Iron-containing nanocomposites based on mesoporous aluminosilicates

Here we report the synthesis and magnetic characterization of iron-containing nanocomposites based on mesoporous aluminosilicates (MAS). MAS with Al:Si ratio equal to 1:15 was synthesized by a two-step hydrothermal treatment. The chemical composition and structure of the matrix were determined by chemical analysis, TGA, SAXS and adsorption of nitrogen at 77 K. The intercalation of iron into the channels was performed either by cationic exchange or by soaking the initial matrix in Fe(CO)₅. Reduction/decomposition of the complex was carried out by thermal treatment in a hydrogen flow. The nanocomposites obtained were characterized by XRD, TEM, Mössbauer spectroscopy and magnetic measurements. It was shown that the first method does not give any magnetic phases at nanolevel. The second method results in the formation of magnetic iron-containing nanowires (with diameter <2 nm and length over 30 nm) in a mesoporous matrix, which possess blocking temperatures over 300 K and coercivity up to 31,000 A/m at room temperature.     

Adsorptive desulfurization of diesel with mesoporous aluminosilicates

Mesoporous aluminosilicates (MAS) bearing microporous zeolite units and mesoporous structures were synthesized by the hydrothermal method. Adsorptive desulfurization ability of model oil and hydrotreated diesel was studied. The effects of template concentration, crystalization time and calcination time were investigated. The desulfurization ability of adsorbents was improved by transitional metal ion-exchanging. The adsorptive desulfurization of diesel was carried out on a fixed-bed system. The results show that the adsorptive capacity is MAS>MCM-41>NaY. The improvement of desulfurization ability of MAS by Cu⁺ is more significant than that of Ag⁺. Supported by National Basic Research Program of China (Grant No. 2006CB202507), National High-tech R&D Program (Grant No. 2006AA02Z209), and the National Natural Science Foundation of China (Grant No. 20806086)     

Properties of mesoporous aluminosilicates prepared with nonionic surfactants

Mesoporous aluminosilicates with specific surface area up to 1030 m²/g and pore diameters from 33 to 43 Å were obtained using hexadecylamine and block copolymers of polyethylene and polypropylene oxides as structural directing agents. The synthesized materials were tested in catalytic cracking of hydrotreated vacuum gas oil at 500°C.     

Hydrogenation of naphthalene on noble-metal-containing mesoporous MCM-41 aluminosilicates

Aromatic saturation of oil fractions is a key process in the refining industry due to increasing demand for cleanest distillates with superior performances. In this study, the behavior of different catalysts containing 1 wt.% of noble-metal inside a mesoporous MCM-41 (Si:Al=20) framework was investigated in the hydrogenation of naphthalene, as preliminary step to investigate bimetallic catalysts. While at atmospheric pressure only Rh and Pd showed a low hydrogenation activity, in the tests performed at 6.0 MPa the catalytic activity grew, exhibiting the following order: Pt>RhPd>>>Ru≈Ir. However, all the catalysts required a large H₂ excess, to avoid a decrease in hydrogenation and ring-opening activity, and gave rise to the best performance for a contact time of 6.8 s, favouring at lower values the partial hydrogenation to tetralin and at higher values cracking reactions. Finally, all the catalysts showed low thio-tolerance, with significant deactivation already feeding 100 ppm wt. of dibenzothiophene (DBT), with a partial reversibility only for the Pt-containing catalyst (CAT 3).     

One-step synthesis of hydrothermally stable mesoporous aluminosilicates with strong acidity

Using tetraethylorthosilicate (TEOS), polymethylhydrosiloxane (PMHS) and aluminium isopropoxide (AIP) as the reactants, through a one-step nonsurfactant route based on PMHS-TEOS-AIP co-polycondensation, hydrothermally stable mesoporous aluminosilicates with different Si/Al molar ratios were successfully prepared. All samples exclusively showed narrow pore size distribution centered at 3.6 nm. To assess the hydrothermal stability, samples were subjected to 100 °C distilled water for 300 h. The boiled mesoporous aluminosilicates have nearly the same N₂ adsorption-desorption isotherms and the same pore size distributions as those newly synthesized ones, indicating excellent hydrothermal stability. The ²⁹Si MAS NMR spectra confirmed that PMHS and TEOS have jointly condensed and CH₃ groups have been introduced into the materials. The ²⁷Al MAS NMR spectra indicated that Al atoms have been incorporated in the mesopore frameworks. The NH₃ temperature-programmed desorption showed strong acidity. Due to the existence of large amount of CH₃ groups, the mesoporous aluminosilicates obtained good hydrophobicity. Owing to the relatively large pore and the strong acidity provided by the uniform four-coordinated Al atoms, the excellent catalytic performance for 1,3,5-triisopropylbenzene cracking was acquired easily. The materials may be a profitable complement for the synthesis of solid acid catalysts.     

