Real-time online study of the mechanism of ABS-catalysed pyrolysis using photoionisation mass spectrometry

Applications: Research into the catalytic pyrolysis mechanisms of copolymers; resource recovery from waste plastics;

Introduction

ABS (acrylonitrile–butadiene–styrene) copolymer is a major component of waste electronic and electrical plastics; its pyrolysis products contain a large amount of nitrogen-containing compounds, which severely affect the quality of the fuel oil. This study utilises vacuum ultraviolet photoionisation mass spectrometry (SVUV-PIMS) technology to systematically investigate the performance of three molecular sieve catalysts—HZSM-5, HUSY and Al-MCM-41—during the catalytic pyrolysis of ABS, thereby providing a scientific basis for the optimisation of catalytic denitrification processes.

‌Materials and Methods

Instrumentation: Vacuum ultraviolet photoionisation mass spectrometry system + tube furnace pyrolysis reactor

Samples‌: ABS copolymer (A:B:S = 22:17:61)

‌Catalysts‌: HZSM-5, HUSY, Al-MCM-41
HZSM-5 (SiO₂/Al₂O₃ = 25, pore size = 0.55 nm)

HUSY (SiO₂/Al₂O₃ = 11, pore size = 0.74 nm)

Al-MCM-41 (SiO₂/Al₂O₃ = 25, pore size = 4–6 nm)

‌Experimental temperature‌:

Isothermal pyrolysis: 500 °C

Programmed temperature ramp: 100–700 °C (heating rate 10 °C/min)

‌Photoionisation energies‌: 8.5, 10.6, 12.0, 14.0 eV

‌Mass spectrometry acquisition interval‌: 5 s

‌Results and Conclusions

1. Comparison of Catalyst Performance

As shown by the NH₃-TPD (ammonia temperature-programmed desorption) curves (Figure 1), the three catalysts exhibited distinct desorption characteristics:

‌HZSM-5 and HUSY‌: The desorption peaks at 150–300 °C correspond to the desorption of ammonia from weak acid sites, whilst those at 300–550 °C correspond to the desorption of ammonia from strong acid sites.‌ Acid site distribution‌: Both weak and strong acid sites are present.‌ HZSM-5 has a higher acid density than HUSY.

‌Al-MCM-41‌: Only a single‌ weak desorption peak‌ is observed, indicating that this mesoporous material‌ primarily contains weak acid sites.‌

Figure 1   NH₃-TPD curves of HZSM-5, HUSY and Al-MCM-41

Upon the onset of ABS thermal decomposition, some primary products with suitable molecular diameters diffuse into the catalyst pores, where resistance is lower, and subsequently undergo a series of cracking, cyclisation and dehydrogenation reactions at the active sites, ultimately leading to coking.

The thermogravimetric analysis (TG) curves indicate that catalysts with larger pore sizes (such as Al-MCM-41 and HUSY) facilitate molecular diffusion and coking, whilst HZSM-5 exhibits better resistance to coking during thermal degradation. Pure ABS leaves virtually no residue at 800 °C, whereas the addition of a catalyst results in a significant amount of residue.

Compared with pure ABS (Figure 2), the DTG curves of the various ABS/catalyst samples show a slight shift of approximately 10 °C towards higher temperatures. This may be due to the heat transfer limitation effect of the solid catalyst, as well as radical termination reactions on the catalyst surface.‌

Figure 2  TG and DTG curves of pure ABS and ABS/catalyst samples

‌2. Product selectivity and denitrogenation efficiency of different catalysts during the pyrolysis of ABS‌

2.1 Major product types

① Nitrogen-containing products‌:

‌HCN‌ (m/z 27),‌ acrylonitrile‌ (m/z 53) and other light nitrogen-containing compounds.‌ Phenylnitrile dimers‌ (e.g.  m/z 145, 157) are the primary source of nitrogen in the pyrolysis oil.

② Aromatic products:

Valuable chemicals such as benzene (m/z 78), toluene (m/z 92), ethylbenzene (m/z 106) and styrene (m/z 104).

