Acidity evolution on highly dispersed chromium supported on mesostructured silica: The effect of hydrothermal treatment and calcination temperature

Microporous and Mesoporous Materials xxx (xxxx) xxx

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso

Acidity evolution on highly dispersed chromium supported on mesostructured silica: The effect of hydrothermal treatment and calcination temperature �nica L. Parentis b, Norberto A. Bonini a Pablo M. Cuesta Zapata a, *, Elio E. Gonzo b, Mo a b

INIQUI – CIUNSa, Fac. de Ciencias Exactas, Avda. Bolivia, 5150–4400, Salta, Argentina INIQUI – CIUNSa, Fac. de Ingeniería, Avda. Bolivia, 5150–4400, Salta, Argentina

A R T I C L E I N F O

A B S T R A C T

Keywords: Cr-MCM-41 Lewis acid sites Brønsted acid sites Pyridine Lutidine

Mesoporous Cr-MCM-41 catalysts were synthesized, under different hydrothermal temperatures, by the sol-gel procedure using tetraethylorthosilicate (TEOS) as silica precursor and cetyltrimethylammonium bromide (CTAB) as a structure-directing agent. Materials were characterized by several techniques (XRD, XPS, TG-DTA, FTIR, UV–Vis, AA and N2 sorptometry). The amount and accessibility of different hydroxyl groups were analyzed by FTIR spectroscopy after heat treatment in vacuum (10 5 mmHg) and subsequent exchange of hydrogen by deuterium using D2O. TG-DTA, FTIR of adsorbed pyridine and 2,6-dimethylpyridine and TPD of pyridine, let to determine the strength, thermal evolution, and amount of the different acid sites present at the surface. Results correlate with catalytic activity against 2-propanol in the gas phase, and let to propose mechanisms for dehy­ dration and dehydrogenation reactions.

1. Introduction The incorporation of transition metals ions into silica was deeply studied for their use as heterogeneous catalysts in selective reactions. Among them, chromium results particularly attractive owing to its redox properties and acid behavior; both applied to diverse catalytic systems such as ethylene polymerization [1–6], dehydrogenation and oxidative dehydrogenation of hydrocarbons [7–11], oxidation of organic sub­ strates [12–14], among others [15–17]. Due to a large number of acid sites catalyzed reactions, it is recog­ nized the importance to characterize solid acids not only regarding their nature (Lewis or Brønsted) but also by their strength, amount and sur­ face distribution. These acidic characteristics can be determined both by “direct” as well as by “indirect” methods. Within “direct” techniques, IR spectroscopy was often used to quantify simultaneously both nature and amount of Brønsted and Lewis acid sites on solid surfaces, by monitoring the absorption bands of adsorbed basic “probe” molecules, such as pyridine (Py) (pKBHþ ¼ 5.2) and ammonia (pKBHþ ¼ 9.24) among others. Besides, many studies have shown that 2,6-dimethylpyridine (2,6-lutidine) (Lu), due to their increased basic character (pKBHþ ¼ 6.7) and the steric hindrance of their

methyl groups, has a higher capability to bond more strongly to the protruding Brønsted acid centers than to those, more occluded, Lewis ones. Therefore, Lu is considered an appropriate probe molecule not only establishing the nature of the sites but also determining their accessibility [18–20]. Unfortunately, quantitative measurements are hampered due to the unavailability of molar absorption coefficients values of hydrogen bonded Lu (HLu), Lu bonded to coordinatively un­ saturated centers (LLu), and lutidinium ion (BLu) molecule. However, thermal desorption techniques can be complementarily used not only to determine the thermal effects associated with acid-base adsorption but also to quantify the total amount of adsorbed molecules, giving a mea­ sure of the strength and distribution of acid sites. On the other hand, determining the acid-base properties of a catalyst from its behavior against 2-propanol decomposition can be considered as an “indirect” method. According to their prevalence to produce acetone or propylene/i-propyl ether at a given temperature, the cata­ lysts can classify as basic or acidic ones, respectively. The last deduced from their tendency toward dehydration or dehydrogenation reactions, respectively, promoted by the presence of an acid or basic/redox centers on their surfaces [21–23]. Although the information provided by this method is complex, it is particularly valuable because it reflects the “real

* Corresponding author. E-mail addresses: [email protected] (P.M. Cuesta Zapata), [email protected] (E.E. Gonzo), [email protected] (M.L. Parentis), [email protected] (N.A. Bonini). https://doi.org/10.1016/j.micromeso.2019.109895 Received 1 August 2019; Received in revised form 7 November 2019; Accepted 15 November 2019 Available online 16 November 2019 1387-1811/© 2019 Published by Elsevier Inc.

Please cite this article as: Pablo M. Cuesta Zapata, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109895

P.M. Cuesta Zapata et al.

Microporous and Mesoporous Materials xxx (xxxx) xxx

state” of the catalyst under reaction conditions. In previous papers [13,24–26], we observed that the incorporation of monodisperse Crnþ in a high surface mesoporous molecular sieve network constitutes a good strategy to develop new catalytic properties. Thus, on amorphous Cr–SiO2 materials, the development of different acid sites (Brønsted and Lewis) was attributed to the Cr2þ ↔ Cr3þ ↔ Cr6þ redox behavior. Their evolution was a function both of the temperature of hydrothermal treatment, as well as the calcination temperature. In the present contribution, it was studied the acid sites evolution, both in nature and amount, over highly dispersed chromium supported on mesoporous silica materials prepared under different hydrothermal conditions. It was performed applying several techniques: TG-DTA, FTIR spectroscopy of adsorbed probe molecules, deuterium-hydrogen ex­ change and pyridine TPD studies. These studies were complemented by an analysis of the catalytic activity against the 2-propanol gas-phase reaction. Finally, mechanisms for both 2-propanol reactions, dehydro­ genation, and dehydration, were proposed.

