Hidetoshi Kita

Gas permeation properties of hyperbranched polyimide membranes

Abstract A series of wholly aromatic hyperbranched polyimides were successfully prepared by condensation polymerization of a triamine monomer, tris(4-aminophenyl)amine (TAPA), and a series of commercially available dianhydride monomers such as 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), 3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride (DSDA) and pyromellitic anhydride (PMDA). Two types of hyperbranched polyimides (amine-terminated and anhydride terminated) were obtained by controlling monomer addition manner and monomer molar ratio. Films of these hyperbranched polyimides were fabricated by crosslinking treatment during film cast. Such a film formation technique is based on the chemical reaction between the terminal functional groups of a hyperbranched polyimide and a difunctional crosslinking agent, which leads to the connection of globular hyperbranched macromolecules via chemical bonds. The amine-terminated hyperbranched polyimides generally exhibited much higher gas permeability coefficients than the corresponding anhydride-terminated ones. Crosslinking had great effects on gas permeation properties: lower crosslinking density resulted in higher gas permeability coefficients, but the ideal selectivity hardly changed. For the same hyperbranched polyimide, the “rigid” crosslinking agent, terephthaldehyde (TPA), gave much higher gas permeability coefficients and very similar (CO 2 /N 2 ) or only a little lower (CO 2 /CH 4 and O 2 /N 2 ) ideal selectivity than the “flexible” one, ethylene glycol diglycidyl ether (EGDE). TPA-crosslinked hyperbranched polyimide membranes displayed better separation performance than that of the linear analogues and many other linear polymeric membranes. The amine-terminated 6FDA-TAPA hyperbranched polyimide membrane crosslinked with TPA, for example, displayed fairly high CO 2 /N 2 separation performance, i.e. P CO 2 =65 barrer and P CO 2 / P N 2 =30 at 1xa0atm and 35°C.

Application of zeolite membranes to esterification reactions

Pervaporation-aided esterification of acetic acid with ethanol was investigated at 343 K using zeolite T membranes. Almost complete conversion was reached within 8 h when initial molar ratios of alcohol to acetic acid were 1.5 and 2. The reaction time courses were well described by a simple model based on the assumptions that the reaction obeyed second-order kinetics and the permeation flux of each component was proportional to its concentration. The influence of operating parameters on variation in conversion with reaction time was investigated by means of the simulation using the model.

Preparation and gas separation performance of zeolite T membrane

Zeolite T membranes were prepared by hydrothermal synthesis on porous mullite tubes seeded with zeolite T crystals, using milk-like aluminosilicate gel with a molar composition of SiO2∶Al2O3∶Na2O∶K2O∶H2O = 1∶0.05∶0.26∶0.09∶14. A zeolite T crystal layer of about 20 µm in thickness was formed on the outer surface of the support after the synthesis at 373 K for 30 h. Single-gas and mixed-gas permeation experiments through zeolite T membranes were carried out by a vacuum method at 303∼473 K using He, H2, CO2, O2, N2, CH4, C2H6 and C3H8 single-component gases and CO2/N2, CO2/CH4 and other CO2/hydrocarbon mixtures, respectively. In single-gas permeation experiments, with increasing kinetic diameter from 0.33 nm for CO2 to 0.43 nm for C3H8, the gas permeance decreased by four orders in magnitude, indicating a kind of molecular sieving behavior for the zeolite T membranes prepared in this study, that is, they had little defects and their permeation behavior was controlled by zeolitic pores of erionite. Permeance of CO2 was much higher than those of N2 and CH4 and the ideal selectivities for CO2/N2 and CO2/CH4 were 31 and 266 at 343 K, respectively. In mixed-gas permeation experiments, zeolite T membranes showed the high selectivities for CO2/N2 and CO2/CH4 pairs of 107 and 400, respectively, at 308 K. The selectivity α decreased with an increase in temperature, but was still in a high level of 20 and 52 for CO2/N2 and CO2/CH4, respectively, even at 473 K. This is due to the synergetic effects of competitive adsorption of CO2 and molecular sieving of zeolitic pores. Because of the increasing effect of single file diffusion, the selectivities for CO2/C2H6 n (α n = 61) and CO2/C3H8 n (α n = 17) were rather low.

