Phosphoric Acid Loaded Covalent Organic Framework And A Process For The Preparation Thereof

Abstract

The present invention is directed to a process for the preparation of phosphoric acid loaded covalent organic framework (PA@Tp-Azo and PA@Tp-Stb) with high stability and high proton conductivity.

Claims

1. A process for preparing phosphoric acid loaded covalent organic framework with high stability and high proton conductivity comprising the steps of: a) dispersing 1,3,5-triformylphloroglucinol (Tp) and diamine compound in solvent by ultrasonication for 10 minutes followed by degassing the solution; b) heating the solution as obtained in step (a) in oven for 3-5 days at temperature in the range of 100 to 120 C. followed by washing and drying under vacuum at temperature in the range of 120 to 150 C. for period in the range of 18 to 12 h to obtain desired covalent organic framework with at least one basic anchoring site; c) immersing the covalent organic framework material of step (b) in H.sub.3PO.sub.4 for period in the range of 1 h to 5 h followed by washing to remove the surface absorbed phosphoric acid and activating overnight for period in the range of 12 to 18 hr at temperature in the range of 60 to 80 C. under vacuum to obtain phosphoric acid loaded covalent organic framework.

2. The process as claimed in claim 1, wherein the diamine compound in step (a) is 4,4-azodianiline (Azo) or 4,4-diaminostilbene (Stb) dihydrochloride.

3. The process as claimed in claim 1, wherein the covalent organic framework formed in step (b) is Tp-Azo or Tp-Stb.

4. The process as claimed in claim 1, wherein phosphoric acid loaded covalent organic framework formed in step (c) is PA@Tp-Azo or PA@Tp-Stb.

5. The process as claimed in claim 1, wherein the solvent used in step (a) is mixture of dimethylacetamide and o-dichlorobenzene in 1:1 ratio.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1A is a crystal structure of 4-[(E)-phenyl-diazenyl] anilinium dihydrogen phosphate phosphoric acid solvate;

[0032] FIG. 1B is a schematic representation of synthesis of Tp-Azo and Tp-Stb by the reaction between 1,3,5-triformylphloroglucinol (Tp) and 4,4-azodianiline (Azo) or 4,4-diaminostilbene (Stb), respectively;

[0033] FIG. 2A is observed PXRD patterns of Tp-Azo (Blue) and Tp-Stb (Green) compared with simulated patterns;

[0034] FIG. 2B is a FT-IR spectra of Tp-Azo and Tp-Stb compared with starting material Tp, Azo and Stb;

[0035] FIG. 2C is a .sup.13C NMR comparison of Tp-Azo (Blue), Tp-Stb (Green) compared against the reference compound Tp-Azo monomer=2,4,6-tris(((4-((E)-phenyldiazenyl)phenyl)amino)methylene)cyclohexane-1,3,5-trione (Red);

[0036] FIG. 3A changes in the UV-Vis spectra of monomer of Tp-Azo with increasing concentration of H.sub.3PO.sub.4;

[0037] FIG. 3B is .sup.31P CP-NMR of H.sub.3PO.sub.4 treated Tp-Azo and Tp-Stb;

[0038] FIG. 3C is PXRD patterns of HCl (Red), boiling water (Orange) treated and as-synthesized Tp-Azo (Blue);

[0039] FIG. 3D is N.sub.2 adsorption isotherms of Tp-Azo (Blue), boiling water (Orange) and acid treated Tp-Azo (Red);

[0040] FIG. 3E is PXRD patterns of HCl treated (Blue) and as-synthesized Tp-Stb (Green);

[0041] FIG. 3F is N.sub.2 adsorption isotherms of as-synthesized Tp-Stb (Green) and 9 N HCl treated Tp-Stb (Blue);

[0042] FIG. 4A is a schematic representation of H.sub.3PO.sub.4 doping in COFs;

[0043] FIG. 4B is proton conductivity of PA@Tp-Azo in anhydrous condition;

[0044] FIG. 4C is hydrous condition;