vacuum and of adsorbed D2O, pyridine (Py), and 2,6-lutidine (Lu) were carried out over self-supported wafer (20 mg approx.) in a cell with KRS5 windows that allows vacuum and heating treatments “in-situ”. Pre­ vious to the adsorption, each sample was evacuated (10 5 mmHg) at the desired temperatures and the spectrum recorded (“baseline”). Gaseous adsorbent (D2O, Py or 2,6-Lu) was introduced, left to equilibrate at room temperature (RT), evacuated at wanted temperatures and then the spectra recorded. Difference spectra were obtained subtracting the corresponding “baseline” spectrum. For quantitative determinations, all the spectra were normalized using, as an internal standard, the over­ tones and combination bands in the 2100 to 1800 cm 1 spectral region. Py temperature programmed desorption (Py-TPD) was performed into a modified equipment in which out-coming gases pass through a methanation reactor previous to an FID detector [27]. Before to Py adsorption, catalysts were pre-treated in situ in an N2 flow at 300 � C for 1 h and then cooling down to room temperature. Subsequently, the sample was saturated by a Py/N2 stream. Pure nitrogen was introduced and temperature increased up to 150 � C until no Py was detected in the out-coming gas from the cell. Typically, the TPD experiments were carried out by heating at 12 � C min 1 from 150 to 750 � C. Catalytic activity against 2-propanol (2-PrOH) was determined as it was previously described (24). Summary, it was evaluated in the gas phase, at atmospheric pressure, using a Pyrex tubular gas flow reactor. Catalyst amount varied from 50 to 100 mg (80–100 mesh) and N2 was used as the carrier gas (180 ml min-1). 2-PrOH was vaporized to obtain a partial pressure of 0.05 atm. The effluent composition was determined by gas chromatography.

2. Experimental 2.1. Catalysts preparation 2.1.1. Reactants Tetraethylorthosilicate (TEOS) (Merck >98%) as silica source; cetyltrimethylammonium bromide (CTAB) (Fluka) as surfactant; Cr (NO3)3⋅9H2O (Anedra >98%) as chromium source; tetramethylammo­ nium hydroxide (TMAOH) (Merck – 20% aqueous solution), ethanol (EtOH) (Merck – 99.9%), and distilled water were used for the samples synthesis. D2O (Anedra), Pyridine (Merck) and 2,6-Lutidine (Merck) were employed as probe molecules.

3. Results and discussions 3.1. Catalysts characterization

2.1.2. Preparation of Cr-MCM-41 materials Materials were prepared as it was described previously [24,26]. So, for Cr-MCM-41 catalyst, a gel with a precursors molar ratio of 1.0 TEOS:0.24 CTAB:105 H2O:25 EtOH: 0.25 TMAOH:0.06 Cr was obtained. Two portions of the gel, placed in a Teflon vessel, were hydrothermally treated at 150 and 220 � C, respectively; and a portion without hydro­ thermal treatment was preserved. The obtained solid samples were filtered, thoroughly washed, dried at 110 � C, heated in a nitrogen flow at 550 � C for 19 h and finally calcined in air flow at 300 � C for 3 h to remove the surfactant. Materials were named as 5MEyyy xxx where xxx and yyy are the hydrotreatment and the in-air calcination temperatures, respectively. The non-hydrotreated sample was named as 5MEyyy nHT. Following an identical procedure, a pure silica material with the same molar ratios but without chromium was synthesized (0ME500 150).

Some characterization results are summarized in Table 1. There, textural properties derived from the N2 adsorption-desorption isotherms (Fig. S1), such as SBET, mean pore diameter (Dp) and pore volume (VSTP) of the different synthesized materials are shown. As it was previously reported [24,26], an increase in hydrotreatment temperature promotes a progressive decrease of SBET. So, it changes from 1344 m2g-1 for the sample without hydrotreatment to 776 m2 g 1 2 -1 � for 5ME300 150 and decreases to 237 m g for the sample treated at 220 C (5ME300 ). The material without hydrotreatment shows a unimodal pore 220 size distribution, centered at Dp ¼ 2.3 nm. A similar distribution, but centered at Dp ¼ 2.9 nm, is conserved in the 5ME300 150. Meanwhile, the material prepared by hydrothermal treatment at 220 � C, presents a wide pore size distribution (with a maximum at 3.2 nm) without an ordered mesoporous structure as it was determined by XRD. Increasing the temperature of hydrothermal treatment produces an increase both in the wall thickness t (nm) and the cell parameter a0 (Table 1). Table 2 shows, from XPS results over 5 ME samples (Fig. S2), the change in the relative amount of Cr3þ and Cr6þ species as temperatures of the hydrothermal treatment and of the in-air calcination were modified. From the XPS results, coincident with those previously reported from FTIR and DRS-UV experiences [24,26], it can be concluded that the Cr/SiO2 system consists of supported Cr3þ and Cr6þ species. Their relative amounts depend on the hydrothermal treatment and calcination temperature. So, the hydrothermal treatment allows the incorporation

2.2. Catalyst characterization Characterization techniques were formerly described [24,26]. Briefly, specific surface areas (SBET), pore size distributions and pore volumes were determined by sorptometry (Micromeritics ASAP-2020) using N2 at 77 K. X-Ray Diffraction (DRX) spectra were carried out in a RIGAKU-DENKI D-Max IIC powder diffractometer with a Cu-Kα emission (40 V) lamp. A VG-Microtech ESCA spectrometer, with a non-monochromatic Al Kα radiation (300 W, 15 kV, hν ¼ 1486.6 eV) combined with a VG-100-AX hemispherical analyzer operating at 25 eV pass energy, was employed for X-Ray photoelectron spectroscopy (XPS) analyses. The Si2p peak at 103.4 eV was taken as the reference for binding energy (BE) calibration. Chromium loading was determined by Atomic Absorption Spectroscopy (AAS) and UV–visible Diffuse Reflec­ tance (DRS-UV-vis) spectra were obtained in GBC-918 equipment with a diffuse reflectance sphere employing BaSO4 as a reference. Thermog­ ravimetric (TG) and differential thermal analysis (DTA) were carried out in air, in a RIGAKU unit, at a heating rate of 10 � C min 1. On the other hand, FTIR spectra were recorded on a Spectrum GXFTIR PerkinElmer spectrophotometer. FTIR measurements, both in

Table 1 Textural properties of 5ME materials [24,26].

2

Sample

d100 (nm)

SBET (m2 g 1)

VSTP (cm3 g 1)

Dp (nm)

a0 (nm)

t (nm)

5ME300 nHT 5ME300 150 5ME300 220

3.8 4.3 –

1344 776 237

0.779 0.796 0.579

2.3 2.9 3.2

4.3 5.0 –

2.0 2.1 –

P.M. Cuesta Zapata et al.

