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  Short Communication Direct synthesis of formic acid by partial oxidation of methane on H - ZSM - 5solid acid catalyst Abul Kalam Md. Lutfor Rahman 1, ⁎ , Masako Kumashiro, Tatsumi Ishihara ⁎ Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan a b s t r a c ta r t i c l e i n f o  Article history: Received 28 January 2011Received in revised form 25 March 2011Accepted 4 April 2011Available online 12 April 2011 Keyword: MethanePartial oxidationDirect synthesisFormic acidH-ZSM-5 catalystSilica – alumina ratio The direct synthesis of formic acid by partial oxidation of methane was studied using hydrogen peroxide(H 2 O 2 ) as oxidant with keeping reaction temperature of 373 K and a pressure of 2.6 MPa. High yield (13.0%)and selectivity (66.8%) of formic acid (HCOOH) an important oxygenated compound in chemical industrywereachievedusingprotonatedpentasil-typezeolite(H-ZSM-5)asasolidacidcatalyst.Tryphenylphosphene(Ph 3 P) was used as a promoter in reaction system. A fairly large amount of CO 2  was also observed as deepoxidation product.© 2011 Elsevier B.V. All rights reserved. 1. Introduction Directconversionofmethane(CH 4 )tooxygenatedcompoundssuchasaceticacid(CH 3 COOH),formicacid(HCOOH)andmethanol(CH 3 OH)under mild reaction conditions is of interest in both organic andbioinorganic chemistry with the special interest in catalysis. Manyresearchers have investigated metal-catalyzed oxidation systems fordirect conversion of methane into useful oxygenates. Most activecatalyststhatoperateatlowtemperaturerequirethepresenceofstrongoxidants such as SO 3  [1, 2], K 2 S 2 O 8  [3-6] or NaIO 4  [7]. However,environmentalandeconomicconcernsfavortheuseofoxidantssuchasH 2 O 2  (the only by-product being H 2 O) or O 2  that are both harmless tothe environment and atom ef  󿬁 cient. A number of studies of the partialoxidation of alkane using H 2 O 2  oxidant have been reported [8-12]. Thepresentworkextendsthesestudiesbyfocusingonthepartialoxidationof methane into oxygenated compound using H 2 O 2  oxidant.Several reports exist on the selective oxidation of methane intoformic acid, but yields are still poor ( ≤ 5%) [13-16]. Galina et al. [13] reportedthe oxidation of methanein the presenceof air and obtainedHCOOHof5.2%.Kuzminetal.[14]foundthat0.08×10 − 3 MofHCOOHby oxidation of methane using H 2 O 2  in the presence of copper(II)peroxocomplexes. Min et al. [15] reported the selective oxygenationof methane into formic acid with 8.7×10 − 2 % conversion of methane.Seki et al. [16] used H 2 O 2  oxidant and H 4 PV  1 Mo 11 O 40  catalyst;reaction gave a total yield of 4.4% including HCOOH. In summary, allthe studies published to date on formic acid synthesis from methanereport yields of just a few percent. Clearly, a strong need exists for anew catalyst for partial oxidation of methane that gives higher yieldsand greater selectivity.ZSM-5,a pentasil-type zeolite,isknownto beanactivecatalyst[17,18].Hanetal.[17]reportedtheproductionofC 5+ liquidhydrocarbonsinthepresenceof C 3+ additivebydirectpartialoxidation of methanewithO 2 overH-ZSM-5catalyst.CH 3 OHwastheprimaryoxygenatedproductin partial oxidation of methane under pressurized conditions [17, 18].We therefore chose to investigate the activity of pentasil-type zeolitecatalystfortheliquid-phaseoxidationofmethaneusingH 2 O 2 oxidant,areaction that has not heretofore been studied in detail. 