Mesoporous Layered Aluminosilicates Prepared From Protozeolitic

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Mesoporous Layered Aluminosilicates Prepared From Protozeolitic Nanoclusters: Synthesis, Physicochemical and Catalytic Properties Katarzyna Onik, Mariusz Gackowski, Miroslaw A. Derewinski, and Bogdan Sulikowski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07748 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

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The Journal of Physical Chemistry

Mesoporous Layered Aluminosilicates Prepared from Protozeolitic Nanoclusters: Synthesis, Physicochemical and Catalytic Properties Katarzyna Onik,† Mariusz Gackowski,† Miroslaw Andrzej Derewinski,*,‡ Bogdan Sulikowski*,† †

Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences,

Niezapominajek St. 8, 30-239 Kraków, Poland ‡

Pacific Northwest National Laboratory, Institute for Integrated Catalysis, 902 Battelle

Boulevard, Richland 99352 WA, USA

ABSTRACT Mesoporous layered aluminosilicates were prepared by a two-stage route using tetrapropylammonium hydroxide (TPAOH) and hexadecylamine (HDA). First, aluminosilicate sols were prepared from very dilute solutions under mild conditions. The optically clear sols contained aluminosilicate nanoclusters of the average size 4 – 5 nm.

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Al NMR studies

demonstrated that tumbling alone of the small aluminosilicate nanoclusters due to Brownian motions was removing effectively chemical shift anisotropy, dipolar interactions and narrowing quadrupolar interactions; thus, magic-angle-spinning was not required for obtaining good quality

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Al and also

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Si NMR spectra of the sols studied. Evolution of the quantitative

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Si NMR

spectra in time provided insight into dynamical behavior of the sols. Second, the sols with the Si/Al ratios of 12 – 70 were used for preparation of the X-ray amorphous aluminosilicate materials. The samples exhibited good sorption properties due to the presence of the ca. 3 nm slit mesopores. They also contained Brønsted and Lewis acid sites. The mesoporous aluminosilicate materials turned out to be active in the liquid phase reaction. The initial reaction rate in the isomerization of α-pinene and the turnover frequency (TOF) estimated at 348 K were comparable to or higher than the reference zeolitic and non-zeolitic materials.

1. INTRODUCTION Chemistry of silicon is very rich. Starting from silicon and aluminum sources amorphous or crystalline aluminosilicates (zeolites) can be formed under proper conditions. An example of the former is a ‘silica garden’ preparation, the experiment known for few centuries. It was shown that when sodium silicate was combined with aluminum nitrate, an amorphous aluminosilicate with a hierarchic substructure was obtained.1 On the other hand, a plethora of zeolites and sometimes non-zeolitic solids (tectosilicates) can be prepared from 4-component systems comprising silicon and aluminum sources, an inorganic base and water at temperatures ranging from 373 to 723 K. The system can be simplified further by using a silicon source, an organic template (e.g., tetrapropylammonium hydroxide, TPAOH) and water. The 3-component system is useful in producing the all-silica end-members of pentasil zeolites.2 We note that mechanisms responsible for formation of zeolites are the much debated topic.3,4 While regular zeolites are used broadly for preparation of catalysts and adsorbents, nanoclusters offer an unique opportunity for producing different aluminosilicate materials


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revealing sometimes a hierarchical organization. Such materials can facilitate transport of reactants due to the reduced mass transfer limitations, thus enabling better accessibility of them to the active sites located in the solids. Therefore, enhanced catalytic performance of materials assembled from nanoclusters might be a priori expected in various reactions.5 For example, it was reported that nanosized amorphous precursors of zeolites were stable and revealed high activity in cracking of alkylbenzenes.6,7 Most often, amorphous aluminosilicates and crystalline zeolites can be prepared starting either from gels or optically clear sols.3,8 In the early stages of preparation X-ray amorphous aluminosilicates (sometimes called protozeolitic nanoclusters) could be formed.9-11 The objective of this paper was to prepare X-ray amorphous aluminosilicate materials, exhibiting good porosity, thermal stability and containing Brønsted acid sites. Due to the presence of the latter, the aluminosilicate could serve as a model catalyst for the liquid-phase isomerization of α-pinene.12 The catalysts were obtained by the two-stage route at low temperature of 323 K. It was of interest to study not only the final solid mesoporous materials, but also to shed some light on the synthesis of aluminosilicate sols. We have dwelled on the properties of the sols and solid preparations by considering results obtained by, inter alia,

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Si

and 27Al NMR spectroscopy. The catalytic properties were discussed in terms of initial reaction rate and turnover frequency (TOF), and these parameters were compared to the performance of a standard bulk aluminosilicate, alumina, ZSM-5, ZSM-12 and ferrierite type zeolites.

