Silica Immobilized Ruthenium Catalyst for Formic Acid Synthesis from Supercritical Carbon Dioxide Hydrogenation Ⅱ: Effect of Reaction Conditions on the Catalyst PerformanceYU, Ying-Min a(于英民) ZHANG, Yi-Ping a, b(张一平) FEI, Jin-Hua a(费金华)ZHENG, Xiao-Ming*,a(郑小明)a Institute of Catalysis, Xixi Campus, Zhejiang University, Hangzhou, Zhejiang 310028, Chinab Department of Chemistry, Zhejiang Education Institute, Hangzhou, Zhejiang 310012, ChinaAbstractThe synthesis of formic acid from carbon dioxide and hydrogen using a silica immobilized ruthenium catalyst as precursor has been studied in different reaction conditions. The results revealed that the TOF of HCOOH achieved 1481.5 h-1 on immobilized ruthenium catalyst near the critical pressure point of CO2 with H2 pressure of 4.0 MPa, reaction temperature of 80 °C and PPh3/Ru ratio of 6:1. The reaction activity of immobilized catalyst was higher than that of homogeneous catalyst, and the immobilized catalyst also offered the practical advantages such as easy separation and reuse.Keywords: immobilized, ruthenium, formic acid, carbon dioxide, supercritical* Tel.: 0086-571-88273417 Fax: 0086-571-88273283 E-mail: xmzheng@Received November 9, 2004; revised February 15, 2005; acceptedProject supported by the National Natural Science Foundation of China (No. 20173048) and the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20030335068) and the Scientific Research Project from Education Department of Zhejiang Province (No. 20040320)IntroductionIn recent years, the efficient conversion and application of carbon dioxide as a chemicalfeedstock has received ongoing attention, because it is a natural and abundant carbon source and hasbeen considered a major atmosphere pollutant, responsible for the greenhouse effect. The catalytichydrogenation of carbon dioxide produces products ranging from alkylanes, alkenes, alcohols,aldehydes and ethers to acids.Among the possible valuable chemical compounds, much attention has been paid to thehydrogenation of carbon dioxide to formic acid,1-6 because it is an atom economic reaction with nowaste produced. But the process is thermodynamically unfavorable. The conversion CO2(g) + H2(g)→HCOOH(l) occurs with ∆G o298= +32.9 kJ·mol-1, the equilibrium conversion of CO2 is 0.72×10-5at 3.0 MPa, 100 °C and 0.14×10-4 at 6.0 MPa, 100 °C, respectively.7 When this reaction was in amixture of scCO2 (supercritical carbon dioxide), H2 and NEt3, a TOF (turnover frequency, mol ofHCOOH/mol of Ru/h ) of 1400 h-1 was obtained using RuH2(PMe3)4 homogeneous catalysts.3However, the separation of the relatively expensive catalysts from the reaction mixture systemis one of the major drawbacks of the homogeneous catalysis. An available solution to this separationproblem is the heterogenisation of homogeneous catalysts. To immobilize homogeneous catalystson solid supports via linkers has become the most often employed method. Herein, we reported thesynthesis of formic acid from CO2 and H2 using ruthenium immobilized on functionalized silica. Itwas found that good performance was achieved when CO2 was in supercritical condition, and thecatalyst could be recovered from the reaction mixture by filtration easily and then, potentially bereused.ExperimentalPreparation of the immobilized ruthenium catalyst precursorIn a typical surface modification process, a mixture of fumed silica (6 g), anhydrous toluene (140 mL) and γ-aminopropyl triethoxysilane (4.65 g) was refluxed for 8 h, then 25 mL of H 2O were added. The mixture was stirred again for 4 h. By reaction of γ-aminopropyl triethoxysilane with the silanol groups, amino ones were grafted onto silica (see Scheme 1). The product was filtered, washed with toluene in order to remove the remaining non-bonding organic amine, and then dried in vacumn at 200 °C for 5 h. The organic amine functionalized silica (Si)-NH 2 was obtained. The amount of amino groups loaded on silica measured by element