Hydrogenation – to treat with hydrogen – is a chemical reaction
between molecular hydrogen (H2) and another compound or element,
usually in the presence of a catalyst such as nickel, palladium or
platinum. The process is commonly employed to reduce or saturate
Hydrogenation typically constitutes the addition of
pairs of hydrogen atoms to a molecule, often an alkene. Catalysts are
required for the reaction to be usable; non-catalytic hydrogenation
takes place only at very high temperatures.
double and triple bonds in hydrocarbons.
1.1 Related or competing reactions
1.4.1 Homogeneous catalysts
1.4.2 Heterogeneous catalysts
1.5 Transfer hydrogenation
1.6 Electrolytic hydrogenation
2 Thermodynamics and mechanism
2.1 Heterogeneous catalysis
2.2 Homogeneous catalysis
3 Inorganic substrates
4 Industrial applications
4.2 Petrochemical industry
4.3 Organic chemistry
Hydrogenation of coal
5.1 Heterogeneous catalytic hydrogenation
5.2 Homogeneous catalytic hydrogenation
6 Metal-free hydrogenation
7 Equipment used for hydrogenation
7.1 Batch hydrogenation under atmospheric conditions
7.2 Batch hydrogenation at elevated temperature and/or pressure
7.3 Flow hydrogenation
7.4 Industrial reactors
8 See also
10 Further reading
11 External links
It has three components, the unsaturated substrate, the hydrogen (or
hydrogen source) and, invariably, a catalyst. The reduction reaction
is carried out at different temperatures and pressures depending upon
the substrate and the activity of the catalyst.
Related or competing reactions
The same catalysts and conditions that are used for hydrogenation
reactions can also lead to isomerization of the alkenes from cis to
trans. This process is of great interest because hydrogenation
technology generates most of the trans fat in foods (see below). A
reaction where bonds are broken while hydrogen is added is called
hydrogenolysis, a reaction that may occur to carbon-carbon and
carbon-heteroatom (oxygen, nitrogen or halogen) bonds. Some
hydrogenations of polar bonds are accompanied by hydrogenolysis.
For hydrogenation, the obvious source of hydrogen is H2 gas itself,
which is typically available commercially within the storage medium of
a pressurized cylinder. The hydrogenation process often uses greater
than 1 atmosphere of H2, usually conveyed from the cylinders and
sometimes augmented by "booster pumps". Gaseous hydrogen is produced
industrially from hydrocarbons by the process known as steam
reforming. For many applications, hydrogen is transferred from
donor molecules such as formic acid, isopropanol, and
dihydroanthracene. These hydrogen donors undergo dehydrogenation to,
respectively, carbon dioxide, acetone, and anthracene. These processes
are called transfer hydrogenations.
An important characteristic of alkene and alkyne hydrogenations, both
the homogeneously and heterogeneously catalyzed versions, is that
hydrogen addition occurs with "syn addition", with hydrogen entering
from the least hindered side. This reaction can be performed on a
variety of different functional groups.
Substrates for and products of hydrogenation
Heat of hydrogenation
large application is production of margarine
−90 to −130
semihydrogenation gives cis-RHC=CHR'
(for full hydrogenation)
often employs transfer hydrogenation
−60 to −65
often employs transfer hydrogenation
−60 to −65
RCH2OH + R'OH
often applies to production of fatty alcohols
−25 to −105
applicable to fatty alcohols
−25 to −75
major application is aniline
With rare exceptions, H2 is unreactive toward organic compounds in the
absence of metal catalysts. The unsaturated substrate is chemisorbed
onto the catalyst, with most sites covered by the substrate. In
heterogeneous catalysts, hydrogen forms surface hydrides (M-H) from
which hydrogens can be transferred to the chemisorbed substrate.
Platinum, palladium, rhodium, and ruthenium form highly active
catalysts, which operate at lower temperatures and lower pressures of
H2. Non-precious metal catalysts, especially those based on nickel
Raney nickel and Urushibara nickel) have also been developed
as economical alternatives, but they are often slower or require
higher temperatures. The trade-off is activity (speed of reaction) vs.
cost of the catalyst and cost of the apparatus required for use of
high pressures. Notice that the Raney-nickel catalysed hydrogenations
require high pressures:
Catalysts are usually classified into two broad classes: homogeneous
catalysts and heterogeneous catalysts. Homogeneous catalysts dissolve
in the solvent that contains the unsaturated substrate. Heterogeneous
catalysts are solids that are suspended in the same solvent with the
substrate or are treated with gaseous substrate.
