Continuous flow experiments

In continuous flow experiments, catalyst was packed into a downflow trickle-bed reactor of 30 cc bed volume. Hydrogen was passed slowly over the catalyst at atmospheric pressme and the temperature was slowly raised to the desired reduction/activation temperature and held for at least four hours. After activation, the reactor was cooled to the desired reaction temperature, the pressure was raised, and flow of an aqueous feed of glycerol and sodium hydroxide initiated along with a corresponding amonnt of hydrogen. A large set of reaction conditions was tested. 

The supported aqueous phase methodology was applied to the system Pd(OAc)2/5 TPPTS, a catalytic precursor for the Trost-Tsuji reaction. The characterization of the solid by 31P MAS NMR confirms the presence of Pd°(TPPTS)3 as the main surface species. The catalytic properties of the solid were tested for the allylic substitution of E-cinnamylethylcarbonate by different nucleophiles such as ethyl acetoacetate, dimethyl malonate, morpholine, phenol, and 2-mercapto-pyridine. The absence of palladium leaching was demonstrated, and having solved the problem of water leaching from the solid to the organic phase, the SAP-Pd catalyst was successfully recycled several times without loss in its activity. It was used in a continuous flow experiment which... 

Figures 7-9 show the fractional conversion of methanol in the pulse as a function of temperature for the three catalysts and the three methanol feeds. Evidently the kinetic isotope effect is present on all three catalysts and over the complete temperature range, indicating that the rate limiting step is the breaking of a carbon-hydrogen bond under all conditions. From these experiments, the effect cannot be determined quantitatively as in the case of the continuous flow experiments, but to obtain the same conversion of CD,0D, the temperature needs to be 50-60° higher. This corresponds to a factor of about three in reaction rate. The difference in activity between PfoCL and Fe.(MoO.), is larger in the pulse experiments compared to tHe steady stateJ results.

First, suitable combinations of catalyst 13 and ILs were determined in batch experiments. Using [EMIM][BARF] as IL complete conversion was obtained with 89% ee of (R)-3-phenyl-l-butene. [EMIM][Tf2N] gave lower ee values in the range of 65%. Nevertheless, better availability and easier hand-hng made this the IL of choice for recychng and continuous-flow experiments. In batch-wise recycling experiments of catalyst 13 in ILs, it was found that the products could be readily isolated by extraction with SCCO2. However, the batch mode led to rapid deactivation of the catalyst within three to four cy-... 

Finally, a monolith-supported version of the Grubbs-Hoveyda catalyst was prepared in an analogous approach using a perfluoroglutaric anhydride-derived ligand (Scheme 19). When used in continuous flow experiments, TOFs of 0.1 s were observed, and TONs were > 500 [ 127,128]. 

In continuous-flow experiments a feasible way is to apply a pH-control unit which continuously doses NaOH. The slight scatter of the pH normally observed will not negative influence on the oxidation results. ... 

Species II and III (Reaction 3) were postulated to account for the catalytic behavior of these systems species II, for the amine-promoted hydrogenation activity, and species III, for continued hydroformylation activity in the presence of amine. In continuous flow experiments on polyDMBA catalysts, rhodium retention was reported to be good (9) with... 

Since conditions are not quite the same for a catalyst used intermittently to study 5-mg. samples in a stream of pure hydrogen as they are for one under normal continuous flow operation, some continuous flow experiments were done for comparison. 

The catalase like activity was tested with (48) as catalyst From the results it is evident that the polymer bond in phthalocyanine led to a lower activation ener due to h%her concentration of not aggregated active centers than with low molecular jAthalocy-anines. Continuous flow experiments in a column show that (48) keeps 60% of its original activity. The polymer is more stable than the low molecular phthalocyanine. 

On-flow or continuous-flow experiments require more sample to analyze on the fly because the resident time in the NMR flow cell is very short (3.6 sec at 1 mL/min) during the chromatographic run. 

The argon, CO2, n-butane, and maleic anhydride responses were collected at m/e values of 40, 44, 58, and 98 respectively. From continuous flow experiments using pure reagents it was determined that the QMS signals at m/e = 40, 58, and 98 are unique to argon, n-butane and maleic anhydride respectively. The area of the transient responses at these mass numbers is directly proportional to the amount of each species. The experimentally observed CO2 response contained contributions from n-butane and maleic anhydride. To obtain the true CO2 response these contributions were subtracted out. N-butane conversion and the relative selectivities to maleic anhydride and CO2 were obtained by measuring the areas of their respective response curves. 

