Xia Qinghua
Email Address
elexq@nus.edu.sg
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Publication H3PW12O40-supported MCM-41 acid catalyst for the gas-phase synthesis of MTBE(2004) Xia, Q.H.; Wee, C.H.; Hidajat, K.; Kawi, S.; CHEMICAL AND PROCESS ENGINEERING CENTRE; CHEMICAL & BIOMOLECULAR ENGINEERINGHPW (H3PW12O40) was impregnated onto the surface of fluorinated Si-MCM-41 with an excellent hydrothermal stability. The ordered hexagonal mesostructure of the Si-MCM-41 support is maintained after the impregnation of 50-wt% HPW. The HPW amount has a remarkable effect on the intensity of the main XRD reflection [100] peak of the Si-MCM-41 support. No visible crystalline diffraction peaks appear above 10° 2θ for these samples. This is assigned to the use of a well-ordered MCM-41 host in this work. A sharp mesoporous inflection is observed at relative pressures between p/po = 0.25 and 0.40 on the isotherms of these samples. The infrared framework vibration bands of Si-MCM-41 are at 1236, 1090, 965, 800, 564 and 465 cm 1, easily overlap with those of Keggin structure of HPW. It is observed that the intensity of the 800-cm-1 (W-O-W) band is almost proportional to the increase of HPW amount on MCM-41. Once Si-MCM-41 is loaded with HPW, Brønsted and Lewis acidities are revealed on the HPW/MCM-41 catalysts, with an ascending order in the surface acidity of Si-MCM-41 ≪ HPW(15%)/MCM-41 < HPW(30%)/MCM-41 < HPW(50%)/ MCM-41. For the gas-phase synthesis of MTBE, the HPW/MCM-41 catalysts show high activity at optimal temperatures, and high on-stream steady activity until 110 h at 100 °C, comparable to that of the Amberlyst-15 catalyst. But the temperature window to achieve an ideal conversion narrows gradually from 80-100 °C for Amberlyst-15, to 90-100 °C for HPW(30-50%)/MCM-41 catalysts, and to 100 °C for HPW(15%)/MCM-41.Publication Characterization and morphological control of β zeolite synthesized in a fluoride medium(2004) Xia, Q.H.; Song, J.; Kawi, S.; Li, L.L.; CHEMICAL AND PROCESS ENGINEERING CENTRE; CHEMICAL & BIOMOLECULAR ENGINEERINGThe formation and morphological control of beta zeolite was investigated in a fluoride medium. 29Si MASNMR spectra showed that the chiral siliceous β framework was free of the connectivity defects. For F-W-β, F-Pt-β and F-Pd-β, a visible mesoporous size distribution at 17.4-17.9 Å was detected. The relative coverage of surface -OH groups decreased in the sequence: H-β > H-ZSM-5 > F-Al-β > F-W-β > F-Pd-β > F-Pt-β > F-Si-β(1) > F-Si-β(2). The synthesis in a fluoride medium resulted in a higher resolution of all the IR vibration environments of framework elements. The spontaneous nucleation in a fluoride medium induced the formation of especially uniform crystalline morphology, while the presence of β seeding resulted in a mixed morphology. Prolonging the crystallization time remarkably increased the crystal size of siliceous β. TEOS as the silica source resulted in the formation of a single-crystal zeolite β, but aerosil-200 silica induced a multi-crystal β.Publication Structure, morphology, and catalytic activity of β zeolite synthesized in a fluoride medium for asymmetric hydrogenation(2003-10-01) Xia, Q.-H.; Shen, S.-C.; Song, J.; Kawi, S.; Hidajat, K.; CHEMICAL & ENVIRONMENTAL ENGINEERING; CHEMICAL AND PROCESS ENGINEERING CENTREA series of beta (β) zeolites has been synthesized in a fluoride medium and characterized by different techniques. XRD showed that the majority of F-Si-β, F-Pt-β, F-Pd-β, and F-W-β materials consisted of polymorph A. W and Al were incorporated into the framework of β, while Pt and Pd existed on the surface of β in the form of PtOx or PdOx oxides. The framework structures of F-Si-β, F-Pt-β, and F-Pd-β were almost identical, as proven by 29Si MAS NMR and FTIR spectra. The relative coverage of surface -OH groups decreased in the sequence