Ultrasound assisted ZnO coating in a microflow based photoreactor for selective oxidation of benzyl alcohol to benzaldehyde

Vaishakh Nair *, Juan Carlos Colmenares * and Dmytro Lisovytskiy
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland. E-mail: vnair@ichf.edu.pl; jcarloscolmenares@ichf.edu.pl

Received 6th October 2018 , Accepted 14th November 2018

First published on 14th November 2018


Heterogeneous photocatalysis in a microflow system for generation of value added chemicals is a novel green chemistry approach requiring the understanding of photocatalysis, microfluidics and reactor design. Research on the development of a low energy and environmentally friendly based photo-microreactor system for photocatalysis is yet to be explored. Commercial ZnO nanoparticles were deposited in the inner wall of a fluoropolymer using a low energy ultrasound bath under mild conditions and later used for selective conversion of aromatic alcohols to aldehydes. The deposition of the nanoparticles on the fluoropolymer is mainly attributed to the physical changes taking place inside the microtubes under the effect of ultrasound.


Green chemistry based research involving the generation of value added chemicals from biomass using efficient photocatalytic reactors requires strategic planning and implementation using the concepts of photocatalysis and chemical engineering. The chemical compounds obtained from biomass especially lignin based molecules are of high value for the pharmaceutical and fine chemical producing industries.1,2 Similarly photocatalysis based studies for selective generation of chemicals are a growing research field considering the fact that these reaction systems can be operated using the renewable solar light.3,4 Among the different types of catalyst based photochemical reactions, homogeneous based photocatalysis has been extensively studied in microfluidic based flow systems for selective organic synthesis.5–7 Moreover, microflow based photoreactors have shown to be an efficient catalytic setup compared to the traditional batch systems.6 In such miniaturized reaction systems, intensified mass and heat transfer, improved mixing and efficient reaction control lead to improved reaction selectivity and higher throughput per unit volume.5 Based on these advantages a number of articles on the mineralization of harmful organic compounds have been published based on heterogeneous photocatalytic reactions in microflow systems.8–11 Research related to selective organic synthesis in heterogeneous photocatalysis in a microfluidic based flow is in an early stage as compared to traditional heterogeneous photocatalytic batch systems.11–14 In a photocatalytic-microreactor the configuration of the photocatalyst in the microreactor along with the light source is a key feature which differentiates it from non-photocatalytic microreactor configurations. In addition, other general parameters like the type of material for the photo-microreactor, the catalyst properties, the reactant–catalyst interaction and the type of light source also decide the reactant conversion and product selectivity.15,16

In general, deposition of nano-sized particles inside a microtube or microchannel can be carried out through three routes, namely packed-bed, monolithic and wall-coated techniques.17 Wall coating of nanoparticles in the inner walls of photo-microreactors is preferred for photocatalytic reactions for the maximum exposure of the catalyst to the irradiating light as well as eliminating the need to separate the photocatalyst from the reaction solution.17 Fluorinated ethylene propylene (FEP) is a fluoropolymer known to have high light transmission to both UV and visible light, flexibility, easy fabrication, and low cost hence making it an ideal material for photo-microreactor fabrication.6,7 Even though the fluoropolymer has superior chemical stability it shows poor adhesion, making traditional coating processes such as sol–gel ineffective.18,19 The feasibility of using photocatalyst immobilized fluoropolymer microtubes as the photocatalytic microreactor was investigated by Ramos et al. by loading TiO2 suspension into the tubes, followed by quick thermal treatment at 285 °C.20 However, the excessive thermal treatment adversely affected the important mechanical properties of the tubes such as flexibility, transparency and mechanical integrity of the channel shape and diameter.

