Part 1.
ALKANES and the PETROCHEMICAL INDUSTRY
1.3 Modification of alkanes by
cracking, isomerisation and reforming -
how to make better use of the chemical feedstock
from the fractional distillation of crude oil, thermal/catalytic cracking, use
of zeolite catalysts
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ALL my advanced A level organic
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All my advanced A level
ALKANE
chemistry notes
Index of GCSE level Oil - Useful Products
Revision Notes
What is cracking? Why is cracking
important? What are the products of cracking? What are the products
of cracking used for? What is reforming? Why is reforming important?
What are the products of reforming? What are the products of
reforming used for?
Sub-index:
Introduction *
Cracking (thermal
and catalytic) *
Isomerisation *
Reforming
See also
1.5
Modification of hydrocarbon fuel
mixtures and alternative fuels
A basic introduction to
the chemistry of
alkanes
Introduction to CRACKING - a
problem of supply and demand and other products.
Introduction
Petrol (gasoline) contains a mixture of
hydrocarbons, with 5 to 10 carbon atoms, but many the products of
fractional distillation do not completely suit the desired mixture
for a fuel.
There are too many larger hydrocarbons to
be used as petrol or diesel, they are mainly alkanes and
there are no alkenes for polymer production and other important
products derived from unsaturated hydrocarbons.
Therefore the secondary processes in the petrochemical
industry address
See basic discussion for the need
of cracking see
CRACKING - a
problem of supply and demand, other products
As well as cracking to produce alkenes and lower alkanes,
this advanced discussion
below does include isomerisation and reforming which are described in
detail below.
TOP OF PAGE
and sub-index
There
are two main types of CRACKING
Cracking is essentially the thermal decomposition of
hydrocarbons such as alkanes into molecules of smaller carbon number, namely
lower alkanes as chemical feedstock for other processes and alkenes for polymer
production.
Cracking does involve breaking a
strong C-C bond in the alkane to produce smaller molecules.
You therefore need to employ high reaction
temperatures and catalysts to effect the decomposition
efficiently.
Cracking and reforming reactions are quite varied in
their products e.g.
larger alkanes and other hydrocarbons can be
converted to smaller linear/branched alkanes, linear/branched alkenes,
cycloalkanes, cycloalkenes and aromatic hydrocarbons (arenes).
Hydrogen
is also produced in some reactions.
Branched and cyclic alkane and aromatic hydrocarbon
compounds are important components in
petrol
mixtures designed to produce the cleanest most efficient burning,
with good antiknock properties, particularly as lead tetraethyl is now
banned.
The hydrogen can be used in other chemical processes e.g.
Haber synthesis of ammonia and hydrogenating vegetable oils to make
margarine - nothing wasted, all helps the economics of the petrochemical
industry.
(a)
Thermal cracking (steam
cracking)
Thermal cracking is done at high temperatures e.g. 450oC to 900oC and high
pressure (up to 7000 kPa, 70 atm = 70 x normal atmospheric pressure), with no catalyst
and the absence of air.
Naphtha (C6-C10) and kerosene
(C10 - C16) are the chemical feedstocks and the vaporised
hydrocarbons are exposed to the high temperature for just a short time.
Steam is added as a diluent to prevent
'coking' (carbon deposit on reactor surface).
Reaction conditions can be set to
maximise alkene
production - remember, alkenes are NOT found in oil but one of the most
important chemical feedstocks for polymer production and other organic
industrial products.
The alkanes are only heated for a few seconds at
these high temperatures, otherwise the hydrocarbons will break down into
carbon (soot) and hydrogen.
Typical products are ethene, ethane, propene, propane and
C4 - C5 alkanes and alkenes.
The products of thermal cracking depend on conditions
i.e. temperature and pressure are the variables.
At lower temperatures the alkane
carbon atom chain breaks nearer the middle of the molecule.
This produces a higher proportion
of medium sized straight chain alkanes and alkenes, all of which are
important raw chemicals (feedstock) for the chemical industry.
This gives less ethene and more high grade
petrol.
At higher temperatures the alkane
carbon atom chain breaks nearer the end of the molecule.
This produces a higher proportion
of smaller alkenes like ethene and propene.
You can get yields of ethene up to 30%, smaller
amounts of propene and up to 25% of high grade petrol.
The products from thermal cracking
are separated by fractional distillation.
(b)
Catalytic cracking
Catalytic cracking involves
passing a fraction like gas oil (C14 - C20 alkanes) over a
zeolite catalyst at 450oC to 500oC. The reactor pressure is slightly above
normal - very different from thermal cracking.
Zeolites are minerals composed of
alumino-silicates (Al, Si, O), found naturally or manufactured to have
specific catalytic properties.
Zeolites have microscopic pores running through the whole of the structure,
giving them a huge surface area (large surface area / volume ratio).
The zeolite is employed as a fluidised bed of solid
particles (fine particles can flow like a liquid) mixed in with the
vapourised hydrocarbons that pass up the 'catalytic cracker', a tall
reaction vessel called a riser.
