1 / 58

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

hija
Download Presentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related