Facile production of vitamin B3 and other heterocyclic carboxylic acids using an efficient Ag/ZnO/graphene-Si hybrid nanocatalyst

Article abstract of DOI:10.1007/s11164-016-2615-7

Abstract: High yield of vitamin B₃ is produced using Ag/ZnO/graphene nanocomposite (1?wt%) as a nanocatalyst after its activation by a silicon precursor such as trimethylsilyl chloride(TMSCl) or tert-butyldimethylsilyl chloride (TBSCl) under visible light. TBSCl has been proved as more efficient activating agent than TMSCl in the oxidation of heterocyclic alcohol derivatives to afford their corresponding carboxylic acids. 3-Pyridinemethanol was selected to be a model substrate to test the ability of Ag/ZnO/graphene-Si nanocatalysts. The oxidation reaction of alcohols was completed in short reaction time (30–60?min) at ambient condition to yield vitamin B₃ and other heterocyclic carboxylic acids in excellent yields (86–99?%). A catalytic oxidation mechanism that is based on the generation of highly active catalytic oxidation species (oxygen radicals) has been proposed. The utility of this inexpensive and recyclable catalytic system without using molecular oxygen or any oxidants make this procedure interesting from a green chemistry point of view. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-016-2615-7

Res Chem Intermed
DOI 10.1007/s11164-016-2615-7
Facile production of vitamin B3 and other heterocyclic
carboxylic acids using an efficient Ag/ZnO/graphene-Si
hybrid nanocatalyst
Yasser A. Attia1,2
1,2
•
•
´ ´
Carlos Vazquez Vazquez
Yasser M. A. Mohamed3
Received: 26 April 2016 / Accepted: 14 June 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract High yield of vitamin B3 is produced using Ag/ZnO/graphene nanocomposite
(1 wt%) as a nanocatalyst after its activation by a silicon precursor such as trimethylsilyl
chloride(TMSCl) or tert-butyldimethylsilyl chloride (TBSCl) under visible light. TBSCl
has been proved as more efficient activating agent than TMSCl in the oxidation of
heterocyclic alcohol derivatives to afford their corresponding carboxylic acids. 3-Pyr-
idinemethanol was selected to be a model substrate to test the ability of Ag/ZnO/
graphene-Si nanocatalysts. The oxidation reaction of alcohols was completed in short
reaction time (30–60 min) at ambient condition to yield vitamin B3 and other hetero-
cyclic carboxylic acids in excellent yields (86–99 %). A catalytic oxidation mechanism
that is based on the generation of highly active catalytic oxidation species (oxygen
radicals) has been proposed. The utility of this inexpensive and recyclable catalytic
system without using molecular oxygen or any oxidants make this procedure interesting
from a green chemistry point of view.
&
&
1
Yasser A. Attia
yasserniles@niles.edu.eg
Yasser M. A. Mohamed
y.m.a.mohamed@outlook.com
Department Physical Chemistry, Faculty of Chemistry, Santiago de Compostela University,
15705 Santiago de Compostela, Spain
2
3
National Institute of Laser Enhanced Sciences, Cairo University, Giza 12631, Egypt
Photochemistry Department, National Research Center, Dokki, Giza 12622, Egypt
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Y. A. Attia et al.
Graphical Abstract
Keywords Heterogeneous nanocatalysts Á Ag/ZnO/graphene Á Nanocomposite Á
Catalytic oxidation Á Photodegradation Á Carboxylic acids and Vitamin B3
Introduction
Production of nictonic acid, also known as vitamin B3, with green procedures has
received great attention due to its pharmaceutical use as a vitamin. Every year, an
almost 20,000 tons of vitamin B3 are produced worldwide. Currently, the
commonly used oxidizing agent in production of vitamin B3 is chromic acid,
which is characterized as corrosive. In addition, several tons of inorganic waste are
produced for every ton of nicotinic acid generated from 3-picoline. Therefore,
developing an efficient catalytic system was mandatory to minimize the inorganic
wastes and obtain vitamin B3 in high yield [1]. Production of vitaimin B3 from
catalytic oxidation of 3-pyridinemethanol [2], 3-pyridinecarboxyaldehyde [3] or
picoline [4] has been reported in several cases, due to its biological importance [5].