2. Experimental High silica zeolite of NaZSM-5 supplied by Tosoh Corporation, Japan was used as catalyst. The elemental composition of zeolitecatalyst is listed in the Supporting information (SI). Protonatedzeolites were obtained via ion exchange of Na-type zeolite. Na-zeolitewas 󿬁 rstion-exchangedintoitsammoniumformbyNH 4 NO 3 aqueoussolution at about 368 K for 2 h. The sample was then dried at 333 Kovernightandcalcinedat773 Kfor3 h.Catalystwasfed intoa 200-mlreactor with H 2 O 2  and H 2 O. The reactor was  󿬂 ushed with N 2  severaltimes to remove air inside it. Pure CH 4  was fed into the reactor vesselat the designated pressure. The temperature of the reaction was kept Catalysis Communications 12 (2011) 1198 – 1200 ⁎  Corresponding authors. Tel.: +81 92 802 2869; fax: +81 92 802 2871. E-mail addresses:  lrahman1973@gmail.com (A.K.M.L. Rahman),ishihara@cstf.kyushu-u.ac.jp (T. Ishihara). 1 Present Address: Department of Chemistry, Jagannath University, Dhaka 1100,Bangladesh. Tel.: +880 1732108451; +880 27176191; fax: +880 7113752.1566-7367/$  –  see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.catcom.2011.04.001 Contents lists available at ScienceDirect Catalysis Communications  journal homepage: www.elsevier.com/locate/catcom  at 373 K. Reaction was run for 5 h. Gas sampling was monitored byGas chromatographs (TCD) with column containing molecular sieve(4 mm×5 m) and active carbon (4 mm×2 m). Yield of formic acidwas measured by DIONEX Ion Chromatograph of DX-120 enclosedwith the IonPacAS9-HC Analytical, 4×250 mm column. CH 3 CHO andCH 3 OH were detected by a Gas chromatography – mass spectroscopy(GC – MS, Shimadzu, GC – MS-QP2010 Plus) containing the column of Stabilwax 0.32 mm×60 m. Acetic acid obtained as by-product wasdetected by DIONEX Ion Chromatography of ICS 1000 fabricated withIonPacICE-AS6 Ion exclusion column (9×250 mm). The amount of H 2 O 2  before and after the reaction was estimated through automaticredox titration. Details in the experiment and analysis were given inthe Supporting information (SI). 3. Results and discussion Ascreeningtestwasperformed inthepresentstudywithdifferentcatalystsystems.Theproductyieldsfrompartialoxidationofmethaneusing H 2 O 2  oxidant and various catalysts are listed in Table 1.We carried out partial oxidations of methane using different typesof protonated zeolite catalysts. Since TS-1- and vanadium-basedcatalysts such as VOSO 4  and H 4 PVMo 11 O 40  are active catalysts for thereaction of methane partial oxidation, we also investigated their useunder similar reaction conditions.For all catalysts examined, the reaction gave HCOOH, CH 3 CHO, asmall amount of CH 3 COOH by-product and the deep oxidationproduct CO 2 . H-ZSM-5 catalyst (SiO 2 /Al 2 O 3 =23.8) gave the highestyield of HCOOH, indicating that H-ZSM-5 shows higher activity to thepartial oxidation of methane using H 2 O 2  oxidant than does theconventional catalyst, even under similar reaction conditions. Wethus conclude that for this catalyst the acid site seems to be the activesite for the partial oxidation of methane.Triphenylphosphine(Ph 3 P)isknowntobeanef  󿬁 cientpromoterof methane oxidation [19, 20]. We therefore investigated the effects of adding Ph 3 P to the reaction mixture. Table 1 shows that the additionof Ph 3 P greatly improves the yields of both HCOOH and CH 3 COOH.With added Ph 3 P, H-ZSM-5 catalyst gives HCOOH in a yield of 13.0%and with a selectivity of 66.8%.Acidity has been suggested to be an important parameter for thepartial oxidation of methane. We therefore investigated the effects of the H-ZSM-5 catalyst's SiO 2 /Al 2 O 3  ratio, which determines its acidity.Fig.1showsplotsofHCOOHyieldandselectivityasafunctionofSiO 2 / Al 2 O 3  ratio. Yield is strongly affected and selectivity is clearly affectedby the ratio: yield is highest at the lowest ratio (23.8) and decreaseswith increasing ratio.The ratio is supposedly related to the number of acid sites that areresponsible for the activation of methane and is thus more importantthan the actual acid strength.Han et al. [17] reported that for reactionusing