Joaquin L. Brito | Instituto Venezolano de Investigaciones Cientificas - IVIC (original) (raw)

Papers by Joaquin L. Brito

Research paper thumbnail of Removal of lead in aqueous solutions using amorphous nanoaluminophosphates

Amorphous aluminophosphates nanoparticles with diameter sizes from 23 to 30 nm were used for the ... more Amorphous aluminophosphates nanoparticles with diameter sizes from 23 to 30 nm were used for the adsorption of lead ions (Pb2+) in aqueous solutions. The materials were
biosynthesized using Saccharomyces cerevisiae as a biological template. Adsorption studies were conducted using the batch method, employing 100 mg of the solid and 200 µg L-1 of
lead ions. The quantitative determination of Pb2+ was carried out by electrothermal atomic absorption spectrometry (ETAAS). The percentage of removal (EF), adsorption capacity
qe (µg g-1
) and removal coefficient Kd (mL g-1
) averaged 40, 90 and 800, respectively. The results showed the potential of these materials for the adsorption of Pb2+ ions, and that these materials are good candidates for use in the remediation of contaminated water

Research paper thumbnail of Influence of toluene on the catalytic activity of NiPdCe catalyst for selective hydrogenation of 1,3-butadiene

Fuel, 2013

h i g h l i g h t s " Conversion of BD increases with toluene addition. " Conversion of BD increa... more h i g h l i g h t s " Conversion of BD increases with toluene addition. " Conversion of BD increases with higher pressure. " Toluene addition enhances 2BE/1BE selectivity and favors the trans-isomer. " 1BE hydrogenation losses are reduced by toluene addition and pressure lowering.

Research paper thumbnail of Theoretical study of CO adsorption and oxidation on Au3–5 clusters supported on silico-aluminophospates

Computational and Theoretical Chemistry, 2014

The Au 3-5 clusters supported on silico-aluminophospates (SAPO-11) were studied using quantum che... more The Au 3-5 clusters supported on silico-aluminophospates (SAPO-11) were studied using quantum chemistry calculations, by means of the ONIOM2 methodology at two different levels of calculation, B3LYP for the high level and UFF for the low level. These calculations were performed on Au 3 /SAPO-11, Au 4 /SAPO-11, Au 5 /SAPO-11, CO-Au 3 /SAPO-11, CO-Au 4 /SAPO-11 and CO-Au 5 /SAPO-11 aggregates to analyze the geometries of small clusters of Au 3 , Au 4 and Au 5 on SAPO-11 support. Au 3 cluster presents a triangle structure in Au 3 /SAPO-11. Au 4 cluster shows a ''Y shaped'' structure in Au 4 /SAPO-11. Au 4 as a rhombus structure is also studied but it is a less stable structure. Au 5 cluster displays a ''pentagon shaped'' structure as the most stable. The CO interaction with Au 3 , Au 4 and Au 5 /SAPO-11 is studied; this CO adsorption is different from that reported in the literature. The formation energy DE F of the aggregates, the CO adsorption energy DE ads on them and the change of energy of the oxidation reaction are presented. Some relevant charges and bond indexes were calculated. On supporting all Au 3-5 there is a charge transfer from the Au cluster to the binding O atoms. On CO adsorption in all Au 3-5 clusters there is a charge transfer from the cluster to the CO. j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m p t c

Research paper thumbnail of Parametric calculations of Mo-allyl complexes anchored on silica

Journal of Molecular Catalysis A: Chemical, 2001

The SiO 2 anchored Mo( 3 -C 3 H 5 ) 4 catalyst was modeled by means of the CNDO-UHF parametrical ... more The SiO 2 anchored Mo( 3 -C 3 H 5 ) 4 catalyst was modeled by means of the CNDO-UHF parametrical method. Two models (two separated centers (TSC) and two adjacent centers (TAC)) were proposed for the silica pre-treated at relatively low temperature. An analysis of total energies, bond indexes, diatomic energies, and parametric diatomic binding energies for selected bonds suggests the existence of both models on the surface. However, TSC is favored with respect to TAC. Furthermore, it is observed an allyl group interaction with the support through a C-O bond that leads to a C-C bond activation. Also, C-H activation is found due to Mo-H interaction. These results support several features of the generally accepted mechanism for olefin metathesis in this type of catalyst. Finally, experimental results of NMR, obtained in previous work by some of the authors, are interpreted by inequivalency of different allyl ligands.

Research paper thumbnail of Theoretical study of olefin oxidation process on a dioxo-μ-oxo Mo catalyst

International Journal of Quantum Chemistry, 2008

The dimeric complex dioxo (-oxo) thiocyanato of molybdenum(VI) bearing a 4,4Ј-di-ter-butyl-2,2Ј-b... more The dimeric complex dioxo (-oxo) thiocyanato of molybdenum(VI) bearing a 4,4Ј-di-ter-butyl-2,2Ј-bipyridine ligand in trans conformation (complex 1) has been used in catalytic reactions of olefins (ethylene) oxidation, giving as intermediate a dimeric complex oxo bis (-oxo) thiocyanato of molybdenum(V) in cis conformation (complex 2). Binding and electronic properties were calculated using the parametric quantum chemistry method for catalytic reactions, CATIVIC. The interaction of an ethylene molecule with complex 3 (cis version of complex 1) leads to the formation of the intermediate named complex 4. This complex is dissociated to form complex 5 plus ethylene oxide. Changes of electronic and bonding properties are analyzed in terms of atomic charge, orbital population, total binding energies, diatomic bond energies, diatomic energies, and Wiberg indexes, and equilibrium bond distances. The energy balance for the complete catalytic process is presented in a schematic way. Results confirm the extraordinary oxo-transfer abilities of this complex, considering that the whole process is exothermic (Ϫ44 kcal/mol). Regeneration of the catalyst is also calculated by the interaction of H 2 O 2 with complex 5.