These methods have some drawbacks such as selectivity in production of nicotinic
acid, long lasting reaction time, the use of high temperature, and the use of
expensive materials as catalysts. Nanostructured catalysts are becoming a more
popular strategy to provide heterogeneous catalysts with improved characteristics
[6–12]. Currently, the use of recyclable nanocatalyst in oxidation reactions has
attracted great attention from an eco-friendly point of view [13, 14]. For this
purpose, we investigated a recyclable and efficient nanocatalytic system for
generating vitamin B3 in excellent yield, with short reaction time and no waste
production.
ZnO/graphene-based nanostructured catalysts are becoming a popular strategy to
provide heterogeneous catalysts with improved characteristics, due to broad band-
gap energy, cost effectiveness, physicochemical stability, easiness of availability
and high oxidative capacity [15–17]. Subsequently, ZnO based nanocomposites
have been widely used in different potential applications such as sensors, solar
energy conversion devices and photocatalysts [18]. It is worth noticing that although
the catalytic behavior of Ag/ZnO/graphene nanocomposite as a photocatalyst for
degradation of dyes has been investigated in several reports [16, 17]. However, its
catalytic activity towards oxidation of alcohols has not been reported. Graphene-
supported noble metal nanoparticles have potential application in catalysis [18].
Furthermore, the adsorption and dissociation of O2 molecule on pristine and silicon-
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Facile production of vitamin B3 and other heterocyclic…
doped graphene was investigated by Chen et al. [19]. It has been reported that
graphene–silicon interactions exhibited much larger interactions than those
normally associated with Wan der Waals forces [20]. In addition, the growth
behavior of ZnO in the presence of different concentrations of silicon has been
ˇˇ
reported by Podbrscek et al. [21]. Based on these results, we developed Ag/
ZnO/graphene-Si hybrid nanocomposite and exploited it in catalytic oxidation
application as a chemical evolution. Hence, due to our continuous interest in the
development of heterogeneous catalysts [22, 23], herein we report on the application
of Ag/ZnO/graphene nanocomposite mixed with tert-butyldimethylsilyl chloride
(TBSCl) or trimethylsilyl chloride (TMSCl) to overcome the aforementioned
limitation in oxidation processes.
Experimental section
Materials
Graphite powder and potassium permanganate (KMnO4) were purchased from Alfa
Aesar. Sulfuric acid (H2SO4), hydrochloric acid (HCl), sodium hydroxide (NaOH),
hydrogen peroxide (H2O2), ethylene glycol(EG), zinc(II) sulfate, zinc(II) acetate,
silver(I) nitrate, tert-butyldimethylsilyl chloride (TBSCl), trimethylsilyl chloride
(TMSCl), methelyene blue dye, methanol, hexane, acetone, Dimethyl Sulfoxide
(DMSO), Chloroform, Ethyl acetate (EtOAc), Diethyl ether (Et2O), 3-Pyridine-
methanol, 2-pyridinemethanol, 2-thiophenemethanol and 2-furanmethanol were
purchased from Sigma-Aldrich. Deionized water was used in all the experiments.
Characterization
UV–Vis spectra were measured with a Perkin Elmer Lambda 40 UV–visible
spectrophotometer using 1-cm path length Hellma quartz cuvettes. Transmission
electron microscopy (TEM) images were obtained with a Joel JEM-1230 electron
microscope operated at 120 kV equipped with GatanUltraScan 4000SP 4K 9 4K
CCD camera. A drop from a diluted sample dispersion was deposited onto an
amorphous carbon film on 400 mesh copper grids and left to evaporate at room
temperature. Scanning electron microscopy (SEM) images were obtained with a
ZEISS FE-SEM ULTRA Plus (equipped with EDX analyzer) microscope with a
Philips CM20 microscope, operating at an accelerating voltage of 200 kV. Several
drops from the sample dispersion were deposited onto an aluminum pin stubs and
left to evaporate at room temperature. X-ray diffraction (XRD) measurements were
performed using a Philips PW1710 X-ray diffractometer using Cu Ka radiation
˚
(k = 1.54186 A). The XRD patterns were recorded from 20° to 70°2H with a step
size of 0.020°2H and collecting 10 s per step. FT-IR spectra were recorded with a
Nicolet 6700 infrared spectrophotometer to determine the specific functional groups
present on the surface. Melting points (m.p.) were uncorrected using IA 9100MK-
Digital Melting Point Apparatus. Thin layer chromatography (TLC) was done by
1
silica gel plates 60 GF254, cellulose plates (20 9 20 cm). H NMR and 13C NMR
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Y. A. Attia et al.
