Comparative studies of Zr-based MCM-41 and MCM-48 mesoporous molecular sieves: Synthesis and physicochemical properties

Two surfactant-templated synthetic routes are developed for the preparation of new types of mesoporous molecular sieves, Zr-MCM-41 and Zr-MCM-48, using different Si sources but keeping the same zirconium precursor (zirconium-n-propoxide). When fumed silica was used as Si precursor, a Zr-MCM-48 material of cubic structure was formed with a surface area of 654.8 m²/g and an unimodal pore diameter distribution. It shows low stability: after calcination at 600 °C, the ordered structure was transformed into a relatively disordered worm-like mesostructure with many defects and silanol groups. The use of tetraethyl orthosilicate as Si source led to the formation of a Zr-MCM-41 mesoporous solid, which had good thermal stability and a highly ordered hexagonal arrangement, with a surface area 677.9 m²/g and an uniform pore diameter distribution. Fourier transform infrared (FT-IR) characterization and ²⁹Si NMR analysis confirm that zirconium ions indeed incorporated into the framework of the solid. The in situ FT-IR spectroscopy of pyridine adsorption reveals that both, Lewis and Brönsted acid sites, were formed on the surface of these mesoporous materials. The strength and number of the Brönsted acid sites of the Zr-MCM-48 solid were greater than those of the Zr-MCM-41, due to a lower degree of condensation reaction during the synthesis that led to more structural defects in the framework and more silanol groups stretching from the solid surface.     

Synthesis and characterization of ionic liquid-functionalized alumino-silicate MCM-41 hybrid mesoporous materials

Ionic liquid-functionalized alumino-silicate MCM-41 hybrid mesoporous materials have been synthesized with two-step approach, by means of in situ skeleton doping with aluminium and post surface grafting with N-methylimidazole ionic liquid groups. The samples were characterized by X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), N₂ adsorption-desorption, Fourier transform infrared (FTIR) spectra, ²⁷Al and ¹³C MAS NMR spectra and temperature-programmed desorption (TPD) of NH₃. The results indicated that the bifunctionalized MCM-41 possessed ordered mesostructure. Aluminium was efficiently introduced into the framework of the mesostructure, generating Lewis and Brönsted acid sites. N-methylimidazole ionic liquid groups were covalently grafted onto the surface of mesoporous materials. The as-synthesized bifunctional MCM-41 showed good catalytic performance in the coupling reaction of CO₂ and propylene oxide.     

H and P solid-state NMR of trimethylphosphine adsorbed on heteropolytungstate supported on silica

Trimethylphosphine (TMP) has been used as an NMR probe in order to determine the acidity of Keggin-type 12-tungstophosphoric heteropolyacid (HPW), pure and supported on silica, dehydrated at 473 K. Adsorption of TMP on pure dehydrated HPW leads to the formation of trimethylphosphonium ions (TMPH⁺) characteristic of the presence of strong Brönsted acid sites. TMP replaces the water molecules lost by dryness and allows the Keggin secondary structure to recover.Silica interacts with TMP by two kinds of acid sites: with weak acid support sites through the isolated silanol groups and with strong Brönsted acid, which lead to the formation of TMPH⁺, through the hydrogen-bonded silanol groups. Silica only interacts with HPW through its isolated silanol groups.     

Catalytic performance of grafted Al-MCM-41 in hydroisomerization of n-dodecane

Pt/Al-MCM-41 samples with constant metal loading but various Si/Al ratios were prepared by reacting pure silica MCM-41 with different concentrations of polyaluminum chloride (PAC) aqueous solutions. It is observed that the molar ratio of Si/Al decreases from 24.3 to 11.2 when increase the PAC concentration from 0.1 M to 2.0 M. A better retention of structural integrity could be seen when the PAC concentration is below 1.0 M. The Lewis acid increases while incorporating aluminum into the framework of MCM-41, while the Brönsted acid reaches a maximum value when the PAC concentration is 1.0 M. Hydroisomerization of n-dodecane was carried out over these Pt/Al-MCM-41 samples. It is demonstrated that increasing the aluminum content generally can trigger a higher n-dodecane conversion, but a lower isomers selectivity due to the increasing strong Brönsted acid sites. And the Pt/Al-MCM-41 post-synthesized by the PAC concentration at 1.0 M shows highest isomers selectivity and yield corresponding to the maximum medium Brönsted acid quantities (24.15 μmol/g).     

