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Closed-vessel microwave heating techniques

time:2008-08-13 07:32:20  View:        

Fundamentals

Closed-vessel microwave heating techniques have been the state of the art for sample preparation in the analytical laboratory for over fifteen years. However, the application of microwaves in the organic synthesis community is only now beginning to receive widespread attention.

The first papers on the use of microwaves for synthesis reactions appeared in the open, peer-reviewed literature in 1986. Since that time, over a thousand articles have been published, numerous conferences have focused on the advance of microwave techniques, and the use of microwave processing is now the hot topic for combinatorial and parallel strategies.

Two forces are cultivating the current interest in microwaves for synthesis. First, technical advances derived from many years' experience with hardware, software, and reaction vessel design have produced microwave labstations with the performance and flexibility to meet the needs of organic chemistry. Second, the open literature is mature enough to demonstrate clearly just how effective microwaves can be at enhancing synthetic reactions.

Microwave enhancement can take several forms. Reaction rates can be accelerated, yields can be improved, and reaction pathways can be selectively activated or suppressed. Fundamentally, microwaves heat things differently than conventional means.

To help the synthetic organic chemist understand what microwave heating is and how it can be used to enhance synthetic reactions, we present the following treatise. Our purpose is to educate, to inform, and to help chemists develop their expectations for this rapidly proliferating technology.

Microwaves Are Energy

Microwaves are a form of electromagnetic energy. Microwaves, like all electromagnetic radiation, have an electrical component as well as a magnetic component. The microwave portion of the electromagnetic spectrum is characterized by wavelengths between 1 mm and 1 m, and corresponds to frequencies between 100 and 5,000 MHz. Milestone microwave labstations use a specific, fixed frequency of 2,450 MHz (2.45 GHz).

It is useful to consider the quantum energy of microwaves in relation to other forms of electromagnetic energy. It is important to recognize that the energy delivered by microwaves is insufficient for breaking covalent chemical bonds. This information can help to narrow speculation on the mechanisms for enhancement in specific reactions.

Microwaves Can Interact with Matter

One can broadly characterize how bulk materials behave in a microwave field. Materials can absorb the energy, they can reflect the energy, or they can simply pass the energy. It should be noted that few materials are either pure absorbers, pure reflectors, or completely transparent to microwaves. The chemical composition of the material, as well as the physical size and shape, will affect how it behaves in a microwave field.

Microwave interaction with matter is characterized by a penetration depth. That is, microwaves can penetrate only a certain distance into a bulk material. Not only is the penetration depth a function of the material composition, it is a function of the frequency of the microwaves. It is not true that microwaves "heat" a bulk material "from the inside out."

Two Principal Mechanisms for Interaction With Matter

There are two specific mechanisms of interaction between materials and microwaves: (1) dipole interactions and (2) ionic conduction. Both mechanisms require effective coupling between components of the target material and the rapidly oscillating electrical field of the microwaves.

Dipole interactions occur with polar molecules. The polar ends of a molecule tend to align themselves and oscillate in step with the oscillating electrical field of the microwaves. Collisions and friction between the moving molecules result in heating. Broadly, the more polar a molecule, the more effectively it will couple with (and be influenced by) the microwave field.

Ionic conduction is only minimally different from dipole interactions. Obviously, ions in solution do not have a dipole moment. They are charged species that are distributed and can couple with the oscillating electrical field of the microwaves. The effectiveness or rate of microwave heating of an ionic solution is a function of the concentration of ions in solution.

Materials have physical properties that can be measured and used to predict their behavior in a microwave field. One calculated parameter is the dissipation factor, often called the loss tangent. The dissipation factor is a ratio of the dielectric loss (loss factor) to the dielectric constant. Taken one more step, the dielectric loss is a measure of how well a material absorbs the electromagnetic energy to which it is exposed, while the dielectric constant is a measure of the polarizability of a material, essentially how strongly it resists the movement of either polar molecules or ionic species in the material. Both the dielectric loss and the dielectric constant are measurable properties.

Microwave Heating Differs from Conventional Means

Conventional Heating Methods
In all conventional means for heating reaction mixtures, heating proceeds from a surface, usually the inside surface of the reaction vessel. Whether one uses a heating mantle, oil bath, steam bath, or even an immersion heater, the mixture must be in physical contact with a surface that is at a higher temperature than the rest of the mixture.

In conventional heating, energy is transferred from a surface, to the bulk mixture, and eventually to the reacting species. The energy can either make the reaction thermodynamically allowed or it can increase the reaction kinetics.

In conventional heating, spontaneous mixing of the reaction mixture may occur through convection, or mechanical means (stirring) can be employed to homogeneously distribute the reactants and temperature throughout the reaction vessel. Equilibrium temperature conditions can be established and maintained.

Although it is an obvious point, it should be noted here that in all conventional heating of open reaction vessels, the highest temperature that can be achieved is limited by the boiling point of the particular mixture. In order to reach a higher temperature in the open vessel, a higher-boiling solvent must be used.

