Kinetic modeling of homogeneous methanol synthesis catalyzed by base-promoted nickel complexes

Abstract

Nickel complexes promoted by alkoxide bases can affect facile conversion of synthesis gas into methanol under thermodynamically favorable temperatures. At 100°C, syngas conversion of 99% with turnover frequency of 66 h-1 is reported. The reaction is truly catalytic in Ni as well as in base. A kinetic study of the Ni(CO)4-KOMe catalyst system in 1,2-bis(2-methoxy ethoxy)ethane (triglyme)-MeOH solvent mixture is presented. The kinetic expression includes terms of zero-order in H2 and first-order in CO with syngas of H2-CO stoichiometry of ~2:1 and less than first-order in Ni (0.03-0.10 M), ~third-order in base (2.0-4.75 M), and an exponential dependence on methanol concentration (13.0-23.7 M at 2.0 M base; 15.8-22.6 M at 3.0 M base). The activation energy of 42.2 kJ mol-1 was estimated from the Arrhenius plot of the data between 374 and 393 K. Gas phase IR spectrum at the end of each run showed an intense signature peak at 2060 cm-1 for Ni(CO)4. Process uncertainties to commercialization of this versatile homogenous catalyst system for CO hydrogenation to methanol are discussed.Key words: homogeneous catalysis, syngas conversion, methanol synthesis, alkoxides, carbonyls of nickel.

Atom-economical reduction of carbon monoxide to methanol catalyzed by soluble transition metal complexes at low temperatures

“…Three common features of all these systems were as follows: (1) the reaction operated in a liquid medium, (2) the operating temperature was below 200°C, and (3) the use of a highly basic medium in the form of an alkali metal alkoxide, notably sodium or potassium methoxide (NaOCH 3 or KOCH 3 ) was essential to the reaction. Interestingly, catalysts based on metals other than Ni, under similar reaction conditions, failed to achieve high turnover numbers reported by us with the Ni(CO) 4 /KOCH 3 /methanol/glyme system [9]. In our system, the reaction could be carried out in methanol alone, but the presence of glyme co-solvent increased the turnover numbers by at least a factor of two compared to those in methanol solvent alone [9].…”

“…Interestingly, catalysts based on metals other than Ni, under similar reaction conditions, failed to achieve high turnover numbers reported by us with the Ni(CO) 4 /KOCH 3 /methanol/glyme system [9]. In our system, the reaction could be carried out in methanol alone, but the presence of glyme co-solvent increased the turnover numbers by at least a factor of two compared to those in methanol solvent alone [9].…”

“…Previous studies from our group have been reported that described Ni-based systems to achieve methanol synthesis under mild conditions [8][9][10]. Several other groups have reported the use of modified Ni and other transition metals to carry out methanol synthesis at low temperatures utilizing systems such as Ni [11,12], Cu [13][14][15], Ru and Re [7,16].…”

“…The major drawback of the earlier reported system was the use of toxic Ni(CO) 4 (OSHA exposure limit is 1 ppb) as a catalyst precursor [9]. Therefore, we recently reported a subsequent study on the use of nickel complexes, other than nickel tetracarbonyl, as versatile catalysts to catalyze CO reduction to methanol under mild conditions of temperature (T: 80-130°C) and pressure (2000-5000 kPa) [10].…”

Carbon Monoxide as a Chemical Feedstock: Carbonylation Catalysis

“…The kinetic expression includes terms of zero-order in H 2 and first-order in CO with syngas H 2 : CO stoichiometry of around 2 : 1 and less than first-order in nickel, approximately third-order in base, and an exponential dependence on methanol concentration. A gas-phase infrared (IR) spectrum recorded at the end of each run showed an intense signature peak at 2060 cm -1 for Ni(CO) 4 [8]. The major problems related with these catalysts are probably the low stability, toxicity and volatility of nickel tetracarbonyl under the reaction conditions.…”