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Letter
Atomically Precise Strategy to a PtZn Alloy Nano-Cluster Catalyst for the Deep Dehydrogenation of n-Butane to 1,3-Butadiene Jeffrey Camacho-Bunquin, Magali S. Ferrandon, Hyuntae Sohn, A. Jeremy Kropf, Ce Yang, Jianguo Wen, Ryan A. Hackler, Cong Liu, Gokhan Celik, Christopher L. Marshall, Peter C. Stair, and Massimiliano Delferro ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02794 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018
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ACS Catalysis
Atomically Precise Strategy to a PtZn Alloy Nano-Cluster Catalyst for the Deep Dehydrogenation of n-Butane to 1,3-Butadiene Jeffrey Camacho-Bunquin,*,† Magali S. Ferrandon,† Hyuntae Sohn,† A. Jeremy Kropf,† Ce Yang,† Jianguo Wen,§ Ryan A. Hackler,ǂ Cong Liu,† Gokhan Celik,† Christopher L. Marshall,† Peter C. Stair,*,†,ǂ Massimiliano Delferro*,† † Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 S Cass Avenue, Lemont, Illinois 60439, United States § Center for Nanoscale Materials, Argonne National Laboratory, 9700 S Cass Avenue, Lemont, Illinois 60439, United States ǂ Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States Supporting Information Placeholder ABSTRACT: The development of on-purpose 1,3-butadiene (BDE) technologies remains an active area in catalysis research due to the importance of BDE in industrial polymer production. Here, we report on a non-oxidative dehydrogenation catalyst for the production of BDE prepared by atomically precise installation of platinum sites on a Znmodified SiO2 support via Atomic Layer Deposition (ALD). In situ reduction x-ray adsorption spectroscopy (XAS) experiments, X-ray photoelectron spectroscopy (XPS), CO chemisorption, and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) imaging of activated PtZn/SiO2 material, revealed the formation of a uniform, well-distributed sub-nano to nanometer PtZn (1.2 ± 0.3 nm) alloy as the active catalytic species.
Keywords: Atomic Layer Deposition, Non-Oxidative Dehydrogenation, Nanoparticles, Alloy, 1,3-Butadiene The accessibility of practical and efficient technologies for the direct conversion of n-butane to 1,3-butadiene (BDE) 1 remains a grand challenge in catalysis science. BDE, a prima2 ry building block of synthetic rubber and a wide range of 3 polymer products, is mainly produced by cryogenic distilla4 tion of C4 fractions from naphtha cracking, or catalytic di5 merization of ethanol. The recent emergence of lighter feedstocks (e.g., shale gas) has created a technology shift focused on upgrading lighter hydrocarbons, which negatively impacts yields of C4 fractions, ultimately raising barriers to the sustainable cracking-based BDE production. Catalytic dehydrogenation of C4 hydrocarbons (e.g., n-butane, butenes) to 6-7 8 BDE, both oxidative and non-oxidative, have long been investigated as on-purpose BDE technologies. Non-oxidative dehydrogenation of light hydrocarbons, an equilibriumlimited, endothermic reaction, requires higher temperature 8 conditions (550 to 700 °C) to achieve practical conversions. These elevated temperatures also promote a range of side reactions which include (1) C–C bond scission via hydrogenolysis or cracking, giving C1-C3 products, (2) isomerization,
forming branched hydrocarbons, and (3) over dehydrogena8 tion that leads to coke deposition and catalyst deactivation. Catalysts selective to BDE via non-oxidative dehydrogena9-14,15 tion on n-butane remain inaccessible; even the most 8 extensively studied Pt catalysts (e.g., PtySnx/Al2O3), are mainly selective to butenes. However, the breadth of fundamental knowledge on Pt-catalyzed dehydrogenation provides insights into the ideal attributes of robust, high-performance catalysts for direct conversion of n-butane to BDE. It has been established that dehydrogenation is insensitive to the 8 structure of Pt, suggesting that all surface-accessible Pt atoms facilitate the reaction. On the other hand, cracking, isomerization and coke deposition are structure-sensitive 16-17 reactions. Acid sites and hydrogenolysis-active Pt sites in nanoparticles are proposed responsible for C-C bond scission, while highly coordinatively unsaturated Pt sites in bulk assemblies promote excessive dehydrogenation to coke deposits. Thus, effective engineering of catalysts by (1) ensuring high Pt dispersity, and (2) enabling