Methanol from CO2 by Organo-Cocatalysis: CO2 Capture and

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Methanol from CO2 by Organo-Co-Catalysis - CO2 Capture and Hy-drogenation in One Process Step Christian Reller, Matthias Pöge, Andreas Lissner, and Florian O.R.L. Mertens Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es503914d • Publication Date (Web): 14 Nov 2014 Downloaded from http://pubs.acs.org on November 16, 2014

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Environmental Science & Technology

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Methanol from CO2 by Organo-Co-Catalysis - CO2

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Capture and Hydrogenation in One Process Step

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Christian Reller, Matthias Pöge, Andreas Lißner and Florian O.R.L Mertens*

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Institut für Physikalische Chemie, Technische Universität Bergakademie Freiberg, Leipziger

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Straße 29, 09599 Freiberg (Germany)

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ABSTRACT: Carbon dioxide chemically bound to alcohol-amines was hydrogenated to

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methanol under retrieval of these industrially used CO2 capturing reagents. The energetics of the

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process can be seen as a partial cancellation of the exothermic heat of reaction of the

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hydrogenation with the endothermic one of the CO2 release from the capturing reagent. The

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process provides a means to significantly improve the energy efficiency of CO2 to methanol

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conversions.

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INTRODUCTION

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In recent years, the regard of CO2 has changed from a mere waste product to an attractive low

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energy chemical feedstock obtainable from fossil fuel and biomass combustion processes or

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biogas production.

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converting CO2 by catalytic hydrogenation into valuable base chemicals or fuels, i.e. the

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conversion to products such as formic acid, methanol, and methane.

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of CO2 to the latter two products is strongly exothermic, a recovery of the heat of reaction and its

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utilization in the overall process is mandatory in order to be energy efficient. Two areas for the

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Great efforts have been devoted to the development of processes for

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Since the hydrogenation

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utilization of the process heat are conceivable, which are the hydrogen production and the CO2

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separation from flue gas or biogas. In respect to the first option, recent projects have focused on

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the development of more efficient high temperature electrolysis (Solid Oxide Electrolysis Cells)

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and the utilization of the recovered heat for preheating and vaporization of water before it is

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converted in the electrolyzer. The second option would be the use of the heat of reaction of the

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CO2 hydrogenation in the CO2 recovery from the absorbing or capturing reagents that were used

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to extract the CO2 from gaseous sources such as flue gas or biogas. As industrial capturing

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reagents, alcoholamines are typically used. It is important to note that these reagents not only

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physically but also chemically bind the CO2 molecule and that the absolute value of the enthalpy

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that is needed to release the CO2 molecule from the capturing reagents again is comparable to the

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one needed for its hydrogenation (for example for mono-ethanolamine (MEA) the CO2 release

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enthalpy is ∆H353K =+88.91 kJ/mol while the reaction enthalpy for the CO2 hydrogenation to

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CH3OH gas and H2O(liq) is ∆H353K= -93.5 kJ/mol. 8,9

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In respect to this situation, the work presented here started out from two questions. First, if CO2

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is separated by conventional means, which is its capture by amines or alcohol-amines, it has

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undergone a chemical fixation that fully changes the nature of its chemical bonds.

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that not be seen as a sort of chemical activation, which should allow a more efficient

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hydrogenation process? Second, if the hydrogenation of CO2 is exothermic and the release of

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CO2 from the capture reagent endothermic, should it not be possible to avoid the strong heat

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effects by directly hydrogenating the CO2-laden capture reagent? In this way the two processes

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CO2 hydrogenation and release from the capture reagent would be joined in one process step and

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an energetic shortcut of the heats of reaction that accompany these two processes would have

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occurred, leading to a strong energy efficiency gain because much less heat is produced during

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If so, can


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this new process. In this contribution, we present a method in which the captured and chemically

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activated CO2 is directly hydrogenated to methanol under retrieval of the capturing reagent und

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thus the energy demanding stripping processes can be omitted, completely (see Scheme 1).

