Activated metallic gold as an agent for direct methoxycarbonylation

We have discovered that metallic gold is a highly effective vehicle for the low-temperature vapor-phase carbonylation of methanol by insertion of CO into the O-H bond to form methoxycarbonyl. This reaction contrasts sharply to the carbonylation pathway well known for homogeneously catalyzed carbonylation reactions, such as the synthesis of acetic acid. The methoxycarbonyl intermediate can be further employed in a variety of methoxycarbonylation reactions, without the use or production of toxic chemicals. More generally we observe facile, selective methoxycarbonylation of alkyl and aryl alcohols and secondary amines on metallic gold well below room temperature. A specific example is the synthesis of dimethyl carbonate, which has extensive use in organic synthesis. This work establishes a unique framework for using oxygen-activated metallic gold as a catalyst for energy-efficient, environmentally benign production of key synthetic chemical agents.     

Sulfur dioxide adsorption and reaction with atomic oxygen on the Ag(110) surface

The adsorption of SO₂ on Ag(110) and the reaction of SO₂ with oxygen adatoms have been studied under ultrahigh vacuum conditions using low energy electron diffraction, temperature programmed reaction spectroscopy and photoelectron spectroscopy. Below 300 K, SO₂ adsorbs molecularly giving p(1×2) and c(4×2) LEED patterns at coverages of one half and three quarter monolayers. respectively. At intermediate coverages, streaked diffraction patterns, similar to those reported for noble gas and alkali metal adsorption on the (110) face of face-centered cubic metals were observed, indicating adsorption out of registry with the surface. A feature at low binding energy in the ultraviolet photoemission spectrum appeared which was assigned to a weak chemisorption bond to the surface via the sulfur, analogous to bonding observed in SO₂-amine charge transfer complexes and in transition metal complexes. SO₂ exhibited three binding states on Ag(110) with binding energies of 41, 53 and 64 kJ mol^?1; no decomposition on clean Ag(110) was observed. On oxygen pretreated Ag(110), SO₂ reacted with oxygen adatoms to form SO_3(a), as determined by X-ray photoelectron spectroscopy. Reacting preadsorbed atomic oxygen in a p(2 × 1) structure with SO₂ resulted in a c(6 × 2) pattern for SO_3(a). The adsorbed SO_3(a) decomposed and disproportionated upon heating to 500 K to yield SO_2(g), SO_4(a) and subsurface oxygen.     

The dynamical origin of non-normal energy scaling and the effect of surface temperature on the trapping of low molecular weight alkanes on Pt(111)

Molecular classical dynamical simulations of alkanes trapping on platinum surfaces were performed to examine the origin of non-normal energy scaling for molecular adsorption. Conversion of normal to parallel translational energy at normal incidence and conversion of parallel translational energy into normal translational energy at glancing angles are the primary mechanisms which produce non-normal energy scaling of alkanes trapping on cold Pt(111). In addition, a tendency to convert rotational energy gained in the first gas-surface collision into normal translational energy for collisions at glancing incidence further increases the degree of non-normal energy scaling. Increasing surface temperature is shown to have little effect on energy transfer processes in the first bounce but increasing influence on subsequent bounces. Despite difficulties in defining trapping at high surface temperatures, simulations indicate that the initial trapping probability of ethane on Pt(111) does not fall by more than a factor of two over the surface temperature range of 100–700 K.     

Strong structure sensitivity in the partial oxidation of styrene on silver single crystals

In contrast to the formation of styrene oxide on Ag(1 1 1), phenylacetaldehyde and phenylketene dominate the partial oxidation of styrene on Ag(1 1 0), even though the reactions follow the same mechanism on both surfaces. The origin of this difference is that on Ag(1 1 0) the activation energy for transformation of the oxametallacycle to the combustion intermediate is much lower than on the (1 1 1) surface, so that ring-closure of the oxametallacycle to form styrene oxide is short circuited. Also the combustion intermediate appears more stable on Ag(1 1 0) than on Ag(1 1 1).     

