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CanpuIational techniques predict thal several microprous zeolitic srmctures have unusual negative ccefhcienfs of lhennal expansion. High-resolution powder diffraction techniques confirm the qualitative a s p & of these simulations.Zeolites are an important class of framework-structured microporous solids which have attracted widespread interest because of their intriguing and varied crystal architectures and their widespread applications as heterogeneous catalysts in cracking, isomerization and other reactions. Typical framework structures are shown in figure 1. The materials are normally aluminosilicates with the negative charge of the framework being compensated by extra framework cations and/or protons present as bridging OH groups. Purely siliceous samples (i.e. Si02 polymorphs) can, however, be prepared for several zeolitic structures.

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The bonding of the oxygen species held on a Ag/a-Al 2 O 3 catalyst has been studied by temperature programmed desorption and their reactivity in ethene epoxidation by temperature programmed reduction using ethene as the reductant. The Ag/a-Al 2 O 3 catalyst was produced by the thermal decomposition of a Ag oxalate/a-Al 2 O 3 precursor. Oxygen desorbs from this Ag/a-Al 2 O 3 catalyst in two states, one (peak maximum temperature 520 K) having a desorption activation energy of 140 kJ mol À1 -oxygen desorbing from Ag(111), and one (peak maximum temperature 573 K) having a desorption activation energy of 155 kJ mol À1 -oxygen desorbing from a highly stepped or defected Ag surface. Temperature programmed reduction of the two oxygen states existing on the surface of the Ag/a-Al 2 O 3 catalyst using ethene as the reductant produced two peaks at 373 and 473 K in which ethene epoxide and CO 2 evolved coincidently. The peak at 373 K derives from the reduction of oxygen atoms adsorbed on Ag(111). The higher temperature peak (473 K) corresponds to the reduction of oxygen atoms adsorbed on highly stepped or defected Ag surface. The selectivity to ethene epoxide for the 373 K peak is $57%, while that of the 473 K peak is 34%. The coincident evolution of ethene epoxide and CO 2 shows that the selective and unselective reaction pathways have a common surface intermediate-probably an oxametallacycle. The higher selectivity of the oxametallacycle formed by the bonding of ethene to the weaker AgAO bond is considered to result from its having a lower activation energy to cyclisation than that produced by ethene bonding to the higher AgAO bond.