Cell transmission

Coleman and Sivy also used an infrared transmission cell to undertake degradation studies under reduced pressure on a series of poly(acrylonitrile) (ACN) copolymers [30-33]. Thin films prepared from a polymer were mounted in the specially designed temperature-controlled cell mounted within the infrared spectrometer. The comparative studies were made on ACN copolymers containing vinyl acetate [30,32], methacrylic acid [30,31] and acrylamide [30,33]. The species monitored was the production of the cyclised pyridone structure. This was characterised in part by loss of C=N stretch (vC = N) intensity at 2,240 cm-1 accompanied by the appearance and increase in intensity of a doublet at 1,610/1,580 cm-1. 

Figure 4.2 Schematic of basic probe designs transmission cell, immersion probe (retroreflecting and transmission), attenuated total internal reflection.

This is a development of the above where a fiber-optic linked hqnid sample transmission cell is integrated with the sample fast loop cabinet (Figures 5.24 and 5.25). There can be multiple sample streams, take-offs and fast loops, each with its own separate fiber-optic transmission cell. The analyzer can either be local with short fiber-optic runs to the sampling cabinet(s), or remote, where a safe area location for the analyzer module may be feasible, but at the cost of longer, potentially less stable fiber-optic runs. This system avoids physical stream switching. 

Alternatively, for nonequilibrium process streams, where a pumped reactor sample recycle line is available, in-line fiber-optic transmission cells or probes (Figures 5.26 and 5.27) can be used to minimize sample transport. It is highly desirable that some form of pumped sample bypass loop is available for installation of the cell or probe, so that isolation and cleaning can take place periodically for background reference measurement. 

Figure 5.26 Example fibre-optic-coupled in-line transmission cell (Reproduced courtesy of Specac Limited, www.specac.co.uk).

One of the most important factors in the selection of the sample handling technique is to attempt to analyze the sample, as it exists, without any form of chemical or physical modification. For gases and certain liquids, simple transmission cells, often with a flow-through configuration meet these requirements. 

HP IR transmission cells can be divided into two broad categories, namely (i) where the contents of the high pressure vessel are observed directly through IR transparent windows and (ii) where the reaction solution is circulated from the autoclave to an auxiliary observation cell. The first type is exemplified by the cell shown in Figure 3.2, developed by Whyman at ICI [2, 3]. The stirred reaction solution surrounds the cell windows in an arrangement that minimises the problem of... 

Figure 3.2 The transmission cell design of Whyman (from Ref [3], reproduced by permission of lOP Publishing Ltd).

In recent years, a number of modifications to HP IR transmission cells have been developed for applications to particular problems. Some of these are considered in detail below. 

In situ HP IR studies of methanol carbonylation reactions were originally undertaken by Forster and co-workers at Monsanto. Using a transmission cell, it was shown that for the rhodium catalyst, the dominant species (at relatively high... 

Cationic Group 11 carbonyl complexes have been implicated in catalytic carbony-lation reactions of alkenes, arenes, alcohols, saturated hydrocarbons and aldehydes under acidic conditions [106]. While the mono- and di-carbonyls [M(CO)] and [M(C0)2] (M = Cu, Ag, Au) can be formed at atmospheric pressure of CO, only Cu(I) forms a tricarbonyl cation under such conditions [107]. Strauss and co-workers reported the observation of [Ag(CO)3] when a Fluorolube mull of Ag[Nb(OTep5)5] was subjected to 13 bar CO in an HP IR transmission cell [108]. 

Figure 5.2 shows a typical IR spectrum for a Rh system in aqueous AcOH obtained from a transmission cell at 180 °C and pressurized to about 30 bar vdth CO. Peaks due to [Rh(CO)2l2] are indicated. 

Figure 5.2 HP IR spectrum from measurement of water gas shift rate and Rh speciation in a transmission cell showing Rh species and COj in AcOH/HjO/HI at 180°C and 30 bar.

Using either a transmission cell or a TIR cell as described above to study MeOH carbonylations means that the IR information is taken under the same conditions as any normal batch autoclave experiment in carbonylation. 

In a later publication, workers at Eastman demonstrated more clearly the role of reductants in maintaining catalyst activity in MeOAc carbonyiation to AC2O. They also used a HP IR transmission cell external to an autoclave to show that a portion of the Rh was present as inactive [Rh(CO)2l4] in batch reactions and that by adding H2 to the reaction it was converted to [Rh(CO)2l2], thus increasing the catalytic activity, (Eq. (47)) [5]. 

The Rh catalysed carbonylation of MeOH to AcOH was studied at Monsanto by HP IR under working reaction conditions using a short path length transmission cell coupled to a stirred reactor [12]. The presence of [Rh(CO)2l2] as the principal Rh species was generally noted. Consistent with the model studies and the kinetics of the carbonylation reaction, which tended to first order in total Rh and Mel, the rate controlling step was of course the reaction of [Rh(CO)2l2r with Mel. 

Transmission Only cell-free transmission Cell-free and cell-to-ceU transmission... 

Figure 17.9 Gas-tight transmission cell for UV-visible spectroelectrochemistry in moderate-melting salts. [From G. Mamantov, V. E. Norvell, and L. Klatt, J. Electrochem. Soc. 727 1768 (1980), with permission.)...

FT-IR has been applied for determining the sucrose content of sugar cane juice [21]. In place of the more familiar transmission cell, an attenuated total reflectance (ATR) cell and clarified sugar cane juice were used to record FT-IR spectra from 800 to 1250 cm"1. In the spectra, significant wavenumbers (927.59, 997.02, 1054.87, 1116.51, and 1137.80 cm"1) have been identified for sucrose. The application of PCR has been proposed for the development of a calibration equation for sucrose content. PCR is basically a MLR applied to scores assessed by PCA. On the basis of FT-IR spectra and sucrose content, an accurate calibration equation could be obtained by the application of PCR. The root mean square difference between predicted FT-IR values and the actual values were 0.12 % (w/v) with a bias of -0.03 % (w/v). The accuracy of FT-IR for determining sugar cane sucrose is almost equal to that of NIR [25]. 

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