Kei Morisato

Sol–gel synthesis of macro–mesoporous titania monoliths and their applications to chromatographic separation media for organophosphate compounds

We have developed a method of independently tailoring the macro- and mesoporous structures in titania (TiO2) monoliths in order to achieve liquid chromatographic separations of phosphorous-containing compounds. Anatase TiO2 monolithic gels with well-defined bicontinuous macropores and microstructured skeletons are obtained via the sol-gel process in strongly acidic conditions using poly(ethylene oxide) as a phase separator and N-methylformamide as a proton scavenger. Aging treatment of the wet gels in the mother liquor at temperatures of 100-200 degrees C and subsequent heat treatment at 400 degrees C allow the formation and control of mesoporous structures with uniform pore size distributions in the gel skeletons, without disturbing the preformed macroporous morphology. The monolithic TiO2 rod columns with bimodal macro-mesoporous structures possess the phospho-sensitivity and exhibit excellent chromatographic separations of phosphorus-containing compounds.

Monolithic silica rod columns for high-efficiency reversed-phase liquid chromatography.

Chromatographic properties of a new type of monolithic silica rod columns were examined. Silica rod columns employed for the study were prepared from tetramethoxysilane, modified with octadecylsilyl moieties, and encased in a stainless-steel protective column with two polymer layers between the silica and the stainless-steel tubing. A 25 cm column provided up to 45,000 theoretical plates for aromatic hydrocarbons, or a minimum plate height of about 5.5 μm, at optimum linear velocity of ca. 2.3 mm/s and back pressure of 7.5 MPa in an acetonitrile-water (80/20, v/v) mobile phase at 40°C. The permeability of the column was similar to that of a column packed with 5 μm particles, with K(F) about 2.4×10(-14) m(2) (based on the superficial linear velocity of the mobile phase), while the plate height value equivalent to that of a column packed with 2.5 μm particles. Generation of 80,000-120,000 theoretical plates was feasible with back pressure below 30 MPa by employing two or three 25 cm columns connected in series. The use of the long columns enabled facile generation of large numbers of theoretical plates in comparison with conventional monolithic silica columns or particulate columns. Kinetic plot analysis indicates that the monolithic columns operated at 30 MPa can provide faster separations than a column packed with totally porous 3-μm particles operated at 40 MPa in a range where the number of theoretical plates (N) is greater than 50,000.

Synthesis of silver nanoparticles confined in hierarchically porous monolithic silica: a new function in aromatic hydrocarbon separations.

Silver nanoparticles (Ag NPs) have been homogeneously introduced into hierarchically porous monolithic silica columns with well-defined macropores and SBA-15-type hexagonally ordered mesopores by using ethanol as the mild reductant. Within the cylindrical silica mesopores treated with aminopropyl groups as the host, monocrystalline Ag NPs and nanorods are obtained after being treated in silver nitrate/ethanol solution at room temperature for different durations of reducing time. The loading of Ag NPs in the monolith can be increased to 33 wt % by the repetitive treatment, which also led to the formation of polycrystalline Ag nanorods in the mesopores. Although the bare silica column cannot separate aromatic hydrocarbons, good separation of those molecules by noncharged Ag NPs confined in the porous structure of the monolith has been for the first time demonstrated with the Ag NP-embedded silica column. The NP-embedded monolithic silica would be a powerful separation tool for hydrocarbons with different number, position, and configuration of unsaturated bonds.

Semi-micro-monolithic columns using macroporous silica rods with improved performance.

Monolithic silica columns in semi-micro-format have been synthesized using poly(acrylic acid) as a phase-separation inducer via a sol-gel route. The absence of a thick skin layer accompanied by deformation of the micrometer-sized gelling skeletons on the outermost part of the macroporous silica rod contributed to improve the efficiency of monolithic silica columns as thick as 2.4 mm in diameter. The kinetic plot analysis revealed that monolithic silica columns with macropore diameter of 1 microm and skeleton thickness of 1 microm with decreased macroporosity behave similarly to columns packed with 3 microm particles with slightly lower back pressure.

Efficiency of short, small-diameter columns for reversed-phase liquid chromatography under practical operating conditions

Prototype small-size (1.0mm I.D., 5cm long) columns for reversed-phase HPLC were evaluated in relation to instrument requirements. The performance of three types of columns, monolithic silica and particulate silica (2μm, totally porous and 2.6μm, core-shell particles) was studied in the presence of considerable or minimal extra-column effects, while the detector contribution to band broadening was minimized by employing a small size UV-detector cell (6- or 90nL). A micro-LC instrument having small system volume (<1μL) provided extra-column band variance of only 0.01-0.02μL(2). The three columns generated about 8500 theoretical plates for solutes with retention factor, k>1-3 (depending on the column), in acetonitrile/water mobile phase (65/35=vol/vol) at 0.05mL/min, with the instrument specified above. The column efficiency was lower by up to 30% than that observed with a 2.1mm I.D. commercial column. The small-size columns also provided 8000-8500 theoretical plates for well retained solutes with a commercial ultrahigh-pressure liquid chromatography (UHPLC) instrument when extra-column contributions were minimized. While a significant extra-column effect was observed for early eluting solutes (k<2-4, depending on column) with methanol/water (20/80=vol/vol) as weak-wash solvent, the use of methanol/water=50/50 as wash solvent affected the column efficiency for most analytes. The results suggest that the band compression effect by the weak-wash solvent associated with partial-loop injection may provide a practical means to reducing the extra-column effect for small-size columns, while the use of an instrument with minimum extra-column effect is highly desirable.

