Ammonia desorbent

Figure 3.52. TPD. spectrum of ammonia desorbing from H-ZSM-5 (Van Bekkum etal, 1991).

When the ammonia storage capacities of Fe-ZSM5 and V205/W03-Ti02 are compared (not shown here), the much higher amount of ammonia desorbed from Fe-ZSM5... 

Bulk amounts of elements were determined by atomic absorption spectrophotometry. The amount of framework A1 was determined by Al MAS NMR. The acidic properties of the metallosilicates were determined by IR and NH3-TPD measurements. Before the IR measurements, the sample wafer was evacuated at 773 K for 1.5 h. In the observation of pyridine adsorbed on metallosilicates, the sample wafer was exposed to pyridine vapor (1.3 kPa) at 423 K for 1 h, then was evacuated at the same temperature for 1 h. All IR spectra were recorded at room temperature. NH3-TPD experiments were performed using a quadrupole mass spectrometer as a detector for ammonia desorbed. The sample zeolite dehydrated at 773 K for 1 h was brought into contact with a 21 kPa of NH3 gas at 423 K for 0.5 h, then evacuated at the same temperature for 1 h. The samples were cooled to room temperature, and the spectra obtained at a heating rate of 10 K min from 314 to 848 K. 

Based on the total amount of ammonia desorbed in the TPD experiment Based on the amount of ammonia desorhed above 300°C in the TPD experiment... 

The gallium content of NH4-[Si,Ga]MAG (0.73 mmol/g) corresponds well to the amount of ammonia desorbed from this sample in the high-temperature step of the TPD process (0.67 mmol/g). Thus, the amount of intercrystalline gallium oxide should not exceed 10 % of the total gallium content. In contrast to NH4-[Si,Al]MAG, the gallium variety seems to be relatively resistant to extraction of the incorporated trivalent framework element by acid leaching (compare curves 1 and 2 in Figure 4 or the respective data in Table 1). 

Sample Thermal treatment Ammonia desorbed mmol/g Methanol conversion at443K wt% m-xylene isomerization at 673K wt% 60 min on stream 150 min on stream... 

Figure 2 TPD pattern of ammonia desorbing from an H-Y zeolite. Explanation for the various maxima Is given In text.

Studies of the acidic properties of the surfaces of samples by the TPDA method were carried out as follows. Samples (0.2 g) with 1-2 mm grain size were placed in a flow reactor (c(=0.6 cm) and were conditioned in a stream of helium (F= 60 ml/min) for 1 h at 550 °C. After decreasing the temperature to 100 °C the sample was saturated with ammonia. Completion of saturation was monitored by titration of the ammonia at the exit of the reactor. The saturated sample was treated with helium at 100 °C to remove the physically adsorbed ammonia (30 min). The sample was then subjected to programmed heating in a stream of helium at a rate of 26°/min. The thermodesorption process was monitored with a catharometer and the amount of ammonia desorbed was determined by titration with HCl. 

When vanadia was added to sulfated Ti-PILC, the amount of NH3 desorbed decreased significantly. R. T. Yang [26] reported that the vanadia create more Bronsted acid sites with the increase of vanadia from 2 to 6%. Thus, the decrease of the acidity observed in our samples may be explained by the assumption that some of the ammonia desorbed from the surface was oxidized by lattice oxygen of the catalysts. It is known that lattice oxygen of V2O5 can oxidize ammonia to N2 and nitrogen oxides at high temperatures (27). 

Water certainly competes with NH3 for the formation of both species I and II, but allows the formation of new species like species III. The competition of water with ammonia causes a decrease in the FT-IR spectra of the band characteristic of species I (5symNH3 mode near 1220 cm ) while it results in an increase of a band at lower frequency (near 1160 cm-1) assigned to species III. In the TPD experiments, treatment with water vapour at r.t. after NH3 adsorption, can lead to the displacement of ammonia by water, thus transforming species II into species III. When the sample is heated, since water desorbs at a temperature lower than ammonia, species III transforms into species I, while species II cannot be formed. This mechanism can explain why TPD experiments following water treatment show ammonia desorbing only at high temperature, while the low temperature signal is completely absent. 

Acidity of the samples were determined by temperature programmed desorption of ammonia. The calcined samples were heated at 450°C for 3 h in helium flow followed by cooling to 100°C. To minimise the amount of physisoibed ammonia adsorption was carried out at 100°C and any physisorbed ammonia was then flushed out in helium flow (50ml/min) for 3 hours. Finally, TPD spectra were recorded by heating from 100 to 600"C at a heating rate of 10 C/min. The amount of ammonia desorbed was monitored using a calibrated thermal conductivity detector. 

Figure 1. Amnonia thermal desorption amount of ammonia desorbed at different temperatures —) HZSM5 (-—) 1.6 ZnHZSMS.

There was a simple relationship between the rate of caprolactam formation over the range of modified aluminas studied and the surface concentration of intermediate strength acid sites, namely those from which ammonia desorbed in the temperature range 200-350°C. This relationship is shown in figure 6 and establishes a link between acidic sites of intermediate strength and caprolactam formation. Based on these data turnover frequencies were all in the range 0.8-1.8 x 10 3 molecules of caprolactam formed per surface site of intermediate acidity per second. 

Description The ExxonMobil Chemical (EMC) process offers commercially proven technologies for efficient recovery and purification of high-purity n-paraffin from kerosine feedstock. Kerosine feedstocks are introduced to the proprietary ENSORB recovery process developed by ExxonMobil Chemical, wherein the long-chain aliphatic normal paraffins are selectively removed from the kerosine stream in vapor phase by adsorption onto a molecular sieve. Isoparaffins, cycloparaffins, aromatics and other components not adsorbed are typically returned to the refinery kerosine pool. The cyclical process uses a low pressure ammonia desorbate to recover the n-paraffins from the sieve for use as LAB-qual-ity product or for further purification. 

Fig. 5-45 Scheme of a TPD spectrum of ammonia desorbing from zeolite [43]... 

The desorption spectnun consists of two broad overlapping peaks. The first (low-temperature LT) peak is assigned to NH3 desorbing from weak acid or non-acidic sites such as be built from sihcalite. The high-temperature peak (HT) is due to ammonia desorbed from strong acid sites. Thus, the peak temperatures can be correlated to die acid strength of the adsorption sites. But it should be mentioned tiiat the technique does not clearly discriminate between Lewis and Bronsted sites. 

Both processes utilize a desorbent to remove the adsorbed n-parafflns from the pores of the adsorbent. Once displaced from the adsorbent, both processes use some form of fractionation to separate the desorbent from the n-paraffln product. In both cases, this desorbent is recycled back to the process. In the case of liquid-phase extraction, the desorbent consists of n-pentane whereas in vapor-phase extraction, hexane or ammonia desorbents are employed. 

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