Al-Fiber for CO Coupling to

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Microfibrous-Structured Pd/AlOOH/Al-Fiber for CO Coupling to Dimethyl Oxalate: Effect of Morphology of AlOOH Nanosheet Endogenously Grown on Al-Fiber Chunzheng Wang, Jia Ding, Guofeng Zhao, Tao Deng, Ye Liu, and Yong Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00889 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 1, 2017

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ACS Applied Materials & Interfaces

Microfibrous-Structured Pd/AlOOH/Al-Fiber for CO Coupling to Dimethyl Oxalate: Effect of Morphology of AlOOH Nanosheet Endogenously Grown on Al-Fiber Chunzheng Wang, Jia Ding, Guofeng Zhao, Tao Deng, Ye Liu, and Yong Lu*

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, China

KEYWORDS: structured catalyst, boehmite nanosheet, palladium, dimethyl oxalate, carbon monoxide

1/36 ACS Paragon Plus Environment


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ABSTRACT We report a green, template-free and general one-pot method of endogenous growth of free-standing boehmite (AlOOH) nanosheets on a 3D-network 60µm-Al-fiber felt through water-only hydrothermal oxidation reaction between Al metal and H2O (2Al + 4H2O → 2AlOOH + 3H2). Content and morphology of AlOOH nanosheets can be finely tuned by adjusting the hydrothermal oxidation time length and temperature. Palladium is highly dispersed on such AlOOH endogenously formed on Al-fiber felt via

incipient wetness impregnation method and as-obtained

Pd/AlOOH/Al-fiber catalysts are checked in the CO coupling to dimethyl oxalate (DMO) reaction. Interestingly, Pd dispersion is very sensitive to the thickness (26 to 68 nm) of AlOOH nanosheet, and therefore the conversion shows strong AlOOH-nanosheet-thickness dependence whereas the intrinsic activity (TOF) is AlOOH-nanosheet-thickness independence. The most promising structured catalyst is the one using a microfibrous-structured composite with the thinnest AlOOH nanosheet (26 nm) to support a small amount of Pd of only 0.26 wt%. This catalyst, with high thermal-conductivity and satisfying structural robustness, delivers 67% CO conversion and 96% DMO selectivity at 150 oC using a feed of CH3ONO/CO/N2 (1/1.4/7.6, vol) and a gas hourly space velocity of 3000 L kg-1 h-1, and particularly, is very stable for at least 150 h without deactivation sign.


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1. INTRODUCTION Among metal oxides, alumina is the most popular catalyst/support in the petrochemical industries and environment protection because of its low cost, large specific surface area (SSA) and excellent hydrothermal stability.1,2 In particular, hierarchical alumina (e.g., Al2O3 nanosheets) with open meso-/macro- pore structure shows significant enhancements in the diffusion and pore accessibility, which has the potential to further extend its industrial applications relative to traditional Al2O3.3,4 Nowadays, plenty of hierarchical alumina with unique morphology has been successfully synthesized through various methods, but its large-scale applications are still challenging owing to the use of costly template, low fabrication efficiency, and formation of worthless byproduct slat and waste water.5,6 Hence, it is particularly desirable to develop a template-free, highly efficient and environmentally-benign route for the fabrication of hierarchical alumina. On the other hand, the Al2O3-produced catalysts in the microgranules or pellets of millimeters are usually loaded in fixed beds with inherent disadvantages, such as poor intraparticle/interbed mass/heat transfer, high pressure drop and non-regular flow pattern.7,8 This thus becomes an important starting point for attempts to develop the composite of Al2O3 and structured substrates (e.g., metal-foam/-fiber and cordierite honeycomb) via conventional coating techniques.9,10 The catalysts using such composites as supports exhibit excellent catalytic performance with enhanced mass/heat transfer.11,12 However, conventional coating techniques suffer from exfoliation and nonuniformity of coating layer as well as binder harmful contamination.13,14 It should also be noted that general coating is not suitable for fiber-based structured substrates (e.g., 3D Al-fiber) because coating procedures cause pore blocking due to the small fibers and pores.15,16 Recently, great efforts have been dedicated to the non-coating strategy of direct activation of metal fiber/foam substrates for 3/36 ACS Paragon Plus Environment


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structured catalyst development, in applications for the highly endo/exothermic and/or high-throughput reaction processes such as foam-structured Ni-based catalysts for syngas methanation,17,18

PdNi(alloy)/Ni-foam

catalyst

for

coalbed

methane

deoxygenation,19

HZSM-5/metal-fiber catalyst for methanol to propylene20 and Au/Ni-fiber catalyst for gas-phase alcohol oxidation21,22. All these catalysts substantially provided unique combination of superior activity/selectivity and promising stability, with high permeability and good thermal conductivity. Aluminum has low price, excellent conductivity and good malleability, and can be readily fabricated into various shapes engineered from micro- to macro-scale, such as foam, fiber and microchannel.23,24 These Al-substrates have a great flexibility in geometric appearance for structured catalyst packings that is paving the way to the “ideal” reactor (e.g., ultralow pressure drop, mal-distribution free and remarkable decrease of hot/cold spot temperature).25,26 In spite of these marked promising features, what to be noteworthy is that the practical applications of such Al-substrates in heterogeneous catalysis are severely restricted by their low SSA and inactive surfaces. To solve this problem, anodic oxidation of aluminum to form Al2O3 layer on the metal Al via electrolysis, has been widely developed.23 Although anodic layer has regular array of nanometer pores and large SSA, the anodic oxidation process is usually limited to foil-like Al substrates and its scaling up for applications is far more difficult.27,28 Moreover, based on the fundamental oxidation reaction between Al metal and H2O, the formation of aluminum hydroxide (including boehmite and bayerite; amorphous or well crystalized) layer under various conditions has been reported.29,30 However, these studies often employed Al powder as substrate, which is too reactive to control the morphology of aluminum hydroxide layer through the Al-H2O reaction, and little attention is paid to the applications in heterogeneous catalysis.29,30


