Selective Adsorption of Propene over Propane on Hierarchical Zeolite

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Applied Chemistry

Selective adsorption of propene over propane on hierarchical zeolite ZSM-58 Carolin Selzer, Anja Werner, and Stefan Kaskel Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00377 • Publication Date (Web): 28 Apr 2018 Downloaded from http://pubs.acs.org on April 29, 2018

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Industrial & Engineering Chemistry Research

Selective adsorption of propene over propane on hierarchical zeolite ZSM-58 Carolin Selzera, Anja Wernera and Stefan Kaskel*a,b a

Department of Inorganic Chemistry, Technische Universität Dresden, Bergstraße 66, 01069

Dresden, Germany b

Fraunhofer Institute for Material and Beam Technology (IWS), Winterbergstraße 28, 01277

Dresden, Germany

ABSTRACT

Hierarchical zeolite ZSM-58 was synthesized via a bottom-up synthesis method using varying amounts of carbon nanotubes (CNTs) as mesopore-generating template. Resulting ZSM-58 samples were analyzed by X-ray powder diffraction, scanning electron microscopy and physisorption experiments with argon, propane and propene. The adsorption of a binary propane/propene mixture was determined by breakthrough experiments. Furthermore, thermal response measurements using InfraSORP technology were applied to investigate the kinetics of propene adsorption on microporous vs. hierarchical ZSM-58 samples. The use of up to 5 wt% CNTs as secondary template in the synthesis gel results in formation of zeolite ZSM-58 with

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mesopore volumes up to 0.06 cm³ g-1, while maintaining a high degree of crystallinity as demonstrated via X-ray powder diffraction. A direct correlation between the relative mesopore volume (Vmeso/Vtotal) and the mass fraction of CNTs applied during zeolite synthesis was found. The enhanced adsorption rates of hierarchical zeolite materials are evidenced by InfraSORP measurements with propene. Microporous and hierarchical ZSM-58 materials exhibit comparable propene adsorption isotherms at 298 K, demonstrating that the additional mesoporosity does not affect their capacity for propene. However, it promotes propane adsorption, as attributed to the higher propane capacity observed for hierarchical ZSM-58 samples with higher mesopore volume. Nevertheless, a total separation of propane/propene mixtures could be achieved for all microporous and hierarchical materials due to kinetic hindrance of propane adsorption, as confirmed by breakthrough experiments. The highly variable synthesis method allows preparation of hierarchical ZSM-58 with tailored adsorption properties. Therefore, these materials are promising candidates for industrial gas separation processes.

KEYWORDS: zeolites, DDR, hierarchical, adsorption, propane, propene, separation

1. INTRODUCTION Propene is one of the most important primary petrochemicals, serving as precursor for the synthesis of industrially relevant compounds such as polypropene, propene oxide, isopropanol and others. The production of propene by cracking processes yields mixtures of propene and propane. Due to their similar boiling points, high energy consuming cryogenic distillation is commonly applied to separate the gases, leading to high costs of operation.1 Additionally considering the constantly growing demand for propene, enhanced energy efficiency for the


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separation process is highly desirable. Separation methods based on adsorption are promising. A material suitable for the use as adsorbent for separation purposes should have a high capacity but more important a high selectivity towards one component based on steric or kinetic effects. Over the last decades, propane and propene adsorption have been studied using various zeolites (e.g. 4A,2–6

