Phase Transformation Processes in Coprecipitated Cu/Zn/Zr Methanol Catalyst Precursors—Insights into Suspension Aging Form Ultrafast Nucleation

Catalyst production for methanol synthesis includes an aging step. By ultrafast nucleation, occurring transient changes in the suspension important for later performance can be assessed. Analytical techniques deliver insights which lead to a mechanism in which zincian malachite is the main product, whereas a sodium species play an important role as transient zinc depot.

Catalyst precursors for methanol synthesis prepared by coprecipitation, such as Cu/Zn-based hydroxycarbonates, are generally formed within two steps. The initial precipitation phase (nucleation) leads to a suspension from which, in a subsequent aging phase, the final solid precursor is formed, thus providing the structural and morphological features required for later use in catalysis. Combing ultrafast continuous nucleation with sampling from batch-wise aging opens up the possibility to follow the evolution and transitions of solid phases during suspension aging. The temporal progression of the existence of the different phases in a Cu/ZnO/ZrO₂-based system is investigated by scanning electron microscopy, X-ray diffraction, and inductive coupled plasma optical emission spectroscopy . According to the findings of this study, the intermediate recrystallization reveals to be a yet unknown two-step process. The presence of an amorphous transient zinc depot of Na₂Zn₃(CO₃)₄ × 3 H₂O greatly influences the formation of the relevant zincian malachite catalyst precursor [(Cu,Zn)₂(OH)₂CO₃], which is partly in contrast to reports on Cu/ZnO/Al₂O₃ systems. Finally, a general mechanism including the relevant transformations during suspension aging in Cu/ZnO/ZrO₂ systems is proposed, relying on a general thermodynamic approach, explaining the transient and final species.

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