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Recently, unique composite of boehmite nanosheets on microfibrous-structured 3D Al-fiber (60 µm in diameter) has been developed by steam-only oxidation; typically, the pre-treated Al-fiber underwent oxidation process in steam flow at 120 oC for 6 h and thus the AlOOH is endogenously formed to a mass fraction of 7.5 wt% (Al-fiber in balance).31-33 Such AlOOH/Al-fiber composite shows substantial potential for fabricating novel structured catalysts and reactors with unique combination of enhanced heat/mass transfer, low pressure drop and promising catalytic performance, which has been verified by several hot-topic reactions such as Pd-catalyzed CO oxidative coupling to dimethyl oxalate (DMO),31,32 Pd-catalyzed volatile organic compounds (VOCs) combustion,34 Fe-Mn-K-catalyzed Fischer-Tropsch synthesis35 and Ni-catalyzed CO methanation31. It is well known, however, the morphology of aluminum hydroxide layer can profoundly affect its surface properties and therefore the catalytic performances of as-produced catalysts supports.4 Hence, it is exceptionally worthwhile to finely tune the morphology of aluminum hydroxides endogenously formed on Al substrates (e.g., thin-sheet Al-fiber felt with 3D network structure) and to investigate whether the activity/selectivity is dependent on their morphology for the catalytic reaction processes such as titled reaction, and if so, to what extent. Herein, we established a green, versatile and general one-pot route to the morphology- and content-controllable endogenous growth of AlOOH nanosheets on Al substrates (e.g., Al-fiber felt and foam) through water-only hydrothermal oxidation between Al metal and H2O. Hydrothermal oxidation time and temperature dependent growth of AlOOH nanosheets were investigated carefully. A series of microfibrous-structured Pd catalysts were prepared by incipient wetness impregnation method using as-prepared AlOOH/Al-fiber composites as supports and were examined in the strongly exothermic reaction of CO coupling to dimethyl oxalate (DMO) (-159 kJ mol-1).



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Interestingly, the catalyst performance was very sensitive to the thickness of AlOOH nanosheet on the Al-fiber felt. Scanning electron microscope (SEM), CO pulse adsorption, intrinsic activity (expressed by turnover frequency) calculation and N2 adsorption-desorption measurements were carried out

to

reveal the

AlOOH-morphology

dependence of

catalytic properties of

Pd/AlOOH/Al-fiber catalyst.

2. EXPERIMENTAL SECTION 2.1. Preparation of AlOOH/Al-fiber and Pd/AlOOH/Al-fiber. A thin-sheet (~1.3 mm in thickness) felt with 3D network structure using 60-µm Al-fiber (99.9 wt% purity) was utilized as the mother substrate, which was taken from Shanghai Xincai Net-structured Material Co. Ltd., China. Palladium acetate was purchased from Shanghai Jiuling Chemical Co., Ltd., China, and other chemicals were from Sinopharm Chemical Reagent Co., Ltd., China. All chemicals were used as received. Typically, circular chips of Al-fiber (16 or 8 mm in diameter equal to the inner diameter of fixed-bed quartz tube reactor) punched down from their large felt were ultrasonically degreased in pure acetone for 10 min, cleaned in diluted aqueous solution of sodium hydroxide (0.1 wt% NaOH) at room temperature for 2 min to remove the aluminum oxide film, and then rinsed thoroughly with deionized water. After that, the pre-treated circular chips (5.0 g) were transferred into a 100 mL Teflon-lined stainless steel autoclave filled with only deionized water of 50 mL. After hydrothermal oxidation in temperature range from 100 to 180 oC for 0.5 to 24 h, the sample was washed thoroughly with deionized water, and then dried at 100 oC for 2 h. As-obtained composite samples are denoted as AlOOH/Al-fiber-(x/y), where “x” and “y” are hydrothermal oxidation temperature (oC) and time length (h), respectively. Note that the content of obtained AlOOH was estimated by the 6/36 ACS Paragon Plus Environment

