IRSAPS Bulletin Vol 1 Issue 2 by J Sharma

Vol. 1, Issue 2

IRSAPS Bulletin

May-August 2011 http://irsaps.org

(A periodical published by Indian Research Scholars’ Association for Promoting Science)

Orbital Pictures of Carbon Nanotube

 Multi-walled Carbon Nanotubes  Alkylation of benzene with ethylene over acidic zeolite  Metal-organic framework (MOF) as electrocatalyst for fuel cell application Copyright @ Indian Research Scholars’ Association for Promoting Science, 2011. All rights reserved. Reproduction in whole or in part for any other purpose except for the educational interest is prohibited without the prior written consent. Contact publication and distribution department for further details. Visit: http://irsaps.org

Scope and Aim of Indian Research Scholars’ Association for Promoting Science Indian Research Scholars’ Association for Promoting Science (IRSAPS) is created to spread brotherhood through scientific research to every part of our country! The aim of the association is to help all research scholars who are desperate to excel in scientific research and to provide necessary support, share knowledge with proper attention in terms of research articles, infrastructure, and guidance. The association will take appropriate steps to highlight and encourage the achievements of talented research scholars.

Since India’s future equally depends on the knowledge pool in basic sciences, the association will endeavor to encourage budding researchers by encouraging them at school and higher secondary levels through the network of volunteers. Though the association is a nonprofit organization, it will encourage entrepreneurship among the members through scientific innovations. The scope of the forum will change with time depending on the requirements of the forum members. No discrimination will be tolerated in terms of regional, ethnic, or any other means. The association will have a governing body constituted by at least one member from each state of India (depending on the availability of volunteers). Any organizational dispute arising in due course will be sorted out through a democratic voting process among the governing body members. For a general dispute, all members of the association will be invited to register their opinion through opinion polls. IRSAPS will carry out online voting only.

Date of establishment: 14 August 2010 Total number of members (September 2011): 337

IRSAPS Bulletin 2011, Vol. 1, Issue 2

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IRSAPS Bulletin Volume 1, Issue 2 Issue Editor: Prof. R. C. Deka Release date: 10th October 2011 This journal is published by Indian Research Scholars’ Association for Promoting Science. To join IRSAPS, please visit: http://irsaps.org 1st Issue: January-April 2nd Issue: May-August 3rd Issue: September-December Statement on journal policy: IRSAPS Bulletin does not have a peer review policy for articles (any changes in this regard will be notified). It is a free open source online journal.

Cover page details: Single walled carbon nanotubes, HOMO and LUMO orbital pictures of carbon nanotube are shown. SWCNT could be used for drug delivery purposes. An antituberculosis drug molecule isoniazid is used for controlled release. Courtesy: Nabanita Saikia and Ramesh C. Deka, Department of Chemical Sciences, Tezpur University, Assam, INDIA. Contact person: Ramesh C. Deka E-mail: ramesh@tezu.ernet.in.

©Indian Research Scholars’ Association for Promoting Science, 2011. All rights reserved. Reproduction in whole or in part of this journal for any other purpose except for the educational interest is prohibited without the prior written consent.

IRSAPS Bulletin 2011, Vol. 1, Issue 2

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Editorial Board Members* 1. Dr. Jadab Sharma

International Advanced Research Institute for Powder

Cookson India Research Centre, Cookson Electronics Bangalore, India E-mail: jadab.s@gmail.com

Metallurgy and Material Processing (ARCI) Balapur, Hyderabad-500005 India E-mail:suresh@arci.res.in

2. Dr. Amit K. Chattopadhyay School of Engineering and Applied Sciences Mathematics (NCRG) Aston University

8. Prof. Ramesh C. Deka Department of Chemical Sciences Tezpur University, Tezpur 784 028 Tel: +91-3712-267008 (extension 5058) E–mail: ramesh@tezu.ernet.in

6. Dr. Sonika Saddar Pulmonary and Vascular Biology

Birmingham B4 7ET, UK E-mail: akchaste@gmail.com

Department of Pediatrics UT Southwestern Medical Center 5323 Harry hines Blvd Dallas, TX 75235 USA E-mail: sonikasaddar@gmail.com

3. Dr. Santosh B. Chavan Jay Biotech, Pune, India E-mail: sbchavan23@gmail.com 4. Dr. Sanjeev Malik

7. Dr. Ujjal Gautam

Department of Mathematics, Indian Institute of Technology, Roorkee, India E-mail: malikdma@gmail.com

ICYS-MANA Research Fellow National Institute for Materials Science, 1-1, Namiki, Sukuba, Japan-3050044 E-mail: ujjalgautam@gmail.com

5. Dr. M. Buchi Suresh

9. Prof. Chandravanu Dash Center for AIDS Health Disparities Research Department of Cancer Biology and Biochemistry Hubbard Hospital BldgCAHDR Meharry Medical College School of Medicine 1005 Dr. DB Todd Jr Blvd, Nashville, TN 37208, USA E–mail: cdash@mmc.edu

Center for Ceramic Processing

Publication and Distribution 1. Dr. Amit Sharma (North Zone) Unite de Catalyse et de Chimie du Solide (UCCS) UMR CNRS 8181 Ecole Centrale de Lille, Cité Scientifique, BP 48 Villeneuve d'Ascq, Lille, Nord, FRANCE 59651 E-mail: amitfrance@gmail.com

2. Mr. Qureshi Ziyauddin (West Zone)

3. Dr. Rupam Jyoti Sarma (East Zone) Department of Chemistry Gauhati University Gopinath Bordoloi Nagar Guwahati, Assam, India E-mail: rup.sarma@gmail.com

4. Dr. P. R. Naren (South Zone)

Institute of Chemical Technology Nathalal Parekh Marg, Matunga, Mumbai 400019, Maharashtra, India

Senior Assistant Professor (SAP) School of Chemical and Biotechnology (SCBT) Shanmugha Arts, Science, Technology and Research Academy (SASTRA)

E-mail: qureshi.ziya@gmail.com

Sastra University, Tirumalaisamudram, Thanjavur, Tamilnadu 613 402 INDIA E-mail: naren_pr@yahoo.com

* List is incomplete

IRSAPS Bulletin 2011, Vol. 1, Issue 2

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Contents

1. Editorial

2. Nobel Prize (Science) 2011

3. Effect of Temperature Modulation on the Synthesis of Multi-walled Carbon Nanotubes:

Morphological Evolution by Bhalchandra A. Kakade

4. Alkylation of benzene with ethylene over acidic zeolite: a quantum chemical study by

Paritosh Mondal and Ramesh C. Deka

5. Metal-Organic Framework as Electrocatalyst for Fuel Cell Application by Pankaj

Bharali

6. Science cartoons by Sumanta Baruah

IRSAPS Bulletin 2011, Vol. 1, Issue 2

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ÂŠ IRSAPS

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Few lines from the editorial desk……………! The year 2011 is being celebrated as

and metal-organic-framework and a science

the international year of chemistry for the

cartoon. In the first article felt-like mats (6-7

achievements of Madeame Curie and her

micron thick) of multiwalled carbon nanotubes

contributions to the well-being of humankind.

(MWCNTs) wrapped into scrolls have been

This year marks the 100th anniversary of the

synthesized. The texture and the structure of

Noble Prize awarded to Madame Curie and the

the carbon nanotubes are determined using the

contributions of women to science have been

electron microscopy techniques. In the second

celebrated worldwide.

The year is also the

article alkylation of benzene with ethylene over

150th birth anniversary of the great Indian

acidic zeolites has been investigated using

Scientist

Ray.

theoretical methods and the third article

Several conferences, workshops and seminars

presents a brief review on the fate of metal-

have been organized to mark these celestial

organic-frameworks (MOF) as heterogeneous

events. Special emphasis has also been given in

catalyst and electrocatalyst for fuel cell

popularizing science. At this juncture it is a

applications.

Acharya

Prafulla

Chandra

welcome move that IRSAPS has started this

In the forthcoming issues we plan to publish

journal which is providing a platform to

articles covering broader subject areas.

budding researchers from all fields of science,

look forward to active participation from the

engineering and medicine for publishing their

scientific community. Your contributions will

research work and for the exchange of

be a source of encouragement in our endeavor.

knowledge.