[0045] FIG. 4D is proton conductivity of PA@Tp-Stb in hydrous condition;

[0046] FIG. 4E is activation energy plot for PA@Tp-Azo in hydrous condition;

[0047] FIG. 5A is SEM images of Tp-Azo;

[0048] FIG. 5B is PA@Tp-Azo;

[0049] FIG. 6A is SEM images of Tp-Stb;

[0050] FIG. 6B is PA@Tp-Stb;

[0051] FIG. 7A is TEM images of Tp-Azo at different magnifications;

[0052] FIG. 7B is TEM images of Tp-Stb at different magnifications;

[0053] FIG. 8A is Nyquist plots for PA@Tp-Azo in anhydrous condition at different temperatures;

[0054] FIG. 8B is a variation of proton conductivity as a function of temperature;

[0055] FIG. 9A is Nyquist plots for PA@Tp-Azo in hydrous condition at different temperatures;

[0056] FIG. 9B is a variation of proton conductivity as a function of temperature;

[0057] FIG. 10A is selected Nyquist plots for PA@Tp-Stb in hydrous condition at different temperatures;

[0058] FIG. 10B is activation energy fitting for PA@Tp-Stb;

[0059] FIG. 11 is stability of Tp-Azo in acid;

[0060] FIG. 12 is stability of Tp-Azo in base;

[0061] FIG. 13 is stability of Tp-Stb in acid; and

[0062] FIG. 14 is stability of Tp-Stb in base.

DETAILED DESCRIPTION OF THE INVENTION

[0063] One aspect of the present invention is to provide a stable loaded covalent organic framework which shows greater stability under ambient conditions as well as towards strong acidic and moderately strong basic conditions even upon isoreticulation and functionalization.

[0064] Another aspect of the present invention is to provide a process for the preparation of phosphoric acid loaded covalent organic framework with high stability and high proton conductivity comprising the steps of:

[0065] a) dispersing 1,3,5-triformylphloroglucinol and diamine compound in solvent by ultrasonication for 10 minutes followed by degassing the solution through three freeze-pump-thaw cycles;

[0066] b) heating of the solution of step (a) in oven for 3 days at 120 C. followed by washing the COFs until it is pure and then drying under vacuum at 150 C. for 12 h to obtain desired covalent organic framework with at least one basic anchoring site;

[0067] c) immersing the covalent organic framework material of step (b) in H.sub.3PO.sub.4 for 2 h followed by washing to remove the surface absorbed phosphoric acid and activating overnight at 80 C. under vacuum to obtain phosphoric acid loaded covalent organic framework.

[0068] The covalent organic framework formed in step (b) is Tp-Azo or Tp-Stb.

[0069] The phosphoric acid loaded covalent organic framework formed in step (c) is PA@Tp-Azo or PA@Tp-Stb.

[0070] The diamine compound in step (a) is 4,4-azodianiline or 4,4-diaminostilbene dihydrochloride and the solvent is (1:1) mixture of dimethylacetamide and o-dichlorobenzene.

[0071] The covalent organic framework formed in step (b) greater stability under ambient conditions as well as towards strong acidic and moderately strong basic conditions even upon isoreticulation and functionalization.

[0072] The present invention provides azo functionalized COF (Tp-Azo) (FIG. 1B) which are prepared by Schiff base reaction between 1,3,5-triformylphloroglucinol (Tp) and 4,4-azodianiline (Azo) and exhibit excellent structural stability, porosity and crystallinity even after acid treatment. PA@Tp-Azo of the instant invention shows proton conductivity in both humid and anhydrous conditions (9.910-4 and 6.710-5 Scm.sup.1, respectively). To elucidate the structure-property relationship, the inventors have synthesized the non-azo counterpart, i.e., stilbene functionalized COF (Tp-Stb) (FIG. 1B) which shows less stability, crystallinity, porosity and much less proton conductivity than Tp-Azo because of the nonavailability of anchoring site.

