Robust Microporous Monoliths with Integrated Catalytically Active Metal Sites Investigated by Hyperpolarized 129 Xe NMR

Robust Microporous Monoliths with Integrated Catalytically ActiveMetal Sites Investigated by Hyperpolarized 129Xe NMRMuhammad Zaheer,† Caroline D. Keenan,‡ Justus Hermannsdorfer,† Ernest Roessler,*,§ Gunter Motz,*,∇

Jurgen Senker,*,‡ and Rhett Kempe*,†

†Department of Inorganic Chemistry II, ‡Inorganic Chemistry III, §Experimental Physics II, and ∇Ceramic Materials Engineering,University of Bayreuth, D-95440 Bayreuth, Germany

*S Supporting Information

ABSTRACT: Robust microporous nanocomposites (specificsurface area ≈ 400 m2/g) containing nickel nanoparticles havebeen synthesized and characterized by thermogravimetricanalysis (TGA), differential thermal analysis (DTA), Fouriertransform infrared (FT-IR) spectroscopy, nitrogen physisorp-tion, powder X-ray diffraction (XRD), transmission electronmicroscopy (TEM), and 13C and 29Si solid-state nuclearmagnetic resonance (NMR) spectroscopy. The commerciallyavailable polysilazane (HTT-1800) is chemically modified usingan N-ligand-stabilized nickel complex that catalyzes the cross-linking of the polymer via hydrosilylation at room temperature. Upon pyrolysis at 600 °C under an inert atmosphere, nickelnanoparticles and micropores are generated in a concerted process. The specific surface area, pore volume, and size of the nickelparticles can be tuned. The materials show excellent shape retention upon pyrolysis providing the possibility to fabricatemonoliths. The composites are stable in the presence of moisture and are both thermally and solvothermally robust, as indicatedby the nitrogen adsorption, FT-IR, and TGA measurements. Continuous-flow, hyperpolarized 129Xe NMR methods were used intandem to evaluate the effects of the nickel content and annealing time on the pore structure of the microporous nanocomposite.The adsorption enthalpy is rather independent of nickel particle inclusion. The interior adsorption sites are lined with methylgroups and the nickel particles seem to be located near the external surface of the composites and within the internal voids. Thenickel nanoparticles were used to catalyze selective hydrogenation reactions indicating applications of the nanocomposites ascatalyst itself or as catalyst support.

KEYWORDS: microporous materials, selective catalytic hydrogenation, nickel nanoparticles, aminopyridinato complex,hyperpolarized Xe NMR

■ INTRODUCTION

The design of microporous materials (MPMs) is of highscientific and technological interest, because of the wide rangeof applications offered by these materials in sorption,separation, and catalysis.1−6 The distribution of the size,shape, and volume of the pores decides the ability of solids toperform a specific function in a particular application.7 Highlycrystalline zeolites are perhaps the most widely used MPMs.They possess pores with molecular dimensions that allow theselective movement of molecules through them.8 Together withthe presence of acidic sites and narrow pore size distribution,zeolites have been used as acid- and shape-selective catalysts9 inpetrochemical industries10 and, more recently, biomassconversion.11,12

Although zeolites offer higher thermal and hydrothermalstabilities,13 compared to the carbon and other silica-basedporous materials,7 they do not facilitate the fabrication ofmonoliths, which afford lower pressure drops at a givengeometric surface area.14 Moreover, the characteristic smallpores, typically in the 0.3−0.7 nm range, impose severe masstransport constraints.15 This limitation led to the development

of other zeotypes (metallophosphates, and related materials16)and metal-organic frameworks (MOFs).17 These microporousmaterials possess larger pores, compared to zeolites, but areusually less stable.7

MOFs have been intensively used as heterogeneouscatalysts18 but are limited only to low-temperature catalytictransformations, because of the poor thermal and hydrothermalstability offered by them.7 In the past decade, amorphousmicroporous materials such as polymers of intrinsic porosity(PIMs),19 porous aromatic frameworks (PAFs),20 covalentorganic frameworks (COFs),21 porous polymer networks(PPNs),22 and sol-gel-derived materials23,24 have beensynthesized and investigated for their potential applications.These materials show very high surface areas and high uptake ofgases (hydrogen, carbon dioxide) but afford less thermal andhydrothermal stability, compared to the conventional micro-

Received: July 27, 2012Revised: September 20, 2012Published: September 24, 2012

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© 2012 American Chemical Society 3952 dx.doi.org/10.1021/cm302379b | Chem. Mater. 2012, 24, 3952−3963

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porous solids, which limits their potential use as catalystsupport materials.We have recently studied the formation of metal nano-

particles in SiCN nanocomposites, as well as catalyticapplications of these hybrid materials.25−28 Unfortunately,most of the generated metal nanoparticles are not accessiblefor catalysis, since they are deeply embedded in the ceramicsupport material. On the other hand, we developed M@MOFcatalysts, metal nanoparticles stabilized in highly porous MOFsthat suffer from low thermal stability of the porous host orcatalyst support.29−32 Here, we report on the synthesis andcharacterization of a new type of microporous nanocompositeswith catalytically active nickel nanoparticles. In addition tostandard characterization methods, 13C and 29Si solid-statenuclear magnetic resonance (NMR) spectropscopic techniquesare used to elucidate the effect of the size and amount of nickelon the chemical structure of the nanocomposite with increasingnickel content and metal particulate size. The materialsmanifest good thermal stabilities and fine-tuning of surfacearea and the size of metal particles are possible. The selectivecatalytic hydrogenation demonstrates the catalytic potential ofthese materials. Furthermore, applications as robust catalystsupports can be expected. The presence of nitrogen sites mayalso provide basic properties to the composites extending theirapplication to base catalysis.33

Continuous-flow hyperpolarized (CF-HP) 129Xe NMR wasemployed as a noninvasive probe of the pore architecture andXe adsorption capacities within the microporous SiCNnanocomposite materials. The observed 129Xe chemical shift(δobs) reflects even small changes in the local environment ofXe, indicating modifications in porosity, enthalpies ofadsorption, as well as pore connectivity and accessibility.34−39

As such, it is an invaluable tool to detect subtle changes inphysical and chemical environments within porous materials,because of the high polarizability of its electron cloud. Thesignificant increase in the signal-to-noise ratio afforded byhyperpolarization methods reduce the experiment time fromhours to seconds.39−42 The resulting spectral enhancement isunique in that it has the capability to allow one to probe specificXe surface interactions, which can yield information aboutmicropore size43 and degree of surface heterogeneity.44 It hasnot only been proven sensitive to the changes in amorphouspolymer structure with cross-linking,45 but also has been shownto be useful in the characterization of paramagnetic metalparticles confined within zeolites with varying cluster size.46,47

Here, we use 129Xe NMR as a tool to monitor the subsequenteffects of Ni-particle inclusion on the internal surfaces of thehybrid nanocomposite material.

