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W tym Artykule

  • Podsumowanie
  • Streszczenie
  • Wprowadzenie
  • Protokół
  • Wyniki
  • Dyskusje
  • Ujawnienia
  • Podziękowania
  • Materiały
  • Odniesienia
  • Przedruki i uprawnienia

Podsumowanie

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.

Streszczenie

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.

Wprowadzenie

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.

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Protokół

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)

  1. Crystallinity Characterization of the Catalyst
    Note: HKUST-1 is a commercially available porous coordination polymer (catalyst). In order to corroborate the crystallinity of the catalyst, samples of HKUST-1 need to be characterized by powder X-ray diffraction (PXRD).
    1. Collect a PXRD pattern of a 0.1 g sample of HKUST-1 (on a diffractometer operating at 160 W (40 kV, 40 mA)) for Cu-Kα1 radiation (λ = 1.5406 Å) in Bragg-Brentano geometry. Record a PXRD pattern from 5° to 60° (2θ) in 0.02° steps and 1 sec counting time44.
  2. Desolvation of HKUST-1
    1. Weigh 0.05 g of the catalyst (HKUST-1).
    2. Clamp a 250 ml two-neck round-bottom flask to a stand and insert a magnetic stirring bar into the round-bottom flask.
    3. Attach a condenser to the round-bottom flask.
    4. Use some vacuum grease or Teflon tape between the joints of the flask and condenser in order to generate a perfect seal.
    5. Connect the condenser, from the top, to a vacuum pump (via a stopcock to a hose).
    6. Make sure that the vacuum generated by the pump is approximately 10-2 bar.
      Note: For convenience, this experimental set up (the two-neck round-bottom flask attached to a condenser, which is connected to a vacuum pump) will be referred as the activation system.
    7. Place the catalyst (0.05 g) inside the 250 ml two-neck round-bottom flask.
    8. Insert a rubber septum in the second neck of the round bottom flask and make sure that it seals (fits) properly.
    9. Carefully, place the activation system into a sand bath.
    10. Start the vacuum pump and carefully turn the stopcock until it is fully open. With a hot plate, heat the activation system up to 100 °C for 1 hr.
    11. Stir at the lowest speed of the hot plate, in order to distribute homogenously the catalyst at the bottom of the round-bottom flask.
    12. Turn off the heat (hot plate) after 1 hr of heating, and let the activation system cool to room temperature (under vacuum).
    13. Once the activation system has cooled down to room temperature, turn the stopcock off (thus, the activation system would be under passive vacuum) and turn the pump off.
    14. Connect a balloon filled with nitrogen (N2), through the septum, to the two-neck round-bottom flask and wait a few seconds to reach the equilibrium pressure.
      Note: After the activation of the catalyst, leave it under an inert atmosphere (N2) since the access to the uncoordinated metal sites (or open metal sites) is the key to obtain an active catalyst.
    15. Remove the balloon filled with N2, when the equilibrium pressure has been achieved.
      Note: A color change from turquoise (as-received HKUST-1) to dark blue (upon activation) is observed.

2. Synthesis of Vanillin via Heterogeneous Catalysis

  1. Degassing of the Organic Solvent
    1. Degas approximately 70 ml of ethanol by bubbling N2 for 5 min.
  2. Preparation of the Catalytic Reaction
    1. Add 10 ml of degassed ethanol to the two-neck round-bottom flask and gently stir the suspension on a hot plate.
    2. Add 5 ml of H2O2 (30 % in H2O) to the suspension.
    3. Add 0.25 ml of acetonitrile to the suspension.
    4. Weigh 0.50 g of ferulic acid and dissolve it in 20 ml of degassed ethanol in a beaker.
    5. Add the dissolved ferulic acid to the suspension.
    6. Wash the beaker with 20 ml of degassed ethanol and add it to the suspension.
  3. Oxidation of Trans-ferulic Acid to Vanillin
    1. Unplug the hose that connects the condenser to the vacuum pump.
    2. Turn on the tap water that goes through the condenser. Preferably, use a water pump.
    3. Heat the suspension up to 100 °C (refluxing) for 1 hr.
    4. Turn off the heat and stirring. Carefully lift the two-neck round-bottom flask (attached to the condenser) and let it cool to room temperature.
  4. Work-up of the Reaction
    1. Filter off the reaction mixture (use a Buchner funnel and flask), recover the catalyst (HKUST-1) and wash it with 200 ml of ethyl acetate.
      Note: In order to speed up the filtration process, use a vacuum (connected to the Buchner flask) to quickly recover and wash the catalyst.
    2. Corroborate the retention of framework crystallinity of the catalyst by PXRD as in Section 1.
    3. Concentrate the combined organic phases (under vacuum with a rotary evaporator) and re-dissolve it with 100 ml ethyl acetate.
    4. Wash the organic phases (use a separation funnel) with a saturated solution of NH4Cl (30 ml).
    5. Recover the organic phases and mix them with anhydrous Na2SO4 (30 g). Let the suspension stand for 15 min.
    6. Filter the suspension off and recover the filtrate.
    7. Concentrate the filtrate (to approximately 20 ml) under vacuum with a rotary evaporator.
  5. Purification of the Residue (Vanillin)
    1. Purify the residue by flash column chromatography44. The stationary phase is silica gel and the mobile phase is a solvent mixture of ethyl acetate-hexane (5:95).
    2. Pack the glass column for chromatography (1 cm x 30 cm) with silica gel (1 cm x 6 cm). Saturate the column with acetate-hexane (5:95) solvent mixture.
    3. Pour carefully the concentrated-filtrate at top of the glass column.
    4. Slowly add the solvent mixture to the glass column and collect all of the fractions until 1,200 ml are collected.
    5. Concentrate the organic fractions (1,200 ml) with a rotary evaporator until dryness.
    6. Recover the final solid powder that is the purified vanillin.

