Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on

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Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on a Mostly Amorphous Alumina: Role of the Local Grafting Environment Fan Zhang, Kai C. Szeto, Mostafa Taoufik, Laurent Delevoye, Régis M. Gauvin, and Susannah L. Scott J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08630 • Publication Date (Web): 30 Sep 2018 Downloaded from http://pubs.acs.org on October 1, 2018

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Journal of the American Chemical Society

Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on a Mostly Amorphous Alumina: Role of the Local Grafting Environment Fan Zhang,†,‡ Kai C. Szeto,§ Mostafa Taoufik,§,* Laurent Delevoye,∥,* Régis M. Gauvin,∥,* and Susannah L. Scott†,‡,* †

Department of Chemical Engineering, and ‡ Department of Chemistry & Biochemistry, University of California, Santa Barbara, CA 93106, United States; § Laboratoire de Chimie, Catalyse, Polymères et Procedés, UMR 5265 CNRS/ESCPE-Lyon/UCBL, ESCPE Lyon, F-308-43, Boulevard du 11 Novembre 1918, F-69616 Villeurbanne Cedex, France; ∥ Université Lille, CNRS, Centrale Lille, ENSCL, Université Artois, UMR 8181, UCCS - Unité de Catalyse et Chimie du Solide, F-59000 Lille, France

KEYWORDS: olefin metathesis, methyltrioxorhenium, amorphous alumina, Lewis acidity, surface hydroxyl groups, active sites

ABSTRACT: Inorganic oxides play a crucial role in the activation of atomically-dispersed metal oxides for catalytic olefin transformations. The inefficient activation processes remain poorly understood. Activation of methyltrioxorhenium (MTO) for propene metathesis via its deposition on the surface of g-Al2O3 results in < 5 % active sites, and these sites deactivate rapidly. Simple substitution of the support by a less crystalline (largely amorphous) alumina (a-Al2O3) results in ca. 4´ more activity and at least 10´ more productivity. On both types of alumina, metathesis is initiated only at specific sites, whose availability limits the catalytic activity. While the two aluminas have similar total numbers of Lewis acid sites, the less crystalline support activates twice as many grafted MTO sites. Interestingly, a-Al2O3 has nearly double the number of strong Lewis acid sites. However, the number of active sites is ca. 10´ lower than the total number of strong Lewis acid sites, and metathesis proceeds even when most are occupied by pyridine. DQSQ and D-HMQC 1H and 27Al solid-state NMR reveal that many Lewis acid sites are co-located with surface hydroxyl groups, which prevent activation and/or cause rapid deactivation. Under-coordinated Al sites on the dominant (110) facets, which retain hydroxyl groups under catalyst preparation conditions, are unlikely to lead to stable active sites. In contrast, the minor (100) facets of g-Al2O3, which are completely dehydroxylated, contain strongly Lewis-acidic five-coordinate Al sites that are necessarily remote from surface hydroxyl groups. Such sites, which are more abundant on less well-crystallized aluminas, are presumed responsible for generating the stable metathesis sites.

INTRODUCTION In the five decades since its discovery, olefin metathesis has been implemented in industrial manufacturing of commodity chemicals, surfactants, polymers, a variety of natural products, therapeutic drugs, and many specialty chemicals.1-4 Heterogeneous catalysts based on highly dispersed Mo and W ions are currently preferred for use in large-scale, continuous processes. Supported Re oxides have attracted attention for their milder operating conditions which are compatible with liquid phase reactions, their much higher activities, and their greater tolerance of functional groups.5 However, the high cost of Re, combined with rapid catalyst deactivation, present considerable barriers to implementation. An approach to improve catalyst efficiency involves the use of molecular complexes such as methyltrioxorhenium (MTO). Although inactive as a single-component metathesis catalyst, its activation by a Lewis acidic support leads to catalysts with even higher initial activities than those based on inorganic perrhenates.6-7 The organometallic system still suffers from low activation efficiency, suggesting that the properties of the activator may be more important than the identity of the metal precursor in this class of supported catalysts. Aluminas exhibit several crystallographic phases with varying

thermal stabilities and surface areas.8-11 The textural differences between these transition aluminas are important for catalysis, since their surface chemical properties play a major role in the formation and stabilization of active sites.9 Our understanding of the nature of the surface, and especially its relationship to the properties of active sites, remains limited.10 MTO is believed to be activated for olefin metathesis by its interaction with the Lewis acidic surface cations of aluminas;12 no metathesis activity is observed for MTO on supports that lack Lewis acidity, such as silica. Higher activities have been reported for MTO grafted onto more highly Lewis acidic supports such as niobia,13-14 silica-alumina,15-16 and organized mesoporous alumina (OMA).17-18 The mechanism by which the Re-alkylidene active sites are initially formed via activation of MTO is poorly understood. The low fraction of active sites in these catalysts, combined with their intrinsic instability, have complicated efforts to identify and observe them. Initially, support-induced tautomerization of MTO was proposed.1920 Although [CH2=Re(O)2OH] is energetically inaccessible in the absence of a cocatalyst,21 a bridging methylene complex (i.e., containing the [Re-CH2-py] motif) forms when MTO is combined with a soluble Lewis acid (SnCl2Me2) and a Brønsted base (Scheme 1a).22 Tautomerization was similarly suggested to generate initiating meth-

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ylene species in MTO grafted on γ-Al2O3 (Scheme 1b).19, 21, 23 However, the terminal alkylidenes that are proposed to form spontaneously in situ have yet to be observed directly in supported MTO (or in any heterogeneous dispersed metal oxide catalyst, involving Re or other transition metal ions). a

py 4 O

CH3 SnMe2Cl2

Re

O

8 py

H 2C

Re

py

O

Al O

Me

CH3 Re

O O

Cl

O

O Al

O O active site precursor

O

O

Al

O

Re

py O

O O O active site (not observed)

Re O

HO

O Al

2 ReO42 CH4

CH2 O

C 2H 4

O

HO

py py

CH2 Re

+ py

O

Sn Me

O O

Cl

O

py

O

b

2+

py

O

O

O

Al

O

O

Al

O

O

O O active site (resting state)

Scheme 1. (a) Lewis acid-induced tautomerization of MTO yields a metathesis-inactive complex with bridging methylene ligands, stabilized by their interaction with pyridine,22 and (b) a related activation mechanism on Lewis acid sites of γ-Al2O3 was suggested to give rise to a transient complex with a terminal methylene ligand, which is trapped by further reaction with an olefin to form a rhenacyclobutane.20-21, 23-24

In an NMR study of MTO/γ-Al2O3, a solid-state 13C NMR signal was tentatively assigned to a methylene group bridging between Re and Al, attributed to a possible active site precursor.25 However, the signal appears only in the spectra of low activity catalysts;26-27 it is completely absent from the spectra of much more active catalysts, such as MTO supported on silica-alumina.15 Similarly, while MTO supported on chlorinated γ-Al2O3 is far more active (34´) and more productive (20´) than MTO supported on unchlorinated γ-Al2O3, and while the fraction of active sites is much higher on the chlorinated support, no 13C solid-state NMR signals for bridging methylene ligands were detected.24 Instead, terminal methylenes were suggested to form only under reaction conditions (i.e., in the presence of olefin), and the resting state of the catalyst was proposed to be a rhenacyclobutane, Scheme 1b. The crystallinity of alumina determines its surface properties, and therefore affects its performance as a catalyst or catalyst support, via changes in surface acidity (both Lewis and Brønsted).28-29 However, the precise nature and evolution of these sites is not yet wellunderstood. For example, an amorphous alumina (a-Al2O3) calcined at 820 K was reported to have fewer Lewis acid sites than a similarly treated γ-Al2O3, as measured by pyridine adsorption.28 A higher proportion of coordinatively-saturated, hexa-coordinated Al3+ ions in an a-Al2O3 was proposed as the reason for its poor performance in methanol dehydration, relative to γ-Al2O3.29 In contrast, ordered, an ordered mesoporous a-Al2O3 calcined at 673 K was reported to have more Lewis acid sites than a γ-Al2O3; a gradual decrease in their number with calcination temperature was attributed to loss of surface area due to crystallization.30 In reality, many γ-Al2O3 materials are not fully crystalline and most “amorphous” aluminas have a small degree of crystallinity. The role of surface hydroxyl groups in metathesis catalyst performance is even less well-explored than the role of Lewis acid sites, although the hydroxyl groups have been previously implicated in the

