Mechanism-Based\nThermodynamic Analysis for One-Step\nand Two-Step Ethanol-to-1,3-Butadiene Conversion Processes

Renewable <i>1,3</i>-butadiene (BD) is essential\nfor\nsustainability of the synthetic rubber sector. This work presents\na comprehensive thermodynamic analysis for one- and two-step ethanol-to-BD\nconversion processes. The two-step process comprises ethanol dehydrogenation,\nfollowed by the condensation of acetaldehyde with another ethanol\nmolecule into BD. The process involves a complex reaction network\nwith a wide range of byproducts depending on the nature of the catalysts\nand operating conditions, lacking unique consensus on the C–C\nbond-forming mechanism. This study elucidates the temperature regime\nfor the spontaneity of the reactions proposed in various mechanisms\nand side reactions based on the standard Gibbs free energy change.\nThe equilibrium conversion and product selectivity were further calculated\nunder a wide temperature and pressure range. The overall reaction\nin the one-step process is thermodynamically spontaneous above 417\nK, while the first and second steps of the two-step process are spontaneous\nabove 550 and 285 K, respectively. Excepting Prins condensation, other\nmechanisms lack the spontaneity of all reaction steps. The equilibrium\nBD selectivity is favorable at elevated temperatures and low pressures.\nThe addition of acetaldehyde in the two-step process has a favorable\nimpact with higher BD selectivity, the maximum being at a 1:1 molar\nratio of ethanol/acetaldehyde. This study elucidates thermodynamic\ninsights into existing mechanisms and drives the evolution of a feasible\nmechanism. This effort will eventually help design novel catalysts\nand optimized processes for sustainable biobased BD production using\nethanol derived from renewable feedstocks, aligning with the global\ncommitment to greener and resource-friendly chemical manufacturing.