Properties of aqueous alkaline sodium borohydride solutions and by-products formed during hydrolysis

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1 Properties of aqueous alkaline sodium borohydride solutions and by-products formed during hydrolysis presented at the HYATT REGENCY SAN FRANCISCO AT EMBARCADERO CENTER BOARD ROOM B, ATRIUM LEVEL 5 EMBARCADERO CENTER SAN FRANCISCO, CA Session 1:00-6:00 PM by Don Gervasio, Michael Xu and Evan Thomas Arizona State University Tempe, AZ AUGUST 17, 2005

2 Outline Review of ASU tasks Identity of product of borohydride hydrolysis, NaB(OH) 4 Solubility of hydrolysis product in aqueous solutions Future Research Characterizing hydrolysis by- products Identity Solution properties Chemical vs electrochemical hydrolysis of borohydride Seeking Polybenzimidazole (PBI) from PEMEAS for fuel cell membrane Conclusions

3 Review of ASU tasks Task 1: ASU will support the Univ. of Illinois (UIUC) in reporting and participate in technical meetings. Task 4.3: Optimize NaBH 4 performance Task 4.3.1: ASU will develop an energy dense (>2100 Wh/liter or Wh/Kg) hydrogen storage solution component consisting of >30% sodium borohydride in aqueous >1M NaOH solution. Task 4.3.2: ASU will develop a means of separating the hydrogen gas from the liquid hydrogen storage solution, so that only the hydrogen gas is supplied to the fuel cell anode. Today s talk relates to Tasks 4.3, -maximizing hydrogen storage & optimizing fluidics, -and is about boron-oxide identity and solution stability. Future work will try to determine - if the boron oxides that form during direct oxidation of borohydride on Pt is different from the oxides that form from heterogeneous catalysis (Ru) of borohydride hydrolysis. -Setting up an electrolysis cell and in situ electrochemical NMR cell for this purpose Applied Nano Bioscience Center at ASU

4 Current Research OVERALL GOAL: Maximize hydrogen storage capacity of aqueous alkaline sodium borohydride solution and solubility of hydrolysis by-product solution. Hydrolysis of Borohydride: NaBH H 2 O 4 H 2 + NaBO 2 SUB-GOALS: * Characterize hydrolysis of sodium borohydride (NaBH 4 ) using: - 11 B Nuclear Magnetic Resonance (NMR) Spectroscopy - X-ray diffraction (XRD) - Fourier Transform Infrared (FTIR) spectroscopy. *Ascertain the chemical structure of the by-product(s) formed during hydrolysis. *Determine the mechanism of the catalytic hydrolysis of NaBH4 in alkaline solution *Evaluate the effect of additives on: - the hydrolysis of sodium borohydride and the - solubility of the resulting boron-oxide by-products.

Alumina-borate solid lump that forms in hydrogen generation reactor from alumina dissolution 30wt% sodium borohydride in aqueous

5 Precipitates during catalytic hydrolysis Ru on alumina Not Stable in aqueous alkaline sodium borohydride hydrogen-storage solution! Alumina-borate solid lump that forms in hydrogen generation reactor from alumina dissolution 30wt% sodium borohydride in aqueous 1M NaOH solution. Ru on various metal supports Ti best support: -stable - Ru adheres. Ru on metal Ti support is stable in aqueous alkaline sodium borohydride solution! Question Do sodium borohydride and hydrolysis by-products stay in solution? Or Precipitate?

6 Answer it depends on hydrolysis by-product(s) Possible By-Products formed during NaBH 4 Hydrolysis The reaction of hydrogen generation from sodium borohydride is nominally written as: NaBH H 2 O 4 H 2 + NaBO 2 Actually many boron oxides can form, dictating the amount of water needed as seen below. Boron Oxide Mole Oxygen H 2 O Needed Volume H 2 O (per NaBH 4 ) (30% sol n) (milliliter) *NaB(OH) 4 4 moles 32 moles 576 NaBO. 2 x H2O 2+x Na2B4O7 7/ Na 2 B4O H2O 17/ Na 2 B 4 O 6 (OH). 2 3 H 2 O 11/ Na 2 B 4 O. 7 5 H 2 O 12/ Na 2 B 4 O 5 (OH). 4 3 H 2 O 12/ Na 2 B 4 O 5 (OH). 4 8 H 2 O 17/ * ASU X-ray diffraction data indicate that NaB(OH) 4 is the by-product of hydrolysis reaction.

7 XRD of Hydrolysis Product Hydrolysis product Same as NaB(OH) 4

8 Synthesis of NaB(OH)4 from boric acid B(OH) 3 + NaOH NaB(OH) 4 Stoichiometric reaction done in water or water tetrahydrofuran (THF) mixtures Precipitate Filter XRD of the product of this synthesis is same as NaB(OH)4 standard which is same as borohydride hydrolysis product last slide

9 NMR of NaB(OH) NaB(OH) ppm Standard ppm (f1) NMR Figure 1: B11 NMR, presumably NaB(OH)4 with BF3 diethyl etherate in external capillary Applied Nano Bioscience Center at ASU

10 NMR of Borohydride Hydrolysis Product ppm ppm (f1) B11 NMR, 30% NaBH4 w/ Ru catalyst in situ after 3 weeks Applied Nano Bioscience Center at ASU -60

