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DFT, Monte-Carlo and classical simulation studies of crystals, surfaces and zeolites

DFT, Monte-Carlo and classical simulation studies of crystals, surfaces and zeolites. Molecular Mechanics Theory Binding of molecules to surfaces Monte-Carlo of Metal Organic F rameworks MC of Hydrocarbons in Zeolites Defect migration QM Methods Theory DMol Lanthanum Catalysis

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DFT, Monte-Carlo and classical simulation studies of crystals, surfaces and zeolites

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  1. DFT, Monte-Carlo and classical simulation studies of crystals, surfaces and zeolites

  2. Molecular Mechanics Theory Binding of molecules to surfaces Monte-Carlo of Metal Organic Frameworks MC of Hydrocarbons in Zeolites Defect migration QM Methods Theory DMol Lanthanum Catalysis Methanol to Gasoline in ZSM-5 CASTEP Surface Binding Catalyst degradation DeSOx and DeNOx Summary Overview

  3. Molecular Mechanics • Assumption: • Classical mechanical description is adequate • Empirical analytical representation of energy • Limitations: • Accuracy limited by empirical parameters • limited to parameterized systems • atom connectivity can not change • Advantages: • Very fast • Works for 1000’s of atoms Typical applications • Applications: • Biological compounds, silicas, zeolites, polymers, glasses • Conformational energies • Crystal morphology • Physisorption energy & properties • Diffusion

  4. Energy Expression Force Field Parameterization

  5. Typical Force Field Interactions Inter and intra molecular interactions are modeled with bonding and non- bond interactions

  6. Modelling of retarders on ettringite: Physisorption.

  7. Modelling of phosphonate retarders • Schlumberger make a range ofphosphonatecement retarders to control the setting of cements in oil wells. • Retarders are believed to workby a chelating to the surface ofettringite • What is the mechanism? How can we design more efficient ones? Reference: J. Chem. Soc. Faraday Trans., 1996, (92), P831

  8. Modelling of phosphonate retarders • Optimized structures of 8 experimentally used compounds • Examined the phosphate-phosphate distances • Calculated the morphology of Ettringite, found the most dominant face is the (001) plane. • Surface structure of plane shows that molecules are likely to bind to the sulphur atoms on the surface.

  9. Modelling of phosphonate retarders • The best retarders are those with phosphate - phosphate distances which match the sulfur-sulfur distances on the (001). • This correlates with experiment

  10. Modelling of phosphonate retarders • Minimisation and Dynamics run with the molecules docked on the surface. • Molecules with more flexible backbones tend to bind better. • Longer chains • Only one phosphonate group on each side is used. Other extends into space due to steric repulsion • Replace with smaller non-polar groups

  11. Modelling of phosphonate retarders • Proposed new structures included • The cyclic compound was synthesized and proved to be a powerful retarding agent.

  12. Sorbent Frameworks: MOF-5

  13. Demo Study: MOF-5 Ar Loading using Sorption • Sorption • Characterizes the sorption behavior a pure sorbate (or mixture of sorbate components) absorbed in a sorbent framework • Uses classical force-field potential to represent framework-molecule interactions with a Monte-Carlo search to calculate properties including: • adsorption isotherms • binding sites and binding energies • global minimum sorbate locations • densityfields

  14. Sorbent Frameworks: MOF-5 Nature (1999) 402, 276-279. • Open metal-organic framework using carboxylate linkers and Zn+2 ions • Possible substrate for gas-storage applications MOF-5 cavity sphere diameter 18.5A

  15. Demo Study: MOF-5 Ar Loading using Sorption • Preliminary demo study using COMPASS forcefield and Sorption to predict the fixed pressure loading of Ar in MOF-5 • The Ar loading at 101 KPa and 79K is 230 /unit cell which agrees with the experimental value of 230 /unit cell • Adsorption density of Ar in MOF-5 (blue) • Calculated the lowest energy binding site for a single Ar; Binding energy = 3.382 kcal/mol MOF-5 crystal structure FM-3M

