Strength of metal chemisorption on gas molecules
1. What are the sources of acidity and the formation of acid centers in solid acids?
(1) Sources: ionization, supply of electrons, acceptance of electrons.
(2) 1) Formation of inorganic acid centers impregnated on carriers
(2) Formation of halide acid centers
3) Formation of metal salt acid center
(4) Formation of acid centers of cation exchange resins
5) Formation of oxide acid-base centers
6) formation of acid centers of heteropolyacid compounds
2. What are the properties of solid acids?
(1) The type of acid center: usually associated with the catalyst role is mostly B acid and L acid.
(2) Concentration of acid centers (number of acid centers): This refers to the number of acid centers per unit of catalyst surface or per unit of catalyst mass.
(3) Strength of acid centers: The strength of the ability to give protons (B-acid) or accept electron pairs (L-acid).
3. What is the mechanism of homogeneous acid-base catalysis?
Generally, it is carried out by ionic mechanism, i.e. the acid-base catalyst interacts with the reactants to form positive or negative carbon ion intermediate species, and these intermediate species interact with another reactant to generate products and release the catalyst, which constitutes the acid-base catalytic cycle.
4. What are the characteristics of molecular sieve catalyst?
①Higher activity;
①have high selectivity and can be selectively catalyzed;
③ It has more flexible modulation;
(iv) good stability.
5. Explain the interaction between metals and carriers?
There are three types of interactions: one is the contact position of metal particles and carriers at the interface site, the dispersed metal can maintain the nature of the cation; the second is the dispersed metal atoms fused to the lattice structure of the oxide carriers or the formation of mixed oxides with the carriers; the third is the surface of the metal particles are coated with oxides from the carriers. It is the metal-carrier interaction that results in the transfer of electron fluids between the two and the displacement of charge. The interaction between the metal and the carrier results in a change in the catalytic process.
6. Explain which properties of metal catalysts have a greater influence on their catalytic action?
(1) Lattice defects of metal catalysts and their influence on catalysis Point defects refer to the lack of atoms or excess atoms on the lattice, and the defects can be caused by three kinds of reasons ① mechanical point defects ② electronic defects ③ chemical defects.
(2) The effect of inhomogeneous surface of metal catalyst on catalysis.
7. Definition of acid-base
Lewis acid-base proton theory: any substance that can provide protons is called B acid, any substance that can accept protons is called B base.
Lewis acid-base electron theory: any substance capable of accepting electron pairs is called acid (L acid), any substance capable of providing electron pairs is called base Hard and soft acid-base theory: acids that are not easy to lose the outer layer of electrons are called hard acids (HA), acids that are easy to lose the outer layer of electrons are called soft acids (SA); bases that are hard to lose electron pairs are called hard bases (HB), and bases that are easy to lose electron pairs are called soft bases (SB).
Reaction characteristics of protonated acid catalysts: electron pair transfer (give OR gain a pair of electrons)
8. Simple solid acids and bases
If M is quite electronegative, it has a strong attraction to the electron pair of the oxygen ion, which weakens the O-H bond and facilitates dissociation according to the a formula;
If the electronegativity of M is small, it is favorable to dissociate according to the b-form to form a base center;
Whether the reaction dissociates according to the a-form or the b-form depends mainly on the reactant of the H+ or OH- acceptor.
M-O-H is similar to amphoteric compounds: dissociation according to a-form if a base is present dissociation according to b-form if an acid is present
9. electronegativity and acidity/base Metal oxides with high electronegativity are mainly acidic; oxides with low electronegativity are mainly basic.
10. amorphous silica-aluminate (electronegativity problem) zeolite is a silica-aluminate formed when silicon (Si4+) is partially replaced by aluminum (Al3+) in a silicate with a skeletal structure.
