Solid acids are crucial heterogeneous catalysts due to their tunable acidity, recyclability, and stability. However, crystalline zeolites, while strongly acidic, suffer from microporosity, limiting their efficiency in reactions involving large molecules. Mesoporous zeolites have been explored to address this, but face stability issues. Mesoporous ASAs, on the other hand, exhibit weak acidity. The acidity in ASAs originates from pseudo-bridging silanols, weaker than the bridging silanols in zeolites. This research aims to synthesize a material combining the strong acidity of zeolites with the high surface area and accessibility of ASAs. The synthesis of ASAs with zeolite-like bridging silanols is a significant challenge and the existence of such a material has been debated. This study addresses this challenge by synthesizing "acidic aluminosilicates" (AAS) nanosponges with the desired properties, aiming to demonstrate their superior catalytic performance compared to existing materials.

The literature extensively discusses the challenges and limitations of existing solid acid catalysts. Crystalline zeolites, while possessing strong acidity, are hindered by microporosity, limiting accessibility for larger molecules. Attempts to create mesoporous zeolites have faced difficulties with phase separation and stability. Amorphous aluminosilicates (ASAs), while offering mesoporosity, suffer from weak acidity. The different sources of acidity in zeolites (bridging silanols) and ASAs (pseudo-bridging silanols) have been identified and characterized. The key difference lies in the shorter silanol O to Al bond distance in zeolites, contributing to their stronger acidity. This review highlights the existing gap in the literature: the lack of a material with both strong zeolite-like acidity and the accessible surface area of ASAs. This study seeks to bridge this gap by developing a new class of materials.

The synthesis of AAS involves controlled hetero-condensation between tetraethylorthosilicate (TEOS) and an aluminum precursor. Two different aluminum precursors were investigated: aluminum acetylacetonate (Al-AC) and aluminum isopropoxide (Al-IP). The reaction was performed in a bicontinuous microemulsion template, using cetyltrimethylammonium bromide (CTAB) as a surfactant and 1-pentanol as a cosurfactant. This soft template approach promotes the formation of a porous structure. The Si/Al ratio was varied to tune the acidity of the resulting AAS. Materials were characterized by scanning and transmission electron microscopy (SEM and TEM), nitrogen sorption analysis (BET), energy-dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (PXRD), and selected area electron diffraction (SAED). Catalytic activity was assessed through several reactions: styrene oxide ring-opening, vesidryl synthesis, Friedel–Crafts alkylation, jasminaldehyde synthesis, *m*-xylene isomerization, and cumene cracking. The plastic degradation experiments utilized low-density polyethylene (LDPE), plastic bottles (PET), centrifuge tubes (polypropylene), and carrier bags (high-density polyethylene) using thermogravimetric analysis (TGA). For CO₂ to dimethyl ether (DME) conversion, a bifunctional Cu-Zn-Al/AAS catalyst was prepared and tested in a fixed-bed reactor. Solid-state nuclear magnetic resonance (NMR) spectroscopy, including 1D and 2D experiments, and dynamic nuclear polarization (DNP) enhanced NMR were used to characterize the acidic sites. Ammonia temperature-programmed desorption (TPD) and pyridine adsorption studies using diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) were also conducted.

The synthesized AAS nanosponges exhibited high surface areas (up to 588 m² g⁻¹) and pore volumes (up to 1.5 cm³ g⁻¹). The choice of aluminum precursor significantly impacted the material's properties and catalytic activity. The AAS synthesized using Al-AC showed superior performance compared to those synthesized with Al-IP, indicating the crucial role of the precursor in dictating the nature and strength of the acid sites. Six different catalytic reactions confirmed the presence of strong Brønsted acid sites similar to zeolites, with superior accessibility due to the nanosponge morphology. Solid-state NMR studies, including DNP-enhanced ¹H-²⁷Al HETCOR experiments, corroborated the presence of zeolite-like bridging silanol sites. The AAS efficiently catalyzed the conversion of waste plastics to hydrocarbons at significantly lower temperatures (T₅₀ as low as 325 °C for LDPE) than in the absence of a catalyst. A Cu-Zn-Al/AAS hybrid exhibited excellent performance for CO₂ to DME conversion, achieving 79% selectivity for DME under optimized conditions. Ammonia TPD and pyridine adsorption studies further confirmed the presence of strong Brønsted acid sites. The recyclability studies showed that the catalyst could be regenerated by oxygen treatment after carbon deposition.

The findings demonstrate the successful synthesis of AAS nanosponges possessing a unique combination of strong acidity and high accessibility. The superior catalytic performance of AAS compared to existing zeolites and ASAs validates the approach. The ability of AAS to efficiently convert waste plastics and CO₂ into valuable products addresses pressing environmental challenges. The detailed characterization using solid-state NMR and other techniques provides fundamental insights into the nature of the active sites. The results suggest that the AAS materials represent a new class of solid acid catalysts that bridge the gap between zeolites and amorphous aluminosilicates. The strong acidity and improved mass transport provided by the nanosponge morphology are key factors driving the enhanced catalytic activity.

This study successfully synthesized acidic aluminosilicate (AAS) nanosponges exhibiting strong Brønsted acidity comparable to zeolites and high surface area and pore volume similar to ASAs. The superior catalytic performance in various reactions, including waste plastic degradation and CO₂ conversion, demonstrates the potential of AAS as a new generation of catalysts. Future research can focus on optimizing the synthesis parameters, exploring different metal loadings for enhanced CO₂ conversion, and investigating the applicability of AAS in other catalytic processes.

The recyclability studies showed a gradual deactivation of the catalyst due to carbon deposition. While regeneration was possible, prolonged use might necessitate more robust methods for carbon removal. The current study focuses primarily on LDPE and a few other polymers; further research is needed to assess the efficacy of AAS in degrading a wider range of plastics. The CO₂ to DME conversion study employed a specific catalyst formulation; investigating other metal combinations and support materials could potentially enhance performance further.