CEM Discover Microwave

Transcription

1 CEM Discover Microwave User Guide and Tutorial for Conducting Microwave Experiments in the Reynolds Research Group 2 nd Edition: April 2012 Georgia Institute of Technology School of Chemistry & Biochemistry School of Materials Science and Engineering Written by: Coralie A. Richard 1

2 Table of Contents 1. Introduction to microwaves and microwave reactors Microwave Hardware Principle of Operation Troubleshooting Contact and references Annexes

3 Microwave Tutorial 1. Introduction to microwaves and microwave reactors A microwave is a radio wave with a frequency ranging from 300 MHz to 300 GHz. 1 They are used in everyday life to heat food, and they have attracted more and more interest in organic and inorganic chemistry these last 15 years. There are three different classes of materials as far as microwave chemistry is concerned: insulators, which are not affected by microwaves, conductors (such as metals) which will reflect the energy, but do not heat, and dielectrics (polar molecules, ions) which will absorb the energy and will be heated. Depending on the material present, superheating can been observed, especially when metals are used. It is important to remember that you will need some polar or ionic solvents or molecules to capture from the microwave heating effect. Conventional heating (e.g. from a hot plate) will heat a reaction by convection, from the outside to the inside, and it will take a long time for the whole flask to be heated. Microwave heating produces a more localized heating, and acts on all parts of the vessel. It has been suggested that as the material more rapidly and directly absorbs the heat, the high temperature can activate a larger percentage of molecules above the required activation energy. 2 Relating this to the Arrhenius equation (equation 1) as you heat more rapidly above the boiling point temperature of the solvents (the system is closed), the activation energy is lowered. The use of a microwave reactor can speed up the reaction time, lead to higher yield, improve the purity of the product and reduce the amount of solvent used. Equation 1: Arrhenius equation, k: rate constant of the chemical reaction (s -1 ), A: preexponential factor (s -1 ), Ea: activation energy (kj.mol -1 ) R: universal gas constant (kj.mol - 1.K -1 ), T: temperature (K). The microwave can also be used for selective heating, ideally to synthesize and extract the products at the same time 1, but for our chemistry, this effect is not very useful. Of course, a microwave reactor is not a black box, and sometimes, weird results are obtained. Consequently, it is important to first perform a reaction using conventional 3

4 heating, and then to apply microwaves. As a rule of thumb, if a reaction works under conventional heating, it is likely to work using the microwave. As always, a well thought issue is half resolved. The microwave reactor in our laboratory (CEM SP Discover) permits experiments to be performed in both closed and open vessels, and also allows reactions to be run under various conditions: fixed power, cycles, maximum temperature/pressure etc. The reactor also keeps track of the parameters (temperature, pressure, time, etc.) used during a reaction and these parameters can be extracted with a USB drive. Finally, the microwave can be operated by a computer instead of manually, using the Synergy software. 2. Microwave Hardware The main components of the microwave reactor are shown in Figure 1: the nitrogen hook up with a regulator (1), the cavity at the top where the vessel is inserted (2), the screen and touch pad where the parameters are chosen (3) and the trap where overflow chemicals go on the back of the microwave (4). Below in Figure 2 is a close up picture of the screen, with a description of the buttons. 4

5 1: Spin bar on/off and speed 2: Microwave power 3: Air Cooling 4: Home (screen in the picture) 5: Start/Pause 6: Arrows to scroll up and down on the menu, as well as to change parameter values 7: Enter 8: Discover button 9: Stop 10: Temperature 11: Pressure 12: Time 3. Principle of Operation Add your reagents and solvent (if needed) to the vessel. Do not fill more than ¾ of the vessel to avoid overflow. Add a spin bar if needed. The minimal amount required is the one which cover totally the spin bar. If you are using metal (catalyst for example), be sure to cover all metals with reagents. Cap the reaction vessel and insert the vial in microwave. Check the nitrogen pressure on the regulator. It should be at 30 PSI. The nitrogen is used to rapidly cool the vessel after the reaction and can be toggled using the Air Cooling feature. Press Enter to exit the screen saver, and press enter again to choose the user profile. Press Discover to see a list of methods. Each method allows you to adjust a different set of parameters: Standard will let you choose the time and the temperature Fixed power will let you choose the power you want, the time, and the maximum temperature. If the maximal temperature is exceeded, the microwave will stop and cool the 5

