Immobilization of chiral oxazaborolidine catalyst over highly ordered 3D mesoporous silica with Ia3d symmetry for enantioselective reduction of prochiral ketone

Here we demonstrate for the first time the encapsulation of a chiral oxazaborolidine complex in the 3D mesoporous channels of an amine functionalized KIT-6 material via covalent bonding through a post-synthetic approach. The physico-chemical properties of the pure and immobilized KIT-6 catalysts were obtained by various techniques such as XRD, nitrogen adsorption, HRSEM, UV-Vis diffuse reflectance spectroscopy, and FT-IR spectroscopy. It has been found that the structural stability of the KIT-6 was not affected even after the immobilization of a significant amount of chiral ligand inside the mesoporous channels of the support. However, the values of structural parameters such as the specific surface area and the specific pore volume of the KIT-6 support was significantly lower than the pure KIT-6 support. The chemical interaction between the chiral ligand inside the mesochannels and the KIT-6 support was also confirmed by UV-Vis and FT-IR spectroscopy. The chiral catalytic performance of the immobilized catalysts for the enantioselective reduction of aromatic prochiral ketones was demonstrated and the results were compared with chiral catalyst immobilized supports with uni-dimensional porous structures, such as MCM-41 and SBA-15. Among the catalysts studied, chiral catalyst immobilized KIT-6 showed the highest performance with a high product yield and a high enantioselectivity due to its 3D porous structure with two continuous and interpenetrating systems of chiral channels and an interwoven 3D cylindrical type pores of Ia3d symmetry. The catalyst also exhibited much better recycling capability than other chiral catalyst supported mesoporous materials used in the study.     

Cobalt(II) acetylacetonate complex immobilized on aminosilane‐modified SBA‐15 as an efficient catalyst for epoxidation of trans‐stilbene with molecular oxygen

From environmental and economic points of view, it is highly desirable to develop a clean and efficient catalytic process to produce epoxides. An attractive approach is to use a solid, recyclable catalyst and molecular oxygen as the oxidant without any sacrificial reductant or other additives. Nonetheless, the catalysts reported up to now still cannot balance catalytic activity with epoxide selectivity. It is of great importance to explore novel catalysts with both high activity and selectivity for the epoxidation of olefins. In this work, cobalt(II) acetylacetonate (Co(acac)₂) was covalently bonded to the silica surface of SBA‐15 molecular sieve by multi‐step grafting using 3‐aminopropytrimethoxysilane (APTS) as coupling agent. Characterizations with nitrogen physisorption, X‐ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis suggested that the metal complex was successfully immobilized on the aminosilane‐modified SBA‐15 surface and the channel structure remained intact. The synthesized Co(acac)₂APTS@SBA‐15 catalyst was used in the epoxidation of trans‐stilbene (TS) with molecular oxygen. Compared to the sample prepared by the impregnation method as well as Co(acac)₂ solutions under the same reaction conditions, the Co(acac)₂ immobilized catalyst exhibited remarkably higher TS conversion and trans‐stilbene oxide (TSO) selectivity. An increase in TS conversion with Co content was observed when the Co loading was lower than 0.70% and the 0.70Co(acac)₂APTS@SBA‐15 sample exhibited the best catalytic performance. Up to 50.1% of TS conversion could be achieved within 6 h, affording TSO selectivity as high as 96.7%. The superior catalytic performance of this particular catalyst is attributed to the high activity of the immobilized Co(acac)₂ species on SBA‐15. Copyright © 2016 John Wiley & Sons, Ltd.     

Cationic ruthenium(II) complexes supported on mesoporous silica as catalyst precursors in the selective oxidative cleavage of 1‐octene

The selective oxidative cleavage of 1‐octene to heptanal and heptanoic acid is reported. A range of model and silica‐immobilized ruthenium(II) systems were evaluated. The MCM‐41 and SBA‐15 immobilized systems were found to show superior activity when compared to their homogeneous counterparts and were found to exert control over the selective formation of aldehydes or carboxylic acids. This could be achieved by varying the reaction times with very high yields being achieved at relatively short reaction times and low metal concentrations. The immobilized catalysts were characterized using nitrogen sorption, powder X‐ray diffraction, transmission and scanning electron microscopies, solid‐state NMR spectroscopy and thermogravimetric analysis.     

