Pore-filling agent

The extensive size of organic amine as structure-directing templates or pore filling agents, coupled with a new gel chemistry resulted in the discovery of a third generation of zeolites containing Al3+ and P5+ as lattice atoms (1982). These aluminophosphate materials are a family of molecular sieves as shown in figure 7.10. 

In the presence of minor amounts of Pr4N+, poorly crystalline ZSM-5 is obtained (6) while in complete absence of template, only limited amounts of ZSM-5 appear after several days of crystallization (7, 8). In that case, hydrated sodium ions constitute a poor replacement as template or pore filling agents for Pr4N+ (8). The crystallization rate of such systems can be considerably increased by addition of amines that then primarily act as pore fillers (5, 9, 10) but not necessarily as specific structure directors during nucleation (5). 

Zeolite ZSM-23 can be readily synthesized with pyrrolidine as organic template and fumed silica as silica source. Pyrrolidine seems to intervene in the nucleation and crystal growth process, in the latter case most probably as pore-filling agent. By proper optimization of the gel composition and the synthesis conditions, ZSM-23 can be synthesized in less than one day. The synthesis of ZSM-23 has some... 

A higher A1 content in the hydrogel is a predominant variable that leads to a differently arranged Al-richer EU-1 framework. HM++ stabilize this framework as counterions along with the alkali cations and also as pore filling agents, but do not initiate its nucleation. The more open, tortuous pore structure that results, accomodates the HM++ ions easily, without much steric constraint, and the EU-1 structure contains a far smaller number of Si-0- defects than ZSM-48. 

Fig. 6. Change in chemical potential of lattice precursors of porous crystals upon sorption of a pore filling agent. Curve (a) represents a reference state different from that of curves (b) and (c). Curve (b) refers to a more strongly held sorbate than curve (c) (after ref. 18).

The definition for catalytic purposes of a zeolite reads as follows a crystalline material with micropores and cation-exchange capacity that is insoluble in water and common organic solvents and has sufficient thermal stability that allows removal of all pore-filling agents present in the as-synthesized materials. This definition is narrower than that of the IZA Constitution, which includes mesoporous solids, metal organic frameworks (MOFs), cationic and anionic clays [3]. 

For obvious catalytic reasons, synthesis of large-pore zeolites has been sought. 14-, 18-, 20-, and even 24-MR materials have been made with different T atoms in the framework [181]. Unfortunately, most of the materials show monodimensional pores and limited stability in catalytic conditions. Aluminophos-phates with 14-MR and 18-MR, denoted as AIPO4-8 (AET) [182] and VPI-5 (VFI) [183] have been synthesized with di-propylamine as pore-filling agent. 

At 250"C catalyst activity decreases exponentially with temperature. This is probably due to formation of ammonium sulfate at the exterior surface of the catalyst at this tow temp> erature. causing severe pore blockage. However, at 400<>C, a temperature above that required for decomposition of ammonium sulfate, deposition of the deactivating agent, ammonium sulfat probably occurs deep inside pores leading to selective pore filling rather than pore blockage. 

In addition, the nanopore filling and nanotube formation mechanism of both polymers and metals is analyzed in detail. The results indicate that many factors affect the deposition morphology including electrochemical bath pH, pore-anchoring agents, nanopore geometry, nanoelectrode morphology, pore-wall functionalization, active ion concentration, as well as electrodeposition potential and/or current density. 

The entire phase inversion process of a polymeric solution is represented by the path from A to D. The original polymeric solution is at point A, where no precipitation agent (nonsolvent) is present in the solution. After the immersion of the polymeric solution into a nonsolvent coagulation bath, the solvent diffuses out of the polymer solution, whereas the nonsolvent diffuses into the solution. In the case when the solvent flux is higher than the nonsolvent flux, the polymer concentration at the interface would increase, and at some point, the polymer starts to precipitate (as represented by point B). The continuous replacement of the solvent by the nonsolvent would result in the solidification of the polymer-rich phase (point C). Further solvent/nonsolvent exchange would cause shrinkage of the polymer-rich phase and finally reach point D, where the two phases (solid and liquid) are in equilibrium. A solid (polymer-rich) phase that forms the membrane structure is represented by point S and a liquid (polymer-poor) phase that constitutes the membrane pores filled with nonsolvent is represented by point L. 

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