1
spectra were determined on Bruker Avance DPX spectrometer at 300 MHZ for H
NMR and 75 MHz for 13C NMR. High-resolution mass (ESI–MS) spectra were
measured on (TOF) LC/MS; 6230 Series Accurate-Mass Time-of-Flight.
Catalyst preparation
Graphene oxide (GO) was prepared using a modified Hummer’s method [24]. In a
typical procedure, H2SO4 (150 mL) was added into a flask containing graphite (6 g)
in an ice bath under stirring. After that, 18 g potassium permanganate (KMnO4) was
slowly added to the above mixture, and the ice bath was removed after several
minutes. The color of the solution turned to dark green under continued stirring for
2 h at 35 °C. Then, water (300 mL) was slowly added to the reaction mixture and
the conical flask was bathed in boiling water; the reaction mixture was kept at this
temperature for another 30 min. After that, the flask was cooled in an ice bath, and
1 L distilled water was added to stop the reaction. 20 mL H2O2 (30 %) was added
to reduce the residual permanganate to soluble manganese ions. Eventually, the
precipitation was centrifuged and washed with 1 M HCl and distilled water several
times, and then vacuum dried at 60 °C for 24 h.
To prepare Ag/ZnO/graphene composite via the co-precipitation method, 0.05 g
GO dissolved in 40 mL of ethylene glycol (EG) was sonicated for 2 h to give a
brown dispersion. To this solution, 0.04 g of zinc acetate and 0.25 g silver nitrate
dissolved in 40 mL of EG were added under vigorous stirring, and the stirring was
continued for 2 h. Subsequently, 0.025 M NaOH was added to the mixture dropwise
followed by stirring for 1 h. Then, the mixture was heated at 160 °C to achieve the
reduction of GO and the deposition of ZnO, then washed with ethanol and deionized
water five times. Finally, the resultant composite was dried at 70 °C in vacuum for
24 h. The obtained composite (40 mg) was mixed with 10 mg of TBSCl or TMSCl
for degradation of methylene blue (MB) dye under low power UV-lamp (Ana-lamp
254 nm) and halogen lamp (HALOPAR 20 75 W 230 V 30° GU10, Italy). The
temperature was kept constant at 28 °C all the experiments to avoid the thermal
degradation.
Production of vitamin B3 (nicotinic acid) (2) using Ag/ZnO/graphene-Si catalyst
In a round flask, Ag/ZnO/graphene nanocomposite (10 mg, 1 wt%) was weighted, a
mixture of MeOH:H2O (1:1, 10 mL) was added, and that was followed by addition
of tert-butyldimethylsiyl chloride (TBSCl) (1.38 g, 9.20 mmol, 1 eq). 3-Pyridine-
methanol (1) (1.0 g, 9.20 mmol, 1 eq) was subsequently added to the reaction
mixture, which was stirred for 30 min at room temperature and monitored by TLC
using eluent system (EtOAc:Hexane, 3:7). The mixture was filtrated and then water
(5 mL) was added, and the mixture was extracted with (CHCl3: MeOH, 1:1)
(5 mL 9 2). The collected methanol/water layer was evaporated to afford vitamin
B3 (2) in quantitative yield.
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Facile production of vitamin B3 and other heterocyclic…
Catalyst recovering
The above catalyst was collected on a Buchner funnel and washed with water
(30 ml), MeOH (30 ml), acetone (30 ml) and Et2O (30 ml). The recovered catalyst
was then used in another oxidation process of 3-pyridinemethanol (1).
