Extended study on the synthesis of amorphous titanium phosphates with tailored sorption properties

Journal of Non-Crystalline Solids 358 (2012) 2943–2950

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Extended study on the synthesis of amorphous titanium phosphates with tailored sorption properties Marina V. Maslova a, Daniela Rusanova b,⁎, Valeri Naydenov c, Oleg N. Antzutkin d, Lidia G. Gerasimova a a Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials, Kola Science Center, Russian Academy of Sciences, Fersman St., 26a, Apatity, Murmansk region, 184209 Russia b School of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom c SunPine AB, Box 76, 941 22 Piteå, Sweden d Division of Chemical Engineering, Luleå University of Technology, 97187 Luleå, Sweden

a r t i c l e

i n f o

Article history: Received 30 April 2012 Received in revised form 21 June 2012 Available online 17 August 2012 Keywords: Titanyl sulphate; Titanium phosphate; Synthesis; Ion-exchanger

a b s t r a c t The influence of concentrations of both TiO2 and H2SO4 in the syntheses of amorphous titanium phosphates (TiP) is reported. IR, XRD, TGA, BET and NMR techniques were used to characterise the isolated TiP products. The concentration of sulphuric acid in the initial synthesis plays a major role in the structural diversity and sorption properties of the final ionites. In the primary solutions, Ti(IV) is in monomeric, polymeric and colloidal forms. Upon addition of H3PO4 the presence of monomeric titanium ensures formation of the Ti(HPO4)2 phase. The polymeric Ti(IV) is responsible for formation of the titanium hydroxo-phosphate phase, Ti(OH)2(HPO4), whilst the colloidal form of Ti(IV) appears to have a role in coagulation of a minor Ti(OH)4 phase in an amorphous TiP. It is found that TiP ion-exchange capacities gradually increase with an increase of both TiO2 and H2SO4 concentrations and reach a maximum value of 3.8 mg-eq g−1 when TiO2 is 70–100 g L−1 and H2SO4 is 480–560 g L−1. Analyses of compositional, structural and sorption data allowed 3D correlation diagrams to be built that can facilitate fabrication of TiP with tailored sorption properties. © 2012 Elsevier B.V. All rights reserved.

1. Introduction In recent years, the synthesis of titanium phosphates (TiP) has attracted considerable attention: carefully chosen synthetic conditions and post-synthesis treatments result in custom-built TiP that have been found to work as ion-exchangers, catalysts, selective membranes, plastic fillers and whiteners in pulp and paper production [1–3]. Notwithstanding the emphasis that has been given to their ion-exchange properties, TiP-based ionites have been shown to be highly effective in purifying various waters containing radio-nuclei and/or a number of heavy d-elements [4–7]. Titanium phosphate materials can be obtained relatively straightforwardly from Ti(IV) solutions and phosphoric acid. An important synthetic aspect is to ensure the presence of discrete and reactive Ti(IV) species upon addition of H3PO4. This is somewhat challenging considering the rather small size of Ti(IV) ion combined with its high charge density, which in aqueous solutions leads to formation of titanium oxo- species of variable complexity, and which can precipitate as hydrated TiO2. To overcome this, TiCl4 and various titanium alkoxides have been mostly utilised as sources of discrete Ti(IV). [8–11].

⁎ Corresponding author. Tel.: +44 161 275 1412; fax: +44 161 275 4598. E-mail address: [email protected] (D. Rusanova). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2012.06.033

In comparison to titanium tetra-halides and alkoxides, titanyl sulphate, TiO(SO4), is more environmentally benign and a low-cost, viable source of Ti(IV). In acidic solutions containing high concentrations of titanyl sulphate the titanium is mainly present as infinite \Ti\O\Ti\O\ chains, whereas low concentration favours monomeric titanyl species [8]. Further to that, studies on the influence of free sulphuric acid concentration on the conversion of titanyl sulphate to hydrated titanium dioxide have shown that high sulphuric acid concentration in the synthetic solutions could entirely prevent Ti(IV) hydrolysis. Thus for highly acidic solutions where de-protonation of Ti(IV)-particles is favoured the highly reactive and discrete TiО2+ groups predominate [12,13], which makes TiO(SO4) a valuable source of titanium. The titanium phosphate exchangers are synthesised from acidic Ti(IV) solutions via precipitation with H3PO4 acid [1]. Variations in the synthetic conditions result in products of different P:Ti ratio and of different levels of crystallinity, and hence different ion-exchange abilities. For example at high P:Ti ratio in the initial solutions the amount of phosphate groups increases [14,15]. In relation to their sorption properties, it has been demonstrated that the cation-exchange capacity of the ionites decreases when the amount of phosphorus (in hydro- and di-hydro-phosphate forms: НРО42−, Н2РО4−) decreases, whilst their anion-exchange capability is solely determined by the number of hydroxo-groups in relation to titanium ions [16,17]. It has also been shown that an increase of synthesis temperature up to the boiling

