Monofunctional catalysts

In addition to production of simple monofunctional products in hydrocarbon oxidation there are many complex, multifimctional products that are produced by less weU-understood mechanisms. There are also important influences of reactor and reaction types (plug-flow or batch, back-mixed, vapor-phase, Hquid-phase, catalysts, etc). 

Monofunctional, cyclohexylamine is used as a polyamide polymerization chain terminator to control polymer molecular weight. 3,3,5-Trimethylcyclohexylamines ate usehil fuel additives, corrosion inhibitors, and biocides (50). Dicyclohexylamine has direct uses as a solvent for cephalosporin antibiotic production, as a corrosion inhibitor, and as a fuel oil additive, in addition to serving as an organic intermediate. Cycloahphatic tertiary amines are used as urethane catalysts (72). Dimethylcyclohexylarnine (DMCHA) is marketed by Air Products as POLYCAT 8 for pour-in-place rigid insulating foam. Methyldicyclohexylamine is POLYCAT 12 used for flexible slabstock and molded foam. DM CHA is also sold as a fuel oil additive, which acts as an antioxidant. StericaHy hindered secondary cycloahphatic amines, specifically dicyclohexylamine, effectively catalyze polycarbonate polymerization (73). 

Polyether Polyols. Polyether polyols are addition products derived from cyclic ethers (Table 4). The alkylene oxide polymerisation is usually initiated by alkah hydroxides, especially potassium hydroxide. In the base-catalysed polymerisation of propylene oxide, some rearrangement occurs to give aHyl alcohol. Further reaction of aHyl alcohol with propylene oxide produces a monofunctional alcohol. Therefore, polyether polyols derived from propylene oxide are not truly diftmctional. By using sine hexacyano cobaltate as catalyst, a more diftmctional polyol is obtained (20). Olin has introduced the diftmctional polyether polyols under the trade name POLY-L. Trichlorobutylene oxide-derived polyether polyols are useful as reactive fire retardants. Poly(tetramethylene glycol) (PTMG) is produced in the acid-catalysed homopolymerisation of tetrahydrofuran. Copolymers derived from tetrahydrofuran and ethylene oxide are also produced. 

The terminal R groups can be aromatic or aliphatic. Typically, they are derivatives of monohydric phenoHc compounds including phenol and alkylated phenols, eg, /-butylphenol. In iaterfacial polymerization, bisphenol A and a monofunctional terminator are dissolved in aqueous caustic. Methylene chloride containing a phase-transfer catalyst is added. The two-phase system is stirred and phosgene is added. The bisphenol A salt reacts with the phosgene at the interface of the two solutions and the polymer "grows" into the methylene chloride. The sodium chloride by-product enters the aqueous phase. Chain length is controlled by the amount of monohydric terminator. The methylene chloride—polymer solution is separated from the aqueous brine-laden by-products. The facile separation of a pure polymer solution is the key to the interfacial process. The methylene chloride solvent is removed, and the polymer is isolated in the form of pellets, powder, or slurries. 

To obtain high molecular weight polymers the tetramer is equilibrated with a trace of alkaline catalyst for several hours at 150-200°C. The product is a viscous gum with no elastic properties. The molecular weight is controlled by CcU eful addition of monofunctional material. 

Dehydrogenase modules within larger assemblies as monofunctional catalysts... 

For reduction of monofunctional ketones, the most effective catalysts include diamine ligands. The diamine catalysts exhibit strong selectivity for carbonyl groups over carbon-carbon double and triple bonds. These catalysts have a preference for equatorial approach in the reduction of cyclohexanones and for steric approach control in the reduction of acyclic ketones.51... 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

As mentioned, cyclopentanes can be formed with monofunctional catalysts, and so even with dual function catalysts, one would expect some of the cyclopentanes to form via mechanisms associated with the platinum reactivity part of the dual functionality. 

It now remains to place the concept of an ester as an active species into a wider chemical context, with special reference to polymerisation catalysts. Sinn and Patat [39] have emphasised the distinction between monofunctional and bifunctional catalytic systems and this distinction is obviously and necessarily related to the idea, explained above, that there is a difference in kind between polarised molecules and the ions which can be formed from them. Whereas the carbonium and other cations as reactive species are monofunctional, the esters evidently belong to the class of bifunctional catalysts their mode of action - the addition of their constituent parts across a double bond - is, in modern terminology, an insertion reaction. In this context, we must note the important... 

