Poisoning by sulfur

Rhenium oxides have been studied as catalyst materials in oxidation reactions of sulfur dioxide to sulfur trioxide, sulfite to sulfate, and nitrite to nitrate. There has been no commercial development in this area. These compounds have also been used as catalysts for reductions, but appear not to have exceptional properties. Rhenium sulfide catalysts have been used for hydrogenations of organic compounds, including benzene and styrene, and for dehydrogenation of alcohols to give aldehydes (qv) and ketones (qv). The significant property of these catalyst systems is that they are not poisoned by sulfur compounds. 

Catalysis by Metal Sulfides. Metal sulfides such as M0S2, WS2, and many others catalyze numerous reactions that are catalyzed by metals (98). The metal sulfides are typically several orders of magnitude less active than the metals, but they have the unique advantage of not being poisoned by sulfur compounds. They are thus good catalysts for appHcations with sulfur-containing feeds, including many fossil fuels. 

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. 

Steam reforming is the reaction of steam with hydrocarbons to make town gas or hydrogen. The first stage is at 700 to 830°C (1,292 to 1,532°F) and 15-40 atm (221 to 588 psih A representative catalyst composition contains 13 percent Ni supported on Ot-alumina with 0.3 percent potassium oxide to minimize carbon formation. The catalyst is poisoned by sulfur. A subsequent shift reaction converts CO to CO9 and more H2, at 190 to 260°C (374 to 500°F) with copper metal on a support of zinc oxide which protects the catalyst from poisoning by traces of sulfur. 

In a penicillin synthesis, the carboxyl group was protected as a / -bromophenacyl ester that was cleaved by nucleophilic displacement (PhSK, DMF, 20°, 30 min, 64% yield). Hydrogenolysis of a benzyl ester was difficult (perhaps because of catalyst poisoning by sulfur) basic hydrolysis of methyl or ethyl esters led to attack at the /3-lactam ring. ... 

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

Nickel. As a methanation catalyst, nickel is presently preeminent. It is relatively cheap, it is very active, and it is the most selective to methane of all the metals. Its main drawback is that it is easily poisoned by sulfur, a fault common to all the known active methanation catalysts. The nickel content of commercial nickel catalysts is 25-77 wt %. Nickel is dispersed on a high-surface-area, refractory support such as alumina or kieselguhr. Some supports inhibit the formation of carbon by Reaction 4. Chromia-supported nickel has been studied by Czechoslovakian and Russian investigators. 

It is highly active but easily poisoned by sulfur and not particularly selective to methane. Oddly enough, carbon monoxide appears to inhibit the rate of methane formation. 

Partially extracted Raney cobalt is very active, but it is easily poisoned by sulfur and tends to lay down carbon more readily than Raney nickel (21). Cobalt is less active than nickel and much less selective to methane... 

Iron is reasonably active, but less selective than nickel. It also tends to lay down carbon and to be poisoned by sulfur (30, 31). 

The above described experiments over atomically clean single crystal catalysts have been extended to studies of the kinetics of various catalytic reactions over chemically modified catalysts. Examples are recent studies Into the nature of poisoning by sulfur of the catalytic activity of nickel, ruthenium, and rhodium toward methana-tlon of CO (11,12) and CO2 (15). ethane (12) and cyclopropane (20) hydrogenolysls, and ethylene hydrogenation (21). 

For catalysts poisoned by sulfur, the metal-sulfur bond is usually broken in the presence of steam, as shown for nickel ... 

Sulfur is a potential problem even at low levels for synthesis gas systems using certain types of catalysts. The production of methanol from synthesis gas, for example, uses catalysts that are poisoned by sulfur. Some tar cracking catalysts are also sulfur sensitive. In those systems, thorough removal of sulfur will be required. Fuel cell systems are also sulfur sensitive. 

Ni-YSZ cermet infiltrated by Mo orW precursors [116] 2005 Still poisoned by sulfur (C4H4S) but to a lesser extent and showed gradual recovery. No Pt current collector layer. Pmax °f 300 mW/cm2 at 750°C in H2. 

Durability of catalyst performance under the harsh environment of a GT combustor is another key issue in the development of the technology. In addition to thermomechanical issues discussed in the previous section, volatility and sintering of the active catalytic species are major concerns in this respect. On the other hand, poisoning by sulfur and other contaminants has been recognized to have a minor effect on the catalyst performance due to the high temperature of this application [8]. 

For a review of catalyst poisoning by sulfur, see Barbier Lamy-Pitara Marecot Boitiaux Cosyns Verna Adv. 

Noble metal catalysts are known to be easily poisoned by sulfur. Tungsten carbide appeared to show noble metal characteristics, yet was found to be sulfur resistant. The n-heptane isomerization reaction described above was repeated with the two catalysts unsupported W2C and 0.3% Pt/Al203. 

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