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Application of phosphorus-31 and aluminum-27 NMR spectroscopic techniques to study aqueous and methanolic solutions of tetraphenylammonium aluminophosphate
Journal of Molecular Structure 1128 (2017) 338e344
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Application of phosphorus-31 and aluminum-27 NMR spectroscopic techniques to study aqueous and methanolic solutions of tetraphenylammonium aluminophosphate Nasser Goudarzi*, Amir H. Amin Faculty of Chemistry, Shahrood University of Technology, P.O. Box 316, Shahrood, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 May 2016 Received in revised form 29 August 2016 Accepted 29 August 2016 Available online 30 August 2016
In this work, aluminum-27 and phosphorus-31 NMR spectroscopic techniques were used to investigate and characterize the distribution of aluminophosphate (AlPO) species soluble in the aqueous and methanolic solutions of tetraphenylammonium (TPhA) chloride. The reaction between hexaaquaaluminum cations, [A1(H2O)6]3þ, and different phosphate ligands such as H3PO4, H2PO 4 , and the acidic dimers H6P2O8 and H5P2O 8 resulted in the formation of the soluble AlPO cations. The effective aluminum-27 and phosphorous-31 NMR spectroscopies can be employed to characterize the species present in a solution. Assignment of the peaks present in the aluminum-27 NMR spectra to the aluminate species or aluminate connectivities was done to acquire information about different AlPO complexes. Some resonance lines were observed in the phosphorus-31 {1H} NMR spectra, indicating the existence of different complexes in the AlPO solutions. Some peaks were observed in the methanolic solutions of AlPO at the chemical shifts of 0.41, 6.4, 7.5, 7.9, 13.1, 13.9, 16.6, 18.1, and 20.6 ppm. Four additional peaks were also observed in the phosphorus-31 {1H} NMR spectra of the methanolic solutions of AlPO, whose intensities changed with changes in the methanol:water volume ratio; they were observed in methanol but not in aqueous AlPO. © 2016 Elsevier B.V. All rights reserved.
Keywords: Aluminophosphate (AlPO) species Tetraphenylammonium (TPhA) Phosphorus-31 NMR spectroscopy 27 Al NMR spectroscopy Methanolic solutions of AlPO species
1. Introduction The aluminophosphate (AlPO) species are highly used as catalysts and molecular sieves in industrial operations [1]. In the early 1980's, a novel family of molecular sieves was introduced by Union Carbide Laboratories based on the AlPO species [2,3]. The AlPO molecular sieves, known as AlPOn 4 (in which n corresponds to a distinct structural type), are built of strictly alternating AlO4 and PO4 tetrahedra. Their general molecular formula is [(AlO2)x(PO2)x]$ yH2O, which implies that, contrary to most zeolites, the AlPO molecular sieves are ordered with an Al/P ratio of unity. Despite this, the AlPO molecular sieves present an enhanced structural variety. Among more than 40 AlPOn 4 structures, some are zeolite analogues, although there are also some new exclusive structures [4]. Despite their structural resemblance, the crystal chemistry of the AlPO4 molecular sieves and zeolites differs considerably. Firstly, the
* Corresponding author. Tel.: þ98 2332395441; fax: þ98 2332395441. E-mail addresses: [email protected], [email protected] (N. Goudarzi). http://dx.doi.org/10.1016/j.molstruc.2016.08.078 0022-2860/© 2016 Elsevier B.V. All rights reserved.
AlPO framework is neutral, contrary to the negatively-charged aluminosilicate one. Secondly, Al atoms in the aluminosilicate framework are always tetrahedrally coordinated in comparison with the four-, five- or six-coordinated aluminum atoms available in the AlPO framework. These facts act as illustrations of the structural varieties of the AlPO4 molecular sieves. On the other hand, a soluble AlPO species, [Al (H2O) 4(OH) (H2PO4)]þ, has been suggested as a nutrient for the growth of AlPO molecular sieves [5]. Thus understanding the impacts of the Al and P concentrations on the formation of soluble AlPO species is necessary. Mortlock et al. [6] have suggested the existence of some complex cations such as [Al(H2O)5(H3PO4)]3þ, [Al(H2O)5(H2PO4)]2þ, and [Al(H2O)4(H2PO4)2]þ in acidic conditions. The effect of pH on the distribution of AlPO cations has also been investigated in the pH range of 2e13 by Mortlock et al. [7]. Akitt et al. [8] have studied aluminum chloride solutions in aqueous phosphoric acid by aluminum-27 and phosphorus-31 NMR spectroscopies. The aluminum-27 NMR spectra showed a broad peak upfield from the peak arising due to the existence of free hexaaquaaluminum cation, [A1 (H2O)6]3þ; this new peak was assigned to a range of species in which A1 was bonded via an oxygen atom to P. The phosphorus-31
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{1H} NMR spectra showed a peak corresponding to the free phosphoric acid and four upfield peaks. The peak appearing at the chemical shift of 7.6 ppm was assigned to [A1(H3PO4)n]mþ (n 2, m was undetermined), a series of polymeric acid ligands; the peak at the chemical shift of 12.3 ppm, to both [Al(H2PO4)]2þ and [A1(H2PO4)2]þ; the peak present at the chemical shift of 15.7 ppm, to [Al(H3PO4)]3þ; and the peaks appearing at the chemical shifts of 16.2 and 17.3 ppm, to the binuclear Al species. All chemical shifts were referenced to 85% (wt/wt%) phosphoric acid. It has been reported by Akitt et al. [8] that the phosphorus-31 NMR chemical shifts depend mainly on the constitution of the ligand, and, to a lesser extent, on the charge of the complex. It has also been reported by Samadi et al. that the soluble AlPO cations are produced by the reaction of [Al(H2O)6]3þ cations with phosphate ligands (i.e. H3PO4, H2PO 4 , and their acidic dimers H6P2O8 and H5P2O 8 ) [9]. Deschamps and Massiot [10] have investigated the AleOePeOeAl networks by means of the 2D correlation (COSY) solid-state NMR techniques. Al is the most plentiful metal available in the earth crust, consisting of approximately 7% of the matter present in an average soil [11]. Aluminum-27 NMR spectroscopy has been frequently used for the solvation and coordination studies in aqueous and nonaqueous solutions [6,7,10,12,13]. Although the 27Al nucleus is quadrupolar (spin ¼ 5/2), the combination of high sensitivity (0.2 relative to protons) and high natural abundance (100%) allows viewing reasonably symmetric complexes at quite low concentrations [13]. There is only one drawback for the aluminum-27 NMR spectroscopy, the quadrupole moment of 0.149 B that gives rise to a rather efficient quadruple relaxation, which, in extreme cases, can give rise to line widths of up to several kHz. 31P nucleus has a wide range of chemical shifts (700 ppm), which ensures satisfactory signal separations in different environments. This nucleus is 100% naturally abundant, and has a high sensitivity for phosphorus-31 NMR spectroscopy, which makes phosphorus-31 NMR this type of spectroscopy a reliable analytical tool, like the 1H and 19F NMR spectroscopies [14,15]. Thus some studies have been performed to