Chemical Precipitation Method for the Synthesis of Nb2O5 Modified Bulk Nickel Catalysts with High Specific Surface Area

Podsumowanie

A protocol for the synthesis of sponge-like and fold-like Ni_1-xNb_xO nanoparticles by chemical precipitation is presented.

Streszczenie

We demonstrate a method for the synthesis of Ni_xNb_1-xO catalysts with sponge-like and fold-like nanostructures. By varying the Nb:Ni ratio, a series of Ni_xNb_1-xO nanoparticles with different atomic compositions (x = 0.03, 0.08, 0.15, and 0.20) have been prepared by chemical precipitation. These Ni_xNb_1-xO catalysts are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The study revealed the sponge-like and fold-like appearance of Ni_0.97Nb_0.03O and Ni_0.92Nb_0.08O on the NiO surface, and the larger surface area of these Ni_xNb_1-xO catalysts, compared with the bulk NiO. Maximum surface area of 173 m²/g can be obtained for Ni_0.92Nb_0.08O catalysts. In addition, the catalytic hydroconversion of lignin-derived compounds using the synthesized Ni_0.92Nb_0.08O catalysts have been investigated.

Wprowadzenie

The preparation of nanocomposites has received increasing attention due to their crucial application in various field. To prepare Ni-Nb-O mixed oxide nanoparticles,^1,2,3,4,5,6 different methods have been developed such as dry mixing method,^7,8 evaporation method,^{9,10,11,12,13} sol gel method,¹⁴ thermal decomposition method,¹⁵ and auto-combustion.¹⁶ In a typical evaporation method⁹, aqueous solutions containing the appropriate amount of metal precursors, nickel nitrate hexahydrate and ammonium niobium oxalate were heated at 70 °C. After the removal of solvent and further drying and calcination, the mixed oxide was obtained. These oxide catalysts exhibit excellent catalytic activity and selectivity towards the oxidative dehydrogenation (ODH) of ethane, which is related to the electronic and structural rearrangement induced by the incorporation of niobium cations in the NiO lattice.¹¹ The insertion of Nb drastically decreases the electrophilic oxygen species, which is responsible for the oxidation reactions of ethane¹². As a result, extensions of this method have been done on the preparation of different types of mixed Ni-Me-O oxides, where Me = Li, Mg, Al, Ga, Ti and Ta.¹³ It is found that the variation of metal dopants could alter the unselective and electrophilic oxygen radicals of NiO, thus systematically tune the ODH activity and selectivity towards ethane. However, generally the surface area of these oxides is relatively small (< 100 m²/g), due to the extended phase segregation and formation of large Nb₂O₅ crystallites, and thus hampered their uses in other catalytic applications.

Dry mixing method, also known as the solid-state grinding method, is another commonly used method to prepare the mixed-oxide catalysts. Since the catalytic materials are obtained in a solvent-free way, this method provides a promising green and sustainable alternative to the preparation of mixed-oxide. The highest surface area obtained by this method is 172 m²/g for Ni₈₀Nb₂₀ at calcination temperature of 250 °C.⁸ However, this solid-state method is not reliable as reactants are not well mixed on the atomic scale. Therefore, for better control of chemical homogeneity and specific particle size distribution and morphology, other suitable methods to prepare Ni-Nb-O mixed oxide nanoparticles are still being sought.⁷

Among various strategies in the development of nanoparticles, chemical precipitation serves as one of the promising methods to develop the nanocatalysts, since it allows the complete precipitation of the metal ions. Also, nanoparticles of higher surface areas are commonly prepared by using this method. To improve the catalytic properties of Ni-Nb-O nanoparticles, we herein report the protocol for the synthesis of a series of Ni-Nb-O mixed oxide catalysts with high surface area by chemical precipitation method. We demonstrated that the Nb:Ni molar ratio is a crucial factor in determining the catalytic activity of the oxides towards the hydrodeoxygenation of lignin-derived organic compounds. With high Nb:Ni ratio above 0.087, inactive NiNb₂O₆ species were formed. Ni_0.92Nb_0.08O, which had the largest surface area (173 m²/g), exhibits fold-like nanosheets structures and showed the best activity and selectivity towards the hydrodeoxygenation of anisole to cyclohexane.

Protokół

Caution: For the proper handling methods, properties and toxicities of the chemicals described in this paper, refer to the relevant material safety data sheets (MSDS). Some of the chemicals used are toxic and carcinogenic and special cares must be taken. Nanomaterials may potentially pose safety hazards and health effects. Inhalation and skin contact should be avoided. Safety precaution must be exercised, such as performing the catalyst synthesis in the fume hood and catalyst performance evaluation with autoclave reactors. Personal protective equipment must be worn.

