falls directly into the reservoir 11 through an opening at the bottom of the cell.

In FIG. 2, 101 is a continuous centrifuge, 102 is an electrolytic cell provided with a stirrer 108 and a circulation pump 106, 103 and 104 are reservoirs for the silver salt solution provided with a circulation pump 107 and the separated silver is removed by a tube 105.

The system operates as follows: in the electrolytic cell 102, which will be described in more detail below with reference to Figs. 3 and 4, the electrolytic silver which is fed as a suspension to the centrifuge 101 by the pump 106 and delivered through the tube 105 The pump 106 also aims to recycle the electrolyte by means of a suitable series of valves, and the electrolyte is directed to the cell 102 from the reservoirs 103 and 104 by the pump 107.

Reservoirs 103 and 104 are used alternately, i. one is used to prepare the electrolyte while the other containing the electrolyte feeds the cell. The reservoirs, of course, are provided with agitators, which, however, are not shown.

The electrolytic cell is shown in detail in FIG. 3 and 4, wherein 109 is an anode, e.g. of perforated graphite, 110 is a cathode, e.g. a silver mesh, III is through terminal terminals for supplying the electrodes with direct current, 112 is a valve which permits removal of silver towards the centrifuge 101, 113 is a recirculation tube for the centrifuge 101, 114 is a recirculation tube for the cell 102, and 115 is a feed solution for fresh solution.

In FIG. 4 it appears that the anode / cathode pairs are interconnected in groups and the groups may e.g. as shown in the drawing each contain 5 pairs.

The power supply scheme of the industrial cell is shown schematically in FIG. 5th

As shown in FIG. 3 and 4, the 30 electrodes are supplied with power in small independent groups of 5 series connected pairs.

Direct current (pulses of 5 to 15 seconds) is effected by an amperostatic generator 123 which, by means of switching units 121a to 121f, successively feeds one of the small, each of 5 element pairs, 112 a to 12 2 f.

First, the switching unit 121a feeds only the small group 122a during the duration of the pulse. Thereafter, the supply to this group stops and the supply to the small group 122b is started by means of the switching unit 121b and lasts as long as the supply to the group 122a and then until the last small group.

The supply cycle starts again as programmed by a logic control unit 120 consisting of a square generator 116, a frequency divider 117, a counting unit in binary code 118 and a binary decimal unit 119.

The direction of the DC supply to the individual small groups 122a to 122f is periodically reversed by means of a switching unit 124 which directly drives the generator 123.

The method according to the invention is further illustrated by the following non-limiting examples.

Example 1 100 g of silver nitrate, 750 g of borax (Na 2 B 2 ammo- 5 141277 niak.

This solution, containing silver as an ammonia complex, is used as the electrolyte in an electrolytic cell (shown in Figure 1) using a silver mesh as cathode (1 x 1 mm mesh) and a tubular or perforated graphite plate as the anode. The electrical contacts are made of wire as gold under the cell operating conditions acts as an inert metal.

The solution is circulated in the embodiment shown in FIG. 1 by means of an external circulation pump and the silver content is kept constant by subsequent additions of silver nitrate in an ammonia solution.

The continuous power supply to the cell is done by means of pulses with a rectangular profile of 10 seconds.

The power supply circuit is arranged so you get 10 seconds of power supply and 60 seconds of power cut. Every 6-12 cycles, the current direction is reversed for the purpose of facilitating the release of the silver separated on the cathode.

The cathodic separation is performed at constant current (amperostatic).

The most important anode process is the oxygen release and the most important cathode process is the silver secretion. For this reason, the electrolytic solution content of silver ions, complexed with ammonia, is reduced while the content of ammonium nitrate is increased.

The operating conditions are as follows:

Temperature Room temperature 2

Apparent cathode surface 10-15 cm 2

Apparent cathodic current density 0.2-0.5 A / cm

Discharge voltage for Ag after deduction -600 to -1200 mV (in advance the ohmic drop to the saturated calcium electrode) 2

Apparent anode surface 10-15 cm

Electrolyte flow through cathode 250-350 l / hr

The total voltage applied to the cell is between 7.5 and 15 V. The faradic efficiency of the silver formed on the cathode is between 80 and 90%.