Research paper thumbnail of Theoretical study of Au/SAPO-11 catalyst and its potential use in thiophene HDS

Journal of Molecular Catalysis A: Chemical, 2010

Quantum chemistry calculations were carried out, using ONIOM2 methodology, in order to investigat... more Quantum chemistry calculations were carried out, using ONIOM2 methodology, in order to investigate the thiophene interaction with gold supported on silicoaluminophospates molecular sieves (Au/SAPO-11) catalysts. Two models were studied, one containing one Au atom per site, and the other with two Au atoms per site. Thiophene adsorption was found to be 1 type. This adsorption presents a H of −13.2 and −9.7 kcal/mol, for the models with one Au atom (Au/SAPO-11), and two Au atoms (Au 2 /SAPO-11), respectively. The partial hydrogenation of the thiophene-Au/SAPO-11 and thiophene-Au 2 /SAPO-11 complexes gives 2,5-dihydrothiophene (DHT), with a H of −23.0 and −36.8 kcal/mol, respectively. 2-Butene production was found in both models with further hydrogenation. Likewise the direct butadiene elimination is achieved, but only with the separated Au dimer ( H = −17.5 kcal/mol).

Research paper thumbnail of Theoretical calculations of silica supported Mo2(η3-C3H5)4 species

Journal of Molecular Structure: THEOCHEM, 2003

Two models are studied for the anchored system Mo 2 (h 3 -C 3 H 5 ) 4 /SiO 2 : united model (UM) ... more Two models are studied for the anchored system Mo 2 (h 3 -C 3 H 5 ) 4 /SiO 2 : united model (UM) and separated model (SM). Total energies, bond orders, diatomic energies, and parametric diatomic binding energies of selected bonds were calculated for Si 8 O 24 H 16 , Mo 2 (h 3 -C 3 H 5 ) 4 , and Mo 2 (h 3 -C 3 H 5 ) 4 /Si 8 O 24 H 16 systems, using a CNDO -UHF parametrical method. From the energetic viewpoint, UM is more stable than SM to explain the immobilizing of Mo 2 (h 3 -C 3 H 5 ) 4 on silica. It is observed Mo -H interaction that leads to C -H activation. These results support several features of the generally accepted mechanism for olefin metathesis. An interpretation of these theoretical results was carried by comparing with PAS -FTIR, 1 H and 13 C solid-state NMR, and XPS trends. q

Research paper thumbnail of ESTUDIO TEORICO DEL ANCLAJE DE Mo 2 (η 3 -C 3 H 5 ) 4 EN SILICA CON EVOLUCION DE PROPENO Y POSTERIOR REDUCCIÓN DE LAS ESPECIES EN LA SUPERFICIE

Resumen Este trabajo forma parte de una serie de cálculos teóricos del anclaje del Mo 2 (η 3 C 3 ... more Resumen Este trabajo forma parte de una serie de cálculos teóricos del anclaje del Mo 2 (η 3 C 3 H 5) 4 en SiO 2 . Abstract This work is one of a series of theoretical calculations of the anchoring of Mo 2 (η 3 C 3 H 5) 4 on SiO 2 . In this case, propene evolution and the formation of resulting species due to reduction are considered. The calculations were done, using the semipirical method CNDO. Interatomic distances, diatomic energies, bond orders, binding diatomic energies will be evaluated.

Research paper thumbnail of Interpretación de Datos Experimentales y Teóricos en el Estudio de Mo 2 (η 3 C 3 H 5 ) 4 Soportado en Sílica

Key Words: Modelaje Molecular, Anclaje de Mo 2 (η 3 -C 3 H 5) 4 Introducción Los catalizadores ba... more Key Words: Modelaje Molecular, Anclaje de Mo 2 (η 3 -C 3 H 5) 4 Introducción Los catalizadores basados en molibdeno son de amplio uso industrial debido a la versatilidad de este elemento, que posee una variedad de estados de oxidación estables y una rica química de coordinación. El estudio de catalizadores obtenidos mediante reacción de grupos OH de soportes y complejos alílicos de Mo, con posterior transformación de las especies superficiales mediante reacciones red-ox, ha recibido considerable atención en la literatura [1]. En nuestro laboratorio, hemos estudiado los complejos alílicos anclados en sílica, sin haber sido sometidos a transformaciones adicionales luego del anclaje [2], debido tanto al interés en la química de las especies superficiales como a su potencial en algunas reacciones de transformación de hidrocarburos. Las especies ancladas a las superficies han sido caracterizadas mediante Espectroscopía Fotoacústica en el Infra-rojo por Transformada de Fourier (PAS-FTIR),...

