Multifunctional Catalyst Combination for the Direct Conversion of CO2 to Propane

Abstract

The production of carbon-rich hydrocarbons via CO₂ valorization is essential for the transition to renewable, non-fossil-fuel-based energy sources. However, most of the recent works in the state of the art are devoted to the formation of olefins and aromatics, ignoring the rest of the hydrocarbon commodities that, like propane, are essential to our economy. Hence, in this work, we have developed a highly active and selective PdZn/ZrO₂+SAPO-34 multifunctional catalyst for the direct conversion of CO₂ to propane. Our multifunctional system displays a total selectivity to propane higher than 50% (with 20% CO, 6% C₁, 13% C₂, 10% C₄, and 1% C₅) and a CO₂ conversion close to 40% at 350 °C, 50 bar, and 1500 mL g^-1 h^-1. We attribute these results to the synergy between the intimately mixed PdZn/ZrO₂ and SAPO-34 components that shifts the overall reaction equilibrium, boosting CO₂ conversion and minimizing CO selectivity. Comparison to a PdZn/ZrO₂+ZSM-5 system showed that propane selectivity is further boosted by the topology of SAPO-34. The presence of Pd in the catalyst drives paraffin production via hydrogenation, with more than 99.9% of the products being saturated hydrocarbons, offering very important advantages for the purification of the products.

Figure 1

(a) CO₂ conversion (filled symbols) and MeOH (for PdZn/ZrO₂) or propane (for PdZn/ZrO₂+SAPO-34 and PdZn/ZrO₂+ZSM-5) selectivity (empty symbols) at several screening conditions. H₂/CO₂ = 3, 12 000 mL g^–1 h^–1. (b) CO₂ conversion (filled symbols) and CO (empty symbols) or propane (half empty symbols) selectivity for the PdZn/ZrO₂+SAPO-34 system at different space velocities and pressures. 350 °C, H₂/CO₂ = 3. MeOH selectivity was lower than 1% at all conditions. (c) Detailed hydrocarbon distribution (CO free) of the PdZn/ZrO₂+SAPO-34 combined system for the CO₂ conversion to hydrocarbons at different space velocities. CO₂:H₂ 1:3, 350 °C, 50 bar.

Figure 2

Catalytic performance of the PdZn/ZrO₂ catalyst and the PdZn/ZrO₂+SAPO-34 combined system: (a) CO₂ and (b) CO conversion to hydrocarbons. CO_x:H₂ 1:3, 350 °C, 30 bar, 3000 mL g^–1 h^–1.

Figure 3

Experimental data fitting of (a) CO₂ to methanol over the PdZn/ZrO₂ catalyst at 300 °C and 30 bar and (b) CO₂ to propane over the PdZn/ZrO₂+SAPO-34 system at 350 °C and 50 bar. (c) Comparison of reaction rates for methanol and CO formation over the PdZn/ZrO₂ catalyst at 250 and 350 °C and 30 bar (12000 mL g^–1 h^–1), and (d) influence of propane formation on these rates over the PdZn/ZrO₂+SAPO-34 system at 350 °C and 30 and 50 bar (6000 mL g^–1 h^–1). Evolution with GHSV of the net formation rate of (e) methanol and (f) CO and propane over the PdZn/ZrO₂+SAPO-34 system at 350 °C.

Figure 4

Pd K-edge, Zn K-edge XANES (main panels), and EXAFS (insets) spectra of PdZn/ZrO₂+ZSM-5 catalyst. XAS spectra for relevant reference compounds are also reported as dashed lines. (a) Pd K-edge for as prepared catalyst. (b) Pd K-edge for catalyst during activation (RT-400 °C) under H₂ gas flow. (c) Pd K-edge for catalyst after activation at 400 °C under H₂ atmosphere. (d) Zn K-edge for as prepared catalyst. (e) Zn K-edge for catalyst during activation (RT-400 °C) under a H₂ gas flow. Top right inset: first derivative of the XANES spectra for as prepared catalyst (black), activated catalyst (red), reference Zn(0) metal foil (light gray). (f) Zn K-edge for catalyst after activation at 400 °C under a H₂ gas flow. For clarity of comparison, the Pd metallic foil EXAFS signal was rescaled by a factor of 1/2. The EXAFS spectra reported in the bottom insets have been obtained by transforming the corresponding k²χ(k) EXAFS function in the 2.5–11.0 Å^–1 range.

Figure 5

(a) FT-IR spectra of CO₂ adsorbed at RT on oxidized and activated PdZn/ZrO₂ at equilibrium pressure of 20 mbar. (b) FT-IR spectra of CO adsorbed at RT on activated PdZn/ZrO₂. Spectra were acquired at increasing dosage of CO up to 20 mbar (from blue to red line) and after outgassing (green dashed line).

Figure 6

Low-magnification HAADF-STEM imaging of PdZn/ZrO₂ catalysts mixed either with (a) SAPO-34 and with (c) ZSM-5 crystals and (b, d) related elemental maps built with Kα emission lines provided by Al, Si, Pd, Zr, and Zn atoms. Magnification of the PdZn/ZnO nanoparticles observed at the edge of the ZrO₂ support when mixed either with (e) SAPO-34 and with (g) ZSM-5 crystals. (f, h) Associated overlaps of elemental maps.

Figure 7

(a) High-resolution HAADF-STEM imaging of a PdZn alloy nanoparticle after CO₂ hydrogenation. Elemental maps of the same area built with Kα emission lines: (b) Zn, (c) Pd, (d) overlap of Pd and Zn maps, and (e) overlap of Pd and O maps.

Figure 8

Reaction mechanism for the CO₂ conversion to propane over the PdZn/ZrO₂+SAPO-34 system.