Zeolites alkyl carbenium ions

Intermolecular hydride transfer (Reaction (6)), typically from isobutane to an alkyl-carbenium ion, transforms the ions into the corresponding alkanes and regenerates the t-butyl cation to continue the chain sequence in both liquid acids and zeolites. 

The nature of intermediates in heterogeneous catalysis by acid form zeolites attracted considerable interest since the first experiments by Derouane et al. on characterization of adsorbed hydrocarbons with CP/MAS NMR [14]. Alkyl carbenium ions, which were... 

The possibility of NMR to follow the intramolecular migration of the selective label in adsorbed hydrocarbons gave the researches a chance to provide an evidence for the formation of alkyl carbenium ions as intermediates in reactions on solid acid catalysts, including zeolites. Carbenium ions in superacids exhibit a unique property to scramble the selective... 

Since the discovery of alkylation, the elucidation of its mechanism has attracted great interest. The early findings are associated with Schmerling (17-19), who successfully applied a carbenium ion mechanism with a set of consecutive and simultaneous reaction steps to describe the observed reaction kinetics. Later, most of the mechanistic information about sulfuric acid-catalyzed processes was provided by Albright. Much less information is available about hydrofluoric acid as catalyst. In the following, a consolidated view of the alkylation mechanism is presented. Similarities and dissimilarities between zeolites as representatives of solid acid alkylation catalysts and HF and H2S04 as liquid catalysts are highlighted. Experimental results are compared with quantum-chemical calculations of the individual reaction steps in various media. 

Only large-pore zeolites exhibit sufficient activity and selectivity for the alkylation reaction. Chu and Chester (119) found ZSM-5, a typical medium-pore zeolite, to be inactive under typical alkylation conditions. This observation was explained by diffusion limitations in the pores. Corma et al. (126) tested HZSM-5 and HMCM-22 samples at 323 K, finding that the ZSM-5 exhibited a very low activity with a rapid and complete deactivation and produced mainly dimethyl-hexanes and dimethylhexenes. The authors claimed that alkylation takes place mainly at the external surface of the zeolite, whereas dimerization, which is less sterically demanding, proceeds within the pore system. Weitkamp and Jacobs (170) found ZSM-5 and ZSM-11 to be active at temperatures above 423 K. The product distribution was very different from that of a typical alkylate it contained much more cracked products trimethylpentanes were absent and considerable amounts of monomethyl isomers, n-alkanes, and cyclic hydrocarbons were present. This behavior was explained by steric restrictions that prevented the formation of highly branched carbenium ions. Reactions with the less branched or non-branched carbenium ions require higher activation energies, so that higher temperatures are necessary. 

Numerous studies suggest that alkyl-aluminumsilyl oxonium ions should be the real intermediates in hydrocarbon reactions over zeolite, whereas carbocations should be just transition states (J). Equilibrium between the alkyl-aluminumsilyl oxonium ion and the carbocation, although suggested in some cases, has never been experimentally or theoretically proven, but recent calculations indicated that the tert-butyl carbenium ion is an intermediate on some specific zeolite structures 6,7). 

Recently, a novel CF MAS NMR-UV/Vis technique (Fig. 17, Section III.B) was applied to characterize the formation of hydrocarbons by the conversion of methanol on a weakly dealuminated zeolite HZSM-5 6S). The C MAS NMR spectrum recorded at 413 K during the continuous conversion of C-enriched methanol (Fig. 37a, left) consists of signals at 51 and 61 ppm attributed to methanol and DME, respectively. The very weak signal at ca. 23 ppm is probably an indication of alkanes or alkylated cyclic compounds. The appearance of the signals at 23 and 61 ppm indicates that the conversion of methanol on weakly dealuminated zeolites HZSM-5 starts even at 413 K. The simultaneously recorded UV/Vis spectrum (Fig. 37a, right) consists of bands at 275, 315, and 375 nm. The band at 275 nm indicates the formation of neutral aromatic compounds 301,302), and those at 315 and 375 nm may be assigned to mono- and dienylic carbenium ions (301,302), respectively. Because the UV/Vis spectrum of the non-dealuminated zeolite HZSM-5, that... 

The weakly dealuminated zeolite HZSM-5 used to convert methanol was subsequently applied to investigate the conversion of ethylene ( C-isotopes in natural abundance) (Fig. 37b). MAS NMR signals, appearing at 14, 23, and 32 ppm during conversion of ethylene at 413 K for 1 h (Fig. 37b, left), are assigned to alkyl groups of small amounts of alkylated cyclic compounds, such as cyclopentene, cyclohexene, cyclohexadiene, and/or benzene. The simultaneously recorded UV/Vis spectrum (Fig. 37b, right) shows bands at 300 and 375 nm, which characterize the formation of neutral cyclic compoimds and dienylic carbenium ions, respectively (301). 

