A Strong Support Effect in Selective Propane Dehydrogenation

Subscriber access provided by University of Sussex Library

Article

A strong support effect in selective propane dehydrogenation catalyzed by Ga(i-Bu)3 grafted onto #-alumina and silica Kai C. Szeto, Zachary R. Jones, Nicolas Merle, Cesar Rios, Alessandro Gallo, Frédéric Le Quéméner, Laurent Delevoye, Régis M. Gauvin, Susannah L Scott, and Mostafa Taoufik ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00936 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

A strong support effect in selective propane dehydrogenation catalyzed by Ga(i-Bu)3 grafted onto γ-alumina and silica. Kai C. Szeto,1 Zachary R. Jones,2 Nicolas Merle,3 César Rios,1 Alessandro Gallo,4,5,6 Frederic Le Quemener,1 Laurent Delevoye,3 Régis M. Gauvin,3* Susannah L. Scott,2,4* Mostafa Taoufik1* 1

C2P2 (CNRS-UMR 5265), Université Lyon 1, ESCPE Lyon, 43 Boulevard du 11 Novembre

1918, 69626 Villeurbanne Cedex, France. 2

Department of Chemistry & Biochemistry, University of California, Santa Barbara 93106-

9510 USA. 3

Université Lille, CNRS, Centrale Lille, ENSCL, Université Artois, UMR 8181 - UCCS -

Unité de Catalyse et Chimie du Solide, F-59000 Lille, France. 4

Department of Chemical Engineering, University of California, Santa Barbara 93106-5080

USA. 5

SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering,

Stanford University, Stanford, California 94305, USA. 6

SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator

Laboratory, Menlo Park, California 94025, USA.

Corresponding authors: Mostafa Taoufik, [email protected] Susannah L. Scott, [email protected] Régis Gauvin, [email protected]

ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

Abstract The reactions of Ga(i-Bu)3 (i-Bu = CH2CH(CH3)2) with the dehydrated and partially dehydroxylated surfaces of alumina (Al2O3-500) and silica (SiO2-700) were studied by IR, high field solid-state NMR and EXAFS spectroscopies, as well as elemental analysis. Grafting onto Al2O3-500 occurs selectively by protonolysis at individual surface hydroxyl groups, resulting in the formation of mononuclear [(≡AlO)Ga(i-Bu)2L] (L = surface oxygen) sites as the major surface organometallic entities. Conversely, grafting on silica affords dinuclear species [(≡SiO)2Ga2(i-Bu)3] by a combination of protonolysis and isobutyl group transfer to Si. Further evidence for the difference in nuclearity was obtained by analysis of the WTEXAFS. The mononuclear alumina-supported Ga sites show much higher activity in propane dehydrogenation

than

their

dinuclear

silica-supported

counterparts.

The

propane

dehydrogenation reaction may require the presence of Al-O-Ga bonds to promote heterolytic C-H bond activation. Comparisons with benchmark catalysts and related systems show that the effect of the catalyst diluent is significant under the reaction conditions, and must be carefully assessed in order to attribute reactivity correctly. Keywords:

Gallium,

surface

organometallic

chemistry,

alumina,

silica,

propane

dehydrogenation, wavelet transform EXAFS, benchmarking


2

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Introduction World demand for propylene has increased steadily during recent decades, and is predicted to continue to grow in coming years, mostly due to rising polypropylene manufacturing. The gap between supply and demand has spurred research into new, on-purpose technologies for propylene production.1 Established catalytic methods for propylene include olefin metathesis,2 methanol-to-olefins3 and propane dehydrogenation.4-6 This last technology is particularly interesting in the current industrial context, due to the availability of a relatively cheap feedstock (propane) and a simple reactor design. Commercial plants typically operate under non-oxidizing conditions, in which only propane is fed to the catalyst bed (e.g., UOP’s Oleflex process). The technology affords high selectivity to propylene and simultaneously produces valuable H2. The main issues are the low equilibrium conversion, the requirement for high reaction temperatures, relatively fast catalyst deactivation, and formation of byproducts due to propane cracking.7 Most propane dehydrogenation catalysts are based on supported transition metals. Supported noble metal particles, with or without promoters, have been widely explored.8-10 However, supported non-noble metals such as chromium can be economically advantageous; the active sites are believed to be isolated tri- or divalent ions.11-13 The mechanisms of C-H activation, and propylene release by β-H elimination, are still matters of active debate.14 Vanadium supported on silica, alumina or titania also shows promising activity in catalytic light alkane dehydrogenation.15 However, similar to chromium, the classical preparation methods involving impregnation or sol-gel processes with oxovanadium(V) complexes typically yield mixtures of monomeric, dimeric and polymeric VOx surface species. Nevertheless, the active species in dehydrogenation are postulated to be isolated vanadium sites.16-19 In classical heterogeneous catalysts, the low concentration of these sites hampers their characterization, as well as mechanistic analysis of the catalytic reaction, including the initiation and deactivation steps. In our on-going study of supported oxometal complexes as models for industrial catalysts,20-30 we prepared well-defined, “single-site” supported oxovanadium species using surface organometallic chemistry. The resulting materials converted propane to propylene with 90 % selectivity and high activity.31 Gallium is another non-noble element known to catalyze propane dehydrogenation. A commercial process for propane aromatization, developed jointly by UOP and BP as the Cyclar Process, uses Ga/H-ZSM-5 as a bifunctional catalyst.32 Alkane dehydrogenation is believed to occur at the Ga sites, while the Brønsted acid sites of the zeolite (SiO(H)Al)


3

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 30

catalyze olefin oligomerization, cyclization and further dehydrogenation to aromatic compounds.33,34 When Hensen and co-workers saturated H-ZSM-5 with GaMe3, the resulting catalyst was active only for propane dehydrogenation.35,36 This selectivity towards olefins rather than aromatics was attributed to complete removal of the Brønsted-acidic protons of the zeolite, which are accessible to the small GaMe3 complex. Recently, we deposited the much larger Ga(i-Bu)3 on a mesoporous H-ZSM-5 to obtain a highly active catalyst that converts propane to aromatics (BTX);37 minor, but significant, amounts of propylene were also observed. Spectroscopic studies (IR, solid state NMR, EXAFS) suggested that Ga(i-Bu)3 reacts selectively with silanol defects located in the mesopores created by post-treatment of H-ZSM-5. Importantly, the Brønsted-acidic protons remain intact in this material, allowing it to operate as a bifunctional catalyst. According to the accepted mechanism for propane aromatization, gallium sites are responsible for catalyzing propane dehydrogenation. At the relatively high temperatures for this reaction, restructuring and migration of the gallium sites (relative to the as-prepared species) in the zeolite may occur. The local structure of the active sites under reaction conditions may therefore include Ga-OSi ligands, Ga-OAl ligands, or a mixture of both. Their precise structure is difficult to assess using spectroscopic techniques such as EXAFS, because of the similarity in X-ray scattering by Si and Al. A straightforward method to gain further insight is to prepare structurally well-defined gallium species supported on amorphous silica and γ-alumina, and to compare the consequences for subsequent catalytic activity in propane conversion. Here, we grafted Ga(i-Bu)3 onto these supports at room temperature. The resulting materials were characterized by several spectroscopic methods (DRIFT, solid-state NMR, XANES and EXAFS) as well as elemental analysis, and their reactivity towards propane measured in a continuous flow reactor.

Results and discussion Grafting of Ga(i-Bu)3 on alumina In order to leverage our previous, extensive characterization of alumina and aluminasupported catalysts,23,38,39 we selected a γ-alumina with a BET surface area of 200 m2 g-1 that had been partially dehydroxylated in vacuum at 500 °C (Al2O3-500). Its remaining surface hydroxyl groups, with a density of ca. 2.0 OH per nm2 (0.65 mmol g-1),40 are potential sites for reaction with organometallic complexes. In addition, thermal treatment of γ-alumina


4

Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

generates coordinatively-unsaturated, Lewis acidic Al sites that are electrophilic and therefore, potentially reactive towards organometallics either during grafting or in subsequent surface-mediated reactions. For example, grafted species such as Al(i-Bu)n (n = 1, 2) have been shown to react with neighboring surface sites by isobutyl group transfer.41 The reaction of Al2O3-500 with a pentane solution containing a small excess of Ga(iBu)3 (relative to the initial surface hydroxyl concentration) at room temperature afforded a white powder consisting of a modified alumina bearing Ga-i-Bu moieties (I). It was washed extensively with pentane and dried under high vacuum. IR spectroscopy reveals that grafting of Ga(i-Bu)3 proceeds at least partially on the surface hydroxyl groups (Figure 1). The reactivity contrasts with that of more reactive Al(i-Bu)3,40 which reacts with all of the surface hydroxyls, and with that of less reactive [W(≡CtBu)(CH2tBu)3], which consumes only terminal hydroxyls present on tetrahedral aluminum sites.42 The unreacted hydroxyl groups may be either triply-bridging, due to their lower O-H stretching frequencies (ca. 3650 cm-1), or unreacted bridging hydroxyls in interaction with alkyl fragments (thus with frequency lower than unperturbed analogues). Vibrational modes typical of C–H stretching and bending are observed at 3000-2800 and 1460-1320 cm-1, confirming the retention of some alkyl groups on the surface.

