University of Manchester · Web viewHong Lia, Ying Menga, Dandan Shua, Zhenyu Zhaoa, Yongjin Yangb,...

a. School of Chemical Engineering and Technology, National Engineering Research Center of Distillation Technology, Collaborative Innovation Center of Chemical Science and Engineering(Tianjin), Tianjin University, Tianjin 300072, China. Email: [email protected].

b. Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China.

c. School of Chemical Engineering and Analytical Science, The University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom. Email: [email protected].

Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x

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Received 00th January 20xx,Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

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Breaking the Equilibrium at the Interface: Microwave-Assisted Reactive Distillation (MARD)Hong Lia, Ying Menga, Dandan Shua, Zhenyu Zhaoa, Yongjin Yangb, Jinsong Zhangb, Xingang Lia, Xiaolei Fan*c and Xin Gao*ac

Microwave irradiation has shown the intensification effect on reactive distillation. Herein, we report a systematic study of microwave-assisted reactive distillation (MARD) relative to its counterpart by the conventional heating, aiming at understanding the intensification mechanism of MARD. Catalytic homogeneous esterification of phthalic anhydride with 2-ethylhexanol for producing dioctyl phthalate (DOP) was studied as the model system, showing the improved performance by MARD. However, the comparative study on the effect of the reaction temperature and microwave power density showed that the reaction system was mainly affected by the system temperature, suggesting the dominant thermal effect in MARD. Phase equilibrium of the model system by different heating methods was investigated, showing an improved water content in the vapour phase promoted by the microwave heating than the conventional heating. Therefore, the rapid selective heating of water content at the liquid-vapour interface in MARD caused the change of the relative volatility of system, facilitating the intensified evaporation of water molecules at the liquid-vapour interface, and hence the improved performance.

IntroductionReactive distillation (RD), a class of integrated reaction/separation processes, serves as an attractive process intensification (PI) strategy to address the chemical equilibrium limited reversible reactions such as etherification1-3 and esterification.4, 5 For esterification, such as ethyl acetate, the chemical equilibrium of reactants and products seriously impedes the production of esters. The coupling of reaction and separation can separate the product (i.e. ethyl acetate, b.p. 77 °C, as the overhead distillate) from the reaction zone, and hence breaking the chemical equilibrium which limits the conversion of substrates. The industrial application of such RD systems can be very effective with high conversions (e.g. 96%

in esterification reaction for producing ethyl acetate at 30kg/h scale6). In the last decades, the microwave-assisted reactive distillation (MARD), as a method of PI, has drawn much attention to the academic and industrial communities due to the improved reaction and separation efficiency of RD by coupling microwave irradiation.7-13 In principle, the introduction of microwave irradiation can lead to the accelerated separation of one or multiple components (via vaporisation) in the reaction system, and hence overcoming the equilibrium limitation, to impel the reaction to the desired direction. For example, an industrially relevant demethylation reaction was investigated to prove the effect of breaking the equilibrium constraint through the elimination of gaseous byproducts (methyl bromide) by the microwave heating.14

Existing literature has expounded different mechanisms between the conventional heating and microwave-induced heating.15-18 In principle, when a microwave field is applied, dielectric heating acts on the substances in the system directly via either dipolar polarisation or ionic conduction.19 For the dielectric heating under the microwave irradiation, substances with different dielectric constants (i.e. different structural formulas) have different microwave absorbing abilities20, 21, and hence being heated with different heating rates. The component with a high dielectric constant can easily convert

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1

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the microwave energy into heat.22, 23 Therefore, in comparison with the conventional heating, the microwave heating has the theoretical advantages, such as volumetric heating, fast heat transfer and more importantly, selective heating,24 which may potentially benefit both reaction and separation processes. For organic synthesis, numerous investigations have been carried out to understand the fundamental difference between the conventional and microwave heating on a given reaction, including both thermal and non-thermal effects induced by the microwave irradiation, aiming at developing the microwave-assist protocols for reactive process intensification.25-27 In general, the thermal effect by the microwave irradiation can accelerate the reaction rate due to the effective and efficient microwave volumetric heating.25, 26 Additionally, the selective heating due to the interaction between the microwave and polar molecules can also overheat polar solvents over apolar ones, tuning the selectivity of a specific reaction.25, 27

