Pressure dehydration, adsorption-reaction

The basis for this procedure for evaluating the concentration of absorbed species at reaction conditions rests upon being able to measure adsorption while a much slower reaction step takes place. If the study is to go beyond the adsorption step, the reaction must be of the type that produces a change in pressure at constant volume and temperature. Figure 4 shows portions of a typical adsorption reaction history for the catalytic dehydration of t-butanol on Alumina lOOS which has been treated or "conditioned" with water (6). The reaction which is endothermic produces one mole of isobutylene and a mole of water for each mole of t-butanoL The steep decrease in pressxore during the first second (approximately) was caused by adsorption, then the slow rise resulted from the reaction. The ratio of adsorption rate to reaction rate for this case was about 1700. The temperature rose during the first three seconds as a result of the heat of adsorption then fell because of the endothermic reaction and heat loss to the reactor. The temperature lag may be due in part to the slower response of the thermocouple. The amount of t-butanol which was measured by the drop in pressure from the initial value to the minimum is considered to be the adsorption at reaction conditions. 

The best catalysts for olefin hydration are not necessarily those which have proved most satisfactory for the reverse reaction. Some of the successful hydration catalysts are not typical dehydration catalysts. The more obvious reasons are (i) different adsorption characteristics of the catalyst is desirable, e.g. stronger adsorption of olefin relative to alcohol, (ii) under the conditions used for the hydration, ether formation cannot be suppressed as readily as in the dehydration, (iii) at high pressures, the olefins tend to polymerise much more than at the low pressures used for the dehydration. 

Adsorption Measurement. The capacities of the molecular sieves to adsorb vapor phase 1,3,5-triisopropylbenzene (97 %, Aldrich) and 1,2,4-triisopropylbenzene (99%, Camegie-Mellon University) were measured at 373 K using a McBain-Bakr balance. The adsorption temperature was chosen such that no chemical reactions of the adsorbates were observed. Prior to the adsorption experiment, the NH4+-forms of the solids (except SAPO-37) were dehydrated at 573 K under a vacuum of 10" 2 Torr. The as-made SAPO-37 was calcined at 793 K in an oxygen flow of 6 L/h in-situ in the adsorption system for removal of organic species and dehydration. The vapor pressure at 296 K of 1,3,5- and 1,2,4-triisopropylbenzene is approximately 0.45 Torr. The adsorption experiments were conducted at this pressure. 

The break-point temperature in dehydration (above which the rate was temperature insensitive) matched the maximum temperature for dehydrogenation, suggesting that a common intermediate exists for each reaction, and that the product selectivity is determined by interactions with other molecules and the surface. Above 650 K, the catalytic dehydration channel dominates, but the rate-determining step changes above 700 K. Below 700 K, the reaction rate is nearly independent of the partial pressure of formic acid (ca. 0.2 order). Above 700 K, the rate of the reaction is essentially independent of temperature, implying that reaction is limited by formic acid adsorption and dissociation thus, above 700 K, the rate becomes first-order with respect to the partial pressure of formic acid. Higher pressures of formic acid over the crystal surface should therefore increase the transition temperature - this behavior was observed by Iwasawa and coworkers, and the turnover frequency for catalytic dehydration approached the collision frequency of formic acid at high... 

The dehydration of thin ciystals of potassim aim [103] in dry air (323 to 343 K) showed different rates of reaction following nucleation of different surfaces. This anisotropy was attributed to the variation in density of packing of water molecules with crystallographic direction. At low pressure [104] the adsorption of water vapour was reversible, but at larger values of KHjO) multilayers were formed and uptake of water was controlled by difhision into the bulk of the crystal ( , < 2 kJ mol ). 

Formation of acetaldehyde via reaction of acetylene and water over Cd-exchanged phillipsite, mordenite, clinoptilolite, erionite, chabazite, and zeolites A, X and Y was investigated by the group of Kallo [907,908], while acetaldehyde adsorption on H-ZSM-5 was studied via FTIR by Diaz et al. [909], which indicated proton transfer with formation of crotonaldehyde and subsequent dehydration. At pressures higher than 400 Pa oligomerization occurred. 

In MAS NMR experiments [25,26] samples were contained inside capsules [27] which could be spun inside the MAS NMR probehead at rates of up to 3 kHz. The design of the capsule allowed the samples to be dehydrated at 400oC under a pressure of 10 mbar before adsorption of the organic. Capsules were then sealed while keeping the sample at liquid nitrogen temperature in order to prevent the onset of chemical reactions. 

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