The Effect of Heating on the Surface Area, Porosity and Surface Acidity of a Bentonite

Abstract

The Hançılı bentonite from Turkey shows significant changes in surface area, micro- and mesoporosity, surface acidity and acid strength with heating from 100 to 900°C for 2 h. The specific surface area (S) and specific micro-mesopore volume (V) of the original and heated samples were evaluated from N2 adsorption and desorption data, respectively, by standard methods. The adsorption of n-butylamine from the solution in cyclohexane on the samples was used to determine the total surface acidity (nm) and the adsorption equilibrium constant (K) was taken as a measure of the acid strength. S, V and nm having initial values of 98 m2g−1, 0.080 cm3g−1 and 4.8 × 10−4 mol g−1, respectively, stayed approximately constant as the temperature increased to 450°C and then decreased almost in parallel with each other, reaching their minimum or zero at 900°C. The total surface acidity, in general, declines with increasing temperature. The most acidic sites, however, increase with heating, and especially at dehydration and dehydroxylation. Acid strength reaches its maximum during the dehydroxylation phase at ∼600°C. It was concluded that the total surface acidity does not necessarily parallel the strength of the most acid sites.

Evolution of nanoporosity in organic-rich shales during thermal maturation

“…As compared to the initial sample, the micro-and mesopore volumes reduce by 86.7% and 83.8% respectively, and its micro-and mesopore surface areas reduce by 50.1% and 71.3% respectively, when it is matured to the VRo value of 4.19 %. The result is in agreement with the heating treatment of clay minerals [41,42]. Noyan et al [42] observed an overall trend of decline in both specific surface areas and micromesopore volumes with increasing heating temperature from100 to 900°C for Hancili bentonite from Turkey.…”

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

“…However, several different results have been obtained by other studies. For example, using a titration method, Mahmoud et al (2003) found that the largest number of acid sites and the highest-strength acid sites appeared after heating at 250°C on Jordanian bentonite, whereas Noyan et al (2006) proposed that the acidity of bentonite declined with increasing temperature (from 25 to 900°C) and found that the highest number of solid acid sites obtained was only 0.45 mmol/g, which is significantly lower than the value obtained in our study. The variation in these results is attributed to the specific features of different montmorillonite samples and the different acidity measurement methods used.…”

“…ions (Lewis acid sites) result from the rupture of Si-O-Al bonds or dehydroxylation at high temperatures (Bhattacharyya and Gupta 2008), and the Lewis acid sites that result from dehydroxylation are of significantly high acid strength Fig. 4 The total number of acid sites of Mt and its heated (a) and cation-exchanged (b) products (determined by n-butylamine titration in nonaqueous system) (Noyan et al 2006). The abnormally low Q L values of Mt-400 and Mt-600 in this study may be explained by the difficulty in detection due to the size of pyridine and the reduction in interlayer spacing, which both prevent basic probes from entering the interlayer to react with unsaturated Al 3?…”

“…Therefore, a complete evaluation of the solid acidity of montmorillonite, including the types and origins of solid acid sites, the acid amount, and the distribution of acid strength, is necessary. The solid acidity of montmorillonite is greatly influenced by heating and cation-exchange processes (Billingham et al 1996;Brown and Rhodes 1997;Heller-Kallai 2006;Liu et al 2011;Noyan et al 2006;Reddy et al 2009;Reddy et al 2007). However, most investigations have focused on acidactivated montmorillonite due to its high acidity and excellent performance in catalytic reactions, such as Friedel-Crafts alkylation and the polymerization of unsaturated hydrocarbons.…”

High-pressure adsorption of methane on montmorillonite, kaolinite and illite

“…Therefore, the S BET values of Mt-400 and Mt-600 reflect only their external surface areas. In light of the fact that the layered structure was retained in Mt-400 but destroyed in Mt-600 by heating (Noyan et al, 2006), the measured S BET of Mt-400 could accurately represent the external surface area of Mt without the interference of adsorbed water.…”