carbon

1. Chemical Engineering Journal 285 (2016) 625–634 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Direct synthesis of formic acid from carbon dioxide and hydrogen: A thermodynamic and experimental study using poly-urea encapsulated catalysts Satish K. Kabraa, Esa Turpeinenb, Mika Huuhtanenb, Riitta L. Keiskib, Ganapati D. Yadava,⇑ aDepartment of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai 400019, India bFaculty 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 g r a p h i c a l a b s t r a c t ? Direct hydrogenation of CO2to formic acid. ? Synthesis of mono and bi-metallic poly-urea encapsulated catalysts. ? EnCap Ru with trihexyl (tetradecyl) phosphonium chloride (IL) as the best system. ? TOF 11,900 h?1144 bar, 70 ?C, H2/CO21, catalyst 0.04 g/cm3, 3.12 ? 10?5mol/cm3IL. a r t i c l e i n f o a b s t r a c t Article history: Received 2 June 2015 Received in revised form 14 September 2015 Accepted 29 September 2015 Available online 9 October 2015 The present work is concerned with direct hydrogenation of CO2to formic acid which takes into account thermodynamic feasibility and experimental studies. Poly-urea encapsulated catalysts were explored and the effect of ionic liquids under supercritical conditions was examined. The monometallic and bimetallic catalysts were prepared, characterized, screened for the hydrogenation of CO2and also compared with a commercially 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 studied and discussed in order to maximize the formation of formic acid. The highest yield of formic acid obtained in terms of turn-over frequency (TOF) was 11,900 h?1at a total pressure of 144 bar, temperature of 70 ?C, mole ratio (H2/CO2) of 1, catalyst (poly urea encapsulated Ru) loading of 0.04 g/cm3and 3.12 ? 10?5mol/cm3of ionic liquid (trihexyl (tetradecyl) phosphonium chloride). Keywords: Hydrogenation Ionic liquid Supercritical CO2 Encapsulated poly-urea catalyst Ruthenium ? 2015 Elsevier B.V. All rights reserved. 1. Introduction emissions in the atmosphere, a number of mitigation routes have been 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 efficient capture and sequestration technologies for huge quantities of CO2is of much interest [3]. As of now, several CO2capture tech- nologies based on physisorption, chemisorption, membrane sepa- ration, carbamation, amine physical scrubbing and mineral carbonation have been developed [4]. All the efforts taken into account till date may not be sufficient to Carbon dioxide emitted by utilization of fossil fuels is causing an increased concern, mainly due to the rapid progression in anthropogenic CO2emissions worldwide which are predicted to rise to 40.2 Gt by 2030 [1]. In order to decrease the effect of CO2 absorption, amine dry ⇑Corresponding author. Tel.: +91 22 3361 1001/1111/2222; fax: +91 22 3361 1002/1020. E-mail addresses: gdyadav@yahoo.com, gd.yadav@ictmumbai.edu.in (G.D. Yadav). http://dx.doi.org/10.1016/j.cej.2015.09.101 1385-8947/? 2015 Elsevier B.V. All rights reserved.

2. 626 S.K. Kabra et al./Chemical Engineering Journal 285 (2016) 625–634 reduce the global warming in the future and more innovative and efficient CO2 capture, storage and utilization technologies are required [5]. The developed routes for syntheses of value added CO2-based chemicals such as methane, formic acid, methanol [6], dimethyl carbonate, cyclic carbonates, etc. could have a great impact on the environment and the economy. The direct synthesis of formic acid from CO2and hydrogen is one of the most encouraging CO2reduction methods [7,8]. The existing literature indicates that many research groups have been working intensively in order to develop and implement technolo- gies on industrial scale, but the processes are still at the laboratory research level and far away from commercialization. Thus, it is worth to note that in spite of the well-known thermodynamic limitations, CO2 can be hydrogenated into many products (Scheme 1) and all of the routes of CO2 hydrogenation can be classified as ‘‘green processes”. The catalytic hydrogenation of CO2to formic acid is a typical atom economic reaction which has received increased attention recently [9]. The hydrogen content in formic acid is 4.4 wt% and formic acid is a very convenient hydrogen carrier for fuel cells among other applications. Synthesis of formic acid has been studied widely in the presence of many effective homogeneous as well However, further studies for the development of heterogeneous, cheap and efficient catalysts are needed. Homogeneous catalysts for CO2hydrogenation to formic acid include various transition metal complexes of Ru, Rh, Ir, Pd, Ni, Fe, Ti, and Mo and the cat- alytic activity is significantly dependent on the pH of the reaction mixture [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 to the 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 conditions with turnover frequencies (TOF) exceeding 4,000 h?1. Hydrogena- tion of CO2has been studied using different solvents and reagents [12] and under supercritical carbon dioxide (scCO2) as a solvent it gives a turn over frequency (TOF) of 95,000 h?1which is ?1000 times greater than the values reported for the non-scCO2 processes. An organometallic iridium complex has been reported as an efficient catalyst for inter-conversion between H2 and HCOOH depending on the pH value [13]. Hydrogenation of CO2 by hydrogen occurs in the presence of a catalyst in weakly basic water (pH 7.5) at around atmospheric pressure and room temperature, whereas formic acid efficiently decomposes to H2 and CO2in 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. The Wilkin- son catalyst, RhCl (PPh3)3was first introduced for hydrogenation of CO2with 125 h?1of TOF using methanol as a solvent. A TOF of 920 h?1was reported, when water was used as a solvent and ionic liquid as a promoter in the presence of a Si(CH2)3NH(CSCH3) [RuCl3(PPh3)] catalyst.ButwhenscCO2wasused