5A,7–9

13X,2,8,10–13

DDR,14–17

Si-CHA,17–19

ITQ-3,17,18

ITQ-12,20

ITQ-3221),

silicoaluminophosphates (e.g. SAPO-3422), metal-organic frameworks (e.g. Ni-MOF-74,23 MgMOF-74,24,25 Co-MOF-74,25 CuBTC26–28) and zeolitic imidazolate frameworks (e.g. ZIF-4,29 ZIF-830,31). Recently, Maghsoudi compared these materials regarding their propene/propane equilibrium selectivity and adsorption capacity.32 His comparative study revealed that the zeolite with the framework type Deca-dodecasil-3R (DDR)33 has the highest selectivity towards propene among all literature known adsorbents. Zeolite ZSM-58, firstly synthesized by Valyocsik in 1987,34 is isostructural to the DDR framework and crystallizes in the space group R-3m with cell parameters a = b = 13.795 Å and c = 40.750 Å. The structure is built up by corner-sharing [SiO4/2]-tetrahedra, which are partially substituted by [AlO4/2]--tetrahedra and arranged into [512]dodecahedra. These dodecahedra are surface linked and form layers with an ABCABC stacking sequence. SiO4-dimers interconnect these layers, leading to formation of two new cage types in form of [435661]-decahedra and [435126183]-polyhedra. The latter are interconnected through 8rings with the dimension 0.45 x 0.36 nm, creating a two-dimensional pore system between the [512]-dodecahedron layers. The size of the pore openings lies between the critical diameter of propene (0.43 nm) and propane (0.45 nm).15 Consequently, propene is adsorbed and propane is nearly excluded, causing the high selectivity towards propene observed for zeolites with DDR topology. However, with adsorption of molecules having a critical diameter in the dimension of the pore openings, diffusivity is small and diffusional constraints arise, leading to low mass


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transfer and adsorption rates.35,36 An intensively studied way to improve adsorption kinetics is the introduction of mesopores in a microporous zeolite framework by either demetallation routes (top-down) or bottom-up approaches with a mesopore-generating template.37–43 In the presence of larger pores, propane adsorption can be enhanced leading to a lower separation efficiency. Therefore, it is beneficial to introduce a rather small mesopore volume in the crystal structure to accelerate adsorption kinetics on the one hand and suppress propane adsorption as far as possible on the other hand. Introduction of mesopores by demetallation is poorly controllable and often leads to a broad pore size distribution and high mesopore volumes. Hence, a bottom-up synthesis method with a secondary template for mesopore formation was chosen in this work to prepare ZSM-58 with hierarchical porosity, i.e. micro- and mesoporosity. Until today, hierarchical ZSM-58 has only been prepared by postsynthetic desilication procedures.44,45 In this work, we report, for the first time, the synthesis of hierarchical ZSM-58 using a bottom-up strategy. The use of carbon materials as secondary templates is popular because they can be removed easily after zeolite synthesis by calcination. Carbon nanotubes (CNTs) were proven to be a suitable template for mesopore formation and have the ability to form interconnected pores with access to the external particle surface area,46,47 which has a positive influence on the mass transfer through a pore system.48,49 In the following we describe the use of CNTs as secondary template for the synthesis of ZSM-58 with hierarchical porosity. Obtained materials were investigated regarding their propane and propene adsorption capacity. Furthermore, the influence of the additional mesoporosity on the adsorption kinetics was studied. To the best of our knowledge, this is the first report about adsorption behavior of propane and propene on hierarchical ZSM-58 materials.


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2. EXPERIMENTAL 2.1 Synthesis of the structure-directing agent methyltropinium iodide (MTI) Methyltropinium iodide was synthesized following a previously described procedure.44 First, 30.0 g tropine were dissolved in 150.0 g ethanol (absolute, VWR, 99.8 %). The solution was cooled with an ice-water bath. Then, 30.15 g methyliodide were added dropwise to the cold solution under intensive stirring. After the mixture was heated up slowly to 368 K, it was refluxed for 72 h. The MTI precipitated as white powder, was filtered off and washed with ethanol. Removal of impurities was carried out by recrystallization of MTI from a water/ethanol solution. Finally, the MTI was washed again with ethanol and dried at 353 K. 2.2 Synthesis of zeolite ZSM-58 The conventional route for the hydrothermal synthesis of zeolite ZSM-58 published by Valyocsik34 was modified, resulting in a higher yield of crystalline material. The synthesis gel was formed by mixing two previously prepared solutions (A and B). Solution A was prepared by dissolving 2.44 g of the structure-directing agent MTI in 15.0 g deionized water followed by addition of 6.2 g Ludox AS-40 (Sigma Aldrich, 40 wt% SiO2 suspension in water) and 0.016 g sodium aluminate (abcr, 92 %), leading to a Si/Al ratio of 200. Solution B consisted of 0.2 g sodium hydroxide dissolved in 4.0 g deionized water. After adding solution B to A, the mixture was stirred for 20 h. Furthermore, 5 mg ZSM-58 seeding crystals were added to assist crystallization. The resulting gel was transferred into a 50 mL teflon-lined stainless steel autoclave. The autoclave was heated up to 433 K for five days while rotating with 180 rph. After


