Comprehensive Vibrational Spectroscopic Characterization of Nylon‐6 Precursors for Precise Tracking of the Beckmann Rearrangement

Abstract

As a key step in nylon‐6 synthesis, the Beckmann rearrangement is an ongoing target of catalytic studies that seek to improve the sustainability of polymer manufacture. Whilst solid‐acid catalysts (predominantly zeotypes) have proven effective for this transformation, the development of more active and selective systems demands an understanding of fundamental catalytic mechanisms. In this undertaking, in situ and operando characterization techniques can be informative, provided rigorous spectroscopic groundwork is in place. Thus, to facilitate mechanistic studies we present a detailed investigation of the vibrational spectra of cyclohexanone, cyclohexanone oxime, ϵ‐caprolactam and their D10‐isotopomers, in the solid state. Variable‐temperature infrared (150–300 K) and Raman (10–300 K) spectra are reported alongside inelastic neutron scattering data. Moreover, where key vibrational modes have been assigned with the aid of periodic density functional theory calculations, it has been possible to include hydrogen‐bonding interactions explicitly.

Integrated Theoretical and Empirical Studies for Probing Substrate–Framework Interactions in Hierarchical Catalysts

“…[20] As there is minimal interaction between the oxime trimers, the system was modelled as as ingle asymmetric unit, and the INS spectrum assignedb y correlating experimental modesw ith computed vibrations, as reportedi nt he literature. [10] Notably,h eating the pure oxime above its meltingp oint (359 K) did not modify the appearance of its INS vibrational spectrum,t hat is, intermolecular hydrogen-bonding interactions in the oxime trimer were not irreversibly modified by thermalt reatment, and the crystal structure of the substrate was retained.…”

“…For reference purposes, the INS spectrum of cyclohexanone oxime was acquired ( Figure 3a). [10] Crystallographic studies indicatedt hat cyclohexanone oxime crystallizes in space group P with asymmetric units that contain ah ydrogen-bonded trimer (Figure 3b), in which individual oxime molecules can both accept and donate hydrogen bonds. [20] As there is minimal interaction between the oxime trimers, the system was modelled as as ingle asymmetric unit, and the INS spectrum assignedb y correlating experimental modesw ith computed vibrations, as reportedi nt he literature.…”

“…[9] More recently,w eused INS to perform adetailed vibrational characterization of the molecular speciesr elevant to the Beckmann rearrangement. [10] Building on these fundamental studies, we now present INS as at ool to probe the atomic-level substrate-framework interactions in the Beckmannr earrangement, with av iew to ascertaining new mechanistic insights that could aid rational catalyst design. Moreover, we demonstrate how ad iverse spectroscopica nalysis can aid in understanding more complexc atalytic systems, [11] that is, hierarchically porous silicoaluminophosphates (HP SAPOs).…”

“…[8] Although opticalv ibrational spectroscopy is at ypical methodf or characterizings urface-bound adsorbates, its sensitivity is often limited by the opacityo ft he zeolite and an inability to access informative low-energy modes in the fingerprint region. [10] Building on these fundamental studies, we now present INS as at ool to probe the atomic-level substrate-framework interactions in the Beckmannr earrangement, with av iew to ascertaining new mechanistic insights that could aid rational catalyst design. Since 1 Hh as am ore than tenfold larger incoherent scattering crosss ectionf or neutrons than the elements making up the zeolite, the framework is effectively transparent.…”

“…Previously, we exploited the properties of neutron‐based techniques in a quasi‐elastic neutron scattering study on the bulk diffusion of cyclohexanone oxime in microporous zeotypes for the Beckmann rearrangement . More recently, we used INS to perform a detailed vibrational characterization of the molecular species relevant to the Beckmann rearrangement . Building on these fundamental studies, we now present INS as a tool to probe the atomic‐level substrate–framework interactions in the Beckmann rearrangement, with a view to ascertaining new mechanistic insights that could aid rational catalyst design.…”