Solar-driven thermochemical water splitting has the potential to transform concentrated solar energy into green hydrogen and other solar fuels. In this work, La0.8Ca0.2MeO3 +/-$ (Me = Co, Ni, Fe and Cu) perovskites have been synthesised by a modified Pechini method and evaluated as materials for hydrogen production by two step thermochemical water splitting cycles. Performing the thermal reduction at temperatures of 1200 and 1000 degrees C, while the oxidation is done at 800 degrees C, allows a remarkable and stable hydrogen production after 5 consecutive cycles. However, the perovskites suffer changes in the structure after each redox cycle, with potential effects in the long-term cyclic operation. On the contrary, the isothermal thermochemical cycles at 800 degrees C produce a stable amount of hydrogen with each consecutive cycle maintaining the perovskite structure. This hydrogen production ranges from 3.60 cm3 STP/gmaterial$cycle for the material with the lowest productivity (La0.8Ca0.2FeO3 +/- d) to 5.02 cm3 STP/gmaterial$cycle for the one with the highest activity (La0.8Ca0.2NiO3 +/- d). Particularly the Ni-based material shows the highest H2 productivity accompanied by very good material stability after 15 consecutive cycles, being possible to combine with current solar thermal facilities based on concentrated solar power technologies like plants with central receivers.(c) 2023 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Solar-driven thermochemical water splitting has the potential to transform concentrated solar energy into green hydrogen and other solar fuels. In this work, La0.8Ca0.2MeO3±δ (Me = Co, Ni, Fe and Cu) perovskites have been synthesised by a modified Pechini method and evaluated as materials for hydrogen production by two step thermochemical water splitting cycles. Performing the thermal reduction at temperatures of 1200 and 1000 °C, while the oxidation is done at 800 °C, allows a remarkable and stable hydrogen production after 5 consecutive cycles. However, the perovskites suffer changes in the structure after each redox cycle, with potential effects in the long-term cyclic operation. On the contrary, the isothermal thermochemical cycles at 800 °C produce a stable amount of hydrogen with each consecutive cycle maintaining the perovskite structure. This hydrogen production ranges from 3.60 cm3 STP/gmaterial·cycle for the material with the lowest productivity (La0.8Ca0.2FeO3±δ) to 5.02 cm3 STP/gmaterial·cycle for the one with the highest activity (La0.8Ca0.2NiO3±δ). Particularly the Ni-based material shows the highest H2 productivity accompanied by very good material stability after 15 consecutive cycles, being possible to combine with current solar thermal facilities based on concentrated solar power technologies like plants with central receivers.

The preparation and optimisation of La 0.8 Al 0.2 NiO 3-delta (LANi82) perovskite shaped as reticulated porous ceramic (RPC) structures for H 2 production by thermochemical water splitting is presented for the first time. The perovskite was first synthesised in powder form following a modified Pechini method. The redox properties of the LANi82 were first tested under N 2 /air flow in a thermogravimetric analyser. After that, the sponge replica method for preparing RPCs was optimised in terms of slurry composition and final thermal treatment to obtain a LANi82-RPC structure with porosity and strength appropriate to enhance heat and mass transfer in further solar reactors. The optimised LANi82-RPC material showed an outstanding hydrogen production of 8.3 cm 3 STP/ g material & sdot; cycle at isothermal conditions (800 degrees C). This production was increased up to 11.5 cm 3 STP/g material & sdot; cycle if the thermal reduction was performed at 1000 degrees C. Additionally, a stable activity with almost constant H 2 production in consecutive cycles was obtained for the optimised LANi82-RPC in both cases. The structure of the reticulated porous materials, with open macroporosity and wide interconnected channels, enhances heat and mass transfer, leading to higher hydrogen productions of the LANi82-RPC as compared to the materials as powder form in the same experimental set-up. These facts reinforce the favourable prospects of LANi82-RPC for large-scale hydrogen production, improving the coupling to current solar thermal concentration technologies developed, such as concentrated solar power tower.

