Ab initio molecular modelling of the

Transcription

1 Énergies renouvelables Production éco-responsable Transports innovants Procédés éco-efficients Ressources durables Ab initio molecular modelling of the mechanisms of dealumination and desilication of relevant zeolite frameworks Marius-Christian laghi a, E. Petracovschi a, T. Kerber a, C. Chizallet a, J. Sauer b, P. Raybaud a a IFP Energies nouvelles - Direction Catalyse et Séparation - BP Solaize, FRANCE b umboldt-universität zu Berlin, Institut für Chemie, Unter den Linden 6, Berlin, GERMANY Atelier Modélisation des xydes Septembre Paris

2 Table of contents Introduction Mechanism and Formation of Mesopores Dealumination Desilication Strategy & Methods Results 2 Perspectives

3 Introduction Aluminosilicates: porous acidic materials ydrocracking, Fluid Catalytic Cracking (FCC) Zeolites (MR, ZSM-5, FAU) strong Brønsted acidity and micropores diffusion limitations, confinement effect (shape transition state selectivity) Mesostructured Aluminosilicates (e.g. MCM-48, MCM-41) amorphous silica: long range ordered framework structure better diffusion but amorphous walls leading to an instability ierarchical Zeolites Introduction of Mesopores - Dealumination: steaming and acid treatment extra-framework Al (EFAL) - Desilication: aqueous basic treatment extra-framework (EFSI) 3 Pérez-Ramírez et al., Chem. Soc. Rev., 2008

4 Mechanism and Formation of Mesopores Dealumination Marcilly s Mechanism Marcilly, Pétrole et Techniques, 1986 Ban et al., J. Phys. Chem., 2010 steamed ZSM-5 displays formation of vast areas of mesopores Karwacki et al., Angew. Chem. Int. Ed., 2011 sinusoidal channels more susceptible to dealumination extraction of EFAL hindered within straight channels 4

5 Motivation understanding the mechanism on the molecular scale why are certain T sites more susceptible to the dealumination? how does the water initiate an Al-/- bond break? how do mesopores form/propagate? only few mechanistic approaches by dint of ab initio calculations for the dealumination/desilication 5

6 Dealumination Reaction Path Zeolitic System: -Chabazite Malola et al., Angew. Chem Int. Ed., 2011 first kinetic reaction path of the dealumination mechanism using periodic DFT (functional: PBE) 6

7 Motivation understanding the mechanism on the molecular scale why are certain T sites more susceptible to the dealumination? how does the water attack initiate a bond break? how do mesopores propagate? only few mechanistic approaches by dint of ab initio calculations for the dealumination/desilication recent periodic DFT analysis only on -CA Malola et al., Angew. Chem Int. Ed., MR, ZSM-5, (FAU) of great interest since largely used in petroleum refining

8 Strategy & Methods ydrolysis/adsorption reactions involving one water molecule Mechanistic approach for the Dealumination/Desilication Theoretical approaches periodic DFT+D (VASP) and QM/QM (QMPT) 8

9 Model System ZSM-5: 12 T sites /Al = 95 Mordenite: 4 T sites /Al = MR sinusoidal 10 MR straight a T8 T11 T7 T9 T3 T10 b T5 T4 T12 T2 T1 T6 moncic et al., Am Mineral, T1 4 b T4 2 5 T2 6 T a Al Al

10 Methods. U U U nu ads zeo n( water ) zeo water Kresse, afner, Phys Rev B, 1993 periodic DFT: VASP structure optimization: DFT+D2: dispersion forces added semi-empirically to PBE Grimme, J. Comput. Chem., 2006 Nudged Elastic Band: PAW / cut-off: 400eV / Γ-point SCF convergence: 10-6 ev / forces on atoms < 0.02 ev/å 2 SCF convergence: 10-4 ev / forces on atoms < 0.03 ev/å 2 estimate reaction barriers (8 intermediate images) Jonsson, Classical and Quantum Dynamics in condensed Phase mulations, post-f: Møller-Plesset Perturbation Theory (MP2) adding a certain dynamic correlation as a perturbative potential ˆ ˆ A ˆ 0

11 Methods QMPT erka, Sauer, J. Chem. Phys., MR: T44 MP2 high level Turbomole: R. Ahlrichs et al., Chem. Phys. Lett., T cluster PBE+D low level VASP: Kresse, afner, Phys Rev B, optimization PBE D 2. ngle Point calculations with Turbomole on 6T cluster F, MP2: cc-pvxz (X = T, Q) for CBS no BSSE correction MP2 SP no CCSD(T) verification possible (not implemented in TM 5.9) E E MP 2 E E E S S T 6 T 6 tot PBE D PBE D MP2, CBS PBE D pbc PBE+D pbc PBE+D; MP kj/mol kj/mol

12 Results 1. ydrolysis/adsorption reactions implicating one water molecule 2. Mechanistic approach for the Dealumination/ Desilication 12

13 1. ydrolysis/adsorption reactions implicating one water molecule non-dissociative water adsorption Al V Al or Al IV Al Bond overlap population analyses Petracovschi et al, Rapport IFPEN 62062, Dec Al- bond break if r(al-) > 2.20 Å 13

14 1. ydrolysis/adsorption reactions implicating one water molecule non-dissociative water adsorption dissociative water adsorption (with hydrolysis) vicinal disilanol 14

15 1. ydrolysis/adsorption reactions implicating one water molecule Malola et al., Angew. Chem Int. Ed., ads on BAS 2 ads on Al in anti to BAS hydrolysis Al- vicinal silanol on Al-- Interaction Geometry -MR -ZSM-5 T13 (8 membered ring) T44 (12 membered ring) T102 (10 membered ring) a) 2 ads on BAS c) 2 ads on Al in anti to BAS Al V i) hydrolysis Al-* - -Al j) vicinal silanol on Al--* vicinal silanol *stability depending on insertion site 15 water adsorption on Al in anti to BAS most exothermic reaction

