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Strongly enhanced acidity and activity of amorphous silica–alumina by formation of pentacoordinated AlV species
Journal of Catalysis 372 (2019) 1–7
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Strongly enhanced acidity and activity of amorphous silica–alumina by formation of pentacoordinated AlV species Zichun Wang a,b, Yijiao Jiang b, Fangzhu Jin a, Catherine Stampfl c, Michael Hunger d,⇑, Alfons Baiker e,⇑, Jun Huang a,⇑ a
Laboratory for Catalysis Engineering, School of Chemical and Biomolecular Engineering, Sydney Nano Institute, the University of Sydney, New South Wales 2006, Australia Department of Engineering, Macquarie University, Sydney, New South Wales 2109, Australia School of Physics, Sydney Nano Institute, the University of Sydney, Sydney, New South Wales 2006, Australia d Institute of Chemical Technology, University of Stuttgart, D-70550 Stuttgart, Germany e Institute for Chemical and Bioengineering, Department of Chemistry and Applied Bioscience, ETH Zürich, Hönggerberg, HCI, CH-8093 Zürich, Switzerland b c
a r t i c l e
i n f o
Article history: Received 23 August 2018 Revised 12 December 2018 Accepted 3 February 2019
Keywords: Amorphous silica–alumina Flame-spray pyrolysis Pentacoordinated aluminum Solid-state NMR Acidity Phenylglyoxal Ethyl mandelate
a b s t r a c t Tailoring high-performance aluminosilicates plays a key role in the efficient and clean production of high-value chemicals. Recent work reveals that pentacoordinated Al (AlV) species can significantly enhance the Brønsted acidity of amorphous silica–alumina (ASA), compared with that typically dominated by tetracoordinated Al species. However, the controlled synthesis of AlV-rich ASAs is challenging. Employing xylene as the solvent in a flame-spray pyrolysis process, we synthesized AlV-rich ASAs successfully. The high combustion enthalpy of xylene (36.9 kJ/ml) results in a high flame temperature, promoting the formation and distribution of metastable AlV species in the silica network forming Brønsted acid sites. This provides a promising route for the controlled synthesis of AlV-rich ASAs with higher Brønsted acidity. As an example, AlV-rich ASAs are shown to exhibit superior catalytic performance in phenylglyoxal conversion to ethyl mandelate in ethanol compared with that achieved with other acid catalysts, attaining an ethyl mandelate yield of 99.8%. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Amorphous silica–aluminas (ASAs) are among the most widely used solid acid catalysts and supports for the production of petrochemicals, fine chemicals, and renewable energy [1–3]. The catalytically active Brønsted acid sites (BAS) originate from aluminum centers, commonly accepted as tetracoordinated aluminum (AlIV) species, flexibly coordinated to neighboring SiOH groups in ASAs [4–6]. However, a very low Si/Al ratio (<7/3) leads to the formation of alumina phases [7,8] and to a decrease of the Brønsted acidity in ASAs [9,10]. Recently, 27Al-{1H} dipolar-mediated heteronuclear multiple quantum correlation (D-HMQC) two-dimensional (2D) NMR studies have evidenced that pentacoordinated aluminum (AlV) species in spatial proximity to SiOH groups contribute to a new type of BAS (BAS-AlV) [11]. Coexisting AlV and AlIV species can elevate the BAS population density in ASA, reaching seven times higher densities than in conventional AlIV-rich ASAs [12]. AlV species are ⇑ Corresponding authors. E-mail addresses: [email protected] (M. Hunger), baiker@ chem.ethz.ch (A. Baiker), [email protected] (J. Huang). https://doi.org/10.1016/j.jcat.2019.02.007 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
proposed to further enhance the acid strength of neighboring moderate BAS (e.g., BAS-AlIV) to zeolitic strength or even higher [13]. This enables AlV-rich ASA to exhibit excellent catalytic performance; as an example, a turnover frequency more than an order of magnitude higher than that of dealuminated zeolite H-Y was achieved in the catalytic conversion of phenylglyoxal (PG) [14]. Moreover, AlV species can act as Lewis acid sites (LAS) and anchor active metal species on the ASA surface [15,16]. Therefore, enrichment of ASA with AlV species not only can enhance their Brønsted acidity, but also can improve their catalytic performance as multifunctional catalysts or excellent support materials for reactions. AlV species are intermediates or metastable species between IV Al and AlVI species [15,17–19]. Various techniques, including classical wet-chemical synthesis [20,21], chemical liquid deposition [6], and atomic layer deposition [22], generally produce AlIV-rich ASAs, rather than AlV–rich ASAs. Flame spray pyrolysis (FSP), which we employed in this work, is a single-step method that can generate multicomposition nanoparticles in milliseconds, combining synthesis and calcination on a large scale [23–25]. Taking advantage of super-high temperatures (up to 2000 K) and fast cooling rates, FSP is able to produce materials dominated by metastable phases and polymorphs [24], and thus, FSP techniques
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are suitable for preparing AlV-enriched ASAs [13,26]. The AlV content in ASAs can be tuned from 0 to 41.6% to promote BAS formation with increasing Al loading up to 70% [13]. At the same Al loading, a higher precursor flow rate results in a higher AlV content, but decreases the number of BAS, which is attributed to the larger particle size of the ASAs [26]. Catalytic conversion of PG in water and alcohols over solid acids enables single-step production of mandelic acid and mandelates [14,27–29], which are valuable intermediates for producing pharmaceuticals and fine chemicals [30,31]. Nonporous ASA catalysts facilitate a turnover frequency an order of magnitude higher than that of microporous dealuminated HY zeolites [14]. The catalytic performance of ASAs can be improved significantly by increasing their Brønsted acidity [14], which provides a much higher ethyl mandelate (EM) yield (98.6%) than that achieved with other solid acids reported earlier [27–29]. To our knowledge, the deliberate generation of AlV species to promote Brønsted acidity in ASAs has so far not been reported, but would be promising for improving the catalytic performance of ASAs for various reactions. With this in mind, we used FSP to prepare ASAs with significantly improved AlV density, resulting in higher Brønsted acidity of the surface sites and superior catalytic performance. A series of AlV-rich ASAs with different Si/Al ratios were synthesized using xylene as the solvent, which has a higher combustion enthalpy (36.9 kJ/ml), resulting in a higher flame temperature than that of the methanol/acetic acid (Me/AA) mixture (volume ratio 1/1) with a combustion heat of ’25.4 kJ/ml employed in previously reported ASA syntheses [13]. It will be shown that the use of xylene as a solvent in the liquid precursor flame feed results in significantly enhanced AlV concentration at the same Al loading and thus in a larger number of BAS, as revealed by quantitative 1H magic-angle spinning (MAS) NMR spectroscopy. Finally, the catalytic performance of AlV-rich ASAs prepared via the above-mentioned route was evaluated for the reaction of PG in ethanol to demonstrate their remarkable catalytic activity compared to other solid acids. 