31P and 27Al NMR studies of aqueous (2-hydroxyethyl) trimethylammonium solutions containing aluminum and phosphorus

Spectrochimica Acta Part A 72 (2009) 382–389

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31

P and 27 Al NMR studies of aqueous (2-hydroxyethyl) trimethylammonium solutions containing aluminum and phosphorus

Abdolraouf Samadi-Maybodi a,∗ , S. Karim Hassani Nejad-Darzi a , Hamidreza Bijanzadeh b a b

Analytical Division, Faculty of Chemistry, University of Mazandaran, Babolsar, Iran Faculty of Science, University of Tarbiat Modarres, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 2 April 2008 Received in revised form 25 August 2008 Accepted 9 October 2008 Keywords: 31 P NMR 27 Al NMR Aluminophosphate (2-Hydroxyethyl) trimethylammonium chloride

a b s t r a c t Phosphorus-31 and aluminum-27 nuclear magnetic resonance techniques have been used to characterize the distribution of soluble aluminophosphate species in aqueous solutions of (2-hydroxyethyl) trimethylammonium chloride (2-HETMACl), phosphoric acid, and aluminum sulfate. Soluble aluminophosphate cations obtain from reactions of hexaaqua aluminum cations [A1(H2 O)6 ]3+ , with phosphate ligands (i.e., H3 PO4 , H2 PO4 – , and acid dimers H6 P2 O8 and H5 P2 O7 – ). 31 P NMR and 27 Al NMR spectroscopies are very powerful techniques for characterization of the species present in the solution. A number of solutions containing different mole ratio of Al/P were prepared. The assignment of the peaks to aluminate connectivities is attempted, clarifying earlier works and producing information on the equilibrium between various aluminum-containing species (different aluminophosphate complexes). At least seven separated resonances were observed by 31 P NMR spectroscopy indicating presence of different complexes in aluminum phosphate solutions. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Alminophosphate (AlPOs) system is one of the useful molecular sieves, and is widely utilized as catalysts and molecular sieves in industrial processes. One of the greatest challenges of experimentalists working in the field of molecular sieve science is the understanding of the principles that determine how porous crystalline materials are formed starting from a precursor gel under hydrothermal conditions [1]. This is far from easy because hydrothermal crystallizations take place in a closed vessel, where many interactions, equilibriums, and chemical processes continuously change with crystallization time [2]. Consequently, to discover mechanism of such materials requires a systematic and intelligent screening of the reaction [3]. As with aluminosilicate zeolites, open-framework aluminophosphate made up of Al–O–P bonds obey Lowenstein’s Rule [4] with an avoidance of Al–O–Al bonds (only one exceptional case was reported in a layered AlPO containing Al–O–Al linkages) [5]. The P–O–P bonds do not appear to be stable in these structures [6]. Thus the avoidance of Al–O–Al and P–O–P bonds provides open-framework AlPOs featured by even-numbered rings. The majority of AlPO4 -n molecular sieves are based on a four connected network of corner sharing tetrahedra, i.e., AlO4b and

∗ Corresponding author. Tel.: +98 1125242002; fax: +98 1125242002. E-mail address: [email protected] (A. Samadi-Maybodi). 1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.10.002

