Expanding Beyond the Micropore: Active-Site Engineering in Hierarchical Architectures for Beckmann Rearrangement

Abstract

ABSTRACT:The ability to engineer discrete solid-acid centers within hierarchically porous architectures that contain micropores with interconnected mesopores, offers the potential to overcome the spatial restraints and diffusional limitations imposed by conventional microporous zeotype catalysts, which often lead to decreased catalyst lifetimes and restricted substrate scope. By employing a one-step soft-templating approach, coupled with detailed physicochemical and spectroscopic characterization, isolated, solidacid sites can be suitably tailored and discretely modulated within the micropores and mesopores. The design strategy facilitates a synergistic enhancement in catalytic activity, selectivity, substrate versatility, and longevity, compared to analogous microporous zeotypes that have been extensively employed in the chemical industry, as solid-acid catalysts, for the production of -caprolactam (precursor for Nylon-6).

Influence of dopant substitution mechanism on catalytic properties within hierarchical architectures

“…The HP catalysts were synthesized by using a soft-templating bottom-up approach [40]. The surfactant, DMOD, was added to the synthesis gel with the microporous structure-directing agent, triethylamine.…”

“…Upon calcination, the microporous SDA and micelles are removed, and an HP catalyst was created, which has silanols lining the mesopores ( figure 1). This methodology has proved versatile and has been used to create HP AlPO-n catalysts, with a range of dopants (Co [42], Cr [43], Fe [44], Si [40,45]), with a large majority focused on the creation of selective oxidation catalysts. As of yet the relationship between the nature and type of dopant species, which lead to different substitution mechanisms resulting in the creation of diverse active sites for catalysis, has not been fully explored.…”

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

“…The alternative bottom‐up approach ensures a high degree of structural control of the SAPOs, with retention of the active sites associated with the microporous analogue . Typically, bottom‐up preparations use a sophisticated surfactant soft‐template (such as an organosilane) that is added to the synthesis gel to template mesoporosity . However, in a recent drive towards low‐cost, sustainable mesoporogens, saccharides have been identified as viable candidates for mesopore formation .…”

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

“…To overcome these drawbacks, hierarchical SAPO‐34, which combines the microporous CHA structure with an additional mesoporous network, has been prepared using both top‐down and bottom‐up, approaches. Generally, top‐down approaches involve post‐synthetic modification, such as demetallation under acidic or basic conditions, to extract framework constituents whilst preserving crystallinity.…”

“…These molecules incorporate a trimethoxysilyl moiety that can form covalent bonds with zeolitic precursors, promoting the crystallization of a single, hierarchical phase . Nonetheless, the bottom‐up approach can have a detrimental impact on material crystallinity and, moreover, the use of amphiphilic organosilane templates leads to the incorporation of siliceous species into the walls of the mesopores, modifying the acid characteristics of the parent framework …”