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Journal of Coordination Chemistry

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Atrane complexes chemistry as a tool forobtaining trimodal UVM-7-like porous silica

M. Dolores Garrido, Carolina García-Llacer, Jamal El Haskouri, María D.Marcos, Juan F. Sánchez-Royo, Aurelio Beltrán & Pedro Amorós

To cite this article: M. Dolores Garrido, Carolina García-Llacer, Jamal El Haskouri, María D.Marcos, Juan F. Sánchez-Royo, Aurelio Beltrán & Pedro Amorós (2018): Atrane complexeschemistry as a tool for obtaining trimodal UVM-7-like porous silica, Journal of CoordinationChemistry, DOI: 10.1080/00958972.2018.1442002

To link to this article: https://doi.org/10.1080/00958972.2018.1442002

Published online: 25 Feb 2018.

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Journal of Coordination Chemistry, 2018https://doi.org/10.1080/00958972.2018.1442002

Atrane complexes chemistry as a tool for obtaining trimodal UVM-7-like porous silica

M. Dolores Garridoa, Carolina García-Llacera, Jamal El Haskouria, María D. Marcosb,c, Juan F. Sánchez-Royoa, Aurelio Beltrána and Pedro Amorósa

ainstitut de Ciencia dels materials (iCmuV), universitat de València, Valencia, spain; bCentro de reconocimiento molecular y desarrollo tecnológico (idm), unidad mixta universitat Politècnica de València, universitat de València, departamento de Química, universitat Politècnica de València, Valencia, spain; cCiBer de Bioingeniería, Biomateriales y nanomedicina (CiBer-BBn), Valencia, spain

ABSTRACTThe use of atrane complexes as hydrolytic precursors enables the homogeneous incorporation of manganese (25  ≤  Si/Mn  ≤  48) throughout the porous walls of the nanoparticles of a surfactant-templated bimodal mesoporous silica (UVM-7). The subsequent leaching of the manganese nanodomains allows adding controlled microporosity to the host silica framework. The resulting final silica material presents three pore systems structured at different length scales: interparticle textural-type macroporosity (ca. 43.2 nm), ordered intraparticle mesoporosity (ca. 2.63  nm; after template removal), and well-dispersed microporosity (< 2  nm; as consequence of the lixiviation of the Mn-rich domains). The good dispersion of the guest element (Mn) in the silica intermediate provided by the atrane route is responsible for the disordered but regular microporosity achieved.

1. Introduction

Interest in the chemistry of atrane-type complexes is far from having fallen [1–4]. The current understanding of this stimulating and intricate field of coordination chemistry is largely consequence of the pioneering results of the scientific schools of M.G. Voronkov and J.G.

© 2018 informa uK limited, trading as taylor & francis Group

KEYWORDSatrane complexes; silica; mesoporous; microporous; etching

ARTICLE HISTORYreceived 3 november 2017 accepted 18 January 2018

CONTACT Pedro amorós pedro.amoros@uv.es

2 M. D. GARRIDO ET AL.

Verkade [5–8]. While the diversity of new atranes (and related complexes) grew continuously, our research group reported in 2000, a generalized overview on how atranes could be con-sidered as “ideal hydrolytic precursors” for synthesizing porous oxides (pure or mixed) by one-pot surfactant-assisted procedures [9]. Such a preparative strategy, the so-called “atrane route” [10], was later refined [11] and applied for obtaining a large diversity of materials [12–15].

In practice, as with other alkoxides, atranes (broadly speaking, complexes including trieth-anolamine-derived ligands; strictly, complexes in which triethanolamine acts as a tripod ligand whose coordination implies an intramolecular N → M bond) are very useful clean oxide precursors. In any case, the most outstanding feature of the atranes, with respect to this point, is that, without prejudice from their inherent thermodynamic instability, they are chemically inert towards hydrolysis. Thus, once obtained in the absence of water, the atrane species may generate aqueous solutions that remain unchanged for long periods [9, 16]. In the surfactant-assisted synthesis of porous oxides, in order to achieve intermediate meso-structured frameworks, it is first necessary to reach an adequate kinetic coupling between the hydrolytic reactions of the inorganic precursors and the subsequent self-as-sembling processes. While attaining this result is not too complicated when managing a single element (as Si) [11], the situation changes when simultaneously dealing with various oxide-forming elements (to grow mixed or doped oxides). In fact, the co-hydrolysis approach is plagued with problems related to the inherently different reactivity of the inorganic pre-cursors, frequently leading to phase segregation [10]. In order to achieve an effective dis-persion of the heteroelements throughout the oxidic framework, it becomes also necessary to harmonize the rates of the hydrolytic processes implying the different inorganic precur-sors. As we have verified, the combination of instability and inertness provided by the very nature of the atrane-like complexes is a powerful tool for adequately matching their hydro-lytic reactivities [9].

