Formation of Micro and Mesoporous Amorphous Silica-Based Materials from Single Source Precursors

inorganics

Article Formation of Micro and Mesoporous Amorphous Silica-Based Materials from Single Source Precursors

Mohd Nazri Mohd Sokri 1,2, Yusuke Daiko 1,3, Zineb Mouline 1, Sawao Honda 1,3 and Yuji Iwamoto 1,3,*

1 Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan; [email protected] (M.N.M.S.); [email protected] (Y.D.); [email protected] (Z.M.); [email protected] (S.H.) 2 Department of Energy Engineering and Advanced Membrane Technology Research Centre, Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru 81310, Malaysia 3 Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan * Correspondence: [email protected]; Tel./Fax: +81-52-735-5276

Academic Editors: Samuel Bernard, André Ayral and Philippe Miele Received: 8 January 2016; Accepted: 1 March 2016; Published: 9 March 2016

Abstract: Polysilazanes functionalized with alkoxy groups were designed and synthesized as single source precursors for fabrication of micro and mesoporous amorphous silica-based materials. The pyrolytic behaviors during the polymer to ceramic conversion were studied by the simultaneous thermogravimetry-mass spectrometry (TG-MS) analysis. The porosity of the resulting ceramics was characterized by the N2 adsorption/desorption isotherm measurements. The Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopic analyses as well as elemental composition analysis were performed on the polymer-derived amorphous silica-based materials, and the role of the alkoxy group as a sacriﬁcial template for the micro and mesopore formations was discussed from a viewpoint to establish novel micro and mesoporous structure controlling technologies through the polymer-derived ceramics (PDCs) route.

Keywords: perhydropolysilazane; alkyl alcohol; single source precursor; polymer-derived ceramics; micro and mesoporous silica

1. Introduction The organometallic precursor route has received increased attention as an attractive ceramic processing method since it has inherent advantages over conventional powder processing methods such as purity control, compositional homogeneity in the ﬁnal ceramic product and lower processing temperatures in the ceramics fabrication [1–3]. Moreover, this route provides alternatives towards the synthesis of advanced silicon-based non-oxide ceramics such as silicon nitride (Si3N4)-based ceramics, particularly starting from silicon-based polymers such as polysilazanes [4–6]. Recently, micro and mesoporous structure formations have been often discussed for the polymer-derived amorphous silicon nitride [7,8], silicon carbide [9–14], silicon carbonitride (Si–C–N) [15], silicon oxycarbide (Si–O–C) [16–18] and quaternary Si–M–C–N (M = B [19,20], Ni [21]). During the crosslinking and subsequent high-temperature pyrolysis under an inert atmosphere of polymer precursors, by-product gases such as CH4, NH3 and H2 were detected [5,22], and the microporosity in the polymer-derived non-oxide amorphous ceramics could be assigned to the release of the small gaseous species formed in-situ [8,22]. Iwamoto et al. [23] reported an approach in synthesizing micro and mesoporous amorphous silica using organo-substituted polysilazane precursors, in which the organic moieties acted as a “sacriﬁcial

Inorganics 2016, 4, 5; doi:10.3390/inorganics4010005 www.mdpi.com/journal/inorganics Inorganics 2016, 4, 5 2 of 14

Inorganics 2016, 4, 5 2 of 14 template” during polymer to ceramic conversion by heat treatment in air, thus allowing the micro and mesoporous“sacrificial template” structure during formation. polymer to ceramic conversion by heat treatment in air, thus allowing the microIn our and recent mesoporous studies, commercially structure formation. available perhydropolysilazanes (PHPS) was used as a starting polymer.In our The recent PHPS studies, contains commerc many reactiveially available Si–H and perhydropolysilazanes N–H groups which can(PHPS) react was with used chemical as a modiﬁersstarting polymer. to yield The novel PHPS ternary contains or quaternary many reactive Si-based Si–H non-oxide and N–H ceramics groups which [24–27 ].can In react addition, with comparedchemical modifiers with polysiloxanes to yield novel often usedternary as aor precursor quaternary for silica,Si-based PHPS non-oxide can be easily ceramics oxidized [24–27]. at low In temperaturesaddition, compared to yield purewith silicapolysiloxanes in high ceramic often yieldused dueas a to precursor the weight for gain silica, as shown PHPS in can the followingbe easily equationoxidized [at28 –low30]. temperatures to yield pure silica in high ceramic yield due to the weight gain as shown in the following equation [28–30].

rSiH[SiH2 ´2–NH]NHsnn `+ nnOO22 →Ñ nnSiOSiO22 +` nNHnNH3 ↑3 Ò (1)(1) InIn ourour previousprevious studies,studies, alkoxyalkoxy groupsgroups werewere introducedintroduced toto PHPS,PHPS, andand thethe resultingresulting chemicallychemically modifiedmodified PHPSPHPS waswas convertedconverted toto microporousmicroporous amorphousamorphous silicasilica byby oxidativeoxidative crosslinkingcrosslinking atat 270270 ˝°CC followedfollowed by heat heat treatment treatment at at 600 600 °C˝ Cin inair air (route (route R1 R1in Figure in Figure 1) [31].1)[ 31Mi].croporous Microporous amorphous amorphous silica silicawas also was synthesized also synthesized through through the two the step two conversi step conversionon route, route,(i) room (i) roomtemperature temperature oxidation oxidation of the ofchemically the chemically modified modified PHPS PHPSto afford to affordan air-stable an air-stable alkoxy alkoxy group-functionalized group-functionalized amorphous amorphous silicasilica-basedbased inorganic-organic inorganic-organic hybrid; hybrid; and (ii) andsubsequent (ii) subsequent heat treatment heat treatment at 600 °C in at air 600 (route˝C in R2 air in Figure (route R21) [32]. in Figure In continuation1)[ 32]. In of continuation our ongoing of studies, our ongoing we herein studies, report we the herein effect report of the the heat effect treatment of the heatatmosphere treatment on atmosphereporous structure on porous development structure developmentof the polymer-derived of the polymer-derived amorphous amorphoussilica-based silica-basedmaterials. The materials. polymer/ceramics The polymer/ceramics thermal conversi thermalon conversion under an underargon anflow argon was flow performed was performed on the onpolymer the polymer precursors precursors and the polymer- and the polymer-derivedderived hybrid silica hybrid (route silica R3 and (route R4 in R3 Figure and R4 1). inThe Figure thermal1). Theconversion thermal conversionbehaviors behaviorswere in- weresitu inanalyzed-situ analyzed by the by thesimultaneous simultaneous thermogravimetry-mass spectrometryspectrometry (TG-MS)(TG-MS) analysis.analysis. TheThe relationshipsrelationships betweenbetween thethe heatheat treatmenttreatment atmosphere,atmosphere, gaseousgaseous speciesspecies formedformed inin--situsitu duringduring thethe thermalthermal conversionconversion andand thethe micro and mesoporosity inin the resulting amorphousamorphous silica-based silica-based materials materials are are discussed discussed and and compared compared with with our ourprevious previous results results [32] in [32 order] in orderto achieve to achieve a better a better understanding understanding of the ofthe micro micro and and mesoporous mesoporous structure structure development development in thethe polymer-derivedpolymer-derived amorphous-silicaamorphous-silica basedbased materials.materials.

