(83k) An Experimental Study on Nitration of Naphthalene Using Dynamically Rotating Axis Microreactor

Microreactor for chemical synthesis is a promising tool which enables high reactivity, high selectivity, process safety, and so on. On the other hand improvement of productivity is essential for industrial production of chemical materials. Such improvement includes acceleration of reaction besides numbering-up or scaling-up of reactors. Area of interface between substrate and reagent is dominant factor to accelerate liquid-liquid multi-phase reaction such as nitration of aromatic compounds. In case of T-shape mixer strong mixing accelerate reaction at impinging point and retention tube. Even emulsions can be generated at higher flow rate, however, large pressure drop is caused by increasing flow rate and long; 1 to 10 m typically; retention tube. In addition, aggregation of emulsions decreases the effect of elongation of reaction tube.

In this study dynamically mixing microreactor was developed and utilized for nitration of naphthalene. Cylindrical rotor generates shared flow in microchannel and makes micro emulsion, which shorten retention time needed for high conversion rate. This fact can also decrease pressure drop of microreactor system.

The photograph of dynamically rotating axis microreactor (DRAM) is shown in Fig. 1. DRAM is composed by three divided components; rotation drive unit, mixing unit, and rotation sensing unit. A motor packed in rotation drive unit rotates a magnet settled on driving shaft. At the mixing unit made of PTFE and ceramic materials, liquid substrate impinges reagent and flow through the gap between inner and outer cylindrical surfaces. The inner cylinder which has a magnet within itself is magnetically coupled and rotates with driving shaft. Shared flow generated at the gap between cylinders brings about strong mixing of the liquids. Minimum gap width is 0.1 mm and total inner volume of the mixing unit is about 3 ml. Rotation speed of the cylinder can be monitored by magnetic sensing device packed in the rotation sensing unit.

Nitration acid and naphthalene solved by dichlorobenzene were pumped to DRAM and mixed under room temperature. Mixed liquid was conducted to short retention time unit; 0.5 mm in I.D and 100 mm long, and recovered into plenty of distilled water in order to dilute acid concentration and quench reaction. The aromatic phase of the recovered liquid was separated and analyzed by gas chromatography. Nitration acid was the mixture of nitric acid and sulfuric acid. Molar ratio of nitric acid and naphthalene was 10. The degree of nitration reaction was investigated by changing rotation speed of the rotor and total flow rate between 0 and 4000 rpm, and 1and 20 ml/min, respectively.

The gas-chromatographs of recovered samples are shown in Fig. 2 obtained at the flow rate of 2 ml/min. Nitration of naphthalene proceeds even at the rotation speed of 0 rpm; a large amount of mononitronaphthalene (about 90% of initial material, naphthalene), dinitronaphthalene (1,5- and 1,8-derivatives, about 10%), and nitrated dichlorobenzene were detected. At the higher rotation speed micro emulsions were observed in the recovered liquid and the amount of dinitronaphthalene increased with the rotation speed. The molar ratio of 1,5-dinitronaphthalne to 1,8-dinitronaphthalene was about 0.5, and other dinitro derivatives were quite small (below 0.5%) and trinitronaphthalene was not detected.

The gas-chromatograph yield of dinitronaphthalene is shown in Fig. 3 with the relationship between total flow rate and rotation speed. The value of yielding is no less than 80% at the small flow rate below 5 ml/min and moderate rotation speed around 1000 rpm. The maximum yield is greater than 90% at 2 ml/min and higher than 1000 rpm. On the other hand, yield gradually decreases below 20% with the increase of flow rate up to 20 ml/min because of the shortening of retention time at the cylinder. Scale-up of the cylindrical parts and increment of rotation speed will work effectively in higher flow rate.

For the case of T-shape mixer 10 m-long tubing was needed for 30% yield of dinitronaphthalene at 10 ml/min. DRAM connected with 100 mm-long tubing accomplishes 30% yield at 1000 rpm. The needless of long retention time unit realizes considerable decrement of pressure drop; about 10 MPa for T-shape with 10 m tubing, 0.06 MPa for DRAM with 0.1m tubing. This brings important advantage in industrial nitration process because of needless to introduce high pressure pump and rigid metal tubing which are not tolerable under corrosive condition.

Acknowledgement

This research was financially supported by the Project of ?Development of Microspace and Nanospace Reaction Environment Technology for Functional Materials? of New Energy and Industrial Technology Development Organization (NEDO), Japan.