The UVM-7 silicas are, very likely, the most versatile materials that we have prepared on the basis of the “atrane route.” The most outstanding feature of these nano-structured bimodal silica (NBS) materials is their very open architecture. This is based on a continuous network constructed from covalently bonded mesoporous nanoparticles, the aggregation of which defines a secondary system of large pores [11, 17]. In fact, the search for materials including hierarchic pore systems was approached as an alternative for reducing the mass diffusion constraints that might occur (pore-blocking) through the unimodal long-mesopore systems in the typical MCM-41 silicas [18, 19]. The decrease of the particle size at the nano-metric range is one way to do it: this necessarily implies shortening of the maximum mes-opore length, which results in a first advantage for enhancing the site accessibility [11].

Inorganic functionalization of the UVM-7 silicas is feasible by introducing some procedural modifications. In any case, it is logical to expect that the very nature in which different het-eroelements are incorporated in the UVM-7 silica walls will depend on the intrinsic reactivity of the specific element, which must be related to, at least, their oxidation sates, sizes, and preferred environments. Very likely, factors like these will influence both the amount of the heteroelement able to enter the silica network and the way (isomorphic substitution, small oligomers, oxidic domains of variable size) it does it. If so, the subsequent elimination of the heteroelement might alter in very different degrees the proper silica network. Here, we explore how Mn elimination from Mn-UVM-7 modifies the pore array in the host support to yield new trimodal porous silicas.

JOURNAL OF COORDINATION CHEMISTRY 3

2. Experimental

2.1. Chemicals

All the synthetic reagents are analytically pure and were used as received from Aldrich (≥ 98.0% tetraethyl-orthosilicate [TEOS], ≥ 99.0% triethanolamine [N(CH2–CH2–OH)3, hereinafter TEAH3], ≥ 98% cetyl-trimethylammonium bromide [CTMABr], HCl (35% wt), and MnCl2).

2.2. Synthesis of trimodal porous UVM-7-like silica

The first preparative stage is the isolation of meso-structured silicas containing variable amounts of Mn (Mn-UVM-7). The procedure for doing so is a modification of the “atrane route,” which has been described in detail elsewhere [11, 20]. In a typical synthesis leading to the Si/Mn = 100 (molar nominal composition) solid, a mixture of TEOS (11 mL), MnCl2 (0.10 g), and TEAH3 (23 mL) was heated at 140 °C under stirring for 10 min. The resulting homogeneous brown solution was cooled to 120 °C, and 4.5 g CTABr were added. Then, 80 mL of water were slowly added with vigorous stirring at 80 °C. After a few minutes, a suspension began to be observed. This was aged at room temperature for 24 h. The resulting meso-structured whitish solid was then separated by centrifugation, washed with water and ethanol, and air dried. The analogous Si/Mn = 50 solid was prepared in the same way, that is to say, in all cases, the reagents molar ratio in the starting solution was adjusted to (2-x) Si: x Mn: 7 TEAH3: 0.52 CTABr: 180 H2O, where x = 0.02 (Si/Mn = 100), 0.04 (Si/Mn = 50).

Once the meso-structured Mn-UVM-7 solids were isolated, the next preparative stage served a dual purpose: to eliminate the surfactant (thus generating intraparticle mesopores) and also proceed to the leaching of the zones rich in Mn. With this aim, in both cases, a suspension of 10 g of the meso-structured silica was refluxed under stirring in a mixture of absolute ethanol (1 L) and concentrated (35% wt) hydrochloric acid (75 mL). After two hours, the suspension was left at room temperature, and the resulting solid was then collected by filtration. The process was repeated again with a reflux time of 16 h. The resulting white solids were separated by centrifugation, washed with water and ethanol, and air dried. The final products (exempt from surfactant and with extremely low Mn contents, Si/M > 200) were labeled as micro-Mn-UVM-7.

