Method of synthesizing aluminum oxide from gallium-aluminum hydrogen evolution reaction

Abstract

A method of synthesizing aluminum oxide includes reacting a gallium and aluminum composite in a hydrogen evolution reaction with water to form an aluminum byproduct having at least one of aluminum hydroxide or aluminum oxyhydroxide. The method also includes removing the aluminum byproduct and calcinating the aluminum byproduct to form aluminum oxide.

Description

METHOD OF SYNTHESIZING ALUMINUM OXIDE FROM GALLIUM- ALUMINUM HYDROGEN EVOLUTION REACTIONCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of, and priority to, U.S. Provisional Patent Application Serial No. 63 / 513,912 filed on July 17, 2023, and U.S. Provisional Patent Application Serial No. 63 / 589,745 filed on October 12, 2023. The entire contents of the foregoing applications are incorporated by reference herein.BACKGROUND

[0002] The development of efficient ways to split water for the generation of hydrogen gas (H2) is essential for the widespread adoption of a hydrogen economy. Currently, over 95% of H2 production relies on fossil fuels, such as steam reforming of natural gas. Unfortunately, this process also produces large quantities of carbon dioxide, greatly offsetting the environmental benefits of H2. The gravimetric energy density of H2 is 120 megajoules per kilogram (MJ / kg) and the sole combustion product is water. Although the volumetric energy density of H2 at 700 bar, which is the typical on-board gas storage pressure for H2 fueled vehicles, is only 5.6 MJ / L, H2 holds immense promise for the widespread production of clean energy. Moreover, H2 generating processes that consume minimal energy and produce value-added byproducts can significantly enhance the viability of this technology.SUMMARY

[0003] The present disclosure provides a method for production of aluminum hydroxides [Al(OH)s] and aluminum oxyhydroxide [AIO(OH)] as byproducts of a liquid metal galliumaluminum (Ga-Al) composite water splitting reaction with the co-generation of hydrogen fuel.The reaction takes place under atmospheric conditions with zero reliance on caustic chemicals or electrolysis. Composites may be produced without an inert atmosphere and with minimal energy input.

[0004] Below 60 °C, all Ga-Al composites (tested Ga:Al atomic ratios of 6: 1, 3:1, 1:1 and 1:2) reacted spontaneously with varying amounts of water (3, 5, 10 and 20 mL), producing bay erite A1(OH)3 exclusively along with hydrogen. Byproduct formation was sensitive to temperature, with boehmite phase AIO(OH) as observed at 60 to 100 °C. The A1(OH)3 was successfully calcined to a-AhCh at 1,100 °C in 24 hours (h), becoming the dominant phase after just 2 h. Ga recovered from the exhausted composites may also be reused without any effect on byproduct fomiation. The reaction produced Ga nanoparticles (NPs) and after 48 h results in Ga-doped A1(OH)3. This value-added byproduct makes the CCb-frcc hydrogen production even more economically viable.

[0005] H2 is generated using aluminum (Al) metal, which is capable of spontaneously splitting water to produce H2 fuel and A1(OH)3 as a byproduct. Common Al products do not undergo this reaction due to its passivating oxide layer that prevents water from contacting the pure Al metal. If this passivating layer is removed, water can freely react with Al to produce hydrogen gas. An exemplary way to remove the passivating oxide layer is to dissolve Al metal in liquid gallium (Ga) to form a gallium-aluminum (Ga-Al) binary composite. Liquid Ga metal is capable of seeping into the grain boundaries of Al, effectively breaking up the metal into Al nanoparticles (Al NPs). These NPs are free of a passivating oxide layer since they are formed and reside within the liquid Ga. The Ga-Al composite can then be used to spontaneously split water at ambient conditions, with the only energy required being very mild heat to melt the Ga (MP = 29.8 °C).

[0006] The byproduct (oxy)hydroxides, e.g., Al(OH)a and AIO(OH), are important industrial commodities, as they are the raw materials for Al metal in addition to alpha alumina (alpha aluminum oxide or (X-AI2O3), which has vast and diverse applications ranging from ceramics and catalysis to packaging and electronics. Currently, Al production relies on the Bayer process for removing A1(OH)3 from bauxite ore. This highly polluting and energy intensive process uses caustic chemicals that produce vast pools of environmentally toxic red mud. The process ultimately isolates Al from bauxite ore as gibbsite phase A1(OH)3, which can then be calcined to industrially valuable (X-AI2O3. Water splitting via a Ga-Al composite offers a non-polluting route to A1(OH)3 from post-consumer Al, in addition to producing economically and environmentally valuable H2 fuel.

[0007] A method of production of aluminum hydroxide [A1(OH)3] and aluminum oxyhydroxide [AIO(OH)] via liquid metal water splitting reactions is disclosed, especially those from simple and efficient binary Ga-Al composites. These binary Ga-Al composites produce H2 and A1(OH) and / or AIO(OH) without the use of corrosive chemicals, electrolytic solutions, electrolyzers, or elevated temperature / pressure. The composites are incredibly efficient, with an 80% H2 yield after only 15 minutes (min) when a Ga:Al ratio of 3:1 was used for the composites.