Sonochemical processes have been successfully used for the synthesis of various semiconductor based nanoparticles in batch reactions which have proved to be highly efficient in terms of selectivity, reaction time, and operational simplicity.21–23 Recently, Colmenares and co-workers were able to demonstrate for the first time ultrasound aided deposition of commercial TiO2 nanoparticles in FEP based microtubes using a probe type ultrasonic system.24 The method is simple to implement and environmentally friendly with low heat generation and has been filed for a patent.25 However, the experiments were carried out for degradation of phenol using a high energy 125 W UV mercury lamp. In addition, during the usage of the probe type ultrasonic system, a rise in temperature to 80 °C was observed at the end of the deposition.24 As a next step, different low energy based ultrasonic sources for deposition of different semiconductor photocatalysts in fluoropolymers have to be used. In addition, high energy consuming and large configuration type lamp sources should be replaced by efficient and convenient light emitting diodes (LEDs) for photocatalytic reactions. A semiconductor UV-LED based light source is a viable alternative to conventional UV lamps, due to its nontoxic nature and narrow emission spectrum.26 As stated before heterogeneous photocatalysis based on a microfluidic flow system for selective generation of value added compounds is still in the early stage. However such research initiatives are stated to revolutionize the prospects of introducing bio-chemical processes for conversion of biomass using low energy consuming flow based photo-microreactors.

Benzyl alcohol (BnOH) is a well-known lignin based model compound which can be used as a source for generation of benzaldehyde (BnAld), which is an important intermediate for the synthesis of other organic compounds.27–29 A number of non-photocatalytic oxidation reactions of BnOH in a microflow system for selective production of BnAld have been reported.30–33 A packed-bed microreactor using TEMPO catalyst immobilized AMBERZYME® oxirane resin was used to oxidize different aromatic alcohols in a biphasic mixture.30 The phases were combined at a Y-junction and passed through a 60 cm channel packed with 300 mg of catalyst submerged in an ice bath. After 9 hours of reaction 81% conversion of BnOH and 81% BnAld selectivity were obtained. Similarly, Zhu et al. prepared BnAld by Swern oxidation of BnOH which was carried out in a continuous flow using dimethyl sulfoxide (Me2SO) as the oxidizing agent and oxalyl chloride as the activating agent.31 In a non-heterogeneous catalytic microflow system the reaction was carried out at 5 °C with a residence time of reactants in microreactors of milliseconds. In another work, a Teflon AF-2400 tube-in-tube microreactor was investigated for the continuous, solvent-free, catalytic oxidation of BnOH with oxygen.32 The highest conversion of BnOH obtained for the range of conditions investigated was 44.1%, with 73.0% selectivity to BnAld, at 120 °C with 20 mg of 1 wt% Au–Pd/TiO2 catalyst particles packed inside a 30 cm Teflon tube. Similarly, Cao et al. used 20 mg of TPAP catalyst which was prepared by impregnating on active acidic aluminum oxide in silicon-glass microchannel reactors.33 The reaction was carried out at 75 °C and a conversion of 30–40% was obtained with residence times below 1 min.

The current work for the first time establishes a novel low energy ultrasound based deposition method for coating of commercial ZnO nanoparticles in the inner walls of FEP microtubes knowing that ZnO is an important material for the catalysis and photocatalysis fields.34 Additionally, this work reports for the first time the selective oxidation of BnOH to BnAld in a photocatalytic microreactor under UV-LED as the light source. In the current work, the ultrasonic power and the deposition time during the catalyst deposition process were optimized for getting the highest amount of catalyst deposition inside the FEP microtubes. In the initial stage of deposition studies, the FEP microtubes were washed with Milli-Q water and later dried in an oven. The ZnO nanoparticles suspended in 20 mL of aqueous medium were injected into the cleaned FEP microtube at different rates and ultrasound powers of the ultrasonic bath. The optimum amount of ZnO nanoparticles to be pumped inside the microtube was based on the initial batch photoreactor experiments (Fig. S1) for benzyl alcohol conversion while also considering the fact that using a higher amount of ZnO could lead to blockage in the microtube. After carrying out deposition of ZnO as shown in Fig. 1, the nanoparticle coated FEP microtubes were dried at 100 °C in a hot air oven for 6 h. After drying, the ZnO coated FEP microtubes were flushed with Milli-Q water to remove loosely attached ZnO nanoparticles and later dried again. All the ZnO modified FEP microtubes were further used for photocatalytic studies for selective conversion of BnOH.


image file: c8gc03131b-f1.tif
Fig. 1 ZnO deposition in the inner wall of the FEP microtube; (1) syringe infusion pump, (2) ultrasonic bath, (3) Teflon tube, (4) cooling water circulation, (5) FEP microtube, (6) Teflon tube and (7) receiver.

