Carbon Composites

Article pubs.acs.org/crystal

Micron-Sized Single-Crystal-like CoAPO-5/Carbon Composites Leading to Hierarchical CoAPO‑5 with Both Inter- and Intracrystalline Mesoporosity Manuel Sánchez-Sánchez,*,† Alicia Manjón-Sanz,†,∥ Isabel Díaz,† Á lvaro Mayoral,‡ and Enrique Sastre† †

Instituto de Catálisis y Petroleoquímica, ICP-CSIC, C/Marie Curie 2, 28049 Madrid, Spain Laboratorio de Microscopías Avanzadas (LMA), Instituto de Nanociencia de Aragon (INA), Universidad de Zaragoza, Mariano Esquillor, Edificio I+D, 50018, Zaragoza, Spain

‡

S Supporting Information *

ABSTRACT: Commercial mesopores-containing carbon (carbon black BP2000) has been used as crystallization host as well as hard template of intracrystalline mesopores in CoAPO-5 materials prepared with N-methyldicyclohexylamine as specific structure-directing agent. As a result, micrometer-sized AFIstructured terminated-tip (pencil-like) hexagonal prims have been obtained. Despite the appearance of single crystals, the crystals are actually composites formed by a large number of elongated-shape CoAPO-5 and carbon nanodomains. Indeed, the removal of carbon by calcination leaves an inorganic Codoped AlPO4-based material having a complex hierarchical pore system, formed by the expected microporosity for an AFIstructured material and both intra- and intercrystalline mesoporosity. In good agreement with the nanocomposite nature of these micrometer-sized crystals, the size of their fibers generated by burning the carbon matrix can be relatively controlled by the conditions of the calcination processes. Under optimized synthesis and calcination conditions, AFI-structured materials with high mesoporous surface area and pore volume can be afforded.

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INTRODUCTION For decades, inorganic microporous materials (zeolites and zeotypes) have attracted a huge attention from the scientific community in different research fields because of their singular properties, including exceptional thermal and hydrothermal stability, their regular pore network, and a relatively high versatility in both composition and topology.1−3 However, these promising expectations have not implied parallel and massive industrial establishments, particularly in heterogeneous catalytic processes. One of the reasons behind this limited real catalytic application is found in the diffusion problems,4 inherently associated to the microporous nature of these materials, that is, to the basis of their own success.5 As a consequence, a great part of the current research effort with these materials points out the reduction of diffusion problems from very different approaches: materials with larger pores,6,7 even sacrificing their crystalline nature;8 reducing crystal size9,10 or a particular crystal dimension;11 delaminated zeolitic materials;12 or introducing mesoporosity in microporous materials.13,14 Mesoporosity within crystalline materials is intracrystalline when it is generated during the synthesis by trapping a chemical substance of mesoscale size (2−50 nm, according to the pore size classification assigned to mesopores by IUPAC) in a given crystal. Such substance acts as hard template and it is removable, leaving a microporous network highly connected by mesopores.13,14 Moreover, intracrystalline © 2013 American Chemical Society

mesoporosity can be also generated through postsynthesis treatment, dissolving crystal regions.15 On the contrary, the meso-sized void enclosed by agglomeration of nanosized crystals is known as intercrystalline mesoporosity.13,14 So far, research on generating mesoporosity in microporous materials has mainly focused on zeolites5,13,16,17 and just a few studies have dealt with the mesoporosity introduced in AlPO4-based crystals by hard templates.14,18 N-Methyldicyclohexylamine (MCHA) is one of the most specific structure directing agents (SDAs) for a particular AlPO4-based material (AFI-structured one).19−23 In addition, it leads to crystalline materials of small crystal size, which can be agglomerated in particles containing well-defined intercrystalline mesoporosity.22,23 The magnitude and type of that mesoporosity strongly depend on the nature of the heteroatom ion to be incorporated into the AlPO4-5 framework.22 Moreover, for a given heteroatom ion, Co2+, we have recently investigated the influence of different experimental synthesis parameters the crystal size and the intercrystalline mesoporosity in these AlPO4-based crystalline materials.23 That study, although significantly contributes to the general understanding of the mechanism of intercrystalline mesoporosity generation, Received: January 29, 2013 Revised: March 21, 2013 Published: April 10, 2013 2476

