Grafted Polymethylhydrosiloxane on Hierarchically Porous Silica

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Grafted Polymethylhydrosiloxane on Hierarchically Porous Silica Monoliths: A New Path to Monolith supported Palladium Nanoparticles for Continuous Flow Catalysis Applications. Carl-Hugo Pélisson, Takahiro Nakanishi, Yang Zhu, Kei Morisato, Toshiyuki Kamei, Ayaka Maeno, Hironori Kaji, Shunki Muroyama, Masamoto Tafu, Kazuyoshi Kanamori, Toyoshi Shimada, and Kazuki Nakanishi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12653 • Publication Date (Web): 14 Dec 2016 Downloaded from http://pubs.acs.org on December 16, 2016

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ACS Applied Materials & Interfaces

Grafted Polymethylhydrosiloxane on Hierarchically Porous Silica Monoliths: A New Path to Monolith Supported Palladium Nanoparticles for Continuous Flow Catalysis Applications. Carl-Hugo Pélisson,a Takahiro Nakanishi,a Yang Zhu,a Kei Morisato,b Toshiyuki Kamei,c Ayaka Maeno,d Hironori Kaji,d Shunki Muroyama,e Masamoto Tafu,e Kazuyoshi Kanamori,a Toyoshi Shimada*,c and Kazuki Nakanishi*,a a

Department of Chemistry, Gradutate School of Science, Kyoto University, Kitashirakawa,

Sakyo-ku, Kyoto 606-8502, Japan b

GL Sciences Inc./Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan

c

Department of Chemical Engineering, Nara National College of Technology, 22 Yata-cho,

Yamatokoriyama, Nara 639-1080, Japan d

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

e

Department of Applied Chemistry and Chemical Engineering, National Institute of Technology,

Toyama College 13 Hongo-machi, Toyama 939-8630, Japan

KEYWORDS silica monolith, mesoporous materials, hydrosylilation, polymer, palladium nanoparticle, catalysis, selective hydrogenation

ACS Paragon Plus Environment

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ABSTRACT

Polymethylhydrosiloxane has been grafted on the surface of a hierarchically porous silica monolith using a facile catalytic reaction between Si-H and silanol to anchor the polymer. This easy methodology leads to the functionalization of the surface of silica monolith, where a large amount of free Si-H bonds remains available for reducing metal ions in solution. Palladium nanoparticles of 15 nm have been synthetized homogeneously inside the mesopores of the monolith without any stabilizers, using a flow of a solution containing Pd2+. This monolith was used as column-type fixed bed catalyst for continuous flow hydrogenation of styrene and selective hydrogenation of 3-hexyn-1-ol, in each case without a significant decrease of the catalytic activity after several hours or days. Conversion, selectivity and stereoselectivity of the alkyne hydrogenation can be tuned by flow rates of hydrogen and the substrate solution, leading to high productivity (1.57 mol g(Pd)-1 h-1) of the corresponding cis-alkene.

INTRODUCTION Hierarchically porous silica has been under the spotlights during the few last decades, due to numerous applications in extended fields such as drug delivery,1-4 bioimaging,5-7 optics,8-9 ion or molecule adsorption,10-12 separation13-14 and catalysis.15-17 In this last field, easily functionalizable materials are generally used as the support for catalytic species like organometallic complexes, enzymes or metallic nanoparticles.18-21 Recently, porous monolithic materials are used in continuous flow catalysis, and efficiency in catalytic reactions has been improved through a better control of reaction parameters as contact time between reactants and catalyst.22-28 As common knowledge, metallic nanoparticles are of great interest in catalysis allowing high activity, selectivity and stability.29-30 Different methodologies of synthesis have


