The γ-cyclodextrin (γ-CD) metal–organic frameworks (CD-MOF-1) consist of γ-CD and potassium (K+) ions through coordinating an eight-coordinated K+ ion with two C5-linked oxygen and C6-linked hydroxyl (C5–O/C6–OH) groups in the primary faces of adjacent γ-CD units and two C2- and C3-linked hydroxyl (C2–OH/C3–OH) groups in the secondary faces. Herein, we found polysaccharide gels with only C2–OH/C3–OH or C5–O/C6–OH groups in pyranoid rings can form four-coordinated K+ ions and then coordinate γ-CD in a KOH solution for CD-MOF-1 growth. Exposure of C2–OH/C3–OH or C5–O/C6–OH groups in polysaccharide gels is important to form active four-coordinated K+ ions. Mechanism supporting this work is that four-coordinated K+ ion sites are first formed after coordinating C2–OH/C3–OH groups in pectin and then coordinating C5–O/C6–OH groups in the primary faces of γ-CD units. Alternatively, four-coordinated K+ ions with C5–O/C6–OH groups in chitosan can coordinate the C2–OH/C3–OH groups in the secondary faces of γ-CD units. Mechanism of CD-MOF-1 growing on pectin and chitosan gels through the proposed four-coordinated K+ ions is also universally applicable to other polysaccharide gels with similar C2–OH/C3–OH or C5–O/C6–OH groups such as alginate gel. Based on this mechanism, we developed pectin and chitosan gel-based CD-MOF-1 composites and exemplified applications of them in antibacterial and organic dye removal. To help future research and applications of this mechanism, we share our theoretical assumption for further investigations that any matrices with an ortho-hydroxyl carbon chain or ortho-hydroxyl ether structures may form four-coordinated K+ ions for CD-MOF-1 growth. The proposed mechanism will broaden the development of novel CD-MOF-1 composites in various fields.
The valence states of Cu site are regarded as an important factor affecting the performance of syngas to higher alcohols (C 2+ OH). Regrettably, the role of Cu 0 and Cu + in CO hydrogenation to higher alcohols remains controversial due to the complexity of the reaction. In this study, Cu 0 and Cu 2 O powders are directly added as the precursors to adjust the ratios of Cu + /(Cu 0 +Cu + ) and Cu particle sizes in CuZnAl catalysts by a complete liquid-phase (CLP) technology. The CAT-Cu 0 catalyst with higher Cu 0 content shows the highest CO conversion (>15%) ascribing to the biggest S Cu , but is also accompanied by the highest CO 2 selectivity (>40%), while the CAT-Cu + catalysts with higher Cu + content displays relative better C 2+ OH selectivity (>60%) and stability due to a higher probability of CO insertion. In-situ DRIFTS experiments reveal that a synergy between larger Cu particle sizes (30～40 nm) and surface Cu + sites, in which larger Cu sizes enhances the probability of bridge CO adsorption, thereby favoring the C–O bond dissociation for CH x formation. Meanwhile, Cu + active sites provide a higher probability for CO/CHO insertion to form CH 3 CHO*/CH 2 CO* intermediates. This study further deepens our understanding of the critical controlling factors for highly selective synthesis of C 2+ OH from syngas over Cu-based catalysts.
Graphical The Al-doped ZnO catalyst has a lower work function, which is conducive to promoting the dissociation of CH x O* to form CH x *; it also facilitates the generation of Zn x+ and oxygen vacancies, which further promotes the generation of C 2+ OH. The direct conversion of syngas into C 2+ OH has attracted significant attention as a potential pathway for energy conversion. However, this process typically requires the use of precious metals or the collaborative action of multi-component catalysts, making it relatively complex. In this paper, simple and economical Al-doped ZnO catalysts were employed for the synthesis of C 2+ OH. The results show that the work function, Zn x+, and the relative content of oxygen vacancies are linearly related to C 2+ OH/ROH. When the doping amount was 7 mol%, the catalyst had the highest C 2+ OH/ROH (82.6 wt%), corresponding to the lowest work function value, the highest Zn x+ content, and the relative content of oxygen vacancies. We reveal that the successful synthesis of C 2+ OH depends not only on the low work function of the Al-doped ZnO but also on the existence of the oxygen vacancies and Zn x+ sites. Specifically, the low work function induced by Al doping facilitates electron transfer in the reaction, facilitating the dissociation of CH x O* into CH x *. Meanwhile, the presence of oxygen vacancies enhances the dissociative adsorption capacity of CO and facilitates the formation of CH x * species. The Zn x+ sites serve as active centers for carbon chain growth, promoting C–C coupling through the insertion of CO or CH x O* into CH x *, thereby achieving chain elongation.
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