Direct Observation of Coordinatively Unsaturated Sites on the Surface

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Direct Observation of Coordinatively Unsaturated Sites on the Surface of a Fluoride-Doped Alumina Catalyst Lukas Ahrem, Gudrun Scholz, Torsten Gutmann, Beatriz Calvo, Gerd Buntkowsky, and Erhard Kemnitz J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 19, 2017

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The Journal of Physical Chemistry

Direct Observation of Coordinatively Unsaturated Sites on the Surface of a Fluoride-Doped Alumina Catalyst Lukas Ahrem,1 Gudrun Scholz,1 Torsten Gutmann,

2,*

Beatriz Calvo,1

Gerd Buntkowsky,2 Erhard Kemnitz 1,*

1

Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-

Strasse 2, 12489 Berlin, Germany 2

Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische

Universität Darmstadt, Alarich-Weiss-Strasse 8, 64287 Darmstadt, Germany

* corresponding authors: Torsten Gutmann, email: [email protected] Erhard Kemnitz, email: [email protected]

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Abstract Coordinatively unsaturated sites (CUS) present a key feature of alumina based catalysts as they are believed to act as Lewis-acid sites in heterogeneously catalyzed reactions. In the present study, the direct observation of active species on a fluoride-doped aluminum oxide catalyst is demonstrated. This new fluoride-doped aluminum oxide exhibits strong Lewis-acid sites and superior catalytic activity as compared to γ-Al2O3. To emphasize the labile state of Lewis-acid sites, two distinctive states of the catalysts surface are addressed using 1H -

27

Al cross

polarization (CP) MAS NMR. On the one hand, the highly dehydrated and active state after calcination at 700 °C, and on the other hand the rehydrated and catalytically inactive surface (produced by contact to air) are probed. These experiments revealed the presence of significant amounts of coordinatively unsaturated sites in form of four- and fivefold coordinated Al-sites on the highly-dehydrated surface. In contrast to this, the rehydrated sample exhibited a severely restructured surface caused by the chemisorption of H2O which is constituted in a manner that was proposed in earlier models for γ-Al2O3 surfaces.


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1 Introduction The nature of coordinatively unsaturated sites (CUS) on alumina surfaces has long been subject to manifold discussions as they are believed to act as Lewis-acid sites in one of the most widely used (co-)catalyst for fluid catalytic cracking.1–3 The general idea of such sites arises from early models for the hydrated γ-Al2O3 surface proposed by several authors.4–7 Thermal treatment leads to partially or highly dehydrated surfaces that consequently have to possess a certain amount of CUS, naturally in form of threefold (AlIII), fourfold (AlIV) and fivefold (AlV) sites. More recent works, mostly based on DFT calculations of the bulk structure of γ-Al2O3, and metastable surface terminations, emphasized the idea that especially the strongest Lewis-acid sites, i.e. AlIII centers, are not only stable but also responsible for catalytic activity of γ-Al2O3.8–10 From an experimental point of view, the characterization of catalytically active species remains an elusive goal. Though, it is possible to target active sites on alumina surfaces with the help of probe molecules e.g. NH3, Pyridine, CO, etc. and vibrational spectroscopy (mostly infrared).11–13 Such methods give insights into key properties of a heterogeneous catalyst as they allow to determine for example its acid-base characteristics. However, as an indirect method they do not provide substantial information of the catalyst structure. An intuitive method for direct characterization of these active sites is solid state magic angle spinning (MAS) NMR spectroscopy and to date there are some approaches made to observe CUS on alumina surfaces. Often, small amounts of fivefold coordinated Al-sites (up to ~5%) were observed in γ-Al2O3 samples that were heat treated at temperatures above 400 °C.14–17 Since their formation is predicted by model, it is reasonable to assume these sites are solely present on the surface, especially because fivefold coordinated Al is not endemic to the structure of transitional alumina.14–17 However, due to direct excitation in the NMR experiment, it is not clear whether these sites are really exclusively formed on the surface. For example, amorphous alumina is a potential source of AlV-species and is also likely to be present in small fractions in such nonequilibrium phases (e.g. by quenching or fast heating rates).18–23 Approaches to selectively observe surface species in alumina using cross polarization (CP) MAS NMR were already made about thirty years ago.24–26 It seems that back at that time common problems resulted mostly from technical limitations and sample preparation. Often, the catalytically active surface (dehydrated) could not be protected from rehydration before or during measurements. Unsurprisingly, most of 3 ACS Paragon Plus Environment