[0008] The present disclosure also analyzed the effects of time, volume of water, temperature, Ga:Al ratio and recycled Ga on aluminum hydroxide and oxyhydroxide production. Chemical and physical nature of the byproducts was analyzed using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), inductively coupled plasma-optical emission spectroscopy (ICP-OES) and powder X-ray diffraction (PXRD). ICP-OES was chosen to determine Al and Ga atomic ratio composition of all solid byproducts and recovered Ga due to the accuracy of the method for determining metal concentrations of bulk samples. Calcination experiments were alsoperformed to determine whether industrially valuable (X-AI2O3 could be produced from the isolated Al(OH)s and A10(0H) byproducts.

[0009] According to one embodiment of the present disclosure, a method of synthesizing aluminum oxide is disclosed. The method includes reacting a gallium and aluminum composite in a hydrogen evolution reaction with water to form aluminum hydroxide. The method also includes removing the aluminum hydroxide and heating the aluminum hydroxide to form aluminum oxide.

[0010] Implementations of the above embodiment may include one or more of the following features. According to one aspect of the above embodiment, reacting the gallium and aluminum composite with water is performed at a temperature from about 60 °C to about 100 °C and forms aluminum oxyhydroxide. The method may also include removing the aluminum oxyhydroxide and heating the aluminum oxyhydroxide to form aluminum oxide. The method may further include removing hydrogen gas formed during the reaction of the gallium and aluminum composite with water. The method may additionally include removing remaining gallium formed during the reaction of the gallium and aluminum composite with water. The method may further include forming the gallium and aluminum composite from the removed gallium. The aluminum hydroxide may be doped with gallium. The aluminum oxide may be also doped with gallium. Heating the aluminum hydroxide may include calcinating the aluminum hydroxide at a temperature from about 1,000 °C to about 1,500 °C and for about 2 hours to about 24 hours. The gallium and aluminum composite may include gallium and aluminum at an atomic ratio from about 1:1 to about 6:1 of gallium to aluminum. Removing the aluminum hydroxide may further include suspending the aluminum hydroxide in water for up to about 48 hours. The aluminum oxide may be a-aluminum oxide.

[0011] According to another embodiment of the present disclosure, a method of synthesizing aluminum oxide is disclosed. The method includes reacting a gallium and aluminum composite in a hydrogen evolution reaction with water to form an aluminum byproduct having at least one of aluminum hydroxide or aluminum oxyhydroxide. The method also includes removing the aluminum byproduct and calcinating the aluminum byproduct to form aluminum oxide.

[0012] Implementations of the above embodiment may include one or more of the following features. According to one aspect of the above embodiment, the method may include adjusting the temperature of the reaction of gallium and aluminum composite with water to adjust the ratio of aluminum hydroxide or aluminum oxyhydroxide of the aluminum byproduct. The temperature may be adjusted to from about 20 °C to about 100 °C. The method may also include removing hydrogen gas formed during the reaction of the gallium and aluminum composite with water. The method may further include removing unreacted gallium formed during the reaction of the gallium and aluminum composite with water. The method may additionally include forming the gallium and aluminum composite from the removed gallium. The aluminum byproduct may be doped with gallium. The aluminum oxide may be a-aluminum oxide doped with gallium.

[0013] According to a further embodiment of the present disclosure, a method of making aluminum hydroxide is disclosed. The method includes reacting a gallium and aluminum composite in a hydrogen evolution reaction with water to form an aluminum hydroxide in a post reaction mixture.

[0014] Implementations of the above embodiment may include one or more of the following features. According to one aspect of the above embodiment, the hydrogen evolution reaction proceeds for at least 15 minutes. The method may further include allowing the post reaction mixture to settle for at least 48 hours. The aluminum hydroxide may include bayerite and thewater may be provided at a temperature of between 25 °C and 80 °C. Reacting the gallium and aluminum composite in the hydrogen evolution reaction with water also forms boehmite, where the water is provided at a temperature above 60 °C. The water may be provided at a temperature above 80 °C. The aluminum hydroxide may include gibbsite, and the water may be provided at 25 °C in a molar ratio of 10:1 or above relative to the gallium and aluminum composite.BRIEF DESCRIPTION OF DRAWINGS

[0015] Various embodiments of the present disclosure are described below with reference to the following figures:

[0016] FIG. 1 shows a method of synthesizing aluminum oxide from aluminum byproducts of a gallium-aluminum hydrogen evolution reaction according to an embodiment of the present disclosure;

[0017] FIG. 2 shows PXRD spectra of bayerite [A1(OH)3] obtained as a byproduct of reactions with Ga:Al composite at different ratios with water according to an embodiment of the present disclosure;

[0018] FIG. 3 shows PXRD spectra of an amorphous byproduct after 24 h and crystalline bayerite after aging for 48 h according to an embodiment of the present disclosure;

[0019] FIGS. 4A shows an SEM image of Ga NPs suspended in a bayerite hydrogel 15 min after water was introduced to the composite according to an embodiment of the present disclosure;