Micro and mesoporous amorphous silica-based materials

R1, R4 R2, R3

NH H Si NH NH N N SiH OR R H2Si 2 3 n R-OH H H Oxidation O N SiH SiH SiH O O O R Si SiH O Si Si -n H2 N NH Si O H Si N N N at r.t. 2 O O O O O HN HSi SiH SiH SiH Si Si 2 2 Name R Si O O NH O O HN P0 None SiH2 R P5 n-C5H11 Perhydropolysilazane (PHPS) P10 n-C H Alkoxy group-functionalized 10 21 amorphous silica

R1 R2 1h R31h R4 1h 1000

800 1h 1h 1h 1h 600

400 (a)

Temperature / ºC / Temperature 200 In air (b) In air (b) Under Ar flow Under Ar flow 0 Time/ min. (a): Cross-linking in air at 270 ºC for 1h. (b): Oxidation at room temperature. Figure 1. Synthesis of micro- and mesoporous amorphous silica-based materials through polymer derivedFigure 1. ceramics Synthesis (PDCs) of micro- routes and investigated mesoporous in this amorphous study. silica-based materials through polymer derived ceramics (PDCs) routes investigated in this study.

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2.2. Results Results and and Discussion Discussion

2.1.2.1. Polymer Polymer Precursors Precursors and and Alkoxy Alkoxy Group-Functionalized Group-Functionalized Amorphous Silica Silica TheThe chemical chemical structures structures of of the the polymer polymer precursorsprecursors and those those of of the the alkoxy alkoxy group-functionalized group-functionalized inorganic-organicinorganic-organic hybrid hybrid silica silica were were assessedassessed based on on their their FT-IR FT-IR spectra. spectra. As As shown shown in inFigure Figure 2a,2 a, thethe as-received as-received PHPS PHPS exhibited exhibited absorption absorption bandsbands at 3400 ( (ννN–H),N–H), 2150 2150 (ν (νSi–H),Si–H), 1180 1180 (δ (N–H)δN–H) and and 840–1020840–1020 cm cm´1−1( δ(δSi–N–Si)Si–N–Si) [[4,31–33].4,31–33]. The PHPSs modified modiﬁed wi withth the the alcohols alcohols (Figure (Figure 2b,c)2b,c) presented presented ´1 additionaladditional absorption absorption bands bands atat 2950–28502950–2850 ( (ννC–H),C–H), 1450 1450 (δ (CHδCH3) and3) and 1090 1090 cm− cm1 (νSi–OR)(νSi–OR) [31,32,34]. [31,32 As,34 ]. Asshown shown in in Figure Figure 2a,2a, after after the the room room temperature temperature oxid oxidation,ation, the theabsorption absorption bands bands corresponding corresponding to the to theas-received as-received PHPS PHPS completely completely disappeared disappeared and andnew newabsorption absorption bands bands appeared appeared at 3400 at (Si–OH) 3400 (Si–OH) and and1090 1090 cm cm−1 (Si–O–Si)´1 (Si–O–Si) [32,34]. [32, 34In]. addition In addition to these to these band bands,s, the chemically the chemically modified modiﬁed PHPSs PHPSs exhibited exhibited C– C–HH absorption bands bands in in the the vicinity vicinity of of2950 2950 to to2850 2850 cm cm−1 (Figure´1 (Figure 2b,c).2b,c).

R2: 600oC-heated (a) R2: 600oC-heated (b) R2: 600oC-heated (c) in air in air in air

R3: 600oC-heated R3: 600oC-heated R3: 600oC-heated in Ar in Ar in Ar * *

Oxidized at. r.t. Oxidized at. r.t. Oxidized at. r.t.

Transmittance / a. u. Transmittance Polymer state Polymer state Polymer state * * *

4000 3000 2000 1000 4000 3000 2000 1000 4000 3000 2000 1000 Wavenumber / cm-1

FigureFigure 2. FT-IR2. FT-IR spectra spectra for polymerfor polymer precursors precursors and the and precursor-derived the precursor-derived samples. samples. Polymer Polymer precursor: precursor: (a) P0; (b) P5 and (c) P10 (* indicating background, absorption due to CO2). (a) P0;(b) P5 and (c) P10 (* indicating background, absorption due to CO2).

The chemical structure of the oxidized samples was further studied by the solid state 29Si magic 29 angleThe spinning chemical (MAS) structure NMR of spectroscopy. the oxidized samplesAs a typical was result, further the studied spectrum by thefor solidthe sample state derivedSi magic angle spinning (MAS) NMR spectroscopy. As a typical result, the spectrum for the sample derived from PHPS chemically modified with n-C10H21OH (P10) is shown in Figure 3. The characteristic peaks fromdue PHPS to the chemically structural units modiﬁed of as-received with n-C 10PHPSH21OH at around (P10) is−35 shown ppm in(SiHN Figure3/SiH3.2N The2) and characteristic −50 ppm peaks(SiH due3N) [35] to thecompletely structural disappeared, units of as-received and the spectrum PHPS presented at around new´35 broad ppm peaks (SiHN assigned3/SiH2 Nto2 )the and ´50units ppm which (SiH 3composedN) [35] completely amorphous disappeared, silica at −78, and −89, the −85 spectrum to −94 and presented −112 ppm new assigned broad peaksto Q1( assigned≡SiO– toSiX the3), units Q2 (( which≡SiO)2SiX composed2), Q3 ((≡ amorphousSiO)3SiX) and silica Q4 (( at≡SiO)´78,4Si))´89, (X ´= 85OH, to OC´9410H and21), respectively.´112 ppmassigned These toresults Q1(”SiO–SiX indicate3 ),that Q2 the ((” as-receivedSiO)2SiX2), PHPS Q3 (( and”SiO) the3 SiX)chemically and Q4 modified ((”SiO) PHPSs4Si)) (X were = OH, successfully OC10H21 ), respectively.converted to These amorphous results indicatesilica, and that alkoxy the as-received group-functionalized PHPS and inorganic-organic the chemically modiﬁed hybrid silica, PHPSs wererespectively. successfully converted to amorphous silica, and alkoxy group-functionalized inorganic-organic hybrid silica, respectively.

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Figure 3. 29Si MAS NMR spectrum for alkoxy group-functionalized amorphous silica synthesized by Figure 3. 29Si MAS NMR spectrum for alkoxy group-functionalized amorphous silica synthesized by room temperature oxidation of P10 (PHPS chemically modified with n-C10H21OH). room temperature oxidation of P10 (PHPS chemically modiﬁed with n-C10H21OH). 2.2. Thermal Conversion to Amorphous Silica-Based Materials 2.2. Thermal Conversion to Amorphous Silica-Based Materials The oxidative cross-linking at 270 °C and subsequent heat treatment at 600 °C in air of the The oxidative cross-linking at 270 ˝C and subsequent heat treatment at 600 ˝C in air of the polymer precursors (route R1) yielded carbon-free amorphous silica (Table 1, route R1, 600 °C). The polymer precursors (route R1) yielded carbon-free amorphous silica (Table1, route R1, 600 ˝C). alkoxy group-functionalized inorganic-organic hybrid silica could be also converted to carbon-free The alkoxy group-functionalized inorganic-organic hybrid silica could be also converted to carbon-free amorphous silica by the 600 °C-heat treatment in air (Table 1, route R2, 600 °C), and the C–H amorphous silica by the 600 ˝C-heat treatment in air (Table1, route R2, 600 ˝C), and the C–H absorption bands completely disappeared in their FT-IR spectra (R2 in Figure 2b,c). On the other absorption bands completely disappeared in their FT-IR spectra (R2 in Figure2b,c). On the other hand, the 600 °C-heat treatment in argon (Ar) resulted in the detection of C–H absorption bands (R3 hand, the 600 ˝C-heat treatment in argon (Ar) resulted in the detection of C–H absorption bands (R3 in Figure 2b,c). The amount of carbon remained in the P5 and P10-derived samples were 6.2 and 9.5 in Figure2b,c). The amount of carbon remained in the P5 and P10-derived samples were 6.2 and wt %, respectively (Table 1, route R3, 600 °C). 9.5 wt %, respectively (Table1, route R3, 600 ˝C).

