Polyhedral Oligomeric Silsesquioxane Modification of Metathesis

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Letter

POSS-modification of Metathesis Catalysts – Improved Recycling and Life-Time in Membrane Separation Anna Falk, Jens Martin Dreimann, and Dieter Vogt ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01429 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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ACS Sustainable Chemistry & Engineering

POSS-modification of Metathesis Catalysts – Improved Recycling and Life-Time in Membrane Separation Anna Falk†, Jens M. Dreimann# and Dieter Vogt*,‡ †,‡ Affiliation: School of Chemistry, The University of Edinburgh, King's Buildings, David Brewster Rd, Edinburgh EH9 3FJ, United Kingdom. E-Mail: [email protected] # Lehrstuhl für Technische Chemie, Technische Universität Dortmund, Emil-Figge-Straße 66, 44227 Dortmund, Germany.

Abstract: We herein report the synthesis, application and recycling of stable, polyhedral oligomeric silsesquioxane (POSS) modified ruthenium complexes. The efficiency of all catalysts was evaluated in the ring closing metathesis of a test system relative to the commercially available Grubbs-Hoveyda 2nd generation catalyst. Recycling experiments were carried out with a ceramic membrane in a cat-in-a-cup setup, where the necessity of two POSS units, one on the N-heterocyclic carbene, as well as one on the benzylidene moiety, was shown. ICP analyses were carried out to prove the low levels of ruthenium contamination in the product solutions and thus the high retention of catalyst by the membrane.

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Keywords: Membranes, Green Chemistry, Metathesis, Ruthenium, Silicon.

Introduction: Olefin metathesis is a powerful tool to rearrange carbon-carbon double bonds between two or more unsaturated molecules (cross metathesis) or within the same molecule (ring opening/closing metathesis. The reaction allows the preparation of chemicals with a wide range of applications, including polymers, natural products and pharmaceutical compounds, from both fossil and renewable sources.[1,2] Nevertheless, whilst the most widely used well-defined, homogeneous ruthenium-catalysts display excellent functional group-, as well as air and moisture tolerance, their high cost and often tedious recovery and separation from the product disfavors their application in industrial processes. One approach to overcome these problems has been to immobilize the metathesis catalyst on inorganic[3,4,5,6,7,8] or organic supports.[5,9,10] Also the direct separation of the molecular catalyst by nanofiltration, where substrates and products are small enough to pass through a membrane and a bulky catalyst is retained has been attempted.[11,12,13,14] In principle membrane separation can be an elegant and cost efficient way to separate and reuse homogeneous catalysts, which is beneficial especially from an environmental point of view, as it uses less resources and creates less waste and cleaner solvents and products. There is another important benefit in this membrane separation approach. As a typical equilibrium reaction metathesis tends to give complex product mixtures if run in batch mode towards full conversion. The decoupling of the residence times of catalyst and reaction mixture offers an additional element of reaction control by in situ product removal. This can be of particular advantage in cross-metathesis reactions. In many cases the hitherto available membranes suffer from insufficient selectivity. In those cases where the size of the catalyst does not sufficiently differ from the size of the reactants,


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molecular weight enlargement (MWE) of the catalyst is an elegant way to increase the retention by nanofiltration.[15,16,17,18] Reports in recent years described the installation of various large organic residues on either the N-heterocyclic carbene (NHC)[11,12,13] backbone or the benzylidene unit[14] of Grubbs-type metathesis catalysts and their recycling in a membrane reactor, although only with moderate success, mostly due to catalyst deactivation and suspected membrane fouling. Polyhedral oligomeric silsesquioxane (POSS) derivatives have a highly defined, rigid structure and are commercially available at low cost, which makes them ideal candidates for MWE. Our group reported the use of a POSS-enlarged triphenylphosphine ligand in a continuous hydroformylation reaction with great success.[19] Nevertheless, other applications in MWE have been relatively scarce.[20,21,22,23,24,25] Very recently, the Grela group reported the successful synthesis and recycling of a Grubbs-type catalyst with a POSS-tagged benzylidene moiety.[26] The system suffered from catalyst deactivation, limiting the TON to 144.