Microporous and Mesoporous Materials xxx (xxxx) xxx

Table 2 XPS parameters determined for 5MEnHT, 5ME150 and 5ME220 samples calcined in air at different temperatures. Sample 5ME300 nHT 5ME350 nHT 5ME400 nHT 5ME450 nHT 5ME300 150 5ME350 150 5ME400 150 5ME450 150 5ME300 220 5ME350 220 5ME400 220 5ME450 220 a b

Cr 2p (3/2) BE (eV)

FWHM (eV)

576.5 578.0 576.9 578.5 575.6 578.1 575.7 578.6 577.0 578.6 577.3 579.3 577.1 579.1 577.0 578.9 577.2 579.0 577.3 579.3 577.1 579.1 577.1 579.1

2.2 2.7 2.2 3.8 2.4 3.8 2.5 3.9 2.4 3.4 2.6 3.7 2.5 3.9 2.2 3.7 2.7 3.8 2.5 3.6 2.4 3.5 2.3 3.6

Oxidation State Assignment

Atomic Percentage (%)

Cr/Si

AF (%) (a)

Total Cr Content (%) (b)

3þ 6þ 3þ 6þ 3þ 6þ 3þ 6þ 3þ 6þ 3þ 6þ 3þ 6þ 3þ 6þ 3þ 6þ 3þ 6þ 3þ 6þ 3þ 6þ

48 52 35 65 36 64 28 72 57 43 36 64 25 75 22 78 75 25 45 55 33 67 26 74

0.048

1.33

3.5

0.046

1.24

3.5

0.062

1.70

3.5

0.063

1.68

3.5

0.071

1.84

4.0

0.071

1.87

4.0

0.065

1.64

4.0

0.051

1.30

4.0

0.115

3.32

4.0

0.124

3.28

4.0

0.117

3.02

4.0

0.111

3.07

4.0

(% of the atomic fraction) Determined by XPS from the areas of O 1s, Si 2p, and Cr 2p signals [28]. (w/w) Determined by Atomic Absorption Spectroscopy.

of Cr3þ ions into the silica structure, generating SiO–Cr linkages into chromasiloxanes cycles, with distorted octahedral coordination. That increases the Cr3þ unsaturation degree, as being incorporated into more strained chromasiloxane rings, increasing the binding energy (BE) observed by XPS (Table 2). These chromium atoms incorporated into the support develop a greater resistance both to Cr6þ oxidation and segre­ gation. On the other hand, only in those materials without hydrothermal treatment and calcined in air at 450 � C, segregation of crystalline α-Cr2O3 was observed (Fig. S3). This Cr (III) oxide forms from poly­ chromates segregated after oxidation of Cr3þ species weakly linked to the support. Contrarily, in the hydrothermally treated sample at 220 � C, the oxidation of Cr3þ to Cr6þ, only observed by DRSUV-Vis but not by DRX, produced mainly monochromate species. The 5ME150 sample showed an intermediate behavior. So, in hydrothermally treated mate­ rials, segregation of crystalline α-Cr2O3 was not observed after calcina­ tion in air at temperatures up to 450 � C. Although the amount of metal added during synthesis was exactly the necessary to obtain 5% w/w of chromium in the catalyst, the final

chromium contents, determined from AA spectrophotometry, were be­ tween 3.5 and 4.0% w/w; with a tendency to increase the metal amount as the temperature of hydrothermal treatment increases. On the other hand, XPS determinations show (Table 2) that the percentage of chro­ mium at the surface is, in all cases, inferior to that corresponding to the total chromium content and that the amount of the exposed metal in­ creases with the temperature of hydrothermal treatment. These obser­ vations suggest that, as a result of the synthesis method employed, a fraction of metal occludes into the bulk, but their exposure turns greater as the samples are hydrothermally treated at higher temperatures. 3.1.1. TG-DTA studies Fig. 1 shows the TG-DTA normalized thermograms corresponding to 110 110 the 5ME110 nHT, 5ME150 and 5ME220 samples and Table 3 summarizes the peaks observed and assigns those associated with the surfactant decomposition. In this manner, weight loss below 150 � C reflects the desorption of physisorbed water molecules both from the outer surface of the particles as well as those retained into the macro and mesopores

110 110 Fig. 1. TG-DTA analysis of 5 ME samples, dried in air at 110 � C after hydrothermal treatment: a) 5ME110 nHT, b) 5ME150 and c) 5ME220.

3

P.M. Cuesta Zapata et al.

Microporous and Mesoporous Materials xxx (xxxx) xxx

Table 3 110 110 Summary of the TG-DTA results for 5ME110 nHT, 5ME150 and 5ME220. Sample