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the spectra either differed only little from NMR spectra obtained by direct excitation of 27Al or showed no features at all.24–27 The most conclusive approach was published by Coster et al. who employed the adsorption of water-free NH3 to different alumina surfaces (γ-Al2O3 and amorphous samples) followed by 1H -

27

Al CP MAS NMR to detect CUS on their samples.26

Nevertheless, chemisorption of the basic molecule NH3 alters the surface and therefore it is questionable whether those spectra are representative of the active surface. In recent works of Taoufik et al.28 and Lee et al.29, it was demonstrated that pentacoordinated Alsites can be located at the surface of Al2O3. Also, a relatively new technique, dynamic nuclear polarization (DNP), was employed in a couple of works for the research of (hydrated) alumina systems.29–32 Even though DNP provides huge improvements in terms of sensitivity and acquisition times compared to CP experiments, certain problems remain. This especially holds true for the observation of active species since prior to measurement samples have to be wetted or impregnated with aqueous solutions of stable biradicals. The contact to water should lead to the rehydroxylation of these active species (CUS) which would consequently make it impossible to probe them. To overcome some of the above-mentioned issues, in the present study we focused on the direct observation of coordinatively unsaturated sites and their behavior at ambient conditions. Therefore, we used 1H -

27

Al CP MAS NMR and direct excitation

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Al MAS NMR in

combination with a catalytic test reaction which is only promoted by strong Lewis-acid sites. As sample, we chose a fluoride-doped alumina (F-Al2O3) with high catalytic activity. This sample can be prepared by calcination of an amorphous aluminum hydroxide fluoride with low fluorination degree. Due to the easily controllable preparation conditions (temperature, exposure to air), the same sample allowed observing it in three distinctive situations: (i) as amorphous precursor, (ii) as catalyst in its active state, and (iii) as rehydrated and inactive sample. With this set of samples, it should be possible to discriminate between the different sample areas (bulk, surface) that were probed. Finally, it should also allow to explain the impact of contact to air on catalytically active surfaces.

2 Experimental 2.1 Sample Preparation 4 ACS Paragon Plus Environment

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Aluminum-sec-butoxide (97 %) and γ-Al2O3 were purchased from Sigma-Aldrich and were used without further purification. Iso-propanol (99.5 %) was purchased from Roth and was dried by standard procedures before use. The hydrogen fluoride (HF) solution was prepared by a procedure described previously using gaseous HF from Solvay.33 The samples were prepared by a modified process of the fluorolytic sol-gel synthesis.34–36 The key steps of the synthesis and characterization of the aluminum hydroxide fluoride (for details see 36) are described in the following. For one batch of powder, 2.463 g (20 mmol) of aluminumsec-butoxide (ASB) were dissolved in 50 mL of iso-propanol (iPrOH) under vigorous stirring in a 120 mL polypropylene beaker. Then, 2.26 ml of a 2.21 M anhydrous iso-propanolic HF solution were added to the reaction mixture (5 mmol), followed by the controlled addition of 10 equivalents (200 mmol) of deionized water. The resulting gel was equilibrated for 4 h and was then dried at ambient conditions, allowing the solvents to evaporate slowly. After 72 h, the remaining solid was weighted and subsequently crushed to obtain the amorphous aluminum hydroxide fluoride precursor (F-Al(OH)3). For calcination, the samples (F-Al(OH)3 or γ-Al2O3) were filled into a long quartz Schlenk tube. The lower end of that tube was inserted in a special furnace. Subsequently, dynamic vacuum was applied to the vessel and calcination was performed for 12 h at temperatures of 300 °C (F-Al2O3-300) and 700 °C (F-Al2O3-700, γ-Al2O3). After cooling down, the Schlenk tube was removed from the oven and was transferred immediately into a glovebox to prevent the sample from rehydration. For sample F-Al2O3-700-Air, prior to all measurements (NMR, XRD, catalytic test reaction) a small amount of the calcined sample was removed from the glovebox and was kept under air for 15 min.