Synthesis of carboxylic acids [pyridine-2-carboxylic acid (7), thiophene-2-
carboxylic acid (8) or furan-2-carboxylic acid (9)
An appropriate alcohol [2-pyridinemethanol (3) (1.0 g, 9.20 mmol, 1 eq), 2-thio-
phenemethanol (4) (1.0 g, 8.78 mmol, 1 eq) or 2-furanmethanol (5) (1.0 g,
10.20 mmol] was added to reaction mixture that contained Ag/ZnO/graphene
nanocomposite (10 mg, 1 wt%), and TBSCl (1.38 g, 9.20 mmol, 1 eq), (1.32 g,
8.78 mmol, 1 eq), (1.53 g, 10.20 mmol, 1 eq) or (1.38 g, 9.26 mmol, 1 eq)
dissolved in MeOH:H2O (1:1, 10 mL). The mixture was stirred for 30, 60 or
60 min, respectively, at room temperature and monitored by TLC using eluent
system (EtOAc:Hexane, 3:7). The mixture was filtrated and then water (5 mL) was
added, and the mixture was extracted with (CHCl3: MeOH, 1:1) (5 mL 9 2). The
collected methanol/water layer was evaporated using rotary evaporator to afford the
residue that was purified by column chromatography using silica gel (Hex-
ane:EtOAc, 7:3, 6:4 and then 1:1) to afford the corresponding carboxylic acids;
pyridine-2-carboxylic acid (7); thiophene-2-carboxylic acid (8) or furan-2-car-
boxylic acid (9) respectively.
The spectroscopic data of the products are shown as following:
Nicotinic acid (2) Yield (0.99 g, 99 %), 1H NMR (300 MHz, DMSO-d6): d (ppm)
12.09 (br, s, 1H), 9.14 (dd, J = 2.1, 0.6 Hz, 1H), 8.79 (d, 1.7 Hz, 1H), 8.27 (dt, 7.4,
2.0 Hz, 1H), 7.52 (dd, 7.4, 4.9 Hz, 1H); 13C NMR (75 MHz, DMSO-d6,): d (ppm)
166.5, 153.8, 150.1, 136.09, 126.4, 124.1; HRMS (ESI): m/z calcd for C6H5O2N:
123.0320, found: 122.0316 [M-1]?.
1
Pyridine-2-carboxylic acid (7) Yield (0.97 g, 97 %), m.p. 136–137 °C, H NMR
(300 MHz, DMSO-d6): d (ppm) 12.13 (br, s, 1H), 8.68 (dd, J = 4.6, 1.6 Hz, 1H),
8.04 (dt, J = 7.6, 1.0 Hz, 1H), 7.91 (dt, J = 4.0, 1.3 Hz, 1H), 7.58 (dd, J = 7.6,
4.0 Hz, 1H); 13C NMR (75 MHz, DMSO-d6): d(ppm) 166.2, 149.0, 137.8, 148.7,
127.9, 125.0. HRMS (ESI): m/z calcd for C6H5O2N: 123.0320, found:
122.0311[M-1]?.
1
Thiophene-2-carboxylic acid (8) Yield (0.92, 92 %), m.p. 129–130 °C. H NMR
(300 MHz, DMSO-d6): d (ppm) 12.09 (br, s, 1H), 7.88 (dd, J = 4.8, 1.2 Hz, 1H),
7.72 (dd, J = 3.6, 1.2 Hz, 1H), 7.21 (dd, J = 4.8, 3.7 Hz, 1H). 13C NMR
(75 MHz,DMSO-d6): d (ppm) 162.5, 134.1, 133.9, 132.9, 128.7. HRMS (ESI): m/z
calcd for C5H4O2S: 127.9932, found: 126.9927 [M-1]?.
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Y. A. Attia et al.
Furan-2-carboxylic acid (9) Yield (0.86, 86 %), m.p. 132–34 °C; 1H NMR
(300 MHz, DMSO-d6): d (ppm) 12.08 (br, s, 1H), 7.94 (dd, J = 3.6., 1.2 Hz, 1H),
7.21 (dd, J = 3.6, 1.2 Hz, 1H), 6.65 (dd, J = 1.2, 2.8 Hz, 1H); 13C NMR
(75 MHz,DMSO-d6): d (ppm) 159.8, 146.8, 144.3, 117.9, 112.6. HRMS (ESI): m/z
calcd for C5H4O3: 112.0160, found: 111.0157 [M-1]?.
Results and discussion
Through the co-precipitation synthesis process, reduction of graphene oxide and
loading of Ag nanoparticles and ZnO nanoparticles on two-dimensional graphene
sheets were achieved to form the visible photoresponsive Ag/ZnO/Graphene
nanocomposites for the photodegradation of organic dyes. The TEM and SEM
images of the obtained products are depicted in Fig. 1. The TEM image shows that
the as-prepared ZnO NPs has a uniform diameter of *86.4 1.8 nm (Fig. 1a).
