Calorimetric and Spectroscopic Investigation of the Acidity of HZSM-5

Abstract

The cal-ad analysis of HZSM-5 provides a novel characterization of the donor-acceptor properties of this solid acid showing 0.0415 mmol per g of a strong Brönsted site and 0.53 mmol per g of a weaker site. Of all studies to date, only cal-ad provides a measure of the extent, Ki ; strength, −ΔHi ; and quantity of sites. The relative K values show why gas phase calorimetry, TPD, TGA, DSC, and isopropylamine decomposition do not resolve the two sites found by cal-ad and incorrectly suggest 0.6 mmol g-1 of strong acid sites. Reaction with pyridine gives an enthalpy of 42.1 and 8.6 kcal mol-1, respectively, for the two sites suggesting the latter is a hydrogen bonding site. Other acidity measures average these sites, and their enthalpies contain dispersion as well as donor-acceptor components. Comparison of these results with other cal-ad results suggests HZSM-5 is a strong solid acid but not a super acid. Titrations with 2,6-lutidine and 2,6-di-tert-butylpyridine indicate that the strong Brönsted sites are located in the 5.5 Å channels with no detectable amounts on the exterior surface.

Acidic Characterization of Copper Oxide and Niobium Pentoxide Supported on Silica–Alumina

“…Slurry microcalorimetric experiments are more sensitive than the TG-DTG technique being able to detect much lower concentrations of the strongest Brönsted sites [24,25]. For the samples with 2 and 5 mass% CuO, the amount of the strongest sites is quite similar, considering the qualitative approximation obtained by the calorimetric method.…”

Solvent effect on the preparation of H3PW12O40 supported on alumina

“…In addition, other solids such as silica gel [26], mordenite [27], Y zeolite [28], ZSM-5 [29], TS-1 [30], sulfated zirconia [31], etc., are among the variety of important solid acids characterized by this method.…”

The Influence of Aluminium Concentration on the Hydration of ZSM-5-Type Zeolites in Aqueous Suspensions

“…In reality, when a high degree of hydration of the surface exists, water not only interacts with primary adsorption sites but also becomes part of the molecular aggregates which contain the Brönsted acid sites and several molecules of water bound to such a site (Zecchina et al 1995;Buzzoni et al 1995). However, only a relatively small number of surface hydroxy groups exhibit a high acidity as demonstrated by calorimetric titration data (Drago et al 1997). With increasing concentration of aluminium in the crystal lattice, the percentage content of such active OH groups decreases and, as a consequence, a decrease in the sizes of the water molecule clusters adsorbed on the surface is also observed.…”

“…Samples of zeolites were synthesized according to the methodology described by Grose and Flanigen (1977) using highly dispersed aluminosilicas prepared via the simultaneous hydrolysis of SiCl 4 and TiCl 4 in the flame of a hydrogen-air burner. Such a procedure allowed us to simplify the synthesis of zeolites since aluminosilica served as a source for the formation of both oxides in the zeolite crystal lattice.…”