Assessment of the Improvement of Effective Diffusivity over Technical

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Assessment of the Improvement of Effective Diffusivity over Technical Zeolite Bodies by Different Techniques Rogeria Bingre, Bruno Vincent, Qiang Wang, Patrick Nguyen, and Benoit Louis J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10914 • Publication Date (Web): 19 Dec 2018 Downloaded from http://pubs.acs.org on December 26, 2018

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The Journal of Physical Chemistry

Assessment of the Improvement of Effective Diffusivity over Technical Zeolite Bodies by Different Techniques Rogéria Bingre,† Bruno Vincent,‡ Qiang Wang,§ Patrick Nguyen,‖ Benoît Louis*,† †ICPEES

– Institute de Chimie et Procédés pour l’Energie, l’Environnement et la Santé,

Energy and Fuels for a Sustainable Environment Team, UMR 7515 CNRS – Université de Strasbourg – ECPM, 25 rue Becquerel, F-67087 Strasbourg cedex 2, France ‡NMR-service

Strasbourg, Institute de Chimie, UMR 7177, Université de Strasbourg, 1 rue

Blaise Pascal, 67000 Strasbourg cedex, France §EFN

– Environmental Functional Nanomaterials Lab, College of Environmental Science and

Engineering, Beijing Forestry University, P.O. Box 60, 35 Qinghua East Road, Haidian District, Beijing 100083, P.R. China ‖Saint-Gobain

C.R.E.E., 550 Avenue Alphonse Jauffret, BP 224, 84306 Cavaillon cedex,

France

ABSTRACT Technical zeolite bodies with additional porosity have been prepared with the intention to improve effective diffusivity. The latter has been assessed by three different techniques, combining a simple one as uptake measurements (TGA) to more sophisticated pulsed-field

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gradient PFG-NMR method. Inverse gas chromatography provided further complement to the aforementioned techniques by mimicking realistic operating conditions (temperature and flow). The three methods assessed an enhancement in the effective diffusivity for the samples where meso- or macropores were created. Useful insights concerning the interactions between the binder and the zeolite were gained, showing a strong impact on the effective diffusivity within the catalyst body.

INTRODUCTION Zeolites are often associated to slow mass transfer through their structure due to the pores and cages of molecular size (< 1 nm). This has been shown to have consequences over the catalyst lifetime, its catalytic performance and on the formation of unwanted by-products. Many researchers have focused their work on the development of strategies to improve transport properties and they can be classified in, mainly, three categories: (i) reduction of the crystal size; (ii) synthesis of structures with larger pores; (iii) incorporation of meso- or macropores into the network of micropores, as an additional path to the diffusion of molecules. The first option raises several drawbacks concerning the limited technology available to the synthesis and handling of nanocrystals.1 Materials with larger pores and MOFs, although improving the mass transport, often penalize the catalytic activity and the selectivity properties with respect to purely microporous structures.2–5 It is then the third option that has been widely studied by researchers. Materials with hierarchical porosities have shown to greatly increase the mass transfer properties, raising the conversion in the cracking of large molecules, and the lifetime of the catalyst in reactions with extensive coke deposition. 6–15 The improvement of the effective diffusivity in these porous materials has been assessed by many different techniques throughout the decades. The methods of diffusion characterization 2 ACS Paragon Plus Environment

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may be divided into two classes: equilibrium and non-equilibrium techniques.16–18 The first category occurs when a distinction between different molecules is possible as it is the case for pulsed-field gradient (PFG)-NMR. This technique involves the measurement of the molecular displacement, by detection of the phase shift caused by a motion of nuclear spin, in this case 1H,

in the presence of an external magnetic field within a short time interval. On the other

