Publication Date

2025

Document Type

Dissertation/Thesis

First Advisor

Ryzhov, Victor

Degree Name

Ph.D. (Doctor of Philosophy)

Legacy Department

Department of Chemistry and Biochemistry

Abstract

In the transport of hydrogen, cryogenic methods are often utilized; however, this method is high in energy cost and not sufficiently safe in terms of release prevention. Liquid organic hydrogen carriers (LOHCs) are one of the many methods being developed for a robust and safe hydrogen energy infrastructure. One aspect of LOHCs with a rich research field as of late is dehydrogenation techniques to liberate stored hydrogen gas for energy consumption. This process currently requires excessive cost (environmental and monetary), specialty catalysts using expensive metals like palladium for the selective dehydrogenation without coking of the catalyst surface. In search for more economically feasible catalysts using early transition metal catalysts, gas–phase experiments with homogeneous analogues of heterogeneous catalysts are employed for high–throughput screening for selectivity and basic mechanistic behavior. In this context, [CpM]+ complexes of iron, cobalt, and nickel have been formed by electrospray ionization in the gas–phase and reacted with multiple model compounds of LOHC motifs such as cyclohexane, pyrrolidine, n–methyl pyrrolidine, and piperidine. Reactions were carried out utilizing ion–molecule reactions via a home–modified helium intake line which allows for direct injection of volatile substrates into the ion trap. The iron complex showed the most promising results where reaction with cyclohexane and pyrrolidine yielded selective dehydrogenation while piperidine and N–methylpyrrolidine yielded significant side reactions. Nickel and cobalt complexes suffered from intensity issues due to instability of the [CpM]+ complex and a lack of suitable precursors for electrospray ionization and could not be fully examined in full with piperidine, N–methylpyrrolidine, and pyrrolidine; however, these complexes show evidence for dehydrogenation of the substrates. In an effort to map similar reactivity of a more relevant to the solution phase analogue of [CpM]+ compounds, [CpFe(CO)2]+ was used in reactions with the same model LOHC compounds as [CpM]+ to varying results. Cyclohexane provided no reactivity with the more coordinated [CpFe(CO)2]+ while reactions with the N–heterocycles provided decarbonylation and dehydrogenation products upon addition of substrate L to the complex giving complexes of the general formulas [CpFe(CO)L]+ and [CpFeL]+. With the addition of CO to the helium dampening gas for the reformation of [CpFe(CO)L]+ after decarbonylation, formal catalytic cycles were established for piperidine and pyrrolidine systems. For modeling heterogeneous catalysts active sites, quinoline complexes of nickel and palladium were utilized in a similar manner to [CpM]+ and [CpFe(CO)2]+ for reactions of cyclohexane with [(8–hydroxyquinolinate)M(II/0)]+/– or [(8–quinolinide)M(II/0)]+/– (M= Ni or Pd). Cationic Ni complexes yielded dehydrogenation products of double and triple dehydrogenation products from reaction of cyclohexane with [(8–hydroxyquinolate)Ni(II)]+ and [(8–quinolinide)Ni(II)]+ respectively. Interestingly, [(8–hydroxyquinolinate)Pd(II)]+ showed no reactivity with cyclohexane while [(8–quinolinide)Pd(II)]+ provided a single dehydrogenation. Anionic complexes yielded no reactivity with cyclohexane due to decreased interaction of C–H bond electrons with the electron rich ground state metal. Lastly, C–H activation is the one common process in all hydrocarbon dehydrogenation reactions studied in this work. As a continuation of previous work done by our group on [(phen)Ni(R)]+ (R = CH3 and H), [(phen)Ni(OH)]+ was examined for C–H activation of various substrates and was found to be successful for those with C–H bond dissociationenergy no larger than 384 kJ/mol. Reactivity with [(phen)Ni(OH)]+ generally fell into two categories: C–H activation with only cracking of the substrate and C–H activation with dehydrogenation and cracking of the substrate. Density functional theory (DFT) calculations were employed to confirm structures of key ions observed during each reaction.

Extent

145 pages

Language

en

Publisher

Northern Illinois University

Rights Statement

In Copyright

Rights Statement 2

NIU theses are protected by copyright. They may be viewed from Huskie Commons for any purpose, but reproduction or distribution in any format is prohibited without the written permission of the authors.

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