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point leads to a decrease of sorption properties as a consequence of Ti(IV) hydrolysis (that initiates at temperatures above 80 °C) [13]. All this indicates a widespread current interest in this topic, but a detailed literature review reveals that only synthesis of crystalline TiP has been explored analytically [4,7,14,15,18–20]. The studies of amorphous TiP ionites remain somewhat isolated and mostly focused on varying the H3PO4 acidity [21–24]. It is worth noting that there is a lack of comprehensive and methodical studies on the influence of primary synthetic solution composition over properties of isolated TiP ionites for both crystalline and amorphous sorbents. Our earlier studies on titanium phosphate synthesis have also been targeting variations of phosphoric acid used for synthesis in combination with the variations of the temperature of synthesis [25]. The aim of this work is to map the composition and sorption properties of amorphous TiP synthesised from a TiO2–H2SO4–H3PO4–H2O system (using TiO(SO4) as the source of titanium) by varying concentrations of both TiO2 and H2SO4, systematically, and to explain the observed differences in properties in terms of TiP structural diversity. Stepwise this means: (i) To elucidate the effect of both (a) TiO2 and H2SO4 concentrations, and (b) Ti(IV) forms (passive, active and colloidal) present in the initial synthetic solutions, on the composition of amorphous TiP ionites obtained; (ii) To interpret the texture and sorption properties of the final TiP products in terms of both (a) TiO2 and H2SO4 concentrations utilised and (b) Ti(IV) speciation; (iii) To combine the knowledge of functional group dissimilarity (H2PO4−, HPO42− and/or OH−) with their accessibility (porosity, particle size) and show that the relationships established in this study can be easily used for synthesis of titanium phosphate ionites with tailored cation-exchange properties. To the best of our knowledge there is no other study that extensively covers the Ti(IV) speciation in TiO2–H2SO4–H3PO4–H2O systems and relates it to the properties of the amorphous titanium phosphate ionites obtained. 2. Experimental 2.1. Synthetic solutions The titanium phosphate products were precipitated from the TiO2–H2SO4–H3PO4–H2O system, where the contents of H2SO4 (10–550 g L −1) and Ti(IV) with respect to TiO2 (5–100 g L −1) were varied widely. An acidic solution of oxo-titanium sulphate (TiO2 — 268.5 g L −1, c.H2SO4 — 302.9 g L −1) obtained by the method of Motov and Maximova [26] was used as the source of titanium. A given amount of this acidic solution was diluted to the desired concentration (with respect to TiO2) and then combined with the corresponding amount of H3PO4 to initiate precipitation. 60% H3PO4 was used in all experiments where the required amount was recalculated to achieve a fixed mole ratio of TiO2:P2O5 = 1:1 within all initial synthetic solutions (although the TiP products obtained showed different ratios, see Compositional aspects — characteristic synthesis zones and Ti(IV) forms in solution section). The total solution volume in all cases was kept at 100 mL. 2.1.1. Synthetic procedure About 60 solutions with different concentration of H2SO4 and TiO2 were prepared as follows: a solution containing oxo-titanium sulphate with a given concentration was heated to 70 °С and slowly combined with the corresponding amount of H3PO4 solution. The synthetic mixture was kept at this temperature for 24 h under constant stirring. The resulting white solid precipitate was filtered, washed with a portion of 5% H3PO4 (for removing excess H2SO4) and then with distilled water

until the washings were at pH 2.8–3.2. The solid products were dried initially in air and then at 60 °С. 2.1.2. Determination of ion-exchange capacity The titanium phosphate products were treated with 0.5 M NaOH for 12 h (solid:liquid = 1:100). After filtration, the solutions were titrated with 0.1000 M HCl (using bromthymolblue as indicator). Titrimetric measurements errors estimated: ±0.05 mL. The exchange capacity E (in mg-eq g −1 sorbent) was calculated using the equation [27]:

E¼

VHCl  MHCl  VSolution VAliquot  mSorbent

2.2. Characterisation of the form of Ti(IV) For quantification of the different forms of Ti(IV) present in the initial solutions, the spectrophotometric-kinetic method of Kozachek was used [28,29]. The method uses the well-known kinetics of reaction between Ti(IV) and hydrogen peroxide in the presence of polyacrylamide that leads to formation of a yellow-coloured complex, wherein hydrolysed forms of Ti(IV) react slowly with H2O2, whilst the colloidal ones remain intact. In detail: – for determination of the total amount of Ti(IV), an aliquot of a given solution was heated to boiling in the presence of H2O2. A photo-colorimeter (KFK-2) was used to measure the λmax of the resultant coloured solutions; – the concentration of monomeric (active form) TiO2 was determined as follows: an aliquot of solution was inserted into a 25 mL volumetric flask containing 5% H2SO4 (nearly filled), 1.0 ml 30% H2O2 and 5 drops of polyacrylamide, and was then filled to the mark with sulphuric acid. The first colorimetric measurement of the solution was made after the solution had been in contact with the H2O2 for 1 min. The cuvette was then kept inside the instrument and measurements were repeated at 2, 4, 6, 8 and 10 min. The data were plotted as [TiO2] in g L−1 as f(t) time in minutes, and were extrapolated back to t equal to 0 min. The concentration of [TiO2] at t =0 represents the amount of active Ti(IV) that is in a non-hydrolysed form; – the amount of polymeric forms of Ti(IV) i.e. the hydroxo(oxo)sulphate complexes (in other words the titanium in passive form) is then calculated as a difference of Ti(IV) at t= tmax min (tmax is the time when the concentration of Ti(IV) stops changing) and the concentration of active titanium at t= 0 min; – the Ti(IV) forms that coagulated with polyacrylamide (the colloidal forms) were determined as a difference from the total amount of titanium in the solution and the amount of Ti(IV) at t = tmax (in min) as determined by the colorimeter. The experimental errors in determination of the Ti(IV) forms were estimated to be less than ± 0.5–1.0% in all cases. 2.3. Characterisation and instrumental techniques The product solid phases (TiPs), dried at 60 °C, were dissolved in a mixture of HF–HNO3–HCl and the concentrations of titanium and phosphorus determined using an ICPE-9000 spectrometer. The resultant concentrations are presented in terms of TiO2 and P2O5 oxides. The corresponding filtrates were also analysed for the presence of titanium, phosphorus, and sodium (in relation to ion-exchange capacity determinations) using an atomic absorption spectrometer, Perkin–Elmer AAS 300. All elemental analysis data was obtained with accuracy of ±0.05%. The thermogravimetric (TG) and differential thermal analysis (DTA) of the samples were carried out in an argon atmosphere using a high-resolution thermogravimetric analyzer (Model Netzch STA 409/ QMS) in a range of 25–900 °C with a heating rate of 10 °C min−1.

