Pedro Castaño

Towards the electrochemical conversion of carbon dioxide into methanol

Various strategies have been proposed to date in order to mitigate the concentration of CO2 in the atmosphere, such as the separation, storage, and utilization of this gas. Among the available technologies, the electrochemical valorisation of CO2 appears to be an innovative technology, in which electrical energy is supplied to establish a potential between two electrodes, allowing CO2 to be transformed into value-added chemicals under mild conditions. It provides a method to recycle CO2 (in a carbon neutral cycle) and, at the same time, a way to chemically store the excess of renewable energy from intermittent sources, thus reducing our dependence on fossil fuels. Among the useful products that can be obtained, methanol is particularly interesting as a platform chemical, and it has gained renewed and growing attention in the research community. Accomplishments to date in the electroreduction of CO2 to methanol have been encouraging, although substantial advances are still needed for it to become a profitable technology able to shift society to renewable energy sources. This review presents a unified discussion of the significant work that has been published in the field of electrocatalytic reduction of CO2 to methanol. It emphasizes the aspects related to process design at different levels: cathode materials, reaction media, design of electrochemical cells, as well as working conditions. It then extends the discussion to the important conclusions from different electrocatalytic routes, and recommendations for future directions to develop a catalytic system that will convert CO2 to methanol at high process efficiencies.

Copper‐Based Metal–Organic Porous Materials for CO2 Electrocatalytic Reduction to Alcohols

The electrocatalytic reduction of CO2 has been investigated using four Cu-based metal-organic porous materials supported on gas diffusion electrodes, namely, (1) HKUST-1 metal-organic framework (MOF), [Cu3 (μ6 -C9 H3 O6 )2 ]n ; (2) CuAdeAce MOF, [Cu3 (μ3 -C5 H4 N5 )2 ]n ; (3) CuDTA mesoporous metal-organic aerogel (MOA), [Cu(μ-C2 H2 N2 S2 )]n ; and (4) CuZnDTA MOA, [Cu0.6 Zn0.4 (μ-C2 H2 N2 S2 )]n . The electrodes show relatively high surface areas, accessibilities, and exposure of the Cu catalytic centers as well as favorable electrocatalytic CO2 reduction performance, that is, they have a high efficiency for the production of methanol and ethanol in the liquid phase. The maximum cumulative Faradaic efficiencies for CO2 conversion at HKUST-1-, CuAdeAce-, CuDTA-, and CuZnDTA-based electrodes are 15.9, 1.2, 6, and 9.9 %, respectively, at a current density of 10 mA cm-2 , an electrolyte-flow/area ratio of 3 mL min cm-2 , and a gas-flow/area ratio of 20 mL min cm-2 . We can correlate these observations with the structural features of the electrodes. Furthermore, HKUST-1- and CuZnDTA-based electrodes show stable electrocatalytic performance for 17 and 12 h, respectively.

Simultaneous coking and dealumination of zeolite H-ZSM-5 during the transformation of chloromethane into olefins

The deactivation pathways of a zeolite H-ZSM-5 catalyst containing bentonite and α-Al2O3 as binder material have been studied during the transformation of chloromethane into light olefins, which is considered as a possible step to valorize methane from natural gas. The reactions have been carried out in a fixed bed reactor, feeding pure chloromethane at 400, 425 and 450 °C, 1.5 bar and with a space-time of 5.4 (gcatalyst) h (molCH2)−1 for 255 min. The properties of the fresh and spent catalysts have been assessed by several techniques, such as N2 physisorption, adsorption/desorption of NH3, XPS and 29Si NMR. Additional measurements of the spent catalysts have been performed to study the nature of the deactivating coke species: TG-TPO analysis, SEM, and FT-IR and UV-vis spectroscopy. With the results in hand, two deactivation mechanisms were proposed: irreversible dealumination at temperatures higher than 450 °C by HCl and reversible coke fouling, while coke formation results from the condensation of polyalkylbenzenes, which are also intermediates in olefin production. The coke deposits grow in size with the addition of Cl to the carbonaceous structure.

Spatial Distribution of Zeolite ZSM‐5 within Catalyst Bodies Affects Selectivity and Stability of Methanol‐to‐Hydrocarbons Conversion

Solid acids, such as zeolites, are used as catalyst materials in a wide variety of important crude oil refinery, bulk chemical synthesis, and green processes. Examples include fluid catalytic cracking (FCC),[1] methanol-to-hydrocarbons (MTH) conversion,[ 2] plastic waste valorization,[3] and biomass catalysis.[4] Industrially, these solid acid catalysts are used as composite materials with the zeolite particles as active components heterogeneously dispersed within a matrix of binders and fillers (alumina or porous solids) then shaped into micro- or millimetersized catalyst bodies.[5] It is known that these matrix materials extend the lifetime of the active zeolite component by providing mechanical strength and protection against poisoning as well as by enhancing heat dissipation and mass transport.