For comparative purposes, aliquots of the Mn-UVM-7 meso-structured intermediate solids were calcined at 500 °C for 4 h under static air. In this case, while the surfactant is thermally removed, Mn remains in the silica walls. The resulting brown solids have been labeled as Mn-UVM-7(c). Summarized in Table 1 are the main synthesis variables and physical data corresponding to the isolated porous materials.

2.3. Characterization techniques

All solids were analyzed for Mn by energy dispersive X-ray analysis (EDX) using an S-4800 (HITACHI) instrument. Si/Mn molar ratio values were averaged from EDX data corresponding to ca. 50 different particles. X-ray powder diffraction (XRD) data at low angles were recorded on a Seifert 3000TT θ–θ using CuKα radiation. Patterns were collected in steps of 0.02° (2θ) over the angular range 1–10° (2θ) for 25 s per step. To detect the presence of some crystalline bulk phase, additional patterns were collected with a bigger scanning step [0.1 (2θ)] over a wide angular range (10–70° (2θ)) and a lower acquisition time (10 s/step). An electron

4 M. D. GARRIDO ET AL.

microscopy study (TEM) was carried out with a JEOL JEM-1010 instrument operating at 100 kV. HRTEM images were recorded using a JEOL-2100F microscope operating at 200 kV. Samples were gently ground in dodecane, and grains were deposited on a holey carbon film supported on a Cu grid. Surface area, pore size and volume values were calculated from nitrogen adsorption–desorption isotherms (−196 °C) recorded on a Micromeritics ASAP-2020 automated analyzer. A fast atomic bombardment (FAB) coupled to Mass Spectrometry (MS) analysis of the precursor solutions was performed to investigate the role played by TEAH3. X-ray photoelectron spectra (XPS) were recorded with an Omicron spectrometer equipped with an EA-125 hemispherical electron multichannel analyzer and an unmonochromated Mg Kα X-ray source having radiation energy of 1253.6 eV.

3. Results and discussion

3.1. Synthesis strategy

As mentioned above, alkoxides (like TEOS) are very useful clean oxide precursors. Notwithstanding, as occurs with Mn, commercial (and/or low cost) alkoxides are not always available with this aim. The usual in these cases is to use different salts as reagents, which, because of kinetic factors, frequently results in phase segregation and/or material hetero-geneity phenomena. Our preparative strategy to overcome these problems (the “atrane route”) lies on using mixtures of atrane-type complexes as hydrolytic precursors. As a rather general rule, in surfactant-assisted syntheses, we have observed that the relative inertness of the atrane species helps to orchestrate the hydrolytic rates involving different inorganic moieties. This, in turn, facilitates the harmonization of the subsequent self-assembling pro-cesses between the inorganic oligomers and the surfactant micelles to give the hybrid com-posites from which the mesostructure grows. While ensuring a good dispersion of the heteroelements in the intermediate framework, surfactant removal usually leads to (porous) mixed oxides showing a high chemical homogeneity. In the case we are dealing with, such homogeneity/dispersion of the Mn-rich nanodomains along the silica walls would constitute a key for achieving a homogeneous generation of micropores after leaching. In this sense, the Mn-oxidic domains might be branded as sacrificial inorganic templates for formation of micropores.

From long time ago, the literature includes references to Mn-containing triethanolamine derivatives [21–23]. However, in our opinion, the reports on this matter cannot be interpreted

Table 1. selected preparative chemical parameters and physical data.

asi/mn nominal values.bsi/mn in the solids before leaching.cfinal si/mn values after leaching.dsurface area values estimated using the Bet model.ePore size and volume values determined through the use of the BJh model.

