High temperature thermal expansion behavior of H[Al]ZSM-5 zeolites: The role of Brønsted sites

High temperature X-ray diffraction technique has been used to measure the thermal expansion behavior of orthorhombic H[Al]ZSM-5 zeolites over a temperature range of 473–1173 K. The presence of Al is found to have a remarkable effect on the negative thermal expansion (NTE) behavior of ZSM-5 even when the total Al2O3 concentration is less than 1 mol%. A low-Al H[Al]ZSM-5 sample with Si:Al ratio of 250:1 shows NTE behavior over the entire temperature range with an average volume expansion coefficient β ∼ −34 × 10−6 K−1. In contrast, samples with an Si:Al ratio of 27.5:1 show an initial positive thermal expansion (β ∼ +39 × 10−6 K−1) between 473 and 673 K and the NTE behavior returns only at higher temperatures (β ∼ −85 × 10−6 K−1) and continues up to 1173 K. The vibrational density of states obtained from MD simulations of orthorhombic ZSM-5 in this temperature range clearly demonstrate for the first time that the bending of Si–O–Si linkages and the rotational modes of SiO4 tetrahedra play an important role in controlling the NTE behavior. MD simulations also indicate weak vibrational coupling between the SiO4 and AlO4 tetrahedra associated with the Si–OH–Al linkages in the H[Al]ZSM-5 lattice which can explain the experimentally observed absence of NTE behavior in H[Al]ZSM-5 with Si:Al ∼ 27.5:1 at low temperatures. High resolution 27Al NMR spectroscopic results indicate that the return of the NTE behavior in this zeolite at high temperatures can be explained to be due to the partial expulsion of Al from the tetrahedral framework.