The conceptual design of a continuous pervaporation membrane reactor for the production of 1,1 diethoxy butane

I. Agirre, M.B. Güemez, A. Motelica, H.M. van Veen, J.F. Vente, P.L. Arias

AIChE Journal, 2011, doi.10.1002/aic.12692, http://onlinelibrary.wiley.com/doi/10.1002/aic.12692/abstract

Acetals are considered as an important bio-based diesel additives. Generally, the catalytic production of these compounds from an alcohol and an aldehyde suffers from a low conversion because of thermodynamic limitations. These limitations can be overcome through the in situ removal of the by-product water using, for example, a water selective membrane. A critical evaluation on the hybrid silica membrane performance, catalyst activity, optimal configuration, and feed composition leads to the conclusion that a combined reaction and separation is unlikely to be advantageous. The HybSi membrane selectivity appeared to be sufficiently high as the ethanol losses were limited. The water permeance of the selected membrane was assessed to be too low in relation with the catalyst activity. A permeance increase with a factor of three was required to come to a reasonable reactor length. An alternative solution can be to uncouple the reaction and separation.

Evaluation of hybrid silica sols for stable microporous membranes using high-throughput screening
R. Kreiter, M.D.A. Rietkerk, H.L. Castricum, H.M. van Veen, J.E. ten Elshof, J.F. Vente
J. Sol-Gel Sci. Techn., Vol. 57, P. 245-252, 2011, doi:10.1007/s10971-010-2208-7

Microporous membranes are a promising option for energy-efficient molecular separations. Long term hydrothermal stability of the membrane material is of prime importance for several industrial processes. Here, a short overview of silica-based membrane materials and their hydrothermal stability is presented. Following this, the development of a series of organic–inorganic hybrid silica sols is described, based on a,x-bis(triethoxysilyl)-precursors with bridging methane, ethane, propane, and benzene groups. High-throughput screening was used to scan a range of sol parameters, followed by membrane preparation from the most promising sols. These organic–inorganic hybrid silica (HybSi® ) membranes were used in dewatering of lower alcohols by pervaporation. Separation factors up to 200 were found for ethanol/water mixtures, and up to 23 for methanol/water mixtures. Modest permselectivity values for hydrogen over nitrogen were found, ranging up to 20.7 for the shortest bridging group. It was concluded that the length of the organic bridge has a clear effect on the pore size distribution and the selectivity of the membrane.

Stable Hybrid Silica Nanosieve Membranes for the Dehydration of Lower Alcohols

R. Kreiter, M. D. A. Rietkerk, H. L. Castricum, H. M. van Veen, J.E. ten Elshof, J. F. Vente

ChemSusChem, 2, 2, Pages 158-160, 2009 WEB http://www3.interscience.wiley.com/journal/121639413/abstract

Organic-inorganic hybrid silica nanosieve membranes with narrow pore size distributions were developed for the separation of binary (bio)alcohol/water mixtures, for example, to remove water from wet biofuels during production. These membranes dehydrate lower alcohols and show a stable performance in the presence of significant amounts of acetic acid.

High-Performance Hybrid Pervaporation Membranes with Superior Hydrothermal and Acid-Stability.
H.L. Castricum, R. Kreiter, H.M. van Veen, D.H.A. Blank, J.F. Vente, J.E. ten Elshof
J. Membr. Sci. 324, 1-2, 111-118, 2008 WEB  http://dx.doi.org/10.1016/j.memsci.2008.07.014

A new organic–inorganic hybrid membrane has been prepared with exceptional performance in dewatering applications. The only precursor used in the sol–gel synthesis of the selective layer was organically linked 1,2-bis(triethoxysilyl)ethane (BTESE). The microporous structure of this layer enables selective molecular sieving of small molecules from larger ones. In the dehydration of n-butanol with 5% of water, the membrane shows a high separation factor of over 4000 and ultra-fast water transport at a rate of more than 20 kg m?2 h?1 at 150 °C. This can be related to the high adsorption capacity of the material and the sub-micron thickness of the selective layer. The selectivity has now remained constant over almost one and a half years under continuous process testing conditions. Apart from the hydrothermal stability, the membrane exhibits a high tolerance for acid contamination. A slow performance decline in flux and separation factor is only observed at a pH lower than 2. The high stability and effective separation indicate a broad industrial application potential of the hybrid membrane material.

Structure of hybrid organic-inorganic sols for the preparation of hydrothermally stabile membranes
H.L. Castricum, A. Sah, J. A. J. Geenevasen, R. Kreiter, D.H.A. Blank, J.F. Vente, J.E. ten Elshof
J. Sol-Gel Sci. Technol. 48, 1-2, 11-17, 2008. Web: doi:10.1007/s10971-008-1742-z

A procedure for the preparation of hybrid sols for the synthesis of organic–inorganic microporous materials and thin film membranes is reported. We describe silane reactivity and sol structure for acid-catalysed colloidal sols from mixtures of either tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES), or bis(triethoxysilyl)ethane (BTESE) and MTES. Early-stage hydrolysis and condensation rates of the individual silane precursors were followed with 29Si liquid NMR and structural characteristics of more developed sols were studied with Dynamic Light Scattering. Condensation was found to proceed at more or less similar rates for the different precursors. Homogeneously mixed hybrid colloids can therefore be formed from precursor mixtures. The conditions of preparation under which clear sols with low viscosity could be formed from BTESE/MTES were determined. These sols were synthesized at moderate water/silane and acid/silane ratios and could be applied for the coating of defect-free microporous membranes for molecular separations under hydrothermal conditions.

Hydrothermally Stable Molecular Separation Membranes from Organically Linked Silica.
H.L. Castricum, A. Sah, R. Kreiter, D.H.A. Blank, J.F. Vente, J.E. ten Elshof
J. Mater. Chem, 18, 2150-2158, 2008.  Web: doi:10.1039/b801972j

A highly hydrothermally stable microporous network material has been developed that can be applied in energy-efficient molecular sieving. The material was synthesized by employing organically bridged monomers in acid-catalysed sol–gel hydrolysis and condensation, and is composed of covalently bonded organic and inorganic moieties. Due to its hybrid nature, it withstands higher temperatures than organic polymers and exhibits high solvolytical and acid stability. A thin film membrane that was prepared with the hybrid material was found to be stable in the dehydration of n-butanol at 150 °C for almost two years. This membrane is the first that combines a high resistance against water at elevated temperatures with a high separation factor and permeance. It therefore has high potential for energy-efficient molecular separation under industrial conditions, including the dehydration of organic solvents. The organically bridged monomers induce increased toughness in the thin film layer. This suppresses hydrolysis of Si–O–Si network bonds and results in a high resistance towards stress-induced cracking. The large non-hydrolysable units thus remain well incorporated in the surrounding matrix such that the material combines high (pore) structural and mechanical stability. The sols mean particle size and degree of branching proved viable parameters to tune the thickness of the membrane layer and thus optimize the separation performance. We anticipate that other hybrid organo-silicas can be prepared in a similar fashion, to yield a whole new class of materials with superior molecular sieving properties and high hydrothermal stability.

Hybrid Ceramic Nanosieves: Stabilizing Nanopores with Organic Links
H.L. Castricum, A. Sah, R. Kreiter, D.H.A. Blank, J.F. Vente, J.E. ten Elshof
Chem. Commun. 1103-1105, 2008.  Web: doi:10.1039/b718082a

Unprecedented hydrothermal stability in functional membranes has been obtained with hybrid organic–inorganic nanoporous materials, enabling long-term application in energy-efficient molecular separation, including dehydration up to at least 150°C.