## 2 The Precursor (P)MCM-22 and MCM-22 Zeolites
The microcalorimetry results showed a concentration of acid sites (for an MCM-22 with Si/Al = 16) of 1052 \(\upmu\)mol\(\cdot\)g\({}^{-1}\), which is modestly higher than the concentration of aluminum ions, 947 \(\upmu\)mol\(\cdot\)g\({}^{-1}\), suggesting that all aluminum ions produce acid sites either by producing an unbalanced charge structure that is balanced by the proton or active as Lewis acid sites. It was assumed that the aluminum ions in the zeolite structure do not act as Lewis acid sites because they are \"protected\" by nearby protonic centers. These aluminum ions may be located on the extra-framework where the structure is relaxed, acting as Lewis acid sites. The author of this study pointed out that the additional concentration of acid sites (105 \(\upmu\)mol\(\cdot\)g\({}^{-1}\)) may be related to silanol groups located at the external surface [23]. Studies employing infrared spectroscopy with adsorbed pyridine have reported that for samples with Si/Al = 10, 14, and 30, most acid sites (50-70%) are located in the supercavities. The other sites are located in the sinusoidal channels (20-30%) or connected to the hexagonal prisms between supercavities, with values of 10% for Si/Al = 10 and 14 and 20% for Si/Al = 30 [24]. In addition,  
Figure 2: Morphologies of MCM-22 with different Si/Al molar ratios = 9, 21, 30, 46. The last image corresponds to pure ferrierite formed under static conditions. Adapted from Reference [19]. Copyright (2009), with permission from Elsevier. a study using density functional theory reported that the favorite placement sites of aluminum ions are the sites T1, T3, and T4, as shown in Figure 1b. The T2 site is presented as less favorite and the acidity of the T1 and T4 sites are equivalent and stronger than that of the T3 site, respectively [15]. Regarding the good performance of the MWW materials for benzene alkylation reactions, and despite the small size of the channel apertures, it is suggested that a significant number of cavities are open on the surface of the crystallites. It was assumed that the \"cups\" of the supercavities have a free diameter of 0.71 nm and the formation of cumene and ethylbenzene must occur in these cavities without any diffusional barrier. This hypothesis is supported when catalytic activity is significantly decreased by deactivation with 2,6-di-tert-butylpyridine, a large molecule that cannot enter in the channels of MCM-22. However, spectroscopic results confirm that benzene could easily enter the supercavities [23; 25]. The hydrothermal crystallization and morphology of MCM-22 can be significantly altered by static or dynamic conditions. The dynamic condition minimizes the excessive aggregation of the crystals (see Figure 3a) when compared with static conditions, as shown in Figure 3b-g. Synthesis under dynamic conditions also induces the formation of zeolite with a higher crystallinity in a shorter time [17]. Figure 3: Morphologies of MCM-22 zeolites obtained under dynamic (**a**) and static conditions (**b–h**). The zeolites under static conditions differ in methodology, molar composition, silicon source, and temperature. Adapted from Reference [9; 26; 27; 28; 29]. The crystallization of MCM-22 is also influenced by the source of silicon used because its degree of dissolution affects nucleation and crystal growth. A study compared three silicon sources with different surface areas: silicic acid (750 m\({}^{2}\cdot\)g\({}^{-1}\)), silica gel (500 m\({}^{2}\cdot\)g\({}^{-1}\)), and precipitated Ultrasil silica (176 m\({}^{2}\cdot\)g\({}^{-1}\)); zeolite with 100% crystallinity was obtained with silicic acid followed by silica gel (90%) and Ultrasil (80%) by aging the gel for 7 days in dynamic conditions [26]. The authors showed high crystallinities obtained under static conditions using silicic acid when compared with the other silicon sources. This indicates that silicon sources with a high surface area are a determining factor in the crystallization of MCM-22. Figure 3a,b show the morphologies of materials synthesized with silicic acid.