Incorporación de Ti(III) en aluminofosfatos cristalinos y su comportamiento catalítico en reacciones de oxidación selectiva

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Química Inorgánica. Fecha de lectura: 20-11-2015This work focuses on the development of a synthesis procedure for the preparation of Ti-doped microporous aluminophosphate (TAPO) materials, by means of the isomorphic substitution of Al(III) atoms in tetrahedral sites by Ti(III) atoms. The main aim consists on modifying the chemical environment of the resultant titanium centers with respect to those in the conventional TAPO materials, prepared with Ti(IV), in an attempt to enhance their catalytic behavior and reduce its difference with the one shown by Ti-zeolites in liquid phase oxidation reactions. The materials have been synthesized using two Ti(III) sources, different titanium contents and different structure directing agents, to generate the so-called Ti(III)APO materials. They have been prepared with several structures, mainly AlPO- 5 (AFI) and to a lesser extent with AlPO-11 (AEL) and AlPO-41 (AFO) structures. The characterization of the materials obtained by hydrothermal treatment of the synthesis gels prepared under nitrogen atmosphere, in order to minimize Ti(III) oxidation before its incorporation in the structure, confirms the presence of those ions. This is also visible at first sight by the purple color of the samples. On the other hand, the characterization shows the changes in the TAPO materials framework caused by the incorporation of Ti(III), as the different crystal morphology, the modification of the phosphorous environment or the high hydration degree of the materials. The analysis of the results allows proposing the incorporation mechanisms of this heteroatom, being favored the formation of environments Ti(OP)4, by substitution of isolated Al(III), which lead to Ti–OH y P–OH species, by oxidation of these Ti(III) atoms when the sample is exposed to air. However, Ti–O–Ti species are also likely if a partial oxidation of Ti(III) atoms to Ti(IV) takes place during the synthesis process. The proportion of both mechanisms is, in some way, regulated by the titanium content. The catalytic behavior of the materials has been studied in the oxidation reaction of cyclohexene with H2O2 as oxidant and acetonitrile as solvent. It has been observed that the presence of water in the reaction media reduces considerably the catalytic activity of the Ti(III)APO materials. This effect has led to the design of a procedure for the removal of the water that accompanies the commercial oxidant H2O2 30 wt. %, by a Soxhlet system and molecular sieve 3A as selective adsorbent. This procedure allows removing around 90 % of the water present. Furthermore, a reproducible quantification method of water and H2O2 has been developed by means of FTIR spectroscopy. The catalytic activity studies show that titanium centers in Ti(III)APO-5 materials are more active than those in the conventional Ti(IV)APO-5, and the first ones can reach similar conversion rates to those of the Ti-beta zeolite under anhydrous reaction conditions. The Ti(III)APO-5 materials are highly selective to the allylic oxidation products, and 2-cyclohexenyl hydroperoxide is the main one. In addition, synthesis strategies to enhance the stability of Ti(III) and favor its incorporation in the AlPO4 framework in that oxidation state have been designed. The synthesis were carried out, on the one hand, with urea as complexing agent of Ti(III) and, on the other hand, with addition of HCl to the initial gel. The former have given rise to the crystallization of AlPO-15 structure instead of AlPO-5. The latter have not had a significant effect on Ti(III) incorporation, but on its textural properties leading to higher conversion of cyclohexene over these materials. On the other hand, an additional modification of the titanium coordination environment, as well as of the hydrophilic degree of the materials, was caused by the addition of a silicon source together with the Ti(III) one. The synthesis gels were prepared in aqueous and in biphasic media, making use of synthesis strategies of SAPO materials. The presence of silicon induces the Ti(III) incorporation in isolated Al(III) sites and, furthermore, some Ti–O–Si are formed, while there is no evidence of the existence of Ti(OSi)4 environments. This synthesis strategy has led to titanium centers with higher intrinsic activity than those in the Si-free Ti(III)APO-5 materials. The crystallization of the AlPO-11 or the AlPO-41 structures seems to be directed by the Ti(III) content in the synthesis gel, being favored the latter at higher content. The intrinsic activity of the titanium centers in the Ti(III)APO-11 and Ti(III)APO-41 materials, especially in the first ones, is higher than that in the Ti(III)APO-5 in the oxidation reaction of cyclohexene. Nevertheless, the chemical nature of the active centers is similar in the three materials, since all of them selectively lead to the allylic oxidation productsConcesión del CSIC de una beca JAE-Predoc para la realización de esta tesis y la financiación de las estancias en el extranjero y de los Ministerios de Ciencia e Innovación y de Economía y Competitividad de los proyectos MAT2009-13569 y MAT2012-31127 que han permitido la financiación de este trabaj

