Pei-Hao Wu

31P Chemical Shift of Adsorbed Trialkylphosphine Oxides for Acidity Characterization of Solid Acids Catalysts

A comprehensive study has been made to predict the adsorption structures and (31)P NMR chemical shifts of various trialkylphosphine oxides (R3PO) probe molecules, viz., trimethylphosphine oxide (TMPO), triethylphosphine oxide (TEPO), tributylphosphine oxide (TBPO), and trioctylphosphine oxide (TOPO), by density functional theory (DFT) calculations based on 8T zeolite cluster models with varied Si-H bond lengths. A linear correlation between the (31)P chemical shifts and proton affinity (PA) was observed for each of the homologous R3PO probe molecules examined. It is found that the differences in (31)P chemical shifts of the R3POH(+) adsorption complexes, when referring to the corresponding chemical shifts in their crystalline phase, may be used not only in identifying Brønsted acid sites with varied acid strengths but also in correlating the (31)P NMR data obtained from various R3PO probes. Such a chemical shift difference therefore can serve as a quantitative measure during acidity characterization of solid acid catalysts when utilizing (31)P NMR of various adsorbed R3PO, as proposed in our earlier report (Zhao; et al. J. Phys. Chem. B 2002, 106, 4462) and also illustrated herein by using a mesoporous H-MCM-41 aluminosilicate (Si/Al = 25) test adsorbent. It is indicative that, with the exception of (TMPO), variations in the alkyl chain length of the R3PO (R = C(n)H(2n+1); n > or = 2) probe molecules have only negligible effect on the (31)P chemical shifts (within experimental error of ca. 1-2 ppm) either in their crystalline bulk or in their corresponding R3POH(+) adsorption complexes. Consequently, an average offset of 8 +/- 2 ppm was observed for (31)P chemical shifts of adsorbed R3PO with n > or = 2 relative to TMPO (n = 1). Moreover, by taking the value of 86 ppm predicted for TMPO adsorbed on 8T cluster models as a threshold for superacidity (Zheng; et al. J. Phys. Chem. B 2008, 112, 4496), a similar threshold (31)P chemical shift of ca. 92-94 ppm was deduced for TEPO, TBPO, and TOPO.

New insights into Keggin-type 12-tungstophosphoric acid from 31P MAS NMR analysis of absorbed trimethylphosphine oxide and DFT calculations.

The acid and transport properties of the anhydrous Keggin-type 12-tungstophosphoric acid (H(3)PW(12)O(40); HPW) have been studied by solid-state (31)P magic-angle spinning NMR of absorbed trimethylphosphine oxide (TMPO) in conjunction with DFT calculations. Accordingly, (31)P NMR resonances arising from various protonated complexes, such as TMPOH(+) and (TMPO)(2)H(+) adducts, could be unambiguously identified. It was found that thermal pretreatment of the sample at elevated temperatures (≥423 K) is a prerequisite for ensuring complete penetration of the TMPO guest probe molecule into HPW particles. Transport of the TMPO absorbate into the matrix of the HPW adsorbent was found to invoke a desorption/absorption process associated with the (TMPO)(2)H(+) adducts. Consequently, three types of protonic acid sites with distinct superacid strengths, which correspond to (31)P chemical shifts of 92.1, 89.4, and 87.7 ppm, were observed for HPW samples loaded with less than three molecules of TMPO per Keggin unit. Together with detailed DFT calculations, these results support the scenario that the TMPOH(+) complexes are associated with protons located at three different terminal oxygen (O(d)) sites of the PW(12)O(40)(3-) polyanions. Upon increasing the TMPO loading to >3.0 molecules per Keggin unit, abrupt decreases in acid strength and the corresponding structural variations were attributed to the change in secondary structure of the pseudoliquid phase of HPW in the presence of excessive guest absorbate.

Syntheses of novel halogen-free Brønsted–Lewis acidic ionic liquid catalysts and their applications for synthesis of methyl caprylate

A series of benign halogen-free ionic liquid (IL) catalysts were synthesized by combining the Bronsted acidic ionic liquid [HSO3-pmim]+HSO4− with ZnO in different composition ratios. The IL catalysts, which possess both Bronsted and Lewis acidities, were employed as acidic catalysts for the esterification of n-caprylic acid to methyl caprylate. The [HSO3-pmim]+(1/2Zn2+)SO42−, prepared by cooperating equimolar amounts of Bronsted and Lewis acid sites, was found to exhibit an optimal catalytic performance and excellent durability. This is attributed to a synergy of Bronsted and Lewis acidities manifested by the catalyst. The response surface methodology (RSM) based on the Box–Behnken design (BBD) was utilized to explore the effects of different experimental variables (viz. catalyst amount, methanol to caprylic acid molar ratio, temperature, and reaction time) on the esterification reaction. Analysis of variance (ANOVA) was also employed to study the interactions between variables and their effects on the catalytic process. Accordingly, the deduced optimal reaction conditions led to a high methyl caprylate yield of 95.4%, in good agreement with experimental results and those predicted by the BBD model. Moreover, a kinetic study performed under optimal reaction conditions revealed an apparent reaction order of 1.70 and an active energy of 33.66 kJ mol−1.

Highly nitrogen-doped mesoscopic carbons as efficient metal-free electrocatalysts for oxygen reduction reactions

A facile method was developed for the synthesis of metal-free, highly N-doped (>7 wt%) mesoscopic carbons (NMCs), which were fabricated by first preparing carbon–silicate (C–Si) composites by co-condensation method using a melamine-formaldehyde resin oligomer as the primary nitrogen and carbon source, and P123 triblock copolymer surfactant and sodium silicate as the soft and hard template, respectively, under microwave irradiation conditions, followed by carbonization and silica-template removal. The NMCs were found to exhibit superior electrocatalytic activity, long-term stability, and excellent tolerance over methanol crossover effect. Such NMCs derived from organic–inorganic hybrids assisted by microwave heating not only possess high surface areas and active quaternary and pyridinic-N species that are favourable for ORR, as verified by DFT calculations, but also render large-scale production and practical applications as cost-effective electrode materials.

Transesterification of soybean oil to biodiesel by tin-based Brønsted-Lewis acidic ionic liquid catalysts

A series of Brønsted-Lewis acidic ionic liquid (BLAIL) catalysts consisting of sulfonated ionic liquid [SO3H-pmim]Cl and Sn(II) chloride have been synthesized and exploited for catalytic transesterification of soybean oil with methanol to biodiesel. The structural and chemical properties of these [SO3H-pmim]Cl-xSnCl2 (x=0-0.8) catalysts were characterized by different analytical and spectroscopic techniques, such as FT-IR, TGA, and NMR. In particular, their acid properties were studied by solid-state 31P NMR using trimethylphosphine oxide as the probe molecule. The BLAIL catalysts were found highly efficient for transesterification reaction due to the introduction of Lewis acidity by SnCl2 in the initially Brønsted acidic [SO3H-pmim]Cl catalyst. The effects of three independent process variables on biodiesel yield were optimized by response surface methodology (RSM). Consequently, an excellent biodiesel yield of 98.6% was achieved under optimized reaction conditions over the BLAIL catalyst with SnCl2 loading (x) of 0.7.