For separation processes, it is found that the microwave irradiation can alter the vapour-liquid equilibrium (VLE) of binary systems, enabling the selective evaporation of the microwave responsive molecules and overcoming the azeotropism of a given system.22, 28 For instance, the azeotrope composition and temperature of the binary ethanol/benzene system under the microwave irradiation (200 W) have shifted clearly from that established by the conventional heating (e.g. ethanol in the vapour phase shows an increase by ca. 17% by comparing the microwave system to the conventional counterpart). This can be attributed to the difference of their dielectric constants (ε’, ε’ethanol = 24.3; ε’benzene = 2.322). Therefore, the microwave irradiation can be particularly beneficial to the coupled reaction/separation system, i.e. RD. RD is a PI method to remove one or multiple products from the reaction stream for breaking the chemical equilibrium. Although RD is effective for certain reactions such as esterification, azeotropism may occur causing the compromise of process efficiency. Microwave irradiation has been recently proved to be effective to intensify RD further12, 28, 29 (i.e. MARD 12), exemplified by the simple esterification of acetic acid with ethanol for ethyl acetate synthesis, showing the reduced reaction time and improved ethyl acetate yield in comparison to the conventional heating systems.12, 29 Recently, we have demonstrated the first example of pilot scale MARD for the production of dioctyl phthalate (DOP) and showed better performance than the conventional RD, especially with the isooctanol stripping.30 However, the intensification mechanism of MARD has not been addressed systematically, especially for MARD with multi-step complex reaction systems.DOP is a common plasticiser, with the advantage of miscibility, high plasticity efficiency, low volatility, excellent low-temperature softness and high electrical performance, for making PVC based articles.31, 32 The industrial production of DOP is based on the two-step esterification of phthalic anhydride with 2-ethylhexanol,31 with the reaction stoichiometry as shown in Scheme 1. The first step of phthalic anhydride reacting with 2-ethylhexanol (to produce mono-2-ethylhexyl phthalate) is non-catalytic, fast and irreversible, which accomplishes when

phthalic anhydride completely dissolves in 2-ethylhexanol at 130 °C, forming the monoester of mono-2-ethylhexyl phthalate. The second step is relatively slow, and hence rate limiting, in which acid catalysts such as sulfuric acid, carboxylic acids and heteropoly acids are used to catalyse the esterification of mono-2-ethylhexyl phthalate with 2-ethylhexanol to produce DOP and water.31, 33, 34 The esterification reaction is limited by the equilibrium and the water produced by the reaction eventually halts the reaction, limiting the yield of DOP. Therefore, processes with in situ water removal are beneficial for improving the yield of DOP.32,

35 The state-of-the-art process (by BASF) employs continuous stirred tanks to break the equilibrium gradually;32 but it is complex with low economic efficiency. Built on the promising results obtained on the pilot MARD,30 in this work, we performed a systematic study of the MARD synthesis of DOP in reference to a conventional RD process, with the focus on the understanding of the intensification mechanism of phthalic anhydride esterification under microwave irradiation. The effect of microwave on intensifying the separation and reaction was decoupled and analysed carefully. Firstly, we performed the comparative study of the influence of microwave and conventional heating on the phase equilibrium of the binary DOP/water system to show the variation in the separation efficiency of the two systems. Subsequently, esterification of phthalic anhydride and 2-ethylhexanol promoted the by conventional heating and microwave irradiation was investigated aiming at elucidating the effect of microwave irradiation on the reaction system.

ExperimentalChemicals

Phthalic anhydride, 2-ethylhexanol and tetrabutyl titanate (the catalyst for DOP synthesis) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemical were analytical grade with the purities > 99.5% and used as received.

RD synthesis of DOP under conventional heating

2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx

Scheme 1. Esterification of phthalic anhydride with 2-ethylhexanol for the synthesis of DOP.