  16. Demo Study: MOF-5 N2 Loading using Sorption • Preliminary demo study using COMPASS forcefield and Sorption to predict the fixed pressure loading of N2 in MOF-5 • The N2 loading at 101 KPa and 79K is 205 /unit cell which agrees with the experimental value of 183 /unit cell • Adsorption of N2 in MOF-5 (blue) • Calculated the lowest energy binding site for a single N2; Binding energy = 3.457 kcal/mol MOF-5 crystal structure FM-3M

  17. Demo Study: MOF-5 H2 Loading using Sorption • Preliminary demo study using Sorption to predict the fixed pressure loading of H2 in MOF-5 • The HC2 loading at 101 KPa and 295K is predicted to be 162 /unit cell • Adsorption of H2 in MOF-5 is shown • Calculated the lowest energy binding site for a single CHCl3; Binding energy = 1.469 kcal/mol MOF-5 crystal structure FM-3M

  18. Molecular Mechanics Theory Binding of molecules to surfaces Monte-Carlo of Metal Organic Frameworks MC of Hydrocarbons in Zeolites Defect migration QM Methods Theory DMol Lanthanum Catalysis Methanol to Gasoline in ZSM-5 CASTEP Surface Binding Catalyst degradation DeSOx and DeNOx Summary Overview

  19. Adsorption of hydrocarbons in microporous materials • Sorption simulation to analyze the adsorption of hydrocarbons on microporous zeolites and on the Pt/g-Al2O3 catalyst • Agreement between docking energy and sorption energy • Pt catalyst displays greater ability to absorb substrate Docking energies increase as heptane < methylcyclohexane, ethylpentane < toluene Szczygiel, J. ; Szyja, B.; Microp. Mesop. Mat. 76 (2004)247.

  20. Adsorption of hydrocarbons in microporous materials Calculation of adsorption isotherms • at low pressure adsorption of toluene molecules is impaired because of high interaction energy • adsorption of heptane molecules preferred • only at the highest pressures adsorption of toluene becomes favourable. Szczygiel, J. ; Szyja, B.; Microp. Mesop. Mat. 76 (2004)247.

  21. Adsorption of hydrocarbons in microporous materials Adsorption sites in the host structure (silicalite) • Ring and branched hydrocarbon accumulate at sites offering sufficient space • Heptane located at entire pore length due to greater flexibilty • Heptane has lower density at sites where other molecules accumulate • Stronger adsorption in channels between intersections caused by proximity of host atoms Szczygiel, J. ; Szyja, B.; Microp. Mesop. Mat. 76 (2004)247.

  22. Defect Migration: Lanthanium Oxide

  23. Defects in La2O3 Doped La2O3 is used as an electrolyte in solid oxide fuel cells and in oxygen sensors. Material is a fast ion conductor. Controlling factor is vacancy migration. Doping with mono and divalent cations increases vacancy concentration. A B Two possible vacancy migration routes; A and B Route A - 0.63 eV Route B - 4.79 eV Low energy for Route A explains fast ion conduction and implies single crystals will show anisotropic behavior Ref. D. J. Ilett and M. S. Islam - J. Chem. Soc. Farad. Trans. 1993, 89 (20), 3833

  24. Defects in La2O3 • Doping with mono and divalent cations increases the number of oxygen vacancies in the lattice to maintain neutrality. • Defect Energy calculations carried out on alkali metals and alkaline earth metals; • Li+, Na+, K+, Rb+ • Mg2+, Ca2+, Sr2+, Ba2+ • Results show Sr2+, has the lowest solution energy is 1.71eV per ion. • Hence doping with Sr ions will improve electrolytic properties of La2O3

  25. Introduction Molecular Mechanics Theory Binding of molecules to surfaces Monte-Carlo of Metal Organic Frameworks Defect migration QM Methods Theory DMol Lanthanum Catalysis Methanol to Gasoline in ZSM-5 CASTEP Surface Binding Catalyst degradation DeSOx and DeNOx Summary Overview

  26. H = E E. Schrödinger, 1926 Need for QM methods • Force Fields give good estimates for • structures, conformations, … • BUT an accurate determination of transition states requires highly sophisticated quantum mechanical methods… • no empirical parameters • work for all elements • dissociate bonds …

