Pyridine Vapor Research Articles

Color-change behavior of a bis(benzimidazole)-coordinated nickel-dichlorido complex induced by the adsorption of pyridine or ammonia vapor

Color-change behavior of a bis(benzimidazole)-coordinated nickel-dichlorido complex induced by the adsorption of pyridine or ammonia vapor

Stimuli-Responsive Properties of a Zinc(II) Salen-Type Schiff-Base Complex and Vapochromic Detection of Volatile Organic Compounds.

This contribution reports, through a combined thermogravimetric analysis, differential scanning calorimetry, UV-vis, powder X-ray diffraction, and Rietveld refinement analysis, on the stimuli-responsive chromic properties of a substituted Zn(salmal) Schiff-base Lewis acidic complex with unique and distinct thermo- and vapochromic characteristics. The solid complex obtained in air or by evaporation of the solvent from their THF solutions shows a marked thermochromism associated with a phase transition, unusually triggered by the reversible desorption/adsorption of one lattice water molecule. In contrast, the anhydrous solid, achieved from THF solutions of the complex by evaporation of the solvent under anhydrous conditions, behaves very differently as it does not show any absorption of water or thermochromism and exhibits varied vapochromic properties. Detection of volatile organic compounds having Lewis basicity is demonstrated by using the anhydrous solid or the related cast films on glass or paper substrates. In both cases, a marked vapochromism is observed upon exposure to vapors of various volatile species and involves well-defined optical absorptions and naked-eye color changes, also allowing the discrimination of primary aliphatic amines. Vapochromic behavior with the formation of stable, stoichiometric adducts is also demonstrated for both the solid obtained in air and the anhydrous solid or for the related cast films after exposure to vapors of pyridine.

Atom-Economical Synthesis of Lewis Acidic Boron Containing Porous Organic Polymers via Hydroboration Polymerization for Basic Chemical Capture.

Atom economy is one of the main concerns for material synthesis. Here, the facile synthesis of Lewis acidic boron-containing porous organic polymers (B-POPs) via hydroboration polymerization reaction of commercially available borane dimethyl sulfide complex (BH3 ∙SMe2 ) with multi-alkynes under mild reaction conditions is presented. This new synthetic method for B-POPs has the advantage of high atom economy. The resulted porous alkenyl borane polymers (PABPs) have unique features such as high boron content, strong Lewis acidity, and high surface areas. Owing to the strong Lewis acid-base interactions, PABPs exhibit excellent adsorptive capacity toward triethylamine (up to 841mgg-1 ) and pyridine (up to 1396mgg-1 ) vapor.

Three heteroleptic copper(I) complexes with [Cu(P˄P)N2]+ structure and their fluorescence sensing for VOCs

The design and research of luminescent and volatile organic compound (VOC) fluorescent sensing materials are of great significance and challenge. We report herein the ligand substitution reaction and VOC sensing of new heteroleptic [Cu(P˄P)N2]+ type copper(I) complexes. Firstly, three new complexes 1–3 were designed by utilizing a chelate diphosphine ligand 4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene (Xantphos) and synthesized by the substitution reaction of different N‐containing ligands of 4‐PBO (1), 3‐PBO (2), and 4,4′‐Bipy (3) (4‐PBO = 2‐(4′‐pyridyl)‐benzoxazole, 3‐PBO = 2‐(3′‐pyridyl)‐benzoxazole, 4,4′‐Bipy = 4,4′‐bipyridine), respectively. Three complexes were characterized by elemental analysis, spectroscopic analysis (IR, UV–Vis), single‐crystal X‐ray diffraction (SCXRD), and photoluminescence study. The SCXRD study revealed that complexes 1 and 2 both exhibit a molecular structure with tetrahedral copper(I) complex cation and hexafluorophosphate anion, while complex 3 differs in that its cation is a binuclear copper(I) structure bridged by 4,4′‐bipyridine. Three complexes 1–3 present supramolecular ribbon, supramolecular dimer, and supramolecular framework structure, respectively. Some differences of their UV–Vis absorption spectra were explained by TD‐DFT calculation and wavefunction analysis. It is found that 1 has an abnormal luminescence blue shift, and a luminescence mechanism through the high‐energy T2 excited state is proposed by TD‐DFT calculation. Based on 3, a fluorescent test strip was developed, and its fast and selective fluorescent sensing of pyridine vapor through quenching mechanism was successfully realized. The fluorescent quenching mechanism of the material was also studied, and it was proposed that the quenching should be attributed to the photoinduced electron transfer (PET) mechanism.

