High pressure IR cell

Figure 7.3 Exploded view of the high pressure Figure 7.4 High pressure IR cell connected in situ NMR flow cell. (From J. A. Iggo, to an autoclave (constructed at ICCOM-CNR,...

IR Data. Transformations of rhodium complexes under hydroformylation conditions were followed by attaching a heated, high-pressure IR cell to the autoclave and periodically sampling the reaction liquid. Solvents varied from pure hexane to actual reacting mixtures of olefin and aldehyde. The data are summarized in Table VI. 

Reaction (78) regenerates Mel from methanol and HI. Using a high-pressure IR cell at 0.6 MPa, complex (95) was found to be the main species present under catalytic conditions, and the oxidative addition of Mel was therefore assumed to be the rate determining step. The water-gas shift reaction (equation 70) also occurs during the process, causing a limited loss of carbon monoxide. A review of the cobalt-, rhodium- and iridium-catalyzed carbonylation of methanol to acetic acid is available.415... 

By use of a high-pressure IR cell, we observed that under our conditions the... 

The stainless steel high pressure IR cell with Irtran-1 windows and associated high pressure equipment and spectrometer has been described elsewhere (4). Air-sensitive iron carbonyl solutions, which were to be examined by IR spectroscopy, were loaded into the high pressure cell under N2 and were then quickly placed under an atmosphere of CO to insure their stability. 

The IR spectra of 1-butanol solutions of Fe(CO)5 and M(CO)e (M = Mo and W) containing aqueous NaOH or KOH were examined under pressure using a stainless steel high pressure IR cell with Irtran I windows (4) in attempts to identify the various metal carbonyl species present in the reaction solutions under catalytic conditions. In the cases of M(CO)e (M = Mo and W), the only metal carbonyl v(CO) frequencies observed at temperatures and pressures comparable with those used for catalysis were those that correspond to the respective metal hexacarbonyl. In addition, a band at 2300 cm gradually appeared in the IR spectra of such metal hexacarbonyl solutions above I10°C under CO pressure. This band can be assigned to the CO2 produced in the water gas shift reaction (Reaction 1) proceeding under these conditions. 

In recent work with ethylene and a phosphine-free rhodium system using a high-pressure IR cell. King and co-workers have assigned bands at 2115 m, 2037 s, and 2019 scm to and bands at... 

It should be noted that the coupling reaction of nitrobenzene to afford azobenzene had been earlier reported to proceed even in the absence of any catalyst, but only under 3000 atm of CO [107]. However, it should also be noted that metallic iron is known to partly react with CO under pressure and the formation of Fe(CO)s by reaction of CO with the stainless steel walls of an high-pressure IR cell has been observed even under only 1 atm of CO [108]. Thus, even in this older experiment, some Fe(CO)s was surely present, which catalysed the reaction. The high pressure is probably required in order to produce significant amounts of the iron carbonyl. Indeed, by replacing Fe(CO)s with finely divided iron powder under the milder conditions of eq. 16, no reaction was observed. 

We now turn to the problem of the eventual cluster nature of the active catalyst. Ru3(CO)i2 is known to be in equilibrium with Ru(CO)5 under high CO pressure and the equilibrium has been studied even from a quantitative point of view, albeit only in isooctane [176, 177]. By use of a high-pressure IR cell, we evidenced that, under our conditions, the equilibrium is essentially completely shifted towards Ru(CO)s. A typical test to identify the real cluster nature of the active catalyst in systems of this kind is to examine the turnover frequency as a function of the total metal concentration [178]. Our preliminary results indicate that Ru(CO)5 is the active species and cluster compounds play no, or at best a very limited, role. 

As well as the CIR-type cells discussed above, the ATR technique has also been employed in other types of HP IR cell. An alternative arrangement to a cylindrical crystal inserted through the autoclave body (Figure 3.8) is to embed the ATR crystal in the base or wall of the autoclave. This approach was used in the HP IR cell developed by Wolf et al., illustrated in Figure 3.9 [43]. An ATR crystal is mounted in the bottom of the reactor, which can operate at pressures up to 200 bar and can be adapted to give a high pressure flow-cell. 

Many catalytic reactions require high pressures of reactant gases. Thus, an in-depth understanding of such catalytic systems requires truly in situ NMR and IR measurements and it has been necessary to develop appropriate High Pressure-spectroscopic cells the development and use of HP-NMR and HP-IR cells are reviewed in chapters 2 and 3 respectively. The use of both of these complementary methods/HP-techniques is probably best illustrated in chapters 5 - carbonyla-tion reactions, chapter 6 - hydroformylation and chapter 7 - alkene/CO copolymerisation, which deal with the recent advances in each of these important areas. 

Historically, high-pressure IR spectroscopy has been one of the most important methods to measure intermediates or resting-state species in catalytic cycles. In 1%8, Wilkinson observed HRh(PPh3)2(CO)2 in the Rh/PPh3 catalyst system by IR spectroscopy where an IR cell was connected via a tube to the autoclave. A related study was performed more recently by Moser et who applied their cylindrical internal reflectance IR cell. They determined the rate-... 

The apparatus used to perform vibrational relaxation experiments in supercritical fluids consists of a picosecond mid-infrared laser system and a variable-temperature, high-pressure optical cell (68,73). Because the vibrational absorption lines under study are quite narrow (<10 cm-1), a source of IR pulses is required that produces narrow bandwidths. To this end, an output-coupled, acousto-optically Q-switched and mode-locked Nd YAG laser is used to synchronously pump a Rhodamine 610 dye laser. The Nd YAG laser is also cavity-dumped, and the resulting 1.06 pm pulse is doubled to give an 600 u.l pulse at 532 nm with a pulse duration of "-75 ps. The output pulse from the amplified dye laser ("-35 uJ at 595 nm, 40 ps FWHM) and the cavity-dumped, frequency-doubled pulse at 532 nm... 

Fig. 9. An SFG-compatible UHV high-pressure reaction cell that allows the performance of 2-IR 1-vis SFG experiments (48,160). Three noncollinear laser beams (two IR, one visible beam) are spatially and temporally overlapped on the adsorbate/substrate interface, which allows simultaneous monitoring of two different spectral regions (reflected pump beams are not shown).

IR spectral studies can be carried out either in the transmission or in the reflectance mode. A high-pressure transmission cell requires IR-transparent windows made of CaF or ZnS, while for the reflectance mode they are made of IR-transmitting crystalline materials of high refractive index such as silicon or zinc selenide. 

Under such conditions, it is possible that some of the cataljdic intermediates that are actually formed under true in situ conditions will not be formed and observed. This problem may be addressed by building high-pressure IR and NMR cells where rapid stirring under high pressure and temperature is possible. 

---