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Fructose dehydration to hydroxyl-methylfurfural in an immobilized catalytic microreactor

Fructose dehydration to hydroxyl-methylfurfural in an immobilized catalytic microreactor In this paper we report a microfluidic platform that allows for high temperature, high pressure conversion with inline spectroscopic measurement for a fast and accurate determination of both reaction rate constant and activation energy. The dehydration of fructose to hydroxyl-methylfurfural has been performed in this immobilized microreactor with both dense zirconia and porous titania layers, as a starting point to probe the potential of abundant metal oxide catalysts. Keywords Microreactor · Catalysis · Fructose to HMF Introduction Future bio-refineries need to produce high value bio based chemicals to be economically competitive. There are Catalysis is ever-present in industry with up to 90% of the two possible strategies for approaching the market. The reaction processes using catalysts. Most of these processes first is to aim for novel products with new and improved comprise fossil based feedstock which entails selective properties such is the case of 2,5-furan dicarboxylic acid functionalization of apolar, unfunctionalized hydrocarbons. which has the prospect of replacing terephthalic acid in For well-known reasons, there is a global interest for more the fabrication of PET [4]. In this context, markets need sustainable resources. The catalytic conversion of biomass to develop. The second scenario is to aim for existing waste is a promising alternative for chemicals, materials, products that utilize existing infrastructure and for which and fuel production [1–3]. Especially for chemical syn- there is already a mature market [5]. Both processing routes thesis, this route is challenging due to the richness and converge towards a few important platform molecules from complexity of the chemical composition of biomass waste. which a myriad of end products can diverge. Dusselier identified that carbohydrates give access to a plethora While oil consists of hydrocarbons, biomass is high in oxy- gen content and consequently hydrophilic. Catalysts will of chemicals, including 5-hydroxymethylfurfural (HMF), have to fulfil completely different requirements in the case levulinic acid (LA) and γ -valerolactone (GVL) [6–8]. of biomass conversion, as the chemical conversion will HMF is a versatile and promising compound derived entail selective defunctionalisation of polar, highly function- from carbohydrates. It can be used as a renewable alized oxygenates. intermediate for the production of polymers, fuels or solvents in the petrochemical industry. The last step in the synthesis of HMF is the dehydration of fructose (Fig. 1). Researchers have developed biocatalysts to produce Electronic supplementary material The online version of fructose from cellulose [9], which is one of the major this article (https://doi.org/10.1007/s41981-020-00087-5) contains component of most plants and agriculture wastes [10]. supplementary material, which is available to authorized users. An environmental-friendly process with a high fructose RobG.H.Lammertink conversion rate is desirable for the overall utilization of r.g.h.lammertink@utwente.nl biomass. Aura Visan The challenge for the effective conversion of biomass a.visan@utwente.nl is to develop catalysts for efficient conversion at low cost [11–14]. Group VIII metals, notably ruthenium, have shown to facilitate the hydrogenation step in liquefaction, but the Soft Matter, Fluidics and Interfaces, University of Twente, Enschede, The Netherlands cost of these precious metals is extremely high. Inexpensive 462 J Flow Chem (2020) 10:46 1–46 8 Fig. 1 Reaction scheme for the dehydration of fructose to hydroxymethylfurfural heterogeneous catalysts, such as zirconium dioxide and Experimental titanium dioxide are attracting increasingly more attention as alternatives [15]. Moreover, due to its high content in Chip fabrication oxygen, biomass is typically processed in aqueous solutions or other polar solvents such as alcohols. These polar The fabrication of microchannels and inlet/outlet holes solvents at high temperatures and pressures and often at in silicon was achieved using photolithography and deep extreme pH challenge the stability of most catalysts. Among reactive ion etching. Anodic bonding of the glass cover the few that can withstand these severe conditions are metals to silicon gives a very strong bond without the need of oxides. excessive heating which could affect the morphology of Solid acids such as, phosphates [18] and chlorides [19, the immobilized catalyst. Having the channels in silicon is 20], but also plain metal oxides [16, 21] have been studied beneficial due to its high thermal conductivity and glass is as catalysts in the dehydration of fructose. It has been the obvious choice to close the microreactor because it gives proven that metal oxides increase their acid site density the possibility of UV irradiation necessary for the surface upon treatment with phosphoric acid which improves the functionalization of the catalyst and the use of microscopic dehydration from fructose to HMF [17, 21]. The treatment techniques for in-situ observations. The meandering channel with phosphoric acid, esterifies -OH groups on the surface (Fig. 2) is 500 μm wide and 50 μm deep and 18 cm long. of TiO into -O-PO(OH ) which increases the HMF We have used two types of catalysts. A dense zirconia 2 2 selectivity [11, 17]. What is more, TiO surfaces exhibit layer which was sputtered during the cleanroom fabrication super - hydrophilicity during and after UV light exposure and a porous titania which was wash coated after closing due to the formation of excess surface -OH groups. The the reactor. The cleanroom fabrication steps include pho- combination of UV light and phosphoric acid treatment