The authors thank Dr. A. Tejeda-Cruz (X-ray; IIM-UNAM). R.Y. thanks CINVESTAV, Mexico for technical support. M.S.S acknowledges the financial support by Spanish Government, MINECO (MAT2012-31127). I.A.I thanks CONACyT (212318) and PAPIIT UNAM (IN100415), Mexico for financial support. E.G-Z. thanks CONACyT (156801 and 236879), Mexico for financial support. Thanks to U. Winnberg (ITAM and ITESM) for scientific discussions.

CAUTION: The chemicals used in this catalytic procedure are relatively low in toxicity and non-carcinogenic. Please use all appropriate safety precautions when performing this experimental procedure such as safety glasses, gloves, lab coat, full length pants and closed-toe shoes. One part of the following procedures involves standard air-free handling techniques.
1. Activation of the Catalyst (HKUST-1)
<ol>
	<li>Crystallinity Characterization of the Catalyst 
	Note: HKUST-1 is a commercially available porou...

<ol>
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The authors have nothing to disclose.

The fundamental step for the catalytic conversion of trans-ferulic acid to vanillin was the activation of the catalyst (HKUST-1). If the catalyst is not activated in situ (under vacuum and at 100 &#176;C), only partial conversion of trans-ferulic acid to vanillin was observed44. In other words, the accessibility to open metal sites is crucial for the catalytic cycle44, and this can be achieved by the elimination of coordinated water to the Cu(II) metal sites within the por...