dx.doi.org/10.1021/cg4001768 | Cryst. Growth Des. 2013, 13, 2476−2485

Crystal Growth & Design

Article

pH arisen from the mixture of the soluble sources, particularly the amine MCHA, provokes the formation of a gel or suspension. Experimental details of the synthesis of CoAPO materials from soluble-in-water sources (samples denoted as MCHAy, where y is the MCHA/P molar ratio) and CoAPO materials within the carbon matrix BP2000 (samples denoted Cx, where x is the C/P molar ratio) are given in Supporting Information. Calcination Methods. Two different calcination methods were studied in the calcination of the sample C64. The sample was distributed on the bottom of a ceramic calcination vessel forming a thin layer (less than 1 mm). Method 1, Sample C64-M1. Calcination in a furnace at 540 °C. The temperature was increased from room temperature to 540 °C in 3 h (2.9 °C/min), and the sample was held at that temperature for 15 h. Method 2, Sample C64-M2. Slow calcination in a furnace at 500 °C. The temperature was increased from room temperature (20 °C) at 1 °C/min, and the sample was held temperature for 2 h at 100, 200, 300, 400, 450, and 500 °C. The total duration of the heating treatment was 21 h and 40 min. Characterization Techniques. Phase purity of the samples was checked by powder X-ray diffraction (XRD). XRD patterns were acquired with a Philips X’PERT diffractometer using Cu Kα radiation. Difractograms were running with steps of 0.0167°, and each point was irradiated for 0.407 s. Crystallite sizes were estimated by applying Scherrer equation to the XRD reflections 210, 002, 211, and 220, at approximately 19.7°, 21.0°, 22.4°, and 25.9°, respectively, taking advantage of their appreciable intensity. XRD peaks were considered as Gaussians for crystal size estimation purposes. The instrumental broadening was evaluated from Cu Kα1 of XRD peaks of LaB6 at different 2θ angles. Low-angle instrumental broadenings (2θ below 21°, which is the position of the lowest-angle reflection of LaB6) was extrapolated from the best fit of the curve width vs 2θ considering all XRD peaks of LaB6 in the 20−90° range, following the bibliographic recommendations.24 Studies by scanning electron microscopy (SEM) were carried out in an ultrahigh resolution FEI-NOVA NanoSEM 230 FESEM instrument. For transmission electron microscopy analysis (TEM), the material was crushed using a mortar and pestle, carefully dispersed in ethanol, and sonicated. A drop of the suspension was placed onto a lacey carbon copper microgrid. TEM observations were performed in a field emission JEOL-2100F microscope operated at 200 kV, equipped with a Gatan CCD camera and an Oxford systems EDS detector for chemical analysis. Nitrogen adsorption/desorption isotherms were measured at −196 °C in a Micromeritics ASAP 2420 device. Before, the previously calcined samples were degassed at 350 °C, respectively, under high vacuum for at least 18 h. Surface areas were estimated by BET method. Micropore and external surface area was estimated from t-plot method. Pore size distributions were obtained by application of BJH method to the desorption branch of the N2 isotherms. The organic content of the samples was studied by elemental analysis with a Perkin-Elmer 2400 CHN analyzer and by thermogravimetric analysis (TGA) using a Perkin-Elmer TGA7 instrument. TG analyses were carried out at a heating rate of 20 °C/min under air flow.

does not imply a real diffusional improvement, since mesopores run parallel to the one-dimensional micropores; in other words, micropores and mesopores are not connected. Trusting in the above-mentioned exceptional specificity of MCHA toward AFI-structured AlPO4-based materials,19−23 a set of synthesis was planned with a considerable excess of carbon black, commonly used for hard templating mesopores inside of the microporous crystals.5,13 Indeed, with the purpose of obtaining AFI nanocrystals for a better reactant and product diffusion, the current method was designed physically limiting the zeolite crystal growing from the solutions (not gels) that fill up the mesopores of a carbon black BP2000 by incipient wetness impregnation. This is the first time that this technique has been applied to AlPO4-based system, probably because of their unusual crystallization from clear solutions, which somehow is developed here. The choice of CoAPO system instead of, for instance, a simple nondoped AlPO4 one, gives us different advantages, such as (i) to demonstrate the right incorporation of heteroatom ions into the AlPO4 framework, making more extended this approach, (ii) to easily follow the presence of crystalline or amorphous phases by the blue or pink color given by Co, respectively, and (iii) unlike some other heteroatom ions,22 the hydrothermal treatment of Co2+containing gels produces well-organized crystal agglomeration in particles that contains an ordered intercrystalline mesoporosity.22,23

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EXPERIMENTAL SECTION

Synthesis. The most relevant experimental details of the synthesis experiments discussed in this article are compiled in Table 1, as well as the nomenclature given to the different samples.

Table 1. Composition of the CoAPO Gels, Crystallization Times, and Obtained Crystallized Phases, Either in Absence or in Presence of Carbon Matrix sample MCHA2.4 MCHA2.8 MCHA3.2 MCHA4.5 MCHA5 C32 C64 C128 C64-2h C64-4h C64-7h C64-42h

xa

ya

za

pH

t (h)

phases

32 64 128 64 64 64 64

2.4 2.8 3.2 4.0 5.0 2.4 2.4 2.4 2.4 2.4 2.4 2.4

25 25 25 25 25 93b 197b 404b 195 195 195 195

2.5 5 5.5 6.5 7 nmc nmc nmc nmc nmc nmc nmc

18 18 18 18 18 18 18 18 2 4 7 42

quartz tridymite AFI + ATS + unkd AFI + ATS AFI AFI + BP2000 AFI + BP2000 TRI + BP2000 AFI + BP2000 AFI + BP2000 AFI + BP2000 AFI + BP2000

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a Molar ratios from the final gel composition: 1.0 P/0.96 Al/0.04 Co/ 2.4 MCHA/x C (BP2000)/ y MCHA/ z H2O. bThis water ratio was not designed but given by the final amount of water required for filling up the carbon mesoporous to reach the wetness impregnation point. c Since it is practically a solid, the pH was not measured. dA nonidentified phase.

