Ammonia adsorption acid site characterization

Recently, Tops e et al. have reconsidered the same system in a quite detailed study. Ammonia and pyridine adsorption have, in particular, been studied. The most active acidic site, characterized by the band at 3600 cm , are thought to be at the channel intersection. Weaker acidic sites, characterized by the band at 3720 cm" are, in contrast with previous assignments, related to terminal groups on the external surface of zeolite and possibly to non-zeolitic impurities. 

Another thermal analysis method available for catalyst characterization is microcalorimetiy, which is based on the measurement of the heat generated or consumed when a gas adsorbs and reacts on the surface of a solid [66-68], This information can be used, for instance, to determine the relative stability among different phases of a solid [69], Microcalorimetiy is also applicable in the measurement of the strengths and distribution of acidic or basic sites as well as for the characterization of metal-based catalysts [66-68], For instance, Figure 1.10 presents microcalorimetry data for ammonia adsorption on H-ZSM-5 and H-mordenite zeolites [70], clearly illustrating the differences in both acid strength (indicated by the different initial adsorption heats) and total number of acidic sites (measured by the total ammonia uptake) between the two catalysts. 

The purpose of the present work is to incorporate aluminum into the framework of SBA-15 during the synthesis in order to create acid sites on the surface of the material directly and to enhance its activity in acid-catalyzed reactions and to study the stability of SBA and AlSBA molecular sieves under various treatments. The influence of these treatments on the pore size, wall thickness and the environment of Al in these materials are investigated in detail. X-ray diffraction (XRD), Electron Microscopy (TEM) and N2 adsorption were used to characterize the structure, the porosity and the stability of these materials. 27Al MAS NMR was used to ascertain the nature and environment of Al, cumene cracking to test the catalytic activity of parent materials and ammonia chemisorption to probe their surface acidity. 

The catalytically active sites of isomorphous substituted MFI structures have been characterized by inirared spectroscopy and microcalorimetric measurements using ammonia and acetonitrile as probe. Due to decreasing heats of NH, adsorption, the NH, TPD peak positions, the positions of the IR OH stretching frequencies and their shifts upon adsorption of acetonitrile the Bronsted acid site strength of the modified MFI decreases from Al>Fe>In> >silicalite. In addition to those strong sites weaker Lewis centres due to the non-framework material have been found. For TS-1 comparatively low heats of adsorption due to coordinatively bonded ammonia have been detected. The amounts of adsorption with heats higher than found for silicalite correlates with the amount of Ti in the sample. 

The use of gas flow techniques are now widely recognized as important for the study of adsorption/desorption - phenomena at solid interfaces. Such studies are particularly important for the characterization of catalytically active sites on the surfaces of solids. One particular example is a study of the adsorption of ammonia onto activated carbon. It can be shown that adsorption of ammonia consists of reversible and irreversible steps and that these steps can be attributed to physisorp-tion for the former and adsorption on particular chemical groups for the latter. The heats of adsorption for various sites can be measured and the data can be used to reveal the existence of a wide distribution of acid sites that are accessible to ammonia to varying degrees. 

The powder X-ray diffraction patterns were measured in a D-500 SIEMENS diffractometer with a graphite seeondary beam monochromator and CuKoj contribution was eliminated by the DIFFRAC/AT software to obtain a monochromatic CuKa,. The Unit Cell Size (UCS) was measured following the ASTM D-3942-90 procedure. The Surface areas were measured by nitrogen adsorption at 75 K on a Micromeritics Accusorb 2100 E equipment using the ASTM method D-3663-78. Temperature Programmed Desorption (TPD) of ammonia and pyridine adsorption by Infrared Spectroscopy (IR) were used to characterize the acidity of the zeolites. For IR-Pyridine the spectra were recorded each 100°C and the characteristic bands of Lewis and Bronsted acid sites (1444 cm" and 1540 cm, respectively) were integrated in order to obtain the total acid sites. 

NHs adsorption microcalorimetry has been used to characterize the acid sites of a H-USY zeolite and another USY sample in which the strong Lewis acid sites were poisoned with ammonia. Poisoning of the Lewis acid sites did not affect the rate of deactivation, the cracking activity, or the distribution of cracked products during 2-methylpentane cracking. Thus, strong Lewis acid sites do not seem to play any important role in cracking reactions [148]. 

Microcalorimetry has been extensively used to characterize the acid centers in ZSM-5 zeolites, and for comparison in its completely dealuminated form, sihcalite-1. No electron donor (basic) sites have been evidenced [167]. Ammonia adsorption experiments have shown that there exists a strong acidity in H-ZSM-5 zeohte which exceeds that present in H-Y zeohte [104]. 

The acidity of H-ZSM-5 zeohtes synthesized with different A1 contents has been characterized by the microcalorimetric measiuement of the differential heats of adsorption of ammonia [51]. The strength of the strongest acid sites... 

Adsorption of ammonia on mordenites has been shown to be significantly influenced by the adsorption temperature [202]. Non-selective adsorption was observed below 373 K, and the measured heats of adsorption were found to be an average between those on the most energetic (acidic) sites and those on the least energetic ones. The molecules adsorbed on the latter migrated at higher temperatures. Consequently, adsorption above 423 K was required to characterize the acidity of mordenites. Since no diffusion Hmita-tion was observed in the adsorption of ammonia on faujasites above room temperature [102] and on ZSM-5 above 373 K, this phenomenon may result from the peculiar channel structure and the opening diameter of mordenites [202]. 

A sample of mordenite (98% degree of ammonium-ion exchange) deam-moniated at various temperatures from 693 K to 923 K was studied at 573 K by NH3 adsorption microcalorimetry by Bankos et al. [203]. On increasing the pretreatment temperature, the number of acid sites passed through a maximum at 753 K as a result of simultaneous decationation and dehydroxylation. The heat of adsorption of NH3 on Bronsted acid sites formed by decationation was 110 -160 kJ mol During dehydroxylation, two types of Lewis sites were formed, characterized by heats of NH3 adsorption of 170-185kjmol and 95-100 kJ mol respectively, and on which dissociative chemisorption of ammonia was evidenced by IR [101]. 

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