US3379626A

US3379626A - Process and apparatus for oxidizing olefins

Info

Publication number: US3379626A
Application number: US368126A
Authority: US
Inventors: Heuse Otto; Boldt Manfred; Wirtz Rudolf; Theilig Gerhard
Original assignee: Hoechst AG
Current assignee: Hoechst AG
Priority date: 1963-05-21
Filing date: 1964-05-18
Publication date: 1968-04-23
Anticipated expiration: 1985-04-23
Also published as: DE1211617B; BE648212A; GB1054119A

Description

PROCESS AND APARATUS FOR OXIDIZING OLEFINS Filed May 18, 1964 2 Sheets-Sheet 1 Qin,

INVENTORS OTTO HEL/5E MA All-REU BOLDT RUDOLF W/RTZ GERHARD THE/U6 ATTORN PY 23, 968 o. HEUSE ETAL 3,379,626

PROCESS AND APPARATUS FOR OXIDIZING OLEFINS Filed May 18, 1964 2 Sheets-Sheet 2 INVENTORS OTTO HEL/5E MAN/W50 BOLT RUDOLF W/RTZ GERHARD THE/[J6 ATTORN United States Patent O 39,8 Claims. (Cl. 2414-80) ABSTRACT OF THE DISCLOSURE An apparatus for oxidizing olefins by electrolytic means has been provided. It consists of an inclined vessel containing the anode, the cathode and a permeable membrane for hydrogen ions and means for conveying the electrolyte through the anode and from the vessel to an external vessel in which an olefin is introduced, and the olefin oxidizing reaction takes place as well as from which the olefin is recovered. The electrolyte is thereafter reconveyed to the electrolytic vessel. The cathode is suitably perforated to allow the hydrogen to pass through and be collected. Other embodiments of the apparatus include using porous oxygen solvent electrode as the cathode and means by which oxygen or air is conveyed therethrough. A further embodiment of the apparatus includes providing for means by which olefin is conveyed through the anode directly.

The process aspect of the invention is based on the proper conveyance of the electrolyte through the anode and the reuse of the electrolyte, the contacting of the olefin with the electrolyte outside the electrolytic vessel as well as preventing the hydrogen ions from contacting catalyst metal ions, or if within the vessel, then by conveying the olefin through the anode and the proper conveying of the reaction products away from the vessel. Another process aspect involves the conveying of oxygen or air through the cathode and the conveyance of the electrolyte to the external reaction vessel. lf slowly reacting olefins are involved, surfactants or solution promoters are employed.

The present invention relates to a process and apparatus for oxidizing olefins. It is known that noble metal compounds, for example, compounds of palladium, platinum, silver or copper, react with olefins to form complex cornpounds. This reaction is made use of in known processes, for example, in the process described by Smidt in a paper entitled, Katalytische Umsetzungen von Olefinen and Platinmetall-Verbindungen, which has been dealt with in Angewandte Chemie, 71, 1959, pages 176 to 182, and in the process described in Belgian patent specification 569,036. In the first of the two aforesaid processes compounds containing olefinic double bonds are in general oxidized in the presence of platinum metal compounds. According to the process described in the above-mentioned Belgian patent olefins are oxidized into aldehydes, ketones or the acids corresponding to the aldehydes by contacting them in a neutral or acid medium in the presence of a redox system with an oxidizing agent, water and a compound of a noble metal of group VIII of the Periodic Table. In that process the olefin and the noble metal compound first undergo a stoichiometrical reaction whereby ice complex compound of palladium and ethylene is decomposed, acetaldehyde and palladium form.

Processes for converting the precipitated metal into the original metal compound have been known. Palladium, for example, can be converted with copper II chloride into palladium chloride and copper I chloride according to the equation Pd+2CuCl2 PdCl2+2CuCl The copper I chloride that has formed can again be oxidized into copper II chloride in a hydrochloric acid solution by means of oxygen.

In this case and in similar other cases CuCl2 or another reducible compound has to .be added to the mixture of metal and olefin as a third component in order to convert the metal into the metal compound in the form of which it has initially been used. Often, however, the addition of the aforesaid third component gives rise to difficulties since in many cases it reacts with the oxidation product of the olefin. This is all the more so when the third component is added in a large excess amount. This is the case in particular when butenes or higher olefins are to be oxidized and the third component, as in the above case, contains chloride ions. In that case chlorination products form, which are ditiicult to separate from the desired oxidation products and, besides, their separation is expensive.