RESULTS CoAPO-5 Materials from Soluble-in-Water Sources. This section discusses the results of the carbon-free synthesis experiments (Table 1), which were essential for the subsequent design of the CoAPO-5/carbon experiments. Figure 1 shows the powder XRD patterns of the samples crystallized from gels prepared with soluble-in-water sources. Gel prepared with the easily soluble-in-water Al salts as Al sources were able to produce pure CoAPO-5 only under high MCHA/P ratio, in spite of the high specificity of MCHA as SDA for AFIstructured AlPO4-based materials. In bibliography, the preparation of these AFI-materials has been described from gels containing MCHA in a relatively wide pH range, at least from 5 to 9.19,20 In the systems of this work, in which AlCl3 is the Al

This synthesis method for CoAPO-5/carbon composites was inspired by that developed by Jacobsen et al.10 for nanocrystalline zeolites, which requires starting from clear solution mixture rather than from gels or suspensions. Because of the insolubility of aluminophosphate-based species in water at the common pH values (3−10), a previous optimized synthesis method had to be developed for C-free CoAPO-5 starting from sources soluble in water. However, this CoAPO-5 crystallizes from a gel and not from a clear solution, as the 2477



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Data of some of the more relevant experiments, denoted as Cx (where x is the C/P ratio), are shown in Table 1. The XRD patterns of the as-prepared sample C64 and its calcined versions C64-M1 and C64-M2 following the different calcination processes described in Experimental Section, are plotted in Figure 3, in comparison with the XRD pattern of the

Figure 1. Powder XRD patterns of the as-prepared C-free CoAPO-5 samples from gels formed by soluble sources with different SDA MCHA content. pH values of the starting gels are indicated.

source, a starting gel with a pH of 5 does not produce any AFIstructured material after 18 h of crystallization, and a pH higher than 5.5 is necessary to avoid the cocrystallization of any dense phase. A MCHA/P ratio as high as 5 was necessary for preventing the crystallization of any impurity. For similar systems in which Al(OH)3 is the Al source, a ten-times lower MCHA/P ratio of 0.5 could become enough.19 The samples that resulted rich in CoAPO-5 phase (samples MCHA3.2, MCHA4.5 and MCHA5) have the typical blue color attributed to the incorporation of Co2+ ions into an AlPO4 framework. Diffuse-reflectance UV−vis spectra (not-shown) indeed supported the right incorporation of the Co2+ ions. According to the very narrow peaks of the AFI phases presented in Figure 1, these CoAPO-5 materials must be formed by relatively large crystals. This was confirmed by the SEM image of some CoAPO-5 crystals shown in Figure 2. The

Figure 3. Powder XRD patterns of the as-prepared samples MCHA5 and C64 and the latter after being calcined by two different methods M1 and M2. An enlargement of the patterns showing in-detailed the 220 reflection is also included.

pure CoAPO-5 (sample MCHA5) prepared in absence of any carbon matrix. The background of the pattern of the asprepared sample C64 is due to the contribution of the carbon matrix, which obviously maintains its amorphous nature after crystallization. Obviating that amorphous contribution, the typical reflections of AFI structure are perfectly identified in the pattern. The comparison between the XRD pattern of the asprepared sample C64 and the C-free sample MCHA5 makes clear some singularities of this new AFI-structured material. The most relevant one is the remarkable broadening of the peaks, unusually found in AFI-structured materials, as they are generally formed by large crystals (of a few micrometers). The broadening is even notable when comparing with those patterns of the samples prepared with MCHA as SDA and with conventional Al sources, which forms nanocrystalline MeAPO-5 materials,22 especially under optimized conditions.23 In order to have an idea of the crystal size quantification, we have applied Scherrer equation. Being aware of the limits of this equation for microporous materials24 and particularly for AlPO4-5-based ones,23 we took these crystal size estimations only for relative comparison rather than as absolute values of crystal sizes. The so-estimated crystal size of the as-prepared sample C64 has an average crystal size of 34 nm, which is smaller than the smallest crystal size of CoAPO-5 prepared with this SD A (40 nm), in absence of carbon and optimized according systematic changes in different synthesis parameters.23 This nanoscaled size of the crystals becomes more remarkable considering that a similar CoAPO gel prepared in absence of any carbon matrix leads to a larger CoAPO-5 crystals (Figure 2). In other words, carbon matrix as host of the AlPO4 gels has a key role in the formation of AlPO4-based crystals of nanoscaled size. On the other hand, the difference in relative intensities of the peaks must be related to a different morphology.25−27 In particular, the low intensity of the 002 reflections (the one at ∼21°) suggests that the crystals are of

Figure 2. SEM image of the as-prepared C-free sample MCHA5.

sample is formed by crystals fused in aggregates of micrometer size having a peculiar shape, which to the best of our knowledge has not been described so far in the literature. That shape could be described like double cone. Therefore, this initial research designed just for obtaining CoAPO-5 materials from gels formed by soluble-in-water sources has been successful, not only in the water solubility aspect but also in creating a new morphology of the generated crystals. Then, this synthesis method is interesting by itself as it could add these morphologic singularities to the high specificity of the SDA. CoAPO-5 from CoAPO Gels within a Carbon Matrix. Once the synthesis conditions of CoAPO-5 from gels with all sources soluble in water were optimized, we proceed with the preparation of the CoAPO-5 over carbon matrix BP2000 (C). 2478