Figure 1. DRIFT spectra of Al2O3-500: (a) before, and (b) after reaction with Ga(i-Bu)3 in pentane at 25 °C, showing the partial disappearance of surface hydroxyl groups as well as the appearance of surface alkyl groups.

According to elemental analysis, I contains 1.97 wt% Ga and 2.85 wt% C, for a C/Ga atomic ratio of 8.4. The amount of grafted Ga corresponds to 0.28 mmol/g γ-alumina, while the original surface hydroxyl concentration was ca. 0.65 mmol/g,43 thus the grafting


5

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 30

stoichiometry, mol Ga/mol OH, is 0.43. This result is consistent with Ga(i-Bu)3 being less reactive toward the various hydroxyl groups of alumina than Al(i-Bu)3, since the latter reacted with a stoichiometry (mol Al/mol OH) of 0.92.40 Analysis of the gaseous products evolved during grafting of Ga(i-Bu)3 revealed the release of 0.9 i-BuH/Ga. Together, these results support grafting via the protonolysis of i-Bu ligands by aluminols, and are in line with formulation of the major species representing I as [(AlO)Ga(i-Bu)2L], where L represents a surface oxygen, Scheme 1.

Ga

Al2O3-500

O O

Al O

O

O

O Al O O Al O

Ga

SiO2-700

O

Ga

Si O

O O

O Si O

O

O

O

O Ga O Si O O Si O O

O

Si O

I

O

II

Scheme 1. Grafting of Ga(i-Bu)3 onto Al2O3-500 and SiO2-700 leads to structurally distinct surface species.

Further spectroscopic characterization provided more detailed information about the Ga coordination environment in Ga(i-Bu)3/Al2O3-500. The 13C CP/MAS spectrum (Figure S1) consists of a dominant signal at 25.4 ppm accompanied by a shoulder at ca. 30 ppm, assigned to CH2/CH3 and CH groups, respectively.44 The 1H MAS NMR contains two main signals consisting of broad peaks at 1.95 and 0.85 ppm (Figure 2a). These signals are assigned to methyne and (undifferentiated) methyl and methylene groups, respectively. The complex surface chemistry of the alumina support (relative to silica) causes structural diversity in the grafted species, resulting in broadening of the 1H NMR signals. A third, weak signal at -0.12 ppm is assigned to alkyl groups transferred from gallium to the alumina surface; the chemical shift corresponds to that of Al-CH2CHMe2 groups, as we reported previously.41 Its signal is more clearly visible as an on-diagonal signal at about -0.1 ppm in the single-quantum dimension in the 1H-1H DQ MAS spectrum (Figure 2b).


6

Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 2. MAS NMR of Ga(i-Bu)3/Al2O3-500: (a) 1H, and (b) 1H-1H DQ spectra (18.8 T, 20 kHz MAS). The 1H-1H DQ MAS spectrum also features two main off-diagonal correlations. The most intense one (labeled A in Figure 2b) associates methyl/methylene and methyne fragments of the same isobutyl ligand. The methyne groups also give rise to (on-diagonal) self-correlation, in agreement with the presence of two alkyl ligands on the grafted Ga(i-Bu)2 fragments. The second off-diagonal signal results from correlation between CH3 groups and protons centered at 3 ppm range (B in Figure 2b). It arises from methyl interactions with residual aluminum hydroxyl groups, the latter being visible in the IR spectrum as a broad peak centered at 3650 cm-1 (Figure 1b). The correlation involving a sharp signal at the methyl chemical shift in the single quantum dimension despite the absence of a corresponding signal in the higher frequency region of the hydroxyl protons signal (centered at ca. 3 ppm) is caused by the low abundance of these residual hydroxyls and the distribution of their signals over a large chemical shift range, hampering their direct observation.39 Consequently, no 1H NMR ACS Paragon Plus Environment

7

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 30

peak is visible in the bridging hydroxyl region, although the 2D spectrum allows its detection due to dipolar interactions involving the methyl groups. The Ga-modified alumina was also studied using material, the

27

27

Al NMR. As expected for this

Al MAS NMR spectrum is typical of γ-Al2O3-500, featuring mostly tetra- and

hexa-coordinated Al in the bulk (signals at ca. 60 and 0 ppm, respectively), as well as a minor amount of near-surface penta-coordinated Al at ca. 30 ppm (Figure 3b). Heteronuclear correlation techniques such as

1

H-27Al D-HMQC (Heteronuclear Multiple Quantum

Correlation)45,46 can efficiently probe the surface of alumina and alumina-containing supports through 1H-27Al dipolar filtering.23,37,39,40 By suppressing the bulk Al signals, this type of experiment can shed light on structural features of the surface, such as (1) the Al coordination numbers (AlT, AlP, and AlH) that are associated with each type of hydroxyl group, and (2) the extent of local distortions, manifested in the quadrupolar coupling constants. The greater robustness of D-HMQC relative to classical CP (Cross Polarization) in terms of efficiency of signal observation has been amply demonstrated.47 In 2D acquisition mode, and given sufficient resolution in both dimensions, 1H-27Al D-HMQC can afford precise assignments of the surface hydroxyl signals. Use of a short recoupling time (600 µs) further ensures that only Al centers close to alkyl groups and residual hydroxyls give rise to signals. Indeed, by suppressing most of the Al signals from the bulk, and thereby facilitating the observation of surface Al in close proximity to protons, the 1H-27Al D-HMQC dipolar heteronuclear filter generates a 1D

27

Al MAS NMR spectrum for Ga(i-Bu)3/Al2O3-500 with a

distinctly different appearance (Figure 3c). Thus, the HMQC MAS NMR signal for tetracoordinated Al species is broader than the corresponding MAS NMR signal, indicating larger second-order quadrupolar coupling, while the signal intensity for penta-coordinated Al species increases relative to the intensity in the MAS NMR spectrum. We conclude that fourcoordinate Al sites at the surface possess a wider range of coordination environments, and that the relative abundance of five-coordinate Al is higher at the surface, relative to the corresponding bulk Al sites.


8

Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 3. MAS NMR of Ga(i-Bu)3/Al2O3-500: (a) 1H, (b) 27Al, (c) 1H D-HMQC-filtered 27Al, and (d) 27Al-1H D-HMQC spectra (18.8 T, 20 kHz MAS). To gain more insight into the surface structure, we recorded the 2D 1H-27Al D-HMQC MAS NMR spectrum of this material (Figure 3d). The spectrum is dominated by strong signals from alkyl moieties (CH2/CH3 and CH at 0.85 and 1.95 ppm, respectively, in the 1H dimension). As expected based on the IR, the highly characteristic, intense signal of terminal hydroxyls (-0.2 ppm) is not observed, nor is there any evidence of signals for HO-µ2-Al2 sites (ca. 2 ppm) (see Figures S2 and S3 for the comparison of the spectra for pristine and galliummodified alumina).39 From the IR data described above, the remaining hydroxyl groups are either triply bridging and/or in interaction with neighboring alkyl moieties. Their assignment as HO-µ3-Al3 sites is supported by the broad cross-peak between the 1H [5-2] ppm region and mostly hexa-coordinated Al (ca. 10-0 ppm), which is strongly reminiscent of the HO-µ3 signal


9

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 30

in γ-Al2O3-500.39 As these protons are the most acidic ones, and thus are more likely to react with gallium alkyl fragments, one may also propose that the signal corresponds to hydroxyl groups (of undetermined coordination type) experiencing strong perturbation of their 1H NMR signal. However, other examples on the contrasted reactivity of alumina’s hydroxyl towards organometallics have shown that not only acidity, but also steric factors must be taken into account.42 Signals for the various alkyl fragments (CH, CH2 and CH3) correlate with signals for all three types of surface Al (tetra-, penta- and hexa-coordinated). Thus, the distribution of surface Al sites is barely affected by the grafting of Ga onto the hydroxyl groups, showing the same distribution and quadrupolar coupling strengths without significant surface reconstruction or alteration. Regarding the alkyl ligands transferred to Al (1H dimension: 0.12 ppm), the corresponding row in the 2D spectrum shows no preferential transfer to any particular type of Al site (Figure S2). The average Ga coordination environment in Ga(i-Bu3)/Al2O3-500 was further probed using X-ray absorption spectroscopy. The Ga K-edge, defined based on the first inflection point in the XANES, is located at 10,370.5 eV, followed by a broad, unstructured peak with a maximum at 10,371.5 eV, Figure 4a. The appearance and intensity of the absorption edge intensity are consistent with tetra-coordinate Ga(III).48-50 The same edge position was reported for monomeric Ga(t-Bu)2(O-C6H2-2,4-6-t-Bu).35,36 The EXAFS in Figure 4b consists of a principal feature at ca. 1.6 Å in non-phase-corrected R-space due to scattering from atoms directly coordinated to Ga, as well as weaker features from 2 - 3 Å due to scattering from nonbonded atoms at well-defined distances from Ga.