■ EXPERIMENTAL SECTION

General Remarks. All reactions were carried out under dryargon using standard Schlenk and glovebox techniques.Solvents were dried and distilled from sodium benzophenonebefore use. Deuterated solvents obtained from CambridgeIsotope Laboratories were degassed, dried using molecularsieves, and distilled prior to use. 2-Amino-4-picoline,chlorotrimethylsilane, phenylacetylene (Sigma−Aldrich), n-butyl lithium, dodecane (ACROS Chemicals), p-tolylacetylene,m-tolylacetylene, 4-tert-butylphenylacetylene, 4-fluorophenyla-cetylene, 4-methoxyphenylacetylene (Alfa-Aesar), and poly-organosilazane (HTT-1800, Clariant GmbH, Sulzbach, Ger-many) were used as-received without any further purification.

The starting materials 4-methyl-2-((trimethylsilyl)amino)-pyridine (ApTMSH), and complex I (see Scheme 1) weresynthesized following reported methods.48,49

Cross-Linking, Metal Transfer, and Pyrolysis. Poly-organosilazane HTT-1800 (3.218 g) was reacted with [Ni-(ApTMS)2]2 (Ni-10: Si/Ni 10/1, 2.08 g, 2.49 mmol; Ni-20: Si/Ni 20/1, 1.04 g, 1.25 mmol; Ni-100: Si/Ni 100/1, 0.20 g, 0.25mmol; Ni-133: Si/Ni 133/1, 0.15 g, 0.18 mmol) in 5 mLTetrahydrofuran (THF). The solvent was evaporated undervacuum and obtained gels were pyrolyzed in a furnace(Nabertherm LH 60/14, Nabertherm, Germany) undernitrogen stream (from room temperature to 300 °C (holdingfor 2 h) in 5 h, heating from 300 °C to 600 °C (holding for 2h) in 5 h and finally cooling back to room temperature.

Solid-State NMR. All data were acquired at roomtemperature and externally referenced to TMS usingadamantine and tris(trimethylsilyl)amine for 13C and 29Sinuclei, respectively. A broad band 1H decoupling with aSPINAL-64 sequence (νnut = 70 kHz) was used for allmeasurements.Solid-state 29Si and 13C NMR spectra of the cross-linked

HTT-1800 were carried out on a DSX 400 Bruker AvanceNMR spectrometer using a Hahn-echo pulse sequence undermagic-angle spinning (MAS). The rotational frequency was setto 12.5 kHz. The spectra were recorded with a recycle delay of600 and 60 s for the 29Si and 13C MAS NMR, respectively. A π/2 pulse length of ∼3.3 μs was used for both.Pyrolyzed samples were characterized by ramped 29Si{1H}

and 13C{1H} cross-polarization (CP) MAS and single-pulseexcitation (HPDEC) 29Si MAS NMR techniques, recorded on acommercial Avance II 300 Bruker spectrometer equipped witha standard triple resonance 7-mm MAS probe.50 Typical recycledelays were 3.0 and 60 s, respectively. Spectra wereaccumulated at a rotation frequency of 4.0 kHz. The average1H field strength was set to 60 kHz. The X channel nutationfrequency was adjusted to 56 kHz to match the Hartmann−Hahn condition. The contact time (τCP) was set between 2 msand 5 ms, depending on the sample.The 29Si MAS spectra from both the CP and HPDEC

experiments were deconvoluted using a scripted MATLABprotocol, fitting Lorentz−Gaussian peaks with upper and lowerbounds for spectral linewidth, chemical shift, and amplitude.Comparison of the two experiments showed little difference inrelative peak amplitudes, although the two peaks furthestupfield exhibit a weaker 1H dipolar interaction, which isexpected (see feature S1 in the Supporting Information).Quantification of 29Si silicate populations from HPDEC andHahn-Echo 29Si MAS NMR measurements were performed onboth the cross-linked HTT-1800 and pyrolyzed samples.Overlapping resonances within 13C CP/MAS spectra were

Scheme 1. Molecular Structure of Polysilazane (HTT-1800)and the Amido Nickel Complex (I)

Chemistry of Materials Article

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also decovoluted in order to help differentiate between −CH3and aliphatic carbon units.Hyperpolarized 129Xe NMR. 129Xe NMR experiments

were performed on a home-built hyperpolarization (HP)generator located at the University of Bayreuth. Specific detailsconcerning the design principles and performance quality are tobe published in a subsequent article. Variable-temperature 129XeNMR spectra were obtained on an Avance II 300 Brukerspectrometer with a static probe head, modified to incorporatecontinuous flow (CF) of HP 129Xe through a 3.0-mm innerdiameter PEEK sample holder. Variable-temperature spectrawere acquired using a single-pulse excitation with a π/2 pulselength of 2.6 us, 2 to 16 transients, and a 10 s recycle delay.Unlike conventional 129Xe NMR methods, here, the recycledelay is limited by the flow rate instead of the relaxation time.51

Applying a hard 90° pulse essentially destroys all nuclearpolarization; the signal is then only observable if theunpolarized Xe atoms are replaced by fresh HP gas. Our gasmixture was comprised of 1:1:98, Xe:N2:He (by vol %) and waspolarized using 15 W of diode laser power (Coherent, Inc.)tuned to the Rb D1 line (λ ≅ 794 nm). The Xe partial pressurewas maintained at 0.06 bar and the flow rate was held relativelyconstant during the experiment time (∼100 sccm). All spectrawere referenced to the gas-phase spectral frequency obtained ina nickel-free environment.Sorption Analysis. The specific surface area measurements

were carried out on a Quantachrome (NOVA 2000 e-Series)surface area and pore size analyzer. The pore width and averagepore volume was calculated using quenched solid densityfunctional theory (QSDFT, adsorption branch) and a carbonkernel (cylindrical/slit/spherical pore geometry) was applied.Powder X-ray Diffraction (PXRD). All powder X-ray

diffractograms were recorded by using a STOE STADI-P-diffractometer (Cu Kα radiation, λ = 1.54178 Å) in θ−2θgeometry and with a position-sensitive detector. All powdersamples were introduced into glass capillaries (diameter of 0.7mm, Mark-tubes Hilgenberg, No. 10) in a glovebox and sealedprior to the measurements.Fourier Transform Infrared (FT-IR) Spectroscopy.

