Investigation of Nanofiber Breakup in the Melt-Blowing Process

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Investigation of Nanofiber Breakup in the Melt-Blowing Process Wanli Han,*,† Gajanan S. Bhat,‡ and Xinhou Wang§,∥ †

Materials and Textile Engineering College, Jiaxing University, Jiaxing 314001, Zhejiang Province, China Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996-2200, United States § College of Textiles and ∥Key Laboratory of Textile Science & Technology, Ministry of Education, Donghua University, Shanghai 201620, China ‡

ABSTRACT: Nanofibers definitely hold a great advantage and can be applied in many areas because they have very high specific surface area. Recently, great attention is being paid to fabricate nanofiber nonwoven webs using melt-blowing technology. In this paper, melt-blown nanofibers have been produced by multihole dies using two kinds of commercial polymers under different processing conditions. The average nanofiber diameters achieved were 780 and 810 nm, respectively. The fiber breakup, which was driven by surface tension when fiber diameters approached nanoscale, was investigated. The Rayleigh instability theory for a melt-blowing fiber was introduced and illustrated as the reason for fiber breakup. Both theory and experimental results reveal that the surface tension, polymer viscosity, fiber diameter, and melt-blowing-process conditions, such as air pressure and temperature, significantly influence the fiber breakup. This research gives a useful understanding for the formation of melt-blown nanofibers and provides a general understanding of the limitations for conditions to achieve nanofibers in a commercial melt-blowing process. Unfortunately, only one image of such small fibers was shown, and there were no details for fiber-diameter distribution or the process conditions. After that, many significant efforts have been made to understand this technology and to improve the melt-blowing equipment. Some of the researches on the meltblowing process focused on the coat-hanger die distributor design,8,9 air drawing of the polymer jet,10,11 the performance of the jet slot dies,12 the fiber movement,13 and the prediction of the fiber diameter.14 However, in their research, melt-blown webs were composed of fibers with average diameters exceeding 5 μm, and no fiber breakup was reported. More recently, Khan et al.15 have studied the fabrication of nanofiber melt-blown nonwovens and their filtration properties. They investigated the characteristics of nano-melt-blown fibrous membranes produced using three different Hills dies and different process conditions. They reported that when the fiber size was reduced below 1 μm, higher filtration quality was achieved at lower basis weights relative to those of conventional melt-blown webs. These results showed significant promise for the use of nano-melt-blown fibers in filtration applications. However, they did not investigate the mechanism of fiber formation in melt blowing. Nayak et al.16,17 also obtained nanowebs by the melt-blowing process with the injection of different fluids (such as air and water) at the vent port of

1. INTRODUCTION Nanofibers, because of their extremely high surface-to-weight ratio, have a wide range of applications such as in filtration, medical, tissue engineering, and wound dressing.1−3 Typically, nanofibers are produced using electrospinning, which produces fiber diameters in the range of 100−500 nm. However, the electrospinning process is inherently slow because of the common requirement of removing residual solvent, and it is very difficult to increase the fiber production rate using this method. There is increasing interest in developing highthroughput fiber-spinning methods to produce nanofibers. Melt blowing is one of the most popular processes to make superfine fibers on the micron or submicron scale. In the meltblowing process, a thermoplastic polymer is extruded through a die and is rapidly attenuated by the hot air stream to finediameter fibers.4 The attenuated fibers are then deposited on a collector screen to form a fine-fibered, self-bonded web. The combination of fiber entanglement and fiber-to-fiber bonding provides enough web cohesion that the web can be used without further bonding. Melt-blown fibers generally have diameters in the range of 2−5 μm. Lately, the focus has been on obtaining nanoscale fiber diameters under the commercial meltblowing-process conditions.5,6 If melt-blowing technology can be extended to nanoscale fiber sizes, this will provide a much easier, faster, and cheaper method to produce nanofibers, unlike the electrospinning process. The melt-blowing process is based on Wente’s original work published in 1956, in which the nanometer-range melt-blown fibers with fiber diameters as small as 500 nm were reported.7 © 2016 American Chemical Society

Received: Revised: Accepted: Published: 3150

November 25, 2015 February 27, 2016 March 2, 2016 March 3, 2016 DOI: 10.1021/acs.iecr.5b04472 Ind. Eng. Chem. Res. 2016, 55, 3150−3156

