Molecular Sieve Catalyst
1.Concept of molecular sieve
Molecular sieve is crystalline silica-aluminate with uniform pore structure. Molecular sieve contains a large amount of crystalline water, which can be vaporized and removed when heated, so it is also called zeolite. Those that exist in nature are often called zeolites, and those that are synthetic are called molecular sieves. Their chemical composition can be expressed as
Mx/n[(AlO2)x(SiO2)y] ZH2O
Where M is the metal cation, n is its valence, x is the number of molecules of AlO2, y is the number of molecules of SiO2, and Z is the number of water molecules, because AlO2 is negatively charged, and the presence of the metal cation can keep the molecular sieve electrically neutral. When the valence of the metal ion n = 1, the number of atoms of M is equal to the number of atoms of Al; if n = 2, the number of atoms of M is half of the number of atoms of Al.
Commonly used molecular sieves mainly include: square sodium zeolite, such as A-type molecular sieve; octahedral zeolite, such as X-type, Y-type molecular sieve; mercerized zeolite (-M-type); high silica-type zeolite, such as ZSM-5 and so on. Molecular sieves can provide high activity and unusual selectivity in a variety of different acid catalysts, and the vast majority of reactions are caused by the acidity of molecular sieves, which also belongs to the solid acid class. They have been widely used in industry in the last 20 years, especially in the oil refining industry and petrochemical industry as industrial catalysts occupy an important position.
2. Structural characteristics of molecular sieves (1) four aspects and three levels:
The structural characteristics of molecular sieves can be divided into four aspects and three different structural levels. The first structural level is also the most basic structural unit silicon oxygen tetrahedron (SiO4) and aluminum oxygen tetrahedron (AlO4), which constitute the skeleton of molecular sieve. Neighboring tetrahedra are connected by oxygen bridges to form rings. Ring is the second level of molecular sieve structure, according to the number of oxygen atoms in the ring, there are four oxygen rings, five oxygen rings, six oxygen rings, eight oxygen rings, ten oxygen rings and twelve oxygen rings. The ring is the channel orifice of the molecular sieve, which plays a sieving role for the passing molecules. Oxygen rings are interconnected through oxygen bridges to form polyhedra with three-dimensional space. Various polyhedra are the third level of molecular sieve structure. The polyhedra have hollow cages, and the cages are an important feature of the molecular sieve structure. The cage is divided into α cage, octahedral zeolite cage, β cage and γ cage, etc.
(2) Cages of molecular sieve:
α cage: it is the main pore of the skeleton structure of A-type molecular sieve, which is an icosahedron consisting of 12 four-membered rings, 8 six-membered rings and 6 eight-membered rings. The average pore size of the cage is 1.14 nm and the cavity volume is 760[]3. The largest window of the α-cage is the octahedral ring with a pore size of 0.41 nm.
Octahedral zeolite cage: it is the main pore cavity that constitutes the skeleton of X-type and Y-type molecular sieves, and consists of 18 four-membered rings, 4 six-membered rings, and 4 dodecahedral rings composed of icosahedra. the average pore diameter of the cage is 1.25 nm, and the volume of the cavity is 850[ ]3. the largest window of the pore is the dodecahedral ring, with a pore diameter of 0.74 nm. the octahedral zeolite cage is also known as the supercage.
β cage: mainly used to constitute the skeleton structure of A-type, X-type and Y-type molecular sieve, is the most important kind of hole, its shape is like about the top of the positive octahedron, cavity volume of 160 []3, the window aperture is about 0.66 nm, only allows NH3, H2O and other smaller molecules into the size.
There are also hexagonal column cages and γ-cages, which are smaller in size and generally do not allow molecules to enter the cage.
Different structures of the cage and then linked to each other through the oxygen bridge to form a variety of different structures of molecular sieves, mainly A-type, X-type and Y-type.
(3) Several representative molecular sieves
A type molecular sieve
Similar to the cubic crystal system structure of NaCl. If the NaCl lattice Na+ and Cl- are all replaced by β cages, and the adjacent β cages are linked with γ cages to obtain the crystal structure of A-type molecular sieves. 8 β cages are linked to form a square natrium structure, such as γ cages as a bridge linkage, you get the structure of A-type molecular sieves. There is a large α cage in the center. α cage between the channel has an eight element ring window, its diameter is 4, so it is called 4A molecular sieve. If 70% of the Na+ on the 4A molecular sieve is exchanged for Ca2+, the octet ring can be increased to 5, and the corresponding zeolite is called 5A molecular sieve. On the contrary, if 70% of Na+ is exchanged for K+, the pore diameter of the eight element ring is reduced to 3, and the corresponding zeolite is called 3A molecular sieve.
