2Hydrothermal CO2 conversion using zinc as reductant: Batch reaction, modeling and parametric analysisof the process

Abstract

Hydrothermal reduction of CO 2 using Zn as reductant to obtain formic acid is a selective and efficient process. This process has the advantage of avoiding the use of gaseous hydrogen with all its safety and environmental concerns, and allowing an easier integration with CO 2 capturing steps such as CO 2 absorption in aqueous NaOH, because the latter solutions can be directly fed to the process as NaHCO 3. In this work, this reaction was studied in batch reactors at temperatures from 275 to 325ºC. Conversions up to 75% were obtained with selectivity towards formic acid near 100%, at residence times between 10 and 180 min. Reactions proceeds fast in the first steps of reaction, and it is slowed down when the oxidation of Zn is completed. The experimental results obtained were used to stablish a model that can explain both experimental data from this work and from literature with an averaged error of 13%. Using both the model and the experimental data the main variables of the process were analyzed: temperature, Zn/HCO 3 ratio, heating rate, Zn particle size, pressure reactor material and use of supercritical conditions. The optimum reaction conditions found were 300ºC with a rapid heating, and particle sizes of 0.75-1 mm. Zn excess dramatically improves the yield, but working with a lower excess can be compensated by working at pressures higher than 300 bar.

Recent Approaches to Design Electrocatalysts Based on Metal–Organic Frameworks and Their Derivatives

“…Conversion of CO 2 into valuable chemical compounds, intermediate industry products and green fuel not only can prevent severe global warming but also can relieve energy crisis. There are plentiful approaches for CO 2 conversion, for example, electrochemical, biochemical, photochemical, chemical and hydrothermal, to name just a few.…”

Hydrothermal CO2 Reduction by Glucose as Reducing Agent and Metals and Metal Oxides as Catalysts

“…In order to reduce CO 2 , several metals have been suggested as CO 2 reductants: Zn [ 18 ], Fe [ 19 ], Mn [ 20 ], Mg [ 20 ] and Al [ 20 ] (efficiency: Al > Zn > Mn > Fe) [ 10 , 21 ] are mentioned as favorable for the process. Metallic catalysts metals such as Ni-ferrite [ 22 ], Ni nanoparticles [ 23 ], Ni [ 24 ], Raney Ni [ 25 ], Cu [ 20 , 21 ], Fe 2 O 3 [ 26 ], Ru/C [ 27 ] and Pd/C [ 12 , 27 ] can be used as-is or coupled with “zero-valent” metals to improve the reaction; however, the reduction of the metal after the process in order to recycle the material should be considered.…”

“…So far, there has been a number of studies in which the use of catalysts (Cu, Ni, Pd/C) in hydrothermal CO 2 reduction was carried out using metals as reductants (Zn, Mg, Al, etc.) [ 12 , 18 , 20 , 21 , 22 , 25 ]. This work studies, for the first time, the influence of different catalysts in the hydrothermal reduction of CO 2 by using an organic (glucose) as a reducing agent.…”

H2‐free Synthesis of Aromatic, Cyclic and Linear Oxygenates from CO2

“…Therefore, iron can be considered as a catalyst and may also play a role in directing secondary reactions resulting in the formation ≥C 2 products. It should be noted that other relatively Earth‐abundant metals with low redox potential, such as zinc, are also useful catalysts for CO 2 reduction . Low‐reduction‐potential metals have been combined with other catalytic substances, particularly nickel and copper, to enhance reaction kinetics and yields.…”

“…C 2 products.I t should be noted that other relatively Earth-abundantm etals with low redox potential, such as zinc, are also useful catalysts for CO 2 reduction. [29,30] Low-reduction-potential metalsh ave been combined with other catalytic substances, particularly nickel [31,32] and copper, [29,33,34] to enhancer eactionk inetics and yields. Additionally,r educing agentss uch as glycerol, [35] biomass derivatives [36] and microalgae [37] have been effective in converting CO 2 to HCOOH.…”