[Show abstract][Hide abstract] ABSTRACT: Hydrophobic Ti-MCM-41 samples prepared by post-synthesis silylation treatment demonstrate to be highly active and selective catalysts in olefins epoxidation by using organic hydroperoxides as oxidizing agents in liquid phase reaction systems. Epoxide yields show important enhancements with increased silylation degrees of the Ti-mesoporous samples. Catalytic studies are combined and correlated with spectroscopic techniques (e.g. XRD, XANES, UV-Visible, 29Si MAS-NMR) and calorimetric measurements to better understand the changes in the surface chemistry of Ti-MCM-41 samples due to the post-synthesis silylation treatment and to ascertain the role of these trimethylsilyl groups incorporated in olefin epoxidation. In such manner, the effect of the organic moieties on solids, and both water and glycol molecules contents on the catalytic activity and selectivity are analyzed in detail. Results show that the hydrophobicity level of the samples is responsible for the decrease in water adsorption and, consequently, the negligible formation of the non-desired glycol during the catalytic process. Thus, catalyst deactivation by glycol poisoning of Ti active sites is greatly diminished, this increasing catalyst stability and leading to practically quantitative production of the corresponding epoxide. The extended use of these hydrophobic Ti-MCM-41 catalysts together with organic hydroperoxides for the highly efficient and selective epoxidation of natural terpenes is also exemplified.
[Show abstract][Hide abstract] ABSTRACT: We report on ultrafast studies of Nile Red (NR) interacting with MCM41 mesoporous materials doped by Al, Ga, Zr and Ti in dichloromethane suspensions. The steady-state results showed a significant red shift and broadening of the diffuse transmittance and the emission spectra upon interaction with the MCM41-based materials. These findings are explained in terms of H-bonds with the host, the different Brønsted/Lewis interactions with the matrix and formation of H- and J-aggregates, in addition to weakly and strongly adsorbed monomers. The pico- to nanosecond time-resolved data support this explanation, showing a significant shortening in the emission lifetimes where NR is interacting with metal-doped MCM41. The femtosecond dynamics of NR loaded into X-MCM41 (X= Si, Al, Ga) indicate that the charge-separated state (CS) is formed at the S1 state in 350 fs. While for Zr- and Ti- MCM41 hosts the intramolecular charge transfer (ICT) occurs in less than 200 fs, and a subsequent electron injection to Ti or Zr trap states happens in 250 fs. Our studies reveal a strong interaction between the NR species and the framework of MCM41 materials at both the S0 and S1 states.
The Journal of Physical Chemistry C 05/2015; 119(23):150513162306006. DOI:10.1021/acs.jpcc.5b02296 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report on UV–visible absorption and emission and ultrafast emission dynamics of 7-hydroxyquinoline (7HQ) encapsulated within titania-doped silicates (mesoporous and unstructured) and purely TiO2 nanomaterials. When titania is incorporated as larger (50 Å) nanoparticles (TiO2-MCM41), the stationary spectra and fluorescence decay times are very similar to those of the regular MCM-41 sample (R-MCM41). In turn, the presence of titania domains in MCM-41(Ti-MCM41) leads to a large increase of the ground state population of anionic (A) and zwitterionic (Z) forms of the adsorbed dye. This is explained in terms of the cooperative effect of Ti–OH groups and the MCM-41 framework interacting with the confined structures. The emission spectra in Ti-MCM41 show a partial fluorescence quenching, whereas a complete quenching occurs when the dye is interacting with pure TiO2 materials (mesoporous TiMeso and nonporous P25). A femtosecond emission study revealed a very short lifetime (10–14 ps) of the Z form. This strong emission quenching is assigned to an electron transfer process from the excited dye to the conduction band of TiO2 material. For 7HQ caged within TiMeso pores, this process occurs in 0.1–0.4 and 0.8–1.2 ps for A and Z forms, respectively. However, for 7HQ/Ti-MCM41, the emission decays become longer (0.3–5 ps). This result is due to the presence of mixed dynamics gated from 7HQ molecules located close to and far from TiO2 domains, reflecting the heterogeneity of the sample and the competition between proton- and electron-transfer processes. Our results suggest negligible contribution of a direct titania excitation in the observed dynamics of the used materials.