A three-coordinate cuprous complex exhibiting both photochromic and fluorescent sensing properties, and some TD-DFT calculations

Photo-functional materials with photochromic and fluorescent properties are very attractive because they have a wide range of applications. The design and research of fluorescent sensing materials for VOCs (volatile organic compounds) are also of great significance and challenge. Herein, we report a three-coordinate cuprous complex, XantphosCuI (1) (Xantphos is 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene), and its reversible photochromism, photoluminescence, and fluorescent sensing for VOCs. The complex 1 was characterized by single-crystal X-ray diffraction as an isolated molecule with a triangle CuIP2 coordination geometry. In solution, complex 1 undergoes a reversible photochromic transition by alternating non-irradiation and irradiation with 365 nm UV light. Under UV light excitation, both the powder and doped thin film exhibit broadband photoluminescence emission. TD-DFT computational studies have been conducted to further understand the electronic transition properties of this compound. Based on 1, a fluorescent test strip was developed, and its fast and selective fluorescent sensing of pyridine vapor through enhanced mechanism was successfully realized. When exposed to vapor, the fluorescence of the test strip quickly becomes extremely bright. The fluorescent enhanced mechanism of the material was studied with the assistance of TD-DFT calculations, and it was proposed that the enhancement should be attributed to the charge transfer mechanism transition of the excited state after the coordination increase.

Optimization of light-analyte interaction in Si3N4/polymer hybrid waveguide for sensitive sensing of pyridine vapor with ppb-level detection limit

High-sensitivity and miniature volatile organic compound (VOC) sensor is considerably significant for environment monitoring. Herein we performed the theoretical and experimental optimization of waveguide sensor to realize the sensitive detection of volatile toxic pyridine vapor with ppb-level detection limit. The miniature sensor is based on the CMOS-compatible silicon nitride (Si3N4) Mach-Zehnder interferometer (MZI) waveguide with dipolar polycarbonate as sensitive cladding material. Theoretical optimization by varying the waveguide width and the thickness of sensitive cladding is carried out to tune the group refractive index and evanescent field. Experimentally, two MZIs with distinct waveguide widths and sensitive cladding thickness are fabricated to compare their performance in pyridine vapor sensing. The sensing results show that the thickness of sensitive layer plays a key role in sensing sensitivity and response time. The final obtained pyridine sensing sensitivity can reach 63 pm/ppm and the detection limit can be as low as 476 ppb. Our demonstration provide a simple and efficient strategy for sensitive sensing of hazardous chemical vapors, which may find application in biochemical sensing.

Acidic property of YNU-5 zeolite influenced by its unique micropore system

YNU-5 zeolite has the YFI -type framework with a 12-12-8-ring system (2-dimensional 12-oxygen membered ring pores 3-dimensionally connected by twin 8-ring channels), and isolated 8-ring channels separated from the system by a thin (mono-atomic silicate) wall. In this study, the acidic property of YNU-5 zeolite was analyzed mainly by means of the ammonia IRMS-TPD (infrared/mass spectroscopy temperature-programmed desorption) method. In addition, the accessibility of acid sites was evaluated through adsorption of pyridine. The number of Brønsted acid sites approximately agreed with the number of Al atoms, and the amount of Lewis acid sites was negligible, indicating that most acid sites have the nature of bridging Si(OH)Al group in the YFI framework. Most of the Brønsted acidic OH groups on the YNU-5 zeolites (including the dealuminated samples) reacted with pyridine vapor at 343 K to form pyridinium cations, indicating that the Brønsted acid sites are highly accessible through the 12-12-8-ring system. Enthalpy of ammonia desorption from the Brønsted acid site, which can be an index of acid strength or reactivity of the acid site with a basic reactant, was in the following order: FAU < MOR (12-ring) ≈ *BEA < MFI < MWW ≈ YFI < MOR (8-ring), indicating the markedly large ammonia desorption enthalpy on the YNU-5 zeolite compared to the other 12-ring zeolites. The DFT calculations suggest that the Brønsted acidic protons with especially high ammonia desorption enthalpy are located in the isolated 8-ring but accessible from the 12-12-8-ring system side. In addition, dealumination at a high temperature under the reflux conditions with nitric acid resulted in the preferential removal of Brønsted acid sites with low ammonia desorption enthalpy, probably in the 12-12-8-ring system. The presence of reactive Brønsted acid sites in the isolated 8-ring, accessibility to them from the 12-12-8-ring system, and preferential removal of the Brønsted acid sites from the 12-12-8-ring system are reasonably consistent with the previously reported catalytic properties for dimethyl ether-to-olefin reaction. • Enthalpy of ammonia desorption (Δ H ) from the Brønsted acid site on the YNU-5 zeolite was high. • The Brønsted acidic protons with high Δ H were located in the isolated 8-ring but accessible from the 12-12-8-ring system. • Dealumination in the reflux conditions of nitric acid preferential proceeded in the 12-12-8-ring system.

