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Summary

A protocol for the synthesis of HNbWO₆, HNbMoO₆, HTaWO₆ solid acid nanosheet modified Pt/CNTs is presented.

Abstract

We herein present a method for the synthesis of HNbWO₆, HNbMoO₆, HTaWO₆ solid acid nanosheet modified Pt/CNTs. By varying the weight of various solid acid nanosheets, a series of Pt/xHMNO₆/CNTs with different solid acid compositions (x = 5, 20 wt%; M = Nb, Ta; N = Mo, W) have been prepared by carbon nanotube pretreatment, protonic exchange, solid acid exfoliation, aggregation and finally Pt particles impregnation. The Pt/xHMNO₆/CNTs are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and NH₃-temperature programmed desorption. The study revealed that HNbWO₆ nanosheets were attached on CNTs, with some edges of the nanosheets being bent in shape. The acid strength of the supported Pt catalysts increases in the following order: Pt/CNTs < Pt/5HNbWO₆/CNTs < Pt/20HNbMoO₆/CNTs < Pt/20HNbWO₆/CNTs < Pt/20HTaWO₆/CNTs. In addition, the catalytic hydroconversion of lignin-derived model compound: diphenyl ether using the synthesized Pt/20HNbWO₆ catalyst has been investigated.

Introduction

Many industrial processes for the manufacture of chemicals involve the use of aqueous inorganic acid. One typical example is the conventional H₂SO₄ process for the hydration of cyclohexane to produce cyclohexanol. The process involves a biphasic system, with the cyclohexane being in the organic phase and the cyclohexanol product being in the acidic aqueous phase, thus making the separation process by simple distillation difficult. Apart from difficulty in separation and recovery, inorganic acid is also highly toxic and corrosive to equipment. Sometimes, the use of inorganic acid generates byproducts that will lower the product yield and must be avoided. For example, the dehydration of 2-cyclohexene-1-ol to produce 1,3-cyclohexadiene using H₂SO₄ will lead to polymerization byproducts¹. Thus, many industrial processes shift towards using solid acid catalysts. Various water tolerant solid acids are used to solve the above problem and to maximize the product yields, such as the use of HZSM-5 and Amberlyst-15. The use of high-silica HZSM-5 zeolite has been shown to replace H₂SO₄ in the production of cyclohexanol from benzene². Since the zeolite is present in the neutral aqueous phase, the product will go to the organic phase exclusively, thus simplifying the separation process. However, due to Lewis acid-base adduct formation of water molecules to the Lewis acid sites, zeolitic materials still demonstrated lower selectivity due to the presence of inactive sites³. Among all these solid acids, Nb₂O₅ is one of the best candidates that contain both Lewis and BrØnsted acid sites. The acidity of Nb₂O₅∙nH₂O is equivalent to a 70% H₂SO₄ solution, due to the presence of the labile protons. The BrØnsted acidity, which is comparable to protonic zeolite materials, are very high. This acidity will turn to Lewis acidity following water elimination. In the presence of water, Nb₂O₅ forms the tetrahedral NbO₄-H₂O adducts, which may decrease in Lewis acidity. However, the Lewis acid sites are still effective since the NbO₄ tetrahedral still have effective positive charges⁴. Such phenomenon has been demonstrated successfully in the conversion of glucose into 5-(hydroxymethyl)furfural (HMF) and the allylation of benzaldehyde with tetraallyl tin in water⁵. Water-tolerant catalysts are thus crucial in biomass conversion in renewable energy applications, especially when the conversions are performed in environmental benign solvents such as water.

Among the many environmental benign solid acid catalysts, functionalized carbon nanomaterials using graphene, carbon nanotubes, carbon nanofibers, mesoporous carbon materials have been playing an important role in the valorization of biomass due to the tunable porosity, extremely high specific surface area, and excellent hydrophobicity^6,7. The sulfonated derivatives are particularly stable and highly active protonic catalytic materials. They can either be prepared by incomplete carbonization of sulfonated aromatic compounds⁸ or by sulfonation of incompletely carbonized sugars⁹. They have proven to be very efficient catalysts (e.g., for the esterification of higher fatty acids) with activity comparable to the use of liquid H₂SO₄. Graphenes and CNTs are carbon materials with a large surface area, excellent mechanical properties, good acid resistance, uniform pore size distributions, as well as resistance to coke deposition. Sulfonated graphene has been found to efficiently catalyze the hydrolysis of ethyl acetate¹⁰ and bifunctional graphene catalysts has been found to facilitate the one-pot conversion of levullinic acid to γ-valerolactone¹¹. Bifunctional metals supported on CNTs are also very efficient catalysts for application in biomass conversion^12,13 such as the highly selective aerobic oxidation of HMF to 2,5-diformylfuran over the VO_2-PANI/CNT catalyst¹⁴.

