One-pot direct synthesis route to self-assembled highly ordered Zn-decorated mesoporous aluminium oxide toward efficient and sustainable metathesis heterogeneous catalyst design

Abdelnasser Abidli*ab
aDepartment of Soil Sciences and Agri-Food Engineering, Laval University, G1V 0A6, Quebec City, Quebec, Canada. E-mail: abdelnasser.abidli.1@ulaval.ca; Fax: +1-418-656-3723; Tel: +1-581-777-0860
bCentre in Green Chemistry and Catalysis (CGCC), H3A 0B8, Montreal, Quebec, Canada

Received 23rd June 2015 , Accepted 22nd October 2015

First published on 22nd October 2015


Abstract

Hexagonally well-organized ZnCl2-OMA materials have been successfully synthesized by a one-pot approach using a sol–gel method at ambient temperature and pressure. These materials were prepared through simultaneous self-assembly process with F127 as directing agent combined with in situ impregnation of ZnCl2. The new approach was found to be compatible with various common aluminium precursors and carboxylic acids used as self-assembly interfacial protectors. The obtained ZnCl2-OMA materials were fully characterized using XRD, N2 adsorption–desorption, TEM, SEM, EDX, XPS and 1H and 27Al MAS-NMR techniques. These materials were compared with the zinc chloride-doped organized mesoporous alumina (ZnCl2-OMA) supports prepared through a conventional two steps process that includes OMA synthesis and then post-synthesis incorporation of ZnCl2. Thus, the one-pot synthesized materials were found to exhibit improved properties compared to the conventional ones, particularly larger BET surface area. The synthesized ZnCl2-OMA materials were then used as catalytic supports for methyltrioxorhenium (MTO), showing enhanced catalytic performance for methyl oleate self-metathesis, demonstrating better activity and selectivity towards desired metathesis products. These features are highly beneficial for large-scale materials synthesis through fast, simple, easy and low-cost introduction of functionalities on mesoporous materials surface without multi-step procedures.


Introduction

Metal oxide-based supported catalysts are considered as an important class of heterogeneous catalysts.1,2 Among these metal oxides, porous systems represent the major share of this family, more particularly mesoporous materials which gathered enormous attention for their wide application spectrum in catalysis.3 These materials were also advantageously used for other applications including energy conversion and storage, gas sensing as well as adsorption and separation applications.4–7 Mesoporous aluminium oxide (e.g. γ-Al2O3) is one of the most commonly used catalytic supports.8 Moreover, a variety of active species including organics, metals, organometallics and metal oxides were grafted on alumina surface generating highly robust heterogeneous catalysts.9–11 Furthermore, for heterogeneous catalytic conversions, alumina supports are usually modified with such species before incorporation of the active components. These species usually offers better stability to the alumina matrix and the catalyst,12 increased dispersion of active components,13 improved acid-base properties,14–16 as well as providing enhanced catalytic performance usually through surface synergetic interactions with those species.17 However, conventionally, the modification of the catalysts supports is conventionally performed via a post-synthesis methodology, through a challenging multistep process. These procedures are usually laborious as well as energy and time consuming. Moreover, these features render the synthesis process costly. In addition, the use of several separation and treatment steps increase both the use and the generation of possible toxic solvents, compounds and gasses. Furthermore, such long processes may hinder the control over the materials synthesis and alter their properties (e.g. textural, morphological, chemical, etc.) as well as the synthesis yield. These drawbacks present enormous limitations towards large scale synthesis of such modified materials and catalysts, as well as their broad application.

Therefore, the most suitable route for the modification and functionalization of these catalyst supports is to perform an in situ incorporation of the desired components, prior (or during) to the formation of the host materials. These one-step procedures will highly increase the efficiency of the synthesis processes by offering a variety of advantages including faster synthesis by shortening the process from several days to few hours or minutes.18–22 These one-step approaches are more cost-effective and leads to higher reaction yields.23 This renders the introduction of desired functionalities easier and simpler with limited pore blocking phenomena.24 Moreover, this methodology offers better control over the materials synthesis, uniformity, purity and homogeneity.22,25,26 Thus, allowing access to catalytic supports with better properties that are key factor for the enhancement of their catalytic performance.18,27 Actually, several reports showed that such synthesized catalysts always exhibit significantly better catalytic properties compared to those prepared via the conventional multistep process.18,27,28

Therefore, in view of these promising features, we decided to adopt this methodology for our study on modified mesoporous alumina-based metathesis catalysts. Previously, we have reported a successful synthesis of a highly active metathesis catalysts using methyltrioxorhenium (MTO) supported on functionalized mesoporous alumina. First, through several optimizations we showed that ZnCl2 is the most suitable metal halide for alumina modification, offering the highest metathesis performance among several other metal halides used for the study.29–31 The MTO supported ZnCl2-modified Al2O3 catalyst was found to exhibit higher metathesis activity compared to the unmodified alumina.29–31 This catalyst showed high performance for both methyl oleate and triolein self-metathesis.29–31 However, this catalyst displayed relatively slow kinetics and unsatisfactory selectivity towards desired metathesis products. Therefore, we replaced the traditional alumina support having wormhole-like mesostructure by a hexagonally ordered mesoporous alumina (OMA). Thus, we have recently reported an enhanced performance of the MTO-based catalysts for methyl oleate self-metathesis using the ZnCl2-modified OMA supports.32 The OMA-based catalysts were successfully tested for their activity for methyl oleate self-metathesis. In fact, metathesis reaction in one of the most interesting catalytic route providing access to a variety of valuable monomers via C[double bond, length as m-dash]C bond formation.33,34 On the other hand, methyl oleate is chosen as a model molecule having both long carbon chain containing C[double bond, length as m-dash]C bond which will be subjected to metathesis catalysis conditions, and also the ester functional groups to evaluate the MTO-based catalyst tolerance for functional groups. In addition, methyl oleate is a bulky molecule which is suitable to evaluate the kinetics and molecular diffusion inside the mesoporous network. In the meantime, this renewable substrate originated form vegetable oils represents an interesting feedstock for the synthesis of value-added compounds via valorization through metathesis pathway.35,36

Therefore, in this study, in order to improve furthermore the MTO-based catalyst design and performance, we developed a new direct one-pot self-assembly strategy for the synthesis of ZnCl2-modified OMA supports. Instead of the traditional methodology that we usually adopted which consist on preparing the OMA supports then a post-synthetic functionalization with ZnCl2 in a laborious process. Thus, we are aiming to improve the synthesis process trough a simpler, faster, easier and cost-effective process. The new synthesis protocol is expected to offer the desired materials with enhanced chemical, morphological, and textural properties which may likely lead to better metathesis catalytic performance. In addition, higher synthesis yields are expected to be reached. Detailed characterizations and comparison with alumina-based supports prepared via the conventional approach and their catalytic performance are provided. This successful methodology is a step forward towards an easy and efficient scalability. Also provide a rapid access route for such desired materials, especially with the recent attention gathered around zinc chloride impregnated on alumina and alumina–silica materials for other applications such as methyl chloride synthesis37,38 and catalytic alkylation reactions.39–42 As well as other zinc chloride-modified materials including mesoporous carbon nitride (ZnCl2/mp-C3N4) and doped graphitic carbon nitride (Zn-g-C3N4) for other catalytic applications (e.g. cycloaddition and transesterification).43,44 Therefore, in addition to the synthesis of modified-alumina, this method represents a promising approach to prepare various functionalized mesoporous materials for broad applications.