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weight increase of the pre-treated circular chips of Al-fiber before and after the hydrothermal oxidation. Palladium, to an appointed loading of 0.3 wt%, was placed onto the as-prepared AlOOH/Al-fiber-(x/y) composite supports by the incipient wetness impregnation with a toluene solution of palladium acetate. After that, the samples were vacuumed to remove toluene solvent, dried at 100 oC for 2 h and calcined in static air at 300 oC for 2 h to obtain the catalyst products, being denoted as Pd/AlOOH/Al-fiber-(x/y). For example, the catalyst using support obtained after water-only hydrothermal oxidation for 12 h at 100 oC is named as Pd/AlOOH/Al-fiber-(100/12). 2.2. Characterizations. The samples were characterized by X-ray diffraction (XRD, Rigaku Ultima IV diffractometer, Cu Kα), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS, Bruker Tensor 27), scanning electron microscope (SEM, Hitachi S-4800), transmission electron microscope (TEM, FEI TECNAI G2 F30 instrument at 300 kV) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES; ICP Thermo IRIS Intrepid II XSP). Specific surface area (SSA) was determined by the N2 adsorption isotherms at -196 oC using standard Brunauer-Emmett-Teller (BET) theory on a Quantachrome Autosorb-3B instrument. The pore size distribution

was

deduced

from

the

adsorption

branch

of

the

isotherms

using

the

Barrett-Joyner-Halenda (BJH) method. CO pulse adsorption was performed on a Quantachrome ChemBET-3000 chemisorption apparatus with a thermal conductivity detector (TCD) to determine the Pd dispersion by assuming a CO/Pd ratio of 1 on account of the systematic errors.32 Typically, sample of 200 mg was pre-reduced with 10 vol% H2 in N2 at 150 oC for 2 h, flushed with helium at that temperature for 1 h and then cooled down to 35 oC. After that, the gas mixture of 10% CO in He was pulsed, with a pulse volume



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of 250 µL, to the reactor every 3 min until the CO peak intensity remained unchanged. In all cases, the flow rate of gas to the reactor was maintained at 40 mL min-1. 2.3. Reactivity tests for CO coupling to DMO. The gas-phase CO coupling to dimethyl oxalate (DMO) over these catalysts was carried out in a fixed-bed quartz tube reactor (760 mm length) under atmospheric pressure. The feed gas of methyl nitrite (MN, CH3ONO) was synthesized by dropping sulfuric acid (50 vol% H2SO4) into an aqueous solution of methanol (CH3OH) and sodium nitrite (NaNO2), collected into a steel cylinder and subsequently mixed with pure N2 to obtain a gas mixture of MN/N2 (1/7.6, vol). Three calibrated mass flow controllers were employed to control the gas flow of N2, CO and MN/N2 mixture, respectively. The circular chips of Pd/AlOOH/Al-fiber-(x/y) catalysts were packed layer-up-layer into the quartz tube reactor and compacted to avoid the appearance of a gap between the reactor wall and catalyst chip edge. Note that the catalyst of 0.80 g packed in a 16-mm (i.d.) quartz tube reactor was used at low gas hourly space velocity (GHSV) of 3000 L kg-1 h-1 while 0.08 g catalyst packed in an 8-mm (i.d.) quartz tube reactor was used at high GHSV of 60000 L kg-1 h-1. The gas mixture of MN/CO/N2 (1/1.4/7.6, vol) was employed as feedstock and reaction temperature was varied in range from 110 to 200 oC. All data were collected when the reaction reached steady-state under the appointed conditions for at least 30 min. Prior to the reaction testing, the catalyst was in-situ activated by performing CO coupling reaction at high temperature of 200 oC for 2 h with the gas mixture of MN/CO/N2 (1/1.4/7.6, vol). The composition of feed gas as well as product effluent gas was quantitatively analyzed by an on-line gas chromatography (GC) equipped with a TCD connected to a ShinCarbon ST packed column (DIKMA) and a flame ionization detector (FID) connected to an Innowax PEG-20M capillary column (HP). The gas pipeline and sampling six-way valve of reactor outlet were heated to 120 oC to prevent products


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condensation. The calculation of turnover frequency (TOF) was presented in the Supporting Information. The main byproducts were identified including dimethyl carbonate (DMC), methyl formate and methanol. The formation rate of byproduct methyl formate and methanol (resulting from the catalytic decomposition of MN) was low enough to be neglected in temperature range from 110 to 170 oC and thus the CO conversion and DMO selectivity were calculated using the following formulas: CO conversion (%) = 100% − (ACOout / AN2out) / (ACOin / AN2in) × 100% DMO selectivity (%) = ADMO / (ADMO + f ×ADMC) × 100% where Acoin and Acoout are the peak areas of CO (determined by TCD) at the inlet and outlet of the fixed-bed reactor, AN2in and AN2out the peak areas of N2 (determined by TCD) at the inlet and outlet of the fixed-bed reactor, ADMO and ADMC the peak areas of DMO and DMC (determined by FID) at the outlet of the fixed-bed reactor, and f the relative correction factor of DMC to DMO.