Ti(III)APO-5 vs. conventional Ti-zeolites: selective allylic oxidation of cyclohexene

Trabajo presentado en el 1st CZM Workshop on Catalysis, celebrado en Clausthal (Alemania) del 22 al 23 de noviembre de 2013.Peer Reviewe

Aluminofosfatos cristalinos microporosos que comprenden ti, procedimiento de obtención y su uso como catalizador

Esta patente reivindica materiales aluminofosfatos microporosos cristalinos que comprenden Ti (TAPOs) partiendo de geles que contienen Ti en estado de oxidación inferior a (IV), así como el procedimiento de obtención de estos materiales. Los materiales pueden comprender dos tipos de centros dependiendo de la atmósfera a la que se expongan tras su preparación. Si se mantienen en ausencia de oxígeno, se obtendrán materiales aluminofosfatos que comprenden Ti(III) (Ti(III)APOs) con propiedades rédox por explotar en catálisis (por ejemplo, producción de plásticos). Se trata de los primeros materiales TAPOs que no contienen centros ácidos de naturaleza Brönsted coexistiendo con los centros rédox. Si se exponen a oxígeno molecular, se obtienen materiales TAPOs que comprenden Ti(IV) (Ti(IV)APOs), pero con un entorno diferente al de los TAPOs conocidos: en lugar de entornos Ti(OAl)4 o Ti(OAl)4-n(OTi)n (con 0<=4), sus entornos son exclusivamente Ti(OP)4, presentando así un comportamiento catalítico propio en reacciones ácidas y de oxidación.Peer reviewedConsejo Superior de Investigaciones Científicas (España)A1 Solicitud de patente con informe sobre el estado de la técnic

New Ti centers in AlPO-5 materials catalytically active and selective in the oxidation of cyclohexene

Trabajo presentado en el 5th Early Stage Researchers Workshop in Nanoscience, celebrado en Madrid (España) del 17 al 18 de junio de 2019.Zeolites and zeotypes are nanoporous crystalline materials , based on a three dimensional T02 (T = Si, Al, P, etc.) network and potentially used as heterogeneous catalysts with shape selectivity to reactants and products. These networks allow the incorporation of a great number of metallic elements by isomorphous substitution , what increased their applications as suitable catalysts far redox or acid-base reactions. In that sense , Ti-containing zeolites have become relevan! industrial heterogeneous catalysts in oxidation processes [1]. On the contrary , their homologous Si-free zeotypes TAPO materials have shown scarce catalytic activity in !hose reactions , likely due to their different titanium environments within the nanoporous networks. However, AIP04 frameworks possess higher versatility !han zeolitic ones in terms of heteroatom incorporation as either Al(lll) and/or P(V) ions could be substituted. The incorporation of Sn(ll) in Al(lll) siles of the AIP0-5 framework , instead of the more conventional and stable Sn(IV), replacing P(V), resulted in a material presenting better catalytic behavior in acid reactions [2]. In this work , we have changed the incorporation mechanism of titanium ions into AIP0-5 framework (Figure) in arder to generate titanium environments different from !hose found in the conventional TAP0-5 , by using Ti(lll) sources [3] instead of Ti(IV) ones in the synthesis gel. The final aim was the modification of the catalytic activity of TAP0-5 in the oxidation of olefins , particularly cyclohexene , using H,O, as the oxidan!. The Ti(lll)AP0-5 materials synthesized by this strategy enhanced the catalytic activity compared to the conventional Ti(IV)AP0-5 ones. Furthermore , they could reach the conversion level of the Ti-beta zeolite under suitable reaction conditions. Unlike the latter, AIP04 type materials are hydrophilyc and, consequently , the catalytic activity of both TAP0-5 tested was greatly affected by the presence of water in the reaction medium. Regarding the selectivity, Ti(lll)AP0-5 catalysts run the reaction preferably through a radical pathway , being highly selective to the allylic oxidation products , rather !han through the epoxidation route almos! exclusively followed by Ti-betaPeer Reviewe

