Correlation of Active Site Precursors and Olefin Metathesis Activity in

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Correlation Of Active Site Precursors And Olefin Metathesis Activity In W-Incorporated Silicates Jianfeng Wu, Anand Ramanathan, Alessandro Biancardi, Amy Jystad, Marco Caricato, Yongfeng Hu, and Bala Subramaniam ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03263 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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ACS Catalysis

Correlation Of Active Site Precursors And Olefin Metathesis Activity In W-Incorporated Silicates Jian-Feng Wu † ‡, Anand Ramanathan‡, Alessandro Biancardi▽, Amy Marie Jystad▽, Marco Caricato▽, *, ,

Yongfeng Hu#, and Bala Subramaniam ‡ ‖ *, ,

† State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China. ‡ Center for Environmentally Beneficial Catalysis, The University of Kansas, Lawrence, KS 66047, USA. ‖Department of Chemical and Petroleum Engineering, The University of Kansas, Lawrence, KS 66045, USA. ▽ Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, Kansas 66045, United States # Canadian Light Source Inc., University of Saskatchewan, 44 Innovation Boulevard, Saskatoon, SK S7N 2V3, Canada.

1

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KEYWORDS: mesoporous materials • tungsten • metathesis • XPS • NMR spectroscopy

ABSTRACT: SiO -supported tungsten catalysts (WO /SiO ) are currently used in industry to make propene via 2

3

2

metathesis of butene and ethylene. Reliable information on structure-activity relationship, aimed at improved catalyst design, continues to be an active area of investigation. Based on ex situ XPS and solid-state NMR spectroscopy of calcined W-incorporated mesoporous silicates (W-KIT-6), complemented by computational studies and activity evaluation in a fixed-bed reactor, we demonstrate that W–O–Si species are the active site precursors for 2-butene + ethylene metathesis to produce propene at 450°C. This conclusion is supported by evidence that the population of W–O–Si species on the calcined W-KIT-6 catalysts is linearly correlated with 2-butene conversion, regardless of W loading. We further show that for a given W loading, the accessible WO-Si species, and thereby the propene yield, can be enhanced by delaying the tungsten source addition during catalyst synthesis. Our results are consistent with previously reported in situ mechanistic studies using complementary analytical tools.

Introduction Tungsten based catalysts have been widely used as heterogeneous catalysts in petroleum refining and chemical manufacturing processes. Among these processes, propylene production from ethylene and butene 1-3

via olefin metathesis is receiving extensive interest in the U.S. due to the increased availability of shale gas and the decreased supply of naphtha as feedstock. Impregnated WO /SiO catalysts are typically used in 4-9

3

2

industry for olefin metathesis, but they suffer from low dispersion of the active W species and the formation 6

of inactive crystalline WO species. Accordingly, numerous efforts have been devoted to developing catalysts 3

10-12

with enhanced W dispersion. These methods include grafting,

7,13-18

spreading,

32-33

flame-spray pyrolysis, aerosol, 34

35-37

sol-gel,

19-26

non-hydrolytic sol-gel,

27-31

thermal

and evaporation-induced self-assembly.

38

2


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To better understand the structure-activity relationship, the Trunschke39, Wachs , Stair and Bell groups 40-41

42

43

have reported mechanistic investigations of WO /SiO and MoO /SiO catalysts. The Trunschke group found 3

2

3

2

that the occurrence of strain on the active sites is correlated to activity on MoO /SiO catalyst. Using Raman 3

39

2

amd XAS techniques, the Wachs group reported that dioxo (O=) WO (dominant species, > 90%) and mono2

2

oxo O=WO species are present on dehydrated WO /SiO catalyst when the W loading is below 8 wt%, while 4

x

2

crystalline WO nanoparticles are present at W loading ≥ 8 wt%. They also speculated that the dioxo species 40

3

are the most active precursors for metathesis. The Stair group reported that a 100-1000-fold increase in the 40

low temperature propene metathesis activity can be achieved by pretreatment of MoO /SiO and WO /SiO 3

2

3

2

catalysts under an olefin-containing atmosphere at elevated temperature. Furthermore, a strong correlation 42

between the population of active sites and monomeric Mo(=O) dioxo species was reported by the Stair group.

42

2

Using Raman, UV-vis and XAS spectroscopies, the Bell group reported that the mono-oxo O=WO species can 4

convert to dioxo (O=) WO species under He or propene atmosphere at reaction condition, with the dioxo 2

2

species being more rapidly activated by propene. From kinetic studies, Lwin and Wachs identified three types 43

of active sites [highly active at ~ 160°C, modestly active at ~ 450°C, and sluggishly active at ~ 600-750°C] on WO /SiO catalyst. x

XPS

2

44-46

41

and solid-state NMR

47-52

spectroscopy have emerged as powerful tools to investigate the nature of the

active sites on heterogeneous catalysts. In this work, we have used these tools to identify three kinds of structures (W–O–W, W–O–Si and Si–O–Si) on calcined W-KIT-6 catalyst, prior to conducting the metathesis reaction (Scheme 1). Results from complementary computational studies of model tungsten silicate structures were used to qualitatively confirm XPS peak assignments as well as chemical shift attributions in solid-state NMR spectroscopy results. Steady-state conversion data for the metathesis of ethylene and 2-butene to form propene were obtained in a fixed-bed reactor and correlated with the measured population of the active site precursors on several fresh W-KIT-6 catalysts with different W loadings. Our investigations were motivated 3


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by our quest to determine if any clear correlation exists between the surface species on the fresh catalyst (calcined in air at 550°C for 10 h) and metathesis activity. Such a correlation, if it exists, should provide convenient guidance for synthesizing catalysts in which the population of the active precursor species is maximized.