11. complex oxides solid acids and solid bases
Molecular sieve: molecular sieve composition is a hydrated silica-aluminate, M2/n▪Al2O3▪mSiO2▪pH2O M: metal or organic cation; n: metal cation valence; m: silica-aluminate ratio; p: moles of water
Skeleton structure: molecular sieve skeleton is negatively charged – electrically balanced cations (usually Na+)
Source of acidity: generally believed to be formed by the molecular sieve weight of the equilibrium sodium ions directly for the proton replacement
12. Molecular sieve properties: adsorption and ion exchange
Reason for acidity: generally believed to be formed by the molecular sieve heavy equilibrium sodium ions directly for the proton substitution and formation
13. Catalytic properties: selective catalysis
(1) Reactant selective catalysis
(2) product shape-selective catalytic reaction products in the molecular critical diameter smaller than the pore diameter can diffuse out of the air, known as the final product, while the molecular critical diameter is larger than the pore diameter can not be escaped from the pore, and produce product selectivity.
(3) Restricted transition state selective catalysis When the effective space in the cavity is smaller than the space required for the transition state, the reaction will be blocked, and then restricted transition state selective catalysis is generated
(4) molecular channel control reactants enter from one channel and exit from another channel e.g. ZSM-5:0.54nm 0.58×0.52nm straight chain alkanes can pass through both channels branched alkanes have selective
14. super-strong acid refers to the acid strength of more than 100% H2SO4 material, its acid strength function H0 <-10.6 solid super-strong acid SbF5, NbF5, TaF5, SO3 in Sb +5, Nb +5, Ta +5, S +6 has a strong ability to accept electrons
15… Catalytic reactions: orthocarbon ions, orthocarbon ion reaction pattern, type of acid center in relation to catalytic activity and selectivity,
(1) Formation of positive carbon ions
① alkanes, cycloalkanes, aromatic hydrocarbons, olefins on the L acid center
② olefins, aromatics and other unsaturated hydrocarbons in the B acid center
③ alkanes, cycloalkanes, olefins, aromatics and R + hydrogen transfer, the generation of new positive carbon ions, and the original positive carbon ions to hydrocarbons
(2) positive carbon ion reaction law
① positive carbon ions can be 1-2 carbon on the hydrogen transfer and change the position of the positive carbon ions, can repeatedly add H + and off H +, and finally off H + generated double bond transfer of olefins, double bond isomerization
② C-C + bond in the positive carbon ion is a single bond, free to rotate, rotate to the two sides of the CH3 group in the opposite position, off H +, resulting in olefin cis-trans isomerization
(iii) The alkyl group in the positive carbon ion can be transferred, resulting in isomerization of the olefin skeleton.
(iv) the positive carbon ion can be added to the olefin to generate a new positive carbon ion, the latter and then off the H +, will produce a dimer, the new positive carbon ion can continue to add with the olefin, resulting in the polymerization of olefins
⑤ The positive carbon ion can be isomerized by hydrogen transfer plus H+ or off H+, and ring expansion and contraction occurs
(6) When the positive carbon ion is large enough, it is easy to undergo β-breakage and turn into an olefin and a smaller positive carbon ion.
(vii) the positive carbon ion is very unstable, easy to internal hydrogen transfer, isomerization or reaction with other molecules, the speed is generally greater than the speed of the formation of the positive carbon ion itself, so the formation of the positive carbon ion is often the reaction control step
(3) for different acid-catalyzed reactions long require different types of acid centers (L or B) Different reactions require different acid strengths
Requirement of acid strength: skeleton isomerization＞aromatic dealkylation＞isomeric alkane cracking, double bond isomerization＞dehydration Many experiments have proved that in a certain range of acid strength, there is a good correspondence between acid concentration and catalytic activity.
Many experiments have proved that there is a good correspondence between acid concentration and catalytic activity in a certain range of acid strength.
16. What are the electronic properties of transition metal oxides?
① transition metal oxides in the d electron layer of the metal cation is easy to lose electrons or capture electrons, with strong redox properties.
② transition metal oxides have semiconductor properties.
