Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
2 Surface Organometallic Chemistry (SOMC) . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . 157
2.1 Various Oxide Supports and Their Functionalities Used in SOMC . . . . . . . . . . . . . . . . 158
3 Surface Organometallic Chemistry of Metal Alkyls/Alkylidene and Alkylidyne . . . . . . . . 160
3.1 Reactivity of Group IV (Zr, Hf, and Ti) Metal Alkyls on Oxide Surfaces . . . . . . . . . 160
4 Metathesis of Alkane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
4.1 Mechanism for Alkane Metathesis Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
4.2 Metathesis of Linear Alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
4.3 Metathesis of Cycloalkanes . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 176
4.4 Branched Alkanes Metathesis: Metathesis of 2-Methylpropane . . . . . . . . . . . . . . . . . . . . 179
4.5 Cross Metathesis Between Two Different Alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
4.6 Hydro-metathesis Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
5 Conclusions . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . 183
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
1 Introduction
The activation of ubiquitous C–H and C–C bonds of paraffins is a long-standing
challenge for chemists because of their presence in petroleum and natural gas and
their intrinsic low reactivity. In 1997, our group reported the catalytic transformation of acyclic alkanes into their lower and higher homologues using silicasupported tantalum hydride(s) [1] in the absence of hydrogen at low temperature
(150
C). This has resulted in the faster development of surface organometallic
chemistry (SOMC), a discipline which has progressively emerged as a new area of
heterogeneous and homogeneous catalysis where one can prepare relatively welldefined “single-site catalysts” [2]. The first results in the area of C–H and C–C bond
activation came from the discovery of single-site catalysts, e.g., [(Si–O–) 3 Zr–H],
which were able to catalyze the low-temperature hydrogenolysis of alkanes [3] and
later of polyethylene [4].
What was interesting and new in the SOMC approach is the fact that it was
bridging the two areas of homogeneous and heterogeneous catalysis which did not
overlap enough in the past. During the last 60 years, homogeneous catalysis played
an important role in the selective organic transformations of lower to higher value
products [5, 6]. Clear understanding of the reaction mechanism at the molecular
level and selective formation of the product by tuning the metal center and its
ligands are the main reasons for the increasing use of homogeneous catalysts. This
resulted from the parallel development of molecular organometallic chemistry [7–
11]. In contrast, the heterogeneous catalysts, which are more commonly used in
industry than homogeneous catalysis, did not lead to a clear understanding of
reaction mechanisms (at least at the atomic and molecular level), although the
parallel development of surface science could allow successful story in the identification of elementary steps (e.g., in ammonia synthesis) [12]. The main reason was
the small amount of active sites and consequently the difficulty to fully characterize
156
M.K. Samantaray et al.
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
2 Surface Organometallic Chemistry (SOMC) . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . 157
2.1 Various Oxide Supports and Their Functionalities Used in SOMC . . . . . . . . . . . . . . . . 158
3 Surface Organometallic Chemistry of Metal Alkyls/Alkylidene and Alkylidyne . . . . . . . . 160
3.1 Reactivity of Group IV (Zr, Hf, and Ti) Metal Alkyls on Oxide Surfaces . . . . . . . . . 160
4 Metathesis of Alkane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
4.1 Mechanism for Alkane Metathesis Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
4.2 Metathesis of Linear Alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
4.3 Metathesis of Cycloalkanes . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 176
4.4 Branched Alkanes Metathesis: Metathesis of 2-Methylpropane . . . . . . . . . . . . . . . . . . . . 179
4.5 Cross Metathesis Between Two Different Alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
4.6 Hydro-metathesis Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
5 Conclusions . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . 183
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
1 Introduction
The activation of ubiquitous C–H and C–C bonds of paraffins is a long-standing
challenge for chemists because of their presence in petroleum and natural gas and
their intrinsic low reactivity. In 1997, our group reported the catalytic transformation of acyclic alkanes into their lower and higher homologues using silicasupported tantalum hydride(s) [1] in the absence of hydrogen at low temperature
(150
C). This has resulted in the faster development of surface organometallic
chemistry (SOMC), a discipline which has progressively emerged as a new area of
heterogeneous and homogeneous catalysis where one can prepare relatively welldefined “single-site catalysts” [2]. The first results in the area of C–H and C–C bond
activation came from the discovery of single-site catalysts, e.g., [(Si–O–) 3 Zr–H],
which were able to catalyze the low-temperature hydrogenolysis of alkanes [3] and
later of polyethylene [4].
What was interesting and new in the SOMC approach is the fact that it was
bridging the two areas of homogeneous and heterogeneous catalysis which did not
overlap enough in the past. During the last 60 years, homogeneous catalysis played
an important role in the selective organic transformations of lower to higher value
products [5, 6]. Clear understanding of the reaction mechanism at the molecular
level and selective formation of the product by tuning the metal center and its
ligands are the main reasons for the increasing use of homogeneous catalysts. This
resulted from the parallel development of molecular organometallic chemistry [7–
11]. In contrast, the heterogeneous catalysts, which are more commonly used in
industry than homogeneous catalysis, did not lead to a clear understanding of
reaction mechanisms (at least at the atomic and molecular level), although the
parallel development of surface science could allow successful story in the identification of elementary steps (e.g., in ammonia synthesis) [12]. The main reason was
the small amount of active sites and consequently the difficulty to fully characterize
156
M.K. Samantaray et al.