The zeolite gets coated in carbon
from hydrocarbon decomposition, so it is collected and the carbon burned
off, this regenerates the zeolite catalyst and the heat is used to heat
up the catalyst which then heats up the incoming hydrocarbon feedstock,
so reducing thermal energy costs.
Catalytic cracking is more efficient than thermal
cracking and doesn't need as higher temperature or pressure.
Catalytic cracking produces more branched alkanes, cyclic
alkanes and cyclic aromatic compounds with a benzene ring (as well as some
alkenes too).
e.g.
branched alkane,
cycloalkane,
aromatic
Catalytic cracking is used to produce
hydrocarbons, like those above, to reduce the octane number of petrol fuels, e.g. you try to
maximise the yield of lower carbon number branched alkanes and some aromatic
hydrocarbons.
See also 1.5
Modification of hydrocarbon fuel
mixtures and alternative fuels
You can minimise the yield of unsaturated alkenes, but some
will still be produced.
Extra
note on the zeolite catalysts
Zeolites can also act as
molecular
sieves - which tells you exactly how small the pores can be!
Unbranched hydrocarbon molecules can enter the pores
but branched molecules cannot - they have a more 'bulky' cross-section!
Therefore zeolites allow the separation of branched
and unbranched hydrocarbons - two sub-types of raw materials.
Unbranched hydrocarbons are used in the manufacture
of detergents.
The detergent industry adds
15-30% zeolites to washing powders to remove calcium ions - this
softens the water giving a more efficient soap action.
Organic substances e.g. from stains, stick on to
the zeolite particles giving a cleaner washing action.
I presume the calcium ions and organic molecules
stick on the large surface of the zeolite pores?
(c)
Equations and examples of three ways in which hydrocarbons (mainly
alkanes) are modified to make them more useful
PLEASE READ my basic notes on
CRACKING - a problem of supply
and demand, other products (I'm NOT
repeating them here)
Included are several structural formula equation examples of cracking alkanes into
alkenes, lower alkanes and hydrogen.
I
won't repeat all the structural formula equations here, but add I've added more
above on the reaction conditions above, and showing the products using skeletal formula equations below.
e.g. cracking to produce alkenes (for polymer production) and lower alkanes for
fuels to reduce the octane number of petrol, and note the 'theoretical'
variety of products possible!
Note that carbon chain structural
isomers of lower alkanes and alkenes can be formed in the products, and
hydrogen too.
(1) cracking hexane ===> products
(a)
+
or
hexane ===> ethene
+ butane/methylpropane
CH3(CH2)4CH3
===> H2C=CH2 + CH3CH2CH2CH3
or CH(CH3)3
(b)
+
hexane ===>
propene + propane
CH3(CH2)4CH3
===> CH3CH=CH2 + CH3CH2CH3
(c)
or
or
+
hexane ===>
but-1-ene/but-2-ene/methylpropene + ethane
CH3(CH2)4CH3
===> CH3CH2CH=CH2 or
CH3CH==CHCH3 or (CH3)2C=CH2
+ CH3CH3
(d) hexane ===> butene +
ethene + hydrogen (note hydrogen is formed instead of
a lower alkane)
CH3(CH2)4CH3
===> CH3CH2CH=CH2 or
CH3CH=CHCH3 +
H2C=CH2
+ H2
(2) cracking octane ===>
products
(a)
+
octane ===>
propane + pent-1-ene
(b)
+
octane ===>
propene + pentane
(c)
2
+ H2
(could also be but-2-ene,
)
octane ===> but-1-ene + hydrogen or other
combinations
(3) Cracking a large alkane molecule to
give mainly alkenes (propene and ethene) and hydrogen
e.g. heptadecane:
CH3(CH2)15CH3
===> 3CH3CH=CH2 + 4CH2=CH2
+ H2
(4) The production of
more branched alkanes
from decane C10H22
e.g. various branched alkanes C7H16
and propene C3H6
,
,
,
,
+
There are at least five C7H14
branched isomers that can be formed - can you name them all?
Branched alkanes increase the octane number of
petrol.
See 1.5
Modification of hydrocarbon fuel
mixtures and alternative fuels
Reaction mechanisms for alkane cracking
reactions
Free radical mechanism for cracking
hydrocarbons (.g. propane) to
give shorter alkanes and alkenes
Ionic mechanism for cracking
hydrocarbons (definitely NOT needed for UK A level
chemistry)
Instead of repeating the mechanism
for propane (link above), I'm quoted what happens with ethane at high
temperatures. The free radical mechanism of converting ethane to
methane, ethene and hydrogen in a cracking reaction at >600oC
is shown below.
