Demonstration of Polymorphic Spacing Strategy against Sintering: Synthesis of Stabilized Calcium Looping Absorbents for High-Temperature CO₂ Sorption

To decrease the sintering deterioration of CaO sorbents in multiple CO₂ capture and release cycles, we synthesized a series of stabilized CaO sorbents incorporated with silica through freeze-drying and heat-drying, the latter of which was referred to as benchmark. The ratio of Ca and Si precursors was varied to control the reactive loadings of CaO (from 70 to 100 wt %) and the fraction of spacers in the sorbents. The characterization results show that the freeze-drying method produces sorbents with higher specific area and larger pore volume than the heat-drying method. Moreover, the stability test of over 30 cycles demonstrated that the freeze-dried samples exhibited better performance with higher stability and total CO₂ uptake. The high-resolution transmission electron microscopy image shows that Ca₂SiO₄ crystallites as spacers are distributed within the matrix of CaO crystallites. The optimal spacer loading was determined to be ∼10 wt %, and the optimal reaction temperature was found to be 700 °C. Finally, the best sorbent was tested under harsh conditions and maintained a stable capture capacity with a CO₂ uptake of 0.21 g of CO₂ g^–1 of sorbent even at the 30th cycle. The performance of the sorbent in this work was then systematically compared to those reported in the literature. The use of a Si-based spacer and freeze-drying have significant potential to enhance the stability of CaO sorbents

Na₂ZrO₃ as an Effective Bifunctional Catalyst–Sorbent during Cellulose Pyrolysis

Na₂ZrO₃ was tested as bifunctional catalyst sorbent using cellulose as model biomass under pyrolytic conditions. Thermogravimetric analyzer connected to a mass spectrometer (TG-MS) was used to study the influence of Na₂ZrO₃ on the gas evolution from cellulose pyrolysis. The weight loss data and gas evolution was analyzed over a temperature range of 200–800 °C. Na₂ZrO₃ showed a clear catalytic influence during cellulose pyrolysis, and it was actively catalyzing tar cracking and reforming reactions at elevated temperatures. A comparison with CaO was conducted under identical conditions and results showed that Na₂ZrO₃ mixed samples were able to produce higher yield of hydrogen from cellulose, mainly due to participating in tar-cracking and reforming reactions at lower temperatures than CaO (500 °C for Na₂ZrO₃, compared to 600 °C for CaO). The study showed that Na₂ZrO₃ can act as catalyst for pyrolysis reactions of cracking and reforming, and subsequently remove CO₂ produced <i>in situ</i>. The results suggest that Na₂ZrO₃ has potential to participate in the gasification of biomass as an effective bifunctional catalyst–sorbent, which may enhance hydrogen yield

Effects of Drying Methods on Wet Chemistry Synthesis of Al-Stabilized CaO Sorbents for Cyclic CO₂ Capture

Al-stabilized CaO sorbents synthesized by wet chemistry methods have demonstrated effectiveness to mitigate CaO sintering during Ca-looping cycles (CaO + CO₂ ⇌ CaCO₃) for CO₂ capture. To further screen the synthesis techniques and recipes, a series of Al-stabilized CaO sorbents, namely, CaO–Ca₉Al₆O₁₈ hybrid materials, derived from cosolutions of calcium acetate and aluminum nitrate were prepared using three different drying methods, i.e., freeze drying, spray drying, and evaporation drying. These sorbents were then characterized by X-ray diffraction, N₂ physisorption, scanning electron microscopy, and energy dispersive spectrometry. The effects of drying methods on the CO₂ capture performance of the sorbents were analyzed comprehensively. Out of the three drying methods, spray drying enabled the optimal textural property and the hard skeleton with sufficient mechanical strength, resulting in the supreme CO₂ capture capacity. Furthermore, it was found that, by spray drying, the inert spacer Ca₉Al₆O₁₈ could play the most significant role in stabilizing the cyclic sorption reactivity of CaO. For spray dried samples, the SD70 sample with 70 wt % CaO and 30 wt % Ca₉Al₆O₁₈ could well balance the capacity and stability under mild conditions. Its advantage was much more pronounced under severe conditions, where SD70 overtook other samples in CO₂ uptake capacity from the fourth cycle and maintained the highest CaO conversion all through the 30 cycles

Enhanced Hydrogen Production from Sawdust Decomposition Using Hybrid-Functional Ni-CaO-Ca₂SiO₄ Materials

A hybrid-functional material consisting of Ni as catalyst, CaO as CO₂ sorbent, and Ca₂SiO₄ as polymorphic “active” spacer was synthesized by freeze-drying a mixed solution containing Ni, Ca and Si precursors, respectively, to be deployed during sawdust decomposition that generated gases mainly containing H₂, CO, CO₂ and CH₄. The catalytic activity showed a positive correlation to the Ni loading, but at the expense of lower porosity and surface area with Ni loading beyond 20 wt %, indicating an optimal Ni loading of 20 wt % for Ni-CaO-Ca₂SiO₄ hybrid-functional materials, which enables ∼626 mL H₂ (room temperature, 1 atm) produced from each gram of sawdust, with H₂ purity in the product gas up to 68 vol %. This performance was superior over a conventional supported catalyst Ni–Ca₂SiO₄ that produced 443 mL H₂ g-sawdust^–1 under the same operating condition with a purity of ∼61 vol %. Although the Ni-CaO bifunctional material in its fresh form generated a bit more H₂ (∼689 mL H₂ g-sawdust^–1), its cyclic performance decayed dramatically, resulting in H₂ yield reduced by 62% and purity dropped from 73 to 49 vol % after 15 cycles. The “active” Ca₂SiO₄ spacer offers porosity and mechanical strength to the Ni-CaO-Ca₂SiO₄ hybrid-functional material, corresponding to its minor loss in reactivity over cycles (H₂ yield reduced by only 7% and H₂ purity dropped from 68 to 64 vol % after 15 cycles)