Figure 1b shows the loading of ZnO on graphene sheets. An important decrease in
the particle size of ZnO nanoparticles is observed (36.1 1.4 nm). This change is
related to the presence of polar groups (–OH, –C(O)OH, –C(O)–, –O–) in the
starting graphene oxide material, that act as nucleation sites for the formation of
ZnO nanoparticles and favor the control of the ZnO growth. Ag/ZnO NPs are
homogeneously dispersed and anchored on the graphene sheets, as shown in TEM
image (Fig. 1c). The larger nanoparticles (*10–50 nm) correspond mainly to Ag
NPs due to their larger electron contrast and also to the larger molar content in the
Ag/ZnO/Graphene nanocomposite synthesis; on the other hand, ZnO NPs are very
small with an average particle size of 1.9 0.5 nm. The SEM image of GO
indicates the product has two-dimensional sheets structure (Fig. 1d). It could be
further confirmed by the corresponding SEM image of the composite (Fig. 1e, f),
considering the oxygen containing groups such as carboxyl in GO serve as
anchoring sites to direct coating process, that the Ag/ZnO NPs were homogeneously
formed on the graphene sheets and free from aggregation. The chemical
composition of Ag/ZnO/graphene was analyzed by an energy dispersive spectrom-
eter (EDS) spectrum via SEM (Fig. 1g) and elemental mapping (Fig. 1h–k). The
results show the peaks of C, O, Zn and Ag, confirming the sample is of high purity.
The absorbance spectra of the prepared samples are shown in Fig. 2. ZnO
nanoparticles showed a maximum absorbance at 298 nm that shifted to longer
wavelength with loading on graphene sheets to 348 nm, and with the addition of Ag
nanoparticles, shifted to 307 nm in addition to the plasmonic band of silver
nantoparticles that appears at 398 nm. In Ag/ZnO/graphene, two peaks are shown at
318 and 402 nm, corresponding to ZnO and Ag nanoparticles respectively.
The prepared Ag/ZnO/graphene nanocomposites mainly show diffraction peaks
as in Fig. 3, located at 38.23°, 44.42°, and 64.60° 2h, corresponding to the presence
of Ag (0) nanoparticles. Very low intensity peaks are observed in the angle range
where ZnO is observed, indicating probably very small particle sizes, low
crystallinity or low content. No evidence of initially formed Ag2O is observed
after the aging step at 150 °C, although it readily forms after addition of NaOH to
Ag (I) solutions. The broad band at 20°–25° 2h is related to rGO. By introducing Si
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Facile production of vitamin B3 and other heterocyclic…
Fig. 1 a TEM images of ZnO NPs, b ZnO/graphene nanocomposites, and c Ag/ZnO/graphene
nanocomposites respectively. d SEM images of graphene oxide, e Ag/ZnO/graphene nanocomposites,
and f Ag/ZnO/graphene-Si nanocomposites respectively. g EDS analysis of Ag/ZnO/graphene
nanocomposites, and h–k elemental mapping
from TBSCl or TMSCl to Ag/ZnO/graphene composites, other peaks located at
70.93°, 69.86°, 58.59°, 49.64°, 36.21° and 33.74° 2h corresponding to Si are
observed.
Photocatalytic activity towards degradation of methylene blue (MB) dye
The photocatalytic performance of the samples (Ag/ZnO/graphene and Ag/
ZnO/graphene-Si) was monitored by degradation of methylene blue dye (MB) in
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Y. A. Attia et al.
0.6
0.5
0.4
0.3
0.2
0.1
0.0
ZnO
ZnO/G
Ag/ZnO
Ag/ZnO/G
200
300
400
500
600
700
800
900
Wavelength/nm
Fig. 2 Absorption spectra of ZnO, ZnO/graphene, Ag/ZnO, and Ag/ZnO/graphene
5500
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
1800
1600
1400
1200
1000
800
38.23
Ag-ZnO-Graphene
a
b
38.380
Ag/ZnO/G-Si
33.740
36.210
600
58.590
44.43
64.740
69.860
70.940
64.59
49.640
400
200
0
0
20
40
60
80
20 25 30 35 40 45 50 55 60 65 70
2θ/degrees
2-Theta/degrees
Fig. 3 a The XRD patterns of Ag/ZnO/graphene composite, and b Ag/ZnO/G-Si composite
aqueous solution under low power UV and visible irradiation. The photodegradation
was followed by measuring UV–visible absorption spectra at different irradiation
times (Fig. 4).
