Chemical Engineering Journal 285 (2016) 625–634

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Chemical Engineering Journal

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Direct synthesis of formic acid from carbon dioxide and hydrogen: Athermodynamic and experimental study using poly-urea encapsulatedcatalysts

http://dx.doi.org/10.1016/j.cej.2015.09.1011385-8947/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +91 22 3361 1001/1111/2222; fax: +91 22 33611002/1020.

E-mail addresses: gdyadav@yahoo.com, gd.yadav@ictmumbai.edu.in (G.D. Yadav).

Satish K. Kabra a, Esa Turpeinen b, Mika Huuhtanen b, Riitta L. Keiski b, Ganapati D. Yadav a,⇑aDepartment of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai 400019, Indiab Faculty of Technology, University of Oulu, Environmental and Chemical Engineering Research Group, POB 4300, FI-90014 Oulu, Finland

h i g h l i g h t s

� Direct hydrogenation of CO2 to formicacid.

� Synthesis of mono and bi-metallicpoly-urea encapsulated catalysts.

� EnCap Ru with trihexyl (tetradecyl)phosphonium chloride (IL) as thebest system.

� TOF 11,900 h�1 144 bar, 70 �C,H2/CO2 1, catalyst 0.04 g/cm3,3.12 � 10�5 mol/cm3 IL.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 June 2015Received in revised form 14 September2015Accepted 29 September 2015Available online 9 October 2015

Keywords:HydrogenationIonic liquidSupercritical CO2

Encapsulated poly-urea catalystRuthenium

a b s t r a c t

The present work is concerned with direct hydrogenation of CO2 to formic acid which takes into accountthermodynamic feasibility and experimental studies. Poly-urea encapsulated catalysts were explored andthe effect of ionic liquids under supercritical conditions was examined. The monometallic and bimetalliccatalysts were prepared, characterized, screened for the hydrogenation of CO2 and also compared with acommercially available poly-urea–Pd catalyst. The effect of reaction temperature, type of the catalyst,promoter, pressure and molar ratio of the feed (H2/CO2) on the yield of formic acid has been studiedand discussed in order to maximize the formation of formic acid. The highest yield of formic acidobtained in terms of turn-over frequency (TOF) was 11,900 h�1 at a total pressure of 144 bar, temperatureof 70 �C, mole ratio (H2/CO2) of 1, catalyst (poly urea encapsulated Ru) loading of 0.04 g/cm3 and3.12 � 10�5 mol/cm3 of ionic liquid (trihexyl (tetradecyl) phosphonium chloride).

� 2015 Elsevier B.V. All rights reserved.

1. Introduction

Carbon dioxide emitted by utilization of fossil fuels is causingan increased concern, mainly due to the rapid progression inanthropogenic CO2 emissions worldwide which are predicted torise to 40.2 Gt by 2030 [1]. In order to decrease the effect of CO2

emissions in the atmosphere, a number of mitigation routes havebeen reported, including reduction in energy consumption by effi-cient energy transformation, use of low carbon fuels and renew-able resources [2]. Moreover, the development of efficientcapture and sequestration technologies for huge quantities ofCO2 is of much interest [3]. As of now, several CO2 capture tech-nologies based on physisorption, chemisorption, membrane sepa-ration, carbamation, amine physical absorption, amine dryscrubbing and mineral carbonation have been developed [4]. Allthe efforts taken into account till date may not be sufficient to

626 S.K. Kabra et al. / Chemical Engineering Journal 285 (2016) 625–634

reduce the global warming in the future and more innovative andefficient CO2 capture, storage and utilization technologies arerequired [5]. The developed routes for syntheses of value addedCO2-based chemicals such as methane, formic acid, methanol [6],dimethyl carbonate, cyclic carbonates, etc. could have a greatimpact on the environment and the economy.

The direct synthesis of formic acid from CO2 and hydrogen isone of the most encouraging CO2 reduction methods [7,8]. Theexisting literature indicates that many research groups have beenworking intensively in order to develop and implement technolo-gies on industrial scale, but the processes are still at the laboratoryresearch level and far away from commercialization. Thus, it isworth to note that in spite of the well-known thermodynamiclimitations, CO2 can be hydrogenated into many products(Scheme 1) and all of the routes of CO2 hydrogenation can beclassified as ‘‘green processes”. The catalytic hydrogenation ofCO2 to formic acid is a typical atom economic reaction which hasreceived increased attention recently [9]. The hydrogen contentin formic acid is 4.4 wt% and formic acid is a very convenienthydrogen carrier for fuel cells among other applications. Synthesisof formic acid has been studied widely in the presence of manyeffective homogeneous as well as heterogeneous catalysts.However, further studies for the development of heterogeneous,cheap and efficient catalysts are needed. Homogeneous catalystsfor CO2 hydrogenation to formic acid include various transitionmetal complexes of Ru, Rh, Ir, Pd, Ni, Fe, Ti, and Mo and the cat-alytic activity is significantly dependent on the pH of the reactionmixture [10]. The activity was found to be at its best in the pres-ence of a base, because it abstracts proton and is in contrast tothe thermodynamically unfavorable ‘‘base-free” reduction. Amines,NaOH or carbonates are most widely used as the base promoters.The reaction can be enhanced by applying supercritical conditions.Jessop and co-workers [11] have shown successful results using[RuCl(OAc)(PMe3)4] as a catalyst under supercritical conditionswith turnover frequencies (TOF) exceeding 4,000 h�1. Hydrogena-tion of CO2 has been studied using different solvents and reagents[12] and under supercritical carbon dioxide (scCO2) as a solvent itgives a turn over frequency (TOF) of 95,000 h�1 which is �1000times greater than the values reported for the non-scCO2