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deactivation of oxide-activated Re-based metathesis catalysts.5, 31-32 In particular, the significantly enhanced stability of MTO supported on a Me3Si-capped silica-alumina,15 or on highly chlorinated alumina,24 was attributed to suppression of the surface hydroxyl population. In this work, we prepared supported metathesis catalysts using two kinds of mesoporous alumina with similar textural properties: mostly crystalline γ-Al2O3, and mostly amorphous a-Al2O3. The latter shows very broad, low intensity reflections in its powder XRD pattern (Figure S1), consistent with its much lower crystallinity. Differences in catalyst performance (activity and stability) are correlated to surface properties for the two types of aluminas assessed by quantitative IR spectroscopy and advanced NMR techniques. Our findings suggest that isolated Lewis sites (i.e., Lewis acid sites remote from hydroxyl groups) which are relatively more abundant on a-Al2O3 are responsible for activating grafted MTO. The result is a catalyst that has much more stable active sites, without the need for other types of extensive surface modification.

RESULTS AND DISCUSSION Effect of Support Crystallinity and Re Loading on Metathesis Activity. The olefin metathesis activity of MTO, supported on and activated by either a-Al2O3 or γ-Al2O3, was assessed in a batch reactor, to facilitate precise measurement of rate constants. Rate constants are preferred over rates (including TOFs) for comparing intrinsic catalytic activities,33 since rates depend strongly on experimental conditions while rate constants do not (with the exception of their readily-quantified temperature dependence). Consequently, rate constants measured under kinetically-controlled conditions in batch experiments can be compared directly with values obtained in flow reactors, and they can be readily used to make quantitative rate predictions.

In a typical experiment, a batch reactor was charged with catalyst (10.0 mg, containing 0.5 – 2.1 µmol Re) and propene (67 mbar, 0.36 mmol) at 0 °C, and the evolution of each metathesis product (ethene, cis- and trans-2-butenes) was monitored over time by GC-FID. The approach to equilibrium (corresponding to 32.4 % propene conversion at 0 °C)24 is pseudo-first-order, Figures 1 and S2. The measured pseudo-first-order rate constants kobs were normalized by the total amount of Re per unit volume to obtain apparent second-order rate constants, k (see the Supporting Information). For MTO/a-Al2O3 (2.0 wt% Re), the value of k is 5.4´ higher than the corresponding value for MTO/γ-Al2O3 at a similar loading (2.4 wt% Re), Table 1. It is still lower than the rate constant for MTO/Cl-γ-Al2O3, but we note that analogous chlorination of the amorphous support results in a further doubling of the activity in MTO/Cl-a-Al2O3 (Figure S3 and Table 1). Initial rates calculated from the measured k values on a mass basis (i.e., normalized by the total mass of catalyst, mcat, including the mass of the support) are useful for comparing the activities of catalysts tested under identical conditions (see the Supporting Information). This approach can be more helpful than the above comparison of apparent rate constants since the specific metathesis activity of MTO/γ-Al2O3 is known to be virtually independent of Re loading in the range 0.5-3.0 wt%.31 A similar result is found here for MTO/aAl2O3, whose initial rates are the same for Re loadings of 2.0 and 3.9 wt% Re (Figure 2a). Even when the Re loading decreased to 0.86 wt%, the activity declined by just 22 %. Thus the maximum activity of MTO/a-Al2O3 remains 4.4´ higher than the maximum activity of


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Figure 2. Comparisons of catalyst performance for MTO/a-Al2O3 (red) and MTO/γ-Al2O3 (black): (a) initial rates for propene metathesis, measured in a batch reactor (10 mg catalyst, 0 °C), normalized by total catalyst mass, and (b) cumulative turnover number (TON) measured in a flow reactor after 21 h on-stream (35 mg catalyst, 1.0 bar propene, 25 mL/min, 10 °C), based on molRe.

Figure 1. Comparison of representative kinetic profiles for propene metathesis catalyzed by MTO supported on a-Al2O3 (red, 2.0 wt% Re) and γ-Al2O3 (black, 2.4 wt% Re) in a batch reactor at 0 °C. Solid lines are nonlinear least-squares fits of the integrated first-order rate equation with two variable fit parameters (x∞, kobs). Rate constants are shown in Table 1.

Table 1. Comparison of propene metathesis activities and productivities, for various aluminas modified with MTO

Reactor Support

Batch a

γ-Al2O3

Re loading

103 kobs

k

Conversion

Initial rate

Initial TOF

wt%

s-1

L molRe-1 s-1

%

µmolC3 gcat-1 s-1

molC3 molRe-1 s-1

2.4

0.63(0.04)

29(4)

23(3)

0.17(0.03)

2.3 b

0.80

39

23

0.19

0.75

b

0.76

114

22

0.55

0.37

b

0.70

210

20

1.01

3.9

2.80(0.11)

80(7)

100(11)

0.48(0.06)

2.0

2.82(0.07)

157(16)

102(10)

0.94(0.13)

0.86

2.13(0.06)

287(32)

79(9)

1.72(0.22)

Cl-γ-Al2O3 b

2.3

15.2 (0.3)

726(65)

540(49)

4.4(0.6)

Cl-a-Al2O3

2.0

24.7 (1.1)

1390(158)

901(103)

8.3(1.0)

55(2)

0.22(0.01)

3.6

a-Al2O3

Flow

c

103 TON e

γ-Al2O3

b

a-Al2O3

Cl-Al2O3

b,d

d

4.7

28(2)

10.2

3.7

95(5)

27.9

146(3)

0.73(0.03)

30.9

3.1

117(7)

27.2

150(3)

0.90(0.05)

38.5

0.88

289(25)

19.6

105(2)

2.22(0.17)

75.9

0.43

375(56)

12.4

67(2)

2.88(0.42)

107

2.4

50(3)

29.9

74(2)

0.57(0.04)

24.0

a Conditions: constant volume (0.120 L) batch reactor, 67 mbar propene, 0 °C, 10.0 mg catalyst. Uncertainties are propagated from the errors in the first-order curvefits. Initial rates and TOFs were calculated for t = 0 s, using the computed k values (see Supporting Information). b Values calculated from data reported in references 24 and 31. c Conditions: continuous flow packed bed reactor, 1.00 bar propene, 25.0 mL min-1, 10 °C, 35.0 mg catalyst. Except as noted, all conversions were recorded after 90 min on-stream, when values were well below equilibrium. The k values were calculated from the corresponding initial rates (see Supporting Information). Uncertainties are propagated from the estimated error in conversion (± 0.1 %). d Conversion recorded after 20 min on-stream. e Cumulative product yield achieved after 21 h on-stream, normalized by total molRe.



