Design And Synthesis Of Nanoporous Materials: Stability Of Catalytically Active Sites

There are a rapid growth in research and development of nanotechnology, especially nanostructured materials, in the last decade. Nanoporous materials as an important class of nanostructured materials possess high specific surface area, large pore volume, uniform pore size, and rich surface chemistry. These materials present great promises and opportunities for a new generation of functional materials with improved and tailorabel properties for applications in adsorption, membranes, sensors, energy storage, catalysis, and biotechnology, etc. For catalysis, catalytically stable and active sites based on nanoporous materials are preferred. Although thermally and hydrothermally mesostructured stability of nanoporous materials is enhanced significantly, the improved stability of catalytically active sites based on nanoporous materials, including introduced heteroatoms in mesoporous wall and encapsulated oxides or biocatalysts in nanoporous materials, is reported rarely.The weaker stability of catalytically active sites in mesoporous materials is attributed to low condensed mesoporous walls with large amounts of terminal hydroxyl groups which make mesostructure unstable. Therefore, it can be expected that the level of silica condensation can be enhanced.The assembly of preformed titanosilicate zeolite precursors with triblock copolymers in strongly acid condition can lead to an ordered mesoporous titanosilicate (MTS-9), which is hydrothermally stable and very active for the oxidations. Howerver, calcinations of the mesoporous titanosilicates leads to significant reduction of catalytic activities towards both small and bulky molecules, which is due to relative low stability of catalytically active 4-coordinated titanium sites in the mesoporous titanosilicates, compared with that of TS-1. The relatively low stability of titanium species in catalysis may be related to imperfectly condensed mesoporous walls. Possibly, the level of condensation for mesoporous walls will be enhanced by increasing the crystallization temperature. More recently, high-temperature synthesis (180-220°C) of mesoporous silica materials templated by fluorocarbon and triblock copolymer mixture, results in more condensed mesoporous walls, showing excellent thermal and hydrothermal stability for these materials. We demonstrate here that highly ordered mesoporous titanosilicates with catalytically both active and stable 4-coordinated Ti species, designated Ti-JLU-20, are successfully assembled from preformed TS-1 nanoclusters at high-temperature (180-220°C). Ti-JLU-20 is very active for the catalytic conversion of phenol and trimethylphenol. Furthermore, the active sites of Ti species in Ti-JLU-20 are much more stable than those in MTS-9 during calcination and hydrothermal treatments. Apparently, stable and active Ti species in ordered mesoporous titanosilicates are very important the recycle of titanosilicate catalysts in oxidations. The catalytically stable and active 4-coordinated titanium sites in Ti-JLU-20 have been also confirmed by UV-vis spectroscopy. The high stability of both mesostructure and 4-coordinated titanium species in Ti-JLU-20 is reasonably attributed to the high condensation of mesoporous walls. The evidence is confirmed by 29Si NMR technique.The synthesis of ordered mesoporous materials under alkaline media has an advantage for simple incorporation of catalytic active sites such as Al3+ and Ti4+ into silica mesoporous walls, compared with acidic media, because the heteroatoms such as aluminum and titanium species in acidic media are general cations. However, flurocarbon-hydrocarbon surfactant mixtures are not ordered micelle under alkaline media. It has been reported other family of high-temperature (>150°C) stable surfactant micelle recently, such as cationic phase transfer catalysts and cationic modified ionic liquids. Therefore, when a cationic phase transfer catalyst named DIHAB is used as a template, ordered mesoporous silica materials with unusual hydrothermal stability, designated JLU-30 is successfully synthesized in alkaline media at relatively high temperatures (160-180°C). Additionally, under alkaline media, heterogeneous atoms of Al and Ti substituted materials, with the goal of introducing catalytically active sites, have been easily introduced into the mesoporous walls of JLU-30 by simple mixture of TEOS with aluminum or titanium source in initial reaction gel. For example, 27Al MAS NMR spectrum of the as-synthesized Al-JLU-30 shows a strong signal centered at near 53 ppm, indicating that Al species have been successfully incorporated into the mesoporous walls with 4-coordinated number, which suggests that Al-JLU-30 may be used as acidic catalysts and ion-exchangers. Particularly, after calcination at 550°C for 5 h and hydrothermal treatment in boiling water for 150 h, in addition to a very small peak at 0 ppm associate with octahedral Al, the peak assigned to tetrahedral Al is almost unchanged, indicating high thermal and hydrothermal stability of tetrahedral Al sites in JLU-30. Furthermore, highly ordered cubic mesoporous aluminosilicates with highly stable tetrahedral aluminum sites, designated as Al-JLU-31, are also synthesized at high temperatures (>150°C) in alkaline media. The high stability of tetrahedral aluminum sites in Al-JLU-30 and Al-JLU-31 can be also reasonably attributed to full condensation of mesoporous walls.It could be still expected that amount of stably catalytically active sites such as tetrahedral Al sites in mesoporous materials can be improved. All the conventional preparation methods used join to one common point that independent and separated heteroatom sources have to be used either to introduce in the silica gel or to contact with the pre-synthesized mesoporous silica and would lead either to relatively low heteroatom content, or to extra framework heteroatom species, or to lower order of mesostructure, or to an anchor of heteroatom species only on the surface of channels. The principle reason of these relevant issues is that a new Si-O-T bond has to be constructed from two independent precursors (Direct-synthesis) or from a secondary precursor with the surface of preformed mesoporous silica framework (Post-synthesis). The different hydrolysis and condensation rates of any two independent precursors and the predomination of homocondensation rate of one source over that of heterocondensation between two different precursors will unavoidably lead to the formation