Hydrodechlorination of Tetrachloromethane Over Supported Platinum Catalysts. Effects of Hydrogen Partial Pressure and Catalyst’s Screening Protocol on the Catalytic Performance

Magdalena Bonarowska,Zbigniew Karpiński

doi:10.1007/s11244-012-9858-3

Abstract

Two series of silica- and alumina-supported platinum catalysts were investigated in the hydrodechlorination (HdCl) of tetrachloromethane. During an initial period of reaction carried out at a lower H2/CCl4 ratio the catalysts, especially those characterized by high metal dispersion, deactivated with time-on-stream. Two catalyst screening protocols were used. The first one concerned a gradually increased hydrogen partial pressure, whereas during the second one the H2 pressure was decreased. Although, in general, the hydrogen-rich reaction conditions resulted in improved catalyst performance (higher overall activity and selectivity to CHCl3), the second protocol led to even better results. Reasons for such a behaviour are suggested. Because of very high activity of a few tested samples, changes in CCl4 conversion with the hydrogen partial pressure do not reflect real reaction orders in hydrogen. The same reason may lead to falsification of apparent activation energies. In certain cases the relation between conversion and hydrogen pressure showed a maximum, suggesting that HdCl undergoes via a Langmuir–Hinshelwood mechanism, when hydrogen and CCl4 compete for metal surface sites. Both carbon- and chlorine-containing deposits were found in the post-reaction catalyst samples.

Highlights

We have shown that catalytic activity of Pt/Al2O3 in CCl4 hydrodechlorination (HdCl) exhibits very strong inverse relationship with metal dispersion; highly dispersed Pt samples exhibit very low turnover frequencies [1]
Two series of silica- and alumina-supported platinum catalysts were investigated in the HdCl of tetrachloromethane at the reaction temperature range 343–363 K, at different H2/CCl4 ratios and very short contact times (B0.26 s)
During an initial period of reaction carried out at a lower H2/CCl4 ratio (6.7 and less), all catalysts deactivated with time-on-stream, the effect was more drastic for highly dispersed Pt catalysts

Summary

Introduction

We have shown that catalytic activity of Pt/Al2O3 in CCl4 hydrodechlorination (HdCl) exhibits very strong inverse relationship with metal dispersion; highly dispersed Pt samples exhibit very low turnover frequencies [1]. A similar, but a much milder trend, was found for silicasupported Pt catalysts. This significant support effect was attributed to an extensive surface chloriding of small Pt particles interacting with Lewis acid sites of c-alumina. Similar in kind metalsupport interactions do not occur, and, in effect, deactivation of Pt/SiO2 catalysts is less marked. These results were in line with previously published data by Zhang and Beard [2] for Pt/Al2O3 catalysts with varied metal dispersions catalysts and by Prati and Rossi [3] who showed an exceptional catalytic behavior of Pt/Vycor glass (96 % silica)

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