Creating Accessible Active Sites in Hierarchical MFI Zeolites for Low‐Temperature Acid Catalysis

Abstract

A versatile desilication design strategy for the creation of hierarchical H-ZSM-5 catalysts with different Si/Al ratio has been demonstrated. The nature, strength and the accessibility of the acid sites after the alkaline treatment was elucidated by employing a range of physico-chemical characterization tools; notably probebased FTIR spectroscopy along with SS MAS-NMR. In addition, structural and textural properties of the hierarchical zeolites were also explored and compared to their corresponding microporous analogues. CO was used to probe the acidic properties of the hierarchical zeolites with the concomitant deployment of a bulky molecular probe, 2,4,6-trimethylpyridine (collidine), which is too large to access the micropores, to specifically investigate the enhanced accessibility of the active sites. The hierarchical zeolites were evaluated in the industrially relevant, acid-catalyzed Beckmann rearrangement of cyclohexanone oxime to -caprolactam, the precursor for Nylon-6, in liquid phase and at low-temperatures. The catalytic findings with the hierarchical catalysts reveal a significant enhancement in the production of -caprolactam, compared with the parent microporous H-ZSM-5 zeolite, thereby highlighting the merits of our design approach in facilitating enhanced diffusion and masstransfer.

Hierarchical SAPO‐34 Architectures with Tailored Acid Sites using Sustainable Sugar Templates

“…The ν 8a C−C stretching mode of 2,4,6‐TMP (which has a liquid‐phase frequency of 1611 cm −1 ) is very sensitive to the Brønsted sites in the hierarchical materials, with its position changing in response to acid‐site strength. When the ν 8a mode appears at wavenumbers >1630 cm −1 , the formation of the protonated species (2,4,6‐TMPH + ) is inferred, whereas a lower wavenumber indicates the formation of a H‐bonded adduct or physisorbed species . Upon 2,4,6‐TMP adsorption on FRU‐S02 and SUC‐S03 (Figures D and 4 E), bands at 1611 and 1575 cm −1 (owing to physisorbed 2,4,6‐TMP) and a signal at 1638 cm −1 (owing to 2,4,6‐TMPH + ) are observed.…”

“…Moreover, quantitative analysis by 1 H MAS NMR indicates that the number of Brønsted acid sites is comparable in all hierarchical SAPO‐34 systems (Table ). Weak signals at 2.6 ppm (attributed to P−OH species) and 1.8 ppm (attributed to Si−OH species) are also present in hierarchical materials, whereas proton resonances beyond 4.5 ppm are attributed to hydrogen‐bonded species …”

“…For this reason, 2,4,6‐trimethylpyridine (2,4,6‐TMP) is widely used to study the enhanced accessibility of acid sites in hierarchical zeolites, exploiting the impeded diffusion through small micropores . By contrasting the interactions of NH 3 and 2,4,6‐TMP probes, it is possible to extract information relating to the accessibility of the Brønsted acid sites …”

Mesoporous Silica Scaffolds as Precursor to Drive the Formation of Hierarchical SAPO‐34 with Tunable Acid Properties

“…Localized collapse of the zeolite framework generates mesoporous voids but simultaneously elicits defect sites, thus irrevocably altering both the textural and acid characteristics of the parent framework . Although demetallation has been widely employed for the production of hierarchical zeolites,, comparably few examples have been reported for silicoaluminophosphates, due to the their relative instability under strongly acidic or alkaline conditions . Often, a bottom‐up approach is employed for the production of hierarchical SAPOs, whereby sophisticated, but sacrificial, surfactant molecules self‐assemble into a supramolecular micelle that guides the formation of the mesoporous network, whilst a structure‐directing agent facilitates the formation of the microporous architecture.…”

“…In addition, the structural and textural properties of the HierSAPO‐34 were evaluated using X‐ray diffraction (XRD) and volumetric analyses. FTIR spectroscopy of adsorbed probe molecules of varying kinetic diameters is a powerful tool in the systematic study of the acid properties of hierarchical zeolites, providing information on the location, content, strength and accessibility of both Brønsted and Lewis sites ,,. In this study, CO was used to assess the acid strength of the HierSAPO‐34, whilst strongly‐basic molecules of increasing kinetic diameter, including ammonia, pyridine, 2,4,6‐trimethylpyridine (collidine), and 2,6‐di‐ tert ‐butylpyridine, were used to probe the accessibility of these sites.…”