Thermal spreading of WO3 onto zirconia support

This study focused on preparation of tungsten oxide supported on zirconia by thermal spreading. The prepared samples were characterized by infrared spectroscopy, UV-vis diffuse reflection spectroscopy, X-ray diffraction, and also by methanol dehydration reaction. It was observed that isolated octahedral tungsten dispersed species and dispersed polytungstate were formed on zirconia surface, although some WO₃ that remained after the thermal treatment could also be detected. The presence of these species led to an increase of the number of Lewis sites and the generation of Brönsted acid sites. High calcination temperatures promoted the creation of Brönsted sites as a consequence of polytungstate species formation. The activity on methanol dehydration was also determined by the concentration of these species, whereas the isolated WO_x species were found poorly active. The correlation observed between the catalytic performance and the tungsten dispersed species, as revealed by spectroscopic techniques, evidenced the occurrence of thermal spreading of WO₃ on ZrO₂. The results presented in this work show that WO₃ thermal spreading on ZrO₂ may be effectively accomplished as predicted by thermodynamics.     

NMR研究超稳Y分子筛水热老化过程中结构与酸性的变化

利用固体核磁共振（NMR）结合探针分子技术，探讨了超稳Y分子筛在水热老化过程中酸中心结构和酸性的变化规律.结果表明，水热老化初期，大量的骨架铝脱出最终形成了非骨架五配位铝，使得丙酮探测到的Brønsted酸（B酸）酸量随着老化时间的增加而减少.而非骨架五配位铝会向表面迁移并形成多聚铝，使得分子筛中Lewis酸（L酸）酸量也随老化时间的增加而减少.在老化过程的前3 h内，酸量的变化幅度较大.整体硅/铝原子比相等的分子筛，其B酸酸量的变化幅度与新鲜剂的晶胞大小呈正比；随着老化时间的继续增加，不同配位的铝物种的含量趋于稳定，B酸和L酸的酸量变化幅度很小.在整个水热老化过程中，超稳Y分子筛中B酸和L酸的协同效应一直存在，由协同效应产生的一种酸性增强的B酸位的稳定性最好.     

Further experimental evidences of thermal spreading of tungsten oxide on zirconia

The effects brought about by the time of thermal treatment as well as the water content in the gas phase during the thermal spreading of WO₃ on zirconia were investigated. Diffuse reflectance UV-vis spectroscopy evidenced the thermal spreading phenomenon and revealed the formation of polymeric tungsten dispersed species. Neither the thermal treatment time nor the water content showed to influence the nature of the dispersed species, which reveal to present thermodynamically preferential molecular structures. Infrared spectroscopy analysis of adsorbed pyridine evidenced that the polytungstate species lead to the generation of Brönsted acid sites. Lewis acid sites stronger than those naturally present on zirconia could also be detected in addition to weaker Lewis sites, which were associated to the WO₃ still present in the catalysts as showed by X-ray diffraction.     

Dehydration of isopropyl alcohol used as an indicator of the type and strength of catalyst acid centres

The subject of our studies was determination of the kind and strength of the catalyst acid centres on isopropyl alcohol conversion. The investigations were carried out for two groups of catalysts: typical Lewis acids γ-Al₂O₃ and ZrO₂ and Brønsted acids: H₃PW₁₂O₄₀ and H₃PMo₁₂O₄₀. Considerable differences between Lewis and Brønsted acids in the conversion of isopropyl alcohol were observed. The influence of Brønsted acid centres was studied for a group of catalysts with different strength: mixtures of two different heteropolyacids. It was observed that the increasing strength of Brønsted acids centres leads to higher catalytic activity but it does not significantly affect the activation energy of dehydration to propene.     

