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

Summary

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.

Abstract

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.

Introduction

The use of porous coordination polymers (PCPs) as heterogeneous catalysts^1-4 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 catalysts^5-6. The generation of catalytically active PCPs has been the main focus of many research groups^7-10. 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 reactio....

Protocol

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-....

Results

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 resolution of .......

Discussion

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
HKUST-1	Sigma-Aldrich	MFCD10567003
Ferulic Acid (trans-4-Hydroxy-3-methoxycinnamic acid)	Sigma-Aldrich	537-98-4
Ethanol	Sigma-Aldrich	64-17-5
Hydrogen peroxide solution	Sigma-Aldrich	7722-84-1
Acetonitrile	Sigma-Aldrich	75-05-8
Ethyl acetate	Sigma-Aldrich	141-78-6
Ammonium chloride	Sigma-Aldrich	12125-02-9
Sodium sulfate anhydrous	Sigma-Aldrich	7757-82-6
Ethyl acetate	Sigma-Aldrich	141-78-6
n-Hexane	Sigma-Aldrich	110-54-3
Silica Gel	Sigma-Aldrich	112926-00-8	Size 70/230
250 ml two-neck round-bottom flask	Sigma-Aldrich	Z516872-1EA	250 ml capacity
Magnetic stirring bar	Bel-Art products	371100002	Teflon, octagon
Condenser	Cole-Parmer	JZ-34706-00	200 mm Jacket length
Vacuum pump (Approx. 10-2 bar)	Cole-Parmer	JZ-78162-00	Vacuum/Pressure Diaphragm Pump
Stopcock	Cole-Parmer	EW-30600-00	with a male Luer slip
Hose	Cole-Parmer	JZ-06602-04	16.0 mm ID and 23.2 mm ED
Rubber septums	Cole-Parmer	JZ-08918-34	Silicone with PTFE coating
Hot plate	Cole-Parmer	JZ-04660-15	10.2 cm x 10.2 cm, 5 to 540 °C
Sand bath	Cole-Parmer	GH-01184-00	Fluidized Sand Bath SBS-4, 50 to 600 °C
N2 gas	INFRA	Cod. 103	Cylinder 9 m3
Ballons (filled with N2 gas)	Sigma-Aldrich	Z154989-100EA	Thick-wall, natural latex rubber
Syringes with removable needles	Sigma-Aldrich	Z116912-100EA	10 ml capacity
Filter paper	Cole-Parmer	JZ-81050-24	Grade No. 235 qualitative filter paper (90 mm diameter disc)
Buchner funnel	Cole-Parmer	JZ-17815-04	320 ml capacity which accept standard paper filter sizes
Buchner flask	Cole-Parmer	JZ-34557-02	250 ml capacity
Rotary Evaporator	Cole-Parmer	JZ-28710-02
Beakers	Cole-Parmer	JZ-34502-(02,04,05)	Pyrex Brand 1000 Griffin; 20, 50 and 100 ml
Separation funnel	Cole-Parmer	JZ-34505-44	Capacity for 125 ml with steam length of 60 mm
Glass column for chromatography	Cole-Parmer	JZ-34695-42	Column with fritted disk, 10.5 mm ID x 250 mm L
PXRD diffractometer	Bruker		AXS D8 Advance XRD
FTIR spectrophotometer	Thermo scientific	FT-IR (JZ-83008-02); ATR (JZ-83008-26)	Nicolet iS5 FT-IR Spectrometer, with KBr Windows and iD5 Diamond ATR