Poisoning solid acids

Capture of Active Catalyst Using Solid Acidic Support with H2 Elution. The limit on the practical life of a catalyst solution may be determined by several factors including the presence of poisons or inhibitors, the buildup of less soluble materials such as the oxidation products of organophosphorus ligands, or an increase in the concentration of heavy aldehyde condensation products in the catalyst solution. In the latter case, there may be substantial amounts of active catalyst, but it is in a solvent that is unsuitable. Alternately, active rhodium catalyst may have been carried over with product. Technology has been disclosed [39] that permits the isolation of an active metal hydride catalyst. Steps include ... 

For the most highly developed processes, maf coal conversion can be as high as 90 to 95 % with a C4+ distillate yield of 60 to 75 wt % and a hydrogen consumption of 5 to 7 wt %. When an external catalyst is used, it is typically some combination of cobalt, nickel, and molybdenum on a solid acid support, such as silica alumina. In slurry hydrogenation processes, catalyst life is typically fairly short because of the large number of potential catalyst poisons present in the system. 

The most widely applied isomerization catalysts are platinum-promoted solid acids, namely chlorinated alumina1,2 and mordenite.3 5 The former catalysts require the continuous addition of chlorine-containing compounds, are moisture-sensitive and are poisoned by sulfur impurities.6 They are also highly corrosive and hence, environmentally hazardous. Zeolite catalysts are significantly less active and have to be operated at higher temperatures (500-550 K). As a consequence of thermodynamic limitations only relatively low yields of the target branched alkanes can be achieved under these conditions. 

Alkane isomerization is carried out under hydrogen pressure on zeohte-type catalysts, which are particularly robust, being insensitive to poisons such as sulfur-containing compounds and water at 200-280°C. The use of so-called dual catalysts, containing both a solid acid species and a transition metal such as... 

According to the open literature, other solid acid alkylation catalysts are generally susceptible to poisoning/deactivation by water and other common feed impurities (e.g., oxygenates, sulfur compounds, dienes, etc.), thus necessitating (potentially costly) feedstock pretreatment for their removal. In some cases, this requirement is further mandated by the potential corrosion problems associated with the use of halogens in the catalyst system. 

The susceptibility of solid acids to these poisons correlates well with the basicity of nitrogen compounds, as shown in Fig. 8.16. In practical feeds, a wide range and variation of types are present. 

Hydroxyethyl)-pyridine was dehydrated to 2-vinyl-pyridine in liquid phase over solid acid catalysts, with very high selectivity and fairly good reaction rate at relatively low reaction temperature (160°C). The catalytic activity is well correlated with the presence on the catalyst surface of medium to weak Bronsted acid sites. The analysis of coke left behind onto the catalyst and the effect of partial poisoning of catalytic activity by CO2 indicate that the reaction takes place through two mechanisms, involving either a Bronsted acid site or a couple of acid-base sites. 

Many reactions in the fine-chemical industry are performed with organic liquids or in organic solvents. When the surface of a solid-acid catalyst is hydrophilic, the presence of small amounts of water can completely block the surface of the catalyst. Carefully dried catalysts and reagents must then be employed. It may be noted that zeolites with a high silica-to-alumina ratio are hydrophobic. The (internal) surface of the zeolite is, therefore, not readily poisoned by accumulation of (traces of) water. With zeolites, moreover, use can be made of the uniform size of the pores to perform shape-selective reactions after passivation of the catalytic sites on the external surface of the zeolite crystallites. In the bulk-chemical industry several interesting reactions have been developed with zeolite catalysts [27] (cf. the review of Tanabe and Hdlderich [28]). 

They also found PbO to be a strong poison, while CaO had a weaker poisoning effect than any alkali metal. They observed that C11SO4 on AI2O3 is about as active as V2O5 on TiC>2 at 300 °C, but much more active than the latter at higher temperatures. They proposed that CUSO4 is a solid acid with Brpnsted acid sites. 

For the double bond isomerization of unsaturated compounds containing hetero atoms such as N and O, solid base catalysts act as an efficient catalyst. Solid acid catalysts would be poisoned by hetero atoms and show no activity. In contrast, the active sites of solid base catalysts interact little with hetero atoms and, thereifore, solid base catalysts act as efficient catalysts. 

Cyanogen ohlorids. — Ttvo polymeric chlorids are known. Gaseous cyanogen chlorid—CNCl—is formed by the action of Cl upon anhydrous hydrocyanic acid or upon mercuric cyanid in the dark. It is a colorless gas, intensely irritating and poisonous. Solid cyanogen chlorid—CsNsCls—is formed, as a crystalline solid, when anhydrous hydrocyanic acid is acted upon by Cl in... 

Figure 5. Time evolution of the catalytic activity, poisoned acetic acid. Points, experimental data solid lines, Model fit...

These reactions require ionic intermediates and are catalyzed by acidic or basic solids hke AI2O3 or CaO and especially mixed oxides such as Al203/Si02 and MgO/ Si02. Electronic effects can also successfully explain the phenomena of catalyst promotion and catalyst poisoning. Solid-state catalysts can be classified according to their electrical conductivity and electron-transfer properties as shown in Table 5-14. 

In contrast, nitrogen-containing bases are contained in many crude-oil fractions. In a typical starting material, 25 35 % of the nitrogen compounds have basic character. The sensitivity of solid acids towards these poisons correlates directly with their basicity. For example, pyridine, quinoline, amines, and indoles are basic, while pyrrole and carbazole are nonbasic. These poisons are best removed by hydrogenation, together with sulfur and most of the heavy metal poisons. 

From a qualitative viewpoint, the decay behavior of sulfated zirconia, and in general of sulfated transition metal oxides, is quite similar to that previously described for zeolites and other solid acids. During the initial decline in olefin conversion, a sharp increase in the concentration of Cg products occurs as a consequence of cracking suppression, followed by a rapid decrease in TMP within the Cg fraction and the concomitant increase in octenes. This trend indicates that the acid sites responsible for cracking are the first to be poisoned, followed by... 

Compounds containing these elements sometimes are toxic but sometimes are promoters, depending on the type of catalyst. It is the character of non-metal compounds. For instance, water is harmful to solid acid catalysts, but is a promoter for hydrogenation reactions on ruthenium catalyst. Hydrogen sulfide can poison a nickel catal3 t, but a sulfate does not. If hydrogen sulfide is oxidized to form a shield-type structure, it is a non-toxic substance. 

Ho et al. developed a correlation for the poisoning effects of nitrogen compounds on FCC catalysts and a scaling law for estimating the HDS reactivities of middle-distillates in terms of three key properties. " For solid acid catalyzed reactions, Sowerby and coworkers developed a method for estimating adsorption equilibrium constants from an integrated form of van Hofi s equation. 

As in the case of other solid catalysts, the deactivation of solid acid and base catalysts occurs during use for catalytic reactions by the following mechanisms. Adsorption of poisonous compounds Basic molecules are poisons or inhibitors for acidic... 

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