Confinement of supported metal catalysts at high loading in the

Dec 28, 2017 - The supported catalyst formed an embedded network of nanowires along the zeolite mesopores. Although tightly filled in the mesopores, t...
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Confinement of supported metal catalysts at high loading in the mesopore network of hierarchical zeolites, with access via the microporous windows Jongho Han, Jangkeun Cho, Jeong-Chul Kim, and Ryong Ryoo ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04183 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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Confinement of supported metal catalysts at high loading in the mesopore network of hierarchical zeolites, with access via the microporous windows Jongho Han†,‡, Jangkeun Cho‡, §, Jeong-Chul Kim‡, and Ryong Ryoo*,†,‡ †

Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon 34141, Korea

‡

Natural Science Research Institute, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea

§

ABSTRACT: High loading on a porous support is important for preparing high-performance metal catalysts, but the increased loading often results in a loss of dispersion and limited mass transfer. We approached this problem by supporting a large amount of metal or metal oxide on a hierarchically porous zeolite. The supported catalyst formed an embedded network of nanowires along the zeolite mesopores. Although tightly filled in the mesopores, the catalyst was readily accessible through microporous windows at the encasing mesopore walls. Cobalt, nickel, and TiO2, supported in this manner, exhibited high catalytic performance in Fischer-Tropsch synthesis, benzene hydrogenation, and furfural-to-γ-valerolactone conversion, respectively.

KEYWORDS: high loading, supported catalyst, mesopore network, hierarchical zeolite, zeolite nanosponge Noble metal (e.g., Pt, Pd and Ru) catalysts have been widely used in the form of supported catalyst on porous materials (e.g., silica, γ-alumina and titania) for various catalytic applications, but they are very expensive.1-6 It is desirable to replace the noble metal catalysts with light transition metal elements (e.g., Fe, Co, Ni, and Cu).7-11 However, light metals normally have low intrinsic catalytic activities, and require much higher loadings (~30 wt%) as compared with noble metals (~1 wt%).12-14 When the metal loading is increased to such a high level, the supported catalyst tends to agglomerate into large particles, resulting in a serious loss of catalyst dispersion.15-16 Moreover, the light transition elements have low polarizability and accordingly weak van der Waals interactions with the surface of the supporting materials. Hence, the light transition metals are more susceptible to agglomeration than noble metals.17 In this regard, high loading of metals with high dispersion is crucial to achieve excellent catalytic performance for light transition metal catalysts. From this perspective, one useful approach is to support the catalyst on materials that provide strong metal-support interactions and high specific surface area, such as γ-Al2O3, TiO2, and CeO2. The strong interactions with the supported metal are effective for stabilization of the metal catalyst into highly dispersed tiny nanoparticles. At high loadings, however, the large amount of supported catalyst on open surfaces can cause large agglomeration of the catalyst species. Another approach is to use ordered mesoporous materials, such as MCM-41, MCM-48, or KIT-6 silica. The

high surface area and highly porous nature of the mesoporous materials are helpful to disperse the catalyst into tiny separated nanoparticles. However, when the catalyst loading increases to more than 10 wt%, the catalyst phase tends to form nanowires or networks that fill along the mesoporous channels. Although the outermost tips of the nanowires are accessible for gas adsorption and catalytic reactions, most portions of the catalysts located on the long lateral surfaces become inaccessible, as they are blocked by the mesopore walls. In this paper, we present a strategy to improve the catalytic activity of mesopore-supported nanowires. Here, a hierarchically mesoporous-microporous zeolite is used as a catalyst support instead of MCM-41 or MCM-48. High loading of the catalyst on the mesoporous zeolite resulted in the formation of nanowires or networks that fill along the mesoporous channels, similar to the case of MCM-41 and MCM-48. The access to the nanowires along the mesopores is blocked except for the tip portion, similarly to the case of MCM-41. Nevertheless, the lateral surfaces become readily accessible for catalytic reactions through microporous zeolite windows existing on the mesopore walls. The accessibility through microporous windows ensures high catalytic activity. Moreover, the nanowires have high resistance to metal sintering, due to interpenetration of the branched network with the mesopore walls. To demonstrate the microporous window effects, we synthesized two kinds of hierarchically mesoporous-microporous zeolites via a hydrothermal route using meso-

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micro dual structure-directing multiammonium surfactants.18-19 One zeolite had a highly mesoporous nanosponge-like morphology built of MFI-type aluminosilicate frameworks of 2.5 nm thickness. The other exhibited a similar nanosponge-like morphology, but the frameworks were beta zeolite of 3.9-nm thickness, as confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) (Figure S1). According to a recent study using electron tomography, the mesopores in these zeolites formed a 3-dimensionally interconnected network. The mesopores were connected to the zeolitic micropores, and were open to the exterior of the nanosponge particles. The mesopores in the MFI nanosponge had a slit shape, whereas the beta zeolite mesopores were of an interconnected cage type.20 The two zeolite nanosponges were supported with Co at various loading levels ranging from 3 to 50 wt% by melt infiltration, where 50 wt% loading amounted to 1g Co per g zeolite (see Section S1 for experimental details in Supporting Information (SI)). For brevity, we denote the Co-supporting MFI and beta zeolite nanosponges as ‘x Co/MFI-NS’ and ‘x Co/beta-NS’, where x indicates the Co wt% and NS refers to nanosponge. The catalytic conversion rate by these samples was measured in the Fischer-Tropsch (FT) synthesis reaction, which converted the syngas (CO + H2) to liquid fuels. The measured catalytic conversion was compared with that of MCM-41 and MCM-48 supporting the same amount of Co (experimental details in Section S1 of SI). The MCM-41 and MCM-48 samples were ordered mesoporous aluminosilicate materials with amorphous frameworks, which are designated as ‘x Co/MCM-41’ and ‘x Co/MCM-48’ according to the Co loading.

Figure 1. The CO conversion rates (μmol s-1 gcat-1) in FischerTropsch synthesis plotted versus the Co loading amount (gCo/gsupport) on beta-NS, MFI-NS, MCM-48, and MCM-41 (reaction conditions: GHSV = 0.3~7.8 L h-1 g-1, H2/CO = 2, 493 K and 20 atm). Each conversion was measured 10 h after starting the reactant feeding.

Figure 1 displays the FT catalytic activity according to the Co loading of each catalyst sample. This result shows that the Co/NS-zeolites gave much higher CO conversion rates,

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when compared with the Co/MCM samples at the same Co loading. The difference in conversion rates increased more dramatically when the Co loading was increased to above 20 wt%. At 30 wt%, both the 30 Co/MFI-NS and 30 Co/beta-NS samples showed catalytic conversion rates higher than 80 μmol s-1 gcat-1. The CO conversion by Co/beta-NS increased continuously until the Co loading was increased to 50 wt%. On the other hand, 30 Co/MCM41 and 30 Co/MCM-48 catalysts exhibited very low conversion rates of less than 6 μmol s-1 gcat-1. Similarly, low conversion rate was obtained with 30 wt% Co supported on SBA15 mesoporous silica (Figure S2). It is particularly noteworthy that the catalytic activity of the Co/MCMs rather sharply decreased with increasing Co loading in the range of 20 ~ 30 wt%. This decrease in catalytic activity was in sharp contrast to the continuous increase of the catalytic conversion in the case of the zeolite nanosponges.

Figure 2. STEM images of (a) 30 Co/beta-NS and (b) 30 Co/MFI-NS. (Inset in (a) and (b): High-magnification STEM image of 30 Co/beta-NS and 30 Co/MFI-NS, respectively.)

To investigate the reason for the difference between zeolite nanosponges and MCMs shown in Figure 1, the catalyst dispersion was investigated by high-resolution scanning transmission electron microscopy (STEM). Figure 2 shows representative STEM images of the 30 Co/beta-NS and 30 Co/MFI-NS samples. As shown in Figure 2a, the 30 Co/beta-NS sample taken before the catalytic reaction exhibited a 3D network of Co nanowires of approximately 4 nm thickness that resembled a pile of tetrapods. The 30 Co/MFI-NS in Figure 2b exhibited 3D network of Co nanoplates of about 4 nm thickness. The thickness of the nanowires and nanoplates was very similar to the mesopore diameter of the zeolite nanosponge (4 nm). XRD and HRTEM analyses confirmed that the nanowires and nanoplates were composed of metallic Co with hcp structure (Figure S3 and S4). In addition, no significant amount of agglomerated Co particles was detected at the exterior of the zeolite nanosponge particles, regardless of whether the STEM images were taken before the reaction or 10 h after the reaction (Figure S5). Due to the presence of the Co nanowires, the Ar adsorption isotherms of the two Co/zeolite samples indicated a distinct decrease in the mesopore volume. However, the micropore volume did not significantly change (Table S1, Figure S6). This indicated that the Co nanowires were selectively located inside the mesopores, but not in the zeolitic micropores. Similar to the case of the

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zeolite nanosponges, the MCM-41 and MCM-48 mesoporous materials also contained Co metal supported in the form of 4-nm nanowires along their mesopores, both before and after the reaction (STEM images in Figure S7 and S10). Hence, when comparing the nanowire diameters and thickness, the only difference between the Co/zeolites and Co/MCMs was the presence of micropores.

Figure 4. STEM images of (a) 30 Ni/beta-NS and (b) 30 TiO2/beta-NS. (Inset in (a): High-magnification STEM image of 30 Ni/beta-NS. Inset in (b): EDX mapping image of Ti element (green) in 30 TiO2/beta-NS.)

Figure 3. Schematic illustration of Co/zeolite nanosponge participating in FT synthesis and the diffusion pathway for reactants and products. Based on the results above, we propose that the supported Co metal could exist in a form of embedded network of nanowires along the zeolite mesopores as shown in Figure 3. If this were the case, the Co nanowires in both MFI-NS and beta-NS would be accessible by the FT reactants through the zeolitic microporous windows. Then, the Co metal in both zeolite nanosponges could exhibit high catalytic performance even at very high Co loadings. The details of the catalytic performance could somewhat depend on the mesopore shape and micropore diameters (discussion in Section S3 of SI). On the other hand, the Co metal supported on the solely mesoporous MCMs at a high Co loading would suffer from severe diffusion problems. Only the tips of the Co nanowires would be open to the mesopores to function as active sites (see Figure S8 in SI). In addition to the high catalytic activity, another notable feature of the Co/NS-zeolite was its high catalytic longevity at high Co loadings. The CO conversion was monitored over 100 h of the FT reaction period (Figure S9). The result with 30 Co/beta-NS showed a gentle decrease in the catalytic conversion, from 79 to 73.2%, during this period. This change was accompanied by a mild sintering of the Co phase embedded within the interconnected 3D network of mesopores. On the other hand, in the case of 30 wt% Co supported on open-surfaced γ-Al2O3, the STEM images showed Co nanoparticles with a wide range of particle diameters. The size of the Co nanoparticles increased markedly during the 100-h reaction period, due to the open nature of the alumina surface (Figure S10). As a result, the catalytic conversion decreased from 65.2 to 22.3%.

The results in Figure 1 are not presented to emphasize the high activity of zeolite-supported FT catalysts, but rather to demonstrate the catalyst design strategy using hierarchical zeolites with mesopore-micropore connectivity. The present concept could be applied to other various supported metals and metal oxide catalysts. For example, 30 wt% Ni could be readily supported on the beta zeolite nanosponge, following the same procedure used for the preparation of Co/beta-NS. It was also possible to support 30 wt% TiO2 on the beta zeolite, using Ti isopropoxide (experimental details in SI). The resultant beta zeolite samples containing Ni and TiO2 are denoted as 30 Ni/beta-NS and 30 TiO2/beta-NS, respectively. Figure 4 shows representative STEM images of these samples. As the image in Figure 4a shows, the Ni metal in the 30 Ni/beta-NS sample existed in the form of a nanowire network. In the case of the 30.TiO2/beta-NS sample, the supported TiO2 phase is difficult to distinguish from the zeolite framework in the highresolution STEM image shown in Fig. 4b. This is due to the low Z-contrast between Ti and Si atoms. For this reason, the location of the TiO2 nanoparticles was probed by EDX mapping. The EDX mapping image showed TiO2 networks within the zeolite nanosponge (inset in Figure 4b), similar to the case of 30 Ni/beta-NS. XRD and HRTEM analyses indicated that the Ni and TiO2 nanowires had fcc and anatase structures, respectively (Figures S3 and S4). According to the pore volume analysis by argon adsorption, the Ni loading caused the mesopore volume to decrease by 0.17 cm3 g-1support. However, the micropore volume only decreased by 0.01 cm3 g-1support (Table S1, Figure S6). In the case of TiO2 loading, the volume changes were 0.33 cm3 g-1support for mesopores, and 0.02 cm3 g-1support for micropores.

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30.TiO2/beta-NS (44% GVL yield) and 30 TiO2/MCM-48 (19%). According to previous studies, the second LAcatalyzed transfer hydrogenation is the rate-determining step for the furfural-to-GVL conversion.21, 23 Therefore, the difference in the GVL yield can be attributed to the larger accessible number of TiO2 LA sites through micropore windows in the case of the zeolite nanosponge.

Figure 5. (a) Rate of benzene conversion over Ni supported catalysts and Pt supported catalyst (reaction conditions: GHSV = 0.25 L h-1 g-1, H2/C6H6 molar ratio = 3.64, reaction time = 1 h, 373 K, 1 atm). (b) GVL yield over TiO2 supported catalyst (reaction conditions: 0.34 mmol of furfural, 1.2 ml of 2-butanol, 50 mg of the catalyst, reaction time = 40 h, 363 K, 1 atm).

The catalytic performance of the 30 wt% Ni supported on beta-NS and MCM-48 was investigated for the benzene hydrogenation reaction. We measured the benzene conversion rate after 1 h of reactant feeding at 373 K. The benzene conversion rate, obtained in this manner, was 4.7 μmol s-1 g-1 in the case of 30.Ni/MCM-48. In the case of 30 Ni/betaNS, the conversion rate was 13 times higher than that of 30 Ni/MCM-48 (Figure 5a). The catalytic conversion rate over 30 Ni/beta-NS was even higher than that of 0.5 wt% Pt supported on a γ-Al2O3 catalyst, which is a well-known catalyst for commercial industrial processes. The high catalytic activity could be attributed to the increased accessibility of benzene to the Ni surface through the micropore windows of the zeolite nanosponge, similar to the case in FT synthesis reaction (Figures 1 and 3). The window effect was further confirmed with decalin hydrogenolysis over 30 Ni/MFI-NS, where the reactant molecular diameter (0.64 nm) was supposed to be larger than the zeolite pore aperture (0.55 nm). In this bulky molecular reaction, the zeolite-supported catalyst exhibited no significant effect of windows (Figure S11). In the case of the 30 TiO2/beta-NS sample, we tested the conversion of furfural-to-γ-valerolactone (GVL) in one pot. The catalytic conversion is known to occur in three steps: transfer-hydrogenation of furfural catalyzed by a Lewis acid (LA), hydrolytic ring opening by a Brønsted acid (BA), and another transfer-hydrogenation by a LA (Scheme S1).2122 . In the TiO2/beta-NS catalyst, the LA sites were provided by the supported TiO2 while the BA sites were given by the aluminosilicate zeolite framework. Hence, the furfural-toGVL conversion took place in one pot. We took the GVL yield after 40 h of reaction at 363 K. The measured GVL yield is presented in Figure 5b. For comparison, the results are compared with the GVL yield obtained with 30 wt% TiO2/MCM-48 having a similar BA concentration to the beta-NS. As Figure 5b shows, the two catalyst samples exhibited a remarkable difference depending on the support:

In summary, we supported a large amount (e.g., 30 wt% loading) of Co, Ni, and TiO2 on a mesoporous zeolite nanosponge. The supporting process was readily controlled that the supported component formed a nanowire network along the mesoporous channels. The catalytic activities of Fischer-Tropsch synthesis, benzene hydrogenation, and furfural-to-GVL conversion indicated that a large portion of the nanowire surfaces was accessible for high catalytic activity through the zeolitic micropore apertures located on the mesopore wall. In addition, the nanowires exhibited resistance to sintering owing to the confinement effect from the 3D-interpenetrated zeolite framework. We believe that the present zeolite window-functioning concept could lend new insight into the design of supported catalysts.

ASSOCIATED CONTENT Supporting Information Experimental procedures and supplementary XRD, TEM, Ar adsorption, and catalytic reaction data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by IBS-R004-D1.

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Figure 1. The CO conversion rates (µmol s-1 gcat-1) in Fischer-Tropsch synthesis plotted versus the Co loading amount (gCo/gsupport) on beta-NS, MFI-NS, MCM-48, and MCM-41 (reaction conditions: GHSV = 0.3~7.8 L h-1 g-1, H2/CO = 2, 493 K and 20 atm). Each conversion was measured 10 h after starting the reactant feeding. 196x147mm (95 x 95 DPI)

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Figure 2. STEM images of (a) 30 Co/beta-NS and (b) 30 Co/MFI-NS. (Inset in (a) and (b): Highmagnification STEM image of 30 Co/beta-NS and 30 Co/MFI-NS, respectively.) 306x154mm (95 x 95 DPI)

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Figure 3. Schematic illustration of Co/zeolite nanosponge participating in FT synthesis and the diffusion pathway for reactants and products. 287x134mm (95 x 95 DPI)

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