side, the non-equilibrium techniques are characterized by transient or boundary conditions that ensure a stationary state. This is, for instance, the case for uptake measurements of a given probe molecule adsorbed in a material. However, this technique fails to give an understanding of the transport mechanisms within such situations giving rise to further development of PFG-NMR technique among others.19,20 Moreover, uptake rate measurements occur in static conditions and suffer from mass transfer resistance at the external surface area of the zeolite crystals.18 Later on, the inverse gas chromatography technique (iGC) was developed, based on the determination of the diffusion coefficient by concentration gradients and the flow of the carrier gas, mimicking realistic operating conditions such as FCC process, or reactions under plug flow regime.16,21 However, the latter macroscopic technique does neither have the capability to distinguish between different types of diffusion mechanisms, nor to consider the total pores present but only the transport-available pores. Furthermore, many contributions report the determination of effective diffusivity in materials without industrial relevance, such as zeolite powders. There is a lack of understanding the influence of the binder in zeolite extrudates, and how to overcome the drawbacks associated with it. Herein, we present the detailed characterization of the diffusion of benzene and toluene over technical zeolite bodies where additional porosity was introduced in the binder, thanks to the addition of pore former agents. This strategy allows leaving the zeolite structure intact, but creating additional pathways for the diffusion of molecules towards the active sites.



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EXPERIMENTAL SECTION Catalysts and materials The technical bodies were obtained by extrusion of commercial ZSM-5 (CBV3020E, Zeolyst) with boehmite (SASOL Disperal P2) in a ratio of 4:1, respectively. Different pore agent formers were added to the mixture to introduce meso- or macropores in the binder. The samples were labelled Catal_PAX (being X a number corresponding to different pore former agents). A reference sample without any addition of pore agent was also prepared (Catal_ref). All the samples were calcined at 600ºC for 4h with a heating ramp of 1ºC/min. Liquid toluene 99.8% used in the preparation of PFG-NMR tubes was purchased from SigmaAldrich. For TGA measurements, toluene was supplied via a gas cylinder of 200 bar in a concentration of 1993 ppm in Ar. Inverse gas chromatography was performed with benzene (99%) purchased from Alfa Aesar.

Characterization methods Focused ion beam scanning electron microscopy (SEM) micrographs of resin-embedded extrudates were acquired on a microprobe JEOL FEG JXA8530F with spectrometers WDS and EDS JEOL. The samples were metallized with Au for 60s. The textural properties involving the BET surface area (SBET) were evaluated from classical nitrogen physical adsorption-desorption isotherms measured at -196°C by means of ASAP2020M equipment (Micromeritics). In addition, mercury intrusion porosimetry was performed in a Micromeritics Autopore IV 9510 operated in the pressure range from vacuum to 414 MPa. Samples were degassed in situ prior to measurement. A contact angle of 140 °


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for mercury and a pressure equilibration time of 10 s were used. The pore size distribution was determined by application of the Washburn equation.

Diffusion measurements For uptake measurements, the samples were tested in extruded form or as a powder with 0.074 < diameter of particles < 0.1 mm, likewise to CBV3020E reference sample. The samples were placed in the TGA sample holder and calcined at 550ºC for 30 min with a heating ramp of 10º/min under a 40 mL/min nitrogen flow. The temperature was then reduced to 35ºC, and the gas was switched for toluene at a concentration of 0.28 mg/min. The mass of the sample was recorded during all the process until saturation. Inverse gas chromatography was performed in a gas chromatograph Clarius 500 from Perkin Elmer. The samples were treated in situ at 110°C overnight under a He flow of 15 mL/min to remove any adsorbed water and other impurities. The temperature of the column was then increased to 350°C and the analysis consisted in the injection of methane and benzene at nearly infinite dilution and different flows of carrier gas. The treatment of the data was performed using Adscientis software. Measurement of self-diffusion coefficients by pulse field gradient (PFG) NMR were performed on a Bruker 600 MHz spectrometer - Avance III, equipped with a high strength z gradient probe DOTY Scientific, developing a pulse field gradient of 50 G/cm/A. The gradient coil is cooled by air flow and the sample was thermostated at 25°C. Diffusion NMR data were acquired using a Stimulated Echo pulse sequence with bipolar z gradients. The gradient strength varied linearly between 16 and 302 G / cm in 40 experiments. The diffusion time and the duration of the sinusoidal gradients were optimized for each sample. Typically, 5 ACS Paragon Plus Environment


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the diffusion time was set between 2 and 5 ms, and the half-gradient delay between 400 and 900 µs. The gradient recovery delay was set to 200 µs. A recycling delay of at least 5 s was respected between scans. DOSY spectra are generated by the DOSY module of the software NMR Notebook, using Inverse Laplace Transform (ILT) driven by maximum entropy, to build the diffusion dimension. Prior to the analyses, samples were dried at 100ºC overnight. Then 300 mg of sample were mixed for 1 h with a determined amount of liquid toluene to allow a loading of 2 molecules per unit cell. The mixture was transferred to an NMR tube along with a capillary tube contained MeOD, capped and sealed.