Scheme 1. Three possible structures (W–O–W, W–O–Si and Si–O–Si) on fresh W-KIT-6 catalyst to make propene from ethylene and 2-butene. The W–O–Si species are the active site precursors. The SSNMR represents solid-state NMR.

Experimental section Chemicals WO (≥ 99%), triblock copolymer Pluronic P123 (EO -PO -EO , average MW = 5800), tetraethyl 3

20

70

20

orthosilicate (TEOS, 98%) were purchased form Sigma-Aldrich. H WO (99+%) and Na WO ·2H O (99.9%) 2

4

2

4

2

were purchased from Acros and Alfa Aesar, respectively. 1-Butanol (HPLC grade) and hydrochloric acid (certified ACS plus) were purchased from Fisher Scientific. Catalyst preparation W-KIT-6 catalyst.26,53 4.5 g P123, 160 mL H2O, and 8.3 g 37.1 wt% HCl were mixed by a hotplate at 38 ± 2°C for 4 h. Then, 4.5 g 1-butanol were added into the resulting mixture and the system was stirred for another 1 h. After that, 9.7 g TEOS and the desired amount of sodium tungstate were added without any delay and the 4


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mixture was kept for stirring for 24 h. The mixture was charged into a autoclave with Teflon-lined and treated at 100°C for 24 h. The obtained solid product was washed by deionized water, then dried at 100°C overnight. The sample was further treated in a muffle furnace to remove the template with flowing air at 550°C for 10 h at a ramp rate of 1°C/min. The resulting sample was denoted as fresh catalyst. These synthesized samples are designated as W-KIT-6 (X) where X represents W wt%. Modified W-KIT-6 catalyst.26 Addition of the tungsten source was delayed in this synthesis method in order to increase the population of accessible W species on the catalyst surface. 4.5 g P123, 160 mL H2O, and 8.3 g 37.1 wt% HCl were mixed by a hotplate at 38 ± 2°C for 4 h. Then, 4.5 g 1-butanol were added into the resulting mixture and the system was stirred for another 1 h. Following that, 9.7 g TEOS was added into the mixture. After 2 h, the desired amount of sodium tungstate (dissolved in 5 mL H2O) was added. After the addition of the tungsten source, the mixture was kept stirred for 24 h. The mixture was charged into a Teflon-lined autoclave and treated at 100°C for 24 h. The resulting solid product was washed with deionized water, then dried overnight at 100°C. The sample was further treated in a muffle furnace with flowing air to remove the template at 550°C for 10 h at a ramp rate of 1°C/min. The resulting sample was denoted as fresh catalyst. The catalyst sample is designated as W-KIT-6 (t, x) where t and x reflect the delayed addition time of the tungsten source (h) and the tungsten weight percent, respectively.

Catalyst characterization X-ray photoelectron spectroscopy (XPS) was performed with a PHI 5000 Versa Probe II instrument equipped with Monochromated Al Kα (50 W, 15 kV) X-ray. The diameter of the X-ray beam is 200 µm. A dual-beam charge neutralizer was adopted to compensate the charging effect. The spherical capacitance analyzer was operated with a pass energy of 46.95 eV for the detailed scan. To obtain information mainly from the catalyst 5


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surface, the sample stage was tilted to 20°.

26,38

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The surface of the catalyst sample was sputtered with Ar ions +

(2.0 keV) for 5 min prior to the analysis to remove the adventitious carbon.

26,54

Another reason for the Ar ion +

sputtering pretreatment is to expose the W species that reside deep in the pores of the mesoporous W-KIT-6 catalyst. During the Ar ion sputtering treatment, the electron flood gun is used to neutralize extra charge on +

the surface. The spectrum fitting was processed with CasaXPS software. All XPS peaks were referenced to the Si peak at 103.3 eV. 2p

The paramagnetic species in the fresh and used catalysts were analyzed by electron paramagnetic resonance (EPR) spectrometer (Bruker EMX PremiumX) equipped with Oxford ITC503 temperature system to monitor and regulate the temperature. The fresh or used catalysts were transferred to the EPR tube at ambient condition. Spectra were recorded at the following settings: attenuation = 20.0 dB, microwave power = 2.00 mW, frequency = 9.34 GHz, center field = 3500 G, sweep width = 1000 G, receiver gain = 30 dB, modulation amplitude = 5.999 G, resolution = 2048 points, modulation frequency = 100 kHz, time constant = 10.24 ms, T = 80 K. X-ray absorption near edge structure (XANES) measurements were used to identify the oxidation state of tungsten. W L -edge spectra were collected at the IDEAS beamline of Canadian Light Source (CLS), using Ge 1

(220) double crystal monochrometer for an energy range of 3.4-13.4 keV. Spectra were recorded in a transmission mode at room temperature. All the XANES data were analyzed by using Athena software.