Range

T1 T2 T3 T4

<150 � C 150–300 � C 300–340 � C 340–450 � C 450–1000 � C 20–1000 � C

Total

5ME110 nHT

5ME110 150

5ME110 220

TM�ax

Weight Loss%

TM�ax

Weight Loss%

TM�ax

Weight Loss%

– – 308 377 – –

6.5 22.5 11.0 11.5 2.0 53.5

– 251 310 405 – –

5.0 11.5 9.0 9.5 2.0 37.0

– 254 313 406 – –

3.0 11.0 6.0 6.0 3.0 29.0

formed by inter-particles aggregation. This weight loss is associated with the small endothermic peak T1, with a maximum at 90 � C, in the DTA diagram (see Fig. 1-b for peaks assignation). For this peak, the weight loss was greater for materials with highest SBET (Table 1). So, it was 6.5% 110 110 (w/w) for 5ME110 nHT, 5% for 5ME150 and diminished to 3% for 5ME220. Similar results were reported in the literature for MCM-41 type materials with different Si/Al ratios [29,30]. Between 150 and 600 � C occurs the greatest loss of weight due to the removal of the surfactant. The amount removed between 150 and 600 � C decreases as the temperatures of hydrothermal treatment increases. So, 110 for 5ME110 nHT the loss was 45% of weight, passed to 30% for 5ME150 and material. This behavior relates to the diminished to 23% for the 5ME110 220 progressive loss of the ordered mesoporous structure and the consequent decrease of the SBET (Table 1). According to these results and coincidently with that proposed in the literature [29–32], surfactant removal occurs in three stages: i) between 150 and 300 � C (T2 peak), ii) between 300 and 340 � C (T3 peak) and iii) between 340 and 600 � C (T4 peak). All of them associated with sharp exothermic peaks into DTA. Peaks are attributed to the presence of surfactant molecules interacting with surface sites, with different acid strength, arising from Si–O–Si, Si–O-M and metal species (M) [29]. Beck et al. [32] suggested that surfactant cations bound to siloxane Si–O–Si groups are removed at a lower temperature than those associated with Si–O-M groups, which decompose at higher temperatures. The DTA diagrams allow distinguishing changes in the decomposi­ tion processes as a function of the hydrotreatment temperature. Thus, 110 � � � for 5ME110 150 (T2 ¼ 251 C and T3 ¼ 310 C) and 5ME220 (T2 ¼ 254 C and T3 ¼ 313 � C), two well-defined peaks were observed; while for 5ME110 nHT both peaks appear overlapped, with a maximum (T3) at 308 � C and a low-temperature shoulder (T2) at 250 � C approximately (dotted line in Fig. 1-a). Both signals correspond to the removal of the surfactant weakly interacting with Si–O–Si groups, both outside (T2) and inside the pores (T3). � On the other hand, the T4 peak shifts from 377 � C (5ME110 nHT) to 405 C 110 � (5ME110 ) and 406 C (5ME ) as the hydrotreatment temperature in­ 150 220 creases. This peak corresponds to the removal of surfactant interacting with Si–O-M groups. The shift evidences the acidity increase as a consequence of the incorporation of the Cr3þ to the support through chromasiloxane rings formation [33]. Additionally, a gradual loss of weight (2–3%), between 450 � C and 1000 � C, was observed for all materials. It corresponds to water release as neighboring silanol groups condense, to give Si–O–Si bridges.

Fig. 2. FTIR of 5 ME samples hydrothermally treated at different temperatures, evacuated at 450 � C (10 5 mmHg).

xerogel) evacuated (10 5 mmHg) at 450 � C, only remains a very lowintensity broad signal attributed to hydroxyl groups disturbed by the environment, inside of the unexposed cavities of the silica [34]. Fig. 2 shows that the intensity of the band centered at 3660-3680 cm 1 rela­ tive to that of the isolated silanols groups (3740-3745 cm 1) increases with the hydrotreatment severity. This behavior is associated with greater metal-support interaction and the generation of hydroxyl groups associated with hydro-silicate type structures [35,36]. From the ratio (R) between the νO-H intensity of isolated hydroxyl groups and SBET (Table 1) of the 5ME samples, it is possible to evaluate the Si–OH surface density. Table 4 shows the evolution of R with hy­ drothermal treatment for 5 ME structured materials. There, it can be observed a progressive increase in the surface density of isolated silanols groups as the temperature of hydrotreatment rises, despite the fall of the SBET (Table 1). 3.1.2.2. D2O exchange. With the aim to evaluate the exposition of the developed OH groups, samples were vacuum treated (450 � C - 10 5 mmHg), exposed at room temperature to D2O vapor pressure and then FTIR spectra, after evacuation at different temperatures between RT to

3.1.2. FTIR studies 3.1.2.1. Vacuum treatments. Fig. 2 shows the FTIR spectra, in the 40002000 cm 1 spectroscopic region, of 5 ME samples hydrothermally treated at different temperatures after evacuation (2 � 10 5 mm Hg) at 450 � C. There, bands corresponding to OH stretching bond (νO-H) of two species resistant to evacuation developed: a sharp band between 3740 and 3745 cm 1 assigned to isolated silanol groups, and a broad one with a maximum between 3680 and 3660 cm 1 assigned to hydroxyl groups associated with chromium ions grafted into the sílica surface. The last assignation is done considering that over pure silica (c.a: aerogel or

Table 4 Ratio between the intensity of the isolated silanol groups and the specific surface area of the samples, depending on the hydrothermal treatment. Sample 5ME300 nHT 5ME300 150 5ME300 220

4

Signal Intensity 3745 cm 1.11 1.10 0.50

1

SBET (m2 g 1)

R ¼ (Iis-OH/SBET) A.U.

1344 776 237

0.82 1.42 2.11

P.M. Cuesta Zapata et al.

Microporous and Mesoporous Materials xxx (xxxx) xxx

450 � C, were determined. Fig. 3 shows, after D2O exposure, the almost total disappearance of the 3742-3745 cm 1 signal; demonstrating that isolated OH groups are exposed at the surface and exchange hydrogen by deuterium. This exchange gives rise to a sharp peak (2760 - 2757 cm 1) characteristic of the SiO-D stretching (νO-D) of isolated deuteroxyl groups. The appearance of a second broad band in the 2640-2617 cm 1 region, confirms the formation of O-D bonds interacting with other neighboring O-D groups. The exchange leaves a weak band around 3620 cm 1, which lets to visualize the amount of occluded and nonexchangeable hydroxyl groups previously discussed. Their relative in­ tensity increases with the temperature of hydrotreatment that promotes the interaction of partially occluded chromium into the support [35,36]. While the “unexposed” OH groups band do not suffer changes by the vacuum treatment, important changes occur in the O-D broad band around 2640 cm 1. On them, the increase of the evacuation temperature causes an intensity drop. This behavior resembles that of the OH groups (Fig. 2). As the intensity of this band decreases, a weak band into the complex structure of the νO-D band appears. Fig. 4 shows a detail of the spectral region corresponding to νO-D. The 300 300 spectra observed in 5ME300 nHT, 5ME150 and 5ME220 materials evacuated for 30 min at 150 � C, after deuterium exchange, show in addition to the bands previously indicated, a new band at 2742-2737 cm 1. This band could be assigned to νO-D of isolated deuteroxil groups interacting with chromium grafted to the silica surface. As a consequence of their interaction with chromium, its acidity increases, causing the signal shifts to 2740 cm 1 approximately. The existence of hydroxyl groups associated both to Cr3þ or Cr6þ species has been proposed by different authors [36–38] without spectroscopic evi­ dence; because it merges within the complex band generated by the OH groups in the region between 3745 and 3200 cm 1. The existence of O-D bonds associated both to Cr3þ as well as to Cr6þ can be visualized by the formation of equilibrium species depicted by Schemes 1 and 2 respectively.

Fig. 4. Samples subjected to D2O exchange, subsequently evacuated at 150 � C, and an amplification of the region between 2770 and 2570 cm 1.

Scheme 1. SiO-D group in strong interaction with Cr3þ ions.