Horvath conducted several other interesting experiments with rhodium SAPCs (Table 1) [12]. Clearly, the water-solubility of the olefins does not limit the performance of the SAPCs since the TOFs (turnover frequencies) are essentially independent of olefin carbon number. This has been shown to be true also for carbon numbers as high as 17 [13]. Additionally, Horvath conducted experiments aimed at observing rhodium loss into the organic phase. He concluded that the SAPC does not leach catalytically active rhodium species under hydroformylation conditions. Another critical test for leaching was performed by Horvath. He conducted a 38 h continuous-flow experiment in a trickle-bed reactor and showed no loss of rhodium by elemental analysis. Thus, the combined data from all work shows that... 

In the experimental studies, tests have been made on the direct steam methanation of coal in two types of equipment. Batch tests have been conducted in small rocking-type high pressure autoclaves. Continuous flow experiments have been made in a 1-in. reactor, 5 ft. long, under Stage 2 conditions using simulated Stage 1 gas. 

Experimental results on only the yield of methane in tests in the continuous flow unit are shown in Figure 12, a plot of the methane yield, expressed as percent of the calorific value of the coal converted into methane, vs. the precent of B.t.u. in coal converted to gas. The methane concentrations obtained in the flow unit are, as stated above, lower than those in the autoclave tests consequently, the points for the continuous flow experiments also lie below the autoclave correlation curve at all... 

Recycling experiments were performed to find the optimum conditions for a continuous flow process. Initially, the reactions were carried out in anhydrous benzonitrile. The reaction was terminated by filtration of the loaded support, which was then washed with benzonitrile and reused with a new batch of substrates. This procedure led to a dramatic loss of activity, for which the loss of palladium was not responsible but rather leaching of water into the organic phase. Thus, the mobility of the immobilized catalyst was reduced combined with a decrease of the activity. The use of benzonitrile/water (v/v=l/l) resulted in a constant level of activity. For a continuous flow experiment, a dry SAPC sample was prepared from Pd(OAc)2 and five equivalents of TPPTS. The dry support was then placed into a reactor. The required amount of water was transferred from water-saturated benzonitrile. The test reaction was the transformation of cinnamyl ethyl carbonate with morpholine. The process achieved a TON of 2,200 and worked continuously for approx. 12 h without loss of activity. 

The results of continuous flow experiments are presented in Figure 6 as the amount of extract as a function of time. 

A study on alkaloid production by nongrowing (owing to the absence of 2,4-D) cell suspension cultures of C. roseus in a continuous flow reactor with cell retention was performed by Pareilleux and Vinas (626). It was shown by the authors that part of the produced alkaloids were excreted into the culture medium. Although the continuous flow system seemed to be suitable for secondary metabolite production, the reported specific production rate was relatively low (0.012 mg ajmalicine/g DW/day). To obtain ajmalicine production 1.2 mM tryptamine had to be added to the infiuent medium. The duration of the continuous flow experiment was only... 

The decision to use LC/NMR involves a decision also as to the mode of NMR detection to be used in the analysis. The alternatives are stopped flow, loop capture, or continuous flow. The latter is probably the most obvious use of LC/ NMR to a chromatographer - the NMR becomes a detector in the way that a photodiode array detector or MS might be used. Structural information will obviously be available from this use but typically the constraints of continuous flow preclude the structural analysis of low-level components. For example, in a typical continuous flow experiment at ImLmin", the need is to acquire as many NMR spectra per unit time as is possible to give sufficient S/N in the NMR spectrum while retaining adequate time resolution in the chromatographic dimension. Thus 32 scans may be acquired per time increment, resulting in... 

As pointed out in the preceding paragraph the recording equipment used for a continuous flow experiment may be simpler and slower in response than that required by relaxation methods operating in real time. As a result, if it is necessary to make a few kinetic measurements, and if the supply of material will permit, it may be much cheaper to set up an unambitious flow apparatus than to attempt other procedures. 

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