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)

Brønsted acid sites of zeolitic strength in amorphous silica-alumina

The most acidic OH groups in silica-aluminas (zeolites, clays, amorphous silica-aluminas) can be made to react selectively with C6D6 to give acidic OD groups; quantification by IR spectroscopy shows that differences in the overall Brønsted acidity of aluminosilicates are dominated by differences in the density of sites of similar acid strength

Dehydration of glucose to 5-Hydroxymethylfurfural using Nb-doped Tungstite

Dehydration of glucose to 5-hydroxymethylfurfural (HMF) remains a significant problem in the context of the valorization of lignocellulosic biomass. Hydrolysis of WCl6 and NbCl5 leads to precipitation of Nb-containing tungstite (WO3⋅H2O) at low Nb content and mixtures of tungstite and niobic acid at higher Nb content. Tungstite is a promising catalyst for the dehydration of glucose to HMF. Compared with Nb2O5, fewer by-products are formed because of the low Brønsted acidity of the (mixed) oxides. In water, an optimum yield of HMF was obtained for Nb–W oxides with low Nb content owing to balanced Lewis and Brønsted acidity. In THF/water, the strong Lewis acidity and weak Brønsted acidity caused the reaction to proceed through isomerization to fructose and dehydration of fructose to a partially dehydrated intermediate, which was identified by LC-ESI-MS. The addition of HCl to the reaction mixture resulted in rapid dehydration of this intermediate to HMF. The HMF yield obtained in this way was approximately 56 % for all tungstite catalysts. Density functional theory calculations show that the Lewis acid centers on the tungstite surface can isomerize glucose into fructose. Substitution of W by Nb lowers the overall activation barrier for glucose isomerization by stabilizing the deprotonated glucose adsorbate.\u3cbr/\u3

The nature of strong Brønsted acidity of Ni-SMM clay

The origin of the high Brønsted acidity of Ni-SMM (Ni-substituted synthetic mica-montmorillonite; beidellite structure) clays was investigated. Ni-SMM clays with varying F content, SMM with F and Ni-SMM without F in the structure were synthesized under hydrothermal conditions. Ni-SMM clays with intermediate F content contained very strong Brønsted acid sites. The optimum Ni-SMM sample outperformed zeolites such as H-ZSM-5 and H-USY (ultrastabilized Y) in alkane hydroisomerization. Infrared spectroscopy with different probe molecules shows that Ni-SMM contains two types of BAS. In addition to acid sites also observed in other clays and amorphous silica-alumina, Ni-SMM contains a small number of acid sites that are stronger than the acid sites in zeolites. The number of such sites does not depend on Ni-SMM reduction. A small amount of strongest Brønsted acid sites positions the catalytic activity of Ni-SMM clay beyond that of zeolites. Periodic density functional theory calculations show that the substitution of octahedral [Al3+-O]+ by [Ni2+-F]+ causes high acidity of the interlayer proton connected to the aluminium-occupied tetrahedron. This explains why Ni-SMM (no F in the structure) and SMM with F (no Ni in the structure; F replaces only structural OH) exhibit conventional clay acidity. The presence of Ni in the octahedral layer leads to isomorphous substitution of bridging O anions that connect the octahedral with the tetrahedral layer by F. The electron-withdrawing nature of the bridging F induces the unusually high acidity of the interlayer protons in Ni-SMM