Polymer-to-catalyst ratio

An important aspect in a future catalytic process for degradation of plastic waste is the amount of catalyst used in such a process. In a batch system, which is the most broadly used so far to test the performance of various catalytic systems, the amount of catalyst is characterized by the polymer-to catalyst mass ratio. Results from initial experiments 

These results have been confirmed by estimating the activation energies at various ratios by a series of experiments with different heating rates [31]. The activation energy of pure thermal degradation of HDPE in the absence of catalyst was considerably higher (61 kcal/mol) than even the one with only 10% of zeolite USY (HDPE US-Y = 9 1), 47 kcal/mol. However at HDPE US-Y ratios 2 1, 1 1, 1 2 the difference of the activation energy was minimal, 25, 24 and 22 kcal/mol respectively. 

All the above results indicated the possible existence of a limiting step over the whole reaction process. It is reasonable to assume that large macromolecules had to react on the external surface of the zeolite catalyst first, which could be the limiting reaction step. 

Smaller cracked fragments diffused subsequently into the zeolite pores and underwent further reactions. It seemed that the addition of more zeolite above a specific quantity, corresponding to a polymer-to-catalyst ratio between 1 1 and 2 1, did not increase the overall degradation rate. 

The melted polymer resided in the voids of the zeolitic bed. When the amount of polymer was high (high polymer-to-catalyst ratio), polymer fiUed these voids fully with the excessive polymer mass not being in contact with zeolite. The more zeolite was added, the more polymer was in contact with it and more polymer participated in the initial degradation step. This was true to a point when the added zeolite was no longer in contact with plastic. In the last case the excessive zeolite did not contribute to the initial degradation step of the large macromolecules. 

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Figure 7.7 Boiling point distribution of liquid fuel formed over US-Y zeolite, a commercial cracking catalyst, a pillared clay (polymer-to-catalyst ratio 2 1) and comparison with a commercial gasoline sample...

However all the samples heated in the presence of US-Y catalyst (polymer-to-catalyst mass ratio 2 1) showed a deviation from the original polymer molar mass distribution in the region of lower molar masses. In the first experiment, the polymer/US-Y-zeolite sample was exposed at a temperature of 378 K, which is below its melting point, for 120 min and then for 30 min at 418 K. No volatile products were initially observed, but traces of isobutane and isopentane were detected when the temperature was raised to 418 K. Although these conditions were much milder than in the equivalent experiment with pure polymer (curve number 2), the molar mass distribution, curve number 5 in Figure 7.5, was different from that of the original polymer. 

In contrast, the carbene complexes 15 and 16 could be used very efficiently in pure liquid or supercritical CO2, giving high yields of polynorbomenamer with very similar cis trans ratios of the double bonds to those obtained with the same catalyst in CH2CI2 [6]. More recent studies showed that the robust complex 16 can be used at substrate to catalyst ratios up to 5300 without deactivation [100]. Fairly broad molecular weight distributions were observed when 16 and norbomene were placed together in the reactor, but narrow distributions could be achieved if the catalyst was injected as a solid or in solution to the supercritical mixture of alkene and CO2. The polymer morphology was very similar to samples prepared with 16 under conventional conditions. 

Aguado et al studied the conversion of polyethylene over MCM-41 with a Si/ Al ratio of 29.4 in a semibatch reactor at 673 K. At a polymer to catalyst mass ratio of 4, the polymer was almost completely converted to low molecular weight products with carbon numbers less than 40. The products were centered in the C5-C12 range (59 wt%), but 20 wt% C1-C4 products and 8 wt% aromatic products... 

Chemical polynKrizations were carried out in 10 ml Schlenck tub. The tubes were treated with trimethylsilyl chloride, washed with three S ml portions of methanol, dried at 120 in a oven for 12 h, flame dried and kept in a desiccator to cool down to room temperature. In a glove bag maintained under nitrogen atmosphere the lactone mixture and the catalyst (0.1 molar solution in toluene, monomer to catalyst ratio was 1 2(X)) were transferred into the polymerization tube ami capped with a rubber septum. The tubes were degassed by several vacuum purge cycles to remove the solvent in the catalyst solution and then placed in an oil bath maintained at 120 for 12 h. At the end of die reaction period, the crude sample was collected to estimate the conversion and the contents of the tubes woe dissolved in chloroform (0.5 ml) and precipated in methanol (30 ml) by vigorous stirring and methanol decanted. The precipitate was further washed with methanol (2 X 20 ml). The polymer was dried in a vacuum oven at 40 °C for 24 h and GPC data were recorded. 

A summary of ADMET polymerization of Ge- and Sn- containing polymer (1) and (2) in terms of polymerization conditions, polymer yield, and molecular weight is given in Table 1. It shows that the electrochemically reduced catalyst is an active system toward ADMET polymerization because quantitative yield for polymer (1) and (2) is 68% and 92% depending on the monomer to catalyst ratio and reaction time. The slightly higher trans polymers were obtained by this catalyst system with high yields in a short periods. 

As the monomer to catalyst ratio was increased, yield of the polymer also increased reaching to a maximum value which is around 100 1 polymer (1) or 90 1 polymer (2), and further increase of the ratio caused a decrease in the yield of the polymer. A maximum yield of 68% and 92% for polymer (1) and (2) was obtained. At high monomer concentrations deactivation of the active catalyst may occur which results in low yield of polymer. 

This group of experiments were performed for different reaction times (180-3200 min). The monomer to catalyst ratio was kept at 100 1 for polymer (1) or 90 1 for polymer (2), and the reaction was quenched by the addition of methanol after a certain time from the start of reactioa Figure 2 shows the influences of different reaction times on the amount of polymers (1) and (2). Polymerization yield first increased with time and almost reached a plateau value around 36-48 h. 

The ADMET polymerization yield of Ge- (1) and Sn (2)-containing polymers reaches a maximum at a total monomer to catalyst ratio of about 100 1 and 90 1, respectively. Polymerization reaction completed at around 36-48 h. 

A representative example of a metal-based catalysis is the Ziegler-Natta process, which has been gradually improved to such an extent that the monomer-to-catalyst ratio is typically in the range 10 -10 . As a matter of fact, removal of the metallic catalyst is not needed and commodity polymers obtained in this way (polyolefins) are directly processed without any purification step. In contrast, the synthesis of specialty polymers used in niche applications requires larger amounts of metallic catalysts (the monomer-to-catalyst ratios being 10 -10 ). To prevent hazards due to the presence of toxic metallic species that can freely migrate out of the polymeric material when in service, a purification process is sometimes required which... 

In a 10 mL Schlenk flask, 0.25 g of the monomer were dissolved in 3 mL of dry toluene and the Pt-NHC complex (catalyst) was added (substrate to catalyst ratio = 1000/1). The reaction mixture was stirred at 60°C for 48 h. After the mixture had cooled to room temperature, the solvent was removed in vacuum. The polymers were obtained as sticky, colorless to brownish liquids to solids, depending on the degree of polymerization. 

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