Ammonia Capture within Isoreticular Metal–Organic Frameworks with Rod Secondary Building Units

Abstract

The efficient removal, capture, and recycling of ammonia (NH₃) constitutes a demanding process; thus, the development of competent adsorbent materials is highly desirable. The implementation of metal–organic frameworks (MOFs), known for their tunability and high porosity, has attracted much attention for NH3 adsorption studies. Here, we report three isoreticular porphyrin-based MOFs containing aluminum (Al-PMOF), gallium (Ga-PMOF), and indium (In-PMOF) rod secondary building units with Brønsted acidic bridging hydroxyl groups. NH₃ sorption isotherms in Al-PMOF demonstrated reversibility in isotherms. In contrast, the slopes of the adsorption isotherms in Ga-PMOF and In-PMOF were much steeper than those of Al-PMOF in lower pressure regions, with a decrease of NH₃ adsorbed amounts observed between first cycle and second cycle measurements. Here, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) suggested that the strength of the Brønsted acidic –OH sites was controlled by the identity of the metal, which resulted in stronger interactions between ammonia and the framework in Ga-PMOF and In-PMOF compared to Al-PMOF.