Surface‐Passivated SBA‐15‐Supported Gold Nanoparticles: Highly Improved Catalytic Activity and Selectivity toward Hydrophobic Substrates

Silanol groups on a silica surface affect the activity of immobilized catalysts because they can influence the hydrophilicity/hydrophobicity, matter transfer, or even transition state in a catalytic reaction. Previously, these silanol groups have usually been passivated by using surface‐passivation reagents, such as alkoxysilanes, bis‐silylamine reagents, chlorosilanes, etc., and surface passivation has typically been found in mesoporous‐silicas‐supported molecular catalysts and heteroatomic catalysts. However, this property has rarely been reported in mesoporous‐silicas‐supported metal‐nanoparticle catalysts. Herein, we prepared an almost‐superhydrophobic SBA‐15‐supported gold‐nanoparticle catalyst by using surface passivation, in which the catalytic activity increased more than 14 times for the reduction of nitrobenzene compared with non‐passivated SBA‐15. In addition, this catalyst can selectively catalyze hydrophobic molecules under our experimental conditions, owing to its high (almost superhydrophobic) hydrophobic properties.     

Epoxidation of styrene with molecular oxygen catalyzed by a novel oxovanadium(IV) catalyst containing two different kinds of ligands

A novel type of oxovanadium(IV) 8‐hydroxyquinoline (8‐Q) complex and oxovanadium(IV) acetylacetonate (acac) complex were synchronously anchored on to amino‐modified SBA‐15 (named VO(acac:8‐Q)‐SBA‐15) and examined as a catalyst for styrene oxidation. The structure of the synthesized catalyst was characterized by means of X‐ray diffraction (XRD), FT‐IR, diffusion reflection UV–visible, inductively coupled plasma atomic emission spectroscopy (ICP‐AES), scanning electron microscopy (SEM), and thermogravimetric analysis (TG). Characterizations with FT‐IR, diffusion reflection UV‐visible, ICP‐AES and TG suggested the incorporation of oxovanadium(IV) complex in amino‐modified SBA‐15. XRD and SEM results indicated that SBA‐15 remained intact throughout the grafting procedure. It was found that VO(acac:8‐Q)‐SBA‐15 catalyst was more active than single‐ligand catalysts VO(acac)₂‐SBA‐15 and VO(8‐Q)₂‐SBA‐15 and that the product selectivity varied in cases of different oxidants. The catalyst VO(acac:8‐Q)‐SBA‐15 with two different ligands showed high yield of styrene oxide (45.8%) and good recoverability when using air as oxidant. Copyright © 2012 John Wiley & Sons, Ltd.     

Dehydrogenation of Isobutane to Isobutene with Carbon Dioxide over SBA‐15‐Supported Chromia‐Ceria Catalysts

A series of SBA‐15‐supported chromia‐ceria catalysts with 3% Cr and 1%–5% Ce (3Cr‐Ce/SBA) were prepared using an incipient wetness impregnation method. The catalysts were characterized by XRD, N₂ adsorption, SEM, TEM‐EDX, Raman spectroscopy, UV–vis spectroscopy, XPS and H₂‐TPR, and their catalytic performance for isobutane dehydrogenation with CO₂ was tested. The addition of ceria to SBA‐15‐supported chromia improves the dispersion of chromium species. 3Cr‐Ce/SBA catalysts are more active than SBA‐15‐supported chromia (3Cr/SBA), which is due to a higher concentration of Cr⁶⁺ species present on the former catalysts. The 3Cr‐3Ce/SBA catalyst shows the highest activity, which gives 35.4% isobutane conversion and 89.6% isobutene selectivity at 570 °C after 10 min of the reaction.     

