Quantifiable Surface Methoxy Groups on Zr(OH) Groups of UiO-66 Metal–Organic Framework: Generation from Methanol-¹³C and Reactivity

Abstract

We herein conduct the first 1 H and 13 C MAS NMR investigations that explore the formation of surface methoxy groups (SMG) on defect Zr(OH) groups of UiO-66 metal−organic frameworks. Loading acetone-2-13 C indicates that UiO-66 contains weakly acidic Zr(OH) groups. These react at room temperature with adsorbed methanol-13 C to SMG. Quantitative formation of SMG and methanol removal are achieved at 473 K. 1 H− 13 C heteronuclear correlation proves close proximity between persistent, inaccessible Zr(OH) groups at δ 1H = 1.2 ppm and SMG groups. SMG react with water to free methanol; however, the SMG cannot methylate hydrocarbons like toluene. This is explained by the weak acidity of Zr(OH) groups hosting the SMG. The 0.17 mmol/g SMG that was formed corresponds to 13% missing linkers, in good agreement with 11% missing linkers calculated from TGA. Thus, the formation and quantification of SMG by 13 C MAS NMR spectroscopy are demonstrated as alternative measures for the amount of Zr(OH) in defects.

Solid-State NMR Probe Molecules for Catalysts and Adsorbents: Concepts, Quantification, Accessibility, and Spatial Distribution

“…Acetone-2- 13 C has in particular been loaded on zeolites and zeotypes ZSM-5, ,,, Y, ,,, MOR, ZSM­12, ZSM-22, MAPO-5, MAPO-34, − amorphous silica-alumina, ,, [Al]­MCM-41, , [Sn]­MCM-41, [Sn]­Beta, [Sn]­SSZ-13, on γ-Al 2 O 3 , , on TiO 2 (anatase), on heteropoly acid H 3 PW 12 O 40 , ,, on pure and sulfated SnO 2 and ZrO 2 , on MoO 3 /SnO 2 , on pure, MoO x - and WO x -loaded ZrO 2 , on acidic metal salts (mainly halogenides at low temperature), and on UiO-66 hydroxyl groups. , Acetone-2- 13 C reacts at room temperature on acid sites mainly via aldol condensation. ,,,,,, The 13 C MAS NMR peaks caused by the reaction products are usually of no further relevance for the interpretation; however, they cause often confusion and shall thus be addressed. In particular, NMR peaks of diacetone were identified around δ 13C = 209 and 73 ppm, of mesityl oxide at δ 13C = 210 and 188 ppm, and of isophorone and mesitylene around δ 13C = 210, 180, 138, and 34 ppm. , On [Sn]­Beta, supported by 119 Sn MAS NMR spectra, gem -diol-type intermediates giving rise to a slim peak at δ 13C = 76 ppm were reported .…”

“…However, based on the available literature, it is suggested that rather strongly adsorbed methanol gave rise to this signal. 1 H MAS NMR chemical shifts of methyl group protons are found at δ 1H = 3.8 ± 0.2 ppm. ,, Methanol interacts surprisingly strong with the surface, and thus the desorption of excess methanol must be performed at elevated temperatures. , As a too high temperature will lead to methanol dehydration to DME, an intermediate methylation and desorption temperature of 393 K or below is suggested, while temperatures above 400 K are discouraged. , Note that recycle delay times up to 30 s were applied for quantitative measurements of SMG. , X-ray powder diffraction data on methanol adsorbed in ZSM-5 micropores are provided elsewhere ,− Physisorbed methyl iodide gives peaks at δ 13C = −18 to −25 ppm, far distant from the chemical shift range of SMG .…”

“…A typical example would be a correlation between the nucleus of a probe molecule and a nearby surface site. Luckily, the strong peak enhancement due to the nonequilibrium polarization enables thereby the indirect measurements of formerly barely accessible nuclei. ,,, Thus, such correlations enable a much better understanding of the location of probe molecules. Especially dynamic nuclear polarization (DNP) provides new insights into the spatial location of sites and great signal enhancements.…”