Measurements were performed at a Perkin−Elmer FTIR-Spectrum 100 over a range from 4400 cm−1 to 650 cm−1.Thermal analysis (TGA/DTA) was performed over

Thermowaage L81 (Linseis, Germany).Transmission Electron Microscopy (TEM). Transmission

electron microscopy (TEM) was performed using a VarianLEO 9220 (200 kV) instrument. The sample was suspended inchloroform and sonicated for 5 min. Subsequently, a drop ofthe suspended sample was placed on a grid (Plano S 166-3) andallowed to dry.Gas Chromatography (GC). Gas chromatography (GC)

analyses were performed using an Agilent 6890N gaschromatograph equipped with a flame ionization detection(FID) device and an Agilent 19091 J-413 FS capillary column,using dodecane as an internal standard.Catalysis. The catalytic-selective hydrogenation of alkynes

was done in a Parr autoclave under hydrogen pressure (20 bar).The catalyst was activated by heating at 100 °C for 24 h under ahydrogen pressure of 20 bar. A glass tube was charged with amagnetic bar and milled catalyst (particle size of <100 μm) andplaced in the reactor, which was then filled with hydrogen atroom temperature. The reactor was heated to respectivetemperature within 20 min and stirring was continued at thistemperature for 24 h. Afterward, the reaction mixture was

diluted with THF and dodecane was added as an internalstandard. The conversion and selectivity were determined byGC.

■ RESULTS AND DISCUSSIONSynthesis. The preceramic polymer HTT-1800 was reacted

with nickel complex (see Table 1). Upon the addition of

polymer to a solution of nickel complex in THF, an exothermicreaction took place with an immediate solidification anddarkening of polysilazane. The gelation time was dependent onthe amount of the complex added and ranged from 2 min (Ni-10, Ni-20) to 2 h (Ni-131). After the removal of solvent, thegels were pyrolyzed in a furnace under an atmosphere ofnitrogen and were found to retain their shape after the pyrolysisat 600 °C (see Figure 1). Abbreviations identifying the specific

loading of nickel complex, with respect to HTT-1800, thesilicon-to-nickel (Si/Ni) ratio, and the loading of nickel in theresulting materials pyrolyzed at 600 °C (N2) are found in Table1. The Si/Ni ratio, which increases with decreasing the amountof added nickel complex, was used to adjust the amount ofnickel in the final materials.

Characterization. Solid-State Magic Angle Spinning−Nuclear Magnetic Resonance Spectroscopy (MAS NMR). TheHahn-Echo 13C and 29Si MAS NMR measurements of the gelsobtained at room temperature by the reaction of nickelcomplex with HTT-1800 provide an explanation about thesolidification of the polysilazane. As shown in Figure 2a, theHahn-Echo 13C MAS NMR spectrum of the Ni-100-RT gelshows characteristic resonance peaks for both Si−CH3 (2.97ppm)52,53 and Si−(CH2)n−Si (9.36 ppm).

54 The latter linkagesarise as a result of addition of Si−H to the CH2CH− bond.The absence of any peak at 132 ppm (sp2 carbon of vinyl)confirms the operative cross-linking via hydrosilylation.After pyrolysis, the signals assigned to methyl or methylene

carbon atoms become broader (Figure 2). The additionalresonance at 24 ppm is assigned to the Si4C environment

Table 1. Indentification of Experimental Parameters forSpecific Nanocomposite Materials

sample ID Ni-complex loading (wt %) Si/Ni (ratio) Ni loadinga (wt %)

Ni-10 64.8 10 8.1Ni-20 32.4 20 4.3Ni-100 6.4 100 1.0Ni-133 2.0 133 0.3

aTheoretical loading of nickel in materials pyrolyzed at 600 °C undernitrogen.

Figure 1. Pictorial presentation of Ni-131 (2 wt % nickel complex) (a)before and (b) after pyrolysis at 600 °C for 2 h.



(feature S3 in the Supporting Information). The presence ofpeaks at 132 and 141 ppm in the materials obtained frompolysilazane show the existence of a vinylic group, whichbecome totally consumed in the case of samples with nickel.The nickel-containing hybrid materials also show the presenceof graphitic carbon, as confirmed by the peak at 135 ppm. Themost intense signal in the Hahn-Echo 29Si NMR spectrum Ni-100-RT (see Figure 2b) at −22.6 ppm is assigned toSiH(Csp3)N2 environment55 originally present in the polymer(HTT-1800 in Scheme 1), while the additional resonance at−31.0 ppm is attributed to SiHC(sp2)N2.

56,57 After hydro-silylation, SiH(Csp3)N2 changes into C(sp3)2SiN2 (see featureS2 in the Supporting Information), which appears in thespectrum at a chemical shift of −8.7 ppm.58 The utilization ofthis function was also confirmed by FT-IR studies of the gels(Figure 3a).After pyrolysis at 600 °C for 2 h, the same sample (Ni-100−2

h) shows an intense peak at −24.0 ppm in its 29Si CP/MASspectrum, corresponding to CSiN3, formed as a result of adehydrocoupling reaction between Si−H and N−H func-tions.58 The additional peaks observed at −8.5 ppm and −43.6ppm are assigned to C2SiN2 and SiN4 environments,respectively. The former peak can be attributed to thehydrosilylation reaction while the latter appears at the expenseof C2SiN2 and CSiN3 (breaking of Si−C bonds and theemission of methane) (see feature S2 in the SupportingInformation). This is confirmed by (i) the decrease in theC2SiN2 and CSiN3 intensities and annealing time, as shown bythe integrated HPDEC 29Si MAS NMR (summarized in Table2) and (ii) the decrease within the FT- IR spectral intensity at1255 cm−1 with the increasing nickel content (see Figure 3b).The C2SiN2 also forms CSiN3 by the expulsion of methane