Experiments, Results and Discussion: Based on the literature precedent, we decided to synthesize ruthenium complexes with a POSS tag on the NHC ligand, as well as those with a POSS tag on both, the NHC- and the benzylidene-unit, in order to achieve stable metathesis catalysts, recyclable by nanofiltration. In order to connect a POSS unit to the NHC ligand and thus incorporate it into the ruthenium catalyst, the precursor 1 was synthesized according to a literature protocol.[27] The allyl unit was hydrosilylated to form the unstable intermediate 2, which was in situ reacted with the corresponding POSS-trisilanol 3 in the presence of Et3N to form the imidazolinium salts of type 4. These salts were deprotonated using KHMDS and immediately combined with Grubbs-Hoveyda 1st generation catalyst (5) to form the corresponding Ru complexes of type 6 (scheme 1). Complexation of the NHC ligand to the 1st


3


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generation Grubbs-Hoveyda catalyst was confirmed by the loss of the imidazolium proton resonance at 9 ppm in the 1H NMR. The compounds were found to be stable towards separation by column chromatography.

Scheme 1. Synthesis of POSS-modified catalysts of type 6.


4

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In order to evaluate the performance of the newly synthesized catalysts and to see if the POSS unit on the NHC influences the reaction, ring closing metathesis (RCM) with test substrate 7 was performed (scheme 2). The Schlenk flasks containing substrate 7, catalyst (0.5 mol%) and toluene[28] were placed in an oil bath, heated to 25 °C and a stream of argon was constantly bubbled through the stirred mixture to eliminate the ethene formed. The reactions were sampled regularly to collect the reaction profiles for the new catalysts, which were then compared to commercially available Grubbs-Hoveyda 2nd generation catalyst (figure 1).

Scheme 2. RCM with test substrate 7.

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60

GH2

50 6a 40 6b

30

6c

20 10 0 0

50

100

150

200

time [min]

Figure 1. Plot of conversion against reaction time for the RCM with test substrate 7 using 0.5 mol% catalyst loading in toluene at 25 °C ([7] = 97.8 mM, [cat] = 488.9 µM).


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The reactions with all catalysts but 6b gave more than 94% conversion after 2 h reaction time. The turnover frequencies (TOFs) at 20% conversion were determined to be: Grubbs-Hoveyda 2nd generation catalyst (20.4 min-1), 6a (14.8 min-1), 6c (9.2 min-1) and the lowest for 6b (6.2 min1

). To further investigate the potential impact of the POSS units on the catalyst performance, the

RCM of another test substrate 9 was investigated (scheme 3).

Scheme 3. RCM with test substrate 9.

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60

GH 2

50 6a 40 6b

30

6c

20 10 0 0

50

100

150

200

time [min]

Figure 2. Plot of conversion against reaction time for the RCM with test substrate 9 using 0.5 mol% catalyst loading in toluene at 25 °C ([7] = 97.8 mM, [cat] = 488.9 µM). With substrate 9 the reaction outcome was similar to the one with substrate 7 (figure 2). All catalysts but 6b gave more than 94% conversion after 2 h, while 6b only led to 38% conversion.


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The TOFs at 20% conversion were determined to be: 6a (29.6 min-1), Grubbs-Hoveyda 2 generation catalyst (27.2 min-1), 6c ( 27.6 min-1), and 6b (4.2 min-1). Plausible reasons for the poor conversion with 6b are its poor solubility in toluene or poisoning by the product 10. While the alkyl-POSS containing complexes 6a and 6c readily dissolve in apolar and aromatic solvents, 6b is very insoluble in apolar solvents and dissolves only poorly in aromatic solvents like toluene. With iBu-POSS trisilanol being most readily available and showing the best results in catalysis, in terms of total conversion and TOF, we decided to go forward with this unit for further molecular weight enlargement. To create an even larger catalyst, we followed Grela’s approach and synthesized the POSS-modified benzylidene ligand 13,[26] which was then combined with the POSS-tagged Grubbs-like complex 12, to yield catalyst 14 (scheme 4).


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Scheme 4. Synthesis of catalyst 14, bearing POSS-units on the NHC and the benzylidene ligand.


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In order to test the performance of the newly synthesized catalyst 14, RCM of substrate 7 with 0.5 mol% catalyst loading was performed under the conditions mentioned above (toluene, 25 °C, figure 3). With a conversion of 98% after 2 h and a TOF of 15.2 min-1 at 20% conversion, 14 displayed an activity very similar to 6a (14.8 min-1). 100 90 80 70

conv. [%]

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60 50

0.5 mol%

40

0.2 mol%

30

0.05 mol%

20 10 0 0

50

100

150

200

time [min]

Figure 3. Plot of conversion against reaction time for the RCM with test substrate 7 ([7] = 97.8 mM) and catalyst 14 using different catalyst loadings ([14] = 489, 196, and 48.9 µM, respectively at 0.5, 0.2 and 0.05 mol%) in toluene at 25 °C. In order to further explore the performance of catalyst 14 the loading was subsequently lowered to 0.2 and 0.05 mol% (figure 3). The same was done for catalyst 6a to compare both systems (figure 4).