PO4b (b: bridging oxygen between Al and P). There is a number of AlPO4 -n with mixed-bonded frameworks containing five- or six-coordinated Al atoms with one or two extra framework oxygen species, such as OH and H2 O [7]. The anionic open-framework AlPOs display a wide range of Al and P coordinations. According to the Lowenstein’s Rule, the number of Al–Ob bonds must be equal to the number of P–Ob bonds in open-framework AlPOs. By applying advanced solid-state NMR techniques, the detailed Al and P coordinations can be determined. Yu and Xu [8] have investigated a series of anionic framework AlPOs with different Al and P coordinations by various solid-state NMR techniques, including 27 Al, 31 P magic angle spinning (MAS), 27 Al → 31 P cross polarization (CP), 27 Al{31 P} rotational echo double resonance (REDOR), and 31 P{27 Al} transfer of population double resonance (TRAPDOR). Furthermore, a new method to determine the Al/P ratio of openframework AlPOs as well as NMR studies has been investigated for understanding of unknown AlPO structures [9]. Aluminum is the most abundant metal ion in the earths crust and the third most abundant element after oxygen and silicon [10]. The earth’s crust contains 8.1% by weight of aluminum compared to 5.0% by weight of iron [11]. Aluminum accumulation is also specifically linked with Alzheimer’s disease, although whether or not it is a causative agent remains controversial [12]. 27 A1 NMR has been employed extensively in the investigation of aluminum complexes in nonaqueous and aqueous solution. Although the 27 A1 nucleus is quadrupolar (spin = 5/2), the combination of high natural abundance (100%) and sensitivity (0.2 relative

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to protons) readily allows observation of reasonably symmetric complexes in the milimolar concentration range [13]. Deschamps and Massiot [14] were investigated Al–O–P–O–Al networks using two-dimensional correlation (COSY) solid-state NMR technique. Aluminophosphate species, such as [Al(H2 O)4 , (OH)(H2 PO4 )]+ , have been suggested as the nutrients for the growth of AlPOs molecular sieves [15]. Mortlock et al. [16] indicated the presence of [Al(H2 O)5 (H3 PO4 )]3+ , [Al(H2 O)5 (H3 PO4 )n ]m+ (n ≥ 2, m is undetermined), [Al(H2 O)5 (H2 PO4 )]2+ and [A1(H2 O)4 (H2 PO4 )2 ]+ cations under acidic conditions. The effects of pH on the distribution of AlPOs cations over the range of 2–13 were investigated by Mortlock et al. [17]. Salmon and co-workers [18–20] were reported the species present in solution, based on analyses of adsorption data obtained using cationic and anionic exchangers. Anion-exchange experiments indicate the presence of [Al(HPO4 )3 ]3− complexes, and cationic-exchange studies indicate the existence of species such as [A1(HPO4 )]+ , [A1(H2 PO4 )]+ , A1(H2 PO4 )2 ]+ and binuclear A1 species. A wide range of chemical shift (700 ppm) for the 31 P nucleus ensures good separation of signals in different environment. 31 P nucleolus is 100% naturally abundant and has high sensitivity of 31 P NMR. These parameters make 31 P NMR technique a reliable analytical tool similar to 19 F and 1 H NMR [21,22]. Recently, one of us (Samadi-Maybodi) has been used aluminum27 NMR spectroscopy for investigation of the species present in the alcoholic 2-HETMA aluminate solutions [23]. To the best of our knowledge, no paper reported within the 27 Al and 31 P NMR spectroscopy of aluminophosphate species using a 2-HETMA cation as a template and aluminum sulfate as an aluminum source. In this work, we prepared series of aluminophosphate solution containing aluminum sulfate, orthophosphoric acid and (2-hydroxyethyl) trimethylammonium chloride (2-HETMACl) as a template for pH ≤1.0. 27 Al and 31 P NMR spectra of corresponding solutions have been recorded in similar spectroscopic conditions. Signals of AlPOs complexes in both spectra were identified and interpreted. The kinetic reaction has also studied using 31 P NMR techniques.

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equilibrium; a recycle delay of 0.2 s was applied. The 27 Al chemical shifts were measured by substitution with a 1.0-M aqueous sample of AlCl3 and are thus considered to be referenced with respect to the octahedral [Al(H2 O)6 ]3+ . In all cases, 350 ␮L of the sample solution was poured into the NMR tube and then added 100 ␮L of deuterium oxide (to give a 2 D field/frequency lock signal). All 31 P NMR spectra were recorded with the same instrument, operating at 202.46 MHz for the phosphorus-31. 31 P NMR spectra were obtained by applying 90◦ pulses (12 ␮s pulse duration) with recycle delay of 2 s. The same above procedure was performed for the frequency lock. The chemical shifts are expressed in ppm relative to an 85% H3 PO4 solution. The probe temperature was 25 ◦ C for qualitative analysis of aluminum phosphate. 1 H decoupling was applied for the recording of 31 P NMR spectra. All 27 Al and 31 P NMR spectra were recorded at least 2 days after preparation of the solutions. 4. Result and discussion 4.1.