2. Experimental methods 2.1. Preparation of ASAs by FSP Aluminum acetylacetonate (Al(acac)3, purity 99.9%), tetraethyl orthosilicate (TEOS) (purity 99.9%), and xylene (purity 98.5%) were all purchased from Sigma-Aldrich and used for producing ASA catalysts. The precursor solutions used for the preparation of ASAs by FSP were obtained by dissolving appropriate amounts of the precursor materials in xylene (0.5 M concentration by metal), followed by filtration using a glass filter. Then the FSP precursor solutions were used in the FSP process as described previously [13]. In brief, the precursor solutions were pumped through a capillary at a rate of 5 ml/min and nebulized at 5 L/ min O2. The resulting spray was ignited by an annular supporting methane/oxygen flame (1.5/0.9 L/min). Particles were collected on a cooled Whatman GF6 filter (diameter 257 mm). A Busch SV 1040C vacuum pump aided in particle recovery. The synthesized silica–alumina powders are designated as SA/Xx, where X is 5, 10, 30, or 70, representing the atom% of Al in the precursor, and x indicates the use of xylene as a solvent to distinguish it from the methanol/acetic acid (denoted as SA/X) used in earlier work [13]. 2.2. Structural characterization N2 adsorption–desorption isotherms measured at 77 K on an Autosorb IQ-C system indicated that the synthesized ASA powder
materials were virtually nonporous. Prior to isotherm measurement, each sample was degassed at 423 K for 12 h under vacuum. The BET surface areas of the ASAs were determined from the N2 adsorption isotherms measured at 77 K using the Brunauer–Emm ett–Teller (BET) method. Powder X-ray diffraction (XRD) studies were performed on a Siemens D5000 instrument with CuKa radiation in the range 10°–70° with scanning steps of 0.02°. 2.3. Solid-state MAS NMR spectroscopy For the 27Al and 29Si MAS NMR investigations, all samples were fully hydrated by overnight exposure to the saturated vapor of a Ca (NO3)2 solution at ambient temperature in a desiccator. 27Al and 29 Si MAS NMR investigations were carried out on a Bruker Avance III 400 WB spectrometer at resonance frequencies of 104.3 and 79.5 MHz, respectively. 27Al MAS NMR spectra were recorded at a sample spinning rate of 8 kHz using 4-mm MAS rotors after single-pulse p/6 excitation with a repetition time of 0.5 s. 29Si MAS NMR measurements were performed with a sample spinning rate of 4 kHz using a 7-mm MAS rotor after single-pulse p/2 excitation, high-power proton decoupling, and a recycle delay of 20 s. Before the 1H and 13C MAS NMR experiments, the samples filled into glass tubes were dehydrated for 12 h at 723 K and at a pressure of less than 102 bar. These dehydrated samples were sealed in glass tubes or directly loaded with ammonia or acetone-2-13C (99.5% 13C-enriched, Sigma-Aldrich) on a vacuum line. Subsequently, the loaded samples were evacuated at 373 K for 1 h (for ammonia) or at room temperature for 2 h (for acetone) to remove weakly physisorbed molecules. Then the samples were transferred into the MAS NMR rotors under dry nitrogen gas inside a glove box. 1 H and 13C MAS NMR investigations were performed on the same spectrometer at resonance frequencies of 400.1 and 100.6 MHz, respectively, with a sample spinning rate of 8 kHz using 4-mm MAS rotors. 1H MAS NMR spectra were recorded after singlepulse p/2 excitation with a repetition time of 20 s. Quantitative 1 H MAS NMR measurements were carried out using a zeolite H, Na-Y (35% ion-exchanged) as an external intensity standard. 13C cross-polarization (CP) MAS NMR spectra were recorded with a contact time of 4 ms and a repetition time of 4 s. 2.4. Catalytic conversion of phenylglyoxal in ethanol The conversion of PG (Sigma-Aldrich, >97%) to EM was utilized to study the catalytic performance of the SA/Xx materials. An amount of 0.05 g of the catalyst was employed and activated overnight under an N2 flow of 50 ml/min at 723 K. After cooling down in flowing N2 gas, the catalyst was transferred into a glass reactor with a volume of 15 ml. Subsequently, 1.25 ml of the ethanol solution containing 0.4 M PG and 0.05 M octane (as GC internal standard) was added and mixed with the activated catalyst under magnetic stirring. The reaction was carried out in a tightly closed glass reactor immersed in an oil bath at 363 K for 6 h. The reaction products were determined using a Shimadzu QP2010 Ultra GC–MS equipped with a Rtx-5MS column (30 m 0.25 mm 0.25 lm) and quantified by a Shimadzu GC2010 equipped with an flame ionization detector and a Rtx-5 column (30 m 0.32 mm 0.25 lm). The selectivity for the specific products i (Si) was calculated as Si (%) = 100 (i)/[(PG)0 (PG)], where (i) is the molar concentration of the product i and (PG)0 and (PG) correspond to the molar concentrations of PG before and after the reaction, respectively. The used catalyst was washed with ethanol three times and dried overnight at 373 K before being calcined at 773 K for 3 h for complete removal of organic residues. The reaction mixture after 6 h reaction was analyzed by atomic emission spectroscopy with an inductively coupled plasma (ICP-AES, Perkin-Elmer, Plasma 400).
Z. Wang et al. / Journal of Catalysis 372 (2019) 1–7
3. Results and discussion 3.1. Catalyst morphology The amorphous nature of the SA/Xx catalysts was confirmed by XRD patterns (Fig. 1). All samples showed a broad peak at 2h = 20°–30° attributed to amorphous silica [32], and no crystalline silica or alumina phases could be detected. This finding indicates that aluminum species are well dispersed in the amorphous silicon oxide network. The surface areas and primary particle sizes of the SA/Xx catalysts are summarized in Table 1. The specific surface areas of these materials were in the range of 241–391 m2/g, significantly higher than those of ASAs previously prepared using Me/AA mixtures as solvent (SA/X materials, 200–377 m2/g) [13]. As observed for the SA/X materials [13], the surface areas of SA/Xx catalysts gradually decreased with increasing Al content. The average particle sizes of SA materials were estimated from BET measurement (Table 1) and confirmed by SEM images (Fig. S1 in the Supporting Information). Interestingly, using xylene instead of Me/AA in the FSP precursor solution did not strongly affect the particle size of SA materials (7–17 vs. 7–20 nm). 3.2. Surface acidity and local structure of SA/Xx materials The effect of using xylene as a solvent in the FSP synthesis of AlV-rich ASA was investigated by 27Al MAS NMR spectroscopy (Fig. 2). In SA/5x, the strong signal at d27Al = 51 ppm and the weak signal at d27Al = 0 ppm represent the commonly observed AlIV and AlVI species in ASAs. With the Al content increased to 10 mol.% (SA/10x), a signal at d27Al = ca. 28 ppm assigned to AlV species was observed [11,13,33], which was strongly enhanced when the Al content was increased to 70 mol% (SA/70x). The molar ratios of Al species are summarized in Table 1. Obviously, using a solvent with higher combustion enthalpy, such as xylene, can enhance the formation of AlV species in ASAs. The AlV species was observed in SA/10x (17.6%), in this research but not in SA/10 in our previous study [13]. At 30 to 70 mol.% Al loading, the fraction of AlV species synthesized by FSP using xylene as solvent was 1.3–1.6 times higher than that of corresponding catalysts produced with Me/AA as solvent. During the FSP synthesis [23,24,34], the metal precursors and the solvent were atomized and sprayed to form the precursor vapor, which in turn was converted to product vapor containing
Fig. 1. XRD patterns of FSP-derived SA/Xx catalysts with different Al contents.
