Effect of impregnation of ZSM-5 with H3BO3 on its acidity: A microcalorimetric study of NH3 adsorption

Sayed, Moein B.; Auroux, A.; Védrine, J.C.

doi:10.1016/s0166-9834(00)81451-7

Cited by 31 publications

(13 citation statements)

References 5 publications

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“…Therefore, a part of the fl peak should also represent desorption from acidic OH groups of the zeolites. This is in agreement with numerous tpd and microcalorimetric studies [2,4,5,9,18,19,21,23] even though the comparison is difficult in some cases.…”

Section: H-mas Nmrsupporting

confidence: 91%

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Temperature-programmed desorption (TPD) of ammonia from HZSM-5 zeolites

Hunger

Hoffmȧnn

Heitzsch

et al. 1990

Journal of Thermal Analysis

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The temperature-programmed desorption (tpd) of the amount of ammonia which is preadsorbed at about 373 K at HZSM-5 zeolites yields a complex desorption curve consisting of two overlapped peaks (fl and ~,lPeak). Parts of the ammonia desorbed can be attributed to SiOHAI groups considering also H-MAS NMR measurements.The course of the desorption of both peaks is describable by a rate equation which considers a dependence of the desorption energy on the degree of coverage or an energy distribution, as could be shown by various methods of evaluation. The calculated dependence of the desorption energy on the ammonia amount adsorbed well agrees with data of literature of adsorption heats determined miero-calorimetrieally.Temperature-programmed desorption (tpd) of bases [3,[4][5][6][7][20][21][22][23][24][25][26][27][28][29][30][31] is, beside of IR spectroscopy [1-9], 1H-MAS-NMR spectroscopy [10][11][12][13][14][15][16] and microcalorimetry [2,8,9,[17][18][19][20] a method frequently used for the characterization of acidic properties of HZSM-5 zeolites.Tpd of ammonia adsorbed at low temperature provides 3 peaks more or less strongly overlapped (a, fl or LT, and 7 or HT peak [7,20]), which are ascribed to acidic centres of different strength. The a peak is ascribed in this case to the physisorption of ammonia. The reason for the ~ peak is the NH3 desorption from strongly acidic OH groups, which can be characterized in the IR spectrum by the vibration band at about 3600 cm -1. The fl peak is explained in different manner by several authors: Tops0e [7] ascribes this peak to desorption from weak acidic centres according to a IR band in the 3720...3740 cm -1 region or to the adsbrption on Na + and Hidalgo [5] to a broad IR band at 3350...3650 cm -1. Lok [3] interpretes the peak by splitting of physisorbed ammonia and Schnabel et al. [6] discuss a desorption from extra-framework aluminium.

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Section: H-mas Nmrsupporting

confidence: 91%

“…Temperature-programmed desorption (tpd) of bases [3,[4][5][6][7][20][21][22][23][24][25][26][27][28][29][30][31] is, beside of IR spectroscopy [1-9], 1H-MAS-NMR spectroscopy [10][11][12][13][14][15][16] and microcalorimetry [2,8,9,[17][18][19][20] a method frequently used for the characterization of acidic properties of HZSM-5 zeolites.…”

mentioning

confidence: 99%

Temperature-programmed desorption (TPD) of ammonia from HZSM-5 zeolites

Hunger

Hoffmȧnn

Heitzsch

et al. 1990

Journal of Thermal Analysis

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“…It is interesting to note that the acidity of H-ZSM-8 is reduced due to its modification with MgO, P,O,, and B,O,. These results are consistent with that observed for the ZSM-5 zeolite modified by boron (Auroux et al, 1985;Sayed et al, 1986) and phosphorous (Lercher et al, 1985;Lercher and Rumplmayr, 1986). In the present case, the sorption is expected to be controlled mostly by the intracrystalline mass transfer because the reduction in the zeolite particle size from 0.2 mm to 0.1 mm ( Figure la) showed no significant effect on the sorption kinetics at 3 13 K. Thus, there was no influence of intercrystalline (i.e., macropore) diffusional mass transfer on the sorption process.…”