以微孔β沸石为硅铝源合成强酸性介孔分子筛

以微孔β沸石为硅铝源,通过碱处理和以十六烷基三甲基溴化铵为模板剂,合成了具有较强酸性的六方结构介孔分子筛材料B-MCM-41,并采用XRD、N₂吸附脱附、FT-IR、²7Al MAS NMR、HRTEM和水热处理等手段对其进行了结构表征,采用NH₃-TPD对其进行了酸性表征。实验结果表明,B-MCM-41具有明显强于常规介孔分子筛的酸性,且在C⁺₁0混合芳烃加氢脱烷基化反应中表现出了良好的催化性能。这主要是由于碱溶液将β沸石降解为沸石结构单元,在表面活性剂作用下五元环次级结构单元被引入了介孔铝硅酸盐B-MCM-41的结构。     

Synthesis of carboxylic acid esters in the presence of micro- and mesoporous aluminosilicates

The catalytic properties of zeolites HY, HBeta, and HZSM-12 and of mesoporous amorphous aluminosilicate in liquid-phase esterification of aliphatic (monobasic C₁–C₁₈, dibasic C₆, C₁₀) and aromatic (benzoic, trimellitic, phthalic) carboxylic acids with butanol were studied. Zeolite HBeta appeared to be the most active catalyst. Procedures were developed for preparing esters in the presence of zeolitic catalyst HBeta, ensuring 100% selectivity of ester formation at 90–98% conversion of the acid.     

The plumber's nightmare: a new morphology in block copolymer-ceramic nanocomposites and mesoporous aluminosilicates

A novel cubic bicontinuous morphology is found in polymer-ceramic nanocomposites and mesoporous aluminosilicates that are derived by an amphiphilic diblock copolymer, poly(isoprene-b-ethylene oxide) (PI-b-PEO), used as a structure-directing agent for an inorganic aluminosilicate. Small-angle X-ray scattering (SAXS) was employed to unambiguously identify the Im(-)3m crystallographic symmetry of the materials by fitting individual Bragg peak positions in the two-dimensional X-ray images. Structure factor calculations, in conjunction with results from transmission electron microscopy, were used to narrow the range of possible structures consistent with the symmetry and showed the plumber's nightmare morphology to be consistent with the data. The samples are made by deposition onto a substrate that imposes a strain field, generating a lattice distortion. This distortion is quantitatively analyzed and shown to have resulted in shrinkage of the crystallites by approximately one-third in a direction perpendicular to the substrate, in both as-made composites and calcined ceramic materials. Finally, the observation of the bicontinuous block-copolymer-derived hybrid morphology is discussed in the context of a pseudo-ternary morphology diagram and compared to existing studies of ternary phase diagrams of amphiphiles in a mixture of two solvents. The calcined mesoporous materials have potential applications in the fields of catalysis, separation technology, and microelectronics.     

Crystalline-like molecularly ordered mesoporous aluminosilicates derived from aluminosilica-surfactant mesophases via benign template removal

We report the preparation of mesoporous aluminosilicate materials that exhibit molecular-scale ordering in their pore wall framework. The materials were derived from mesoporous aluminosilica-surfactant mesophases via benign template removal methods, which allowed the retention of molecular ordering in surfactant-free materials. The molecularly ordered aluminosilica-surfactant mesophases were obtained from hydrothermal crystallization of cetyltrimethylammonium hydroxide/Al,Si/H2O systems at 135 degrees C for 12 days. Benign template removal via H2O2-mediated oxidation of the surfactant at room temperature was found to be the most effective method in generating surfactant-free materials with molecular ordering, high textural properties (depending on Al content), and high acidity. The Al in the resulting aluminosilicates was entirely incorporated in framework (tetrahedrally coordinated) sites. Template extraction in acidified ethanol also generated molecularly ordered materials but compromised the Al content and acidity. Template removal via conventional calcination generated porous materials with high textural properties but which exhibited only limited molecular ordering and had relatively low acidity and significant amounts of nonframework Al. This work demonstrates that molecular ordering in mesoporous silicate-surfactant mesophases is due to crystallographic ordering within inorganic frameworks rather than the arrangement/packing of surfactant molecules.     

