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

Abstract

The activity of iodine-modified MgO in transfer hydrogenation of acrolein with alcohols can be significantly higher than that of unmodified MgO. Data from literature and our previous studies indicate that the initial source of a halide influences the surface properties and hence the activity of the obtained catalytic system. The aim of this paper is to present a systematic study on the influence of the initial source of a halide on the activity and surface properties of MgO-based catalysts. For the sake of comparison, results for an MgO sample subjected to the same procedure as the iodine-modified catalysts (MgO-CH 3 OH) are included in the manuscript. The following order of activity was observed: MgO-I 2 > MgO-MgI 2 > MgO-HI >> MgO-KI > MgO = MgO-CH 3 OH. The second part of the study consisted of characterization studies aimed at determining the strengths of Brønsted basic and acidic sites, total concentrations of acidic and basic sites, topography of the surfaces, elemental composition, differences in the chemical environment of the surface ions, as well as relative amounts of different bands in an attenuated total reflectance-Fourier transform infrared spectrum, which correspond to the bonds present on the surface of the samples. Some parameters were independent of the iodine precursor used (eg, the breadth of the Mg 2p signal), whereas others strongly varied from one sample to another (eg, the range of strengths of the Brønsted acidic and basic sites). The ratio of 2 carbonate bands of the attenuated total reflectance-Fourier transform infrared spectra of the modified catalysts shows a trend which corresponds to the order of activity. KEYWORDS acrolein, ATR-FTIR, catalytic transfer hydrogenation, ethanol, methanol, modified MgO catalysts 1 | INTRODUCTION Magnesium oxide is a solid that exhibits good catalytic activity in transfer hydrogenation of various carbonyl compounds 1-4 due to the wide range of strengths of basic sites. In fact, it is one of the strongest solid bases. 5 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. 6 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. Therefore, methanol/thermal treatment was considered to have a negligible effect on the surface of MgO. Since it has been demonstrated that the properties of a modified catalyst depend not only on the halide but also the initial compound introduced onto the surface of MgO, we have extended our studies by looking at new iodine precursors and incorporating other characterization techniques, such as attenuated total reflectance-Fourier transform infrared (ATR-FTIR), which is another method mentione...

Catalytic Transfer Hydrogenation Performance of Magnesium-Doped ZrO2 Solid Solutions

“…There are many methods of assessing the strength or concentration of basic sites found on the surface of a solid oxide. Some methods can assess only the former, e.g., the use of indicators [46], whereas other techniques can assess only the latter, e.g., titration with benzoic acid [46]. However, Temperature-Programmed Desorption of carbon dioxide can be used to determine both the strength of the sites, which is indicated by the temperature of the desorption peak, and the concentration of the basic sites, which can be obtained by integration of the area under the desorption peak [17,28,30,[47][48][49].…”

Catalytic Activity of High-Surface-Area Amorphous MgO Obtained from Upsalite

“…The high basicity of the MgO surface makes it an attractive catalyst that is used in numerous chemical processes, such as the transesterification of fats with methanol to produce fatty acid methyl esters (FAME) [12][13][14], the alkylation of phenol with methanol to 2,6-dimethylphenol [15,16], or the isomerization of a carbon-carbon double bond in the Shell Higher Olefin Process (SHOP) [17,18]. It is noteworthy that magnesium oxide is one of the most active metal oxides in the transfer hydrogenation reaction to carbonyl compounds using alcohols as hydrogen donors [19,20], although many other transfer hydrogenation catalysts continue to be widely investigated [21][22][23][24][25]. The reaction was shown to occur on supported metal and alloy catalysts, such as Au, Pt, Pd, Ru, Ag-Pd, etc.…”

“…In contrast to the supported cobalt and copper systems studied in this reaction [24] (and references therein), magnesia is a single-component system, which does not require a multi-step synthesis and is environmentally benign. Moreover, an advantage of this oxide is that it exhibits numerous basic sites along with weak Lewis acid sites on its surface [25], which are responsible for this transfer hydrogenation and can lead to high selectivity. A study of different catalytic systems conducted by Feng et al [26] showed that by-products were formed on those systems that had strong acidic sites, namely Pd/SiO 2 , Pd/Al 2 O 3 , and Pd/TiO 2 , whereas high selectivity of Pd/C and Pd/NaY was observed, which was attributed to the lack of numerous strong acid sites.…”