Structure evolution of char obtained from hydrothermal treatment of glucose

Abstract: Hydro-char can be widely used in fuel, soil improvement, pollutant removal, functional nano-materials preparation, CO2 fixation and other areas. Moreover, the hydrothermal carbonization process is accomplished under mild and simple conditions and is adaptable for wide feedstocks. As a result, conversion of biomass into hydro-char is very promising. In order to promote the progress of the hydrothermal synthesis of carbon microspheres and hydrothermal utilization of biomass, in this study, glucose was carbonized in a subcritical hydrothermal environment, and the process and results were studied. All the experiments were carried out in a 500 mL autoclave of 316 L stainless steel that was heated by an electrical heater. For each test, a certain amount of glucose and 100 g of deionized water were fed to the autoclave and sealed. Then, air was purged out of the reactor with argon and the initiative pressure was set at 0 MPa. After each reaction in a certain temperature and residence time, the gas was discharged and the reactor was rapidly cooled to below 90℃ with internal cooling by a U-loop and external fan. When the temperature dropped to room temperature, the hydro-char was collected by filtration, washed with deionized water and acetone several times, and dried in 105℃ for 12 h at last. The obtained hydro-chars were examined by means of Elemental Analysis, X-ray Diffraction, Scanning Electron Microscope, Fourier Transform Infrared Spectroscopy, and X-ray Photoelectron Spectroscopy.The results showed that there were a lot of carbon spheres in the hydro-char which were distributed between 0.6-7 μm. Moreover, the physicochemical properties of hydro-char were influenced by the temperature, glucose addition and residence time. Within the HTC condition of temperature (170-280℃), residence time (0.5-12 h) and addition of glucose (1-12 g/100mL), suitable temperature (220℃) and residence time (4 h), and small addition of glucose (<6 g/100mL) favored obtaining carbon microspheres with smooth surface and good sphericity. Higher temperature (<280℃), larger addition of glucose and longer residence time were conductive to improving the yield of hydro-char and carbon fixation in it. Hydro-char with excellent morphology was obtained under conditions of 220℃ and 6 g/100ml for 4 h from glucose. The hydro-char contained a large number of microspheres which had a smooth surface, good spherictiy and distributed in the range of 0.7~1.9 μm; At the same time, the hydro-char was an amorphous carbon with a tiny BET surface area and only few mesopores and macropores; The hydro-char contained a lot of oxygen-containing functional groups which were mainly -OH and -C=O; The degree of carbonization on the surface was higher than that in the core. The formation of hydro-char was done like this: , the feedstock was firstly hydrolyzed into many intermediate products which would be transformed into microsphere cores through dehydration and polymerization, the microsphere cores then formed into dispersed colloidal particles by the adsorption of surrounding molecules and eventually evolved into carbon microspheres or lump coke.The research results laid out a data reference for the process control of hydrothermal microspheres preparation and hydrothermal utilization of biomass.