55

The chemical environment of Si in the fresh catalysts was studied by solid-state NMR. The fresh catalyst sample was packed into a 4 mm Bruker rotor at ambient condition. The sample was then investigated by Si 29

high-power proton decoupling (HPDEC) MAS NMR spectroscopy on a Bruker Avance III WB 400 MHz NMR instrument at a spinning rate of 10.0 kHz. The Si chemical shift was referenced to tetramethylsilane (TMS). 29

The deconvolution was performed on TopSpin 3.5pl6 software.

6


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Measurement of metathesis activity Details for evaluation of the metathesis activity of W-KIT-6 catalysts may be found elsewhere. Briefly, the 26

2-butene+ethylene metathesis reaction was performed in a fixed-bed reactor loaded with activated W-KIT-6 catalyst (treated at 550°C for 1 h with N ) at T = 450°C, P = 1 atm, WHSV (ethylene and 2-butene) = 2.0 h , -1

2

n(ethylene)/n(2-butene) = 3/1. The reaction products were analyzed by an on-line Varian CP-3800 gas chromatograph equipped with a six-way valve, an Agilent GS-gaspro column (30 m x 0.320 mm), and a flame ®

ionization detector (FID). The following equations were used to assess the performance (2-butene conversion) of the W-KT-6 catalysts and the mass balance closure: 𝑋"#$%&'(' =

[2– 𝑏𝑢𝑡𝑒𝑛𝑒]4( − [2– 𝑏𝑢𝑡𝑒𝑛𝑒]6%& × 100% [2 − 𝑏𝑢𝑡𝑒𝑛𝑒]4(

𝑀𝑎𝑠𝑠 𝑏𝑎𝑙𝑎𝑛𝑐𝑒 𝑐𝑙𝑜𝑠𝑢𝑟𝑒 = where X

2-butene

[𝑇𝑜𝑡𝑎𝑙 𝑐𝑎𝑟𝑏𝑜𝑛 𝑖𝑛 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠] × 100% [𝑇𝑜𝑡𝑎𝑙 𝑐𝑎𝑟𝑏𝑜𝑛 𝑖𝑛 𝑓𝑒𝑒𝑑 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡𝑠]

represents 2-butene (the limiting reactant) conversion. The mass balance closure at steady state was

100 ± 2.5%, which is within experimental error. The conversions were confirmed to be free from internal and external transport limitations as explained elsewhere.

26

DFT Calculations W 4f XPS binding energy. Three different types of clusters were proposed: (i) single site tetrahedral coordinated W [W–(O–Si) ], (ii) polymeric tetrahedral coordinated W [W–(O–W) , polytungstate], and (iii) n

m

octahedral coordinated W clusters [W–(O–W) , WO ]. In the case of the single site tetrahedral coordinated W m

3

clusters, the binding energies were obtained as an average over 10 cluster structures taken from the Metaldoped Amorphous Silicate Library (METASIL). In the case of the polymeric tetrahedral coordinated W, six 56

types of tetrahedral coordinated clusters with two, three or four W atoms were considered, while in the case of

7


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Page 8 of 28

the octahedral coordinated clusters, three types of clusters with [(WO ) , n = 3, 4, 5], four and eight W atoms 3 n

were considered. All the structural optimizations were performed by using the B3LYP functional and the Def2TZVP basis 57

set for the W atoms, where the core electron were represented by using Stuttgaurt’s pseudopotentials . The 658

59

31++G(d,p) basis set was used for the Si, O, and H atoms. In case of octahedral coordinated W clusters, the structures were built by using the experimental lattice parameters, i.e.

a

= 3.77 and β = 90° as in the cubic

crystal. A slightly distorted octahedral structure was built by using the experimental lattice parameters of the 60

monoclinic WO crystal, i.e. a = 7.31, b = 7.54, c = 7.69, and β = 90.9°,

61-62

3

as an ideal monoclinic structure

(without distorting the position of the oxygen atoms of the cubic crystal). The binding energies were obtained within the framework of the Koopmans' theorem, i.e., by monitoring the Kohn-Sham (KS) eigenvalues of the W 4f core electrons. All the binding energy calculations were performed by using the B3LYP functional, and the full-electrons UGBS basis set for the W atoms and the 6-31++G(d,p) 57

63

basis set for the Si, O, and H atoms. All calculations were performed with a development version of the GAUSSIAN suite of programs.

64

Si NMR chemical shifts. The calculated Si NMR chemical shifts of ten W-incorporated amorphous silica

29

29

clusters (~175 non-hydrogen atoms) are presented here. W has the oxidation state of VI and a coordination number of four, i.e., W(=O) (O–Si) . The cluster geometries are also taken from METASIL as in the case of 2

2

XPS 4f binding energy simulations. The calculations were performed with the B3LYP functional and D3 56

dispersion correction. The basis sets used for the various elements are: Def2TZVP and Stuttgaurt’s 65

pseudopotentials for W, 6-311+g(2d,p) for 100 atoms nearest to the metal, and 3-21g for the rest. Si NMR 58 29

chemical shifts are calculated relative to tetramethylsilane (TMS), which was also treated with B3LYP-D3/6311+g(2d,p).

66-67

Only the Si treated with 6-311+g(2d,p) were used to calculate the chemical shifts, which are

8


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ACS Catalysis

presented here as averages. All calculations were performed with a development version of the GAUSSIAN suite of programs.