address the structure of the ions available in the silicate, aluminate, and aluminosilicate solutions [16e31]. It is noteworthy that organic amines or quaternary ammonium salts show a critical structuredirecting role in the synthesis of aluminophosphate-based molecular sieves. They have both an electronic and a steric effect [22]. In their absence, no crystalline porous AlPO is produced. More than 80 amines and quaternary ammonium salts have been used as the template species. Their structure-directing specificity varies widely [32]. We have recently used aluminum-27 NMR spectroscopy to study the species available in alcoholic TOA aluminate solutions [33]. To the best of our knowledge, no paper has yet been published on the aluminum-27 and phosphorus-31 NMR spectroscopies of the AlPO species using the tetraphenylammonium (TPhA) cation as a template. The aim of this work was to identify the complexes available in the aqueous and methanolic solutions having aluminum chloride, aluminum sulfate, and ortho-phosphoric acid or TPhA chloride as a template for pH 1.0 using the aluminum-27 and phosphorus-31 NMR spectroscopies. The aluminum-27 and phosphorus-31 {1H} NMR spectra of the corresponding solutions were then recorded under similar experimental and instrumental conditions. The signals for the AlPO complexes in both spectra were recognized and interpreted. 2. Experimental 2.1. Materials and method To prepare the stock aluminate solution, 8.662 g of aluminum
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sulfate hexadecahydrate, [Al2(SO4)3]$16H2O, (supplied from Fluka) and 8.443 g of TPhACl (purchased from Merck) were dissolved in diluted sulfuric acid (25 mL). The concentrations of H2SO4, aluminum, and TPhACl in the stock solution were 0.95, 0.95, and 0.55 mol L1, respectively. The daily working solutions were prepared in 5.0-mL volumetric flasks by diluting proper amounts of the stock aluminate solution and ortho-phosphoric acid, H3PO4 (supplied from the Fluka) in deionized water to the mark. The stock aluminate solution (1.0 mol L1) was also prepared by dissolving a proper amount of aluminum sulfate hexadecahydrate in methanol (50 mL). The solutions thus prepared were very acidic, having different Al/P mole ratios with the same concentrations of Al (0.45 mol L1), H2SO4 (0.25 mol L1), and TPhACl (0.45 mol L1). The working solutions were prepared in 5.0-mL volumetric flasks by adding proper amounts of the different stock aluminate solutions and ortho-phosphoric acid, and dilution with different methanol/H2O volume ratios to the mark. All the solutions thus prepared were stable, that is neither gel formation nor precipitation was found during the experiments; the solutions were clear when used to record the NMR spectra. The phosphorus-31 {1H} and aluminum-27 NMR spectra of the corresponding solutions were recorded under the same spectral conditions at the ambient temperature (ca. 25 C). 2.2. Instrumentation Aluminum-27 NMR spectra were recorded on a Bruker DRX-500 FT-NMR spectrometer operating at 130.3 MHz for the aluminum-27 nuclei. These spectra were acquired by applying 90 pulses (of 9.1 ms pulse duration), the time between pulses being enough to let a complete re-equilibration of the magnetization; a recycle delay of 3.0 s was applied. The aluminum-27 chemical shifts were expressed in ppm from the signal for an external 0.1 mol L1 aqueous aluminum chloride hexahydrate, AlCl3$6H2O. In all instances, D2O (0.1 mL) was added to the sample solution (0.6 mL) in the NMR tube to give a 2D field/frequency lock signal. Phosphorus-31 {1H} NMR spectra were recorded on a Bruker DRX-500 FT-NMR spectrometer operating at 202.41 MHz for the phosphorus-31 nuclei. These spectra were acquired by applying 90 pulses (of 8-ms pulse duration) with a recycle delay of 2.0 s. The same procedure, as mentioned earlier, was carried out for the frequency lock. The chemical shifts were recorded in ppm relative to an 85% H3PO4 solution. The probe temperature for the qualitative analysis of AlPO was 25.0 ± 0.1 C. 1H-decoupling was applied to record the phosphorus-31 {1H} NMR spectra. The aluminum-27 and phosphorus-31 {1H} NMR spectra were recorded at least three days after preparing the solutions. 3. Results and discussion 3.1. Studying aluminate solutions using aluminum-27 NMR spectroscopy Fig. 1a shows the 27Al NMR spectrum for a 0.40 mol L1 aqueous aluminum sulfate solution. It is obvious in this spectrum that merely two signals were present, situated at the chemical shifts of 0.0 and 3.0 ppm. The signal at 0.0 ppm was attributed to the [Al (H2O)6]3þ cations, and that at 3.0 ppm was assigned to the [Al(H2O)5(SO4)]þ complex. It is noteworthy that the signal intensity for the second peak enhances with increase in the sulfate concentration [34]. On the other hand, Fig. 1b illustrates the aluminum27 NMR spectrum for the ALPO solution with an Al/P mole ratio of 1 and the same concentration of aluminum and phosphorus (0.40 mol L1). This spectrum shows four peaks and one shoulder at roughly 3.3 ppm. As one may note in this figure, there are two
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Fig. 1. Aluminum-27 NMR spectra of (a) an aqueous aluminum sulfate solution (in H2SO4 and TPhACl) and (b) an aqueous ALPO solution of the same Al and P concentration (0.40 mol L1). (Aluminum-27 NMR resonances were referenced to aluminum-27 peak in a 1.0 mol L1AlCl3 solution.)
additional signals at lower frequencies, at the chemical shifts of 6.2 and 7.7 ppm. As pointed out above, the peak at 0.0 ppm can be assigned to the octahedral [Al(H2O)6]3þ cations. On the basis of the previous reports [15,22], a small shoulder is present on the right hand side of the second peak with the chemical shift of 3.0 ppm, which can be attributed to the species [Al(H2O)5(SO4)]þ and [Al(L)]mþ (L is the Hbonded polymeric phosphoric acids like H6P2O8 and H5P2O 8 , and m is undetermined), respectively. The peak at the chemical shift of 6.2 ppm was assigned to the [Al(H2O)5(H3PO4)]3þ complex [9]. Ultimately, the last broad peak, centered at the chemical shift of 7.7 ppm, was attributed to the two species [Al(H2O)5(H2PO4)]2þ and trans-[Al(H2O)4(H2PO4)2]þ[9]. It is noteworthy that these complexes were formed by the reaction between the octahedral [Al(H2O)6]3þ cations and phosphoric acid.
and higher, these peaks vanished. It is noteworthy that in the presence of this template (TPhA), the last two signals in the spectrum vanished, and these peaks were present at an Al/P mole ratio higher than the other templates like (2-hydroxyethyl) trimethyl ammonium [9]. Therefore, it can be stated that the structural directing role of this template is significant. However, this phenomenon is anticipated since with decrease in the concentration of P, the formation probability of the AlPO species like [Al(H2O)5(H3PO4)]3þ, [Al(H2O)5(H2PO4)]2þ, and trans-[Al(H2O)4(H2PO4)2]þ decreases. It can now be stated that a number of aluminate oligomers corresponding to the NMR signals are too broad to be detected by the aluminum-27 NMR spectroscopy [8,34e37].
