Large molecules, production

Figure B2.5.18 compares this inter molecular selectivity with intra molecular or mode selectivity. In an IR plus UV, two-photon process, it is possible to break either of the two bonds selectively in the same ITOD molecule. Depending on whether the OFI or the OD stretching vibration is excited, the products are either IT -t OD or FIO + D [24]- hr large molecules, mirmnolecular selectivity competes with fast miramolecular (i.e. unimolecular) vibrational energy redistribution (IVR) processes, which destroy the selectivity. In laser experiments with D-difluorobutane [82], it was estimated that, in spite of frequency selective excitation of the...

Membrane Sep r tion. The separation of components ofhquid milk products can be accompHshed with semipermeable membranes by either ultrafiltration (qv) or hyperfiltration, also called reverse osmosis (qv) (30). With ultrafiltration (UF) the membrane selectively prevents the passage of large molecules such as protein. In reverse osmosis (RO) different small, low molecular weight molecules are separated. Both procedures require that pressure be maintained and that the energy needed is a cost item. The materials from which the membranes are made are similar for both processes and include cellulose acetate, poly(vinyl chloride), poly(vinyHdene diduoride), nylon, and polyamide (see AFembrane technology). Membranes are commonly used for the concentration of whey and milk for cheesemaking (31). For example, membranes with 100 and 200 p.m are used to obtain a 4 1 reduction of skimmed milk. 

Activated carbons for use in Hquid-phase appHcations differ from gas-phase carbons primarily in pore size distribution. Liquid-phase carbons have significantly more pore volume in the macropore range, which permits Hquids to diffuse more rapidly into the mesopores and micropores (69). The larger pores also promote greater adsorption of large molecules, either impurities or products, in many Hquid-phase appHcations. Specific-grade choice is based on the isotherm (70,71) and, in some cases, bench or pilot scale evaluations of candidate carbons. 

Nylon, also a linear polymer, is made by a condensation reaction. Two different kinds of molecule react to give a larger molecule, and a by-product (usually HjO) the ends of large molecules are active, and react further, building a polymer chain. Note how molecules of one type condense with those of the other in this reaction of two symmetrical molecules... 

Elimination reactions of fluorine compounds are not limited to the removal of simple molecules Frequently, large molecules or combination of smaller ones are formed as by-products, especially in pyrolytic reactions For example perhalo genated acid chlorides lose not only carbon monoxide but also chlorine fluoride [106, 107] (equations 74 and 75)... 

The ability to remove particulates has made RO indispensable in the production of ultra-pure water for microchip washing. Its ability to remove large molecules enables it to produce pyrogen-free water for the pharmaceuticals industry. In the USA and elsewhere RO is permitted for producing the water used in making up injectable preparations. The European Pharmacopoeia still insists on distillation for this, but the larger amounts of water needed for ampoule washing, etc. are often purified by RO. 

In conclusion, there appears to be some supporting evidence, other than these tracer studies, that the synthesis of HMX and RDX molecules can be accomplished thru a build-up from single methylene-containing spedes or other small molecules, and that this route can also take place via a total degradation and resynthesis from molecules such as Hexamine. However, die development of an economical process for the large scale production of these expl nitramines, in particular HMX, via a method precluding the use of Hexamine, is vet to be accomplished... 

The small molecules used as the basic building blocks for these large molecules are known as monomers. For example the commercially important material poly(vinyl chloride) is made from the monomer vinyl chloride. The repeat unit in the polymer usually corresponds to the monomer from which the polymer was made. There are exceptions to this, though. Poly(vinyl alcohol) is formally considered to be made up of vinyl alcohol (CH2CHOH) repeat units but there is, in fact, no such monomer as vinyl alcohol. The appropriate molecular unit exists in the alternative tautomeric form, ethanal CH3CHO. To make this polymer, it is necessary first to prepare poly(vinyl ethanoate) from the monomer vinyl ethanoate, and then to hydrolyse the product to yield the polymeric alcohol. 

A significant proportion of the fine chemicals are complex, multifunctional large molecules. These molecules are labile, unstable at elevated temperature, and sensitive towards (occasionally even minor) changes in their environment (e.g. pH). Therefore, processes are needed with inherent protective measures (e.g. chemical or physical quenching) or a precise control system to operate exactly within the allowable range. Otherwise the yield of the desired product can drop to nearly zero. 

This chapter discusses the synthesis, characterization and applications of a very unique mesoporous material, TUD-1. This amorphous material possesses three-dimensional intercoimecting pores with narrow pore size distribution and excellent thermal and hydrothermal stabilities. The basic material is Si-TUD-1 however, many versions of TUD-1 using different metal variants have been prepared, characterized, and evaluated for a wide variety of hydrocarbon processing applications. Also, zeolitic material can be incorporated into the mesoporous TUD-1 to take the advantage of its mesopores to facilitate the reaction of large molecules, and enhance the mass transfer of reactants, intermediates and products. Examples of preparation and application of many different TUD-1 are described in this chapter. 

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