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From blue hydrochars to activated carbons: Hydrothermal carbonization, chemical activation and gas adsorption
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).ORCID iD: 0000-0001-8225-9670
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Hydrothermal carbonization (HTC) of carbohydrates and biomass is a straightforward method for preparing hydrochars at low temperatures of 180-250 °C. Hydrochars are more carbonized than their precursors. Increasing the carbonization degree of hydrochars at hydrothermal temperatures is a scientific quest that is addressed in this thesis. Hydrochars are known to have a spherical or irregular morphology. Here we address thin film hydrochars for the first time.  Hydrochars themselves are carbon precursors for preparing activated carbons. Activated carbons are porous materials that can be used for gas adsorption applications. In this thesis, enhanced adsorption of VOCs at low pressures is addressed by using iron phosphate impregnated activated carbons. Finaly, any chemical process or product including those in this thesis such as HTC, activation, hydrochar and activated carbons may contribute to the issue of environmental degradation positively or negatively. Such environmental impacts are addressed by life cycle assessment of processes of HTC and activation and their related products in the last paper of this thesis. Briefly mentioned, in my first study (Paper I), I focused on the HTC of glucose in the presence of iron (II) sulfate. By changing the concentration of iron (II) sulfate, with a catalytic amount, blue hydrochars were formed at the bottom of the autoclave. The blueness was related to thin film interference. The thin film hydrochars were more carbonized than spherical hydrochars and the yield of HTC has increased in the presence of iron (II) sulfate. The second study (Paper II) is focused on the activation of hydrochars with H3PO4 and H3PO4+FeCl3. We showed that ultramicroporosity and impregnated iron phosphate species enhance the adsorption of VOCs at low pressure. The ACs were impregnated with Fe (PO3)2 and it was shown that Fe (PO3)2 acts as an activation agent which opens up for future studies. In the third study (Paper III), H3PO4-activated carbons were prepared and modified with FeS and FeSe and it was shown that the ACs were also impregnated with Fe2P, in the case of AC-FeS/Fe2P. FeSe and FeS were not detected by XRD. Only large crystals of Fe2P were detected in the sample AC-FeS/Fe2P. In the last study (Paper IV), prickly pear seed biomass from the agro sector in Tunisia was hydrothermally carbonized. The hydrochars were then activated into ACs by CO2 activation. The life cycle assessment of the HTC and activation process was investigated.  

Place, publisher, year, edition, pages
Department of Materials and Environmental Chemistry, Stockholm University , 2024. , p. 51
Keywords [en]
hydrochars, activated carbons, hydrothermal carbonization, chemical activation, gas adsorption
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-228549ISBN: 978-91-8014-797-2 (print)ISBN: 978-91-8014-798-9 (electronic)OAI: oai:DiVA.org:su-228549DiVA, id: diva2:1853327
Public defence
2024-06-14, online via Zoom, public link is available at the department website, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2024-05-22 Created: 2024-04-22 Last updated: 2024-05-13Bibliographically approved
List of papers
1. Blue hydrochars formed on hydrothermal carbonization of glucose using an iron catalyst
Open this publication in new window or tab >>Blue hydrochars formed on hydrothermal carbonization of glucose using an iron catalyst
2022 (English)In: Carbon Trends, ISSN 2667-0569, Vol. 8, article id 100172Article in journal (Refereed) Published
Abstract [en]

We hypothesized that the morphology of the hydrochar from hydrothermal carbonization of glucose would be affected by Fe2+; and indeed, with such ions, large pieces of hydrochar formed that comprised aggregated spherical particles and blue and thin films. Thin carbonized films formed at the bottom of the autoclave liners or on TeflonTM tape used as a template. Free-standing films could be prepared by stretching the TeflonTM tape after the synthesis. The carbonized films that formed at the bottom of the autoclave adhered to spherical hydrochar particles. The blueness was ascribed to thin-film interference under white-light irradiation and related to the film thickness, which was about 200 nm. Analysis of transmission electron microscopy (TEM) images showed that the films consisted of a layered amorphous carbon. The amorphous and thin films were more carbonized than the amorphous carbon of the TEM grid, as derived via electron energy loss spectroscopy (EELS). Additional analysis of one of the thin films by X-ray photoelectron spectral analysis showed a higher carbon fraction than for bulk hydrochar, supporting the EELS analysis. We believe that the synthesis of thin films of hydrochar can open up new colloidal processing pathways, which could be useful in the preparation of carbon-based materials and alike.

