Characteristics and applications of catalysts in petrochemicals
Petrochemical Catalysts
Catalysts are an important product in the catalyst industry, used in the chemical processing of petrochemical products. There are many varieties of these catalysts, mainly oxidation catalysts, hydrogenation catalysts, dehydrogenation catalysts, hydroformylation catalysts, polymerization catalysts, hydration catalysts, dehydration catalysts, alkylation catalysts, isomerization catalysts, disproportionation catalysts, etc. According to the catalytic function, the first five types of catalysts are used in large quantities. Today Seven takes you through the characteristics and applications of these catalysts for your reference!
Oxidation catalysts
The vast majority of petrochemical manufacturing processes containing oxygenated products are selective oxidation processes. Selective oxidation products account for 80% of the total organic chemical products; the catalysts used are required to have high catalytic selectivity in the first place. Selective oxidation catalysts can be divided into gas-solid phase oxidation catalysts and liquid phase oxidation catalysts.
Taking the production of ethylene glycol as an example, the unit cost of oxygen and ethylene accounts for 85-90% of the production cost of ethylene glycol, and the unit cost of both depends mainly on the selectivity of the catalyst. Therefore, the core competition of ethylene glycol plant is the competition of catalyst. High selectivity catalyst not only directly determines the unit cost of raw materials such as ethylene and oxygen, but also generates less by-products and impurities, and higher quality of ethylene glycol and ethylene oxide products.
Gas-solid phase oxidation catalysts
Gas-solid phase oxidation catalysts consist of silicon carbide or α-alumina carrier and vanadium-titanium oxides as the active component, which are divided into the following five categories.
(1) Silver catalysts for ethylene oxidation to ethylene oxide, using silicon carbide or α-alumina as the carrier (with a small amount of barium oxide as a co-catalyst). After continuous improvement of the catalyst and process conditions, the weight yield of ethylene has exceeded 100%.
(2) Catalysts made from vanadium-titanium oxides as the active component, sprayed on silicon carbide or corundum, for the oxidation of phthalic anhydride from o-xylene. The catalyst is made of vanadium-molybdenum oxide active component sprayed on corundum for the oxidation of benzene or butane to maleic anhydride.
Oxidation of o-xylene to phthalic anhydride reaction
These catalysts have been improved by the development of multi-component catalysts, and eight-component catalysts have been developed. The shape of the carrier has also changed from spherical to annular, semi-circular, etc. to facilitate heat transfer. The general trend is to pursue high loading, high yield and high purity of the product.
(3) Oxidation of alcohols to aldehydes or ketones, such as silver-pumice (or alumina), iron oxide-molybdenum oxide and electrolytic silver catalysts for the oxidation of methanol to formaldehyde.
(4) ammonia oxidation catalysts, developed in the 1960s with bismuth-molybdenum-phosphorus composite oxide catalytic component loaded on silicon oxide, on this catalyst pass propylene, ammonia, air, can be one-step synthesis of acrylonitrile.
Synthesis reaction of acrylonitrile
In order to improve selectivity and yield and reduce environmental pollution, this catalyst is being improved continuously, and some new catalysts contain up to 15 kinds of elements.
(5) Oxychlorination catalysts, copper chloride-alumina catalysts were developed in the 1960s, and dichloroethane can be obtained by passing ethylene, hydrogen chloride and air or oxygen in a boiling bed reactor. Dichloroethane is thermally cracked to yield vinyl chloride monomer. This method is advantageous for the development of PVC in areas where electricity is expensive and petrochemicals are developed.
Liquid Phase Oxidation Catalyst
Catalysts used for oxidation of aromatic side chains to aryl acids, such as p-xylene in acetic acid solution with cobalt acetate and a small amount of ammonium bromide heated and oxidized by air to produce terephthalic acid.
Ethylene, propylene oxidation to acetaldehyde, acetone (Wacker method), with a small amount of copper chloride solution containing palladium chloride catalyst, pass olefins, air or oxygen, after a one-step or two-step reaction to obtain the desired oxygen-containing compounds.
Reaction for the production of propylene oxide by the chlorhydrin method
The liquid-phase oxidation catalyst method is very corrosive to the reaction equipment and has been gradually replaced by the organic peroxide method, which is still used only for the preparation of propylene oxide.
Hydrogenation catalysts
These catalysts are used in the production of products and are also widely used in the refining of raw materials and products. They are divided into three categories depending on the hydrogenation situation.
