Defects tune the acidic strength of amorphous aluminosilicates

Abstract

Crystalline zeolites have high acidity but limited utility due to microporosity, whereas mesoporous amorphous aluminosilicates offer better porosity but lack sufficient acidity. In this work, we investigated defect engineering to fine-tune the acidity of amorphous acidic aluminosilicates (AAS). Here we introduced oxygen vacancies in AAS to synthesize defective acidic aluminosilicates (D-AAS). ¹H, ²⁷Al, and ¹⁷O solid-state nuclear magnetic resonance (NMR) studies indicated that defects induced localized structural changes around the acidic sites, thereby modifying their acidity. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy studies substantiated that oxygen vacancies alter the chemical environment of Brønsted acidic sites of AAS. The effect of defect creation in AAS on its acidity and catalytic behavior was demonstrated using four different acid-catalyzed reactions namely, styrene oxide ring opening, vesidryl synthesis, Friedel-Crafts alkylation, and jasminaldehyde synthesis. The defects played a role in activating reactants during AAS-catalyzed reactions, enhancing the overall catalytic process. This was supported by in-situ FTIR, which provided insights into the molecular-level reaction mechanism and the role of defects in reactant activation. This study demonstrates defect engineering as a promising approach to fine-tune acidity in amorphous aluminosilicates, bridging the porosity and acidity gaps between mesoporous amorphous aluminosilicates and crystalline zeolites.

Fig. 1. Structural and surface characterization of AAS and defective AAS.

SEM and TEM images of a, d AAS, b, e D-AAS-12, and c, f D-AAS-25. Inset: optical images; Nitrogen sorption isotherms of g AAS, h D-AAS-12, and i D-AAS-25, Brunauer–Emmett–Teller (BET) surface areas are written in the plots; Barrett–Joyner–Halenda (BJH) adsorption pore-size distributions of j AAS, k D-AAS−12, and l D-AAS-25. Error bars: Standard deviation calculated from data of at least three repeated experiments.

Fig. 2. Structural analysis by solid-state NMR of AAS and defective AAS.

Solid-state NMR spectra of AAS, D-AAS-12, and D-AAS-12-¹⁷O. a 1D ¹H MAS spectra of AAS, D-AAS-12, and D-AAS-12-¹⁷O. b 2D ²⁷Al → ¹H D-RINEPT spectra of AAS and D-AAS-12. c Overlay of 2D ¹H–¹H spin diffusion spectra of D-AAS-12 (blue) and D-AAS-12-¹⁷O (orange) acquired with a 50 ms spin diffusion time. d 1D ¹⁷O solid-state NMR spectrum. e 2D ¹⁷O multiple quantum magic angle spinning (MQMAS) spectrum of D-AAS-12-¹⁷O. Spinning sidebands in the 1D ¹⁷O NMR spectrum are indicated. The shoulder at ca. 60 ppm likely arises from satellite transitions. In the MQMAS spectrum, the F₁ dimension shows the isotropic quadrupolar frequency (δ_iso) obtained after shearing. f Structures of various sites present in AAS and D-AAS.

Fig. 3. Catalytic performance and kinetic analysis of styrene oxide ring-opening by AAS and defective AAS.

a Styrene oxide ring-opening catalyzed by AAS, D-AAS-12, and D-AAS-25, The solid lines are guidelines, not fit lines. b Order of the reaction, c Arrhenius plot for the activation energy, and d Eyring plot for entropy change for styrene oxide ring-opening reaction catalyzed by AAS and D-AAS-12. Error bars: Standard deviation calculated from data of at least three repeated experiments for all the figures.

Fig. 4. Catalytic activity of AAS and D-AAS-12 in various reactions.

a Friedel−Crafts alkylation, b jasminaldehyde, and c vesidryl synthesis reactions catalyzed by AAS, and D-AAS-12. Error bars: Standard deviation calculated from data of at least three repeated experiments for all the figures.

Fig. 5. FTIR analysis and proposed mechanism of styrene oxide ring-opening.

a FTIR spectra of AAS and D-AAS, Deconvulated FTIR spectra of b AAS, c D-AAS-12, the dashed lines indicate the peak corresponding to stretching vibrations of Al–O–Si (1131 cm⁻¹) and Si–O–Si (1070 cm⁻¹). d, e FTIR spectra of styrene oxide pure and adsorbed on D-AAS-12, the vertical dashed line shows the appearance of a shoulder in the 875 cm⁻¹ peak. f Proposed interactions of styrene oxide with the defects centers of D-AAS-12. g Time-dependent in-situ FTIR of styrene oxide ring-opening reaction catalyzed by D-AAS-12, the shaded region in the spectra highlights the stretching vibrations of the epoxide ring at 1250 and 875 cm⁻¹, and the changes in these features over time. h Proposed reaction mechanism of styrene oxide ring-opening reaction catalyzed by D-AAS-12.