Single sites in heterogeneous catalysts: separating myth from reality

Single sites in heterogeneous catalysis correspond to an ultimate form of dispersed metal sites with well-defined environments and uniform activity towards the reactants. A surface organometallic chemistry (SOMC) approach aims at the formation of single-site catalysts by leveraging controlled reactivity of coordination complexes with surfaces of supports. The intrinsic complexity associated with the surfaces of oxidic materials is a huge obstacle in realizing the ultimate single site. Attempts at minimizing surface disorder can counterintuitively increase complexity and vice versa. Single site is a concept and constraint that fosters creativity and challenges our views on and understanding of surfaces of materials. Heterogeneous catalytic processes are a staple of the sustainable chemical industry. One of the holy grails of contemporary catalysis science is the formation of the so-called single-site catalysts. The single-site character implies that the catalytically active species are structurally uniform, do not interact with each other, and exhibit identical affinity towards the substrates in the desired transformation. Achieving such characteristics in real-life materials is a challenge due to the complex nature of chemical transformations at surfaces. Surface organometallic chemistry (SOMC) has emerged as a powerful approach to form well-defined species on the surface of materials. In this short review, we discuss the myth and reality of single-site heterogeneous catalyst formation, with a primary focus on application of SOMC methodology. Heterogeneous catalytic processes are a staple of the sustainable chemical industry. One of the holy grails of contemporary catalysis science is the formation of the so-called single-site catalysts. The single-site character implies that the catalytically active species are structurally uniform, do not interact with each other, and exhibit identical affinity towards the substrates in the desired transformation. Achieving such characteristics in real-life materials is a challenge due to the complex nature of chemical transformations at surfaces. Surface organometallic chemistry (SOMC) has emerged as a powerful approach to form well-defined species on the surface of materials. In this short review, we discuss the myth and reality of single-site heterogeneous catalyst formation, with a primary focus on application of SOMC methodology. catalytic reaction between two olefinic fragments, during which a formal scission and rearrangement of the double bonds occur. As a result, a new set of olefins is formed. chromium-based heterogeneous catalyst for olefin polymerization, developed by Phillips Petroleum. The catalyst is obtained by impregnation of silica with chromium precursors, followed by calcination under oxidizing conditions. Polydispersity index of polyethylene obtained with this catalyst typically ranges from 8 to 65 [5.McDaniel M.P. A Review of the Phillips Supported Chromium Catalyst and its Commercial Use for Ethylene Polymerization.in: Gates B.C. Knözinger H. Advances in Catalysis. 53. Elsevier, 2010: 123-606Google Scholar]. the polydispersity index a measure of molecular mass distribution of a polymer obtained by dividing weight average molecular weight by number average molecular weight. a class of olefin metathesis catalysts that are typically comprised of a high-valent, early-transition metal center and a nucleophilic alkylidene fragment. The coordination sphere is commonly completed with π-donor ligands. industrially relevant class of heterogeneous olefin polymerization catalysts with typical formulations consisting of titanium chlorides supported on MgCl2 and activated with alkylaluminum species. Polydispersity index of polyethylene obtained with this catalyst is typically 4.0, a number closer to values obtained with single-site homogeneous metallocene catalysts (approximately 2.0) [5.McDaniel M.P. A Review of the Phillips Supported Chromium Catalyst and its Commercial Use for Ethylene Polymerization.in: Gates B.C. Knözinger H. Advances in Catalysis. 53. Elsevier, 2010: 123-606Google Scholar].