(iii) The inner valence orbitals and foreign orbitals of metal ions in transition metal oxides can be cleaved.
④ transition metal oxides and transition metals can be used as catalysts for redox reactions, and the former due to its heat resistance, strong resistance to toxicity, and light-sensitive, heat-sensitive, impurity-sensitive, more conducive to the catalyst performance modulation, so more widely used.
17. What is the redox mechanism of transition metal oxides?
A: In the process of catalytic oxidation reaction of transition metal oxide catalysts, especially for hydrocarbon oxidation reaction, the oxygen in the reaction products is often not directly from the oxygen in the gas phase, but from the lattice oxygen in the metal oxides, and the oxygen in the gas phase is only used to replenish the lattice oxygen consumed by the catalyst in the reaction. The redox process is:
18. What processes are involved in the design and development of industrial catalysts?
1） Exploration stage
According to the characteristics of the reaction, review the relevant literature analysis, theoretical guidance, etc., from the technical and Jiji design and preliminary development. After measurement, screening, improvement, to determine the catalyst for further amplification of the study. This stage is usually carried out in the laboratory. Generally only the synthesis method and catalyst activity, selectivity evaluation test. The laboratory usually conducts the test on a small device that can reflect the intrinsic activity of the catalyst.
(2) Simulation and amplification stage The catalyst screened in the first stage is taken in a certain quantity and examined for activity, selectivity, lifetime, reproducibility and other performances under industrial production conditions, and then improved and perfected to obtain the comprehensive performance data of the catalyst, and to propose the data that can be applied in the industrial production scale as well as the data for the industrial production. The tests are usually carried out on an industrial plant of a certain scale (according to the production conditions), either in a scale-up laboratory or on the sideline of an industrial plant. In this stage should also be the reaction kinetics law research, for industrial scale-up to provide data and information.
3）Industrial production stage Design and set up the industrial production of the reaction device and catalyst production plant, after the test run formally industrialized production. It should be especially pointed out that, in each of the above stages, great attention should be paid to the economic feasibility, to carry out a comprehensive evaluation and analysis of technology and economy, otherwise the production is meaningless.
19. What are the principles of selection of raw materials and ingredients?
A: (1) the choice of raw materials.
1. Meet the performance requirements of the product;
2. Contained impurities are easy to remove, or limited to a certain range;
3. Easily available, inexpensive and easy to process;
4. There is no pollution of the environment or can be removed after treatment of pollution
(2) Principles of selection of ingredients:
1. When formulated into a solution, water-soluble substances can be used as ingredients with water, but attention should also be paid to its acidity.
2. When the raw material is insoluble in water, other solvents should be easily removed when used as ingredients, or its presence does not affect the catalytic properties of the catalyst.
3. The ratio of raw materials can be weight ratio or atomic ratio, the key is accurate and meet the requirements of the catalyst.
20. What are the characteristics and differences between the precipitation method and the gel method?
(1) Advantages: (1) It is good for the removal of impurities; (2) It can get the product with high dispersion of active components; (3) When preparing multi-component catalyst, it is good for the close bonding of components and the formation of suitable active structure; (4) The active components are more tightly bonded in the carrier and are not easy to be lost.
(2) Disadvantages: (i) The mechanism of precipitation process is complicated and not easy to master; (ii) The uniformity is not easy to be guaranteed in the preparation of multi-component catalysts; (iii) The manufacturing process is longer and more complicated, and the production cost is large.
(3) Difference: compared with the precipitation method, the gel method can prepare more uniform multi-component catalyst, but the equipment of the gel method is bigger and more expensive.
21. What are the characteristics of the impregnation method?
①It can use the shape and size of the carrier that has already been molded, eliminating the need for subsequent catalyst molding operations;
② The impregnation method can load one or several active components on the carrier, the active components are distributed on the carrier surface, the active components are highly utilized, and the dosage is small, which is significant for precious metals;
③ The structure of the carrier basically determines the pore structure and specific surface size of the prepared catalyst. Then the carrier with suitable structure and specific surface can be selected to provide various mechanical and physical properties suitable for the catalyst.
