Surface studies of magnesium oxide‐based catalysts modified with X₂ or MgX₂ (X = Br, I)

Abstract

The introduction of a halide (Br, I) onto MgO leads to obtaining catalysts whose catalytic activity in transfer hydrogenation of acrolein with ethanol is significantly higher than that of unmodified MgO. In data found in literature, there is no concise view on how the modification of the surface of MgO with a halide affects its surface properties. Although most of the literature studies indicate a dependence of activity on the initial halide‐containing compound and type of halide, no study comprising both these aspects has been performed. The aim of the present study was to conduct measurements of MgO modified with two halides and with two types of initial halide‐containing compounds with surface‐sensitive techniques to determine how each of these factors influences the surface properties of the catalyst. Titration experiments showed that all of the modified catalysts have a smaller diversity of basic site strengths than unmodified MgO. They also revealed that for all studied systems, the modification of the surface leads to the formation of Brønsted acidic sites, which MgO does not possess. XPS spectra indicate that bromine‐modified catalysts give a broader range of surface species than the appropriate iodine‐modified ones. Moreover, they showed that in the case of both iodine‐modified and bromine‐modified catalysts, the one that exhibited the higher concentration of the appropriate halogen on the surface showed a lower activity in catalytic transfer hydrogenation of acrolein with ethanol. Copyright © 2015 John Wiley & Sons, Ltd.

Magnesium oxide modified with various iodine-containing compounds--Surface studies

“…In fact, it is one of the strongest solid bases . As our previous studies have shown, the modification of the surface with the deposition of bromine or iodine by impregnation of the MgO with a methanol solution of the halide precursor can influence the strength of active sites, as well as introduce new types of active sites . The iodine‐modified catalysts exhibit narrow XPS Mg 2p signals, whereas those of unmodified MgO and bromine‐modified catalysts exhibit wide signals, which is in line with the fact that iodine is more selective than bromine and hence has a less varied chemical environment.…”

“…When the counterion is changed from potassium to hydrogen, the activity of thus obtained system is significantly higher, ie, 2.93. Values of 3.30 and 3.50 were obtained for MgO‐MgI 2 and MgO‐I 2 , respectively (data taken from our previously published results). The modified catalysts contain 0.3% to 0.6% of iodine on the surface as determined by XPS analysis.…”

“…Strength of Brønsted acidic and basic sites as determined using Hammett indicators; * data taken from our previous paper…”

“…The preparation of magnesium oxide has been described in detail in our previous paper . In brief, magnesium nitrate (Mg(NO 3 ) 2 · 6 H 2 O, pa, puriss, Fluka) and an aqueous solution of NH 3 (25%, pa, POCh) were used to obtain magnesium hydroxide.…”

“…The following Hammett indicators were used to determine the strength of acidic and basic sites on the surface of the catalysts: dicinnamylideneacetone, crystal violet, methyl red, bromothymol blue, phenolphthalein, 2,4‐dinitroaniline, 4‐nitroaniline, diphenylamine, 4‐chloraniline, and triphenylmethane. Their properties as well as the procedure are summarized in our previous article . In short, 30 mg of a catalyst were mixed with 0.5 cm 3 of anhydrous toluene (pa, POCh; distilled under normal pressure, refluxed with metallic sodium and benzophenone under an atmosphere of nitrogen and distilled again) and 0.15 cm 3 of 0.1% solution of the appropriate indicator in anhydrous toluene.…”