Cyclohexene oxidation by TAPO-5 catalysts prepared with Ti(III) source

Trabajo presentado en la 17th International Zeolite Conference (17 IZC), celebrada en Moscú (Rusia) del 7 al 12 de julio de 2013.Peer Reviewe

Oxidación selectiva de ciclohexeno sobre materiales TAPO-5 preparados con incorporación de Ti(III)

Trabajo presentado en el I Encuentro de Jóvenes Investigadores de la Sociedad Española de Catálisis (SECAT), celebrado en Málaga (España) del 22 al 24 de junio de 2014.En las últimas décadas . las zeolitas que contienen Ti en su estructura han adquirido gran importancia debido a su alta actividad catalítica en procesos de oxidación de interés industrial, éxito que no ha sido compartido por sus homólogos. los materiales TAPO. Sin embargo. las estructuras AIPO. presentan la ventaja de que tanto AI(lI!) como P(V) pueden sustituirse por otros heteroátomos. En ese sentido. la introducción de Ti(I1I) en la estructura de AIPO-S dirige su incorporación a través de un mecanismo distinto al que tiene lugar en los materiales sintetizados con Ti(lV) y da lugar a catalizadores denominados Ti(I1IlAPO-S. en los que se generan entornos de Ti diferentes de aquellos presentes en los materiales convencionales Ti(lV)APO-S, con el objetivo final de modificar su actividad catalítica en reacciones de oxidación.A. Alfayate agradece la concesión de una beca de doctorado JAE-Predoc del CSIC. Los autores agradecen la financiación de este trabajo al MINECO (MAT-2012-31127).Peer Reviewe

Incorporation of Ti(III) into the AlPO4-5 framework by direct synthesis

This work focuses on the synthesis and characterization of a conceptually innovative series of TAPO materials, particularly centered in the AFI-structured TAPO-5. Unlike the conventional TAPOs, the so-called Ti(III)APO-5 are prepared by using Ti(III) sources. This strategy aims to avoid the presence of both Ti–O–Al and Ti–O–Ti bonds in TAPOs, undesired for catalytic purposes. The thermodynamic stability of this oxidation state of Ti in water and in the presence of air is low, but the preparation of the gel and the filling of the autoclaves under N2 atmosphere allowed Ti to maintain its 3+ oxidation state in a considerable proportion in the hydrothermally-crystallized solids. The materials were characterized by powder XRD, ICP chemical analysis, SEM, DR-UV–visible spectroscopy, TGA and 31P and 27Al MAS NMR. DR-UV–visible spectroscopy certified the presence of Ti(III) in both tetrahedral and octahedral environments in the as-made samples before being exposed to air, whereas 31P MAS NMR indicated the vicinity of Ti in the second coordination sphere of P in the already oxidized sample. Although once dried and calcined in air Ti(III) is entirely oxidized to Ti(IV), Ti(III)APO-5 materials present key singularities against conventional TAPO-5. Thus, apart from the presence of Ti(OP)4 environments, the charge transfer transition detected by DR-UV–vis appears at lower wavelength, whereas both TGA and 27Al NMR indicates a peculiar hydration of the as-prepared samples, which is unprecedented in conventional TAPO-5. Based on these characterization features, the possible nature of the Ti centers generated by this synthesis approach is discussed.Authors thank Dr. Gloria Berlier from University of Torino for allowing us to register DR-UV–vis spectra under controlled atmosphere. They acknowledge Spanish Ministry for the financial support of this project (MAT-2012-31127), and A. Alfayate acknowledges CSIC for a JAE-predoc fellowship.Peer Reviewe

The interaction of H2O2 with TiAlPO-5 molecular sieves: Probing the catalytic potential of framework substituted Ti ions