-Ramesh C. Deka

In this issue there are three scientific articles covering multiwalled carbon nanotube, zeolites 1

Following the science community's fallout, Shechtman's quasicrystals have been seen in mineral samples from a Russian river and a certain form of steel, where the crystals reinforce the material like armour. Quasicrystals are currently being investigated for use in products like frying pans and diesel engines.

Nobel Prize (Science) 2011 Courtesy: http://www.nobelprize.org Total 7 researchers in the fields of chemistry, physics and medicine received the 2011 Nobel Prize for their contributions to the sciences. Chemistry

Physics The 2011 Nobel Prize for physics has been awarded to three U.S.-born scientists for "the discovery of the accelerating expansion of the Universe" through observations of exploding stars. The Nobel Prize in Physics 2011 was divided, one half awarded to Saul Perlmutter, the other half jointly to Brian P. Schmidt and Adam G. Riess "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae".

Dan Shechtman The 2011 Nobel Prize for Chemistry has been awarded to Israeli scientist Daniel Shechtman for the discovery of "quasicrystals." The Royal Swedish Academy of Sciences said Shechtman's work has "altered how chemists conceive of solid matter." It said Shechtman showed that the atoms in a crystal could be packed in a pattern that could not be repeated,'' contrary to previous thinking. In the world of chemistry, Irsraeli researcher Dan Shechtman's discovery ran so counter to the laws of nature that he faced years of controversy, attacks from other scientists and was even asked to leave his research group for bringing disgrace to the team. But now, the 70-year-old chemist from Technion Israel Institute of Technology in Haifa received 2011's Nobel Prize in Chemistry for his disruptive discovery of seemingly impossible atomic structures called quasicrystals. Back in 1982 he popped a crystal under an electron microscope and peered deep into the material's structure. There he found atoms packed tightly in patterns that never repeated themselves. Before this, atoms were believed to be packed inside crystals in symmetrical patterns that were repeated periodically over and over again.

Saul Perlmutter

Brian P. Schmidt

Adam G. Riess The Royal Swedish Academy of Sciences said the discoveries by the scientists "have helped to unveil a universe that to a large extent is unknown to science" Three astrophysicists might have given us a sneak peek at the final fate of the universe, when they discovered that it's expanding at an everaccelerating rate. If the expansion continues to speed

up, the cosmological bodies will drift away from each other into a cold expanse of isolated galaxies, until the temperature reaches absolute zero. According to the Nobel jury, the universe "will end in ice, if we are to believe this year's Nobel Laureates in Physics". They used ground and space telescopes to hunt down type Ia supernovae -- the explosion of a star as heavy as the Sun but as small as the Earth, were the resulting bang can emit as much light as a whole galaxy. This allowed the different teams to infer the amount of expansion of the universe at different times in its 13-or-so billion year history, and discovered that the universe's expansion -- contrary to expectations of the time -- was speeding up. That increasing acceleration has been blamed on dark energy, an enigmatic presence in the universe that remains a mystery to physicists.

They revolutionized our understanding of the immune system. Bruce Beutler and Jules Hoffmann won for their discoveries concerning the activation of innate immunity, while Ralph Steinman discovered a new cell type that controls adaptive immunity. "The three were lauded for their work on the body's complex defence system in which signalling molecules unleash antibodies and killer cells to respond to invading microbes," the Nobel jury said in a statement. Sadly, Canadian researcher Ralph Steinman received his Nobel Prize posthumously. He died from pancreatic cancer on Friday 30 September, days before his prize was announced on Monday 3 October 2011.

Medicine

(Courtesy: Online news http://www.nobelprize.org.)

Bruce A. Beutler

Compiled by: Jadab Sharma source

and

Jules A. Hoffmann

Ralph M. Steinman The Nobel Prize in Physiology or Medicine 2011 was divided, one half jointly to Bruce A. Beutler and Jules A. Hoffmann "for their discoveries concerning the activation of innate immunity" and the other half to Ralph M. Steinman "for his discovery of the dendritic cell and its role in adaptive immunity".

Effect of Temperature Modulation on the Synthesis of Multi-walled Carbon Nanotubes: Morphological Evolution Bhalchandra A. Kakade* Chemical Resources Laboratory, Tokyo Institute of Technology, R1-17, 4259 Nagatsuta, Midori-ku,Yokohama, Japan 226-8503. Fax: (+81) 45-924-5254; Tel: (+81) 45-924-5253. E-mail: kakade.b.aa@m.titech.ac.jp KEYWORDS: Carbon nanotubes, scrolled mats, thermal expansion effects Felt-like mats (6-7 micron thick) of multiwalled carbon nanotubes (MWCNTs) wrapped into scrolls have been synthesized by chemical vapor deposition from a toluene-ferrocene mixture using a temperature ramp from 680 oC to 550oC in hydrogen-argon atmosphere. The texture and the structure of the carbon nanotubes are determined using the electron microscopy techniques. Thermal expansion effects and scroll mechanisms are proposed based on the electron microscopy results. 1. Introduction After a landmark report by Iijima1, Carbon nanotubes (CNTs) have attracted tremendous attention from scientific community due to their unique structural, electronic, mechanical and nonlinear optical properties2, tunable at various levels. Consequently, CNTs have been considered as promising materials for a variety of applications such as field effect transistor3-5, random access memory6, and atomic force microscopy7. While most of the efforts are focused in the production of single walled carbon nanotubes (SWCNTs), a single graphene tube, there are enumerable difficulties to reproduce them into proper geometry, high purity and consistent chirality since their synthesis needs an accurate control of many parameters like temperature, nature and extent of catalysts, partial pressures of gaseous reactants, etc. On the other hand, considerable progress has been made in the synthesis of multiwalled carbon nanotubes (MWCNTs), which comprise several graphene sheets leading to a stiffer and robust carbon structure, due to its reproducibility at relatively low temperatures. One of the important steps during CNT synthesis is the precise control of temperature of the reaction zone because of its direct influence on the CNT diameter-distribution8. Conventional synthetic procedures for CNT growth by chemical vapor deposition (CVD) and its variants enable researchers to deal at relatively lower temperature range, viz. 500-1200oC, using various hydrocarbons in presence of suitable catalysts9-12. For example, Andrews et al.10 have reported an efficient and selective growth of aligned MWCNTs at 675oC rather than at elevated temperature although the hydrocarbon conversion is only 25%. Similarly, Singh et al. have suggested a perpendicular growth of aligned MWCNTs on micron sized quartz flakes at 760oC using a toluene-ferrocene mixture13. More interesting is a report on tree-like structures of carbon by Ajayan et al, achieved in flash CVD method using methane as a source, demonstrating the effect of rapid heating and cooling cycles14 on CNT morphology. In addition, maintaining a constant temperature gradient throughout the duration of synthesis has been proven to be effective to obtain carbon nanostructures with varying morphology like trees14 cones15, onions16, and filaments17. A ―kite-mechanism‖ has been proposed recently in a ‗fast-heating‘ CVD process in order to synthesize long and oriented SWCNTs on a surface, where different heating rates on the surface and in the surroundings plays a crucial role for the growth18. On the contrary, despite the well-demonstrated importance of temperature in controlling the morphology, no report exists on the synthesis of carbon nanostructures under a decreasing temperature ramp. Here, we report a method for synthesizing highly pure scrolls of MWCNTs, by subjecting a reaction mixture of toluene and ferrocene in hydrogen-argon environment to a decreasing temperature gradient facilitating the formation of scrolls of felt-like carbonaceous layers. These few micron thick layers comprising intertwined MWCNTs (outer diameter of 10-150 nm) with a very low

catalyst content (less than ca. 2 wt%) have been characterized by many techniques Based on these results, a growth mechanism has been discussed in order to explain the effect of the decreasing temperature gradient on the formation of the scroll morphology. 2. Experimental Section

Figure 1 (a). Low resolution scanning electron micrographs of ―as synthesized‖ scrolls of MWCNTs; (b) SEM of magnified portion of (a).

