Offretite 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]. 

Offretite-erionite / chabazite Offr. Hex., P6m2 Erion Hex., P6s/mmc... 

ZSM-34 seems to be another excunple of offretite-erionite intergrowth. The highest SiOj/AljOj molar ratio that has been reported for this zeolite is about 15. ... 

Liang et al. carried out a study on SAPO-34, erionite and erionite-offretite zeolites comprising 55 regenerative cycles. The catalysts were regenerated at 530 C in air after reaction at 450 °C and 5 h WHSV. With SAPO-34 the initial conversion of methanol after 55 regenerations was still 100%, while the total hydrocarbon yield and content of Cj-C olefins in the hydrocarbon fraction were also constant. The offretite-erionite type zeolite used in this work suffered an appreciable deterioration after repeated regenerations. 

In various zeolites in the hydrogen form (mordenite, faujasite, offretite, erionite, ZSM-5) ultraviolet spectroscopy showed the presence of two types of A1 species. A band at 240 nm is related to framework Al-0 units and the band at 320 nm may be connected to oxoaluminum stmctures inside the zeolite matrix (34). 

The framework Si/Al ratio of zeolite Q (synthetic mazzite) was increased by treatment with SiCl4 at 500 °C from 4.24 to 6.00 without significant loss in crystalHnity [177]. The dealumination reaction was accompanied by a sfight increase in the hexagonal lattice parameter a while c remained unaffected. This unusual phenomenon, i.e., cell expansion upon isomorphous substitution of silicon for aluminum, as well as AL MAS NMR spectroscopic results, pointed to a redistribution of aluminum on at least two crystallographically different framework T-sites. Si MAS NMR spectra of offretite, erionite and zeofite Q all dealuminated with SiCl4 were presented in [178]. 

Figure C2.12.4. Typical polyhedra found in zeolites (a) sodalite cage found in sodalite, zeolite A or faujasite (b) cancrinite or a-cage found in cancrinite, erionite, offretite or gmelinite (c) the 5-ring polyhedron found in ZSM-5 and ZSM-11 (d) the large cavity of the faujasite stmcture and (e) the a-cage fonning the large cavity in zeolite A.

One of the earliest direct bonuses of imaging zeolitic catalysts by HRTEM was the discovery (10) that the nominally phase-pure ZSM-5 (structure code MFI) contained sub-unit-cell coherent intergrowths of ZSM-11 (MEL). It soon became apparent (46) that, depending on the mode of synthesis of these and other pentasil (zeolitic) catalysts, some specimens of ZSM-5 contained recurrent (regular) intergrowths of ZSM-11. It also emerged that intergrowths of offretite and erionite are features of both nominally phase-pure erionite and of pure offretite and of many members of the so-called ABC-6 family of zeolites (47). 

There is no systematic nomenclature developed for molecular sieve materials. The discoverer of a synthehc species based on a characteristic X-ray powder diffraction pattern and chemical composihon typicaUy assigns trivial symbols. The early syn-thehc materials discovered by Milton, Breck and coworkers at Uruon Carbide used the modem Lahn alphabet, for example, zeoHtes A, B, X, Y, L. The use of the Greek alphabet was inihated by Mobil and Union Carbide with the zeoHtes alpha, beta, omega. Many of the synthetic zeoHtes which have the structural topology of mineral zeoHte species were assigned the name of the mineral, for example, syn-thehc mordenite, chabazite, erionite and offretite.The molecular sieve Hterature is replete with acronyms ZSM-5, -11, ZK-4 (Mobil), EU-1, FU-1, NU-1 (ICI), LZ-210, AlPO, SAPO, MeAPO, etc. (Union Carbide, UOP) and ECR-1 (Exxon). The one pubHcaHon on nomenclature by lUPAC in 1979 is Hmited to the then-known zeoHte-type materials [3]. 

Zeolites are crystalline aluminosilicates with a regular pore structure. These materials have been used in major catalytic processes for a number of years. The application using the largest quantities of zeolites is FCC [102]. The zeolites with significant cracking activity are dealuminated Y zeolites that exhibit greatly increased hydrothermal stability, and are accordingly called ultrastable Y zeolites (USY), ZSM-5 (alternatively known as MFI), mordenite, offretite, and erionite [103]. 

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. 

Hedron (hexagonal prism) 2 6-rings 6 4-rings 12 2.3 in plane of 6-rings faujasite, ZK-5, chabazite erionite, offretite, levynite... 

Because of the high surface free energy at the liquid-solid interface, it is suggested that the stages of nucleation, transport of species by surface diffusion, and crystallization occur at the interface in the boundary layer. Culfaz and Sand in this volume (48) propose a mechanism with nucleation at the solid-liquid interface. This mechanism should be most evident in more concentrated gel systems where interparticle contact is maximized for aggregation, coalescence, or ripening processes. The epitaxy observed by Kerr et al. (84) in cocrystallization of zeolites L, offretite, and erionite further supports a surface nucleation mechanism. 

Ion-Exchanged Forms of Zeolite L, Erionite, and Offretite and Sorption of Inert Gases... 

It was also of interest to compare sorption by the ion-exchanged forms of zeolite L with corresponding forms of erionite and offretite, in view of the structural similarities and differences among the three zeolites (3, 8) and the growing interest in erionite and offretite. Previous sorption... 

For the erionites and for H-offretite a decreased and c increased mono-tonically with increasing temperature. The overall result of these changes was a marginal diminution of the unit cell volumes in A3 as follows K-enriched erionite 2299 (20° C) to 2237 (500° C)... 

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.

Curves of isosteric heats as functions of amount sorbed were obtained for Ar and Xe in Ba-, Na-, and K-enriched forms of zeolite L, for Kr in H-enriched forms of zeolite L, or erionite, and offretite, and for Kr in H-, Li-, Na-, K-, Cs-, Ba-, and La-enriched forms of L. Some results are shown in Figures 6a, b, and c, respectively. The abcissae give the amounts sorbed, rj, in cm3 at STP per cm3 of intracrystalline free volume, using the micropore volumes W0 of Table III to evaluate rj. This scale provides satisfactory means of comparing the sorbents. 

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