M.V. Maslova et al. / Journal of Non-Crystalline Solids 358 (2012) 2943–2950

The powder X-ray diffraction studies of as-synthesised and calcined (160–900 °C) samples were performed using a Siemens D 5000 diffractometer with monochromatic Cu Kα radiation (λ = 1.5418 Å). A scanning rate of 2.4°min−1 was used for a 2θ diffraction angle range of 10–50°. A Micrometrics ASAP 2000 analyzer was used to characterise the surface properties and porosity of the product TiP ionites. All samples were degassed at 373 K for 4 h prior to N2 adsorption/desorption measurements. This relatively low degassing temperature was selected to avoid any structural changes in the materials. The specific surface area was calculated using the BET equation. The total pore volumes of samples were determined by converting the amount of adsorbed dinitrogen at a relative pressure of 0.995 to the volume of liquid dinitrogen. The external surface areas and micropore volumes were determined by the t-plot method. Pore size distributions were determined by the BJH method using the desorption branch of corresponding isotherms [30]. The size and shape of titanium phosphate agglomerates were studied using an optical microscope (Leica DM 2500R) and phase homogeneity was deduced using a polarised light source. The density of TiP agglomerates was obtained using a Nova-1100 densitometer. The size of particles included in the agglomerates was calculated using the following formula: S = 6/dρ where S is the surface area (cm 2 g−1), d is the diameter of particles (cm) and ρ is the density of products (g cm−3) [30]. Infrared (IR) spectra were recorded on a Fourier transform IR spectrometer (Perkin–Elmer FT-IR 2000). A resolution of 4 cm −1 was used with 100 scans averaged to obtain a wavenumber spectrum ranging from 4000 to 370 cm −1. The spectra were recorded in IR-grade KBr discs at room temperature. Solid-state 31P NMR experiments were recorded at 145.70 MHz on a Varian/Chemagnetics InfinityPlus CMX-360 (B0 = 8.46 T) spectrometer. The spectra were acquired using a Varian 4 mm MAS probe and samples were packed in standard ZrO2 rotors. 10 kHz spinning frequency was used. One pulse experiments with proton-decoupling were utilised. The pulse width was 5.2 μs and 3 s relaxation delays were used. 148–284 transients were averaged for all cases. All spectra were externally referenced to 85% H3PO4. Cross polarisation experiments were also performed, but no additional information was gathered. Deconvolutions of all NMR spectra were performed using Spinsight (software provided with the spectrometer). The resonance lines were simulated with a 75% Gaussian/25% Lorenzian ratio that is close to the previously reported G/L ratio for titanium phosphate systems [31,32]. All spectra were well-fitted with a maximum of 3 resonance lines (the fits were of poor quality when a model with 2-resonance lines was used). The deconvolution errors were ±0.5–1.5% for all cases.

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compositional distribution diagram to be built that shows the TiPs' compositions (expressed in TiO2:P2O5 ratio) as functions of both concentrations of TiO2 and of H2SO4 used, Fig. 1. (Fig. S1 in the Electronic supporting material displays this compositional distribution diagram in Descartes coordinates.) The iso-lines in Fig. 1 represent the TiO2: P2O5 mole ratios determined for the amorphous TiP ionites and it can be seen that variation of the H2SO4 concentration more strongly affects the composition of the final TiP products in comparison to variation of TiO2 concentration. The following general trends are also recognised: (i) When low concentration of H2SO4 is used (up to 100 g L −1) the TiO2:P2O5 mole ratio for recovered solids is 1:0.38–0.44. An increase of initial TiO2 concentration in these acidic conditions leads to an insignificant increase of Ti(IV) in the solid phase products and the TiO2:P2O5 mole ratio only changes to 1:0.44–0.47. (ii) For highly acidic synthetic solutions (H2SO4 more than 400 g L−1) the TiO2:P2O5 mole ratio is higher than 1:0.52 and practically does not depend on the initial TiO2 concentration. Thereupon comparison and analysis of all data and trends allow us to distinguish more specifically five characteristic synthesis zones (for clearer visualisation of zones see Fig S1 in ESM). Zone I: TiO2 up to 40 g L −1 and H2SO4 up to 100 g L −1. In this zone, the synthesis of amorphous TiP is carried out at low concentration of Ti(IV) and low concentration of H2SO4. The TiO2:P2O5 mole ratio of solids is 1:0.38–0.44. In such conditions of low acidity Ti(IV) is very liable to hydrolysis forming TiOH3+, TiO2+ and Ti(OH)22+, and the established “phosphorus deficiency” points towards the fact that not all Ti(IV) centres are connected to phosphorus. Hence a Ti(OH)4 phase can be anticipated along with titanium hydro- and hydroxo-phosphate phases in the final amorphous products. Zone II: TiO2 up to 100 g L−1 and H2SO4 in the range of 100– 250 g L−1. The synthesis was performed at relatively low concentration of H2SO4 and high concentration of Ti(IV) as oxo-titanium sulphate solution. The TiO2:P2O5 mole ratio varies between 1:0.42 and 1:0.48. Zone III: TiO2 is in the range of 15–100 g L−1 and H2SO4 is in the range of 150–450 g L−1. The TiO2:P2O5 mole ratio of TiP products from this zone increases to 1:0.50–0.51 suggesting that all Ti(IV) is

