Microcatalytic Chromatographic Technique

The microreactor is probably the most direct link between GC and catalysis and has been widely used

Carrier gas

Flow controller

Flow controller

Figure 3 Schematic of a microcatalytic chromatographic reactor. TCD, thermal conductivity detector.

for catalyst research since it was first described in 1955. A schematic of the system is shown in Figure 3. The principle of this 'one shot' technique is simple. A small catalytic reactor is placed as near as possible to the entrance of a GC column. Small pulses of reactant are injected into an inert or reactive carrier gas stream just before the reactor. Reaction takes place in the catalyst bed and the unconverted reactant pulse together with volatile products are swept into the GC column where the components are separated in the usual way and then detected by a rapid response microthermal conductivity detector. A useful adjunct, employed even in the earliest studies, is to place a Geiger detector in the exit stream from the thermal conductivity detector so that the radioactivity of each peak can be determined. This extends the method to include tracer experiments using 14C- and tritium-labelled compounds. The advantages and disadvantages of the microreactor are listed below.

Advantages

1. Rapid surveys of new or modified catalyst are possible, enabling rapid assessment of whether the changes in catalyst formulations have been beneficial or detrimental (but see item 6 below). The rate of acquisition of results ultimately depends on the speed of analysis; because of its relatively slow speed, the interval between successive pulses can be tens of minutes in packed columns but less in the high speed wall coated open-tubular columns (WOTC) now available.

2. The high sensitivity of GC enables very small reac-tant pulses to be used with a consequent reduction in the risk of poisoning active sites by trace impurities.

3. Radioactive tracer experiments are straightforward, enabling the path of individual reaction sites in reactant molecules to be tracked into the product molecules.

4. By careful analysis of the changes in product yields with changes in flow rate (contact time) and sample size, the rate expression of the reaction can be established and rate constants and Arrhenius parameters estimated (but see Disadvantage 2).

5. The technique is particularly well suited for kinetic isotope experiments. Alternate pulses of a deuterated and protonated reactant can be injected into the microreactor so that their reactions are compared on exactly the same catalyst sample under identical conditions. Very accurate measures of primary and secondary kinetic isotope effects (the ratio kD/kH) can obtained and subtle mechanistic detail unravelled.

6. Poisoning effects can sometimes be monitored if the activity of the catalyst is found to diminish with successive pulses, an aspect that can be enhanced by using longer contact times or larger pulses.

7. The developments in data acquisition, storage and processing, coupled together with the use of highly reproducible and readily automated loop sampling valves, benefit and facilitate the use of the micro-catalytic reactor for mechanistic and kinetic studies and for catalyst screening.

Disadvantages

1. Steady-state conditions are often not achieved in the short contact times used in microreactors. This can be useful for kinetic studies, but is a disadvantage for screening tests of a new catalyst or a new formulation since the catalyst will usually operate commercially in a continuous flow configuration, where steady-state or pseudo steady-state conditions prevail.

2. Accurate measures of the residence time of the reactant pulse in the catalyst bed are essential for reliable kinetic measurements. The time cannot be calculated simply from the outlet gas volumetric flow rate and the bed volume because of the considerable pressure drop between the reactor and the end of the GC column, which varies with flow rate. The corrected residence tine tr is determined from the following equation derived by application of the gas laws to the exiting carrier gas flow rate:

where L is the reactor length and a its cross-section; V0, P0 and T0 are the volume flow rate, pressure and temperature at the end of the GC column; Pi and Pr are the pressures at the inlet and outlet of the reactor.

Nonlinear first-order plots result if the uncorrec-ted residence time is used, whereas excellent linearity is obtained by using corrected residence times.

3. If the carrier gas is itself a reactant, its depletion in the gas pulse leaving the reactor can result in artefacts because of changes in detector sensitivity when the pulse components pass through the detector. This can be avoided by split stream techniques. In microreactor studies of the dehydroch-lorination of 1,2-dichloroethane, air was used as the carrier gas to eliminate the builid-up of carbonaceous poisons when inert carrier gases were used. The problem of different amounts of oxygen feeding into the flame ionization detector (FID) flame at different flow rates was overcome by splitting the stream leaving the GC column, so that the air supply to the flame could be held constant by venting different amounts of excess air through a control valve in the column exit stream. Peak broadening and overlapping occurs in the catalyst bed and in the dead volume between the end of the reactor and the beginning of the GC column, leading inevitably to some loss of resolution.

4. The data obtained are inherently integral despite the small contact times used, so that the technique is not readily adapted to give differential reaction kinetics.

5. Most catalyst laboratories will probably use microcatalytic chromatographic reactors in the near to medium-term future, especially since high speed numerical computer methods are now available to model the differential equations of flow and reaction in these reactors. These results place rigorous and stringent tests on the reaction mechanisms proposed and reliable estimates of rate parameters for some of the rate-determining steps can result. These numerical modelling techniques are much better than the use of the approximate though complicated rate expressions, which contain functions of many rate parameters often conjoined and requiring much ingenuity and chemical intuition to untwine. However, the necessity for intricate valving, pressure and temperature corrections, and computer control and data acquisition and processing, leads to a loss of the essential simplicity of the microreactor technique. These elaborations may lead researchers into using conventional catalytic reactors with proscribed switching of the reacted gas streams into multiloop sampling valves linked to GC and GC-MS analysers.

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