16 1. ydrolysis/adsorption reactions implicating one water molecule water adsorption on Al in anti position to BAS favored if 1. sterical constraints: unhindered anti attack on Al proton occluded in small cavities 2. hydrogen bonding: adsorbed 2 and formed silanol stabilized by framework oxygen atoms elongation of the Al- bond for certain T sites Al- bond break -MR T Mordenite ZSM-5 T2 T3 Al IV - T28 (ral- = 2.27 Å) T3 A IV - T34 (ral- = 2.28 Å) T4 - T34 (ral- = 2.91 Å) T5 strongly weakened Al... bond: - T52 (ral- = 2.18 Å) Al V - T44 (ral- = 2.12 Å) T10 - T102 (ral- = 2.19 Å) 16

17 ΔU ads (kj/mol) 1. ydrolysis/adsorption reactions implicating one water molecule mechanism of the first water splitting and Al- bond break: in -MR T TS 1.1 E A =100 kj/mol TS 1.2 E A =7 kj/mol kinetic product thermodynamic product adsorption on Al TS 2.1 E A =89 kj/mol TS 2.2 E A =6 kj/mol 17

18 1. ydrolysis/adsorption reactions implicating one water molecule mechanism of the first water splitting and Al- bond break: in -MR T44 water adsorption/splitting: thermodynamic and kinetic products depend on which oxygen atom water splitting took place no descriptor found predicting preferred splitting site first water adsorption determines extraction direction of EFAL in cavity: e.g. 12MR (see full mechanism) 18

19 2. Mechanistic approach for the Dealumination/Desilication simple Dealumination: e.g. pathway in -MR at T adsorption 1 2 bond break Al- 19 ΔUads [kj/mol] number of water molecules -MR T44

20 2. Mechanistic approach for the Dealumination/Desilication simple Dealumination: e.g. pathway in -MR at T adsorption bond break Al- adsorption bond break Al- 20 ΔUads [kj/mol] number of water molecules -MR T44

21 2. Mechanistic approach for the Dealumination/Desilication simple Dealumination: e.g. pathway in -MR at T adsorption bond break Al- adsorption bond break Al- adsorption bond break Al- 21 ΔUads [kj/mol] number of water molecules -MR T44

22 1 2 adsorption 2. Mechanistic approach for the Dealumination/Desilication simple Dealumination: e.g. pathway in -MR at T bond break Al- 2 2 adsorption 2 2 bond break Al- 3 2 adsorption 3 2 bond break Al- 4 2 EFAL creation Al() 3 ( 2 ) 4 2 EFAL desorption Al() 3 ( 2 ) ΔUads [kj/mol] number of water molecules -MR T44 22

23 1 2 adsorption 2. Mechanistic approach for the Dealumination/Desilication simple Dealumination: e.g. pathway in -MR at T bond break Al- 2 2 adsorption 2 2 bond break Al- 3 2 adsorption 3 2 bond break Al- 4 2 EFAL creation Al() 3 ( 2 ) 4 2 EFAL desorption Al() 3 ( 2 ) ΔUads [kj/mol] MR T44 -MR T28 number of water molecules 23

24 2. Mechanistic approach for the Dealumination/Desilication T13 in -MR: local sterical effects (curvature) of 8MRc vicinal disilanol: further dealumination is blocked nb of water molecules ΔUads [kj/mol] MR 12MR -MR: T13 -MR: T

25 2. Mechanistic approach for the Dealumination/Desilication T34 in -ZSM-5: EFAL Al() 3 ( 2 ) in 10MR stabilizing effect of the 10MR nb of water molecules ΔUads [kj/mol] MR 12MR -MR: T13 -MR: T MR -ZSM-5: T

26 2. Mechanistic approach for the Dealumination/Desilication Dealumination (at T44)/Desilication (at T2) in -MR nb of water molecules hydrolysis -50 Al- bond break ΔUads [kj/mol] bond break

27 2. Mechanistic approach for the Dealumination/Desilication Dealumination (at T44)/Desilication (at T2) in -MR ΔUads [kj/mol] nb of water molecules EFSI: () 4 Al- bond break - bond break combined Al- and - bond break EFAL: Al() hydrolysis framework (no defect site) defect sites - scission -10 to 98 kj/mol -100 to -60 kj/mol 27

28 Summary Al V Al Al IV Al 28 adsorption of water on Al in anti position to BAS most exothermic reaction first water adsorption predicts extraction direction of EFAL EFAL stability depends on pore size highest stability in 10MR of -ZSM 5 thermodynamical preference of a combined (consecutive/simultaneous) dealumination/desilication Marcilly, Pétrole et Techniques, 1986 Marcilly s Mechanism desilication preferentially takes place at defect sites

29 Perspectives reaction barriers for entire dealumination/desilication path: especially for first water dissociation increasing water amount: local hydration + 2 decreasing /Al ratio 29

30 Énergies renouvelables Production éco-responsable Transports innovants Procédés éco-efficients Ressources durables

31 1. Water adsorption on BAS vs LAS Adsorption on BAS Anti attack of a water molecule on LAS: Rapport IFPEN 62062, Dec Al Al Al V Al or Al IV Al r() [Å] FAU -MR -ZSM r(al-) [Å] T44 T28 T34 r() [A] T T T r(al-) [A] 31 color code: ΔUads(LAS) ΔUads(BAS) [kj/mol] > < 0

32 Model System Faujasite: 1 T site; /Al ratio = 47 Al exagonal Prism Supercage Sodalite Cage 32