This is an example of thermal
decomposition or pyrolysis reaction. These can be quite
complicated, there are always several options in hydrocarbon cracking
mechanisms, but each step is easy to follow:
Initiation step
(formation of the first free radicals to initiate the chain reaction,
endothermic - bond breaking)
CH3CH3
===> 2CH3 (homolytic C-C bond fission
of the ethane molecule)
The C-C bond is weaker than the
C-H bond, so it is the first to break on collision of high kinetic
energy ethane molecules.
Propagation steps
(one free radical facilitates formation of another radical to keep the
chain reaction going)
CH3 +
CH3CH3 ===> CH4 +
CH3CH2 (the methyl radical
abstracts a proton to yield methane,
and an ethyl radical is
left to continue the chain)
CH3CH2
===> H2C=CH2 + H
(the ethyl radical loses a hydrogen atom to yield ethene,
and a hydrogen atom radical can
continue the chain reaction)
H + CH3CH3
===> H2 + CH3CH2
(the hydrogen radical abstracts a proton from ethane,
to give hydrogen and an ethyl radical can
continue the chain reaction)
Termination steps
(these remove radicals slowing or terminating the reaction, exothermic -
bond formation)
2CH3 ===>
CH3CH3 (two methyl radicals
form ethane)
2CH3CH2
===> CH3CH2CH2CH3
(two ethyl radicals form butane, evidence of mechanism)
2CH3CH2
===> CH3CH3 + H2C=CH2
(two ethyl radicals exchange a hydrogen atom to give ethane and
ethene)
TOP OF PAGE
and sub-index
ISOMERISATION (isomerization),
in some cases this is the first stage of reforming described
in the next section)
Converting linear alkane molecules into
branched alkane molecules is an example of an isomerisation
reaction because the reactant and products have the same
molecular formula. Branched alkanes increase the
octane number of petrol
fuels. The chemical feedstock is usually C4 to C6
alkanes.
Typical reaction conditions: Pt/Al2O3
at 150oC
Examples of isomerisation reactions - good
for making branched isomers.
(i) butane ===> methylpropane (all
C4H10)
(structural formula equation)
(skeletal formula equation)
(ii) pentane ===> methylbutane
(2-methylbutane) or dimethylpropane (2,2-dimethylpropane,
numbers not strictly needed)
or
or
(all C5H12)
(iii) hexane ===>
2-methylpentane
or 3-methylpentane
or 2,2-dimethylbutane or
2,3-dimethylbutane (all C6H14)
or
or
or
or
or
or
(iv)
heptane ===> 3-ethylpentane
or 2,2-dimethylpentane or 2,3-dimethylpentane
or 2,4-dimethylpentane
or
3,3-dimethylpentane or 2,2,3-trimethlybutane
or
or
or
or
or
or
By changing the reaction conditions you can vary,
and to some extent change, the ratio of products to select those
most desired from an isomerisation reaction.
See also 1.5
Modification of hydrocarbon fuel
mixtures and alternative fuels
TOP OF PAGE
and sub-index
REFORMING
Converting linear alkane molecules into
cyclic hydrocarbons (cycloalkanes and aromatic molecules - have a
benzene ring).
The products have the same number of carbon atoms
as the reactants, but less hydrogen atoms and a ring structure.
Naphtha is the chemical feedstock (C6
- C10 alkanes) and passed over a Pt/Al2O3
catalyst at 500oC. The hydrogen formed is recycled to
minimise 'coking' (thermal decomposition to give a carbon deposit on
the catalyst).
e.g
(i) hexane ===>
1st cyclohexane ===> 2nd benzene
1st
+
H2
+
H2
then 2nd
+
3H2
+
3H2
This reforming reaction probably goes through the
dehydrogenation
sequence ?
hexane ==> cyclohexane ==> cyclohexene ==>
cyclohexa-1,3-diene ==> benzene
or
finally
in old 'Kekule' style
(ii) heptane
or 2-methylhexane
or 3-methylhexane ===>
methylcyclohexane ===> methylbenzene
or
or
+ 3H2
methylbenzene is usually shown as
the benzene ring is 'skeletal' and the methyl group 'structural'
in terms of skeletal formulae this reforming reaction
would be shown as ...
or
or
+
H2
+
3H2 (dehydrogenation reactions)
If asked to write the full equation don't
forget the hydrogens!
and check whether structural formulae or
skeletal formulae are required.
(iii)
You can convert pentane to
cyclopentane:
===>
+ H2
Extra note on an example of an aromatic
disproportionation reaction
Disproportionation is said to occur, when a
reactant is transformed into two or more dissimilar products with no other
products and both products are directly derived from the original
molecule.
2
+
,
,
(mixture of three isomeric products)
This is not a redox disproportionation
reaction, but it does involve two identical molecules reacting together to give
two different products with no other reactant or product involved. The reaction
is a way of producing a mixture of 1,2-dimethylbenzene, 1,3-dimethylbenzene and
1,4-dimethylbenzene from methylbenzene. If you need less methylbenzene and more
benzene or dimethylbenzenes, this is the way to do it.
See also 1.5
Modification of hydrocarbon fuel
mixtures and alternative fuels
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