  Mesopore Macropore

Sample Si/Ma Si/Mb Si/McArea

(m2 g−1)dSize

(nm)eVolume

(cm3 g−1)eSize

(nm)eVolume

(cm3 g−1)e

1 uVm-7 ∞ ∞ ∞ 1045 2.94 0.99 46.8 1.652 micro-mn-uVm-7 100 48 ± 4 263 ± 15 1014 2.59 0.84 39.1 0.953 micro-mn-uVm-7 50 25 ± 2 210 ± 45 1003 2.68 0.81 47.2 0.514 mn-uVm-7(c) 100 43 ± 7 43 ± 7 1034 2.86 0.99 53.2 0.56

JOURNAL OF COORDINATION CHEMISTRY 5

unequivocally concerning the ultimate nature of these atrane-type species [21–25]. To ana-lyze in more detail the Mn-TEAH3 interactions, we have followed, by means of mass-spectral analysis (FAB-MS), the formation and stabilization processes of the corresponding atrane-type species. These FAB-MS analyses have been carried out working with separated solutions containing the Si and Mn reagents in molar concentrations identical to those used in the preparation of the porous materials described in this work: 2 M: 7 TEAH3: y H2O (y = 0, 180). In the case of silatranes, we have previously demonstrated the presence of Si(TEA)(TEAH2) as majority species (where TEA stands for the fully deprotonated ligand) before and after water addition (together with other polynuclear complexes, such as Si2(TEA)3H or Si3(TEA)4, under anhydrous conditions) [9, 26]. Here, we have performed a similar analysis to identify the character of the Mn-atrane complexes. The principal complexes detected in this way are gathered in Table 2. Prior to water addition, the analysis shows the presence of very different entities, from mono- to tri-nuclear species, even incorporating chlorides (from the MnCl2 used as Mn source). After water addition, the mononuclear Mn/TEA = 1/1 species is clearly the dominant one. Once more, the inertness towards hydrolysis of the atrane species is confirmed by this analysis.

In accord with the above, insofar as the mother liquors we have used in our syntheses contain a significant excess of TEAH3 (with regard to the amount necessary for coordinating both Si and Mn), it is reasonable to assume that the precursor solutions mainly consist of random intimate mixtures of independent Mn and Si molecular atrane complexes. If so, it is not surprising that such a similar environment for both elements (Si and Mn), in light of the resulting inertness, will restrict their differences in reactivity. In other words, the role of TEAH3 is to harmonize the hydrolytic reactions which generate the inorganic entities involved in the subsequent self-assembling processes with the surfactant. In this way, the “atrane route” also could allow modulation of the heteroelement content in the resulting mixed oxide.

As shown below, the final meso-structured whitish solids contain silica-based porous walls with well-dispersed Mn-rich zones (as small oligomers or oxide nanodomains). Obviously, included into the mesostructure are the templating CTMA+ moieties. When these are pyrolytically removed, the Mn-rich domains consolidate in the pore walls (Mn-UVM-7(c) brown solids). In contrast, when the surfactant is removed by acidic (HCl) attack (via chemical CTMA+↔H+ exchange), a practically complete leaching of Mn occurs (micro-Mn-UVM-7 white solids). In both cases, the surfactant removal by itself leads, as expected, to a mesopore system (acting as sacrifice supramolecular template). However, the chemical etching of the transition metal domains generates additional microporosity (as noted above, in some sense, these nanodomains are sacrificial inorganic templates able to form, after evolution, new pores).

Table 2. majority species detected by faB and relative intensities without considering the peak associ-ated to the dissolvent (teah3

+ (m/e = 150)) for mixtures with 2 mn: 7 teah3: y h2o (y = 0, 180) molar ratios. in this table, the tea acronym corresponds to the fully deprotonated ligand.

m/e Species Relative intensity % (y = 0) Relative intensity % (y = 180)203 mn(tea)h2

+ 100 100239 mn(tea)h3Cl+ 28 12352 mn(tea)2h5

+ 43 35405 mn2(tea)2h3

+ 50 –607 mn3(tea)3h4

+ 25 –

6 M. D. GARRIDO ET AL.

The pale color (whitish) of the meso-structured solids suggests that manganese is incor-porated in the silica walls as Mn(II) species (although the presence of atoms with other oxidation states cannot be fully discarded according to preliminary XPS results). Under the strongly acidic conditions used during the chemical exchange, the solubility of the oxidic silicon and manganese species is very different. While silica remains practically insoluble (pH close to the zpc value of silica [27]), the Mn-rich domains are easily solubilized [28]. Very likely, in this leaching process (mainly driven by the pH conditions), a certain cooperative effect due to the formation of chloro-complexes cannot be discarded.