[0020] FIGS. 4B-D show EDS maps of Al (FIG. 4B), Ga (FIG. 4C), O (FIG. 4D) of Ga NPs of FIG. 4A;

[0021] FIGS. 5 A shows an SEM image of bayerite collected 48 h after combining water and the composite according to an embodiment of the present disclosure;

[0022] FIGS. 5B-D show EDS maps of Al (FIG. 5B), Ga (FIG. 5C), O (FIG. 5C) of Ga NPs of FIG. 5 A;

[0023] FIG. 6 shows PXRD spectra of bayerite showing the effects of the volume of water according to an embodiment of the present disclosure;

[0024] FIG. 7 shows PXRD spectra of byproduct samples illustrating the transition from bayerite to boehmite [AIO(OH)] byproduct as the reaction temperature is increased according to an embodiment of the present disclosure;

[0025] FIG. 8 shows PXRD spectra of four samples of bayerite showing that recycled Ga metal has no effect on the production of bayerite under standard conditions after four reaction cycles according to an embodiment of the present disclosure;

[0026] FIG. 9 shows PXRD spectra of a) theoretical a-AhCh; b) calcination product of commercially purchased A1(OH)3; c) calcination product of byproduct A1(OH)3; and d) calcination product of byproduct AIO(OH);

[0027] FIG. 10A shows an SEM image bayerite produced under standard conditions according to an embodiment of the present disclosure;

[0028] FIG. 10B shows an SEM image of boehmite produced at 80 °C according to an embodiment of the present disclosure;

[0029] FIG. 10C shows an SEM image of the resultant (X-AI2O3 produced by calcination of bayerite at 1,100 °C for 24 h according to an embodiment of the present disclosure; and

[0030] FIG. 10D shows an SEM image of the resultant 01-AI2O3 produced by calcination of boehmite synthesized at 80 °C according to an embodiment of the present disclosure.DETAILED DESCRIPTION

[0031] As used herein, the term “nanoparticle” denotes a particle having any shape and a maximum dimension in any direction of from about 1 nm to about 100 nm. As used herein the temis “about”, “approximately”, and other relative terms, denote a range of ± 5% of the stated value.

[0032] The present disclosure provides for a method for producing aluminum hydroxide using a hydrogen evolution reaction (HER) of water with a Ga-Al composite catalyst. H2 gas is collected and Ga is also collected for use in subsequent HER. Aluminum hydroxide products are dried and crystallized and may then be subjected to a calcination process to form aluminum oxide.

[0033] With reference to FIG. 1 a method for synthesizing aluminum oxide from byproducts of a gallium-aluminum hydrogen evolution reaction includes forming a Ga-Al composite catalyst at step 10. The liquid metal catalyst may be a binary composite alloy of Ga and Al. Various atomic ratios of the Ga-Al composites may be used to optimize the production of hydrogen. Ga-Al composites having a higher Ga ratio produce larger amounts of hydrogen, compared to Al-rich composites. In embodiments, Ga and Al may be present in the composite at an atomic ratio from about 1 : 1 to about 6:1. In further embodiments, the ratio may be from about 2.5:1 to about 4: 1 of Ga to Al. In additional embodiments, the ratio may be from 2.5:1 to about 3.5:1, or about 3:1 of Ga to Al.

[0034] Ga-Al composites have a melting point from about 20 °C to about 25 °C. Ga-Al composite may be formed by mechanically combining Ga and Al, such as by folding the two metals at a temperature from about 20 °C to about 30 °C. Folding the two metals results in production of Al nanoparticles, which may have an average particle size from about 5 nm toabout 30 nm. Folding may include any application of pressure on the two metals that results in formation of the composite.

[0035] Suitable aluminum may be obtained from any source such as foil, or recyclable aluminum (e.g., cans, packaging, trays, etc.). Suitable aluminum foil may have a thickness of less than 0.5 mm. In embodiments, aluminum foil may have a thickness of less than 0.4 mm. Aluminum may also be provided as particles having an average diameter from about 0.01 mm to about 1 mm, and in embodiments about 0.5 mm and in further embodiments about 0.04 mm. The disclosed range has been found to be more effective mixing of the two metals.

[0036] At step 12, the Ga-Al composite is added to a reactor with water or conversely, water may be added to the reactor containing the Ga-Al composite. The reactor may be equipped with a gas collector to suction H2 gas generated during the HER, which occurs upon the Ga-Al composite contacting water. In addition to H2 gas, Ga is also produced by the reaction as well as aluminum (oxy)hydroxide by products such as various polymorphs of A1(OH)3 as well as AIO(OH).

[0037] Any suitable amount water may be added which may be from 10:1 mole ratio of water to Ga-Al composite or above. The amount of water used during the reaction affects the ratio of polymorphs of A1(OH)3 (e.g., of bay erite to gibbsite) that are produced by the HER. In particular, the ratio of bayerite to gibbsite increases as the amount of water decreases. In other words, smaller volumes of water promote formation of the more dehydrated phase of A1(OH)3, gibbsite.