Experimental

One-pot synthesis of zinc chloride (ZnCl2)-modified ordered mesoporous alumina (OMA)

Well-organized ZnCl2-doped hexagonal mesoporous alumina materials denoted as ZnCl2-OMA were prepared by a facile one-pot synthesis through a combined sol–gel process and in situ impregnation method. All the reagents were used as received without further purification. Various aluminium precursors were used including Al(OBus)3, Al(OPri)3, Al(OBut)3 and Al(NO3)3·9H2O, with distinct carboxylic acids (citric, tartaric, fumaric, oxalic, malonic, maleic and acetic acid) in order to generalize this methodology and to verify the reproducibility of the newly designed synthesis approach. For a typical synthesis of these modified ordered mesoporous materials; Pluronic F127 (2.17 g) was dissolved in anhydrous ethanol (20 mL) under magnetic stirring at room temperature. Then, the investigated carboxylic acid (2.6 mmol) was added, while the pH of the solution was adjusted below 2 by adding hydrochloric acid (1.0–1.5 mL), giving a clearer solution. Subsequently, the investigated aluminium-precursor (10 mmol) was added under violent magnetic stirring. Simultaneously, zinc chloride (170 mg, 1.25 mmol) was dissolved in ethanol (10 mL) by applying a 30 s vortex mixing. The dissolved zinc chloride was then slowly added dropwise to the above Pluronic F127-containing solution under gentle stirring. The resulting mixture was then covered with polyethylene film, and continuously stirred for an additional 5 h at room temperature to homogenize the content. Afterwards, the mixture was transferred for aging into a drying oven at 60 °C, allowing slow and compete evaporation of solvents within 48 h. Finally, the resulting dried solid was annealed at 400 °C with a slow ramping rate (1 °C min−1), and calcined for 4 h in air to achieve the final product (ZnCl2-OMA materials) with optimized Al[thin space (1/6-em)]:[thin space (1/6-em)]Zn molar ratio of 8[thin space (1/6-em)]:[thin space (1/6-em)]1.

Catalysts preparation and metathesis reaction

3 wt% of methyltrioxorhenium (MTO) was impregnated on the prepared ZnCl2-OMA catalytic supports. Various supports those synthesized using different aluminium precursors and carboxylic acids were used, and numerous tests were performed for each catalyst composition (support). The evaluation of the catalyst was carried out on a batch reactor. After impregnation of MTO (6.2 mg) on ZnCl2-OMA supports (200 mg) in hexane under N2 atmosphere at 45 °C, the prepared 3 wt% MTO/ZnCl2-OMA catalyst was loaded in the glass reactor. Afterwards, 750 mg of methyl oleate in 2 mL of hexane was injected. The reacting mixture was heated to the desired temperature (45 °C) with stirring under atmospheric pressure. After reaction was complete, the reactor was cooled, and the catalyst was removed by centrifugation. The methyl oleate conversion as well as reaction products identification and quantification were conducted through GC and GC-MS analysis using highly pure commercial standards.

Characterization of the prepared materials and surface properties

The textural properties of calcined samples including the BET specific surface area, total pore volume and BJH pore diameter distribution have been measured by nitrogen adsorption–desorption using a volumetric adsorption analyzer (Model Autosorb-1, Quantachrome Instruments). The hexagonal crystalline structure of the calcined zinc chloride-modified alumina samples were characterized by powder X-ray diffraction (XRD). Small-angle X-ray scattering (SAXS) patterns were recorded using Ultima III Rigaku monochromatic diffractometer (Model D/MAX-2200). Transmission electron microscopy (TEM, JEM-1230 electron microscope) was performed in order to examine the structure of the materials' porous network. The samples microstructures and morphologies were observed using a scanning electron microscope (SEM, JEOL model JSM-840A), while the elemental composition and the Al[thin space (1/6-em)]:[thin space (1/6-em)]Zn atomic ratio in the synthesized materials was determined using energy dispersive X-ray spectrometry (EDX, JEOL model JSM-840A). X-ray photoelectron spectroscopy (XPS, AXIS ULTRA from Kratos Analytical) measurements were conducted to identify the materials surface species and their oxidation/coordination states, as well as to confirm the elemental composition and the Al[thin space (1/6-em)]:[thin space (1/6-em)]Zn atomic ratios analyzed by EDX. The catalyst framework was investigated by proton solid-state NMR spectroscopy (1H MAS NMR), while the form, coordination state and environment of aluminium, as well as the surface and intrinsic Lewis acidic site concentrations were studied by means of aluminium solid-state magic-angle spinning NMR spectroscopy measurements (27Al MAS NMR). See also S1 in the ESI for more details on characterization experiments and instrumentations.

Results and discussion

Characterization of the alumina-based materials

Nitrogen adsorption–desorption measurements. Fig. 1A shows the N2 adsorption–desorption isotherms of the prepared ZnCl2-OMA samples. All obtained isotherms exhibit a typical IV-type curve accompanied with a H1-type hysteresis loop according to IUPAC classification. This results correlate with the capillary condensation of nitrogen inside the confined mesoporous solids, indicating that all samples have organized mesoporous network with uniform and well-defined cylinder-like pore geometry.45,46
image file: c5ra12057h-f1.tif
Fig. 1 (A) Nitrogen adsorption–desorption isotherms and (B) pore size distributions of the prepared ZnCl2-OMA samples. All samples were calcined at 400 °C.

Table 1 summarizes the Brunauer–Emmet–Teller (BET) surface areas (SBET), total pore volumes (PV) average pore size (dBJH) data obtained by N2 adsorption–desorption at 77 K illustrating the textural properties and porosity parameters of the synthesized ZnCl2-OMA materials. Relatively large BET surface are and pore volume were obtained, up to 358 m2 g−1 and 0.67 cm3 g−1, respectively. In addition, nearly all the samples displayed uniform narrow pore size distribution with a mean diameter from 3.4 to 6.5 nm. However, some analyzed samples present some secondary porosity (Table 1). These results suggest that the presence of ZnCl2 did not hamper the formation of the desired hexagonally ordered mesoporous materials.