In the photocatalytic experiments, 1 mM BnOH in different solvents like acetonitrile (ACN) and Milli-Q water was pumped in the photocatalytic-microreactor at a flow rate of 0.053 mL min−1 which was established from the batch photocatalytic experiments. Prior to light experiments under UV-LED, dark adsorption studies were carried out and the adsorption equilibrium reached after 15 min of the adsorption studies. The ZnO tubes were placed in a UV-LED light chamber as shown in Fig. 2, where the temperature was maintained below 30 °C using an air cooling system. The photocatalytic experiments in the microflow and batch photoreactors were repeated again and the relative error was less than 5%. Later, the stability of the catalyst in the single coated FEP microtube was checked for five repeated reaction cycles.


image file: c8gc03131b-f2.tif
Fig. 2 Photocatalytic reaction of BnOH in the ZnO-US-FEP microtube; (1) syringe infusion pump, (2) Teflon tube, (3) the metal covering containing the LED and the photocatalytic microreactor, (4) Teflon tube, (5) sample collector, (6) UV-LED, (7) ZnO-US-FEP microtube and (8) air cooling tube.

During the 6 h photocatalytic experiment, the highest conversion and selectivity for BnAld formation was observed after 15 min of light experiment. Moreover, at higher reaction time the conversion of BnOH and BnAld selectivity was found to decrease. The ZnO coated FEP microtube (ZnO-US-FEP) having a single layer deposition time of 75 min using an ultrasound power of 70% showed 98% BnAld selectivity with a specific conversion rate of 1.8 mol m−2 h−1 in ACN after 15 min of light experiment as shown in Fig. 3. In the case of the photocatalytic reaction of BnOH in water, the specific conversion rate of 288 mol m−2 h−1 and 14.6% of BnAld selectivity were observed after 15 min of light experiment. It was also observed that the amount of ZnO deposited in ZnO-US-FEP microtubes during single deposition was 0.4 mg compared to other ZnO deposited FEP microtubes (Table S1). Compared to the photocatalytic microreactor, the specific conversion rate of BnOH of 0.07 mol m−2 h−1 and 0.6 mol m−2 h−1 in ACN and water, respectively, was much lower in the batch photocatalytic reactor. Moreover the batch photocatalytic reaction had a lower BnAld selectivity of 23.7% and 20.5% in ACN and water, respectively, clearly showing the effect of the reactor configuration on the BnAld selectivity. It should be noted that the BnOH conversions especially in ACN show lower conversions than those reported in other publications.35,36 The lower performance is mostly due to the catalyst used in the photocatalytic microreactor. Fig. S2 shows the XRD pattern of the commercial ZnO. The characteristic peaks at 31.7°, 34.4°, 36.2°, 47.5°, 56.5°, 62.8° and 67.9° are identified to be the (010), (002), (011), (012), (110), (013), and (112) diffraction peaks of the commercial ZnO possessing a hexagonal crystal structure. The sharp and intense peaks show that ZnO has good crystallinity.37 The average size of the crystallites was evaluated using the Scherrer equation. The mean grain size of ZnO is calculated to be 34.7 nm using the (011) plane diffraction peak (2θ = 36.2°). The band gap for ZnO was calculated to be 3.2 eV (Fig. S3). However the ZnO nanoparticles are poly-dispersed (Fig. S4) and have a specific surface area of 13.35 m2 g−1 (Fig. S5). The non-uniformity in size and shape and low specific surface area of the photocatalyst lower the number of active sites for the BnOH reaction. In addition, neither external oxygen nor an oxidizing agent like hydrogen peroxide was used in our studies as seen in the majority of BnOH selective conversion studies.38–40


image file: c8gc03131b-f3.tif
Fig. 3 (a) Photocatalytic conversion of BnOH into BnAld in (a) ACN and (b) water as solvents in a microflow reactor and batch photoreactor.

