Textural characterization of native and n-alky-bonded silica monoliths by mercury intrusion/extrusion, inverse size exclusion chromatography and nitrogen adsorption

Native and n-alkyl-bonded (n-octadecyl) monolithic silica rods with mesopores in the range between 10 and 25 nm and macropores in the range between 1.8 and 6.0 microm were examined by mercury intrusion/extrusion, inverse size exclusion chromatography (ISEC) and nitrogen sorption. Our results reveal very good agreement for the mesopore size distribution obtained from nitrogen adsorption (in combination with an advanced NLDFT analysis) and ISEC. Our studies highlight the importance of mercury porosimetry for the assessment of the macropore size distribution and show that mercury porosimetry is the only method which allows obtaining a combined and comprehensive structural characterization of macroporous/mesoporous silica monoliths. Our data clearly confirm that mercury porosimetry hysteresis and entrapment have different origin, and indicate the intrinsic nature of mercury porosimetry hysteresis in these silica monoliths. Within this context some silica monoliths show the remarkable result of no entrapment of mercury after extrusion from the mesopore system (i.e. for the first intrusion/extrusion cycle). The results of a systematic study of the mercury intrusion/extrusion behavior into native silica monoliths and monoliths with bonded n-alkyl groups reveals that the macro (through) pore structure, which controls the mass transfer to and from the mesopores, here mainly controls the entrapment behavior. Our data suggest that mercury intrusion/extrusion porosimetry does not only allow to obtain a comprehensive pore structure analysis, but can also serve as a tool to estimate the mass transport properties of silica monoliths to be employed in liquid-phase separation processes.     

Comprehensive pore structure characterization of silica monoliths with controlled mesopore size and macropore size by nitrogen sorption, mercury porosimetry, transmission electron microscopy and inverse size exclusion chromatography

The porosity of monolithic silica columns is measured by using different analytical methods. Two sets of monoliths were prepared with a given mesopore diameter of 10 and 25 nm, respectively and with gradated macropore diameters between 1.8 and 7.5 microm. After preparing the two sets of monolithic silica columns with different macro- and mesopores the internal, external and total porosity of these columns are determined by inverse size-exclusion chromatography (ISEC) using polystyrene samples of narrow molecular size distribution and known average molecular weight. The ISEC data from the 4.6 mm analytical monolithic silica columns are used to determine the structural properties of monolithic silica capillaries (100 microm I.D.) prepared as a third set of samples. The ISEC results illustrate a multimodal mesopore structure (mesopores are pores with stagnant zones) of the monoliths. It is found by ISEC that the ratio of the different types of pores is dependent on the change in diameter of the macropores (serve as flow-through pores). The porosity data achieved from the mercury penetration measurement and nitrogen adsorption as well of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) pictures are correlated with the results we calculated from the ISEC measurements. The ISEC results, namely the multimodal pore structure of the monoliths, reported in several publications, are not confirmed analyzing the pore structures of the different silica monoliths using all other analytical methods.     

Pore structural characteristics, size exclusion properties and column performance of two mesoporous amorphous silicas and their pseudomorphically transformed MCM-41 type derivatives

Highly ordered mesoporous silicas such as, mobile composition of matter, MCM-41, MCM-48, and the SBA-types of materials have helped to a large extent to understand the formation mechanisms of the pore structure of adsorbents and to improve the methods of pore structural characterization. It still remains an open question whether the high order, the regularity of the pore system, and the narrow pore size distribution of the materials will lead to a substantial benefit when these materials are employed in liquid phase separation processes. MCM-41 type 10 microm beads are synthesized following the route of pseudomorphic transformation of highly porous amorphous silicas. Highly porous silicas and the pseudomorphically transformed derivatives are characterized by nitrogen sorption at 77 K and by inverse size-exclusion chromatography (ISEC) employing polystyrene standards. Applying the network model developed by Grimes, we calculated the pore connectivity n(T) of the materials. The value of n(T) varies between the percolation threshold of the lattice and values of n(T) > 10, the latter being the limiting value above which the material can be considered to be almost infinitely connected such that the ISEC behavior of the material calculated with the pore network model is the same when calculated with a parallel pore model which assumes an infinite connectivity. One should expect that the pore connectivity is reflected in the column performance, when these native and unmodified materials are packed into columns and tested with low molecular weight analytes in the Normal Phase LC mode. As found in a previous study on monolithic silicas and highly porous silicas, the slope of the plate height (HETP) - linear velocity (u) curve decreased significantly with enhanced pore connectivity of the materials. First results on the pseudomorphically transformed MCM-41 type silicas and their highly porous amorphous precursors showed that (i) the transformation did not change the pore connectivity (within the limits detectable by ISEC) from the starting material to the final product and (ii) the slope of the HETP versus u curve for dibutylphtalate did not change significantly after the pseudomorphic transformation.     