  27. Quantum Mechanical Methods • Solution of Schrödinger’s equation, ab initio • Disadvantages: • Potentially slow • Applicable to ~100 atoms • Advantages: • Applicable to any element • Tunable accuracy • Models bond breaking • Predicts absolute energies • Applications: • molecular geometry • chemisorption • chemical reactivity • UV & IR spectra • Solubility and thermodynamic properties

  28. DMol: Understanding Catalysis

  29. DMol • DFT program for molecules, crystals, surfaces • Uses Localized numerical basis sets • DMol3 has been one of the main QM engines of Biosym/MSI/Accelrys since 1988 • Successful applications include: • polymerization catalysis (metallocenes) • metal oxides • zeolites • CVD • molecular organic crystal structure • Platforms • NT, Linux, Irix, Windows 2000

  30. Rcut DMol3: Linear Combination of Atomic Orbitals Periodic and a periodic systems Radial portion atomic DFT eqs. numerically Angular Portion Good for molecules, clusters, zeolites, molecular crystals, polymers "open structures"

  31. Case Study: Lanthanide Catalysts using DMol3

  32. Lanthanide Catalysts using DMol3 • La2O3, LaOCl, LaCl3 used in commercial reactions, • Production of vinyl chloride, alkane conversion to chloride • Studied model reaction of • CCl4 + 2 H2O → CO2 + 4 HCl • A collaboration between Dow Chemical and several Universities • Use experiment & theory to link surface properties with catalytic activity • Detailed work from can be found in JPC:B108 (2004) 15770; Chem. Euro. J10 (2004) 1637; JPC:B109 (2005) 11634

  33. Lanthanide Catalysts using DMol3 • Use DFT to study decomposition reaction on surface of La2O3 • Rate determining step: • La3+surf + O2-surf CCl4 → La-Clsurf + O-CCl3 surf • Acidic La site initiates split by polarizing one of Cl atoms • Base site (typically surface oxygen) stabilizes CCl3 fragment • Study first reaction step on surface of LaOCl, LaCl3, and La2O3

  34. Lanthanide Catalysts using DMol3 Reaction on La2O3 Reaction on LaOCl Reaction on LaCl3 • Initial and final configuration same as La2O3 • Stronger interaction with acid site • No activation barrier • Intermediate reaction mechanism: • Similar to La2O3 before transition state • Similar to LaOCl after transition state • Activation barrier is 109 KJ mol-1 • Chlorine become anion and CCl3 loses charge • Stabilized above O site • Activation barrier is 147 KJ mol-1

  35. Lanthanide Catalysts using DMol3 Conclusions • Bond breaking CCl4 → CCl3+ + Cl- is rate limiting • Activation energy consistent with expt activity LaOCl > LaCl3 (with partial dechlorination of surface) > La2O3 • Explains activity in terms of surface features: • C-Cl bond activated by acid site • CCl3 fragment stabilized by O-atom base site • Best catalyst will be characterized by both • Strong acid & base sites • Geometrically favorable arrangement • Experiment provides “raw results” like relative ordering of sites, ordering of catalytic activity • Modeling provides critical insight for improved catalyst engineering

  36. Case Study: Methanol to Gasoline Conversion

  37. Zeolite-Catalyzed Hydocarbon Formation from Methanol • Reaction studied: conversion of methanol to gasoline (MTG) developed by Mobil in the 1970s • Study of mechanisms of C-O bond cleavage and formation of first C-C bond Open questions • Clusters of H-bonded methanols form in zeolite cage leading to dimethylether (DME) formation ? • Is C-O bond cleaved through formation of methoxyl or by surface ylide ? •  Study of reaction mechanism using periodic models Govind, N.; Andzelm, J. ; Reindel, K.; Fitzgerald, G. ; Int. J. Mol. Sci. 3 (2002) 423.

  38. Zeolite-Catalyzed Hydocarbon Formation from Methanol • Single methanol adsorbs via H-bonds • Surface methoxyl formation occurs via concerted reaction of C-O bond breaking in methanol and C-O bond formation on the surface • Energy barrier of 54 kcal/mol Govind, N.; Andzelm, J. ; Reindel, K.; Fitzgerald, G. ; Int. J. Mol. Sci. 3 (2002) 423.