(feature S2 in the Supporting Information). This explains whythe abundance of the latter environment shows negligiblechange with increasing nickel content (samples pyrolyzed for 2h; see Table 2).Quantitative analysis of the relative signal intensities within

the acquired 29Si HPDEC MAS spectra yields additional insightinto compositional changes between the various nano-composite materials. The results of which are summarized inTable 2. The most notable trend is the growth in the SiN4 peakintensity with increasing nickel content and annealing time,from 7.5% in the pure polysilazane (HTT-1800−2 h) to 56.0%in the case of Ni-20−6 h. The corresponding C2SiN2 andCSiN3 intensities decrease from 30.5% to 14.2% and 43.6% to19.9%, respectively (see Table 2). The materials obtained frompure polysilazane and Ni-133−2 h (low nickel content) showedincorporation of oxygen when stored under air most probablyby the hydrolysis of Si-NH bonds by moisture, as indicated bythe appearance of signals at −67.0 ppm (CSiO3) and −110.0ppm (SiO4). The small amount of oxygen might have come bythe exposure of the samples to the air during theirtransportation to the furnace, as seen in the case of Ni-20.The percent composition of each material, summarized in

Table 2, is based on the relative intensities extracted fromintegrals of their individual HPDEC 29Si MAS NMR spectra,using quantitative methods. The composition of hybridmaterials obtained at 600 °C was then calculated assumingthe C2SiN2, CSiN3, and SiN4 as the major silicon environments,following the procedure as discussed elsewhere.54 Whereas theN/Si ratio in the materials obtained from pure polysilazane isalmost equal to unity (same as in the polymer), the materialswith nickel contain more nitrogen (SiC0.2N1.2, in the case of Ni-133) and it increases with the increase in the amount of nickel

Figure 2. Quantitative (a) 13C and (b) 29Si Hahn-Echo MAS NMR spectra of cross-linked HTT-1800 (Ni-100-RT) is compared to CP/MAS NMRspectra of pyrolyzed materials with varying Ni content and annealing times. Asterisks denote spinning side bands while the dashed lines are thedeconvolution. The annealing time is specified by the addition of −2 h (2 h) or −6 h (6 h) to the sample name. The intensity of the Si-CHCHenvironment (95−230 ppm) is enlarged by a factor of n times (n×). The index (denoted by the caron symbol, “∨”) indicates an erroneous solventpeak (for dimethylsulfoxide) introduced after pyrolysis.



(SiC0.3N1.3 in the case of Ni-100-RT) as well as the holdingtime at 600 °C (SiC0.25N1.38 in the case of Ni-20). Theincreased nitrogen content suggests that transaminationreactions59 during pyrolysis are not favored and the loss ofnitrogen in the form of ammonia is avoided. On the other hand,the C/Si ratio decreases from 1.4 in the polymer to 0.2−0.3 inthe pyrolyzed materials. This trend might be attributed to theloss of methane from the polymeric network during pyrolysis,which reduces the carbon content of the Si−C−N network butincreases the amount of free carbon (graphitic carbon) in thematerials.

Fourier Transform Infrared Spectroscopy (FT-IR). Ni-100-RT sample pyrolyzed at different temperatures under a nitrogenatmosphere was analyzed via FT-IR (Figure 3a). At roomtemperature, the characteristic stretching vibrations assigned toSi−H functions (2117 cm−1) decrease in intensity, which isindicative of the hydrosilylation reaction. Moreover, thestretching frequency for the N−H bond (3382 cm−1) shiftsslightly toward higher wavenumbers.The amido-metal bond is formed as a result of the transfer of

the metal from aminopyridinato complexes to the nitrogenfunctions of the polysilazane, producing a metal-modifiedpolymer.25 The stretching vibration for vinylic linkage (3049,1404 cm−1) became totally absent at 300 °C, confirming thecompletion of cross-linking reactions. In particular, almostcomplete utilization of Si−H (2117 cm−1) and N−H (3382,1158 cm−1) functions at a pyrolyzing temperature of 300−600°C is contradictory to the results of Trassl et al., who observedthe existence of N−H bonds at 800 °C in the same type ofpolymer having much fewer N−H bonds, compared to HTT-1800.57 The utilization of these functions provides a clearindication of dehydrocoupling reactions between Si−H and N−H functions,60 along with the operative hydrosilylation. At500−600 °C, the materials show a small peak at 1255 cm−1

(Si−CH3) and a broad band at 1200−600 cm−1 due to thestretching of Si−C and Si−N bonds. When samples were heldat this temperature for 6 h, the peak at 1255 cm−1 almostvanished which is in accordance to 13C MAS NMR, where theintensity of Si-CH3 decreased with increase in amount of nickeland holding time (see Figure 2b and feature S2 in theSupporting Information). In the case of sample without nickel(HTT-1800), the same peak was quite prominent. Thissuggests the formation of more ceramic like materials (SiCNceramics) at as low temperature as 600 °C, because of thepresence of nickel. At and above 700 °C, the infrared (IR)spectra show only broad bands characteristic of Si−C−Nceramics.

Thermogravimetric Analysis (TGA) and Differential Ther-mal Analysis (DTA). Thermogravimetric analysis (TGA) of thesample (Figure 4) with a lower loading of nickel complex (Ni-100) shows a mass loss of 16% from 25 °C to 600 °C and anadditional 6% loss between 600 °C and 1100 °C, providingmaterials in a good yield (86% and 80% at 600 °C and 1100 °C,respectively). The mass loss between 25 °C and 450 °C wasrelated to the loss of hydrogen and oligomeric silazanefragments, while that above 530 °C is due to the loss ofmethane. The ceramic yield of Ni-100 (80%) at 1100 °C was

Figure 3. (a) FT-IR spectra of Ni-100-RT at room temperature(spectrum II), 300 °C (spectrum III), 500 °C (spectrum IV), 700 °C(spectrum V), 900 °C (spectrum VI), and 1100 °C (spectrum VII), incomparison to the spectrum of HTT-1800 (spectrum I). (b) FT-IRspectra of materials pyrolyzed at 600 °C under nitrogen for 6 h.