Microwave Heating
Microwave heating occurs somewhat differently from conventional heating. First, the reaction vessel must be substantially transparent to the passage of microwaves. The selection of vessel materials is limited to fluoropolymers and only a few other engineering plastics such as polypropylene, or glass fiber filled PEEK (poly ether-ether-ketone). Heating of the reaction mixture does not proceed from the surface of the vessel; the vessel wall is almost always at a lower temperature than the reaction mixture. In fact, the vessel wall can be an effective route for heat loss from the reaction mixture.

Second, for microwave heating to occur, there must be some component of the reaction mixture that absorbs the penetrating microwaves. Microwaves will penetrate the reaction mixture, and if they are absorbed, the energy will be converted into heat. Just as with conventional heating, mixing of the reaction mixture may occur through convection, or mechanical means (stirring) can be employed to homogeneously distribute the reactants and temperature throughout the reaction vessel.

The Microwave Effect
To understand how microwave heating can have effects that are different from conventional heating techniques, one must focus on what in the reaction mixture is actually absorbing the microwave energy. One must recognize the simple fact that materials or components of a reaction mixture can differ in their ability to absorb microwaves. Differential absorption of microwaves will lead to differential heating and localized thermal inhomogeneities that cannot be duplicated by conventional heating techniques.

To illustrate the consequences, several examples are presented wherein we consider microwave absorption by a bulk solvent and/or by the minor concentration of reactants in the solvent.

Example 1: Solvent and Reactants Absorb Microwaves Equally
If the bulk solvent and reactants absorb microwaves equally, then energy transfer and heating will occur to the allowed depth of penetration into the bulk mixture. Homogeneous reaction conditions can be established with thorough mixing, and at equilibrium (chemical and thermal), the temperature of the reactants will be the same as that of the bulk solvent.

In this case, reaction rates can be increased by increasing the temperature of the reaction mixture. This can easily be achieved using closed-vessel microwave techniques, using the same reaction chemistry and solvent. Alternatively, using conventional heating techniques, higher reaction temperatures can be achieved in a closed reactor system, or by using a higher-boiling solvent in an open vessel.

Example 2: Solvent Absorbs Microwaves, Reactants Much Less So
If the bulk solvent absorbs microwaves, but the reactants do not absorb (or absorb to a lesser extent than the solvent), then energy transfer and heating of the solvent will occur to the allowed depth of penetration. The bulk solvent will, in turn, heat the reactants by conduction. Homogeneous reaction conditions can be established with thorough mixing, and at equilibrium the temperature of the reactants will be the same as that of the bulk solvent.

This case is little different from conventional heating techniques. Reaction rates can be increased by increasing the temperature of the reaction mixture. This can easily be achieved using closed-vessel microwave techniques, using the same reaction chemistry and solvent. Alternatively, using conventional heating techniques, higher reaction temperatures can be achieved in a closed-vessel reactor system, or by using a higher-boiling solvent in an open vessel.

Example 3: Reactants Absorb Microwaves, Solvent Much Less So
If the bulk solvent does not absorb microwaves, but the reactants do, then direct energy transfer and heating of the reactant molecules will occur to the allowed depth of penetration. The bulk solvent will, in turn, be heated by conduction from the reactants. Although homogeneous reaction conditions can be established with thorough mixing, the temperature of the reactants will always be higher than that of the solvent, as long as the solvent continues to lose heat to the environment through the vessel wall.

This case is significantly different from conventional heating techniques. Reaction rates can be increased by increasing the temperature of the reactants, delivering microwave energy faster than the heat can be transferred to the bulk solvent and radiated to the environment. For this effect to be sustainable, careful attention must be paid to vessel design and vessel cooling. This effect can be achieved using microwave reflux techniques. It must be recognized that only the bulk temperature can be measured by direct insertion of a monitoring probe, so there really isn't any practical way of measuring a temperature differential between the reactants and the solvent. Multimode cavities, with their higher output power (1,000 watts or more), are best suited to creating the necessary conditions for obtaining this unique microwave effect.

Example 4: Catalysts on Microwave Absorbing Supports
Some unusual reaction conditions can be created in a microwave field when catalysts are present in the mixture, particularly when the catalyst is deposited on a microwave-absorbing material. Palladium on carbon is a common catalyst in some reaction mechanisms. Carbon or graphite is an excellent absorber of microwave energy, with a dissipation factor significantly higher than most solvents.

An unexpected effect of the microwave field is that it can directly heat some catalyst supports, and create a condition where the catalyst is at a substantially higher temperature than the rest of the bulk mixture. The catalyst support will transfer heat to the bulk mixture by conduction. There really isn't any practical way to measure the temperature at the surface of the catalyst support. The enhanced reactivity, however, can be quite dramatic, as evidenced by the reaction products. This superheating of the catalyst cannot be duplicated by conventional means. Multimode cavities, with their higher output power (1,000 watts or more), are better suited to creating the necessary conditions for obtaining this unique microwave effect.

Single-Mode and Multimode Microwave Cavities
The working compartment where materials are exposed to microwaves is variously called a cavity, a resonator, an applicator, etc. Cavities are available in many sizes and shapes, depending on the manufacturer and the application.