3. Results and discussion 3.1. Compositional aspects — characteristic synthesis zones and Ti(IV) forms in solution In this study, targeting amorphous TiPs with high ion-exchange capacities, we have used a fixed TiO2:P2O5 mole ratio of 1:1 in the synthetic solutions. The choice of concentration range (for H2SO4 up to 550 g L −1 and for TiO2 a maximum of ca. 100 g L −1) is based on technological specifics of TiP synthesis: if a higher concentration of H2SO4 is used, washing of TiP products become incomplete and sulphuric acid can still be found in the solid products, whereas if the concentration of TiO2 is higher than 100 g L −1 the subsequent filtration processes become inefficient. Within the specified concentration ranges the degree of Ti(IV) precipitation was found to be 99.3–99.8% (with regard to TiO2) as probed by ICP and AAS, and it appears to be independent of both TiO2 and sulphuric acid concentrations in the initial solutions. Analyses of all samples from the TiO2–H2SO4–H3PO4–H2O systems allow a 3D

Fig. 1. Correlation diagram: TiP composition (in mole ratio) as a function of concentrations of TiO2 and H2SO4 in the synthetic solutions. The numbers 1:0.XY represents the TiO2:P2O5 mole ratio.

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Table 1 Elemental analysis data (±0.05%) for TiP products from Zones I–V. Zones

I II III IV V

Solid phase content, %

TiO2:P2O5, mole ratio

TiO2

P2O5

Н2О

41.21 40.53 38.83 37.86 37.81

28.02 33.09 35.15 35.60 36.24

30.77 29.34 26.22 26.59 30.43

1:0.38 1:0.46 1:0.51 1:0.53 1:0.54

in the forms of titanium hydro- and hydroxo-phosphate compounds. This ratio is an indicator of synthesis of amorphous TiP solids with optimal cation exchange abilities. Zone IV: TiO2 up to 100 g L −1 and H2SO4 in the range of 300– 560 g L −1. The TiP products are obtained at high concentration of both H2SO4 and TiO2. The elemental composition in terms of TiO2: P2O5 ratio for TiP solids is 1:0.52–0.54. Zone V: TiO2 up to 50 g L −1 and H2SO4 in the range of 250– 500 g L −1. Amorphous TiP solids within this zone are obtained at conditions of very high concentration of H2SO4 and low concentration of TiO2. The measured TiO2:P2O5 mole ratio of these TiP products is generally greater than 1:0.52. From every zone a representative data point was chosen and the corresponding final products were subjected to thorough characterisation by means of IR, NMR, XRD and TG techniques. The elemental analysis data (expressed as corresponding element oxides) along with the determined TiO2:P2O5 mole ratios for the five TiP samples for Zones I–V are listed in Table 1. All other data described in this work refer to these TiP products. An important aspect of TiP synthesis was revealed when titanium forms – active, passive and colloidal – present in the initial solution, were identified prior to addition of phosphoric acid (see: Characterisation of the form of Ti(IV) in the Experimental section). These forms were quantitatively determined at ambient temperature and at the elevated synthetic temperature, 70 °C. In Fig. 2 it can be seen that distribution of Ti(IV) forms in the initial acidic titanium oxo-sulphate solutions at ambient temperature differs significantly from the pattern obtained for heated solutions for Zones I–V. In ambient conditions, independent of the amounts of TiO2 and of H2SO4, more than 80% of Ti(IV) is in an active form (valid for all zones). The elevated solution temperature, at which the syntheses were carried out, has a stronger effect on the distribution of Ti(IV) forms (Fig. 2b): in solutions of low acidity the active form of Ti(IV) drops down to 24–41% (Zones I and II), whilst the fractions of passive and colloidal titanium substantially increase. This consequential drop of active Ti(IV) is most likely related to insufficient solution acidity that promotes Ti(IV) hydrolysis followed