Enhancing stability and oxidation activity of cytochrome C by immobilization in the nanochannels of mesoporous aluminosilicates

Hydrothermally stable and structrurally ordered mesoporous and microporous aluminosilicates with different pore sizes have been synthesized to immobilize cytochrome c (cyt c): MAS-9 (pore size 90 A), MCM-48-S (27 A), MCM-41-S (25 A), and Y zeolites (7.4 A). The amount of cyt c adsorption could be increased by the introduction of aluminum into the framework of pure silica materials. Among these mesoprous silicas (MPS), MAS-9 showed the highest loading capacity due to its large pore size. However, cyt c immobilized in MAS-9 could undergo facile unfolding during hydrothermal treatments. MCM-41-S and MCM-48-S have the pore sizes that match well the size of cyt c (25 x 25 x 37 A). Hence the adsorbed cyt c in these two medium pore size MPS have the highest hydrothermal stability and overall catalytic activity. On the other hand, the pore size of NaY zeolite is so small that cyt c is mostly adsorbed only on the outer surface and loses its enzymatic activity rapidly. The improved stability and high catalytic activity of cyt c immobilized in MPS are attributed to the electrostatic attraction between the pore surface and cyt c and the confinement provided by nanochannels. We further observed that cyt c immobilized in MPS exists in both high and low spin states, as inferred from the ESR and UV-vis studies. This is different from the native cyt c, which shows primarily the low spin state. The high spin state arises from the replacement of Met-80 ligands of heme Fe (III) by water or silanol group on silica surface, which could open up the heme groove for easy access of oxidants and substrates to iron center and facilitate the catalytic activity. In the catalytic study, MAS-9-cyt c showed the highest specific activity toward the oxidation of polycyclic aromatic hydrocarbons (PAHs), which arises from the fast mass transfer rate of reaction substrate due to its large pore size. For pinacyanol (a hydrophilic substrate), MCM-41-S-cyt c and MCM-48-S-cyt c showed higher specific activity than NaY-cyt c and MAS-9-cyt c. The result indicated that cyt c embedded in the channels of MCM-41-S and MCM-48-S was protected against unfolding and loss of activity. By increasing the concentration of the spin trapping agent, 5,5-dimethyl-1-pyrroline N-oxide (DMPO) in ESR experiments, we showed that cyt c catalyzes a homolytic cleavage of the O-O bond of hydroperoxide and generates a protein cation radical (g = 2.00). Possible mechanisms for MPS-cyt c catalytic oxidation of hydroperoxides and PAHs are proposed based on the spectroscopic characterizations of the systems.     

Charge separation in acridine- and phenothiazine-modified DNA

The formation of the long-lived, charge-separated state in DNA upon visible light irradiation is of particular interest in molecular-scale optoelectronics, sensor design, and other areas of nanotechnology. However, the efficient generation of the charge-separated state is hampered by fast charge recombination within a contact ion pair, which limits the application of DNA for photoelectrochemical sensors and devices. In this study, a series of protonated 9-alkylamino-6-chloro-2-methoxyacridine (Acr+)- and phenothiazine (Ptz)-modified DNAs were synthesized for the further understanding of the mechanism of charge separation in DNA to generate a long-lived, charge-separated state with a high quantum yield (Phi). The Acr+ serves as a photosensitizer to produce a hole on guanine (G), and the G-C base pairs were used as a hole-transporting pathway to separate a hole from Acr* (the one-electron-reduced form of Acr+) to be trapped at Ptz. Since Acr+ oxides only G upon photoexcitation, the A-T base pair can be used as a spacer between Acr+ and the G-C base pair to avoid the formation of a contact ion pair. The charge injection dynamics was investigated by steady-state fluorescence spectra and fluorescence lifetime measurements, and the Phi and the lifetime of the charge-separated state produced upon photoirradiation were assessed by nanosecond laser flash photolysis of the Acr+- and Ptz-modified DNA. A long-lived, charge-separated state was successfully formed upon visible-light irradiation, and the Phi was the highest for the DNA having a single intervening A-T base pair between Acr+ and the G-C base pair. These results clearly demonstrated that the charge separation process in DNA can be refined by putting a redox-inactive intervening base pair as a spacer between a photosensitizer and the nucleobase to be oxidized to slow down the charge recombination rate.     

