Noemi Linares

Synthesis, characterization and magnetism of monodispersed water soluble palladium nanoparticles

Water soluble, monodispersed Pd nanoparticles with a narrow particle size distribution have been successfully synthesized by controlled reduction of [PdCl4]2−. The resulting aqueous colloids are stable over extended periods of time and can be prepared at high nanoparticle loading (20 g/L of Pd) with no agglomeration. The size of the nanoparticles can be reduced from the nanometer (ca. 3.5 nm) to the sub-nanometer size range (ca. 0.9 nm). Detailed magnetic characterization indicated that the larger, 3.5 nm nanoparticles show ferromagnetic properties at room temperature, while the sub-nanometric ones lose this magnetic behavior.

Synthesis of mesoporous metal complex-silica materials and their use as solvent-free catalysts

Incorporation of various Pd(II) complexes into the framework of MSU-X mesoporous silica has been achieved by co-condensation using a facile solvent-free one-pot synthesis. The use of ligands with triethoxysilyl terminal groups permitted the synthesis of three different metallosilanes precursors (metal complexes with ligands containing trialkoxysilane terminal groups), which allow for the homogeneous in situ incorporation of metal complexes covalently bonded to the porous support. Inorganic precursor tetraethylorthosilicate was used both as silica source and as solvent for the synthesis of the complexes, avoiding the use of other organic co-solvents, making the synthesis environmentally benign. The gentle synthesis conditions used such as neutral pH, room temperature and mild ethanol extraction of the surfactant, allowed a cleaner route for the immobilization of homogeneous Pd(II) catalysts in mesoporous silica, while protecting the structural and chemical integrity of the metal complexes. For comparison purposes, monomer complexes [trans-PdCl2L2] (L = NH2(CH2)3Si(OEt)3, 4-C5H4N–(CH2)2Si(OEt3), PPh2(CH2)2Si(OEt)3) were synthesized using the same aerobic reaction conditions to those use for the co-condensation processes and fully characterized before their incorporation in the mesoporous silica. The catalytic performance of these materials was tested for the Suzuki-Miyaura reaction under solvent-free conditions. Efficient mixing of all the components was accomplished by applying either magnetic stirring or ball milling. The good yields obtained, even at room temperature, confirmed the catalytic activity of the metal complexes once incorporated into the mesoporous silica framework. The possibility to work under solvent-free conditions even with solid starting reactants, is a significant step forward in the Suzuki-Miyaura coupling reaction because its benefits in terms of cost and impact of the environment.

Sol–Gel Coordination Chemistry: Building Catalysts from the Bottom‐Up

The development of synthetic routes for the tailoring of efficient silica‐based heterogeneous catalysts functionalized with coordination complexes or metallic nanoparticles has become a important goal in chemistry. Most of these techniques have been based on postsynthetic treatments of preformed silicas. Nevertheless, there is an emerging approach, so‐called sol–gel coordination chemistry, based on co‐condensation during the sol–gel preparation of the hybrid material of the corresponding complex or nanoparticle modified with terminal trialkoxysilane groups with a silica source (such as tetraethoxysilane) and in the presence of an adequate surfactant. This method leads to the production of new mesoporous metal complex‐silica materials, with the metallic functionality incorporated homogeneously into the structure of the hybrid material, improving the stability of the coordination complex (which is protected by the silica network) and reducing the leaching of the active phase. This technique also offers the actual possibility of functionalizing silica or other metal oxides for a wider range of applications, such as photonics, sensing, and biochemical functions.

PdNP@Titanate Nanotubes as Effective Catalyst for Continuous‐Flow Partial Hydrogenation Reactions

Pd nanoparticles were easily immobilized onto titanate nanotubes by a straightforward procedure. The material (0.50 wt % Pd) was used as catalyst in the continuous‐flow, liquid‐phase hydrogenation reaction of unsaturated C−C bonds and it showed excellent performance and durability under very mild conditions (room temperature, 1–2 bar H2, residence time 13–36 s). In particular, very high productivity was obtained in the synthesis of the perfumery component cis‐3‐hexen‐1‐ol (40.6 mol gPd−1 h−1) without additives or metal contamination, with clear benefits in terms of process economy and environmental impact compared with conventional catalysts. The catalyst performance is discussed in the light of comparable systems.

Ultrasmall Zeolite L Crystals Prepared from Highly Interdispersed Alkali‐Silicate Precursors

The preparation of nanosized zeolites is critical for applications where mass-transport limitations within microporous networks hinder their performance. Often the ability to generate ultrasmall zeolite crystals is dependent upon the use of expensive organics with limited commercial relevance. Herein, we report the generation of zeolite L crystals with uniform sizes less than 30 nm using a facile, organic-free method. Time-resolved analysis of precursor assembly and evolution during nonclassical crystallization highlights key differences among silicon sources. Our findings reveal that a homogenous dispersion of potassium ions throughout silicate precursors leads to the formation of a metastable nonporous phase, which undergoes an intercrystalline transformation to zeolite L. The generation of highly interdispersed alkali-silicate precursors is seemingly critical to enhancing the rate of nucleation and facilitating the formation of ultrasmall crystal.