27 Al

NMR of stock aluminates solution

Fig. 1a presents 27 Al NMR spectrum of the aqueous aluminum sulfate solution. As can be seen, this spectrum consists of two signals where appeared at chemical shift of 0.0 (band A) and −3.6 (band B) ppm, The signal at 0.0 ppm is denoted to the Al(H2 O)6 3+ and band B is assigned to a [Al(H2 O)5 (SO4 )]+ complex. Akitt et al. [24] reported that the peak area of the latter signal is roughly increased by increasing of sulfate concentration. Fig. 1b 27 Al NMR spectrum of solution with mole ratio of phosphor to aluminum from Al/P = 1 with the same concentration of aluminum and phosphor (0.35 M). In contrast to the earlier report [25] this spectrum exhibits four distinct peaks (designated as A, B, C, and D). As can be seen in Fig. 1b, two new signals are appeared at lower frequencies, i.e. ıAl = −6.6 and −8.2 ppm (bands C and D). As mentioned above, peak centered at 0 ppm (i.e. band A) is assigned to [Al(H2 O)6 ]3+ . The band B where located at chemical

2. Materials and methods Stock aluminate solution was prepared by dissolving 7.10 g of aluminum sulfate hexadecahydrate (Al2 (SO4 )3 ·16H2 O, Fluka Co.) and 3.14 gr of (2-hydroxyethyl) trimethylammonium chloride (2-HETMACl) (Merck Co.) in 25 mL of diluted sulfuric acid. The concentration of aluminum, H2 SO4 and 2-HETMACl in stock solution was 0.90, 0.90 and 0.45 M, respectively. Working solutions were prepared in 5.0 ml volumetric flasks by addition of appropriate amounts of stock aluminate solution, orthophosphoric acid (H3 PO4 , Fluka Co.) and diluted by deionized water to the mark. All solutions were highly acidic containing different mole ratios of Al:P with the same concentrations of aluminum (0.35 M), H2 SO4 (0.20 M) and 2HETMACl (0.35 M). All solutions are stable, i.e. neither precipitation nor gel formation have been found during experiments and always were clear when they used for recording of the NMR spectra. 27 Al and 31 P NMR spectra of the corresponding solutions were recorded in the same spectral conditions at ambient temperature (ca. 25 ◦ C). 3. Instrumentation 27 Al NMR spectra were collected on a Bruker DRX-500 Fourier transform NMR spectrometer, operating at 130.32 MHz for aluminum frequency. 27 Al NMR spectra were obtained by applying 90◦ pulses (21 ␮s pulse duration), the time elapsing between pulses being sufficient to allow a complete return of the magnetization to

Fig. 1. 27 Al NMR of (a) stock aluminum sulfate solution in H2 SO4 , 2-HETMACl and H2 O and (b) an aqueous aluminophosphate solution with the same concentration of aluminum and phosphor (0.35 M). 27 A1 NMR resonances are referenced to the 27 A1 peak in a 1.0-M AlCl3 solution (a.u. is the arbitrary unit).