  39. Zeolite-Catalyzed Hydocarbon Formation from Methanol • Second methanol lowers barrier to 44 kcal/mol • Methoxonium forms spontaneously by capture of proton from Bronsted acid site Govind, N.; Andzelm, J. ; Reindel, K.; Fitzgerald, G. ; Int. J. Mol. Sci. 3 (2002) 423.

  40. Zeolite-Catalyzed Hydocarbon Formation from Methanol • Ethanol formation: New pathway with water as spectator • Concerted reaction: Methanol gives up proton to Bronsted site • Barrier (50 kcal/mol) similar to previously reported scheme (competing reactions). Overall reaction exothermic. Govind, N.; Andzelm, J. ; Reindel, K.; Fitzgerald, G. ; Int. J. Mol. Sci. 3 (2002) 423.

  41. Zeolite-Catalyzed Hydocarbon Formation from Methanol • Ylide species formation has substantially higher barrier (78 kcal/mol) • Substantial lattice distortion around Bronsted site and zeolite cage • Rules out possibility of ylide formation Govind, N.; Andzelm, J. ; Reindel, K.; Fitzgerald, G. ; Int. J. Mol. Sci. 3 (2002) 423.

  42. Molecular Mechanics Theory Binding of molecules to surfaces Monte-Carlo of Metal Organic Frameworks Defect migration QM Methods Theory DMol Lanthanum Catalysis Methanol to Gasoline in ZSM-5 CASTEP Surface Binding Catalyst degradation DeSOx and DeNOx Summary Overview

  43. CASTEP Facts at a Glance Technology First-principles plane-wave pseudopotential code Periodic Boundary Conditions Application Solids and surfaces, all material types (metals, semiconductors and insulators) Origin Mike Payne’s group Cambridge University plus world-wide developers club Information Wealth of applications including defects, surface chemistry, zeolites, diffusion 240+ publications Platforms SGI, Linux, NT, Windows 2000 CASTEP=CAmbridge Serial Total Energy Program

  44. Plane Wave Basis Set • Block’s theorem states • The cell-periodic part can then be expanded using a basis set consisting of a discrete set of plane waves. Then each electronic wave function can be written as a sum of plane waves G – reciprocal lattice vectors Wavelike part Cell-periodic part

  45. Case Study: DeSox Catalysts

  46. DeSOx Catalyst Design using Simulation • Back ground • SO2 is a major air pollutant arising from sulfur in fuels • Causes acid rain with negative impact on ecosystem, human health and buildings and monuments • Oxides can be used to catalyze DeSOx reactions • Key goal is activation of the S-O bond in SO2 • Simulations and experiments have been used to understand the chemistry of SO2 on oxide surfaces Claus reaction 2H2S + SO2 -> 2H2O + 3Ssolid J. A. Rodriguez et al. JACS, 122, 12362 (2000). Reduction of SO2 by CO SO2 + 2CO -> 2CO2 + Ssolid

  47. SO2 Conversion (%) Cr2O3 Cr0.06Mg0.94O MgO DeSOx Catalyst Design SO2 + 2CO  2C2O + Ssolid at 500 C O Mg Cr • Why is Cr0.06Mg0.94O so active? • What metal could be user to replace Cr? • Health hazard, environmental impact, cost

  48. Activation of S-O bonds h2-O,O h1-S Mg Cr Cr Calculation of SO2 Adsorption on Surfaces h1-O Cr

  49. State can hybridize with the SO2 LUMO Origins of SO2 Activation • Cr0.06Mg0.94O is a good catalyst for the reduction of SO2 by CO because • Occupied electronic states appear well above the valence band edge of MgO • The Cr atoms in Cr0.06Mg0.94O are in a lower oxidation state than the atoms in Cr2O3 Design rule that can now be applied to look for alternative dopants to chromium!

  50. How can Cr be replaced? • Candidates to replace Cr: Mn, Fe, Co, Ni, Zn & Sn • INMATEL rank according to non-chemical factors • Approach: compute electronic structure and measure dopant level position above Valence Band (VB) edge Mn < Ni < Co < Sn < Zn < Fe Worst Best Iron looks like best choice! Dopant position above VB edge in eV

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