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Figure 3a, consistent with our previous results for MTO/γ-Al2O3.24, 31 Furthermore, the maximum number of active sites is constant at ca. 4.0 μmol gcat-1, Figure 3b and Table 2. The results are consistent with the observed invariance of metathesis activity at higher Re loadings, as well as higher apparent rate constants (k) for supported MTO catalysts with lower Re loadings, Table 1. On a-Al2O3, the fraction of active sites is 26 % at the lowest Re loading, decreasing to just 3 % of total Re as the metal loading increases (Figure 3a). Nevertheless, the maximum number of active sites remains constant, at ca. 8 μmol gcat-1 (Figure 3b). This value is double that for MTO/γ-Al2O3, and strongly suggests that the number of active sites on each alumina is limited by the availability of specific surface sites. Furthermore, it confirms that the abundance of such sites capable of activating MTO depends on the degree of crystallinity of the support.

Figure 3. Variation with MTO loading of (a) the active site fraction, and (b) the total number of active sites, on a-Al2O3 (red) and γAl2O3 (black).

MTO/γ-Al2O3 over a wide range of Re loadings, while the ratio of the apparent second-order rate constants k is a strong function of the Re loading. Flow experiments were conducted to obtain meaningful assessments of catalyst productivity. The flow reactor kinetics were also checked for consistency with the batch reactor results. Initial rates were calculated from propene conversion after 90 min on-stream, when the initial conversions were all significantly lower than the equilibrium value (33.5 % at 10 °C).34 After this time, some catalyst deactivation had already occurred. Nevertheless, the second-order rate constants (k) are very similar under both batch and flow conditions, Table 1, despite the different temperatures (0 °C vs. 10 °C) and pressures (67 mbar vs. 1 bar). In the flow reactor, the initial activity of MTO/a-Al2O3 (3.7 wt% Re) is 3.6´ higher than that of MTO/γ-Al2O3 (4.7 wt% Re). When the Re loading of MTO/a-Al2O3 decreased to 3.1 wt%, the initial activity remained unchanged. It declined only when the loading was below 1 wt% Re. Catalyst productivity is reflected in the cumulative turnover number (TON), Table 1. After 21 h on-stream, MTO/γ-Al2O3 (4.7 wt% wt% Re) achieves a modest TON of 3.6´103. For MTO/a-Al2O3 (3.7 wt% Re), the TON is an order of magnitude higher, 3.1´104, in the same time period. The TON is inversely proportional to Re loading (Figure 2b). Thus the catalyst with the lowest Re loading (0.43 wt%) is particularly effective, achieving a cumulative TON of 1.07´105 in the same time. This value is 5´ greater than the TON recorded for MTO/a-Al2O3 at the highest Re loading tested here, and it is more than 30´ higher than the value for MTO/γ-Al2O3. Compared with MTO/Cl-g-Al2O3,24 the productivity of the unchlorinated catalyst is much higher, even though the initial TOF for MTO/a-Al2O3 recorded in the batch reactor is lower. Thus in the flow reactor, their initial activities are comparable, and after 21 h on-stream, MTO/a-Al2O3 catalysts achieve even larger turnover numbers (TON, Table 1). Active site numbers . The effect of alumina crystallinity and Re loading on the number of active sites was investigated. Active site counting was performed at 0 °C, following a literature method.35 On γ-Al2O3, the fraction of active sites varies from 1-5 % of total Re,

Role of Lewis acid sites. In order to explore the nature of the surface sites that activate MTO, we investigated the acidities of both aluminas via the 31P{1H} MAS NMR spectra of adsorbed PMe2Ph (dimethylphenylphosphine, DMPP). Low temperature detection is necessary due to rapid molecular exchange at higher temperatures, in particular, exchange between physisorbed phosphine and phosphine interacting with weak- or medium-strength Lewis acid sites.36 This exchange is frozen out by cooling to 100 K.

A precise amount of DMPP (1.00 mmol g-1) was adsorbed on both aluminas at room temperature, without further desorption. The two overlapping signals in the 31P{1H} NMR spectrum at -46 and -36 ppm (Figure S4) are assigned to physisorbed phosphine and phosphine bound to Lewis acid sites (i.e., coordinatively-unsaturated Al sites), respectively.37-38 Protonated phosphine (DMPPH+, signal expected at approx. 0 ppm) was not observed on either support. The absolute number of Lewis acid sites was obtained by deconvoluting the spectrum (Figure S4, inset). For both aluminas, the same fraction of phosphine is adsorbed on Lewis acid sites, (0.18 ± 0.01), and the total number of Lewis acid sites is the same, (180 ± 10) μmol/g (Table 2). The distribution of Lewis acid sites on the two aluminas was probed by IR spectroscopy of adsorbed pyridine. The IR spectra in Figure 4a were recorded after desorption at 150 °C. Most hydrogenbonded pyridine is removed by desorption at 150 °C, since its characteristic n8a mode at 1590-1600 cm-1 is barely detectable. The lack of significant strong Brønsted acidity for either support is confirmed by the absence of bands at 1640 and 1540 cm-1 for adsorbed pyridinium ions. Bands at ca. 1620, 1580, 1490 and 1450 cm-1 correspond to the n8a, n8b, n19a and n19b modes, respectively, of pyridine interacting with various Lewis acid sites.39-40 The total number of Lewis acid sites was estimated from the amount of residual pyridine after desorption at 150 °C,41 via integration of the n19b ring vibration (Figure S5). The amounts are remarkably similar for both supports (ca. 200 μmol g-1, Table 2), and are fully consistent with the 31P NMR results described above. The precise frequencies of the n8a and n19b bands for pyridine bound to weak Lewis acid sites are 1452 and 1618 cm-1, respectively, while bands for pyridine interacting with strong Lewis acid sites are shifted slightly to higher frequencies (1456 and 1624 cm-1).42 For γAl2O3, the resolved splitting of the n8a band indicates two distinct populations of Lewis acid sites with different acid strengths, consistent with previous reports.43-44 The blue-shifts of the n8a and n19b


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Table 2. Quantitative comparison of surface chemical properties of a-Al2O3 and γ-Al2O3, after dehydration at 450 °C

Abundance (μmol g-1)

Property

Surface density (nm-2)

γ-Al2O3

a-Al2O3

γ-Al2O3

a-Al2O3

4.0 (0.2)

8.3 (0.1)

0.014 (0.001)

0.023 (0.001)

209 (8)

0.78 (0.03)

0.58 (0.02)

180 (10)

180 (10)

0.65 (0.04)

0.50 (0.03)

Strong Lewis acid sites b

51 (2)

89 (3)

0.18 (0.01)

0.25 (0.01)

Total OH content b,d

860 (31)

575 (21)

3.1 (0.1)

1.6 (0.06)

Metathesis active sites

a

Total Lewis acid sites Estimated by pyridine adsorption b 217 (8) Estimated by DMPP adsorption

c

a

Measured in propene metathesis at 0 °C, with adsorbed MTO (1.4-5.2 wt% Re). Each value is the average measurement for three different Re loadings, as shown in Figure 3. The uncertainty is the standard deviation of the average. b The uncertainty represents the error in the IR calibration plot (Figure S5). c The uncertainty represents the measurement error in the micro-syringe used to deliver DMPP. d Calculated by combining the pyridine/OH ratio, measured by solid-state NMR, with the total pyridine uptake, measured by IR. Solid-state NMR spectra were recorded for both aluminas after modification by adsorbed pyridine and its desorption at 350 °C, in order to observe the adducts formed between pyridine and the strong Lewis acid sites. While IR monitoring of pyridine adsorption is commonly used for exploring acidity in inorganic supports,45 corresponding studies using 1H and 27Al NMR have attracted much less attention.46 Instead, NMR studies have focused primarily on the 13C-, 15 N- and 2H-nuclei of the adsorbed pyridine itself.47-50 Recent technological and methodological advances have provided the resolution enhancements to allow us to probe details of the alumina itself at the atomic level.51 H NMR spectra of g-Al2O3 and a-Al2O3 with and without adsorbed pyridine are presented in Figure 5a-b. The pyridine molecules give rise to three sharp signals, indicating considerable dynamic behavior (most likely, pyridine rotation around the Al-N bond).49 The highest frequency chemical shifts belong to the ortho-protons (8.86 and 8.58 ppm for g-Al2O3 and a-Al2O3, respectively). All of the pyridine signals appear at slightly higher frequencies on g-Al2O3 compared to a-Al2O3. This finding is consistent with the greater Lewis acidity of a-Al2O3, and/or with stronger interactions between the adsorbed pyridine molecules and the surface hydroxyls for g-Al2O3 compared to a-Al2O3 (as is evident in the IR spectra in Figure S6). The NMR spectra of both supports also show significant differences in the region of the hydroxyl signals (discussed in greater detail below). 1

