In Kyu Song

Scanning Tunneling Microscopy (STM) and Tunneling Spectroscopy (TS) of Heteropolyacid (HPA) Self-Assembled Monolayers (SAMS): Connecting Nano Properties to Bulk Properties

Nanoscale investigation of Keggin-type heteropolyacid (HPA) self-assembled monolayers (SAMs) was performed by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) in order to relate surface properties of nanostructured HPA monolayers to bulk redox and acid properties of HPAs. Cation-exchanged, polyatomsubstituted, and heteroatom-substituted HPAs were examined to see the effect of different substitutions. HPA samples were deposited on HOPG surfaces in order to obtain images and tunneling spectra by STM before and after pyridine adsorption. All HPA samples formed well-ordered monolayer arrays, and exhibited negative difference resistance (NDR) behavior in their tunneling spectra. NDR peaks measured for fresh HPA samples appeared at less negative potentials for higher reduction potentials of the HPAs. These changes could also be correlated with the electronegativities of the substituted atoms. Introduction of pyridine into the HPA arrays increased the lattice constants of the two-dimensional HPA arrays by ca. 6 A. Exposure to pyridine also shifted NDR peak voltages of HPA samples to less negative values in the tunneling spectroscopy measurements. The NDR shifts of HPAs obtained before and after pyridine adsorption were correlated with the acid strengths of the HPAs. This work demonstrates that tunneling spectra measured by STM can fingerprint acid and redox properties of HPA monolayers on the nanometer scale.

Synthesis and characterization of mesoporous alumina as a catalyst support for hydrodechlorination of 1,2-dichloropropane: Effect of catalyst preparation method

A mesoporous alumina was synthesized by a posthydrolysis method. The prepared mesoporous alumina was found to have randomly ordered pores, and retained relatively high surface area with narrow pore size distribution centered at ca. 4 nm. Nickel precursors were then supported on the mesoporous alumina by an impregnation (Ni-IMP) and vapor deposition (Ni-VD) method. Several characterizations were carried out in order to investigate physical and chemical properties of mesoporous alumina and supported Ni catalysts. TPR, XPS, and UV-DRS measurements revealed that the Ni-IMP catalyst retained much more amounts of surface nickel aluminate-like species than the Ni-VD sample. TPD experiments also showed that nickel aluminate species affected the adsorption amounts of reactant (1,2-dichloropropane). In the hydrodechlorination of 1,2-dichloropropane (DCPA), DCPA conversion over the Ni-VD catalyst was about two times higher than that over the Ni-IMP catalyst at 300 °C. It is probably due to the fact that the Ni-VD catalyst, which had low contents of nickel aluminate species compared to the Ni-IMP catalyst, exhibited higher degree of reduction than the Ni-IMP catalyst at pretreatment conditions. The difference in DCPA conversion between two catalysts was closely related to the degree of reduction of nickel species and the amounts of adsorption of DCPA onto the catalyst as well.

Hydrogen production by steam reforming of liquefied natural gas (LNG) over mesoporous nickel–alumina aerogel catalyst

Abstract A mesoporous nickel–phosphorus–alumina aerogel catalyst (NPAA) was prepared by a single-step epoxide-driven sol–gel method and a subsequent supercritical CO2 drying method for use in the hydrogen production by steam reforming of liquefied natural gas (LNG). In order to investigate the effect of drying method of nickel–phosphorus–alumina catalysts on their physicochemical properties and catalytic activities, a mesoporous nickel–phosphorus–alumina xerogel catalyst (NPAX) was also prepared by a single-step epoxide-driven sol–gel method and a subsequent evaporative drying method for comparison purpose. It was found that supercritical CO2 drying method was effective for enhancing textural properties of NPAA catalyst. Although both NPAX and NPAA catalysts retained surface nickel aluminate phase, NPAA catalyst showed stronger metal-support interaction than NPAX catalyst. XRD patterns of reduced NPAX and NPAA catalysts revealed that NPAA catalyst retained smaller metallic nickel crystallite than NPAX catalysts. It was also observed that the reduced NPAA catalyst exhibited high nickel dispersion, large amount of strong hydrogen-binding sites, and large amount of methane adsorption compared to the reduced NPAX catalyst. In the steam reforming of LNG, NPAA catalyst with high affinity toward methane showed a better catalytic performance than NPAX catalyst.