processes. An organometallic iridium complex has been reportedas an efficient catalyst for inter-conversion between H2 andHCOOH depending on the pH value [13]. Hydrogenation of CO2

Scheme 1. Possible reaction products fro

by hydrogen occurs in the presence of a catalyst in weaklybasic water (pH 7.5) at around atmospheric pressure and roomtemperature, whereas formic acid efficiently decomposes to H2

and CO2 in the presence of a catalyst in acidic water (pH 2.8) [13].Many researchers have reported lower yields of formic acid by

direct hydrogenation of CO2. (Table 1) The studies have been per-formed with various catalysts, solvents and promoters. TheWilkin-son catalyst, RhCl (PPh3)3 was first introduced for hydrogenation ofCO2 with 125 h�1 of TOF using methanol as a solvent. A TOF of920 h�1 was reported, when water was used as a solvent and ionicliquid as a promoter in the presence of a Si(CH2)3NH(CSCH3)[RuCl3(PPh3)] catalyst. Butwhen scCO2was used as a solvent a dras-tic change in the yield was seen; TOF of 4,000 and 95,000 h�1 werereported for [RuCl(OAc)(PMe3)4] and RuCl2 (PMe3)4, respectively at50 �C. High rates of hydrogenationwere obtained by using homoge-nous RuCl(PMe3)4 catalysts under scCO2 conditions [11] and theseworkers have discussed several factors, including easy separation,improved mass and heat transfer rates and high solubility of H2

with scCO2. Further, they reported that the kinetics of reaction ofscCO2 hydrogenation is first order [11,12,14–18].

Ionic liquids (IL) have certain exclusive properties, such asexceptional thermal stability, wide liquid regions, and promisingsolvation properties for a number of substances [16–18]. CO2 ishighly soluble in many ILs. Zang et al. [19] described that the com-bination of a basic imidazolium based ionic liquid with a supportedruthenium catalyst have given satisfactory activity and selectivityfor the hydrogenation of CO2.The solubility of CO2 in imidazoliumbased ILs has been thoroughly discussed [20]. ILs which chemicallyare likely to complex with CO2 have a huge potential to increasethe CO2 solubility. Proper scCO2 reaction conditions for the directsynthesis of formic acid from CO2 and hydrogen should be depen-dent upon the initial ratio of carbon dioxide to hydrogen and thereaction extent [21].

Homogeneous catalysts are efficient for CO2 hydrogenation toformic acid. However, there are problems related to the separationof products and recycling the catalyst. Therefore, there is a need forcatalyst immobilization which will increase the reusability andstability. The immobilized ruthenium complexes over amine func-tionalized silica have been prepared in situ for CO2 hydrogenationto formic acid [22]. The catalyst exhibits high activity and 100%selectivity with the practical advantages such as easy separationand recycling [22].

m hydrogenation of carbon dioxide.

Table 1The effect of various catalysts, solvents and promoters on the direct hydrogenation of CO2.

Catalyst Solvent Promoter P H2/CO2 (bar) T (�C) TOF (h�1) References

RhCl (PPh3)3 Methanol PPh3, NEt3 20/40 25 125 [12]Si(CH2)3NH(CSCH3)(RuCl3(PPh3)) H2O IL 88/88 80 920 [13]Si(CH2)3 NH(CSCH3) Ru C2H5OH PPh3, NEt3 39/117 80 1,384 [14][RuCl(OAc)(PMe3)4] scCO2 H2O, CH3OH 85/85 50 4,000 [9][RuCl2(tppms)2]2 H2O NaHCO3 60/35 80 9,600 [15]PNP–Ir (III) H2O KOH, THF 29/29 120 73,000 [16]RuCl2 (PMe3)4 scCO2 NEt3, C6F5OH 70/120 50 95,000 [10]EnCap Ru scCO2 IL 72/72 70 11,900 Present work

TOF = Turn-over frequency, moles of formic acid/mole of active metal site/hour.