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Scheme 1. Corresponding CCU (carbon capture and utilization) process based on DEEA. The

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stripping process was replaced by in-situ hydrogenation. The scrubbing/capturing reagent was

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recovered after the distillation of water and methanol.

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In order to increase the life time of the technical Cu/ZnO-Al2O3 catalysts in the classical

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methanol syngas synthesis, there is a general attempt to lower the reaction temperature and to

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work directly with syngas in the liquid phase, i.e. in mineral oils or alcohols as solvents, to

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improve the heat conductivity of the system. 10-15

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As part of the attempt to combine syngas chemistry with fossil energy power plant operations by

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the so called Integrated Gasification Combined Cycle (IGCC), the so called LPMeOHTM process

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was implemented on pilot plant scale by Air Products in the 1990ies and intensively studied.

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In this process the catalyst is suspended in mineral oils and the reaction takes place in slurry

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bubble column reactors These studies demonstrated that in the case of methanol from syngas

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liquid phase processes can have the potential to compete economically with the standard gas

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phase process because of the much improved heat removal from the reaction zone. Beside some

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specific benefits connected to syngas chemistry, like being able to directly process syngas which

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is rich in carbon oxides (CO and CO2), the process demonstrates possible benefits that will also

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hold for the proposed process such as compact system construction (small heat exchanger

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volume), continuous catalyst exchange, and robustness to sharp transient operations.10,11

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In processes in which alcohol is added, it known that it esterifies with the surface-bound formic

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acid species (b-HCOO-) to form alkyl formates, which were catalytically hydrogenated to

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methanol and the corresponding alcohol in a subsequent reaction step.

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temperature in the proposed procedure is reduced because the overall heat of reaction is much

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lower, it nevertheless somewhat resembles these known procedures because many amine based

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effective CO2 capturing/scrubbing agents also possess OH groups (alcohol-amines), which may

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also react with the (b-HCOO-) species to formic acid esters. In addition, amines have also been

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used as thermodynamic auxiliary reagents in the homogeneously conducted hydrogenation of

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CO2 to formic acid (see Figure S1). The driving force, i.e. the additional gain in free energy, in

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these reactions was clearly attributable to the formation of the corresponding ammonium salts.16

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EXPERIMENTAL SECTION

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General Methods: All operations were carried out using standard Schlenk and glove box

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techniques under an atmosphere of argon 5.0. The CO2 hydrogenation processes were conducted

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in a PTFE-lined 50 ml stainless steel pressure vessel under magnetic stirring. In all time-

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dependent experiments, the sample extraction of the liquid phase was done with a riser without

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depressurizing the autoclave. Gases: Hydrogen 5.0 (Praxair), carbon dioxide 4.8 (Praxair) were

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Although the


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used without further purification. All solvents were also used without further purification. The

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1

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solvent CDCl3. Off-line GC analysis was performed on an Agilent 6890 N gas chromatograph

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equipped with a HP 7694 E headspace sampler. A 30 m Supelco SPB™-1 (0.32 mm diameter;

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film: polymethyl-silanes) column was used for separation. The GC samples were thermostated at

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353 K for 20 min before they were transferred by a helium stream (ß=1.2 ml/min) into the

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injector manifold. The injector temperature was 523 K and a 1:5 split was used. A linear

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temperature program from 313 K to 453 K (ß=10 K/min) was chosen for the analyte separation

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and the latter temperature was held for 2 min. GC calibration and further details are described in

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the Supporting Information, SI. The composition investigation of the supernatant gas phase of

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the reaction procedure was performed using a Pfeiffer Vacuum Omnistar residual gas analyzer.

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The calorimetric measurements were carried out with a Setaram C80 calorimeter. The

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calorimetric cell with a volume of 7.5 ml was filled with 19.9 mmol DEEA and evacuated to its

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vapor pressure. Afterward the setup was left to equilibrate for 5 hrs (separate tests showed that

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the mass loss due to the evacuation procedure is less than 1 percent). Subsequently, the cell was

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connected to a CO2 filled gas reservoir (1953 mbar, 15.7735 ml) resulting in a pressure drop to

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1188 mbar from which an overall gas volume of 25.93 ml (gas reservoir, calorimetric cell and

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tubing) was calculated.

