Sebastian Werner

Ionic Liquids in Chemical Engineering

The development of engineering applications with ionic liquids stretches back to the mid-1990s when the first examples of continuous catalytic processes using ionic liquids and the first studies of ionic liquid-based extractions were published. Ever since, the use of ionic liquids has seen tremendous progress in many fields of chemistry and engineering, and the first commercial applications have been reported. The main driver for ionic liquid engineering applications is to make practical use of their unique property profiles, which are the result of a complex interplay of coulombic, hydrogen bonding and van der Waals interactions. Remarkably, many ionic liquid properties can be tuned in a wide range by structural modifications at their cation and anion. This review highlights specific examples of ionic liquid applications in catalysis and in separation technologies. Additionally, the application of ionic liquids as working fluids in process machines is introduced.

Rhodium–Phosphite SILP Catalysis for the Highly Selective Hydroformylation of Mixed C4 Feedstocks

The hydroformylation of alkenes catalyzed by dissolved rhodium complexes is not only one of the largest applications of homogeneous catalysis in industry, but also an established benchmark reaction for testing immobilization concepts for homogeneous catalysts. In recent years, ionic liquids (ILs) as non-aqueous solvents for liquid–liquid biphasic hydroformylation catalysis have been the subject of intensive study. Important features of ILs compared to the industrial aqueous–organic biphasic catalysis (Ruhrchemie–Rh ne–Poulenc process), are their much better solubility for higher alkenes and their compatibility with phosphite ligands, which readily decompose by hydrolysis in water. Despite these attractive features, we know of no largescale industrial application of ionic liquids in biphasic hydroformylation catalysis to date. Two important drawbacks of the biphasic ionic liquid systems are the relatively high amounts of expensive IL that are required and its intrinsically high viscosity, which leads to slow mass transport between the two liquid phases. To overcome these limitations, we, among others, have in recent years developed the supported ionic liquid phase (SILP) concept. SILP materials are prepared by dispersing a solution of the catalyst complex in an ionic liquid as a thin, physisorbed film on the large internal surface area of a porous solid material. Since the film thickness of the ionic liquid is within the nanometer range, diffusion problems are minimized by the extremely small diffusion distances. Excellent ionic liquid utilization is achieved; that is, the same catalytic performance can be achieved with a much smaller total IL amount compared to liquid–liquid biphasic systems. Because ionic liquids typically have extremely low vapor pressures, catalysis with SILP materials is particularly attractive in continuous gas-phase contact. During catalysis the immobilized catalytic ionic liquid film comes into contact solely with gaseous reactants and products. For the continuous gas-phase hydroformylation of pure 1-alkene feedstock, such as, propene and 1-butene, this concept has been demonstrated quite successfully with good catalytic activity (turnover frequencies (TOFs) up to 500 h 1 in the case of propene and 564 h 1 in the case of 1-butene) and excellent catalyst stability (up to 200 h time-on-stream in the case of propene and 120 h in the case of 1-butene) as was demonstrated using a Rh-SILP catalyst modified with the sulfonated phosphine ligand sulfoxantphos (1). The sulfoxantphos–rhodium catalyst is, however, unable to react with internal alkenes such as 2butenes in either hydroformylation or isomerization. Thus, to convert 1-butene and 2-butenes from a mixed technical C4 feedstock from steam-cracker into the desired linear pentanal, a different catalyst system is required. Rhodium–phosphite complexes are known to be capable of selective isomerization/hydroformylation activity, which converts internal alkenes in a classical monophase homogeneous catalysis into linear aldehydes with good to excellent selectivity. Most of these ligands, however, are highly airand moisture-sensitive, making it difficult to handle and use them in large quantities and a real challenge to recycle rhodium– phosphite systems. Herein, we show how the new diphosphite ligand 2 in form of a SILP catalyst system is applied in the continuous gas-phase hydroformylation of an industrial mixed C4 feedstock as illustrated in Scheme 1. Synthesizing 2 and using it in

Ultra‐Low‐Temperature Water–Gas Shift Catalysis using Supported Ionic Liquid Phase (SILP) Materials*