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Esterification of phthalic anhydride by the conventional RD was performed in a laboratory setup (Fig. S1). The system consists of a 250 mL three-necked round-bottom flask as the reactor which is heated by a heating mantle (maximum heating power: 300W) with the accuracy of ±1.0 °C in temperature control. The temperature of the system was measured by a laboratory glass thermometer, which was inserted into the reaction mixture (depth = 30 mm, the thermometer was insured to submerge in the liquid). A spiral condenser with the water-oil separator was connected with the reactor to condense the vapour (the lowest azeotrope) to the mixture liquid of oil and water. The oil phase was separated from the mixture using the water-oil separator and recirculated back to the reactor. The water phase was extracted. An overhead stirrer was used to mix the reaction fluid with the stirring speed of 1,500 rpm. In order to monitor the progress of the reaction, reaction samples were taken regularly, via a sampling port and syringes, for quantitative analysis.In a typical reaction under conventional heating, the following procedure was used: the reactor was charged with phthalic anhydride (40 g) and 2-ethylhexanol (100 mL) and heated to the desired reaction temperature (at 180, 200, 210, 220 and 230 °C, respectively) under vigorous stirring; then a homogeneous catalyst (i.e. tetrabutyl titanate, 0.6 g) was introduced to the mixture to initiate the esterification reactions at a required constant temperature; over the course of the reaction, the sampling of the liquid phase (0.1 mL) was performed every 20 min to monitor the progress of the reaction. Microwave reactive distillation (MARD) synthesis of DOP

For MARD synthesis of DOP, a three-necked flask reactor (200 mL, maximum diameter = 7 cm) was placed in an in-house developed microwave applicator (monomode) which has a cylindrical cavity (inner diameter = 200 mm, height = 150 mm) connected with the microwave generator/controller via the rectangular waveguide, as shown in Fig. 1. The microwave generator consists of a voltage-controlled microwave oscillator which oscillates at 2,450 MHz and an MW amplifier with the variable power output of 50–150 W. The temperature of the reaction mixture was measured using a fibre optic temperature sensor (FISO Technologies, FOTL-BA). The reactor was also connected to a sampler and a spiral condenser with a water-oil separator. A magnetic stirrer was used during the reaction 1,500 rpm. It is worth noting that the maximum horizontal depth of the liquid system in the flask was about 3 cm. Therefore, considering the half-power penetration depth of alcohols at 2,450 MHz.36 we estimate that the electromagnetic field can penetrate the whole reaction system and to be absorbed for an effective microwave heating.In a typical reaction under microwave irradiation, the following procedure was used: the reactor was charged with phthalic anhydride (40 g), 2-ethylhexanol (100 mL) and tetrabutyl titanate (0.6 g) and placed in the microwave cavity; the fibre optic sensor and condenser were then installed prior to the reaction; the reaction was initiated by the microwave-induced

heating at the desired temperatures of 180, 200, 210, 220 and 230 °C, respectively. During the reaction under the microwave irradiation process, the temperature of the mixture was maintained at a target temperature (the accuracy is ±2 °C) by manually adjusting the microwave power. During the reaction under the microwave, same sampling procedure and frequency were used in the reaction under conventional heating to monitor the progress of the reaction.Phase equilibrium measurement

Details of the experimental rig and the experimental procedure used for the phase equilibrium measurement under the microwave irradiation have been described elsewhere in our previous publications.22, 23 Specifically, for this work, the homogeneous mixture of 2-ethylhexanol and water (total 70 mL with the water content adjusted to 0.5%, 1% and 2%, respectively, for different experiments) was added into the vapour-liquid steady state (VLSS) chamber in this test. The microwave power was regulated as 0 (conventional heating), 120, 240 and 360 W. When the mixture reached gas-liquid equilibrium under atmospheric pressure, samples (0.1 mL) were taken from both the gas phase and liquid phase for analysis.Sample analysis

Samples from the experiments were analysed according to the GB1668-81 method30 to quantify the content of monoester. To perform the analysis, the sample (monoester, 1.0 g) was dissolved in anhydrous ethanol (50 mL), then the analysis indicator of 1% phenolphthalein was added (2~3 drops). The mixture was then neutralised with 0.1 mol/l potassium hydroxide until the pink colour appeared and persisted for a minimum 15 s.

Results and discussionPhase equilibrium under the conventional heating and microwave irradiation

In esterification, 2-ethylhexanol was used in excess (also as the solvent) to improve the conversion of phthalic anhydride.


Fig. 1. Experimental rig for reactive distillation by microwave irradiation. 1. Spiral condenser with water-oil separator, 2. Three-necked flask, 3. Sampler, 4. Microwave cavity, 5. Rectangular waveguide, 6. Microwave generator, 7. Microwave cavity cover, 8 and 9. Fibre optic temperature sensor and display.