Table 2. Percent Contribution of Different Silicon Environments Extracted from Deconvoluted HPDEC 29Si MAS NMR Spectra(See Feature S1 in the Supporting Information)

Percent Contribution

pyrolysis conditions sample (Csp3)Si(H)N2 (Csp3)2SiN2 C(sp2)Si(H)N2 C3SiN Si−Si Si(H)N2 compositiona

25 °C Ni-100-RT 51.6 32.6 6.7 5.1 1.9 1.2(Csp3)SiN3 SiN4 (Csp3)2SiN2 (Csp3)SiO3 C3SiN SiO4

600 °C (2 h) HTT-1800 43.6 7.6 30.5 7.8 4.8 5.8 SiC0.32N0.91

Ni-133 42.8 21.7 14.9 9.2 9.0 4.8 SiC0.29N1.23

Ni-100 51.9 29.5 11.7 <1 <1 1.8 SiC0.38N1.31

600 °C (6 h) HTT-1800 31.7 20.0 22.7 6.9 10.1 8.4 SiC0.26N1.17

Ni-133 35.1 28.6 32.9 <1 <1 3.3 SiC0.26N1.09

Ni-20 19.9 56.1 14.2 <1 <1 9.7 SiC0.25N1.38

aCalculated from the percentage intensities of C2SiN2, CSiN3, and SiN4 peaks (the major silicon environments) following the method discussed inref 54. The contribution from oxygenated silicon environments (CSiO3, SiO4) and nickel is not shown.



found to be higher than the ceramics obtained from pure HTT-1800 (78%) cross-linked with the addition of dicumylperoxide(DCP). Increasing the amount of loaded nickel complexdecreases the yield of the materials obtained at differenttemperatures. For instance, Ni-10 (64.8 wt % nickel complex)shows a mass loss of 29% and 4% in the temperature ranges of25−600 °C and 600−1100 °C, respectively (see Figure 4).DTA of the gel (Ni-10) showed broad endothermic peaks at25−600 °C and were attributed to the loss of oligomericsilazane species (25−200 °C) and methane (600 °C) from thepolymer. Powder XRD analysis of the Ni-10 pyrolyzed at 1100°C showed the crystallization of silicon nitride (see Figure 5). A

sharp and intense endothermic peak above 1400 °C wasassigned to the melting of nickel. A weight loss of 13% (TGA)is associated with this peak.61 Powder XRD analysis of thematerials analysis of the materials (initially pyrolyzed at 1100°C under nitrogen) post-heated to 1500 °C (argon) indicatescrystallization of silicon carbide (see feature S4 in theSupporting Information).

Powder X-ray Diffraction (PXRD). The evolution of phasesin Ni-10 at different pyrolyzing temperatures was investigatedby powder XRD studies. The sample remains X-ray amorphousup to 500 °C, above which the graphitic carbon arises at a 2θvalue of 26.4° (see Figure 5). At 1100 °C, the characteristicreflection pattern of β-Si3N4 appears in the diffractogram, inaccordance to the HPDEC MAS NMR studies, which show anincrease in the SiN4 sites with nickel content (see Table 2) andDTA studies (Figure 4). The samples pyrolyzed at 600 °C(under nitrogen) show a broad peak at ∼26° (2θ), which wasassigned to the amorphous carbon and is in accordance withthe 13C MAS NMR studies. A broad reflection peak at 44.6°corresponds to the (111) plane of cubic nickel and thebroadening is due to the small size of the particles, as seen inTEM images (see Figures 6d and 6f). The size of the nickel

nanoparticles decreases with decreasing nickel content and inthe case of samples with lower loadings (Ni-20 and Ni-100-RT), the particles are too small to be detected via powder XRD.

Nickel Particle Size Distribution. The microstructure of thenanocomposites was analyzed by transmission electronmicroscopy (TEM). The materials obtained at 600 °C showhomogeneous distribution of nickel particles whose sizeincreased as the nickel content increased (see Figure 6). Thehybrid materials with lower loading of nickel (Ni-100) show anarrow distribution of particle size (2−3 nm), which somehowbroadened with increasing nickel content (6−9 nm in the caseof Ni-10).In order to have an idea about the formation and growth of

nickel particles with temperature, Ni-100 was pyrolyzed atdifferent temperatures, followed by the analysis of the productswith TEM. The formation of the metal nanoparticles seems tostart above 300 °C, because they were not observed in thesamples pyrolyzed below this temperature. The nickel complexdecomposes in the temperature range of 200−300 °C andresults in the generation of Ni ions, which are then reduced inthe reductive pyrolysis atmosphere, because of the emission ofhydrogen and methane.25 At 500 °C, small nickel nanoparticleswere seen which seem to be loosely bonded to the surface asthey moved to the copper grid used for TEM analysis (seefeature S5 in the Supporting Information). The size of theparticles increased as the pyrolyzing temperature increased.

Figure 4. Thermogravimetric analysis (TGA) (5 K/min, N2) of Ni-100 (black line) and Ni-10 (blue line). Percentages are expressed as arelative yield within set temperature ranges described within the text.DTA of Ni-10 is characterized by a sharp endothermic at 1406 °C(melting point of Si, bottom blue line).

Figure 5. Powder XRD patterns of Ni-10 pyrolyzed at 600 °C (N2/2h) shows a reflection pattern characteristic of cubic phase of nickel(International Centre for Diffraction Data (ICDD) Powder DiffractionFile (PDF) Card No. 00-001-1260) and graphite (ICDD PDF CardNo. 00-003-0401), as indicated by red and gray vertical bars,respectively. Crystallization of hexagonal phase (black vertical bars)of silicon nitride (ICDD PDF Card No. 00-041-0360) takes place at1100 °C (N2).

Figure 6. TEM micrographs of (a and b) Ni-10, (c and d) Ni-20, and(e and f) Ni-100 pyrolyzed at 600 °C (2 h), along with the particle sizecount (red bars). The size of the particles increased as the amount ofnickel complex contained by the materials increased.