3. RESULTS AND DISCUSSION 3.1. Endogenous growth of AlOOH on Al-fiber felt through water-only hydrothermal oxidation 3.1.1 Feature of AlOOH/Al-fiber composite. Figure 1 shows a top-down preparation strategy, geometry, morphology and structure feature of the representative AlOOH/Al-fiber-(120/2) composite engineered from nano- to macro-scale in one step. Figure 1A schematically illustrates the endogenous growth of AlOOH nanosheets on a 3D-network Al-fiber felt (Figure 1B) to obtain the AlOOH/Al-fiber-(120/2) composite (Figure 1C) through water-only hydrothermal oxidation. A thin-sheet microfibrous structure with entirely open 3D porous network (Figure 1D), consisting of 10 vol% 60-µm Al-fiber and 90 vol% voidage, was employed as the mother substrate. The pristine Al-fiber has a smooth surface observed by the SEM image (Figure 1E). After water-only 9/36 ACS Paragon Plus Environment


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hydrothermal oxidation in an autoclave at 120 oC for 2 h through the oxidation reaction between Al metal and H2O (2Al + 4H2O → 2AlOOH + 3H2), leaf-like boehmite nanosheets are endogenously formed and disorderedly aligned to a uniform nanosheet array (shell) along with the Al-fiber (core) (Figure 1F). The average thickness of densely packed shell is about 1.0 µm, which corresponds to an AlOOH content of 9.5 wt% estimated by the weight increase of the pre-treated Al-fiber before and after the hydrothermal oxidation. As shown in Figure 1G, in comparison with the pristine Al-fiber, as-obtained AlOOH/Al-fiber-(120/2) composite shows the formation of boehmite phase (JCPDS no. 74-1895) solidly evidenced by XRD pattern at 2 theta of 14.5, 28.2, 38.4 and 49.3o. DRIFTS spectra in Figure 1H indicate that the pristine Al-fiber shows clean surface with no detectable aluminum oxide bands, while the AlOOH/Al-fiber-(120/2) composite delivers four clear bands at 3298, 3105, 1144 and 1080 cm-1, which are assigned to the boehmite.36,37 The SSA and total pore volume of pristine Al-fiber are too low to be determined by N2 adsorption-desorption isotherms while as-obtained AlOOH/Al-fiber-(120/2) composite offers a large SSA of 11 m2 g-1 and total pore volume of 0.014 cm3 g-1 (including the Al-fiber mass) (Figure 1I); after the Al-fiber content of 90.5 wt% is subtracted, it is easy to calculate that the AlOOH shell contributes a large SSA of ~117 m2 gAlOOH-1 and total pore volume of ~0.147 cm3 gAlOOH-1. Moreover, such composite shows a mesopore feature evidenced by the appearance of a clear capillary condensation step at a P/P0 region of 0.4-0.6 on the N2 adsorption-desorption isotherms (Figure 1I). 3.1.2 Effect of hydrothermal oxidation time length. Figure 2 shows the effect of water-only hydrothermal oxidation time length on AlOOH content of the AlOOH/Al-fiber composite at 120 oC in time range from 0.5 to 24 h. Clearly, the AlOOH content is increased progressively from 5.9 wt% to 15.6 wt% (shell thickness: from ~0.5 to ~1.5 µm) with prolonging the hydrothermal oxidation time


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from 0.5 to 12 h and then plateaus at 15.8 wt% with further increasing the hydrothermal oxidation time even up to 24 h. As well known, the formation of AlOOH results from the oxidation reaction between the Al and water,38,39 and according to the Wagner’s theory,40 the inward diffusion of water across the aluminum hydroxide layer is the rate-determining step. Associated with the formation of AlOOH, the water molecules become more and more difficult to penetrate the thickening AlOOH shell thereby terminating the Al-H2O reaction, being consistent with the fact that growth of AlOOH is almost frozen with time from 12 to 24 h (Figure 2). Note that the ultimate amount of AlOOH shell obtained by water-only hydrothermal oxidation is larger than that through steam-only oxidation in our previous work (15.6 wt% at 120 oC for 12 h vs. 8.4 wt% at 120 oC for 12 h),31 mostly because the absolute pressure (~0.3 MPa) in the hydrothermal process is higher than that (~0.1 MPa) in steam process, which facilitates the water diffusion across the AlOOH shell; indeed, the obtained AlOOH content clearly shows an increased trend with elevating the absolute pressure in the hydrothermal synthesis (Figure S1; e.g., 19.6 wt% can be achieved at ~1.3 MPa). 3.1.3 Effect of hydrothermal oxidation temperature. The effect of water-only hydrothermal oxidation temperature was investigated in temperature range from 100 to 180 oC with a fixed time length of 12 h by XRD and SEM, with the results as shown in Figures 3 and 4. Clearly, highly homogeneous and dense shells of aluminum hydroxide nanosheets are all formed on the surface of Al-fiber through water-only hydrothermal oxidation process in the temperature range studied. At a low hydrothermal oxidation temperature of 100 oC, nano-sheet boehmite (AlOOH, JCPDS no. 74-1895) and some sporadic bulky bayerite37,41 (Al(OH)3, JCPDS no. 74-1119) are co-generated (Figure 3A, pattern a; Figure 4A,B); however, only boehmite nanosheets are observed except Al substrate in the hydrothermal temperature range from 120 to 180 oC (Figure 3A, patterns b-d; Figure



