Microporous crystalline molecular sieves, such as zeolites and SAPOs, are useful as adsorbents and catalysts in a wide variety of commercial applications. For example, zeolites are formed by a crystal lattice of TO4 tetrahedra which share all their vertices giving rise to a three-dimensional structure containing channels and/or cavities of molecular dimensions. They have variable chemical compositions, with T generally representing atoms with a formal oxidation state of +3 or +4, such as for example Si, Ge, Ti, Al, B or Ga, among others. Depending on the nature and ratio of the different T atoms, zeolitic materials can exhibit significant acid activity.
The existence of channels and cavities of uniform dimension in the interior of microporous crystalline molecular sieves gives rise to a high specific area. In addition, the fact that the pore size is uniform and with a narrow distribution permits these crystalline materials to selectively adsorb molecules of differing size depending on the dimensions of the channels.
Moreover, the crystal structure of each zeolite or SAPO, with its specific system of channels and cavities, gives rise to a characteristic X-ray diffraction pattern. Hence, microporous crystalline molecular sieves can be differentiated from each other by their range of chemical composition and their X-ray diffraction pattern. Both characteristics (crystal structure and chemical composition) also determine the physico-chemical properties of each molecular sieve and hence its possible application in different industrial processes.
Most zeolitic materials have a pore size of less than 13 Å which often limits their ability to process very large molecules. Also known are mesoporous crystalline molecular sieves, such as MCM-41 (see U.S. Pat. No. 5,098,684), which have an ordered arrangement of uniformly sized pores having a diameter between 15 and greater than 100 Å. Moreover, the pore size of a given MCM-41 material is very narrow and the mean pore diameter can be tailored by the method used for its synthesis. However, the low acidity of MCM-41 materials has limited their utility in many catalytic processes.
Recently, interest has refocused on amorphous or substantially amorphous materials since these materials offer the possibility of providing catalysts with reasonably high surface area. However, traditional amorphous materials have significantly lower acid strength than microporous crystalline molecular sieves and hence one area of interest in current research is the development of amorphous materials with enhanced acid strength.
For example, in an article entitled “Structural, compositional and acidic characteristics of nanosized amorphous or partially crystalline ZSM-5 zeolite-based materials”, Microporous and Mesoporous Materials 75 (2004), pages 89-100, Triantafyllidis et al. report that the synthesis of X-ray amorphous and partially crystalline nanosized zeolite-based materials can be accomplished by lowering the reaction temperature used to produce ZSM-5 from a typical hydrothermal synthesis mixture containing tetrapropylammonium ions as a structure directing agent. Based on NH3-TPD and 27Al MAS NMR data, the resultant X-ray amorphous materials were reported to possess tetrahedral aluminum atoms that were more zeolitic in nature and their associated protons more acidic than a conventional amorphous silica-alumina standard. For example, the material synthesized at 50° C., termed NAS-50, had an acid site density of between 0.05 and 0.19 mmol/g, depending on degree of ion exchange, based on NH3-TPD data, together with a total pore volume of 1.084 cm3 g−1, a micropore volume of 0.011 cm3 g−1 and specific surface area of 142 m2 g−1. European Patent Application No. 1 783 099 A1 discloses a microporous amorphous material having a chemical composition in the calcined and anhydrous state obeying the following empirical formula:x(M1/nXO2):y YO2:SiO2 wherein x has a value of less than 0.2, and can be equal to zero, y has a value of less than 0.2, and can be equal to zero, M is selected from among H+, one or more inorganic cations having a charge +n and a mixture of same; X is one or more chemical elements in oxidation state +3; and Y is one or more elements in oxidation state +4, and wherein the microporous amorphous material has a uniform pore distribution and a micropore volume of greater than or equal to 0.05 cm3 g−1 and which has a specific surface area of greater than 100 m2 g−1.
According to the invention, a novel porous aluminosilicate-based material has now been synthesized which, although substantially X-ray amorphous in its calcined form, exhibits significant Bronsted acid activity and an alpha value at or near one, making the material attractive for certain catalytic applications, such as fluid catalytic cracking, hydroprocessing and reforming.