Table 1. Chemical composition of polymer-derived amorphous silica-based materials synthesized in Table 1. Chemical composition of polymer-derived amorphous silica-based materials synthesized in this study. this study. Conversion Flow Temp. Composition / wt% Polymer Conversion Flow Composition/wt % Empirical Ratio Polymer Route Gas Temp.// °C ˝C Si C O N H Empirical Ratio Route Gas Si C O N H R1 air 600 68.1 0.0 30.2 0.7 0.0 Si1.0C0.00O0.78N0.02H0.00 R1 air 600 68.1 0.0 30.2 0.7 0.0 Si1.0C0.00O0.78N0.02H0.00 R2 air 600 59.1 0.0 39.3 0.6 0.0 Si1.0C0.00O1.17N0.02H0.00 R2 air 600 59.1 0.0 39.3 0.6 0.0 Si1.0C0.00O1.17N0.02H0.00 R3R3 Ar Ar 600 600 54.5 54.5 0.0 42.442.4 1.91.9 0.2 0.2 Si1.0CSi0.001.0CO0.001.37ON1.370.07N0.07H0.10H0.10 P0 P0 R4 Ar 600 65.8 0.9 4.8 26.9 0.6 Si1.0C0.03O0.13N0.82H0.25 R4 Ar 600 65.8 0.9 4.8 26.9 0.6 Si1.0C0.03O0.13N0.82H0.25 R3 Ar 1000 58.1 0.0 40.2 0.5 0.2 Si1.0C0.00O1.21N0.02H0.10 R3R4 Ar Ar 1000 1000 58.1 63.4 0.0 0.9 5.040.2 29.10.5 0.6 0.2 Si1.0CSi0.031.0CO0.000.14ON1.210.92N0.02H0.26H0.10

R4R1 Ar air 1000 600 63.4 64.7 0.9 0.0 34.2 5.0 0.129.1 0.0 0.6Si 1.0CSi0.001.0CO0.030.93ON0.140.003N0.92H0.00H0.26 R2 air 600 60.7 0.0 38.2 0.1 0.0 Si1.0C0.00O1.10N0.003H0.00 P5 R1 air 600 64.7 0.0 34.2 0.1 0.0 Si1.0C0.00O0.93N0.003H0.00 R3 Ar 600 55.6 6.2 36.2 0.6 0.4 Si1.0C0.26O1.13N0.02H0.20 R2R3 air Ar 600 1000 60.7 52.7 0.0 4.1 41.838.2 0.30.1 0.1 0.0 Si SiC 1.0CO0.00ON1.10N0.003H H0.00 P5 1.0 0.18 1.39 0.01 0.05 R3R1 Ar air 600 600 55.6 64.7 6.2 0.0 34.336.2 0.00.6 0.0 0.4 Si1.0CSi0.001.0CO0.260.93ON1.130.00N0.02H0.00H0.20 R2 air 600 63.1 0.0 35.9 0.0 0.0 Si C O N H R3 Ar 1000 52.7 4.1 41.8 0.3 0.1 1.0 Si0.001.0C0.181.00O1.390.00N0.010.00H0.05 R3 Ar 600 51.9 9.5 36.6 0.1 0.9 Si1.0C0.43O1.24N0.004H0.48 1.0 0.00 0.93 0.00 0.00 P10 R1R4 air Ar 600 600 64.7 57.7 0.0 6.7 30.234.3 3.00.0 1.4 0.0 Si1.0CSi0.27CO0.92ON0.10NH0.68H R2R2 air air 600 1000 63.1 63.6 0.0 35.435.9 0.00.0 0.0 0.0 Si1.0CSi0.001.0CO0.000.98ON1.000.00N0.00H0.00H0.00 R3 Ar 1000 49.4 6.1 42.3 0.0 1.2 Si1.0C0.29O1.50N0.00H0.68 R3 Ar 600 51.9 9.5 36.6 0.1 0.9 Si1.0C0.43O1.24N0.004H0.48 R4 Ar 1000 59.6 8.4 14.9 14.4 1.7 Si1.0C0.33O0.44N0.48H0.79 P10 R4 Ar 600 57.7 6.7 30.2 3.0 1.4 Si1.0C0.27O0.92N0.10H0.68

R2 air 1000 63.6 0.0 35.4 0.0 0.0 Si1.0C0.00O0.98N0.00H0.00

R3 Ar 1000 49.4 6.1 42.3 0.0 1.2 Si1.0C0.29O1.50N0.00H0.68

R4 Ar 1000 59.6 8.4 14.9 14.4 1.7 Si1.0C0.33O0.44N0.48H0.79

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2.3. TG-MSInorganics Analysis 2016, 4, 5 5 of 14 To2.3. study TG-MS the Analysis effect of atmosphere on the pyrolytic behaviors more extensively, TG-MS analysis was performedTo study on the the effect selected of atmosphere samples. on The the results pyroly weretic behaviors summarized more extensively, and shown TG-MS in Figures analysis4 and 5. The PHPS-derivedwas performed amorphouson the selected silica samples. synthesized The result bys were the room summarized temperature and shown oxidation in Figures showed 4 and a 5. slight weightThe loss PHPS-derived at 150 to 400 amorphous˝C (Figure silica4(Aa)). synthesized The MS spectrumby the room measured temperature at 200 oxidation˝C was showed composed a slight of m/z ratiosweight at 106, loss 91, at 77, 150 63, to 51 400 and °C 39, (Figure which 4(Aa)). was identicalThe MS spectrum to that reported measured for at xylene 200 °C (Figurewas composed4(Ab)) [of36 ,37]. Sincem the/z ratios molecular at 106, 91, ion 77, (m/z 63, =51106) and 39, and which the tropyliumwas identical ion to (thatm/z reported= 91) were for xylene detected (Figure at 400 4(Ab))˝C and below[36,37]. (Figure Since4(Ac)), the molecular the observed ion (m/z weight = 106) loss and isthe mainly tropylium due ion to the(m/z residual= 91) were xylene detected [ 32 at]. 400 When °C the and below (Figure 4(Ac)), the observed weight loss is mainly due to the residual xylene [32]. When PHPS was directly heat-treated under the inert atmosphere, the weight loss up to 400 ˝C increased the PHPS was directly heat-treated under the inert atmosphere, the weight loss up to 400 °C increased˝ to approximatelyto approximately 20 20 wt wt % % (Figure (Figure 44(Ba)).(Ba)). A Atypical typical MS MSspectrum spectrum measured measured at 200 to at 250 200 °C tois shown 250 C is + + shownin Figure in Figure 4(Bb).4(Bb). The m The/z ratiosm/ zatratios 76 and at 30 76 were and assigned 30 were to assignedSiH2–NH–SiH to SiH3+ and2–NH–SiH SiH2+, respectively.3 and SiH 2 , + + respectively.The smaller The species, smaller m species,/z ratiosm at/z 28ratios and at17 28 were and assigned 17 were assignedto N2+ and to NH N23+, andrespectively. NH3 , respectively. These Thesefragment fragment ions ions could could be formed be formed in-situin -bysitu theby thermal the thermal decomposition decomposition of the silazane of the silazanerings. The rings. The evolutionsevolutions of these these gaseous gaseous species species were were found found to be to completed be completed at around at around 400 °C 400(Figure˝C 4(Bc)). (Figure 4(Bc)).

