In Situ ¹³C Solid‐State NMR and Ex Situ GC–MS Analysis of the Products of tert‐Butyl Alcohol Dehydration on H‐ZSM‐5 Zeolite Catalyst

Abstract

The hydrocarbon products that are formed upon dehydration at 296-673 K of tert-butyl alcohol (tBuOH), adsorbed on H-ZSM-5 zeolite in concentrations equal to that of active Al-OH-Si sites in the catalyst, have been analyzed by 13C solid-state MAS NMR and GC-MS. To facilitate 13C NMR analysis, the alcohol selectively labeled with 13C isotope in the COH group was used. It was found that tBuOH transforms to the adsorbed C, butene dimers plus a trace amount of alkanes at 296 K. Butene dimers exist inside H-ZSM-5 pores in the form of interconverting adsorbed octene, octyl silyl ether, and octyl carbenium ion; octyl silyl ether is the main adsorption state. Fluxionality of the carbenium ion form provides a pathway for isomerization of the highly branched hydrocarbon skeleton of the initial alcohol to the predominantly linear one in the adsorbed butene dimer. The driving force for the isomerization into the linear structure is the shape selectivity induced by the small size of the zeolite channels. At 373 K the adsorbed butene dimers further crack into species that contain an average of about 6.5 carbon

Mechanism Studies of the Conversion of 13C‐Labeled n‐Butane on Zeolite H‐ZSM‐5 by Using 13C Magic Angle Spinning NMR Spectroscopy and GC–MS Analysis

“…The product distribution for the n-butane conversion on zeolite H-ZSM-5, which is revealed by 13 C MAS NMR spectroscopy, appears to be similar to that typically observed earlier for conjunct polymerization of olefins and alkanes in concentrated sulfuric acid, [38][39][40] for olefins and alcohols on acidic zeolites, [14,27,42] as well as for n-pentane and n-butane conversion on sulfated zirconia. [16,17] It should be emphasized that isobutane (as well as propane) is produced in the course of all stages of the conjunctpolymerization process rather than only through the pure oligomerization-cracking process (Scheme 2) observed earlier (e.g., for the n-butane isomerization on SZ).…”

“…The resonances at d = 16.9 and 18.0 ppm belong to the methyl and methylene groups of propane, respectively. [14,16] The intensities of the propane and isobutane signals increase with the reaction time. Propane is the main product at the end of the reaction (Figure 1d).…”

“…573 K. [11] However, according to unambiguous data obtained by using 13 C solid-state NMR spectroscopy, alkene oligomers should undergo further transformation into alkylsubstituted cyclopentenyl cations (CPC) on zeolites, even at lower temperatures. [14,27,28] Thus, the nature of the adsorbed products responsible for zeolite deactivation during alkane conversion on acidic zeolite needs further clarification.…”

“…Another approach to the discrimination between mono-(intra) and bi-(inter) molecular mechanisms of hydrocarbon conversion on solid catalysts, which provides more direct information about the reaction mechanism, uses 13 C-labeled reactants, both for monitoring the redistribution of a number of 13 C labels per molecule by GC-MS analysis [3,6,9,10,[12][13][14] and for following the migration of the selective 13 C labels during the reaction by means of solid-state 13 C NMR spectroscopy. [15][16][17] In some cases, only integrated studies using complementary techniques (e.g., the combined Abstract: By using 13 C MAS NMR spectroscopy (MAS = magic angle spinning), the conversion of selectively 13 Clabeled n-butane on zeolite H-ZSM-5 at 430-470 K has been demonstrated to proceed through two pathways: 1) scrambling of the selective 13 C-label in the n-butane molecule, and 2) oligomerization-cracking and conjunct polymerization.…”

Sorption and activation of hydrocarbons by molecular sieves

“…Despite claims that a small fraction of 1-octene adsorbed at ambient temperature on H-MFI exhibits carbenium ion properties, evidence from IR [28] and NMR [29] experiments has shown that the proton was .6 -6.…”