Comparison of Products by Type of Hydrocarbon

Table 7.1 compares the products of thermal cracking and catalytic cracking of different type of hydrocarbons. Notably, high yields of C₁ and C₂ gaseous products (methane, ethane, and ethylene) from thermal cracking are contrasted with high yields of C₃- C₆, with small quantities of methane and essentially no olefins heavier than butylene, from catalytic cracking. Significant for the octane number of the gasoline fraction from the catalytic cracking of aliphatic hydrocarbons are the abundance of i-alkanes and significant concentration of aromatic compounds (BTX) that increase the octane number.

Table 7.1: Catalytic Cracking vs. Thermal Cracking
Hydrocarbons	Thermal Cracking	Catalytic Cracking
n-alkanes (e.g., C₁₆)	C₂is major product C₁in large quantities C₄-C₁₅olefins in moderate abundance	C₃-C₆are major light prods C₁in small quantities No olefins > C₄
Aliphatic	Little aromatization at 500^ºC No branched – chain alkanes present	Significant aromatization Abundance of branched – chain alkanes
n-0lefins	Slow double bond isomerization Little skeletal isomerization	Rapid isomerization C=C–C–C→ C–C=C–C Rapid skeletal isomerization $C–C=C–C \to C= \begin{matrix} C \\ \overset{\|}{C} \end{matrix} –C$
Alkylaromatics	ß – scission	$α – scission (dealkylation)$
Naphthenes	Crack more slowly than n-paraffins	Crack at comparable rates with n-paraffins

As discussed in Lesson 6, the slow isomerization of free radicals (moving the unpaired electron from an edge atom to the interior atoms) results in the production of shorter straight-chain alkanes and straight-chain olefins in thermal cracking, thus leading to low octane numbers of the gasoline product. In contrast to free radicals, the isomerization of carbocations is very fast because of the thermodynamic driving force, shown in Table 7.2. One can see in Table 7.2 that the isomerization of a primary propyl carbenium ion to a secondary propyl carbenium ion releases (19.1- 1.5) = 17.6 kcal/mol. This is a very large thermodynamic driving force for the isomerization of a primary ion to a secondary ion, and further to a tertiary ion, with even a larger driving force. Isomerization of the secondary propyl ion to the tertiary propyl ion, releases 1.5 kcal/mol of energy. It is, therefore, clear that the initiation and propagation of carbocations in catalytic cracking chain reactions on the catalyst surfaces will be dominated by the formation of secondary and tertiary carbocations. The reactions of these carbocations lead to the formation of branched-chain alkanes and olefins with high octane numbers.

Table7.2: ΔH_f of Carbenium Ions
Carbenium Ions	ΔH_f(relative) (kcal/mol)
C₁+ primary $C–C– C_{\|}^{\|} \oplus$	19.1
C₂+ secondary $C– \overset{\oplus}{C} - C$	1.5
C₃+ tertiary $C– C_{\oplus}^{\begin{matrix} C \\ \| \end{matrix}} - C$	0

Another important feature of carbocation formation is the differences in the enthalpy of formation which favors the formation of carbocations > C₃ versus C₁ and C₂ ions (Table 7.3), because C₁ and C₂ ions are primary ions. This explains the low yields of C₁ and C₂ species obtained from catalytic cracking.

Table 7.3: ΔH_f of Carbenium Ions
Carbenium Ions	ΔH_f(relative) (kcal/mol)
CH₃⊕	258
C₂H₅⊕	225
n-C₃H₇⊕	218
i-C₃H₇⊕	198
n-C₄H₉⊕primary	211
t-C₄H₉⊕ tertiary	174

Comparison of Products by Type of Hydrocarbon