The Journal of Physical Chemistry C 07/2012; 116(29):15385–15395. DOI:10.1021/jp302932t · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Metal catalysts based on Pt and Pd nano-particles supported on structured microporous Beta zeolites and mesoporous MCM-41 materials, as well as specially designed γ-Al2O3 samples, were synthesized following a simple and economic procedure. Physico-chemical characterization of metal-composites (by XRD, IR spectroscopy, TEM, N2 adsorption, ICP, among others) indicated that both Pt and Pd nano-particles were adequately supported and homogeneously dispersed on supports. These metal/solid acid composites were applied as efficient catalysts under mild reaction conditions in the selective reductive amination of ketones, a useful industrial reaction for the synthesis of substituted amines and N-heterocycles. Results obtained showed that Pt/Al-Beta catalyst possesses the best catalytic activity (TON=1610, with amine selectivities >95%) superior to that observed with commercial Pt/C and Pt/Al2O3 (TON≈1000). Enhancements in Pt/Al-Beta samples were achieved by optimizing the Pt loading, and mainly the Si/Al molar ratio in solids. On the contrary, inferior catalytic activities were encountered with the Pt/Si-MCM-41 and Pt/Al-MCM-41 materials. The Pd incorporation on MCM-41 materials produced more active catalysts than the commercial Pd/C and Pd/Al2O3 samples. Finally, the study of Pt/ and Pd/γ-Al2O3 materials demonstrated that the treatments of support prior to the metal impregnation and the posterior calcinations processes were essentials to obtain efficient catalysts.
[Show abstract][Hide abstract] ABSTRACT: A triphenylamine dye (TPC1) encapsulated in titanium-doped mesoporous silica structures as alternative materials for dye-sensitized solar cells has been studied by means of stationary absorption and emission as well as ultrafast emission spectroscopy. For the samples prepared by a grafting method, a TPC1 complex with titanium atoms within the mesoporous silica in dichloromethane (DCM) solution is formed, having a red shift of the visible absorption band by about 1300 cm−1 with respect to that of the TPC1 in DCM (from 455 to 485 nm). For the complexes, multi- exponential emission quenching of the relaxed singlet excited state occurs with time constants from 300 fs to 30 ps and is assigned to the confined electron injection process into the Ti–O chromophore. The averaged electron injection rate from the higher energy levels gets smaller values for less energetic probing, from 2.7 × 1012 s–1 at 600 nm to 1.5 × 1012 s–1 at 700 nm. However, in the titanium-doped samples prepared by an impregnation method, we observed about 2–3 times slower injection. The difference is explained by different coupling between TPC1 and titania domains. As a reference to the confining effect on the dynamics, we also studied the behavior of TPC1 when interacting with amorphous silica and purely siliceous MCM-41 material in the same solvent. In amorphous silica, an equilibrium between neutral and anion structures of TPC1 is found to be shifted toward the anion form. For the MCM-41 material, the presence of a new absorption band at around 690 nm is revealed, assigned to the spontaneously created and remarkable stable TPC1 radical cation. The lifetimes of the normal and anion forms in both materials were found to be similar to those in solution. The femtosecond relaxation dynamics in these materials is also similar to that in solution (dominated by the solvation), but additional emission quenching in the TPC1/MCM-41 sample is observed, probably due to intermolecular energy transfer. The rate of energy transfer was estimated to decrease gradually when increasing the observation wavelength, from 1.11 × 1012 s–1 at 500 nm to 0.13 × 1012 s–1 at 700 nm. We believe that our results interrogating ultrafast dynamics of an efficient dye interacting with titania within the mesoporous materials will help in a better understanding and improvement of dye-sensitized solar cells.
The Journal of Physical Chemistry C 04/2011; 115(17). DOI:10.1021/jp201627t · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Organic structure-directing agents (OSDAs) are used to guide the formation of particular types of pores and channels during the synthesis of zeolites. We report that the use of highly versatile OSDAs based on phosphazenes has been successfully introduced for the synthesis of zeolites. This approach has made possible the synthesis of the elusive boggsite zeolite, which is formed by 10- and 12-ring intersecting channels. This topology and these pore dimensions present interesting opportunities for catalysis in reactions of industrial relevance.