As shown in Fig. 4, MB dye is photodegraded within 60 min with Ag/
ZnO/graphene photocatalyst under UV-light. However, only 10 min was enough for
complete degradation of MB with Ag/ZnO/graphene-Si as a new nanocatalyst.
However, under visible light, we can observe that loading of Ag/ZnO on graphene
sheets enhanced the photocatalytic activity for MB dye degradation, but with longer
irradiation time of 180 min, and only 30 min was enough for complete degradation
of MB under visible light with Ag/ZnO/graphene-Si composites. The photodegra-
dation rate order under UV and visible light irradiation of the samples is: Ag/
ZnO/graphene-Si[Ag/ZnO/graphene. These results indicate that the photocatalytic
performance can be enhanced by incorporation Ag/ZnO on graphene sheets and it
becomes highly effective by introducing Si to Ag/ZnO/graphene composite. The
enhanced photocatalytic performance suggests that the ternary composite (Ag/
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Facile production of vitamin B3 and other heterocyclic…
Fig. 4 Absorption spectra showing the MB dye photodegradiation using Ag/ZnO/graphene and Ag/
ZnO/graphene-Si composites under UV and visible irradiation
ZnO/graphene-Si) can be served as highly efficient photocatalyst for degradation of
organics in aquatic environmental.
The mechanisms of photodegradation of MB by Ag/ZnO/graphene under UV and
visible light have been proposed [16, 17, 25–29]. It can be summarized as follows:
when the photoreaction vessel (MB solution with Ag/ZnO/graphene powder) is
irradiated under UV/visible light, photoinduced electrons and holes are formed due
to the excitation of ZnO. Then, radicals can capture the electrons or holes, the
organic compounds will be degraded [29]. However, from our results, Si from
TBSCl or TMSCl further enhanced the photocatalytic activities of the doped Ag/
ZnO/graphene samples, because it effectively improves the charge separation
efficiency. In Ag/ZnO/graphene-Si composites, photoinduced electron due to
excitation of ZnO by visible light with narrow band gap of Ag/ZnO (3.24 to
3.03 eV) [16] can be transferred to graphene, silver NPs, Si and to the model
organic dye. Finally, the electrons generated react with dissolved oxygen molecules,
producing oxygen peroxide radicals. The hole can react with the hydroxide ion from
water to form hydroxyl radicals. These generated radicals from Ag/ZnO/graphene-
Si composite can cause the oxidative decomposition of dyes like MB [28]. Also,
when we start with graphene oxide and produce reduced GO, there are still some –
OH and –COOH groups in the graphene; in particular, more in the edges. Cl– can be
easily removed from TBSCl or TMSCl by these –OH groups forming C–O–Si
bonds, as shown in FT-IR spectra of reduced graphene oxide, Ag/Zn/graphene and
Ag/ZnO/graphene-Si composites (Fig. 5). The –OH group that appears in the
Fourier transform infrared spectroscopy (FT-IR) spectrum of Ag/ZnO/G at
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Y. A. Attia et al.
Fig. 5 FT-IR of reduced graphene oxide, Ag/ZnO/graphene and Ag/ZnO/graphene-Si hybrid
nanocomposites
3345 cm-1 disappeared in Ag/ZnO/G-Si and another band was observed at
1131 cm-1 corresponding to –Si–O–C– bond. Moreover, it is reported that TBSCl
or TMSCl are common protecting reagents for acids and alcohols [30–32]. This
protecting ability seems to make the rGO sheets more hydrophobic due to the alkyl
chains, but maybe it favors the adsorption of aromatic molecules on the rGO sheets,
explaining the increased efficiency.