Research paper thumbnail of Estudio Teórico de una Zeolita Au/SAPO11 y su Reactividad Catalítica en Oxidación de CO

En este estudio teórico realizamos cálculos químico cuánticos usando el método ONIOM a dos nivele... more En este estudio teórico realizamos cálculos químico cuánticos usando el método ONIOM a dos niveles de cálculos. Estos cálculos se llevaron a cabo sobre agregados de SAPO11, Au/SAPO11, CO-Au/SAPO11 y en la reacción de oxidación en este último sistema. Igualmente se realizaron cálculos sobre Au2/SAPO11, 2CO-Au2/SAPO11 y la reacción de oxidación. Los resultados revelan que Au/SAPO11 es muy estable teniendo un ΔE de –81 kcal/mol, que la adsorción de CO presenta un ΔE de –38 kcal/mol y la reacción de oxidación de CO un ΔE= -29.2 kcal/mol. En el caso de Au2/SAPO11 se obtiene un ΔE= -163.7 kcal/mol, el 2CO-Au2/SAPO11 un ΔE= -60.6 kcal/mol, y la reacción de oxidación de 2CO-Au2/SAPO11 un ΔE= -103 kcal/mol. En ambos modelos la reacción de oxidación presenta un mecanismo tipo Eley-Rideal con el CO adsorbido. Los resultados de los cálculos revelan que este tipo de material resulta interesante para dispersar Au, y por ende para su uso como catalizadores en diferentes reacciones.

Research paper thumbnail of Probing silica-supported Mo 2+ by means of FT-IR of adsorbed CO

Catalysis letters, 1998

In a recent work, López et al. studied the interaction of CO with silica-supported molybdenum ato... more In a recent work, López et al. studied the interaction of CO with silica-supported molybdenum atoms by means of density functional theory calculations. They considered different reactive sites, with Mo atoms in various formal oxidation states (O-S), ranging from +II to +VI, and using both molecular and cluster models of the Mo/SiO 2 surface sites. According to their calculations, models with neutral Mo atoms in any O-S from +II to +V result in negative ("red") shifts of ν(CO), the frequency of the adsorbed CO molecule, with respect to that of gas phase CO. Thus, these models can not account for the IR peak at 2170 cm −1 ("blue" shifted by 27 cm −1 ) which had been observed experimentally [2] for allyl-derived Mo/SiO 2 catalysts reduced in hydrogen at 823 K. This peak had been assigned [2] to Mo(II) (or Mo 2+ , see below) species, which were detected by XPS in >90% concentration with respect to total Mo in the surface of the catalyst reduced at 823 K [3]. On the other hand, calculations by López et al.

Research paper thumbnail of Estudio teórico del efecto del H2O sobre CO adsorbido sobre catalizadores Au/SAPO-11. ¿Es viable la reacción WGS en este sistema?

Estudio teórico del efecto del H 2 O sobre CO adsorbido sobre catalizadores Au/SAPO-11. ¿Es viabl... more Estudio teórico del efecto del H 2 O sobre CO adsorbido sobre catalizadores Au/SAPO-11. ¿Es viable la reacción WGS en este sistema? Resumen En este estudio se llevaron a cabo cálculos químico cuánticos, usando la metodología ONIOM2 en 2 niveles, para determinar los cambios de energía en la interacción de los silico-aluminofosfatos intercambiados con oro (Au/SAPO-11) con CO y H 2 O. Se estudiaron 2 modelos, uno que contiene un átomo de Au por sitio (Au/SAPO-11) y otro con 2 átomos de Au por sitio. Los resultados revelan la formación de un posible intermediario tal como HO=C=OH en ambos modelos es un proceso energéticamente posible. Así como la formación del ácido fórmico es un proceso enérgicamente favorable. Este trabajo muestra por primera vez la habilidad potencial de los sistemas catalíticos Au/SAPO-11 para realizar la reacción de desplazamiento de agua o WGS a través del intermediario HO=C=OH -Au/SAPO-11 y HO=C=OH –Au 2 /SAPO-11. Palabras claves: ONIOM; Catálisis Computacional; ...

Research paper thumbnail of Anclaje y caracterizaciòn de tetra--alildimolibdeno sobre sìlica

Research paper thumbnail of Estudio teórico de la oxidación catalítica de CO en agregados pequeños Au 3-5 sobre catalizadores Au/SAPO-11

Introducción Se han hecho innumerables estudios experimentales y teóricos sobre las propiedades e... more Introducción Se han hecho innumerables estudios experimentales y teóricos sobre las propiedades electrónicas y estructurales de los agregados de Au, debido a sus potenciales aplicaciones para su uso en dispositivos electrónicos, en nano-materiales y como catalizadores, entre otros [1]. Algunos autores [1-3] reportan las estructuras más estables de los agregados de Au que contienen de 2 a 20 átomos. Häkkinen y Landman [4] estudiaron agregados de Au neutros y aniónicos Au 2−10 . H-Y Wang [5] ha publicado estudios con agregados de Au n donde n<14. Asimismo se han hecho estudios experimentales y teóricos sobre la oxidación de CO para revelar el origen de la gran actividad catalítica en los agregados de Au [6-7]. En este trabajo preliminar presentamos un estudio teórico de clusters pequeños de Au 3-5 soportados en SAPO-11, su interacción con CO y la reacción de oxidación del CO. Detalles computacionales y modelos moleculares Todas las optimizaciones de geometrías y cálculos fueron rea...