asa solventa dras- tic change in the yield was seen; TOF of 4,000 and 95,000 h?1were reported for [RuCl(OAc)(PMe3)4] and RuCl2(PMe3)4, respectively at 50 ?C. High rates of hydrogenation were obtained by using homoge- nous RuCl(PMe3)4catalysts under scCO2conditions [11] and these workers 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 of scCO2hydrogenation is first order [11,12,14–18]. Ionic liquids (IL) have certain exclusive properties, such as exceptional thermal stability, wide liquid regions, and promising solvation properties for a number of substances [16–18]. CO2is highly soluble in many ILs. Zang et al. [19] described that the com- bination of a basic imidazolium based ionic liquid with a supported ruthenium catalyst have given satisfactory activity and selectivity for the hydrogenation of CO2.The solubility of CO2in imidazolium based ILs has been thoroughly discussed [20]. ILs which chemically are likely to complex with CO2have a huge potential to increase the CO2solubility. Proper scCO2reaction conditions for the direct synthesis of formic acid from CO2and hydrogen should be depen- dent upon the initial ratio of carbon dioxide to hydrogen and the reaction extent [21]. Homogeneous catalysts are efficient for CO2hydrogenation to formic acid. However, there are problems related to the separation of products and recycling the catalyst. Therefore, there is a need for catalyst immobilization which will increase the reusability and stability. The immobilized ruthenium complexes over amine func- tionalized silica have been prepared in situ for CO2hydrogenation to formic acid [22]. The catalyst exhibits high activity and 100% selectivity with the practical advantages such as easy separation and recycling [22]. as heterogeneous catalysts. Scheme 1. Possible reaction products from hydrogenation of carbon dioxide.

3. 627 S.K. Kabra et al./Chemical Engineering Journal 285 (2016) 625–634 Table 1 The effect of various catalysts, solvents and promoters on the direct hydrogenation of CO2. TOF (h?1) T (?C) Catalyst Solvent Promoter P H2/CO2(bar) References RhCl (PPh3)3 Si(CH2)3NH(CSCH3)(RuCl3(PPh3)) Si(CH2)3NH(CSCH3) Ru [RuCl(OAc)(PMe3)4] [RuCl2(tppms)2]2 PNP–Ir (III) RuCl2(PMe3)4 EnCap Ru Methanol H2O C2H5OH scCO2 H2O H2O scCO2 scCO2 PPh3, NEt3 IL PPh3, NEt3 H2O, CH3OH NaHCO3 KOH, THF NEt3, C6F5OH IL 20/40 88/88 39/117 85/85 60/35 29/29 70/120 72/72 25 80 80 50 80 120 50 70 125 920 1,384 4,000 9,600 73,000 95,000 11,900 [12] [13] [14] [9] [15] [16] [10] Present work TOF = Turn-over frequency, moles of formic acid/mole of active metal site/hour. There is a dearth of literature on direct hydrogenation of CO2to form formic acid over heterogeneous catalysts. Mainly because of the unfavorable reaction conditions compared to homogeneous catalysis 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 used in the reaction including Raney nickel, Au, Au/TiO2, Pd, Pd/C, Ru and Ru/C under 200–400 bar hydrogen pressure and 80–150 ?C. However, an equivalent amount of base is required to shift the thermodynamic equilibrium towards the formic acid formation [28,29]. Only a few immobilized as well as supported catalysts have been investigated. Microencapsulation is a process of entrapping material or metal into a polymeric shell or coating. The use of microencapsulation in catalysis has also been reported [30,31]. These catalysts are termed as ‘‘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 technique was exploited in the preparation of polystyrene entrapped OsO4 [32], Pd(Ph3) and Sc(OTf)3[33]. Ramarao et al. [34] have reported the use of interfacial microencapsulation to immobilize homoge- neous catalysts solving problematic limitations of previous approaches. In order to have a proficient entrapment of transition metals, the design of systems having ligating functionality is very important. Most importantly those should be physically strong and chemically inactive in the reaction conditions used and also cost effective [35]. Poly-urea microcapsules are appropriate to held metal species such as Pd(OAc)2 with their quality of chemical structure [30]. Poly-urea encapsulated (EnCap) catalysts have been used 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 and process of catalyst synthesis [34]. Yadav and Lawate [39,40] have reported promising use of poly-urea encapsulated catalysts for hydrogenation reactions. Based on the reported results the direct synthesis of formic acid from hydrogen and scCO2over various basic and organometallic catalysts is promising. In addition, the yield of formic acid can be increased with the addition of a catalyst promoter and dehydrating agent. From the foregoing it was concluded that it is possible to develop a new and sustainable approach for the direct synthesis of formic acid from CO2. The aim of this work was to develop and test novel poly-urea encapsulated heterogeneous catalysts for the hydrogenation of CO2to formic acid. were purchased from Sigma Aldrich, Finland. Liquid CO2 and hydrogen 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) phosphoniumhexafluoro phosphate (IL-110) and tetrabutylphosphonium bromide (IL-163) were provided by Cytec Industries Inc., Canada. All the chemicals were of analytical grade with high purity (>99%) and used without further purification. 