Sulfur modified catalyst

Paixao and coworkers showed that a sulfur-modified catalyst C5 was able to efficiently catalyse the Michael addition of malonates to enals in EtOH/ brine mixture (Scheme 8.7). The sulfur-containing catalyst was efficient for the Michael addition of aldehydes to nitroalkenes in polyethylene glycol (PEG). The most important feature of this system was its reusability. After extraction of the reaction products, the PEG solution containing the organocatalyst was reused at least four times with no adverse effects on yield or enantioselectivity. ... 

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). 

The Union Oil selective sulfur oxidation catalyst is the basis for many modified sulfur plant designs announced in recent years 7>18. This system may be ideal for a synfuels facility because of the low H2S/CO2 ratio of synfuel raw-gas streams. If a physical solvent is employed for acid-gas removal, some hydrocarbon will be lost to the acid-gas stream. With the selective sulfur oxidation catalyst, this fuel is not oxidized, rather it is available for tail-gas reduction over cobalt-molybdenom, prior to final treatment. 

Finally, it should be realized that CO is not the only fuel (or fuel-derived) contaminant expected to affect anode performance in the PEFC. In a test of other possible contaminants that could result, in principle, from methanol reforming, Seymour et al. [27] reported strong and irreversible effects of formic acid at a PEFC platinum (high-loading) anode, whereas methanol, formaldehyde, and methyl formate were found to have much smaller and reversible effects. The fuel impurity aspects of coupling between natural gas (or gasoline) reformers of various types and a PEFC stack are even wider, and make it essential to probe and address, either by removal upstream or by use of modified catalysts, the possible detrimental effects of low levels of sulfur, H2S, COS, and NH3 [28]. 

Apparently, even for very different composition and dispersion values, the Mo-modified catalysts have much higher thioresistance. Sulfur resistance of nickel is apparently independent of composition and of dispersion, as can be concluded from the curves for the monometallic systems (Catalysts A and C). Extrapolating Curves A and C to a=0, 6 values of approximately 0.4 0.5 are obtained. In the Mo modified systems, the extrapolation gives 0 values of 0.6S and 0.95 for D and B respectively. In the monometallic system and considering... 

Environmental issues as well as process optimization requires the replacement of liquid acids by more friendly and safe solid-acid catalysts. The use in organic syntheses of clays or clay-modified catalysts or reagents recommends itself by its simple implementation, the small amount of catalyst required (in comparison to standard Lewis acids, such as AICI3, or Brpnsted acids, such as concentrated sulfuric or nitric acids), the presence of the acidity internally (making them safe to handle), the use of more environmentally compatible conditions and/or substances, the simple recovery and/or reuse and, finally, the harmless disposal. Furthermore, clay microreactors may display enzyme-like characteristics. 

The behavior of modified Rli/Ce02-Al203 catalyst is shown in Fig.5. Contrarily to what was found on AI2O3, for modified catalysts supported on ceria-alumina, chlorine has only a sliglit effect on the OSC values. This difference would be result of the presence of ceria that could promote a better distribution of chlorine at the carrier surface. Sulfates would increase the oxygen storage capacity of the modified catalyst. This effect would only be apparent and would be due partly to the sulflir reduction by CO which masks the inliibitor effect of sulfur on OSC. This can be observed by comparison with the oxygen mobilities. 

The first supported molten salt catalyst systems date from 1914, where BASF filed a patent on a silica-supported V20s-alkali pyrosul te sulfur dioxide oxidation catalyst [48], which even today - as a slightly modified catalyst system - is still the preferred catalyst for sulfuric acid production [49]. However, it took many years to realize in the 1940s [50,51], that the catalyst system actually was a molten salt SLP-type system which is best described by a mixture of vanadium alkah sulfate/hydrogensulfate/pyrosulfate complexes at reaction conditions in the temperature range 400-600 °C with the vanadium complexes playing a key role in the catalytic reaction [49]. 

To quantify the elements deposited on the catalyst surface as a function of exposure time in the flue gas duct, their relative atomic fractions x were recorded with exposure time (Fig. 4). An increase in sulfur and arsenic contents with exposure time can be observed. Both elements were first detected after 48 h. Depending on exposure time, these elements cover increasingly large areas, 10% after 96 h, 20% after 3270 h. At the same time, all samples were observed to determine whether the detected Xjj decreases with exposure time (Fig. 5). This reduction is particularly pronounced between 24 h and 48 h. After about 400 h of exposure time, the titanium fraction has decreased to about one third of its initial value. Since it does not change after this time, it is assumed that the catalyst surface contamination process is completed. From this time on, a constant balance should be achieved between abrasion and contamination. Surprisingly, this does not hold for sulfur and arsenic. It is assumed that the modified catalyst surface involved in this abrasion/contamination balance forms a further reactive component for these elements. There is other interesting evidence that the arsenic deposited originates, at least from that point on, exclusively in the gas phase, since the arsenic entrained by the flue dust would otherwise result in an increase in Xgj. 