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4C-H). Moreover, the calculated crystallite size of AlOOH nanosheet at 2 theta of 14.5o is increased gradually with the increase in hydrothermal oxidation temperature: 12, 16, 19 and 24 nm at 100, 120, 150 and 180 oC, respectively, according to the Scherrer-Debye equation; the SEM observations also show a similar increase trend of nanosheet thickness with hydrothermal temperature but bigger values at each temperature point (about 26, 37, 47 and 68 nm at 100, 120, 150 and 180 oC) compared to the results derived from XRD data.37 Possible explanation of the thickening indicative of AlOOH nanosheet with hydrothermal temperature lies in two reasons: (1) both the water diffusion and Al-H2O reaction are favorable at elevated temperature and thus the formation rate of AlOOH is promoted at the local metal/hydroxide interface, which causes the thickness increase of generated boehmite nanosheets; (2) thin AlOOH nanosheets are possibly destroyed at high temperature while thick AlOOH nanosheets are formed instead through the conventional Ostwald ripening.42,43 After calcination at 600 oC, not surprisingly, both bayerite and boehmite are completely transformed into γ-Al2O3 (JCPDS no. 80-0956, Figure 3B); moreover, their leaf-like morphologies with disorderedly aligned nanosheet array are all well preserved on various AlOOH/Al-fiber composites prepared at different hydrothermal oxidation temperatures (SEM images in Figure S2). 3.1.4 Formation mechanism of AlOOH/Al-fiber composite. On the basis of the above results, our water-only hydrothermal oxidation process involves the following steps (H2O → H+ + OH-; 2Al + 6H+ → 2Al3+ + 3H2↑; Al3+ + 3OH- → Al(OH)3↓ (only occurring at or below 100 oC); Al3+ + 3OH- → AlOOH↓ + H2O). During this process, H2O migrates inward across the aluminum hydroxide layer, which is the rate-determining step as previously stated.40 At elevated temperature and pressure, the ionization of water and dissociation of Al proceed easily, and the preferential formation of boehmite (AlOOH) rather than bayerite (Al(OH)3) mainly occurs at the hydroxide/Al interface.36 Since


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boehmite possesses a lamellar structure within an orthorhombic symmetry,44,45 the AlOOH is prone to generate nanosheets. With prolonged hydrothermal oxidation time, abundant AlOOH nanosheets intercrossed with a side-by-side growth and finally assembled into a nanosheet array on the Al-fiber surface. 3.1.5 Extension to other Al substrates. According to the growth mechanism of AlOOH nanosheets on the Al-fiber, our water-only hydrothermal oxidation strategy can be readily extended to any Al substrate on demand, including particle, wire, foil, tube, foam and microchannel. For example, Al-foam (99.9 wt%) and Al-tube (6061 Al alloy) are first cleaned in diluted aqueous solution of sodium hydroxide and washed thoroughly with deionized water. After water-only hydrothermal oxidation at 120 oC for 2 h, leaf-like AlOOH within nanosheet array is homogenously formed on both Al-foam and Al-tube, solidly evidenced by SEM images (Figure 5) and XRD patterns (Figure S3). Moreover, the boehmite endogenously grown on both the Al-foam and Al-tube can be routinely transformed into γ-Al2O3 after calcination at 600 oC. We anticipate that such structured AlOOH and γ-Al2O3 nanosheets possess substantial potential for vast applications in industrial and environmental catalysis.46,47 3.2. Pd/AlOOH/Al-fiber catalyst for CO coupling to DMO The gas-phase CO oxidative coupling to DMO (2CO + 2CH3ONO → (COOCH3)2 + 2NO; -159 kJ mol-1) is the crucial step from syngas (a mixture of CO and H2) to ethylene glycol (hydrogenation of DMO).48,49 The generated NO from CO coupling reaction can be recycled to react with methanol and oxygen (without catalyst) to yield MN (2NO + 2CH3OH + 1/2O2 → 2CH3ONO + H2O; -148 kJ mol-1) and thus the overall reaction (2CO + 2CH3OH + 1/2O2 → (COOCH3)2 + H2O; -307 kJ mol-1) is a chemical-looping and environmentally benign route.50 Although this process has been



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successfully developed and industrialized, the industrial Pd/α-Al2O3 catalyst in the fixed-bed reactor suffers from high Pd loading of 2-3 wt% (the state of the art) and poor intraparticle/interbed mass/heat transfer.11,49 Thanks to abovementioned unique AlOOH/Al-fiber composite with the macroscopic thin-sheet 3D network and microscopic core-shell structure, it was expected the catalysts using such composites as supports could provide pleasurable combination of low Pd content and high performance on the titled reaction, with inherently enhanced mass/heat transfer and large permeability. 3.2.1 Morphology feature. Catalysts using the AlOOH/Al-fiber supports obtained by water-only hydrothermal

oxidation

for

12

h

at

100,

120,

150

and

180

o

C

are

named

as

Pd/AlOOH/Al-fiber-(100/12), -(120/12), -(150/12) and -(180/12). Figure 6 shows the morphology and structure of the representative Pd/AlOOH/Al-fiber-(100/12) catalyst. After loading Pd, the leaf-like morphology is well preserved (Figure 6A,B). The XRD pattern (Figure 6C) and DRIFTS spectrum (Figure S4) of this catalyst clearly indicate that the boehmite phase is well retained and the bayerite phase is totally transformed into boehmite during the catalyst calcination. Notably, no diffraction peaks of Pd (JCPDS no. 01-1201) are detectable, indicating a high dispersion feature (Figure 6C); indeed, the TEM image obviously shows that Pd nanoparticles are highly dispersed on the AlOOH nanosheets with average size of 2.0 ± 0.4 nm (Figure 6D). 3.2.2 Morphology-dependent catalytic performance. Figure 7 shows the CO conversion and DMO selectivity against reaction temperature for the CO coupling to DMO using a gas mixture of MN/CO/N2 (1/1.4/7.6, vol) and a GHSV of 3000 L kg-1 h-1. The CO conversion of Pd/AlOOH/Al-fiber-(100/12), -(120/12), -(150/12) and -(180/12) catalysts all shows a volcano-type evolution behavior against the temperature ranged from 120 to 170 oC and reaches a maxima at 150