Charge separation in DNA via consecutive adenine hopping

Charge transfer in DNA is of current interest because of the involvement of charge transfer in oxidative DNA damage and electronic molecular devices. We have investigated the charge separation process via the consecutive adenine (A)-hopping mechanism using laser flash photolysis of DNA conjugated with naphthaldiimide (NDI) as an electron acceptor and phenothiazine (PTZ) as a donor. Upon the 355-nm laser flash excitation of NDI, the charge separation and recombination process between NDI and PTZ was observed. The yields of the charge separation via the consecutive A-hopping were slightly dependent upon the number of A bases between the two chromophores, while the charge recombination rate was strongly dependent upon the distance. The charge-separated state persisted over 300 micros when NDI was separated from PTZ by eight A bases. Furthermore, the rate constant of the A-hopping process was determined to be 2 x 10(10) s(-1) from an analysis of the yield of the charge separation depending on the number of A-hopping steps.     

表面相结结构在促进光催化电荷分离中的应用

二十世纪八十年代以来,特别是近十年,光催化研究在利用可再生能源太阳能的道路上飞速发展。越来越多的研究表明,相结结构的构筑是有效提高半导体光催化剂性能的重要策略。其中, TiO2作为重要的模型光催化剂,其相关研究成果呈现出指数增长的趋势。本综述围绕TiO2模型光催化剂,主要介绍TiO2表面相结的研究成果,包括TiO2表面相的表征、锐钛矿：金红石TiO2相结用于光催化产氢研究、TiO2相结在光催化中作用的最新认识等。在表征方面,通过表面灵敏的紫外拉曼光谱研究了TiO2相变过程中表面相结构的变化,结合可见拉曼以及XRD表征揭示了TiO2独特的相变过程,即相变始于锐钛矿粒子的界面处,小粒子逐渐团聚为大粒子,致其相变从大粒子体相开始最终扩展到整个粒子。使用CO, CO2探针红外光谱,根据锐钛矿和金红石表面吸附物种的差异,进一步证实了锐钛矿：金红石表面相结结构,为紫外拉曼光谱的表面表征特性提供坚实证据。同时,利用发光光谱观察到锐钛矿晶相的可见发光带和金红石晶相的近红外发光带,并基于此给出了TiO2材料表面相结结构的荧光表征新方法。此外荧光光谱还提供了锐钛矿、金红石相中载流子动力学信息,揭示了束缚态在光催化中的作用。在光催化应用方面,观察到混相结构TiO2较单独锐钛矿及金红石相具有更高的光催化产氢活性,通过在较大金红石颗粒上担载纳米锐钛矿粒子,证明了相结结构在提高光催化活性中的核心作用,并首次提出了锐钛矿：金红石表面异相结结构概念,推断其对电荷分离的促进作用是最终提高反应活性的原因。之后将此概念应用到改善商品TiO2(P25)光催化活性中,通过可控热处理精细调控P25的表面相结构,在光催化重整生物质衍生物产氢实验中,成功将P25光催化产氢活性提高3?5倍。之后发展了新的TiO2表面控制方法,通过加入Na2SO4等相变控制剂,延缓了TiO2从锐钛矿向金红石的相变过程,在较高温度下实现TiO2相结结构的调控,最终可将P25光催化重整甲醇制氢的活性提高6倍,同时通过高分辨电镜清晰观察到锐钛矿：金红石相结的原子层生长接触。在相结作用机理方面,多种时间分辨光谱技术以及理论计算被用作探索锐钛矿：金红石相结处的电子转移机理。通过时间分辨红外光谱对TiO2表面相结结构作用的研究,特别是利用锐钛矿、金红石不同的瞬态吸收光谱特征,证明了锐钛矿：金红石相结处的载流子转移过程,存在锐钛矿向金红石的电子转移过程。模型光催化剂TiO2相结的研究成果,加深了对光催化机理的认识,促进新型高效光催化体系的设计合成。     

