Highlighting the composition of each type of catalyst
Structure, catalytic behavior and catalytic mechanism. Five major types of catalysts are included: solid acid-base catalysts, molecular sieve catalysts, metal catalysts, metal oxide and metal sulfide catalysts, and complexed catalysts. The focus is on the basic knowledge, basic concepts, typical representatives, industrial applications and the latest progress of each type of catalysts.
I. Acid-Base Catalysts
Petroleum refining and petrochemical industry is the largest application field of catalysts, which occupies an important position in the national economy. In petroleum refining and petrochemical industry, acid catalysts occupy an important position. The catalytic cracking of hydrocarbons, alkylation of aromatics and olefins, zwitterionization, copolymerization and polymerization of olefins and diolefins, hydration of olefins to make alcohols and catalytic dehydration of alcohols and other reactions are carried out under the action of acid catalysts. Most of the acid catalysts used in industry are solid. since 1960s, some new types of solid acid catalysts have been found, among which the most influential ones are molecular sieve type catalysts, followed by sulfate type acid catalysts.
1、Definition and classification of solid acids and bases
Solid acid: generally regarded as a solid that can chemisorb alkali, can also be understood as a solid that can make the alkaline indicator change color on it. Solid acids are further divided into Bronsted (Brφnsted) acid and Lewis (Lewis) acid. The former is referred to as B acid, the latter is referred to as L acid. B acid B base is defined as: can give protons are acids, can accept protons are bases, so B acid B base is also known as proton acid base. L acid L base is defined as: can accept electron pairs are acids, can give electron pairs are bases, so L acid L base is also known as non-protonic acid base.
2, solid acid-base strength and acid-base quantity
B acid strength, is the ability to give protons; L acid strength is the ability to accept electron pairs. Acid strength is usually expressed by the Hammeett function H0, defined as follows:
If a solid acid surface is capable of adsorbing an undissociated base and transforming it into the corresponding conjugate acid, and the transformation is by means of proton transfer from the surface of the solid acid to the adsorbed base, i.e., Eqs. [B]a and [BH+]a are the concentrations of the undissociated base (base indicator) and the conjugate acid, respectively. pKa is the negative logarithm of the equilibrium constant for the dissociation of BH+ from the conjugate acid and is analogous to pH. If the transformation is by means of the adsorbed base’s electron pair moves to the surface of the solid acid, i.e.
where [A:B] is the concentration of the complex AB formed by the adsorbed base B and the electron pair acceptor A. The smaller the H0 the stronger the acidity.
Acid quantity: the amount of acid on a solid surface, usually expressed as the number of millimoles of acid sites per unit weight or per unit surface area, i.e., m mol/wt or m mol/m2. acid quantity, also known as acidity, refers to the concentration of acid.
The strength of a solid base is defined as the ability of an acid adsorbed on a surface to be converted to a conjugate base, and also as the ability of a surface to give electrons for an adsorbed acid. The amount of base expressed in terms of millimoles of base per unit weight or per unit surface area, i.e., m mol/wt or m mol/m2. The amount of base, also called alkalinity, refers to the concentration of the base center.
Acid-base pair synergistic sites: Certain reactions, known to be catalyzed by acid sites on the surface of the catalyst, are more or less synergistic with the base sites. Catalysts with such acid-base pair synergistic sites sometimes show better activity, even if their acid-base strength is lower than that of the individual acid or base sites. ZrO2, for example, is a weak acid and and a weak base, but the activity of splitting C-H bonds is higher than that of the more acidic SiO2-Al2O3 and higher than that of the more basic MgO. This synergistic action of the acid and base sites is advantageous for certain specific reactions and thus provides higher selectivity. These catalysts are called acid-base bifunctional catalysts.
Solid Ultra-strong Acids and Ultra-strong Bases: A solid acid is called ultra-strong if its strength exceeds the strength of 100% sulfuric acid. Since the acid strength of 100% sulfuric acid is expressed as H0 = -11.9 using the Hammeett acid strength function, a solid acid with strength H0 < -11.9 is called a solid super-strong acid or super-acid.
Solid super-strong alkali means that its alkali strength is higher than +26 when expressed by the alkali strength function H-. Solid super-strong base is mostly alkaline earth metal oxides, or alkaline earth metal and alkali metal composite oxides.
3.Important solid acid-base catalysts
The alkaline earth metal oxides MgO, CaO and SrO2 are typical solid base catalysts, AlCl3 and FeCl3 are typical solid acid catalysts, and Al2O3 has many different crystalline variants. The most important as catalysts are γ- Al2O3 and η- Al2O3, both of which are defective spinel structures. The most stable is the anhydrous α- Al2O3, which is a hexagonal close-packed body of O= ions, with the Al3+ ions occupying 2/3 of the positive octahedral sites.The various variants of Al2O3 are melted above 1470°C and eventually transformed into α- Al2O3.
