PLATINUM METALS REVIEW A quurterly survey of reseurch on the platinum metuls urrd of dwelopments in their applications in industry VOL. 5 OCTOBER 1961 NO. 4 Contents PaUadium-on-Charcoal Catalysts 122 Continuous Melting of Optical Glass 126 Removal of Chloride Contaminants from Nitric Acid 128 Synthesis of Penicillin Derivatives 131 The Contamination of Platinum Metal Thermocouples 132 The Gaseous Oxides of the Platinum Metals 134 A High Temperature Research Microscope 139 Internal Nucleation of Glass I 4'3 Corrosion Resistance of Chromium 141 Miniature Moving Coil Relay 143 Creep Tests on Platinum Alloys 144 The Discovery of Iridium and Osmium 1466 Abstracts 149 New Patents Index to Volume 5 159 Communications should be addressed to The Editor, Platinum Metals Review Johnson, Rlutthey & Co., Limited, Hatton Garden, London, E.C.1 Palladium-on-Charcoal Catalysts SOME EFFECTS OF VARIABLES ON HYDROGENATION ACTnTITY By J. C. Chaston, Ph.D., and E. J. Sercombe, B.SC. Research Laboratories, Johnson Matthey & Co Limited The performance of charcoal-based catalysts cannot be described simply in terms of their mesh site and noble metal content, as many factors can cause wide variations in activity, selectivity and resistance to poisoning. It is possible, however, by close collaboration between catalyst manufac- turer and user, and by suitable choice of base and production technique, to provide a range of such catalysts tailored to meet the needs of a particular industrial reaction. Catalysts of the type commonly described tion catalysts, in their selectivity and in their as palladium-on-charcoal find extensive use resistance to particular forms of poisoning in in the chemical and pharmaceutical manufac- use. It cannot be emphasised too strongly turing industries as hydrogenation catalysts that the activity of any palladium hydrogena- for a wide variety of reactions. tion catalyst cannot be expressed generally Basically, charcoal supported catalysts are but only as it is related to specific hydro- comprised of powdered activated charcoal genation reactions or groups of reactions. having a particle size of the order of ZOO mesh impregnated with a solution of a palladium Nature of Charcoal Base salt which is subsequently reduced to metallic Charcoal bases for catalysts are nearly form. always selected from the group known as Since charcoal is essentially a natural pro- “activated” charcoals. The raw material from duct, wide variations in its physical structure, which “activated” charcoal is made is a care- in its purity, in its content of trace elements, fully selected natural product which may be and indeed in its properties generally are to be hard wood, nutshells, peat, or a blend of par- expected. More than this, the activity of ticular grades of coal and anthracite. After palladium-on-charcoal catalysts is, somewhat carbonising by heating in retorts to about surprisingly, susceptible to extraordinarily 600”C,the charcoal is “activated” to develop wide variation according to the method chosen the required pore structure by heating again to introduce palladium into the structure and in a controlled atmosphere, usually of steam. to absorb it on the surface of the charcoal The control of this stage is normally by means particles. In addition, the resistance of a of an absorption or decolourising test using a palladium-on-charcoal catalyst to certain suitable dyestuff such as methylene blue. types of poisoning can be influenced both by The extent to which the activity of pal- method of manufacture and by various after- ladium-on-charcoal catalysts in specific re- treatments. actions is dependent on the nature of the It thus becomes possible, by suitable choice charcoal base may be illustrated by a simple of charcoal and by the use of appropriate example I manufacturing techniques, to make palladium- Two charcoals, impregnated with the same on-charcoal catalysts that differ widely in their proportion of palladium by the same tech- performance, in their activity as hydrogena- nique, were used as catalysts in two well- Platinum Metals Rev., 1961, 5, (4), 122-125 122 Determining the activity of a palladium-on-charcoal catalyst in the Johnson Matthey research laboratories by measuring the rate of hydrogen absorption in a hydrogenation reaction at room temperature known hydrogenation reactions-one of cro- this means is well shown by the following test tonic acid and the other of nitrobenzene- results, all relating to catalysts containing 3 both at room temperature and atmospheric per cent of palladium on the same type of pressure. The relative rates of uptake of charcoal. hydrogen were found to be as follows: Relative Activity in Activity in an Industrial Hydro- Hydro- Crotonic Acid Nitrobenzene genation of genation Charcoal A I00 55 Nitrobenzene Reaction Charcoal B I05 3 Made by Procedure A 100 Good ,, B 46 Poor It is clear that these two charcoals are ,, C 24 Good fundamentally different in their performance ,, D 118 Good as catalyst bases. A catalyst made with Charcoal B is slightly more active