Organic chemistry Part 10: Reactivity and reaction mechanisms

10.2.4 An ionic mechanism for catalytic cracking of alkanes

(Not needed for pre-university/college courses)

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10.2.4 An ionic mechanism for catalytic cracking of alkanes

(a) Catalytic cracking occurs at lower temperatures over a ceramic/zeolite catalytic material based on silica and/or aluminium oxide (I've called it Z in the mechanisms). The mechanism is believed to be ionic and some examples of reaction steps are given below. Z represents a matrix element of the alumino-silicate catalyst and can act in a variety ways e.g. Lewis acid or base, Bronsted-Lowry acid or base. R₁, R₂, R₃  = alkyl groups.

Step 1 Initiation: This can occur via an alkane or an alkene previously formed from an alkane.

(i) R₁-CH₂-CH₂-R₂ + Z⁺ ==> R₁-CH₂-CH⁺-R₂ + HL 

(ii) R₁-CH=CH-R₂ + HZ ==> R₁-CH₂-CH⁺-R₂ + Z^- 

Step 2 Propagation: This example shows chain scission forming a 'smaller' alkene and a 'smaller' carbocation to continue the chain.

R₁-CH₂-CH⁺-R₂ ==> R₁⁺ + CH₂=CH-R₂ 

Step 3 Termination: A lower alkene or alkane is formed.

R₁⁺ + Z^- ==> R₃CH=CH₂  + ZH

(R₃ < R₁ in alkene, but no scission, so same chain length overall)

R₁⁺ + ZH ==> R₁H  + Z⁺ 

(no bond scission, so R₁ same length in carbocation or alkane formed)

(b) A catalytic 'ionic' cracking cycle via alkenes:

I found this example in the literature, and the aluminosilicate/zeolite matrix (Z) acts alternately as an acid in step 1 and a base in step 4. Unfortunately it does involve starting with a rather a 'big' alkene called 2,4,4-trimethylpent-1-ene (it was the only example I could find in a local university library).

Step 1 initiation

(CH₃)₃C-CH₂-C(CH₃)=CH₂ + HZ ==> (CH₃)₃C-CH₂-C⁺(CH₃)₂ + Z^- 

The 'big' alkene is protonated by the catalyst and forms a 'big' carbocation (tertiary, most stable).

Step 2 propagation:

(CH₃)₃C-CH₂-C⁺(CH₃)₂ ==> (CH₃)₃C⁺ + CH₂=C(CH₃)₂ 

The 'big' carbocation from step 1 splits into 2-methylpropene (a cracking product) and a 'smaller' carbocation that continues the chain in step 3.

Step 3 propagation:

(CH₃)₃C-CH₂-C(CH₃)=CH₂ + (CH₃)₃C⁺ ==> (CH₃)₃C-CH₂-C⁺(CH₃)₂ + CH₂=C(CH₃)₂ 

From another 'big' starter alkene, and the carbocation from step 2, another molecule of 2-methylpropene is formed and the 'big' carbocation that was also formed in step 1, so allowing the 'ionic chain' reaction to continue via steps 2 and 3.

Step 4 termination:

e.g. (CH₃)₃C⁺ + Z^- ==> CH₂=C(CH₃)₂ + HZ 

The 'smaller' carbocation from step 2 is deprotonated to form another molecule of 2-methylpropene, and HZ is ready for step 1 again.

(c) FURTHER COMMENTS

I found little help for ionic cracking mechanisms in either textbooks or the internet, I'm quite happy with the catalytic cycle in (b), but frankly I found the descriptions for (a) not easy to follow and cannot vouch for their absolute authenticity. I just my best to make sense of it and illustrate how the ionic mechanism might operate. (I wonder what does the IB syllabus require? Can anyone help me on this one?)