3.1.2.3. Pyridine adsorption (FTIR). Fig. 5 shows the spectra of adsor­ � bed Py on pure mesoporous silica material 0ME500 150 evacuated at 350 C before pyridine adsorption at room temperature. After the evacuation at 50 � C, the amount of adsorbed Py was very low (see the absorbance scale in Fig. 5) in spite of its high specific surface area (SBET ¼ 620 m2 g 1). Two principal bands, at 1596 and 1446 cm 1 assignable to H-bonded

Scheme 2. SiO-D group in strong interaction with Cr6þ ions.

300 300 � Fig. 3. FTIR spectra of 5ME300 nHT, 5ME150 and 5ME220 samples evacuated at 450 C and exposed to D2O vapor. After deuterium exchange, samples were evacuated (10 5 mmHg) by 1 h at a) 100 � C, b) 150 � C, c) 250 � C, d) 350 � C and e) 450 � C.

5

P.M. Cuesta Zapata et al.

Microporous and Mesoporous Materials xxx (xxxx) xxx

These HPy species are almost eliminated at 100 � C and disappear after evacuation at 150 � C, with similar behavior to that observed on the support (Fig. 5). The BPy species, with bands at 1640-1634 cm 1 (ν8a-8b), 1545 cm 1 (ν19b) and 1485 cm 1 (ν19a), decrease their intensity with the evacuation temperature. On the other hand, LPy species, with bands at 1611 cm 1 (ν8a), 1485 cm 1 (ν19a) and 1450 cm 1 (ν19b), remain present in all the solids analyzed. They have the strongest interaction retaining their intensity after evacuation at 150 � C. From these results, we conclude that chro­ mium incorporation to the silica generates both Brønsted and Lewis acid centers, been the last the more acidic ones. Fig. 7 shows the spectra of adsorbed Py on 5 ME materials calcined between 300 and 450 � C. Prior to Py adsorption at RT, samples were evacuated at 350 � C. To ensures the elimination of HPy and PPy species, whose bands interfere in the determination of Py adsorbed on Lewis and Brønsted acid centers, the evacuation was carried out at 100 � C for 1 h [31,40–42]. It can be seen (Fig. 7) that, besides Lewis sites, Brønsted centers develop as the calcination temperature increases from 300 to 450 � C; but decreases as the hydrotreatment temperature increases. Above 350 � C, a decrease of Lewis centers occurs due to the oxidation of a fraction of Cr3þ to Cr6þ, as it was determined by XPS (Table 2). These Cr6þ species interact with Py developing the observed pyridinium ions (Fig. 7). In the case of the 5ME220 material, due to the increase of the hydrothermal treatment temperature, the Cr3þ ions strongly fixed to the support present a greater resistance to oxidation; consequently, only a slight decrease of the Lewis centers produces after calcination. The last, consequence of an increasingly higher metal-support interaction as it was observed by XPS. This BPy species, in a similar way that was observed over uncalcined samples, almost disappear after evacuation at 150 � C. From the above results, it can be concluded that the amount and nature of acid centers depend on the calcination and the hydrothermal treatment temperature. To quantify and compare the evolution of LPy species, the area of the ν19b band at 1450 cm 1 was measured and the Lewis acid superficial sites density (area of the LPy band at 1450 cm 1 per m2 g 1 (SBET)) determined. Fig. 8 shows the evolution of LPy density for the 5 ME samples calcined in the air from 300 to 450 � C. From this Figure, it can be observed an increase of the Lewis acid sites density as the hydrothermal treatment temperature increases passing through a maximum, between 300 and 350 � C, with the in-air

Fig. 5. FTIR spectra of 0ME500 150 material, after adsorption of Py and subsequent evacuation at different temperatures for 60 min: a) evacuation at 50 � C and b) evacuation at 100 � C.

pyridine (HPy) were developed, besides two weak signals at 1579 cm 1 and 1626 cm 1. The first weak band corresponds to physisorbed pyri­ dine (PPy) inside the pores, hard to be evacuated at low temperatures, and the last to Lewis pyridine (LPy) formed by interaction with coor­ dinative unsaturated centers on the silica surface. These weak Lewis sites form by calcination at 500 � C [39]. After the evacuation at 100 � C, the removal of almost all Py occurs leaving a slight remnant of the main signals at 1597 and 1446 cm 1. Fig. 6 shows the IR spectra of adsorbed Py on 5 ME materials pre­ viously treated in vacuum at 350 � C. The spectra show that the amount of remnant Py, chemisorbed after 1-h evacuation at temperatures be­ tween 50 and 150 � C, was higher than that observed over the support (Fig. 5). Besides, on them, three chemisorbed Py species were distin­ guished: HPy, LPy, and one linked to Brønsted acid sites (BPy). HPy signals at 1598 cm 1 (ν8a) and 1447 cm 1 (ν19b) are the most � intense ones on the 5ME300 nHT material evacuated a 50 C, but they decrease as the hydrotreatment temperature increases. So, over 5ME300 220 material, their intensity is similar to that of LPy. This behavior correlates with the structural changes (SBET and the porous structure) observed in the 5 ME materials (Table 1). Thereby, at a higher SBET, the greater is the amount of SiO–H groups capable of hydrogen-bond interaction (Fig. 2).

Fig. 6. FTIR of 5 ME samples calcined at 300 � C after Py adsorption and subsequent evacuation at different temperatures for 1 h: a) evacuation at 50 � C, b) evacuation at 100 � C and c) evacuation at 150 � C. Samples were evacuated at 350 � C before Py adsorption. 6

P.M. Cuesta Zapata et al.

Microporous and Mesoporous Materials xxx (xxxx) xxx

Fig. 7. FTIR of adsorbed Py on 5 ME samples with different hydrothermal treatment and calcined at a) 300 � C, b) 350 � C, c) 400 � C and d) 450 � C. After Py adsorption, samples were evacuated at 100 � C for 1 h.

Fig. 8. 1450 cm 1 band area per unit of specific surface area (SBET) variation as a function of the calcination temperature, for 5ME samples obtained by different hydrothermal treatments, after adsorption of pyridine and subsequent evacuation at 100 � C for 1 h 5MEnHT, 5ME150 and 5ME220.

calcination. This behavior reflexes not only the high Cr3þ superficial exposition after the hydrotreatment but also their resistance to being oxidized to Cr6þ after grafting to form chromium-siloxane bonds [26]. Fig. 9. FTIR spectra of adsorbed Lu onto the 5ME300 nHT sample, previously evacuated at a) 250, b) 350, c) 400 and d) 450 � C. Samples evacuated at 100 � C after Lu adsorption.

