Surface organometallic chemistry (SOMC) is an established concept, associated with specific tools for the characterization of a well-defined single-site catalysts. Primarily, chemical industry preferred heterogeneous catalyst over homogeneous catalyst for various reasons. But the difficulty in development of a well-defined heterogeneous catalyst stalled by the presence of various kinds of active sites as well as their low concentration on surfaces. To develop a well-defined heterogeneous catalyst which can possess hundred percent active sites like that of homogeneous catalyst a new branch was developed called surface organometallic catalyst (SOMCcat). This is a well-proven concept where a homogeneous catalyst could graft on an oxide support to form SOMCcat. These surface species can be well-characterized by modern NMR techniques apart from EXAFS, IR, and gas quantification methods. In this review the application of SOMC strategy for the design and preparation of catalyst for industrial relevant processes are discussed.
Heterogeneous catalysis is a crucial discipline to solve the global fields of energy and environment which are presently challenging all the scientists worldwide. Heterogeneous catalysis is moving from a situation of “ill-defined materials” to a situation of “well defined single site catalysis”. In particular “single atom catalysis” is becoming now a mature area, because it was discovered that in many cases it is not necessary to have an ensemble of atoms (on oxides or on metals). There are many examples to illustrate this evolution in catalysis [1,2]. But there is a particular case of single atom catalysis, which is called surface organometallic chemistry or surface organometallic catalysis (both abbreviated as SOMC) [3]. This area started when it was discovered that any type of organometallic compounds, and by extension coordination compounds, can react with any kind of surface such as oxides, MOF, zeolites, metal nanoparticles to give a new kind of hybrid material which contains both the solid state material and the molecular organometallic fragment which is called Surface Organometallic Fragment (SOMF) [4], [5], [6], [7], [8], [9], [10], [11].
Progressively it appeared that in most cases these well-defined SOMFs are reaction intermediates in heterogeneous catalysis [12]. This discovery was due to the fact that molecular chemistry was progressing quite rapidly with identification of the elementary steps of homogeneous catalysis. A theory has emerged based on this overlap of concepts, which lead to the discovery of new catalytic reactions (e.g. alkane metathesis cleavage of alkanes by methane, coupling of methane to ethane and hydrogen [13], [14], [15], [16], [17], [18], [19], [20], [21] as well as improvement of existing reactions (e/g. olefin metathesis). [22]
This theory is predictive: one has just to postulate a mechanism based on elementary steps of molecular chemistry and enter this reaction by this postulated reaction intermediate [12].
In this review article we will consider the new various reactions discovered and the known reactions improved via this strategy
Olefin metathesis can be defined as a reaction where redistribution of the olefinic bond takes place in the presence of a catalyst [23]. The first observation of the metathesis of propene at high temperature was reported in 1931 [24], whereas the earliest catalyzed metathesis reactions were found in the 1950s by scientists at DuPont, Standard Oil, and Phillips Petroleum [25]. They reported that propene led to ethylene and 2-butene in the presence of molybdenum oxide catalyst [26]. Although the
Propane was converted into ethane and butane in the presence of a tantalum surface complex [(Si−O−)2 TaH] at 150 o C as it resembles to the olefin metathesis reaction. For the first time, the name “alkane metathesis” was given for this type of reaction [13] although this reaction was observed before by Burnett and Hughes in 1973 and termed as “disproportionation of alkane” [65]. But in contrast to a single site catalyst, they used a dual catalytic system for this conversion at a higher
The first well−defined SOMC complex used for the hydrogenolysis reaction was [(Si−O−)Zr(Np)3] [76,77]. During the hydrogenolysis of [(Si−O−)Zr(Np)3], it was observed that gaseous methane and ethane were produced instead of neopentane: this suggested that neopentane could undergo hydrogenolysis [76]. Subsequently, the hydride of [(Si−O−)Zr(Np)3] was prepared by treating it with H 2 at 150 o C to obtain [(Si−O−)(4−x) Zr(H)x] (x=1 or 2), which were subsequently used for hydrogenolysis [78]. This
Dwindling of petroleum products with time forced researchers to find an alternative way for the synthesis of diesel range alkanes. The Fischer-Tropsch alkane metathesis is the primary alternative for synthetic fuel. In alkane metathesis reaction, it was observed that the TONs are not very high until the synthesis of a bimetallic [W/Ti] surface complex where TONs have risen to 10,000. During the alkane metathesis reaction, it was observed that catalysts like [Ta−H] and [W−H] are not able to
Hydroamination and hydroaminoalkylation are the reactions where the N−H bond of an amine or α−C−H bond of an alkylamine is added across an unsaturated C−C bond (Scheme 20). These reactions open up new paths for functionalizing cyclic or acyclic olefins for pharmaceuticals and agrochemicals [87].
Hydroamination reaction is an economical atom reaction where one can obtain a range of organic molecules incorporating imines, enamines, or N−containing heterocycles [88], [89], [90]. Though the
The first imine metathesis reaction was reported by Ingold in 1922 [98]. Though there are many reports of homogeneous catalysts in the field, only a few reports are present in the literature using SOMC complexes for this reaction. The first SOMC catalyst which was utilized for imine metathesis reaction was a surface zirconium catalyst [(Si−O−)Zr(=NEt)NEt 2] [99]. This zirconium catalyst was prepared by the reaction of partially dehydroxylated silica (SiO 2−700) with Zr(NEt 2)4 in pentane after
Surface Organometallics Chemistry plays more and more an important role in heterogeneous catalysis for many fundamental and applied reasons:
The Author declare no competing financial interests.
We Thank King Abdullah University of Science and Technology for generous financial support.