20 100 20 100 a) a) 0 80 0 80 -20 60 -20 60

-40 40 -40 40 DTAµV / -60 20 -60 20 µV DTA / -80 0 -80 0 Weight change / % Weight change / % -100 -100 0 200 400 600 800 1000 0 200 400 600 800 1000 Temperature / °C Temperature / °C

0.4 15 b) 91 b) 30 / a. u. 0.3 / a. u. 3 4 10

0.2 39 51 106 5 0.1 63 77 17 28 76 0 0 Intensity x 10 xIntensity 10 30 50 70 90 110 10 x Intensity 10 2030 40 50 60 70 80

m/z m/z 0.3 140 c) m/z c) m/z + 91 (C7H7 ) 120 17 (NH +) + 3 / a. u./ a. 106 (C8H10 ) 28 (N +) / a. u. 2 4 100

0.2 4 + 30 (SiH2 ) + 80 76 (SiH2-NH-SiH3 )

60 0.1 40

Intensity x 10 xIntensity 20 Intensity x 10 xIntensity 0 0 0 200 400 600 800 1000 0 200 400 600 800 1000 Temperature / °C Temperature / °C (A) (B)

FigureFigure 4. Thermal 4. Thermal behavior behavior of of (A ()A PHPS) PHPS ( P0(P0)-derived)-derived amorphous silica silica under under oxidative oxidative atmosphere atmosphere (route(route R2); andR2); ( Band) as-received (B) as-received PHPS PHPS under under an inert an atmosphere inert atmosphere (route (route R4). (a )R4). TG-DTA; (a) TG-DTA; (b) monitoring (b) monitoring of gaseous species by mass spectrometry at 200 °C and (c) continuous in-situ monitoring of gaseous species by mass spectrometry at 200 ˝C and (c) continuous in-situ monitoring of gaseous of gaseous species by mass spectrometry. species by mass spectrometry. As shown in Figure 5(Aa), under the oxidative atmosphere (route R2), the P10-derived hybrid Assilica shown having in n Figure-C10H21O5(Aa), groups under exhibited the oxidative a weight loss atmosphere of approximately (route R2),50% theat 150P10 to-derived 600 °C, and hybrid ˝ silicathe having differentialn-C10H thermal21O groups analysis exhibited (DTA)a resulted weight lossin the of detection approximately of a dominant 50% at 150 exothermic to 600 C,peak and the differential thermal analysis (DTA) resulted in the detection of a dominant exothermic peak centered at 320 ˝C. The MS spectrum measured at 320 ˝C is shown in Figure5(Ab). The sequential peaks 14 mass Inorganics 2016, 4, 5 6 of 14 Inorganics 2016, 4, 5 6 of 14 centered at 320 °C. The MS spectrum measured at 320 °C is shown in Figure 5(Ab). The sequential peaks 14 mass units apart at m/z = 71, 57 and 43 can be assigned to the fragment ions derived from units apart at m/z = 71, 57 and 43 can be assigned to the fragment ions derived from hydrocarbons hydrocarbons formed in situ by the typical α-cleavage in the n-C10H21O group, followed by the α formed in situ by the typical -cleavage in the n-C10H21O group, followed by the sequential C–C bond●+ sequential C–C bond cleavage to release methylene units [36,38]. The m/z ratios+ at m/z = 70 (C5+H10 ) cleavage to release methylene units [36,38]. The m/z ratios at m/z = 70 (C5H10 ) and 55 (C4H7 ) could and 55 (C4H7+) could be derived from traces of unreacted n-C10H21OH [36,39]. The smaller species, m/z be derived from traces of unreacted n-C H OH [36,39]. The smaller species, m/z ratios at 44 and 18 ratios at 44 and 18 were assigned to CO102+ and21 H2O+, respectively. By monitoring the fragment ions were assigned to CO + and H O+, respectively. By monitoring the fragment ions with m/z ratios at with m/z ratios at 85,2 71, 57 2and 43, it was confirmed that the decomposition of n-C10H21O group 85,mainly 71, 57 occurred and 43, it at was 250 conﬁrmed to 500 °C (Figure that the 5(Ac)). decomposition Simultaneously, of n-C 10combustionH21O group of mainlythe hydrocarbon occurred at ˝ 250species to 500 startedC (Figure at around5(Ac)). 250 Simultaneously, °C, and the evolution combustion of CO of2 and the hydrocarbonH2O continued species to approximately started at around 650 ˝ ˝ 250°C C,(Figure and the5(Ad)). evolution of CO2 and H2O continued to approximately 650 C (Figure5(Ad)).

20 200 20 200 20 200 a) a) a) 0 160 0 160 0 160 -20 120 -20 120 -20 120

-40 80 -40 80 -40 80 DTAµV / DTAµV / -60 40 DTAµV / -60 40 -60 40

0 Weight change / % -80 0 -80 0 Weight change / % -80 Weight change / %

0 200 400 600 800 1000 0 200 400 600 800 1000 0 200 400 600 800 1000 Temperature / °C Temperature / °C Temperature / °C 25 25 25 b) 44 b) b) 43 / a. u.

/ a. u. 20 20 20 / a. u. 4

3 41 4 15 15 43 15

43 29 10 41 10 29 10 57 18 55 57 15 5 57 70 5 71 5 71 85 71 15 85 0 0 0 Intensity x 10 x Intensity Intensity x 10 xIntensity 10 30 50 70 1030 50 70 90 110 130 10 xIntensity 10 3050 70 90 110 130

m/z m/z m/z 14 140 140 c) m/z c) m/z c) m/z 120 + + 12 + 15 (CH3 ) 120 15 (CH3 )

43 (C H ) / a. u. 3 7 + / a. u. + 4

+ 29 (C H ) 4 29 (C H ) / a. u. 100 2 5 2 5 10 57 (C4H9 ) 100 4 + + 71 (C H +) 41 (C3H5 ) 43 (C3H7 ) 8 5 11 80 + 80 +

85 (C H +) 43 (C3H7 ) 57 (C4H9 ) 6 13 60 + + 6 57 (C4H9 ) 60 71 (C5H11 ) + + 4 40 71 (C5H11 ) 40 85 (C6H13 ) + 85 (C6H13 ) Intensity x 10 Intensity x 2 20 10 xIntensity 20 Intensity x 10 x Intensity 0 0 0 0 200 400 600 800 1000 0 200 400 600 800 1000 0 200 400 600 800 1000

