Hybrid organosilica membranes and processes: Status and outlook
I. Agirre, P.L. Arias, H.L. Castricum, M. Creatore, J.E. ten Elshof, G.G. Paradis, P.H.T. Ngamou, H.M. van Veen, J.F. Vente
Separation and purification technology 121 (2014) 2-12,

In the past, the research in molecular separation membranes prepared through sol–gel technologies has been dominated by ceramic membranes. Especially, silica membranes have been studied in great depth. Steps towards hybrid organosilica membranes were taken by using pendant organic groups. However, only with the appearance of organically bridged silica, stable and reliable membranes that are suitable for large scale industrial utilization have become available. In this paper, we provide an overview of recent developments of hybrid silica membranes that contain organic bridges. The freedom of choice in precursor allows for a flexible approach towards tailoring of the membrane properties. New support materials can be used by applying alternative deposition methods, such as expanding thermal plasma chemical vapor deposition. The robustness of the membrane concept allows for the design of novel separation process concepts in which the demonstrated stability is required.

Acetal production process assisted by pervaporation membranes

From hydrophilic to hydrophobic HybSi® membranes: a change of affinity and applicability
G.G. Paradis, D.P. Shanahan, R. Kreiter, H.M. van Veen, H.L. Castricum, A Nijmeijer, J.F. Vente
J. Membrane Science, 428 (2013) 157-162,

The article describes the effect of the presence of terminating alkyl groups on the performance of organic–inorganic hybrid silica membranes. By incorporation of different R-triethoxysilanes (R=C1 to C10 alkyl) into 1,2-bis(triethoxysilyl)ethane(BTESE) based materials the affinity could be tailored from hydrophilic to hydrophobic. These separation properties of the membrane based on these materials were found to be strongly dependent on the length of the R-group. Gas permeance measurements indicated reduced molecular sieving properties and an enhanced affinity for CO2 for longer R-groups. Longer R-groups also resulted in lower permeate water purity, falling from >99 wt% for BTESE to 40 wt% for C10 in the dehydration of n-butanol/water (95/5wt%) by pervaporation. Concomitantly, the n-butanol permeate purity for the pervaporation of n-butanol/water mixtures (6.8 wt% of n-butanol) increased to a value of >40 wt% for R=C10. This membrane exhibited constant separation factors over a large range of temperatures (30 to 90oC) and n-butanol feed concentrations (0.5 to 6.8 wt%). By increasing the temperature from 30 to 90oC, the n-butanol flux reached a value as high as 3 kg/m2h for a feed mixture containing 4.5 wt% of n-butanol.

Permeate concentration as function of the number of C atoms in the R-group in pervaporation for butanol/water feed mixtures of 95/5 and 5/95wt%. The feed concentrations are normalized to 5wt% for direct comparison.

On the enhancement of pervaporation properties of plasma-deposited hybrid silica membranes
P.H.T. Ngamou, J.P. Overbeek, H.M. van Veen, J.F. Vente, P.F. Cuperus and M. Creatore
RSC Advances 2013, 3, 14241-14244,

Applying an rf bias to a porous polymeric substrate during the deposition enables densification of hybrid silica layers which takes place by formation of siloxane functionalities at the expense of the organic moieties. The densification helps improving the separation performance towards the dehydration of a water-butanol mixture as noticed by the increase in water selectivity and flux.

Plasma-deposited hybrid silica membranes with a controlled retention of organic bridges
P.H.T. Ngamou, J.P. Overbeek, R. Kreiter, H.M. van Veen, J.F. Vente, I.M. Wienk, P.F. Cuperus and M. Creatore
J. of Materials Chemstry A 2013, 1, 5567-5576,

Hybrid organically bridged silica membranes are suitable for energy-efficient molecular separations under harsh industrial conditions. Such membranes can be useful in organic solvent nanofiltration if they can be deposited on flexible, porous and large area supports. Here, we report the proof of concept for applying an expanding thermal plasma to the synthesis of perm-selective hybrid silica films from an organically bridged monomer, 1,2-bis(triethoxysilyl)ethane. This membrane is the first in its class to be produced by plasma enhanced chemical vapor deposition. By tuning the plasma and process parameters, the organic bridging groups could be retained in the separating layer. This way, a defect free film could be made with pervaporation performances of an n-butanol–water mixture comparable with those of conventional ceramic supported membranes made by sol–gel technology (i.e. a water flux of 1.8 kg m2 h1, a water concentration in the permeate higher than 98% and a separation factor of >1100). The obtained results show the suitability of expanding thermal plasma as a technology for the deposition of hybrid silica membranes for molecular separations.