Table 1 Textural properties and porosity parameters obtained by N2 adsorption–desorption at 77 K for the synthesized zinc chloride-modified organized mesoporous alumina (ZnCl2-OMA) materials using different aluminium precursors and carboxylic acids
ZnCl2-OMA samplesa Acid Aluminium precursor Synthesis approach SBETb (m2 g−1) PVc (cm3 g−1) dBJHd (nm)
a Al[thin space (1/6-em)]:[thin space (1/6-em)]Zn atomic ratio of 8[thin space (1/6-em)]:[thin space (1/6-em)]1 determined by EDX analysis.b BET specific surface area (m2 g−1).c BJH pore volume (cm3 g−1) determined at P/P0 = 0.997.d BJH average pore diameter (nm).e BET–BHJ results obtained previously.32 ND: not determined.
ZnCl2-OMA-1 Tartaric Al(OBus)3 One-pot 203 0.30 3.8 and 5.6
Two-stepe 154 0.26 3.8
ZnCl2-OMA-2 Fumaric Al(OBus)3 One-pot 299 0.41 3.8
Two-stepe 214 0.31 3.8
ZnCl2-OMA-3 Citric Al(OBus)3 One-pot 307 0.42 3.8
Two-stepe 316 0.44 3.8
ZnCl2-OMA-4 Oxalic Al(OBus)3 One-pot 282 0.36 3.8
Two-stepe ND ND ND
ZnCl2-OMA-5 Maleic Al(OBus)3 One-pot 238 0.27 3.4
Two-stepe ND ND ND
ZnCl2-OMA-6 Malonic Al(OBus)3 One-pot 252 0.27 3.4
Two-stepe ND ND ND
ZnCl2-OMA-7 Oxalic Al(OPri)3 One-pot 234 0.46 3.8 and 6.5
Two-stepe ND ND ND
ZnCl2-OMA-8 Malonic Al(OPri)3 One-pot 181 0.23 3.4
Two-stepe ND ND ND
ZnCl2-OMA-9 Acetic Al(OPri)3 One-pot 199 0.32 3.8 and 4.9
Two-stepe ND ND ND
ZnCl2-OMA-10 Maleic Al(OPri)3 One-pot 159 0.34 3.8 and 6.5
Two-stepe ND ND ND
ZnCl2-OMA-11 Tartaric Al(OPri)3 One-pot 219 0.38 3.8 and 4.9
Two-stepe 190 0.39 5.7
ZnCl2-OMA-12 Citric Al(OPri)3 One-pot 232 0.47 7.7
    Two-stepe 241 0.38 3.8 and 4.9
ZnCl2-OMA-13 Citric Al(OBut)3 One-pot 358 0.67 3.8 and 5.6
Two-stepe 239 0.41 4.9
ZnCl2-OMA-14 Tartaric Al(OBut)3 One-pot 307 0.44 3.8
Two-stepe 257 0.44 5.6
ZnCl2-OMA-15 Malonic Al(OBut)3 One-pot 224 0.41 3.8
Two-stepe ND ND ND
ZnCl2-OMA-16 Citric Al(NO3)3·9H2O One-pot 201 0.39 3.8 and 4.9
Two-stepe 139 0.40 3.8 and 6.5
ZnCl2-OMA-17 Tartaric Al(NO3)3·9H2O One-pot 226 0.41 3.8 and 5.6
Two-stepe 222 0.46 3.8
ZnCl2-OMA-18 Malonic Al(NO3)3·9H2O One-pot 183 0.33 3.8
Two-stepe ND ND ND


As mentioned above, the one-pot synthesis was performed using distinct aluminium precursors and carboxylic acids in order to generalize the synthesis procedure and verify it's reproducibly.

Therefore, we used the same precursors (Al(OBus)3, Al(OPri)3, Al(OBut)3 and Al(NO3)3·9H2O) and acids (citric, tartaric, fumaric, oxalic, malonic, maleic and acetic acid) that we have previously used for the synthesis of catalytic ZnCl2-OMA materials through conventional two-steps process.32

As depicted in Fig. 1 and Table 1, we successfully prepared various ZnCl2-OMA samples using different carboxylic acids, while no significant difference was observed on their textural properties and their porosity parameters. Furthermore, the targeted organized ZnCl2-OMA materials were successfully obtained upon the use of different aluminium precursors as illustrated in Table 1 and Fig. 2 with no significant shift. This similarity is ascribed to the identical behavior of the investigated precursors and acids, suggesting that the different carboxylic acids tested exhibit similar coordination abilities with the metal centers (Al) via monodentate or bridging bidentate modes during the mesophase formation pathway.32,47 On the other hand, the investigated aluminium precursors proved also similar chelation abilities of their metallic centers (Al) with both the templating agent (F127) fragments and the added carboxylic acids used as organic–inorganic interfacial protectors assisting the ordered self-assembly process. This kind of organized assembly is tailored by means of several interfacial interactions including covalent and hydrogen bonding as well as van der Waals interactions, which plays a pivotal role in the mesophase construction.48,49


image file: c5ra12057h-f2.tif
Fig. 2 (A) Nitrogen adsorption–desorption isotherms and (B) BJH pore size distributions curves of the prepared ZnCl2-OMA sample. All samples were calcined at 400 °C.

In comparison with the materials synthesized via the conventional two-steps process,32 as depicted in Table 1, almost all the newly prepared ZnCl2-OMA samples (one-pot) possess larger BET surface area. As we reported previously, we believe that the lower BET surface area obtained through the two-steps synthesis is primarily ascribed to the additional calcination step during the ZnCl2 impregnation, as well as the filling of added ZnCl2 inside the already formed mesopores.32 These troublesome effects were advantageously avoided when adopting the one-pot synthesis process were in situ impregnation of ZnCl2 is occurred with no need for subsequent annealing step that may alter the materials surface.

Meanwhile, both synthesis procedures led to ZnCl2-OMA materials with relatively similar total pore volume and pore size distributions (Table 1), suggesting that independently from the synthesis process, similar porosity features can be obtained using the same aluminium precursors and carboxylic acids having identical ligands and carbon chain length, respectively.

