Concerning
the environmental and health problems related to SO
x
emission, the environmental protection agencies
are incessantly tightening the permissible sulfur level in fuels such
as gasoline and diesel. This gives a greater challenge for the traditional
hydrodesulfurization (HDS), which is quite inefficient for removing
refractory sulfur (RS) compounds (e.g., 4,6-dimethyldibenzothiophene).
Consequently, the new efficient HDS catalysts for substituting the
industrial HDS catalysts (Ni(Co)Mo/Al2O3) have
received much attention. Zeolite-containing HDS catalysts are of particular
importance because zeolites could alleviate the HDS of RS compounds
via isomerization and cracking abilities of their Brønsted acid
sites. The microporous zeolites slow down the diffusion of reactants
and products, leading to uncontrolled cracking reactions that promote
the coke formation and thus deactivate the HDS catalysts. Therefore,
the application of zeolite-based catalysts was limited for HDS reactions
in the early years. But, due to the introduction of hierarchical porous
structure in zeolites, the cracking reactions catalyzed by strong
Brønsted acid sites are controlled and uninterrupted molecular
diffusion becomes highly possible. As a sequel, a better selectivity
and improved HDS efficiency are attainable without any coke formation
or with a reduced coke formation. Besides the porosity and acidity
of zeolites, the synthetic methods and precursors of zeolite-based
HDS catalysts and the mixing of zeolites with other supports like
Al2O3 have impacts on the efficiency of zeolite-based
HDS catalysts. In this review, all these factors are discussed along
with the preparation methods of hierarchical zeolites and tuning methods
of zeolite acidity. The process and catalyst details of traditional
HDS are briefly explained at the outset. The types of metal active
component present in various zeolite-based HDS catalysts and how their
properties are influenced by zeolites are also summarized.
Heterogeneous catalysis plays a key role in promoting green chemistry through many routes. The functionalizable reactive silanols highlight silica as a beguiling support for the preparation of heterogeneous catalysts. Metal active sites anchored on functionalized silica (FS) usually demonstrate the better dispersion and stability due to their firm chemical interaction with FSs. Having certain functional groups in structure, FSs can act as the useful catalysts for few organic reactions even without the need of metal active sites which are termed as the covetous reusable organocatalysts. Magnetic FSs have laid the platform where the effortless recovery of catalysts is realized just using an external magnet, resulting in the simplified reaction procedure. Using FSs of multiple functional groups, we can envisage the shortened reaction pathway and, reduced chemical uses and chemical wastes. Unstable bio‐molecules like enzymes have been stabilized when they get chemically anchored on FSs. The resultant solid bio‐catalysts exhibited very good reusability in many catalytic reactions. Getting provoked from the green chemistry aspects and benefits of FS‐based catalysts, we confer the recent literature and progress focusing on the significance of FSs in heterogeneous catalysis. This review covers the preparative methods, types and catalytic applications of FSs. A special emphasis is given to the metal‐free FS catalysts, multiple FS‐based catalysts and magnetic FSs. Through this review, we presume that the contribution of FSs to green chemistry can be well understood. The future perspective of FSs and the improvements still required for implementing FS‐based catalysts in practical applications have been narrated at the end of this review.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.