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This post was last edited by LQ198619 on 2017-5-29 at 18:45. Section 1: The dual-functional properties of catalytic reforming catalysts. The most fundamental reactions in catalytic reforming are dehydration and isomerization; the dehydrocyclization of alkanes can also be regarded as a combination of these two reactions. The mechanisms of these two types of reactions and the catalytic active substances required are different: hydrogenation-dehydrogenation reactions require metal catalysts, whereas isomerization reactions that proceed via a carbocation mechanism require acidic catalysts. This requirement means that catalytic reforming catalysts must possess both of these functions simultaneously: metal active centers for dehydrogenation and acidic active centers for isomerization, namely the so-called dual functionality. Taking the reaction of converting methylcyclopentane into benzene as an example, it can be seen that the catalyst for catalytic reforming must be bifunctional. The reaction mechanism is roughly as follows: the metal components in the reforming catalyst enable alkanes to be catalytically dehydrogenated to alkenes, cycloalkanes to be catalytically dehydrogenated to aromatics; it can also catalyze the hydrogenation of isomeric alkenes, and it plays a role in dehydrogenation, cyclization, and isomerization as well. The acid functionality provided by the carrier (Al2O3-halogen) catalyzes isomerization, cyclization, and hydrocracking via a carbocation mechanism. These two functions operate through olefins (key intermediates). Mills and others were the first to propose a scheme for the reforming reaction, which is as follows: Currently, it is generally believed that on the surface of catalytic reforming catalysts, metal and acidic active centers do not exist separately and independently; rather, they form active groups together. In active groups, the number and reactivity of metal centers and acidic centers, as well as their relative numbers and reactivities, determine the properties of the active group they form. The coordination between metal centers and acidic centers on the reforming catalyst is an important factor in ensuring the full exertion of its catalytic performance. If the metal’s functionality is too strong, carbon deposits are likely to form, causing the catalyst to become inactive and reducing its stability ； It can also cause severe hydrolysis reactions, resulting in a decrease in liquid yield and poorer catalyst selectivity. If the acidic functionality of the catalyst is too strong, it will accelerate the hydrocracking of alkanes or cycloalkanes, resulting in a decrease in the liquid yield and the selectivity for conversion to aromatics ； At the same time, catalysts with overly strong acidic properties will also see their stability decrease due to accelerated carbon deposition. Section 2: Composition of Catalytic Reforming Catalysts Catalytic reforming catalysts are of the supported type, that is, they consist of metal components supported on an alumina carrier modified with halogens. The following discusses these three elements: the metal components, the carrier, and the halogens. 1. Metal components: Metal catalysts are a major category of heterogeneous catalysts. Transition metals are effective hydrogenation-dehydrogenation catalysts, among which group VIII metals are the most widely used. The common feature of these metals is that the number of electrons in their d orbitals is less than 10; they all possess one or more unpaired d electrons. They exhibit a strong chemical adsorption capacity for hydrogen, which gives them high catalytic activity for hydrogenation-dehydrogenation reactions. At present, although there are many types of catalytic reforming catalysts, platinum is invariably the main metal in them all. Initially, only platinum was used as the metal; after 1968, multi-metallic compositions emerged, such as platinum-rhodium, platinum-iridium, and platinum-tin. (1) The platinum series – platinum/Al2O3-halogen – was industrialized in 1949. Previously, the catalysts used for catalytic reforming were molybdenum oxide/alumina, chromium oxide/alumina, etc.; the use of platinum catalysts made catalytic reforming an important process in the petroleum refining industry. Generally, the dehydrogenation activity, stability, and resistance to poisoning of catalysts increase as the platinum content increases. However, platinum is a precious metal, and the manufacturing cost of platinum catalysts is primarily determined by its content. Highly active platinum reforming catalysts require an appropriate amount of platinum, full utilization of the surface of the support, and highly dispersed and uniform distribution of platinum. ①Dispersion of platinum grains: Platinum exists in the form of crystals in reforming catalysts, and its unit cell is a face-centered cubic structure composed of 14 platinum atoms. The smaller the platinum grains, the larger the interface area with the support, resulting in more active clusters, and consequently higher activity and stability. On the carrier, the size of the platinum grains can be measured directly using an electron microscope, or it can be determined by X-ray diffraction. When determined using gas chemical adsorption, the dispersion degree of platinum can be defined as follows: a dispersion degree of 1.0 indicates that the grains are extremely small, with each platinum atom located on its surface ； A dispersion degree of 0.1 means that 1 out of every 10 platinum atoms is located on the surface of the grains, with an average grain size of approximately 10 nm in this case. It is generally required that the average diameter of platinum grains be between 0.8 