Adsorption of ammonia on vanadium-antimony mixed oxides

We analyzed the adsorption of ammonia (NH₃) on the VSbO₄(1 1 0) catalyst surface using density functional theory (DFT) calculations. We followed the evolution of the chemical bonds between different atoms of the resulting NH₃/VSbO₄ system and the changes in the electronic structure of the catalyst. NH₃ preferential adsorption geometries were analyzed through the crystal orbital overlap population (COOP) concept and the density of states (DOS) curves. The VSbO₄(1 1 0) surface exhibits Lewis and Brønsted acid sites on which the ammonia molecule can interact. On the Lewis acid site, NH₃ adsorption resulted in the interaction between the N and a surface V-isolated cation. On Brønsted acid site, N interacted with a surface H coming from the chemical dissociation of water. The COOP analysis indicate that NH₃ interaction on the VSbO₄(1 1 0) surface is weak. In addition, the DOS curves show more developed electronic interactions for NH₃ adsorption on Lewis acid site than over Brønsted acid site.     

The acid-base properties of the surface of native zinc oxide layers: An XPS study of adsorption of 1,2-diaminoethane

The acid-base properties of native zinc oxide surfaces have been studied using X-ray photoelectron spectroscopy (XPS). The native layers of zinc oxide have been obtained by ageing mechanically polished pure zinc disks in a glass dryer for 1 month. Such a treatment lead to the formation of an unstable oxide layer and dehydroxylation has been observed during storage in vacuum. By following adsorption in ultrahigh vacuum of 1,2-diaminoethane (DAE) several types of active sites have been evidenced. Zinc cations react with the probe molecule following a Lewis acid/base interaction, while the hydroxyl and the carbonate-like species react following a Brønsted acid/base reaction. Although initial interaction via the Brønsted-like mechanisms is favoured, it has been shown that the resulting complexes are not stable. Under vacuum conditions, the adsorbed DAE molecules either partly desorb or modify their interaction mode with the surface to form additional Lewis-like bonded stable complexes. In addition, a cleaning effect of the molecule has been observed which lead to partial removal of the carbonate-like contamination.     

Solid-state nuclear magnetic resonance investigations of the nature, property, and activity of acid sites on solid catalysts

Further progress in the field of heterogeneous catalysis depends on our knowledge of the nature and behavior of surface sites on solid catalysts and of the mechanisms of chemical reactions catalyzed by these materials. In the past decades, solid-state NMR spectroscopy has been developed to an important tool for routine characterization of solid catalysts. The present work gives a review on experimental approaches and applications of solid-state NMR spectroscopy for investigating Brønsted and Lewis sites on solid acids. Studies focusing on the generation of surface sites via post-synthesis modification routes of microporous and mesoporous materials support the development of new and the improvement of existing catalyst systems. High-temperature and flow techniques of in situ solid-state NMR spectroscopy allow a deeper insight into the mechanisms of heterogeneously catalyzed reactions and open the way for studying the activity of acidic surface sites. They help to clarify the activation of reactants on Brønsted and Lewis acid sites and improve our understanding of mechanisms affecting the selectivity of acid-catalyzed reactions.     

Molybdophosphoric acid in sol-gel titania: Physico-chemical properties

Titania mixed with molybdophosphoric acid, HMoP, in various proportions (1, 15, 25 and 50 wt.%) was obtained by the sol-gel method. The gels were dried and calcined in air at 400, 600 and 800 °C. The crystalline phases were identified by X-ray diffraction (XRD), whereas the physico-chemical properties were characterized by Raman spectroscopy and ³¹P MAS-NMR spectroscopy. FTIR was a technique used to determine acidic properties by pyridine adsorption. XRD showed that HMoP does not decompose until 700 °C. The stabilization of the anatase phase and the Keggin HMoP crystalline structure were evidenced by Raman spectroscopy; whereas FTIR-pyridine adsorption spectra showed that titania-HMoP present Brönsted and Lewis acid sites. A correlation between the acid sites and the amount of HMoP was observed by ³¹P MAS-NMR.     