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Industrial & Engineering Chemistry Research commercial melt-blowing equipment using polypropylene. The properties of nano-melt-blown fibrous membranes were studied, and the results of this research showed the feasibility of fabricating nanofibers by melt blowing, which can close the gap between melt-blowing and electrospinning processes. In addition, Bates et al.18 studied the nanoscale melt-blown fibers using a single-orifice melt-blowing apparatus and analyzed the fiber diameter and diameter distribution. They observed the fiber breakup, which was driven by surface tension, and these instabilities represented the onset of an underlying limit to the process. The research illustrated that the polymer viscosity and some melt-blowing process conditions could influence the fiber breakup. However, in their experiment, a single-hole meltblowing apparatus was used, and it is significantly different from the multihole die used in a commercial production setup because of the fiber movement during the melt-blowing process. Besides, they also did not explain the theoretical reasons for the fiber breakup. A fine polymer jet could be unstable and break up into a series of small droplets via Rayleigh instability under melt-blowing circumstances. This phenomenon may present a fundamental limit on the smallest fibers achievable by melt blowing and needs to be further investigated. Given all that, in this paper, we are interested in melt-blowing nanofiber breakup based on the Rayleigh instability theory and explain the reasons for the fiber breakup. Furthermore, we also studied the influence of processing conditions for the fiber breakup, wherein webs were produced with a multihole die using commercially practiced processing conditions.

Figure 1. Melt-blowing fiber-drawing model: (a) schematic diagram of the melt-blowing process; (b) air-stream action on the polymer.

is kept low because of the higher melt temperature and the hot air blowing as soon as it comes out of the orifice. Thus, it may be assumed that the viscosity changes are small within the region of interest, although the viscosity increases tremendously afterward. We assume that the fiber speed V(z) is independent of fiber diameter r(z). From Bernoulli’s theory at points A and B: 1 1 ρV0 2 + ρgz + PA = ρV 2(z) + PB (1) 2 2 The polymer pressures within the jet at points A and B can simply be related to that of the ambient, P0: σ σ PA ≈ P0 = , PB ≈ P0 + (2) a r

2. FIBER BREAKUP THEORY The study of unstable liquid jets dates back to Rayleigh’s pioneering work.19,20 In his research, only axisymmetric disturbances can grow and dominate the breakup of a jet and a small disturbance can be magnified exponentially with time. This phenomenon, called Rayleigh instability, was later studied by numerous researchers, both theoretically and experimentally. Tomotika21 investigated the effect of the viscosity of the surrounding fluid and showed an optimal ratio of the viscosities of the jet. Weber22 implemented the study of a viscous liquid jet, which gave more details for the instability of a Newtonian liquid jet. Many additional phenomena have been investigated, such as the cascade structure in a drop falling from a faucet,23 steady capillary jets of submicrometer diameters,24 and generation of structured spheres.25 Besides, the application of instability for fiber breakup is also broadly investigated in fiber spinning. In these research papers,26,27 the flow inside the filament was purely extensional. Polymer fiber is stretched continuously and would be unstable and break up into series or arrays of small droplets via Rayleigh instability.28 However, the melt-blowing process is different from melt spinning. The primary difference between melt blowing and conventional melt spinning is that a draw roll, rather than a gas stream, provides the attenuating force in melt spinning. In the meltblowing process, high-velocity gas is heated approximately to the polymer temperature and attenuates the molten stream of polymer into a superfine fiber, as shown in Figure 1. From Figure 1a, one part of the fiber polymer is taken as the control volume for analysis, as shown in Figure 1b. Considering a circular fiber with volumetric flow Q, polymer density ρ, and velocity v, with high-velocity air drawing, the polymer is influenced by both the drawing force acceleration and the surface tension σ. In the melt-blowing process viscosity

Then 1 σ 1 σ ρV0 2 = ρgz + P0 + = ρV 2(z) + P0 + 2 a 2 r

(3)

So 1/2 2gz V (z ) ⎡ a ⎞⎟⎤ 2σ ⎛⎜ ⎥ 1− = ⎢1 + 2 + V0 r ⎠⎦ V0 ρV0 2a ⎝ ⎣

(4)

