Dynamic properties of protons in solid acids as studied by variable temperature [formula omitted] MAS NMR

Abstract

The temperature dependence of the line width of 1 H MAS NMR reveals that the acidic protons in H-ZSM-5 become mobile at temperature as low as ∼370 K, though not mobile at 298 K. Substitution of a small part of protons with Na + or K + ions sharply decreases the mobility as well as the catalytic activity of the remaining protons, suggesting that a long-range interaction exists among the acid sites. In H-[B,Al]-ZSM-5, two types of protons from the bridging hydroxyl groups, B–OH–Si and Al–OH–Si are observed separately, indicating that the nature of protons are determined by the local environment of the acid sites. However, the local migration of the protons around Al-based acid sites is suppressed by boron introduction to the framework. Two types of protons are observed at 6.4 and 9.3 ppm when Ag 3PW 12O 40 is reduced with hydrogen. The protons at 6.4 ppm exist only when hydrogen is present in the gas phase. They are mobile even at room temperature and catalytically more active than the protons at 9.3 ppm. Upon raising temperature, the exchange of protons between the two kinds of protons proceeds.