We report a new and simple one-pot synthetic method to produce mesoporous silica and nanoporous solid acid catalyst capable of catalyzing pinacole-pinacolone rearrangement and esterification reactions, by preparing a solvent washable phosphonated triblock copolymer template and self-assembling it in the presence of alkoxysilane

Asefa, Tewodros

Biradar, Ankush V.

Duncan, Cole T.

Mishler, Richard E., II

Schiff, Eric A

English

Syracuse University Research Facility and Collaborative Environment

Syracuse University SURFACE Physics College of Arts and Sciences 2009 Solvent-Washable Polymer Templated Synthesis of Mesoporous Materials and Solic-Acid Nanocatalysts in One-Pot Richard E. Mishler II Syracuse University Ankush V. Biradar Syracuse University Cole T. Duncan Syracuse University Eric A. Schiff Syracuse University Tewodros Asefa Syracuse University Follow this and additional works at: https://surface.syr.edu/phy  Part of the Physics Commons Recommended Citation "Solvent-washable polymer templated synthesis of mesoporous materials and solic-acid nanocatalysts in one-pot", Richard E. Mishler II, Ankush V. Biradar, Cole T. Duncan, Eric A. Schiff, and Tewodros Asefa, Chem. Comm. 6201-6203 (2009). This Article is brought to you for free and open access by the College of Arts and Sciences at SURFACE. It has been accepted for inclusion in Physics by an authorized administrator of SURFACE. For more information, please contact surface@syr.edu. Solvent-washable polymer templated synthesis of mesoporous materialsand solid-acid nanocatalysts in one-potwRichard E. Mishler II,a Ankush V. Biradar,a Cole T. Duncan,a Eric A. Schiﬀ*b andTewodros Asefa*aReceived (in Berkeley, CA, USA) 1st July 2009, Accepted 1st September 2009First published as an Advance Article on the web 18th September 2009DOI: 10.1039/b913035gWe report a new and simple one-pot synthetic method to producemesoporous silica and nanoporous solid acid catalyst capable ofcatalyzing pinacole-pinacolone rearrangement and esteriﬁcationreactions, by preparing a solvent washable phosphonatedtriblock copolymer template and self-assembling it in thepresence of alkoxysilane.In the early 1990’s, a family of mesoporous silicas (MCM41,MCM48 and MCM50) were discovered that had a broadrange of uses including sensor applications, catalysis, drugdelivery, and use in solar cells.1 Depending upon the inorganicmaterial, surfactant template, and synthetic conditionsemployed, mesoporous materials with widely varyingcomposition, structure and pore sizes can be synthesized.The ﬁrst report in 1998 on the synthesis of SBA15 mesoporoussilica with very large pore diameter by using triblockcopolymer templates was also a signiﬁcant advance in the ﬁeldof mesoporous materials chemistry.2 The SBA15 materialallows the immobilization via ‘‘host–guest’’ inclusion processesof larger molecules that are unable to diﬀuse in the MCM41mesoporous material.3The synthesis of well-ordered mesoporous structuresrequires an extraction step to remove the templating agent.The most common method is calcination, which involvesheating the material to a temperature suﬃcient to oxidizethe organic or polymer template. Calcination is widely usedin SBA15-type materials.1,2 Although extraction of polymertemplates by calcination can completely remove the template,the fairly high temperature is known to promote condensationof surface hydroxyl (–OH) groups on the mesoporousframework, reducing the number of hydroxyl groups utilizedfor further organic functionalization of the material viagrafting and post-modiﬁcation. Calcination also modiﬁes theframework, typically by shrinkage, and of course the template,which could be potentially recycled, is lost.The second method is solvent-extraction,4 which is oftenable to remove ionic template molecules, but has been shownto be less successful in removing non-ionic templates such astriblock copolymers that are hydrogen-bonded to andoccluded within micropores of the framework.5 Othermethods include Soxhlet extraction,6 stirring in supercriticalﬂuids7 and stirring under reﬂux conditions.8 Here we reportthe synthesis of SBA15-type mesoporous material andsolid acid nanostructures by using phosphonated pluronicsP123 (designated P123P), as a solvent-extractable template(Scheme 1).Acid cleavable, phosphonated