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Hydrothermal synthesis of open-framework borophosphates with tunable micropore sizes, crystal morphologies and thermal stabilities Wei Sun, Ya-Xi Huang, Sergiy Nokhrin, Yuanming Pan, and Jin-Xiao Mi Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg5017438 • Publication Date (Web): 14 May 2015 Downloaded from http://pubs.acs.org on May 21, 2015

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Crystal Growth & Design

Hydrothermal synthesis of open-framework borophosphates with tunable micropore sizes, crystal morphologies and thermal stabilities Wei Suna,b, Ya-Xi Huanga, Sergiy Nokhrinc, Yuanming Panb, and Jin-Xiao Mia,* a

Fujian Provincial Key Laboratory of Advanced Materials, Department of Materials Science and

Engineering, College of Materials, Xiamen University, Xiamen 361005, Fujian Province, People’s Republic of China b

Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2,

Canada c

Department of Chemistry, University of Toronto, Toronto, ON M5S 3H6, Canada

KEYWORDS: Improved hydrothermal route; zeolitic borophosphate; fundamental building unit tune; morphology control; thermal stabilities.

ABSTRACT

Thermal stabilities of zeolitic frameworks are important parameters for many applications. Two decades of research have produced only a very small number of zeolitic borophosphates such as Na2[VO(B2O)(PO4)2(HBO3)]·xH2O (x ≈ 2.92) (denoted as B3P2), which shows the onset

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dehydration and a complete decomposition at 200ºC and 400ºC, respectively. In order to enhance thermal stabilities of borophosphate frameworks, a water-deficient hydrothermal route with phosphoric acid as the sole solvent has been developed and led to controlled syntheses of B3P2 and a new vanadium borophosphate, K1.33Na0.67[VO(B2O)(PO4)2(HPO4)]·xH2O (x ≈ 1.63) (denoted as B2P3). The latter is the first-ever borophosphate possessing the zeolite RHO-type net and is characterized by super-large spherical cages, including 16-ring and 8-ring channels along the axes, and 12-ring channels along the diagonals of the cubic cell. The new compound B2P3 has larger structural cages and higher thermal stability than B3P2, where the enhanced thermal stability is attributable to different bonding arising from the substitution of [BO2(OH)] by [PO3(OH)] in the framework. This is the first demonstration that the micropore size, crystal morphology and thermal stability of zeolitic borophosphates can be tuned by changing the fundamental building units of their frameworks via adjusting the B/P ratios in the starting materials.

1. Introduction. Inorganic materials with open frameworks have attracted enormous research owing to their wide applications for sorption and separation, heterogeneous catalysis, and ion exchange.1 Following

extensive

research

on

silicates,

aluminosilicates

and

aluminophosphates,

borophosphates have been suggested to be another potential class of open-framework materials, due to the diverse connections between B–O and P–O groups.2-7 In the past two decades, a few open-framework borophosphates with various structural types have been synthesized by using different techniques, such as organic templating, hydrothermal and boric acid flux methods.8-15


2

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However, most of these borophosphate frameworks comprise transition-metal atoms, only a minority of them possesses 3-D anionic partial structures.16-25 In comparison with those frameworks linked by metal atoms, these frameworks consisting of 3-D anionic partial structures are known to have the potential to form super-large channels.20 Therefore, synthesis of borophosphates with 3-D anionic partial structures represents an interesting avenue for the preparation of novel open-framework materials, although it (and especially controlled synthesis) remains a great challenge (see below). Among the previously reported borophosphates, Na2[VB3P2O12(OH)]·2.92H2O (denoted as B3P2) is distinguished for its interesting spherical cavity and complex channels, especially the biggest 16-ring channel.20 One notable disadvantage of B3P2 is its relatively low thermal stability: i.e., up to only 400 °C on the basis of a TG analysis.20 Therefore, it is important to develop new synthesis routes for the production of new open-framework borophosphates with improved thermal stabilities. In this context, the strategies used in the controlled synthesis of aluminosilicate zeolites by using variable Si/Al ratios are worth noting.26-28 Our working hypothesis was that a favorable method for improving the thermal stabilities of borophosphates could be achieved by adjusting the B/P ratio. This hypothesis was based on structural chemistry, the fundamental building unit (FBU) of B3P2 is [B2O(PO4)2(BO2(OH))] (simplified as △4□:□□ according to the scheme of Kniep et al),15 which is similar to another pentameric FBU [B2O(PO4)2(PO3(OH))] (simplified as 5□:□□). The only difference between these two FBUs lies in the terminals, and the transformation between [BO2(OH)] and [PO3(OH)] may be achieved by adjusting the B/P ratio. A new open-framework borophosphate with the [B2O(PO4)2(PO3(OH))] FBU is expected to have different thermal stability and other properties (e.g., catalytic properties tunable by morphology control).29-33


