Johannes Lercher#
Laudatio by Helmut Knözinger #
Johannes Lercher is Professor of Chemistry at the catalysis institute at the Technische Universitat München. He received the degrees of Diplom Ingenieur (1978) and Dr. techn, (1980) from the Technische Universitat Wien (both with summa cum laude), He spent a year (1982) at Yale University under Profs. Haller and Fenn’s guidance as postdoctoral fellow and lecturer. After his return to the Technische Universitat Wien, he received the venia docendi in 1985 and became acting Chairman of the Institute of Physical Chemistry (1989- 1993). In 1998, he joined the Technische Universitat Miinchen, after spending five years (1993-1998) as Professor of Chemical Technology at the University of Twente (the Netherlands).
Prof Lercher’s group works on the fundamental aspects of industrially relevant catalyzed reactions with the aim to understand the reaction steps on the surface of solid catalysts on an elementary level. This knowledge is used to design and synthesize nanoscopically well-defined chemically functionalized surfaces and materials. The synthesis and modification of the target materials is controlled on the level of the individual chemical reactions during the genesis of the (nanoscopic) particles and the assembly of the pre- functionalized entities. The materials explored include primarily highly structured micro- and meso-porous materials containing protons, metal ions as well as metal and metal oxide clusters. Advanced characterization methods (in situ XRD and X-ray absorption spectroscopy as well as IR, Raman and inelastic neutron scattering spectroscopy) are used to characterize these materials in stages of preparation and during/after sorption and catalysis. Catalytic target reactions are the low temperature acid-catalyzed activation, functionalization, and transformation of alkanes, the oxidative activation of light alkanes including methane, the hydrogenation and cracking of heavy biogenic and fossil molecules (such as lignin and multinuclear aromatic molecules). The elementary steps and selective control of sorption and diffusion in molecular sieves form a strong focus of work outside the catalysis domain. Prof. Lercher’s group has pioneered the use of in situ molecular spectroscopy to characterize surface reactions and to develop novel complex catalysts, Successful examples are e,g., xylene isomerization and methylamine synthesis.
Prof. Lercher has carried out groundbreaking work on the combination of spectroscopy, calorimetry and kinetic experiments to unravel the nature of the elementary catalytic steps. His approach and work directed towards understanding the interactions and transformations of organic molecules on solid catalysts have lead to unprecedented insight into acid—base-catalyzed reactions.
The contributions of Prof Lercher to the understanding of how hydrocarbons adsorb, are activated, and finally converted on zeolites and oxide catalysts must be emphasized. His work has stimulated not only experimental and theoretical studies in these areas, the results also demonstrated novel catalytic chemistry in activating and utilizing alkanes, In consequence, in several instances the insight gained can be used to develop new approaches to convert alkanes at low temperatures. Examples can be taken hom his work on the activation and conversion/functionalization of alkanes to illustrate his approach to combine various sites in a catalyst to achieve such targets.
Let us start with his investigations on the interactions of hydrocarbons in molecular sieves. As alkanes sorb in a zeolite with a particular pore structure and diameter, the heat of adsorption proportionally increases with the number of carbon atoms of the hydrocarbon, showing that physisorption dominates the interactions with the pores. By comparing Brønsted acidic and neutral silica zeolites, he demonstrated that the direct contribution of Brønsted acid sites is surprisingly small (ca 10 kJ/mol), comparable to the interaction strength provided by a single CH2 group in an alkane chain.[1] It is important to note also this weak interaction is only possible because of the flexibility of the alkanes and that a more rigid and bulky, but also more basic, hydrocarbons, such as benzene, are sterically so constrained in medium pore zeolites (ZSM-5) that the interact with the Brønsted acid sites only with very weak agnostic interactions.[2] Recently, Lercher’s group used the detailed knowledge on the elementary steps of sorption processes in using a porous overlayer on ZSM-5 to enhance the sorption kinetics of benzene indicating that surface modification of zeolites works not only in restricting access to zeolite pores, but also in enhancing sorption of molecules.[3] Using this approach it is possible for the first time to separate aromatic molecules on the basis of their radius of gyration rather than on their minimum kinetic diameter.[4]
While the interaction between Brønsted acid sites and alkanes provides sufficient energy to localize them at the Brønsted acid sites during sorption in zeolite pores, it does not suffice to significantly polarize the alkane C-H groups. In consequence the direct (protolytic ) cracking of alkanes following the mechanism proposed by Haag and Dessau, has a very high true energy of activation of approximately 200 kJ/rnol nearly independent of the length of the alkane chain length? [5] While the entropy change in the sorption process is the larger, the higher the heat of adsorption is it is important to note that the entropy change in this process increases the further the smaller the pores of the zeolite are. This means that the loss of entropy stems from to contributions, the tight binding to the sorption sites and the confinement through loss of configurational entropy. With narrow pore zeolites and at low coverages the effects of confinement (and the associated loss in entropy) may outweigh the stronger sorption in the pores and molecules are sorbing on the outer surface rather than in the pores.[6]This case has been demonstrated for sterically hindered alkanes that cannot penetrate fully into zeolite pores. The finding explains why with some materials operating conditions exist in which the catalytic conversions take place only on the pore mouth rather than in the inner of the pores.