It is also known that noble metals which are applied to an anode in an electrolyte can be dissolved by applying an electric tension between the anode and a neighbouring cathode. When, for example, palladium is applied to an anode and when an aqueous sulphuric acid solution is used as the electrolyte, palladium can be dissolved in the form of palladium sulphate which is very suitable for use as a catalyst in the oxidation of olefins. However, in the oxidation of olefins the aforesaid process of regenerating the noble metal catalyst by electrolysis cannot 'be applied without special measures being taken, because the metal ions or a part of them either deposit directly on the cathode in the form of the metal or are reduced by the hydrogen forming at the cathode and are precipitated in the form of the metal. This can -be seen very clearly when metallic palladium is converted by electrolysis into Pd++ ions. As soon as a current flows in an electrolytical cell of the aforesaid kind the acid solution turns greenish owing to the Pd++ ions contained in it. But as soon as the Pd++ ions come into the cathode space metallic palladium precipitates in the form of a black slime.4

The present invention provides a process for the oxidation of olefins by reacting an olefin with an acid solution containing metal ions of the group of platinum and/or of Group I (b) or Group II (b) of the Periodic Table as a catalyst and by subsequently re-oxidizing the catalyst metals that have precipitated in the course of the oxidation into the metal ions acting as a catalyst in the oxidation reaction. According to the process of the present invention the catalyst metal that has precipitated in the course of the oxidation is applied to the surface of a porous electrode of an electrolytic cell, which electrode is insoluble and has the function of an anode and the cata lyst ions that have formed by anodic oxidation and have entered the electrolyte of the electrolytc cell are applied again for the oxidation of the olefins. The gaseous hydrogen forming at the counter-electrode which has the function of a cathode is prevented lfrom mixing with the electrolyte containing the metal ions serving as a catalyst by separating the spaces around the electrodes, which in this specification are called anode space and cathode space, by means of a membrane that is permeable to hydrogen ions but largely impermeable to the catalyst metal ions and/ or lby using an oxygen solvent electrode as the catalyst.

According to the first-mentioned method of realizing the process according to the invention a membrane is used which is arranged between the anode and the cathode and which is permeable to hydrogen ions and impermeable to the metal ions. For this purpose there may be used in particular in the process of the invention the socalled cation exchange membranes which have been described, for example, by Helfferich in Ionenaustauschen volume I, pages 55 to 56, published by Verlag Chemie Weinheim (1959). It is known that these membranes are impermeable to anions but permeable to cations. But in addition to this fact it has now `been found that they are much less permeable to heavy than to light cations. When hydrogen ions and metal ions are present practically only the hydrogen ions pass through the membrane. It has been observed that in a cell of the aforesaid kind the anode space is coloured by the action of the metal ions as soon as current is switched in whereas the cathode space remains colourless. Nor does any metal precipitate at the cathode.

Advantageously, the membrane separating the electrode spaces is at least 5 times, preferably more than 20 times, as permeable to hydrogen ions as to the metal ions serving as the catalyst and formed in the electrolyte at the anode.

Another possibility to avoid the detrimental effect of the hydrogen which forms is to use a cathode in the form of a so-called oxygen solvent electrode as is known from fuel cells. In that case the hydrogen is oxidized by the oxygen dissolving at the cathode while it simultaneously absorbs an electron and is thus converted into water.

An oxygen solvent electrode is an electrode which has the form of a porous membrane and to the one side of which an oxidizing agent, for example, gaseous oxygen, if desired in the form of air, is fed and the other side of which is wetted by the electrolyte. The oxygen solvent electrode may be made up of a metal oxide, for example, nickel oxide, or coal that has been impregnated with a special catalyst, for example, a spinel or a noble metal. Finely divided metals, for example, Raney metals, in particular Raney silver are also suitable for this purpose.

The mode of carrying out the process of the invention, in which an oxygen solvent electrode is used, offers the advantage that the energy which is set free when hydrogen is lburnt with oxygen is available in the form of electric energy, so -that under certain conditions no additional electric energy is required in the whole course of the reaction. No additional electric energy is required when in the cell the energy required for the formation of the metal ion plus the losses due to electrical resistance and the other losses is smaller than the energy set free 'by the oxidation of hydrogen. Even if this is not the case the amount of electric energy to be supplied from outside is smaller than in the first-mentioned mode of carrying out the process of the invention in which a membrane is used.