11 NMR Spectrum of Sodium Metaborate, Na BO Not NaBO is not close to BF ppm (f1) B NMR, neat BF 3 diethyl etherate in external capillary and 0.05M NaBO 2 4H 2 O, 5% D 2 O in sample tube. Boric acid: 0 ppm

12 Conclusion on Identification of Borohydride Hydrolysis Product NMR like XRD indicates hydrolysis product if NaB(OH)4 Will do thermal analysis next to confirm match

13 Oxide Identity and Solubility Tests Since solution NMR is in agreement with X-ray scattering of solids, that both techniques indicate that NaB(OH)4 is the principal oxide from the hydrolysis of 10 to 30% NaBH4 in aqueous 1 M NaOH, we accelerate studies of solubility by direct synthesis of NaB(OH)4 and testing of its solubility. Continuing solubility tests of NaB(OH)4 - air dry & weigh solid NaB(OH)4 - place in solvent - warm, stir - filter - air dry and reweigh - solubility is weight loss/volume of solvent

14 Synthesis of NaB(OH)4 from boric acid B(OH) 3 + NaOH NaB(OH) 4 Stoichiometric reaction done in water or water tetrahydrofuran (THF) mixtures Precipitate Filter XRD of the product of this synthesis is same as NaB(OH)4 standard which is same as borohydride hydrolysis product last slide

15 Hydrogen Content of alkaline sodium borohydride solutions - Goal: use as high a concentration of borohydride in solution as possible Gravimetric storage density: % H2 by weight Solid NaBH % NaBH4-20 solution (20 wt% NaBH4, 3 wt% NaOH, 77 wt% H 2O) 4.3 % NaBH4-25 solution (25 wt% NaBH 4, 3 wt% NaOH, 72 wt% H2O) 5.3 % NaBH4-30 solution (30 wt% NaBH 4, 3 wt% NaOH, 67 wt% H2O) 6.4 % NaBH4-35 solution (35 wt% NaBH4, 3 wt% NaOH, 62 wt% H2O) 7.5 % Volumetric storage density: 1 Liter NaBH4-20 solution 44 grams or 526 Standard Liters 1 Liter NaBH 4-25 solution 55 grams or 658 Standard Liters 1 Liter NaBH4-30 solution 66 grams or 789 Standard Liters 1 Liter NaBH4-35 solution 77 grams or 921 Standard Liters From Millenium Cell website,

16 Solubility of Sodium Borohydride in Various Solvents From Rohm and Haas Borohydride digest For all liquid reactor, solubility of NaB(OH)4 not NaBH4 may be critical!

17 Solubility Testing of NaB(OH)4 At Room Temp. Sample Solvent Amt. (ml) Solid Wt. (g) Undissolved Solid Collected (g) Solubility Limit (g/50ml) Solubility Limit (g/l) H 2 O M NH 4 OH wt% NH 4 OH M NaOH At Elevated Temp. (40ºC) Sample Solvent Amt. (ml) Solid Wt. (g) Undissolved Solid Collected (g) Solubility Limit (g/10ml) Solubility Limit (g/l) H 2 O M NH 4 OH wt% NH 4 OH M NaOH H 2 O w/ 2% 90k CMC polymer M NaOH w/ 2% 90k CMC polymer M NH 4 OH 20wt% NH 4 OH Solubility of NaB(OH) 4 in Various Solvents NEXT

18 Status: Hydrolysis of 30% NaBH 4 to NaB(OH) 4 Recall: in 1000 g of fuel (NaBH4-30), there are: g NaOH or 0.07 moles of NaOH per kilo of solution g H 2 O or 39 moles of free water per kilogram of solution g NaBH 4 or 8 moles of NaBH 4 per kilo of solution. NaBH H 2 O 4 H 2 + NaB(OH) 4 After the above hydrolysis reaction of 30wt% borohydride solution, there are: - 32 moles or 571 g of water consumed - 8 moles or g of NaB(OH) 4 formed or 2.8 g of NaOH remain ml of free water remains. Questions for Future Work Q1: Is NaB(OH) 4 the only hydrolysis by-product? A: Yes Q2: Can 800 g of NaB(OH) 4 dissolve in 125 ml of pure water? With 0.07 moles of NaOH? A: Water or base does not seem to matter but this is work in progress Q3: Does the hydrolysis process influence identity of by-product(s)? - heterogeneous catalytic hydrolysis over Ru? (NMR ex-situ and in-situ) - electrolysis on Pt electrode? (NMR ex-situ and in-situ) A: work in progress Applied Nano Bioscience Center at ASU

19 Conclusions! A key problem with the hydrogen generation reactor is keeping the boron oxide (borohydride hydrolysis by-product) from precipitating and obstructing the micro-fluidics in system.! Boron oxide is definitively established through NMR spectroscopy and XRD to be Na B(OH) 4! Additives being tested to keep the boron oxide in solution.

Heng-Pan Yang, Sen Qin, Ying-Na Yue, Li Liu, Huan Wang* and Jia-Xing Lu**

Heng-Pan Yang, Sen Qin, Ying-Na Yue, Li Liu, Huan Wang* and Jia-Xing Lu** Electronic Supplementary Material (ESI) for Catalysis Science & Technology. This journal is The Royal Society of Chemistry 2016 Supplementary information for Entrapment of a pyridine derivative within