Catalysts in hydrodesulfurization

Olguin, E., Vrinat, M., Cedeno, L., Ramirez, I., Borque, M., and Lopez-Agudo, A. The use of Ti02-A1203 binary oxides as supports for Mo-based catalysts in hydrodesulfurization of thiophene and dibenzothiophene. Applied. Catalysis. A, General, 1997, 165, 1. 

Nagai, M. Kabe, T. Selectivity of molybdenum catalyst in hydrodesulfurization, hydrodenitrogenation, and hydrodeoxygenation effect of additives on dibenzothiophene hydrodesulfurization. J. Catal. 1983, 81, 440-449. 

Titanium disulfide, T1S2, is a desirable cathode material for lithium batteries [102]. It is also used as a catalyst in hydrodesulfurization processes [103] and as a lubricating component in titanium alloys [104a]. Chemical vapor deposition has been employed widely for the preparation of TiS2 thin films [104b]. 

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

Delmon, B. "Recent Approaches To The Anatomy and Physiology of Cobalt Molybdenum Hydrodesulfurization Catalysts" in preprints of Third International Conference on the Chemistry and Uses of Molybdenum, Univ. of Michigan, Ann Arbor, 1979 (Climax Molybdenum Company). 

A Molecular Approach to Synergy Generation in Co Mo Binary Sulfide Catalysts for Hydrodesulfurization... 

Minaev, V. Z. Zaidman, N. M. Spirina, G. A., et al., Effect of Pore Structure of Alumina-Cobalt-Molybdenum Catalyst on Activity and Stability in Hydrodesulfurization of Heavy Feedstocks. Chemistry and Technology of Fuels and Oils, 1975. 11(6) pp. 436-39. 

Trickle Hydrodesulfurization A process for removing sulfur-, nitrogen-, and heavy-metal-compounds from petroleum distillates before catalytic cracking. The preheated feed is hydrogenated, without a catalyst, in an adiabatic reactor at 315 to 430°C. Developed by Shell Development Company. As of 1978, 91 units had been installed. 

From a separation-process point of view, a fluid-fluid reaction is intended to enhance separation (e.g., preparation of feed for a subsequent process step, product purification, or effluent control for environmental protection). Examples include the use of ethanolamines for the removal of H2S and C02 (reactions (A) and (B) in Section 9.2), the removal of SO, by an aqueous stream of a hydroxide, and absorption of 02 by blood or desorption of C02 from blood. A solid catalyst may be involved as a third phase, as in hydrodesulfurization in a trickle-bed reactor. 

If reaction (2-13) follows reaction (2-12) instantaneously, the effect will not be noticeable in the H2 signal [12]. In spite of these limitations, we conclude that TPS with mass spectrometric detection is a highly useful technique for studying the sulfidation of hydrotreating catalysts. We shall return to the sulfidation of molybdenum oxides in the chapters on photoemission (Chapter 3), ion spectroscopy (Chapter 4), and in a case study on hydrodesulfurization catalysts in Chapter 9. 

Mossbauer spectroscopy is one of the techniques that is relatively little used in catalysis. Nevertheless, it has yielded very useful information on a number of important catalysts, such as the iron catalyst for Fischer-Tropsch and ammonia synthesis, and the cobalt-molybdenum catalyst for hydrodesulfurization reactions. The technique is limited to those elements that exhibit the Mossbauer effect. Iron, tin, iridium, ruthenium, antimony, platinum and gold are the ones relevant for catalysis. Through the Mossbauer effect in iron, one can also obtain information on the state of cobalt. Mossbauer spectroscopy provides valuable information on oxidation states, magnetic fields, lattice symmetry and lattice vibrations. Several books on Mossbauer spectroscopy [1-3] and reviews on the application of the technique on catalysts [4—8] are available. 

Measurements of supported catalysts in diffuse reflection and transmission mode are in practice limited to frequencies above those where the support absorbs (below about 1250 cm-1). Infrared Emission Spectroscopy (IRES) offers an alternative in this case. When a material is heated to about 100 °C or higher, it emits a spectrum of infrared radiation in which all the characteristic vibrations appear as clearly recognizable peaks. Although measuring in this mode offers the attractive advantage that low frequencies such as those of metal-oxygen or sulfur-sulfur bonds are easily accessible, the technique has hardly been explored for the purpose of catalyst characterization. An in situ cell for IRES measurements and some experiments on Mo-O-S clusters of interest for hydrodesulfurization catalysts have been described by Weber etal. [11],... 