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been developed in the last decades to obtained size and/or shape-controlled particles. Their synthesis or deposition on inorganic support have greatly improved the stability of NPs, mostly obtained through a bottom-up pathway using salts or metallic complexes in the case of catalytic systems.31 However, those methodologies have to be modified (supercritical media, sonication…) to ensure homogeneous size and spatial of NPs inside macro-mesoporous monoliths, where diffusion effects are a vital parameter.16 In this context, utilization of reductive pore surface is proved to be an excellent solution to the preparation of supported metal nanoparticles inside mesopores via liquid phase containing desirable metal ions.32 Hierarchically porous monoliths of hydrogen silsesquioxane (HSQ or hydridosilica) obtained through a combination of spinodal decomposition and sol-gel transition of trialkoxysilane (HSi(OR)3) have been developed as such an interesting alternative.33 The HSQ materials contain free Si-H group, which is highly efficient for the reduction of different metal ions into nanoparticles, on the pore surface and inside the network.34-35 However, since the controllability of macro/mesopores in the HSQ monolith is still rather limited, it will be of significant use if the Si-H groups are introduced on the pore surface of silica monoliths, in which the extended pore controllability is well established.36 The functionalization of silica gels is a key technology in separation and catalysis, and several processes using different kinds of silanes are well known. Various chlorosilanes, alkoxysilanes, vinylsilanes, allylsilanes, and other protocols have been extensively studied to anchor organic functional moieties to the silanol groups on the surface of silica.37-38 Reported about two decades ago by Piers and co-workers, the use of hydrosilane can lead to the reduction of ketones or esters,39-40 once activated by a strong Lewis acid (tris(pentafluorophenyl)borane, B(C6F5)3). As an activator of Si-H groups by coordination, the catalyst decreases the electron density of Si,


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allowing attacks by electron-rich substrates.39,41 As extensions of Piers-Rubinsztajn reaction were also reported the condensations between hydrosilanes and alkoxysilanes,42-43 and more recently, silanol groups,44 which is highly interesting for grafting reactions on various material surfaces. Recently, we applied this reaction to the surface modification of silica using hydrosilanes containing at least one Si-H group under the presence of the strong Lewis acid B(C6F5)3 as the catalyst. This process typically allows fast (~5 min) modifications at mild conditions (at room temperature), and high loadings of various functional groups are already achieved.44 Other advantages include higher robustness of hydrosilane compounds as compared to classical alkoxysilanes, aminosilanes and halosilanes frequently used for silica surface modification. Many hydrosilane compounds are also commercially available, including few polymeric hydrosilanes. Herein we present the grafting of a polymeric hydrosilane, polymethylhydrosiloxane (PMHS), on the surface of a silica monolith by using the above-mentioned methodology developed in our group. The modification of the silica monolith has been performed as a flow reaction. This grafting process leads to an introduction of Si-H groups on the silica monolith with controlled porosity. The resulting surface Si-H groups are available for the reduction of palladium ions into metallic nanoparticles directly inside the mesopores. The obtained Pd NPs supported on the monolithic silica was tested as a catalyst for the hydrogenation of styrene and the selective hydrogenation of 3-hexyn-1-ol in a continuous flow setup. This catalyst proved to be highly active and stable in time. In the case of the selective hydrogenation, remarkable selectivity and cis-selectivity at high conversion have been observed in comparison to Pd catalysts reported in


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the literature. The productivity of cis-alkene is shown superior to the Lindlar catalyst with a space-time yield (STY) of 354 g L-1 h-1, without any additives to palladium species.

MATERIALS AND METHODS Synthesis of silica monolith: A 10-cm long straight meso-macroporous silica monolith was synthetized using a well-known methodology of this laboratory,36 with a diameter of 0.4 cm. Synthesis of Grafted-PMHS (GPMHS) monolith: In a PTFE heat-shrinkable tube was inserted a 2-cm of the previous monolith, as well as the tip of a syringe, which was then heated at 120°C for 15 min, followed by cooling to room temperature. Using a syringe, a solution of 2 mL of polymethylhydrosiloxane (PHMS-40, 190 mg), 10 mL of dehydrated toluene and 0.15 mg of tris(pentafluorophenyl)borane B(C6F5)3 was passed through the monolith 3 times at 0.1 mL.min1

. The column was washed 3 times with 10mL of acetone, and then dried at 40°C for 12 h.

