Structural and Mechanistic Differences in Mixed-Linker Zeolitic Imidazolate Framework Synthesis by Solvent Assisted Linker Exchange and de Novo Routes

Abstract

Mixed-linker zeolitic imidazolate frameworks (ZIFs) are a subclass of metal-organic frameworks (MOFs) amenable to significant property tuning by altering the functional groups on the imidazolate linkers. Solvent assisted linker exchange (SALE) and de novo synthesis of mixed-linker ZIFs have been demonstrated, but the differences in structural properties-most importantly the linker distributions-and synthesis mechanisms of these two different types of hybrid ZIFs are unknown. In this work, a combination of H NMR combined rotation and multiple pulse spectroscopy (CRAMPS), water adsorption, and nitrogen measurements reveal distinct differences in linker mixing between SALE and de novo ZIF-8-90 hybrids. Native-fluorescence confocal microscopy is shown to provide a direct means to visualize these differences. The effects of crystal size, temperature, and SALE duration were studied in detail, and a generalizable mechanism for SALE processes in ZIFs is proposed. The SALE process is found to follow a diffusion-limited behavior leading to core-shell morphologies. Under harsher SALE conditions, deviations from diffusion-limited behavior are found due to etching and partial dissolution of the initial ZIF-8 crystals. With the selection of appropriate reaction conditions, SALE processes appear to be capable of generating controlled core-shell ZIF structures of good morphological quality that complement the well-mixed structures obtained by de novo methods.

Vapor-Phase Linker Exchange of the Metal-Organic Framework ZIF-8: A Solvent-Free Approach to Post-Synthetic Modification

“…As CHO-Im is prone to side-reactions, [13][14][15] VPLE trials (24 h) were performed at different temperatures.R eaction at 353 and 373 Kresulted in ayellow ZIF powder, similar to what is obtained through direct synthesis or PSLE with CHO-HIm in solution. [16,17] X-ray diffraction (XRD) confirms that the sod topology is maintained after VPLE. 1 HNMR analysis of the acid-digested samples revealed the incorporation of intact CHO-Im ( Figure S6).…”

“…Ac lear dependence of the linker exchange kinetics on the particle size would indicate diffusion limitations during the exchange process. [16] In that case,a fter 24 ho f exchange,t he CHO-Im linker fraction was 70 %f or nanocrystals compared to only 26 %for micron-sized crystals. FTIR spectroscopy ( Figure S11) confirms the progressive decrease of mIm bands (e.g.,a t9 91 cm À1 ), accompanied by increasing signals characteristic of CHO-Im incorporation (e.g., at 1667 cm À1 ).…”

“…[18][19][20][21] Foreach linker, VPLE was performed below the melting point and the thermal stability limit (Table S2). [16] Error bars correspond to the standard deviation of three experimental points. More bulky linkers and linkers that can form ZIF topologies other than sod seem to be incorporated less.Nevertheless,only the sod topology was observed, thus supporting the notion of asingle-crystal-to-single-crystal transformation (Figures S20-21).…”

“…We hypothesize that, similar as for acid gases in ZIFs,t he incoming linker and water form am ore reactive complex. [16] Since VPLE is not diffusion-limited (see above), other factors must determine the linker distribution. Solid-state 1 HNMR spin-diffusion measurements were performed for VPLE samples with aZ IF-8 49 -90 51 composition (ZIF VPLE ).…”

Cross-dehydrogenative coupling of coumarins with Csp3–H bonds using an iron–organic framework as a productive heterogeneous catalyst

“…[24][25][26] If two linkers are present in the frameworks, attractive properties might be achieved. 27 A variety of organic transformations utilizing iron-based MOFs as heterogeneous catalysts were previously reported in the literature.…”