2.2. Experimental set up A known quantity of catalyst was charged into the reactor. The reactor (Fig. S1, Supplementary information) was sealed, flushed and filled with hydrogen to the desired pressure. The reactor was heated to the set temperature and then carbon dioxide was pumped in slowly; a supercritical phase was maintained through- out the experiment. In the control experiment the total pressure was 144 bar (at 50 ?C); with partial pressure of hydrogen, 72 bar in the scCO2phase occupying the entire volume of the reactor. The reaction was conducted by stirring the mixture for a desired time. The reactor was then cooled down to room temperature and gases (both unreacted reactants and formed products) were then carefully vented and the formed liquid mixture was separated from the catalyst by filtration and then analyzed by HPLC. Scheme 1 gives the desired reaction where CO2and H2react to give formic acid 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 of adsorbed products from the catalysts, after completion of the reac- tion 5 ml of deionized water was added to dissolve the products for analyses. The reaction mixture was analyzed with HPLC for liquid samples (equipped with a Coregel 87 H3 column) and a gas chro- matography equipped with an HP-5 capillary column (0.20 mm internal diameter, 25 m length and 0.33 lm film thickness) for gaseous samples. 2.4. Catalyst preparation Both, single metal and bimetallic EnCapPd, EnCapPd–Cu, EnCa- pRu, EnCapRu–Cu and EnCapPd–Ru catalysts were prepared as discussed below. In addition, commercial EnCatPd (Sigma Aldrich) was used as a reference catalyst. 2. Experimental 2.4.1. Synthesis of poly-urea encapsulated metal catalyst The catalysts were prepared with some modifications using the method shown by Yadav and Lawate [39,40]. At the first stage the desired 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 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

4. 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 stirred for 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 important role in the encapsulation process. Into this solution 0.68 g of ethy- lene diamine and 0.72 g of diethylene triamine (1:1 ratio) were added. Amines are known to increase the cross linking and subse- quently functional ligation of the metal. Increasing the amines number (i.e. diamine, triamine, pentaamine, etc.) enhances the cross linking which in turn increases the mechanical strength of the polymer beads. The aqueous solution was homogenized by stirring it for 1 h. The temperature of solution was cooled down to 5 ?C. Then the first solution was added into the prepared aque- ous solution drop-wise over 1 h maintaining the temperature below 5 ?C to avoid onset of polymerization before stable oil-in- water microemulsion was formed. The final solution was stirred at a desired speed of agitation for 10 min to obtain a stable microemulsion of oil in water until the required drop size was obtained. The temperature was then increased and maintained at 50 ?C for 3 h to complete the polymerization reaction. Microcap- sules of poly-urea were formed by the interfacial polymerization process. After the polymerization process was completed, the catalyst was filtered with a G4 sintered disc – Buchner filter with the aver- age pore size <20 lm. The filtered catalyst was washed with 100 ml of deionized water for three times, then with ethanol and hexane, respectively. For the efficient wash-off of surfactant, other unreacted monomers, and free metal particles the use of solvents are necessary. The catalyst was then reduced in hydrogen atmosphere using ethanol for 3 h at 15 bar and 100 ?C. Reduction was ensured visu- ally by following the color change from pale yellow or brown to black. 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 of monomer. palladium in which ruthenium or copper is infused between the polymer 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 solubilized in 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 Transform Infrared (FTIR) spectroscopy, textural determination of encapsu- lated species by scanning electron microscopy (SEM), and surface elemental 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 CO2to formic acid in terms of TOF over dif- ferent catalysts prepared. The primary aim here was to find out the most active and suitable catalyst for the synthesis of formic acid by direct hydrogenation of CO2and 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 acid was lower and byproducts were analyzed as summarized in a qual- itative analysis, the analysis showed maximum formation of acetic acid with some methyl formate, ethyl formate and traces of etha- nol/methanol. In the case of a commercial catalyst EnCatPd, similar results were obtained maximizing the acetic acid formation. When EnCap 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 of formic acid was obtained but at the same time Cu metal was found to leach out. With EnCap Pd–Ru, a fairly low yield of formic acid was detected and in addition, the catalyst showed a good selectiv- ity towards acetic acid. However, in the case of EnCap Ru, there was no formation of acetic acid at all under all reaction conditions 2.4.2. Poly-urea encapsulated bimetallic catalyst Addition of the second metal into a monometallic catalyst can alter the selectivity and activity of the reaction positively due to the synergistic effect of both metals. Poly-urea encapsulated Table 2 Activity for different catalysts. Catalyst Turn over Frequency (h?1) Remarks EnCap Pd EnCap Pd + IL EnCap Pd–Cu EnCap Pd–Cu + IL EnCap Ru EnCap Ru + IL EnCap Ru–Cu EnCap Ru–Cu + IL EnCap Pd–Ru EnCap Pd–Ru + IL EnCat Pd 40 commercial EnCat Pd 40 commercial + IL Ionic liquid 950 1,050 620 700 7,000 7,690 4,600 4,690 1,550 1,850 1,400 1,550 Byproducts such as acetic acid, methyl formate and methanol were analyzed The metal leaches out from the encapsulated catalyst, very low CO2conversion and also byproducts were formed Best suited catalyst for the hydrogenation of CO2(no acetic acid was detected and no leaching of metal) Leaching of copper from the catalyst and formation of byproducts Byproducts formation, mainly acetic acid Byproducts formation, mainly acetic acid 0 No yield of formic acid, but when used with a catalyst IL acts as a promoter TOF = moles of formic acid/mole of active metal site/hour. Reaction conditions: total pressure = 108 bar, partial pressure of H2= 36 bar, partial pressure of CO2= 72 bar, temperature = 50 ?C, catalyst = 0.04 g/cm3and ionic liq- uid = 3.12 ? 10?5mol/cm3and reaction time 4 h.