Synthesis of Pd-GPMHS monolith: Using the same system of column and syringe, a 9 mL solution of acetone with 5 vol.% of water and 2 mmol L-1 of PdCl2 were passed through the previous monolithic column at 0.2 mL.min-1 repeatedly until the decolouration of the solution was completed. The monolith was dried under air for 16 h, and then at 40°C for 1 d. Catalytic system and procedures: The catalytic system was built accordingly to the already reported system,24-28 using a HPLC pump (GL Sciences PU712), a gas flow controller (Bronkhorst Hi-Tec model F200CV-002-RGD-11-V-MFC, a PEEK T-shapped mixer and a column oven -GL Sciences CO631A) .The previously obtained column was fixed using a heatshrinkable PTFE tube to glass tubes (4 mm inner diameter) connected to the catalytic system via


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Swagelok fittings, then washed by a flow of MeOH at 0.2mL.min-1 overnight at 80°C. The system is used with variable flow parameters at a fixed temperature at 40°C, under the monitored hydrogen gas pressure. The products were analyzed by GC, using a GL Sciences GC-4000 and an InertCap Pure-WAX column (length of 30 m, ID of 0.32 mm and df of 0.25µm). Analytic procedures: The observation of microstructures of the fractured surfaces of the samples was conducted under a scanning electron microscope (SEM, JSM-6060S, JEOL Ltd. (Japan)). Meso- and microstructure of the samples were characterized by nitrogen adsorption– desorption (BELSORP-mini II, Bel Japan Inc., Japan), where samples were degassed at 200° C for 6 h before each measurement. The Fourier transform-IR (FT-IR) spectra of the samples were recorded on an FT-IR spectrometer (IRAffinity-1, Shimadzu Corp. (Japan)) using ground samples that were mixed with KBr. Solid-state

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Si cross-polarization/magic angle spinning

(CP/MAS) NMR experiment was performed at room temperature in a magnetic field of 7 T on an OPENCORE NMR spectrometer operating at 299.52 MHz (for 1H) and 59.507 MHz (for

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using a 5-mm MAS probe. The sample spinning frequency was 5 kHz and the contact time was 10 ms. We found no appreciable change in the resonance-line shape between the spectra obtained by the CP and single-pulse techniques. To characterize the size of the nanoparticles and their spatial distribution within the monoliths, the obtained composite samples were crushed into powders and then deposited on micro-grids covered by carbon films. The high-resolution TEM observations were performed using a 120 kV JEM-1400 Plus (JEOL Ltd. (Japan)). The quantitative measurement of palladium loading into the monoliths was determined by ICP-OES (Varian 720-ES, Agilent Technology Inc., U.S.). Calibration was carried out with standard solutions of the elements. RESULTS AND DISCUSSIONS


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As commonly known and reported in the literature,36 the hierarchically macro-mesoporous silica monoliths are obtained by sol-gel accompanied by phase separation starting from tetraethoxysilane in diluted acidic media in the presence of polyethylene oxide (PEO). This methodology leads to silica monoliths with tunable hierarchical porosity, high surface area and pore volume. Herein we synthesized a silica monolith with macropores of 5 µm, BET specific surface area of 220 m² g-1, total mesopore volume of 0.9 cm3 g-1 and mesopores of 20 nm (Figure 1, orange). Grafting PMHS40 (polymethylhydrosiloxane of 40 subunits) on the surface of the silica monolith were performed through the reaction between Si-H groups and silanol groups under the presence of B(C6F5)3 as a catalyst as mentioned above. To ensure homogeneous grafting inside the pores, the solution of PMHS and catalyst (1 wt.%) was slowly introduced by a syringe through the monolith repetitively. Elemental analysis shows a low amount of polymer grafting in the resultant monolith (denoted as GPMHS40) with 4 wt.% of carbon corresponding to a polymer loading of 0.07 mmol g-1 and a maximum of 2.73 mmol g-1 of free Si-H bonds. Nitrogen adsorption-desorption isotherms (Figure 1, grey) show a slight decrease of surface to 200 m² g-1, mesopore size to 18 nm and a decrease of microporosity after the grafting process, observed between 0 to 0.2 P/P0. With a lower c-value of 21, the GPMHS monolith shows a higher hydrophobicity than the starting materials (c-value of 85), directly related to the introduction of the polymer. In addition, an expected decrease of pore volume is observed, from 0.9 to 0.6 cm3 g-1, caused by the introduction of polymers into the pores of the material.


