“…The interplay of different factors, such as the framework structure or its chemical composition, particularly the SiO 2 /Al 2 O 3 ratio, with respect to the catalytic properties is still a matter of intensive research. [1][2][3][4] These examinations concentrate on H-ZSM-5, because this structure type can be synthesized over a wide range of SiO 2 /Al 2 O 3 ratios, ranging from 20 to 1. It has been found that the catalytic conversion increases with increasing aluminium content up to SiO 2 /Al 2 O 3 % 30, suggesting chemical equivalency of Brønsted sites, while a further increase in the aluminium content has only a minor effect on the catalytic activity.…”
Activation barriers and jump rates for translational proton motion in zeolite H-ZSM-5 are calculated by a combined quantum mechanics-interatomic potential function approach (QM-Pot). The potential energies of the stable intermediate proton positions and the transition structures for proton jumps between two neighboring Brønsted-sites, spatially separated by 14 A ˚, show an almost symmetrical trend, which reaches a maximum value midway between these sites. The highest barrier is 210 kJ mol À1 above the initial and final state energies. The energy barrier for the initial step of the translational motion (leaving the AlO 4 À site) is 127, 119 and 83 kJ mol À1 for Al-Al distances of 14, 8 and 6 A ˚, respectively. Thus, decreased separation of Brønsted sites leads to decreased energy barriers for proton jumps due to increased interaction of their Coulomb potentials. Proton jump rates calculated by classical transition state theory vary over wide ranges, 10 À5 to 10 8 s À1 and 10 3 to 10 10 s À1 , at temperatures of 373 and 673 K, respectively, depending on the particular proton path and the Al-Al distance.
“…The interplay of different factors, such as the framework structure or its chemical composition, particularly the SiO 2 /Al 2 O 3 ratio, with respect to the catalytic properties is still a matter of intensive research. [1][2][3][4] These examinations concentrate on H-ZSM-5, because this structure type can be synthesized over a wide range of SiO 2 /Al 2 O 3 ratios, ranging from 20 to 1. It has been found that the catalytic conversion increases with increasing aluminium content up to SiO 2 /Al 2 O 3 % 30, suggesting chemical equivalency of Brønsted sites, while a further increase in the aluminium content has only a minor effect on the catalytic activity.…”
Activation barriers and jump rates for translational proton motion in zeolite H-ZSM-5 are calculated by a combined quantum mechanics-interatomic potential function approach (QM-Pot). The potential energies of the stable intermediate proton positions and the transition structures for proton jumps between two neighboring Brønsted-sites, spatially separated by 14 A ˚, show an almost symmetrical trend, which reaches a maximum value midway between these sites. The highest barrier is 210 kJ mol À1 above the initial and final state energies. The energy barrier for the initial step of the translational motion (leaving the AlO 4 À site) is 127, 119 and 83 kJ mol À1 for Al-Al distances of 14, 8 and 6 A ˚, respectively. Thus, decreased separation of Brønsted sites leads to decreased energy barriers for proton jumps due to increased interaction of their Coulomb potentials. Proton jump rates calculated by classical transition state theory vary over wide ranges, 10 À5 to 10 8 s À1 and 10 3 to 10 10 s À1 , at temperatures of 373 and 673 K, respectively, depending on the particular proton path and the Al-Al distance.
“…The results of MCH conversion for the acid catalysts prepared using different HZSM-5 zeolites are compared in Figure in the 250−450 °C range. It is observed that as temperature and Al content of the zeolite (acidity) are increased, conversion increases significantly as has already been observed. , 3 Effect of temperature on methylcyclohexane conversion for catalysts with HZSM-5 zeolites of different Si:Al ratio. WHSV = 2.8 h -1 .…”
Section: Resultsmentioning
confidence: 59%
“…The selectivity of the hydrotreatment, in either one or two steps, is conditioned by the ring opening, which is slower and more complex than the hydrogenation reaction. In the one-step reaction, n -alkane and isoalkane formation occurs mainly over the acidic surface of the bifunctional catalyst. − The HZSM-5 zeolite is considered a suitable acid function because of its moderate acid strength, shape selectivity, and hydrothermal stability. With the aim of optimizing C 2+ n -alkanes selectivity and composition for their use as a feed for the steamcracker, several studies had been carried out on the modification of the HZSM-5 zeolite by incorporating Ga or by treating it with tetraethoxysilane (TEOS). , In the one-step reaction process, the performance of the Pd/HZSM-5 zeolite has been improved by substituting Pd by Pt given that the latter accelerates two key steps of the process: the saturation of CC bonds and the hydrogenolysis of C−C bonds. − …”
Section: Introductionmentioning
confidence: 99%
“…Raichle et al. proved indirectly the importance of acidity when they were studying the effect of the HZSM-5 zeolite Si:Al ratio on the product distribution of methylcyclohexane (MCH) cracking …”
In this work, the effect of HZSM-5 zeolite acidity on hydroconversion of methylcyclohexane and toluene has
been studied. These are test reactions for the second step and the single step of aromatic valorization process,
respectively, with the aim of obtaining C2+
n-alkanes and isoalkanes. Monofunctional HZSM-5 zeolite catalysts
(Si:Al ratio between 15 and 140) have been studied in methylcyclohexane ring opening while bifunctional
catalysts (hybrid Pt/Al2O3-HZSM-5, same zeolites) have been used in the hydrocracking of toluene. Runs
have been carried out in a fixed bed reactor under 250−450 °C and 20−80 bar. A positive effect of HZSM-5
zeolite acidity on methylcyclohexane conversion and C2+
n-alkane selectivity is evident at certain conditions,
whereas the maximum selectivity to isoalkanes requires an intermediate value of acidity. On the basis of the
relationship between conversion and the Si:Al ratio of the HZSM-5 zeolite, the hydrogenolytic cracking of
methylcyclohexane is proposed as a test reaction to determine the Si:Al ratio. Acidity has a highly favorable
effect in the hydrocracking of toluene given that it avoids the thermodynamic restrictions for toluene
hydrogenation and enhancing all the cracking steps during methylcyclohexane (MCH) transformation, which
increases selectivity to C2+
n-alkanes and isoalkanes.
“…In H-form zeolites protons compensate the negative charge of the [AlO 4 ] tetrahedra by forming bridging Si–OH–Al groups, so-called Brønstedt sites, leading to various technical applications in catalysis owing to the resulting acidic properties [1]. The acidity depends on the Si/Al ratio, called the silica modulus of a zeolite, and it can be measured by different measuring techniques, such as infrared (IR) spectroscopy [2], NMR spectroscopy [3], thermal analysis [4–6] and indicator method [7], as well as the amine titration method [8] and temperature-programmed desorption of ammonia (NH 3 -TPD) [9–11].…”
SummaryIn a proof-of-concept study we demonstrate in situ reaction monitoring of DeNOx-SCR on proton-conducting zeolites serving as catalyst and gas sensor at the same time. By means of temperature-dependent impedance spectroscopy we found that the thermally induced NH3 desorption in H-form and in Fe-loaded zeolite H-ZSM-5 follow the same process, while a remarkable difference under DeNOx-SCR reaction conditions was found. The Fe-loaded catalyst shows a significantly lower onset temperature, and time-dependent measurements suggest different SCR reaction mechanisms for the two catalysts tested. These results may help in the development of catalysts for the reduction of NOx emissions and ammonia consumption, and provide insight into the elementary catalytic process promoting a full description of the NH3-SCR reaction system.
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