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metastable species (e.g., AlV) through combustion at high temperature, and the resulting well-mixed product molecules and clusters underwent nucleation and grew to nanoparticles. The high cooling rate facilitated the freezing of metastable species in the final product. Compared with Me/AA, xylene has a higher combustion enthalpy, providing a higher flame temperature, and thus a more severe temperature drop, which enhanced the freezing of metastable AlV species. Recently, the formation of BAS based on AlV species has been evidenced by 27Al-{1H} D-HMQC 2D NMR experiments [11]. Therefore, the effect of increasing the AlV content on the surface Brønsted acidity of the ASAs was studied. 1H MAS NMR spectroscopy is a powerful tool for the characterization of surface hydroxyl protons [5]. As shown in Fig. 3a–e, top, the signals of nonacidic and acidic surface OH groups are strongly overlapping, and only a strong peak at d1H = 1.8 ppm assigned to SiOH groups can be observed in the 1H MAS NMR spectra of dehydrated ASA catalysts [5,13]. Adsorption of a strong base, such as ammonia, is a useful technique for identifying BAS on ASA [5,13]. Upon ammonia loading, a signal arises at d1H = ca. 6.7 ppm (Fig. 3a–e, bottom), which indicates the formation of ammonium ions via ammonia protonation at BAS. The number of BAS can be estimated by integration of the ammonium signal (d1H = ca. 6.7 ppm) in Fig. 3 [5], leading to the results summarized in Table 1. Since AlVI species mainly afford surface Lewis acidity [35–38], the enhancement of BAS (both density and fraction) with increasing Al content (from 5 to 70%) obviously depends on the significant increase in AlV concentration (from 0 to 55.5%) rather than on the strong decrease in AlIV concentration (from 97 to 16.3%). Moreover, the larger amount of AlV species contributes to the higher BAS density (6.8–19.8 102 mmol/g) obtained for SA/Xx using xylene as solvent compared with ASAs prepared by other methods [12,39], and with FSP-derived SAs prepared with Me/AA as solvent (9.8–15.1 102 mmol/g) [13]. In ASAs, the formation of BAS is accepted as being caused by Al atoms in the vicinity of SiOH groups. The correlation between Al atoms and neighboring SiOH groups was probed by a 1H/27Al TRAPDOR (transfer of population in double resonance) MAS NMR experiment (Fig. 4) [4,40,41]. In a TRAPDOR experiment, the dipolar dephasing of 1H–27Al pairs can be suppressed by continuous 27Al irradiation, which influences the intensity of 1H spin–echo MAS NMR signals caused by OH groups in the vicinity of Al species. Thus, the difference spectrum of SA/70x recorded without and with 27 Al irradiation (Fig. 4 bottom) evidenced the interaction between SiOH groups (d1H = 1.8 ppm) and neighboring Al atoms [4,5]. Moreover, a strong low-field shift of the main peak in the 29Si MAS NMR spectra of SA/Xx catalysts with increasing Al content (Fig. 5) demonstrates a continuous incorporation of Al into the silica network [13,42]. It correlates well with the increasing number of BAS, which indicates that increasing aluminum incorporation contributes to the formation of these surface sites on ASA. Since ASA catalysts are often utilized under anhydrous conditions, the study of the correlation between the number of BAS on the dehydrated materials and the AlV species as a function of Al loading is of great importance (Fig. 6). The concentrations of AlV species were determined by decomposing the 27Al MAS NMR spectra of corresponding dehydrated SA/Xx catalysts as shown in the Supporting Information (Fig. S1 and Table S2). Compared to SAs prepared with Me/AA as solvent, the increase of the BAS density from using xylene as solvent correlates well with the increase of AlV species (44% vs. 39%, 52% vs. 48%, 57% vs. 51%, and 61% vs. 55% for Al loadings of 10%, 30%, 50%, and 70%, respectively) while the AlIV fraction decreased (54% vs. 59%, 43% vs. 51%, 38% vs. 42%, and 32% vs. 38% for Al loadings of 10%, 30%, 50%, and 70%, respectively). Clearly, using xylene as a solvent yields a larger population of AlV species, and thus promotes the connection between
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Table 1 Properties of FSP SA/Xx catalysts with Al content X of 5–70 atom%: BET surface areas (ABET), particle sizes (DBET), total densities of surface OH groups (TOH), densities of Brønsted acid sites (NBAS), and fractions of Al species with different oxygen coordination. Sample SA/5x SA/10x SA/30x SA/50x SA/70x
ABET (m2/g) 391 369 289 267 241
DBET (nm)a 7 8 9 12 17
TOH (mmol/g)b 1.4 1 1.3 1.2 1.6
NBAS (mmol/g)b 2
6.8 10 7.4 102 14.8 102 15.8 102 19.8 102
AlIV (atom%)c
AlV (atom%)c
AlVI (atom%)c
AlV (atom%)d
97 56.7 23 20 16.3
— 17.6 43 55 55.5
3 25.7 34 25 28.2
— 0 20.3 40.5 41.6
a
The particle diameter (DBET) of SA/Xx materials derived from BET surfaces according to DBET (nm) = 6000/(BET surface area in m2/g) (density in g/cm3). Total number of OH groups (TOH) and population of Brønsted acidic OH groups (NBAS). Determined from quantitative 1H MAS NMR using NH3 as a probe molecule as shown in Section 3.2. c The atomic concentrations of Al species were determined from Fig. S2. d AlV concentration obtained from Ref. [13] using Me/AA as a solvent. b
Fig. 2. 27Al MAS NMR spectra of FSP-derived SA/5x (a), SA/10x (b), SA/30x (c), SA/ 50x (d), and SA/70x (e).
AlV species and SiOH groups in their vicinity [11] to generate more BAS. Notably, both SA/30x and SA/50, as well as SA/50x and SA/70, which contain similar AlV and AlIV fractions but higher BAS densities, are obtained for SA prepared using xylene at a much lower Al content. More significantly, SA/30x has a BAS density similar to that of SA/70 having a factor of 2.33 higher Al content at lower AlV concentration. This finding indicates that using a solvent with higher combustion enthalpy (e.g., xylene) not only increases the AlV concentration, but can also promote the distribution of Al species over the silica network and thus the formation of BAS. Therefore, this approach provides a potential route for the synthesis of ASA materials with higher acidity at lower Al content. The effect of AlV species on the acid strength of ASA was scaled by the 13C NMR chemical shift of the carbonyl groups of acetone2-13C (CH313COCH3) as a probe molecule [5]. A greater adsorbateinduced low-field shift of the 13C CP/MAS NMR signals indicates a higher Brønsted acid strength of surface OH groups and vice versa. As shown in Fig. 7, the 13C CP/MAS NMR spectra of SA/Xx samples are all dominated by a signal at 214–216 ppm, assigned to acetone-2-13C adsorbed onto BAS with moderate strength. The broad hump at ca. 225 ppm in Fig. 7b and 7c indicates a wide distribution of acidic OH groups with higher strength on SA/Xx, while the weak signal at ca. 240 ppm in SA/70x (Fig. 7c) was caused by acetone adsorbed onto Lewis acidic sites [13]. Other d13C signals are due to aldol reaction of the acetone-2-13C on SA/x catalysts and nonenriched carbon atoms in methyl groups, respectively [43–45].
Fig. 3. 1H MAS NMR spectra of dehydrated FSP-derived SA/5x (a), SA/10x (b), SA/ 30x (c), SA/50x (d), and SA/70x (e), recorded before (top) and after (bottom) loading with NH3 and subsequent evacuation at 373 K for 1 h.
Obviously, the surface OH groups of the SA/Xx catalysts exhibit a Brønsted acid strength from moderate to comparable to or even stronger than that of zeolites H-Y (d13C = 220 ppm) [45] and HZSM5 (d13C = 223 ppm) [44,45], which is similar to that of FSP-derived ASA synthesized using Me/AA as a solvent [13]. Particularly, the spectrum of SA/10x shows a hump at d13C ca. 225 ppm, which was not observed in the spectrum of SA/10 using Me/AA as solvent and which was dominated by a signal at d13C = 213 ppm only [13]. In terms of the slightly lower BAS density, this indicates that the
Z. Wang et al. / Journal of Catalysis 372 (2019) 1–7
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Fig. 4. 1H/27Al TRAPDOR spectra of dehydrated SA/70x.
Fig. 7. 13C CP/MAS NMR spectra of SA/5x (a), SA/10x (b), and SA/70x (c) recorded after loading with acetone-2-13C and evacuation at room temperature for 1 h.
3.3. Catalytic conversion of phenylglyoxal to ethyl mandelate
Fig. 5. 29Si MAS NMR spectra of FSP-derived SA/5x (a), SA/10x (b), SA/30x (c), and SA/70x (d).
Fig. 6. Correlation between the concentration of Brønsted acidic OH groups (BAS, solid line) and AlV species (dashed line) as a function of the Al content in dehydrated FSP-derived SA/Xx prepared with xylene ( ) and MeAA (j) as solvents. The density of AlV species was obtained by simulation of the corresponding 27Al NMR spectra of dehydrated SA/Xx samples using the parameters obtained in MQMAS NMR investigations (Fig. S1). The simulation parameters are summarized in Table S1.
higher AlV concentration of SA/10x causes a higher Brønsted acid strength. With increasing Al content up to 70%, both the density and strength of BAS are promoted by the increase of AlV species in SA/Xx compared with other ASA catalysts. Therefore, the FSP-derived ASA synthesized using xylene as solvent is expected to exhibit higher catalytic performance for the target reaction than ASA catalysts prepared via other routes.