Amino-functionalized microporous hybrid silica membranes
G.G. Paradis, R. Kreiter, M.M.A. van Tuel, A. Nijmeijer, J.F. Vente
J.Mat.Chem., Vol. 22, 7258-7264, 2012,

The present study describes the effect of the incorporation of amino-functionalized terminating groups on the behavior and performance of an organic–inorganic hybrid silica membrane. A primary amine, a mixed primary and secondary amine, and an imidazole functionality were selected. The molar ratio of the amino-functionalized precursors in the matrix forming 1,2-bis(triethoxysilyl)ethane (BTESE) precursor was varied in the range of 25–100 mol%. Strong water adsorption, which remains at temperatures up to 523 K, was found for all membranes. The observed low gas permeances and contrasting high water fluxes in pervaporation were explained in relation to the strong water adsorption. XPS measurements indicate a relation between the concentration of amino functional groups in the hybrid layers and the starting amine concentration of the sols. XPS measurements also revealed the existence of a maximum loading of the amino-functionalized precursor. Depending on the precursor, a maximum N/Si element ratio between 0.07 and 0.45 was found. At amine concentrations higher than a precursor dependent threshold value, membrane selectivity is constant over the range of amine concentrations. For alcohol/water (95/5 wt.%) feed mixtures, the observed water concentrations in the permeate were over 90 wt.% for EtOH and 95 wt.% for n-BuOH dehydration.

Overview of the precursors

A techno-economic comparison of various process options for the production of 1,1-diethoxy butane
I. Agirre, M.B. Guemez, A. Motelica, H.M. van Veen, J.F. Vente, P.L. Arias
J.Chem. Technol. Biotechnol. (2012), dio.10.1002/jctb.3704,

Acetals can be considered important bio-based diesel additives. The production of most of these compounds, from an alcohol and an aldehyde, suffers from low conversion due to thermodynamic limitations. These limitations can be overcome through the removal of the by-product water. Previous studies showed that the in situ dehydration options of reactive distillation and pervaporation membrane reactor integration offer little advantage or at least not at reasonable unit dimensions. The aim of the present work is the development of a membrane based process and comparison with other alternatives (based on experimental data).
Three different membrane processes were developed. The one in which the reaction mixture is recycled over a first dehydration membrane module and subsequently through a simple distillation column, was found to give the highest overall conversion (100%) at low recycle rates and reasonable membrane area. This process was techno-economically compared with other possible alternatives: (1) a process based on a conventional tubular reactor and several distillation columns; and (2) a process based on reactive distillation.
Efficient water removal by membranes avoids possible azeotropes in downstream distillation units making them much simpler, reducing considerably the unit sizes and the energy demand (40% lower).

ProcessConventionalReactive distillationPervaporation membrane
Steam at 3.5 bar, kgsteam/kgacetal1.803.761.12
Cooling water, kgH2O/kgacetal33.6878.2019.51
Electricity, kW·h/kgacetal000.09
Total capital investment, M€2.012.853.76
Total utility costs, M€/y3.788.502.53
Total utility costs, €/L0.070.140.04
Total raw material cost, €/L0.720.720.72
Total production costs, €/L0.790.860.76

Pushing membrane stability boundaries with HybSi® pervaporation membranes
H.M. van Veen, M.D.A. Rietkerk, D.P. Shanahan, M.M.A. van Tuel, R. Kreiter, H.L. Castricum, J.E. ten Elshof, J.F. Vente
J.Membrane Sci., Vol. 380, 124-131, 2011,

To overcome the limitations of currently available commercial polymer and zeolite membranes for pervaporation applications, a hybrid silica membrane (HybSi®) has been developed. In this paper the unprecedented stability of HybSi® membrane technology for the dehydration of organic solvents is reported. It is shown that the HybSi® membranes are suitable for demanding separations using pervaporation at temperatures up to at least 190oC, in aggressive aprotic solvents including N-methyl-2-pyrrolidone (NMP), and down to a pH value of 2. The high hydrothermal and chemical stability was proven in continuous measurements that lasted for periods of months to several years. The longest test, on the dehydration of n-butanol at 150oC, lasted for 1000 days. The high stability parallels high fluxes and selectivities that meet current industrial demands and expectations. After a period of stabilization, fluxes and selectivities become constant. The presented results show that HybSi® membranes are widely applicable in the dehydration of organic solvents by pervaporation.

