In actual production and use, sometimes platinum catalysts may lose their activity, turn black, and other situations. When faced with these problems, even experienced production veterans express helplessness and can only repeat operations or even give up raw materials. Since platinum catalysts often fail, let's summarize the reasons for platinum catalyst deactivation:
1. The inactivation caused by poisoning is divided into three types: temporary poisoning (reversible poisoning), permanent poisoning (irreversible poisoning), and selective poisoning.
2. Deactivation caused by coking and blockage
3. Sintering and thermal deactivation (solid-state transformation)
Of course, the reasons for catalyst deactivation are complex and intricate, and each catalyst deactivation is not only caused by one of the above classifications, but often by two or more reasons.
For platinum catalysts, there are three main characteristics:
1. Catalyst carbon deposition. Whether it is the catalyst for aging deactivation or the catalyst for industrial trial deactivation, the carbon deposition is greater than 6% (by weight). It has been found that catalytic reforming reactions are very complex, with two main types of reactions occurring simultaneously. One is the upgrading reaction that increases the octane value of gasoline, which includes dehydrogenation of hexacyclic hydrocarbons to produce aromatic hydrocarbons, isomerization dehydrogenation of pentacyclic hydrocarbons to produce aromatic hydrocarbons, cyclization dehydrogenation of alkanes to produce aromatic hydrocarbons, and isomerization of n-alkanes to produce isoparaffins. The other type is side reactions, which can be divided into two types. One is cracking reactions, including dealkylation of aromatic hydrocarbons and cycloalkanes, ring opening of cycloalkanes, and excessive hydrocracking of alkanes to generate gaseous pathways; Another type is condensation reaction, such as the dehydrogenation and condensation of aromatic hydrocarbons to produce polycyclic aromatic hydrocarbons, and the polymerization and alkylation of acid catalyzed alkene intermediates to produce high boiling point substances. These substances are further converted into black solid or semi-solid charcoal containing a certain amount of hydrogen, which is generally referred to as carbon deposition and deposited on the surface of the catalyst, covering the active center of the catalyst. If each carbon atom in the carbon deposit is regarded as a carbon atom in graphite, occupying a cross-sectional area of 4 square angstroms, the above carbon deposit amount will cover more than 1/2 of the surface area of the catalyst. Fortunately, with different reaction conditions, the composition and structure of the carbon deposit are different, and the carbon deposit exists as particles of a certain diameter, not in the form of single molecules. However, if we consider that carbon deposition first occurs on the active surface, which only accounts for less than L% of the catalyst surface, then it can be considered that carbon deposition is the main factor contributing to catalyst deactivation.
2. Agglomeration of platinum grains: Determination of the dispersion state of platinum metal in fresh and deactivated catalysts using gas pulse chromatography. The results showed that the platinum dispersion of the deactivated catalyst was lower than that of the fresh catalyst, and the platinum dispersion of the aged deactivated catalyst decreased from 0.48 of the fresh catalyst to 0.19. The dispersion size of platinum reflects the average particle size of platinum metal. Observing with an electron microscope, it was found that the platinum grains of the deactivated catalyst condensed from the average particle size of 10-20 of the fresh catalyst to 40-50, and some samples also found platinum grains of 10. If we assume that the platinum grains on the catalyst surface are all ideal cubes of the same size, with one surface in contact with the support and the other five surfaces exposed, and the side length of the cube calculated based on the average particle size, the surface area of platinum metal decreases from 156 to 52 square meters per gram of platinum. The aggregation of platinum grains leads to a decrease in the surface area of platinum. There have been literature reports on the effect of platinum surface area on reaction activity, and the degree of influence varies depending on the type of reaction. When the platinum surface area decreases, the activity of various reactions on the platinum center decreases, especially in the dehydrocyclization reaction where the activity decreases the most.
3. Loss of chlorine components: The chlorine content of both deactivated catalysts decreased from 1.27% to around 0.58% by weight. Chlorine is a component added to regulate the strength of the acidic center of the catalyst. The acid function of the catalyst is mainly determined by the chlorine content, and there is also a certain coordination relationship between the solid acid amount and the platinum surface area. Therefore, the loss of chlorine not only reduces the total acidity of the catalyst and changes the strength distribution of solid acids, but also causes bifunctional dysfunction, reducing the activity and selectivity of the catalyst. In addition, certain changes in the macroscopic structure of deactivated catalysts were also observed. If the specific surface area and pore volume slightly decrease, the number of small pores below 30 decreases, and the number of pores above 30 increases to varying degrees. These changes also affect the activity and selectivity of catalysts from different aspects.