Temperature / °C Temperature / °C Temperature / °C 12 120 120 d) m/z d) m/z d) m/z + + + 10 18 (H2O ) 100 18 (H2O ) 100 17 (NH3 ) + + + / a. u. / a. u. 44 (CO2 ) / a. u. 28 (CO ) 18 (H2O ) 4 4 8 4 80 80 + + 28 (N2 ,CO )

6 60 60 4 40 40 2 20 20 Intensity x 10 Intensity x Intensity x 10 x Intensity Intensity x 10 x Intensity 0 0 0 0 200 400 600 800 1000 0 200 400 600 800 1000 0 200 400 600 800 1000 Temperature / °C Temperature / °C Temperature / °C (A) (B) (C)

Figure 5. Thermal behavior of n-C10H21O group-functionalized hybrid silica under (A) oxidative Figure 5. Thermal behavior of n-C10H21O group-functionalized hybrid silica under (A) oxidative atmosphere (route R2); (B) inert atmosphere (route R3) and (C) that of PHPS functionalized with n- atmosphere (route R2); (B) inert atmosphere (route R3) and (C) that of PHPS functionalized with C10H21O groups (P10) under inert atmosphere (route R4). (a) TG-DTA; (b) monitoring of gaseous n-C10H21O groups (P10) under inert atmosphere (route R4). (a) TG-DTA; (b) monitoring of gaseous species by mass spectrometry at 320 °C; (c,d) continuous in situ monitoring of gaseous species by mass species by mass spectrometry at 320 ˝C; (c,d) continuous in situ monitoring of gaseous species by spectrometry. mass spectrometry. Under the inert atmosphere (route R3) (Figure 5(Ba)), the weight loss up to 600 °C of the P10- ˝ derivedUnder hybrid the inert silica atmospherewas decreased (route to 20%. R3) The (Figure MS spectrum5(Ba)), themeasured weight atloss 320 °C up was to 600composedC of of the ˝ P10-thederived above hybridmentioned silica species was decreased derived from to 20%. n-C The10H21 MSO groups spectrum and measured those with at 320the mC/z wasratios composed of 41 + + + of(C the3H above5 ), 29 (C mentioned2H5 ) and species15 (CH3 derived) (Figure from5(Bb)).n-C The10H gaseous21O groups species and formed those in with situ the werem/ detectedz ratios of + + + 41mainly (C3H5 at), 100 29 (Cto2 H6005 )°C and (Figure 15 (CH 5(Bc)).3 ) (Figure When 5the(Bb)). P10 Thewas gaseousdirectly speciesheat-treated formed underin an situ inertwere detectedatmosphere mainly (route at 100R4), tothe 600weight˝C loss (Figure up to5(Bc)). 600 °C When was rather the suppressedP10 was directly and measured heat-treated to be 38%. under an inert atmosphere (route R4), the weight loss up to 600 ˝C was rather suppressed and measured to be 38%. The thermal decomposition behavior of the P10 was similar to that characterized for the Inorganics 2016, 4, 5 7 of 14 Inorganics 2016, 4, 5 7 of 14 Inorganics 2016, 4, 5 7 of 14 The thermal decomposition behavior of the P10 was similar to that characterized for the n-C10H21O- The thermal decomposition behavior of the P10 was similar to that characterized for the n-C10H21O- functionalizedfunctionalized hybridhybrid silica,silica, andand itit waswas founfound that the relative volume fraction of the inin situ formedformed nC-C3H107+H (m21/O-functionalizedz = 43) remarkably hybrid increased silica, (Figure and it was 5C). found that the relative volume fraction of the in situ C3H7+ (m/z = 43) remarkably increased (Figure 5C). + formed C3H7 (m/z = 43) remarkably increased (Figure5C). 2.4. N2 Adsorption/Desorption Isotherm and Porosities Evaluated for the Polymer-Derived Amorphous 2.4. N2 Adsorption/Desorption Isotherm and Porosities Evaluated for the Polymer-Derived Amorphous 2.4.Silica-Based N2 Adsorption/Desorption Materials Isotherm and Porosities Evaluated for the Polymer-Derived Amorphous Silica-Based Materials The textural properties of the polymer-derived amorphous silica-based materials were studied The textural properties of the polymer-derived amorphous silica-based materials were studied by N2 physisorption at −196 °C. The adsorption/desorption isotherms are presented in Figure 6. Each by N2 physisorption at −196 °C.˝ The adsorption/desorption isotherms are presented in Figure 6. Each by N2 physisorption at ´196 C. The adsorption/desorption isotherms are presented in Figure6. sample exhibited an N2 uptake above p/po = 0.9, which was thought to be related to the porosity Eachsample sample exhibited exhibited an N an2 uptake N uptake above above p/pop /= p0.9,= 0.9,which which was was thought thought to tobe be related related to to the the porosity generated by agglomeration2 of the powdered sampleo [31]. generated by agglomeration of the powdered samplesample [[31].31].

160

-1 160 -1 g 600oC-heated (a) o (b) o (c) o (d) g

▪ 600 C-heated 600 C-heated 1000 C-heated 600oC-heated (a) o (b) o (c) o (d)

▪ 140 600 C-heated 600 C-heated 1000 C-heated 140 Polymer precursor: P0 Polymer precursor: P5 Polymer precursor: P10 P0 (R3, in Ar) Polymer precursor: P0 Polymer precursor: P5 Polymer precursor: P10 P0 (R3, in Ar) P0 (R4, in Ar)

(STP) 120 P0 (R4, in Ar)

(STP) 120 P5 (R3, in Ar) 3 P5 (R3, in Ar) 3 P10 (R2, in air) 100 P10 (R2, in air) 100 P10 (R3, in Ar) P10 (R3, in Ar) P10 (R4, in Ar) 80 P10 (R4, in Ar) 80 R1, in air R1, in air 60 R1, in air R1, in air 60 R2, in air R2, in air R2, in air R2, in air R3, in Ar R3, in Ar R1, in air 40 R3, in Ar R3, in Ar R1, in air 40 R4, in Ar R2, in air R4, in Ar R2, in air 20 R3, in Ar Volume adsorbed / adsorbedcm Volume 20 R3, in Ar Volume adsorbed / adsorbedcm Volume R4, in Ar R4, in Ar 0 0 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 p/p p/p p/p p/p p/po p/p o p/p o p/po o o o o

Figure 6. N2 sorption isotherms of polymer-derived samples. Figure 6. N2 sorption isotherms of polymer-derived samples. Figure 6. N2 sorption isotherms of polymer-derived samples.

Regardless of the thermal conversion condition investigated in this study, the PHPS (P0)-derived)-derived Regardless of the thermal conversion condition investigated in this study, the PHPS (P0)-derived amorphous silica generated a type III isotherm according to the International Union of Pure and amorphous silica generated a type III isotherm according to the International Union of Pure and Applied Chemistry (IUPAC) classification method [40,41] (Figure 6a), and the P0-derived-derived samplessamples Applied Chemistry (IUPAC) classiﬁcation method [40,41] (Figure6a), and the P0-derived samples were characterized as non-porous (Figure 7a). Consequently, it was clarified that the evolution below were characterized as non-porous (Figure7a). Consequently, it was clariﬁed that the evolution below 400 °C of the relatively large gaseous species (shown inin FigureFigure 4)4) diddid notnot contributecontribute toto thethe porousporous 400 ˝C of the relatively large gaseous species (shown in Figure4) did not contribute to the porous structure formation. structure formation.