10

Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 4. Comparison of (a) Ga K-edge XANES (with inset showing the first derivatives), and (b) Ga K-edge EXAFS, for Ga(i-Bu)3/Al2O3-500 (black) and Ga(i-Bu)3/SiO2-700 (red). The EXAFS data for Ga(i-Bu)3/Al2O3 were analyzed using a model similar to one used for Ga(i-Bu)3/meso-H-ZSM5. In addition to the short Ga-C1 path associated with the methylene carbon of the isobutyl ligands at 2.07 Å (N = 2), the curvefit shown in Figure 5 includes paths for an anionic oxygen donor ligand at 1.89 Å (Ga-O1) and a neutral oxygen donor ligand at 2.51 Å (Ga-O2), Table 1. Longer (non-bonded) paths involving the methyne carbons of the isobutyl ligands (Ga-C2, 3.01 Å) and a surface Al (3.23 Å) were also successfully refined. All distances are physically reasonable. The EXAFS therefore supports a formulation of the major species in I as isolated, four-coordinate [Ga(i-Bu)2(OAl)(L)] sites, where L is a neutral oxygen donor ligand derived from the alumina surface.


11

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 30

Figure 5. Ga K-edge EXAFS (points) for Ga(i-Bu)3/Al2O3, I, shown in (a) k-space, and (b) Rspace (FT magnitude and imaginary component). The data are k3-weighted and not phasecorrected. The curve-fit (lines) was generated using the parameters in Table 1. Table 1. Curve-fit parameters a for Ga K-edge EXAFS of Ga(i-Bu)3/Al2O3, I

a

Path

N

R/Å

103 σ2 / Å2

Ga-O1

1

1.89 (1)

1.3 (7) b

Ga-C1

2

2.07 (2)

1.3 (7) b

Ga-O2

1

2.51 (3)

7 (2) c

Ga-C2

2

3.01 (5)

7 (2) c

Ga-Al

1

3.23 (3)

7 (2) c

The data ranges used in the fit are 3.0 ≤ k ≤ 13.0 Å-1 and 1.0 ≤ R ≤ 3.2 Å. S02 was fixed at 1

and N was fixed at the integer values shown; ∆E0 was refined as a global fit parameter, returning a value of (1 ± 2) eV. The σ2 values were constrained in order to decrease the number of fit parameters and the correlations between them. The total number of variable parameters in the fit is 8, out of a total of 13.8 independent data points. Uncertainties in the last digit are shown in parentheses. The R-factor for this fit is 0.022. b Constrained to the same value. c Constrained to the same value.

In sum, the evidence from several characterization methods suggests that the reaction of Ga(i-Bu)3 with Al2O3-500 results in a major surface species derived from reaction with a surface hydroxyl group that results in protonolysis of a single isobutyl ligand giving isolated, mononuclear grafted sites attached via a single aluminoxy ligand, Scheme 1. A minor surface species may have two aluminoxy ligands, resulting from a small extent of isobutyl ligand transfer from Ga to Al.

Grafting of Ga(i-Bu)3 onto silica To complement our study of the interaction of Ga(i-Bu)3 with Al2O3-500, we also investigated silica as a support. The surface of the pyrogenic silica (Aerosil 200 from Degussa, partially dehydroxylated at 700 °C under high vacuum) features non-interacting silanols (average density: 0.70 OH/nm2; 0.27 mmol/g)51 available to anchor metal complexes. Upon reaction of a small excess of Ga(i-Bu)3 (relative to the surface hydroxyl concentration) with a pentane suspension of SiO2-700 at room temperature, a white powder (II) was recovered. It was washed extensively with pentane and dried under high vacuum. ACS Paragon Plus Environment

12

Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

DRIFT spectroscopy showed that the silanols are completely consumed, and that alkyl ligands are present, as evidenced by the appearance of peaks in the range 3000-2800 cm-1 (Figure S4). Elemental analysis revealed Ga and C contents of 3.34 wt% and 5.37 wt%, respectively, representing a C/Ga atomic ratio of 9.3. The amount of grafted Ga, 0.48 mmol/g silica, represents a grafting stoichiometry (mol Ga/mol OH) of 1.8. The amount of gas released during grafting is 0.50 i-BuH/Ga. These results suggest that, while grafting by ligand protonolysis does take place, the major surface species is dinuclear in gallium. This is similar to findings in reports on the grafting of GaMe3 on highly dehydroxylated silicas (Scheme 1).52,53 Nevertheless, the structure resulting from grafting Ga(i-Bu)3 on silica differs from that of Ga(i-Bu)3 on alumina (vide supra) or mesoporous H-ZSM-5.37 Hydrolysis of II resulted in the release of only 1.45 i-BuH/Ga, suggesting that ca. 1 iBu ligand per Ga was transferred to Si, presumably by reaction with an adjacent siloxane bond, to form a protolytically-insensitive isobutylsilane. A similar alkyl transfer reaction occurred when Al(i-Bu)3 was grafted onto SiO2-700.41 The linewidths in the 1H MAS NMR spectrum of II (Figure 6a) are very similar to those of Ga(i-Bu)3 supported on mesoporous HZSM-5, implying greater uniformity of the surface species present on silica and the zeolite, relative to alumina.37 The main signals appear at 2.0 and 0.9 ppm, corresponding to CH and CH3 protons, respectively. A high field shoulder on the methyl peak (ca. 0.75 ppm) is assigned to Ga-CH2 protons. The 1H-1H DQ MAS NMR spectrum confirms the presence of isobutyl groups, via an off-diagonal correlation between the (undifferentiated) CH2/CH3 signals and the CH signal (Figure 6b). Curiously, the on-diagonal correlation observed for the methyne groups is very weak, in contrast to the spectrum of Ga(i-Bu)3/alumina.


13

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 30

Figure 6. MAS NMR of Ga(i-Bu)3/SiO2-700: (a) 1H, and b) 1H-1H DQ spectra (18.8 T, 20 kHz MAS). The

29

Si CP/MAS spectrum (Figure S5) is dominated by a signal from the silica at -

105 ppm (Si(OSi)4), however a weak contribution at -58 ppm (i-BuSi(OSi)3) confirms the transfer of isobutyl ligands from Ga to Si (as already indicated by the mass balance).41 The SiCH2 protons signal, expected at 0.9 ppm,41,54 cannot be distinguished within the 1H MAS NMR spectrum. Additional evidence for isobutyl ligand transfer was obtained by

13

C

CP/MAS NMR, facilitated by the very different reactivity of Ga-R and Si-R towards O2 and D2O. The

13

C CP/MAS spectrum (Figure S6) consists of two signals at 31.2 and 25.6 ppm,

assigned to CH and CH2/CH3 groups, respectively.41 After exposure of II to dry O2, the spectrum (Figure S7) contains an intense peak at 25 ppm, attributed to the unresolved signals of SiCH2CH(CH3)2,54 as well as three resolved signals at 17.5, 31.0 and 71.8 ppm, assigned to the GaOCH2CH(CH3)2 moiety.44 Exposure of II to D2O resulted in elimination of the isobutyl ligands bound to Ga, and a 13C CP/MAS NMR spectrum consisting solely of a signal at 24.8 ppm (SiCH2CH(CH3)2) with an upfield shoulder due to SiCH2CH(CH3)2 (Figure S8).54 ACS Paragon Plus Environment

