The H/D exchange reaction occurring at low temperature between small alkanes and D2O exchanged solid acids. III. The role of alkenes and carbenium ions as reaction intermediates

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

“…The participation of only a limited number of acid sites in the reaction is in contrast with our previous findings on the H/D exchange reaction observed between deuterated zeolite and isobutane [10,11,13] for which all the acid sites of the zeolite are involved in the hydrogen exchange with isobutane. In that case, the carbenium ion intermediates are involved in a catalytic cycle (ion deprotonation ± alkene reprotonation) for which very strong acid sites are not necessary.…”

“…[4±6] These acids are generally prepared from alkenes in sulfuric acid (Koch reaction); [7,8] however, we have recently shown that they can also be selectively synthesized from an olefin and CO on an acidic zeolite under mild temperature and pressure conditions. [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.…”

Isomerization of cycloheptane, cyclooctane, and cyclodecane catalyzed by sulfated zirconia—comparison with open-chain alkanes

“…22 Sommer and co-workers studied the intermolecular H/D exchange occurring between isobutane and D 2 O-exchanged solid acids, and the replacement by deuteriums at the only primary hydrogens was explained by a mechanism involving the reprotonation (redeuteronation) of isobutylene, generated as an intermediate on the solid acids, through the monodeuterated trimethyl carbenium ion in accordance with the Markovnikov rule. 17,23 In the rearrangement of alkyl groups catalyzed by a superacidic site, it is known that the reaction proceeds through a protonated cyclopropane intermediate after the formation of a carbenium ion. On the basis of the number of carbon atoms of the carbenium ion, two feasible mechanisms have been proposed, monomolecular and bimolecular reactions.…”

Room‐Temperature Alkane Reactivity in Zeolites: An H/D Exchange Study

“…[28] During the last two decades the improvements in highfield multinuclear NMR, MAS-NMR, FTIR, UV/Vis, and MS techniques have facilitated the experimental reinvestigation of H/D exchange between various small alkanes (C 1 to C 7 ) and deuteriated solid acids. The reaction has been extensively studied with zeolites, [11,[29][30][31][32][33][34] sulfated zirconias, [35][36][37] and heteropolyacids. [38] The results have consistently shown that, at temperatures around 100-150 8C, hydron exchange occurs readily with branched alkanes whereas linear alkanes are practically unreactive under the same conditions.…”

“…[10,32,37,49] The exchange takes place exclusively with the hydrons next to the tertiary center, as suggested in Scheme 3. …”