the suppression of surface sites responsible for C-C scission and coking, are key to the development of on-purpose BDE catalyst technologies. The ability to tune the electronic properties of Pt is also critical for imparting high BDE selectivity. Optimal catalytic sites should (1) bind strong enough with C4 substrates, butane and butenes, to facilitate dehydrogenation to BDE, but (2) the Pt– BDE interaction should be labile enough to facilitate product desorption and prevent further dehydrogenation. In this report, we describe the development of atomically precise surface-supported Pt-alloy catalysts for deep dehydrogenation of n-butane to BDE under non-oxidative conditions. Specifically, high-surface-area silica (SiO2) was 2+ equipped with a sub-monolayer of Zn anchoring sites via ALD; these cationic zinc sites were shown to stabilize and influence the reactivity of Pt, giving rise to a sub-nanometer PtZn/SiO2 catalyst with unprecedented activity, BDE yields, and stability under practical conditions. The ability of PtZn 18 19 alloys or other catalytic PtZn phases to selectively dehydrogenate hydrocarbons has been reported in the 20 literature. These include (i) stable PtZn spinels on oxide 21 supports, and (ii) PtZn nanoalloys on SiO2 (~ 9 wt% Pt, 2.5 ± 22 0.6 nm). It has been proposed that zinc increases the
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ACS Catalysis Scheme 1. Stepwise ALD synthesis of PtZn alloy nanocluster catalysts for the deep dehydrogenation of n-butane to 1,3-butadiene. CH3
CH3 OH OH Si O Si O Si O O O OO O OH
Zn
Zn(CH2CH 3)2
O
O
O
Pt
H
H 3C
Si O Si O Si O O O OO O
H2O
CH3
CH3
90 °C
ALD
ALD
Zn O
Zn
Pt
O
O
H2
O
O
Si O Si O Si O Si O Si O O O O O OO O O
550 °C, 2 h
O O O Si O O Si O Si Si O O O OO O O O
1 23
electron denstity at the platinum sites in PtZn alloys, weakening Pt–olefin binding and facilitating olefin 24 desorption and faster dehydrogenation rates. Highly dispersed organoplatinum sites were deposited on25-26 to high-surface-area Zn(II)-modified SiO2 (10 wt.% Zn) at 27 28 90 °C via one-cycle ALD employing [(MeCp)PtMe3] as the metal precursor ((MeCp)PtMex/Zn/SiO2; Scheme 1). The sample was then reduced at 550 °C for 2 h under flowing 29 hydrogen to yield the 0.5 wt.% PtZn/SiO2 (1). The same synthetic procedure was used for the deposition of platinum 30-31 on SiO2 (2; 0.5 wt.%). In situ Pt L3-edge XAS reduction experiments on the as-deposited organoplatinum precursor on Zn/SiO2 ((MeCp)PtMex/Zn/SiO2; Figure 1) shows the complete
Normalized Absorption
a)
2.0
1.5 Pt4+
4+
0
reduction of Pt to Pt and the detection of EXAFS features attributable to Pt-Zn (2.57 ± 0.02 Å) and Pt-Pt (2.66 ± 0.02 Å) (see Table S1 for fitting and Figures S1-3), with a Pt-Zn vs PtPt ratio of 15 to 85, suggesting the formation of a Pt-rich PtZn alloy. Note also a shift of ~0.2 eV in the Pt-L3 edge (Figure S1) is observed from 1 to 2, supporting the possible formation of a PtZn alloy. Additionally, X-ray photoelectron spectroscopy (XPS) measurements showed a significant shift of ~0.3 eV (1, PtZn@SiO2 = 71.3 eV) in the Pt 4f7/2 electron binding energy from 2 (71.0 eV), further corroborating the formation of an 32-34 alloyed structure with zinc (Figure S4-S6). Note also that the binding energy of nanocluster 1 (71.3 eV) is very similar to the binding energy of PtZn alloy nanocluster encapsulated 32 in zeolites (71.4 eV). The particle sizes of species 1 and 2 were investigated using the high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) imaging technique and energy-dispersive X-ray spectroscopy (EDS) elemental mapping. The STEM images of 1 showed homogenously dispersed Pt particles (bright dots) with an averaged PtZn particle size of 1.2 ± 0.3 nm (Figure 2).
Pt0
1.0
0.5
[(MeCp)PtMe3] reference (MeCp)PtMex/Zn/SiO2 as-prepared 1
0.0 11540
11560
11580
11600
11620
Photon Energy keV
b) 1.0
[(MeCp)PtMe3] reference (MeCp)PtMex/Zn/SiO2 as-prepared 1
0.8
|χ χ (R)| (Å-3)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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0.6 0.4 0.2
Pt-Zn + Pt-Pt
0.0 0
1
2
3
4
5
6
Radial Coordinate (Å)
Figure 1. a) Pt L3-edge in situ reduction XANES spectra of 4+ (MeCp)PtMex/Zn/SiO2 confirming the Pt state of the pre0 catalysts as prepared and Pt after H2 treatment at 550 °C (1). b) in situ reduction EXAFS spectra of (MeCp)PtMex/Zn/SiO2 as prepared and after H2 treatment at 550 °C (1).