H-ZSM-5 and H-Beta zeolites ion-exchanged with alkali (Na+ and K+) and alkaline-earth (Mg2+) metals have been explored for the catalytic fast pyrolysis of lignin. Incorporating these metals led to a significant change in the acidic properties of the parent zeolites turning into mostly Lewis-type acidity. Catalytic fast pyrolysis experiments of lignin were performed in a fixed bed reactor with exsitu configuration operating at 550 degrees C (thermal zone) and 450 degrees C (catalytic zone), atmospheric pressure and under a nitrogen flow. Moreover, two catalysts to lignin mass ratios (C/L = 0.2 and 0.4) were studied. Compared with non-catalytic tests, the use of parent zeolites caused a decrease in the bio-oil* (water-free basis) yield due to enhanced production of gases, water, and the coke deposition on the catalyst. In addition, the quality of bio-oil* was improved since it presents a lower oxygen content regarding the thermal test. H-Beta zeolite showed a higher deoxygenation degree than H-ZSM-5, but the latter exhibited a higher share of light components in the bio-oil* that can be detected by GC-MS analyses. Both catalysts promoted the production of light oxygenates, aromatics, and oxygenated aromatics. Regarding the effect of the incorporation of metals, oxygenated aromatic compounds were the predominant family in the bio-oil* obtained with all ion-exchanged zeolites. Likewise, significant differences were observed among the catalysts regarding the main components of this family (alkylphenols, guaiacols, syringols, catechols, and methox-ybenzenes), achieving guaiacols concentrations in bio-oil* near to 24 wt.% for NaH-ZSM-5 and KH-ZSM-5 catalysts, and alkylphenols concentrations close to 16 wt.% for MgH-Beta and KH-Beta zeolites.

Cyclohexylphenols are valuable intermediate chemicals applied in the manufacture of dyes, resins, and pharmaceutical drugs.

The catalytic properties of K-grafted USY zeolite in fast pyrolysis of acid-washed wheat straw have been investigated using the parent USY sample as a reference. Potassium was incorporated into the zeolite by replacing H+ protons from framework hydroxyl groups with K+ cations. CO2-TPD confirmed the presence of basic sites, with mainly weak-mild strength, in the resulting material. Catalytic tests were performed in a down-flow fixed bed reactor with two furnaces. The thermal zone operated at 550 degrees C, whereas the temperature of the catalytic bed was varied from 400 to 500 degrees C. The catalysts were tested at two different catalyst to biomass ratios (C/B = 0.2 and 0.6). The K-USY sample was quite more active in bio-oil* (bio-oil in a water-free basis) conversion and upgrading than the parent zeolite, as denoted by the increased production of gases and the lower oxygen content of the organic liquid phase, although it also suffered higher coke deposition. Sharp changes were observed in the bio-oil* composition when using the K-USY catalyst. In particular, the presence in this material of a high population of weak-mild basic sites favoured the conversion of lignin oligomers and the occurrence of ketone-forming reactions from holocellulose. As a result, phenols and ketones were the main components (with an overall concentration up to 31.5 wt%) in the bio-oil* obtained over K-USY, which is an interesting result due to their potential use for the preparation of fine chemicals with high-added value.

Aluminum based perovskites, La0.8Al0.2MeO3-6 (Me = Co, Ni, Fe, Cu), were synthesized following the Pechini method at different pHs and evaluated for hydrogen production by water splitting in a two-step thermochemical cycle. The pH of the synthesis medium showed a critical influence in the redox properties of the aluminum-based perovskites during the thermochemical cycle. Both the thermal reduction and the hydrolysis step produce irreversible changes in the crystalline structure of La0.8Al0.2MeO3-6 perovskites prepared at acid pH, avoiding the cyclability of the material and a stable production of hydrogen during consecutive cycles. However, these changes are not observed when the perovskites were synthesized at basic pH and operated at isothermal conditions at 800 & DEG;C. In this case, the materials keep the crystalline structure leading to stable hydrogen production during consecutive cycles. Among all the studied materials, the nickel-based La0.8Al0.2MeO3-6 perovskites exhibited the best hydrogen productivity per cycle, 4.4 cm3 STP /g(material).cycle (Standard Temperature and Pressure conditions), which is a remarkable result considering that the thermochemical water splitting is conducted under isothermal conditions at just 800 ?. Moreover, this result is accompanied with a good performance of the perovskite in terms of solar to fuel efficiency (being 0.46 the ratio between the potential energy recovery from the produced hydrogen and the solar heat required for its production), comparable or even higher than values reported in the literature for other metal oxides. These results confirm the La0.8Al0.2MeO3-6 perovskite as an auspicious material for a full-scale H2 production from water splitting by solar-driven thermochemical cycles at low temperatures.