tolithography, opening the SiO mask, deep reactive ion would increase the density of surface phosphate species, etching of the microchannels, lift-off, second photolithog- thereby enhancing its catalytic performance [11]. raphy on the back, deep reactive ion etching the inlet and Microreactors form an attractive platform for kinetic outlet holes, as well as the gap separating the heated area investigations for heterogeneous catalysis. Their small from the fluidic connections, again lift off, wet etching dimensions provide a laminar flow profile and, conse- of the SiO mask, photolithography for the third time by quently, a well-defined mass transport. They also allow spray coating the photoresist to homogeneously cover the for fast inline measurement without the need to quench 3D structures, sputtering the catalyst (see below), lift off the reaction for sample collection and analysis. The cur- in acetone bath, anodic bonding and dicing (Figure S1). rent project investigates one of the key reactions in the The success of this patterning is due to the thick resist conversion of biomass waste, namely fructose to hydrox- that is obtained by spray coating. The 5 μm layer ensures ymethylfurfural (Fig. 1), in a microreactor device at elevated that no edge connection takes place upon sputtering. The pressures and temperatures. second type of microreactor does not require a third Fig. 2 a Microreactor CAD design. b The actual microreactor with the sputtered ZrO layer (visible as the purple colour) J Flow Chem (2020) 10:46 1–46 8 463 photolithography step and it was closed after deep reactive ion etching the back. Catalyst deposition and characterization Zirconium dioxide was deposited by reactive magnetron sputtering with a zirconium target using a dc power source. The film was sputtered for 40 min at 200W using a reactor gas mixture of 92.5 vol% Ar and 7.5 vol% O at a process −3 pressure of 5 × 10 mbar. The layer was annealed for 4 h at 500 C in air. The heating and cooling rates were kept at 2 C/min. High resolution scanning electron microscopy revealed a nonporous film with significant roughness. The elemental stoichiometry of the metal oxide was investigated using Energy Dispersive X-ray spectroscopy. The chemical composition was also confirmed by X-Ray Photoelectron Spectroscopy. The thickness and roughness were determined by Spectroscopic Ellipsometry using the Fig. 3 Chipholder CAD design displaying the configuration of the method described by Visan et al. [22]. X-ray Diffraction TEC element was used to investigate the degree of crystallinity and detect the crystalline phases. The orientation of the crystallites was visualized by TEM. The second microreactor was wash coated with a porous titania layer using a monodisperse commercial suspension removed according to the design of Samuel Marre [23]. The (VP Disp. W 2730 X, Evonik). The suspension was used temperature difference between the reaction zone and the port side has been assessed by using a second temperature without dilution at the initial 30% (wt.) solid content. The aqueous dispersion is pumped through the microchannel sensor in order to check if active cooling is required on the compression side. The system proved to be leakage free and flushed afterwards at a constant displacement velocity to ensure a constant thickness along the channel. The until 50 bar. We opted for silicone o-rings due to their higher resulting layer was sintered for 2 h at 500 Cinair. flexibility compared to Karlez. Karlez and Valco gave good The heating and cooling rates were 2 C/min. The quality sealing when newly installed. of the suspension gave a homogeneous layer according The heating was performed locally on the reaction to high resolution scanning electron microscopy. The side using a Peltier element which allowed for accurate narrow size distribution and absence of aggregates of temperature control up to 200 C. The chipholder has two the starting suspension was confirmed by light scattering separate top parts, the fluidic connection and the aluminum measurements using a Zetasizer. The roughness and plate that pushes down the Peltier element which also works as a heat sink to ensure the heat flux through the element. porosity was determined by spectroscopic ellipsometry [22]. The crystalline composition is provided by the This separate top prevents heat dissipation to the rest of the chipholder. The commercial temperature control system manufacturer. After sintering the wash coated layer, TiO was treated does not go higher than 120 - 150 C for thermoelectric with phosphoric acid under UV. The microchannels were (TEC) cooling elements, so individual components were put flushed with 1M H PO solution for 15 min at 50 μl/min. together in house. Two separate temperature measurements 3 4 While filled with H PO solution, the microreactors were are possible. On the reaction side this is done with a Pt100 3 4 fixed under UV light (Dr. Grobel ¨ UV light source HP- sensor that provides a very high accuracy in the order of −3 ◦ 120, 180 mW/cm ) for 4 hours. After treatment, the 10 C, while a NPT sensor monitors the compression side. The thermocouple tip is positioned very close to the microchannels were rinsed thoroughly with distilled water. microchannels inside a separate pocket. A 24 watt resistive heater can replace the Peltier within the same control unit to Modular packaging extend the temperature range up to 250 C. The chipholder The chipholder design is illustrated in Fig. 3. We placed was fabricated entirely from PEEK in order to minimize the heating element inside the chipholder to keep a compact the heat loss from the Peltier element. It also has a window design. To limit the heating at the connection side, a part of on the glass side of the chip which allows microscopic the silicon between the connection zone and heated zone is observation. 