Research paper thumbnail of Estudio teórico de la oxidación catalítica de CO en agregados pequeños Au 5-6 sobre catalizadores Au/SAPO-11. Theoretical study of catalytic oxidation of CO on small clusters Au 5-6 on Au/SAPO-11 catalysts

Resumen: En este estudio teórico se realizaron cálculos químico cuánticos usando la metodología O... more Resumen: En este estudio teórico se realizaron cálculos químico cuánticos usando la metodología ONIOM(B3LYP:uff). Estos cálculos se efectuaron sobre agregados Au 5 /SAPO-11, Au 6 /SAPO-11, CO-Au 5 /SAPO-11, CO-Au 6 /SAPO-11 y de la oxidación de estas dos últimas especies. Se presentaran resultados de cambio de energía ΔE y de frecuencia de CO en los agregados. Palabras claves: Au/SAPO; ONIOM; Cálculos teóricos; DFT. Abstract: In this theoretical study, quantum chemistry calculations were done, using the ONIOM method at two different levels of calculation. These calculations were performed on Au 5 /SAPO-11, Au 6 /SAPO-11, CO-Au 5 /SAPO-11, CO-Au 6 /SAPO-11 clusters and on the oxidation of the two last species. Energy changes of the clusters and CO frequency are presented. Introducción El estudio teórico y experimental de agregados de Au ha alcanzado una gran importancia y trascendencia debido a que sus propiedades estructurales y electrónicas los han conducido a tener una gran aplica...

Research paper thumbnail of Theoretical study of small clusters Au 3−4 on Au/SAPO-11 catalysts and their interactions with CO

Quantum chemistry calculations were done, using the ONIOM2 methodology at two different levels of... more Quantum chemistry calculations were done, using the ONIOM2 methodology at two different levels of calculation, B3LYP for the high level and UFF for the low level. These calculations were performed on Au3/SAPO-11, Au4/SAPO-11, CO-Au3/SAPO-11 and CO-Au4/SAPO-11 aggregates to analyze the geometries of small clusters of Au3 and Au4 on SAPO-11 support. Au3 cluster present a triangle structure in Au3/SAPO-11. Au4 cluster shows a "Y shaped" structure in Au4/SAPO-11. Au4 as a rhombus structure is also studied but it is an unstable intermediate to the "Y shaped" structure. The CO interaction with Au3 and Au4/SAPO-11 is studied, this CO adsorption is different from reported in the literature. The formation energy ΔEF of the aggregates and the CO adsorption energy ΔE ads on them are presented.

Research paper thumbnail of Estudio teórico de la oxidación catalítica de CO en agregados pequeños Au3-6 sobre catalizadores Au/SAPO-11

In this theoretical study, quantum chemistry calculations were done, using the ONIOM(B3LYP:uff) m... more In this theoretical study, quantum chemistry calculations were done, using the ONIOM(B3LYP:uff) method at two different levels of computation. These calculations were performed on Au3-6/SAPO-11, CO-Au3-6/SAPO-11 clusters and on the oxidation of these species. The supported Aun (n=3-6) structures are presented and compared to the planar, similarly published structures of pristine Au aggregates. Energy changes of the formation of the clusters and CO frequency are presented. ΔE results indicate that Au clusters formation on SAPO-11 are stable from the energetically point of view. Likewise the CO-Au3-6/SAPO-11 oxidation calculations show that they are highly exothermic reactions. The CO adsorption is moderated due to the Au aggregates stability. Keywords: Au/SAPO; ONIOM; Theoretical calculations; DFT.

Research paper thumbnail of Theoretical study of small clusters Au 5−6 on Au/SAPO-11 catalysts and their interactions with CO

Journal of Computational Methods in Sciences and Engineering

Quantum chemistry calculations were done, using the ONIOM2 methodology at two different levels of... more Quantum chemistry calculations were done, using the ONIOM2 methodology at two different levels of calculation, B3LYP for the high level and UFF for the low level. These calculations were performed on Au5/SAPO-11, Au6/SAPO-11, CO-Au5/SAPO-11 and CO-Au6/SAPO-11 aggregates to analyze the geometries of small clusters of Au5 and Au6 on SAPO-11 support. Au5 cluster presents a pentagonal structure in Au5/SAPO-11. Au6 aggregate shows a "multi triangular" structure (as from a trapezoidal "W shaped" Au5) in Au6/SAPO-11. Au5 is also obtained as an "X shaped" structure. Similarly another Au6 aggregate configuration is obtained also multi triangular but as from X shape Au5 cluster. The CO interaction with Au5 and Au6/SAPO-11 is studied. The formation energy ΔEF of the aggregates, the CO adsorption energy ΔE ads on them and CO frequency are presented.

Research paper thumbnail of Theoretical study of the CO catalytic oxidation on Au/SAPO-11 zeolite

International Journal of Quantum Chemistry, 2010

Quantum chemistry calculations were carried out, using ONIOM2 methodology, to investigate the CO ... more Quantum chemistry calculations were carried out, using ONIOM2 methodology, to investigate the CO adsorption and oxidation on gold supported on Silicoaluminophospates (SAPO) molecular sieves Au/SAPO-11 catalysts. Two models were studied, one containing one Au atom per site (AuASAPO-11), and the other with two Au atoms per site (Au 2 ASAPO-11). The results reveal that the CO adsorption and oxidation are exothermic on Au/SAPO11 with an DE of À41.0 kcal/mol and DE ¼ À52.0 kcal/mol, for the adsorption and oxidation, respectively. On the Au 2 ASAPO-11 model, the CO adsorption and oxidation reaction occur, with a DE of À29.7 kcal/mol and À52 kcal/mol, respectively. According to our results, the oxidation reaction exhibits an Eley-Rideal type mechanism with adsorbed CO. The theoretical calculations reveal that this type of material could be interesting to disperse Au and consequently to strengthen its catalytic use for different reactions.

