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.     

Effects of template and precursor chemistry on structure and properties of mesoporous TiO2 thin films

Mesoporous TiO2 thin films were synthesized by sol-gel processing using an aqueous-based, inexpensive, and environmentally friendly precursor and cationic surfactants as templates under mild reaction conditions. The films were prepared by spin-coating on glass substrates followed by calcination to remove the surfactant. N2 sorption, X-ray diffraction, and transmission electron microscopy were used to characterize the porosity, pore size, and pore structure before and after calcination. Films were found to have wormlike pore structures after calcination and surface areas on the order of 200 m2/g. These results show that the mesostructure and porosity of the thin films can be controlled by the surfactant template chemistry such as surfactant/Ti ratio, pH, and rate of solvent evaporation.     

Influence of the composition of the reaction mixture and the synthesis conditions on the formation of mesomorphous substances in the presence of glucose oxidase

The possibility of synthesizing mesoporous substances in silicate and aluminosilicate systems in the presence of combinations of glucose oxidase and cetyltrimethylammonium bromide has been demonstrated. Highly crystalline mesoporous molecular sieves of type MCM-41 with a high aluminum content were obtained from the aluminosilicate system with a reaction mixture pH close to neutral. L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Prospekt Nauki, Kiev 252039, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 3, 190–193, May–June, 1999.     

Catalytic degradation of polyethylene in the presence of synthetic aluminosilicates

Degradation of polyethylene in the presence of synthetic amorphous aluminosilicates as catalysts to form petroleum-like hydrocarbons was studied.     

Effect of drying on the mesoporous structure of sol-gel derived silica with PPO-PEO-PPO template block copolymer

The effects of drying method on the pore structure of mesoporous silica were studied from the viewpoint of enhancing closed porosity in mesoporous silica. The mesoporous silica was prepared via a sol-gel process using polyethyleneoxide-polypropyleneoxide-polyethyleneoxide (PEO-PPO-PEO) triblock copolymer (Pluronic P123) as the structure-directing template. The closed porosity was evaluated from the apparent mass density of the sample measured by a helium pycnometer. These mesoporous silicas were also characterized by transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and nitrogen adsorption. The drying method was shown to be responsible for the finally templated mesoporous structure of the silica. More rapid drying is more preferable for enhancing the closed porosity of the mesoporous silica. The closed pores were formed by immediate immobilization of copolymer molecular assemblies in the silica matrix due to the instant removal of the solvent and solidification at higher temperatures. The drying method, mainly affecting the drying rate, is highly influential on the finally replicated mesoporous structure in silica.     

Effect of colloidochemical properties of template on the formation of the structure of mesoporous silica molecular sieves

A “homologous” series of mesoporous silica materials of MSM-41 type whose X-ray characteristics change in proportion to the length of hydrocarbon radical of template surfactant is synthesized using alkylpyridinium halides and sodium silicate. The correspondence principle of the state of the aggregation of micellar templates is used on synthesis for the first time. The influence of colloidochemical properties of surfactant solubilizer and solubilizate on the structuring ability of bitemplate (solubilization) composition is analyzed.     

Effect of nature of templates on formation mechanism of aluminum oxide mesoporous structure

Mesoporous aluminum oxide is produced by the sol-gel method from aluminum isopropoxide using molecular structures of a cationic polyelectrolyte (polyethylenimine) and dodecylamine micelles as nanotemplates. The mechanism for the formation of mesophases of hybrid materials in colloidal systems upon template synthesis is discussed.     

Effect of the nature of quaternary ammonium surfactants on the structure and properties of mesoporous molecular sieves

The influence of the nature of surfactant cations (dodecyltrimethylammonium bromide and tosylate and ethonium chloride) on the structure and adsorbent properties of mesoporous molecular sieves has been studied. The possibility of spatial self-organization of silica-surfactant systems without hydrothermal treatment has been observed. The influence of the surfactant counterions on the structure formation process has also been noted. L. V. Pisarzhevskii Institute of Physical Chemistry, Ukrainian Academy of Sciences, Ukraine, 252039 Kiev, Science Prosp., 31. Translated from Teoreticheskiya i éksperimental’naya Khimii, Vol. 33, No. 1, pp. 53–57, January–February, 1997.     

Effects of template sequence and secondary structure on DNA-templated reactivity

DNA-templated organic synthesis enables the translation, selection, and amplification of DNA sequences encoding synthetic small-molecule libraries. As the size of DNA-templated libraries increases, the possibility of forming intramolecularly base-paired structures within templates that impede templated reactions increases as well. To achieve uniform reactivity across many template sequences and to computationally predict and remove any problematic sequences from DNA-templated libraries, we have systematically examined the effects of template sequence and secondary structure on DNA-templated reactivity. By testing a series of template sequences computationally designed to contain different degrees of internal secondary structure, we observed that high levels of predicted secondary structure involving the reagent binding site within a DNA template interfere with reagent hybridization and impair reactivity, as expected. Unexpectedly, we also discovered that templates containing virtually no predicted internal secondary structure also exhibit poor reaction efficiencies. Further studies revealed that a modest degree of internal secondary structure is required to maximize effective molarities between reactants, possibly by compacting intervening template nucleotides that separate the hybridized reactants. Therefore, ideal sequences for DNA-templated synthesis lie between two undesirable extremes of too much or too little internal secondary structure. The relationship between effective molarity and intervening nucleic acid secondary structure described in this work may also apply to nucleic acid sequences in living systems that separate interacting biological molecules.     