analysis was 1.76 mmol per gram of catalyst.Scheme 1SiO 2OHOHOH OH +(C 2H 5O)3Si(CH 2)3NH2SiO 2OH O O OSiNH 2Then a mixture of RuCl 3·3H 2O (0.035 g), (Si)-NH 2(1 g) and ethanol (40 mL) was stirred at room temperature for 1 h. The resultant solid, identified as (Si)-NH 2-RuCl 3, was filtered, washed with ethanol and dried in air. The ruthenium content in (Si)-NH 2-RuCl 3 was measured by ICP instrument, to be 3.38×10-2 mmol per gram of catalyst.The active ruthenium complex catalyst responsible for the HCOOH formation was synthesized in situ during the catalytic reaction with (Si)-NH 2-RuCl 3 and triphenylphosphine.Catalytic reactionThe hydrogenation of CO 2 was carried out in a stainless steel 100 ml autoclave. In a typical run, 0.6 g of the immobilized catalyst (Si)-NH 2-RuCl 3 was used. Then solvent (ethanol, 20 mL), ligand(triphenylphosphine, 0.06 g) and base (triethylamine, 5 mL) were added into the autoclave. The vessel was flushed with H2 three times to remove air and pressurized up to 4.0 MPa with H2. Then the liquid CO2 was pumped into the autoclave from a cooled(-5 °C) reservoir using a syringe pump to reach the desired pressure. The vessel was heated to the reaction temperature (80 °C) and the pressure became 16.0 MPa after temperature equilibration, and the reaction was stirred for the desired reaction time. At the end of the reaction period, the reactor was cooled rapidly by being placed in a cold water bath and vented carefully. The reaction solution collected was acidified and the formic acid esterified completely for analysis. Qualitative and quantitative analyses were conduced both by GC-MS equipped with TC-wax column and GC equipped with poropak QS column and FID detector. The two results were consistent with each other.Results and discussionPhase behaviorThe critical point of CO2 is at a T of 33.1 °C and a P of 7.23 MPa. Under the normally applied condition, the reaction proceeded at 80 °C, H2 pressure 4.0 MPa with the total pressure 16.0 MPa and in the supercritical CO2 region.Although the reaction was performed under scCO2 condition, the system contained also other components. The upper phase was predominantly gaseous H2, while the lower phase, which was the reaction phase, included the solvent, the base, the catalyst, also the dissolved CO2 and H2. A large amount of CO2 was present in the lower liquid phase.3 It is likely that the large amount of CO2 in the lower phase increased the solubility of H2 in this phase relative to the solubility of H2 in liquid solvent, thus making scCO2 a good medium for its own hydrogenation.Effect of CO 2 pressureThe effect of CO 2 pressure on the reaction rate is shown in Figure 1. It can be found that at the beginning, the TOF of HCOOH was increased with increasing CO 2 pressure and a maximum value (1197 h -1) appeared at 7.5 MPa. When the CO 2 pressure was further up to 8.5, 9.5 and 12 MPa, the TOF of HCOOH was slightly decreased to 1192, 1180 and 1161 h -1, but those were still higher than that at 6.0 MPa. The 7.5 MPa of CO 2 pressure, which was just the point a little exceeding the critical pressure of CO 2 (7.23 MPa), led to reduce the phase transfer barrier and increase the dissolvability of H 2 in scCO 2, so the high concentration of reactants and the immobilized catalyst were in the same phase to increase the rate of reaction. However, dilution effect might play a role to decrease the reaction rate at a further higher CO 2 pressure. Because at pressure higher than the critical pressure, CO 2 was already in supercritical condition, the addition of more CO 2 did not increase the dissolvability of H 2. On the contrary, the addition of more CO 2 would decrease the probability of contact and impact between catalyst and H 2, therefore the TOF of HCOOH was a bit reduced.4681012600700800900100011001200T O F /h-1CO 2 pressure /MPaFigure 1 The effect of CO 2 pressure on the rate of formic acid production.Reaction conditions: ruthenium