Research paper thumbnail of Evaluación de catalizadores a base de carburos y nitrus de circonio en la reacción de hidrodesulfuración de tiofeno

Research paper thumbnail of Removal of lead in aqueous solutions using amorphous nanoaluminophosphates

Amorphous aluminophosphates nanoparticles with diameter sizes from 23 to 30 nm were used for the ... more Amorphous aluminophosphates nanoparticles with diameter sizes from 23 to 30 nm were used for the adsorption of lead ions (Pb2+) in aqueous solutions. The materials were
biosynthesized using Saccharomyces cerevisiae as a biological template. Adsorption studies were conducted using the batch method, employing 100 mg of the solid and 200 µg L-1 of
lead ions. The quantitative determination of Pb2+ was carried out by electrothermal atomic absorption spectrometry (ETAAS). The percentage of removal (EF), adsorption capacity
qe (µg g-1
) and removal coefficient Kd (mL g-1
) averaged 40, 90 and 800, respectively. The results showed the potential of these materials for the adsorption of Pb2+ ions, and that these materials are good candidates for use in the remediation of contaminated water

Research paper thumbnail of Influence of toluene on the catalytic activity of NiPdCe catalyst for selective hydrogenation of 1,3-butadiene

Fuel, 2013

h i g h l i g h t s " Conversion of BD increases with toluene addition. " Conversion of BD increa... more h i g h l i g h t s " Conversion of BD increases with toluene addition. " Conversion of BD increases with higher pressure. " Toluene addition enhances 2BE/1BE selectivity and favors the trans-isomer. " 1BE hydrogenation losses are reduced by toluene addition and pressure lowering.

Research paper thumbnail of Theoretical study of CO adsorption and oxidation on Au3–5 clusters supported on silico-aluminophospates

Computational and Theoretical Chemistry, 2014

The Au 3-5 clusters supported on silico-aluminophospates (SAPO-11) were studied using quantum che... more The Au 3-5 clusters supported on silico-aluminophospates (SAPO-11) were studied using quantum chemistry calculations, by means of the ONIOM2 methodology at two different levels of calculation, B3LYP for the high level and UFF for the low level. These calculations were performed on Au 3 /SAPO-11, Au 4 /SAPO-11, Au 5 /SAPO-11, CO-Au 3 /SAPO-11, CO-Au 4 /SAPO-11 and CO-Au 5 /SAPO-11 aggregates to analyze the geometries of small clusters of Au 3 , Au 4 and Au 5 on SAPO-11 support. Au 3 cluster presents a triangle structure in Au 3 /SAPO-11. Au 4 cluster shows a ''Y shaped'' structure in Au 4 /SAPO-11. Au 4 as a rhombus structure is also studied but it is a less stable structure. Au 5 cluster displays a ''pentagon shaped'' structure as the most stable. The CO interaction with Au 3 , Au 4 and Au 5 /SAPO-11 is studied; this CO adsorption is different from that reported in the literature. The formation energy DE F of the aggregates, the CO adsorption energy DE ads on them and the change of energy of the oxidation reaction are presented. Some relevant charges and bond indexes were calculated. On supporting all Au 3-5 there is a charge transfer from the Au cluster to the binding O atoms. On CO adsorption in all Au 3-5 clusters there is a charge transfer from the cluster to the CO. j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m p t c

Research paper thumbnail of Parametric calculations of Mo-allyl complexes anchored on silica

Journal of Molecular Catalysis A: Chemical, 2001

The SiO 2 anchored Mo( 3 -C 3 H 5 ) 4 catalyst was modeled by means of the CNDO-UHF parametrical ... more The SiO 2 anchored Mo( 3 -C 3 H 5 ) 4 catalyst was modeled by means of the CNDO-UHF parametrical method. Two models (two separated centers (TSC) and two adjacent centers (TAC)) were proposed for the silica pre-treated at relatively low temperature. An analysis of total energies, bond indexes, diatomic energies, and parametric diatomic binding energies for selected bonds suggests the existence of both models on the surface. However, TSC is favored with respect to TAC. Furthermore, it is observed an allyl group interaction with the support through a C-O bond that leads to a C-C bond activation. Also, C-H activation is found due to Mo-H interaction. These results support several features of the generally accepted mechanism for olefin metathesis in this type of catalyst. Finally, experimental results of NMR, obtained in previous work by some of the authors, are interpreted by inequivalency of different allyl ligands.

Research paper thumbnail of Theoretical study of olefin oxidation process on a dioxo-μ-oxo Mo catalyst

International Journal of Quantum Chemistry, 2008

The dimeric complex dioxo (-oxo) thiocyanato of molybdenum(VI) bearing a 4,4Ј-di-ter-butyl-2,2Ј-b... more The dimeric complex dioxo (-oxo) thiocyanato of molybdenum(VI) bearing a 4,4Ј-di-ter-butyl-2,2Ј-bipyridine ligand in trans conformation (complex 1) has been used in catalytic reactions of olefins (ethylene) oxidation, giving as intermediate a dimeric complex oxo bis (-oxo) thiocyanato of molybdenum(V) in cis conformation (complex 2). Binding and electronic properties were calculated using the parametric quantum chemistry method for catalytic reactions, CATIVIC. The interaction of an ethylene molecule with complex 3 (cis version of complex 1) leads to the formation of the intermediate named complex 4. This complex is dissociated to form complex 5 plus ethylene oxide. Changes of electronic and bonding properties are analyzed in terms of atomic charge, orbital population, total binding energies, diatomic bond energies, diatomic energies, and Wiberg indexes, and equilibrium bond distances. The energy balance for the complete catalytic process is presented in a schematic way. Results confirm the extraordinary oxo-transfer abilities of this complex, considering that the whole process is exothermic (Ϫ44 kcal/mol). Regeneration of the catalyst is also calculated by the interaction of H 2 O 2 with complex 5.