The SEM characterization of unmodified FEP microtubes, ultrasound modified FEP (US-FEP) and ZnO-US-FEP was carried out to understand the effect of ultrasound on the deposition procedure. Physical changes in the form of the etched surface are seen in US-FEP (Fig. 4b) and are absent in the unmodified FEP microtube (Fig. 4a). The SEM image of the ZnO-US-FEP microtube shows the presence of ZnO nanoparticles along with physical changes on the FEP surface (Fig. 4c). It is also noticeable that agglomerates of ZnO nanoparticles are present (Fig. 4d). During the ultrasound treatment the water present in the FEP microtube promotes acoustic cavitation which gives rise to regions with high and low pressure which further results in the formation of acoustic bubbles.23 The bubbles when in contact with the FEP surface explode to produce microjets thereby changing the surface free energy and resulting in structural transformation of the inner wall of the FEP microtube.23,24 This process is further enhanced due to the presence of ZnO nanoparticles in the aqueous suspension which involves high frequency of mutual collisions between the nanoparticles and the inner wall of the FEP microtube. As a result of these physical interactions, deposition of ZnO nanoparticles takes place on the etched surface of the inner walls of the FEP microtube. Moreover in the AT-IR spectra of the ZnO-US-FEP and US-FEP microtubes (Fig. S6), the bands at 2870 and 2969 cm−1 related to the CH stretching in the unmodified FEP microtube are absent, confirming that ultrasound brings some chemical changes in the inner walls of the FEP microtube. The ZnO-US-FEP microtube was further coated to have a double and triple layer of the ZnO catalyst which was also tested for the photocatalytic studies. It was observed that the increase in the layers of coating showed a small increase in the BnOH specific conversion rates, though the BnAld selectivity was reported to be much lower than that of the single coated FEP microtube (Table S1).


image file: c8gc03131b-f4.tif
Fig. 4 SEM images of (a) the unmodified FEP microtube, (b) US-FEP microtube, (c) ZnO-US-FEP microtube, (d) ZnO nanoparticle and images of (e) the unmodified FEP microtube and ZnO-US-FEP microtube.

To test the stability of the ZnO deposition inside the FEP microtube, the photocatalytic studies were repeated five times on the single coated ZnO-US-FEP microtube. After the completion of each run, the catalyst coated FEP microtube was washed, dried and utilized for the next runs. The respective specific conversion rate of BnOH and BnAld selectivity as a function of different reaction cycles is shown in Fig. 3. It was observed that the catalytic efficiency of the photocatalytic microreactor did not show major changes after five runs for reactions carried out in both ACN and water. However some changes in the BnAld selectivity were observed for reactions carried out in ACN stating the presence of unreacted BnOH and possible formed by-products on the photocatalyst surface. It can be hence confirmed that the ZnO nanoparticles are well deposited inside the FEP microtube. Furthermore, the EDXRF measurements recorded for the samples after photocatalytic studies in Fig. 5(a) did not indicate the presence of Zn2+ ions indicating the absence of leaching of the photocatalyst under applied experimental conditions. Based on the above experiments, and from the literature on photocatalytic BnOH conversion to BnAld, the probable mechanism for the photocatalytic selective oxidation of alcohols inside the photocatalytic microreactor has been proposed, as illustrated in Fig. 5(b). In the first possible mechanism (Mechanism I), during UV light irradiation, electron–hole pairs are generated from the surface of the ZnO nanoparticles. The photogenerated electrons are trapped by the dissolved oxygen molecules in the reaction solvent to form superoxide radicals (O2˙), which also inhibit the recombination of electron–hole pairs.41 It should be noted that no external oxidizing reagents were used and the oxygen present in the ACN solvent was involved in the conversion of BnOH molecules. In addition, the photocatalytic reactions were carried out using degassed ACN and aerated ACN solution (Fig. S7) which confirms changes in the conversion of BnOH and BnAld selectivity due to the naturally dissolved oxygen in the commercially available ACN solvent. In Mechanism II, the UV-light excited holes are taken by BnOH. The BnOH molecule then turns into an oxidized state, and subsequently transforms into BnAld by releasing the H atom and an electron.42 In the photocatalytic studies in water, the higher conversion and lower BnAld selectivity are due to the presence of hydroxyl radicals formed by the reaction of water molecules with the holes. Hydroxyl radicals attack both benzyl alcohol and BnAld to form undesired by-products like 2- and 4-hydroxy benzyl alcohols, and 2- and 4-hydroxy benzaldehydes.29


