Comments on "nature of initial transient period during enantioselective hydrogenation on Pt and Pd" by T. Mallat, Z. Bodnar, B. Minder, K. Borszeky, and A. Baiker

Journal of Catalysis

Volumn 176, Issue 1, 1998, Pages 267-270

  Blackmond, Donna G ^a  

^a MAX PLANCK INSTITUT FÜR KOHLENFORSCHUNG   (Germany)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0006523075     PISSN: 00219517     EISSN: None     Source Type: Journal    
DOI: 10.1006/jcat.1998.2080     Document Type: Editorial

References (16)

1. Mallat, T., Bodnar, Z., Borszeky, K., and Baiker, A., J. Catal. 168, 183 (1997).
2. Blaser, H. U., Jalett, H.-P., Müller, M., and Studer, M., Catal. Today, in press. [See for a recent comprehensive review]
3. Sutherland, I. M., Ibbotson, A., Moyes, R. B., and Wells, P. B., J. Catal. 125, 77 (1990). [This first published observation of a rising reaction rate was made using in-situ hydrogen uptake measurements]
4. Singh, U. K., Landau, R. N., Sun, Y., LeBlond, C., Blackmond, D. G., Tanielyan, S. K., and Augustine, R. L., J. Catal. 154, 91 (1995).
5. Wang, J., Sun, Y., LeBlond, C., Landau, R. N., and Blackmond, D. G., J. Catal. 161, 752 (1996).
6. Sun, Y., Wang, J., LeBlond, C., Landau, R. N., Laquidara, J., Sowa, J. R., Jr., and Blackmond, D. G., J. Mol. Catal. A 115, 495 (1997).
7. Margitfalvi, J. L., Hegedus, M., and Tfirst, E., Tetrahedron: Asymm. 7, 571 (1996).
8. Augustine, R. L., and Tanielyan, S. K., J. Mol. Catal. 112, 93 (1996).
9. The Experimental section of our paper cited as Ref. (25) by Mallat et al. (Ref. (4b) in this letter) specifically notes that the subtraction of racemic ethyl lactate impurity was carried out. Figure 1 of that paper reproduced the reaction conditions of Fig. 1 in the paper cited as Ref. (24) (Ref. (4a)), showing that the subtraction was carried out although not specifically noted in Ref. (4a). It should also be clarified for readers that the authors of Ref. (1) present uncorrected enantioselectivity values in all data presented prior to Fig. 7 of Ref. (1).
10. The electrochemical measurements of catalyst potential under hydrogen shown in Fig. 3 in Ref. (1) show, however, that after 2 h in the reaction mixture the catalysts exhibit identical potentials, regardless of treatment history and whether hydrogen was introduced at the beginning or end of this 2-h period.
11. The conclusion in Ref. (1) that hydrogen surface concentration changes over the course of a diffusion-limited reaction supports our observation of similar changes in solution hydrogen concentration during diffusion-limited reactions (see Fig. 5 in Sun, Y., Wang, J., LeBlond, C., Landau, R. N., and Blackmond, D. G., J. Catal. 161, 759 (1996)). However, extension of this interpretation of surface coverages to experiments carried out under kinetically controlled conditions is not straightforward.
12. r = volume of reactor contents. Monitoring in-situ hydrogen uptake provides reaction progress calculated from the reaction stoichiometry and the ideal gas law. GC analysis of the reaction solution gives compositional information as a function of conversion. Any contribution to the observed rate from side reactions must be negligible in a case, where these different methods give identical reaction rates, as we showed in Ref. (4b) and also in LeBlond, C., Wang, J., Larsen, R. D., Orella, C. J., Forman, A. J., Landau, R. N., Laquidara, J., Sowa., J. R., Jr., Blackmond, D. G., and Sun, Y.-K., Thermochim. Acta 289, 189 (1996).
13. A number of groups report initial rate values or define integral rate parameters, such as the time required to achieve a specific conversion or a specific number of turnovers. For a very recent report of detailed rate measurements in this reaction system see Kohler, J., and Bradley, J. S., Catal. Lett. 45, 203 (1997).
14. Wang, J., LeBlond, C., Orella, C. F., Sun, Y., Bradley, J. S., and Blackmond, D. G., in "Proceedings 4th Intl. Symp. on Het. Catal. and Fine Chem., Basel, 1996."
15. It may be calculated from Fig. 6 of Ref. (1) that the (corrected) instantaneous enantioselectivity increases by approximately 10% over the range of conversion shown.
16. A general conclusion of our work is the recommendation that reaction rate data receive more prominent consideration in any discussion of selectivity, since selectivity is a derived property of reaction rates.