Metal-Substituted Microporous Aluminophosphates

This chapter aims to present the zeotypes aluminophosphates (AlPOs) as a complementary alternative to zeolites in the isomorphic incorporation of metal ions within all-inorganic microporous frameworks as well as to discuss didactically the catalytic consequences derived from the distinctive features of both frameworks. It does not intend to be a compilation of either all or the most significant publications involving metal-substituted microporous aluminophosphates. Families of AlPOs and zeolites, which include metal ion-substituted variants, are the dominant microporous materials. Both these systems are widely used as catalysts, in particular through aliovalent metal ions substitution. Here, some general description of the synthesis procedures and characterization techniques of the MeAPOs (metal-contained aluminophosphates) is given along with catalytic properties. Next, some illustrative examples of the catalytic possibilities of MeAPOs as catalysts in the transformation of the organic molecules are given. The oxidation of the hardly activated hydrocarbons has probably been the most successful use of AlPOs doped with the divalent transition metal ions Co2+, Mn2+, and Fe2+, whose incorporation in zeolites is disfavoured. The catalytic role of these MeAPOs is rationalized based on the knowledge acquired from a combination of the most advanced characterization techniques. Finally, the importance of the high specificity of the structure-directing agents employed in the preparation of MeAPOs is discussed taking N,N-methyldicyclohexylamine in the synthesis of AFI-structured materials as a driving force. It is shown how such a high specificity could be predicted and how it can open great possibilities in the control of parameters as critical in catalysis as crystal size, inter-and intracrystalline mesoporosity, acidity, redox properties, incorporation of a great variety of heteroatom ions or final environment of the metal site (surrounding it by either P or Al)

Spectroscopic and computational insights on catalytic synergy in bimetallic aluminophosphate catalysts

A combined electronic structure computational and X-ray absorption spectroscopy study was used to investigate the nature of the active sites responsible for catalytic synergy in Co-Ti bimetallic nanoporous frameworks. Probing the nature of the molecular species at the atomic level has led to the identification of a unique Co-O-Ti bond, which serves as the loci for the superior performance of the bimetallic catalyst, when compared with its analogous monometallic counterpart. The structural and spectroscopic features associated with this active site have been characterized and contrasted, with a view to affording structure property relationships, in the wider context of designing sustainable catalytic oxidations with porous solids

Structure Directing Effect of (1S,2S)-2-Hydroxymethyl-1-benzyl-1-methylpyrrolidinium in the Synthesis of AlPO-5

A pure diastereoisomer of a new chiral organic molecule with two asymmetric atoms, (1S, 2S)-2-hydroxymethyl-1-benzyl-1-methylpyrrolidinium, has been prepared and used as structure directing agent (SDA) for the crystallization of the one-dimensional AlPO-5 microporous aluminophosphate. Our main objective is to induce chiral supramolecular arrangements of the SDA molecules within the microporous structure. The SDA molecules can be arranged as monomers or dimers within the nanochannels of AlPO-5, the aggregation being driven by pi-pi type interactions between the aromatic rings. It has been found that increasing the crystallization time leads to a higher organic content (and lower water incorporation) together with a higher formation of dimers, which indicates that the incorporation of the organic molecules as dimers is thermodynamically favored. We also observe a partially reversible photoinduced rearrangement of the SDAs, the aggregation is enhanced by exciting the occluded monomers with the appropriate wavelength. A recently developed computational model has been applied to determine the stable loading of SDAs and water molecules, which has been found to be of 1.0 SDA and 7 water molecules per unit cell, with water forming stable 6-ring clusters between consecutive dimers, in good agreement with the experimental observations

Cooperative structure-directing effect of fluorine-containing organic molecules and fluoride anions in the synthesis of zeolites

Dibenzyldimethylammonium (DBDM) cations, singly fluorinated at the ortho, meta, and para positions of the aromatic ring, have been investigated as structure-directing agents (SDAs) for the synthesis of all-silica zeolites in fluoride media and compared with the nonfluorinated molecule. The nonfluorinated DBDM cation can direct the synthesis either to an all-silica zeolite beta or to an all-silica zeolite ZSM-50 with the EUO framework topology. Under the same conditions that DBDM gives zeolite beta, benzyl-orthofluorobenzyldimethylammonium (o-FDBDM) readily directs the crystallization of pure all-silica ZSM-50 [Cmma, a = 13.726(3) angstrom, b = 22.171(5) angstrom, c = 20.254(4) angstrom]. Replacing o-FDBDM by the para fluoro derivative (p-FDBDM) results in a much slower crystallization of ZSM-50 whereas no crystalline product results if the meta fluoro derivative (m-FDBDM) is added. Single-crystal diffraction studies of selected ZSM-50 crystals prepared using DBDM and o-FDBDM determine the location of the template and of charge-balancing fluoride atoms bound to framework silicon atoms and included within [41516 2] cages in the framework. Furthermore, the o-FDBDM is found to order within the structure, with the fluorine-containing aromatic rings occupying channel, rather than cavity, locations. Computer simulations indicate how the position of fluorine in the SDA influences the template ordering.</p

Cooperative structure-directing effect of fluorine-containing organic molecules and fluoride anions in the synthesis of zeolites

Dibenzyldimethylammonium (DBDM) cations, singly fluorinated at the ortho, meta, and para positions of the aromatic ring, have been investigated as structure-directing agents (SDAs) for the synthesis of all-silica zeolites in fluoride media and compared with the nonfluorinated molecule. The nonfluorinated DBDM cation can direct the synthesis either to an all-silica zeolite beta or to an all-silica zeolite ZSM-50 with the EUO framework topology. Under the same conditions that DBDM gives zeolite beta, benzyl-orthofluorobenzyldimethylammonium (o-FDBDM) readily directs the crystallization of pure all-silica ZSM-50 [Cmma, a = 13.726(3) angstrom, b = 22.171(5) angstrom, c = 20.254(4) angstrom]. Replacing o-FDBDM by the para fluoro derivative (p-FDBDM) results in a much slower crystallization of ZSM-50 whereas no crystalline product results if the meta fluoro derivative (m-FDBDM) is added. Single-crystal diffraction studies of selected ZSM-50 crystals prepared using DBDM and o-FDBDM determine the location of the template and of charge-balancing fluoride atoms bound to framework silicon atoms and included within [41516 2] cages in the framework. Furthermore, the o-FDBDM is found to order within the structure, with the fluorine-containing aromatic rings occupying channel, rather than cavity, locations. Computer simulations indicate how the position of fluorine in the SDA influences the template ordering.</p