triblock copolymer (P123P)template containing terminal phosphate ester groups wassynthesized by converting the terminal hydroxyl groups ofthe PEG blocks of Pluronic P123 or poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblockcopolymer with dimethylphosphite (DMP) using a similarmethod as reported by Gitsov and co-workers for synthesisof block-copolymers via phosphorylation.9 The resultingP123P polymer was then self-assembled with tetraethoxysilane(TEOS) in aqueous solution to produce mesostructured silica,SBA15P, under conditions similar to those reported formaking SBA15 with P123 (see ESI for details of the synthesisand characterizationw).The template extraction was performed under variousconditions with special attention to solvent extraction withthe expectation that the P123P template would be easilyremoved through a low temperature solvent washing byundergoing hydrolysis at its terminal phosphate ester groups.For comparison purposes, a material templated withas-received P123 was also synthesized and subjected to similarextraction conditions. The samples were then characterizedwith transmission electron microscopy (TEM), gas adsorption,small angle X-ray scattering (SAXS), powder X-ray diﬀraction(XRD), elemental analysis, thermogravimetric analysis andsolid-state NMR and FT-IR spectroscopy (FTIR).Scheme 1 Synthesis of hexagonal mesoporous material and solid acidcatalyst with solvent extractable phosphonated pluronics template,P123P.aDepartment of Chemistry, 111 College Place, Syracuse University,Syracuse NY 13244, USA. E-mail: tasefa@syr.edu;Fax: +1-315-443-4070; Tel: +1-315-443-3360bDepartment of Physics, Syracuse University, Syracuse, NY 13244,USA. E-mail: easchiﬀ@syr.edu; Fax: +1-315-443-9103;Tel: +1-315-443-2566w Electronic supplementary information (ESI) available: Synthesis,experimental details, catalysis results, and solution NMR and FT-IRspectra. See DOI: 10.1039/b913035gThis journal is c The Royal Society of Chemistry 2009 Chem. Commun., 2009, 6201–6203 | 6201COMMUNICATION www.rsc.org/chemcomm | ChemCommFig. 1 shows SAXS for material made with P123P in itsas-synthesized state (SBA15P), after solvent washing (SBA15P-sw),and after calcination (SBA15P-cal). All three samples yield apeak near 0.6 nm1; higher order peaks are also discernible.The scattering for sample SBA15P-sw is similar to that ofSBA15P, which supports the idea that the gentler approachof solvent extraction will not change the mesostructure.SBA15P-cal showed slight unit cell shrinkage indicating thathigh temperature calcination reduced the unit cell of thematerial. The XRD patterns also exhibited that the materialshave ordered mesostructures with unit cell dimensions of 11.5,11.5, and 10.3 nm for SBA15P, SBA15P-sw, and SBA15P-cal,respectively. The TEM images are consistent with anhexagonally ordered mesostructure (Fig. 1).Fig. 2 illustrates nitrogen gas adsorption isotherms forseveral template-extracted samples; they all have type IVisotherms, which is indicative of the presence of mesoporousstructures in the materials. Compared with SBA-15, thesamples prepared from P123P showed two desorption stepsindicating that they have bottle-neck shaped pores or poreclogging.10 This may be due to the presence of residualphosphate ester groups on the walls of the materials aftersolvent washing or possible residues of phosphorus oxidesfrom oxidation of phosphate esters inside the mesopores uponcalcination.11 The pore size distribution in Fig. S7 (ESIw) thatwas calculated from the isotherms revealed that sampleSBA15P-sw, SBA15P-cal and SBA15P have major porediameters of 8.5, 8.5, and 8.0 nm, respectively. It is worthnoting that SBA-15 has insigniﬁcant complementary orinterconnecting small pores with only 0.09 cc/g pore volume,while samples synthesized with P123P have signiﬁcantamounts of such pores with an average size of 2.3 nm andwith 40.2 cc/g pore volume (Table S1 and Fig. S7, ESIw).The thermogravimetric analysis (TGA) traces (Fig. 3) forthe mesoporous samples show two regions of major weightloss. In the ﬁrst B25–100 1C, a distinct drop in weight isobserved which is attributed to evaporation of physisorbedwater. In the second drop between B200 1C–600 1C, anotherdistinct weight loss is observed due to the oxidation of theorganic template. Upon