Optimization of Light-Analyte Interaction in Si3n4/Polymer Hybrid Waveguide for Sensitive Sensing of Pyridine Vapor with Ppb-Level Detection Limit

Optimization of Light-Analyte Interaction in Si3n4/Polymer Hybrid Waveguide for Sensitive Sensing of Pyridine Vapor with Ppb-Level Detection Limit

ZnO-Activated Carbon Blended as a Catalyst for Oxidative Desulfurization of Dibenzothiophene

The problem of sulfur content in heavy oil is a challenge for researchers to meet the needs of environmentally friendly fuels. The catalyst preparation plays an important role in the desulfurization process. The synthesis of ZnO-activated carbon as a catalyst and its activity in oxidative desulfurization (ODS) reaction has been successfully carried out. In this work, the ZnO and activated carbon (AC) were blended by a solid-solid reaction. The ZnO, AC, and ZnO-AC were then characterized using acidity test with pyridine vapor adsorption, Fourier Transform Infra-Red (FTIR), X-ray Diffraction (XRD), Scanning Electron Microscope-Energy Dispersive X-Ray (SEM-EDX), and Surface Area Analyzer (SAA). ODS of dibenzothiophene (DBT) reaction was performed by using H2O2 under variation of the reaction time (30, 60, 120, and 150 min) for the ZnO-AC catalyst. The efficiency of ODS-DBT was analyzed by a UV-Visible spectrophotometer. The XRD analysis result showed that ZnO-AC blended displays new crystal peaks of Zn in the AC diffractogram. The surface area (734.351 m2/g) and acidity (4.8780 mmol/g) of ZnO-AC were higher than ZnO and AC themselves. ZnO-AC produced the highest efficiency of ODS-DBT which was 93.83% in the reaction time of 120 min. Therefore, the simple procedure of this physical blending was proved effective to homogenize between ZnO and AC into ZnO-AC so that it has good physicochemical properties as an ODS-DBT catalyst. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Luminescent Soft Crystals That Exhibit Color Changes in Response to Vapor

It is well-known that square-planar Pt(II) complexes exhibit characteristic luminescence when the complex units are stacked with Pt···Pt short contacts. Such assembled systems have interesting properties that the luminescence color can change sensitively in response to the surroundings such as temperature, pressure, and vapor. Therefore, the Pt(II) complexes are attractive as flexible and ordered sensing materials called soft crystals. In this presentation, the latest topics of vapochromic Pt(II) complexes are introduced.1. Wide-range color tuning of luminescence based on the fine control of the Pt···Pt interactions.1) A series of assembled Pt(II) complexes comprising N-heterocyclic carbene and cyanide ligands was constructed using different substituent groups (Figure 1). All the complexes exhibited highly efficient photoluminescence with an emission quantum yield in the solid state at room temperature, originating from the triplet metal-metal-to ligand charge transfer (3MMLCT) state. Their emission colors cover the entire visible region from red to blue by changing the substituents. It is notable that blue 3MMLCT emission was realized firstly in this system. In addition, vapochromic response was observed based on the single-crystal-to-single-crystal transformation.2. Fast and stable vapochromic response induced through nanocrystal formation of a luminescent platinum(II) complex on periodic mesoporous organosilica.2) A hybrid vapoluminescent system exhibiting fast and repeatable response was constructed using periodic mesoporous organosilica with bipyridine moieties (Bpy-PMO) and a Pt(II) complex bearing a potentially luminescent 2-phenylpyridinato ligand. An intense red luminescence appeared when the Pt(II)-complex immobilized Bpy-PMO was exposed to methanol vapor and disappeared on exposure to pyridine vapor. The ON-OFF vapochromic behavior occurred repeatedly in a methanol/ pyridine/heating cycle. Interestingly, a rapid response was achieved in the second cycle and cycles thereafter. The unique vapoluminescence triggered by the unprecedented protodesilylation on exposure to protic solvent vapor at room temperature is attributable to Bpy-PMO being a giant ligand and an effective vapor condenser. Consequently, this hybrid system presents a new strategy for developing sensors using bulk powdery materials. 1) D. Saito et al., Angew. Chem. Int. Ed. 2020, 59, 18723–18730. 2) H. Matsukawa et al., Sci. Rep. 2019, 9:15151. Figure 1