2. EXPERIMENTAL SECTION 2.1 Preparation of Aluminosilicate Sols


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The suspensions of aluminosilicate nanoclusters (sols) were prepared by mixing tetraethyl orthosilicate (TEOS, Sigma Aldrich, 98 wt%) with aluminum sec-triisobutylate (Merck, 97 wt%) and a tetrapropylammonium hydroxide solution (TPAOH, Sigma Aldrich, 20 wt%). First, TPAOH was added to doubly distilled water, followed by addition of aluminum sectriisobutylate. Then TEOS was added dropwise to this solution under vigorous stirring; the final mixture was stirred further for 15 minutes at room temperature (RT) and then aged under stirring for 2 - 48 h at 323 K. The sols were prepared from different amounts of reactants specified in Table 1 to obtain the Si/Al ratios ranging from 12 to 70. During preparation, the H2O/SiO2 = 46.3 and OH-/SiO2 = 0.2 molar ratios were kept constant. The sols were studied extensively by NMR. They were probed just after mixing the reactants (the sample labeled ‘0 h’), and then after 2, 5, 24 and 48 h, respectively. The sol with Si/Al = 25 labeled ‘48 h’ was kept in a closed rotor at ambient temperature and studied by NMR after 2, 27 and 270 days. Dynamic light scattering (DLS) and NMR were used for controlling formation of nanoclusters. For characterization methods requiring solid samples, the sols were lyophilized first. 2.2 Synthesis of Mesoporous Samples from the Aluminosilicate Sols The mesoporous samples were prepared from the aged aluminosilicate sols and a second organic template, hexadecylamine (HDA). First, the template was dissolved in a water-diluted ethanol (53 wt% of alcohol) to obtain a concentration of 7.7 wt% HDA. The chosen concentration of HDA guaranteed formation of the layer micelles. The aluminosilicate sol was then added slowly to the HDA solution and the cross-linking of the nanoclusters and small (alumino)silica oligomers on the surface of micelles to yield a final aluminosilicate was carried out at room temperature for 20 h under intensive stirring. We note that the yield of protozeolitic species is low, which means that part of the (alumino)silica species is still present in a form of


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smaller oligomers. The latter during the second step of synthesis contribute to preparation of a lamellar material acting as a ‘glue’ for binding together the larger nanoclusters. After condensation of the X-ray amorphous aluminosilicate nanoclusters and the remaining (alumino)silicate oligomers, the final materials were washed with diluted ethanol (20 vol%), centrifuged and dried by lyophilization. As during preparation of sols no inorganic bases were used and hence no sodium or potassium ions were present, no additional ammonium ions exchange was required. The method of preparation used was preventing significantly eventual further condensation and irreversible cross-linking of the neighboring layers by forming the SiO-Si chemical bonds. Before adsorption and catalytic studies the samples were carefully calcined in air to remove occluded organic molecules. Calcination was carried out first at 773 K for 20 h and finally at 823 K for 20 h to yield carbon-free materials. 2.3 Physicochemical Characterization 2.3.1 Dynamic Light Scattering Dynamic Light Scattering (DLS) measurements were carried out using a Malvern Zetasizer Nano ZS instrument. 2.3.2 XRD Powder X-ray diffraction (XRD) patterns in the range of 2Ɵ = 5–50o were recorded on a Siemens D5005 automatic diffractometer using CuKα radiation. Silicon powder was used as the internal standard. 2.3.3 FT IR Spectra in the infrared region were obtained using sample powder (2 wt%) embedded in KBr by Nicolet 6700 FT-IR Thermo Scientific instrument, with 64 scans for each measurement and with 4 cm-1 of resolution. The concentration of both Brønsted and Lewis acid sites was


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determined by quantitative IR studies of pyridine sorption. The samples were pressed into the form of self-supporting discs (ca. 6-9 mg/cm2) and evacuated in a quartz IR cell at 723 K under vacuum for 1 h. After cooling, the acid sites were neutralized by pyridine for 15 minutes at room temperature. Physisorbed pyridine was removed by evacuation at RT and 423 K. The concentration of Brønsted and Lewis sites was calculated from the intensities of 1545 cm˗1 and 1450 cm˗1 bands of pyridinium ions (PyH+) and pyridine interacting with Lewis sites (PyL).13 2.3.4 Nitrogen sorption Nitrogen sorption was carried out using Quantachrome Nova 2000. Prior the N2 adsorption the samples were outgassed at 573 K for 12 h. 2.3.5 NMR NMR spectra were acquired using a Bruker Avance III 500 MHz spectrometer operating at a magnetic field of 11.7 T. Before measurements, the solid samples were fully hydrated in a desiccator over the saturated solution of magnesium nitrate.

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Si MAS NMR spectra were

acquired at 99.37 MHz with single-pulse excitations of 5 µs (π/7), repetition times of 10 s, using MAS of 8 kHz. Different conditions were applied for acquiring 27Al spectra of sols and mesoporous samples at 130.32 MHz. The sols were recorded using either soft 5.6 µs pulses (π/2) and repetition time 0.3 s without rotation, or short 0.5 µs pulses (π/18) under magic-angle-spinning (MAS) rotation of 4 kHz and 2 s delay between pulses.