A study of the hydrogen production by thermochemical water splitting with a commercial perovskite La0.8Sr0.2CoO3-$(denoted as LSC) under different temperature conditions is pre-sented. The experiments revealed that high operational temperatures for the thermal reduction step (>1000 degrees C) implied a decrease in the hydrogen production with each consecutive cycle due to the formation of segregated phases of Co3O4. On the other hand, the experiments at lower thermal reduction operational temperatures indicated that the material had a stable behaviour with a hydrogen production of 15.8 cm3 STP/gmaterial.cycle during 20 consecutive cycles at 1000 degrees C, being negligible at 800 degrees C. This results comparable or even higher than the maximum values reported in literature for other perovskites (9.80-10.50 STP/gmaterial.cycle), but at considerable lower temperatures in the reduction step of the thermochemical cycle for the water splitting (1000 vs 1300-1400 degrees C). The LSC keeps the perovskite type structure after each thermochemical cycle, ensuring a stable and constant H2 production. An energy and exergy evaluation of the cycle led to values of solar to fuel efficiency and exergy efficiency of 0.67 and 0.36 (as a percentage of 1), respectively, which are higher than those reported for other metal oxides redox pairs commonly found in the literature, being the reduction temperature remarkably lower. These facts point out to the LSC perovskite as a promising material for full-scale applications of solar hydrogen pro-duction with good cyclability and compatible with current concentrating solar power technology. (c) 2022 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY license (http://creativecommons.org/ licenses/by/4.0/).

In this work, we present for the first time the preparation and evaluation of Ceria-based mixed oxides reticulated porous ceramic (RPC) structures for H-2 production by thermochemical water splitting. After appropriate screening of the powder materials, ceria-based materials modified with Co, Mn and Zr were discarded due to their low cyclability and/or hydrogen productivity, derived from segregation of active phases or sintering during the thermal reduction and reoxidation. Sponge replica method has been optimized to allow obtaining a Ce(0.9)Fe(0.1)Oy RPC sponge structure with an outstanding hydrogen production of 15 STPcm(3/)g(material).cycle at a maximum temperature of 1300 degrees C. This better performance, comparing to the powder, can be attributed to the open macroporosity of the reticulated porous structure which enhances both heat and mass transfer. The H-2 production is maintained along several consecutive cycles without loss of activity, reinforcing the favorable prospects for large-scale hydrogen production. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Fractional pyrolysis of lignocellulosic biomass, by staged thermal treatment, has been assessed as an in-situ speciation method of the bio-oil components that could be highly beneficial for extracting valuable compounds or for their subsequent catalytic upgrading. Wheat straw and pine woodchips were used as representative biomasses. Based on the results of TG analyses in an inert atmosphere, 350 and 700 degrees C were selected, respectively, as operational temperatures for the fractional pyrolysis. Compared to single-step pyrolysis, fractional thermal treatment of both biomasses led to some reduction of the bio-oil yield but with improved properties due to their lower oxygen content. Sharp differences were observed in the bio-oil composition obtained at the two steps of fractional pyrolysis. GC-MS analyses revealed that most of the compounds detected in the bio-oil obtained at 350 degrees C were products formed by the decomposition of polysaccharides, such as carboxylic acids, furans, sugars, and light oxygenates. In contrast, the organic liquid phase obtained during the subsequent treatment at 700 degrees C was rich in aromatic oxygenated compounds, coming from the lignin conversion. The content of oligomeric and heavy species, not detected by GC-MS, was much higher in the bio-oils obtained in the hightemperature step of fractional pyrolysis, denoting that they are largely formed from lignin. Significant changes were also observed in the relative contribution of the deoxygenation pathways during the two steps of fractional pyrolysis. Thus, dehydration was the predominant deoxygenation route during the degradation of the holocellulose biopolymers at the low-temperature step, whereas the decomposition of the lignin-rich solid at the high-temperature treatment proceeded with a significant contribution of decarbonylation and decarboxylation. These results evidence the great potential of lignocellulose fractional pyrolysis to generate bio-oil streams with high speciation of the components, facilitating sharply their further processing and upgrading.

