Hexanal, conversion

Sachtler WMH, Somoijai GA. 1983. Influence of ensemble size on CO chemisorption and catalytic n-hexane conversion by Au-Pt(l 11) bimetallic single-crystal surfaces. J Catal 81 77. 

Keywords acidity, activity, zeolites, hexane conversion... 

Table 1. Activity of bimolecular hexane conversion of zeolites and Av0h values,...

Figure 2 shows the evolution in n-hexane conversion with time-on-stream for the three bifunctional I, E and M samples. Large differences in behaviour can be observed. However with all samples, there is a very fast initial decrease, but this decrease is followed by a plateau with Pt/MCM-22E and I samples, and by a slower decrease with sample M. The initial decrease can be related to n-hexane transformation within the inner micropores and especially the supercages, with trapping of the bulkiest products. 

Co-exposure to approximately equal concentrations of xylene or toluene (Nylen et al. 1989) has also prevented 77-hexane-induced testicular atrophy in Sprague-Dawley rats. The protective effects of xylene and toluene on peripheral neuropathy and testicular atrophy caused by 77-hexane may result from competition for metabolism, resulting in a slowing of 77-hexane conversion to 2,5-hexanedione. 

Introduction of Pt significantly enhances zeolite isomerization catalyst stabiUty and alters the reaction pathways. The Pt/acid ratio not only changes the isomeriza-tion/cracking ratio, but also changes the ratio of mono/di-branched isomers in Pt/Y [14]. High Pt dispersion and close proximity to acid sites correlate with high n-hexane conversion as well as high isomerization selectivity [20, 21]. 

The preparation of precious metal supported catalysts by the HTAD process is illustrated by the synthesis of a wide range of silver on alumina materials, and Pt-, Pt-Ir, Ir-alumina catalysts. It is interesting to note that the aerosol synthesis of alumina without any metal loading results in a material showing only broad reflections by XRD. When the alumina sample was calcined to 900°C, only reflections for a-alumina were evident. The low temperature required for calcination to the alpha-phase along with TEM results suggest that this material was formed as nano-phase, a-alumina. Furthermore, the use of this material for hexane conversions at 450°C indicated that it has an exceptionally low surface acidity as evidenced by the lack of any detectable cracking or isomerization. 

Fig. 10. Selectivities in hexane conversions versus temperature for benzene formation (Be), hydrogenolysis (Hy), methylcyclopentane formation (MCP), isomerization (ISOM), and dehydrocyclization (Dehy) (9 wt. % Pt on inert Si02).

Before starting the measurements the catalysts were treated in synthetic air at 823 K for lh. The hexane conversion rates on the catalysts were determined at 673 K, 723 K and 773 K after a steady state at each temperature had been reached. 

Fig. 12. Reaction parameters of H-hexane conversion by nickel and Ni-Cu alloys. A, = log c at 330 C, A2 = log rs at 330 C, activation energy of the overall reaction fission parameter M, selectivity parameter S all as a function of alloy composition (in at. % Cu). r, is rate per cm2, rw rale per gram catalyst. From Ponec and Sachtler (14).

Another difference between WC and Mo2C catalysts in n-hexane-H2 reactions is their selectivity-conversion relationship as shown in Figure 21.3. Isomerization selectivity over WC did not show any dependence on n-hexane conversion. It depended only on the oxygen treatment temperature. In contrast, isomerization selectivity over Mo2C showed a linear decrease with increasing -hexane conversion. 

Figure 21.1 Change in n-hexane conversion and isomerization selectivity with time over WC catalyst. The flow rate of 6% n-hexane in H-. was 15 [imol s-1 for 2 g of the catalyst. T = 623 K. Symbols WC/fresh (O), WC/RT ( ), WC/473 ( ), and WC/673 ( ).

Figure 21.3 The relationship between n-hexane conversion and isomerization selectivity in n-hexane-H2 reactions at 623 K. (a) WC and (b) Mo2C catalysts. Conversions were varied by changing flow rates of 6% n-hexane-H2 mixture. Top panel , WC/fresh , WC/RT . WC/200 O, WC/400 Botton panel , Mo2C/RT , Mo2C/fresh.

A fragment of n-hexane conversions on supported Pt catalysts is re-presented as... 

A sample of the n-hexane conversions on supported platinum catalysts can be represented by the scheme... 