CVD set-up, capable of attaining 1100oC, comprising of a dual zone furnace was used to synthesize the scrolls of MWCNTs, similar to the one used by Andrews et al10. In the beginning, a stream of argon was passed through the quartz tube (i.d. 34 mm) at a flow rate of 100 sccm to drive out all impurities till both the zones achieved their respective temperatures (preheater zone; 200oC, main zone; 680oC). A solution of approximately 6 mol% of ferrocene in toluene was used as a precursor. This precursor solution was injected at a flow rate of 1.5 ml/hr for 16 minutes into the first zone of the dual furnace, maintaining the temperature at 200oC to ensure the complete sublimation of ferrocene and vaporization of toluene. The sublimed precursors were then passed into the main zone by means of a mixture of hydrogen and argon (ratio 1:9) as a carrier gas with a flow rate of 200 sccm. The temperature of the main furnace was programmed such that it starts decreasing as soon as the vaporized gases entered the reaction zone without any external cooling aid. The scrolls were obtained by subjecting the system to a temperature ramp from 680-

550oC at a natural cooling rate (~8oC/min). Almost 5 cm of the inner wall of quartz tube was spread by plenty of needles like (length: 0.5-2.0 cm, diameter: ~10 m) scrolls of MWCNTs. 3. Results and discussion 3.1. Electron microscopy study and growth mechanism 3.1.1. Topology Fig. 1(a) shows a low magnification SEM image of as-synthesized scrolls of MWCNTs with an average thickness of 6-7 m for the mats mounted on a copper substrate using a conducting carbon tape. Fig. 1b is a magnified image of the cross-section edge of the scrolled mat. Fig. 2 (a-b) are Field Emission Scanning Electron Microscopy (FESEM) images of the multi-wall carbon nanotubes located at the bottom and at the top of the scrolled mat. These Figures show a variation in the MWCNT diameter distribution between the bottom and the top of the mat (compare Figures 2a and b). A large dispersion in outer diameter of the MWCNTs is evident, varying generally from 10 to 150 nm either at the top or at the bottom of the scrolled mat. However, it appears that the average diameter of the MWCNTs at the top is lower than that at bottom. In addition, the FESEM images show that the large diameter MWCNTs are often distorted at the surface perhaps, due to dynamic temperature variations. The nut-like ends with catalyst at the tip indicates that the MWNCTs grow following the ―tip growth‖ mechanism.

Figure 3. Low magnification TEM micrographs showing the MWCNTs constituting the scrolled mat sample; catalyst particles are abundantly present.

High-resolution transmission electron microscopic (HRTEM) investigation (Fig 4 a-c) shows that majority of the MWCNTs exhibit a herringbone texture in which all graphenes are oblique to the tubular axis. The angle between the graphene orientations and the nanotubes axis is generally small and is of about ~5-10°. It indicates that the MWCNT surfaces are constituted of successive and free graphene edges as shown in Fig 4. However, no significant difference has been observed between the bottom and the top of the MWCNTs according to the graphene display.

Figure 2. FESEM images of the multi-wall carbon nanotubes located at the (a) bottom surface of the scrolled mat (b) top of the scrolled mat cross-section

3.1.2. Texture Fig. 3 shows low magnification TEM images of as-synthesized MWCNTs after ultrasonication in toluene for 10 minutes. A detailed investigation of a scrolled mat sample reveals a comparatively wide diameter distribution of MWCNTs (10-150 nm). However, these MWCNTs exhibit always a central tube with a diameter ranging from 4 to 50 nm. Generally, the diameter of the inner tube varies locally within the same MWCNT. These images also reveal that the majority of the nanotubes contain Fe at the tip in addition to their presence in the walls of very few tubes. The purity of the sample is found to be high based on TEM observations by scanning almost every region of the sample on the copper grid.

Figure 4 (a-c). HRTEM images showing examples of MWCNTs, forming the scrolled mat sample, exhibiting the herringbone texture.

On the other hand, in many cases and especially concerning MWNTs exhibiting large diameters (Ф> 50 nm), graphenes are found to be frequently distorted close to the MWNT surfaces as it is shown in Figure 5. Performing the high resolution mode on the catalyst particles encapsulated at the tip of the MWNTs allowed, in many cases, observing the lattice fringes of the catalysts as it is shown in Figure 6. In addition, the catalyst particles contributing to the growth of the large diameter MWNTs (Ф>40 nm) are generally facetted (see Figure 6). However, those contributed to the growth of the small diameter MWNTs (Ф<40 nm) are generally not as it is shown in Figure 6. 5

2a-b), since the cracking rate of the carbon species is directly proportional to the prevailing temperature. This will make the top surface of the mats smaller than the bottom surface that will favor the form of scrolls.

Figure 5 (a-c). HRTEM micrographs showing the distortion of the graphenes close to the large diameter MWCNT surfaces.

3.1.4. Growth and scroll mechanisms Usually two growth mechanisms have been proposed in the literature for the nanotube growth in CVD process, viz. ‗top growth‘ and ‗root growth‘, where, latter mechanism has been well studied in case of both SWCNTs and MWCNTs19. While, the former one well demonstrated for certain MWCNTs20-22. In the present case, a ‗top growth‘ is demonstrated based on electron micrograph shown in Figure 2c, where the presence of Fe at the tip of the nanotubes clearly supports the growth hypothesis. In case of scrolls of MWNTs, the intertubular interaction (due to the decreasing effect of thermal expansion coefficient with time) would play an important role in enhancing the stability of the rolling structure. In order to explain the scrolling feature observed, under our experimental conditions, a tentative mechanism is proposed. Two effects are presumably responsible for the scrolling feature: (i) the variation in the average diameter of nanotubes between the top and the bottom (ii) the variation in the distance between graphenes which is induced by the variation in the thermal expansion coefficient (TEC) between the top and the bottom due to temperature modulation. Both effects are induced by the specific cooling down from 680 to 550°C during the MWCNT growth. Since our growth is predominantly through the tip growth mechanism, the bottom of the MWCNT mat corresponding to the growth already had taken place at the early stages, (i.e. at the highest temperature) means that the MWCNT diameters are at maximum due to thermal expansion coefficient (TEC). As long as MWCNTs proceed towards the top of the mat corresponding to the end of the growing event, (i.e. which had occurred at the lowest temperature), these tubes could get closer and closer to each other because of the decreasing effect of TEC. Then, because the amount of tubes is obviously the same at top and at bottom of the mat, the effect of TEC upon the decreasing temperature ramp makes the top surface of the mat to be smaller inducing the scrolling effect. On the other hand, because, the initial growth of nanotubes at the bottom of mat occurs at a higher temperature than that at the top, the MWCNTs constituting the bottom will exhibit higher average diameter compared to those at the top (for example, see Figures

Figure 6 (a-b). Lattice fringes images of the catalyst particles located at the MWCNT tips; catalyst particles are not facetted for MWCNTs exhibiting ~20 nm in diameter.

4. Conclusions MWCNTs in the form of microsized felt-like mats have been synthesized from a mixture of toluene and ferrocene using a temperature ramp from 680°C to 550°C in H2 - Ar atmosphere. Since the properties of carbon nanostructures depend on the local reaction temperature, it is possible to tune their morphology by the simple approach of using a controlled temperature gradient. These experimental conditions would result in high yield and pure MWCNTs in the form of thick scrolls. HRTEM microscopy investigations show that the MWCNTs exhibit the herringbone texture. The nanotubes grow following the "tip growth" mechanism because catalyst particles are always present at the MWCNT tips. On the other hand, the thermal expansion effects have been shown to be responsible for the possible scrolling mechanism. Acknowledgement Author sincerely thanks Dr. K. Vijayamohanan for his constant encouragement and valuable scientific discussions. Author also thanks Dr. Hatem Allouche for the FESEM and HRTEM.