Laser-Induced Fluorescence of Base Adsorbed on Surface Sites in Solid Catalysts

The interaction of quinoline with the surface of Nafion-H and Nafion-H (13 wt%)/silica gel composites, silica gel itself, HY zeolite, dealuminated or not, and two types of alumina is studied by laser-induced fluorescence. Strong Brønsted acid sites interact with quinoline to form the quinolinum, that ion being revealed by a band peaking at 390 nm. When Lewis acid sites are present, a complex formed between these sites and quinoline is observed. A dimeric species and the diffusion of the adsorbates on the surface of the Nafion-H polymer are discussed.     

The n-butyl amine TPD measurement of Brönsted acidity for solid catalysts by simultaneous TG/DTG-DTA

The method for Brönsted acidity measurement based on TPD of alkyl amines desorption by gas-chromatography or thermogravimetry was adapted for simultaneous TG/DTG-DTA analysis. The acidity measurements were focused on the 12-tungstophosphoric acid (H₃PW₁₂O₄₀) and its salts, especially with Cesium since these posses the highest Brönsted acidity and they are among the most interesting catalysts. The n-butyl amine (NBA) desorption takes place in three steps for Cs_xH_3−xPW₁₂O₄₀, x = 0-2, and four steps for the Cs_2.5H_0.5PW₁₂O₄₀. The steps of desorption correspond to the release of NBA molecules in stages, as NBA or butene molecules resulted from the Hofmann elimination reaction and NH₃ + H₂O formed by decomposition of ammonium salt. The quantities of desorption products, C₄H₈ and NH₃ + H₂O, corresponding to the stages with the maximum desorption rates at 400-420 °C, respectively 560-600 °C, are in the stoichiometric ratio with the Brönsted acidity.     

Surface states associated with acid sites on solids

It is shown that present models of Lewis acidity or basicity and of surface states on ionic solids have substantial overlap, although the former is designed to describe chemical interactions at the surface site, and the latter electron capture. The site requirements for a Lewis acid are compared to the requirements for an acceptor surface state to suggest under what conditions a site should exhibit both strong acidity and a deep surface state: The expected influence of electronic properties of the solid on acid strength, and the influence of adsorbed water, acids or bases on surface state energies are discussed. Experimental measurements are reported where both chemical acidity measurements and electrical surface state measurements are made on a series of Lewis acids. A positive correlation is found. Where there is correlation, it is concluded that chemical interaction measurements should provide a valuable tool to describe the energy distribution of surface states on a semiconductor surface. Also, the use of the surface state models and measurements from semi-conductor physics should help in understanding and classifying acid and basic sites on ionic semiconductors and insulators.     

杭锦2#土的光谱特征及非均相Fenton反应机理

杭锦2#土是内蒙古鄂尔多斯杭锦旗地区发现的层状含铁天然矿物,利用Ｘ射线衍射、吡啶吸附红外光谱及Ｘ射线光电子能谱技术对样品的性质进行了表征。X射线光电子能谱表明杭锦2#土骨架结构中Si和Al原子结合能与标准硅氧四面体和铝氧八面体中Si和Al结合能相比明显增加,表面存在Lewis酸位和Brönsted酸位,且杭锦2#土中铁物种以Fe(Ⅲ)和Fe(Ⅱ)形式存在于骨架结构中;非均相Fenton反应中杭锦2#土的Fe(Ⅱ)可与H₂O₂反应生成自由基(·OH)与Fe(Ⅲ),但反应速率慢且难以循环。酸活化后杭锦2#土中Si和Al的结合能进一步增加,铁物种部分转变为非结构铁并以Fe3+与Fe2+转移到样品表面;X射线光电子能谱、吡啶红外和氨气程序升温表征表明酸活化杭锦2#土表面Lewis酸位和Brönsted酸位增多;非均相Fenton反应中,酸活化杭锦2#土表面Fe3+与Fe2+可与H₂O₂循环反应,不断生成·OH并对甲基橙进行降解,且活化杭锦2#土表面Brönsted酸能够提供质子将H₂O₂包围,抑制其分解生成HO-₂并提供更多的·OH,Lewis酸能增加杭锦2#土表面吸附氧（O_ad）含量,而Fe2+可被O_ad氧化为Fe3+,促进Fe2+/Fe3+之间的循环,同时在氧化过程中电子转移到O_ad形成O·-₂,O·-₂能够与Brönsted酸提供的质子反应形成·OH,·OH与O·-₂均为氧化性自由基,能够提升活化杭锦2#土非均相Fenton反应活性。此外,X射线衍射表明酸活化使杭锦2#土中CO2-₃转化为对Fenton反应负面影响更小的SO2-₄进而提升其非均相Fenton反应活性。     

