Article pubs.acs.org/JPCC
Direct Spectroscopic Evidence for Isolated Silanols in SiOx/Al2O3 and Their Formation Mechanism Aidan R. Mouat,† Takeshi Kobayashi,‡ Marek Pruski,*,‡,§ Tobin J. Marks,*,† and Peter C. Stair*,† †
Department of Chemistry and the Center for Catalysis and Surface Science, Northwestern University, Evanston, Illinois 60208, United States ‡ Ames Laboratory, U.S. Department of Energy, Ames, Iowa 50011, United States § Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States S Supporting Information *
ABSTRACT: The preparation and unambiguous characterization of isolated Brønsted-acidic silanol species on silica−alumina catalysts presents a key challenge in the rational design of solid acid catalysts. In this report, atomic layer deposition (ALD) and liquid-phase preparation (chemical liquid deposition, CLD) are used to install the SiOx sites on Al2O3 catalysts using the same Si source (tetraethylorthosilicate, TEOS). The ALD-derived and CLD-derived SiOx sites are probed with dynamic nuclear polarization (DNP)enhanced 29Si−29Si double-quantum/single-quantum (DQ/SQ) correlation NMR spectroscopy. The investigation reveals conclusively that the SiOx/Al2O3 material prepared by ALD and CLD, followed by calcination under an O2 stream, contains fully spatially isolated Si species, in contrast with those resulting from the calcination under static air, which is widely accepted as a postgrafting treatment for CLD. Insight into the formation mechanism of these sites is obtained via in situ monitoring of the TEOS + γ-Al2O3 reaction in an environmental diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell. Upon calcination, the DRIFTS spectra of SiOx/Al2O3 reveal a signature unambiguously assignable to isolated Brønsted-acidic silanol species. Surprisingly, the results of this study indicate that the method of preparing SiOx/Al2O3 catalysts is less important to the final structure of the silanol sites than the post-treatment conditions. This finding should greatly simplify the methods for synthesizing site-isolated, Brønstedacidic SiOx/Al2O3 catalysts.
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INTRODUCTION
Alternative approaches to preparing well-defined, tunable acid surface structures include thermolytic preparation of AlxOy deposited on silica (Al@SiO2),10 templating with organic species,11 and grafting of TEOS on γ-Al2O3 under anhydrous conditions (chemical liquid deposition, CLD).9,12−14 This laboratory recently reported an atomic layer deposition (ALD) process that selectively installs highly dispersed, Brønsted-acidic silanol sites on γ-Al2O3 surfaces.15 The use of ALD proved critical to achieving higher catalytic performance and better active site homogeneity than SiOx/Al2O3 prepared via an analogous literature CLD method. However, the origin of the catalytic performance differences resulting from the two preparation methods remains unclear. Indeed, understanding the cause of performance differences between seemingly identical materials represents an ongoing challenge in heterogeneous catalysis. In this report, we analyze the spatial distribution of Si species in ALD- and CLD-prepared SiOx/Al2O3 catalysts using
The nature and distribution of acid sites on silica−alumina catalysts determines their catalytic performance, especially in fuel production and biomass upgrading.1−3 The optimization of “mildly” acidic silica−alumina materials is particularly important due to their proven high activity in catalytic hydrocracking processes.1,2 The Brønsted-acid sites in amorphous silica− alumina are known to be milder than those of zeolites such as H-ZSM-5.3 Conventionally, amorphous silica−alumina is prepared via sol−gel or coprecipitation methods. While these methods typically employ a metal silicate precursor as the Si source,4 methods using tetraethylorthosilicate (TEOS) as the silicon source have also been reported.5−7 The Brønsted acid sites of relevance are postulated to involve the protons of silanol groups interacting with Lewis acid sites on the solid surface; however, such sites present a distribution of acid strengths, challenging spectroscopic and reactive characterization.8,9 Because of the simultaneous polymerization of both Al and Si precursors under hydrothermal conditions, the nuclearity and structure of the precipitated or deposited Si structures are poorly defined. © XXXX American Chemical Society
Received: November 7, 2016 Revised: February 27, 2017 Published: February 27, 2017 A