14

Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

The average coordination environment of Ga(i-Bu)3/SiO2 was also probed using X-ray absorption spectroscopy. The location of the Ga K-edge edge, 10,370.4 eV, is just slightly lower than that of Ga(i-Bu)3/Al2O3, Figure 4a. However, the appearance of the XANES is clearly different, with not one but two resolved peaks just beyond the edge, at 10,371.5 and 10,373.5 eV. The EXAFS is also subtly different, particularly in the region from 2 - 3 Å in Rspace, Figure 4b. Initially, it was analyzed with a mononuclear model analogous to that used for Ga(i-Bu)3/Al2O3 (with appropriately modified coordination numbers, reflecting the slightly different grafting stoichiometry). A reasonable fit with similar fit parameters was obtained (Figure S9 and Table S1). However, the expected combination of long (non-bonded) Ga-O, Ga-C and Ga-Si paths gave a less than satisfactory fit in the region 2 – 3 Å in R-space. Based on prior EXAFS fit results for GaMe3/SiO2, as well as the elemental analysis reported above for Ga(i-Bu)3/SiO2, we also attempted a fit using the digallium model shown in Scheme 1, which includes a a pair of four-coordinate Ga(i-Bu)2 sites and a Ga-Ga path. The fit is shown in Figures 7 and 9, with fit parameters given in Table 2. The short Ga-O path at 1.92 Å represents the terminal silanolate ligands. Paths representing the Ga-C and Ga-O contributions of the isobutyl and bridging silanolate ligands were combined into a single contribution at 2.04 Å, because they have very similar scattering properties and the expected difference in their path lengths is less than the resolution of the EXAFS dataset (ca. 0.1 Å). The path involving the methyne carbons of the isobutyl ligands appears at 3.03 Å, as expected. The Ga-Ga path refined at 2.96 Å, with a coordination number of (0.8 ± 0.1). The distance is consistent with a Ga2O2 ring formed by bridging silanolates, as reported for [Me2Ga(OSiPh3)]2 and silica-supported [Me2Ga(OSi≡)],52,53 and the coordination number suggests that the majority (or possibly all, considering the uncertainty of EXAFS coordination numbers) of the grafted Ga sites are dinuclear. The data agree very well with the curvefit over the full R range.


15

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 30

Figure 7. Ga K-edge EXAFS (points) for Ga(i-Bu)3/SiO2, II, shown in R-space, as the FT magnitude and imaginary component. The data are k3-weighted and not phase-corrected. The curve-fit (lines) obtained using a digallium model was generated using the curve-fit parameters in Table 2. Table 2. Curve-fit parametersa for Ga K-edge EXAFS of Ga(i-Bu)3/SiO2 (II)

a

Path

Nb

R (Å)

103 σ2 (Å2)

Ga-O1

1

1.92 (2)

4.7 (5) b

Ga-C1/O2

3

2.04 (3)

4.7 (5) b

Ga-C2

1.5

3.03 (1)

4.7 (5) b

Ga-Ga

0.8(1)

2.97 (1)

8 (1)

-1

The data ranges used in the fit are 3.0 ≤ k ≤ 13.0 Å and 1.0 ≤ R ≤ 3.2 Å. S02 was fixed at 1

and N was fixed at the values shown (except for the Ga-Ga path); ∆E0 was refined as a global fit parameter, returning a value of (6 ± 1) eV. The σ2 values were constrained in order to decrease the number of fit parameters and the correlations between them. The number of variable parameters is 8, out of a total of 13.8 independent data points. Uncertainties in the last digit are shown in parentheses. The R-factor for this fit is 0.007. b Constrained to the same value.

The subtle differences in the FT-EXAFS curvefits using models either long-range GaGa or Ga-X (X = Al, Si) paths (Figures 5 and S10 for Ga(i-Bu)3/Al2O3, and Figures 7 and S9 for Ga(i-Bu)3/SiO2) led us to seek additional information via the wavelet transform of the


16

Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

EXAFS (WT-EXAFS) about the identity of the non-bonded scatterers that give rise to the features between 2 and 3 Å in R-space.55 This approach was instrumental in establishing the dimeric nature of GaMe3/SiO2.52,53 The results for I and II are compared in Figure 8. For both materials, the short-range features appear in non-phase-corrected R-space as maxima at 1.48 Å, due to scattering by directly coordinated light atoms (C, O) which give rise to maxima in kspace at 5.7 Å-1. However, the maxima for I and II differ with respect to the long-range scatterers. For Ga(i-Bu)3/SiO2 (II), the maxima in R-space and k-space (2.66 Å and 7.5 Å-1, respectively) are both significantly higher than those for Ga(i-Bu)3/Al2O3 (I, 2.52 Å and 6.8 Å-1, respectively). In particular, the locations of the k-space maxima are consistent with major contributions from a heavier scatterer (Ga) for II and a combination of lighter scatterers (Al, C) for I, based on WT-EXAFS simulations for each type of path,52,53 as well as a combination of paths (Figure S11).

a

b

c

d


17

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

Figure 8. Ga K-edge WT-EXAFS, for (a,b) Ga(i-Bu)3/Al2O3, I; and (c,d) Ga(i-Bu)3/SiO2, II. The k-space data for I and II are distinctly different in the range 9 ≤ k ≤ 11.5 Å-1, Figure 9a. The EXAFS of Ga(i-Bu)3/Al2O3 shows a single peak in this region, while the spectrum of Ga(i-Bu)3/SiO2 shows a double peak. Examination of the curvefit paths revealed that there are two major contributors to the intensity of this peak in the EXAFS of Ga(iBu)3/Al2O3: the short Ga-O1 and Ga-C1 paths, which are in-phase (Figure 9b). The contributions of the longer Ga-O2 and Ga-C2 paths, as well as the Ga-Al path, are much weaker. However, the EXAFS of Ga(i-Bu)3/SiO2 has an additional strong contribution from the Ga-Ga path which is out-of-phase with the other two paths, resulting in a splitting of the feature (Figure 9c). Notably, the curvefit obtained using a mononuclear Ga model does not show this peak splitting (Figure S9a). This analysis, in combination with the WT-EXAFS shown above, strongly suggests that the second coordination spheres of the two grafted sites are indeed different.


18

Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 9. (a) Comparison of Ga K-edge EXAFS (points) in k3-weighted k-space for Ga(iBu)3/Al2O3, I, and Ga(i-Bu)3/SiO2, II. The curve-fits (lines) were obtained using a monogallium model for I and a digallium model for II. The region of major difference at high k values is indicated by the dashed lines. Major contributing curvefit paths in the region 9.0 ≤ k ≤ 11.5 Å-1, for (b) Ga(i-Bu)3/Al2O3, I; and (c) Ga(i-Bu)3/SiO2, II.

Catalytic propane dehydrogenation The catalytic behaviors of I and II in propane dehydrogenation were evaluated in a continuous fixed-bed type reactor at 550 °C and a total flow rate of 5 mL/min (20 vol% C3H8 in Ar). When propane was fed to a bed of I, the conversion initially reached a maximum of 24 % at early times on-stream (Figure 10). Simultaneously, traces of isobutane were detected, most likely due to C-H bond activation of propane on the Ga-i-Bu sites via a σ-bond metathesis pathway. Next, a gradual deactivation was observed. After 1500 min, the conversion was 8 %. At this time, the products were comprised mainly of propene (90 %), with very small amounts of methane (3.6 %), ethene (3.1 %), ethane (1.0 %), as well as traces (< 1 %) of isobutene and benzene (Figure 11). The other products originate from propane cracking (vide infra) and a very small degree of aromatization.


19

ACS Catalysis

35 30

Conversion (%)

25 20 15 10 5 0 0

200

400

600

800

1000

1200

1400

Time on stream (min)

Figure 10. Comparison of propane conversion at 550 °C catalyzed by Ga(i-Bu)3/Al2O3-500 (1.97 wt% Ga, 80 mg, I, solid black symbols), Ga(i-Bu)3/SiO2-700 (3.34 wt% Ga, 50 mg, II, open black symbols), and thermally treated Ga[OSi(O-t-Bu)3]3/SiO2 (1.53 wt% Ga, 100 mg, red), all diluted in SiC (2.5 g).