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Al spectra of the solids, that is lyophilized sols and the

mesoporous layered aluminosilicates, were acquired using very short 0.2 µs pulses (π/44) under MAS = 12 kHz. Short pulses are necessary to ensure reliable, quantitative results for the

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spectra.14 Special care was put on applying relaxation delays long enough for ensuring


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quantitative measurements.15 For this purpose, inversion recovery experiments were used for both nuclei studied. 27Al and 29Si chemical shifts are quoted in parts per million from external 1 M aqueous Al(NO3)3 solution and tetramethylsilane (TMS), respectively. The NMR spectra of sols, lyophilized samples and the mesoporous aluminosilicates were normalized to the same sample weight. Deconvolution of the spectra was carried out using the Bruker TopSpin 3.1 software. 2.3.6 Isomerization of α-Pinene The transformation of α-pinene was carried out in a batch-type glass reactor equipped with a reflux condenser, a stirrer, and a temperature controller. The tests were performed under isothermal conditions at 348 K and atmospheric pressure under intensive stirring. In a typical run, the catalyst (0.1 - 0.25 g) was dried in the reactor at 423 K for 1 h, and then 5 mL (4.29 g) of α-pinene (Fluka, 98 %) was added. After selected times, aliquots of the reaction mixture were obtained for analysis by a Hewlett-Packard HP 6890 gas chromatograph equipped with a thermal conductivity detector and the 3 m home-made packed column (2.0 mm i.d., filled with 8 % bentone-34, 6 % didecyl phthalate, and 1 % silicon oil A on Chromosorb W).16 The column was calibrated using the following standard references: α-pinene, camphene, limonene, α-terpinene, γ-terpinene and p-cymene.

3. RESULTS AND DISCUSSION The mesoporous layered aluminosilicates were prepared from the optically clear sols containing protozeolitic nanoclusters and smaller (alumino)silica oligomers. The chemical compositions of the sols are listed in Table 1. It was of interest to follow the changes taking place in sols during their preparation. Thus, the diameters of the nanospecies formed were


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estimated by DLS, while their composition was monitored using 29Si and 27Al NMR. The latter measurements were carried out using both the sols and the solid materials obtained from them by lyophilization. 3.1 Characterization of Sols In general, the diameters of nanoclusters in the sols should not exceed few nanometers due to expected layers thickness of the porous materials prepared from them in the second synthesis step. The suspensions of nanoparticles should also be monodispersed, stable in time, and importantly, aggregation of nanoparticles should be avoided. Preliminary experiments aimed at the optimization of the sols’ preparation were performed at room temperature and at 323 K. Better quality of nanospecies, in terms of nanoparticle size distribution and lack of aggregation, was obtained at 323 K, therefore all the sols were normally aged at this temperature. Our aim was to monitor the sols by

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Si and

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Al NMR using a MAS probehead for studying solids in

order to obtain, in one experiment, information on the species present in liquid and in nanoclusters. Aluminum spectra of the sols were therefore acquired under static and magicangle-spinning conditions. Histograms visualizing distribution of particles’ dimensions in the sols after 48 h, versus both number of nanospecies and their volume, are shown in Figure S1. As it is seen, all the samples are highly monodispersed, with the maxima found at ca. 4 – 5 nm. Interestingly, such behavior is not affected by the amount of aluminum used, within the range of Si/Al ratios studied here (Si/Al = 12 to 70, Table 1), and the traces overlap largely. Analysis of distribution of nanoparticles volume versus size excluded the presence of larger particles in the sols (Figure S1 b). Long-time stability of sols was checked and the results are shown in Figure S2. At the beginning the


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particles are growing and after about 48 h their size becomes stabilized. Upon prolongation of the experiment up to 180 h no aggregation of nanoclusters was evidenced. 29

Si NMR spectra were recorded using aliquots probed at a different time, labelled from 0 to

48 h, and the results are summarized in Figure 1. As it is seen, the silica source - tetraethyl orthosilicate gives a single sharp signal at -82 ppm (Figure 1 a). In the sample labeled 0 h (trace b), recorded after 15 minutes of mixing the reagents, only small amounts of TEOS can be detected, while for all the other samples the signal disappears completely (traces c – e). Under the conditions applied, hydrolysis of TEOS towards silicon species and ethanol is clearly a fast reaction, in accord with other observations.17 In the sample labeled 0 h (Figure 1 b) four new silicon signals appeared. The signals at -71.9 and -80.6 ppm can be assigned to monomeric and dimeric silicon species, denoted as Q0 and Q1. After mixing the silicon and aluminum sources in the presence of TPAOH two condensation reactions commence to proceed simultaneously. The first is formation of a Si-O-Si silicate dimer from silicic acid and its deprotonated form. The second is condensation of Si(OH)4 and Al(OH)4- to yield a Si-O-Al aluminosilicate dimer. The overall free energy barrier for the former reaction is 75 kJ mol-1, while for the latter 70 kJ mol-1. Theoretical calculations therefore predict that aluminum is preferentially incorporated into the Al-O-Si dimer during the very initial steps of the reaction.18 Accordingly, the Q0 and Q1 signals decay with time to reach approximately 50 % of their initial values after 48 h, because these species constitute reactants used to form oligomers and then nanoclusters. Further grow of dimers to (alumino)silicate oligomers followed by their cross-linking, also with silicon species formed by hydrolysis of TEOS, leads finally to formation of small nanoclusters. The slightly broader third signal at -89 ppm can be assigned to a prismatic hexamer (double-3ring, D3R).17,19,20 Because of the presence of three membered rings, the chemical shift of the


