Activation of Small Alkanes on Solid Acids. An H/D Exchange Study by Liquid and Solid-State NMR: The Activation Energy and the Inhibiting Effect of Carbon Monoxide

Formation of Carboxylic Acids from Small Alkanes in Zeolite H-ZSM-5

“…[9, 18±21] The position of this signal at d 191 is characteristic of the carbonyl group of a carboxylic acid adsorbed onto H-ZSM-5. [9] Thus, the appearance of the signal at d 191 is in agreement with a reaction between propane, carbon monoxide, and water during which 13 CO was transformed into a 13 C-labeled carbonyl group of a carboxylic acid. To elucidate the reaction pathway further, we then used 13 C-labeled propane for the coadsorption.…”

“…[9] Thus, the appearance of the signal at d 191 is in agreement with a reaction between propane, carbon monoxide, and water during which 13 CO was transformed into a 13 C-labeled carbonyl group of a carboxylic acid. To elucidate the reaction pathway further, we then used 13 C-labeled propane for the coadsorption. When the reaction was carried out with [2-13 C]propane, unlabeled CO and water, a weak signal at d 7.3 from ethane [16] and a signal at d 36.4 were observed in the 13 C MAS NMR spectrum, in addition to the intense signal at d 18.2 from the 13 C-labeled carbon of the methylene group in propane (Figure 1 b).…”

“…573 K), no conversion of propane was reported at 473 K and below. [13] However, in analogy with the superacid-catalyzed protolytic activation of propane, [5,6] the Haag and Dessau mechanism (Scheme 1) would rationalize the formation of the same products.…”

“…[9] On the other hand, on the basis of regioselective H/D exchange between isoalkanes and zeolites, Sommer et al [10,11] and Hall and Engelhardt [12] have suggested that alkenes and carbenium ions occur as reaction intermediates on solid acids at 373 ± 473 K. We also reported that carbon monoxide blocked this exchange by trapping the carbenium ion intermediates. [13] Taking these two results into account, we could expect that carbon monoxide and alkanes should interact on zeolite at temperatures greatly below the usual working temperatures at which cracking is observed.…”

Low‐Temperature Alkane CH Bond Activation by Zeolites: An In Situ Solid‐State NMR H/D Exchange Study for a Carbenium Concerto

“…These values are close to the H/D exchange reaction between the isobutane molecule on the methyl position and H-ZSM5 (50 kJ mol À1 ), [13,36] much closer than to the value for the methane and H-Beta reaction (138 kJ mol À1 ). [37] The difference of 88 kJ mol À1 between methane and isobutane has been imputed to the difference in chemical pathways with these respective molecules for which the former favors carbonium chemistry and the latter carbenium chemistry.…”