3.1.2.4. 2,6-Lutidine adsorption (FTIR). Fig. 9 shows the spectra of adsorbed 2,6-Lutidine (Lu) on the 5ME300 nHT sample, previously evacuated (10 5 mmHg - 1 h) at different temperatures. Lu adsorption over the catalyst evacuated at 250 � C produces BLu (ν8a-8b ¼ 1644 cm 1) and LLu (ν8a ¼ 1610 cm 1) species. The Brønsted acid centers observed are a consequence of the in-air calcination process (3 h at 300 � C) done to remove the surfactant. Vacuum treatments, above 350 � C, prior to Lu absorption, cause a gradual decrease of the Brønsted acid centers due to the in-vacuum reduction of chromate centers at these temperatures. So, in vacuum at T > 300 � C, Cr6þ species are gradually reduced to Cr3þ and at 450 � C mainly to Cr2þ. The change was accompanied by color variations of the sample going from the characteristic yelloworange color of the samples with Cr6þ species, passing through a brown-green at intermediate evacuation temperatures and reaching a dark blue color (Cr2þ) in samples evacuated at 450 � C. From this behavior, we conclude the existence of a reversible redox process Cr2þ Cr3þ - Cr6þ. Such a redox process shows that on the catalyst surface chromium centers, strongly interacting with the support, can modify its oxidation state as a function of the temperature and the atmosphere of

the environment. On the other hand, as it was analyzed for LPy species, Fig. 10 shows the evolution of the Brønsted acid site density (1645 cm 1 band area/ SBET). There, it is observed that hydrothermal treatment produces a higher density of Brønsted acid sites than that produced over materials without hydrothermal treatment (5MEnHT); despite hydrotreated mate­ rials have a higher resistance to Cr3þ oxidation. On these materials, the density of the Brønsted sites doesn’t increase proportionally to de amount of Cr6þ formed, but it depends on the amount of monochromate species formed decreasing as the polychromate ones are produced. 3.1.3. Pyridine TPD The temperature programmed desorption of a base, such as Py, al­ lows knowing both the number and also the strength of acid sites. In Fig. 11, the thermograms (TPD) of adsorbed Py onto 5 ME samples are presented. 7

P.M. Cuesta Zapata et al.

Microporous and Mesoporous Materials xxx (xxxx) xxx

HPy was strongly dependent on the structural properties of the materials (SBET, Dpores, and Vpores), while that corresponding to BPy and LPy showed a correlation with the hydrothermal treatment temperature. 3.1.4. Catalytic activity against 2-propanol The catalytic activity of the 5ME mesostructured materials was studied in the 2-propanol decomposition reactions. The 2-propanol transformation, catalyzed by a solid in the gas phase, allows to study the reaction rate at which this secondary alcohol is converted to different products (propene, acetone and i-propyl ether), and also to correlate the acid-base properties of the material with the products type and his amount. This makes them a complementary tool for the acid/ base characterization results obtained by other techniques. Fig. 12-a shows the variation of the 2-propanol dehydration rate as a function of the hydrothermal treatment temperature. It is observed that these structured materials show an increase in the dehydration rate by the hydrothermal treatment effect. Thus, hydrothermally treated cata­ 300 lysts, 5ME300 150 and 5ME220, show a 2-propanol dehydration rate one order of magnitude higher compared with that shown by the nonhydrothermally treated material 5ME300 nHT. This behavior correlates with the acidity increase produced by the Cr3þ incorporation to the silica structure, and the formation of coordinatively unsaturated Lewis centers associated with high stressed chromasiloxane cycles [1,33,43,44]. On the other hand, Fig. 12-b shows, at 350 � C, a maximum in the dehydration rate on 5ME150 calcined in air at different temperatures. This maximum agrees with the evolution of the Lewis acidity density determined by pyridine adsorption (Fig. 8). Hence, the in-air calcination at 450 � C produces a drop in dehydration activity associated with a decrease in the Lewis acid sites, as a consequence of the Cr3þ partial oxidation. Table 6 shows the reaction rates and selectivity obtained against the 2-propanol dehydration reaction at 280 � C, as well as their apparent

Fig. 10. Ratio of the band area at 1645 cm 1 per unit of specific surface area (SBET), for 5 ME samples after adsorption of 2,6-lutidine and subsequent evacuation at 100 � C for 1 h 5MEnHT and 5ME220.

From the observed profiles, it is possible to point out the existence of Py interacting with sites of different natures that co-exist on the catalyst surface. Additionally, the combined analysis of Py-TPD profiles and the FTIR spectra identifying the different Py species, allows us to propose 5 peaks to perform the deconvolution adjustment of the thermograms. 1 Table 5 shows the normalized amounts (mmol Py. g 1. SBET ) of Py corresponding to each peak. The desorbed Py corresponding to the first combined peak, (deter­ mined as the sum of the A and B peaks), is correlated with the pore volume of the materials (Table 1). It corresponds to the physisorbed Py (PPy) remaining inside the pores (peak A) and the hydrogen bonded Py (peak B). The last bonded to OH groups both into the pores and in the external surface of the materials. The proposal of these two types of Py species is based on the composed profile of the band and the almost negligible variation of the desorption maxima for the different hydro­ treated materials. According to FTIR Py chemisorption experiments (Fig. 7), the BPy species are more easily desorbed than LPy; therefore, it was proposed that peak C corresponds to BPy. The normalized BPy areas are similar for 5MEnHT and 5ME150; however, the normalized amount of BPy (sites. m 2) was almost four times greater in the 220 � C hydrothermally syn­ thesized material (5ME300 220) reported in Fig. 10. Accordingly, the sum of D and E peaks reflects the presence of Py interacting with Lewis centers. They arise because the different strengths of the sites on the surface previously determined by FTIR of adsorbed Py (Fig. 7). From the area of these peaks, we determined a higher density of Lewis acid sites on hydrothermally treated materials than on the not hydrotreated ones. From the obtained results, it is observed that the amount of PPy and

Table 5 Summary of the results obtained from the deconvolution of the pyridine TPD over 5 ME samples. 5ME300 nHT

Signal

A B C D E Total

5ME300 150

5ME300 220

TMAX (� C)

mmol Py m 2 x 10 5

TMAX (� C)

mmol Py m 2 x 10 5

TMAX (� C)

mmol Py m 2 x 10 5

221 298 443 623 677 –

2.158 5.580 2.604 0.670 0.595 11.61

241 302 467 622 692 –

5.026 11.21 2.706 1.160 0.258 20.36

229 305 451 643 696 –

10.55 18.99 9.705 1.266 0.844 41.35

300 300 Fig. 11. Py-TPD of 5 ME samples prepared by different hydrothermal treatments and calcined at 300 � C. a) 5ME300 nHT, b) 5ME150 and c) 5ME220.