Figure 4. IR spectra of a-Al2O3 (red) and γ-Al2O3 (black), recorded after dehydration at 450 °C and pyridine adsorption at room temperature, followed by desorption for 20 min at either (a) 150 °C, or (b) 350 °C. All spectra were recorded at room temperature, and are normalized by sample mass. The pairs of spectra are offset vertically for clarity.

bands for pyridine on a-Al2O3 indicate a higher fraction of strong Lewis acid sites for the amorphous support, while the lack of splitting in the n8a band suggests that weak acid sites are much less abundant. To quantify the strong Lewis acid sites, pyridine was desorbed at 350 °C, removing molecules adsorbed on the weak Lewis acid sites. After this thermal treatment, the n19b peak shifted slightly, to 1455 cm-1, Figure 4b.42 Integration reveals a significantly higher fraction (1.8´) of strong Lewis acid sites on a-Al2O3 (corresponding to 43 % of the total Lewis acid sites) compared to γ-alumina (24 % of total Lewis acid sites), Table 2. Interestingly, this ratio is similar to the ratio of active MTO sites for the two supports (2.1´, Figure 3), implicating the strong Lewis acid sites in the activation of grafted MTO. However, the number of active sites on a-Al2O3 is still an order of magnitude smaller than the total number of strong Lewis acid sites. Thus only a small number of very specific strong Lewis acid sites can actually convert adsorbed MTO to stable metathesis-active sites.

Effect of alumina crystallinity on the surface hydroxyl population. The two aluminas used in this study also differ in their hydroxyl populations. IR spectra of the partially dehydrated aluminas in the O-H stretching region are compared in Figure S7. Assignment of the characteristic modes at ca. 3790 and 3773 cm-1 to terminal (basic) OH groups9, 52 was confirmed in combined NMR/IR studies of the reaction of g-Al2O3 with CO2.53 The band at 3773 cm-1, assigned to terminal hydroxyls bound to tetra-coordinated Al sites (µ1-HOAlT),53 is significantly weaker in the IR spectrum of a-Al2O3, while the intensity at 3794 cm-1 (due to terminal hydroxyls bound to pentaand/or hexa-coordinated Al sites, µ1-HO-AlP and/or µ1-HO-AlH) is very similar in the spectra of both types of alumina. Bridging (acidic) OH groups appear at 3730, 3690 and 3673 cm-1 for g-Al2O3. For aAl2O3, the 3730 cm-1 band is weaker, while the signals at 3690 and 3673 cm-1 appear as a single, broad maximum at 3670 cm-1.



Page 6 of 16

with an AlT site. Finally, µ3-OH sites (i.e., hydroxyls with the highest Brønsted acidity) are relatively more abundant on the surface of aAl2O3.

Figure 5. Comparison of 1H MAS NMR spectra for g-Al2O3 (black) and a-Al2O3 (red): (a) after dehydration at 450 °C; (b) after adsorption of excess pyridine followed by desorption at 350 °C; (c) after adsorption of MTO (3.0 wt% Re, without pyridine). All spectra were recorded at 800.13 MHz, and a spinning speed of 20 kHz.

The 1H MAS NMR spectra of a-Al2O3 and g-Al2O3 contain peaks in the regions associated with terminal hydroxyls (-0.5 to 0.5 ppm) and doubly-bridging hydroxyls (0.5 to 2.5 ppm), Figure 5a. A major difference is that terminal hydroxyl groups are much less abundant on the surface of a-Al2O3 compared to g-Al2O3, consistent with the IR spectra (Figure S7). For g-Al2O3, the majority of the terminal hydroxyls are bonded to tetra-coordinated surface aluminum sites, i.e., µ1-HO-AlT.53 Their characteristic 1H NMR signals are significantly weaker in the spectrum of a-Al2O3, compared to the intensity of bridging hydroxyls (µ2,3-OH, seen more clearly in Figure 6 below). In addition, the broad lineshape for the µ1-OH proton signals of aAl2O3 suggests several overlapping contributions. The maximum intensity at ca. 0.5 ppm is consistent with a lower average Brønsted basicity than the µ1-OH groups of g-Al2O3, whose maximum signal appears at -0.2 ppm. The maximum signal of the µ2,3-OH protons is also shifted to higher frequency (by 0.3 ppm) for a-Al2O3 relative to g-Al2O3. It is broader for the amorphous material, as expected for the more heterogeneous distribution of sites. More detailed insight into the surface hydroxyl distribution was obtained via 1H-27Al D-HMQC NMR,54 Figure 6. For g-Al2O3, the characteristic signals of the terminal hydroxyls are mostly associated with µ1-HO-AlT groups, and they span a wide range, from 70 to 30 ppm in the 27Al dimension, due to the large second-order quadrupolar coupling. There are only minor contributions from µ1-HO-AlP and traces of µ1-HO-AlH (see the extracted slice in Figure 6b). For aAl2O3, the terminal hydroxyls are much less abundant and include a broad distribution of µ1-HO-AlT sites, as well as a larger proportion of µ1-HO-AlP and µ1-HO-AlH sites (see the extracted slice in Figure 6e). The µ2,3-OH sites show similar overall correlation patterns for both aluminas: the µ2-OH sites associated with one tetracoordinated Al center (i.e., µ2-OH-AlT,AlP,H) give rise to signals with higher 1H chemical shifts (by ca. 0.4 ppm) than those which are not associated

Figure 6. Comparison of g-Al2O3 (top) and a-Al2O3 (bottom), both treated at 450 °C: (a,d) 1H-27Al HMQC MAS NMR spectra; (b,e) sums of the 27Al NMR signals in the 1H region from 0.3 to -0.6 ppm; and (c,f) projections in the 27Al dimension for the 1H region from 8 to -2 ppm. Spectra were recorded at 18.8 T, with a spinning speed of 20 kHz. Effect of pyridine adsorption on surface hydroxyls. Upon adsorption of similar amounts of pyridine (ca. 45 μmol/g) on either gAl2O3 or a-Al2O3, the IR spectra show two major changes. The intensities of some hydroxyl stretching modes decrease, especially those associated with the µ1-OH at 3775 and 3740 cm-1, Figure S6. At the same time, a broad new signal appears at lower frequency (ca. 3500 cm-1). Its frequency and shape suggest hydrogen-bonding between pyridine and surface OH groups.55 The changes in the IR spectrum are larger for g-Al2O3 relative to a-Al2O3.