6 vessel. You can choose PowerMax, which will blow nitrogen continuously to eliminate any heating while still subjecting the vial to microwaves. SPS will let you choose the power, the time and the temperature range. Load will let you download a saved method Dynamic will let you choose the time, the maximum temperature, the maximum power, and the maximum pressure your reaction will be run at. Once one of the maximum parameters is attained, the system will cool and vent. You can also choose the spin bar speed (none, low, high), and choose to premix your reagents for a certain time at RT. Power Cycling will let you run cycles of power. You choose the voltage as well as the minimal and maximal temperatures. You can also choose the number of cycles and the time between the cycles. Data Review is to be used when you want to recover the temperature, pressure and power evolution as a function of time. Install an USB drive and follow the instructions on the screen. Save will allow you to save the method. I will advise to choose the program first, and then it will automatically ask you if you want to save it or not. Pressing the discover key when you are trying to fix the parameter will bring you to a method venting setup where you can set the pressure at which you want the system to vent. It is great option when you generate gas as a product and you want to drive the reaction to the products side by eliminating the gas formed. In this case, choose a low pressure. The delta pressure should be of 25 PSI (recommended by CEM). NB: You can enter a temperature superior to the boiling point of your solvent as the vessel is closed and can take up to 250 PSI. This is one way to speed up your reaction. To change the values of the parameters, use the arrow buttons (up/down, right/left). You can also press the temperature/pressure/etc buttons on the pad and change the values with the arrows. When you are done, press enter and then play. The screen will ask you if you want to save your set up. If you will use the same parameters repeatedly, save it and you can recall it next time. Otherwise, do not save it as we do not want to overload the library. The user must stay for the first few minutes to be sure that the automatic trap is moving to close the opening; also, check the pressure of your system. You can manually stop the reaction at any time by pressing the red octagonal button. Once the time is over, the vessel will automatically be cooled, and the trap will automatically open. Cleaning: Tubes can be cleaned in the base bath, and then in the dishwasher. Caps can be cleaned with acetone, or other solvents, by taking a kim wipe loaded with your solvent, and 6

7 brushing to clean the surface. Sometimes, the reaction will overflow. To clean, take out the white recipient, clean it with acetone, open the trap in the back of the microwave and clean it with acetone. For air sensitive reactions, reactants and solvent need to be inserted in the vial in a glove box. Then, the vial can be taken outside of the box, and in the microwave. Keep in mind that the cap is not entirely hermetic, and that oxygen/air can enter during the reaction in the microwave. With an open vessel reaction, you can add a condenser. Remember that in this case, you will not be able to go higher than the boiling point temperature of the solvent. Some people say that this feature just transforms your microwave into an expensive hotplate, but some others tell you that you still have better results than conventional heating. You can also use this open vessel configuration in conjunction with a vacuum pump to put your system under vacuum. This is useful for reaction when you need to pull formed products to push your reaction to the products side, like ADMET polymerization. 4. Troubleshooting As in most electrical devices, when something weird happen, try to switch off the instrument, remove the plug, leave it for a minute, plug it back and switch on. That works 90% of the time. Overflows: overflow can happen when the vessel is too full, or a lot of gas is generated. Just clean both the plastic cup and the metallic tray on the back of the microwave. 5. Contact and references Our contact for the microwave is Greg Barlow from CEM: Greg Barlow CEM Corporation ext 402 greg.barlow@cem.com CEM website is full of information, videos and a good references for papers. 1 : Wikipedia, microwave and microwave chemistry 2: CEM website 3: 7

8 6. Annexes List of interesting articles Vaibhav P. Mehta and Erik V. Van der Eycken Chem. Soc. Rev., 2011,40, 4925 Robert C. Coffin, Jeff Peet, James Rogers & Guillermo C. Bazan, Nat. Chem., 2009, 1, 657 Kristian Kempe, C. Remzi Becer, and Ulrich S. Schubert, Macromolecules, 2011, 44 (15),