The use of porous coordination polymers (PCPs) as heterogeneous catalysts1-4 is a relatively new research field. Due to very interesting properties that PCPs show, e.g., porous regularity, high surface area and metal access, they can offer new alternatives for heterogeneous catalysts5-6. The generation of catalytically active PCPs has been the main focus of many research groups7-10. A porous coordination polymer is constituted by metal ions and organic linkers and thus, the catalytic activity of these materials is provided by any of these parts. Some PCPs contain unsaturated (active) metals that can catalyze a chemical reaction11. However, the generation of unsaturated metal sites (open metal sites) within coordination polymers is not a trivial task and it represents a synthetic challenge that can be summarized in: (i) the generation of vacant coordination by removal of labile ligands7-11; (ii) the generation of bimetallic PCPs by incorporating organometallic ligands (previously synthesized)8,12-13; (iii) the post-synthetic variation of the metal ions9,14-15 or to the organic ligands10, 16-17 within the pores of the PCPs. Since the methodology (i) is the simplest thus, it is the most frequently used. Typically, the generation of open metal sites has been used for enhancing the affinity of PCPs towards H218-19, as well as for designing active heterogeneous catalysts20-27. In order to achieve good catalyst properties, PCPs need to show, additionally to the accessibility of open metal sites, retention of the crystallinity after the catalytic experiment, relatively high thermal stability and chemical stability to the reaction conditions.
HKUST-1 (Cu3(BTC)2(H2O)3, BTC = 1,3,5-benzene-tricarboxylate)7 is a well-investigated porous coordination polymer constructed with Cu(II) cations, that are coordinated to the carboxylate ligands and water. Interestingly, these water molecules can be eliminated (by heating) and this provides a square planar coordination around the copper ions which exhibit hard Lewis acid properties11. Bordiga and co-workers28 showed that the elimination of these H2O molecules did not affect the crystallinity (retention of the regularity) and the oxidation state of the metal ions (Cu(II)) was not affected. The use of HKUST-1 as a catalyst has been extensively investigated29-33 and in particular (very relevant for the present work) the oxidation with hydrogen peroxide of aromatic molecules34.
Vanilla is one of the most widely used flavoring agents in the cosmetic, pharmaceutical and food industries. It is extracted from the cured beans of the orchid Vanilla planifolia, Vanilla tahitiensis and Vanilla pompon. The Mayan and Aztec civilizations (pre-Columbian people) first realized the enormous potential of vanilla as a flavoring agent since it improved the chocolate flavor35-37. Vanilla was first isolated in 185838 and it was not until 187439 that the chemical structure of vanillin was finally determined. The natural extraction of vanillin (from the orchid Vanilla planifolia, Vanilla tahitiensis and Vanilla pompon) represents only 1% of the worldwide production and since this process is expensive and very long40, the rest of vanillin is synthesized40. Many biotechnological approaches can be used for the synthesis of vanillin from lignin, phenolic stilbenes, isoeugenol, eugenol, guaicol, etc. However, these approaches have the disadvantage of harming the environment since these processes use strong oxidizing agents and toxic solvents41-43. Herein, we report a synthetic strategy for the production of vanillin by the oxidation of trans-ferulic acid using HKUST-1 as a catalyst.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>HKUST-1</td>
			<td>Sigma-Aldrich</td>
			<td>MFCD10567003</td>
			<td></td>
		</tr>
		<tr>
			<td>Ferulic Acid (trans-4-Hydroxy-3-methoxycinnamic acid)</td>
			<td>Sigma-Aldrich</td>
			<td>537-98-4</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethanol</td>
			<td>Sigma-Aldrich</td>
			<td>64-17-5</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrogen peroxide solution</td>
			<td>Sigma-Aldrich</td>
			<td>&#160;7722-84-1</td>
			<td></td>
		</tr>
		<tr>
			<td>Acetonitrile</td>
			<td>Sigma-Aldrich</td>
			<td>75-05-8</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethyl acetate</td>
			<td>Sigma-Aldrich</td>
			<td>141-78-6</td>
			<td></td>
		</tr>
		<tr>
			<td>Ammonium chloride</td>
			<td>Sigma-Aldrich</td>
			<td>12125-02-9</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium sulfate anhydrous</td>
			<td>Sigma-Aldrich</td>
			<td>7757-82-6</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethyl acetate</td>
			<td>Sigma-Aldrich</td>
			<td>141-78-6</td>
			<td></td>
		</tr>
		<tr>
			<td>n-Hexane</td>
			<td>Sigma-Aldrich</td>
			<td>110-54-3</td>
			<td></td>
		</tr>
		<tr>
			<td>Silica Gel</td>
			<td>Sigma-Aldrich</td>
			<td>112926-00-8</td>
			<td>&#160;Size 70/230</td>
		</tr>
		<tr>
			<td>250 mL two-neck round-bottom flask</td>
			<td>Sigma-Aldrich</td>
			<td>Z516872-1EA</td>
			<td>250 mL capacity</td>
		</tr>
		<tr>
			<td>Magnetic stirring bar</td>
			<td>Bel-Art products</td>
			<td>371100002</td>
			<td>Teflon, octagon</td>
		</tr>
		<tr>
			<td>Condenser</td>
			<td>Cole-Parmer</td>
			<td>JZ-34706-00</td>
			<td>200 mm Jacket length</td>
		</tr>
		<tr>
			<td>Vacuum pump (Approx. 10X-2 bar)</td>
			<td>Cole-Parmer</td>
			<td>JZ-78162-00</td>
			<td>Vacuum/Pressure Diaphragm Pump</td>
		</tr>
		<tr>
			<td>Stopcock</td>
			<td>Cole-Parmer</td>
			<td>EW-30600-00</td>
			<td>with a male luer slip</td>
		</tr>
		<tr>
			<td>Hose</td>
			<td>Cole-Parmer</td>
			<td>JZ-06602-04</td>
			<td>16.0 mm ID and 23.2 mm ED</td>
		</tr>
		<tr>
			<td>Rubber septums</td>
			<td>Cole-Parmer</td>
			<td>JZ-08918-34</td>
			<td>Silicone with PTFE coating</td>
		</tr>
		<tr>
			<td>Hot plate</td>
			<td>Cole-Parmer</td>
			<td>JZ-04660-15</td>
			<td>10.2 cm x 10.2 cm, 5 to 540 &#176;C</td>
		</tr>
		<tr>
			<td>Sand bath&#160;</td>
			<td>Cole-Parmer</td>
			<td>GH-01184-00</td>
			<td>Fluidized Sand Bath SBS-4, 50 to 600 &#176;C</td>
		</tr>
		<tr>
			<td>N2 gas</td>
			<td>INFRA</td>
			<td>Cod. 103</td>
			<td>Cylinder 9m &#179;</td>
		</tr>
		<tr>
			<td>Ballons (filled with N2 gas)</td>
			<td>Sigma-Aldrich</td>
			<td>Z154989-100EA</td>
			<td>Thick-wall, natural latex rubber</td>
		</tr>
		<tr>
			<td>Syringes with removable needles</td>
			<td>Sigma-Aldrich</td>
			<td>Z116912-100EA</td>
			<td>10 mL capacity</td>
		</tr>
		<tr>
			<td>Filter paper</td>
			<td>Cole-Parmer</td>
			<td>JZ-81050-24</td>
			<td>Grade No. 235 qualitative filter paper (90 mm diameter disc)</td>
		</tr>
		<tr>
			<td>Buchner funnel</td>
			<td>Cole-Parmer</td>
			<td>JZ-17815-04</td>
			<td>320 mL capacity which accept standard paper filter sizes&#160;</td>
		</tr>
		<tr>
			<td>Buchner flask</td>
			<td>Cole-Parmer</td>
			<td>JZ-34557-02</td>
			<td>250 mL capacity</td>
		</tr>
		<tr>
			<td>Rotary Evaporator</td>
			<td>Cole-Parmer</td>
			<td>JZ-28710-02</td>
			<td></td>
		</tr>
		<tr>
			<td>Beakers</td>
			<td>Cole-Parmer</td>
			<td>JZ-34502-(02,04,05)</td>
			<td>Pyrex Brand 1000 Griffin; 20, 50 and 100 mL</td>
		</tr>
		<tr>
			<td>Separation funnel&#160;</td>
			<td>Cole-Parmer</td>
			<td>JZ-34505-44</td>
			<td>Capacity for 125 mL with steam lenght of 60 mm</td>
		</tr>
		<tr>
			<td>Glass column for chromatography</td>
			<td>Cole-Parmer</td>
			<td>JZ-34695-42</td>
			<td>Column with fritted disk, 10.5 mm ID x 250 mm L</td>
		</tr>
		<tr>
			<td>PXRD diffractometer</td>
			<td>Bruker</td>
			<td></td>
			<td>AXS D8 Advance XRD</td>
		</tr>
		<tr>
			<td>FTIR spectrophotometer</td>
			<td>Thermo scientific</td>
			<td>FT-IR (JZ-83008-02); ATR (JZ-83008-26)</td>
			<td>Nicolet iS5 FT-IR Spectrometer, with KBr Windows and iD5 Diamond ATR</td>
		</tr>
	</tbody>
</table>