Stability Studies. The porous hybrid materials are stableup to 600 °C, with regard to mass loss under an inertatmosphere, as depicted by the TGA analysis (Figure S6 in theSupporting Information). The composites (Ni-133 and Ni-10,600 °C/6 h) were heated up to 600 °C in air and exhibitsurface area retention up to 500 °C. Increasing the nickelcontent in the materials seemed to enhance the stability as theNi-10 showed only a small change in surface area when heatedto 600 °C. FT-IR spectra of the composites at 500−600 °Cshow the appearance of Si−O and Si−O−Si bonds in the rangeof 1000−800 cm−1, which is due to the incorporation of asufficient amount of oxygen. Ni-133 was stirred in water andTHF at room temperature for 24 h, followed by nitrogenadsorption studies, which show only a small decrease in surfacearea, indicating the stability of materials in water and organicsolvents (see feature S7 in the Supporting Information). Theremarkable resistivity in an oxidative environment and thermalstability of our nickel-containing hybrid material might beattributed to the richness in SiN4 sites, which providesrobustness (ceramic property) to the material.Porosity and Surface Studies. There are few reports

about the synthesis of nanoporous metal-free materials fromoligo or polysilazanes,62−65 but the resulting silicon imidontridestructures are highly reactive and prone to hydrolysis, due tothe presence of Si−NH functions.49 The hybrid materialsreported here offer better stability (hydro (solvo)thermal),compared to the aforementioned materials, because of therestructuring catalyzed by nickel nanoparticles (NPs). Recently,the synthesis of nanoporous SiOCN ceramics from thepyrolysis of a polysilazane modified with a nickel aminoethanolcomplex was reported.66 Although the generation of metalnanoparticles and the porosity was achieved in a single step, ahigher amount of oxygen (∼30%) coming from the complexcould not be avoided.The nanocomposites were subjected to nitrogen physisorp-

tion studies in order to evaluate the generation of porosityunder pyrolyzing conditions. It was found that the apparentsurface area increased with the pyrolyzing temperature up to500 °C (400 m2/g) and then decreased to 18 m2/g at 1100 °C.The generation of porosity at 500−600 °C may be related tothe loss of methane from the three-dimensional (3D) networkof cross-linked polymer. The hybrid materials obtained at 600°C show typical Type I adsorption isotherms (Figure 7) andboth surface area and the average pore volume of the materialsincrease as the amount of nickel complex decreases (see Figure7 and Table 3).The dwelling time at 600 °C seems to affect the pore volume

as well as the surface area, both of which increased with anincrease in holding time, in the case of the materials with highnickel content (Ni-10, Ni-20). However, the pore widthsslightly decreased when the dwelling time was increased from 2h to 6 h. It is worth mentioning here that the carbon kernelused for the calculation of pore widths would be less polar thanSiCN, thus a slight underestimation of the pore widths isexpected.

129Xe NMR Studies. The static CF-HP 129Xe NMR spectrafor selected nanocomposite materials at 298 K are displayed inFigure 8. Xe adsorbed on the Ni-free polymer, pyrolyzed at 600°C for 6 h (HTT-1800−6 h) shows a narrow gas resonancecentered at 0 ppm and a broader, yet symmetric resonance at150.3 ppm, demonstrating the accessibility of the pore volumeto Xe. As a general rule, the larger the observed shift (δobs), thesmaller the pore size.42,67 Briefly, δobs represents a sum of

potential interactions:68,69 δobs = δref + δS + δXe−Xe + δSAS + δE,M,where δref is the gas-phase Xe shift extrapolated to zero pressure(0 ppm) and δS is the component arising from Xe surfaceinteractions; δXe−Xe refers to contributions from Xe−Xecollisions, δSAS arises from its interaction with strong adsorptionsites (SAS), and δE,M manifests from electric and/or magneticfields induced by the presence of metal particles acting as SAS.HP methods typically employ dilute Xe densities whicheffectively limits the Xe−Xe contribution to the δobs at ambienttemperature (298 K). It should be noted that reducing thesample temperature leads to an increase in either δXe−Xe or δS,depending on whether micropores or mesopores arepresent.38,70

Because the HTT-1800−6 h sample does not contain anynickel, the observed 129Xe chemical shift is attributed to Xeconfined within internal voids. However, this shift is too largeto reflect Xe occupying the pore diameters indicated by N2adsorption measurements. As such, we suggest that this shiftarises from Xe interacting with organic moieties71 (e.g., −CH3groups) lining the pore surface, which solid-state 13C and FTIRmeasurements show are in abundance. Preferred adsorption

Figure 7. Nitrogen adsorption−desorption isotherm of Ni-100pyrolyzed at 600 °C for 2 h. The inset shows the pore size distributioncalculated by the Quenched Solid Density Functional Theory(QSDFT) model for the same sample held at 600 °C for 2 and 6 h.

Table 3. Specific Surface Area, Average Pore Width, andPore Volume of Hybrid Material Pyrolyzed at 600 °C (N2/2h or 6 h)

samplespecific surface area,

SSAa (m2/g)pore widthb

(nm)pore volume(cm3/g)

HTT-1800

2 h 294 1.0 0.119

6 h 382 0.9 0.168Ni-10 2 h 83 1.1 0.046

6 h 160 1.0 0.085Ni-20 2 h 232 1.1 0.101

6 h 330 0.9 0.139Ni-100 2 h 323 1.0 0.126

6 h 217 0.9 0.104Ni-133 2 h 374 0.9 0.159

6 h 200 1.1 0.095aBET equivalent specific surface area calculated by the Brunauer−Emmett−Teller (BET) method. bPore size distribution calculated bythe Quenched Solid Density Functional Theory (QSDFT) method.



within organic macrocycles has been shown to exhibit similareffects.72,73

The addition of 2 wt % nickel complex (Ni-133−2 h) has anegligible effect on the δobs value. The observed decrease in theδobs line width from 701 Hz to 530 Hz may indicate a morehomogeneous structure of internal voids, compared to thepyrolyzed HTT-1800−6 h (see Figure 8). A subsequentincrease in the nickel content (Ni-100−2 h) induces noticeablebroadening in both the adsorbed and gas-phase resonances,indicating a strong influence due to the inclusion of Niparticles. The signal for the adsorbed Xe remains rathersymmetric, while the resonance centered near 0 ppm starts toexhibit some asymmetry and a new resonance near 14.6 ppm,indicating the presence of larger void space. These featuresbecome more pronounced with longer annealing times.The resonance attributed to the adsorbed Xe in Ni-133−6 h

is further broadened to 13 kHz and shifted downfield to 157ppm (Figure 8). The downfield shift may be due to the increasein SiN4 units (compared to both Ni-133−2 h and Ni-100−2 hmatrices); the removal of larger amorphous voids may promotethe formation of more ordered phases with condensed voidspace. As shown previously by TEM images, longer annealingtimes lead to larger Ni-particle aggregates (Figure 6). Moreagglomeration of the nickel particles within the nanocompositesupport material would likely increase the probability of surfaceroughness, which have been shown to promote the formationof Xe clusters.74,75 The strong local dipole fields induced by thesuperparamagnetic nature of the nickel particles may beresponsible for the severe broadening of the resonances dueto the shortening of the 129Xe spin−lattice relaxation. Theparamagnetic nature of the nickel particles near the externalsurface would have an additional depolarizing effect on the HP129Xe. The higher metallic character of the particles willeventually limit the detectable amount of Xe sampling theinternal voids, because of an increase in the depolarizationpotential and broadening. This justifies our observations on theNi-20 sample, which contains the highest nickel content and

