RESUMEN
The applied pyrolysis temperature was found to strongly affect composition, structure, and oxidation behavior of pure and iron oxide nanoparticle (NP)-loaded carbon materials originating from hydrothermal carbonization (HTC) of cellulose. A strong loss of functional groups during pyrolysis at temperatures beyond 300 °C of the HTC-derived hydrochars was observed, resulting in an increase of the carbon content up to 95 wt% for the carbon materials pyrolyzed at 800 °C and an increase of the specific surface area with a maximum of 520 m2 g-1 at a pyrolysis temperature of 600 °C. Devolatilization mainly took place in the range from 300 to 500 °C, releasing light pyrolysis gases such as CO, CO2, H2O and larger oxygen-containing molecules up to C11. The presence of iron oxide NPs lowered the specific surface areas by about 200 m2 g-1 and resulted in the formation of mesopores. For the iron oxide-containing composites pyrolyzed up to 500 °C, the oxidation temperature was decreased by about 100 °C, indicating tight contact between the iron oxide NPs and the carbon matrix. For higher pyrolysis temperatures, this catalytic effect of iron oxide on carbon oxidation vanished due to carbothermal reduction to iron and iron carbide, which, however, catalyzed the graphitization of the carbon matrix. Thus, the well-controlled two-step synthesis based on a biomass-derived precursor yielded stably embedded iron NPs in a corrosion-resistant graphitic carbon matrix.
RESUMEN
Flexible metal-organic frameworks (MOFs), also referred to as soft porous crystals (SPCs), show reversible structural transitions dependent on the nature and quantity of adsorbed guest molecules. In recent studies it has been reported that covalent functionalization of the organic linker can influence or even integrate framework flexibility ("breathing") in MOFs. However, rational fine-tuning of such responsive properties is very desirable but challenging as well. Here we present a powerful approach for the targeted manipulation of responsiveness and framework flexibility of an important family of pillared-layered MOFs based on the parent structure [Zn(2)(bdc)(2)(dabco)](n) (bdc = 1,4-benzenedicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane). A library of functionalized bdc-type linkers (fu-bdc), which bear additional dangling side groups at different positions of the benzene core (alkoxy groups of varying chain length with diverse functionalities and polarity), was generated. Synthesis of the materials [Zn(2)(fu-bdc)(2)(dabco)](n) yields the respective collection of highly responsive MOFs. The parent MOF is only weakly flexible; however, the substituted frameworks of [Zn(2)(fu-bdc)(2)(dabco)](n) contract drastically upon guest removal and expand again upon adsorption of DMF (N,N-dimethylformamide), EtOH, or CO(2), etc., while N(2) is hardly adsorbed and does not open the narrow-pored form. These "breathing" dynamics are attributed to the dangling side chains that act as immobilized "guests", which interact with mobile guest molecules as well as with themselves and with the framework backbone. The structural details of the guest-free, contracted form and the gas sorption behavior (phase transition pressure, hysteresis loop) are highly dependent on the nature of the substituent at the linker and can therefore be adjusted using our approach. Combining our library of functionalized linkers with the concept of mixed-component MOFs (solid solutions) offers very rich additional dimensions of tailoring the structural dynamics and responsiveness. Implementation of two differently functionalized linkers in varying ratios yields multicomponent single-phased [Zn(2)(fu-bdc')(2x)(fu-bdcâ³)(2-2x)(dabco)](n) MOFs (0 < x < 1) of increased inherent complexity, which feature a non-linear dependence of their gas sorption properties on the applied ratio of components. Hence, the responsive behavior of such pillared-layered MOFs can be extensively tuned via an intelligent combination of functionalized linkers.