Time-temperature reduction

The products prepared at temperatures below 450° and above 600 0 have a lower activity and require a greater length of time for reduction to platinum black than those prepared at temperatures between 450° and 6oo°. 

Catalyst Activation Gas phase activation of supported DENs was examined using in-situ FTIR spectroscopy and FTIR spectroscopy of adsorbed CO. For in-situ dendrimer decomposition studies, the spectra were collected under a gas flow composed of 20% 02/He or 20% H2/He. The supported DEN sample was pressed into a self-supporting wafer, loaded into a controlled atmosphere IR cell, and collected as the sample background. The temperature was raised stepwise and spectra were collected at each temperature until little or no change was observed. After oxidation, the sample was reduced in 20% H2/He flow with various time/temperature combinations. The sample was then flushed with He for lhr at the reduction temperature. After cooling under He flow, a background spectrum was collected at room temperature. A 5% CO/He mixture was flowed over the sample for 15 minutes, followed by pure He. IR spectra of CO adsorbed on the catalyst surface were collected after the gas phase CO had been purged from the cell. 

Besides the main depolymerization reactions, side reactions should also be considered in the kinetic description of a PET recycling process. This is emphasized by the results obtained from a PET extrusion model [85] shown in Figures 2.19-2.23. The complete set of reactions summarized below in Table 2.10 have been used, but shear effects have not been taken into account. Chain degradation, accompanied by a significant reduction of intrinsic viscosity, occurs even within residence times of a few minutes. Carboxyl end groups, vinyl end groups and acetaldehyde are formed in amounts depending on residence time, temperature and initial moisture content of the PET flakes. 

This conversion is directed so as to create the most favorable conditions for the delocalization of the nnpaired electron within the aromatic nucleus. It is worth noting here that thermal treatment (150—190°C) also initiates isomerization of the initial neutral molecule of norcara-diene into the benzotropylidene system. At the same time, the reductive transformation of Scheme 6.33 proceeds smoothly even at negative temperatures. Under comparable reaction conditions (25°C), the rate of conversion of the neutral molecule is 15 orders lower than that of the anion-radical. 

By working with vinyl ester (specifically DERAKANE 411-C50), Michaud [9] observed a drastic influence of the fibers (E-glass) on the induction time (i.e., the time required for the reaction to become observable at a specific temperature). An induction time of 90 minutes and 5.5 hours was necessary for RTM runs at 53°C and DSC runs at 50°C, respectively. Lee and Lee [33] affirmed that the age of the resin can also alter the induction time A reduction time of 50 percent was found after 10 months of storage at 70°C. All these effects are more evident at low temperatures. 

The distilled product slowly deposits mercury. In an effort to determine whether extent of this deposition is reduced by extending the time of reduction, the checkers found that stirring the crude alcohol with Celite for 15 hours, followed by filtration and distillation, did not diminish the amount of mercury deposited. However, in a typical run where the yield of distilled alcohol was 24.6 g. (72.0%), after standing for 24 hours, the distillate was decanted from the deposited mercury and redistilled to give 21.4 g. of 1-methylcyclohexanol which did not deposit mercury upon standing for one week at room temperature. The yield of twice-distilled alcohol was 02.6%. 

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