by polymerisation processes, which are accelerated as expected at elevated temperatures in comparison to room temperature. Upon addition of H3PO4, the passive Ti(IV) may still be able to react with phosphoric acid forming a range of titanium hydroxophosphates (though with TiO2:P2O5 ratios lower than 1:0.5) whilst the colloidal forms of titanium when formed are expected to have very low or no reactivity towards H3PO4. Furthermore, being a weak electrolyte, the phosphoric acid itself may also have a role in coagulation of polymeric forms of titanium, which would lead to titanium hydroxide formation instead. These remarks are addressed further in Section 3.2, where individual phases of the multi-phase amorphous TiP products are identified. It is worth noting here that TiP solids of Zone III appear to contain the maximum percentage of active Ti(IV) species at a “minimum” level of acidity. This is imperative for the targeted Ti(HPO4)2 phase to form once H3PO4 is supplied. Thus, from technological and economical points of view this zone should be most appealing when producing amorphous TiP with tailored sorption properties. 3.2. Structural aspects of TiPs' diversity of functional groups The amorphous TiP products obtained from the TiO2–H2SO4– H3PO4–H2O systems studied here are expected to be multi-phase solids comprised of titanium hydroxo-phosphate, Ti(OH)2(HPO4), titanium hydro-phosphate (mono-hydrogen phosphate), Ti(HPO4)2 and titanium hydroxide, Ti(OH)4, in ratios solely depending on the initial synthetic conditions. The IR spectra for TiP products obtained from Zones I–V are shown in the ESM. It can be seen that in all cases spectra of ‘typical’ amorphous titanium phosphates are obtained. The broad band around 3400 cm−1 is due to overlapping asymmetric and symmetric \OH stretching vibrations of adsorbed water molecules and P\OH groups, and the band at 1615–1625 cm−1 is generally related to the \OH bending vibration of adsorbed water. The absorption band at 1039–1047 cm−1 is assigned to the Ti\O\P stretching modes of HPO42− groups whilst the band at 439–462 cm−1 is attributed to the P\O bending vibrations [7,4]. For TiP ionites of Zone I, a weak vibration at ca. 850 сm −1 is also seen. This band has been assigned to Ti\O\Ti vibrations by various authors [33,34]. The presence of these Ti\O\Ti bridges correlates well with the observed differences in the composition of initial solutions i.e. increased fractions of passive and colloidal titanium forms in the initial solutions and lower than 1:0.50 TiO2:P2O5 mole ratio in the recovered TiP solids. The ionites from this Zone I also display a weak vibration at 723 сm −1. The Ti\O stretching vibrations that are characteristic of titanium oxide and hydroxides have been usually observed in the region of 700–750 сm −1. Different authors have attributed these frequencies to vibrations of titanium connected to non-bridging oxygen atoms [35], which in this instance indicates the presence of a terminal \OH

Fig. 2. Forms of Ti(IV) in TiOSO4 solutions: active (in blue), passive (in red) and colloidal (in green) at (a) ambient conditions, and at (b) elevated temperatures.

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group. Thus it can be suggested that titanium hydroxo-phosphate, Ti(OH)2(HPO4), and Ti(OH)4 phases are characteristic ones for Zone I-TiP ionites. When the acidity of initial synthetic solutions increases (from Zones II to V) the vibration at about 720 becomes less-resolved, which indicates that the amount of titanium hydroxo-phosphate (and/or titanium-hydroxide) formed decreases and it becomes negligible in Zones III–V. Accordingly, the Ti\О\Р vibrations in the lowfrequency region become more distinguishable, suggesting more of these bonds and an increase of titanium directly bonded to phosphate groups. The latter process takes place with a redistribution of electron density where the P\OH bond becomes weaker as is seen from the changes in P\O bending vibrations (from 462 to 439 cm−1). Deconvolution results of the 31P MAS NMR data for TiP products of Zones I–V are shown in Table 2 (whilst the deconvolution plots are shown in the ESM). All products displayed a very broad 31P resonance peak between 10 and − 40 ррm attributed to overlapping signals of similar phosphorus sites. This is in agreement with the amorphous nature of TiP materials. Deconvolutions of the data, using reported 31 P-isotropic shifts of various titanium phosphates, show that for all samples the species: P(OTi)x(OH)4−x, where x = 1 or 2, and of P(OTi)3OH type are present in different amounts [22,32,36]. The ratio of these species depends on the synthetic conditions (Zones I–V), but in all cases it is observed that the 31P resonance in a hydro-phosphate moiety (P(OTi)3OH) dominates in the data. Furthermore, according to various authors the 31P resonance of P(OTi)4, if present, should appear in the range of − 26 up to − 31 ppm and this would correspond to the existence of PO43 − species [22,36]. The IR data of these samples also did not reveal vibrations due to PO43−, which if present would have appeared at 956 and 630 сm−1 [37]. Smiutz et al. have also found that 31P resonance peaks of P(OTi)3OH species or HPO42− in amorphous TiP materials would appear between −15 and −22 ppm in a solid state 31P MAS NMR spectrum [22]. Based on these facts, we tentatively assign the 31 P resonances at about −24 ppm, observed for all Zones (I–V), to the presence of P(OTi)3OH*-species (i.e. species with slightly different Ti\O\P bond angles and bond distances) rather than to a single P(OTi)4-species. The trends in the data show that with an increase of H2SO4 acidity (from Zone I to Zone V), and upon addition of phosphoric acid, a substitution of Ti\O\Ti (Ti\OH) for HPO42− groups occurs and the amount of phosphorus in the P(OTi)x(OH)4−x (x= 1 to 3) moiety slightly decreases, whilst the amount of P(OTi)3OH* species gradually increases (as shown in Table 2). In other words: increasing the acidity leads to substitution of shorter Ti\O bonds (corresponding to terminal Ti\OH groups) to longer P\O\Ti bonds and most of the phosphorus (in summation more than 80% for all samples) is in Р(OTi)3(OH)surroundings in the form of НРО42 − sites. XRD patterns of product TiPs and of calcined (at 700 °C) ionites (from Zones I–V) did not show any long-range order and patterns typical for amorphous solids were recorded (not shown). Samples annealed at 900 °C show XRD patterns (see Fig. 3) that are consistent with patterns of Ti2O(PO4)2 and TiP2O7 phases in different ratios (formed from phases of Ti(OH)2(HPO)4 and Ti(HPO4)2, respectively, in the non-calcined samples). For ionites of Zones I and II the Table 2 Deconvolution results of Zones

I II III IV V

31

P MAS NMR data for TiP ionites.