S.K. Kabra et al. / Chemical Engineering Journal 285 (2016) 625–634 627

There is a dearth of literature on direct hydrogenation of CO2 toform formic acid over heterogeneous catalysts. Mainly because ofthe unfavorable reaction conditions compared to homogeneouscatalysis and low chemo-selectivity of heterogeneous processes,formic acid can be hydrogenated further to methanol or methane[23–27]. Various heterogeneous catalysts are reported to be usedin the reaction including Raney nickel, Au, Au/TiO2, Pd, Pd/C, Ruand Ru/C under 200–400 bar hydrogen pressure and 80–150 �C.However, an equivalent amount of base is required to shift thethermodynamic equilibrium towards the formic acid formation[28,29]. Only a few immobilized as well as supported catalystshave been investigated.

Microencapsulation is a process of entrapping material or metalinto a polymeric shell or coating. The use of microencapsulation incatalysis has also been reported [30,31]. These catalysts are termedas ‘‘supported” or ‘‘polymer-anchored” homogeneous catalysts.The entrapping of homogeneous catalysts within a polymeric coat-ing has been developed by Kobayashi et al. [32,33]. This techniquewas exploited in the preparation of polystyrene entrapped OsO4

[32], Pd(Ph3) and Sc(OTf)3 [33]. Ramarao et al. [34] have reportedthe use of interfacial microencapsulation to immobilize homoge-neous catalysts solving problematic limitations of previousapproaches. In order to have a proficient entrapment of transitionmetals, the design of systems having ligating functionality is veryimportant. Most importantly those should be physically strongand chemically inactive in the reaction conditions used and alsocost effective [35]. Poly-urea microcapsules are appropriate to heldmetal species such as Pd(OAc)2 with their quality of chemicalstructure [30]. Poly-urea encapsulated (EnCap) catalysts have beenused in reductive Suzuki reactions [30,36,37] Heck coupling [30],hydrogenation [38], and transfer hydrogenation [30]. These cata-lysts have good leaching resistance, which depends on cross link-ing, solvent used, monomers used, reaction conditions andprocess of catalyst synthesis [34]. Yadav and Lawate [39,40] havereported promising use of poly-urea encapsulated catalysts forhydrogenation reactions.

Based on the reported results the direct synthesis of formic acidfrom hydrogen and scCO2 over various basic and organometalliccatalysts is promising. In addition, the yield of formic acid can beincreased with the addition of a catalyst promoter and dehydratingagent. From the foregoing it was concluded that it is possible todevelop a new and sustainable approach for the direct synthesisof formic acid from CO2. The aim of this work was to developand test novel poly-urea encapsulated heterogeneous catalystsfor the hydrogenation of CO2 to formic acid.

2. Experimental

2.1. Chemicals

Palladium acetate, ruthenium tri-chloride, copper acetate,toluene di-isocyanate (TDI), sodium dodeca sulfonate (SDS), ethy-lene diamine, diethylene triamine, toluene, ethanol, and hexane

were purchased from Sigma Aldrich, Finland. Liquid CO2 andhydrogen with 99.95% purity were purchased from Oy AGA Ab,Finland. Phosphonium based ionic liquids, namely, trihexyl(tetradecyl) phosphonium chloride (IL-101), trihexyl (tetradecyl)phosphonium bromide (IL-102), trihexyl (tetradecyl) phosphoni-umdecanoate (IL-103), trihexyl (tetradecyl) phosphoniumhexafluorophosphate (IL-110) and tetrabutylphosphonium bromide (IL-163)were provided by Cytec Industries Inc., Canada. All the chemicalswere of analytical grade with high purity (>99%) and used withoutfurther purification.

2.2. Experimental set up

A known quantity of catalyst was charged into the reactor. Thereactor (Fig. S1, Supplementary information) was sealed, flushedand filled with hydrogen to the desired pressure. The reactor washeated to the set temperature and then carbon dioxide waspumped in slowly; a supercritical phase was maintained through-out the experiment. In the control experiment the total pressurewas 144 bar (at 50 �C); with partial pressure of hydrogen, 72 barin the scCO2 phase occupying the entire volume of the reactor.The reaction was conducted by stirring the mixture for a desiredtime. The reactor was then cooled down to room temperatureand gases (both unreacted reactants and formed products) werethen carefully vented and the formed liquid mixture was separatedfrom the catalyst by filtration and then analyzed by HPLC. Scheme 1gives the desired reaction where CO2 and H2 react to give formicacid as well as the undesired reactions occurring at the same time.

2.3. Sampling and analysis

As it was very difficult to separate a very small quantity ofadsorbed products from the catalysts, after completion of the reac-tion 5 ml of deionized water was added to dissolve the products foranalyses. The reaction mixture was analyzed with HPLC for liquidsamples (equipped with a Coregel 87 H3 column) and a gas chro-matography equipped with an HP-5 capillary column (0.20 mminternal diameter, 25 m length and 0.33 lm film thickness) forgaseous samples.