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Hydrogenation: 20 ml DEEA and 300 mg Cu/ZnO-Al2O3 catalyst were filled in a 50 ml PTFE

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lined autoclave under inert conditions. The mixture was heated to the desired temperature and

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pressurized with 20 bars of CO2 until the solvent became saturated. After the system reached its

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equilibrium, gaseous hydrogen was introduced (pH2=50-70 bar). The reaction procedure was

H and 13C NMR spectra were recorded on a Bruker Avance III 500 spectrometer using the NMR


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surveyed by internal pressure measurement and by time resolved sample extraction over a riser

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followed by subsequent GC and NMR analysis.

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Catalyst preparation: The preparation of the catalyst Cu/ZnO-Al2O3 follows mainly the method

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given in Ref. [17]. A 0.1M metal salt solution was prepared by dissolving 8.92 g Zn(NO3)2·4H2O

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(0.03 mol), 14.449 g Cu(NO3)2·3H2O (0.06 mol), and 3.75 g Al(NO3)3·9H2O (0.01 mol) in 100

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ml distilled H2O. The co-precipitation agent was a 1.6M solution of Na2CO3 (17 g/ 100 ml). The

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pH and temperature controlled co-precipitation was performed at T=65 °C and pH=6.8 in 400 ml

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distilled water. The slurry was aged under continuous stirring for 60 min. The precipitate was

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filtered off and washed free from alkaline ions before it was dried at T=80 °C overnight. For the

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calcination the catalyst was heated with 2K/min from ambient temperature to T=330 °C in an

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Ar/O2 80/20% mixture and kept there for 3 h. The catalyst was then slowly cooled in a mixture

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of 10% H2 in Ar to 280 °C (1 K/min) and kept at that temperature for 2 hours. The reduction

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properties of the Cu, Zn, and Al oxides were investigated by temperature programmed reduction

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(for further details see Supporting Information).

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RESULTS AND DISCUSSION

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Since N,N-Diethylethanolamine (DEEA) is capable of O-activating CO2 by zwitterion formation

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under anhydrous conditions, the chemical modification of CO2 affects the course of the

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hydrogenation mechanism and may lead to a more attractive synthesis route to methanol.

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However, it has to be mentioned that not all alcoholamines can promote a chemical activation of

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CO2 and that the procedure is limited to non-aqueous solvents because of the zwitterion

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decomposition to HCO3- in the presence of water.

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during the hydrolysis of carbamates in the case of MEA and DEA (diethanolamine).

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Figure S8). In the work presented here, the alcoholamine DEEA was predominantly used as

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A similar phenomenon is also observed 19

(see


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scrubbing and activation agent, because it proved to be stable over a wide temperature range (25

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°C- 170 °C) in the applied hydrogenation procedure.

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At first, we investigated the hydrogenation of CO2 in the pure scrubbing solvent DEEA (20 ml,

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300 mg Cu/ZnO-Al2O3) at two different temperatures, 100 °C and 170 °C. After reaching the

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desired reaction temperature the autoclave was pressurized with 10 bar of CO2. After the

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absorption equilibrium was reached and the solvent was saturated, the autoclave was further

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pressurized with 60 bar hydrogen resulting in a total pressure of 70 bar. The subsequent pressure

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drop resulting from the reaction was recorded as a function of time. Figure 1 illustrates the time-

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resolved GC-analysis of the product concentrations at different reaction stages of a

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hydrogenation performed at T=100 °C. Gas chromatographic measurements were used for

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product quantification.

