Fischer–Tropsch Synthesis: Effect of the Promoter’s Ionic Charge and Valence Level Energy on Activity

Mirtha Z Leguizamón León Ribeiro,Michela Martinelli,Muthu Kumaran Gnanamani,Gary Jacobs,Gabriel F Upton,Joice C Souza,Mauro C Ribeiro

doi:10.3390/reactions2040026

Mirtha Z Leguizamón León Ribeiro, Michela Martinelli + Show 5 more

Open Access

https://doi.org/10.3390/reactions2040026

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Abstract

In this contribution, we examine the effect of the promoter´s ionic charge and valence orbital energy on the catalytic activity of Fe-based catalysts, based on in situ synchrotron X-ray powder diffraction (SXRPD), temperature-programmed-based techniques (TPR, TPD, CO-TP carburization), and Fischer–Tropsch synthesis catalytic testing studies. We compared the promoting effects of K (a known promoter for longer-chained products) with Ba, which has a similar ionic radius but has double the ionic charge. Despite being partially “buried” in a crystalline BaCO3 phase, the carburization of the Ba-promoted catalyst was more effective than that of K; this was primarily due to its higher (2+) ionic charge. With Ba2+, higher selectivity to methane and lighter products were obtained compared to the K-promoted catalysts; this is likely due to Ba´s lesser capability of suppressing H adsorption on the catalyst surface. An explanation is provided in terms of a more limited mixing between electron-filled Ba2+ 5p and partially filled Fe 3d orbitals, which are expected to be important for the chemical promotion, as they are further apart in energy compared to the K+ 3p and Fe 3d orbitals.

Highlights

Iron-based catalysts are an interesting choice for Fischer–Tropsch synthesis (FTS), where CO is hydrogenated to a mixture of hydrocarbons that can be upgraded and used as transportation fuels, lubricants, waxes, and other value-added products [1]
Many efforts [1,3,4,5] have been dedicated to investigating the nature of the effect of potassium, which has been observed to increase the carburization rate during catalyst activation [5], resulting in the formation of a mixture of metastable iron carbides that seem to coexist in a dynamic equilibrium on the catalyst surface under FTS conditions
The CO conversion trend for the Ba-promoted Fe catalysts followed that of unpromoted Fe but was 5–10% lower. This indicates that the K promoter influences the catalyst deactivation rate moreso than Ba does at the promoter loadings used in this work (5Me:100Fe, atomic ratio)

Summary

Introduction

Iron-based catalysts are an interesting choice for Fischer–Tropsch synthesis (FTS), where CO is hydrogenated to a mixture of hydrocarbons that can be upgraded and used as transportation fuels, lubricants, waxes, and other value-added products [1]. A residual Fe oxide phase that is present in the active catalyst enhances H2 production via its intrinsic water–gas shift (WGS) activity, enabling the use of H2 depleted, coal-derived syngas as a feed. Structural promoters, such as Al, Mg, and Si, positively impact catalyst performance by minimizing both the crystallite sintering and filter-clogging mechanical disintegration of the catalyst during working conditions [2].

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