Carbon Dioxide Adsorption in Betulin-Based Micro- and Macroporous Polyurethanes

Microporous polymers, that is polymers having pores of molecular size (D<2 nm), attracted a lot of interest during the last few years. They are promising materials for gas storage or separation but could also find use in heterogeneous catalysis. Recently, the separation of the green house gas CO2 evoked large interest in the scientific community. Among other microporous materials, such as activated carbon, metal-organic frameworks, or zeolites, nanoporous polymers have been suggested as adsorbents, as they can combine large surface areas together with the well-known processability of common polymers. Within this contribution, we present microporous polyurethane with good CO2 over N2 selectivity. The state of CO2 adsorbed in the micropores is analyzed by NMR spectroscopy in order to get a better picture of the gas–polymer interactions, which have not been subject to intense investigations so far. In combination with the analysis of the adsorption data, this allows a statement on the underlying basics of the high selectivity. One of the various ways to synthesize microporous polymers starts from the concept of intrinsic microporosity. Using stiff and contorted molecules, it is possible to prevent dense packing of the polymer chains, leaving an extraordinary high free volume behind. If the free volume is connected and can be accessed from the environment, the polymer is considered to be intrinsically microporous. Recently, we developed microporous polyesters with promising CO2/N2 selectivity based on betulin. Betulin, a triterpene that can be extracted from birch bark in up to 30% yield with respect to the dry weight of the bark, can be used as a building block for microporous polymers. Two hydroxy groups point out of the otherwise stiff hydrocarbon plane, thus introducing frustrated packing (=high free volume) upon polymerization. Consequently, we envisaged the synthesis of microporous polyurethanes as a next step, making use of the diol nature of betulin. Polyurethanes based on betulin have already been described in the 1980s by Russian and Finnish researchers, but no investigations were performed on the porosity of the products. Here, we targeted the synthesis of microporous networks, as these can typically result in higher porosities. Hence, a commercial triisocyanate (20% triphenylmethane triisocyanate in ethyl acetate, Desmodur RE, Bayer AG) was used as a crosslinker in an A3-B2 polyaddition reaction to prepare microporous polymer networks (see Scheme 1). Betulin was extracted from birch bark and recrystallized from ethanol before use.