31

P isotopic chemical shift (±0.2 ppm) and peak deconvolution data (in %)

P(OTi)x(OH)4−x, ppm x=1 or 2

%

−5.1 −5.0 −5.2 −5.1 −5.1

16.7 16.7 16.0 15.8 12.8

P(OTi)3OH, ppm

%

−13.5 −14.6 −15.5 −15.2 −14.5

71.7 68.3 66.4 65.4 63.2

P(OTi)3OH*, ppm

%

−24.0 −24.3 −24.6 −24.5 −24.2

11.6 15.0 17.6 18.8 24.0

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Fig. 3. XRD diffractograms of TiP products from Zone I (a) to Zone V (e), respectively. For clarity only the main peaks of anatase (*), Ti2O(PO4)2 (+) and TiP2O7 (−) are indicated in the plots.

presence of Ti2O(PO4)2 dominates over the amount of TiP2O7. For Zones III–V the amounts of Ti2O(PO4)2-phases decrease, whilst the pyrophosphate phase (TiP2O7) increases. This trend was also confirmed by the 31P NMR data of the samples annealed at 900 °C (not shown). It is worth noting that for solids of Zones I and II, samples synthesised under conditions of lower acidity, the anatase phase of TiO2 is also observed in the XRD data. This is correlated to the presence of titanium hydroxide, Ti(OH)4, in the non-calcined TiP products. For these two zones colloidal Ti(IV) was determined in the primary synthetic solutions (see Fig. 2b) hence the calcined final products can be expected to contain a TiO2-phase (vice versa the lack of it would correspond to absence of colloidal forms in the primary solutions). This fact is also in line with our other observations for these two zones related to lower TiO2:P2O5 ratios and evidence for Ti\O\Ti vibrations in the corresponding IR spectra. The data from thermogravimetric analysis for TiP ionites, in the temperature range of 50–900 °С, are displayed in Fig. 4. Table 3 summarises weight losses detected to 700 °С. For all samples a characteristic broad endothermic peak in the region of 131–138 °С is observed. It is assigned to the removal of adsorbed and coordinated water molecules. The mass loss to 160 °С is 10–7.5% for all samples from Zones I to V. The total weight loss for samples from Zones I and II is slightly higher than for Zones III–IV. This is most likely related to the presence of Ti(OH)4 releasing water under thermal treatment: TiðOHÞ4 →TiO2 þ 2H2 O Condensation of hydroxo- and hydroxo-phosphate groups takes place simultaneously in the temperature range of 160–700 °С. The mass loss due to this process is 21–17% for all zones. No mass loss is observed at temperature above 700 °С and two exothermic peaks

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TG,%

772.3 C

a

100

DTA, mW.mg-1 0.0

95 -0.5 90 -1.0 85 879.9 C

80

DTA, mWmg-1

TG,% 100

b

0.0 -0.2

770.6 C

95

-0.4

90

873.3 C -0.6

85

-0.8 -1.0

-1.5

-1.2 80

75

-1.4

-2.0 75

131.0 C

100

200

300

400

500 600

700

800

-1.6

137.5 C

-1.8

100 200 300 400 500 600 700 800

T, C

T, C Fig. 4. TG data for TiP products of Zone I (a) and Zone V (b).

are detected in the regions of 770–775 °С and 870–880 °С. These peaks (700–900 °С) correspond to phase transitions leading to formation of Ti2O(PO4)2 and TiP2O7, respectively. For all Zones I–V the two phases are formed as follows: −1:74H2 Oð50160 CÞ −3H2 Oð160700 CÞ 2TiðOHÞ2 ðHPO4 Þ→ Ti2 P2 O9 2TiðOHÞ2 ðHPO4 Þ•0:86H2 O→

ð700900 CÞ Ti2 P2 O9 → Ti2 OðPO4 Þ2 The theoretical mass loss is 28.52% and is in a good agreement with the experimental TG data. −H2 Oð50160 CÞ −H2 Oð160700 CÞ ð700900 CÞ TiðHPO4 Þ2 •H2 O→TiðHPO4 Þ2→TiP2 O7→TiP2 O7

It has been reported that formation of a titanium pyrophosphate phase, TiP2O7, takes place at about 880 °С [38]. In this study, the intensity of this peak (at ca. 880 °С) increases for samples from Zone I to Zone V (as shown in Fig. 4). This can be correlated to different amounts of Ti(HPO4)2 present in the samples, which lead to formation of TiP2O7 to a different extent when more acidic conditions are used in the syntheses. This is in an agreement with the XRD and 31P NMR data for these samples, wherein the amount of pyrophosphate in Zone I was found to be low whilst it was predominant in Zone V. 3.3. Sorption aspects — texture, porosity and capacity The amorphous TiP sorbents obtained are white powders composed of particles with different shapes and sizes. The morphologies of these solids were studied with SEM (not shown), but no noticeable differences were observed for samples from Zones I–V. Certain differences in the phase homogeneity were noticed when samples were probed with polarised light under an optical microscope. Fig. 5a shows an agglomerated particle from a TiP product obtained from a low-acid solution (H2SO4 less than 150 g L −1, Zone II). An increase of the sulphuric acid concentration (up to 450 g L −1), whilst the concentration of TiO2 is kept almost unchanged (as in Zone IV), results in Table 3 Weigh loss data for TiP products of Zones I–V for the temperature interval 20–600 °C. Zones