of materials with above relevant defects. The question can then arise as to whether a single-source molecular precursor containing already the Si-O-T bonds can be used in the synthesis. We can expect a well organized framework with a Si/T ratio as low as possible up to unit, in another word, the T content as high as possible. Heteroatoms can molecularly and homogenously distribute because the heteroatoms are initial part of the same molecule and their presence in the framework is owing to the existence of Si-O-T linkage in this single molecular source. Here, aluminosilicate ester ((BusO)2-Al-O-Si-(OEt)3) was used as a single source of silicon and aluminum elements for the synthesis of ordered mesoporous aluminosilicates. The use of aluminosilicate ester containing Al-O-Si linkage on the molecular scale renders possible the precise control of the structure and active sites in the mesoporous materials. The present work demonstrates the fine tuning of the hydrolytic rate by using acetone as inhibitor to prevent the Si-O-Al bond cleavage. The highly ordered mesoporous Aluminosilicates with the unprecedentedly lowest Si/Al ratio of 1, designated as MAS-C, is synthesized. More importantly, tetrahedral Al sites in MAS-C are highly stable after calcinations, long period immersion in boiling water and steaming treatment, which is quite essential for catalysis. The high stability of tetrahedral aluminum sites in MAS-C can be reasonably attributed to molecular order and full condensation of mesoporous walls, which is further evidenced by 29Si MAS NMR spectroscopy.The synthesis of new nanostructured materials with tailored porosity is a major challenge in advanced materials science. Core-shell structure of nanoporous materials with high surface areas and uniform pore size have been received a lot of recent attention due to their potential applications in drug-delivery, adsorption, chromatography, biomolecular-release systems, and confined-space catalysis. Notably, directed synthesis of bimodal nanoporous core-shell structure is still not easy due to the complexity in the synthesis and limitation of various synthetic routes, although these core-shell structures are successfully prepared. Furthermore, most work in this area has only been focused on the development of synthetic methodologies. Very little attention has been directed toward the functionalization of bimodal porous materials such as nanoreactor. In this study, Aluminosilicate ester ((BusO)2-Al-O-Si-(OEt)3) used for the synthesis of bimodal nanoprous aluminosilicates with hierarchically macroporous core@ mesoporous shell. The use of aluminosilicate ester containing various hydrolysis rates renders possible the precise control of this novel structure. In fact, the hydrolysis of Al-(BusO)2 side is very rapid to easily self-aggregate to form interior macroporous structure, even under template-free condition, and then the hydrolysis of Si-(OEt)3 side on the other hand is slow and can suitably assemble together with TMOS, a supplementary silica source around surfactant micelles (CTAB) to form order mesopoous structure at the surface of the cores. Therefore the novel nanosized spheres with foam-like macroporous core and ordered mesoporous shell can be obtained. Furthermore, the synthetic method of core-shell structure was precisely applied in nanoreactor conception by encapsulation and crystallization of metal oxide nanoparticles inside of core-shell structure. The advantage of this new core-shell structure is that mesoporous shell allows the diffusion of chemical reagents towards inside of structure and prevents the leaching of the active metal oxide nanocrystals as formed in macroporous core. The encapsulated Fe2O3 nanoparticles are stable, reusable, and active for the catalytic conversion of phenol. For example, after 5 recycles, the encapsulated Fe2O3 still exhibits 90% of the initial activity. The high efficiency of the encapsulated Fe2O3 nanoparticles can be partly attributed to nanosize, high dispersant and unique structure of support, which minimizes Fe2O3 nanoparticles aggregation, and preventing leaching of the active sites, and supports large mesoporous channels and high surface area. It is believable that the strategy developed here provides a general, facile, and unique approach for bimodal nanoporous materials with core-shell structure and its application of nanoreactor with long term and extraordinary recycling stability, and high activity.There is currently great interest in enzyme immobilization to enhance enzyme stability, reusability, and easy separation from the reaction mixture. Here we report a general and facile approach for the encapsulation of enzymes of various sizes in ordered mesoporous silica, where the enzymes are entrapped in macroporous cages connected by uniform mesoporous channels. These encapsulated enzymes show good activity, long term stability, and excellent recycling characteristics. The conception for the"Fish-in-Net"encapsulation of enzymes in ordered mesoporous silica under mild conditions is first time was demonstrated. During the interaction between enzymes and the preformed precursors with ordered mesostructured silica particles, active enzymes as"Fish"were gradually entrapped in the"Net"formed by polymerization and condensation of order mesostructured silica particles. The encapsulated enzymes in mesoporous silica are typical"nanoreactors", which combine advantages of native enzymes and mesoporous channels. When water is introduced into the cages, the chemical environment for the enzymes in the cages will be similar to that of native enzymes in aqueous solution. This is beneficial for protein rotation and conformational transitions, and provides high biocatalytic activity. Moreover, the ordered mesoporous channels play a very important role of both protecting the enzymes from leaching and providing a path for the diffusion of reactants and products for biocatalytic processes. It is notable that all of the encapsulated enzymes retain over 90% of the activities of the native enzymes. Especially, after 30 recycles, the encapsulated trypsin still exhibits 95% (±10%) of the initial activity.The high activity and stability, and excellent recycling behavior of the encapsulated enzymes in the samples should be attributed directly to the unique"Fish-in-Net"encapsulation under mild conditions. It is believable that the strategy developed here provides a general, facile, and unique approach for the encapsulation of enzymes with long term and extraordinary recycling stability, and high activity.