Can We Replace Cr(VI) as a Homogeneous Catalyst in the Chlorate Process?

Abstract

The chemistry of hypochlorous acid in the presence of metal impurities is of high importance for many areas of chemistry ranging from water purification and disinfection to sea water splitting and the electrosynthesis of chlorate. The production of chlorate comprises one of the most important electrochemical processes and relies on the Cr­(VI) catalyzed transformation of HOCl to chlorate. Since the use of Cr­(VI) has been restricted in the European Union due to its toxicity, mutagenicity, and carcinogenicity, alternative catalysts need to be developed. Building on the recently identified mechanism for the chromate catalyzed oxidation of HOCl to chlorate, we performed a screening for potential alternative catalysts using density functional theory modeling. Based on these results, a volcano plot, which combines all previously reported reaction paths, was constructed and the thermodynamic limitations were identified. Our results indicate that, in principle, many materials are able to catalyze the chlorate formation at much higher rates. However, in practice, the reaction is limited by the strong correlation between the pKa of the active site and the activity and the limitation to pH neutral conditions. This renders chromate, despite its overall poor performance, effectively the most active material. Furthermore, we show that none of the remaining considered molecules is able to replace Cr­(VI) as the homogeneous catalyst due to side reactions or process limitations. Thus, these results will have a significant impact in the political decision of banning Cr­(VI) in the chlorate process and also clearly show that alternative approaches for chlorate synthesis need to be developed if Cr­(VI) is to be removed from the process.

Can Telluric Acid Replace Chromate as a Homogeneous Catalyst in the Chlorate Process?

“…In Figure 6a the molar flow of oxygen is shown, the change at low telluric acid addition is not significant however the addition of 33.5 mM telluric acid results in a lower oxygen formation which is in line with previous theoretical and experimental studies [39,46] . It is also clearly shown that the reference system with sodium dichromate reduces the oxygen formation significantly.…”

“…Amongst these compounds deprotonated telluric acid ([Te(OH) 5 (O)] À ) has been identified as a possible catalyst candidate. [39] These predictions were also confirmed by the experimental work of Szabo et al, [46] who showed, that indeed a real catalytic process takes place. [16,46] The comparison of the results obtained using Cr(VI) versus deprotonated telluric acid reveals that the former is a more active catalyst.…”

“…[39]). The performance of the catalysts candidates in the screening study is ranked graphically in a volcano plot using the thermodynamically most demanding or chemical potential determining step (pds) [39] . Most materials are far away from the top which is well in line with the experimentally observed difficulties in finding a suitable replacement to Cr(VI) [17] .…”

“…[6,14,16,36,37] Both routes have reaction rates in the same order of magnitude. [7,25,38,39] Also the competing decomposition of hypochlorite to oxygen (Reaction 2), proceeds through an uncatalyzed [6,25,36] and a catalyzed [7,38] reaction path.…”

“…Based on these data, linear scaling relationships between key reaction intermediates have been established and used to construct volcano plots to explore the limitations of the catalyzed reaction (for details see Ref. [39]). The performance of the catalysts candidates in the screening study is ranked graphically in a volcano plot using the thermodynamically most demanding or chemical potential determining step (pds).…”