Pore formation using chemical etching strategies is a well-known and established meth-odology [29, 30]. Desilication or dealumination are usual techniques for generating porosity (i.e. mesoporosity in microporous zeolites) via selective chemical attack to Si or Al sites (and subsequent solubilization), respectively. In general, chemical etching methods have been used for pore formation in a large variety of solids, as for example, silicon wafers. However, until now, the common feature of all the described procedures is their application for pro-ducing large pores (in the range of meso or macropores). In contrast, in our case, we are implementing microporosity in a previously bimodal mesoporous (siliceous) matrix. Here, the formation of new small micropores takes advantage of two factors: the good dispersion of the heteroelement (guaranteed by the “atrane route”) and the selection of species with very different hydrolytic profiles under the experimental pH conditions used.

3.2. Materials characterization

We have used EDX to assess the stoichiometry and chemical homogeneity of the solids, given that a good dispersion of Mn in the silica walls of the intermediate mesostructure is an essential requirement of our work. All samples were analyzed before and after acidic attack in HCl/ethanol medium. Summarized in Table 1 are the Si/Mn molar ratio values averaged from data corresponding to ca. 50 different particles. In all cases, the small standard deviation estimated is consistent with the chemical homogeneity of the solids at the scale spot area (ca. 1 μm). This supports that Mn and Si should be regularly distributed throughout the inor-ganic walls (i.e. the “atrane route” actually results in a good dispersion of the heteroelement). The real Mn content after the first preparative step (whitish Mn-UVM-7 meso-structured samples) is approximately twice the nominal one (in the mother liquor). This enrichment of the net in Mn must be viewed as consequence of the relatively high solubility of silica (when compared to that of Mn-oxidic species) at the moderately basic conditions (pH ca. 9) provided by the TEAH3 solution. After chemical etching (micro-Mn-UVM-7), the Mn content abruptly drops, as anticipated by the disappearance of any appraisable indication of color (the samples become practically white). Indeed, in all cases, the final Si/Mn molar ratios become higher than 200, which allow us to consider our final materials as virtually pure silicas.

The preservation of the UVM-7 typical mesostructure is confirmed by XRD and TEM. Shown in Figure 1 are the XRD patterns, at low- and high-angle (2θ) values, corresponding to the micro-Mn-UVM-7 silicas and the pure UVM-7 material, for comparative purposes. All patterns display a single peak and a wide low-intensity signal that can be assigned to the (100) and the overlapped (110) and (200) peaks, respectively, assuming a MCM-41-related hexagonal cell [31]. These features are typical of partially disordered hexagonal arrays of mesopores (generated by the elimination of the templating surfactant micelles). This confirms that the original mesopore system is preserved in the micro-Mn-UVM-7 materials. On the other hand,

JOURNAL OF COORDINATION CHEMISTRY 7

the absence of XRD peaks in the high-angles zone is consistent with the amorphous character of the porous walls. This (XRD) absence of ordered domains occurs also in the Mn-UVM-7 intermediate meso-structured solids. It seems therefore reasonable to assume the disordered nature and/or the small (nanometric) size of these inorganic sacrificial templates. Representative TEM images of selected samples are gathered in Figure 2. TEM micrographs fully correlate to XRD observations, thus ratifying that the hierarchic bimodal porosity char-acteristic of the pure UVM-7 silica is preserved in the micro-Mn-UVM-7 samples. All samples display a morphology based on the existence of clusters of nanoparticles. The primary nan-oparticles, in the 30–60 nm size range, show a typical partially ordered distribution of white spots (mesopores), whose organization level is in accord with the XRD data. Together with these intraparticle mesopores, TEM images show also the presence of textural interparticle macroporosity, which corresponds to the voids that remain among the primary nanoparticles when they aggregate. This large pore system, similar to that observed in silica xerogels has, as expected, a completely disordered character. On the other hand, HRTEM images allow discarding the presence of crystalline nanodomains in the Mn-UVM-7 meso-structured phases (Figure 3). As commented below, the small size of the generated micropores (< 2 nm) in the micro-Mn-UVM-7 solids (and its disordered distribution) makes them undetectable at the magnification level we have achieved.

