CHEE 323J.S. Parent1 Projected Trends in U.S. Liquid Fuels Demand With world oil prices in the range of USD/bbl, liquid fuel chemistry is the single most valuable catalytic technology in practice. Within the gasoline fraction, fuel blend quality is especially important, with acid catalyzed processes playing the central role.

CHEE 323J.S. Parent2 Gasoline Blend Formulations

CHEE 323J.S. Parent3 Performance Value of Gasoline Components

CHEE 323J.S. Parent4 Refinery Processes - Applications of Acid Catalysis

CHEE 323J.S. Parent5 Acid Catalysis vs. Metal/Organometallic Catalysis Where selectivity is concerned, using acid catalysis is akin to performing surgery with farm implements.  wide range of products accessible from carbenium ion intermediates  selectivity for given compounds is very often quite poor Why acid catalysis for fuel production?  Relatively cheap initiators compared to organometallic compounds  Solid “superacids” allow for heterogeneous, gas-phase chemistry  Fuel performance is insensitive to structure in comparison with drug action, polymer performance.

CHEE 323J.S. Parent6 Estimated Refining Catalysts/Usage in 1987

CHEE 323J.S. Parent7 General Kinetics of Acid Catalyzed Processes The acid catalyzed processes utilized for fuel production can be classified as truly catalytic (closed sequences without loss of active species) or as chain reactions (closed sequences requiring initiation and experiencing large-scale deactivation). Processes generally involve: 1. Carbenium ion generation 2. Transformation  olefin isomerization  rearrangement  alkylation  fragmentation (cracking)  hydride abstraction 3. Chain processes terminate by  hydride abstraction  collapse of ion pair

CHEE 323J.S. Parent8 Carbenium Ion Generation 1. Protonation of an unsaturated hydrocarbon  If the olefin is not strongly basic, a strong acid is required to generate the carbocation 2. Protonation of a saturated hydrocarbon  predominant mechanism in the cracking of alkanes carbonium ion 3. Electron removal from a neutral molecule

CHEE 323J.S. Parent9 Basicity of Unsaturated Hydrocarbons The ease of substrate protonation, while easily measured for relatively good bases (ketones, amines, etc), is difficult to estimate for hydrocarbons. Theoretical estimates of proton affinity for various olefins point to increasing basicity with increased carbenium ion substitution. Direct measurements of these equilibria are not possible, because several catalytic reactions (polymerization, isomerization, hydride transfer) take place as soon as the carbenium ion is formed in strongly acidic media.

CHEE 323J.S. Parent10 Superacids Combinations of Lewis and Bronsted acids can create extremely acidic conditions that are needed for hydrocarbon conversions. Acid H o HSO 3 F-12.8 BF 3 / H 2 O-11.4 H 2 SO 4 (98%)-9.36 HF-10.2 BF 3 / HF-16.8 TaF 5 (10mol%) / HF-18.9 SbF 5 (10mol%) / HF-18.9 SbF 5 / HSO 3 F-18.9 The term Friedel-Crafts Acids is often used. These are metal halides, organometal halides or organometals that coordinatively stabilize conjugate bases (anions, conjugate bases of Bronsted acids, halogens) by complexation. AlCl 3 activated by HCl is the best known example.

CHEE 323J.S. Parent11 Relative Stability of Alkyl Cations The stability of alkyl cations is influenced by:  inductive effects (substituents)  hyperconjugation

CHEE 323J.S. Parent12 Hammond’s Postulate The transition state for alkene protonation closely resembles the carbocation. Therefore, the factors that contribute to the relatively high energy of the carbocation contribute to the instability of the transition state.  Hammond’s Postulate is the assumption that the transition state for the formation of reactive intermediates resembles the intermediates themselves. Protonation of isobutylene to give the tertiary carbocation has the more stable transition state, and is therefore the kinetically preferred reaction pathway.