Pseudomorphic Transformation Research Articles

Synthesis, characterization, and evaluation of a superficially porous particle with unique, elongated pore channels normal to the surface

In recent years, superficially porous particles (SPPs) have drawn great interest because of their special particle characteristics and improvement in separation efficiency. Superficially porous particles are currently manufactured by adding silica nanoparticles onto solid cores using either a multistep multilayer process or one-step coacervation process. The pore size is mainly controlled by the size of the silica nanoparticles and the tortuous pore channel geometry is determined by how those nanoparticles randomly aggregate. Such tortuous pore structure is also similar to that of all totally porous particles used in HPLC today. In this article, we report on the development of a next generation superficially porous particle with a unique pore structure that includes a thinner shell thickness and ordered pore channels oriented normal to the particle surface.The method of making the new superficially porous particles is a process called pseudomorphic transformation (PMT), which is a form of micelle templating. Porosity is no longer controlled by randomly aggregated nanoparticles but rather by micelles that have an ordered liquid crystal structure. The new particle possesses many advantages such as a narrower particle size distribution, thinner porous layer with high surface area and, most importantly, highly ordered, non-tortuous pore channels oriented normal to the particle surface. This PMT process has been applied to make 1.8–5.1μm SPPs with pore size controlled around 75Å and surface area around 100m2/g. All particles with different sizes show the same unique pore structure with tunable pore size and shell thickness. The impact of the novel pore structure on the performance of these particles is characterized by measuring van Deemter curves and constructing kinetic plots. Reduced plate heights as low as 1.0 have been achieved on conventional LC instruments. This indicates higher efficiency of such particles compared to conventional totally porous and superficially porous particles.

Pseudomorphic Transformation of Layered Simple Hydroxides into Prussian Blue Analogue Nanoplatelets

AbstractThe reactivity of potassium or sodium hexacyanoferrate(II) with transition‐metal layered simple hydroxides (LSHs) has been investigated to evaluate the possibility of inserting Prussian Blue analogues (PBAs) into magnetic copper(II) LSHs. This report shows how such a reaction led, unexpectedly, to the direct formation of PBA nanoparticles, the layered hydroxide serving as a metal‐ion reservoir. The lamellar structure of the starting LSH collapses and copper–iron PBA nanoparticles were obtained as well‐defined cubic structures stacked in layers. In addition to structural and magnetic characterization, the reaction mechanism was investigated by 57Fe Mössbauer spectrometry and X‐ray photoemission spectroscopy. The layered copper hydroxide acetate turns out to be a smart precursor of well‐shaped PBA nanoparticle layers through pseudomorphic replication.

Recent advances in the synthesis of hierarchically porous silica materials on the basis of porous glasses

The methods for integrating an additional pore system in glass-based monoliths are reviewed with an emphasis on sintering and pseudomorphic transformation routes.

Hierarchical porous silica monoliths: A novel class of microreactors for process intensification in catalysis and adsorption

Abstract Silica monoliths with hierarchical porosity (macro-/mesoporous), prepared by a combination of phase separation (spinodal decomposition) and sol–gel process, have demonstrated remarkable potential as supports for catalysts and adsorbents with improved efficiency and productivity of a number of applications. This is due to perfect homogeneous interconnected porous networks enabling an exceptional mass transfer and a fine control of contact times. Silica monoliths have been functionalized by an important variety of moities and techniques, such as grafting with versatile species (acidic, basic), by alumina coating, immobilization of ionic liquids, in-situ synthesis of nanoparticles of Pd, MOF, and NiMoS 2 , pseudomorphic transformation of the skeleton into MCM-41 or zeolites (SOD, LTA). These functional materials reveal great opportunities for process intensification.

Crystallization of ikaite and its pseudomorphic transformation into calcite: Raman spectroscopy evidence

