Method to process borate by-products from sodium borohydride hydrolysis

Abstract

The present disclosure relates to a method for processing a liquid by-product of sodium borohydride hydrolysis to obtain a borate compound, the method comprising the following steps: separating the liquid by-product by sedimentation, to obtain a borate-rich supernatant; drying the borate-rich supernatant under vacuum to obtain a solid composition comprising a borate compound. An aspect of the present disclosure relates composition obtainable by the disclosed method comprising at least 90% (w/w) of sodium boron hydroxide and its use as a source of borate in the production of sodium borohydride and/or hydrogen.

Claims

1. A method for processing a liquid by-product of sodium borohydride hydrolysis to obtain a borate compound, the method comprising the following steps: separating the liquid by-product by sedimentation, to obtain a borate-rich supernatant; drying the borate-rich supernatant under vacuum to obtain a solid composition comprising a borate compound; wherein the borate compound is sodium boron hydroxide (NaB(OH).sub.4).

2. The method according to claim 1, wherein the crystal form of the borate compound has an XRD pattern essentially the same as shown in FIG. 6B having a melting point ranging from 53 C. to 58 C.

3. The method according to claim 1, wherein the solid composition comprises at least 90% (w/w) of the borate compound (NaB(OH).sub.4).

4. (canceled)

5. The method according to claim 1, wherein the borate-rich supernatant is dried under vacuum for 6 to 8 days.

6. The method according to claim 1, further comprising a step of rehydrogenating the borate compound into sodium borohydride.

7. The method according to claim 6, wherein the rehydrogenation step is a thermochemical process, a mechanochemical process or an electrochemical process, preferably electrochemical.

8. The method according to claim 1, wherein the sedimentation occurs by natural sedimentation, or by centrifugation.

9. The method according to claim 8, wherein the sedimentation occurs for up to 12 hours, or the centrifugation occurs for up to 5 minutes.

10. A crystalline sodium boron hydroxide (NaB(OH).sub.4) obtainable by the method of claim 6, wherein the crystalline hydroxide form has an XRD pattern essentially the same as shown in FIG. 6B having a melting point ranging from 53 C. to 58 C.

11. The crystalline sodium boron hydroxide (NaB(OH).sub.4) according to claim 10 comprising the absence of peaks at diffraction angles (2) of 21.4-21.6, 32.2-32.4 and 37.8-37.9.

12. A composition obtainable by the method described in claim 1 comprising at least 90% (w/w) of sodium boron hydroxide (NaB(OH).sub.4), and thermonatrite.

13. A method for production of sodium boron hydroxide (NaB(OH).sub.4) and/or hydrogen comprising using a composition as described in claim 12 as a source of borate in the production of sodium borohydride and/or hydrogen.

14. A method for obtaining hydrogen comprising a step of processing a liquid by-product of sodium borohydride hydrolysis as described in claim 1.

15. The method according to claim 14 further comprising the following steps: adding a catalyst into a reactor; injecting a mixture of sodium borohydride and aqueous sodium hydroxide into the reactor; hydrolysing the sodium borohydride into hydrogen with formation of a liquid by-product.

16. The method according to claim 14, wherein the catalyst is a metallic catalyst, preferably wherein the metallic catalyst is a bimetallic catalyst, more preferably NiRu.

17. (canceled)

18. The method according to claim 14, wherein the concentration of sodium borohydride ranges from 5 to 20% (w/w), preferably from 10 to 15% (w/w).

19. The method according to claim 14, wherein the concentration of sodium hydroxide ranges from 0.5 to 70% (w/w).

20. The method according to claim 14, wherein the mass ratio between sodium borohydride, sodium hydroxide and catalyst ranges from 10.0:7.0:4.0 to 10.0:7.0:6.3.

21. The method according to claim 14 wherein the hydrolysis step occurs at a temperature ranging from 18 to 27 C.

22. The method according to claim 14, wherein the hydrolysis step starts at a pressure ranging from 60 to 102 kPa.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The following figures provide preferred embodiments for illustrating the disclosure and should not be seen as limiting the scope of invention.

[0040] FIG. 1: Schematic representation of an embodiment of a hydrogen synthesis method comprising the disclosed method to recover the borate by-product (1).

[0041] FIG. 2: Embodiment of the separation of the by-product by sedimentation of the catalyst.

[0042] FIG. 3: Embodiment of the by-product appearance after drying.

[0043] FIG. 4: Embodiment of results of by-product comparison (in mass %) between air exposure or vacuum drying.

[0044] FIG. 5: Embodiment of the hydrolysis' by-product composition (in mass %) under vacuum drying, for four different samples (numbered as 1,2,3 and 4).

[0045] FIG. 6: Embodiment of X-ray powder diffraction spectra for NaB(OH).sub.4 obtained by common drying process (common drying) or by the method described in the present disclosure (present disclosure).

DETAILED DESCRIPTION

[0046] The present disclosure relates to a method for processing a liquid by-product of sodium borohydride hydrolysis to obtain a borate compound, the method comprising the following steps: separating the liquid by-product by sedimentation, to obtain a borate-rich supernatant; drying the borate-rich supernatant under vacuum to obtain a solid composition comprising a borate compound, wherein the borate compound is sodium boron hydroxide.

[0047] The present disclosure relates to a method to obtain sodium borohydride from a liquid mixture comprising borate, the method comprising the following steps: separating the borate compound from the mixture by sedimentation; drying the borate compound to obtain a pure borate compound; and rehydrogenating the pure borate compound into sodium borohydride. An aspect of the present disclosure relates to a method to obtain hydrogen comprising the step of obtaining sodium borohydride from a liquid mixture comprising borate disclosed.