Selective preparation of macroporous monoliths of conductive titanium oxides Ti(n)O(2n-1) (n = 2, 3, 4, 6)

Monolithic conductive titanium oxides Ti(n)O(2n-1) (n = 2, 3, 4, 6) with well-defined macropores have been successfully prepared as a single phase, via reduction of a macroporous TiO(2) precursor monolith using zirconium getter. Despite substantial removal of oxide ions, all the reduced monoliths retain the macropore properties of the precursor, i.e., uniform pore size distribution and pore volume. Furthermore, compared to commercial porous Ebonex (shaped conductive Ti(n)O(2n-1)), the bulk densities (1.8 g cm(-3)) are half, and the porosities (60%) are about 3 times higher. The obtained Ti(n)O(2n-1) (n = 2, 3, 4, 6) macroporous monoliths could find applications as electrodes for many electrochemical reactions.     

Synthesis and characterization of SiC materials with hierarchical porosity obtained by replication techniques

Porous silicon carbide monoliths were obtained using the infiltration of preformed SiO(2) frameworks with appropriate carbon precursors such as mesophase pitch. The initial SiO(2) monoliths possessed a hierarchical pore system, composed of an interpenetrating bicontinuous macropore structure and 13 nm mesopores confined in the macropore walls. After carbonization, further heat treatment at ca. 1,400 degrees C resulted in the formation of a SiC-SiO(2) composite, which was converted into a porous SiC monolith by post-treatment with ammonium fluoride solution. The resulting porous SiC featured high crystallinity, high chemical purity and showed a surface area of 280 m(2) g(-1) and a pore volume of 0.8 ml g(-1).     

Determination of the interparticle void volume in packed beds via intraparticle Donnan exclusion

Interparticle void volumes and porosities of packed capillaries have been determined using intraparticle Donnan exclusion of a small, unretained, co-ionic tracer (nitrate ions). The operational domain of this approach has been characterized for bare silica, reversed-phase, and strong cation-exchange materials (with different particle sizes and intraparticle pore sizes) in dependence of the mobile phase ionic strength. Interparticle porosities agree well with those analyzed by inverse size-exclusion chromatography (ISEC). Limitations to the use of Donnan exclusion (electrostatic exclusion) and ISEC (mechanical exclusion) arise as either type of exclusion becomes noticeable also in the cusp regions between the particles, or as the intraparticle pores are so large that complete electrostatic and size-exclusion are difficult to realize. Our data confirm that intraparticle Donnan exclusion presents a most simple, fast, and reliable approach for the analysis of packing densities.     

Liquid-phase synthesis and application of monolithic porous materials based on organic–inorganic hybrid methylsiloxanes, crosslinked polymers and carbons

Our recent progress in porous materials based on organic–inorganic hybrids, organic crosslinked polymers, and carbons is summarized. Flexible aerogels and aerogel-like xerogels with the polymethylsilsesquioxane (PMSQ) composition are obtained using methyltrimethoxysilane (MTMS) as the sole precursor. Preparation process and the flexible mechanical properties of these aerogels/xerogels are overviewed. As the derivative materials, hierarchically macro- and mesoporous PMSQ monoliths and marshmallow-like soft and bendable porous monoliths prepared from dimethyldimethoxysilane /MTMS co-precursors have been obtained. Organic crosslinked polymer monoliths with well-defined macropores are also tailored using gelling systems of vinyl monomers under controlled/living radical polymerization. The obtained polymer monoliths are carbonized and activated into activated carbon monoliths with well-defined pore properties. The activated carbon monoliths exhibit good electrochemical properties as the monolithic electrode. Some possibilities of applications for these porous materials are also discussed.     

