Zeolite Application and Production

Many people Though both of silica/alumina and zeolites are the same. Although, these substances are made of an aluminosilicate framework, there are some key differences in regard to their morphologies, physical properties, and acidity. For instance silica-alumina is an amorphous material, and zeolites on the other hand are crystalline in nature. A key physical difference is that zeolites are typically microporous (i.e. very small pores) and silica-alumina are mesoporous (i.e. moderate sized pores). Lastly the Bronsted acid sites on zeolites have been found to significantly stronger than those on silica-alumina, due to the homogeneity of the zeolite.

Applications of zeolites are mainly used as adsorbents, catalysts and ion exchange agents. Zeolites are extensively used as catalysts in the petroleum refining, petrochemical productions, coal and fine chemical industries. Also, they are used in the detergent industry and treatment of liquid waste as ion exchange agents. Zeolite largest economic impact comes from the use in the fluid catalytic cracking (FCC) processes. FCC catalyst account for more than 95% of the zeolite catalyst consumption. The two most important zeolites used in the FCC are Zeolite Y and ZSM-5. Moreover, ZSM-5 is used as octane number booster. Different other zeolites also used to boost the octane number such as mordenite and zeolite beta.

Recently, technology of hydrocarbon production from methanol was commercialized to produce gasoline, aromatics and olefins. Zeolite based materials are the core of this technology. The new approach of producing high octane gasoline by co-catalytic reaction of methanol and naphtha(or sweet condensate) or methaforming technology utilize ZSM-5 and Zn metal as the catalyst. Zeolites are also used in the treatment of car exhaust gases such as NOx removal . Furthermore, zeolites are being explored for their use in biomass catalysis.

Example of different zeolites used in catalytic processes:

Zeolite Y: Catalytic cracking, alkylation, hydrocracking, hydro isomerization of light alkanes

ZSM-5: Alkylation of aromatics, disproportionation of toluene, isomerization, methanol to olefins reaction, dewaxing, alkylation by methanol, trans alkylation of toluene and c9 aromatics, ethylbenzene isomerization to p-xylene

Beta: Tran alkylation, benzene alkylation, hydrocracking

ZSM-22: Hydro isomerization of alkenes

MCM-22: Benzene alkylation

SAPO-34: Methanol to olefin

Zeolites are three dimensional, microporous, crystalline solids with well-defined structures containing aluminium, silicon, and oxygen in their regular framework. Also, cations and water molecules are located in the pores. The history of zeolites began when the first natural zeolite (Stilbite) was discovered by the Swedish mineralogist Cronstedt in 1756. He classified zeolites as new class of hydrated aluminosilicates including alkali and alkaline earth elements. The word zeolite is originated from two Greek words (Zeo) and (lithos) which means to (boil) and (stone) since when heated in a blowpipe they form a frothy mass. The microporous properties of natural zeolites and their advantages, such as water adsorption, were gradually identified during the 19th century. More than 40 types of natural zeolite have been discovered and some of them are widely used in the fields of drying and liquid and gas separation, softening of hard water and sewage treatment. The high industrial demand on natural zeolites required the use of synthesized zeolites. The first hydrothermal synthesis of zeolites copying the geothermal formation of natural zeolites was introduced in the 1940s. In 1948, Barrer reported the first definitive synthesis of zeolites including the synthetic correspondent for the zeolite mineral mordenite [4]. Between 1949 and 1954 Milton and Breck discovered several commercially significant zeolites, which are zeolites A, X and Y. In 1962, Mobil Oil introduced the use of zeolite X (FAU) as a refinery cracking catalyst. since then, the use of zeolites as catalysts became a key factor in the refining industry which significantly increased the production efficiency. After this point, the use of zeolites was not limited to the petroleum and petrochemical industry and was introduced to the laundry detergents markets by 1974. In 1978, their selective ion exchange properties were used to clean up water contaminated by radiation in the united states. The period from 1954 to 1980 is considered to be the “golden age” in zeolites development. During this time, zeolites with low, medium and high Si/Al ratios were discovered. Zeolite Y was synthesized by Breck to increase the thermal stability and acidity of zeolites which played a very important role in the catalysis of hydrocarbon conversion processes. In 1972, the most important member of the pentasil zeolite family, ZSM-5, was synthesized by Argauer and Landelt. Later the high Silica Zeolite beta (BEA) was synthesized by Wadlinger and Kerr. Zeolite structure Zeolites consist of TO4 tetrahedra units where T is silica or alumina with oxygen atoms connecting the neighboring tetrahedral. Zeolites structure contains pores and cavities that are occupied by cations and water molecules. These two molecules move freely inside the cavities allowing ion exchange to take place.