Chemical passivation of the perovskite layer and its real-time effect on the device performance in back-contact perovskite solar cells

The back-contact architecture for perovskite solar cells (PSCs) offers the possibility of positioning both electrodes on one side of the absorber layer and shining light directly on the perovskite photoactive layer. This helps us to avoid the occurrence of transmission losses caused by the charge collecting transparent conductive oxide electrode in the conventional sandwich structure for PSCs. The back-contact device architecture is also useful for conducting fundamental studies as it has an exposed photoactive area, permitting in situ measurements on the effects of chemical treatment, passivation, and annealing. A successful application of back-contact PSCs in studying the effect of chemical passivation of the perovskite photo-absorber layer trap states with pyridine and its influence on the device performance have been studied. The real-time effect of pyridine vapor treatment on the device performance is visualized by monitoring the maximum power output of the devices under operation conditions. The device performance enhancement by ∼12% owing to the surface passivating effect of pyridine is demonstrated.

Unique vapochromism of a paddlewheel-type dirhodium complex accompanied by dynamic structural and phase transitions.

The one-dimensional coordination polymer [Rh2(HA)4]n (1G; HA = hexanoate) exhibits a drastic vapochromic color change from green to red upon exposure to pyridine (py) vapor. Heating the red discrete complex [Rh2(HA)4(py)2] (1R) at 338 K affords the purple discrete tetrarhodium complex [Rh2(HA)4(py)]2 (1P), which is an intermediate species in the vapochromic transformation of 1G to 1R. The obtained complexes 1G, 1R, and 1P differ not only in their color in the solid state, but also in their temperature-dependent phase transition properties.

Surface Acidity of Sulfuric Acid Activated Maredan Clay Catalysts with Boehm Titration Method and Pyridine Adsorption-FTIR

The acid or base of the solid surface is a very interesting aspect at the structure of surface and important in the field of ion exchange and heterogeneous catalysts. The use of clay as a heterogeneous catalyst is due to its acidity. In this paper, the acidity of heterogeneous catalysts Maredan clay activated by sulfuric acid was examined by the Boehm titration and adsorption pyridine method. In this study, Maredan clay was activated with several variation concentration of sulfuric acid (0.5, 1, 2 and 3 M) by impregnation method. The mixture was refluxed at 80°C for 3 hours, washed with double deionised water, dried and calcined at 500 °C for 3 hours. The results showed that activation of Maredan clay by sulfuric acid produced the total acid amount of 0.6483 mmol/g catalyst by using Boehm titration method and 1.1137 mmol/g catalyst by using pyridine vapor adsorption method. FTIR analysis of the spectra of pyridine adsorption was seen that the acid site of the Maredan clay catalyst activated by sulfuric acid dominant was Lewis acid site, with wave numbers at 1449.57; 1490.07; 1573.98 and 1616.42 cm−1.

Fast and stable vapochromic response induced through nanocrystal formation of a luminescent platinum(II) complex on periodic mesoporous organosilica

A hybrid vapoluminescent system exhibiting fast and repeatable response was constructed using periodic mesoporous organosilica with bipyridine moieties (BPy-PMO) and a Pt(II) complex bearing a potentially luminescent 2-phenylpyridinato (ppy) ligand. An intense red luminescence appeared when the Pt(II)-complex immobilised BPy-PMO was exposed to methanol vapour and disappeared on exposure to pyridine vapour. The ON-OFF vapochromic behaviour occurred repeatedly in a methanol/pyridine/heating cycle. Interestingly, a rapid response was achieved in the second cycle and cycles thereafter. Scanning and transmission electron microscopies (SEM/TEM), absorption and emission, and nuclear magnetic resonance spectroscopies, mass spectrometry, and powder X-ray diffraction indicated that methanol vapour induced Si-C cleavage and thus liberated [Pt(ppy)(bpy)]Cl (bpy = 2,2′-bipyridine) from the BPy-PMO framework. Furthermore, the self-assembling properties of the Pt(II) complex resulted in the formation of highly luminescent micro/nanocrystals that were homogeneously dispersed on the porous support. The unique vapoluminescence triggered by the unprecedented protodesilylation on exposure to protic solvent vapour at room temperature is attributable to BPy-PMO being a giant ligand and an effective vapour condenser. Consequently, this hybrid system presents a new strategy for developing sensors using bulk powdery materials.