Research paper thumbnail of Theoretical study of Au/SAPO-11 catalyst and its potential use in thiophene HDS

Journal of Molecular Catalysis A: Chemical, 2010

Quantum chemistry calculations were carried out, using ONIOM2 methodology, in order to investigat... more Quantum chemistry calculations were carried out, using ONIOM2 methodology, in order to investigate the thiophene interaction with gold supported on silicoaluminophospates molecular sieves (Au/SAPO-11) catalysts. Two models were studied, one containing one Au atom per site, and the other with two Au atoms per site. Thiophene adsorption was found to be 1 type. This adsorption presents a H of −13.2 and −9.7 kcal/mol, for the models with one Au atom (Au/SAPO-11), and two Au atoms (Au 2 /SAPO-11), respectively. The partial hydrogenation of the thiophene-Au/SAPO-11 and thiophene-Au 2 /SAPO-11 complexes gives 2,5-dihydrothiophene (DHT), with a H of −23.0 and −36.8 kcal/mol, respectively. 2-Butene production was found in both models with further hydrogenation. Likewise the direct butadiene elimination is achieved, but only with the separated Au dimer ( H = −17.5 kcal/mol).

Research paper thumbnail of Theoretical calculations of silica supported Mo2(η3-C3H5)4 species

Journal of Molecular Structure: THEOCHEM, 2003

Two models are studied for the anchored system Mo 2 (h 3 -C 3 H 5 ) 4 /SiO 2 : united model (UM) ... more Two models are studied for the anchored system Mo 2 (h 3 -C 3 H 5 ) 4 /SiO 2 : united model (UM) and separated model (SM). Total energies, bond orders, diatomic energies, and parametric diatomic binding energies of selected bonds were calculated for Si 8 O 24 H 16 , Mo 2 (h 3 -C 3 H 5 ) 4 , and Mo 2 (h 3 -C 3 H 5 ) 4 /Si 8 O 24 H 16 systems, using a CNDO -UHF parametrical method. From the energetic viewpoint, UM is more stable than SM to explain the immobilizing of Mo 2 (h 3 -C 3 H 5 ) 4 on silica. It is observed Mo -H interaction that leads to C -H activation. These results support several features of the generally accepted mechanism for olefin metathesis. An interpretation of these theoretical results was carried by comparing with PAS -FTIR, 1 H and 13 C solid-state NMR, and XPS trends. q

Research paper thumbnail of ESTUDIO TEORICO DEL ANCLAJE DE Mo 2 (η 3 -C 3 H 5 ) 4 EN SILICA CON EVOLUCION DE PROPENO Y POSTERIOR REDUCCIÓN DE LAS ESPECIES EN LA SUPERFICIE

Resumen Este trabajo forma parte de una serie de cálculos teóricos del anclaje del Mo 2 (η 3 C 3 ... more Resumen Este trabajo forma parte de una serie de cálculos teóricos del anclaje del Mo 2 (η 3 C 3 H 5) 4 en SiO 2 . Abstract This work is one of a series of theoretical calculations of the anchoring of Mo 2 (η 3 C 3 H 5) 4 on SiO 2 . In this case, propene evolution and the formation of resulting species due to reduction are considered. The calculations were done, using the semipirical method CNDO. Interatomic distances, diatomic energies, bond orders, binding diatomic energies will be evaluated.

Research paper thumbnail of Interpretación de Datos Experimentales y Teóricos en el Estudio de Mo 2 (η 3 C 3 H 5 ) 4 Soportado en Sílica

Key Words: Modelaje Molecular, Anclaje de Mo 2 (η 3 -C 3 H 5) 4 Introducción Los catalizadores ba... more Key Words: Modelaje Molecular, Anclaje de Mo 2 (η 3 -C 3 H 5) 4 Introducción Los catalizadores basados en molibdeno son de amplio uso industrial debido a la versatilidad de este elemento, que posee una variedad de estados de oxidación estables y una rica química de coordinación. El estudio de catalizadores obtenidos mediante reacción de grupos OH de soportes y complejos alílicos de Mo, con posterior transformación de las especies superficiales mediante reacciones red-ox, ha recibido considerable atención en la literatura [1]. En nuestro laboratorio, hemos estudiado los complejos alílicos anclados en sílica, sin haber sido sometidos a transformaciones adicionales luego del anclaje [2], debido tanto al interés en la química de las especies superficiales como a su potencial en algunas reacciones de transformación de hidrocarburos. Las especies ancladas a las superficies han sido caracterizadas mediante Espectroscopía Fotoacústica en el Infra-rojo por Transformada de Fourier (PAS-FTIR),...

