HKUST-1 as a Heterogeneous Catalyst for the Synthesis of Vanillin

The conversion of trans-ferulic acid to vanillin was achieved by heterogeneous catalysis. HKUST-1 was employed in this synthesis and the essential step in the catalytic process was the generation of unsaturated metal sites. Thus, when the catalyst was activated under vacuum, full vanillin conversion (yield of 95%) was obtained.

Transcript

The overall goal of this methodology is the conversion of trans ferulic acid to vanillin by heterogeneous catalysis using the porous coordination polymer HKUST-1. The essential step in the catalytic process is the generation of unsaturated metal sites in HKUST-1. These methods can help us answer key questions in the heterogeneous catalysis field, such as in the production of vanillin, which is one of the most widely employed flavoring agents in the world.

The main advantage of this technique is that we provide an experimental protocol to catalytically convert ferulic acid to vanillin under mild oxidation conditions. The implications of this technique extend toward the use of porous coordination polymers as catalysts. Generally, individuals new to this method will struggle, because activation of the HKUST-1 material is fundamental.

Demonstrating the procedure will be Rebeca Yepez, a graduate student from my laboratory. To characterize the crystallinity of the catalyst, collect the powder x-ray diffraction pattern of a 0.1 gram sample of HKUST-1, as described in the text protocol. Record a powder x-ray diffraction pattern from five degrees to 60 degrees in 0.02 degree steps, and one second counting time.

Then, perform dissolvation of HKUST-1 by first weighing 0.05 grams of the catalyst. Next, prepare the activation system by clamping a 250 milliliter two neck round bottom flask to a stand and inserting a magnetic stirring bar into the round bottom flask. Attach a condenser to the round bottom flask.

Use some vacuum grease or Teflon tape between the joints of the flask and condenser to generate a perfect seal. Connect the condenser from the top to a vacuum pump. Make sure that the vacuum generated by the pump is approximately 0.1 bar.

Place the catalyst inside the 250 milliliter two neck round bottom flask. Insert a rubber septum in the second neck of the round bottom flask, and make sure that it seals properly. Carefully place the activation system into a sand bath.

Start the vacuum pump and carefully turn the stopcock until it is fully opened. With a hot plate, heat the activation system up to 100 degrees Celsius for one hour. Stir at the lowest speed, the hot plate to homogeneously distribute the catalyst at the bottom of the round bottom flask.

When the catalyst is activated, a color change must be clearly observed. This is an indication of the removal of coordinated water molecules, and therefore the generation of coordinately unsaturated copper to metal sites within the catalyst. A color change from turquoise to dark blue upon activation is observed.

Turn off the heat after one hour of heating and let the activation system cool to room temperature. Once the activation system is cooled down to room temperature, turn the stopcock off, and turn the pump off. Connect a balloon filled with nitrogen though the septum to the two neck round bottom flask, and wait a few seconds to reach the equilibrium pressure.

After the activation of the catalyst, leave it under an inert atmosphere, since the access to the uncoordinated metal sites is the key to obtaining an active catalyst. Remove the balloon filled with nitrogen when the equilibrium pressure has been achieved. De-gas approximately 70 milliliters of ethanol by bubbling nitrogen for five minutes.

To prepare the catalytic reaction, add 10 milliliters of de-gassed ethanol to the two neck round bottom flask and gently stir the suspension on a hot plate. Then, add five milliliters of hydrogen peroxide followed by 25 milliliters of acetonitrile to the suspension. Weigh 0.5 grams of ferulic acid and dissolve it in 20 milliliters of de-gassed ethanol in a beaker.

Add the dissolved ferulic acid to the suspension, then wash the beaker with 20 milliliters of de-gassed ethanol and add it to the suspension. For oxidation of trans ferulic acid to vanillin, first unplug the hose that connects the condenser to the vacuum pump. Turn on the tap water, or preferably, the water pump that goes though the condenser.

Heat the suspension up to 100 degrees Celsius and reflux for one hour. After turning off the heat and stirring, carefully lift the two neck round bottom flask attached to the condenser and let it cool to room temperature. Next, filter off the reaction mixture using a Buchner funnel and flask.

Recover the catalyst and wash it with 200 milliliters of ethyl acetate. Concentrate the combined organic phases under vacuum with a rotary evaporator and re-dissolve it with 100 milliliters of ethyl acetate. Wash the organic phases in a separation funnel with a 30 milliliter saturated solution of ammonium chloride.

Then, recover the organic phases and mix them with 30 grams of anhydrous sodium sulfate. After letting the suspension stand for 15 minutes, filter the suspension off, and recover the filtrate. Then, concentrate the filtrate to approximately 20 milliliters under vacuum with a rotary evaporator.

Purify the residue by flash column chromatography. The stationary phase is silica gel, and the mobile phase is a solvent mixture of ethyl acetate and hexane. Pack the glass column for chromatography with a silica gel.

Then, saturate the column with the ethyl acetate hexane solvent mixture. Carefully pour the concentrated filtrate on top of the glass column. Slowly add the solvent mixture to the glass column and collect all of the fractions until 1, 200 milliliters are collected.

Concentrate the 1, 200 milliliters of the organic fractions with a rotary evaporator until dry. Recover the final solid powder that is the purified vanillin. Shown here are fourier transform infrared spectra of the HKUST-1 catalyst at 25 degrees Celsius.

The spectrum of the non-activated catalyst is shown in green. The spectrum of the activated catalyst in a conventional oven is shown in purple, and the spectrum of the activated catalyst under vacuum is shown in orange. Shown here is a proton NMR spectra of vanillin after column chromatography purification on silica gel when the catalyst was activated under vacuum and 100 degrees Celsius for one hour.

Once mastered, this technique can be done in four hours, if it is performed properly. Though this method can provide insight into the mechanism for the oxidation of trans ferulic acid, it can also be applied to other systems, such as oxidation of other alpha beta unsaturated carboxylic acid. While attempting this procedure, it's important to remember to emphasize on the correct purification of vanillin by column chromatography.

Following this procedure, other methods like the oxidation of other alpha beta unsaturated carboxylic acid can be performed in order to answer additional questions, like oxidation mechanisms. After its development, this technique paved way for researchers in the field of porous coordination polymers to explore these materials as heterogeneous catalyst to synthesize molecules of high chemical relevance. After watching this video, you should have a good understanding of how to carry out the oxidation of ferulic acid to vanillin.

Don't forget that working with a high volatility solvents under reflexive conditions can be extremely hazardous, and precautions such as wearing a lab coat, safety glasses, and working in a fume hood, should always be taken while performing this procedure.

Read Article

Journal (Chemistry)

Journal (Chemistry)

HKUST-1 as a Heterogeneous Catalyst for the Synthesis of Vanillin

Chapters

Transcript

Related Videos

Get cutting-edge science videos from JoVE sent straight to your inbox every month.

Your free access has ended.