nm and 10 nm. To deposit platinum in a highly dispersed manner on an alumina carrier, the impregnation method using chloroplatinic acid (H2PtCl6) solution is commonly employed, followed by steps such as drying, calcination, and reduction. The drying temperature is generally set between 100 and 120°C. The purpose of calcination is to convert the metal salts impregnated onto the material into their corresponding oxides, such as PtO2, so that they can be reduced to active metal components. The baking temperature should not be too high, otherwise it will cause the grains of platinum after reduction to become too large. The catalytic reforming catalyst is treated at high temperature in a hydrogen atmosphere, during which the platinum grains gradually coalesce. As the platinum grains grow larger and their specific surface area decreases, the overall conversion rate of heptane declines progressively, while the conversion rate for dehydrocyclization drops even more significantly, indicating that both the activity and selectivity of the catalyst deteriorate over time. ②Within a certain range for the platinum content in the catalyst, the activity and stability of the reforming catalyst increase as the platinum content rises. The effect of platinum content on the rate of n-heptane dehydrocyclization shows that industrial single-platinum catalysts contain 0.3–0.7% platinum; too low a platinum content makes the catalyst prone to deactivation, while too high a content merely increases the cost of the catalyst without significantly improving its performance. Due to the high cost of platinum, there is a tendency to minimize its content as much as possible. ⑵Platinum-rhodium series (rhodium as a co-catalyst): Early catalytic reforming catalysts contained only platinum as the metal component. To further improve the activity and stability of reforming catalysts and enable reforming to be carried out under more stringent conditions, the so-called bimetallic reforming has been widely adopted since the late 1960s; in this approach, in addition to platinum remaining as the main metal component in the catalyst, a second metal component is also added. The commonly used second metal components are rhenium, tin, and iridium, which are used to create bimetallic reforming catalysts in the forms of platinum-rhenium, platinum-tin, and platinum-iridium catalysts. Among these, the first two types are the most widely used. Compared with platinum catalysts, platinum-rhenium catalysts do not offer a significant improvement in initial activity; however, their activity stability is improved, and their carbon tolerance is enhanced. This enables reforming units to operate for extended periods under harsh conditions (low pressure, low hydrogen-to-oil ratio, and high temperatures). Why does the introduction of rhenium improve the stability of the catalyst? There are several explanations: ① The combination of platinum and rhenium causes platinum to be dispersed into finer crystals (i.e., atomic clusters), which prevents their aggregation and growth, thereby enhancing the stability of its activity ； ②Platinum and rhenium form alloys, and these alloys have a slightly poorer dehydrogenation capacity than pure platinum; this prevents severe dehydrogenation reactions, reduces carbon deposition, and thus enhances the stability of the catalyst. ③The carbon deposition reaction often requires two, three, or more adjacent metal atoms to form multi-centered structures; the introduction of rhenium disperses these multi-centered structures, thereby suppressing the carbon deposition reaction, while the useful reforming reactions can still take place at those centers. (The finer the platinum crystals, the easier it is for them to form the active complexes required for dehydrogenation and cyclization in conjunction with acidic components; this makes it less favorable for the polymerization and coking reactions of dimolecules.) ④ Others believe that, under the influence of the carrier, rhenium is not completely reduced to its metallic state, and a portion remains as Re+4. Re+4 can promote the hydrogenation of precursors to carbon deposits, thereby reducing such deposits and extending the catalyst’s lifespan. In platinum-rhodium series reforming catalysts, the Re/Pt ratio has an impact on their reaction performance: ① As the Re/Pt ratio increases, the dehydroisomerization of methylcyclopentane is promoted, while the dehydrocyclization of straight-chain alkanes is inhibited. This also reduces the amount of carbon deposition during reactions, thereby improving the stability of the catalyst and allowing for an **extended operating cycle ； ②Catalysts with a high Re/Pt ratio are more sensitive to sulfur; therefore, stricter control over the sulfur content in the reforming feed oil is required. ③ When Rt/Pt is constant, an appropriate amount of sulfur introduced into the catalyst can suppress the formation of carbon deposits. Furthermore, an increase in Re/Pt does not alter the anti-sintering ability of the metal on the catalyst. The Re/Pt ratio is generally controlled at 1/1 to 3.5/1. Currently, the platinum content in industrial platinum-rhodium catalysts is generally between 0.2 and 0.4 m%, with some having been reduced to 0.15 m%. Role of tin: In catalysts, tin exists mainly in the form of Sn+4 oxides, with a small portion present in the Sn0 state. Due to the presence of SnO2 and Sn atomic clusters, the platinum atomic clusters are diluted, making it difficult for multiple centers to form; this thereby suppresses carbon deposition. At the same time, it also prevents the highly dispersed platinum grains from aggregating. Furthermore, there is a generally agreed view that tin acts as an inhibitor; it prevents the catalyst from