Introduction to Zeolites

A three-letter code system has been developed for naming, assigned to specific structure types independent of their actual chemical compositions e.g., the code LTA is used for Zeolite A and is derived from the name Linde Type A.

Bronsted Acidity Al – OH – Si Terminal Silanol group Lewis Acidity AlO+ , Al(OH)3 x H2O Metal Cations

Few common STRUCTURES

Cation exchange properties of zeolite is function of its intrinsic properties : Framework topology Ion size and shape Charge density of anionic framework Ion valence Electrolyte concentration in aq. Phases

Characteristics and Physicochemical properties of Zeolites High surface area, Uniform micropore size, High hydro thermal stability, Intrinsic acidity, Ability to accommodate active metal species, Introducing constraints to undesired species by molecular sieving effect (shape selectivity), Environmentally harmless, Non-corrosive, Show ease of separation from reaction mixture compared with homogeneous catalysts.

Zeolites used as FCC catalysts Zeolites typically applied as FCC catalysts are Type X, Type Y and ZSM-5 X zeolite has a lower silica-alumina ratio, which makes it less stable. ZSM-5 is a versatile zeolite that helps in increasing the yield of olefins from FCC. Zeolite Y Zeolite X

Synthesis of FCC Catalysts

Inside FCC Catalyst

Major Physical Characteristics of FCC Catalysts Attrition Resistance : High gas flow rates and the harsh temperature  attrition of the catalyst particles and produce fines. An increased zeolite content of the catalyst, reduction of the zeolite crystal size and better dispersion of the zeolite within the matrix Pore Size Distribution and Pore Volumes : Small pores  greater tendency to get clogged by coke greater diffusional resistance Surface Area: The surface area of the catalyst comes from the zeolite and also the matrix. It ranges from 600 – 800 m2/gm in zeolites

Size Constraints or Molecular sieving or Shape selectivity Due to rigid skeletal system, zeolite window has intact window size due to which molecules having size larger than this window dimensions often experience constraints in accessing the intrinsic sites. Thus, prevents undesired large molecules to enter network. Shape selective

(1) Reactant Shape selectivity This refines the accessibility molecules inside pore geometry of zeolite by imparting the fixed window opening, thus limits the diffusion of molecules with kinetic diameter larger than this window. This allows shape selective entrance of molecules, activity of such molecules is often hindered. Fig Shows the shape selectivity of n- butane and iso-butane over zeolites. Fig. Shape selectivity of Hydrocarbons Reactants on to zeolite framework

(2) Transition Shape selectivity Transition shape selectivity refers to curbing of intermediate compounds formed therin the framework whose size is larger than skeleton size. Fig. shows formation of small sized isomer compared to bulky ones. Fig. Shape selective transition compound formation restriction

(3) Product Selectivity Due to restricted pore size products diffusing through these materials experience constraints and so product formed is also selective in nature. But however sometimes this bulky products accumulate over a position of zeolite blocking the accessibility of virgin molecules resulting into coking or deactivation of catalyst. Fig. 8 Shape selective product formation.

Mesoporous M41S Materials Lamellar MCM-50 Hexagonal MCM-41 Cubic MCM-48 8

More resistant to pore blockages What makes MCM-48 interesting candidate ? Three dimensional interwoven structure More resistant to pore blockages High surface area, pore volume and thermal stability Higher catalytic activity than one dimensional counterpart, MCM-41. Structures of MCM-41 and MCM-48 12

Zeolites used as FCC catalysts Zeolites typically applied as FCC catalysts are Type X, Type Y (Faujasite Type) and ZSM-5 X zeolite has a lower silica-alumina ratio, which makes it less stable. ZSM-5 is a versatile zeolite that helps in increasing the yield of olefins from FCC. Zeolite Y Zeolite X

Acidity Characterization Name of reaction Description Solid-acid catalyst used Cracking/ hydrocracking Crack large molecules in petroleum oils FCC additives for more C3 and octane Silica-alumina; ZeoliteY ZSM-5 Dewaxing Crak n-paraffins (waxes) in petroleum oils ZSM-5 Isodewaxing Isomerization of waxy molecules. SAPO-11 Xylene isomerisation p- and o-xylenes from m-xylene. ZSM-5; Mordenite Naphtha reforming Isomerization reactions for aromatization of paraffins. Chlorided alumina Hydrotreating Remove N and S from petroleum oils Alumina support Hydration Hydrate olefins to alcohols. Ion-exchange resin; ZSM-5;

achieve maximum selectivity for the desired reaction It is important to know the strength of the acid catalyst to achieve maximum selectivity for the desired reaction