References

1. Iijima S. Nature 1991, 354, 56- 58. 2. Baughman, R. H.; Zakhidov, A. A.; Heer, W. A. Science 2002, 297, 787- 92. 3. Heer, W. A.; Chatelain, A.; Ugarte, D. Science 1995, 270, 1179- 80. 4. Shim, M.; Javey, A.; Kam, N.; Dai, H. J. J. Am. Chem. Soc. 2001, 123, 11512- 13. 5. Wang, Q. H.; Sethur, A. A.; Lauerhaas, J. M.; Dai, J. Y.; Seeling, E. W.; Chang, R. P. Appl. Phys. Lett. 1998, 72, 2912- 13. 6

6. Rueckes, T.; Kim, K.; Joselevich, E.; Tseng, G. Y.; Cheung C. L.; Lieber, C. M. Science 2000, 289, 94- 97. 7. Hafner, J. H.; Cheung, C. L.; Woolley, A.T.; Lieber, C. M. Progr. Biophys. Mol, Bio 2001, 77, 73- 110. 8. Singh, C.; Shaffer, M.; Kinloch, I.; Windle, A. Physica B: Condensed Matter 2002, 323, 339- 40. 9. Dai, H.; Kong, J.; Zhou, C.; Franklin, N.; Tombler, T.; Cassell, A.; et al., J. Phys. Chem. B 1999, 103, 11246- 55. 10. Andrews, R.; Jacques, D.; Rao, A. M.; Derbyshire, F.; Qian, D.; Fan, X.; et al., Chem. Phys. Lett. 1999, 303, 467- 74. 11. Delzeit, L.; Nguyen, C. V.; Chen, B.; Stevens, R.; Cassell. A.; Han, J. J. Phys. Chem. B 2002, 106, 5629- 35. 12. Hoffmann, S.; Ducati, C.; Robertson, J.; Kleinsorge, B.; Appl. Phys. Lett. 2003, 83, 135- 37. 13. Singh, C.; Shaffer, M. S.; Kozial, K. K.; Kinloch, I. A.; Windle, A. H. Chem. Phys. Lett. 2003, 372, 860- 65. 14. Ajayan, P. M.; Nugent, J. M.; Siegel, R. W.; Wei, B.; KohlerRedlich, Ph. Nature, 2000, 404, 243- 43. 15. Blanck, V. D.; Polyakov, E. V.; Kulnitskiy, B. A.; Nuzhdin, A. A. Alshevskiy, Y. L. Thin Solid Films 1999, 346, 86- 90.

16. Ugarte, D. Nature 1992, 359, 707- 9. 17. Motojima, S.; Kawaguchi, M.; Nozaki, K.; Iwanaga, H. Appl. Phys. Lett. 1990, 56, 321- 23. 18. Huang, S.; Woodson, M.; Smalley, R.; Liu, J. Nano Lett. 2004, 4, 1025-1028. 19. Li, Y. M.; Kim, W.; Zhang, Y. G.; Rolandi, M.; Wang, D. W.; Dai, H. J. J. Phys. Chem. B 2001, 105, 11424-11431. 20. Baker, R. T. K. Carbon 1989, 27, 315. 21. Sinnott, S. B.; Andrews, R.; Qian, D.; Rao, A. M.; Mao, Z.; Dickey, E. C.; Derbyshire, F. Chem. Phys. Lett. 1999, 315, 25-30. 22. Han, J.; Yoo, J. -B.; Park, C. Y.; Kim, H. -J.; Park, G. S..; Yang, M.; Han, I. T.; Lee, N.; Yi, W.; Yu, S. G.; Kim, J. M. J. Appl. Phys. 2002, 91, 483-486.

Alkylation of benzene with ethylene over acidic zeolite: a quantum chemical study Paritosh Mondal*1, Ramesh C. Deka, 2 1

Department of Chemistry, Assam University, Dargakona, Silchar – 788 011, INDIA

Department of Chemical Sciences, Tezpur University, Napaam, Tezpur – 784 028, INDIA

KEYWORDS: Alkylation of Benzene, zeolites, epoxide A quantum chemical investigation of alkylation of benzene with ethylene over acidic zeolites has been performed using PW91/DNP method. A 5T cluster model has been used to represent the zeolite. Stepwise and concerted mechanisms of alkylation reaction of benzene with ethylene are considered. In the stepwise mechanism, ethoxide intermediate is first formed due to adsorption of ethylene on Brönsted site of zeolite followed by the reaction of ethoxide with benzene molecule to produce ethylbenzene. In concerted mechanism, alkylation of benzene takes place in single step without prior formation of ethoxide. The concerted mechanism has the activation energy in between the two activation energies of stepwise mechanism. 1. Introduction Ethylbenzene is an important raw material in petrochemical industry for the manufacture of styrene, which is one of the most important industrial monomers. Worldwide capacity of ethylbenzene production is about 23 million metric tons per year.1 Conventional methods for industrial production of ethylbenzenes follow the Friedel-Crafts alkylation reaction with AlCl3 or supported H3PO4 as catalysts. Unfortunately, these commercial processes present serious ecological problems such as corrosion and disposal of spent harmful catalysts. Now-a-days zeolite-type catalysts mostly replace the aluminum halides, as they offer environmental and economical advantages. The microporous structure of zeolites provides large internal surface area and most important selectivity effects are related to diffusion of reactants and products inside the pore and steric constrains of transition states. In alkylation of benzene, zeolite based catalysts also offer additional advantage to suppress formation of undesired diisopropylbenzenes, n-propylbenzenes and chlorinated products. Experimentally these reactions have been studied extensively.2-7 From industrial point of view, elucidation of reaction mechanism of benzene alkylation on zeolite catalysts is of great interest. However, the reaction mechanism of alkylation of aromatics with short-chain olefins on zeolites is not yet clearly understood. Venuto et al.8 and Weitkamp9 suggested that alkylation of benzene with ethylene over acidic faujasite and ZSM-5 zeolites followed Eley–Rideal mechanism. Corma et al.10 reported the Eley–Rideal mechanism for alkylation of benzene with propylene over MCM-22. While the Langmuir–Hinshelwood mechanism of alkylation of benzene with short-chain olefins has also been reported.11,12 Recently, Smirniotis and Ruckenstein13 suggested that the size of the pores of zeolites in combination with the size of the aromatics and the alkylating agents could regulate the alkylation mechanism. In case where the size of the aromatic molecule is comparable to the pores of the zeolite, alkylation proceeds via the Langmuir–Hinshelwood mechanism. For large-pore zeolites such as faujasite and beta zeolites, alkylation occurs via the Eley– Rideal mechanism. To understand the reaction mechanism, theoretical studies can offer a practical tool that provides insight to the reaction mechanism complementing experimental investigations or, in certain cases, offer an understanding that is not possible by experimental investigations. Numerous theoretical models, including the periodic calculations, have been proposed to study the crystalline zeolites.14-20 The large number of atoms present in the unit cell of zeolites makes the use of periodic ab initio calculations computationally too expensive and even impractical when very large zeolites are concerned. Therefore, electronic properties of zeolites are usually modelled with quantum chemical methods

for relatively small clusters where only the most important part of zeolites is focused.21,22 It has been proved that the course of acid zeolite catalysed reactions can be studied qualitatively using cluster approximations.23,24 The cluster approach is well-suited to describe local phenomena such as interactions of molecules with active sites or bond breaking and bond formation processes which allows use of high quality theoretical methods on it. Limtrakul et al.25, in their recent studies, investigated reaction mechanism of ethylation of benzene over acidic faujasite zeolites using quantum chemical methods. Clark et al26 calculated the overlap between Fukui functions of the aromatic and the electrophilic alkyl group as a measure for the tendency of electrons to transfer from the aromatic to the alkyl group. This gives an indication for the probability of reaction. In this work, density functional calculations were performed to study stepwise and concerted mechanism of alkylation of benzene with ethylene on a 5T cluster of acidic zeolites. 2. Computational Details 5T model cluster were taken from the lattice structure of fauzasite to represent active site of the zeolite. Central silicon atom in the zeolite cluster is substituted by an aluminum atom, and a proton is added to one of the oxygen atoms bonded directly to the aluminum atom to preserve electrostatic neutrality. The dangling bonds are saturated with hydrogen atoms. All geometry optimizations were performed without imposing any geometrical constrain. During the geometry optimizations, we look for a local minimum for reactants and products and for a first order saddle point for transition structures. On stationary states, vibrational frequencies were calculated to ensure that the obtained structures have correct number of imaginary frequencies: none for minima and one for transition structures. All calculations were carried out with DMol3 program package, using PW91 functional and DNP basis set. 3. Results and discussion 3.1. Alkylation of benzene with ethylene We performed DFT calculation to study the reaction mechanism of ethylation of benzene with acidic zeolites. The reaction can be considered to occur either by stepwise mechanism or concerted mechanism. In this work thermodynamics of these two mechanisms for the reaction is studied. 3.1.1. Stepwise mechanism for benzene alkylation with ethylene In this pathway, ethylene molecule is initially adsorbed on the acidic site of the zeolite, and an ethoxide intermediate is formed. Subsequently, involves the interaction of benzene with ethoxide intermediate, ethoxide leave the zeolite wall and a C―C 8