The catalytic performance of the SA/Xx materials was evaluated using the conversion of PG in ethanol to EM at 363 K. The catalytic results are summarized in Table 2. The conversion of PG and the selectivity to EM as a function of time are shown in Fig. 8 (also see Fig. S3 with error bars). Pure silica is inactive for this reaction [14]. Already, SA/5x, having mainly Brønsted acidity, showed a PG conversion of 61.6% and a selectivity to EM of 98.7% after a reaction time of 6 h. The enhancement of the Brønsted acidity of SA/Xx materials with increasing Al content up to 70% significantly improved the PG conversion from 61.6% to 100% under the same conditions, and the selectivity of EM remained higher than 98.6% for all samples. With SA/70x, an EM yield of 99.8% was achieved, which to our knowledge is the highest reported so far with solid acid catalysts [29,46–50]. Compared with AlV–rich FSP-derived ASA materials synthesized using Me/AA as the solvent [13], a significant increase of the EM yield (89.8% vs. 56.5%, 95.2% vs. 67%, and 99.8% vs. 97% for SA/10x, SA/30x, and SA/70x, respectively) was acquired on SA/Xx using xylene as the solvent. The major difference between these two batches is the remarkable increase in the AlV concentration, which improves the Brønsted acidity. The effect of AlV species on the catalytic activity of the BAS was evaluated by comparing the rate constants k of PG conversion (Table 2, columns 5 and 6). SA/10x showed a 2.1 times higher k value (0.0054 s1 vs. 0.0026 s1) with a slightly smaller number of BAS (7.4 102 mmol/g vs. 9.8 102 mmol/g) than SA/10. This catalytic property is attributed to the enhanced BAS strength (d13C = 214–225 ppm for SA/10x vs. 213 ppm for SA/10), which facilitates the activation of the C@O bond in PG as the initiating step of the reaction. The high BAS densities of the AlV–rich SA/30x and SA/70x catalysts resulted in 2.8 and 1.3 times higher k values than for the FSP-derived ASA using Me/AA as solvent (0.0097 s1 vs. 0.0035 s1 and 0.0141 s1 vs. 0.0106 s1). Moreover, SA/30x, which has a number of BAS similar to that for SA/70, afforded a similar PG conversion and k value, but at half the Al content, demonstrating the preparation of high-performance ASA materials with much lower Al content. Therefore, the efficiency of the catalytic conversion of PG to EM can be significantly improved by enhancing the Brønsted acidity of ASA via increasing the AlV concentration. The novel route for preparing FSP-derived ASA using xylene as a solvent represents a promising way to enhance the Brønsted acidity of ASAs and to improve their catalytic performance.
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Table 2 Catalytic properties of FSP-derived SA/x catalysts with different Al content.a
a b c d e f g
Sample
CPGb (%)
YEMc (%)
SEMd (%)
ke (s1)
k0 f (s1)
TOFg (h1)
TOF0 g, (h1)
SA/5x SA/10x SA/30x SA/50x SA/70x
61.6 90.1 95.4 98.9 100
61 89.8 95.2 98.7 99.8
98.7 98.8 98.6 99.8 99.8
0.0023 0.0054 0.0097 0.0126 0.0141
— 0.0026 0.0035 0.0051 0.0106
24.4 35.2 20.5 35.3 28.4
— 27.5 19.8 31.6 25.3
Conditions: 1.25 ml of ethanol solution containing 0.4 M PG and 0.05 g catalyst at 363 K for 6 h with stirring. CPG = conversion of PG. YEM = yield of ethyl mandelate in mol.%. SEM = selectivity to ethyl mandelate at 50% conversion. k = reaction rate from fitting kinetic data. k0 obtained from the kinetic data of FSP SA catalysts using the mixture of methanol and acetic acid (1:1 by volume) as solvent. TOF and TOF0 were calculated on the basis of 8–10% PG conversion and the number of BAS on SA/Xx and SA/X, respectively.
significant activity loss (Fig. S4). Furthermore, no Al leaching could be detected in the reaction mixture, and the 27Al MAS NMR spectra of SA/70x indicated no significant change before and after five recycling runs (Fig. S5), confirming the high stability of SA/70x in the liquid-phase reaction under the current conditions.
4. Conclusions
Fig. 8. Catalytic conversion of PG in ethanol (closed symbols) and selectivity to ethyl mandelate (open symbols) over SA/5x (j), SA/10x (d), SA/30x (▲), SA/50x (r), and SA/70x ( ) as a function of reaction time. Conditions: 1.25 ml of ethanol solution containing 0.4 M PG, 0.05 g catalyst, at 363 K for 6 h with stirring.
The catalytic performance of individual active sites on the surface was evaluated by turnover frequency (TOF) measurements. The TOFs of SA/Xx at 8–10% PG conversion, summarized in Table 2, are in the range of 20.5–38.3 h1, slightly higher than those obtained with SA/X (19.8–31.6 h1, TOF0 in Table 2) at the same Al content. Thus, increasing the AlV concentration is able to boost the catalytic performance of individual sites (TOF) mainly at low Al content, similarly to the enhancement of the strength of BAS observed with SA/10x compared with SA/10 due to greater acid strength, as detected by 13C NMR experiments (Fig. 7). Flame-derived SA catalysts exhibit much higher catalytic performance than microporous and mesoporous silica–alumina catalysts reported in the literature [14,28,29]. For example, SA/70x affords a PG conversion of 95% and an EM yield of 94% after 4 h reaction, while with dealuminated HY (de-Al-HY) zeolite and [Al] MCM-41 (Si/Al = 15) PG conversions of only 82% and 84% and EM yields of 74% and 80% were achieved. Remarkably, the TOFs obtained in this work are 3–6 times higher than those achieved with de-Al-HY zeolite (6.2 h1 at 9% PG conversion), the best catalyst for this reaction reported so far in the literature [14,29]. Since de-Al-HY zeolite has much higher density and strength of acid sites, the enhanced catalytic performance of flame-made nonporous catalysts may be ascribed partly to better diffusional mass transfer than with the diffusional constraints inherent in the microporous zeolite. Finally, the recycling behavior of the best-performing catalyst (SA/70x) was tested in five recycling runs, which showed no
AlV-rich ASA catalysts with different Al content have been successfully synthesized by FSP using a solvent with high combustion enthalpy, xylene, for the precursor solution fed to the flame. The local structure and acidity of as-prepared ASAs have been characterized using suitable probe molecules and various solid-state NMR methods. The studies show that the concentration of AlV species on the ASA surface can be tuned by applying solvents with different combustion enthalpies, resulting in different flame temperatures. The higher combustion enthalpy of xylene used as solvent causes a higher flame temperature and a sharper temperature drop during cooling than with previously used solvents, which enhances the freezing of metastable AlV species in the final product. High AlV concentration promotes the formation of BAS in the ASA. The AlV concentration in as-prepared ASAs is much higher than those in ASAs prepared by other methods, where AlIV species is predominant [12,39]. Moreover, as-prepared ASAs exhibit comparable BAS density at significantly lower Al content than previously prepared ASAs. The FSP-derived ASAs using xylene as solvent for the precursor solution showed outstanding catalytic performance in the conversion of PG in ethanol to EM, affording 99.8% yield, which to our knowledge is the highest yield of EM reported so far with a solid acid catalyst. The present work shows that the concentration of AlV species and the Brønsted acidity of FSP-synthesized ASAs can be controlled by the choice of the solvent used in the FSP process, providing together with the optimization of the Al content a promising strategy for tuning the acidity of ASAs.
Acknowledgments This work was supported by Australian Research Council Discovery Projects (DP150103842 and DP180104010), the SOAR Fellowship, and the Sydney Nano Grand Challenge from the University of Sydney from the University of Sydney.
Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.02.007.