Production of high purity hydrogen from fossil fuels or renewable feedstocks requires efficient water–gas shift (WGS) catalysts to remove traces of carbon monoxide, a catalyst poison for, for example, proton-exchange membrane fuel cells (PEMFCs) and ammonia catalysts. State of the art heterogeneous WGS catalysis is performed in a combination of high-temperature shift (HTS; Fe2O3/Cr2O3, T=450 8C, P=3 MPa) and low-temperature shift (LTS, CuO/ZnO/Al2O3, T=200 8C), reducing the CO level to 0.1–0.3 wt%. This level is still higher than acceptable for most direct applications of the obtained hydrogen and therefore additional purification steps, such as selective oxidation, methanization, or adsorption, are required. To reduce the equilibrium CO level in the exothermic WGS reaction further, catalysts are required that efficiently operate at temperatures below 180 8C (ULT, ultra-low temperature), a goal that could never be achieved with the known commercial heterogeneous catalytic systems. A promising alternative to heterogeneous WGS catalysts is the clever application of homogeneous transition metal complexes in multiphase systems. Some examples of homogeneous WGS catalysis, such as, for example, [Ru3(CO)12] in trimethylamine/water by Pettit et al. or the pyridine-modified ruthenium systems in the group of Pakkanen, have been reported. However, moderate activities, the requirement for high total pressures (typically 2.5 MPa) and catalyst recycling problems have been major drawbacks of these systems. For future hydrogen production scenarios based on decentralized biogas conversion, high WGS activity at atmospheric pressure is mandatory. Recently, we have shown that WGS catalysts based on supported ionic liquid phase (SILP) technology are active under very mild reaction conditions below 160 8C and 0.1 MPa. The SILP WGS catalyst previously reported by our group consisted of a RuCl3 catalyst precursor dissolved in the ionic liquid (IL) 1butyl-2,3-dimethylimidazolium trifluoromethanesulfonate [BMMIM][OTf], highly dispersed as a thin film on silica gel. Using a model feedstock of H2O and CO in a continuous screening rig, an activity of 3 molH2mol 1 Ru h 1 was attained at 160 8C and 0.1 MPa after a prolonged induction period of more than 20 h. This moderate activity already exceeded that of a commercial copper-based catalyst under the same ULT conditions (0.5 molH2mol 1 Cu h ). However, this first-generation, proof-of-principle SILP WGS catalyst suffered from a number of severe drawbacks. At higher temperatures the formation of volatile Ru-carbonyl species caused ruthenium losses from the SILP material. Furthermore, no activity was observed when using the technically relevant diluted syngas mixture (13% CO2, 8% CO, 4% N2, 75% H2). A detailed screening of several metal complexes by our group revealed that only ruthenium-based homogeneous catalysts exhibited reasonable long-term activity in WGS below 200 8C. Herein we present a significantly improved ULT SILP WGS catalyst system that shows besides high activity and stability even with a technical relevant, diluted syngas feed a unique re-start behavior after shutdown. Efficient restart is one critical criterion for on-demand hydrogen production in dynamic applications that, to our knowledge, has never been fulfilled with any commercial or academically reported, stable and productive WGS catalyst to date. This remarkable success was rendered possible by focusing on three different issues: a) Elimination of the catalyst induction period; b) introduction of basicity via the support, additives or IL anion; c) activity tests of the so-optimized systems using typical real syngas mixtures. These aspects will be highlighted herein point by point. To explain the long induction period (20 h) required for with the RuCl3-based systems, we performed IR spectroscopy on the SILP catalyst prior to and after use in a WGS experiment. The obtained spectra were compared to spectra recorded for various ruthenium–carbonyl–chloro complexes. Whereas the spectrum of a freshly prepared SILP RuCl3 catalyst shows no absorption (Figure 1a, dashed line) in the region between 2100 and 1700 cm 1 (as there is no CO coordinated to the ruthenium), the spectra obtained from the same SILP WGS catalyst after 48 h operation in the WGS reactor show two bands at 2047 and 1970 cm 1 (Figure 1a, solid line) indicating in situ formation of a Ru–carbonyl complex. This interpretation is further supported by earlier work of Roberto et al. , who reported the formation of dimeric [{Ru(CO)3Cl2}2] from RuCl3 under a carbon monoxide-containing atmosphere. Indeed, by using separately synthesized [Ru(CO)3Cl2] for the preparation of our SILP material, a catalyst was obtained that showed high WGS activity right from the first minutes’ timeon-stream, whereas the respective catalyst prepared with RuCl3 required 95 h time-on-stream to obtain the same level of activity of 4.6 molCO2mol 1 Ru h 1 (Figure 1b, Table 1, entry 10). These [a] S. Werner, Dr. M. Haumann, Prof. Dr. P. Wasserscheid Lehrstuhl f r Chemische Reaktionstechnik Friedrich-Alexander-Universit t Erlangen-N rnberg Egerlandstrase 3, 91058 Erlangen (Germany) Fax: (+49)9131-8527421 E-mail : wasserscheid@crt.cbi.uni-erlangen.de [b] Dr. N. Szesni, M. Kaiser, Dr. R. W. Fischer Catalytic Technologies R&D, S d-Chemie AG Waldheimer Str. 13, 85052 Bruckm hl (Germany)