9


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conv. [%]

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60 50

0.5 mol%

40

0.2 mol%

30

0.05 mol%

20 10 0 0

50

100

150

200

time [min]

Figure 4. Plot of conversion against reaction time for the RCM with test substrate 7 ([7] = 97.8 mM) and catalyst 6a using different catalyst loadings ([6a] = 489, 196, and 48.9 µM, respectively at 0.5, 0.2 and 0.05 mol%) in toluene at 25 °C. Lowering the catalyst loading to 0.2 mol% still led to a conversion of 89% for 6a and 97% for 14 after 2 h, respectively. At 0.05 mol%, both catalysts show a similar reaction profile with conversions after 2 h of 91% for 14 and 85% for 6a. For further studies a catalyst loading of 0.2 mol% was chosen since it delivered high conversions after a reasonable time with a TOF at 20% conversion of 24.0 min-1 for 6a and 26.0 min-1 for 14. Membrane reactors often require specialized equipment, inconvenient for a conventional laboratory. To avoid these difficulties, we aimed at using a “cat-in-a-cup” approach with a ceramic membrane first described by Rothenberg[29] and later also successfully applied by our own group.[30] A solution of 0.2 mol% of the respective catalyst was prepared and inserted into a commercially available ceramic membrane tube (0.9 nm TiO2, 450 Da weight cut-off)[31] fitted into a stainless steel holder. The holder was then placed into a Schlenk tube filled with substrate


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solution, and was left to stand for 24 h to allow the substrate to diffuse into the membrane tube and undergo RCM and the product to diffuse out again (figure 5).

Figure 5. Cat-in-a-cup setup for the recycling experiments (left) and picture of the tube in a stainless-steel holder (right). After 24 h, the product-containing solution from the Schlenk tube was removed, analyzed by GC and replaced by fresh substrate solution. This was repeated three times (figure 6). The Grubbs-Hoveyda 2nd generation catalyst was used as benchmark and, although showing a higher initial conversion, dropped in conversion significantly faster than the POSS-tagged equivalents 6a and 14, leading to an overall conversion of 41% over 4 cycles (total TON = 825). Catalyst 6a with only one POSS-unit exhibited a similar, albeit less dramatic reduction in activity with an overall conversion of 49% (total TON = 980). Only 14, containing a POSS-unit on the NHC ligand as well as on the benzylidene unit, still showed a conversion of nearly 50% in the fourth


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cycle, which corresponds to a total reaction time of 4 days for the catalyst in solution. The overall conversion with catalyst 14 reached 69% (total TON = 1380). 100 90 80 70

conv. [%]

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60

GH2

50

6a

40

14

30 20 10 0 1

2

3

4

Figure 6. Repetitive batch recycling of catalysts in the RCM of 7 using 0.2 mol% catalyst loading in toluene at room temperature. These results provide further support for the assumption that it is highly important to enlarge both the NHC- and the benzylidene moiety of metathesis catalysts, so that catalyst decomposition is not driven forward by the removal of stabilizing components during filtration, which may have occurred in previous literature systems.[11,12,13,14,26] The Ru-content of the post reaction solutions were measured by ICP OES (table 1).

Table 1. Catalyst leaching levels in post reaction media with different catalysts at 0.2 mol% catalyst loading in ppm and as percentage of the total amount of Ru originally placed in the membrane tube.[a]


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Run

Ru [ppm] with 14

[%] of total Ru with 14

Ru [ppm] with 6a

[%] of total Ru with 6a

Ru [ppm] with GrubbsHoveyda 2nd gen.

% of total Ru with GrubbsHoveyda 2nd gen.

1

0.64

3.2

1.58

7.8

2.51

12.4

2

0.56

2.8

0.84

4.2

0.98

4.9

3

0.41

2.0

0.46

2.3

0.59

2.9

4

0.27

1.3

0.21

1.0

0.20

1.0

[a] Determined by ICP OES analysis.

As expected, the quantities of leached ruthenium in the samples where catalyst 14 was used were well below 1 ppm in all 4 runs. With catalyst 6a Ru leaching was significantly higher in the first two cycles, probably due to the loss of the small benzylidene moiety through membrane diffusion, leaving a vulnerable catalyst species more prone to decomposition. The highest leaching was observed for Grubbs-Hoveyda 2nd generation catalyst. In addition to the loss of the stabilizing benzylidene moiety, the catalyst itself might show a low retention, contributing to a high overall Ru level in the permeate solution. Finally, to show that the reaction is truly controlled by substrate and product diffusion through the membrane and not by small amounts of catalyst leaching, samples of the reaction mixture with catalyst 14 in the cat-in-a-cup setup were taken over time. The obtained reaction profile is shown in figure 7 and compared to the profile of the reaction carried out homogeneously in a Schlenk tube.


