Date of Award

Summer 8-22-2025

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science in Chemical Engineering (MSChE)

Department

Chemical Engineering

First Committee Advisor

Thomas J. Schwartz

Second Committee Member

M. Clayton Wheeler

Third Committee Member

Brian G. Federick

Abstract

The sustainable conversion of bioethanol to 1,3-butadiene offers a renewable route to a high-demand petrochemical traditionally derived from fossil sources. This study investigates the complex reaction network of ethanol-to-butadiene (ETB) conversion over MgO–SiO₂ catalysts, with a particular focus on elucidating product formation sequences and mechanistic pathways using Delplot analysis. The MgO–SiO₂ catalyst system, characterized by its bifunctional acid–base properties, plays a crucial role in orchestrating the key steps required for butadiene formation, including ethanol dehydrogenation, aldol condensation, Meerwein–Ponndorf–Verley (MPV) reduction, and subsequent dehydration. Experimental investigations from literatures together with those conducted our research laboratory were collected across a range of ethanol conversions (10–98%), weight hourly space velocities (0.2–21 h⁻¹), and reaction temperatures (678 and723 K) under atmospheric pressure. Reaction products that this research focused on were acetaldehyde, ethylene, and 1,3-butadiene due to limited data for other intermediates and side products of this reaction. To systematically classify these products, Delplot analysis was applied to experimental selectivity and conversion data. First-rank Delplots (S vs. X) were used to identify primary products, those formed directly from ethanol while second-rank Delplots (S/X vs. X) revealed secondary and tertiary species formed through sequential transformations. The results from first rank delplot showed that acetaldehyde and ethylene were largely primary products, formed via dehydrogenation and dehydration pathways, respectively. Butadiene, the target molecule, was confirmed to be a secondary or tertiary product formed through the sequential reduction and dehydration of intermediate C₄ oxygenates. These classifications validate the accepted multi-step reaction mechanism for ETB conversion and reinforce the role of the MgO– SiO₂ surface in facilitating stepwise transformations involving both Lewis base and acid functionalities. Additionally, past studies from literature observed fractional reaction orders with respect to ethanol which aligns with those gotten in our research group, suggesting site inhibition effects likely due to strong adsorption of intermediates. These kinetic phenomena were reflected in the delplot trends, as product selectivities shifted with conversion. The findings established the value of delplot analysis as a rapid diagnostic tool for product classification and reaction pathway inference even though it also comes with its own limitations which could be incorrect product identification from the delplot trend because of limited low conversion data or extrapolation to the axis. However, to quantitatively interpret these trends and understand the interplay of surface coverages, kinetic parameters, and rate-determining steps, the integration of a microkinetic model is recommended. Such a model would complement delplot analysis, enabling rigorous validation of proposed mechanisms and supporting the rational design of catalyst formulations and operating conditions for improved butadiene selectivity. In conclusion, this work provides a comprehensive kinetic and mechanistic investigation of the ethanol-to-butadiene process over MgO–SiO₂ catalysts. Through the combined use of delplot analysis and catalytic insights, the study advances our understanding of product evolution and lays the groundwork for future kinetic modeling efforts aimed at optimizing bioethanol valorization pathways.

Download

Share