3.2. Effect of Al/P mole ratio on aluminum-27 NMR spectra of AlPO solutions
In this section of the work, the effect of Al/P mole ratio on the distribution of different ALPO species was investigated. For this goal, the phosphorus-31 {1H} NMR spectra for the AlPO solutions with the Al/P mole ratios of (a) 1.0, (b) 2.0, (c) 8.0, and (d) 15.0 were recorded. Fig. 3a shows the phosphorus-31 {1H} NMR spectrum of an AlPO solution with Al/P ¼ 1 and a constant Al and P concentration of 0.40 mol L1. Five signals (IeV) are present in this figure, all of which are situated at lower frequencies relative to the reference signal for H3PO4 (85%) (with the chemical shift of 0.0 ppm). Based on some previous reports [6,8,36], assignment of the peaks in the phosphorus-31 {1H} NMR spectra can be discussed as follows. The peak at the chemical shift of about 0.0 ppm (peak I) can be assigned to the four species H3PO4, H2PO 4 , H6P2O8, and H5P2O8 . Peak III at the chemical shift of 7.6 ppm can be attributed to the species [Al(L)]þ m, where L is the H-bonded polymeric phosphoric acids like H6P2O8 and H5P2O 8 , and m is undetermined [6]. Peak IV, located at the chemical shift of 13.3 ppm, can be assigned to the species [Al(H2O)5(H2PO4)]2þ and trans-[Al(H2O)4(H2PO4)2]þ. Ultimately, peak V, with the chemical shift of 16.1 ppm, was assigned
The Al/P mole ratio is an important parameter that can affect the formation of different ALPO species. In order to study this parameter, the aluminum-27 NMR and phosphorus-31 NMR spectroscopic techniques were utilized. Fig. 2a displays the aluminum-27 NMR spectrum for the AlPO solution with the Al/P mole ratio of 1 and a constant Al concentration (0.40 mol L1). As indicated in the previous section, four signals can be seen in this spectrum, whose chemical shifts are 0.0, 3.0, 6.2, and 7.7 ppm; they can be þ attributed to the Al(H2O)3þ 6 , [Al(H2O)5(SO4)] , [Al(H2O)5(H3PO4)]3þ,[Al(H2O)5(H2PO4)]2þ, and trans-[Al(H2O)4(H2PO4)2]þ complexes, respectively. Fig. 2(bee) show the aluminum-27 NMR spectra of the AlPO solutions with the Al/P mole ratios of 2.0, 5.0, 10.0, and 15.0. It is clear in these figures that with increase in the Al/ P mole ratio (that is decrease in the P concentration), the intensities of the third (chemical shift of 6.4 ppm) and fourth (chemical shift of 7.7 ppm) peaks decreased, so that at the Al/P mole ratio of 10:1
3.3. Influence of Al/P mole ratio on phosphorus-31 {1H} NMR spectra of AlPO solutions
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Fig. 2. Aluminum-27 NMR spectra of aqueous AlPO solutions with a constant Al concentration (0.40 mol L1) and different Al/P ratios of (a) 1.0, (b) 2.0, (c) 5.0, (d) 10.0, and (e) 15.0. (27A1 NMR resonances were referenced to aluminum-27 peak in a 1.0 mol L1AlCl3 solution.)
to the [Al(H2O)5(H3PO4)]3þ complex. It is clear in Fig. 3 that with decrease in the concentration of P (i.e. increase in the Al/P mole ratio), the intensities of the two peaks VII and VIII in the phosphorus-31 {1H} NMR spectrum decreased, and at the Al/P mole ratios equal to 10:1 and higher, the VII signal vanished. However, the presence of signal VIII (with the chemical
shift of 16.3 ppm) accounts for the presence of [Al(H2O)5(H3PO4)]3þ (see Fig. 3d). This means that the existence of some AlPO species such as [Al(H2O)5(H2PO4)]2þ, trans-[Al(H2O)4(H2PO4)2]þ, and [Al(H2O)5(H3PO4)]3þ is not important. The intensity of signal VI at the chemical shift of 7.6 ppm decreases with decrease in the concentration of phosphoric acid. It has been
Fig. 3. Phosphorus-31 {1H} NMR spectra of an aqueous AlPO solution with a constant Al concentration (0.40 mol L1) and different P concentrations (Al/P 1) for Al/P of (a) 1.0, (b) 2.0, (c) 8.0, and (d) 15.0. (Phosphorus-31 {1H} NMR resonances were referenced to phosphorus-31 {1H} peak in 85% (wt/wt) of H3PO4.).
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reported in an earlier work [6] that at a low concentration of phosphoric acid, existence of the dimeric phosphoric acid species such as H6P2O8 and H5P2O 8 is not of significance. Therefore, the formation probability of the complexation of Al with these species is not important. It is noteworthy that the intensity of signal II at the chemical shift of 8.3 ppm decreases with decrease in the P/Al mole ratio. Ultimately, on the basis of the results acquired (Fig. 3), with decrease in the concentration of P, the probability of formation of some AlPO species decreases. On the other hand, it has been suggested by Samadi et al. that peak III can be assigned to the complex formed between hexaaqualuminum and phosphoric acid, where the two species are linked through hydrogen bonding [9]. By looking at Figs. 2 and 3, corresponding to the 27Al and phosphorus-31 {1H} NMR spectra in the same AlPO solution, a satisfactory agreement can be reached between the two results. For instance, the last peak in the aluminum-27 NMR spectrum (Fig. 2) and peak IV in the phosphorus-31 {1H} NMR spectrum (Fig. 3), assigned to [Al(H2O)5(H2PO4)]2þ, are reduced in intensity with decrease in the concentration of P. Also the peak at the chemical shift of 6.7 ppm in the aluminum-27 NMR spectrum and peak V in the phosphorus-31 {1H} NMR spectrum of [Al(H2O)5(H3PO4)]3þ suggest a decrease in the peak intensities with decrease in the P concentration. Peak III in the phosphorus-31 {1H} NMR spectra and the shoulder in the second peak (3.2 ppm) in the aluminum-27 NMR spectra vanish at the Al/P mole ratio of 8.0:1.0. 3.4. Effect of existence of methanol in aluminum-27 NMR spectra of AlPO species Fig. 4a displays the aluminum-27 NMR spectrum for the methanolic aluminum sulfate solution. As it is clear in this figure, there is only one signal existing at the chemical shift of 0.60 ppm, which can be assigned to the hexaaquaaluminum cations, [Al(H2O)6]3þ. Fig. 4bed displays the aluminum-27 NMR spectra for the solutions of the same P and Al concentrations (0.55 mol L1), and also having the CH3OH: H2O volume ratios of 150:1, 25:1, and 1:1, respectively. All Al complexes are octahedral, and some water
molecules can be replaced by methanol. However, replacing CH3OH and H2O molecules with H3PO4 and the related species results in broadening of the observed Al resonances in the aluminum-27 NMR spectrum of the AlPO solution. This phenomenon is anticipated because the symmetry of the complexes is reduced, and/or exchange processes occur among the species [9]. According to some previous reports [6,9,34], the broad peak centered at the chemical shift of 7.5 ppm can be attributed to the species [Al(H2O)5(H2PO4)]2þ, trans-[Al(H2O)4(H2PO4)2]þ, and [Al(H2O)5(H3PO4)]3þ. An additional signal came into view at the chemical shift of 2.5 ppm with decrease in the CH3OH: H2O volume ratio. This peak can be assigned to the species [Al(L)]mþ (where L is a Hbonded polymeric phosphoric acid like H6P2O8 and H5P2O 8 , and m is undetermined) or [Al(H2O)4(CH3OH) (H6P2O8)]3þ and [Al(H2O)4(CH3OH)(H5P2O8)]2þ[6,9,34]. 3.5. Studying phosphorus-31 {1H} NMR spectra of AlPO species in the presence of methanol To study the effect of methanol on the formation of different AlPO species in the methanolic solutions of AlPO, a number of experiments were performed by adding different volume ratios of water and methanol to the AlPO solutions with the same P and Al concentrations (0.55 mol L1). Fig. 5a displays the phosphorus-31 {1H} NMR spectrum of a solution of AlPO with the CH3OH: H2O volume ratio of 200:1. Fig. 5bed shows the phosphorus-31 {1H} NMR spectra for the solutions with the same P and Al concentrations (0.55 mol L1) and the CH3OH:H2O volume ratios of 100:1, 40:1, and 1:1, respectively. On the basis of the number and intensity of the signals available in the spectra, we can see that the peak locations as well as the peak intensities highly depend on the methanol concentration. One may note that the existence of sulfate ions in the solution (AlCl3 was used as the aluminum source in this work) does not affect the distribution of AlPO complexes [6,34]. As it can be seen in Fig. 5, all the signals existing in the spectra are situated at frequencies lower than the reference signal for H3PO4 (85%) at 0.0 ppm. Pople [38,39] has indicated that with increase in
Fig. 4. Aluminum-27 NMR spectra of (a) aluminate solution in methanol ([Al] ¼ 0.43 mol L1) and AlPO solutions with a constant Al and P concentration (0.55 mol L1) and different volume ratios of CH3OH: H2O: (b) 150:1, (c) 25:1, and (d) 1:1.
