A waste-derived basic silicate catalyst for lactose to lactulose isomerisation Victoria Elmes, Nichola J. Coleman School of Science, Faculty of Engineering &Science, University of Greenwich, Chatham Maritime, ME4 4TB, UK In 2020, the UK recycled approximately 74% of the 2.4 million tonnes of glass that entered the waste stream [1] . Despite being 100% recyclable, poor collection infrastructure and colour mismatch limit the extent to which discarded container glass is actually reutilised. Hence, alternative processing routes to up-cycle post- consumer glass into value-added products such as zeolites, ion-exchangers, ceramics and geopolymers are currently sought [2,3] . In this respect, the present study considers the hydrothermal synthesis of tobermorite (Ca 5 Si 6 O 16 (OH) 2 .4H 2 O) from green container cullet with a view to its application as a catalyst for the industrial isomerisation of sugars. The hypothesis is that the basic silanol (-OH) groups of the silicate chains of tobermorite will present as catalytic sites for the heterogeneous conversion of lactose to lactulose. A mixed product of tobermorite, with minor proportions of calcium hydroxide and calcium carbonate, was obtained by hydrothermal synthesis from post-consumer green container glass in 4 M NaOH (aq) at 125 °C [4] . The reaction products were confirmed by powder X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy. Lactulose is a synthetic disaccharide that is used to treat constipation and hepatic encephalopathy [5] . Its alkaline isomerisation from lactose is characterised by low yields, unwanted by-products and the generation of caustic waste liquors. Accordingly, the use of environmentally benign solid basic catalysts has been proposed as a potential alternative to traditional homogeneous isomerisation. The ability of the waste-derived tobermorite to catalyse the conversion of lactose to lactulose was evaluated at 50, 60, 70 and 80 °C, under reflux, using a 10 wt% aqueous solution of lactose (containing both α and β anomers). The isomerisation reaction, in the presence and absence of the catalyst, was monitored by 1 H nuclear magnetic resonance spectroscopy at intervals up to 3 h. Under the selected experimental conditions, the rate of lactose to lactulose conversion increased markedly with increasing temperature in the presence of tobermorite, whereas no reaction was observed in the corresponding controls. In summary, the findings of this preliminary study indicate that waste glass-derived tobermorite is a promising basic heterogeneous catalysts for the isomerisation of industrially relevant sugars. References 1. GOV.UK, 2022, UK Statistics on Waste [online], https://www.gov.uk/government/statistics/uk-waste-data/uk-statistics-on- waste#packaging-waste [date accessed: 22/04/2023] 2. M. Sayehi, G. Delahay, H. Tounsi, Synthesis and characterisation of eco-friendly materials zeolite from waste glass and aluminium scraps using hydrothermal technique, J. Environ. Chem. Eng. 10 (2022) 108561 3. V.K. Elmes, A.P. Hurt, N.J. Coleman, Mixed-phase ion-exchangers from waste amber container glass, Mater. 14 (2021) 4887 4. J.H. Taylor, V.E. Elmes, A.P. Hurt, N.J. Coleman, Synthesis of feldspathoids and zeolite K-F from waste amber container glass. Mater. Chem. Phys. 246 (2020) 122805 5. T.K. Nahla, The chemical isomerisation of lactose to lactulose by using sodium hydroxide as batch reaction, Pak. J. Chem. 5 (2016) 24-30
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