Ethanol condensation to butanol at high temperatures over basic heterogeneous catalysts: how relevant is acetaldehyde self-aldolisation?

Transcription

1 Ethanol condensation to butanol at high temperatures over basic heterogeneous catalysts: how relevant is acetaldehyde self-aldolisation? PII-4: Unravelling the mechanism of catalytic reactions through combined kinetic and thermodynamic analyses Julien Scalbert a, Fred Thibault-Starzyk a, Fred Meunier a,b a Laboratoire Catalyse et Spectrochimie, Université de Caen, CNRS, ENSICAEN, Caen. b IRCELYON, Université Lyon 1, CNRS, Villeurbanne. 1

2 Outline: 1. Why studying ethanol condensation to butanol? 2. Suggested reaction mechanisms in the literature 3. Methods: - Thermodynamic calculations - Reactor + analytics 4. Ethanol condensation over a transition metal-free hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 Scalbert et al., J. Catal. 311 (2014) 28 2

3 Oxo process: propene, CO, H 2 (Co, Rh, Ni) Solvent, chemicals Butanol Aldol condensation: Base + Metal M-free basic solids! ABE Fermentation Ethanol 3

4 Outline: 1. Why studying ethanol condensation to butanol? 2. Suggested reaction mechanisms in the literature 3. Methods: - Thermodynamic calculations - Reactor + analytics 4. Ethanol condensation over a transition metal-free hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 Scalbert et al., J. Catal. 311 (2014) 28 4

5 Metal-free ethanol condensation to butanol: mechanism? «Guerbet» mechanism (Acetaldehyde self-aldolisation) ethanol (éthanal) (3-hydroxybutenal) (butenal) Direct dimerisation (butanal) Ogo et al., Appl. Catal. A 402 (2011) 188 ethanol ethanol Semi-direct dimerisation Yang and Meng, J. Catal. 142 (1993) 37 Yang and Meng, J. Catal. 142 (1993) 37 ethanol acetaldehyde 5

6 Ethanol condensation mechanism over Mg x AlO y butanol Direct Acetaldehyde Self-aldolisation Di Cosimo, Apesteguia, Gines, Iglesia J. Catal. 190 (2000) 261 6

7 Outline: 1. Why studying ethanol condensation to butanol? 2. Suggested reaction mechanisms in the literature 3. Methods: - Thermodynamic calculations - Reactor + analytics 4. Ethanol condensation over a transition metal-free hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 Scalbert et al., J. Catal. 311 (2014) 28

8 Molar Gibbs energy of reaction: r G = r G ø + RT ln Q = RT ln Q/K where Q = reaction quotient and K = equilibrium constant (éthanal) (3-hydroxybutenal) (butenal) (butanal) 2 acetaldehyde + 2 H 2 butanol + water Q = P butanol P water ( P acetaldehyde P ) 2 H2 Calculating Q requires quantifying: butanol, acetaldehyde, H 2 O and H 2 8

9 Q = P butanol P water ( P acetaldehyde P H2 ) 2 During a catalytic test, Q goes from a value of 0 to K or less, but not higher r G(T) = RT ln Q/K Q/K = η = progress to equilibrium Mechanism validity criterion: Q/K = η 1 9

10 ethanol Reactor: quartz plug flow reactor Catalyst: Hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 from Accros Operating conditions : 350 C < T < 450 C 3.8 < P ethanol < 19.1 kpa 1.4 < WHSV < 56 g ethanol g cata -1 h -1 Gas Chromatograph + Mass Spectrometer + FT-IR gas cell ethanol acetaldehyde H 2 butanol H 2 O acetaldehyde ethene butenol butadiene 10

11 Outline: 1. Why studying ethanol condensation to butanol? 2. Suggested reaction mechanisms in the literature 3. Methods: - Thermodynamic calculations - Reactor + analytics 4. Ethanol condensation over a transition metal-free hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 Scalbert et al., J. Catal. 311 (2014) 28 1

12 Sélectivités (%) Hydroxyapatite: selectivity vs. temperature %Ethanol = 15.2 % ; WHSV = 14 h % 90% 80% 70% 60% 50% 40% 30% 20% 10% Autres butenol éthylene butadiene acétaldéhyde butanol 0% Température de réaction ( C) most abundant product is butanol 12

13 Pression partielle (kpa) Effect of contact time at 400 C Butanol Acétaldéhyde butanol acetaldehyde secondary butanol primary butanol W/F (g cata.h.mol -1 Ethanol) Two routes to butanol? one direct, one involving acetaldehyde? 13

14 HSC equilibrium calculation based on self-aldolisation intermediates % (molaire) H 2 (g) H 2 O(g) Butanol(g) Acetaldehyde(g) Butenal(g) Température ( C) Butenol(g) At 400 C: acetaldehyde and butenal should be more abundant than butanol 14

15 Computing Q/K for: 2 Acetaldehyde + 2 H 2 Butanol + H 2 O Q = P butanol P water ( P acetaldehyde P H2 ) 2 Temperature %EtOH WHSV K Q η = Q/K / C ,2 % 28 h ,2 % 28 h ,2 % 28 h ,6 % 1,4 h ,6 % 2,1 h ,6 % 4,2 h ,6 % 7,0 h Q/K >> 1 15

16 Suggested reaction mechanisms: Main: direct reaction mechanism Q/K < 1 - H 2 O 2 OH OH ethanol butanol Minor: semi-direct reaction mechanism Q/K < 1 ethanol acetaldehyde O O OH H OH - H 2 + H OH - H 2 O Butenol OH + 2 H butanol OH Final hydrogenation: H-tranfer from sacrifical ethanol (not via H 2 dissociation) Minor route is intrinsically less selective: one ethanol is sacrificed 16

17 Conclusion 1/2: Kinetics and Thermodynamics The comparison of the reaction quotient (Q) to The thermodynamic equilibrium ratio (K) can be very useful in supporting or rejecting a reaction mechanism. 17

18 Other cases with useful insight from thermodynamics CH 3 OH + H 2 O = 3 H 2 + CO 2 over Cu/Zn/Zr/Al: CO not reaction intermediate, secondary reaction product Breen et al., Chem. Commun. 1999, 2247 C 3 H 6 -SCR of NO over Ag/ -Al 2 O 3 : NO 2 not formed via NO + O 2, but via CxHyNzO +O 2 Meunier et al., Chem. Commun.1999, 259 n-alkane hydroisomerisation over MoOx: bifunctional mechanism, RDS is different for C 4 and C 5 Meunier, Chem. Commun. 2003,

19 Conclusion 2/2: ethanol condensation at high T over hydroxyapatite without metals O - 2 H OH H Indirect + ethanol -acetaldehyde -H 2 O -H 2 Direct - H 2 O - H 2 O Aldol selfcondensation O OH + 2 H 2 H Target? try to hinder the less selective mechanism that involves acetaldehyde, contrary to the case of low T metal-promoted reactions! Scalbert et al., J. Catal. 311 (2014) 28 19

20 Acknowledgments Julien Scalbert