Zeolites, or molecular sieves, are described as materials formed by TO4 tetrahedra (T=Si, Al, P, Ge, B, Ti, Sn, etc.), interconnected by oxygen atoms, to create pores and cavities of uniform size and shape over the molecular range. These zeolite materials have important applications as catalysts, adsorbents or ion exchangers, amongst others.
Zeolites may be classified on the basis of the size of their channels and pores. In this regard, zeolites with channels limited by 8-T atoms are called “small-pore zeolites” (openings of about 4 Å), zeolites with channels limited by 10-T atoms are “medium-pore zeolites” (openings of about 5.5 Å), those with channels limited by 12-T atoms are “large-pore zeolites” (openings of about 7 Å), and, finally, zeolites with channels limited by more than 12-T atoms are called “extra-large-pore zeolites” (openings greater than 7 Å).
Amongst the more than 200 zeolite structures accepted by the International Zeolite Association (IZA), the chabazite crystal structure is one of the most interesting, due to its use in many diverse applications, most noteworthy as a heterogeneous catalyst in methanol-to-olefins processes (MTO) and in the selective catalytic reduction (SCR) of NOx.
The IZA has assigned the code CHA to the molecular sieve chabazite, which has a crystal structure formed by a tri-directional system of small pores interconnected by large cavities. The CHA structure has been synthesised with various chemical compositions, most noteworthy as an aluminosilicate (“SSZ-13”; Zones, U.S. Pat. No. 4,544,538, 1985, assigned to Chevron) or silicoaluminophosphate (“SAPO-34”; Lok et al., U.S. Pat. No. 4,440,871, 1984, assigned to UOP).
In general, it may be said that aluminosilicates show higher hydrothermal stability and better acidic properties than homologous silicoaluminophosphates (Katada et al., J. Phys. Chem. C., 2011, 115, 22505). Consequently, the synthesis of the CHA structure in aluminosilicate form, in an economical manner and with good physical-chemical properties, would be of great interest for application in industrial processes.
Chabazite is a natural zeolite that has the following chemical composition: Ca6Al12Si24O72. In addition to the natural form of chabazite, this zeolite structure has been synthesised in the laboratory using different inorganic alkaline cations as inorganic structure-directing agents (SDAs). Thus, the following syntheses have been disclosed: zeolite K-G (J. Chem. Soc., 1956, 2822), which is a chabazite synthesised in the presence of potassium cations and has an Si/Al ratio of 1.1-2.1; zeolite D (British Patent 868846, 1961), which is a chabazite synthesised in the presence of sodium-potassium cations and has an Si/Al ratio of 2.2-2.5; and zeolite R (U.S. Pat. No. 3,030,181, 1962, assigned to Union Carbide), which has an Si/Al ratio of 1.7-1.8.
Most likely, the first use of organic structure-directing agents (OSDAs) in the synthesis of the zeolite chabazite was disclosed by Tsitsishrili et al. (Soobsch. Akad. Nauk. Cruz, S S R, 1980, 97, 621), who show the presence of tetramethylammonium (TMA) cations in the reaction mixture K2O—Na2O—SiO2—Al2O3—H2O. However, the Si/Al ratio obtained in the final solid is very low (Si/Al˜2.1). The article discloses that the presence of TMA in the synthesis medium seems to affect the crystallisation of CHA, but said organic molecule is not incorporated into the synthesised material.
In general, aluminosilicates with a low Si/Al ratio (lower than 5) exhibit low hydrothermal stability. Consequently, in order to increase said Si/Al ratio in the synthesis of CHA, larger OSDAs, such as N,N,N-tri-alkyl-1-adamantylammonium, N-alkyl-3-quinuclidinol and/or N,N-tri-alkyl-exoaminonorbornane (Zones, U.S. Pat. No. 4,544,538, 1985, assigned to Chevron), were introduced into the synthesis medium. Using these OSDAs, the zeolite CHA is obtained with Si/Al ratios ranging between 4-25, which is called SSZ-13.
