CH4 Chlorination with Cl2 using zeolites having different surface polarities: Catalysis descriptors explaining the electrophilic pathway

Abstract

• CH 4 chlorination with Cl 2 is investigated using zeolites ion exchanged with various cations. • Electrophilic chlorination is predominant at the early stage of reaction. • As the reaction proceeds, radical-mediated chlorination becomes predominant. • Electrophilic chlorination is crucial for giving CH 3 Cl more selectively than polychloromethanes. • EA, SRP, physisorption enthalpy and NBO charge can be the catalysis descriptors. CH 4 chlorination with reactive Cl 2 occurs via two competing mechanisms: (i) a free-radical-mediated chain reaction resulting in a statistical, non-selective thermodynamic product distribution including polychloromethanes, or (ii) an electrophilic ionic chlorination affording the more selective formation of a desired monochlorinated product (CH 3 Cl) with inhibition of poly-chlorination. Herein, the balance between the two competing pathways was controlled using zeolite acid catalysts containing various cations, of which the cations were introduced to the framework simply by ion-exchange that changes the surface acidities and hence the polarities. This resulted in remarkable changes in CH 3 Cl yield depending on the type of the cations. From the reaction data and computational calculations, four physicochemical properties of the catalysts can be proposed as the catalysis descriptors describing the electrophilic chlorination pathway: electron affinity, standard reduction potential, physisorption enthalpy, and natural bond orbital charge. CH 3 Cl yield increased proportionally with electron affinity and standard reduction potential due to increased polarizability of Cl 2 . In addition, the large physisorption enthalpy to CH 3 Cl from the cations on the zeolite surface resulted in a decreased CH 3 Cl yield due to the increased probability of poly-chlorination. The natural-bond-orbital charges of the cations in the zeolite framework derived by DFT calculations further suggested that strong electrostatic interaction between CH 3 Cl and the cations in the zeolite cluster model could increase the residence time of CH 3 Cl on the surface, resulting in the increased production of poly-chloromethanes and decreased CH 3 Cl yield.