Ontrafelen van de katalyse voor selectieve depolymerisatie van polyolefines

Plastics, predominantly derived from fossil-based resources, are deeply embedded in modern society due to their versatility across applications such as packaging, construction, automotive, and electronics. However, the widespread use of single-use plastics, especially polyethylene (PE) and polypropylene (PP) in packaging, has led to immense volumes of plastic waste that are often improperly managed via linear strategies, for instance landfilling or incineration. Recycling forms a sustainable alternative where plastic materials are converted to low-value recyclates (mechanical recycling), or to monomeric units or base chemicals (chemical recycling), keeping the carbon in the plastic lifecycle. Both recycling strategies act complementary, where streams too contaminated for mechanical recycling are destined for chemical recycling. Among chemical recycling, in-situ catalytic pyrolysis is a promising strategy where the polymer melt is directly contacted with a heterogeneous acid catalyst, for example zeolites, enabling an effective and selective molecular weight reduction to valuable hydrocarbons. While analogies can be drawn with fluid catalytic cracking (FCC) in petroleum refining, key differences in feedstock characteristics necessitate dedicated catalytic systems tailored to plastic waste. Such catalytic studies are preferably executed in a high-throughput manner, as novel materials are often costly. Therefore, testing on small (milligram)-scale is advised, avoiding time consuming test in lab- or pilot-scale setups. Thermogravimetric analysis (TGA) and micropyrolysis coupled to GC-analysis have been widely used for collecting preliminary information on catalyst performance for plastic conversion, which involves assessment of catalyst activity, stability and selectivity. Unfortunately, methodologies are scattered over literature reports with a wide range of screening conditions, incomplete assessments of catalyst performance, and inconsistent data analysis and interpretation. This complicates comparison of research outcomes over the community. Moreover, the extent to which results obtained at small (milligram)-scale are transferable to lab- or pilot-scale setups remains unexplored. This work presents a high-throughput screening methodology for evaluating catalyst performance for in-situ catalytic pyrolysis of PE and PP melts. The approach integrates 2D-TGA to assess catalyst activity and stability, quantified by well-thought metrics ΔTmidpoint and Coke content, respectively. Selectivity was probed using micropyrolysis-GCxGC-FID/MS, with the Average carbon number introduced as a selectivity descriptor. The transferability of the results is validated via lab-scale tests in a mechanically stirred semi-batch slurry reactor with continuous condensing of products, increasing the scale x1000. 2D-TGA was used for screening of more than sixty commercial catalysts (e.g. zeolites, amorphous and mesoporous silica-alumina, clays) with varying properties (crystallinity, pore dimensionality and size, Si/Al ratio). Based on their high activity and high stability, ZSM-5 (25) and ASA [40] HPV were found optimal for LDPE conversion, while USY (40) and ASA [70] HPV were chosen for PP conversion, highlighting polymer-specific optimal catalysts. Micropyrolysis studies identified the selectivity of medium pore ZSM-5 (25) for light hydrocarbons (C3-C10), while large pore faujasite zeolite and amorphous silica-alumina produced more liquid range products (C5-C12). Validation of selected catalysts in the lab-scale reactor semi-batch slurry reactor confirmed the transferability of qualitative trends, approving TGA and micropyrolysis for generating preliminary data on catalyst performance. Quantitative trends were not directly transferable as a result of distinct local reaction conditions upon scale-up, but could be improved by using higher heating rates to simulate more heat transport limitation. The distinct selectivity patterns of catalysts formed a starting point for two case studies. First, the methodology is applied for screening of hierarchical and nanostructured ZSM-5 as catalyst with enhanced activity for direct polyolefin conversion to light olefins. High mesopore volume and external surface area led to increased content of accessible Brønsted acid sites (BAS), leading to enhanced catalytic activity while preserving high stability to coke formation, as obtained from the 2D-TGA tests. The activity increase was more prominent for PP, which poses more stringent accessibility requirements than LDPE due to its bulky structure. Higher cracking susceptibility of the branched structure showed less dependency to strong BAS, while LDPE benefit from the presence of strong BAS. Micropyrolysis tests revealed a slight shift to higher carbon numbers upon hierarchization, likely due to improved diffusion and less hydride transfer control that reduced cracking. However, the selectivity remained well below the liquid selectivity of large pore benchmark catalysts. Enhanced activity, preserved stability and selectivity pattern of hierarchical ZSM-5 from the small-scale study was confirmed from the lab-scale tests, giving light olefin yields of 13 wt% (LDPE) and 16 wt% (PP), which was lower than 29 wt% (LDPE) and 22 wt% (PP) with ZSM-5 (140) benchmark. This case study demonstrated the relevance of small-scale tools in a context of catalyst design and optimization. The second case study focused on amorphous silica-alumina (ASA) and large pore faujasite zeolites as selective catalysts for producing naphtha range products (C5-C12) from LDPE. 2D-TGA identified distinct activity trends in both classes, which for ASA strongly correlated with the content of BAS. Except for USY (40)*, faujasite zeolites showed high coke formation, which influenced the interpretation of activity results due to catalyst deactivation. Tests in the semi-batch reactor showed excellent agreement of qualitative activity trends for ASA, while stability predictions were less consistent. Faujasite zeolites with high coke content and strong deactivation also complicated the transferability study, partially confirming activity trends when excluding high coke zeolites. Selectivity results from the lab-scale tests revealed highest C5-C12 yields for ASA [20] HPV (76 wt%) and USY (40)* (73 wt%), which followed from the combination of sufficient mesoporosity with moderate content of BAS, which prevent excessive secondary reactions to gas or coke. Besides the catalyst aspect, the impact of reaction conditions was studied, showing preference for conditions that limit the residence time of products in the reactor (semi-batch configuration, atmospheric pressure, optimal isothermal temperature, controlled catalyst content). Finally, USY (40)* proved to be valuable in a multistep process for producing naphtha steam cracker feedstock via consecutive in-situ catalytic pyrolysis (69 ± 1.4 wt% liquid yield) and mild hydrogenation. Naphtha steam cracking gave a final 42 wt% light olefin yield (LDPE based) over the whole process chain, which theoretically increased to 48 wt% when optimal condensation of pyrolysis vapors would be assumed. In conclusion, this study provides a comprehensive high-throughput methodology for catalyst evaluation in in-situ catalytic pyrolysis of polyolefins, offering robust insights into catalyst performance across multiple scales. It bridges the gap between preliminary screening and practical application, accelerating catalyst development for sustainable plastic waste valorisation. The validation of high-throughput small-scale tools paves the way toward scalable, data-rich approaches for circular plastic economy innovations.