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

Abstract

Isotopic H/D exchange has been monitored by in situ MAS NMR spectroscopy of 2-[D(14)]methylpentane with H-USY to probe the controversy over the alkane conversion mechanism. The probe molecule has distinct exchangeable sites with different accessibility to the zeolite surface. In the early stages of the process, the regioselectivity of exchange demonstrates that the slow step of the mechanism is controlled by a carbenium ion intermediate. At a later stage of exchange, intramolecular hydride migrations, typical of carbenium chemistry, replace D by H also on other carbon atoms, resulting in a loss of regioselectivity. Therefore, the first and the subsequent steps of the H/D exchange proceed at this temperature through a carbenium intermediate species.

Nanoscale metal fluorides: a new class of heterogeneous catalysts

“…83,84 Deuterated compounds play a fundamental role in isotope labelling reactions in pharmaceutical, medical and spectroscopic applications. [85][86][87][88][89] Besides the novel Shilov or Periana type precious metal catalysts 90,91 and others, also solid Lewis acids like zeolites and dehydroxylated alumina are described to catalyse to some extent H/D-exchange reactions above 150°C [92][93][94][95][96][97][98][99][100][101] whereas molecular Lewis acids like AlBr 3 or MoCl 5 catalyse H/D exchange between arenes only. 102,103 Thus, both very strong Lewis acids ACF and HS-AlF 3 were tested for their catalytic activity in H/D exchange reactions between aliphatic and aromatic C-D/H bonds.…”

Insights into reaction mechanisms in heterogeneous catalysis revealed by in situ NMR spectroscopy

“…27,29 The activation mechanism of branched alkanes, especially isobutane, on Brønsted acid sites of solid catalysts at ambient or lower temperature is also controversial. 26,[30][31][32][33] In situ solid state 1 H NMR spectroscopy was used to study the H/D exchange of deuterated isobutane on zeolite H-ZSM-5 in the absence of impurities of alkenes. 26,30 The results are described in Fig.…”

“…31 Less symmetric branched alkane molecules, 2-methylpentane and 3-methylpentane, giving products exchanged in different carbon atoms depending on the activation mechanism have been studied very recently, suggesting an alternative mechanism involving carbenium ions for the activation of branched alkanes. [31][32][33] The approach followed in ref. 31 and 32 for the H/D isotope exchange experiments is not strictly in situ, but they are relevant in this context.…”

“…32 In situ solid state NMR spectroscopy has been also used to study the activation of 2-methylpentane-d 14 on H-Y zeolite by H/D exchange. 33 The regioselective exchange was checked by 13 C NMR spectroscopy providing a much better resolution than 1 H. A 13 C{ 1 H} DEPT-45 pulse sequence in which the polarization of protons is transferred to carbons through scalar 1 J C-H coupling was applied to assign the carbon sites and the proton attached. Fig.…”

Catalytic CH Bond Activation at Nanoscale Lewis Acidic Aluminium Fluorides: H/D Exchange Reactions at Aromatic and Aliphatic Hydrocarbons

“…The substrate loadings and conversions are usually low, even after prolonged reaction times. [31][32][33][34][35][36][37] However, dehydroxylated metal oxides, especially g-Al 2 O 3 , catalyse H/D exchange, for example, at methane, at room temperature, when treated beforehand at 400 8C. [38] Lewis acids such as AlBr 3 , EtAlCl 2 or MoCl 5 catalyse efficiently the H/D exchange between arenes, only.…”