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Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Strongly enhanced acidity and activity of amorphous silica–alumina by formation of pentacoordinated AlV species Zichun Wang a,b, Yijiao Jiang b, Fangzhu Jin a, Catherine Stampfl c, Michael Hunger d,⇑, Alfons Baiker e,⇑, Jun Huang a,⇑ a
Laboratory for Catalysis Engineering, School of Chemical and Biomolecular Engineering, Sydney Nano Institute, the University of Sydney, New South Wales 2006, Australia Department of Engineering, Macquarie University, Sydney, New South Wales 2109, Australia School of Physics, Sydney Nano Institute, the University of Sydney, Sydney, New South Wales 2006, Australia d Institute of Chemical Technology, University of Stuttgart, D-70550 Stuttgart, Germany e Institute for Chemical and Bioengineering, Department of Chemistry and Applied Bioscience, ETH Zürich, Hönggerberg, HCI, CH-8093 Zürich, Switzerland b c
a r t i c l e
i n f o
Article history: Received 23 August 2018 Revised 12 December 2018 Accepted 3 February 2019
Keywords: Amorphous silica–alumina Flame-spray pyrolysis Pentacoordinated aluminum Solid-state NMR Acidity Phenylglyoxal Ethyl mandelate
a b s t r a c t Tailoring high-performance aluminosilicates plays a key role in the efficient and clean production of high-value chemicals. Recent work reveals that pentacoordinated Al (AlV) species can significantly enhance the Brønsted acidity of amorphous silica–alumina (ASA), compared with that typically dominated by tetracoordinated Al species. However, the controlled synthesis of AlV-rich ASAs is challenging. Employing xylene as the solvent in a flame-spray pyrolysis process, we synthesized AlV-rich ASAs successfully. The high combustion enthalpy of xylene (36.9 kJ/ml) results in a high flame temperature, promoting the formation and distribution of metastable AlV species in the silica network forming Brønsted acid sites. This provides a promising route for the controlled synthesis of AlV-rich ASAs with higher Brønsted acidity. As an example, AlV-rich ASAs are shown to exhibit superior catalytic performance in phenylglyoxal conversion to ethyl mandelate in ethanol compared with that achieved with other acid catalysts, attaining an ethyl mandelate yield of 99.8%. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Amorphous silica–aluminas (ASAs) are among the most widely used solid acid catalysts and supports for the production of petrochemicals, fine chemicals, and renewable energy [1–3]. The catalytically active Brønsted acid sites (BAS) originate from aluminum centers, commonly accepted as tetracoordinated aluminum (AlIV) species, flexibly coordinated to neighboring SiOH groups in ASAs [4–6]. However, a very low Si/Al ratio (<7/3) leads to the formation of alumina phases [7,8] and to a decrease of the Brønsted acidity in ASAs [9,10]. Recently, 27Al-{1H} dipolar-mediated heteronuclear multiple quantum correlation (D-HMQC) two-dimensional (2D) NMR studies have evidenced that pentacoordinated aluminum (AlV) species in spatial proximity to SiOH groups contribute to a new type of BAS (BAS-AlV) [11]. Coexisting AlV and AlIV species can elevate the BAS population density in ASA, reaching seven times higher densities than in conventional AlIV-rich ASAs [12]. AlV species are ⇑ Corresponding authors. E-mail addresses: [email protected] (M. Hunger), baiker@ chem.ethz.ch (A. Baiker), [email protected] (J. Huang). https://doi.org/10.1016/j.jcat.2019.02.007 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
proposed to further enhance the acid strength of neighboring moderate BAS (e.g., BAS-AlIV) to zeolitic strength or even higher [13]. This enables AlV-rich ASA to exhibit excellent catalytic performance; as an example, a turnover frequency more than an order of magnitude higher than that of dealuminated zeolite H-Y was achieved in the catalytic conversion of phenylglyoxal (PG) [14]. Moreover, AlV species can act as Lewis acid sites (LAS) and anchor active metal species on the ASA surface [15,16]. Therefore, enrichment of ASA with AlV species not only can enhance their Brønsted acidity, but also can improve their catalytic performance as multifunctional catalysts or excellent support materials for reactions. AlV species are intermediates or metastable species between IV Al and AlVI species [15,17–19]. Various techniques, including classical wet-chemical synthesis [20,21], chemical liquid deposition [6], and atomic layer deposition [22], generally produce AlIV-rich ASAs, rather than AlV–rich ASAs. Flame spray pyrolysis (FSP), which we employed in this work, is a single-step method that can generate multicomposition nanoparticles in milliseconds, combining synthesis and calcination on a large scale [23–25]. Taking advantage of super-high temperatures (up to 2000 K) and fast cooling rates, FSP is able to produce materials dominated by metastable phases and polymorphs [24], and thus, FSP techniques
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are suitable for preparing AlV-enriched ASAs [13,26]. The AlV content in ASAs can be tuned from 0 to 41.6% to promote BAS formation with increasing Al loading up to 70% [13]. At the same Al loading, a higher precursor flow rate results in a higher AlV content, but decreases the number of BAS, which is attributed to the larger particle size of the ASAs [26]. Catalytic conversion of PG in water and alcohols over solid acids enables single-step production of mandelic acid and mandelates [14,27–29], which are valuable intermediates for producing pharmaceuticals and fine chemicals [30,31]. Nonporous ASA catalysts facilitate a turnover frequency an order of magnitude higher than that of microporous dealuminated HY zeolites [14]. The catalytic performance of ASAs can be improved significantly by increasing their Brønsted acidity [14], which provides a much higher ethyl mandelate (EM) yield (98.6%) than that achieved with other solid acids reported earlier [27–29]. To our knowledge, the deliberate generation of AlV species to promote Brønsted acidity in ASAs has so far not been reported, but would be promising for improving the catalytic performance of ASAs for various reactions. With this in mind, we used FSP to prepare ASAs with significantly improved AlV density, resulting in higher Brønsted acidity of the surface sites and superior catalytic performance. A series of AlV-rich ASAs with different Si/Al ratios were synthesized using xylene as the solvent, which has a higher combustion enthalpy (36.9 kJ/ml), resulting in a higher flame temperature than that of the methanol/acetic acid (Me/AA) mixture (volume ratio 1/1) with a combustion heat of ’25.4 kJ/ml employed in previously reported ASA syntheses [13]. It will be shown that the use of xylene as a solvent in the liquid precursor flame feed results in significantly enhanced AlV concentration at the same Al loading and thus in a larger number of BAS, as revealed by quantitative 1H magic-angle spinning (MAS) NMR spectroscopy. Finally, the catalytic performance of AlV-rich ASAs prepared via the above-mentioned route was evaluated for the reaction of PG in ethanol to demonstrate their remarkable catalytic activity compared to other solid acids. 