Dolores Esquivel

Covalent triazine-based frameworks (CTFs) from triptycene and fluorene motifs for CO2 adsorption

Two microporous covalent triazine-based frameworks (CTFs) with triptycene (TPC) and fluorene (FL) backbones have been synthesized through a mild AlCl3-catalyzed Friedel–Crafts reaction, with the highest surface area of up to 1668 m2 g−1 for non-ionothermal CTFs. CTF-TPC and CTF-FL show an excellent carbon dioxide uptake capacity of up to 4.24 mmol g−1 at 273 K and 1 bar.

100% thiol-functionalized ethylene PMOs prepared by “thiol acid–ene” chemistry

A novel thiol functionalized bis-silane PMO precursor was synthesized by highly efficient thiol acid-ene chemistry between the double bonds of 1,2-(E)-bis(triethoxysilyl)ethene and thioacetic acid. After aminolysis the self-assembly process of the formed SH-precursor with Pluronic P123 under acidic conditions yields the first 100% thiol-PMO material with good structural ordering.

Thermal behaviour, sulfonation and catalytic activity of phenylene-bridged periodic mesoporous organosilicas

The structural and surface changes in a phenylene-bridged periodic mesoporous organosilica after thermal treatment under different conditions are reported. Organic moieties in the pore walls are stable at temperatures close to 500 °C but calcination in the presence of oxygen leads to C–Si bond cleavage as well as the formation of oxidized groups of phenolic and carbonyl type. The oxidized materials are functionalized by reaction with chlorosulfonic acid in a higher extension than those calcined under nitrogen but all of them preserve their structure and surface properties. All sulfonated organosilicas were active as catalysts for the esterification of acetic acid with ethanol, particularly those calcined in the air. Remarkably, some of them can be even more active than Amberlyst-15 when water is used as reaction medium which is of a high interest for the design of sustainable acid-catalyzed chemical processes.

A photoluminescent covalent triazine framework: CO2 adsorption, light-driven hydrogen evolution and sensing of nitroaromatics

A highly photoluminescent (PL) porous covalent triazine-based framework (PCTF-8) is synthesized from tetra(4-cyanophenyl)ethylene by using trifluoromethanesulfonic acid as the catalyst at room temperature. Due to triazine units in the framework, the PCTF-8 exhibits excellent thermal stability (>400 °C). The Brunauer–Emmett–Teller (BET) specific surface area of PCTF-8 is 625 m2 g−1 which is lower than the one obtained from the synthesis under Lewis acid conditions (ZnCl2). At 1 bar and 273 K, the PCTF-8 adsorbs a significant amount of CO2 (56 cm3 g−1) and CH4 (17 cm3 g−1) which is highly comparable to nanoporous 1,3,5-triazine frameworks (NOP-1-6, 29–56 cm3 g−1). This nitrogen rich framework exhibits good ideal selectivity (61 : 1 (85% N2 : 15% CO2) at 273 K, 1 bar). Thus, it can be used as a promising candidate for potential applications in post-combustion CO2 capture and sequestration technologies. In addition, photoluminescence properties as well as the sensing behaviour towards nitroaromatics have been demonstrated. The fluorescence emission intensity of PCTF-8 is quenched by ca. 71% in the presence of 2,4,6-trinitrophenol (TNP). From time-resolved studies, a static quenching behaviour was found. This high photoluminescence property is used for hydrogen evolving organic photocatalysis from water in the presence of a sacrificial electron donor and a cocatalyst.

Au@UiO-66: a base free oxidation catalyst

We present the in situ synthesis of Au nanoparticles within the Zr based Metal Organic Framework, UiO-66. The resulting Au@UiO-66 materials were characterized by means of N2 sorption, XRPD, UV-Vis, XRF, XPS and TEM analysis. The Au nanoparticles (NP) are homogeneously distributed along the UiO-66 host matrix when using NaBH4 or H2 as reducing agents. The Au@UiO-66 materials were evaluated as catalysts in the oxidation of benzyl alcohol and benzyl amine employing O2 as oxidant. The Au@MOF materials exhibit a very high selectivity towards the ketone (up to 100%). Regenerability and stability tests demonstrate that the Au@UiO-66 catalyst can be recycled with a negligible loss of Au species and no loss of crystallinity. In situ IR measurements of UiO-66 and Au@UiO-66-NaBH4, before and after treatment with alcohol, showed an increase in IR bands that can be assigned to a combination of physisorbed and chemisorbed alcohol species. This was confirmed by velocity power spectra obtained from the molecular dynamics simulations. Active peroxo and oxo species on Au could be visualized with Raman analysis.