2.4. Catalyst preparation

Both, single metal and bimetallic EnCapPd, EnCapPd–Cu, EnCa-pRu, EnCapRu–Cu and EnCapPd–Ru catalysts were prepared asdiscussed below. In addition, commercial EnCatPd (Sigma Aldrich)was used as a reference catalyst.

2.4.1. Synthesis of poly-urea encapsulated metal catalystThe catalysts were prepared with some modifications using the

method shown by Yadav and Lawate [39,40]. At the first stage thedesired quantity of palladium acetate or copper acetate or ruthe-nium tri-chloride was solubilized in 5 ml of toluene. 0.05 g of Ali-quat 336 was added as surfactant with stirring at 80 �C. Aliquat

628 S.K. Kabra et al. / Chemical Engineering Journal 285 (2016) 625–634

336 also worked as a wall modifier [41]. Then toluene di-isocyanate (TDI) (3.66 g) was added and the solution was stirredfor 1 h to make a homogeneous mixture.

Secondly, an aqueous solution was prepared separately contain-ing 50 ml of deionized water in which sodium dodeca sulfonate(SDS) (72 mg) and PEG 400 (50 mg) were added at room tempera-ture. The quantity and nature of the surfactant plays an importantrole in the encapsulation process. Into this solution 0.68 g of ethy-lene diamine and 0.72 g of diethylene triamine (1:1 ratio) wereadded. Amines are known to increase the cross linking and subse-quently functional ligation of the metal. Increasing the aminesnumber (i.e. diamine, triamine, pentaamine, etc.) enhances thecross linking which in turn increases the mechanical strength ofthe polymer beads. The aqueous solution was homogenized bystirring it for 1 h. The temperature of solution was cooled downto 5 �C. Then the first solution was added into the prepared aque-ous solution drop-wise over 1 h maintaining the temperaturebelow 5 �C to avoid onset of polymerization before stable oil-in-water microemulsion was formed. The final solution was stirredat a desired speed of agitation for 10 min to obtain a stablemicroemulsion of oil in water until the required drop size wasobtained. The temperature was then increased and maintained at50 �C for 3 h to complete the polymerization reaction. Microcap-sules of poly-urea were formed by the interfacial polymerizationprocess.

After the polymerization process was completed, the catalystwas filtered with a G4 sintered disc – Buchner filter with the aver-age pore size <20 lm. The filtered catalyst was washed with100 ml of deionized water for three times, then with ethanol andhexane, respectively. For the efficient wash-off of surfactant, otherunreacted monomers, and free metal particles the use of solventsare necessary.

The catalyst was then reduced in hydrogen atmosphere usingethanol for 3 h at 15 bar and 100 �C. Reduction was ensured visu-ally by following the color change from pale yellow or brown toblack. The catalyst was dried at 115 �C for 3 h after reduction.The key parameters which govern the polymerization are the stir-ring speed during the emulsion and polymerization, type of surfac-tant and its concentration, and nature and concentration ofmonomer.

2.4.2. Poly-urea encapsulated bimetallic catalystAddition of the second metal into a monometallic catalyst can

alter the selectivity and activity of the reaction positively due tothe synergistic effect of both metals. Poly-urea encapsulated

Table 2Activity for different catalysts.

Catalyst Turn overFrequency (h�1)

Remarks

EnCap Pd 950 Byproducts such as acetic acid, mEnCap Pd + IL 1,050EnCap Pd–Cu 620 The metal leaches out from the enEnCap Pd–Cu + IL 700EnCap Ru 7,000 Best suited catalyst for the hydrogEnCap Ru + IL 7,690EnCap Ru–Cu 4,600 Leaching of copper from the catalEnCap Ru–Cu + IL 4,690EnCap Pd–Ru 1,550 Byproducts formation, mainly aceEnCap Pd–Ru + IL 1,850EnCat Pd 40 commercial 1,400 Byproducts formation, mainly aceEnCat Pd 40 commercial + IL 1,550Ionic liquid 0 No yield of formic acid, but when

TOF = moles of formic acid/mole of active metal site/hour.Reaction conditions: total pressure = 108 bar, partial pressure of H2 = 36 bar, partial puid = 3.12 � 10�5 mol/cm3 and reaction time 4 h.

palladium in which ruthenium or copper is infused between thepolymer matrixes was synthesized. The process of catalyst synthe-sis is as follows:

At the beginning the desired quantities of ruthenium tri-chloride and copper acetate or palladium acetate were solubilizedin 5 ml of toluene and the rest of the process was repeated simi-larly as described above in the case of single metal catalyst.

2.5. Catalyst characterization

The prepared catalysts were characterized using several tech-niques. The characterization studies carried out include the deter-mination of crystalline nature of polymer by X-ray diffraction(XRD), determination of adsorbed species by Fourier TransformInfrared (FTIR) spectroscopy, textural determination of encapsu-lated species by scanning electron microscopy (SEM), and surfaceelemental analysis by energy dispersive X-ray spectroscopy (EDS).