DOI: 10.1021/acs.jpcc.6b11196 J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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DNP-ENHANCED 2D 29Si−29Si DQ/SQ CORRELATION NMR SPECTROSCOPY In a previous study, this laboratory used 1D DNP-enhanced 29 Si{1H} cross-polarization magic-angle spinning (CPMAS) SSNMR to detect and identify SiOx sites on γ-Al2O3 prepared by ALD before and after calcination.15 The implementation of DNP proved essential because conventional (i.e., non-DNP) CPMAS measurements could not elicit any 29Si NMR response from the ALD-derived SiOx/Al2O3 sites, even after 40 000 scans. Here the signal-enhancing power of DNP is used to selectively detect the 29Si−29Si pairs in a 2D DQ/SQ correlation measurement. Note that with DQ/SQ correlation spectroscopy the challenge is elevated to a far higher level because only ∼5% of the Si atoms are NMR-active. Only 1 of ∼450 Si−Si pairs will include a pair of 29Si nuclei; furthermore, the spectra must be acquired in a 2D fashion. However, these correlations become detectable due to much higher enhancements achieved by replacing the previously used polarizing agent AMUPol17 dissolved in water with a tetrachloroethane solution of TEKPol.18 Importantly, the DQ/SQ spectrum features only the sites that are in close spatial proximity (∼4 Å or less19,20), whereas the Si sites that are isolated in this sense remain NMR-silent due to DQ filtering. The cross-peaks representing pairs of coupled nuclei appear at the sum of their respective frequencies in the indirect (DQ) dimension.21 Accordingly, the resonances appearing on the diagonal represent pairs of interacting sites of the same kind (or at least with the same chemical shift), whereas sites with different chemical shifts produce off-diagonal peaks, which here are difficult to resolve due to line broadening. The samples chosen for the DNP-enhanced measurements include: (i) two ALD-prepared SiOx/Al2O3 materials with the lowest and highest Si loadings of 0.6 and 1.4 Si/nm2 labeled 1 and 4, respectively (see SI, Table S5), and (2) a CLD-prepared sample 5 with a loading closely matching sample 4 (1.7 Si/nm2; see SI, Table S5). The 1D CPMAS spectra of 1 and 4 (Figure S6) are consistent with those previously reported.15 Because of the low Si loading of 1, a DNP-enhanced 29Si−29Si DQ/SQ correlation spectrum could not be obtained. However, the DNP-enhanced DQ/SQ 29Si−29Si correlation spectra of 4 and 5 taken before calcination (Figure 1A,B) show correlation signals from 29Si species in close proximity to one another, most likely the previously postulated Si(2Al,Si,OH) (see Scheme 1C) and Si(Al,Si,2OH) structures, which are both expected at around −85 ppm. Upon calcination under an O2 stream, no 29Si−29Si correlations are detected in either sample (thus the spectra cannot be shown), consistent with the dispersion of the Si species on the γ-Al2O3 surface and the formation of spatially isolated SiOx sites. On the basis of the signal-to-noise ratio observed in the spectrum in Figure 1B, we estimate that calcination under an O2 stream results in an at least a 7-fold decrease in the concentration of aggregated Si species or else we would have detected the DQ/SQ correlation. In contrast, CLD-prepared 5, after calcination under static air, which is the generally accepted postgrafting treatment for CLD,13 shows detectable 29Si−29Si correlation signals (Figure 1C). Note that the 1D 29Si{1H} CPMAS spectra of calcined samples show a similar downfield shift of the center of gravity of the 29Si signal regardless of the calcination procedure (Figure 1D). It is only through the use of 2D Si−Si correlation spectroscopy that we are able to unequivocally demonstrate a fundamental difference between the products of these two
dynamic nuclear polarization (DNP)-enhanced 2D 29Si−29Si double-quantum/single-quantum (DQ/SQ) correlation NMR spectroscopy. Initially, NMR reveals spatial proximity between Si species in the CLD-derived SiOx/Al2O3 catalyst and no detectable spatial proximity between Si species in the ALDderived SiOx/Al2O3 catalyst. However, note that the literature preparation for CLD involves calcination of the precatalyst at 550 °C in static air,13 whereas the ALD precatalysts are calcined at 550 °C in flowing O2.15 Because the moisture in static air has been previously observed to promote aggregation of surface species,16 an experiment is then performed in which the CLDprepared precatalyst was calcined at 550 °C in the same manner as the ALD-prepared precatalyst, resulting in complete spatial isolation of the SiOx species. This new result answers two questions directly: (1) The claim of “isolated” SiOx species in the conventional CLD-prepared catalyst calcined at 550 °C is inaccurate and (2) the method of silica deposition is ultimately a secondary factor to the calcination method. The mechanism of surface SiOx species formation during the ALD process on γ-Al2O3 is illuminated using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). In situ DRIFTS of the γ-Al2O3 reveals the formation of welldefined, stable Si surface structures during deposition and full hydrolysis upon exposure to water. The intermediate surface structures formed during this process can be positively identified by their IR signatures. After calcination at 550 °C in flowing O2, a single narrow band appears at 3747 cm−1. The NMR data enable this species to be unambiguously assigned to isolated silanol species interacting with Lewis acid sites on the surface. The appearance of this lone feature confirms that surface changes leading to the formation of the isolated SiOx species occur during the calcination process, not during the deposition process itself, as suggested in previous studies.9,12,13
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Article
SiOx/Al2O3 CATALYSTS PREPARED BY ALD AND CLD
SiOx species were deposited on γ-Al2O3 (Alfa-Aesar, 242 m2/g, bimodal pores,