Recently, Lercher’s group has shown that the direct interactions of alkanes with acid sites in zeolites can be dramatically enhanced using large, highly charged cations such as La3+ at ion exchange positions. In the presence of such cations, the C-H groups of the alkanes are highly polarized. Detailed analyses of the IR and UC MAS-NMR spectra show the C-H groups of secondary and tertiary carbon atoms are highly polarized, while the C-H groups at primary carbon atoms are contracting. This strong interaction induces dehydrogenation, as well as carbeniurn ion formation at near ambient conditions. Provided that additional alkane molecules are present, cracking and isomerization takes place already at ambient temperatures.[7] However, the results are reaching beyond the immediate discovery of LaX zeolites as being highly active to convert alkanes. The experimental evidence for localized C-H polarization in sorbed alkanes allows predicting catalytic chemistry in zeolite pores.
In a related piece of work, the group showed rigorously that alkanes can also be activated by combining controlled oxidation using sulfated zirconias with chemisorbed SOBS with conversion on neighboring Brønsted acid sites, This chemisorbed SO3 [8] is shown to result from the partial decomposition of sulfate groups during activation explaining why drastically different activities in sulfated zirconias have been observed. In this process, e.g., butane is converted to butene which forms a sec-butylcarbenium ion on Brønsted acid sites. The sec-butylcarbenium isomerizes and is removed by hydride transfer from the catalyst surface generating a new alkoxy group. Each olefin generated leads to the activation of about 500 alkanes via hydride transfer, similar to the induction of the catalytic cycles described for the polarization induced activation of alkanes. Thus, the catalytic activity depends upon the product of the concentration of olefins generated in the oxidative step, the concentration of Brønsted acid sites, and the rate constant for isornerization of the carbenium ion.
Prof Lercher’s group has also explored a new generic route to functionalize methane via electrophilic substitution. Together with collaborators at DOW, he showed that hydrogen in methane can be exchanged for positively charged chlorine atoms leading to methyl ch1oride.[9] The catalysts are based on LaCl3, which is during the catalytic cycle partly oxidized and forms hypochloride anions. After exchanging the positively charged chlorine, the formed hydroxy group is eliminated together with hydrogen from another hydroxy group as water. The vacancy generated is subsequently filled by HCl. The rate determining step is the Cl-H exchange, which has a true energy of activation of approximately l00 kJ/mol, which is very low for activating methane.
Prof. Lercher has coauthored in total over 370 publications (322 appear in the web of sciences with 6,494 citations leading to an h-index of 41, with an average number of citations of 20.17/paper) and is co-inventor of several patents.
Noteworthy is that the catalysis community has asked him to organize the next International Congress on Catalysis which will take place in Munich in 2012.
[1] F. Eder and J.A. Lercher, J. Phys. Chem., 101, 1273 (1997). F. Eder and J.A. Lercher, J. Phys. Chem.,101, 1273 (1997).
[2] R. R. Mukti, A. Jentys and J. A. Lercher, J. Phys. Chem. C, 111, 3973 (2007).
[3] S. J. Reitmeier, D. Bulichen, O. C. Gobin, A. Jentys and J. A. Lercher, Angewandte Chemie, 48, 533 (2009)
[4]S.J.Reitrneier, O. C. Gobin, A. Jentys and J. A. Lercher, J. Phys. Chem. C, 113, 15355 (2009).
[5]S Th. F. Narbeshuber, H. Vinek and J.A. Lercher, J, Catal., 157, 388 (1995).
[6] J.A.Z. Pieterse, S. Veefkind-Reyes, K. Seshan, and J. A. Lercher, J. Phys. Chem. B, 104, 5715 (2000).
[7] C. Sievers, A. Onda, A. Guzman, K. S. Otillinger, R. Olindo, and J. A. Lercher, J. Phys. Chem. C, 111, 210 (2007).
[8] X. Li, K. Nagaoka, L. J. Simon, A. Hofmann , J. Sauer and J. A. Lercher, J. Am Chem. Soc., 127,
16159 (2005).
[9] S. G. Podkolzin, E. E. Stangland, M. E. Jones, E. Peringer and J. A. Lercher, J, Am. Chem. Soc., 129, 2569 (2007).
Any further pages in alphabetic order of their title as created by you.
#
Just click at "Create new page", then type a short title and click OK, then add information on the empty page presented to you (including maybe a picture from your harddisk or a pdf-file by using the "Upload" Button) and finally click at "Save".
...no Data available yet!
