Atmospheric CO2 emissions need to be reduced drastically to mitigate the impact of global warming. CO2 capture and utilization technologies are the only viable option towards a more sustainable society. Currently, the development of technologies to directly transform this greenhouse gas into valuable chemicals receives a lot of interest. Promising results in the CO2 reductive valorization are obtained with a combination of conventional metallic catalysts and acid zeolites. [1-3] The metallic catalyst is used to convert CO2 and H2 via the reverse water gas shift reaction into H2O and CO, which is in turn converted into hydrocarbons through classical Fischer-Tropsch synthesis. The acid zeolite increases the light olefin and aromatics yield by transformation of these hydrocarbons through cracking, aromatization, isomerization reactions, … Alternatively, the metallic catalyst can convert CO2 in methanol, which is further processed by C-C coupling reactions in the zeolite framework through the methanol-to-hydrocarbons (MTH) chemistry. To optimize these bifunctional catalysts, fundamental insight into the governing reaction mechanisms is essential.

Figure 1. Illustration of the oxide-zeolite bifunctional catalyst to convert CO2 into light olefins. [4]

By selecting a proper zeolite catalyst, a higher light olefin yield or a higher aromatics yield can be achieved. One of the main challenges is to tune the product distribution towards the desired products such as propene. This is complicated by the many competitive reactions (cracking, hydrogenation, alkylation …) taking place simultaneously and by the presence of CO, CO2, H2O and H2 in the zeolite pores which can affect reactions between hydrocarbons. For example, carbonylation reactions have similarities with C-C coupling reactions in the methanol-to-hydrocarbons process. Ketenes and surface formates are proposed as important intermediates to control the selectivity of this process. [4-5] However, these are very reactive, short-lived species and their role in the formation of hydrocarbons is not yet unravelled. Nevertheless, a full understanding of the reaction mechanism is essential to progress towards more efficient CO2 conversion catalysts.

Figure 2. Proposed ketene-based mechanism for the formation of light olefins during the zeolite catalyzed MTH process. [4]

Goal

The goal of this master thesis is to investigate how conventional MTH reactions (both in the initiation phase and steady state operation) are affected by the presence of CO, CO2, H2 and H2O. First, you will evaluate the stability of various reactive intermediates (ketene, surface formate …) which can be formed in a hydrogenative environment. Afterwards, key pathways involving these intermediates in the C-C coupling and hydrocarbon transformation mechanism will be simulated to assess the importance of the presence of CO in the reaction mixture. To discriminate between competitive pathways and to take into account the complex molecular environment, first-principle molecular dynamics simulations at operating conditions will be performed. As the role of the zeolite is crucial for fine-tuning of the product distribution towards light olefins or aromatics, you will perform this study in zeolites with different pore topologies. For example, channel structured zeolites like MFI and cage zeolites like CHA will be compared. This analysis will yield insight in the role of CO and H2 on hydrocarbon transformations in zeolites and will assist in the improvement of light olefin selectivity of CO2 conversion processes.

The Center for Molecular Modeling has ample experience in modeling of zeolite catalysis with advanced simulation techniques. The proposed topic is situated in a very active research field. The student will be actively coached to get acquainted with the multitude of available techniques to tackle the proposed problem. The CMM has access to large scale computational infrastructure for studying this complex system. The student will be actively involved in the running collaboration with prof. Jorge Gascon (KAUST) on this topic.

Aspects
Master of Science in Chemical Engineering:
Chemistry aspects: chemical insight into the reactivity of different intermediates that can occur during the transformation of CO2 into valuable chemicals over zeolite catalysts. Engineering aspects: exploration of hydrocarbon formation pathways in a complex environment, i.e. in the presence of CO, H2O and H2, to identify reaction routes for enhancing the light olefin selectivity.