Preparation of Faujasite membranes and their permeation properties

NaX and NaY zeolite membranes were grown hydrothermally on the surface of a porous cylindrical support. The molar compositions of the starting gel of NaX and NaY zeolite membranes were SiO 2 /Al 2 O 3 =3.6-5.3 (X), 25 (Y), Na 2 O/SiO 2 =1.2-1.4 (X), 0.88 (Y), H 2 O/Na 2 O=30-50. X-ray diffraction (XRD) patterns of membranes consist of peaks corresponding to the support and NaX or NaY zeolite. The Si/Al ratio of these NaX and NaY zeolites determined by atomic absorption spectrophotometry was 1.3 and 2.1, respectively. The outer-surface of the porous support was completely covered with randomly oriented, intergrown NaX or NaY zeolite crystals and the thickness of the membrane was about 20-30 μm, judging from the scanning electron microscopy (SEM) observation. Pervaporation and vapor permeation of organic mixtures through NaY and NaX zeolite membranes was investigated. The zeolite membranes showed high alcohol selectivity for several feed mixtures with methanol or ethanol. Furthermore, high benzene selectivity was observed for benzene/cyclohexane and benzene/n-hexane separation. High permselectivity of the zeolite membrane can be attributed to the selective sorption into the membrane.

Application of zeolite T membrane to vapor-permeation-aided esterification of lactic acid with ethanol

Zeolite T membranes were applied to vapor-permeation-aided esterification of lactic acid with ethanol. The hybrid process provided almost complete conversion within a short reaction time by removing water from the reaction mixture. Zeolite T membrane worked steadily for a long time. The reaction time-courses were described by a model based on the assumptions that the esterification obeyed second-order kinetics and the permeation flux of each component was proportional to its concentration in the reaction mixture. The final reaction liquid mixtures consisted mostly of ethyl lactate and ethanol with little ester of polylactic acids, although concentrated lactic acid solution was used as a source.

Preparation and gas permeation properties of carbon molecular sieve membranes based on sulfonated phenolic resin

Abstract Effects of pyrolysis and preparation conditions on the gas permeation properties of pyrolytic membranes derived from sulfonated phenolic resin were investigated. Pyrolysis temperature, dip-coating conditions and coating/pyrolysis (C/P) cycle had significant influence on the gas permeation properties of pyrolytic membranes. Membrane obtained under optimum preparation conditions exhibited O 2 permeance of 30xa0GPU and ideal O 2 /N 2 separation factor of 12 at 35xa0°C, which are comparable to the O 2 /N 2 separation performances of carbon molecular sieve (CMS) membranes on the upper-bound line in the plots of permeance versus selectivity. N 2 adsorption and desorption at 77xa0K suggested that the pore structures of CMS membranes pyrolyzed around 500xa0°C seemed to be made up of interconnected pores with different size and shape.

Olefin/paraffin separation performance of asymmetric hollow fiber membrane of 6FDA/BPDA¿DDBT copolyimide

Abstract Gas permeation properties of asymmetric hollow fiber membrane of copolyimide prepared from equimolar portion of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) with 3,7-diamino-2,8(6)-dimethyldibenzothiophene sulfone (DDBT) were investigated for single-component light gases, olefins and paraffins and for mixed components of C3H6/C3H8 and C4H6/C4H10. The gas permeability of the copolyimide film was close to that of 6FDA–DDBT polyimide and much larger than that of BPDA–DDBT. Gas permeance of the asymmetric copolyimide hollow fiber membrane decreased significantly in the first several months and leveled off after about 10 months of aging. The skin layer thickness calculated from the gas permeability and permeance was in the range of 0.6 (for H2) to 1.7xa0μm (for C3H6) and about 10 times larger than the thickness estimated from the SEM observation. These results indicated that the significant densification of the skin layer was caused by physical aging. The silicone rubber coating hardly changed the selectivity for light gas pairs such as H2/CH4 and O2/N2, but enhanced that for C3H6/C3H8 and C4H6/C4H10 significantly especially at low temperatures. The evaluation of membrane quality based on the resistance model suggested that the extremely small surface porosity of defect pores significantly reduced the selectivity for the larger gas pairs. The asymmetric copolyimide hollow fiber membrane displayed better performance for C3H6/C3H8 and C4H6/C4H10: for example, permeances of C3H6 and C4H6 of 3.6 and 7.4xa0GPU, respectively, and separation factors of 15 and 69 for C3H6/C3H8 and C4H6/C4H10 (50/50xa0mol% in feed) at 373xa0K and 1xa0atm.