[Show abstract][Hide abstract] ABSTRACT: We report on steady-state and time (ns to fs regime) resolved studies of H-bonding interactions and proton-transfer reaction dynamics of silica-based mesoporous material MCM-41 with an H-bond donor and acceptor guest aromatic molecule (7-hydroxyquinoline, 7HQ). We observed the ground state reaction which leads to the formation of intermediates and products of the confined molecular probe. We compare this behavior with the observed one for the dye adsorbed on the surface of silica particles, lacking the nanotubes. The result clearly shows that the formation of keto (or zwitterionic) tautomers at the ground state is enhanced by the confinement provided by the channels of MCM-41. Introduction of hydrophobic groups (by silylation of the OH groups in regular MCM-41 host) changes the ground state tautomeric equilibria and the emission behavior. A new lifetime (3.19 ns, suggested being due to a more stabilized anion of the guest) was observed in addition to the ones due to confined bound enol (0.26 ns), anion (1.5 ns), and zwitterionic (5.5 ns) structures. Both steady-state and ps-data show the importance of solvation of 7HQ structures inside MCM-41, when compared with the solid-state result. We investigated the intermolecular proton-transfer reaction dynamics in the confined structures using femtosecond-resolved emission spectroscopy, and we got the reaction times needed to produce the anion intermediates (0.3 ps) and zwitterion products (3 ps) upon electronic excitation of bound enol forms of the guest, in addition to the cooling times of the final zwitterionic form. We believe that our results might be useful for designing new nanophotonics sensors based on mesoporous materials, and open the window for further studies to better understand the chemical reactivity of silica-based nanohosts, at a short time scale.
The Journal of Physical Chemistry C 03/2010; 114(14). DOI:10.1021/jp911942d · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
[Show abstract][Hide abstract] ABSTRACT: Monodispersed, nanosized gold-mesoporous silica nanospheres in the size range 80–280 nm with specific optical properties are prepared in two steps: i) controlled aggregation of gold nanoparticles (GNP) in nanoclusters embedded in a uniform coating of amorphous silica, and ii) development of a porous regular and ordered silica shell by pseudomorphic transformation of the external layer. By careful manipulation of the synthesis variables, i.e., the concentration of co-solvent, surfactant and alkali, it is possible to obtain nanoparticles with controlled morphological and textural characteristics and desired optical behaviour. An optimized sample with most of the nanoparticles containing a multinucleus core made of 10 or more GNP shows a strong absorption cross-section in the near-infrared (NIR) region and a significant photothermal effect when irradiated with a λ = 808 nm laser diode module.
[Show abstract][Hide abstract] ABSTRACT: A photoactive, hexagonally structured mesoporous material (Tyl-MCM41) is produced through co-hydrolysis of SiO(2) and a trialkoxysilane-functionalized trityl cation as photoactive species. The formation of the corresponding triphenylmethanol precursor in the material is confirmed by spectroscopic techniques. The hybrid material has an ordered structure with a narrow pore size distribution typical of ordered mesoporous MCM-41-type materials. The triarylcarbinol molecules incorporated into this hybrid structure (Triphol-MCM41) generate trityl cations under acidic conditions, which are photoactive and able to promote the photosensitized dimerization of 1,3-cyclohexadiene in a heterogeneous solid-liquid system. The material is stable and can be recycled without loss of the photochemical activity.
[Show abstract][Hide abstract] ABSTRACT: The Lewis acid properties of a series of isolated and well-defined Sn centers in different micro- and mesoporous materials have been investigated by means of DFT calculations and IR spectroscopy of probe molecules, and the results have been related to the experimentally measured catalytic activity of these materials for different reactions. Different types of Sn centers have been detected and modelled: inactive fully coordinated Sn atoms in framework positions that weakly interact with Lewis bases, and highly active strong Lewis acid sites associated with Sn centers with one or two hydrolyzed Sn-O-Si bridges. The relative amount of strong and weak sites depends on the zeolite structure, and can be modified by the catalyst pre-treatment conditions. On the other hand, it has been established that Sn-containing mesoporous materials obtained either by direct synthesis or by post-synthesis incorporation of Sn, show a similar distribution of centers. Finally, the influence of solvent and free space available around the active site on the diastereoselectivity of the cyclization of citronellal to isopulegol were investigated, and some hints were obtained on how to improve the catalyst performance.