X-ray diffraction (XRD) analysis. The prepared ZnCl2-OMA materials calcined at 400 °C were analyzed by powder X-ray diffraction to verify their mesoscopic order. Fig. 3 shows the small-angle X-ray scattering (SAXS) patterns of the calcined ZnCl2-OMA samples. The patterns exhibit a major sharp and well-resolved peak at around a 2θ value of 1.0° indexed as the (100) Bragg reflection of the 2-D hexagonal p6mm structure (space group), with two minor peaks observed at around 2θ = 1.8 and 2.1° those corresponding to the higher order (110) and (200) reflections, respectively.50,51 These results suggest the well-ordered mesoporous network of the analyzed ZnCl2-OMA samples with hexagonal arrays without any lattice shrinkage.
image file: c5ra12057h-f3.tif
Fig. 3 Small-angle X-ray scattering (SAXS) patterns of zinc chloride-modified ordered mesoporous alumina samples. All samples were calcined at 400 °C.

As illustrated from the BET results, similarly, the XRD analysis revealed no significant shift in the XRD patterns when using distinct carboxylic acids (Fig. 3i) or different aluminium precursors (Fig. 3ii). It is worth to note that the obtained XRD analysis results were similar to those obtained for the ZnCl2-OMA samples prepared using the conventional two-steps process where the 2-D hexagonal p6mm structure was also detected for the prepared samples.32 Furthermore, no crystalline alumina phase was formed at 400 °C, as shown form the wide angle XRD patterns recorded for the calcined ZnCl2-OMA samples (Fig. S1 in the ESI). These results indicate the formation of an amorphous network. More interestingly, the data obtained is a clear evidence for the absence of any additional Zn-containing crystalline phases (e.g. ZnAl2O4, ZnO), which are usually obtained in this range of calcination temperatures.52,53

Morphology and structure of ZnCl2-OMA: TEM and SEM analysis. After organic molecules (surfactant and carboxylic acid) removal at 400 °C, the samples were also analyzed by transmission electron microscopy (TEM). The observed images are represented in Fig. 4 along the 001 and 110 directions for the obtained ZnCl2-OMA materials. In a good agreement with the results illustrated from the SAXS and N2 adsorption–desorption analyses (Fig. 1–3), the TEM micrographs (Fig. 4) confirm the presence of hexagonal ordering in the prepared mesoporous ZnCl2-OMA materials. Furthermore, as evidenced by SAXS and N2 adsorption–desorption measurements, highly ordered 2-D hexagonal mesostructure was successfully obtained upon the use of different aluminium precursors and diverse carboxylic acids as depicted in Fig. 4. The extremely well-ordered hexagonal arrangement of tubular mesopores along the [001] direction, and the alignment of uniform and non-interconnected cylindrical mesopores along the [110] direction are observed in all analyzed ZnCl2-OMA samples.
image file: c5ra12057h-f4.tif
Fig. 4 TEM images of the zinc chloride-modified ordered mesoporous alumina materials synthesized using Al(OBus)3 with different carboxylic acids: (a) citric, (c) tartaric, (d) malonic, (e and f) maleic, (g) fumaric and (i) oxalic acid, viewed along the [110] orientation, and (b) citric, (h) fumaric and (j) acetic acid, viewed along the [001] orientation. And using different aluminium precursors; Al(OPri)3 for (k) fumaric and (n) tartaric acid; Al(NO3)3·9H2O for (l) citric and Al(OBut)3 for (m) citric and (o) tartaric acid, viewed along the [110] orientation. All samples were calcined at 400 °C.

In addition, the diameter of the cylindrical mesopores as observed from the TEM micrographs was around 5 nm in consistency with the obtained N2 adsorption–desorption data. The observed organized structure of the materials prepared through one-pot process was similar to that observed for the ZnCl2-OMA samples prepared using the conventional two-steps process.32 The TEM data confirm again that the presence of ZnCl2 during the mesophase formation does not alter the organized assembly of alumina during the templating and the calcination process, and preserves the organized alumina framework. Furthermore, the micrographs displayed in Fig. 4 indicate the absence of any aggregation of Zn species/phases.

To probe into the morphological features of the synthesized ZnCl2-OMA materials, the prepared samples were examined by SEM. Fig. 5a–d shows plane-view SEM images of the ZnCl2-OMA samples. The mesoporous ZnCl2-OMA microparticles exhibit a non-uniform three-dimensional (3D) architecture ranging from cuboid-like to plate-like shaped morphologies resulting from irregular micelles aggregation during the EISA process. Thus, the observed microparticles have a non-uniform particle size distribution varying from 20 to 50 μm with gaps between crystals (Fig. 5a–d). However, a relatively smooth particles surface is obtained, suggesting the excellent incorporation of ZnCl2. In our previous work, the ZnCl2-OMA materials prepared using the conventional two-steps process have smaller microparticles with less regular shape and morphology, which was ascribed to the mechano-chemical dislocation effect occurred during the ZnCl2 impregnation step.32 Unlike the two-steps conventional approach, the newly designed one-pot process led to efficient and homogeneous incorporation of ZnCl2 particle in the OMA matrix. However, we believe that the presence of ZnCl2 in the surfactant micelle before thermal treatment is among the reasons that led to relatively larger particles (compared to two-steps process) where ZnCl2 is entirely embedded in OMA phase.


image file: c5ra12057h-f5.tif
Fig. 5 Representative SEM images (a–d) and energy dispersive X-ray (EDX) spectra (e–h) obtained for ZnCl2-modified OMA microparticles prepared using different aluminium precursors and carboxylic acids: (a and e) Al(OBus)3–oxalic acid; (b and f) Al(OPri)3–tartaric acid; (c and g) Al(OBut)3–fumaric acid and (d and h) Al(NO3)3·9H2O–citric acid. All samples were calcined at 400 °C.

Furthermore, as shown from previous analysis data (XRD, BET and TEM), the use of various carboxylic acids and aluminium precursors did not result in any significant morphology changes (Fig. 5a–d).