Methane decomposition (DeCH(4)) over solid catalysts is an interesting route for the production of hydrogen free of CO2 emissions. Moreover, it could lead to a negative carbon balance if biogas/biomethane is used as feedstock. However, it is limited by the huge amounts of carbon that are deposited over the catalyst causing its deactivation and hindering its regeneration, which makes necessary the development of low-cost and durable catalytic systems. This work reports the use of different silica materials fully produced from rice husk, i.e. without incorporating any external phase or component, as DeCH(4) catalysts. The highest catalytic activity has been found for the silica samples showing large BET surface area and amorphous nature. These properties favor the generation of the actual DeCH(4) active sites (-Si-C- species), shortening the induction time detected at the beginning of the reaction tests. The nano-silica materials produced from acid-washed rice husk exhibit a remarkable resistance against deactivation, affording an almost constant reaction rate at long times on stream. This fact is assigned to the presence of large mesopores that facilitate the growth of the carbons deposits towards the outer part of the catalyst particles. The results here reported show the great potential of rice husk-derived nano-silica to overcome several of the most relevant limitations that currently exist for the commercial deployment of hydrogen production by catalytic DeCH(4), as a consequence of the low cost and durable activity of these sustainable materials.

The oxides Ca0.5Sr0.5CoO3-delta and SrCoO3-delta, which present perovskite or perovskite-related phases in different temperature domains, have been tested as materials for thermochemical energy storage. The first one, Ca0.5Sr0.5CoO3-delta, experiences a reversible phase transition upon consecutive cycles under an airflow at a maximum operating temperature of 1196 K. Unfortunately, the heat stored in this process, associated with an oxygen loss/gain and a structural phase transition, is very small, hindering its use for thermochemical heat storage. The as-prepared oxide SrCoO(3)(-)d, which displays a brownmillerite structure like the Ca-containing compound, in the first heating step irreversibly segregates some Co3O4 at 823 K to yield a 2H hexagonal perovskite. This phase reversibly transforms at 1073 K into a cubic perovskite. These 2H reversible arrow C transitions occur from the 2nd to, at least, 30th cycle. The average absorbed and released heat is similar to 104.1 +/- 0.06 and similar to 68.8 +/- 1.8 J/g, respectively, and therefore, SrCoO3-delta presents a high exo/endo ratio. The exergy efficiency is, on average for the 30 cycles performed, as high as 63.9 +/- 1.2%. The mechanism of the phase 2H reversible arrow C transition of SrCoO3-delta explains the good performance of this material for thermochemical energy storage.

Herein, two novel isostructural metal-organic frameworks (MOFs) M-URJC-4 (M = Co, Ni; URJC = "Universidad Rey Juan Carlos") with open metal sites, permanent microposity, and large surface areas and pore volumes have been developed. These novel MOFs, with polyhedral morphology, crystallize in the monoclinic P21/c space group, exhibiting a three-dimensional structure with microporous channels along the c axis. Initially, they were fully characterized and tested in hydrogen (H2) adsorption at different conditions of temperature and pressure. The physisorption capacities of both materials surpassed the gravimetric H2 uptake shown by most MOF materials under the same conditions. On the basis of the outstanding adsorption properties, the Ni-URJC-4 material was used as a catalyst in a one-pot reductive amination reaction using various carbonyl compounds and primary amines. A possible chemical pathway to obtain secondary amines was proposed via imine formation, and remarkable performances were accomplished. This work evidences the dual ability of M-URJC-4 materials to be used as a H2 adsorbent and a catalyst in reductive amination reactions, activating molecular H2 at low pressures for the reduction of C═N double bonds and providing reference structural features for the design of new versatile heterogeneous materials for industrial application.