1. Preparation of Ni_0.97Nb_0.03O Catalysts where Nb:(Ni+Nb) molar ratios equal to 0.03

1. Combine 0.161 g of niobium (V) oxalate hydrate with 2.821 g of nickel nitrate in 100 mL of deionized water in a 250-mL three-necked round bottom flask equipped with a stir bar.
2. Stir the solution at 50 rpm and 70 °C to dissolve the compounds until the disappearance of precipitate using a heating magnetic stirrer.
3. Raise the temperature rapidly to 80 °C at a rate of 2 °C/min.
4. Add a mixed basic solution [aqueous ammonium hydroxide (50 mL, 1.0 M) and sodium hydroxide (50 mL, 0.2 M)] into the reaction mixture dropwise until the pH of the Ni/Nb solution reaches 9.0.
5. While stirring the reaction mixture, raise the temperature to 120 °C at 2 °C/min.
6. Stir the reaction mixture overnight at 50 rpm at 120 °C until the complete disappearance of the green color of the solution.
7. Perform inductively coupled plasma-optical emission spectrometry (ICP-OES) analysis for the solution to evaluate the concentration of remaining Ni²⁺ and Nb⁵⁺ ions in the solution and ensure the complete precipitation of remaining nickel nitrate.
8. Collect the solid by filtration using Büchner flask. Wash the solid by adding 2 L deionized water repeatedly within 20 min to remove the residual Na⁺ cation.
9. Collect the solid in a watch glass. Dry the solid at 110 °C for 12 h in dry oven.
10. Calcine by heating the solids in synthetic air (20 mL/min O₂ and 80 mL/min N₂) at 450 °C for 5 h in tube furnace. Check all glassware for defect prior to use the high temperature of reaction.
11. After the calcination, obtain 1 g of Ni_0.97Nb_0.03O catalyst. Use appropriate protective equipment such as safety glasses, gloves, lab coat, and fume hood to perform the nanocrystal reaction due to potential safety hazards and health effects of the nanomaterials.

2. Preparation of Ni_0.92Nb_0.08O Catalysts where Nb:(Ni+Nb) molar ratios equal to 0.08

1. This procedure is similar to that of 1 except for the first two steps:
   1. Dissolve 0.43 g of niobium (V) oxalate hydrate in 100 mL of deionized water.
   2. Separately, dissolve 2.675 g of nickel nitrate in 100 mL of deionized water.

3. Preparation of Ni_0.85Nb_0.15O Catalysts where Nb:(Ni+Nb) molar ratios equal to 0.15

1. The procedure is similar to that of 1 except for the first two steps:
   1. Dissolve 0.807 g of niobium (V) oxalate hydrate in 100 mL of deionized water.
   2. Separately, dissolve 2.472 g of nickel nitrate in 100 mL of deionized water.

4. Preparation of Ni_0.80Nb_0.20O Catalysts where Nb:(Ni+Nb) molar ratios equal to 0.20

1. The procedure is similar to that of 1 except for the first two steps:
   1. Dissolve 1.076 g of niobium (V) oxalate hydrate in 100 mL of deionized water.
   2. Separately, dissolve 2.326 g of nickel nitrate in 100 mL of deionized water.

5. Preparation of Nb₂O₅ using chemical precipitation method

1. Calcine niobic acid (Nb₂O₅·nH₂O) in synthetic air for 5 h at 450 °C to obtain pure Nb₂O₅ particles.
   NOTE: Confirm the completion of reaction using X-ray powder diffraction (XRD) analysis, where Nb₂O₅·nH₂O is Amorphous and Nb₂O₅ is crystalline. According to the analysis, the calcination for 5 h at 450 °C was enough to complete the reaction.

6. Synthesis of β-O-4 lignin model compound, 2-(2-methoxyphenoxy)-1-phenylethan-1-one

1. Dissolve bromoacetophenone (9.0 g, 45 mmol) and 2-methoxyphenol (6.6 g, 53 mmol) in 200 mL of dimethylformamide (DMF) in a 500-mL conical flask with a magnetic stirrer. Use appropriate protective equipment and fume hood to perform the reaction using corrosive and carcinogenic chemicals and reagents.
2. Mix the above DMF solution with potassium hydroxide (3.0 g, 53 mmol) and stir the mixture overnight at 50 rpm at room temperature using magnetic stirrers.
3. Extract the product with the mixture solution of 200 mL of H₂O and 600 mL of diethyl ether (1:3, v/v) using separation funnel. Obtain the upper diethyl ether layer of the solution.
4. Add MgSO₄ (10 g) to absorb moisture of the diethyl ether solution. Filter the MgSO₄ to obtain the diethyl ether solution by using filter paper and funnel.
5. After removal of the diethyl ether solution under reduced pressure at 0.08 MPa using rotary evaporator, dissolve the residue in 5 mL of ethanol.
6. Slowly evaporate the ethanol solvent to recrystallize the product in a 10-mL beaker. Obtain the product (11.5 g) as yellowish powder and the yield of product is 90% based on bromoacetophenone. From the ¹H NMR analysis, ¹H NMR (DMSO): δ 3.78 (s, 3H, OCH₃), 5.54 (s, 2H, CH₂), 6.82-8.01 (m, 9H, aromatic) ppm.¹⁷