The two above-described methods of realizing the process of the invention may be combined and a membrane of the kind described above and an oxygen solvent electrode serving to burn the hydrogen that has formed may be used simultaneously.

In either of the two embodiments of the invention acid or neutral aqueous solutions as are usually applied in electrolysis may be used as electrolyte solutions, aqueous sulphuric acid, aqueous phosphoric acid and aqueous ammonium sulphate solutions being preferably used. The normality of these solutions is in general within the range of 1 to 10, preferably l to 5. Solutions having a conductivity of more than 0.05 ohm*1-cm.-1, preferably of more than 0.2 ohmfl-cml, are preferred. Electrolyte mixtures may also 'be used.

The electrode consists of a material which is generally used for the preparation of electrodes. The material must, of course, 'be inert towards the reactants. Both the anode and the cathode are suitably made of porous coal, porous graphite or another porous conductor such as a sintered metal, for example, sintered nickel. The cathode may likewise be constituted by a metal grid.

All those noble metals may be used that are capable of forming complex compounds with olens, in particular the noble metals of Group VIII, but also those of Groups I and II of the Periodic Table. Of the noble metals of Group ViII, palladium is particularly suitable while silver and mercury are the most suitable noble metals of Groups I and II of the Periodic Table.

As materials -for the preparation of the membrane which may be used in the process of the invention there are particularly suitable cation exchange resins based on polystyrene or copolymers of styrene.

As olelins which can be oxidized by the process according to the invention there may be mentioned aliphatic monoand polyoletins, cycloaliphatic or araliphatic olefins, for example, cyclohexene or styrene and their homologues, substituted olens, for example, unsaturated carboxylic acids and other unsaturated oxygen compounds such as aldehydes, esters, ethers and ltetones, as have been mentioned in the paper by Smidt cited above. The olenic compounds may be substituted, for example, by halogen atoms, amino groups, nitrile groups, nitro groups or amide groups. Mixtures of these compounds may also be used. The process according to the invention may also be used for converting olens into aldehydes, ketones or the acids corresponding to the aldehydes by the process described in Belgian Patent 569,036 according to which in particular low aliphatic olefms containing 2 to l2, preferably 2 to 6 carbon atoms, for example, ethylene, propylene, butene-l, butene-Z or pentene, are used as starting compounds.

The process according to the invention may be carried out in such a manner that only the oxidation of the catalyst metal which results in the formation of the metal ions accelerating the oxidation reaction takes place in the electrolytic cell and the oxidation of the oleins by means of the ions of the catalyst metal takes place in a reaction vessel which is separate from the electrolytic cell.

T he oxidation of the olefins in the presence of the catalyst metal ions may also be carried out in the electrolytic cell itself for example by pressing the olen to be oxidized through the pores of a porous anode, reacting it with the catalyst metal deposited on the anode and withdrawing the oxidation products formed from the cell.

In all the embodiments of the process of the invention it is often advantageous to add solution promoters or surfactants to the electrolyte in the electrolytic cell. Such compounds are added in particular in cases in which slowly reacting olerins are to be reacted. As solution promoters may be used, for example, low aliphatic alcohols, ketones or ethers and as surfactants may be used taurines or similar substances.

If the conductivity of the electrolyte should be diminished by oleiins that are dissolved or by oxidation products thereof or if these dissolved substances should initiate the formation of an emulsion the disturbing substances may be eliminated by an appropriate known measure carried out at an appropriate place in the cycle, for example, by stripping, heating or extraction.

The invention will now be described in greater detail by way of example only with reference to the accompanying drawings of which:

FIG. 1 is an elevation in section of a rst embodiment.

FIGS. 2 and 3 are elevations in section of further embodiments of the invention.

With reference to the drawings, FIGURES l to 3 illustrate various methods of carrying out the process of the invention. A feature that is common to all three of the methods is that the electrodes are inclined to the horizontal. Such a disposition of the electrodes is suitable because in this way the maintenance of the cycle and the Withdrawal of the gas are best ensured. The aforesaid arrangement is, however, not absolutely necessary for the success ofthe process. The angle of inclination between the electrodes and the horizontal is preferably within the range of about to 70. When the angle is smaller than 10 0r wider than 70 it is suitable to provide for a mechanical mixing of the electrolyte solution in order to prevent undissolved reactants from accumulating at the bottom of the electrolytic cell.