2.2 Catalytic test reaction The reactions were performed under argon atmosphere using standard Schlenk techniques. For each reaction, a Young NMR tube was charged with 25 mg of catalyst and 0.6 ml of dry, deuterated benzene (C6D6) were added. Subsequently, the tube was filled with about 1.2 bar of ethylene. The formation of the product (styrene) was studied after 5 min, 5 h and 24 h by 1H NMR spectroscopy. For more details see also Calvo et al.37

2.3 Elemental Analysis



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The fluoride contents were determined with a fluoride sensitive electrode after chemical decomposition of the solids with Na2CO3/ K2CO3. The weight percentage was related to the molar mass of the nominal composition AlF0.25O1.375 to give an estimation of the molar F-content in each sample.

2.4 XRD For X-ray powder diffraction measurements, a Seiffert XRD 3003 TT equipment (Freiberg, Germany) with Cu Kα radiation was applied.

2.5 Thermogravimetric Analysis The thermal behavior was studied by conventional thermal analysis (TA) with a NETZSCH thermoanalyzer STA 409 C Skimmer, being additionally equipped with a conventional hightemperature SiC oven. A DTA-TG sample carrier system with platinum crucibles and Pt/PtRh10 thermocouples was used. Samples of 50−65 mg each were measured versus an empty reference crucible. A constant purge gas flow of 80 mL/min and a constant heating rate of 10 °C/min were applied.

2.6 Nitrogen Adsorption The surface areas of the samples were determined using nitrogen absorption by means of a micromeritics ASAP 2020 instrument at 77 K. Before each measurement, the samples were degassed at 5 × 10−5 mbar for 12 h at 120 °C. The isotherms were analyzed employing the Brunauer−Emmett−Teller (BET) model.

2.7 Solid State NMR To avoid contact to air all sensitive samples were packed into ZrO2 rotors in a glovebox and sealed with appropriate Vespel driving caps. 27

Al solid-state NMR spectra were measured on a Bruker Avance III HD spectrometer equipped

with a 4 mm 1H/X broad band probe. Spectra were recorded at B0= 14 T corresponding to a frequency of 600.13 MHz for 1H and 156.38 MHz for 27Al, and at a spinning rate of 12 kHz. 27Al MAS direct polarization experiments were acquired employing a selective pulse of 17 µs and a recycle delay of d1= 2s.

27

Al CP MAS experiments were performed following the procedure 6 ACS Paragon Plus Environment

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described by Massiot and co-workers20 taking a spin-lock field of 15 kHz on

27

Al. The CP

condition was optimized employing a linear ramp on 1H. The contact time was set to 1 ms and spinal 64 was employed for 1H decoupling.38 All spectra were referenced to AlCl3 in H2O (0 ppm)