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o

C. Once the reaction temperature is higher than 150 oC, the useless decomposition of reactant MN

becomes favorable thereby leading to a decline of CO conversion.51 In addition, with the increase in the reaction temperature, their DMO selectivity decreases gradually, because the elevated reaction temperature is detrimental to the formation of target DMO product but inversely facilitates the formation of DMC byproduct.32 Notably, their CO conversion is increased with the decrease in the hydrothermal oxidation temperature for AlOOH/Al-fiber synthesis from 180 to 100 oC while their DMO selectivity remains almost equal (Figure 7), at each reaction temperature point in the whole range from 120 to 170 oC. As shown in Figure 4, increasing the hydrothermal oxidation temperature from 100 to 180 oC makes the boehmite nanosheets on the AlOOH/Al-fiber supports thickened from 26 to 68 nm while the AlOOH nanosheets preserve well even after loading Pd (Figure 6). Figure 8 show the textural properties of the used Pd/AlOOH/Al-fiber-(100/12), -(120/12), -(150/12) and -(180/12) catalysts. Clearly, SSA and total pore volume of only AlOOH shell (not including the mass of Al-fiber; Pd mass was ignored) show a decline trend as well as their BJH mesopore amount, along with the increase in the hydrothermal oxidation temperature from 100 to 180 oC. In addition, the AlOOH contents of Pd/AlOOH/Al-fiber-(100/12), -(120/12), -(150/12) and -(180/12) catalysts are 18.0 wt%, 15.6 wt%, 16.8 wt% and 20.2 wt%, corresponding to the AlOOH shell SSA of 140, 121, 104 and 92 m2 gAlOOH-1. From the above results, it is clear that the boehmite nanosheet thickness is well in line with the catalyst activity. Thickened boehmite nanosheets reduce the SSA and BJH mesopore amount of AlOOH shell thereby leading to a reduction of catalyst activity. 3.2.3 Insight into the morphology-performance dependence. By CO pulse chemisorption experiments, Pd dispersion (D) of Pd/AlOOH/Al-fiber-(100/12), -(120/12), -(150/12) and -(180/12)



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is determined to be 56%, 47%, 41% and 32%, corresponding to the Pd nanoparticle size (d) of 2.0, 2.4, 2.7 and 3.5 nm according to the equation d = 1.12 / D52 (Table 1). Clearly, this observation is in well agreement with their decreasing trend of the AlOOH SSA and BJH mesopore amount with the hydrothermal oxidation temperature for AlOOH/Al-fiber synthesis from 100 to 180 oC (Figure 8). It is thus believed that the Pd dispersion is favored with the increase in the SSA and BJH mesopore amount of the AlOOH shell. The Pd nanoparticle size of 2.0 nm for the representative Pd/AlOOH/Al-fiber-(100/12) catalyst is consistent with the counterpart observed by TEM (2.0 ± 0.4 nm; Figure 6D). Furthermore, TOF (namely intrinsic activity) was determined by controlling the DMO yield with CO conversion below 10%, with the results as shown in Table 1. Interestingly, the Pd/AlOOH/Al-fiber-(100/12), -(120/12), -(150/12) and -(180/12) catalysts all indicate comparable intrinsic activity because of the equivalent TOF of 0.29-0.40 s-1, unlike the SSA and BJH mesopore amount of AlOOH shell (Figure 8), Pd dispersion (Table 1) and CO conversion (Figure 7) that all show an increasing indicative well in line with the declining trend of the boehmite nanosheet thicknesses (Figure 4) in the order of Pd/AlOOH/Al-fiber-(180/12) < -(150/12) < -(120/12) < -(100/12). High CO conversion of the Pd/AlOOH/Al-fiber-(100/12) catalyst is thus contributed to the enhanced Pd dispersion (i.e., providing more surface Pd active sites) rather than the improved intrinsic activity, in comparison with the other catalyst samples. 3.2.4 Stability. Stability is an important consideration for a heterogeneous catalyst in its practical application. A long-term test was carried out over our representative Pd/AlOOH/Al-fiber-(100/12) catalyst for a feed gas of MN/CO/N2 (1/1.4/7.6, vol) at a GHSV of 3000 L kg-1 h-1 and 150 oC. As shown in Figure 9A, the catalyst shows the promising conversion/selectivity maintenance, delivering the CO conversion of 67.3 ± 0.1% and the DMO selectivity of 96.3 ± 0.1% throughout the entire 150


