Enhancing the Catalytic Activity of Zn-Containing Magnetic Oxides in a Methanol Synthesis: Identifying the Key Factors

Abstract

A new family of Ni-, Co-, and Cr-doped Zn-containing magnetic oxide nanoparticles (NPs) stabilized by polyphenylquinoxaline (PPQ) and hyperbranched pyridylphenylene polymer (PPP) has been developed. These NPs have been synthesized by thermal decomposition of Zn and doping metal acetylacetonates in the reaction solution of preformed magnetite NPs, resulting in single-crystal NPs with spinel structure. For the PPQ-capped NPs, it was demonstrated that all three types of metal species (Fe, Zn, and a doping metal) reside within the same NPs, the surface of which is enriched with Zn and a doping metal, while the deeper layers are enriched with Fe. The Cr-doped NPs at the high Cr loading are an exception due to favored deposition of Cr on magnetite located in the NP depth. The PPP-capped NPs exhibit similar morphology and crystallinity; however, the detailed study of the NP composition was barred due to the high PPP amount retained on the NP surface. The catalyst testing in syngas conversion to methanol demonstrated outstanding catalytic properties of doped Zn-containing magnetic oxides, whose activities are dependent on the doping metal content and on the stabilizing polymer. The PPP stabilization allows for better access to the catalytic species due to the open and rigid polymer architecture and most likely optimized distribution of doping species. Repeat experiments carried out after magnetic separation of catalysts from the reaction mixture showed excellent catalyst stability even after five consecutive catalytic runs.

Efficient Furfuryl Alcohol Synthesis from Furfural over Magnetically Recoverable Catalysts: Does the Catalyst Stabilizing Medium Matter?

“…Along with PPQ, Pd‐containing magnetite NPs were synthesized in the presence of PPP as capping molecules (Table ). This was motivated by the results of our preceding work on doped Zn‐containing magnetic oxides NPs, where hyperbranched PPP allowed for a much higher catalytic activity . TEM images for PPP stabilized Pd‐containing magnetite NPs presented in Figure S8 (SI) show similar morphology as that in the case of the PPQ NP stabilization (see Figure S4, SI).…”

“…The XRD pattern of Pd‐6 (Figure S9, SI) contains typical reflections of magnetite and a full set of reflections of Pd 0 . XPS was not performed because polymer shielded the NP surface and no reliable signal could be obtained …”

“…Better stabilization of catalytic and magnetic species can be carried out using a polymer as stabilizer. In our preceding work thermally stable polyphenylquinoxaline (PPQ) and a hyperbranched pyridylphenylene polymer (PPP), whose structures are presented in Scheme , were employed as efficient stabilizers of Zn‐containing magnetic oxides designed for a syngas‐to‐methanol transformation . Both polymers are thermally stable up to 400–500 °C, allowing for a wide temperature range of catalytic reactions.…”

Magnetically Recoverable Catalysts: Beyond Magnetic Separation

“…However, no ZnO phase was detected in Zn-containing magnetic oxides, although the activities of these catalysts in the methanol synthesis were much higher than those of conventional catalysts. To further enhance the catalytic activity, we studied the influence of such doping metals as Ni, Co, and Cr on the structure of Zn-containing magnetic oxides and their catalytic properties in the methanol synthesis (Baird et al, 2017 ). Two thermally stable capping polymers have been explored: (i) linear PPQ (Singh et al, 1995 ; Keshtov et al, 2001 ) and a hyperbranched pyridylphenylene polymer (PPP) (Kuchkina et al, 2015 ).…”

“…At low doping metal contents, a significant increase of catalytic activity has been observed (the most pronounced for hyperbranched PPP). In neither case, however, Zn or doping metal formed a separate phase and the Fe 2+ :Fe 3+ atomic ratio of magnetite was preserved, both on the NP surface and in subsurface layers (by X-ray photoelectron spectroscopy, XPS) (Baird et al, 2017 ). XPS also showed that there is a gradient in Zn and doping metal contents with clear surface enrichment.…”