RESULTS & DISCUSSION Structural properties Nitrogen adsorption-desorption isotherms were obtained for the extrudates and for the commercial zeolite powder. Table 1 summarizes these textural properties, namely, specific surface area, SBET, micropore area, Smicro, and pore volume and size, Vp and Dp, respectively. For the cases where the sample did not exhibit mesopores, mercury intrusion porosimetry (MIP) was performed to assess the presence of macropores.


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Table 1. Textural properties of as-prepared samples Sample

SBET (m2/g)

Smicro (m2/g)

Vp (cm3/g)

Dp (nm)

Boeh_ext

237

-

0.36

4.5

CBV3020E

369

222 (60%)a

0.23

3.5 – 6.5

Catal_ref

337

189 (56%)a

0.25

3.5 – 6.5

Catal_PA1

344

189 (55%)a

0.25

400-1000b

Catal_PA2

348

194 (56%)a

0.32

12

apercentage

of micropore surface area in the sample; bdetermined by MIP

Extruded boehmite exhibited lower specific surface area than the commercial zeolite, with mesopores of 4.5 nm. This is in accordance with previous studies of γ-alumina (structure obtained after calcination of boehmite at 600ºC).22–26 Both specific surface area and micropore area of the catalytic bodies decreased upon extrusion that is associated to the presence of the binder.27 The BJH pore profile was performed to verify the dimensions of the pores. All the samples exhibited at least two peaks in the zone of 3.5 to 6.5 nm characteristic of intercrystalline voids between the zeolite crystals. Only Catal_PA2 presented a peak around 12 nm, while Catal_PA1 exhibited macropores sizes ranging between 400 – 1000 nm determined by MIP ( Figure 1). This can be attributed to the presence of the pore former agent added during the extrusion step, that upon calcination is burned out leaving voids in the catalytic body. Concerning Catal_PA2 sample, macropores in the range of 10 to 1000 µm are evidenced by MIP. However, the latter macropores can be attributed to the presence of cracks in the 7 ACS Paragon Plus Environment


technical body rather to macropores formed by the presence of pore former agents. Further analysis of microprobe SEM ( Figure 1) reveals the presence of macropores in the sample Catal_PA1 that are not present in Catal_ref and the presence of cracks Catal_PA2.

Catal_PA1

Catal_PA2

100 µm

100 µm

0.8

CBV3020E Catal_ref Catal_PA1 Catal_PA2

0.7 0.6 0.5 0.4 0.3 0.2 0.1

100 µm

Incremental intrusion (mL/g)

Catal_ref

dV/dlogD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.05

0.03 0.02 0.01

0 10

100 Pore diameter (Å)

Catal_ref Catal_PA1 Catal_PA2

0.04

1000

0 0.001

0.01

0.1 1 10 Pore diameter (µm)

100

Figure 1. Microprobe SEM images of as-prepared extrudates, revealing the presence of macropores in Catal_PA1 formed by the presence of the pore former agent during the extrusion process. BJH profile pores of the samples by N2 isotherms (left) and MIP (right)

Diffusion studies


1000

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Uptake measurements Uptake measurements were carried out under non-equilibrium conditions, based on the observation of macroscopic phenomena, like the change in the overall mass or pressure. One has to keep in mind that uptake measurements detect not only the adsorption within the zeolitic pores but also the sorbate permeation through the bed of crystallites, the transport resistances at the external surface of the crystallites, and the dissipation of the heat of adsorption.28 This may imply that uptake measurements are controlled by processes different from intracrystalline diffusion, and to deduce it from uptake measurements, it is mandatory to demonstrate that these aspects are negligibly small or to accurately consider them. If it is the first case, the intracrystalline diffusion controls the process and there is no significant change in the adsorbate phase concentration. The transient diffusion can then be expressed by Eq. 1, for a spherical particle of radius 𝑟:

(

)

∂𝑞 ∂2𝑞 2∂𝑞 = 𝐷𝑒𝑓𝑓 2 + ∂𝑡 𝑟 ∂𝑟 ∂𝑟

(1)

The initial and boundary conditions for a gravimetric uptake experiment are: 𝑡 < 0, 𝐶 = 𝐶0, 𝑞 = 𝑞0 (independent of 𝑟 and 𝑡) 𝑡 ≥ 0, 𝐶 = 𝐶∞, 𝑞(𝑟𝑐,𝑡) 𝑡→∞, 𝐶 = 𝐶∞, 𝑞(𝑟,𝑡)

|

∂𝑞

∂𝑟 𝑟 = 0

= 0 for any 𝑟

The solution, in terms of the uptake of sorbate by the solid, is given as follows (Eq. 2):



𝑞 ― 𝑞0 𝑞∞ ― 𝑞0

=1―

6

∞

∑

1

(

𝑒𝑥𝑝 ― 𝜋2𝑛 = 1𝑛2

𝑛2𝜋2𝐷𝑒𝑓𝑓𝑡 𝑟2

)

(2)

The linear part of the uptake curves can then be expressed by Eq. 3: 𝑞 ― 𝑞0 𝑞∞ ― 𝑞0

=

1/2

( )

6 𝐷𝑒𝑓𝑓 𝜋 𝑟2

𝑡

(3)

The uptake curves are presented in the following plots. The y-axis represents the percentage of mass of catalyst as a function of time, 𝑞∞ being the mass of catalyst saturated, and 𝑞0 the initial mass of catalyst.

q/q∞ (mg/mg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

20

40 CBV3020E

60 80 Time (min)

100

120

140

Catal_ref

Figure 2. Uptake curves for the different zeolite samples

It is noteworthy that the uptake of CBV3020E extruded with the binder (Catal_ref) is slower than pristine powder commercial zeolite (Figure 2), thus indicating that the access of the toluene molecules within the zeolite structure is hindered by the presence of the binder. After introduction of porosity in the catalyst body, the slope of the constant uptake becomes higher 10 ACS Paragon Plus Environment

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than for the reference sample, which may be explained by the presence of additional pathways in the binder. Indeed, those additional diffusion paths render easier the diffusion of the molecules throughout the catalytic body until reaching the active sites. Table 2 summarizes the effective diffusivity as well as the toluene adsorption capacity within all samples. The catalytic bodies where it was assessed the presence of meso- or macroporosity exhibit higher Deff values when compared to the reference sample, Catal_ref. Surprisingly, Catal_PA2 showed faster mass transport than Catal_PA1, as it was expected that macropores would contribute to an improved diffusion than mesopores. In fact, by looking indepth to the pore profile obtained by mercury intrusion of the samples Catal_PA2 and related SEM images, large macropores can be observed, being attributed to poor extrusion efficiency. The toluene adsorption capacity was also increased in the porous samples, indicating that the accessibility to the active sites is enhanced by either the presence of meso- or macropores. Table 2. Effective diffusivity and toluene uptake capacity of the samples obtained by the gravimetric method CBV3020E

Catal_ref

Catal_PA1

Catal_PA2

Deff (m2/s)

1.5 x 10-13

4.8 x 10-11

6.6 x 10-11

9.8 x 10-11

Capacity

69.7

68.5

72.5

83.0

(mg/g) It is important to note that transient diffusion defined in Eq. 1 depends on the particle size. Since the equivalent diameter of the extrudates was used, the two orders of magnitude difference observed over CBV3020E powder with an average particle size of 87 µm can be easily explained.