Techniques for direct experimental evaluation of structure–transport relationships in disordered porous solids

Determining structure–transport relationships is critical to optimising the activity and selectivity performance of porous pellets acting as heterogeneous catalysts for diffusion-limited reactions. For amorphous porous systems determining the impact of particular aspects of the void space on mass transport often requires complex characterization and modelling steps to deconvolve the specific influence of the feature in question. These characterization and modelling steps often have limited accuracy and precision. It is the purpose of this work to present a case-study demonstrating the use of a more direct experimental evaluation of the impact of pore network features on mass transport. The case study evaluated the efficacy of the macropores of a bidisperse porous foam structure on improving mass transport over a purely mesoporous system. The method presented involved extending the novel integrated gas sorption and mercury porosimetry method to include uptake kinetics. Results for the new method were compared with those obtained by the alternative NMR cryodiffusometry technique, and found to lead to similar conclusions. It was found that the experimentally-determined degree of influence of the foam macropores was in line with expectations from a simple resistance model for a disconnected macropore network.     

Highly porous titania network: fabrication,characterization and photocatalytic activity

Highly porous titania network (CPTN) has been prepared using the protein entrapped cellulose gel as structural template via a template-assisted sol–gel process. The key point toward highly porous titania network lies in the entrapped proteins with template. To elucidate this, the effect of protein loading of template on the structure of final material was investigated. It reveals that high protein loading in cellulose gel gives rise to both large macroporosity and large surface area of final titania network. Specially, highly porous titania network is possessed of bimodal pore system and withstands high compressive pressure over 19 MPa. The highly porous titania network is found to have higher catalytic activity for the photodegradation of methylene blue than its counterpart of commercial P25 and microporous titania one (CTN) that derived from the pure cellulose gel. As a result, the macropores of titania network serve as the leading role in improving the photocatalytic activity. The proposed method might be applied to fabricate other inorganic network with desired macropore structure.     

Preparation of multilevel macroporous materials using natural plants as templates

A series of two-dimensionally (2D) ordered macroporous silica materials have been prepared by using eight natural plants as templates. The macroporous materials replicate the complicated morphologies of natural plants precisely, and retained the original pore shape of plants. Meanwhile, these macroporous materials showed roughly similar morphologies and pore structure by the same part of plants, while the distribution of macropore diameters is ca. 8–1,000 μm. It may provide a effective approach to prepare macroporous materials with different 2D pore and complicated morphologies. These 2D ordered macropore silica materials may have potentially application for tissue repairing and templates materials to produce other kinds of macropores or hierarchically porous materials.     

Comparison between the loading capacities of columns packed with partially and totally porous fine particles. What is the effective surface area available for adsorption?

The adsorption isotherms of phenol, caffeine, insulin, and lysozyme were measured on two C(18)-bonded silica columns. The first one was packed with classical totally porous particles (3 microm Luna(2)-C(18)from Phenomenex, Torrance, CA, USA), the second one with shell particles (2.7 microm Halo-C(18) from Advanced Materials Technology, Wilmington, DE, USA). The measurements were made at room temperature (T=295+/-1K), using mainly frontal analysis (FA) and also elution by characteristic points (FACP) when necessary. The adsorption energy distributions (AEDs) were estimated by the iterative numerical expectation-maximization (EM) procedure and served to justify the choice of the best adsorption isotherm model for each compound. The best isotherm parameters were derived from either the best fit of the experimental data to a multi-Langmuir isotherm model (MLRA) or from the AED results (equilibrium constants and saturation capacities), when the convergence of the EM program was achieved. The experiments show than the loading capacity of the Luna column is more than twice that of the Halo column for low-molecular-weight compounds. This result was expected; it is in good agreement with the values of the accessible surface area of these two materials, which were calculated from the pore size volume distributions. The pore size volume distributions are validated by the excellent agreement between the calculated and measured exclusion volumes of polystyrene standards by inverse size exclusion chromatography (ISEC). In contrast, the loading capacity ratio of the two columns is 1.5 or less with insulin and lysozyme. This is due to a significant exclusion of these two proteins from the internal pore volumes of the two packing materials. This result raises the problem of the determination of the effective surface area of the packing material, particularly in the case of proteins. This area is about 40 and 30% of the total surface area for insulin and for lysozyme, respectively, based on the pore size volume distribution validated by the ISEC method. The ISEC experiments showed that the largest and the smallest mesopores have rather a cylindrical and a spherical shape, respectively, for both packing materials.     

