Aluminophosphate-type zeolites

The catalysts for xylene isomerization with EB dealkylahon are dominated by MFI zeolite. The de-ethylation reaction is particularly facile over this zeolite. There have been several generations of catalyst technology developed by Mobil, now ExxonMobil [84]. The features in their patents include selectivation and two-catalyst systems in which the catalysts have been optimized separately for deethylation of EB and xylene isomerization [85-87]. The crystallite size used for de-ethylation is significantly larger than in the second catalyst used for xylene isomerization. Advanced MHAI is one example. The Isolene process is offered by Toray and their catalyst also appears to be MFI zeoUte-based, though some patents claim the use of mordenite [88, 89]. The metal function favored in their patents appears to be rhenium [90]. Bimetallic platinum catalysts have also been claimed on a variety of ZSM-type zeolites [91]. There are also EB dealkylation catalysts for the UOP Isomar process [92]. The zeolite claimed in UOP patents is MFI in combination with aluminophosphate binder [93]. 

Significant Parameters in the Synthesis of Large Alkaline-Free MFI-Type Zeolites and AFI-Type Aluminophosphates... 

The entrapment-type nanocomposites can be prepared from zeolites and they are of two types zeolite-inorganic and zeolite-organic. Zeolite crystals are three-dimensionally linked network structures of aluminosilicate, aluminophosphate (ALPO), and silicoaluminophosphate (SAPO) composition and are porous, the pores being in the range of 2.8 to 10 A. Many of the highly siliceous, ALPO, and SAPO zeolites have been synthesized using organic templates such as tetrapropyl... 

AEL (n defines a number indicating a specific material) zeolite structure types aluminophosphate eleven (A1P04-11)... 

In the metal aluminophosphate (MeAPO) family the framework composition contains metal, aluminum and phosphorus [27]. The metal (Me) species include the divalent forms of Co, Fe, Mg, Mn and Zn and trivalent Fe. As in the case of SAPO, the MeAPOs exhibit both structural diversity and even more extensive composihonal variation. Seventeen microporous structures have been reported, 11 of these never before observed in zeoUtes. Structure types crystallized in the MeAPO family include framework topologies related to the zeolites, for example, -34 (CHA) and -35 (LEV), and to the AIPO4S, e.g., -5 and -11, as well as novel structures, e.g., -36 (O.Snm pore) and -39 (0.4nm pore). The MeAPOs represent the first demonstrated incorporation of divalent elements into microporous frameworks. 

Many books, reviews and treatises have been pubUshed on related subjects [1-7]. Thus the objective of this chapter is the deUneation of the key features of the catalytic surface and the process conditions which enable better control of the reaction pathways for more efficient and environmentally friendly processes and minimal utiHzation of precious natural resources. As it stands today, hundreds of known framework types have been synthesized and scaled-up [8], but only a handful have found significant application in the hydrocarbon processing industries. They are zeolite Y and its many variants, ZSM-5, Mordenite and zeohte Beta. Other very important crystalline materials (including aluminophosphates (ALPOs),... 

Other elements, such as Ga and Ge, can substitute for Si and A1 in the zeolitic framework, and there are claims that many other elements can also do so. New classes of nonsilicate zeolite-type crystalline aluminophosphates (31) and silicoaluminophosphates (SAPO) (65) have been reported but relatively little is known about their chemical behaviour. 

The search for new zeolite-like structures was initially extended to aluminophosphate-based molecular sieves,3 and these explorations produced a variety of exotic compounds with open-framework structures, which include besides metal phosphates,4 carboxylates,5 sulfates,6 selenites, and selenates.7 Growth of this area has been rapid in the past decade necessitating new editions of the Atlas of Zeolites.2 It is noteworthy that metal—organic framework (MOF) solids have been included as zeolite types in the latest edition of the Atlas of Zeolites. 

A novel class of crystalline, microporous aluminophosphate phases has been discovered. It represents the first class of molecular sieves with framework oxide compositions free of silica. The new class of materials encompasses some fourteen reported three-dimensional microporous framework structures, and six two-dimensional layer-type structures. The three-dimensional structures include structural analogues of the zeolites sodalite and erionite-offre-tite. The novel phases can be synthesized hydro-thermally in the presence of organic amines and quaternary ammonium templates. The template is entrapped or clathrated within the crystallizing aluminophosphate network. After thermal decomposition of the template the three-dimensional molecular sieves have the general composition of Al303 1.0 ... 

Alumipophosphate molecular sieve membranes. In addition to zeolites, Haag Tsikoyiannis [1991] have also briefly described another type of molecular sieve membranes consisting of AIPO4 units whose aluminum or phosphorous constituent may be substituted by other elements such as silicon or metals. These membranes are made from aluminophosphates, silico-aluminophosphates, metalo-aluminophosphates or metalo-aluminophosphosilicates. Like zeolites, these materials have ordered pore structures that can discriminate molecules based on their molecular dimensions. Their separation and catalytic properties can also be tailored with similar techniques employed for zeolites. The procedures for calcining the membranes or separating them from non-porous subsuates are essentially the same as those described earlier for zeolites. 

Stereoselective catalysis in zeolites is still one of the ultimate goals in zeolite science. Earlier work in this field was summarized recently [4]. More recently, Mahrwald et al. [95] reported that the addition of aluminophosphate molecular sieves in the liquid phase alkylation of a-chiral benzaldehydes by butyllithium results in an increased proportion of the so-called Cram product in the diastereomeric mixture. It is argued that in this Grignard type reaction the adsorption of the reactants on the molecular sieves favors the attack at the sterically less hindered position of the molecule. This shape selectivity effect is even observed when the reactant is adsorbed at the outer crystal surface, as demonstrated for the case of the small-pore AIPO4-I7. 

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