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Hydrogenations and Other Reactions on Titanium Mixed Metal Oxides
Published in John R. Kosak, Thomas A. Johnson, Catalysis of Organic Reactions, 2020
Gether Irick, Patricia N. Mercer
Hydrogenolysis of esters to alcohols is a reaction of considerable commercial importance for production of fatty alcohols, 1,4-cyclohexanedimethanol, and 1,4-butanediol. Copper-chromium oxides have proven to be the catalysts of choice, but are becoming increasingly difficult to use because of the toxic nature of the spent catalyst (related to chromium and sometimes barium contents). Mixed oxides of copper and titanium, either neat or supported on silica were found to be excellent catalysts for the vapor-phase hydrogenolysis of methyl acetate to methanol and ethanol (Table 4). Hydrogenolysis activities increased with increasing temperature in the 190–289°C range and were accompanied by an increase in transesterification activity. The silica-supported catalyst was a much more active transesterification catalyst than the mixed oxide catalyst at all temperatures and also produced more methane and ethane.
Feedstock Purification
Published in Martyn V. Twigg, Catalyst Handbook, 2018
In general, the rate of hydrogenolysis is first-order with respect to the sulphur compound. The order of reaction with respect to the partial pressure of hydrogen lies between zero-order and first-order, depending on the nature of the sulphur compounds present. The equilibrium constants for the hydrogenolysis of organic sulphur compounds are large and positive, even at temperatures as high as 500°C. The rate of hydrogenolysis increases with temperature, and under the usual process conditions between 350°C and 400°C with an excess of hydrogen the hydrogenolysis is normally complete. Figure 4.2 shows the variation of the equilibrium constants for the hydrogenolysis of some sulphur compounds with temperature. There is a marked difference in the rate of hydrogenolysis of thiophene and other commonly found sulphur compounds. This is illustrated in Figure 4.3, which shows the ease of hydrogenolysis of a number of sulphur compounds using a heptane feed at atmospheric pressure and 370°C and 250°C.
Study on activity and stability of Pt-Ru/WOx/Al2O3 catalyst in the glycerol selective hydrogenolysis to 1,3-propanediol
Published in Chemical Engineering Communications, 2023
Yinglin Wen, Shiyu Liu, Weihua Shen, Yunjin Fang
Several reaction mechanisms have been proposed for the selective hydrogenolysis of glycerol to 1,3-PDO. Two primary mechanisms have been identified as dehydration-hydrogenation and direct hydrogenolysis (Wang et al. 2020). Combined with multiple literature (Chen et al. 2020; Zhou et al. 2019; Wang, Lei, et al. 2016; Wang, Zhao, et al 2016), a direct hydrogenolysis mechanism has been proposed and shown in Figure 8. Initially, a large amount of H2 is activated by the metal site of the Pt-Ru reduction catalyst. Two hydrogen atoms dissociated from hydrogen on the metal sites overflow onto the support. One of the hydrogen atoms will form H+ after releasing an electron. The (WOx)-H+ (Bronsted acid site) is generated when H+ attaches on WOx. Another hydrogen atom on the metal sites will turn into H−in situ with taking one electron. Second, a glycerol will adsorb on W = O of WOx (Zhou et al. 2019; Wang and Chen 2019). Glycerol’s internal hydroxyl group will be attacked by Bronsted acid sites, resulting in dehydration and formation of carbocation. Stabilization of this product is possible with the use of Pt-(WOx)nδ− species (David et al. 1999). Third, carbocation will be attacked by H- adsorbed on Pt-Ru metal site, resulting in the formation of 1,3-PDO that adsorb on catalyst surface. Finally, 1,3-PDO will dissociate from the surface and the catalyst will be decreased throughout the process.
Waste tire pyrolysis and desulfurization of tire pyrolytic oil (TPO) – A review
Published in Journal of the Air & Waste Management Association, 2023
Moshe Mello, Hilary Rutto, Tumisang Seodigeng
The most common and generally accepted reaction mechanism followed during the HDS process was first proposed by Houalla et al. (1978; 1980). There are two main reaction mechanism pathways suggested for desulfurization of DBT and 4,6-DMDBT using the conventional HDS technique; Pathway (1) allows sulfur to be removed without abetting the aromatic rings. The reaction occurs through direct desulfurization or hydrogenolysis, where the carbon-sulfur single bond is cleaved by hydrogen. Pathway (2) follows the hydrogenation process whereby aromatic rings of DBT containing compounds are hydrogenated to 4H-or 6H-DBT intermediates and are subsequently desulfurized. The two pathways are represented in Figure 2 (Houalla et al. 1978). Pathway 1, which involves direct desulfurization, shows that through the C-S bond, hydrogenolysis of DBT gives biphenyl (BiPh). The subsequent reaction involves hydrogenation of the BiPh compound which produces cyclohexylbenzene (CHB).
An expedient carbon–sulfur bond formation explored through the cellulose sulfonic acid (CSA) catalyzed dithioacetal protection of carbonyl compounds
Published in Journal of Sulfur Chemistry, 2020
The carbonyl group is one of the most studied and highly appraised functional group in organic synthesis. Due to its’ high reactivity and wide range of transformations, it has been contributing more than any other functional group to organic synthesis [1]. In multi-step synthesis, its high reactivity often becomes an obstacle, especially where poly-functional groups are involved. In such cases it needs to be temporarily protected in a suitable form. Among the existing protecting groups, 1,1-dithioacetal protection is recognized as the most versatile due to its ease of formation, inherent stability towards acidic or basic conditions [2], and ability to revert the polarity of the carbonyl group [3]. Dithioacetals have been used as the precursor in alkylation [4], olefination [5], fluorination [6] hydrogenolysis [7], autoxidation [8] hydrodesulfurization [9] as well as in C–H or C–S bond activation [10,11]. Sulfur-containing compounds have found an important place in the synthesis of natural products [12], pharmaceuticals [13], structural diversifications [14] and in drug discovery [15] as well.