Metathesis of 1‐Butene to Propene over Mo/Al2O3@SBA‐15: Influence of Alumina Introduction Methods on Catalytic Performance

A series of Mo‐based catalysts for 1‐butene metathesis to propene were prepared by supporting Mo species on SBA‐15 premodified with alumina. The effects of the method of introduction of the alumina guest to the host SBA‐15 on the location of the Mo species and the corresponding metathesis activity were studied. As revealed by N₂ adsorption isotherms and TEM results, well‐dispersed alumina was formed on the pore walls of SBA‐15 if the ammonia/water vapor induced hydrolysis (NIH) method was employed. The Mo species preferentially interacted with alumina instead of SBA‐15, as evidenced by X‐ray photoelectron spectroscopy, time‐of‐flight secondary‐ion mass spectrometry, and IR spectroscopy of adsorbed pyridine. Furthermore, new Brønsted acid sites favorable for the dispersion of the Mo species and low‐temperature metathesis activity were generated as a result of the effective synergy between the alumina and SBA‐15. The Mo/Al₂O₃@SBA‐15 catalyst prepared by the NIH method showed higher metathesis activity and stability under the conditions of 120 °C, 0.1 MPa, and 1.5 h^?1 than catalysts prepared by other methods.     

Encapsulation of Homogeneous Catalysts in Mesoporous Materials Using Diffusion‐Limited Atomic Layer Deposition

The heterogenization of homogeneous metal complex catalysts has attracted great attention. The encapsulation of metal complexes into nanochannels of mesoporous materials is achieved by coating metal oxides at/near the pore entrance by diffusion‐limited atomic layer deposition (ALD) to produce a hollow plug. The pore size of the hollow plug is precisely controlled on the sub‐nanometer scale by the number of ALD cycles to fit various metal complexes with different molecular sizes. Typically, Co or Ti complexes are successfully encapsulated into the nanochannels of SBA‐15, SBA‐16, and MCM‐41. The encapsulated Co and Ti catalysts show excellent catalytic activity and reusability in the hydrolytic kinetic resolution of epoxides and asymmetric cyanosilylation of carbonyl compounds, respectively. This ALD‐assisted encapsulation method can be extended to the encapsulation of other homogeneous catalysts into different mesoporous materials for various heterogeneous reactions.     

Axially coordinated chiral salen Mn(iii) anchored onto azole onium modified ZnPS-PVPA as effective catalysts for asymmetric epoxidation of unfunctionalized olefins

A series of chiral salen Mn(iii) immobilized onto azole onium modified zinc poly(styrene-phenylvinyl phosphonate)-phosphate (ZnPS-PVPA) were prepared. The catalysts were characterized by FT-IR, diffusion reflection UV-vis, AAS, N(2) volumetric adsorption, SEM, TEM, XPS, XRD, TG and elemental analysis. The results showed that the chiral salen Mn(iii) complex was successfully immobilized onto ZnPS-PVPA. These prepared catalysts were evaluated in the asymmetric epoxidation of unfunctionalized olefins with m-CPBA and NaIO(4) as oxidants and demonstrated higher catalytic activities than those of the corresponding homogeneous chiral salen Mn(iii) catalyst under the same conditions. Moreover, these heterogeneous catalysts were stable and could be recycled nine times without significant loss of activity. Furthermore, this novel type of catalyst could also be validly used in large-scale reactions with superior catalytic disposition being maintained at the same level, which indicated the potential for applications in industry.     

Alkane Oxidation by an Immobilized Nickel Complex Catalyst: Structural and Reactivity Differences Induced by Surface‐Ligand Density on Mesoporous Silica

Immobilized nickel catalysts SBA*‐ L ‐x/Ni ( L =bis(2‐pyridylmethyl)(1H‐1,2,3‐triazol‐4‐ylmethyl)amine) with various ligand densities ( L content (x)=0.5, 1, 2, 4 mol % Si) have been prepared from azidopropyl‐functionalized mesoporous silicas SBA‐N₃‐x. Related homogeneous ligand LtBu and its Ni^II complexes, [Ni( LtBu )(OAc)₂(H₂O)] ( LtBu /Ni) and [Ni( LtBu )₂]BF₄ (2 LtBu /Ni), have been synthesized. The L /Ni ratio (0.9–1.7:1) in SBA*‐ L ‐x/Ni suggests the formation of an inert [Ni L ₂] site on the surface at higher ligand loadings. SBA*‐ L ‐x/Ni has been applied to the catalytic oxidation of cyclohexane with m‐chloroperbenzoic acid (mCPBA). The catalyst with the lowest loading shows high activity in its initial use as the homogeneous LtBu /Ni catalyst, with some metal leaching. As the ligand loading increases, the activity and Ni leaching are suppressed. The importance of site‐density control for the development of immobilized catalysts has been demonstrated.     

