Zeolites are metastable, as they form under hydrothermal conditions as a transitional phase of quartz.135 Therefore, the continued application of hydrothermal conditions eventually leads to the collapse of their framework structure. In the presence of steam and at temperatures above 400 °C framework Si–O–Al bonds will be gradually hydrolysed and aluminium will be expelled from the framework, forming extra-framework aluminium (EFAl) species (Scheme 3).104,105,136–138 During this process, silicon can also be extracted from the framework, leading to the formation of extra-framework silica-alumina.139 When Si and Al atoms are extracted from the zeolite lattice, mesopores form and amorphisation of the crystal lattice can take place.5,137 Due to the removal of negatively charged [AlO4]− units from the zeolite lattice, this so-called dealumination will reduce the amount of counter-cations that the zeolitic framework can retain. In the case of H-zeolites, this means that the active sites for catalysis are lost.