[0038] The HER may be performed at a temperature from about 20 °C to about 100 °C by heating the reactor vessel and / or the supplied water. Increasing the temperature of the reaction toabout 60-100 °C may also produce aluminum oxyhydroxide, e.g., boehmite AIO(OH), as a byproduct of the HER.

[0039] The HER may continue from about 15 minutes to about 48 hours. In addition, to H2 gas, Ga remains unreacted by the reaction and may be collected at step 14 any time after the bulk of the HER is completed, e.g., 15 minutes. Ga may be recycled to form additional Ga-Al composite for use in subsequent HER reactions as described above at step 10.

[0040] Aluminum (oxy)hydroxides (Al(OH)a and / or AIO(OH), e.g.) are also produced as a byproduct of the HER and may be collected as part of a supernatant also at step 14 that is formed during the reaction of Ga-Al composite with water. Supernatant includes dissolved aluminum (oxy)hydroxides, which may be left in the reactor or transferred to another vessel for up to 48 hours to set and to allow the aluminum (oxy)hydroxides to crystallize.

[0041] During HER, Ga nanoparticles are formed which are retained in the aluminum (oxy)hydroxide hydrogel (i.e., supernatant). As the supernatant suspension is aged, initially formed Ga NPs oxidize to Ga3+over the duration of 48 hours and become incorporated into the aluminum (oxy)hydroxide crystal structure, thereby forming Ga doped aluminum (oxy)hydroxides since Ga3+can substitute for Al3+in the bayerite to fomi Ga-doped bayerite due to their similar chemical nature and ionic radii.

[0042] At step 16 the crystalline aluminum (oxy)hydroxides are filtered, which may be done via vacuum filtration, and the solid is dried, e.g., in a vacuum oven connected to house vacuum (about 30 Torr) at about 100 °C or above for about 15 minutes or more to form solid (e.g., powder) aluminum (oxy)hydroxides.

[0043] At step 18, Al(OH)s can be subjected to calcination to form a-aluminum oxide (a-AbO,).Calcination refers to thermal treatment of aluminum (oxy)hydroxides whereby the compoundsare heated without melting under a restricted supply of ambient oxygen, for the purpose of removing impurities or volatile substances and / or to incur thermal decomposition. Calcination may be carried out in a container heated inside an oven at a temperature of about 1,000 - 1,500 °C, which in embodiments may be 1,100 °C for a period of time from about 2 to 24 hours. Since the aluminum (oxy)hydroxides were doped with Ga, the resulting a-aluminum oxide is also similarly doped with Ga.

[0044] The following Examples illustrate embodiments of the present disclosure. These Examples are intended to be illustrative only and are not intended to limit the scope of the present disclosure. Also, parts and percentages are by weight unless otherwise indicated. As used herein, “room temperature” or “ambient temperature” refers to a temperature from about 20C to about 25 °C.EXAMPLESEXAMPLE 1

[0045] This example describes preparation of liquid metal catalyst Ga-Al Composite.

[0046] All chemicals and reagents were purchased and used without further purification. Ga (99.99% metal basis) was purchased from Alfa Aesar (Tewksbury, MA, USA). Commercially available off-the-shelf aluminum foil, free of any non-stick chemicals, was used.

[0047] Production of a Ga:Al composite having atomic ratio of about 3: 1 was performed by adding loosely crumpled aluminum foil (0.108 g, 4 mmol) to a 50-mL round-bottom flask. Gallium metal (0.837 g, 12 mmol) was briefly heated using a heat gun until it became liquid and was added using a syringe to the same flask. The aluminum spheres were then mixed into the gallium with a combination of pressing and stirring for 3 min or until the composite appeared as a homogeneous shiny coating on the bottom of the flask. Mixing over a heat gun on low heat ensured the composite remained liquid during preparation. This procedure was also used to prepare Ga:Al composites of 6: 1, 1 : 1 and 1 :2 atomic ratios.EXAMPLE 2

[0048] This example describes monitoring the effects of Ga-Al composite atomic ratio on the nature of Al oxide-hydroxide byproduct.

[0049] To a 50-mL flask containing 6: 1 (24 mmol, 4 mmol), 3: 1 (12 mmol, 4 mmol), 1 : 1 (4 mmol, 4 mmol) or 1 :2 (2 mmol, 4 mmol) Ga:Al composites, 10 mL of deionized (DT) water was added at 25 °C without stirring. Hydrogen gas evolved rapidly upon the addition of water and the recovered Ga from the composites was removed after 15 min by supernatant decanting. Isolated Ga was rinsed with DI water and stored in a separate container. The samples were then filteredand the solid was dried in a vacuum oven connected to house vacuum (30 Torr) at 100 °C for a minimum of 15 min.