X-type and Y-type molecular sieves
Dense stacked hexagonal crystal system structure similar to diamond. If the β-cage is used as the structural unit, replacing the carbon atom nodes of diamond, and the two adjacent β-cages are linked by hexagonal column cages, i.e., four hexagonal column cages are used to link five β-cages together, with one β-cage at the center and the remaining four β-cages located at the apex of the ortho-tetrahedron, then an octahedral zeolite-type crystal structure is formed. The X- and Y-type molecular sieve structures were obtained by continuing the linkage with this structure. In this structure, the large cage formed by the β-cage and the hexagonal column cage is an octahedral zeolite cage, and the window holes through which they are connected are dodecahedral rings with an average effective pore size of 0.74 nm, which is the pore size of the X-type and Y-type molecular sieves. The difference between these two types each other is mainly Si/Al ratio is different, X-type is 1~1.5; Y-type is 1.5~3.0.
Mercerized zeolite type molecular sieve
The structure of this kind of zeolite has no cage but a laminated structure. The structure contains a large number of five-membered rings, and linked together in pairs, each pair of five-membered rings through the oxygen bridge and then linked with another pair. The linkages form four-membered rings. This structural unit is further linked to form a layered structure. The layers contain eight-membered and twelve-membered rings, the latter of which are elliptical in shape with an average diameter of 0.74 nm, and are the main pores of the silky zeolite. This pore channel is one-dimensional, i.e., a straight-through channel.
High Silica Zeolite ZSM (Zeolite Socony Mobil) Type Molecular Sieves
There is a series of this zeolite, the widely used is ZSM-5, with the same structure of ZSM-8 and ZSM-11; another group is ZSM-21, ZSM-35 and ZSM-38, etc. ZSM-5 is often referred to as a high silica type zeolite, and its Si/Al ratio can be as high as 50 or more, and ZSM-8 can be as high as 100, and this group of molecular sieves also shows water-repellent properties. Their structural unit is similar to that of silica zeolite, which consists of paired five-membered rings without cage-like cavities, only channels.ZSM-5 has two sets of crossed channels, one is straight through and the other is zigzag perpendicular to each other, both formed by ten-membered rings. The channels are elliptical, and their window diameters are (0.55-0.60) nm. zeolites belonging to the high-silica group also have the all-silica type Silicalite-1, which has the same structure as that of ZSM-5, and Silicalite-2, which is the same as that of ZSM-11.
Aluminum phosphate molecular sieve
This series of zeolite is the third generation of new molecular sieves that appeared in the 1980s after the Y-type molecular sieves in the 1960s and the high silica molecular sieves in the 1970s ZSM-5. Including large-pore AlPO-5 (0.1-0.8nm), medium-pore AlPO-11 (0.6nm) and small-pore AlPO-34 (0.4nm) and other structures and MAPO-n series and AlPO path chemically modified by Si into the SAPO series.
4、Catalytic mechanism of molecular sieve catalysts
Molecular sieve has clear pore cavity distribution, very high internal surface area (600m2/s), good thermal stability (1000℃), and tunable acid site center. The acidity of molecular sieves mainly comes from the three-coordinated aluminum atoms and aluminum ions (AlO)+ on the skeleton and in the pores. The OH group on the molecular sieve HY obtained by ion exchange shows the acid site center, and the aluminum ions outside the skeleton will strengthen the acid site and form the L acid site center. Multivalent cations such as Ca2+, Mg2+, La3+, etc. can be exchanged to show the acid site center. reduction of transition metal ions such as Cu2+ and Ag+ can also form the acid site center. Generally the higher the Al/Si ratio, the higher the specific activity of OH group. The modulation of the acidity of molecular sieves can be achieved by direct exchange of protons into dilute hydrochloric acid. Since this approach often leads to de-aluminization of the molecular sieve skeleton. So NaY has to be changed to NH4Y and then to HY.
