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 AcidityAlO+ , Al(OH)3 x H2O Metal Cations

Few common STRUCTURES

• Cation exchange properties of zeolite is function of

its intrinsic properties :

1)Framework topology

2)Ion size and shape

3)Charge density of anionic framework

4)Ion valence

5)Electrolyte concentration in aq. Phases

Characteristics and Physicochemical properties of Zeolites

• 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 X Zeolite Y

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.

Hexagonal MCM-41 Cubic MCM-48 Lamellar MCM-50

Mesoporous M41S Materials

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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

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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 X Zeolite Y

Acidity CharacterizationName of reaction

Description Solid-acid catalystused

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;

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