Biological versus Synthetic Polymers as Templates for Calcium Oxide for CO₂ Capture

When CaO absorbs CO₂ in pre-combustion systems, it can greatly increase the H₂ yield from pyrolysis and gasification reactions. However, natural sources of CaO, such as limestone (CaCO₃), produce sorbents that quickly lose reactivity, and more stable sorbents are therefore desired. Although both synthetic and biopolymers can induce the formation of a wide variety of CaCO₃ morphologies, only a few synthetic polymers and perhaps no natural polymers have been applied to the synthesis of CO₂ sorbents. We prepared CaO precursors templated on three natural polysaccharides (chitosan, agar, and carrageenan) and three synthetic polymers [poly­(acrylic acid), poly­(ethylene glycol), and poly­(ethylene oxide-<i>b</i>-propylene oxide-<i>b</i>-ethylene oxide)] and tested these as CO₂ sorbents. The sorbents templated on biopolymer films had interesting reactivity that was affected by residual biopolymer, even after several calcination–carbonation cycles at high temperatures. However, these sorbents were not more effective than CaO derived from commercial CaCO₃. Thus, although biopolymers can direct CaCO₃ formation by binding to Ca²⁺, the biopolymer-templated CaCO₃ samples tested here were inferior CO₂ sorbents. Two of the sorbents templated on synthetic polymers, on the other hand, gave better CO₂ uptake activity and stability than CaO derived from commercial CaCO₃. These results, in combination with the vast array of existing synthetic polymers, suggest a promising future for synthetic polymers in this field

Novel CaO–SiO₂ Sorbent and Bifunctional Ni/Co–CaO/SiO₂ Complex for Selective H₂ Synthesis from Cellulose

Catalysis- and sorption-enhanced biomass gasification is a promising route to high-purity hydrogen (H₂); however, most CaO-based sorbents for CO₂ capture have poor surface area and mechanical properties, lose carrying capacity over multiple uses, and have insufficient porosity to accommodate extra catalyst sites. We aimed to develop a high-surface-area CaO–SiO₂ framework onto which catalysts could be grafted. The best CaO–SiO₂ sorbent (<i>n</i>_Ca/<i>n</i>_Si = 2:1) maintained a CaO conversion of 65% even after 50 carbonation–decarbonation cycles, better than commercial micrometer-sized CaO or tailored CaO, because of stabilization via Ca–O–Si interactions and an ordered porous structure. Bimetallic catalyst grains (Ni/Co alloy, <20 nm) could be evenly loaded onto this structure by impregnation. The resulting bifunctional complex produced H₂ at nearly the same rate as a mixture of catalyst and commercial CaO while using less total sorbent/catalyst. Furthermore, this complex was much more durable due to its higher coking resistance and stable structure. After 25 carbonation–decarbonation cycles, the new catalyst–sorbent complex enhanced the H₂ yield from cellulose far more than a mixture of catalyst and commercial CaO did following the same treatment

A Simple Gas–Solid Route To Functionalize Ordered Carbon

The reaction of nitric oxide (NO) and carbonaceous materials generates nitrogen functionalities on and in graphitic carbons and oxidizes some of the carbon. Here, we have exploited these phenomena to provide a novel route to surface-functionalized multiwalled carbon nanotubes (MWCNTs). We investigated the impacts of NO on the physical and chemical properties of industrially synthesized multiwalled carbon nanotubes to find a facile treatment that increased the specific surface area (<i>S</i>_BET) of the MWCNTs by ∼20%, with only a minimal effect on their degree of graphitization. The technique caused less material loss (∼12 wt %) than traditional gas-based activation techniques and grafted some nitrogen functional groups (1.1 at. %) on the MWCNTs. Moreover, we found that Ni nanoparticles deposited on NO-treated MWCNTs had a crystallite size of <i>d</i>_Ni = 13.1 nm, similar to those deposited on acid-treated MWCNTs (<i>d</i>_Ni = 14.2 nm), and clearly much smaller than those deposited under the same conditions on untreated MWCNTs (<i>d</i>_Ni = 18.3 nm)