comparing the graphs in Fig. 3A(mesoporous samples made using the new P123P template)to graphs in Fig. 3B (P123 template), a signiﬁcant diﬀerenceparticularly between the two diﬀerent solvent washeswas observed. For the samples synthesized from P123P, thecalculated diﬀerence between the weight losses in the range ofB200 1C–600 1C for unextracted material and solvent washedfor 24 h was 14%; when extracted for 48 h, an additionalweight loss of 6% was observed (Fig. 3A). For the SBA15sample synthesized from P123, the solvent washed sample lostonly about 3% mass compared to the untreated sample. Thisclearly indicates that a simple solvent washing that is unable toremove P123 template does remove most of the mass of P123Ptemplate. We attribute the latter to the hydrolytic cleavage ofterminal phosphate ester groups of P123P, facilitating bulkremoval of the template leaving residual phosphate orphosphonic acid groups intact on the walls of the material.To conﬁrm this, we characterized the sample with elementalanalysis, FT-IR, which showed residual phosphorus aftersolvent washing (Fig. S10 and Table S2, ESIw). Becausethe content of phosphorus in the ﬁnal samples is small(Table S2, ESIw), 31P solid-state NMR and X-ray photo-emission spectroscopy (XPS) did not show peaks correspondingto phosphorus. The low amount of residual phosphorus mayFig. 1 Small angle X-ray scattering (SAXS) of mesoporous silica assynthesized from phosphonated template, P123P (SBA15P) andfollowing solvent washing (SBA15P-sw) and calcination (SBA15P-cal).Inset: transmission electron micrograph of SBA15P-sw.Fig. 2 Nitrogen gas adsorption/desorption isotherms of SBA15P(from P123P) and SBA15 (from P123) after template extraction withsolvent washing and calcination.Fig. 3 Thermogravimetric traces before and after solvent washing ofmesoporous materials made with (a) P123P and (b) as-received P123.6202 | Chem. Commun., 2009, 6201–6203 This journal is c The Royal Society of Chemistry 2009be a result of hydrolysis of phosphate esters over severalminutes,9c upon self-assembly at low pH.Besides its ease of extraction by hydrolytic cleavage, the useof the P123P template allowed the placement of residualphosphate ester, which was converted to phosphonic acidgroups on the mesoporous framework in one-pot, creating asolid acid nanocatalyst (see ESI for detailsw).z We tested thesolid acid catalytic properties of the material by ﬁrst acidifyingits phosphate esters with dilute HCl solution and thenusing the resulting material as a solid acid catalyst in thebenzopinacole–benzopinacolone rearrangement12,13 (Scheme 2)as well as esteriﬁcation reactions.14 The acidiﬁed SBA15Pmaterial showed nearly a 100% conversion of benzopinacolewith 100% selectivity towards benzopinacolane in 12 h at120 1C (Table 1). Furthermore, the catalyst was recycled threetimes without loss of its catalytic activity. The product wasconﬁrmed by GC, GC-MS, FT-IR, and 1H NMR spectro-scopy. To perform the control reaction, we used an acidiﬁedSBA15 (see ESIw). This sample resulted in no reaction underthe same conditions. It is also worth noting that this catalyticreaction is dependent on the reaction temperature, wherehigher temperatures favored this catalytic transformation(Table 1).We also used this catalyst successfully for esteriﬁcationreaction of acetic acid with ethanol. The catalyst produced40% conversion with 100% selectivity to ethyl acetate at75 1C in 24 h whereas in a control run with SBA15 only 25%conversion in the same time was observed. Also this catalyst wassuccessfully recycled 5 times with only marginal loss in activity.This slight loss in activity was mainly due to handling losses ofcatalyst (see ESIw). These experiments suggest that we havegenerated an important catalyst with a systematic design andsynthetic approach using easily extractable polymer template.In conclusion, we have synthesized mesoporous silica bypreparing and using a phosphonated template, P123P, anddemonstrated its extraction by a simple solvent washing; thesolvent washing procedure had little eﬀect on materialprepared using the P123 template. The residual phosphateester groups could be converted into phosphonic acids,making the material a solid acid nanocatalyst; its catalyticactivity was demonstrated by acid-catalyzed pinacole