Taking advantage of the unique properties of Nb₂O₅ solid acid, functionalized CNTs and bifunctional metal supported on CNTs, we report the protocol for the synthesis of a series of Nb(Ta)-based solid acid nanosheet modified Pt/CNTs with a high surface area by a nanosheet aggregation method. Furthermore, we demonstrated that Pt/20HNbWO₆/CNTs, as a result of the synergistic effect of well-dispersed Pt particles and strong acid sites derived from HNbWO₆ nanosheets, exhibit the best activity and selectivity in converting lignin-derived model compounds into fuels by hydrodeoxygenation.

Protocol

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 care must be taken. Nanomaterials may potentially pose safety hazards and health effects. Inhalation and skin contact should be avoided. Safety precautions 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. Pretreatment of CNTs13 Immerse 1.0 g of CNTs into 50 mL of nitric acid in a 100 mL beaker. Sonicate the solution at 25 °C for 1.5 h to remove surface impurities and to enhance the anchoring effect of the catalyst. Transfer the solution into a 100 mL round bottom flask. Reflux the solution in a mixture of nitric acid (65%) and sulfuric acid (98%) at 60 °C for overnight. Set the volume ratio at 3:1. This will create surface defects on the CNTs. Filter the solution to obtain the multiwall carbon nanotube solid. Wash the solid with deionized water. Dry the solid at 80 °C for 14 h. 2. Preparation of HNbWO6 solid acid nanosheets15 by protonic exchange followed by exfoliation Mix stoichiometric amounts of Li2CO3 (0.9236 g) and metal oxides Nb2O5 (3.3223 g) and WO3 (5.7963 g) at a molar ratio of 1:1:2. Calcine the solid mixture at 800 °C for 24 h with one intermediate grinding. Place 10.0 g of LiNbWO6 powder in 200 mL of 2 M HNO3 aqueous solution at 50 °C and stir the solution mixture for 5 days (120 h) with one replacement of the acid at 60 h. Exchange the acid liquid every day and repeat step 2.3. Filter the solid and wash the solid with deionized water 3x. Dry the solid at 80 °C overnight. Add an amount of 25 wt.% TBAOH (tetra (n-butylammonium) hydroxide) solution to 150 mL of deionized water solution with 2.0 g of protonated compound obtained in step 2.6 until the pH reaches 9.5 – 10.0. Stir the above solution for 7 days. Centrifuge the above solution and collect the supernatant solution that contains the dispersed nanosheets. 3. Preparation of HNbMO6 solid acid nanosheets NOTE: The procedure is similar to that of step 2 except for the first and third steps. Mix stoichiometric amounts of Li2CO3 and metal oxides Nb2O5 and MoO3 in a molar ratio of 1:1:2. Calcine the above solid mixtures at 800 °C in air for 24 h with one intermediate grinding. Place 10.0 g of LiNbMoO6 powder in 200 mL of 2 M HNO3 aqueous solution at 50 °C and stir the solution mixture for 5 days (120 h) with one replacement of the acid at 60 h. 4. Preparation of HTaWO6 solid acid nanosheets NOTE: The procedure is similar to that of step 2 except for the first and third steps. Mix stoichiometric amounts of Li2CO3 and metal oxides Ta2O5 and WO3 in a molar ratio of 1:1:2. Calcine the above solid mixtures at 900 °C in air for 24 h with one intermediate grinding. Place 10.0 g of LiTaWO6 powder in 200 mL of 2 M HNO3 aqueous solution at 50 °C and stir the solution mixture for 5 days (120 h) with one replacement of the acid at 60 h. 5. Preparation of HNbWO6/MWCNTs by the nanosheet aggregation method Add 2.0 g of multiwall CNTs obtained in step 1 to a 100 mL solution of HNbWO6 nanosheets in a 250 mL round bottom flask. Add 100 mL of 1.0 M HNO3 aqueous solution into the round bottom flask dropwise. This will aggregate the nanosheets samples. Continue to stir the solution at 50 °C for 6 h. Filter the solid and wash the solid with deionized water 3x. Dry the solid at 80 °C overnight. Weigh the dried solid and record the % loading of the solid acid on the MWCNT. 6. Preparation of Pt/20HNbWO6/CNTs by the impregnation method Dissolve the H2PtCl6∙H2O into water (1.0 g/100 mL). Impregnate the as-prepared nanosheets modified CNTs materials with 1.34 mL of the above Pt aqueous solution. Dry the nanosheets CNTs materials at 80 °C, and calcinate the materials at 400 °C for 3 h. Obtain the Nb(Ta)-based solid acid nanosheets modified Pt/CNTs catalysts. 7. Hydrodeoxygenation of lignin-derived aromatic ether NOTE: The chosen lignin-derived aromatic ether is diphenyl ether in this experiment. The chosen lignin-derived aromatic ether is diphenyl ether in this experiment. The activity of Pt/20HTaWO6/CNTs (88.8% conversion, not shown in this paper) is lower than Pt/20HNbWO6/CNTs (99.6%), thus the yield of cyclohexane decreases. Hence, although, higher selectivity of cyclohexane was obtained over Pt/20HTaWO6/CNTs, lower conversion of diphenyl ether limits its utilization. Using appropriate protective equipment and fume hood to perform the reaction using carcinogenic reagents. Dilute 0.05 grams of catalyst in 5 milliliters of quartz sand. Load the solution in the middle of a fixed bed reactor between two pillows of quartz wool. Reduce the catalyst in H2 (40 mL/min) at 300 °C for 2 h. Pump the diphenyl ether feedstocks (including 5.0 wt.% reactant in n-decane and 2.0 wt.% n-dodecane as an internal standard for gas chromatography analysis) into the fixed bed reactor at different flow rates (0.05-0.06 mL/min) Collect the products at different space times defined as the ratio between the mass of catalyst W (g) and the flow rate of the substrate F (g/min). Identify the liquid products by an GC (HP-5, 30 m x 0.32 mm x 0.25 μm) with 5977A MSD and analyze off-line by gas chromatography (GC 450, FID, FFAP capillary column 30 m x 0.32 mm x 0.25 μm). Determine the conversion of reactants (conv.%), selectivity towards product I (Si %), and yield of product i (Yi %) using the following equations: (1) (2) (3)

