Mordenite, surface acidity

H+). The order suggests that surface acidity is a major factor in the catalysis, but the mordenite... 

Y-zeolite, ZSM-5 and mordenite are three important industrial zeolites because of their pore structure and surface acidity. The coke extraction from the strongly coked HYZ catalyst has already been investigated under supercritical conditions [4,5], However, investigations on ZSM-5 and mordenite under supercritical conditions can not be found in literatures. 

Figure 5.10 Correlation between the surface acidity of mordenite measured by XPS and the selectivity in the dismutation of ethylben/ene.

Formation of byproducts may partly be caused by the outer surface acidity of the zeolites. If so, deactivation of the outer surface should lead to better selectivities. The outer surfaces of H-mordenite and HB were deactivated by poisoning with triphenylphosphine. For checking outer surface deactivation we have developed a chemical probe molecule [6]. In this way, it was found that the poisoning with triphenylphosphine resulted in a totally inactive outer surface within the time scale of the reactions. 

An acid treated zeolite beta with a 6 M HNO3 acid solution is a shape-selective catalyst for the alkylation of biphenyl with propene. This is however not the case for the alkylation of naphthalene. The reaction is highly mass transfer limited and the selectivity mechanism is attributed to a product selectivity. The major effect of the acid treatment is to deactivate the external surface area so that the intrinsic micropore properties can come out. Contrary to zeolite mordenite, the acid treatment does not reduce deactivation and the formation of highly aromatic coke remains important at high temperatures. 

The surface acidity of three commercial mordenite (MOR) zeolites with Si/Al ratios of 10, 60, and 80 has been evaluated by adsorption micro calorimetry at 423 K, using pyridine as a probe molecule [210]. As could be expected from the Si/Al ratios, the total pyridine uptakes varied in the order MOR-10 >MOR-60 >MOR-80. The initial differential heats of adsorption were in the range 215-220 kJ mol After a sudden drop, Qdiff changed slightly and stepwise over a relatively wide range of pyridine uptake (at least for MOR-10) and then steeply decreased. The site-energy distributions and the thermokinetic parameters versus coverage were also determined [210]. 

Recently, more complex heterocyclic nitrogen bases such as A-methylpyrrole, 2-chloropyridine, quinoline, pyrazine, pyrimidine, and pyridazine have been tested as reliable probes to obtain information about the surface acidity of Si02, Ti02, Zr02, Si02-Al203, H-mordenite, and sepiolite [22-25]. 

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. 

Adsorption Capacities. The adsorption capacities of the various molecular sieves and the amorphous silica-alumina for 1,3,5- and 1,2,4-triisopropylbenzene are listed in Table I. 1,3,5-Triisopropylbenzene does not adsorb into die channels of the acid form of S APO-5, mordenite and offretite at an adsorption temperature of 373 K. The small adsorption capacity of around 0.005 g/g most likely is indicative of some surface and/or intercrystalline adsorption of 1,3,5-triisopropylbenzene in these three samples. On the other hand, all the other samples studied in this work possess cavities and/or channels large enough to accommodate 1,3,5-triisopropylbenzene. In addition, all the sieves used here are capable of adsorbing 1,2,4-triisopropylbenzene. 

Chemical vapor deposition of silanes, along with a subsequent calcination using steam, can be utilized to deposit silica (Si(>2) inside the pore system. By variation oF the temperature, the partial pressure of the silane and the duration of the treatment, location and amount of the deposited material can be controlled [104]. When, for example, tetraethoxy- or tetrame-thoxysilane are used as reacting agents on a mordenite, ZSM-5, or /1-zeolite, then a controlled deactivation of only the external cristallite surface is possible [23, 44]. This is because these are rather bulky molecules which are not able to diffuse into the pore system of the crystallite. Alternatively, an irreversible adsorption of bulky bases may serve to destroy the undesired external acidity. Suitable basic compounds are 4-methylquino-line for ZSM-5 [2] and tributylphosphite for mordenites [71]. 

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