Mesoporous materials controlled pore size

The dimensions and accessibility of pores of zeohtes and microporous solids are confined to the subnanometer scale (<1.5 run), which hmits their applications when processing bulky molecules. Mesoporous materials with pore sizes ranging from 2 to 50 nm overcome these limitations. In contrast with microporous zeolites, these materials lack atomic ordering (crystallinity) in their silica walls as these are usually amorphous. The attractive properties of ordered mesoporous materials include well-defined pore system high surface area and pore sizes narrow pore size distribution tunable up to 100 nm existence of micropores in the amorphous wall (for thicker wall materials) existence of various wall (framework) compositions obtained from direct synthesis, or posttreatment or modification high thermal and hydrothermal stabilities if properly prepared or treated and various controllable regular morphologies on different scales from nanometers to micrometers. 

Materials with controlled pore sizes and functionality, particularly in three dimensions would have many uses (53-59). Numerous totally inorganic microporous and mesoporous materials have been subject of thousands of papers, and applications of the former (e.g., zeolites) have a sizable impact on the global economy at present (myriad uses from production of gasoline to a host of chemicals) (60-66). However, the use... 

It is our intention to present strategies based on chemically induced phase separation (CIPS), which allow one to prepare porous thermosets with controlled size and distribution in the low pm-range. According to lUPAC nomenclature, porous materials with pore sizes greater than 50 nm should be termed macroporous [1]. Based on this terminology, porous materials with pore diameters lower than 2 nm are called microporous. The nomination mesoporous is reserved for materials with intermediate pore sizes. In this introductory section, we will classify and explain the different approaches to prepare porous polymers and to check their feasibility to achieve macroporous thermosets. A summary of the technologically most important techniques to prepare polymeric foams can be found in [2,3]. 

The biggest advantage of ordered mesoporous materials is their uniform mesopores pore control is very important for theses mesoporous materials. The mesopore system (pore shape and array of pores) can be controlled by varying different mesostructures. In this section, the general methods to control pore size will be discussed. 

Zeolite materials with tunable size and volume of mesopores can be prepared by using dispersed carbon black particles with narrow distribution of their sizes as inert mesoporous matrix or as secondary template. In such confined space for synthesis the crystallization of zeolite gel occurs inside the interparticle voids of carbon matrix [10,11,12]. In the case of generation of mesopores by secondary templating by means of addition of carbon black into the reaction mixture, zeolite crystals are formed around carbon particles [13]. After burning off a carbon matrix or carbon particles, zeolite crystals with a controlled pore size distribution and a crystalline micro-mesoporous hierarchical structure are prepared. 

Other specific areas of micellar catalysis in which industry has expressed interest are in micellar phase-transfer catalysis and in the synthesis of mesoporous molecular sieves [92]. In the first example of the latter application, investigators at Mobil were able to control pore size and properties by synthesizing the desired mesoporous material in the presence of appropriately sized, structured, and charged micelles [96]. The burst of research activity in this area that occurred in the next few years after this discovery has been reviewed by Huo et al. [97]. 

Zeolites are crystalline aluminosilicates which assemble into well-defined three-dimensional structures comprised of microporous channels that interconnnect cavities which approach molecular scale dimensions ranging from 2 to 12 A. Mesoporous silica materials with pore sizes over 40 nm have also been made. They are relatively easy to synthesize and offer outstanding control over the pore size as well as its three-dimensional pore architecture, which makes them ideal for gas separations and shape-selective catalysis. Zeolites are perhaps the most widely recognized catalytic material. They are found in nature in over 50 distinct mineral forms. In addition, over 140 man-made zeolites have also been synthesized, with a total of 165 different framework structmesl l. 

Mesoporous silica was recently investigated as an excipient for formulations of molecules with low water solubility. These materials have very high specific surface area and small pore size. The customized template synthesis produces highly porous silica materials which can enhance the drug dissolution of hydrophobic molecules. Due to the porous nature and the controlled pore size volume of these materials, surface adsorption of the molecules to the mesoporous silica not only enhances the dissolution but also prevents the recrystallization of the amorphous materials. Due to the relatively finite space available to the amorphous molecules, the probability to align with their crystalline counterparts is low to negligible, resulting in amorphous... 

As mentioned earlier, mesoporous materials possess special physical properties such as high surface areas above 1000m /g, high pore volumes above l.Ocm /g and controlled pore sizes between 2 and 50 nm. Apart from these physical properties, mesoporous materials offer other unique properties that make them suitable for applications in pharmaceutical sciences and, in particular, as functional excipients. It is also clear that the tensile strength of mesoporous materials under different tableting pressures as well as any thermal stresses will vary considerably depending on particle size and shape, as has been observed by Vialpando et al. (2011). 

Template-directed synthetic methods have been widely employed to create ordered mesoporous materials, in particular SiO and TiO. Mesoporous materials have unique features, for example, controllable pore size, high surface area, and regularly arranged channel systems [93]. 

Recent reports describe the use of various porous carbon materials for protein adsorption. For example, Hyeon and coworkers summarized the recent development of porous carbon materials in their review [163], where the successful use of mesoporous carbons as adsorbents for bulky pollutants, as electrodes for supercapacitors and fuel cells, and as hosts for protein immobilization are described. Gogotsi and coworkers synthesized novel mesoporous carbon materials using ternary MAX-phase carbides that can be optimized for efficient adsorption of large inflammatory proteins [164]. The synthesized carbons possess tunable pore size with a large volume of slit-shaped mesopores. They demonstrated that not only micropores (0.4—2 nm) but also mesopores (2-50 nm) can be tuned in a controlled way by extraction of metals from carbides, providing a mechanism for the optimization of adsorption systems for selective adsorption of a large variety of biomolecules. Furthermore, Vinu and coworkers have successfully developed the synthesis of... 

One of the most promising applications of enzyme-immobilized mesoporous materials is as microscopic reactors. Galameau et al. investigated the effect of mesoporous silica structures and their surface natures on the activity of immobilized lipases [199]. Too hydrophilic (pure silica) or too hydrophobic (butyl-grafted silica) supports are not appropriate for the development of high activity for lipases. An adequate hydrophobic/hydrophilic balance of the support, such as a supported-micelle, provides the best route to enhance lipase activity. They also encapsulated the lipases in sponge mesoporous silicates, a new procedure based on the addition of a mixture of lecithin and amines to a sol-gel synthesis to provide pore-size control. 

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