Surface reactions temperature conversions

The difluoroamine radical produced dimerizes to N2F4, the reversible equilibrium resembling that between NOz and N204. The reactivity of the copper, which acts as a defluorinating agent in this reaction, decreases as its surface becomes coated with the involatile fluoride. This difficulty was overcome when it was replaced by arsenic, antimony, or bismuth, the fluorides of which are volatile at the reaction temperature. Conversion of NF3 to N2F4 in these reactions was of the order of 40-60%. There was a similar conversion when the trifluoride was passed over liquid mercury at 320-330°C (91). Alternatively, the trifluoride could be defluorinated by mercury vapor in an electrical discharge (118). 

Conversions obtained on films ( 0.3-0.2 cm ) reduced at various temperamres are summarized in Table 1. At low reaction temperatures, conversion decreases with increasing reduction temperamre and then levels off at about 24% for reduction temperatures < 450°C. At the higher temperatures the conversion is insensitive to the surface strucmre b ause of mass-transfer control of the reaction rate. 

In order to confirm the hypothesis made on the role of catalyst components, we carried out the reaction with a ratile-type V/Sb/0 catalyst, having V/Sb atomic ratio equal to 1/1 (Table 40.1). This catalyst was prepared with the conventional sluny method, and therefore had a surface area of 10 mVg, lower than that obtained with the Sn/V/Nb/Sb/0 catalysts prepared with the co-precipitation method. However, despite this difference, with V/Sb/0 the conversion of n-hexane was similar to that one obtained with Sn/V/Nb/Sb/0. This is shown in Figure 40.7, which reports the conversion of n-hexane, the selectivity to CO2, to / -containing compounds and the carbon balance as a function of the reaction temperature. 

These assumptions are partially different from those introduced in our previous model.10 In that work, in fact, in order to simplify the kinetic description, we assumed that all the steps involved in the formation of both the chain growth monomer CH2 and water (i.e., Equations 16.3 and 16.4a to 16.4e) were a series of irreversible and consecutive steps. Under this assumption, it was possible to describe the rate of the overall CO conversion process by means of a single rate equation. Nevertheless, from a physical point of view, this hypothesis implies that the surface concentration of the molecular adsorbed CO is nil, with the rate of formation of this species equal to the rate of consumption. However, recent in situ Fourier transform infrared (FT-IR) studies carried out on the same catalyst adopted in this work, at the typical reaction temperature and in an atmosphere composed by H2 and CO, revealed the presence of a significant amount of molecular CO adsorbed on the catalysts surface.17 For these reasons, in the present work, the hypothesis of the irreversible molecular CO adsorption has been removed. 

Recently, cross-aldol condensation of benzaldehyde with n-heptaldehyde to give jasminaldehyde (Scheme 13) has been reported a mesoporous molecular sieve Al-MCM-41 with supported MgO was the catalyst. The reactions were carried out in a stirred autoclave reactor with a molar benzaldehyde/heptanal ratio of 10 at 373-448 K (236). The results show that Al-MCM-41 is catalytically active, and its activity is significantly increased by the deposition of MgO (Table V). Increasing the amount of deposited MgO on Al-MCM-41 decreases the surface area but enhances the catalyst basicity. The basicity is well correlated with the catalytic activity, although the selectivity to jasminaldehyde is not the selectivity is essentially independent of temperature, pressure, time of the reaction, and conversion. 

The optimal distribution of silver catalyst in a-Al203 pellets is investigated experimentally for the ethylene epoxidation reaction network, using a novel single-pellet reactor. Previous theoretical work suggests that a Dirac-delta type distribution of the catalyst is optimal. This distribution is approximated in practice by a step-distribution of narrow width. The effect of the location and width of the active layer on the conversion of ethylene and the selectivity to ethylene oxide, for various ethylene feed concentrations and reaction temperatures, is discussed. The results clearly demonstrate that for optimum selectivity, the silver catalyst should be placed in a thin layer at the external surface of the pellet. 

Reaction temperature can also affect the order of a reaction. It is generally agreed that the rate constant for the conversion (or desorption) of the surface-oxygen complex has a higher activation energy than the rate constant for the formation of the complex. Therefore, a reaction which is zero order at low temperatures and a given pressure can become first order at the same pressure and a sufficiently high temperature. 

Generally, several protocols are used for the characterization of sohd-catalyzed reactions under batch reaction conditions by NMR spectroscopy. In ex situ experiments, the conversion of reactants adsorbed on the catalyst is carried out in an external oven and stopped after a given reaction time by quenching, for example, in liquid nitrogen. Subsequently, the reaction products formed on the catalyst surface are investigated at room temperature by use of a standard MAS NMR probe. This protocol is repeated with a stepwise increment of the reaction time at the same temperature or with a stepwise increment of the reaction temperature for the same duration. In an in situ experiment, the catalytic conversion of the reactants is measured inside the NMR spectrometer by use of a high-temperature MAS NMR probe. 

The left-hand side of Figs 28a-c shows C CF MAS NMR spectra which were recorded during the conversion of pure C-enriched methanol on zeolite CsOH/ Cs,NaY under CF conditions at reaction temperatures of 473—523 K (242). As was observed previously (234-236), the conversion of methanol (49 ppm) on the basic zeolite catalysts caused the formation of surface formate species, leading to a C MAS NMR signal at 166 ppm. Upon cessation of the methanol flow at the reaction... 

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