Designing Heterogeneous-Based Cascade Catalytic Systems for Carbon Dioxide Hydrogenation.

Abstract

The search for sustainable and economical chemical processes is a key priority for the chemical industry. Cascade catalysis presents a promising technology to meet this need by combining multiple catalysts to drive a sequence of catalytic steps in one reactor. These catalysts work cooperatively to achieve desired products with higher selectivity/atom efficiency than from using any single catalyst alone. Consequently, this cascade approach enables efficient use of feedstock and reduces the operational work-up between catalytic sub-steps, leading to both time and cost benefits. Research described in this dissertation investigates the use of heterogeneous-based cascade catalytic systems to advance carbon dioxide hydrogenation for producing value-added products. Carbon dioxide is an abundant and nontoxic carbon source; its conversion provides sustainable routes for chemical synthesis and opportunities to balance green-house gas emissions that are linked to climate change. In this dissertation, copper- and molybdenum carbide-based heterogeneous catalysts were evaluated for the hydrogenation of carbon dioxide and subsequent intermediates, i.e. formic acid and formate esters, to produce methanol. Catalysts identified from these sub-reactions were combined to devise several cascade systems for carbon dioxide hydrogenation. A series of mixed homo-/hetero-geneous cascade systems were first targeted via pathways at both Lewis acidic and basic conditions. Surface interactions between homogeneous and heterogeneous catalysts seemed to play an important role in catalytic performance, ultimately leading to catalyst deactivation and ineffective cascade systems. Several heterogeneous-based cascade systems were developed to eliminate these interactions; these systems worked cooperatively to produce methanol and/or dimethyl ether. For example, a system containing an iridium- and a copper-based catalyst enhanced the methanol production by threefold (via the formic acid intermediate) compared to employing each catalyst alone. The study was further extended to evaluate several molybdenum carbide supported metal catalysts to produce alcohols and hydrocarbons. The role of temperature and metal type on reaction performances and pathways was also elucidated. In addition to developing novel heterogeneous cascade systems for carbon dioxide hydrogenation, research described in this dissertation also provides useful insights for designing similar types of catalytic systems for other challenging and multi-step chemical transformations of interest.