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Inverse gas chromatography The inverse gas chromatography at infinite dilution (iGC-ID) measures the effective diffusivity of a material based in gas chromatography but where the mobile phase and the stationary phase are inverted. Here, the stationary phase is replaced by the zeolite to be analyzed, while the mobile phase consists in a carrier gas (helium) and probe molecules chosen specifically for the study. The measurements are made by injecting individually and in very small quantities (infinite dilution) each probe molecule together with methane. The later acts as a reference to determine the dead time of the column (𝑡𝐶𝐻4). By subtracting this value to the retention time of the molecule, the contact time between the molecule and the stationary phase (𝑡𝑅) is obtained. By doing several injections at different gas probe flows and by determining the height equivalent to a theoretical plate [𝐻𝐸𝑇𝑃 (cm)] in each condition, the results can be represented as a function of the linear velocity of the carrier gas [𝜇 (cm/s)]. This representation corresponds to a van Deemter curve,29 and can be described by Eq. 4: 𝐻𝐸𝑇𝑃 = 𝐴 + 𝐵 µ + 𝐶𝜇

(4)

This equation contains terms corresponding to different types of diffusion: 𝐴 = Eddy diffusion; 𝐵 = longitudinal diffusion; and 𝐶 = mass-transfer resistance in the stationary phase. While using high values of carrier gas velocities, the term 𝐵 µ becomes negligible and the equation is simplified to: 𝐻𝐸𝑇𝑃 = 𝐴 + 𝐶𝜇

(5)

where 𝜇 can be calculated taking into account the diameter of the column (𝑑𝑐𝑜𝑙), the flow of the carrier gas (𝐹), the room temperature and the temperature of the column (𝑇0 and 𝑇𝑐𝑜𝑙, 12 ACS Paragon Plus Environment

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repectively), and the compressibility factor of James-Martin (𝑗), proposed by J. V. Hinshaw30 in Eq. 6:

𝜇=

( )

4𝑗𝐹 𝑇𝑐𝑜𝑙

(6)

𝜋𝑑2𝑐𝑜𝑙 𝑇0

The compressibility factor is obtained by the pressure at the entrance and the exit of the column (𝑃𝑖 and 𝑃0,respectively). 2

( 𝑃 ) ―1 𝑗 = 1.5 (𝑃 𝑃 ) ― 1 𝑃𝑖

0

(7)

3

𝑖

0

𝐻𝐸𝑇𝑃 is calculated by the length of the column (𝐿), the variance of the peak (𝜎) and the corresponding retention time (𝑡𝑟), that takes into account the asymmetry of the chromatograms (Eq. 8): 31 𝜎2 𝐻𝐸𝑇𝑃 = 𝐿 2 𝑡𝑟

(8)

Finally, with all those data in hand, the diffusion coefficient (𝐷) can be easily determined by the relation given by Eq. 9: 16𝑑2𝑚 𝑘 𝐷= 𝜋 𝐶 1+𝑘

where 𝑑𝑚 corresponds to the average size of the particles and 𝑘 =

(9)

(𝑡𝑅 ― 𝑡𝐶𝐻4)

𝑡𝐶𝐻4.

One of the constraints of this technique is the slowness of the process, with each injection analysis reaching up to 20 min. This induces long tailing with significant “uncertainties” in



the values while treating the peaks detected by the GC, especially due to the background noise associate to infinite dilution. It was then necessary to change the probe molecule toluene to benzene, to minimize the errors of the measurements. Figure 3 presents the van Deemter curves associated to the different samples.

0.31 0.29 0.27

HETP (cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.25 0.23 0.21 0.19 0.17 0.15 25

30

35

40

45

50

55

60

65

µ (cm²/s) Catal_ref

Catal_PA1

Catal_PA2

Figure 3. Comparison of van Deemter curves of benzene for different samples The reference sample exhibited an effective diffusivity value of 1.0x10-4 m2/s. At the first sight, one may notice the huge discrepancy between orders of magnitude comparing with uptake measurements, however, some facts have to be considered: (i) the use of benzene instead of toluene – the first having a smaller molecular diameter is expected to diffuse easier through the porous structure; (ii) higher analysis temperature leads to higher effective diffusivity values; (iii) the sample in the gravimetric method may suffer external mass transfer limitations, as it is present in static conditions, while in iGC this factor can be neglected due to the turbulence existence around the catalyst. Meanwhile, the samples containing hierarchical porosity exhibited higher values: 1.8 and 1.5 x10-4 m2/s for Catal_PA1 and Catal_PA2, respectively. This improvement was also observed


