Carbon Dioxide Hydrogenation into Methanol with CuO-ZnO-ZrO2 Catalyst Prepared By a Continuous Flow Microfluidic-Assisted Synthesis

Carbon dioxide hydrogenation into methanol with CuO-ZnO-ZrO₂ catalyst prepared by a continuous flow microfluidic-assisted synthesis

 

Valentin L’hospital¹, Ksenia Parhomenko¹, Christophe Serra², Anne-Cécile Roger¹

 

¹ICPEES (UMR7515), Université de Strasbourg, 25 rue Becquerel, 67087 Strasbourg France

²ICS (UPR22-CNRS), 23 rue du Loess BP 84047 67034 Strasbourg Cedex 2, France

E-mail :lhospital.valentin@gmail.com

 

Numerous measures to reduce anthropogenic greenhouse gas emissions and in particular CO₂ already exist such as its capture and storage as well as its conversion into valuable chemical compounds (methanol, methane, dimethyl ether and others). Currently, methanol is synthesized from syngaz (originally from oil or natural gas) but in this project, methanol is synthesized from pure CO₂ and green H₂, produced by water electrolysis with renewable energies. Methanol produced by CO₂ hydrogenation is considered as a platform molecule of great interest, for example for formaldehyde production, as a precursor in the plastic polymere formation (MTO, methanol to olefins) or it can be used in the hydrocarbons production (MTG, methanol to gasoline). This project is a promissing way of energy storage via CO₂ valorization and an important process for the overall World energy transition scenario.

               During the methanol synthesis from CO2, several reactions may take place: CO2 hydrogenation (1), reverse water-gas shift (2) and possible CO hydrogenation (3). The majority of catalysts of this reaction are based on copper and the couple Cu/ZnO is considered as the base of the methanol synthesis catalyst, as it permits to increase the selectivity of methanol production versus competitive reactions[1]. Different supports have been studied, usually it is not inert, it has several roles in catalysis, like for example, increase of the Cu dispersion, creation of some active species for additional CO2 adsorption. The use of ZrO2 instead of alumina as catalyst support can improve the copper reductibility and so, increase the active copper phases and finally the catalyst sintering may be avoided[2].

               Batch coprecipitation is a classical and easy way to prepare such catalytic materials but the main disadvantage of this method is the pH and concentration variations during the synthesis and variation of the composition during the coprecipitation and so, heterogeneity in the final catalyst. In this work, we propose to synthesize for the first time Cu-based catalytic materials by a continuous flow microfluidic-assisted method[3] and compare it to the classical way. During the microfluidic-assisted synthesis the precipitation zone is much smaller and the different variation of pH and concentration are greatly reduce. All the synthesis parameters are better controlled which may be beneficial in terms of kinetic control of the reactant mixing thus allowing improving the homogeneity of the final mixed oxide materials[4].

               The metal oxides obtained (CZZ2) were characterized by various techniques and tested, after reducing treatment, as catalysts for CO2 hydrogenation into methanol. The results obtained were compared to those of a catalyst of same composition prepared by classical co-precipitation in batch (CZZ1) and to those of an industrial copper-based catalyst (INDUS). 

 

Figure 1: TEM pictures of (a) CZZ1(batch) and (b) CZZ2 (microfluidic)

Compared to the batch coprecipitation, the catalytic materials synthesized by a microfluidic system are more homogeneous (Figure 1) with much higher specific and metallic surfaces area (BET and TPD-N₂O, results not shown here).  

The in depth study of the precursors generated by batch or by continuous microfluidic coprecipitation have been carried out. Single precursors (Cu, Zn or Zr carbonate/hydroxycarbonate), binary precursors (Cu-Zn, Cu-Zr or Zn-Zr carbonate/hydroxycarbonate and ternary precursors (Cu-Zn-Zr carbonate/hydroxycarbonate) have been prepared by both syntheses and have been characterised as prepared (XRD, TGA) and after calcination, under the oxide form (XRD, BET, microscopy, N₂O chemisorption, thermo-programmed reduction). It has been shown that no ternary species are formed in the precursors prepared by batch coprecipitation. On the contrary, a new ternary Cu-Zn-Zr precursor is obtained through continuous coprecipitation due to a better local control of parameters (lower composition gradient, pH). The formation of this ternary precursor is responsible for the better local homogeneity of the final material after calcination.

After synthesis and characterization, the catalytic materials were reduced under H₂ at 300°C and the CO₂ hydrogenation tests were carried out in a fixed bed stainless steel reactor at 240-300 °C, 50 bar, GHSV = 25 000 h^-1 (STP) with H₂/CO₂ = 3.9^{[3, 5].} The gas phase is analyzed online with a µ-GC, and the liquid phase is condensed, collected and analyzed by GC at the end of the reaction. The results obtained at 280 °C are shown in Table 1 and Figure 2.

 

    Table 1. Catalytic results at 280 °C, 50 bar, 25 000 h^-1 (STP), H₂/CO₂=3.9

Catalyst	H2 conv.(%)	CO2 conv.(%)	CH3OH sel.(%)	CH3OH productivity (g·kgcat-1·h-1)
INDUS	11.6	25.1	34	439
CZZ1 (batch)	7.4	17.9	36	725
CZZ2 (microfluidic)	9.9	21.4	33	1135

 

Figure 2: Methanol productivity from CO2 hydrogenation

It can be seen that all the synthesized catalysts present lower conversions but better MeOH productivity compared to the INDUS catalyst.  The microfluidic system (CZZ2) helps to increase the CO₂ and H₂ conversions and methanol productivity compared to batch (CZZ1).

We managed to control the physico-chemical characteristics of synthesized materials by tuning process parameters (the nature of vector fluids) which resulted in the improvement of the catalytic activity. The modification of other parameters (temperature of coprecipitation, pH precipitation and the residence time) have been shown to enhance even more the homogeneity and thus the activity of these catalytic materials.

               The possible production of dimethylether from CO2 has been studied. The coupling of the most performant Cu-based catalyst with a catalyst with tunable acidity (zeolithe ZSM5) has been performed. The presence of the second acid catalyst clearly modifies the composition of the outlet flow. The selectivity to DME reaches almost 50% and the selectivity to CO is strongly decreased compared to “pure methanol catalyst”. As a consequence, excellent dimethylether yields were obtained, up to 700 g(DME+MeOH).kgcata-1.h-1 at 20 bar and 280 °C. 

               The whole study, synthesis, precursor characterization, catalytic activity for methanol synthesis and catalytic activity for DME synthesis, when combined to ZSM5, will be presented.

 

Funding: National Research Agency of France, project DIGAS ANR-14-CE05-0012

Acknowledgement: Prof Sébastien Paul and Dr Svetlana Heyte for performing the catalytic experiments on the RealCat platform (Lille, France).

 

 

References

[1]      M. Behrens et al., J. of Inorg. and Gen. Chem. 639, 2013, 2683–2695

[2]      C. Li et al., Appl. Catal. A Gen., 469, 2014, 306–311

[3]      L. Angelo et al., Catal. Today, 270, 2015, 59–67

[4]      Y-T. Yang et al., Adv. Powder Technol., 26, 2015, 156–162

[5]      L. Angelo et al., Comptes Rendus Chim., 18, 2015, 250–260