bond is formed, resulting in the formation of ethylbenzene adsorbed on acidic zeolite. Figure 1 and Figure 2 show the stationary points for the formation of ethoxide from ethylene and reaction of ethoxide intermediate with benzene, respectively. The selected geometric parameters of ethoxide intermediate formation and its reaction are summarized in Table 1. The interaction of ethylene double bond with Brönsted acid site of zeolite results in the formation of π-complex. Due to adsorption of ethylene molecule on zeolite, minor changes of geometrical parameters are noticed. It is seen from Table 1 that the acidic O1―H1 bond distance is increased from 0.97 to 0.99 Å and C―C double bond distance of ethylene increases from 1.33 to 1.34 Å. This increase of bond distances indicates that adsorption of ethylene molecule on acidic zeolites weakens the C―C double bond and O1―H1 bond. A smaller deviation of Si1―O1―Al and Si2―O2―Al bond angles are also noticed on adsorption of ethylene molecule. The adsorption energy of -7.08 kcal/mol is calculated for adsorption of ethylene molecule on acidic zeolite. Namunangruk et al.25 reported an adsorption energy of -8.73 kcal/mol using ONIOM method. In the transition state for ethoxide formation, the acidic proton is about halfway between the zeolite oxygen atom (O1) and ethylene carbon atom (C2). Simultaneously, the other carbon atom (C1) of ethylene is moving closer to another zeolite oxygen atom (O2) and hence, the C1―O2 bond of ethoxide is formed. At the transition state one imaginary frequency of -583 cm-1 is obtained which corresponds to movement of zeolite proton towards ethylene carbon (C2) atom while C2―C1 bond is increased from 1.34 to 1.41Å and carbon C1 is moving towards zeolite oxygen atom O2 to form a covalent bond. The activation energy for the protonation of ethylene is found to be 21.52 kcal/mol and apparent activation energy for this step is 14.44 kcal/mol. The activation energy as well as apparent activation energies reported by Namunangruk et al.25 are higher than that of our results (30.6 and 21.33 kcal/mol, respectively). However, the activation energy reported by Svelle et al.27 (23.18 kcal/mol) is in reasonable agreement with the activation energy calculated in this work. On formation of ethoxide, a significant geometrical change of the zeolite cluster is noticed. The Al―O1 bond distance is decreased from 1.93 to 1.72Å, while Al―O2 distance increased from 1.72 to 1.93Å (Table 1). The Si1―O1―Al and Si2―O2―Al angles are also change significantly due to the formation of ethoxide. In the next step, benzene is adsorbed on zeolite cluster next to ethoxide group, interacting with the cluster as well as ethoxide. The energy profile diagram of benzene adsorption on ethoxide is shown in Figure 2, and selected geometrical parameters are listed in Table 2. The transition state of ethylation of benzene involves formation of new C―C bond between ethoxide carbon (C1) and benzene carbon (CAr) and cleavage of CAr―HAr bond, giving the proton back to zeolite framework oxygen. The O2―C1 bond, which connects the ethoxide with the cluster, is gradually stretched and at the same time C1 moves closer to CAr carbon of benzene. The activation energy for the step in which new C―C bond is formed is calculated to be 53.52 kcal/mol. This value is higher than that of the corresponding energy reported by Namunangruk et al.25 3.1.2. Concerted mechanism for benzene alkylation with ethylene In the one step mechanism, ethylene and benzene molecules are co-adsorbed on acidic zeolite without the formation of ethoxide intermediate. The energy profile of the concerted reaction is shown in Figure 3 and the selected geometrical parameters of the reactant, product and the transition state are presented in Table 3. The reaction is initiated by co-adsorption of benzene and ethylene at the acid site of the zeolite. The acidic proton of

Figure 1: Energy profile diagram for the first step of the stepwise mechanism of ethylation of benzene.

Table 1: Optimized geometrical parameters of ethane adsorption, transition state and alkoxide intermediate. Distances are in Å and angles are in degrees

Parameters

Isolated cluster

Adsorption complex (R1)

Transition state (TS1)

Products (P1)

AlO1

1.95

1.93

1.83

1.72

AlO2

1.72

1.80

1.93

O1H1

0.97

0.99

1.38

4.41

H1C2

----

2.22

1.28

1.10

H1C1

----

2.20

2.14

2.16

C1C2

----

1.34

1.41

1.51

Si1O1Al

121.2

123.2

128.8

150.0

Si2O1Al

150.5

149.3

128.7

119.0

state. However, the adsorption energy reported by Namunangruk et al. 25 is higher than the values reported in this study. It is seen from Table 3 that no substantial deformation of the zeolites cluster is noticed on co-adsorption of ethylene and benzene. In the adsorption complex the C2―H1 and C1―H1 distances are found to be 2.22 Å. The benzene H-atom (HAr) that is closest to zeolite cluster is 2.60 Å far from the oxygen (O2) atom. This is in agreement with the empirically known van der Waals radii of the two atoms. In the transition state, the acidic proton H 1 attacks an ethylene carbon atom C2, and simultaneously, the other ethylene carbon C1 starts forming a bond with benzene carbon atom CAr. In the transition state O1H1 distance is elongated from 0.99 to 1.62Å. The C1―C2 distance increases from normal double bond distance 1.34 to 1.43 Å, and C 1―CAr distance changes from the noninteracting distance 3.92 to 2.17Å. This distance is much longer than the normal C―C double bond, but within the van der Waals distance. However, structure of benzene molecule does not significantly differ from that of the co-adsorbed structure except that the distance between HAr and O2 atoms. In the transition state, slight deviation of the structure of the zeolite cluster is noticed. The Al―O1 (1.81 Å) distance is found almost equal to Al―O2 (1.77Å). The transition state structure obtained in this study is similar to that reported by Arstad et al.29 and

Figure 2: Energy profile diagram for the second step of the stepwise mechanism of ethylation of benzene. Table 2: Optimized geometrical parameters of benzene-alkoxide adsorption complex, transition state and product adsorbed ethylbenzene. Distances are in Å and angles are in egrees. Parameters

Adsorption complex (R2)

Transition state (TS2)

Products (P2)

AlO2

1.92

1.78

1.73

AlO1

1.73

1.79

1.94

O2C1

1.48

2.85

3.87

C1C2

1.51

1.48

1.54

C1CAr

3.97

2.38

1.51

CArHAr

1.09

1.12

2.53

HArO1

2.55

1.85

0.99

Si1O1Al

144.8

146.0

122.1

Si2O1Al

118.6

140.2

146.0

zeolite is in interaction with π-electrons of ethylene. In the coadsorbed reactant, the ethylene is adsorbed on the acidic site and the benzene is adsorbed edge on. When the proton attacks C2 carbon of ethylene molecule, the C1 atom becomes electron deficient and thus undergoes an electrophilic attack with benzene. The co-adsorption energy evaluated for the reaction, 10.06 kcal/mol, is slightly higher than the values reported by Vos et al.28 and Arstad et al.29 (7.3 and 7.8 kcal/mol, respectively). The energy difference might be due to different geometries of the adsorbed

Table 3: Optimized geometrical parameters of isolated cluster, adsorption complex, transition state and products of concerted reaction of ethylation of benzene. Distances are in Å and angles are in degrees. Parameters

Adsorption complex (R)

Transition state (TS)

Products (P)

AlO1

1.93

1.81

1.73

AlO2

1.73

1.77

1.93

O1H1

0.99

1.62

2.88

H1C2

2.22

1.16

1.10

H1C1

2.22

2.30

2.19

C1C2

1.34

1.43

1.53

C1CAr

3.92

2.17

1.51

CArHAr

1.09

1.11

2.64

HArO2

2.60

2.47

0.99

Si1O1Al

122.3

139.5

144.9

Si2O1Al

143.3

137.4

122.3

slightly different from values reported by Vos et al.28 in which the acidic proton is completely attached to the ethylene molecule (C2―H1 distance is 1.09Å) and the distance between acidic proton (H1) and nearest oxygen atom (O1) on zeolite is 3.69Å. In this work, the corresponding distances are 1.16 and 1.62 Å. However, C1―CAr distance is found to be 2.17Å which is closer to the value of Vos et al., (2.12 Å) but smaller than that reported by Arstad et al. (2.39Å).

mind, one could argue that concerted mechanism should dominate the overall ethylation reaction. However, the stepwise mechanism could also contribute significantly, because activation energy for ethoxide formation is relatively low, hence, ethoxide forms easily. Thus every acidic site of the zeolite is occupied by an ethoxide in the first step of two-step mechanism blocking the route for concerted mechanism. Once the ethoxide intermediate is formed, the stability of the adsorbed benzene ethoxide adduct makes the reverse reaction more difficult and thus leads the formation of ethylbenzene.

