James F. Haw

Zeolite acid strength and reaction mechanisms in catalysis

The heterogeneous catalysts of greatest practical importance are acidic zeolites, and these cannot be studied using the traditional methods of UHV surface science. As recently as 1994 there was near universal agreement that zeolites were superacids, and that many of the most important reaction mechanisms in catalysis were based upon simple carbenium ions and other exotic high-energy intermediate species. This Paper reviews a series of investigations, many of them using solid-state NMR, that led to the reclassification of zeolite acidity and a theoretical basis for understanding carbenium ion stability in these fascinating solids. Building on this better understanding of zeolite acid strength and the types of carbenium ions that are reasonable intermediates in zeolites, it was possible to elucidate many of the features of a challenging mechanistic problem: methanol to hydrocarbon catalysis.

In - situ spectroscopy in heterogeneous catalysis

Overview of In Situ Methods in Catalysis. In Situ Catalysis and Surface Science Methods. In Situ NMR. Theoretical Catalysis: Methods, Applications, and Future Directions. In Situ Ultraviolet Raman Spectroscopy. In Situ Infrared Methods. In situ XAS Characterization of Heterogeneous Catalysts. In Situ Measurement of Heterogeneous Catalytic Reactor Phenomena using Positron Emission. TAP Reactor Studies. Subject Index.

Synthesis of the Heptamethylbenzenium Cation in Zeolite-β: in situ NMR and Theory

We synthesized the heptamethylbenzenium cation on zeolite-β by co-feeding excess methanol and benzene into a flow rector at 250 °C. Experimental isotropic 13C chemical shifts are in excellent agreement with theoretical (GIAO-MP2/tzp/dz) values calculated for the theoretical (B3LYP/6-311G*) structure of the cation. These results, along with a previous study of the pentamethylbenzenium cation on HZSM-5, afford an example of zeolite topology controlling the substitution pattern of persistent carbenium ions.

Polycyclic Aromatics Formation in HSAPO-34 During Methanol-to-Olefin Catalysis: Ex Situ Characterization After Cryogenic Grinding

Cryogenic grinding using a freezer mill is shown to be a useful adjunct to acid digestion for the ex situ analysis of aromatic hydrocarbons entrained in microporous catalysts. In the course of converting methanol to olefins, HSAPO-34 contains first methylbenzenes and later methylnaphthalenes. As the catalyst deactivates, significant amounts of phenanthrene and pyrene form as well.

Zeolite-supported organorhodium fragments: essentially molecular surface chemistry elucidated with spectroscopy and theory.

Structures of zeolite-anchored organorhodium complexes undergoing conversions with gas-phase reactants were characterized by infrared spectra bolstered by calculations with density functional theory and analysis of the gas-phase products. Structurally well-defined zeolite-supported rhodium diethylene complexes were synthesized by chemisorption of Rh(C(2)H(4))(2)(acac) (acac = CH(3)COCHCOCH(3)) on dealuminated Y zeolite, being anchored by two Rh-O bonds, as shown by extended X-ray absorption fine structure (EXAFS) spectroscopy. In contrast to the nonuniformity of metal complexes anchored to metal oxides, the near uniformity of the zeolite-supported species allowed precise determination of their chemistry, including the role of the support as a ligand. The anchored rhodium diethylene complex underwent facile, reversible ligand exchange with deuterated ethylene at 298 K, and ethylene ligands were hydrogenated by reverse spillover of hydrogen from support hydroxyl groups. The supported complexes reacted with CO to form rhodium gem-dicarbonyls, which, in the presence of ethylene, gave rhodium monocarbonyls. The facile removal of ethylene ligands from the complex in H(2)-N(2) mixtures created coordinatively unsaturated rhodium complexes; the coordinative unsaturation was stabilized by the site isolation of the complexes, allowing reaction with N(2) to form rhodium complexes with one and with two N(2) ligands. The results also provide evidence of a new rhodium monohydride species incorporating a C(2)H(4) ligand.