In the method represented by FIGURE 1 an ion exchange membrane is used and the reaction of the metal compound with the olefin is carried out in a separate reaction vessel. In the method illustrated by FIGURE 2 an oxygen solvent electrode serving for the oxidation of the hydrogen is used. In the method illustrated by FIGURE 3 no separate reaction vessel is used, the reaction of the metal compound with the olen taking place between the electrodes. In the three figures equal numbers refer to parts of the apparatus having equal functions.

FIGURE l shows a vessel 1 for electrolysis which is made of an inert material and in which two electrodes, viz an anode Z and a cathode 3, are arranged opposite one another. The cathode 3 is suitably perforated in order to enable the hydrogen 4 that has formed to pass through it in the form of bubbles. There are arranged electric conductors 5 and 6 for the cathode and the anode. Between the anode and the cathode is arranged a membrane 7 which is permeable to H+ ions but impermeable to the metal ions. First, the metal 8 is applied to the anode. The space 9 between the anode 2 and the membrane 7, the space 10 below the anode, the connecting

tubes

11, 12 and 13, the pump 14 and the reaction vessel 15 are filled with the aqueous electrolyte solution. The space 10 between the membrane 7 and the upper part of the vessel is likewise lilled with an electrolyte. Both electrolytes may be identical with regard to their kind and concentration, but this is not necessary. If between the anode 2 and the cathode 3 an electric tension is applied which is sutiicient to dissolve the metal, the metal that has been applied to the anode dissolves in the form of ions. If the metal used is, for example, palladium and the electrolyte used is aqueous sulphuric acid the aqueous solution contains palladium sulphate. This solution is sucked off through the pores of the anode 2 by means of the pump 14 and conveyed into the reaction vessel 15. The oleiin to be oxidized is introduced through an opening 17 into the reaction vessel 1S through which it passes in the form of bubbles. It reacts with the metal compound whereby the metal is reduced, the metal precipitating in general in the form of a ne slime. The oxidation product of the olefin is withdrawn through an outlet 18. The electrolyte charged with the metal slime passes through the connecting pipe 13 and arrives in the space 9 in which the slime deposits anew on the anode. The cycle is closed. The gaseous hydrogen formed at the cathode rises in the space 16 in the form of bubbles, it is withdrawn through an outlet 19 and may be applied for any desired purpose.

The aforesaid method oters the particular ladvantage that the nascent hydrogen which forms at the cathode and which, as is known, is particularly reactive can directly be used in space 16 for hydrogenation purposes. Its Ehydrogenating property Can 'be further improved by using a cathode made of an appropriate material, for example, fby using a cathode made of a material having a catalytic ttect in the hydrogenation, for example, platinum or palladium. In that case the electrolyte in space 16 has to be brought into contact at the cathode with the substance to be hydrogenated.

Such a hydrogenation reaction which takes place in the cathode space does not disturb the oxidation reaction which takes place below the membrane 7, provided that the membrane 7 is impermeable to the material to be hydrogenated and to the hydrogenation product, which in general is the case.

In the method illustrated by FIGURE 2 the cathode 20 used is a porous oxygen solvent electrode. The oxidizing agent which advantageously is oxygen or air is introduced through an inlet 22 into the space 21. It passes through the pores of the membrane-like electrode 2) and at the limit between space 9 and the cathode 2t) it reacts with the hydrogen ions arriving there. This reaction is promoted by suitable catalyst of known type with which the electrode is provided. Since the hydrogen reacts with the oxygen passing through the pores of the membrane and yields Iwater no hydrogen forms at the cathode which would reduce the metal salt that has formed.

In the method shown yby FIGURE 3 the space 23 below the anode Z, which is also made of a porous conductive material is filled with the olelin to be oxidized. The oleiin is pressed in through an inlet 25 and passes in the form of bubbles through the pores of the anode into a space 26 situated between the anode 2 and the membrane 7 and lled with electrolyte Vand metal compound. In this space 26 both the migration of ions, which is caused by electrolysis and leads to the formation of the metal cornpounds, and the reaction which, on the one hand, leads to the reduction of the metal and, on the other hand, to the oxidation of the olefin takes place. The olefin and its oxidation product a-re withdrawn through an outlet 27. The apparatus in which the aforesaid method is carried out may be provided with devices serving to remove portions of the olefin or its oxidation product that are dissolved in the electrolyte, if such are present, by heating, stripping or another method, such devices being, however, not shown in FIGURE 3.