7. Hydrodeoxygenation of Lignin-derived Aromatic Ether

NOTE: The chosen lignin-derived aromatic ether is anisole in this experiment and the catalyst is Ni_0.92Nb_0.08O. Use appropriate protective equipment and fume hood to perform the reaction using carcinogenic reagents.

1. Equip a 50-mL stainless steel autoclave reactor with a heater and a magnetic stirrer.
2. Reduce the Ni_0.92Nb_0.08O catalyst (1 g) obtained from step 2 in the autoclave reactor in H₂ atmosphere at 400 °C for 2 h, and then passivate the catalyst under Argon (50 mL/min) overnight.
3. Dissolve anisole (1.1712 g, 8 wt%) into n-decane (20 mL) with the use of n-dodecane (0.2928 g, 2 wt%) as an internal standard for quantitative gas chromatography (GC) analysis.
4. Introduce the reduced catalysts (0.1 g) in into the autoclave reactor rapidly to avoid long exposure time with air (< 5 mins).
5. Seal the autoclave reactor, purge with H₂ repeatedly (3 times, at 3 MPa pressure) to eliminate air, and then the reaction mixture at atmosphere pressure.
6. Set the stirring speed at 700 rpm.
7. After heating to the desired temperature at 160-210 °C at 2 °C/min, pressurize the autoclave reactor to 3 MPa and set the zero-time point (t = 0).
   NOTE: The temperature range of 160-210 °C is appropriate in this report.
8. Subsequently, cool the mixture to room temperature at 10 °C/min immediately and analyze the deoxygenated products using gas chromatography with mass selective detector.¹⁷
9. Determine the conversion of lignin model compound according to the following equation:
   
10. Determine the product selectivity according to the following equation:
    

Wyniki

X-ray diffraction (XRD) patterns (Figure 1 and Figure 2), BET surface areas, temperature-programmed reduction of hydrogen with hydrogen (H₂-TPR), scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray (EDX) analyzer, X-ray photoelectron spectroscopy (XPS) were collected for the nanoparticles NiO, Ni-Nb-O and Nb₂O₅ oxides¹⁷ (Figure 3...

Dyskusje

One of the common methods to prepare the nickel-doped bulk niobium oxide nanoparticles is rotary evaporation method.⁹ By employing various pressure and temperature conditions during the process of rotary evaporation, the precipitation of Ni-Nb-O particles commerce with the slow removal of the solvent. In contrast to the rotary evaporation method, the chemical precipitation method reported in this study has received increasing attention to prepare the nanoparticles as this do not require the remova...

Materiały

Name	Company	Catalog Number	Comments
Niobium(V) oxalate hydrate, 98%	Alfa	L04481902
Nickel nitrate hexahydrate, 99%	Aladdin	N108891
Sodium hydroxide, 98%	Aladdin	S111501
Ammonium hydroxide, 23-25%	Aladdin	A112077
Anisole, 99%	Sinopharm	81001728
Diphenyl ether, 98%	Aladdin	D110644
Phenol, 98%	Sinopharm	100153008
2-Methoxyphenol, 98%	Sinopharm	30114526
Vanillin, 99.5%	Sinopharm	69024316
Potassium hydroxide, AR	Aladdin	P112284
N,N-Dimethylformamide, 99.5%	Sinopharm	40016462
2-Bromoacetophenone,98%	Aladdin	B103328
Diethyl ether,99.5%	Sinopharm	10009318
Decane,98%	Aladdin	D105231
Dodecane,99%	Aladdin	D119697
Niobic acid	CBMM	1313968
Heating and Drying Oven			DHG Series (shanghai jinghong laboratory instrument co. ltd)
Autoclave Reactor			CJF-0.05—0.1L (Dalian Tongda Equipment Technology Development Co., Ltd)
Tube furnace			SK2-1-10/12 (Luoyang Huaxulier Electric Stove Co., Ltd)
Heating magnetic stirrer			DF-101 (Yu Hua Instrument Co. Ltd.)
Rotary evaporator			RE-3000A (Shanghai Yarong Biochemical Instrument Factory)
Synthetic air
Hydrogen gas
Argon gas