Bimetallic Ir-based Catalysts for Ring Opening and Hydrodesulfurization Reactions

• Author / Creator

  Ziaeiazad, Hessam

• Activity, selectivity and stability enhancements often observed in bimetallic catalysts make them an inseparable part of today’s chemical industry. However, it is usually problematic to understand the real interactions resulting in such improvements due to the lack of control over the bimetallic particle’s surface structure. The main goal of the current thesis is to realize and explain this type of functioning, which is usually referred as synergism. A method, i.e. colloid chemistry, is applied for the preparation of catalytic materials, which allows manipulating the surface structure and enables the calculation of catalytic activity per specific surface atoms so that an explanation for bimetallic interactions is substantiated. Refining industry is targeted for the current study, which nowadays faces significant challenges of growing demand for premium quality fuels as well as stringent environmental standards limiting the maximum allowable pollutants (like sulfur) in the final products. Two processes are discussed: ring opening (RO) of indane and hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT). Due to the limited resources of Pt as well as the high demand for this metal in different areas, a Pt-free approach is applied here. The structure-controlled synthesis of polyvinylpyrrolidone (PVP)-stabilized Ir-Pd and Ir-Ni bimetallic nanoparticles of alloy and core-shell is explained in the current thesis. Various particles in the range of 1.6 – 3.5 nm were synthesized by the colloid chemistry method and supported on a weak acidic material, i.e. γ-Al2O3. The nanoparticles and supported catalysts were characterized by several techniques including TEM, UV-vis spectroscopy, TPR, XPS, CO-DRIFTS, ISS, CO chemisorption, XANES and CO2-TPD, iii which verified the formation of new bimetallic particles and showed structural differences between the catalysts of alloy and core-shell structures. Removal of PVP residues from the surface of supported catalysts was studied thermally by calcining the samples under air. Only part of the PVP remnants was removed after calcination at 200oC, while treatment at 400oC completely eliminated all the residual polymers. Minor structural changes as well as sintering and size increase of samples with Pd or Ni enriched surfaces occurred at 400oC. The catalytic activity of supported Ir-Pd and Ir-Ni catalysts was studied in ring opening of indane at 336oC and atmospheric pressure. Monometallic Ni or Pd showed insignificant activity, whereas enhancement of activity was observed for bimetallic catalysts compared to monometallic Ir. However, calculating activity values per surface atoms revealed no intrinsic changes in catalytic behaviour of Ir atoms, which indicates the role of Pd or Ni as only being the surface diluent in bimetallic alloys or the dispersing agent in core-shell catalysts, providing a 100% surface coverage for Ir. Having investigated the effect of PVP residues on catalytic activity of Ir-Pd catalysts for the SRO of indane, a more pronounced activity improvement was observed after the complete removal of PVP from Pd-rich samples, an effect that was sluggish though for Ir-rich ones. Furthermore, the HDS of 4,6-DMDBT was studied over Ir-Pd catalysts at 300oC and 5 MPa. Providing the necessary hydrogenation properties, monometallic Pd suffered from severe sintering after 400oC calcination, which dramatically reduced its dispersion. Addition of Ir enhanced the activity and selectivity toward sulfur-free products, also improved the selectivity contribution of the direct desulfurization (DDS) pathway by five times because of its proven hydrogenolysis tendency. Similar calculations for finding the iv activity values per surface atoms indicated the superior performance of bimetallic Ir-Pd catalysts to be a consequence of the observed synergism and not the significant changes in the intrinsic behaviour of Pd atoms. This means that Ir has helped to disperse Pd atoms and prevented and/or reduced their agglomeration. The high pressure hydrogenation of indane at 280-320oC and 5 MPa was also briefly studied, where the addition of Ir similarly improved the activity of Pd catalysts. However, when a mixture of indane and 4,6- DMDBT was fed to the system, the hydroconversion behaviour of bimetallic Ir-Pd catalysts was manipulated by the HDS of 4,6-DMDBT, an insignificant conversion was observed for hydrogenation of indane. The catalyst stability was studied for 3 days under a mixture containing 300 ppm sulfur as 4,6-DMDBT.

• Subjects / Keywords

  • Ring Opening
  • Iridium
  • Hydrodesulfurization
  • Bimetallic Catalysts

• Graduation date

  Fall 2014

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R35Q4RR80

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.