(1) Molecular sieves have the property of selective catalysis
Because molecular sieve structure has uniform small internal holes, when the molecular linearity of the reactants and products is close to the pore size within the crystal, the selectivity of the catalytic reaction often depends on the molecules and the corresponding size of the pore size. This selectivity is called selective catalysis. There are two mechanisms leading to the selectivity of selectivity, one is caused by the difference in diffusion coefficients of the molecules involved in the reaction in the pore cavities, which is called mass-transfer selectivity, and the other is caused by the spatial confinement of the transition state of the catalytic reaction, which is called transition-state selectivity. There are four forms of selective catalysis:
Reactant selective catalysis
When certain molecules in the reaction mixture that can react are too large to diffuse into the catalyst pore cavity, only those molecules with diameters smaller than the inner pore diameter can enter the inner pore and react in the catalytically active part.
Product selective catalysis
Product selectivity is formed when certain molecules in the product mixture are too large to diffuse out of the inner pore window of the molecular sieve catalyst.
Transition state-limited selectivity
Some reactions, the reactant molecules and product molecules are not subject to the diffusion limitations of the catalyst window aperture, only because of the need for a larger space in the inner hole or cage cavity, in order to form the corresponding transition state, or else it will be restricted so that the reaction can not be carried out; on the contrary, some reactions only need a smaller space for the transition state is not subject to such limitations, and this constitutes the restriction of the transition state of the shape-selective catalytic.
ZSM-5 is commonly used to catalyze such transition state-selective reactions, with the greatest advantage of preventing coking. Because ZSM-5 has smaller internal pores than other molecular sieves, it is not conducive to the formation of large transition states required for the polymerization of coke-generating precursors. Thus, it has a longer lifetime than other molecular sieves and amorphous catalysts.
Molecular Traffic-Controlled Shape-Selective Catalysis
In molecular sieves with two different shapes and sizes of pores, reactant molecules can easily enter the active part of the catalyst through one type of pore to carry out the catalytic reaction, while the product molecules diffuse out of the other pore to minimize back-diffusion and increase the reaction rate from the surface. This molecular traffic-controlled catalytic reaction is a special form of selectivity called molecular traffic-controlled selective catalysis.
(2) Modulation of selectivity
It can be done by poisoning the active center of the outer surface; modifying the size of the entrance of the window pore, and the commonly used modifier is tetraethyl protosilicate; or changing the grain size.
The greatest practical value of selective catalysis lies in the use of it to characterize the difference in pore structure, which is one of the methods to distinguish acidic molecular sieves. Selective catalysis has been widely used in refining process and petroleum industry production, such as molecular sieve dewaxing, selective isomerization, selective reforming, methanol synthesis of gasoline, methanol to ethylene, selective alkylation of aromatics and so on.
Metal catalysts and their catalytic mechanism
1. Overview of metal catalysts
Metal catalysts are an important class of industrial catalysts. They mainly include lump catalysts, such as electrolytic silver catalysts, molten iron catalysts, platinum mesh catalysts, etc.; dispersed or loaded metal catalysts, such as Pt-Re/-Al2O3 reforming catalysts and Ni/Al2O3 hydrogenation catalysts;
6.3 Metal catalysts and their catalytic mechanism of action
Metal intercalation catalysts, such as LaNi5 can catalyze the conversion of syngas to hydrocarbons, is a new class of catalysts developed in the 1970s, but also magnetic materials, hydrogen storage materials; metal clusters catalysts, such as olefin hydrogen aldolization of carbonyl compounds of the multinuclear Fe3(CO)12 catalysts, at least two or more metal atoms to meet the catalyst activation initiation necessary. Of these five categories of metal catalysts, the first two are dominant and the last three have seen new developments since the 1970s.
Almost all metal catalysts are transition metals, which is related to the structure of the metal, surface chemical bonding. The type of catalyst for which a metal is suitable depends on its compatibility with the reactants. When a catalytic reaction occurs, the catalyst and the reactants have to interact. It does not penetrate deeply into the body except on the surface, which is called compatibility. For example, transition metals are good catalysts for hydrogenation and dehydrogenation because H2 is easily adsorbed on their surfaces and the reaction does not proceed below the surface. But only “precious metals” (Pd, Pt, also Ag) can be used as catalysts for oxidation reactions, because they can resist oxidation at the corresponding temperature. Therefore, the in-depth understanding of metal catalysts, to understand its adsorption properties and chemical bonding properties.
2. Chemical bonding of metals and metal surfaces
There are three theoretical approaches to study the chemical bonding of metals: energy band theory, valence bond theory and coordination field theory, each from a different point of view to illustrate the characteristics of the chemical bonding of metals, and each of these theories provides some useful concepts. Each of the three theories, which can be used to correlate specific covariates with the chemisorption and catalytic properties of metals, are complementary to each other.
