Reaction Pathways over ZnZrO2‑Based Catalysts and Catalytic Sorbents

TL;DR

This paper studies how CO2 is converted to methanol using a special catalyst under a new process that combines capture and conversion, revealing different reaction pathways compared to traditional methods.

Contribution

The study identifies distinct reaction pathways for CO2 hydrogenation under transient RCC conditions versus steady-state conditions using a ZnZrO2-based catalyst and catalytic sorbent.

Findings

Under steady-state conditions, ZnZrO2 produces methanol via sequential hydrogenation of HCOO* and CH3O* intermediates.

In RCC, monodentate carbonate species (m-CO3^2–) form and follow two competing routes: hydrogenation to methane or migration to ZnZrO2 for methanol synthesis.

RCC enables carbonate hydrogenation routes not observed under conventional steady-state cofeed conditions.

Abstract

Reactive capture and conversion (RCC) is a process intensification
approach that integrates CO2 capture and hydrogenation
within a single unit, removing the CO2 purification and
storage steps of traditional process flow schemes. This alters the
catalytic step from a traditional steady-state (SS) flow process to
a transient capture and conversion cycle, which could lead to product
distributions distinct from those observed in conventional SS experiments.
Such differences are investigated in the combined capture and hydrogenation
of carbon dioxide to methanol over a ZnZrO2 catalyst and
a ZnZrO2 + NaNO3/Mg3AlO
x
catalytic sorbent (CS) using fixed-bed kinetic measurements, in situ diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS), and steady-state isotopic transient kinetic analysis-DRIFTS
(SSITKA-DRIFTS). Under SS conditions, ZnZrO2 produced methanol
through sequential hydrogenation of HCOO* and CH3O* intermediates.
On the contrary, CO was attributed primarily to CO2 dissociation
at oxygen vacancies, as supported by isotopic shifts and measured
reaction orders. For the CS, isotopic switching experiments suggested
that monodentate carbonate species (CO3
2–, abbreviated as m-CO3
2–) act as active
intermediates that can be hydrogenated to HCOO* and subsequently to
CH3O. Under RCC conditions, in situ DRIFTS
and isotopic experiments reveal that m-CO3
2– species formed during the CO2 capture step follow two
competing routes upon H2 exposure: (i) direct hydrogenation
to methane on the sorbent domain or (ii) migration of m-CO3
2– to the ZnZrO2 domain, where they
are hydrogenated to methanol through the HCOO pathway. Overall, RCC
enables carbonate hydrogenation routes not observed under SS cofeed
conditions. Thus, the reaction pathways and rates during RCC can be
different from operation under conventional SS conditions, and the
product distribution is determined here by competition between carbonate
hydrogenation on sorbent sites and migration to ZnZrO2 for
methanol synthesis.

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