Heterogeneous Catalysis

In chemistry, Heterogeneous Catalysis is catalysis where the phase of the catalyst is different from that of the reactants [1] or products . This process is in contrast to homogeneous catalysis where the reactant, product and catalyst are present in a single step. The phase differentiates not only between solid , liquid , and gas components, but also between immiscible mixtures (such as oil and water ), or anywhere an interface exists. Catalysts are useful because they increase the rate of the reaction [2] without being themselves consumed and are therefore reusable.

Heterogeneous catalysts usually involve solid phase catalysts and gas phase reactants. [3] In this case, a cycle of molecular adsorption, reaction and desorption occurs on the catalyst surface. Thermodynamics, mass transfer and heat transfer affect the rate (kinetics) of the reaction .

Heterogeneous catalysis is very important as it enables fast, large-scale production and selective product manufacturing. [4] About 35% of the world’s GDP is affected by catalysis. [5] The production of 90% of chemicals (by volume) is facilitated by solid catalysts. [3] The chemical and energy industries rely heavily on heterogeneous catalysis. For example, the Haber–Bosch process uses a metal-based catalyst in the synthesis of ammonia , which is an important component in fertilizer; In 2016, 144 million tonnes of ammonia were produced.

Adsorption

Adsorption is an essential step in heterogeneous catalysis. Adsorption is the process by which a gas (or solution) phase molecule (adsorbent) binds to solid (or liquid) surface atoms (adsorbents). The reverse of adsorption is desorption , the division of adsorption by adsorption. In a reaction facilitated by heterogeneous catalysis, the catalyst is the adsorbent and the reactants are the adsorbents.

Type of adsorption

Two types of adsorption are recognized: physical adsorption, weakly bound adsorption, and chemistry , strongly bound adsorption. Many processes in heterogeneous catalysis occur between the two extremes. The Lennard Jones model provides a basic framework for predicting molecular interactions as a function of atomic separation.

physical adsorption

In physical adsorption, a molecule is attracted to surface atoms through van der Waals forces . These include dipole–dipole interactions, induced dipole interactions, and the London dispersion force. Note that no chemical bonds are formed between the adsorbent and the adsorbent, and their electronic state remains relatively unaffected. The specific energy for physical adsorption ranges from 3 to 10 kcal/mol. [3] In heterogeneous catalysis, when a reactant molecule physisorbs a catalyst, it is usually said to be in a precursor state, an intermediate energy state before chemisorption, a more strongly bound adsorption. [7] From the preceding state, a molecule can undergo either chemical adsorption, desorption, or migration to the surface. [8]The nature of the precursor state may influence the reaction dynamics.

chemistry

When a molecule gets so close to surface atoms that their electron clouds overlap, chemiluminescence can occur. In chemical adsorption, the adsorbent and the adsorbent share electrons which shows the formation of chemical bonds . The specific energy for chemistry ranges from 20 to 100 kcal/mol. [3] There are two cases of chemical adsorption:

• Molecular Adsorption: The adsorption remains intact. An example is alkene binding by platinum.
• Dissociation Adsorption: With adsorption, one or more bonds break. In this case, the impedance of dissociation affects the rate of adsorption. An example of this is the binding of H2 to a metal catalyst , where the H-H bond is broken upon adsorption.

surface reactions

Most metal surface reactions occur by chain diffusion in which catalytic intermediates are cyclically produced and consumed. [9] Two main mechanisms can be described for surface reactions for A + B → C.

• Langmuir–Hinselwood Mechanism: The reactant molecules, A and B, both adsorb on the catalyst surface. While adsorbing to the surface, they combine to form product C, which then denatures.
• Eli-Riddle Mechanism: A reactive molecule, A, is adsorbed on the catalyst surface. Without adsorbing, B reacts with adsorbed A to form C, which then descends from the surface.

The most asymmetrically catalyzed reactions are described by the Langmuir–Hinschelwood model.