Pd(OAc)2@SBA‐15/PrEn nanoreactor: a highly active,reusable and selective phosphine‐free catalyst for Suzuki–Miyaura cross‐coupling reaction in aqueous media

A series of ordered mesoporous organic–inorganic hybrid material was designed by using the amine‐functionalized SBA‐15 (PdX₂@SBA‐15/N_Y, Y = 1, 2) as solid support for palladium complexes. Among them, the Pd(OAc)₂/ethylenediamine complex encapsulated into SBA‐15 (Pd(OAc)₂@SBA‐15/PrEn or Pd(OAc)₂@SBA‐15/PrNHEtNH₂) exhibits higher activity and selectivity toward Suzuki cross‐coupling reaction under aerobic conditions and water solvent mixture. The SBA‐15/PrEn supported palladium pre‐catalyst could be separated easily from reaction products and used repetitively several times, showing its superiority over homogeneous catalysts for industrial and chemical applications. Copyright © 2013 John Wiley & Sons, Ltd.     

Copper(II)–diaminosarcophagine‐functionalized SBA‐15: a heterogeneous nanocatalyst for the synthesis of benzimidazole,benzoxazole and benzothiazole derivatives under solvent‐free conditions

Solvent‐free organic reactions were studied over periodic mesoporous silica (SBA‐15) containing a Cu(II) organometallic complex. This heterogeneous catalyst was achieved by coordination of Cu(II) ions with the diaminosarcophagine ligand and then its grafting onto the surface of SBA‐15. This catalyst displayed ordered mesoporous channels, which implies an extremely high dispersion of the Cu(II) complex and the convenient diffusion of reactant molecules into the pore channels. Therefore, this catalyst can offer high activity and also facile separation or recycling when compared with its homogeneous counterparts. Copyright © 2015 John Wiley & Sons, Ltd.     

Epoxidation of cyclohexene with H2O2 over efficient water‐tolerant heterogeneous catalysts composed of mono‐substituted phosphotungstic acid on co‐functionalized SBA‐15

A series of Keggin‐type heteropolyacid‐based heterogeneous catalysts (Co‐/Fe‐/Cu‐POM‐octyl‐NH₃‐SBA‐15) were synthesized via immobilized transition metal mono‐ substituted phosphotungstic acids (Co‐/Fe‐/Cu‐POM) on octyl‐amino‐co‐functionalized mesoporous silica SBA‐15 (octyl‐NH₂‐SBA‐15). Characterization results indicated that Co‐/Fe‐/Cu‐POM units were highly dispersed in mesochannels of SBA‐15, and both types of Brønsted and Lewis acid sites existed in Co‐/Fe‐/Cu‐POM‐octyl‐NH₃‐SBA‐15 catalysts. Co‐POM‐octyl‐NH₃‐SBA‐15 catalyst showed excellent catalytic performance in H₂O₂‐mediated cyclohexene epoxidation with 83.8% of cyclohexene conversion, 92.8% of cyclohexene oxide selectivity, and 98/2 of epoxidation/allylic oxidation selectivity. The order of catalytic activity was Co‐POM‐octyl‐NH₃‐SBA‐15 > Fe‐POM‐octyl‐NH₃‐SBA‐15 > Cu‐POM‐octyl‐NH₃‐SBA‐15. In order to obtain insights into the role of ‐octyl moieties during catalysis, an octyl‐free catalyst (Co‐POM‐NH₃‐SBA‐15) was also synthesized. In comparison with Co‐POM‐NH₃‐SBA‐15, Co‐POM‐octyl‐NH₃‐SBA‐15 showed enhanced catalytic properties (viz. activity and selectivity) in cyclohexene epoxidation. Strong chemical bonding between ‐NH₃⁺ anchored on the surface of SBA‐15 and heteropolyanions resulted in excellent stability of Co‐POM‐octyl‐NH₃‐SBA‐15 catalyst, and it could be reused six times without considerable loss of activity.     