3 Results and Discussion 3.1 Sample Description and General Aspects In this study three different samples were investigated (F-Al2O3-300, F-Al2O3-700 and F-Al2O3700-Air). All have a nominal F-content of 0.25 eq. with respect to the total Al content (related formula: AlF0.25O1.375). After preparation, the precursor was heated under dynamic vacuum to 300 °C (F-Al2O3-300) and 700 °C (F-Al2O3-700, F-Al2O3-700-Air), respectively. The difference between the latter two samples is related to the treatment after calcination. Whereas F-Al2O3-700 was handled without contact to atmospheric gases, sample F-Al2O3-700-Air was purposely exposed to air for 15 min. In previous works, we already showed that these phases retain most of the initial fluoride amounts at mild calcination conditions.36 At higher calcination temperature, mixed aluminum oxide fluorides can suffer from pyrohydrolysis which leads to a loss of fluorine.35,41 The elemental analysis of samples F-Al2O3-300 and F-Al2O3-700 yielded values of 6.3 and 5.7 wt% fluorine content, respectively. This equals about 22 mol% for fluoride in FAl2O3-300 and 21 mol% in F-Al2O3-700, which is only a slight decrease as compared to the nominal F-content of 25 mol% in the precursor. Thus, the calcination in dynamic vacuum prevents the sample from pyrohydrolysis and large quantities of the initial fluoride are retained. An obvious difference between the samples arises in their (crystalline) structure. Confirmed by XRD, F-Al2O3-300 remains completely amorphous after heating (Figure S1), whereas the samples calcined at 700 °C (F-Al2O3-700 and F-Al2O3-700-Air) show reflections that can be assigned to two transitional aluminum oxide phases (γ-Al2O31,42 and κ-Al2O343) (Error! Reference source not found.). The nature of transitional alumina phases is complex and due to structural similarities and lattice irregularities a precise assignment of the structure is often not possible. In the current study, additionally the presence of small amounts of fluoride has to be considered. Therefore, no distinct assignment can be made. Nevertheless, the XRD confirms the presence of a phase that is closely related to transitional aluminum oxides. Additionally, within the limits of the sensitivity of XRD measurements, it confirms that no pure crystalline aluminum 7 ACS Paragon Plus Environment


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fluoride phase like α-AlF3 is formed. Also the formation of other metastable AlF3 phases is unlikely, as they normally feature strong reflections at around 15° and 25° 2 Theta, which are not observed in our XRD data.44

Figure 1: Powder XRD patterns of the samples F-Al2O3-700 (black line) and F-Al2O3-700-Air (red line). For comparison, the most 45 significant reflections of γ-Al2O3 (dark grey, compare Figure S1), κ’-Al2O3 (light green, pdf 26-31) and α-AlF3 (light blue, pdf 8045 1007) are shown, as well.

With the help of thermogravimetric analysis (TGA), it is possible to give an estimation of the remaining water contents (or OH groups) (Figure 2). In the TGA, a stream of N2 gas was applied to avoid interactions with atmospheric gas (O2, H2O) and to model the calcination conditions in a reasonable way. As shown previously, the dehydration of the F-Al(OH)3 already proceeds at temperatures of 80 °C and is finished to large extends at around 300 °C (~90 % of total mass loss),36 which results in a hydroxyl content in sample F-Al2O3-300 of about 0.5 OH mol-1.


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Figure 2: TGA of the material starting from the F-Al(OH)3-precursor at room temperature. At 300 °C the sample lost 35.8 % and at 700 °C about 40.5 % of its initial weight.

Looking at sample F-Al2O3-700, the aluminum oxide-like structure is confirmed by XRD measurement. Interestingly, the DTA curve does not reveal a sharp signal for a first-order phase transition but a broad exothermic regime between 600 °C and 700 °C. In this sample, only 0.05 OH groups per mole remain which equals an OH coverage of 2.3 OH nm-2, well in agreement with values from literature.10,46,47 Since numerous studies confirmed that OH groups on alumina surfaces are stable up to very high temperatures (> 700 °C), it is reasonable to allocate the remaining hydroxyl groups at the surface.7,12,48,28,49 A fact that underlines this statement is that sample F-Al2O3-700 has significant Brønsted acidity, and selectively catalyzes the reaction of citronellal to isopulegol.50

3.2 Catalytic Test Reaction To demonstrate the catalytic activity of the samples, a catalytic test reaction was employed. Therefore we chose a reaction that is only mediated by the presence of strong Lewis-acid centers as for example present in aluminum chlorofluoride (ACF)51 or HS-AlF3.34 The hydrophenylation



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of ethene on such Lewis acid centers was recently published by Calvo et al. and allows for C-C bond formation at mild conditions. This reaction results in the formation of styrene (Scheme 1).37

Scheme 1: Reaction of benzene with ethene to form styrene with the addition of a heterogeneous catalyst.

