Facilitated by redox catalysts capable of catalytic reactions and reactive separation, chemical looping offers exciting opportunities for intensified chemical production.

Global warming and climate change are most likely linked to the increasing concentration of the greenhouse gas carbon dioxide (CO2) in the atmosphere. Additionally, the consumption of fossil fuels is predicted further to increase in the coming decades, particularly due to the rapid development of populous countries such as Brazil, India, and China. Therefore, it is imperative to develop and implement processes that avoid the emission of anthropogenic CO2. One possible midterm solution is carbon‐dioxide capture and storage (CCS). In this context, the so‐called chemical‐looping combustion (CLC) process, that is, an emerging 3rd‐generation CCS technology, is particularly attractive due to its very low predicted CO2‐capture costs compared to the currently available technology (i.e., amine scrubbing). In CLC, lattice oxygen from a solid‐state oxygen carrier is used to combust a hydrocarbon fuel, which yields, after the condensation of steam, a pure stream of CO2 suitable for sequestration. To allow the application of CLC to solid fuels, chemical looping with oxygen uncoupling (CLOU) has been proposed. Here molecular oxygen is provided by using the decomposition reaction of the oxygen carrier, thus, effectively the solid fuel is combusted in an oxyfuel mode. Importantly, a cornerstone of the CLOU process is the development of suitable oxygen carriers. In the first part of the review we discuss the thermodynamic properties of various CLOU materials. Subsequently, recent advances in the development of novel oxygen carriers are summarized. In particular, we focus on the physical and chemical properties of the new materials and the synthesis strategies employed. The review is concluded with an outlook on the CLOU process.

Calcium looping, a CO2 capture technique, may offer a mid-term if not near-term solution to mitigate climate change, triggered by the yet increasing anthropogenic CO2 emissions. A key requirement for the economic operation of calcium looping is the availability of highly effective CaO-based CO2 sorbents. Here we report a facile synthesis route that yields hollow, MgO-stabilized, CaO microspheres featuring highly porous multishelled morphologies. As a thermal stabilizer, MgO minimized the sintering-induced decay of the sorbents’ CO2 capacity and ensured a stable CO2 uptake over multiple operation cycles. Detailed electron microscopy-based analyses confirm a compositional homogeneity which is identified, together with the characteristics of its porous structure, as an essential feature to yield a high-performance sorbent. After 30 cycles of repeated CO2 capture and sorbent regeneration, the best performing material requires as little as 11 wt.% MgO for structural stabilization and exceeds the CO2 uptake of the limestone-derived reference material by ~500%.

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A bifunctional catalyst for the sorbent-enhanced steam methane reforming (SE-SMR) reaction was derived from a hydrotalcite-based precursor synthesized via a coprecipitation technique. The material contained both the Ni reforming catalyst and the Ca-based CO 2 sorbent and was characterized using X-ray diffraction, H 2 chemisorption, N 2 physisorption, transmission electron microscopy, and temperature-programmed reduction. Reduction of the calcined hydrotalcite converted the (Al:Ca:Mg:Ni)O x mixed oxide into nickel and CaO particles supported on an (Al:Mg)O x matrix with a surface area of 54 m 2 •g −1 . The high CO 2 absorption capacity and its stability with carbonation cycles was attributed to the high dispersion of CaO on the porous and thermally stable (Al:Mg)O x network, whereas for naturally occurring limestone, a rapid decay in the CO 2 absorption capacity was observed. Under SE-SMR conditions, the recorded mole fraction of hydrogen in the effluent stream was 99 vol % (dry and without inert component); that is, thermodynamic equilibrium calculated to be 99 vol % (without inert component) was reached. The CO 2 uptake of the bifunctional material averaged 0.074 g CO 2 /g sorbent over 10 cycles. After approximately seven cycles, the CO 2 capture capacity stabilized, resulting in an average decay rate of only 0.3% per cycle over the last three cycles. The bifunctional material developed here produced a larger amount of high-purity H 2 than limestone mixed with Ni−SiO 2 or a Ca-free, nickel hydrotalcite-derived catalyst, making the new material an interesting candidate for the SE-SMR process.

The commercially dominating technology for hydrogen production (i.e. steam methane reforming) emits large quantities of CO2 into the atmosphere. On the other hand, thermochemical water-splitting cycles allow to produce high purity H2 while simultaneously capturing CO2.

Sorbent-enhanced steam methane reforming (SE-SMR) is an emerging technology for the production of high-purity hydrogen from hydrocarbons with in situ CO2 capture. Here, SE-SMR was studied using a mixture containing a Ni-hydrotalcite-derived catalyst and a synthetic, Ca-based, calcium aluminate supported CO2 sorbent. The fresh and cycled materials were characterized using N2 physisorption, X-ray diffraction, and scanning and transmission electron microscopy. The combination of a Ni-hydrotalcite catalyst and the synthetic CO2 sorbent produced a stream of high-purity hydrogen, that is, 99 vol % (H2O- and N2-free basis). The CaO conversion of the synthetic CO2 sorbent was 0.58 mol CO2/mol CaO after 10 cycles, which was more than double the value achieved by limestone. The favorable CO2 capture characteristics of the synthetic CO2 sorbent were attributed to the uniform dispersion of CaO on a stable nanosized mayenite framework, thus retarding thermal sintering of the material. On the other hand, the cycled limestone lost its nanostructured morphology completely over 10 SE-SMR cycles due to its intrinsic lack of a support component.

Polyethylene terephthalate (PET) was depolymerized to monomer bis(2-hydroxyethyl) terephthalate (BHET) using excess ethylene glycol (EG) in the presence of metal oxides that were impregnated on different forms of silica support [silica nanoparticles (SNPs) or silica microparticles (SMPs)] as glycolysis catalysts. The reactions were carried out at 300 degrees C and 1.1 MPa at an EG-to-PET molar ratio of 11:1 and a catalyst-to-PET-weight ratio of 1.0% for 40-80 min. Among the four prepared catalysts (Mn3O4/SNPs, ZnO/SNPs, Mn3O4/SMPs, and ZnO/SMPs), the Mn3O4/SNPs nanocomposite had the highest monomer yield (> 90%). This high yield may be explained by the high surface area, amorphous and porous structure, and existence of numerous active sites on the nanocomposite catalyst. The BHET yield increased with time and reached the highest level where equilibrium was established between BHET and its dimer. The catalysts were characterized by their SEM, TEM, and BET surface areas, and via XRD, whereas the monomer BHET was characterized by HPLC and FT-IR. The glycolysis with the Mn3O4/SNPs nanocomposite as the glycolysis catalyst produced a maximum BHET in a short reaction time.