464 J Flow Chem (2020) 10:46 1–46 8 Fig. 4 Schematic illustration of the used setup, combining temperature and flow control with inline UV-Vis analysis Setup and operation standard operating conditions for the conversion of fructose to hydroxymethylfurfural (HMF) are 7 bar and 130 C. The setup allows to independently vary residence time, The conversion is quantified by measuring the product pressure and temperature while monitoring conversion concentration via inline UV-Vis spectroscopy that tracks using inline analysis. The liquid handling system comprises the absorption in the UV range of HMF. The maximum a Fluigent flow controller equipped with a thermal sensor absorption peak of HMF is located at 284 nm. The that is connected to a control unit to achieve the flowrate calibration was carried out for different concentrations of set point. A back-pressure regulator using an active valve HMF solutions (from 2 to 15 mg/L). The fitted calibration controls the pressure, decoupling in this way the flowrate curve is presented by: A = 0.1174·c [mg/L]. The 284nm HMF from the pressure. Figure 4 shows the schematic of the product molar yield was evaluated according to [h]/[f ] · set up used. After the pressure reached the set point, the 100, where [f ] is the initial fructose concentration and (0) microreactor was heated to the desired temperature. The [h] is the HMF concentration. Fig. 5 a High resolution SEM of a 250 nm ZrO layer. b Higher SEM magnification revealing the dense structure. c TEM of sputtered ZrO displaying its polycrystalline morphology J Flow Chem (2020) 10:46 1–46 8 465 Fig. 8 HMF absorbance showing the increase in HMF production for Fig. 6 Deposition rate dependency on volumetric gas composition higher residence times. Reaction performed at 130 C and 7 bar using the porous TiO layer Results and discussion The sputtered zirconia layer has a dense structured (Fig. 5), as sputtering is a high energy process which does as visualized by HRSEM (Fig. 5). The 1:2 elemental not give the possibility for preferential orientation. stoichiometry for Zr:O is constant for a wide range of The wash coated titania film displays a high degree of O concentrations used during the sputtering process. homogeneity (Fig. 7). While the initial suspension shows The elemental content was measured with both XPS and already a narrow size distribution (Figure S3), 157 ± 70 EDX. The observed drop in deposition rate for high O nm, there are small aggregates comprising of monodisperse concentrations is attributed to the oxidation of the target particles of 21 nm which are the building blocks of the prior to sputtering (Fig. 6)[24]. For the final recipe, a final coating as it is shown in Fig. 7. A porosity of 45% 7.5% (vol.) O was chosen to ensure a high deposition rate and roughness similar to the particle radius (∼ 8nm) which results in a 250 nm layer for a 40 min deposition was measured by Spectroscopic Ellipsometry. The specific time as shown in Fig. 5a and b XRD shows a mixture of surface area of the porous film given the particle size and 3 2 tetragonal and monoclinic crystalline phases (Figure S2). the density of anatase (3895 kg/m ) is about 50 m /g. The The annealing process increases the crystallinity of the film, crystalline phase is not affected by the sintering process. with the previous amorphous phase transitioning to the The 80% anatase to 20% rutile composition specified by tetragonal structure which shows up in an increase in the the manufacturer is preserved. The high quality of the T(1,1,1) peak intensity. TEM revealed a polycrystalline film commercial suspension and the constant displacement rate Fig. 7 High resolution SEM of TiO showing. a the porous structure and b the monodisperse particles. c Cross-section of the wash coated uniform layer 466 J Flow Chem (2020) 10:46 1–46 8 ensured a uniform thickness of the wash coated catalyst intrinsic surface reaction rate constant can be extracted from which can be observed in the cross section of the channel the fitted volumetric rate constant k. (Fig. 7c). A displacement rate of 17 mm/s led to a thickness d[f ] u =−k[f ] (3) of 5 ± 0.5 μm. dx The reaction was performed at 7 bar and 130 C with velocity u [m/s] and fructose concentration [f ].The for different flowrates. The product (HMF) absorption velocity u corresponds to the residence time t = L/u, with peak was monitored at each corresponding residence time channel length L, which gives: (Fig. 8), after steady state was reached. Given the small [f ] [f ] −[h] product yield (Fig. 9), the conversion is reaction rate limited ln = ln =−kt (4) [f ] [f ] 0 0 and external mass transfer does not have to be taken into account. This assumption is valid when the second [h] −kt = 1 − e (5) −1 Damkohler ¨ number, Da < 10 , which varies for the II [f ] −3 −2 fitted reaction constants between 4 × 10 and 1.3 × 10 . We take into account only the reaction pathway to HMF, as the present spectroscopic measurement allows for HMF Da = · H (1) II detection only. Mass spectrometry is required to identify other possible secondary products and quantify the absolute where k is the volumetric reaction rate constant [1/s] for a conversion of fructose and its selectivity to HMF. first order reaction rate, H is the height of the channel. D The sputtered ZrO displayed a low activity (k = 1.43 × is the diffusion coefficient of fructose in water corrected for −3 10 1/min) which referenced to the geometric surface area the change in temperature and pressure by using the Stokes of the film converts to a surface reaction rate constant of Einstein equation: −9 k = 1.192 × 10 m/s. The slow overall kinetics can be k T attributed to the obvious low available surface area, but also D = (2) 6πη a to a low density of active sites. k and η are the Boltzmann constant and the viscosity of The next attempt tried to improve both aspects, increase pure water at temperature T and the corresponding pressure the specific surface area and alter the surface