From volumetric-flow conservation: Q = 2π

∫0

r

V (z) r(z) dr =πa 2V0 = πr 2V (z)

(5)

1/4 ⎡ V (z) ⎤1/2 ⎡ 2gz a a ⎞⎟⎤ 2σ ⎛⎜ ⎥ 1− =⎢ ⎥ = ⎢1 + 2 + r(z) ⎣ V0 ⎦ r ⎠⎦ V0 ρV0 2a ⎝ ⎣

(6)

From analysis of eq 6, we can infer that the surface tension force increases dramatically with decreasing fiber diameter in the melt-blowing process. When the fiber diameter reaches the nanoscale range, the surface tension force is so large that it should be taken into consideration. From Figure 1, it is obvious that the high-velocity air stream is around the polymer melt jet and impinges on the polymer melt. Because the polymer fiber in the blowing process is a free end without a draw roll, there are perturbations for the polymer jet.29 The stream of the fiber polymer can become unstable for a small undulation of its surface. In Xie and Zeng’s study, fiber motion was obtained by analysis of the high-speed photographs and showed that the fiber motion amplitude could jump to maximun lateral displacements.30 This can lead to an increase in the surface 3151

DOI: 10.1021/acs.iecr.5b04472 Ind. Eng. Chem. Res. 2016, 55, 3150−3156

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Industrial & Engineering Chemistry Research tension to a critical value, and the stream of the polymer fiber becomes unstable and breaks up into a series of small droplets under these melt-blowing process conditions. Figure 2 shows that there are some droplets intermingled among the fibers. Bates et al.18 emphasized that these droplets

because ε is small and linear is appropriate. When the order ε2 terms are neglected, the following equations can be obtained:

∂ur̃ 1 ∂p ̃ =− ∂t ρ ∂r

(11)

∂uz̃ 1 ∂p ̃ =− ∂t ρ ∂z

(12)

The continuity equation for cylindrical coordinates is ∼

∂ur̃ u + r + uz̃ = 0 ∂r r

(13)

We defined the disturbance of the velocity. If the disturbance of the pressure is similar to the disturbance of the fiber diameter, then Figure 2. Melt-blown fibers and the Rayleigh instability model.

ur̃ = R(r ) e ωt + ikz p ̃ = P(r ) e ωt + ikz

were different from the so-called “shot” formation in melt blowing because shot refers to larger particles of the polymer (>several tens of microns in size) in the fiber mat. These observed phenomena are a result of fiber breakup and are driven by instabilities caused by the surface tension. These surface-tension-driven instabilities are termed Rayleigh instabilities. Rayleigh19,20 first gave a mathematical equilibrium, and according to his theory, the fiber has cylindrical symmetry, the Rayleigh instability arises at the interface of the fiber, and the tension increases with decreasing viscosity and identifies that the surface tension and temperature act on the jet breakup. The perturbation period (wavelength) is λ = 2 π/k. Lee et al.31 have given the perturbation, a variation pressure equation for the difference across a curved interface, as Ap − A0 =

4π 2r0 b2 2 2 (k r0 − 1) k 4r0 2

r2

R(x) = CI1(kr ) + C1K1(kr )

(17)

Here, C is a constant, which is determined by the boundary conditions. The initial phase of the fiber diameter is R0, the disturbance amplitude is ε, the perturbation growth rate is ω, and the diameter of the disturbance of the fiber is εω. Then, eq 17 can be written as εω C= I1(kR 0) (18) Figure 3a is the steady-state fiber, the initial fiber diameter is R0, the density is ρ, and the surface tension is σ. When the inertia

where R0 is the steady-state fiber diameter, ε is the disturbance amplitude, ω is the perturbation growth rate, and k is the disturbance wavenumber. We supposed that the disturbance velocities along the radial and axial components of the fiber are ur̃ anduz̃ , respectively. The perturbation pressure is p̃. Assuming axisysmmetry of the fiber and negligible viscous and body forces, based on the cylindrical coordinates of Stokes equation, the following formula can be obtained:

(10)

(16)

R(r ) = CI1(kr )

(8)

∂u ̃ ∂u ̃ ∂uz̃ 1 ∂p ̃ + ur z + uz z = − ∂z ∂r ∂t ρ ∂z

(15)

where C and C1 are the coefficients of a related solution and I1 and K1 are independent functions. According to the definition of Bessel’s equation, K1(kr) is divergent when the fiber diameter size is nanoscale (r → 0) and C1 can be inferred as zero. Therefore,

