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)

Influence of the size, order and topology of mesopores in bifunctional Pd-containing acidic SBA-15 and M41S catalysts for n-hexadecane hydrocracking

The catalytic performance in n-hexadecane hydrocracking of a set of bifunctional catalysts composed of acidic mesoporous silicas, i.e., SBA-15, MCM-41, MCM-48, and amorphous silica-alumina (ASA), and Pd as acid and (de)hydrogenation components, respectively, was investigated. The selectivity to cracked products and the occurrence of secondary cracking depended on the pore topology, acidity, and Pd dispersion. The Si/Al ratio and the mesopore order of SBA-15 were modified by changing the pH in the synthesis step. Al was introduced in the M41S materials by post-synthesis grafting. All materials including ASA exhibited low acidity compared to crystalline zeolites. Increasing Al content led to a decrease of the order of mesopores. Secondary cracking of n-hexadecane was more pronounced for catalysts containing long one-dimensional cylindrical pores (SBA-15 and MCM-41) in comparison with catalysts containing three-dimensional ordered (MCM-48) or disordered (ASA) mesopores. The selectivity difference is attributed to differences in residence time of intermediates in the mesopores. The distance between acid sites located in mesopores and Pd nanoparticles primarily located outside these pores also influences the product distribution. Ideal hydrocracking operation is approached for ASA, MCM-48, and SBA-15 prepared at a high pH containing disordered mesopores

Mechanistic aspects of n-hexadecane hydroconversion:Impact of di-branched isomers on the cracked products distribution

The mechanism of n-hexadecane (n-C16) hydroisomerization/hydrocracking was studied over bifunctional catalysts employing Pd and Pt as (de)hydrogenation components and large-pore zeolites and (ordered) mesoporous materials as acidic supports. Zeolite Y and Beta supports were compared to amorphous silica-alumina as well as aluminium-containing ordered mesoporous MCM-41, MCM-48 and SBA-15 materials to cover a wide range of pore sizes and topologies. Products were analyzed in terms of mono- and di-branched C16 isomers and cracked products as a function of the n-C16 conversion. All samples followed a similar mechanism in which, at low conversion, di-branched isomers with methyl groups toward the ends of the tetradecane chain were observed. At higher conversion, the products shifted to isomers with methyl groups closer to the center at higher n-C16 conversion. Consistent with these differences, the typical ‘M’ shape distribution of cracked products was obtained at low conversion, which evolved into ideal symmetric cracking patterns at higher conversion. The unusual di-branched isomer distribution at low conversion and the corresponding ‘M’ shaped cracked products distribution have earlier been associated with pore mouth catalysis induced by restricted diffusion in medium-pore zeolites. Here we show that such reaction pathways also occur in catalysts with large enough pores to exclude diffusion restrictions

Influence of polyvinylpyrrolidone as stabilizing agent on Pt nanoparticles in Pt/H-BEA catalyzed hydroconversion of n-hexadecane

Colloidal Pt particles prepared using polyvinylpyrrolidone (PVP) as a capping agent were loaded on acidic Beta zeolite, with the aim of obtaining bifunctional catalysts with controllable dispersion for the hydroconversion of n-hexadecane (n-C16). Among the different approaches to remove PVP, reduction was the most effective whilst maintaining the initial Pt particle size after deposition on the zeolite. The presence of small amounts of nitrogen- and carbon-containing products from PVP decomposition affected the acidity stronger than the Pt metal function. Therefore, it was not possible to establish the effect of Pt particle size on the performance of the Pt/Beta zeolites in n-C16 hydroconversion and the performance trends were dominated by the acidity differences. Small Pt particles on zeolite obtained by using high PVP/Pt ratios during the colloidal synthesis step presented lower Brønsted acidity than catalysts containing larger Pt particles. The resulting variations in Pt/H+ ratios led to a transition of observed ideal cracking behavior for weakly acidic catalysts (small Pt particles, larger amount of PVP residuals) to overcracking behavior for catalysts with stronger acidity. We find that the Pt/H+ ratio and the number of acid-catalyzed steps extrapolated to zero conversion are better indicators of ideal cracking behavior than the isomers yield