functionality (7 bar). a = 0.365 nm is the effective hydrodynamic for an increase in the number of acid sites. The initial radius of fructose in water at small concentrations [9]. The concentration of fructose was lowered to 0.5 g/L in the case variation in a with temperature is small, less than 3%, of TiO due to the absorption signal saturation beyond HMF for the experimental conditions involved. A homogeneous concentrations of 15 mg/L. Figure 8 shows the full spectrum concentration profile can be assumed in the transversal of the steady state for each residence time. direction which simplifies the system to a plug flow reactor The porous titania layer displayed a higher volumetric (PFR) model. This gives a steady state 1D advection rate constant due to the higher available surface area, k = −3 reaction balance, where we opt for a first order reaction 6.34 × 10 1/min. The extracted value is averaged with rate expression for fructose. We will later explain how the respect to the volume of the channel. This value needs to Fig. 9 HMF yield dependence on residence time for a. dense ZrO and b. porous TiO . The data points represent the experimental measurements. 2 2 −3 −3 The continuous line represents equation 5 where a k = 1.43 × 10 1/min and b k = 6.34 × 10 1/min J Flow Chem (2020) 10:46 1–46 8 467 be rescaled in order to obtain the volume averaged value corresponding to the catalyst layer. The relevant length scales are the height of the channel, H , and the catalyst thickness, δ. The following scale relation converts the rate constant per volume of channel to per volume of catalyst: k = k ·H/δ. To be able to validate the volume averaging TiO of the reaction rate constant, we need to exclude internal mass transfer limitations, such that the catalyst layer is utilized evenly throughout its thickness. Two parameters can be used in this respect: thiele modulus, φ,and the internal effectiveness factor, η. Thiele modulus evaluates the reaction time scale with respect to the diffusion time scale: TiO φ = · δ (6) eff Where D = D · is the effective diffusion eff coefficient,  = 0.45 is the porosity and τ =1.35isthe Fig. 10 The dependency of k with temperature. The data points −3 tortuosity [22]. φ values up to 6 × 10 for the extracted represent the experimental measurements. The continuous line 19.4 kinetics confirmed the reaction driven regime for which the represents Eq. 8 where E = 80 kJ/mol and k = e 1/min a 0 −1 formal criterion is φ< 10 . The effectiveness factor gives the ratio between the net reaction rate and the rate in the to be a useful tool for rapid investigation of catalytic absence of concentration and temperature gradients, which performance. Especially for higher activities when mass for the present system is almost unity: transport becomes limiting and accurate modelling is crucial tanh φ η = (7) to decouple the kinetics from reactor design, microreactors provide a reliable option due to their well defined fluid −1 The conversion from k per unit volume [s ]to k TiO dynamics. per unit surface area [m/s] can be derived from: k = −4 2 k /(S ρ(1 − )),where S = 5 × 10 m /kg is the TiO a a specific surface area of TiO , ρ = 3895 kg/m is the anatase 2 Conclusion density and  = 0.45 is the porosity. This gave a lower −12 surface reaction rate constant, k = 9.87 × 10 m/s, than A microfluidic platform was developed for high tempera- the corresponding value for the ZrO layer. ture, high pressure conversion with an inline UV-Vis spec- The dependency of the reaction rate constant k on troscopic measurement that facilitates the fast screening of temperature is typically expressed by the Arrhenius catalytic materials. The well-defined mass transport char- equation: acteristic for immobilized catalytic layers in microchannels −E /RT allows for accurate kinetic investigation. The dehydration k = k e (8) of fructose to 5-hydroxymethyl-2-furaldehyde (HMF) was where E is the activation energy, k is the pre-exponential a 0 studied using both sputtered ZrO andwashcoatedTiO 2 2 −1 −1 factor and R = 8.31446 Jmol K is the ideal gas layers. The kinetics were determined for each catalyst. For constant. We investigated experimentally the dependency the TiO layer, that showed higher conversion, the depen- of k on temperature for the porous TiO layer for which dency on temperature was also investigated, revealing an 19.4 we obtained E = 80 kJ/mol and k = e 1/min a 0 activation energy of 80 kJ/mol. Surface functionalization (Fig. 10). These values are slightly higher than what Carnity of TiO using phosphoric acid treatment under UV light et al. measured for the same reaction using a niobium proved to increase the catalyst reactivity, likely by enhanc- 15.7 phosphate catalyst (E = 65.8 kJ/mol and k = e 1/min) a 0 ing the density of active sites. [25]. For these measurements, the surface functionalization procedure using H PO acid under UV exposure was 3 4 extended from 4 to 6 h. This change led to an improvement Acknowledgments This work was supported by the Netherlands −3 Center for Multiscale Catalytic Energy Conversion (MCEC), an NWO in the reaction rate constant, from 6.34 × 10 1/min to Gravitation programme funded by the Ministry of Education, Culture −3 ◦ 11 × 10 1/min at 130 C. and Science of the government of the Netherlands. R.G.H.L. also While the catalysts investigated in this work displayed acknowledges the Vici project STW 016.160.312, financed by the rather low activities, the microfluidic platform proved Netherlands Organisation for Scientific Research (NWO). 468 J Flow Chem (2020) 10:46 1–46 8 Compliance with Ethical Standards 8. Dusselier M, Mascal M, Sels BF (2014). In: Selective catalysis for renewable feedstocks and chemicals. Springer, Cham, pp 1–40, https://doi.org/10.1007/128-2014-544 Conflict of interests The authors declare that they have no conflict of 9. 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Fructose dehydration to hydroxyl-methylfurfural in an immobilized catalytic microreactor

