Evaluation of kinetic data. From simple plots to advanced computations

Transcription

1 Evaluation of kinetic data From simple plots to advanced computations

2 Intrinsic kinetics What is intrinsic kinetics? Real kinetics, free from mass and heat transfer effects Kinetics of universal value, independent of equipment, laboratory, experimentalist The goal of fundamental research The sound basis of applied research Great responsibility in reporting kinetic data!

3 Turnover number (TON) & turnover frequency (TOF) What they are? TON in enzymatic catalysis: maximum number of molecules of substrate (=reactant) that an enzyme can convert to product per catalytic site per time TON in organometallic catalysis: number of moles of reactant that a mole of catalyst can convert before becoming deactivated TOF is turnover per time

4 Relative concentration Initial rate What is initial rate and how to determine it in a reliable way? What we dear to publish? The rate at t=0 Example: Glucose hydrogenation (A.Aho) Best performing catalysts glucose (Catalyst A) sorbitol (Catalyst A) glucose (RuO2/NCNT) sorbitol (RuO2/NCNT) glucose (Ru/NCNT small NP) sorbitol (Ru/NCNT small NP) glucose (Ru/NCNT-PVP) sorbitol (Ru/NCNT-PVP) glucose (Olga DP non-reduced) sorbitol (Olga DP non-reduced) time*mcat (min*g)

5 Example:Hydrolysis of hemicelluloses O-Acetylgalactoglucomannane our molecule -a branched hemicellulose (GGM) Hydrolysed with the aid of homogeneous and heterogeneous catalysts

6 Heterogeneous catalysts Smopex-101 Fibrous and non-porous: d=0.01 mm I=4 mm Polymer, polyethene-graft-polystyrene with sulfonic acid functional group Capacity 3.6 mmol eq /g 20 μm

7 Autocatalysis detected- initial rate? TOF? very difficult! c/(mg/l) data set mannose galactose and glucose time/s

8 Relative concentration Initial rate how to determine Which is a good method to determine the initial rate? The rate at t=0 Model the kinetics properly, then calculate the initial rate from the model Model the kinetic curve empirically (e.g. by polynomial), calculate the initial rate from the model Make a test plot (logarithmic or other), extrapolate to zero Take a difference between to first experimental points (terrible!) the accuracy is questionable and depends on the reaction order! Avoid this method! Best performing catalysts glucose (Catalyst A) sorbitol (Catalyst A) glucose (RuO2/NCNT) sorbitol (RuO2/NCNT) glucose (Ru/NCNT small NP) sorbitol (Ru/NCNT small NP) glucose (Ru/NCNT-PVP) sorbitol (Ru/NCNT-PVP) glucose (Olga DP non-reduced) sorbitol (Olga DP non-reduced) time*mcat (min*g)

9 Reaction order What is apparent reaction order is it oldfashioned? Definition: the exponent or sum of exponents above the concentrations/activities appearing in the rate equation (Apparent) reaction order is good to be determined as one of the first diagnosis of kinetic data

10 Example: Lactose hydrogenation The reaction scheme is complicated Detailed kinetic analysis is needed A B C D

11 Effect of catalyst amount

12 Effect of hydrogen pressure

13 Linear plots obtained Reaction is of first order with respect to lactose (on Ni and Ru catalysts), the apparent reaction order with respect to hydrogen is SUGGEST a mechanism and rate equation!

14 Parallel reactions kinetic analysis Two products are formed, P1 and P2 from A (and other reactants) is the reaction scheme parallel? If yes, what is the ratio between rate constants? Procedure: Plot P1 vs time and P2 vs time; then plot P1 vs P2 is the plot linear, is the plot dependent on temperature QUESTION: if the plot P1-P2 plot is linear, how does the selectivity vs conversion plot look?

15 conversion isoselectivity Example: Selectivity vs conversion Influence of ligand mass on (a) conversion and (b) selectivity in hydroformylation of propene on Rh/CHDPP (100 C, 10 bar, 250 ppm Rh) (A.Bernas) wt-% CHDPP (a) wt-% CHDPP (b) 10 wt-% CHDPP wt-% 1 wt-% 2 wt-% 3.34 wt-% 10 wt-% wt-% CHDPP 0.5 wt-% 1 wt-% 2 wt-% 3.34 wt-% 10 wt-% time (min) conversion

16 Simultaneous reactions kinetic analysis A typical case when a mixture of similar type of molecules react in a mixture (e.g. hydrogenation of a mixture of molecules) A typical question: Do they follow a similar type of reaction kinetics? PROPOSE A PROCEDURE TO CHECK THIS!

17 Example: Hydrogenation of A & G mixtures (V. Sifontes) Mixture hydrogenation - Molar ratio 1, 40 bar.

18 Modeling principles Langmuir-Hinshelwood model Non-competitive hydrogen adsorption Simple reaction networks Parameter estimation and simulation carried out with numerical software (ModEst)

19 Modeling mixtures (A&G) Modeling results at 105 C and 40 bar pressure. Ratio 3.6:1. L-arabinose, L-arabitol, D-galactose, D-galactitol. Lines denote regression values. Modeling results at 105 C and 50 bar pressure. Ratio 3.6:1.. L-arabinose, L-arabitol, D-galactose, D-galactitol. Lines denote regression values..

20 Sensitivity analysis ( 31) Sensitivity analysis for L- arabinose activation energy. Sensitivity analysis for D-galactose activation energy.

21 Product distribution analysis A double logarithmic plot should give a straight line!

22 Product distribution analysis ( 32) Hydrogenation of (D-gal:L-ara=0.5) solution at 120 C and 40 bar Hydrogenation of (D-gal:L-ara=5) solution at 120 C and 40 bar

23 Final message Remember the big responsibility in reporting initial rate, TOF, TON... Determine apparent reaction orders as the first step Make a simple preliminary analysis of complex systems, before throwing everything to regression analysis The best way to do industrially relevant kinetics is to perform experiments under industrially irrelevant conditions! broad experimental domain needed to make reliable kinetics

24 Here we are!