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species is classified as Q3∆. The most intense, still broader signal at ca. -98 ppm shifts its position with time and becomes even broader. It can be assigned to the cubic octamer D4R and not to the prismatic decamer.17,21 Such an assignment is consistent with earlier results, because hydrolysis of TEOS yields ethanol, and it is known that alcoholic solutions contain high proportions of the cubic octamer.21 The signal at -98 ppm is increasing with time (Table 2). Apart from the four relatively sharp signals of silicon species in solution, a broad hump can be found in the traces b – e of Figure 1, in the region from about -80 to -125 ppm. The hump can be deconvoluted using four signals. An example of deconvolution is visualized in Figure S3. We assign these signals to the small nanoparticles of ca. 4 – 5 nm in size, as was estimated by DLS. Note than the full width at half-maximum (FWHM) of the signals, 5 to 10 ppm, is much larger than the four sharp signals related to the liquid phase and considered earlier. Such an assignment is supported further by 29Si NMR spectra of the solid samples obtained by lyophilization of the sols (cf. infra). The four broader signals forming not well-resolved hump can be assigned tentatively to the Si(3Al), Si(2Al), Si(1Al), and Si(0Al) groupings in the aluminosilicate nanoclusters, respectively. (The eventual fifth signal which would correspond to Si(4Al) was not found in the spectra.) Gaussian-Lorentz deconvolution of the sol signals in Figure 1 is summarized in Table 2, and the analysis of the data leads to the following conclusions. Just after mixing the components of the sol all the sites related to the liquid and to the aluminosilicate nanoclusters are well seen, however, their intensities are changing in time. The Q0 and Q1 signals of monomeric and dimeric silicon species exhibit a similar behaviour – they decrease about 50 % of the initial value, from 5-6 % in the sample 0 h to 3 % in the sample acquired after one month. Less pronounced changes were observed in the intensity of the D3R signal at -89 ppm, it was essentially constant (ca. 10 %), and decreased insignificantly to 7 % after one month. The


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contribution from cubic octamer (D4R) at -98 ppm reveals a growing tendency, from 8 to 20 % observed after one month (Table 2). On the other hand, the two signals assigned to the nanoclusters, Si(3Al) and Si(2Al), either disappeared or decreased significantly from 14 to 6 after 48 h. The last two signals at -101 and -110 ppm, corresponding to Si(1Al) and Si(0Al) environments, are the dominating ones, and are changing in irregular way (Table 2). We also note that, similar to other silicon-containing materials (zeolites, niobosilicates), some contribution from Si(3Si1OH) and Si(2Si2OH) environments is expected at -101 and -92 ppm (cf. infra). Clearly, a dynamical picture emerged from these observations: elimination of Si(3Al) and Si(2Al) groupings coupled with reorganization of the remaining ones with time was evidenced in nanoclusters. The (alumino)silicate nanoclusters, present in the final sol after one month, are composed predominantly of the Si(1Al) and Si(0Al) groupings. Aluminum has spin quantum number I = 5/2 and therefore possesses a nuclear quadrupole moment which gives rise to quadrupolar broadening of NMR spectra in solids, in addition to chemical shift anisotropy (CSA) and dipolar interactions. To overcome this, routinely magicangle-spinning (MAS) is used for solids, which removes CSA and dipolar interactions, as well as narrows signals of quadrupolar nuclei. Other, less common methods can also be applied for studying quadrupolar nuclei.22,23 On the other hand, 100 % abundance of

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relaxation times all contribute to obtaining good quality spectra in a reasonable time. It was of interest to compare aluminum spectra of the sols obtained under MAS with those recorded under static conditions. The representative spectra of the sol with Si/Al = 25 are shown in Figure 2. The spectra were normalized to the sample weight; higher intensity of the upper traces is due to different excitation parameters applied. The lower traces of each pair were acquired under


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conditions used normally for solid-state measurements. For this purpose, 0.5 µs pulses were used, corresponding to π/18 flip angle. Alternatively, the upper spectra were recorded using soft 5.6 µs pulses (π/2 flip angle) typical for liquids. As it is seen, the static and MAS spectra are essentially the same, giving the same chemical shifts and the same width at half-maximum. This is especially well seen when a comparison of the two spectra at the same intensity was made; the identical contour was obtained visualized in Figure S4. The same behaviour was observed for the spectra of the sols with different Si/Al ratios (not shown). It is therefore clear that tumbling alone of the small, 4 – 5 nm aluminosilicate nanoclusters due to Brownian motions are removing effectively CSA, dipolar interactions and narrows quadrupolar interactions, hence no further broadening of the aluminum signal in the static spectra acquired here was observed. Apart from MAS and other methods mentioned earlier, Brownian motions were used for studying the ultrafine particles of aluminum fluoride.24 As an alternative to MAS, the use of ultra-sound excitations for recording solid-state NMR spectra was also suggested.25 To conclude, it was shown that magic-angle-spinning rotation is not required for obtaining the good quality

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spectra of the aluminosilicate sols studied here. Tetrahedrally coordinated aluminum signals characteristic for aluminosilicates are essentially the only ones observed since the beginning of the experiments (Figure 2). With the nanoparticles growth, the chemical shift of this signal changes insignificantly from 56.6 to 53.2 ppm. There is no further change in chemical shifts observed after additional 27 days (Figure 2, traces g, h). Interestingly, essentially the same signal from tetrahedrally coordinated aluminum at 52.5 ppm was found in the bulk, amorphous aluminosilicate with Si/Al = 7 (Figure S5). Upon close inspection, a very weak broad hump can be discerned in the