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by uptake measurements, thus suggesting that the presence of macropores induced a higher effective diffusion than in the presence of mesoporosity. Moreover, it is also confirmed that the presence of additional porosity in the catalytic body leads to an increase in the diffusion of toluene/benzene molecules throughout the catalyst as expected. PFG-NMR Pulsed Field Gradient – Nuclear Magnetic Resonance technique is a suitable tool to study static properties of matter and dynamic properties like self-diffusion, flow and relaxation, being extensively studied during the past decades.32–35 The precession of a proton in a magnetic field is described by the Larmor equation (Eq. 10): 𝜔0 = 𝛾𝐵0

(10)

𝛾 – gyromagnetic ratio; 𝐵0 – strength of the external static magnetic field If an additional magnetic field is applied, 𝐵𝑧 , the effective frequency becomes: 𝜔𝑒𝑓𝑓 = 𝜔0 + 𝛾𝐵𝑧

if the gradient is constant: 𝑔 =

∂𝐵𝑧 ∂𝑧

(11)

, then

𝜔𝑒𝑓𝑓 = 𝜔0 + 𝛾𝑔𝑧

(12)

The measurement of diffusion implies a pulse magnetic field gradient. This gradient causes the spin in different positions in the samples to process differently. If the spin maintains its position throughout the experiment, it will refocus completely into a spin echo by the pulse sequence. If it changes position, the refocusing will be incomplete, resulting in a decrease in the intensity of the spin echo.



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At the first pulse gradient, the phase shift is given by Eq. 13:

∅1 = ∅0 +

∫𝛾𝑔𝑧 𝑑𝑡 = ∅

0

+ 𝛾𝑔𝑧1𝛿

(13)

𝑧1- position of the spin After the second pulse: (14)

∅2 = ∅0 + 𝛾𝑔𝛿(𝑧1 ― 𝑧2)

A typical measurement of a self-diffusion consists in acquiring a set of spectra employing to different values of the field gradient strength, 𝑔 , or the length of the gradient pulse, 𝛿 , while the other parameters are kept constant. The decay of the echo intensity will be given by Eq. 15:

Ψ=

𝐼

[

(

)]

𝜏 𝛿 2 2 2 𝐼0 = 𝑒𝑥𝑝 ― 𝛾 𝛿 𝑔 𝐷𝑒𝑓𝑓 Δ ― 2 ― 3

(15)

Δ- time interval between the first gradient pulse of the two bipolar pulses; 𝜏- time between the two gradient pulses in a bipolar pulse (echo time). The effective diffusivity is deduced from the slope of the echo attenuation curves 𝑙𝑛Ψ = 𝑓(𝑔2 ), and the spectra are treated by DOSY software. Figure 4 presents the DOSY spectra of samples tested in PFG-NMR. The most surprising observation is the presence of two signals in Catal_ref and Catal_PA1, while this is not evident in the commercial zeolite (Fig. 4a). In fact, for the latter, the effective diffusivity exhibited a sole value of 16x10-9 m2/s. Although, the diffusion in zeolites determined by this technique showed to have an order of magnitude of 10-9, higher values are obtained for nanocrystals, commonly accepted to be attributed to intercrystalline mesoporosity.36,37 16 ACS Paragon Plus Environment

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a

b

c

2

-1

Deff (µm .s )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ppm

Figure 4. DOSY spectra obtained from (PFG)-NMR for a) CBV3020E; b) Catal_ref; c) Catal_PA1

Regarding the extrudate samples, the presence of the binder may reduce the microporosity, although it has its own diffusion behaviour. In fact, it is expected that the molecules diffuse differently in the binder than within the zeolite micropores, which can be proved by the spectra presented in Figure 4b: (1.2 and 7.9) x10-9 m2/s are associated to the two signals. By testing an extrudate of pure γ-alumina, a value of 0.85x10-9 m2/s was obtained, suggesting that the previous spectra are indeed detecting the diffusion in two different phases. The same behaviour is observed in Catal_PA1 sample, presenting a signal with a value of 4.0x10-9 m2/s, being attributed to the diffusion within the zeolite structure. However, the second signal presents an astonishing value of 1700x10-9 m2/s. This is characteristic of gas phase diffusion38, which can be assigned to the macropores of up to 1 µm present in this sample. Regarding the mesoporous Catal_PA2 sample, different signals can also be distinguished with values concordant with the zeolite diffusion (3.8x10-9 m2/s) and slightly higher values of 17 ACS Paragon Plus Environment


