Closing the loop: Valorizing pyrolyzed waste tyre residue into functional carbon materials, SiO2 with exceptionally high silanol groups, and Zn salt

Abstract

This study aims to identify suitable processing conditions for converting pyrolytic solid residue from off-the-road tyres (OTR) to improve carbon materials properties that can be used in multiple applications and the recovery of minerals from OTR. Pyrolysis of OTR at 800 °C and a heating rate 2 °C.min−1 gave a carbon material with the highest surface area, most defective carbon structures, and the highest micro-porosity. This operating condition was used to compare the conventional three-step carbonization approach, which involves a demineralization stage that produces high volumes of toxic wastewater, with a two-step approach that bypasses this stage. Analysis of the carbon structures showed that the quality of the carbon material from the two-step approach is similar to the three-step approach. This two-step approach resulted in a solid and a liquid phase, in which ∼ 93.4% of Zn was selectively fractionated to the liquid phase. The wastewater from the acid wash of the carbonized OTR was neutralized to recover the SiO2, of which 55.5% was reactive SiO2. The SiO2 was found to have an exceptionally high cross-linking ratio of 5.94, achievable only when SiO2 is reacted with silane groups. The study demonstrated that the engineered carbon material from OTR has a H2 uptake of 1.03 wt% at 77 K and 1.2 bar, and the sulfonated counterpart was an effective catalyst (64% conversion) for the Aldol condensation of levunilic acid to two dimer products [tetrahydro-2- methyl-5,γ-dioxo-2-furanpentanoic acid (TMDFA) and 3-(2-methyl-5-oxo- tetrahydrofuran-2-yl)-4-oxopentanoic acid (MOTOA)] that are precursors for fuels and chemicals.