Elemental analysis: energy-dispersive X-ray spectroscopy (EDX). Details on the chemical composition of the ZnCl2-OMA materials are provided. Thus, the successful one-pot incorporation of ZnCl2 with mesoporous organized alumina is further evidenced by EDX analysis. As depicted in Fig. 5e–h displaying the EDX spectra obtained for the prepared ZnCl2-OMA samples, O and Al sharp signals were detected along with Zn and Cl signals which belong to Al2O3 and ZnCl2 species, respectively. These elements were detected for all the analyzed samples synthesized using various carboxylic acids and aluminium precursors. These results prove again the efficiency of the in situ incorporation of ZnCl2 into OMA. Additionally, the Au, Pd and C signals were detected (Fig. 5e–h), which is due to residuals of the Au/Pd and C films used for the specimens preparation for analysis. For all analyzed samples, the Zn/Cl ratios calculated from the EDX data higher than 1/2 (Table S1 in the ESI) that corresponds to the composition on ZnCl2, indicating the presence of further Zn–Cl species.
X-ray photoelectron spectroscopy (XPS) measurements. To precisely determine the elemental composition of the ZnCl2-OMA as well as the electronic state and environment of the detected elements (Al, O, Zn and Cl) and the bonding configurations of the related species, X-ray photoelectron spectroscopy (XPS) measurements were conducted. The XPS spectra obtained are displayed in Fig. 6 and were carefully studied. The survey spectrum (Fig. 6a) exhibits four main peaks centered at around 75 eV, 200 eV, 530 eV and 1020 eV, which are assigned to Al, Cl, O and Zn species, respectively. These elements are attributed to ZnCl2 and Al2O3 phase. Similar compositions were found in the analyzed samples prepared using different carboxylic acids and aluminium precursors without any significant effect of these experimental conditions (Fig. S2 in the ESI). The measured Al/Zn atomic ratios values from the XPS data are found to be around 8 (Table S2 in the ESI). These values are close to the desired and optimized nominal Al/Zn ratio of 8 for the synthesized ZnCl2-OMA samples. Moreover, in all analyzed samples, the calculated Cl[thin space (1/6-em)]:[thin space (1/6-em)]Zn ratios from the survey spectra obtained were always found to be lower than the nominal 2[thin space (1/6-em)]:[thin space (1/6-em)]1 atomic ratio for the incorporated ZnCl2, where higher Zn amounts were detected compared to Cl (Cl/Zn ∼ 0.61–0.68), similarly to the data obtained by EDX measurements (Table S1 in the ESI). These results suggest that in addition to ZnCl2 and Zn–Cl species, the samples contain further Zn-containing species like bridging Al–O–Zn–O–Al or Zn(OH)2 species. However, thermal decomposition of zinc chloride may partially occur at higher annealing temperatures.32 The deconvolution of each element spectrum is then necessary to investigate the exact species formed on the surface and in the framework of the prepared ZnCl2-OMA materials.
image file: c5ra12057h-f6.tif
Fig. 6 XPS analysis of ZnCl2-OMA sample synthesized using malonic acid with Al(OBus)3 aluminium precursor: (a) survey spectrum and (b) Al 2p, (c) O 1s, (d) Zn 2p3/2 and (e) Cl 2p3/2–Cl 2p1/2 deconvoluted spectra. All shifts for the samples were corrected by normalization of the C 1s binding energy to 285.0 eV.

The Al 2p spectrum displayed in Fig. 6b revealed the presence of one component, the peak at 74.4 eV corresponds to Al present in the Al2O3 phase.54,55 Subsequently, the O 1s spectrum (Fig. 6c) can be deconvoluted into two components. Two peaks with similar intensities are observed at 531.2 eV and 532.5 eV, which are attributed to bulk oxygen present in the Al2O3 lattice (Al–O–Al) and higher concentration hydroxyl groups (Al–O–H) present in the Al2O3 materials surface, respectively.55–58 Unlike the conventionally prepared ZnCl2-OMA materials,32 the absence of a third peak at higher BE indicates that no molecular H2O is physisorbed at this temperature.58 These binding energy values are slightly higher compared to those we reported for the pure alumina materials.32 This could be due to the presence of ZnCl2 and its interactions with the species present in the alumina phase, which leads to a slight change in the electronic environment of these elements. The hydroxyl groups peak at 532.5 eV may also correspond to the Zn(OH)2 species that results from ZnCl2 interaction with alumina surface.59 The O 1s XPS data obtained previously for the two steps-prepared ZnCl2-OMA materials showed a lower intensity (amount) of Al–O–H peak compared to that of Al–O–Al.32 On the other hand, herein, the two peak intensities are quite equal. This could be due to elimination on the second calcination step which helps avoiding further dehydroxylation of alumina surface.

The zinc species bonding configurations were also investigated. The Zn 2p3/2 spectrum (Fig. 6d) shows one component with a sharp peak at 1022.1 eV, which could be assigned to distinct Zn2+ species including Zn–O, ZnCl2 or Zn(OH)2.32,60,61 We then studied the Cl spectrum (Fig. 6e).

This spectrum can be deconvoluted into two components; a well-defined peak at 198.9 eV and a smaller peak at 200.5 eV correspond to Zn–Cl 2p3/2 and Zn–Cl 2p1/2 species, respectively.32,60,62 In addition to the possible Zn-containing species like bridging Al–O–Zn–O–Al or Zn(OH)2 species, the ZnCl2/Zn–Cl species are likely bonded to the alumina surface through interactions with surface hydroxyl groups. With the presence of Zn precursors in the reaction mixture before the formation of the alumina phase, we logically suspected the formation of other phases besides alumina. Zinc aluminate (ZnAl2O4) and zinc oxide (ZnO) are the most likely phases to be obtained under such similar conditions. Fortunately, the XPS measurements are clear evidence that no ZnAl2O4 phase was formed along with the ZnCl2-modified OMA phase. If ZnAl2O4 phase was present, the XPS analysis would show a single O 1s component associated with a peak at 531.4 eV, while Zn 2p3/2 and Zn 2p1/2 peaks would appear at 1044.9 eV and 1021.5–1021.9 eV, respectively.63,64 Similarly, no evidence of formation of zinc oxide phase (ZnO) was found, which is characterized by Zn 2p3/2 and Zn 2p1/2 peaks at 1021.5 eV and 1044.6 eV.64 It is worth to mention that even if zinc chloride thermal decomposition is possible, the resulted Zn is likely converted into the species cited above, while no trace of elemental Zn were found in the analyzed samples which can result in two Zn 2p3/2 and Zn 2p1/2 peaks at lower BE values; 1021.4 eV and 1044.5 eV, respectively, as reported previously.64