In the 1H NMR spectrum, pyridine adsorption on g-Al2O3 causes dramatic changes in the hydroxyl region, Figure 5b. A slight decrease


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in the µ1-OH signal at -0.20 ppm is accompanied by a new signal at 0.5 ppm assigned to µ1-OH near adsorbed pyridine. Another, sharp signal appears at 2.2 ppm, assigned to a new type of µ2-OH group. The origin of the two new signals will be discussed further below, making use of information extracted from the homo- and heteronuclear correlation spectra. For pyridine adsorbed on a-Al2O3, the hydroxyl region of the 1H NMR spectrum is strongly broadened due to the amorphous nature of the surface.

dination, the very low intensity of the corresponding cross-peaks is likely a consequence of the low pyridine/OH ratio (ca. 0.05).

Quantitative information can be extracted from these spectra due to the delay between consecutive scans (>120 s), chosen because the longitudinal relaxation delay T1 is long (> 60 s) for both pyridinemodified aluminas. Integration of the 1H NMR signals reveals that the ratio of pyridine to hydroxyl groups is significantly different: 0.05 py/OH for g-Al2O3, vs. 0.08 py/OH for a-Al2O3 (Table S1). Since the amounts of pyridine adsorbed on g-Al2O3 and a-Al2O3 are known from the quantitative IR measurements to be 43 and 46 μmol/g, respectively, we can obtain the surface hydroxyl density for each alumina. The largely amorphous support contains one-third fewer hydroxyls (575 µmol g-1) and half the surface hydroxyl density (1.6 nm-2) compared to the largely crystalline support (860 µmol g-1 and 3.1 nm-2), Table 2. This finding is consistent with the lower overall intensity for the O-H stretching vibrations in the IR spectrum of the amorphous support, Figure S7. We also note that the number of surface hydroxyls present after thermal treatment of each alumina at 450 °C is an order of magnitude higher than the number of strong Lewis acid sites. Further information regarding the local structure of the hydroxyl network and adsorbed pyridine was obtained using 1H-1H DQSQ MAS NMR, which relies on dipolar interactions. The spectra reveal proximity between nuclei with (1) similar chemical environments, giving rise to self-correlating (i.e., on-diagonal) signals, and (2) distinct chemical environments, giving rise to off-diagonal signals. The off-diagonal cross-peaks appear as two spots at both chemical shifts (d1 and d2) in the single quantum (direct) dimension, and at (d1 + d2) in the double quantum (indirect) dimension.56 In the case of g-Al2O3, the 1D spectra confirm that the main hydroxyl signals (from -0.5 to 5 ppm, Figure 5a-b) do not change upon pyridine adsorption. The network of complex, well-defined hydroxyl interactions identified in our previous study53 on the surface of partially dehydrated g-Al2O3 is therefore largely retained in the presence of pyridine. Other cross peaks (discussed below) have low intensity. The 1H-1H DQSQ MAS NMR spectrum of g-Al2O3 with adsorbed pyridine is shown in Figure 7. In the aromatic region, the para-H of pyridine adsorbed on g-Al2O3 show no self-correlation, in contrast to the meta- and ortho-H signals. In addition, ortho- and meta-H show well-defined, off-diagonal correlations with two of the main types of hydroxyl protons: (1) µ1-HO-AlT sites (-0.30 ppm, interaction A), and (2) trinuclear Al-(µ2-OH)-Al-(µ2-OH)-Al sites, more precisely, those which give rise to higher chemical shift signals compared to pristine g-Al2O3 (1.75 ppm, interaction B). It is important to note that the cross-peaks have very low intensity compared to the on-diagonal signals, indicating either long distances between the correlated sites, or (more likely) dynamic processes that lead to poor dipolar transfer efficiency. The new hydroxyl signals at 0.5 and 2.2 ppm in the 1H NMR spectrum of pyridine-modified gAl2O3 give rise to even weaker correlations with pyridine ortho-H. Since these new hydroxyl sites appear due to nearby pyridine coor-

Figure 7. Top: 1H-1H DQSQ MAS NMR spectrum of g-Al2O3 with adsorbed pyridine, following desorption at 350 °C (18.8 T, 20 kHz spinning speed). Bottom: Principal assignments and relative proximities deduced from the spectral data.53 1 H-27Al HMQC MAS NMR spectra for pyridine adsorbed on gAl2O3 and a-Al2O3 are compared in Figure 8, where the 1H domain is limited to the region containing the hydroxyl signals. The full spectra, including the pyridine 1H signals, show additional weak, sharp correlations between the ortho- and meta-H signals and AlT and AlH centers from the bulk, inferred from their narrow lineshapes (Figure S8). In the case of pyridine-modified g-Al2O3, the 1H-27Al correlation spectrum in Figure 8a affords further information. The signal at -0.2 ppm in the 1H dimension is mostly due to µ1-HO-AlT, and it does not differ significantly from the signal of the pristine support (compare Figures 6b and 8d). The new signal at 0.35 ppm in the 1H dimension correlates with all three types of Al coordination, as shown in the corresponding slice (Figure 8c). Although the signal-to-noise ratio is low, the relative contributions are clearly AlT > AlH ≈ AlP.

The new signal at 2.2 ppm corresponds to µ2-HO sites that are less shielded in the presence of adsorbed pyridine compared to the pristine support (where they appear at 1.7 ppm, and where they remain a major component in this region after pyridine modification). The new signal arises mostly from protons close to AlT and AlP sites (in similar amounts), as well as a smaller number of AlH sites (Figure 8b). Therefore we conclude that it arises principally from µ2-HO-(AlT,AlP) sites (Figure 8 inset). The chemical shift perturbation of ca. 0.5 ppm due to the proximity to adsorbed pyridine parallels the shift observed in the terminal hydroxyl region: thus, the new signal at 0.35 ppm



may be due to protons resonating at -0.20 ppm in the spectrum of the pristine support, whose signals shift to higher frequency by 0.65 ppm upon adsorption of pyridine. Both shifts resemble previous observations on the effect of adsorbed pyridine on the OH groups of g-Al2O3, which led to the suggestion that Lewis acid sites are located near specific OH groups.55 Pyridine-modified a-Al2O3 was also studied using 1H-27Al HMQC MAS NMR (Figure 8f). Adsorption results in a severe loss of resolution compared to the spectrum of pristine a-Al2O3 (Figure 6d), rendering the spectrum significantly less informative than that of pyridine-modified g-Al2O3. Local disorder stemming from the amorphous nature of a-Al2O3 is most likely responsible, since the nature of the interactions are expected to be more heterogeneous than in the case of crystalline g-Al2O3.

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4 ppm. This region includes overlapping signals for the [Re-CH3] group (expected at ca. 3 ppm)24 and µ2,3-HO sites, as well as other hydroxyls perturbed by their interaction with grafted MTO. Such interactions could include coordination to Re (e.g., µ2-HO-AlT,Re), as well as medium-to-long range perturbation of hydroxyls that are too distant to coordinate to Re but yet still close enough to interact. Grafting MTO onto a-Al2O3 induces a similar, dramatic drop in the intensity of the µ1-HO signals, and an increase in the intensity of the broad, featureless signal in the region 7.0-2.5 ppm. The latter likely corresponds to signals of [Re-CH3] and bridging hydroxyl groups, either µ3-HO sites or perturbed µ2-HO configurations (see below).

Spectroscopic changes resulting from MTO adsorption. Differences in the structure of adsorbed MTO due to the crystallinity of the support were initially investigated by Re L3-edge EXAFS, for MTO grafted onto g-Al2O3 and a-Al2O3 at similar Re loadings (ca. 2 wt% Re). The k3-weighted spectra are compared in Figure S9. For both catalysts, the general shape and intensity of the main peak in the Fourier transform magnitude at ca. 1.2 Å are very similar, and resemble our previously reported spectra for MTO/g-Al2O3.24 This finding indicates that the major Re sites have similar structures on both supports. Notably, the curvefits require an interaction between Re and a surface oxygen derived from the alumina support (possibly a hydroxyl group).24 The insensitivity of the EXAFS technique to structural differences between the two supports is not surprising considering the small fractions of active sites (< 10 % for both supports).