22. What are the causes of catalyst deactivation?
1) Sintering:At high temperatures, the particles migrate, lowering the surface to make the system proceed in the direction of stabilization, which leads to sintering of the catalyst.
2) Poisoning: The catalyst is deactivated by the presence of poisons.
(3) Carbon accumulation (coking): At high temperature, carbon or coke generated by decomposition, polymerization and other reactions of reaction materials are deposited on the catalyst surface, which reduces the active surface of the catalyst, blocks the pores, and affects the activity of the catalyst, resulting in its decline.
(4) Change of catalyst phase composition
23. Carbon ion reaction law: Acid-base catalysts have catalytic activity for many hydrocarbon molecules. Acid catalyst can provide protons or accept electron pairs, so that the hydrocarbon molecules are transformed into positively charged positive carbon ions, and catalytic conversion is carried out according to the positive carbon ion reaction mechanism; whereas the alkaline catalyst tends to seize the protons or give the hydrocarbon molecules electron pairs to make the reaction carry out according to the negative carbon ion reaction mechanism after attacking the hydrocarbon molecules.
24. Catalyst life: It refers to the use time of catalyst to maintain certain reaction activity and selectivity under certain reaction conditions.
25. Catalyst poisoning: refers to the loss of activity and selectivity of catalyst under the action of trace poison.
Short Answer
1. Comparison between acid-base catalysis and redox catalysis
① Acid-base catalytic reaction The reaction mechanism is due to the catalyst and the reactant molecules through the acceptance of electron pairs and coordination, or polarization, the formation of ionic active intermediate species catalytic reaction.
The reaction mechanism is due to the catalytic reaction between the catalyst and the reactant molecules through the transfer of individual electrons and the formation of active intermediate species.
2. The source and regulation of acidity and alkalinity of solid acid-base catalysts
(1) Source : ① structural differences, in the same coordination number, with different valence of the substitution of atoms, resulting in charge imbalance and produced; the same when the same valence but different coordination number, will make its structure imbalance, resulting in acidity. (ii) For metal salts, when containing a less structured amount of water, the metal ions polarize the H2O and produce a B acid center. (My answer: ionization, accepting electrons, providing electrons)
(2) Regulation: ① Synthesize zeolites with different silica-alumina ratios, or pass low-silica zeolites through silica-alumina ratios by de-alumina; ② Regulate the acid strength or acid concentration by adjusting the type and number of exchange cations to improve the activity and selectivity; ③ Reduce or weaken their strong acid centers by high-temperature roasting, high-temperature hydrothermal treatment, pre-cumulation of carbon, or alkali-poisoning to change the activity and selectivity; ④ By changing the reaction atmosphere, thus Change the activity.
1, expository questions: try to analyze the precipitation method and the gel method of catalyst preparation of the factors affecting the preparation of the advantages and disadvantages of their preparation methods (in the previous Chapter V, Chapter 7) to comment on.
Answer: (1) precipitation method: its chemical composition and structure, the properties of the precipitate is in turn determined by the state of precipitation and conditions (concentration, temperature, PH value, etc.). Concentration, temperature, agitation, PH .
(2) Gel method: in addition to the PH value, the acidity of the introduced substance as well as its ionizability (electrolyte nature) and the time the gel is left in place have a large influence. Also, the strength of the condensation reaction has an effect on the gelation process.
Catalysts and Catalysis Reflection Questions
1. do catalyzed reactions have the same heat of reaction and activation energy as non-catalyzed reactions? Why?
(1) Does not change the heat of reaction: Because the catalyst only reduces the activation energy by changing the course of chemical reaction, and the energy change before and after the chemical reaction is determined by the relative energy positions of the reactants and products in the reaction system, and the structure of the reactants and products determines their relative energy positions, i.e., does not change the enthalpies of the reactants and the moles of the products, so adding the catalyst does not change the heat of reaction.
