Investigação de metais suportados em silicatos lamelares e sua aplicação em reações de descarboxilação, por cálculos ab initio

Abstract

Magadiite is one of the hydrous layered silicate composed of siloxane (SiO4) and silanols/silanolates (SiO3OH/SiO3O - ) groups. This compound is well known for producing modified materials that improve acidity properties, leading to catalysts with wide application in adsorption and heterogeneous catalysis. Thus, this thesis aims to develop new bifunctional metal catalysts of the M/H-magadiite, M/H-[AlTd]-magadiite, and M/H-[AlOh]-magadiite (M = Au, Ag, Pd, Pt) systems through computational calculations based on Density Functional Theory (DFT). The simulations were performed in the QUANTUM-ESPRESSO package, with periodic boundary conditions and a set of plane wave basis sets. The PBE-GGA functional and ultrasoft Vanderbilt pseudopotentials were chosen for electronic density. Hmagadiite, H-[AlTd]-Magadiite, and H-[AlOh]-magadiite surfaces were simulated at a vacuum of 19 Å to test noble metal atoms (Ag, Au, Pd, Pt) at different deposition sites. The optimized models showed metal atoms remained close to the silanol groups for M/H-magadiite, between OSiOH and Hextra for M/H-[AlTd]-magadiites, and between OAlOH and Hextra for M/H-[AlOh]- magadiites. Aluminum provided significant improvement for metal deposition, mainly for H- [AlOh]-magadiites, where chemisorption was observed. The Pt/H-[AlOh]-magadiite system demonstrated the best metallic stability, suggesting that this material may be promising in preventing sintering processes. Then, the most stable M/surface system were selected for the insertion of metal clusters. Charge density calculations showed that the decreasing order of interaction with the surfaces was Pt12 > Pd12 > Au12 > Ag12. Again, the aluminum site of the H- [AlOh]-magadiite was highlighted for stabilizing clusters near the acid site, where Pt12 cluster chemisorbed in the surface (O–Pt–H), turning this system the most promising catalysts. Thus, decarboxylation reactions of propanoic acid were simulated with in the Pt12/H-[AlOh]- magadiite through the CI-NEB (Climbing Image-Nudging Elastic Band) method. The reaction was proposed in nine steps. First, the adsorption of carboxylic acid on the surface and its dissociation in the Pt12 cluster, with formation of a H2 molecule, were simulated. These steps produced small energy barriers, with ΔE = 33.55 kJ.mol-1. Then, there was desorption of H2 for conversion of the alkyl group into ethane and CO2 in the decarboxylation reaction. In this case, the barriers found were 259.92 kJ.mol-1 and 24.42 kJ.mol-1, for ethane and CO2 removal, respectively. The CI-NEB results indicated that the steps may be favorable under adequate temperature conditions, corroborating the catalytic efficiency of the Pt12/H-[AlOh]- magadiite model indicated by the structural analysis.