Catalytic oxidation activity towards organic substrates: novel commercially
production method for vitamin B3
It is noteworthy that the Ag/ZnO/graphene catalyst has not been reported in the
oxidation of organic substrates. Our initial attempts are to optimize the reaction
conditions using 3-pyridinemethanol (1) as the model substrate (Scheme 1). When
Ag/ZnO/graphene (1 wt%) was employed for oxidation reaction of 3-pyridine-
methanol (1) at room temperature after 24 h, the starting material was recovered and
there was no conversion of alcohol 1 (entry 1, Table 1). Since our aim is to develop
affordable and relatively accessible methodology, performing the catalytic
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Facile production of vitamin B3 and other heterocyclic…
Scheme 1 Catalytic oxidation of 3-pyridinemethanol (1) to nicotinic acid 2 (vitamin B3)
Table 1 The concentration of TBSCl that mixing with Ag/ZnO/graphene versus the yield of nicotinic
acid (2)
Entry
Catalysta
Additives
Time of reaction
Yieldb (%)
1
2
3
4
5
6
7
a
Ag/ZnO/graphene
Ag/ZnO/graphene
Ag/ZnO/graphene
Ag/ZnO/graphene
Ag/ZnO/graphene
Ag/ZnO/graphene
Ag/ZnO/graphene
In dark
24 h
24 h
24 h
24 h
24 h
24 h
30 min
–
hm (360–700 nm)
TBSCl (1 mol%)
TBSCl (10 mol%)
TBSCl (0.25 eq.)
TBSCl (0.5 eq.)
TBSCl (1 eq.)
20
5
12
40
70
99
Ag/ZnO/graphene (1 wt%)
Isolated yield
b
oxidation reaction using visible light was developed. The reaction mixture that
consisted of 3-pyridinemethanol (1) and the catalyst (1 wt%) in methanol/water was
irradiated using a halogen lamp in the visible region (360–00 nm), but disappointing
yield (20 %) of nicotinic acid (2) after 24 h was observed (entry 2, Table 1). Hence,
we started to study the influence of adding different concentrations of TBSCl as a
precursor of a silicon substrate on the Ag/ZnO/graphene nanocomposite catalyst
(1 wt%) under normal conditions. In the absence of the catalyst, TMSCl or TBSCl
was initially added to a solution of 3-pyridinemethanol (1) dissolved in a mixture of
methanol/water with stirring at room temperature for 24 h, and no reaction was
observed. When one equivalent of TBSCl was added to the Ag/ZnO/graphene
nanocomposite catalyst (1 wt%), 5 % isolated yield of nicotinic acid (2) was
produced (entry 3, Table 1). By increasing the amount of TBSCl using 10 mol%,
acid 2 was obtained with 12 % isolated yield (entry 4, Table 1). However, adding
0.25 equivalent of TBSCl produced acid 2 in 40 % isolated yield (entry 5, Table 1)
after chromatographic purification. An improved catalytic activity of the catalyst
was observed by adding 0.5 equivalents of TBSCl. The acid (2) was formed in 70 %
isolated yield (entry 6, Table 1). Pleasingly, addition of one equivalent of TBSCl
resulted in full conversion of alcohol (1) to afford the corresponding acid (2) in
99 % isolated yield after 30 min at room temperature (entry 7, Table 1). However,
when one equivalent of trimethylsilyl chloride (TMSCl) was used for activation of
Ag/ZnO/graphene nanocomposite in the catalytic oxidation reaction of alcohol (1),
nicotinic acid (2) was only obtained in 74 % isolated yield. That might because the
TBS ether could be formed during the oxidation reaction, which had been proven
earlier to be more stable than TMS ether (ca *104 times) [33]. In addition, other
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Y. A. Attia et al.
studies confirmed our investigation by using TBS ether in selective oxidation
processes in combination with Bi(OTf)3 [34] and with CrO3 [35]. The catalytic
activity of graphene oxide (GO) or reduced graphene oxide (r-GO) was examined at
oxidation of 3-pyridinemethanol, but the starting material was recovered. Again,
when graphene oxide mixed with TBSCl, the alcohol (1) was recovered. However,
when graphene (G) was mixed with TBSCl, nicotinic acid (2) was formed in trace
amount with 10 % isolated yield after 24 h. Therefore, we concluded that the most
efficient catalytic system was Ag/ZnO/graphene-Si system.
Recyclability of the catalyst was tested for five cycles (see Table 2).
The first recycled catalyst (1 wt%) with one equivalent of TBSCl was employed
in oxidation of 3-pyridinemethanol (1) to afford acid (2) in 99 % isolated yield.
Nicotinic acid (2) was yielded in 98 % isolated yield in the second recycled batch.
However, in the third recycled batch, acid (2) was afforded in 98 % isolated yield.
The fourth and fifth recycled catalyst was employed in oxidation reaction to produce
97 and 95 % isolated yield. These results emphasize the ability to reuse the
developed catalytic oxidation system in this report for a great number ofcycles,
producing excellent yield in each recycle.