-1 1.2

-1 1.2 600oC-heated (a) 600oC-heated (b) 600oC-heated (c) nm o (a) o (b) o (c)

▪ 600 C-heated 600 C-heated 600 C-heated nm ▪ g Polymer precursor: P0 Polymer precursor: P5 Polymer precursor: P10 ▪

g 1.0 Polymer precursor: P0 Polymer precursor: P5 Polymer precursor: P10 3 ▪ 1.0 3 R2, in air R1, in air R1, in air R2, in air R1, in air R1, in air R3, in Ar R2, in air R2, in air R3, in Ar R2, in air R2, in air 0.8 R4, in Ar R3, in Ar R3, in Ar 0.8 R4, in Ar R3, in Ar R3, in Ar R4, in Ar R4, in Ar 0.6 0.6

0.4 0.4

0.2 0.2 Pore volume distribution / cm distribution Pore volume Pore volume distribution / cm distribution Pore volume 0 00.4 0.8 1.2 1.6 2.0 10 100 0.4 0.8 1.2 1.6 2.0 10 100 0.4 0.8 1.2 1.6 2.0 10 100 0.4 0.8 1.2 1.6 2.0 10 100 0.4 0.8 1.2 1.6 2.0 10 100 0.4 0.8 1.2 1.6 2.0 10 100 Pore size / nm Pore size / nm Porre sizize // nmnm Pore size / nm Pore size / nm Figure 7. PorePore size size distribution distribution of of the the polymer-derive polymer-derivedd samples samples synthesized synthesized by byheat heat treatment treatment at 600 at Figure˝ 7. Pore size distribution of the polymer-derived samples synthesized by heat treatment at 600 600°C forC 1 for h. 1 h. °C for 1 h.

Inorganics 2016, 4, 5 8 of 14 Inorganics 2016, 4, 5 8 of 14

The isotherm of the P5-derived sample synthesized through through the route R1 exhibited a type I without distinct hysteresis loops. The slight N2 uptake at the relative pressure lower than p/po = 0.2 without distinct hysteresis loops. The slight N2 uptake at the relative pressure lower than p/po = 0.2 was related to the micropore filling (Figure 6b), and the micropore sizes evaluated by using N2 as was related to the micropore filling (Figure6b), and the micropore sizes evaluated by using N 2 as probe molecule were inin thethe rangerange ofof 0.430.43 toto 1.21.2 nmnm [[31]31] (Figure(Figure7 7b).b). The isothermisotherm ofof the theP10 P10-derived-derived sample sample synthesized synthesized through through the routethe route R1 also R1 presented also presented a similar a similar type (Figure 6c). The N2 uptake below p/po = 0.2 was apparently increased by increasing the type (Figure6c). The N 2 uptake below p/po = 0.2 was apparently increased by increasing the carbon carbonnumber number in the alkoxy in the groupalkoxy from group 5 to from 10, and 5 to the 10,resulting and the resulting micropore micropore size distribution size distribution plot peaked plot at peaked0.43 nm at and 0.43 extended nm and toextended approximately to approximately 1.6 nm in size1.6 nm [31] in (Figure size [31]7c). (Figure 7c). On the other hand, when the heat heat treatment treatment of the the P5P5-derived-derived hybrid hybrid silica silica was was performed performed under under flowing Ar, the N2 uptake below p/po = 0.2 apparently decreased (R3 in Figure 6b). The micropore flowing Ar, the N2 uptake below p/po = 0.2 apparently decreased (R3 in Figure6b). The micropore sizes sizesevaluated evaluated for the for resulting the resultingP5-derived P5-derived sample sample were belowwere below 1.0 nm, 1.0 and nm, thus and almost thus almost unchanged, unchanged, while whilethe micropore the micropore volume volume decreased decreased (R3 in Figure (R3 in7b). Figure In addition 7b). In to addition the similar to the microporosity similar microporosity change (R3 changein Figures (R36c in and Figures7c, the 6cP10 and-derived 7c, the sample P10-derived exhibited sample an increase exhibited in the an volume increase of mesoporesin the volume having of mesoporesa size range having of approximately a size range 2 of to approximately 20 nm (R3 in Figure 2 to 207c). nm (R3 in Figure 7c). The heat treatment under an Ar flow flow of the P10 also resulted in the micro and mesoporosity changes similar to those observed for the P10-derived-derived hybrid hybrid silica silica (R4 (R4 in in Figures Figures 66cc and7 7c.c. However,However, regardless of the atmospheric condition, the porosity in the P5 and P10-derived samples was lost during further heat treatment from 600 toto 1000 ˝°CC (Figure6 6d).d). Figure8 a8a presents presents the the relations relations between between the number the number of the carbonof the incarbon the alkoxy in the group alkoxy introduced group introducedto PHPS, the to conversion PHPS, the route conversion and the route resulting and microporethe resulting volume micropore of the heat-treatedvolume of the samples. heat-treated In our previoussamples. In studies our previous on the alkoxystudies group-functionalizedon the alkoxy group-functionalized PHPS [31] and PHPS hybrid [31] silicaand hybrid [32], the silica alkoxy [32], thegroup alkoxy with group C3 hydrocarbon with C3 hydrocarbon did not contribute did not cont toribute the porous to the porous structure structure development development [31], while [31], whilethose havingthose having a carbon a carbon chain chain longer longer than than or equal or equal to C5 to hydrocarbonC5 hydrocarbon were were found found to beto be effective effective as asa sacrificial a sacrificial template template for generating for generating microporosity micropor underosity theunder thermal the thermal conversion conversion in air [32 ].in Moreover, air [32]. Moreover,the micropore the micropore volume increased volume increased with increasing with incr theeasing total volumethe total ofvolume the released of the released gaseous gaseous species specieshaving ahaving kinetic a diameter kinetic belowdiameter 0.45 below nm. As 0.45 a result, nm. As the a micropore result, the volume micropore of the volumeP10-derived of the sample P10- 3 derivedreached 0.17sample cm 3reached/g (R2 in 0.17 Figure cm 8/ga), (R2 and in this Figure value 8a), was and compatible this value with was thatcompatible of the amorphous with that of silica the 3 amorphousfabricated without silica fabricated room temperature without room oxidation temperature (route R1 oxidation in Figure (route8a, 0.173 R1 in cm Figure3/g). It8a, should 0.173 cm also/g). be 2 Itnoted should that also the be specific noted surface that the area specific (SSA) surface of this area sample (SSA) was of 321.2 this sample m2/g, andwas almost321.2 m the/g, same and almost as that 2 theachieved same as by that the sampleachieved fabricated by the sample through fabricated the route through R1 (328 the m2 /g)route [32 R1]. (328 m /g) [32].

