Fig.1

Proposed models for Brønsted acid sites (BASs) in silica-alumina catalysts[7]. (a) BAS consisting of a bridging silanol site bonded to AlIV site (SiOHAl) in zeolites; (b) BAS consisting of PBS interacting with AlIV site; (c) BAS consisting of the flexible coordination between silanol oxygen and neighbouring AlIV. In the latter two cases, the dotted line does not denote a covalent bond but only the close proximity between O and Al atoms

Fig.2

27Al MQMAS spectra of SA/10 (a, b) and SA/30 (c, d) after dehydrated at 723 K (a, c) and after rehydration (b, d), recorded at a magnetic field of 16.4 T, and corresponding EDX images of SA/10 (e, f) and SA/30 (g, h)[6]. (i) 3D-APT reconstruction of isolated SA/10 nanoparticles[8]

Table 1

Al concentration and population density of BASs in AlV-enriched ASAs[6, 9, 10]

Fig.3

27Al-{1H} D-HMQC 2D spectra of SA/50. (a) SA/50 dehydrated at 723 K for 12 h under vacuum, and (b) after ammonia loading and evacuated at 373 K for 1 h. The spectra were recorded at 18.8 T with a MAS frequency of νR=20 kHz, and τrec=1.0 ms for dehydrated and τrec=900 μs for ammonia-loaded sample, respectively[7]

Fig.4

Optimized structure of AlV species in dehydrated states (a, c and e) and in corresponding rehydrated states (b, d, and f), both calculated at B3LYP/6-31g theoretical level[6]; (g) 17O{27Al} TRAPDOR curves obtained from 17O{1H, 27Al} PRESTO-TRAPDOR experiment were simulated to determine the Al-OH distance, for discriminating the structure of AlV-BASs, corresponding to bridging silanols (black line, O-Al distance of 2 Å) and pseudo-bridging silanols (gray shaded area, O-Al distance range of 3 to 4.4 Å)[41], the corresponding 17O chemical shifts were at -60 and 15 ppm

Fig.5

13C MAS NMR spectra of acetone-2-13C-loaded AlV-enriched ASAs[10]: (a) SA/0; (b) SA/10; (c) SA/70

Fig.6

(a)~(e) 3D APT reconstruction of two isolated SA/50 nanoparticles showing all atoms; (f) 27Al DQ-SQ 2D NMR spectrum recorded at 18.8 T with νR=20 kHz of dehydrated SA/50, and (g)~(k) rows extracted from the 2D spectrum corresponding to the various autocorrelation and cross-peaks[8]

Fig.7

Proposed models for BAS on ASAs generated by (a) one Al center per SiOH for moderate BAS, and (b) two Al centers per SiOH group, leading to zeolitic acid strengths[8]

Fig.8

Catalytic performance of H-ZSM-5 and AlV-enriched ASAs in H/D exchange with d6-benzene at 313 K. 1H MAS NMR spectra at 9.4 T of (a) zeolite H-ZSM-5 and (b) SA/50 (stack plot spectra). (c) Kinetics and H/D exchange rates k for H-ZSM-5 (top), SA/50 (middle), and SA/10 (bottom)[8]

Table 2

Catalytic data of PG conversion to ethyl mandelate over ASAs and De-Al-HY

Fig.9

Catalytic conversion of C3 sugars over ASAs. (a) Catalytic conversion of phenylglyoxal (―) and selectivity to alkyl mandelate (---) as a function of reaction time in MeOH (■), EtOH (●), i-PrOH (▼), n-PrOH (◆), n-BuOH (▲) over SA/70 and reaction in i-PrOH (★) over De-Al-HY[26]. (b) Catalytic glyceraldehyde conversion in ethanol over fresh De-Al-HY and SA/50 and after five recycle uses. Conditions: 1.25 mL of alcohol solution containing 0.4 mol/L phenylglyoxal or glyceraldehyde, 0.05 g catalyst, at 363 K for 6 h with stirring[8]

Table 3

Catalytic performance of ASAs and pertinent reference catalysts in the conversion of glucose to HMF

Fig.10

(a) 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 ASAs prepared by using methanol/acetic acid (1:1 by volume, ■) and xylene (★), respectively; (b) Catalytic conversion of PG in ethanol. The PG conversion (CPG), EM selectivity (SEM), reaction rates k, and TOFs were obtained from Ref. [9], respectively. Same reaction conditions as shown in Fig. 9

Fig.11

27Al MAS NMR spectra of (a) ASA, (b) Pt/ASA, (c) Al2O3 (yellow dash line), and Pt/Al2O3 (blue solid line). Static 27Al NMR of wet samples obtained after: (d) H2PtCl6 mixed with ASA, (e) mixing (d) with NH3•H2O, and (f) after heating mixture (e) at 90 ℃ for 1 h, recorded at 11.7 T[27]

Fig.12

27Al (a~d) and 1H (e~h) MAS NMR spectra of the as-synthesized Pt/ASA dried at different temperatures: room temperature (Pt/ASA-RT) (a, e), 100 ℃ (b, f), 200 ℃ (c, g) and 350 ℃ (d, h) [27]

Fig.13

Schematic presentation of the proposed pathways for the formation of Pt single sites at AlV sites on ASA support[27]