Research paper thumbnail of Estudio Teórico de una Zeolita Au/SAPO11 y su Reactividad Catalítica en Oxidación de CO

En este estudio teórico realizamos cálculos químico cuánticos usando el método ONIOM a dos nivele... more En este estudio teórico realizamos cálculos químico cuánticos usando el método ONIOM a dos niveles de cálculos. Estos cálculos se llevaron a cabo sobre agregados de SAPO11, Au/SAPO11, CO-Au/SAPO11 y en la reacción de oxidación en este último sistema. Igualmente se realizaron cálculos sobre Au2/SAPO11, 2CO-Au2/SAPO11 y la reacción de oxidación. Los resultados revelan que Au/SAPO11 es muy estable teniendo un ΔE de –81 kcal/mol, que la adsorción de CO presenta un ΔE de –38 kcal/mol y la reacción de oxidación de CO un ΔE= -29.2 kcal/mol. En el caso de Au2/SAPO11 se obtiene un ΔE= -163.7 kcal/mol, el 2CO-Au2/SAPO11 un ΔE= -60.6 kcal/mol, y la reacción de oxidación de 2CO-Au2/SAPO11 un ΔE= -103 kcal/mol. En ambos modelos la reacción de oxidación presenta un mecanismo tipo Eley-Rideal con el CO adsorbido. Los resultados de los cálculos revelan que este tipo de material resulta interesante para dispersar Au, y por ende para su uso como catalizadores en diferentes reacciones.

Research paper thumbnail of Probing silica-supported Mo 2+ by means of FT-IR of adsorbed CO

Catalysis letters, 1998

In a recent work, López et al. studied the interaction of CO with silica-supported molybdenum ato... more In a recent work, López et al. studied the interaction of CO with silica-supported molybdenum atoms by means of density functional theory calculations. They considered different reactive sites, with Mo atoms in various formal oxidation states (O-S), ranging from +II to +VI, and using both molecular and cluster models of the Mo/SiO 2 surface sites. According to their calculations, models with neutral Mo atoms in any O-S from +II to +V result in negative ("red") shifts of ν(CO), the frequency of the adsorbed CO molecule, with respect to that of gas phase CO. Thus, these models can not account for the IR peak at 2170 cm −1 ("blue" shifted by 27 cm −1 ) which had been observed experimentally [2] for allyl-derived Mo/SiO 2 catalysts reduced in hydrogen at 823 K. This peak had been assigned [2] to Mo(II) (or Mo 2+ , see below) species, which were detected by XPS in >90% concentration with respect to total Mo in the surface of the catalyst reduced at 823 K [3]. On the other hand, calculations by López et al.

Research paper thumbnail of Estudio teórico del efecto del H2O sobre CO adsorbido sobre catalizadores Au/SAPO-11. ¿Es viable la reacción WGS en este sistema?

Estudio teórico del efecto del H 2 O sobre CO adsorbido sobre catalizadores Au/SAPO-11. ¿Es viabl... more Estudio teórico del efecto del H 2 O sobre CO adsorbido sobre catalizadores Au/SAPO-11. ¿Es viable la reacción WGS en este sistema? Resumen En este estudio se llevaron a cabo cálculos químico cuánticos, usando la metodología ONIOM2 en 2 niveles, para determinar los cambios de energía en la interacción de los silico-aluminofosfatos intercambiados con oro (Au/SAPO-11) con CO y H 2 O. Se estudiaron 2 modelos, uno que contiene un átomo de Au por sitio (Au/SAPO-11) y otro con 2 átomos de Au por sitio. Los resultados revelan la formación de un posible intermediario tal como HO=C=OH en ambos modelos es un proceso energéticamente posible. Así como la formación del ácido fórmico es un proceso enérgicamente favorable. Este trabajo muestra por primera vez la habilidad potencial de los sistemas catalíticos Au/SAPO-11 para realizar la reacción de desplazamiento de agua o WGS a través del intermediario HO=C=OH -Au/SAPO-11 y HO=C=OH –Au 2 /SAPO-11. Palabras claves: ONIOM; Catálisis Computacional; ...

Research paper thumbnail of Anclaje y caracterizaciòn de tetra--alildimolibdeno sobre sìlica

Research paper thumbnail of Estudio teórico de la oxidación catalítica de CO en agregados pequeños Au 3-5 sobre catalizadores Au/SAPO-11

Introducción Se han hecho innumerables estudios experimentales y teóricos sobre las propiedades e... more Introducción Se han hecho innumerables estudios experimentales y teóricos sobre las propiedades electrónicas y estructurales de los agregados de Au, debido a sus potenciales aplicaciones para su uso en dispositivos electrónicos, en nano-materiales y como catalizadores, entre otros [1]. Algunos autores [1-3] reportan las estructuras más estables de los agregados de Au que contienen de 2 a 20 átomos. Häkkinen y Landman [4] estudiaron agregados de Au neutros y aniónicos Au 2−10 . H-Y Wang [5] ha publicado estudios con agregados de Au n donde n<14. Asimismo se han hecho estudios experimentales y teóricos sobre la oxidación de CO para revelar el origen de la gran actividad catalítica en los agregados de Au [6-7]. En este trabajo preliminar presentamos un estudio teórico de clusters pequeños de Au 3-5 soportados en SAPO-11, su interacción con CO y la reacción de oxidación del CO. Detalles computacionales y modelos moleculares Todas las optimizaciones de geometrías y cálculos fueron rea...