Despite the variety of cavities available, there are in reality only two basic types: single-mode (often referred to as monomode), and multimode. It is important to recognize the essential difference between the two types. Far more important is for synthetic chemists to recognize how cavities are the same in every important aspect with regard to chemical reactions. The distinction between cavity types is determined by the design characteristics, not by the processes that can be performed inside the cavity. The design characteristics are determined by many factors that have nothing to do with the reaction chemistry.

Single-Mode Cavities
The essential characteristic of single-mode cavities is the deliberate creation of a standing wave pattern inside the cavity. To do this, the dimensions of the cavity must be carefully controlled to correspond in some systematic way to the characteristic wavelength of the microwaves. Just as the resonant frequency of a standing wave on a violin string is a function of the length of the string, so too is the maintenance of that standing wave determined by the stability of the string dimensions.

For 2.45 GHz microwaves, the length of a single full wave is 12.2 cm. A single-mode cavity for 2.45 GHz microwaves must be dimensioned such that a whole-number multiple of the full or half wavelength fits inside the confines of the cavity.

Several consequences of single-mode cavity design must be appreciated. First, microwave-absorbing materials placed inside such a cavity will, in fact, absorb microwaves and be heated. Of course, if the intensity of the field is controlled, then the heating rate can be controlled, some sort of temperature monitoring and feedback control scheme can be implemented, etc.

The second consequence is far more important. There are specific positions inside the single-mode cavity where items to be heated must be placed. The intensity of the field is greatest at the peaks of the standing wave, and actually goes to zero at the nodes of the standing wave. There are positions in the single-mode cavity (the nodes) where no heating will occur. Only in the volume of the peak envelopes can transfer of microwave energy to a target absorbing material occur. This substantially limits the physical dimensions of objects (such as reaction vessels) that can be placed in a single-mode cavity and be heated effectively.

The third consequence is probably the most important. Anything placed inside a single-mode cavity can disrupt the standing wave pattern. For this reason, it is quite uncommon for single-mode cavities to be designed to accept more than one target object (such as a vessel) to be heated at a time.

Single-mode cavities do not lend themselves to simultaneous processing of multiple targets (such as reaction vessels). Also, processing materials in single-mode cavities can result in inconsistent heating due to variation in the volume, total absorbing mass, or physical shape (in the cavity) of the material to be heated.

A final consequence of single-mode cavity design is the fact that there is no practical way to scale up processes. One must recognize from the outset that microwave processes developed on a small scale may need to be transferred to a larger scale for production. One cannot extend the dimension of a standing wave cavity indefinitely, nor can one simply purchase components off the shelf to generate a standing wave of just any delivered power.

Multimode Cavities
The essential characteristic of multimode cavities is the deliberate avoidance and/or disruption of any standing wave pattern inside the cavity. The engineering goal is to produce as much chaos inside the cavity as possible.

There are two principal approaches to achieving this goal. First, the dimensions of the cavity must be carefully controlled to avoid whole-number multiples of the microwave full or half wavelength. Second, some means must be employed to physically disrupt any standing waves that may form as a consequence of items placed in the cavity. This is best performed with a mechanical mode stirrer, typically a periodically moving metal vane that continuously changes the instantaneous field pattern inside the cavity. The shape of the vane and its movement is such that the microwave field is continually stirred, and therefore the field intensity is homogeneous in all directions and all locations throughout the entire cavity.

Several consequences of multimode cavity design must be appreciated. First, microwave-absorbing materials placed anywhere inside the cavity will, in fact, absorb microwaves and be heated. Of course, if the intensity of the field is controlled, then the heating rate can be controlled, some sort of temperature monitoring and feedback control scheme can be implemented, etc.

The second consequence is far more important. In a properly designed multimode cavity, there is no specific position inside the cavity where items to be heated must be placed. Because the field is homogeneous everywhere and in all directions, every position in the cavity is like every other position.

The third consequence is perhaps the most important. Since the field in a multimode cavity is continuously re-homogenized (stirred), nothing placed inside the cavity will permanently affect the distribution of the field intensity. This means that there are no limitations on the size and shape of the objects placed inside the cavity. If they will physically fit inside the cavity and if they absorb microwaves, then they will be heated.

This also means that multiple objects (for example, multiple reaction vessels) can be processed simultaneously, just as effectively as single objects. This is a dominant advantage of multimode cavity design.

A final consequence of multimode cavity design is the fact that practical scale-up of processes is easy. Any process that is developed in a bench-top multimode microwave labstation can be scaled up to production or pilot plant capacities. A multimode cavity can, in theory, be infinitely extended in every dimension. With a multimode design, additional field intensity to fill that extended cavity can easily be had by simply adding additional magnetrons to the system.

These facts of cavity design can be important considerations when first exploring the microwave advantage. The user must carefully assess how the microwave system will be used in the lab today, and the possible applications and configurations for the future. Only a multimode cavity provides the opportunity to go beyond a single reaction format to multiple reactions simultaneously and uniformly. Only a multimode cavity allows you to progress from discovery through to production, in any form factor, with confidence.

  

 
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