AU470570B2 - Process forthe electrochemical manufacture of silver containing catalysts - Google Patents

Description

44,47272 This invention relates to the /FlRtr.nGR ical

manufacture of silver particles suitable for use as, or

incorporation in, a catalyst.

More particularly, the present invention relates

to the manufacture of silver particles for use as, or

incorporation in, a catalyst useful for the catalytic

production of ethylene oxide from ethylene. Many methods are known for the manufacture of

silver-coating catalysts suitable for use in the production

of ethylene oxide. It is also known that the structure

of the.silver and the operating conditions used for its

preparation are very important in order to obtain a high

activity and selectivity. The chemical methods previously

proposed essentially involve the decomposition of a

silver salt or other silver compound to obtain finely divided silver. Such methods have as a main drawback their

poor reproducibility and inevitable losses of silver

which can arise in the course of the method, which require

expensive processes of recovery. It should be possible to produce silver in

high yields and in a reproducible way by using electro-

chemical deposition of the metal; however, it is known

that the electrolysis of silver solutions produces a

metal having a compact structure or, in any case, al

particle size; such a metal has too poor an activity

and selectivity to be employed in an industrial-scale

process for producing ethylene oxide by ethylene oxidation. According to the present invention, there is

provided a method of electrochemically manufacturing

silver particles, which comprises subjecting a solution

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44, 472 /2 of a silver salt to a succession of cycles, each of which

cycles includes a period of from 3 to 10 seconds during

which electric current is passed in one direction through

the solution and a period of from 3 to 60 seconds in which

no electric current is passed through the solution.

The present invention makes it possible to produce silver catalysts having a particle size less than o 1500 A, preferably between 300 o o A and 1500 A, which when employed in the ethylene oxide synthesis by ethylene

oxidation, give high activity and selectivity.

Preferably the process according to the present invention is carried out in the presence of a complexing

agent, for example ammonia.

The stopping of the current constitutes, in effect, a pulsed electrolysis,that is an electrolysis wherein

the current supply is broken off at intervals. This stopping of the current can in some cases be followed by a reversal

of the supply of current, i.e. a current in the opposite

direction.

Preferably in an eeeeLeeed cycle the current

is passed through the solution in the opposite direction

to the said one direction. Preferably the jee.aene4d cycle IS occadiond is one in every 10-e cycles. Preferably in the e~accinodm

cycle, the period of passing the current in the opposite direction is for from 1 to 60 seconds, followed by a 6 aseaat4op period AAA hO £&t 4 it i~u~i

Solutions of silver salts advantageously used Ln the method according to the present invention are silver

litrate, silver chloride, silver sulphate, silver acetate

ind silver oxalate, complexed for example with ammonia.

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44,472 72 Preferably the concentration of the solution per of silver salt is from 0.1 g to 10 g. of silver

litre of solution. The amount of complexing agent when present moles per gram is preferably in the range from 3 to 50 atom of silver employed in the salt solution. It is preferred to employ abut solution

electrolytic to keep approximately constant the pH of the advantageously solution. The buffer solutions which can be and 12.5. used are those which keep the pH between 9 glycine and sodium Examples of such buffer solutions are and sodium hydroxide; as well as disodium phosphate have been obtained with hydroxide. Also, very good results

a mixture of borax and sodium hydroxide. The electrolysis is advantageously carried 800 C, preferably out at a temperature in the range from O to 0 from 10 to 40 C. The potential of the cathode can be kept respect to the between -500 and -1500 mV measured with current density saturated calomel electrode. The apparent range from 0.1 to at the cathode surface can be in the 2 amp/cm 2 preferably from 0.2 to 0.3 amp/cm The electrolytic cell anode can be constituted titanium or any by graphite, platinum, platinum-rhodium, be attacked by alkaline other good conductor which cannot by silver, material, while the cathode can be constituted the same stainless steel, graphite and generally

materials as used for the anode. Preferably during the electrolytic deposition vigorous stirring. the silver salt solution is kept under

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44,472 72 The obtained silver, in the form of a powder, can after

washing be directly used either as it is or preferably

supported on a ceramic material, as a catalyst for the

production of ethylene oxide.

For a better understanding of the present invention and to show how the same may be carried into

effect, reference will now be made, by way of example to

the accompanying drawings, in which:-

Figure 1 represents an electrolytic cell for

carrying out the silver deposition according to the

present invention;

Figure 2 illustrates schematically a unit suitable

for the production, according to the present invention, of

silver on an industrial scale;

Figure 3 is a vertical cross-sectional view

through an industrial unit as shown in Figure 2;

Figure 4 is a plan of part of the industrial

unit of Figure 2; and

Figure 5 shows the current supply scheme

of the cell shown in Figures 3 and 4.