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Fig. 5. Phosphorus-31 {1H} NMR spectra for AlPO solutions with a constant Al and P concentration (0.55 mol L1) and different CH3OH: H2O volume ratios: (a) 200:1, (b) 100:1, (c) 40:1, and (d) 1:1.
the partial charge of P atom, its nucleus becomes more shielded, and causes more negative of the chemical shift. According to the previous studies [6e9,36], the following assignment can be given for the spectrum illustrated in Fig. 5a. The peak at the approximate chemical shift of 0.20 ppm (peak I) can be assigned to the species H3PO4, H2PO 4 , H6P2O8, and H5P2O8 . The intensity of peak II located at the chemical shift of 6.4 ppm decreases with decrease in the CH3OH: H2O volume ratio; this peak cannot be seen in the absence of methanol. We suggest that peak II can be attributed to the species [Al(H2O)4(CH3OH) (H6P2O8)]3þ and [Al(H2O)4(CH3OH)(H5P2O8)]2þ. Peak III, located at the chemical shift of 8.2 ppm, can be attributed to a complex between [Al(H2O)6]3þ and H3PO4, linked through hydrogen bonding; this has been suggested by Samadi et al. [9] (See below for the structure of this complex [9].). A reduction in the intensity of peak II and an increase in the intensity of peak III can be seen when the volume ratio of CH3OH: H2O decreases. When this ratio was decreased to 100, a small signal (peak X) was seen at the chemical shift of 7.4 ppm, assigned to [Al(L)]mþ; it is noteworthy that L is a H-bonded polymeric phosphoric acid such as H6P2O8 and H5P2O 8 , and m is undetermined [3,4]. At the CH3OH: H2O volume ratio of 1:1, peak II vanishes (see Fig. 5d). The relative broad peak located at the chemical shift of 13.6 ppm (peak IV) can be assigned to the species [Al(H2O)5(H2PO4)]2þ and trans-[Al(H2O)4(H2PO4)2]þ. On the basis of the calculation of the partial charge of the P atoms, carried out by Mortlock et al. [6], the cis-[Al(H2O)4(H2PO4)(H3PO4)]2þ and cis[Al(H2O)4(H2PO4)2]þ complexes should show phosphorus-31 {1H} NMR peaks at the rough chemical shift of 14.5 ppm. Therefore, peak IV may also have contributions from these complexes. Peak V, situated at the chemical shift of 17.5 ppm, can be attributed to the complexes [Al(H2O)5(H3PO4)]3þ and [Al(H2O)4(H3PO4)2]3þ[6,7,9]. It is noteworthy that a new signal appeared at the chemical shift of 18.7 ppm (labeled as VI), which was not seen in the aqueous medium (solution without methanol) [9]. As one may note in Fig. 5, with decrease in the CH3OH: H2O volume ratios, the intensity of this peak decreases. We propose that peak VI can be attributed to
the complex [Al (H2O)5(OP (OCH3) (OH) 2)]3þ, where an OeH unit of H3PO4 is replaced by an eOCH3 group. It is noteworthy that Blackwell and Patton [40] have reported that the signals located at the rough chemical shift range of 19 to 31 ppm in the -n phosphorus-31 {1H} NMR spectra of crystalline AlPOn 4 and SAPO materials. We can now state that the peak at the chemical shift of 21.1 ppm should be assigned to {(OH)2ePe[OeAl(H2O)5]2}5þ. This peak can be seen in the AlPO sol-gels, although it is not recognizable in the aqueous AlPO solution [4]. As a result, we suggest that peak VII at the chemical shift of 21.1 ppm in the CH3OH: H2O mixture is associated with {(OH)2ePe[OeAl(H2O)5]2}5þ. We can come into this conclusion that the binuclear aluminum complexes are available in the mixtures of CH3OH with H2O. One may expect this because oligomerization (i.e. condensation) of the ionic species can occur in non-aqueous media. As we may note in Fig. 5, with decrease in the CH3OH: H2O volume ratio, the intensity of peak I increases. We can say that H3PO4, H2PO 4 , H6P2O8, and H5P2O8 are dominant at lower concentrations of methanol. The intensity of peak III (at the chemical shift of 8.2 ppm) increases at lower CH3OH: H2O volume ratios, suggesting increase in the concentration of the dimeric phosphoric acids such as H6P2O8 and H5P2O 8 (see Fig. 5aed). Also the intensity of peak X (at the chemical shift of 7.4 ppm) increases with increase in the concentration of water since the presence of complex X (see the above figure) is preferred at lower CH3OH: H2O volume ratios. 4. Conclusion In this work, the aluminum-27 and phosphorus-31 NMR spectroscopic techniques were utilized to characterize the species available in the aqueous and methanolic solutions of aluminophosphate (AlPO). Also, for the first time, (TPhA) chloride was utilized as a template. Formation of a bond between P and Al via an oxygen atom available in the ALPO species is affected by the P/Al ratio. The phosphorus-31 {1H} NMR spectra of alcoholic AlPO solutions exhibit seven peaks at lower frequencies relative to the
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reference signal of H3PO4 (85%). The results obtained revealed that some peaks were new and that they were not seen in the aqueous solution of AlPO (compare Fig. 3 with Fig. 5). The intensities of the peaks changed with variations in the CH3OH:H2O volume ratio. It is noteworthy that the new observed peaks are evidenced from the conducted studies. These results can yield a better understanding of the synthesis of the AlPO molecular sieves in non-aqueous media.