The IR spectrum of MTO adsorbed on either g-Al2O3 and a-Al2O3 contains bands in the C-H stretching region at 2990 and 2910 cm−1, Figure 9, which are assigned to asymmetric and symmetric methyl stretching modes, respectively. In the O-H stretching region, the intensity of the hydroxyl signals (both terminal and bridging) decreases, while a broad new signal appears at much lower frequency (ca. 3500 cm-1). A similar feature is observed for MTO/a-Al2O3. To explore the origin of the observed changes in the O-H stretching region, IR spectra were recorded for MTO/g-Al2O3 with various Re loadings. As the Re content increases, the intensities of the original signals for the surface hydroxyls decrease, while the intensities of the broad lower frequency signals increase steadily, Figure S10. Previously, increased intensity in the O-H stretching region for MTO/gAl2O3 was attributed to new surface hydroxyls formed by tautomerization of grafted [Re-CH3] sites.26 However, the active site counting results reported above (Figure 3) show that the number of active sites does not increase with Re loading. Therefore the IR spectra are quantitatively inconsistent with formation of new hydroxyls by deprotonation of [Re-CH3] sites. A more likely explanation is that adsorption of MTO alters the hydrogen-bonding network of the alumina support. This perturbation could involve direct interactions between adsorbed MTO and surface hydroxyl groups, or adsorbateinduced reorganization of the surface hydroxyl network. Both possibilities were explored using solid-state NMR. The H NMR spectra of MTO grafted onto g-Al2O3 and a-Al2O3 are compared in Figure 5c. For MTO/g-Al2O3, a significant decrease in the µ1-HO signal intensity at -0.2 ppm relative to the spectrum of g-Al2O3 is consistent with interaction of these Brønsted basic sites with MTO. A large increase in signal intensity is observed from 2 to 1

Figure 8. Comparison of g-Al2O3 (top) and a-Al2O3 (bottom), both exposed to pyridine followed by desorption at 350 °C: (a,f) 1H-27Al HMQC MAS NMR spectra, with (b) 27Al NMR signals in the range 2.5 to 2.0 ppm in the 1H dimension, (c) 27Al NMR signals in the range 0.8 to 0 ppm in the 1H dimension, (d) 27Al NMR signals in the range 0 to 1 ppm in the 1H dimension, and (e,g) 27Al NMR signals in


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the range 8 to -2 ppm in the 1H dimension. All spectra were recorded at 18.8 T with a spinning speed of 20 kHz.

Figure 9. Transmission IR spectra before and after adsorption of MTO on (a) g-Al2O3 (2.8 wt% Re), (b) a-Al2O3 (3.9 wt% Re), and (c) difference spectra. All spectra are normalized to the same sample mass.

The 1H-1H DQSQ homonuclear dipolar correlation spectrum of MTO/g-Al2O3 in Figure 10a shows a principal correlation between ca. 2 and 1 ppm in the single-quantum dimension, assigned to a pair of µ2-HO groups bonded to a common Al site (illustrated in the blue frame in Figure 10), on the basis of the characteristic intense correlation associating these two specific sites (the same correlation was observed for unmodified g-Al2O3, as described above). The [Re-CH3] signal at 3.0 ppm is correlated with one of the two hydroxyls in this pair, namely the one responsible for the signal centered at 2 ppm (slice shown in Figure 10b). This specific binding of MTO on the alumina surface is similar to that observed for adsorbed pyridine, whose protons are correlated with the same µ2-HO-Al2 site (Figure 7). A major change in the 1H-1H DQSQ homonuclear dipolar correlation spectrum for g-Al2O3 observed in the presence of MTO is the absence of self-correlation for the µ1-HO signals, and of correlations between µ1-HO and µ2,3-HO signals. This may be a result of the lower number of µ1-HO species compared to pristine g-Al2O3, resulting in their greater isolation. It is consistent with the lower intensity of the µ1-HO signal in the 1H NMR spectrum of MTO/g-Al2O3, compared to that of g-Al2O3 (compare Figures 5a and 5c). The 27Al-1H HMQC dipolar correlation spectrum for MTO/gAl2O3 is shown in Figure 10d. Comparison of the 27Al projection for the entire spectrum (Figure 10f) with that for the MTO region (3.42.7 ppm in the 1H dimension, Figure 10e) reveals no clear correlations that might suggest a preferential binding site for MTO, in contrast to the case of MTO/Cl-Al2O3.24 Residual µ1-HO-AlT and µ2-OH sites appear to be mostly unaffected by MTO grafting. However, a broad signal at higher frequency (ca. 3-6 ppm) suggests new µ3-OH and/or

Figure 10. NMR data for MTO/g-Al2O3 (3.0 wt% Re): (a) 1H-1H DQSQ MAS NMR spectrum; with (b) slice at 5.1 ppm in the doublequantum dimension, illustrating the correlation between [CH3Re] sites and specific µ2-OH groups; (c) 1H MAS NMR spectrum; (d) 1H-27Al D-HMQC MAS NMR spectrum; with (e) slice at 3.0 ppm in the 1H NMR dimension; and (f) projection in the 27Al dimension. All spectra were recorded at 18.8 T and a spinning speed of 20 kHz. The top scheme illustrates principal assignments as well as relative proximities deduced from the spectra.



Page 10 of 16

Figure 11. (a) IR spectra of a-Al2O3 with various amounts of adsorbed pyridine; (b) Comparison of kinetic profiles for propene metathesis catalyzed by MTO supported on a-Al2O3 pre-dosed with various amounts of pyridine (solid lines are nonlinear least-squares fits of the integrated first-order rate equation with two variable fit parameters: x∞, kobs); and (c) dependence of the apparent second-order rate constants for propene metathesis (kobs/molRe) on the fraction of total or strong Lewis acid sites occupied by pyridine. Conditions: 67 mbar propene (C3/Re = 300), 10 mg catalyst (ca. 2 wt% Re), 0 °C, constant-volume batch reactor.

H-bonded µ2-OH groups, consistent with the changes observed in the IR spectra (Figure 9). For MTO/a-Al2O3, the 27Al- 1H HMQC dipolar correlation spectrum (Figure S11) confirms the quasiabsence of µ1-OH sites. Grafting of MTO induces a large shift of the main OH signal (centered at 2 ppm in pristine a-Al2O3) to higher frequency (3-7 ppm), with correlations to all three types of Al coordination. The projection of the 1H dimension of the spectrum confirms the position of the [CH3Re] signal at ca. 3 ppm, however, the low resolution does not allow us to infer specific correlations between the methyl protons and surface Al sites. Effect of pyridine adsorption on the metathesis activity of grafted MTO. To explore which Lewis acid sites promote olefin metathesis activity, MTO was grafted onto a-Al2O3 pre-dosed with various amounts of pyridine, quantified by IR (Figure 11a). Pyridine is not released from the surface upon deposition of MTO (Figure S12). As expected, the activity decreases as the amount of adsorbed pyridine increases. The corresponding propene metathesis reaction profiles, measured in a batch reactor at 0 °C, are shown in Figure 11b, with rate constants in Table S2. When enough pyridine is present to block all of the Lewis acid sites, no metathesis activity remains. These findings further implicate Lewis acid sites in the activation of MTO.

