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In the mid-1950s, U.S. demand for gasoline increased year by year, while demand for diesel and fuel oil declined year by year; the product mix was not able to adapt to these changes in demand patterns. Although secondary processing technologies such as thermal cracking, catalytic cracking, and delayed coking could increase gasoline production at that time, the quality of gasoline did not meet the requirements for higher octane numbers in vehicle fuel. As the compression ratio of car engines increases, gasoline with a high content of heteroalkanes and aromatics is required to prevent knocking in vehicles. Therefore, a new processing technology is needed to convert heavy oil products into light oil products. Based on their experience in the development of catalytic cracking catalysts, as well as Germany’s experience in producing gasoline and diesel through high-pressure catalytic hydrogenation of coal and coal tar, many oil companies have, through experimental research, identified certain special irreversible reaction processes. They have also developed fixed-bed hydrocracking processes and catalysts that enable individual hydrocarbons to react as desired, thereby controlling the conversion of the entire mixture. In 1959, the American company Chevron was the first to announce the development of the Isocracking hydrocracking technology. In 1960, UOP announced the development of the Lomax hydrocracking technology, while Union announced the development of the Unicracking hydrocracking technology. Later on, H-G from Gulf Research and Development Company, Shell from Shell International Petroleum Group, IFP from the French Petroleum Institute, DHC from German company BASF, and BP from the United Kingdom were all successful in developing hydrocracking technologies. In November 1961, UOP’s Lomax hydrocracking technology and Chevron’s Isocracking hydrocracking technology were combined to form Isomax hydrocracking (with the hydrocracking catalysts still being supplied by each of the two companies separately). The hydrocracking technologies developed by various companies use different catalysts, but their process flows are similar. The development of hydrocracking technology over the past 40 years can be summarized as follows: In the early 1960s, hydrocracking technology was primarily used to convert CGO, LCO, and AGO into gasoline. Since the conversion rate in catalytic cracking was low at that time and some feedstocks could not be converted, hydrocracking was mainly used to convert oils that were difficult to crack in catalytic cracking units, in order to increase gasoline production. At that time, hydrocracking units all employed a two-stage process: first, in the first stage, the feed oil was refined using hydrogenation catalysts to remove impurities such as sulfur and nitrogen; thereafter, it proceeded to the second stage, where selective cracking catalysts were used to crack the oil and produce gasoline. The hydrocracked light gasoline obtained had a high octane rating, and could be used directly as a component in gasoline blending ； Catalytic reforming of heavy gasoline containing naphthenes yields high-octane gasoline and hydrogen in high yields. This two-stage hydrocracking process is still in use today; it is employed in refineries with large amounts of catalytic cycle oil and high demand for gasoline (such as in the United States), as well as in refineries that use VGO as a raw material to produce gasoline and reforming feedstocks. It is also used in refineries that primarily produce middle distillates from VGO and have large hydrocracking capacity. With the advancement of catalytic cracking technology (lift tube technology and molecular sieve catalysts), catalytic cracking has become capable of producing large quantities of high-octane gasoline. Meanwhile, due to the rapid increase in demand for jet fuel and diesel in the oil market – especially after the 1970s – new catalysts with high activity, strong selectivity, good stability, and the ability to convert heavier crude oils have become more mature. In terms of hydrocracking processes, single-stage processes focused on producing middle distillates have emerged, as well as single-stage cascade processes that offer greater flexibility by enabling both the production of middle distillates and naphtha. Most newly built hydrocracking units in refineries are now designed primarily to process VGO for the production of jet fuel and diesel. By 1975, 60% of the processing capacity of the newly built hydrocracking units was used for producing jet fuel and diesel, and this percentage continued to increase over the years. Since the 1980s, the trend in the development of hydrocracking technology has been, in addition to producing more middle-distillate oils, to use the unconverted, alkane-rich tail oil from hydrocracking as feedstock for catalytic cracking, or as material for cracking to produce ethylene, or as a base oil for high-viscosity index lubricants. Since the 1990s, 90% of the processing capacity of newly built hydrocracking units has been used to primarily produce middle distillates. There are units that use a single-stage process or a single-stage in-series process, as well as large-scale units that employ a two-stage process. The Isocracking and Lomax two-stage hydrocracking