Substrate induced phase

Identification and Structural Characterization of the Substrate-Induced Phase... 

These results strongly suggest that this new structural arrangement is stabilized in the vicinity of the substrate surface (less than 30 nm distance), and co-exists with the bulk phase. Accordingly, the new phase will hereafter be referred to as the substrate-induced phase, SIP. 

After having determined the structure of the SIP and the morphological changes associated with its appearance, we sought to decipher the reasons responsible for the formation of the substrate-induced phase. To gain some insights into these intriguing questions we modified the interfaces and studied its effect on the SIP (Fig. 9.15). 

The thickness of the SIP is expected to be a function of the dielectric constant of the substrate. This fact is established by studies done on thin films of pentacene and polystyrene [15, 39, 40]. In our scenario, this theory does not apply, as the SIP is observed on Si/SiOx substrates (composed of a thin SiOx layer of 2 nm on top of silicon) having a dielectric constant of e = 11.9 as also on glass (Si02), PVP and AlOx with a dielectric constant of s = 3.9, 5.0 and 9.9, respectively. On the top, the substrate-induced phase exists in contact with air or a sacrificial layer, and the bulk phase. The rectangular lattice of the bulk DLC-phase is incommensurable with the tetragonal phase of the SIP. The substrate-induced phase thus forms regardless of the thickness and the structure of the bulk phase as a function of time due to nucleation events initiated by the solid substrate. 

This evidence suggests that neutral TCNQ is formed in a solid-state field induced phase transition when electric fields are applied to crystalline films of Cu-TCNQ grown on copper substrates. 

The Ruhrchemie/Rhone-Poulenc process is performed annually on a 600,000 metric ton scale (18). In this process, propylene is hydroformylated to form butyraldehyde. While the solubility of propylene in water (200 ppm) is sufficient for catalysis, the technique cannot be extended to longer-chain olefins, such as 1-octene (<3 ppm solubility) (20). Since the reaction occurs in the aqueous phase (21), the hydrophobicity of the substrate is a paramount concern. We overcame these limitations via the addition of a polar organic co-solvent coupled with subsequent phase splitting induced by dissolution of gaseous CO2. This creates the opportunity to run homogeneous reactions with extremely hydrophobic substrates in an organic/aqueous mixture with a water-soluble catalyst. After C02-induced phase separation, the catalyst-rich aqueous phase and the product-rich organic phase can be easily decanted and the aqueous catalyst recycled. 

With the neutral [(RCN)2PdCl2] pro-catalyst system (Fig. 12.3, graph iv), computer simulation of the kinetic data acquired with various initial pro-catalyst concentrations and substrate concentrations resulted in the conclusion that the turnover rates are controlled by substrate-induced trickle feed catalyst generation, substrate concentration-dependent turnover and continuous catalyst termination. The active catalyst concentration is always low and, for a prolonged phase in the middle of the reaction, the net effect is to give rise to an apparent pseudo-zero-order kinetic profile. For both sets of data obtained with pro-catalysts of type B (Fig. 12.3), one could conceive that the kinetic product is 11, but (unlike with type A) the isomerisation to 12 is extremely rapid such that 11 does not accumulate appreciably. Of course, in this event, one needs to explain why the isomerisation of 11 now proceeds to give 12 rather than 13. With the [(phen)Pd(Me)(MeCN)]+ system, analysis of the relative concentrations of 11 and 13 as the conversion proceeds confirmed that the small amount of... 

SCCO2 is delivered to the top of the pre-mixer, where it is mixed with the substrates and any other reactants that are needed for the reaction. The flow of reactants will then be carried by the SCCO2 over the heated catalyst bed where reaction occurs. The products and SCCO2 are then passed through the apparatus to the point where they are expanded to a pressure below the critical point of CO2, to induce phase separation of products [26]. The products may then be separated by gravity from the gaseous CO2, which may be recycled or vented. Clearly, the exact conditions required for reactions within these types of reactors depend on the chemistry. The important point is that the use of the SCF enables the reaction conditions to be controlled more precisely. The presence of the SCF does not necessarily make the catalyst more active, nor does an SCF normally alter the catalytic cycle. But in many cases, overall conversions and selectivities have been found to be at least equal to, if not higher than those obtained by conventional routes. 

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