Three novel flexible sulfur-containing MOF materials named Co-URJC-5, Cu-URJC-6 and Zn-URJC-7, based on the 5,5'-thiodiisophthalic acid linker have been synthesized through solvothermal methods and characterized by different physicochemical techniques. Hydrogen adsorption analysis at room temperature reveals that these compounds display a gate-opening type adsorption mechanism at low pressures, attributed to the flexible nature of the H4TBTC ligand. This behavior is even more noticeable for Cu-URJC-6, since the layer arrangement by p-p stacking interactions between the aromatic layers could contribute to the flexibility of the structure. These results can be considered as a representative example to elucidate how MOF structures are built using flexible ligands and more significantly as a promising route for designing materials with selective gas sorption properties.

The high temperature required for hydrogen production by solar driven thermochemical cycles is a critical factor hindering full-scale applications. The thermochemical cycle based on Mn3O4/MnO redox pair is one of the most studied despite the high operating temperature required for complete de reduction step (1623-1723 K). The combination of Mn3O4 with Co3O4, a metal oxide with lower reduction temperature than Mn3O4 that cannot be used for hydrogen production due to thermodynamic limitations, is presented as a way to decrease significantly the energy demand of the cycle. In this work, a complete thermodynamic study of thermochemical cycles with different Mn/Co mixed oxides (Mn3-xCoxO4, 0.9 < x < 2.7) for hydrogen production has been performed. The study of the variation of Gibbs energy with temperature allowed to determine that the thermal reduction of the metal oxide (Mn3-xCoxO4) takes place at temperatures between 1048 and 1173 K, which can be achieved by conventional solar concentration technologies. Unfortunately, the oxidation of the reduced metal oxide (Mn3-xCoxO3) with water to produce H-2 is not feasible from a thermodynamically point of view, so a stronger oxidizing agent, as sodium hydroxide, is required. The optimum temperature for the oxidation with NaOH was found to be 1373 K, meaning that this reaction takes place at higher temperatures than those actually required for the reduction, something uncommon in thermochemical cycles for water splitting. On the other hand, after the study of the variables affecting the equilibrium like the inert gas/solid ratio, it can be concluded that in both reactions, thermal requirements can be reduced by operating at lower temperatures by means of a higher inert gas/Mn3-xCoxO4 ratio. Finally, energy and exergy analysis of the system based on the solar absorption efficiency and the energy requirements predicts a solar-to-fuel efficiency and exergy efficiency of 40% and 23%, respectively. These values are comparable or even higher than those found in literature for other metal oxides thermochemical cycles for water splitting, thus with the advantage of working at a considerable lower temperature (1373 K).

This work investigates the effects of both zeolite accessibility and reaction temperature on the conversion of phenol assisted by 2-propanol over the tandem system Raney Ni + ZSM-5 zeolite. Two different zeolite samples, containing similar SiAI ratios, were used: nanocrystalline ZSM-5 (n-ZSM-5) and hierarchical ZSM-5 (h-ZSM-5), operating at temperatures of 125, 150, and 175 degrees C. When working with the Raney Ni + n-ZSM-5 system, at low and intermediate temperatures, the main products were mostly benzene and cyclohexene formed by phenol deoxygenation. In contrast, on increasing the temperature up to 175 degrees C, a shift in the reaction pathways was observed, leading toward a significant share of valuable alkylphenols (mostly cresols and cyclohexylphenols) in the product distribution, generated by the occurrence of alkylation reactions catalyzed by the zeolite acid sites. This effect was enhanced in the case of the h-ZSM-5 sample, due to its improved accessibility and larger mesopore/external surface area.

