Marta Navarro

Probing the Relationship between Silicalite-1 Defects and Polyol Adsorption Properties

The relationship between polyol adsorption affinity and silanol defect density was investigated through the development of vapor and aqueous adsorption isotherms on silicalite-1 materials which vary in structural and surface properties. Silicalite-1 crystals prepared through alkaline synthesis, alkaline synthesis with steaming post-treatment, and fluoride synthesis routes were confirmed as crystalline mordenite framework inverted (MFI) by SEM and XRD and were shown to contain ~8.5-0 silanol defects per unit cell by (29)Si MAS, (1)H MAS, and (1)H-(29)Si CPMAS NMR. A hysteresis in the Ar 87 K adsorption isotherm at 10(-3)P/P0 evolved with a decrease in silanol defects, and, through features in the XRD and (29)Si MAS NMR spectra, it is postulated that the hysteresis is the result of an orthorhombic-monoclinic symmetry shift with decreasing silanol defect density. Gravimetric and aqueous solution measurements reveal that propylene glycol adsorption at 333 K is promoted by silanol defects, with a maximum 20-fold increase observed for aqueous adsorption at ~10(-3) g/mL with an increase from ~0 to 8.5 silanols per unit cell. A comparison of vapor and aqueous propylene glycol adsorption isotherms on defect-free silicalite-1 at 333 K, both of which exhibit the Type-V character, indicates that water enhances adsorption by a factor of ~2 in the Henrys Law regime. Henrys constants for aqueous C2-C4 polyol adsorption (concentrations below 0.004 g/mL) at 298 K are shown to have a linear dependence on the silanol defect density, demonstrating that these molecules preferentially adsorb at silanol defects at dilute concentrations. This systematic study of polyol adsorption on silicalite-1 materials highlights the critical role of defects on adsorption of hydrophilic molecules and clearly details the effects of coadsorption of water, which can guide the selection of zeolites for separation of biomass-derived oxygenates.

ZIF-8 micromembranes for gas separation prepared on laser-perforated brass supports

ZIF-8 is an imidazolate-based metal–organic framework (MOF). ZIF-8 micromembranes of 20–32 μm diameter are prepared by synthesizing the MOF on Nd:YAG laser-perforated 75 μm thick brass sheets (63/37 Cu/Zn). The laser irradiation activates the brass support, promoting ZIF-8 growth. A thick and continuous ZIF-8 membrane is crystallized on the laser irradiation outlet side of the support, while the inlet side and the inner surface of the microperforations are also coated with ZIF-8 intergrowth crystals. Laser perforated brass supports are not only cheap, flexible, strong, and easy to handle and to process as membrane materials; they are also chemically compatible with the ZIF-8 composition because of the shared Zn element. The ZIF-8 membranes obtained are characterized by XRD, SEM, EDX, TGA and N2 sorption analysis. Furthermore, the membranes are applied to the separation of equimolar H2–CH4, He–CH4, CO2–CH4 and O2–N2 mixtures confirming the expected molecular sieving effect due to the MOF microporosity.

In situ solid-state NMR and XRD studies of the ADOR process and the unusual structure of zeolite IPC-6

The assembly–disassembly–organization–reassembly (ADOR) mechanism is a recent method for preparing inorganic framework materials and, in particular, zeolites. This flexible approach has enabled the synthesis of isoreticular families of zeolites with unprecedented continuous control over porosity, and the design and preparation of materials that would have been difficult—or even impossible—to obtain using traditional hydrothermal techniques. Applying the ADOR process to a parent zeolite with the UTL framework topology, for example, has led to six previously unknown zeolites (named IPC-n, where n = 2, 4, 6, 7, 9 and 10). To realize the full potential of the ADOR method, however, a further understanding of the complex mechanism at play is needed. Here, we probe the disassembly, organization and reassembly steps of the ADOR process through a combination of in situ solid-state NMR spectroscopy and powder X-ray diffraction experiments. We further use the insight gained to explain the formation of the unusual structure of zeolite IPC-6. The assembly–disassembly–organization–reassembly (ADOR) process has recently enabled the synthesis of unusual — and sometimes previously inaccessible — inorganic materials. Further insight into its complex mechanism has now been gained that explains the unexpected formation and structure of such a zeolite.