Compositional Insights and Valorization Pathways for Carbonaceous Material Deposited During Bio-Oil Thermal Treatment

This work analyses the composition, morphology, and thermal behavior of the carbonaceous materials deposited during the thermal treatment of bio-oil (thermal pyrolytic lignin-TPL). The bio-oil was obtained by flash pyrolysis of lignocellulosic biomass (pine sawdust), and the TPLs were obtained in the 400-700 °C range. The TPLs were characterized by performing elemental analysis; (13)C NMR, Raman, FTIR, and X-ray photoelectron spectroscopy; SEM; and temperature-programmed oxidation analyzed by differential thermogravimetry and differential scanning calorimetry. The results are compared to a commercial lignin (CL). The TPLs have lower oxygen and hydrogen contents and a greater aromaticity and structural order than the CL material. Based on these features, different valorization routes are proposed: the TPL obtained at 500 °C is suitable for use as a fuel, and the TPL obtained at 700 °C has a suitable morphology and composition for use as an adsorbent or catalyst support.

Designing supported ZnNi catalysts for the removal of oxygen from bio-liquids and aromatics from diesel

This work describes the effect of the support (TiO2, hybrid 2TiO2–SiO2, SBA-15 and SBA-15 decorated with TiO2 particles) on the catalytic activity of ZnNi catalysts in the gas-phase hydrodeoxygenation (HDO or O-removal) of phenol and the liquid-phase hydrodearomatization (HDA) of synthetic diesel. These reactions are representative of the two major challenges of the hydrotreating unit embedded in a sustainable refinery: (i) decreasing oxygen content of bio-oils (produced in the pyrolysis of lignocellulosic biomass); and (ii) decreasing aromatics content in diesel. The fresh and deactivated catalysts were characterized by XRD, N2 adsorption–desorption, TPR, MS/TPD-NH3, XPS, SEM, HRTEM and coke combustion. Under steady-state conditions, the ZnNi catalyst supported on SBA-15 decorated with TiO2 particles displayed the highest activity in the hydrodeoxygenation of phenol (selectivity toward deoxygenated products > 95%) whereas the ZnNi/SBA-15 catalyst displayed the highest activity in the hydrodearomatization of synthetic diesel. It has been shown that dispersion of the active ingredient is favoured on the SBA-15 substrate. The relationship between structure and activity demonstrated that HDO and HDA reactions require optimized metal dispersion and acid function, metal dispersion being more important for HDA than for HDO reactions.

Role of oxygenates and effect of operating conditions in the deactivation of a Ni supported catalyst during the steam reforming of bio-oil

This work investigates the correlation of the reaction conditions (temperature and steam-to-carbon ratio (S/C)) and the reaction medium composition with the deactivation behavior of a Ni/La2O3-αAl2O3 catalyst used in steam reforming of bio-oil, aiming at sustainable hydrogen production from lignocellulosic biomass. The reaction was performed in an in-line two-step system consisting of thermal treatment of bio-oil at 500 °C for retaining the thermal pyrolytic lignin and in-line steam reforming of the remaining oxygenates in a fluidized bed catalytic reactor. The reforming step was conducted at 550 and 700 °C and S/C ratios of 1.5 and 6. Fresh and deactivated catalyst samples were characterized using XRD, SEM, TEM, TPO, XPS, Raman and FTIR spectroscopy. The catalyst deactivation was mainly due to the amorphous and encapsulating coke deposition whose formation is attenuated when both the temperature and S/C ratio are increased. Although the highest catalyst stability is attained at 700 °C and/or an S/C ratio of 6, Ni sintering is noticeable under these conditions. The encapsulating coke is highly oxygenated, in contrast with the more aromatic and condensed nature of filamentous coke. Based on the correlation between the composition of the coke and the reaction medium, it was established that bio-oil oxygenates are the precursors of the encapsulating coke, particularly phenols and alcohols, whereas CO and CH4 are the possible precursors of the coke fraction made of filaments whose contribution to catalyst deactivation is hardly significant.