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The above hypothesis has led us to develop a new hydrothermal route for producing a new vanadium borophosphate K1.33Na0.67[VO(B2O)(PO4)2(HPO4)]·xH2O (x ≈ 1.63) (denoted as B2P3 hereafter),

with

an

expected

3-D

open-framework

structure

comprising

the

[B2O(PO4)2(PO3(OH))] FBU. Results reported herein show that this new hydrothermal route is also capable of synthesizing the previously reported B3P2 as well, which was prepared by the boric acid flux method.20 In fact, the new hydrothermal route not only can be tuned to produce the desired crystal structures (i.e., B2P3 and B3P2) but also is shown to be capable of controlling the crystal morphology, which helps to discriminate between compounds B2P3 and B3P2. The B2P3 and B3P2 compounds obtained from the new hydrothermal route have been characterized by thermogravimetric (TG), scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDS), Fourier transform infrared (FTIR),

11

B and

31

P magic

angle spinning nuclear magnetic resonance (MAS NMR), powder X-ray diffraction (PXRD) and in-situ PXRD analyses. The valence state of V4+ in B2P3 was also investigated by X-ray photoelectron spectroscopy (XPS) and magnetic susceptibility analysis. In addition, the reversible water-absorption property of B2P3 was investigated as well and reported herein.

2. Experimental methodology 2.1. Hydrothermal synthesis Hydrothermal experiments for the synthesis of pure B2P3 used 1.50 g CH3COOK (KAC 15.28 mmol), 0.30 g V2O5 (1.65 mmol), 1.50 g Na2B4O7·10H2O (3.93 mmol), 0.80 g H3BO3 (12.94 mmol), 1.00 g K2HPO4·3H2O (4.38 mmol), and 5.00 mL concentrated H3PO4 (85 %, 73.15

mmol).

These

reactants

correspond

to

the

molar

K:Na:V:B:P

ratio

of


4

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7.28:2.38:1:8.68:22.2. Experiments for the preparation of pure B3P2 used 0.60 g CH3COONa·3H2O (NaAC·3H2O 4.41 mmol), 0.30 g V2O5 (1.65 mmol), 3.00 g Na2B4O7·10H2O (7.86 mmol) and 3.00 mL concentrated H3PO4 (85 %, 43.89 mmol), with the molar Na:V:B:P ration of 6.1:1:9.53:13.3. All reagents were analytically pure, and were added into 25 mL Teflon lined stainless steel autoclaves directly. The autoclaves were heated to and held at 190 ºC for 3 days and then naturally cooled down to room temperature. After thorough washing with distilled water, light blue crystals with rhombic dodecahedral shapes (up to 300 µm in diameter) were obtained for both B2P3 and B3P2. High-quality crystals selected on the basis of morphological and optical examinations were used for single-crystal X-ray diffraction analyses, and the remaining samples were dried at 90 ºC for other analyses.

2.2. Characterization. Scanning electron microscopy (SEM) images and energy dispersive X-ray spectrometry (EDS) spectra were collected on a LEO-1530 field emission scanning electron microscope. Powder X-ray diffraction (PXRD) patterns were taken on a PANAlytical X’pert PRO diffractometer with Cu Kα radiation (λ = 1.54056 Å), operated at 40 kV and 30 mA. In-situ PXRD measurements were also performed on the same instrument, using a constant nitrogen flow with the heating rate of 5 ºC /min. Fourier transform infrared (FTIR) spectra were collected on a Nicolet Avatar 360 FTIR spectrometer equipped with a Smart Endurance diamond attenuated total reflectance (ATR) accessory, under both transmittance and reflectance modes. The transmittance spectra were obtained by using the KBr pellet method in the range of 400– 4000 cm-1, while the reflectance spectra were measured by the diamond ATR method over the


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range of 500–4000 cm-1. Thermogravimetric (TG) analyses were carried out on a NETZSCH TG 209 F1 TG/DTA analyzer in the nitrogen atmosphere with a heating rate of 10 ºC/min from room temperature to 1000 ºC. Compositional analyses were conducted on a Varian Vista RL spectrometer by using the inductively coupled plasma-optical emission spectroscopy (ICP-OES) method. X-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI quantum 2000 apparatus with monochromatic Al Kα radiation (hν = 1486.6 eV). Magnetic susceptibility was measured at a field of 1 kOe over the temperature range of 2–300 K, on a Quantum Design MPMS XL-7 SQUID magnetometer. Argon adsorption and desorption isotherms were measured at 87 K on an automatic Ar Accelerated Surface Area and Porosimetry analyzer (Micromeritics ASAP 2020 system, USA). 31P and 11B MAS NMR measurements of B2P3 and B3P2 were made on an Agilent DD2 700 MHz spectrometer equipped with a 1.6 mm T3-HX MAS Solids Balun probe, at the University of Toronto. The 31P MAS NMR spectra collected with a relaxation delay of 10s, 8 scans, and a spin rate 18 kHz, are referenced to the -148.89 ppm line of NaPF6. The 11B MAS NMR spectra collected with a relaxation delay of 2s and 16 scans were referenced to the 42.06 ppm line of NaBH4. Two additional borates (i.e., boric acid and borax) were also analyzed as independent reference materials. In addition, 11B spectra of B2P3, B3P2 and NaBH4 were also collected on a Bruker Advance III 400 NMR spectrometer at Xiamen University for comparison.