[0050] A composite of 3:1 (Ga:Al atomic ratio) had the best hydrogen generating ability at 25 °C. For completeness, the effects on byproduct formation of lesser performing Ga:Al ratios at 25 °C were also investigated. The byproduct collected was bayerite Al(0H)3. It was also determined that the Ga:Al composite ratio did not affect the nature of the byproduct formed. FIG. 2 shows PXRD spectra of bayerite Al(0H) formed using different Ga-Al composites. PXRD evidence showed minimal effect of Ga:Al atomic ratio on byproduct formation. In FIG. 2, the inset shows the Al and Ga atomic ratio of the byproduct bayerite as determined by ICP-OES. Since the 3: 1 Ga:Al composite evolved the largest volume of hydrogen, this composite ratio was selected for further investigating byproduct formation.EXAMPLE 3

[0051] This example describes monitoring the effects of reaction time on the nature of Al oxidehydroxide byproduct.

[0052] To a flask containing a Ga:Al (3: 1 atomic ratio, 0.837 g:0.108 g, 12 mmol:4 mmol) composite, DI water (10 mb, 555 mmol) was added at 25 °C without stirring. The unreacted Ga from the reaction mixture was removed 15 min after the addition of water to the composite and rinsed with copious amounts of DI water and stored in a separate container for reuse. The supernatant suspension was then set aside, without stirring, at 25 °C. Aliquots were withdrawn from the suspension after 24 h and 48 h. The aliquots were then filtered and the solid was dried in a vacuum oven connected to house vacuum at 100 °C and dried for a minimum of 15 min.

[0053] HER was about 80% complete within 1 min after water was added to the 3: 1 (Ga:Al atomic ratio) composite at 25 °C. In order to scale up this reaction, one would need to remove the unreacted Ga from the reaction mixture as quickly as possible so it can be recycled for subsequent reactions and continue to generate hydrogen. For this reason, water was allowed to react with the composite for 15 min before removing unreacted Ga and the supernatant was allowed to set for 48 h at the initial temperature of the reaction. At no point during the progress of the reaction was mechanical stirring employed since it had no effect on the nature of the byproduct formed. Since these conditions incorporate the highest performing composite ratio for hydrogen generation and allow for the quick recovery of Ga in the absence of any stirring, these conditions are referred herein as “standard conditions.” Under standard conditions with 10 mL of water and a constant temperature of 25 °C, it took about 48 h for the initially produced Al(0H)3 hydrogel to assemble into crystalline bay erite phase Al(OH)s. Consequently, all supernatant reaction mixtures were allowed to set for 48 h after the addition of water to the Ga-Al composite before isolating the crystalline Al(OH)s byproduct. FIG. 3 shows PXRD spectrum of an amorphous byproduct after 24 h and converting into crystalline bayerite after aging for 48 h. The peaks highlighted by a vertical bar arise from the Al sample holder.

[0054] Immediately upon the addition of water to the Ga: Al composite, the supernatant became a dark grey colored suspension. Hydrated alumina, which includes Al(OH)s and A10(0H) that are white solids and thus do not account for the grey color of the reaction mixture, gradually turned white over approximately 48 h. The grey solids were analyzed using SEM-EDS. It was determined the color did not originate from any Al-based byproduct but is due to Ga nanoparticles (Ga NPs) as evidenced by SEM-EDS shown in FIGS. 4A-D. SEM micrographsshowed distinct NPs with a wide range of sizes suspended in the bayerite hydrogel collected 15 min after water was added to the composite.

[0055] As the supernatant suspension was allowed to age, it gradually transformed from grey to white and the SEM-EDS imaging showed the initial Ga NPs were absent in the supernatant after 48 h as shown in FIGS. 5A-D. FIG. 5A shows an SEM image of bayerite, Al(0H)3, collected 48 h after combining water and the composite. FIGS. 5B-D show accompanying EDS maps showing Ga does not exist as individual crystals but is evenly distributed amongst the bayerite Al(0H)3 due to Ga doping (Al in FIG. 5B; Ga in FIG. 5C; and O in FIG. 5D). EDS mapping showed Ga evenly distributed among the byproduct bayerite. EDS is a reliable technique for mapping Al, Ga, and O since electronic transitions associated with the elements that are detected by EDS do not overlap and thus each can be clearly resolved.

[0056] Without being bound by a particular theory, it is believed that the initially formed Ga NPs oxidize to Ga3+over the duration of 48 h and become incorporated into the bayerite structure. Ga metal does not readily oxidize under ambient conditions but since it exists as Ga NPs in the reaction mixture, oxidation is readily achievable due to the greatly increased surface area. This is also supported by the change in color of the supernatant suspension from grey to white over the course of 48 h and the even distribution of Ga amongst the resultant bayerite Al(0H)3. Ga3+can substitute for Al3+in the bayerite to form Ga-doped bayerite due to their similar chemical nature and ionic radii. Based on the above explanation, it has been shown that Ga dopes bayerite. Subsequent examples further investigate the key variables that affect the byproduct of the Ga-Al water splitting reaction, which include: i) volume of water introduced to the composite; ii) reaction temperature; and iii) effect of recycled Ga for composite construction.EXAMPLE 4

[0057] This example describes monitoring the effects of volume of water on the nature of Al oxide-hydroxide byproduct.