Figure 3  Mass spectrometry spectra of pyrolysis products from pure ABS and ABS/catalyst samples at 500 °C under different photon energies

Note: Peak intensities marked in red indicate mass spectrometry peaks exceeding the range‌

2.2 Effect of different catalysts on product distribution‌

‌HZSM-5 and HUSY‌ (zeolite catalysts): significantly suppress the formation of nitrogen-containing dimers/trimers, whilst promoting the formation of light nitrogen-containing gases (such as NH₃ and HCN) and aromatic hydrocarbons. ‌Al-MCM-41‌ (mesoporous material): has a minor effect on the product distribution, but tends to favour the formation of polycyclic aromatic hydrocarbons.

Figure 4: Effect of different catalysts on the relative intensities of nitrogen-containing products (a), monoaromatic products (b) and diaromatic products (c)

2.3 Temperature-dependent evolution curves of typical products and analysis of the catalytic mechanism‌

Figure 5 shows the temperature-dependent peak area curves for eight aromatic products in pure ABS and ABS/catalyst samples.‌

①‌Pure ABS (Figures 5a–c)‌: All aromatic products are formed almost simultaneously, following a radical chain reaction mechanism.

② HZSM-5 catalytic system: Benzene is formed at a relatively low temperature; the formation temperature of benzene (~420 °C) precedes that of styrene (~460 °C), indicating that it is the primary catalytic cracking product, with the reaction pathway involving proton attack on the ortho position of the benzene ring to form a carbocation, followed by β-cleavage to directly yield benzene. The formation of styrene and α-methylstyrene is significantly delayed and yields are reduced, suggesting that they are primarily formed via non-catalytic pathways. Indene (C₉H₈) and naphthalene (C₁₀H₈) exhibit a two-stage evolution: the first stage arises from the cyclisation of intermediates following benzene decomposition, whilst the second stage is attributed to secondary reactions involving primary products (such as styrene).

③ HUSY catalytic system: Benzene and ethylbenzene are formed synergistically in the low-temperature region, and the ethylbenzene yield is significantly higher than that on HZSM-5, demonstrating that intermolecular hydrogen transfer reactions occur preferentially. The yields of styrene and α-methylstyrene drop sharply, confirming that the cage-like structure of HUSY significantly promotes hydrogen transfer.

④ Al-MCM-41 catalytic system: The temperature-dependent product yield curves are similar to those of pure ABS, with only a slight variation in intensity, indicating that its weak acidity has a limited influence on the reaction pathway. The macroporous structure leads to delayed product release (curve tailing), which may be related to coke blocking the pore channels.

Figure 5 Temperature evolution curves of the main aromatic products for pure ABS (a–c), ABS/HZSM-5 (d–f), ABS/HUSY (g–i) and ABS/Al-MCM-41 (j–l). The styrene curves for each sample are shown as dashed lines in the corresponding figures.

Figure 6 shows the temperature-dependent changes in the peak areas of seven nitrogen-containing products in pure ABS and ABS/catalyst samples.‌‌

①‌ Non-catalysed pyrolysis (pure ABS)‌: All nitrogen-containing products (e.g. acrylonitrile, HCN, NH₃) are generated simultaneously, consistent with the radical degradation mechanism of the PAN unit.

②‌ HZSM-5 catalytic system:‌‌ The maximum peak temperatures for light nitriles (e.g. C₃H₃N) and HCN‌ occur earlier, indicating that they are formed via protonation and cracking.

③‌ HUSY catalytic system:‌‌ The yields of NH₃ and HCN‌ increase significantly, demonstrating that strong Brønsted acid sites (BAS) promote the secondary cracking of nitriles.

④‌Al-MCM-41 catalytic system:‌ The temperature-dependent curves for nitrogen-containing products are essentially consistent with those of the non-catalytic system, but the yields of NH₃ and HCN are slightly higher, indicating that its weak acidity triggers only a limited number of secondary reactions.

Figure 6  Temperature evolution curves of the main nitrogen-containing products for ABS (a–c), ABS/HZSM-5 (d–f), ABS/HUSY (g–i) and ABS/Al-MCM-41 (j–l).

This study systematically analysed the pyrolysis behaviour of ABS under the action of different zeolite catalysts using SVUV-PIMS technology. This technique provides important technical support for research into the mechanisms of catalytic pyrolysis of complex copolymers and holds significant practical value in the field of resource recovery from waste plastics.

References

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