[Show abstract][Hide abstract] ABSTRACT: Monodispersed, uniform, gold encapsulatedâmesoporous silica nanocomposites of 100â350 nm diameter with a centered metal core at about 15 or 30 nm and high gold occupancy are prepared by pseudomorphic transformation of preformed goldâsilica nanospheres. The characterization by X-ray powder diffraction, transmission electron microscopy, 29Si and 13C magic angle spinning NMR, and N2 adsorption isotherms indicates that these materials present the same morphology and particle size distribution of the parent amorphous silica but with an ordered mesoporous shell of complex wormhole-like pore structure and specific surface area of about 1000 m2 g-1. This method is particularly advantageous for both stabilizing gold nanoparticles and tailoring structural and textural parameters of the outer shell, thus, offering an exceptional support for particle functionalization.
Chemistry of Materials 04/2007; 19(8):1979-1983. DOI:10.1021/cm0629457 · 8.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The activity of the titanium mesoporous molecular sieve Ti-MCM-41 and amorphous Ti-silicate materials for carbon–carbon bond formation in the Mukaiyama aldol-type reaction between benzaldehyde and methyl trimethylsilyl dimethylketene acetal was studied and the reaction network established. Only tetrahedral TiIV is catalytically active, where TiVI and highly dispersed TiO2 are not active or only slightly active for the Mukaiyama reaction and have no effect on selectivity. Whereas in Ti-zeolites only a some of the active sites are accessible to reactants, all of them are accessible in Ti-MCM-41. The structured Ti-MCM-41 offers a clear catalytic advantage over amorphous Ti-silicalites. Water has a negative effect on catalytic activity and selectivity, and catalyst preactivation and hydrophobicity play an important role in the final catalytic behavior. The influence of different solvents on conversion was explored.
Journal of Catalysis 07/2005; 233(2-233):342-350. DOI:10.1016/j.jcat.2005.04.035 · 6.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Laser flash photolysis of germanium-containing ITQ-17 zeolite (Ge/ITQ-17, a single polymorph of beta zeolite) at 266 nm generates a transient spectrum decaying in the sub-millisecond time scale that is compatible with the formation of two transient species. The shorter lived transient (tau approximately 45 micros under nitrogen) has been assigned to trapped electrons due to the characteristic spectroscopic absorption (single band at 480 nm) and its quenching by typical electron scavengers such as N(2)O and CH(2)Cl(2). The second longer lived transient (lambda(max) = 500, 540, and 600 nm; tau approximately 390 micros) is not quenched by O(2) or electron scavengers, but it is quenched by methanol as hole scavenger and has been assigned to positive holes. Also there is a remarkable similarity of the transient spectrum of the Ge/ITQ-17 with the optical spectrum reported previously for electron-hole pairs in ZSM-5 zeolite. Under the same irradiation conditions, photoejection of electrons and photogeneration of positive holes has not been observed for conventional aluminosilicate zeolites, all-silica zeolites, or GeO(2)-impregnated zeolites. Therefore this photochemical behavior has been ascribed to the presence of framework germanium atoms opening the way for photoresponsive zeolites. The ability of Ge/ITQ-17 to generate photochemically electrons and holes has been confirmed by adsorbing naphthalene and propyl viologen sulfonate as electron donor and acceptor, respectively, and observing the generation of the corresponding radical ions.