Imaging the Profiles of Deactivating Species on the Catalyst used for the Cracking of Waste Polyethylene by Combined Microscopies

The catalytic cracking of high-density polyethylene (HDPE) is an attractive process to valorize wastes throughout the production of the original monomers or fuels. The cracking catalyst based on zeolites is able to drive the scission of the polymeric chain, while controlling the final selectivity of monomers or fuels. The disadvantage of using a cracking catalyst is the deactivation caused by coke fouling, which hinders the cracking of heavy hydrocarbons and reduces the lifetime of the catalyst. Amongst the reactor designs able to directly feed solid HDPE without clogging problems, fluidizedand spouted-bed reactors are the most promising. 4] In such reactors, the mechanism of cracking of HDPE involves several steps: 1) the polymer is fed into the reactor as a solid and melts, coating the surface of the catalytic particles; 2) the melted plastic is cracked through a thermal mechanism involving radicals, and forms waxes; 3) the waxes diffuse through the macropores and the mesopores of the catalyst and eventually reach the acid sites were they react through carbocation chemistry, most probably through protonated cyclopropanes; 4) the cracked products can diffuse through the micropores of the catalyst, reacting to form lighter products or reacting to form coke. The formation of coke on zeolites during the cracking of heavy hydrocarbons involves steps at multiple scales: at the nanoscale, the acid sites catalyze reactions of condensation, cyclization, and hydrogen transfer to form aromatics with lower H/C ratio values; at the microscale, coke molecules can flow, be trapped, and grow in the mesoand macropores of the catalyst leading to site blockage. In a previous work, we demonstrated that the coke deposited on an MFI catalyst during the cracking of HDPE grows in the interior and the exterior of the zeolite crystals simultaneously. The external coke grows faster than the internal coke and it is directly responsible for the final collapse of the activity of the catalyst. Despite the vast bibliography dealing with the mechanisms of creation and growth of the internal coke, the same pathways corresponding to the external coke remain much less studied. In this sense, a fundamental question in the cracking of HDPE is to understand the impact and composition of external coke, and the location (profiles) within the catalytic particle. This issue is critical for understanding the deactivation and future strategies for preparation and regeneration of the catalyst. Visualizing the way a catalyst is prepared, activated, used, or deactivated is a critical topic for rationally designing a catalyst with enhanced properties. The most advanced methods for imaging (that are non-invasive and can analyze by an operando approach) are tomographic energy dispersive diffraction (TEDDI), UV/vis spectroscopy, magnetic resonance imaging (MRI), 12] and fluorescence microscopy, with great potential for elucidating the mechanisms of preparation and deactivation at the level of the individual zeolite crystal, in a time-resolved manner. FTIR and Raman imaging are amongst the simpler imaging methodologies, with the reported successes in the steps of preparation, 15] or the deactivation of the dehydrogenation catalyst, or the hydrotreating catalyst. These methodologies are inevitably intrusive and require dissection of the catalytic particle. However, they require a less expensive infrastructure and, especially, enable the characterization of macroscopic external coke presumably formed in the cracking of HDPE. In this work, we have imaged the external (macroscopic) coke deposited on catalytic particles containing MFI (HZSM-5) zeolite during the cracking of HDPE in a spouted bed (pilot plant) reactor. Summarized in Figure 1 are the steps followed in the study. The catalytic particles consisted of zeolite crystals agglomerated into an inert matrix of bentonite and a-Al2O3. In this way the zeolite is dispersed and the catalyst has better flu-

Pathways of coke formation on an MFI catalyst during the cracking of waste polyolefins

A study has been carried out on the deposition kinetics of carbonaceous-species on an acid catalyst (containing an MFI zeolite) in the cracking of high-density polyethylene and polypropylene. The initiation of coke deposition occurs on the acid sites, followed by aromatic and aliphatic growth in the micro- and mesopores, respectively.

Catalytic Cracking of Plastic Pyrolysis Waxes with Vacuum Gasoil: Effect of HZSM-5 Zeolite in the FCC Catalyst

Catalytic cracking of waste plastics is an interesting option for selectively recovering raw materials or for obtaining fuels. In this paper, a new recycling strategy is proposed, which consists of upgrading the waxes obtained by flash pyrolysis of polyolefins in a FCC (Fluidized Catalytic Cracking) unit. The waxes have been obtained by flash pyrolysis of polypropylene at 500 ºC and they have been dissolved (20 wt% wax) in the vacuum gasoil (VGO) of a FCC unit. The runs have been carried out in a CREC-UWO Riser Simulator Reactor (atmospheric pressure; 500-550 ºC; C/O = 5.5; contact times, 3-12 s). A commercial catalyst and a hybrid one (containing HZSM-5 zeolite) have been used. The cracking of the mixture leads to higher yield of gasoline than in the cracking of VGO with a higher content of olefins. The results of the effect of the operating conditions (temperature and contact time) are qualitatively similar to those corresponding to standard feed. Consequently, no difficulties inherent to the presence of waxes in the feed are expected in the treatment of mixtures at industrial conditions. The presence of HZSM-5 zeolite in the catalyst causes a significant increase in the amount of LPG (especially C3-C4 olefins), at the expense of a decrease in the gasoline fraction, whose RON is 1-2 points higher than that corresponding to the commercial catalyst. The gasoline obtained also has a higher content of olefins (especially C5-C7) and benzene at the expense of a decrease in the amount of C6-C10 i-paraffins.