higher alumina-content (lower ratio, more acid sites) H-ZSM-5catalyst, the yields of C 2+ and C 5+ liquid hydrocarbons from the partialoxidation of methane are higher than those for reaction using loweralumina-content H-ZSM-5 catalyst. Farizul et al. [21] also reportedthat higher conversions of palm oil were achieved using H-ZSM-5with lower SiO 2 /Al 2 O 3  ratio.We investigated the effect of reaction temperature on HCOOHyield and H 2 O 2  conversion. Fig. 2 shows plots of HCOOH yield andselectivityand H 2 O 2  conversionasa function of reactiontemperature.Both yield and conversion increase with increasing temperature andreach the highest at 373 K. Since simple decomposition of H 2 O 2 (H 2 O 2 → H 2 O+½O 2 )increaseswithincreasingtemperatureandH 2 O 2 conversion reaches nearly 100% at 373 K. Although, production of molecular O 2  which is a by-product of H 2 O 2  increases withtemperature and remain in the gas phase but no effect observed formolecular oxygen in the yield of HCOOH. It suggests that activeoxygenspecieswhichisresponsibleforthepartialoxidationcomenotfrom the O 2  in gas phase but rather from H 2 O 2 . Anyway, based on thesimple decomposition of H 2 O 2 , the optimum reaction temperatureseems to be 373 K.We examined the effect of the amount of H 2 O 2  on HCOOH yield.Fig.3showsplotsofHCOOHyieldasafunctionoftheamountofH 2 O 2 .  Table 1 Product yields for the partial oxidation of methane using H 2 O 2  oxidant and various catalysts.Catalyst Yield (%) Selectivity toHCOOH (%)Conv. Of H 2 O 2 (%)Ef  󿬁 ciencyof H 2 O 2 (%)HCOOH CO 2  CH 3 CHO CH 3 OH CH 3 COOHVOSO 4  0.4  – – –  0.03 87.4 100 0.4H 4 PVMo 11 O 40  1.3 3.2  – –  0.18 26.4 100 1.9TS-1 0.6  –  0.1  –  0.09 59.6 79.8 0.9H-Ferrierite 0.2 1.1 0.1 0.02 0.07 11.9 12.7 0.2H-Y 0.2 1.0 0.1  –  0.03 13.4 25.5 0.4H-ZSM-5 13.0 5.6 0.3 0.02 0.14 66.8 98.0 14.0H-ZSM-5 ⁎ 7.9 6.4  –  0.01  –  55.0 100 9.0Catalyst 1.5 g; Ph 3 P 0.3 g; H 2 O 2  121.88 mmol; H 2 O 70 ml; CH 4  pressure 26 atm (118 mmol); temperature 373 K and reaction time — 5 h. ⁎  Reaction without Ph 3 P. Selectivity was calculated based on the total amount of detected carbon containing products. 1101001000020406080100 Selectivity to HCOOH(%)Yield of HCOOH (%)    Y   i  e   l   d ,   S  e   l  e  c   t   i  v   i   t  y  o   f   H   C   O   O   H   (   %   ) SiO 2  /Al 2 O 3 Fig. 1.  Yield and selectivity of HCOOH as a function of SiO 2 /Al 2 O 3  ratio of H-ZSM-5catalyst. 32034036038040042002468101214020406080100120 Yield of HCOOH (%)H 2 O 2 Conv. (%)Selectivity(%)    Y  e   i   l   d  o   f   H   C   O   O   H   (   %   ) Reaction temperature (K)    S  e   l  e  c   t   i  v   i   t  y   (   %   ) ,   H    2    O    2    C  o  n  v .   (   %   ) Fig. 2.  Yield and, selectivity of HCOOH and H 2 O 2  conversion as a function of reactiontemperature.1199  A.K.M.L. Rahman et al. / Catalysis Communications 12 (2011) 1198 – 1200  Yield increases with increasing amounts of H 2 O 2 , reaches a maximumof 13.0% with a selectivity of 66.8% at 121.88 mmol of H 2 O 2 , anddecreases slightly at higher amounts of H 2 O 2  because of oxidation of HCOOH with H 2 O 2  or gaseous O 2 . Under the reaction conditions used,the decomposition of H 2 O 2  is 98% with the ef  󿬁 ciency to produceHCOOH of 14.0%, estimated by mole of HCOOH per mole of H 2 O 2  withsubtraction of the O 2  produced from initial amount of H 2 O 2 . On theother hand, selectivity decreases gradually with increasing H 2 O 2 amounts.Acetaldehyde (CH 3 CHO) and CH 3 OH were observed by GC – MS inthereactionproduct.LargeramountsofCH 3 CHOwereassociatedwithlarger amounts of HCOOH product obtained. HCOOH has beenspeculated to form by the oxidation of CH 3 CHO [22]. Bar-Nahun etal. [23] reported that CH 4  can be transformed to CH 3 CHO