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hydrothermal treatment, the zeolite was collected by centrifugation, washed several times with deionized water and dried at 353 K. To remove the structure-directing agent, the zeolite was calcined at 823 K (2 K/min heating rate) under static air atmosphere for 16 h. The obtained microporous zeolite ZSM-58 is labeled as Z58 in following sections. 2.3 Synthesis of mesoporous zeolite ZSM-58 The synthesis of mesoporous ZSM-58 was performed using a bottom up method with carbon nanotubes (CNTs) as mesopore-generating template. Before synthesis, CNTs (MWCNTs, NC7000 from nanocyl, 99 %) were refluxed in HNO3 (Sigma Aldrich, 65 %) for 3 h. Subsequently, they were washed with deionized water until neutrality. This oxidative treatment was applied to achieve enhanced dispersion of CNTs in water. The synthesis route for preparing hierarchical ZSM-58 was the same as described above for Z58, but a varying mass fraction of CNTs, ranging from 1 wt% to 5 wt% CNT related to the amount of SiO2 in the gel, was added to the final synthesis gel before hydrothermal treatment. The mixture of gel and CNTs was treated in an ultrasonic bath for 30 min and then stirred for 24 h at room temperature. The following synthesis, washing and calcination steps were the same as reported for Z58. The resulting zeolite samples were named ‘Z58-x’, where x corresponds to the mass fraction of CNTs in wt% in the synthesis gel. 2.3 Material characterization Powder X-ray diffraction (PXRD) patterns were obtained in reflection mode on a PANalytical X’Pert PRO diffractometer operating at 40 kV and 40 mA with monochromatic Cu Kα1 radiation and a step size of 2θ = 0.026°.


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Argon physisorption isotherms were measured on an Autosorb iQ (Quantachrome Instruments) at 87 K. Zeolite samples were activated for 24 h at 503 K under dynamic vacuum (10-3 bar) before measurement. Calculation of specific surface areas (SBET) was conducted by using the multi-point Brunauer-Emmett-Teller (BET) equation50 in consideration of a linear BET-plot and a positive C-value.51 Non-local density functional theory52–54 (NLDFT) was applied to determine pore size distributions using the kernel for argon adsorbed on zeolites with cylindrical/spherical pore shape and the adsorption branch. NLDFT calculations were further used to determine micropore volumes (Vmicro, dpore < 2 nm), total pore volumes (Vtotal, 0 < dpore ≤ 50 nm) and mesopore volumes (Vmeso = Vtotal - Vmicro). Scanning Electron Microscope (SEM) images were obtained with an SU8020 from Hitachi equipped with an X-maxN 80 (Oxford Instruments) detector with an acceleration voltage of 1.0 kV. Before recording images, zeolite samples were sputtered with gold to increase conductivity. Propane (Air Liquide, 99.95 %) and Propene (Air Liquide, 99.95 %) isotherms were measured with a BELSORP-max from BEL Japan at 298 K. Before measurement, samples were activated at 503 K for 24 h under dynamic vacuum (10-3 bar). Breakthrough curves of propane/propene mixtures were measured with a customized stainless steel fixed-bed adsorber (L = 4.5 cm, ø = 1 cm). Each experiment was performed with 1 g activated zeolite powder diluted with inert sand to achieve the same volume for each measurement. Activation of zeolite samples was the same as described above. Breakthrough measurements were conducted at room temperature with a gas flow of 80 mL min-1 consisting of 5 vol% propene, 5 vol% propane and 90 vol% nitrogen. The composition at the adsorber outlet was analyzed using an EcoSys-P mass spectrometer from European Spectrometry Systems Ltd.


