# Stationary Phases for Green Liquid Chromatography
## 3 Stationary Phases Used in Pure Water Conditions
### Instability of Stationary Phases--A Motivation to Search for Solutions
#### 3.1.3. Zirconia-Based Stationary Phases  
Silica, due to the low pH of the point of zero charge (pH\({}_{\text{pzc}}\)), allows only cation exchange under neutral pH conditions. The dissociation of silanols then occurs, and the silica surface acquires a negative charge. Therefore, chromatography of basic compounds on silica is troublesome, because it requires a significant change in pH--which adversely affects the attached ligands or the silica itself--or the use of ion-exchange displacers or anionic ion-pairing agents [112; 113; 114].  
Metal oxides--especially zirconium, aluminum, and titanium oxides--have much higher pH\({}_{\text{pzc}}\), which means they do not have a negative surface charge, so no electrostatic interactions will occur; they behave as amphoteric ion exchangers, so anionic or cationic exchange can occur depending on the pH. Due to the presence of Lewis acid sites, the additional ligand exchangeability works to the advantage of using these supports [112; 113].  
[PERSON] et al. were the first research group to examine the use of zirconium oxide as a potential packing material for chromatography columns [113; 115]. This raised issues about modifying its surface with alkyl ligands. The modification required special conditions and additives. Finally, it turned out that among metal oxides, only zirconia could be used as effective chromatographic material.  
Zirconium oxide has many adsorption sites, so it can be easily modified. This action favors a wide adaptation of the use of zirconium-based stationary phases. In general, there are three types of zirconium oxide modification: (1) dynamic, where the mobile phase contains a strongly interacting Lewis base; (2) permanent, where the zirconium oxide surface is permanently silylated as a result of binding to adsorption sites; and (3) physical screening, e.g., coating the zirconium surface with polymer or carbon. The most commonly used approaches are zirconia encapsulated by polybutadiene (PDB)--which is also known as a ZirChrom-PDB [116; 117; 118; 119; 120; 81; 82; 83]--and zirconia encapsulated by polystyrene (PS), called ZirChrom-PS [54; 66; 120]. Other zirconia-based packings include carbon-coated zirconia (CARB)--ZirChrom-CARB [117; 81; 123]--or a secondary bonded C\({}_{18}\) zirconia-based stationary phase (DiamondBond) e.g., ZirChrom-DB-C18 [83]. All zirconium- and aluminum-based stationary phases are listed in Table 3.  
Zirconium oxide used as a support in chromatographic analyses has good thermal stability and is stable under pure water conditions. For ZirChrom-PDB, the manufacturer recommends an upper temperature limit of 150 \({}^{\circ}\)C; however, the thermal stability of this material has been observed up to 200 \({}^{\circ}\)C. ZirChrom-PDB was tested by [PERSON] et al. [116], who showed that the upper limit of the column's thermal stability reaches 260 \({}^{\circ}\)C. The ZirChrom-PDB and ZirChrom-CARB columns tested by [PERSON] et al. [117] reached thermal stability of 370 \({}^{\circ}\)C and 300 \({}^{\circ}\)C, respectively, using a pressure of 75.8 MPa. Compounds from the phenol and alkylbenzene groups were successfully separated on both stationary phases.  
It is also important to note that complete removal of CO\({}_{2}\) from the mobile phase is necessary before working in pure water, as it binds to the Lewis acids sites, completely changing the surface character of the stationary phase. Therefore, it is necessary to boil the water before use as a mobile phase, degas it, or use a scrubber or pre-column [113].