Figure 2. a) HAADF STEM image of 1 showing PtZn nanoparticles (bright dots). b) The histograms show the particle size distribution including averaged PtZn particle size and standard deviation. Furthermore, EDS mapping confirmed that Pt and Zn are homogeneously dispersed in these particles (Figure S7). Several studies have reported nano/sub-nanometer 29, monometallic and bimetallic particles synthesized by ALD 35-36 37-38 and Surface Organometallic Chemistry (SOMC). −1 CO adsorption (2100–1950 cm region; linearly bonded CO on Pt atom) was used to determine the effect of zinc on the electronic structure of the platinum phase. The presence of −1 Zn in 1 results in a red-shift of ~9 cm of the CO stretching −1 frequency compared to 2 (from 2072 to 2063 cm ; Figure S10), due to an increased electron density of surface Pt metal, 24 which further suggests the formation of a PtZn alloy phase. Catalysts 1, 2, and PtSn/Al2O3 were tested for the nonoxidative n-butane dehydrogenation reaction employing
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ACS Catalysis Table 1. Summary of catalytic performance observed for 1, 2, 3, 4, 5 and PtSn/Al2O3.a
Catalyst 1 2 PtSn/Al2O3 3 4 22 5
Zn (wt.%) 9.9
2.5 2.8 8.8
Pt (wt.%) 0.6 0.5 1 0.5 0.07 0.4
[Zn]/[Pt] 52 --16 120 69
Conv. b (%) 79.0-51.7 19.1-8.8 51.2-7.1 36.2-17.7 22.0-13.5 7.5-7.0
Selectivity c (%) C4 Olefin
BDE
92.5–99.0 88.5-77.4 96.4–98.8 99.0–99.2 99.3–99.5 99.6-99.3
12.9–18.4 33.6-27.6 17.5–36.7 21.5–30.8 31.1–32.9 19.5-18.1
BDE d Yield (%) 9.7–9.1 6.4-2.4 8.5-1.6 7.8-5.4 6.9-4.4 1.5-1.3
Initial Specific Activity -1 e (s ) C4 Olefin -1
1.8 x 10 -2 3.3 x 10 -2 6.8 x 10 -2 9.6 x 10 -1 4.2 x 10 -2 2.5 x 10
BDE -2
2.3 x 10 -2 1.3 x 10 -2 1.2 x 10 -2 2.1 x 10 -1 1.3 x 10 -3 4.9 x 10
Kd -1 f (h ) 0.06 0.05 0.13 0.05 0.03 0.01
a
Catalyst pre-activation: 550 °C, 5% H2/Ar (30 mL/min), 1 h; Dehydrogenation conditions: 500 °C, 5% n-butane/Ar (20 -1 b c mL/min), GHSVaverage = 2,800 h . Conversion at 20 h = (initial amount n-butane / Σ all products)/100. Selectivity (%) = (Σ C4 d e -1 products / Σ all products)/100. BDE yield (%) = (% conversion) x (% BDE selectivity). Initial Specific Activity (s ) = (mol prodf -1 uct)/(mol Pt)/s. Kd (h ) = [ln((1-χend)/χend))-ln((1-χstart)/χstart)]/t; where χstart and χend are the conversion at the start and end of 8 the experiment, and t is the time length of catalyst evaluation. a 16-parallel-reactor Flowrence® system. Initially, nonoxidative n-butane (1% n-butane/Ar feed) dehydrogenation was carried out under differential conditions. Catalysts were investigated under varied reaction conditions, including temperature (from 450 to 600 °C; Figure S11), time-on-stream (up to 90 h to monitor catalyst stability), and H2 co-feed (1:1 H2/n-butane; Figure S13). Optimal catalytic performance was observed between 500 and 550 °C, while aggressive deactivation occurred at higher temperatures (≥ 600 °C). Co-feeding with H2 is detrimental to BDE selectivity, while catalyst preactivation with a 10% H2/Ar feed at 550 °C for 2 h resulted in higher activity (Figure S12). Optimal reaction parameters (pre-activation with H2 at 550 °C, catalyst loading of 100 mg, dehydrogenation temperature at 500 °C, identified from the high-throughput screening, Table 1) were employed in all individual fixed-bed micro-reactor experiments with a 5% n-1 butane/Ar feed, employing an average GHSV of 2,800 h . Catalyst 1 outperforms 2 and PtSn/Al2O3, with initial specific -1 -2 -2 -1 activity for C4 olefins of 1.8 x 10 , 3.3 x 10 , and 6.8 x 10 s , respectively. Note the calculated n-butane equilibrium 39 conversion at 503 °C is 83.8% (Table S2). Under identical 26 conditions, the Zn/SiO2 support afforded significantly lower n-butane conversion to butenes (~99% selectivity), confirming that the dehydrogenation activity of 1 is mainly due to Pt. Note also that PtZn nanoparticles on γ-Al2O3 exhibit negligible activity under these reaction conditions (Figure S14). All three catalysts gave trace quantities (