New hierarchically porous titania monoliths for chromatographic separation media

Separation media based on hierarchically porous titania (TiO(2)) monoliths for high-performance liquid chromatography (HPLC) have been successfully fabricated by the sol-gel process of titanium alkoxide in a mild condition utilizing a chelating agent and mineral salt. The as-gelled TiO(2) monoliths were subjected to a simple solvent exchange process from ethanol (EtOH) to H(2)O followed by drying and calcination. The resultant monolithic TiO(2) columns consist of anatase crystallites with the typical specific surface area of more than 200 m(2)/g. The resultant monolithic TiO(2) column calcined at 200 and 400°C exhibited a good separation performance for organophosphates as well as for polar benzene derivatives in the normal-phase mode.

Synthesis of robust hierarchically porous zirconium phosphate monolith for efficient ion adsorption

Hierarchically porous monolithic materials are advantageous as adsorbents, catalysts and catalyst supports due to the better accessibility of reactants to the active sites and the ease of recycle and reuse. Traditional synthetic routes, however, have limitations in designing hierarchical porosity as well as the mechanically stable monolithic shape in inorganic phosphate materials, which are useful as adsorbents and catalysts. We present a low-temperature, one-step liquid phase synthesis of hierarchically porous zirconium phosphate (ZrP) monoliths with tunable compositions (from Zr(HPO4)2 (Zr : P = 1 : 2) to NaSICON (Na super ionic conductor)-type ZrP (Zr : P = 1 : 1.5)) as well as macropore size (from 0.5 to 5 μm). The as-synthesized ZrP monolith with a high reactive surface area (600 m2 g−1) and relatively high mechanical strength (Youngs modulus 320 MPa) was applied to ion adsorption. A simple syringe device inserted tightly with the ZrP monolith as a continuous flow setup was demonstrated to remove various toxic metal ions in aqueous solutions, which shows promising results for water purification.

Grafted Polymethylhydrosiloxane on Hierarchically Porous Silica Monoliths: A New Path to Monolith-Supported Palladium Nanoparticles for Continuous Flow Catalysis Applications

Polymethylhydrosiloxane has been grafted on the surface of a hierarchically porous silica monolith using a facile catalytic reaction between Si-H and silanol to anchor the polymer. This easy methodology leads to the functionalization of the surface of a silica monolith, where a large amount of free Si-H bonds remain available for reducing metal ions in solution. Palladium nanoparticles of 15 nm have been synthesized homogeneously inside the mesopores of the monolith without any stabilizers, using a flow of a solution containing Pd2+. This monolith was used as column-type fixed bed catalyst for continuous flow hydrogenation of styrene and selective hydrogenation of 3-hexyn-1-ol, in each case without a significant decrease of the catalytic activity after several hours or days. Conversion, selectivity, and stereoselectivity of the alkyne hydrogenation can be tuned by flow rates of hydrogen and the substrate solution, leading to high productivity (1.57 mol g(Pd)-1 h-1) of the corresponding cis-alkene.

High‐performance liquid chromatography separation of unsaturated organic compounds by a monolithic silica column embedded with silver nanoparticles

The optimization of a porous structure to ensure good separation performances is always a significant issue in high-performance liquid chromatography column design. Recently we reported the homogeneous embedment of Ag nanoparticles in periodic mesoporous silica monolith and the application of such Ag nanoparticles embedded silica monolith for the high-performance liquid chromatography separation of polyaromatic hydrocarbons. However, the separation performance remains to be improved and the retention mechanism as compared with the Ag ion high-performance liquid chromatography technique still needs to be clarified. In this research, Ag nanoparticles were introduced into a macro/mesoporous silica monolith with optimized pore parameters for high-performance liquid chromatography separations. Baseline separation of benzene, naphthalene, anthracene, and pyrene was achieved with the theoretical plate number for analyte naphthalene as 36,000 m(-1). Its separation function was further extended to cis/trans isomers of aromatic compounds where cis/trans stilbenes were chosen as a benchmark. Good separation of cis/trans-stilbene with separation factor as 7 and theoretical plate number as 76,000 m(-1) for cis-stilbene was obtained. The trans isomer, however, is retained more strongly, which contradicts the long- established retention rule of Ag ion chromatography. Such behavior of Ag nanoparticles embedded in a silica column can be attributed to the differences in the molecular geometric configuration of cis/trans stilbenes.

Iron(III) oxyhydroxide and oxide monoliths with controlled multiscale porosity: synthesis and their adsorption performance

Iron(III) oxyhydroxide and oxide monoliths with controlled multiscale porosity have been successfully fabricated via the sol–gel process accompanied by phase separation. The size of macropores was controlled by synthesis parameters such as starting compositions. The as-dried iron(III) oxyhydroxide monoliths were amorphous and possessed surface areas over 340 m2 g−1, of which mesostructures could be further controlled by a heat-treatment at 250–350 °C without collapse of macrostructures and monolithic forms. When the as-dried gel was heated at 300 °C, the resultant gel transformed to crystalline α-Fe2O3 and exhibited a specific surface area of 124 m2 g−1. Heat-treatment at 350 °C resulted in the broadened size distribution of mesopores. The adsorption behavior of Congo red has revealed that the interconnected macroporous structure contributed to faster diffusion and better accessibility in a continuous flow-through set up, and the crack-free monolithic forms accounted for an advantageous use of the flow-through adsorbents.