Chiral poly-rare earth metal complexes in asymmetric catalysis

Asymmetric catalysis is a powerful component of modern synthetic organic chemistry. To further broaden the scope and utility of asymmetric catalysis, new basic concepts for the design of asymmetric catalysts are crucial. Because most chemical reactions involve bond-formation between two substrates or moieties, high enantioselectivity and catalyst activity should be realized if an asymmetric catalyst can activate two reacting substrates simultaneously at defined positions. Thus, we proposed the concept of bifunctional asymmetric catalysis, which led us to the design of new asymmetric catalysts containing two functionalities (e.g. a Lewis acid and a Brønsted base or a Lewis acid and a Lewis base). These catalysts demonstrated broad reaction applicability with excellent substrate generality. Using our catalytic asymmetric reactions as keys steps, efficient total syntheses of pharmaceuticals and their biologically active lead natural products were achieved.     

Surface characterization of acidic ceria-zirconia prepared by direct sulfation

Acidic ceria-zirconia (SCZ) solid acid catalysts with a nominal surface density of ca 2 SO₄²⁻/nm² were prepared by a simple route consisting in soaking high specific surface area Ce_xZr_1−xO₂ (with x = 0.21 and 0.69) mixed oxides solutions in 0.5 M sulphuric acid. Characterizations by TPD-MS, TP-DRIFTS and FT-Raman revealed that most of surface structures generated by sulfation are stable at least up to 700 °C under inert atmosphere and consist mainly as isolated sulfates located on defects or crystal planes and to a lesser extent as polysulfates. Investigations by pyridine adsorption/desorption have stated that: SCZ possess both strong Brønsted (B) and Lewis (L) acid sites, some of them being presumably superacidic; the B/L site ratio was found to be more dependent on the temperature and hydration degree than on the composition of the ceria-zirconia. By contrast, the reactivity of the parent Ce_xZr_1−xO₂ materials towards pyridine is mostly driven by redox properties resulting in the formation of Py-oxide with the participation of Lewis acid sites of moderate strength (cus Ce^x+ and Zr^x+ cations). Basicity studies by CO₂ adsorption/desorption reveal that SCZ surfaces are solely acidic whereas the number and strength of Lewis basic sites increases with the Ce content for the parent Ce_xZr_1−xO₂ materials.     

FT-IR study of the acid sites on the surface of silica supported ruthenium oxide

FT-IR spectra of adsorbed CO, pyridine and acetone indicate that on the surface of RuO₂/SiO₂ are present Brønsted and Lewis acid sites and different reduced metal sites.     

SSZ-13分子筛上甲醇转化过程中甲氧基物种的生成与活性研究

本文使用固体核磁共振（NMR）技术研究了SSZ-13分子筛上甲醇制烯烃反应过程中表面甲氧基物种的生成以及反应活性.通过二维¹³C-²⁷Al HMQC NMR方法确证了甲醇在分子筛骨架Brønsted酸位上生成的甲氧基物种,以及在Lewis酸位上生成的另外一种表面甲氧基物种.¹³C NMR结合气相色谱-质谱（GC-MS）实验结果表明,这两种甲氧基物种在甲醇制烯烃反应中均具有较高的反应活性,既可以导致烃池物种的生成,也可以参与烃池反应生成碳氢化合物.