Zeolite and alumina catalysts: Bronsted and Lewis acidity, adsorption property, and catalytic performances

The Bronsted and Lewis acidity, adsorption property, and catalytic performances of steamed and calcined acid zeolites ZSM-5, Y, and mordenite as well as of Al{dollar}sp{lcub}rm V{rcub}{dollar} enriched transition alumina catalyts have been studied.; Thermal activation (calcination or steaming) dislodges aluminum from framework to nonframework positions. From the correlation between the unit cell volume contraction measured by XRD and the micropore volume decrease measured by N{dollar}sb2{dollar} adsorption, NFAl species were found to spread into the whole lattice in H-mordenite dealumated by calcination, while in the cases of calcined dealuminated HZSM-5 and H-Y, the NFAl species are located in the micropores available to N{dollar}sb2.{dollar}; A Bronsted acid center is an OH group bridging a Si and an Al in the framework. The strength of Bronsted acid sites depends on the environment of the OH groups. From the viewpoint of the aluminum, the strongest Bronsted sites associate with the framework aluminum with zero next nearest neighbor (0 NNN), or FAl1, which can be calculated by Monte Carlo simulation. From the viewpoint of Si, the strongest Bronsted acid sites associate with the Si with one aluminum in the first coordination shell, or Q1, which can be obtained from the deconvelution of {dollar}sp{lcub}29{rcub}{dollar}Si MAS NMR. Q1 corresponds to the amount of ammonia chemisorbed at 175{dollar}spcirc{dollar}C (ASTM 4824-88), while FAl1 corresponds the chemisorbed CO stretching band at 2175 cm{dollar}sp{lcub}-1{rcub}{dollar} and to the amount of NH{dollar}sb3{dollar} with heat of adsorption larger than 100 kJ/mol. Lewis acid centers are associated with the NFAl species and they are either perturbed tetrahedral aluminum or pentahedral aluminum.; The catalytic behaviors in n-pentane conversion at 285{dollar}spcirc{dollar}C on these zeolites are quite different. In dealuminated H-Y zeolites, a maximum in activity has been observed at Si/FAl {dollar}sim{dollar}5, in agreement with literature data. The activity of DHZ and DHM seem to increase linearly with the number of Q1 and FAl1. The TOF sequence DHM {dollar}>{dollar} DHZ is unexpected, due to a larger fraction of FAl1 in DHZ. The TOF sequence DHM {dollar}>{dollar} DHZ is unexpected, because the fraction of FAl1 in DHZ is larger. Maybe the enhanced efficiency of the Bronsted acid sites demonstrates quantitatively the synergistic interaction between the Lewis and Bronsted acid sites in steamed or calcined dealuminated H-mordenites.; l-butene isomerization was used to study the Lewis acid sites associated with penta-coordinated aluminum species (Al{dollar}sp{lcub}rm V{rcub}){dollar} in aluminas. Due to the fact that there are at least 5 kinds of acid/basic sites on alumina, and both acidic and basic sites are capable of isomerizating 1-butene at room temperature, there is no direct correlation between activity and acidity. However, from the analysis of selectivity (cis/trans ratio), Al{dollar}sp{lcub}rm V{rcub}{dollar} seems to be associated with stronger Lewis acid sites.