In heterogeneous catalysis, the reactants diffuse from the bulk liquid state to the catalyst surface from adsorption . The adsorption site is not always an active catalytic site, so the reactant molecules must move to an active site on the surface. At the active site, the reactant molecules will react to form the product molecule(s) by following a more energetically accessible path through the catalytic intermediate (see figure at right). The product molecules then descend from the surface and disperse away. The catalyst itself remains intact and free to mediate further reactions. Transport phenomena such as heat and mass transfer also play a role in the observed reaction rate.

trigger design

Catalysts are not reactive towards reactants over their entire surface; Catalytic activity occurs only in specific locations, called active sites . The surface area of ​​a solid catalyst has a strong effect on the number of active sites available. In industrial practice, solid catalysts are often porous to maximize surface area, typically achieving 50–400 m2 / g. [3] Some mesoporous silicates, such as MCM-41, have surface areas in excess of 1000 m2 / g. [11] Porous materials are cost-effective due to their high surface area-to-mass ratio and enhanced catalytic activity.

In many cases, a solid catalyst is spread over a supporting material to increase the surface area (spread out the number of active sites) and provide stability. [3] Catalyst supports are usually inert, high melting point materials, but they can also be catalysts themselves. Most catalyst supports are porous (often carbon, silica, zeolite, or alumina-based) [5] and are chosen for their high surface area-to-mass ratio. For a given reaction, porous supports must be selected so that the reactants and products can enter and exit the material.

Often, substances are intentionally added to the reaction feed or catalyst to affect catalytic activity, selectivity and/or stability. These compounds are called promoters. _{For example, alumina (Al 2} O ₃ ) is added during ammonia synthesis to provide greater stability by slowing down the sintering processes on the Fe-catalyst .

The Sabatier principle can be considered one of the cornerstones of the modern theory of catalysis. ^[12] Subtear theory states that the surface-adsorption interaction should be an optimal amount: not too weak to be inert towards reactants and not too strong to poison the surface and avoid absorption of products. ^[13] The statement that the surface-absorbent interaction must be an optimum is a qualitative one. Usually the number of adsorbates and transition states associated with a chemical reaction is a large number, thus the optimum has to be found in many-dimensional space. Catalyst design is not a computationally feasible task in such a multi-dimensional space. Additionally, such an optimization process would not be intuitive. Scaling relations are used to reduce the dimensionality of the space of the catalyst design.^[14] Such relationships are correlations between adsorbates binding energies (or between adsorbate binding energies and transition states also known as BEP relationships) ^[15] which are “similar enough”, e.g., OH versus OOH scaling. ^[16] Applying scaling relations to catalytic design problems greatly reduces the dimensionality of the space (sometimes as small as 1 or 2). ^[17] Micro-kinetic modeling based on such scaling relationships can also be used to take into account the kinetics associated with the adsorption, reactivity, and desorption of molecules under specific pressure or temperature conditions. ^[18]Such modeling then leads to well-known volcano-plots at which the optimum qualitatively described by the subtear theory is known as the “top of a volcano”. Scaling relations can be used to link not only the energetics of radical surface-adsorbing groups (e.g., O*, OH*), ^[14] but also the energetics of closed-shell molecules to each other or equivalently. Linking to radical adsorbates can also be done. , ^[19] A recent challenge for researchers in catalytic science has been to “break” the scaling relationship. ^[20]The correlations that appear in scaling relationships limit the catalyst design space, preventing one from reaching the “top of the volcano”. Breaking scaling relationships can refer to either designing surfaces or motifs that do not obey a scaling relationship, or that follow a different scaling relationship (compared to the general relation for related adsorbates) in the right direction: a Which may bring us closer to the top of the reactive volcano. ^[17] In addition to studying catalytic reactivity, scaling relationships can be used to study and screen materials for selectivity for a particular product. ^[21]There are particular combinations of binding energies that favor specific products over others. Sometimes a set of binding energies that can change the selectivity of a specific product “scale” with each other, thus breaking certain scaling relationships to improve selectivity; An example of this is the scaling between methane and methanol oxidative activation energies that leads to a lack of selectivity in the direct conversion of methane to methanol. 

catalytic deactivation

Catalytic inactivation is defined as the loss in catalytic activity and/or selectivity over time.