Gas permeation properties of thianthrene-5,5,10,10-tetraoxide-containing polyimides

Abstract A series of thianthrene-5,5,10,10-tetraoxide-containing polyimides with great chain stiffness were prepared and their gas permeation properties were investigated. TADATO/DSDA(1/1)–DDBT copolyimide, which was prepared from thianthrene-2,3,7,8-tetracarboxylic dianhydride-5,5,10,10-tetraoxide (TADATO), 3,3′,4,4′-diphenylsulfonyltetracarboxylic dianhydride (DSDA) and 3,7-diamino-2,8(6)-dimethyldibenzothiophene sulfone (DDBT), displayed much higher CO 2 permeability coefficient ( P CO 2 ) than both DSDA–DDBT and TADATO–DDBT homopolyimides, but their ideal selectivity ( P CO 2 / P N 2 ) were similar. The higher CO 2 permeability coefficient of TADATO/DSDA(1/1)–DDBT was attributed to the higher diffusion coefficient as well as a little higher solubility coefficient. The separation performance of TADATO/DSDA(1/1)–DDBT toward CO 2 /N 2 fell above the “upper bound” line, indicating the highest separation performance among the glassy polymer membranes developed so far. Furthermore, this polyimide showed a significantly higher selectivity of CO 2 over N 2 for mixed gas permeation than for single gas permeation (single gas: P CO 2 / P N 2 =35, mixed gases: P CO 2 / P N 2 =49, at 35°C and 2xa0atm). The “dual-mode” model was used to describe gas sorption and transport behaviors in the membrane.

Preparation and gas separation properties of zeolite T membrane

Zeolite T membranes were synthesized on tubular porous mullite tubes by hydrothermal synthesis. The membranes selectively permeated carbon dioxide from CO2/CH4 and CO2/N2 mixtures with high separation performances, which were due to combined effects of molecular sieving and competitive adsorption.

Positron annihilation properties of copolyimides and copolyamides with microphase-separated structures

The positron annihilation lifetime (PAL) of a series of copolyimides and copolyamides with microphase-separated structures was measured to investigate the effects of different hard-segment polymers on the PAL properties of soft-segment domains of poly(dimethyl-siloxane) (PDMS) and poly(ethylene oxide) (PEO). The lifetime (τ3) and intensity (I3) of the long-lived component are given as a function of the PDMS or PEO content for a series of copolymers, of which the density roughly obeys the additive rule except for the PDMS-segmented copolyamides. The PDMS-segmented copolyimides and copolyamides show much smaller I3 values than those estimated from the additive rule. The lifetime distribution of the long-lived component for the PDMS-segmented copolyamides is composed of two components. The longer-lifetime component is attributed to pure PDMS domains, and the shorter-lifetime component is attributed to the polyamide domains, intermediate phases, and PDMS domains containing small amounts of short amide blocks. Despite the high PDMS content, the latter component is rather large. Thus, the positronium formation in the PDMS domains of the copolyimides and copolyamides is effectively reduced. This can be explained by the combination of the difference in the electron affinity of the PDMS and polyimide or polyamide segments and the incomplete phase separation. The PEO-segmented copolyimides show much smaller I3 values than those predicted from the additive rule. This is likely attributable to the effects of the intermediate phases.