content=0.02 mmol, reaction temperature=80 °C, H 2 pressure=4.0 MPa, reaction time=1 h, PPh 3/Ru(mol ratio)=6:1, stirring rate=250 r·min -1.Effect of H 2 pressureWith a CO 2 pressure of 12.0 MPa, the effect of H 2 pressure on the reaction rate is shown in Figure 2. The rate of HCOOH production was nearly first-order dependent on the H 2 pressure, and no formic acid was obtained in the absence of H 2, while TOF of 2019 h -1 was obtained at 8.0 MPa of H 2.T O F /h-1H 2 pressure /MPaFigure 2 The effect of H 2 pressure on the rate of formic acid production under the same reaction conditions as those of Figure 1 except CO 2 pressure=12.0 MPa instead of H 2 pressure=4.0 MPa.Effect of temperatureThe effect of the reaction temperature on the rate of formic acid production under CO 2 pressure of 4.0 MPa and scCO 2 is showed in Figure 3 and Figure 4, respectively. The results displayed that the highest TOF of HCOOH was obtained at 110 and 80 °C under CO 2 pressure of 4.0 MPa and scCO 2, respectively.It was unfavorable to chemical adsorption and activation of reactants when the reaction temperature was too low, hence the reaction activity was also low. With the increase of temperature, the average kinetic energy of reactants was increased, which was favorable to its diffusion, thus thereaction rate quickened. But when the temperature was raised to some extent, the dissolvability of CO 2 in the reaction solution and the dissolvability of H 2 in CO 2 began to decrease. On the other hand, CO 2 hydrogenation to HCOOH was an exothermic reaction, it was unfavorable to the reaction when temperature increased, and hence it was unfavorable to the undergoing of reaction iftemperature was too high.20040060080010005080110140Reaction temperature /℃T O F /h -1Figure 3 The influence of reaction temperature on the rate of formic acid production under high pressure CO 2 condition.Reaction conditions: ruthenium content=0.02 mmol, H 2 pressure=4.0 MPa, total pressure=8.0 MPa, reaction time=3 h, PPh 3/Ru(mol ratio)=6:1, stirring rate=250 r·min -1.400800120016005080110Reaction temperature /℃T O F /h -1Figure 4 The influence of reaction temperature on the rate of formic acid production under scCO 2 condition. Reaction conditions: ruthenium content=0.02 mmol, H 2 pressure=4.0 MPa, total pressure=16.0 MPa, reaction time=1 h, PPh 3/Ru(mol ratio)=6:1, stirring rate=250 r·min -1.From Figure 3 and Figure 4, it can be seen that under scCO 2 condition, the optimum reaction temperature was 80 °C , which was 30°C lower than that of high pressure CO 2 condition. And the TOF of HCOOH was higher than that of high pressure CO 2 condition with the value being 1161 h -1and 826.5 h -1, respectively. It indicated that the use of supercritical fluid as reactant could lower the reaction temperature, and the high dissolvability of H 2 in scCO 2 was responsible for it.Effect of reaction timeFigure 5 and Figure 6 show the effect of reaction time on the HCOOH production under high pressure CO 2 condition (4.0MPa) and scCO 2 (12.0MPa) at 80°C, H 2 pressure of 4.0 MPa, respectively. From Figure 5, the yield of HCOOH reached a constant after 3 h running under high pressure CO 2 condition and the value was 17.50-18.00 mmol.0123454681012141618H C O O H y i e l d /m m o lReaction time /hFigure 5 The influence of reaction time on the HCOOH yield under high pressure CO 2 condition.Reaction conditions: ruthenium content=0.02 mmol, reaction temperature=80 °C, H 2 pressure=4.0 MPa, total pressure=8.0 MPa, PPh 3/Ru(mol ratio)=6:1, stirring rate=250 r·min -1.4812162024H C O O H y i e l d /m m o lReaction time /hFigure 6 The influence of reaction time on the HCOOH yield under scCO 2 condition.Reaction conditions: ruthenium content=0.02 mmol, reaction temperature=80 °C, H 2 pressure=4.0 MPa, total pressure=16.0 MPa, PPh 3/Ru(mol ratio)=6:1, stirring rate=250 r·min -1.From scCO 2 hydrogenation condition, as showed in Figure 6, the yield of HCOOH