Catalytic hydrotreatment of waste tire pyrolysis oil: Insights into transformation of heavy fractions and heteroatomic species

Licentiate thesis, 2026

The accumulation of end-of-life tires poses significant environmental challenges due to their non-biodegradable nature and continuously increasing quantities generated. Among the available waste-tire management strategies, pyrolysis is considered one of the most effective methods, producing waste tire pyrolysis oil (WTPO) as a potential feedstock for chemical and energy applications. However, the direct utilization of WTPO for fuel production remains limited because of several physicochemical constraints. This thesis explores the upgrading of WTPO by catalytic hydrotreatment and investigates the transformation pathways of heavy components and heteroatomic compounds during the process. The findings of this thesis are presented in two manuscripts, namely Manuscript I and Manuscript II.

In Manuscript I, a reduced NiMo/Al2O3 catalyst was utilized to systematically investigate the catalytic upgrading of WTPO in a batch reactor. Screening of reaction conditions revealed that temperature, reaction time, hydrogen pressure, and catalyst-to-oil mass ratio strongly influenced the upgrading performance. Hydrogenation (HYD) proved essential for effective hydrotreatment, as evidenced by a remarkable decline in olefinic hydrogen and heavy fractions in the hydrotreated products. Sulfur speciation by GC-SCD indicated that hydrodesulfurization (HDS) was governed by both the reaction conditions and molecular structures of the sulfurous compounds. A liquid product with a yield of 96.7 wt.% with HDS efficiency of 63% was achieved under the reaction conditions: 360 oC, 4 h, 900 rpm stirring, 1:10 catalyst-to-WTPO mass ratio with continuous hydrogen replenishment maintaining 70 bar. Catalyst recyclability tests demonstrated high reusability, with the catalyst maintaining strong performance over three consecutive hydrotreatment cycles.

Manuscript II focuses on the influence of sulfidating-agent loading during catalyst pre-sulfidation on the upgrading process. In this study, dimethyl disulfide (DMDS) was used to sulfide the NiMo/Al2O3 catalyst prior to hydrotreatment. Catalyst characterization by temperature-programmed reduction/desorption, X-ray photoelectron spectroscopy (XPS) revealed a stepwise transformation of the catalyst’s active sites as the DMDS loading increased, leading to the formation of new active sites after sulfidation. The emergence of these new active sites enhanced the catalyst’s HYD capacity and created a more favorable balance between HYD and cracking reactions, resulting in improved upgrading efficiency. The catalyst sulfided with a stoichiometric amount of DMDS achieved an HDS efficiency of 86.4%. Furthermore, MALDI FT-ICR MS served as an excellent complement to conventional gas chromatography analysis, providing a comprehensive representation of molecular-level transformations occurring during WTPO upgrading. Overall, this work elucidates the fundamental mechanisms governing WTPO hydrotreatment and emphasizes the critical interplay between reaction conditions and upgrading efficiency, which is highly relevant for industrial applications.