Theoretical study of amine degradation University of Texas, 10 January 2008

1. Theoretical study of amine degradationUniversity of Texas, 10 January 2008 Eirik F. da Silva, Karl A. Hoff, Kristin Rist Sørheim, Odd Gunnar Brakstad and Hallvard F. Svendsen SINTEF Materials and Chemistry, NTNU and University of Florida

2. Outline • Application of Computational Chemistry • Toxicity and Biodegradation • Mechanistic Interpretation of Results • Carbamate Degradation

3. Computational Chemistry • Quantum Mechanical calculations (HF, DFT, MP2..) • Potentially accurate results without any experimental input. • Cost of calculations increase with the quality of results and the size of the system being studied. • Solvation models (PCM, SM, Cosmo-RS ….) • Simplified representation of interactions in a liquid • Classical simulations (Molecular Dynamics, Monte Carlo) • Simplified “ball-and-stick” representation of molecules. • Can represent a liquid phase explicitly. Quality of results depend on parameterization.

4. Exploring Reactions • Computational Chemistry can be used to explore any reaction. • No general answers concerning the stability of species. • Experimental guidance is important. Illustration from: helix.nih.gov

5. Snøhvit and REACT

6. OSPAR ConventionClassification of chemicals • Commercial chemical solvents like MDEA/piperazine are classified as red • Red Category: Chemicals shall be phased out and/or substituted

7. Absorption Chemistry • Tertiary amines • Secondary amines • Sterically hindered amines • Primary amines • Some cyclical amines

8. Solvent selection criteria • Regeneration energy requirement • Rate of reaction/Mass transfer • Cyclic capacity • Molecular weight (per active site) • Foaming properties • Water solubility • Molecular transport properties • Corrosion • Chemical stability • Toxicity and Biodegradation • Cost

9. REACT • Determine experimentally the ecotoxicity and biodegradability of a wide set of process chemicals. • Develop understanding of degradation in both process and environmental conditions. • Chemicals identified as promising shall be characterized by measurement of thermodynamical and kinetic data. • New solvents will be implemented in a process modeling tool and simulations performed to assess the process performance and energy requirements.

10. REACT Solvent characterization Ecotoxicity/biodegradablity Degradationmechanisms Molecular modeling/QSAR studies Process modeling New solvent with thermal and chemical stability at process conditions Classified as green or yellow

11. Selection of chemicals for testing • 28 Candidates for first test campaign selected from • Alkanolamines known to be in commercial operation (MEA, MDEA, AMP, DEA etc.) • New candidates deemed promising based upon earlier experience + molecular modeling studies • Cyclic amines • Linear polyamines • Sterically hindered amines • A larger set of data is required in order to correlate results with molecular structure

12. Ecotoxicity studies • Ecotoxicity tests recognized by OSPAR and by Norwegian Pollution authorities • Phytoplankton: Skeletonema costatum (ISO/DIS 10253) – all chemicals • Marine biodegradation test (OECD 306) – all chemicals • Bioaccumulation testing – calculations • Other bioassay studies • Microtox assay – all chemicals • Response studies in Calanus finnmarchicus – method development on selected chemicals

13. Skeletonema – EC-50 results

14. BOD results

15. Bioaccumulation test • Chemical test to determine the distribution of a chemical between two immiscible phases; octanol and water • The calculations were based on the difference between free energy of solvation in water (dGswater) and in the water-immiscible solvent octanol (dGsoctanol). • LogPOW = • Results: No tested chemicals were bioaccumulating (all water-soluble)

16. Toxicity and log Pow correlation

17. Observations on degradation • Sterically hindered amines degrade slowly. • No obvious trends between different classes of amines. • Initial search for correlation between calculated bond breaking energies and degradation rates gave no correlation. • EPI suite of models fail to predict trends in biodegradability. • Most of the amines displaying high degradability are known enzyme substrates1: • 8/10 Known enzyme substrates had BOD over 20 • 2/14 Amines not known be enzyme substrates have BOD over 20 1: BRENDA database (www.brenda.uni-koeln.de)

18. Proposed Mechanism for Copper Amino Oxidase: Prabhakar and Siegbahn J. Comp. Chem. (2003)

19. Proposed Mechanism for Trimethylamine Dehydrogenase: Basran, Sutcliffe and Scrutton J. Bio. Chem. (2001)

20. Proposed Mechanism for Ethanolamine Oxidase: Warncke Biochemistry (2005)

21. Degradation Products

23. How to understand and predict biodegradation • Can degradation be accounted for by a single enzyme? • Determine similarity with known amine substrates. • Develop QSAR based on likely reaction path(s). • Quantum Mechanical calculations on key intermediates. • Experimental work to determine intermediates and final products

24. Carbamate Degradation

25. Transition State for Carbamate Degradation Relative barriers: • MEA: 55 kcal/mol • DEA: 60 kcal/mol • MPA: 53 kcal/mol • EDA: 1700 kcal/mol

26. Conclusions • The amines tested show a relatively large span in both ecotoxicity and biodegradability • Most tertiary and sterically hindered amines are red • Of the candidates identified as yellow, several are promising solvents. • For natural gas CO2 removal the gas is not oxidative. It may therefore be easier to find a candidate solvent fulfilling the requirements of process stability and biodegradability • The relevance of these results for large scale post-combustion CO2 capture needs to be investigated further

27. Acknowledgements: Thank you for your attention! • The REACT project is funded by the Research Council of Norway, through the strategic PETROMAKS program • Co-funded by Shell Technology Norway and Statoil ASA