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Kinetics, Catalysis, and Reaction Engineering

Microfibrous-structured Pd/AlOOH/Al-Fiber with Hydroxyl-Enriched Surfaces for The Catalytic Semi-Hydrogenation of Acetylene Song Wang, Guofeng Zhao, Ye Liu, and Yong Lu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02784 • Publication Date (Web): 08 Aug 2019 Downloaded from pubs.acs.org on August 10, 2019

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Industrial & Engineering Chemistry Research

Microfibrous-structured Pd/AlOOH/Al-Fiber with Hydroxyl-Enriched Surfaces for The Catalytic SemiHydrogenation of Acetylene Song Wang, Guofeng Zhao*, 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.

* Corresponding authors. E-mail: [email protected](G.F.Z.); [email protected](Y.L.) Tel. & Fax: (+86)21-62233424

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ABSTRACT: Thin-sheet Al-fiber-felt structured Pd/AlOOH/Al-fiber catalysts are developed for the back-end process of semi-hydrogenation of acetylene. A series of AlOOH/Al-fiber substrates were synthesized by endogenous growth and subsequent annealing in air. Palladium was then dispersed onto the as-obtained substrates by impregnation. The catalyst activity and stability show interesting substrate annealing temperature dependence. The preferred catalyst, obtained by dispersing lower than 0.05 wt% Pd onto the AlOOH/Al-fiber annealed at 100 oC, exhibits a high specific activity with the turnover frequency (TOF, measured at 40 oC) of 0.0167 s-1, being two times as high as that (0.0083 s-1) for the catalyst using a high substrate annealing temperature of 600 oC. Low AlOOH/Al-fiber annealing temperature enables the catalysts with hydroxyl-rich surfaces that not only promote the hydrogen activation on Pd but also strengthen the adsorption of acetylene. Notably, the Pd-hydroxyl interaction obviously suppresses carbonaceous deposition thereby getting the stability improved by a large extent.

KEYWORDS: structured catalyst; process intensification; Pd-based catalyst; acetylene hydrogenation; hydroxyl modification


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1. INTRODUCTION Ethylene (C2H4) is regarded as the most important petrochemical platform molecule to produce diverse commodity chemicals such as ethanol, acetaldehyde, polyethylene, ethylene oxide, vinyl chloride, and polystyrene, and its annual production reached higher than 150 million tons with continuous growth

every year.1 Nowadays, the

thermal cracking of naphtha is still one of the predominant mode in C2H4 production, but minute amount of acetylene (C2H2, ca. 1 % in the feed gas) is inevitably generated in such process.3,4 Due to their close physical properties, traditional fractional distillation is uneconomic in eliminating the impurity, as it requires high energy and capital expenditures. More notoriously, acetylene even with very low content will severely poison the Ziegler-Natta catalyst, which is commonly used for the ethylene polymerization process. Therefore, the raw product of ethylene must be pre-treated to diminish acetylene content to ppm level.5 From the technical perspective, selective adsorption of acetylene with metal-organic frameworks (MOFs) and semi-hydrogenation of acetylene to ethylene are regarded as the promising methods to solve this problem. However, it is still a grand challenge to design MOFs that possess both high adsorption capacity and selectivity, and the complicated process and high cost in the preparation of MOFs limit their industrial applications.6 In contrast, the semi-hydrogenation of acetylene to ethylene is relatively easy to implement and it is feasible to tune the activity and selectivity of catalysts for this process.2-5


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Palladium (Pd) based catalysts are commonly used for the semi-hydrogenation of acetylene to ethylene, because of their relatively higher catalytic activity in comparison to other metals such as Ni, Pt, and Au.3,7-9 Nevertheless, in industrial process, overhydrogenation of ethylene to ethane at high acetylene conversion is unavoidable. In order to alleviate the over-hydrogenation of ethylene, Pd nanoparticles are usually modified with a second metal such as Ag,10 Au,11 and Cu12, which endows Pd nanoparticles with improved ethylene selectivity but deteriorates the catalytic activity due to the alloying effect of Pd with these second metals. For instance, compared with the Pd/ Al2O3 catalyst, the CuPd/Al2O3 catalyst markedly enhanced the ethylene selectivity from almost 0 to 70%, but the C2H2 conversion was reduced from 85% to 10% at 50 oC.13 Pei et al. reported that use of high Ag/Pd ratio enables the AgPd catalysts with high ethylene selectivity up to 90% while leading to dramatic decline of the activity.14 Mechanistically, the second metals affect catalysts in both electronic and geometric aspects so as to weaken the adsorption of acetylene and ethylene thereby resulting in the high selectivity but low activity.3,15-17 Particularly in the back-end process, there is only minute amount of hydrogen in the feed gas, fully converting acetylene is impossible if the catalyst activity is not high enough.18 Besides the second-metal modification, supports also play the vital roles in tuning the catalyst performance because the electronic and geometric structures of catalytic components can be affected by supports. For the Pd/ZnO catalyst, the PdZn alloy