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materials. In addition, it is observed the disappearance of the broad background attributed to the carbon BP2000 as well as a narrowing of the AFI peaks, which should be necessarily related to crystal sintering at some extent. Certain sintering was also detected in nanocrystalline zeolites grown within the BP2000 after carbon removal by calcination.10 Figure 4B−D show different SEM images of the calcined sample C64-M1. Carbon matrix disappears during the calcination process, the sample being then formed mainly by AFI crystals, which are only contaminated by some minor impurities (not shown), probably of amorphous nature as they were not detected by XRD. The pencil-like crystals are somehow damaged during the calcination process. Probably, the as-prepared crystals contain a substantial amount of carbon that, under calcination conditions, is forced to get out aggressively from the crystals, provoking the abrupt rupture of only certain regions of the crystal-like surface. This fact exposes the real nature of these particles, which, as-synthesized, had the typical semblance of single compact crystals (Figure 4A). SEM images of the calcined sample C64-M1 shown in Figure 4B−D indicate that the each crystalline particle is actually formed by hundreds of AlPO4-based nanofibers, observed in great detail by HRTEM (Supporting Information). Presumably, these nanofibers would come from the sintering of the AlPO4-based nanodomains homogenously mixed with C-based nanodomains in the asprepared sample C64. Calcination would provoke the combustion of the C-based nanodomains, whose exothermic character would incite the sintering process of the AFIstructured crystals. That feature and its suggested interpretation in principle resolves the apparent disagreement between Figure 3 and Figure 4A, certifying the indication given by XRD pattern about the nanosized nature of the AFI crystals of the samples even before calcination. Interestingly, the sintering extent is (at least relatively) controllable by modifying the calcination conditions. Thus, the estimated average crystal size of the sample C64-M1 was 60 nm, whereas the calcination program specially designed for minimizing the exothermicity of the carbon combustion (slow ramp temperature of 1 °C/min combined with temperature maintenance steps every 100 °C, and with a maximum temperature of 500 °C) resulted counterproductive, giving larger crystals of 67 nm (sample C64-M2). That difference in peak broadening is more evident in the enlargement of Figure 3, only showing the reflection 220, which must be exclusively sensitive to the nanofibers width and insensitive to their fibers length.24 It opens the possibility of preserving the original crystal nanosize by some other calcination strategies (different heating rate, atmosphere and/or flow control and etcetera). Complementing Figure 4B−D, Figure 5 contains highmagnification SEM images of different regions of a couple of representative particles of the calcined sample C64-M1. Figure 5B and 5D are enlargements of the Figures 5A and 5C, respectively. Apart from Supporting Information Figure 4 on the real nature of the CoAPO-5/C composites, Figure 5 makes clear that those fibers are also not compact but contains some mesoscaled holes, whose magnitude and abundance in the crystalline fibers seem to be diverse in different particles and even in different regions of a given particle. Thus, Figure 5D shows isolated mesopores, whereas the statistically less abundant particles shown in Figure 5B possess a more complex network of mesoporosity, even becoming dominant against the nanofibrous nature of the particle. These differences could be due to very diverse factors, such as carbon content and its

accentuated elongated shape. That elongated shape, but not so marked, is probably the most common morphology of the AlPO4-5/MeAPO-5 crystals as they crystallize in a hexagonal space group. These suggested features from XRD should be supported by electronic microscopy studies. Figure 4A shows a representative

Figure 4. SEM images of the as-prepared (A) and calcined (B−D) sample C64. Picture C is an enlargement of picture B.

SEM image of the as-prepared C64 sample. A well-defined pencil-like crystal emerging from the carbon matrix is clearly seen. The more intense brightness of the crystals with respect to the darker carbon matrix because of the sensitivity to the atomic weight of the used backscattered SEM detector, suggests that the crystal is effectively formed by the heaviest inorganic atoms (Al, P, and Co). Such assumption was corroborated by STEM-EDS analysis (see Supporting Information). Both the morphology and width of the pencil-like crystals resemble those observed for the C-free CoAPO-5 aggregates (Figure 2), but markedly more elongated, as suggested by its XRD pattern. However, surprisingly and contrarily to the XRD evidence, the sample is apparently formed, apart from the amorphous carbon, by isolated crystals of a size of a few micrometers. Unlike the aggregates of the C-free sample MCHA5, this crystal does not seem to be formed by aggregation of smaller crystals. The differences in crystal dimensions between the estimations from XRD broadenings and the observed by SEM is of some orders of magnitude, and therefore, it cannot be attributed to any analytical error. In other words, although SEM images show the “real” crystals, the nature of these crystal-like particles cannot be micrometer-sized single crystals. To shed light on this apparent inconsistence between both individually trustable characterization techniques (XRD and SEM), we studied the calcined samples with the same techniques. The calcination of the black sample produces a less abundant sample of a bluish green color, which is distinctive of the calcined CoAPO-5 materials. Figure 3 evidence that the XRD pattern of the calcined samples C64M1 and -M2 are the typical ones of calcined AlPO4-based materials having AFI topology. By comparison with the pattern of the uncalcined sample, it is obvious the well-known reversal of relative intensity between peaks at ∼12.9° (reflection 110) and 14.9° (200), characteristic of the calcined AFI-structured 2479



