Chemistry of Catalytic Cracking

Chemistry of Catalytic Cracking rak189 Thu, 06/06/2013 - 15:07

As opposed to thermal cracking governed by free radicals, catalytic cracking proceeds through the formation of ionic species on catalyst surfaces, and produces shorter, but branched-chain (not straight-chain) alkanes by cracking the long straight-chain alkanes. The formation of branched-chain alkanes, or iso-alkanes, leads to the production of gasoline with high octane numbers. This is the fundamental reason why catalytic cracking has replaced thermal cracking as the central process in a refinery geared to maximize gasoline production. A high octane number of gasoline is needed for current spark-ignition engines to run at high compression ratios without knocking. High compression ratios in spark-ignition engines translate to high power and high efficiency.

Figure 7.1 introduces the two types of ionic species, carbocations, that are active in catalytic cracking reactions as carbenium, and carbonium ions, using the IUPAC terminology. Carbocations are the positively charged ions made from hydrocarbons. Figure 7.1 shows that removing a hydride ion (H^-, a hydrogen atom with an additional electron) from an alkane (e.g., methane) produces carbenium ions (path 1a). Also, adding a proton (H⁺, a hydrogen atom without the electron) to an olefin (e.g., ethylene) can produce carbenium ions, as shown in path 1b.

Formation of Carbocations. See accessible description below

Figure 7.1. Definition and formation of carbocations from hydrocarbons.

Click here for a text alternative to the figure above

Formation of Carbocations

IUPAC terminology

-Carbenium ions: CH₄ (arrow) H^- + CH3⁺ (alkane – hydride ion)

-C₂H₄ + H⁺ (arrow) C₂H₅⁺. (olefin + proton)

-Carbonium Ions: CH₄ + H⁺(arrow) CH4

KEY: H^-(hydride ion), H⁺ (proton)

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Analogous to the terminology used for free radicals, C⁺H₃ is called methyl carbenium ion, and C₂⁺H₅ is called an ethyl carbenium ion. Carbonium ions are produced by adding a proton to an alkane, say methane, as shown in Figure 7.1. The resulting ion C⁺H₅ is called methanium. Note that there is some confusion in the literature about naming the carbocations. Carbenium ions used to be called carbonium ions in some sources, including your textbook [2]. All references to carbonium ions in Section 6.3 Cracking Reactions in the textbook should be corrected as carbenium ions.

Bronsted and Lewis Acid Sites

Bronsted and Lewis Acid Sites azs2 Wed, 05/07/2014 - 15:57

Carbocations are formed from hydrocarbons on two different acid sites: Bronsted acid sites and Lewis acid sites. You should remember that Bronsted acid sites donate protons, while Lewis acid sites accept electrons to form carbocations from hydrocarbons. Figure 7.2 illustrates how an olefin (e.g., ethylene, C=C) produces an ethyl carbenium ion (C⁺₂H₅) by reacting with a proton donated from Bronsted acid site. Alternatively, also seen in Figure 7.2, a Lewis acid site accepts an electron (or a hydride ion, H^-) from an alkane (e.g., ethane, C-C) to produce the same ethyl carbenium ion (C⁺₂H₅). These two reactions that take place on the acid sites of catalysts, along with the formation of carbonium ions by protonation of hydrocarbons on Bronsted sites, function as the initiation steps in the ionic chain reactions that lead to the products obtained from catalytic cracking.

Formation of carbocations on different catalysts. Bronsted acids donate protons, lewis acid sites accept electrons. More info in text above

Figure 7.2. Formation of carbocations on different acid sites of catalysts.

Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Distribution of Products

Distribution of Products azs2 Wed, 05/07/2014 - 15:57

Figure 7.3 compares the distribution of products from thermal cracking (free radical chain reactions) and catalytic cracking (ionic chain reactions). Short chain paraffins constitute the principal products in both cases, with one important difference – an abundance of iso-alkanes (branched-chain alkanes) in catalytic cracking products. One can also note in Figure 7.3 that catalytic cracking products contain higher concentration of aromatic compounds. High octane number of gasoline produced by catalytic cracking can be attributed to high concentrations of i-alkanes and relatively more abundant aromatics present in the crackate (catalytic cracking product). Having no olefins larger than butylene (C₄) from catalytic cracking processes, also distinguishes catalytic cracking products from thermal cracking products obtained from gas oil.

Products of Thermal and Catalytic Cracking. See text description below

Figure 7.3. Differences in the distribution of hydrocarbons obtained from thermal and catalytic cracking reactions.

Click here for an alternative text description of the image above

Products of Thermal and Catalytic Cracking

Thermal Cracking – Free radical reactions

P araffins

O lefins

N aphthenes

A romatics

Catalytic Cracking – Carbocation reaction

High-octane gasoline because of branching in alkanes

P araffins

I -paraffins

A romatics

N aphthenes

O lefins

Comparison of Products by Type of Hydrocarbon

Comparison of Products by Type of Hydrocarbon azs2 Wed, 05/07/2014 - 15:59

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: ∆H_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

Ionic Chain Reactions

Ionic Chain Reactions azs2 Wed, 05/07/2014 - 16:00

Figure 7.4 illustrates the ionic chain reactions that govern catalytic cracking of hydrocarbons. The initiation step includes the formation of a carbonium ion by proton donation from a Bronsted acid site and/or the formation of a carbenium ion through hydride ion abstraction by a Lewis acid site. In a propagation step, the carbonium ion goes through cracking to produce an alkane product and a carbenium ion, while the carbenium ion produced on the Lewis acid site goes through a β-scission to produce an olefin product and another carbenium ion. In additional propagation reactions, carbenium ions (secondary, or tertiary) react with alkanes to produce i-alkane products and other carbenium ions, which can go through isomerization reactions generating more stable ions. Finally, in termination steps, carbenium ions donate a proton to restore a Bronsted acid site and produce an olefin as final product, or they abstract a hydride ion to restore a Lewis acid site producing an i-alkane product, and the ionic chain reaction continues. Other reactions during catalytic cracking include dehydrocyclization and dehydrogenation reactions to produce aromatic compounds. One should note that thermal cracking reactions also take place during catalytic cracking because of the sufficiently high temperatures used in the process. Some claim that initial thermal cracking of alkanes to produce olefins should also be considered as an initiation step in ionic chain reactions [2].

Diagram of carbocationic chain reactions in catalytic cracking. More info in text above

Figure 7.4. Carbocationic chain reactions in catalytic cracking.