Bifunctional catalysis is one of the most important routes to green (more economical and more environmentally friendly) processes. Indeed, the deactivation of bifunctional catalysts by coking is much slower than that of monofunctional catalysts and their selectivity generally higher (e.g., hydrocracking compared to... 

To date, only a few iridium catalysts have been applied to industrially relevant targets, especially on the larger scale. It is likely that several types of Ir catalyst are, in principle, feasible for technical applications in the pharmaceutical and agrochemical industries. At present, the most important problems are the relatively low catalytic activities of many highly selective systems and the fact, that relatively few catalysts have been applied to multifunctional substrates. For this reason, the scope and limitations of most catalysts known today have not yet been explored. For those in academic research, the lesson might be to employ new catalysts not only with monofunctional model compounds but also to test functional group tolerance and-as has already been done in some cases-to apply the catalysts to the total synthesis of relevant target molecules. 

C10-C14 long paraffin dehydrogenation is a key-step for linear alkyl benzene (LAB) production. However, this reaction, which requires monofunctional catalysis, is implemented on Pt-Sn catalysts deposited on controlled acidity alumina. It is generally associated with several secondary reactions, among which aromatic formation is extremely problematic it is catalyzed by a metallic phase (M) or by residual support (A) activity. Indeed, on the one hand, these arylaromatics are very good coke precursors and are consequently responsible for a large part of the... 

In contrast to monofunctional (thio)urea organocatalysts, bifunctional catalyst structures enable simultaneous coordination, activation, and suitable relative orientation of both reaction components (the electrophile and the nucleophile) resulting in high... 

Maruoka reported the use of the didentate catalyst 8 for double electrophilic activation of carbonyl compounds [70], but since no comparison with monofunctional phenolates was given it is not clear whether having two aluminium centres in the same catalyst offers any special advantages. They used this catalyst to effect transfer hydrogenation between remote aldehyde and alcohol groups in the same molecule [71], but again it is not clear whether the transfer is truly intramolecular or in any way different from that of reduction by an external alcohol using 8 or a monuclear aluminium catalyst. 

The terms in Equation 1.3 (Malkin s autocatalytic model) are described in Nomenclature. In Malkin s autocatalytic model, the concentration of the activator, [A], is defined as the concentration of the initiator times the functionality of the initiator. For a difimctional initiator [e.g., isophthaloyl-bis-caprolactam, the concentration of the activator (acyllactam) is twice the concentration of the initiator]. The term [C] is defined as the concentration of the metal ion that catalyzes the anionic polymerization of caprolactam. In a magnesium-bromide catalyzed system, the concentration of the metal ion is the same as the concentration of the caprolactam-magnesium-bromide (catalyst) because the latter is monofunctional. 

Grafting of CTA segments Monofunctional CTA segments were attached to HPL by dissolving both components in ethylene chloride (ca. 20% concentration) and heating the mixture for 1 hour in the presence of T9 catalysts (Union Carbide). The reaction product was poured onto a... 

Aromatization, however, may also be envisaged as taking place via stepwise dehydrogenation of an unbranched hydrocarbon molecule followed by ring closure of the polyunsaturated intermediates. In fact, the formation of dienes was proved during the aromatization of C6 and C7 alkanes to the corresponding aromatics over monofunctional metal oxides and metal black, and bifunctional catalysts.307 308 Radiotracer studies even allowed the detection in very low concentration of hexatriene during the aromatization of n-hexane over Pt black.309 It was also proposed that aromatics are formed from the cis isomers, whereas trans isomers may be coke precursors.213 Direct experimental evidence has recently been... 

Additional evidence to this scheme was reported applying temporal analysis of products. This technique allows the direct determination of the reaction mechanism over each catalyst. Aromatization of n-hexane was studied on Pt, Pt—Re, and Pd catalysts on various nonacidic supports, and a monofunctional aromatization pathway was established.312 Specifically, linear hydrocarbons undergo rapid dehydrogenation to unsaturated species, that is, alkenes and dienes, which is then followed by a slow 1,6-cyclization step. Cyclohexane was excluded as possible intermediate in the dehydrocyclization network. 

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