0.20 0.06 -1 -1 g

g (a) (b)

0.18 ▪ ▪ 3 3 Polymer ● R1: 600ºC in air 0.05 0.16 precursor: ● R2: 600ºC in air P10 0.14 ● R3: 600ºC in Ar 0.04 0.12 ● R4: 600ºC in Ar

0.10 ○ R2: 1000ºC in air 0.03 0.08 ○ R3: 1000ºC in Ar 0.06 ○ R4: 1000ºC in Ar 0.02 0.04

Mesopore volume / cm volume Mesopore 0.01 Micropore volume / cm volume Micropore 0.02 0 0 010192345678 R1 R2 R3 R4 R2 R3 R4 600°C 600°C 600°C 600°C 1000°C 1000°C 1000°C Number of carbon atom in alkoxy group in air in air in Ar in Ar in air in Ar in Ar

Figure 8. PorosityPorosity of polymer-derived amor amorphousphous silica-based materials. ( a) micropore micropore volume of the synthesized samples and (b) mesopore volumevolume ofof P10-derived-derived samples.samples.

Under the present thermal conversion in Ar, the gaseous species formed inin situ were C4–C1 hydrocarbons and CO with a kinetic diameter below 0.45 nm [[42],42], which contributed to the microporosity formation in the P5 and P10-derived samples. However, the total volume of the microporosity formation in the P5 and P10-derived samples. However, the total volume of the released gaseous species essentially decreased due to the lack of CO2 evolution, which led to a released gaseous species essentially decreased due to the lack of CO2 evolution, which led to a decrease decrease in the micropore volume and a considerable amount of carbon remaining in the resulting amorphous silica-based material. Moreover, it was found that the decrease in the micropore volume

Inorganics 2016, 4, 5 9 of 14 in the micropore volume and a considerable amount of carbon remaining in the resulting amorphous silica-based material. Moreover, it was found that the decrease in the micropore volume became much Inorganics 2016, 4, 5 9 of 14 more pronounced when the thermal conversion of P10 was performed after the room temperature oxidationbecame (R3 much at the more number pronounced of carbon when =the 10 thermal in Figure conversion8a). Simultaneously, of P10 was performed the mesopore after thevolume room in this sampletemperature turned oxidation out to (R3 be muchat the number higher of (0.055 carbon cm =3 10/g) in than Figure those 8a). Simultaneously, in other samples the (approximately mesopore 0.01–0.02volume cm 3in/g) this (Figure sample8b). turned One possibleout to be reason much forhigher the (0.055 decrease cm3 in/g) the than micropore those in volumeother samples associated with(approximately the increase in 0.01–0.02 mesopore cm volume3/g) (Figure may 8b). be One attributed possible to reason the pore for the growth decrease from in micro- the micropore to mesopore size rangevolume within associated the with amorphous the increase silica in mesopore matrix immediately volume may be densiﬁed attributed at to the the early pore growth stage of from the heat micro- to mesopore size range within the amorphous silica matrix immediately densified at the early treatment approximately below 400 ˝C. stage of the heat treatment approximately below 400 °C. Figure9 presents the Raman spectra for the P10-derived amorphous silica synthesized by heat Figure 9 presents the Raman spectra for the P10-derived amorphous silica synthesized by heat ˝ treatmenttreatment in air in air (route (route R2), R2), andand their assignments assignments areare listed listed in Table in Table 2. The2 spectra. The spectra for the 600 for °C the heat- 600 C heat-treatedtreated sample sample exhibited exhibited broad broad bands bands related related to th toe O–Si–O the O–Si–O and Si–O–Si and Si–O–Si asymmetrical asymmetrical vibrations vibrations at ´1 ´1 at 800800 and and 978 978 cm cm−1,, respectivelyrespectively [43]. [43]. The The weak weak broad broad bands bands above above 1000 1000 cm− cm1 werewere thought thought to be to be attributedattributed to theto the SiO SiO4 asymmetrical4 asymmetricalvibration vibration [44–46]. [44–46]. On On the the other other hand, hand, the thelower lower wavenumber wavenumber bandsbands below below 700 700 cm ´cm1 −were1 were attributed attributed to the Si–O–Si Si–O–Si symmetric symmetric vibrations vibrations originated originated from from structural structural defectsdefects such such as 3-, as 4-3-, and4- and 6-membered 6-membered Si–O–Si Si–O–Si rings within within the the amorphous amorphous silica silica matrix matrix [47–49]. [47– 49].

Polymer precursor: P10

R2: 1000°C in air

R2: 600°C in air

200 400 600 800 1000 1200 Raman shift / cm-1

FigureFigure 9. Raman 9. Raman spectra spectra for forP10 P10-derived-derived amorphous amorphous silica silica synthesized synthesized byby heat treatment in in air air (route (route R2). R2). Table 2. Assignment for the Raman spectra shown in Figure9. Table 2. Assignment for the Raman spectra shown in Figure 9.

´1 NumberNumber of of ν, (Si–O–Si)/cm −1 Notes Reference ν, (Si–O–Si) /cm SiO4 in Rings Notes Reference SiO4 in Rings 381 (ν ) 6 Observe in α-Carnegieite Matson et al.[48] 381s (νs) 6 Observe in α-Carnegieite Matson et al. [48] 463 (νs) 6 Observe in moganite Kingma and Hemley [49] 463 (νs) 6 Observe in moganite Kingma and Hemley [49] 465 (νs) 6 Observe in α-quartz Kingma and Hemley [49] 498 (465νs) (νs) 4 +6 6 Observe Observe in inα-quartz Leucite Kingma Matsonand Hemleyet al .[[49]48 ] 606 (498νs) (νs) 34 + 6 Observe in -Leucite Matson Uchino et al. et[48] al .[48] O–Si–O vibration (asymmetric 800 (606ν )-(νs) 3 - UchinoLi et etal. al [48].[43 ] as stretching mode) O–Si–O vibration 800 (νas) - Si–O–Si bond due to defect by Li et al. [43] (asymmetric stretching mode) 978 (νas)- surface silanol group Li et al.[43] Si–O–Si(asymmetric bond due stretching to defect mode) by Apopei et al.[44] 978 (νas) - Asymmetricsurface silanol stretching group Li et al. [43] 1000–1200 (ν ) - Makreski et al.[45] as (asymmetricSiO stretchingvibrations mode) 4 Ventura et al.[46] Apopei et al. [44] Asymmetric stretching SiO4 1000–1200 (νas) - Makreski et al. [45] vibrations ˝ On the other hand, after the high-temperature heat treatment up toVentura 1000 et alC,. [46] the spectrum presented one distinct band at 465 cm´1 due to the six-membered Si–O–Si ring [49]. Since the intensity

Inorganics 2016, 4, 5 10 of 14 of the characteristic bands related to the Si–O–Si and O–Si–O vibrations remarkably decreased, the 1000 ˝C heat-treated sample was thought to be composed of a highly rigid silica network. It has been suggested that PHPS contains 2-, 3- and 4-membered Si–N–Si ring clusters as microporous units [5,22]. Upon oxidation at room temperature, the ring clusters were readily oxidized to form Si–O–Si linkages, which could serve as nucleation sites for subsequent growth of micropores. Finally, after the 1000 ˝C-heat treatment, the six-membered Si–O–Si rings remained to form the intrinsic microporosity in amorphous silica. The estimated diameter of the six-membered ring is approximately 0.3 nm [50,51], which is too small for the N2 probe molecule to access (0.364 nm) [42]. Consequently, the resulting 1000 ˝C heat-treated sample was characterized as non-porous. On the other hand, the walls of micro- and mesopores formed by the thermal decomposition of the alkoxy groups are thought to be composed of disordered silica having considerable amount of dangling bonds, and the thermal stability of the polymer-derived micro- and mesoporous amorphous silica-based materials synthesized in this study is estimated to be around 600 ˝C.