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Figure 1. Nitrogen sorption isotherms and mesopores size distributions of bare silica monolith (orange) and GMPHS40 (grey), SEM images and picture of the GPMHS40 column. The partial grafting of polymeric hydrosilane leads to the preservation of Si-H groups inside the monolith, proved by FT-IR spectroscopy and 29Si CPMAS NMR (Figure 2). The IR spectrum of GPMHS40 shows identical peaks at 900 and 2160 cm-1 between PMHS40 and GPMHS40, corresponding respectively to bending and stretching of the Si-H bonds, which is not observed in the bare silica monolith. The grafting of polymeric hydrosilane was also evidenced by the new stretching vibrations of Si-C and C-H bonds in the GPMHS40 at 1380 and 2970 cm-1, respectively. In addition, the grafting by the covalent bonds is confirmed by 29Si CPMAS NMR, where peaks at -35.6, -57.5 and 65.4 ppm are observed in addition to the Q3 and Q4 peaks of the silica monolith between -90 and -120 ppm. The major peak at 35.6 ppm corresponds to CH3Si(OSi)2H (D2H) from the grafted PMHS and the peak at -57.5 ppm to CH3Si(OSi)2OH


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groups (D2OH) derived from the hydrolysis of the Si-H in D2H. The peak at -65.4 ppm corresponds to CH3Si(OSi)3 (T3) formed by the grafting reaction on the surface of the monolith.45-46

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Figure 2. a) FT-IR spectroscopic characterization of bare silica monolith (, PMHS40 and GPMHS40, and b) 29Si CPMAS NMR of GPMHS40. The Si-H groups introduced by grafting the polymeric hydrosilane are available on the pores for further functionalization. The Si-H groups are commonly used for hydrosilylation and also reported to reduce metal ions into nanoparticles simultaneously with the oxidation of Si-H to Si-OH groups.34 Using the similar liquid-phase methodology as the grafting reaction, a solution of palladium salt was slowly pushed through the GPMHS40 monolith by the syringe (Figure 3). 2

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Figure 3. Images of synthesis of Pd NPs in flow inside GPMHS40, nitrogen sorption isotherm at 77 K and mesopores size distributions (GPMHS40 in grey, Pd@GPMHS40 in black). An instant reduction of Pd2+ can be confirmed during the process by a fast colouration in black throughout the monolith. The process was repeated until no more decolouration of the solution could be observed, and then the obtained Pd@GPMHS40 was washed by pumping acetone repeatedly. Post-synthesis nitrogen sorption isotherms (Figure 3) show no significant change in surface area, pore volume and mesopores size, implying negligible pore blocking after synthesis of nanoparticles. Moreover, TEM images at several locations of the monolith (Figure 4) show palladium nanoparticles with an average size of 15 nm, homogeneously dispersed inside the mesopores and onto the silica skeleton. 80 % of those NPs are between 10 and 20 nm, with no particles larger than 22 nm. With mesopore sizes still larger than NPs and no blocked pores (Figure 3, no change of hysteresis), this synthesis by flow is highly promising to ensure optimal diffusion in the whole monolith during continuous flow catalysis. Finally, elemental analyses by EDX and ICP-AES show an average loading of 0.9 wt.% of Pd inside the monolith (0.95 and 0.86 wt.% at the top and bottom of the monolith respectively).

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Size of NPs (nm)

Figure 4. TEM images and NPs size distribution of Pd@GPMHS40.