Journal of Flow Chemistry , Volume 10 (2) – Jun 18, 2020

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Abstract

In this paper we report a microfluidic platform that allows for high temperature, high pressure conversion with inline spectroscopic measurement for a fast and accurate determination of both reaction rate constant and activation energy. The dehydration of fructose to hydroxyl-methylfurfural has been performed in this immobilized microreactor with both dense zirconia and porous titania layers, as a starting point to probe the potential of abundant metal oxide catalysts. Keywords Microreactor · Catalysis · Fructose to HMF Introduction Future bio-refineries need to produce high value bio based chemicals to be economically competitive. There are Catalysis is ever-present in industry with up to 90% of the two possible strategies for approaching the market. The reaction processes using catalysts. Most of these processes first is to aim for novel products with new and improved comprise fossil based feedstock which entails selective properties such is the case of 2,5-furan dicarboxylic acid functionalization of apolar, unfunctionalized hydrocarbons. which has the prospect of replacing terephthalic acid in For well-known reasons, there is a global interest for more the fabrication of PET [4]. In this context, markets need sustainable resources. The catalytic conversion of biomass to develop. The second scenario is to aim for existing waste is a promising alternative for chemicals, materials, products that utilize existing infrastructure and for which and fuel production [1–3]. Especially for chemical syn- there is already a mature market [5]. Both processing routes thesis, this route is challenging due to the richness and converge towards a few important platform molecules from complexity of the chemical composition of biomass waste. which a myriad of end products can diverge. Dusselier identified that carbohydrates give access to a plethora While oil consists of hydrocarbons, biomass is high in oxy- gen content and consequently hydrophilic. Catalysts will of chemicals, including 5-hydroxymethylfurfural (HMF), have to fulfil completely different requirements in the case levulinic acid (LA) and γ -valerolactone (GVL) [6–8]. of biomass conversion, as the chemical conversion will HMF is a versatile and promising compound derived entail selective defunctionalisation of polar, highly function- from carbohydrates. It can be used as a renewable alized oxygenates. intermediate for the production of polymers, fuels or solvents in the petrochemical industry. The last step in the synthesis of HMF is the dehydration of fructose (Fig. 1). Researchers have developed biocatalysts to produce Electronic supplementary material The online version of fructose from cellulose [9], which is one of the major this article (https://doi.org/10.1007/s41981-020-00087-5) contains component of most plants and agriculture wastes [10]. supplementary material, which is available to authorized users. An environmental-friendly process with a high fructose RobG.H.Lammertink conversion rate is desirable for the overall utilization of r.g.h.lammertink@utwente.nl biomass. Aura Visan The challenge for the effective conversion of biomass a.visan@utwente.nl is to develop catalysts for efficient conversion at low cost [11–14]. Group VIII metals, notably ruthenium, have shown to facilitate the hydrogenation step in liquefaction, but the Soft Matter, Fluidics and Interfaces, University of Twente, Enschede, The Netherlands cost of these precious metals is extremely high. Inexpensive 462 J Flow Chem (2020) 10:46 1–46 8 Fig. 1 Reaction scheme for the dehydration of fructose to hydroxymethylfurfural heterogeneous catalysts, such as zirconium dioxide and Experimental titanium dioxide are attracting increasingly more attention as alternatives [15]. Moreover, due to its high content in Chip fabrication oxygen, biomass is typically processed in aqueous solutions or other polar solvents such as alcohols. These polar The fabrication of microchannels and inlet/outlet holes solvents at high temperatures and pressures and often at in silicon was achieved using photolithography and deep extreme pH challenge the stability of most catalysts. Among reactive ion etching. Anodic bonding of the glass cover the few that can withstand these severe conditions are metals to silicon gives a very strong bond without the need of oxides. excessive heating which could affect the morphology of Solid acids such as, phosphates [18] and chlorides [19, the immobilized catalyst. Having the channels in silicon is 20], but also plain metal oxides [16, 21] have been studied beneficial due to its high thermal conductivity and glass is as catalysts in the dehydration of fructose. It has been the obvious choice to close the microreactor because it gives proven that metal oxides increase their acid site density the possibility of UV irradiation necessary for the surface upon treatment with phosphoric acid which improves the functionalization of the catalyst and the use of microscopic dehydration from fructose to HMF [17, 21]. The treatment techniques for in-situ observations. The meandering channel with phosphoric acid, esterifies -OH groups on the surface (Fig. 2) is 500 μm wide and 50 μm deep and 18 cm long. of TiO into -O-PO(OH ) which increases the HMF We have used two types of catalysts. A dense zirconia 2 2 selectivity [11, 17]. What is more, TiO surfaces exhibit layer which was sputtered during the cleanroom fabrication super - hydrophilicity during and after UV light exposure and a porous titania which was wash coated after closing due to the formation of excess surface -OH groups. The the reactor. The cleanroom fabrication steps include pho- combination of UV light and phosphoric acid treatment tolithography, opening the SiO mask, deep reactive