Outline of Conventional GC in Catalysis Research

Since many of the developments in catalysis listed in Table 2 predate the invention of GC in 1952, its contributions have inevitably been since then. However, all the important characteristics of catalysts defined in Table 1 require analysis of complex gas mixtures. Thus inevitably the use of this powerful analytical technique in catalyst research is now endemic and pervasive; so much so that it is now used almost routinely in catalyst laboratories and on catalyst testing rigs. It is the technique sine qua non for multicomponent analysis, especially when allied to mass spectrometry and spectroscopic methods. It has revolutionized the analysis of complex volatiles in the same way that liquid and paper chromatography have caused a sea change in the analysis of complex involatile bioorganic molecules. It has replaced previous tedious and time-consuming methods such as fractional distillation, which have largely been abandoned since its advent (but see later for a discussion of the online infrared methods now in an advanced state of development). The complex nature of catalysis product streams is illustrated by the chromatograph of a gasoline in Figure 1.

The acceleration of reaction rates by catalysts arises because of the lowering of the energy barriers through the formation of surface compounds. The energy released by the making of surface bonds facilitates the scission or rearrangement of bonds in the substrate molecule, as depicted in Figure 2. The surface bonds must be of intermediate strength; if they are too strong permanent bonding to the surface occurs, and if they are too weak the perturbations are too small. This gives rise to an optimum value of the enthalpy of formation of the surface intermediate and the so-called 'volcano' plots when catalyst activity is plotted against bond enthalpy.

GC cannot give direct information about these crucial surface intermediates, although ingenious experiments using radiolabelling experiments and normal kinetic analysis and numerical modelling can lead to information about their nature. Major advances in surface spectroscopic techniques have occurred since the 1960s allow the observation and quantitation of minuscule amounts (10~3 mono-layers) of stable and metastable surface species.

However, there has sometimes been a short-sighted overemphasis of the results of these powerful and expensive techniques and a neglect of those from cheaper methods of final product analysis such as GC.

Figure 1 Chromatogram of a gasoline fraction (courtesy of Dr R. Malpas, Shell Research Ltd, Thornton Research and Technology, Centre).

There is the danger that the moieties seen by surface spectroscopies are mere spectators and not the active participants. Prudent researchers interlink surface science results with those from stable final product time profiles and conventional rate studies on the same catalyst. This problem has been highlighted in an elegant demonstration that the rates of ethene hydrogenation were invariant to the levels of the much studied surface ethylidenes. This underlines the essential importance of stable product analytical methods in catalyst research and evaluation. These are integral components for the study of catalysis in

Figure 2 Energy diagram for a reaction occurring homogeneously and heterogeneously.

the multifarious reactor types in use, namely: batch, continuous stirred tank slurry, fixed and fluid bed, trickle bed, catalytic gauze and spinning basket.

The high sensitivity of GC (detection limits of 0.1 to 1 ppm) enables very low conversions to be studied. This reduces the problems of poisoning by products of secondary reactions. GC is used routinely and extensively in laboratory research to find new catalysts and processes for existing and new products. It is also used in all the scale-up stages to full plant operation. The output of the plant is constantly monitored by GC and other methods of analysis and the results used to trim operating conditions to meet product specifications when there are changes in feedstock composition, flow rates, and reactor temperatures and pressures.

Conventional reactor studies can be readily automated by using multiple sample loop valves activated by signals from minicomputers, which also acquire and process the compositional data and GC conditions in digital modes. A semiautomatic pulsed small reactor system with a simple timing device for sampling at prescribed times and early data acquisition and processing methods was described as early as 1960.

A commercial laboratory rig incorporating GC is now marketed for catalytic studies at pressures of several hundred atmospheres and temperatures up to 350°C (Chemical Data Systems, Pittsburgh, USA). The pulse of products from the high pressure micro-reactor is depressurized in a multiple loop valve so that its pressure is compatible with that of the carrier gas stream into which it is injected. The unit is semiautomatic and data acquisition, processing and control is by a personal computer (PC).

The very ease of using GC routinely often means that the technique is not mentioned in the title of research papers in the open literature, in much the same way that the use of NMR is often assumed and not noted in papers on synthetic organic chemistry. Perusal of recent issues of journals such as the Journal of Catalysis and the Journal of Applied Catalysis indicate that about 40% have used GC for product analysis, while a BIDS search of the years 1991-97 shows the linking of GC with catalysis in about 90 papers each year.

It is worth emphasizing that process improvement is more common than purposeful or serendipitous invention and discovery. This is because large companies prefer to expand and advance by steady or even marginal changes associated with low risk rather than to initiate new high risk processes requiring large capital investment and many years of negative cash flows and long payback periods. An example is hydrocracking, which was not considered worthwhile by most of the oil companies until Chevron (California, USA) brought the first hydrocracking plant on stream in the mid-1960s - competition is as much the mother of invention as necessity! The scale of operations provides the motivation for marginal improvements; a catalyst that increases the yields of a product by 1% can earn £1 million per annum if product turnover is £100 million p.a. It is in such marginal improvements that GC can prove so valuable because of the ease of measuring such small changes in product yields.

GC has played a role in the development of devices to protect the environment from the complex chemical wastes from the chemical semiconductor and electroplating industries. It was used in the development of the remarkable 'three-way' catalytic converter fitted into motor vehicle exhausts. The simultaneous oxidation of carbon monoxide and unburnt hydrocarbons alongside the reduction of nitric oxide in the same catalyst bed exposed to widely different exhaust compositions and temperatures from ambient to 1000°C) is a major chemical and engineering achievement. Even the humble kitchen stove has been rendered self-cleaning by incorporating manganese dioxide/zeolites/ferrites into its glass enamel coatings. The application of catalysis and GC in environmental protection is increasing in importance.

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