Behaviour of Cuznal and Cuznzr Ferrierite-Based Catalysts to Produce DME By CO2 Hydrogenation

Behaviour of CuZnAl and CuZnZr ferrierite-based catalysts to produce DME by CO₂ hydrogenation.

Fabio Salomone^a, Giuseppe Bonura^b, Francesco Frusteri^b, Samir Bensaid^a, *, Raffaele Pirone^a

^a Department of Applied Science and Technology (DISAT), Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy.

^b ITAE - Istituto di Tecnologie Avanzate per l'Energia "Nicola Giordano", Consiglio Nazionale delle Ricerche, Salita Santa Lucia Sopra Contesse, 5 - 98126 Messina, Italy.

* Corresponding author. E-mail address: samir.bensaid@polito.it

 

The direct hydrogenation of CO₂ into dimethyl-ether (DME) has been studied in presence of copper-based methanol (MeOH) selective catalysts and a commercial ferrierite-type zeolite [1 - 5]. These Cu-based systems are Cu-Zn-Al/ferrierite (CZA/FER) and Cu-Zn-Zr/ferrierite (CZZ/FER). Both hybrid catalysts, obtained through gel-oxalate coprecipitation (OX) and wet impregnation (WI), and physical mixed catalysts with 1:2 and 2:1 weight ratio [2] were explored to compare the activity and the selectivity of these different systems. MeOH-selective catalysts were prepared with a 6:3:1 atomic ratio.

All the samples were properly characterized with different physic-chemical techniques in order to determine the textural and the morphological nature of the surface of the catalysts. Firstly, physical adsorption measurements of N₂ were elaborated to assess the surface area and the porosity of the samples. Secondly, X-rays diffraction (XRD) patterns were analysed to identify the nature of the phases of the catalysts. In addition, FESEM and EDS techniques were carried out to investigate the morphology and the atomic composition of the surfaces. Eventually, temperature programmed reduction (TPR) measurements were performed for observing the surface and bulk reduction of metal oxides.

The experimental campaign was carried out in a fixed bed reactor using 250-500 µm catalyst particles. According to TPR measurements, the samples were activated by means of a reducing process at a temperature of 350 °C, at atmospheric pressure for 3 h. The reducing atmosphere was obtained with a H₂/N₂ molar ratio equal to 1:9. After the activation process, a degreening test was performed (2.5 MPa, 275 °C and 13.6 Nl/kg_cat/h) to stabilize the catalyst before the activity tests. This procedure was helpful in order to put in evidence if a fresh catalyst was affected by a strong and fast deactivation in reacting conditions [3]. This method highlighted that hybrid catalysts (i.e. CZZ/FER OX 1:2) seem to be very active for few hours, then their activity decreases up to 50 % before reaching a stable operating condition. On the contrary, commercial CZA/FER-based catalysts were not affected by fast deactivation. The diminishing of performances may be probably due to a rearrangement of the catalyst structures at high temperatures. For these reasons, both reduced and aged catalysts has also to be characterized in order to identify the causes of the deactivation and improve catalyst performance and stability.

The activity tests were carried out operating at 2.5 MPa with a temperature ranging from 200 °C to 300 °C with steps of 25 °C. A hydrogen (H₂), carbon dioxide (CO₂) and nitrogen (N₂) gas mixture with a H₂/CO₂/N₂ ratio of 3:1:1 was fed to the reactor. Besides, three different weighted hourly space velocity (WHSV) were investigated from 6.6 to 20.0 Nl/kg_cat/h. The results of the activity tests highlighted that hybrid catalysts are more selective towards methanol and dimethyl-ether also at higher temperatures, because the reverse water gas shift (WGS) reaction is more inhibited. Indeed, CZA/FER catalysts were less selective and produce non-negligible amounts of by-products (i.e. C₂ hydrocarbons) at temperatures higher than 275 °C. At low WHSV (6.6 Nl/kg_cat/h) the thermodynamic equilibrium was approached from 250 °C, but the cumulative MeOH/DME yield is lower than 80 g/kg_cat/h. Whereas, the cumulative yield increases at higher WHSV (20 Nl/kg_cat/h) reaching a cumulative yield of 256 g/kg_cat/h. The CO₂ conversion always rises as the reaction temperature grows up due to the endothermic reverse WGS reaction, which is favoured at higher temperatures. As illustrated in Fig. 1 (CZA/FER 1:2, 13.6 Nl/kg_cat/h), DME selectivity is extremely higher than CO selectivity at low CO₂ conversions, which implies a lower reaction temperature. In literature [3], hybrid CZZ/FER systems could reach extremely high DME yield of about 600 g/kg_cat/h, but the stability of these catalysts is a crucial aspect, which has to be improved.

These tests showed that hybrid CZZ/FER catalysts might to be improved exploring different synthesis methods in order to increase their activity at lower temperature and possibly also at lower pressures. However, at the same time, selectivity could be improved at higher temperature for increasing the yield through MeOH and DME. These goals could be achieved by trying to add promoters to the structure of these catalysts, or also changing the active phase of the catalysts [1].

Figure 1 Activity tests on hybrid catalysts CZA/FER 1:2.

Conditions: H₂/CO₂/N₂ ratio of 3:1:1, 2.5 MPa, space velocity of 13.6 Nl/kg_cat/h.

Left axis: CO₂ conversion. Right axis: selectivity to CO, Methanol and DME.

 

References

[1]    Andrea Álvarez et al., “Challenges in the Greener Production of Formates/Formic Acis, Methanol, and DME by Heterogeneously Catalyzed CO₂ hydrogenation Processes”, Chemical Reviews, 117, 9804-9838, 2017.

[2]    G. Bonura, C. Cannilla, L. Frusteri, A. Mezzapica, F. Frusteri, “DME production by CO₂ hydrogenation: Key factors affecting the behaviour of CuZnZr/ferrierite catalysts” Catalysis Today, 281, 337-344, 2017.

[3]    F. Frusteri et al., “Direct CO₂-to-DME hydrogenation reaction: New evidences of a superior behaviour of FER-based hybrid systems to obtain high DME yield”, Journal of CO₂ Utilization, 18, 353-361, 2017.

[4]    G. Bonura et al., “Catalytic features of CuZnZr-zeolite hybrid systems for the direct CO₂-to-DME hydrogenation reaction”, Catalysis Today, 277, 48-54, 2016.

[5]    G. Bonura, “Acidity control of zolite functionality on activity and stability of hybrid catalysts during DME production via CO₂ hydrogenation”, Journal of CO₂ Utilization, 24, 398-406, 2018.