Thin-Film Nanocomposite Membrane with the Minimum Amount of MOF by the Langmuir–Schaefer Technique for Nanofiltration

An innovative procedure for positioning a monolayer of hydrophilic metal organic framework (MOF) MIL-101(Cr) (MIL, Materials of Institute Lavoisier) nanoparticles (NPs) in thin-film nanocomposite (TFN) membranes has been implemented by transferring a Langmuir-Schaefer (LS) film of the MOF in between the polyamide thin layer at the top and the cross-linked asymmetric polyimide (P84) support at the bottom. The presence and layout of the LS-MIL-101(Cr) monolayer in the TFN membrane was confirmed by scanning transmission electron microscopy imaging with a high-angle annular dark-field detector images and X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy, and atomic force microscopy analyses. This methodology requires the smallest amount of MOF reported to date, 3.8 μg cm-2, and promotes the formation of a defect-free ultrathin MOF film. Although conventional TFN membranes tend to show MOF agglomerates that could contribute to the formation of unselective defects, LS-TFN membranes, characterized by a homogeneous and continuous MOF coating, exhibit an optimal membrane performance, without a significant decrease in selectivity. Outstanding methanol permeances, one of the best results reported to date, of 10.1 ± 0.5 L m-2 h-1 bar-1 when filtering sunset yellow and of 9.5 ± 2.1 L m-2 h-1 bar-1 when filtering rose bengal have been achieved in LS-TFN membranes with a rejection higher than 90% in all cases. Methanol permeates through the polyamide and the LS-MIL-101(Cr) monolayer, greatly enhanced by the MOF pore system, in comparison to thin-film composite and conventional TFN membranes (7.5 ± 0.7 and 7.7 ± 1.1 L m-2 h-1 bar-1 when filtering sunset yellow), respectively, in which polyamide areas free of MOF NPs are present.

Insight into the synthesis and characterization of zeolite millimeter-sized crystals

The effect of time, composition and structure-directing agents on the synthesis of silicalite-1 and octadecasil zeolite millimeter-sized crystals has been studied. Crystals up to about 3 mm size range were obtained and special attention was paid to their aspect ratio, thermal behavior, monocrystallinity and textural properties by means of electronic and optical microscopy, powder and single-crystal X-ray diffraction (XRD), thermogravimetry and Ar adsorption. For the silicalite-1 crystals obtained after 34 days of hydrothermal synthesis, more than one thousand specimens from two repeated batches were measured along a, b and c axes. Reproducibility was thus assessed simultaneously with a statistical analysis to obtain cumulative and differential distributions of crystal dimensions. Single-crystal XRD evidenced a high degree of monocrystallinity, while thermal analyses suggested that crystals with sizes above 125 μm have serious difficulties in activation, giving rise to high external specific surface area values upon Ar adsorption.

Laser control of zeolite nucleation.

Precursor solutions for the synthesis of zeolites are irradiated by means of a Nd-YAG laser. These solutions are subsequently submitted to a hydrothermal treatment and the results analyzed by X-ray diffraction and electron microscopy. Laser irradiation promotes the formation of silica nanoparticles that nucleate into zeolite (silicalite-1), following a hydrothermal treatment. The average crystal size (in the 0.6-3.6 μm range) of the zeolite exponentially decreases as a function of laser irradiation time. In addition, a longer irradiation time results in a narrower crystal size distribution.

Activation of giant silicalite-1 monocrystals combining rapid thermal processing and ozone calcination

A rapid thermal processing (RTP) technique combined with ozone/oxygen calcination was applied to activate and heal the structure of a 1.1 mm2 section of a silicalite-1 giant monocrystal. Crystallinity was monitored by single crystal X-ray diffraction and high sensitive thermogravimetry (TGA) was performed to prove the complete removal of the structure directing agent.

Microwave heating and the fast ADOR process for preparing zeolites

The microwave assisted ADOR process was applied to quickly disassemble zeolites IWW and UTL in acidic media and obtain the novel IPC-5 zeolite from IWW and a variant of the IPC-6 structure from UTL. XRD and Cs aberration-corrected STEM were used to characterise the structures. The results show that the speed of the process can be vastly increased using microwave heating.