Sol-Gel Synthesis of Non-Silica Monolithic Materials

Interaction of HPC with CTAB and Tween 40 at Water/Air and Water/Soya Oil Interfaces

The bulk and interfacial shear rheological behavior of aqueous solutions of biocompatible polymer HPC has been investigated in the presence of cationic CTAB and nonionic Tween 40 having the same chain length but different head groups. Steady-state bulk experiments depict two distinct regions in the rheogram (Newtonian followed by pseudoplastic). Dynamic experiments suggest that the stability of HPC hydrogels decreases with the increase in surfactant concentration. Interfacial steady shear tests of 2D monolayers of 1 wt % HPC and 1 wt % HPC with varying concentrations of Tween 40/CTAB show a non-Newtonian dilatant behavior at the solution-air interface. However, two distinct dilatant regions separated by a Newtonian region were observed for the same films at the solution-soya oil interface. The strength of films formed at the two interfaces decreases with the increase of surfactant concentration as observed from oscillatory interfacial tests. HPC interacts more strongly with CTAB than Tween 40 both in bulk as well as at the interfaces studied.

Nanoporous–Crystalline Poly(2,6-dimethyl-1,4-phenylene)oxide Aerogels with Selectively Sulfonated Amorphous Phase for Fast VOC Sorption from Water

This paper describes the preparation and characterization of poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) highly porous monolithic aerogels with a hydrophobic nanoporous–crystalline phase and a hydrophilic sulfonated amorphous phase. The sulfonated aerogels were obtained by the sulfonation of PPO physical gels, followed by the supercritical CO₂ extraction of solvents. WAXD and FTIR analysis showed that the nanoporous–crystalline phase was preserved for a degree of sulfonation up to c.a. 35%, allowing a highly volatile organic compound (VOC) sorption capacity. The sulfonated PPO aerogels exhibited a high water sorption capacity, with a water uptake of up to 500 wt%, and faster VOC sorption kinetics from water with respect to unsulfonated aerogels.

Advancements in the preparation and application of monolithic silica columns for efficient separation in liquid chromatography

Fast and efficient separation remains a big challenge in high performance liquid chromatography (HPLC). The need for higher efficiency and resolution in separation is constantly in demand. To achieve that, columns developed are rapidly moving towards having smaller particle sizes and internal diameters (i.d.). However, these parameters will lead to high back-pressure in the system and will burden the pumps of the HPLC instrument. To address this limitation, monolithic columns, especially silica-based monolithic columns have been introduced. These columns are being widely investigated for fast and efficient separation of a wide range of molecules. The present article describes the current methods developed to enhance the column efficiency of particle packed columns and how silica monolithic columns can act as an alternative in overcoming the low permeability of particle packed columns. The fundamental processes behind the fabrication of the monolith including the starting materials and the silica sol-gel process will be discussed. Different monolith derivatization and end-capping processes will be further elaborated and followed by highlights of the performance such monolithic columns in key applications in different fields with various types of matrices.

Towards Macroporous α-Al2O3-Routes, Possibilities and Limitations

This article combines a systematic literature review on the fabrication of macroporous α-Al₂O₃ with increased specific surface area with recent results from our group. Publications claiming the fabrication of α-Al₂O₃ with high specific surface areas (HSSA) are comprehensively assessed and critically reviewed. An account of all major routes towards HSSA α-Al₂O₃ is given, including hydrothermal methods, pore protection approaches, dopants, anodically oxidized alumina membranes, and sol-gel syntheses. Furthermore, limitations of these routes are disclosed, as thermodynamic calculations suggest that γ-Al₂O₃ may be the more stable alumina modification for A_BET > 175 m²/g. In fact, the highest specific surface area unobjectionably reported to date for α-Al₂O₃ amounts to 16-24 m²/g and was attained via a sol-gel process. In a second part, we report on some of our own results, including a novel sol-gel synthesis, designated as mutual cross-hydrolysis. Besides, the Mn-assisted α-transition appears to be a promising approach for some alumina materials, whereas pore protection by carbon filling kinetically inhibits the formation of α-Al₂O₃ seeds. These experimental results are substantiated by attempts to theoretically calculate and predict the specific surface areas of both porous materials and nanopowders.