Charge recombination versus charge separation in donor-bridge-acceptor systems

Optimizing the ratio of the rates for charge separation (CS) over charge recombination (CR) is crucial to create long-lived charge-separated states. Mastering the factors that govern the electron transfer (ET) rates is essential when trying to achieve molecular-scale electronics, artificial photosynthesis, and also for the further development of solar cells. Much work has been put into the question of how the donor-acceptor distances and donor-bridge energy gaps affect the electronic coupling, V(DA), and thus the rates of ET. We present here a unique comparison on how these factors differently influence the rates for CS and CR in a porphyrin-based donor-bridge-acceptor model system. Our system contains three series, each of which focuses on a separate charge-transfer rate-determining factor, the donor-acceptor distance, the donor-bridge energy gap, and last, the influence of the electron acceptor on the rate for charge transfer. In these three series both CS and CR are governed by superexchange interactions which make a CR/CS comparative study ideal. We show here that the exponential distance dependence increases slightly for CR compared to that for CS as a result of the increased tunneling barrier height for this reaction, in accordance with the McConnell superexchange model. We also show that the dependence on the tunneling barrier height is different for CS and CR. This difference is highly dependent on the electron acceptor and thus cannot solely be explained by the differences in the frontier orbitals of the electron donor in these porphyrin systems.     

One-step direct synthesis of mesoporous aluminosilicates Al-SBA-15 with cage-like macropores by using micrometer-sized aluminum balls

Ordered mesoporous aluminosilicate Al-SBA-15 materials with cage-like macropores have been synthesized by using micrometer-sized aluminum balls as an Al source, tetraethyl orthosilicate (TEOS) as a silica source, and triblock copolymer Pluronic P123 as a template. The resulting materials were fully characterized by XRD, N₂ adsorption, SEM, TEM, ICP-AES, and ²⁷Al MAS-NMR. The products (Al-SBA-15) have ordered two-dimensional (2-D) hexagonal mesostructures (space group p6mm). The calcined Al-SBA-15 materials exhibit disordered macropores with diameters of about 70–80 nm and ordered mesopores with a diameter of ∼5 nm, a BET surface area of about 500 m²/g, Si/Al ratio of 40–80, and a ratio of tetrahedral Al to octahedral Al sites of about 2:1. This combination of properties gives these materials potential applications in areas such as adsorption, catalysis and separation. Supported by the National Natural Science Foundation of China (Grant Nos. 20890123 & 20721063), Shanghai Science & Technology Committee (08DZ2270500), and Shanghai Leading Academic Discipline Project (No.B108)     

Charge separation in ground-state 1,2,4,5-tetra-substituted benzene derivatives

The conditions required for a formal biradical to exist in a zwitterionic form in the ground state are discussed following the recent experimental observation of zwitterionic structure in the ground state of a quinoid molecule (di-tert-butyl derivative of 2,5-diamino-1,4-benzoquinonediimine, I). A unique characteristic of molecules of this class is the fact that they may be considered as being formed by the union of two radicals, each having an odd number of pi electrons. In the case of I, one fragment carries the two amino group having 7 pi electrons; it acts as the electron donor. The other fragment carries the two oxygen atoms (carrying 5 pi electrons) and acts as an electron acceptor. A model that predicts the properties of these systems is presented, based on previous work on non-Kekule hydrocarbons(2,3) and on the electron donating and attracting properties of the donor and acceptor groups, respectively. The zwitterion is formed by an electron transfer leading to two subunits carrying 6 pi electrons each and may become more stable than the triplet biradical even in the gas phase (i.e., in the absence of an external field) if the ionization potential of the donor is small (of the order of 3-4 eV). In some cases solvation in a polar solvent is required to make the zwitterionic form the lowest energy species on the ground-state surface. The 'spacer' between the donor and acceptor groups (which need not be necessarily derived from an aromatic structure) can be varied and influences the overall dipole moment that is calculated in some cases to be quite large (over 20 D in the gas phase).