Vibhu

Hydrogenation Using Precious Metal Catalysts
KETAN PATEL
Practicals India Enterprises, 11, My-Nest, Sector 2, Vashi, Navi Mumbai 400 703.
Tel/Fax: +91 22 27826487; E-mail: info@practicalsindia.com
Typically 5% palladium/platinum/rhodium on ac Also, India depends totally on the international market
tivated carbon/alumina and 10% palladium/ for the purchase of palladium. This makes things more platinum/ rhodium on carbon/alumina catalysts difficult for the consumer. So what can to be done to finds application in a variety of reactions like hydro- keep things under control? The best method is to mini- genation, hydrosilation, dehydrogenation, hydro- mize the consumption and maximise the output of the genolysis, dehalogenation, reduction, debenzylation, catalyst. Some of the techniques that can be followed Rosenmund reduction, alkylation, methanation, carbon- to ensure optimization of catalyst use are: oxygen cleavage, carbon-nitrogen cleavage, etc.
1. Carry out strict analysis of the fresh catalyst sup- Precious (nobel) metal catalysts has many advan- plied. It would be better if proper sampling and ho- tages as compared to nickel based catalysts in terms of mogenizing is done before the analysis. The sam- reaction parameters, selectivity and applications. The pling procedure should include preconditioning, dry- filtration rate is also faster and the catalysts can be ing, grinding, sieving, homogenization, bulk homo- recycled in many cases. The above chemical reactions lead to manufacture of critical life saving drugs, fine 2. The reactants should de-aerated by purging with an chemicals and intermediates. Some of these include: inert gas for 5 to 15 minutes. This is best suited fora majority of reactions.
3. Minimise palladium leaching (re-deposition) during • Antibiotics like azithromycin, clarithromycin.
and at the end of the reaction. Palladium leaching • Carvedilol (congestive heart failure and hyperten- generally correlates significantly with the progress of the reaction, the properties of reactants, the sol- • Isoxsuprine (vascular diseases such as arterioscle- 4. In cases of leaching, palladium re-deposition can be rosis, Buerger’s disease, Raynaud’s disease and men- achieved by: Further treatment at a slightly increased temperature; addition of reducing agents; and/or • Cisplatin (metastatic testicular, ovarian, tumors, 5. Efforts should be made to study: the activity/selec- • Pentazocine (interstitial cystitis).
tivity of the catalyst under ambient conditions; and • Theobromine (energizer and weight loss).
the complete separation (filtration) from the product • Theophylline and salbutamol (anti-asthama).
• Lisinopril (high blood pressure and heart failure).
6. The atmospheric conditions should be preferably inert • Cyanocobalamine (neurological disorders).
(e.g. argon, helium, etc.). In some cases, compressed inert gases have been found to increase catalyst ac- • Benzathene penicillin (diphtheria).
7. The spent catalyst, many a times, may not be spent.
A detailed analysis should be done of the spent cata- lyst before sending the same for reprocessing.
ROLE OF A SOLVENT IN CATALYTIC
REACTIONS
The only major disadvantage of these catalyst sys- tems is the price factor, which makes it a very expen- Solvents play a vital role in some hydrogenation reac- sive inventory to carry. Moreover the price of palla- tions involving a catalyst, but not necessarily in all reac- dium keeps on fluctuating from very low to very high.
Among the important reactions carried out by 2. It is necessary to steam distil the amine from the iron manufacturers of dyes and intermediates are reduc- sludge, which consumes very high energy.
tion of nitro-aromatics to amines. The reactions 3. Dumping and disposal of the sludge is a problem.
should be carried in the molten state, in the absence 4. If a solvent is used, the iron sludge has to be filtered of solvent, in a stirred tank reactor. In case solvents are desired, then isopropanol or ethyl acetate is rec-ommended. The catalyst loadings depend on the re- The advantages of hydrogenation using palladium/ actant purity with respect to the catalyst poisons and the desired reaction time. The catalyst exhibits highselectivity and activity. Hence, it is not necessary to add bases to neutralize the liberated hydrochloric acid, which can reduce the catalyst activity. The 3. Recycling of the catalyst is possible.