3. Results and discussion

3.1. Catalyst activity

Experiments were carried out with different types of catalysts;Table 2 shows the activity of the catalyst towards the desired pro-duct and the yields of CO2 to formic acid in terms of TOF over dif-ferent catalysts prepared. The primary aim here was to find out themost active and suitable catalyst for the synthesis of formic acid bydirect hydrogenation of CO2 and hence different poly-urea encap-sulated catalysts were screened.

When the poly-urea encapsulated palladium catalyst (EnCapPd)was used under supercritical conditions, the yield of formic acidwas lower and byproducts were analyzed as summarized in a qual-itative analysis, the analysis showed maximum formation of aceticacid with some methyl formate, ethyl formate and traces of etha-nol/methanol. In the case of a commercial catalyst EnCatPd, similarresults were obtained maximizing the acetic acid formation. WhenEnCap Pd–Cu was used, similar results were obtained with forma-tion of byproducts coupled with leaching of Cu metal from the cat-alyst. Similarly in the case of EnCap Ru–Cu catalyst, high yield offormic acid was obtained but at the same time Cu metal was foundto leach out. With EnCap Pd–Ru, a fairly low yield of formic acidwas detected and in addition, the catalyst showed a good selectiv-ity towards acetic acid. However, in the case of EnCap Ru, therewas no formation of acetic acid at all under all reaction conditions

ethyl formate and methanol were analyzed

capsulated catalyst, very low CO2 conversion and also byproducts were formed

enation of CO2 (no acetic acid was detected and no leaching of metal)

yst and formation of byproducts

tic acid

tic acid

used with a catalyst IL acts as a promoter

ressure of CO2 = 72 bar, temperature = 50 �C, catalyst = 0.04 g/cm3 and ionic liq-

S.K. Kabra et al. / Chemical Engineering Journal 285 (2016) 625–634 629

studied. EnCap Ru showed a high yield of formic acid with TOF of7000 h�1 and some by-products like methanol and methyl formatewere detected.

Catalytic activities of phosphonium based ionic liquids alonewere also evaluated and no formation to formic acid was foundout. Ionic liquids have shown a very good promoting effect onthese catalysts as ionic liquids are good solvents for CO2 [20].The yield of formic acid was further increased when ionic liquidwas used as a promoter with the catalysts. Zhang et al. [42] havereported the mechanism for hydrogenation of CO2 to formic acidpromoted by a diamine-functionalized ionic liquid. A similarmechanism and promoting effect was seen. In the case of theEnCap Ru with IL as a promoter a TOF of 7690 h�1 was observed,which is approximately 10% more than without the promoter.The different ionic liquids studied showed the same promotingeffect. So trihexyl (tetradecyl) phosphonium chloride was ran-domly chosen and used as a promoter for further studies.

Fig. 1c. SEM image of the used EnCap Ru catalyst.

Fig. 1b. SEM image of the EnCap Ru catalyst.

3.2. Scanning electron microscopy (SEM)/Energy dispersive X-rayspectroscopy (EDS)

SEM images of encapsulated catalysts were taken by Zeiss UltraPluss Field emission scanning electron microscope (FESEM)equipped with an energy-dispersive X-ray unit. Prior to SEManalysis the samples were prepared by coating them with a carbonfilm in order to prevent charging at higher magnifications. Thesamples were examined by using various magnifications.Figs. 1a–c and 2a, 2b show the SEM images of fresh and used EnCapRu catalyst. The SEM–EDS studies reveal plenty of morphology andsurface related information of the encapsulated catalyst. Overallthe catalyst is made up of a comparatively rough surface ofpolymers in which metals are encapsulated. This rough surfaceis due to the high rate of polymerization between aromaticdi-isocyanate (TDI) and mixture of EDA/DETA as explained by Hongand Park [43]. The poly-urea material encapsulated by the aliphaticdi-isocyanate shows a comparatively smooth and macroglobulinslike structure. As the reactivity at the interface is lower it showsfewer disturbances and so the external surface is comparativelysmooth. The difference in the di-isocyanate reactivity induced fromthe chemical structure brings about the various membranemorphologies, which can significantly determine the permeability,crystallinity, and thickness of the resultant microcapsules. This isbecause of the addition of the second metal which is infused inthe poly-urea coating making it spongier.

Fig. 1a. SEM image of the EnCap Ru catalyst.

Fig. 2a. SEM image of EnCap Pd–Ru bimetallic catalyst.

The surface elemental composition of the encapsulated catalystwas obtained by EDS. Poly-urea encapsulation shows O, N, Cl andrespective metals. It is important to note that the element percent-ages shown in the respective Tables are values only for the surface

Fig. 2b. SEM image of EnCap Pd–Ru bimetallic catalyst.

Fig. 3. FTIR of EnCap Ru catalyst.