Mass loss to 160 °С, %

Mass loss, 160–700 °С, %

Total mass loss, %

I II III IV V

10.30 8.51 9 7.5 8

18.22 20.74 17.73 19.02 17.11

28.52 29.25 26.72 26.52 25.11

formation of more homogeneous agglomerates with reference to phase distribution as evident from Fig. 5b. In addition, size distribution histograms of particle-agglomerates for solids from Zones I, III and V (See Electronic supplementary material Fig. S3) show that an increase of solution acidity leads to an increase of population of agglomerates with sizes less than 100 μm. This fact correlates well with the estimations of average particle size, which show that the size of particle-agglomerates decreases from 74 down to 7.7 nm when the solution acidity increases through Zones I–V (Table 4). The surface properties and porosity of TiP ionites from Zones I–V were probed by dinitrogen adsorption–desorption measurements. A representative example of an isotherm valid for all samples is shown in ESM, Fig. S4. All isotherms are attributed to type IV according to the Brunauer classification, which is typical for mesoporous materials. The hysteresis loops observed are in the high relative pressure region (р/р0 > 0.6) that is commonly assigned to a capillary condensation associated with large pore cavities in the mesopore range [30]. All data obtained from the dinitrogen adsorption–desorption measurements are summarised in Table 5. Samples characterised with the highest surface areas can be found within Zones II and III. It is worth noting that most of this surface area is an external area, which is a prerequisite for accessible active sites. The values of total pore volume for the samples from Zones I–V show a distinctive maximum (for TiP solids from Zone III), whereas the fraction of micropore volume is negligible for all samples in Zones I–V. In contrast, the values for average pore diameter show a slightly different trend — similar values for TiPs from Zones I and II with a gradual increase for solids in Zones III–V (although values for Zones IV and V are comparable). Fig. 6 shows the pore size distribution plots, for ionites of Zones I, III and V (where Zones II and IV are omitted for clarity). In line with the lowest total pore volume, the samples from Zone I display a broad pore size distribution curve of very low intensity. The pore size distribution curve for samples corresponding to Zone III is characterised by a strong relatively narrow peak centred at about 50 nm i.e. at the transition from meso- to macro-pores. The relatively narrow peak suggests agglomerates composed of particles with nearly uniform particle sizes. The pore size distribution curve for samples from Zone V broadens and decreases in intensity compared to the curve for samples from Zone III. Capacity is one of the most important parameters of an ionite/sorbent. In this study the capacity towards sodium ions (Na+) was chosen in order to probe the sorption properties of the amorphous TiP obtained from the TiO2–H2SO4–H3PO4–H2O system. The 3D correlation diagram in Fig. 7 shows the relationship between the composition of initial solutions (amounts of TiO2 and H2SO4) and the capacity of TiP ionites analysed. (This diagram in Descartes coordinates is shown in the ESM, Fig. S5). The iso-lines represent the sorption capacity of TiP ionites towards sodium ions (in mg-eq g−1). The position of sorption contour lines shows that at low concentrations of both TiO2 (20 g L−1) and

M.V. Maslova et al. / Journal of Non-Crystalline Solids 358 (2012) 2943–2950

2949

Fig. 5. Optical microscope images (magnification ×500) for TiP products of Zones II (a) and IV (b).

H2SO4 (to 150 g L−1), Zone I, the capacity of sorbent does not exceed 2.5 mg-eq g−1. It can also be seen that the capacity gradually increases with an increase of both concentrations and reaches a maximum value of 3.8 mg-eq g−1 when TiO2 is 70–100 g L−1 and H2SO4 is 480– 560 g L−1, that is characteristic of Zone IV (Compare Figs. 1 and 7). Supplementing the solid-state 31P NMR data, the sorption capacity data point towards assignment of the 31P resonance at ca. −24 ppm to P(OTi)3OH* species that contribute to the sorption ability of TiP towards sodium ions (rather than to P(OTi)4 species, which if present would have not been expected to increase the sorption capacity of the TiP ionites). Considering the compositional and structural diversity of the prepared TiP ionites, the following can be summarised for the textural and sorption properties of sorbents from each zone: Zone I: The sorption capacity of these TiP products towards sodium ions does not exceed 2.6 mg-eq g−1. This somewhat lower ion exchange capacity is related to the presence of titanium hydroxophosphate Ti(OH)2(HPO4) combined with a minor phase of Ti(OH)4 and a low degree of porosity (amorphous solids with lowest surface area and pore volume). Zone II: The sorption ability of these TiP products gradually increases from 2.7 to 2.9 mg-eq g−1, when the concentration of TiO2 reaches 100 g L −1. However, it should be noted, that the sorption capacity of these materials increases only slightly compared to solids from Zone I despite the dramatic increase in porosity. This can be expected considering the fact that building block species that have been identified were similar in types and quantities to the species characteristic for amorphous solids within Zone I. Zone III: The sorption capacity towards Na+ is 2.8–3.2 mg-eq g −1 and is higher than the sorption affinity found for TiP sorbents of Zones I–II. This is related to dominance of the titanium hydrophosphate Ti(HPO4)2 phase that is governed by the optimal TiO2: P2O5 ratio of ca. 1:0.5 in the prepared ionites. The relatively high sorption capacity of TiP products from this zone is complemented by the high surface area (though comparable to the surface area for solids from Zone II) and also by the largest pore volume out of Table 4 Average particle size and sorption capacity of TiP products for Zones I–V. Zones