64

Results and discussion To understand the structure of the W species, XPS analysis was performed on two fresh W-KIT-6 catalysts

26

synthesized by different methods. The XPS spectra of the W 4f region for the fresh (a) W-KIT-6 (8.7) catalyst containing 8.7 wt% W, and (b) W-KIT-6 (2 h, 9.2) catalyst, containing 9.2 wt% W in which the addition of 68

the W source was delayed by 2 h to obtain better W accessibility, are shown in Figure 1. After deconvolution 26,68

of the XPS spectra (Figure 1, Tables 1 and S1), two groups of W signals, similar to those previously reported on W-modified mesoporous silicates, are identified.

20,23,69

Species 1 is assigned to W–(O–W) moiety (W in m

polytungstate and WO , where W is in W oxidation state) rather than simple W species reported previously. 6+

6+

20,23,69

3

Further, the prevalence of species 1 is greater in W-KIT-6 (8.7) than in W-KIT-6 (2 h, 9.2) catalyst (Table 1). This trend is consistent with previously reported XRD patterns and UV-Vis spectra that reveal higher peak 26

26

intensities for WO species in W-KIT-6 (8.7) catalyst. We assign Species 2 to the W–(O–Si) moiety rather than 3

n

W species suggested in previous reports. 5+

20,23,69

This assignment is supported by both EPR and XANES results

that reveal only W species and no W species on W-KIT-6 (Figures S1 and S2). As shown in Figure 1, the 6+

5+

binding energy of the W–(O–Si) species is downshifted by 1.5-1.7 eV, attributed to differences in the chemical n

environments (bond lengths, bond angles, and electronegativity) of –O–Si compared to –O–W. Several W–(O–W) (WO and polytungstate) and W–(O–Si) structures were considered for theoretical m

3

n

calculations (Figure 2). Density functional theory (DFT) calculations based on these models are shown in Table 2. The calculations indicate that the binding energy is significantly smaller for the single-site structure (1), compared to the polytungstate models (2a-2c and 5-10). The difference varies in a range of 0.5-1.1 eV, which qualitatively confirms the experimental assignment. The difference between the single-site and polytungstate 9


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species increases with the number of W centers (going from models 5-6 towards 9-10), which suggests that longer chains are present in the material compared to the models used here. We note that a quantitative agreement with the experimental values is hard to achieve for a variety of reasons. First, we used a relatively simple model chemistry to describe the binding energy: a relatively small basis set, no relativistic corrections, and no-orbital relaxation effects on the ionization calculations. Second, and more importantly, the amorphous nature of the surface and the unknown number of W centers in the polytungstate chains make the precise modeling of these species particularly difficult. The binding energy of the octahedral species (3 and 4) are only slightly larger (0.3-0.4 eV) than the single site species 1. The difference with the ideal monoclinic model is even smaller, 0.2 eV. However, these models represent idealized, symmetric structures that are unlikely to be present on the amorphous surface treated at such high temperatures and low loadings as in the current experiments. Thus, it is likely that more disordered WO model structures would provide higher values of the 3

binding energy, similar to the polytungstate models. The sampling of such structures would also require a very intensive computational effort that is beyond the scope of the present work, where the focus is on a qualitative comparison with experiment. Despite these limitations, the simulations persuasively indicate that W–(O–W)

m

species display higher W 4f binding energy than W–(O–Si) species, thus qualitatively confirming the n

experimental trends.

10


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Figure 1. XPS spectra of the W 4f region for (a) fresh W-KIT-6 (8.7) and (b) fresh W-KIT-6 (2 h, 9.2) catalysts (the black lines are the original spectra; the purple lines are the fitted spectra; the red and blue lines are fitted components). The deconvolution of the W 4f region was performed following reported procedures.

69-70

Table 1. Peak-fitting results of W4f XPS spectra for fresh W-KIT-6 (8.7) and W-KIT-6 (2 h, 9.2) catalysts. Binding energy for W 4f (eV) Catalyst

Species 1

Species 2

Peak area of Peak area of Species 1 Species 2 /% /%

W 4f 5/2

W 4f 7/2

W 4f 5/2

W 4f 7/2

W-KIT-6 (8.7)

38.8

36.7

37.1

35.0

44.9

55.1

W-KIT-6 (2 h, 9.2)

38.7

36.6

37.2

35.1

35.0

65.0

11


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Figure 2. Several models for the W–(O–W) (WO and polytungstate) and W–(O–Si) structures used for the m

3

n

binding energy calculations. Although the picture of structure 1 only includes the Si centers directly bonded to the W center for clarity, we actually use the ten METASIL clusters (which include ~175 non-H atoms) for the binding energy calculations. The W, Si, O, and H atoms are labeled with cyan, gray, red, and white color, respectively.