Green Silver Nanoparticles Confined in Monolithic Silica Disk-packed Spin Column for Human Serum Albumin Preconcentration

Background:

In recent times many new uses have been found for nanomaterials that have undergone homogenous immobilization within porous supports. For this paper, immobilization of SNPs on a thiol-functionalized silica monolith using a fast, easy, environmentally friendly and costeffective process was performed. This was achieved by modifying the surface of a silica-based monolith using thiol groups, and then we fabricated green SNPs in situ, reducing an inorganic precursor silver nitrate solution (AgNO3) by employing tangerine peel extract as a reducing reagent, with Ag-thiol bonds forming along the monument. Doing this allows monoliths to be prepared in such a way that, as TEM analysis demonstrated, SNPs are evenly distributed along the rod's length. Once the materials had been fabricated, they were employed as a sorbent by being placed in a centrifuge. The SNP-thiol functionalized silica monolith was then tested using a standard protein (HSA).

Methods:

The process involves creating monolithic materials by employing a two-part sol-gel technique before modifying the surface of the silica-based monolith using thiol groups for hosting purposes. Homogenous surface coverage was achieved through the use of a non-toxic "green" reducing reagent (tangerine peel extract) to reduce a silver nitrate solution in place to create SNPs joined to the pore surface of a thiol-functionalized silica monolith, employing bonds of Ag-thiol. Once these materials were synthesized, they were classified by utilizing a number of methods based on SEM coupled with EDAX, TEM, AFM and BET analysis. The silica-based monolith, embedded with constructed SNPs, was employed as a sorbent in the preconcentration of human serum albumin (HSA).

Results:

The performance of the fabricated materials was measured against a silica-based monolith with no SNPs. Also, a silica monolith with constructed SNPs embedded was employed to capture HSA within a sample of human urine mixed with a double detergent concentrate (SDS). Such a monolith containing functionalized SNPs can be a highly effective sorbent for preconcentration of proteins in complex samples.

Conclusion:

It was shown to have superior performance compared to a bare silica-based monolith. Additionally, it was shown that a monolithic column modified by SNPs could preconcentrate spiked HSA in urine samples.

Low-Temperature Continuous-Flow Dehydration of Xylose Over Water-Tolerant Niobia-Titania Heterogeneous Catalysts

The sustainable conversion of vegetable biomass-derived feeds to useful chemicals requires innovative routes meeting environmental and economical criteria. The approach herein pursued is the synthesis of water-tolerant, unconventional solid acid monolithic catalysts based on a mixed niobia-titania skeleton building up a hierarchical open-cell network of meso- and macropores, and tailored for use under continuous-flow conditions. The materials were characterized by spectroscopic, microscopy, and diffraction techniques, showing a reproducible isotropic structure and an increasing Lewis/Brønsted acid sites ratio with increasing Nb content. The catalytic dehydration reaction of xylose to furfural was investigated as a representative application. The efficiency of the catalyst was found to be dramatically affected by the niobia content in the titania lattice. The presence of as low as 2 wt % niobium resulted in the highest furfural yield at 140 °C under continuous-flow conditions, by using H₂ O/γ-valerolactone as a safe monophasic solvent system. The interception of a transient 2,5-anhydroxylose species suggested the dehydration process occurs via a cyclic intermediates mechanism. The catalytic activity and the formation of the anhydro intermediate were related to the Lewis acid sites (LAS)/Brønsted acid sites (BAS) ratio and indicated a significant contribution of xylose-xylulose isomerization. No significant catalyst deactivation was observed over 4 days usage.

Holographic sol-gel monoliths: optical properties and application for humidity sensing

Sol-gel monoliths based on SiO2, TiO2 and ZrO2 with holographic colourful diffraction on their surfaces were obtained via a sol-gel synthesis and soft lithography combined method. The production was carried out without any additional equipment at near room temperature and atmospheric pressure. The accurately replicated wavy structure with nanoscale size of material particles yields holographic effect and its visibility strongly depends on refractive index (RI) of materials. Addition of multi-walled carbon nanotubes (MWCNTs) in systems increases their RI and lends absorbing properties due to extremely high light absorption constant. Further prospective and intriguing applications based on the most successful samples, MWCNTs-doped titania, were investigated as reversible optical humidity sensor. Owing to such property as reversible resuspension of TiO2 nanoparticles while interacting with water, it was proved that holographic xerogels can repeatedly act as humidity sensors. Materials which can be applied as humidity sensors in dependence on holographic response were discovered for the first time.