The conversion of ethylene on a fresh zeolite HZSM-5 catalyst, which had not been used beforehand for methanol conversion, led to the spectra shown in Fig. 37c. The MAS NMR spectrum consists of signals at 14, 24, and 34 ppm caused by alkyl groups of cyclic compounds. Furthermore, a broad signal in the chemical shift range of alkenic and aromatic compounds appeared at ca. 120 ppm. The UV/Vis spectrum consists of bands similar to those shown in Fig. 37b and an additional weak band at ca. 450 nm. The latter may be attributed to condensed aromatics or trienylic carbenium ions (301). A weak shoulder observed at ca. 400 nm is an indication for the formation of hexamethylbenzenium ions (302). 

The simultaneous investigation of the methanol conversion on weakly dealuminated zeolite HZSM-5 by C CF MAS NMR and UV/Vis spectroscopy has shown that the first cyclic compounds and carbenium ions are formed even at 413 K. This result is in agreement with UV/Vis investigations of the methanol conversion on dealuminated zeolite HZSM-5 performed by Karge et al (303). It is probably that extra-framework aluminum species acting as Lewis acid sites are responsible for the formation of hydrocarbons and carbenium ions at low reaction temperatures. NMR spectroscopy allows the identification of alkyl signals in more detail, and UV/Vis spectroscopy gives hints to the formation of low amounts of cyclic compounds and carbenium ions. 

Propene on HY was, therefore, selected for the first in situ variable-temperature study using the CAVERN method. These experiments were carried out in early 1988 and published in 1989 (93). The central features of the CAVERN experiments were that the propene was introduced into the zeolite at cryogenic temperature and the sample was manipulated so that spectral acquisition could commence with an unreacted sample. Additional spectra were then acquired as the sample was slowly raised to room temperature. Detailed experiments of this sort were carried out for propene-2-l3C and propene-7-13C and less extensive experiments were performed for propene-3-13C. These experiments showed, among other things, that the 250 ppm peak was formed coincident with a second peak at ca. 156 ppm and the relative intensities of these peaks were 2 1. A careful study of the literature of carbenium ion chemistry in sulfuric acid and superacid solution media suggested the assignment of these resonances (250 and 156 ppm) to alkyl-substituted cyclopentenyl cations similar to 4. 

Moreau et al.56 obtained unexpected results in the alkylation of naphtalene with 2-propanol over H-Beta in the liquid phase at 200°C. Here a cyclic compound 1 was formed with a selectivity around 40% at 28.5% conversion. When applying HY as the catalyst alkylation to di- and trialkylnaphthalenes was faster but the cyclic compound was not observed. These results illustrate the more confined space within the zeolites Beta channels. The cyclic compound is assumed to be formed through iso-propylation of naphthalene followed by a hydride abstraction giving a carbenium ion, reaction with a propylene and finally ring-closure. 

Friedel Crafts type alkylations of benzene by alkenes involve the initial formation of a lattice associated carbenium ion, formed by protonation of the sorbed olefin. The chemisorbed alkene is covalently bound to the zeolite in the form of an alkoxy group and the carbenium ion formed exists only in the transition state. As would be expected fixjm conventional Friedel Crafts alkylation, the reaction rate over acidic molecular sieves also increases with the degree of substitution of the aromatic ring (tetramethyl > trimethyl > dimethyl > methyl > unsubstituted benzene). The spatial restrictions induced by the pore size and geometry frequently inhibit the formation of large multisubstituted products (see also the section on shape selectivity). 

Possibly the sequence of reaction steps first involves reaction between the aniline and the benzaldehyde to form an imine, which adds- as carbenium ion- to the dihydrofuran or- pyran to form an oxocarbenium ion that alkylates the aromatic ring. In view of the size of the product molecules, the reaction is assumed to occur at the outer surface of the zeolite. In the product the aromatic ring is positioned trans or cis with respect to the oxygen-containing ring. The trans cis ratio varies from 74 26 to 92 8... 

The peak at 250 ppm is indicative of three-coordinate carbenium ions [64]. A resonance at 250 ppm, observed in an in situ study of the oligomerization of propene on zeolite HY at low temperatures, was assigned to alkyl-substituted eyclopentenyl cations [37]. In that paper, the 250 ppm resonance was assigned to... 

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