12


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Table 2. Calculations based on several models the W 4f binding energy of W–(O–W) (WO and polytungstate) m

3

and W–(O–Si) structures at the B3LYP/UGBS[W],6-31++G**[H,O,Si] level of theory.

a

n

Structure W–(O–Si)

n

W–(O–W) , WO m

3

Model

Averaged Binding Energy (eV)

1 W in silica

46.8

2a, 2b, 2c (WO ) , n = 3, 4, 5

47.9

3 Cubic W4

47.2

4 Cubic W8

47.1

3 n

[b]

Ideal Monoclinic W8

5 (WO ) OSi(OH) 3 2

47.4

2

6 (WO ) [OSi(OH) ]

47.3

7 (WO ) OSi(OH)

47.5

3 2

2 2

3 3

W–(O–W) , polytungstate

47.0

2

m

8 (WO ) [OSi(OH) ]

47.5

9 (WO ) OSi(OH)

47.8

3 3

3 4

2 2

2

10 (WO ) [OSi(OH) ] 3 4

2 2

47.6

Several cluster models for evaluating the binding energy of W 4f in W doped silicates were proposed. The three types of clusters considered here include single site tetrahedral coordinated W [W–(O–Si) ], polymeric tetrahedral coordinated W [W–(O–W) , polytungstate], and octahedral coordinated W clusters [W–(O–W) , WO ]. All the [W–(O–W) , WO ] models show higher binding energy than W–(O–Si) species. When the size of the W cluster (5-10) increases, the averaged binding energy of W 4f increases and becomes close to the range of model 2. The calculated results indicate that the averaged binding energies of W–(O–W) species (WO and polytungstate) are higher than the W–(O–Si) species. No picture is provided, as the structures of Cubic W8 and Ideal Monoclinic W8 are very similar. a

n

m

3

m

m

3

n

m

3

n

b

To further probe the structure of Species 2 in the XPS spectra, Si high-power proton decoupling (HPDEC) 29

NMR technique was employed. The deconvoluted NMR spectra are shown in Figure 3 (the raw spectra without deconvolution are shown in Figure S3). As shown in Figure 3a, a shoulder peak around -103.7 ppm is clearly observed in W-KIT-6 (8.7). The intensity of this peak increases significantly in the W-KIT-6 (2 h, 9.2) catalyst (Figure 3b and Table 3). The chemical shift of this peak (-103.7 ppm) is different from those seen in neat KIT13


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6 silicates at -99.2 ppm for Si–(OH)(O–Si) species and -91.3 ppm for Si–(OH) (O–Si) species.

71-73

3

2

2

Klepel et al.

assigned the peak around -103.7 ppm to the Si(3Si,W) structure, in which the silicon is flanked by three Si atoms and one W atom in tungsten incorporated W-MCM-41. Similar observations were reported for Ga74

MCM-41 and aluminosilicates. 75

72-73

In addition, the Taoufik group directly observed the W– O–Si structure by 18

17

O NMR spectroscopy through O labelled studies. To further confirm the assignment above, W–O–Si–(O–

17

17

Si) and Si–(O–Si) structures were built to compare the difference in chemical shift for those structures (Figure 3

4

4). DFT calculations (Table 4) support the assignment of the W–O–Si–(O–Si) structure. The shifts observed 3

experimentally (d

W-O-Si-(O-Si)3

-d

Si-(O-Si)4

= 6.4 ppm) and those predicted by DFT computations (4.7 ppm) are closely

matched. Accordingly, the peak around -103.7 ppm is assigned to W–O–Si–(O–Si) structure, which is 3

consistent with the inferred structure of Species 2 (W–O–Si) based on the XPS method (Figure 1).

Figure 3. Si HPDEC NMR spectra of W-KIT-6 and KIT-6 catalysts. (a) W-KIT-6 (8.7), (b) W-KIT-6 (2 h, 29

9.2), and (c) KIT-6 catalysts. For the deconvolutions, the blue lines are the original spectra, the red lines are the fitted spectra, and the green, purple as well as yellow lines are the deconvoluted components.

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Figure 4. An example of the W–O–Si–(O–Si) structure and the Si–(O–Si) structure are shown in the picture. 3

4

The W, Si, O, and H atoms are labeled with cyan, gray, red, and white color, respectively. The ball-and-stick atoms indicated the local W structure, the surrounding one-hundred tube atoms are treated with 6-311+g(2d,p) basis set, the remaining wireframe atoms are treated with 3-21g basis set. Table 3. Peak-fitting results of Si HPDEC NMR spectra for fresh W-KIT-6 (8.7) and W-KIT-6 (2 h, 9.2) 29

catalysts.

Catalyst

Chemical shift / ppm

Peak area / %

W–O–Si–(O–Si)

Si–(O–Si)

W–O–Si–(O–Si)

Si–(O–Si)

-103.7

-110.1

55.8

44.2

W-KIT-6 (2 h, 9.2) -103.8

-110.1

72.9

27.1

3

W-KIT-6 (8.7)

4

3

4

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Table 4. Comparison of chemical shift results between the W–O–Si–(O–Si) and the Si–(O–Si) structures. 3

d

W–O–Si–(O–Si)3

d

Si–(O–Si)4

Δ [d

Experimental value

-103.7

-110.1

6.4

Averaged calculated value

-116.5

-121.2

4.7

W–O–Si–(O–Si)3

4

-d

]