Sol-gel synthesis of monolithic materials with hierarchical porosity

The development of synthetic routes to hierarchically organized porous materials containing multiple, discrete sets of pores having disparate length scales is of high interest for a wide range of applications. One possible route towards the formation of multilevel porous architectures relies on the processing of condensable, network forming precursors (sol-gel processes) in the presence of molecular porogens, lyotropic mesophases, supramolecular architectures, emulsions, organic polymers, or ice. In this review the focus is on sol-gel processing of inorganic and organic precursors with concurrently occurring microscopic and/or macroscopic phase separation for the formation of self-supporting monoliths. The potential and the limitations of the solution-based approaches is presented with special emphasis to recent examples of hierarchically organized silica, metal oxides and phosphates as well as carbon monoliths.

Morphological Properties of Methacrylate-Based Polymer Monoliths: From Gel Porosity to Macroscopic Inhomogeneities

Shaping chemical interfaces of hard and soft matter materials into physical morphologies that guarantee excellent transport properties is of central importance for technologies relying on adsorption, separation, and reaction at the interface. Polymer monoliths with a hierarchically structured pore space, for example, are widely used in flow-driven processes, whose efficiency depends on the morphology of the support material over several length scales. Compared with alternative support structures, particularly silica monoliths, polymer monoliths yield lower efficiency, which suggests a suboptimal morphology. Based on physical reconstruction by serial block-face scanning electron microscopy we evaluate the structural features of a methacrylate-based polymer monolith from the pore scale to the column scale. The morphological data reveal a homogeneous polymer skeleton with a solute-impenetrable core-porous shell architecture and a heterogeneous macropore space that suffers from inhomogeneities at the short-range and the transcolumn scale. Although the morphology of the polymer phase is favorable to efficient mass transport, the performance of the polymer monolith is limited by severe transcolumn gradients in macroporosity and macropore size. We propose to overcome these morphological limitations by pursuing a preparation strategy that involves active rather than passive shaping of the macropore space, for example, by using silica monoliths as templating structures for polymer monolith preparation.

Tough Polymer Aerogels Incorporating a Conformal Inorganic Coating for Low Flammability and Durable Hydrophobicity

Both inorganic and polymeric aerogels are well-known in the materials field. Inorganic aerogels are generally susceptible to brittle fracture, while polymeric aerogels tend to exhibit low modului and high flammability. To overcome these disadvantages, we introduce a new approach to the design of aerogels. A microporous poly(vinyl alcohol) (PVA) aerogel/silica nanocomposite was prepared by growing a silica conformal coating onto a PVA aerogel scaffold. Such aerogel/silica nanocomposites show significant improvement in their mechanical properties over either individual component. The nanocomposites show excellent fire resistance since the silica conformal coating serves as a barrier for heat transfer and mass loss of the coated organic materials. After a fluorocarbon silane treatment, the nanocomposites also show durable superhydrophobicity.

Monolithic polymeric aerogels with VOCs sorbent nanoporous crystalline and water sorbent amorphous phases

Monolithic syndiotactic polystyrene (s-PS) aerogels, formed by highly crystalline nanofibrils with a hydrophobic nanoporous-crystalline phase and a hydrophilic amorphous phase have been prepared and characterized. These aerogels, with a high degree of sulfonation of the amorphous phase and nearly negligible sulfonation of the crystalline phase, are obtained by treating physical gels exhibiting δ-clathrate form. With respect to unsulfonated nanoporous-crystalline polymeric aerogels, these new selectively sulfonated aerogels present the great advantage of a high water diffusivity and water uptake up to 600 wt %. This water uptake increases greatly the sorption kinetics of organic pollutants by the hydrophobic nanopores of the crystalline phase. For instance, for aerogels with a sulfonation of 10%, the diffusivity of a volatile organic compound (1,2-dichloroethane, DCE) from 10 ppm aqueous solution is more than 3 orders of magnitude higher than that for the unsulfonated aerogel and is very close to the DCE diffusivity in water.

