Polymerization on CO-Reduced Phillips Catalyst initiates through the C–H bond Activation of Ethylene on Cr–O Sites

Abstract

Investigation of the polymerization of ethylene on CO-reduced Phillips catalyst (1 wt% chromium) by infrared spectroscopy reveals the presence of new OH bands. In particular, an OH-band appears at 3,605 cm-1 , consistent with the interaction of the SiOH group with an adjacent Lewis acidic chromium center, Si-(l-OH)-Cr. Polymerization with d 4-ethylene leads to the formation of the isotopically shifted band at 2,580 cm-1 , consistent with heterolytic C-H activation of ethylene over a Cr-O bond to generate the first Cr-C bond in ethylene polymerization with Phillips catalyst, as recently observed on well-defined Cr(III) silicates.

The Nature of Surface CrOx Sites on SiO2 in Different Environments

“…Similar high initial polyethylene polymerization activity was also found for the mononuclear (:SiO) 3 Cr ?3 model compound consistent with the role of Cr ?3 sites for ethylene polymerization [28]. Furthermore, the same initial ethylene polymerization rate was also obtained with a traditional CO-activated supported CrO x /SiO 2 catalyst consistent with the role of surface Cr ?3 sites for ethylene polymerization [32]. These new findings further support the role of Cr ?3 sites on silica as the active sites, but may not necessarily be identical to the traditional catalyst.…”

Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Redox Processes Leading to the Active Site

“…Our results indicate that the angle is larger in these xerogel materials than for materials made by grafting CrO 2 Cl 2 onto non-porous silica, consistent with interactions imposed by the xerogel pore system. 13 CO band could be predicted.…”

“…Recently, minor Cr III sites present in the reduced catalyst were proposed to be responsible for forming organoCr III active sites without a redox reaction, by invoking ethylene deprotonation. 13,14 This suggestion, which explicitly excludes participation of Cr II in the activation process is based on flawed analyses of IR and DFT results, 15 and is contrary to a large body of prior results. 2 In this study, we report a step-wise investigation of the CO reduction pathway leading to the activated Cr II precatalyst.…”

Polyethylene with Reverse Co‐monomer Incorporation: From an Industrial Serendipitous Discovery to Fundamental Understanding

“…[23] In af irst series of experiments,t he catalyst and its polymerisation behaviour were studied using in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy.T he obtained DRIFT spectra measured as af unction of time-on-stream for the Cr/Ti/SiO 2 catalyst under study are shown in Figure 2a.T he initial spectrum corresponds with ah ighly dehydroxylated catalyst as testified by the sharp silanol and titanol stretching bands located at 3747 cm À1 and 3722 cm À1 ,respectively.During the first 5min, the injection of the TEAl co-catalyst in heptane,for aAl:Cr molar ratio of 2, can be noted by the increase of the methyl and methylene stretching bands of these compounds in the 2800-3000 cm À1 CH stretching region. [24][25][26][27][28] It must be noted that the addition of TEAl did not lead to as ignificant decrease of the silanol and titanol groups,s uggesting that the added TEAl was also consumed for the reduction of Cr 6+ to Cr 2+ ,C r 3+ and Cr 5+ species, [29][30][31][32][33][34][35][36][37][38] for the transformation of ap ortion of the polymerisation into the oligomerisation active sites as well as for the scavenging of poisons. [1] Flushing of heptane revealed complex vibrational features of TEAl in the CH stretching region, suggesting alkylation of some of the Cr sites.U V-Vis-NIR DRS measurements ( Figure 2d)o ft he catalyst at this point show that TEAl is inducing as mall decrease in the intensity of mono-and polychromate O!Cr 6+ CT bands at 36 000, 28 000 and 21 500 cm À1 ,a ccompanied by the appearance of the d-d bands at 16 000 and 10 000 cm À1 due to the 4 A 2g !…”