image file: c8gc03131b-f5.tif
Fig. 5 (a) EDXRF measurement of the liquid sample after photocatalytic conversion of BnOH under UV light. The absence of Zn2+ ions in the liquid sample shows the strong attachment of ZnO nanoparticles in the inner walls of the FEP microtube. (b) Schematic diagram of different mechanisms for the selective oxidation of BnOH to BnAld over ZnO nanoparticles deposited inside the FEP microtube under UV light irradiation.41,42

At the end it can be summarized that the results from this work provide the initial path for developing sustainable photocatalytic-microreactors which can be used for selective organic synthesis. However, a deeper analysis and discussions are required to optimize such novel reactors. Experiments related to catalyst coating and related to photocatalytic studies have to be designed using statistical software. The experimental design software can be used to optimize more parameters included in catalyst deposition like the concentration of catalyst, the type of solvent for deposition and parameters related to photocatalytic studies like reactant flow rate and microreactor length. The catalyst deposition can be further enhanced by carry out a second round coating in absence of ultrasound. Considering the fact that the synthesis and usage of a catalyst with specific properties can provide enhanced catalytic activity compared to commercially available catalysts, the deposition of mono-dispersed nanoparticles with a higher surface area will be the key factor for obtaining a higher catalytic activity in photo-microreactors. Later this can be further extended to in situ deposition of semiconductor oxide nanoparticles in the inner walls of the microtubes using traditional synthesis methods like ultrasound assisted sol–gel. Moreover, photocatalytic studies of other lignin based model compounds like coniferyl alcohol, eugenol, under the optimized reaction conditions will be a major step towards commercializing biomass based chemical reactions for generation of organic compounds. These synthesized chemicals are stated to be of high value in the pharmaceutical and fine chemical industries.

Conclusions

This work combines photocatalysis, nanomaterials, and ultrasonic and microfluidic flow processes to develop a novel Green Chemistry based method for ZnO nanoparticle deposition inside a photo-microreactor for its application for selective production of value added chemicals from biomass based model compounds. The deposition of ZnO nanoparticles is mainly attributed to the physical changes in the FEP microtube surface which occur during the usage of ultrasound. The selective oxidation of BnOH to BnAld in the photocatalytic microreactor showed a better specific conversion rate and BnAld selectivity compared to the batch photocatalytic reactor. The photocatalytic microreactor showed a lower change in the catalytic efficiency after multiple reaction runs as no leaching of the nanoparticle was observed. Further studies for developing photocatalytic microreactors with a better catalyst configuration and optimized reaction conditions are needed for operating selective oxidation reactions of other biomass based model compounds. This work provides the foundation for the establishment of a novel energy and environment friendly concept based on using chemicals from nature in a low energy based reactor operating on solar energy.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

J. C. Colmenares and V. Nair are very grateful for the support from the National Science Centre in Poland within Sonata Bis Project No. 2015/18/E/ST5/00306.

Notes and references

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8gc03131b

This journal is © The Royal Society of Chemistry 2019