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Table 1 27 Al NMR band assignments from Fig. 2. Band

Chemical shift (ppm)

Assignments

A B

0.0 to −1.0 –3.0 to –5.0

C

–6.0 to –7.5

D

–7.5 to –9.0

Free [A1(H2 O)6 ]3+ [Al(H2 O)5 (SO4 )]+ and [Al(L)]m+ (L is H-bonded polymeric phosphoric acid such as H6 P2 O8 and H5 P2 O8 – , m is undetermined) Al complexed to H3 PO4 ligands, [Al(H2 O)5 (H3 PO4 )]3+ Al complexed to H2 PO4 – ligands, [Al(H2 O)5 (H2 PO4 )]2+ and trans-[Al(H2 O)4 (H2 PO4 )2 ]+

ratio of Al/P), the band intensity of C and D are decreased. At the mole ratio of Al/P equal 5:1 and higher, bands C and D are disappeared. This phenomenon is expected since by decreasing concentration of phosphor, formation probability of the aluminophosphate species such as [Al(H2 O)5 (H3 PO4 )]3+ , [Al(H2 O)5 (H2 PO4 )]2+ and trans-[Al(H2 O)4 (H2 PO4 )2 ]+ is negligible. It is proposed that the some oligomers of aluminate species such as [(H2 O)–Al–(OH)2 –Al–(H2 O)4 ]4+ can be presented in the solution but the corresponding NMR signals are too broad to be detected by 27 Al NMR spectroscopy [26,27]. 4.3.

shift of −3.6 ppm contains a sharp signal plus a shoulder (in the right-hand side) can be denoted to the species of [Al(H2 O)5 (SO4 )]+ and [Al(L)]m+ (L is H-bonded polymeric phosphoric acid such as H6 P2 O8 and H5 P2 O8 − , m is undetermined), respectively (see Table 1) [16,24]. The peak at chemical shift of ıAl = −6.6 ppm is assigned to the complex of [Al(H2 O)5 (H3 PO4 )]3+ . Finally, the band D where centered at ıAl = −8.2 ppm is correlated to the species of [Al(H2 O)5 (H2 PO4 )]2+ and trans-[Al(H2 O)4 (H2 PO4 )2 ]+ . 4.2.

27 Al

NMR spectra of AlPOs with Al/P ≥ 1

Fig. 2(a–f) exhibits 27 Al NMR spectra of aluminophosphate solutions with mole ratio of phosphor to aluminum from Al/P = 1 to Al/P = 20 with the constant aluminum concentration (0.35 M). By decreasing concentration of phosphor (i.e. with higher mole

31 P

NMR spectra of AlPOs with Al/P ≥ 1

Fig. 3(a–e) shows the 31 P NMR spectra of the same solution for which the 27 Al NMR spectra are presented in above section. Fig. 3a presents the 31 P NMR spectrum of [Al] = [P] = 0.35 M. As can be seen, all four bands (E–H) are located at lower frequency with respect to the reference signal of H3 PO4 (85%) (i.e. 0.0 ppm). On the basis of previous reports [16,25,25,28] the following assignment can be presented: The peak at chemical shift of about 0.0 ppm (band E), is assigned to species of H3 PO4 , H2 PO4 − , H6 P2 O8 and H5 P2 O8 − . Band F at ıP = −7.5 is denoted to the species of [Al(L)]m+ (L is H-bonded polymeric phosphoric acid such as H6 P2 O8 and H5 P2 O8 − , m is undetermined) [16]. Band G that centered at chemical shift of −13.0 ppm can be associated to the species of [Al(H2 O)5 (H2 PO4 )]2+ and trans-[Al(H2 O)4 (H2 PO4 )2 ]+ . Finally, band H where appears at chemical shift of −16.1 ppm denoted to the complexes of [Al(H2 O)5 (H3 PO4 )]3+ . Data compositions of the solu-

Fig. 2. 27 Al NMR spectra of an aqueous aluminophosphate solution with constant aluminum concentration (0.35 M) and different phosphor concentration (Al/P ≥ 1): (a) Al/P = 1.0, (b) Al/P = 2.0, (c) Al/P = 5.0, (d) Al/P = 10.0, (e) Al/P = 15.0, and (f) Al/P = 20.0. 27 A1 NMR resonances are referenced to the 27 A1 peak in a 1.0-M AlCl3 solution.

