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What are Zeolites?Zeolites are complex crystalline aluminosilicates that are made of oxygen tetrahedrons, each encasing either a Si or Al atom. The oxygen atoms can be shared by only two tetrahedrons, and no two aluminum atoms can share the same oxygen atom. This latter restriction means that the Al/O ratio is always equal to or less than one. The crystal structures that result are complex 3-D frameworks of aluminosilicates with precisely dimensioned channels (typically 2.5 to 8 Angstrom units) running through them. These channels enable the zeolite crystals to be selectively permeable to various gases or liquids. Thus, they are “molecular sieves.”
For every aluminum atom in the unit cell made up of these tetrahedrons, there will be one free electron and hence a negative charge. The negative charges are compensated for by incorporating cations (usually, but not necessarily, Na+, K+, Ca2+, Mg2+). These cations are not part of the actual framework of tetrahedrons, but reside at the internal channels. When the crystals absorb fluids in the channels, it is possible for one type of cation to be exchanged for another without affecting electrical neutrality or crystal structure. This ion exchange ability gives zeolites the ability to act as selective absorbents and catalysts in industrial processes. If the charge balancing is effectuated by H+ ions in the channels, then the zeolites act as an acid and become particularly effective in catalytic cracking of petroleum. This has become one of the most important industrial uses of zeolites, maximizing the production of various petrochemicals, especially gasoline.
Natural zeolites have been known and studied for about 250 years. In the 1850s, the ion exchange ability of natural zeolites was commercially exploited for water softening (removing Ca or Mg from water passing through a zeolite bed). Presently, the ion exchange property of zeolite minerals is used to extract radioactive isotopes from fluids in the nuclear industry. The molecular sieve behavior of the mineral chabazite has been used to mitigate SO2 pollution. However, the largest use of natural zeolites is for pet litter.
It was not until the 1930s and 1940s that scientists started to synthesize zeolites. Since then, hundreds of zeolites have been synthesized, the vast majority of which do not exist in nature. It is these synthetic zeolites that are used in petrochemical applications. The zeolites most commonly used as petrochemical catalysts have a faujasite type structure, the so-called Y zeolites—either rare earth exchanged, REY or ultrastable Y (USY)—or a compound known as ZMS-5.
Zeolites in Petroleum CrackingThere is a strong economic (and strategic) desire to extract more gallons of gasoline (or diesel fuel) per barrel of oil. There may always be 42 gallons of oil in a barrel, but how the refinery processes it will determine if that barrel yields 12, 20 or more gallons of gasoline. In the early 1960s, refineries using fluid catalytic cracking (FCC) processing (the most widespread process) with amorphous aluminosilicate catalysts yielded ~14 gallons per barrel. By the late 1980s, similar processes using USY zeolites were yielding ~20 gallons per barrel—about a 42% increase.
Since lead additives used to control engine knock were banned, increasing octane numbers has become important. The low aluminum to silicon ratio in USY helps here, as does the small pore size of the ZSM-5 zeolite, which filters out the highly branched molecules that increase knock. Today, FCC catalysts are frequently composite particles containing both USY and ZSM-5 zeolites. As each FCC facility is a unique design, virtually each one uses a customized zeolite catalyst composition/composite. Since the pore sizes of zeolites are too small to catalyze the cracking of large molecules, most FCC catalysts are composites of an amorphous aluminosilicate matrix (~100 Angstrom pores) with mixed zeolite particles.
A growing alternative to FCC is hydrocracking, where cracking is carried out at higher hydrogen pressures. Zeolite catalysts have led to reduced temperatures and hydrogen pressures, thus significantly improving the economics of the process.