Isobutene Protonation in H-FAU, H-MOR, H-ZSM-5, and H-ZSM-22

Abstract

Kinetics and thermodynamics of isobutene protonation in H-FAU, H-MOR., H-ZSM-5, and H-ZSM-22 have been studied in a temperature range of 300-800 K, combining PW91-D//PW91 periodic density functional :theory calculations with statistical thermodynamics. At temperatures relevant for industrial zeolite-catalyzed processes (500-800 K), the tert-butyl carbenium ions more stable than the tert-butoxy in H-MOR, H-ZSM-5, and H-ZSM-22. Entropy contributions govern the standard Gibbs free energy stability of the chemisorbed intermediates. Due to the absence of a C-O covalent bond, formation of the tert-butyl carbenium ion is accompanied by a lower entropy :Loss and, consequently, has a higher stability than the tert-butoxy in H-MOB., H-ZSM-5, and H-ZSM-22. At 800 K, the protonation toward tert-butoxy in H-FAU, H-MOB, and H-ZSM-5 and to the tert-butyl carbenium ion in H-ZSM-22 is 5 to 7 orders of magnitude faster than the protonation toward isobutoxy. Among the four zeolites, the lowest activation energy is found in H-ZSM-22

Revisiting carbenium chemistry on amorphous silica-alumina: Unraveling their milder acidity as compared to zeolites

“…[36], in which no direct correlation between CO adsorption and lutidine protonation was reported, the stability of the p-complexes cannot be deduced from the analysis of BAS strength probed by lutidine or CO molecule because it results from multicontribution effects. Note that regarding mordenite, our energy results compare well with previous ab initio reports in the literature [78,94], more particularly those at DFT + D level [59,66,67,95-97].…”

“…Note that the carbocation is positioned, after geometry optimization, above the aluminum atom, each of the methyl groups being oriented towards framework oxygen atoms, similarly to previous results reported for H-ZSM-5 [74]. The energetics are comparable to most recent reports on various zeolite frameworks using similar levels of theory [78,94] (although D r U being more negative than most advanced hybrid MP2/DFT results [66,67]). On ASA, however, it appeared to be very difficult to obtain carbenium as local energy minimum, as the geometry optimizations very often led to the restitution of the proton to the surface, to regenerate isobutene and the acid site, or to the formation of a CÀO surf bond to provide the alkoxide (see next section).…”

“…In a first approach, we kept unchanged the vibrational contribution to enthalpy H and entropy S as compared to gas phase species. Although some of the approximations performed here differ from previously published calculations dealing with isobutene reactivity in zeolites [77,78], it can be noted that difference in Gibbs free energy corrections between the tertiary alkoxides and carbenium species is of the same order of magnitude: (D r G À D r U) carbenium À (D r G À D r U) alkoxide equals to 70 kJ mol À1 at 500 K in our case, whereas it was evaluated around 70 kJ mol À1 in mordenite by Nguyen et al [78], and about 80 kJ mol À1 from extrapolated values after Tuma and Sauer [77]. This validates the D r G values which will be presented in the following, even if refinements could be brought to check the assumptions made for their calculation.…”

“…Indeed, taking into account the electrostatics induced by the porosity leads to a stabilization of charged species, whereas the stability of the neutral species remains almost unchanged. Temperature effects are also shown to affect the alkoxide existence probability over that of carbenium, according to DFT calculations [76][77][78].…”

A facile synthesis of racemic aggregation pheromones of palm pests, Rhinoceros beetle and Rhynchophorus weevil

“…Synthesis of O. rhinoceros pheromone In our initial synthetic work to the O. rhinoceros pheromone, 20 2,6-dimethyl-2-decene was prepared from natural citronellol then converted by a one-step oxidation using KMnO4-FeCl3 into 4-methyloctanoic acid, which was then esterified with microwave-assistance, giving 1 in an overall yield of over 60% (Figure 2). However, this reaction generated a small content of minor products that complicated the purification process.…”