Materials microporous

Porous polymeric biomaterials and implants composed of biodegradable polymers have been presented in numerous papers and patents. A number of techniques that have been proposed to build such porous polymer structures are collected in Table 8.8. 

Melt blending of two immiscible polymers (51) Using solid porogens (52) 

For forming biocompatible foam structures, by solidifying a solution of a solvent and a bioabsorbable pol5mcier to form a solid, then subliming the solvent out of the solid. 

A microporous biodegradable pol5mcieric material is prepared from a biodegradable polymer (A), another polymer (B), no matter whether biodegradable or not. However, the biodegradable polymer should be least partially immiscible with (A). Further, a polymeric compatibiUzer (C) for the polymers (A) and (B) is needed (56). 

The components are then melt blended. Thereby a compatibilized polymer blend is prepared, wherein the pol5nners (A) and (B) have an essentially continuous morphology. At cooling the blend to room temperature, retaining its morphology should be retained. 

In substances in which the cavities (cages/voids) are interconnected, it has become customary to refer to them as microporous materials with pore structures. In these, the pores are typically 3-20 A diameter (but not necessarily circular or spherical) with a very narrow pore size distribution. They have large internal surface areas, typically 300 m /g and void volnmes 0.1 cc/g. The sizes and uniformity of the pores often enable them to be used to separate unlike molecules by virtue of their different sizes and shapes. 

Methods of synthesis of these materials generally involve a wide range of conditions, usually invoking elevated temperatures (up to 250°C) and increased pressnres (up to -1000 bars). 

Structural features found among space-containing compounds, which are of interest both to the chemist and to the materials scientist, include 

Occurrence of M atoms in partially reduced valency states 

Condensation of PO4 into P2O7, P3O10 or larger groups which may act as bidentate or tridentate ligands around M 

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Lewis D W, Sankar G, Wyles J K, Thomas J M, Catlow C R A and Willock D J 1997 Synthesis of a small-pore microporous material using a computationally designed template Angew. Chem. Int. Ed. Engl. 36 2675-7... 

Burban J FI, Fie M and Cussler E L 1995 Organic microporous materials made by bicontinuous microemulsion polymerization AlChE J. 41 907-14... 

Karge FI G 1997 Post-synthesis modification of microporous materials by solid-state reactions Stud. Surf. Sol. Catal. 105 1901-48... 

Volume 84 Zeolites and Related Microporous Materials State of the Art 1994. 

Volume 104 Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces edited by W. Rudzihski, W.A. Steele and G. Zgrablich Volume 105 Progress in Zeolite and Microporous Materials... 

ITie BET method is the most widely used procedure for determining the surface area of porous materials. In this chapter, BET results were obtained from single point measurements using a Micromeritics Flowsorb II 2300 surface area analyzer. A mixture of nitrogen in helium (30 70 mole percentage) was used. Although this simple method is not quantitative for the microporous materials studied in section 5, it still allows qualitative comparisons to be made. 

To improve the yield of mono- and dimethylamines, a shape selective catalyst has been tried. Carhogenic sieves are microporous materials (similar to zeolites), which have catalytic as well as shape selective properties. Comhining the amorphous aluminum silicate catalyst (used for producing the amines) with carhogenic sieves gave higher yeilds of the more valuable MMA and DMA. ... 

Corma, G.A., Eduardo Domine, M., Susarte, R.M., and Rey, G.F. (2002) MCM-41 type microporous materials containing titanium and their utilization as catalysts in a- pinene oxidation, Patent W00054880. 

Other microporous materials have been synthesized using the porogen polyethylene glycol in polyethylene oxide-urethane gels [27]. Micropores were formed in the gel, and it was found that the diffusion of larger species, vitamin B12, was enhanced relatively more than that of a smaller species, proxyphylline. This result is in qualitative agreement with that found for electrophoretic transport by RiU et al. [322] discussed earher, where the mobility of larger species was preferentially enhanced in the templated media. 

Volume 94 Catalysis by Microporous Materials. Proceedings of ZEOCAT 95, Szombathely,... 

