Exchange and hydrogenolysis reactions of hydrocarbons on rhodium-titania and platinum-iridium-alumina catalysts
Faro, Arnaldo da Costa.Jr.
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A Monte Carlo method was developed which allows the calculation of theoretical cyclopentane exchange patterns for a mechanism involving interconversion between monoadsorbed and a,ß.- diadsorbed cyclopentane combined with rollover or a,a- diadsorption processes, Theoretical patterns were compared with experimental results for cyclopentane exchange on Rh, Ir, Pt, Pd and Ni catalysts. It was concluded that several parallel exchange processes have to be invoked in order to explain the experimental patterns. Samples obtained from exchange experiments on supported Rh, Ir and Pt catalysts were examined by deuterium- n.m.r. spectroscopy, Several resonances were observed in the spectra and the corresponding frequencies were interpreted in terms of isotopic shifts associated with deuterium atoms in different positions in the molecule. The values of the isotopic shifts were found to be a = -18 p,p.b., ac = - 6 p.p.b., ßT = -9 p.p.b., Yc = -3 p.p.b., 'T = 0 p.p.b. (C = cis, T = trans). Special attention was given to the elucidation of the nature of the large d2 component frequently found in cyclopentane exchange on Rh. It was concluded that the d2 componente arises largely from cyclopentane - 1,2 - d2.A 0.6 wt % Rh /Tí02 catalyst was prepared and characterized by pulse chemisorption /titration techniques. After reduction at 473 K (LTR treatment) the catalyst showed normal chemisorptive behaviour but after reduction at 773 K (HTR treatment) its chemisorption capacity was suppressed indicating that a strong metal- support interaction (SMSI) was induced by HTR, The activity of the catalyst after both LTR and HTR was measured for a number of reactions such as methane and cyclopentane exchange,neopentane hydrogenolysis and methylcyclopentane (MCP) reactions and compared to that of a 3 wt % Rh /Sí02 catalyst subjected to the same pretreatments. Suppression of activity after HTR was only found with Rh /Tí02, Different reactions displayed different sensitivities to the SMSI effect, decreasing in the order; neopentane hydrogenolysis > cyclopentane exchange > MCP multiple hydrogenolysis > methane exchange -- MCP ring- opening -MCP aromatization -- MCP demethanation > MCP dehydrogenation, The results were discussed in terms of electronic,ensemble and structural effects and it was concluded that an ensemble effect correlates a greater number of results.series of Pt -Ir catalysts containing 0,5 wt % of metal supported on y - A1203 was characterized by temperature -programmed reduction (TPR) and related techniques. The effect of pretreatments such as calcination at 523 K and 773 K was examined, The catalysts showed a high dispersion and gave evidence of having a single metallic phase although phase separation occurred on calcination at 773 K. In all cases two peaks were found during TPR: the first with a maximum at ca, 400 K was attributed to reduction of the metallic phase and the second, which was centred at 650 -700 K was thought to result from reduction of hydroxyl groups on the surface of the support.The exchange reactions of neopentane and benzene with D2 and neopentane and MCP hydrogenolysis were examined on the series of Pt - Ir catalysts. Isobutane, isopentane and n- hexane hydrogenolysis were examined on the monometallic catalysts, The results suggested that Pt and Ir interact in the bimetallic catalysts and that little, if any, surface enrichment in Pt is likely to have occurred. The activity patterns observed with neopentane in the 480 K to 520 K temperature range are consistent with a model wereby 3 -5 atom ensembles participate in the reaction and mixed Pt - Ir ensembles have an activity intermediate between those of pure Pt and pure Ir ensembles, Evidence was presented that cleavage of disecondary bonds involves intermediates different from the ones involved in neopentane hydrogenolysis under the experimental conditions selected (ca, 480 K, 5 -10 kPa total pressure, ca, 10 :1 hydrogen to hydrocarbon),