Modifications by impregnation and
grafting are commonly used for
the preparation of amine-functionalized MCM-41. A comprehensive evaluation
of the advantages and disadvantages of the two methods was performed
in this work. MCM-41 was synthesized by the hydrothermal method, setting
the amine-loading mass fraction at 40, 50, and 60 wt %. Three amine-modified
adsorbents were prepared by impregnating polyethylenimine (PEI), and
the three other adsorbents were prepared by grafting 3-aminopropyltriethoxysilane
(APTS) onto MCM-41. The as-prepared adsorbents were characterized
by X-ray diffraction, Fourier transform infrared spectroscopy, scanning
electron microscopy, thermogravimetric analysis, and N2 adsorption–desorption techniques. CO2 adsorption
capacities were measured, and the experimental data were fitted with
adsorption kinetic models. The cyclic stability of the adsorbents
prepared by the two kinds of amine-modified methods was compared using
the cyclic adsorption–desorption experiments. The characterization
results showed that the target adsorbents were prepared successfully.
The thermal stability of the adsorbents modified by grafting was better
than the thermal stability of the adsorbents modified by the impregnation.
Maximum CO2 adsorption capacities of 3.53 mmol g–1 (50% PEI–MCM-41) and 2.41 mmol g–1 (50%
APTS–MCM-41) could be reached at 25 °C and 1 atm, which
were 4.7 and 3.2 times greater than that of MCM-41. The Avrami model
fitted the experimental data well, indicating a variety of interactions
between the adsorbents and CO2. CO2 adsorption
capacity after 5 adsorption–desorption cycles decreased by
14.22 and 5.19% for the adsorbents prepared by impregnation and grafting,
respectively. It was concluded that MCM-41 modified by impregnation
and grafting followed the same kinetic model. The absorbents modified
by impregnation showed higher CO2 adsorption capacity and
amine-loading efficiency, while those prepared by grafting had better
thermal and cyclic stabilities.
Hemicellulose is a carbohydrate biopolymer second only to cellulose, which is rich and has a broad application prospect. The limitation of high-value utilization of hemicellulose has been a long-standing challenge due to its complex and diversified structure. The extraction and subsequent modification of hemicellulose from lignocellulosic biomass represent a promising pathway toward this goal. Herein, the extraction processes including physical pretreatment, chemical pretreatment, and combined pretreatment for separating hemicellulose from lignocellulosic biomass were introduced, and the advantages and disadvantages of various extraction procedures were also described. The chemical modification of hemicellulose such as etherification, esterification, grafting, and cross-linking modification was reviewed in detail. The separation and modification of hemicellulose in the future are prospected based on the earlier studies.
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