The Journal of Physical Chemistry B 04/2005; 109(8):3696-700. DOI:10.1021/jp0471047 · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 2-Methoxynaphthalene (2-MN) has been acylated with acetic anhydride on polymorph C of Beta (ITQ-17), and 2-acetylmethoxynaphthalene (2-AMN) and 1-acetylmethoxynaphthalene (1-AMN) have been obtained. The diffusivity of the two isomer products, as determined by molecular dynamics, shows that the ratio of the diffusion coefficients for 2-AMN/1-AMN is one order of magnitude larger for polymorph C than for polymorphs A and B (Beta zeolite). This corresponds very well with experimental acylation results that show a higher selectivity for 2-AMN on polymorph C when reaction is carried out in batch as well as in a fixed-bed continuous reactor.
Journal of Catalysis 02/2003; 217(2):406-416. DOI:10.1016/S0021-9517(03)00053-8 · 6.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Referencia OEPM: P200101608.-- Fecha de solicitud: 03/07/2001.-- Titulares: Universidad Politécnica de Valencia (UPV), Consejo Superior de Investigaciones Científicas (CSIC). En la presente invención, se presenta un material cristalino microporoso, que en estado calcinado tiene una composición equivalente a la ITQ-17, que no contiene fluoruros y que en su forma sintetizada sin calcinar tiene una composición molar en forma de óxidos y en su forma anhidra: xX203 : (1-z)Y02 : zGe02 : r/n Rn0, en la que X es un elemento trivalente tal como A1, B, Fe, In, Ga, Cr, o mezclas de éstos, Y es uno o más elementos tetravalentes distinto/s de germanio, y preferentemente silicio, y R es un compuesto orgánico director de estructura y preferentemente el catión 1-metil-4- Aza,1-azoniabiciclo [2.2.2] octano (DABMe+) o el catión 1,4- bis[N-(4-aza,1-azoniabiciclo [2,2,2] octano) metil]benceno (d- DABBz)2+. Peer reviewed
[Show abstract][Hide abstract] ABSTRACT: Referencia OEPM: P9901638.-- Fecha de solicitud: 20/07/1999.-- Titular: Sumitono Chemical Company, Limited (Japón). Un catalizador de óxido de silicio que contiene titanio que satisface todas las condiciones siguientes (1) a (4): (1) Un tamaño medio de poro de 10 angstron o más; (2) Un tamaño de poro del 90 % o más del volumen total de poros de 5 a 200 angstron; (3) Un volumen específico de poro de 0,2 cm3/g o más, y (4) Se utiliza un ion amonio cuaternario representado por la siguiente fórmula general (I) como plantilla y luego se elimina dicha plantilla: [NR^1R^2R^3R^4]+ OH- (I), donde R^1 representa una cadena hidrocarbonada lineal o ramificada de 2 a 36 átomos de carbono y R^2 a R^4 representan un grupo alquilo de 1 a 6 átomos de carbono. Peer reviewed
[Show abstract][Hide abstract] ABSTRACT: Ge-directed zeolites. Forcefield atomic simulations are used to investigate the location of the Ge atoms and the structure-directing agents (SDAs) in the Ge containing ITQ-17 zeolite (see picture). The Ge atoms are found to locate preferentially at the T1 sites which form double four-ring (D4R) units. A combined theoretical and experimental treatment has allowed us to prove the preferential location of Ge atoms in the double four-member rings of the polymorph C of Beta zeolite, and its corresponding structure-directing effect to be demonstrated.
[Show abstract][Hide abstract] ABSTRACT: Two strategies to improve the catalytic activity of Ti-MCM-41 materials in the epoxidation of olefins are described; the first approach involves silylation of the surface of Ti-MCM-41 which produces a very hydrophobic catalyst whereas the second approach is based on removal of water from the reaction media; the increase in activity is not due to a change in the intrinsic activity of the Ti sites, but rather to a decrease of the catalyst deactivation by reducing the formation of diols produced by ring opening of the epoxide.
Chemical Communications 10/1998; DOI:10.1039/A806702C · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ti-MCM-41 materials with methylated silicons have been prepared in one step synthesis; the resultant organic-inorganic composites are highly stable and present the highest activity and selectivity ever reported on Ti containing mesoporous catalysts for the epoxidation of olefins using organic peroxides.
Chemical Communications 09/1998; 17(17-17):1899-1900. DOI:10.1039/A804801K · 6.83 Impact Factor