Diameter of particles, nm

Na+ capacity, mg-eq g−1

I II III IV V

74 17.5 9.4 8.7 7.7

2.52 2.84 3.18 3.82 3.16

all solids isolated (Zones I–V). Furthermore, the pore size distribution of these TiP ionites appears on the narrow side, which might be beneficial in cases where ions with different ionic radii are absorbed. Zones IV–V: Overall, the products in these zones show similar compositional and structural features: similar TiO2:P2O5 ratios and a rather homogeneous Ti(HPO4)2 phase, combined with comparable porosity parameters (surface area and pore volumes). Even if not economically beneficial, the TiP ionites from these zones are characterised with the highest values for their sorption capacities (in comparison to products from preceding zones); the capacity of TiPs from Zone IV (3.2–3.8 mg-eq g−1) are slightly higher compared to Zone V (2.7–3.2 mg-eq g−1) and so that would make them good sorbents to target. 4. Conclusions The syntheses of amorphous titanium phosphate (TiP) ionites with regard to the influence of initial concentrations of both TiO2 and H2SO4 in the TiO2–H2SO4–H3PO4–H2O preparative system were studied in great detail. The sorbents obtained were systematically analysed and data are displayed in 3D correlation diagrams. Analysis of all data shows that the composition of these multi-phase amorphous solids largely depends on the concentration parameters of their initial synthetic solutions. In particular, titanium hydroxo-phosphate Ti(OH)2(HPO4), titanium hydro-phosphate Ti(HPO4)2 and titanium hydroxide Ti(OH)4 phases in different ratios were identified in the final TiP sorbents. It was shown that in the initial acidic Ti(IV) sulphate solutions, the Ti(IV) is in monomeric-active, polymeric-passive and colloidal forms that determine the compositional diversity in the amorphous TiPs. The amount of these three types of titanium forms largely depends on the concentration of both TiO2 and H2SO4 in the primary synthetic solutions. Reaction of monomeric titanium with phosphoric acid leads to formation of the targeted Ti(HPO4)2 phase. The polymeric

Table 5 Surface properties (SA—surface area, Vp—total pore volume, Vu—micropore volume and Sexternal—external surface area) of titanium phosphate solid products of Zones I–V. Zones

SA, m2g−1

Vp, сm3g−1

Vu, сm3g−1

Daveragea, nm

Sexternal, m2g−1

I II III IV V

27.9 88.2 87.8 74.4 73.1

0.1298 0.3504 0.6717 0.5027 0.5102

– 0.0018 0.0013 0.0015 0.0022

15.2 14.1 18.6 22.5 22.3

28.2 81.8 82.6 69.2 66.4

a

Average pore diameter calculated as 4Vp/SA.

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1.8

in this study may lead to direct fabrication of titanium phosphate ionites with tailored structural and sorption characteristics.

dV/dlog(D1/D2), cm3 g-1

1.6

Acknowledgements

1.4

M.V. Maslova would like to thank the Swedish Institute for supporting this work. The authors are very grateful to Miss Amber-Marie Pearce and Prof. David Collison for linguistic and grammar corrections.

1.2 1 0.8

Appendix A. Supplementary data

0.6

The correlation diagrams in this study presented in Descartes coordinates, the IR data, the adsorption–desorption isotherm for TiP of Zone V, the histograms of TiP from Zones I, II and V and the 31P NMR deconvolution data can be found in the online version of the paper at http://dx.doi.org/10.1016/j.jnoncrysol.2012.06.033.

0.4 0.2 0 1

10

100

1000

Average pore diameter, nm Fig. 6. Pore size distributions for TiP-s for samples from Zone I (○), Zone III (▲) and Zone V (□).

Ti(IV) is responsible for formation of the titanium hydroxo-phosphate Ti(OH)2(HPO4) phase. The colloidal form of Ti(IV) is non-reactive towards H3PO4 and appears to play a role in the coagulation of the Ti(OH)4 phase. In all cases it was observed that precipitation of Ti(IV) exceeds 99% with respect to TiO2, irrespective of the initial concentrations of TiO2 and H2SO4 and it was proved that the change in the acidity has a greater impact on the final TiP ionites composition than the change of TiO2 concentration. Upon addition of H3PO4, an increase of a solution's acidity leads to a progressive substitution of Ti\O\Ti (Ti\OH) groups by phosphate groups, which in turn contributed to an increased amount of ion-exchange phosphate sites. It is shown that TiP ionite's capacity gradually increases and reaches a maximum value of 3.8 mg-eq g −1 when TiO2 is 70–100 g L −1 and H2SO4 reaches 480–560 g L −1. All these findings reveal the impact of the initial synthetic solutions' compositions onto the phase diversity of the TiP products, their structural characteristics and sorption properties. These systematic data greatly contribute to the general knowledge of amorphous TiP ionites formation and properties, and are comprehensive guide for optimising the syntheses of TiP ionites. Using the 3D correlation diagrams shown

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Fig. 7. Sorption capacity correlation diagram for TiP ionites. The numbers X.Y represent the sorption capacity of TiP towards Na+ in mg-eq g−1.

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