8

P.M. Cuesta Zapata et al.

Microporous and Mesoporous Materials xxx (xxxx) xxx

300 300 Fig. 12. Variation of 2-propanol dehydration reaction rate (280 � C) with: a) hydrothermal treatment temperature (5ME300 nHT, 5ME150 and 5ME220), and b) calcination 350 450 temperature on 5ME150 material (5ME300 150, 5ME150 and 5ME150).

activation energies. All materials of the 5ME series calcined at 300 � C exhibit, clearly, an acid behavior evidenced by the high selectivity to­ wards propene formation at 280 � C. However, calcination at 450 � C (5ME450 150) led to selectivity values showing an increase in the dehydro­ genation reaction rate to acetone as a consequence of an increase in the number of Cr6þ centers (Table 2). Note that the drop in selectivity does not necessarily relate to a decrease in the 2-propanol dehydration rate, but to an increase of the dehydrogenation reaction rate. Accordingly, the 5ME350 150 material (Table 4) exhibited a higher dehydration rate than 5ME300 150 but a lower selectivity. On the other hand, the Ea values allow concluding that the changes in dehydration rate relate to the changing in the number and not on the nature of the active sites. Gervasini and Auroux [23] determined that neither SiO2 nor α-Cr2O3 have 2-propanol dehydrating activity, at least up to 250 � C. This lets to conclude that the incorporation of chromium to the support produces Lewis acid sites, observed both by Py (Figs. 7) and 2,6-lutidine (Fig. 9) adsorption, with dehydrating capacity. Moreover, considering that the polychromate species do not present significant catalytic activity to dehydrogenation reaction, the acetone formation must be attributed to isolated (monodispersed) Cr6þ centers. Burwell et al. [45,46] determined, from the crystalline field energy of d3 ions, that Cr3þ reaches its maximum stabilization because of the presence of ligands in an octahedral geometry. However, Cr3þ ions located on the surface of a solid have low coordination, showing a tendency to complete their coordination sphere through new ligands. The unsaturation of these centers depends on the geometry of the sur­ face ligands. On silica, ligands are partially provided by SiO bonds (internal coordination sphere ligands) forming chromasiloxanes cycles, and fulfilled by external labile ligands such as water molecules or oxy­ gen atoms from farther siloxane groups, which complete the octahedral coordination. Thus, the coordinative unsaturated Cr3þ centers are suit­ able sites for the 2-propanol adsorption as electronic density donor

ligand and subsequently, it can be dehydrated by an Hα elimination mechanism (Scheme 3. On the other hand, after the in-air calcination (T > 300 � C) Cr6þ monochomate centers form linked to the silica surface by siloxane groups. They have the ability to chemisorb 2-PrOH to form isopropyl chromate esters, release an acetone molecule to auto-reduce and finally liberate H2 to conform to a redox cycle that justifies the dehydrogena­ tion activity shown by these materials (Scheme 4). 4. Conclusions The incorporation of highly dispersed chromium ions to silica is responsible for the generation of both Lewis and Brønsted acid sites. Their relative amount is strongly dependent on calcination and

Table 6 Dehydration activity for 5ME materials against 2-propanol. Sample

TCALCINATION (� C)

5ME300 nHT 5ME300 150 5ME350 150 5ME450 150 5ME300 220

300 300 350 450 300

r280� C (mol ene/ h 1m 2mol Cr 1) 6.859 3.386 5.825 1.801 4.684

� 10 � 10 � 10 � 10 � 10

5 4 4 4 4

Propene Selectivity (280 � C)

EaPropene (Kcal mol 1)

78 84 66 51 89

22.70 22.31 23.06 23.38 20.46

Scheme 3. 2-PrOH dehydration over Cr3þ centers. 9

P.M. Cuesta Zapata et al.

Microporous and Mesoporous Materials xxx (xxxx) xxx

- References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

Scheme 4. 2-PrOH dehydrogenation over Cr6þ centers.

[20] [21] [22] [23] [24]

hydrothermal treatment temperature. So, while the hydrothermal treatment modifies the concentration and strength of the Lewis sites, the in-air calcination promotes the Brønsted acid sites formation. Thereby, Cr3þ species, forming strained chromasiloxane rings, correlate with the coordinately unsaturated Lewis acid sites; whereas Cr6þ species with the Brønsted ones. Considering that polymeric Cr6þ species segregation decreases the Brønsted centers, it is possible to conclude that they are linked to monochromate species formation. Hydrogen exchange by deuterium confirms the presence of isolated O-D bonds, associated with the presence of grafted chromium atoms, which are related to chromasiloxane rings formation. These chromium centers are involved in a reversible redox cycle Cr2þ - Cr3þ - Cr6þ. Their relative amount depends on the hydrothermal treatment, the calcination temperature, and vacuum treatment. The catalytic behavior towards 2-propanol decomposition shows that the Cr3þ Lewis centers are responsible for dehydrating activity whereas the Cr6þ relates to the dehydrogenating activity through a redox behavior.

[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]

Declaration of Competing interest

[38] [39]

There are no conflicts of interest to declare.

[40] [41]

Acknowledgments

[42] [43]

This work was performed under the auspices and the financial sup­ �n de la Universidad port of INIQUI-CONICET and Consejo de Investigacio Nacional de Salta (CIUNSa).

[44] [45] [46]

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109895.