1
Selective hydrogenation catalysts
When petroleum hydrocarbons are cracked to produce ethylene and propylene for polymerization, they must first be selectively hydrogenated to remove trace impurities such as alkynes, dienes, carbon monoxide, carbon dioxide, oxygen, etc., without loss of olefins. The catalyst used is generally palladium, platinum or nickel, cobalt, molybdenum, etc. loaded on alumina. Controlling the amount of active substance, the carrier and the method of catalyst manufacture results in selective hydrogenation catalysts with different properties. Other catalysts such as refining of cracked gasoline and hydroreduction of nitrobenzene to aniline are also used for selective hydrogenation.
2
Non-selective hydrogenation catalysts
Catalysts used for deep hydrogenation to saturated compounds. For example, nickel-alumina catalysts for benzene hydrogenation to cyclohexane, skeletal nickel catalysts for phenol hydrogenation to cyclohexanol and adiponitrile hydrogenation to hexanediamine.
3
Catalysts for hydrogenolysis
The process of hydrogenation of fats and oils to produce high grade alcohols using copper chromite as catalyst.
Dehydrogenation Catalysts
High temperature dehydrogenation catalysts
Iron-chromium oxide-potassium oxide can dehydrogenate ethylbenzene (or n-butene) to styrene (or butadiene) at high temperatures and in the presence of large amounts of water vapor.
Low Temperature Dehydrogenation Catalysis
Since dehydrogenation generally requires high temperatures, reduced pressure or the presence of large amounts of diluent, energy consumption is high. In recent years, oxidative dehydrogenation catalysis at lower temperatures has been developed. For example, polyethylene is produced by oxidative dehydrogenation of butadiene with bismuth-molybdenum metal oxide catalysts.
Hydroformylation catalysts
These catalysts are the first complexation catalysts used in industrial production. Olefins are reacted with synthesis gas (CO+H2) in the presence of the catalyst to produce aldehydes with one more carbon atom. For example, propionaldehyde and butyraldehyde are produced from ethylene and propylene by hydroformylation (commonly known as carbonyl synthesis).
Various transition metal carbonyl complexes have catalytic effect on the hydroformylation reaction. However, only the carbonyl complexes of cobalt and rhodium are used in industrial production. The hydroformylation process used to use carbonyl cobalt complexes as catalysts under high temperature and pressure in the liquid phase. In recent years, the reaction pressure has been reduced from 20 MPa to 5 MPa by using carbonyl-rhodium-phosphine complex catalysts, and the selectivity of ortho-formaldehyde has been improved, energy has been saved, and the cost has been reduced.
At present, we are studying the recovery method of rhodium and searching for other inexpensive and efficient catalysts instead of rhodium, and studying loaded complex catalysts to simplify the separation process.
Polymerization catalysts
Polyethylene is mainly divided into two types: low density and high density.
Polyethylene made from Ziegler-type catalysts
In the past, the former is mostly produced by high-pressure method (100-300 MPa), using oxygen and organic peroxide as catalysts. The latter is mostly produced by medium-pressure method or low-pressure method, which uses chromium-molybdenum oxide loaded on silica-alumina gel as catalyst, while the low-pressure method uses Ziegler-type catalysts (represented by titanium tetrachloride and triethylaluminum system) for polymerization at low temperature and low pressure.
In recent years, new high efficiency catalysts have been developed, although each plant has its own unique new catalysts, but mostly with magnesium compounds as the carrier of the titanium-aluminum system catalyst, which has reached the level of hundreds of thousands of grams of polyethylene per gram of titanium, because the residual catalyst in the polymer is very little, the polymer can be free of purification treatment, reducing costs. In addition, processes have been developed to produce linear low density polyethylene at low pressures.
Loaded titanium-aluminum systems with high efficiency catalysts have also been developed for polypropylene production, yielding more than 1000 kg of polypropylene per gram of titanium.
Hydration catalysts
Hydration is a reaction process in which water is combined with a molecule of another substance to become one molecule. The water molecule with its hydrogen and hydroxyl group and the unsaturated bond of the substance molecule add up to generate new compounds, and the substance that plays a catalytic role in this process is called hydration catalyst, which is used in organic chemical production.
Reaction of ethylene hydration to ethanol
Catalysts used in the industrial production of ethylene to ethanol
The hydration process is one of the organic synthesis methods, but as an important production method, it is still limited to a few types of products, such as ethanol and diols.