H-β>H-ZSM-5>F-Al-β>F-W-β>F-Pd-β> F-Pt-β>F-Si-β(1)>F-Si-β(2). SEM revealed an especially uniform crystalline morphology of F-Si-β(2) nucleated spontaneously and a mixed morphology of F-Si-β(1), F-W-β, F-Pt-β, and F-Pd-β induced by seeding H-β. The reduced F-Pt-β and F-Pd-β were catalytically active for the asymmetric hydrogenation of tiglic acid with an appreciable enantiomeric excess value of about 9-11% without adding chiral modifiers. © 2003 Elsevier Inc. All rights reserved.Publication Enhancement of thermal and hydrothermal stability of MCM-41 by TiO 2 deposition(2004) Xia, Q.H.; Kawi, S.; Hidajat, K.; Li, L.L.; CHEMICAL AND PROCESS ENGINEERING CENTRE; CHEMICAL & BIOMOLECULAR ENGINEERINGTiO2 was deposited onto the mesoporous surface of siliceous MCM-41 by chemical liquid deposition. The deposition of TiO2 on MCM-41 was found to largely enhance the thermal stability of mesoporous framework, and the calcination treatment at 1173 K remarkably improved the hydrothermal stability of mesostructure of TiO2/MCM-41. The fraction of anatase phase to rutile phase on TiO2/MCM-41 was controllable through varying heating rate, and the thermal treatment using a fast heating rate of 65 °C/min benefited the growth of rutile phase. The lower TiO 2 loading ≤19.7-wt% was beneficial to the formation of TiO 2/MCM-41 with a high BET surface area of >900 m2/g; however, the higher TiO2 loading up to 45.2 wt% led to an abrupt drop of BET surface area of the resulting material to 544 m2/g due to the partial blockage of mesopores.Publication Improvement of the hydrothermal stability of siliceous MCM-48 by fluorination(2001) Xia, Q.-H.; Hidajat, K.; Kawi, S.; CHEMICAL & ENVIRONMENTAL ENGINEERING; CHEMICAL AND PROCESS ENGINEERING CENTREThe siliceous MCM-48 molecular sieve, a member of the M41s family, is prepared for the first time in the presence of fluoride ions, and the hydrothermal stability of fluorinated MCM-48 framework is found to be remarkably improved as compared with those synthesized in other media.Publication Catalytic oxidation of pyridine on the supported copper catalysts in the presence of excess oxygen(2004-07-01) Zhou, J.; Xia, Q.-H.; Shen, S.-C.; Kawi, S.; Hidajat, K.; CHEMICAL AND PROCESS ENGINEERING CENTRE; CHEMICAL & BIOMOLECULAR ENGINEERINGThe catalytic oxidation of pyridine pollutant on a series of supported metals/zeolites catalysts in the presence of excess oxygen was studied. All the catalysts were further characterized by means of BET, XRD, H2-TPR, XPS, and FTIR techniques. The catalyst that could be reduced at lower temperatures had a better oxidation activity. The better NOx control ability could be attributed to the importance of the Lewis acidity of the samples, rather than to that of Brønsted acidity. On supported copper catalysts, Cu(H2O)6 2+ ions had higher activity for the NOx control but poorer activity for the pyridine oxidation. The cointeraction of some factors, such as the surface acidity of catalysts, the structure of supports, and the amount of Cu(H2O)6 2+ ions on the supports, played a deciding role in the NOx control ability of the catalysts. Comparatively, Cu/beta was the most active for the pyridine oxidation and NOx control, possibly being a potential catalyst for the catalytic removal of pyridine pollutant. © 2004 Elsevier Inc. All rights reserved.Publication Effect of surface modifications on the adsorption and hydrothermal stability of MCM-41 material(2004) Xia, Q.H.; Hidajat, K.; Kawi, S.; CHEMICAL AND PROCESS ENGINEERING CENTRE; CHEMICAL & BIOMOLECULAR ENGINEERINGFor mesoporous materials (M1, M2, M3, and M4) synthesized in different ways, the α values (hydrophobic index) were found to decrease in the following order: M1(4.57) > M2(2.57) > M3(1.36) > M4(0.92) > ZSM-5 (0.63). The surface -OH groups on mesoporous materials M1 and M2 were removed through different surface treatments, such as fluorination, silylation, alkoxylation, and