[20]
[21]
[22]
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Application of phosphorus-31 and aluminum-27 NMR spectroscopic techniques to study aqueous and methanolic solutions of tetraphenylammonium aluminophosphate Nasser Goudarzi*, Amir H. Amin Faculty of Chemistry, Shahrood University of Technology, P.O. Box 316, Shahrood, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 May 2016 Received in revised form 29 August 2016 Accepted 29 August 2016 Available online 30 August 2016
In this work, aluminum-27 and phosphorus-31 NMR spectroscopic techniques were used to investigate and characterize the distribution of aluminophosphate (AlPO) species soluble in the aqueous and methanolic solutions of tetraphenylammonium (TPhA) chloride. The reaction between hexaaquaaluminum cations, [A1(H2O)6]3þ, and different phosphate ligands such as H3PO4, H2PO 4 , and the acidic dimers H6P2O8 and H5P2O 8 resulted in the formation of the soluble AlPO cations. The effective aluminum-27 and phosphorous-31 NMR spectroscopies can be employed to characterize the species present in a solution. Assignment of the peaks present in the aluminum-27 NMR spectra to the aluminate species or aluminate connectivities was done to acquire information about different AlPO complexes. Some resonance lines were observed in the phosphorus-31 {1H} NMR spectra, indicating the existence of different complexes in the AlPO solutions. Some peaks were observed in the methanolic solutions of AlPO at the chemical shifts of 0.41, 6.4, 7.5, 7.9, 13.1, 13.9, 16.6, 18.1, and 20.6 ppm. Four additional peaks were also observed in the phosphorus-31 {1H} NMR spectra of the methanolic solutions of AlPO, whose intensities changed with changes in the methanol:water volume ratio; they were observed in methanol but not in aqueous AlPO. © 2016 Elsevier B.V. All rights reserved.
Keywords: Aluminophosphate (AlPO) species Tetraphenylammonium (TPhA) Phosphorus-31 NMR spectroscopy 27 Al NMR spectroscopy Methanolic solutions of AlPO species
1. Introduction The aluminophosphate (AlPO) species are highly used as catalysts and molecular sieves in industrial operations [1]. In the early 1980's, a novel family of molecular sieves was introduced by Union Carbide Laboratories based on the AlPO species [2,3]. The AlPO molecular sieves, known as AlPOn 4 (in which n corresponds to a distinct structural type), are built of strictly alternating AlO4 and PO4 tetrahedra. Their general molecular formula is [(AlO2)x(PO2)x]$ yH2O, which implies that, contrary to most zeolites, the AlPO molecular sieves are ordered with an Al/P ratio of unity. Despite this, the AlPO molecular sieves present an enhanced structural variety. Among more than 40 AlPOn 4 structures, some are zeolite analogues, although there are also some new exclusive structures [4]. Despite their structural resemblance, the crystal chemistry of the AlPO4 molecular sieves and zeolites differs considerably. Firstly, the
* Corresponding author. Tel.: þ98 2332395441; fax: þ98 2332395441. E-mail addresses: [email protected], [email protected] (N. Goudarzi). http://dx.doi.org/10.1016/j.molstruc.2016.08.078 0022-2860/© 2016 Elsevier B.V. All rights reserved.
AlPO framework is neutral, contrary to the negatively-charged aluminosilicate one. Secondly, Al atoms in the aluminosilicate framework are always tetrahedrally coordinated in comparison with the four-, five- or six-coordinated aluminum atoms available in the AlPO framework. These facts act as illustrations of the structural varieties of the AlPO4 molecular sieves. On the other hand, a soluble AlPO species, [Al (H2O) 4(OH) (H2PO4)]þ, has been suggested as a nutrient for the growth of AlPO molecular sieves [5]. Thus understanding the impacts of the Al and P concentrations on the formation of soluble AlPO species is necessary. Mortlock et al. [6] have suggested the existence of some complex cations such as [Al(H2O)5(H3PO4)]3þ, [Al(H2O)5(H2PO4)]2þ, and [Al(H2O)4(H2PO4)2]þ in acidic conditions. The effect of pH on the distribution of AlPO cations has also been investigated in the pH range of 2e13 by Mortlock et al. [7]. Akitt et al. [8] have studied aluminum chloride solutions in aqueous phosphoric acid by aluminum-27 and phosphorus-31 NMR spectroscopies. The aluminum-27 NMR spectra showed a broad peak upfield from the peak arising due to the existence of free hexaaquaaluminum cation, [A1 (H2O)6]3þ; this new peak was assigned to a range of species in which A1 was bonded via an oxygen atom to P. The phosphorus-31
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{1H} NMR spectra showed a peak corresponding to the free phosphoric acid and four upfield peaks. The peak appearing at the chemical shift of 7.6 ppm was assigned to [A1(H3PO4)n]mþ (n 2, m was undetermined), a series of polymeric acid ligands; the peak at the chemical shift of 12.3 ppm, to both [Al(H2PO4)]2þ and [A1(H2PO4)2]þ; the peak present at the chemical shift of 15.7 ppm, to [Al(H3PO4)]3þ; and the peaks appearing at the chemical shifts of 16.2 and 17.3 ppm, to the binuclear Al species. All chemical shifts were referenced to 85% (wt/wt%) phosphoric acid. It has been reported by Akitt et al. [8] that the phosphorus-31 NMR chemical shifts depend mainly on the constitution of the ligand, and, to a lesser extent, on the charge of the complex. It has also been reported by Samadi et al. that the soluble AlPO cations are produced by the reaction of [Al(H2O)6]3þ cations with phosphate ligands (i.e. H3PO4, H2PO 4 , and their acidic dimers H6P2O8 and H5P2O 8 ) [9]. Deschamps and Massiot [10] have investigated the AleOePeOeAl networks by means of the 2D correlation (COSY) solid-state NMR techniques. Al is the most plentiful metal available in the earth crust, consisting of approximately 7% of the matter present in an average soil [11]. Aluminum-27 NMR spectroscopy has been frequently used for the solvation and coordination studies in aqueous and nonaqueous solutions [6,7,10,12,13]. Although the 27Al nucleus is quadrupolar (spin ¼ 5/2), the combination of high sensitivity (0.2 relative to protons) and high natural abundance (100%) allows viewing reasonably symmetric complexes at quite low concentrations [13]. There is only one drawback for the aluminum-27 NMR spectroscopy, the quadrupole moment of 0.149 B that gives rise to a rather efficient quadruple relaxation, which, in extreme cases, can give rise to line widths of up to several kHz. 31P nucleus has a wide range of chemical shifts (700 ppm), which ensures satisfactory signal separations in different environments. This nucleus is 100% naturally abundant, and has a high sensitivity for phosphorus-31 NMR spectroscopy, which makes phosphorus-31 NMR this type of spectroscopy a reliable analytical tool, like the 1H and 19F NMR spectroscopies [14,15]. Thus some studies have been performed to address the structure of the ions available in the silicate, aluminate, and aluminosilicate solutions [16e31]. It is noteworthy