10

E. Groppo, G.A. Martino, A. Piovano, C. Barzan, ACS Catal. 8 (2018) 10846–10863. M. McDaniel, Appl. Catal. Gen. 542 (2017) 392–410. M. Gierada, J. Handzlik, J. Catal. 352 (2017) 314–328. C. Brown, A. Lita, Y. Tao, N. Peek, M. Crosswhite, M. Mileham, J. Krzystek, R. Achey, R. Fu, J.K. Bindra, M. Polinski, Y. Wang, L.J. van de Burgt, D. Jeffcoat, S. Profeta, A.E. Stiegman, S.L. Scott, ACS Catal. 7 (2017) 7442–7455. M.P. McDaniel, K.S. Clear, Appl. Catal. Gen. 527 (2016) 116–126. K.C. Potter, C.W. Beckerle, F.C. Jentoft, E. Schwerdtfeger, M.P. McDaniel, J. Catal. 344 (2016) 657–668. A. Burri, M.A. Hasib, Y.-H. Mo, B.M. Reddy, S.-E. Park, Catal. Lett. 148 (2018) 576–585. V.Z. Fridman, R. Xing, Appl. Catal. Gen. 530 (2017) 154–165. L. Li, W. Zhu, L. Shi, Y. Liu, H. Liu, Y. Ni, S. Liu, H. Zhou, Z. Liu, Chin. J. Catal. 37 (2016) 359–366. Y. Cheng, L. Zhou, J. Xu, C. Miao, W. Hua, Y. Yue, Z. Gao, Microporous Mesoporous Mater. 234 (2016) 370–376. S. Kilicarslan, M. Dogan, T. Dogu, Ind. Eng. Chem. Res. 52 (2013) 3674–3682. M. Kaur, A. Ratan, S. Kunchakara, M. Dutt, V. Singh, J. Porous Mater. 26 (1) (2018) 239–246. J.F. Miranda, P.M. Cuesta Zapata, E.E. Gonzo, M.L. Parentis, L.E. Davies, N. A. Bonini, Catal. Lett. 148 (2018) 2082–2094. M. Selvaraj, D.W. Park, S. Kawi, I. Kim, Appl. Catal. Gen. 415–416 (2012) 17–21. D. Macina, A. Opioła, M. Rutkowska, S. Basąg, Z. Piwowarska, M. Michalik, L. Chmielarz, Mater. Chem. Phys. 187 (2017) 60–71. W.K. J� o�zwiak, W. Ignaczak, D. Dominiak, T.P. Maniecki, Appl. Catal. Gen. 258 (2004) 33–45. F. Hao, Y. Yang, W. Xiong, P. Liu, K. You, H.a. Luo, React. Kinet. Mech. Catal. 125 (2018) 199–211. J.F. Le Page, D. Avnir, E. Taglauer, M. Guisnet, G. Moretti, M. Che, F. BozonVerduraz, M. Anpo, E. Roduner, H. Kn€ ozinger, Handbook of Heterogeneous Catalysis, Wiley-VCH Verlag GmbH, 2008, pp. 582–689. Y. Leng, Materials Characterization, John Wiley & Sons (Asia) Pte Ltd, 2008, pp. 253–300. C.A. Emeis, J. Catal. 141 (1993) 347–354. W. Turek, A. Krowiak, Appl. Catal. Gen. 417–418 (2012) 102–110. W. Turek, J. Haber, A. Krowiak, Appl. Surf. Sci. 252 (2005) 823–827. A. Gervasini, A. Auroux, J. Catal. 131 (1991) 190–198. P.M. Cuesta Zapata, M.S. Nazzarro, M.L. Parentis, E.E. Gonzo, N.A. Bonini, Chem. Eng. Sci. 101 (2013) 374–381. P.M. Cuesta Zapata, M.L. Parentis, E.E. Gonzo, N.A. Bonini, Appl. Catal. Gen. 457 (2013) 26–33. P.M. Cuesta Zapata, M.S. Nazzarro, E.E. Gonzo, M.L. Parentis, N.A. Bonini, Catal. Today 259 (2016) 39–49. S.C. Fung, C.A. Querini, J. Catal. 138 (1992) 240–254. C.R. Wagner, L. Davis, J. Moulder, G. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy, Perkin Elmer Corporation, Minessota, 1978. S. Biz, M.L. Occelli, Catal. Rev. 40 (1998) 329–407. R. Mokaya, W. Jones, Chem. Commun. (1996) 981–982. S. Kawi, M. Te, Catal. Today 44 (1998) 101–109. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T. W. Chu, D.H. Olson, E.W. Sheppard, J. Am. Chem. Soc. 114 (1992) 10834–10843. C.A. Demmelmaier, R.E. White, J.A. van Bokhoven, S.L. Scott, J. Catal. 262 (2009) 44–56. B.A. Morrow, A.J. McFarlan, The Colloid Chemistry of Silica, American Chemical Society, 1994, pp. 183–198. N.A. Bonini, E.E. Gonzo, S. Locatelli, Lat. Am. Appl. Res. 24 (1994) 149–158. S. De Rossi, G. Ferraris, S. Fremiotti, E. Garrone, G. Ghiotti, M.C. Campa, V. Indovina, J. Catal. 148 (1994) 36–46. S. Lillehaug, K.J. Børve, M. Sierka, J. Sauer, J. Phys. Org. Chem. 17 (2004) 990–1006. S. Lillehaug, V.R. Jensen, K.J. Børve, J. Phys. Org. Chem. 19 (2006) 25–33. M.I. Zaki, M.A. Hasan, F.A. Al-Sagheer, L. Pasupulety, Colloid. Surf. Physicochem. Eng. Asp. 190 (2001) 261–274. A. Corma, C. Rodellas, V. Fornes, J. Catal. 88 (1984) 374–381. Y.-M. Liu, W.-L. Feng, L.-C. Wang, Y. Cao, W.-L. Dai, H.-Y. He, K.-N. Fan, Catal. Lett. 106 (2006) 145–152. E. Selli, L. Forni, Microporous Mesoporous Mater. 31 (1999) 129–140. E. Groppo, C. Lamberti, S. Bordiga, G. Spoto, A. Zecchina, Chem. Rev. 105 (2005) 115–184. A. Zecchina, E. Groppo, A. Damin, C. Prestipino, in: C. Cop� eret, B. Chaudret (Eds.), Surface and Interfacial Organometallic Chemistry and Catalysis, Springer Berlin Heidelberg, 2005, pp. 1–35. S.R. Ely, R.L. Burwell, J. Colloid Interface Sci. 65 (1978) 244–253. R.L. Burwell, G.L. Haller, K.C. Taylor, J.F. Read, in: D.D. Eley, H. Pines, P.B. Weisz (Eds.), Advances in Catalysis, Academic Press, 1969, pp. 1–96.