Hideaki Yoshitake

Synthesis of amino-functionalized MCM-41 via direct co-condensation and post-synthesis grafting methods using mono-, di- and tri-amino-organoalkoxysilanes

Amino-functionalized mesoporous MCM-41 materials were synthesized directly by co-condensation of 3-aminopropyltrimethoxysilane (N silane), [1-(2-aminoethyl)-3-aminopropyl]trimethoxysilane (NN silane) or 1-[3-(trimethoxysilyl)propyl]diethylenetriamine (NNN silane) with tetraethyl orthosilicate. The maximum proportions of N, NN and NNN silanes in the Si sources for obtaining the hexagonally structured functionalized silica were 0.5, 0.4 and 0.4, respectively. The 29Si MAS NMR measurements of the materials thus obtained indicate that the Si atoms containing Si–C bonds accounted for 26.0, 17.0 and 7.8 Si mol% of the total silicons, respectively. By comparing with the results from the elemental analyses and the argentometric titrations of the surfactant-extracted samples, it was revealed that all the amino moieties incorporated were not present on the surface, but some of them were in the wall of the hexagonal channels. Amino-functionalized MCM-41 samples were also synthesized via a post-synthesis grafting on dehydrated MCM-41. At the maximum level of silylation with N, NN and NNN silanes, Si atoms containing Si–C bonds of about 20.6, 19.2 and 17.9 Si mol%, respectively, were obtained, indicating that the surface coverage of amino-organoalkoxysilanes anchored to silanol groups on the surface of MCM-41 were almost the same, irrespective of the size of the amino-organoalkoxysilanes. The adsorption experiments of Co2+ and Fe3+ were conducted in order to comparatively investigate the activity and the location of the amino-organic moieties introduced via the two methods, and some differences in the activity of amino groups derived from N, NN and NNN silanes were found. The cations adsorbed on the amino-functionalized samples via the co-condensation method were increased with an increase in the surface density of amino groups regardless of the amino-organoalkoxysilanes. When N and NN silanes were grafted onto the surface of MCM-41, the adsorption capacity was increased with an increase in the surface density of amino groups. However, when NNN silane was anchored to the surface, the adsorption capacity was decreased with an increase in the surface density of the amino groups. These results imply that the locations of the amino groups introduced to the silica via the direct co-condensation and the post-synthesis grafting methods are clearly different.

Solubilities of Nickel Oxide in Molten Carbonate

Cathode dissolution is a major problem for the development of the molten carbonate fuel cell. In order to evaluate the stability of the cathode, the solubility of NiO has been measured for several compositions of molten alkaline carbonates in the CO 2 pressure range from 10 -5 to 1 atm and the temperature range from 813 to 1023 K. The solubility depended on the CO 2 pressure, although the solubility of NiO was independent of either O 2 or H 2 O pressure

Highly-controlled synthesis of organic layers on mesoporous silica: their structure and application to toxic ion adsorptions

Since the discovery of ordered mesoporous silica, control of the structure of organic layers at the mesoscopic level has become a topic of growing interest for the modification of silica surfaces. In addition, the microstructure of functional groups on the surface also produces new problems specific to mesoporous silica. Our interest in this field consists in revealing potential problems in the synthesis of modified mesoporous silica for achieving a uniform structure, developing new synthetic routes to highly-functionalized solids and elucidating the mechanism where they work as an adsorbent. All these objectives will be described both on the mesoscopic and microscopic scales. The comparison of in-advance silane synthesis with post on-surface modification is related mainly to the uniformity in the molecular structure of the functional groups. On the other hand, the differences emerging from the comparison between grafting and direct co-condensation are often attributed to the uniformity in the dispersion of the organic groups. The density of the functional groups, which is often regarded as an important specification of the material, is discussed in detail, especially in terms of the mesoscopic uniformity, the method of dispersion (explored by changing the numbers of the functional groups per organic chain or per unit surface area and by changing the stage of synthesis where the modification is carried out) and the distribution on the solid. A refined direct co-condensation method, functionalization associated with templating, has several advantages in the exposure of all functional groups as well as in selective modification of the internal pore surface. The performance of ion adsorption is discussed in relation to the structural differences caused by the choice of synthetic method, small differences in the adsorption sites and the mesopore framework structures.