Dehydration of 5 wt.% water in ethanol and 1.5 wt.% HAc at 70ºC by a BTESM membrane

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,

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.

Molar fraction profile on the feed side vs. reactor length. Conditions: feed temperature: 70ºC, 500 g/L of A70 catalyst, 500 Pa in the permeate and stoichiometric feed ratio, adiabatic reactor

Tailoring the Separation Behavior of Hybrid Organosilica Membranes by Adjusting the Structure of the Organic Bridging Group
H.L. Castricum, G.G. Paradis, M.C. Mittelmeijer-Hazeleger, R. Kreiter, J.F. Vente, and J.E. ten Elshof
Adv. Funct. Mater., Vol. 21, 2319–2329, 2011,

Hybrid organically linked silica is a highly promising class of materials for the application in energy-efficient molecular separation membranes. Its high stability allows operation under aggressive working conditions. Herein is reported the tailoring of the separation performance of these hybrid silica membranes by adjusting the size, flexibility, shape, and electronic structure of the organic bridging group. A single generic procedure is applied to synthesize nanoporous membranes from bridged silsesquioxane precursors with different reactivities. Membranes with short alkylene (CH2 and C2H4) bridging groups show high H2/N2 permeance ratios, related to differences in molecular size. The highest CO2/H2 permeance ratios, related to the affinity of adsorption in the material, are obtained for longer (C8H16) alkylene and aryl bridges. Materials with long flexible alkylene bridges have a hydrophobic surface and show strongly temperature-dependent molecular transport as well as a high n-butanol flux in a pervaporation process, which is indicative of organic polymerlike properties. The versatility of the bridging group offers an extensive toolbox to tune the nanostructure and the affinity of hybrid silica membranes and by doing so to optimize the performance towards specific separation challenges. This provides excellent prospects for industrial applications such as carbon capture and biofuel production.

Frontispiece of the article

Acetalization reaction of ethanol with butyraldehyde coupled with pervaporation. Semi-batch pervaporation studies and resistance of HybSi® membranes to catalyst impacts
I. Agirre, M.B. Güemez, H.M. van Veen, A. Motelica, J.F. Vente, P.L. Arias
J. Membrane Sci., Vol. 371, p 179-188, 2011,

HybSi® pervaporation membranes can selectively remove water from a reaction mixture containing ethanol, butyraldehyde, 1,1-diethoxy butane and water including an Amberlyst solid catalyst. The conversion of this batch acetalization reaction can be increased from about 40% to 70% because of the shift of the reaction equilibrium. The mechanical resistance of the membrane against the solid catalyst was good and the membrane performance remained constant over the 4 months test period even in the quite aggressive butyraldehyde. It was thus proven that the HybSi® membranes can be used for dehydration in membrane reactors. Besides the formation of these acetals – that can be used as bio-based diesel additives – applications could be esterification reactions. For more information see the article in J. Membrane Science by the Engineering School of Bilbao in cooperation with ECN.

Effect of the temperature and time on conversion. Conditions: ratio EtOH/Butyraldehyde 2:1 in moles, catalyst loading 0.5 wt%

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.

Figures: Hydrodynamic diameter against [H2O]/[OEt] and [H+]/[Si] for BTESM (a), BTESE (b), BTESP (c), and BTESB (d); Data points as well as their projections on the axes planes are shown

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

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.

The cover picture of ChemSusChem 2, 2 shows the membrane separation of water (blue arrow) from crude ethanol (red arrows) as a key enabling technology in the production of ethanol from lignocellulosic biomass

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

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.

Long-term separation performance (stars: water content of permeate; squares: water flux; diamonds: n-butanol flux) of a hybrid silica membrane towards dehydration of n-butanol (calculated for 5 wt% water) at various acid concentrations and 95°C

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.

Colloid size distribution plotted against reflux time for a sol from a BTESE/MTES mixture

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.

FTIR spectrum of thermally treated unsupported mixed hybrid film (‘mixture’), compared with those of films prepared from TEOS, MTES, and BTESE, showing the presence of organic bridges in the relevant films

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.

Artist impression of a HybSi® membrane