Three Series of Sulfo‐Functionalized Mixed‐Linker CAU‐10 Analogues: Sorption Properties, Proton Conductivity, and Catalytic Activity

Abstract

Ten mixed-linker metal-organic frameworks [Al(OH)(m-BDC-X)(1-y)(m-BDC-SO3H)y] (H2BDC = 1,3-benzenedicarboxylic acid; X = H, NO2, OH) exhibiting the CAU-10-type structure were synthesized. The compounds can be grouped into three series according to the combination of ligands employed. The three series of compounds were obtained by employing different ratios of m-H2 BDC-X and m-H2BDC-SO3Li. The resulting compounds, which are denoted CAU-10-H/Sx, -N/Sx and -O/Sx, show exceptionally high thermal stability for sulfonated materials of up to 350 °C. Detailed characterization with special focus on polarity and acidity was performed, and the impact of the additional SO3H groups is clearly demonstrated by changes in the sorption affinities/capacities towards several gases and water vapor. In addition, selected samples were evaluated for proton conductivity and as catalysts for the gas-phase dehydration of ethanol to ethylene. While only very low proton conductivities were observed, a pronounced increase in catalytic activity was achieved. Although reactions were performed at temperatures of 250 and 300 °C for more than 40 h, no desulfonation and no loss of crystallinity were observed, and stable ethanol conversion resulted. This demonstrates the high stability of this material.

Screening of mixed-linker CAU-10 MOF materials for humidity sensing by impedance spectroscopy

“…Hydrophilic modification leads to an increase in water adsorption affinity at low relative humidity values which in turn shifts pore filling to lower p/p 0 values [17]. This correlation is partly represented by the comparison of our sulfonic acid-modified samples with the unmodified samples [9,13]. For all three title compounds the correlation holds with increasing Henry constant, which can be expected for stronger adsorbateeadsorbent interactions.…”

“…Water vapour sorption measurements smallest deviation [9,13]. This is explained by the considerably smaller sulfonate content of this sample.…”

“…The resulting precipitate was filtered off, washed with water, and dried in air. Further synthesis details have been reported elsewhere (N. Reimer et al [13]). …”

“…The additional functional group has a strong influence on the sorption properties of the respective material. Based on that fact, a fine-tuning of this property was performed using a mixed-linker approach [12,13]. Recently Reimer et al synthesized the mixed-linker compounds CAU-10-H/SO 3 H, CAU-10-NO 2 /SO 3 H and CAU-10-OH/SO 3 H containing different ratios of the two linker molecules, respectively.…”

“…Recently Reimer et al synthesized the mixed-linker compounds CAU-10-H/SO 3 H, CAU-10-NO 2 /SO 3 H and CAU-10-OH/SO 3 H containing different ratios of the two linker molecules, respectively. With increasing amounts of eSO 3 H groups the hydrophobic/hydrophilic character of these compounds could be varied [13]. The strong affinity towards water vapour in combination with a high thermal stability up to 350e400 C, which is considerably higher than typical operating temperatures of commonly used polymer-based humidity sensor devices, turns these compounds into promising candidates even for hightemperature humidity sensing.…”