Abstract
ABSTRACT Chromium(III) complexes with sulphur, nitrogen and phosphorus tridentate ligands were synthesized and characterized. These complexes were supported on SiO2 and characterized by BET surface area measurements, XRF, SEM-EDX and FTIR. The complexes were tested for activity and selectivity in the trimerization of ethylene. The substituent's effect and influence on the sulphur on the supported catalysts were studied using the ethyl and the decyl substituted catalysts. The influence of temperature on catalytic performance was evaluated using the PPP supported system. The most active supported catalyst, the decyl substituted SNS catalyst, showed good activity of up to 19 500 g/g Cr h-1 and selectivity of 97.3 % to C6 products (98.2 % 1-hexene). This activity and selectivity were comparable to the homogeneous counterpart's performance that achieved 22 000 g/g Cr h-1 and 98.2 % C6 products (96.7 % 1-hexene), which surpassed the ethyl substituted catalyst, which was not supported, under the same reaction conditions. The supported PPP catalyst activities showed it was sensitive to higher temperatures, but this depends on the supporting technique. Keywords: Ethylene trimerization, sulfanyl ligands, phosphine ligands, supported catalysts.
Highlights
Supported metal complexes that are well known for ethylene conversions are metallocenes, which are immobilized for ethylene polymerization reactions.[1,2] As a result of the different routes of immobilization and strategies that have been proposed, different catalyst activities and/or polymer properties are obtained
Multiplets revealed that the protons on the carbons directly bonded to the nitrogen atom were more deshielded (2.75– 3.20 ppm) relative to the protons on the carbons bonded to the phosphorus atom (1.38–1.61 ppm)
The supported catalyst SC3 achieved the highest catalytic activity of 14 998 g/g Cr h–1 at 60 °C, which decreased with increasing temperature to 7944 g/g Cr h–1 at 90 °C
Summary
Supported metal complexes that are well known for ethylene conversions are metallocenes, which are immobilized for ethylene polymerization reactions.[1,2] As a result of the different routes of immobilization and strategies that have been proposed, different catalyst activities and/or polymer properties are obtained. The trend that has been observed is that the supported complexes exhibit lower catalytic activities than their homogeneous counterparts This decrease in the activities can be attributed to many factors, viz. Some researchers have noted that homogeneous catalysts are much more active and selective than their heterogeneous counterparts[22–24], Monoi and Sasaki[25] reported a new, highly active and selective silica-supported ethylene trimerization catalyst. Nenu and Weckhuysen[26] discovered another supported system that proved successful for trimerization of ethylene This catalyst that consisted of a Cr centre attached via two silanol groups to an amorphous silica carrier was more active than its homogeneous parallel. The 1,3,5-tribenzylhexahydro-1,3,5-triazine (TAC) ligand was anchored on a Cr/SiO2 surface This ligand was reported to form active trimerization complexes with different metals. Results, so we hoped to improve these by supporting them or gaining a separation advantage
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