∼

(9)

d2R dR +r − [1 + (kr )2 ]R = 0 dr dr 2

Equation 15 is Bessel’s equation, and the solution of the equation can be written as

(7)

∂u ̃ ∂u ̃ ∂ur̃ 1 ∂p ̃ + ur r + uz r = − ∂z ∂r ∂t ρ ∂r

(14)

When eq 14 is plugged into eqs 11, 12 and 13 and z and p are eliminated, the differential equation for R(r) is

where Ap is the perturbed area over the drawing period, A0 is the unpertrubed area, r is the fiber diameter, and b is the amplitude of the wave at time t. If kr0 > 1, the surface energy increases and the perturbation will tend to die out. If kr0 < 1, the perturbation will tend to grow. At the beginning, the varicose disturbance is infinitesimal (ε ≪ r0). Under the disturbed-state conditions, the fiber diameter R̃ can be calculated by the following formula: R = R 0 + εe ωt + ikz

uz̃ = Z(r ) e ωt + ikz

Figure 3. Steady- and unsteady-state fiber models.

θ=0 3152

DOI: 10.1021/acs.iecr.5b04472 Ind. Eng. Chem. Res. 2016, 55, 3150−3156

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Industrial & Engineering Chemistry Research force g is ignored, the internal pressure P is a constant based on the relationship between the surface tension and normal stress. Then σ p0 = σ ∇·n ⇒ p0 = R0 (19)

Figure 4. The melt-blown web samples were produced using two different dies: a Hills die that has 365 holes with a hole density of 24 holes/cm and a AGR die that has 213 capillaries with a hole density of 14 holes/cm. For both dies, the die tip is sharp and the angle between each of the air slots and the face of the die is 60°. The orifice diameter of the Hills die is 0.01778 mm, and the orifice diameter of the AGR die is 0.1254 mm. Both of them have smaller diameters and high L/D ratios that generate high pressure drops at low polymer throughput and hence increase the flow uniformity. A schematic illustration of the die is shown in Figure 5. In this process, PP is fed to the extruder through the hopper. The motor-driven screw pushes the material through the extruder, while five independently controlled heaters melt the polymer to the desired temperatures. The heating temperatures of the five zones were 175, 190, 200, 230, and 230, respectively. The speed of the motor that turns the screw is controlled by a dial located on the control panel. A digital readout of the motor speed is displayed on the control panel. The desired flow rate of polymer exiting through the die was achieved by controlling the motor speed and by monitoring the readout of the melt pressure sensor. A Dynisco pressure sensor measured the pressure that the polymer was exerting on the tube walls (melt pressure) between zones 4 and 5, and the readout from the sensor was used to monitor and ensure that a constant polymer flow rate (throughout) was being supplied to the die while the samples were being collected. 3.3. Characterization. PP melt-blown webs prepared from two different polymers were sectioned from melt-blown fiber mats and imaged using scanning electron microscopy (SEM; ETEC Autoscan) at 12 keV after coating with a gold layer. In the SEM photographs obtained at high magnification, one can see submicron fibers and observe the fibers’ morphologies.

Figure 3b is the unsteady-state fiber, and the pressure P of the internal fiber is ⎛1 1 ⎞ p0 + p ̃ = σ ∇·n = σ ⎜ + ⎟ R2 ⎠ ⎝ R1

(20)

R1 and R2 are the principal radii of curvature for the internal and external circular arcs, respectively. They can be written as ε ωt + ikz 1 1 1 = ≈ − e 2 ωt + ikz R1 R R R 0 + εe 0 0

(21)

1 = εk 2e wt + ikz R2

(22)

Therefore, p0 + p ̃ =

σ εσ − (1 − k 2R 0 2)e ωt + ikz 2 R0 R0

(23)

Then p̃ = −

εσ (1 − k 2R 0 2)e ωt + ikz R 02

(24)

At last, the perturbation growth can be obtained ω2 = σ

k I1(kR 0) (1 − k 2R 0 2) ρR 0 2 I1(kr )

(25)

From eq 25, we can see that the perturbation growth rate ω is influenced by the surface tension and fiber diameter. The disturbance of the fiber surface curvature increases with increasing pressure gradient. From eq 24, the pressure gradient in the fiber increases with increasing perturbation growth rate. The surface tension force of the fiber increases dramatically with decreasing fiber diameter. It is closely related with the fiber diameter and can be influenced by the polymer viscosity and melt-blowing-process conditions, such as the air pressure, air temperature, and so on. It also generates the perturbation for the fiber in the melt-blowing process. With increasing disturbance, the fiber can break up and form a droplet.

