Zeolites in Microsystems for Chemical Synthesis and Energy Generation

Abstract

Zeolites were incorporated as membrane and catalyst in chemical microsystems for portable energy generation and fine chemical synthesis. Microfabricated HZSM-5 micromembrane was used as a proton-exchange membrane in a miniature direct methanol fuel cell (l-DMFC). The good proton conductivity of HZSM-5 micromembrane was attributed to a Grotthus-like diffusion of protons along the water molecules bridging neighboring aluminum sites in the hydrated HZSM-5. The 6-lm thick HZSM-5 micromembrane exhibited comparable proton flux as Nafion Ò 117 and delivered a P max of 2.9 mW cm -2 (E = 0.33 V) at room temperature. This is smaller compared to 16.5 mW cm -2 (E = 0.23 V) for a Nafion Ò -based l-DMFC and was believed to be caused by adsorbed methanol molecules interrupting the proton transport along the water bridge. A Cs-exchanged NaX on NaA bilayer catalyst-membrane incorporated in microreactor channels was used for the Knoevenagel condensation reactions between benzaldehyde and (1) ethyl cyanoacetate, (2) ethyl acetoacetate (EAA) and (3) diethyl malonate. Microreactor and membrane microreactor gave higher conversion compared to fixed-bed and batch reactors, but the reaction of benzaldehyde and EAA in the microreactor had poorer selectivity due to the slow diffusion of the product molecules in the microchannel that allowed their further reactions to form undesired byproducts.

Zeolite Proton Conducting Membrane for Micro Fuel Cell Applications

“…Miniaturization is shown to benefit membrane processes and their use has been demonstrated for various reactions including selective oxidations [68][69][70], direct hydroxylation [71,72] and Knoevenagel condensation reactions [73][74][75][76]. Membrane research in fuel cell mainly concerns hydrogen separation [77][78][79][80][81][82] and proton-exchange membranes [40,41]. This work demonstrates that zeolites are promising materials for proton-exchange membranes, but further improvements in membrane transport by selecting large pore zeolite of higher aluminum contents (e.g., faujasite, zeolite Beta) and by decreasing the diffusion pathways through the zeolite (e.g., thinner membrane and b-oriented HZSM-5 are needed to yield a higher fuel cell performance.…”

“…1b). The ZSM-5 film was grown by hydrothermal regrowth at 423 K for 48 h from a synthesis solution containing 40 SiO 2 :2 Al 2 O 3 :0.5 TPA 2 O:5 Na 2 O:20,000 H 2 O [37][38][39][40][41]. The self-supporting micromembranes were obtained after etching away the remaining 50 lm Si layer and removing the TPA ?…”

“…We had reported on the synthesis and preparation of supported and self-supporting zeolite membranes in our recent works [27][28][29][30][31][32][33][34][35][36][37][38][39] and had successfully assembled a microfabricated HZSM-5 zeolite micromembrane unit into operational micro fuel cells (i.e., H 2 l-PEMFC [40] and l-DMFC [41]). This work investigates the properties and performance of supported and self-supporting zeolite membranes for H 2 PEMFC operation.…”

Zeolites and Molecular Sieves in Fuel Cell Applications

“…Although most zeolites have lower proton conductivity than the commercial PFSA, they have considerably better mechanical strength and could be fabricated into ultrathin membranes. Recent works by the group of Yeung investigated HZSM-5 as proton conducting membrane for microfabricated miniature hydrogen PEMFCs [75] and DMFCs [76]. They chose HZSM-5 as an experiments model, and calculations indicated that the proton mobility in ZSM-5 was among the highest in zeolites [33,38,77].…”

Post-Synthesis Modification of SSZ-35 Zeolite to Enhance the Selectivity in p-Xylene Alkylation with Isopropyl Alcohol

“…The mentioned areas substantially contributed to a much deeper insight into the zeolite chemistry. Recently, also novel applications of zeolites like gas storage microsystems for chemical synthesis and energy generations were discussed [18,19].…”