Acidic Properties of Hierarchical Zeolites

Proton Activity in Nanochannels Revealed by Electron Paramagnetic Resonance of Ionizable Nitroxides: A Test of the Poisson–Boltzmann Double Layer Theory

“…Acid–base properties of organic and inorganic porous materials are of great interest from perspectives of fundamental understanding of molecular processes in heterogeneous catalysis and chemical adsorption as well as numerous practical applications including chromatography. − These processes occur at the channel surfaces and are known to be affected by both the chemical nature of solutions and surface properties of these materials. In the past, the classic method of acid strength measurements based on monitoring the color changes of adsorbed dye indicators (Hammett indicators) upon titration as well as microcalorimetry and thermal programmed desorption measurements of other basic and/or acidic probes of different strength absorbed on catalysts was employed to determine the concentration and strength of the acid sites. − A number of spectroscopic methods including Fourier-transform infrared spectroscopy (FTIR), magic angle spinning, NMR, IR, photoluminescence, Raman, UV–vis, X-ray photoelectron spectroscopy, and electron paramagnetic resonance (EPR) have been also successfully applied to some of catalytic mesoporous materials in order to identify and characterize acid–base properties of the surface sites. − While each of these spectroscopic methods has its own pros and cons, EPR spectroscopy of ionizable nitroxides − has the benefits of being applicable to nontransparent/turbid materials. Also, spin-probe EPR generally exhibits better concentration sensitivity than solid-state NMR.…”

Shaped Hierarchical H-ZSM-5 Catalysts for the Conversion of Dimethyl Ether to Gasoline

“…This is explained by both changes in the environments and the nature of strong AS influencing NH 3 adsorption strength as well as by an improved acid site accessibility due to an increased mesoporosity. 104 Concerning acid strength, it has been reported that the intrinsic Brønsted-acidity of framework Al, i.e., bridging Si(OH) Al groups, is unaffected by short alkaline treatment under similar conditions to the ones employed herein, i.e., 0.2 M NaOH at 65 °C for 30 min 101 and 0.05−1.0 M NaOH at 85 °C for 60 min. 87 This was evidenced by low-temperature CO sorption FTIR experiments, where both position (red-shift, Δν OH•••CO ) and shape of the bands of Si(OH)Al groups interacting with CO were unaffected by alkaline treatment.…”

“…Another factor, which may impact the nature and average strength of strong AS in Z28.3 and Z40.3, is the formation of additional acid sites via realumination. , These Al atoms were found to be able to form protonic sites with distinct, albeit quite weak Brønsted-acidic character comparable to amorphous or mesoporous aluminosilicates. , Such sites are prone to dehydroxylation, i.e., formation of strong Lewis-acid sites . Both of these effects combined with the Lewis-acidic nature of EFAl species located on the external and mesopore surfaces of alkaline-treated hierarchical zeolites ,, are expected to contribute to the shift and broadening evident in the high-temperature NH 3 -TPD peaks of Z28.3 and Z40.3 (Figure ).…”

Hierarchy in zeolite catalysis: The influence of enhanced mesoporosity on the synthesis of renewable fuels and bio-based platform chemicals

“…Nevertheless, HMOR10 zeolite exhibited similar conversion of FFA compared with parent zeolite despite the diffusion-restricted one-dimensional channel pockets (8-MR) for bulky molecules such as oleic acid. [52,54] The catalytic activity of spent zeolite catalysts did not change significantly after five consecutive runs, confirming that it is possible to recycle these catalysts for several times in esterification reaction. The information on the catalytic conditions for the HFAU2.5 zeolite catalyst is presented in (Figure 6).…”

“…[51] Most importantly, all the IR spectroscopy data reported in this work proved that the surface defects formation upon the treatment by surfactant in a basic media is linked with the loss of some of Brønsted acid sites, but increased the Lewis:Brønsted ratio, suggesting formation of hydroxyl nests (silanol terminal group ~ 3740cm -1 ) see (Figure 5). [42,52] No. The conversion of oleic acid on various zeolite materials using excess methanol at 100 o C in very shorter time (up to 30 min.)…”