100

80

Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

m ethane ethane ethene propene

60

40

20

0 0

200

400

600

800

1000

1200

1400

Time on stream (min)

Figure 11. Selectivity of Ga(i-Bu)3/Al2O3-500 in propane conversion at 550 °C. Ga(i-Bu)3/SiO2 (II) exhibited far lower activity, reflected in an initial conversion of ca. 8 % under the same conditions (Figure 10). The conversion profile again shows gradual deactivation, stabilizing at ca. 2.5 % after 1500 min on-stream. Interestingly, the selectivity varies as a function of conversion (or time on-stream). Initially, the products are comprised of propene (89.5 %), methane (5.3 %), and ethene (4.2 %), along with traces (< 1 %) of ethane and benzene. However, after 1500 min, the methane and ethane selectivities increased to 14.2


20

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

and 13.2 %, respectively, while the selectivity for propene decreased to 72.3 % (Figure S12). The dramatic change prompted us to perform a blank test with 2.5 g of the “inert” SiC diluent under the same conditions (Ptotal = 1 bar, T = 550 °C, total flow rate = 5 mL⋅min-1 of 20 vol% propane in Ar). This amount of SiC exhibited a constant propane conversion of 0.9 % with time on-stream (Figure S13). The product selectivity at steady-state is propene (41.0 %), methane (30.1 %), and ethene (28.9 %), Figure S14. Hence, the product selectivity for silicasupported gallium catalysts diluted in SiC appears to be altered by the catalytic contribution of the SiC diluent at low conversions or after significant deactivation of the active gallium species. This finding is not surprising in light of reports where SiC can show catalytic activity in non-oxidative dehydrogenation reaction.56,57 Additional blank tests with alumina, silica and the empty reactor alone are given in the Supporting Information (Figures S15, S16 and S17). Alumina alone is known to catalyze isobutane dehydrogenation.58 Under the reaction conditions used here (550 °C, 80 mg, 20 % propane in Ar, 1 bar), alumina converted 4 % of the propane to propene (81.5 %), methane (9.5 %), ethene (7.5 %), ethane (1.5 %) and trace of benzene. On the other hand, propane conversion over silica (550 °C, 50 mg, 20 % propane in Ar, 1 bar) and over the empty reactor was 0.6 %, with propene (39.5 %), methane (28.0 %), ethene (28.5 %) and ethane (4.0 %) as products, originating from thermal cracking. Our findings contrast with a recent report59 of high propane dehydrogenation activity for a Ga/silica catalyst prepared by thermal decomposition of Ga[OSi(O-t-Bu)3]3 immobilized on silica. That catalyst was reported to have mononuclear Ga sites, in contrast to the dinuclear sites found here for Ga(i-Bu)3/SiO2. To explore whether the different active site structures might be responsible for the different activities, we prepared Ga[OSi(O-t-Bu)3]3/SiO2 according to published procedure and tested its activity. Surprisingly, it showed virtually the same (i.e., low) propane dehydrogenation activity (Figure 10, Table S5) as Ga(i-Bu)3/SiO2, and similar selectivity (Figure S18) under similar conversion. From these findings, we infer that the striking difference in propane dehydrogenation activity for Ga(i-Bu)3 grafted onto the two supports is apparently not a consequence of the different nuclearities of the Ga sites, but of their Ga-support interactions. Previously, we reported that Ga(i-Bu)3/meso-HZSM5 exhibits high propane dehydrogenation activity with selectivity for aromatics.37 In comparison, GaiBu3/γ-Al2O3 shows significantly lower selectivity to aromatics. The difference is caused by the absence of strong Brønsted acid sites in the alumina, precluding the aromatization reaction observed for Ga(i-Bu)3/meso-HZSM5. Nevertheless, I appears to give a more active and selective catalyst than II for propane dehydrogenation. Thus the alumina surface strongly influences the activity of supported Ga in ACS Paragon Plus Environment

21

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

propane dehydrogenation. Since the mechanism is believed to involve C-H activation across a Ga-O bond,35 we postulate that this C-H activation is more favorable at [-Ga-O-Al-] sites relative to [-Ga-O-Si-] sites. However, it is also possible that Lewis acid sites on the alumina surface play a role in stabilizing highly dispersed Ga sites under the reaction conditions, or even in the propane activation itself. Another proposed mechanism involves the presence of Ga-H moiety as the active species.60 The structure of the actual active sites in the Ga system is still a matter of debate, as no clear conclusion has yet been widely accepted. The nature of the active species of the herein-described catalysts, including the gallium oxidation state and the metal-bound ligands, is under currently investigation by using in-situ and in-operando techniques under the reaction conditions, and will be the subject of a forthcoming study.

Conclusions Silica and alumina were modified with Ga(i-Bu)3 using surface organometallic chemistry in order to study the effect of the nature of the support on the structure of the grafted sites and its implications for activity in propane dehydrogenation. Complementary IR, high field solid-state NMR and X-ray absorption spectroscopies, in addition to elemental analysis, reveal that grafting on alumina occurs mostly on surface hydroxyl groups, resulting predominantly in mononuclear gallium sites, described as [(≡AlO)Ga(i-Bu)2L] (L = surface oxygen). Conversely, grafting on silica results in dinuclear species with the empirical formula [(≡SiO)3Ga2(i-Bu)3], via a combination of ligand protonolysis by surface hydroxyls and isobutyl group transfer from Ga to Si via reactions with strained siloxane bridges. Thus Ga(i-Bu)3 exhibits significantly different reactivity towards alumina and silica, leading to structurally distinct gallium surface sites. Moreover, the subsequent reactivity of each material towards propane is markedly different. The alumina-supported catalyst shows promising activity and high selectivity toward propylene at 550 °C, while the silica-supported material is far less active under the same reaction conditions, despite a recent report59 to the contrary. These results demonstrate the importance of proximity between Al and Ga in the mechanism of alkane dehydrogenation. They also illustrate the necessity of careful blank experiments to establish the identity of the catalytically active material. Future work will describe in more detail the structures of the active sites as they evolve under the reaction conditions.

Experimental Methods


22

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

General Procedures. All experiments were carried out under a controlled atmosphere, using Schlenk and glove box techniques for organometallic synthesis. The synthesis and handling of supported species were carried out using high-vacuum lines (ca. 1 mPa) and glove boxes. Diethyl ether and pentane were distilled from Na-benzophenone and NaK-benzophenone, respectively, and degassed using freeze-pump-thaw cycles. [Ga(i-Bu)3], Ga[OSi(O-t-Bu)3]3 and Ga{(OSi(O-t-Bu)3}3/SiO2 were synthesized according to published methods.59,61,62 OH titration was performed by reaction between the support and excess AliBu3, (which reacts quantitatively with hydroxyl groups from Al2O3-50040 and from SiO2-70041) and quantification of released isobutane by GC using a KCl/Al2O3 column and a FID.

Preparation and characterization of Ga(i-Bu)3/γγ-Al2O3-500 (I). A mixture of Ga(iBu)3 (200 mg, 0.80 mmol) and γ-Al2O3-500 (1.0 g) in pentane (10 mL) was stirred at 25 °C for 4 h. After filtration, the solid was washed 5 times with pentane and the washing fractions and the vapors were transferred to another reactor in order to quantify the isobutane evolved during grafting by GC. The resulting white powder was dried under vacuum (1 mPa) at 25 °C for 15 h. Elemental analysis: Ga 1.97 wt%, C 2.85 wt%. Preparation and characterization of Ga(i-Bu)3/SiO2-700 (II). A mixture of Ga(i-Bu)3 (200 mg, 0.80 mmol) and SiO2-700 (1.0 g) in pentane (10 mL) was stirred at 25 °C for 4 h. After filtration, the solid was washed 5 times with pentane and the washing fractions and the vapors were transferred to another reactor in order to quantify the isobutane evolved during grafting by GC. The resulting white powder was dried under vacuum (1 mPa) at 25 °C for 15 h. Elemental analysis: Ga 3.34 wt%, C 5.37 wt%. Analytical and Spectroscopic Procedures. Elemental analyses were carried out under air-free conditions by Mikroanalytisches Labor Pascher, Remagen (Germany). Gas analyses were performed on a Hewlett-Packard 5890 series II gas chromatograph, equipped with a flame ionization detector and an HP PLOT KCl/Al2O3 column (50 m × 0.32 mm). Diffuse reflectance IR spectra were collected in an air-tight IR cell equipped with CaF2 windows, using a Nicolet 6700 FT-IR spectrophotometer and recording 64 scans at 4 cm-1 resolution. Solid-state NMR spectra were acquired on Bruker Avance 500 and Bruker Avance III 800 spectrometers (1H: 800.13 MHz, 27Al: 208.50 MHz). For 1H NMR experiments (18.8 T), the spinning frequency was 20 kHz, the recycle delay was 10 s, and 16 scans were collected using a 90 ° pulse excitation of 2.25 µs. The