Therefore, the understanding of the phase equilibrium of 2-ethylhexanol/water binary mixtures by different heating methods is necessary before MARD. Fig. 2. depicts the results of phase equilibrium of 2-ethylhexanol/water binary mixtures (with different water contents) under the different heating conditions. In all experiments regarding the phase equilibrium, the system temperature was maintained at its bubble point temperature regardless of the water content. The bubble point temperature for this system increased gradually with the increase of the water content in the system. When the content water was adjusted to 0.5%, 1%, 2%, the system temperature was 180.05°C, 178.61°C, 176.78°C, respectively (Fig. S2). It is noticed that, under the conditions used in this work, the liquid composition of systems (xl, water, solid scatters in Fig. 2) was barely affected by neither the heating method nor the initial composition. A water composition of ca. 0.25% water in the liquid phase (SD = 0.05%) was always achieved at the gas-liquid equilibrium state. Water in the vapour phase (yv, water) is a function of the initial composition (or the system temperature), regardless of the heating methods. However, the microwave heating promoted higher water compositions in the vapour phase than the conventional heating (open scatters in Fig. 2), proving the selective heating effect induced by the microwave irradiation (due to the relatively high loss tangent of water, δ = 0.12337). We found that, under microwave irradiation, the value of yv, water increases by increasing the microwave power regardless of the initial feed composition, e.g. for experiments with an initial water content of 2%, yv, water

increased by about 86% by raising the power from 120 W to 360 W.

Effect of temperature on the conversion of monoester

In esterification of phthalic anhydride with 2-ethylhexanol, the formation of monoester (i.e. mono-2-ethylhexyl phthalate in Step one, Scheme 1) is instantaneous during the dissolution of phthalic anhydride in 2-ethylhexanol. However, the conversion of the monoester is a function of temperature. For the two systems under study, the system temperature is believed to be established quickly and homogeneously by both heating

methods. In order to ensure the homogeneity of the temporal and spatial temperature of the MARD system, temperatures were measured at various times and locations during the reaction. We found that the deviation in temperatures was insignificant in the MARD system. For example, with a set temperature of 180 °C, an average temperature of 180.1±0.3 °C was measured in MARD, as shown in Fig. S3. Under conventional heating, the temperature showed a positive effect on the conversion of phthalic anhydride (Fig. 3a). By increasing the reaction temperature from 180 to 230 °C, the time required for achieving 90% conversion has been significantly reduced from 108 to 40 min. Similar temperature effect on the system was also measured under microwave irradiation (as shown in Fig. 3b) under comparable reaction conditions, showing the accelerated reaction rate promoted by increasing the system temperature. Koshima et al. conducted the microwave-assisted esterification of octanoic acid with 1-octanol (hydrochloric acid as catalyst in a monomode microwave reactor) in both glass (microwave transparent) and silicon carbide (SiC, microwave absorbing) reactor tubes.38

Protonated octanoic acid was proposed as the molecular microwave absorber in the glass reactor to promote the chemistry at 150 °C, while, in the SiC reactor, the reactor absorbed microwave for conductive heating. However, 1-octanol could also be rapidly heated to 150 °C in about 2 min. Thus the thermal effect on the chemistry could not be decoupled in this case.38 Under microwave heating, the constant reaction temperature was maintained by adjusting the microwave power constantly during the reaction. More power was required during the process of ramping the temperature towards the set temperature than that under steady state, i.e. about two-fold. However, in general, the laboratory MARD system only needed relatively low power input during the reaction (e.g. 60 W at 180 °C, whereas the conventional heating required 300 W), showing the energy efficient aspect of the MARD system.Effect of the microwave power


) and the vapor phase (yv, water) at the gas-liquid phase equilibria under the conventional and microwave heating.



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The influence of the microwave power density on the system was investigated by conducting the reaction at the 0.5, 1, 1.5 kW/L, respectively. Fig. 4a shows the conversion of monoester as a function of time-on-stream at different microwave power densities, showing that the completion time of the reaction gradually decreases by increasing the microwave power

density (i.e. about 110 min at 0.5 kW/L versus 85 min at 1.5 kW/L). By measuring the system temperature during the reaction, it was found that as the microwave irradiation proceeded, the system was rapidly heated (inset, Fig. 4b), especially for the high microwave power density. By correlating the conversion data with the system temperature, the linear relationship was established for all cases, as shown in Fig. 4b. Therefore, we can conclude that the microwave power density has impacted on the DOP esterification reaction mainly through the thermal effect via the rapid volumetric heating of the liquid phase.