[0073] The synthesis of Tp-Azo and Tp-Stb begins by reacting 1,3,5-triformylphloroglucinol with 4,4-azodianiline or 4,4-diaminostilbene using an organic solvent. The organic solvent is preferably 1:1 mixture of dimethylacetamide and o-dichlorobenzene as solvent. The reactants were first dispersed in the solvent by ultrasonication for 10 minutes and then degassed through three freeze-pump-thaw cycles. The tubes were then vacuum sealed, placed in isotherm oven for 3 days at 120 C. Finally, the material was filtered out and washed with dry acetone and dried under vacuum at 180 C. for 12 h to obtain Tp-Azo and Tp-Stb respectively.

[0074] The PXRD patterns of Tp-Azo and Tp-Stb indicate an intense peak at 2=3.2 which corresponds to 100 plane reflections (FIG. 1A), with minor peaks at 5.5, 6.4, 8.4 and 27 (001 plane). The - stacking distance between COF layers was calculated as 3.4 from the d spacing between 001 planes. In order to elucidate the structure of these COFs and to calculate the unit cell parameters, a possible 2D model was built with eclipsed structure in hexagonal space group (P6/m) and staggered structure in P1 space group using the software Crystal 09. The experimental PXRD pattern matches well with the simulated pattern of the eclipsed stacking model (FIG. 3A). The unit cell values were found to be (a=b=31.5 , c=3.3 ) for Tp-Azo and (a=b=30.500 , c=3.49 ) for Tp-Stb.

[0075] FT-IR spectra of Tp-Azo and Tp-Stb indicate total consumption of starting materials due to the disappearance of NH stretching bands (3100-3300 cm.sup.1) of Azo or Stb and carbonyl stretching bands (1639 cm.sup.1) of Tp (FIG. 2B). Strong peak at 1578 cm.sup.1 arises due to the CC stretching in keto-form similar to the FT-IR spectrum of the reference compound 2,4,6-tris((phenyldiazenyl)phenylamino-methylene)cyclohexane-1,3,5-trione, monomer of Tp-Azo [product of the reaction of 1,3,5-triformylphloroglucinol (Tp) and 4-aminoazobenzene]. Most of the FT-IR peaks of Tp-Azo and Tp-Stb match well with that of the reference compound (FIG. 2B). The CO peaks (1619 cm.sup.1) of Tp-Azo and Tp-Stb get merged with CC stretching band (1582 cm.sup.1). The isolation of Tp-Azo and Tp-Stb as keto form was confirmed by .sup.13C CP-MAS solid state NMR. Both COFs show carbonyl (CO) carbon signals at =181 and 186 ppm for Tp-Azo and Tp-Stb, respectively. In the starting material, trialdehyde carbonyl (CO) carbon resonate at a downfield position around =192 ppm. The absence of peak at =192 ppm in .sup.13C CP-NMR spectrum also indicates the total consumption of the starting materials (FIG. 3C). To investigate the protonation of azo bond by phosphoric acid in PA@Tp-Azo, .sup.31P CP-NMR was conducted which shows two distinct peaks at 1.31 ppm and 14.3 ppm. The .sup.31P resonance peak at 1.31 ppm attribute to the undissociated H.sub.3PO.sub.4 and the shoulder at 14.3 ppm correspond to the H.sub.2PO.sub.4.sup. anion which indicate the protonation of azo bond. However in PA@Tp-Stb only single intense peak at 0.88 ppm correspond to undissociated H.sub.3PO.sub.4 observed and the peak at 14.3 ppm was absent which conclude the absence of H.sub.2PO.sub.4 anion due to the lack of protonation site in Tp-Stb (FIG. 4B).