4. RESULTS AND DISCUSSION Figure 6 shows that, for the average fiber diameters, which have been generated for PP under the same melt-blowing conditions with different dies (the air temperature is 260 °C, the air pressure is 0.1 MPa, the throughout is 0.025g/h·min, the air gap is 0.76 mm, and the die-to-collector distance is 25 cm). From the statistical analysis of the fiber diameter histograms, it is clearly evident that the average fiber diameter decreases to a nanometer scale with both of the multihole dies under commercially viable melt-blowing-process conditions. The fiber diameters are 0.78 and 0.81 μm for the AGR and Hills dies, respectively. The fiber diameter size is nearly a symmetric distribution and the same with Shambaugh’s study.32 From the nanofiber morphology figures, it should be noted that there are many droplets on melt-blown webs, especially the fiber diameter decreasing nanoscale meters. As mentioned previously, these droplets were different from the “shot” formation in melt blowing. “Shot” refers to larger particles of the polymer (greater than several tens of microns in size) in the fiber mat, which is formed by the elastic “snapback” of the fiber ends upon breaking or the excess volume of the polymer melt.33 We believe that these observed phenomena are a result of fiber breakup, which is driven by instabilities caused by the surface tension. The instability results in disturbances that generate pinched and bulged sections on the surface of the polymer jet. From eq 25, with the pressure gradient conditions, the pinched areas can rupture when the disturbance amplitude is similar to the fiber diameter scale.

3. EXPERIMENTAL SECTION 3.1. Materials. Two different commerical polypropylene (PP) resins were used in this melt-blowing study. The physical properties of the resins are shown in Table 1. 3.2. Processing. Melt blowing was performed on the 15.2cm (6 in.)-wide melt-blowing pilot line at the University of Tennessee Nonwovens Reasearch Labotatory, Knoxville, TN. A schematic diagram of the melt-blowing process is shown in Table 1. Physical Properties of PP Used for Melt Blowing company

name

ExxonMobil

PP6936G

Basell

MF560Y

features narrow MWD narrow MWD

morphology

MI (g/10 min)

white granular

1550

white microspheres

1800

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DOI: 10.1021/acs.iecr.5b04472 Ind. Eng. Chem. Res. 2016, 55, 3150−3156

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Figure 4. Schematic diagram of the melt-blowing pilot line.

for the MF560Y polymer. This is different from Figure 6 because there are fewer breakups in the PP6936G polymer melt-blown web under 0.103 MPa air pressure and 260 °C. When the air pressure decreases to 0.034 MPa, no fiber breakup is observed in the web. This observed difference is due to the polymer viscosity. The MFR of the PP6936G polymer is 1550 g/10 min, while the MFR of the MF560Y polymer is 1800 g/10 min. This means that the viscosity of the MF560Y polymer is lower than that of the PP6936G polymer. When the viscosity is lower, the flow property of the polymer is better and the polymer jet is easier to attetuate. The instability can appear with increasing surface tension of the polymer. At the same time, the airflow velocity is modified with the fiber motion, and the instability on the surface of air jet is easier transferred and increases dramatically for the lower viscosity of the polymer. It is easy to achieve fiber breakup with Rayleigh instability. This inference is the same as that from the results of Bate et al.’s14 study and is consistent with eq 25. Besides, we also studied the influence of different temperatures on the fiber breakup. The MF560Y polymer was chosen to study under 260 and 232 °C. The melt-blown fiber morphology is shown in Figure 9. The results indicate that the temperature can also influence the fiber breakup. Under the 260 °C air temperature, some breakups appear on the fiber surface. When the temperature decreases to 232 °C, there are no fiber breakups. The reason for these observations also can be attributed to the polymer viscosity and fiber diameter. With lower air temperature, the fibers have larger diameter under the same melt-blowing-process conditions. Besides, the polymer viscosity is much higher at lower processing temperature. Both of these factors reduce the fiber breakup. It is evident from Figure 8 that the fiber breakup disappears under lower surface tension and higher viscosity at 232 °C air temperature.