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Fig. 3. 31 P NMR spectra of an aqueous aluminophosphate solution with constant aluminum concentration (0.35 M) and different phosphor concentration (Al/P ≥ 1): (a) Al/P = 1.0, (b) Al/P = 2.0, (c) Al/P = 5.0, (d) Al/P = 10.0 and (e) Al/P = 15.0. 3l P NMR resonances are referenced to the 31 P peak in 85 wt% H3 PO4 .

tions as well as peak assignments of the 31 P NMR spectra are presented in the Table 2. By decreasing concentration of phosphor, the band intensity of G and H are decreased. At the mole ratio of Al/P equal 10:1 and higher, the band G completely disappeared. Nevertheless, the band H (ıP = −16.1 ppm is still exists indicating presence

of the [Al(H2 O)5 (H3 PO4 )]3+ (see Fig. 3d). This means that the aluminophosphate species such as [Al(H2 O)5 (H2 PO4 )]2+ , trans[Al(H2 O)4 (H2 PO4 )2 ]+ and [Al(H2 O)5 (H3 PO4 )]3+ are negligible. The band F at ıP = −7.5 ppm is decreased by decreasing concentration of phosphoric acid. In our investigation as well previous reports [16] showed that in low concentration of phosphoric acid

Table 2 Peak assignments for 31 P resonances from Figs. 3 and 6. Band

Chemical shift (ppm)

Assignments

E F

0.4 to–0.5 –7.2 to–7.6

Free phosphoric acid molecules and ions such as H3 PO4 , H2 PO4 – , H6 P2 O8 and H5 P2 O8 – [Al(L)]m+ (L is H-bonded polymeric phosphoric acid such as H6 P2 O8 and H5 P2 O8 – , m is undetermined)

F G H K L

–8.0 to–8.5 –13.0 to–13.6 –15.9 to–16.8 –20.6 –23.5

Al complexed to H2 PO4 – ligands, [Al(H2 O)5 (H2 PO4 )]2+ and trans-[Al(H2 O)4 (H2 PO4 )2 ]+ Al complexed to H3 PO4 ligands, [Al(H2 O)5 (H3 PO4 )]3+ Complexes which are binuclear Al, {(OH)2 –P–[O–Al(H2 O)5 ]2 }5+ Complexes which are trinuclear Al, {(OH)2 –P–[O–Al(H2 O)5 ]3 }7+

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Fig. 4. 27 Al NMR spectra of an aqueous aluminophosphate solution with constant aluminum concentration (0.35 M) and different phosphor concentration (Al/P ≤ 1): (a) Al/P = 1.00, (b) Al/P = 0.50, (c) Al/P = 0.20, (d) Al/P = 0.10, (e) Al/P = 0.067, and (f) Al/P = 0.05. 27 A1 NMR resonances are referenced to the 27 A1 peak in a 1.0-M AlCl3 solution.

the existence of the dimeric phosphoric acid such as H6 P2 O8 and H5 P2 O8 − are insignificant. Consequently, formations of complex Al with those species are negligible. It is noticeable to be mentioned that the signal ıP = –8.2 ppm is labeled F nearly has the same signal intensity, this signal even in very low concentration of phosphoric acid in comparison with the other signals (except free phosphoric acid, i.e. at ıP = 0.0 ppm) considerably has intense signal (see Fig. 3(a–d)). We suggest that band F ascribes to species of complex between hexaaqua aluminum and phosphoric acid that two species are bounded through hydrogen bound. By looking Figs. 2 and 3 corresponding to the 27 Al NMR and 31 P NMR spectra of the same aluminophosphate solution we can deduce a good agreement between two results. For instance, bonds D in spectrum of 27 Al NMR (in Fig. 2) and G in spectrum of 31 P NMR (in Fig. 3) that ascribed to the species of [Al(H2 O)5 (H2 PO4 )]2+ are decreased by decreasing P concentration. Also, bonds C in spectrum of 27 Al NMR and H in spectrum of 31 P NMR denoting to the species of [Al(H2 O)5 (H3 PO4 )]3+ are decreased by decreasing P concentration. The band F (in 31 P NMR spectra) and the shoulder in band B (in 27 Al NMR spectra) are gradually disappeared at the mole ratio of Al/P = 10:1. It is supposed that the signal ascribed to the species of “I” is very broad to be detectable by 27 Al NMR spectroscopy, but it is visible in 31 P NMR spectra (complex “I” is shown in Table 2).