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Al NMR spectra

(Figures 2, S4, and S6). A large FWHM points to the siting of this aluminum species in the


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nanoclusters. It can be assigned tentatively to Al located at various sites in the nanoclusters, characterized by different Si-O-Al bond angles and/or different coordination. In order to estimate behavior of the sols formed, the silicon and aluminum spectra were recorded after much longer time – up to few months. For example, silicon spectra of the sols aged in the rotors at room temperature were recorded after 27 days and 270 days, respectively (Figure S7). As it is seen, the spectra recorded after prolonged time do not differ significantly from the spectrum obtained after 2 days. It can be therefore concluded that the majority of changes in the sols took place within 48 h of ageing. After two days the sols become stable despite of their aluminum content. The sols prepared under different conditions were lyophilized to yield samples amenable to XRD studies. The representative X-ray diffraction patterns of the solid materials are visualized in Figure 3. The sol with Si/Al = 40 after ageing at 323 K for 48 h exhibits a broad hump from 18 to 32o 2θ, characteristic of an amorphous aluminosilicate phase (trace a). The same behaviour was observed for all the materials studied despite the aluminum content – no any long-range ordering could be discerned by XRD. It was reported that in order to observe the XRD reflection at 2ϴ = 10o with a half-band-width of 1o, crystallite size of ca. 8 nm are required;26 thus, the 4-5 nm size nanoparticles present in the sols studied here should be X-Ray amorphous. It is known that crystallization of a high-silica zeolite requires much higher temperature of synthesis 413 – 453 K.27 To check whether a zeolitic material would be eventually crystallized from the same sol prepared as above, the temperature of 373 K was chosen for ageing the sol further. The corresponding diffractograms of the lyophilized samples obtained after aging at 323 K for 48 h followed by additional treatment at 373 K for 6 – 18 h are shown in Figure 3 b-e. First traces of a crystalline phase were found after 9 h, while the zeolitic material containing more


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reflexes which could be identified as the MFI phase, was obtained after 18 h (Figure 3 e). No other phases were found. For comparison purposes, the diffractogram of a standard, bulk ZSM-5 is shown as a trace f. The reflexes of standard ZSM-5 are in general better developed and much stronger. The experiments have shown unequivocally that the samples aged at 323 K are completely X-ray amorphous. On the other hand, only after prolonged ageing at much higher temperature of 373 K, the materials containing some ZSM-5 phase could be prepared from the same sol. For comparison purposes FT IR spectra of amorphous silica, bulk, standard ZSM-5 and lyophilized sol (Si/Al = 40) were recorded (Figure S8). In the spectrum of amorphous silica (Cabosil) three bands are present at 1104, 809 and 473 cm-1. These are due to νas , νs and δ vibrations inside TO4 tetrahedra, respectively. The hump at ca. 1220 cm-1 is assigned to asymmetric vibrations between the SiO4 tetrahedra. FT IR spectrum of a standard ZSM-5 zeolite reveals significant differences. Apart from the signals due to internal vibrations of TO4 , where T = Si or Al, that is at 1095, 790 and 450 cm-1, the new strong bands of vibrations between the tetrahedra (1223 cm-1) and of the 5-membered rings (546 cm-1) can be discerned (Figure S8 b). In general, shift of the bands in ZSM-5 towards lower values is due to isomorphous substitution of some silicon atoms by aluminum ones. The upper spectrum of lyophilized sol Figure S8 c reveals the structurally insensitive bands at 1093, 790 and 452 cm-1 which are accompanied by the signals at 1218 and 565 cm-1. The two latter bands can be related to the presence of protozeolitic nanoclusters, resembling the ordered structure of standard ZSM-5 with the crystal size higher than ca. 100 nm. The band at 565 cm-1 might be assigned to small protozeolitic units, containing several T atoms, and growing mostly along the a- and b-axis.28 The bands of water


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are present at 3430 and 1647 cm-1, while the signals at 2973, 2939, 2878, 1475 and 1389 cm-1 are due to the organic template molecules and ions (TPAOH/TPA+).29

3.2 Characterization of Mesoporous Aluminosilicates The sols described above were used for synthesis of the final solid aluminosilicates. Sorption properties probed by nitrogen gave the adsorption-desorption isotherms of type 4 with the H4 hysteresis loops. The H4 hysteresis loops found pointed to the presence of slit pores, including the wedge-shaped ones. Three samples exhibited total pore volume close to 1 cm3/g, and only the most aluminous sample was characterized by lower adsorption (Table 3). Earlier experiments have shown that increasing the amount of aluminum in the sponge-like amorphous aluminosilicates containing worm-hole mesopores led to decreased sorption capacity.30,31 The average diameter of pores of ca. 3 nm was affected by the materials compositions (Table 3). The lamellar character of the samples was also confirmed by TEM microphotographs and low-angle X-ray diffraction patterns (not shown). The status of silicon and aluminum in the mesoporous aluminosilicates was studied by solidstate NMR. In Figure 4

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Si MAS NMR spectra are visualized. Three broad signals typical for

aluminosilicates were found in the spectra, at around -110, -101 and -90 ppm, respectively. The line at -110 ppm is due to the Si(0Al) environments. The second line at ca. -101 ppm corresponds to the superposition of two silicon environments, namely the Si(3Si1Al) and Si(3Si1OH) groups.32 For the more siliceous sample the signal gain better resolution and intensity, pointing to the larger contribution of the Si(3Si1OH) groups. Finally, the last signal at -90 ppm corresponds to the superposition of Si(2Si2Al) and Si(2Si2OH) groupings; its intensity is low, in accord with the low intensity observed in nanoclusters and shown in Table 2.