The Lewis acid sites do not contribute equally to the metathesis activity. Sites with stronger acidity are more effective, since the decrease in activity with pyridine loading is sub-linear, Figure 11c. MTO may simply bind to weaker Lewis acid sites to give less active metathesis sites. In addition, MTO is known to bind pyridine,22, 57-58 and may be capable of removing it from weaker Lewis acid sites on the alumina surface, thereby freeing up such sites to activate other MTO molecules. Inhibition of MTO activation by surface hydroxyl groups. Stabilization of under-coordinated [CH3Re] sites may be critical in activating MTO for olefin metathesis. In benzene-d6, spontaneous MTO tautomerization occurs in the presence of both a Lewis acid

promoter (to increase the acidity of the methyl group) and a Brønsted base (to act as proton acceptor), resulting in H-D exchange between the methyl and phenol-d6.23 However, direct formation of Lewis acid-base adducts CH3ReO3(L) disfavors tautomerization.23 For g-Al2O3, the 1H-1H DQSQ MAS NMR spectra show that (1) many of the strong Lewis acid sites are located close to surface hydroxyl groups (Figure 7), and (2) MTO binds to the same sites as pyridine (Figure 10). Furthermore, both IR and 2D NMR spectra provide evidence of interactions between adsorbed MTO and the surface hydroxyls of alumina. The most Lewis basic sites on alumina, µ1-HO-AlT, will suppress tautomerization simply by coordinating to Re.23 Furthermore, any Brønsted acid sites present nearby or arising from such hydroxyl coordination (µ2-(HO)AlT,Re) would be capable of protonating an alkylidene. Consequently, stable active sites are likely formed from MTO grafted onto strong Lewis acid sites that are remote from hydroxyl groups, while MTO grafted onto Lewis acid sites near hydroxyl groups may not contribute to the activity (Scheme 2). Similarly, the ability of the dehydrated metal-organic framework d-NU-1000 to activate MTO contrasts with the complete lack of metathesis activity for MTO interacting with NU-1000 containing [Zr-OH] groups.59 According to a sophisticated DFT model for g-Al2O3,60 a small number of three-coordinate Al sites may appear on the predominant (110) facet of g-Al2O3 when the hydroxyl coverage is very low. Although very high temperatures are required to dehydrate these surfaces, the resulting sites would be very strongly Lewis acidic, and have been postulated to be capable of dissociating H2 and CH4, as well as binding N2.61-62 They have also be suggested to be the sites which activate MTO.25 However, at the alumina dehydration temperature used here (450 °C), the surface density of these sites was reported to be only ca. 0.004 nm-2,61 which is considerably smaller than the surface density of active metathesis sites (0.014 nm-2). In addition, it was reported that the number of active sites does not change appreciably when MTO is grafted onto g-Al2O3 pretreated at


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(a)

(b) relatively more abundant on γ-Al2O3

relatively more abundant on a-Al2O3

H O Al

O O

O O

O

py O

O

Al

N O Al O

O O

Al

O

O

O

OH O

O

Al O

O

O

MTO

O

Al

(c)

Re

O

O O O

O

O

Al

O

O

O

O

HO O Al O O

O

O

O

Al O

O Al O

O

O

Al O

O

O

O

OH

HO

Al

O

OH O

O

O

py

Al O

Al O

O Al

H O

O

O

O

O

O H O

O O

Al

O O

Al

Al O

O

O H O

O O

O

O

O

Re

MTO

O

O O

O Al O

Al O

Re

Al H O

O

Al

O

O

O

O O

HO

O O O

O

O

O

H O

O O

CH2

O

O

Al

O

O

O

O CH3

O

Al

O

O

O

O Al

O

O O

O

O O O

HO O

O O O

Re

O

Al

Al O

O

O

Re

O

O

Al H O

O

CH2

O

O

O

O

O

Al

Al

CH3

O H O

Re

O

O

N

Al

O

O

O

O

O

relatively more abundant on γ-Al2O3

O

O

CH3

CH2

CH3

O

py

MTO

O Re

Al

O

N

H O

O

O

Al O

Al O

O Al O H O

O O

O O

O

Al O O

Scheme 2. Adsorption of pyridine and MTO onto Lewis acid sites (shown as under-coordinated Al sites with plausible coordination numbers determined by NMR analysis, in combination with computational studies of g-Al2O3 surface terminations): 60, 66 (a) AlP remote from all hydroxyl groups, allowing tautomerization to generate the active sites in the presence of olefin, (b) distorted AlT adjacent to a basic hydroxyl group, µ1-HO-AlT, whose coordination inhibits methyl tautomerization, and (c) distorted AlT adjacent to a pair of acidic hydroxyl groups, µ2-HOAlT,AlH, one of which protonates the methylene tautomer.

only 200 or 300 °C,25 despite the absence of three-coordinate Al sites on g-Al2O3 dehydrated at temperatures below 400 °C.61 Mild dehydration of g-Al2O3 results in the appearance of AlP defect sites on its surface. They have been shown to interact strongly with atomically dispersed metal atoms and metal oxides.63-65 The DFTmodel described above predicts that the minor (100) facet of gAl2O3, which is completely dehydrated above 323 °C, exposes AlP sites whose Lewis acidity is similar to that of some of the AlT sites present on the (110) facet.60, 66 Such AlP sites, remote from any surface hydroxyl groups, could be mostly responsible for activating MTO. This hypothesis is consistent with the low ratio of active sites to strong Lewis acid sites (Table 2), and with the larger number of activating sites in in less well-crystallized aluminas (due to their

higher fraction of AlP sites67-70 and lower surface density of hydroxyl groups) relative to g-Al2O3. It is also consistent with a recent report that shows a correlation between the abundance of AlP sites and ethanol dehydration activity.71 Since most g-Al2O3 and a-Al2O3 materials vary in their degree of crystallinity, their activating ability towards MTO is likely to be a sensitive function of the preparation conditions of the support, due to changes in the abundance of these particular Lewis acid sites.

CONCLUSIONS Supported CH3ReO3 (MTO) catalysts were prepared using aluminas with similar textural properties but different degrees of crystallinity. The catalyst made with a largely amorphous alumina (a-Al2O3)



has ca. 2´ more active sites and it is ca. 4.4´ more active in propene homo-metathesis than a catalyst made with a more crystalline γAl2O3. On both supports, the number of active sites remains constant as the Re loading increases, suggesting that activation occurs on specific Lewis acid sites whose low abundance limits the number of sites that eventually participate in metathesis. Interestingly, a-Al2O3 has nearly double the number of strong Lewis acid sites, while the surface density of hydroxyl groups is about half, relative to γ-Al2O3. Although it is perilous to oversimplify these complex systems, multi-modal spectroscopic investigations can rationalize and guide the development of more efficient heterogeneous catalysts. MTO binds to the same Lewis acid sites as pyridine, and many of these sites are located in proximity to surface hydroxyl groups. The a-Al2O3 support, which contains more strong Lewis acid sites and fewer hydroxyl groups than γ-Al2O3, activates a much higher fraction of the grafted MTO. Thus strong Lewis acid sites that are remote from surface hydroxyls appear to be the key to generating materials with enhanced performance. Specifically, the absence of hydroxyl groups in the vicinity of the Lewis acid sites that adsorb MTO is critical. Such sites are present on the minor (100) facet of g-Al2O3, and are therefore candidates for the activating sites. They also appear to be significantly more abundant on the surfaces of less well-crystallized, “amorphous” aluminas. This information will enable more in-depth spectroscopic investigations, and facilitate further catalyst development based on tailor-made supports.