Section: Resultssupporting

confidence: 92%

Diffusion of cumene in H‐ZSM‐8 and modified H‐ZSM‐8 Zeolites

Choudhary

Mamman

1990

AIChE Journal

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ZSM-8 zeolite is a medium-pore (pore size = 0.6 nm) high-silica zeolite, belonging to a pentad family of zeolites, introduced by Mobil Oil Corporation in 1971. The X-ray diffraction spectrum of ZSM-8 is somewhat similar to that of ZSM-5, which is a very widely investigated zeolite, except for a splitting in the highest peak in the region 28 = 22-25O observed for ZSM-8 (Lechert, 1984). Earlier studies (Akolekar and Choudhary, 1987) have indicated that the influence of degree of Hi -exchange and calcination temperature on strong acid sites and catalytic activity and paraselectivity (in hydrocarbon conversion and alcohol-to-aromatics conversion reactions) of ZSM-8 are quite similar to those observed for ZSM-5. For ZSM-8, however, the number of strong sites per framework A1 (Choudhary and Akolekar, 1989) and the rate of catalyst deactivation due to deposition of large hydrocarbon molecules in zeolite channels (Akolekar and Choudhary, 1987;Akolekar, 1987) are found to be higher. Also the diffusion of cyclohexane (Chon et al., 1985) in H-ZSM-8 is found to be faster.Extensive studies have been reported on ZSM-5-type zeolites for the sorption/diffusion properties (Ruthven, 1984; Choudhary and Srinivasan, 1986a, b) and also on the zeolite modified with MgO, P,O,, and B,O, (Kaeding and Butter, 1980;Kaeding et al., 1984;Mamman, 1988). For ZSM-5, intracrystalline species like MgO can produce a two order of magnitude change in diffusivity and also significant changes in catalytic properties (Olson and Haag, 1984).No detailed investigation on the diffusion in ZSM-8 and modified ZSM-8 zeolites has been reported so far. The present investigation was undertaken with the objective of studying the influence of cations ( H + or N a + ) and modification by MgO, P,O,, and B,O, on the diffusion in ZSM-8-type zeolite, using a model sorbate (viz., Cumene).Correapndcncc concerning this paper should be addrcsd to V. R. Choudhary. Experimental StudiesThe preparation and characterization of H-ZSM-8 and H . N a -Z S M -8 zeolites (Si/AI = 29.6 and crystal size = 3.5pm) have been described earlier (Akolekar and Choudhary, 1987). The degrees of H+-exchange of the H-ZSM-8 and H . Na-ZSM-8 zeolites were 0.96 and 0.08, respectively. The magnesium, phosphorous and boron modified H-ZSM-8 zeolites have been prepared by contacting the H-ZSM-8 zeolite with aqueous solutions of magnesium acetate, ortho-phosphoric acid, and boric acid (all BDH AnalaR), respectively, at room temperature for two weeks while shaking the flask containing the zeolite and the solution occasionally. The zeolite was then filtered using a Gooch crucible and washed quickly to remove the material impregnated on the external surface of the zeolite crystals. The zeolite was then dried in a vacuum oven at 373 K for 12 h, calcined in air at 773 K for 12 h and analyzed for its magnesium, phosphorous or boron content by atomic absorption spectroscopy or spectrophotometry. All the zeolites were pressed (without any binder) and crushed into particles of about 0.2 mm size. This has not cau...

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“…Considering ZSM-5, para-selectivity can be further improved by increasing the crystallite size or by selectivation of the external surface, which avoids unselective reactions to occur, but also by using zeolites with larger crystallites [266] and/or by eliminating the acid sites at the external surface through selective surface dealumination, chemical deposition of bulky organosilicon compounds, or coke-selectivation treatments [267][268][269][270][271].…”

Section: Figure 15: Initial Isomerization/disproportionation Ratio (Amentioning

confidence: 99%