A Site-Isolated Rhodium-Diethylene Complex Supported on Highly Dealuminated Y Zeolite: Synthesis and Characterization

The reaction of Rh(C2H4)2(acac) with the partially dehydroxylated surface of dealuminated zeolite Y (calcined at 773 K) and treatments of the resultant surface species in various atmospheres (He, CO, H2, and D2) were investigated with infrared (IR), extended X-ray absorption fine structure (EXAFS), and 13C NMR spectroscopies. The IR spectra show that Rh(C2H4)2(acac) reacted readily with surface OH groups of the zeolite, leading to loss of acac ligands from the Rh(C2H4)2(acac) and formation of supported mononuclear rhodium complexes, confirmed by the lack of Rh-Rh contributions in the EXAFS spectra; each Rh atom was bonded on average to two oxygen atoms of the zeolite surface with a Rh-O distance of 2.19 A. IR, EXAFS, and 13C NMR spectra show that the ethylene ligands remained bonded to the Rh center in the supported complex. Treatment of the sample in CO led to the formation of site-isolated Rh(CO)2 complexes bonded to the zeolite. The sharpness of the nu(CO) bands in the IR spectrum gives evidence of a nearly uniform supported Rh(CO)2 complex and, by inference, the near uniformity of the mononuclear rhodium complex with ethylene ligands from which it was formed. The supported complex with ethylene ligands reacted with H2 to give ethane, and it also catalyzed ethylene hydrogenation at 294 K.

Nuclear Magnetic Resonance Study of Autohydrolyzed and Organosolv-Treated Lodgepole Pinewood Using Carbon-13 with Cross Polarization and Magic-Angle Spinning

Nuclear Magnetic Resonance Study of Autohydrolyzed and Organosolv-Treated Lodgepole Pinewood Using Carbon-13 with Cross Polarization and Magic-Angle Spinning By James F. Haw and Gary E. Maciel* Department of Chemistry, Colorado Stade University, Fort Collins, Colorado 80523 and James C. Linden and Vincent G. Murphy Department of Agricultural and Chemical Engineering, Colorado State University, Fort Collins, Colorado 80523

Characterization of poly(carbon monofluoride) by 19F and 19F to 13C cross polarization MAS NMR spectroscopy

High speed 19F MAS NMR and 13C MAS NMR with 19F to 13C cross polarization allows spectroscopic identification of monofluorinated and geminally difluorinated carbon species in poly(carbon monofluoride).

A Zeolite-Supported Molecular Ruthenium Complex with η6-C6H6 Ligands: Chemistry Elucidated by Using Spectroscopy and Density Functional Theory

An essentially molecular ruthenium-benzene complex anchored at the aluminum sites of dealuminated zeolite Y was formed by treating a zeolite-supported mononuclear ruthenium complex, [Ru(acac)(eta(2)-C(2)H(4))(2)](+) (acac=acetylacetonate, C(5)H(7)O(2)(-)), with (13)C(6)H(6) at 413 K. IR, (13)C NMR, and extended X-ray absorption fine structure (EXAFS) spectra of the sample reveal the replacement of two ethene ligands and one acac ligand in the original complex with one (13)C(6)H(6) ligand and the formation of adsorbed protonated acac (Hacac). The EXAFS results indicate that the supported [Ru(eta(6)-C(6)H(6))](2+) incorporates an oxygen atom of the support to balance the charge, being bonded to the zeolite through three Ru-O bonds. The supported ruthenium-benzene complex is analogous to complexes with polyoxometalate ligands, consistent with the high structural uniformity of the zeolite-supported species, which led to good agreement between the spectra and calculations at the density functional theory level. The calculations show that the interaction of the zeolite with the Hacac formed on treatment of the original complex with (13)C(6)H(6) drives the reaction to form the ruthenium-benzene complex.

Methanol to Hydrocarbon Catalysis on Sulfated Zirconia Proceeds Through a Hydrocarbon-Pool Mechanism

Aromatic hydrocarbons and other cyclic organic species have recently been identified as the locus of carbon--carbon bond forming and breaking in the conversion of methanol to olefins (MTO) on microporous solid acids such as HZSM-5 and HSAPO-34. In order to ascertain whether or not this hydrocarbon-pool mechanism for MTO catalysis is unique to zeotype solid acids, we studied the stronger solid acid sulfated zirconia. This study establishes a greater generality for the hydrocarbon-pool mechanism.