[0058] To four 50-mL flasks containing Ga:Al (3: 1 atomic ratio, 0.837 g:0.108 g, 12 mmol:4 mmol) composite, 3, 5, 10 or 20 mL of water was added at 25 °C without stirring. Hydrogen gas evolved rapidly and the unreacted Ga from the composites was removed after 15 min, rinsed with DI water, weighed and stored in a separate container. The resulting supernatant suspension was then set aside, without stirring at 25 °C for 48 h, filtered, and the solid was dried in a vacuum oven at 100 °C for a minimum of 15 min.

[0059] The volume of water used was varied (3, 5, 10 and 20 mL) and any effects on the structure, crystallinity and percent Ga doping of the resultant Al(OH)s was recorded as PXRD spectra of bayerite Al(0H)3 in FIG. 6, which showed the effects of the volume of water on the nature of the byproduct Al(OH)i produced under standard conditions at 25 °C. In FIG. 6, the inset shows the Ga and Al atomic ratio of the byproduct bayerite as determined by ICP-OES.

[0060] The addition of water was immediately followed by the release of hydrogen that bubbled through the grey supernatant suspension. Solid byproduct was isolated as described earlier and the PXRD spectrum confirmed that bayerite Al(OH)s was the major product for each trial. PXRD of the solid byproduct produced from the 3 mL water sample and to a lesser extent from the 5 mL sample, however, showed a peak associated with gibbsite Al(OH)s at 18.3° (29) as a shoulder on the lowest angle bayerite peak at 18.9° (20). Thus, it is believed that smaller volumes of water promote formation of the more dehydrated phase of Al(OH)s, gibbsite.EXAMPLE 5

[0061] This example describes monitoring the effects of hydrolysis temperature on the composition of Al oxide-hydroxide byproduct.

[0062] To five 50-mL flasks containing Ga:Al (3: 1 atomic ratio, 0.837 g:0.108 g, 12 mmol:4 mmol) composites and maintained at 25, 40, 60, 80 or 100 °C, DI water (10 mL) was added without stirring. The unreacted Ga from the reaction mixture was removed after 15 min, rinsed with copious amounts of DI water and stored in a separate container. The supernatant suspensions were then set aside, without stirring, at 25, 40, 60, 80 or 100 °C for 48 h, filtered, and the solid was dried in a vacuum oven at 100 °C for a minimum of 15 min.

[0063] These experiments systematically investigated the effect of reaction temperature on the composition and level of metal doping of the Al(OH)s byproduct. These experiments were performed by introducing 10 mL of water to the composite under standard conditions at 40, 60, 80 and 100 °C. In previous examples, all experiments were carried out at 25 °C and all produced bay erite Al(0H)3 as the major byproduct. Under no conditions at 25 °C was boehmite AIO(OH) ever produced. Boehmite, however, was readily synthesized as the exclusive byproduct by elevating the reaction temperatures to 80-100 °C. When hydrolysis was carried out at 40 °C, bayerite was still the dominant byproduct with gibbsite present in minor amounts. For the hydrolysis reaction at 60 °C, bayerite and gibbsite were both present in addition to the initial formation of broad peaks associated with boehmite as shown in the spectra of FIG. 7, which illustrate the transition from bayerite A1(OH)3 to boehmite AIO(OH) byproduct formation as the reaction temperature is increased. The inset of FIG. 7 shows the Ga and Al atomic ratio of the aluminum hydroxide and aluminum oxyhydroxide byproducts as determined by ICP-OES.

[0064] As a control, previously synthesized bayerite was introduced to water at 100 °C for 48 h to observe whether it would convert into boehmite. After 48 h, the collected product remained bayerite with only minor blips associated with boehmite arising. This also shows that the temperature of water used during the composite's introduction and the 48 h aging effect on the formation of boehmite.EXAMPLE 6

[0065] This example describes isolating the unreacted Ga from the reaction mixture.

[0066] The unreacted Ga collects at the bottom of the reaction flask as a liquid and was removed from the reaction mixtures at 25 °C by decanting the supernatant. The Ga remaining in the flask was transferred to a glass vial and was rinsed with DI water (~10 mL) until it became a shiny solid. Additional scratching of the liquid Ga with a metal spatula was occasionally necessary to induce solidification.EXAMPLE 7

[0067] This example describes the recyclability of unreacted Ga.

[0068] To a 50-mL flask containing Ga:Al (3: 1 atomic ratio, 0.837 g:0.108 g, 12 mmol:4 mmol) composite, DI water (10 mL) was added at 25 °C without stirring. The unreacted Ga from the composite was removed 15 min after water was added and rinsed with DI water as described above. The supernatant suspension was then set aside, without stirring at 25 °C for 48 h and was isolated by vacuum filtration and dried in a vacuum oven at 100 °C. Unreacted Ga was recycled from three separate experiments and was used to make three additional Ga:Al (3:1 atomic ratio) composites. These composites made from the recycled Ga were used in the hydrolysis of waterto show proof of concept for Ga recyclability; no effect was observed on the production of byproduct bayerite.