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formed during the reduction process provides appropriate spatial arrangement of Pd sites in PdZn alloy, which facilitates the acetylene adsorption on catalyst surface and promotes the ethylene desorption from the surface,19 achieving an ethylene selectivity of >90% at almost 100% acetylene conversion. TiO2 can be partially reduced to form Ti3+ species on its surface, thereby promoting the Pd dispersion on its surface and donating electrons to Pd particles.20 As a result, a high ethylene selectivity of 92% could be obtained at full acetylene conversion over the Pd/TiO2 catalyst.20 In addition to these single oxide supports, some complex materials such as fiberglass were also applied as promising supports. The catalyst of 0.02 wt% Pd supported on modified fiberglass showed nearly 100% acetylene conversion and 56% ethylene selectivity at 20 bar, because the modified fiberglass not only allows the high Pd dispersion to improve the catalyst activity, but also selectively adsorbs acetylene to increase the ethylene selectivity even at high acetylene conversion.21 Li et al. synthesized a series of Pd catalysts supported on hydrotalcites and mixed oxides, and the catalyst based on mixed MgO-TiO2 achieved a nearly 100% conversion and 83% selectivity at 70 oC. 22-26 They demonstrated that the acidity/basicity of support would result in a significant effect on Pd dispersion and electronic structure thereby leading to further improvement of the catalyst activity and selectivity.26,27 Despite above interesting advances, the real-world use of these catalysts still remains a challenge as their poor thermal conductivity is detrimental to rapid dissipation


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of reaction heat released in this strongly exothermic reaction (ΔH = -174 kJ/mol), which causes severe hotspots in the catalyst bed and therefore leads to the acetylene excessive hydrogenation in association with more heat releasing.28 Recently, development of structured catalysts based on the monolithic metal-fibers/foams has been attracting significant interests in heterogeneous catalysis due to the intensified heat transfer, which is favorable to tailor catalysts for strongly exothermic reactions, including syngas methanation,29-31 exhaust purification,32-35 oxy-methane reforming,36 hydrogenolysis of dimethyl oxalate to ethylene glycol,37,38 etc. However, to develop qualified

metal-fibers/-foams

thermodynamic

reaction

catalysts

processes,

for highly

applications effective

in and

strongly efficient

endo/exocatalytic

functionalization of their surface is particularly desirable but is a new challenging area. The commercial washcoating method is not suitable to the metal-foam/-fiber substrates, because of the shortcomings, especially, the nonuniformity and easy exfoliation of coatings. Recently, Lu and co-workers39-41 have developed a series of non-dip-coating fabrication methods to overcome the washcoating limitations, including wet chemical etching, galvanic deposition, direct growth, and self-organization. As-prepared structured catalysts have been qualified in several strongly exo-/endo-thermic and/or high throughput reaction processes.39-41 As a whole, non-dip-coating catalytic functionalization of metal-fibers/-foams substrates provides a promising solution for the heat/mass transfer limitations encountered in the packed bed with oxidesupported catalysts (such as Pd) for the selective hydrogenation of acetylene.


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Herein, a thin-sheet 60m-Al-fiber felt was treated with steam to endogenously grow an AlOOH layer along with Al-fiber surface (to form AlOOH/Al-fiber), which was employed to tailor a Pd-based catalyst, and this catalyst exhibited two times higher intrinsic activity than Pd/α-Al2O3 in CO oxidative coupling to dimethyl oxalate.42,43 The high catalytic activity is attributed to the Pd-hydroxyl synergistic interaction, effectively facilitating the adsorption of active CO-species on nano-Pd surface. Allowing for the excellent heat/mass transfer properties of Al-fiber and the special surface characteristic of AlOOH, we used the AlOOH/Al-fiber as catalyst support to fabricate the Pd-based catalysts for the selective hydrogenation of acetylene. Surprisingly, the catalyst exhibited a high acetylene conversion of >99% at 70 oC with only 0.05 wt % Pd loading. To reveal the nature for the high catalytic activity, a series of supports annealed at various temperatures were adopted to prepare the contrastive catalysts, which were characterized by N2 adsorption isotherm, scanning electron microscopy (SEM), inductively coupled plasma atomic emission spectrometry (ICP-AES), hydrogen temperature programmed reduction (H2-TPR), and Infrared Fourier transform spectroscopy techniques. It was found that the active Pd sites are attributed to the Pdhydroxyl synergistic interaction mainly in electronic aspect, which can be tuned by the amount of the surface hydroxyl over AlOOH/Al-fiber. 2. EXPERIMENTAL SECTION


