Cobalt based catalysts

An example of such recychng in a parallel reaction system is in the Oxo process for the production of C4 alcohols. Propylene and synthesis gas (a mixture of carbon monoxide and hydrogen) are first reacted to ra- and isobutyraldehydes using a cobalt-based catalyst. Two parallel reactions occur ... 

The reaction conditions are approximately 200°C and 30 atmospheres over a cobalt-based catalyst. 

A cobalt-based catalyst, prepared by reducing Co(acac)3 with diethylalumi-num chloride in the presence of the bidentate ligand l,2-bis(triphenylphosphi-no)ethane, accelerates [87] the cycloadditions of norbornadiene (88) with a variety of acetylenes (Equation 3.30). 

Figure 8.17. Hydrocarbon distribution of the products formed by Fischer-Tropsch synthesis over cobalt-based catalysts and by additional hydrocracking, illustrating how a two-stage concept enables optimization of diesel fuel yield. [Adapted from S.T. Sie,...

The catalysts used in hydroformylation are typically organometallic complexes. Cobalt-based catalysts dominated hydroformylation until 1970s thereafter rhodium-based catalysts were commerciahzed. Synthesized aldehydes are typical intermediates for chemical industry [5]. A typical hydroformylation catalyst is modified with a ligand, e.g., tiiphenylphoshine. In recent years, a lot of effort has been put on the ligand chemistry in order to find new ligands for tailored processes [7-9]. In the present study, phosphine-based rhodium catalysts were used for hydroformylation of 1-butene. Despite intensive research on hydroformylation in the last 50 years, both the reaction mechanisms and kinetics are not in the most cases clear. Both associative and dissociative mechanisms have been proposed [5-6]. The discrepancies in mechanistic speculations have also led to a variety of rate equations for hydroformylation processes. 

Rhodium and rhodium-cobalt based catalysts using silanes as the stoichiometric reductant were initially reported in 1992 to reductively couple enyne substrate (Eq. 30) [90,91]. Further investigation showed this reaction to be an effective method for the cyclization of enyne substrates 129 to... 

Currently the reaction is carried out using cobalt based catalysts with severe penalties in terms of harsh operating conditions (80 bar CO/H2, 200°C). In addition, substantial loss of substrate (ca. 10%) to hydrogenation makes the overall selectivity to the linear alcohol ca. 80% [15]. Rhodium based systems are capable of giving higher selectivities (>90%) to the desired linear aldeyde product under milder conditions (20 bar, 100°C) [13]... 

Krishnamoorthy, S., Tu, M., Ojeda, M. P., Pinna, D., and Iglesia, E. 2002. An investigation of the effects of water on rate and selectivity for the Fischer-Tropsch synthesis on cobalt-based catalysts. J. Catal. 211 422-33. 

An Overview of Reported Claims of Bulk Cobalt Carbide Being Observed after/when Performing Fischer-Tropsch Synthesis over Supported Cobalt-Based Catalysts... 

Borko, L., Horvath, Z. E., Schay, Z., and Guczi, L. 2007. The role of carbon nanospecies in deactivation of cobalt based catalysts in CH4 and CO transformation. Stud. Surf. Sci. Catal. 167 231-36... 

Xiong, J., Ding, Y., Wang, T., Yan, L., Chen, W., Zhu, H., and Lu, Y. 2005. The formation of Co2C species in activated carbon supported cobalt-based catalysts and its impact on Fischer-Tropsch reaction. Catal. Lett. 102 265-69. 

Jacobs, G., Das, T.K., Li, J., Luo, M., Patterson, P.M., and B.H. Davis. 2007. Fischer-Tropsch synthesis Influence of support on the impact of co-fed water for cobalt-based catalysts. In Fischer-Tropsch synthesis Catalysts and catalysis, ed. B.H. Davis and M.L. Occelli, 217-53 Amsterdam, The Netherlands Elsevier. 

Detailed Kinetic Study and Modeling of the Fischer-Tropsch Synthesis over a State-of-the-Art Cobalt-Based Catalyst... 

In this work, a detailed kinetic model for the Fischer-Tropsch synthesis (FTS) has been developed. Based on the analysis of the literature data concerning the FT reaction mechanism and on the results we obtained from chemical enrichment experiments, we have first defined a detailed FT mechanism for a cobalt-based catalyst, explaining the synthesis of each product through the evolution of adsorbed reaction intermediates. Moreover, appropriate rate laws have been attributed to each reaction step and the resulting kinetic scheme fitted to a comprehensive set of FT data describing the effect of process conditions on catalyst activity and selectivity in the range of process conditions typical of industrial operations. 