(2) Lowering the activation energy: because catalysts make the reaction proceed along an easier pathway by changing the course of the reaction.
2. Can a catalyst change chemical equilibrium? No, because the chemical properties of the catalyst do not change at the end of the reaction and do not affect the change in the standard free energy of the reaction system, ΔG0 (a state function, which is determined at the beginning and end of the process and is independent of the process), and thus the equilibrium constant K.
3. What are the factors affecting the lifetime of a catalyst?
Chemical stability: the chemical composition and chemical state are stable, and the active components and additives do not react or lose;
② Thermal stability: no sintering, microcrystalline growth and crystalline phase change;
③ Anti-toxic stability: resistance to inactivation of adsorbed active poisons;
④Mechanical stability: resistance to abrasion rate, crushing strength, and thermal shock. Determine the crushing and abrasion during the catalyst use process
4. What is the role of carrier in catalyst?
①Dispersing active components: providing large surface and pore structure, saving active components, improving resistance to poisoning, improving resistance to sintering (heat emission and removal), improving mechanical strength (binder)
② Provide catalytic activity (bifunctional catalyst)
③ modulation of the structure, morphology, size of the active phase nanoparticles
5、What conditions should a good industrial catalyst fulfill?
(1) High activity: moderate reaction conditions low pressure, low temperature, high air velocity, large processing capacity; reduce the cost of the device; reduce operating costs; improve process economics
(2) good stability: long life, long service time, slow decline in activity; resistance to sintering, poisoning, coking, pollution ability; reduce catalyst costs; improve process economics
(3) Fluid flow: fluid distribution, pressure drop, diffusion effect catalyst utilization and process control; fluid distribution: uniform, turbulent, close to the piston flow reactor and catalyst particles diameter ratio of 5 ~ 10; reactor length and catalyst particles diameter ratio of > 50 ~ 100
(4) Pressure drop: as small as possible to improve the catalyst bed gap
6、What is conversion rate, yield and specific activity?
Conversion rate: the amount of material that has been converted as a proportion of the total feed size
Yield: the ratio of the amount of target product generated to the initial total amount of reactants
Selectivity: the proportion of the destination product to the total product (m%, mol%)
7. Explain why the heat of adsorption, activation energy of adsorption, and activation energy of desorption vary with surface coverage? And draw a graph of the relationship between the changes.
(1) Law: as the surface coverage increases, the heat of adsorption is seen to decrease and the activation energy of adsorption increases
(2) Reasons: 1) surface inhomogeneity: the heat of adsorption decreases linearly with increasing degree of coverage, the adsorption sites on the surface are not energy-equalized, there are interactions between adsorbed molecules, and the molecule that is adsorbed first is always adsorbed at the most active and easily adsorbed site. 2) Interaction between adsorbed species 3) Occupation of different energy levels in the solid
8. What is the use of heat of adsorption data in catalyst research?
In some cases, the activity of a catalyst can be inferred from the heat of adsorption data. Solid surfaces with moderately strong adsorption of reacting molecules usually have good catalytic activity. For example, the volcano model of the heat of adsorption can be used to select a suitable catalyst.
9. What is a control step? What is the use of studying the control step of a catalytic reaction for selecting and improving catalysts? How can the effects of internal and external diffusion be eliminated?
(1) In a multi-step reaction, a step is a control step if the resistance is much greater than the other steps.
(3) The practice of internal diffusion elimination experiment is to change the particle size of catalyst under a certain catalyst loading (mass or volume), and determine the elimination of internal diffusion by comparing the change of catalyst activity under different particle sizes, i.e., as the particle size decreases, the activity will no longer change, then the particle size corresponding to the activity no longer changing can be regarded as free of internal diffusion, or the effect of internal diffusion can be ignored.
10. What steps are involved in a surface catalyzed reaction?
(1) Diffusion of reactants from the main body of the gas phase to the outer surface of the catalyst.