The catalytic behavior of the respective samples was tested in the above-mentioned reaction (Table 1). To give a general estimate of the catalytic activity a γ-Al2O3 sample, preheated and handled exactly as F-Al2O3-700, was tested as well. Table 1: Performance of the different catalysts tested in the hydrophenylation of ethene. The value for ACF was taken from 37 Ref. [ ]

catalyst

conversion

time

F-Al2O3-300

0%

24 h

F-Al2O3-700

100 %

5h

F-Al2O3-700-Air

0%

24 h

γ-Al2O3

0%

24 h

ACF

100 %

5 min

Sample F-Al2O3-300 shows no catalytic activity towards the C-C bond formation. Thus, it obviously lacks the presence of strong Lewis-acid sites on its surface. This could be due to the comparably mild calcination conditions with temperatures of only 300 °C. From related γ-Al2O3 systems it is known that thermal activation, in order to create Lewis-acid sites, is usually carried out at temperatures well above 400 °C.


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In contrast, F-Al2O3-700 promotes a 100 % conversion of educts within 5 hours reaction time. Since strong Lewis-acid sites seem to be mandatory for the activation of educts, such sites have to be present in the sample directly taken from the calcination under vacuum. As outlined in the introduction, these strong Lewis-acid sites are regarded as coordinatively unsaturated Al-sites on the surface of the catalyst (AlIII, AlIV, AlV) and are generated by dehydroxylation of surface OHgroups. Finally, the complete loss of catalytic activity is observed once the sample was exposed to air for 15 min (F-Al2O3-700-Air, Table 1). Hence, exposure to air seems to cause immediate vanishing of strong Lewis-acid sites from the samples surface. This is supposedly caused by the chemisorption of water which is acting as Lewis-base. This dissociative rehydration should also go in hand with an overall lowering of the coordination of Al-sites Interestingly, γ-Al2O3 is not capable of catalyzing the same reaction under such mild conditions even though it was handled as sample F-A2O3-700 and should also possess CUS on its surface. However, partly fluorinated [AlFOx](x= 3-5)-sites should have increased Lewis-acidity compared to their non-fluorinated counterparts due to stronger polarization effects of the highly electronegative fluoride. This case illustrates that additional fluoride-doping can lead to superior catalytic activity (at least in this case).

3.3 NMR Spectroscopy 3.3.1 Sample F-Al2O3-300 To explain the findings from the catalytic test reaction, comprehensive solid-state NMR experiments were performed. First of all, sample F-Al2O3-300 was tested in a single pulse

27

Al

VI

experiment. The spectrum shows three signal groups, belonging to sixfold (Al -sites: ~ 15 ppm), fivefold (AlV-sites: ~ 38 ppm) and fourfold coordination (AlIV-sites: ~ 77 ppm) of aluminium, respectively (Figure 3). This finding, i.e. the presence of AlV-species, is quite characteristic for amorphous alumina (aluminum oxides19,22 and aluminum hydroxides52) and closely related systems (aluminum (hydr)oxide fluorides36,53). It is notable that in this spectrum all the three sites roughly show equal intensities.



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27

1

27

Figure 3: Al MAS NMR spectra of sample F-Al2O3-300. 1P stands for the directly excited spectrum and CP for the H - Al CP experiment (see also Error! Reference source not found.).

Signals at the same chemical shift values are also present in the 1H -

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Al CP MAS spectrum.

