On positive carbon ions and free radicals

Thermal cracking proceeds according to a free radical reaction mechanism. At 400–600°C, large molecular alkanes break down into smaller molecular alkanes and alkenes ； Naphthenes break down into smaller molecules or undergo dehydrogenation to form aromatics, whose side chains are more prone to breaking ； The rings of aromatic hydrocarbons are difficult to break; side chain cleavage is the main process that occurs. Thermal cracking gases are characterized by a high content of methane and ethane-ethylene components. The catalytic cracking reaction can be explained by the positive carbocation mechanism. Positive carbocations refer to hydrocarbon ions formed when a carbon atom lacks a pair of valence electrons; such positive carbocations cannot exist freely and must adsorb on the catalyst surface to carry out reactions. Carbocations are formed as a result of the interaction between a catalyst and an alkene molecule. In the presence of an acidic catalyst, much less energy is required to generate carbocations compared to the energy needed for thermal cracking to produce radicals. In the absence of a catalyst, thermal cracking is a gas-phase thermal reaction, and in this case much more energy is required to form carbocations than to break down the molecules into radicals; as a result, the hydrocarbon molecules break down evenly into radicals, following the mechanism of radical reactions. At this point, the active sites of the catalyst provide protons, which protonate the alkene to form carbocations. Two conditions must be met for carbocations to be formed: first, there must be an alkene (either from the raw materials or from the products of thermal cracking), and second, there must be an acidic catalyst that can provide protons. After the formation of the carbocation, a series of parallel and sequential reactions take place; the reaction process is complex. The main characteristics of these reactions are as follows: (1) The formation of the carbocation can occur through the reaction between an alkene and a proton ； The smaller positive carbocation recombines with the alkene to form a larger positive carbocation ； (2) Primary carbocations can undergo spontaneous isomerization; primary carbocations can be converted into secondary carbocations, which in turn can be converted into tertiary carbocations. The stability order of carbocations is: tertiary > secondary > primary, and ultimately isomeric hydrocarbons are formed ； (3) The carbocation encounters a hydrocarbon molecule, takes away its hydrogen, forming a new carbocation and thus initiating a chain reaction ； (4) A carbocation can lose a proton to form an alkene; this proton is either returned to the acidic center of the catalyst or given to another alkene, resulting in the formation of a new carbocation while the original carbocation becomes the alkene product ； (5) The large positive carbocation decomposes to yield an alkene and a small positive carbocation, that is, a cleavage reaction occurs ； (6) The proparylium ion undergoes self-reaction, resulting in a cyclization reaction.

Carbocation mechanism: A proton is donated (either by a hydrocarbon that has already formed a carbocation or by another source); the hydrocarbon gains a proton and becomes a carbocation, which then reaches a stable state and releases a proton. Radical mechanism: Chain initiation, chain propagation, chain termination