that organic amines or quaternary ammonium salts show a critical structuredirecting role in the synthesis of aluminophosphate-based molecular sieves. They have both an electronic and a steric effect [22]. In their absence, no crystalline porous AlPO is produced. More than 80 amines and quaternary ammonium salts have been used as the template species. Their structure-directing specificity varies widely [32]. We have recently used aluminum-27 NMR spectroscopy to study the species available in alcoholic TOA aluminate solutions [33]. To the best of our knowledge, no paper has yet been published on the aluminum-27 and phosphorus-31 NMR spectroscopies of the AlPO species using the tetraphenylammonium (TPhA) cation as a template. The aim of this work was to identify the complexes available in the aqueous and methanolic solutions having aluminum chloride, aluminum sulfate, and ortho-phosphoric acid or TPhA chloride as a template for pH 1.0 using the aluminum-27 and phosphorus-31 NMR spectroscopies. The aluminum-27 and phosphorus-31 {1H} NMR spectra of the corresponding solutions were then recorded under similar experimental and instrumental conditions. The signals for the AlPO complexes in both spectra were recognized and interpreted. 2. Experimental 2.1. Materials and method To prepare the stock aluminate solution, 8.662 g of aluminum
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sulfate hexadecahydrate, [Al2(SO4)3]$16H2O, (supplied from Fluka) and 8.443 g of TPhACl (purchased from Merck) were dissolved in diluted sulfuric acid (25 mL). The concentrations of H2SO4, aluminum, and TPhACl in the stock solution were 0.95, 0.95, and 0.55 mol L1, respectively. The daily working solutions were prepared in 5.0-mL volumetric flasks by diluting proper amounts of the stock aluminate solution and ortho-phosphoric acid, H3PO4 (supplied from the Fluka) in deionized water to the mark. The stock aluminate solution (1.0 mol L1) was also prepared by dissolving a proper amount of aluminum sulfate hexadecahydrate in methanol (50 mL). The solutions thus prepared were very acidic, having different Al/P mole ratios with the same concentrations of Al (0.45 mol L1), H2SO4 (0.25 mol L1), and TPhACl (0.45 mol L1). The working solutions were prepared in 5.0-mL volumetric flasks by adding proper amounts of the different stock aluminate solutions and ortho-phosphoric acid, and dilution with different methanol/H2O volume ratios to the mark. All the solutions thus prepared were stable, that is neither gel formation nor precipitation was found during the experiments; the solutions were clear when used to record the NMR spectra. The phosphorus-31 {1H} and aluminum-27 NMR spectra of the corresponding solutions were recorded under the same spectral conditions at the ambient temperature (ca. 25 C). 2.2. Instrumentation Aluminum-27 NMR spectra were recorded on a Bruker DRX-500 FT-NMR spectrometer operating at 130.3 MHz for the aluminum-27 nuclei. These spectra were acquired by applying 90 pulses (of 9.1 ms pulse duration), the time between pulses being enough to let a complete re-equilibration of the magnetization; a recycle delay of 3.0 s was applied. The aluminum-27 chemical shifts were expressed in ppm from the signal for an external 0.1 mol L1 aqueous aluminum chloride hexahydrate, AlCl3$6H2O. In all instances, D2O (0.1 mL) was added to the sample solution (0.6 mL) in the NMR tube to give a 2D field/frequency lock signal. Phosphorus-31 {1H} NMR spectra were recorded on a Bruker DRX-500 FT-NMR spectrometer operating at 202.41 MHz for the phosphorus-31 nuclei. These spectra were acquired by applying 90 pulses (of 8-ms pulse duration) with a recycle delay of 2.0 s. The same procedure, as mentioned earlier, was carried out for the frequency lock. The chemical shifts were recorded in ppm relative to an 85% H3PO4 solution. The probe temperature for the qualitative analysis of AlPO was 25.0 ± 0.1 C. 1H-decoupling was applied to record the phosphorus-31 {1H} NMR spectra. The aluminum-27 and phosphorus-31 {1H} NMR spectra were recorded at least three days after preparing the solutions. 3. Results and discussion 3.1. Studying aluminate solutions using aluminum-27 NMR spectroscopy Fig. 1a shows the 27Al NMR spectrum for a 0.40 mol L1 aqueous aluminum sulfate solution. It is obvious in this spectrum that merely two signals were present, situated at the chemical shifts of 0.0 and 3.0 ppm. The signal at 0.0 ppm was attributed to the [Al (H2O)6]3þ cations, and that at 3.0 ppm was assigned to the [Al(H2O)5(SO4)]þ complex. It is noteworthy that the signal intensity for the second peak enhances with increase in the sulfate concentration [34]. On the other hand, Fig. 1b illustrates the aluminum27 NMR spectrum for the ALPO solution with an Al/P mole ratio of 1 and the same concentration of aluminum and phosphorus (0.40 mol L1). This spectrum shows four peaks and one shoulder at roughly 3.3 ppm. As one may note in this figure, there are two
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Fig. 1. Aluminum-27 NMR spectra of (a) an aqueous aluminum sulfate solution (in H2SO4 and TPhACl) and (b) an aqueous ALPO solution of the same Al and P concentration (0.40 mol L1). (Aluminum-27 NMR resonances were referenced to aluminum-27 peak in a 1.0 mol L1AlCl3 solution.)
additional signals at lower frequencies, at the chemical shifts of 6.2 and 7.7 ppm. As pointed out above, the peak at 0.0 ppm can be assigned to the octahedral [Al(H2O)6]3þ cations. On the basis of the previous reports [15,22], a small shoulder is present on the right hand side of the second peak with the chemical shift of 3.0 ppm, which can be attributed to the species [Al(H2O)5(SO4)]þ and [Al(L)]mþ (L is the Hbonded polymeric phosphoric acids like H6P2O8 and H5P2O 8 , and m is undetermined), respectively. The peak at the chemical shift of 6.2 ppm was assigned to the [Al(H2O)5(H3PO4)]3þ complex [9]. Ultimately, the last broad peak, centered at the chemical shift of 7.7 ppm, was attributed to the two species [Al(H2O)5(H2PO4)]2þ and trans-[Al(H2O)4(H2PO4)2]þ[9]. It is noteworthy that these complexes were formed by the reaction between the octahedral [Al(H2O)6]3þ cations and phosphoric acid.
and higher, these peaks vanished. It is noteworthy that in the presence of this template (TPhA), the last two signals in the spectrum vanished, and these peaks were present at an Al/P mole ratio higher than the other templates like (2-hydroxyethyl) trimethyl ammonium [9]. Therefore, it can be stated that the structural directing role of this template is significant. However, this phenomenon is anticipated since with decrease in the concentration of P, the formation probability of the AlPO species like [Al(H2O)5(H3PO4)]3þ, [Al(H2O)5(H2PO4)]2þ, and trans-[Al(H2O)4(H2PO4)2]þ decreases. It can now be stated that a number of aluminate oligomers corresponding to the NMR signals are too broad to be detected by the aluminum-27 NMR spectroscopy [8,34e37].
