Albarracin-Suazo, Sandra Cecilia

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    DFT study on the effect of aluminum atom distribution in zn-exchanged mfi zeolites for methane c-h bond activation
    (2018) Albarracin-Suazo, Sandra Cecilia; Pagán-Torres, Yomaira J.; College of Engineering; Curet-Arana, María C.; Cardona-Martínez, Nelson; Martínez-Iñesta, María M.; Department of Chemical Engineering; Neco-Valle, Juan C.
    The direct conversion of methane to useful chemicals is economically attractive owing to the vast availability of natural gas resources. Nevertheless, a fundamental limitation is the chemical inertness and stability of the C-H bond of the CH4 molecule. Recently, metal-ion exchanged aluminosilicates (i.e., Zn-MFI,) have been shown to be active in the C-H bond activation of methane, however elucidation of the effect of the Al atom array within the rings of MFI on the formation of the active site remains unknown. In this work, we use Density Functional Theory (DFT) with specific treatment of Van der Waals forces to systematically analyze the effect of Al atom distribution within the α-ring of Zn-MFI on the activation of methane. In these studies, we have: (1) identified the preferred site for Zn2+ exchange in the α-ring of MFI as a function of Al atom arrangement, (2) developed a relative energy diagram for C-H bond activation in CH4 as a function of the Al atom array in the α-ring of Zn-MFI, and (3) developed correlations based on electrostatic and electron transfer interactions between CH4, the Zn metal center, and the MFI α-ring. Among the clusters analyzed, we determined that the lowest energy barrier for C-H bond activation of CH4 is obtained for the Zn-MFI-5 configuration in which the Al atoms are located at the T8 and T5 sites of the α-ring. Our work unravels that Al atoms distribution produces a variation in the energy barrier in the detachment of a hydrogen atom from methane. Furthermore, the most energetically stable Al configuration does not yield the lowest energy barrier for methane activation.
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    Selective hydrodeoxygenation of biomass-derived oxygenates over heterogeneous Mo-based catalysts
    (2021-07) Albarracin-Suazo, Sandra Cecilia; Pagán-Torres, Yomaira J.; College of Engineering; Curet-Arana, María C.; Cardona-Martínez, Nelson; Martínez-Iñesta, María M.; Department of Chemical Engineering; Ortiz-Ríos, Gloribell
    The development of catalytic pathways to produce commodity chemicals from lignocellulosic biomass is promising for substituting non-renewable fossil feedstock use. This dissertation studied the catalytic performance of molybdenum-based materials for the selective cleavage of C-O bonds in biomass-derived compounds (1,4-anhydroerythritol, tartaric acid, and glycerol) to produce value-added chemicals. The reactions studied include the transformation of (i) 1,4-anhydroerythritol to tetrahydrofuran, (ii) tartaric acid to succinic acid, and (iii) glycerol to 1,2-propanediol and 1,3-propanediol.
    Hydrodeoxygenation reactions represent a transformation by which C-O bonds can be selectively cleaved. A heterogeneous MoOx-Pd/TiO2 catalyst was active, selective, and stable in the hydrodeoxygenation of 1,4-anhydroerythritol to tetrahydrofuran. Results obtained from catalyst characterization studies (Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Diffuse Reflectance Infrared Fourier Transform Spectroscopy) and reaction kinetics indicate that hydrogen atoms dissociated over Pd nanoparticles promote the reduction of supported MoOx species from a Mo6+ to Mo5+/Mo4+ oxidation state. The reduced Mo5+ and Mo4+ centers facilitate the cleavage of vicinal hydroxyls in 1,4-anhydroerythritol to produce the intermediate 2,5-dihydrofuran, which undergoes subsequent hydrogenation to tetrahydrofuran.
    The hydrodeoxygenation of tartaric acid to succinic acid over MoOx-Pd/TiO2 was also examined. The MoOx-Pd/TiO2 catalyst exhibited the highest succinic acid production rates among supported oxophilic metal (MoOx, ReOx, WOx) and noble metal (Pd, Pt, Rh) combinations studied. Insights from catalyst characterization and kinetics indicate that an active Mo5+ center is formed, cleaving internal C-O bonds from tartaric acid while leaving carboxylic acid end-groups intact.
    Another catalytic reaction reported in this dissertation is the hydrodeoxygenation of glycerol over Mo2C, and Cu promoted Mo2C. These catalysts were studied under continuous flow reaction conditions to determine reaction orders, apparent activation energy barrier, product selectivity, and catalyst stability. Reaction kinetic studies show selectivity trends for Mo2C and Cu-Mo2C powders that agree with the literature reported surface science and computational studies. Mechanistic insights suggest that the reaction proceeds through a concerted bond cleavage and formation pathway. The modification of the catalyst with Cu modifies the oxophilicity of the Mo site, and can be tailored to control the number of glycerol C-O bonds which are cleaved.