New Insights in the Working Principles of Europium-Based Methane Oxychlorination Catalysts
Authors:   , Terlingen Bas Johan Pieter 
Published: January 2023

The aim of this PhD thesis is to design new catalytic materials for the CH4 oxychlorination (MOC) reaction and to study these solid catalysts under real working conditions by applying light, i.e., operando spectroscopy. In this way, the activity and spectroscopic information can be coupled, yielding crucial mechanistic insights to improve the design of the catalytic materials. The presented work reveals that redox properties, bulk and synergistic effects play a crucial role in the working mechanism of bulk MOC catalysts.
In chapter 2, a set of lanthanide oxychloride catalysts were synthesized, characterized and tested. EuOCl came out as the most promising candidate as it possessed the highest MOC activity, the best activity/selectivity relation and was proven to be stable. Moreover, the catalytic performance of EuOCl could be tuned by increasing the HCl concentration in the feed, thereby mitigating the unwanted catalytic destruction of chloromethanes to COx. Operando Raman spectroscopy revealed that the chlorination of the catalyst was rate limiting as an increase in the HCl concentration in the feed resulted in enhancement of activity, but did not cause bulk chlorination of the catalyst.
While EuOCl appeared promising as a MOC catalyst, its catalytic performance was limited by the rate of chlorination. The catalyst design was improved in chapter 3 by adding a chlorinating agent for Eu3+. The activity of all La3+-Eu3+ bimetallic catalysts was significantly higher than anticipated for the linear combination of LaOCl and EuOCl. More important was that a higher CH3Cl selectivity could be achieved without giving in on the excellent CO selectivity obtained for monometallic EuOCl. Operando luminescence spectroscopy revealed that La3+ acted as a chlorinating agent for Eu3+, even when the phases were completely segregated as in the case for the physical mixture.
By applying operando catalyst thermometry in chapter 4, the observed catalytic performance of EuOCl could be linked to the actual catalyst temperature, gaining more insights in arguably one of the most important reaction parameters. A catalyst temperature of maximum 16 °C higher compared to the oven temperature was recorded, due to the exothermic nature of the MOC reaction. Heat dissipation by means of radiation was identified as the main heat loss mechanism, resulting in a uniform catalyst bed temperature.
The concept that irreducible elements can make up a very active catalyst for the MOC reaction is proven by the design of irreducible Mg2+-Al3+ mixed-metal oxide (MMO) catalysts. The Mg4Al MMO possessed one of the highest reported activities under the tested reaction conditions, which was all the more surprising considering that MgO and -Al2O3 did not show any significant activity. Operando Raman spectroscopy revealed that Mg2+ acted as a Cl- buffer and as a chlorinating agent for Al3+, which was the active metal in the CH4 activation step. Adding 2 atom-% of the redox active Eu3+ in the material doubled the activity, made the activity/selectivity relation tuneable and preserved the activity under high HCl concentrations. These results indicate that both redox activity as well as synergistic effects are required to obtaining benchmark performance.