Acid Site Strength Distribution Research Articles

Acid sites strength distribution in heteropoly acids by differential scanning calorimetry

Acid sites strength distributions (ASSD) in the heteropoly acids (HPAs) silicotungstic acid (STA), phosphotungstic acid (PTA) and phosphomolybdic acid (PMA) are investigated using differential scanning calorimetery (DSC). The strengths of the acid sites in these HPAs are grouped into weak, intermediate and strong, according to the temperature region in which presorbed triethylamine (TEA) desorbs. Such desorption appears in a DSC thermogram for a single group of acid sites with a definite strength as one or more exothermal effects. The Δ H value corresponding to these effects is taken to evaluate the relative quantities of acid sites. The percentage of strong acid sites in the HPAs studied is found to increase as the total number of acid sites increases, whereas the percentage of intermediate-strength acid sites is found to decrease.

Acid site strength distribution in mordenites by differential scanning calorimetry

Using differential scanning calorimetry (DSC), the acid site strength distribution in Na- and H-mordenites is investigated. Triethylamine (TEA) and ammonia are presorbed into the mordenites and then desorbed via a DSC programmed temperature increase from 50 to 600 °C. Various presorbing contact periods are used to throw light on the rate of accessibility for the different types of acid sites in the mordenites by the two bases under study. A new DSC technique is used to nullify the zeolitic DSC effects such that the effects due to TEA and NH 3 desorption from the acid sites appear only in the thermograms. The Δ H values obtained may evaluate the relative proportions of the acid Site types.

Desorption of presorbed ammonia, triethylamine and pyridine from the acid sites of mordenites via differential scanning calorimetry

Differential scanning calorimetry (DSC) was used to desorb and detect presorbed bases having different strengths, i.e. ammonia, triethylamine (TEA) and pyridine, from the different acid sites of Na- and H-mordenites. A method recently adopted by the author was used to nullify the zeolitic water DSC effect such that base desorption effects only appear in the thermograms. The Lewis acid sites in Na-mordenite were found to be stronger than in H-mordenite, whereas the Brönsted acid sites in H-mordenite were stronger than in Na-mordenite. Ammonia desorption from the mordenites gave satisfactory information about the acid sites strength distribution. TEA desorption from the Brönsted acid sites takes place at higher temperatures than for ammonia. Pyridine desorption from most of the Lewis acid sites showed a shift to the high temperature region owing to the strengthening of their binding forces via interaction of the ring π-electrons with positively charged aluminium in the mordenite zeolitic structure.

Catalytic and physical properties of phosphorus-modified ZSM-5 zeolite

H-ZSM-5 zeolite modified by phosphorus (1.1 and 2.0 wt%) was studied by means of infrared spectroscopy and microcalorimetry for acidity characterization, by ESCA for determining elementary composition in the outer layers (depth ∼- 50 Å) with respect to the bulk (φ ∼- several μm), and lastly by catalytic reaction analysis of methanol conversion. It was found that phosphorus neutralizes acidic sites primarily at the entrance of the channels of the zeolite particles in the same manner as do carbonaceous residues after some time of reaction. However, the strongest acid sites remain unmodified, which leads to the suggestion that the aluminium distribution and subsequently the acid site strength distribution along the zeolite channels is heterogeneous. Phosphorus-modified zeolites gave a higher yield of light olefins (C 2 =C 4 =) and subsequently a smaller yield of saturated aliphatics and aromatics than the parent zeolite in the methanol conversion reaction. The phosphorus-modified zeolite gave similar yields of the meta + para isomers of xylene and of ethyltoluene and a smaller yield of the ortho isomers. Lastly the yield of heavier aromatics A 9 + was greatly decreased by the addition of phosphorus. The changes in selectivity were tentatively assigned to slight modifications in channel size and to increased tortuosity due to phosphorus compounds bonding to the zeolite framework, rather than to changes in acid strength.