Perspectives on some challenges and approaches for developing the next generation of selective, low temperature, oxidation catalysts for alkane hydroxylation based on the CH activation reaction
Challenges and approaches to the de novo, rational development of the next generation of organometallic, alkane functionalization catalysts based on the CH activation reaction.
Introduction
The conversion of fossilized hydrocarbons to energy and materials is a foundational technology. While it is important that we consider a switch to future alternatives, such as the proposed hydrogen-based economy, it is critical that as a bridge to this long term future, we develop more environmentally benign, greener technologies for these essential fossil fuel based processes that will continue to be important in the next decade. As shown in Fig. 1, the key objectives of such greener processes must be to minimize emissions and capital while maximizing energy and materials output. Importantly, reducing dependence on petroleum and increasing use of under-utilized, abundant natural gas would facilitate this movement to these greener technologies while extending the life time of these limited fossilized resources.
Alkanes from natural gas and petroleum are among the world’s most abundant and low-cost feedstocks. Currently petrochemical technologies to convert these feedstocks to energy, fuel and chemicals operate at high temperatures and utilize multiple steps that lead to inefficient, capital intensive processes. The development of low temperature, selective, direct alkane oxidation chemistry could lead to a new paradigm in petrochemical technology that is environmentally cleaner, economically superior and allow the large reserves of untapped remote natural gas to be valorized as primary feedstocks for fuels and chemicals [1]. Alcohols are among the highest volume commodity chemicals and most versatile feedstocks [1b]. A primary reason that technologies for direct, selective hydroxylation of alkanes to alcohols remain a challenge is that the current commercial catalysts for alkane oxidation (typically solid metal oxides) are not sufficiently active for the functionalization of alkane CH bonds and high temperatures and harsh conditions must be employed that lead to low reaction selectivity [1a].
The development of next generation catalysts that would allow the selective hydroxylation of methane and higher alkanes to alcohols at low temperatures (∼200–250 °C) in inexpensive reactors, with fewer steps and in high yields could provide a basis for this paradigm change in the petrochemical industry. Examples of the products that could be dramatically impacted by such low temperature, hydroxylation catalysts are shown in Fig. 2.
Significant advances in the chemistry of the hydrocarbons have been made since the 1970s. Particularly relevant to the development of low temperature, selective, heteroatom hydrocarbon functionalization catalysts has been the discovery of homogeneous metal complexes that cleave the CH bonds of unactivated hydrocarbons at low temperatures and with extraordinary selectivity, Fig. 3 [2], [3]. Thus, studies have shown that primary CH bonds are more reactive than tertiary, aromatic more reactive than aliphatic and important to the challenge of selective oxidation of methane to methanol, the CH bonds of alcohols are less reactive than those of the parent alkanes. Since this discovery, there has been and continues to be intense interest in incorporating the CH activation reaction into catalytic cycles to convert hydrocarbon to more useful functionalized products. However, to date relatively few catalyst systems that are based on the CH activation reaction have been developed that allow the functionalization of hydrocarbons [4], [5], [6], [7], [8], [9], [10] and there are still large gaps in our fundamental knowledge of how to design such catalysts [2], [3]. In this article, the focus is discussion of some of the challenges and approaches to developing the next generation of alkane hydroxylation catalysts based on the CH activation reaction with emphasis on our research.
Section snippets
General catalyst requirements
In considering the de novo design of any new catalyst it is important to note that to be useful, all catalysts must meet some minimum performance requirements related to catalyst stability, rate and selectivity. Importantly, as illustrated in Fig. 4, effective catalysts must simultaneously meet all three of these performance requirements; meeting any one or two would not lead to useful catalysts. This represents a key challenge to the rational design of any efficient catalyst because these
CH activation as an inner-sphere reaction
Homogeneous transition metal catalysis has had a substantial impact on organic chemistry. From polymerizations to hydrogenations there are few aspects of organic chemistry that have not been touched by this field of research. The majority of these catalytic reactions take place in the inner or first coordination sphere of the homogeneous metal catalyst and in many cases lead to the formation of organometallic, MC, intermediates. The advantage of these inner-sphere, organometallic reactions is
Key challenge is simultaneous design of rapid, stable CH activation systems coupled to oxidative functionalization reaction
If we assume that catalysts based on CH activation can be selective and rapid, the key challenges that remain are to identify the requirements for designing stable catalysts while maintaining high alkane oxidation rates at low temperatures. These general requirements can be broken into more specific requirements as follows: (A) A reactive species must be generated that reacts rapidly with the CH bond via the CH activation reaction without the need for high energy conditions (as this is
Some performance guidelines for homogeneous catalysts for selective low temperature oxidation of methane to methanol
Meeting the challenge of developing the next generation of catalysts based on the CH activation reaction (or other pathways) for methane (or other alkane) conversion will require continued fundamental knowledge to be acquired by basic research and to be used in the design of these new catalysts. An important additional consideration is that while the main thrust of research to develop new catalysts is still largely in the area of fundamental research, as the field advances it could be of
Acknowledgements
We thank Dr. W.L. Schinski, Dr. D. Driver, Dr. J. Oxgaard and Prof. W.A. Goddard III for helpful discussion. Our work on alkane oxidation is supported by NSF grant CHE-0328121 and the ChevronTexaco Energy Research and Technology Company.
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