Selective hydrogenation of 5-hydroxymethylfurfural (HMF) to 2, 5-dimethylfuran (DMF) over Ru, Ni, and Co mono and bimetallic catalysts supported on carbon and carbon nanotube

• Main Author: Endot, N. B.
• Published: University of Liverpool 2017
• Subjects: 540
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.733808

The main objective of this thesis is to develop better catalysts for the hydrogenation of 5- hydroxymethylfurfural, HMF into high quality liquid fuel 2, 5- dimethylfuran (DMF) or partly hydrogenating compounds. This could be achieved by exploiting the support effect and using CNTs to improve activity and developing mono- and bimetallic systems utilising less expensive metals like Nickel, Cobalt or Iron compared to noble metals like ruthenium and rhodium. Succeeding at this could be an economic incentive for the scale-up production of DMF. In addition, we envisaged the opportunity to be able to produce the metal catalyst and support in one simple step using a sugar and the metal salt and we studied the utilisation of carbon synthesised hydrothermally from glucose as a catalyst support using microwave techniques. The physical and chemical properties of the catalysts were characterised using such techniques as X-ray diffraction (XRD), temperature programmed reduction (TPR), inductively coupled plasma emission spectroscopy (ICP), hydrogen and carbon monoxide chemisorption, transition electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). Hydrogenation of HMF to DMF was examined over monometallic and bimetallic Ru, Ni and Co supported on carbon and CNT at 150 °C (20 bar of H2). Among the monometallic catalysts supported on carbon, Ru catalysts exhibited the highest DMF yield up to 80 % in 3 hours followed by Ni and Co catalyst. This is because Ni and Co are not as good as Ru in the hydrogenolysis step in order to get to DMF. As a comparison to carbon, CNT as a support shows a remarkable improvement in the HMF conversion and DMF yield in a shorter reaction time. The effect is consistence for all the catalysts. A control test with only CNT showed a negligible activity which confirms that the enhancement is due to the presence of metal catalyst. HMF conversion of 100 % with 84 % DMF yield in 1 hr was achieved over 5 wt % Ru/CNT. The improvement of reactivity is attributed to the electronic effect of CNT derived from the curvature shape of CNT. This consequently enhanced the electron density of metal thus improving the adsorption of C=O bonds resulting in higher reactivity. We found that the promoting effect of the CNT support was universal to all catalyst tested, so that reasonably good Co and Ni catalysts could be obtained, particularly for the initial step of the reaction. The bimetallic system of RuCo and RuNi with specific molar ratios showed a significant improvement in reactivity compared to their monometallic counterparts, particularly considering the lower loading used. High yield of DMF was obtained even at low Ru content in a bimetallic catalyst without losing much DMF yield. 3.5 % RuCo/C 1:5 (0.6 % Ru) and 3.2 % RuNi 1:3 (0.7 % Ru) have better specific DMF yield as compared to 5% Ru/C and 5 % Ru/CNT. 4.2 % RuCo/CNT 1:2 with only 0.2 % loading of Ru showed the highest specific DMF yield. This finding is a positive outcome in order to reduce the dependent on the expensive noble metal without compromising the activity and the yield of desired product, in our case DMF. This proved the synergistic effect of this system. The only difference when different supports were used was that CNT improved the reaction rates however this is also lead to the formation of ring hydrogenation and ring opening products. Carbon has lower reaction rate however it gives better DMF yield. Finally, we demonstrated that one step microwave assisted synthesis of carbon supported catalysts is a promising technique to simultaneously synthesise catalyst as well as hydrothermal carbon in a shorter amount of time compared to the conventional hydrothermal and incipient wetness impregnation method.