Improving porosity and water uptake of aluminum metal-organic frameworks (Al-MOFs) as graphite oxide (GO) composites

Abstract

Composites of the aluminum metal-organic frameworks (Al-MOFs) MIL-100(Al) and CAU-10-H and graphite oxide (GO) have been synthesized (by in-situ MOF formation) with four different GO loadings (from 2 to 16 wt%) for enhanced porosity and water uptake capacity. These composite materials MIL-100(Al)-GO and CAU-10-H-GO were examined by powder X-ray diffraction to show the retention of crystallinity and by scanning electron microscopy to show the retention of the morphology. Nitrogen and water sorption isotherms indicate no loss of mass-weighted surface areas, porosity and uptake capacity. In case of MIL-100(Al)-GO composites, the BET surface areas and porosities from nitrogen sorption data even reveal an increase of 8–12% in surface area with the GO increase from 2 to 9 wt% and an increase of 7–10% in total pore volume with GO loading from 9 to 2 wt%, in comparison to neat MIL-100(Al). In the 2 and 5 wt% CAU-10-H-GO composites, a 4% and 7% increase, respectively, in total pore volume was observed. The water uptake in the MIL-100(Al)-GO composites increased 23% over neat MIL-100(Al) with only 2 wt% GO but dropped again to a small 7% increase upon higher GO loadings up to 16 wt%. To the contrary, in the CAU-10-H-GO composites, the water uptake increases only slightly by 4–6% with 2–8 wt% GO. In particular, the MIL-100(Al)-GO composites showed an increase in the porosities and water uptake due to the synergistic effect from the interaction of MOF and GO with the formation of an additional interface with surface area and void volume. Furthermore, a hydrophilic shift of the water uptake to lower relative pressure p∙p0−1 was observed in the 2, 5, 8, 15 wt% CAU-10-H-GO composites. The MOF-GO composites showed essentially unchanged water sorption data after five water ad- and desorption cycles together with retention of crystallinity, morphology and porosity as verified by powder X-ray diffraction, scanning electron microscopy and by nitrogen sorption isotherm analysis, respectively.