A gas–liquid interface synthesis in polyoxometalate chemistry: potential bag filter for volatile organic amines

Diffusion of piperidine and pyridine into an acidified aqueous solution of $$\hbox {Na}_{2}\hbox {MoO}_{4}$$ , yield compounds $$[(\hbox {pipH})_{4}][\hbox {Mo}_{8}\hbox {O}_{26}]\cdot 4\hbox {H}_{2}\hbox {O}$$ $$(\hbox {pip} = \hbox {piperidine} = \hbox {C}_{5}\hbox {H}_{11}\hbox {N})$$ (1) and $$[(\hbox {pyH})_{4}][\hbox {Mo}_{8}\hbox {O}_{26}]$$ $$(\hbox {py} = \hbox {pyridine} = \hbox {C}_{5}\hbox {H}_{5}\hbox {N})$$ (2), respectively. Compound 1 possesses supramolecular 3-D network and the relevant connectivity pattern generates channels of approximate dimensions of $$10.76 \times 11.57 \acute{{\AA }}^{2}$$ , in which the piperidinium cations are located as guests. Multidimensional supramolecular frameworks (3-D in compound 1 and 2-D layer type of network in compound 2) have been made possible, as the organic cations and polyoxometalate (POM) anions are glued together by significant hydrogen bonding interactions. The synthesis of compounds 1 and 2 provides a unique ‘gas–liquid’ synthetic route in POM chemistry that result in organic–inorganic hybrid materials with structural diversities. This synthetic approach, first time in POM chemistry, can be described as a potential bag filter for volatile organic amines. Synopsis: Diffusion of piperidine and pyridine vapours into acidified aqueous molybdate solution leads to the formation of ion pair compounds having supramolecular framework structures. These organic amines are trapped as guest or as part of the framework making the present system a potential bag filter for the volatile organic amines.

Surface holograms for sensing application

Surface gratings with periodicity of 2 μm and amplitude in the range of 175 and 240 nm were fabricated in a plasticized polyvinylchloride doped with a metalloporphyrin (ZnTPP), via a single laser pulse holographic ablation process. The effect of the laser pulse energy on the profiles of the fabricated surface structure was investigated. The sensing capabilities of the fabricated diffractive structures towards amines (triethylamine, diethylamine) and pyridine vapours were then explored; the holographic structures were exposed to the analyte vapours and changes in the intensity of the diffracted light were monitored in real time at 473 nm. It was demonstrated that surface structures, fabricated in a polymer doped with a metalloporphyrin which acts as analyte receptor, have a potential in sensing application.

Quantitative infrared and near-infrared gas-phase spectra for pyridine: Absolute intensities and vibrational assignments

Using vetted methods for generating quantitative absorption reference data, broadband infrared and near-infrared spectra (total range 11,000–600 cm−1) of pyridine vapor were recorded at 0.1 cm−1 spectral resolution, with the analyte thermostatted at 298 K and pressure-broadened to 1 atmosphere using N2 ballast gas. The quantitative spectrum is reported for the first time, and we have re-assigned some of the 27 fundamental modes. Fundamental assignments were confirmed by IR vapor phase band shapes, FT-Raman measurements and comparison with previous analyses. For the 760-Torr vapor-phase IR data several bands show resolved peaks (Q-branches). We have also assigned for the first time many combination and overtone bands in the mid- and near-IR. Assignments were made via comparison to theoretically calculated frequencies and intensities: The frequencies were computed with Gaussian03 with the anharmonic option, using MP2 and the ccpvtz basis set. The intensities were taken from a VSCF calculation in GAMESS using Hartree–Fock (for overtones and combination bands) or from the harmonic MP2 for fundamentals. Overtone and combination band harmonic and anharmonic frequencies, as well as intensities were also calculated using the CFOUR program. It is seen in the NIR spectrum near 6000 cm−1 that the very strong bands arise from the C-H first overtones, whereas only much weaker bands are observed for combination bands of C-H stretching modes. Certain features are discussed for their potential utility for atmospheric monitoring.