Research paper thumbnail of Estudio teórico de la oxidación catalítica de CO en agregados pequeños Au 5-6 sobre catalizadores Au/SAPO-11. Theoretical study of catalytic oxidation of CO on small clusters Au 5-6 on Au/SAPO-11 catalysts

Resumen: En este estudio teórico se realizaron cálculos químico cuánticos usando la metodología O... more Resumen: En este estudio teórico se realizaron cálculos químico cuánticos usando la metodología ONIOM(B3LYP:uff). Estos cálculos se efectuaron sobre agregados Au 5 /SAPO-11, Au 6 /SAPO-11, CO-Au 5 /SAPO-11, CO-Au 6 /SAPO-11 y de la oxidación de estas dos últimas especies. Se presentaran resultados de cambio de energía ΔE y de frecuencia de CO en los agregados. Palabras claves: Au/SAPO; ONIOM; Cálculos teóricos; DFT. Abstract: In this theoretical study, quantum chemistry calculations were done, using the ONIOM method at two different levels of calculation. These calculations were performed on Au 5 /SAPO-11, Au 6 /SAPO-11, CO-Au 5 /SAPO-11, CO-Au 6 /SAPO-11 clusters and on the oxidation of the two last species. Energy changes of the clusters and CO frequency are presented. Introducción El estudio teórico y experimental de agregados de Au ha alcanzado una gran importancia y trascendencia debido a que sus propiedades estructurales y electrónicas los han conducido a tener una gran aplica...

Research paper thumbnail of Theoretical study of small clusters Au 3−4 on Au/SAPO-11 catalysts and their interactions with CO

Quantum chemistry calculations were done, using the ONIOM2 methodology at two different levels of... more Quantum chemistry calculations were done, using the ONIOM2 methodology at two different levels of calculation, B3LYP for the high level and UFF for the low level. These calculations were performed on Au3/SAPO-11, Au4/SAPO-11, CO-Au3/SAPO-11 and CO-Au4/SAPO-11 aggregates to analyze the geometries of small clusters of Au3 and Au4 on SAPO-11 support. Au3 cluster present a triangle structure in Au3/SAPO-11. Au4 cluster shows a "Y shaped" structure in Au4/SAPO-11. Au4 as a rhombus structure is also studied but it is an unstable intermediate to the "Y shaped" structure. The CO interaction with Au3 and Au4/SAPO-11 is studied, this CO adsorption is different from reported in the literature. The formation energy ΔEF of the aggregates and the CO adsorption energy ΔE ads on them are presented.

Research paper thumbnail of Estudio teórico de la oxidación catalítica de CO en agregados pequeños Au3-6 sobre catalizadores Au/SAPO-11

In this theoretical study, quantum chemistry calculations were done, using the ONIOM(B3LYP:uff) m... more In this theoretical study, quantum chemistry calculations were done, using the ONIOM(B3LYP:uff) method at two different levels of computation. These calculations were performed on Au3-6/SAPO-11, CO-Au3-6/SAPO-11 clusters and on the oxidation of these species. The supported Aun (n=3-6) structures are presented and compared to the planar, similarly published structures of pristine Au aggregates. Energy changes of the formation of the clusters and CO frequency are presented. ΔE results indicate that Au clusters formation on SAPO-11 are stable from the energetically point of view. Likewise the CO-Au3-6/SAPO-11 oxidation calculations show that they are highly exothermic reactions. The CO adsorption is moderated due to the Au aggregates stability. Keywords: Au/SAPO; ONIOM; Theoretical calculations; DFT.

Research paper thumbnail of Theoretical study of small clusters Au 5−6 on Au/SAPO-11 catalysts and their interactions with CO

Journal of Computational Methods in Sciences and Engineering

Quantum chemistry calculations were done, using the ONIOM2 methodology at two different levels of... more Quantum chemistry calculations were done, using the ONIOM2 methodology at two different levels of calculation, B3LYP for the high level and UFF for the low level. These calculations were performed on Au5/SAPO-11, Au6/SAPO-11, CO-Au5/SAPO-11 and CO-Au6/SAPO-11 aggregates to analyze the geometries of small clusters of Au5 and Au6 on SAPO-11 support. Au5 cluster presents a pentagonal structure in Au5/SAPO-11. Au6 aggregate shows a "multi triangular" structure (as from a trapezoidal "W shaped" Au5) in Au6/SAPO-11. Au5 is also obtained as an "X shaped" structure. Similarly another Au6 aggregate configuration is obtained also multi triangular but as from X shape Au5 cluster. The CO interaction with Au5 and Au6/SAPO-11 is studied. The formation energy ΔEF of the aggregates, the CO adsorption energy ΔE ads on them and CO frequency are presented.

Research paper thumbnail of Theoretical study of the CO catalytic oxidation on Au/SAPO-11 zeolite

International Journal of Quantum Chemistry, 2010

Quantum chemistry calculations were carried out, using ONIOM2 methodology, to investigate the CO ... more Quantum chemistry calculations were carried out, using ONIOM2 methodology, to investigate the CO adsorption and oxidation on gold supported on Silicoaluminophospates (SAPO) molecular sieves Au/SAPO-11 catalysts. Two models were studied, one containing one Au atom per site (AuASAPO-11), and the other with two Au atoms per site (Au 2 ASAPO-11). The results reveal that the CO adsorption and oxidation are exothermic on Au/SAPO11 with an DE of À41.0 kcal/mol and DE ¼ À52.0 kcal/mol, for the adsorption and oxidation, respectively. On the Au 2 ASAPO-11 model, the CO adsorption and oxidation reaction occur, with a DE of À29.7 kcal/mol and À52 kcal/mol, respectively. According to our results, the oxidation reaction exhibits an Eley-Rideal type mechanism with adsorbed CO. The theoretical calculations reveal that this type of material could be interesting to disperse Au and consequently to strengthen its catalytic use for different reactions.

Research paper thumbnail of Evaluación de catalizadores a base de carburos y nitrus de circonio en la reacción de hidrodesulfuración de tiofeno