Heteropolyacid (HPA)-polymer composite films as heterogeneous catalysts and catalytic membranes

Preparation, characterization, and catalytic application of membrane-like heteropolyacid (HPA)-polymer composite films as heterogeneous catalysts and catalytic membranes were reported in this work. HPA-polymer composite film catalysts were prepared by a membrane preparation technique by blending HPA with polymer using a common or a mixed solvent. HPA-polymer composite film catalysts were cut into small pieces, and they were applied as heterogeneous catalysts to the liquid-phase tert-butyl alcohol (TBA) synthesis, to the vapor-phase ethyl tert-butyl ether (ETBE) synthesis, and to the vapor-phase ethanol conversion. It was observed that HPA catalysts were finely dispersed throughout the polymer supports, and the blending patterns of the composite film catalysts were different depending on the identity of solvent and polymer material used. The HPA-polymer composite films showed enhanced catalytic activities in these reactions compared to the mother HPAs. Pore characteristics of the composite film catalysts could be controlled by a conventional membrane preparation technique, and they were related to the catalytic performance of the film catalysts. Shell- and tube-type membrane reactors comprising inert polymer membranes and HPAs were designed and applied to the vapor-phase methyl tert-butyl ether (MTBE) decomposition. Simulation and experimental results revealed that polyphenylene oxide (PPO)-based membrane reactor showed the best performance, among some polymer membranes examined. Selective removal of methanol through the polymer membrane inhibited the unfavorable reverse reaction (MTBE synthesis), and at the same time accelerated the MTBE decomposition. Shell- and tube-type catalytic membrane reactors (CMRs) utilizing HPA-polymer composite catalytic membranes were also designed for the vapor-phase MTBE decomposition. Experimental results showed that HPA-polymer composite catalytic membranes not only showed catalytic reactivity for the reaction, but also were perm-selective for methanol with respect to isobutene and MTBE. It has been demonstrated that HPA-polymer composite catalytic membranes have potential application in a membrane reactor for the MTBE decomposition.

Heterogeneous liquid-phase hydration of isobutene by heteropoly acid–polymer composite film catalyst

H3PMo12O40-polymer composite film catalysts were prepared by blending these two materials using a methanol–chloroform mixture by a membrane preparation technique. Polyphenylene oxide (PPO), polyethersulfone (PES) and polysulfone (PSF) were used as blending polymers. A H3PMo12O40-PPO composite catalyst coated on Al2O3 was also prepared. These catalysts were used as heterogeneous catalysts for the liquid-phase tert-butanol (TBA) synthesis from isobutene and water. It was found that all the composite film catalysts showed higher catalytic activities than homogeneous H3PMo12O40 in the TBA synthesis. Among the composite film catalysts, H3PMo12O40-PPO showed the best catalytic performance. Not only high absorption capability of H3PMo12O40-PPO for isobutene but also stability of H3PMo12O40-PPO in the reaction medium was responsible for such a catalytic performance. The H3PMo12O40-PPO/Al2O3 also showed a higher activity than a homogeneous solution of H3PMo12O40.

Heterogeneous liquid-phase lactonization of 1,4-butanediol using H3+xPMo12−xVxO40 (x=0–3) catalysts immobilized on polyaniline

Abstract In order for successful application of heteropolyacids (HPAs) as heterogeneous catalysts to liquid-phase lactonization of 1,4-butanediol, H 3+ x PMo 12− x V x O 40 ( x =0–3) HPAs were immobilized on polyaniline (PANI) by one-step and two-step methods. Aniline was polymerized in the presence of HPA in the one-step preparation method, while HPA was immobilized on the ready-made PANI support in the two-step method. It was found that HPAs were molecularly dispersed and strongly immobilized in/on the PANI support as charge-compensating components. Most HPAs in the two-step HPAPANI catalysts were immobilized only on the surface of PANI support. The surface areas of the two-step HPAPANI catalysts were much higher than those of the one-step catalysts, which is important from the practical point of applications. Thermal stability of PANI support was much enhanced by the binding with HPA, and thermal stability of the two-step HPAPANI catalysts was superior to the one-step catalysts. In the lactonization of 1,4-butanediol, catalytic activities were in the following order: two-step HPAPANI>one-step HPAPANI>unsupported HPA; H 6 PMo 9 V 3 O 40 PANI>H 5 PMo 10 V 2 O 40 PANI>H 4 PMo 11 V 1 O 40 PANI>H 3 PMo 12 O 40 PANI. High activity of V-containing two-step catalysts and easiness of catalyst recovery in the liquid-phase reaction make them good candidates for an energy-saving lactonization process of 1,4-butanediol.