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Thermal response measurements55,56 were recorded at atmospheric pressure and room temperature using an InfraSORPone instrument developed by Fraunhofer IWS Dresden. About 15 mg of an activated zeolite material (degassed at 503 K for 24 h under vacuum) were placed in a measuring cell and purged with nitrogen until the sample temperature remained constant. Subsequently, the zeolite sample was exposed to a propene flow (100 %, 80 mL min-1) at 1 bar for 400 s. The heat release caused by the exothermic adsorption of propene on the porous material leads to a temperature increase, which is monitored as a function of time by an infrared sensor located above the sample. The resulting graph is called thermal response curve. 3 RESULTS AND DISCUSSION The PXRD patterns of microporous zeolite ZSM-58 (Z58) and ZSM-58 received by templating with different amounts of carbon nanotubes (Z58-x, 1 ≤ x ≤ 5) are shown in Figure 1. They confirm phase pure formation of the DDR framework structure57 and high crystallinity for all investigated samples. The use of CNTs as mesopore-generating template in the synthesis gel had no negative influence on the crystallinity of resulting ZSM-58 materials as it has been also reported for other zeolite types.48,49


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Figure 1. Measured PXRD patterns of microporous ZSM-58 and templated ZSM-58 materials compared with the pattern of the DDR structure57. Physisorption measurements with argon at 87 K were conducted to analyze pore architectures of the purely microporous and templated zeolite samples (Figure 2). Textural properties calculated from these data are listed in Table 1. Z58 exhibits a IUPAC type I(a) isotherm with a steep increase of adsorbed argon at low relative pressure p/p0 typical for microporous materials.58 The specific micropore volume Vmicro is 0.22 cm3 g-1. All isotherms of CNT-templated zeolites show an additional argon uptake in the mesopore region, being rather small for sample Z58-1 and getting higher with increasing amount of CNTs applied during zeolite synthesis. Isotherms of these materials are combinations of IUPAC type I and IV isotherms, confirming hierarchical porosity consisting of micro- as well as mesopores.58 The introduced mesopore volume Vmeso is rather small (ranging from 0.01 cm3 g-1 to 0.06 cm3 g-1), which is important to preserve


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separation efficiency as discussed in the introduction. The relative mesopore volume Vmeso/Vtotal, serving as an indicator for the grade of hierarchy that is achieved with the templated synthesis procedure, is correlated with the mass fraction of CNTs applied during synthesis (Figure 3). The direct correlation clearly demonstrates that higher CNT amounts lead to zeolite samples with enhanced hierarchical porosity in the examined range of 1-5 wt% CNTs. For all hierarchical samples, rather small hysteresis loops are observed in the isotherm, indicating a relatively low hindrance of argon desorption, which can be attributed to the shape of the introduced pores being cylindrical as it was expected from the use of the CNT-template. Specific surface areas of about 380 m2 g-1 and micropore volumes of about 0.22 cm3 g-1 are determined for microporous as well as hierarchical porous ZSM-58 materials, which is in agreement with their high crystallinity proven by PXRD measurements. Hence, results of argon physisorption experiments clearly evidence that the introduction of mesopores in zeolite ZSM-58 by using CNTs as template during synthesis is feasible and controllable by the applied template amount.


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Figure 2. Argon physisorption isotherms (offset 5, 30, 50, 70 cm3 g-1) of microporous and hierarchical ZSM-58 materials measured at 87 K. Table 1. Textural properties calculated from argon isotherms at 87 K. SBETa /

Vmicrob /

Vmesoc /

Vtotald /

m2 g-1

cm3 g-1

cm3 g-1

cm3 g-1

Z58

385

0.22

-

0.22

0.00

Z58-1

380

0.22

0.01

0.23

0.04

Z58-2

378

0.22

0.02

0.24

0.08

Z58-3

376

0.20

0.03

0.23

0.13

Z58-5

380

0.20

0.06

0.26

0.23

Sample

a

multi-point BET method

b

c

Vmeso/Vtotal

NLDFT, dpore< 2 nm

Vmeso = Vtotal - Vmicro

d

NLDFT, 0 < dpore ≤ 50 nm

Figure 3. Correlation of relative mesopore volume Vmeso/Vtotal and CNT mass fraction.