Dual template synthesis of a highly mesoporous SSZ-13 zeolite with improved stability in the methanol-to-olefins reaction

The dual template synthesis of zeolite SSZ-13 by use of trimethyl-adamantanammonium hydroxide and a diquaternary-ammonium mesoporogen induces considerable mesoporosity without impeding zeolite microporosity. The strongly improved accessibility of Br?nsted sites in mesoporous SSZ-13 increases its stability during application as an acid catalyst in the methanol-to-olefins reaction.     

The effect of composition of the reaction medium on the structural-textural characteristics of mesoporous silicon dioxide

The conditions for the preparation of mesoporous silicon dioxide in water-alkali, alcohol-alkali, water-ammonia, alcohol-ammonia, and acid media were determined and its textural characteristics were studied.     

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.     

The heterogeneous catalytic oxidation of propylene in the presence of singlet oxygen in the reaction mixture

Three methods for the introduction of singlet oxygen into the reaction mixture were tested, including thermal generation of singlet oxygen on the catalyst itself, the introduction of singlet oxygen from an external source, and photogeneration of singlet oxygen on the catalyst. Zeolites with admixtures of Mo, Bi, V, and Ni and SiO₂ with deposited Mo, V, and Bi were used. Common to all reactions was an increase in the yield of deep oxidation products in the presence of singlet oxygen. A sharp increase in the yield of mild oxidation products was observed in the oxidation of propylene on a Bi/SiO₂ catalyst. The generation of singlet oxygen under irradiation at 240–260 nm was found to cause deep oxidation only. Mild oxidation products could only form under the action of total mercury lamp light.     

Catalytic activity of layered aluminosilicates for VOC oxidation in the presence of NOx

Raw and variously modified layered aluminosilicates have been used as catalysts in the reaction of ethanol oxidation both in the presence and absence of NO_x. In this study, we clearly showed that the conversion of VOC on the modified layered aluminosilicates decreases slightly in the presence of NO_x. However, the presence of NO_x in the reaction mixture did not affect the stability of the used catalysts. Only a small change of selectivity depending on the carrier type as well as on the way of modification was found.     

Catalytic hydrogenation of a binary substrate mixture with pseudo-zero reaction order: Dependence of the overall rate on mixture composition

An abnormal dependence of overall hydrogenation rate on the composition of a cyclopentadiene-isoprene mixture for the case of pseudo-zero reaction order has been interpreted in terms of a model accounting for the inhomogeneity of active sites.

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模板剂和制备条件对介孔CeO2材料性能的影响

在碱性条件下,采用Brij35、Brij35 CTAB、Brij35 PEG20000为模板剂合成了m-CeO2,考察了模板剂种类、合成温度、老化温度及焙烧温度等因素对m-CeO2性能的影响,运用XRD、FT-IR、TEM、N2吸附-脱附和比表面-孔径测定等手段对m-CeO2进行了表征。结果表明,表面活性剂的种类、合成温度、老化温度及焙烧温度对介孔材料的性能有较大影响。以Brij35 CTAB为复合模板剂,合成温度、老化温度及焙烧温度分别为40℃、60℃及450℃时,所制备的m-CeO2-BC,其比表面积、孔容、平均孔径和孔壁厚度分别为137.9m2/g、0.194 cm3/g、4.9nm和0.010nm,且孔径分布较集中,晶粒尺寸为5.9nm,为纳米介孔材料。     

Influence of composition of reaction mixture on selectivity in oxidation of aromatic compounds on oxide catalysts

A general outline is given of a kinetic model of oxidation of a hydrocarbon under the conditions of coexistence on the catalyst surface of sections of different oxidation levels. An analytical dependence has been obtained of the selectivity of the process and conversion on the composition of the reaction mixture. A qualitative agreement has been established between the theoretical and experimental dependences of selectivity and conversion on the ratio of the benzene and oxygen concentrations in the reaction mixture.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 1, pp. 105–108, January–February, 1985.     

Oxidative photo-decarboxylation in the presence of mesoporous silicas

FSM-16, a mesoporous silica, was found to catalyze oxidative photo-decarboxylation of alpha-hydroxy carboxylic acid, phenyl acetic acid derivatives and N-acyl-protected alpha-amino acids to afford the corresponding carbonyl compounds. Furthermore, FSM-16 proved to be re-usable by re-calcination at 450 degrees C after the reaction.