Design of functionalization and structural analysis of organically-modified siliceous oxides with periodic structures for the development of sorbents for hazardous substances

In this review the preparation and adsorption phenomena specific to functionalized mesoporous silicas are discussed for designing the optimum structures of adsorbents for the removal of aqueous pollutants. The topics mainly described are control of spacing of functional groups on the surface, selective functionalization of characteristic sites in the mesostructure and the effects of mesopore confinement. Comparisons with synthetic pendant polymers and other organo-siliceous oxides in a periodic structure are performed in order to illustrate the adsorption features of functionalized silicas clearly.

Cooperative behavior of two kinds of reaction sites and reaction mechanisms for deuteration of acrolein on SMSI-Pt/Nb2O5 catalyst

Abstract The mechanism of the deuteration of acrolein in the formation of propanal and allyl alcohol over normal (LTR-) and SMSI-Pt/Nb 2 O 5 was investigated in order to clarify the structure and chemical environment of the reaction sites and their microscopic behavior in the working state, in relation to general promoter/poisoning phenomena. The reaction on the LTR catalyst follows a conventional associative mechanism involving η 2 (CC) or η 2 (CO) species. In contrast, as is seen from the reaction rates as a function of the reduction time of the catalyst, together with the isotope distributions in the products, both propanal and allyl alcohol on the SMSI catalyst are formed through η 4 (CCCO)-intermediates on the peripheral sites of migrated Nb x , islands, where deuterium atoms are supplied from bare metal sites on which deuterium is dissociatively adsorbed. A large isotope effect (one-tenth) on the rate of allyl alcohol formation when d 0 -acrolein was replaced by d 4 -acrolein was observed with the SMSI catalyst, suggesting a hydrogen-induced γCH breaking mechanism. The kinetic parameters were compared with those for deuteration of 3,3,3-trifluoropropene, which has a CF 3 group with a Hammetts parameter similar to that of the CHO group of acrolein, in order to examine the contribution of the conjugation of CC and CO ion acrolein to the adsorption and to the reaction taking place on the SMSI surface. The reaction mechanisms for the LTR and SMSI catalysts are described and discussed in relation to electronic and geometric factors for the origin of cooperative catalysis. The differences in catalytic behavior among the SMSI metal (Pt, Ir, and Rh) catalysts are also discussed in terms of their electronic structures.

Solubility of Cobalt Oxide in Molten Carbonate

The molten carbonate fuel cell (MCFC) is expected to be one of the most promising power generation systems for the coming century owing to its high efficiency, reduced quantity of air pollution, and flexibility to a variety of fuels. In this system, durability is the key point for practical operation. The stability of the cathode material under cathodic conditions and corrosion resistance of the metallic components in the presence of molten carbonate are considered to be very important factors for commercialization. The solubility of Co oxide in (Li{sub 0.62}K{sub 0.38}){sub 2}CO{sub 3} melt has been studied in Co{sub 2}-O{sub 2} atmospheres at 873--1,073 K. The stable phase of Co oxide in the melt was LiCoO{sub 2} under O{sub 2} and CO{sub 2}. At high CO{sub 2} pressure (P{sub CO{sub 2}}>0.1 atm) the acid dissolution was observed and the solubility of Co oxide was proportional to P{sub CO{sub 2}}. Although the solubility was lower than that for NiO, the pressure dependence was larger. At low CO{sub 2} pressure (P{sub CO{sub 2}}<0.01 atm) basic dissolution was observed.

Study of the density of the d-state and structure transformation of Pt fine particles dispersed on carbon electrodes by in situ X-ray absorption spectroscopy

Abstract The structure and density of the d-state of Pt fine particles on a carbon electrode were investigated in H 2 SO 4 and NaOH solutions by in situ X-ray absorption fine structure spectroscopy. The applied potential was changed between −0.2 V and 1.3 V vs. Ag/AgCl for 0.26 M H 2 SO 4 solution. The total density of the unoccupied d-state h T increased according to the potential and was always larger in the negative sweep than in the positive sweep. Around 0.0 V vs. Ag/AgCl in H 2 SO 4 , h T agreed in both sweep directions, that is, a kind of hysteresis was observed in this voltammogram. The coordination number and the disorder factor were determined for the first Pt-Pt shell. The disorder of the structure of the particles (decreasing N and increasing σ) increased in the course of the positive sweep and disappeared at the hydrogen region after reversal of the sweep. The relation of the structural change to the hysteresis of the d-density is discussed with respect to cyclic voltammograms of Pt.