2.3. Single-crystal structure determination. Crystals of B2P3 and B3P2 were selected under a polarized microscope for single-crystal Xray diffraction analyses. All data were collected at 173 K on an Oxford Gemini S Ultra CCD diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å), at 50 kV and 40


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mA. Data processing was accomplished with the CrysAlis processing program. The crystal structures were solved by the direct method and refined by full matrix least-squares methods implemented in the SHELXTL crystallographic software package.34 The V, B, P, K, Na1, O, OW1 and OW2 atoms of the synthetic compounds could be unambiguously located. The Na2 and OW3 atoms were located from the difference Fourier map, and the occupancy of Na2 atom was determined according to the requirement of charge balance as well as the compositional analyses. Experimental details for crystal determination are listed in Table 1.

Table 1. Crystal data and structure refinement of B2P3 Compound

B 2P 3

Formula sum, formula weight

H8.52B4K2.67Na1.33O31.26P6V2, 974.56 g/mol

Crystal size (mm), color

0.15 × 0.15 × 0.15, light blue

Crystal system, space-group

Cubic, I23 (No. 197)

Unit cell dimensions, cell volume

a = 19.985(2) Å, 7982(3) Å3

Calc. density, Z

2.433 g cm-3, 12

Rad., wavelength, temp.

MoKα, 0.71073 Å, 173(2)

-1

µ (mm ), F(0 0 0)

1.633, 5759

θmax (°), Npara

30.899, 225

Miller-index

-26 ≤ h ≤ 12, -28 ≤ k ≤ 26, -28 ≤ l ≤ 16

Rint, R1, wR2

0.017, 0.028, 0.078

S, N, N(I > 2σ(I))

1.093, 3920, 3873

3. Results and Discussion. 3.1. Synthesis and characterization. In order to tune the B/P ratio of the zeolitic framework, we have performed a systematic investigation on synthesis conditions, including the hydrous pressures, oxidation state, and


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species and ratio of starting reactants. Firstly, the experimental route adopted in this study is a “water-deficient” hydrothermal technique, using water-bearing reactants but phosphoric acid as the sole solvent. This “water-deficient” hydrothermal method has been proven to be successful in the synthesis of novel compounds in other systems.35,36 This unconventional route was intended to maintain low water-gas pressure, high solute-saturation and activities during the synthesis experiments to prompt the polymerization of phosphate and borate groups.37 Particularly, low water-gas pressure is crucial for the formation of zeolitic frameworks. Secondly, acetates took as a balancing reactant in the redox reaction, their amounts in the starting mixtures played a critical role in ensuring vanadium to be tetravalent in the final products (see below). At 190 ºC, 0.4–3.5 g KAC and 0.2–1.4 g NaAC·3H2O were suitable for the synthesis of B2P3 and B3P2, respectively. Insufficient acetates as the reducing agent resulted in the production of pentavalent vanadates, whereas excess acetates led to the formation of KV[BP2O8(OH)] and NaV[BP2O7(OH)3] with trivalent vanadium. Moreover, the amounts of acetates should be reduced at higher temperatures. At 240 ºC, for example, KAC should be limited to 1.2 g in order to avoid the appearance of KV[BP2O8(OH)], whereas 0.2 g or more NaAC·3H2O would yield NaV[BP2O7(OH)3] . Thirdly, the ratio of Na/K in the starting material is also crucial for the synthesis. Attempts to obtain the Na endmember of B2P3 by using Na2HPO4·12H2O (up to 20.0 g) were not successful. Similarly, we were unable to synthesize the K endmember of B2P3 by using K2B4O7·4H2O instead of Na2B4O7·10H2O, even the amounts of H3BO3, K2B4O7·4H2O and K2HPO4·3H2O were varied in the range of 0–3.0 g, 0–4.0 g and 0–11.0 g, respectively. These systematic experiments not only yielded two more new compounds (Na2VP2O9H2 and


