Low temperature methanol synthesis catalysed by a copper nanoparticle-alkoxide system
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Methanol (MeOH) synthesis at low temperature in a liquid medium presents the possibility of achieving full syngas conversion per pass. The Low temperature MeOH synthesis (LTMS) process is advantageous over the current technology for MeOH production since the former is thermodynamically favourable and gives a high yield per pass. The LTMS involves two main steps, (i) MeOH carbonylation to form methyl formate and (ii) hydrogenolysis of methyl formate to form MeOH. The initial aim of the present work was to develop, characterize and evaluate the catalyst system involved in the LTMS process. A once-through catalyst system involving copper (II) acetate and methoxide was used to obtain up to 92 % conversion (> 94 % selectivity to MeOH) per batch at 20 bar syngas pressure and 100 oC temperature within 2 h. XRD and TEM characterization of the slurry catalyst system revealed that about 10 ± 5 nm Cu2O/Cu0 nanoparticles were involved in the catalytic process. Decreasing Cu nanoparticles sizes led to increased MeOH production due to an increase in active Cu surface area, which enhanced methyl formate hydrogenolysis. Agglomeration of the Cu nanoparticles in the course of MeOH production was identified as a major cause for the deactivation of the Cu nanoparticle component of the LTMS catalyst system. Furthermore, with the aim of investigating the role of solvents polarity on the LTMS, MeOH production maximized for solvents with dielectric constant (ɛ) around 7.2, similar to the polarity of diglyme. A probe of possible side reactions of the main intermediate revealed that, in the presence of methoxide, low polar solvents enhanced decarbonylation of methyl formate while high polar solvents enhanced a nucleophilic substitution to form dimethyl ether and sodium formate. Relatively moderate polar solvents such as diglyme appeared to give a good balance in minimizing possible side reactions of methyl formate and therefore enhanced MeOH production. In addition, the spinning disk reactor (SDR) was used to synthesize on-purpose Cu nanoparticles with predefined particle sizes for catalysing the LTMS reaction. By maintaining the same chemical recipe, Cu nanoparticle sizes were tuned down to 3 nm when physical conditions were varied to shorten for example micromixing time, mean residence time and relative residence time distribution. This subsequently led to uniform nucleation and ultimately formation of smaller Cu nanoparticle sizes with narrow particle size distribution. At the end, a model was proposed for a complete LTMS process with the help of Aspen HYSYS simulation tool, using an air-blown autothermal reformer, for a full conversion per pass at 60 bar syngas (0.31 CO: 0.62 H2: 0.07 N2) and 100 oC MeOH synthesis temperature.
Består avPaper 1: Ahoba-Sam, C., Olsbye, U. & Jens, K.-J.: Low temperature methanol synthesis catalysed by copper nanoparticles. Catalysis Today 299, (2018), 112-119. https://doi.org/10.1016/j.cattod.2017.06.038
Paper 2: Ahoba-Sam, C., Olsbye, U. & Jens, K.-J.: The role of solvent Polarity on Low-Temperature Methanol Synthesis Catalyzed by Cu Nanoparticles. Frontiers in Energy Research 5, (2017). https://doi.org/10.3389/fenrg.2017.00015
Paper 3: Ahoba-Sam, C., Boodhoo, K.V., Olsbye, U. & Jens, K.-J.: Tailoring Cu nanoparticles catalyst for methanol synthesis using the spinning disk reactor. Materials 11(1), 154 (2018). https://doi.org/10.3390/ma11010154
Paper 4: Ahoba-Sam, C., Øi, L.E. & Jens, K.-J.: Process Design of a Novel Low Temperature Methanol Synthesis Process Using an Air-blown Autothermal Reformer. Submitted to Linköping Electronic Conference Proceedings