In general zeolites can be described by the following formula: Mx/n [(AlO2 - )x (SiO2)y] zH2O Eq. 2-1 Where M is a cation to balance the negative charge of the framework and n is the valence for the cation while x, y and z are the total number of aluminate, silicate and water molecules in the unit cell. Many of the zeolite properties could be explained and clarified once the framework type is known. In addition to describing the topology of the framework tetrahedral atoms, the type of zeolite framework defines the pore opening size and shape, the dimensionality of the channel systems and the volume, and organization of the cages. Zeolites are composed of primary and secondary building units. The primary building block of a zeolite is tetrahedron of silicon or aluminum ion surrounded by four oxygen anions. These tetrahedra are arranged in a way where each one of the four oxygen anions is shared with another silicon or aluminum tetrahedron. Each silica tetrahedron is electrically neutral as every silicon ion has +4 charge balanced with four tetrahedral oxygen anions. On the other hand, aluminum has +3 charge but bonding to four oxygen anions will lead to a tetrahedron with a charge of -1, so each aluminum tetrahedron needs to be neutralized by a cation with a charge of +1, such as Na+. The silica and alumina tetrahedra connect to form more complicated secondary units which form the building blocks of a zeolite crystal structure (secondary building unit, SBU). SBUs can be in different simple polyhedral forms such as hexagonal prisms, cubes or octahedra. All known zeolite structures can be described by nine different SBUs. SBUs can be made up of 4, 6 and 8-membered single rings, 4-4, 6-6 and 8-8-member double rings, and 4-4-1 branched rings. The final zeolite structure consists of grouped SBUs. For example, FAU zeolites are made of sodalite framework which is built of 4-member ring and 6-member ring building units .

Zeolites with different structures are used for different applications and reactions. They range in pore size from 0.3 nm to 0.8 nm making them molecular sieves for molecules with different sizes. As a result of this characteristic zeolites exhibit an important catalytic function called shape selectivity where steric restrictions are imposed on the formation of bulky transition states. Zeolites in literature are categorized based on their pore size into five categories. They are small pore zeolites with 8 ring pores, medium pores with 10 rings, large pores with 12 membered rings, zeolites with dual pore systems (e.g. mordenite with 12- and 8 – membered rings channels), and mesoporous systems. Medium pore zeolites Medium pore zeolites exhibit high shape selectivity compared to other zeolites due to their pores openings consisting of 10 membered rings. Also, they have high resistance to coking as their structure does not have any cages and prevents reactions with bulky intermediates. The shape selectivity of medium pore zeolites is measured by the constraint index where values are obtained from n-hexane and 3-methylpentane cracking. Medium pore zeolites usually exhibit constraint index values between 3-12 whilst zeolites with large pores have indices between 1-3 or even less than 1. For instance, ZSM-5 showed a value of 8.3 while the value for zeolite Y and mordenite obtained was 0.5. The content of aluminum in ZSM-5 can vary over a wide and hence has a wide range of Si/Al ratios range from around 10 to infinity. The configuration of ZSM-5 has not been reported in any other type of zeolites. ZSM-5 possesses unique shape selectivity because of its 10-membered ring channel system making it a favorite candidate over other zeolites for the cracking of long chain, low-octane number normal paraffins and some olefins in the gasoline fraction. The absence of cages combined with a small entrance in ZSM-5 gives it special coke resistance properties. ZSM-5 is one of the widely utilized catalysts in industry, especially in 27 petroleum and petrochemical industries such as in isomerization of xylenes, alkylation of toluene with methanol and toluene disproportionation. ZSM-5 zeolite is highly favorable for selective toluene disproportionation reaction to produce p-xylene as a result of the shape selectivity imposed by the pore size (0.51-0.56) nm which is close to the kinetic diameter of para-xylene (0.58 nm) compared to the other xylene isomers (ortho- and meta-) which possess larger kinetic diameters (0.68 nm) hindering their formation and slowing their diffusion through the channels. Depending on the size of the molecule, the pore openings and channels will control the diffusion of the reactants, products and transition states. Thus, zeolites are called molecular sieves. Zeolites have been used in industry as acid catalysts in hydrocracking, isomerization, alkylation and catalytic reforming due to their acidic properties. To describe the acidity of a zeolite properly it is important to understand the nature of the acid sites (Br?nsted acid sites and Lewis acid sites) and their strength. The strength of acid sites is reliant on their distribution and location in the framework. The generation of active sites is a very important step for the zeolite to be used as a catalyst. This is performed by ion exchanging the zeolite with ammonium hydroxide followed by calcination. The catalytic activity of zeolites is related to the number of Br?nsted acid sites in the framework. However, the role of Lewis acid sites cannot be neglected and is still under investigation in the literature. Almutairi et al [28] studied the effect of extra framework aluminum (EFAL) by adding aluminum species to zeolite Y through impregnation method and ion exchange with Al (NO3)3 and they were compared with commercial USY zeolites. The modification resulted only in small amount of stabilized cationic EFAL species at the exchangeable sites of the zeolite. Testing the modified catalysts for propane cracking, it was found that the higher the ration of cationic EFAL species and Br?nsted acid the higher the rate of propane cracking. Br?nsted acid sites are proton donors while Lewis sites accept electrons. The Br?nsted acid sites are recognized by the bridging hydroxyl groups formed from the oxygen in the framework and the proton (H+ ). On the other hand, Lewis acid sites are formed by heating the zeolite to an elevated temperature where protons are lost as water molecules. It was reported that the amount of aluminum in the structure is proportional to the available number of acid sites. The strength of the Br?nsted acid sites depends on the number of aluminum atoms that are incorporated in the tetrahedral sites of the framework. Evidence is provided in the literature that the strongest Br?nsted acid sites occur in isolated AlO4 tetrahedra and the strength decreases with increasing adjacent Al atoms. This is why dealumination is used to increase the catalytic activity of zeolites in some cases to create super acid sites.

