Studies on the solid acidity of heated and cation-exchanged montmorillonite using n-butylamine titration in non-aqueous system and diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy

Abstract

The effects of heating and cation exchange on the solid acidity of montmorillonite were investigated using n-butylamine titration in non-aqueous system and diffuse reflectance Fourier transform infrared spectroscopy. The number of total, Brønsted, and Lewis acid sites showed the same modulation tendency with increasing heating temperature, reaching a maximum at 120°C and subsequently decreasing until it reaches a minimum at 600°C. The Lewis acid sites result from unsaturated Al 3? cations, and their number increased with the heating temperature due to the dehydration and dehydroxylation of montmorillonite. The generation and evolution of Brønsted acidity were mainly related to interlayer-polarized water molecules. Water adsorbed on the unsaturated Al 3? ions also acted as a Brønsted acid. The acid strength of the Brønsted acid sites was dependent on the polarization ability of the exchangeable cation, the amount of interlayer water, and the degree of dissociation of the interlayer water coordinated to exchangeable cations. All cation-exchanged montmorillonites exhibited different numbers of acid sites and various distributions of acid strength. Brønsted acidity was predominant in Al 3? -exchanged montmorillonite, whereas the Na ? -and K ? -exchanged montmorillonites showed predominantly Lewis acidity. Moreover, Mg 2? -and Li ? -exchanged montmorillonites exhibited approximately equal numbers of Brønsted and Lewis acid sites. The Brønsted acidity of cation-exchanged montmorillonite was positively correlated with the charge-to-radius ratios of the cations, whereas the Lewis acidity was highly dependent on the electronegativity of the cations. The acid strengths of Al 3? -and Mg 2? -exchanged montmorillonites were remarkably higher than those of monovalent cationexchanged montmorillonites, showing the highest acid strength (H 0 B -3.0). Li ? -and Na ? -exchanged montmorillonites exhibited an acid strength distribution of -3.0 \ H 0 B 4.8, with the acid strength ranging primarily from 1.5 to 3.3 in Li ? -exchanged montmorillonite, whereas only weaker-strength acid sites (1.5 \ H 0 B 4.8) were present in K ? -exchanged montmorillonite. The results of the catalysis experiments indicated that montmorillonite promoted the thermal decomposition of the model organic. The catalytic activity showed a positive correlation with the solid acidity of montmorillonite and was affected by cation exchange, which occurs naturally in geological processes.

Improving the Stability and Efficiency of Dimeric Fatty Acids Production by Increasing the Brønsted Acidity and Basal Spacing of Montmorillonite

“…[ 16 ] Therefore, natural MMT usually shows Brønsted acidity rather than Lewis acidity. The pyridine infrared results of Reddy [ 16 ] and Liu [ 27 ] showed that only MMT treated with strong Lewis acid (NiCl 2 , ZnCl 2 , FeCl 3 , AlCl 3 ) showed strong PyL and PyH + vibration peaks. However, PyL peak was much weaker and even negligible compared with PyH + pak in the native MMTs.…”

“…The water content was determined by measuring the mass loss of samples dried at 120°C for 24 h, which was mainly attributed to surface‐adsorbed water. [ 27 ] The sufficient surface‐adsorbed water indicated that all of the interlayer cations were coordinated by interlayer water. Since Brønsted acidity mainly depended on the hydration ability of interlayer cations, the sufficient surface‐adsorbed water in the MMTs caused negligible effect of water content on the Brønsted acidity.…”

“…The Brønsted acidity and Lewis acidity were determined by Fourier transform infrared (FT‐IR) spectrum using pyridine as a basic probe molecule. [ 27 ] 0.1 g dried MMT was paved into a 0.1 mm thick layer. Pyridine was adsorbed at 25°C for 24 h. The samples were kept in a hot air oven at 120°C for 1 h to remove physically adsorbed pyridine and then analyzed by FT‐IR spectroscopy.…”

“…The interlayer cations, mass ratio of silica to alumina (Si/Al), and water content of MMT can affect dimerization activity through influence on acidity. [ 26–28 ] Huang [ 29 ] found hexadecyl trimethyl ammonium bromide (CTAB) intercalated MMT showed better dimerization activity than native MMT. The result was attributed to the change in surface acidity, basal spacing and Si/Al ratio of MMT.…”

A recent review of non-biological remediation of aflatoxin-contaminated crops

“…The effectiveness of some binders is debatable because of the complex process involving a combination of porosity characteristics, surface acidity, temperature, pH and distribution of exchangeable cations . For instance, Johnston et al conducted a study that investigated glucomannon (MTB‐100) as a possible agent for sequestering aflatoxin in contaminated dried distillers grains (DDGs), resulting in an 80% reduction in total aflatoxin levels.…”