The synthesis, structure, and acid function of solid phosphoric acid (SPA) catalyst were studied in detail. 31P and 29Si MAS NMR and X-ray powder diffraction identified the following crystalline silicon phosphate phases in SPA: Si5O(PO4)6, hexagonal-SiP2O7, Si(HPO4)2·H2O, and SiHP3O10. The acidity of SPA is due to a liquid or glassy solution of phosphoric acid oligomers supported on the silicon phosphate phases. 15N MAS NMR of adsorbed pyridine-15 N and 13C MAS NMR of adsorbed acetone-2-13 C showed Brønsted acid sites and no Lewis acid sites. 1H→15N→31P and 1H→13C→31P double cross polarization MAS NMR of the probe molecules provided a rare opportunity to use NMR to unambiguously localize chemisorption sites; the probe molecules are complexed to phosphoric acid and pyrophosphoric acid but not to the silicon phosphate phases. In situ NMR of the oligomerization of propene on SPA suggests that propene quantitatively reacts with phosphoric acid and its oligomers to form isopropyl phosphate, and formation of this very stable intermediate accounts for the lower olefin oligomerization activity of SPA relative to acidic zeolites. Theoretical calculations including geometries at B3LYP/6-311+G(d,p) and chemical shifts at GIAO-MP2/tzp/dz were used to model complexation of acetone or propene to SPA, and these support our conclusions.
The observation and mechanistic significance of carbenium ions in acidic zeolites is one of the central problems in heterogeneous catalysis. 1-3 Early speculation included suggestions of superacidic zeolites and carbenium intermediates of all types. It is now generally recognized that zeolite acid strength is more in line with conventional strong acids, and the characterization of a long-lived, free carbenium ion within a zeolite is a rare achievement. Previously, only three types of long-lived carbenium ions have been unambiguously identified in zeolites, mostly by 13 C solidstate NMR with magic angle spinning. [4][5][6][7][8][9] In 1989 Haw et al. identified alkyl-substituted cyclopentenyl cations similar to 1 which formed in the low-temperature reactions of propene on zeolite HY. 4 Cyclopentenyl cations have since been shown to form on several zeolites from a variety of olefins and their precursors. 5,6,9 In 1994 Xu and Haw 7 identified several indanyl cations, including 2, which formed from styrene on zeolite HZSM-5. In 1995 Tao and Maciel 8 synthesized the trityl cation, (C 6 H 5 ) 3 C + , in zeolite HY from the Friedel-Crafts reaction of CCl 4 and benzene. Cano et al. recently reported the synthesis of several trityl cation derivatives inside zeolites Y and . 10 Previous NMR studies of carbenium ions in zeolites (and many similar, but unsuccessful, attempts) used sample preparation methods that differ in significant ways from the conditions used in catalytic flow reactors. In particular, the organic precursor(s) and the zeolite were invariably sealed such that nothing could escape the zeolite.We prepared the benzenium cation 3 in zeolite HZSM-5 by a novel experimental procedure. The key to the preparation of 3 was the use of flow reactor conditions for sample preparation; 11 we pulsed an aromatic hydrocarbon and an excess of methanol onto a zeolite bed at 573 K with continuous He carrier gas flow, allowed them to react for 4 s, and then rapidly quenched the sample temperature to ambient. In a flow reactor, the coproduct (water) diffuses out of the zeolite crystallites and is swept out of the sample by carrier gas, leaving cation 3 trapped in the zeolite. Our various attempts to observe 3 in sealed MAS NMR rotors were uniformly unsuccessful; in the presence of water the equilibrium concentration of 3 in the zeolite is negligible.We synthesized 3 in zeolite HZSM-5 a number of times, pulsing either benzene or toluene into the flow reactor with methanol, and we permuted the 13 C label in the reactants as an aid to spectral assignment. Representative 13 C MAS spectra showing 3 prepared from various precursors are summarized in Figure 1. Signals due to the cation were also enhanced by cross polarization (not shown). Verification of the identity of 3 was obtained with theoretical methods and by comparison with 13 C shifts of model carbenium ions such as heptamethylbenzenium ion. 12 We optimized the geometry of 3 at the B3LYP 13 level using the 6-311G** basis set 14 and the program NWChem. 15 GIAO-MP2 16 13 C che...
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