13

C CP-MAS NMR spectrum was obtained at


23

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

11.74 T using the CP-MAS pulse sequence and a high power 1H decoupling at an RF field amplitude of 70 kHz. The spinning frequency was set to 10 kHz. 27Al MAS NMR spectra at 18.8 T were acquired at a spinning frequency of 20 kHz (3.2 mm rotor diameter). The DHMQC experiments were set up with a 27Al spin echo selective to the central transition, with pulses of 8 and 16 µs, and a 1H π/2 pulse of 3.3 µs on either side of the

27

Al π pulse. The

number of scans for each t1 increment was set to 1024. The dipolar recoupling scheme (SR421)63 was applied for 500 µs. Two-dimensional (2D) 1H-1H Double Quantum MagicAngle Spinning spectra were acquired at 20 kHz spinning speed using the R1225 symmetrybased recoupling scheme64 applied for 200 µs at an RF field strength of 40 kHz. The recycle delay was set to 10 s and 16 transients were added for each of the 300 t1 increments. Chemical shifts are given in ppm with respect to TMS as external reference for 1H and

13

C, and to

Al(H2O)63+ for 27Al. X-ray absorption spectra at the Ga K-edge (10,367 eV) were collected on beamline 4-1 at the Stanford Synchrotron Radiation Lightsource, which operates at 3.0 GeV with a current of 500 mA. The spectrum was acquired under an inert atmosphere in a He cryostat at 15 K to minimize sample decomposition. Data were collected in fluorescence mode using an Ar-filled Lytle detector, and analyzed using the Demeter software package.65 The data were processed using Athena software, by subtraction of a linear pre-edge and normalization by the edge jump. The χ(k) data were isolated by subtracting a smooth, third-order polynomial approximating the absorption background of an isolated atom. The data were k3-weighted prior to Fourier-transform. The distance to the scattering atom (R) and the mean-squared displacement (σ2) were obtained by non-linear fitting with least-squares refinement to the EXAFS equation of the Fourier-transformed data in R-space, using Artemis software.66 To investigate the stability of the fits, the coordination numbers (N) were then refined while keeping the R-values fixed at their previously refined values. To avoid exceeding the number of allowed variable parameters, the first coordination sphere of Ga was required to be tetracoordinated, and the short and long Ga-C paths belonging to the same isobutyl ligand were required to have the same value of N. Morlet Wavelet Transforms (MWTs) were computed from k-space EXAFS data using IGOR Pro (WaveMetrics), following a previously described procedure.55 They were generated using κ = 10, σ = 1 as Morlet parameters. For MWT simulations, back-scattering amplitude functions and phase shifts were calculated using FEFF8.


24

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Reactivity testing. The reactivity of the supported catalysts in the dehydrogenation of propane (99.95% Air Liquide, < 20 ppm propene) was evaluated in a stainless steel, continuous flow reactor packed with 50 – 100 mg catalyst (depending on the Ga loading). The catalysts were diluted in SiC (Strem), affording a final catalyst bed mass of 2.5 g. Catalysts were loaded into the reactor inside a glovebox, and protected from the ambient atmosphere by a 4-way valve. Lines were purged with the reaction gas extensively prior to initiating the reaction. In addition, the catalyst bed was heated to 550 °C in flowing Ar (30 mL/min) and held for 30 min before admitting propane to the reactor. The molar flow rate was constant at 1.7 molC3⋅molGa-1⋅min-1, at Ptotal = 1 bar, T = 550 °C, total flow rate = 5 mL min-1 (20 vol% propane in argon). The feed gas was purified by passage through a column containing molecular sieves and activated Cu2O/Al2O3, and the flow rate controlled by a Brooks mass flow controller. Products were determined by online GC (HP 6890, equipped with 50 m KCl/Al2O3 column and an FID). Conversion was calculated based on peak areas normalized by carbon number.

Supporting Information Available. The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional NMR and IR spectra; EXAFS curvefits; analyses of propane conversion and selectivity for various supports.

Acknowledgments. Financial support of the U.S. Department of Energy, Office of Science, Division of Basic Energy Sciences, under the Catalysis Science Initiative (DEFG-02-03ER15467) is gratefully acknowledged. Chevreul Institute (FR 2638), Ministère de l’Enseignement Supérieur et de la Recherche, Région Nord – Pas de Calais, FEDER and CNRS are acknowledged for supporting and funding partially this work. Financial support from the TGIR RMN THC Fr3050 for conducting the research is gratefully acknowledged. 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.


25

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

References

(1) Tullo, A. H. Propylene On Demand - A Tight Market for Propylene Globally Is Breathing New Life into Propane Dehydrogenation. Chem. Eng. News 2003, 81, 15-16. (2) Mol, J. C. Industrial Applications of Olefin Metathesis. J. Mol. Catal. A 2004, 213, 39-45. (3) Olsbye, U.; Svelle, S.; Bjørgen, M.; Beato, P.; Janssens, T. V. W.; Joensen, F.; Bordiga, S.; Lillerud, K. P. Conversion of Methanol to Hydrocarbons: How Zeolite Cavity and Pore Size Controls Product Selectivity. Angew. Chem. Int. Ed. 2012, 51, 58105831. (4) Chen, D.; Holmen, A.; Sui, Z. J.; Zhou, X. G. Carbon Mediated Catalysis: A Review on Oxidative Dehydrogenation. Chin. J. Catal. 2014, 35, 824-841. (5) Nawaz, Z. Light Alkane Dehydrogenation to Light Olefin Technologies: A Comprehensive Review. Rev. Chem. Eng. 2015, 31, 413-436. (6) Sattler, J.; Ruiz-Martinez, J.; Santillan-Jimenez, E.; Weckhuysen, B. M. Catalytic Dehydrogenation of Light Alkanes on Metals and Metal Oxides. Chem. Rev. 2014, 114, 10613-10653. (7) Chalupka, K.; Thomas, C.; Millot, Y.; Averseng, F.; Dzwigaj, S. Mononuclear Pseudo-tetrahedral V Species of VSiBEA Zeolite as the Active Sites of the Selective Oxidative Dehydrogenation of Propane. J. Catal. 2013, 305, 46-55. (8) Kumar, M. S.; Chen, D.; Holmen, A.; Walmsley, J. C. Dehydrogenation of Propane over Pt-SBA-15 and Pt-Sn-SBA-15: Effect of Sn on the Dispersion of Pt and Catalytic Behavior. Catal. Today 2009, 142, 17-23. (9) Nawaz, Z.; Wei, F. Hydrothermal Study of Pt-Sn-based SAPO-34 Supported Novel Catalyst Used for Selective Propane Dehydrogenation to Propylene. J. Ind. Eng. Chem. 2010, 16, 774-784. (10) Sun, P.; Siddiqi, G.; Vining, W. C.; Chi, M.; Bell, A. T. Novel Pt/Mg(In)(Al)O Catalysts for Ethane and Propane Dehydrogenation. J. Catal. 2011, 282, 165-174. (11) Lillehaug, S.; Børve, K. J.; Sierka, M.; Sauer, J. A. Catalytic Dehydrogenation of Ethane over Mononuclear Cr(III) Surface Sites on Silica. Part I. C-H Activation by Sigma-bond Metathesis. J. Phys. Org. Chem. 2004, 17, 990-1006. (12) Olsbye, U.; Virnovskaia, A.; Prytz, O.; Tinnemans, S. J.; Weckhuysen, B. M. Mechanistic Insight in the Ethane Dehydrogenation Reaction over Cr/Al2O3 Catalysts. Catal. Lett. 2005, 103, 143-148. (13) Weckhuysen, B. M.; Schoonheydt, R. A. Alkane Dehydrogenation over Supported Chromium Oxide Catalysts. Catal. Today 1999, 51, 223-232. (14) Cheng, L.; Ferguson, G. A.; Zygmunt, S. A.; Curtiss, L. A. Structureactivity Relationship for Propane Oxidative Dehydrogenation by Anatase-supported Vanadium Oxide Monomers and Dimers. J. Catal. 2013, 302, 31-36. (15) Rossetti, I.; Fabbrini, L.; Ballarini, N.; Oliva, C.; Cavani, F.; Cericola, A.; Bonelli, B.; Piumetti, M.; Garrone, E.; Dyrbeck, H.; Blekkan, E. A.; Forni, L. V-Al-O catalysts Prepared by Flame Pyrolysis for the Oxidative Dehydrogenation of Propane to Propylene. Catal. Today 2009, 141, 271-281. (16) Barman, S.; Maity, N.; Bhatte, K.; Ould-Chikh, S.; Dachwald, O.; Haessner, C.; Saih, Y.; Abou-Hamad, E.; Llorens, I.; Hazemann, J. L.; Kohler, K.; D'Elia, V.; Basset, J. M. Single-Site VOx Moieties Generated on Silica by Surface Organometallic