Chemisorption-Induced Activation of MgCl2 Film as Realistic Route for Heterogeneous Ziegler–Natta Surfaces under Ultrahigh Vacuum

The preparation of model Ziegler–Natta (ZN) surfaces under ultrahigh vacuum necessitates the activation of MgCl2 films deposited on a substrate, since these films unexceptionally expose the energetically most stable but catalytically inactive (001) face. Here, we present a method to activate MgCl2 films with the inactive (001) termination, which was based on the chemisorption of a Lewis base in an analogous way to industrial powder catalyst preparation. We found that a (001)-terminated MgCl2 film on Cu(110) was activated into a MgCl2·Lewis base adduct when exposed to a pyridine vapor at a nearly saturated vapor pressure. A dynamic response of the MgCl2 film to the chemisorption and desorption of a Lewis base was uncovered using low-energy electron diffraction and temperature-programmed desorption.

Reaction between Pyridine and CH3NH3PbI3: Surface-Confined Reaction or Bulk Transformation?

Pyridine molecules have been used to passivate surface Pb2+ sites of CH3NH3PbI3, to recrystallize CH3NH3PbI3, and to bleach CH3NH3PbI3. However, these results contradict each other, as recrystallization and optical-bleach require transformation of bulk CH3NH3PbI3, but surface passivation demands the confinement of the reaction at the surface region. The underlying mechanism for these seemly contradicting results is not yet understood. In this paper, we show, at 25 °C, partial pressure of pyridine vapor is a determining factor for its reaction behaviors with CH3NH3PbI3: one can modify the surface region of CH3NH3PbI3 by using pyridine vapor of pressure 1.15 torr or lower but can transform the whole bulk CH3NH3PbI3 film with a pyridine vapor of 1.3 torr or higher. Our result is the first demonstration that the reaction modes, i.e., surface-confined reaction and bulk transformation, are very sensitive to the partial pressure of under-saturated pyridine vapor. Despite the different reaction behaviors, it is interesting that in all pressure ranges, pyridinium ion is a main product from the reaction between pyridine and CH3NH3PbI3. The bulk transformation is due to the formation of a liquid-like film, which increases the mobility of species to catalyze the reaction between pyridine and CH3NH3PbI3. It is important to note 1.3 torr is much smaller than the saturated vapor pressure of pyridine (20 torr at 25 °C). These findings provide a guidance in applying pyridine and other amines to functionalize and transform CH3NH3PbI3 and other hybrid halide perovskites. It also highlights the critical role of fundamental studies in controllably modifying CH3NH3PbI3.

γ-Alumina and Amorphous Silica–Alumina: Structural Features, Acid Sites and the Role of Adsorbed Water

The adsorption of water, pyridine, CO and CO2 on a commercial γ-Al2O3 and a commercial amorphous silica alumina (ASA) cracking catalyst are reinvestigated by means of FTIR spectroscopy. It is concluded that the real active hydroxyl groups on γ-Al2O3 are those absorbing at 3790, 3770 and 3730 cm−1 and that they are significantly acidic. The strongest Lewis acid sites are identified as Nuclear Magnetic Resonance (NMR)-silent (or still undetected) tricoordinated Al3+, becoming tetracoordinated when interacting with basic adsorbates. More than one pyridine molecule may adsorb over such highly coordinatively unsaturated sites at high pyridine vapour pressure. It has also been confirmed that the existence of Bronsted acid sites sufficiently strong to protonate pyridine on ASA is its intrinsic property, not due to coadsorbed water. A “stuffed amorphous silica” model is proposed for low-alumina ASA, where part of Al3+ ions compensate the charge in-balance due to the substitution of Al for Si in a silica-like tetrahedral framework. Interstitial Al3+ occupy 4-, 5- or 6-coordination interstices in the amorphous silica-like framework as do alkali cations in “stuffed silicas” aluminosilicates. Such “interstitial” Al3+ ions when located accidentally in opened interstices exposed at the surface play the role of strong Lewis sites. When these ions are located near a silanol group, they increase the Bronsted acidity of the terminal silanols by neutralizing the negative charge produced by their dissociation when interacting with bases.