[0076] Thermogravimetric analysis (TGA) of the activated Tp-Azo and Tp-Stb show thermal stability up to 350 C., with a gradual weight loss of 50% after 360 C. due to the decomposition of the framework. Permanent porosity of Tp-Azo and Tp-Stb are evaluated by N.sub.2 adsorption isotherm at 77 K, which show reversible type IV adsorption isotherm. Surface area of the activated COFs calculated using BET model was found to be 1328 and 422 m.sup.2/g for Tp-Azo and Tp-Stb, respectively (FIGS. 3D and 3F). The lower surface area of Tp-Stb may be due to the poor crystallinity and not so uniform channels resulted from the lower solubility of Stb precursors in organic solvents. Pore size distribution of Tp-Azo and Tp-Stb shows a narrow pore size distribution between the ranges of 1.6-2.5 nm.

[0077] The stability of Tp-Azo and Tp-Stb were assayed by immersing 50 mg of COFs in either 20 ml boiling water, or standing in 20 ml strong mineral acids (9 N HCl/1.5 M H.sub.3PO.sub.4) and bases (3-6 N NaOH) (FIGS. 11-14). Interestingly, both Tp-Azo and Tp-Stb remain stable, crystalline and porous while directly submerged in boiling water for several days (7 days), as verified by PXRD, FT-IR spectra and N.sub.2 adsorption isotherm. Further, these COFs also exhibit strong acid (9 N HCl) stability with almost retention of molecular crystallinity. However, base treated (3-6 N NaOH) COFs show moderate base stability with partial retention of crystallinity. The N.sub.2 adsorption isotherm of the 9 N HCl treated Tp-Azo indicates the retention of its intrinsic porosity, while Tp-Stb shows decrease in porosity after the acid treatment. However, 1.5 M H.sub.3PO.sub.4 treatment shows significant loss of porosity in both cases, which alludes to H.sub.3PO.sub.4 loading. H.sub.3PO.sub.4 exhibit high proton conductivity (10.sup.1 Scm.sup.1) due to its low volatility (>158 C.) and high proton mobility resulted from extended hydrogen bonding utilizing three ionizable OH bonds. The significant loss of porosity indicates that azo/stilbene groups are susceptible for the protonation and can stabilize the counter anions like phosphate or dihydrogenphosphate.

[0078] The PA@Tp-Azo and PA@Tp-Stb covalent organic frameworks are prepared by H.sub.3PO.sub.4 loading in Tp-Azo and Tp-Stb (FIG. 4A), which is achieved by simply immersing the evacuated COF materials in 1.5 M H.sub.3PO.sub.4 for 2 h. Further, COFs are washed with copious amount of water and activated overnight at 353K under dynamic vacuum to obtain H.sub.3PO.sub.4 loaded PA@Tp-Azo and PA@Tp-Stb. It is significant that the H.sub.3PO.sub.4 loaded COFs exhibit identical IR spectra and .sup.13C NMR spectra along with moderate crystallinity and porosity compared to the parent COFs Tp-Azo and Tp-Stb. Tp-Azo possess a higher acid loading (5.4 wt %) compared to Tp-Stb (2.8 wt %) as evident from TGA analysis.

[0079] Further, the proton conductivities of Tp-Azo, Tp-Stb, PA@Tp-Azo and PA@Tp-Stb were measured in both hydrous (FIG. 4C) and (FIG. 4B) anhydrous condition. The conductivities are determined from the semicircles in the Nyquist plots (FIGS. 8-10). Interestingly, both Tp-Azo and Tp-Stb exhibit almost zero conductivity, which signifies that the COF backbones are acting as a support. The proton conductivity of PA@Tp-Azo and PA@Tp-Stb were measured from 295K to 415K. Conductivity values gradually increased upon heating which reaches a maximum at a temperature of 332-340 K and then decreased gradually upon increasing temperature. The proton conductivity of PA@Tp-Azo is measured as 6.710-5 S cm.sup.1 at 340K at anhydrous condition. This value is highly humidity-dependent and increases upon humidification. Finally, PA@Tp-Azo exhibit proton conductivity of 9.910-4 S cm.sup.1 at 332K under 98% relative humidity (RH). Surprisingly, PA@Tp-Stb shows almost zero proton conductivity in anhydrous condition, while exhibiting poor proton conductivity value of 2.310-5 S cm.sup.1 at 332 K under 98% RH. Notably, PA@Tp-Azo exhibit an activation energy value of 0.11 eV, which is very low when compared to Nafion (0.22 eV) and its MOF counterparts operating under humid conditions. However, Tp-Azo exhibits distinct color change (from red to black) upon H.sub.3PO.sub.4 treatment, while color of Tp-Stb remains almost unchanged (gray) upon H.sub.3PO.sub.4 loading. The UV-Visible spectra of phosphoric acid treated monomer of Tp-Azo shows a red shift of the peak from 380 nm to 496 nm due to the protonation of the azo bond.

