(95b) Advanced Catalytic Microstructured Reactor for Continuous Chemical Synthesis Integrated with a Separation Step

Microstructured reactors are nowadays regarded as a separate class of chemical reactors with their own specific characteristics. Microreactors have a channel diameter of 50 - 500 micron, which offers numerous advantages in processing of fine chemicals when compared with traditional batch-scale synthesis. The improved heat and mass transfer properties typical of microfluidic systems enable the use of more intensive reaction conditions that result in higher yields than those obtained with conventional size reactors. Residence time and heat management in microreactors can be exactly tuned to avoid secondary reactions. This leads to a higher selectivity to desired products, especially for a series of sequential reactions. Finally, microreactors offer the capability to carry out flexible on-site production of fine chemicals at the point of demand.

The synthesis of a large number of fine chemicals, particularly in the field of flavor and fragrance chemistry and pharmaceuticals, involves the selective hydrogenation of ,-unsaturated carbonyl intermediates to the unsaturated alcohols as a critical step. At present, 2-propanol is used as a solvent in this type of hydrogenations. Owing to the safety problems inherent in the use of volatile organic materials as well as environmental concerns, it has become important to reduce the dependency on organic solvents in favor of a reaction medium that is ?green? (low volatility, low toxicity, easy recyclability, and sufficient or superior abilities as a reaction medium compared with existing organic solvents). Fluorous (i.e. highly fluorinated saturated organic) solvents offer attractive features due to their excellent characteristics for dissolving diatomic gases. Although hydrogenation reactions can be carried out under liquid/liquid/gas conditions, from a reaction rate standpoint it is advantageous to operate under two phase liquid/gas conditions. The phase transition temperature is a function of the phase molar ratio. For this reason it is important to know at what temperatures various fluorous and non-fluorous solvents become miscible. For a number of binary and ternary solvent systems, a consolute temperature above which phase separation can not occur, whatever the composition, was determined. Unfortunately, binary systems of 2-propanol with common fluorous solvents have consolute temperatures above 383 K, which is too high for most selective hydrogenation reactions. On the contrary, ternary systems with 2-propanol have a wide range of phase transition temperatures, which depends strongly on composition. Figure 1 shows the consolute temperature in a system 2-propanol ? hexafluoro-2-propanol ? perfluorodecalin. The addition of an organic substrate slightly increases the phase transition temperature: e.g. an addition of 1 mol.% of citral to a system corresponding to the red line in Figure 1 increased the consolute temperature by 3 K from 318 to 321 K. In order to extract the products from reactions involving fluorous solvents in a rational way, partition coefficients between the fluorous and organic phases were determined (Table 1). It can be seen that a molar composition of 2-propanol/HFP/PFD of 27.2/9.0/63.8 allows effectively extracting the major reaction products of citral hydrogenation to a 2-propanol ? HFP mixture after cooling to 296 K.

Citral hydrogenation requires a long time for the GC product analysis, therefore a fast heterogeneous reaction, hydrogenation of cyclohexene on a Pd catalyst, was performed to model a reacting gas-liquid flow. Optical measurements of the reacting two-phase flow were performed using a high speed camera with a maximum frame rate of 10.000 frames per second and a minimum shutter time of 2 µs. A T-mixer (Figure 2) introduces the flow into a sinusoidal channel with a rectangular cross-section of 150x50 µm2 and a length of 40 cm etched in a glass wafer by dry reactive ion etching (DRIE). The reaction was investigated in the range of superficial hydrogen velocities of 0.1-1.0 m/s (ReG = 0.6-6) and superficial liquid velocities of 0.01-0.1 m/s (ReL = 0.67-6.7) corresponding to Taylor flow [1, 2]. The Pd catalyst was deposited as a 50 nanometer thick film on the side and bottom walls of the microchannel by atomic layer deposition using sequential exposures of Pd(II) hexafluoroacetylacetonate (Pd(hfac)2) and formalin. After catalyst deposition, the wafer with microchannel was sealed with a flat glass wafer by adhesive bonding.

Steady-state conversion of cyclohexene at the channel outlet was measured by a gas chromatograph. The product and unreacted reactant dissolved in an organic solvent were separated from hydrogen using a passive gas-liquid microstructured separator positioned downstream of the microreactor. The separator chip is fabricated in a 500 micron thick glass wafer and a 100 micron thick membrane with 12 micron wide holes for the gas release was fabricated in silicon. After teflonizing with a hydrophobic polymer and subsequent Piranha cleaning, the silicon wafer was anodically bonded to the glass wafer. The difference between the system and the hydrogen disposal side was kept below 0.025 bar at 298 K by controlling the flow rate.

With the implementation of a separation step, the whole microstructured device consists of three individual modules which are connected to each other (Figure 3). The mixer module is used to warm up a gas-liquid mixture to the reaction temperature. Then, a single liquid phase with high hydrogen solubility enters the reactor module where a hydrogenation reaction take places on a catalytic coating deposited on the channel walls. A separator module is positioned downstream of the reactor module. To prevent the G/L separator chip from high pressure at the exit of the microreactor, a fast safety valve (release time: 5 ms) and a 2 m safety capillary is inserted between the microreactor and the microseparator (Figure 4). If the pressure at the reactor outlet exceeds the critical pressure of 2 bar, the safety valve is opened and the pressure is released to the environment. The safety capillary is required to provide enough time for the valve to be opened and to prevent that a high pressure peak reaches the microseparator.

In the paper, the results with respect to different modules of the microreactor/microseparator system will be presented.

References [1] V. Haverkamp, V. Hessel, H. Löwe, G. Menges, M.J.F. Warnier, E.V. Rebrov, M.H.J.M. de Croon, J.C. Schouten, M. Liauw, Chem. Eng. Technol., 29 (2006) 1015-1026. [2] M.J.F. Warnier, E.V. Rebrov, M.H.J.M. de Croon, V. Hessel, J.C. Schouten, Gas hold-up in Taylor flow in rectangular micro channels, Chem. Eng. J., submitted.