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Al MAS NMR spectra of the as-synthesized samples prepared using hexadecylamine (HDA)

as the second template are shown in Figure 5. The spectra are essentially identical, the only difference is in their intensity, due to different amounts of aluminum used for the preparation of samples. The signals at 53 ppm are assigned to tetrahedrally coordinated aluminum, that is to Al atoms giving rise to the presence of Brønsted acid sites. Not even traces of octahedral aluminum were found in the materials. Very similar spectra were obtained for the solid samples of lyophilized sols (not shown). It can be therefore concluded that assembling of the aluminosilicates from the sols in the presence of HDA did not affect the local environment of aluminum atoms. The presence of Brønsted acid sites (BAS) in catalysts is a prerequisite for their activity in the isomerization of α-pinene.33 Quantitative estimation of the amount of acid sites was made by IR spectroscopy using pyridine as a probe molecule (Figure S9). Desorption of the pyridine excess was done at 423 K, because the samples were pretreated at this temperature before the catalytic tests. The mesoporous samples exhibited remarkable Brønsted and Lewis acidity (Table 3). The meso-12Al sample contained 338 µmol/g of BAS and this amount decreases continuously with diminishing the aluminum content. For comparison purposes, Brønsted acidity of a standard, bulk ZSM-5 (Si/Al = 35) was estimated to be 360 µmol/g. The sample meso-40Al of a very similar Si/Al ratio had however lower amount of Brønsted acid sites. 3.3 Isomerization of α-Pinene The isomerization of α-pinene is a useful process performed currently on an industrial scale on acidic TiO2, to give the main products: bicyclic camphene and monocyclic limonene. Camphene and limonene have found numerous applications as raw materials for manufacturing a number of


16

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products used broadly in chemical, food and pharmaceutical industries. Isomerization of α-pinene is also used, in addition to other test reactions (e.g., isomerization of o- and m-xylene), as a versatile model reaction for assessing the activity of different classes of catalysts, like oxides, clays, zeolites and heteropolyacids.12,34-36 Transformations of α-pinene can be monitored both in the liquid and gas phase. Figure 6 summarizes representative results of the conversion of α-pinene in the liquid phase on the two mesoporous aluminosilicates in comparison with various catalysts used as a reference. Pure alumina is inactive, while the standard, bulk ZSM-5 and the amorphous aluminosilicate reveal after 8 h relatively low conversion of 11 and 24 %, respectively. On the contrary, two mesoporous aluminosilicates, meso-12Al and meso-40Al, exhibit much higher activity (Figure 6, traces d - f). Catalytic activity of the samples can be conveniently discussed in terms of the initial rate of α-pinene transformation (ro) and turnover frequency (TOF). In Table 4 such a comparison is made at 348 K. (The values for ferrierite samples correspond to 363 K.) Analysis of the initial reaction rates over non-zeolite and zeolite catalysts reveals a broad range of values, from 0.06 to 6.06 mmol α-pinene / gcat min (Table 4). Out of medium-pore zeolite catalysts, the initial reaction rates were in the order: ferrierite (4.2 x 5.4 Å) > ZSM-12 (5.6 x 6.0 Å) > H-ZSM-5 (5.3 x 5.6 Å)37. Clearly, the reaction rates did not follow the dimensions of channel systems given in brackets. The reaction rates on natural ferrierite and its dealuminated form were high, despite the relatively narrow pores present in these materials. On the other hand, in natural, narrow-pore clinoptilolite type zeolite (3.0 Å × 7.6 Å), the initial reaction rates above 2 and close to 6 s-1 were indeed observed, however at much higher temperatures (≥ 413 K).38


17


Page 18 of 37

Activity can also be estimated by comparing the number of molecules transformed per one active site. In this reaction Brønsted acid sites are of primary importance.33,39 We have therefore calculated TOF per one Brønsted site, based on the data shown in Table 3 and on our earlier works.12,40 The results obtained are interesting. TOF over ZSM-5, ZSM-12 and ferrierite, were in the range of 0.02 - 0.16 s-1. In ferrierite, very mild dealumination increased TOF by 50 %. The TOF’s observed on the mesoporous aluminosilicate samples meso-12 and meso-40Al were relatively high, equal to 0.15 and 0.27 s-1 (Table 4). Interestingly, the performance of ZSM12 in terms of ro and TOF was comparable to the meso-40Al sample. The amount of aluminum in three ZSM-5, ZSM-12 and meso-40Al materials was very close. The two ZSM zeolites were characterized by fully crystalline, well-ordered structures. On the contrary, the meso-40Al sample was X-ray amorphous and contained aluminosilicate layers with ca. 3 nm mesopores stacked between them (Table 3). These results might be rationalized by assuming that pore system of ZSM-12 is large enough to give transport properties comparable to meso-40Al containing mesopores. To conclude, the mesoporous, layered aluminosilicates meso-12Al and meso-40Al revealed very good transport properties of reactants. The dense, regular arrangement of slit mesopores in the whole aluminosilicate solid seemed to be of primary importance for generating promising catalytic properties in the liquid phase isomerization of α-pinene.