4. Conclusions The ethylation of benzene over acidic zeolite has been investigated with 5T cluster using PW91/DNP method. Both concerted (simultaneous protonation and C―C bond formation) and stepwise (via ethoxide formation) mechanism have been evaluated. Formation of ethoxide has the lowest activation barrier of the investigated reaction steps. The barrier of the second step of the stepwise mechanism, i.e. C―C bond formation step, has the higher activation energy than the single barrier of the concerted mechanism. In the stepwise mechanism, ethylation starts with the protonation of the adsorbed ethylene which leads to the formation of ethoxide intermediate. The ethylation takes place due to the interaction of benzene and ethoxide intermediate. The second step is found to be the rate determining step with the activation energy of 53.52 kcal/mol. In the concerted mechanism ethylation of benzene takes place in a single reaction step with an activation energy barrier of 46.25 kcal/mol. Acknowledgement The author (PM) thanks Department of Science and Technology, New Delhi for financial support (SR/FT/CS-86/2010). Figure 3: Energy profile diagram for the concerted mechanism of ethylation of benzene. The activation energy for this reaction is calculated to be 46.25 kcal/mol. This energy is higher than the corresponding value reported by Vos et al. Error! Bookmark not defined. (31.6 kcal/mol), Arstad et al. Error! Bookmark not defined. (31.3 kcal/mol) and Namunangruk et al. Error! Bookmark not defined. (33.41 kcal/mol). The reaction ends when the bond between ethyl fragment and benzene is formed (C1―CAr =1.51Å) and benzene releases a proton, HAr, (CAr―HAr= 2.64Å) which form a bond with a bridging oxygen atom, O2, of the cluster (HAr―O2=0.99Å). Ethylbenzene is now adsorbed on an acidic site of the zeolite cluster. The adsorption energy of ethyl benzene on acidic zeolite is -34.82 kcal/mol. The adsorption energy of ethylbenzene reported by Namunangruk et al. Error! Bookmark not defined. is close to the value presented in this study. 3.1.2. Concerted versus stepwise mechanism for benzene alkylation with ethylene The first barrier of the stepwise mechanism (formation of ethoxide) is lower than the single barrier of the concerted mechanism, whereas the second barrier of the stepwise mechanism is considerably higher than that of the concerted mechanism. In the stepwise mechanism, the ethoxide formation has smaller activation energy of 21.52 kcal/mol and the reaction between ethoxide intermediate with benzene has the activation energy 53.52 kcal/mol, which is the rate determining step. However, the activation barrier for the concerted mechanism is 46.25 kcal/mol, which is in between the barriers of the stepwise mechanism. With this in

References

23. Degnan, Jr. T. F.; Smith, C. M.; Venkat, C. R.; Appl. Catal. A 2001, 221, 283. 24. Keading, W. W.; Young, B. L.; Chu C. –C., J. Catal. 1984, 89, 267. 25. Keading, W. W., J. Catal. 1985, 95, 512. 26. Cejka, J.; Wichterlova, B.; Bednarova, S. Appl. Catal. 1991, 79, 215. 27. Wichterlova, B.; Cejka, J., J. Catal. 1994, 146, 523. 28. Ivanova, I. I.; Brunel, D.; Nagy, J. B.; Derouane, E. G. J. Mol. Catal. A 1995, 95, 243. 29. Corma, A.; Martinez-Soria, V.; Schnoeveld, E. J. Catal. 2000, 192, 163. 30. Venuto, P.B.; Hamilton, L.A.; Landis, P.S. J. Catal. 1966, 5, 484. 31. Weitkamp, J. Int. Sympos. in Zeolite Catalysis, Siofok, Hungary 1985, 217. 32. Corma, A.; Martinez-Soria, V.; Schnoeveld, E. J. Catal. 2000,192, 163. 33. Morita, Y.; Matsumoto, H.; Kimura, T.; Kato, F.; Takayasu, M. Bull. Jpn. Pet. Inst. 1973, 15, 37. 34. Becker, K. A.; Karge, H.G.; Streubel, W.-D. J. Catal. 1973, 28, 403. 35. Smirniotis, P.G.; Ruckenstein, E. Ind. Eng. Chem. Res. 1995, 34, 1517. 36. Shah, R.; Gale, J.D.; Payne, M.C. J. Phys. Chem. B 1997, 101, 4787. 37. Jeanvoine, Y.; Angyan, J. G.; Kresse, G.; Hafner, J., J. Phys. Chem. B 1998, 102, 5573. 11

38. Hillier, I. H. J. Mol. Struct. (THEOCHEM) 1999, 463, 45. 39. Limtrakul, J.; Jungsuttiwong, S.; Khongpracha, P. J. Mol. Struct. 2000, 525, 153. 40. Sauer, J.; Sierka, M. J. Comput. Chem. 2000, 21, 1470. 41. Sauer, J.; Ugliengo, P.; Garrone, E.; Saunders, V. R. Chem. Rev. 1994, 94, 2095. 42. Sauer, J. Chem. Rev. 1989, 89, 199. 43. Limtrakul, J. Chem. Phys. 1995, 193,79. 44. Limtrakul, J.; Treesukol, P.; Ebner, C.; Probst, M. Chem. Phys. 1997, 215, 77. 45. Van Santen, R. A.; Kramer, G. J. Chem. Rev. 1995, 95, 637.

46. Blaszkowski, S. R.; van Santen, R. A. J. Am. Chem. Soc. 1997, 119, 5020. 47. Namuangruk, S.; Pantu, P.; Limtrakul, J. J. Catal. 2004, 225, 523. 48. Clark, L. A.; Snurr, R. Q. Stud. Surf. Sci. Catal. 2001, 135. 49. Svelle, S.; Kolboe, S.; Swang, O. J. Phys. Chem. B 2004, 108, 2953. 50. Vos, A. M.; Schoonheydt, R. A.; De Proft, F.; Geerling, P. J. Phys. Chem. B 2003, 107, 2001. 51. Arstad, B.; Kolboe, S.; Swang, O. J. Phys. Chem. B 2004, 108, 2300.