expected since the possibility of the reaction between phosphoric acid and [Al(H2 O)6 ]3+ is increased by increasing concentration of phosphoric acid. Although it is difficult to say exactly which species can be present in the solution, however by observation of the 27 Al NMR spectra illustrated in Fig. 4, it can be imagine species in the solution undergo exchange processing and consequently a broad band can be detected by 27 Al NMR spectroscopy. Fig. 5(a–f) shows the 31 P NMR spectra of the same solution that mentioned above. By increasing concentration of phosphor (i.e. with lower mole ratio of Al/P), band E is enhanced. This occurrence is expected since by increasing concentration of phosphoric acid, existence of the H6 P2 O8 and H5 P2 O8 − are more significant. By increasing concentration of phosphoric acid (1 ≤ Al/P ≤ 0.2) bands F and G are enhanced but the intensity of bands H and F almost do not change. It can be said, by increasing concentration of phosphor, polymerization of phosphoric acid is occurred and therefore the band F corresponding to the species of [Al(L)]m+ is improved (see Table 2). Meanwhile, in this region, the concentration of H2 PO4 − is increased [24] and consequently the signal intensity of band G related to the species of [Al(H2 O)5 (H2 PO4 )]2+ is intensified. On the basis of previous report [24] which indicated that by increasing the concentrations of phosphoric acid (Al/P ≤ 0.1), the existence of the species H3 PO4 and H2 PO4 − in expense formation of H6 P2 O8 and H5 P2 O8 − is decreased. As a result, the intensity of bands F , G and H is decreased (see Fig. 5 and Table 2).

4.4. Study of 27 Al and 31 P NMR spectroscopy of AlPOs with Al/P ≤ 1

4.5. NMR study of aluminophosphte sol–gel

Fig. 4(a–f) exhibits 27 Al NMR spectra of aluminophosphate solutions with mole ratio of phosphor to aluminum from Al/P = 1 to Al/P = 1:20 with the constant aluminum concentration (0.35 M). By increasing concentration of phosphor (i.e. with lower mole ratio of Al/P), the intensity of band A is decreased. This phenomenon is

The synthesis sol–gel with molar compositions of Al:P:2HETMACl:20H2 O was prepared by adding aluminum sulfate hexadecahydrate to a phosphoric acid 85% with stirring at 50 ◦ C until the solution was clear. After 2 h mixing, corresponding amounts of water and 2-HETMACl were added with stirring. The

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Fig. 5. 31 P NMR spectra of an aqueous aluminophosphate solution with constant aluminum concentration (0.35 M) and different phosphor concentration (Al/P ≤ 1): (a) Al/P = 1.00, (b) Al/P = 0.50, (c) Al/P = 0.20, (d) Al/P = 0.10, (e) Al/P = 0.067, and (f) Al/P = 0.05. 3l P NMR resonances are referenced to the 31 P peak in 85 wt% H3 PO4 .