Si–(O–Si)4

As shown in Table 5, the ratio (1.24) of the experimentally observed 2-butene conversions with W-KIT6 (2 h, 9.2) and W-KIT-6 (8.7) catalysts based on fixed-bed reactor studies (see Supporting Information for details) closely track the ratio of the peak areas of the W–O–Si structures in these catalysts obtained from XPS results (1.18) and solid-state NMR results (1.31). As shown in Figure 5, the 2-butene conversion values (Table S2) and the peak area percentages (based on Solid-state NMR measurement) corresponding 26,76

to the W–O–Si structure in the W-KIT-6 catalysts (Figure S4-S6, Tables S2 and S3) display a linear relationship, regardless of W loading. Therefore, we conclude that the W–O–Si species are active site precursors for metathesis of 2-butene and ethylene to propene at 450 °C. To our knowledge, such a clear correlation has not been reported previously. Figure 5 also shows that the population of W–O–Si species is not proportional to the W loading, decreasing in fact at the higher W loadings (8.7 and 13.7 wt%). Also, the population of W–O–Si species increases sharply at 9.2 wt% W loading when the addition of the W source is delayed by 2 h following the addition of the Si source. The trend of overall W loading and 2butene conversion thus clearly indicates that maximizing W utilization (i.e., maximizing the fraction of active site precursors on the fresh catalyst) depends not only on the W loading but also on the synthesis technique. Solid-state NMR- rather than XPS-derived W-O-Si peak areas are used in Figure 5 for the following reasons: 1) The dispersion of W species in the W-KIT-6 catalysts is not homogeneous; 2) The XPS measurement is a semiquantitative method with the photoelectron emissions being confined to only several nanometers below the incident surface. In contrast, the solid-state NMR analysis provides a quantitative measurement of the W-O-Si content in the sample and hence used to reliably represent the correlation in Figure 5. The amount of W-O-Si species in the catalyst sample is given by the product of the ACS Paragon Plus Environment

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SSNMR peak area % corresponding to W-O-Si species and the weight of catalyst. Since we used a constant amount (1 g) of catalyst in each catalytic run, the linear correlation in Figure 5 is preserved. Table 5. The relationship between the metathesis activity (2-butene conversion) and peak area of W–O–Si structure measured with either XPS or solid-state NMR.

2-Butene conversion / %

Peak area / %

W-KIT-6 (8.7)

54.9

55.1

55.8

W-KIT-6 (2 h, 9.2)

68.3

65.0

72.9

Ratio

1.24

1.18

1.31

Catalyst

a

a

XPS

b

SSNMR

c

Value corresponding to [W-KIT-6 (2 h, 9.2)] catalyst/value corresponding to [W-KIT-6 (8.7)] catalyst.

b

The XPS value represents Area (W-O-Si)/[Area (W-O-W) + Area (W-O-Si)].

c

The SSNMR value represents Area (W-O-Si)/[Area (W-O-W) + Area (Si-O-Si)].

Figure 5. Linear correlation between 2-butene conversion and peak area percentage of W–O–Si species on various W-KIT-6 catalysts (based on solid-state NMR results). The values in parentheses denote W wt%. For the catalyst with 9.2 wt% W loading, the W source addition was delayed by 2 h. The metathesis reaction was performed at T = 450°C, P = 1 atm, WHSV (ethylene and 2-butene) = 2.0 h , n(ethylene)/n(2-butene) -1

= 3/1. ACS Paragon Plus Environment

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The types of oxide structures reported in previous in situ studies are shown schematically in Scheme 2.8,40,43,77 The Wachs group reported that dioxo (O=)2WO2 (dominant species, > 90%) and mono-oxo O=WO4 species are present on dehydrated WOx/SiO2 catalyst when the W loading is below 8 wt%, while crystalline WO3 nanoparticles are present at W loading ≥ 8 wt%.40 They also speculated that the dioxo species are the most active precursors for metathesis.40 The Bell group reported that the mono-oxo O=WO4 species can convert to dioxo (O=)2WO2 species under He or propene atmosphere at reaction conditions, with the dioxo species being more rapidly activated by propene.43 The Copéret group reported the synthesis of the mono-oxo O=WO4 and dioxo (O=)2WO2 species (a 50-50% mixture) using a surface grafting method,77 and report low initial activity in batch reactor studies. More recently, Taoufik group reported the preparation of the dioxo (O=)2WO2 species with a new grafting reagent.78 The dioxo species give similar TOF values (0.045 s-1) as those estimated by the Wachs group for the active sites of industrial catalysts but only after a long induction period (~100 h). Hence, our correlation (Figure 5) depicting a linear relationship between 2-butene conversion and the surface W-O-Si species (characterized prior to metathesis reaction) is consistent with these earlier reports. We therefore postulate that the mono-oxo O=WO4 and dioxo (O=)2WO2 species (orange-colored W atoms flanked by either two or four -O-Si moieties) are the active precursors while the blue-colored W species in polytungstate and tungsten oxide, flanked by two or more O-W moieties, exhibit sluggish propene metathesis activity at 450°C (see Scheme 2). We further speculate that the yellow-colored W atoms in the polytungstate species (flanked by one -O-Si moiety and one -O-W moiety) are the precursors for the active sites with intermediate activity reported by Lwin and Wachs.41 Our speculation is based on the differing electron effects of –O-Si and –O-W moieties linked to the central W atom. However, this hypothesis needs to be verified by further systematic investigations.


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Scheme 2. Structures of mono-oxo O=WO , dioxo (O=) WO , polytungstate, and tungsten oxide on calcined 4

2

2

W-KIT-6 catalyst showing W-O-Si and W-O-W species. The polytungstate structure is proposed based on previous work, and represents a simple model meant to show that the central W atom is flanked by two 79

O-W moieties.

