Variable contact-time

Figure 8. STDP over various HZSM-5 catalysts. Temperature 550°C. Pressure 41 bar. H2/Tol 4, variable contact time.

In circulating fluidized beds, the clusters move randomly. Some clusters are swept from the surface, while others stay on the surface. Thus, the heat transfer between the surface and clusters occurs via unsteady heat conduction with a variable contact time. This part of heat transfer due to cluster movement represents the main part of particle convective heat transfer. Heat transfer is also due to gas flow which covers the surface (or a part of surface). This part of heat transfer corresponds to the gas convective component. 

In order to conceive the contributions of the various species to every band, the spectra are deconvoluted. After deconvolution of the recorded spectrum into a sum of Gaussian curves, the different contributing signals can be integrated separately. For quantitative assessment some contact time and cross-polarization criteria have to be met. Optimal conditions are obtained from a variable contact time experiment. Full details have been worked out by Pfleiderer23 and Caravajal et al.21,22 In general, quantitative correlations should only be made for species within one spectrum and not... 

Figure 9.35 29Si CP MAS NMR spectra of silica gel after pretreatment at 973 K, deuteration and modification with APTS, measured with variable contact time, (a) 8 ms, (b) 5 ms, (c) 2.5 ms, (d) same sample before APTS modification at same scale,with 5 ms contact time.

The H- Si cross-polarisation time constants (Ts,h) associated with the Q species of both the 5% V-MCM and the standard MCM-41 calcined samples have been derived from variable-contact-time experiments. These values can be viewed as an indication of how fast the transfer of polarisation from protons to silicons occurs. The value for the 5% V-MCM sample is of the order of 0.15 ms, whereas for the standard sample it is 20 times greater, about 3 ms. 

There is also a number of sequences that allow the dynamics to be probed. The CP curve itself can be mapped out by using variable contact time, and fitted to determine Tis and Tip. Other sequences for determining the various relaxation times (X T and Tip, H T and Tip) are shown in Figure 3.27. The H T can be detected via the carbon which sometimes allows the Ti of the different protons to be distinguished. Even if T i is long, it can be determined via the proton magnetisation, provided Ti > > T (Torchia 1978). 

Structural studies of two complexes with dimethylformamide (DMF) and dimethylacetamide (DMA) were reported. In the former, three types of solvent molecules are embedded in the crystal lattice first, disordered isolated molecules located just above the cavities formed by the resorcinol units second, clusters of four DMF molecules located between the calixarene layers and third, single DMF molecules located inside the layers close to the alkyl chains. The challenge for solid state NMR is to recognize and locate guest molecules in the lattice. CP/MAS, dipolar dephasing and variable contact-time exper-... 

More information on the chemical structure of bone apatite can be obtained from H—> P CP. First, consider the dependence of peak intensity J(f) on contact time t (also denoted CT) in the conventional variable-contact time experiment (Fig. 11). Such an experiment monitors the CP kinetics, which is very specific for a particular material [12]. There are two models of the CP kinetics, I-T-S and I-S [12]. In this notation, the spin polarization is transferred from spins I to spins S, in our case from protons to P, respectively. 

Fig. 13 Deconvolution of the variable-contact time CP curve (A) for human trabec-...

In the solid state, high-resolution Sn NMR spectra can be readily obtained, in most cases, by using CP/MAS techniques, based on an optimized contact time (see Fig. 4) or by using variable contact times. Owing to the high NMR sensitivity, even single-pulse techniques will provide meaningful Sn NMR spectra within reasonable time. 

Normal CP/MAS, variable contact time and dipolar dephasing NMR measurements have been used to derive a set of 12 parameters related to the carbon skeletal structure of carbonaceous materials. From the structural... 

Figure 34.3 shows the results of a variable contact time CP experiment, in which the Si CP contact period (tcp) is varied in order to elucidate the Si CP (relaxation) time constant (ThsO for each Si peak. The early (small tcp) part of such curves is typically dominated by the rate of CP transfer, as characterized by the rate constant and the latter part of such curves is usually determined by the rate constant of the rotating frame spin-lattice relaxation of the protons responsible for polarization transfer to the observed silicons, as characterized by the time constant, (assuming > Thsi). These curves can be analyzed mathematically in terms of well-known equations [17]. 

FIGURE 34.3 Variable contact time Si CP-MAS plots for silica. From reference [2a]. With permission. 

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