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From the 13C NMR analysis (Figure S2) in the time period from 30 min to 360 min after start of

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reaction, one can conclude that the hydrogenation of CO2 performed at T=100 °C leads to an

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initial formation of 2-diethylaminoethyl formate (δ=160 ppm, 13C NMR) indicated by the proton

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resonance at δ =8 ppm (see 1H NMR Figure S2). It is further illustrated that the physically

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dissolved CO2 content simultaneous decreased (see δ = 125 ppm,

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with the pressure drop in the reactor. The corresponding

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were extracted after 3h, 12h, and 24h show an increasing amount of methanol (δ =49.2 ppm)

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C NMR) which coincides

C NMR spectra of samples which

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Figure 1. Hydrogenation experiment at T=100 °C, p(CO2)=10 bar , p(H2)=60 bar, catalyst

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loading 300 mg. Chromatograms of the DEEA solvent as received and the reaction solution

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measured after 3h, 12h and 24h reaction time. The corresponding 1H NMR spectrum measured

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after 3h is displayed in Figure S2.

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and of 2-diethylaminoethyl formate (δ =160 ppm). The chromatogram of a sample that was

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extracted after 3h indicates the formation of methanol as the main product. The peak areas were

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calibrated using an external standard (see Supporting Information). It is also noticeable that even

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at longer reaction periods, the methanol content does not remarkably increase, but the peak of the

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2-diethylaminoethyl formate species continues to grow. The product quantification from the

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corresponding GC analysis after 3h of reaction resulted in a yield to methanol of 5.4 %. From the

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corresponding 1H NMR integration we determined the yield to 2-diethylaminoethyl formate to be

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49 %. All yield numbers are calculated in respect to the total amount of CO2 added to the

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autoclave at the beginning of the experiment. Due to the simultaneous occurrence of solution of

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CO2, in physical and chemical form, and the reaction, we did not determine the amount of the

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reactant CO2 after the reaction was terminated. Consequently, we can only state a minimal

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conversion of about 54 % in this experiment. To improve the measurements in this respect


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extensive CO2 and hydrogen solubility studies with pure DEEA are needed. The methanol

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activity (MTY) of the catalyst at T=100 °C was determined to be 0.542 mmol/ (kg-cat*h). In a

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second experiment we raised the temperature to 170 °C and determined the composition of the

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reaction mixture after 6h by NMR and GC-analysis (see Figure 2). The yield to methanol was

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determined to be 30 % which resulted in a methanol activity (MTY) of 1640 mmol/ (kg-cat*h).

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The characteristic signals of 2-diethylaminoethyl formate (13C NMR; δ =160 ppm and 1H NMR δ

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=8 ppm) noticeably disappeared which leads to the conclusion that the formate species was fully

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converted to methanol at this temperature. Only small amounts of the formate were detected by

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gas chromatography (see Figure 2b). Running the experiment for 12 h resulted in a yield to

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methanol of 60%. In the early beginning (first 3 h), an initial formation of the ester species 2-

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diethylaminoethyl formate was also observed, as it was previously mentioned for the 100 °C

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experiment. The NMR spectroscopic investigation (Figure 2a) also confirmed that at T=170 °C

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the DEEA itself was not hydrogenated to Et3N over the monitored reaction period. At higher

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temperatures, T>200 °C, DEEA hydrogenation products became detectable. In addition, the

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supernatant gas phase present after the reaction (170 °C, 6h) was also investigated by mass

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spectrometry to exclude a significant formation of volatile by- products. Only traces of CO, CH4,

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and methanol could be


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Figure 2. Hydrogenation experiment of CO2 captured by DEEA at T=170 °C, p(CO2)=20 bar,

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p(H2)= 60 bar a) 1H NMR (CDCl3) spectrum b) gas chromatogram of the corresponding sample.

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The samples were taken after 12h. For the analysis of the supernatant gas phase see Figure S3.

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qualitatively identified in the gas phase of the autoclave (see Figure S3). Regarding the fact that

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in the 170 °C experiment methanol is essentially the sole product after 6h and thus the selectivity

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is close to 1, the given yield number equals the one for the conversion.