3. Experimental Procedures

3.1. Precursor Synthesis Commercially available perhydropolysilazane (PHPS, Type NN110, 20% xylene solution, AZ Electronic Materials, Tokyo, Japan) was used as the starting polymer. n-Pentanol (n-C5H11OH) and n-decanol (n-C10H21OH) were used for the chemical modification of the PHPS. According to the published procedures [32], the reaction between the as-received PHPS and each alcohol was carried out under a dry Ar atmosphere using Schlenk techniques. A PHPS (Si basis)/ROH molar ratio of 4:1 was applied in all cases. In each synthesis, the alcohol was added dropwise to a xylene solution of as-received PHPS with magnetic stirring at room temperature, followed by the addition of toluene to decrease the PHPS concentration from 20 to 1 wt %. After the addition was complete, the mixture was refluxed for 1 h under an Ar flow and cooled to room temperature. The xylene and toluene were removed from the reaction mixture under vacuum to afford the alcohol adduct as a viscous liquid.

3.2. Conversion of Polymer Precursor to Amorphous Silica-Based Materials In this study, the polymer precursor was converted to amorphous silica-based material through the following four different routes shown in Figure1.

R1: Oxidative crosslinking and subsequent heat treatment in air [31] Polymer precursor was cured by heating at 270 ˝C in air to promote oxidative crosslinking with a heating rate of 100 ˝C/h and dwell time of 1 h in an alumina tube furnace. After cooling down to room temperature, the crosslinked polymer was obtained as white solid. The crosslinked polymer precursor was ground to a fine powder using a mortar and pestle, then heat-treated in an alumina tube furnace under an air flow by heating from room temperature to 600 ˝C in 6 h, maintaining the temperature at 600 ˝C for an additional 1 h, and finally cooling down to room temperature to give amorphous silica as white powders.

R2: Room temperature oxidation followed by heat treatment in air [32] Polymer precursor was converted to amorphous silica-based hybrid by exposing the polymer precursor to vapour from aqueous ammonia (NH3) according to the procedure reported by Kubo et al. [28,29]. In this process, 15 mL of aqueous NH3 was placed in a 200 mL beaker with a slightly open lid and the polymer precursor (ca. 600 mg) was suspended over the aqueous NH3 until the silica-based hybrid formed as white powder. The resulting amorphous silica-based hybrid was ground to a fine powder using a mortar and pestle, then heat-treated in an alumina tube furnace under an air flow up to 600 ˝C for a period of time of 6 h. Then the temperature was maintained at 600 ˝C for an additional hour, and finally cooled down to room temperature to afford amorphous silica as white powders. Inorganics 2016, 4, 5 11 of 14

R3: Room temperature oxidation followed by heat treatment in Ar The powdered sample of polymer-derived amorphous silica-based hybrid was heat-treated in an alumina tube furnace under an Ar flow up to 600 ˝C at a heating rate of 10 ˝C/min. Then the temperature was maintained at 600 ˝C for an additional hour, and finally cooled down to room temperature.

R4: Heat treatment in Ar Polymer precursor was heat-treated in an alumina tube furnace under Ar ﬂow up to 600 ˝C for a period of time of 1 h (heating rate of 10 ˝C/min). Then the temperature was maintained at 600 ˝C for an additional 1 h, and ﬁnally cooled down to room temperature. As shown in Figure1, 1000 ˝C-heat treatment was also performed for the polymer/ceramics conversion (R2, R3 and R4).

3.3. Characterization FT-IR spectra of the polymers and the polymer-derived amorphous silica-based materials were recorded using KBr pellets over the range of 4000 to 400 cm´1 (FT/IR-4200 IF, Jasco, Tokyo, Japan). Solid-state 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of the amorphous silica-based hybrids were measured at 79.45 MHz, 6.0 µs of 90˝ pulse length, 120 s of decay time between pulses, and spinning rate of 3500 Hz (Unity 400 plus, Varian, Palo Alto, CA, USA). Raman spectra of the amorphous silica-based materials were measured using 532 or 785 nm solid laser (Renishaw, Wotton-under-Edge, UK). The laser power of 5% was applied for the measurements. The thermal behaviors up to 1000 ˝C were studied by simultaneous TG and MS analyses (Model TG/DTA6300, Hitachi High Technologies Ltd., Tokyo, Japan/Model JMS-Q1050GC, JEOL, Tokyo, Japan). The measurements were performed under ﬂowing mixed gas of helium (He) and oxygen (O2) ˝ (He:O2 = 4:1, 100 mL/min) or He with a heating rate of 20 C/min. To avoid the presence of dominant + fragment peaks at around m/z = 32 (O2 ) related to the presence of O2, the m/z ratios in the range of 20 to 35 were excluded from the MS spectra recorded under the ﬂowing mixed gas of He and O2. Elemental analyses were performed on the heat-treated samples for oxygen, nitrogen and hydrogen (inert-gas fusion method, Model EMGA-930, HORIBA, Ltd., Kyoto, Japan), and carbon (non-dispersive infrared method, Model CS744, LECO Co., St Joseph, MI, USA). The silicon content in the samples was calculated as the difference of the sum of the measured C, N, O and H content to 100 wt % [35]. The pore size distribution for the heat-treated samples was determined using a nitrogen (N2) sorption technique with the relative pressure of the N2 gas ranging from 0 to 0.99 (Belsorp Max, BEL Japan Inc., Osaka, Japan). The micropores (rpore < 2.0 nm) and mesopores (2.0 nm ď rpore < 50 nm) of the polymer-derived amorphous silica-based materials were characterized by the SF [52] and BJH [53] methods, respectively.

4. Conclusions In this study, micro and mesoporous amorphous silica-based materials were successfully synthesized from single source precursors, PHPS functionalized with alkoxy groups (OR, R = n-C5H11, n-C10H21). The textural properties evaluated for the polymer-derived amorphous silica-based materials suggested the potential of the polymer precursors employed in this study to develop a novel microporous structure controlling technology by manipulating the carbon chain in the alkoxy groups introduced to PHPS. It is also interesting to note that the PDC routes investigated in this study show some potential to synthesize mesoporous amorphous silica-based materials by the two-step conversion process, the room temperature oxidation followed by the thermal treatment under an inert atmosphere. These porous structure control technologies are expected to be useful to synthesize amorphous silica-based materials for energy application such as a highly efﬁcient catalyst support and a gas separation membrane having a hyper-organized porous structure both in the micro- and mesopore size ranges. Inorganics 2016, 4, 5 12 of 14

Acknowledgments: This work was supported by CREST (Core Research for Evolutional Science and Technology) of Japan Science and Technology Corporation (JST). Author Contributions: Mohd Nazri Mohd Sokri performed experiments and wrote the paper; Yusuke Daiko contributed NMR and FT-IR spectroscopic analyses; Sawao Honda contributed TG-MS analysis; Zineb Mouline contributed nitrogen adsorption/desorption measurements; Yuji Iwamoto conceived and designed experiments and wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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