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60 50 membrane 40 no membrane

30 20 10 0 0

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time [min]

Figure 7. Plot of conversion against reaction time for the RCM with test substrate 7 in toluene with and without 0.2 mol% of catalyst 14 being contained in a membrane. The vast difference in the reaction profiles and the fact that the conversion rises in a linear instead of a curved fashion in the membrane setup clearly shows that the product formation in this case is solely controlled by steady diffusion through the pores of the membrane.

Conclusion: In conclusion, we herein describe an easy-to-use setup with a POSS-modified metathesis catalyst suitable for recycling at least four times. Our research highlights for the first time the necessity for molecular weight enlargement of both, the NHC and the benzylidene ligand to achieve high catalyst retention. Even with low catalyst loadings, good conversions can be observed and the product solution contains only traces of ruthenium and thus is easy to further purify or use in subsequent reactions. The application of catalyst 14 in other metathesis reactions and in a more appropriate cross flow membrane filtration setup is currently under way in our


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laboratories. This will avoid the rate limitation by diffusion and allow a more efficient removal of ethene from the reaction mixture.

Materials and Methods: General procedure for recycling experiments: n-Dodecane (255.6 mg, 1.5 mmol) as internal standard for GC analysis and 7 (360.3 mg, 1.5 mmol) were dissolved in dry, purified toluene (15 mL) and a sample was taken (t = 0 min). A solution of the relevant catalyst (0.003 mmol, 0.2 mol%) in dry, purified toluene (1.5 mL) was added to a membrane tube fitted into a stainless-steel holder in the glovebox. The holder was subsequently placed into a Schlenk tube which was then sealed and removed from the glove box. The substrate solution was added to the Schlenk tube and the reaction vessel was kept at room temperature. After 24 h, the product containing solution was removed and analyzed, and the reaction tube was refilled with fresh substrate solution, to be left again for 24 h. This catalyst recycling procedure was repeated three times. Further details on experimental procedures and analytical data are available in the Supporting Information.

AUTHOR INFORMATION Corresponding Author *Prof. Dr. D. Vogt, Lehrstuhl für Technische Chemie, Technische Universität Dortmund, EmilFigge-Straße 66, 44227 Dortmund, Germany, E-mail: [email protected]. Present Addresses † Dr. A. Falk, School of GeoSciences, The University of Edinburgh, King's Buildings, Alexander Crum Brown Road, Edinburgh EH9 3FF, UK.


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Prof. Dr. D. Vogt, Dr. Jens M. Dreimann, Lehrstuhl für Technische Chemie, Technische

Universität Dortmund, Emil-Figge-Straße 66, 44227 Dortmund, Germany. ORCID Dieter Vogt: 0000-0002-8514-5326 Jens M. Dreimann: 0000-0002-8466-0799 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources We thank the Alexander von Humboldt foundation for granting a postdoctoral Feodor-Lynen fellowship to Dr. Anna Falk.

ACKNOWLEDGMENT We like to thank Dr. Lorna Eades for ICP analyses. We further thank Evonik Industries and Dr. Renat Kadyrov for helpful advice and a generous gift of ruthenium complexes. We also thank Hybrid Catalysis for gifting us POSS-trisilanol.

SUPPORTING INFORMATION Catalyst syntheses and analytical data, detailed catalysis results and NMR spectra.

