Ethanol Decomposition and Dehydrogenation for Hydrogen Production: A Review of Heterogeneous Catalysts

Global Vision of the Reaction and Deactivation Routes in the Ethanol Steam Reforming on a Catalyst Derived from a Ni-Al Spinel

Ethanol steam reforming (ESR) over a Ni/Al2O3 catalyst prepared by reduction of a NiAl2O4 spinel is a promising alternative route to produce H2 from biomass. This work deepens into the effect of reaction conditions (450-650 °C, a steam/ethanol (S/E) ratio of 3-9, and a weight space time up to 1.3 h) and evaluates the time on stream evolution of the yields of H2, gaseous byproducts (CO, CO2, CH4, C2H4, C2H4O), and formed carbon/coke. The results are explained taking into consideration the thermodynamics, the extent of each individual reaction, and the catalyst deactivation. Up to 600 °C, the predominant intermediate in the H2 formation is C2H4 (formed by ethanol dehydration) with the preferential formation of nanostructured carbon (nanotubes/filaments) by C2H4 decomposition. The deposition of this type of carbon partially deactivates the catalyst, mainly affecting the extent of the C2H4 decomposition causing a sharp decrease in the H2 and carbon yields. Nevertheless, the catalyst reaches a pseudosteady state with an apparent constant activity for other reactions in the kinetic scheme. At 650 °C, C2H4O (formed by the ethanol dehydrogenation) is the main intermediate in the H2 formation, which is the precursor of an amorphous/turbostratic carbon (coke) formation that initially causes a rapid deactivation of the catalyst, affecting the ethanol dehydration and, to a lower extent, the reforming and water gas shift reactions. The increase in the S/E ratio favors the H2 formation, attenuates the catalyst deactivation due to the suppression of the ethanol dehydration to C2H4, and promotes the reforming, water gas shift, and carbon/coke gasification reactions. A H2 yield of 85% stable for 48 h on stream is achieved at 600 °C, with a space time of 0.1 h and an S/E ratio of 9.

Production of Acetaldehyde via Oxidative Dehydrogenation of Ethanol in a Chemical Looping Setup

A novel chemical looping (CL) process was demonstrated to produce acetaldehyde (AA) via oxidative dehydrogenation (ODH) of ethanol. Here, the ODH of ethanol takes place in the absence of a gaseous oxygen stream; instead, oxygen is supplied from a metal oxide, an active support for an ODH catalyst. The support material reduces as the reaction takes place and needs to be regenerated in air in a separate step, resulting in a CL process. Here, strontium ferrite perovskite (SrFeO_3-δ) was used as the active support, with both silver and copper as the ODH catalysts. The performance of Ag/SrFeO_3-δ and Cu/SrFeO_3-δ was investigated in a packed bed reactor, operated at temperatures from 200 to 270 ^°C and a gas hourly space velocity of 9600 h^-1. The CL capability to produce AA was then compared to the performance of bare SrFeO_3-δ (no catalysts) and materials comprising a catalyst on an inert support, Cu or Ag on Al₂O₃. The Ag/Al₂O₃ catalyst was completely inactive in the absence of air, confirming that oxygen supplied from the support is required to oxidize ethanol to AA and water, while Cu/Al₂O₃ gradually got covered in coke, indicating cracking of ethanol. The bare SrFeO_3-δ achieved a similar selectivity to AA as Ag/SrFeO_3-δ but at a greatly reduced activity. For the best performing catalyst, Ag/SrFeO_3-δ, the obtained selectivity to AA reached 92-98% at yields of up to 70%, comparable to the incumbent Veba-Chemie process for ethanol ODH, but at around 250 ^°C lower temperature. The CL-ODH setup was operated at high effective production times (i.e., the time spent producing AA to the time spent regenerating SrFeO_3-δ). In the investigated configuration with 2 g of the CLC catalyst and 200 mL/min feed flowrate ∼5.8 vol % ethanol, only three reactors would be required for the pseudo-continuous production of AA via CL-ODH.

Catalytic Pt/Al2O3 Monolithic Foam for Ethanol Reforming Fabricated by the Competitive Impregnation Method

Here, the synthesis of Pt/Al₂O₃ catalysts on a monolithic foam employing the competitive impregnation method is presented. NO₃ ^- was used as a competitive adsorbate at different concentrations in order to delay the adsorption of Pt, minimizing the formation of Pt concentration gradients throughout the monolith. The catalysts' characterization includes the BET, H₂-pulse titration, SEM, XRD and XPS techniques. The catalytic activity evaluation was performed under partial oxidation and autothermal reforming of ethanol in a short contact time reactor. The competitive impregnation method was able to produce better dispersion of the Pt particles through the Al₂O₃ foams. XPS analysis indicated the catalytic activity of the samples, by the presence of metallic Pt and Pt oxides (PtO and PtO₂) in the internal regions of the monoliths. Compared to other Pt catalysts reported in the literature, the catalyst produced by the competitive impregnation method was revealed to be selective toward H₂. Overall, the results showed that the competitive impregnation method employing NO₃ ^- as the co-adsorbate is a promising technique to synthesize well dispersed Pt catalysts over α-Al₂O₃ foams.

The Route from Green H2 Production through Bioethanol Reforming to CO2 Catalytic Conversion: A Review

Currently, a progressively different approach to the generation of power and the production of fuels for the automotive sector as well as for domestic applications is being taken. As a result, research on the feasibility of applying renewable energy sources to the present energy scenario has been progressively growing, aiming to reduce greenhouse gas emissions. Following more than one approach, the integration of renewables mainly involves the utilization of biomass-derived raw material and the combination of power generated via clean sources with conventional power generation systems. The aim of this review article is to provide a satisfactory overview of the most recent progress in the catalysis of hydrogen production through sustainable reforming and CO2 utilization. In particular, attention is focused on the route that, starting from bioethanol reforming for H2 production, leads to the use of the produced CO2 for different purposes and by means of different catalytic processes, passing through the water–gas shift stage. The newest approaches reported in the literature are reviewed, showing that it is possible to successfully produce “green” and sustainable hydrogen, which can represent a power storage technology, and its utilization is a strategy for the integration of renewables into the power generation scenario. Moreover, this hydrogen may be used for CO2 catalytic conversion to hydrocarbons, thus giving CO2 added value.