amount of catalyst recommended is 0.5 weight % of 4. Optimization of reaction conditions and good house the nitro compound. In general, increase in catalyst keeping will ensure minimal catalyst consumption.
loading, pressure and/or reaction temperature will 5. The noble metal can be recovered from the spent increase the rate and decrease the selectivity.The gas- catalyst and reprocessed to a fresh catalyst. The sub- liquid-solid mixing also has an impact on the reac- sequent purchases are only small after the initial tion rate and catalyst selectivity. Optimization of the agitation rate will ensure a maximum yield of thedesired product.
Two reactions where both palladium and platinum Comparison between catalytic hydrogenation and
• Conversion of alkylated dihydroxybenzene to the iron/hydrochloric acid reduction
corresponding alkylated cyclohexanol.
• Conversion of alkylated guaicols to the correspond- The disadvantages of Fe/HCl reduction are: 1. Problems are often experienced in sourcing iron pow- der of desired specifications and quality.
For both the above reactions, palladium catalysts REACTION CONDITIONS FOR REDUCTION REACTIONS
Reactant
Temp. (°C)
Pressure (Bars)
Catalyst
Yield (%)
USE OF RUTHENIUM & RHODIUM CATALYSTS
require high temperatures and pressures to achieve good The packing should be done in durable and leak- yields of the product. Platinum catalyst gives lesser yields due to the hydrogenolysis of the carbon-oxygenbond. Rhodium and ruthenium based catalysts over- Sampling procedure
come these problems to a good extent. Rhodium can beused under mild conditions also. Ruthenium catalysts The sampling should be carried out in the presence are also good, but require slightly higher temperatures of representatives of the consumer and vendors. The fines of the catalyst also should be mixed for sampling.
After the collection is done, the spent has to be weighed, The other problem in rhodium based catalysts is that stored, recorded and attested in presence of representa- Rhodium prices have been fluctuating a lot since re- cent times, as compared to other platinum group met-als. This can hence sometimes increase / decrease the 1. Each drum containing the catalyst should be num- overall cost of the manufacturing process.
2. Gross weight of each drum should be recorded.
THE IMPORTANCE OF SAMPLING
3. Each drum should be emptied and its tare weight The petrochemical industry also conducts dehydro- 4. Contents of each drum should be sieved through a genation reactions using platinum based catalysts and BS mesh to remove fines. Sampling should be done chemicals in large quantities. A typical example is while screening for obtaining composite sample.
chloroplatinic acid. It contains 39-42% platinum and 5. Weight of material retained on and passing through is a red-brown crystalline solid. Some companies use the mesh should be recorded for each drum sepa- 20% chloroplatinic acid containing 20% platinum.
Chloroplatinic acid is also used in electroplating, etch- 6. Fines collected as at step 5 should be mixed and heaped together for all drums. With the method ofconing and quartering, 1 kg samples of fines should Spent chloroplatinic acid is in the form of non-uni- be collected in bottles and sealed. The remaining fines form crystals and partly in powder form (fines). Hence, should be transferred to separate empty drums (pre- care should be taken to perform the sampling proce- viously weighed and recorded). The number of 1 kg dure for spent chloroplatinic acid before dispatching it samples will be determined by the number of parties to the manufacturer for recovery and regeneration/re- processing. Discrepancies in sampling may lead to huge 7. The entire quantity of samples collected as per the financial losses for the consumer. The samples of step 4 should be mixed and heaped for obtaining the chloroplatinic acid (CPA) should be drawn as per the composite sample. With the method of coning and quartering 1 kg sample of this catalyst should be SAMPLING OF CHLOROPLATINIC ACID
Sample No.
To be retained by CONS duly sealed by CONS for their analysis.
Bearing CONS code no. and to be sealed by CONS for sending the sample to the third To be handed over to the vendor i.e. MANU for carrying out analysis in their laboratory.
Jointly to be sealed by MANU and CONS to be retained by the vendor as reference.