In order to show the merit of Ag/ZnO/G-Si hybrid nanocompasite with recently
reported protocols, some homogeneous and heterogeneous catalysts in the oxidation
of 3-pyridinemethanol (3-PM) and 3-picoline to afford vitamin B3 are presented in
Table 3.
From the results in Tables 2 and 3, we can say that the loading of Si on the Ag/
ZnO/graphene nanocomposite enhanced the oxidation formation of vitamin B3 from
3-pyridinemethanol (1). This procedure can be considered as a new production
method for vitamin B3 with an excellent yield (99 %) in a short reaction time.
Moreover, the catalyst is recyclable and reusable in further oxidation reactions.
Other heterocyclic alcohols were employed as substrates (scheme 2). 2-Pyr-
idinemethanol (3), 2-thiophenemethanol (4) and 2-furanmethanol (5) were oxidized
using Ag/ZnO/graphene nanocomposite catalyst (1 wt%) with one equivalent of
TBSCl to produce pyridine-2-carboxylic acid (7), thiophene-2-carboxylic acid (8)
and furan-2-carboxylic acid (9) in 97, 92, and 86 % isolated yield, respectively,
after chromatographic purification (Table 4). The 1H NMR for compound 2
revealed a broad peak at d = 12.09 ppm, which characterized for carboxylic acid
group. In addition, the 13C NMR showed a characteristic peak for (COOH) group at
166.5 ppm. The spectroscopic data for all prepared compounds were in accordance
with the assigned structures [36–38].
Table 2 Recyclability of the
Entry
Recycling
Time of reaction (min)
Yield (%)
used catalyst in the oxidative
reaction of 3-pyridinemethanol
(1) to nicotinic acid (2) (vitamin
B3)
1
2
3
4
5
1st
30
30
30
30
30
99
98
98
97
95
2nd
3rd
4th
5th
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Facile production of vitamin B3 and other heterocyclic…
123

Y. A. Attia et al.
Scheme 2 Catalytic oxidation of alcohol derivatives (3–5) to the corresponding carboxylic acids (7–9)
Table 4 Catalytic oxidation of heterocyclic alcohols to corresponding carboxylic acids
Substrates
Products
Reaction time (min)
30
Yield (%)a
97
60
60
92
86
a
Isolated yield after chromatographic purification
Scheme 3 Catalytic oxidation mechanism by Ag/ZnO/graphene-Si hybrid system
Hence, adding Si activates Ag/ZnO/graphene nanocomposite and then promotes
the spilling of oxygen over the surface of the catalyst under visible light due to the
formation of Ag/ZnO/graphene-Si hybrid catalyst. The contrasting catalytic
behaviors afforded by the Ag/ZnO/graphene nanocomposite catalyst with Si could
be represented in Scheme 3.
Conclusions
Herein, we report a green procedure for oxidation of 3-pyridinemethanol and other
heterocyclic alcohols using Ag/ZnO/graphene-Si nanocatalytic system without
using molecular oxygen or any source of oxidants. The catalyst selectively attacks
the hydroxyl group in the primary alcohol group in 3-pyridinemethanol without
affecting on the nitrogen group in pyridine ring to produce nicotinic acid (vitamin
123

Facile production of vitamin B3 and other heterocyclic…
B3) in quantitative yield. In order to generalize the oxidation procedure, other
heterocyclic alcohols such as 2-pyridinemethanol, 2-thiophenemethanol and
2-furanmethanol acid were subjected to the oxidation reaction condition to afford
their corresponding carboxylic acid derivatives in excellent yields. The hydroxyl
ions generated in situ due to presence of electrons and holes at the surface of the
catalyst (Ag/ZnO/graphene) after activation by TBSCl or TMSCl provide fast
reaction with selectivity. Herein, we investigated the catalytic property of Ag/
ZnO/graphene in oxidation of organic compounds to produce pharmaceutical
products with a simple procedure, which could be considered to be an environ-
mentally benign method. The new oxidation methodology is characterized by its
simplicity, ease to work up, works efficiency without adding oxidative species, and
eco-friendliness, and could be applied industrially.
Acknowledgments This work was supported by MINECO, Spain (MAT2012-36754-C02-01 and
MAT2015- 67458-P), Xunta de Galicia, Spain (GRC2013-044, FEDER Funds).
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