EXPERIMENTAL METHODS Catalyst preparation. Two commercial, mesoporous aluminas (MSU-X wormhole-type, Aldrich 517747, Lot 13309KUV, 239 m2 g-1, denoted a-Al2O3) and γ-Al2O3 (Strem 13-2525, Lot 29481900, 186 m2 g-1) were used to prepare supported catalysts. Both were pretreated by calcination in air at 500 °C for 4 h, followed by evacuation (10-4 Torr) at 450 °C for 12 h prior to synthesizing supported MTO catalysts under strictly anaerobic conditions. Volatile MTO (Sigma-Aldrich) was deposited onto the desired dry support by sublimation under vacuum (ca. 10-4 Torr) at room temperature, following a previously described procedure.24 The solid was shaken vigorously during the procedure to promote uniform deposition. After grafting, the reactor was evacuated at room temperature for 2 h to remove physisorbed MTO. Re loading was determined by dissolving the catalyst completely in 0.1 M HNO3, followed by ICP-AES analysis of the solution (Thermo iCAP 6300).31 Physicochemical characterization of aluminas and aluminasupported catalysts. N2 adsorption and desorption isotherms (Figure S13) were measured at -196 °C using a Micromeritics TRISTAR 3000 porosimeter, using 0.143 nm2 as the appropriate cross-section for the N2 molecule adsorbed on a hydroxyl-terminated surface.72 Both aluminas have similar textural properties, with mesopores in the 8-11 nm range, mesopore volumes of ca. 0.5 cm3 g-1, and negligible micropore volumes (< 0.01 cm3 g-1), Table S3.

Powder X-ray diffractograms (XRD) were obtained using a Phillips X’PERT MPD diffractometer with Cu Ka radiation. Samples were sealed in quartz capillaries without exposure to air or moisture. XRD patterns scanned over the 2q range from 10 to 50 °, with a step size of 0.1 ° and a count time of 2 s. Wide-angle powder X-ray diffraction patterns for both of the dry supports show principal reflections at 2q = 37 and 46 °, characteristic of γ-Al2O3 (JCPDS card 29-0063), Figure S1. However, the lines are broader and far less intense for a-

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Al2O3, consistent with its lower crystallinity and largely amorphous character, compared to the well-crystallized γ-Al2O3. IR spectra were acquired using 5-mm diameter self-supporting pellets (ca. 10 mg), pressed inside an Ar-filled glove-box. Spectra were recorded using a Bruker Alpha FTIR spectrometer over the range 4000-400 cm-1 at a resolution of 2 cm-1, accumulating 32 scans. The absorbance was normalized by the precise sample mass. For solid-state NMR analysis, the highly air-sensitive samples were packed into 3.2-mm zirconia rotors inside an Ar-filled glove-box, and sealed with tightly-fitting Kel-F caps. 1H MAS NMR, 1H-1H DQ-SQ MAS NMR and 1H-27Al D-HMQC MAS NMR spectra were acquired on a Bruker Avance III 800 spectrometer (1H: 800.13 MHz, 27 Al: 208.50 MHz). For 1H NMR experiments, the spinning frequency was 20 kHz, the recycle delays were ≥ 120 s, and 16 scans were collected using a 90 ° pulse excitation of 2.25 μs. 27Al MAS NMR spectra were acquired at a spinning frequency of 20 kHz. D-HMQC experiments were set up with a 27Al spin echo selective to the central transition, with pulses of 7 and 14 μs, and a 1H π/2 pulse of 3.3 μs on either side of the 27Al π pulse. A recycling delay of 2 s was used. The number of scans for each t1 increment varied from 512 to 2048, depending on the sample. The SR421 dipolar recoupling scheme73 was applied for 600 μs, unless specified otherwise. Two-dimensional (2D) 1 H-1H Double Quantum Magic-Angle Spinning (DQ-MAS) experiments were performed at 20 kHz spinning speed using the R1225 symmetry-based recoupling scheme,74 applied for 108 μs at a radiofrequency field strength of 40 kHz. A total of 16 transients were summed for each of the 160 t1 increments, with a recycle delay of 120 s. X-ray absorption spectra were recorded at the Re L3-edge on beamline 7-3 at the Stanford Synchrotron Radiation Lightsource (SSRL). It operates at 3.0 GeV with a current of 75-100 mA. In order to prevent adsorption of atmospheric moisture, catalysts were packed into slotted sample plates inside a glovebox, sealed under N2, and mounted in a liquid He-cooled cryostat to prevent thermal decomposition, as previously described.75-76 For each sample, at least three data sweeps were averaged to improve the signal-to-noise ratio. Characterization of Lewis acidity. Dimethylphenylphosphine (DMPP, 99 %, Aldrich) was added directly to a known amount of the dry alumina in a N2-filled glove box using a 25 µL syringe, to achieve a phosphine loading of (1.000 ± 0.035) mmol/g. The mixture was immediately sealed in a 3.2-mm zirconia solid-state NMR rotor (Wilmad) and allowed to equilibrate for 1 h before spectra were acquired. The rotor was placed in a N2-filled vial, removed from the glove-box, and transferred directly to the NMR probe. Low temperature 31P {1H} MAS NMR experiments36 were performed at 100 K using liquid N2 boiloff to cool and pneumatically spin the rotor at 10 kHz. Spectra were recorded using a triple resonance broadband X/Y/H probe and a Bruker 400 MHz (9.4 T) NMR spectrometer, operating at 161.61 MHz. Chemical shifts are referenced to 85 % H3PO4 as external standard, at 0 ppm.

For pyridine adsorption experiments, each alumina sample was first weighed and then dried by evacuation (10-4 Torr) at 450 °C for 2 h. A known pressure of anhydrous pyridine vapor (99 %, Aldrich) was added at room temperature, with the aid of a capacitance manometer (MKS Instruments). After 15 min exposure, the sample was heated to either 150 or 350 °C and evacuated for 20 min. IR spectra were recorded in transmission mode at room temperature in an Arfilled glovebox. To quantify the amount of adsorbed pyridine, a cali-


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bration curve was constructed (Figure S5) following a literature method.41 Propene metathesis activity, productivity, and active site counting. Catalyst activity and productivity in propene homo-metathesis was evaluated in both batch and continuous flow reactors, following previously described procedures.24 Active site counting was performed in a 120 mL batch reactor, following the dual olefin method of Chauvin and Commereuc.35 More details can be found in the Supporting information.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Experimental details on catalyst activity testing and active site counting; sample calculations of rates and rate constants; B.E.T. isotherms and XRD powder patterns for the alumina supports; calibration curve for pyridine adsorption; additional kinetic profiles; additional IR, solidstate NMR, and X-ray absorption spectra of grafted MTO.

AUTHOR INFORMATION Corresponding Authors MT: [email protected] LD: [email protected] RMG: [email protected] SLS: [email protected]

ACKNOWLEDGMENTS Financial support was provided by the U.S. Department of Energy, Office of Science, Division of Basic Energy Sciences, under the Catalysis Science Initiative (DE-FG-02-03ER15467). F.Z. is grateful for an iChem scholarship from the China Scholarship Council. The Chevreul Institute (FR 2638), Institut de Chimie de Lyon, Ministère de l’Enseignement Supérieur et de la Recherche, Région Nord – Pas de Calais, FEDER and CNRS are acknowledged for support and partial funding of this work. The financial support of the TGIR RMN THC Fr3050 is gratefully acknowledged. Portions of this work made use of the facilities of the Materials Research Laboratory (MRL) at UC Santa Barbara, supported by the NSF MRSEC Program under award DMR1121053. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract DE-AC02-76SF00515.

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Table of Contents graphic

O

CH3 O Re O OH O O Al O Al O O

O

CH3

Re O OO O O Al O O Al O O O

O


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Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on

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