Synthesis of zeolites with micro- and meso- porosity is a subject of high interest as diffusion limitations and “molecules traffic” control can enhance the performance of the catalytic and separation processes. In 1948, the first synthesis of mordenite zeolite was confirmed by Barrer followed by the synthesis of zeolite A, at low temperatures, by Milton and Beck. In the 1960s, Barrer and Denny replaced the use of inorganic bases with organic templates in the synthesis of zeolites. Later, ZSM-5 zeolite was synthesized using an organic template, such as tetrapropylammonium hydrxide (TPAOH), in the synthesis mixture. In the last decade, different new synthetic approaches have been developed to synthesize zeolites with specific properties and structures based on the use of pre-designed organic structure-directing agents . On the other hand, the advancement of computer simulation techniques has improved the ability of researchers to hypothetically predict the zeolite structures and obtain structure solutions of complex zeolite structures . The advancement in structures determining and synthesis resulted in discovering new zeolite framework. The international zeolite association approved 50 new framework types since 2007.

Zeolite synthesis is affected by many factors and includes the gel composition, temperature, mineralizer, cation source, solvent, silica and alumina sources, and aging. During zeolite synthesis, amorphous phase containing all the synthesis raw materials mixed (Si-source, Al-source, cation and water) forms crystalline aluminosilicate through a sequence of complex chemical reactions. The crystallization mechanism via hydrothermal synthesis was extensively discussed in the literature. Monomers and oligomers are provided by the mineralizing agent formed by dissolving silica and alumina sources in water. These monomers and oligomers arrange into intermediate structure which is followed by the formation of a crystalline phase. Ageing of the gel at room temperature is a crucial step to accelerate the crystallization. Crystallization of zeolites involves three steps which are supersaturation, nucleation and crystal growth. Supersaturation occurs during ageing process where the aluminosilicate precursor species concentration increase as the amorphous solid phase is given longer time to dissolve. Aluminosilicate precursor species chemically aggregate which triggers the nucleation process and nucleation is either homogenous or heterogeneous. Heterogeneous nucleation comprises the addition of crystals to the gel via seeding method. This helps in decreasing the crystallization time and growing crystals with high surface area.