Supercritical Carbon Dioxide:  An Inert Solvent for Catalytic Hydrogenation?

Abstract

Various surface species originating from the reaction between CO2 and H2 over Al2O3-supported Pt, Pd, Rh, and Ru model catalysts were investigated by attenuated total reflection infrared (ATR-IR) spectroscopy under high-pressure conditions. Two different spectroscopic cells were used: a variable-volume view cell equipped with ATR-crystal and transmission IR windows (batch reactor) and a continuous-flow cell also equipped with a reflection element for ATR-IR spectroscopy. The study corroborated that CO formation from dense CO2 in the presence of hydrogen occurs over all Pt-group metals commonly used in heterogeneous catalytic hydrogenations in supercritical CO2 (scCO2). In the batch reactor cell, formation of CO was detected on all metals at 50 and 90 degrees C, with the highest rate on Pt. Additional surface species were observed on Pt/Al2O3 at 150 bar under static conditions. It seems that further reaction of CO with hydrogen is facilitated by the higher surface concentration at higher pressure. In the continuous-flow cell, CO coverage on Pt/Al2O3 was less prominent than that in the batch reactor cell. A transient experiment in the continuous-flow cell additionally revealed CO formation on Pt/Al2O3 at 120 bar after switching the feed from a H2-ethane to a H2-CO2 mixture. The in situ ATR-IR measurements indicate that CO formation in CO2-H2 mixtures is normally a minor side reaction during hydrogenation reactions on Pt-group metal catalysts, and dense ("supercritical") CO2 may be considered as a relatively "inert" solvent in many practical applications. However, blocking of specific sites on the metal surface by CO and consecutive products can affect structure sensitive hydrogenation reactions and may be at the origin of unexpected shifts in the product distribution.

Hydrogenation of phenol with supported Rh catalysts in the presence of compressed CO2: Its effects on reaction rate, product selectivity and catalyst life

“…If the competitive adsorption of CO 2 and phenol is important, the catalyst deactivation should occur shortly after the start of reaction at any CO 2 pressures since the adsorption usually reaches its equilibrium rapidly irrespective of the pressure. It was previously reported that the amount of adsorbed CO produced from CO 2 gradually increased with time [37,40]. This can explain the induction period for the complete deactivation of Rh/Al 2 O 3 catalyst.…”

“…11) that the adsorbed CO was produced from CO 2 -H 2 , although no gaseous CO was detected in the gas phase after the hydrogenation runs in the presence of CO 2 . Several research groups reported, by FTIR (not in situ), the formation of adsorbed CO from CO 2 -H 2 mixtures via reverse water gas shift reaction over Pt [37][38][39][40], Rh [40], Ru [40,41], and Pd [40,41] catalysts even at such a low temperatures as 323 K. Some authors have also shown that adsorbed CO suppresses hydrogenation of ethyl pyruvate over Pt/Al 2 O 3 [38] and dechlorination of chloroaniline over Pt/C [39]. Hence, the formation and adsorption of CO should retard the phenol hydrogenation by blocking Rh active site.…”

“…These CO 2 -dissolved expanded liquid phases are demonstrated to be interesting reaction media for reactions including gaseous reactants like O 2 , H 2 , and CO. Moreover, it is reported that CO may be formed in hydrogenation reactions with supported noble metal catalysts in scCO 2 and this has negative impacts on the catalytic activity and, subsequently, the reaction rate and selectivity [37][38][39][40][41][42].…”

“…Furthermore, in situ high-pressure transmittance FTIR measurements have been made to study the molecular interactions of the reacting species with CO 2 and the formation and adsorption of CO on the catalysts at high pressures. In situ high-pressure measurement of the formation of CO from pressurized CO 2 and H 2 over supported metal catalysts was studied only by Baiker et al, who used attenuated total reflection infrared (ATR-IR) spectroscopy [37,38,40].…”

Multiphase reaction media including dense phase carbon dioxide and/or water: A case study for hydrogenation of phenol with a Pd/Al2O3 catalyst

“…The FTIR of CO adsorbed on the catalysts was made in the presence of organic substrate (phenol or cyclohexanone) and/or water to examine the competitive adsorption of substrate vapor, water vapor, and CO on the catalysts. Generally, CO could be formed in hydrogenation reactions in the presence of dense phase CO 2 and H 2 at high pressures and would affect the outcome of these reactions [16][17][18]. Fig.…”