5. 629 S.K. Kabra et al./Chemical Engineering Journal 285 (2016) 625–634 studied. EnCap Ru showed a high yield of formic acid with TOF of 7000 h?1and some by-products like methanol and methyl formate were detected. Catalytic activities of phosphonium based ionic liquids alone were also evaluated and no formation to formic acid was found out. Ionic liquids have shown a very good promoting effect on these catalysts as ionic liquids are good solvents for CO2 [20]. The yield of formic acid was further increased when ionic liquid was used as a promoter with the catalysts. Zhang et al. [42] have reported the mechanism for hydrogenation of CO2to formic acid promoted by a diamine-functionalized ionic liquid. A similar mechanism and promoting effect was seen. In the case of the EnCap Ru with IL as a promoter a TOF of 7690 h?1was observed, which is approximately 10% more than without the promoter. The different ionic liquids studied showed the same promoting effect. So trihexyl (tetradecyl) phosphonium chloride was ran- domly chosen and used as a promoter for further studies. Fig. 1b. SEM image of the EnCap Ru catalyst. 3.2. Scanning electron microscopy (SEM)/Energy dispersive X-ray spectroscopy (EDS) SEM images of encapsulated catalysts were taken by Zeiss Ultra Pluss Field emission scanning electron microscope (FESEM) equipped with an energy-dispersive X-ray unit. Prior to SEM analysis the samples were prepared by coating them with a carbon film in order to prevent charging at higher magnifications. The samples were examined by Figs. 1a–c and 2a, 2b show the SEM images of fresh and used EnCap Ru catalyst. The SEM–EDS studies reveal plenty of morphology and surface related information of the encapsulated catalyst. Overall the catalyst is made up of a comparatively rough surface of polymers in which metals are encapsulated. This rough surface is due to the high rate of polymerization between aromatic di-isocyanate (TDI) and mixture of EDA/DETA as explained by Hong and Park [43]. The poly-urea material encapsulated by the aliphatic di-isocyanate shows a comparatively smooth and macroglobulins like structure. As the reactivity at the interface is lower it shows fewer disturbances and so the external surface is comparatively smooth. The difference in the di-isocyanate reactivity induced from the chemical structure brings about the various membrane morphologies, which can significantly determine the permeability, crystallinity, and thickness of the resultant microcapsules. This is because of the addition of the second metal which is infused in the poly-urea coating making it spongier. using various magnifications. Fig. 1c. SEM image of the used EnCap Ru catalyst. Fig. 2a. SEM image of EnCap Pd–Ru bimetallic catalyst. The surface elemental composition of the encapsulated catalyst was obtained by EDS. Poly-urea encapsulation shows O, N, Cl and respective metals. It is important to note that the element percent- ages shown in the respective Tables are values only for the surface Fig. 1a. SEM image of the EnCap Ru catalyst.

6. 630 S.K. Kabra et al./Chemical Engineering Journal 285 (2016) 625–634 Fig. 3. FTIR of EnCap Ru catalyst. Fig. 2b. SEM image of EnCap Pd–Ru bimetallic catalyst. then MDI. Other aliphatic di-isocyanates show the same result as MDI. metals. The ruthenium content, in the EnCap Ru was found to be 4.57 wt% (Table 3) and in case of EnCap Pd–Ru, the Ru content was 1.77% wt% with the Pd content of 2.57 wt% (Table 4) for the same catalyst. So it can be concluded that, some ruthenium must be within the poly-urea matrix, and thus not so easy to be analyzed by the EDS analysis. 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 diffract meter. The broad diffraction peak in the low angle region (2h = 14–30?) is visible indicating that a long chain carbon was formed (Fig. 4) and the ruthenium metal peak was obtained between 2h = 37–42?. The XRD pattern shows that poly-urea has a semi-crystalline structure. Yadav et al. [29] postulate that when polymer molecules form and precipitate out at high rates, they do not get sufficient time to arrange themselves in an ordered lat- tice. So, the poly-urea formed with higher rate of reaction shows less crystalline structure than the one formed with lower rate of reaction. However, it is possible to vary the structure of these poly- mer films through the conditions employed in their preparation [29]. 3.3. Fourier transform infrared spectroscopy (FTIR) Infrared spectra of the samples pressed in KBr pellets were obtained at a resolution of 2 cm?1between 4000 and 400 cm?1. Spectra were collected with a Perkin–Elmer instrument and in each case the samples were referenced against a blank KBr pellet. Fig. 3 shows the FTIR spectra of the poly-urea encapsulated ruthenium catalyst obtained from the mixture of di-isocyanate and amines. The sample prepared in this experiment has also strong band known for the ?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 the di-isocyanate at 2270 cm?1disappears [40]. Two carbonyl stretch- ing bands are observed: a hydrogen bonded urea carbonyl at 1550 cm?1 and a free urea ?1645 cm?1. From these characteristic peaks, it is evident that the poly-urea microcapsules were successfully prepared. The very reactive aromatic TDI can produce many more hydrogen-bonded N–H groups than the other aliphatic di-isocyanates. This is corrob- orated by the high intensity of the peak at 3306 cm?1for TDI and N–H stretching vibration at 3.5. BET surface area analysis carbonyl absorption band at The accessible surface area and pore size distribution are impor- tant parameters for any catalyst. Surface area measurement was done by nitrogen adsorption at temperature ?196 ?C using a Micromeritics ASAP 2020 instrument, after pretreating the sample under high vacuum at 150 ?C for 3 h. The typical BET surface area was found to be 3.69 m2/g for EnCap Ru and 3.43 m2/g for EnCapPd–Ru. The surface area of the Table 3 EDS surface analysis of poly-urea encapsulated ruthenium based catalyst. Elements Fresh catalyst (wt%) Used catalyst (wt%) N O Cl Ru 61.99 30.86 3.59 4.57 60.48 31.43 3.83 4.26 Table 4 EDS of EnCap Pd–Ru bimetallic catalyst. Elements Fresh catalyst (wt%) Used catalyst (wt%) N O Cl Ru Pd 54.37 38.42 2.87 1.77 2.57 53.88 39.37 2.63 1.64 2.48 Fig. 4. XRD of EnCap Ru catalyst.