The organometallic Li-Crown ether species, formed by the complexation of lithium cation with the hydrophobic 18Crown6 ether, has been included in three Metal-Organic-Frameworks (MOF) structures with different pore size: Cr-MIL-101, Fe-MIL100 and Ni-MOF-74. X-ray powder diffraction, thermogravimetric analysis, proton nuclear magnetic resonance, infrared spectroscopy and inductively coupled plasma atomic emission spectroscopy measurements have proved the successful incorporation of the organometallic units to the three MOFs without altering their crystalline structure. Hydrogen adsorption properties of the post-synthesis modified materials have been evaluated in a wide temperature (77-298 K) and pressure (1-170 bar) range conditions. The post-synthetic modification method used based on the MOF impregnation with a Li-Crown ether complex solution produced a partial pore blocking effect on the microporous Ni-MOF-74, reducing its hydrogen adsorption capacity. However, the inclusion of the crown-ether and particularly the Li-Crown ether complex resulted in an increase of the volumetric hydrogen adsorption capacity at room temperature for Cr-MIL101 and Fe-MIL-100, due to the pore volume reduction, higher confinement of H-2 molecules in the cavities and the formation of new specific binding sites for H-2 molecules. The inclusion of Li-Crown ether complex also enhances the H-2 interaction with the mesoporous MOF structures, attributed to the additional electrostatic interactions produced by the presence of Li+ ions complexed to the crown ether molecules. Further work following this strategy to improve hydrogen adsorption capacity of mesoporous MOFs at room temperature should be extended to other MOF materials, checking its influence on their capacity for gas separation purposes. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

An expanded Co-IRMOF-74 material, with a similar structure to the well-known Co-MOF-74 but with wider pores has been developed using a larger organic linker containing two benzene rings instead of one. Moreover, the partial isomorphic substitution of cobalt for nickel in the inorganic cluster of the MOF was performed in gradation, obtaining a series of materials with mixed-metal Co/Ni sites in different proportion. This partial isomorphic substitution of the OMS displayed a significant enhancement of their hydrogen adsorption capacity. The presence of nickel increased the hydrogen uptake at cryogenic and ambient temperature, particularly when nickel and cobalt are present in the structure in the same ratio. This behaviour demonstrated a synergetic contribution, between both transition metal cations, related to electronic effects. This new series of expanded materials evidence the great versatility of MOF structures in tuning their porosity and chemistry, which can let eventually hosting different active species inside the pores to enhance specific properties, particularly their affinity to hydrogen molecules.

A novel URJC-3 material based on cobalt and 5,5'-(diazene-1,2-diyl)diisophthalate ligand, containing Lewis acid and basic sites, has been synthesized under solvothermal conditions. Compound URJC-3, with polyhedral morphology, crystallizes in the tetragonal and P43 21 2 space group, exhibiting a three-dimensional structure with small channels along a and b axes. This material was fully characterized, and its hydrogen adsorption properties were estimated for a wide range of temperatures (77-298 K) and pressures (1-170 bar). The hydrogen storage capacity of URJC-3 is quite high in relation to its moderate surface area, which is probably due to the confinement effect of hydrogen molecules inside its reduced pores of 6 Å, which is close the ionic radii of hydrogen molecules. The storage capacity of this material is not only higher than that of active carbon and purified single-walled carbon nanotubes, but also surpasses the gravimetric hydrogen uptake of most MOF materials.

Biomass catalytic pyrolysis has been investigated using wheat straw, previously subjected to acid washing to remove the mineral matter, as raw material with the goal of producing upgraded bio-oils with potential applications as liquid biofuels or as a source of bio-based chemicals. The catalyst employed consisted in zeolite ZSM-5 having nanocrystalline properties to enhance the accessibility towards the active sites, which was partially ion exchanged with K+ cations (KH-ZSM-5). Characterisation of this catalyst denoted that, although most of the strong acid sites were removed during the ion exchange treatment, it possesses a well-balanced combination of acid and basic properties. The biomass catalytic pyrolysis tests were performed in a two-step (thermal/catalytic) fixed bed reactor operating with temperatures of 550 and 450 °C, respectively, and different catalyst to biomass ratios (C/B). Compared with the parent acidic zeolite, the KH-ZSM-5 sample exhibited lower activity for the conversion and upgrading of the bio-oil. However, the removal of most of the strong acid sites by ion exchange with K+ resulted in a more efficient deoxygenation pathway, with lower contribution of decarbonylation. Moreover, the KH-ZSM-5 catalyst showed a reduced extension of cracking reaction into gases and of coke formation, whereas it promoted the conversion of the oligomeric species present in the bio-oil. As a consequence, the KH-ZSM-5 sample led to a more favourable relationship between the bio-oil oxygen content and the bio-oil yield when varying the C/B ratio in respect to the parent H-ZSM-5 zeolite.