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The non-local density functional theory (NLDFT) was used to calculate pore size distributions plotted in Figure 4. It is shown that mesopore volumes (with a pore diameter of 2 nm ≤ dpore ≤ 50 nm) are small relative to micropore volumes (dpore < 2 nm) generated by the ZSM-58 architectures. The majority of the introduced mesopores has a pore diameter between 3 and 20 nm. With higher amounts of CNTs, the mesopore volume increased. Furthermore, the amount of pores with sizes larger than 20 nm also increased because agglomeration of CNTs in an aqueous gel is promoted with increasing CNT concentration, creating larger voids during zeolite synthesis.

Figure 4. NLDFT pore size distribution of microporous and hierarchical ZSM-58 materials. SEM images (Figure 5) were recorded to acquire information about size and surface morphology of the ZSM-58 crystals. The microporous sample Z58 exhibits crystals with a size of about 3 µm and a smooth surface. In contrast, hierarchical zeolite particles with a size between 4 and 5 µm appear as intergrown crystallites with sharp edges but also a smooth surface. The images reveal that CNTs inhibit uniform growth of the ZSM-58 crystals, resulting in intergrowths. SEM


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images of hierarchical Z58-2 and Z58-3 are exemplarily shown in Figure 5 but the described morphology also applies to the other hierarchical ZSM-58 samples.

Figure 5. SEM images of samples Z58 (left), Z58-2 (middle) and Z58-3 (right). The aim of introducing larger mesopores in the microporous zeolite structure is an enhancement of the adsorption rate by increasing the effective diffusivity of molecules through the pore system. The InfraSORP technology is a rapid measurement technique providing initial insights into mass transfer and adsorption properties of various porous materials such as activated carbons, metal-organic frameworks and zeolites.55,56,59–62 It is based on the detection of temperature changes during adsorption of a test gas on a porous sample. The area under the resulting thermal response curve (temperature T vs. time t) serves as a measure for the adsorption capacity of the measured material,59 while its shape reflects the adsorption kinetics59,60. In this work, propene (100 %) was used as test gas. Since propene capacity was shown to be similar for all investigated ZSM-58 materials and the same sample mass was used for each measurement, the integrals of the thermal response curve (supporting information, Table S1) are of the same order of magnitude. Hence, the peak temperature of the thermal response only depends on the amount of heat release at a point in time. An enhanced diffusivity of propene molecules through the crystal structure leads to a faster adsorption rate and accordingly a higher amount of total


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adsorption heat released at one point in time, resulting in an increased peak temperature. In Figure 6, thermal response curves of microporous and hierarchical ZSM-58 samples are plotted. The instantaneous temperature increase ∆T caused by adsorption of propene on each CNTtemplated zeolite sample is higher compared to ∆T observed for microporous zeolite Z58. Because of similar textural properties, ∆T of samples Z58 and Z58-1 are quite similar. In Figure 7, the temperature increase ∆T is correlated with the relative mesopore volume Vmeso/Vtotal of the samples. With enhanced hierarchy (i.e. higher Vmeso/Vtotal), ∆T significantly increased, which is attributed to a more effective diffusivity of propene molecules through the ZSM-58 structure and higher adsorption rates. This is also reflected in the time ∆t needed to cool down again to the baseline temperature. The temperature decrease is caused by the continuous gas flow through the measurement cell. As depicted in Figure 7, ∆t decreases with higher Vmeso/Vtotal, demonstrating that samples with enhanced hierarchy cool down faster even though their peak temperature is higher compared to samples with fewer mesopores. Again, this is attributed to the hierarchical porosity, enabling unrestricted gas transfer. The results of the thermal response measurements also indicate that CNT-templates probably produce an interconnected pore system with access to the external surface of ZSM-58 crystals, since the differences in ∆T and ∆t are assumed to be significantly smaller if the introduced pores were isolated inside the zeolite crystals.


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Figure 6. Thermal response curves observed for propene adsorption on microporous and hierarchical ZSM-58 samples at 1 bar and 298 K.