630 S.K. Kabra et al. / Chemical Engineering Journal 285 (2016) 625–634

metals. The ruthenium content, in the EnCap Ru was found to be4.57 wt% (Table 3) and in case of EnCap Pd–Ru, the Ru contentwas 1.77% wt% with the Pd content of 2.57 wt% (Table 4) for thesame catalyst. So it can be concluded that, some ruthenium mustbe within the poly-urea matrix, and thus not so easy to be analyzedby the EDS analysis.

3.3. Fourier transform infrared spectroscopy (FTIR)

Infrared spectra of the samples pressed in KBr pellets wereobtained at a resolution of 2 cm�1 between 4000 and 400 cm�1.Spectra were collected with a Perkin–Elmer instrument and in eachcase the samples were referenced against a blank KBr pellet.

Fig. 3 shows the FTIR spectra of the poly-urea encapsulatedruthenium catalyst obtained from the mixture of di-isocyanateand amines. The sample prepared in this experiment has alsostrong band known for the N–H stretching vibration at�3306 cm�1. C–H stretching vibration is shown at �2923 cm�1.By the reaction of di-isocyanate and EDA, the NCO peak in thedi-isocyanate at 2270 cm�1 disappears [40]. Two carbonyl stretch-ing bands are observed: a hydrogen bonded urea carbonyl at1550 cm�1 and a free urea carbonyl absorption band at�1645 cm�1. From these characteristic peaks, it is evident thatthe poly-urea microcapsules were successfully prepared. The veryreactive aromatic TDI can produce many more hydrogen-bondedN–H groups than the other aliphatic di-isocyanates. This is corrob-orated by the high intensity of the peak at 3306 cm�1 for TDI and

Table 3EDS surface analysis of poly-urea encapsulated ruthenium based catalyst.

Elements Fresh catalyst (wt%) Used catalyst (wt%)

N 61.99 60.48O 30.86 31.43Cl 3.59 3.83Ru 4.57 4.26

Table 4EDS of EnCap Pd–Ru bimetallic catalyst.

Elements Fresh catalyst (wt%) Used catalyst (wt%)

N 54.37 53.88O 38.42 39.37Cl 2.87 2.63Ru 1.77 1.64Pd 2.57 2.48

then MDI. Other aliphatic di-isocyanates show the same result asMDI.

3.4. X-ray diffraction

Polymeric materials generally show an amorphous structure.The crystallographic structure of the poly-urea encapsulated cata-lysts synthesized was determined with a Siemens D5000 diffractmeter. The broad diffraction peak in the low angle region(2h = 14–30�) is visible indicating that a long chain carbon wasformed (Fig. 4) and the ruthenium metal peak was obtainedbetween 2h = 37–42�. The XRD pattern shows that poly-urea hasa semi-crystalline structure. Yadav et al. [29] postulate that whenpolymer molecules form and precipitate out at high rates, theydo not get sufficient time to arrange themselves in an ordered lat-tice. So, the poly-urea formed with higher rate of reaction showsless crystalline structure than the one formed with lower rate ofreaction. However, it is possible to vary the structure of these poly-mer films through the conditions employed in their preparation[29].

3.5. BET surface area analysis

The accessible surface area and pore size distribution are impor-tant parameters for any catalyst. Surface area measurement wasdone by nitrogen adsorption at temperature �196 �C using aMicromeritics ASAP 2020 instrument, after pretreating the sampleunder high vacuum at 150 �C for 3 h.

The typical BET surface area was found to be 3.69 m2/g forEnCap Ru and 3.43 m2/g for EnCapPd–Ru. The surface area of the

Fig. 4. XRD of EnCap Ru catalyst.

Fig. 5. Amount of formic acid formed as a function of temperature (at pressure of1 bar and stoichiometric feed ratio (CO2:H2 = 1)).

Table 6Thermodynamic data for the formic acid synthesis.

[�C] DH [kJ] DS [J/K] DG [kJ] K Log(K)

0 15.41 �93.78 41.03 1.42E�8 �7.8425 14.89 �95.60 43.39 2.49E�8 �7.6050 14.40 �97.18 45.80 3.93E�8 �7.4075 13.95 �98.53 48.25 5.74E�8 �7.24

100 13.53 �99.68 50.73 7.89E�8 �7.10125 13.16 �100.62 53.23 1.03E�7 �6.98150 12.81 �101.50 55.76 1.30E�7 �6.88175 12.49 �102.23 58.31 1.59E�7 �6.79200 12.21 �102.85 60.87 1.90E�7 �6.72225 11.95 �103.39 63.45 2.21E�7 �6.65250 11.71 �103.85 66.04 2.54E�7 �6.59275 11.50 �104.24 68.64 2.87E�7 �6.54300 11.31 �104.58 71.25 3.20E�7 �6.49

S.K. Kabra et al. / Chemical Engineering Journal 285 (2016) 625–634 631

single metal catalyst was comparatively higher than that for thebimetallic poly-urea encapsulated catalysts, which is in goodagreement with the results reported by Smith [38]. The higherspecific surface area could be due to the rough surface of micro-capsule which was synthesized by the reactive TDI and aliphaticamines. The surface area of the used catalyst was found to be3.87 m2/g for EnCap Ru, which is somewhat higher than the surfacearea of the fresh catalyst. The reason could be broken catalyst par-ticles, as there is attrition because of mechanical agitation in thereactor.

