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Hierarchical Ga-MFI Catalysts for Propane Dehydrogenation Wun-gwi Kim,† Jungseob So,† Seung-Won Choi,† Yujun Liu,‡ Ravindra S. Dixit,‡ Carsten Sievers,† David S. Sholl,† Sankar Nair,*,† and Christopher W. Jones*,† †
School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100, United States Engineering & Process Sciences, The Dow Chemical Company, Freeport, Texas 77541, United States
‡
S Supporting Information *
ABSTRACT: We report the synthesis, characterization, and enhanced propane dehydrogenation properties of hierarchical Ga-MFI zeolite catalysts synthesized by two different methods: (i) repetitive branching and (ii) utilization of a long chain alkyl SDA. Structural, compositional, and morphological characterizations confirm that the Ga-MFI catalyst materials have hierarchical structures including micropores and mesopores in a single particle and show that Si/Ga ratios comparable to bulk Ga-MFI catalysts can be obtained. Acid site analysis using NH3-TPD and pyridine adsorption followed by in situ IR spectroscopy allowed quantification of the number and types of acid sites present and show that the hierarchical catalysts have considerably higher Lewis acid site concentrations than the bulk catalysts. The hierarchical Ga-MFI catalysts show superior PDH performance (at 600 °C) compared to bulk Ga-MFI catalysts. Propane conversion rates are increased 2−6-fold and propylene selectivities are 10−100% higher. We also examine the effects of synthesis variationssuch as addition of 3-mercaptopropyltrimethoxysilane during synthesis and H+ ionexchangeand find that both these steps have a beneficial effect on PDH properties. Calculations suggest that PDH in both the bulk and the hierarchical Ga-MFI is not diffusion-limited. Therefore, the superior performance of hierarchical Ga-MFI is due to intrinsically higher activity and selectivity. Potential reasons for this behavior are outlined. The present findings showing enhancement of PDH in hierarchical Ga-MFI catalysts suggests that the utility of the hierarchical zeolites is not limited to diffusion-limited reactions involving large, bulky molecules and that they may be useful in more diverse applications involving gas-phase reactions with small molecules.
■
INTRODUCTION
increased catalyst lifetime by providing faster transport paths for removal of products and byproducts.9,10 Synthesis methods for hierarchical zeolites can be classified as “top-down” and “bottom-up” processes.8 Top-down processes include postsynthesis dealumination and desilication by chemical hydrolysis to create mesopores in the crystal structure. However, this route requires relatively harsh conditions and carries the risk of losing microporosity due to partial amorphization of the crystal structure during silica leaching from the zeolite.11−13 Bottom-up processes include several different routes. One involves the use of polyquaternary ammonium surfactants that can simultaneously template the formation of both micropores and mesopores.14,15 Another method involves the assembly of microporous zeolite nanocrystals into hierarchical materials that contain interstitial mesopores.14,15 A third methodwhich is of interest in this workuses long-chain alkylammonium templates (also referred to as structure directing agents, SDAs) and the repetitive branching method of microporous slabs.16 Among
Zeolites are microporous aluminosilicate materials that are widely used as molecular sieves,1 catalysts,2 and adsorbents.3 Zeolite catalysts can provide different types of catalytic sites, e.g., protonic acid sites formed due to the unbalanced charge of heteroatoms (often Al) in the structure or metal oxide sites (such as Ga, Fe, Ti, Sn) in the form of either structural heteroatoms or extraframework clusters. These characteristics, combined with exceptional thermal and mechanical stability, allow their use in a number of industrial catalytic reaction systems.4 However, the subnanometer (2 nm) in the same particle.7,8 The introduction of mesopores improves accessibility of reactants to catalytic sites, leading to higher activity and capability for processing molecules that are larger than the micropores, as well as an © 2017 American Chemical Society
Received: April 16, 2017 Revised: August 1, 2017 Published: August 2, 2017 7213
DOI: 10.1021/acs.chemmater.7b01566 Chem. Mater. 2017, 29, 7213−7222
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Chemistry of Materials
a homogeneous solution. Then, 0.09 g of gallium nitrate (Ga(NO3)3· xH2O (99.9%, Sigma-Aldrich) was added, followed by 26.6 g of deionized (DI) H2O. After 3 h of stirring, 0.07 g of 3-mercaptopropyltrimethoxysilane (MPS, 95%, Sigma-Aldrich) was added to the solution. To obtain a synthesis gel with a Si/Ga ratio of ∼100, the molar composition of the reactants was 1 SiO2:0.01 Ga(NO3)3:0.32 TPAOH:45.4 H2O:0.01 MPS. To obtain Si/Ga ∼ 50, the molar composition of Ga(NO3)3 was increased to 0.02. Reaction gels were also prepared without MPS addition, all other concentrations being unchanged. To obtain the Na+ form of Ga-MFI rather than the H+ form, sodium hydroxide (NaOH, Sigma-Aldrich) was added with the same molar concentration as Ga(NO3)3. The reaction gels were aged for 1 day and then transferred to Teflon-lined autoclaves, followed by 4 days of hydrothermal reaction in an oven at 150 °C without rotation. The synthesized zeolite crystals were centrifuged, dried overnight at 80 °C, and calcined in air at 550 °C for 10 h. Hierarchical Ga-MFI by Repetitive Branching (Self Pillared Pentasil, SPP Ga-MFI). SPP Ga-MFI32 was prepared by repetitive crystal twinning at 120 °C with a tetrabutylammonium hydroxide (TBAOH) SDA. The bulkier size of TBAOH (relative to TPAOH) and the lower reaction temperature have been reported to result in continuous orthogonal branching of the growing MFI slab-like crystals leading to the formation of a hierarchical morphology.32 Typically, 0.2 g of gallium nitrate was added to 16.6 g of TEOS. Then, 15.6 g of TBAOH (40% in H2O, Sigma-Aldrich) was added dropwise followed by 5.1 g of DI water and 0.16 g of MPS. The molar composition of the reaction gel was 1 SiO2:0.3 TBAOH:0.01 Ga nitrate:10 H2O:0.01 MPS. Reaction gels were also prepared without MPS addition, all other concentrations being unchanged. The gels were stirred for 1 day, transferred to a Teflon-lined autoclave, and hydrothermally reacted for 5 days at 120 °C. The zeolite crystals were centrifuged at 12 000 rpm, dried overnight at 80 °C, and calcined in air at 550 °C for 10 h. Hierarchical Ga-MFI with Layered Form (Layered Ga-MFI). Layered Ga-MFI was synthesized using a long-chain diquaternary ammonium SDA C22−N+−C6−N+−C6Br2 that can form lamellar micelles during hydrothermal reaction.16 The SDA was synthesized by two steps of organic SN2 reactions, as described in previous literature.16 The solid SDA (1.6 g) was dissolved in 33.47 g of DI H2O followed by addition of 0.56 g of NaOH, 0.4 g of H2SO4, 0.12 g of Ga(NO3)3, and then dropwise addition of 4.86 g of TEOS. The final molar composition of the reaction gel was 30 Na2O:100 SiO2:10 C22−6−6Br2:2 Ga(NO3)3:8000 H2O:2 MPS. Reaction gels were also prepared without MPS addition, all other concentrations being unchanged. The gels were stirred for 1 day, transferred to a Teflonlined autoclave, and hydrothermally reacted for 5 days at 150 °C. The zeolite crystals were centrifuged at 12 000 rpm, dried overnight at 80 °C, and calcined in air at 550 °C for 10 h. Proton Exchange. The Na+ forms of bulk Ga-MFI and layered Ga-MFI were exchanged to their H+ form. Typically, 1 g of the zeolite powder was dispersed in 100 mL of 1 M NH4NO3 solution and stirred at 80 °C for 3 h. These steps were repeated 3 times, followed by a final calcination at 550 °C for 4 h. Characterization. Powder X-ray diffraction (XRD) patterns were measured with a PANalytical XPert PRO diffractometer using Cu Kα radiation at a scan step size of 0.0167° in a 2θ range of 5−50°. Scanning electron microscopy (SEM) was conducted with a Hitachi SU-8010 operating at 1 kV. Samples were sputter-coated with carbon to prevent surface charging effects. High resolution TEM (HTEM) was performed on a FEI Tecnai G2 F30 TEM at 300 kV. For sample preparation, the powder material was dispersed in water, sonicated for 5 min, and transferred to a lacey carbon grid followed by drying in ambient air. N2 physisorption isotherms were obtained at 77 K with a Micrometrics Tristar II. The samples were degassed at 150 °C for 12 h. The external surface area was calculated by the t-plot method, and the mesopore volume was estimated using the BJH model. The DFT model was used to obtain the textural characteristics from the isotherms. Elemental analysis of Ga and Si was performed using inductively coupled plasma−optical emission spectroscopy (ICP-OES) by ALS Environmental (Tucson, AZ). Solid-state 29Si NMR and 71GaNMR spectra were measured with a Bruker Avance III 400
several examples of catalysts prepared by the latter method,19−23 a hierarchical Sn-MFI material has been applied to the isomerization of glucose and lactose, as well as the Baeyer−Villiger oxidation of cyclic ketones. The Sn atoms incorporated in the hierarchical MFI zeolite act as Lewis acid sites.17−19 The hierarchical materials also showed improved catalytic performance compared to a conventional microporous (“bulk”) Sn-MFI by overcoming diffusion limitations of glucose in the MFI micropores. Although most reports on hierarchical zeolite catalysts focus on their capability to handle larger reactants, it is also interesting to investigate their use in small-molecule catalysis wherein diffusion limitations are not the primary concern. In this case, one is mainly interested in the question of whether the redistribution of catalytic sites on the zeolitic slab-like surfaces of a hierarchical material (as opposed to a “bulk” microporous zeolite material) leads to changes in reactivity that affect conversion, selectivity, or deactivation. Here, we investigate these issues in the context of propylene production by propane dehydrogenation (PDH), which is receiving a large amount of recent interest due to the abundant new supplies of propane from shale gas.20 There are several recent works reporting new PDH catalysts with improved stability, activity, and selectivity.21−25 Among conventional microporous zeolite materials, MWW-type zeolites have been used as supports for gallium oxide PDH catalysts,26 and the redox properties of HFe-MFI catalysts have also been investigated for PDH.27,28 However, hierarchical zeolites have so far only been used as supports for metallic or metal oxide PDH catalysts. For example, hierarchical MFI (in the aluminosilicate form, ZSM-5) has been used as a support to impregnate PtSnNa catalysts with improved PDH conversion and selectivity and lower deactivation compared to bulk ZSM-5 supports.29 Hierarchical H-ZSM-5 has also been used as a support for impregnation of Ga2O3 for oxidative dehydrogenation reactions.30 The above reports are encouraging for the application of hierarchical zeolites to PDH and related reactions. In this work, we focus on direct incorporation of gallium (Ga) sites during the formation of silica MFI hierarchical zeolites. This route has two hypothesized advantages. First, it allows the creation of more well-defined catalytic sites in situ during synthesis, rather than clusters resulting from postsynthetic impregnation/deposition. Second, it eliminates the strong Brønsted acid sites that result from incorporation of heteroatoms such as Al in aluminosilicate ZSM-5, thereby potentially suppressing side reactions that lead to aromatization and coke formation. Two different synthesis methods, (i) repetitive branching and (ii) use of long-chain alkylammonium SDAs, have been modified for the synthesis of well-defined hierarchical Ga-MFI catalysts. To our knowledge, this is the first direct synthesis and demonstration of hierarchical Ga-MFI catalysts for PDH. We also report detailed characterization and acid site analysis of the hierarchical GaMFI materials and their comparison to bulk Ga-MFI that was also synthesized using the 3-mercaptopropylsilane route.31
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EXPERIMENTAL METHODS
Synthesis of Bulk Ga-MFI and Hierarchical Ga-MFI. Bulk GaMFI and hierarchical Ga-MFI were synthesized by hydrothermal reactions. Detailed synthesis procedures for each type of material are described as follows. Bulk Ga-MFI. A total 7.7 g of tetraethylorthosilicate (TEOS, 98% reagent grade, Sigma-Aldrich) and 6.0 g of tetrapropylammonium hydroxide (TPAOH, 40% in H2O, Sigma-Aldrich) was mixed to make 7214
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Chemistry of Materials spectrometer at a sample spinning rate of 10 kHz and a time delay of 10 s. The reference chemicals for 29Si NMR and 71Ga-NMR spectra were 3-(trimethylsilyl)-1-propanesulfonic acid and aqueous Ga(NO3)3 solution, respectively. The strength and concentration of total acid sites of catalysts were determined by NH3 temperature-programmed desorption (NH3-TPD) using a Micromeritics Autochem II. Typically, 100 mg of powder sample was placed in a U-shaped fixed-bed reactor, preheated at 530 °C for 1 h, and cooled to 100 °C. Then NH3 gas was injected to saturate the sample, followed by introduction of a He carrier gas to purge the excess NH3. After stabilization for 1 h, the sample was heated to 600 °C at a ramping rate of 10 °C/min. The desorption profile of NH3 in the temperature range of 100−600 °C was measured using a thermal conductivity detector (TCD). The NH3-TPD profiles were deconvoluted by peak-fitting to quantify the numbers of acid sites at different temperatures. The concentrations of Brønsted and Lewis acid sites were measured by Fourier transform infrared spectroscopy (FTIR) of pyridine-loaded samples using a Thermo Scientific Nicolet 8700 spectrometer. The powder sample was fabricated into a 1 cm size pellet and loaded into a sealed cell for in situ transmission to the FTIR cell. The sample was activated at 500 °C under vacuum for 6 h. A background spectrum was measured after cooling down to 150 °C. Pyridine was then dosed for 1 h to reach equilibrium, and the physisorbed pyridine was removed overnight under vacuum. The FTIR spectra of the pyridine-loaded samples were measured at evacuation temperatures of 150, 250, 350, and 450 °C. Propane Dehydrogenation (PDH). PDH is an equilibrium limited, highly endothermic reaction (ΔH = 120 kJ/mol) that requires a high-temperature reaction setup.33 PDH was conducted at 600 °C and 1 atm total pressure. All the Ga-MFI catalyst samples were pelletized, crushed, and sieved to obtain catalyst particles in the size range of 200−400 μm. This helps to maintain similar pressure drops for each sample and remove any effects of differing external mass transfer resistances. The sieved catalyst was placed in a quartz tube fixed-bed reactor and supported with quartz wool at the top and bottom. The reactor was placed in fluidized bath (FB-08, Bibby Scientific) and heated to 600 °C under N2 flow of 19 mL STP/min. A feed stream of 5 mol % C3H8/95 mol % N2 was then introduced at 20 mL STP/min. The product stream was continuously analyzed by an online GC (Shimadzu GC2014). A flame ionization detector (FID) and a thermal conductivity detector (TCD) were used to simultaneously detect hydrocarbon products and H2, respectively. The GC was calibrated with a 5% C3H8/95% N2 stream before and after PDH. The conversion and selectivity were calculated using the following equations:
C3H 8conversion(%) ≡
C3H8in − C3H8out C3H8in
× 100
Table 1. Bulk and Hierarchical Ga-MFI Materials Synthesizeda Si/Ga ratio Ga-MFI material
(gel)
(actual: ICP)
H+ exchange
MPS addition
#1: bulk Ga-MFI #2: bulk Ga-MFI-MPS #3: bulk H-Ga-MFI #4: bulk H-Ga-MFI-MPS #5: SPP Ga-MFI #6: SPP Ga-MFI-MPS #7: layered H-Ga-MFI #8: layered H-Ga-MFIMPS
100 100 50 50 100 100 50 50
124 123 61 69 128 140 75 72
no no yes yes no no yes yes
no yes no yes no yes no yes
a Materials #3, #4, #7, and #8 were prepared from gels containing NaOH and required proton exchange before use as catalysts.
Figure 1 shows SEM images of the synthesized bulk and hierarchical Ga-MFI materials. The particle sizes of the bulk
(1)
out
Product selectivity (%) ≡
■
Product × 100 C3H8 in − C3H8out
(2)
RESULTS AND DISCUSSION Structure, Morphology, and Textural Properties. Table 1 shows the eight materials (bulk and hierarchical Ga-MFI) synthesized for the present study. The Si/Ga ratios in the reaction gels and the actual Si/Ga ratios obtained in the crystallized materials are shown. In general, the actual Si/Ga ratios are higher than in the gel. The incorporation of Ga is less favorable than that of other heteroatoms like Al,34 and the addition of higher concentrations of Ga in the reaction gel resulted in amorphous products. Nevertheless, the materials in Table 1 could be successfully crystallized and contain significant amounts of Ga. While we were unable to synthesize SPP GaMFI with Si/Ga ratios less than 128, the layered Ga-MFI materials (#7 and #8) reached lower Si/Ga ratios of ∼75. Materials prepared with and without MPS addition did not show significant differences in the Si/Ga ratio.
Figure 1. (a−h) SEM images of the Ga-MFI materials #1−#8, respectively, from Table 1. Scale bars are 500 nm (a−f), 1 μm (g), and 3 μm (h). Scale bars on insets in e and f are 100 nm.
Ga-MFI materials (Figure 1a−d) are similar and range from 200 to 300 nm. The surfaces are rougher at higher Ga content, probably due to irregular twinning of crystallites in the presence of Ga heteroatoms. The SPP Ga-MFI materials (Figure 1e,f) show uniform particle sizes of roughly 100 nm. Unlike the bulk Ga-MFI materials, their surfaces are highly roughened due to the repetitive twinning (“self-pillaring”) of the MFI crystallite 7215
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Chemistry of Materials slabs during hydrothermal reaction. The mesoporosity of the particles is visible in the higher-magnification insets shown in (Figure 1e,f). The SEM images of layered Ga-MFI also show highly roughened particle surfaces due to twinning of the layered slabs. The average particle size of the material made without MPS (Figure 1g) is around 400 nm, and it increases to ∼1 μm when the synthesis gel includes MPS (Figure 1h). Figure 2 shows the powder XRD patterns of the bulk and hierarchical Ga-MFI materials. All the patterns show the
Figure 2. Powder XRD patterns of the eight Ga-MFI materials from Table 1. The patterns are vertically offset for clarity.
Figure 3. (a) Low-magnification TEM and (b) high-magnification HTEM images of SPP-Ga-MFI (material #6 in Table 1).
characteristic features of the MFI crystal structure. However, the XRD patterns of the hierarchical materials (#5−#8) reveal a number of missing or very weak peaks in relation to the bulk MFI as well-known from previous reports.17,18 This is because the MFI slabs comprising the hierarchical (SPP and layered) MFI materials possess long-range crystallinity along only two crystallographic directions, whereas growth along the third direction is prevented either by repetitive branching (SPP MFI) or by the long-chain alkyl groups of the SDA (layered MFI). Direct visualization of the pore structure of the hierarchical materials is carried out by HTEM imaging (Figures 3 and 4). In the case of SPP-Ga-MFI, the low-magnification TEM image (Figure 3a) shows well-dispersed particles with highly roughened surfaces. The high-magnification HTEM image (Figure 3b) shows both the MFI micropores as well as the mesoporous spaces between the MFI nanoslab that comprise the material. In the case of layered Ga-MFI, the lowmagnification image (Figure 4a) also shows particles with highly roughened surfaces, comprised of thin nanoslabs with large lateral dimensions of several hundred nanometers. The high-magnification HTEM image (Figure 4b) shows the microporosity of the nanoslabs (which are in the range of 2− 10 nm in thickness) as well as the mesoporous spaces existing between the nanoslabs. It is therefore clear that Ga incorporation in the structure of the SPP and layered MFI materials does not significantly affect their unique hierarchical morphologies. Figure 5 compares the textural properties of the SPP and layered Ga-MFI materials (#6 and #7) with those of bulk GaMFI materials with similar Si/Ga ratios (#2 and #3), as obtained from N2 physisorption. Table 2 summarizes the textural properties of all eight materials. In Figure 5a, all the materials show similar uptake curves at low partial pressure
Figure 4. (a) Low-magnification TEM and (b) high-magnification HTEM images of Layered Ga-MFI (material #7 in Table 1).
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Table 2. Surface Areas (total BET and external) and Micropore/Mesopore Volumes of Bulk Ga-MFI and Hierarchical Ga-MFI Materials, Obtained from N2 Physisorption Isotherms Ga-MFI materials
BET SA (m2 g−1)
external SA (m2 g−1)
micropore vol. (cm3 g−1)
mesopore vol. (cm3 g−1)
#1: bulk Ga-MFI #2: bulk GaMFI-MPS #3: bulk H-GaMFI #4: bulk H-GaMFI-MPS #5: SPP Ga-MFI #6: SPP GaMFI-MPS #7: layered HGa-MFI #8: layered HGa-MFI-MPS
396 402
125 116
0.13 0.14
0.087 0.075
409
135
0.13
0.087
402
130
0.13
0.090
659 608
400 367
0.14 0.12
0.83 0.59
541
316
0.11
0.64
436
194
0.12
0.57
differences. Layered Ga-MFI shows considerable hysteresis, thereby suggesting more interconnected mesopore spaces created by the long-chain SDA. SPP Ga-MFI shows little hysteresis, likely due to its “ink-bottle” shaped dead-end mesopores35 created by repetitive branching (twinning). The DFT-derived cumulative pore volumes and pore size distributions (PSD) are shown in Figure 5b and Figure 5c, respectively. The combined mesoporosity and microporosity of the hierarchical materials result in much greater pore volumes than in the exclusively microporous Ga-MFI. While bulk GaMFI shows negligible mesoporosity, the mesopores in both types of hierarchical Ga-MFI materials are mostly in the size range of 2−10 nm. However, both materials also show a significant fraction of mesopores >10 nm in size (Figure 5c). The total BET and external surface areas, as well as the micropore and mesopore volumes of all eight materials, are summarized in Table 2. The hierarchical Ga-MFI materials have higher total BET surface areas compared to bulk Ga-MFI, mainly due to the much higher contribution from the external surface area. As expected, the micropore volumes of all the MFI materials are very similar and characteristic of the MFI structure. However, the mesopore volumes of the hierarchical Ga-MFI materials are about an order of magnitude higher than those of the bulk Ga-MFI materials. The coordination environments of Si and Ga species are analyzed by 29Si NMR and 71Ga NMR (Figure 6) spectroscopy. The 29Si NMR spectra of all the bulk Ga-MFI materials (Figure 6a, #1−#4) show peaks at −113 ppm (framework Q4 Si connected to 4 other Si) and −105 ppm (framework Q4 Si connected to 3 Si and 1 Ga). The 29Si NMR spectra of all the hierarchical Ga-MFI materials (Figure 6a, #5−#8) show similar features. However, the peak at −105 ppm is broadened due to the presence of significant numbers of surface Q3 Si atoms (connected to 3 Si and a hydroxyl group, ca. −103 ppm). Figure 6b shows a more detailed comparison of the 29Si NMR spectra of a bulk Ga-MFI (#2) and a SPP-Ga-MFI (#6) material. Besides the broader peak at ca. −105 ppm (explained above), the SPP-Ga-MFI also shows a broadened Q4 peak, which is due to the presence of “distorted” Q4′ Si.36 These are thought to be Q4 Si atoms close to the surface having slightly distorted coordination environments due to surface energy effects.
Figure 5. (a) N2 physisorption isotherms at 77 K, (b) cumulative pore volumes, and (c) pore size distributions of hierarchical Ga-MFI materials #6 and #7 and bulk Ga-MFI materials #2 and #3.
ranges, due to the microporosity of MFI. However, the N2 uptake of bulk Ga-MFI materials quickly reaches a saturation value, whereas the hierarchical materials show rapidly increasing uptakes at higher relative pressures due to adsorption in the mesopores. The SPP Ga-MFI (#6) and layered Ga-MFI (#7) materials show different shapes of the adsorption curves that indicate different pore structures due to their morphological 7217
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hierarchical materials, which have a much lower density than the bulk materials (and hence a lower sample mass packed into the sample rotor). The strong peak at +150 ppm seen for all the materials belongs to tetrahedral framework Ga37,38 and is strong evidence of the incorporation of Ga into the zeolitic framework. The bulk Ga-MFI materials also show a weak peak at 0 ppm, which is due to octahedral extra-framework Ga. However, the hierarchical materials do not show this peak due to the low signal-to-noise ratio. Acid Site Analysis and PDH Performance. Incorporation of Ga into MFI generates Brønsted acid sites (framework Ga) and Lewis acid sites (extra-framework Ga).31 The acid site concentration and distribution of Brønsted and Lewis acid sites has been characterized by NH3-TPD (Figure 7) and pyridine
Figure 7. NH3-TPD profiles of the eight materials.
adsorption followed by IR spectroscopy (Figure S1). Figure 7 shows that all the bulk Ga-MFI and hierarchical Ga-MFI materials display NH3 desorption peaks around 150 °C (physisorption of weakly held NH3) and 280 °C (acid site I, Brønsted acid sites generated by framework Ga) and a shoulder around 450 °C (acid site II, commonly assigned to strong Lewis acid sites generated by extraframework Ga).39,40 These features were deconvoluted by peak-fitting, and the calculated acid site concentrations are summarized in Table S1 (Supporting Information). The total acid site concentration in both types of materials is inversely proportional to the Si/Ga ratio. Materials made with and without MPS addition show similar acid site concentrations. The total acid site concentration can be estimated by the Si/Ga ratio for each sample. In this work, the NH3-TPD measured acid site quantification suggests that 60−90% of acid sites estimated from the Si/Ga ratio are present in these samples. Although NH3-TPD analysis is a well-known method to estimate the total acid site concentration, a complementary quantification of the Brønsted and Lewis acid sites can be accomplished with pyridine adsorption followed by FTIR spectroscopy.41 Figure S1 compares FTIR spectra of pyridine on bulk (#2), SPP (#6), and layered (#7) Ga-MFI materials, each measured after evacuation at different temperatures of 150, 250, 350, and 450 °C. Three peaks are of interest: 1540 cm−1 (PyH+ pyridinium ions at Brønsted acid sites), 1450 cm−1 (pyridine adsorbed on Lewis acid sites39,40), and 1445 cm−1 (weakly adsorbed hydrogen bonded pyridine39). As expected,
Figure 6. (a) 29Si NMR spectra of the eight materials and (b) detailed comparison of the 29Si NMR spectra of the bulk material #2 and the SPP material #6. (c) 71Ga NMR spectra of the eight materials.
Figure 6c shows the 71Ga-NMR spectra of the eight materials. The signal-to-noise ratio is somewhat lower for the 7218
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Table 3. Summary of Calculations for Acid Site Analysis Based upon NH3-TPD (§) and Pyridine Adsorption FTIR (†) Methods Ga-MFI materials #1: #2: #3: #4: #5: #6: #7: #8:
bulk Ga-MFI bulk Ga-MFI-MPS bulk H-Ga-MFI bulk H-Ga-MFI-MPS SPP Ga-MFI SPP Ga-MFI-MPS layered H-Ga-MFI layered H-Ga-MFI-MPS
Si/Ga ratio
total sites§ (μmol g−1)
total sites† (μmol g−1)
B/L ratio†
B sites† (μmol g−1)
L sites† (μmol g−1)
124 123 61 69 128 140 75 72
104 80 239 214 86 88 181 176
65 53 150 124 45 65 117 160
3.9 2.2 3.9 3.4 2.5 1.5 1.2 1.4
51 37 119 96 33 39 64 93
13 17 30 28 13 26 54 67
similar propane conversions for the bulk Ga-MFI material and its hierarchical analogue. Table 4 summarizes the PDH results
the intensity of the latter peak decreased significantly at higher evacuation temperatures. The Brønsted and Lewis acid site concentrations were calculated using integrated molar extinction coefficients for MFI zeolites.42 Table S2 (Supporting Information) shows detailed results of the pyridine FTIR acid site analysis at different evacuation temperatures for the three materials in Figure S1. The total acid site concentration in the SPP Ga-MFI material is significantly higher than that of the bulk Ga-MFI, which has a similar Si/Ga ratio. This difference is mainly caused by the larger concentration of Lewis acid sites in the SPP material, whereas the concentration of Brønsted acid sites is similar. The layered Ga-MFI material also shows a high concentration of Lewis acid sites and furthermore a higher overall concentration of both types of sites, since it has a much lower Si/Ga ratio than the other two materials. Table 3 summarizes the main results of the characterziation of acid sites, including the total acid site concentrations obtained from both the NH3-TPD and pyridine adsorption followed by FTIR spectroscopy. For the latter method, the results are based on an evacuation temperature of 150 °C. The total acid site concentrations obtained by the two methods, while quantitatively different due to the significantly different kinetic diameters of NH3 (3.0 Å) and pyridine (5.7 Å) as probe molecules, obey the same trends. The B/L ratios of the hierarchical materials are much lower than those of the bulk materials, mainly due to the considerably higher concentrations of Lewis sites (and also slightly lower Brønsted site concentrations) in the hierarchical materials. There are more Lewis acid sites created by extraframework Ga, perhaps because the formation of Ga clusters is easier in the mesopores. Based upon all the characterization results obtained, we hypothesize that hierarchical Ga-MFI catalysts should show signifcant differences in reactivity in propane conversion compared to bulk Ga-MFI catalysts, leading to significant changes in PDH efficiency and selectivity. Due to the widely varying morphology, Ga content, and acid site content in the different catalysts (Tables 1−3), it is less meaningful to compare the PDH performance of all the materials together. Furthermore, the widely varying catalytic activities make it a challenge to achieve exactly the same propane conversion for all the materials. Therefore, we first compare the PDH performance of pairs of materials (one bulk and one hierarchical) that are the most similar/analogous to each other in terms of synthesis method, composition, and Si/Ga ratio. Based upon Tables 1−3, such a comparison can be readily made for the pairs (#1, #5), (#2, #6), (#3, #7), and (#4, #8), with the materials in each pair measured at similar conversion levels to allow a reliable comparison of catalyst activity and propylene selectivity. To accomplish this, the W/F values (W: weight of catalyst in g, and F: feed flow rate in cm3 s−1) had to be adjusted in a rather wide range of 0.15−0.9 g s cm−3 to obtain
Table 4. Summary of Measured PDH Characteristics of the Eight Ga-MFI Materialsa Ga-MFI materials
W/F
conversion (%)
activity
selectivity (%)
TOF
#1: bulk Ga-MFI #2: bulk Ga-MFIMPS #3: bulk H-GaMFI #4: bulk H-GaMFI-MPS #5: SPP Ga-MFI #6: SPP Ga-MFIMPS #7: layered H-GaMFI #8: layered H-GaMFI-MPS
0.9 0.9
4.2 ± 0.4 4.6 ± 0.5
0.3 ± 0.0 0.4 ± 0.0
29 ± 3 35 ± 4
3 5
0.3
7.3 ± 0.4
1.8 ± 0.1
63 ± 2
8
0.3
10.0 ± 0.4
2.5 ± 0.1
75 ± 5
12
0.3 0.15
6.3 ± 1.1 5.1 ± 0.4
1.5 ± 0.1 2.5 ± 0.0
50 ± 9 79 ± 6
18 28
0.15
8.6 ± 0.9
4.2 ± 0.4
75 ± 7
23
0.15
12.2 ± 0.6
6.0 ± 0.3
82 ± 3
34
a
Units are W/F: g s cm−3, activity: mmol h−1 g−1, TOF: h−1.
for all eight materials, along with calculation of the catalyst activity (by normalizing the total moles of propane converted with time-on-stream and catalyst mass) and turnover frequency (TOF, by normalizing the activity with total acid site concentration shown in Table 3 as obtained from NH3TPD). We first discuss the pairwise comparison of bulk versus hierarchical materials and then discuss a more generalized comparison of multiple materials. Figure 8 shows the performance comparison of an example material pair (#2, #6), and Figure S2 (Supporting Information) presents the comparison for the other three pairs of materials, all over an on-stream time period of 5.5 h. It is seen that adjustment of the W/F values allows similar levels of conversion (Figure 8a and Figure S2a−c) to be obtained between hierarchical Ga-MFI and its bulk Ga-MFI counterpart with negligible catalyst deactivation over the time on stream. It is noteworthy that each of the bulk Ga-MFI catalysts #1−#4 required considerably higher W/F values (0.9, 0.9, 0.3, and 0.3 g s cm−3 respectively, Table 4) to obtain conversions similar to their hierarchical Ga-MFI analogues #5−#8 (0.3, 0.3, 0.15, and 0.15 g s cm−3, respectively), indicating that the latter catalysts are more active. Furthermore, Figure 8b and Figure S2d−f show the propylene selectivities obtained under the above conditions. Under the operating conditions employed, with 5% propane feed at 873 K, the equilibrium conversion is about 80% and hence the PDH reactions in this paper (4−12%) are conducted very far from thermodynamic limitations. Each hierarchical Ga-MFI catalyst showed superior propane activity, TOF, and propylene selectivity when compared to the 7219
DOI: 10.1021/acs.chemmater.7b01566 Chem. Mater. 2017, 29, 7213−7222
Article
Chemistry of Materials
terms of an increase in Lewis acid sites or a lower B/L ratio. This is likely due to different ranges of Si/Ga ratios attained in the two studies. In this study, the lowest Si/Ga ratio is 61 (Table 1), which is almost twice as high as the lowest Si/Ga ratio of 34 obtained previously in the bulk Ga-MFI materials.18 It is possible that MPS addition achieves easily measurable structural effects only at higher Ga concentrations, although their catalytic effects are still clearly evident as shown here. Similar trends can be discerned in relation to the ion-exchange step. For example, we consider the materials #3, #4, #7, and #8 which are all prepared in Na+ form and then H+ ion exchanged. Comparing the bulk materials (#3, #4) and the hierarchical materials (#7, #8) at similar conversion levels (albeit with a somewhat greater spread of conversion values compared to the previous cases discussed), we find that the hierarchical (layered) materials show a clear improvement in selectivity and activity (much lower W/F) than the bulk materials. Similarly, the materials #1, #2, #5, and #6 do not require ion exchange (since they contain no Na+). Again, the hierarchical (SPP) materials show a clear improvement in selectivity at lower W/F than the bulk materials at similar conversion levels (within 1−2%). Looking at the overall results in Table 4, we also find that the ion-exchanged materials show higher enhancements in activity and selectivity than the non-ionexchanged materials. This is possibly because the ionexchanged materials had lower Si/Ga (50, target) ratios than non-ion-exchanged samples (100, target). Since the hypothesized bifunctional mechanism for PDH utilizes both B and L acid sites, a higher acid site concentration (lower Si/Ga ratio) may be more effective for this reaction system in the range studied. Based on Tables 3 and 4, it is also clear that the superior PDH performance of hierarchical Ga-MFI materials is not simply due to an increase in the Lewis (or total) acid site concentrations. To investigate this issue further, we examined the potential role of diffusion limitations in explaining the enhanced PDH performance of the hierarchical Ga-MFI materials. In particular, we estimated the dimensionless Weisz−Prater (W−P) number44 for a representative set of the materials used in this study. The Supporting Information and Table S3 give details of these calculations. The W−P numbers for all the Ga-MFI materials are in the range of 10−7 and are very far from the diffusion-limited regime (which requires a W−P number >0.3). We therefore rule out diffusion effects for small molecules like propane, unlike the case of largemolecule conversions wherein diffusion issues are critical in explaining the large differences in catalytic activity between hierarchical and bulk zeolites.18,32 Instead, it appears that the observed enhancements in PDH should be better explained by reactivity differences arising from the different environments of the catalytic sites in bulk and hierarchical Ga-MFI. One possible difference relates to the fact that the catalytic sites in the bulk zeolite are almost exclusively confined inside nanoscale (