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The Pd@GPMHS40 monolith was then evaluated as a catalyst for hydrogenation reactions under a continuous flow of hydrogen (0 to 1.6 mL.min-1, 1.2 to 1.6 bar) and a solution of substrate in methanol (0.2 M, 0.1 to 0.4 mL.min-1), using the already described catalytic system.24-28,47 The efficiency and stability through time of this catalyst was first tested by the hydrogenation of

Conversion of styrene into ethylbenzene (%)

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Figure 5. Hydrogenation of styrene (0.2 M in MeOH) through the catalytic monolith Pd@GPMHS40 (0.9 wt Pd %) at 40°C: a) substrate flow rate of 0.2 mL min-1 and variable H2 flow rate (variable H2/substrate ratio) b) variable substrate flow rate and constant H2/substrate ratio of 1.2 (adapted H2 flow rate) c) and d) fixed flow rates of H2 (0.8 mL min-1, 1.4 bar) and substrate (0.2 mL min-1) over time (H2/substrate = 2)


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As expected, at a 0.2 mL min-1 of fixed substrate flow rate (Figure 5a), tuning up the hydrogen flow rate leads to an increase of conversion at 40°C. The reaction was complete with a H2 flow rate of 1.4 mL min-1, corresponding to a H2/substrate ratio of 2.1 in the catalytic flow. On the other hand, increasing the contact time of the substrate with the catalyst through a decrease of substrate flow rate also results in higher conversion (Figure 5b); total conversion is observed below 0.15 mL min-1 of the substrate solution (0.2M of substrate into MeOH). Finally, a constant 80% conversion of styrene into ethylbenzene was observed over 8 hours at 40°C at 0.2 mL min¬1 of the substrate solution and 0.8 mL min-1 of H2 (1.4 bar), corresponding to the H2/substrate molar ratio of 1.2 (Figure 5c). This stability of conversion has been observed over more than 5 days with no significant loss of activity (Figure 5d). Another reaction investigated with Pd@GPMHS40 as a catalyst was the stereo-selective hydrogenation of 3-hexyn-1-ol. This catalytic reaction in liquid phase can be used as a guide for the selective hydrogenation of acetylene into ethylene, extensively studied for the great interest of purifying an ethylene stream with acetylene contaminant (up to 3%) in petroleum industries.4852

In addition, the selective hydrogenation of 3-hexyn-1-ol is generally used in the fragrance

industries to obtain the corresponding cis-alkene known as the leaf alcohol, with an annual production over 400 tons (including ester analogues).53 This important reaction is usually performed in industry with the Lindlar catalyst (Pd poisoned by Pb additives on CaCO3) in batch to achieve 89 % selectivity toward alkenes. The major drawback being the use of toxic Pb additives, a lot of research has been focused on alternatives with non-toxic additives for the Pd catalyst (organic compounds, carbon monoxide adsorption, metal ions), achieving high selectivities toward alkenes with complex and expensive catalysts and/or pre-treatment by gas.4850,54

However, a continuous flow reaction with a Pd-doped silica monolith, Pd@MonoSil


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synthetized by adsorption of a salt precursor in a basic solution and reduction under H2 after drying, as catalytic reactor was reported in the literature and a very promising result was shown without any additives as well as an increased productivity over time.26 In the present study, Pd@GPMHS40 was used under the same conditions, with tunable feeds of hydrogen to control conversion. Substrate flow rate was fixed here, as its impact on conversion

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Figure 6. Hydrogenation of 3-hexyn-1-ol (0.2 M in MeOH) through the catalytic monolith Pd@GPMHS40 (0.9 wt Pd %) at 40°C with a substrate flow rate of 0.2 mL min-1 and a variable H2 flow rate (variable H2/substrate ratio). As shown in Figure 6, the conversion and selectivity vary with the flow rate of gas and substrate solution. The increase of H2 flow rate not only results in the increase of conversion by adding more H2 in contact with the substrate and the catalyst as expected, but also leads to a decrease of selectivity toward the formation of alkene as well as the cis-selectivity. In the case of Pd@GPMHS40, almost complete conversion was obtained by increasing the H2 feed at 1.4 mL min-1 but the selectivity is maintained above 80 % (100% conversion and 40 %