ion would increase the density of surface phosphate species, etching of the microchannels, lift-off, second photolithog- thereby enhancing its catalytic performance [11]. raphy on the back, deep reactive ion etching the inlet and Microreactors form an attractive platform for kinetic outlet holes, as well as the gap separating the heated area investigations for heterogeneous catalysis. Their small from the fluidic connections, again lift off, wet etching dimensions provide a laminar flow profile and, conse- of the SiO mask, photolithography for the third time by quently, a well-defined mass transport. They also allow spray coating the photoresist to homogeneously cover the for fast inline measurement without the need to quench 3D structures, sputtering the catalyst (see below), lift off the reaction for sample collection and analysis. The cur- in acetone bath, anodic bonding and dicing (Figure S1). rent project investigates one of the key reactions in the The success of this patterning is due to the thick resist conversion of biomass waste, namely fructose to hydrox- that is obtained by spray coating. The 5 μm layer ensures ymethylfurfural (Fig. 1), in a microreactor device at elevated that no edge connection takes place upon sputtering. The pressures and temperatures. second type of microreactor does not require a third Fig. 2 a Microreactor CAD design. b The actual microreactor with the sputtered ZrO layer (visible as the purple colour) J Flow Chem (2020) 10:46 1–46 8 463 photolithography step and it was closed after deep reactive ion etching the back. Catalyst deposition and characterization Zirconium dioxide was deposited by reactive magnetron sputtering with a zirconium target using a dc power source. The film was sputtered for 40 min at 200W using a reactor gas mixture of 92.5 vol% Ar and 7.5 vol% O at a process −3 pressure of 5 × 10 mbar. The layer was annealed for 4 h at 500 C in air. The heating and cooling rates were kept at 2 C/min. High resolution scanning electron microscopy revealed a nonporous film with significant roughness. The elemental stoichiometry of the metal oxide was investigated using Energy Dispersive X-ray spectroscopy. The chemical composition was also confirmed by X-Ray Photoelectron Spectroscopy. The thickness and roughness were determined by Spectroscopic Ellipsometry using the Fig. 3 Chipholder CAD design displaying the configuration of the method described by Visan et al. [22]. X-ray Diffraction TEC element was used to investigate the degree of crystallinity and detect the crystalline phases. The orientation of the crystallites was visualized by TEM. The second microreactor was wash coated with a porous titania layer using a monodisperse commercial suspension removed according to the design of Samuel Marre [23]. The (VP Disp. W 2730 X, Evonik). The suspension was used temperature difference between the reaction zone and the port side has been assessed by using a second temperature without dilution at the initial 30% (wt.) solid content. The aqueous dispersion is pumped through the microchannel sensor in order to check if active cooling is required on the compression side. The system proved to be leakage free and flushed afterwards at a constant displacement velocity to ensure a constant thickness along the channel. The until 50 bar. We opted for silicone o-rings due to their higher resulting layer was sintered for 2 h at 500 Cinair. flexibility compared to Karlez. Karlez and Valco gave good The heating and cooling rates were 2 C/min. The quality sealing when newly installed. of the suspension gave a homogeneous layer according The heating was performed locally on the reaction to high resolution scanning electron microscopy. The side using a Peltier element which allowed for accurate narrow size distribution and absence of aggregates of temperature control up to 200 C. The chipholder has two the starting suspension was confirmed by light scattering separate top parts, the fluidic connection and the aluminum measurements using a Zetasizer. The roughness and plate that pushes down the Peltier element which also works as a heat sink to ensure the heat flux through the element. porosity was determined by spectroscopic ellipsometry [22]. The crystalline composition is provided by the This separate top prevents heat dissipation to the rest of the chipholder. The commercial temperature control system manufacturer. After sintering the wash coated layer, TiO was treated does not go higher than 120 - 150 C for thermoelectric with phosphoric acid under UV. The microchannels were (TEC) cooling elements, so individual components were put flushed with 1M H PO solution for 15 min at 50 μl/min. together in house. Two separate temperature measurements 3 4 While filled with H PO solution, the microreactors were are possible. On the reaction side this is done with a Pt100 3 4 fixed under UV light (Dr. Grobel ¨ UV light source HP- sensor that provides a very high accuracy in the order of −3 ◦ 120, 180 mW/cm ) for 4 hours. After treatment, the 10 C, while a NPT sensor monitors the compression side. The thermocouple tip is positioned very close to the microchannels were rinsed thoroughly with distilled water. microchannels inside a separate pocket. A 24 watt resistive heater can replace the Peltier within the same control unit to Modular packaging extend the temperature range up to 250 C. The chipholder The chipholder design is illustrated in Fig. 3. We placed was fabricated entirely from PEEK in order to minimize the heating element inside the chipholder to keep a compact the heat loss from the Peltier element. It also has a window design. To limit the heating at the connection side, a part of on the glass side of the chip which allows microscopic the silicon between the connection zone and heated zone is observation. 