3.6. Thermodynamics and simulations

The thermodynamic aspects of the reaction was studied byusing computer based simulation software Aspen and HSC Chem-istry, which are designed for various kinds of chemical reactionsand equilibrium calculations. Thermochemical calculations arebased on enthalpy (H), entropy (S), heat capacity (Cp) or Gibbsenergy (G) values for chemical species. The chemical equilibriumfor CO2 direct hydrogenation to formic acid (Eq. (1)) was analyzedbased on the components’ thermodynamic parameters.

CO2 þH2 $K1 HCOOH ð1Þeffects of reaction temperature, pressure and molar feed ratio

(H2/CO2) on the yield of formic acid in the system was studied.H0, S0 and Cp are standard molar enthalpy of formation, standardmolar entropy and molar heat capacity, respectively (Table 5).The standard Gibbs free energy (G0) can be calculated by the meansof enthalpy and entropy:

G0 ¼ H0 � TS0 ð2ÞThe temperature dependence of heat capacity (Cp) at elevated

temperatures can be predicted by the Kelley equation (3):

Cp ¼ Aþ B � 10�3 � Tþ C � 105 � T�2 þ D � 10�6 � T2 ð3Þwhere A, B, C and D are coefficients estimated from experimen-

tal data.Temperature dependence of the enthalpy can be derived from

Eq. (4)

HðTÞ ¼ H0 þZ T

298:15CpdT ð4Þ

Temperature dependence of the entropy can be calculated fromEq. (5) as follows:

SðTÞ ¼ S0 þZ T

298:15

Cp

TdT ð5Þ

The enthalpy, entropy and Gibbs energy functions for a chemi-cal reaction are calculated as the difference between the productsand reactants. For instance, the enthalpy of reaction is described as:

DrH ¼ DHproducts � DHreactants ð6ÞThe relationship between the equilibrium constant (K) and the

free energy of the reaction is as follows:

lnK ¼ DG=ð�RTÞ ð7Þ

Table 5Thermodynamic properties of species involved in synthesis of formic acid.

Substance H0 [kJ/mol] S0 [J/mol�K]

CO2(g) �393.50 213.76H2(g) 0.00 130.67HCOOH(g) �378.61 248.84

Thermodynamic data for the formic acid synthesis werecalculated and listed (Table 6). Where the enthalpy (DH) valuesare positive indicating that the reaction is endothermic at alltemperatures. Moreover, the Gibbs free energy values arepositive through the temperature range studied showing thereaction to be non-spontaneous. Also the really small value ofthe equilibrium constant (K � 1) proves the forward reaction tobe unfavorable.

Fig. 5 shows the influence of reaction temperature on theamount of formic acid when reaction pressure is atmosphericand the molar feed ratio is stoichiometric (CO2:H2 = 1).

The curve in Fig. 5 indicates that the amount of formic acidincreases when the reaction temperature increases. Thus, hightemperature favors the formation of formic acid. In the studiedconditions the amount of formic acid reaches its maximum(3.9 � 10�5 mol-%) at 500 �C.

Fig. 6 shows the influence of reaction pressure on the amount offormic acid formed when temperature is 25 �C and the molar feedratio is stoichiometric (CO2:H2 = 1).

Cp = A + B�10�3�T + C�105�T�2 + D�10�6�T2 [J/mol�K]A B C D

29.31 39.97 �2.48 �14.7825.85 4.83 1.58 �0.3724.72 100.63 �5.19 �37.52

Fig. 6. Amount of formic acid as a function of pressure (stoichiometric CO2:H2 feedratio).

Fig. 7. Amount of formic acid formed as a function of amount of CO2 in the feed(Temperature: 25 �C, Pressure: 1 bar and H2 in the feed: 1 mol).

Table 7Effect of studied parameters on EnCap Ru catalyst performance.

Parameter

H2 pressure (bar) 183672

Catalyst loading (g/cm3) 0.020.030.04

Temperature (�C) 305070

Catalyst reusability Fresh1st reuse2nd reuse

TOF = moles of formic acid/mole of active metal site/hour.

632 S.K. Kabra et al. / Chemical Engineering Journal 285 (2016) 625–634

The increase in the reaction pressure improves the evolving offormic acid. The amount of formic acid formed increases linearlyas a function of pressure. In the studied conditions the amount offormic acid reaches its maximum (2.5 � 10�4 mol%) at 200 bar.Fig. 7 shows the influence of molar feed ratio on the amount of for-mic acid formed at temperature of 25 �C and pressure of 1 bar.