Mutual consistency between simulated and measured pressure drops in silica monoliths based on geometrical parameters obtained by three-dimensional laser scanning confocal microscope observations

The geometrical properties of co-continuous macroporous silica monoliths have been studied by laser scanning confocal microscopy (LSCM) and a comparison has been made with those obtained by conventional mercury intrusion method. Tetrahedral skeleton model (TMS), which mimics the gel skeleton shape of monoliths, was compared with real monoliths in terms of macropore and porosity using the geometrical parameters extracted from the LSCM observations. Liquid flow behavior through the macroporous silica monoliths was examined in comparison with those simulated using TSM, based on the geometrical properties obtained from LSCM observations. Heterogeneity in macropore topology and connectivity in pores and skeletons are suggested to contribute to the improvement of the model structure for macroporous monoliths.     

Effects of molecular weight on macropore sizes and characterization of porous hydroxyapatite ceramics fabricated using polyethylene glycol: mechanisms to generate macropores and tune their sizes

Hydroxyapatite (HAp) is a raw material used to fabricate scaffolds. Scaffolds are required to be porous to facilitate nutrient flow and vascularization. This study aims to produce porous HAp ceramics with macropores (>200 μm) and bioactivity and tune their macropore size. Polyethylene glycol (PEG) with molecular weights of 400, 3,400, and 8,300 was used to generate macropores. The macropore size increased as the molecular weight of PEG increased. In this method, emulsions were formed by hydrophobic PEG binding HAp nanoparticles during chemical syntheses. Water foams, as a core of the emulsion, were transformed into steam, and the steam expanded under heat treatment. Macropores were generated by the evaporation of the steam and consolidation of HAp nanoparticles. The difference in the molecular weight of PEG did not affect cell adhesion to the porous HAp ceramics. Cells adhered well to and spread widely on the HAp ceramics regardless of macropore size.     

Nondestructive technique for the characterization of the pore size distribution of soft porous constructs for tissue engineering

Polymer scaffolds tailored for tissue engineering applications possessing the desired pore structure require reproducible fabrication techniques. Nondestructive, quantitative methods for pore characterization are required to determine the pore size and its distribution. In this study, a promising alternative to traditional pore size characterization techniques is presented. We introduce a quantitative, nondestructive and inexpensive method to determine the pore size distribution of large soft porous solids based on the on the displacement of a liquid, that spreads without limits though a porous medium, by nitrogen. The capillary pressure is measured and related to the pore sizes as well as the pore size distribution of the narrowest bottlenecks of the largest interconnected pores in a porous medium. The measured pore diameters correspond to the narrowest bottleneck of the largest pores connecting the bottom with the top surface of a given porous solid. The applicability and reproducibility of the breakthrough technique is demonstrated on two polyurethane foams, manufactured using the thermally induced phase separation (TIPS) process, with almost identical overall porosity (60-70%) but very different pore morphology. By selecting different quenching temperatures to induce polymer phase separation, the pore structure could be regulated while maintaining the overall porosity. Depending on the quenching temperature, the foams exhibited either longitudinally oriented tubular macropores interconnected with micropores or independent macropores connected to adjacent pores via openings in the pore walls. The pore size and its distribution obtained by the breakthrough test were in excellent agreement to conventional characterization techniques, such as scanning electron microscopy combined with image analysis, BET technique, and mercury intrusion porosimetry. This technique is suitable for the characterization of the micro- and macropore structure of soft porous solids intended for tissue engineering applications. The method is sensitive for the smallest bottlenecks of the largest continuous pores throughout the scaffold that contributes to fluid flow.     

Hierarchically macro/mesoporous silica monoliths constructed with interconnecting micrometer-sized unit rods

Under typical dilute reactant compositions (3 ~ 5 wt% of surfactant template concentration) and conventional hydrothermal conditions for mesoporous materials synthesis, successful preparation of hierarchically macro/mesoporous silica monoliths was reported in this paper. The resultant materials were characterized by a series of techniques including powder X-ray diffraction, N₂ adsorption–desorption, SEM, TEM/EDS, and Hg porosimetry. A new kind of stable and hierarchically porous pure silica monoliths was confirmed, which are featured with highly ordered mesoporous structures, rod-shaped unit particles, large specific surface area of 492 m²/g, continuous macropores of about 4.0 μm in size and high macropore volume of about 13.1 cm³/g. Moreover, using the resultant silica monoliths as hard templates, carbon monoliths have been successfully replicated, which inherit the structural characters of parent silica materials. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