3.2. Effect of Al/P mole ratio on aluminum-27 NMR spectra of AlPO solutions
In this section of the work, the effect of Al/P mole ratio on the distribution of different ALPO species was investigated. For this goal, the phosphorus-31 {1H} NMR spectra for the AlPO solutions with the Al/P mole ratios of (a) 1.0, (b) 2.0, (c) 8.0, and (d) 15.0 were recorded. Fig. 3a shows the phosphorus-31 {1H} NMR spectrum of an AlPO solution with Al/P ¼ 1 and a constant Al and P concentration of 0.40 mol L1. Five signals (IeV) are present in this figure, all of which are situated at lower frequencies relative to the reference signal for H3PO4 (85%) (with the chemical shift of 0.0 ppm). Based on some previous reports [6,8,36], assignment of the peaks in the phosphorus-31 {1H} NMR spectra can be discussed as follows. The peak at the chemical shift of about 0.0 ppm (peak I) can be assigned to the four species H3PO4, H2PO 4 , H6P2O8, and H5P2O8 . Peak III at the chemical shift of 7.6 ppm can be attributed to the species [Al(L)]þ m, where L is the H-bonded polymeric phosphoric acids like H6P2O8 and H5P2O 8 , and m is undetermined [6]. Peak IV, located at the chemical shift of 13.3 ppm, can be assigned to the species [Al(H2O)5(H2PO4)]2þ and trans-[Al(H2O)4(H2PO4)2]þ. Ultimately, peak V, with the chemical shift of 16.1 ppm, was assigned
The Al/P mole ratio is an important parameter that can affect the formation of different ALPO species. In order to study this parameter, the aluminum-27 NMR and phosphorus-31 NMR spectroscopic techniques were utilized. Fig. 2a displays the aluminum-27 NMR spectrum for the AlPO solution with the Al/P mole ratio of 1 and a constant Al concentration (0.40 mol L1). As indicated in the previous section, four signals can be seen in this spectrum, whose chemical shifts are 0.0, 3.0, 6.2, and 7.7 ppm; they can be þ attributed to the Al(H2O)3þ 6 , [Al(H2O)5(SO4)] , [Al(H2O)5(H3PO4)]3þ,[Al(H2O)5(H2PO4)]2þ, and trans-[Al(H2O)4(H2PO4)2]þ complexes, respectively. Fig. 2(bee) show the aluminum-27 NMR spectra of the AlPO solutions with the Al/P mole ratios of 2.0, 5.0, 10.0, and 15.0. It is clear in these figures that with increase in the Al/ P mole ratio (that is decrease in the P concentration), the intensities of the third (chemical shift of 6.4 ppm) and fourth (chemical shift of 7.7 ppm) peaks decreased, so that at the Al/P mole ratio of 10:1
3.3. Influence of Al/P mole ratio on phosphorus-31 {1H} NMR spectra of AlPO solutions
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Fig. 2. Aluminum-27 NMR spectra of aqueous AlPO solutions with a constant Al concentration (0.40 mol L1) and different Al/P ratios of (a) 1.0, (b) 2.0, (c) 5.0, (d) 10.0, and (e) 15.0. (27A1 NMR resonances were referenced to aluminum-27 peak in a 1.0 mol L1AlCl3 solution.)
to the [Al(H2O)5(H3PO4)]3þ complex. It is clear in Fig. 3 that with decrease in the concentration of P (i.e. increase in the Al/P mole ratio), the intensities of the two peaks VII and VIII in the phosphorus-31 {1H} NMR spectrum decreased, and at the Al/P mole ratios equal to 10:1 and higher, the VII signal vanished. However, the presence of signal VIII (with the chemical
shift of 16.3 ppm) accounts for the presence of [Al(H2O)5(H3PO4)]3þ (see Fig. 3d). This means that the existence of some AlPO species such as [Al(H2O)5(H2PO4)]2þ, trans-[Al(H2O)4(H2PO4)2]þ, and [Al(H2O)5(H3PO4)]3þ is not important. The intensity of signal VI at the chemical shift of 7.6 ppm decreases with decrease in the concentration of phosphoric acid. It has been
Fig. 3. Phosphorus-31 {1H} NMR spectra of an aqueous AlPO solution with a constant Al concentration (0.40 mol L1) and different P concentrations (Al/P 1) for Al/P of (a) 1.0, (b) 2.0, (c) 8.0, and (d) 15.0. (Phosphorus-31 {1H} NMR resonances were referenced to phosphorus-31 {1H} peak in 85% (wt/wt) of H3PO4.).
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reported in an earlier work [6] that at a low concentration of phosphoric acid, existence of the dimeric phosphoric acid species such as H6P2O8 and H5P2O 8 is not of significance. Therefore, the formation probability of the complexation of Al with these species is not important. It is noteworthy that the intensity of signal II at the chemical shift of 8.3 ppm decreases with decrease in the P/Al mole ratio. Ultimately, on the basis of the results acquired (Fig. 3), with decrease in the concentration of P, the probability of formation of some AlPO species decreases. On the other hand, it has been suggested by Samadi et al. that peak III can be assigned to the complex formed between hexaaqualuminum and phosphoric acid, where the two species are linked through hydrogen bonding [9]. By looking at Figs. 2 and 3, corresponding to the 27Al and phosphorus-31 {1H} NMR spectra in the same AlPO solution, a satisfactory agreement can be reached between the two results. For instance, the last peak in the aluminum-27 NMR spectrum (Fig. 2) and peak IV in the phosphorus-31 {1H} NMR spectrum (Fig. 3), assigned to [Al(H2O)5(H2PO4)]2þ, are reduced in intensity with decrease in the concentration of P. Also the peak at the chemical shift of 6.7 ppm in the aluminum-27 NMR spectrum and peak V in the phosphorus-31 {1H} NMR spectrum of [Al(H2O)5(H3PO4)]3þ suggest a decrease in the peak intensities with decrease in the P concentration. Peak III in the phosphorus-31 {1H} NMR spectra and the shoulder in the second peak (3.2 ppm) in the aluminum-27 NMR spectra vanish at the Al/P mole ratio of 8.0:1.0. 3.4. Effect of existence of methanol in aluminum-27 NMR spectra of AlPO species Fig. 4a displays the aluminum-27 NMR spectrum for the methanolic aluminum sulfate solution. As it is clear in this figure, there is only one signal existing at the chemical shift of 0.60 ppm, which can be assigned to the hexaaquaaluminum cations, [Al(H2O)6]3þ. Fig. 4bed displays the aluminum-27 NMR spectra for the solutions of the same P and Al concentrations (0.55 mol L1), and also having the CH3OH: H2O volume ratios of 150:1, 25:1, and 1:1, respectively. All Al complexes are octahedral, and some water
molecules can be replaced by methanol. However, replacing CH3OH and H2O molecules with H3PO4 and the related species results in broadening of the observed Al resonances in the aluminum-27 NMR spectrum of the AlPO solution. This phenomenon is anticipated because the symmetry of the complexes is reduced, and/or exchange processes occur among the species [9]. According to some previous reports [6,9,34], the broad peak centered at the chemical shift of 7.5 ppm can be attributed to the species [Al(H2O)5(H2PO4)]2þ, trans-[Al(H2O)4(H2PO4)2]þ, and [Al(H2O)5(H3PO4)]3þ. An additional signal came into view at the chemical shift of 2.5 ppm with decrease in the CH3OH: H2O volume ratio. This peak can be assigned to the species [Al(L)]mþ (where L is a Hbonded polymeric phosphoric acid like H6P2O8 and H5P2O 8 , and m is undetermined) or [Al(H2O)4(CH3OH) (H6P2O8)]3þ and [Al(H2O)4(CH3OH)(H5P2O8)]2þ[6,9,34]. 3.5. Studying phosphorus-31 {1H} NMR spectra of AlPO species in the presence of methanol To study the effect of methanol on the formation of different AlPO species in the methanolic solutions of AlPO, a number of experiments were performed by adding different volume ratios of water and methanol to the AlPO solutions with the same P and Al concentrations (0.55 mol L1). Fig. 5a displays the phosphorus-31 {1H} NMR spectrum of a solution of AlPO with the CH3OH: H2O volume ratio of 200:1. Fig. 5bed shows the phosphorus-31 {1H} NMR spectra for the solutions with the same P and Al concentrations (0.55 mol L1) and the CH3OH:H2O volume ratios of 100:1, 40:1, and 1:1, respectively. On the basis of the number and intensity of the signals available in the spectra, we can see that the peak locations as well as the peak intensities highly depend on the methanol concentration. One may note that the existence of sulfate ions in the solution (AlCl3 was used as the aluminum source in this work) does not affect the distribution of AlPO complexes [6,34]. As it can be seen in Fig. 5, all the signals existing in the spectra are situated at frequencies lower than the reference signal for H3PO4 (85%) at 0.0 ppm. Pople [38,39] has indicated that with increase in
Fig. 4. Aluminum-27 NMR spectra of (a) aluminate solution in methanol ([Al] ¼ 0.43 mol L1) and AlPO solutions with a constant Al and P concentration (0.55 mol L1) and different volume ratios of CH3OH: H2O: (b) 150:1, (c) 25:1, and (d) 1:1.
