Heterogeneous catalysts are promising for the transesterification reaction of vegetable oils to produce biodiesel and have been studied intensively over the last decade. Unlike the homogeneous catalysts, heterogeneous catalysts can be easily separated from reaction mixture and reused for many times. They are environmentally benign and could be easily operated in continuous processes. This review classifies the solid catalysts into two categories based on their catalytic temperature, i.e. high temperature catalysts and low temperature catalysts. The nature of the catalysts can be specified into solid bases and solid acids. Three aspects, catalyst activity, catalyst life and oil flexibility, will be reviewed. Two kinds of heterogeneous catalysts, reported by IFP Inc. and by WSU, respectively, show a high catalytic activity, long catalyst life and low leaching of catalyst components. These two catalysts also show ability to simultaneously catalyze esterification and transesterification, and can be used in half-refined or crude oil system which provide a potential for greatly decrease the feedstock cost.

No abstract

No abstract

No abstract

We have studied the adsorption and reaction of oxygen and CO on a stepped Pt surface with varying amounts of Au, using temperature-programmed desorption and reaction (TPD and TPR), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy, and steady-state reaction measurements. When the surface is fully covered with Au it is inert to oxygen adsorption and to CO oxidation, and supports only a single weakly bound CO adsorption state. The surface covered with 0.7 ML Au, however, exhibits properties different from either bare Pt or bare Au. Our TPD and LEED results suggest the coexistence of completely Au-covered regions and regions with Au on the step edges but not on the terraces. Dissociative oxygen adsorption is reduced by 90%, and the remaining oxygen is confined to Pt sites near the Au/Pt boundaries. The Au-covered regions support weakly bound CO adsorption states with desorption temperatures of 120, 190, and 240 K. CO in these states can diffuse rapidly and react efficiently with adsorbed atomic oxygen at temperatures as low as 150 K. In low-temperature TPR experiments the reaction is limited by the availability of adsorbed oxygen under almost all conditions. Under steady-state conditions, however, it is limited by the availability of CO even at low temperatures and CO partial pressures up to 10-6 Torr. Adding CO partial pressure does not inhibit the reaction. Consequently, adsorbed CO does not completely block all the sites at which oxygen dissociates on this surface, unlike on bare platinum.

Adsorption and reaction of acetaldehyde with clean and oxygen-predosed Pt(S)-[6(l 11)X(100)] were studied by electron energy loss vibrational spectroscopy (EELS) and temperature-programmed reaction spectroscopy (TPRS). Acetaldehyde adsorption at 95 K on the clean surface produced a multilayer structure with vibrational bands characteristic of solid acetaldehyde. This condensed phase desorbed near 133 K leaving an adsorbed layer showing a major binding state of 58 kJ g-mol'1 2and a minor state of 70 kJ g-mol"1. The vibrational spectra and characteristic pattern of H2 evolution obtained upon heating the chemisorbed layer above 300 K indicated the conversion of the acetaldehyde surface complex to ?)3-ethylidyne. The ethylidyne species dehydrogenated to gaseous hydrogen and surface carbon above 450 K. In a separate reaction path, acetaldehyde underwent decarbonylation near 290 K to produce adsorbed CO and methane, which desorbed in a reaction-limited peak at 317 K. Vibrational features suggestive of a surface acetyl species were observed below 290 K. Evidence was also obtained for acetaldehyde adsorbed molecularly in 1 configurations. Similar results were obtained for acetaldehyde on the Pt-(S)-[6(l 11)X(100)] surface in the presence of preadsorbed atomic oxygen, indicating the absence of direct attack of acetaldehyde by oxygen. This contrasts with the nucleophilic attack on the carbonyl carbon observed previously on copper and silver surfaces.

No abstract

No abstract