2. Experimental methods 2.1. Preparation of ASAs by FSP Aluminum acetylacetonate (Al(acac)3, purity 99.9%), tetraethyl orthosilicate (TEOS) (purity 99.9%), and xylene (purity 98.5%) were all purchased from Sigma-Aldrich and used for producing ASA catalysts. The precursor solutions used for the preparation of ASAs by FSP were obtained by dissolving appropriate amounts of the precursor materials in xylene (0.5 M concentration by metal), followed by filtration using a glass filter. Then the FSP precursor solutions were used in the FSP process as described previously [13]. In brief, the precursor solutions were pumped through a capillary at a rate of 5 ml/min and nebulized at 5 L/ min O2. The resulting spray was ignited by an annular supporting methane/oxygen flame (1.5/0.9 L/min). Particles were collected on a cooled Whatman GF6 filter (diameter 257 mm). A Busch SV 1040C vacuum pump aided in particle recovery. The synthesized silica–alumina powders are designated as SA/Xx, where X is 5, 10, 30, or 70, representing the atom% of Al in the precursor, and x indicates the use of xylene as a solvent to distinguish it from the methanol/acetic acid (denoted as SA/X) used in earlier work [13]. 2.2. Structural characterization N2 adsorption–desorption isotherms measured at 77 K on an Autosorb IQ-C system indicated that the synthesized ASA powder
materials were virtually nonporous. Prior to isotherm measurement, each sample was degassed at 423 K for 12 h under vacuum. The BET surface areas of the ASAs were determined from the N2 adsorption isotherms measured at 77 K using the Brunauer–Emm ett–Teller (BET) method. Powder X-ray diffraction (XRD) studies were performed on a Siemens D5000 instrument with CuKa radiation in the range 10°–70° with scanning steps of 0.02°. 2.3. Solid-state MAS NMR spectroscopy For the 27Al and 29Si MAS NMR investigations, all samples were fully hydrated by overnight exposure to the saturated vapor of a Ca (NO3)2 solution at ambient temperature in a desiccator. 27Al and 29 Si MAS NMR investigations were carried out on a Bruker Avance III 400 WB spectrometer at resonance frequencies of 104.3 and 79.5 MHz, respectively. 27Al MAS NMR spectra were recorded at a sample spinning rate of 8 kHz using 4-mm MAS rotors after single-pulse p/6 excitation with a repetition time of 0.5 s. 29Si MAS NMR measurements were performed with a sample spinning rate of 4 kHz using a 7-mm MAS rotor after single-pulse p/2 excitation, high-power proton decoupling, and a recycle delay of 20 s. Before the 1H and 13C MAS NMR experiments, the samples filled into glass tubes were dehydrated for 12 h at 723 K and at a pressure of less than 102 bar. These dehydrated samples were sealed in glass tubes or directly loaded with ammonia or acetone-2-13C (99.5% 13C-enriched, Sigma-Aldrich) on a vacuum line. Subsequently, the loaded samples were evacuated at 373 K for 1 h (for ammonia) or at room temperature for 2 h (for acetone) to remove weakly physisorbed molecules. Then the samples were transferred into the MAS NMR rotors under dry nitrogen gas inside a glove box. 1 H and 13C MAS NMR investigations were performed on the same spectrometer at resonance frequencies of 400.1 and 100.6 MHz, respectively, with a sample spinning rate of 8 kHz using 4-mm MAS rotors. 1H MAS NMR spectra were recorded after singlepulse p/2 excitation with a repetition time of 20 s. Quantitative 1 H MAS NMR measurements were carried out using a zeolite H, Na-Y (35% ion-exchanged) as an external intensity standard. 13C cross-polarization (CP) MAS NMR spectra were recorded with a contact time of 4 ms and a repetition time of 4 s. 2.4. Catalytic conversion of phenylglyoxal in ethanol The conversion of PG (Sigma-Aldrich, >97%) to EM was utilized to study the catalytic performance of the SA/Xx materials. An amount of 0.05 g of the catalyst was employed and activated overnight under an N2 flow of 50 ml/min at 723 K. After cooling down in flowing N2 gas, the catalyst was transferred into a glass reactor with a volume of 15 ml. Subsequently, 1.25 ml of the ethanol solution containing 0.4 M PG and 0.05 M octane (as GC internal standard) was added and mixed with the activated catalyst under magnetic stirring. The reaction was carried out in a tightly closed glass reactor immersed in an oil bath at 363 K for 6 h. The reaction products were determined using a Shimadzu QP2010 Ultra GC–MS equipped with a Rtx-5MS column (30 m 0.25 mm 0.25 lm) and quantified by a Shimadzu GC2010 equipped with an flame ionization detector and a Rtx-5 column (30 m 0.32 mm 0.25 lm). The selectivity for the specific products i (Si) was calculated as Si (%) = 100 (i)/[(PG)0 (PG)], where (i) is the molar concentration of the product i and (PG)0 and (PG) correspond to the molar concentrations of PG before and after the reaction, respectively. The used catalyst was washed with ethanol three times and dried overnight at 373 K before being calcined at 773 K for 3 h for complete removal of organic residues. The reaction mixture after 6 h reaction was analyzed by atomic emission spectroscopy with an inductively coupled plasma (ICP-AES, Perkin-Elmer, Plasma 400).
Z. Wang et al. / Journal of Catalysis 372 (2019) 1–7
3. Results and discussion 3.1. Catalyst morphology The amorphous nature of the SA/Xx catalysts was confirmed by XRD patterns (Fig. 1). All samples showed a broad peak at 2h = 20°–30° attributed to amorphous silica [32], and no crystalline silica or alumina phases could be detected. This finding indicates that aluminum species are well dispersed in the amorphous silicon oxide network. The surface areas and primary particle sizes of the SA/Xx catalysts are summarized in Table 1. The specific surface areas of these materials were in the range of 241–391 m2/g, significantly higher than those of ASAs previously prepared using Me/AA mixtures as solvent (SA/X materials, 200–377 m2/g) [13]. As observed for the SA/X materials [13], the surface areas of SA/Xx catalysts gradually decreased with increasing Al content. The average particle sizes of SA materials were estimated from BET measurement (Table 1) and confirmed by SEM images (Fig. S1 in the Supporting Information). Interestingly, using xylene instead of Me/AA in the FSP precursor solution did not strongly affect the particle size of SA materials (7–17 vs. 7–20 nm). 3.2. Surface acidity and local structure of SA/Xx materials The effect of using xylene as a solvent in the FSP synthesis of AlV-rich ASA was investigated by 27Al MAS NMR spectroscopy (Fig. 2). In SA/5x, the strong signal at d27Al = 51 ppm and the weak signal at d27Al = 0 ppm represent the commonly observed AlIV and AlVI species in ASAs. With the Al content increased to 10 mol.% (SA/10x), a signal at d27Al = ca. 28 ppm assigned to AlV species was observed [11,13,33], which was strongly enhanced when the Al content was increased to 70 mol% (SA/70x). The molar ratios of Al species are summarized in Table 1. Obviously, using a solvent with higher combustion enthalpy, such as xylene, can enhance the formation of AlV species in ASAs. The AlV species was observed in SA/10x (17.6%), in this research but not in SA/10 in our previous study [13]. At 30 to 70 mol.% Al loading, the fraction of AlV species synthesized by FSP using xylene as solvent was 1.3–1.6 times higher than that of corresponding catalysts produced with Me/AA as solvent. During the FSP synthesis [23,24,34], the metal precursors and the solvent were atomized and sprayed to form the precursor vapor, which in turn was converted to product vapor containing
Fig. 1. XRD patterns of FSP-derived SA/Xx catalysts with different Al contents.