8

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K6(VO)4[B4P8O32(OH)2]·xH2O, to be reported elsewhere), but also suggested that both K and Na are essential components for the formation of B2P3. PXRD and FTIR analyses demonstrate that most of our synthesis experiments yielded a mixture of B2P3 and B3P2. Our systematic experiments showed that pure B3P2 can be prepared only by using large amounts of Na2B4O7·10H2O, whereas pure B2P3 cannot be synthesized without K or Na. Indeed, the transformation from B3P2 to B2P3 occurs only when K and Na are introduced simultaneously. However, the proportion between B3P2 to B2P3 is controlled mainly by the concentration of B. Also, the runs for B2P3 often yielded BPO4 and K2(VO)2(HPO4)3·1.125H2O as impurities, which can be decreased and completely eliminated by increasing Na2B4O7·10H2O in the starting mixture. The optimal routes for pure B2P3 and B3P2 were detailed in the Hydrothermal Synthesis Section. Figure 1a shows representative experimental and simulated PXRD patterns of B2P3 and B3P2. Here the simulated PXRD pattern of B3P2 was calculated from the CIF file of VBPOCJ2720. One notable difference in the PXRD patterns of B3P2 and B2P3 is that the latter is characterized by a strong peak at 2θ = 19.87°, which is absent in the former.


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Figure 1 (a) Calculated and experimental powder X-ray diffraction (PXRD) patterns of B2P3 and B3P2. (b) The Fourier transform infrared (FTIR) spectra of B2P3 (black) and B3P2 (pink) by KBr pellet method.

B2P3 and B3P2 are also readily distinguished by FTIR spectra (Figure 1b). Figure 1b shows that the FTIR spectra of the two compounds obtained from the KBr pellet method consist of similar absorption bands at 1634/1648 cm–1 (arising from the stretching and bending vibrations of the O–H groups) and those in the region 3600–3400 cm–1 (from the H2O molecules) and in the region 1220–500 cm–1 (assigned to the stretching and bending vibrations of PO4, BO4, and B–O– P groups). One notable difference is the presence of the band at 1431 cm–1 in the B3P2 spectrum (Figure 1b), which is caused by the stretching vibrations of BO3 groups.38 This difference is also evident in the spectra measured by the diamond ATR method (Figure S1). Therefore, the absence of the absorption band at 1431 cm–1 in the FTIR spectra can be taken as a criterion to verify the


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purity of B2P3 samples, and all of the pure B2P3 samples used for other analyses (e.g., ICP-OES, XPS, TG, MAS NMR and magnetic susceptibility) were carefully examined by both PXRD and FTIR analyses. The energy dispersive X-ray spectrometer (EDS) spectra of B2P3 are shown in Figure S2. More accurate compositions of B2P3 were obtained by ICP-OES analyses. ICP-OES analyses yielded the contents of V, B, P, K and Na at 10.28 (0.02), 4.56 (0.06), 19.28 (0.19), 10.68 (0.18) and 3.07 (0.06) wt %, respectively. The result is in excellent agreement with the values of 10.45(V), 4.44(B), 19.07(P), 10.67(K) and 3.16(Na) wt % obtained from single-crystal structure analysis (K1.33Na0.67[VB2P3O13(OH)]·1.63H2O).

3.2. Crystal structure Single-crystal X-ray data collections and structure analyses have been made for more than 15 crystals of various morphologies synthesized from different conditions. These structure analyses are necessary in this case to unambiguously distinguish the similar B2P3 and B3P2 phases that occur together in many synthesis experiments. Our analyses reveal that B2P3 crystallizes in the space group I23 (No. 197) with a = 19.985(2) Å, V = 7982(3) Å3 and Z = 12, different from the space group I4ത3m (No. 217) of B3P2. The structure of B2P3 consists of a macro-anionic framework formed of the [VB2P3O13(OH)]2– asymmetric units. Each asymmetric unit contains one crystallographically unique V site, two unique B sites, and three unique P sites. The V atom is octahedrally coordinated by six O atoms: four equatorial µ-O atoms, one µ3-O atom and one terminal O atom. The four µ-O atoms, shared with the adjacent P atoms, have approximately equal V–O bond lengths (2.007(2)–2.023(2) Å). The µ3-O atom, connected to the


