Structural Chemistry of Zeolites

Abstract

IntroductionZeolites are tetrahedrally connected framework solids, based on silica, with intricate structures that possess channels and cages large enough to contain extra-framework cations and to permit the uptake and desorption of molecules varying from hydrogen to complex organics up to 1 nm in size. Their crystalline structure directly controls their properties and consequently their performance in applications such as ion exchange, separation, and catalysis, and is therefore of great interest to academics and technologists alike. A ''ball and stick'' representation of the most widely used zeolite A, with tetrahedral Al and Si atoms linked by O atoms and with chargebalancing Na + cations, is given in Figure 7.1.Originally discovered as aluminosilicate minerals, synthetic zeolites with a range of compositions are now widely prepared and subsequently modified for a wide range of applications. Many excellent articles, reviews, and books describe the structures of these solids, often in well-illustrated texts [1-3] and online resources [4]. Here we start by summarizing their structural chemistry, beginning with the key features of the best known and most widely used zeolites A and Y. Besides covering the periodic structures of these solids, other features such as secondary mesoporosity are also important to their performance and are discussed. This is followed by a summary of the structural chemistry of some of the important zeolite types prepared using inorganic and simple organic cations prior to the 1990s.Over the last 20 years there has been a major international effort to prepare new zeolitic materials, in the search for improved adsorbents and catalysts. Much of this has focused on the exploration of the use of complex organic alkylammonium ions, synthesized specifically as potential ''templates,'' giving high-silica-content zeolites or pure silica polytypes. The diversity of structures has also been increased by the inclusion of elements other than Al for Si in the framework, which may be either aliovalent (2+ or 3+) or isovalent (4+), and the search for new materials is encouraged by the tantalizing arrays of hypothetical structures that have been shown to be energetically feasible [5,6]. The remarkable products of this ongoing odyssey continue to show new structural features that are both intriguing and of practical importance or potential: increased crystallographic complexity leading to structures with novel

Kinetic Modeling of Catalytic Olefin Cracking and Methanol-to-Olefins (MTO) over Zeolites: A Review

“…In the meantime, other materials with similar structural features have been included in the definition. All have a common crystalline and tetrahedral structure [135]. Two of their properties are especially important in the context of catalysis.…”

“…Secondly, they contain acid sites which is why they are also called solid acids. Brønsted acidity arises when aliovalent cations such as Al 3+ and Si 4+ are connected via oxygen [135]. The resulting negativity of the framework is balanced by additional cations.…”

“…Within a zeolite crystal, the tetrahedral units represent the primary building units. Their systematic arrangement leads to a block consisting of several tetrahedra which is referred to as a secondary building unit (SBU) [135]. The SBU is characteristic of a certain zeolite because it is found periodically within the framework.…”

“…Another important property is the channel opening which is defined by the number of cations: a ten-membered ring means that the opening is formed by ten cations connected via ten oxygen atoms. Eight-, ten-and twelve-membered rings are classified into small-, medium-and large-pore zeolites, respectively [135]. The Structure Commission of the International Zeolite Association presents an overview of the different zeolites online [141].…”

Core–Shell Crystals of Porous Organic Cages

“…The preparation of new functional porous materials is an important goal in materials chemistry,w ith potential applications in gas storage,m olecular separations,c atalysis,a nd sensing. [1] Established classes of porous materials include extended networks and frameworks such as zeolites, [2] metalorganic frameworks (MOFs), [3] covalent organic frameworks (COFs), [4] and crosslinked polymers. [5] More recently,porous molecular solids have emerged as anew materials platform.…”