(Figure 3). As shown before, the dehydroxylation of the bulk of the amorphous precursor (FAl(OH)3) is linked to the lowering of the aluminum coordination.36 This behavior should result in decreasing proton concentration around lower coordinated Al-species and consequently we would expect a more nuanced distribution of intensity. This trend is clearly visible in the CP spectrum showing decreasing signal intensity going from AlVI to AlV to AlIV. Although, it has to be mentioned that CP experiments are not necessarily quantitative so that the decreasing signal intensity could also stem from the variation of signal transfer efficiency for the different Al-sites. Regarding the nuanced distribution of intensities , the amorphous nature of sample F-Al2O3-300 and the nonexistent catalytic activity, most probably hydroxyl groups are likely distributed throughout both, the bulk and the surface. Therefore, it is not possible to spectroscopically draw a distinction between certain OH groups in this sample. However, these observations illustrate that possessing high amounts of AlV and AlIV-sites (as in F-Al2O3-300), does not necessarily link with high catalytic activity. Thus, to consider a site as (active) CUS, one has to regard not only the coordinative state but also the level of (de)hydration of that site and whether it is actually located at or near to the surface or in the bulk.


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3.3.2 Samples F-Al2O3-700 and F-Al2O3-700-Air Sample F-Al2O3-300For observing CUS on the surface, both samples, F-Al2O3-700 and F-Al2O3700-Air were probed in a single pulse and a 1H -

27

Al cross polarization experiment. The single

pulse spectra represent all excitable Al-sites within the sample (Figure 4). Since the vast majority of Al-sites is located in the bulk, in approximation, surface species can be neglected so that these spectra are representative for the bulk structure of the samples. The F-Al2O3-700-1P spectrum shows two signals, one for the AlVI and one for the AlIV coordination, typical for transitional alumina and also in agreement with the results from XRD. The intensity ratio of the signal groups allows to further discriminate between the different transitional Al2O3 phases. With a ratio of about 3 : 1 (AlVI : AlIV), the structure shows the closest similarity to γ-Al2O3 and κ-Al2O3, which is in agreement with results from XRD analysis.27,54,55 Furthermore, it should be noted that no AlV-coordination is present in the F-Al2O3-700 sample. As discussed above, AlV-sites usually are a characteristic feature of amorphous alumina.18,20,56 With this result, we can rule out the presence of an amorphous phase in sample F-Al2O3-700, which was not possible considering solely the XRD results.

Figure 4: 27Al MAS NMR spectra of F-Al2O3-700 (black) and F-Al2O3-700-Air (red).

For comparison, Figure 4 also shows the single pulse MAS NMR spectrum of the sample that was exposed to air (F-Al2O3-700-Air) for 15 minutes. As obtained from the superposition, 13 ACS Paragon Plus Environment


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differences between the two spectra are only marginal. This emphasizes that the structure of the bulk alumina is not altered by the exposure to air. For information on the surface of alumina, the 1H - 27Al CP MAS NMR spectra were analyzed. In Figure 5a, the spectrum of the calcined sample without contact to air (F-Al2O3-700) is depicted. Due to the low coverage of surface OH-groups (2.3 OH nm-2) the signal to noise ratio of the spectrum is poor. Even though, three resolved signal groups are visible with maxima at around 6 ppm, 32 ppm and 63 ppm, assigning to AlVI, AlV and AlIV respectively. While the four- and sixfold coordinated Al-sites are characteristic for the structure of transitional alumina, AlV-sites are certainly not. Since AlV-sites are not detected in the single pulse experiments (Figure 4) they seem to be present only at the surface and not in the bulk of the material.

1

27

1

27

Figure 5: a) H - Al CP MAS NMR spectrum of sample F-Al2O3-700. b) H - Al CP MAS NMR spectrum of sample F-Al2O3-700-Air (red) and F-Al2O3-700 (black). Note: For comparison, spectra were normalized to the identical number of scans used for both experiments.