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metastable species (e.g., AlV) through combustion at high temperature, and the resulting well-mixed product molecules and clusters underwent nucleation and grew to nanoparticles. The high cooling rate facilitated the freezing of metastable species in the final product. Compared with Me/AA, xylene has a higher combustion enthalpy, providing a higher flame temperature, and thus a more severe temperature drop, which enhanced the freezing of metastable AlV species. Recently, the formation of BAS based on AlV species has been evidenced by 27Al-{1H} D-HMQC 2D NMR experiments [11]. Therefore, the effect of increasing the AlV content on the surface Brønsted acidity of the ASAs was studied. 1H MAS NMR spectroscopy is a powerful tool for the characterization of surface hydroxyl protons [5]. As shown in Fig. 3a–e, top, the signals of nonacidic and acidic surface OH groups are strongly overlapping, and only a strong peak at d1H = 1.8 ppm assigned to SiOH groups can be observed in the 1H MAS NMR spectra of dehydrated ASA catalysts [5,13]. Adsorption of a strong base, such as ammonia, is a useful technique for identifying BAS on ASA [5,13]. Upon ammonia loading, a signal arises at d1H = ca. 6.7 ppm (Fig. 3a–e, bottom), which indicates the formation of ammonium ions via ammonia protonation at BAS. The number of BAS can be estimated by integration of the ammonium signal (d1H = ca. 6.7 ppm) in Fig. 3 [5], leading to the results summarized in Table 1. Since AlVI species mainly afford surface Lewis acidity [35–38], the enhancement of BAS (both density and fraction) with increasing Al content (from 5 to 70%) obviously depends on the significant increase in AlV concentration (from 0 to 55.5%) rather than on the strong decrease in AlIV concentration (from 97 to 16.3%). Moreover, the larger amount of AlV species contributes to the higher BAS density (6.8–19.8 102 mmol/g) obtained for SA/Xx using xylene as solvent compared with ASAs prepared by other methods [12,39], and with FSP-derived SAs prepared with Me/AA as solvent (9.8–15.1 102 mmol/g) [13]. In ASAs, the formation of BAS is accepted as being caused by Al atoms in the vicinity of SiOH groups. The correlation between Al atoms and neighboring SiOH groups was probed by a 1H/27Al TRAPDOR (transfer of population in double resonance) MAS NMR experiment (Fig. 4) [4,40,41]. In a TRAPDOR experiment, the dipolar dephasing of 1H–27Al pairs can be suppressed by continuous 27Al irradiation, which influences the intensity of 1H spin–echo MAS NMR signals caused by OH groups in the vicinity of Al species. Thus, the difference spectrum of SA/70x recorded without and with 27 Al irradiation (Fig. 4 bottom) evidenced the interaction between SiOH groups (d1H = 1.8 ppm) and neighboring Al atoms [4,5]. Moreover, a strong low-field shift of the main peak in the 29Si MAS NMR spectra of SA/Xx catalysts with increasing Al content (Fig. 5) demonstrates a continuous incorporation of Al into the silica network [13,42]. It correlates well with the increasing number of BAS, which indicates that increasing aluminum incorporation contributes to the formation of these surface sites on ASA. Since ASA catalysts are often utilized under anhydrous conditions, the study of the correlation between the number of BAS on the dehydrated materials and the AlV species as a function of Al loading is of great importance (Fig. 6). The concentrations of AlV species were determined by decomposing the 27Al MAS NMR spectra of corresponding dehydrated SA/Xx catalysts as shown in the Supporting Information (Fig. S1 and Table S2). Compared to SAs prepared with Me/AA as solvent, the increase of the BAS density from using xylene as solvent correlates well with the increase of AlV species (44% vs. 39%, 52% vs. 48%, 57% vs. 51%, and 61% vs. 55% for Al loadings of 10%, 30%, 50%, and 70%, respectively) while the AlIV fraction decreased (54% vs. 59%, 43% vs. 51%, 38% vs. 42%, and 32% vs. 38% for Al loadings of 10%, 30%, 50%, and 70%, respectively). Clearly, using xylene as a solvent yields a larger population of AlV species, and thus promotes the connection between
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Z. Wang et al. / Journal of Catalysis 372 (2019) 1–7
Table 1 Properties of FSP SA/Xx catalysts with Al content X of 5–70 atom%: BET surface areas (ABET), particle sizes (DBET), total densities of surface OH groups (TOH), densities of Brønsted acid sites (NBAS), and fractions of Al species with different oxygen coordination. Sample SA/5x SA/10x SA/30x SA/50x SA/70x
ABET (m2/g) 391 369 289 267 241
DBET (nm)a 7 8 9 12 17
TOH (mmol/g)b 1.4 1 1.3 1.2 1.6
NBAS (mmol/g)b 2
6.8 10 7.4 102 14.8 102 15.8 102 19.8 102
AlIV (atom%)c
AlV (atom%)c
AlVI (atom%)c
AlV (atom%)d
97 56.7 23 20 16.3
— 17.6 43 55 55.5
3 25.7 34 25 28.2
— 0 20.3 40.5 41.6
a
The particle diameter (DBET) of SA/Xx materials derived from BET surfaces according to DBET (nm) = 6000/(BET surface area in m2/g) (density in g/cm3). Total number of OH groups (TOH) and population of Brønsted acidic OH groups (NBAS). Determined from quantitative 1H MAS NMR using NH3 as a probe molecule as shown in Section 3.2. c The atomic concentrations of Al species were determined from Fig. S2. d AlV concentration obtained from Ref. [13] using Me/AA as a solvent. b
Fig. 2. 27Al MAS NMR spectra of FSP-derived SA/5x (a), SA/10x (b), SA/30x (c), SA/ 50x (d), and SA/70x (e).
AlV species and SiOH groups in their vicinity [11] to generate more BAS. Notably, both SA/30x and SA/50, as well as SA/50x and SA/70, which contain similar AlV and AlIV fractions but higher BAS densities, are obtained for SA prepared using xylene at a much lower Al content. More significantly, SA/30x has a BAS density similar to that of SA/70 having a factor of 2.33 higher Al content at lower AlV concentration. This finding indicates that using a solvent with higher combustion enthalpy (e.g., xylene) not only increases the AlV concentration, but can also promote the distribution of Al species over the silica network and thus the formation of BAS. Therefore, this approach provides a potential route for the synthesis of ASA materials with higher acidity at lower Al content. The effect of AlV species on the acid strength of ASA was scaled by the 13C NMR chemical shift of the carbonyl groups of acetone2-13C (CH313COCH3) as a probe molecule [5]. A greater adsorbateinduced low-field shift of the 13C CP/MAS NMR signals indicates a higher Brønsted acid strength of surface OH groups and vice versa. As shown in Fig. 7, the 13C CP/MAS NMR spectra of SA/Xx samples are all dominated by a signal at 214–216 ppm, assigned to acetone-2-13C adsorbed onto BAS with moderate strength. The broad hump at ca. 225 ppm in Fig. 7b and 7c indicates a wide distribution of acidic OH groups with higher strength on SA/Xx, while the weak signal at ca. 240 ppm in SA/70x (Fig. 7c) was caused by acetone adsorbed onto Lewis acidic sites [13]. Other d13C signals are due to aldol reaction of the acetone-2-13C on SA/x catalysts and nonenriched carbon atoms in methyl groups, respectively [43–45].
Fig. 3. 1H MAS NMR spectra of dehydrated FSP-derived SA/5x (a), SA/10x (b), SA/ 30x (c), SA/50x (d), and SA/70x (e), recorded before (top) and after (bottom) loading with NH3 and subsequent evacuation at 373 K for 1 h.
Obviously, the surface OH groups of the SA/Xx catalysts exhibit a Brønsted acid strength from moderate to comparable to or even stronger than that of zeolites H-Y (d13C = 220 ppm) [45] and HZSM5 (d13C = 223 ppm) [44,45], which is similar to that of FSP-derived ASA synthesized using Me/AA as a solvent [13]. Particularly, the spectrum of SA/10x shows a hump at d13C ca. 225 ppm, which was not observed in the spectrum of SA/10 using Me/AA as solvent and which was dominated by a signal at d13C = 213 ppm only [13]. In terms of the slightly lower BAS density, this indicates that the
Z. Wang et al. / Journal of Catalysis 372 (2019) 1–7
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Fig. 4. 1H/27Al TRAPDOR spectra of dehydrated SA/70x.
Fig. 7. 13C CP/MAS NMR spectra of SA/5x (a), SA/10x (b), and SA/70x (c) recorded after loading with acetone-2-13C and evacuation at room temperature for 1 h.
3.3. Catalytic conversion of phenylglyoxal to ethyl mandelate
Fig. 5. 29Si MAS NMR spectra of FSP-derived SA/5x (a), SA/10x (b), SA/30x (c), and SA/70x (d).
Fig. 6. Correlation between the concentration of Brønsted acidic OH groups (BAS, solid line) and AlV species (dashed line) as a function of the Al content in dehydrated FSP-derived SA/Xx prepared with xylene ( ) and MeAA (j) as solvents. The density of AlV species was obtained by simulation of the corresponding 27Al NMR spectra of dehydrated SA/Xx samples using the parameters obtained in MQMAS NMR investigations (Fig. S1). The simulation parameters are summarized in Table S1.
higher AlV concentration of SA/10x causes a higher Brønsted acid strength. With increasing Al content up to 70%, both the density and strength of BAS are promoted by the increase of AlV species in SA/Xx compared with other ASA catalysts. Therefore, the FSP-derived ASA synthesized using xylene as solvent is expected to exhibit higher catalytic performance for the target reaction than ASA catalysts prepared via other routes.