Metal-Organic Framework as Electrocatalyst for Fuel Cell Application Pankaj Bharali Department of Chemical Sciences, Tezpur University, Napaam, Tezpur 784 028, Assam, India Fax: (+91) 3712-267005; Tel: (+91)-3712-267008 E-mail: pankajb@tezu.ernet.in KEYWORDS: Metal-Organic Frameworks, Heterogeneous Catalysis, Fuel Cell, Electrocatalysis This article briefly reviews the background and fate of Metal-Organic Frameworks (MOFs) as heterogeneous catalyst and electrocatalyst for fuel cell application. 1. Metal-Organic Frameworks Metalâ&#x20AC;&#x201C;organic frameworks (MOFs) also known as coordination polymers are extended porous structures composed of transition metal ions (or clusters) that are linked by organic bridges. They have infinite crystalline lattices which generally involve two main components of inorganic vertices (metal ions or clusters) and organic linkers/struts. The two main components are connected to each other by coordination bonds, together with other intermolecular interactions, to afford a network having definite topology with pores usually occupied by solvent molecules as shown in Figure 1.1 They are prepared as crystalline solids by solution reactions of metal ion salts with organic linkers. A wide variety of metal ions and organic reagents have been used leading to over 2,000 MOF structures with compositional and architectural diversity unparalleled by any other class of materials. Among these are the original MOFs with very open structures and ultrahigh porosity that recently have been shown to be important for gas (hydrogen, methane and carbon dioxide) storage and separation, among many other applications.2 However, from its very beginning, the chemistry of MOFs had a real limitation, or so it seemed: when long organic links are used in building certain nets, especially those of primitive cubic topology, the resulting frameworks are entangled over the entire crystal structure.3 In other words, two or more independent frameworks are found completely interpenetrated to fill what otherwise would have been large pores. Often, this precludes binding of guests. This is natureâ&#x20AC;&#x2DC;s way of filling up space; an aspect that many considered a drawback to porosity. Recently, a number of studies showed that interpenetration is not quite as bad as once thought, especially with respect to increasing hydrogen volumetric density uptake in MOFs. Indeed even highly entangled MOFs can be designed to be highly porous.4

Figure 1 A general scheme of MOF synthesis.1 Robson and coworkers reported the first MOF in the early 1990s where they described that the single metal ions were connected by simple tetrahedral or rod-shaped ligands like tetracyanophenylmethane or 4,4/-bipyridine.5,6 In the past decade exponential developments in porous coordination networks have been reported by using multi-topic carboxylates as anionic ligands to

support charge-neutral network.7,8 Various research groups have been developing series of MOFs. This series is abbreviated as MOF-n, as for example MOF-5 and MOF-177 were developed by Yaghi and coworkers.9,10 Generally, 3d divalent (Mn2+, Ni2+, Co2+, Cu2+, Zn2+ etc.) and trivalent (Sc3+, V3+, Cr3+, Fe3+ etc.) transition metals or rare earth metals are used as metallic centers.11 The perfect symmetry of MOF-5 structure is shown in Figure 2.

Figure 2 Perfect symmetry of MOF-5 structure. 2. Heterogeneous Catalysis by MOFs Heterogeneous catalysis is one of the most important, earliest proposed (20 years before) and developed applications (15 years before) of MOFs.1 The wide range of adaptable metal centers and their surrounding environments as well as functionizable crystalline frameworks provide multiple opportunities to create desirable active sites for catalysis. The comparison with other porous materials is helpful in predicting the possible catalytic applications of MOFs.12 It is more or less apparent what is the utilization of zeolites and mesoporous aluminosilicates in catalysis â&#x20AC;&#x201D; often commercially employed by the industry; the fate of MOFs is not obvious yet. In any case, we can speculate what are possible applications. Due to their high thermal stability and limited pore size, acidic zeolites are widely used in gas phase reaction under harsh conditions and industrial processes such as cracking, isomerization, oligomerization, and alkylation, whereas if an active center such as a transition metal, a metal oxide or sulfide, or a metal complex is present, they are used in oxidation and reduction reactions.13 On the other hand, the highly dispersed active sites and large channels make mesoporous materials such as MCMs and SBAs more suitable for the encapsulation of metal nanoparticles or molecular catalysts to perform reactions with relatively large 13

substrates.14 Since MOFs feature high crystallinity and do not have theoretical pore size limitations, they provide unique opportunities. This means that it is possible to have a homogeneous distribution of one or more active sites due to the high crystallinity of the material and, at the same time, to overcome diffusion and pore size limitations. Moreover, the fine structure and the nature of the active site can be controlled. However, MOFs have still issues concerning stability and cost, due to the nature of the components of the materials.

commercially viable industrial products. These problems have often been associated with the lack of appropriate materials or manufacturing routes that would enable the cost of electricity per kWh to compete with the existing technology.

Figure 4 Summary of fuel-cell types.16

Figure 3 Successful and potential applications of zeolites, mesoporous silica alumina compounds, and MOFs.12 The right combination between cost, stability, and chemical application should be selected accordingly. As shown in Figure 3, the status how things appear at the moment, MOF catalysts tend to be more suitable for fine chemical synthesis than for bulk chemistry and it is quite obvious that the examples in the literature reflect this. The known strategies for the building of specific catalytic sites in crystalline MOFs may be categorized into the following three classes: (i) framework activity, (ii) encapsulation of active species and (iii) post-synthetic modification. Many catalytic reactions have now been carrying out utilizing wide variety of MOFs. Some of the very useful reactions arehydrogenation, isomerization, oxidation of organic substrates, CO oxidation, photocatalysis, carbonyl cyanosilylation, hydrodesulfurization, and various acid catalyzed reactions.

The types of fuel cells under active development are summarized in Figure 4. The oxidation reaction takes place at the anode (+) and involves the liberation of electrons (for example, O2– + H2 = H2O + 2e– or H2 = 2H+ + 2e–). These electrons travel round the external circuit producing electrical energy by means of the external load, and arrive at the cathode (–) to participate in the reduction reaction (for example, ½ O2 + 2e– = O2– or ½ O2 + 2H+ + 2e– = H2O). It should be noted that as well as producing electrical energy and the reaction products (for example, H2O and CO2), the fuel-cell reactions also produce heat. The reaction products are formed at the anode for solid-oxide fuel cells (SOFC), moltencarbonate fuel cells (MCFC) and alkaline fuel cell (AFC) types, and at the cathode for phosphoric-acid fuel cell (PAFC) and polymeric-electrolyte-membrane fuel cell (PEMFC) types. This difference has implications for the design of the entire fuel-cell system, including pumps and heat exchangers. To maintain the composition of the electrolyte component in the MCFC system, CO2 has to be recirculated from the anode exhaust to the cathode input. Additionally, the composition of the polymeric-membrane electrolyte has to be carefully controlled during operation by an appropriate ‗water management‘ technology.

3. Fuel Cells Fuel cells are electrochemical devices that directly convert chemical energy into electrical energy with high efficiency and low emission of pollutants. They consist of an electrolyte medium sandwiched between two electrodes (Figure 4).15,16 One electrode (called the anode) facilitates electrochemical oxidation of fuel, while the other (called the cathode) promotes electrochemical reduction of oxidant. Ions generated during oxidation or reductions are transported from one electrode to the other through the ionically conductive but electronically insulating electrolyte. The electrolyte also serves as a barrier between the fuel and oxidant. Electrons generated at the anode during oxidation pass through the external circuit (hence generating electricity) on their way to the cathode, where they complete the reduction reaction. The successful conversion of chemical energy into electrical energy in a primitive fuel cell was first demonstrated over 160 years ago. However, in spite of the attractive system efficiencies and environmental benefits associated with fuel-cell technology, it has proved difficult to develop the early scientific experiments into

Figure 5 Fuel-cell types and fuel processing.16 The AFC, PEMFC and PAFC stacks essentially require relatively pure hydrogen to be supplied to the anode. Accordingly, the use of hydrocarbon or alcohol fuels requires an external fuel processor to be incorporated into the system. This item not only increases the complexity and cost of the system, but also decreases the overall efficiency as indicated in Figure 5. While selected fuels can be introduced directly into the anode of the high14