The silver content is kept constant by adding 2.0 to 2.4 g of silver nitrate, complexed with ammonia, every two hours.

The total consumption of electric power is between 2.3 and 4.5 kwh / kg of silver formed.

The silver powder thus formed was recovered partly from the solution and partly from the surface of the electrode, filtered, washed with distilled water and then dried in an oven for 3 hours.

Productivity has been increased to 20 to 24 grams of silver for 24 hours of continuous work, ie. to approx. 2 g daily for each cm apparent cathode surface. With the silver thus prepared, an experiment on catalytic activity has been carried out.

The silver is deposited on a ceramic support and the catalyst thus containing containing 15% Ag is introduced into a tube of 2.4 cm diameter and provided with an outer casing for circulating a thermostating liquid. The height of the catalyst layer is approx. The reactor is heated under reflux and atmospheric pressure with a gaseous mixture of the following composition: 5% ethylene, 6.5% carbon dioxide, 5% oxygen and 83.5% nitrogen.

The power supply is 210 Nl / hour and the contact time is approx. 4.6 seconds. At 20 ° C a good conversion of ethylene to ethylene oxide was obtained with a selectivity (mole of ethylene oxide / 100 mole of converted ethylene) of 80% and a conversion of 25%.

Example 2

The example relates to an industrial process for preparing a silver based catalyst by pulsating electrochemical separation using a special electrolytic cell and silver nitrate as starting material.

As an electrolyte a solution of the following composition is used: Silver nitrate 0.5 - 5 g / l

Borax 5 -20 g / l

Carboxymethyl cellulose 0.1 - 0.5 g / l

Ammonia 5 -50 g / l

Sodium hydroxide 3.5 -10 g / l in an electrolytic cell (shown in Figures 3 and 4) using a silver mesh (1 x 1 mm mesh) as the cathode and perforated compact graphite as the anode. A complete plan of the unit is shown in FIG. 3. The solution is kept under continuous agitation by a mechanical stirrer 108 and passes through the electrodes which, as shown in FIG. 3 and 4 are located radially at the bottom of the cell. The power supply to the electrolytic cell is effected by means of pulses with a rectangular profile of 5 to 15 seconds. The power supply circuit is arranged to provide 5 to 15 seconds of power supply and 20 to 60 seconds of power failure. For every 10 to 20 cycles, the current direction is reversed for the purpose of detaching the silver separated on the cathode.

The total current for each pulse is 700 to 900 amp.

The operating conditions are as follows:

Temperature Room temperature

Apparent cathode surface (30 single- 2 elements of 60 x 60 cm) 10 m 2

Cathodic current density 0.2 - 0.25 A / cm

Discharge voltage for Ag after deduction of the 250 -350 mV (pre-ohmic hold to the saturated calomel electrode)

Apparent anode surface (30 individual 60 x 60 cm elements) 10 m 141277 2 7

Anodic current density '0.2 - 0.3 A / cm

Stirrer speed 40 -1000 rpm.

The most important anode process is the release of oxygen and the most important cathode process is the secretion of silver.

The faradic efficiency of the silver secretion is 80-90% and the voltage applied to the cell is between 5 and 15 V.

The consumption of electric power is between 1.5 and 4.5 kwtime / kg of silver formed.

Since the main anode product is oxygen, the content of the electrolytic solution of silver becomes less and less while the content of ammonium nitrate is increased.

Therefore, a feed system is provided by which fresh solution is fed, continuously recycled and adjusted for the silver content with a highly concentrated solution of silver nitrate, complexed with ammonia (see Figure 3).

The solution is no longer considered useful when the molar concentration of ammonium nitrate is 100 times greater than the molar concentration of silver nitrate.

From this degraded solution, silver is extracted by means of another electrolytic cell,! substantially identical to that of FIG. 1. Here, approx. 1% silver compared to the first quantity of silver.

The highest productivity for this electrolytic cell is 300 kg of catalytically active silver daily for 24 hours of continuous work using 2 an apparent electrode surface of 10 m, which corresponds to a total production of more than 75 fps metallic silver per year.

Using the silver formed as a catalyst in the manner set forth in Example 1, ethylene oxide conversion and selectivity values which were virtually identical to the values set forth in the previous example were obtained.