Lloyd S. White

Properties of a polyimide gas separation membrane in natural gas streams

Abstract Carbon dioxide and methane permeabilities have been measured for dense film and asymmetric membrane prepared from an aromatic polyimide (6FDA/DMB). Selectivities for CO 2 CH 4 of up to 55 were determined for mixture oof these gases. Permeability of gases through the membrane was found to be dependent upon the carbon dioxide and higher hydrocarbon concentrations that can also be present in natural gas streams. In addition to carbon dioxide having an impact on methane permeability, the presence of hexane or toluene cut CO 2 CH 4 selectivities in half. Lowered selectivities from ideal test conditions are a result of plasticization of the polyimide by these components. Results for the polyimide are contrasted with values obtained from cellulose acetate films which are less impacted by hydrocarbon impurities. The polyimide depends more upon diffusivity factors than cellulose acetate to generate high selectivity. Thermal and physical properties of 6FDA/DMB polymer and membrane are also described.

Observations on solvent flux and solute rejection across solvent resistant nanofiltration membranes

Abstract Organic solvent nanofiltration is an emerging technology made possible by the recent development of solvent resistant nanofiltration (SRNF) membranes. These membranes have many potential applications from continuous operation over many months in refinery systems [1,2] to short term operation for a few hours in batch chemical processes [3]. In this paper, solvent flux decline and membrane separation properties are investigated (including their dependence on pressure), using methanol with quaternary alkyl ammonium bromide salts with molecular weights (MW) in the range 322 to 547 Daltons as solutes. The membranes are characterised in terms of an equivalent uniform pore size using three simple pore flow models: Ferry model, Steric Hindrance Pore (SHP) model and Verniory model.

Semi-continuous nanofiltration-coupled Heck reactions as a new approach to improve productivity of homogeneous catalysts

Abstract Substantial increase in homogeneous catalyst productivity for a well known Heck coupling was achieved by nanofiltration-coupled catalysis. The use of nanofiltration membranes enabled catalyst separation and allowed subsequent catalyst recycle and reuse. This new technique demonstrated potential for general applicability to homogeneously catalysed organic syntheses.

Membrane Separation in Green Chemical Processing

Abstract: This paper describes ideas together with preliminary experimental results for applying solvent nanofiltration to liquid phase organic synthesis reactions. Membranes for organic solvent nanofiltration have only recently (during the 1990s) become available and, to date, have been applied primarily to food processing (vegetable oil processing, in particular) and refinery processes. Applications to organic synthesis, even at a laboratory feasibility level, are few. However, these membranes have great potential to improve the environmental performance of many liquid phase synthesis reactions by reducing the need for complex solvent handling operations. Examples that are shown to be feasible are solvent exchanges, where it is desired to swap a high molecular weight molecule from one solvent to another between separate stages in a complex synthesis, and recycle and reuse of homogeneous catalysts. In solvent exchanges, nanofiltration is shown to provide a fast and effective means of swapping from a high boiling point solvent to a solvent with a lower boiling point—this is a difficult operation by means of distillation. Solvent nanofiltration is shown to be able to separate two distinct types of homogeneous catalysts, phase transfer catalysts and organometallic catalysts, from their respective reaction products. In both cases the application of organic solvent nanofiltration allows several reuses of the same catalyst. Catalyst stability is shown to be an essential requirement for this technique to be effective. Finally, we present a discussion of scale‐up aspects including membrane flux and process economics.

Homogeneous catalyst separation and re-use through nanofiltration of organic solvents

Abstract The separation of reaction products from catalysts is a major problem existing in many forms of homogeneous catalysis, including phase transfer catalysis and transition metal catalysed reactions. This study describes a new and generic approach to solving this separation problem using solvent resistant nanofiltration (SRNF) membranes. Data for SRNF separation of a phase-transfer catalyst (PTC), and a Heck reaction transition metal catalyst (TMC), from their respective reaction media is presented. SRNF was used to retain each catalyst from the reaction solvent and, when applied in a coupled reaction-separation system, allowed several subsequent catalyst recycles and re-uses. PTC rejection was high (99+%) whether SRNF was applied to pre- or post-reaction mixtures, and no reaction rate decline was observed for two consecutive catalyst recycles. TMC rejection, while 96% for pre-reaction mixtures, dropped to 90% for post-reaction mixtures and reaction rate decline was observed for four consecutive catalyst recycles. The clear difference between the two systems is the much higher stability of the PTC due to a less demanding catalytic cycle. The need for stable catalytic systems for effective generic use of this technology in homogeneous catalysis is highlighted.

Increased catalytic productivity for nanofiltration-coupled Heck reactions using highly stable catalyst systems

The recycle of homogeneous Heck catalysts from post-reaction mixtures using solvent resistant nanofiltration (SRNF) membranes is known to increase catalytic productivity. However, the technique is sensitive to catalyst stability when applied to conventional catalysts: catalyst deactivation will cause declining reaction rates and higher reactor occupancy with an increasing number of catalyst recycles. In this study, a conventional Heck catalyst {bis(acetato)bis(triphenylphosphine)palladium(II)}, was recycled six times, before the reaction rate dropped below 20% of the original value. A cumulative turnover number (TON) of 690 in 120 h was obtained, whilst providing a product substantially lowered in organometallic impurities. Significant improvements in system performance were realised by employing, at identical Pd loading, state-of-the-art catalysts with greater chemical stability. An imidazolylidene catalyst, bis(1,3-dibenzylimidazoline-2-ylidene)diiodopalladium(II), yielded an equal TON for six recycles in 40 h with substantially less reaction rate decline. Palladium(II) acetate stabilised by the quaternary phosphonium salt (quat) tetraphenylphosphonium bromide even gave six recycles in under 30 h after careful solvent selection. In all cases, the membrane showed good selectivity against the catalyst (up to 96% Pd rejection), while allowing the reaction product to permeate completely. Through the sustainable high reaction rates, reactor occupancy can be minimised, and waste generated by downstream processing reduced. Indeed, the imidazolylidene and the quat-stabilised catalyst systems surpassed 10 catalyst recycles with TONs >1000 within 60 and 45 h, respectively.

Phase-transfer catalyst separation and re-use by solvent resistant nanofiltration membranes

This communication describes the use of nanofiltration (NF) membranes for efficient separation and recycling of phase-transfer catalysts, using the conversion of bromoheptane into iodoheptane with tetraoctylammonium bromide as the PT catalyst, as an example; a solvent flux of >10 L m−2 h−1 was achieved with >99% catalyst recycle and no loss in PT catalyst activity over a cycle of three consecutive reactions.