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Figure 5. SEM images of the calcined sample C64. Images B and D are enlargements of A and C ones, respectively. Figure 6. Low-angle powder XRD pattern of the calcined sample C64. 2θ positions and interplanar distances d of the two detected peaks are indicated. A SEM micrograph helping to the interpretation of the XRD pattern is inserted.

distribution along the CoAPO/C composites. The spherical shape of the mesopores shown in Figure 5B, as well as their size range (20−50 nm) leads us to guess that they could have been templated by isolated BP2000 particles. Although it is commonly accepted an averaged particle size of 12 nm for the commercial BP2000 (ASTM D-3249),14,28,29 superior values with a range of 20−50 nm have been reported in the publications in which this carbon matrix has been studied by TEM.30−32 In the same direction, Jacobsen et al.33 also found that BP2000 templates mesopores in zeolites, whose size is also larger than 12 nm (in particular, they claimed 20 nm), and the maximum of the mesopore size distribution of an AlPO4-5 prepared in presence of the carbon BP2000 as hard template was at ∼30 nm.14 Moreover, both the relatively ordered arrange of nanofibers and the presence of nanoholes suggest that these samples contain certain mesoporosity, which is confirmed by N2 isotherms at −196 °C (see Kinetics of Crystallization of the CoAPO-5/C Composite section) and by low-angle XRD pattern (Figure 6). More importantly, the so-generated mesoporosity is of different type, as the former should be considered intercrystalline, whereas the latter is undoubtedly intracrystalline. The low-angle XRD pattern of the calcined sample C64-M1 shows a well-defined peak corresponding to an interplanar distance d of 19 nm suggests the presence of wellordered mesoporosity. This high order is supported by the presence of a broad and low-intense peak of d = 41 nm, which could be indexed as 200 reflection if the more intense peak would correspond to the 100 reflection of a hexagonal arrangement of mesopores. Because of the irregular distribution of the meso-holes, presumably templated by BP2000 particles, we think that the order causing those diffraction peaks must be the well-ordered arrangement of the fibers inside of the CoAPO-5/C composites. To support this assignment, an extra SEM picture illustrating that idea is inserted in Figure 6. In that composite, whose configuration actually predominates along the sample, the nanofibers runs parallel leaving elongated void mesoporous between them. The mesopore diameter should be somehow related to the BP2000 particle size, as suggested by the XRD pattern of the Figure 6.

Kinetics of Crystallization of the CoAPO-5/C Composites. The potential interest of the above-described CoAPO-5 materials obtained through calcination of the samples Cx lead us to investigate the kinetics of formation of those composites. Figure 7 compiles de XRD patterns of the C64 system after

Figure 7. Powder XRD patterns following the kinetics of crystallization of the sample C64. Detected diffraction peaks that could not be assigned to the AFI-structured phase are marked with asterisks. Representative SEM images of the CoAPO-5/C composites of the samples C64-2h and -42h are shown.

being treated for 2, 4, 7, and 42 h (Table 1). Two SEM images corresponding to the sample crystallized at the shortest and the longest crystallization time are also shown in Figure 7. The SEM image from Figure 4A could be considered as intermediated crystallization times as they belong to the asprepared sample C64 but crystallized during 24 h. After only two hours, some low-intense and already-broad diffraction peaks attributed to AFI-structured materials could be identified. 2480



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Figure 8. (A) N2 adsorption/desorption isotherms of the calcined samples crystallized from the kinetics following of the system C64. The isotherm of the calcined sample MCHA5 is also shown for comparative purposes. The inserted plot shows a more reduced p/p0 range (0−0.15) of the isotherms. (B) Evolution of surface areas (squares read on the left Y-axis) and pore volume (diamonds on the right Y-axis) with time as estimated by BET method and their breakdown in micropore (circles) and external (triangles) surface area as calculated by t-plot method. (C) Pore size distribution plots by application of BJH method. Values of the maxima for the shortest (2 h) and the longest (42 h) crystallization time, as well as TSE artifact, are indicated.

the former which obviously has to be accompanied by certain amount of N2 adsorbed at low pressures (to form the monolayer). Another important feature is that the fullcrystallized sample in presence of C matrix (42 h) does not reach the expected value of the microporosity of a standard CoAPO-5 (for instance, MCHA5). It must be due to the impurities found in the sample (see above) or the accepted decrease in microporosity of nanocrystalline materials in comparison with their homologue materials formed by micrometer-sized crystals,9 and in a lesser extension, to the remaining carbon (0.7 wt. % according to the elemental analysis) surviving the calcination process. Nevertheless, any of the samples from the kinetic series C64 has higher surface area than the reported AlPO4-5 samples templated by BP2000.14 Similarly to the increase of microporosity, the presence of mesoporosity in the samples was also expected based on the nanosized dimension of the fibers forming the calcined CoAPO-5/C composites combined to their more or less ordered arrange and on the presence of meso-holes in the nanofibers (Figures 4 and 5). The evolution of the mesoporosity with the crystallization time was not, however, as expected. That mesoporosity, which is effectively notable in highly crystallized samples, becomes exceptional at short crystallization times. For instance, after 2 h of crystallization time, the volume of the adsorbed N2 at pressures corresponding to mesopore region is at least ten times higher than in micropore region (Figure 8A). Moreover, the real mesoporosity/microporosity ratio is even very much higher as the adsorbed N2 at low pressures is overall due to the first monolayer of the mesopores. Figure 8C shows the pore size distribution (PSD) plots of the five samples studied in the kinetics. The PSD plot of the sample MCHA5 (not shown) has not any peak, as expected from its negligible amount of adsorbed N2 in the mesopore p/ p0 region (Figure 8A) The pore volume reduction with crystallization time, plotted in Figure 8B, is evidenced in Figure 8C by the systematic decrease of the area under the PSD