Monodisperse crosslinked pHEMA particle‐supported chiral Mn(III)salen catalyst for asymmetric epoxidation of olefin

Monodisperse crosslinked poly(hydroxyethyl methacrylate) particles (pHEMA) were synthesized for immobilization of the chiral Mn(III)salen homogeneous catalyst by axial coordination. The pHEMA‐Mn(III)salen catalyst was subsequently characterized by FT‐IR, UV and scanning electron microscopy. The results showed that, the heterogeneous Mn(III)salen catalysts also exhibited high activity and enantioselectivity compared to the homogeneous catalyst for the disubstituted cyclic indene and 6‐cyano‐2,2‐dimethylchromene. Moreover, the catalysts were easily separated from the reaction systems and could be renewed several times without significant loss of catalytic activity. Meanwhile, the enantiomeric excess (ee) value remained at 80% in the eighth cycle. The pHEMA support, immobilized by Mn(III)salen, probably acted as a mediator of the reaction between the substrate and the oxidant, and enhanced the stability of the Mn(III)salen compound. Copyright © 2012 John Wiley & Sons, Ltd.     

Ruthenium Carbenes Supported on Mesoporous Silicas as Highly Active and Selective Hybrid Catalysts for Olefin Metathesis Reactions under Continuous Flow

In the search for a highly active and selective heterogenized metathesis catalyst, we systematically varied the pore geometry and size of various silica‐based mesoporous (i.e., MCM‐41, MCM‐48, and SBA‐15) and microporous (ZSM‐5 and MWW) versus macroporous materials (D11‐10 and Aerosil 200), besides other process parameters (temperature, dilution, and mean residence time). The activity and, especially, selectivity of such “linker‐free” supports for ruthenium metathesis catalysts were evaluated in the cyclodimerization of cis‐cyclooctene to form 1,9‐cyclohexadecadiene, a valuable intermediate in the flavor and fragrance industry. The optimized material showed not only exceptionally high selectivity to the valuable product, but also turned out to be a truly heterogeneous catalyst with superior activity relative to the unsupported homogeneous complex.     

喹啉及其衍生物的多相不对称氢转移反应

喹啉及其衍生物的多相不对称氢转移是制备杂环手性化合物的理想策略.多相手性催化体系具有催化剂可循环利用及产物分离提纯容易等优势.然而,喹啉及其衍生物的多相手性高效催化体系鲜有报道.这主要是由于多相手性氢转移为水-油-固三相反应,在反应的过程中,传质问题极大影响固体催化剂的催化性能.因此,发展具有相转移功能的手性催化材料,是提高多相氢转移体系催化效率的有效途径.本文采用一锅法合成策略,通过离子液体(ILs)为连接基团实现了TsDPEN手性配体在SBA-15介孔孔道中的嫁接.与Rh盐配位后,获得手性固体催化剂SBA-ILBF4-TsDPEN-Rh.FI-IR光谱和13C NMR结果表明,手性催化活性中心成功负载在SBA-15中,随着手性活性中心负载量的增加,SBA-ILBF4-TsDPEN-Rh的比表面积、孔径和孔容逐渐降低.在喹啉衍生物不对称氢转移反应中,SBA-ILBF4-TsDPEN-Rh系列催化剂催化得到产物的ee值为91％,表明多相手性催化剂具有较高的手性选择性.多相手性催化剂的催化活性随着活性中心负载量的上升而呈现下降的趋势,这主要是由于活性中心负载量较低的多相催化剂具有更高的比表面积和孔容,更有利于催化过程中的传质.与均相手性催化剂相比,优化后的多相手性催化剂表现出更高的催化活性(TOF值分别为75和92 h-1).作为对比,本文还合成了采用烷基链为连接基团的SBA-TsDPEN25-Rh,并以其为基础进一步嫁接了ILs基团,得到SBA-TsDPEN20-ILBF4-Rh.在相同的反应条件下,SBA-ILBF4-TsDPEN50-Rh表现出更高的催化活性.上述结果证实了ILs基团在反应过程中起到相转移以及富集氢源甲酸盐的作用,极大促进了喹哪啶不对称氢转移多相催化体系的活性,并且ILs基团和手性活性中心在空间距离上的接近更有利于催化活性的提高.此外,本文还研究了反应体系pH值对固体催化剂上反应速率的影响,随着反应的进行,反应溶液的pH会呈现明显上升的趋势,导致反应速率减缓以及底物转化受限.通过在反应过程中加入适量甲酸或者选用浓度更高的缓冲溶液可以有效防止催化过程中反应速率的减慢.综上可见,负载手性催化剂中的连接基团对多相手性催化剂的催化性能有重要影响.通过改变手性配体的连接基团提高手性固体催化剂的催化活性和手性选择性的策略可以拓展到其他多相手性催化体系.     