464 J Flow Chem (2020) 10:46 1–46 8 Fig. 4 Schematic illustration of the used setup, combining temperature and flow control with inline UV-Vis analysis Setup and operation standard operating conditions for the conversion of fructose to hydroxymethylfurfural (HMF) are 7 bar and 130 C. The setup allows to independently vary residence time, The conversion is quantified by measuring the product pressure and temperature while monitoring conversion concentration via inline UV-Vis spectroscopy that tracks using inline analysis. The liquid handling system comprises the absorption in the UV range of HMF. The maximum a Fluigent flow controller equipped with a thermal sensor absorption peak of HMF is located at 284 nm. The that is connected to a control unit to achieve the flowrate calibration was carried out for different concentrations of set point. A back-pressure regulator using an active valve HMF solutions (from 2 to 15 mg/L). The fitted calibration controls the pressure, decoupling in this way the flowrate curve is presented by: A = 0.1174·c [mg/L]. The 284nm HMF from the pressure. Figure 4 shows the schematic of the product molar yield was evaluated according to [h]/[f ] · set up used. After the pressure reached the set point, the 100, where [f ] is the initial fructose concentration and (0) microreactor was heated to the desired temperature. The [h] is the HMF concentration. Fig. 5 a High resolution SEM of a 250 nm ZrO layer. b Higher SEM magnification revealing the dense structure. c TEM of sputtered ZrO displaying its polycrystalline morphology J Flow Chem (2020) 10:46 1–46 8 465 Fig. 8 HMF absorbance showing the increase in HMF production for Fig. 6 Deposition rate dependency on volumetric gas composition higher residence times. Reaction performed at 130 C and 7 bar using the porous TiO layer Results and discussion The sputtered zirconia layer has a dense structured (Fig. 5), as sputtering is a high energy process which does as visualized by HRSEM (Fig. 5). The 1:2 elemental not give the possibility for preferential orientation. stoichiometry for Zr:O is constant for a wide range of The wash coated titania film displays a high degree of O concentrations used during the sputtering process. homogeneity (Fig. 7). While the initial suspension shows The elemental content was measured with both XPS and already a narrow size distribution (Figure S3), 157 ± 70 EDX. The observed drop in deposition rate for high O nm, there are small aggregates comprising of monodisperse concentrations is attributed to the oxidation of the target particles of 21 nm which are the building blocks of the prior to sputtering (Fig. 6)[24]. For the final recipe, a final coating as it is shown in Fig. 7. A porosity of 45% 7.5% (vol.) O was chosen to ensure a high deposition rate and roughness similar to the particle radius (∼ 8nm) which results in a 250 nm layer for a 40 min deposition was measured by Spectroscopic Ellipsometry. The specific time as shown in Fig. 5a and b XRD shows a mixture of surface area of the porous film given the particle size and 3 2 tetragonal and monoclinic crystalline phases (Figure S2). the density of anatase (3895 kg/m ) is about 50 m /g. The The annealing process increases the crystallinity of the film, crystalline phase is not affected by the sintering process. with the previous amorphous phase transitioning to the The 80% anatase to 20% rutile composition specified by tetragonal structure which shows up in an increase in the the manufacturer is preserved. The high quality of the T(1,1,1) peak intensity. TEM revealed a polycrystalline film commercial suspension and the constant displacement rate Fig. 7 High resolution SEM of TiO showing. a the porous structure and b the monodisperse particles. c Cross-section of the wash coated uniform layer 466 J Flow Chem (2020) 10:46 1–46 8 ensured a uniform thickness of the wash coated catalyst intrinsic surface reaction rate constant can be extracted from which can be observed in the cross section of the channel the fitted volumetric rate constant k. (Fig. 7c). A displacement rate of 17 mm/s led to a thickness d[f ] u =−k[f ] (3) of 5 ± 0.5 μm. dx The reaction was performed at 7 bar and 130 C with velocity u [m/s] and fructose concentration [f ].The for different flowrates. The product (HMF) absorption velocity u corresponds to the residence time t = L/u, with peak was monitored at each corresponding residence time channel length L, which gives: (Fig. 8), after steady state was reached. Given the small [f ] [f ] −[h] product yield (Fig. 9), the conversion is reaction rate limited ln = ln =−kt (4) [f ] [f ] 0 0 and external mass transfer does not have to be taken into account. This assumption is valid when the second [h] −kt = 1 − e (5) −1 Damkohler ¨ number, Da < 10 , which varies for the II [f ] −3 −2 fitted reaction constants between 4 × 10 and 1.3 × 10 . We take into account only the reaction pathway to HMF, as the present spectroscopic measurement allows for HMF Da = · H (1) II detection only. Mass spectrometry is required to identify other possible secondary products and quantify the absolute where k is the volumetric reaction rate constant [1/s] for a conversion of fructose and its selectivity to HMF. first order reaction rate, H is the height of the channel. D The sputtered ZrO displayed a low activity (k = 1.43 × is the diffusion coefficient of fructose in water corrected for −3 10 1/min) which referenced to the geometric surface area the change in temperature and pressure by using the Stokes of the film converts to a surface reaction rate constant of Einstein equation: −9 k = 1.192 × 10 m/s. The slow overall kinetics can be k T attributed to the obvious low available surface area, but also D = (2) 6πη a to a low density of active sites. k and η are the Boltzmann constant and the viscosity of The next attempt tried to improve both aspects, increase pure water at temperature T and the corresponding pressure the specific surface area and alter the surface functionality (7 bar). a = 0.365 nm is the effective