Adsorption erionite

The adsorption microcalorimetry has been also used to measure the heats of adsorption of ammonia and pyridine at 150°C on zeolites with variable offretite-erionite character [241]. The offretite sample (Si/Al = 3.9) exhibited only one population of sites with adsorption heats of NH3 near 155 kJ/mol. The presence of erionite domains in the crystals provoked the appearance of different acid site strengths and densities, as well as the presence of very strong acid sites attributed to the presence of extra-framework Al. In contrast, when the same adsorption experiments were repeated using pyridine, only crystals free from stacking faults, such as H-offretite, adsorbed this probe molecule. The presence of erionite domains in offretite drastically reduced pyridine adsorption. In crystals with erionite character, pyridine uptake could not be measured. Thus, it appears that chemisorption experiments with pyridine could serve as a diagnostic tool to quickly prove the existence of stacking faults in offretite-type crystals [241]. 

Among the early investigations of methanol adsorption and conversion on acidic zeolites, most of the H and C MAS NMR experiments were performed under batch reaction conditions with glass inserts in which the catalyst samples were fused. Zeolites HZSM-5 76a,204,206,264-272), HY 71,72), H-EMT 273), HZSM-12 274), HZSM-23 275), H-erionite 275), H-mordenite 271,272), and H-offretite 275,276), silicoaluminophosphates H-SAPO-5 271,274), H-SAPO-11 274), and H-SAPO-34 76,277,278), as well as montemorillonite 279) and saponite 279) were investigated as catalysts. 

Figure 3. Effect of temperature upon and comparison of isotherms of Kr in (a) H-L, (6) H-Off, and (c) H-Eri. The unit cell of offretite has half the volume of the unit cells of zeolite L or of erionite. o = adsorption points A = desorption points.

The adsorption of oxygen is used here as another measure of total crystallinity (Table III). LZ-105, H-zeolon, erionite, NH4,TMA-fi, NH4,K-L and NH4Y all show 10-20% loss in their total oxygen capacities. There are at least two explanations for such a reduction. First, the total pore volume is lowered because the total crystallinity after fluorination decreases slightly. Second, the fluorine treatment results in the entrapment of fluoride compounds such as MF, AIF3, A1F2(0H), A1F(0H)2, etc. in the pore systems. 

Substantial increase in hydrophobicity is found in fluorine-treated and subsequently 600°C-calcined samples. Such an increase in hydrophobicity is reflected directly in the water adsorption data, delta hexane loading and n-butanol shake test results. The OH region I. R. spectra for these treated materials also show substantial to total loss of hydroxyl groups. The treated erionite sample, however, fails to show either an increase in delta hexane... 

The examples above demonstrate satisfactory agreement between the calculated results and the experimental data. This shows that the initial approximate assumptions are reasonable. In most cases, the one-term Equation 5 is applicable for the description of adsorption equilibria on zeolites, particularly for zeolites with small voids (zeolite L, chaba-site, erionite, mordenite) for which, in adsorption of hydrocarbons, n = 3 as a rule. The concept of the volume filling of micropores makes it possible to describe adsorption equilibria over sufficiently wide ranges of temperatures and pressures (using fs instead of Ps) with the use of only 3 experimentally determined (usually from 1 adsorption isotherm for the average temperature) constants. Wo, A, and n. The constant n requires only a tentative estimation, since it is expressed by an integer. 

Synthetic zeolites of various types differ in the number of cations in their voids which are accessible for direct interaction with the molecules adsorbed. Table I lists, for typical examples of zeolites, the numbers of accessible cations Na per zeolite void and their number Z in mmole/ gram for dehydrated zeolites. When passing from zeolite NaA to zeolite L, the number of accessible cations Z—i.e., the number of adsorption centers in the void—decreases almost by a factor of 10. Therefore, in the case of zeolite L, the relative role of interactions among cations and molecules adsorbed, conventionally called electrostatic, will be approximately one order lower than for zeolite NaA. In adsorption on this zeolite of substances with slightly pronounced nonuniformity of distribution of electron density in molecules— for instance, saturated hydrocarbons— one may expect that electrostatic interactions will not play the decisive role. As a result, we obtain the limiting case of adsorption on zeolites like zeolite L and erionite with a weak electrostatic interaction. 

In Figure 1, the continuous curves depict adsorption isotherms calculated from Equation 4. Experimental points are denoted by circles. The calculation and experimental results are in good agreement. A similar example is illustrated in Figure 2, showing experimental and calculated (from Equation 4) adsorption isotherms of carbon dioxide on Na,K-erionite. The data used in calculation were E = 5250 cal/mole and = 12.4% at to = 80°C. Thus, the general nature of gas and vapor adsorption on zeolites at weak electrostatic interactions is similar to adsorption on active carbons with the finest micropores (3). 

