Kinetic Characterisation of Zeolite Catalysts Using Cracking, Alkylation and Other Chemical Reactions

Transcription

1 Haldor Topsøe Catalysis Forum Munkerupgaard, August 2015 Kinetic Characterisation of Zeolite Catalysts Using Cracking, Alkylation and Other Chemical Reactions Dmitry B. Lukyanov Catalysis & Reaction Engineering Group Department of Chemical Engineering University of Bath, Bath, BA2 7AY, UK

2 To the Memory of Tanya Vazhnova Colleague, Co-Author, Friend and Wife

3 Contents Background Objectives Experimental experience Alpha-activity test (protolytic cracking activity) Beta-activity test (hydrogen transfer activity) Alpha-activity and (forgotten) product selectivity Location of acid sites Probing meso-porous zeolites Enhanced activity sites: unexpected (?) results Alkylation of aromatics with alkanes (methane & ethane) Acknowledgements Conclusions

4 Background Why zeolite catalysts? Why kinetic method? It provides quantitative information about working catalysts (and is really good in doing this) and could see some features that are hidden from IR and NMR How/Why did I get there? Genes? Destiny? M.I. Temkin, S.L. Kiperman & L.I. Lukyanova. Flow-circulation method for studies of kinetics of heterogeneous catalytic reactions (First paper on gradientless reactor), Doklady Akademii Nauk SSSR, 74 (1950) Karpov Institute of Physical Chemistry, Moscow Laboratory of Chemical Reactors Laboratory of Catalyst Testing Mobil influence? C.D. Chang & A.J. Silvestri. Conversion of methanol and other O-compounds to hydrocarbons over zeolite catalysts, J. Catal. 47 (1977) 249. W.O. Haag & R.M. Dessau. Duality of mechanism of acid-catalyzed paraffin cracking. Proc. 8 th ICC, Berlin, 1984, vol. 2, p.305, Dechema, Berlin, R.M. Lago, W.O. Haag, et al. The nature of the catalytic sites in HZSM-5 Activity enhancement. Proc. 7 th IZC, p.677, Kodansha, Tokyo, 1986.

5 Objectives of This Talk To consider Kinetic Method as a tool for characterisation of zeolite catalysts and, hopefully, to demonstrate its (not very well understood) usefulness for catalyst characterisation To present some interesting data regarding a few catalytic reactions, including cracking, alkylation, MTH

6 Experimental Experience You have to be very careful and follow the procedure exactly (this would not always eliminate all mistakes/errors but, at least, will bring their number down considerably) Check quantitatively your data against the literature data (have to be careful with the literature source) Do not get too excited when you see unexpected results these could lead to a discovery, but more likely something was wrong with your rig Conversion (%) Time on stream (hours) Effect of time on stream on the conversion of ethene over H-ZSM-5 in a continuous flow reactor Activity of two H-ZSM-5 zeolites Parent Steamed n-hexane cracking Ethene oligomeris Toluene alkylation with methanol MTH Note that mild steaming creates Enhanced Activity Sites (Lago, Haag, et al 1986)

7 Alpha-activity test Mobil 1965 n-hexane cracking in a continuous flow reactor at 1000 F (538 o C) and n- hexane concentration of 13 mol% with the measurement of the 1 st order rate constant (measure of α-activity) Has been used for characterisation of catalyst acidity and is used by a few companies nowadays for this and other (?) purposes Haag & Dessau, 1984 Protolytic cracking mechanism Hydrogen transfer mechanism

8 α-activity test and mechanism of n-hexane cracking Autocatalysis in n-hexane conversion over H-ZSM-5 at 400 and 500 o C. Experimental data (points) and kinetic modelling curves. Note: autocatalysis is common for alkane cracking reactions D.B. Lukyanov, V.I. Shtral & S.N. Khadzhiev, J. Catal., 146 (1994) 87.

9 α-activity test Why 1000 F and what does it measure? Alpha-activity is the measure of the protolytic cracking activity! First order plots for simulated n-hexane conversion (T = 538 o C, C o C6 = 13 mol%) over Z-240, Z-240(1) and Z-240(2) catalysts. Points correspond to the kinetic modelling results. D.B. Lukyanov, V.I. Shtral & S.N. Khadzhiev, J. Catal., 146 (1994) 87.

10 Beta-activity test Lukyanov 1994 Z-34ST Z-240 HY-1 HY-2 Z-34 Zeolite HZSM-5 HY Hydrogen transfer Protolytic cracking Effect of n-hexane conversion on the rate of isobutane formation (A) and on the isobutene concentration (B) over three H-ZSM-5 and two HY zeolites. D.B. Lukyanov, J. Catal., 145 (1994) 54.

11 Alpha-activity and (forgotten) product selectivity Molar selectivities (%) in n-hexane cracking over different zeolites at 400 o C Zeolite HZSM-5 H 100 -MOR H 45 -MOR HY C 6 + Z C 5= Z + C C 6 + Z C 4= Z + C C 6 + Z C 3= Z + C C 6 + Z C 2= Z + C MR 12MR & 8MR 12MR only 12MR only Can one use selectivity to probe the location of active sites?