REFERENCES


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[23] Ropartz, L.; Foster, D. F.; Morris, R. E.; Slawin, A. M. Z.; Cole-Hamilton, D. J. Hydrocarbonylation reactions using alkylphosphine-containing dendrimers based on a polyhedral oligosilsesquioxane core. J. Chem. Soc. Dalton Trans. 2002, 9, 1997-2008, DOI 10.1039/B200303A. [24] Vautravers, N. R.; Cole-Hamilton, D. J. Diazaphospholidine terminated polyhedral oligomeric silsesquioxanes in the hydroformylation of vinyl acetate. Chem. Commun. 2009, 1, 92-94, DOI 10.1039/B814582B. [25] Cho, M. H.; Weissman, H.; Wilson, S. R.; Moore, J. S. A Mo(VI) Alkylidyne Complex with Polyhedral Oligomeric Silsesquioxane Ligands: Homogeneous Analogue of a SilicaSupported Alkyne Metathesis Catalyst. J. Am. Chem. Soc. 2006, 128, 14742-14743, DOI 10.1021/ja065101x. [26] Kajetanowicz, A.; Czaban, J.; Krishnan, G. R.; Malińska, M.; Woźniak, K.; Siddique, H.; Peeva, L. G.; Livinston, A. G.; Grela, K. Batchwise and Continuous Nanofiltration of POSSTagged Grubbs–Hoveyda-Type Olefin Metathesis Catalysts. ChemSusChem 2013, 6, 182-192, DOI 10.1002/cssc.201200466. [27] Allen, D. P.; Van Wingerden, M. W.; Grubbs, R. H. Well-Defined Silica-Supported Olefin Metathesis Catalysts. Org. Lett. 2009, 11, 1261-1264, DOI 10.1021/ol9000153. [28] Toluene was chosen as a solvent as preliminary experiments showed that the use of CH2Cl2 leads to a faster catalyst degradation.


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[29] Gaikwad, A. V.; Boffa, V.; ten Elshof, J. E.; Rothenberg, G. Cat-in-a-Cup: Facile Separation of Large Homogeneous Catalysts. Angew. Chem., Int. Ed. 2008, 47, 5407-5410, DOI 10.1002/anie.200801116. [30] Janssen, M.; Müller, C.; Vogt, D. ‘Click’ Dendritic Phosphines: Design, Synthesis, Application in Suzuki Coupling, and Recycling by Nanofiltration. Adv. Synth. Catal. 2009, 351, 313-318, DOI 10.1002/adsc.200900058. [31] www.inopor.com, accessed on the 24th of January 2017.


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FOR TABLE OF CONTENTS USE ONLY SYNOPSIS The synthesis, application and recycling of stable, polyhedral oligomeric silsesquioxane (POSS) modified Ruthenium complexes in a cat-in-a-cup-setup is shown.


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Scheme 1. Synthesis of POSS-modified catalysts of type 6. 164x163mm (300 x 300 DPI)



Scheme 2. RCM with test substrate 7. 106x23mm (300 x 300 DPI)


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Figure 1. Plot of conversion against reaction time for the RCM with test substrate 7 using 0.5 mol% catalyst loading in toluene at 25 °C ([7] = 97.8 mM, [cat] = 488.9 µM). 54x35mm (300 x 300 DPI)



Scheme 3. RCM with test substrate 9. 105x23mm (300 x 300 DPI)


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Figure 2. Plot of conversion against reaction time for the RCM with test substrate 9 using 0.5 mol% catalyst loading in toluene at 25 °C ([7] = 97.8 mM, [cat] = 488.9 µM). 53x35mm (300 x 300 DPI)



Scheme 4. Synthesis of catalyst 14, bearing POSS-units on the NHC and the benzylidene ligand. 206x247mm (300 x 300 DPI)


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Figure 3. Plot of conversion against reaction time for the RCM with test substrate 7 ([7] = 97.8 mM) and catalyst 14 using different catalyst loadings ([14] = 489, 196, and 48.9 µM, respectively at 0.5, 0.2 and 0.05 mol%) in toluene at 25 °C. 53x35mm (300 x 300 DPI)



Figure 4. Plot of conversion against reaction time for the RCM with test substrate 7 ([7] = 97.8 mM) and catalyst 6a using different catalyst loadings ([6a] = 489, 196, and 48.9 µM, respectively at 0.5, 0.2 and 0.05 mol%) in toluene at 25 °C. 54x35mm (300 x 300 DPI)


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Figure 5. Cat-in-a-cup setup for the recycling experiments (left) and picture of the tube in a stainless-steel holder (right). 42x25mm (300 x 300 DPI)



Figure 6. Repetitive batch recycling of catalysts in the RCM of 7 using 0.2 mol% catalyst loading in toluene at room temperature. 54x35mm (300 x 300 DPI)


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Figure 7. Plot of conversion against reaction time for the RCM with test substrate 7 in toluene with and without 0.2 mol% of catalyst 14 being contained in a membrane. 53x35mm (300 x 300 DPI)



Synopsis: The synthesis, application and recycling of stable, polyhedral oligomeric silsesquioxane (POSS) modified Ruthenium complexes in a cat-in-a-cup-setup is shown. 89x58mm (300 x 300 DPI)


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Polyhedral Oligomeric Silsesquioxane Modification of Metathesis

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