The First Vanadium-Oxide-Based UHMWPE Catalyst Supported on Chemically Modified Silica Gel

Abstract

Novel vanadium‐oxide‐based catalysts supported on alumina, zirconia, or titania‐modified silica, which are able to synthesize ultrahigh molecular weight polyethylene (UHMWPE), are successfully developed. Compared with the unmodified silica supported catalyst, the activities of the modified catalysts are substantially enhanced. By changing the polymerization temperature (Tp) from 90 to 60 °C, the molecular weight of the produced UHMWPE can be easily regulated from 2 × 106 to more than 6 × 106 g mol−1. It is the first time that chloride‐free nonsingle site catalysts have been developed to synthesize UHMWPE within such a broad range of Tp. Catalyst characterization by NH3‐temperature‐programmed desorption and pyridine‐Fourier transform infrared spectroscopy reveals that the modified catalysts have increased acidity derived from both Lewis and Brønsted acid sites. The X‐ray photoelectron spectroscopy characterization indicates that the vanadium on the modified catalysts is more electron deficient. Thus, the acidity of the catalysts and the electronic state of the vanadium play a critical role in determining the activity of the vanadium active sites for ethylene polymerization.

Remarkable Promotion Effect of Sulfation over the SiO2‐Supported Vanadium‐Oxide‐Based Catalysts for UHMWPE

“…The effects of sulfation on VO x /SiO 2 –MO y (M = Al, Zr, or Ti) catalysts for the synthesis of UHMWPE are investigated systematically. Under the same polymerization conditions, the sulfated catalysts show activity about twice as high as that of the ones modified by metal oxide only, and even four times higher than that of the SiO 2 supported one . The characterization results of NH 3 –TPD indicate that the acidity of the catalysts is improved largely after sulfation, while the results of pyridine‐adsorption‐FTIR imply that a large amount of Lewis acid sites and a small amount of Brønsted acid sites are available on the surface of the catalysts.…”

“…It is obvious that all the catalysts show an optimum Al/V at which the maximum activity is reached, as the cocatalysts act as alkylation reagent for active centers and scavenger for reaction system, but overdose of cocatalyst would result in deactivation through overreduction of the active centers . The optimum Al/V for VO x /SiO 2 –Al 2 O 3 –SO 4 2− , VO x /SiO 2 –ZrO 2 –SO 4 2− and VO x /SiO 2 –TiO 2 –SO 4 2− are 60, 60, and 70, respectively, and the maximum activity is similar for each catalyst at 2200 kg PE mol −1 V, which is equal to or lower than the activity of some supported V‐based Ziegler‐Natta catalysts, but is definitely higher than that of VO x /SiO 2 –MO y (M = Al, Zr, or Ti) at 1200 kg PE mol −1 V . This confirms that the sulfation indeed helps to improve the activity of the vanadium‐oxide‐based catalysts for ethylene polymerization.…”

“…The catalysts were prepared according to the literature, except that the needed amount of (NH 4 ) 2 SO 4 was added together with NH 4 VO 3 during the impregnation step, and the calcination temperature was set at 500 °C, considering the facile decomposition and evaporation features of sulfates. A series of catalysts denoted as VO x /SiO 2 –Al 2 O 3 −SO 4 2− , VO x /SiO 2 –ZrO 2 –SO 4 2− , VO x /SiO 2 −TiO 2 –SO 4 2− was prepared, corresponding to the vanadium‐oxide‐based catalysts supported on Al 2 O 3 /SO 4 2− , ZrO 2 /SO 4 2− and TiO 2 /SO 4 2− modified SiO 2 .…”

“…In our previous work, several bimodal PE products were synthesized by a series of bimetallic catalysts, in which the vanadium‐oxide species were adopted to devote the high MW fractions . Very recently, a series of vanadium‐oxide‐based catalysts supported on MO y modified SiO 2 (VO x /SiO 2 –MO y , M = Al, Zr, or Ti) was also prepared in our group, indicating that UHMWPE with M v of 2–6 × 10 6 g mol −1 could be obtained by regulating T p , and the activities of the modified catalysts were enhanced largely, possibly due to the improved acidity of the catalysts and the enhanced electron deficiency of the V center . This also inspired us to find another way to further improve the activity of this catalyst.…”

Effect of Alkyl Aluminums on Ethylene Polymerization Reactions with a Cr‐V Bimetallic Catalyst

“…Christman has reported Cl 3 CCOOEt or chlorinated hydrocarbons to be an effective reagents for maintaining the higher (active) oxidation state of vanadium systems, thus improving the catalytic activity of the vanadium‐based catalysts . Jin et al studied the effect of cocatalyst types on the activity of the sulfate‐modified VO x /SiO 2 –TiO 2 catalyst, also observing that the cocatalyst dependence on catalytic activity had the priority of DEAC > TIBA > TEA . It is suggested that the chloride of Al‐alkyl with weak reducibility and strong oxidizability retard the over‐reduction of vanadium active centers and the V–Cl–Al bridging bond also accounts much for the higher activity than TEA and TIBA .…”