units that were industrialized in the early 1960s used Ni-SiO2-Al2O3 catalysts for the second-stage hydrocracking. At that time, it was a significant achievement in the development of hydrocracking catalysts, one that drew on synthetic aluminum silicate catalysts used in catalytic cracking and metal sulfide catalysts used for the hydrogenation of coal and coal tar gases, by combining the cracking activity of aluminum silicate with the hydrogenation activity of metal sulfides. However, practical usage results show that this catalyst not only requires thorough desulfurization and denitration of the feed oil, but also has low activity and fails to achieve the desired results, especially when the feed oil has a high sulfur content. At the same time that Mobil Oil first used synthetic molecular sieve catalysts in catalytic cracking units, Rowland Hansford at Union Company was also working on the development of hydrocracking catalysts using molecular sieves as carriers; these catalysts were first employed in the Unicracking hydrocracking unit in 1964. Practice has shown that this molecular sieve hydrocracking catalyst exhibits significantly higher activity and better selectivity compared to amorphous hydrocracking catalysts. Although this molecular sieve hydrocracking catalyst was not the first type of catalyst used in modern hydrocracking, it laid the foundation for the development of hydrocracking catalysts and represented the direction in which such catalysts evolved. The Unicracking hydrocracking unit, which came online in 1964 at Unocal’s refinery in Los Angeles, was the first industrial plant to utilize Unocal’s hydrocracking technology. Its processing capacity was 800 kt/year (16,000 barrels per day); it operated in two stages, with the second stage using the HC-11 hydrocracking catalyst (palladium-Y type molecular sieve) to produce gasoline from LVGO and LCO as raw materials. Its main feature is that the crude oil is first subjected to hydroprocessing, particularly hydrodenitration (to preserve the activity of the molecular sieve catalyst), before proceeding to the second stage for hydrocracking. Over the past 40 years, the development of Unocal hydrocracking catalysts has essentially been part of the history of molecular sieve hydrocracking catalysts. To date, the hydrocracking catalysts developed by Unocal can be divided into the following 5 series. Series I are precious metal molecular sieve hydrocracking catalysts used to produce the largest amount of gasoline (naphtha) and some jet fuel; they are suitable for two-stage units as well as single-stage cascade units. The earlier ones were HC-11, HC-18 ； The third generation is HC-28, which was put into industrial use in 1987. HC-28 has higher activity than HC-18, with the reaction temperature being 10–15 degrees lower℃ ； The fourth generation is HC-35, which was introduced for industrial use in 1996. HC-35 has a similar activity to HC-28, but higher selectivity, resulting in a 4% higher yield of jet fuel. Series II are non-precious metal molecular sieve hydrocracking catalysts used to produce the largest amount of gasoline (naphtha) and some jet fuel; they are suitable for two-stage units as well as for single-stage tandem units. The first generation is HC-14 ； The second generation is HC-24, which was put into industrial use in 1992. HC-24 has higher activity than HC-14, with a reaction temperature that is 5.6–11.2 degrees lower℃ ； The third generation is HC-34; no reports of its industrial application have been seen yet. Pilot-scale test results show that HC-34 has higher activity than HC-24, with a reaction temperature 6.7°C lower. Series III are non-precious metal molecular sieve hydrocracking catalysts for the flexible production of gasoline (naphtha), jet fuel, and diesel; they can be used in single-stage series units as well as in two-stage units. The first generation is HC-16 ； The second generation is HC-26, which was put into industrial use in 1990. HC-26 has higher activity than HC-16, with a reaction temperature that is 17–19.5 degrees lower℃ ； The third generation is HC-33, which was put into industrial use in 1994 ； The fourth generation is HC-43, and no reports of industrial application have been seen yet. Series IV are non-precious metal molecular sieve hydrocracking catalysts that produce the largest amount of middle distillate oil along with a small amount of gasoline (naphtha), and are mainly used in single-stage series units. The first generation was the HC-22, which saw industrial use in the mid-1980s ； The second generation is the DHC-32, which was put into industrial use in 1993 ； The third generation is the DHC-39, which was put into industrial use in 1996 ； The fourth generation is the DHC-41, and no reports of industrial application have been seen yet. Series V are hydrogenation pretreatment catalysts designed to be used in conjunction with molecular sieve hydrocracking catalysts. It has evolved to the 8th generation since the mid-1950s. Those that were developed and put into industrial use in the early stages include N-2, HC-A, HC-B, HC-D, and HC-F. The sixth generation is HC-K/H, which was first used industrially in the early 1980s; if the relative denitrification activity of the first-generation N-2 is set at 100, then that of HC-K/H is 650 ； The seventh generation is HC-P/R, which