[0069] Previous examples exploring the varying conditions discussed thus far utilized fresh Ga metal for synthesizing the binary composites. In order for this process to be industrially feasible and environmentally friendly, the Ga collected from previous reactions was tested to confirm Ga is reusable. To test if Ga recovered from the reaction could be used to construct new composites without influencing the byproduct, Ga collected from previous experiments was used to construct new 3:1 (Ga:Al atomic ratio) composites. These composites, made from recycled Ga metal, were then introduced to 10 mb of water at 25 °C under standard conditions. The Ga metal was collected again to make another recycled Ga composite to repeat the reaction and determine whether there was any change in the nature of the byproduct. This process was repeated three times for proof of concept and the bayerite byproducts were analyzed using PXRD. FIG. 8 shows PXRD spectra of bayerite obtained after four uses without any substantial changes, which shows that recycled Ga metal has no effect on the production of bayerite under standard conditions after four reaction cycles. As can be seen, recycled Ga metal from previous reactions can be collected and recycled to make new composites without influencing the amount of hydrogen generated nor the nature of the resultant bayerite Al(0H)3. Additionally, recovered Ga is essentially pure, -0.1% Al in most samples, as observed by ICP-OES.EXAMPLE 8

[0070] This example describes calcination of Al(0H)3 and A10(0H) to a-AhCE.

[0071] Approximately 50 mg of solid were placed in a ceramic vessel in a tube furnace at 1,100°C. The solid was spread out in the vessel to maximize the surface area and all samples were pre-ground using a mortar and pestle. Samples were not further ground when PXRD was used to check the status of calcination at 2 and 24 h.

[0072] The dehydration of bayerite / gibbsite Al(0H)3 through calcination is used industrially to produce valuable a-AhCh with applications ranging from catalytic supports to electronics and abrasives. Obtaining CC-AI2O3 is still a highly polluting process since it is manufactured from Al(0H)3 through the Bayer process. The Ga-Al composite described in this disclosure produces environmentally friendly Al(0H)3 or AIO(OH) and thus, the resulting 01-AI2O3 produced is highly green (i.e., sustainable) in nature. When byproduct A1(OH)3 and AIO(OH) is calcined for 24 h at 1,100 °C in a tube furnace, the original bayerite structure is converted to (X-AI2O3. FIG. 9 shows PXRD spectra of a) theoretical CX-AI2O3; b) product after 24 h of calcination at 1,100 °C for commercially purchased Al(0H) ; c) product of calcination of byproduct Al(0H) under the same conditions as (b); and d) product of calcination of AIO(OH) byproduct under the same conditions as (b). It was observed the bayerite and boehmite phases were transformed to majority phase OC-AI2O3 after only 2 h.

[0073] SEM was employed to observe microstructural differences between (X-AI2O3 synthesized from bayerite under our standard conditions and boehmite obtained from reactions performed at 80 °C. SEM micrographs revealed that the (X-AI2O3 morphology varied depending on the starting material (see FIGS. 10A-D).

[0074] FIG. 10A shows an SEM image of starting bayerite produced under standard conditions while FIG. 10C shows an SEM image of resultant CX-AI2O3 produced by calcination of bayerite at 1,100 °C for 24 h. The starting bayerite had multiple jagged regions protruding from a smooth and non-porous surface. The resultant 0C-AI2O3 no longer had a smooth underlayer like the starting A1(OH)3 but has been transformed to a porous solid in which jagged features emerge.

[0075] In FIG. 10B shows an SEM image of the starting boehmite produced at 80 °C and FIG. 10D shows an SEM image of resultant CX-AI2O3 produced by calcination of boehmite synthesized at 80 °C. The starting boehmite had a highly fine, fibrous surface with few features visibly extending outward. There are, however, valleys and hills on the surface which become more apparent in the product (X-AI2O3. The surface of the (X-AI2O3 is porous, similar to (X-AI2O3 derived from A1(OH)3, while the surface of CC-AI2O3 from boehmite is significantly smoother. Its surface lacks the jagged features that are ubiquitous on 0C-AI2O3 derived from bay erite. The highly topographical bayerite produces a similar OC-AI2O3 surface, while the smoother, highly fibrous boehmite leads to a more even 0C-AI2O3 surface with less microstructural diversity.

[0076] EDS analysis provided confirmation that Ga and Al are again evenly dispersed in the samples of CC-AI2O3, much like they are in the initial bayerite and boehmite. EDS was employed to estimate the Ga and Al atomic ratio of the samples rather than ICP-OES due to the difficulty of digesting (X-AI2O3. The results demonstrated that the Ga and Al atomic ratio values of the resultant 0C-AI2O3 are identical to those of the starting Al(0H)3 and AIO(OH). The process of calcination thus influences neither the distribution nor the atomic ratios of Ga and Al compared to the initial A1(OH)3 and AIO(OH).