Figure 5. Schematic illustration of the sectional and end-on views of the melt-blowing dies: (a) Hills die; (b) AGR die.

Figure 7 shows the nanofiber morphology under different air pressures for the MF560Y polymer. It is clearly seen that there are many droplets on melt-blown fibers when the air pressure is 0.103 MPa at 260 °C air temperature. The role of higher air pressure is to decrease the fiber diameter. Because the fiberforming polymer is extruded from the die, two jets of hot air rapidly attenuate the molten polymer into ultrafine fibers. During this process, the fiber undergoes bending instability under high air pressure and air temperature conditions. With decreasing fiber diameter, especially when the diameter reaches nanoscale dimensions, the surface tension will increase significantly. Therefore, the droplets are formed due to the instability conditions and surface tension. From eq 25, we can see that perturbation will tend to grow with decreasing fiber diameter. When the air pressure decreases to 0.034 MPa, the fiber breakup also decreases, as is evident from Figure 6a. These results strongly suggest that the fiber diameter and air pressure are important factors in affecting the fiber breakup. Figure 8 shows the PP6936G polymer melt-blown fiber morphology under the same process conditions as those used

Figure 6. Melt-blown nanofiber morphology and diameter distribution. 3154

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Figure 7. SEM photographs of MF560Y webs at 260 °C at different air pressures: (a) 0.034 MPa; (b) 0.103 MPa.

Figure 8. SEM photographs of PP6936G webs melt-blown at 260 °C: (a) air pressure 0.034 MPa; (b) air pressure 0.103 MPa.

Figure 9. SEM photographs of the MF560Y polymer at 0.034 MPa air pressure: (a) air temperature 260 °C; (b) air temperature 232 °C.

5. CONCLUSIONS In this research, we investigated the melt-blowing nanofiber breakup based on the Rayleigh instability theory. The theory showed that the polymer surface tension could increase dramatically with decreasing fiber diameter and influence the fiber breakup significantly. The polymer melt viscosity also played an important role in the fiber breakup and was affected by air temperature. Higher melt viscosity could reduce the fiber breakup. Meanwhile, the air presssure and air temperature, which influenced the fiber diameter, could be used to avoid fiber breakup during the melt-blowing process. This study presented a way to find the limit on the smallest fiber diameter achievable by melt blowing under specific conditions, thus allowing selection of the appropriate processing conditions for successful melt blowing to achieve nanofiber webs.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support provided by the Natural Science Foundation of China (Grant 51506075) and Jiaxing City Bureau of Technology (Grant 2015AY11025). Assistance from Stephen Sheriff in the sample preparation and from Lyondell-Basell and ExxonMobil Chemical Companies for providing the resins is appreciated.

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NOMENCLATURE a = fiber cross-sectional area at point A, m2 A0 = fiber unpertrubed area, m2 Ap = fiber perturbed area, m2 b = amplitude of the wave, m g = gravitational acceleration, m/s2 k = disturbance wavenumber PA = polymer pressure at point A

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 3155

DOI: 10.1021/acs.iecr.5b04472 Ind. Eng. Chem. Res. 2016, 55, 3150−3156

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Industrial & Engineering Chemistry Research PB = polymer pressure at point B P0 = ambient pressure, atm p̃ = perturbation pressure, atm Q = volumetric flow, m3/s r = fiber diameter, m r0 = steady-state fiber diameter, m R0, R1, R2 = fiber diameter; see Figure 3 R̃ = diameter for unsteady-state fiber, m t = time, s z = Cartesian coordinate; see Figures 1 and 2 ur̃ = disturbance velocity along the radial component of the fiber, m/s uz̃ = disturbance velocity along the axial component of the fiber, m/s V = fiber velocity, m/s Greek Letters

ρ = polymer density, g/cm3 σ = surface tension, mN/m ε = disturbance amplitude, m ω = perturbation growth rate, m λ = wavelength, m

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DOI: 10.1021/acs.iecr.5b04472 Ind. Eng. Chem. Res. 2016, 55, 3150−3156