was first used industrially in the mid-1990s, with a relative denitrification activity of 750 ； The eighth generation is HC-T, which has just been put into industrial use. UOP’s hydrocracking technology was the first hydrocracking technology to be applied industrially in the world. The first industrial plant to use this technology (Lomax) was put into operation in August 1961 at the Los Angeles refinery of Bowelin Oil Company in California, USA, with a processing capacity of 110 kt/year (2,200 barrels per day). It employed a two-stage process to produce gasoline using AGO as raw material. The development of UOP’s hydrocracking technology began with amorphous hydrocracking catalysts; after the 1970s, efforts were started on developing molecular sieve hydrocracking catalysts. Since the early 1990s, UOP has collaborated with Unocal to develop hydrocracking catalysts. In 1995, UOP acquired Unocal’s hydrogenation technology division, and from then on all intellectual property related to Unocal’s hydrocracking technology became the property of UOP; however, the name of the Unicracking technology and the catalyst codes continued to be used. After nearly 40 years of development, its catalysts have formed two main series. Series I consists of amorphous hydrogenation cracking catalysts that are non-precious metal-based and primarily used for producing middle distillate oils; they are mainly applicable to single-stage units but can also be used in two-stage units. The first generation was the DHC-2, the second generation was the DHC-6, and the third generation was the DHC-8; it was put into industrial use in 1983. Series II consists of non-precious metal molecular sieve hydrocracking catalysts primarily used for producing middle distillate oils; they are mainly applied in single-stage series units, but can also be used in two-stage units. The first generation was the DHC-100, which was used in industrial applications in the mid-1980s ； The second generation is the DHC-200, which was put into industrial use in 1990. In addition, UOP developed the hydrocracking catalyst HC-8 for producing the largest amount of gasoline (naphtha), and later introduced the HC-100 catalyst as a replacement for HC-8, which was put into industrial use in 1980. There is also the HC-101 catalyst for producing liquefied gas. The hydrocracking catalysts developed by UOP in collaboration with Unocal are DHC-32, DHC-39, and DHC-41. Chevron’s hydrocracking technology is the second-most widely used hydrocracking technology in industry, after UOP’s. The first industrial plant to use this technology (Isocracking) came online in January 1962 at the Toledo Refinery in Ohio, with a processing capacity of 375 kt/year (7,500 barrels per day). It operated on a two-stage process, using AGO, LCO, and CGO as feedstocks to produce gasoline (naphtha). The development of Chevron’s hydrocracking technology also began with amorphous hydrocracking catalysts; molecular sieve hydrocracking catalysts were developed after the 1970s. Through nearly 40 years of development, the following 5 main series have been established: Series I consists of non-precious metal amorphous catalysts that produce the largest amount of middle distillate oil, and can be used in both single-stage and two-stage units. The first generation is ICR-102 ； The second generation is ICR-106 ； The third generation is ICR-120, which was used in industrial applications in the mid-1980s ； The fourth generation is ICR-150, which was used in industrial applications in the late 1990s. Series II are non-precious metal molecular sieve catalysts that produce the largest amount of middle distillate oil, mainly used in single-stage units. The first generation was ICR-126, with industrial use in the mid-1980s ； The second generation is ICR-136, which was used in industrial applications in the early 1990s ； The third generation is ICR-142, which was used in industrial applications in the late 1990s. Series III are non-precious metal molecular sieve catalysts for the flexible production of gasoline, jet fuel, and diesel, mainly used in single-stage units. The first generation is ICR-117, the second generation is ICR-139/141, which were used in industry in the early 1990s; the third generation is ICR-147, which was used in industry in the late 1990s. Series IV are non-precious metal molecular sieve catalysts used to produce the largest amount of gasoline (naphtha) and some jet fuel, primarily in two-stage hydrocracking units ； The first generation is ICR-204 ； The second generation is ICR-208, which was used in industrial applications in the early 1990s ； The third generation is ICR-210, which was introduced for industrial use in 1991. Series V are precious metal molecular sieve catalysts used to produce the largest amounts of gasoline (naphtha) and jet fuel, primarily in two-stage hydrocracking units. The first generation was ICR-207, which was introduced for industrial use in 1988 ； The second generation is ICR-209, which was used in industrial applications in the early 1990s ； The third generation is ICR-220, which was introduced for industrial use in 1999; it is capable of producing the largest amount of jet fuel, with a yield that exceeds that of ICR-120. In addition to Unocal, UOP, and Chevron, there are also IFP, as well as Criterion Catalysts and Akzo, which specialize in