7. 631 S.K. Kabra et al./Chemical Engineering Journal 285 (2016) 625–634 single metal catalyst was comparatively higher than that for the bimetallic poly-urea encapsulated catalysts, which is in good agreement with the results reported by Smith [38]. The higher specific surface area could be due to the rough surface of micro- capsule which was synthesized by the reactive TDI and aliphatic amines. The surface area of the used catalyst was found to be 3.87 m2/g for EnCap Ru, which is somewhat higher than the surface area of the fresh catalyst. The reason could be broken catalyst par- ticles, as there is attrition because of mechanical agitation in the reactor. Table 6 Thermodynamic data for the formic acid synthesis. [?C] DH [kJ] DS [J/K] DG [kJ] K Log(K) 1.42E?8 2.49E?8 3.93E?8 5.74E?8 7.89E?8 1.03E?7 1.30E?7 1.59E?7 1.90E?7 2.21E?7 2.54E?7 2.87E?7 3.20E?7 0 15.41 14.89 14.40 13.95 13.53 13.16 12.81 12.49 12.21 11.95 11.71 11.50 11.31 ?93.78 ?95.60 ?97.18 ?98.53 ?99.68 ?100.62 ?101.50 ?102.23 ?102.85 ?103.39 ?103.85 ?104.24 ?104.58 41.03 43.39 45.80 48.25 50.73 53.23 55.76 58.31 60.87 63.45 66.04 68.64 71.25 ?7.84 ?7.60 ?7.40 ?7.24 ?7.10 ?6.98 ?6.88 ?6.79 ?6.72 ?6.65 ?6.59 ?6.54 ?6.49 25 50 75 100 125 150 175 200 225 250 275 300 3.6. Thermodynamics and simulations The thermodynamic aspects of the reaction was studied by using computer based simulation software Aspen and HSC Chem- istry, which are designed for various kinds of chemical reactions and equilibrium calculations. Thermochemical calculations are based on enthalpy (H), entropy (S), heat capacity (Cp) or Gibbs energy (G) values for chemical species. The chemical equilibrium for CO2direct hydrogenation to formic acid (Eq. (1)) was analyzed based on the components’ thermodynamic parameters. Thermodynamic data for the formic acid synthesis were calculated and listed (Table 6). Where the enthalpy (DH) values are positive indicating that the reaction is endothermic at all temperatures. Moreover, the Gibbs free energy values are positive through the temperature range studied showing the reaction to be non-spontaneous. Also the really small value of the equilibrium constant (K ? 1) proves the forward reaction to be unfavorable. Fig. 5 shows the influence of reaction temperature on the amount of formic acid when reaction pressure is atmospheric and the molar feed ratio is stoichiometric (CO2:H2= 1). The curve in Fig. 5 indicates that the amount of formic acid increases when the reaction temperature increases. Thus, high temperature favors the formation of formic acid. In the studied conditions the amount of formic acid reaches its maximum (3.9 ? 10?5mol-%) at 500 ?C. Fig. 6 shows the influence of reaction pressure on the amount of formic acid formed when temperature is 25 ?C and the molar feed ratio is stoichiometric (CO2:H2= 1). K1HCOOH CO2þ H2$ effects of reaction temperature, pressure and molar feed ratio (H2/CO2) on the yield of formic acid in the system was studied. H0, S0and Cpare standard molar enthalpy of formation, standard molar entropy and molar heat capacity, respectively (Table 5). The standard Gibbs free energy (G0) can be calculated by the means of enthalpy and entropy: ð1Þ G0¼ H0? TS0 The temperature dependence of heat capacity (Cp) at elevated temperatures can be predicted by the Kelley equation (3): ð2Þ Cp¼ A þ B ? 10?3? T þ C ? 105? T?2þ D ? 