Zeolites ammonia temperature

Lok, B.M., Marcus, K.K., and AngeU, C.L (1986) Characterization of zeolite addity. 11. Measurement of zeolite acidity by ammonia temperature programmed desorption and FTIR spectroscopy techniques. Zeolites, 6, 185-194. 

Calorimetric Measurements of the Systems Zeolite—Ammonia and Zeolite—/ -Heptane in a Range of Temperature from 0° to 300° C... 

Long straight paraffins from Cio to C15 eontained in kerosene find applieations in the detergent industry. Although these paraffins are strongly adsorbed on the 5A zeolite, a temperature-switch cyclic desorption process cannot be used, beeause it would require temperatures of over 350°C. at whieh eoking inevitably oeeurs. The only possibilities, therefore, are vacuum desorption or displacement with a desorbent. The possible desorbents include hydrogen, n-pentane, or ammonia. 

Joly, J.P. and Perrard, A. (2000) Determination of the heat of adsorption of ammonia on zeolites from temperature-programmed desorption experiments. Langmuir 1538-42. 

Ammonia temperature programmed desorption (NH3-TPD) curves were recorded by placing 1.5 g of zeolite particles (60x140 mesh) into a tube furnace and heated to 125°C. Aimnonia was supplied to the zeolite in dry air at a concentration of 1,000 mg/m (1,450 ppm) and a flow rate of 2 L/min. The aimnonia flow was terminated once the effluent concentration equaled the feed concentration, and clean air was subsequently purged through the zeohte imtil ammonia was no longer detected in the effluent. The temperature was then... 

Figure 4 TPD spectra on H-mordenite (SiOa/AlaOa 15.0) and H-ZSM-5 (Si02/Al203 = 23.8), I- and h-peaks due to low and high temperatures of desorption. (Reproduced with permission from Niwa M and Katada N (1997) Measurement of acidic property of zeolites by temperature programmed desorption of ammonia. Catalysis Surveys from Japan 1 215-226.)...

Niwa, M. and Katada, N. Measurements of acidic property of zeolites by temperature programmed desorption of ammonia. Catal. Surv. Jpn. 1997, 1, 215-226. 

In order to get the pore system of zeolites available for adsorption and catalysis the template molecules have to be removed. This is generally done by calcination in air at temperatures up to 500 °C. A careful study (ref. 12) of the calcination of as-synthesized TPA-containing MFI-type single crystals by infrared spectroscopy and visible light microscopy showed that quat decomposition sets in around 350 °C. Sometimes special techniques are required, e.g. heating in an ammonia atmosphere (ref. 13) in the case of B-MFI (boron instead of aluminum present) to prevent loss of crystallinity of the zeolite during template quat removal. 

Temperature Programmed Desorption (TPD). Chemisorbed molecules are bonded to the surface by forces dependent on the nature of the sites. For instance, ammonia will be strongly adsorbed on acid sites, whereas it is only weakly adsorbed on basic sites. Consequently, the adsorbate complex formed with the basic sites will decompose at lower temperatures than that formed with the acid sites. The following example regarding the NH.i-zeolite H-ZSM-5 system will illustrate this. 

Pyridine sorption studies on EDTA-dealuminated Y zeolites at various temperatures (54,58), as well as measurements of differential heats of adsorption of ammonia on aluminum-deficient Y zeolites (57,59) have led to the conclusion that aluminum-deficient Y zeolites have stronger acid sites than the parent zeolite. 

Differential heats of NH adsorption were measured for the samples outgassed at different temperatures ranging from 400 to 800°C. Ammonia was chosen as a basic probe because its size is small, which may limitate diffusion effects in small pore zeolite materials. The variations of the differential heats of adsorption are plotted in fig. 3 as a function of the successive pulses of... 

Katada, N., Igi, H., Kim, J.H., and Niwa, M. (1997) Determination of the acidic properties of zeolite by theorecti-cal analysis of temperature programmed desorption of ammonia based on adsorption equilibrium. J. Phys. Chem. B, 101, 5969-5977. 

It is known that the activation temperature can influence the acid strength distribution. For example, measurements of the differential heats of ammonia adsorbed at 150°C for a HY zeolite have led to the conclusion that stronger acid sites, in the 150-180 kJ/mol range, are formed upon increasing the activation temperature from 300 to 650°C. Dehydroxylation at high temperature resulted in the formation of strong Lewis acid sites and the disappearance of intermediate and weak Brpnsted sites [62]. 

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