Vycor® process

The Vycor process described in Chapter 8 uses the principle of phase separation. The resulting glass is 96% SiOz and 4% pores and is used as a filter and a bioceramic where porosity is important. It can be densified (after shaping) to allow processing of a pure SiOz shape at a lower temperature than for pure quartz glass. 

As already mentioned, first attempts have been made to prepare binary oxide zeolitic structures with a combination of pore sizes, for example, micro- and mesopores to enhance diffusion within the material. In addition to the templating approaches in which hard or soft objects/molecules/aggregates are applied, phase separation as already seen in the Vycor process plays an important role, especially in the formation of the macroporous structure. In this section, we will... 

Besides etching of masked glass substrates, see Sect. 6.1, the leaching of phase-separated glasses, see for instance Figs. 1.14-1.16, is completely an other process and also allows for the fabrication of microstructured glass bodies. A schematic of the process to fabricate a porous glass membrane, which was developed from the classical VYCOR process, is shown in Fig. 6.4. 

Vycor glass (Corning, 7930) is porous silica processed in the same way as CPG except that the last step of base etching is missing. Therefore the surface is rough. Only a 40-A pore is available. When porous silica with a pore size smaller than 75 A is needed, Vycor glass is the choice. Bulk pieces of Vycor glass are available commercially and need to be crushed into small particles before use. 

Into a size 6A porcelain boat (88 X 13 X 10 mm.) is placed 10.0 g. (0.0113 mol) of Ag2ReBr6 (as much as 15 g. may be processed readily), and the boat is positioned near the sealed end of a 25 X 300 mm. Pyrex tube fitted with an inner 29/44 S.T. sleeve joint. The tube is connected to a mechanical vacuum pump (capable of reducing the pressure to ca. 50 p) via either a trap cooled by liquid nitrogen or a trap cooled by a Dry Ice-isopropyl alcohol slush, which is in turn connected to an 11-mm. Vycor combustion tube packed with copper gauze and maintained at 600° by means of a furnace. (Either protective system between the tube and the pump will serve to remove bromine, one of the reaction products.)... 

The Vycor glass tube used in the membrane reactor experiments was filled with 3.2 g of catalyst. As the accessible area was ca. 30 cm2, the ratio of membrane area to catalyst mass is in the range specified by Eq. (35). Consequently, there should be sufficient membrane area available to remove significant amounts of hydrogen and, therefore, to have an effect on the reaction process. 

In addition, Vycor can be shaped and/or formed while in its borosilicate state before it is transformed into Vycor. Thus, molded, pressed, tapered, and other shapes that would otherwise be very difficult, expensive, and/or impossible in fused silica can be done (relatively) easily with the pre-Vycor material with much less energy. Once the manufacturing is complete, the glass can then be processed to Vycor. 

The pyrolysis reactors were similar to those used earlier (1,2) they were 1.1 to 1.26-cm i.d. tubes that were heated in an electrical resistance furnace over a length of about 48 cm. The materials of construction in the four reactors used in this investigation were as follows Incoloy 800, stainless steel 304, Vycor glass, and alonized Incoloy 800. The latter reactor was prepared by Alon Processing, Inc. of Tarentum, Pennsylvania. 

In smaller pores where the majority of the molecules are in the proximity of the walls, the presence of an immobile superficial layer limits even more the diffusion coefficient by reducing the effective pore diameter as a result in saturated Vycor (pore size 29 A) the movement of particles is about two times slower than the one in bulk phase, while in MCM-41-S (pore diameter 25 A) and MCM-48-S (pore diameter 22 A) the movement of particles is about four to ten times slower. The limit case is an in-file diffusion process, where particles are unable to pass each other in a channel. 

---