From the results of the catalytic test reaction, it is evident that the surface of F-Al2O3-700 holds catalytically active species. As discussed above, these active species are linked to coordinatively unsaturated Al-species on highly dehydrated surfaces. Therefore, any Al-species at the highlydehydrated surface not having the coordination number of six, should be considered as coordinatively unsaturated site. As shown in Figure 5a, this holds true for AlV- and AlIV-sites. Regarding the fivefold coordinated Al-sites, their presence on the surface was predicted in earlier works.15,17 As discussed in the introduction, in these former works often only single pulse 14 ACS Paragon Plus Environment

27

Al

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MAS NMR experiments were employed.14–17,57 Since AlV-sites are not endemic to the structure of transitional alumina, it is reasonable to assume that they are formed exclusively on the surface. However, single pulse 27Al MAS NMR does not allow to discriminate between surface und bulk species. This lead to the problem that especially fourfold coordinated Al-sites were often not discussed as surface species, because of their omnipresence in the bulk. Although, the schematic approach of Knözinger and Ratnasamy only predicted the presence of AlIII- and AlV-sites on dehydrated γ-Al2O3 surfaces,6 in the present work it is shown that AlIV-species have to be considered as active sites on the alumina catalyst. In a previous work, it was demonstrated that the dehydroxylation of bridging OH-groups directly leads to the formation of (highly dehydrated) 4- and 5- fold coordinated Al-sites which would sufficiently explain their formation on the surface.36 Additionally, some theoretical studies showed that highly dehydrated AlIV-sites are stable on the γ-Al2O3 surface.9,13,49 As in all other works, there is no observable signal for expected threefold coordinated Al-species even though they are predicted to be the most reactive species at alumina surfaces.10 A possible explanation for this was recently given by Wischert et al. who calculated quadrupolar coupling constants (CQ) for such sites.20 With values well above 20 MHz, AlIII-sites suffer from significant line broadening even in high magnetic fields (>600 MHz) so that it is so far impossible to resolve their signal from the baseline. Of importance is also the fact, that no segregated [AlF6] sites are visible in any of the recorded 27

Al MAS NMR spectra, normally visible at -16 to -18 ppm.15,44,58,59 This is also visible in

19

F

MAS NMR spectra of F-Al2O3-700 and F-Al2O3-300, which show only two vastly broadened signal groups with maximum intensity at around -140 ppm and -190 ppm containing several subsignals (Figure S2). In contrast, pure aluminum fluorides phases exhibit a single signal at -170 to -175 ppm, which is not detected.44 Therefore, considering the high structural disorder in all samples, as also indicated by XRD, this is a strong indication that fluoride is distributed throughout the sample.35,53 With regard to the low F-content, [AlF1O5], [AlF1O4] or [AlF1O3] are reasonable species and are also likely to be present at the surface.58,60,61 Nevertheless, with the resolution of the

19

F spectra it is presently not feasible to clearly assign certain species. The

additional fluoride may influence the total number of CUS generated at the surface as compared to undoped γ-Al2O3, as the different charge and the slightly smaller ionic radius introduce structural constraints that most probably lead to a reorganization of the surface.15,62–64 15 ACS Paragon Plus Environment


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For comparison, Figure 5b shows samples F-Al2O3-700-Air-CP and F-Al2O3-700-CP with their normalized intensities. The contact of 15 min to air leads to a significant boost in the signal intensity of the peaks at 6 ppm (AlVI) and 63 ppm (AlIV), while the signal at 32 ppm (AlV) only shows a small increase. These changes are not pronounced in the single pulse NMR experiments (compare Figure 4), which emphasizes that exclusively the surface is probed by the CP experiments. It is visible from the 1H MAS NMR spectra of the two samples (Figure S3) that the increased signal intensities in the CP spectrum are related to the amount of protons which has increased after exposure to air. Furthermore, exposure to air is accompanied by a loss of catalytic activity of the F-Al2O3-700 sample. This observation can be explained by chemical changes at the surface. Since strong Lewis-acid sites or CUS are prone to react with a Lewis-base like H2O, the contact to air should lead to a rehydration of the surface. Thus, the observed changes in the CP experiments may be attributed to the chemisorption of water molecules. The uneven increase of signal intensities of the species indicates that a major reorganization of the surface takes places. This would certainly involve dissociative adsorption of H2O forming new surface hydroxyl groups. The driving force for this reorganization seems to be the stabilization of the highly dehydrated, metastable surface of sample F-Al2O3-700.10,65 The relative intensities of the