Some well known homogeneous catalysts are indicated below. These are
coordination complexes that activate both the unsaturated substrate
and the H2. Most typically, these complexes contain platinum group
metals, especially Rh and Ir.
Homogeneous hydrogenation catalysts and their precursors
Dichlorotris(triphenylphosphine)ruthenium(II) is a precatalyst based
Crabtree's catalyst is a highly active catalyst featuring iridium.
Rh2Cl2(cod)2 is a precursor to many homogeneous catalysts.
(S)-iPr-PHOX is a typical chelating phosphine ligand used in
hydrogenation of propylene with Wilkinson's catalyst
Homogeneous catalysts are also used in asymmetric synthesis by the
hydrogenation of prochiral substrates. An early demonstration of this
approach was the Rh-catalyzed hydrogenation of enamides as precursors
to the drug L-DOPA. To achieve asymmetric reduction, these catalyst
are made chiral by use of chiral diphosphine ligands. Rhodium
catalyzed hydrogenation has also been used in the herbicide production
of S-metolachlor, which uses a Josiphos type ligand (called
Xyliphos). In principle asymmetric hydrogenation can be catalyzed
by chiral heterogeneous catalysts, but this approach remains more
of a curiosity than a useful technology.
Heterogeneous catalysts for hydrogenation are more common
industrially. In industry, precious metal hydrogenation catalysts are
deposited from solution as a fine powder on the support, which is a
cheap, bulky, porous, usually granular material, such as activated
carbon, alumina, calcium carbonate or barium sulfate. For example,
platinum on carbon is produced by reduction of chloroplatinic acid in
situ in carbon. Examples of these catalysts are 5% ruthenium on
activated carbon, or 1% platinum on alumina. Base metal catalysts,
such as Raney nickel, are typically much cheaper and do not need a
support. Also, in the laboratory, unsupported (massive) precious metal
catalysts such as platinum black are still used, despite the cost.
As in homogeneous catalysts, the activity is adjusted through changes
in the environment around the metal, i.e. the coordination sphere.
Different faces of a crystalline heterogeneous catalyst display
distinct activities, for example. This can be modified by mixing
metals or using different preparation techniques. Similarly,
heterogeneous catalysts are affected by their supports.
In many cases, highly empirical modifications involve selective
"poisons". Thus, a carefully chosen catalyst can be used to
hydrogenate some functional groups without affecting others, such as
the hydrogenation of alkenes without touching aromatic rings, or the
selective hydrogenation of alkynes to alkenes using Lindlar's
catalyst. For example, when the catalyst palladium is placed on barium
sulfate and then treated with quinoline, the resulting catalyst
reduces alkynes only as far as alkenes. The
Lindlar catalyst has been
applied to the conversion of phenylacetylene to styrene.
Selective hydrogenation of the less hindered alkene group in carvone
using a homogeneous catalyst (Wilkinson's catalyst).
Partial hydrogenation of phenylacetylene using the Lindlar catalyst.
Hydrogenation of an imine using a
Raney nickel catalyst, a popular
Partial hydrogenation of a resorcinol derivative using a Raney-Nickel
Hydrogenation of maleic acid to succinic acid.
Hydrogen also can be extracted ("transferred") from "hydrogen-donors"
in place of H2 gas.
Hydrogen donors, which often serve as solvents
include hydrazine, dihydronaphthalene, dihydroanthracene, isopropanol,
and formic acid.
In organic synthesis, transfer hydrogenation is useful for the
asymmetric reduction of polar unsaturated substrates, such as ketones,
aldehydes, and imines. The hydrogenation of polar substrates such as
ketones and aldehydes typically require transfer hydrogenation, at
least in homogeneous catalysis. These catalysts are readily generated
in chiral forms, which is the basis of asymmetric hydrogenation of
Transfer hydrogenation catalyzed by transition metal complexes
proceeds by an "outer sphere mechanism."
Polar substrates such as nitriles can be hydrogenated
electrochemically, using protic solvents and reducing equivalents as
the source of hydrogen.
