Obtaining Olefins by Dehydration of Alcohols on Laminar Materials

Mesoporous materials with acid-base properties are relevant for processes aimed at the valorization of biomolecules and the production of high value-added compounds. Among these processes, the dehydration of ethanol to obtain ethylene represents a strategic alternative within the framework of biorefineries. Structured mesoporous silicates have catalytic properties that are highly dependent on the presence and nature of surface acidic sites. In this sense, the incorporation of metallic species in the siliceous matrix is an effective strategy to modify both the morphology and acidity of the material, generating active sites such as Brønsted and/or Lewis that favor dehydration reactions. Hafnium has aroused interest as a dopant in catalytic systems due to its ability to form oxides with acidic properties and high thermal stability by inducing structural and electronic changes that directly impact catalytic performance. However, existing literature on the effect of hafnium on laminar mesoporous silicates and their specific application in ethanol dehydration at moderate temperatures remains sparse. The present work aims to synthesize a pure laminar mesoporous silicate (SM) and its hafnium-modified version (HfSM), evaluate its morphological characteristics, composition and its catalytic performance in the dehydration of ethanol into ethylene. It seeks to establish the relationship between the structural modification induced by hafnium, the generation of active acid sites and the catalytic activity of the material, thus contributing to the design of more efficient catalysts for sustainable processes of transformation of alcohols. A laminar mesoporous support was synthesized using cetyl-trimethyl-ammonium bromide (Aldrich; 99%) mixed with distilled water for 15 min, then a known amount of sodium silicate (Na2O7Si3; Aldrich) was added and homogenized for 30 min at room temperature. The pH of the alkaline solution was adjusted from 12 to 9 by adding 2M sulfuric acid (H2SO4: 2M). A white precipitate was formed, then filtered, washed and finally dried at 80°C for 48 h. The resulting solid was pulverized and calcined at 600°C for 6 h in extra-dry air flow, obtaining a pure laminar mesoporous silicate denoted as SM. A portion of this material was impregnated with a known amount of hafnium by the slow dripping of a hafnium tetrachloride solution (HfCl4, Aldrich, 98%) and homogenized under constant agitation for 4 h at room temperature. The filtering, washing, drying and calcination procedures were carried out under the same conditions as SiO2, the resulting material was denoted as HfSM. The materials were characterized by scanning electron microscopy using a JEOL model 5800LV microscope at an operating voltage of 6 and 10 kV. Both materials were catalytically evaluated in ethanol dehydration reactions to obtain olefins, using a catalyst mass of 100 mg and a reaction temperature of 300 °C. The reactions were carried out in a system consisting of three sections: saturation, reaction and analysis. Figure 1 shows the morphology of the SM and HfSM materials, in both cases a porous configuration is observed consisting of agglomerates of particles with varied and irregular sizes. The incorporation of Hafnium in the synthesis stage of the SM material caused a change in the surface conformation of the silicate, observing compaction in the agglomerates, suggesting a more homogeneous surface and probably an increase in the specific area of the HfSM material. On the other hand, the EDS analysis of the HfSM material (Figure 2) confirmed the presence of the doping metal by quantifying the elements present in the sample and determining that the Hafnium content used for the silicate modification was 3.98% mol, which is a value very close to the theoretical content calculated. The ethanol decomposition profile on the evaluated catalysts is presented in Figure 3. The catalytic activity observed in the SM material was less than 1%, attributed to the absence of active acid sites, which limits the ability to promote ethanol dehydration. On the contrary, the presence of HfO2 on the SM caused a morphological change, as observed in Figure 1, and also promoted an increase in active acid sites such as Brönsted or Lewis, or both, due to the interaction of Si-O-Hf-OH [1] bonds. In the HfSM material, under the same reaction conditions used for the pure material (SM), a conversion of 40% ethanol was obtained, with a selectivity of 85% towards ethylene. These results are consistent with the reported literature for acid catalysts used in the dehydration of alcohols [2,3] In conclusion, the presence of Hafnium in the structure of the laminar-type silicate was active in the dehydration of ethanol to produce ethylene at low reaction temperature, without generating dehydrogenation products, which confirms the absence of strong basic sites, or, as the case may be, the basic sites are too weak to produce acetaldehyde or acetylene. It is worth mentioning that the catalysts were stable with a negligible deactivation during the first 100 min of reaction. Likewise, when the reaction temperature increases, the ethanol conversion percentage increases considerably and the selectivity is towards ethylene, which is a high added-value product in the petrochemical industry, used as raw material in the production of plastics, solvents and other chemical derivatives of interest to biorefineries [9]. Microscopy SEM: a) SM and b) HfSM EDS Analysis of HfSM Material Obtaining olefins. (a) SM, b) HfSM