largest nickel particle size (among the measured materials)where only a multicomponent asymmetric line broadeningaround the gas phase is observed. A more-detailed discussion isgiven in the Supporting Information.Recent work by Clewett et al.76 on the surface properties of

carbon nanotubes laced with paramagnetic particles attributesthe complex structure around the gas-phase resonance to thepresence of additional adsorption sites that are not detectable,because of strong paramagnetic relaxation effects. This wouldeffectively reduce the polarization of Xe adsorbed on the nickelparticles and average the remaining polarization with that of thegaseous Xe (within the fast exchange regime). The weightingfactor of Xe adsorbed into these paramagnetic voids is likely tobe small due to a very short time T1 (on the order of a fewnanoseconds),77 resulting in an averaged frequency closer to 0ppm. As such, we believe that the higher nickel content createsadditional domains that manifest as asymmetric line broadeningand additional peaks observed near the gas phase.The temperature dependence of the CF-HP 129Xe NMR

spectra (Figure 9) was investigated for many samples, with the

purpose of extracting more-specific information on physicalparameters typically hidden within possible exchange processes.Reducing the sample temperature not only increases the Xedensity within the pore space, but it also slows the diffusionprocesses and increases residence times within the microporousmaterials. This makes the identification of additional cavitiesmuch more likely. Samples Ni-133−2 h and Ni-100−2 h werechosen for variable-temperature (VT) analysis, because of theirsimilar temperature treatment (2 h annealing time) andcomparable surface-to-volume ratios. Monitoring the changesdue to the inclusion of larger nickel particles is of particularinterest as Ni-100−2 h possesses almost three times morenickel, compared to Ni-133−2 h (see Table 1). VT CF-HP129Xe spectra for Xe adsorbed within Ni-133−2 h are shown inFigure 9. The broad, symmetric line shape of the adsorbed Xeshifts and broadens significantly with cooling. Only a moderateshift of the signal is detected between 290 and 240 K. Belowwhich, a stronger shift and a multimodal line occurs. Spectral

Figure 8. CF-HP 129Xe NMR spectra obtained at room temperaturefor nanocomposite materials having different Ni-loading and holdingtimes. The addition of either “−2 h” or “−6 h” indicates 2 or 6 h ofpyrolysis.

Figure 9. Variable-temperature CF-HP 129Xe NMR spectra data of Ni-133−2 h.



deconvolution of the adsorbed 129Xe resonance suggests thepresence of three distinct adsorption sites at 185 K.The amount of Xe within the gas and pore environments are

directly proportional to their relative integrated spectralintensities, assuming a sufficiently long recycle delay andnegligible depolarization effects. The extraction of quantitativethermodynamic parameters under low loading conditions (hightemperatures) can be obtained by plotting the adsorbed-to-gasphase ratio (Iads/Ig) as a function of temperature.70 Theintegrated signal intensities extracted from Ni-133−2 h and Ni-100−2 h are shown in Figure 10. The equilibrium constant

describes the activities of the total amount of adsorbed Xe withrespect to the gas phase and can be used to relate Iads/Ig to thestandard Gibbs free energy of the adsorption process. Resultsare evaluated according to

= Δ ° −Δ⎛

⎝⎜⎜

⎞⎠⎟⎟

II

SR

HRT

ln ads

g

ads

where ΔHads is the differential enthalpy of adsorption, R theuniversal gas constant, and ΔS° the temperature-independentstandard entropy change.The validity of this model to evaluate the thermodynamics of

Xe adsorption is limited to fast diffusion conditions and oftenmanifests as a positive, linear slope in the logarithmic plot. Assuch, only the linear segment located within the high-temperature region is used to extract pertinent thermodynamicparameters. The initial slope observed in the Ni-100−2 hsample yields a ΔHads value of −10.0 kJ/mol Xe, and a standardentropy change (ΔS°) of −41.8 J/(mol K), which is

characteristic of physisorption (Figure 10). The turning pointat 255 K reflects a diffusion limiting process wherein longerresidence times limit the replacement of adsorbed gas withfreshly HP Xe.78 The fast depolarization observed in Ni-100−2h is likely due to the elevated nickel content, while thesubstantial broadening of the resonance from 1 kHz at roomtemperature, to 2.6 kHz at 185 K is attributed to paramagneticrelaxation effects that are due to increased interactions betweenXe and nickel particles with cooling.Different behavior is observed for Ni-133−2 h, where

multiple linear segments are shown to span over severaltemperature intervals. Similar to Ni-100−2 h, the first segmentreflects Henry’s law-type behavior (Figure 10, Region I),yielding adsorption enthalpy and entropy of −9.0 kJ/mol Xeand −13.9 J/(mol K), respectively. Here, the higher ΔS° value(−13.9 J/(mol K) versus −41.8 J/(mol K)) reflects the largernumber of open end groups (like −CH3), which enhances theflexibility of Ni-133−2 h, compared to Ni-100−2 h. Thesudden decrease in the Iads/Ig ratio at 240 K with cooling(Figure 10, Region II) suggests a possible change within theinternal structural arrangement. An additional, larger reductionin the integrated signal intensities is detected at 210 K (Figure10, Region III). Interestingly, previously discussed asymmetriccontributions to the adsorbed 129Xe line shape correspond toeach of these two observed transitions. As shown in the threedeconvoluted 129Xe spectra shown in Figure 10 (Regions, I, II,and III), each depression and subsequent buildup within Iads/Igis associated with the emergence of an additional resonance,suggesting some change within Xe exchange processes. TheIads/Ig ratios measured near these two observed transitions werefound to be both reversible and reproducible. These trends arelikely more reflective of the relative lifetime of adsorbed HP Xewithin these domains, rather than its actual distribution. Thetemperature dependence of the adsorption behavior was alsoanalyzed using chemical shift analysis (see feature S8 in theSupporting Information). The adsorption enthalpies werefound to be comparable to those obtained above.