The catalytic performance of the SA/Xx materials was evaluated using the conversion of PG in ethanol to EM at 363 K. The catalytic results are summarized in Table 2. The conversion of PG and the selectivity to EM as a function of time are shown in Fig. 8 (also see Fig. S3 with error bars). Pure silica is inactive for this reaction [14]. Already, SA/5x, having mainly Brønsted acidity, showed a PG conversion of 61.6% and a selectivity to EM of 98.7% after a reaction time of 6 h. The enhancement of the Brønsted acidity of SA/Xx materials with increasing Al content up to 70% significantly improved the PG conversion from 61.6% to 100% under the same conditions, and the selectivity of EM remained higher than 98.6% for all samples. With SA/70x, an EM yield of 99.8% was achieved, which to our knowledge is the highest reported so far with solid acid catalysts [29,46–50]. Compared with AlV–rich FSP-derived ASA materials synthesized using Me/AA as the solvent [13], a significant increase of the EM yield (89.8% vs. 56.5%, 95.2% vs. 67%, and 99.8% vs. 97% for SA/10x, SA/30x, and SA/70x, respectively) was acquired on SA/Xx using xylene as the solvent. The major difference between these two batches is the remarkable increase in the AlV concentration, which improves the Brønsted acidity. The effect of AlV species on the catalytic activity of the BAS was evaluated by comparing the rate constants k of PG conversion (Table 2, columns 5 and 6). SA/10x showed a 2.1 times higher k value (0.0054 s1 vs. 0.0026 s1) with a slightly smaller number of BAS (7.4 102 mmol/g vs. 9.8 102 mmol/g) than SA/10. This catalytic property is attributed to the enhanced BAS strength (d13C = 214–225 ppm for SA/10x vs. 213 ppm for SA/10), which facilitates the activation of the C@O bond in PG as the initiating step of the reaction. The high BAS densities of the AlV–rich SA/30x and SA/70x catalysts resulted in 2.8 and 1.3 times higher k values than for the FSP-derived ASA using Me/AA as solvent (0.0097 s1 vs. 0.0035 s1 and 0.0141 s1 vs. 0.0106 s1). Moreover, SA/30x, which has a number of BAS similar to that for SA/70, afforded a similar PG conversion and k value, but at half the Al content, demonstrating the preparation of high-performance ASA materials with much lower Al content. Therefore, the efficiency of the catalytic conversion of PG to EM can be significantly improved by enhancing the Brønsted acidity of ASA via increasing the AlV concentration. The novel route for preparing FSP-derived ASA using xylene as a solvent represents a promising way to enhance the Brønsted acidity of ASAs and to improve their catalytic performance.
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Z. Wang et al. / Journal of Catalysis 372 (2019) 1–7
Table 2 Catalytic properties of FSP-derived SA/x catalysts with different Al content.a
a b c d e f g
Sample
CPGb (%)
YEMc (%)
SEMd (%)
ke (s1)
k0 f (s1)
TOFg (h1)
TOF0 g, (h1)
SA/5x SA/10x SA/30x SA/50x SA/70x
61.6 90.1 95.4 98.9 100
61 89.8 95.2 98.7 99.8
98.7 98.8 98.6 99.8 99.8
0.0023 0.0054 0.0097 0.0126 0.0141
— 0.0026 0.0035 0.0051 0.0106
24.4 35.2 20.5 35.3 28.4
— 27.5 19.8 31.6 25.3
Conditions: 1.25 ml of ethanol solution containing 0.4 M PG and 0.05 g catalyst at 363 K for 6 h with stirring. CPG = conversion of PG. YEM = yield of ethyl mandelate in mol.%. SEM = selectivity to ethyl mandelate at 50% conversion. k = reaction rate from fitting kinetic data. k0 obtained from the kinetic data of FSP SA catalysts using the mixture of methanol and acetic acid (1:1 by volume) as solvent. TOF and TOF0 were calculated on the basis of 8–10% PG conversion and the number of BAS on SA/Xx and SA/X, respectively.
significant activity loss (Fig. S4). Furthermore, no Al leaching could be detected in the reaction mixture, and the 27Al MAS NMR spectra of SA/70x indicated no significant change before and after five recycling runs (Fig. S5), confirming the high stability of SA/70x in the liquid-phase reaction under the current conditions.
4. Conclusions
Fig. 8. Catalytic conversion of PG in ethanol (closed symbols) and selectivity to ethyl mandelate (open symbols) over SA/5x (j), SA/10x (d), SA/30x (▲), SA/50x (r), and SA/70x ( ) as a function of reaction time. Conditions: 1.25 ml of ethanol solution containing 0.4 M PG, 0.05 g catalyst, at 363 K for 6 h with stirring.
The catalytic performance of individual active sites on the surface was evaluated by turnover frequency (TOF) measurements. The TOFs of SA/Xx at 8–10% PG conversion, summarized in Table 2, are in the range of 20.5–38.3 h1, slightly higher than those obtained with SA/X (19.8–31.6 h1, TOF0 in Table 2) at the same Al content. Thus, increasing the AlV concentration is able to boost the catalytic performance of individual sites (TOF) mainly at low Al content, similarly to the enhancement of the strength of BAS observed with SA/10x compared with SA/10 due to greater acid strength, as detected by 13C NMR experiments (Fig. 7). Flame-derived SA catalysts exhibit much higher catalytic performance than microporous and mesoporous silica–alumina catalysts reported in the literature [14,28,29]. For example, SA/70x affords a PG conversion of 95% and an EM yield of 94% after 4 h reaction, while with dealuminated HY (de-Al-HY) zeolite and [Al] MCM-41 (Si/Al = 15) PG conversions of only 82% and 84% and EM yields of 74% and 80% were achieved. Remarkably, the TOFs obtained in this work are 3–6 times higher than those achieved with de-Al-HY zeolite (6.2 h1 at 9% PG conversion), the best catalyst for this reaction reported so far in the literature [14,29]. Since de-Al-HY zeolite has much higher density and strength of acid sites, the enhanced catalytic performance of flame-made nonporous catalysts may be ascribed partly to better diffusional mass transfer than with the diffusional constraints inherent in the microporous zeolite. Finally, the recycling behavior of the best-performing catalyst (SA/70x) was tested in five recycling runs, which showed no
AlV-rich ASA catalysts with different Al content have been successfully synthesized by FSP using a solvent with high combustion enthalpy, xylene, for the precursor solution fed to the flame. The local structure and acidity of as-prepared ASAs have been characterized using suitable probe molecules and various solid-state NMR methods. The studies show that the concentration of AlV species on the ASA surface can be tuned by applying solvents with different combustion enthalpies, resulting in different flame temperatures. The higher combustion enthalpy of xylene used as solvent causes a higher flame temperature and a sharper temperature drop during cooling than with previously used solvents, which enhances the freezing of metastable AlV species in the final product. High AlV concentration promotes the formation of BAS in the ASA. The AlV concentration in as-prepared ASAs is much higher than those in ASAs prepared by other methods, where AlIV species is predominant [12,39]. Moreover, as-prepared ASAs exhibit comparable BAS density at significantly lower Al content than previously prepared ASAs. The FSP-derived ASAs using xylene as solvent for the precursor solution showed outstanding catalytic performance in the conversion of PG in ethanol to EM, affording 99.8% yield, which to our knowledge is the highest yield of EM reported so far with a solid acid catalyst. The present work shows that the concentration of AlV species and the Brønsted acidity of FSP-synthesized ASAs can be controlled by the choice of the solvent used in the FSP process, providing together with the optimization of the Al content a promising strategy for tuning the acidity of ASAs.
Acknowledgments This work was supported by Australian Research Council Discovery Projects (DP150103842 and DP180104010), the SOAR Fellowship, and the Sydney Nano Grand Challenge from the University of Sydney from the University of Sydney.
Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.02.007.
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