10?6? T2 where A, B, C and D are coefficients estimated from experimen- tal data. Temperature dependence of the enthalpy can be derived from Eq. (4) ZT Temperature dependence of the entropy can be calculated from Eq. (5) as follows: ZT The enthalpy, entropy and Gibbs energy functions for a chemi- cal reaction are calculated as the difference between the products and reactants. For instance, the enthalpy of reaction is described as: ð3Þ ð4Þ HðTÞ ¼ H0þ CpdT 298:15 Cp TdT ð5Þ SðTÞ ¼ S0þ 298:15 DrH ¼ DHproducts? DHreactants The relationship between the equilibrium constant (K) and the free energy of the reaction is as follows: ð6Þ Fig. 5. Amount of formic acid formed as a function of temperature (at pressure of 1 bar and stoichiometric feed ratio (CO2:H2= 1)). lnK ¼ DG=ð?RTÞ ð7Þ Table 5 Thermodynamic properties of species involved in synthesis of formic acid. Cp= A + B?10?3?T + C?105?T?2+ D?10?6?T2[J/mol?K] A B Substance H0[kJ/mol] S0[J/mol?K] C D CO2(g) H2(g) HCOOH(g) ?393.50 0.00 ?378.61 213.76 130.67 248.84 29.31 25.85 24.72 39.97 4.83 100.63 ?2.48 1.58 ?5.19 ?14.78 ?0.37 ?37.52

8. 632 S.K. Kabra et al./Chemical Engineering Journal 285 (2016) 625–634 The increase in the reaction pressure improves the evolving of formic acid. The amount of formic acid formed increases linearly as a function of pressure. In the studied conditions the amount of formic acid reaches its maximum (2.5 ? 10?4mol%) 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 CO2in the feedstock increases, the amount of formic acid increases having the highest value at the (CO2/H2) feed ratio of 1. After that point the formation of formic acid starts to decrease smoothly. Thus, the stoichiometric feed ratio yields the best results at 25 ?C. The calculations showed that the hydrogenation of CO2to for- mic acid is thermodynamically unfavorable reaction. Even in the severest conditions the formation of formic acid is practically neg- ligible. To have an efficient conversion of CO2to formic acid ther- modynamic limitations must be circumvented somehow. Fig. 6. Amount of formic acid as a function of pressure (stoichiometric CO2:H2feed ratio). 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 formic acid were seen. When the pressure of hydrogen was increased to 72 bar, the yield of formic acid increased further. But when 18 bar of hydrogen was used the yield decreased proportionately, which agrees with the thermodynamic study. According to thermodynamics of the reaction, higher pressures favor the forward direction reaction yielding formic acid. When total pressure of the system was studied at different temperatures, it was observed that with the progress in the reaction there is a decrease in pressure as CO2and hydrogen get consumed. When the CO2 pressure further increased (in supercritical region) by keeping the temperature constant, there was a considerable differ- ence in the yield (TOF) values. Further studies of the pressure effect were not possible because of limitations in the equipment design. When the catalyst loading was increased, there was a change in the yield of formic acid as the number of active sites increased. Different reaction temperatures were studied by keeping all the other parameters constant. The effect of temperature on the yield of formic acid was studied in the range of 30–70 ?C. As anticipated the formation rate increased as a function of temperature which Fig. 7. Amount of formic acid formed as a function of amount of CO2in the feed (Temperature: 25 ?C, Pressure: 1 bar and H2in the feed: 1 mol). Table 7 Effect of studied parameters on EnCap Ru catalyst performance. TOF (h?1) Parameter Reaction conditions H2pressure (bar) 18 36 72 3,750 7,690 9,249 Total Pressure = 108 bar, Partial pressure of CO2= 72 bar, Temperature = 50 ?C, Catalyst 0.04 g/cm3 Ionic liquid = 3.12 ? 10?5mol/cm3 Total Pressure = 108 bar, Partial pressure of H2= 36 bar, Partial pressure of CO2= 72 bar, Temperature = 50 ?C, Ionic liquid = 3.12 ? 10?5mol/cm3 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?5mol/cm3 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?5mol/cm3 Catalyst loading (g/cm3) 0.02 0.03 0.04 2,897 5,238 7,690 Temperature (?C) 30 50 70 8,597 9,249 11,900 Catalyst reusability Fresh 1st reuse 2nd reuse 11,900 11,648 11,268 TOF = moles of formic acid/mole of active metal site/hour.