Accordingly, scarce number of pencil-like composite crystals emerging from the carbon matrix could be detected by SEM. The tip of the crystals is not completely formed by then, although the width of the crystals (of a few micrometers) is quite close to that reached by the final crystal-like composites. With the increasing of the crystallization time, the tip of the crystals is progressively adopting a more geometric shape (Figure 4A) and the amount of the CoAPO-5 crystalline materials increases, as it is indicated by the enhanced intensity of the peaks attributed to the AFI phase with respect to the amorphous part (Figure 7). At the end of crystallization, more abundant and elongated crystal-like particles are obtained. Moreover, their tip has practically disappeared, and consequently the pencil-like crystals becomes practically perfect hexagonal prisms. Figure 8A shows the adsorption/desorption isotherms of N2 at −196 °C of the calcined samples whose XRD patterns of their as-prepared forms were shown in Figure 7. The isotherms of the calcined sample C64-M1 (crystallization time of 24 h) and that of the sample crystallized in absence of C, MCHA5, are also included for a better comparison. Besides, Figure 8A contains an enlargement (p/p0 range of 0−0.15) of the main plot to clarify the trends in microporosity region as a function of crystallization time. The most general features in the kinetics are the increase of the microporosity and a parallel decrease of mesoporosity, both evolutions quantified and plotted in Figure 8B. These numerical values of BET surface area, micropore and external surface areas estimated by t-plot method, pore volume (plotted in Figure 8B) and the pore size distribution maxima (Figure 8C) are compiled in Table S1 (Supporting Information). However, there is no significant variation of the total surface area, which has an averaged value slightly higher than 200 m2/g. The increase of the microporous character of the sample with time was expected from Figure 7, as the crystallization of a microporous materialis is being followed. The apparent discontinuity between the samples crystallized after 2 and 4 h can be explained by the large mesoporosity of 2481



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curves. Ignoring the TSE (Tensile Strength Effect) peak near 4.0 nm,34 the PSD curves are characterized by the presence of two different maxima at ∼20−25 and 40−60 nm. The latter is less well-defined and domains the PSD at shorter crystallization times, whereas the intensity of the former predominates in the PSD curves of the more crystalline samples, being the only one after 42 h. Consequently, the PSD maximum at 40−60 nm is attributed to the mesopores of the amorphous phase and the narrower one at 20−25 nm is assigned to the mesoporosity contained in the CoAPO-5 obtained from the calcination of the CoAPO-5/C composites. This pore size distribution is in good agreement with the low-angle XRD patterns (Figure 6) and the SEM pictures (Figures 4−6) of the sample C64. Separation of the CoAPO-5/C Composites from the Carbon Matrix. The described isolating of CoAPO-5 nanofibers agglomerates/aggregates from their physical mixture with carbon matrix by simple calcination in air entails certain disadvantages, such as the presence of some inorganic amorphous impurities or the presumably significant contribution of the carbon matrix burning to the exothermic medium created during the calcination process, which could favor the sintering of the AlPO4-based nanodomains. Therefore, a preliminary separation of CoAPO-5-containing composites and carbon matrix is desirable before the calcination process. In this section, we present some preliminary results evidencing that, although the method could be improvable, such separation is possible. To have a higher proportion of the inorganic phase in the starting sample, the sample C32 was selected for this experiment. The developed method is based on the capacity of carbon, which is practically insoluble in any solvent, to form relative stable suspensions in some organic solvents. Following some preliminary tests, we selected acetone as a relatively good carbon disperser and widely available solvent. The sample C32 was ultrasonically suspended for 30 min in an amount ten times higher in weight of acetone, at room temperature, and in a closed vessel. After that, the suspension was keeping static for a few minutes (0.5−5 min), provoking that some particles, probably the densest or the most hydrophilic ones, settled on the vessel bottom whereas some other particles, which resulted to be practically free of CoAPO-5 crystalline phase, were maintained under suspension. The suspended part was excluded by simple decantation, and the settled part was undergone to a subsequent suspension/decantation cycle. After five cycles, the still-black sample (called CoAPO-5 composite) was exclusively formed by CoAPO-5 composites, as checked by SEM and XRD characterization (Figure 9). SEM images of the sample C32, either before or after the separation process, show that its composites are actually aggregates/agglomerates of pencil-like crystals instead of morphologically similar isolated crystals characterizing the composites of the sample C64 (Figure 4A). The higher concentration of aluminophosphate species in this sample probably provokes the aggregation of the crystals simply by proximity between them. On the contrary, diluting the concentration of both the inorganic species and SDA within the same amount of carbon matrix does not generated AFI-structured materials, but a dense AlPO4-based one (tridymite phase was the only crystalline phase in the sample C128, Table 1). Probably, a high dilution of the species required to form the AlPO4-based microporous material would hinder an effective assembly of them. This handicap is more serious considering that the impregnation of such species is