Thermodynamics and mechanism
The addition of hydrogen to double or triple bonds in hydrocarbons is
a type of redox reaction that can be thermodynamically favorable. For
example, the addition of hydrogen to an alkene has a Gibbs free energy
change of -101 kJ·mol−1. However, the reaction rate for most
hydrogenation reactions is negligible in the absence of catalysts.
Hydrogenation is a strongly exothermic reaction. In the hydrogenation
of vegetable oils and fatty acids, for example, the heat released is
about 25 kcal per mole (105 kJ/mol), sufficient to raise the
temperature of the oil by 1.6–1.7 °C per iodine number drop.
The mechanism of metal-catalyzed hydrogenation of alkenes and alkynes
has been extensively studied. First of all isotope labeling using
deuterium confirms the regiochemistry of the addition:
RCH=CH2 + D2 → RCHDCH2D
On solids, the accepted mechanism is the Horiuti-Polanyi
Binding of the unsaturated bond, and hydrogen dissociation into atomic
hydrogen onto the catalyst
Addition of one atom of hydrogen; this step is reversible
Addition of the second atom; effectively irreversible under
In the second step, the metallointermediate formed is a saturated
compound that can rotate and then break down, again detaching the
alkene from the catalyst. Consequently, contact with a hydrogenation
catalyst necessarily causes cis-trans-isomerization, because the
isomerization is thermodynamically favorable. This is a problem in
partial hydrogenation, while in complete hydrogenation the produced
trans-alkene is eventually hydrogenated.
For aromatic substrates, the first bond is hardest to hydrogenate
because of the free energy penalty for breaking the aromatic system.
The product of this is a cyclohexadiene, which is extremely active and
cannot be isolated; in conditions reducing enough to break the
aromatization, it is immediately reduced to a cyclohexene. The
cyclohexene is ordinarily reduced immediately to a fully saturated
cyclohexane, but special modifications to the catalysts (such as the
use of the anti-solvent water on ruthenium) can preserve some of the
cyclohexene, if that is a desired product.
In many homogeneous hydrogenation processes, the metal binds to
both components to give an intermediate alkene-metal(H)2 complex. The
general sequence of reactions is assumed to be as follows or a related
sequence of steps:
binding of the hydrogen to give a dihydride complex via oxidative
addition (preceding the oxidative addition of H2 is the formation of a
LnM + H2 → LnMH2
binding of alkene:
LnM(η2H2) + CH2=CHR → Ln-1MH2(CH2=CHR) + L
transfer of one hydrogen atom from the metal to carbon (migratory
Ln-1MH2(CH2=CHR) → Ln-1M(H)(CH2-CH2R)
transfer of the second hydrogen atom from the metal to the alkyl group
with simultaneous dissociation of the alkane ("reductive elimination")
Ln-1M(H)(CH2-CH2R) → Ln-1M + CH3-CH2R
The hydrogenation of nitrogen to give ammonia is conducted on a vast
scale by the
Haber-Bosch process, consuming an estimated 1% of the
world's energy supply.
displaystyle ce underset nitrogen N equiv N + underset
hydrogen atop (200atm) 3H2 ->[ ce Fe catalyst ][350-550^ circ
ce C ] underset ammonia 2NH3
Oxygen can be partially hydrogenated to give hydrogen peroxide,
although this process has not been commercialized
Catalytic hydrogenation has diverse industrial uses. Most frequently,
industrial hydrogenation relies on heterogeneous catalysts.
Types of fats in food
Essential fatty acid
Conditionally essential fatty acid
The largest scale application of hydrogenation is for the processing
of vegetable oils. Typical vegetable oils are derived from
polyunsaturated fatty acids (containing more than one carbon-carbon
double bond). Their partial hydrogenation reduces most, but not all,
of these carbon-carbon double bonds. The degree of hydrogenation is
controlled by restricting the amount of hydrogen, reaction temperature
and time, and the catalyst.
Partial hydrogenation of a typical plant oil to a typical component of
margarine. Most of the C=C double bonds are removed in this process,
which elevates the melting point of the product.
Hydrogenation converts liquid vegetable oils into solid or semi-solid
fats, such as those present in margarine. Changing the degree of
saturation of the fat changes some important physical properties, such
as the melting range, which is why liquid oils become semi-solid.