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selectivity for Pd@MonoSil). Moreover, at half-conversion, the selectivity toward alkenes in the case of Pd@GPMHS40 is maintained over 90%. In the case of the GPMHS40, hydrophobicity is increased by the grafted polymers inside the mesopores, improving diffusion of reactants by suppressing negative effects from silanol groups, as expected in a previous work.26 Pd NPs of 10 to 20 nm imply Pd atoms in edges or angles and more on the plane surfaces of nanoparticles as compared to 6 nm NPs of Pd@MonoSil, impacting directly the activity and selectivity of Pd NPs toward alkenes. Indeed, those plane surfaces greatly promote the selective hydrogenation to cisalkene,55 while reactivity is slightly sacrificed, and are more suitable for formation/desorption of cis-alkene product, explaining the high chemo- and cis- selectivities at high conversion. The localization of NPs might also be responsible for those catalytic results, as the particles on the skeleton are also be linked to the increase of selectivity towards alkenes.54 Finally, the best results are obtained with a flow rate of 1.0 mL min-1, a conversion of 80 %, a selectivity of 88 % toward alkenes and a stereoselectivity of 80 %, giving a productivity of 13.9 mmol h-1 g(GPMHS)-1 and 1.57 mol h-1 g(Pd)-1. Those results, activities and both selectivities, were superior to already reported monoliths such as Pd@MonoSil (selectivity toward alkenes about 70% and a cis-selectivity of 70%)26 and Pd@TiO2 (selectivity toward alkenes of 63% and a cisselectivity about 87%).47 Compared to Pd@MonoSil, the calculated productivity per gram of palladium is twofold increased.9c The calculated Space-time yield (STY) of the Pd@GPMHS40 catalytic column is 354 g L-1 h-1, more than ten times higher than the benchmark value of the Lindlar Catalyst (26 g L-1 h-1) and 30% over the Pd@MonoSil (270 g L-1 h-1).26,56 CONCLUSION In summary, we have demonstrated a methodology to functionalize the silica monolith with a polymeric hydrosilane PMHS and a Lewis acid catalyst B(C6F5)3, by surface modification in a


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flow. With remaining Si-H groups available on the surface, a second flow of solution containing palladium ions has successfully led to supported Pd nanoparticles with 15 nm, which are homogeneously distributed inside the monolith without blocking pores. The obtained materials have been used as column-type monolith catalysts for hydrogenation reactions using an HPLC-type continuous flow system, showing high activities and long-time stability over days. They also show high chemo- and stereo- selectivities in the case of the selective hydrogenation of alkynes into alkenes, and are proved to be highly productive compared to the Lindlar catalyst benchmark but also to the already reported bared Pd@MonoSil and Pd@TiO2 monoliths.

AUTHOR INFORMATION Corresponding Author [email protected] [email protected]

ACKNOWLEDGMENT Financial support by Advanced Low Carbon Technology Research and Development Program (ALCA) from Japan Science and Technology Agency (JST) is gratefully acknowledged. ABBREVIATIONS NPs, Nanoparticles; HSQ, Hydrogen Silsesquioxane; PMHS, Polymethylhydrosiloxane; GPMHS, Grafted Polymethylhydrosiloxane. REFERENCES (1) Chen, Y.; Chen, H.; Shi, J. In Vivo Bio-Safety Evaluations and Diagnostic/Therapeutic Adv. Mater. 2013, 25, 3144-3176.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


Graphical Abstract



Figure 1

3

600

2,5

dVp/dlog(dp)

700

500

V ads

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 22 of 27

400

2 1,5

300 1 200 0,5 100 0

0 0

0,2

0,4 Paragon 0,6 0,8Environment 1 ACS Plus

P/P0

0

10

20

30

dP (nm)

40

50

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


Figure 2

D2H GPMHS40

D2OH PMHS40

T3 Q3 / Q4

SiO2 monolith

4000

3500

3000

2500

2000

1500

1000

500

0

-20

-40

-60

ppm

Wavenumber / cm-1


-80

-100

-120


Figure 3

1,8

dVp/dlog(dp)

450 400 350 300

Vads

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 24 of 27

250

1,6 1,4 1,2 1 0,8 0,6

200

0,4

150

0,2

100

0

50

0

10

20

0 0

0,2

0,4

0,6

0,8

P/P0


1

dP (nm)

30

40

50

Page 25 of 27

Figure 4

Size of NPs (nm)


>22

20-22

18-20

16-18

14-16

12-14

10-12

8-10

16 14 12 10 8 6 4 2 0

Grafted Polymethylhydrosiloxane on Hierarchically Porous Silica

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