(510e) Generation of CO Free Hydrogen by Liquid-Phase Bicatalytic WGS Reaction At 40°C Via Formic Acid Intermediate

About 90% of our current hydrogen gas demands are met
through reforming reactions and the water-gas-shift (WGS) reaction where carbon
monoxide is the reactant. On the other hand, formic acid is produced
industrially from carbon monoxide through a combination of carbonylation and
acid hydrolysis reactions. Equations below show that formation of formic acid
from carbon monoxide and water is to a degree more thermodynamically favorable
than that from carbon dioxide and hydrogen.

                                                                                 DG                                   DH

CO₂ (g) + H₂ (g)          à         HCOOH
(l)           33 kJ/mole                  - 32 kJ/mole   
         (1)

CO (g) + H₂O (l)          à        HCOOH (l)         12.9 kJ/mole                 - 29 kJ/mole   
          (2)

We have made a novel approach to realize liquid phase
WGS reaction through sequential continuous reactions of i)
catalytic carbon monoxide hydration using a homogeneous Ru-edta complex [1] and
ii) heterogeneous formic acid dehydrogenation using a PtRuBi_xO_y
which was reported by our group previously [2]. The approach put forward and
the process discussed here is to use carbon monoxide to produce hydrogen
through a liquid phase water-gas-shift reaction. Formic acid is formed through
a catalyzed reaction (2) shown above and then decomposed catalytically giving
out hydrogen and carbon dioxide as the reverse of reaction (1).

Conventional water gas shift reaction with water in
gas phase is given by

CO (g) + H₂O (g)    à       H₂
(g) + CO₂ (g)   DG =
-28.8kJ/mole; DH = -41.2kJ/mole           (3)      

Combining reaction (2) and reverse of reaction (1) gives

CO (g) + H₂O (l)     à       H₂ (g) + CO₂ (g)   DG = -20kJ/mole; DH = 2.9kJ/mole                  (4)

Reaction (4) thus shows the possibility of a liquid
phase WGS reaction. Conventional WGS reaction of (3) is catalyzed by oxides of
iron and chromium occur at temperatures between 250-600 ⁰C. The hydrogen produced still contains carbon
monoxide which has to be removed further removed for majority of applications.
Studies on WGS reaction reported in literature suggest three dominant
mechanisms through formates, carbonates, or redox reactions. There have been
many reports on both homogeneous and heterogeneous catalysts for forward and
backward reactions of (1). With increasing attention on the formic acid
dehydrogenating catalysts, attempts are made to device processes which can lead
to energy efficient, zero-carbon hydrogen production.[3]

Figure 1 is a schematic of
the setup used for the liquid phase WGS reaction. Catalysts were synthesized
and characterized using scanning electron microscopy and transmission electron
microscopy and thermal gravimetric analysis. Qualitative analysis of formic
acid intermediate was done using HPLC from Perkin Elmer with a packed Aminex column and refractive index detector (RID).The
product gas was analyzed by gas chromatograph from Perkin Elmer with a thermal
conductivity detector (TCD).

Figure 1-
Schematic of the reactor

The gas chromatogram of the
product gas showed presence of hydrogen and carbon dioxide only. Concentrations
of hydrogen gas over time in 250 µL of product gas sampled intermittently over
5 hours of a reaction run is shown in Figure 2

Figure 2-
Reaction profile at 40⁰C; 5mM hydration catalyst, 0.1g dehydrogenation
catalyst, 10atm-pCO.

This approach caters to the need of an energy
efficient process for carbon monoxide containing exhaust treatment or hydrogen
production under milder and easier operating conditions. Through further
modifications and scaling, this method also has the potential to turn out as a
sustainable means for looping the carbon sequestration and hydrogen storage
processes.  

References:

1.     
R.S.Shukla,
S.D.Bhatt, R.B.Thorat, R.V.Jasra, Applied Catalysis A: General 294 (2005) 111

2.     
S.W. Ting,
S.A. Cheng, K.Y. Tsang, N. van der Laak, and K.Y. Chan, Chem. Commun., (2009), DOI:
10.1039/B916507J

3.      S.Enthaler,
ChemSusChem, 1(2008) 801