Zinc and Yttrium single sites were introduced into the silanol nests of dealuminated BEA zeolite to produce Zn-DeAlBEA and Y-DeAlBEA. These materials were then investigated for the conversion of ethanol to 1,3-butadiene. Zn-DeAlBEA was found to be highly active for ethanol dehydrogenation to acetaldehyde and exhibited low activity for 1,3-butadiene generation. By contrast, Y-DeAlBEA was highly active for 1,3-butadiene formation but exhibited no activity for ethanol dehydrogenation. The formation of 1,3-butadine over Y-DeAlBEA and Zn-DeAlBEA does not occur via aldol condensation of acetaldehyde but, rather, by concerted reaction of coadsorbed acetaldehyde and ethanol. The active centers for this process are ≡Si–O–Y­(OH)–O–Si≡ or ≡Si–O–Zn–O–Si–O≡ groups closely associated with adjacent silanol groups. The active sites in Y-DeAlBEA are 70 times more active than the Y sites supported on silica, for which the Y site is similar to that in Y-SiO2 but which lacks adjacent hydroxyl groups, and are 7 times more active than the active sites in Zn-DeAlBEA. We propose that C–C bond coupling in Y-DeAlBEA proceeds via the reaction of coadsorbed acetaldehyde and ethanol to form crotyl alcohol and water. The dehydration of crotyl alcohol to 1,3-butadiene is facile and occurs over the mildly Brønsted acidic ≡Si–OH groups present in the silanol nest of DeAlBEA. The catalysts reported here are notably more active than those previously reported for both the direct conversion of ethanol to 1,3-butadiene or the formation of this product by the reaction of ethanol and acetaldehyde.

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 .

Noncatalytic thermal conversion of light olefins proceeds at industrially relevant rates at temperatures above 450 °C and pressures above 50 bar. The discovery of solid acid oligomerization catalysts permitted the use of milder conditions (<300 °C) and significantly improved the octane rating. However, Brønsted acid catalysts deactivate and must be regenerated frequently. In this study, at reaction temperatures of about 250–450 °C and pressures of 1 to 40 bar, olefins react on γ-alumina to form higher molecular weight products. The rate of propylene is about ten times higher than that of ethylene. The products, however, are not a simple olefin oligomerization distribution, and many nonoligomer products are formed. The primary products undergo secondary reactions, including double bond isomerization and H-transfer, giving moderate selectivities for saturated products. Depending on the conversion, temperature, and pressure, the rate of ethylene conversion on alumina is more than 100 times that of thermal, noncatalytic conversion. The apparent activation energy for ethylene conversion is 55–75 kJ/mol, which is much lower than ∼244 kJ/mol observed for the thermal gas-phase reaction. On alumina, some reactants and products undergo disproportionation reactions. For example, propylene forms equal molar amounts of ethylene and iso-butene even at very low conversions. Lewis acid sites on γ-alumina have previously been proposed as the active site for double bond isomerization and H–D exchange. Thus, it seems likely that Lewis acid sites are also catalytic for olefin oligomerization and disproportionation reactions. With the γ-alumina catalyst, high liquid yields can be achieved with little formation of coke and minimal deactivation for at least several days.