Simple cycles are those that do not contain any repeated points except the initial one. All simple cycles for the most complex of the above graphs, i.e. the graph of ra-hexane conversions, are presented in Fig 4. 

Non-Hamiltonian graphs of composite mechanisms are widespread, e.g. the graphs of vinyl chloride synthesis and n-hexane conversion [Fig. 3(d) and (f) and Fig. 5(c) and (d)]. The simplest non-Hamiltonian graph is that of the two-step mechanism supplemented by a "buffer step yielding a non-reactive substance. For the mechanism... 

Dautzenberg et al. (65) tested a number of unsupported PtSn alloys as well as a number of alumina supported PtSn catalysts. n-Hexane conversion was effected at atmospheric pressure for the unsupported alloy catalysts and for some supported catalysts other supported catalyst studies were at 3 bar. These authors reported that the addition of tin decreased the amount of methylcyclopentane and that coke was dramatically reduced during the conversion of n-hexane. 

The samples in Table I were tested in the reaction of n-hexane at 733 K and atmospheric pressure. Figure 3 shows the selectivity to benzene formation (calculated as the yield of benzene divided by the conversion of n-hexane) as a function of n-hexane conversion for Pt-LI and Pt-VI as circles and squares, respectively. Results recently reported for a 0.88 wt% Pt (H/Pt = 0.49) catalyst prepared by impregnation of aqueous Pt(NH3)4Cl2 are included for comparison (8). However, the results in (8) were obtained at 750 K, H2/n-hexane = 6 (6 kPa n-hexane) and diluent He at atmospheric total pressure which are slightly different from the experimental conditions used in the current work. 

Figure 3. Selectivity to benzene formation as a function of n-hexane conversion at 733 K, H2/C6H14 molar ratio of 6, and atmospheric total pressure. Circles and squares correspond to results for Pt-LI and Pt-VI, respectively. Catalytic results at 750 K for Pt/Mg(Al)0 prepared from Pt(NH3)4Cl2 are included as triangles for comparison (Adapted from ref. 8).

FIG. 18. Reaction parameters for n-hexane conversion by Ni and Ni-Cu alloys at 330°C Ai = log rw (rate per gram of catalyst) A2 = log rs (rate per square centimeter of total surface) Eact is activation energy of the overall process S is the selectivity for producing Cg products M is a fission parameter whose value inversely reflects the degree of multiple fragmentation to methane (102). 

The experimental results on the evolution of the n-hexane conversion, XH, as a function of the coke content, Cc, are shown on Figure 4 ... 

Figure 4 n-Hexane conversion as a function of coke content for five experiments at 450 °C with different space times. 

The rate of 2-Me-pentane formation as a function of the n-hexane conversion is shown in Figure 5. It is seen how in experiment E3 e.g, the n-hexane conversion XH decreases from 13.15% to 4.17% while the corresponding coke content increases from 0.38 to 0 88 wt%. To obtain this rate as a function of the coke content only, interpolation at constant n-hexane conversion was carried out, as indicated by the dashed lines in Figure 5. This interpolation was carried out at four different conversion levels. 

Combining the information from Figures 4 and 5 allows the evolution of the rate of 2-Me-pentane formation to be represented in Figure 6 as a function of the coke content for a fixed n-hexane conversion. 

Figure 5 Rate of 2-Me-pentane formation as 2 function of hexane conversion for five experiments at 450 °C.

The same procedure as outlined in Figures 4 to 6 was followed for the determination of the separate influence of coke and of n-hexane conversion on the product select vities, which are ratios of reaction rates. The results are given in Tables 3a and b. Clearly, both the hexane conversion and coke formation induce selectivity changes. The selectivity for coke formation was determined as the ratio of the moles of hexane converted to coke to the total amount of n-hexane consumed. 

Table 3a Influence of the coke content on the product selectivities in n-hexane cracking at 7 % hexane conversion (mole/100 moles hexane reacted).

If the effect of the coke content on the rate of one of the reactions of the network has to be expressed, the partial pressures of the related reactants have to be kept constant. For any of the primary reactions of n-hexane cracking, this is ensured by keeping the hexane conversion constant, but not for the other reactions, however. For secondary reactions, Table 3a does not only reflect the effect of coke, but also of concentration. 

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