Solid-state NMR spectroscopy analysis. After performing several characterizations that evidenced the formation of hexagonally organized mesoporous alumina materials doped with zinc chloride species, further characterization must be conducted to investigate the nature of interactions and intimate contact occurring on the alumina–zinc chloride interface. Therefore, we performed 1H MAS NMR experiments to evaluate the zinc chloride bonding with alumina surface. Fig. 7 shows the 1H MAS NMR spectra of several ZnCl2-OMA samples in which three broad resonance peaks were displayed. The first peak at 1.8 ppm is attributed to the protons of the basic terminal hydroxyl groups present on the ZnCl2-OMA surface.65 The second peak at 3.9 ppm is assigned to the protons of the acidic bridging hydroxyl groups also present on the ZnCl2-OMA surface.65 Both basic and acidic groups present moderate strength.65,66 However, unlike the XPS measurement which showed no evidence of the presence of physisorbed water on the ZnCl2-OMA surface (O 1s spectrum, Fig. 6c), the 1H MAS NMR analysis revealed the presence of moisture trace on the analyzed samples. Therefore, the third peak at 4.2 ppm corresponds to the hydrogen-bonded water which is physisorbed on the ZnCl2-OMA surface.65,67 The use of various aluminium precursors and diverse carboxylic acids for the synthesis of the ZnCl2-OMA materials led to no significant effect on the surface composition as confirmed by 1H MAS NMR analysis (Fig. 7, see also Fig. S3–S5 in the ESI). Furthermore, unlike the two step impregnation of ZnCl2 that we have recently reported demonstrating a selective interaction of ZnCl2 with basic terminal hydroxyl groups leading to the formation of various Al–O–Zn–Cl species,32 the in situ incorporation of ZnCl2 did not lead to total neutralization of the basic terminal hydroxyl groups (at 1.8 ppm). These results do not exclude the usual preferential interaction of the ZnCl2 with basic terminal hydroxyl groups, which may lead to a loss in Brønsted acidity.29–32,67,68 However, this may suggest that the elimination of a second calcination step at 400 °C may enhance the concentration of the surface hydroxyl groups, as a result of avoiding further dehydroxylation of alumina surface under such high thermal treatment conditions. On the other hand, the elimination of this second calcination step may be the reason behind the detection of the physisorbed water trace in almost all analyzed samples (Fig. 7, see also Fig. S3–S5 in the ESI). Ultimately, combination of XPS and 1H MAS NMR data suggests that various Al–O–Zn–Cl/Al–O–Zn–O–Al surface species can be formed through interaction of ZnCl2/Zn–Cl/Zn components with either basic terminal hydroxyl groups, acidic bridging hydroxyl groups or both.
image file: c5ra12057h-f7.tif
Fig. 7 1H MAS NMR spectra of ZnCl2-OMA samples prepared using citric acid with different aluminium precursors. Indicating (a) basic terminal hydroxyl groups (b) acidic bridging hydroxyl groups, and (c) hydrogen bonded water physisorbed on alumina surface. All samples were calcined at 400 °C.

27Al MAS NMR experiments were conducted to investigate the Al environment and the changes related to the in situ ZnCl2 incorporation. Fig. 8 shows the various coordination geometries detected for Al species in the analyzed as-prepared (uncalcined) and calcined ZnCl2-OMA samples. A single sharp resonance peak was observed at around 0 ppm for the as-prepared sample (Fig. 8a). This suggests that the uncalcined sample mainly contains octahedral Al3+ species. This peak can be associated with the presence of the AlOOH boehmite intermediate phase,69,70 which will be then converted to alumina phase after annealing. Afterwards, the calcined ZnCl2-OMA samples were analyzed. The 27Al MAS NMR spectra displayed in Fig. 8b–e show a change of Al species coordination. We then observed the rise of one sharp and two broad resonance peaks (Fig. 8b–e). The major well-defined resonance peak centered at 3 ppm is attributed to the octahedral Al3+ species in the alumina phase (AlO6).70,71 Subsequently, the two broad minor resonance peaks centred at 32 and 63 ppm (Fig. 8b–e) correspond to the penta- and tetrahedral Al3+ species in the alumina phase (AlO5, AlO4), respectively.70 As reported recently in our work, this conversion of the octahedral Al3+ species upon calcination to pentahedral and tetrahedral Al3+ species is attributed to the partial substitution of oxygen ions by hydroxyl groups in octahedral Al centers during the calcination process.14,32,72 All 27Al MAS NMR spectra for the samples prepared using different carboxylic acids and diverse aluminium precursors show approximately the same profile regardless of the experimental conditions (Fig. 8, see also Fig. S6–S8 in the ESI).


image file: c5ra12057h-f8.tif
Fig. 8 27Al MAS NMR spectra of ZnCl2-OMA samples.

It is worth to mention that unlike alumina phase, the 27Al MAS NMR spectra of the ZnAl2O4 spinel obtained under similar thermal conditions (300–500 °C) exhibits a single peak around 0 ppm which is characteristic of Al3+ ions occupying AlO6 sites, as reported previously.52

In comparison, the ZnCl2-OMA samples prepared using the two step process resulted in AlO5 and AlO6 resonance peaks with relatively similar intensities.32 Herein, we observe that the AlO5 resonance peaks have lower intensities compared to those of the AlO6 resonance peaks (Fig. 8, see also Fig. S6–S8 in the ESI), indicating the lower number of AlO5 sites. This suggests that the in the one-pot synthesized ZnCl2-OMA materials the zinc chloride was preferably doped on these sites. This suggestion is in a good agreement with the earlier speculation about the distribution of the Al3+ species in the alumina matrix; where it has been proposed that AlO6 sites are found in the bulk of the alumina inorganic framework walls, while the AlO4 and AlO5 sites are likely on the surface.71 A similar phenomenon was recently observed for the Cu–Al2O3 system.73

One-pot formation of the ZnCl2-doped OMA

In our previous work we showed that the organized self-assembly of the alumina mesophase in governed by the complexation effect of the interfacial protectors (carboxylic acids).32 Briefly, the uniform aggregation of a liquid-crystal phase of surfactant micelles during the EISA process occurs upon complexation of bifunctional carboxylic acids (carboxylate groups) with Al cations on one side, as well as with the nonionic surfactant (F127) via van der Waals forces and hydrogen bonding on the other side (different carboxylate group). Subsequently, and based on the characterizations data, particularly XPS and MAS NMR, herein we propose a similar mechanism for the formation of well-dispersed zinc species within the OMA channels via a one-pot process. The HCl is required to adjust the solution pH; however the strong interaction of Cl with Al sites can lead the organized assembly to collapse during the sol–gel process. Thus, carboxylic acids play a major role to prevent this phenomenon by interaction of their carboxylate fragments with the Al centers during the self-assembly process though monodentate or bridging bidentate modes. Meanwhile, the added zinc precursor migrates insides the inner hydrophobic core of the F127 surfactant micelles during the evaporation-induced self-assembly process.74,75 Therefore, the removal of the organic molecules via calcination at high temperatures lead to simultaneous mesoporous channels formation and grafting of zinc species on formed alumina surface hydroxyl groups as demonstrated by 1H MAS NMR analysis, while maintaining the well-ordered internal mesopore architecture. This tentative formation process is illustrated in Scheme 1.
image file: c5ra12057h-s1.tif
Scheme 1 Schematic representation of the one-pot evaporation induced self-assembly process for the formation of ZnCl2 embedded in well-organized mesoporous alumina (OMA) materials.

