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

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

email doc brown - comments - query?

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. 450^oC to 900^oC 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 450^oC to 500^oC. 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.

(i) 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

 

(ii)  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

(iii) 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

(iv) 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 >600^oC 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  ===> 2•CH₃  (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 150^oC

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 

(v)  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

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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 500^oC. 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 in old 'Kekule' style

(ii)  heptane  or  2-methylhexane  or  3-methylhexane ===> methylcyclohexane ===> methylbenzene

or 

or

 +  H₂        +  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

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.

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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