Figure 9. Powder XRD patterns and SEM images fo the sample C32 before and after the separation of CoAPO-5/C composites from carbon matrix.

being carried out sequentially to avoid their precipitation outside the carbon matrix. Unfortunately, this nonoptimized separation method implies a very important loss of the CoAPO-5 phase if high purity of the remaining sample is required. That is why the amount of this sample was far from being enough to do a trustable characterization by some techniques, such as N2 adsorption/ desorption isotherms. Apart from the interest of this method as phase purifier, extra valuable information could be deduced from thermogravimetric characterization of this sample: the amount of carbon (and the amount of inorganic material) present in the single-crystal like CoAPO-5 nanocomposites. Figure 10 shows the thermograms of the sample CoAPO-5 composite separated from the sample C32 by the described

Figure 10. TGA plots of the as-received carbon matrix BP2000, asprepared sample C32, calcined sample C32-M1 and separated CoAPO-5/C composites from the as-prepared sample C32.

ultrasonic suspension/decantation method compared with those of the as-prepared C32, the calcined sample C32-M1 and the carbon matrix BP2000. The TGA curve of the pure carbon matrix basically has a unique weight loss (centered at ∼740 °C) that corresponds to the practical loss of the material. The as-prepared sample C32 has a more complex TGA, with at least three weight loss: (i) the first one, observed at ∼225 °C, should correspond to the SDA MCHA molecules not incorporated into the AFI-structured material; (ii) the second 2482



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weight loss occurred at ∼360 °C and it should due to the MCHA inside of the AFI pores; and (iii) the quantitatively most relevant one means a loss of ∼66 wt. %, takes place at temperatures around 650 °C, and it is undoubtedly attributed to the carbon matrix. The calcined sample C32 is typical of a calcined AFI-structured AlPO4-based material, with the only remarkable weight loss (at ∼15 wt. %) at temperatures below 100 °C, attributable to the water filling up the micropores, basically certifying a complete calcination of the sample. Finally, the TGA plot of the separated sample CoAPO-5 composite has also three weight loss: (i) due to residual acetone (at ∼53 °C), (ii) due to MCHA inside of the pores, the loss of the MCHA free molecules not being detected, and finally, (iii) in the temperature region of the carbon matrix burning (445−900 °C), a weight loss of ∼34 wt. % is detected (and confirmed by elemental analysis), whereas the remaining not-burnable material, obviously of inorganic CoAPO nature, is around 57 wt.%. Therefore, around one-third of the weight of the CoAPO5 composites is formed by carbon matrix, introducing mesoporosity in the AlPO4-based phase either by separating the CoAPO-5 domains or by hardly templating intracrystalline mesoporosity. It is also remarkable that the weight loss due to carbon matrix is centered at ∼650 °C for separated CoAPO-5 composites and 670 °C for the nonseparated C32 sample, that is, 90 and 70 °C lower than for as-received carbon BP2000. The reason for that difference remains unclear.

method not only to zeotypes but also to a great number of zeolites, since not so many zeolites can crystallize from clear solutions. Likewise, the goodness of this method could be not reduced to the preparation of crystalline materials. Thanks to the following of the kinetics of crystallization carried out for the CoAPO-5/C composites within the carbon matrix, it was possible to isolate an amorphous aluminophosphate material with notable textural properties (BET surface are of 288 m2/g and pore volume of 1.34 cm3/g, Table S1 of Supporting Information), which are enough to beat those of the materials resulting by very new methods37 or the conventional highsurface amorphous AlPOs prepared through coprecipitation of soluble Al and P sources in water.38−40 These amorphous aluminophosphate materials, directly39,40 or conveniently modified,41,42 have found important catalytic applications in Fine Chemistry. They could be even competitive to the mesoporous AlPOs catalysts templated by surfactants,43,44 because of the elevated price of the formers, their thermal instability45,46 and limited control of their acidic−basic properties.43 Considering that our sample could be further optimized in terms of textural properties, there is no doubt about the scientific interest of this strategy to prepare amorphous inorganic materials with high surface area. Finally, the here-described synthesis of the aluminophosphates into the carbon matrix leads to exceptional materials, hardly obtained by any other method. Thus, the synthesis of the unique reported AlPO4-5 in presence of BP200014 did generated morphology changes and morphology heterogeneity in comparison to their homologue C-free AlPO4-5, and the presence of nanosized crystals was discarded.14 On the other hand, when zeolites instead of AlPOs crystallize within BP2000, nanocrystalline materials formed by more or less isolated crystals are formed, which in principle lacks any intracrystalline carbon exerting the role of hard template.10,47 That is, the function of the carbon is only to limit the zeolite crystal growing by confinement.10,47 The same research group has also developed similar methods to produce large (micrometersized) zeolitic crystals enclosing carbon matrix, which once calcined generates intracrystalline mesoporosity.10,47 Our micrometer-sized hierarchical CoAPO-5/C composites containing both intra- and intercrystalline mesoporosity are in practice very different to all of them. The combination of both kinds of mesoporosity is particularly relevant in the case of materials with monodimensional pore system, which is the case of the AFI-structured materials. Its intercrystalline mesoporosity because of the ordered agglomeration generally runs along the same direction to the pores,22,23 making it useless in terms of avoiding diffusion problems of reactants and products in catalytic applications. In other words, the generation of some intracrystalline mesoporosity in this CoAPO-5 system would not only interconnect the different 1-D micropores within a given AFI crystal but would also “reactivate” the role of intercrystalline mesoporosity, doubly facilitating the accessibility of reactants to the active sites located inside of micropores. In addition, the nanosize of the crystals practically guarantees that every intracrystalline mesopore will be accessible to the surface. And, to finish, the relatively large size of the crystal-like composites in comparison to the almost isolated crystals of zeolites obtaining under similar conditions10,47 or even compared to the real single crystals,33,48 in addition to confer to the materials intercrystalline mesoporosity, has allowed us to develop a method to separate

