Interaction Of Probe Molecules Research Articles

What Is Measured When Measuring Acidity in Zeolites with Probe Molecules?

Based on theoretical calculations of CO, NH3, and pyridine adsorption at different sites in MOR and MFI zeolites, we analyze how confinement effects influence the measurement of acidity based on the interaction of probe molecules with Brönsted acid sites. Weak bases, such as CO, form neutral ZH–CO adducts with a linear configuration that can be distorted by spatial restrictions associated with the dimensions of the pore, leading to weaker interaction, but can also be stabilized by dispersion forces if a tighter fitting with the channel void is allowed. Strong bases such as NH3 and pyridine are readily protonated on Brönsted acid sites, and the experimentally determined adsorption enthalpies include not only the thermochemistry associated with the proton transfer process itself, but also the stabilization of the Z––BH+ ion pair formed upon protonation by multiple interactions with the surrounding framework oxygen atoms, leading in some cases to a heterogeneity of acidities within the same zeolite structure.

Interaction of Probe Molecules with Bridging Hydroxyls of Two-Dimensional Zeolites: A Surface Science Approach

Bridging hydroxyls (Si–OH–Al) in zeolites are catalytically active for a multitude of important reactions, including the catalytic cracking of crude oil, oligomerization of olefins, conversion of methanol to hydrocarbons, and the selective catalytic reduction of NOx. The interaction of probe molecules with bridging hydroxyls was studied here on a novel two-dimensional zeolite model system consisting of an aluminosilicate forming a planar sheet of polygonal prisms, supported on a Ru(0001) surface. These bridging hydroxyls are strong Bronsted acid sites and can interact with both weak and strong bases. This interaction is studied here for two weak bases (CO and C2H4) and two strong bases (NH3 and pyridine), by infrared reflection absorption spectroscopy, in comparison with density functional theory calculations. Additionally, ethene is the reactant in the simplest case of the olefin oligomerization reaction which is also catalyzed by bridging hydroxyls, making the study of this adsorbed precursor state part...

The role of surface acidity in adsorption of aromatic sulfur heterocycles from fuels

The objective of this work is to investigate the relationship between sulfur adsorption capacity and surface acidity of supported silver oxide adsorbents from liquid hydrocarbon fuels. Inherently acidic supports such as TiO2 and Al2O3 developed sulfur adsorption capacity following activation in air at temperatures above 350°C. Addition of Ag to these supports further increased sulfur adsorption capacity by ∼40%. Efforts were made to understand the thermal activation process and the sulfur adsorption mechanism. Breakthrough characteristics of various hetero aromatic molecules revealed that the strengths of adsorption increased with electronegativities of the heteroatoms. Sulfur adsorption capacities of these adsorbents were progressively reduced when poisoned with increasingly basic molecules. Surface acidity was measured using NH3 chemisorption and titration with 2,6-lutidine. Interactions of probe molecules on acid centers were also observed from IR spectroscopy where both Bronsted and Lewis centers were observed. However interactions with 2,6-lutidine and trimethyl chlorosilane indicated that the active centers involved in sulfur adsorption were primarily Bronsted in nature. A clear correlation between activation temperature, surface acidity, and sulfur adsorption capacity was established for both blank and Ag loaded samples.

Evidence of Hydrophobic Interactions Controlling Mobile Ions Release from Smart Hydrogels

A water soluble cationic metal complex (tris(2,2′-bipyridine)ruthenium(II)) can be loaded into hydrogels containing acrylamide and/or acrylic acid units. The cationic complex is retained in polymer containing acrylic acid units at high pH and released at low pH. This is likely due to electrostatic interactions of the cation with the carboxylate anions, present at high pH, which are converted into neutral carboxylic acid at low pH, releasing the metal complex. Since the gel contract at low pH, the water soluble cation is also released with the water expelled from the gel. However, a strong retention of the cation inside the gels is observed when acrylamide units are present. A possible explanation is a hydrophobic interaction of the large metal complex with the polyacrylamide network. Using the counteracting electrostatic and hydrophobic interactions of probe molecules with smart hydrogel matrixes it is possible to tune the pH of maximum release away from the pKa of the ionizable group.