Jointly to be sealed by MANU and to be retained by CONS as reference sample carrying Jointly to be sealed by MANU & CONS, and retained by CONS as reference sample.
CONS: Consumer Representative (i.e. user of CPA); MANU: Manufacturers/Vendors (of CPA) representative.
collected separately in bottles and sealed. Out of the redispersion, many complex phenomena take place: the balance quantity of the samples, containers for the particle sizes decrease and surface areas increase. In samples of 1 kg shall be prepared both for fines and particular, the interaction between oxygen and precious coarse catalyst and remaining quantity shall be trans- metals may lead to the formation of species that are ferred to drums and weight recorded.
mobile on the surface and reverse the process of ag- 8. From the above steps, the following are obtained: Sintering is normally physical, rather than chemi- • Net weight of ‘Off-Spec’ catalyst + the mesh cata- cal, in nature and, therefore, the magnitudes of thermal activation are quite different. Furthermore, ageing time • Weight of fines passed through mesh for each is important because it correlates both with sintering • Total weight of the ‘Spec Catalyst’ (fines and The kinetics of catalyst deactivation is a function of • 1 kg samples to be given as per Part (1).
temperature, time, pressure and the concentrations ofdifferent substances.
It is also important to ensure that the procedures are done by experienced and reliable technical personnel.
Deactivation by poisoning
This is applicable both for the consumer and the ven-dor.
Poisoning is defined as a loss of catalytic activity due to the chemisorption of impurities on the active CATALYST DEACTIVATION MECHANISMS
sites of the catalyst. Usually, a distinction is made bet-ween poisons and inhibitors.
If we could stop or minimize the deactivation pro- cess of a noble metal catalyst, it will save us huge reve- Poisons are substances that interact very strongly nues. The manufacturing cost of the end product also and irreversibly with the active sites, whereas the ad- would be less, with prospects of better markets. This sorption of inhibitors on the catalyst surface is weak will be the ideal scenario and everyone in the chemical and reversible. In the latter case, the catalytic activity industry would prefer to achieve such goals.
can be at least partly restored by regeneration. Thisirreversible/reversible or permanent/temporary nature Unfortunately, nothing in this world is indispensa- of deactivation and the regeneration possibility of a cata- ble. Everything has a particular life span, a deprecia- lyst are the main differences between poisoning and tion period and prone to get deactivated. Active pre- cious metals (palladium, platinum, rhodium) are well-known catalysts for gas purification. Among the active However, the distinction between permanent and tem- metals, rhodium is known to be the most sensitive metal porary poisoning is not always so clear, since strong towards sintering at high temperatures. This leads to poisons at low temperatures may be less harmful in poor activity, especially in the reduction of nitro group.
The use of a bimetallic catalyst, such as Pd-Rh or Catalyst poisons can also be classified as selective Pt-Rh, gives a better catalytic activity at high tempera- or non-selective. The description of a poison as selec- tures. The operating conditions also affect the sintering tive or non-selective is related to the nature of the sur- of active metals; for example, ageing atmosphere and face and the degree of interaction of the poison with the the oscillation between oxidizing and reducing atmos- surface. A poison can also be selective in one reaction, Redispersion
Poisoning of a catalyst as a result of the accumula- tion of impurities on the active sites is typically a slow This is an opposite process to sintering. During and irreversible phenomenon. The accumulation of poi- sons on the active sites blocks the access of reactants First, this blocks out the active compounds from reach- to these active sites. As a result of poisoning, the cata- ing the surface sites, and second, the deposits block the lytic activity may be decreased without affecting the internal pores in the catalyst. In many cases, the reac- selectivity, but often selectivity is also changed since tants and aromatic materials are primarily responsible some of the active sites are deactivated while others are for such type of scenario. Among these other deactiva- tion mechanisms, pore blocking is probably one of themost important mechanisms. Pore blocking is often con- In some cases, depending on the adsorbed poison, nected to fouling and is high on the catalyst’s surface.
the poisoned catalyst can be regenerated and its acti-vity can be at least partly restored. Active metal cata- At high temperatures, catalysts may suffer from the lysts are preferred in controlling of gas emissions, be- loss of active phase through volatilization. Metal loss cause they are less liable to sulfur poisoning than metal through direct volatilization is generally an insignifi- oxide catalysts. Precious metals have different types of cant route of catalyst deactivation. By contrast, metal resistance against poisoning. Palladium is more sensi- loss through the formation of volatile compounds is tive than platinum and rhodium to chemical deactiva- important over a wide range of conditions.
tion — in particular to poisoning by sulfur and lead.