reached equilibrium only after 1 h running, and the value was enhanced to 23.20-23.70 mmol. While carbon dioxide was in supercritical region, its diffusion coefficient and the dissolvability of hydrogen were both high, therefore CO 2 could react with H 2 rapidly, and the reaction could also reach equilibrium rapidly. That is the reason why the time of reaching equilibrium under scCO 2 condition was much shorter than at high pressure CO 2, and why the yield of HCOOH under scCO 2 condition was increased greatly than at high pressure CO 2.Effect of the ratio of PPh 3/RuFigure 7 exhibits the effect of the ratio of PPh 3/Ru on the performance of the catalyst for formic acid formation. It can be observed that there was no formic acid formed in the absence of PPh 3 ligand. When the PPh 3/Ru ratio was changed from 3:1 to 6:1, TOF of HCOOH increased from 686 to 1161 h -1. The further increase of PPh 3/Ru ratio showed a decrease of the catalytic activity, and at PPh 3/Ru ratio of 10:1, TOF was 656 h -1. Hence, the optimum amount of PPh 3 ligand should obey 6:1 of the PPh 3/Ru ratio.20040060080010001200T O F /h-1PPh 3:RuFigure 7 The influence of PPh 3:Ru ration on the rate of formic acid production under scCO 2 condition.Reaction conditions: ruthenium content=0.02 mmol, reaction temperature=80 °C, H 2 pressure=4.0 MPa, total pressure=16.0 MPa, reaction time=1 h, stirring rate=250 r·min -1.It indicated that the molar ratio of ligand to metal ion effected on the structure of the complex, the formation of active species and its stability. The proposed structure of the catalytic active ruthenium complex formed in situ identified by the method of XPS and IR as Scheme 2 described:8Scheme 2HPPh 3Ph 3The coordination number of PPh 3 in the complex was 3, but the catalytic activity reached the highest value at the PPh 3/Ru ratio of 6:1. The reason why the formation of active ruthenium complex needed more PPh 3 might be that besides some of PPh 3 was used as ligand, the other part was consumed by the oxidation-reduction reaction with RuCl 3.9 The presence of excessive PPh 3 was disadvantageous to the reaction, and this maybe related to the possible mechanism.8 In the mechanism, a PPh 3 ligand in the complex was replaced by the solvent of ROH to generate the catalytic cycle species as Scheme 3 described, and the excess of PPh 3 affected itself dissociation, thus inhibiting the catalytic activity.Scheme 3Ph 3H +PPh 3 Effect of stirring speed Figure 8 depicts the influence of stirring speed on the reaction rate under scCO 2 condition. It can be found that the TOF of HCOOH was increased with the stirring speed, and the value was 1439 h -1 at stirring speed of 500 r·min -1 while that was 1161 h -1 at stirring speed of 250 r·min -1. But when the stirring speed was above 500 r·min -1, this extent of increase was not so obvious, and the TOF of HCOOH was 1481.5 h -1 at stirring speed of 750 r·min -1. According to the analysis of phase behavior, although under scCO 2 condition, the reaction really occurred in the lower phase including the solvent, the base, the catalyst, the dissolved CO 2 and H 2. Hence, the stirring speed had an effect on the reaction activity, but external diffusion became unimportant to the reaction rate when the reaction occurred with high stirring speed state. 0500100015002000250500750Stirring rate /r·min -1T O F /h -1 Figure 8 The influence of stirring speed on the rate of formic acid production under scCO 2 condition. Reaction conditions: ruthenium content=0.02 mmol, reaction temperature=80 °C, H 2 pressure=4.0 MPa, total pressure=16.0 MPa, reaction time=1 h, PPh 3/Ru(mol ratio)=6:1.Effect of immobilizationThe catalytic activity of the ruthenium catalyst before and after immobilization has been compared in Table 1.Comparing the homogeneous system (entry 1) with the immobilized ruthenium system (entry 2), it can be found that the TOF of HCOOH was enhanced from 838 h-1 of the homogeneous