Catalytic performance of the ZnCl2-OMA-based catalysts

The newly synthesized 3 wt% MTO/ZnCl2-OMA catalysts were evaluated for their catalytic metathesis activity. The self-metathesis reaction was carried out as a model reaction, one of the representative Re-catalyzed reactions (Scheme 2). Also, we have chosen methyl oleate as the model substrates to verify both; the catalyst tolerance for functionalized molecules as well as the mesoporous network for bulky molecules diffusion abilities (Scheme 2). The experiments were carried out under similar conditions of temperature (45 °C) and pressure (atmospheric pressure) during 90 min of reaction time, to those we previously have optimized for the same class of catalyst and reaction.30–32
image file: c5ra12057h-s2.tif
Scheme 2 Methyl oleate self-metathesis observed products.

In order to highlight the advantages of the newly prepared ZnCl2-OMA materials as catalytic supports for MTO, here we present an extended comparison with other alumina-based catalytic support those we have previously synthesized and used for the design of the MTO metathesis catalyst (Table 2). Several one-pot prepared supports were used and numerous experiments were performed for each supports in ordered to investigate their performance and verify the reproducibility of the process. The data reported in Table 2 (Entry 1, 3, 5 and 7) are average values for several metathesis experiments conducted for similar catalysts supported over ZnCl2-OMA supports prepared with the same aluminium precursor and carboxylic acid.

Table 2 Catalytic performance for methyl oleate self-metathesis over heterogeneous 3 wt% MTO-based catalysts supported on organized mesoporous alumina (ZnCl2-OMA) prepared via two-steps or one-pot process. And comparison with metathesis performance of 3 wt% MTO/ZnCl2meso-Al2O3 (wormhole-like), MTO/meso-Al2O3 (wormhole-like) and commercial homogeneous 2nd generation Grubbs catalyst
Entry Catalysta Conv.b (%) Metathesis productsc (%) DMPd (%) Se Ref.
(1) trans (1) cis (2) trans (2) cis (3)
a The investigated 3 wt% MTO/ZnCl2-OMA catalysts supports were prepared using different aluminium precursors and carboxylic acids, while meso-Al2O3 represents the wormhole-like mesoporous alumina.b Conv.: conversion, expressed as Conv. (%) = ([methyl oleate]0 − [methyl oleate]t)/[methyl oleate]0 × 100.c The numbers appearing for the metathesis products 1, 2, and 3 (Scheme 1) are expressed in terms of individual product yield (yieldi), defined as: yieldi (%) = [producti]/([methyl oleate]0 − [methyl oleate]t) × 100, where [producti] is the molar concentration of the product i = 1 to 3, and [methyl oleate]0 and [methyl oleate]t are the molar concentrations of methyl oleate initially and at a given time t, respectively.d DMP: desired metathesis products (1 + 2) yield, which is the summation of the individuals yields of 1 and 2.e The selectivity (S) is defined as the yield of all desired products over the yields of all undesired products, which represents; S = yield (1 and 2)/yield of (3). op: ZnCl2-OMA supports prepared through one-pot process. ts: ZnCl2-OMA supports prepared through two-steps process.
1 3 wt% MTO/ZnCl2-OMA-1 (Al(OBus)3 + citric, op) 80.2 13.7 12.7 14.9 6.9 32.0 48.2 1.51 This work
2 3 wt% MTO/ZnCl2-OMA-1 (Al(OBus)3 + citric, ts) 78.2 15.5 14.3 9.1 4.3 35.0 43.2 1.23 Abidli et al.28
3 3 wt% MTO/ZnCl2-OMA-2 (Al(OBus)3 + tartaric, op) 93.6 48.8 17.1 5.4 2.4 19.9 73.7 3.70 This work
4 3 wt% MTO/ZnCl2-OMA-2 (Al(OBus)3 + tartaric, ts) 88.2 29.3 8.5 12.9 4.6 32.9 55.3 1.68 Abidli et al.28
5 3 wt% MTO/ZnCl2-OMA-3 (Al(OPri)3 + tartaric, op) 86.7 34.8 9.4 9.1 2.5 30.9 55.8 1.81 This work
6 3 wt% MTO/ZnCl2-OMA-3 (Al(OPri)3 + tartaric, ts) 87.4 31.8 8.9 14.1 2.4 30.2 57.2 1.89 Abidli et al.28
7 3 wt% MTO/ZnCl2-OMA-4 (Al(OBut)3 + tartaric, op) 79.8 14.1 13.2 13.6 6.4 32.5 47.3 1.46 This work
8 3 wt% MTO/ZnCl2meso-Al2O3 (ts) 86.7 24.4 8.3 16.2 3.6 34.3 52.4 1.53 Pillai et al.25–27
9 3 wt% MTO/meso-Al2O3 2.4 0.2 0.3 0.2 0.3 1.4 1.0 0.71 Pillai et al.25–27
10 2nd generation Grubbs catalyst 89.6 20.1 3.9 21.0 4.2 40.4 49.2 1.22 Pillai et al.25–27


The other alumina supports used for comparison include the zinc chloride-promoted (Table 2, Entry 8) and unpromoted (Table 2, Entry 9) conventional wormhole-like alumina that we have reported earlier;29–31 as well as the well-ordered hexagonal ZnCl2-OMA prepared via two steps process that we have also recently reported (Table 2, Entry 2, 4 and 6).32 In addition, the newly one-pot prepared ZnCl2-OMA-based heterogeneous metathesis catalyst performance (Table 2, Entry 1, 3, 5 and 7) has been compared to that of the homogeneous commercially available and widely used 2nd generation Grubbs catalyst (Table 2, Entry 10).

Briefly, the addition of zinc chloride to alumina supports was found to offer a great catalytic promoting effect for the metathesis reaction.29–31 The unmodified wormhole-like alumina supports showed the weakest metathesis performance among all tested catalysts, while their promoted counterparts showed much more enhanced catalytic performance (Table 2, Entry 8).29–31 Afterwards, we recently showed that the zinc chloride-promoted hexagonally organized mesoporous alumina (ZnCl2-OMA) materials are the most suitable support of MTO catalysts showing relatively better methyl oleate conversions.32 In all cases methyl oleate self-metathesis leads to the formation of both desired metathesis products including 9-octadecene and dimethyl-9-octadecene-1,18-dioate (Scheme 2) as well as undesired product (methyl elaidate, Scheme 2). However, using the ordered alumina supports, improved selectivity was reached towards the desired metathesis products (Table 2, Entry 2, 4 and 6). More interestingly, we demonstrated that the new catalyst design based on organized mesoporous network has an enormous effect on improving the reaction rate for the conversion of such bulky molecule (methyl oleate).32 This enhancement is attributed to the absence of diffusion limitations of substrates and metathesis products within the ordered cylindrical mesopores of these supports. Compared to the disordered and interconnected pores of the wormhole-like mesoporous alumina supports where molecular diffusion limitations and blocking effects may occurs. All these features were also provided using similar one-pot synthesized ZnCl2-OMA supports, which offered also better activity than the MTO-based catalysts supported over zinc chloride-modified and unmodified wormhole-like alumina (Table 2).