Ikaite (CaCO3·6H2O) is a metastable phase that crystallizes in nature from alkaline waters with high phosphate concentrations at temperatures close to 0°C. This mineral transforms into anhydrous calcium carbonate polymorphs when temperatures rise or when exposed to atmospheric conditions. During the transformation in some cases the shape of the original ikaite crystal is preserved as a pseudomorph. Pseudomorphs after ikaite are considered as a valuable paleoclimatic indicator. In this work we conducted ikaite crystal growth experiments at near-freezing temperatures using the single diffusion silica gel technique, prepared with a natural aqueous solution from the polymictic lake Laguna Potrok Aike (51°57′S, 70°23′W) in Patagonia, Argentina. The ikaite crystals were recovered from the gels and the transformation reactions were monitored by in situ Raman spectroscopy at two different temperatures. The first spectra collected showed the characteristic features of ikaite. In successive spectra new bands at 1072, 1081 and 1086cm−1 and changes in the intensity of bands corresponding to the OH modes were observed. These changes in the Raman spectra were interpreted as corresponding to intermediate stages of the transformation of ikaite into calcite and/or vaterite. After a few hours, the characteristics of the Raman spectrum were consistent with those of calcite. While ikaite directly transforms into calcite at 10°C in contact with air, at 20°C this transformation involves the formation of intermediate, metastable vaterite. During the whole process the external shape of ikaite crystals was preserved. Therefore, this transformation showed the typical characteristics of a pseudomorphic mineral replacement, involving the generation of a large amount of porosity to account for the large difference in molar volumes between ikaite and calcite. A mechanism involving the coupled dissolution of ikaite and crystallization of calcite/vaterite is proposed for this replacement.

Synthesis of spherical mesoporous silica materials by pseudomorphic transformation of silica fume and its Pb2+ removal properties

Spherical mesoporous silica material (SMSM) was hydrothermally synthesized using cetyltrimethylammonium bromide (CTAB) as the template agent and silica fume as the silica source. High-quality SMSMs were successfully prepared by pseudomorphic transformation. The structure and morphology of the SMSMs are investigated by X-ray diffraction, N2 sorption/desorption, Fourier-transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy analyses. The effects of synthetic parameters, such as crystallization time, crystallization temperature, n(NaOH)/n(SiO2) molar ratio, and n(CTAB)/n(SiO2) molar ratio, are quantitatively investigated. The optimal synthetic conditions for high-quality SMSMs are a crystallization time of 48 h, crystallization temperature of 363 K, n(NaOH)/n(SiO2) molar ratio of 0.2–0.3, and n(CTAB)/n(SiO2) molar ratio of 0.15. The qualities of Si–OH are tested by NaOH titration and IR. The results of the both methods confirm that the amount of Si–OH in SMSM is higher than silica fume. Furthermore, SMSMs are employed as effective adsorbents for removing Pb2+ from the static, competitive and column experiments. The capacity for Pb2+ removal demonstrates great improvement.

Positron-Annihilation-Lifetime-Spectroscopy (PALS) for the characterization of bimodal silica-gel synthesized by pseudomorphic transformation

In this study the results of the characterization of bimodal silica-gel samples with nitrogen adsorption and Positron-Annihilation-Lifetime-Spectroscopy (PALS) are compared. The bimodal silica-gel samples were synthesized by pseudomorphic transformation of a mesoporous silica gel. A set of eight samples was generated using different volumes of 0.07 M cetyltrimethylammonium hydroxide (5–42 ml/g) to obtain bimodal pore systems with controlled proportions of 15 nm pores of the original silica-gel and 4 nm pores formed by the MCM-41 phase. The aim was to proof if it is possible to determine the pore volumes directly from the PALS experiment. The results of the PALS measurements clearly indicate the bimodal pore structure in the materials. Two individual pore systems can be detected with a good resolution. The peak area of the signals can be calculated and compared with the pore volumes determined by nitrogen adsorption. The results show a correlation between the pore volume determined by nitrogen adsorption and the peak area calculated from PALS. Some differences between the measured pore volumes of the starting material can be an indication for the formation of a closed porosity in the starting material and after the processes of pseudomorphic transformation. The PALS measurements enable a complementary characterisation of pore systems in the micro- and mesopore range. Bimodal structured porous materials can be studied and the individual pore systems are well separated. The observed correlation between intensities (respectively peak areas) from PALS and pore volumes determined by nitrogen adsorption for the different pore classes allows the direct estimation of the pore volumes by PALS via calibration and the quantification of a potential closed porosity.