undergoing deep dehydrogenation (by slowing down the formation of the pentacyclic diene, which is a precursor to carbon deposits), thereby enhancing the stability of the catalyst. In catalytic reforming reactions, an important pathway for the formation of carbon deposits is: In catalytic reforming reactions, an important pathway for the formation of carbon deposits is: Unlike rhenium, since tin itself has an inhibitory effect on hydrogenolysis reactions, platinum-tin-based reforming catalysts do not require pre-sulfurization before use. The platinum content in the platinum-tin catalysts used in industry currently ranges from 0.35% to 0.6%, while the tin content ranges from 0.25% to 0.5%. ⑷The platinum-iridium series: Iridium, like platinum, is an active component; similar to rhenium, the addition of iridium to platinum catalysts enhances their ability for dehydrogenation and cyclization. Iridium has a stronger hydrogenolysis capability than rhenium; therefore, for platinum-iridium catalysts, not only pre-sulfurization is required, but other metal components must also be added to suppress their hydrogenolysis capability and improve their selectivity. Some believe that, similar to rhodium, iridium can reduce carbon deposition and improve catalyst stability by causing the precursors of carbon deposition (pentacyclic compounds) to undergo ring-opening cleavage. It is different from tin, which reduces carbon buildup by inhibiting the dehydrogenation reaction. Due to its high tendency to agglomerate, limited availability, and higher price than platinum, iridium has gradually been replaced in recent years by platinum-rhodium and platinum-tin catalyst series. ⑸To further improve the performance of reforming catalysts, other metal components are sometimes added in addition to the aforementioned second metal components, such as titanium, aluminum, cerium, etc., resulting in so-called multimetallic reforming catalysts. The introduction of titanium into platinum-rhodium catalysts can improve their activity, selectivity, and stability, resulting in increased yields of both liquids and aromatics. The effect of aluminum incorporation on the reaction performance of platinum-rhodium and platinum-iridium catalysts. 2. Both bulk alumina η-Al2O3 and γ-Al2O3 can be used as the bulk material for reforming catalysts. At higher temperatures, η-Al2O3 has a larger specific surface area and stronger chlorine retention capacity, but it suffers from poor thermal stability and water resistance; therefore, γ-Al2O3 is currently preferred as the support in reforming catalysts. The carrier should have an appropriate pore structure. An excessively small pore size hinders the diffusion of raw materials and products, leading to coking at the micro-pore openings; this prevents the inner surface from being utilized fully and results in a rapid decline in activity. When bimetallic or multimetallic catalysts are used, the operating pressure is lower, requiring the catalyst to have a high carbon capacity and slightly larger pore sizes. For example, the bulk density of platinum catalysts ranges from 0.65 to 0.8 g/cm3, while that of multimetal catalysts ranges from 0.45 to 0.68 g/cm3. The pore size is preferably in the range of 3 to 10 nm; the particle size is 1 to 3 mm. Both η-Al2O3 and γ-Al2O3 possess only weak acidity on their own, which is not sufficient to meet the acidicity requirements of reforming catalysts; however, the addition of chlorine or fluorine increases both the amount and strength of acidity in alumina, thereby meeting the requirements imposed by these catalyst functions. 3. Halogens: The dehydrogenation-hydrogenation and acidic functions of the reforming catalyst must be well coordinated in order to achieve satisfactory results. Although alumina, as a carrier, possesses some acidity, its acidity is too weak to ensure that the catalyst has sufficient ability to promote positive carbocation reactions such as isomerization, which in turn limits the yield of aromatics. To increase the acidity of reforming catalysts, halogen components with high electronegativity such as chlorine and fluorine are generally added. Due to the strong catalytic effect of fluorine on cracking, chlorine is currently more widely used. Its addition amount must be appropriate; if too much is added, the acidity of the catalyst becomes too high, resulting in excessive cracking activity and a decrease in the liquid yield ； If the amount added is too small, the acidity of the catalyst becomes too weak, resulting in poor isomerization capacity, a low yield of aromatics, and a reduced octane number of the products. Generally, the amount of halogen added is 0.4–1.2 m% of the catalyst. Trace amounts of water are inevitably present in the feed oil, and it causes the chlorine on the reforming catalyst to gradually be lost. Therefore, on the one hand, it is necessary to strictly control the water content in the feed oil, and on the other hand, chlorine must be replenished appropriately during operation in order to maintain an adequate acidity level of the catalyst. The method of adding chlorine is to inject dichloroethane, trichloroethane, or carbon tetrachloride into the feed oil. This so-called control of the water-chlorine balance is an important method for adjusting the acidic functionality of the reforming catalyst to match its metallic functionality. The effect of halogens on increasing the acidity of reforming catalysts is generally attributed to an induction effect that enhances the activity of protonic acids on the surface of the alumina support; some also attribute it to an increase in the number of non-protonic acid sites. For Al2O3: ① increase the strength of Brönsted acid centers ② increase the number of Lewis acid centers