Zeolite (In)Stability under Aqueous or Steaming Conditions

Abstract

zeolites, modification of zeolite acidity for use as fluid catalytic cracking (FCC) catalysts or even for the preparation of novel zeolite frameworks. [1] Considering the significance of zeolite applications at the industrial level, a thorough understanding of zeolite (in)stability under aqueous conditions has been identified as a very important problem in catalysis and zeolite science. Zeolite (in)stability in water or under steaming conditions has been investigated by a number of experimental techniques, in particular magic angle spinning (MAS) NMR, IR, powder X-ray diffraction (PXRD), calorimetry and adsorption. Experimental studies have often been augmented by computational modeling and a large number of relevant theoretical papers can be found in the literature. While the main focus of this review is on reactive interactions of water with zeolites, it is also important to review the most important observations obtained for nonreactive water adsorption in zeolites. It is not surprising that all-silica zeolites, which exhibit negligible water uptake are stable even under conditions where Alcontaining zeolites lose crystallinity: the effective framework hydrolysis can only take place when sufficient water is present in the zeolite channel system. The amount of water inside the zeolite channels depends critically on the concentration of heteroatoms and/or framework defects, such as silanol nests, which show much higher affinity toward water than hydrophobic SiOSi bridges. Zeolite hydrophobicity increases with increasing Si/Al ratio and incomplete pore filling by water is commonly observed for zeolites with high Si/Al at standard pressure. It has been suggested already in the 1950s by Young that water can only adsorb on surface silanol groups, while the parts of the silica surfaces formed by SiOSi bridges are hydrophobic. [2] Both water adsorption in zeolites and zeolite (in)stability under aqueous conditions depend on the zeolite composition (Si/Al ratio, charge-compensating cations, and defect concentration in particular). Our primary goal is to review the most critical aspects of zeolite (in)stability for a broad range of temperature and partial water pressure (p 0), focusing on the framework zeolite composition (Si/Al ratio and presence of Ge, Sn, and Ti heteroatoms) and defect concentration. Zeolites (in)stability in water is discussed for increasing extent of framework degradation, starting from the nonreactive interaction with water, reversible hydrolysis of TO bonds, mild zeolite demetallation, mesopore formation, and total amorphization (Figure 1). Zeolites are among the most environmentally friendly materials produced industrially at the Megaton scale. They find numerous commercial applications, particularly in catalysis, adsorption, and separation. Under ambient conditions aluminosilicate zeolites are stable when exposed to water or water vapor. However, at extreme conditions as high temperature, high water vapor pressure or increased acidity/basicity, their crystalline framework can be destroyed....

The Role of Water Loading and Germanium Content in Germanosilicate Hydrolysis

“…Similar vicinal mechanisms have been reported earlier for aluminosilicate zeolites by Malola et al, 17,52 albeit with much higher reaction barriers when forming either vicinal disilanols or vicinal aluminol and silanol pairs. From the vicinal product, the hydrolysis can proceed via the standard equatorialinversion mechanism (Figure S14b), but the overall effective barriers (70 kJ/mol) are lower and the reaction energies (-49 kJ/mol) more favorable than in Ge-poor UTL.…”

“…We analyzed the mechanism of Ge-O bond hydrolysis for three different locations of Ge atoms, always considering all four adjacent framework oxygen atoms (Figure S12). Hydrolysis starts via an equatorial step (Figure 4), as previously described in the literature for the hydrolysis of Si-O bonds, 17,[48][49][50][51] with reaction barriers of at least 65 kJ/mol. The reaction product is an unstable Q 3 Ge species (Figure 5).…”

“…However, water-water interactions are much stronger than water-(pristine)-zeolite interactions (see also section 3.2.1). 17,46 Therefore, water should form clusters rather than remain homogeneously distributed throughout the channel system, thus leading to heterogenous water density in pristine Ge-UTL samples in which the water density may locally cross the threshold necessary for (collective) mechanisms. The extent of irreversibility, i.e., how many waters are irreversibly bound to the framework, is positively correlated with the germanium content.…”

17O NMR spectroscopy of crystalline microporous materials

“…It was suggested that this process could aid the regeneration (de-coking) of the zeolite during de-NO x conversion. The reactivity of zeolites with H 2 O (both in liquid or gaseous form) has long been of interest, as it has important implications for the structural integrity of these systems in a variety of industrial processes 112. In recent work using17 O NMR spectroscopy39,40 the synthesis of new zeolites via the ADOR 40,41 process was investigated.…”

Reverse ADOR: reconstruction of UTL zeolite from layered IPC-1P

“…12,13 The ADOR process exploits labile Ge-rich double-four ring (D4R) units in germanosilicates, such as UTL or *CTH. 14,15 The structure of these zeolites consists of Si-rich layers connected by Ge-rich D4Rs. Upon selective hydrolysis of D4Rs, layered materials are formed, thus preserving their original structure.…”