However, the detailed description of the FT product distribution together with the reactant conversion is a very important task for the industrial practice, being an essential prerequisite for the industrialization of the process. In this work, a detailed kinetic model developed for the FTS over a cobalt-based catalyst is presented that represents an evolution of the model published previously by some of us.10 Such a model has been obtained on the basis of experimental data collected in a fixed bed microreactor under conditions relevant to industrial operations (temperature, 210-235°C pressure, 8-25 bar H2/CO feed molar ratio, 1.8-2.7 gas hourly space velocity, (GHSV) 2,000-7,000 cm3 (STP)/h/gcatalyst), and it is able to predict at the same time both the CO and H2 conversions and the hydrocarbon distribution up to a carbon number of 49. The model does not presently include the formation of alcohols and C02, whose selectivity is very low in the FTS on cobalt-based catalysts. 

The observed H2/CO effect on CO conversion can be explained considering the strong CO adsorption ability on cobalt-based catalysts. At the typical FT process conditions, in fact, it has been reported that the catalyst surface is almost... 

The data available for heterogeneous Fischer-Tropsch catalysts indicate that with cobalt-based catalysts the rate of the water gas-shift reaction is very slow under the synthesis conditions (5). Thus, water is formed together with the hydrocarbon products [Eq. (14)]. The iron-based catalysts show some shift activity, but even with these catalysts, considerable quantities of water are produced. 

The earliest theory, advanced by Fischer and Tropsch in 1926 (84), proposed that the reaction proceeded via formation of intermediate metal carbides which react on the catalyst surface to form methylene groups. It was then suggested that these methylene groups polymerize on the surface to form hydrocarbon chains, which desorb as saturated and unsaturated hydrocarbons. In 1939 Craxford and Rideal expanded the carbide theory, proposing (85), for cobalt-based catalysts, the following reaction sequence ... 

Water is the main reaction product of the FTS over cobalt-based catalysts, the water is formed on the surface, and it is not surprising that the water has an influence on the kinetics of the FTS. Although the water effect was documented early, in a recent review,5 none of the reported kinetic equations describing the FTS over cobalt catalysts include a water-term, van Steen and Schulz41 have since suggested a common rate-equation for Fe and Co-based catalysts, including terms for water. 

E. van Steen and H. Schulz, Polymerisation kinetics of the Fischer-Tropsch CO hydrogenation using iron and cobalt based catalysts, Appl. Catal. A, 1999, 186, 309-320. 

S. Krishnamoorthy, M. Tu, M. P. Ojeda, D. Pinna and E. Iglesia, An Investigation of the Effects of Water on Rate and Selectivity for the Fischer-Tropsch synthesis on Cobalt-Based Catalysts, J. Catal., 2002, 211, 422-433. 

N. O. Elbashir, P. Dutta, A. Manivannan, M. S. Seehra and C. B. Roberts, Impact of cobalt-based catalyst characteristics on the performance of conventional gas-phase and supercritical-phase Fischer-Tropsch synthesis, Appl. Catal. A, 2005, 285, 169-180. 

All metals in the neighborhood of rhodium on the periodic table are known to be active in hydroformylation. Rhodium is by far the most active metal being used in concentrations of 10-100 mg/kg, usually at temperatures below 140 °C. Typical concentrations of cobalt-based catalysts are in the range of 1 -10 g/kg at temperatures up to 190 °C to get sufficient space-time yield. Apart from some specialized applications, other metals are only of scientific interest because of their low activity. A proper comparison of metal activity is difficult because of the different requirements on the reaction conditions. A generally accepted rough order of activity is given in Table 1. 

Shell Gas B.V. constructed a 1987 mVd (12,500 bbl/d) F-T plant in Malaysia that started operations in 1994. The Shell Middle Distillate Synthesis (SMDS) process uses natural gas as the feedstock to fixed-bed reactors containing cobalt-based catalyst. The heavy hydrocarbons from the F-T reactors are converted to distillate fuels by hydrocracking and hydroisomerization. The quality of the products is very high, the diesel fuel having a cetane number in excess of 75 with no sulfur. 

FT is most compatible with existing distribution for conventional diesel and only minimal adjustments are required to obtain optimal performance from existing diesel engines. Physical properties of FT are very similar to No. 2 diesel fuel, and its chemical properties are superior in that the FT process yields middle distillates that, if correctly processed (as through a cobalt-based catalyst), contain no aromatics or sulfur compounds. 

Rhodium-phosphine catalysts are unable to hydroformylate internal olefins, so much that in a mixture of butenes only the terminal isomer is transformed into valeraldehydes (see 4.1.1.2). This is a field still for using cobalt-based catalysts. Indeed, [Co2(CO)6(TPPTS)2] -i-lO TPPTS catalyzed the hydroformylation of 2-pentenes in a two-phase reaction with good yields (up to 70%, but typically between 10 and 20 %). The major products were 1-hexanal and 2-methylpentanal, and n/i selectivity up to 75/25 was observed (Scheme 4.12). The catalyst was recycled in four mns with an increase in activity (from 13 to 19 %), while the selectivity remained constant (n/i = 64/36). 

Our results on the catalytic conversion of other alcohols are given in Table 7. The data indicate that Co(III)-CMS4 acts as an efficient catalyst for the oxidation of other benzylic as well as secondary alcohols to the corresponding carbonyl compoimds. In the process these results compare very well with other cobalt-based catalysts [71]. 

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