[0048] In an embodiment, NaBH.sub.4 was used to produce hydrogen, in particular molecular hydrogen, via hydrolysis at room pressure and temperature.

[0049] For the scope and interpretation of the present disclosure, room pressure is defined as normal air pressure ranging from 60-102 kPa, preferably 101.325 kPa; and room temperature is defined as a temperature ranging from 15 to 27 C., preferably 18 to 25 C.

[0050] In an embodiment, a catalyst was used to obtain hydrogen from sodium borohydride, preferably a metallic catalyst, more preferably a bimetallic catalyst such NiRu.

[0051] In an embodiment, 10% (m/m) of NaBH.sub.4 was injected together with 7% (m/m) NaOH aqueous solution in a stainless-steel batch reactor, using 0.40-0.63 mass of catalyst per mass (mg) of sodium borohydride. After hydrolysis, the application of the disclosed method assures the maximum conservation of the byproduct stability in the form of NaB(OH).sub.4, which is the best viable compound to be integrated in the rehydrogenation process (borate with higher cost-benefit).

[0052] In an embodiment, the disclosed process comprises the extraction of a borate by-product in a liquid form following the production of H.sub.2, separation of the catalyst used in H.sub.2 production by natural sedimentation or forced sedimentation through centrifuge for up to 5 minutes, and drying the resulting supernatant under vacuum, preferably in an enclosed desiccator or similar equipment (for example a glove box) for 6 to 8 days. Pure or close to pure NaB(OH).sub.4 is obtained after drying, which is ready to be rehydrogenated. This is the best borate to rehydrogenate (lower energy demand by not requiring water evaporation) while also generating and storing pure H.sub.2.

[0053] In an embodiment, the NiRu catalyst can be reused. After sedimentation, the formed pellet can be collected and transferred to a glass beaker, washed, preferably at least three times, at room temperature and dried at 80 C. for 1 hour to be reused.

[0054] In an embodiment, the liquid by-product of reaction (supernatant resulting from sedimentation) is transferred to a container with a size adapted to the volume in consideration. For example, the container can be a glass petri dish. The container is then placed in an enclosed environment, e.g., a desiccator, under vacuum using a vacuum pump. For an average of 6 to 8 days the desiccator must remain closed during drying to avoid unnecessary contact with air. The by-product is dried when no liquid and only crystals are visible in the petri dish. As comparative example, the liquid by-product of reaction was also dried by air exposure at room temperature without any pressure control (no vacuum).

[0055] In an embodiment, X-ray powder diffraction (XRD) was used for phase identification of the obtained crystals. Briefly, this method allows the identification and quantification of the compounds present in a crystalline sample. Each compound reflects the x-rays that cross the sample in a different angle and intensity and, with the use of XRD databases, this compound can be identified. In an embodiment, the XRD patterns were recorded at room temperature using monochromatic Cu K- radiation (=1.5406 ). The range of the XRD patterns were 4<2021 70.

[0056] FIG. 4 shows an embodiment of results of by-product comparison (in mass %) between air exposure or vacuum drying, showing that the vacuum drying results in an higher mass percentage of NaB(OH).sub.4 (99.3%) than Na.sub.2CO.sub.3 (0.7%)), as compared to the results obtained with air drying (68.6% of NaB(OH) 4 and 31.4% of Na.sub.2CO.sub.3). FIG. 5 shows an embodiment of the hydrolysis' by-product composition (in mass %) under vacuum drying, for four different samples (numbered as 1,2,3 and 4).

[0057] In an embodiment, the XRD analysis revealed that the crystalline sodium boron hydroxide obtained from the disclosed method comprises the absence of peaks at diffraction angles (2) of 21.4-21.6, 32.2-32-4 and 37.8-37.9 (FIG. 6B), which can be observed for the crystalline material obtained with common drying (FIG. 6A).

[0058] In an embodiment, after the vacuum drying, as described in the present disclosure, it is obtained a solid composition comprising at least 90% (w/w) of the borate compound, preferably NaB(OH).sub.4. This composition has great advantages for the process of rehydrogenation of sodium borohydride, and therefore its use on the production of hydrogen via sodium hydroxide hydrolysis, since it is a non-hydrated compound, thus not requiring dehydration upon rehydrogenation. Also, the obtained composition comprises at least 90% (w/w) of the borate compound, thus increasing the effectiveness of the rehydrogenation method by not giving rise to other unwanted compounds.

[0059] In an embodiment, crystals formed can be milled with a pestle and mortar to store the solid composition comprising the borate compound in powder. This step can be performed open to air, however minimal contact to air is preferred. When in no use, the solid composition comprising the borate compound can be stored in the same container, under vacuum.

[0060] In the state of the art, the regeneration of sodium borohydride may occur by different methods, such as thermochemical, mechano-chemical, or electrochemical processes. Thermochemical processes are based on reactions that involve high pressure and/or temperature; the mechano-chemical processes are similar to the thermochemical ones, but the source of energy used in this type of process relies on mechanical forces; the electrochemical processes use electric energy to produce sodium borohydride by reducing or oxidizing other borates.

[0061] After regeneration, the obtained sodium borohydride can be used on the production of hydrogen, via sodium borohydride hydrolysis, thus closing the NaBH.sub.4H.sub.2 cycle.

[0062] The term comprising whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0063] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof. The above described embodiments are combinable.

[0064] The following claims further set out particular embodiments of the disclosure.