4. CONCLUSIONS The mesoporous layered aluminosilicates were prepared by a two-stage route using tetrapropylammonium hydroxide and hexadecylamine as the organic templates. In the first stage optically clear aluminosilicate sols were prepared from very dilute solutions under mild conditions. The sols were extensively studied by DLS and NMR, revealing formation of


18

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homogeneous aluminosilicate nanoclusters of the average size 4 – 5 nm. It was shown than formation of the sols was essentially finished within first 48 h. We considered however their long-term stability, up to 270 days. By acquiring the

27

Al NMR spectra in the solid-state

probehead under different conditions it was demonstrated that tumbling alone of the small aluminosilicate nanoclusters due to Brownian motions was removing effectively chemical shift anisotropy, dipolar interactions and narrowing quadrupolar interactions. It was demonstrated that magic-angle-spinning was therefore not required for obtaining good quality 27Al and

29

Si NMR

spectra of the sols studied. It was possible to obtain signals from a sol sample characterizing the soluble silicon species and, simultaneously, those arising from the small aluminosilicate nanoclusters. The lyophilized nanoclusters and the final aluminosilicate samples prepared from the sols were completely X-ray amorphous. NMR spectra disclosed very similar 29Si and 27Al local ordering of the lyophilized nanoclusters and the mesoporous materials. Final aluminosilicate samples exhibited good sorption properties due to the presence of the ca. 3 nm slit mesopores. They also revealed the presence of Brønsted and Lewis acid sites quantified by FT IR. Finally, the mesoporous samples were tested in the liquid phase isomerization of α-pinene. The results were compared to performance of the zeolite and non-zeolite materials. The mesoporous aluminosilicate materials turned out to be active catalytically - the initial reaction rate ro was higher than observed over standard amorphous aluminosilicate and ZSM-5, and comparable to or surpassing the values obtained for ZSM-12 and ferrierite. Turnover frequencies were close to ZSM-12 and higher than those found for ZSM-5 and ferrierite. To conclude, mesoporous layered aluminosilicates exhibited very good transport properties of reactants, resulting in better efficiency of the acid centres. The dense, uniform arrangement of


19


Page 20 of 37

mesopores present in the aluminosilicate solids was most probably of primary importance for enhancing the catalytic properties.


20

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Figure 1. The

29

Si NMR spectrum of tetraethyl orthosilicate (TEOS) (a), and evolution of the

silicon NMR spectra with time for the sol of Si/Al = 25 after: (b) stirring the reactants for 15 mins at RT, and the samples aged at 323 K for 5 h (c), 24 h (d), and 48 h (e), respectively. The traces b–e are normalized to the sample weight. Intensity of spectrum of TEOS (a) is adjusted to fit figure.


21


Page 22 of 37

Figure 2. A comparison of 27Al NMR spectra of the reaction mixture with Si/Al = 25 after: (a,b) mixing the reactants and aged the sample under intensive stirring for 15 mins at RT – 0 h; and aged further at 323 K without stirring for 2 h (c,d), and 48 h (e,f). The sample labeled 2 h (traces c,d) was further kept at RT for 27 days and its corresponding spectra are shown in (g,h). The less intense spectra of each pair refer to the experiments made with rotation at magic-angle-spinning of 12 kHz (lower traces). The upper spectra were acquired without rotation. For the parameters of pulses used cf. the Experimental Section.


22

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Figure 3. X-ray diffraction patterns of lyophilized aluminosilicate nanoclusters, obtained from the sol with Si/Al = 40 prepared at 323 K for 48 h (a), and followed by additional ageing at 373 K for 6 to 18 h (traces b – e). The diffractogram of a standard, bulk ZSM-5 is also shown for comparison (f).

Figure 4.

29

Si MAS NMR spectra of the as-synthesized mesoporous layered aluminosilicates

with the Si/Al ratios of: (a) 70, (b) 40, (c) 25, and (d) 12.


23


Figure 5.

27

Page 24 of 37

Al MAS NMR spectra of the as-synthesized mesoporous layered aluminosilicates

obtained from the sols of different Si/Al ratios (for labels cf. Fig. 4).

Figure 6. Conversion of α-pinene on the reference samples (0.25 g) at 348 K: (a) aluminum oxide, (b) standard ZSM-5, and (c) the amorphous aluminosilicate, in comparison with the catalytic activity of the layered mesoporous materials: (d) meso-40Al, 0.1 g, (e) meso-12Al, 0.1 g, and (f) meso-12Al, 0.25 g.


24

Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. The Composition of Solutions (mol%) Used for the Synthesis of Aluminosilicate Sols. The H2O/SiO2 = 46.3 and OH-/SiO2 = 0.2 Molar Ratios Were Constant

a

Sample

TEOSa

sec-(C4H9O)3Al

TPAOHb

H2O

nano-12

12

1

2.4

556

nano-25

25

1

5

1158

nano-40

40

1

8

1853

nano-70

70

1

14

3243

Tetraethyl orthosilicate. bTetrapropylammonium hydroxide.