Features of silicon– and titanium–polyethylene glycol precursors in sol–gel synthesis of new hydrogels

The formation of organic/inorganic hydrogels based on silicon and titanium polyethylene glycolates, new biocompatible water-soluble precursors in sol–gel processing, was investigated.

Liquid Foam Templates Associated with the Sol-Gel Process for Production of Zirconia Ceramic Foams

The unique properties of ceramic foams enable their use in a variety of applications. This work investigated the effects of different parameters on the production of zirconia ceramic foam using the sol-gel process associated with liquid foam templates. Evaluation was made of the influence of the thermal treatment temperature on the porous and crystalline characteristics of foams manufactured using different amounts of sodium dodecylsulfate (SDS) surfactant. A maximum pore volume, with high porosity (94%) and a bimodal pore size distribution, was observed for the ceramic foam produced with 10% SDS. Macropores, with an average size of around 30 μm, were obtained irrespective of the SDS amount, while the average size of the supermesopores increased systematically as the SDS amount was increased up to 10%, after which it decreased. X-ray diffraction analyses showed that the sample treated at 500 °C was amorphous, while crystallization into a tetragonal metastable phase occurred at 600 °C due to the presence of sulfate groups in the zirconia structure. At 800 and 1000 °C the monoclinic phase was observed, which is thermodynamically stable at these temperatures.

Mechanism of structural networking in hydrogels based on silicon and titanium glycerolates

Formation of organic/inorganic hydrogels based on silicon- and titanium-glycerol precursors synthesized by transesterification of alkoxy derivatives in excess of glycerol was investigated. The precursors in excess of glycerol and obtained gels were studied by chemical and physical methods including gelation kinetics, IR spectroscopy, XRD, dynamic and electrophoretic light scattering, mechanical deformation, which disclosed the basic difference in the gelation mechanism and structure of network in the hydrogels. Due to this difference, the gelation time of silicon- and titanium-glycerol precursors depended on pH or electrolyte addition in an opposite way. In the wide pH range, silicon-glycerol hydrogel was a polymeric single-phase system formed by the polymeric network homogeneously swollen in liquid water/glycerol medium. Flory-Rehner theory applied to the elastic modulus of these gels gave 40-180 monomer base units in the subchains of the network depending on water content in the gel. The mechanism of networking was three-dimensional polycondensation promoted by the electrically charged functional groups attached to the flexible polymeric chains. Electrolyte solutions provided the gelation according to Hofmeister series. Titanium-glycerol hydrogels were heterogeneous colloid systems at pH>1.5 and single-phase polymeric gels at lower pH. Electrolyte solutions provided the gelation according to Schultze-Hardy rule.

Hierarchically porous titania thin film prepared by controlled phase separation and surfactant templating

Poly(propylene glycol) (PPG) of moderately high molecular weight (M(n) = 3500 Da) exhibits amphibious behavior in aqueous solution in that it is hydrophilic at low temperature but hydrophobic at high temperature. This property is utilized to generate porous titania thin films with a hierarchical structure consisting of macroporous voids/cracks in films with mesoporous walls. The smaller mesopores result from the self-assembly of the Pluronic block copolymer P123 to form micellar templates in well-ordered arrays with hexagonal symmetry. The larger pores are generated from the phase separation of PPG during aging of the films. The PPG acts to a limited degree as a swelling agent for the P123 micelles, but because the films are aged at a low temperature where PPG is hydrophilic, much of the PPG remains in the polar titania phase. Upon heating, the PPG phase separates to form randomly dispersed, large pores throughout the film while retaining the ordered mesoporous P123-templated structure in the matrix of the material. TEM and SEM imaging confirm that calcined titania thin films have interconnected hierarchical porous structures consisting of ordered mesopores 4-12 nm in diameter and macroporous voids >100 nm in size. The density and size of the voids increase as more PPG is added to the films.