26

Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Chemistry: A Way To Enhance the Catalytic Activity in the Oxidative Dehydrogenation of Propane. ACS Catal. 2016, 6, 5908-5921. (17) Fukudome, K.; Ikenaga, N.-o.; Miyake, T.; Suzuki, T. Oxidative Dehydrogenation of Propane Using Lattice Oxygen of Vanadium Oxides on Silica. Catal. Sci. Technol. 2011, 1, 987-998. (18) Fukudome, K.; Ikenaga, N.-o.; Miyake, T.; Suzuki, T. Oxidative Dehydrogenation of Alkanes over Vanadium Oxide Prepared with V(t-BuO)(3)O and Si(OEt)(4) in the Presence of Polyethyleneglycol. Catal. Today 2013, 203, 10-16. (19) Gao, X. T.; Bare, S. R.; Weckhuysen, B. M.; Wachs, I. E. In Situ Spectroscopic Investigation of Molecular Structures of Highly Dispersed Vanadium Oxide on Silica under Various Conditions. J. Phys. Chem. B 1998, 102, 10842-10852. (20) Bouhoute, Y.; Del Rosal, I.; Szeto, K. C.; Merle, N.; Grekov, D.; De Mallmann, A.; Le Roux, E.; Delevoye, L.; Gauvin, R. M.; Maron, L.; Taoufik, M. Modification of Silica-supported Tungsten Neosilyl Oxo Precatalysts: Impact of Substituted Phenol on Activity and Stability in Olefin Metathesis. Catal. Sci. Technol. 2016, 6, 8532-8539. (21) Bouhoute, Y.; Garron, A.; Grekov, D.; Merle, N.; Szeto, K. C.; De Mallmann, A.; Del Rosal, I.; Maron, L.; Girard, G.; Gauvin, R. M.; Delevoye, L.; Taoufik, M. Well-Defined Supported Mononuclear Tungsten Oxo Species as Olefin Metathesis Pre-Catalysts. ACS Catal. 2014, 4, 4232-4241. (22) Bouhoute, Y.; Grekov, D.; Szeto, K. C.; Merle, N.; De Mallmann, A.; Lefebvre, F.; Raffa, G.; Del Rosal, I.; Maron, L.; Gauvin, R. M.; Delevoye, L.; Taoufik, M. Accessing Realistic Models for the WO3-SiO2 Industrial Catalyst through the Design of Organometallic Precursors. ACS Catal. 2016, 6, 1-18. (23) Gallo, A.; Fong, A.; Szeto, K. C.; Rieb, J.; Delevoye, L.; Gauvin, R. M.; Taoufik, M.; Peters, B.; Scott, S. L. Ligand Exchange-Mediated Activation and Stabilization of a Re-Based Olefin Metathesis Catalyst by Chlorinated Alumina. J. Am. Chem. Soc. 2016, 138, 12935-12947. (24) Grekov, D.; Bouhoute, Y.; Szeto, K. C.; Merle, N.; De Mallmann, A.; Lefebvre, F.; Lucas, C.; Del Rosa, I.; Maron, L.; Gauvin, R. M.; Delevoye, L.; Taoufik, M. Silica-Supported Tungsten Neosilyl Oxo Precatalysts: Impact of the Podality on Activity and Stability in Olefin Metathesis. Organometallics 2016, 35, 2188-2196. (25) Larabi, C.; Szeto, K. C.; Bouhoute, Y.; Charlin, M. O.; Merle, N.; De Mallmann, A.; Gauvin, R. M.; Delevoye, L.; Taoufik, M. Solvent-Free Ring-Opening Metathesis Polymerization of Norbornene over Silica-Supported Tungsten-Oxo Perhydrocarbyl Catalysts. Macromol. Rapid Commun. 2016, 37, 1832-1836. (26) Mazoyer, E.; Merle, N.; de Mallmann, A.; Basset, J.-M.; Berrier, E.; Delevoye, L.; Paul, J.-F.; Nicholas, C. P.; Gauvin, R. M.; Taoufik, M. Development of the First Well-defined Tungsten Oxo Alkyl Derivatives Supported on Silica by SOMC: Towards a Model of WO(3)/SiO(2) Olefin Metathesis Catalyst. Chem. Commun. 2010, 46, 8944-8946. (27) Merle, N.; Girard, G.; Popoff, N.; de Mallmann, A.; Bouhoute, Y.; Trebosc, J.; Paul, J.-F.; Nicholas, C. P.; Del Rosal, I.; Maron, L.; Gauvin, R. M.; Delevoye, L.; Taoufik, M. On the Track to Silica-supported Tungsten Oxo Metathesis Catalysts: Input from 17O Solid-state NMR. Inorg. Chem. 2013, 52, 10119-10130. (28) Merle, N.; Le Quemener, F.; Bouhoute, Y.; Szeto, K. C.; De Mallmann, A.; Barman, S.; Samantaray, M. K.; Delevoye, L.; Gauvin, R. M.; Taoufik, M.; Basset, J. M. Well-Defined Molybdenum Oxo Alkyl Complex Supported on Silica by Surface Organometallic Chemistry: A Highly Active Olefin Metathesis Precatalyst. J. Am. Chem. Soc. 2017, 139, 2144-2147.


27

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

(29) Merle, N.; Le Quemener, F.; Barman, S.; Samantaray, M. K.; Szeto, K. C.; De Mallmann, A.; Taoufik, M.; Basset, J. M. Well-defined Silica Supported Bipodal Molybdenum Oxo Alkyl Complexes: A Model of the Active Sites of Industrial Olefin Metathesis Catalysts. Chem. Commun. 2017, 53, 11338-11341. (30) Larabi, C.; Merle, N.; Le Quemener, F.; Rouge, P.; Berrier, E.; Gauvin, R. M.; Le Roux, E.; de Mallmann, A.; Szeto, K. C.; Taoufik, M. New Synthetic Approach towards Well-defined Silica Supported Tungsten bis-oxo, Active Catalysts for Olefin Metathesis. Catal. Commun. 2018, 108, 51-54. (31) Szeto, K. C.; Loges, B.; Merle, N.; Popoff, N.; Quadrelli, E. A.; Jia, H. P.; Berrier, E.; De Mallmann, A.; Deleyoye, L.; Gauvin, R. M.; Taoufik, M. Vanadium Oxo Organometallic Species Supported on Silica for the Selective Non-oxidative Dehydrogenation of Propane. Organometallics 2013, 32, 6452-6460. (32) Mowry, J. R.; Anderson, R. F.; Johnson, J. A. Process Makes Aromatics from LPG. Oil & Gas J. 1985, 83, 128-131. (33) Bhan, A.; Delgass, W. N. Propane Aromatization over HZSM-5 and Ga/HZSM-5 Catalysts. Catal. Rev. Sci. Eng. 2008, 50, 19-151. (34) Price, G. L.; Kanazirev, V.; Dooley, K. M.; Hart, V. I. On the Mechanism of Propane Dehydrocyclization over Cation-containing, Proton-poor MFI Zeolite. J. Catal. 1998, 173, 17-27. (35) Getsoian, A.; Das, U.; Camacho-Bunquin, J.; Zhang, G. H.; Gallagher, J. R.; Hu, B.; Cheah, S.; Schaidle, J. A.; Ruddy, D. A.; Hensley, J. E.; Krause, T. R.; Curtiss, L. A.; Miller, J. T.; Hock, A. S. Organometallic Model Complexes Elucidate the Active Gallium Species in Alkane Dehydrogenation Catalysts based on Ligand Effects in Ga Kedge XANES. Catal. Sci. Technol. 2016, 6, 6339-6353. (36) Rane, N.; Overweg, A. R.; Kazansky, V. B.; van Santen, R. A.; Hensen, E. J. M. Characterization and Reactivity of Ga+ and GaO+ Cations in Zeolite ZSM-5. J. Catal. 2006, 239, 478-485. (37) Szeto, K. C.; Gallo, A.; Hernandez-Morejudo, S.; Olsbye, U.; De Mallmann, A.; Lefebvre, F.; Gauvin, R. M.; Delevoye, L.; Scott, S. L.; Taoufik, M. Selective Grafting of Ga(i-Bu)(3) on the Silanols of Mesoporous H-ZSM-5 by Surface Organometallic Chemistry. J. Phys. Chem. C 2015, 119, 26611-26619. (38) 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. (39) Taoufik, M.; Szeto, K. C.; Merle, N.; Del Rosal, I.; Maron, L.; Trebosc, J.; Tricot, G.; Gauvin, R. M.; Delevoye, L. Heteronuclear NMR Spectroscopy as a SurfaceSelective Technique: A Unique Look at the Hydroxyl Groups of gamma-Alumina. Chem. Eur. J. 2014, 20, 4038-4046. (40) Mazoyer, E.; Trebosc, J.; Baudouin, A.; Boyron, O.; Pelletier, J.; Basset, J. M.; Vitorino, M. J.; Nicholas, C. P.; Gauvin, R. M.; Taoufik, M.; Delevoye, L. Heteronuclear NMR Correlations To Probe the Local Structure of Catalytically Active Surface Aluminum Hydride Species on gamma-Alumina. Angew. Chem. Int. Ed. 2010, 49, 9854-9858. (41) Pelletier, J.; Espinas, J.; Vu, N.; Norsic, S.; Baudouin, A.; Delevoye, L.; Trebosc, J.; Le Roux, E.; Santini, C.; Basset, J. M.; Gauvin, R. M.; Taoufik, M. A Welldefined Silica-supported Aluminium Alkyl Through an Unprecedented, Consecutive Twostep Protonolysis-alkyl Transfer Mechanism. Chem. Commun. 2011, 47, 2979-2981. (42) Joubert, J.; Delbecq, F.; Sautet, P.; Le Roux, E.; Taoufik, M.; Thieuleux, C.; Blanc, F.; Coperet, C.; Thivolle-Cazat, J.; Basset, J. M. Molecular Understanding of