SiO2阶层多孔结构搭建及块体材料制备机理

借助溶胶-凝胶结合相分离和模板法进行了阶层多孔结构的搭建及二氧化硅多孔块体材料的制备,表征了阶层多孔块体的显微结构及孔结构特性,分析了阶层多孔结构的搭建机理。研究结果表明,三嵌段共聚物聚环氧乙烷-聚环氧丙烷-聚环氧乙烷（P123）的加入不仅诱导共混体系发生相分离,调控大孔结构的形成,同时形成球形胶束并作为模板剂进入骨架,而1,3,5-三甲基苯（TMB）的加入使P123形成的胶束膨胀且更加稳定,在骨架上成功引入了球形介孔,骨架中凝胶粒子相互聚集形成微孔,从而搭建贯通大孔-球形介孔-微孔同时分布的阶层多孔结构,并获得相应的多孔块体材料;当正硅酸甲酯（TMOS）：P123：TMB摩尔比为1：0.015：0.353时,多孔块体材料的阶层多孔结构最优,大孔孔径为0.5-1.5 μm,介孔孔径为3-4 nm,显气孔率66.1%,比表面积为616 m²·g^-1。     

Structure and performance of silica-based monolithic HPLC columns

Silica-based monolithic columns were prepared for HPLC with systematic variations of the tetramethoxysilane (TMOS) and polyethylene oxide (PEO) content as reactants in a sol-gel process accompanied by phase separation. The resulting monoliths showed differences in the macropore and silica skeleton diameter as well as in the corresponding domain sizes (the sum of macropore and skeleton diameter). All monoliths were synthesized with a diameter of 4.6 mm and cladded with a suitable polyaryletheretherketone (PEEK) polymer in a standardized and optimized manner for the subsequent chromatographic evaluation of the resulting monolithic HPLC columns. The columns were tested under normal phase conditions using n-heptane/dioxane (95:5 v/v) as a mobile phase and 2-nitroanisole as a test compound for the determination of separation efficiency and permeability. Two different sets of columns were prepared: the first one in which the amount of PEO was stepwise decreased to yield monoliths with identical macropore volumes and variations in the domain sizes. The second group of materials was synthesized adjusting both TMOS and PEO quantities to yield monolithic columns with identical macropore diameters of about 1.80 microm but different skeleton diameters and macropore volumes. The chromatographic results suggest that an increase in the column performance cannot be achieved by just arbitrarily decreasing the domain size of a given column. From a certain point of "downsizing" the monolithic structure a loss of structural homogeneity can be observed, which is apparently responsible for a lower chromatographic performance.     

Total pore blocking as an alternative method for the on-column determination of the external porosity of packed and monolithic reversed-phase columns

An alternative method to determine the interstitial void volume and the external porosity inside a packed or a monolithic column was developed. The method is based on the total blocking of the mesopores of a porous support by filling them with a hydrophobic solvent. The strong interaction of the latter with the hydrophobic coating inside the pores keeps the solvent in position during the subsequent measurements. With the pores of the stationary phase material completely inaccessible for any type of polar molecules, the method allows to perform interstitial void measurements using small molecular weight (MW) molecules instead of the large MW molecules that need to be used in inverse size exclusion chromatography (ISEC). These small MW molecules are able to penetrate every corner of the interstitial volume and therefore lead to a very accurate determination of the external porosity. Since only one type of molecules needs to be injected, the often troublesome regression analysis needed in ISEC is omitted as well. In the present contribution, the method has been applied to a packed bed and a monolithic column to investigate the optimal conditions of flow velocity, liquid compositions, and unretained marker selection. The robustness and the repeatability of the method are discussed as well.     

Metal palladium dispersed inside macroporous ion-exchange resins: the issue of the accessibility to gaseous reactants

Commercial macroporous ion-exchange resins (Lewatit SPC 118 and UCP 118) were used to support Pd metal by a two step ion-exchange and reduction procedure. The textural features of the resins were determined by Inverse Steric Exclusion Chromatography (ISEC) measurements. TEM characterization of the obtained Pd/resin composite showed the presence of uniformly sized Pd crystallites located at the macropore “surface”. Pulse chemisorption analysis gave evidence for the lack of accessibility of the crystallites when the resin composite is in the dry state. This suggests that the metal particles are in fact embedded in a gel-type resin layer at the “surface” of the macropores and therefore practically unaccessible to the molecules of a gaseous phase unless the support is in the swollen state.