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In order to elucidate the role of the alcohol function of the alcoholamine in respect to the surface

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formic acid (b-HCOO-) esterification and zwitterion formation, pure formic acid pre-esterified

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alcoholamine (2-diethylaminoethyl formate) was used as starting material and subjected to the

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catalytic hydrogenation in a test reaction. The analyses of the 1H and 13C NMR spectra in Figure

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S4 provided evidence that the catalyst Cu/ZnO-Al2O3 facilitates the direct hydrogenation of the

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ester at T=150 °C to methanol. At 170 °C the result indicates only faster kinetics. In addition,

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we tried to clarify the role of the zwitterion in the hydrogenation mechanism. Since in pure

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DEEA, the zwitterion is the only ionic species produced by CO2 absorption, its formation can be

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monitored by electric conductivity measurements which we carried out at different temperatures


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(30 °C, 50 °C, and 100 °C) and CO2 partial pressures (see Figure 3). The obtained results suggest

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that the formation of the zwitterion mainly depends on the selected temperature range. At T=100

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°C no conductivity increase was found even at high CO2 partial pressures p(CO2)=45 bar. While

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at T=50 °C only a small increase in conductivity occurred with the increase of the CO2 partial

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pressure, at T=30 °C a much stronger one caused by the increase of the zwitterion concentration

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was observed. From these results one can conclude that the activation mechanism based on

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zwitter-ionic species should be relevant only at temperatures below 100 °C. In the temperature

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range between 20-50 °C 2-diethylaminoethyl formate is, thus, on one hand formed by the known

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esterification of surface bound formic acid with the alcohol function,

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the other hand by direct hydrogenation of the zwitterion. The ester therefore represents a key

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compound in the described CO2 hydrogenation pathway to methanol. In a second test experiment

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it was examined whether or not the surface formic acid can be neutralized directly by the

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alcoholamine which may finally result in the formation of an ionic liquid (IL). To ascertain the

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role of the ionic liquid [DEEAH+HCOO-] in the CO2 activation procedure, we examined if the

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catalytic hydrogenation of pure [DEEAH+HCOO-] is possible. During the attempt of the direct

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hydrogenation of the pure ionic liquid (for its preparation see Supporting Information), a

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significant pressure increase in the autoclave has been registered, suggesting the decarboxylation

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of the compound.

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here of DEEA, and on


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Figure 3. Conductivity measurements of non-aqueous DEEA exposed to a CO2 atmosphere

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(150 ml, T=30 °C, 50 °C, and 100 °C) indicating the formation of the zwitter-ionic species.

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With the help of 1H NMR spectroscopy after a reaction period of 24 h (see Figure S5a), only

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pure DEEA was found in the liquid phase. In order to determine the decomposition temperature

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of the ionic liquid and to identify the gaseous decomposition products, we subjected it to TG-

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DSC measurements and simultaneously analyzed the emitted gas species by coupled FT-IR

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spectroscopy. In the decomposition experiment of the ionic liquid, an endothermic signal

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(decarboxylation) was observed at T= 120 °C (see Figure S5b) and only the products DEEA and

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CO2 were detected in the exit gas stream (see Figure S5c). By these measurements, the formation

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of undesired CO during the decomposition of the ionic liquid can be ruled out for the desired

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temperature range (120-170 °C). In respect to the hydrogenation reaction, it can be concluded

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that the formation of the ionic liquid is not essential for the formation of methanol at T>120 °C

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due to its decomposition behavior.

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Similarly, it can also be concluded that there will be a noticeable concentration of the IL at

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reaction temperatures below 100 °C, because the thermal decomposition does not occur. Based

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on the performed hydrogenation experiments and particularly on the temperature variation


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experiments, it became clear that a wide variety of CO2 conversion pathways for the

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alcoholamine DEEA CO2 system (Figure 4) does exist. From thermodynamic calculations it is

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known that the formation of the ionic liquid provides an energetic contribution (thermodynamic

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driving force) to the initial formation of formic acid from CO2 at temperatures T

Methanol from CO2 by Organo-Cocatalysis: CO2 Capture and

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