## 2 Zeolites
### Physical Properties  
Knowledge of zeolite properties is important in understanding the performance of the material, which depends on the dimension of its channels, the accessibility of pore volume, the diameter of the pore opening, and the number and location of extra-network species, that is, located outside the structure. Currently, the composition of zeolites has been replaced by heteroatoms, such as those with high silica content. The electronic structure of the component elements tends to affect the role of the counterions in counterbalancing the charges in the elementary mesh. For example, Al, which concentrates high electronic density, can be compensated by exchangeable extra-network cations [58]. The dimensionality in the zeolite structure is important to determine the ease of mobility of the reacting or adsorbed molecules in the pores, giving the molecular sieve character to the structure. Structures can be one-dimensional, two-dimensional, and three-dimensional, and the greater the dimensionality, the better the molecular mobility within the structure. Thus, it can also be an indication of the ease of deactivating the structure [59]. The International Union of Pure and Applied Chemistry determines the nomenclature of zeolites using three letters, regardless of chemical composition, and also determines that according to the size of the pores, they can be classified as small (<4 A), medium (from 4 A to 6 A), large (from 6 A to 8 A), and super large (>8 A) [53]. According to the pore size, a determined number of T-atoms define the smallest pore opening, channels delimited by 8 rings are small, 10 rings are medium and 12 rings are large, and an elevated number of ring members gives an extralarge pore opening. The rings may be distorted depending on the symmetry and T-atoms surrounding them. Moreover, the channel system is defined by channel directions relative to crystallographic axes [42] (p. 8). Certain physicochemical properties may be achieved by structural and chemical modification, preserving the layer integrity. Among these properties, the increasing of surface area to ease molecular diffusion, transport, and access of substrates in active sites could be achieved with techniques such as swelling and pillaring. Many methods have been proposed for postsynthesis modification of interlamellar space in 2D zeolites. Among them are detemplation by thermal combustion and chemical extraction, intercalation into interlamellar space, delamination/exfoliation, and colloidal suspensions. The main property upgraded in modification of 2D zeolites is the increase in sorption capacities by the increasing of external surface area and presence of auxiliary mesoporous [60]. Some examples that demonstrate the best values of external surface area may be unilamellar MFI (BET 710 m\({}^{2}\cdot\)g\({}^{-1}\)) and MIT-1 (BET 513 m\({}^{2}\cdot\)g\({}^{-1}\)). It is important to note the creation of mesopores influencing access to acid sites on the layer surface or inside micropores. The external surface has a high concentration of silanol groups in these conditions, bringing good hydrophilic interactions [54] (p. 9). The layered zeolites are 2D structures with silanol groups linked to each other with hydrogen bonding, making possible ionic/covalentbonding between template molecules present in the interlayer space. In these structures, crystal growth in a third direction is constrained. An outstanding physical property of this structure is that the constructing layers is highly plastic [61; 62]. The stacking mode of layers in postsynthesized materials has a great impact on the accessibility of layers [63]. In one of the most important zeolite structures for catalytic purposes (MFI), the layers may have the template as an integral part, so detemplation is crucial for further transformations of structure. In addition, the interlayer spacing is less visualized than other structures because of a lack of interlayer reflections.