Single-pass transformation of syngas into ethanol with high selectivity by triple tandem catalysis

Abstract

Synthesis of ethanol from non-petroleum carbon resources via syngas (a mixture of H₂ and CO) is an important but challenging research target. The current conversion of syngas to ethanol suffers from low selectivity or multiple processes with high energy consumption. Here, we report a high-selective conversion of syngas into ethanol by a triple tandem catalysis. An efficient trifunctional tandem system composed of potassium-modified ZnO-ZrO₂, modified zeolite mordenite and Pt-Sn/SiC working compatibly in syngas stream in one reactor can afford ethanol with a selectivity of 90%. We demonstrate that the K⁺-ZnO-ZrO₂ catalyses syngas conversion to methanol and the mordenite with eight-membered ring channels functions for methanol carbonylation to acetic acid, which is then hydrogenated to ethanol over the Pt-Sn/SiC catalyst. The present work offers an effective methodology leading to high selective conversion by decoupling a single-catalyst-based complicated and uncontrollable reaction into well-controlled multi-steps in tandem in one reactor.

Fig. 1. Routes for conversion of syngas to ethanol.

Route A, traditional and direct route based on a single catalyst. Route B, indirect route including three processes, i.e., methanol synthesis, methanol carbonylation with CO and acetic acid (AA) hydrogenation. Route C, indirect route including three processes, i.e., syngas to dimethyl ether (DME), DME carbonylation with CO and methyl acetate (MA) hydrogenation. Route D, Tandem catalytic route of this work.

Fig. 2. Catalytic behaviours and reaction pathways.

a Metal oxides alone. b Combinations of K⁺–ZnO–ZrO₂ and zeolites. c Combinations of metal oxides and H-MOR–DA–12MR. d Combinations of K⁺–ZnO–ZrO₂, H-MOR–DA–12MR and hydrogenation catalysts. e Reaction pathways for direct synthesis of ethanol from syngas. C₂₊: C₂₊ hydrocarbons; DME: dimethyl ether; C_2–4⁼: C₂–C₄ olefins; C_2–4⁰: C₂–C₄ paraffins; C₅₊: C₅₊ hydrocarbons; MA: methyl acetate; AA: acetic acid; C₂₊ oxy.: ethyl acetate and methyl acetate. Reaction conditions: weights of metal oxide, zeolite and hydrogenation catalyst = 0.66, 0.66 and 0.66 g; H₂/CO = 1:1; P = 5.0 MPa; T = 583 K; F = 25 mL min⁻¹; time on stream, 20 h. The selectivity was calculated on a molar carbon basis. Carbon balances were 95–99%. The experiments in each case were performed for three times. The error bar represents the relative deviation, which is within 5%.

Fig. 3. Characterization of tandem catalysts.

a TEM micrograph for K⁺–ZnO–ZrO₂ particles with particle size distribution. Scale bar: 20 nm. b HRTEM micrograph for K⁺–ZnO–ZrO₂ particles. Scale bar: 2 nm. c TEM micrograph for H-MOR with particle size distribution. Scale bar: 100 nm. d TEM micrograph for H-MOR–DA–12MR with particle size distribution. Scale bar: 100 nm. e ¹H MAS NMR for H-MOR and H-MOR–DA–12MR zeolites. The peak at 4.0 ppm can be attributed to the Brønsted acid site. f FT-IR spectra of H-MOR and H-MOR–DA–12MR zeolites. Assignments of deconvoluted bands: 3625, 3617 and 3609 cm⁻¹: Brønsted acid sites in 12-MR; 3599 cm⁻¹: Brønsted acid sites in intersections between 8-MR and 12-MR; 3590 and 3581 cm⁻¹: Brønsted acid sites in 8-MR. g TEM micrograph for Pt–Sn/SiC with particle size distribution. Scale bar: 20 nm. h HRTEM micrograph for a Pt–Sn particle. Scale bar: 2 nm.

Fig. 4. Reaction kinetics for the triple tandem system.

a Effect of reaction temperature. Reaction conditions: H₂/CO = 1:1; P = 5.0 MPa; F = 25 mL min⁻¹; time on stream, 20 h. b Catalyst stability. Reaction conditions: H₂/CO = 1:1; P = 5.0 MPa; F = 25 mL min⁻¹; T = 543 K. c Effect of H₂/CO ratio. Reaction condition: P = 5.0 MPa; F = 25 mL min⁻¹; T = 583 K; time on stream, 20 h. d Effect of total pressure. Reaction condition: H₂/CO = 1:1; F = 25 mL min⁻¹; T = 583 K; time on stream, 20 h. e Effect of CO partial pressure at a fixed H₂ pressure. Reaction conditions: P(H₂) = 2.5 MPa; T = 583 K; F = 40 mL min⁻¹; P = 5 MPa; time on stream, 20 h. f Effect of H₂ partial pressure at a fixed CO pressure. Reaction conditions: P(CO) = 2.0 MPa; T = 583 K; F = 40 mL min⁻¹; P = 5 MPa; time on stream, 20 h. C₂₊ HC: C₂₊ hydrocarbons; EA: ethyl acetate; AA: acetic acid; r(CO): the rate of CO conversion. In all cases, the weights of K⁺–ZnO–ZrO₂, H-MOR–DA–12MR and Pt–Sn/SiC catalysts were 0.66, 0.66 and 0.66 g, respectively. The experiments in each case were performed for three times. The error bar represents the relative deviation, which is within 5%.

Fig. 5. Catalyst compatibility in syngas stream.

a Catalytic performances of H-MOR–DA–12MR for methanol carbonylation in CO and syngas streams. AA: acetic acid. Reaction conditions: H-MOR–DA–12MR, 1.0 g; P = 5.0 MPa; T = 583 K; F(CH₃OH) = 1.48 mmol h⁻¹; F(95%CO–5%Ar) or F(48%CO–48%H₂–4%Ar) = 12 or 24 mL min⁻¹. b Catalytic performances of Pt/SiC and Pt–Sn/SiC for acetic acid hydrogenation in H₂ and syngas streams. EA: ethyl acetate; HC: hydrocarbons; AA: acetic acid. Reaction conditions: Pt/SiC or Pt–Sn/SiC, 0.66 g; P = 5.0 MPa; T = 583 K; F(AA) = 1.05 mmol h⁻¹; F(H₂) or F(48%CO–48%H₂–4%Ar) = 12 or 25 mL min⁻¹; time on stream, 20 h. The experiments in each case were performed for three times. The error bar represents the relative deviation, which is within 5%.