Synthesis and Characterization of Mo−SBA-1 Cubic Mesoporous Molecular Sieves

A series of Mo−SBA-1 (Pm3n) cubic mesoporous molecular sieves with various H2O/HCl and Si/Mo molar ratios have been synthesized at 0 °C using cetyltriethylammonium bromide, tetraethyl orthosilicate, ammonium heptamolybdate and hydrochloric acid. It was found that the Mo−SBA-1 particles synthesized at 0 °C for 4 days were of a characteristic shape having fifty-four, seventy-four, or more crystal faces. Synthesis conditions have been optimized to introduce high percentage of Mo into the well-ordered cubic mosoporous structure. The microstructure of Mo centers has been investigated by diffuse reflectance UV−visible and laser Raman spectroscopies. Both techniques reveal that, in the absence of water, Mo species consist of tetrahedrally and octahedrally coordinated monomers and polymers. The tetrahedral Mo centers were converted to octahedral Mo centers in the presence of moisture partially in directly synthesized Mo−SBA-1 samples and completely in impregnated Mo/SBA-1 samples. It is concluded that the impreg...

Estimation of spacing between 3-bromopropyl functions grafted on mesoporous silica surfaces by a substitution reaction using diamine probe molecules

We present here the results of the substitution of 3-bromopropyltriethoxysilane-grafted mesoporous SBA15 (pore size 7.6 nm) with a series of diamines, CH3NH(CH2)nNHCH3 (C2DA, C3DA and C6DA for n = 2, 3 and 6, respectively) or NH2(CH2)nNH2 (C4DA and C5DA for n = 4 and 5, respectively). The outcome of the reactions was closely related to the spacing of the surface bromopropyl groups. Amount of bromine that remained after the reaction decreased linearly with the amount of C2DA, and it disappeared completely when C2DA : Br = 1 : 1 in the initial reactant mixture. This result indicates the complete conversion of C2DA and Br by a one-to-one substitution (i.e. formation of a linear species). By contrast, the rate of decrease of Br was twice as fast during substitutions with amines other than C2DA when diamine/Br < 0.5 (in the initial mixture), suggesting a one-to-two substitution and the formation of a bridge species. We show that the C/N ratios in the elemental analysis after the complete substitution of Br are a simple and reliable indicator of the distributions of the pair spacings of the bromopropyl groups. The results are compared by a geometric calculation in which the configurations and the minimum limits of neighbouring distances are ignored. The formation of linear and bridge forms were confirmed by 13C-NMR analysis. Reactions using MCM41 (pore size 2.4 nm) and fumed silica were also explored for comparison. We found that the order of the pair spacings of the bromopropyl groups grafted onto these silicas was MCM41 < SBA15 ≪ fumed silica. This result disagrees with the outcome that was expected from the number of bromopropyl groups per unit nm2, 0.8, 1.1 and 1.2 for MCM41, SBA15 and fumed silica, respectively, which was derived from the BET specific surface area and bulk elemental analyses. All of these results are consistent with a mode of grafting where the reaction with silanols competes with diffusion into the pore channels.

Polymerisation of aminopropyltrialkoxysilane in the presence of carboxylate: a new layered organosilica mesocomposite built up using intermolecular interactions with LB film-type self-assembly

A new layered organosilicate with a multi-layer LB film-like structure, in which the carboxyl groups are connected to each other via a 3-aminopropylsilicate layer, is successfully synthesized by a self-assembly process using alkanoic acid (CnH2n+1COOH, n > 10) combined with the co-condensation of 3-aminopropyltriethoxysilane. The structure is elucidated by elemental analysis, X-ray diffraction, 13C and 29Si MAS-NMR, infrared spectroscopy and thermogravimetry. The combined results of these techniques demonstrate that the alkyl groups are not packed in a crystalline state, but exist in a liquid-like state in a well-developed lamellar structure with the formula CnH2n+1COOHNH2CH2CH2CH2SiO1.5. In the extremely thin siloxane layer that is developed in the lamellar structure, nearly all of the silicon atoms are bound to one carbon atom, three -OSi groups and no -OH groups. This structure (one silicon atom bound to one 3-aminopropyl group) possibly contains the most highly-populated aminopropyl group among all of the aminopropylsilicates that have ever been reported. The LB film-like interlayer can be exchanged with other alkanoic acids to form another lamellar structure in the same series that possesses the appropriate layer spacing. A fast delamination (<5 min) process is observed after the addition of Fe3+, which implies a rapid diffusion of transition metal cations, and which is also consistent with the liquid-like nature of the alkyl group assembly. The specific amount of Fe3+ sorption reaches 5.1 mmol g−1, probably the highest adsorption capacity ever reported among the aminopropylsilicates.