Pollutec Recycling

Polymertocatalyst 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 carried out on a TGA equipment Figure 7.1 [9] using high-density polyethylene (HDPE) as raw material at different polymer to catalyst mass ratios showed the existence of a limit below which the addition of more catalyst does not change the degradation pattern. In the absence of catalyst, the polymer (HDPE) degradation pattern showed a very steep decrease at about 773 K. The degradation set up very late at quite high temperature, but it progressed very rapidly after initiation. In the presence of catalyst, the polymer degradation occurred at much lower temperatures and more gradually. Even with the smallest catalyst amount (polymer: catalyst = 9 : 1) the degradation commenced at much lower temperature than in the absence of catalyst. As more catalyst was added the reaction proceeded at enhanced rates. At polymer-to-catalyst mass ratios 1:2, 1:1 and 2:1 the polymer degradation curves were very similar.

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.

Figure 7.1 TGA graphs of HDPE at various polymer-to-US-Y zeolite ratios. Heating rate, 5 K/min; nitrogen flow, 50 mLN/min. (From [8]. Reproduced by permission of the American Chemical Society)

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Figure 7.1 TGA graphs of HDPE at various polymer-to-US-Y zeolite ratios. Heating rate, 5 K/min; nitrogen flow, 50 mLN/min. (From [8]. Reproduced by permission of the American Chemical Society)

300 400 500 600 700 800

300 400 500 600 700 800

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 filled 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.

TGA experiments however do not reveal the whole picture. They show only the overall change of the polymer mass without any indication of product distribution. Therefore further experiments were carried out with linear low-density polyethylene (LLDPE), this time using a laboratory semi-batch reactor with quantities at gram levels rather than milligrams. In these experiments various liquid fractions were collected at different stages. Surprisingly, no difference in the liquid yield, neither conversion nor liquid selectivity, was observed in an even wider range of USY-to-polymer ratio (Figure 7.2).

However this figure shows only part of the truth. If the results are plotted for all three stages of the temperature programme used (Figure 7.3), the systems with the most zeolite (US-Y), i.e. zeolite-to-polymer ratio of 1:1 and 1:2, form more liquid (indicative here of higher conversion) than the other two systems at the first stage, i.e. lower temperature (T = 573 K).

However this is compensated by the temperature increase. At the second stage (T = 633 K) the two systems with the lowest zeolite amounts, i.e. zeolite-to-polymer ratio of 1:3 and 1:4, form the highest liquid fraction amount. At the third stage (T = 673 K) the degradation had been already completed for all systems. Minimal liquid amounts were formed.

In order to confirm these results, a linear heating programme has been applied where the relatively mild final temperature was reached only at the end of the experiment, not leaving further time at the top temperature to react. The presence of more catalyst has indeed formed more liquid products (Figure 7.4), confirming the above findings.

Polymer to catalyst ratio

Figure 7.2 Conversion and liquid selectivity during degradation of LLDPE over US-Y zeolite at different ratios and a step temperature programme (0-5 min:573 K, 5-10 min: 633 K, 10-15 min:673 K)

Conversion -*— Selectivity

• t
• t

MICROPOROUS MATERIALS 100

80 60 40

1 20 LL

0 - 5 min (573 K) 5 - 10min (633 K) 10 - 15min (673 K)

Figure 7.3 Liquid formation during degradation of LLDPE over US-Y zeolite at different ratios

□	Pol:Cat =	1
s	Pol:Cat =	2
m	Pol:Cat =	3
a	Pol:Cat =	4

Figure 7.4 Conversion and liquid selectivity during degradation of LLDPE over US-Y zeolite at different ratios and a linear temperature programme (16 K/min to 633 K)

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