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h test. Whereas no Pd diffraction peaks (JCPDS no. 01-1201) are detectable for the used catalyst after 150-h test (Figure S5), the Pd dispersion of such used catalyst determined by CO pulse adsorption is decreased slightly to 48% from 56% for the fresh catalyst, corresponding to a slight increase in the Pd particle size to ~2.3 nm from ~2.0 nm.52 The above results indicate the remarkable sintering resistance of Pd nanoparticles highly dispersed on the AlOOH nanosheets, possibly due to the interaction of Pd and hydroxyls of AlOOH nanosheets that can decrease the mobility of Pd nanoparticles evidenced by the density functional theory calculations.53 Moreover, the SEM images clearly show that the leaf-like nanosheet array is well preserved even after 150-h test (Figure 9B,C) while the boehmite phase remains unchanged as evidenced by XRD pattern (Figure S5). 3.2.5 Structure robustness. As previously noted, conventional coating techniques suffer from the detachment of coating layer due to the poor adhesion,13,14 and it is thus expected that our AlOOH/Al-fiber composite obtained by in-situ endogenous growth can possess strong adhesion with excellent mechanical stability. The adhesion ability of the representative Pd/AlOOH/Al-fiber-(100/12) catalyst was evaluated using ultrasound test10 by immersing the catalyst (5.0 g) into petroleum ether (15.0 g) and exposing to ultrasound with power of 200 W for 30 min at room temperature. After that, the catalyst was dried at 150 oC for 2 h and then the weight loss was measured to assess its mechanical stability. Interestingly, the total weight loss is only 0.04 wt%, indicating the AlOOH shell is strongly adhered on the Al-fiber core. The Pd/AlOOH/Al-fiber-(100/12) catalyst after ultrasound test was calcined again at 300 oC for 2 h and examined in the CO coupling to DMO. Not surprisingly, as shown in Figure S6, the Pd/AlOOH/Al-fiber-(100/12) catalysts before and after ultrasound test show comparable catalytic performance in the whole temperature range from 120 to 170 oC: at 150 o

C, for example, 67.3% CO conversion and 96.3% DMO selectivity for the former vs. 66.9% and



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96.2% for the latter. These above results indicate the superior structure robustness of our Pd/AlOOH/Al-fiber-(100/12) catalyst.

4. CONCLUSIONS We demonstrated a facile, green and scalable one-pot method of endogenous growth of free-standing boehmite (AlOOH) nanosheets on a 3D-network 60µm-Al-fiber felt, with the aid of water-only hydrothermal oxidation reaction between Al metal and H2O (2Al + 4H2O → 2AlOOH + 3H2). Content and morphology of AlOOH can be finely tuned by adjusting the hydrothermal oxidation time length and temperature. Such AlOOH/Al-fiber composites have substantial potential for applications in development of novel structured catalysts and reactors. As an example, microfibrous-structured Pd/AlOOH/Al-fiber catalysts have been developed by highly dispersing Pd nanoparticles onto the as-prepared AlOOH/Al-fiber composites through the incipient wetness impregnation method. This approach effectively and efficiently shows an engineering of Pd/AlOOH nanocatalyst from nano- to macro-scale of both porosity and structure in one step. As-prepared catalysts, with high thermal-conductivity and satisfying mechanical robustness, are active, selective and stable for the strongly exothermic CO coupling to dimethyl oxalate (DMO). Pd dispersion and CO conversion show an increasing indicative well in line with the declining trend of the boehmite nanosheet thickness (from 68 to 26 nm) of the AlOOH/Al-fiber supports, while the pristine activity (TOF) of such catalyst shows AlOOH-thickness independence. The most promising catalyst is the one using the Al-fiber-structured 26-nm-thickness AlOOH nanosheets (obtained through hydrothermal oxidation treatment at 100 oC) to support only 0.26 wt% Pd. Over such catalyst, a high CO conversion of 67% is obtained with a high DMO selectivity of 96% and nearly complete conversion of CH3ONO at 150 oC for a feed of CH3ONO/CO/N2 (1/1.4/7.6, vol) with a gas hourly 18/36 ACS Paragon Plus Environment

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space velocity of 3000 L kg-1 h-1, and is maintained almost unchanged for at least 150 h.

ASSOCIATED CONTENT Supporting Information TOF calculation, effect of pressure on obtained AlOOH content, SEM images of various composites calcined at 600 oC, XRD patterns of Al-foam and Al-tube, DRIFTS spectrum, XRD pattern of the catalyst after 150-h lifetime test, and catalytic performance of the catalyst before and after ultrasound test. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *Tel. & Fax: (+86)21-62233424, E-mail: [email protected] (Y. Lu). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was funded by the NSF of China (21473057, U1462129, 21273075, 21076083) and the “973 program” (2011CB201403) from the MOST of China.

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Table 1. Catalyst characteristics and TOF for the CO coupling to DMO a Catalyst

a

Pd

D

(wt%) b Pd/AlOOH/Al-fiber-(100/12)

d

CO conv.

DMO sel.

TOF

(%) c (nm) c

(%) d

(%) d

(s-1) e

0.26

56

2.0

9.5

98.1

0.40

Pd/AlOOH/Al-fiber-(120/12)

0.25

47

2.4

6.7

98.4

0.34


0.27

41

2.7

5.2

98.0

0.29


0.23

32

3.5

4.5

98.9

0.33

Reaction conditions: 0.08 g catalyst, 110 oC, GHSV of 60000 L kg-1 h-1, MN/CO/N2 (1/1.4/7.6, vol),

0.1 MPa.

b

Pd loading determined by ICP-AES.

c

Pd dispersion (D) measured by CO pulse

adsorption assuming a CO/Pd ratio of 1 on account of the systematic errors32 and Pd particle size (d) calculated by the equation (d = 1.12 / D)52. d CO conversion and DMO selectivity. e TOF (detailed calculations in Supporting Information) based on DMO yield and the amount of surface Pd atoms, calculated with a CO conversion below 10% using ultra-high GHSV and low reaction temperature.


