Missing Building Blocks Defects in a Porous Hydrogen-bonded Amide-Imidazolate Network Proven by Positron Annihilation Lifetime Spectroscopy

Abstract

In situ imidazolate‐4,5‐diamide‐2‐olate (L2) linker generation under conventional electrical (CE) heating and microwave (MW)‐assisted conditions leads to the formation of the porous three‐dimensional hydrogen‐bonded molecular building block (MBB) [Cd14(L2)12(O)(OH)2(H2O)4(DMF)4] based network, named as HIF‐3. In the MBB a Cd6 octahedron is inscribed in a Cd8 cube. The evacuated materials show good N2, CO2, and H2 gas sorption, in comparison with other H‐bonded networks. The desorption branches exhibit a broad hysteresis, suggesting that the materials show flexible behavior during gas uptake. Positron annihilation lifetime spectroscopy (PALS) shows inherent crystal defects of the materials. PALS indicates that the contribution of mesopores (10 and 14 % for HIF‐3‐CE and ‐MW, respectively) due to missing building blocks, makes the frameworks structurally flexible.

Defective Metal‐Organic Frameworks

“…In 2016, Mondal et al applied PALS to study mesopores in hydrogen‐bonded imidazolate framework (HIF‐3) . The authors assigned missing building blocks in HIF‐3 as reason for mesopores that are responsible for structural flexibility during gas uptake.…”

Metal–organic frameworks in Germany: From synthesis to function

“…Already in 2010 PALS was used by M. Liu et al for this purpose [230]. However, in 2016 S. S. Mondal et al [226] applied PALS to study mesopore formation in hydrogenbonded imidazolate framework (HIF-3). The authors assigned missing building blocks in HIF-3 as reason for mesopores which are responsible for structural flexibility during gas uptake.…”

Shedding New Light on Nanostructured Catalysts with Positron Annihilation Spectroscopy

“…In crystalline microporous solids like zeolites, MOFs, or COFs, the observation of short‐lived o Ps species (τ ≈ 1–20 ns) is increasingly recognized as a promising means to gain structural insight into the crystal size, pore topology, and presence of defects. [38a,40‐49] This is of particular interest, since the nanostructuring of these materials, for example, to improve transport properties or expand the functionality in catalytic and other applications, often increases the complexity of structure determination due to the reduced size and/or order of crystalline domains. [9b] The sensitivity of PALS to the pore size was supported by an early comparison of different porous materials that demonstrated a general trend between the o Ps lifetime and the average pore diameter .…”

“…Beyond the study of pore networks, positron annihilation measurements can provide valuable information about various other important properties of nanostructured catalysts. Prominent examples include the analysis of acidic centers in aluminosilicate‐based compounds like zeolites, of defect structures such as missing building blocks in microporous organic materials, vacancies in strategically engineered metal (hydr)oxides, substitutional defects, and interfacial properties in composite materials …”

“…Studies of structural imperfections in nanostructured catalysts by PAS cover different length scales in diverse materials, from missing building blocks in MOFs and HOFs to atomic vacancies in bismuth‐vanadate layers, titanium oxide, and layered double hydroxides (LDHs) . The analysis shares many common features with the study of micropore topology (Section ), since these types of defects essentially lead to voids reducing the electron density, thereby increasing the lifetime of the associated events, but for smaller features the focus may shift to the analysis of positron rather than o Ps lifetimes.…”