Lactic Acid Synthesis From One-pot Conversion Of Cellulose Over Sulfonated KIT-6 Catalysts

Cellulose is the most abundant renewable biomass resources in the world.Hence,catalytic conversion of cellulose into value-added chemicals and liquid fuels has been regarded as one of the promising routes to meet with the shortage of conventional fossil fuels.In this paper,KIT-6 molecular sieves were modified using various met al precursors(SnCl₄·5H₂O and/or C₁₀H₅NbO₂₀).The doped sieves were further sulfonated by?3-mercaptopropyl?trimethoxysilane?MPTMS?to synthesize the solid acid catalysts with Lewis and Br?nsted acid sites.The catalytic performance of the as-prepared catalysts for lactic acid production from cellulose hydrolysis were deeply investigated.Typically,a series of characterization technique including N₂ adsorption-desorption isotherm,Boehm acid density titration,X-ray diffraction?XRD?,scanning electron microscopy?SEM?,X-ray photoelectron spectroscopy?XPS?,Fourier transform infrared spectroscopy?FT-IR?,Thermogravimetric analysis?TG?,Inductively coupled plasma?ICP?etc were conducted to determine the physicochemical features and/or morphology structure of the solid acid catalysts.The results indicated that the investigated samples possessed the uniform mesoporous structure with the high surface area.In addition,the unsaturated coordination of Sn or Nb element led to the formation of Lewis acid sites?L acidity?and the measured total acid density firstly increased upon increasing met al doping and then declined in the presence of more doped met al.Bi-functional catalysts?Sn-KIT-6-Pr-SO₃H and Nb-KIT-6-Pr-SO₃H?were obtained after the successful incorporation of-SO₃H groups.The results of elemental mapping revealed that the larger specific surface area facilitated the high dispersion of dopants and sulfonate groups on the catalyst surface.Meanwhile,TG test depicted the satisfactory thermal stability of the incorporated sulfonate groups,which enabled the catalyst to be used at below 300^oC.The catalytic activity and stability of the catalyst were studied using lactic acid synthesis from one-pot conversion of cellulose as a model reaction.Herein,the experimental results showed that for Sn-KIT-6-Pr-SO₃H sample,cellulose conversion was ca.88.9%with the yields of glucose,HMF and lactic acid as high as 30.1%,14.6%and 16.2%respectively under the optimum reaction conditions(210°C,5MPa N₂,0.1g catalyst,1h and m_H2O/m_cellulose=100/1mass ratio).On the other hand,with respect to Nb-KIT-6-Pr-SO₃H sample,the conversion of cellulose was around 85.9%and the yields of glucose,HMF and lactic acid were 33.9%,12.1%and 18.4%respectively,under the 150/1 ratio reactants and the other same reaction conditions.Interestingly,no obvious deactivation were observed even after four consecutive cycles,indicating its high stability.Hence,the developed solid acid catalyst presented the potential application in the field of catalytic conversion of renewable biomass resources into high valued chemicals.