Influence of Terephthalic Acid Substituents on the Catalytic Activity of MIL‐101(Cr) in Three Lewis Acid Catalyzed Reactions

Abstract

Six isostructural MIL‐101(Cr)‐X (X: H, NO2, SO3H, Cl, CH3, and NH2) materials have been prepared directly by the reaction of CrIII salts and the corresponding terephthalic acid or by postsynthetic treatments of preformed MIL‐101(Cr) following reported procedures. The materials were characterized by using XRD (crystallinity and coincident diffraction pattern), isothermal N2 adsorption (specific surface areas range from 2740 m2 g−1 for MIL‐101(Cr)‐H to 1250 m2 g−1 for MIL‐101(Cr)‐Cl), thermogravimetry (thermal stability up to 400 °C), and IR spectroscopy (detection of the corresponding substituents), and the results were all in agreement with the reported data for these materials. The MIL‐101(Cr) materials were tested as heterogeneous catalysts for epoxide ring opening by methanol, benzaldehyde acetalization by methanol, and Prins coupling, observing a clear influence of the substituent that in general follows a linear relationship with the Hammett σmeta constant of the substituent: the catalytic activity increases as the electron‐withdrawing ability of the substituents increases. An up to three orders of magnitude enhancement in the presence of the NO2 substituent was found for some of these reactions. The present study illustrates the versatility that metal–organic frameworks offer as heterogeneous catalysts that allow the design of actives sites with adequate properties tuned for each reaction.

Tuning the Microenvironment of Gold Nanoparticles Encapsulated within MIL‐101(Cr) for the Selective Oxidation of Alcohols with O2: Influence of the Amino Terephthalate Linker

“…Isothermal N 2 adsorption for MIL‐101(Cr)‐H allows estimation of the Brunauer–Emmett–Teller (BET) surface area and pore volume of 2800 m 2 g −1 and 1.7 cm 3 g −1 , respectively (Figure S4 a in the Supporting Information). In agreement with previous studies, the presence of amino groups in the MIL‐101(Cr)‐NH 2 solid decreases the BET surface area (1550 m 2 g −1 ) and pore volume (1.15 cm 3 g −1 ) with respect to the parent material (Figure S4 b in the Supporting Information) owing to the space needed to accommodate the organic substituents in the cavities of the MOF. ICP‐AES analysis of previously acid‐digested MIL‐101(Cr)‐X (X: H or NH 2 ) samples confirms the expected chromium content according the theoretical formulae (Cr 3 F(H 2 O) 2 O(C 8 H 4 O 4 ) 3 and Cr 3 Cl(H 2 O) 2 O(C 8 H 3 O 4 ‐NH 2 ) 3 .…”

“…FTIR spectroscopy allows observation of the expected functional groups presenti nt he solids, namely carboxylates, aromatic rings, and/or amino groups in the case of MIL-101(Cr)-NH 2 (Figure S1 bi nt he Supporting Information).X PS furtherc onfirms the composition and oxidation state of the elements presenti nt he MIL-101(Cr)-H (Figure S2 in the Supporting Information) and MIL-101(Cr)-NH 2 ( Figure S3 in the Supporting Information). Isothermal N 2 adsorptionf or MIL-101(Cr)-H allows estimation of the Brunauer-Emmett-Teller (BET) surfacea rea and pore volume of 2800 m 2 g À1 and 1.7 cm 3 g À1 ,r espectively ( Figure S4 ai nt he Supporting Information).I na greement with previous studies, [12][13][14] the presenceo fa mino groups in the MIL-101(Cr)-NH 2 solid decreasest he BET surface area (1550 m 2 g À1 )a nd pore volume( 1.15 cm 3 g À1 )w ith respectt ot he parent material ( 3 .I ncorporation of gold NPs on these two MOFs was carried out by following the double solvent method( DSM) procedure as previously reported. [15] Chemical analysiso fg old for samples previ-ously digested in acid medias hows that complete gold incorporation hast aken place under DSM conditions.…”

Encapsulation of Metal Nanoparticles within Metal–Organic Frameworks for the Reduction of Nitro Compounds

“…Metal–organic frameworks (MOFs) are solid porous crystalline materials constituted by metallic nodes coordinated to rigid organic linkers, typically bi- or multipodal aromatic carboxylic acids or nitrogenated heterocycles [1,2,3,4,5,6,7,8,9]. MOFs are currently under intense investigation as solid catalysts, mainly due to the Lewis acidity of metal ions at the nodes [10,11,12,13,14,15], but also by possible acid [16] and basic [17] groups present at organic linker (Figure 1). Another possibility to use MOFs in heterogeneous catalysis is as support of metal nanoparticles (MNPs) and other types of guests that could act as active sites in catalysis [18,19,20,21,22,23,24,25,26,27,28,29,30,31,32].…”

Aerobic Homocoupling of Arylboronic Acids Catalyzed by Regenerable Pd(II)@MIL‐88B‐NH2(Cr)

“…[15] MOFs are crystalline polymers made up of metallic nodes linked by polytopic organic linkers through strong coordination bonds. [16] They can act as catalysts as synthesized, [17] since the presence of Lewis acid/base sites at metal nodes or linker substituents respectively are reported to promote a wide variety of reactions. [18,19] They are also widely used as supports for other catalysts, [20] such as single-site transition-metal complexes or metal nanoparticles (NPs).…”