Catalyst dehydrogenation 201 - poisoning

Example 11-11 An ethylene stream is fed to a polymerization reactor in which the catalyst is poisoned by acetylene. The ethylene is prepared by catal3d ically dehydrogenating ethane. Hence it is important that the dehydrogenation catalyst be selective for dehydrogenating rather than C2H4. The first-order reactions are... 

Xhe presence of CO2 causes various kinetic effects it accelerates the reaction rate, enhances the selectivity, alleviates the chemical equilibrium, suppresses the unwanted total oxidation products, prevents the hot spots on the catalyst surface, poisons the non-selective sites of the catalysts, and the equilibrium yield of styrene dehydrogenation is much higher in the presence of CO2 than in that of steam. 

Dehydrogenation activities of the oxides of selenium and tellurium were observed by the pulse technique.Thus, the zirconium oxides which are loaded with selenic acid or telluric acid and calcined in air can dehydrogenate 2-propanol to acetone and hexane to benzene.In a typical reaction of 2-propanol, the conversion into acetone decreases continuously after the third pulse, probably owing to a decrease in the amount of oxygen on the catalyst surface. Poisoning experiments with injection of CO2, H2O or butylamine at 523 K before reaction had no effect on the yield of acetone. Thus this dehydrogenation process appears to be an oxidative dehydrogenation. No studies on the acidic or basic character of oxides of selenium and tellurium oxides have been reported. 

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. 

Vanadium also promotes dehydrogenation reactions, but less than nickel. Vanadium s contribution to hydrogen yield is 20% to 50% of nickel s contribution, but vanadium is a more severe poison. Unlike nickel, vanadium does not stay on the surface of the catalyst. Instead, it migrates to the inner (zeolite) part of the catalyst and destroys the zeolite crystal structure. Catalyst surface area and activity are permanently lost. 

Nickel in the feed is deposited on the surface of the catalyst, promoting undesirable dehydrogenation and condensation reactions. These nonselective reactions increase gas and coke production at the expense of gasoline and other valuable liquid products. The deleterious effects of nickel poisoning can be reduced by the use of antimony passivation. 

The catalytic system used in the Pacol process is either platinum or platinum/ rhenium-doped aluminum oxide which is partially poisoned with tin or sulfur and alkalinized with an alkali base. The latter modification of the catalyst system hinders the formation of large quantities of diolefins and aromatics. The activities of the UOP in the area of catalyst development led to the documentation of 29 patents between 1970 and 1987 (Table 6). Contact DeH-5, used between 1970 and 1982, already produced good results. The reaction product consisted of about 90% /z-monoolefins. On account of the not inconsiderable content of byproducts (4% diolefins and 3% aromatics) and the relatively short lifetime, the economics of the contact had to be improved. Each diolefin molecule binds in the alkylation two benzene molecules to form di-phenylalkanes or rearranges with the benzene to indane and tetralin derivatives the aromatics, formed during the dehydrogenation, also rearrange to form undesirable byproducts. 

The Raney nickel is a very efficient catalyst for the dehydrogenation of 2-butanol into butanone (Scheme 45) with a good selectivity (90%). But, for industrial applications selectivities as high as 99% are required. This can be achieved by poisoning some sites by reaction with Bu4Sn (the best results are obtained with a Sn/Ni ratio of 0.02), which probably occurs first on the sites responsible for the side reactions. The consequence is a slight decrease of the catalytic activity and an increase of the selectivity in 2-butanone which can reach 99%. This catalyst, developed by IFF, has been used commercially in Japan for several years [180]. 

Under dehydrogenation conditions (385 °C ratio H2/HC = 4), an increase in the selectivity for aromatics with PtSn,(/Si02 catalyst has been observed. The increase in aromatic selectivity with tin content seems to be due to a geometric effect, favoring aromatic desorption. When the catalyst contains only small amounts of tin, an important poisoning by coke has been observed. As a consequence, it is possible that coke comes from adsorbed aromatic degradation. If aromatic formation starting from olefins had already and previously been proposed in the literature, their formation mechanism was still unknown. The coexistence of two possible dehydrocycHzation mechanisms has been proposed (Scheme 3.24). 

There are numerous indications in the literature on catalyst deactivation attributed to over-oxidation of the catalyst (3-5). In the oxidative dehydrogenation of alcohols the surface M° sites are active and the rate of oxygen supply from the gas phase to the catalyst surface should be adjusted to that of the surface chemical reaction to avoid "oxygen poisoning". The other important reason for deactivation is the by-products formation and their strong adsorption on active sites. This type of... 

Hydroprocessing and special absorption techniques are utilized to remove sulfur and nitrogen from the reformer. If not removed through hydroprocessing, feedstock sulfur will be converted to H2S in the reformer. The H2S will then serve as a poison to the platinum reformer catalyst and diminish the dehydrogenation and dehydrocyclization reactions. When present, H2S can neutralize the acid sites on the catalyst diminishing the ability of the catalyst to promote isomerization, dehydrocyclization, and hydrocracking reactions. 

We have shown that the nature of the solvent strongly modifies the activity. The large activity variation is explained oy a partial surface poisoning oy the dehydrogenation products of alcohol solvents on Raney nickel. The effect of the nydrogen pressure in pretreatment suggests that cyclohexane can also react with the catalyst surface. Therefore cyclohexane can no longer be retained as an inert solvent. 

A major problem in noble metal catalyzed liquid phase alcohol oxidations -which is principally an oxidative dehydrogenation- is poisoning of the catalyst by oxygen. The catalytic oxidation requires a proper mutual tuning of oxidation of the substrate, oxygen chemisorption and water formation and desorption. When the overall rate of dehydrogenation of the substrate is lower than the rate of oxidation of adsorbed hydrogen, noble metal surface oxidation and catalyst deactivation occurs. 

We have been able to identify two types of structural features of platinum surfaces that influence the catalytic surface reactions (a) atomic steps and kinks, i.e., sites of low metal coordination number, and (b) carbonaceous overlayers, ordered or disordered. The surface reaction may be sensitive to both or just one of these structural features or it may be totally insensitive to the surface structure, The dehydrogenation of cyclohexane to cyclohexene appears to be a structure-insensitive reaction. It takes place even on the Pt(l 11) crystal face, which has a very low density of steps, and proceeds even in the presence of a disordered overlayer. The dehydrogenation of cyclohexene to benzene is very structure sensitive. It requires the presence of atomic steps [i.e., does not occur on the Pt(l 11) crystal face] and an ordered overlayer (it is poisoned by disorder). Others have found the dehydrogenation of cyclohexane to benzene to be structure insensitive (42, 43) on dispersed-metal catalysts. On our catalyst, surfaces that contain steps, this is also true, but on the Pt(lll) catalyst surface, benzene formation is much slower. Dispersed particles of any size will always contain many steplike atoms of low coordination, and therefore the reaction will display structure insensitivity. Based on our findings, we may write a mechanism for these reactions by identifying the sequence of reaction steps ... 

More important, the carbonaceous deposit on the platinum catalyst surfaces was often ordered, and ordering imparted to it unique properties. The presence of an ordered overlayer eliminated the poisoning of dehydrogenation reactions (C6H10 to C6H6). The dehydrocyclization of -heptane to... 

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