Figure 7. Correlation of relative mesopore volume Vmeso/Vtotal with temperature increase ∆T and the time ∆t that is needed to cool down to baseline temperature. DDR structured zeolites are known as suitable materials for separation of propene/propane gas mixtures but, until today, there are no studies on hierarchical ZSM-58 regarding this topic. To investigate the influence of introduced mesopores on the adsorption behavior of ZSM-58 materials, propene and propane isotherms were measured at 298 K and plotted in Figure 8. All


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materials have propene capacities close to 37 cm3 g-1. Hence, the small mesopore volumes have no negative influence on the propene capacity of hierarchical ZSM-58 materials. This positive characteristic is in accordance with argon physisorption measurements showing similar micropore volumes for all ZSM-58 samples (Table 1). However, the capacity for propane is changed by additional mesopores, as it is shown in Figure 8 (bottom). As a consequence of the introduced larger pores, adsorption of propane is less inhibited in comparison to purely microporous ZSM-58, resulting in an increase of propane capacity from 4 cm3 g-1 (Z58) to 11 cm3 g-1 (Z58-5) with higher mesopore volume. It should be mentioned that the presented propane isotherms could not be fully equilibrated. Based on its critical diameter (0.45 nm15), propane is restricted from entering the narrow pore openings of the ZSM-58 structure. As a consequence, propane diffusion through the ZSM-58 structure and adsorption on the inner surface is too slow to reach equilibrium state within a reasonable time. Therefore, the propane capacity after ideal equilibration of hierarchical ZSM-58 could be in principle higher than described above. The corresponding desorption isotherms of propane are available in the supporting information (Figure S1). Nevertheless, Zhu et al. reported that the mass uptake of a binary propane/propene mixture on DDR structured zeolite is the same as that of pure propene at similar partial pressure.14 They concluded that the presence of propane hardly affects propene adsorption. Furthermore, the kinetically hindered adsorption of propane renders ZSM-58 materials ideal for kinetic separation processes, as their dynamic propane uptake is negligible. Hence, even though hierarchical ZSM-58 materials exhibit higher propane capacities, they may be beneficial for the separation of propene and propane gas mixtures because of their enhanced adsorption rates and preserved propene capacity.


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Figure 8. Propene isotherms (top) and propane isotherms (bottom, not equilibrated) of microporous and hierarchical ZSM-58 samples measured at 298 K. Adsorption behavior of a binary propane/propene gas mixture can differ from the single component adsorption. Therefore, breakthrough curves were recorded to investigate the adsorption behavior of a propane/propene gas mixture (1:1) on microporous and hierarchical ZSM-58. Breakthrough profiles of propane and propene, which are shown in Figure 9a-e, confirm the separation of both gases for all microporous and hierarchical samples. Higher propane capacities of hierarchical ZSM-58 materials have no negative influence on the separation efficiency, because of very slow kinetics for propane adsorption. With rising mesopore volume of the zeolite samples, increasing breakthrough times (tb) can be observed


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(Fig. 9f). Since propene capacities are similar for all samples, this behavior is attributable to improved kinetics and accordingly smaller mass transfer zones for hierarchical ZSM-58. Breakthrough times of samples Z58-3 and Z58-5 are similar, indicating that a further increase in mesopore volume did not affect breakthrough times. The improved kinetics of hierarchical materials are further reflected in steeper breakthrough profiles compared to microporous ZSM-58. The time between the breakthrough point and reaching 50 % of the initial propene concentration (ts,50%) was used to quantify the steepness of propene profiles. With steeper profiles, the time ts,50% gets shorter. As it is shown in Figure 9f, the time ts,50% is decreasing with increasing mesopore volume of the zeolite materials. These results are in good agreement with the kinetic investigations by thermal response measurements. Again, there is no difference between hierarchical sample Z58-3 and Z58-5, like it was observed for the breakthrough times. Accordingly, a mesopore volume of 0.03 cm3 g-1 is enough to improve mass transport through the crystal structure of ZSM-58 sufficiently. A further increase of the mesopore volume has no further benefit for the separation performance. Propane breakthrough curves show, that propane is adsorbed on ZSM-58 materials, but is pushed out by preferential propene adsorption, leading to a roll-up effect in propane profiles. This effect becomes more pronounced with rising mesopore volume and corresponding higher propane capacities of hierarchical materials.