Representative Results

X-ray diffraction patterns (XRD) have been studied for the precursor LiNbWO6 and the corresponding proton-exchanged catalyst sample HNbWO6 to determine the phase (Figure 1 and Figure 2). NH3-temperature programmed desorption (NH3-TPD) was used to probe the surface acidity of the catalyst samples (Figure 3). Scanning electron microscopy (SEM) with X-ray microanalysis and…

Discussion

Pretreatment of CNTs with nitric acid does increase the specific surface area (S_BET) significantly. Raw CNTs have a specific surface area of 103 m²/g while after treatment, the surface area was increased to 134 m²/g. Therefore, such pretreatment to create defects on the CNT surface will have a positive effect on the specific surface area on the catalysts after solid acid modification and platinum particle impregnation. Since the surface area will decrease after incorporation of nanosheets…

Materials

Carbon nanotubes (multi-walled)	Sigma Aldrich	724769
Nitric acid (65%)	Sigma Aldrich	V000191
sulphuric acid (98%)	MERCK	100748
Lithium carbonate (>99%)	Aladdin	L196236
Niobium pentaoxide (99.95%)	Aladdin	N108413
Tungsten trioxide (99.8%)	Aladdin	T103857
Molybdenum trioxide (99.5%)	Aladdin	M104355
Tantalum oxide (99.5%)	Aladdin	T104746
Chloroplatinic acid hexahydrate, ≥37.50% Pt basis	Sigma Aldrich	206083
tetra (n-butylammonium) hydroxide 30-hydrate	Aladdin	D117227
Diphenyl ether, 98%	Aladdin	D110644
2-Bromoacetophenone,98%	Aladdin	B103328
Diethyl ether,99.5%	Sinopharm	10009318
n-Decane,98%	Aladdin	D105231
n-Dodecane,99%	Aladdin	D119697
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)