Thermal, noncatalytic conversion of light olefins (C 2 = −C 4 =) was originally utilized in the production of motor fuels at several U.S. refineries in the 1920s to 1930s. However, the resulting fuels had relatively low octane number and required harsh operating conditions (T > 450 °C, P > 50 bar), ultimately leading to its succession by solid acid catalytic processes. Despite the early utilization of the thermal reaction, relatively little is known about the reaction products, kinetics, and initiation pathway under liquid-producing conditions. In this study, thermal ethylene oligomerization was investigated near industrial operating conditions, i.e, at temperatures between 300 and 500 °C and ethylene pressures from 1.5 to 43.5 bar. Nonoligomer products such as propylene and/or higher odd carbon products were significant at all reaction temperatures, pressures, and reaction extents. Methane and ethane were minor products (<1% each), even at ethylene conversions as high as 74%. The isomer distributions revealed a preference for linear, terminal C 4 and C 5 . The reaction order was found to be second-order with a temperature-dependent overall activation energy ranging from 39.4 to 58.3 kcal mol −1 . Four bimolecular initiation reaction steps for ethylene were calculated using DFT. Of these, simple H-transfer to yield vinyl and ethyl radicals was found to have a free energy activation energy barrier higher (about 10 kcal mol −1 ) than the other three initiation steps forming either cyclobutane, 1-butene, or tetramethylene. The importance of diradical species in generating free radicals during a two-phase initiation process was proposed. The reaction chemistry for ethylene, which has only strong, vinyl C−H bonds, starkly contrasted with propylene, which possesses weaker allylic C−H bonds and showed a preference for dimeric C 6 products over C 2 −C 8 nonoligomers. The resulting C 4 and C 5 nonoligomers from propylene contained more iso-olefins compared to linear C 4 and C 5 .

Control of the spatial proximity of Brønsted acid sites within the zeolite framework can result in materials with properties that are distinct from materials synthesized through conventional crystallization methods or available from commercial sources. Recent experimental evidence has shown that turnover rates of different acid-catalyzed reactions increase with the fraction of proximal sites in chabazite (CHA) zeolites. The catalytic conversion of oxygenates is an important research area, and the dehydration of methanol to dimethyl ether (DME) is a well-studied reaction as part of methanol-to-olefin chemistry catalyzed by solid acids. Published experimental data have shown that DME formation rates (per acid site) increase systematically with the fraction of proximal acid sites in the six-membered ring of CHA. Here, we probe the effect of acid site proximity in CHA on methanol dehydration rates using electronic structure calculations and microkinetic modeling to identify the primary causes of this chemistry and their relationship to the local structure of the catalyst at the nanoscale. We report a density functional theory-parametrized microkinetic model of methanol dehydration to DME, catalyzed by acidic CHA zeolite with direct comparison to experimental data. Effects of proximal acid sites on reaction rates were captured quantitatively for a range of operating conditions and catalyst compositions, with a focus on total paired acid site concentration and reactant clustering to form higher nuclearity complexes. Next-nearest neighbor paired acid sites were identified as promoting the formation of methanol trimer clusters rather than the inhibiting tetramer or pentamer clusters, resulting in large increases in the rate for DME production due to the lower energy barriers present in the concerted methanol trimer reaction pathway. The model framework developed in this study can be extended to other zeolite materials and reaction chemistries toward the goal of rational design and development of next-generation catalytic materials and chemical processes.