Facile synthesis of tungsten carbide nanorods and its application as counter electrode in dye sensitized solar cells

Tungsten carbide nanorods (WC-NRs) are synthesized by pseudomorphic transformation of chemically synthesized W3O8 nanorods using a high-temperature method. The WC-NRs was introduced into dye sensitized solar cell (DSSC) as counter electrode (CE) catalyst to replace the expensive platinum (Pt). The synthesized WC-NRs were characterized by field emission scanning electron microscopy (FESEM), BET surface area analysis and powder X-ray diffraction (PXRD) measurements. The electrochemical properties of WC-NRs counter electrode were studied using electrochemical impedance spectroscopy (EIS) techniques. The photovoltaic performance of the DSSC with WC-NRs counter electrode was evaluated under simulated standard global AM 1.5G sunlight (100mW/cm2). The solar to electrical energy conversion efficiency (η) of the WC-NRs with binder and binder free based DSSC was found to be 1.92% and 0.59% respectively. The cell performance can be attributed to the WC-NRs network, catalytic redox activity and 1-D efficient charge-transfer network. Such WC-NRs configuration as CE provides a potential feasibility for counter electrodes in DSSC applications.

Low-Cost Synthesis of Bimodal Mesoporous Silica-Based Materials by Pseudomorphic Transformation.

Nanoparticulate bimodal porous silica-based materials have been prepared through a surfactant-assisted procedure by using a simple template and starting from inexpensive sodium silicate as silicon source. Different procedural variables, such as pH or the nature and concentration of the surfactant, have been explored to optimize the preparative protocol, which allows, in turn, improved understanding of the formation process. The final bulk materials (called UVM-10 or M-UVM-10) are formed by pseudomorphic transformation of fresh silica-based xerogels under mild basic conditions. The UVM-10 architecture is constructed from small mesoporous nanoparticles, the aggregation of which generates a disordered secondary 3 D pore system (large-meso/macropores) defined by interparticle voids. Modulation of the intraparticle mesopore size is achieved by using surfactants with variable tail lengths, while maintaining the same head group. Textural porosity has been handled independently by hydrothermally modifying the particle size of the reactive silica-based xerogel. By simply adding metal alkoxides to the initial reaction mixture, the preparative protocol allows for functionalizing the silica walls by incorporating relatively high proportions of homogeneously dispersed heteroelements, such as Al and Ti.

Low temperature pseudomorphic synthesis of nanocrystalline carbide aerogels for electrocatalysis

Low-temperature pseudomorphic transformation of nanocrystalline TiC (NbC) aerogels with high surface areas and electrocatalytic activities is realized for DSSCs and the HER.

Bimodal mesoporous silica with bottleneck pores.

Bimodal mesoporous silica consisting of two sets of well-defined mesopores is synthesized by a partial pseudomorphic transformation of an ordered mesoporous starting material (SBA-15 type). The introduction of a second set of smaller mesopores (MCM-41 type) establishes a pore system with bottlenecks that restricts the access to the core of the bimodal mesoporous silica particles. The particle size and shape of the starting material are retained, but micropores present in the starting material disappear during the transformation, leading to a true bimodal mesoporous product. A varying degree of transformation allows the adjustment of the pore volume contribution of the two mesopore domains. Information on the accessibility of the mesopores is obtained by the adsorption of fluorescence-labeled poly(amidoamine) dendrimers and imaging by confocal laser scanning microscopy. This information is correlated with nitrogen sorption data to provide insights regarding the spatial distribution of the two mesopore domains. The bimodal mesoporous materials are excellent model systems for the investigation of cavitation effects in nitrogen desorption isotherms.

Pseudomorphic transformation of amorphous calcium carbonate films follows spherulitic growth mechanisms and can give rise to crystal lattice tilting

Tuning the pseudomorphic transformation of calcium carbonate allows for the generation of crystal lattice tilting similar to that found in calcareous biominerals.

Synthesis of Mesoporous Silica Microspheres by PICA and Pseudomorphic Transformation Using Silica Fume as Silica Source

Monodisperse mesoporous silica microspheres (MSM-SF) were successfully prepared through PICA and pseudomorphic transformation using silica fume as original silica source. The structure and morphology of spheres were quantitatively investigated by Zata potential and particle size analyzer, N2 sorption isotherms, XRD, SEM and TEM. The results showed that the monodisperse spherical mesoporous silica microspheres (MSM-SF) exhibited uniform spherical morphology, worm-like mesoporous, large BET surface area of 559.9m2/g; its BJH average pore size diameter and total pore volume are 3.3nm and 0.12cm3/g, respectively. This preparation method provides a new synthetic strategy to control the particle morphology and structure simultaneously, meanwhile this method can also significantly reduce the cost of synthesis of mesoporous silica microspheres.

Validity of the t-plot method to assess microporosity in hierarchical micro/mesoporous materials.