The catalytic performance of the successfully prepared ZnCl2-OMA supports via a one-pot synthesis process studied in this work was also compared to the performance of the ZnCl2-OMA-based catalysts previously prepared through two steps process. Interestingly the one-pot prepared ZnCl2-OMA materials were found to be the best catalytic support for MTO. Metathesis reaction data displayed in Table 2 (Entry 1, 3, 5 and 7) shows higher catalytic performance including; methyl oleate conversion (up to 93.6%), desired metathesis products yield (up to 73.7%) and selectivity (up to 3.7), using these catalytic supports. For the structure-activity correlation, we believe that this enhanced performance may arise from various factors.

Firstly, the newly one pot-prepared ZnCl2-OMA supports exhibits larger BET surface area (Table 1) compared to the two steps-prepared ZnCl2-OMA supports. The lower BET surface area of these later is due to the subsequent calcination step and pore filling with zinc chloride upon post-modification, which alters their textural properties. The BET specific surface area was dramatically dropped upon incorporation of zinc chloride.32 On the other hand, during the one pot synthesis of ZnCl2-OMA supports this additional annealing step was avoided with in situ impregnation of zinc chloride. This is in a good agreement with several studies showing that supported catalysts' activity is often proportional to the specific surface area of these catalysts.76–83 Furthermore, higher BET surface area favors better dispersion of supported MTO species, which improves the concentration and availability of metathesis active species and intermediates allowing more efficient use of exposed supported MTO molecules. Secondly, unlike the two steps synthesis process, avoiding a second calcination step during the one-pot preparation process will reduce the dehydroxylation of the alumina surface as illustrated above in XPS and MAS NMR sections. Along with this, we showed that zinc chloride is incorporated on the alumina via interactions with surface hydroxyl groups. As a result, inhibited dehydroxylation may allow better incorporation of Zn active intermediates which are crucial for the MTO metathesis catalytic action, displaying a key synergetic effect. Finally, we believe that the single-step approach allow simultaneous zinc species deposition and mesopores walls formation, releasing more homogeneous and well-dispersed zinc species on the OMA surface. Moreover, this simultaneous process may result in a confinement effect of ordered mesochannels of OMA inhibiting Zn species aggregation and growth. This agglomeration phenomenon may likely occur during the two steps synthesis which restricts Zn species interactions with alumina surface and limits their availability for the catalytic process. It is also worth to mention that the separate wet impregnation method during the two-steps synthesis was found to alter the textural and morphological properties of the OMA (mechano-chemical effect), along with formation of zinc chloride aggregates, as revealed by SEM images.32

These results are very exciting showing higher performance, even better than those obtained with the traditional highly active 2nd generation Grubbs catalyst (Table 2, Entry 10), with slightly better methyl oleate conversion (93.6% vs. 89.6%), but exhibiting much more selectivity towards desired metathesis products (73.7% vs. 49.2%). It is worth to mention that we previously showed that the OMA-supported MTO catalyst was found to exhibit fast kinetics,32 comparable to that observed over the Grubbs catalyst,31 with the possibility of recycling and reuse for further metathesis runs.

In addition to the enhanced catalytic activity of the MTO-based metathesis catalysts using the newly prepared ZnCl2-OMA materials, this one-pot synthesis process presents further advantages. Economically, compared to the two-steps process, this process allowed a reduced use of solvents. Also, it reduces energy consumption through elimination of additional evaporation and calcination steps, while reducing enormously the preparation time. More interestingly, using the one-pot process we reached higher synthesis yields, obtaining higher amounts of ZnCl2-OMA materials, almost 75% higher in weight than those obtained using the two steps process. This was mainly attributed to limitation of weight loss during the two subsequent calcination processes.

Conclusions

In summary, we have developed a novel straightforward one-pot sol–gel synthesis approach for zinc-doped well-organized mesoporous alumina preparation. In situ zinc chloride incorporation was successfully achieved along with the formation of ordered alumina mesoporous network. Based on XPS data, zinc chloride was impregnated via interaction with surface hydroxyl groups. Furthermore, the XPS measurements suggested that Al–O–Zn–Cl may be the major surface species formed upon these interactions. Moreover, in order to establish an efficient low-cost production process, this rapid and sustainable methodology was found to be suitable even when using inexpensive aluminium precursors such as aluminium nitrate nonahydrate. The use of various aluminium precursors and carboxylic acid did not result in any significant difference between the prepared materials due to the similar self-assembly pathway during the sol–gel process. Furthermore, the materials prepared via the one-pot route exhibited higher BET surface area compared to the conventionally two steps-synthesized materials. Outstanding arrangement of the mesopores was observed by TEM micrographs. Not surprisingly, these materials with such enhanced features exhibited better catalytic performance for methyl oleate self-metathesis when used as support for the rhenium-based catalyst. Avoiding the second calcination step, which is required during the conventional synthesis, led to a limited dehydroxylation phenomena. Thus, materials with enhanced hydroxyl group concentration and availability on the alumina surface were obtained, which was a key factor for an effective grafting of zinc chloride and methyltrioxorhenium species. Therefore, a better catalytic performance was reached. This efficient synthesis may open-up further catalytic applications of these well ordered supports. On the other hand, rather than modified-alumina, the successful one-pot strategy could be extended and then stimulate further attempts to prepare other functionalized materials that are conventionally prepared through laborious multistep processes. Especially, doped or modified functionalized mesoporous materials that are widely used in catalysis. This successful methodology is a step forward towards an easy and efficient scalability.

Acknowledgements

Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and Canadian Foundation for Innovation (CFI) is greatly appreciated. A. Abidli would like to thank Prof. Khaled Belkacemi for the N2 adsorption–desorption analysis and for providing his laboratory facility at Laval University where the materials/catalysts synthesis and the metathesis experiments were conducted. A. Abidli acknowledges Alain Adnot, Pierre Audet, Prof. Safia Hamoudi, Ronan Corcuff, André Ferland and Richard Janvier for the XPS, MAS NMR, XRD, GC-MS, SEM and TEM analyses, respectively. A great appreciation is dedicated to Dr K. T. Venkateswara Rao for his valuable suggestions and advices.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12057h

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