Conclusions In summary, we have used XPS and solid-state NMR spectroscopic techniques with complementray spectral modeling to understand the structure of active site precursors in fresh W-KIT-6 catalysts. We established that the observed 2-butene conversion correlates linearly with the population of the W–O–Si species on the fresh catalysts. We demonstrate that the accessible W-O-Si species can be enhanced by simple variations in the catalyst synthesis technique. We have also confirmed by EPR and EXAFS analysis that no W5+ species exist on the fresh WO3/SiO2 catalyst. Our conclusions are consistent with those reported previously based on in situ Raman, XAS and UV-vis techniques; viz., that mono-oxo O=WO and dioxo 4

(O=) WO species are active site precursors for olefin metathesis. These techniques and insights should be 2

2

applicable in general to rationally generate and understand active M(metal)–O–Si structures in mesoporous silicates involving metals such as Mo, Nb and Zr.


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ASSOCIATED CONTENT Supporting Information. XPS fitting parameters, EPR results, XANES results, and Solid-state NMR results for fresh WKIT-6 catalysts (PDF). AUTHOR INFORMATION Corresponding Authors *J.F.W: e-mail, [email protected]; *B.S.: e-mail, [email protected]; tel, 785-864-2903; fax, 785-864-6051. ORCID Jian-Feng Wu: 0000-0003-4444-2639 Bala Subramaniam: 0000-0001-5361-1954 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge support from the joint National Science Foundation and Environmental Protection Agency program Networks for Sustainable Material Synthesis and Design (NSF-EPA1339661). A.M.J. and M.C. acknowledge support from the National Science Foundation under Grant OIA-1539105. We thank Canadian Light Source (CLS) for the access to IDEAS beamline and Aimee Maclennan and David Muir for their technical help with XANES


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ACS Catalysis

measurements. We thank Prof. Franklin (Feng) Tao of the University of Kansas for helpful discussions regarding XPS results. We thank Prof. Zhehong Gan of the National High Magnetic Field Laboratory, Florida for helpful discussions regarding NMR analysis. Finally, we are grateful to the anonymous reviewers for their valuable suggestions. REFERENCES (1) Thomas, C. L. M. Catalytic Processes and Proven Catalysts; New York, Academic Press: New York, 1970. (2) Satterfield, C. N. Heterogeneous Catalysis in Practice; New York: McGraw-Hill: New York, 1980. (3) Thomas, J. M.; Thomas, W. J. Principles and Practice of Heterogeneous Catalysis; VCH Publishers, Inc.: New York, 1997. (4) İmamoğlu, Y., Zümreogammalu-Karan, B., Amass, A. J. Olefin Metathesis and Polymerization Catalysts: Synthesis, Mechanism and Utilization; Kluwer Academic Publishers: Dordrecht, Boston and London, 1990; Vol. 326. (5) Ivin, K. J.; Mol, J. C. Olefin Metathesis and Metathesis Polymerization; Academic Press: San Diego, London, Boston, New York, Sydney and Toronto, 1997. (6) Mol, J. C. Industrial Applications of Olefin Metathesis, J. Mol. Catal. A: Chem. 2004, 213, 39-45. (7) Popoff, N.; Mazoyer, E.; Pelletier, J.; Gauvin, R. M.; Taoufik, M. Expanding the Scope of Metathesis: A Survey of Polyfunctional, Single-Site Supported Tungsten Systems for Hydrocarbon Valorization, Chem. Soc. Rev. 2013, 42, 9035-9054. (8) Lwin, S.; Wachs, I. E. Olefin Metathesis by Supported Metal Oxide Catalysts, ACS Catal. 2014, 4, 2505-2520. (9) Gholampour, N.; Yusubov, M.; Verpoort, F. Investigation of the Preparation and Catalytic Activity of Supported Mo, W, and Re Oxides as Heterogeneous Catalysts in Olefin Metathesis, Cat. Rev. Sci. Eng. 2016, 58, 113-156. (10) Thomas, R.; Moulijn, J. A.; Debeer, V. H. J.; Medema, J. Structure-MetathesisActivity Relations of Silica Supported Molybdenum and Tungsten-Oxide, J. Mol. Catal. 1980, 8, 161-174. (11) Spamer, A.; Dube, T. I.; Moodley, D. J.; van Schalkwyk, C.; Botha, J. M. The Reduction of Isomerisation Activity on a WO /SiO Metathesis Catalyst, Appl. Catal. A: Gen. 2003, 255, 153-167. (12) Hua, D. R.; Chen, S. L.; Yuan, G. M.; Wang, Y. L.; Zhao, Q. F.; Wang, X. L.; Fu, B. Metathesis of Butene to Propene and Pentene over WO /MTS-9, Microporous Mesoporous Mater. 2011, 143, 320-325. (13) Le Roux, E.; Taoufik, M.; Copéret, C.; de Mallmann, A.; Thivolle-Cazat, J.; Basset, J. M.; Maunders, B. M.; Sunley, G. J. Development of Tungsten-Based Heterogeneous Alkane Metathesis Catalysts through a Structure-Activity Relationship, Angew. Chem. Int. Ed. 2005, 44, 6755-6758. 3

