618
carbohydrate content [36–39]; (2) non-food values, so that they do not compete with
the food (3); and abundant availability for sustainable supply and rapid growth rate.
Since acidity plays a key role in biomass to HMF/furfural production, many
techniques have been used to establish a tool to measure acidity, but the accuracy of
the results relies solely on the measurement procedure [40, 41]. For example, the
results were observed to be sensitive to impurities when the UV-Vis probe was used
[41]. Iglesia and his co-workers have reported a computational method using ab
initio DFT calculation to measure deprotonation energy (DPE) of the heteropoly
acids. They have established a trend between the activity of various heteropoly acids
and their corresponding DPEs in acid-catalyzed reactions. These findings were consistent with the experimental observations, as reported in the activity-acidity plot
[42, 43]. Lower values of the DPE of heteropoly acid were suggested to show higher
acidic content, resulting in the high activity of the catalysts. Most recently, Alam
et al. have studied the role of Brønsted acidity of the ILs in HMF and EMF formation [23]. The study includes DPEs of SO 3 H-functionalized ILs containing different
cations (DMA
+
, NMP
+
, and BBIM-BuSO 3 H
+
); anions, e.g., [(CH 3 SO 3
−
) (OTh
−
)],
[(CF 3 SO 3
−
) (OTf
−
)], [(CH 3 SO 3 ) 2 N
−
(NTh 2
−
)], and [(CF 3 SO 3 ) 2 N
−
(NTf 2
−
)]; and their
corresponding activities in terms of HMF yield. Computationally measured values
of DPEs were observed to be linearly correlated to experimentally obtained
HMF yield.
3.1 Determination of HMF Yields
HMF and some of its derivatives can be analyzed through UV-Visible spectroscopy
[24, 44],
1
HNMR spectroscopy [8, 9, 24], HPLC [7], and GC [33]. For
1
H NMR
spectroscopic analysis, HMF was extracted from the reaction mixture with ethyl
acetate or dimethyl ether. Upon rotary evaporation, resultant HMF was obtained.
UV-Vis method: The UV-Vis spectrum of pure HMF solution shows a distinct
peak at 284 nm and the corresponding extinction coefficient (e) value of
1.66×10
4
M
−1
cm
−1
. The percentage of HMF in each reaction product can be calculated from the measured absorbance values at 284 nm wavelength and the extinction
coefficient value [24]. The yield of HMF was calculated in wt% concerning the total
mass of the starting substrates.
1
H NMR method: To quantify HMF yield by
1
H NMR method, a known concentration of mesitylene as internal standard and HMF product solution were charged
in a glass tube. The resultant mixture was solubilized in solvents like chloroform-d
or DMSO-d 6 . Remember that the solvent’s peak should not merge with the corresponding product or internal standard. The percentage of HMF yield was typically
measured by the integrated values of the aldehydic peak of HMF (d = 9.58 ppm) and
aromatic protons of mesitylene (d = 6.79 ppm) as reported by Saha and co-workers
[23, 24].
M. I. Alam
carbohydrate content [36–39]; (2) non-food values, so that they do not compete with
the food (3); and abundant availability for sustainable supply and rapid growth rate.
Since acidity plays a key role in biomass to HMF/furfural production, many
techniques have been used to establish a tool to measure acidity, but the accuracy of
the results relies solely on the measurement procedure [40, 41]. For example, the
results were observed to be sensitive to impurities when the UV-Vis probe was used
[41]. Iglesia and his co-workers have reported a computational method using ab
initio DFT calculation to measure deprotonation energy (DPE) of the heteropoly
acids. They have established a trend between the activity of various heteropoly acids
and their corresponding DPEs in acid-catalyzed reactions. These findings were consistent with the experimental observations, as reported in the activity-acidity plot
[42, 43]. Lower values of the DPE of heteropoly acid were suggested to show higher
acidic content, resulting in the high activity of the catalysts. Most recently, Alam
et al. have studied the role of Brønsted acidity of the ILs in HMF and EMF formation [23]. The study includes DPEs of SO 3 H-functionalized ILs containing different
cations (DMA
+
, NMP
+
, and BBIM-BuSO 3 H
+
); anions, e.g., [(CH 3 SO 3
−
) (OTh
−
)],
[(CF 3 SO 3
−
) (OTf
−
)], [(CH 3 SO 3 ) 2 N
−
(NTh 2
−
)], and [(CF 3 SO 3 ) 2 N
−
(NTf 2
−
)]; and their
corresponding activities in terms of HMF yield. Computationally measured values
of DPEs were observed to be linearly correlated to experimentally obtained
HMF yield.
3.1 Determination of HMF Yields
HMF and some of its derivatives can be analyzed through UV-Visible spectroscopy
[24, 44],
1
HNMR spectroscopy [8, 9, 24], HPLC [7], and GC [33]. For
1
H NMR
spectroscopic analysis, HMF was extracted from the reaction mixture with ethyl
acetate or dimethyl ether. Upon rotary evaporation, resultant HMF was obtained.
UV-Vis method: The UV-Vis spectrum of pure HMF solution shows a distinct
peak at 284 nm and the corresponding extinction coefficient (e) value of
1.66×10
4
M
−1
cm
−1
. The percentage of HMF in each reaction product can be calculated from the measured absorbance values at 284 nm wavelength and the extinction
coefficient value [24]. The yield of HMF was calculated in wt% concerning the total
mass of the starting substrates.
1
H NMR method: To quantify HMF yield by
1
H NMR method, a known concentration of mesitylene as internal standard and HMF product solution were charged
in a glass tube. The resultant mixture was solubilized in solvents like chloroform-d
or DMSO-d 6 . Remember that the solvent’s peak should not merge with the corresponding product or internal standard. The percentage of HMF yield was typically
measured by the integrated values of the aldehydic peak of HMF (d = 9.58 ppm) and
aromatic protons of mesitylene (d = 6.79 ppm) as reported by Saha and co-workers
[23, 24].
M. I. Alam