Design of a Cobalt–Zeolite Catalyst for Semi‐Linear Higher‐Olefin Synthesis

Abstract

Olefins that contain 6-16 carbon atoms are important chemical intermediates in the synthesis of plasticizers and surfactants. [1] They are produced on an industrial scale in oligomerization processes, mainly starting from ethylene, propylene, and butenes. [2] The value of a long olefin depends on its skeletal structure; linear olefins are the most valuable. In industrial olefin-oligomerization processes, dissolved nickel complexes and supported-nickel catalysts are used to produce semi-linear oligomers. Homogeneous cobalt catalysts show better selectivity for linear oligomers than nickel catalysts and are more resistant to poisons, such as thiols, thiophenes, and dihydrogensulfide impurities that are contained in industrial feedstocks. [3] Solid catalysts allow reactions to be performed in fixed-bed reactors. Heterogeneous cobalt catalysts have been reported in the literature dating back to the 1960s. [5][6] The development of a solid cobalt catalyst for olefin-oligomerization is, however, not straightforward. [2][3][4][5][6] By building on the knowledge of the placement of transition-metal-and alkaline-earth-metal cations in zeolites and by exploiting the concept of framework-stabilization, we designed the first zeolite-based cobalt catalyst for olefin-oligomerization and demonstrated its excellent selectivity for the synthesis of quasi-linear octene from butenes. The active site, in which alkyl chains are attached to the cobalt atom, was uniquely visualized by X-ray diffraction and structure-refinement.Zeolite catalysts have shown great potential in the olefin-oligomerization reaction. ZSM-57 zeolite in its acid form is an excellent butene-dimerization catalyst; [7] the octene products are mainly comprised of dimethylhexenes and the number of chain branches (NCB) of the dimer fraction is around 2. ZSM-22 zeolite, with its narrow tubular pores, has shown the ability to reduce the NCB of propene oligomers, but it is less active in the conversion of butenes. [8] Oligomers with low degrees of branching can be obtained by using a heterogeneous nickel catalyst in a process called OCTOL. [9] Here, the NCB of the octene-product fraction out of n-butenes is about 1.1. The mechanism of coordinative oligomerization by nickel on homogeneous as well as heterogeneous catalysts involves Ni I species and can be divided into three steps: [10][11] 1) the formation of a p-complex between the olefin and the transition-metal ion followed by transformation into a s-bonded metalÀ alkyl complex; 2) insertion of olefin molecules, in which a pbonded olefin is inserted into the metalÀcarbon bond of the metalÀalkyl complex; and 3) the termination of chain-growth by b-H-transfer or b-elimination. Starting from terminal alkenes, this mechanism leads to linear and monobranched dimerization products. On the targeted heterogeneous cobalt catalyst, on analogy with nickel, a similar reaction mechanism could be followed provided that Co I atoms with free coordination sites can be generated at the surface of a solid support.The synthetic faujasite...

Cobalt Oxide on N-Doped Carbon for 1-Butene Oligomerization to Produce Linear Octenes

“…These catalysts are the most selective heterogeneous catalysts for production of linear olefins that have been reported in the literature with over 70% of the oligomers being linear. In contrast, the linear oligomer selectivity is less than 30% for other heterogeneous catalysts including solid acid catalysts, , supported nickel oxide, and supported cobalt oxide. , …”

“…These catalysts are the most selective heterogeneous catalysts for production of linear olefins that have been reported in the literature with over 70% of the oligomers being linear. In contrast, the linear oligomer selectivity is less than 30% for other heterogeneous catalysts including solid acid catalysts, 5,6 supported nickel oxide, and supported cobalt oxide. 7,8 The reaction was carried out at different WHSV's over 600A-CoOx/N-C and 2A-CoOx/N-C. Table 3 tabulates the inlet and outlet concentrations, conversion, and selectivities at a similar number of turnovers.…”

“…The doubly ammoniated cobalt oxide on carbon catalyst (2A-CoOx/N-C) was reported to exhibit high linear C8 olefin selectivity (70−85%). This linear oligomer selectivity is almost two times as high as that of other heterogeneous catalysts including solid acid catalysts, 5,6 supported nickel oxide, and cobalt oxide on other supports. 7,8 The low linear selectivity (less than 30%) of solid acid catalysts is due to the existence of Brønsted acid sites that create tertiary and secondary carbocations.…”

Recent developments in catalyst systems for selective oligomerization and polymerization of higher α-olefins

“…32 These authors suggested that the low linear selectivity in the case of solid acid catalysts (Co/Ca-Y-zeolite) was due to the existence of Brønsted acid sites that create tertiary and secondary carbocations. 35 Apparently, higher selectivity with Co/N–C was postulated to follow the Cossee–Arlman insertion mechanism for 1-butene oligomerization. Recognizing that pore shape, size and acidity affect the product distribution, Catani and coworkers investigated the catalytic behaviour of MCM-41 type mesoporous aluminosilicates, with and without small amounts of metals inside the structure.…”

Influence of Remaining Acid Sites of an Amorphous Aluminosilicate on the Oligomerization of n-Butenes after Impregnation with Nickel Ions

“…A strong metal-support interaction in such catalyst systems leads to the mutual influence of the active nickel and acid sites providing the observed positive trends in catalytic activity and selectivity. Novel catalyst systems based on cobalt oxide on N-doped carbon [31] or cobalt-zeolite catalysts [32] show promising results for a high selectivity to linear products. However, preparation of these catalysts on an industrial scale tends to pose various challenges.…”