Selectivity structure insensitive reactions

Two structure insensitive reactions have been selected cyclohexene hydrogenation [6] oil surface Pt sites upon silica and but-l-ene isomerisation on acidic sites in bentonite. Both reactions were studied in differential reactors The former was investigated at 273-313K and lOlkPa Samples (5-lGmg) of catalyst were flushed with N2, pre-reduced in H at 423K for lh, flushed with N2 and then the reactant stream (lOlkPa total pressure cyclohexene N2 H2 = 1,7 89.5 10 1 200cm3.min 1 total flow rale) was introduced and analysed... 

The yield of citronellol is given in Figure 5 over catalysts with different metal loading. The hydrogenation rate of citrai and the selectivity to citronellol increased with increasing support surface area and metal dispersion (Table 4), characteristic for a structure insensitive reaction. The highest initial hydrogenation rate and 92% selectivity to citronellol (at maximum yield) was achieved over the... 

Different model reactions were used in order to study the interaction between the modifier and the parent metal. It was observed that an inert additive introduced by a redox reaction generally poisons, more or less, the activity of the parent metal or strongly modifies the selectivity of the reaction, which indicates a deposition of the additive on the parent metal. For example, a decrease in activity for structure insensitive reactions, such as toluene hydrogenation [41] or cyclohexane dehydrogenation [43, 78] proves the existence of bimetallic nanoparticles. Likewise, in the case of the 2,2-dimethylpropane reaction, the modification of both the selectivity and the apparent activation energy, demonstrates an interaction between Pd and Au introduced by direct redox reaction. Conversely, no modification was observed on the catalysts prepared by incipient wetness co-impregnation [75]. 

Although it may seem at first sight that the values of the selectivities are almost random, closer inspection reveals some important regularities. (1) The Z-2-butene selectivity is almost always the smallest in the case of palladium at about room temperature (under which conditions the j8-hydride phase may be formed) it is 5% or less. (2) 1-Butene is almost always the major product, and with copper, manganese and cobalt (Type A) it is more than 85%. Only with zirconium, and with iron, cobalt and nickel as Type B, is -2-butene formed as major product. (3) Except for the base metals of Groups 8 to 10, product distributions under comparable conditions show little significant dependence on physical form, as befits a structure-insensitive reaction. 

The difference between the two definitions is that the rates vary with time or position, while the TOF only depends on the availability of the sites present. The activity of the catalysts and the selectivity of the reaction depend on the characteristics of the material. Some important features of these reactions should be remembered. The first one is related to the sensitivity of the chemical reaction to the catalyst structure. According to Boudart s theory, the supported catalysts consist of metal particles of different and variable sizes. With the increase of particle diameter, the concentrations of metal atoms and exposed sites vary significantly, indicating that no changes in surface structure have happened. The structure sensitive reactions (SSR) are those in which the intrinsic reaction rate relative to the number of surface active sites, i.e., the frequency of the reaction varies with the particle sizes, which does not happen in structure insensitive reactions (SIR). This means that in the structure sensitive reactions, the frequency of... 

It has been seen that transport resistances and catalyst deactivation can have a significant effect on selectivity. It has also been seen that the effect of transport resistances is not necessarily detrimental, even though the selectivity is, in general, adversely affected by diffusion for consecutive reactions. The selectivity is independent of deactivation for structure-insensitive reactions when the same sites are involved for all reactions, provided that the concentration and temperature do not change with deactivation. This condition is rarely met in reactor operation and therefore, the selectivity changes with the extent of deactivation. 

Karpinski et al. (189) found that the reaction of neopentane with H2 over a series of Pd/Si02 catalysts is structure insensitive and exhibits very mild changes in activity and selectivity. The same reaction performed by... 

Oyama and Somorjai have studied the oxidation of ethanol and ethane over vanadia supported on silica.33 Ethane oxidation yielded ethylene and acetylene with considerable CO2 for highly dispersed catalysts. Ethanol oxidation to acetaldehyde, on the other hand was shown to be structure insensitive. Conversions and selectivities for reaction of ethanol are shown in Table III.33... 

Recently, FT synthesis reactions were shown to be independent of metal dispersion on Si02-supported catalysts with 6-22% cobalt dispersion (103). Turnover rates remained nearly constant (1.8-2.7 x 10 s ) over the entire dispersion range. Dispersion effects on reaction kinetics and product distributions were not reported. These tests were performed at very low reactant pressures (3 kPa CO, 9 kPa H2), conditions that prevent the formation of higher hydrocarbons and lead to methane with high selectivity and to CO hydrogenation turnover rates 10 times smaller than those obtained at normal FT synthesis conditions and reported here. These low reactant pressures also lead to kinetics that become positive order in CO pressure. Thus, the reported structure insensitivity (103) may agree only coincidentally with the similar conclusions that we reach here on the basis of our results for the synthesis of higher hydrocarbons on Co. 

Boudart et al. for supported metal catalysts to indicate whether the rate of reaction is nonlinearly or linearly proportional to the total surface area of the metal, but the concept can be extended to other types of catalysts as well. Thus, for acid catalysts, the rate of formation of a particular species may depend upon the total number of acid sites, or upon the number of a particular type, e.g., a particular range of acid strengths. In general, the deactivation process may diminish the number of a particular type of active sites, while the rate of formation of each product may depend upon other types of active sites (which may include part or all, or none, of the first type). The interrelation between these types denotes whether the various reactions are structure-sensitive or structure-insensitive, and determines how the selectivities change with... 

Catalytic reactions can be classified into structure-insensitive (hydrogenation, dehydrogenation, isomerization, hydrogen exchange) or structure-sensitive (hydrogenolysis, CO/H2, NH3 synthesis) reactions depending on the extent to which the rate of the reaction and its selectivity are affected by the size and morphology of the metal particles. 

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