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Project 8 Measuring Zeolitic Diffusion by the Frequency Response Technique Andreas Jentys Johannes A. Lercher Lehrstuhl II für Technische Chemie Technische Universität München Goals ■ Explore the diffusivity in molecular sieves as function of − Pore size and connectivity • AFI (uni-dimensional channels) • Silicalite-1, Ferrierite (two intersecting channels) • LTA, NaX (three-dimensional network) − Pressure − Temperature ■ Explore the diffusivity in the presence of more than one sorbing component as function of − Pore size and connectivity − Pressure − Temperature Means Means • Frequency Response technique accessed via pressure variations • Determination of averaged coverages by IR spectroscopy and TGA Frequency response technique ■ Response of a closed system to a periodic perturbation Volume modulation Pressure response − Empty system: the pressure modulation varies at the same frequency as the volume perturbation (180° out of phase) − Porous sample: change in the phase and the amplitude of the pressure response to volume modulation is observed ■ Measuring the pressure response as function of the frequency of the pressure perturbation Æ Determination of diffusion coefficients Experimental setup Square-wave volume modulation TurboPump P − − − − − − Sample in powder form (~ 50 mg) UHV Vacuum system (base pressure < 10-7 mbar) Thermal activation up to 600 °C Magnetically driven separator plate, UHV bellows Frequency range 0.001 – 5 Hz High precision differential pressure gauge (MKS-Baratron) Frequency response Method 1.06 5.5 1.05 4.5 1.04 3.5 1.03 2.5 1.02 1.5 1.01 0.5 1 0 50 100 Time (seconds) 150 -0.5 200 Magnet Signal (Volts) Pressure (mbar) Square-wave volume modulation Frequency response method Experimental data Theoretical model pB/pZ = amplitude ratio δc, δs = f(f,r,D) φZ - φB = phase lag K = (RT0/Ve)(dBe/dPe) (pB/pZ)cos(φZ- φB) – 1 = Kδc (pB/pZ)sin(φZ- φB) = Kδs ■ Planar sheet model δ1c = (1/η)(sinh η + sin η)/(cosh η + cos η) η = (ωL2/D)1/2 δ1s = (1/η)(sinh η - sin η)/(cosh η + cos η) ω = 2πf ■ More complex models − − − − Spherical particles Two (or more) independent diffusion processes Non-isothermal behavior Diffusion rearrangement Frequency response method Function 0,3 0,25 Out of phase function 0,2 (pB/pZ)cos(φZ- φB)–1 =∑Kδc (p0,15 B/pZ)sin(φZ- φB) = ∑Kδs 0,1 In phase function Fast diffusion 0,05 0.0 0,001 Slow diffusion 0,01 0,1 Frequency (Hz) 1 10 K = (RT0/Ve)(dBe/dPe) δ1c = (1/η)(sinh η + sin η)/(cosh η + cos η) η = (ωL2/D)1/2 δ1s = (1/η)(sinh η - sin η)/(cosh η + cos η) ω = 2πf Relationship between frequency range, particle size and diffusion coefficient Diffusion coefficient (m2/s) 10-6 10-8 10 Hz 1 Hz 10-10 0.1 Hz 0.01 Hz 10-12 0.001 Hz 10-14 10-16 10-18 10-8 10-7 10-6 10-5 Particle radius (m) 10-4 10-3 Advantages of the frequency response method ■ Small perturbation to the sorption equilibrium − Concentration of the sorbed phase remains almost constant − Determination of diffusion coefficients as function of coverage ■ Compression and expansion cycles − Adsorption and desorption pressure response − Differences in the diffusion mechanisms for the two processes ■ Investigation of multiple diffusion processes − Separation of processes differing by half order of magnitude of D ■ Using higher harmonics to study faster processes − Larger diffusion coefficients can be measured Diffusion of toluene in H/ZSM-5 H/ZSM-5 (3 µm) 403 K 0,3 Function 0,25 0,2 0,15 0,1 2.6×10-13 m2/s 2×10-14 m2/s 0,05 0.0 0,001 ZSM-5 0,01 0,1 Frequency (Hz) 1 10 Application of the frequency response method Diffusion coefficients as function of coverage 1000 m /s) 403 K Diffusivity (10 -15 2 373 K 100 10 1 0 0.5 1 1.5 Toluene coverage (molecule/u.c) 2 2.5 Work programme ■ Diffusion in non uni-dimensional pore systems − Study sorption of benzene and alkanes in • AFI (uni-dimensional channels) • Silicalite-1, Ferrierite (two intersecting channels) • LTA, NaX (three-dimensional network) − effects of pore diameter, channel intersections and transport barriers − Link with other techniques within the project ■ Multi-component diffusion in zeolites − Develop multi component detection of two (or more) molecules in the gas phase using IR spectroscopy − Study sorption of • Alkyl substituted aromatic molecules • n- and iso-alkanes (deuterated molecules) − Describe multi-component diffusion pathways and molecular passages − Differentiate between single-file and normal diffusion (passages possible) Square-wave volume modulation Multi component detection by IR spectroscopy TurboPump P Analysis of gas composition by IR spectroscopy − Multiple reflection IR gas cell (1 m path length) − Rapid scan mode − Vacuum IR spectrometer (high S/N) IR Beam Work programme ■ Diffusion in non uni-dimensional pore systems − Study sorption of benzene and alkanes in • AFI (uni-dimensional channels) • Silicalite-1, Ferrierite (two intersecting channels) • LTA, NaX (three-dimensional network) − effects of pore diameter, channel intersections and transport barriers − Link with other techniques within the project ■ Multi-component diffusion in zeolites − Develop multi component detection of two (or more) molecules in the gas phase using IR spectroscopy − Study sorption of • Alkyl substituted aromatic molecules • n- and iso-alkanes (deuterated molecules) − Describe multi-component diffusion pathways and molecular passages − Differentiate between single-file and normal diffusion (passages possible)