28

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Alumina Supported Single-site Catalysts by a Combination of Experiment and Theory. J. Am. Chem. Soc. 2006, 128, 9157-9169. (43) Le Roux, E.; Taoufik, M.; Coperet, 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. (44) Valet, M.; Hoffman, D. M. Synthesis of Homoleptic Gallium Alkoxide Complexes and the Chemical Vapor Deposition of Gallium Oxide Films. Chem. Mater. 2001, 13, 2135-2143. (45) Massiot, D.; Fayon, F.; Alonso, B.; Trebosc, J.; Amoureux, J. P. Chemical Bonding Differences Evidenced from J-coupling in Solid State NMR Experiments Involving Quadrupolar Nuclei. J. Magn. Reson. 2003, 164, 160-164. (46) Trebosc, J.; Hu, B.; Amoureux, J. P.; Gan, Z. Through-space R-3-HETCOR Experiments between Spin-1/2 and Half-integer Quadrupolar Nuclei in Solid-state NMR. J. Magn. Reson. 2007, 186, 220-227. (47) Tricot, G.; Lafon, O.; Trebosc, J.; Delevoye, L.; Mear, F.; Montagne, L.; Amoureux, J. P. Structural Characterisation of Phosphate Materials: New Insights into the Spatial Proximities between Phosphorus and Quadrupolar Nuclei Using the D-HMQC MAS NMR Technique. Phys. Chem. Chem. Phys. 2011, 13, 16786-16794. (48) Behrens, P.; Kosslick, H.; Tuan, V. A.; Froba, M.; Neissendorfer, F. X-Rayabsorption Spectroscopic Study on the Structure and Crystallization of Ga-containing MFI-type Zeolites. Microporous Mater. 1995, 3, 433-441. (49) Higby, P. L.; Shelby, J. E.; Phillips, J. C.; Legrand, A. D. EXAFS Study of Alkali Galliosilicate Glasses. J. Non-Cryst. Solids 1988, 105, 139-148. (50) Nishi, K.; Shimizu, K.; Takamatsu, M.; Yoshida, H.; Satsuma, A.; Tanaka, T.; Yoshida, S.; Hattori, T. Deconvolution Analysis of Ga K-edge XANES for Quantification of Gallium Coordinations in Oxide Environments. J. Phys. Chem. B 1998, 102, 10190-10195. (51) Millot, N.; Santini, C. C.; Lefebvre, F.; Basset, J. M. Surface Organometallic Chemistry: A Route to Well-defined Boron Heterogeneous Co-catalyst for Olefin Polymerisation. C. R. Chim. 2004, 7, 725-736. (52) Fleischman, S. D.; Scott, S. L. Evidence for the Pairwise Disposition of Grafting Sites on Highly Dehydroxylated Silicas via Their Reactions with Ga(CH3)(3). J. Am. Chem. Soc. 2011, 133, 4847-4855. (53) Taha, Z. A.; Deguns, E. W.; Chattopadhyay, S.; Scott, S. L. Formation of Digallium Sites in the Reaction of Trimethylgallium with Silica. Organometallics 2006, 25, 1891-1899. (54) Bassindale, A. R.; Liu, Z. H.; MacKinnon, I. A.; Taylor, P. G.; Yang, Y. X.; Light, M. E.; Horton, P. N.; Hursthouse, M. B. A Higher Yielding Route for T-8 Silsesquioxane Cages and X-ray Crystal Structures of Some Novel Spherosilicates. Dalton Trans. 2003, 2945-2949. (55) Funke, H.; Chukalina, M.; Scheinost, A. C. A New FEFF-based Wavelet for EXAFS Data Analysis. J. Sync. Rad. 2007, 14, 426-432. (56) Ba, H.; Liu, Y. F.; Mu, X. K.; Doh, W. H.; Nhut, J. M.; Granger, P.; PhamHuu, C. Macroscopic Nanodiamonds/beta-SiC Composite as Metal-free Catalysts for Steam-free Dehydrogenation of Ethylbenzene to Styrene. Appl. Catal. A 2015, 499, 217226. (57) Liu, H. Y.; Diao, J. Y.; Wang, Q.; Gu, S. Y.; Chen, T.; Miao, C. X.; Yang, W. M.; Su, D. S. A Nanodiamond/CNT-SiC Monolith as a Novel Metal Free Catalyst for Ethylbenzene Direct Dehydrogenation to Styrene. Chem. Commun. 2014, 50, 7810-7812.


29

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

(58) Rodemerck, U.; Kondratenko, E. V.; Otroshchenko, T.; Linke, D. Unexpectedly High Activity of Bare Alumina for Non-oxidative Isobutane Dehydrogenation. Chem. Commun. 2016, 52, 12222-12225. (59) Searles, K.; Siddiqi, G.; Safonova, O. V.; Coperet, C. Silica-supported Isolated Gallium Sites as Highly Active, Selective and Stable Propane Dehydrogenation Catalysts. Chem. Sci. 2017, 8, 2661-2666. (60) Mansoor, E.; Head-Gordon, M.; Bell, A. T. Computational Modeling of the Nature and Role of Ga Species for Light Alkane Dehydrogenation Catalyzed by Ga/HMFI. ACS Catal. 2018, 8, 6146-6162. (61) Kovar, R. A.; Derr, H.; Brandau, D.; Callaway, J. O. Preparation of Organogallium Compounds from Organolithium Reagents and Galium Chloride - Infrared, Magnetic-resonance, and Mass-spectral Studies of Alkylgallium Compounds. Inorg. Chem. 1975, 14, 2809-2814. (62) Dombrowski, J. P.; Johnson, G. R.; Bell, A. T.; Tilley, T. D. Ga[OSi((OBu)-Bu-t)(3)](3)center dot THF, a Thermolytic Molecular Precursor for High Surface Area Gallium-containing Silica Materials of Controlled Dispersion and Stoichiometry. Dalton Trans. 2016, 45, 11025-11034. (63) Brinkmann, A.; Kentgens, A. P. M. Proton-selective O-17-H-1 Distance Measurements in Fast Magic-angle-spinning Solid-state NMR Spectroscopy for the Determination of Hydrogen Bond Lengths. J. Am. Chem. Soc. 2006, 128, 14758-14759. (64) Carravetta, M.; Eden, M.; Zhao, X.; Brinkmann, A.; Levitt, M. H. Symmetry Principles for the Design of Radiofrequency Pulse Sequences in the Nuclear Magnetic Resonance of Rotating Solids. Chem. Phys. Lett. 2000, 321, 205-215. (65) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-ray Absorption Spectroscopy Using IFEFFIT. J. Sync. Rad. 2005, 12, 537541. (66) Sayers, D. E.; Stern, E. A.; Lytle, F. W. New Technique for Investigating Noncrystalline Structures - Fourier Analysis of Extended X-ray-absorption Fine Structure. Phys. Rev. Lett. 1971, 27, 1204-1207.

TOC Graphic


30

A Strong Support Effect in Selective Propane Dehydrogenation

Recommend Documents