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Figure 9. Propane/propene breakthrough curves at 298 K (a-e) and correlation of CNT mass fraction with tb and ts, 50% (f). 4 CONCLUSION Hierarchical zeolite ZSM-58 was received using a bottom-up synthesis method with CNTs as secondary template, leading to highly crystalline zeolite materials with additional mesopores with pore diameters mainly in the range from 3 to 20 nm. The introduced mesopore volume is rather small, which is important to preserve the outstanding propene/propane separation efficiency of ZSM-58. The specific mesopore volume increased with higher amounts of CNT template applied during synthesis up to 0.06 cm3 g-1 observed for 5 wt% CNTs. A direct correlation between the relative mesopore volume Vmeso/Vtotal and the applied CNT amount was found, clearly demonstrating that the use of higher template amounts leads to enhanced hierarchical porosity. PXRD patterns point out that CNTs have no negative effect on the crystallinity of the resulting hierarchical zeolite materials, which is in agreement with similar micropore volumes observed for all investigated samples. Nevertheless, the presence of the template significantly affected the crystal morphology as shown by SEM images. Hierarchical samples consist of highly intergrown crystallites whereas microporous ZSM-58 appear as uniform single crystals. The additional mesoporosity accelerates mass transfer through the adsorbent, leading to enhanced adsorption rates, as evidenced by thermal response measurements using InfraSORP technology and propene as test gas. This positive effect becomes even more striking with enhanced hierarchy, as it was proven by the correlation of the relative mesopore volume Vmeso/Vtotal with the thermal response. The significant improved mass transfer indicates that introduced mesopores are interconnected and have access to the external zeolite surface. Furthermore, the influence of additional mesopores on propane and propene adsorption


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properties was investigated by physisorption measurements. It was shown that the propene capacity is similar for both microporous and hierarchical ZSM-58 zeolites, implying small mesopore volumes do not affect propene capacity negatively. However, larger pores promote propane adsorption, as could be seen from the higher propane capacity observed with increasing mesopore volume. Nevertheless, the kinetics of propane adsorption are very slow in comparison to propene adsorption. Therefore, ZSM-58 materials can be used for a kinetic separation process, higher propane capacities for hierarchical ZSM-58 are negligible. Propane-propene breakthrough measurements clearly reveal a highly efficient separation of both gases for microporous and hierarchical ZSM-58 materials. In addition, propene breakthrough profiles confirm better kinetics of hierarchical samples which is in agreement with thermal response measurements. The improved mass transfer zone and enhanced breakthrough time for hierarchical ZSM-58 materials renders them as promising candidates for industrial separation of propane/propene gas mixtures.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. -

Propane isotherms (adsorption and desorption) of microporous and hierarchical ZSM-58 materials

-

Peak areas of thermal response curves

AUTHOR INFORMATION Corresponding author


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*E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors thank Sven Grätz for physisorption measurements with argon and Sebastian Ehrling for recording SEM-images. REFERENCES (1) (2) (3)

(4) (5)

(6) (7) (8) (9) (10)

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(13)

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List of figure captions Figure 1. Measured PXRD patterns of microporous ZSM-58 and templated ZSM-58 materials compared with the pattern of the DDR structure.57 Figure 2. Argon physisorption isotherms (offset 5, 30, 50, 70 cm3 g-1) of microporous and hierarchical ZSM-58 materials measured at 87 K. Figure 3. Correlation of relative mesopore volume Vmeso/Vtotal and CNT mass fraction. Figure 4. NLDFT pore size distribution of microporous and hierarchical ZSM-58 materials. Figure 5. SEM images of samples Z58 (left), Z58-2 (middle) and Z58-3 (right). Figure 6. Thermal response curves observed for propene adsorption on microporous and hierarchical ZSM-58 samples at 1 bar and 298 K. Figure 7. Correlation of relative mesopore volume Vmeso/Vtotal with temperature increase ∆T and the time ∆t that is needed to cool down to baseline temperature. Figure 8. Propene isotherms (top) and propane isotherms (bottom, not equilibrated) of microporous and hierarchical ZSM-58 samples measured at 298 K. Figure 9. Propane/propene breakthrough curves at 298 K (a-e) and correlation of CNT mass fraction with tb and ts, 50% (f).


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Selective Adsorption of Propene over Propane on Hierarchical Zeolite

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