The t-plot method is a well-known technique which allows determining the micro- and/or mesoporous volumes and the specific surface area of a sample by comparison with a reference adsorption isotherm of a nonporous material having the same surface chemistry. In this paper, the validity of the t-plot method is discussed in the case of hierarchical porous materials exhibiting both micro- and mesoporosities. Different hierarchical zeolites with MCM-41 type ordered mesoporosity are prepared using pseudomorphic transformation. For comparison, we also consider simple mechanical mixtures of microporous and mesoporous materials. We first show an intrinsic failure of the t-plot method; this method does not describe the fact that, for a given surface chemistry and pressure, the thickness of the film adsorbed in micropores or small mesopores (< 10σ, σ being the diameter of the adsorbate) increases with decreasing the pore size (curvature effect). We further show that such an effect, which arises from the fact that the surface area and, hence, the free energy of the curved gas/liquid interface decreases with increasing the film thickness, is captured using the simple thermodynamical model by Derjaguin. The effect of such a drawback on the ability of the t-plot method to estimate the micro- and mesoporous volumes of hierarchical samples is then discussed, and an abacus is given to correct the underestimated microporous volume by the t-plot method.

Synthesis, characterisation and hydrogenation performance of ternary nitride catalysts

Synthesis of phase pure Co3Mo3N and Fe3Mo3N by temperature programmed ammonolysis has been established by XRD and elemental analysis. The ternary nitrides are characterised by a η-6 structure and low surface area (4–9m2g−1). Pseudomorphic transformation of cobalt molybdate prepared using cobalt nitrate generated rod-shaped crystals while the use of iron chloride resulted in Fe3Mo3N aggregates with irregular morphology and wide size distribution. XPS measurements have revealed surface N enrichment relative to the bulk where the passivated samples show a range of oxidation states; Co3Mo3N exhibited Mo2+ and Con+ (0≤n≤3) whereas Fe3Mo3N was characterised by higher oxidation states (Fe3+ and Mo3+). Temperature programmed reduction (TPR) to 823K served to remove the passivation layer where subsequent H2 chemisorption and temperature programmed desorption (TPD) has demonstrated greater uptake on Fe3Mo3N relative to Co3Mo3N, resulting in a higher nitrobenzene hydrogenation rate (to aniline). Fe3Mo3N promoted selective reduction of –NO2 in p-chloronitrobenzene to generate p-chloroaniline as sole product whereas Co3Mo3N favoured C-Cl scission with the formation of nitrobenzene (in addition to p-chloroaniline). Hydrodechlorination properties were further established for Co3Mo3N in the conversion of chlorobenzene (to benzene) under conditions where Fe3Mo3N was inactive. A temporal deactivation of both nitrides is associated with Cl poisoning of Co3Mo3N and structural changes to Fe3Mo3N.

Mesoporous Hybrid Materials by Simultaneous Pseudomorphic Transformation and Functionalization of Silica Microspheres

Non-agglomerated amino-functionalized mesoporous silica microspheres are synthesized by a one-pot synthesis from a parent silica material. Narrow pore size distributions in the range from 3 to 5 nm are obtained with alkyltrimethylammonium structure-directing agents (SDAs). By following the pseudomorphic transformation pathway, the particle size distribution and spherical morphology of the parent silica are retained during the synthesis. The products contain accessible and uniformly distributed amino groups. The average pore size and the ratio of small uniform mesopores (<5 nm) to larger mesopores and macropores can be controlled by choosing the appropriate SDA and by adjusting the concentration of the amino-functionalized alkoxysilane precursor, leading to a variety of meso-macroporous hybrid materials.

Micro- to nano-scale characterization of martite from a banded iron formation in India and a lateritic soil in Brazil

The pseudomorphic transformation of magnetite into hematite (martitization) is widespread in geological environments, but the process and mechanism of this transformation is still not fully understood. Micro- and nano-scale techniques—scanning electron microscopy, focused ion bean transmission electron microscopy, and Raman spectroscopy—were used in combination with X-ray diffraction, Curie balance and magnetic hysteresis analyses, as well as Mössbauer spectroscopy on martite samples from a banded iron formation (2.9 Ga, Dharwar Craton, India), and from lateritic soils, which have developed on siliciclastic and volcanic rocks previously affected by metamorphic fluids (Minas Gerais, Brazil). Octahedral crystals from both samples are composed of hematite with minor patches of magnetite, but show different structures. The Indian crystals show trellis of subhedral magnetite hosting maghemite in sharp contact with interstitial hematite crystals, which suggests exsolution along parting planes. Grain boundary migrations within the hematite point to dynamic crystallization during deformation. Dislocations and fluid inclusions in hematite reflect its precipitation related to a hydrothermal event. In the Brazilian martite, dislocations are observed and maghemite occurs as Insel structures and nano-twin sets. The latter, typical for the hematite, are a transformation product from maghemite into hematite. For both samples, a deformation-induced hydrothermally driven transformation from magnetite via maghemite to hematite is proposed. The transformation from magnetite into maghemite comprises intermediate non-stoichiometric magnetite steps related to a redox process. This study shows that martite found in supergene environment may result from earlier hypogene processes.

