Mixed-Linker Metal-Organic Frameworks as Catalysts for the Synthesis of Propylene Carbonate from Propylene Oxide and CO2

A series of mixed-linker metal-organic frameworks (MIXMOFs) of the general formula Zn4O(BDC)(x)(ABDC)(3-x) has been synthesized and tested as catalyst in the reaction of propylene oxide (PO) and carbon dioxide. Based on MOF-5 a new synthetic route was developed which allows the partial substitution of benzene-1,4-dicarboxylate (BDC) linkers in the material by functionalized 2-aminobenzene-1,4-dicarboxylate. In that way the number of catalytically active amino groups can be tuned using the desired BDC/ABDC ratio. The presence of MIXMOFs (instead of a mechanical mixture of MOF-5 and IRMOF-3) was proven by high-resolution X-ray diffraction and DTG. XRD and TG/MS analysis revealed that pure MIXMOF materials can be obtained up to an ABDC loading of 40%. The thermal stability in air is decreasing with increasing ABDC content from 450 degrees C for pure MOF-5 (0% ABDC) to ca. 350 degrees C for the 40% MIXMOF Zn4O(BDC)(1.8)(ABDC)(1.2). Consequently, MIXMOF materials represent a promising class of materials for catalytic applications in the temperature range at least up to 300 degrees C which is proven using the synthesis of propylene carbonate (PC) from propylene oxide and carbon dioxide as a test reaction. The catalytic results indicate that solid MIXMOF catalysts in combination with tetraalkylammonium halides (NR4X) as promoters are as active as the corresponding homogeneously dissolved reference compounds (H(2)ABDC, H2BDC and Zn salts) and that they can be recycled with only moderate loss in activity. A comparison of MIXMOFs from 0 to 40% of ABDC revealed a dependence of activity on the number of amino groups. Due to the fact that even pure MOF-5 catalyzed the reaction to some extent, surface Zn centers with free coordination sites might also contribute to substrate activation. Using optimized reaction conditions, a PC yield of up to 63% can be obtained after 3 h in the presence of low catalyst concentration (0.045 mol-% of 40% MIXMOF, containing 0.05 mol-% of NH2 groups). ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)