Intensification the synthesis of DOP by microwave irradiation

Based on the comparative study of the two systems, the intensification of the reaction system by the microwave irradiation was confirmed, reflected by the improved apparent activity of the MARD. The apparent activation energy (Ea) of the two systems was graphically determined, as shown in Fig. 5 (kinetics of the reaction system are presented in ESI, Fig. S4). MARD shows a lower value of Ea (42.2 kJ/mol) than the system by conventional heating (Ea = 93.9 kJ/mol), implying a higher efficiency of MARD to produce DOP.By comparing the time required to achieve the 95% conversion of the two systems (Fig. 6), we observed faster reaction rates promoted by the microwave irradiation than the conventional heating, especially at low temperatures. For example, at 180 °C for reaching 95% conversion, the time required by MARD was reduced by 22% compared to that under the conventional heating, while for the systems at 230 °C, the difference in time was insignificant. We believe that the microwave irradiation mainly affects the phase separation of the system, especially at low temperatures, but being less influential on the homogeneous reaction in the liquid phase. In an open system under conventional heating, the driving force of water

evaporation is related to thermodynamics. Evaporation of water from the reaction system occurs in order for the system to reach thermodynamic equilibrium. Therefore, the higher of the system temperature, the higher the evaporation of water from the reaction mixture, and hence moving the reaction equilibrium towards the production of DOP.In MARD, water at the surface interacts with the microwave, receiving the energy transmitted by the microwave selectively and being readier to leave the thermodynamic equilibrium into the vapour phase (i.e. a change in the relative volatility of the system). Therefore, at the interface, the net flow of water molecules from the liquid phase to the vapour phase is positive, breaking the thermodynamic equilibrium and pulling the reaction even further to DOP production. The microwave effect on the phase change of water is expected to be more effective at the relatively low temperatures than the high ones because, at the relatively high temperatures, water molecules move about more rapidly, leading to the high speed of evaporation regardless of the microwave irradiation. PI by microwave via selective heating was also reported in other multiphase systems. For example, in a fluidised gas-solid system (i.e. catalytic oxidation of n-butane over vanadium phosphorous oxide for maleic anhydride production), the selective heating of solid catalyst by microwave intensified the solid phase reactions over ones in the gas phase, thus significantly enhance the overall performance of the reactor in comparison with the conventional heating, where solids, bulk, and bed temperatures are identical.

ConclusionsCatalytic homogeneous esterification of phthalic anhydride with 2-ethylhexanol was studied comparatively under the microwave irradiation and conventional heating. The effect of the reaction temperature and the microwave power density on the conversion was investigated in detail, showing that the


Fig. 6. Time required by the system to achieve 95% conversion at different temperatures under the conventional heating and microwave irradiation.

Fig. 4. (a) the effect of the microwave power density and (b) the effect of the system temperature (inset: temperature (<97%)-time-on-stream plot) on the conversion of monoester in MARD.




temperature is the dominant parameter for the liquid phase reactions. However, the microwave-assisted process (i.e. MARD) demonstrated better performance than the system by the conventional heating which was confirmed by a lower apparent activation value of MARD than the conventional heating case.In order to elucidate the promotional effect of the microwave irradiation on the system under study, a study on the phase equilibrium of 2-ethylhexanol/water binary mixtures by different heating methods was performed. It was found that the water content in the vapour phase was improved by the microwave heating than that by the conventional heating, suggesting the intensification of water vaporisation at the liquid-vapour interface promoted by microwave irradiation. In summary, the microwave irradiation does not seem to have a non-thermal effect on the liquid phase esterification reaction studied. The measured enhancement in MARD is mainly due to the rapid selective heating of water content at the liquid-vapour interface, which results in the change of the relative volatility of system and fast evaporation of water molecules into the vapour phase, and hence shifting the liquid phase equilibrium towards the DOP production.

Conflicts of interestThere are no conflicts to declare.

AcknowledgementsThe authors acknowledge financial support from National Nature Science Foundation of China (Nos. 21306128, 21776202, 21336007). XF thanks the partial support by The Royal Society International Exchange Award (IE161344) to enable the collaboration. XG thanks the China Scholarship Council (CSC) for his academic visiting fellowship at The University of Manchester in the UK (file no. 201706255020).

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