The preferred OSDA for the synthesis of the zeolite SSZ-13 is the N,N,N-tri-methyl-1-adamantammonium (TMAdA) cation. However, said OSDA has a high cost. This high cost may limit the commercial use of the zeolite SSZ-13 in industrial processes. Therefore, the synthesis of the zeolite SSZ-13 using more economical OSDAs would be of great interest for potential commercial applications of said zeolite.
An alternative for reducing the content of the TMAdA cation in the preparation of the zeolite SSZ-13 involves introducing mixtures of TMAdA with another, more economical OSDA, such as benzyltrimethylammonium (Zones, U.S. Patent 2008/0075656, 2008, assigned to Chevron). In this invention, the TMAdA content is significantly reduced by introducing the benzyltrimethylammonium cation into the synthesis medium. Despite the cost reduction when preparing the zeolite SSZ-13 using these mixtures of OSDAs, the presence of the TMAdA cation, which has a high cost, is still necessary.
Similarly, the use of mixtures of the OSDAs TMAdA and tetramethylammonium (TMA) in the synthesis medium has been proposed to synthesise the aluminosilicate form of CHA (Bull et al., WO2011/064186, 2011, assigned to BASF). Despite the cost reduction when preparing the zeolite SSZ-13 using these mixtures of OSDAs, the presence of the TMAdA cation, which has a high cost, is still necessary.
Recently, the synthesis of the aluminosilicate form of CHA using new, more economical organic molecules than the original OSDA TMAdA as the only OSDAs in the synthesis medium has been disclosed. Said organic molecules are benzyltrimethylammonium (Miller et al., U.S. Pat. No. 8,007,764, 2011, assigned to Chevron), cycloalkyl ammoniums (Cao et al., U.S. Patent 2008/0045767, 2008, assigned to ExxonMobil; Feyen et al., WO2013/182974, 2013, assigned to BASF), N,N-dimethylpiperidinium (Yilmaz et al., WO2013/035054, 2013, assigned to BASF), and N-alkyl-1,4-diazabicyclo[2.2.2]octane cations and derivatives thereof (Zones, WO2010/114996, 2010, assigned to Chevron).
In addition to the OSDAs described above, recently the synthesis of the aluminosilicate form of CHA using choline has also been disclosed (Chen et al., Environ. Sci. Technol., 2014, 48, 13909). In said publication, the authors claim that the use of choline allows for an economical pathway to synthesise CHA. However, for the efficient synthesis of a material, and its subsequent commercial application in industry, not only the sources used in the preparation thereof must be economically appealing, but the material preparation process must also exhibit good yields. In this case, the starting Si/Al ratio of the material is 20 (as may be calculated from the experimental synthesis process of SSZ-13 described in the publication); however, the final Si/Al ratio of the crystalline solid obtained is 6.5. Said difference suggests that the synthesis yield is less than 30% (crystalline solid obtained as a function of the inorganic oxides introduced during preparation of the gel). This low yield would prevent the use of said synthesis process in potential industrial applications.
In recent years, it has been disclosed that zeolite materials with the CHA crystal structure wherein Cu cations have been incorporated (Cu-CHA) are efficient heterogeneous catalysts for the selective reduction of NOx in transport-related emissions. These catalysts show high hydrothermal stability thanks to the presence of the small pores of the CHA structure, and the stabilisation of the Cu cations in the CHA cavities. These catalysts are capable of tolerating temperatures greater than 700° C. in the presence of water.
Despite the progress observed in recent years in the synthesis of the zeolite SSZ-13 using more economical OSDAs, there is clearly still a need for the chemical industry to improve the synthesis of said crystal structure, with a view to its application in various catalytic applications, and, more particularly, its use as a catalyst and/or support in the treatment of NOx in gas emissions from automobiles.
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