(2) Diffusion of reactants from the outer surface of the catalyst to the inner surface of the catalyst
(3) Chemical adsorption of reactants
(4) Chemical reaction of adsorbed reactants to produce products
(5) Desorption of products
(6) Diffusion of products from the inner surface of the catalyst to the outer surface of the catalyst
(7) Diffusion of products from the outer surface of the catalyst to the main body of the gas phase where steps (1) and (7) are external diffusion, steps (2) and (6) are internal diffusion, and steps (3), (4) and (5) are chemical kinetic processes.
11. How is the heat of generation related to the activity of a metal catalyst?
Relationship between chemisorption and catalytic activity of metal catalysts When the surface of a metal catalyst chemisorbs with a reactant molecule, it is often considered that a surface intermediate species is generated, and there is a direct relationship between the strength of the chemisorption bond or the stability of the surface intermediate species and the catalytic activity. The greater the heat of generation of the intermediate species, the higher its stability. It is usually considered that the chemisorption bond is moderate, i.e., the heat of generation of the surface intermediate species is moderate, so that the metal catalyst has the best catalytic activity.
12. What are the factors affecting the chemisorption properties of metal catalysts? What are the basic factors affecting their activity and selectivity?
(1) Factors:
① The height of the Fermi energy level: for a certain reactant, the height of the Fermi energy level determines the strength of chemisorption.
② Change in the escape work of the metal metal after chemisorption
(iii) the effect of d%: as d% increases, the adsorption amount decreases, and d% decreases, the adsorption amount increases.
(iv) the bond strength of the chemisorbed species depends on the difference in orbital configuration
(2) For metal-loaded catalysts, there are three factors affecting activity:
(i) The effect of particle size and single crystal orientation in the critical range;
(ii) The effect of the formation of an alloy from an active group VIII metal with a less active group IB metal, such as Ni-Cu;
(iii) the effect of substitution from one group VIII metal to another metal in the same group
Factors affecting its selectivity are:
① diversity of active centers, which can catalyze several competing reactions at the same time, thus reducing the selectivity of the intended reaction
(ii) The level at which electrons can fill the energy bands. It is directly related to the activity and selectivity of the catalyst.
③ The method of adding additives is used so as to achieve the purpose of increasing the activity and selectivity of the catalyst.
13 What is the meaning of acidity and acid strength? How are they measured? What is the experimental method for distinguishing between B acid and L acid?
(1) Acid strength is the ability to give protons (B acid strength) or accept electron pairs (L acid strength) expressed as a function H0.
(2) Test method: n-butylamine indicator titration method is the total acidity and acid strength Gaseous base adsorption desorption method (
(2) Impregnation: Common methods for preparing loaded catalysts Excess solution impregnation, equal volume impregnation, multiple impregnation, vapor phase impregnation.
(3) Thermal decomposition method (or solid-phase reaction method): the use of heat decomposition of salts: nitrate, carbonate, oxalate, etc. 1) the impact of raw materials (alkaline earth metal nitrate can only be obtained nitrite) 2) thermal decomposition conditions of the decomposition products: decomposition temperature ↑ and decomposition time ↑ → particle size ↑
(4) melting method
(5) reduction method: mostly used in the preparation of metal oxides
21. What are the classifications of poisoning? What are the mechanisms of poisoning?
Definition: A small amount of impurities in the fluid contacted by the catalyst is adsorbed on the active site of the catalyst, which significantly reduces or even disappears the activity of the catalyst, which is called poisoning.
(1) Classification of poisoning:
(1) According to the strength of the poisoning effect: strong, medium, weak
(2) According to the characteristics of the poisoning effect: permanent, temporary
(3) according to the mechanism of poisoning: chemical adsorption, chemical action
22. What are the reasons for catalyst deactivation? How to avoid catalyst deactivation?
Three aspects: chemical reasons (poisoning, coking) heat (high temperature sintering) mechanical reasons (pulverization, loss)