The mesoscopic domain of real catalysts is mostly covered by the typical catalysis periodicals, such as Applied Catalysis, the Journal of Catalysis, Catalysis Letters, Topics in Catalysis, Catalysis Today, Microporous Materials and Zeolites, although occasionally articles also appear in Journal of Physical Chemistry and Physical Chemistry-Chemical Physics, and many others. 

Here the phenomenon of capillary pore condensation comes into play. The adsorption on an infinitely extended, microporous material is described by the Type I isotherm of Fig. 5.20. Here the plateau measures the internal volume of the micropores. For mesoporous materials, one will first observe the filling of a monolayer at relatively low pressures, as in a Type II isotherm, followed by build up of multilayers until capillary condensation sets in and puts a limit to the amount of gas that can be accommodated in the material. Removal of the gas from the pores will show a hysteresis effect the gas leaves the pores at lower equilibrium pressures than at which it entered, because capillary forces have to be overcome. This Type IV isotherm. 

Braun PV, Wiltzius P (1999) Microporous materials. ElectrochemicaUy grown photonic crystals. Nature 402 603-604... 

Another recent new application of a microporous materials in oil refining is the use of zeolite beta as a solid acid system for paraffin alkylation [3]. This zeolite based catalyst, which is operated in a slurry phase reactor, also contains small amounts of Pt or Pd to facilitate catalyst regeneration. Although promising, this novel solid acid catalyst system, has not as yet been applied commercially. 

H.H. Mooiweer, K.P. de Jong, B. Kraushaar-Czarnetzki, W.H.J. Stork and B.C.H. Krutzen, "Zeolites nd Related Microporous Materials State of The Art 1994", Studies in Surface Science and Catalysis, Elsevier, 1994, Eds. 

While our discussion will mainly focus on sifica, other oxide materials can also be used, and they need to be characterized with the same rigorous approach. For example, in the case of meso- and microporous materials such as zeolites, SBA-15, or MCM materials, the pore size, pore distribution, surface composition, and the inner and outer surface areas need to be measured since they can affect the grafting step (and the chemistry thereafter) [5-7]. Some oxides such as alumina or silica-alumina contain Lewis acid centres/sites, which can also participate in the reactivity of the support and the grafted species. These sites need to be characterized and quantified this is typically carried out by using molecular probes (Lewis bases) such as pyridine [8,9],... 

When a microporous material, e.g. a zeolite, is used as a catalyst, only those molecules whose diameters are small enough to enter or pass through the pores can react and leave the catalyst. This is the basis for so-called shape-selectivity (Fig. 3.40). Reactant selectivity is encountered when a fraction of the feed molecules can enter the zeolite, whereas the other fraction cannot. For the molecules produced in the interior the same reasoning applies. The favoured products are the less bulky ones, i.e. those with diameters smaller than the pores of the zeolites. For instance, in the zeolite represented in Fig. 3.40 the production of p-xylene is favoured over the production of o- and m-xylenes. Also the bulkiness of the transition state can lead to a different. selectivity, as shown in the last example in Fig 3.40. 

For microporous materials the 5bet values obtained are usually much higher than the real surface area, because in the region where the BET equation is applied (this equation assumes multilayer adsorption but not condensation) conden.sation already takes place. 

Of course, diffusion limitations may affect the TPD pattern especially for microporous materials such as zeolites. Furthermore, no information is acquired on the nature of the acid sites. For instance, the technique does not discriminate between Lewis and Brbnsted sites. 

J. B. Nagy, P. Bodart, I. Hannus, I. Kiricsi, Synthesis, Characterization and Use ofZeolitic Microporous Materials, Z. Konya, V. Tubak (eds.), DecaGen Ltd, Szeged, Hungary, 1998. 

We may thus conclude after this short overview on DeNO technologies that NH3-SCR using catalysts based on V-W-oxides supported on titania is a well-established technique for stationary sources of power plants and incinerators, while for other relevant sources of NO, such as nitric acid tail gases, where emissions are characterized from a lower temperature and the presence of large amounts of NOz, alternative catalysts based on transition metal containing microporous materials are possible. Also, for the combined DeNO -deSO, alternative catalysts would be necessary, because they should operate in the presence of large amounts of SO,.. Similarly, there is a need to develop new/improved catalysts for the elimination of NO in FCC emissions, again due to the different characteristics of the feed with respect to emissions from power plants. 

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