Zeolites

When stones hold potential in their voids

Zeolites briefly

Zeolites – boiling stones – were discovered in 1756 by the mineralogist Cronstedt who observed by heating a mixture of stilbite and stellerite, the first two natural zeolites, the release of water vapor ; even though natural zeolites were used in ancient times to soften water in wells. Defined as crystalline microporous aluminosilicates, the zeolite family has expanded today to encompass other crystallized microporous solids known as zeotypes. The latter are synthesized using other chemical elements such as gallium, phosphorus, and germanium. It took more than 100 years for the first synthesis of zeolite (Levynite) to be carried out by Ste-Claire de Ville. 80 years later (in the 1940s) the hydrothermal synthesis of zeolites was initiated by Barrer who is considered the founding father of zeolite synthesis. Then, it was the teams of Milton, Breck, and Flanigen who, among others, developed the industrial applications around the 1950s by synthesizing the zeolites LTA (A), FAU (X and Y), and MFI. Today, more than 250 different zeolite structures have been listed on the website of the international zeolite association (IZA) using a three-letter code for each topology. Although the first zeolite was synthesized 160 years ago, this area of research continues to grow exponentially. The most important zeolites topologies used nowadays were first synthesized between 1940 and 1970, 1948: Mordenite (MOR) and Chabazite (CHA), 1949: Linde Type A (LTA) and Faujasite (FAU) and 1967: Beta (BEA) and Mobil Five (MFI). Note that Clinoptilolite (CLI) is the most prevalent natural zeolite. We have to note the introduction of the zeotype family by Wilson in 1982. Alumunuphosphates (AlPOs) and silico-aluminophospahtes (SAPOs) present very important features as well.

Structure of zeolites

The structure of zeolites corresponds to an open periodic three-dimensional sequence of TO4 tetrahedra where the T element is usually silicon or another hetero-element like aluminium, Titanium, etc. Their framework has microporous channels and cavities (diameter < 20 Å according to the IUPAC) periodically distributed in space and of perfectly controlled dimension. The Si/Al ratio divides the zeolites between those called "high silica zeolites" for a ratio ranging from 10 to 100 and "Aluminium rich zeolites" for a ratio between 1 and 5. Note that Si/Al cannot be lower than 1, Al-O-Al bridges are forbidden by Lowenstein rule. The diversity of zeolite topologies and structures comes from the large number of Secondary Building Units (SBUs) formed by TO4 tetrahedra known to be the primary building units (PBUs).

The cages of zeolites were classified according to the pore size expressed in number of Membered Rings (MR). Thus, zeolites can be distinguished as: (i) 'small pores' (8 MR (3 – 4.5 Å)), (ii) 'medium pores' (10 MR (4.5 – 6.0 Å)), (iii) 'large pores' (12 MR (6.0 – 8.0 Å)). The nature and flexibility of the cations structuring the framework, are very important parameters that play a decisive role during the synthesis. These latter are structuring the inorganic network by occupying the inner space of zeolite pores that adapt more or less to their geometry. Thus, they are called 'structure directing agents' or SDA. However, a single SDA can lead to several zeolite topologies and a single topology can be obtained using several structuring agents.

New trends continue to emerge aiming to control and improve the catalytic properties of zeolites. One example is the work devoted to the acceleration of diffusion kinetics by reducing crystallite sizes to a nanometric size or by promoting a certain spatial orientation compared to others during the growth phase, hence the name nanozeolites. Being solid catalysts, their selectivity in the reactions of interest, where they involve, is also the subject of intense research. It has been recognized that this is highly dependent on the location or distribution of active sites within the siliceous network, thus the importance of spectroscopic techniques able to probe the atomic ordering like NMR spectroscopy.

Hydrothermal treatment

The hydrothermal synthesis of zeolites consists of heating at high temperature (between 80 and 200 °C), under autogenous pressure of water vapor, a mixture composed of inorganic (alkaline or alkaline earth cations) or organic (amines, quaternary amines, alcohols, etc.) structuring agents, mineralizing agents (OH-, F-), silicon precursors and / or other chemical elements dissolved in an aqueous solution. The mechanisms of formation are complex, presenting different stages of dissolution and condensation simultaneously. While there is no single mechanism describing the formation of zeolites, several models have been proposed. Their crystallization takes place in three consecutive stages i.e. induction, nucleation and growth. Induction is the period that separates the time of onset of the reaction from the time when the first construction units are observed. Nucleation is the process of the genesis of the first crystallographic meshes, while the final crystals are obtained at the end of growth. Among the questions that have been debated in the scientific community, remains the place of nucleation: in the liquid phase, in the solid phase or at the solid-liquid interface.

Properties and applications

Particular properties due to the topology, the presence of hetero-elements, and extrframework elements, induce a wide variety of applications such as ion exchange, separation, adsorption, and catalysis. The use of zeolites in catalysis is related to the presence of active sites. Those sites result from the insertion of various types of hetero-elements (Al, B, Ga, Ti...). The size of pores and their topology are also essential criteria for a zeolite catalyst, acidity and hydrophobicity are also criteria that play an essential role.