Large amounts of catalytic materials can be trans- Climate conditions also affect the catalyst’s chemi- ported to either substrate where they can react, or into cal deactivation. Especially in countries, where the cold the gas phase where they are lost in the effluent gas weather and environment keep the catalyst’s tempera- stream. High volatility limits the selection of otherwise ture low during a long time period. This accelerates the useful catalytic materials. For example, the oxides of Pt, Pd and Rh formed during the reaction cycles arenot as volatile as other noble metal oxides, such as The stability against thermal and chemical deacti- vation can be improved by a proper choice of the cata-lyst material. In addition, the placement of the active The thermodynamics of volatilization and thermody- material in separate layers improves the durability.
namic equilibrium calculations are useful in the evalua-tion of the volatility of metals and metal oxides in order Other mechanisms of deactivation
to assess which materials are stable over long periodsat high temperatures. Thermodynamic equilibrium cal- There are other essential forms of deactivation of culations of the oxidation/reduction behaviour of pal- catalysts. For example, pore blockage, encapsulation ladium have shown that phase stability in a Pd/PdO of metal particles, volatilization of active compounds, system changes as a function of temperature and oxy- fouling, and metal-metal interactions.
gen partial pressure. The lower the pressure and thehigher the temperature, the more likely is the Pd phase.
High temperature ageing may result in deep encap- The vapour pressures of Pt, Pd and Rh as metals and sulation of sintered precious metal particles as the sur- metal oxides increases with temperature, and is also face area of the catalyst layer decreases. This is a seri- strongly dependent on the composition of the surround- ous type of deactivation because of its permanent na- ing atmosphere. Pd is volatile at temperatures around ture. The encapsulated metal particles cannot partici- 850°C and above, depending on the surrounding envi- pate in catalysis since they are inaccessible to the ronment. The orders of vapour pressures of active molecules. Fouling covers all phenomena where the surface is covered with a deposit, e.g. with reactantresidues or with mechanical wear. There are probably as many other mechanisms of fouling as there are reac- tions where such phenomenon is encountered with pre-cious metal catalysts. During this period carbonaceous Hence the vapour pressure of metallic Pd is clearly residues cover the active surface sites and decrease the several magnitudes higher than the vapour pressures of Pt and Rh, while as oxides, the situation is the reverse.
CONCLUSIONS
SEMINAR ON ‘OPTIMISING USAGE OF
PRECIOUS METAL CATALYSTS’
The main causes for catalyst deactivation and selec- There are many procedures which can be em- ployed in industry to optimise the usage of precious Reduction of active area i.e. sintering or migration.
This paper is intended to provide only a brief out- line of the applications and the challenges in the use of precious metal catalysts. More information on the subject of catalyst optimisation & recovery and Impurities in the feedstocks (reactants, hydrogen gas, the relevant reactions involving precious metal cata- lysts will be provided during a Seminar to be organ- If the reaction product is strongly adsorbed, then the ised by Practicals India Enterprises as per the fol- catalyst can get self-poisoned or self-inhibited.
The surrounding climate conditions, level of atmos-pheric pollution, etc.
Poisoning of the catalyst can lead to alteration in the Venue: Gujarat Bhavan, Sector 15, Vashi, Navi bond strength of the product through its electronic effects.
Also, the precious metal content in a poisoned pre- Contact: Mr. Ketan Patel at info@practicals cious metal catalyst is found to be lesser by around 8-10% than in a non-poisoned catalyst.

Source: http://chemicalweekly.com/news/2004-04-13/12177.pdf