The influence of draw‐induced structural changes on the activity of a semicrystalline polymer‐supported catalyst

Abstract

The activity of a solid, polymer‐supported catalyst (a semicrystalline polyethylene film containing grafted, sulfonated styrene) was shown to be altered by cold drawing. The catalytic activity was measured by a test reaction, the dehydration of isopropyl alcohol to give propylene. Catalytic reaction rates were measured with variously drawn films clamped in a differential flow reactor operated at 100°C and 1 atm. The catalytic activity increased with the elongation of the polymer up to a draw ratio of 2.5; the activity decreased upon further drawing. The drawn films were characterized by x‐ray diffraction, dynamic mechanical measurements, electron microscopy, birefringence, and density measurements. Since no evidence was found for the formation of surface sites by creation of microcracks, the changes in activity are attributed to modifications in the polymer structure induced by drawing. The kinetics of the catalytic reaction and the data giving percentage crystallinity and crystalline and amorphous orientation factors suggest that, in the undrawn polymer, the catalytically active SO3H groups form a hydrogen‐bonded network, which is excluded from the crystalline regions. Initial elongation partially breaks up the network, allowing more SO3H groups to bond to alcohol and become catalytically engaged. Elongation beyond a draw ratio of 2.5 leads to a separation of SO3H groups greater than that required for the formation of the reaction intermediate, which involves the alcohol hydrogen bonded to several SO3H groups. The combined results of the catalytic kinetics experiments and structural characterizations imply that the preliminary deformation response of the semicrystalline polymer occurs predominantly within the noncrystalline regions and is accommodated by the rigid slip and tilting of crystal lamellae. The hydrogen bonding among the SO3H groups in the noncrystalline regions hinders lamellar breakup and suppresses the formation of highly aligned fibrillar morphologies.

Bruce Gates: A Career in Catalysis

“…The structure was manipulated to maximize catalytic potential, the rate data for isopropanol dehydration demonstrating sensitivity to changes induced by drawing (elongation under tensile stress): as the draw ratio was increased from 1 to 2.5, the activity increased but then decreased at higher draw ratios. 153 The changes in catalytic activity induced by drawing did not result from formation of new surface sites.…”

“…In collaboration with polymer science faculty at Delaware, Gates conducted novel research on structure–property relations of solid polymeric catalysts. This involved evaluation of how the catalytic properties depend upon intrinsic polymeric structure. − Gates’s previous research was mostly on solid acidic sulfonated poly­(styrene-divinylbenzene) catalysts, − not films; therefore, this was an early example of his willingness to explore a new field in collaboration with experts outside of catalysis.…”

“…In the film work, a semicrystalline polymeric film was used to support the −SO 3 H groups; it was prepared by γ-radiation grafting of styrene onto commercial polyethylene, followed by sulfonation of the styrene. , The polymer lamellae (plate-like crystals) were 20–60 nm thick and several hundred nm in length and width (Figure ). The structure was manipulated to maximize catalytic potential, the rate data for isopropanol dehydration demonstrating sensitivity to changes induced by drawing (elongation under tensile stress): as the draw ratio was increased from 1 to 2.5, the activity increased but then decreased at higher draw ratios . The changes in catalytic activity induced by drawing did not result from formation of new surface sites.…”