topinacolone transformation.We gratefully acknowledge the ﬁnancial support by theEmpire State Development Corporation through the SyracuseCenter of Excellence in Environmental and Energy Systems.We thank Prof. I. Gitsov of SUNY-ESF, Prof. MietekJaroniec at Kent State University and Dr. Maura Weathersat Cornell University for valuable discussion.Notes and referencesz A solid acid catalyzed pinacole–pinacolone reaction was conductedby using the mesoporous material, SBA15P-sw, as a catalyst.Typically, 0.1 g of benzopinacol was mixed and stirred with 0.1 gSBA15P-sw catalyst and 10.0 mL of toluene at 80 1C. Samples werecollected at 0, 6, 12 and 24 h after starting the reactions for threeseparate runs using the same catalyst to show recyclability. Threediﬀerent temperatures (80, 100, and 120 1C) were also tried close theboiling point of solvent used (tolueneB 110 1C) to determine the mosteﬃcient temperature for the transformation of benzopinacol intobenzopinacolone. Control experiment using SBA15 as a catalyst wasalso conducted in the same way. The reaction mixture was taken with aﬁlter syringe and characterized by 1H NMR spectroscopy.1 (a) C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli andJ. S. Beck, Nature, 1992, 359, 710; (b) S. Inagaki, Y. Fukushimaand K. Kuroda, J. Chem. Soc., Chem. Commun., 1993, 680.2 D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson,B. F. Chmelka and G. D. Stucky, Science, 1998, 279, 548.3 (a) A. Katiyar and N. G. Pinto, Small, 2006, 2, 644; (b) J. Lu,M. Liong, S. Sherman, T. Xia, M. Kovochichm, A. E. Nel,J. I. Zink and F. Tamanoi, NanoBiotechnology, 2007, 3, 89;(c) M. Vallet-Regı´, Chem.–Eur. J., 2006, 12, 5934.4 (a) T. Asefa, M. J. MacLachlan, N. Coombs and G. A. Ozin,Nature, 1999, 402, 867; (b) S. Huh, J. W. Wiench, J.-C. Yoo,M. Pruski and V. S.-Y. Lin, Chem. Mater., 2003, 15, 4247.5 (a) M. Kruk, M. Jaroniec, C. H. Ko and R. Ryoo, Chem. Mater.,2000, 12, 1961; (b) C.-M. Yang, H.-A. Lin, B. Zibrowius,B. Spliethoﬀ, F. Schu¨th, S.-C. Liou, M.-W. Chu andC.-H. Chen, Chem. Mater., 2007, 19, 3205.6 (a) Y. Liang, M. Hanzlik and R. Anwander, Chem. Commun.,2005, 525; (b) N. Leventis, S. Mulik, X. Wang, A. Dass,V. U. Patil, C. Sotiriou-Leventis, H. Lu, G. Churu andA. Capecelatro, J. Non-Cryst. Solids, 2008, 354, 632.7 W.-H. Zhang, X.-B. Lu, J.-H. Xiu, Z.-L. Hua, L.-X. Zhang,M. Robertson, J.-L. Shi, D.-S. Yan and J. D. Holmes, Adv. Funct.Mater., 2004, 14, 544.8 X. Wang, K. S. K. Lin, J. C. C. Chan and S. Cheng, J. Phys. Chem.B, 2005, 109, 1763.9 (a) K. Troev, I. Tsatcheva, R. Georgieva and I. Gitsov, J. Polym.Sci., Part A: Polym. Chem., 2007, 45, 1349; (b) R. Georgieva,R. Tsevi, K. Kossev, R. Kusheva, M. Balgjiska, R. Petrova,V. Tenchova, I. Gitsov and K. Troev, J. Med. Chem., 2002, 45,5797; (c) I. Gitsov and F. E. Johnson, J. Polym. Sci., Part A:Polym. Chem., 2008, 46, 4130.10 E. B. Celer, M. Kruk, Y. Zuzek and M. Jaroniec, J. Mater. Chem.,2006, 16, 2824.11 D. Carta, D. M. Pickup, J. C. Knowles, M. E. Smith andR. J. Newport, J. Mater. Chem., 2005, 15, 2134.12 (a) M. Jurado-Gonzalez, D. L. Ou, B. Ormsby, A. C. Sullivan andJ. R. H. Wilson, Chem. Commun., 2001, 67; (b) B.-Y. Hsu andS. Cheng, Microporous Mesoporous Mater., 1998, 21, 505.13 D. J. Upadhyaya and S. D. Samant, Appl. Catal., A, 2008, 340, 42.14 Y.-M. Liu, J. Xu, L.-C. Wang, Y. Cao, H.-Y. He and K.-N. Fan,Catal. Lett., 2008, 125, 62.Scheme 2 Catalytic transformation of benzopinacol to benzopinaco-lone using P123P as a catalyst.Table 1 % Conversion data for benzopinacol to benzopinacolonetransformationaEntry Sample t/h T/1C % Conversion1 SBA15p 6 80 33.02 SBA15p 12 80 39.63 SBA15p 24 100 66.34 SBA15p-1b 6 80 33.05 SBA15p-1b 12 80 47.66 SBA15p-1b 24 120 B1007 SBA15p-2c 6 120 94.88 SBA15p-2c 12 120 98.39 SBA15p-2c 24 120 B100a % Conversion determined by 1H NMR. b The catalyst afterrecycling once. c The catalyst after recycling twice.This journal is c The Royal Society of Chemistry 2009 Chem. Commun., 2009, 6201–6203 | 6203

Solvent-Washable Polymer Templated Synthesis of Mesoporous Materials and Solic-Acid Nanocatalysts in One-Pot

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Solvent-Washable Polymer Templated Synthesis of Mesoporous Materials and Solic-Acid Nanocatalysts in One-Pot

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