Real-time online study of the catalytic pyrolysis of polypropylene using photoionisation mass spectrometry

Areas of Application: Research into the mechanisms of polymer pyrolysis; evaluation of catalyst performance;

Introduction

The volume of waste polymers is increasing rapidly in line with rising consumption, and pyrolysis technology offers broad prospects for application as a means of fuel recovery. In this study, single-photon ionisation time-of-flight mass spectrometry (SPI–TOFMS) was employed to investigate in situ the pyrolysis and catalytic decomposition of polypropylene on HZSM-5 molecular sieves, thereby providing a new technical approach for studying the mechanisms of polymer pyrolysis and evaluating catalysts.

Experimental Materials and Methods

‌Instrumentation:‌ Tubular furnace coupled with SPI−TOFMS system

‌Reactor:‌ Quartz tube

‌Samples:‌ Pure polypropylene, HZSM-5 zeolite catalyst (Si/Al = 36)

‌Catalyst content:‌ 20%, 30%, 40%, 50% (weight percentage)

‌Carrier gas: N₂

Flow rate: 200 SCCM

‌Temperature settings:‌ 300, 400, 500, 600, 700 °C

Results and Conclusions

1. Identification of pyrolysis products

Three main categories of pyrolysis products were detected by SPI-TOFMS:‌ alkenes: C3–C6 compounds (propane, butene, etc.);‌ dienes: including 1,3-butadiene, etc.;‌ aromatic compounds:‌ benzene, toluene, xylene (BTX).

Figure 1: Mass spectra of volatile pyrolysis products from pure polypropylene and polypropylene/50% HZSM-5

2. Effect of temperature on pyrolysis products

2.1 Pyrolysis of pure polypropylene

The relative intensities of low-molecular-weight alkenes and alkenes increase with rising temperature (Figure 2a); the intensities of high-molecular-weight products (m/z > 110) decrease with rising temperature; aromatic compounds are only detectable at 700 °C.

2.2‌ Catalysed Pyrolysis (Polypropylene/50% HZSM-5)‌

The onset temperature of pyrolysis was reduced to 300 °C (Figure 1f); low-molecular-weight alkenes were predominant at all reaction temperatures (Figure 2b), alkenes were produced in negligible amounts, and aromatic compounds were detectable as early as 300 °C.

Figure 2 Effect of reaction temperature on the intensity of major pyrolysis products of pure polypropylene (a) and polypropylene/50% HZSM-5 (b)

3. Trends in the signal intensity of pyrolysis products over time

‌Under conditions ranging from 500 to 700 °C, the formation of all pyrolysis products was completed within 150 s; increasing the reaction temperature accelerated the reaction rate (Figures 3 and 4). A comparison of Figures 3 and 4 reveals that catalytic decomposition takes longer than non-catalytic decomposition. From a catalytic mechanism perspective, this may be because catalytic decomposition can be broadly divided into two stages:

‌Stage 1: The pore size of the HZSM-5 molecular sieve is 0.6 nm, which prevents large molecules from entering the acid sites within the cavities; consequently, large polymer fragments decompose on the outer surface of the catalyst.

‌Stage 2: The products enter the micropores to participate in secondary reactions.‌

Figure 3: Curves showing the variation of signal intensity over time for the main pyrolysis products of pure polypropylene

Figure 4 Time-dependent signal intensity curves for the main pyrolysis products of polypropylene/50% HZSM-5

4. Effect of different catalyst loadings on the polypropylene pyrolysis process

As the catalyst-to-polymer ratio increases, sufficient acid centres are provided for the polymer decomposition process, making non-catalytic processes less significant compared to catalytic ones. Consequently, as the catalyst content increases, the shape of the time-dependent product intensity profile becomes smoother, the decomposition process accelerates, and there is a positive correlation between catalyst content and product intensity.

Figure 5: Time-dependent curves of the main product intensities from the pyrolysis of polypropylene catalysed by HZSM-5 at 300 °C at different catalyst loadings

Note: (a and b) polypropylene/20% HZSM-5, (c and d) polypropylene/30% HZSM-5, (e and f) polypropylene/40% HZSM-5

Figure 6: Time-dependent intensity curves of the main products from the pyrolysis of polypropylene catalysed by HZSM-5 at 400 °C

Note: (a and b) polypropylene/20% HZSM-5, (c and d) polypropylene/30% HZSM-5, (e and f) polypropylene/40% HZSM-5

This study successfully utilised SPI-TOFMS technology to conduct real-time, in-situ investigations into the pyrolysis and catalytic decomposition processes of polypropylene. Through a comparative analysis of the product distribution and temporal evolution characteristics at different temperatures and catalyst contents, the significant effectiveness of HZSM-5 in reducing the reaction temperature and enhancing product selectivity was verified. This technique not only provides an efficient and reliable method for polymer pyrolysis research but also lays a solid foundation for catalyst performance evaluation and pyrolysis process optimisation.

References

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