In Figure 1, there is shown a circulating pump 1

for the electrolyte, an anode 2, for instance of graphite,

a cathode 3, for instance of silver net, a cell 4 wherein

the electrolysis takes place, of which the bottom consists

of the silver net, electrolyte 5 contained in a reservoir

11, voltmeter 6 to measure the direct voltage of the current

supplied to the cell, a timer 7 for the reversal of the current

flow to the cell, a direct current generator 8, an ammeter

9 for measuring the direct current supplied to the cell, and

a timer 10 for the pulsed electrodeposition.

470.5 7",

44,472/72 The cell is operated in the following manner;

the electrolyte, prepared aside in a container not shown,

is stored in the reservoir 11, withdrawn by the pump

1 and is introduced into the cell at the top thereof.

The electrolyte is periodically renewed when its silver

content decreases below a predetermined value.

The cell is supplied with direct current by

a generator 8 via a timer 10 which periodically cuts off

the supply of current to the cell. Also periodically the

direction of the current is reversed by the timer 7. The

electrodeposited silver which is detached from the

electrode 3 falls directly into the reservoir 11 through

the opening in the bottom of the cell.

In Figure 2 there is shown a continuous centrifuge

101, an electrolytic cell 102 provided with a stirrer 108

(shown in Figure 3) and a circulation pump 106; also

shown are storage reservoirs 103 and 104 of the silver

salt solution., which are equipped with a circulating pump

The centrifuge.101 is provided with a duct 105 from 107.

which the deposited silver is recovered.

The apparatus is operated as follows: in the detail electrolytic cell 102, which is described in greater

in the description relating to Figures 3 and 4, electrolytic means silver is deposited which is fed as a suspension by

of the pump 106 to the centrifuge 101 and discharged through

the duct 105. The pump 106 also serves to recycle the

electrolyte via a suitable series of valves, electrolyte 104 being fed to the cell 102 from reservoirs 103 and

by means of the pump 107.

The reservoirs 103 and 104 work alternately;

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44,472/72 thus one is used for preparing the electrolyte while the

other containing the electrolyte, feeds the cell.

Naturally, in the reservoirs stirring means are provided

which have not been shown. The details of the electrolytic

cell 102 are shown in Figures 3 and 4, wherein 109 shows

the anode, for instance perforated graphite, 110 is the

cathode, for instance of silver net, 111 are terminals for

feeding the electrodes with direct current, 112 is a valve

which controls the output towards the centrifuge 105, 113

is the recycling duct from.the centrifuge, 114 is the

recycling duct to the cell, and 115 is the feed duct of

fresh solution.

In Figure 4 it can be seen that the anode-cathode

pairs are connected to each other in series in groups: in

the drawing, each group consists of 5 pairs.

The electric circuit for feeding the industrial

cell is diagrammatically represented in Figure 5. As can be-.

seen in Figure 4, the 30 pairs of electrodes are fed in

small independent groups of 5 pairs in series. The feeding

with direct current (which is supplied in impulses with a cGndnZ aawit a duration of 3 to 10 sec.) is effected byl/n .mpzr-tti--

generator 123 which feeds, by means of commutation units

121a to 121f, each group, respectively 122a to 122f of the

pairs of elements, one after the other. Thus, at the

start the commutation1l21a feeds only the small group

122a for the duration of the impulse, thereafter the feed-

ing of the small group 122a stops and the feeding of the next

small group 122b is started by the commutation unit 121b,

for the same time, and so on until the last small group,

122f, has been fed.' The feeding cycle then starts again

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44,472 /2 as programmed by a logical control unit 120 which consists

of a square wave generator 116, a frequency divider 117,

a counting unit in/bia code 118 and a binary-decimal unit

119. Periodically the direction of the fed direct current

of the single small groups.122a to 122f is reversed by

means of a reversing unit 124 which directly operates

the /ampoo ati- generator 123. The following Examples illustrate the present

invention but are not intended to be limitative thereof.