via CH 3 OH.Therefore, the CH 3 COOH obtained in our experiments might be fromfurther oxidation of CH 3 CHO. CH 3 OH is considered to be theintermediate product in direct methane oxidation [16, 17]. Althoughreaction mechanism on H-ZSM-5 is not clear in details, however,CH 3 OH might be a possible intermediate in synthesis of HCOOH.Considering the fairly large amount of CO 2  formed, we tentativelypropose Scheme 1 for reaction pathways on H-ZSM-5 catalyst.Anywayitcouldbe proposedthatthestrongsolidacid ofH-ZSM-5is highly active in synthesis of HCOOH by direct partial oxidation of CH 4  using H 2 O 2  as oxidant.  Acknowledgements We gratefully acknowledge the  󿬁 nancial support of the NanoEnvironmental Catalyst Project from the Ministry of Education,Culture, Sports, Science and Technology, (MEXT) Japan.  Appendix A. Supplementary data Supplementary data to this article can be found online atdoi:10.1016/j.catcom.2011.04.001. References [1] R.A. Periana, D.J. Taube, E.R. Evitt, D.G. Lof  󿬂 er, P.R. Wentrcek, G. Voss, T. Masuda,Science 259 (1993) 340 – 343.[2] R.A. Periana, D.J. Taube, S. Gamble, H. Taube, T. Satoh, H. Fujii, Science 280 (1998)560 – 564.[3] N. Basickes, T.E. Hogan, A. Sen, J. Am. Chem. Soc. 118 (1996) 13111 – 13112.[4] K.Nakata,Y.Yamaoka,T.Miyata,Y.Taniguchi,K.Takaki,Y.Fujiwara,J.Organomet.Chem. 473 (1994) 329 – 334.[5] D.G.Piao,K.Inoue,H.Shibasaki,Y.Taniguchi,T.Kitamura,Y.Fujiwara,J.Organomet.Chem. 574 (1999) 116 – 120.[6] M. Asadullah, T. Kitamura, Y. Fujiwara, Angrew. Chem. Int. Ed. 39 (2000)2475 – 2478.[7] T. Osako, E.J. Watson,A. Dehestani, B.C. Bales, J.M.Mayer, Angew Chem, Int.Ed. 45(2006) 7433 – 7436.[8] G. Suss-Fink, S. Stanislas, G.B. Shul'pin, G.V. Nizova, Appl. Organometal. Chem. 14(2000) 623 – 628.[9] G.V. Nizova, B. Krebs, G. Suss-Fink, S. Schindler, L. Westerheide, L.G. Cuervo, G.B.Shul'pin, Tetrahedron 58 (2002) 9231 – 9237.[10] G.B. Shul'pin, G.V. Nizova, Y.N. Kozlov, L.G. Cuervo, G. Suss-Fink, Adv. Synth. Catal.346 (2004) 317 – 332.[11] Q. Yuan, W. Deng, Q. Zhang, Y. Wang, Adv. Synth. Catal. 349 (2007) 1199 – 1209.[12] A.B. Sorokin, E.V. Kudrik, D. Bouchu, Chem. Commun. (Camb). 22 (2008)2562 – 2564.[13] G.V. Nizova, G. Suss-Fink, G.B. Shul'pin, Tetrahedron 53 (1997) 3603 – 3614.[14] A.O. Kuzmin, G.L. Elizarova, L.G. Matvienko, E.R. Savinova, V.N. Parmon,Mendeleev Communications Electronic Version 6 (1998) 207 – 208.[15] J.S.Min, H.Ishige, M. Misono, N.Mizuno,Journal ofCatalysis 198 (2001) 116 – 121.[16] Y. Seki, J.S. Min, M. Misono, N. Mizuno, J. Phys. Chem. B 104 (2000) 5940 – 5944.[17] S. Han, D.J. Martenak, R.E. Palermo, J.A. Pearson, D.E. Walsh, Journal of Catalysis136 (1992) 578 – 583.[18] D.E. Walsh, S. Han, R.E. Palermo, J. Chem. Soc., Chem. Commun. (1991) 1259 – 1260.[19] K.X. Wang, H.F. Xu, W.S. Li, C.T. Au, X.P. Zhou, Applied Catalysis A: General 304(2006) 168 – 177.[20] Y. Fan, M. Ding, X. Bao, Catal. Lett. 130 (2009) 286 – 290.[21] H.K. Farizul, N.A.S. Amin, D. Suhardy, A.S. Saiful, S.M. Nazry, Jurnal Teknologi,Keluaran Khas. Dis 47 (2007) 55 – 67.[22] J.B. Conant, C.O. Tongberg, Converse Memorial Laboratory of Harvard University,Cambridge, 1930 (http://www.jbc.org/ ).[23] I. Bar-Nahum, A.M. Khenkin, R. Neumann, J. Am. Chem. Soc. 126 (2004)10236 – 10237. 1002003000481216020406080 Selectivity (%)Yield (%)    S  e   l  e  c   t   i  v   i   t  y   t  o   H   C   O   O   H   (   %   )   Y   i  e   l   d  o   f   H   C   O   O   H   (   %   ) Amount of H 2 O 2  (mmol) Fig. 3.  Yield and selectivity of HCOOH as a function of the amount of H 2 O 2 . Scheme 1.  Proposed pathway for synthesis of formic acid from partial oxidation of methane.1200  A.K.M.L. Rahman et al. / Catalysis Communications 12 (2011) 1198 – 1200
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