12 Location of acid sites FTIR spectroscopy, for a change Absorbance H-MOR H 82 Na 18 -MOR Absorbance H-MOR H 82 Na 18 -MOR 1.0 H 63 Na 37 -MOR 3 2 H 63 Na 37 -MOR 0.5 H 45 Na 55 -MOR 1 H 45 Na 55 -MOR Wavenumbers (cm -1 ) Wavenumbers (cm -1 ) 3540 IR spectra (left) and FSD traces of four MOR zeolites T. Vazhnova & D.B. Lukyanov, Anal. Chem., 85 (2013) D.B. Lukyanov, T. Vazhnova, etc., J. Phys. Chem. C, 118 (2014)

13 Location of acid sites O8 T1 O6 O5 O2 O3 O7 O9 I IV O10 T4 T3 O1 T2 VI O4 T atoms Oxygen atoms Structure of MOR zeolite with three ion-exchange positions (I, IV and VI) Six IR bands have been assigned to specific oxygen atoms. D.B. Lukyanov, T. Vazhnova, etc., J. Phys. Chem. C, 118 (2014)

14 Probing meso-porous zeolites Hierarchical zeolite catalysts Stability enhancement CH 3 OH Alkenes Coke CH 3 OCH 3 Aromatics Alkanes 80 A B Coke D Steamed HZSM-5 Micro- & Meso-pores Conversion of methanol into hydrocarbons over parent and two steamed zeolites Conversion (%) Steamed HZSM-5 Micro- & Meso-pores Parent HZSM-5 Micro-pores Time on stream (h) D.B. Lukyanov, 2015, in preparation.

15 Probing meso-porous zeolites Hierarchical zeolite catalysts Selectivity enhancement CH 3 OH Alkenes Coke CH 3 OCH 3 Aromatics Alkanes Catalyst Maximum yield of alkenes (%) Parent HZSM-5 27 Steamed HZSM-5-2h 36 Steamed HZSM-5-4h 37 D.B. Lukyanov, 2015, in preparation.

16 Probing meso-porous zeolites Schematics of bulk modification, i.e. external surface is not shown. Microporous Zeolite MMP Zeolite 1 MMP Zeolite 2 Cracking (activity & selectivity) and FSD-FTIR?

17 Enhanced activity sites: unexpected(?) results Parent H-ZSM-5 Activity of two H-ZSM-5 zeolites Parent Steamed n-hexane cracking Ethene oligomeris Toluene alkylation with methanol Mildly steamed H-ZSM-5 MTH There is principal difference between hydrocarbon reactions and formation of C C bond from methanol! D.B. Lukyanov, Zeolites, 13 (1993) 64.

18 Alkylation of aromatics with alkanes (ethane) In principle, direct alkylation of benzene with ethane into ethylbenzene (EB) can be carried out over a bifunctional catalyst by coupling two reactions: Me C 2 H 6 C 2 H 4 + H 2 + C 2 H 4 H + C 2 H 5 T. Vazhnova & D.B.Lukyanov, J. Mol. Catal., 279 (2008) 128. T. Vazhnova & D.B.Lukyanov, J. Catal., 257 (2008) 382.

19 Alkylation of aromatics with alkanes (ethane) Benzene conversion (%) Time on stream (h) Contact time (WHSV -1 ) ( ) = 0.32 h ( ) = h ( ) = h Effect of TOS on benzene conversion at different contact times.

20 Alkylation of aromatics with alkanes (ethane) EB selectivity in aromatic products (mol.%) Time on stream (h) Initial benzene conversion ( ) X = 8% ( ) X = 12.5% ( ) X = 20% Effect of TOS on EB selectivity in aromatic products at different benzene conversions.

21 Alkylation of aromatics with alkanes (methane) Benzene and Methane Conversions vs. Time on Stream 8 Conversion (%) Benzene PtH-MFI catalyst at 370 o C. Methane to benzene molar ratio in the feed was 9: Conversion (%) Methane Time on stream (h) T. Vazhnova & D.B.Lukyanov, J. Mol. Catal. (2009).

22 Alkylation of aromatics with alkanes (methane) Benzene and Methane Conversions vs. Time on Stream Selectivities to all carbon containing products observed over PtH-MFI catalyst at 370 o C and TOS of 4 hours Catalyst PtH-MFI Methane conversion (%) 0.53 Benzene conversion (%) 4.5 Selectivity (mol.%) Ethane 2.7 Toluene 96.1 Ethylbenzene 0.15 Xylenes 1.05

23 Acknowledgements People Tanya Vazhnova Vsevolod Timoshenko John Dwyer Organisations Karpov Institute of Physical Chemistry, Moscow University of Bath EPSRC, BP Chemicals, ICI (different divisions), Johnson Matthey Haldor Topsøe

24 Conclusions Hopefully, we can draw some conclusions during this session!