9. 633 S.K. Kabra et al./Chemical Engineering Journal 285 (2016) 625–634 can be attributed to the higher surface reaction rate constant with an increase in temperature as well as the improved diffusivity and reduced 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 high temperatures selectivity towards formic acid was reduced and increased formation of byproducts was detected. Although ther- modynamic study shows that higher temperatures favor higher conversion of CO2, it also gives raise in byproducts formation. [8] H. Zhong, Y. Gao, G. Yao, X. Zeng, Q. Li, Z. Huo, F. Jin, Highly efficient water splitting and carbon dioxide reduction into formic acid with iron and copper powder, Chem. Eng. J. 280 (2015) 215–221. [9] X. Wenjuan, M. Liping, H. Bin, C. Xia, N. Xuekui, Z. Hang, Thermodynamic analysis of formic acid synthesis from CO2 hydrogenation, in: 2011 International Conference on Materials for Renewable Energy & Environment (ICMREE), IEEE, 2011, pp. 1473–1477. [10] M. Yadav, Q. Xu, Liquid-phase chemical hydrogen storage materials, Energy Environ. Sci. 5 (2012) 9698–9725. [11] P.G. Jessop, Y. Hsiao, T. Ikariya, R. Noyori, Homogeneous catalysis in supercritical fluids: hydrogenation of supercritical carbon dioxide to formic acid, alkyl formates, and formamides, J. Am. Chem. Soc. 118 (1996) 344–355. [12] P. Munshi, A.D. Main, J.C. Linehan, C.-C. Tai, P.G. Jessop, Hydrogenation of carbon dioxide catalyzed by ruthenium trimethylphosphine complexes: the accelerating effect of certain alcohols and amines, J. Am. Chem. Soc. 124 (2002) 7963–7971. [13] Y. Maenaka, T. Suenobu, S. Fukuzumi, Catalytic interconversion between hydrogen and formic acid at ambient temperature and pressure, Energy Environ. Sci. 5 (2012) 7360–7367. [14] Y. Inoue, H. Izumida, Y. Sasaki, H. Hashimoto, Catalytic fixation of carbon dioxide to formic acid by transition-metal complexes under mild conditions, Chem. Lett. 863–864 (1976). [15] S.C. Tsang, C.D.A. Bulpitt, P.C.H. Mitchell, A.J. Ramirez-Cuesta, Some new insights into the sensing mechanism of palladium promoted tin (IV) oxide sensor, J. Phys. Chem. B 105 (2001) 5737–5742. [16] O. Krocher, R.A. Koppel, A. Baiker, Highly active ruthenium complexes with bidentate phosphine ligands for the solvent-free catalytic synthesis of N, N- dimethylformamide and methyl formate, Chem. Commun. 453–454 (1997). [17] J. Elek, L. Nádasdi, G. Papp, G. Laurenczy, F. Joó, Homogeneous hydrogenation of carbon dioxide and bicarbonate in aqueous solution catalyzed by water- soluble ruthenium (II) phosphine complexes, Appl. Catal. A: Gen. 255 (2003) 59–67. [18] Y. Himeda, N. Onozawa-Komatsuzaki, H. Sugihara, K. Kasuga, Recyclable catalyst for conversion of carbon dioxide into formate attributable to an oxyanion on the catalyst ligand, J. Am. Chem. Soc. 127 (2005) 13118–13119. [19] Z. Zhang, Y. Xie, W. Li, S. Hu, J. Song, T. Jiang, B. Han, Hydrogenation of carbon dioxide is promoted by a task-specific ionic liquid, Angew. Chem. Int. Ed. Engl. 47 (2008) 1127–1129. [20] Z. Cai, S. Zhao, C. Xu, Z. Xu, X. Sun, Catalysis and mechanism of ionic liquids for direct synthesis of dimethyl carbonate, Chem. Ind. Eng. Progr. 25 (2006) 546. [21] X.C. Guo, Z.F. Qin, G.F. Wang, J.G. Wang, Critical temperatures and pressures of reacting mixture in synthesis of dimethyl carbonate with methanol and carbon dioxide, Chin. Chem. Lett. 19 (2008) 249–252. [22] Y. Zhang, J. Fei, Y. Yu, X. Zheng, Silica immobilized ruthenium catalyst used for carbon dioxide hydrogenation to formic acid (I): the effect of functionalizing group and additive on the catalyst performance, Catal. Commun. 5 (2004) 643–646. [23] G. Chinchen, P. Denny, J. Jennings, M. Spencer, K. Waugh, Synthesis of methanol: part 1. Catalysts and kinetics, Appl. Catal. 36 (1988) 1–65. [24] H. Nakano, I. Nakamura, T. Fujitani, J. Nakamura, Structure-dependent kinetics for synthesis and decomposition of formate species over Cu (111) and Cu (110) model catalysts, J. Phys. Chem. B 105 (2001) 1355–1365. [25] A. Kiennemann, H. Idriss, J. Hindermann, J. Lavalley, A. Vallet, P. Chaumette, P. Courty, Methanol synthesis on Cu/ZnAl2O4 and Cu/ZnO?Al2O3 catalysts: influence of carbon monoxide concentration of formate species, Appl. Catal. 59 (1990) 165–184. [26] M. Bowker, R. Hadden, H. Houghton, J. Hyland, K. Waugh, The mechanism of methanol synthesis on copper/zinc oxide/alumina catalysts, J. Catal. 109 (1988) 263–273. [27] P. Taylor, P. Rasmussen, C. Ovesen, P. Stoltze, I. Chorkendorff, Formate synthesis on cu (100), Surf. Sci. 261 (1992) 191–206. [28] H. Wiener, J. Blum, H. Feilchenfeld, Y. Sasson, N. Zalmanov, The heterogeneous catalytic hydrogenation of bicarbonate to formate in aqueous solutions, J. Catal. 