Example 1 100 G. of silver nitrate, 750 g. of borax

(Na2 B 40 7 .IOH 400 g. of sodim hydroxide and 10 g. of in presolubilized carboxymethylcellulose, were dissolved distilled water up to a total volume of solution of 100

litres, and 600 cc of an aqueous solution of NH 3 containing

37% by weight of ammonia were added to the first-mentioned

solution. The resulting solution, containing silver as an an ammonia complex, was utilized as an electrolyte in

electrolytic cell (shown in Figure using silver net as

cathode (1X1 mm. mesh) and tubular or perforated plate

graphite as anode. The electrical contacts were made

with gold wire which under the operating conditions of the cell behaves as an inert metal. The solution was circulated in the manner indicated

in Figure 1 by means of an external circulating pump and

the silver content of the electrolyte was kept constant by

subsequent additions of silver nitrate in an ammonia

solution. The current fed to the cell was delivered by each of a period impulses having a rectangular profile,

470.570 of 10 seconds. 44.472/72 The cell feeding circuit was arranged in such a way as to give 10 seconds of delivering and seconds of rest. 6 Once every Q-12 cycles, the direction of the current was reversed for one cycle only in order to facilitate the detachment of the silver deposited on the cathode.

The cathodic deposition was carried out at a

constant current amperostatically). The main anodic process resulted in the discharge of oxygen, and the main cathodic process resulted in the discharge of silver. For this reason, the electrolytic solution contains a

decreasing content of silver ion (ammonia complexed) and an increasing content of ammonium nitrate.

The operating conditions were as follows:-

temperature room temperature

apparent cathodic surface 10 15 cm 2 apparent cathodic current

density 0.2 0.5 A/cm 2

discharge voltage of Ag,

after deducting the

ohmic drops -600 to -1200 mV (with respect to

the saturated calomel electrode)

apparent anodic surface 10 15 cm 2 electrolyte flow through

the cell 250 350 I/hour The total voltage across the cell was in the range from 7.5 V to 15 V. The Faraday efficiency with

respect'to the produced silver.was in the range from

470.570 to 90%. .44,472/72 The silver content was kept approximately constant by adding every 2 hours from 2.0 to 2.4 g.

of ammonia-complexed silver nitrate.

The total consumption of electric power was

therefore between 2.3 and 4.5 kwh per kg. of produced

silver.

The silver powder, obtained by the method

according to the invention was recovered partially from the

solution and partially from the surface of the electrode,

filtered, washed with distilled water and then dried in an

oven for 3 hours.

The productivity was in the range from 20 to 24 g.

of silver for 24 hours of continuous working i.e. of about 2 2 g. each day per cm of apparent cathodic surface.

A test of catalytic activity was carried out

on the silver produced according to the above-mentioned

process. The silver was deposited on a ceramic carrier

and the resulting catalyst containing 15% of Ag was introduced

into a vertical tube having a diameter of 2.4 cm, equipped

with an external jacket for the circulation of a thermo-

stating liquid. The height of the catalyst bed was about

1 metre. Through the tube was passed at atmospheric pressure

a gaseous mixture having the following composition:

ethylene CO 2 02 5% and N 2 83.5%. The flow rate

was 210 litres (measured under normal conditions) per hour

and the contact time about 4.6 sec. At 200°C. a good

conversion of ethylene to ethylene oxide was obtained with

a selectivity moles of ethylene oxide produced per

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44,472 72 100 moles of reacted ethylene) of 80%, and with a

conversion of

Example 2

This Example illustrates an industrial process

for the production of a silver-based catalyst by pulsed

electrochemical deposition, which uses silver nitrate as

starting material.

A solution having the following composition:

AgNO 3 0.5 5 g/1

Borax 5 20 g/ Carboxymethyl- cellulose 0.1 0.5 g/1 Ammonia 5 50 g/1 NaOH 3.5 10 g/1 was employed as electrolyte in an electrolytic cell as illustrated in Figures 3 and 4, using as a cathode a silver net (mesh with a size of 1X1 mm.) and as an anode perforated compact graphite. The solution was kept continuously stirred by means of the mechanical stirrer 108 and was passed through the electrodes which, as is apparent in Figures 3 and 4, were disposed radially in the bottom of the cell. The current fed to the electrolytic

cell was delivered as impulses having a rectangular 5-15 profile of-40 sec. The enmpehenev cycle consisted 5-I1 20-60 second impulses of feeding and/~ second periods of rest. Once every 10-20 cycles, the direction of the

electrical current was reversed for one -cycle only in

order to allow the detachment of the silver deposited on the cathode. The total current for any single impulse was

700 900 A.

44,472/ 2 The other operating conditions of this

cell were as follows:

temperature room temperature

total apparent cathodic

surface (30 single elements

of 60 X 60 cm) 10 m 2 cathodic current density 0.2 0.25 A/cm2

discharge voltage of Ag

after deducting the omhic

drops 250 350 mV (with respect to

the saturated calomel electrode)

apparent anodic surface 2 single elements of 60X60 cm) 10 m 2 anodic current density 0.2 0.3 A/cm

rotating speed of the

stirrer 40 1000 revolutions/min.