An FTIR study of the bonding of methoxy on Ni(100): effects of coadsorbed sulfur, carbon monoxide and hydrogen

The vibrational spectra of CH₃O_(a), CD₃O_(a), CDH₂O_(a) and CD₂HO_(a) on Ni(100) are analyzed and interpreted in terms of resonances between fundamental modes and either combinations or overtones. Analysis of the symmetry of the modes observed suggests that methoxy binds normal to the surface with C_s symmetry, at least at low coverages. Two distinct vibrational bands emerge in the vibrational spectrum of methoxy in the v(CO) region as the coverage increases which are attributed to bonding in four-fold hollow sites and bridging sites. These bands exhibit blue shifts of about 25 cm⁻¹ with increasing coverage up to the saturation coverage. The vibrational bands in the v(CH) region appear concomitantly at all coverages and shift down 12 cm⁻¹ as the coverage is increased. These shifts are attributed to changes in the metal-oxygen bond which are reflected in changes in the strength of the C---O and C---H bonds. Affects on the bonding also appear to occur with the coadsorption of hydrogen or CO with methoxy. Coadsorption of 0.36 ML hydrogen with 0.04 ML methoxy induces blue shifts of 15 and 7 cm⁻¹ for the v(CO) bands at 949 and 984 cm⁻¹, respectively. Adsorbing 0.43 ML of CO with 0.04 ML methoxy (and 0.04 ML hydrogen) causes a red shift of 20 and 12 cm⁻¹ for these bands. A drastic drop in mode intensities for methoxy when CO is coadsorbed suggests that the methoxy tilts away from the surface normal. Pre-adsorbing sulfur on the Ni(100) surface reduces the amount of methoxy formed from methanol, but the v(CO) methoxy bands are unshifted in frequencies relative to their position for the same methoxy coverage on the clean surface.     

XPS,UPS and thermal desorption studies of alcohol adsorption on Cu(110): II. Higher alcohols

Preceding work dealing with the adsorption of methanol on Cu(110) has been extended to include ethanol, n- and iso-propanol and a diol, ethylene glycol. In common with the simplest alcohol, all these molecules are able to form a stable alkoxy species on the surface, that is, the alcohol dissociated at the O-H group. However, in contrast to methanol on the clean surface for which the dissociated methoxy and hydrogen recombined to desorb as methanol, all the higher alcohols reacted further with the surface, dehydrogenating to yield the corresponding aldehyde or ketone in the gas phase. Ethylene glycol reacted to form the most strongly bound intermediate of all, decomposing near 390 K to produce the dialdehyde, glyoxal, with little evidence of monoaldehyde formation or C-C bond breakage. The influence of pre-adsorbed oxygen on these reactions was to generally increase the amount of alkoxy formed on the surface by enhancing the amount of dissociative adsorption (water is formed by the deprotonation of adsorbed alcohol molecules by oxygen atoms). The alkoxide decomposition peaks were shifted to slightly higher temperatures and considerably broadened in such experiments. The decomposition peak temperatures of the different surface alkoxides correlate fairly well with literature values of the αC-H bond strength, which is weaker in iso-propanol than in methanol. XPS showed broad O(1s) spectra for all the molecules adsorbed at 140 K, probably due to hydrogen-bonding effects in the adlayer, with peak emissions at around 533 eV. When the surface was warmed to 250 K, the O(1s) spectra narrowed to close to instrumental linewidths with a concomitant shift to a lower binding energy near 531 eV. C(1s) spectra showed little change between the adsorbed alcohol and alkoxy species. The UPS showed low temperature spectra similar to the gas phase, but the highest occupied orbitals, which are essentially O(2p) orbitals, showed a chemisorption bonding shift of several tenths of an electron volt. UPS for these molecules is shown to have considerable less utility than for the simplest molecule, methanol, due to the masking of possible orbital shifts during chemical changes on the surface by the presence of overlapping emissions in the spectra.