Formation and functionalization of surface Diels–Alder adducts on ethenylene-bridged periodic mesoporous organosilica

The Diels–Alder cycloaddition of two common dienes (cyclopentadiene and anthracene) to the double carbon–carbon bonds of an ethenylene-bridged periodic mesoporous organosilica was studied and compared to that of benzocyclobutene. The resulting materials were characterized by several techniques such as X-ray and thermal analyses, DRIFT, 13C and 29Si MAS NMR and porosimetry. They showed that the mesopores were decorated with the Diels–Alder adducts with negligible structural degradation and with the concomitant reduction of the surface area and pore size. The formation of surface adducts was even successful with a relatively hindered diene such as anthracene. The surface Diels–Alder adducts were stable and susceptible to further functionalization. Thus, after being sulfonated, the resulting solids were used as acid catalysts in the esterification of acetic acid with ethanol, a process in which they proved to be very active. In fact, each acid site of these hybrid materials was up to three times more active than a similar site in Amberlyst-15, a commercial acid resin.

Metal-Exchanged β Zeolites as Catalysts for the Conversion of Acetone to Hydrocarbons

Various metal-β zeolites have been synthesized under similar ion-exchange conditions. During the exchange process, the nature and acid strength of the used cations modified the composition and textural properties as well as the Brönsted and Lewis acidity of the final materials. Zeolites exchanged with divalent cations showed a clear decrease of their surface Brönsted acidity and an increase of their Lewis acidity. All materials were active as catalysts for the transformation of acetone into hydrocarbons. Although the protonic zeolite was the most active in the acetone conversion (96.8% conversion), the metal-exchanged zeolites showed varied selectivities towards different products of the reaction. In particular, we found the Cu-β to have a considerable selectivity towards the production of isobutene from acetone (over 31% yield compared to 7.5% of the protonic zeolite). We propose different reactions mechanisms in order to explain the final product distributions.

Silanol‐Assisted Aldol Condensation on Aminated Silica: Understanding the Arrangement of Functional Groups

Free silanol groups are known to promote the activity of aminated silica. In this work the effect of the silanol‐to‐amine ratio on the aldol condensation of 4‐nitrobenzaldehyde and acetone is investigated in a range from 0 to 2.4. Irrespective of the amine density, identical, moderate turnover frequencies are obtained if the silica exclusively has amines on its surface. The turnover frequency increases with increasing silanol‐to‐amine ratio until an upper limit is reached at a silanol‐to‐amine ratio of 1.7. At this upper limit the turnover frequency is a factor 5 higher than the turnover frequencies obtained with the monofunctional amine‐based catalysts. This increase is ascribed to hydrogen‐bridge interactions between the silanols and the carbonyl moiety of the reactants that provoke a more easy interaction between the carbonyl moiety and the amine as required for the aldol condensation. The observation that higher values than one for the silanol‐to‐amine ratio are required is rationalized by computer simulations. It was found that amine groups were grafted on the silica surface in a clustered manner, originating from positive deviations from ideality in the synthesis mixture, that is, from clustering of the amine precursor in the liquid phase.

Evaluation of phenylene-bridged periodic mesoporous organosilica as a stationary phase for solid phase extraction

A periodic mesoporous organosilica, in particular, a phenylene-bridged material (Ph-PMO), was evaluated for the first time as a sorbent for retaining and eluting fenuron, simazine, atrazine, carbaryl and terbutryn in grape must by solid phase extraction (SPE) prior to their determination with capillary electrophoresis coupled with ultraviolet detection (CE-UV). The analytes were used as model compounds to demonstrate the potential of Ph-PMO for increasing the sensitivity of CE. Under optimal conditions, the limits of detection for the analytes ranged from 0.6 to 4 μg/L, and their limits of quantitation from 2 to 10 μg/L. These values were comparable and, in some cases, even better than those obtained with C18 and HLB materials. Ph-PMO was characterized physicochemically by X-ray diffraction analysis, N2 adsorption-desorption measurements and laser diffraction particle sizing. The sorbent afforded the extraction of atrazine, carbaryl and terbutryn from grape must with mean recoveries ranging from 86 to 105%. Therefore, periodic mesoporous organosilicas possess a high potential as SPE materials.

Eu3+@PMO: synthesis, characterization and luminescence properties

A periodic mesoporous organosilica (PMO) functionalized with dipyridyl-dihydropyridazine units has been successfully prepared by a hetero Diels–Alder reaction between the double bonds of an ethenylene-bridged PMO material and a substituted tetrazine. The ordering and mesoporosity of the parent material is maintained after the post-modification process, and the surface Diels–Alder adducts are clearly observable in the pores. These dipyridyl-dihydropyridazine moieties can form interesting chelates with lanthanide ions. Thus, two novel organic–inorganic luminescent hybrid materials have been obtained by linking of Eu3+ compounds to an ethene-PMO functionalized with dipyridyl-dihydropyridazine. Both materials have been studied in depth by photoluminescence spectroscopy and luminescence decay time measurements. Our results reveal the key role of surface Diels–Alder adducts as suitable sensitizing ligands for europium ions.