hkust-1 as a heterogeneous catalyst for the synthesis of vanillin

Vanillin (4-hydoxy-3-methoxybenzaldehyde) is the main component of the extract of vanilla bean. The natural vanilla scent is a mixture of approximately 200 different odorant compounds in addition to vanillin. The natural extraction of vanillin (from the orchid Vanilla planifolia, Vanilla tahitiensis and Vanilla pompon) represents only 1% of the worldwide production and since this process is expensive and very long, the rest of the production of vanillin is synthesized. Many biotechnological approaches can be used for the synthesis of vanillin from lignin, phenolic stilbenes, isoeugenol, eugenol, guaicol, etc., with the disadvantage of harming the environment since these processes use strong oxidizing agents and toxic solvents. Thus, eco-friendly alternatives on the production of vanillin are very desirable and thus, under current investigation. Porous coordination polymers (PCPs) are a new class of highly crystalline materials that recently have been used for catalysis. HKUST-1 (Cu3(BTC)2(H2O)3, BTC = 1,3,5-benzene-tricarboxylate) is a very well known PCP which has been extensively studied as a heterogeneous catalyst. Here, we report a synthetic strategy for the production of vanillin by the oxidation of trans-ferulic acid using HKUST-1 as a catalyst.

Three representative samples of HKUST-1 were analyzed by infrared spectroscopy: non-activated, activated at 100 &#176;C for 1 hr in an oven (exposed to air), and activated under vacuum (10-2 bar) at 100 &#176;C for 1 hr. Thus, Fourier transform infrared (FTIR) spectra were recorded using a spectrometer with a single reflection diamond ATR accessory (Figure 1). For all spectra, 64 scans in the 4,000 to 400 cm-1 range were recorded with a spectral reso...

Watch this Scientific Journal Video about HKUST-1 as a Heterogeneous Catalyst for the Synthesis of Vanillin at JoVE.com

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.

Vanillin (4-hydoxy-3-methoxybenzaldehyde) is the main component of the extract of vanilla bean. The natural vanilla scent is a mixture of approximately ...

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.

hkust-1-as-a-heterogeneous-catalyst-for-the-synthesis-of-vanillin

Research

JoVE Journal

Chemistry

10.0K visualizações.  Autonomous Metropolitan University-Azcapotzalco. 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 tech...