(1) The energy band model of the electronic structure of metals and the “d-band hole” generalization.
Each electron in the metal lattice occupies a “metal orbital”. Each orbital has its own energy level within the metal crystal field. Since there are N orbitals and N is large, these energy levels are continuous. As the orbitals interact, the energy levels split in two, so that N metal orbitals form 2N energy levels. Occupancy of energy levels by electrons follows the principle of lowest energy and Pauli’s principle (i.e., pairwise occupancy of electrons). Therefore, at absolute zero, electrons in pairs from the lowest energy level has been filled upward, only half of the energy level with electrons, known as the full band, the higher half of the energy level without electrons, called the empty band. The highest energy level occupied by electrons at the boundary between the empty and full bands is called the Fermi energy level.
The s orbitals form the s-band and the d orbitals form the d-band, with an overlap between the s and d bands. This is especially true and important for transition metals. s energy level is a single heavy state, which can only hold 2 electrons; d energy level is a 5-heavy merged state, which can hold 10 electrons. For example, the electronic group state of copper is [Cu](3d10)(4s1), so the d-band electrons in the copper metal is full, for the full band; and the s-band occupies only half. Nickel atom’s electronic group state for [Ni] (3d5) (4s2), so the metal nickel d-band in some energy levels are not filled, called “d-band hole”. The concept of “d-band holes” is crucial to the understanding of chemisorption and catalysis of transition metals, because when an energy band is fully filled with electrons, it is difficult to form bonds.
(2) Valence bonding model and the concept of d characteristic percent (d%)
Valence bonding theory suggests that transition metal atoms combine in hybridized orbitals. Hybridized orbitals are usually linear combinations of s, p, and d atomic orbitals, called spd or dsp hybridization. The percentage of d atomic orbitals in a hybridized orbital is called the d characteristic percentage and is represented by the symbol d%. It is a characteristic parameter used by valence bond theory to relate the catalytic activity and other physical properties of a metal.
The larger the d% of a metal is, the more electrons are filled in the corresponding d energy bands, and the fewer d holes there are. d% and d holes are coefficients that reflect the electronic structure of a metal from different perspectives and are opposite electronic structure characterizations. They are somehow related to the chemisorption and catalytic activity of metal catalysts, respectively. For widely used metal hydrogenation catalysts, a d% of 40~50% is appropriate.
(3) Coordination Field Modeling
The concept of coordination field is borrowed from the bonding treatment in complex chemistry. In an isolated metal atom, five d-orbital energy levels are briefly merged, and after the introduction of a face-centered cubic octahedral symmetric coordination field, the briefly merged energy levels split into t2g orbitals and eg orbitals. The former includes dxy, dxz, and dyz, and the latter includes and . The d energy band splits in a similar form in the coordination field into the t2g energy band and the eg energy band. the eg energy band is high and the t2g energy band is low.
Because of their spatial directivity, the bonding of surface metal atoms is distinctly domain-specific. These orbitals intersect the surface at different angles, and this difference affects the effectiveness of orbital fitness. With this model, chemisorption on metal surfaces can in principle be explained. Not only that, it can also explain differences in chemical activity between different crystalline surfaces; pattern differences between different metals and alloying effects. For example, the heat of adsorption decreases with increasing coverage, which is most satisfactorily explained by the non-uniformity of the adsorption sites, which is in agreement with the fixed-domain bonding model.The different crystalline surfaces of Fe catalysts are differently active for NH3 synthesis, e.g., if we take the activity of the [110] surface to be 1, the activity of the [100] surface is 21 times as much as it is; and that of the [111] surface is even higher, being 440 times as much as it is. This has been confirmed experimentally.
3 The bulk structure, surface structure, lattice defects and dislocations of metals (1) The bulk structure of metals
Except for a few metals, almost all metals belong to three crystal structures, namely, face-centered cubic lattice (F.C.C.), body-centered cubic lattice (B.C.C.) and hexagonal close-packed lattice (H.C.P.). Some structural parameters of the three lattices are listed in Table 6.3.1
Crystals can be understood as different crystal planes. For example, the body-centered cubic lattice of the metal Fe has (100), (110), and (111) crystal planes. The geometrical arrangement of the metal atoms on the different crystal planes is not the same and the spacing of the atoms is not equal, see Fig.
