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

Abstract

Soft templatingw ith siliceous surfactant is an established protocol for the synthesis of hierarchically porous silicoaluminophosphates (HP SAPOs) with improved mass transport properties. Motivated by the enhanced performance of HP SAPOsi nt he Beckmannr earrangement of cyclohexanoneo xime to the nylon 6p recursor e-caprolactam, an integrated theoretical and empirical study was carried out to investigate the catalytic potential of the siliceous mesopore network. Inelastic neutron scattering( INS) studies, in particular,p rovideduniquei nsight into the substrate-framework interactions in HP (Si)AlPOs and allowed reactive species to be studied independent of the catalystm atrix. The spectroscopic (INS, FTIR spectroscopy,M AS NMR spectroscopy) andc omputationala nalyses revealed that in the organosilane-templated SAPO, the interconnectivity of micro-a nd mesopores permits cooperativity between their respective silanol and Brønsted acid sites that facilitates the protonation of cyclohexanone oxime in ap hysical mixture at ambienttemperature.[a] S.

Hierarchically Structured Zeolites: From Design to Application

“…Since the discovery of highly ordered mesoporous molecular sieves (M41S), soft templating methods are widely used for the synthesis of hierarchically porous zeolites. , Soft templates, including silylated polymers, − hydrophilic cationic polymers, − cationic organosilanes, − and surfactants, ,− ,,− exhibit inherent flexibility and advantages, such as tunable size and adjustable functionality, and are more versatile than hard templates. During the crystallization process, these templates not only serve as physical supports but also directly interact with the silica species. − …”

Down the Microporous Rabbit Hole of Silicoaluminophosphates: Recent Developments on Synthesis, Characterization, and Catalytic Applications

“…SAPOs are generally well-defined materials but often have multiple acid sites, which must be resolved to understand their individual behaviors . Infrared (IR) spectroscopy is a commonly used technique for probing acid sites, specifically DRIFTS (diffuse reflectance infrared spectroscopy), as little sample preparation is needed, to focus on surface interactions. ,,,, As the energy of a signal is a function of the bonds’ force constant and the atoms involved, one can resolve different O–H stretches, loosely based on the acid strength. The hydroxyl region of SAPO materials (4000–3000 cm –1 ) commonly shows many features, with ever-present defect hydroxyl sites, Si–OH and P–OH, occurring at 3750 and 3680 cm –1 , respectively. , However, Brønsted acid sites associated with Si substitution (Al–OH–Si) occur at 3650–3500 cm –1 .…”

“…However, small channels can hinder diffusion, preventing reactants to reach the active sites, hindering activity. Worse still, it can stop products leaving the active site, triggering further reactions, leading to byproducts, and ultimately coke. − Many researchers sought to overcome this by incorporating mesopores into the microporous networks, therefore having different pore sizes over multiple length scales, creating hierarchically porous (hierarchical) systems. ,,,,,, Such systems have improved catalyst lifetime for a range of reactions, as mesopores aid product diffusion away from the active site and act as a sacrificial site for coke build-up, protecting the micropores . A variety of methods have been used to incorporate mesopores into SAPOs, which can be divided into two categories.…”

“…196 Infrared (IR) spectroscopy is a commonly used technique for probing acid sites, specifically DRIFTS (diffuse reflectance infrared spectroscopy), as little sample preparation is needed, to focus on surface interactions. 16,138,154,197,198 As the energy of a signal is a function of the bonds' force constant and the atoms involved, one can resolve different O−H stretches, loosely based on the acid strength. The hydroxyl region of SAPO materials (4000− 3000 cm −1 ) commonly shows many features, with ever-present defect hydroxyl sites, Si−OH and P−OH, occurring at 3750 and 3680 cm −1 , respectively.…”

“…248−250 Many researchers sought to overcome this by incorporating mesopores into the microporous networks, therefore having different pore sizes over multiple length scales, creating hierarchically porous (hierarchical) systems. 15,144,154,197,198,207,251 Such systems have improved catalyst lifetime for a range of reactions, as mesopores aid product diffusion away from the active site and act as a sacrificial site for coke build-up, protecting the micropores. 252 A variety of methods have been used to incorporate mesopores into SAPOs, which can be divided into two categories.…”