The first direct evidence of the presence of micropores in the micro-Mn-UVM-7 solids is provided by the N2 adsorption-desorption curves. Indeed, these curves clearly display the two adsorption steps characteristic of the UVM-7 solids, at medium (0.2 < P/P0 < 0.4) and high (P/P0 > 0.8) partial pressure values (Figure 4). These correspond to capillary nitrogen condensation inside the intraparticle mesopores and the interparticle macropores, respec-tively. Included in Table 1 are the BET areas and the pore sizes and volumes estimated for these two pore systems using the BJH model. We can additionally observe a marked differ-ence (at low partial pressure values) among the isotherms of the UVM-7 and micro-Mn-UVM-7 materials (inset in Figure 4). The curves corresponding to micro-Mn-UVM-7 samples exhibit an additional abrupt adsorption step at P/P0 < 0.1, and this new effect is more impor-tant as the Mn content in the intermediate Mn-UVM-7 phase increases.

In order to determine the differences in microporosity among the synthesized solids, we have applied the DFT model to the adsorption data in the low-pressure range (Figure 5). The cumulative pore volume for micropores (size < 2 nm) in the UVM-7 and Mn-UVM-7(c) samples

Figure 1. low- (a) and high-angle (b) Xrd patterns of samples 1–4.

8 M. D. GARRIDO ET AL.

is similar (ca. 0.05 cm3 g−1), and only a small and single peak in both differential pore volume curves is observed at pore sizes around 1.66 nm. So, when the included heteroelement remains in the final silica (as occurs for the calcined samples), there is no change in the very low original microporosity of the UVM-7 silica. On the contrary, in the case of the

Figure 3. representative hrtem images of sample 3 at relatively low (a) and high (b) magnification.

Figure 4. n2 adsorption-desorption isotherms of samples 1–4. shown in the inset is the low partial pressure range for samples 2 and 3. Curves are y-shifted for clarity.

Figure 2. representative tem images of samples 2 (a) and 3 (b).

JOURNAL OF COORDINATION CHEMISTRY 9

micro-Mn-UVM-7 samples, the cumulative pore volume in the micropore range increases up to 0.29 and 0.33 cm3 g−1 for samples 2 and 3, respectively. This increase must be associated to new micropores that appear at ca. 0.7 and 1.23 nm as intense and weak signals, respec-tively, in the differential pore volume curves. The size and shape of these micropores could be thought of as the “photo negative” of the metal-rich domains initially included by the “atrane method.” Their prevailing sub-nanometric size is in accord with the XRD and TEM data. Lastly, it should be noted that the meso and macropore systems reduce their respective volumes as microporosity seems to increase. Very likely, the origin of this tendency resides in the alteration of the silica oligomer condensation (subsequently influencing the self-as-sembling processes) as the proportion of heteroelement increases. This behavior has been frequently described in the bibliography (regardless of the specific heteroelement involved). For this reason, we have used intermediate Mn proportions (in fact, it is possible to incor-porate Mn in Mn-UVM-7 meso-structured solids up to molar ratio values of Si/Mn close to 3) in order to achieve a certain balance among the three hierarchic pore systems.

4. Conclusion

This work shows for the first time how the incorporation of heteroelements in silica matrices, widely studied for various purposes, can be also used to modify the porosity of mesoporous frameworks, in general, and, in particular, the UVM-7 material. In this context, the added heteroelement rich domains might be considered as inorganic sacrifice templates. Its size, dispersion, and nature obviously will depend on the selected metal, which must be related to the ability of the different elements to enter the silica framework ranging from isolated sites (acting as formers or modifier cations) to small oxidic nanodomains. The possibility of modulating the heteroelement content in a wide compositional range (by the “atrane route”) could consequently allow us to adapt, in turn, the extension and features of the construction of new pore systems.

Figure 5. differential pore volume vs. pore width distributions for samples 1–4. Curves are y-shifted for clarity.

10 M. D. GARRIDO ET AL.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This work was supported by the Spanish Ministerio de Economia y Competitividad and the European Feder Funds [grant number MAT2015-64139-C4-2-R].

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