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two B atoms, has the longest V–O bond length of 2.338(2) Å. The terminal O atom has the shortest V–O bond length of 1.589(2) Å, typical of the V=O bond in distorted VO6 octahedra.39 Bond-valence-sum (BVS) calculation for V yields a value of 4.06 valence units (v.u.).40 Both of the two B atoms are tetrahedrally coordinated by the mentioned µ3-O atom, and three µ-O atoms. The B–O bond lengths vary in the range 1.448(4)–1.504(4) Å. All of the three P atoms are tetrahedrally coordinated. P(1) and P(2) atoms are coordinated by four µ-O atoms, with bond lengths varying from 1.518(2) to 1.554(2) Å. The P(3) atom is only coordinated by two µ-O atoms (P(3)–O(11), 1.552(2) Å, P(3)–O(12), 1.562(2) Å) and two terminal O atoms (P(3)–O(13), 1.485(2) Å for the P=O bond, P(3)–O(14), 1.558(2) Å for the OH bond). The BVS of P(3) is 4.79 v.u.. Selected bond lengths and angles of B2P3 are summarized in Table S1. Figure 2 shows that each [VB2P3O13(OH)]2– asymmetric unit connects to four neighboring counterparts by PO4 tetrahedra, to form a 3-D open framework. The framework of B2P3 is featured with super-large spherical cages, containing 16-ring and 8-ring openings along the directions, as well as 12-ring openings along the and directions. The spherical cages are located at the vertices and the body center of the cubic cell, interconnected through the 12-ring openings along the directions, forming the complex channel system of B2P3. Na+ and K+ ions as well as water molecules are accommodated in the channels. Na+ and K+ ions in the channels provide the required charge balances and neutralize the 3-D framework structure, while water molecules fill the remaining interstices of the channels and further stabilize the overall structure via hydrogen bonds.


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Figure 2 Illustration of the improved hydrothermal route and the crystal structure of B2P3.

3.3. 11B and 31P MAS NMR spectra of B3P2 and B2P3 Samples of synthetic B3P2 appear to contain minor amounts of magnetic impurities, which are not detectable in PXRD analyses but give rise to significant peak broadening in the 11B and 31P MAS NMR spectra (Fig. 3). Nevertheless, experimental 11B and 31P MAS NMR spectra (Fig. 3)


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confirm the structural difference in the fundamental building unit between B2P3 and B3P2 (Fig. 2a) In particular, the characterized by a

11

11

B NMR spectra show that B3P2 contains the [BO3] planar triangle

B chemical shift of ~17 ppm and nuclear quadrupole parameters CQ = 2.6

MHz and η = 0.1,41-43 which is absent in B2P3 (Fig. 3a). Interestingly, the 11B MAS NMR signals of the [BO4] groups in B2P3 and B3P2 are centered at ~19 and ~23 ppm, respectively (Fig. 3a), rather than the normal positions of ~1-2 ppm in borates.42,43 We emphasize that these anomalous 11

B chemical shifts are not analytical artifacts, because they have been reproduced at two

independent laboratories (Fig. S3) that also yielded diagnostic

11

B NMR spectra for two well

characterized borates (i.e., borax and boric acid).41-43 However, the origin of these anomalous 11B chemical shifts for the [BO4] groups in borophosphates remains unclear and is the subject of an ongoing research.


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Figure 3 a) 11B and b) 31P MAS NMR spectra of B2P3 and B3P2.

3.4. Topological analysis A topological approach has been applied to better understand the nature of the intricate framework of B2P3. In this topological analysis each [VB2P3O13(OH)]2– unit is taken as a node. Using the standard simplification method, the topological type of B2P3 is RHR (4/4/c3; sqc5544), a semiregular net according to the topological analysis by using TOPOS software.44,45 According to the edge net simplification method, the topological type of B2P3 is zeolite RHO (4/4/c4; sqc11215).46,47 The relationship between RHR and RHO is in accordance with Yaghi’s report.48 Inspired by this topological result, we removed the V, B and P(3) atoms, and connected P(1) and P(2) atoms. Indeed, the linkage of the P(1) and P(2) atoms exhibits an obvious zeolite RHO net (Fig. 4). Therefore, the framework of B2P3 should be best described as a zeolite RHO net, consisting of P(1) and P(2) atoms, and decorated with V, B and P(3) atoms. To the best of our knowledge, this is the first ever report of borophosphates with the zeolite RHO topology.


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Figure 4 Topology analysis of the crystal structure of B2P3. It is also interesting to note that B2P3 and B3P2 possess similar open-frameworks but comprise different building units. Specifically, B2P3 contains a similar 3-D anionic partial structure as B3P2, featured by 16- and 20-ring openings along the crystal axes and 12-ring openings along the diagonal directions of the cubic cell. Nevertheless, the anionic partial structure of B2P3 and B3P2 are constructed by different pentameric fundamental building units (FBU). The FBU of B2P3 is [B2O(PO4)2(HPO4)], which is composed of two BO4 tetrahedra and three PO4 tetrahedra, simplified as 5□:□□ according to the scheme of Kniep et al15. The FBU of B3P2, on the other hand, is [B2O(PO4)2(HBO3)], consisting of two BO4 tetrahedra, two PO4 tetrahedra, and one BO2(OH) planar triangle, denoted as △4□:□□. The major


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difference is that the terminal BO2(OH) planar triangle in the FBU of B3P2 is substituted by the PO3(OH) tetrahedron in the FBU of B2P3 (highlighted by a dashed ellipse in Figure 2).

