Powder neutron and X-ray diffraction studies of benzene adsorbed in zeolite ZSM-5

The whereabouts of benzene molecules adsorbed at two loadings in zeolite ZSM-5 at room temperature have been studied with powder neutron and synchrotron X-ray diffraction. The structures have been refined by the Rietveld method in space group Pnma by fitting simultaneously to the X-ray and neutron data. With approximately 3.4 molecules per unit cell, the best agreement is obtained with a model which has two molecules in mutually exclusive positions in the intersection of the straight and sinusoidal channels. The two positions have occupancies of 2.10(5) and 1.32(5) molecules per unit cell, respectively. At the higher benzene coverage of around 7.6 molecules per unit cell, molecules are found at three locations: in the intersection, in the sinusoidal channel, and in the straight channel. These have occupancies of 4.0, 2.45(4), and 1.15(4) molecules per unit cell, respectively. The intersection is essentially full. There appear to be favorable interactions between the molecules in the intersection and those in the sinusoidal and straight channels that help stabilize the arrangement.