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DISCUSSION This section emphasizes the relevance of the results presented in this work. Although this article has been mainly focused on the novelty of synthesizing CoAPO-5/C nanocomposites, in which both materials are closely mixed (at “nano” level) to the point of looking like real micrometer-size compact crystals, some other secondary but also important goals have been achieved. These goals, more or less underlined thorough the manuscript, deserve to be compiled together to not be relegated to the background. First of all, a new method for preparing (heteroatom-doped) AlPO4-5 materials based on the use of soluble P, Co, and Al sources in water has been developed. In spite of AFI-structured materials is probably the most widely studied AlPO4-based systems, the resultant morphology consisting in crystals fused in aggregates of shape, has not been described so far. In this context, the use of Al soluble salts, such as AlCl3 used in this work, as alternative Al sources to those based on (hydro)oxides for the preparation of AlPO4-based microporous materials, could generally lead to unknown and novel morphologies. Morphology control is becoming relevant since, apart from the already-conventional applications of these AlPO4-based materials in catalysis and adsorption/ separation process, their emerging applications, such as membranes, sensor devices, optics or magnetic materials1 require accurate control of the crystal shape and size.26,27,35,36 Furthermore, the successful method for synthesizing nanocrystalline zeolites from clear solutions taking advantage of the mesoporous nature of the carbon matrix that acts as host of the starting gels,10 has been extended to microporous materials that crystallize from gels and not only from clear solutions. The unique condition is that all sources forming the gel are individually soluble (or at least able to be suspended in particles of size smaller than the mesopore diameter of the carbon matrix) in the solvent or mixture of solvents used in the impregnation process. This could spread the application of this 2483



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them from the lighter BP2000 particles alternative to the direct calcination. These isolated composites should overcome the widely characterized sample C64 in so many different aspects, such as textural properties making these CoAPO-5 materials even more relevant.

CONCLUSIONS A new method to prepare both nanocrystalline and hierarchical AlPO4-based (in the described case, Co-doped AlPO4-5) materials having mesoporosity is presented. The method is based on other one described in literature for certain zeolites. With our approach, this method could be also extendable to microporous materials crystallized from gels (not only from clear solutions) formed by soluble sources. As preliminary step, taking advantage of the high specificity of MCHA as SDA for directing the crystallization of AFI-structured AlPO4-based materials, we have developed a method for preparing CoAPO-5 starting from inorganic sources that are soluble in water. The successive impregnation of carbon matrix BP2000 with the previously dissolved sources and the subsequent solvothermal treatment lead to the formation of micrometer-sized nanocomposite crystals formed by large amount of closely mixed AlPO4-based and carbon matrix nanodomains. Removal of the carbon by calcination generates CoAPO-5 nanofibers having both inter- (thanks to their well-ordered arrangements or simply to their nanosized nature) and intracrystalline (thanks to the carbon particles acting as hard templates) mesoporosity. The introduction of intracrystalline mesoporosity could be transcendental for these materials having monodimensional pore system, as it reactivates the otherwise useless intercrystalline mesoporosity running parallel and unconnected to the micropores. Certain control of the micro- and mesoporosity of the resultant materials could be achieved, as the former increases and the latter decreases with crystallization time. In addition, this method could be potentially of high interest to prepare amorphous AlPO4-based materials with high surface area and high mesoporosity. ASSOCIATED CONTENT

S Supporting Information *

Detailed synthesis procedures, STEM/EDS analysis of the calcined C64 samples, HRTEM micrographs of AlPO4-based nanofibers, and N2 adsorption/desorption isotherms data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +34-915854795. Fax: +34-915854760. E-mail: manuel. [email protected]. Present Address ∥

Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD U.K. Funding

Spanish Ministry (MAT-2009−13569 and MAT-2012−31127). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors thank Dr. Carlos Márquez-Á lvarez and Prof. Joaquiń Pérez-Pariente for useful discussion. Cabot Corporation is acknowledged for providing carbon BP2000. 2484



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