Page 18 of 25

19x10-8 m2/s. It seems obvious that the diffusion behaviour of toluene molecules is similar in mesopores, whether they are of inter- or intracrystalline nature. The presence of two mechanisms of diffusion has been previously assessed by Coasne et al. In this study, the authors attributed such behaviour to non-interconnectivity between the two types of pores, thus confirming the presence of two signals. 19 Table 3 summarizes the results of effective diffusivity obtained by the three distinct techniques described in the present manuscript. Table 3. Summary of the effective diffusivities obtained by the three techniques. Gravimetric method of toluene performed at 35ºC over extrudate samples, inverse gas chromatography of benzene performed at 350ºC over powder samples, and PFG-NMR of toluene performed at 25ºC over powder samples.

Deff (m2/s)

Gravimetric method

iGC

PFG-NMR

(x10-11)

(x10-4)

(x10-9)

CBV3020E

0.015

-

16

Catal_ref

2.2

1.0

1.2

1.9

Catal_PA1

3.6

1.8

4.0

1700

Catal_PA2

2.4

1.5

3.8

19


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The three techniques show different orders of magnitude. This is not surprising since the values of iGC are related to the analysis conditions. At higher temperatures, the effective

(

𝐸𝑎

)

diffusivity is higher, as proved by Arrhenius equation: 𝐷𝑒𝑓𝑓 = 𝐷0exp ― 𝑅𝑇 , being 𝐷0 the temperature-independent pre-exponential factor, 𝐸𝑎 the activation energy for diffusion, 𝑅 =8.314 J/mol.K, and 𝑇 temperature in Kelvin. Moreover, the probe molecule used in this case, benzene, suffers a faster diffusion in the zeolite structure due to its smaller diameter, leading to higher values of effective diffusivity. Concerning the experiments performed under similar conditions, both in temperature and with the same probe molecule, the difference of two orders of magnitude has also been observed in former studies, being attributed to different experimental technologies.17,18 For example, uptake measurements solve Fick’s law of diffusion to acquire the diffusion coefficients, while PFG-NMR uses Einstein’s equation to compute the self-diffusion coefficient. Moreover, one has to keep in mind that uptake measurements detect not only the adsorption within the zeolite pores but also the sorbate permeation through the bed of crystallites, the transport resistances at the external surface of crystallites, and the dissipation of heat adsorption. This may imply that uptake measurements are controlled by processes different from intracrystalline diffusion, leading to smaller diffusivities than PFG-NMR. Nonetheless, in both cases, Catal_PA1 and Catal_PA2 exhibited higher values than Catal_ref.

CONCLUSIONS In this study, three different techniques already known academically for the measurement of the effective diffusivity in porous solids were applied to technical bodies. In all cases, an



Page 20 of 25

increase in the effective diffusivity was observed when additional porosity (meso- or macropores) was introduced into pristine bodies. By uptake measurements and inverse gas chromatography, it was possible to assess the effective diffusivity often used in industrial reactors design. In fact, these values showed to be adequate as they can be obtained at temperatures close to common reaction temperatures and tested in their extruded forms. In addition, PFG-NMR acted as a powerful tool to accurately measure the diffusion behavior indicating two different mechanisms associated with the zeolite structure and the binder (with and without additional porosity), giving an insight on the influence of binder presence in technical zeolite bodies. Finally, the different techniques allowed reproducible measurements under different conditions, either for extruded or powdered samples, at low or high temperatures. However, it remains at the sake of the researcher to select the sound technique to measure the effective diffusivity that better suits the conditions of the sample application (reaction temperature, extruded form, dynamic system, equilibrium). AUTHOR INFORMATION Corresponding Author: *Email : [email protected] Tel. : +33368852766 ACKNOWLEDGMENT This work has been supported by Grand-Est Region and Saint-Gobain / Norpro. The authors are grateful to the French Ministry of Foreign Affairs and the Chinese Science Council for financially supporting the PHC Cai Yuanpei (N°38892 SL).


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TOC GRAPHIC

No pores

Macropores


Assessment of the Improvement of Effective Diffusivity over Technical

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