Modification of 12-molybdophosphoric acid catalyst by blending with polysulfone and its catalytic activity for 2-propanol conversion reaction

H3PMo12O40 embedded in a polysulfone film was prepared by blending H3PMo12O40 with polysulfone using dimethylformamide as a common solvent and its catalytic activity for 2-propanol conversion reaction was examined. The prepared film catalyst showed a higher yield for acetone but a lower yield for propylene than H3PMo12O40 itself. The decrease of acidic function of the film catalyst was mainly due to dimethylformamide strongly adsorbed on the acid sites of H3PMo12O40, while the increase of oxidation function was due to uniformly and finely distributed H3PMo12O40 in the film. When the film catalyst was used as a membrane, it showed higher ratio of acetone to propylene than H3PMo12O40 itself.

Performance of shell and tube-type membrane reactors equipped with heteropolyacid-polymer composite catalytic membranes

Abstract Experimental studies on the performance of shell and tube-type membrane reactors equipped with heteropolyacid-polymer composite catalytic membranes were carried out for the vapor-phase decomposition of MTBE (methyl tert -butyl ether). Three types of catalytic membranes comprised of 12-tungstophosphoricacid (H 3 PW 12 O 40 , PW) and polyphenylene oxide (PPO) were designed in this work. The PW-PPO/Al 2 O 3 (type-1), PW-PPO/PPO/Al 2 O 3 (type-2) and PW/PPO/Al 2 O 3 (type-3) catalytic membranes not only showed catalytic reactivity for the reaction, but also were perm-selective for the reaction species. The selective removal of methanol through the catalytic membrane led to an equilibrium shift in a direction favorable to MTBE decomposition. Among three types of catalytic membrane reactor, the PW-PPO/PPO/Al 2 O 3 (type-2) catalytic membrane reactor showed the best performance. The enhanced performance of the PW-PPO/PPO/Al 2 O 3 (type-2) catalytic membrane reactor can be attributed to the intrinsic perm-selective capabilities of the PW-PPO catalytic membrane and the sub-layered PPO membrane.

Design of novel catalyst imbedding heteropoly acids in polymer films: Catalytic activity for ethanol conversion

Abstract Membrane-like H 3 PMo 12 O 40 or H 5 PMo 10 V 2 O 40 -imbedded polymer films were prepared by blending these materials using a chloroform-methanol mixture as solvent, and they were tested as catalysts for the ethanol conversion reaction in a continuous flow fixed-bed reactor. It was found that the chloroform-methanol mixture itself gave no influence on the catalytic activity of bulk H 3 PMo 12 O 40 and H 5 PMo 10 V 2 O 40 . Polysulfone, polyethersulfone and polyphenylene oxide were used as blending polymers in this study. H 3 PMo 12 O 40 -imbedded polymer films showed a higher catalytic activity than H 3 PMo 12 O 40 itself due to uniform and fine dispersion of H 3 PMo 12 O 40 through polymer films, but the extent of activity and selectivity was varied according to a kind of polymer materials. It was revealed that H 3 PMo 12 O 40 or H 5 PMo 10 V 2 O 40 -imbedded polyphenylene oxide film showed much higher oxidation catalytic activity than the corresponding bulk heteropoly acids. This may be due to the interaction (or bonding) of proton of heteropoly acid with polyphenylene oxide, judging from the fact that glass transition temperature of polyphenylene oxide was increased after blending with heteropoly acid. The catalytic activity of the heteropoly acid imbedded in the same polymer was greatly affected by the composite of heteropoly acid. The permeabilities of reactants through polymer films also affected the product selectivity.

Design of heteropoly compound-imbedded polymer film catalysts and their application

Heteropolyacid-imbedded polymer film catalysts were applied for the vapor-phase catalytic reaction. Thin film catalysts were prepared by membrane technology using homogeneous heteropolyacid-polymer solutions. It was found that heteropolyacid was finely and uniformly dispersed throughout the polymer matrix. Heteropoly-acid catalyst could be designed by this method to enhance acid or oxidation catalysis. Future aspects of the film catalyst were also described in this work.