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Wyniki

Three representative samples of HKUST-1 were analyzed by infrared spectroscopy: non-activated, activated at 100 °C for 1 hr in an oven (exposed to air), and activated under vacuum (10-2 bar) at 100 °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...

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Dyskusje

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 °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...

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Ujawnienia

The authors have nothing to disclose.

Podziękowania

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.

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Materiały

NameCompanyCatalog NumberComments
HKUST-1Sigma-AldrichMFCD10567003
Ferulic Acid (trans-4-Hydroxy-3-methoxycinnamic acid)Sigma-Aldrich537-98-4
EthanolSigma-Aldrich64-17-5
Hydrogen peroxide solutionSigma-Aldrich7722-84-1
AcetonitrileSigma-Aldrich75-05-8
Ethyl acetateSigma-Aldrich141-78-6
Ammonium chlorideSigma-Aldrich12125-02-9
Sodium sulfate anhydrousSigma-Aldrich7757-82-6
Ethyl acetateSigma-Aldrich141-78-6
n-HexaneSigma-Aldrich110-54-3
Silica GelSigma-Aldrich112926-00-8Size 70/230
250 ml two-neck round-bottom flaskSigma-AldrichZ516872-1EA250 ml capacity
Magnetic stirring barBel-Art products371100002Teflon, octagon
CondenserCole-ParmerJZ-34706-00200 mm Jacket length
Vacuum pump (Approx. 10-2 bar)Cole-ParmerJZ-78162-00Vacuum/Pressure Diaphragm Pump
StopcockCole-ParmerEW-30600-00with a male Luer slip
HoseCole-ParmerJZ-06602-0416.0 mm ID and 23.2 mm ED
Rubber septumsCole-ParmerJZ-08918-34Silicone with PTFE coating
Hot plateCole-ParmerJZ-04660-1510.2 cm x 10.2 cm, 5 to 540 °C
Sand bathCole-ParmerGH-01184-00Fluidized Sand Bath SBS-4, 50 to 600 °C
N2 gasINFRACod. 103Cylinder 9 m3
Ballons (filled with N2 gas)Sigma-AldrichZ154989-100EAThick-wall, natural latex rubber
Syringes with removable needlesSigma-AldrichZ116912-100EA10 ml capacity
Filter paperCole-ParmerJZ-81050-24Grade No. 235 qualitative filter paper (90 mm diameter disc)
Buchner funnelCole-ParmerJZ-17815-04320 ml capacity which accept standard paper filter sizes
Buchner flaskCole-ParmerJZ-34557-02250 ml capacity
Rotary EvaporatorCole-ParmerJZ-28710-02
BeakersCole-ParmerJZ-34502-(02,04,05)Pyrex Brand 1000 Griffin; 20, 50 and 100 ml
Separation funnel Cole-ParmerJZ-34505-44Capacity for 125 ml with steam length of 60 mm
Glass column for chromatographyCole-ParmerJZ-34695-42Column with fritted disk, 10.5 mm ID x 250 mm L
PXRD diffractometerBrukerAXS D8 Advance XRD
FTIR spectrophotometerThermo scientificFT-IR (JZ-83008-02); ATR (JZ-83008-26)Nicolet iS5 FT-IR Spectrometer, with KBr Windows and iD5 Diamond ATR

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