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Figure 1. All-in-one for the representative AlOOH/Al-fiber-(120/2) composite. (A) The schematic illustration for top-down preparation. Optical photographs of (B) pristine Al-fiber felt and (C) AlOOH/Al-fiber-(120/2)

composite.

SEM

images

of

(D,E)

pristine

Al-fiber

and

(F)

AlOOH/Al-fiber-(120/2). (G) XRD patterns and (H) DRIFTS spectra of pristine Al-fiber and AlOOH/Al-fiber-(120/2). N2 adsorption-desorption isotherms (I, insert: Barrett-Joyner-Halenda (BJH) mesopore

size

distribution

determined

by

adsorption

branch

of

the

isotherms)

of

AlOOH/Al-fiber-(120/2).



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Figure 2. Relationship between AlOOH content and hydrothermal oxidation time length as well as the fit, for the AlOOH endogenously formed on Al-fiber felt through water-only hydrothermal oxidation at 120 oC in time range from 0.5 to 24 h, and SEM images of corresponding AlOOH/Al-fiber composites.


♣ ♣

12 nm

♥

♠ ♠

16 nm

19 nm 24 nm

20

♥

♥ ♣ ♣

(A)

♣

♥ ♣

a b ♣ c d

40 60 2 theta (degree)

80

Intensity (a. u.)

♣ ♣ ♣Al ♥AlOOH ♠Al(OH)3

(020)

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Intensity (a. u.)

Page 29 of 36

♣Al ♦γ-Al2O3

(B)

♣ ♣ ♣ ♣

♣ ♦ ♣ ♣

20

♦ a

♦ ♣

b ♣c d

40 60 2 theta (degree)

80

Figure 3. XRD patterns of various AlOOH/Al-fiber composites prepared by water-only hydrothermal oxidation for 12 h at different temperatures (A) before and (B) after calcination at 600 o

C for 2 h: (a) 100 oC, (b) 120 oC, (c) 150 oC and (d) 180 oC. Note that crystallite size (D) was

calculated at 2 theta of 14.5o (AlOOH(020)) using Scherrer-Debye equation: D = K λ / (B cos θ), where K = 0.89, λ = 0.154 nm and B is the full width at half maximum (FWHM) of the (020) peak.



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Figure 4. Low- and high-magnification SEM images (insert: thickness distribution of boehmite nanosheet determined by SEM images) of various AlOOH/Al-fiber composites fabricated by water-only hydrothermal oxidation for 12 h at different temperatures: (A,B) 100 oC, (C,D) 120 oC, (E,F) 150 oC and (G,H) 180 oC.


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Figure 5. Macroscopic photographs and high-magnification SEM images of (A,B) Al foam and (C,D) Al tube after water-only hydrothermal oxidation at 120 oC for 2 h.



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Figure 6. All-in-one for the representative Pd/AlOOH/Al-fiber-(100/12) catalyst. The low- and high-magnification SEM images (A,B), XRD pattern (C), and TEM image (D, insert: Pd particle size distribution) of used Pd/AlOOH/Al-fiber-(100/12) catalyst.


80 CO conv. (%)

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100

70

90

60 50 40

(A) (B) (C) (D)

120 130 140 150 160 170 o Reaction temperature ( C)

80

DMO sel. (%)

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70

Figure 7. CO conversion and DMO selectivity against reaction temperature for CO coupling to DMO over Pd/AlOOH/Al-fiber-(100/12) (A), -(120/12) (B), -(150/12) (C) and -(180/12) (D) catalysts. Other conditions: 0.80 g catalyst, GHSV of 3000 L kg-1 h-1, MN/CO/N2 (1/1.4/7.6, vol), 0.1 MPa.



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Figure 8. N2 adsorption-desorption isotherms (insert: BJH mesopore size distribution determined by adsorption branch of the isotherms) of used Pd/AlOOH/Al-fiber-(100/12) (A), -(120/12) (B), -(150/12) (C) and -(180/12) (D) catalysts. Note that SSA and V (total pore volume) of only AlOOH shell (not including the Al-fiber mass; Pd mass was ignored) were calculated based on the AlOOH content of the catalyst, which was determined by the weight increase of the pre-treated Al-fiber before and after the hydrothermal oxidation. The SSA of Pd/AlOOH/Al-fiber-(100/12), -(120/12), -(150/12) and -(180/12) catalysts is respectively 25, 19, 18 and 19 m2 g-1; V is 0.019, 0.013, 0.011 and 0.011 cm3 g-1.


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Figure 9. (A) Lifetime test of the representative Pd/AlOOH/Al-fiber-(100/12) catalyst, and SEM images of the catalyst (B) before and (C) after 150-h test. Reaction conditions: 0.80 g catalyst, 150 o

C, GHSV of 3000 L kg-1 h-1, MN/CO/N2 (1/1.4/7.6, vol), 0.1 MPa.



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TOC


Al-Fiber for CO Coupling to

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