Chemistry
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Tuning the Acidity of Pt/ CNTs Catalysts for Hydrodeoxygenation of Diphenyl Ether
Chapters
Summary August 17th, 2019
A protocol for the synthesis of HNbWO6, HNbMoO6, HTaWO6 solid acid nanosheet modified Pt/CNTs is presented.
Transcript
Functionalized carbon nanomaterials using graphene, carbon nanotubes, carbon nanofibers, and the mesoporous carbon materials play an important role in the valorization of biomass due to the tunable porosity, extremely high specific surface area and excellent hydrophobicity. This protocol demonstrates a general method to tune the acidity of solid acid nanosheet modified platinum carbon nanotubes for biomass valorizations. Acidity of the solid acid can be modified by lowering the number of defects, specific area of the carbon nanotube, and types of solid acid nanosheets so that during biomass conversion, the catalyst can be fine-tuned to produce different products depending on different product's requirements.
First, immerse one gram of carbon nanotubes or CNTs in 15 milliliters of nitric acid in a 100 milliliter beaker. Sonicate the solution at 25 degrees Celsius for one and a half hours to remove surface impurities and to enhance the anchoring effect of the catalyst. Then, transfer the solution to a 100 milliliter round-bottom flask.
Reflux the solution in a mixture of nitric acid and sulfuric acid at 60 degrees Celsius overnight to create surface defects on the CNTs. After cooling to room temperature, filter the solution to obtain the multi-wall carbon nanotube solid. Wash the solid once with deionized water.
Dry the solid at 80 degrees Celsius for 14 hours. Next, weigh kilometric amounts of lithium carbonate and metal oxides niobium oxide and tungsten trioxide in the molar ratio of one to one to two. Calcine the solid mixture at 800 degrees Celsius in air for 24 hours, with one intermediate grinding.
Following calcination, place 10 grams of the cold lithium niobium tungstate powder in 200 milliliters of a two-molar nitric acid solution at 50 degrees Celsius and stir the solution for five days, with one replacement of the acid every 24 hours. During the five days, exchange the acid liquid every day and repeat the previous step. Filter the solid and wash it with deionized water three times.
Then, dry the solid at 80 degrees Celsius overnight. Now, add 25%tetra and butyl ammonium hydroxide solution to 150 milliliters of a deionized water solution containing two grams of the prepared protonated compound, until the pH reaches 9.5 to 10. Then, stir the solution for seven days.
After seven days, centrifuge the solution. Collect the supernatant that contains the dispersed nanosheets. Next, weigh kilometric amounts of lithium carbonate and metal oxides niobium oxide and molybdenum trioxide in a molar ratio of one to one to two.
Calcine the solid mixture at 800 degrees Celsius in air for 24 hours, with one intermediate grinding. After calcination, place 10 grams of the cooled lithium niobium molybdate powder in 200 milliliters of a two-molar nitric acid solution at 50 degrees Celsius and stir the solution for five days, with one replacement of the acid at 60 hours. Next, weigh kilometric amounts of lithium carbonate and metal oxides tantalum pentoxide and tungsten trioxide in a molar ratio of one to one to two.
Calcine the solid mixture at 900 degrees Celsius in air for 24 hours, with one intermediate grinding. Following calcination, place 10 grams of the cooled lithium tantalum tungstate powder in 200 milliliters of a two-molar nitric acid solution at 50 degrees Celsius and stir the solution for five days, with one replacement of the acid at 60 hours. Add two grams of the prepared multi-wall CNTs to a 100 milliliter solution of the niobium tungstic acid nanosheets in a 250 milliliter round-bottom flask.
Add 100 milliliters of a one molar nitric acid solution to the round-bottom flask dropwise to aggregate the nanosheet samples. Continue to stir the solution at 50 degrees Celsius for six hours. Following this, filter the solid and wash with deionized water three times.
Dry the solid at 80 degrees Celsius overnight. The next day, weigh the dried solid and record the percent floating of the solid acid on the multi-wall CNTs. Prepare a one gram per 100 milliliter solution of chloroplatinic acid and water.
Then, impregnate the as prepared nanosheet modified CNTs with 1.34 milliliters of the aqueous platinum solution. After drying the nanosheet CNTs, calcinate in air at 400 degrees Celsius for three hours. Obtain the niobium tantalum-based solid acid nanosheet modified platinum CNT catalysts.
Dilute 05 grams of catalysts in five 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 hydrogen at 300 degrees Celsius for two hours.
Pump the diphenyl ether feedstocks into the fixed bed reactor at different flow rates from 05 to 06 milliliters per minute. Collect the products at different space times, defined as the ratio between the mass of catalyst and the flow rate of the substrate. Identify the liquid products, using a gas chromatograph, equipped with a 5977A mass selective detector and analyze offline by gas chromatography.
Finally, determine the conversion of reactants'selectivity towards product and yield of product, using the appropriate equations. The x are depattern of the precursor lithium niobium tungstate has three distinctive diffraction peaks, which represent a well ordered layered structure and is in good agreement with the tetragonal orthorhombic phase observed for lithium niobium tungstate. After the protonic exchange reaction, a diffraction peak at 6.8 degrees was observed, which agreed with the patterns observed in niobium tungstic acid and indicates the existence of a layered structure.
The x are depattern after exfoliation and mixing with CNTs has peaks attributed to carbons 002 and the 110 and 200 lattice plane of the niobium tungstic acid nanosheets. After exfoliation, the diffraction peak at 6.8 degrees almost entirely disappeared, indicating that the layered compounds were completely transformed into a nanosheet structure. SEM of the platinum impregnated on 20%niobium tungstic acid with carbon nanotubes, and the corresponding elemental mapping analysis of the different elements of the catalysts are shown here.
The analysis directly illustrated the distribution of the platinum particles, demonstrating that these particles, as well as niobium and tungsten, are uniformly dispersed on the surface of the catalysts. All the nanosheet modified platinum catalysts have weak acid characteristic sites that are depicted by the peaks centered at 210 degrees Celsius. Two peaks indicating medium acid strength are centered at 360 and 450 degrees Celsius.
Don't forget to mix the right ratio of nitric acid and sulfuric acid. And maintain an accurate temperature to ensure reproducibility of surface diffracts creations. Following this procedure, all the methods like preparing catalysts with different metal centres can be performed to answer questions like how to optimize the hydro conversion activity of diphenyl ether and other derived model compacts.
After demonstrating that, the as prepared solid acid can convert biomass-derived small molecules, such as diphenyl ether. The next question will be whether this catalyst can be used to convert a real biomass micromolecules to small molecules. This may require the further turning of the acidity of solid catalysts.
Don't forget that working with niobium oxide and related mixed metal oxides can be extremely toxic, and precautions should always be taken while handling nitric acid and sulfuric acid. High pressure reactors and
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