The type of construction which fhas just been described and which can be applied analogously to the apparatus shown in FIGURE 2 has the advantage that in the most favourable case the metal does not leave its place lat the anode at all Ibut is reduced again immediately after its oxidation. This happens when the reduction takes place very quickly and the electrolytic oxidation and the detachment of the metal compound from the electrode take place relatively slowly. This is often the case in the oxidation of oletins.

The apparatus according to FIGURE 3 offers the further advantage that if metal suspensions are present that iare still separated from the anode, these suspensions cannot be lost in tubes, reaction vessels or other parts of the apparatus but after a short time come again into contact with the anode. This is particularly important when valuable noble metals are used.

The apparatus shown in FIGURE 3 may be varied in such a manner that the olen is not introduced through the anode 24 itself but through another inlet, for example, through a frit arranged, for example, at the lower left side of the electrolytic space 26.

We claim:

1. A process for oxidizing oleiins by reacting the oleiin with an acid solution containing as a catalyst ions of at least fone metal selected from the group consisting of metals of the platinum group and metals of Groups I(a) and Il(b) of the Periodic Table according to Mendeleff and mixtures thereof and re-oxidizing the catalyst metals precipitated Iduring the oxidation reaction into the metal ions serving as the catalyst lfor the oxidation, which process comprises conveying the catalyst metal precipitated during the oxidation reaction to the surface of the electrode of an electrolytic cell, which electrode is porous and insoluble and serves as an anode, re-utilizing the catalyst ions formed by anodic oxidation and which have entered the electrolyte of the electrolytic cell in the oxidation reaction of the olefin, conveying said electrolyte to a reaction vessel separate from the electrolytic cell wherein the oxidized olelin is recovered and then recirculating the electrolyte to the electrolytic cell and preventing the gaseous hydrogen forming at the counteraelectrode serving as a cathode from mixing lwith the electrolyte containing the ycatalyst metal ions by separating the two electrode spaces by means of a membrane permeable to hy- `drogen ions but largely impermeable to the catalyst metal lons.

2. A process as claimed in claim 1 wherein the membrane separating the electrode spaces is a cation exchange membrane which is at least live times Ias permeable to hydrogen ions as to the catalyst metal ions formed in the electrolyte at the anode.

3. A process as claimed in claim 1 wherein the membrane separating the electrode spaces is a cation exchange membrane which is lmore than twenty times as permeable to hydrogen ions as to the catalyst metal ions formed in the electrolyte at the anode.

4. A process as claimed in claim 1 wherein the electrolyte used in the electrolytic cell is a member selected from the group consisting of aqueous sulphuric acid, aqueous phosphoric acid and aqueous ammonium sulphate.

5. A process as claimed in claim 1 wherein only the oxidation of the catalyst metal which results in the formation of the metal ions accelerating the oxidation reaction is carried out in the electrolytic cell and the oxidation of the olefins in the presence of the catalyst metal ions is carried out in a reaction vessel which is separated from the electrolytic cell.

6. A process as claimed in claim 1, wherein the oxidation of the olefins with the catalyst metal ions is carried out in the electrolytic cell itself by pressing the olefin to be oxidized through the pores of a porous anode and reacting it with the catalyst metal conveyed to the anode, and the oxidation products that have formed are withdrawn from the cathode space of the cell.

7. A process as claimed in claim 1 wherein a member of the group consisting of a solution promoter and a surfactant is added to the electrolyte in the electrolytic cell.

