Sonia Bocanegra

Ir and IrSn catalysts with different Sn contents (0.5, 0.7 and 0.9 wt%) were prepared using MgAl₂O₄ supports synthesized using two different techniques (the citrate–nitrate combustion and coprecipitation methods). Both supports, with a spinel structure, presented low acidity and good textural properties. However, the support prepared by coprecipitation had higher specific surface area and pore volume than the one prepared by combustion, which would favor the dispersion of the metals to be deposited. Likewise, during the preparation of the catalytic materials, a very good interaction was achieved between the metals and both supports, which was confirmed by the presence of sub-nanometer atomic clusters in the mono- and bimetallic catalysts. Regarding the catalytic properties, while the monometallic Ir/MgAl₂O₄ samples lead to a very low conversion of n-butane and a selectivity towards hydrogenolysis products, the addition of Sn to Ir increases the conversion, decreases hydrogenolysis and therefore sharply increases the selectivity towards the different butenes. Catalysts with higher Sn loadings present better catalytic behavior. One of the roles of the Sn promoter would be to geometrically modify the Ir clusters, drastically decreasing the hydrogenolytic activity. This effect, added to the strong electronic modification of the Ir sites by the action of Sn, with probable Ir-Sn alloy formation, is responsible for the high catalytic performance of these bimetallic catalysts.

In order to determine the catalytic stability, PtSn structured catalysts supported on thin films of MgAl₂O₄ or ZnAl₂O₄ deposited on α‐Al₂O₃ spheres were studied through five reaction‐regeneration cycles in the n‐butane dehydrogenation. Bimetallic catalysts show good catalytic stability along the five reaction‐regeneration cycles. Among the different synthesized catalysts, the PtSn/Sp‐Zn‐CN catalyst showed the best catalytic behaviour, showing very good values of yields to butenes and excellent catalytic stability along the cycles, even better than the one of a structured commercial catalyst used industrially for the dehydrogenation of paraffins. The characterization results by temperature‐programmed reduction and X‐ray photoelectron spectroscopy showed some modifications in the metallic phase of the catalysts after the cycles, mainly in the Sn/Pt surface ratios and in segregation effects in some catalysts. However, the transmission electron microscopy (TEM) results are conclusive in the sense that, after the cycles, the bimetallic catalysts maintained a very high proportion of particles with sizes between 1–2 nm, and therefore, preserved a high metallic dispersion.

This work analyzes the deposition conditions of thin layers of γ‐Al₂O₃ by dip‐coating on a spherical substrate of α‐Al₂O₃ spheres. Results showed that under controlled deposition conditions, a structured support of α‐Al₂O₃ spheres coated by a uniform γ‐Al₂O₃ layer (thickness ≈ 15 µm) of good adherence can be attained. Optimal preparation conditions for the coating, that involved pretreatment of the substrate, gel formation, deposition conditions, and thermal treatments, were obtained. Pt catalysts supported on this structured material can be used as promising dehydrogenation systems. In this sense, a monometallic structured catalyst showed both good metallic dispersion and catalytic performance in n‐butane dehydrogenation reaction compared to another conventional catalytic system.

Structured materials composed of α-Al₂O₃ spheres covered with a porous and thin layer of MgAl₂O₄ or ZnAl₂O₄ were obtained. The innovative coating method developed consists in the formation of a precursor gel of MgAl₂O₄ or ZnAl₂O₄ by citrate-nitrate combustion method. The alumina spheres are submerged in this gel and then submitted to a drying process in vacuum and to calcinations with N₂ and N₂/O₂. SEM results showed that a very rough and porous layer uniformly covers the spheres. The thickness of the layer reaches the maximum value of 16 µm in the sample coated with two layers of ZnAl₂O₄. XRD results confirmed the MgAl₂O₄ or ZnAl₂O₄ spinel formation in the layered material. The structured materials were used as Pt catalysts supports, showing a good catalytic activity in the n-butane dehydrogenation reaction.

The catalytic performance in n-butane dehydrogenation of bimetallic PtSn, PtGa and PtIn, and trimetallic PtSnIn and PtSnGa catalysts (with low metal contents) supported on a MgAl2O4 prepared by a nove