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Fig. 5. Phosphorus-31 {1H} NMR spectra for AlPO solutions with a constant Al and P concentration (0.55 mol L1) and different CH3OH: H2O volume ratios: (a) 200:1, (b) 100:1, (c) 40:1, and (d) 1:1.
the partial charge of P atom, its nucleus becomes more shielded, and causes more negative of the chemical shift. According to the previous studies [6e9,36], the following assignment can be given for the spectrum illustrated in Fig. 5a. The peak at the approximate chemical shift of 0.20 ppm (peak I) can be assigned to the species H3PO4, H2PO 4 , H6P2O8, and H5P2O8 . The intensity of peak II located at the chemical shift of 6.4 ppm decreases with decrease in the CH3OH: H2O volume ratio; this peak cannot be seen in the absence of methanol. We suggest that peak II can be attributed to the species [Al(H2O)4(CH3OH) (H6P2O8)]3þ and [Al(H2O)4(CH3OH)(H5P2O8)]2þ. Peak III, located at the chemical shift of 8.2 ppm, can be attributed to a complex between [Al(H2O)6]3þ and H3PO4, linked through hydrogen bonding; this has been suggested by Samadi et al. [9] (See below for the structure of this complex [9].). A reduction in the intensity of peak II and an increase in the intensity of peak III can be seen when the volume ratio of CH3OH: H2O decreases. When this ratio was decreased to 100, a small signal (peak X) was seen at the chemical shift of 7.4 ppm, assigned to [Al(L)]mþ; it is noteworthy that L is a H-bonded polymeric phosphoric acid such as H6P2O8 and H5P2O 8 , and m is undetermined [3,4]. At the CH3OH: H2O volume ratio of 1:1, peak II vanishes (see Fig. 5d). The relative broad peak located at the chemical shift of 13.6 ppm (peak IV) can be assigned to the species [Al(H2O)5(H2PO4)]2þ and trans-[Al(H2O)4(H2PO4)2]þ. On the basis of the calculation of the partial charge of the P atoms, carried out by Mortlock et al. [6], the cis-[Al(H2O)4(H2PO4)(H3PO4)]2þ and cis[Al(H2O)4(H2PO4)2]þ complexes should show phosphorus-31 {1H} NMR peaks at the rough chemical shift of 14.5 ppm. Therefore, peak IV may also have contributions from these complexes. Peak V, situated at the chemical shift of 17.5 ppm, can be attributed to the complexes [Al(H2O)5(H3PO4)]3þ and [Al(H2O)4(H3PO4)2]3þ[6,7,9]. It is noteworthy that a new signal appeared at the chemical shift of 18.7 ppm (labeled as VI), which was not seen in the aqueous medium (solution without methanol) [9]. As one may note in Fig. 5, with decrease in the CH3OH: H2O volume ratios, the intensity of this peak decreases. We propose that peak VI can be attributed to
the complex [Al (H2O)5(OP (OCH3) (OH) 2)]3þ, where an OeH unit of H3PO4 is replaced by an eOCH3 group. It is noteworthy that Blackwell and Patton [40] have reported that the signals located at the rough chemical shift range of 19 to 31 ppm in the -n phosphorus-31 {1H} NMR spectra of crystalline AlPOn 4 and SAPO materials. We can now state that the peak at the chemical shift of 21.1 ppm should be assigned to {(OH)2ePe[OeAl(H2O)5]2}5þ. This peak can be seen in the AlPO sol-gels, although it is not recognizable in the aqueous AlPO solution [4]. As a result, we suggest that peak VII at the chemical shift of 21.1 ppm in the CH3OH: H2O mixture is associated with {(OH)2ePe[OeAl(H2O)5]2}5þ. We can come into this conclusion that the binuclear aluminum complexes are available in the mixtures of CH3OH with H2O. One may expect this because oligomerization (i.e. condensation) of the ionic species can occur in non-aqueous media. As we may note in Fig. 5, with decrease in the CH3OH: H2O volume ratio, the intensity of peak I increases. We can say that H3PO4, H2PO 4 , H6P2O8, and H5P2O8 are dominant at lower concentrations of methanol. The intensity of peak III (at the chemical shift of 8.2 ppm) increases at lower CH3OH: H2O volume ratios, suggesting increase in the concentration of the dimeric phosphoric acids such as H6P2O8 and H5P2O 8 (see Fig. 5aed). Also the intensity of peak X (at the chemical shift of 7.4 ppm) increases with increase in the concentration of water since the presence of complex X (see the above figure) is preferred at lower CH3OH: H2O volume ratios. 4. Conclusion In this work, the aluminum-27 and phosphorus-31 NMR spectroscopic techniques were utilized to characterize the species available in the aqueous and methanolic solutions of aluminophosphate (AlPO). Also, for the first time, (TPhA) chloride was utilized as a template. Formation of a bond between P and Al via an oxygen atom available in the ALPO species is affected by the P/Al ratio. The phosphorus-31 {1H} NMR spectra of alcoholic AlPO solutions exhibit seven peaks at lower frequencies relative to the
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reference signal of H3PO4 (85%). The results obtained revealed that some peaks were new and that they were not seen in the aqueous solution of AlPO (compare Fig. 3 with Fig. 5). The intensities of the peaks changed with variations in the CH3OH:H2O volume ratio. It is noteworthy that the new observed peaks are evidenced from the conducted studies. These results can yield a better understanding of the synthesis of the AlPO molecular sieves in non-aqueous media.
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