S. A process for oxidizing olefins by reacting the olefin with an acid solution containing as a catalyst ions of at least one metal selected from the group consisting of metals of the platinum group and metals of Group I(b) and II(b) of the yPeriodic Table according to Mendeleff and mixtures thereof and re-oxidizing the catalyst metals precipitated during the oxidation reaction into the metal ions serving as the catalyst for the oxidation, which process comprises conveying the catalyst metal precipitated during the oxidation reaction to the surface of the electrode of an electrolytic cell, which electrode is porous and insoluble and serves as an anode, re-utiiizing the catalyst ions formed by anodic oxidation and which have entered the electrolyte of the electrolytic cell in the oxidation reaction of the olefin, conveying said electrolyte to a reaction vessel separate from the electrolytic cell wherein the oxidized olefin is recovered and then recirculating the electrolyte to the electrolytic cell and preventing the gaseous hydrogen forming at the counterelectrode serving as a cathode from mixing with the electrolyte containing the catalyst metal ions by the use of an oxygen solvent electrode as the cathode.

9. A process for oxidizing olefins by reacting the olefin with an acid solution containing as a catalyst ions of at least one metal selected from the group consisting of metals of the platinum group and metals of Groups 1(1)) and Il(b) of the `Periodic Table according to Mendeleff and mixtures thereof and re-oxidizing the catalyst metals precipitated during the oxidation reaction into the metal ions serving as the catalyst for the oxidation, which process comprises conveying the catalyst metal precipitated during the oxidation reaction to the surface of the electrode of an electrolytic cell, which electrode is porous and insoluble and serves as an anode, re-utilizing the catalyst ions formed by anodic oxidation and which have entered the electrolyte of the electrolytic cell in the oxidation reaction of the olefin, conveying said electrolyte to a reaction vessel separate from the electrolytic cell wherein the oxidized olefin is recovered and then recirculat-ing the electrolyte to the electrolytic cell and preventing the gaseous hydrogen forming at the counterelectrode serving as a cathode from mixing with the electrolyte containing the catalyst metal ions by separating the two electrode spaces by means of a membrane permeable to hydrogen ions but largely impermeable to the catalyst metal ions and by the use of an oxygen solvent electrode as the cathode.

19. An apparatus for oxidizing olefins which comprises -an electrolytic Vessel; a porous anode which extends over the whole cross-sectional area of the electrolytic vessel; a cathode which is arranged above the anode and which likewise extends over the whole cross-sectional area of the electrolytic vessel; a membrane which is arranged between the anode and the cathode, which likewise extends lover the whole cross-sectional area of the electrolytic vessel and which is permeable to hydrogen ions -but largely impermeable to the catalyst metal ions; a reaction vessel serving for the oxidation of the olefins and arranged so as to be separate from the electrolytic vessel, the lower end of which reaction vessel is connected via a conduit with the space limited by the anode and the membrane and the upper end of which is connected via a conduit and a circulating pump in the conduit with the space below the anode and which reaction vessel is provided with an inlet serving for the introduction of the olefin to be oxidized and an outlet serving for the withdrawal of the oxidation products of the olefin.

11. The apparatus according to claim 10 in which the membrane is made of a cation exchange resin based on polystyrene.

12. The apparatus according to claim `1t) in which the membrane is made of a cation exchange resin based on a copolymer of styrene.

13. The apparatus according to claim 10 in which each of the anode and the cathode is made of porous carbon.

14. The apparatus according to claim 10 in which each of the anode and the cathode is made of a porous graphite.

15. An apparatus for oxidizing olefins which comprises an electrolytic vessel; a porous anode which extends over the whole cross-sectional area of the electrolytic vessel; a cathode membrane co-mprised of a porous oxygen solvent electrode and arranged above the anode a-nd extending over the whole cross-sectional area of the electrolytic vessel; an inlet for introducing air or oxygen into the cathode space; an external reaction vessel for the oxidation of the olefins and arranged so as to be joined to the electrolytic vessel by conduits, the lower end of the external reaction vessel lbeing connected Via a conduit with the space limited by the cathode, and the anode and the upper end of the external reaction vessel being connected via a conduit and a circulating pump in the conduit with the space below the anode, said vessel being provided with an inlet for introduction of the olefin to be oxidized and an outlet for the withdrawal of the olefin oxidation product.

References Cited UNITED STATES PATENTS 2,636,851 4/1953 Juda 204-98 3,147,203 9/1964 Klass 204-80 `3,247,085 4/1966 Worsham 204--80 3,248,312 4/1966 Young 204--80 yHOWARD S. WILLIAMS, Primary Examiner.

JOHN H. MACK, Examiner.

I. PODGORSKI, H. M. FLOURNOY,

Assistant Examiners.