Solid or semi-solid fats are preferred for baking because the way the
fat mixes with flour produces a more desirable texture in the baked
product. Because partially hydrogenated vegetable oils are cheaper
than animal fats, are available in a wide range of consistencies, and
have other desirable characteristics (such as increased oxidative
stability and longer shelf life), they are the predominant fats used
as shortening in most commercial baked goods.
A side effect of incomplete hydrogenation having implications for
human health is the isomerization of some of the remaining unsaturated
carbon bonds, resulting in the trans isomers, which have been
implicated in circulatory diseases including heart disease. The
conversion from cis to trans bonds is favored because the trans
configuration has lower energy than the natural cis one. At
equilibrium, the trans/cis isomer ratio is about 2:1. Many countries
and regions have introduced mandatory labeling of trans fats on food
products and appealed to the industry for voluntary
In petrochemical processes, hydrogenation is used to convert alkenes
and aromatics into saturated alkanes (paraffins) and cycloalkanes
(naphthenes), which are less toxic and less reactive. Relevant to
liquid fuels that are stored sometimes for long periods in air,
saturated hydrocarbons exhibit superior storage properties. On the
other hand, alkene tend to form hydroperoxides, which can form gums
that interfere with fuel handing equipment. For example, mineral
turpentine is usually hydrogenated.
Hydrocracking of heavy residues
into diesel is another application. In isomerization and catalytic
reforming processes, some hydrogen pressure is maintained to
hydrogenolyze coke formed on the catalyst and prevent its
Hydrogenation is a useful means for converting unsaturated compounds
into saturated derivatives. Substrates include not only alkenes and
alkynes, but also aldehydes, imines, and nitriles, which are
converted into the corresponding saturated compounds, i.e. alcohols
and amines. Thus, alkyl aldehydes, which can be synthesized with the
oxo process from carbon monoxide and an alkene, can be converted to
1-propanol is produced from propionaldehyde, produced
from ethene and carbon monoxide. Xylitol, a polyol, is produced by
hydrogenation of the sugar xylose, an aldehyde. Primary amines can be
synthesized by hydrogenation of nitriles, while nitriles are readily
synthesized from cyanide and a suitable electrophile. For example,
isophorone diamine, a precursor to the polyurethane monomer isophorone
diisocyanate, is produced from isophorone nitrile by a tandem nitrile
hydrogenation/reductive amination by ammonia, wherein hydrogenation
converts both the nitrile into an amine and the imine formed from the
aldehyde and ammonia into another amine.
Hydrogenation of coal
Main article: Bergius process
Heterogeneous catalytic hydrogenation
The earliest hydrogenation is that of platinum catalyzed addition of
hydrogen to oxygen in the Döbereiner's lamp, a device commercialized
as early as 1823. The French chemist Paul Sabatier is considered the
father of the hydrogenation process. In 1897, building on the earlier
work of James Boyce, an American chemist working in the manufacture of
soap products, he discovered that traces of nickel catalyzed the
addition of hydrogen to molecules of gaseous hydrocarbons in what is
now known as the Sabatier process. For this work, Sabatier shared the
1912 Nobel Prize in Chemistry.
Wilhelm Normann was awarded a patent in
Germany in 1902 and in Britain in 1903 for the hydrogenation of liquid
oils, which was the beginning of what is now a worldwide industry. The
commercially important Haber–Bosch process, first described in 1905,
involves hydrogenation of nitrogen. In the Fischer–Tropsch process,
reported in 1922 carbon monoxide, which is easily derived from coal,
is hydrogenated to liquid fuels.
In 1922, Voorhees and Adams described an apparatus for performing
hydrogenation under pressures above one atmosphere. The Parr
shaker, the first product to allow hydrogenation using elevated
pressures and temperatures, was commercialized in 1926 based on
Voorhees and Adams' research and remains in widespread use. In 1924
Murray Raney developed a finely powdered form of nickel, which is
widely used to catalyze hydrogenation reactions such as conversion of
nitriles to amines or the production of margarine.
Homogeneous catalytic hydrogenation
In the 1930s, Calvin discovered that copper(II) complexes oxidized H2.