than one Percentage of Palladium made with Charcoal A in promoting the The amount of palladium incorporated in hydrogenation of crotonic acid, yet it is very a catalyst mass of the type under discussion is, nearly inactive for the hydrogenation of in general, of less importance than the way in nitrobenzene, for which the catalyst made which it is applied or the nature of the char- with Charcoal A is over 17 times more active. coal base. Provided that more than a mini- Similar wide variations in the character- mum amount is present, the actual palladium istics of palladium-on-charcoal catalysts may content is generally not very critical. be deliberately achieved by modifying the The effect of varying the palladium content procedure by which the charcoal is impreg- on the properties of one type of palladium-on- nated with palladium. The extent to which charcoal catalyst (all other factors being kept activity can be varied in different reactions by constant) is shown in the table over page. Platinum Metals Rev., 1961, 5, (4) 123 vigorous interaction between the catalyst, the Effects of Variations in Palladium Conten! liquid and the gaseous hydrogen) that it is on the Activity of a Charcoal-base Catalys! not possible to compensate for a low pal- Relative Rates of ladium content simply by using more cata- Hydrogenation of lyst. The activity of the catalyst in this and in Nitrobenzene many other reactions appears to rise sharply Using Using constant sufficient to a maximum corresponding to around 4 to 5 Per cent weight of catalyst to per cent of palladium and then to remain pal lad ium catalyst contain in catalyst (I .Dog.) 0.03 g. Pd constant or to decline slowly as the palladium content is increased. This conclusion applies 0.5 22 44 equally whether activity is judged in terms of I .o 59 84 the weight of catalyst or of the weight of 2.0 88 88 palladium in the circuit. 3.0 I00 I00 Poisoning of Catalysts 4.0 I12 I10 General poisoning of charcoal-base cata- 5.0 I I6 I I5 lysts, resulting in an overall loss of activity, 6.5 I I8 97 may of course often occur in scrvice. It is not 8.0 I I8 96 so generally appreciated that poisoning may occur during storage in industrial atmo- It follows from these results (at least under spheres. Traces of sulphur dioxide, in par- the test conditions employed, in which a ticular, may render an active catalyst mechanical shaker was used to maintain completely inactive in the course of a few minutes, and for this reason the protection of catalysts in storage needs particular care and considera- tion to ensure that their initial high activity is maintained until the time comes for their use. Charcoal- base catalysts are accordingly packed in airtight containers and these should be stored in clean, dry and uncontaminated surround- ings. The original packings should not be opened until immediately A constant temperature autoclave for the investigation of hydrogenation reactions over charcoal-base catalysts at elevated temperatures and pressures Platinum Metals Rev., 1961, 5, (4) 124 Equipment for the impregna- tion of charcoal supports in the production of Johnson Matthey catalysts to meet special require- ments in the pharmaceutical industry before the catalyst is required for use. to poisoning in service increased, by selec- In some instances it is of advantage to tively pre-poisoning it during manufacture. supply the catalyst as a damp paste rather The original activity is by this treatment than as a dry powder, for ease in transport and somewhat reduced, but an appreciably more handling. stable catalyst is produced. It is sometimes possible to develop charcoal- The examples quoted above have been con- base catalysts which will selectively promote fined to palladium-on-charcoal catalysts, but one of two or more possible hydrogenation the discussion applies with equal force to reactions in certain systems. This may be platinum-on-charcoal and to charcoal-base achieved by selectively poisoning the catalyst, catalysts impregnated with ruthenium or other presumably thereby occupying those active platinum metals. The performance of none sites on the catalyst surface which would of these is to be described simply in such normally be responsible for the unwanted general terms as mesh size of the charcoal and hydrogenation reactions. Controlled quan- the noble metal content. There is an almost tities of base metal salts, of quinoline or of unlimited field for the development of an certain sulphur compounds, added during immense range of catalysts differing in catalyst manufacture, are examples of selec- activity, selectivity, and service life, and one tive poisons which have been used for the which can be explored only by the closest control of industrial hydrogenation processes. collaboration between the manufacturer of Finally, the active life of a charcoal-base catalysts on the one hand and the user- catalyst in some hydrogenation reactions has whether research chemist, development sometimes been prolonged, and its resistance engineer or plant operator-on the other.