Density Of Bronsted Sites Research Articles

Adsorption of Small Alkanes on ZSM-5 Zeolites: Influence of Brønsted Sites

The adsorption of a series of small alkanes was studied experimentally on H-ZSM-5 zeolites using calorimetric measurements in order to determine their interactions with the Bronsted sites. Differential heats measured on four ZSM-5 samples with different Si/Al2 ratio and with different defect concentrations were found to depend strongly on the Bronsted-site density but not on the presence of defects. The interactions for CH4 with the Bronsted sites were minimal but the effect was significant (up to 11 ± 2 kJ/mol extra heats) for larger alkanes, such as n-C6H14. The affinity of the alkanes with the Bronsted sites increased with the gas-phase proton affinity of the alkanes and the calculated affinity of the alkanes for the strong acid, fluorosulfonic acid. The extra heats of adsorption in H-ZSM-5 over its siliceous counterparts can therefore be associated with the strength of hydrogen bonding between the adsorbed alkane and the Bronsted sites, which in turn increases with molecular size. Specifically, extra ...

The influence of surface acidity on NO2 reduction by propane under lean conditions

This investigation focuses on the influence of surface acidity on the continuous catalytic reduction of NOx by propane under lean conditions. The acidity was varied by studying noble metal free alumina, silica and co-precipitated aluminium-silicates with varying alumina-silica ratio. The catalysts were characterised by isopropylamine temperature programmed desorption (TPD), with respect to NOx reduction (NO2 and propane) and by Fourier Transform Infrared (FTIR) spectroscopy. The isopropylamine TPD was used to determine the sample acidity. The amount of Bronsted acid sites increased with increasing amount of alumina. The sample activity for NOx reduction with propane was found to correlate with the Bronsted-site density. Furthermore, no N2O was formed during the activity studies. The FTIR measurements implied formation of isocyanate species on the surface of the catalyst sample when NO/NO2, propane and oxygen were present in the gas phase. FTIR data also reveal an increasing amount of adsorbates on the surface with increasing amount of alumina in the sample. Moreover, with propane present, NO-species were consumed to a higher extent over samples containing a larger amount of alumina. Our results suggest a connection between the Bronsted-site density and the NOx reduction with propane under lean conditions.

Simultaneous determination of Lewis and Bronsted site densities on silica alumina using infrared techniques

When a pyridine−covered surface of a silica−alumina catalyst is contacted with water, there is an inversion of the population of coordinately bound pyridine to species bound by proton exchange. We have used this fact along with a standard ’’titration’’ technique to determine in a single series of measurements on the same catalyst the number of Lewis and Bronsted sites both before and after water exposure. This modified titration technique was applied to a series of experimental silica−alumina catalysts. The total number of acid sites and the number of Lewis and Bronsted sites per unit area of catalyst were determined as a function of the silica content of the catalyst. The Bronsted−site concentrations agree excellently with previously published data by Clark and Holm (J. Catal., 1963) on the catalytic isomerization of o−xylene over similar silica−alumina catalysts.