The substances which reduce the reaction rate are called poisons . The venom moves the chemosorbent to the surface of the catalyst and reduces the number of active sites available to bind the reactant molecules. ^[23] Common poisons include group V, VI, and VII elements (such as S, O, P, Cl), some toxic metals (such as As, Pb), and adsorbent species with multiple bonds (such as CO, unsaturated hydrocarbons). Are included. ^[7] [23] For example, sulfur inhibits the production of methanol by poisoning the Cu/ZnO catalyst. ^[24] The substances which increase the reaction rate are called promoters . For example, in ammonia synthesis from the presence of alkali metals _N2The rate of separation increases. 

The presence of the venom and promoter can alter the activation energy of the rate-limiting step and affect the selectivity of the catalyst to form certain products. Depending on the quantity, a substance can be favorable or unfavorable for a chemical process. For example, in the production of ethylene, a small amount of chemisorbed chlorine will act as a promoter by improving the Ag-catalyst selectivity toward ethylene over Co ₂ , while too much chlorine will act as a poison. ^[7]

Other mechanisms of catalytic inactivation include:

• Sintering: When heated, the dispersed catalyst metal particles can migrate to the support surface and form crystals. This results in a reduction in the catalyst surface area.
• Fouling: The deposition of material from the liquid phase onto the solid phase catalyst and/or support surfaces. This results in clogging of the active site and/or the pore.
• Coking: The deposition of heavy, carbon-rich solids on surfaces due to the decomposition of hydrocarbons ^[23]
• Vapor-solid reactions: Formation of an inert surface layer and/or formation of a volatile compound that precipitates out of the reactor. ^[23] This results in a loss of surface area and/or catalyst material.
• Solid-State Transformation: Solid-state diffusion of the catalyst supports atoms on the surface, followed by a reaction that forms an inactive phase. This results in a loss of catalyst surface area.
• Erosion: The continuous decay of catalyst materials common in liquid-bed reactors. ^[25] This results in a loss of the catalyst material.

In industry, the cost of catalytic deactivation due to process shutdown and catalyst replacement is in the billions each year. ^[23]

industrial example

In industry, many design variables must be considered, including reactor and catalyst design at many scales ranging from subnanometers to tens of meters. Traditional heterogeneous catalysis reactors include batch, continuous, and fluidized-bed reactors, while more recent setups include fixed-bed, microchannel, and multi-functional reactors. ^[7] Other variables to consider are reactor dimensions, surface area, catalyst type, catalyst support, as well as reactor operating conditions such as temperature, pressure, and reactant concentration.

Some large-scale industrial processes involving heterogeneous catalysts are listed below.

other examples

• Raney nickel catalyst and hydrogen ammonia with phenethylamine in the synthesis of reduction nitriles :

• Cracking, isomerization and reformation of hydrocarbons to produce suitable and useful mixtures of petrol.
• In automobiles, catalytic converters are used to catalyze three main reactions:
  • Oxidation of carbon monoxide to carbon dioxide:2CO(g) + O ₂ (g) → 2CO ₂ (g)
  • Reduction of nitrogen monoxide back to nitrogen :2NO(g) + 2CO(g) → N ₂ (g) + 2CO ₂ (g)
  • Oxidation of hydrocarbons to water and carbon dioxide :2 C ₆ H ₆ + 15 O ₂ → 12 CO ₂ + 6 H ₂ O
• This process can happen with any hydrocarbon, but is mostly done with petrol or diesel.
• Asymmetric heteroatomic catalysis facilitates the production of pure enantiomer compounds using chiral heteromeric catalysts. ^[27]
• Most heterogeneous catalysts are based on metals ^[28] or metal oxides; ^[29] [30] However, some chemical reactions can be catalyzed by carbon-based materials, eg, oxidative dehydrogenation ^[31] or by selective oxidation. ^[32]
  • Ethylbenzene + 1/2 O ₂ → Styrene + H ₂ O
  • acrolein + 1/2 O ₂ → acrylic acid

solid-liquid and liquid-liquid catalyzed reactions

Although the majority of heterogeneous catalysts are solid, there are some differences that are of practical importance. For two immiscible solutions (liquid) one carries the catalyst while the other carries the reactant. This set-up is the basis for biphasic catalysis as implemented in the industrial production of butyraldehyde by hydroformylation of propylene. ^[33]