In petroleum refining, the mechanism of isomerization catalysts involves the rearrangement of hydrocarbon molecules without altering their molecular formula. This process is primarily aimed at improving the octane rating of gasoline or converting linear paraffins into branched-chain paraffins to enhance cold flow properties and combustion efficiency.

Types of Catalysts

Commonly used isomerization catalysts include:

• Platinum-based catalysts: Such as Pt/Al2O3 (platinum on alumina support), which are typically employed for the isomerization of normal paraffins.
• Bifunctional catalysts: Containing both metallic and acidic sites, like platinum combined with zeolites (e.g., Pt/H-USY). The metallic component provides dehydrogenation activity, whereas the acidic sites promote the formation and migration of carbocation intermediates leading to molecular structure rearrangements.

Working Mechanism

1. Adsorption and Activation: Initially, hydrocarbon molecules are adsorbed onto the catalyst surface where they undergo preliminary activation by a metal center, usually platinum. This may involve partial dehydrogenation reactions to produce corresponding olefins or cyclic compounds.

2. Protonation and Carbocation Formation: Subsequently, activated hydrocarbons pick up a proton at acidic sites, forming carbocation intermediates. These cations are unstable and tend to undergo structural rearrangements to achieve more stable configurations.

3. Rearrangement and Isomerization: Through a series of hydrogen shifts, methyl migrations, or skeletal rearrangements, carbocation intermediates evolve into isomers with different branching structures. The key aspect here lies in the mobility and stability of these carbocations.

4. Desorption and Product Formation: Once the desired isomers are formed, they desorb from the catalyst surface and are collected as final products. Unreacted feedstocks or byproducts also leave the catalyst bed during this phase.

Influencing Factors

• Temperature and Pressure: Operating conditions must be meticulously controlled because both excessively high or low temperatures can impact the catalyst's activity and selectivity.
• Hydrogen Environment: To prevent coke formation and maintain catalyst activity, an adequate hydrogen atmosphere is maintained throughout the reaction process.
• Feedstock Properties: The composition of the feedstock also influences catalyst efficiency; for instance, feeds containing significant impurities might lead to catalyst poisoning.

Overall, isomerization catalysts facilitate the structural rearrangement of hydrocarbons within petroleum fractions, producing higher-value products with improved performance characteristics. In practice, optimizing catalyst design and operational parameters is crucial for achieving optimal economic outcomes and technical specifications.

This explanation provides a foundational understanding of how isomerization catalysts work in the context of petroleum refining, highlighting key steps and considerations involved in the process.