Heterogeneous Catalysis in (Bio)Ethanol Conversion to Chemicals and Fuels: Thermodynamics, Catalysis, Reaction Paths, Mechanisms and Product Selectivities

Recent developments in (bio)ethanol conversion to fuels and chemicals over heterogeneous catalysts

Bioethanol is one of the most important bio-resources produced from biomass fermentation and is an environmentally friendly alternative to fossil-based fuels as it is regarded as renewable and clean. Bioethanol and its derivatives are used as feedstocks in petrochemical processes as well as fuel and fuel additives in motor vehicles to compensate for the depletion of fossil fuels. This review chronicles the recent developments in the catalytic conversion of ethanol to diethyl ether, ethylene, propylene, long-chain hydrocarbons, and other important products. Various heterogeneous catalysts, such as zeolites, metal oxides, heteropolyacids, mesoporous materials, and metal-organic frameworks, have been used in the ethanol conversion processes and are discussed extensively. The significance of various reaction parameters such as pressure, temperature, water content in the ethanol feed, and the effect of catalyst modification based on various kinds of literature are critically evaluated. Further, coke formation and coke product analysis using various analytical and spectroscopic techniques during the ethanol conversion are briefly discussed. The review concludes by providing insights into possible research paths pertaining to catalyst design aimed at enhancing the catalytic conversion of (bio)ethanol.

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.

Theoretical investigation of the olefin cycle in H-SSZ-13 for the ethanol-to-olefins process using ab initio calculations and kinetic modeling

The formation of the hydrocarbon pool (HCP) in the ethanol-to-olefins (ETO) process catalyzed by H-SSZ-13 is studied in a kinetic model with ab initio computed reaction barriers.

Transition Metal B-Site Substitutions in LaAlO3 Perovskites Reorient Bio-Ethanol Conversion Reactions

LaAlO3 perovskites, as such and with 25% molar Al substitution by Cu, Co, or Ga, have been prepared by sol-gel methods and tested as heterogeneous catalysts in the gas-phase conversion of ethanol. LaAlO3 presented a significant acidic character, with high formation of ethylene by ethanol dehydration. B-site substitutions increased the basicity of the catalysts, favoring the dehydrogenation of ethanol to acetaldehyde. The most reducible Cu- and Co-substituted materials, characterized by easier formation of surface oxygen vacancies, promoted the self-condensation of acetaldehyde by the Tishchenko mechanism, with formation of acetone and odd-carbon number products. Aldol coupling of acetaldehyde, favored on pure and Ga-substituted LaAlO3, led to the formation of butadiene and hexadiene. The role of Ga insertion, favoring both dehydrogenation of ethylene and dehydration of higher alcohols, corresponds to an amphoteric character. The formation of olefins and diolefins on all catalysts suggests that LaAl-based materials present the most acidic character among La-perovskites.