The Acid Strength of Surfaces
01 Mar 1950-Journal of the American Chemical Society (American Chemical Society)-Vol. 72, Iss: 3, pp 1164-1168
About: This article is published in Journal of the American Chemical Society.The article was published on 1950-03-01. It has received 203 citations till now. The article focuses on the topics: Acid strength.
TL;DR: In this article, in situ FTIR spectra of py adsorbed at room (RT) and higher temperature regimes were measured, and compared with RT-spectra of liquid and gas phase Py obtained under identical spectroscopic conditions.
Abstract: Exposure of strong Lewis (coordinatively unsaturated metal atoms) and Bronsted (proton donor OH-groups) acid sites on solid surfaces is a prime demand for potential adsorptive and catalytic applications. In situ FTIR spectroscopy of small adsorbed base molecules, often NH 3 , pyridine, CH 3 CN, NO or CO molecules, has been well established as a powerful surface analytical technique for characterization of nature, strength and concentration of acid sites. Pyridine (Py) has been preferred as an IR probe molecule of finely divided metal oxide surfaces at room (RT) and higher temperature regimes, since it is (i) more selective and stable than NH 3 ; (ii) much more strongly adsorbed than CO and CH 3 CN; and (iii) relatively more sensitive to the strength of Lewis acid sites than NO. In the present work, in situ IR spectra of Py adsorbed at ≥RT on characterized alumina, silica, silica–alumina, titania, zirconia and ceria were measured, and compared with RT-spectra of liquid and gas phase Py obtained under identical spectroscopic conditions, in order to characterize spectral consequences of mutual Py–Py interactions in the adsorbed phase. It has been concluded that the availability of Lewis acid sites can be unequivocally monitored by formation of coordinated Py molecules giving rise to IR-absorption(s) due to the ν 8a mode of ν CCN vibrations at 1630–1600 cm −1 , where the higher the frequency assumed, the stronger the acidity of the site. Formation of pyridinium surface species (PyH + ) is identifiable by (i) an ν 8a -absorption at ≥1630 cm −1 ; (ii) an ν 19b -absorption at 1550–1530 cm −1 ; as well as (iii) ν N + H and δ N + H absorptions occurring, respectively, near 2450 and 1580 cm −1 , and, thus, the availability of Bronsted acid sites. Moreover, products and IR-characteristics of Py surface reactions at >RT have been identified, and used to imply nature of surface base sites (OH − and O 2− ) involved in formation of acid–base site pairs.
TL;DR: Adsorption, therefore, appears to be one of the major factors affecting the interactions occurring between pesticides and soil colloids.
Abstract: Seven factors are known to influence the fate and behavior of pesticides in soil systems: (1) chemical decomposition, (2) photochemical decomposition, (3) microbial decomposition, (4) volatilization, (5) movement, (6) plant or organism uptake, and (7) adsorption. The phenomenon of adsorption-desorption directly or indirectly influences the magnitude of the effect of the other six factors. Adsorption, therefore, appears to be one of the major factors affecting the interactions occurring between pesticides and soil colloids.
TL;DR: This material has a Hammett acidity function H0 ≤ -14.5 and is thus identified as a superacid, providing the first evidence for superacidity in MOFs, attributed to the presence of zirconium-bound sulfate groups structurally characterized using single-crystal X-ray diffraction analysis.
Abstract: Superacids, defined as acids with a Hammett acidity function H0 ≤ −12, are useful materials, but a need exists for new, designable solid state systems. Here, we report superacidity in a sulfated metal–organic framework (MOF) obtained by treating the microcrystalline form of MOF-808 [MOF-808-P: Zr6O5(OH)3(BTC)2(HCOO)5(H2O)2, BTC = 1,3,5-benzenetricarboxylate] with aqueous sulfuric acid to generate its sulfated analogue, MOF-808-2.5SO4 [Zr6O5(OH)3(BTC)2(SO4)2.5(H2O)2.5]. This material has a Hammett acidity function H0 ≤ −14.5 and is thus identified as a superacid, providing the first evidence for superacidity in MOFs. The superacidity is attributed to the presence of zirconium-bound sulfate groups structurally characterized using single-crystal X-ray diffraction analysis.
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