Catalytic Studies. Selective hydrogenation of phenyl-acetylene to styrene is a reaction of great industrial importanceas the small amount of it present in styrene feed can deactivatethe polymerization catalysts.79 Among the other supportednoble-metal catalysts,80−82 those based on palladium83−87 arethe most active and selective and are used industrially for theremoval of phenylacetylene from styrene feed.88 Although othercatalysts containing inexpensive metals have also been tried,they suffer the problem of being less active and specific,89 andattempts have been made to increase their selectivity byalloying them with silicon.90,91 Nickel nanoparticles supportedby porous hybrid materials reported here were found to behighly selective in the hydrogenation of phenylacetlylene tostyrene. The hydrogenation of phenylacetylene in differentorganic solvents was studied at 25 °C under a hydrogenpressure of 5 bar. Whereas the tetrahydrofuran (THF) anddichloromethane (DCM) afforded better selectivity to styrene,hexane and toluene provided better conversions.By the adjustment of catalyst amount, hydrogen pressure,

volume of toluene, and stirring speed, it was possible to getalmost 100% conversion of alkynes with high selectivity (89%−99%) to the alkenes (see Table 4). No catalytic activity wasdetected by applying a SiCN material without nickel. The effectof various substituents on the phenyl ring of phenylacetylenewas also studied (see Figure 11a). Whereas an electron-donating groups (such as methyl) at the para position

Figure 10. Integrated intensity ratios extracted from VT CF-HP 129XeNMR spectra data of Ni-100−2 h (open symbol) and Ni-133−2 h(closed symbol). Inset: spectra on the upper portion of the summaryplot refer to the first (Region I), second (Region II), and third (RegionIII) linear segments within Ni-133−2 h, as described in the text.Regions I, II, and III correspond to temperature ranges of 333−235 K,238−210 K, and 212−180 K, respectively.



decreased the activity, the electron-withdrawing groups (fluoroand methoxy) and the bulky (tertiary butyl) group at the sameposition were found to increase the activity by many folds. Anelectron-releasing methyl group at the meta position was foundto increase the conversion. All of the aforementioned substrateswere hydrogenated to olefins with high selectivity (82%−98%).The catalyst was consecutively used up to five runs and did notshow any prominent change in the conversion and selectivity(see Figure 11b).After every catalytic run, the catalyst was removed and the

filtrate was checked for its catalytic activity under the sameexperimental conditions with the addition of fresh substrate.No activity was observed, which suggests either no or very lowleaching of nickel.

■ CONCLUSIONSSimultaneous generation of the porosity and nickel nano-particles were achieved by the pyrolysis (600 °C/N2) of achemically modified commercial polysilazane. An amido nickelcomplex catalyzes the hydrosilylation reaction, leading to thecross-linking of polysilazane at room temperature, which, inturn, affords better yields. Dehydrocoupling reactions lead tothe complete utilization of N−H bonds and an enhancedstability of the composites. Increasing the nickel contenttogether with the holding time at 600 °C increases thepopulation of SiN4 sites at the expense of C2SiN2 and CSiN3sites. In other words, much of the methyl and methylenegroups bonded to silicon (C2SiN2 and CSiN3 environments)are lost as methane, producing a SiN4 network that renders

higher thermal stability to the materials. The surface area andthe size of the metal particles can be altered by adjusting theamount of added nickel complex. Moreover, the presence ofnickel lowers the crystallization of silicon nitride (at 1100 °C/N2). The fabrication of monolithic microporous materials ispossible, which are thermally robust until 500 °C in anoxidative environment and are selective hydrogenationcatalysts.CF-HP 129Xe NMR methods were used to probe the

influence of specific thermal and Ni-complex treatments on theformation of microporous nanocomposites with catalyticallyactive nickel particles. Stronger Xe-nickel interactions areverified by the limited detection of adsorbed Xe and appearanceof additional peaks near the gas phase with increasing metalparticle size and concentration. Substantial broadening of bothadsorbed and gaseous Xe resonances with higher nickel contentindicate that nickel particles are not only embedded within theSiCN porous network, but located near the external surface aswell. The negligible change in the adsorption enthalpy of Xewith the nickel particle inclusion suggests minimal catalystsupport effects. As such, the catalytic activity would bedominated by the nickel particles.

■ ASSOCIATED CONTENT*S Supporting InformationSchemetic presentation of various silicon environments, atablular presentation of 13C chemical shift values, powder XRD

Table 4. Hydrogenation of Alkynes at 25 °C Using Ni-10Catalysta

aConditions: 0.5 mmol alkyne, 50 mg of catalyst, 0.2 mL of toluene,hydrogen pressure of 20 bar, and stirring velocity = 1250 rpm.Reaction time = 24 h (except where noted by an asterisk (*), forwhich the reaction time was 32 h). bSelectivity is expressed in terms ofmol % to olefin product.

Figure 11. (a) Hydrogenation of alkynes (0.5 mmol), catalyst (20mg), toluene (0.2 mL), hydrogen pressure (20 bar), stirring velocity(750 rpm): the conversion and selectivity (olefin) are given in mol %units. Legend: PA, phenylacetylene; Me, methyl; TB, tertiary butyl; F,fluoro; MeO, methoxy. (b) Recycling of the catalyst (Ni-10, 20 mg)for five consecutive runs (0.5 mmol of phenylacetylene in toluene (0.2mL) at 25 °C for 24 h under 5 bar of hydrogen).



of the materials treated at 1500 °C, TEM micrographs of thematerials at various pyrolyzing temperatures, and a brief textdiscussion about the variation of Xe chemical shift withtemperature, along with figures. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail addresses: [email protected], [email protected], [email protected], [email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors thank the Deutsche Forschungsgemeinschaft(DFG) and Sonderforschungsbereich (SFB) 840 Bayreuth forfinancial support, and they gratefully acknowledge Dr. Wolf-gang Milius and Kathrin Inzenhofer for PXRD and TGA/DSCmeasurements. M.Z. acknowledges the Higher EducationCommission of Pakistan (HEC) and Deutscher AkademischerAustausch Dienst (DAAD) for the fellowship.

■ ABBREVIATIONSPXRD, powder X-ray diffraction; TGA, thermogravimetricanalysis; DTA, differential thermal analysis; TEM, transmissionelectron microscopy; FT-IR, Fourier transform infrared spec-troscopy; MPMs, microporous materials; BET, Brunauer−Emmett−Teller; NPs, nanoparticles; MAS NMR, magic anglespin nuclear magnetic resonance spectroscopy; HP, hyper-polarized; CP, cross-polarization; CF, continuous flow; HP,hyperpolarization

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