hydrodynamic for an increase in the number of acid sites. The initial radius of fructose in water at small concentrations [9]. The concentration of fructose was lowered to 0.5 g/L in the case variation in a with temperature is small, less than 3%, of TiO due to the absorption signal saturation beyond HMF for the experimental conditions involved. A homogeneous concentrations of 15 mg/L. Figure 8 shows the full spectrum concentration profile can be assumed in the transversal of the steady state for each residence time. direction which simplifies the system to a plug flow reactor The porous titania layer displayed a higher volumetric (PFR) model. This gives a steady state 1D advection rate constant due to the higher available surface area, k = −3 reaction balance, where we opt for a first order reaction 6.34 × 10 1/min. The extracted value is averaged with rate expression for fructose. We will later explain how the respect to the volume of the channel. This value needs to Fig. 9 HMF yield dependence on residence time for a. dense ZrO and b. porous TiO . The data points represent the experimental measurements. 2 2 −3 −3 The continuous line represents equation 5 where a k = 1.43 × 10 1/min and b k = 6.34 × 10 1/min J Flow Chem (2020) 10:46 1–46 8 467 be rescaled in order to obtain the volume averaged value corresponding to the catalyst layer. The relevant length scales are the height of the channel, H , and the catalyst thickness, δ. The following scale relation converts the rate constant per volume of channel to per volume of catalyst: k = k ·H/δ. To be able to validate the volume averaging TiO of the reaction rate constant, we need to exclude internal mass transfer limitations, such that the catalyst layer is utilized evenly throughout its thickness. Two parameters can be used in this respect: thiele modulus, φ,and the internal effectiveness factor, η. Thiele modulus evaluates the reaction time scale with respect to the diffusion time scale: TiO φ = · δ (6) eff Where D = D · is the effective diffusion eff coefficient,  = 0.45 is the porosity and τ =1.35isthe Fig. 10 The dependency of k with temperature. The data points −3 tortuosity [22]. φ values up to 6 × 10 for the extracted represent the experimental measurements. The continuous line 19.4 kinetics confirmed the reaction driven regime for which the represents Eq. 8 where E = 80 kJ/mol and k = e 1/min a 0 −1 formal criterion is φ< 10 . The effectiveness factor gives the ratio between the net reaction rate and the rate in the to be a useful tool for rapid investigation of catalytic absence of concentration and temperature gradients, which performance. Especially for higher activities when mass for the present system is almost unity: transport becomes limiting and accurate modelling is crucial tanh φ η = (7) to decouple the kinetics from reactor design, microreactors provide a reliable option due to their well defined fluid −1 The conversion from k per unit volume [s ]to k TiO dynamics. per unit surface area [m/s] can be derived from: k = −4 2 k /(S ρ(1 − )),where S = 5 × 10 m /kg is the TiO a a specific surface area of TiO , ρ = 3895 kg/m is the anatase 2 Conclusion density and  = 0.45 is the porosity. This gave a lower −12 surface reaction rate constant, k = 9.87 × 10 m/s, than A microfluidic platform was developed for high tempera- the corresponding value for the ZrO layer. ture, high pressure conversion with an inline UV-Vis spec- The dependency of the reaction rate constant k on troscopic measurement that facilitates the fast screening of temperature is typically expressed by the Arrhenius catalytic materials. The well-defined mass transport char- equation: acteristic for immobilized catalytic layers in microchannels −E /RT allows for accurate kinetic investigation. The dehydration k = k e (8) of fructose to 5-hydroxymethyl-2-furaldehyde (HMF) was where E is the activation energy, k is the pre-exponential a 0 studied using both sputtered ZrO andwashcoatedTiO 2 2 −1 −1 factor and R = 8.31446 Jmol K is the ideal gas layers. The kinetics were determined for each catalyst. For constant. We investigated experimentally the dependency the TiO layer, that showed higher conversion, the depen- of k on temperature for the porous TiO layer for which dency on temperature was also investigated, revealing an 19.4 we obtained E = 80 kJ/mol and k = e 1/min a 0 activation energy of 80 kJ/mol. Surface functionalization (Fig. 10). These values are slightly higher than what Carnity of TiO using phosphoric acid treatment under UV light et al. measured for the same reaction using a niobium proved to increase the catalyst reactivity, likely by enhanc- 15.7 phosphate catalyst (E = 65.8 kJ/mol and k = e 1/min) a 0 ing the density of active sites. [25]. For these measurements, the surface functionalization procedure using H PO acid under UV exposure was 3 4 extended from 4 to 6 h. This change led to an improvement Acknowledgments This work was supported by the Netherlands −3 Center for Multiscale Catalytic Energy Conversion (MCEC), an NWO in the reaction rate constant, from 6.34 × 10 1/min to Gravitation programme funded by the Ministry of Education, Culture −3 ◦ 11 × 10 1/min at 130 C. and Science of the government of the Netherlands. R.G.H.L. also While the catalysts investigated in this work displayed acknowledges the Vici project STW 016.160.312, financed by the rather low activities, the microfluidic platform proved Netherlands Organisation for Scientific Research (NWO). 468 J Flow Chem (2020) 10:46 1–46 8 Compliance with Ethical Standards 8. Dusselier M, Mascal M, Sels BF (2014). In: Selective catalysis for renewable feedstocks and chemicals. Springer, Cham, pp 1–40, https://doi.org/10.1007/128-2014-544 Conflict of interests The authors declare that they have no conflict of 9. 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Published: Jun 18, 2020

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