CHEE 32326.3 Zeolite Structure Zeolites are a well-defined class of crystalline aluminosilicate minerals whose 3-dimensional structure is derived from a framework of [SiO 4 ] 4- and [AlO 4 ] 5- coordination polyhedra. Usually zeolites are classified according to common structural units (secondary building units, sbus) Tetrahedra are arranged to yield an open framework structure, as shown below in the most important FCC catalyst, zeolite Y or faujasite. This class of zeolite has 0.74nm apertures, and the supercage of the structure has a radius of approx. 1.2 nm. A range of compositions exist, with a unit cell formula typically being Na j [(AlO 2 ) j (SiO 2 ) 192-j ]zH 2 O where z is about 250 and j is between 48 and 76.
CHEE 32326.4 Zeolite Structure Zeolite structure depends on the Si/Al ratio, template agents and preparation, but all have unique micropore structures. Virtually all acid catalyzed reactions can be conducted with acidic form of zeolites, provided the reactant is small enough to enter the pores. The acidic sites in HZSM-5 are strong enough to protonate paraffins, leading to widespread use as an industrial cat. cracking catalyst.
CHEE 32326.5 Structure of ZSM-5 The zeolite ZSM-5 is finding greater application in FCC as an octane enhancing catalyst, as it cracks/isomerizes low octane components in the gas boiling range to higher octane value while generating propylene and butylene for subsequent alkylation. Zeolites are usually crystallized from alkaline aqueous gels at temperatures between 70°C and 300°C to produce a sodium salt. In addition to structure, key properties are the Si/Al ratio, the particle size and the nature of the (exchanged) cation. These primary structure/composition factors influence acidity, thermal stability and overall catalytic activity.
CHEE 32326.6 Acidity of Zeolites The acidic properties of zeolites are dependent on the method of preparation, form, temperature of dehydration and the Si/Al ratio. Bronsted Acid Sites: generated by ion exchange followed by calcination Lewis Acid Sites: At 550°C, water loss from Bronsted sites leads to unstable Lewis sites, leading to so-called ‘true’ Lewis sites through expelling an Al species.
CHEE 32326.7 Cracking Catalyst Formulations In addition to acidity, physical characteristics must be considered, including: 1. Mechanical stability function of zeolite, matrix and binder composition, degree of zeolite dispersion and bulk density. 2. Pore volume, pore size distribution and surface area determined by matrix and zeolite composition, effects activity and yield through introduction of diffusion effects. 3. Thermal and hydrothermal stability Recrystallization and structure collapse 4. Particle size distribution Fluidization and entrainment specifications require 60-80 m particle diameter 5. Bulk Density
CHEE 32326.8 Synthesis of FCC Catalysts Utilizing Zeolites
CHEE 32326.9 Paraffin Cracking Catalyzed by ZSM-5
CHEE 32326.10 Shape Selectivity Imposed by Zeolite Structure Variable channel and pore sizes of zeolites can create unique selectivity effects. Reactant Selectivity: Products Selectivity: Restricted transition state selectivity Transalkylation of a dialkylbenzene
CHEE 32326.12 Schematic View: Short Contact Time FCC Unit A modern FCC unit is a short- residence time, adiabatic process where atomized feed is contacted with hot catalyst (500°C) in a relatively narrow riser. The reaction riser is a fluidized bed, with mixing promoted by differential particle-gas velocity and large scale turbulence. Upon exiting the riser, the fluid velocity drops, and entrained catalyst settles. The overhead product stream is isolated through a cyclone to remove smaller particles. Large-scale coke formation deactivates the catalyst, limiting the single-pass activity.
CHEE 32326.13 Schematic View: Catalyst Regenerator Coke formation during FCC blocks access to acidic sites within the active zeolite. While limiting the lifetime of the catalyst, regeneration by coke combustion is very efficient. The heat of combustion drives the endothermic cracking process by heating the catalyst prior to reintroduction to the riser. As much as 30 tons per minute of catalyst is regenerated in a full-scale FCC unit.
CHEE 32326.14 Really, Really Big Reactors This FFC installation has the regeneration unit constructed above the cracking riser. Side- by-side configurations are also used. A typical plant can run continuously for several hundred days, processing millions of barrels of oil. In the foreground of the photo is a heating furnace.
CHEE 32326.15 Bifunctional Catalysis Hydrocracking Catalytic cracking and olefin hydrogenation are combined processes in hydrocracking units. FCC Zeolites, combined with dispersed metals (Ni, Pt, Pd) on a standard matrix generates a bifunctional catalyst capable of utilizing reforming by-product hydrogen. Naptha Reforming The low octane number of small paraffins (naptha) can be improved by isomerization without concurrent cracking or alkylation. Pt/SiO 2 -Al 2 O 3 is a bifunctional catalyst preparation, wherein the metal catalyzes dehydrogenation and /hydrogenation and the acidic support catalyzes skeletal isomerization.