A combination of EPR, ENDOR and HYSCORE experiments has been used to investigate the reactivity of hydrogen peroxide with TiAlPO-5 materials under both hydrated and anhydrous conditions. Superoxide radical anions, generated upon reaction, are used as paramagnetic probes to investigate the nature and reactivity of framework incorporated Ti ions. Super hyperfine interactions with 27Al, 31P and 1H are resolved, which allow a detailed mapping of the local environment of the adsorbed O2− ions. Evidence is provided for the first time of a specific redox activity associated with Ti ions incorporated at framework Al sites of TiAlPO materials.A.A. acknowledges CSIC for a PhD JAE-predoc fellowship and for financing her stay in Torino.Peer Reviewe

Función de los metales rédox de los catalizadores M-MOF-74 nanocristalinos en la oxidación de ciclohexeno

Trabajo presentado en el 1er Simposio sobre propiedades y aplicaciones de MOFs y COFs, celebrado en Granada (España) del 9 al 10 de abril de 2015.Las aplicaciones de los MOFs cubren una parte importante de los campos de interés industrial, entre los que se encuentra el de la catálisis heterogénea. No obstante, la bibliografía científica en este ámbito adolece de la sistematicidad y profundidad requeridas para plasmar y esclarecer el potencial catalítico de algunas singularidades de los MOFs, tales como la enorme versatilidad de iones metálicos, que abarca la práctica totalidad de la Tabla Periódica, o la presencia de centros metálicos expuestos e insaturados, hasta ahora inéditos en los materiales microporosos convencionales. Además , raramente se han hecho esfuerzos en síntesis orientados a potenciar la actividad catalítica en materiales MOFs ya conocidos, como pueden ser la reducción del tamaño de partícula o la introducción de porosidad jerarquizada. En este estudio , se presenta la preparación y actividad catalítica en la oxidación de ciclohexeno de una serie ísoestructural de materiales M-MOF-74, donde se ha variado sistemáticamente la naturaleza del metal rédox divalente (M = Mn, Co, Ni o Cu). Además , estos materiales se han preparado a temperatura ambiente, lo que, además de contribuir a la sosteníbilidad de su proceso de preparación, garantiza la obtención de materiales nanocristalínos (Figura 1A) con mesoporosidad intercristalina [1, 2) , que en principio facilita la difusión de reactivos y productos. Todos los catalizadores M-MOF-74 con un M de carácter rédox resultaron significat ivamente activos en la oxidación de ciclohexeno [3), de forma que su actividad global se pudo relacionar con su superficie específica. Sin embargo, el mecanismo de oxidación, que puede tener lugar por vía de epoxidación o por v ía radicalaria, fue determinante para esclarecer el papel catalítico del metal. En particular, la oxidación radicalar ia es no selectiva a la naturaleza del metal, pero la epoxidación se pudo correlacionar con su potencial rédox, de forma que el metal ideal para epoxidar es aquel que tiene carácter oxidante (Cu, Co y Mn) y no reductor (Ni), y cuyo potencial rédox, siendo evidentemente positivo, apenas está por encima de cero para poder completar fácil y rápidamente el ciclo de oxidación-reducción requerido (mejor Cu que Mn, y éste mejor que Co) (Figura 1B).Peer Reviewe

Nanocrystalline M-MOF-74 as heterogeneous catalysts in the oxidation of cyclohexene: Correlation of the activity and redox potential

In some aspects, the potential of metal–organic framework (MOF) materials as heterogeneous catalysts has been realized, at least in an academic context. However, one of their most promising catalytic properties, that is, the presence of open metal sites, is far from understood properly. In this work, a series of M–MOF‐74 (M=Mn, Co, Ni, Cu, Zn) materials, prepared under sustainable conditions, was tested systematically in the oxidation of cyclohexene, which can proceed by either radical or epoxidation routes. Under the optimized reaction conditions, the radical route is spontaneous to some extent and it is enhanced in the presence of any M–MOF‐74 that has a metal with a redox character but not Zn. However, the epoxidation of cyclohexene is also promoted by a redox catalyst in such a way that the conversion correlates qualitatively with the redox potential of the metal. Thus, for the first time, a chemical property of M is correlated with the catalytic activity of the M–MOF‐74 family.The authors acknowledge the Spanish Government, MINECO (MAT2012‐31127). A.A. acknowledges CSIC for a PhD JAE‐predoc fellowship. Dr. E. Sastre is acknowledged for his help with the GC analysis.Peer Reviewe