temperature fuel cells (SOFC and MCFC), better thermal management of the stack can usually be achieved by having separate reformer compartments that are thermally integrated within the stack to produce a mixture of fuel and syngas (H2 and CO). For the lower-temperature fuel cells (PAFC and PEMFC), external reformers are required. Some of the fuel has to be consumed in these external reformers to maintain the operating temperature. Moreover, dilution of the H2 fuel reduces performance of the cells, resulting in significant efficiency losses compared with operation on pure H2. It should be noted that the AFC stack cannot be operated on reformate fuels because of the presence of CO2 in these gases.15,16 In contrast, MCFCs and SOFCs operating at higher temperatures have the advantage that both CO and H2 can be electrochemically oxidized at the anode. Moreover, the fuel-processing reaction can be accomplished within the stack, which enables innovative thermal integration/management design features to provide excellent system efficiencies (~50%). 4. Electrocatalysis by MOFs The unique properties of MOFs that make them good candidates as catalysts for the above-mentioned specific reactions are  (i) they possess various pore sizes (3–35 Å) and surface areas (500–6500 m2g1), (ii) their surface properties can be manipulated using a variety of organic ligands, affording unique functionalities on the channel surface and (iii) since each constituent of an MOF is exposed, they can provide unique catalytic sites for chemical reactions. Finally, adsorbent molecules can be confined in the nanospaces. When molecules are confined in such spaces and undergo stress caused by the deviation from the thermodynamically and kinetically stable structures of the ambient surroundings, this stress brings about an effective energy conversion, and new chemical reactions occur. However, MOFs have been little exploited as electrocatalyst for electrocatalytic reactions to date. This lack is mainly due to the small number of MOF materials that display high electron conductivity, which is essential for electrocatalytic reactions.17 Doménech-Carbó et al. has been reported the pioneering work on the electrochemistry of MOFs.18 For the case of Cu- and ZnMOFs with BTC (=benzene-1,3,5-tricarboxylate) ligands, a range of processes including reductive cation insertion, metal nucleation, and partial dissolution have been published. In situ electrochemical AFM measurements were reported showing interesting metal structures, which grow upon conversion of the MOF to metal nuclei. However, apart from these studies there is only very little information about the electrochemical reactivity of MOF systems. The reactivity of porous metal compounds is of considerable interest due to their potential application for electrocatalytic processes. A considerable body of work exists, for example, on porous Prussian blues which are known to act as versatile electrocatalysts and ion insertion electrodes. It is therefore important to develop new classes of porous solids with electrochemical reactivity and the family of MOF materials appears uniquely suitable based on their structural diversity and known absorption properties. Commercially available Fe(BTC) MOF (BasoliteTM F300) has been recently investigated for electrocatalytic activity.19 It may structurally and electrochemically be compared with Cuand Zn-MOFs and Fe-trimellitates. A further interesting comparison is possible with the well-known Fe-based Prussian blue materials with cyanide bridging ligands. Prussian blue has found many applications and due to facile cation insertion and expulsion processes it has been studied intensely. It is shown that Fe(BTC) in contrast to Prussian blue does not allow facile cation insertion

and that the electrochemical processes of the solid immersed in aqueous electrolyte media are dominated instead by the reductive dissolution (in acidic media) and reductive transformation (in alkaline media).19 In another seminal work, the MOF material, N,N/-bis(2hydroxyethyl)dithiooxamidatocopper(II) [(HOC2H4)2dtoaCu], which is a two-dimensional framework composed of dimeric Cu units and bridging ligands (HOC2H4)2dtoa2 has been employed as catalyst for ethanol electrooxidation reactions (EERs).17 This material is a good proton and electron conductor. Its proton conductivity is 3.3  104 Scm1. The effective pore size is around 7.8 Å, as estimated from the N2 adsorption isotherm. It is insoluble, even in 1M sulfuric acid at 80 C, and is thermally stable below 165 C. Furthermore, the active copper species have free binding sites in the nanospaces, which facilitates the formation of adducts with electroactive molecules. Such unique properties might provide benefits that make [(HOC2H4)2dtoaCu] a good candidate for ethanol oxidation in fuel cells. The oxygen reduction reaction (ORR) at the cathode of a PEM fuel cell represents a very important electrocatalytic reaction. At present, the catalyst materials of choice are platinum group metals (PGMs).20 The high costs and limited reserves of PGMs, however, created a major barrier for large-scale commercialization of PEMFCs. Intensive efforts have been dedicated to the search of low-cost alternatives. The discovery of ORR activity on cobalt phthalocyanine stimulated extensive investigations of using Co– N4 or Fe–N4 macromolecules as precursors for preparation of transition metal (TM) based, non-PGM catalysts. The ORR activity over a cobalt–polypyrrole composite was observed, of which a Co ligated by pyrrolic nitrogens was proposed as the catalytic site. Activation in an inert atmosphere of the similar TM–polymer composite through pyrolysis further improved the catalytic activity. More recently, significant enhancement in ORR activity was demonstrated in a carbon-supported iron-based catalysts, and it was suggested that micropores (width <20 Å) have critical influence on the formation of the active site with an ionic Fe coordinated by four pyridinic nitrogens after high-temperature treatment. The onset potential for an Fe-based catalyst is found to be 0.1 V higher than that of a Co-based system although the latter is more stable under PEMFC operating condition. These previous studies proposed the nitrogen-ligated TM entities either as the precursors or the active centers for the catalytic ORR process. Another challenge for non-PGM ORR catalysts is their relatively low turn-over-frequency in comparison with Pt. To compensate low activity without using excessive amount of catalyst, thus causing thick electrode layer and poor mass transport, it is desirable to produce the highest possible catalytic-site density, that are evenly distributed and accessible to gas diffusion through a porous framework. Ma el. al. reported the first experimental demonstration of porous MOF as a new class of precursor for preparing ORR catalysts. Different from previous approaches, MOFs have the following advantages when used to prepare non-PGM electrocatalysts: MOFs have clearly-defined three-dimensional structures. The initial entities such as TM–N4 can be grafted into MOFs with the highest possible volumetric density through regularly arranged cell structure. The MOF surface area and pore size are tunable by the length of the linker. The organic linkers would be converted to carbon during thermal activation while maintaining the porous framework, leading to catalysts with high surface area and uniformly distributed active sites without the need of a second carbon support or pore forming agent. Furthermore, the TM–ligand composition can be rationally designed with wide selection of metal–linker combinations for systematical investigation on the relationship between precursor structure and catalyst 15

activity.20 These recent advances in the field of electrocatalysis has shown that there is a lot of room for research on MOF as electrocatalyst for fuel cell application. Acknowledgement Author sincerely thanks Prof. R.C. Deka for his constant encouragement and valuable scientific discussions.

5. References (1) Jiang, H.-L.; Xu, Q. Chem. Commun. 2011, 47, 3351–3370. (2) Li, H.; Eddaoudi, M.; O‘Keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276–279. (3) Batten, S. R.; Robson, R. Angew. Chem. Int. Ed. 1998, 37, 1460– 1494. (4) Reineke, T. M. et al. J. Am. Chem. Soc. 2000, 122, 4843–4844. (5) Hoskins, B. F.; Robson, R. J. Am. Chem. Soc. 1989, 111, 5962– 5964. (6) Hoskins, B. F.; Robson, R. J. Am. Chem. Soc. 1990, 112, 1546– 1554. (7) Eddaoudi, M.; Moler, D. B.; Li, H.; Chen, B. M.; Reineke, T.; O‘Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319–330.

(8) Corma, A.; Garcia, H.; Llabres i Xamena, F. X. Chem. Rev. 2010, 110, 4606–4655. (9) Li, H.; Eddaoudi, M.; O‘Keeffe, M.; Yaghi, O. M. Science 1999, 402, 276–280. (10) Chae, H. K.; Siberio-Perez, D. Y.; Kim, J.; Go, Y. B.; Eddaoudi, M.; Matzger, A. J.; O‘Keeffe, M.; Yaghi, O. M. Nature 2004, 427, 523–527. (11) Das, S. K.; Bhunia, M. K.; Seikh, M. M.; Dutta, S.; Bhaumik, A. Dalton Trans. 2011, 40, 2932–2939. (12) Ranocchiari, M.; van Bokhoven, J. A. Phys. Chem. Chem. Phys. 2011, 13, 6388–6396. (13) Cejka, J.; Corma, A.; Zones, S. Zeolites and Catalysis: Synthesis, Reactions and Applications, Wiley-VCH, Weinheim, 2010. (14) Wight, A.; Davis, M. Chem. Rev. 2002, 102, 3589–3614. (15) Wikipedia, http://en.wikipedia.org/ (16) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345 – 352. (17) Yang, L. et. al. Angew. Chem. Int. Ed. 2010, 49, 5348 –5351. (18) Doménech-Carbó, A. Electrochemistry of porous materials, CRC Press, London, 2010 p. 95. (19) Firoz Babu, K.; Anbu Kulandainathan, M.; Katsounaros, I.; Rassaei, L.; Burrows, A. D.; Raithby, P. R.; Marken, F. Electrochem. Commun. 2010, 12, 632–635. (20) Ma, S.; Goenaga, G. A.; Call, A. V.; Liu, D.-J. Chem. Eur. J. 2011, 17, 2063 – 2067.

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IRSAPS Bulletin 2011, Vol. 1, Issue 2

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