3.5. Control of Crystal Structure and Morphology Our synthesis experiments show that the formation of the compounds B3P2 and B2P3 with different FBUs can be controlled by adjusting the B/P ratio in the starting materials. This controlled synthesis of B3P2 and B2P3 is particularly interesting, because the spherical cages with a diameter of ~8.2 Å in the latter are significantly larger than those (~7.9 Å) in the former. It is well known that many properties and applications of zeolitic materials are directly related to their micropore sizes.2,27 SEM examinations followed by single-crystal structural analyses also show that B3P2 crystallizes in our synthesis conditions with the rhombic dodecahedral morphology only, whereas the new B2P3 compound grows into different crystal forms from cube to rhombic dodecahedron (Fig. 5). In particular, systematical experiments (Table 2) show that the crystal morphologies of B2P3 can be controlled by synthesis conditions. Specifically, experiments with the fixed amounts of KAC, V2O5 and K2HPO4·3H2O at 1.00 g, 0.30 g and 2.00 g, respectively, but variable amounts of H3BO3, Na2B4O7·10H2O and H3PO4 (Table 2) can be used for the synthesis of B2P3 with specific crystal morphologies. Our experiments (Table 2) show that Na2B4O7·10H2O is the apparent source for B, because B2P3 can be synthesized in runs without H3BO3. Also, B2P3 cannot be synthesized in runs with H3BO3 but no Na2B4O7·10H2O. Nevertheless, the amount of H3BO3 added does affect the size and morphology of the B2P3 crystals. Structural analyses show that the B2P3 crystals with distinct morphologies have essentially constant contents of K+, Na+ and water molecules in the channels.


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Figure 5 Morphologies and insight into the mechanism of morphology evolution of B2P3. (a) Cube, (b) {110}-truncated cube, (c) {100}-truncated rhombic dodecahedra, (d) rhombic dodecahedra. Illustration of the growth of {110} and {100} faces associated with the growth of the P–O–V and B–O–B chains. Table 2 clearly demonstrates the crystal morphology of B2P3 from cube to rhombic dodecahedron is controlled by the increasing B/P ratio. This morphological evolution can be reasonably explained by crystal structural consideration: i.e., the faces of crystals are determined by the relative growth of the structural chains according to the theory of periodic bond chain (PBC)49-51. Figure 5 shows the growth rates of the {110} and {100} faces are associated with the P–O–V and B–O–B chains, respectively. As such, the B/P ratio has a determining influence on the relative growth rates between the P–O–V and B–O–B chains. At higher B/P ratios, the B–O– B chains grow faster than the P–O–V chains, resulting in better developed {110} faces than the {100} faces. Therefore, B2P3 crystallizes as simple cubes at low B/P ratios but dodecahedra at high B/P ratios. Increasing in the B/P ratio leads to a combined form of dominant cube and subordinate dodecahedron, then a combined form of subordinate cube and dominant


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dodecahedron, and finally a dodecahedron (Figure 5). These results show that the crystal morphology of B2P3 can be tuned by adjusting the B/P ratio. Table 2 Summary of starting mixtures and corresponding products from selected series of synthesis experiments No. H3BO3(g) †Na2B4O7 H3PO4(mL) Products No. H3BO3 Na2B4O7 H3PO4(mL) Products A1 1.60 1.40 5.00 (d) D3 0.40 0.80 5.00 (a)+(e) A2 1.60 1.10 5.00 (c)+(e) D4 0.40 0.50 5.00 (e) A3 1.60 0.80 5.00 (b)+(e) E1 0.20 1.40 5.00 (b)+(e) A4 1.60 0.50 5.00 (a)+ (e) E2 0.20 1.10 5.00 (a)+ (e) B1 1.20 1.40 5.00 (d) E3 0.20 0.80 5.00 (e) B2 1.20 1.10 5.00 (c)+(e) E4 0.20 0.50 5.00 (e) B3 1.20 0.80 5.00 (a)+ (e) F1 0 1.40 5.00 (c)+(e) B4 1.20 0.50 5.00 (a)+(e) F2 0 1.10 5.00 (b)+(e) C1 0.80 1.40 5.00 (d) F3 0 0.80 5.00 (e) C2 0.80 1.10 5.00 (c)+(e) F4 0 0.50 5.00 (e) C3 0.80 0.80 5.00 (a)+(e) G1 1.20 0.80 3.00 (d) C4 0.80 0.50 5.00 (a)+ (e) G2 1.20 0.80 4.00 (b)+ (c)+(f) D1 0.40 1.40 5.00 (d) G3 1.20 0.80 6.00 (e) D2 0.40 1.10 5.00 (b)+(e) G4 1.20 0.80 7.00 (e) † Na2B4O7 represents Na2B4O7·10H2O. Symbols: (a) cube B2P3, (b) {110}-truncated cube B2P3, (c) {100}truncated rhombic dodecahedra B2P3, (d) rhombic dodecahedra B2P3 and BPO4, (e) BPO4 and K2(VO)2(HPO4)3·1.125H2O, and (f) BPO4.