the development and supply of hydrocracking catalysts. However, the technologies developed by these companies and the catalysts they supply have not played a dominant role in the advancement of hydrocracking technology nor in the international market. In the late 1970s and early 1980s, as demand for heavy fuel oil in the international market declined while demand for middle distillates, particularly diesel fractions, increased, many large foreign companies successively introduced Mild Hydrocracking technology. Initially, the existing VGO hydrodesulfurization unit (referred to as indirect desulfurization in Japan) was used, with simple modifications made and the catalyst replaced; hydrocracking was carried out at a low conversion rate while keeping the operating pressure unchanged, in order to increase diesel production. At that time, this technology received widespread attention and emphasis from countries such as Japan and Western Europe. Since most of these refineries are of the light processing type, and the demand for middle distillates – especially diesel fractions – is much greater than that for gasoline, a number of VGO hydrodesulfurization units have been converted into mild hydrocracking units. Due to the limitations imposed by the existing equipment in the mild hydrocracking unit, the operating pressure is generally between 5.6 and 7.0 MPa, which restricts improvements in product quality; as a result, the smoke point of jet fuel and the cetane number of diesel are not high. Later, Mobil Oil introduced the Moderate Pressure Hydrocracking technology. This technology operates in a single stage and single pass under conditions of a pressure of 7.0–10.5 MPa and a temperature of 343–427°C. It was first put into industrial use in 1983, and there are currently 3 industrial units in total ； The second set of units was newly built at the Jurong Refinery in Singapore, with a processing capacity of 1.45 Mt/year. (29,000 barrels/day), put into operation in 1990, with an operating pressure of 9.1 MPa; it uses sulfur-containing VGO as raw material, achieving a conversion rate of 50%–60% to produce light distillate oils, while the unconverted residue is used to produce low-viscosity diesel through isomerization and dewaxing. Mobil once conducted medium-pressure hydrocracking tests on LCO using this technology, obtaining experimental data with conversion rates of 40% and 62%, but there have been no reports of industrial installations using it to date. Recently, Chevron announced that it has developed a new process for the hydrogenation reforming of LCO using single-stage partial conversion technology, which features low hydrogen consumption and high yields of middle-distillate oil, with the first industrial plant set to come online by the year 2000. Mobil-Akzo-Kellog also claims to have developed a process that utilizes single-stage sequential hydrocracking to carry out selective partial hydrocracking, thereby saturating one of the aromatic rings in polycyclic aromatics (such as alkylnaphthalenes), followed by selective ring-opening hydrocracking. This process features mild reaction conditions, with no excessive cracking or dealkylation reactions that result in the production of naphtha and gases. Its advantages include high diesel yield, low hydrogen consumption, a decrease in density, and a significant increase in cetane number. Since the 1970s, amid a significant increase in the international demand for diesel, Mobil Oil Company, building on its discovery of the synthetic mesoporous zeolite ZSM-5 in the late 1960s and utilizing its selective cracking ability for n-paraffins, developed the Medium Distillate Dewaxing (hydrodesulfurization) technology (MDDW). Following the first industrial application of the first-generation catalyst in 1974, two further generations of catalysts were put into industrial use. Meanwhile, Mobil introduced the catalytic dewaxing technology for producing lubricant base oils (MLDW), and by 1996, four generations of catalysts had been put into industrial use. After the industrialization of the catalytic dewaxing (hydrodewaxing) technology for producing low-viscosity diesel (MDDW), the ZSM-5 catalyst was modified to develop a bifunctional noble-metal catalyst, which enabled the successful production of low-viscosity diesel from middle distillate oils through isomerized dewaxing (MIDW). The first industrial plant was put into operation in 1990. On this basis, Mobil developed another technology for producing lubricant base oils through heteroisomerization (MSDW). By using a new synthetic zeolite catalyst containing precious metals and exhibiting better selectivity than ZSM-5, both isomerization and selective cracking reactions are carried out, resulting in II/III type lubricant base oils with a significantly increased viscosity index. The first generation of this catalyst was put into industrial use in 1997, while the second generation was introduced in 1999. To meet the need for upgrading its lubricant products, Chevron was the first to develop isomerization dewaxing technology for producing Group II/III lubricant base oils. The key technology involves precious metal catalysts supported on mesoporous zeolites SAPO; the first generation of such catalysts was put into industrial use in 1993, the second generation in 1996, and the third generation in 1999.