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2.1. Catalyst Preparation. The circular chips (14 mm in diameter and 2 mm in thickness) formed by 60-μm Al-fibers with a voidage volume of 85 vol% (purchased from Shanghai Xincai Net-structured Material Co. Ltd.) were dipped in NaOH (0.1 wt%) aqueous solution for 5 min and washed with deionized water thoroughly. Subsequently, these chips were transferred into a 120 mL hydrothermal reactor, and aged in deionized water at 100 oC for 10 h. The obtained supports, donated as AlOOH/Al-fiber, were respectively annealed from 100 to 600 oC in air, and labeled as AAF-100 to AAF-600. Active component of Pd was then embedded onto the as-obtained AlOOH/Al-fiber substrates, by incipient wetness impregnation method. The prepared supports were impregnated with a certain amount of acetone solution of palladium acetate. After that, the samples were dried at 100 oC for 2 h, and calcined in air at 400 oC (if not specified) for 2 h. Finally, the catalysts were respectively donated as xPd-CAT-100/400 to CAT600/400 (“x” is related to the nominal Pd loading, i.e., Pd weight percentage in whole catalyst; “100 to 600” is related to the corresponding annealed temperature of the AlOOH/Al-fiber substrates; “400” is related to the catalyst calcination temperature). Prior to reaction, the catalysts were reduced in H2 flow at 200 oC for 2h if not specified. 2.2. Catalyst Characterization. The geometry and morphology of catalysts were characterized by scanning electron microscope (SEM, Hitachi S-4800), transmission electron microscopy (TEM, FEI-Tecnai G2F30 spectrometer), and high-angle annular dark field transmission electron microscopy (HAADF–STEM, FEI-Tecnai G2F30


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spectrometer). X-ray diffraction (XRD) patterns were obtained by a Rigaku Ultra IV diffractometer using the Cu Kα radiation at 35 kV and 25 mA in the 2θ scanning range of 10-80° and at the scanning rate of 30° min−1.Thermogravimetric analysis (TGA, NET2SCH STA449F3) and inductively coupled plasma atomic emission spectroscopy (ICP-AES; ICP Thermo IRIS Intrepid II XSP) were applied to quantitatively determine the contents of hydroxyl group and Pd component. Specific surface area (SSA) was determined from N2 adsorption isotherm at -196 oC according to standard BrunauerEmmett-Teller (BET) theory. The X-ray photoelectron spectroscopy (XPS) was conducted on an Escalab 250xi spectrometer, employing a standard Al K X-ray source (300 W) and analyzer pass energy of 30.0 eV. All binding energies were referenced to the adventitious C1s line at 284.8 eV. The Pd dispersion was determined by CO pulse adsorption experiments conducted on a Quantachrome ChemBET 3000 chemisorption apparatus with a pulse volume of 250 μL. 150 mg sample was pre-reduced with 10 vol% H2 in Ar (50 mL/min) at 200 oC for 1 h, and then flushed with He at 200 oC for 30 min. Subsequently, the reactor was cooled down to 40 oC and stabilized for 1 h to ensure the TCD signal steady. Then, 250 μL gas mixture of 10% CO in He was pulsed to the sample every 3 min until the CO peak intensity remained unchanged. The CO/Pd average stoichiometry was assumed as 1. H2-temperature programmed reduction (H2-TPR) was performed using the same equipment with 150 mg of sample (loaded with 1 wt% Pd to obtain obvious signals) in


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each trial. Before the reduction, the sample was purged in He at 400 oC for 1 h to remove the adsorbed species on the surface. When the reactor was cooled down to room temperature, the sample was exposed to a gas mixture of 5% H2 in Ar for 1 h. After that, the reactor was heated to 500 oC at the ramp of 10 oC/min. In all cases, the flow rate of gas fed to the reactor was maintained at 50 mL/min.

In-situ infrared Fourier transform spectroscopy (FTIR) of pyridine, CO2, CO and C2H2 was recorded using a Nicolet Nexus 670 FT-IR spectrometer. The sample was prepared with powder taken from corresponding catalysts. Thin self-supported wafers with a diameter of 13 mm were squashed with 15 mg powder and placed inside a controlledenvironment infrared transmission cell sealed with CaF2 windows. Then the samples were pre-reduced in H2 at 200 oC for 1 h and flushed with N2 for 30 min, followed by the record of a background at room temperature. Thereafter, the sample was exposed to the corresponding gas for 30 min. Then the spectra of adsorbed pyridine species were collected after evacuation in 10−2 Pa vacuum for 1 h, and the spectra of CO, CO2 and C2H2 absorbed species were collected after flushed in the constant nitrogen flow for 1 h. All the spectra were collected from 4000 to 400 cm-1 at a resolution of 4 cm-1. 2.3. Semi-hydrogenation of Acetylene. Semi-hydrogenation of acetylene was carried out in a fixed-bed micro-reactor with 0.50 g catalyst under atmospheric pressure. The feed gas was consisted of 0.66% H2, 0.33% C2H2, 33.33% C2H4 and N2 (balance gas). The catalytic reaction was performed over 20 to 100 oC using a gas hourly space


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velocity (GHSV) of 9,000 mL g-1 h-1. The effects of H2/acetylene molar ratio and GHSV were investigated from 1.0 to 4.0 and 4500 to 18000 mL g-1 h-1, respectively. All data were collected after the reaction remained stable for at least 20 min under the appointed conditions. The turnover frequency (TOF) to the acetylene was measured at 40 oC under a low acetylene conversion (

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