27

Al CP MAS signals of F-Al2O3-700-Air (Figure 5b), which

represent the rehydrated surface, show a distribution that is similar to the intensities of the

27

Al

MAS signals obtained for this sample (Figure 4), which represents the bulk material. The spectrum in Figure 5b is dominated by AlVI- and AlIV-sites. Accordingly, these sites seem to be more stable on the hydrated the surface. In a (simple) schematic approach, the surface of γAlumina was described by Knözinger and Ratnasamy, who proposed the presence of five different AlX-OH(-AlX) species (x= VI or IV, bridging or terminal) at ambient conditions.6 Generally, our findings are in agreement with their model, as primarily AlVI- and AlIV-sites are obtained in the 27Al CP MAS spectrum. A noticeable difference, however, is the presence of also small amounts of AlV-sites (Figure 5b). Due to the lack of catalytic activity these (hydroxylated) AlV-sites should not be related to CUS even though they are present at the surface. This clearly demonstrates that the often used schematization, that AlV-sites in a transition alumina equals coordinatively unsaturated sites and therefore catalytic activity, has to be considered with care.


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4 Conclusions In the present study, the direct observation of catalytically active species in an F-doped alumina catalyst by 1H - 27Al CP MAS NMR experiments is demonstrated. This direct method allowed us to reveal AlIV- and AlV-sites as coordinatively unsaturated sites (CUS) at the catalyst’s surface without any alterations to it; as for example caused by the use of probe molecules. The presence of these CUS is directly linked to the activity in the catalytic test reaction. It is also shown that in this reaction, the F-doped alumina shows superior catalytic activity as compared to γ-alumina. This enhancement seems to be an effect of mixed [AlFOx](x=

3-5)

sites that have higher Lewis-

acidity as compared to pure AlO4-6 species and are therefore able to promote the hydrophenylation of ethene at mild conditions. Furthermore, it was feasible to monitor the reorganization of the highly dehydrated, metastable surface that is caused by the exposure to air. The chemisorption of H2O decreased the number of active Lewis-acid sites and the rehydroxylated surface is more constituted in a manner that is similar to the bulk structure as already predicted by prior models. In contrast to that, it is shown that also AlV-sites can be stable on hydrated alumina surfaces. Hence, their presence does not automatically result in high catalytic activity which is much more related to the degree of dehydration of the surface. These results not only emphasize the sensitive nature of active Lewis-acidic catalysts but also underline that it is important to discriminate between hydrated and dehydrated surfaces in alumina research.

5 Supporting Information Description The supporting information shows additional XRD patterns of F-Al2O3-300 and γ-Al2O3 (Figure S1), 19F MAS NMR spectra of samples F-Al2O3-300 and F-Al2O3-700 (Figure S2) and 1H MAS NMR spectra of samples F-Al2O3-700 and F-Al2O3-700-Air (Figure S3).

6 Acknowledgements We thank the Deutsche Forschungsgemeinschaft (DFG) for financial support of this work through the Collaborative Research Center 1109, Understanding of Metal Oxide/Water Systems at the Molecular Scale: Structural Evolution, Interfaces and Dissolution. This work has also been supported by the Deutsche Forschungsgemeinschaft under contract Bu-911-20/1. Dr. M. Feist (HU Berlin) is kindly acknowledged for performing DTA-TG measurements.



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Table of Contents Graphic


Direct Observation of Coordinatively Unsaturated Sites on the Surface

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