Hierarchically Structured MCM‐41 Silica Beads via Nanocasting in Combination with “Pore‐protected” Pseudomorphic Transformation

AbstractThe following study discusses the synthesis of macroporous glass beads, featuring variable pore sizes, and their application as starting material for a double templating route according to the nanocasting principle. In the first step, the initial porous glass was filled with the carbon precursor, a mesophase pitch, which after the subsequent carbonization and dissolution of the glass matrix results in an inverse macroporous carbon replica. Afterwards, the carbon beads were filled with amorphous silica by a typical sol‐gel‐process. The next step can be divided into two phases. The silica gel phase was first “structured” inside the macropores of the inverse carbon replica by converting it into an ordered mesoporous phase via pseudomorphic transformation. This process demanded alkaline conditions and a surfactant, that finally converts the silica into hierarchically structured beads with the dimension and the pores of the initial glass and MCM‐41 pores inside the walls. The final step comprised the template removal via calcination. The obtained materials were characterized by mercury intrusion, nitrogen adsorption, scanning electron microscopy, x‐ray powder diffraction and particle size analysis. In comparison to the previously reported approaches, the new method allows a higher flexibility in the texture properties of the resulting hierarchically structured materials including a variable ratio between ordered mesopores and additional macropores by parallel control of the total porosity and wall thickness in the starting porous glass.

Fabrication of hierarchically porous silica nanospheres through sol–gel process and pseudomorphic transformation

Hierarchically porous silica nanospheres with well-defined morphology and uniform particle size had been synthesized through a multistep sol–gel method combined with pseudomorphic transformation in the presence of polyvinylpyrrolidone (PVP) and cetyltrimethylammoium bromide (CTAB) as dual template. The prepared materials were characterized by small-angle X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and nitrogen physical adsorption techniques. The preparation process and the origin of hierarchical structure were also investigated. It had been shown that the hierarchical structure of synthesized materials comprises wormlike framework mesopores with diameters of about 3 nm and bubble-like pores with diameters of 20–30 nm. CTAB and PVP behave as dual-template and are responsible for the formation of the mesopores and big pores, respectively. Not only the porous structure, but also the morphology and particle size of hierarchical materials can be adjusted by controlling the addition of PVP. In addition, the formation process of hierarchically porous silica was investigated by transmission electron microscopy, FT-IR spectra and thermo-gravimetric curves and a possible synthetic mechanism had been proposed.

Magnetic iron oxide nanoparticles coated by hierarchically structured silica: a highly stable nanocomposite system and ideal catalyst support

A well-defined core–shell nanocomposite with magnetic iron oxide nanoparticles coated by a hierarchically structured silica shell has been synthesized through a sol–gel process and pseudomorphic transformation. The prepared materials were characterized by means of transmission electron microscopy, small and wide angle X-ray diffraction, Mossbauer spectroscopy and N2 physical adsorption–desorption. It has been shown that the core–shell nanocomposite possesses a magnetic core in the form of Fe3O4 or γ-Fe2O3 and a unique hierarchically structured silica shell consisting of an inner nonporous shell and outer shell with hierarchical pores. As a result, this nanocomposite exhibits high stability (acid resistance) and a large surface area (447 m2 g−1), which will be especially suitable for use as catalyst supports. To demonstrate this point, a functional catalyst consisting of small Pt nanoparticles well-dispersed on the porous surface of the magnetic core–shell nanocomposite was fabricated. In the hydrogenation of nitrobenzene and 2-nitrochlorobenzene to the corresponding aniline compounds, the hierarchically porous catalyst showed superior performances to its counterpart with a monomodal porous structure. In addition, the used catalyst could be separated conveniently from the reaction system with an external magnetic field. Subsequent recycling tests further confirmed the outstanding reusability and regeneration ability of the composite catalyst.