Catalysts for dehydration
Dehydration can take place under the action of heat or catalyst, or in reaction with a dehydrating agent. The dehydration reaction is the reverse of the hydration reaction and is usually a heat-absorbing reaction, generally at high temperature and low pressure. In addition, most of the dehydration process must be carried out in the presence of a catalyst. The catalysts used in the hydration process – acid catalysts are also suitable for dehydration, commonly used are sulfuric acid, phosphoric acid, aluminum trioxide, etc.
Dehydration process of alcohols
(1) The reaction of dehydration of ethanol to ethylene
Sulfuric acid or γ-alumina is used as the catalyst.
(2) The reaction of dehydration of butanol to olefin
Different catalysts, different main products, catalysts with very high selectivity.
Alkylation catalyst
Alkylation is the process of transferring alkyl groups from one molecule to another. It is a reaction in which alkyl groups (methyl, ethyl, etc.) are introduced into the molecule of a compound. Alkylating agents commonly used in industry are olefins, haloalkanes, alkyl sulfates, etc.
In a standard refining process, the alkylation system combines low molecular weight olefins (consisting mainly of propylene and butene) with isobutane in the presence of a catalyst (sulfonic acid or hydrofluoric acid) to form alkylates (consisting mainly of higher octane, side chain alkanes).
Alkylation reactions can be divided into two types: thermal alkylation and catalytic alkylation.
Due to the high temperature of thermal alkylation reaction, it is easy to produce pyrolysis and other side reactions, so the catalytic alkylation method is used in industry. The main catalytic alkylations are
①Alkylation of alkanes, such as the use of isobutylene to alkylate isobutane to obtain high-octane gasoline components.
②Alkylation of aromatic hydrocarbons, such as the alkylation of benzene with ethylene.
Anhydrous aluminum trichloride – hydrogen chloride catalyst for the reaction of benzene and ethylene to produce ethylbenzene
(iii) Alkylation of phenols, such as alkylation of p-cresol with isobutene.
Since both sulfuric acid and hydrofluoric acid are highly acidic, the corrosion of equipment is quite serious. Therefore, from the perspective of safe production and environmental protection, both catalysts are not ideal catalysts. Solid super acid is more studied as alkylation catalyst, but it has not reached the stage of industrial application so far.
Isomerization catalysts
The action or process by which one isomer is interconverted with another isomer. The process of changing the structure of a compound without altering its composition or molecular weight. Generally refers to a change in the position of an atom or group in the molecule of an organic compound. It is often carried out in the presence of a catalyst.
The main types of catalysts are as follows.
①Fred-Kleifert type catalysts, commonly used are aluminum trichloride – hydrogen chloride, boron fluoride – hydrogen fluoride, etc. These catalysts have high activity and require low reaction temperature, and are used for liquid phase isomerization, such as isomerization of n-butane to isobutane, isomerization of xylene, etc.
②Precious metal catalysts with solid acids as carriers, such as platinum-alumina, platinum-molecular sieve, palladium
-alumina, etc. These catalysts are bifunctional catalysts, in which the metal component plays the role of hydrogenation
These catalysts are bifunctional, in which the metal component plays the role of hydrogenation and dehydrogenation, and the solid acid plays the role of isomerization.
When these catalysts are used, the reaction has to be carried out in the presence of hydrogen, so they are also called pro-hydrogen isomerization catalysts, which are used for gas phase isomerization. Isomerization of alkanes, olefins, aromatics and cycloalkanes can also be used. Especially, the isomerization of ethylbenzene into xylene and cycloalkanes is only effective with this type of catalyst. The advantages are less coking and long service life.
③Non-precious metal catalysts with solid acid carriers, such as nickel-molecular sieves, which generally also require the presence of hydrogen, are used for gas-phase isomerization, but cannot isomerize ethylbenzene into xylene.
④ZSM-5 molecular sieve catalysts, mainly used for gas-phase or liquid-phase isomerization of xylenes.
Disproportionation catalyst
Disproportionation is one of the most important methods to regulate the supply and demand of hydrocarbons in industry, since the application of disproportionation process can transform one hydrocarbon into two different hydrocarbons. The most important applications are the disproportionation of toluene to produce additional xylenes and simultaneously produce high purity benzene, and the disproportionation of propylene to produce polymeric grade ethylene and high purity butenes (mainly cis and trans-2-butene) in a triolefin process.
Toluene disproportionation reaction
The conversion of toluene to benzene and xylene is generally carried out using silica-alumina catalysts, and the more popular catalysts studied are molecular sieve catalysts, such as filamentous zeolite-type filamentous molecular sieves.