system to 1161 h-1 of immobilized ruthenium system. The homogeneous catalyst showed the lower activity due to the aggregation of catalytic sites to multinuclear complexes.10 On the immobilization catalyst, the active sites could be anchored to the support, and it held back the aggregation of catalytic sites with each other. Thus, the fact is that the activity of the catalyst increased after immobilization can be ascribed to so-called site-isolation or well dispersion.Table 1 Catalytic activity of the ruthenium catalyst before and after immobilization under scCO2 condition aHCOOHEntry Catalyst/%TOF /h-1 Selectivity1 RuCl3, PPh3 838 1002 (Si)-NH2-RuCl3, PPh3 1161 100a Reaction conditions: ruthenium content=0.02 mmol, reaction temperature=80 °C, Hpressure=4.0 MPa, total2pressure=16.0 MPa, reaction time=1 h, PPh3/Ru(mol ratio)=6:1, stirring rate=250 r·min-1.The reuse of the catalystOne of the advantages of using immobilized catalysts is recyclability. After reaction, the catalyst was filtered and dried before it was used again, and the recycle occurred in the absence of triphenylphosphine ligand under the same reaction condition as that of fresh catalyst. Table 2 indicates the catalytic results with reused catalysts under scCO2 condition. To the reused catalyst,formic acid could be obtained while the recycle was performed in the absence of triphenylphosphine ligand (Table 2, Entries 2 and 3). A worthy noting detail was that there was no formic acid formed when the catalyst was first used in the absence of triphenylphosphine ligand, as shown in Figure 7. It was attributed to the formation of the catalytic active species from the immobilized ruthenium and triphenylphosphine ligand under reaction conditions in the first run, the structure of which is described in Scheme 2 as above.In addition, 1009 h-1 of TOF was maintained after it was used three times, indicating that the drop of ruthenium from amine modified silica was almost negligible and the recycling of the catalyst was possible. To prove this, after filtering the liquid collected at reaction temperature, the filtrate was used to react further, and it exhibited no activity, indicating there was no leaching of ruthenium in the catalytic run.Table 2 The catalytic results with reused catalysts under scCO2 conditionHCOOHEntry Catalyst PPh3/Ru(mol ratio)/%TOF /h-1 Selectivity1 (Si)-NH2-RuCl3, PPh3 6:1 1161 1002 Recycle-1 0 1054 1003 Recycle-2 0 1009 100 Reaction conditions: ruthenium content=0.02 mmol, reaction temperature=80 °C, H2 pressure=4.0 MPa, total pressure=16.0 MPa, reaction time=1 h, stirring rate=250 r·min-1.ConclusionThe catalyst of amine functionalized silica immobilized ruthenium complexes showed higher activity than homogeneous catalyst for formic acid production from CO2 and H2, and could berecyclable. The reaction could occur at the lower temperature with a shorter reaction time of reaching equilibrium under scCO2 than under high pressure CO2. The optimum CO2 partial pressure was near its critical point, and high H2 partial pressure afforded high reaction rate. The TOF of HCOOH achieved 1481.5 h-1 on immobilized ruthenium catalyst under scCO2 with H2 pressure of 4.0 MPa at reaction 80°C, PPh3/Ru ratio of 6:1 and stirring speed of 750 r·min-1.References1 Inoue,Y.; Izumida, H.; Sasaki,Y.; Hashimoto, H. Chem. Lett.1976, 863.2 Jessop, P. G.; Ikariya, T.; Noyori, R. Chem. Rev.1995, 95, 259.3 Jessop, P. G..; Hsiao,Y.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 344.4 Gassner, F.; Leitner, W. J. Chem. Soc., Chem. Commun.1993, 1465.5 Kröcher, O.; Köppel, R. A.; Baiker, A. J. Chem. Soc., Chem. Commun.1997, 453.6 Zhang, J. Z.; Li, Z.; Wang, H.; Wang, C. Y. J. Mol. Catal.1996, 112, 9.7 Cao, F. H.; Liu, D. H.; Hou, Q. S.; Fang, D. Y. J. Nat. Gas. Chem.2001, 10, 24 (in Chinese).8 Zhang, Y. P. Ph.D. Thesis, Zhejiang university, Hangzhou, 2003 (in Chinese).9 Wang, X. Y. 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