Shifts with ZSM

The results on both catalysts A and B show the previously reported yield and octane shifts with ZSM-5 (2,3,4) an increase in gasoline motor octane, a reduction in the gasoline yield, an increase in C3 and C4 olefins, and isobutane, and no change in coke, C2-, LCO or MCB yields. For both catalysts, the motor octane gain and also the motor octane gain per gasoline yield loss are essentially the same within experimental error. Note, that several motor octanes were determined to lend significance to the results. 

Figure 8. Iso/Normal Paraffins Ratio Shift with ZSM-5...

The reaction was also run with ZSM-5 (WHSV=24) at 400 and 500°C (Fig. 4). Deactivation at 400nC is rather slow in contrast to reaction at 500°C. The selectivities at 400°C remain nearly constant (Fig. 5). During the pronounced deactivation at 500°C, the product selectivity shifts towards methane (Fig. 6). 

It can be seen from Figure 3 that all mesophases demonstrate narrow pore size distribution with mean mesopore diameter 0meso) close to 2.6 nm. On the other hand the pore size distributions obtained from the desorption branch of the isotherms (not shown) are broadened and 0 kso is shifted with increasing ZSM-5 crystallinity to higher values. This difference, caused by tensile strength effects, indicates mesopores partially blocked [15]. 

Figure 5 shows a series of infrared spectra taken during the TPR of NO with CH4. At temperatures less than 350 °C the spectra in Figure 5 are virtually identical to those for NO TPR seen in Figure 4. Above 350 °C the nitrosyl bands are more intense in the presence of CH4. A new peak appears at 2270 cm when the temperature is raised to 400 °C and above, and another one appears at 2173 cm at 450 °C. Neither of these bands was observed when the reaction mixture was passed over Na-ZSM-5. When 5NO was substituted for NO, the two bands appeared at 2256 cm- and 2144 cm, and when 3CH4 was substituted for 12CH4, the bands shifted to 2237 cm and 2132 cm-. Based on previous studies [32-34], the band at 2270 cm- is best attributed to NCO species adsorbed at Al + sites. The observed shift in the...

The ZSM-4 sample was prepared following the previously described procedures.[20] The elemental analysis showed that the Si/Al ratio was 3.0. 29Si MAS NMR spectra were recorded at 11.7 T MHz on a Varian InfinityPlus 500 spectrometer on a sample loaded in a 7.5 mm MAS rotor spinning at 4 kHz using a rc/2 rad pulse length and a recycle delay of 360 s. The 29Si chemical shifts are referenced with respect to an external solution of TMS (5Si = 0.0 ppm). 

Diffraction patterns and FTIR spectra of skeletal vibrations of the ZSM-5 and ferrierite zeolites indicated high crystallinity of the analyzed samples. The strong band with a chemical shift of about 55 ppm in the 27Al MAS NMR spectra of hydrated zeolites indicated the presence of more than 97 % Al in the framework in tetrahedral coordination the very low intensity of the peak at 0 ppm indicated less than 3 % rel. of Al in octahedral coordination. 

DRIFT spectroscopy was used to determine Av0h shifts, induced by adsorption of N2 and hexane for zeolite H-ZSM-5 (ZSM-a and ZSM-b, Si/Al=15.5 and 26), H-mordenite (Mor-a and Mor-b, Si/AI— 6.8 and 10) and H-Y (Y-a and Y-b, Si/Al=2.5 and 10.4) samples. Catalysts were activated in 02 flow at 773 K in situ in the DRIFTS cell and contacted than with N2 at pressures up to 9 bar at 298 K or with 6.1% hexane/He mixture at 553 K, i.e., under reaction conditions. Catalytic activities of the solids were measured in a flow-through microreactor and kapp was obtained as slope of -ln(l-X0) vs. W/F plots. The concentration of Bronsted acid sites was determined by measuring the NH4+ ion-exchange capacity of the zeolite. The site specific apparent rate constant, TOFBapp, was obtained as the ratio of kapp and the concentration of Bronsted acid sites. 

The intrinsic acidities of zeolite samples are correlated with the Av0h values induced by adsorption of N2 (Av0h,n2), which can be determined by subtracting the frequency of the shifted OH-band from the frequency of the unperturbed OH-band as shown in fig.l. One shifted OH-band was observed for the ZSM-5 and mordenite samples, while the... 

The shift of the acidic OH-band due to the interaction of the Bronsted acid sites with hexane reactant is clearly accompanied by the appearance of the absorption bands in the vCh region (fig. 2). The Av0h, C6 values listed in table 1 suggest that the apparent acidity of the ZSM-5 and mordenite samples is distinctly different (113-116 and 85-94 cm 1 shifts, respectively) and significantly higher than that of the zeolite Y samples (41-52 cm"1). 

Fig. 2 Experimental analysis of the chemical shift anisotropy of high-silica ZSM-5 zeolite, (a) 29Si MAS NMR and (b) extracted CSA lineshapes from a two-dimensional CSA recoupling sequence dashed lines are simulated lineshapes. Adapted with permission from [79]. Copyright 2008 American Chemical Society...

The detection of Brpnsted acid sites, SiO(H)Al, is the most recent achievement of 170 NMR of zeolites [119-121]. High magnetic fields and double resonance techniques have allowed the observation of this important species in zeolite HY [120]. Chemical shifts of 21 and 24 ppm have been reported for zeolite HY for the Brpnsted sites in the supercage and sodalite cage, respectively [119]. Quadrupole interaction parameters are Cq = 6.0 and 6.2 MHz and r] = 1.0 and 0.9, respectively. Signal enhancement by 1H-170 cross-polarization has also permitted the detection of the acid sites in zeolite ZSM-5 [119], where they exist with lower abundance than in HY. 

As a result of steric constraints imposed by the channel structure of ZSM-5, new or improved aromatics conversion processes have emerged. They show greater product selectivities and reaction paths that are shifted significantly from those obtained with constraint-free catalysts. In xylene isomerization, a high selectivity for isomerization versus disproportionation is shown to be related to zeolite structure rather than composition. The disproportionation of toluene to benzene and xylene can be directed to produce para-xylene in high selectivity by proper catalyst modification. The para-xylene selectivity can be quantitatively described in terms of three key catalyst properties, i.e., activity, crystal size, and diffusivity, supporting the diffusion model of para-selectivity. 

Figure 2a shows the 29Si MAS NMR spectra of calcined NbS-1, TaS-1 and ZSM-5 The spectral shapes of NbS-1 and TaS-1 are similar to those of aluminum-containing ZSM-5 with ns/nAi = 45, Three lines are observed for NbS-l(41) and TaS-l(74) whose chemical shifts are -103, -112 and -115 ppm. The line at -103 ppm is due to the presence of [SiO (OH)] units in defect sites within the silicalite-1 structure This assignment was confirmed by H - 29Si crosspolarization (CP) experiments in which the intensity of the signal at -103 ppm increased... 

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