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(59) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Energy-Adjustedab initio Pseudopotentials for the Second and Third Row Transition Elements, Theor. Chim. Acta 1990, 77, 123-141. (60) Crichton, W. A.; Bouvier, P.; Grzechnik, A. The First Bulk Synthesis of ReO type Tungsten Trioxide, WO , from Nanometric Precursors, Mater. Res. Bull. 2003, 38, 289-296. (61) Loopstra, B.; Rietveld, H. Further Refinement of the Structure of WO , Acta Cryst. 1969, 25, 1420-1421. (62) Wang, F. G.; Di Valentin, C.; Pacchioni, G. Electronic and Structural Properties of WO : A Systematic Hybrid DFT Study, J. Phys. Chem. C 2011, 115, 8345-8353. (63) de Castro, E. V. R.; Jorge, F. E. Accurate Universal Gaussian Basis Set for All Atoms of the Periodic Table, J. Chem. Phys. 1998, 108, 5225-5229. (64) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H. L., X. ; Caricato, M.; Marenich, A.; Bloino, J.; Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D. D., F.; Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson, T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.; Liang, W. H., M. ; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M. K., M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian 09, Revision A. 02; Gaussian, Inc: Wallingford, CT, 2016 2015. (65) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate ab initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu, J. Chem. Phys. 2010, 132, 154104-154119. (66) Gauss, J. Accurate Calculation of NMR Chemical-Shifts, Berichte der Bunsengesellschaft für physikalische Chemie 1995, 99, 1001-1008. (67) Cheeseman, J. R.; Trucks, G. W.; Keith, T. A.; Frisch, M. J. A Comparison of Models for Calculating Nuclear Magnetic Resonance Shielding Tensors, J. Chem. Phys. 1996, 104, 5497-5509. (68) The value “8.7” in W-KIT-6 (8.7) catalyst means the tungsten loading is 8.7 wt%. The W-KIT-6 (8.7) catalyst was synthesized by the co-addition of W and Si sources. The identifier “2 h” in W-KIT-6 (2 h, 9.2) catalyst means that during catalyst synthesis, the W source was added 2 h after adding the Si source. (69) Yang, X.-L.; Dai, W.-L.; Gao, R.; Fan, K. Characterization and Catalytic Behavior of Highly Active Tungsten-Doped SBA-15 Catalyst in the Synthesis of Glutaraldehyde using an Anhydrous Approach, J. Catal. 2007, 249, 278-288. (70) Xie, F. Y.; Gong, L.; Liu, X.; Tao, Y. T.; Zhang, W. H.; Chen, S. H.; Meng, H.; Chen, J. XPS Studies on Surface Reduction of Tungsten Oxide Nanowire Film by Ar Bombardment, J. Electron. Spectrosc. Relat. Phenom. 2012, 185, 112-118. (71) Maciel, G. E.; Sindorf, D. W. Silicon-29 NMR Study of the Surface of Silica Gel by Cross Polarization and Magic-Angle Spinning, J. Am. Chem. Soc. 1980, 102, 7606-7607. (72) Engelhardt, G.; Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites; John Wiley and Sons: Chichester, New York, Brisbane, Toronto and Singapore, 1987. 3

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(73) Hunger, M.; Wang, W. Solid-State NMR Spectroscopy, Handbook of heterogeneous catalysis 2008. (74) Klepel, O.; Bohlmann, W.; Ivanov, E. B.; Riede, V.; Papp, H. Incorporation of Tungsten into MCM-41 Framework, Microporous Mesoporous Mater. 2004, 76, 105-112. (75) Cheng, C. F.; He, H. Y.; Zhou, W. Z.; Klinowski, J.; Goncalves, J. A. S.; Gladden, L. F. Synthesis and Characterization of the Gallosilicate Mesoporous Molecular Sieve MCM-41, J. Phys. Chem. 1996, 100, 390-396. (76) 2-Butene conversion values (rather than TOF values) are used here to correlate with the potential active site precursors, as they are more representative of the active W species at all W loadings. In contrast, the TOF values normalize the 2-butene conversion values with all the W sites (active and non-active) present on the catalyst and hence are not representative of just the active sites. (77) Mougel, V.; Chan, K. W.; Siddiqi, G.; Kawakita, K.; Nagae, H.; Tsurugi, H.; Mashima, K.; Safonova, O.; Coperet, C. Low Temperature Activation of Supported Metathesis Catalysts by Organosilicon Reducing Agents, ACS Cent. Sci. 2016, 2, 569-576. (78) Larabi, C.; Merle, N.; Le Quéméner, F.; Rouge, P.; Berrier, E.; Gauvin, R. M.; Le Roux, E.; de Mallmann, A.; Szeto, K. C.; Taoufik, M. New Synthetic Approach towards Welldefined Silica Supported Tungsten Bis-oxo, Active Catalysts for Olefin Metathesis, Catal. Commun. 2018, 108, 51-54. (79) Adam, F.; Iqbal, A. The Liquid Phase Oxidation of Styrene with Tungsten Modified Silica as A Catalyst, Chem. Eng. J. 2011, 171, 1379-1386.


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TOC Graphic

Solid-state NMR spectroscopic investigations of W- KIT-6 catalysts, complemented by computational studies and activity measurements, demonstrate that W–O–Si species are active site precursors for 2-butene + ethylene metathesis to produce propene.


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Solid-state NMR spectroscopic investigations of W- KIT-6 catalysts, complemented by computational studies and activity measurements, demonstrate that W–O–Si species are active site precursors for 2-butene + ethylene metathesis to produce propene. 82x44mm (300 x 300 DPI)


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Correlation of Active Site Precursors and Olefin Metathesis Activity in

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