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.
Crystalline manganese
oxides have attracted the most attention
in aqueous zinc-ion batteries due to their diverse nanostructures
and low cost. However, extensive studies on amorphous manganese oxides
are lacking. Herein, we report a mesoporous amorphous manganese oxide
(UCT-1-250) as a cathode material with high capacity (222 mAh g–1), good cyclability (57% capacity retention after
200 cycles), and an acceptable discharge plateau (between 1.2 and
1.4 V). An approach to mechanistic studies was performed by comparison
of UCT-1-250 and other crystalline manganese oxides through electrochemical,
elemental, and structural analyses. An in situ conversion to ZnMn2O4 spinel phase after initial cycling contributes
to the high performance. The irreversible capacity fading is due to
the formation of the woodruffite phase.
Propane and propene oxidations on M1 phase MoVTeNb mixed oxide catalysts exhibit relatively high selectivity to acrolein and acrylic acid. We probe the ability of the reactant molecules to access the catalytic sites inside the heptagonal pores of these oxides and analyze elementary steps that limit selectivity. Measured propane/cyclohexane activation rate ratios on MoVTeNbO are nearly an order of magnitude higher than non‐microporous VOx/SiO2, which suggests significant contribution of M1 phase pores to propane activation because both molecules react via homologous rate‐limiting C−H activation. Density functional theory suggests that desired C3H8 dehydrogenation and C3H6 allylic oxidation to acrolein and acrylic acid are limited by C−H activation steps, while less valuable oxygenates form via steps limited by C−O bond formation. Calculated activation barriers for C−O formation are invariably higher than C−H activation when these activations occur inside the pores, suggesting that reactions restricted within the pores are highly selective to desired products. These results demonstrate the role of pore confinement and a framework to assess selectivity limitation in hydrocarbon oxidations involving a complex network of sequential and parallel steps.
The Cover Feature shows sequential conversions of propane to propene and acrolein inside one‐dimensional pores of M1 phase MoVTeNb mixed oxide. In their Full Paper, Y. Liu, A. Twombly et al. show that the pores tightly confine linear alkanes and alkenes and enhance rates of C−H activations via van der Waals stabilization, but steric hindrance to forming C−O bonds at the M‐O‐M bridging lattice O‐atoms accessible within these pores diminishes O‐insertion near C=C bonds in alkenes. This C−H activation enhancement and C−O formation diminution together improves selectivity to dehydrogenation and allylic oxidation products. More information can be found in the Full Paper by Y. Liu, A. Twombly et al.
Two gas phase deuterium/hydrogen exchange reactions are described utilizing a simple inexpensive glass catalyst tube containing 0.5% Pd on alumina through which gas mixtures can be passed and products collected for analysis. The first of these exchange reactions involves H 2 + D 2 , which proceeds at temperatures as low as 77 K yielding a mixture that includes HD. Products are analyzed by 1 H NMR spectrometry. At low temperatures, this reaction requires a catalyst, but it proceeds without a catalyst at high temperature of a gentle flame. The second deuterium/hydrogen exchange reaction involves CH 4 + D 2 producing a series of isotopologues, methane-d x , x = 0−4, with product analysis by GC−MS and 1 H NMR spectrometry. This reaction only takes place in the presence of a catalyst at elevated temperatures due to the large energy of activation of the sp 3 -carbon-to-hydrogen bond. Two outcomes have been observed in the literature regarding D/H exchange and methane. Some catalysts and temperature conditions yield a single-exchange result, methane-d 1 . Others yield multiple exchange results, such as we observe with our catalyst. The single exchange outcome is associated with lower temperatures. Two mechanisms, one by Kemball (1959) and one by Frennet (1974), have been put forth to explain single and multiple exchange outcomes. We discuss our results in the context of these mechanisms. Interested readers could develop a research experience for undergraduate chemistry students based on the openended experiments presented here.
The catalytic hydrogenation of alkenes
and alkynes is an important
part of the undergraduate chemistry curriculum and is a fundamental
process in chemical industry. Inquiry-based laboratory activities
are presented that investigate the hydrogenation of alkynes on a nanoparticle
palladium surface to form alkenes, which go on to form alkanes. Alkyne
hydrogenation using H2 and/or D2 proceeds via
a vinyl–palladium intermediate to form a π-bonded alkene–Pd
species that can desorb or remain on the palladium surface and undergo
further hydrogenation via the Horiuti–Polanyi mechanism, associated
with extensive deuterium–hydrogen exchange. Central to the
experiments is an inexpensive, easy-to-build glass tube containing
palladium nanoparticles on alumina beads that can be used indefinitely.
A total of seven inquiry-based questions are discussed regarding hydrogenation
of alkynes. A similar number of open questions are discussed for further
investigations by interested persons. These activities are suitable
as guided research projects for science majors. Each experiment is
performed by groups of two or three students in about an hour including
analysis by mass spectrometry. An additional hour is allowed for student
analysis and discussion of the mass spectral results, writeup, and
future planning followed by about 30 min with the mentor for group
presentation and discussion of the results. Results often lead to
additional questions, either for clarification or for new exploration
and form the basis for inquiry-based learning and problem-solving.
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