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.There are many different crystalline variants of Al2O3. Of importance as catalysts are γ-Al2O3 and η-Al2O3, both of which are defective spinel structures. The more stable is the anhydrous α-Al2O3, which is a hexagonal, very close-packed mass of O= ions, with the Al3+ ions occupying 2/3 of the positive octahedral sites.Various variants of Al2O3 are generally converted to α-Al2O3 by melting above 1470°C. The composite oxides SiO2-Al2O3 and η-Al2O3 are also used as catalysts.
The composite oxides SiO2-Al2O3, TiO2-SiO2 with TiO2 as the main component, and SiO2-TiO2 with SiO2 as the main component are all acidic catalysts, and ZnO-SiO2 is not acidic, no matter who is the main component.The Al2O3 series of binary oxides, MoO3/Al2O3 is more widely used, and the catalysts of hydrodesulphurization (HDS) and hydrogen denitrification (HND), which are catalysts with Co or Al3+ ions, are the most widely used. 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 has only L-acidic sites in Al2O3, and MoO3 is introduced to form the B-acidic sites. The introduction of Co or Ni is to prevent the formation of L-acidic sites, and the coexistence of medium-strength L-acidic sites and B-acidic sites facilitates the activity of HDS. 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 acid, but the presence of acid 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.
Homogeneous Acid-Base Catalyzed Reaction Mechanisms and Rate Equations
Acid-base catalysis generally proceeds through ionic intermediate compounds, i.e., through positive or negative carbon ions.
For example, the reaction of benzene with halogenated hydrocarbons in the presence of AlCl3 (L-acid) (the Fock reaction), AlCl3 is a Lewis acid that accepts electron pairs to produce positive carbon ions, which are included in acid catalysis to transfer 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 the 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. For base-catalyzed reactions, the base catalytic constant should be proportional to the dissociation constant of the base. Taking the generalized acid catalysis as an example, the general mechanism is that the reactant S first interacts with the generalized acid HA to produce the protonated substance SH+, then the proton is transferred to obtain the product P and a new proton is produced, SH++H2O P+HO
The rate equation, since the protonator is an active intermediate, can be treated with the steady state method, which can be used as a constant value since it is a dilute solution. 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 this as the resulting rate equation, which can distinguish between two extreme cases:
If k2>>k-1cA-, i.e., k2cH+>>k-1KHAcHA (6.1.8), the intermediate is reacting extremely fast. The reaction is controlled by step 1 and the rate is proportional to the concentration of the generalized acid, which is known as generalized acid catalysis.
If k2<<><k-1ca-<>, the intermediate reacts extremely slowly, and the reaction is controlled by step 2 at a rate proportional to the concentration of hydrogen ion, and is so-called hydrogen ion catalyzed. It is a first-order reaction at a certain pH, as exemplified by the hydrolysis of sucrose in dilute acid solution.
