The hydrogenation of alkenes by heterogeneous catalysts has been studied for 80 years. The foundational mechanism was proposed by Horiuti and Polanyi in 1934 and consists of three steps: (i) alkene adsorption on the surface of the hydrogenated metal catalyst, (ii) hydrogen migration to the β-carbon of the alkene with formation of a σ-bond between the metal and α-C, and finally (iii) reductive elimination of the free alkane. Hundreds of papers have appeared on the topic, along with a number of variations on the Horiuti–Polanyi mechanism. The second step is highly reversible, leading to extensive deuterium–hydrogen exchange when D2(g) is used. This paper describes the investigation of gas-phase reactions between deuterium and 1-butene using a supported palladium catalyst under ambient laboratory conditions and how the results are consistent with the Horiuti–Polanyi mechanism. An Excel spreadsheet for modeling the extent and distribution of deuteration within butane-d x is described. Interested readers could develop a laboratory or research experience based on results presented here. The results are also suitable for inclusion in an upper-division chemistry course in which organometallic chemistry or reaction mechanisms involving heterogeneous catalysts are discussed. The catalyst tubes are inexpensive and easy to construct. Analysis of the butane produced by 1H NMR and GC–MS leads to numerous conclusions in support of the Horiuti–Polanyi mechanism.
Nine gas-phase reactions that can be accomplished with an inexpensive, commercially-available glass-encased, heterogeneous palladium catalyst tube are described. The catalyzed reactions are suitable for demonstrating gas-phase reactions in the classroom or teaching laboratory. The reactions described include air or oxygen oxidations (CH4 + O2, CO + O2, NH3 + O2), hydrogenation of ethene, thermal decomposition of nitrous oxide, oxidations involving nitrogen dioxide (CH4 + NO2, CO + NO2), and two oxidation reactions involving nitrous oxide (N2O + NH3, N2O + CO). Several of the reactions demonstrate processes that take place in an automotive catalytic converter. In all cases, the products can be tested by simple chemical methods.
This article describes two organic reactions involving gas-phase heterogeneous catalysis suitable for use as an advanced undergraduate synthesis and spectroscopy laboratory experiment. In the first reaction, 2-propanol is converted to propene using heated alumina beads as the catalyst. The product gas is purified with a dry ice–propanol cold trap. In the second reaction, the propene is hydrogenated to propane using a palladium catalyst in nearly 100% yield. Both propene and propane are characterized by proton nuclear magnetic resonance spectroscopy and both give elegant spectra that are ideally suited for interpretation by undergraduate students. Students may use proton decoupling to assign the spectrum and COSY to confirm chemical shift assignments. This laboratory experiment is original in that gas-phase organic reactions are not usually experienced in chemistry laboratory programs. The experiment demonstrates continuous-flow, closed-system gas phase reactions of fundamental importance in chemical industry.
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