110 (1988) 184–190. [29] D. Preti, C. Resta, S. Squarcialupi, G. Fachinetti, Carbon dioxide hydrogenation to formic acid by using a heterogeneous gold catalyst, Angew. Chem. Int. Ed. 50 (2011) 12551–12554. [30] S.V. Ley, C. Ramarao, R.S. Gordon, A.B. Holmes, A.J. Morrison, I.F. McConvey, I. M. Shirley, S.C. Smith, M.D. Smith, Polyurea-encapsulated palladium (II) acetate: a robust and recyclable catalyst for use in conventional and supercritical media, Chem. Commun. 1134–1135 (2002). [31] S. Yadav, K.C. Khilar, A. Suresh, Microencapsulation in polyurea shell: kinetics and film structure, AICHE J. 42 (1996) 2616–2626. [32] S. Nagayama, M. Endo, S. Kobayashi, Microencapsulated osmium tetraoxide. A new recoverable and reusable polymer-supported osmium catalyst for dihydroxylation of olefins, J. Org. Chem. 63 (1998) 6094–6095. [33] S. Kobayashi, M. Endo, S. Nagayama, Catalytic asymmetric dihydroxylation of olefins using a recoverable and reusable polymer-supported osmium catalyst, J. Am. Chem. Soc. 121 (1999) 11229–11230. [34] C. Ramarao, S.V. Ley, S.C. Smith, I.M. Shirley, N. DeAlmeida, Encapsulation of palladium in polyurea microcapsules, Chem. Commun. 1132–1133 (2002). [35] S. Kobayashi, R. Akiyama, Renaissance of immobilized catalysts. New types of polymer-supported catalysts, ‘microencapsulated catalysts’, which enable 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 CO2to formic acid. There was leaching of Cu metal from poly-urea encapsulation in case of EnCap Cu, EnCap Ru–Cu and EnCap Pd–Cu, whereas no leaching of metal was observed in case of EnCapPd and EnCapRu. The direct synthesis of formic acid from CO2and H2is thermo- dynamically not a favorable reaction at ambient conditions. The thermodynamic study shown high pressures and low temperatures are favorable for the process, which was confirmed by experimen- tal studies. The highest yield of formic acid was observed at 70 ?C under supercritical conditions (144 bar) using EnCap Ru catalyst along with trihexyl (tetradecyl) phosphonium chloride ionic liquid as 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 Dioxide as Feedstock (GreenCatCO2)” supported by the Department of Science and Technology, Government of India (DST-GOI) and the Academy of Finland (No:140122). GDY received support from R.T. Mody Distinguished Professor Endowment and J.C. Bose National Fellowship from DST-GoI. pretreatment on the formation and Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2015.09.101. References [1] I. Statistics, CO2emissions from fuel combustion-highlights, IEA, Paris, <http:// www.iea.org/co2highlights/co2highlights.pdf>, Cited July 2011. [2] H. Hashim, P. Douglas, A. Elkamel, E. Croiset, Optimization model for energy planning with CO2emission considerations, Ind. Eng. Chem. Res. 44 (2005) 879–890. [3] Z. Zhao, H. Dong, X. Zhang, The research progress of CO2capture with ionic liquids, Chin. J. Chem. Eng. 20 (2012) 120–129. [4] M. Hasib-ur-Rahman, M. Siaj, F. Larachi, Ionic liquids for CO2 capture— Development and progress, Chem. Eng. Process. 49 (2010) 313–322. [5] L. Li, N. Zhao, W. Wei, Y. Sun, A review of research progress on CO2capture, storage, and utilization in Chinese Academy of Sciences, Fuel 108 (2013) 112– 130. [6] A.A. Kiss, J.J. Pragt, H.J. Vos, G. Bargeman, M.T. de Groot, Novel efficient process for methanol synthesis by CO2hydrogenation, Chem. Eng. J. 284 (2016) 260– 269. [7] A. Behr, K. Nowakowski, Catalytic hydrogenation of carbon dioxide to formic acid, CO2, Chemistry 66 (2013) 223.

10. 634 S.K. Kabra et al./Chemical Engineering Journal 285 (2016) 625–634 environmentally benign and powerful high-throughput organic synthesis, Chem. Commun. (2003) 449–460. [36] P.G. Jessop, W. Leitner, Chemical Synthesis using Supercritical Fluids, John Wiley & Sons, 2008. [37] R.S. Gordon, A.B. Holmes, Palladium-mediated cross-coupling reactions with supported reagents in supercritical carbon dioxide, Chem. Commun. 640–641 (2002). [38] N. Bremeyer, S. Ley, C. Ramarao, I. Shirley, S. Smith, Synlett 2002, 1843, CAS, Web of Science?Times Cited 37. [39] G.D. Yadav, Y.S. Lawate, Selective hydrogenation of styrene oxide to 2-phenyl ethanol over polyurea supported Pd–Cu catalyst in supercritical carbon dioxide, J. Supercrit. Fluids 59 (2011) 78–86. [40] G.D. Yadav, Y.S. Lawate, Hydrogenation of styrene oxide to 2-phenyl ethanol over polyurea microencapsulated mono-and bimetallic nanocatalysts: activity, selectivity, and kinetic modeling, Ind. Eng. Chem. Res. 52 (2013) 4027–4039. [41] D.A. Pears, S.C. Smith, Polyurea-encapsulated palladium catalysts: the development and application of homogeneous-catalyst technology, Aldrichim. Acta 38 (2005) 23–33. [42] Z. Zhang, S. Hu, J. Song, W. Li, G. Yang, B. Han, Hydrogenation of CO2to formic acid promoted by a diamine-functionalized ionic liquid, ChemSusChem 2 (2009) 234–238. [43] K. Hong, S. Park, Polyurea microcapsules with different structures: Preparation and properties, J. Appl. Polym. Sci. 78 (2000) 894–898. a new and versatile immobilized-