Well-Defined Heterogeneous Catalytically Active Ion Pairs on the Surface of Oxides
Industry often prefers heterogeneous catalysts because solids are easier to remove from the reaction mixture, less likely to foul the reactor, easier to recycle, and produce unique products. However, establishing structure-activity relationships for heterogeneous catalysts is often challenging. Surface organometallic chemistry utilizes techniques similar to homogeneous chemistry to synthesize and characterize heterogeneous catalysts. Ion pairs are common catalysts for chemical transformations and in solution there is a vast library to choose from. Heterogeneous ion pairs are equally important but have been examined in far less detail. The copolymerization of ethylene and polar monomers to yield copolymers with high molecular weights and narrow polydispersities is a challenge that has not been approached by heterogeneous catalysts. Part of this work presents the synthesis of a single-site (α-diimine)PdMe+ catalyst on the surface of sulfated zirconium oxide (SZO) for the copolymerization of methyl acrylate and ethylene.
Heterogeneous Lewis acids are commonly used in catalytic transformations; however, the concentration of active sites is usually very low, and the structure is ambiguous. This work approaches this challenge by synthesizing a well-defined silylium-like ion on the surface of SZO ([iPr3Si][SZO]), the first example on an oxide. [iPr3Si][SZO] hydrodefluorinates several inert sp3 C-F bonds in the presence of excess triethylsilane. This study showed that SZO is not as weakly coordinating as common homogeneous weakly coordinating anions, such as carborane and BArF4- anions. Therefore, we wanted to design a weaker coordinating heterogeneous anion on the surface of an oxide. The strong Lewis acid PhF-Al(OC(CF3)3)3 coordinates to -OH sites on the surface of partially dehydroxylated silica forming -O(H)-Al(OC(CF3)3)3 which upon deprotonation forms weakly coordinating ion pairs. One of the ion pairs [iPr3Si][-OAl(OC(CF3)3)3] has a 29Si NMR chemical shift 17 ppm downfield of [iPr3Si][SZO], supporting that the -OAl(OC(CF3)3)3- anion is weaker coordinating than the sulfate anions on SZO. The last part of this work examines the structure of the active site in the ternary catalyst: Cpb2ZrCl2 + iBu3Al + Al2O3, a Ziegler-Natta type ethylene polymerization catalyst. Model catalysts, activity studies, and solid-state 2H NMR studies show that the active catalyst structure is a Cpb2Zr-H+ cation on the surface of Al2O3.