A covalently immobilized zinc pyridine bromide complex was investigated in the solvent-less synthesis of propylene carbonate (PC) by fixation of carbon dioxide (CO2) in propylene oxide (PO). The reaction was optimized using phase behavior studies based on video monitoring, transmission and attenuated total reflection (ATR) infrared measurements. At first the immobilization process was monitored by DRIFTS which confirmed the successful fixation of the ZnBr2(APY)(2)-complex on the silica matrix. Phase behavior studies of the reaction mixture showed three different regions that influenced strongly the reaction rate: (i) At low overall density a biphasic region with small liquid phase (high molar ratio of PC) was observed. The catalytic activity of this system was poor. (ii) A decreasing CO2 content at the same density resulted in an expanded liquid phase, which was up to a certain PO/PC ratio beneficial for the catalytic activity. However, at too low content of CO2 (ratio CO2/(PO + PC) < 4) the rate decreased again, because CO2 is not only solvent but also reactant in the synthesis of PC. (iii) Finally, experiments at higher pressure led to a single-phase system ("supercritical conditions"). Despite of optimal mass transfer properties the lowest conversion was achieved under these reaction conditions. These observations provided important insight into the reaction system. In situ ATR-IR studies of the catalyst surface during reaction revealed drastic changes in the phase behavior in the first minutes and gave important insight into the solid/fluid interface under reaction conditions. (c) 2006 Elsevier B.V. All rights reserved.