Keywords
Hydrothermal carbonization, Glucose, Iron sulfate, Carbon spheres, Thin films, Thin film interference
National Category
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-206189 (URN)10.1016/j.cartre.2022.100172 (DOI)2-s2.0-85128535062 (Scopus ID)
Funder
Mistra - The Swedish Foundation for Strategic Environmental Research, 2015/31
Available from: 2022-06-22 Created: 2022-06-22 Last updated: 2024-04-22Bibliographically approved
2. Adsorption of volatile organic compounds on activated carbon with included iron phosphate
Open this publication in new window or tab >>Adsorption of volatile organic compounds on activated carbon with included iron phosphate
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2023 (English)In: Carbon trends, ISSN 2667-0569, Vol. 11, article id 100259Article in journal (Refereed) Published
Abstract [en]

Volatile organic compounds (VOCs) are often hazardous and need commonly to be removed from gas mixtures. Capture on activated carbon (AC) is one approach to achieving this. We hypothesized that the smallest pores on ACs and the inclusion of inorganic phosphates could enhance the low gas pressure uptake of two typical VOCs (acetone and isopropanol). To test this hypothesis, ACs were prepared by chemical activation of hydrochars with H3PO4 or a mixture of FeCl3 and H3PO4. The hydrochars had been prepared by hydrothermal carbonization of glucose. The ACs were characterized by XRD, IR, TGA, and the adsorption of N2, CO2, H2O, acetone, and isopropanol. The results showed that the ACs had comparably high adsorption of acetone and isopropanol at low vapor pressures. The low-pressure uptake (at 0.03 kPa) of isopropanol and acetone had values of up to 3.4 mmol/g and 2.2 mmol/g, respectively. This suggests that ACs containing iron phosphate could be of relevance for adsorption driven removal of VOC. It was also observed that the external surface area of the ACs containing iron phosphates increased upon secondary heat treatment in N2.

National Category
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-228544 (URN)10.1016/j.cartre.2023.100259 (DOI)001030948100001 ()2-s2.0-85151239392 (Scopus ID)
Funder
Mistra - The Swedish Foundation for Strategic Environmental Research, 2015/31Knut and Alice Wallenberg Foundation
Available from: 2024-04-22 Created: 2024-04-22 Last updated: 2024-04-22Bibliographically approved
3. Activated carbons impregnated with FeS/FeSe/Fe2P derived from hydrochars and chemical activation with H3PO4
Open this publication in new window or tab >>Activated carbons impregnated with FeS/FeSe/Fe2P derived from hydrochars and chemical activation with H3PO4
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(English)Manuscript (preprint) (Other academic)
National Category
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-228546 (URN)
Available from: 2024-04-22 Created: 2024-04-22 Last updated: 2024-04-22
4. Activated Carbons Produced from Hydrothermally Carbonized Prickly Pear Seed Waste
Open this publication in new window or tab >>Activated Carbons Produced from Hydrothermally Carbonized Prickly Pear Seed Waste
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2022 (English)In: Sustainability, E-ISSN 2071-1050, Vol. 14, no 21, article id 14559Article in journal (Refereed) Published
Abstract [en]

The agro-sector generates organic waste of various kinds, which potentially could be used to prepare functional materials, lessen environmental problems, and enhance circularity. In this context, the hypothesis that was put forward in this work is that prickly pear seed waste from the Tunisian agro-food industry could be used to prepare activated carbons. The prickly pear seed waste was first hydrothermally carbonized and the resulting hydrochar was activated in CO2 at 800 °C. The yield of the hydrothermal carbonization process is of importance, and it was the highest at intermediate dwell times and temperatures, which was ascribed to the re-precipitation of hydrochar particles on the heat-treated biomass. The hydrochars and activated carbons were characterized with scanning electron microscopy, thermogravimetry, Raman spectroscopy, and N2 and CO2 adsorption/desorption analyses. The activated carbons had micro- (<2 nm) and mesopores (2–50 nm), and specific surface areas and total pore volumes of about 400 m2 −1 and 0.21 cm3 g−1. The study showed that the prickly pear seed waste could be effectively transformed into both hydrochars and activated carbons and that is advisable to optimize the hydrothermal process for the mass yield. A life cycle analysis was performed to assess the environmental impact of the production of typical activated carbons using the approach of this study. Further studies could be focused on enhancing the properties of the activated carbons by further optimization of the activation process.

Keywords
bio-waste, prickly pear seeds, hydrothermal carbonization, activated carbons, CO2 activation, life cycle assessment, Green & Sustainable Science & Technology
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-212591 (URN)10.3390/su142114559 (DOI)000885860700001 ()
Available from: 2022-12-13 Created: 2022-12-13 Last updated: 2024-04-22Bibliographically approved

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