Hydrogenation Using Precious Metal Catalysts
Practicals India Enterprises, 11, My-Nest, Sector 2, Vashi, Navi Mumbai 400 703.
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ypically 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
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
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-
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
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,
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.
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.
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