[0080] SEM (FIGS. 5, 6) and TEM (FIG. 7) images shows that Tp-Azo and Tp-Stb crystallize with a flower like morphology with aggregation of large number of petals with an average length of 40-50 nm.

Examples

[0081] Examples are given by way of illustration and therefore should not be construed to limit the scope of the invention.

Example 1: Synthesis of Tp-Azo and Tp-Stb

[0082] In the typical synthesis, a pyrex tube (o.d.i.d.=108 mm.sup.2 and length 18 cm) is charged with 1,3,5-triformylphloroglucinol(63 mg, 0.3 mmol) and 4,4-azodianiline (96 mg, 0.45 mmol) or 4,4-diaminostilbene dihydrochloride (128 mg, 0.45 mmol) in (1:1) dimethylacetamide and o-dichlorobenzene as solvent (3 mL) by ultrasonication for 10 minutes and then degassed through three freeze-pump-thaw cycles. Tubes were then vacuum sealed, placed in isotherm oven for 3 days at 120 C. Finally, the material was filtered out and washed with dry acetone and dried under vacuum at 180 C. for 12 h to obtain Tp-Azo and Tp-Stb respectively.

[0083] FT-IR (Tp-Azo, powder, cm-1): 1619 (w), 1568 (s), 1450 (m), 1284 (w), 1240 (s), 1147 (s), 987 (w), 839 (m). Elemental Analysis; Anal. Calcld. For C9ON2H6: C, 68.35; H, 3.79; N, 17.72. found: C, 48.12; H, 5.27; N, 11.12. FT-IR (Tp-Stb, powder, cm-1): 1574 (s), 1518 (w), 1450 (s), 1255 (m), 991 (w), 958 (w), 824 (m). Elemental Analysis; Anal. Calcld. For C10H6ON: C, 76.92; H, 3.84; N, 8.97; found. C, 69.84; H, 4.50; N, 7.89.

Example 2: Synthesis of H.SUB.3.PO.SUB.4 .Loaded Tp-Azo and Tp-Stb (PA@Tp-Azo and PA@Tp-Stb)

[0084] PA@Tp-Azo and PA@Tp-Stb covalent organic frameworks were prepared by H.sub.3PO.sub.4 loading in Tp-Azo and Tp-Stb. The H.sub.3PO.sub.4 loading was achieved by simply immersing the evacuated COF materials (about 150 mg) as obtained in example 1 in 10 ml of 1.5 M H.sub.3PO.sub.4 for 2 h. Further, COFs were washed with copious amount of water and activated overnight (12 hr) at 353K under dynamic vacuum to obtain H.sub.3PO.sub.4 loaded PA@Tp-Azo and PA@Tp-Stb.

Advantages of the Invention

[0085] Covalent organic framework is lighter and metal free

[0086] Wide variety of functionality

[0087] Higher thermal stability

[0088] High proton-conducting ability

[0089] The highly ordered one-dimensional channels in COFs offer potential pathways for proton conduction.

[0090] The COF may be used as high-performance proton-conducting material in fuel cell applications.

[0091] While the present invention has been illustrated by description of various embodiments and while those embodiments have been described in considerable detail, it is not the intention of Applicants to restrict or in any way limit the scope of the appended claims to such details. Additional advantages and modifications will readily appear to those skilled in the art. The present invention in its broader aspects is therefore not limited to the specific details and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of Applicants' invention.