Quantitative determination of olefins in pyrolysis oils from waste plastics and tires using selective adsorption by Ag–SiO2 followed by GC×GC-FID

Importance of the Topic
Pyrolysis oils from waste plastics and tires contain high olefin contents, which negatively affect thermal and oxidative stability, increase coke formation, and promote fouling during steam cracking. Quantifying olefins in these oils is vital to optimize chemical recycling processes, enable advanced valorization, and support circular economy goals.

Study Objectives and Overview
Develop and validate an affordable method using selective adsorption on Ag–SiO2 combined with GC×GC-FID to determine olefin content in pyrolysis oils without costly detectors (TOF-MS or VUV). Method requirements: minimal sample (50 μL), short separation (<15 min), reliable quantification across various oil types and after hydrotreatment.

Analytical Methodology and Instrumentation

• Adsorbent: Silica gel impregnated with AgNO3 (Ag–SiO2), activated at 160 °C.
• SPE setup: 3 mL cartridge with 1.45 ± 0.03 g Ag–SiO2 (particle size 0.040–0.063 mm).
• Sample: 15 μL pyrolysis oil injected onto top frit. Elution by gravity with 1.8 mL dichloromethane (DCM).
• Detection: Comprehensive two-dimensional gas chromatography with flame ionization detector (GC×GC-FID).
• Peak normalization: External internal standard (3-chlorothiophene) added before and after adsorption.

Optimization of Olefin Separation

• Mobile phase: DCM provided complete elution of saturates and aromatic hydrocarbons, with selective adsorption of olefins and styrenes.
• Solvent volume: 1.8 mL DCM efficiently eluted all non-olefinic compounds and fully retained olefins.
• Adsorbent storage: Ag–SiO2 must be used immediately after activation; storage in evacuated bags significantly reduces activity.
• Ag–SiO2 load: 1.45 g ensures consistent performance; slight mass variations had negligible effect.
• Particle size: 0.040–0.063 mm provides optimal balance between elution time (≈10 min) and adsorption efficiency.
• Low-boiling fractions: Cooling the collection vial (− 20 °C) is essential to minimize loss of C5–C6 hydrocarbons.

Validation and Performance

• Model mixtures (MM-4) spiked into kerosene: Accurate (±3 %) and precise (RSD < 5 %) results for olefins >5 wt%. Accuracy and repeatability deteriorated for <5 wt%.
• Dilution tests: Reliable quantification down to ~5 wt% olefins; LOQ ~5 wt%.
• Hydrotreated oil spiking: Good agreement for 8.5 and 35 wt% olefins; slight overestimation of styrenes due to partial retention of some alkylbenzenes.
• Real samples: Analysis of plastics and tire pyrolysis oils and hydrotreatment fractions showed excellent repeatability (RSD < 5 %) for total olefins >1 wt%.

Benefits and Practical Applications

• Low-cost: Only requires GC×GC-FID and simple Ag–SiO2 SPE setup.
• Rapid: Total analysis <15 min; minimal sample size (50 μL).
• Accessible: No need for mass spectrometry or VUV detectors.
• Versatile: Applicable to crude pyrolysis oils, distillation fractions, and hydrotreated products.

Future Trends and Potential Applications

• Extend to qualitative olefin profiling: Desorb adsorbed olefins from Ag–SiO2 with solvents (THF, ethyl acetate) and analyze by GC×GC-MS.
• Combine with advanced detectors: Validate method with emerging GC-VUV and soft ionization MS for broader compound classes.
• Automation and high-throughput: Integrate multi-cartridge SPE racks and parallel GC×GC sampling for large screening studies.
• Broader feedstocks: Apply to pyrolysis oils from biomass, mixed municipal wastes, and blended plastic/tire streams.

Conclusions
The developed Ag–SiO2-GC×GC-FID method enables reliable quantification of olefins (>5 wt%) in pyrolysis oils using affordable instrumentation. It was thoroughly optimized and validated across model mixtures, kerosene, real pyrolysis oils, and hydrotreated samples. The approach supports circular economy objectives by facilitating routine quality control and process optimization in plastic and tire recycling.