As the amount of CO2 in the feedstock increases, the amount offormic acid increases having the highest value at the (CO2/H2) feedratio of 1. After that point the formation of formic acid starts todecrease smoothly. Thus, the stoichiometric feed ratio yields thebest results at 25 �C.

The calculations showed that the hydrogenation of CO2 to for-mic acid is thermodynamically unfavorable reaction. Even in theseverest conditions the formation of formic acid is practically neg-ligible. To have an efficient conversion of CO2 to formic acid ther-modynamic limitations must be circumvented somehow.

3.7. Effect of different parameters (experimental)

Table 7 shows the effect of different parameters studied exper-imentally. Initially 36 bar of hydrogen was used and at this pres-sure a good conversion of CO2 and selectivity towards formicacid were seen. When the pressure of hydrogen was increased to72 bar, the yield of formic acid increased further. But when18 bar of hydrogen was used the yield decreased proportionately,which agrees with the thermodynamic study.

According to thermodynamics of the reaction, higher pressuresfavor the forward direction reaction yielding formic acid. Whentotal pressure of the system was studied at different temperatures,it was observed that with the progress in the reaction there is adecrease in pressure as CO2 and hydrogen get consumed. Whenthe CO2 pressure further increased (in supercritical region) bykeeping the temperature constant, there was a considerable differ-ence in the yield (TOF) values. Further studies of the pressure effectwere not possible because of limitations in the equipment design.

When the catalyst loading was increased, there was a change inthe yield of formic acid as the number of active sites increased.Different reaction temperatures were studied by keeping all theother parameters constant. The effect of temperature on the yieldof formic acid was studied in the range of 30–70 �C. As anticipatedthe formation rate increased as a function of temperature which

TOF (h�1) Reaction conditions

3,750 Total Pressure = 108 bar,Partial pressure of CO2 = 72 bar,Temperature = 50 �C,Catalyst 0.04 g/cm3

Ionic liquid = 3.12 � 10�5 mol/cm3

7,6909,249

2,897 Total Pressure = 108 bar,Partial pressure of H2 = 36 bar,Partial pressure of CO2 = 72 bar,Temperature = 50 �C,Ionic liquid = 3.12 � 10�5 mol/cm3

5,2387,690

8,597 Total pressure = 144 bar,Partial pressure of H2 = 72 bar,Partial pressure of CO2 = 72 bar,Catalyst 0.04 g/cm3,Ionic liquid = 3.12 � 10�5 mol/cm3

9,24911,900

11,900 Total pressure = 144 bar,Partial pressure of H2 = 72 bar,Partial pressure of CO2 = 72 bar,Temperature = 70 �C,Catalyst 0.04 g/cm3,Ionic liquid = 3.12 � 10�5 mol/cm3

11,64811,268

S.K. Kabra et al. / Chemical Engineering Journal 285 (2016) 625–634 633

can be attributed to the higher surface reaction rate constant withan increase in temperature as well as the improved diffusivity andreduced viscosity of supercritical CO2. Temperature had a pro-nounced effect on selectivity in the studied range. At low temper-atures the formation of formic acid is favored and at hightemperatures selectivity towards formic acid was reduced andincreased formation of byproducts was detected. Although ther-modynamic study shows that higher temperatures favor higherconversion of CO2, it also gives raise in byproducts formation.

4. Conclusions

Various poly-urea encapsulated EnCap Cu, EnCapPd, EnCap Ru,EnCap Cu–Ru, EnCapPd–Ru and EnCapPd–Cu (mono and bi- metal-lic) catalysts were synthesized, characterized and screened.

EnCap Ru catalyst was found to be the best for the direct hydro-genation of CO2 to formic acid. There was leaching of Cu metalfrom poly-urea encapsulation in case of EnCap Cu, EnCap Ru–Cuand EnCap Pd–Cu, whereas no leaching of metal was observed incase of EnCapPd and EnCapRu.

The direct synthesis of formic acid from CO2 and H2 is thermo-dynamically not a favorable reaction at ambient conditions. Thethermodynamic study shown high pressures and low temperaturesare favorable for the process, which was confirmed by experimen-tal studies. The highest yield of formic acid was observed at 70 �Cunder supercritical conditions (144 bar) using EnCap Ru catalystalong with trihexyl (tetradecyl) phosphonium chloride ionic liquidas promoter.

Conflict of interest

The authors declare no conflict of interest.

Acknowledgements

This work was proposed under the collaborative project ‘‘Sus-tainable Catalytic Syntheses of Chemicals using Carbon Dioxideas Feedstock (GreenCatCO2)” supported by the Department ofScience and Technology, Government of India (DST-GOI) and theAcademy of Finland (No:140122). GDY received support from R.T.Mody Distinguished Professor Endowment and J.C. Bose NationalFellowship from DST-GoI.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cej.2015.09.101.

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