TiO2 deposition. The variation of hydrophobicity could be placed in the descending order: silylated M1(17.43) > carbonized M1(14.20) > fluorinated M1(11.71) > fresh M1(4.57) > TiO 2/M1(3.55) ≫ ZSM-5(0.63). Surface fluorination largely increased the hydrophobic property of the material, but strikingly retarded the hydrothermal stability in boiling water. The hydrothermal stability of fresh M1 and carbonized SM1 were behaving similar. The enhanced hydrothermal stability of SM1 in boiling water has been ascribed to the hindrance of surface Si-O-Si(CH3)3 units on MCM-41 for the silicate hydrolysis.Publication Adsorption and catalytic combustion of aromatics on platinum-supported MCM-41 materials(2001-07-01) Xia, Q.-H.; Hidajat, K.; Kawi, S.; CHEMICAL & ENVIRONMENTAL ENGINEERING; CHEMICAL AND PROCESS ENGINEERING CENTREAmong a series of Pt-supported catalysts prepared by impregnation, the most hydrophobic Pt-supported on MCM-41 (synthesized in the presence of fluoride anions) with large surface area and pore size is found to be the most active catalyst for the combustion of toluene in air, and its activity is maintained even in the presence of water. However, Pt/ZSM-5 with micropores, the smallest pore volume and the least hydrophobicity has the worst catalytic activity. The high oxidation activity of the catalyst depends mainly on its high hydrophobicity (characterized by selective adsorption for H2O and toluene and by FTIR spectra of surface hydroxyl groups) and partly on its large pore size and high Pt loading. The order of the oxidation activity of a series of aromatics with different molecular weight and different boiling point on Pt-supported MCM-41 catalyst is found to be as follows: toluene > benzene > cumene > ethyl benzene > mesitylene. © 2001 Elsevier Science B.V. All rights reserved.Publication Effect of ZrO2 loading on the structure, acidity, and catalytic activity of the SO4 2-/ZrO2/MCM-41 acid catalyst(2002) Xia, Q.-H.; Hidajat, K.; Kawi, S.; CHEMICAL & ENVIRONMENTAL ENGINEERING; CHEMICAL AND PROCESS ENGINEERING CENTREThe regular hexagonal structure of siliceous MCM-41 with a uniform mesopore size is still maintained after 48.5 wt% ZrO2 loading. Under high loadings (≥26.7%) of ZrO2, a very small amount of ZrO2 clusters (present as tetragonal ZrO2 phase) might be formed inside or outside the MCM-41 structure. The hydrophobicity of the SO4 2-/ZrO2/MCM-41 catalysts is stepwise enhanced by increasing the ZrO2 loading. The covalent S=O band 1378-cm-1 for the SO4 2-/ZrO2 catalyst shifts to 1363 cm-1 for SO4 2-/ZrO2/MCM-41 materials. Under identical ZrO2 loading, SO4 2-/ZrO2/MCM-41 has stronger Brønsted and Lewis acidities than PMSZM/ZrO2 (physical mixture of SO4 2-/ZrO2 and MCM-41). The SO4 2-/ZrO2/MCM-41 catalysts show high activity with a selective conversion of >95 mol% ButOH to MTBE at low temperatures below 160°C, further showing the excellent on-stream stability of these catalysts for this reaction. The rather poor activity of PMSZM/ZrO2 catalysts has been ascribed to their weak acidity. © 2002 Elsevier Science.Publication Comparative studies on alkylation of phenol with tert-butyl alcohol in the presence of liquid or solid acid catalysts in ionic liquids(2004-04-02) Shen, H.-Y.; Judeh, Z.M.A.; Ching, C.B.; Xia, Q.-H.; CHEMICAL AND PROCESS ENGINEERING CENTRE; CHEMICAL & BIOMOLECULAR ENGINEERINGAlkylation of phenol with tert-butyl alcohol (TBA) was carried out in ionic liquids [bmim]PF6, [omim]BF4, and [hmim]BF 4. Comparative studies on the catalytic properties of ionic liquids, H3PO4 and some solid acidic catalysts, such as HZSM-5, H-β, HPW, and HPW/MCM-41, were carried out under identical reaction conditions. Solvent effect has been as well studied. The use of ionic liquids was found to enhance the catalytic properties of the catalysts used. © 2003 Elsevier B.V. All rights reserved.