The Characterization of Phospholipid Functional Group Probe Species on Respirable Silicon-Containing Dusts by Solid-State 13C and 31P Nuclear Magnetic Resonance Spectroscopy

Solid-state nuclear magnetic resonance (NMR) spectroscopic studies are reported for the interactions of probe molecules with respirable silicon-containing dusts as experimental evidence complementing computational studies reported by Snyder and Madura recently in J. Phys. Chem. B 112, 7095 (2008). The selected probe molecules represent the individual functional groups of a model lung surfactant dipalmitoylphosphatidyl choline (DPPC) deposited on a respirable silica and kaolin from water solution. (13)C and (31)P solid-state NMR spectroscopies were employed to detect chemical shift, line width, and chemical shift anisotropy, providing experimental evidence of mobility and relaxation changes describing the site and orientation of surface-associated species. NMR results confirm that only the phosphate and adjacent carbons are immobilized by surface hydroxyls on kaolin, while these and the carbons of the cationic head group are likewise immobilized by surface silanols on Miu-U-Sil 5. The phosphates in phosphoryl- and phosphatidyl-cholines were the primary interaction sites, with additional weak coordination with the trimethylammonium cation species. Covalent Al-O-P formation is not likely a factor in in vivo or in vitro toxicity mechanisms of respirable silicon-containing materials, but is rather the result of dehydration or demethoxylation reactions occurring over time or during heating or reduced pressure used in preparing materials for NMR spectroscopic study. Hydration is a critical factor in the formation and preparation for spectroscopic observation of coated dusts. Care must be taken to ensure that products formed and studied correspond to species formed in vivo under suitable concentration and hydration conditions.

Brönsted and Lewis Acidity of the BF3/γ-Al2O3 Alkylation Catalyst as Revealed by Solid-State NMR Spectroscopy and DFT Quantum Chemical Calculations

Multinuclear solid-state NMR techniques and DFT quantum chemical calculations were employed to investigate the detailed structure of acid sites on the BF3/gamma-Al2O3 alkylation catalyst. The NMR experiment results indicate that gaseous BF3 is able to react with the hydroxyl groups present on the surface of gamma-Al2O3, leading to the formation of new Brönsted and Lewis acid sites. The 1H/11B and 1H/27Al TRAPDOR (TRAnsfer of Population in DOuble Resonance) experiments suggest that the 3.7 ppm signal in 1H NMR spectra of the BF3/gamma-Al2O3 catalyst is due to a bridging B-OH-Al group that acts as a Brönsted acid site of the catalyst. On the other hand, a Lewis acid site on the surface of the catalysts, as revealed by 31P MAS and 31P/27Al TRAPDOR NMR of adsorbed trimethylphosphine, is associated with three-coordinate -OBF2 species. 13C NMR of adsorbed 2-13C-acetone indicates that the Brönsted acid strength of the catalyst is slightly stronger than that of zeolite HZSM-5 but still weaker than that of 100% H2SO4, which is in good agreement with theoretical prediction. In addition, DFT calculations also reveal the detailed structure of various acid sites formed on the BF3/gamma-Al2O3 catalyst and the interaction of probe molecules with these sites.

Interaction of probe molecules with active sites on cobalt, copper and zinc-exchanged SAPO-18 solid acid catalysts

Novel cobalt (Co), copper (Cu) and zinc (Zn) exchanged chabazite related SAPO-18 solid acid catalysts were prepared and characterised using techniques such as BET (nitrogen sorption), FTIR, ICP-OES, XPS and XRD. A detailed in situ FTIR investigation undertaken on the adsorption and disproportionation of NO and CO over Co-, Cu- and Zn-SAPO-18 molecular sieve catalysts indicated the formation of various NO/CO species or complexes with cationic Lewis acid and/or active metal sites. NO adsorption at room temperature leads to the formation of up to seven bands attributed to adsorbed nitrous oxide (N2O), chemisorbed nitrogen dioxide (NO2), nitrite (M-NO2) (M, metal), mononitrosyl (M-NO) and dinitrosyl [M-(NO)2] complexes. CO adsorption at room temperature leads to the formation of up to six bands attributed to physi-sorbed carbon dioxide (CO2), cationic Lewis acid carbonyl moieties (L-CO) and transition metal carbonyl complexes (M-CO). The concentration and distribution of NO/CO species varies with pressure, reaction temperature and evacuation.