The 1960s witnessed the development of well defined homogeneous
catalysts using transition metal complexes, e.g., Wilkinson's catalyst
(RhCl(PPh3)3). Soon thereafter cationic Rh and Ir were found catalyze
the hydrogenation of alkenes and carbonyls. In the 1970s,
asymmetric hydrogenation was demonstrated in the synthesis of L-DOPA,
and the 1990s saw the invention of Noyori asymmetric
hydrogenation. The development of homogeneous hydrogenation was
influenced by work started in the 1930s and 1940s on the oxo process
and Ziegler–Natta polymerization.
For most practical purposes, hydrogenation requires a metal catalyst.
Hydrogenation can, however, proceed from some hydrogen donors without
catalysts, illustrative hydrogen donors being diimide and aluminium
isopropoxide, the latter illustrated by the
Meerwein–Ponndorf–Verley reduction. Some metal-free catalytic
systems have been investigated in academic research. One such system
for reduction of ketones consists of tert-butanol and potassium
tert-butoxide and very high temperatures. The reaction depicted
below describes the hydrogenation of benzophenone:
A chemical kinetics study found this reaction is first-order in
all three reactants suggesting a cyclic 6-membered transition state.
Another system for metal-free hydrogenation is based on the
phosphine-borane, compound 1, which has been called a frustrated Lewis
pair. It reversibly accepts dihydrogen at relatively low temperatures
to form the phosphonium borate 2 which can reduce simple hindered
The reduction of nitrobenzene to aniline has been reported to be
catalysed by fullerene, its mono-anion, atmospheric hydrogen and UV
Equipment used for hydrogenation
Today's bench chemist has three main choices of hydrogenation
Batch hydrogenation under atmospheric conditions
Batch hydrogenation at elevated temperature and/or pressure
Batch hydrogenation under atmospheric conditions
The original and still a commonly practised form of hydrogenation in
teaching laboratories, this process is usually effected by adding
solid catalyst to a round bottom flask of dissolved reactant which has
been evacuated using nitrogen or argon gas and sealing the mixture
with a penetrable rubber seal.
Hydrogen gas is then supplied from a
H2-filled balloon. The resulting three phase mixture is agitated to
Hydrogen uptake can be monitored, which can be useful
for monitoring progress of a hydrogenation. This is achieved by either
using a graduated tube containing a coloured liquid, usually aqueous
copper sulfate or with gauges for each reaction vessel.
Batch hydrogenation at elevated temperature and/or pressure
Since many hydrogenation reactions such as hydrogenolysis of
protecting groups and the reduction of aromatic systems proceed
extremely sluggishly at atmospheric temperature and pressure,
pressurised systems are popular. In these cases, catalyst is added to
a solution of reactant under an inert atmosphere in a pressure vessel.
Hydrogen is added directly from a cylinder or built in laboratory
hydrogen source, and the pressurized slurry is mechanically rocked to
provide agitation, or a spinning basket is used. Heat may also be
used, as the pressure compensates for the associated reduction in gas
Flow hydrogenation has become a popular technique at the bench and
increasingly the process scale. This technique involves continuously
flowing a dilute stream of dissolved reactant over a fixed bed
catalyst in the presence of hydrogen. Using established HPLC
technology, this technique allows the application of pressures from
atmospheric to 1,450 psi (100 bar). Elevated temperatures
may also be used. At the bench scale, systems use a range of
pre-packed catalysts which eliminates the need for weighing and
filtering pyrophoric catalysts.
Catalytic hydrogenation is done in a tubular plug-flow reactor (PFR)
packed with a supported catalyst. The pressures and temperatures are
typically high, although this depends on the catalyst. Catalyst
loading is typically much lower than in laboratory batch
hydrogenation, and various promoters are added to the metal, or mixed
metals are used, to improve activity, selectivity and catalyst
stability. The use of nickel is common despite its low activity, due
to its low cost compared to precious metals.
Gas Liquid Induction Reactors (Hydrogenator) are also used for
carrying out catalytic hydrogenation.
Carbon neutral fuel
Hydrodesulfurization, hydrotreater and oil desulfurization
Timeline of hydrogen technologies
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Wikiquote has quotations related to: Hydrogenation
"The Magic of Hydro" Popular Mechanics, June 1931, pp. 107–109 –
early article for the general public on hydrogenation of oil produces
in the 1930s