The composite oxides SiO2-Al2O3, TiO2-SiO2 with TiO2 as the main component and SiO2- TiO2 with SiO2 as the main component are acidic catalysts. znO-SiO2 is not acidic no matter who is the main component. among the binary oxides in the Al2O3 series, MoO3/Al2O3 is more widely used, and the catalysts of hydrodesulfurization (HDS) and hydrogen denitrification (HND) are Co or Al3+ ions. Co-MoO3/Al2O3 or Ni-MoO3/Al2O3 catalysts are Al2O3-MoO3 binary sulfide systems modified with Co or Ni. Co-MoO3/Al2O3 or Ni-MoO3/Al2O3 catalysts have only L-acidic sites in Al2O3, and B-acidic sites are formed by the introduction of MoO3. The introduction of Co or Ni is to prevent the formation of L-acidic sites, and the coexistence of moderately strong L-acidic sites and B-acidic sites facilitates the activity of hydrodesulfurization. The coexistence of L- and B-acid sites is sometimes synergistic; sometimes the presence of L-acid sites in the vicinity of the B-acid site mainly enhances the strength of the B-acid site, and therefore increases its catalytic activity. Some reactions are not catalyzed by acids, but the presence of acids affects the selectivity and rate of the reaction.
There are strong and weak acid sites on the surface of γ- Al2O3; the strong acid site is the active site that catalyzes the isomerization reaction, and the weak acid site is is the active site that catalyzes the dehydration reaction. The presence of more than one active site on the surface of solid acid catalysts is responsible for their selective properties. Generally, reactions involving C-C bond breaking, such as catalytic cracking, skeletal isomerization, alkyl transfer and disproportionation reactions, require strong acid centers; reactions involving C-H bond breaking, such as hydrogen transfer, hydration, cyclization, alkylation, etc., require weak acid centers.
4. Homogeneous acid-base catalyzed reaction mechanism and rate equation
Acid-base catalysis is generally carried out through ionic intermediate compounds, i.e., through positive or negative carbon ions. For example
or
For example, the reaction of benzene with halogenated hydrocarbons in the presence of AlCl3 (L acid) (F. K. reaction), the reaction mechanism is
AlCl3 is a Lewis acid that accepts electron pairs to produce positive carbon ions, which then react as follows
In acid catalysis contains the transfer of protons from the catalyst molecule to the reactants. Therefore the efficiency of the catalyst is often related to the acid strength of the catalyst. In acid catalysis, the tendency of an acid to lose protons can be measured by its dissociation constant, so the acid catalysis constant should be proportional to the dissociation constant of the acid. Experiments show that the two have the following relationship
Where and are constants, which are determined by the type and conditions of the reaction.
For base-catalyzed reactions, the base catalytic constant should be proportional to the dissociation constant of the base. The relationship between the two is as follows
In the case of generalized acid catalysis, for example, the general mechanism is that the reactant S first interacts with the generalized acid HA to produce the protonator SH+, and then the proton is transferred to give the product P, and a new proton is produced, and
SH+ + H2O P + HO
The rate equation, since the protonator is an active intermediate, can be handled by the steady state method, i.e., (6.1.3)
where, since it is a dilute solution, it can be taken as a constant value. The reaction rate is
(6.1.5)
Consider further the dissociation equilibrium of the generalized acid HA, HA + H2O = H3O+ + A-, (6.1.6)
KHA is the dissociation equilibrium constant. Substituting into equation (6.1.5) gives (6.1.7)
This is the resulting rate equation, and it is possible to distinguish between two extreme cases:
If k2 >> k-1cA-, i.e., k2cH+ >> k-1KHAcHA, the intermediate reacts extremely fast
(6.1.8)
The reaction is controlled by step 1 and the rate is proportional to the concentration of the generalized acid, which is called generalized acid catalysis.
The reaction is controlled by step 1 and the rate is proportional to the concentration of the generalized acid, and is so-called generalized acid-catalyzed.
If k2 << k-1cA-, the intermediate reaction is extremely slow, and the reaction is controlled by step 2 at a rate proportional to the hydrogen ionization concentration, so-called hydrogen ion catalysis. It is a first-order reaction at a certain pH, as exemplified by the hydrolysis of sucrose in dilute acid solution.
More generally, the generalized acid HA and the hydrogen ion H+ have an effect, as do the generalized base A- and the hydroxide ion OH-, and the reaction can even proceed to some extent in the absence of an acid-base catalyst. The rate constant can be expressed more generally as
Eq. i.e., the non-catalyzed reaction is equivalent to that in Eq. (6.1.9), which is equivalent to that in Eq. (6.1.8). The overall reaction rate is then
The above analysis and derivation can be useful in studying the mechanism and rate equations of multiphase acid-base catalyzed reactions.