25


Table 2.

Page 26 of 37

Deconvolution of the 29Si NMR Spectra of the Samples Shown in Fig. 1. The First

Four Columns Pertain to Dissolved Silicon Species, and the Next Ones to the Aluminosilicate Nanoclusters Signals related to solution

-80.5

D3R prismatic hexamer -89.2

D4R cubic octamer -98.5

5

6

10

5h

4

3

24 h

4

48 h 27 da

Q0

Q1

-71.9

Sample 0h

σSi [ppm]

a

Signals related to nanoclusters Si(3Al)

Si(2Al)

Si(1Al)

Si(0Al)

-84.2

-92.4

-100.7

-110

8

6

14

21

29

9

14

2

8

36

25

4

10

14

3

7

35

24

3

3

10

12

0

6

40

26

3

3

7

20

0

4

32

32

The spectrum of sample ‘48 h’ recorded after 27 days.


26

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Table 3.

Characterization of the Pore System and Brønsted Acidity of the Mesoporous

Aluminosilicate Samples

12

Total pore volume [cm3/g] 0.42

Average diameter of pores [nm] 2.4

Brønsted acid sites [µmol/g] 338

Lewis acid sites [µmol/g] 369

meso-25Al

25

0.91

2.7

273

310

meso-40Al

40

1.05

3.0

246

239

meso-70Al

70

1.00

3.3

132

122

Sample

Si/Al

meso-12Al


27


Table 4.

Page 28 of 37

Comparison of the Initial Reaction Rate (ro) of α-Pinene and Turnover Frequency

(TOF) on the Mesoporous Aluminosilicates, Reference Amorphous Aluminosilicate, Alumina and Zeolites: ZSM-5, ZSM-12 and Ferrierite (FER)

Initial reaction rate Sample

Si/Al

of α-pinene at 348 K, ro [mmol α-pinene / (gcat min)]

TOF [s-1]

meso-12Al

12

6.06

0.27

meso-40Al

40

2.27

0.15

ASAa

7

0.06

n.d.

Al2O3

-

0

0

H-ZSM-5

35

0.45

0.02

H-ZSM-12

40

1.97

0.16

H-FER

9.0

4.28b

0.06b

Reference

This work

[40]

[12] H-FER dealuminated

12.5

a

Bulk, amorphous aluminosilicate.

b

At 363 K.

5.01b


0.09b

28

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ASSOCIATED CONTENT Supporting Information

Distribution of the hydrodynamic diameters of aluminosilicate nanoclusters species; evolution of diameter of nanoclusters for the Si/Al = 40 sample versus time; an example of LorentzianGaussian deconvolution of a

29

Si spectrum of the sol labeled ’48 h’; a comparison of the

27

Al

NMR spectra of the sol with Si/Al = 25 recorded under static and magic-angle-spinning conditions;

27

Al MAS NMR spectrum of the amorphous aluminosilicate with Si/Al = 7; a

comparison of static 27Al NMR spectra of the sol with Si/Al = 25 after stirring the reactants and after ageing; the static 29Si NMR spectra of the sol with Si/Al = 25 aged at 323 K for 48 h, 27 days and 270 days; FT IR spectra of amorphous SiO2, ZSM-5 type zeolite and the lyophilized nanocluster nano-40Al; FT IR spectra of pyridine adsorbed on the calcined meso-12Al sample (PDF)

AUTHOR INFORMATION Corresponding Authors *B.S.: e-mail, [email protected]; tel, 48-12-6395-159; fax, 48-12-4251-923. ORCID Bogdan Sulikowski: 0000-0003-0963-0570

*M.D.: e-mail, [email protected]; tel, 1-509-375-3856; fax, 1-509-391-6498. ORCID Miroslaw A. Derewinski: 0000-0003-1738-2247


29


Page 30 of 37

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We

thank

the

National

Science

Centre,

Kraków,

Poland,

for

the

project

No.

2012/07/B/ST5/00771. B.S. gratefully acknowledges the Ministry of Science and Higher Education for the solid-state NMR 500 MHz spectrometer Investment Grant (project No. 75/E68/S/2008-2). M.A.D. acknowledges support by the Materials Synthesis and Simulation Across Scales (MS3 Initiative) conducted under Laboratory Directed Research & Development Program at PNNL. Thanks are also due to Dr. Łukasz Mokrzycki for catalytic data on ZSM-12.

ABBREVIATIONS TEOS,

tetraethyl

orthosilicate;

TPAOH,

tetrapropylammonium

hydroxide;

HDA,

hexadecylamine; DLS, dynamic light scattering; XRD, X-ray powder diffraction; TOF, turnover frequency; FWHM, full-width-at-half-maximum; Si(0Al), Si(4Si0Al); Si(1Al), Si(3Si1Al); Si(2Al), Si(2Si2Al); Si(3Al), Si(1Si3Al).

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TOC Graphic


36

Page 37 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


TOF = 0.27 s-1

mesoporous layered aluminosilicate

ZSM-5 zeolite

TOF = 0.09 s-1

camphene

limonene TOF = 0.02

s-1

α-pinene


FER zeolite

Mesoporous Layered Aluminosilicates Prepared From Protozeolitic

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