[0077] The use of Ga metal to activate Al metal for water splitting provides an effective and sustainable method for the production of Al(0H)3 and A10(0H), in addition to green hydrogen. It was shown that the 3: 1 Ga-Al binary metal composite produces an amorphous aluminum hydrogel when reacted with water and assembles into pure bayerite over 2 d under ambient conditions. Bayerite also formed for Ga-Al composite ratios of 6: 1, 1 : 1 and 1 :2. Volumes of water ranging from 3 to 20 mL also produced bayerite when reacted with a 3: 1 composite under standard conditions at 25 °C. The hydrated aluminum byproduct phase was dependent ontemperature, with boehmite being the sole phase between 80 and 100 °C. The present disclosure also demonstrated that the Al(0H)3 and AIO(OH) byproducts could be fully converted to a- AI2O3 at 1,100 °C in 24 h, with (X-AI2O3 peaks dominating the PXRD spectra after just 2 h. The Ga metal used for the composites can be collected from spent reaction mixtures in its pure state and can be recycled to make additional binary metal composites without any influence on the reaction. The reaction requires no energy input and produces only hydrogen and value-added Ga- doped Al(0H)3 or A10(0H), as precursors to CX-AI2O3.

[0078] It will be appreciated that of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims. Unless specifically recited in a claim, steps or components of claims should not be implied or imported from the specification or any other claims as to any particular order, number, position, size, shape, angle, or material.

Claims

WHAT IS CLAIMED IS:

1. A method of synthesizing alpha aluminum oxide, the method comprising: reacting a first gallium and aluminum composite in a hydrogen evolution reaction with water to form an aluminum hydroxide; removing the aluminum hydroxide; and heating the aluminum hydroxide to form alpha aluminum oxide.

2. The method according to claim 1, further comprising reacting the first gallium and aluminum composite with water at a temperature from about 60 °C to about 100 °C to form aluminum oxyhydroxide.

3. The method according to claim 2, further comprising: removing the aluminum oxyhydroxide; and heating the aluminum oxyhydroxide to form alpha aluminum oxide.

4. The method according to any of claims 1-3, further comprising: removing hydrogen gas formed during the reaction of the first gallium and aluminum composite with water.

5. The method according to any of claims 1-4, further comprising: removing free gallium remaining after the reaction of the first gallium and aluminum composite with water.

6. The method according to claim 5, further comprising: forming a second gallium and aluminum composite from the free gallium.

7. The method according to any of claims 1-6, wherein the aluminum hydroxide is doped with gallium.

8. The method according to any of claims 1-7, wherein the alpha aluminum oxide is doped with gallium.

9. The method according to any of claims 1-8, wherein heating the aluminum hydroxide includes calcinating the aluminum hydroxide at a temperature from about 1,000 °C to about 1,500 °C and for about 2 hours to about 24 hours.

10. The method according to any of claims 1-9, wherein the first gallium and aluminum composite includes gallium and aluminum at an atomic ratio from about 1: 1 to about 6: 1 of gallium to aluminum.

11. The method according to any of claims 1-10, wherein removing the aluminum hydroxide further includes suspending the aluminum hydroxide in water for up to about 48 hours.

12. A method of synthesizing alpha aluminum oxide, the method comprising: reacting a first gallium and aluminum composite in a hydrogen evolution reaction with water to form an aluminum byproduct including at least at least one of aluminum hydroxide or aluminum oxyhydroxide; removing the aluminum byproduct; and calcinating the aluminum byproduct to form alpha aluminum oxide.

13. The method according to claim 12, further comprising: adjusting a temperature of reacting the first gallium and aluminum composite with water to adjust a ratio of aluminum hydroxide to aluminum oxyhydroxide in the aluminum byproduct.

14. The method according to claim 13, wherein the temperature is adjusted from about 60 °C to about 100 °C.

15. The method according to any of claims 12-14, further comprising: removing hydrogen gas formed during the reaction of the first gallium and aluminum composite with water.

16. The method according to any of claims 12-15, further comprising: removing free gallium remaining after the reaction of the first gallium and aluminum composite with water.

17. The method according to claim 16, further comprising: forming a second gallium and aluminum composite from the free gallium.

18. The method according to any of claims 12-17, wherein the aluminum byproduct is doped with gallium.

19. The method according to claim 13, wherein the alpha aluminum oxide is doped with gallium.

20. A method of making aluminum hydroxide, comprising: reacting a gallium and aluminum composite in a hydrogen evolution reaction with water to form an aluminum hydroxide in a post reaction mixture.

21. The method according to claim 20, wherein the hydrogen evolution reaction proceeds for at least 15 minutes.

22. The method according to any of claims 20 or 21, further comprising allowing the post reaction mixture to settle for at least 48 hours.

23. The method according to any of claims 20-22, wherein the aluminum hydroxide includes bayerite, and the water is provided at a temperature of between 25 °C and 80 °C.

24. The method according to any of claims 20-22, wherein reacting the gallium and aluminum composite in the hydrogen evolution reaction with water forms boehmite, and the water is provided at a temperature above 60 °C.

25. The method according to claim 24, wherein the water is provided at a temperature above 80 °C.

26. The method according to any of claims 20-22, wherein the aluminum hydroxide includes gibbsite, and the water is provided at 25 °C in a molar ratio of 10: 1 or above relative to the gallium and aluminum composite.

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