11
HDN, and hydrogenation of unsaturated hydrocarbons. Industrial HDT catalysts
usually consist of molybdenum supported on γ-alumina and promoted with either
cobalt or nickel. Although CoMo- and NiMo-based hydrotreating catalysts had
been the mainstay in the commercial units, there was a need to improve the activity
of these catalysts due to the introduction of stringent fuel quality regulations, especially with respect to sulfur during the 1990s spearheaded by the USA and Europe.
Refiners were forced to comply with the specifications of ultralow sulfur fuels such
as diesel and gasoline (sulfur: 10–15 ppmw) in several countries following the mandate in developed countries such as the USA and Europe.
The activity of traditional hydrotreating catalysts was not sufficient to meet the
new specifications, and severe process conditions such as high reactor temperatures,
low space velocity, and high hydrogen partial pressure were required in the
hydrotreaters to meet the stringent specifications. The use of low-activity catalysts
under severe process conditions resulted in higher catalyst deactivation rates with
shorter cycle lengths and lower unit throughput. Catalyst suppliers and researchers
were continuously striving to develop highly active and stable catalysts for reducing
sulfur content to ultralow levels from hydrocarbon streams such as diesel and gasoline. Improved understanding of the important aspects of hydrotreating catalysts
such as nature of the active phase and their structure, support effects, and the textural characteristics of the supports paved the way in developing high-activity catalysts with improved performance.
Better catalyst characterization tools helped in unraveling the mystery shrouded
with hydrotreating catalysts, and a scientific basis was explained for the performance of new-generation catalysts in terms of their activity, selectivity, and longterm stability. Catalyst scientists used some of the advanced catalyst characterization
methods such as high-resolution transmission electron microscopy (HR-TEM),
Mössbauer emission spectroscopy (MES), and extended X-ray absorption fine
structure (EXAFS) along with molecular modeling tools to comprehend the nature
of active phase present in sulfided hydrotreating catalysts [11]. A comprehensive
review of the hydrotreating catalyst developments for deep hydrodesulfurization of
diesel was written focusing on the nature of active sites, effect of support and additives, improvements in catalyst preparation techniques, etc. by Stanislaus et al. [4].
6.1 Nature of Active Phase in HDT/HDS Catalysts
Different models have been proposed in the literature to explain the nature of active
sites in the unpromoted and promoted molybdenum oxide-based HDT catalysts.
Some of the features of the proposed models are listed below:
• Coordinately unsaturated (CUS) sites or exposed Mo ions with sulfur vacancies
located at the edges and corners of MoS 2 structures were found to be active for
promoting HDT reactions in the case of unpromoted MoS 2 catalysts.
• Basal planes are usually not active in the adsorption of molecules and do not usually contribute to catalytic activity.
Recent Advances in Hydrotreating/Hydrodesulfurization Catalysts: Part I: Nature…
HDN, and hydrogenation of unsaturated hydrocarbons. Industrial HDT catalysts
usually consist of molybdenum supported on γ-alumina and promoted with either
cobalt or nickel. Although CoMo- and NiMo-based hydrotreating catalysts had
been the mainstay in the commercial units, there was a need to improve the activity
of these catalysts due to the introduction of stringent fuel quality regulations, especially with respect to sulfur during the 1990s spearheaded by the USA and Europe.
Refiners were forced to comply with the specifications of ultralow sulfur fuels such
as diesel and gasoline (sulfur: 10–15 ppmw) in several countries following the mandate in developed countries such as the USA and Europe.
The activity of traditional hydrotreating catalysts was not sufficient to meet the
new specifications, and severe process conditions such as high reactor temperatures,
low space velocity, and high hydrogen partial pressure were required in the
hydrotreaters to meet the stringent specifications. The use of low-activity catalysts
under severe process conditions resulted in higher catalyst deactivation rates with
shorter cycle lengths and lower unit throughput. Catalyst suppliers and researchers
were continuously striving to develop highly active and stable catalysts for reducing
sulfur content to ultralow levels from hydrocarbon streams such as diesel and gasoline. Improved understanding of the important aspects of hydrotreating catalysts
such as nature of the active phase and their structure, support effects, and the textural characteristics of the supports paved the way in developing high-activity catalysts with improved performance.
Better catalyst characterization tools helped in unraveling the mystery shrouded
with hydrotreating catalysts, and a scientific basis was explained for the performance of new-generation catalysts in terms of their activity, selectivity, and longterm stability. Catalyst scientists used some of the advanced catalyst characterization
methods such as high-resolution transmission electron microscopy (HR-TEM),
Mössbauer emission spectroscopy (MES), and extended X-ray absorption fine
structure (EXAFS) along with molecular modeling tools to comprehend the nature
of active phase present in sulfided hydrotreating catalysts [11]. A comprehensive
review of the hydrotreating catalyst developments for deep hydrodesulfurization of
diesel was written focusing on the nature of active sites, effect of support and additives, improvements in catalyst preparation techniques, etc. by Stanislaus et al. [4].
6.1 Nature of Active Phase in HDT/HDS Catalysts
Different models have been proposed in the literature to explain the nature of active
sites in the unpromoted and promoted molybdenum oxide-based HDT catalysts.
Some of the features of the proposed models are listed below:
• Coordinately unsaturated (CUS) sites or exposed Mo ions with sulfur vacancies
located at the edges and corners of MoS 2 structures were found to be active for
promoting HDT reactions in the case of unpromoted MoS 2 catalysts.
• Basal planes are usually not active in the adsorption of molecules and do not usually contribute to catalytic activity.
Recent Advances in Hydrotreating/Hydrodesulfurization Catalysts: Part I: Nature…