埃洛石表面修饰并负载salen钼高效催化烯烃环氧化反应

采用温和的化学表面改性和自组装方法成功制备了埃洛石纳米管负载salen钼(HNTs-SL-Mo)催化剂,运用透射电镜、X射线衍射、红外光谱、诱导偶合等离子体谱和X射线光电子能谱表征了催化剂的形态、大小和分散性等性质。结果证明了salen结构的存在和埃洛石配位钼催化剂的成功制备。制备的催化剂在各种烯烃的环氧化反应中均有很好的活性,且活性高于均相催化剂。对比实验表明,在固定MoO(O2)2(DMF)2时, salen结构发挥了重要作用,不能用N原子作为单一配体来代替。本文还推测了钼和salen配体可能的连接方式和该催化剂催化烯烃环氧化反应的机理。该催化剂在重复使用8次后其活性未见明显下降,表现出优异的重复使用性能。由于埃洛石是一种廉价易得的材料,因此它可为设计效果独特的催化剂提供一个选择。     

Supported Palladium Oxide Nanoparticles in SBA‐15 as a Heterogeneous Catalyst for the Aerobic Oxidation of Alcohols

Palladium nanoparticles were first synthesized through the thermal decomposition method and subsequently immobilized on ordered mesoporous silica material, SBA‐15, to afford PdO/SBA‐15 catalyst. The synthesized catalyst was characterized by X‐ray diffraction, nitrogen adsorption‐desorption measurement, transmission electron microscopy, and inductively coupled plasma atomic emission spectrometry. The catalytic activity was tested for the aerobic oxidation of alcohols. Easy recovery, high yeilds and relatively short reaction times were observed for the mentioned catalyst.     

Facile and rapid immobilization of copper(II) bis(oxazoline) catalysts on silica: application to Diels-Alder reactions, recycling, and unexpected effects on enantioselectivity

Two chiral copper(II) bis(oxazoline) complexes have been immobilized on silica via electrostatic interactions using a remarkably straightforward procedure. The immobilized catalysts were tested in a standard Diels-Alder reaction and gave surprising results. Where the immobilized Cu((S,S)-phenyl-box)(OTf)₂ catalyst was used, the predominant enantiomer formed was the opposite of that produced using the same catalyst in a homogeneous reaction. This is a startling result given that the only difference is the electrostatic immobilization of the catalyst on amorphous silica. The activity of the catalyst in a hetero Diels-Alder reaction was also tested. This catalyst was also recycled, successfully maintaining a similar activity to the homogeneous analogue through a number of cycles.     

Encapsulation of a chiral MnIII(salen) complex in ordered mesoporous silicas: an approach towards heterogenizing asymmetric epoxidation catalysts for non-functionalized alkenes

Two immobilized chiral Mn^III(salen) complexes covalently anchored on modified MCM-41 (50 Å) and SBA-15 (75 Å) were prepared using 3-aminopropyltriethoxysilane as a reactive surface modifier to afford comparable or even higher enantioselectivity than homogeneous catalysts for the enantioselective epoxidation of a series of smaller to bulkier alkenes. The catalyst immobilized in silica with larger pore diameters was found to be more active. Compared to homogeneous catalysts, the heterogenized catalysts are more stable and can be recycled four times with retention of enantioselectivity.