Optimum catalysts in use today for hydrodesulfurization typically consist of Mo or W metals promoted with Co and supported on weakly acidic alumina supports. Increasingly stringent regulatory requirements have pushed sulfur specifications for transportation fuels to les s than 10 ppm for diesel fuels and 30 ppm for gasoline... 

Electron-transfer and intramolecular redox reactions (related to 82 complexes). The redox behavior of 82 complexes is of particular interest because it can probably provide a foundation for understanding the course of reactions involved in relevant enzymes and catalysts (especially hydrodesulfurization catalysts). Intramolecular redox reactions related to type la 82 ligands can be summarized as follows ... 

Ruthenium carbonyl-derived catalytic systems have also been studied in hydrodesulfuration [118, 119], Highly active catalysts for the hydrodesulfuration of diben-zothiophene have been obtained by supporting on alumina MHRu3(CO)n (M = group 1 metal), which was the product of the reaction between Ru3(CO)i2 and MOH. The activity increased from Li to Cs [119]. 

The chapter Fundamental Studies of Transition-Metal Sulfide Catalytic Materials by Chianelli, Daage, and Ledoux reviews current understanding of the relationship between structural and other properties of these catalysts and their catalytic activity and selectivity in hydrodesulfurization. In view of increasing environmental demands, this field has been heavily researched. The authors show how systematic studies and applications of novel methods can provide considerable understanding of these important catalysts. 

The olefin (369) is thought to arise by reaction of the intermediate diradical (368) with hydrogen before subsequent hydrogenation of 369 to 370.767-769 When benzo[6]thiopheno[3,2-6]benzo[6]thiophene (154) is hydrodesulfurized, bibenzyl, (rans-stilbene, and 2-phenyl-benzo[6]thiophene are obtained in amounts determined by the catalyst and reaction conditions.768 This result lends support to the view that removal of sulfur precedes hydrogenation in hydrodesulfurization reactions. 

One of the more important uses of molybdena catalysts is in hydrodesulfurization processes. In operation, the catalyst is usually presulfided with hydrogen sulfide/hydrogen or other suitable sulfiding agents. Even when presulfiding is not employed, the catalysts become... 

The picture here is even less clear than for reduced catalysts. In spite of the fine studies by Delmon and workers on the bulk mixed sulfides, and that of Schuit and De Beer and workers on various hybrid catalysts, it is not conclusively proved that bulk sulfides are the active ingredients for hydrodesulfurization reactions for the mildly sulfided catalysts employed in industry. 

The hydrodesulfurization process is essentially the reaction of hydrogen with a predominantly hydrocarbon feedstock to produce a desulfurized hydrocarbon product and hydrogen sulfide. In a very simplified process (Figure 5-1), the feedstock is first pressurized to a pressure which is a little higher than that of the reactor section, mixed with hot recycle gas and preheated to the temperature of the reactor inlet. The hot feedstock (and the recycle gas) is then introduced to the catalyst in the reactor where temperatures on the order of 290 to 455°C (550 to 850°F) and pressures in the range 150 to 3000 psi prevail. 

In the same manner as in hydrocracking (Dolbear, 1997), hydrogen is added at intermediate points in hydrodesulfurization reactors. This is important for control of reactor temperatures. The mechanical devices in the reactor, called reactor internals, which accomplish this step are very important to successful processes. If redistribution is not efficient, some areas of the catalyst bed will have more contact with the feedstock. This can lead to three levels of problems ... 

The hydrodesulfurization of low boiling (naphtha) feedstocks is usually a gas-phase reaction and may employ the catalyst in fixed beds and (with all of the reactants in the gaseous phase) only minimal diffusion problems are encountered within the catalyst pore system. It is, however, important that the feedstock be completely volatile before entering the reactor as there may be the possibility of pressure variations (leading to less satisfactory results) if some of the feedstock enters the reactor in the liquid phase and is vaporized within the reactor. 

In summary, the presence of large proportions of asphaltic materials in hydrodesulfurization feedstocks can adversely affect the process in many ways, not the least of which are reduced catalyst life and hydrogen requirements. It appears that there may be a case for deasphalting the feedstock prior to the hydro-... 

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