3.6. Thermal stabilities of B3P2 and B2P3 Figure 5a shows that the TG curve of the B3P2 compound synthesized from the new hydrothermal route is similar to that of its counterpart grown from the previous boric acid flux method and is characterized by a single step with an almost complete removal of both molecular and hydroxyl water at below 400 ºC. The new B2P3 compound, on the other hand, features a TG curve with two distinct steps, correspondingly a DTG curve with two sharp peaks at 229 and 474 ºC (Figure 6). The amounts of weight loss during the first and second steps (i.e., 5.62 and 1.80 wt%) of the as-is sample matches closely to those of the molecular water (6.03 wt%) and hydroxyl (1.84 wt%) determined from single-crystal structure analyses. These TG results are further supported by in-situ PXRD data. Figure 7 shows that the framework of B3P2 collapses


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completely before 500 ºC, whereas that of B2P3 is retained up to at least 550 ºC. Therefore, both TG and in-situ PXRD experiments confirm our hypothesis that B2P3 has a relatively higher thermal stability than B3P2 and that the thermal stabilities of borophosphates can be controlled by the B/P ratios. Specifically, the low B/P ratios in the starting mixtures led to the formation of B2P3 with a higher thermal stability than B3P2. This enhanced thermal stability of B2P3 over B3P2 is attributable to different bonding arising from the substitution of the [PO3(OH)] group for [BO2(OH)] in the FBU (Fig. 2a).


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Figure 6 (a) Thermogravimetric (TG) curves of B2P3 (red) and B3P2 (brown), the corresponding DTG curves of B2P3 (blue) and B3P2 (green). (b) TG curves of B2P3 as-prepared (red) and after reversible water absorption (blue). (c) PXRD patterns of as-prepared, water removed and water reabsorbed samples of B2P3.

3.7. Reversible water absorption. Figure 6b compares the TG curves of the as-is B2P3 compound and its counterpart that has been subjected to annealing and water re-sorption treatments. The sample for the latter was first heated to and held at 240 ºC for 24 hours, in order to remove the molecular water completely. After cooling down to room temperature in air atmosphere, the sample was then immersed in deionized water for 6 hours to resorb water. The TG analyses of this treated B2P3 sample was measured after it was dried at 90 ºC for 12 hours. It is interesting to note that the TG curve of the treated B2P3 sample is also characterized by similar two steps of weight loss. Moreover, the nearly identical second step suggests that the FBU has not been affected by annealing at 240 °C. PXRD measurements also confirm the crystal structure is unchanged after water desorption and resorption (see Figure 6c). However, the 3.27 wt% weight loss from the first step of the treated sample is significantly lower than that observed for its as-is counterpart. The different weight loss is readily attributable to the fact that the resorption experiment was made at ambient pressure and temperature, in comparison with the as-is sample synthesized at elevated pressure and temperature. Figure S4 shows the Ar adsorption and desorption isotherms of B2P3. Prior to the measurement, the sample was pretreated first at 573 K for 3h and then degassed in situ at 473 K for 2h. No obvious adsorption of Ar was observed at low pressure, indicating the channels are blocked by the Na+ and K+ ions.


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Figure 7 In-situ PXRD patterns of B2P3 (a) and B3P2 (b).

3.8. XPS and magnetic susceptibility of B2P3 Figure 8a shows the V 2p XPS core-level spectrum obtained from B2P3, using C 1s = 284.80 eV for calibration. A single peak of V 2p3/2 is observed with a binding energy (BE) of 516.7 eV, which is in excellent agreement with the reported value of V4+ in VOHPO4·0.5H2O,52 confirming that the valence state of V in B2P3 is +4. Figure 8b shows the temperature dependence of the reciprocal molar magnetic susceptibility 1/χ for B2P3 obeys the Curie-Weiss law in the whole temperature range investigated, revealing that this compound is paramagnetic. The linear fitting of the 1/χ-T data gives a Weiss constant of -5.0 K and a Curie constant of 0.37 emu·K/mol. The effective magnetic moment per V atom


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calculated from the Curie constant is 1.72 µB, which is typical for the V4+ ion (3d1, S = 1/2), further supporting the V4+ valence state determined from bond valence calculation and XPS analysis.

Figure 8. (a) X-ray photoelectron spectroscopy (XPS) core level of V 2p from B2P3. (b) The χ vs T and 1/χ vs T plots for B2P3. The blue line represents the linear fitting for the data according to the Curie-Weiss law.

