Catalytic effect

Due to the involvement of catalysts and accelerate or slow down The rate of chemical reaction The phenomenon is called catalysis. stay catalytic reaction In the catalyst and Reactant The chemical, change the reaction path, thereby reducing the reaction activation energy This is the catalyst can be improved. reaction rate The reason. Such as the chemical reaction of A+B and AB, the activation energy was E, catalyst C, reaction in two steps, the required activation energy were F, G, F, G were less than E.
Catalytic effect

Catalytic effect Catalytic principle

A+C, AC
AC+B, AB+C
The two step activation energy The value is much smaller than E. according to Arrhenius formula K=Ae-E/RT, the catalysts reacted decrease E, so that reaction rate Improve.
There are also some reaction catalyst after the reaction, the activation energy of the E value did not change, but The pre exponential factor A was significantly increased (or interpreted as activation entropy increases), also lead to faster reaction rate.

Catalytic effect Catalysis type

Catalysis can be divided into the following types: 1. Homogeneous catalysis . The catalyst and reactants are in the same phase catalysis, such as homogeneous acid catalysis, homogeneous complex catalysis. Most of the homogeneous catalyst in liquid phase. Homogeneous catalyst The active center Uniform, high selectivity, side reaction Less, but it is difficult to separate the catalyst, recovery and regeneration.
2 Heterogeneous catalysis . The catalytic effect occurred on the interface of. Usually for porous solid catalyst, Reactant For liquid or gas. stay Heterogeneous catalytic reaction In the chemistry of solid catalyst reactant molecules Adsorption And the reactant molecules are activated, reducing reaction activation energy So, reaction rate Speed up. The solid catalyst surface is not uniform, only part of the reactant molecules chemical adsorption , known as the activity center. The catalytic role in industrial production are mostly heterogeneous catalysis.
3 Biocatalysis . Are internal to the organism of enzyme catalytic reaction . The catalysis of enzyme with high selectivity and high catalytic activity , mild reaction conditions, but by the temperature, the pH value of solution, ionic strength The impacts of factors.
The auto catalytic. Self catalysis reaction. In some cases, some products or intermediate reaction with catalytic function, the reaction after a period induction period Back rate Accelerate. Autocatalysis Is there Chemical oscillation One of the necessary conditions.
There are other Electrocatalysis , Photocatalytic , Photoelectrocatalysis Etc.......

Catalytic effect acid-base catalysis

Catalytic effect Summary

English Name: acid-base catalysis
To do is acid-base catalyst. Reactant Molecular and acid-base contact, or adsorbed on the catalyst surface on solid acid-base sites, there will be an acid-base reaction, the formation of active intermediate complexes, then decomposition products, the catalyst recovery.

Catalytic effect category

The homogeneous acid catalytic reaction Mainly has: hydrolysis And hydrate, condensation Esterification, Alkylation And rearrangement (mostly liquid). For example, ethylene In sulfuric acid catalyst for ethanol; Epichlorohydrin In the role of water solution of sodium hydroxide catalyst glycerol Benzene; and Halogenated hydrocarbon stay Alchlor The presence of a catalyst for alkylation Alkylbenzene .
2 Multi phase PH: mainly catalytic reaction olefin Polymerization, catalytic cracking Olefine alkane The isomerization The condensation and addition, Disproportionation (such as solid phase). For example, alkanes in REY molecular sieve And (or) Alusil Catalytic cracking gasoline and C3, benzene and ethylene in C4 gas; Solid phosphoric acid For alkylation ethylbenzene ; propylene stay Nickel sulfate On the solid base (in olefin oligomerization; sodium carbonate or potassium carbonate Two) under the catalysis of poly etc..
Some reactions can be Acid catalysis Can also base catalysis, only product is not exactly the same. For example, aromatic products and single ene reaction in the presence of acid catalyst is added to generate alkyl Aromatic ring On the formation, and the catalysis of alkali to aromatic hydrocarbons Side chain Alkylation . For example, in acid or alkali catalysis, olefin May occur in cis and trans isomerization and double bond Isomerization reaction But, still only in the acid catalyzed isomerization occurs.
The Brnsted acid catalytic reaction
Luis acid catalyzed reaction

Catalytic effect According to the classification of the catalyst

If the catalyst is acid or acid center is Reactant Molecules must have easy to accept proton Or give Electron pair The atoms or groups; if the center for catalyst containing alkali alkali or, then the reactant molecules must be easy to give or accept electrons to protons. In addition, according to the different catalytic reaction In the adsorption of reactant molecules, and the ability of the catalyst surface catalytic activity Also varies with the acid-base properties of the catalyst. Acid base catalysis to acid-base center in specific types, and need a certain range of acid strength. In the acid-base catalysis Homogeneous catalysis or Heterogeneous catalysis In application, the most common is the catalysis of liquid phase catalysis, a very wide application in chemical industry.

Catalytic effect Solid acid-base Center

Solid acid There are two kinds of solid acid center center: Lewis acid The center and Brnsted acid sites, in addition to base center. Lewis as Lewis acid as acid molecules, has not held on track. Electron pair Have high electron affinity. Lewis base A high center energy level There are electronic, and if it is a adsorbed electron acceptor two only on the electronic, energy will be reduced. Brnsted acid sites with the release of a proton The tendency. Solid acid variety, such as liquid contained in the solid acid, Metallic oxide (e.g. alumina , magnesium oxide , Titanium oxide , Zinc oxide ), sulfides (such as Zinc sulfide , Calcium sulfide ) and chloride ( Mercuric chloride , Copper chloride , Aluminium chloride ), mixed oxide (such as alumina Silicon oxide Silicon oxide, Magnesium Oxide), zeolite And metal sulfate and phosphate. The alumina, energy-saving ion zeolite as example, it illustrates their causes of Lewis acid and Brnsted acid and alkali center. Alumina is produced by heating dehydration of hydrous oxide:
this Lewis acid The center of absorbent into Brnsted acid center: