Molybdenum-Incorporated Mesoporous Silica: Surface Engineering

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Molybdenum-Incorporated Mesoporous Silica: Surface Engineering toward Enhanced Metal-Support Interactions and Efficient Hydrogenation Ting Chen, Zhangping Shi, Guanghui Zhang, Hang Cheong Chan, Yijin Shu, Qingsheng Gao, and Yi Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16496 • Publication Date (Web): 20 Nov 2018 Downloaded from http://pubs.acs.org on November 21, 2018

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ACS Applied Materials & Interfaces

Molybdenum-Incorporated

Mesoporous

Silica:

Surface Engineering toward Enhanced MetalSupport Interactions and Efficient Hydrogenation Ting Chen,† Zhangping Shi,‡ Guanghui Zhang,§ Hang Cheong Chan,† Yijin Shu,† Qingsheng Gao*,† and Yi Tang‡ †Department

of Chemistry, College of Chemistry and Materials Science, Jinan University,

Guangzhou 510632, P. R. China ‡Department

of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative

Materials, Laboratory of Advanced Materials and Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200433, China. §Davidson

School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA.

KEYWORDS: metal-support interactions, mesoporous silica, surface engineering, iridium, hydrogenation

ABSTRACT: In heterogeneous catalysis, strong metal-support interactions are highly desired to improve catalytic turnover on metal catalysts. Herein, uniform molybdenum-incorporation is introduced into mesoporous silica (KIT-6) to accomplish the strong interactions with iridium catalysts, and consequently the active and selective hydrogenation of carbonyl compounds. The Mo-incorporated KIT-6 (Mo-KIT-6) affords electronic interactions to improve the proportion of

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metallic Ir0 species, avoiding the easy surface oxidation of ultrafine metals in silica meso-cavities. Owing to the effective H2 activation and following hydrogenation on metallic Ir0 sites, the optimal Ir/Mo-KIT-6 with a high Ir0/Irδ+ ratio delivers prominent performance in the hydrogenation of amides to amines and α,β-unsaturated aldehydes to unsaturated alcohols. As for N-acetylmorpholine hydrogenation, the Ir/Mo-KIT-6 achieves efficient turnover toward Nethylmorpholine with a high selectivity (> 99%), and demonstrates the activity relying on the engineered chemical state of Ir sites. Such promotion is further proved universal in cinnamaldehyde hydrogenation. This work will provide new opportunities for catalyst design through surface/interface engineering.

INTRODUCTION

Mesoporous silica has received much concern as catalyst supports because of its large surface, uniform porosity, size/shape selectivity, and significant thermal stability.1-4 In particular, the well-defined porosity can provide confined growth of loading metals toward highly dispersed nanoparticles (NPs) or nanoclusters that are active in hydrogenation.5-6 However, along with the decreased size, metal NPs usually suffer from surface oxidation owing to their high surface energy.7-9 The oxidized surface unfortunately makes it difficult for activated hydrogen desorption due to the up-shifted d-band center of metals,10 causing low kinetics. Thereby, effective engineering on inert silica surface is desired to promote metal-support interactions and consequently vary the chemical states of metal centers (e.g., size, atom arrangement, charge distribution, etc.).11-12 As evidenced, (3-aminopropyl)-trimethoxysilane functionalized SBA-15 can immobilize highly dispersed Au and Au-In NPs (~ 2 nm), and more importantly the electrontransfer from N to metal enables active and selective hydrogenation of crotonaldehyde.13


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However, the grafting by organosilane requires fussy procedures, and the stability and recyclability in catalytic applications are usually unsatisfied. Alternatively, surface decoration employing inorganic dopants emerges as a rational option, in which precise control is highly desired to prevent catalysts from blocking mesopores and diminishing surface area.14-16 To this end, uniform metal doping, particularly in the form of single-site doping, is a promising way.17-19 Molybdenum oxides (MoO3) are reactive supports owing to their reducible nature.20-22 The Mo element possesses multiple valence states (+6, +5, +4, +3, and +2), which ensures the engineering on the electronic configuration and surface acidity of its oxides.23-25 As been hydrogen-doped or partially reduced by H2 spillover during preparation or catalysis, the resulting hydrogenated MoOx (H-MoOx) delivers strong interactions with loading metals.21,26 As indicated in our recent work, such electronic metal-support interactions are feasible to vary charge distribution on metal (e.g., Pt, Pt-Sn, and Ir) surface and thereby promote the hydrogenation of α,β-unsaturated aldehyde and functionalized nitroarenes.27-28 Meanwhile, the enriched acidic surface and defects on H-MoOx are able to chemisorb and activate substrate molecules.19,21,29 Therefore, incorporating Mo into mesoporous silica is envisioned as a feasible strategy to engineer the internal surface from inert to active, accomplishing metal-support interactions and interfacial synergy for efficient catalysis. In comparison with bare Mo-based materials, silica mesoporous frameworks further provide large surface to immobilize highly dispersed Mo-sites, and thereby amplify interactions with loading metals. In this regard, Mo-incorporated silica serves as versatile supports for catalysts, different from previous reports using Mo as active centers in biomass pyrolysis and propane oxidation.30-31 Herein, Mo-incorporation is successfully introduced to activate the surface of mesoporous silica (KIT-6) via facile co-assembly procedures, achieving efficienct hydrogenation on loading


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Ir (Scheme 1). The Mo-incorporated KIT-6 (Mo-KIT-6) affords strong electronic interactions to improve the proportion of metallic Ir0 species, avoiding the easy surface oxidation of ultrafine metals in silica meso-cavities. To indentify the interactions and functionalities, the hydrogenation of amides and α,β-unsaturated aldehydes are taken as probing reactions, which are of great importance to produce valuable chemicals used in manufacturing durg, perfumes, polymers, and flavorings.32-33 The efficient and selective reduction of carbonyl is highly chanllenging to metal catalysts, and requires optimized chemical configuration on active-sites.34 As expected, Ir/Mo-KIT-6 delivers efficient turnover toward N-ethylmorpholine with a high selectivity (> 99%) in N-acetylmorpholine hydrogenation, which is correlated to the enriched metallic Ir0 sites on Mo-incorporated silica surface. And the promotion by such metal-support interactions is further identified in the chemoselective hydrogenation of cinnamaldehyde to cinnamyl alcohol.

Scheme 1. Schematic illustration for the fabrication of Mo-KIT-6 and the metal-support interactions to boost catalytic hydrogenation.


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RESULTS AND DISCUSSION Mo-incorporated KIT-6 with three-dimensional (3D) ordered mesoporous structures are synthesized via a one-pot co-assembly method (Scheme 1). The slow assembly involving silicate and molybdate oligomers in presence of P123 surfactant enables uniform Mo incorporation into KIT-6 pore-wall. Thereby, the inert KIT-6 surface becomes active and affords enhanced metalsupport interactions toward efficient hydrogenation. For convenience, they are named as xMoKIT-6, and x represents the Mo/Si molar ratio (Mo/Si = x/100).

Figure 1. XRD patterns of xMo-KIT-6 and 3.0Mo/KIT-6 in (a) Low-angle and (b) wide-angle ranges. (c) HAADF-TEM of KIT-6, 3.0Mo-KIT-6 and 3.0Mo/KIT-6. (d) FT magnitudes of the k2[x(k)] EXAFS spectra of xMo-KIT-6 (x = 0.5 ~ 8.0) and the MoO3 reference. (e) N2 adsorption-desorption isotherms and (f) pore-size distribution of Mo-KIT-6 with various Mo contents.


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Figure 1a depicts the X-ray diffraction (XRD) patterns of xMo-KIT-6, identifying a cubic mesostructure consistent with that of bare KIT-6.35 The peak at 2θ = 0.80 ~ 1.00o corresponds to the (211) plane and a hump for the (220). Noticeably, the (211) peak moves to low angles and decreases in intensity with increasing Mo-incorporation in KIT-6 frameworks.36 In comparison, the KIT-6 supported Mo species (3.0Mo/KIT-6) identifies negligible shift in XRD patterns as compared with that of KIT-6, owing to the failure in Mo-incorporation into pore-wall. Moreover, there are no detectable peaks in the wide-angle XRD patterns of xMo-KIT-6 (Figure 1b), which is different from the 3.0Mo/KIT-6 that shows sharp peaks of MoO3 at 2θ = 13.4 and 27.5º. This verifies the highly dispersive Mo in Mo-KIT-6. Accordingly, high-angle annular dark-field transition electronic microscopy (HAADF-TEM) confirms a mesoporous structure of 3.0MoKIT-6 without observable bright dots, the same as bare KIT-6 (Figure 1c). In sharp contrast, bright nanoclusters, ascribed to Mo-based nanoclusters, are visible in 3.0Mo/KIT-6. Such different states of Mo species in Mo-KIT-6 and Mo/KIT-6 will further lead to the varied metalsupport interactions and consequently catalytic performance. The successful Mo-incorporation into KIT-6 was further confirmed by Mo K-edge extended X-ray absorption fine structure (EXAFS). Because the measurement on samples with extremely low Mo content (e.g., 0.1Mo-KIT-6) is too time consuming, we collected the data for xMo-KIT6 (x = 0.5 ~ 8.0) and the MoO3 reference. The Mo K-edge spectra show gradual changes with different Mo concentrations in xMo-KIT-6 (Figure S1 in Supporting Information). Figure 1d further gives the corresponding R space curves after k2[x(k)] functions Fourier transform. The peaks between 1.0 and 2.0 Å correspond to Mo–O scattering and the ones at ~ 3 Å are ascribed to Mo–Mo scattering.37 The Mo–O and Mo–Mo bonds present a slight increase in xMo-KIT-6, indicating a decreased oligomerization degree of Mo species.38-39 Accordingly, the peak for Mo–


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Mo bonds in xMo-KIT-6 decreases in intensity, as compared with the referring MoO3 samples, which is owing to the interrupted Mo-O-Mo chains in silica framework. Furthermore, the mesoporosity of xMo-KIT-6 was investigated by mean of N2 sorption isotherms. As displayed in Figure 1e, the samples exhibit type-IV isotherms with H1-type hysteresis loops. It’s revealed that the mesoporous structures are well-mantained after Moincorporation, agreeing with the observation in XRD. Nevertheless, the hysteresis loop moves to a higher pressure range as x increases from 0 to 6.0, suggesting the gradually enlarged pore-sizes. Accordingly, the pore-size distribution presents a visible increase along with introducing Mo (Figure 1f). The observation can be interpreted by the substitution of Mo atoms into silica framework,40 in which Mo atoms with larger sizes and longer Mo-O bonds, in comparison with those of Si, lead to enlarged meso-periodic structures. This is in line with XRD analysis (Figure 1a). In the case of 8.0Mo-KIT-6, the capillary condensation step in isotherms turns to dispersive, and the corresponding pore-size distribution locates at a wider range, indicating the destruction of mesotructures. Moreover, the BET surface of xMo-KIT-6 decreases from 745 to 539 m2 g-1 as x increases from 0 to 6.0 (Table S1 in Supporting Information), and drastically drops to 397 m2 g-1 in 8.0Mo-KIT-6. It’s indicated that excessive Mo introduction (e.g., 8.0Mo-KIT-6) will block some of the mesopores and reduce the surface area. The corresponding evolutions in pore volume, pore-wall thickness and unit cell are further summarized in Table S1 of Supporting Information, showing the clear dependence on varied Mo-incorporation. Via typical impregnation procedures, Ir species is loaded into mesoporous Mo-KIT-6 and KIT-6. Figure 2 displays the TEM images of Ir/xMo-KIT-6 (x = 0 ~ 8.0) with a consistent Ir loading (4.9 ± 0.1 wt%). Highly dispersive Ir NPs with a small size (2.1 ± 0.1 nm) are observed, indicating the negligible influence by different Mo-incorporation. The mesoporous framework in


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Ir/8.0Mo-KIT-6 shows a partial destruction (Figure 2f), agreeing with the observation on 8.0MoKIT-6 in XRD and N2 sorption isotherms (Figure 1). By contrast, as Mo/KIT-6 is adopted as supports, the obviously larger Ir NPs (~ 3.9 nm) are observed (Figure 2g), even with the same Ir loading. And the aggregation of Ir on Mo-species nanodomains is owing to the uneven Mo dispersion on KIT-6 surface. The structural features of above samples are summarized in Table S2 of Supporting Inforamtion. And the above observation is confirmed by HAADF-TEM invesitgation (Figure S2 in Supporting Information), in which the bright and nanosized dots are ascribed to the well-dispersed Ir. In addition, the XRD pattern and high-resolution TEM (HRTEM) image of Ir/Mo-KIT-6 are showed in Figure S3 of Supporting Information. Because of the ultrafine Ir NPs (~ 2.1 nm), the XRD presents negligible diffraction peaks for Ir. However, the HR-TEM clearly identifies the fcc phase of Ir embedded in Mo-KIT-6 mesopores.

Figure 2. TEM images of (a) Ir/KIT-6, (b) Ir/0.1Mo-KIT-6, (c) Ir/0.5Mo-KIT-6, (d) Ir/3.0MoKIT-6, (e) Ir/6.0Mo-KIT-6, (f) Ir/8.0Mo-KIT-6, (g) Ir/3.0Mo/KIT-6, and (h) corresponding elemental mapping of Ir/3.0Mo-KIT-6. The insets of panels are the size distributions of Ir NPs. The Ir loading is fixed as 4.9 ± 0.1 wt%.


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As a model sample, the Ir/3.0Mo-KIT-6 was investigated by energy-dispersive spectroscopy (EDS) and the corresponding elemental mapping. The EDS results confirm the presence of Ir, Mo, Si, and O in the 3D mesoporous networks, and the aside signal for Cu is owing to the supporting grid used in TEM (Figure S4 in Supporting Information). Accordingly, the elemental mapping shows the uniform distribution of elements (Figure 2h), agreeing with the good dispersion of Ir in Mo-KIT-6.

Figure 3. (a) H2-TPR of H2IrCl6 on KIT-6 and xMo-KIT-6, and (b) Ir 4f XPS profiles of Ir/xMoKIT-6 catalysts obtained via a reduction by H2/Ar at 300 oC. (c) Surface composition of Ir and relative CO desorption associated with Mo-incorporation in KIT-6. (d) CO-TPD profiles of Ir/xMo-KIT-6. To investigate the metal-support interactions in Ir/xMo-KIT-6, H2 temperature-programed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS) and CO temperature-programed desorption (CO-TPD) were conducted. The samples have a fixed Ir loading of 4.9 ± 0.1 wt%, while the Mo-incorporation is varied from x = 0 to 8.0. As depicted in H2-TPR (Figure 3a), the


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reduction of Ir4+ species on xMo-KIT-6 shifts to lower temperature after introducing Mo (x = 0 ~ 6.0), indicating the enhanced metal-support interactions on Mo-incorporated internal surface. As Mo is excessive (x = 8.0), the move to slightly higher temperature suggests the weakened interactions. Remarkably, the XPS investigation further reveals the electronic interactions with loading Ir. The Ir 4f profiles in Figure 3b can be deconvoluted to four peaks. The peaks at 62.0 and 64.9 eV are owing to the 4f7/2 and 4f5/2 of Irδ+,41 respectively, and those at 61.0 and 63.9 eV are correlated with metallic Ir0 species.42-43 On Ir/KIT-6, the Irδ+ species is dominant, suggesting the partially oxidized surface of ultrasmall Ir particles. Visibly, the overall peaks red-shift to lower binding energy in Ir/xMo-KIT-6 (x = 0.1 ~ 3.0), along with the improved proportion of metallic Ir0 species. Such varied chemical states on Ir surface results from the electronic interactions with Mo-KIT-6 that experiences the visible reduction of Mo6+ to Mo5+ by H2 spillover during preparation (Figure S5 in Supporting Information).20 As previously evidenced by experimental and theoretical investigations, the H-doping into MoO3 can enrich band states around the EF.23,44 Because of the downshifting d-band center of Mo sites, the Mo-incorporated surface will facilitate electron transfer to Ir,44-45 thereby retaining rich metallic Ir0 species. As Mo is excessive (x = 6.0 and 8.0), the Irδ+ species increases again, which should be ascribed to the weakened interactions on unevenly Mo-incorporated KIT-6 surface. When we plot the Ir0/Irδ+ molar ratio on catalyst surface against the x value (Figure 3c), the increase of Ir0/Irδ+ ratio is observed with x from 0 to 3.0, and a decline in the range of x = 3.0 ~ 8.0. As suggested, the maximized metal-support interactions can be achieved on 3.0Mo-KIT-6 with uniform Moincorporation. Such varied chemical state of Ir in Mo-KIT-6 can be further confirmed by CO-TPD analysis (Figure 3d). Because of the similar dispersity of Ir on various xMo-KIT-6 supports (Table S2 in


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Supporting Information), the different chemisoprtion in CO-TPD can be ascribed to their varied chemical state on surface. Typically, the binding of CO to Ir sites involves electron donation from σC=O to dIr, and the opposite feedback from dIr to 2π*C=O, causing the CO chemisorption sensitive to the electronic state of Ir sites. The Ir/KIT-6 shows the low intensity associated with CO desorption, suggesting the few CO binding to Ir NPs that are excessively oxidized on surface. With increasing Mo in KIT-6, the signals for CO desorption becomes obvious as x ranging from 0 to 3.0. And they suffer decline as x increases from 3.0 to 8.0. This trend is quite consistent with that of surface Ir0/Irδ+ (Figure 3c). It’s indicated that the enriched metallic Ir0 is beneficial for CO chemisorption due to its largely-expanded d orbitals, and the Irδ+ species coupling with oxygen has not enough empty orbitals to bond with CO.

Figure 4. (a) N-acetylmorpholine hydrogenation on a series of Ir/xMo-KIT-6 catalysts (Ir loading: 4.9 ± 0.1 wt%) obtained after H2 reduction at 300 oC, accompanied with the evolution of Ir surface concentration. Reaction conditions: N-acetylmorpholine (0.5 mmol), DME (1,2dimethoxyethane 10 mL), catalyst (0.1 g), H2 (3 MPa), temperature (130 oC), time (4 h), stirring rate (600 rpm). (b) Correlation of initial reaction rates with H2 pressure and N-acetylmorpholine


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concentration. (c) Schematic illustration for N-acetylmorpholine hydrogenation on Ir/xMo-KIT-6. (d) N-acetylmorpholine hydrogenation over 4.9% Ir/3.0Mo-KIT-6 for five successive runs. To identify the functionality of Ir/xMo-KIT-6, the hydrodeoxygenation of Nacetylmorpholine to N-ethylmorpholine, a typical reduction of amide, was employed as a probe reaction. Generally, amide reduction to amine is among the most difficult hydrogenation of carboxylic acid derivatives because amides are the least reactive motifs.46 Figure 4 describes the N-acetylmorpholine hydrogenation on Ir/xMo-KIT-6. They can catalyze the reduction to Nethylmorpholine with a high selectivity (~ 99%). By contrast, the Mo-KIT-6 exhibits negligible activity (Table S3 in Supporting Information), confirming the highly dispersed Ir as the main active species for hydrogenation. However, the Ir on commercial MoO3 and SiO2 with smaller surface areas affords low activity, pointing out the contribution by mesoporous Mo-KIT-6 with large surface area. Observably, Ir/xMo-KIT-6 shows the N-acetylmorpholine conversion correlated with the Mo content in KIT-6, which makes influences on the metal-support interactions. On Ir/KIT-6, a lower conversion of only 6% is observed. The Mo-incorporation into KIT-6 leads to the obviously promoted N-acetylmorpholine reduction on Ir. The conversion reaches the maximum of 85% on Ir/3.0Mo-KIT-6, but declines with excessive Mo-incorporation in KIT-6 (e.g., Ir/6.0Mo-KIT-6 and Ir/8.0Mo-KIT-6). As evidenced by kinetics analysis (Figure 4b), the hydrogenation is a first-order reaction relative to PH2, but nearly zero-order to Nacetylmorpholine concentration. This situation is quite consistent with previous investigations.4748

Therefore, the activity is highly correlated with the adsorbed H (Hads) on Ir surface, and the

chemisorption/activation of N-acetylmorpholine is in rapid-equilibrium. The metallic Ir0 sites can effectively dissociate H2 to Hads that is highly active for hydrogenation. By contrast, the Irδ+


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bonded with oxygen suffers down-shifting d-band centers and consensequently the difficulty to desorb Hads.10,20 As a result, the conversion of N-acetylmorpholine on Ir/xMo-KIT-6 shows a consistent evolution with surface Ir0/Irδ+, confirming the importance of metal-support interactions on Mo-KIT-6 (Figure 4c). Meanwhile, the enriched vacancies on Mo-incorporated surface due to H-doping can attract the C=O of N-acetylmorpholine, resulting in highly selective hydrodeoxidation.46 This functionality by Mo-incorporated silica surface is further confirmed by Mo-KIT-6 supported Pt and Pd catalysts, which show the obviously improved activity in Nacetylmorpholine hydrogenation, in comparison with their counterparts supported by bare KIT-6 (Figure S6 in Supporting Information). Additionally, a recycle experiment was conducted to test the service life of Ir/xMo-KIT-6. The optimal Ir/3.0Mo-KIT-6 affords consistent activity and selectivity at a high level in five successive cycles (Figure 4d), demostrating its satisfied durability. It needs notice that the slight decrease in conversion can be ascribed to the inevitable loss of catalysts during the recovery by centrifugation. Moreover, the TEM investigation on Ir/3.0Mo-KIT-6 after cycling tests clearly confirms the well-retained Ir NPs embedded in Mo-KIT-6 framework (Figure S7 in Supporting Information), in good accordance with the high catalytic performance.

Figure 5. (a) Mo 3d and (b) Ir 4f XPS profiles of Ir/3.0Mo-KIT-6 (Ir loading: 4.9 ± 0.1 wt%) obtained after reduction by H2/Ar at 250, 300, 400 and 500 oC, and (c) surface concentrations of Mo and Ir relative to reduction temperature and N-acetylmorpholine conversion. Reaction


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conditions: N-acetylmorpholine (0.5 mmol), DME (10 mL), catalyst (0.1 g), temperature (130 oC),

H2 (3 MPa), time (4 h), and stirring rate (600 rpm). Varying the H-doping in Mo-based supports can modulate the electronic configuration and

consequently engineer the metal-support interactions with loading metals.20,27 This effect is available on Ir/3.0Mo-KIT-6 via changing the temperature (250 ~ 500 oC) for reducing H2IrCl6 to Ir on xMo-KIT-6 surface. When a relatively lower temperature (250 oC) is adopted, the received samples affords dominant Mo6+ and negligible Mo5+ in XPS analysis, along with the lower Ir0/Irδ+ (Figures 5a and 5b). This indicates the insufficient H-doping into Mo species is unable to afford strong electronic interaction to maintain the metallic Ir0 species. The catalyst delivers a low conversion (18%) in N-acetylmorpholine hydrogenation (Figure 5c). In comparison, the Ir/3.0Mo-KIT-6 received at 300 oC presents the obviously enriched Mo5+ and Ir0 species, and thereby is highly active (conv.: 85%). The further reduction of Mo5+ to Mo4+ at elevated temperature (e.g., 400 and 500 oC) will degrade the reactive H-MoOx to inert MoO2, resulting in weak metal-support interactions with Ir. Evidently, the conversion of Nacetylmorpholine decreases on Ir/3.0Mo-KIT-6 that are obtained at 400 and 500 oC.

Figure 6. Cinnamaldehyde hydrogenation performance on (a) Ir/xMo-KIT-6 (Ir loading: 4.9 ± 0.1 wt%) reduced by H2/Ar at 300 oC, and (b) Ir/3.0Mo-KIT-6 (Ir loading: 4.9 ± 0.1 wt%) prepared in H2/Ar at 250, 300, 400 and 500 oC. Reaction conditions: cinnamaldehyde (0.8 mmol),


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H2O (9 mL), EtOH (21 mL), catalyst (30 mg), H2 pressure (2 MPa), temperature (30 oC), time (3 h), and stirring rate (600 rpm). The Ir/xMo-KIT-6 further delievers active and selective hydrogenation of cinnamaldehyde to cinnamyl alcohol. The hydrogenation activity visibly relies on the amount of Moincorporation in KIT-6 (Figure 6a), showing the similar trend with that in N-acetylmorpholine hydrogenation. And the activity reaches the maximum on Ir/3.0Mo-KIT-6, further verifying the metal-support interactions as the key to activate H2 and consequently hydrogenate substracts. Meanwhile, the cinnamyl alcohol selectivity is obviously improved as Mo is introduced, which is consistent with the previous reports on H-MoOx supported Ir.28 The H-doping into Mo-sites results in enhanced surface acidity that would selectively activate the C=O moiety via MoO=C or Mo-OHO=C,49-50 leading to the favored C=O hydrogenation toward unsaturated alcohols. When the reduction temperature to fabricate Ir/3.0Mo-KIT-6 increases from 250 oC to 500 oC, the cinnamaldehyde conversion and cinnamyl alcohol selectivity also present a volcano-type tendency (Figure 6b), reaching the maximum at 300 oC. The Ir/3.0Mo-KIT-6 affords effective metal-support interactions to fulfil the efficient and selective hydrogenation. In comparison, Ir/MoO3 and Ir/Mo/KIT-6 with a similar Ir loading (4.9 ± 0.1 wt%) showed relatively lower cinnamaldehyde conversion and cinnamyl alcohol selectivity (Table S4 in Supporting Information), highlighting the enhanced metal-support interactions on Mo-KIT-6 to boost chemoselective hydrogenation.

CONCLUSION In summary, Mo-incorporation is successfully introduced to engineer the internal surface of mesoporous silica (KIT-6), accomplishing the strong metal-support interactions with ultrafine Ir


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NPs, and the high efficiency for catalytic hydrogenation. The effective Mo-incorporation into KIT-6 is clearly evidenced, which enhances metal-support interactions to maintain metallic species on ultrafine Ir NPs. It’s clear that the sufficient Mo5+ species in Mo-KIT-6 will result in enriched metallic Ir0 species for H2 activation and hydrogenation. Thereby, the optimal Ir/xMoKIT-6 affords the efficient and selective hydrogenation of N-acetylmorpholine to Nethylmorpholine, and of cinnamaldehyde to cinnamyl alcohol. Their performance is highly related to the interactions between Ir and Mo-incorporated silica surface. Demonstrating a operational modulation on support surface and thereby metal centers, this work will pave the way for catalyst design by means of surface/interface engineering. EXPERIMENTAL SECTION Catalyst preparation Synthesis of Mo-KIT-6: Pluronic P123 (0.8 g) was dissolved into 7.8 mL of HCl (2 mol L-1) under stirring at 35 oC to form a clear solution. Meanwhile, ammonium heptamolybdate (AHM) with an acquired amount was dissolved in 28.5 mL of deionized water. The two solutions were mixed under stirring for 2 hours, and then 1.0 mL of BuOH was loaded. After aging for 1 hour, 2.2 mL of tetraethoxysilane (TEOS) was introduced. The mixture was stirred at 35 oC for 24 hours, prior to the hydrothermal reaction in a Teflon-lined stainless-steel autoclave (100 oC, 24 hours). Afterwards, the solid products were filtered and washed thoroughly with distilled water, and dried overnight at 100 oC. The Mo-KIT-6 was finally obtained after a calcination at 550 oC under air. Synthesis of Mo/KIT-6: KIT-6 supported molybdenum species (Mo/KIT-6) was fabricated by a conventional impregnation method. A certain amount of AHM was dissolved in distilled


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water. And then KIT-6 was introduced into the resulting aqueous solution. After stirring at room temperature, the solid was dried at 100 oC and then calcined in air at 550 oC for 4 hours. Synthesis of Ir/xMo-KIT-6 and Ir/Mo/KIT-6: Ir/xMo-KIT-6 and Ir/Mo/KIT-6 catalysts were prepared by incipient wetness impregnation. The catalyst supports Mo-KIT-6 or Mo/KIT-6 were mixed with H2IrCl6 aqueous solution, and the solutions were heated and dried in oil bath at 80 oC for 4 hours. The solids were further dried at 50 oC overnight, and then reduced by a 5 vol% H2/Ar flow at 300 oC for 2 hours. Catalyst characterization XRD analysis was conducted on a diffractometer (Bruker D8) with Cu Kα radiation (λ=1.54056 Å). SEM and TEM were performed on a Zeiss ULTRA55 and a JEOL JEM 2100F, respectively. XPS analysis was measured on Thermo Scientific Escalab 250Xi, using C 1s (284.6 eV) as a reference. The metal loading was measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). N2 sorption isotherms were collected on Micromeritics Tristar 3000 at -196 oC (77 K). The Brunauer–Emmett–Teller (BET) specific surface areas were calculated from adsorption data, and the pore distribution was analyzed by the Barrett–Joyner– Halenda (BJH) method. H2-TPR and CO-TPD were undertaken on Quantachrome ChemBET Pulsar. X-ray absorption spectroscopy measurements at the molybdenum K-edge were performed on the 10-ID beamline of the Materials Research Collaborative Access Team (MRCAT) at the Advanced Photon Source (APS), Argonne National Laboratory. Catalytic performance measurements N-acetylmorpholine hydrogenation was carried out in a 100 mL stainless-steel autoclave (Parr 4848), in which N-acetylmorpholine (0.5 mmol), catalyst (0.1 g), 4 Å molecular sieve (0.4 g), and 1,2-dimethoxyethane (DME, 10 mL) were introduced. To remove the air, the reactor was


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sealed and purged five times with H2. Then, the reactor was purged with H2 (3 MPa) and heated to the desired temperature. The products were analyzed by a Shizumadu GC-2014C Cinnamaldehyde hydrogenation was carried out via the same procedure. Catalyst (30 mg), cinnamaldehyde (0.8 mmol), EtOH (9 mL), and H2O (21 mL) were placed together. Next, the H2 (2 MPa) was purged into the reactor, then the reaction was stirred at 30 oC . The products were characterized by a Shizumadu GC-2014C. ASSOCIATED CONTENT Supporting Information. Mo K-edge XANES and textural features of xMo-KIT-6, textural properties, HAADF-TEM, EDS and XPS of Ir/xMo-KIT-6, and catalytic performance on various Ir, Pt and Pd catalysts. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail for Q. S. G.: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors appreciate the financial support from National Natural Science Foundation of China (21773093

and

21433002),

Natural

Science

Foundation

of

Guangdong

Province

(2015A030306014) and Guangzhou Science and Technology Program (201707010268).


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Table of content


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Molybdenum-Incorporated Mesoporous Silica: Surface Engineering

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