Erionite has been synthesized at i00°-I50°C from a (Na,K) aluminosilicate gel with Si02/AUOs = 10. X-ray and electron diffraction results on the product show intergrowths of the related offretite structure, which is a large-pore zeolite. Adsorption capacity for n-hexane is consistent with the density but adsorption rates are far slower than for zeolite A. Adsorption rates for n-octane are even slower but still better than for natural erionite. Hydrocracking tests on a C /Cq naphtha show strong selectivity for converting normal paraffins to Cf gas, particularly propane. As temperature is increased, other components of the naphtha feed are cracked and selectivity decreases. 

Since offretite is a large-pore structure, intergrowth of offretite in the erionite phase would be expected to affect the adsorption properties. Table II compares adsorption capacities for natural and synthetic erionite with Zeolite A (Ca) and synthetic faujasite (Na) (4.8 Si02/Al203). As expected, the more dense erionite structure shows lower capacity (5). There is substantial agreement between natural and synthetic erionite capacity the difference shows in adsorption rates (D/r ). The low apparent diffusivity of n-parafBns in erionite is somewhat a mystery since there does not appear to be that much difference in pore dimensions between erionite and zeolite A as predicted from their structures (6). The difference cannot be attributed to crystallite size since the natural erionite sample (laths, 0.5 /x diameter or less) has finer crystallite size than any of the synthetic materials (1-5 /x). 

The difference is more notable in n-octane adsorption which is shown in the last 2 columns of Table II. Zeolite A shows substantially the same capacity and adsorption rate for n-octane as for n-hexane. But for erionite, both natural and synthetic, n-octane capacities, and particularly the adsorption rates are substantially reduced. Here the difference between synthetic and natural erionite adsorption rate is quite large. It is possible that this is an effect of residual cations. However, simple exchange of Na" and for H" showed little change. We believe the more probable explanation is the intergrowth of offretite in the erionite crystal. The large offretite channels could give more rapid distribution of the sorbate molecule within the synthetic erionite crystal. 

From the previous discussion, it follows that the intracrystalline volume in zeolites is accessible only to those molecules whose size and shape permits sorption through the entry pores thus, a highly selective form of catalysis, based on sieving effects, is possible. Weisz and coworkers 7) have conclusively established that the locus of catalytic activity is within the intracrystalline pores when Linde 5A sieve ( 5 A pore diameter) was used, selective cracking of linear paraffins, but not branched paraffins, was observed. Furthermore, isoparaffin products were essentially absent. With the same catalyst, -butanol, but not isobutanol, was smoothly dehydrated at 230-260°. At very high temperatures, slight conversion of the excluded branched alcohol was observed, suggesting catalysis by a small number of active sites located at the exterior surface. Similar selectivity between adsorption of n-paraffins and branched-chain or aromatic hydrocarbons is shown by chabazite and erionite (18). 

Adsorption measurements of nitrogen (-196°C) and n-hexane (0°C) on USHY, H-MOR, HZSM-5 and H-Erionite that were coked with n-heptane at 450°C, were carried out. The inaccessible pore volume in each case (Va), and the volume occupied by coke (Vr) were calculated. Typically, for low coke content the value is close to 1 (except for H-ERI), and at higher coke content it drops. The comparison of nitrogen and n-hexane results made it possible to discuss whether the coke inhibited the access of reactants to the active sites or not. 

IR framework spectra were used as a diagnostic tool by Occelli et al. [260] in detecting the presence of offretite (via a band at 600-610 cm ) and erionite (bands at 410-425,550-610,655-685 cm ) in mixtures of these two structures. Roessner et al. [261 ] considered, in their IR spectroscopic work on the cation distribution in dehydrated calcium-exchanged erionite, also the framework vibrations of Ca-erionite besides OD vibrations, CO adsorption and DRIFT spectroscopy in the NIR region. They were able to show that the Ca + cations were selectively located in the supercages in front of the six-membered rings. Similar to the features encountered with Y-type zeolites and mordenite (vide supra), also with offretite a sufficiently linear relationship was found between the wave-numbers of the asymmetric and symmetric T-0 vibrations and the number of framework Al atoms per unit cell [262]. 

In particular, the combined application of acetonitrile and the bulkier adamantanecarbonitrile (diameter greater than 0.6 mn) in FTIR experiments proved to be suitable for discriminating internal from external acid sites of medium pore zeolite crystallites such as ZSM-5, chabazite, erionite and ferrierite. For the same purpose, Trombetta et al. [695] employed 2,2-dimethylproprionitrile (pivalonitrile, PN) instead of adamantanecarbonitrile. They studied B- and L-sites on the (external) surface of ferrierite and the total surface of [Si]MCM-41 and [ Al] MCM-41. Upon adsorption of PN, the C = N band was shifted from 2236 to about 2250 cm. The weak B-sites of [Al]MCM-41 interacted with pivalonitrile and were also associated with an OH band at 3745 cm. ... 

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. 

Methanol adsorption on erionites containing K" ", Na, and Li" " cations has also been studied by calorimetry at 303 K and IR spectroscopy. An increase in the content of Na ions in the crystals and the substitution of Na" " ions by Li" ions were shown to result in the increase of the heat of CH3OH adsorption in a wide range of coverage [215]. 

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