Rare earth compounds are also used in numerous catalytic reactions in petrochemical industry. One example is the use of rare earth salts to stabilize zeolites used for the catalytic cracking of crude oils to gasoline. Rare earth doping increases the activity of these zeolites with the consequence of higher gasoline yields. In addition, these rare earth-modified catalysts have found expanded application as a consequence of the refineries use of residual or heavy crude oils which contain high levels of nickel, vanadium, and sulfur which attack zeolites and reduce their activity rare earths are more resistant to these catalytic poisons. ... 

Solid-supported versions of the Buchwald-Hartwig reaction are known too. In 2012, Al-Amin et al. [42] reported a stable heterogeneous sulfur-modified gold-supported palladium material (SAPd), which was used for the amination of aryl bromides and chlorides. The catalyst was employed at a loading of about 0.2 mol%, and can be used for a minimum of 10 reaction cycles without loss of catalytic activity, affording very good yields for a large cross-section of substrates. 

Montassier, C., Menezo, J.C., Hoang, L.C., Renaud, C., Barbier, J., 1991. Aqueous polyol conversions on ruthenium and on sulfur-modified ruthenium. Journal of Molecule Catalyst 70,99-110. 

Tetrahydronaphthalene is produced by the catalytic treatment of naphthalene with hydrogen. Various processes have been used, eg, vapor-phase reactions at 101.3 kPa (1 atm) as well as higher pressure Hquid-phase hydrogenation where the conditions are dependent upon the particular catalyst used. Nickel or modified nickel catalysts generally are used commercially however, they are sensitive to sulfur, and only naphthalene that has very low sulfur levels can be used. Thus many naphthalene producers purify their product to remove the thionaphthene, which is the principal sulfur compound present. Sodium treatment and catalytic hydrodesulfuri2ation processes have been used for the removal of sulfur from naphthalene the latter treatment is preferred because of the ha2ardous nature of sodium treatment. 

Other THF polymerization processes that have been disclosed in papers and patents, but which do not appear to be in commercial use in the 1990s, include catalysis by boron trifluoride complexes in combination with other cocatalysts (241—245), modified montmorrillonite clay (246—248) or modified metal oxide composites (249), rare-earth catalysts (250), triflate salts (164), and sulfuric acid or Aiming sulfuric acid with cocatalysts (237,251—255). 

The choice of catalyst is based primarily on economic effects and product purity requirements. More recentiy, the handling of waste associated with the choice of catalyst has become an important factor in the economic evaluation. Catalysts that produce less waste and more easily handled waste by-products are strongly preferred by alkylphenol producers. Some commonly used catalysts are sulfuric acid, boron trifluoride, aluminum phenoxide, methanesulfonic acid, toluene—xylene sulfonic acid, cationic-exchange resin, acidic clays, and modified zeoHtes. 

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

Another sulfur dioxide appHcation in oil refining is as a selective extraction solvent in the Edeleanu process (323), wherein aromatic components are extracted from a kerosene stream by sulfur dioxide, leaving a purified stream of saturated aHphatic hydrocarbons which are relatively insoluble in sulfur dioxide. Sulfur dioxide acts as a cocatalyst or catalyst modifier in certain processes for oxidation of o-xylene or naphthalene to phthaHc anhydride (324,325). 

The Claus process is the most widely used to convert hydrogen sulfide to sulfur. The process, developed by C. F. Claus in 1883, was significantly modified in the late 1930s by I. G. Farbenindustrie AG, but did not become widely used until the 1950s. Figure 5 illustrates the basic process scheme. A Claus sulfur recovery unit consists of a combustion furnace, waste heat boiler, sulfur condenser, and a series of catalytic stages each of which employs reheat, catalyst bed, and sulfur condenser. Typically, two or three catalytic stages are employed. 

ButylatedPhenols and Cresols. Butylated phenols and cresols, used primarily as oxidation inhibitors and chain terrninators, are manufactured by direct alkylation of the phenol using a wide variety of conditions and acid catalysts, including sulfuric acid, -toluenesulfonic acid, and sulfonic acid ion-exchange resins (110,111). By use of a small amount of catalyst and short residence times, the first-formed, ortho-alkylated products can be made to predominate. Eor the preparation of the 2,6-substituted products, aluminum phenoxides generated in situ from the phenol being alkylated are used as catalyst. Reaction conditions are controlled to minimise formation of the thermodynamically favored 4-substituted products (see Alkylphenols). The most commonly used is -/ fZ-butylphenol [98-54-4] for manufacture of phenoHc resins. The tert-huty group leaves only two rather than three active sites for condensation with formaldehyde and thus modifies the characteristics of the resin. 

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