Metal-organic frameworks have emerged as one of the key materials of interest over the last decade, with the number of publications and MOF structures registered with the Cambridge Crystallography database increasing year on year. One of the key reasons for this interest is the potential for very large internal surface areas of MOFs and the ability to tune pore sizes in order to target materials for the intended applications. The general range of surface areas of MOFs vary greatly, although a majority of MOFs exhibit surface areas of over 1000 m²g⁻¹, with an expected potential of possibly greater than 10000 m²g⁻¹ and large internal volumes, which in turn results in a number of useful properties for applications such as gas storage and separation, heterogeneous catalysis and even for use as medical devices. MOFs can also mimic some of the useful properties of zeolites such as molecular sieving, with the potential for tuneable pore sizes allowing a more optimal pore size for the selected application. However, MOFs are rarely produced at a large scale and are yet to become a viable alternative to current technologies. Here we have selected two archetypal MOFs to investigate, with the aim of producing continuous synthesis routes for these MOFs. We selected two MOFs with well documented syntheses in the literature in order to allow us to quickly ascertain the relative performance of our materials and optimise these. We also explore the possibilities of using unconventional heating methods such a microwave heating in order to exploit the potential benefits of this heating technique. MOF-5, with structure Zn₄O(BDC)₃ (BDC = benzene dicarboxylate) is one of the most commonly synthesized MOFs with potential applications such as hydrogen storage and catalysis. Here we showed that the formation mechanism of MOF-5 from solution is actually very complex and features multiple metastable crystalline phases. Importantly, we show that for MOF-5 formed through a common synthetic technique will always transition through at least one metastable crystalline phase before formation of MOF-5. Parameters affecting the synthesis of MOF-5 were then analysed and optimised in order to gain a deeper understanding of the process chemistry, how process intensification affects the final product and create a continuous MOF-5 synthetic procedure. We have demonstrated that MOF-5 can be formed continuously with high Langmuir Surface Areas (>2000 m² g⁻¹) while also producing yields of greater than 80%, giving space-time yield (STY) of 50 kg m³ day⁻¹ suggesting that MOF-5 should be scalable to a high degree. HKUST-1, Cu-BTC (BTC = benzene tricarboxylate), has the potential to be used in gas applications such as short chain hydrocarbon separation, hydrogen storage or purification and H₂S sequestration. Here we demonstrate a scalable continuous synthesis of HKUST-1 with a space time yield of 80 kg m³ day⁻¹ while maintain a Langmuir surface area of >2000 m² g⁻¹. Further optimisation of this system by varying the solid concentrations and the residence time was investigated. We then show that microwave heating can be used to produce HKUST-1 in several orders of magnitude faster than by conventional heating. The use of microwave technology for continuous production system of HKUST-1 enabled STY of 80000 kg m³ day⁻¹ and surface area of >1900 m² g⁻¹, thus strongly suggesting the significant benefits of combining continuous manufacture with microwave heating. We have shown that continuous production of two archetypal MOFs is possible, and optimised these systems while also comparing a number of key parameters in order to provide an overview of the potential benefits of process intensification and scale up. Further, we have highlighted clearly the potential benefits of using techniques such as microwave heating in order to exploit the beneficial changes to process chemistry of this heating method.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:668884 |
| Date | January 2015 |
| Creators | McKinstry, Colin |
| Publisher | University of Strathclyde |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://oleg.lib.strath.ac.uk:80/R/?func=dbin-jump-full&object_id=25921 |
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