The Energetics of Surfactant‐Templating of Zeolites

Mesoporosity can be conveniently introduced in zeolites by treating them in basic surfactant solutions. The apparent activation energy involved in the formation of mesopores in USY via surfactant-templating was obtained through the combination of in situ synchrotron XRD and ex situ gas adsorption. Additionally, techniques such as pH measurements and TG/DTA were employed to determine the OH evolution and the CTA uptake during the development of mesoporosity, providing information about the different steps involved. By combining both in situ and ex situ techniques, we have been able, for the first time, to determine the apparent activation energies of the different processes involved in the mesostructuring of USY zeolites, which are in the same order of magnitude (30 – 65 kJ mol) of those involved in the crystallization of zeolites. Hence, important mechanistic insights on the surfactanttemplating method were obtained. For more than 25 years now, cationic surfactants have been widely applied to prepare mesoporous amorphous materials. Since then, researchers have made great efforts to extend the use of these surfactant-templating techniques for the preparation of mesoporous zeolites. Among the different methods used to impart secondary porosity within zeolites, the post-synthetic treatment with cationic surfactants has proved to be an effective tool to introduce tunable intracrystalline mesoporosity while maintaining the key features of the zeolite including strong acidity and excellent hydrothermal stability. Recently, we reported the first time-resolved observation of the development of mesoporosity in zeolites through surfactanttemplating by in situ XRD. Indeed, the combination of experimental data with theoretical calculations provided new insights on the formation of intracrystalline mesoporosity featuring short-range order in zeolites. Moreover, the use of LiqTEM rendered the first in situ real time visualization of this process. In spite of the numerous reports dealing with surfactanttemplating of zeolites and the fact that these materials are already a commercial reality, the driving forces responsible for this process are still unknown. Their investigation is one of the objectives of this paper. A systematic study of the apparent activation energies of the different processes involved in surfactant-templating in zeolites was undertaken to increase our understanding of this approach. With this aim, in situ synchrotron XRD and SAXS studies of this process were performed using the experimental setup shown in Figure 1a. This apparatus was specifically designed and built to get detailed information about the kinetics of the surfactanttemplating process, as it allows for the in situ monitoring of the pH and temperature while performing the XRD and SAXS measurements. The reaction mixture, containing the basic surfactant solution ([NaOH] = 0.09 M; [CTAB] = 0.07 M) was placed in a sealed reactor while the temperature was carefully controlled (± 0.1 oC) in the 45 – 90 oC range. The USY zeolite (CBV 720, Si/Al = 15) was introduced in the addition vessel on top of the reactor. The programmed opening of the lid between the reactor and the vessel allowed for the addition of the zeolite at once and thus the collection of data from the beginning of the reaction. Both pH and temperature probes were fitted in the reactor in order to continuously monitor both parameters. A peristaltic pump was used to circulate the mixture through a capillary where both the XRD and SAXS measurements were realized (see Figure S1 for further details). The evolution of the pH was in situ monitored (Figure S2a) providing kinetic information on the OH consumption. These experiments were carried out after calibrating the pH glass electrode with titrated NaOH solutions at the different temperatures studied, see ESI and Figure S3 for details. The decrease of the OH concentration is due to both the cleaving of Si-O-Si bonds and the ion exchange between H and Na/CTA ions in the zeolite. To distinguish between both processes, experiments in which the zeolite was firstly converted into its Na-form were carried out (Figure S2b) thus avoiding the ion exchange by H ions during surfactant-templating. Similar results were obtained in both cases (Figure S2) and thus experiments obtained from the Na-form of the zeolite were employed in this case (Figure 1b). As deduced from the consumption of OH the cleavage of the Si-O-Si bonds is a very fast process, occurring within the first 5 min of the treatment (Figure 1b). As previously described, the opening of the Si-OSi bonds and the formation of SiO sites is required for the mesostructuring to occur. Indeed, the process does not take place if base is not added to the reaction media (Figure S4) and the initial [OH] determines the amount of mesoporosity that can be formed within the zeolite. In order to obtain the apparent activation energy of the cleavage of the Si-O-Si bonds, its rate constant, k, at different temperatures was determined from the slope of the linear part of the evolution of [OH] versus time. These values were used to depict the Arrhenius plot, from where the Arrhenius apparent activation energy was obtained, Ea = 35 kJ mol (Figure 2b, OH data). After that, the step related to the surfactant uptake by the zeolite was investigated by TG to determine the amount of CTA incorporated. [a] Dr. N. Linares, Dr. E. O. Jardim, Dr. A. Sachse, Dr. E. Serrano, and Prof. Dr. J. Garcia-Martinez Laboratorio de Nanotecnología Molecular, Departamento de Química Inorgánica Universidad de Alicante Ctra. San Vicente-Alicante s/n, E-03690 Alicante, Spain. E-mail: j.garcia@ua.es; www.nanomol.es [b] Prof. Dr. J. Garcia-Martinez Rive Technology, Inc. 1 Deer Park Drive, Monmouth Junction, New Jersey 08852, United States Supporting information for this article is given via a link at the end of the document. 10.1002/ange.201803759 A cc ep te d M an us cr ip t Angewandte Chemie This article is protected by copyright. All rights reserved.