4 Conclusions B2P3, a new borophosphate with a 3-D open-framework structure of the zeolite RHO-type net, has been synthesized by using a novel hydrothermal route. This hydrothermal route featuring hydrous starting materials but no addition of any free water reduces the water activity to promote the polymerization of phosphate and borate groups. The crystal structure of B2P3 contains super-


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large spherical cages and complex channels, including 16-ring and 8-ring channels along the crystal axes as well as 12-ring channels along the diagonal of the cubic cell. The framework of B2P3 is similar but not identical to that of the previously reported B3P2, which is also readily synthesized by the hydrothermal route. These two similar compounds can be distinguished by EDS, PXRD, MAS NMR and FTIR analyses. TG and in-situ PXRD measurements show that the modification of the framework yields a thermally more stable B2P3 relative to B3P2. Systematic synthesis experiments demonstrate that the structural transformation between B2P3 and B3P as well as the crystal morphology of B2P3 can be controlled by adjusting the B/P ratio in the starting materials. In addition, the tetravalence of V in B2P3 has been confirmed by XPS and magnetic susceptibility analyses. TG analyses also show that B2P3 exhibits a reversible water-absorption property. This report opens a new way to tune the physico-chemical properties of zeolitic borophosphates by the adjusting the B/P ratio in the starting materials.

Supporting Information. Figures of IR, SEM/EDS, MAS NMR and adsorption/desorption isotherms, and a table of selected bond angles and distances, as well as a crystallographic input file (CIF)(csd No. 428777) are supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author * Jin-Xiao Mi, email: [email protected]. Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We thank the National Natural Science Foundation of China (Nos. 21233004 & 21201144), the Fundamental Research Funds for the Central Universities (2013121020), and Natural Science and Engineering Research Council of Canada for financial support. ABBREVIATIONS FBU, fundamental building unit; B3P2, Na2[VO(B2O)(PO4)2(HBO3)]·xH2O (x ≈ 2.92); B2P3, K1.33Na0.67[VO(B2O)(PO4)2(HPO4)]·xH2O (x ≈ 1.63); TG, thermogravimetric; SEM, scanning electron microscopy; EDS, energy dispersive X-ray spectrometer; FTIR, Fourier transform infrared; PXRD, powder X-ray diffraction; XPS, X-ray photoelectron spectroscopy; MAS NMR, magic angle spinning nuclear magnetic resonance. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27)

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For Table of Contents Use Only

Hydrothermal synthesis of open-framework borophosphates with tunable micropore sizes, crystal morphologies and thermal stabilities Wei Sun, Ya-Xi Huang, Sergiy Nokhrin, Yuanming Pan, and Jin-Xiao Mi*

A water-deficient hydrothermal route has been developed to tune the micropore size, crystal morphology and thermal stability of zeolitic borophosphates via adjusting the B/P ratios in the starting materials.


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Figure 1 (a) Calculated and experimental powder X-ray diffraction (PXRD) patterns of B2P3 and B3P2. (b) The Fourier transform infrared (FTIR) spectra of B2P3 (black) and B3P2 (pink) by KBr pellet method. 928x1423mm (96 x 96 DPI)


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Figure 2 Illustration of the improved hydrothermal route and the crystal structure of B2P3. 800x1475mm (96 x 96 DPI)



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Figure 3 a) 11B and b) 31P MAS NMR spectra of B2P3 and B3P2. 725x1374mm (96 x 96 DPI)


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Figure 4 Topology analysis of the crystal structure of B2P3. 805x1319mm (96 x 96 DPI)



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Figure 5 Morphologies and insight into the mechanism of morphology evolution of B2P3. (a) Cube, (b) {110}-truncated cube, (c) {100}-truncated rhombic dodecahedra, (d) rhombic dodecahedra. Illustration of the growth of {110} and {100} faces associated with the growth of the P–O–V and B–O–B chains. 800x700mm (96 x 96 DPI)


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Figure 6 (a) Thermogravimetric (TG) curves of B2P3 (red) and B3P2 (brown), the corresponding DTG curves of B2P3 (blue) and B3P2 (green). (b) TG curves of B2P3 as-prepared (red) and after reversible water absorption (blue). (c) PXRD patterns of as-prepared, water removed and water reabsorbed samples of B2P3. 1105x2113mm (96 x 96 DPI)



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Figure 7 In-situ PXRD patterns of B2P3 (a) and B3P2 (b). 899x1362mm (96 x 96 DPI)


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Figure 8. (a) X-ray photoelectron spectroscopy (XPS) core level of V 2p from B2P3. (b) The χ vs T and 1/χ vs T plots for B2P3. The blue line represents the linear fitting for the data according to the Curie-Weiss law. 1054x1401mm (96 x 96 DPI)



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1613x772mm (96 x 96 DPI)


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Hydrothermal Synthesis of Open-Framework ... - ACS Publications

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