solution was stirred for 1 h. All 31 P and 27 Al NMR spectra were recorded after 2 days of solution preparation. Fig. 6a and b illustrates 31 P NMR spectra of aluminophosphte solution (sample 1) and sol–gel (sample 2) with Al/P = 1, respectively. Clearly, two new peaks can be observed in Fig. 6b (labelled K and L). Blackwell and Patton [29] reported that signals of microporous crystalline AlPO4 -n and SAPO-n materials in the range from −19 to −31 ppm observed by 31 P NMR spectroscopy are associated to the binuclear and trinuclear aluminum. We can suggest that band K is corresponding to the species of {(OH)2 –P–[O–Al(H2 O)5 ]2 }5+ at ıP = −20.6 ppm, and band L belongs to the {(OH)2 –P–[O–Al(H2 O)5 ]3 }7+ at ıP = −23.5 ppm. One can visualize that those species may act as a precursor species for building block of the AlPO4 zeolite structures. Therefore it can provide some more information about the synthesis of AlPO4 zeolite. It is pertinent to compare the 31 P NMR spectra of aluminophosphte solution and sol–gel is presented in Fig. 6a and b. As can be seen, all of the signals (except band E) in AlPOs sol–gel are more enhanced. This announces that the species of [Al(L)]m+ , [Al(H2 O)5 (H2 PO4 )]2+ , trans-[Al(H2 O)4 (H2 PO4 )2 ]+ and [Al(H2 O)5 (H3 PO4 )]3+ are more concentrated. The significant growth of the band G in the 31 P NMR spectrum in Fig. 6b indicates that the oligomerization processing is occurred in the sol–gel. In spite of the high natural abundance of aluminum-27 nucleus, duo to high asymmetric species such as {(OH)2 –P–[O–Al(H2 O)5 ]2 }5+

Fig. 6. 31 P NMR spectra of (a) an aqueous aluminophosphate solution ([Al] = [P] = 2-HETMACl = 0.35 M) and (b) aluminuphosphate sol–gel ([Al] = [P] = 2HETMACl = 0.7 M). 3l P NMR resonances are referenced to the 31 P peak in 85 wt% H3 PO4 .

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Fig. 7. Proposed mechanism for the reaction between hexaaqua aluminum cation and phosphoric acid (and its related compound).

and {(OH)2 –P–[O–Al(H2 O)5 ]3 }7+ and also because aluminum is a quadrupole nucleus those species do not detectable by 27 Al NMR in sol–gel [30]. Although samples 1 and 2 have the same ratio of Al:P but the concentration of the latter is twice of the former. It can be realized that in sample 2 (sol–gel) interaction between phosphoric acid (and related compounds) and [Al(H2 O)6 ]3+ cation is more taken place. Consequently, it can be expected that signal intensity of band E is reduced but instead of intensity of the bands F, F , G and H are enhanced. Proposed mechanism for the reaction between hexaaqua aluminum cation and phosphoric acid (and its related compound) is depicted in Fig. 7.

5. Conclusions

4.6. Kinetic studies

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The aim in this study is to provide some insight into aluminate/phosphate replacement processes. An aluminophosphate solution was made by adding phosphoric acid to a freshly prepared aqua-aluminate solution to achieve a P/Al ratio of 1. This solution contained of Al (0.35 M), H2 SO4 (0.20 M), 2-HETMACl (0.35 M) and phosphor (0.35 M). To study the rate of reaction between aluminate and phosphoric acid (and its congeners deprotonated), following procedure were performed, all at a temperature of 25 ◦ C: (1) Recording the of 31 P spectrum soon after mixing the solutions. (2) Recording the of 31 P spectrum 20 min mixing the solutions. (3) Recording the of 31 P spectrum two days after mixing the solutions. Although 31 P spectra were recorded at different times but there were no different between them. The spectra were the same as the spectrum shown in Fig. 3a. This results show that all of five signals are appeared soon after mixing. Consequently, it can be realized that the rate of reaction is very fast.

The work here presented 27 Al and 31 P NMR spectroscopy of acidic aluminophosphate solutions using (2-hydroxyethyl) trimethylammonium chloride as a template. 31 P and 27 Al NMR spectra reveal evidence for Al bound to P atoms through oxygen atoms. The production of P and Al sites present in aluminophosphate species is affected by the P/Al ratio. Results obtained from 31 P NMR spectra indicated that the reaction between aluminate and phosphate is very rapid. References

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