Processes where zeolites are involved could be divided into three main categories including more or less carbon: 

• C-zero- water treatments

• C-one+ gas treatments

• C-two+ hydrocarbon treatments

Among the processes where the most important zeolites are involved:

• MOR Hydro-isomerization of light alkanes, hydrocracking, aromatic alkylation, olefin oligomerization.

• CHA NOx reduction, gas capture, methanol valorization.

• LTA gas purification, gas storage.

• FAU Catalytic cracking, hydrocracking, aromatic alkylation, acetylation.

• BEA Benzene alkylation, aliphatic alkylation, acetylation, etherification.

• MFI Dewaxing, methanol to olefins, hydrocracking, olefins and xylene isomerization, oxydation.

• CLI Ion exchange, wastewater treatment.

Catalysis - a combination of Energies 

The term catalysis, proposed in 1835 by Berzelius, comes from the Greek words kata meaning down and lyein meaning loosen. Berzelius wrote that by the term catalysis he meant “the property of exerting on other bodies an action which is very different from chemical affinity. By means of this action, they produce decomposition in bodies, and form new compounds into the composition of which they do not enter”.

Thermodynamics gives the energy balance between reactants and products |Ereactant – Eproduct| on the free energy axis. Once the reactants and products selected, DG is a constant quantity. To displace the thermodynamic equilibrium, in addition to T and P, separation plays an important role (Le Chatelier principle). [DG = DH – TDS = -RTlnK] (K is the equilibrium constant).

Kinetics translates the energy needed to go from reactants to products through an intermediate state. The less we need energy, the faster the reaction, this is the role of a catalyst:to reduce Ea and accelerate the reaction events. Ea is the energy to overcome to go from reactant to products through intermediate states. [Ea = -RTlnk ] (k is the rate constant).

Quantum physics explains why transition metals are efficient catalysts. Their variable oxidation states allow them to lend and take electrons from molecules. The d-band model is an approximate description of the electronic interaction between adsorbed molecules and transition metal surfaces.

To bring the reactants and the catalysts together, we need a fine-tuning of the adsorption and desorption processes i.e. physisorption and chemisorption energies and diffusion rates. In addition to electrical-level interactions, sorption phenomena explain why a specific metal acts as an active site for a specific reaction. Another important parameter is diffusion mechanisms i.e. internal and external. This diffusion is highly important and needs to be controlled for a better selectivity. Considering all these parameters, the importance of zeolites pores can be understood in a better way.

The definition of catalysts evolved with time

• 1836, Berzelius - Catalysts are substances that promote the conversion of the component parts of the body they influence into other states, without necessarily participating in the process with their own component parts; the body effecting the change does not take part in the reaction and remains unaltered through the reaction.

• 1894, Ostwald - Catalyst do not alter the equilibrium between the reaction and product molecules, but they enhance or suppress the rate of their reaction.

• 1913, Sabatier - In a catalytic reaction, reacting molecules form intermediate complexes with the catalyst’s surface. These complexes should be of intermediate stability. If they are too stable, they will not decompose to achieve product formation. If they are too unstable, reagent molecules will not be activated and surface reaction intermediate complexes will not be formed.

The importance of catalysis and zeolites in the modern era goes beyond chemical industry and economy. Indeed, the occurrence of chemical reactions in the pores of zeolites brings together a considerable amount of knowledge developed by humanity. The industrial revolution happened after the development of thermodynamics and kinetics concepts by knowledgeable scientists who succeeded to manipulate heat and produce energy. Meanwhile, Boltzmann succeeded to prove the existence of energy levels and particles with statistical thermodynamics, paving the way for the theory of quantum physics. This revolution allowed the understanding of the behavior of subatomic particles e.g. electrons by another family of scientists. It is enough to consider an acid catalyzed reaction occurring at high temperature and pressure in a pore of zeolite to see the global nature of interactions involving thermodynamics, reaction kinetics, diffusion, adsorption, desorption processes, and electronic interactions to understand the complexity and the span of catalysis. Then, catalysis in zeolites is nothing than the summum of human knowledge of mater, accumulated along the last 500 years. 

The importance of catalysis lies in its gathering of inter-alia thermodynamics, kinetics, adsorption, and diffusion concepts together with quantum, mineral, and organic chemistry. This mixture of expertises combined with the impact of catalysis on economy and society make it difficult to grasp and control. We note that catalysis controls a considerable amount of Energy worldwide. In addition to Solvay and Bayer processes, three are very important in the last century.

• 1911 Haber-Bosch process for industrial production of ammonia.

• 1924 Fischer-Tropsch process for the conversion of syngas into hydrocarbons.

• 1940 Campbell-Martin-Murphree-Tyson for Fluid Catalytic Cracking (FCC) process for the conversion of crude oil into gasoline.

While these processes concern catalysis the most, other chemical industries of high importance may be added here e.g. tires, polymers, steel, textile, metals, alloys, ceramics, paints, glass, cement, paper, detergents.  They are all inter-connected and intimately concerned by the question of Energy.

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