The main anodic process was the discharge

of oxygen, and the main cathodic process was the

discharge of the silver. The Faraday efficiency of the

discharge of Ag was 80-90% and the voltage of the

comprehensive feeding was in the range from 5 to 15 V.

The consumption of electric power was therefore between

and 4.5 kwh per kg. of produced silver.

Since the main anodic product is oxygen, the

electrolyte contained a decreasing amount.of Ag and an

increasing amount of NH 4 NO 3 Therefore a feeding system

was provided to introduce fresh solution which was

continuously recycled and adjusted with regard to its

Ag content by means of a highly concentrated silver

.nitrate ammonia solution (as shown in Figure The

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44,472/7 solution was deemed unsuitable when the ammonium nitrate

concentration (in moles) became 100 times greater than

that of the silver nitrate (in moles).

From this exhausted solution, silver was

recovered by means of a second electrolytic cell

substantially identical to that shown in Figure 1. The amount of silver to be recovered was about 1% with

respect to the amount of produced silver.

The producitivity of this electrolytic cell,

for catalytically active silver, was 300 kg/day for

a continuous working of 24 hours (by using an apparent

cathodic surface of 10 mm2), giving a total yearly

production of more than 75 tons of metallic silver.

The produced silver, used as a catalyst according to the

procedure described in Example 1, gave values of conversion

and selectivity in the ethylene oxide manufacture practically

identical to those of the preceding Example.

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Claims (1)

1. 44,472/ 72 The claims defining the invention are as follows: 1. A method of electrochemically manufacturing

   silver particles, which comprises subjecting a solution

   of a silver salt to a succession of cycles, each of which

   cycles includes a period of from 3 to 10 seconds during

   which electric current is passed in one direction through

   the solution and a period of from 3 to 60 seconds in

   which no electric current is passed through the solution.

   2. A method according to Claim 1, wherein in an occasional cycle, the current is passed through the solution in the opposite direction to the said one direction. 3. A method according to Claim 2, wherein said occasional cycle is one in every 10-50 cycles. 4. A method according to Claim 3, wherein in said occasional cycle, the period of passing the current in the opposite direction is for from 1 to 60 seconds. A method according to any preceding claim, wherein the silver salt is silver nitrate, silver chloride,. silver sulphate, silver acetate or silver oxalate, in the presence of a complexing agent. 6. A method according to Claim 5, wherein the complexing agent is ammonia. 7. A method according to Claim 5 or 6, wherein the amount of complexing agent is in the range from 3 to moles per gram atom of silver in the solution.

   8. A method according to any preceding claim, wherein the concentration of the silver salt solution is in the range from 0.1 to 10 g. of silver per litre of solution.

   470.570

   44,472/2 9. A method according to any preceding claim,

   wherein the solution of the silver salt is buffered so as

   to keep the pH of the silver salt solution in the range

   -from 9 to 12.5. A method according to any preceding claim, wherein the electrolysis iscarried out at a temperature in the range from 10 to 400 C. 11. A method according to any preceding claim, wherein the cathodic potential during electrolysis is between -500 and -1500 mV with respect to a saturated calomel electrode. 12. A method according to any preceding claim, wherein the current density is in the range from 0.1 to amp/cm2 of cathode surface. 13. A method according to Claim 12, wherein the current density is 'in the range from 0.2 to 0.3 amp/cm 2 of cathode surface. 14. A method according to any preceding claim, wherein the anode (with the current in the said one direction) of the cell in which electrolysis is being effected is formed of. graphite, platinum, platinum- rhodium, or titanium. A method according to any preceding claim, wherein the cathode (with the current in the said one direction) of the cell in which electrolysis is being effected is formed of silver, stainless steel, graphite, platinum, platinum-rhodium, or titanium. 16. A method according to any preceding claim, wherein

   the solution is subjected to agitation during the method.

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   44, 472 2 17. A method according to Claim 1, substantially

   as hereinbefore described with reference to Figure 1 or

   Figures 2, 3, 4 and 5 of the accompanying drawings.

   18. A method according to claim 1, substantially

   as described in either of the foregoing Examples 1 and 2.

   19. Silver particles having a particle size of

   less than 1500 R whenever produced by a method according

   to any preceding claim.

   A catalyst comprising a carrier and, as

   active ingredient, silver as claimed in Claim 19.

   21. A process for producing ethylene oxide, which

   comprises oxidizing ethylene in the presence of a

   catalyst as claimed in Claim 20 or of silver as claimed

   in Claim 19.

   22. Ethylene oxide whenever produced by a process

   according to Claim 21.

   DATED: 16 December, 1975.

   PHILLIPS ORMONDE FITZPAT.I

   ATTORNEYS FOR:

   SNAM PROGETTI S.p.A.

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