Plasma-assisted immobilization of a phosphonium salt and its use as a catalyst in the valorization of CO2.

Abstract The first plasma‐assisted immobilization of an organocatalyst, namely a bifunctional phosphonium salt in an amorphous hydrogenated carbon coating, is reported. This method makes the requirement for prefunctionalized supports redundant. The immobilized catalyst was characterized by solid‐state 13C and 31P NMR spectroscopy, SEM, and energy‐dispersive X‐ray spectroscopy. The immobilized catalyst (1 mol %) was employed in the synthesis of cyclic carbonates from epoxides and CO2. Notably, the efficiency of the plasma‐treated catalyst on SiO2 was higher than those of the SiO2 support impregnated with the catalyst and even the homogeneous counterpart. After optimization of the reaction conditions, 13 terminal and four internal epoxides were converted with CO2 to the respective cyclic carbonates in yields of up to 99 %. Furthermore, the possibility to recycle the immobilized catalyst was evaluated. Even though the catalyst could be reused, the yields gradually decreased from the third run. However, this is the first example of the recycling of a plasma‐immobilized catalyst, which opens new possibilities in the recovery and reuse of catalysts.


General considerations
All chemicals were purchased from commercial sources in purities of ≥95% and used without further purification. The TiO2 nanopowder (21 nm particle size, >99.5%) and FeO (10 mesh, 99.8% trace metals basis) were purchased from Sigma-Aldrich Chemie GmbH, SiO2 (60, 230-400 mesh) was purchased from Roth GmbH. Deuterated solvents were ordered from Deutero GmbH and stored over molecular sieves (3 Å). NMR spectra were received using Bruker 300 Fourier, Bruker AV 300 and Bruker AV 400 spectrometers. Chemical shifts are reported in ppm relative to the deuterated solvent. Coupling constants are expressed in Hertz (Hz). The following abbreviations are used: s= singlet, d= doublet, t= triplet and m= multiplet. NMR yields were determined by using mesitylene as internal standard.
Elementary analysis was performed on a TruSpec CHMS Micro from Leco. IR spectra were recorded on a Nicolet iS10 MIR FT-IR-spectrometer from Thermo Fisher Scientific. performed on selected grains with surfaces uniformly exposed into the direction of the detector for 300 seconds using a mapping window of 800x600 pixels, in addition an elemental spectrum with 210 6 counts was recorded on the grain surface area at higher magnification. SEM images shown in the manuscript ( Fig. 5 and 9) are depicted in later section of the supporting information with a scale (SI6.1-

Amorphous hydrogenated carbon (a-C:H) thin films
Amorphous hydrogenated carbon coatings (a-C:H) are characterized by an irregular network of C atoms, which are partially saturated with hydrogen atoms. It is known that a-C:H coatings have good chemical resistance and undergo degradation only at temperatures greater than 350 °C. A high proportion of hydrogen in the layer leads to large sp 3 C-H bond fractions and rather causes soft layers. By reducing the hydrogen content, the proportion of sp 3 C-C bonds is increased resulting in harder layers (diamondlike). When using methane as a precursor it can be assumed that due to the relatively high H / C ratio, especially soft layers are produced. This is an important feature for the use of the a-C: H coatings as a polymer for encapsulating the catalysts, since this conserves the mechanical parts of the equipment (e.g., circulation pumps) for chemical catalysis and leads to less abrasion.
Amorphous hydrogenated carbon coatings were deposited directly onto the powdery support that has been impregnated with the catalyst before. The polymer deposition was performed in capacitively coupled radio frequency (rf) plasma generated by a rod electrode in vacuum chamber ('Piccolo', Plasma Electronic GmbH, Neuenburg/Germany). A schematic view of the experimental setup is shown in Figure   S1.
In this way, the catalyst was partially encapsulated by the plasma polymer layer.   (Table 2 and Table 3): A 45 cm 3 stainless-steel autoclave was charged with the impregnated or plasma treated catalyst (500 mg, 1.0 mol% or 2.0 mol%) and 1,2-butylene oxide (1a, 1.00 g, 13.9 mmol, 1.0 equiv). The autoclave was purged with CO2 and the reactor was heated to 45 °C or 90 °C for 3-24 h, while p(CO2, 90 °C) was kept constant at 1.0 MPa. The reactor was cooled with an ice bath below 20 °C and CO2 was released slowly. The conversion of the epoxide 1a and the yield of the carbonate 2a were determined by 1 H NMR spectroscopy from the reaction mixture using mesitylene as internal standard.
Subsequently the reactor was cooled to ≤20 °C with an ice bath and CO2 was released slowly. The reaction mixture was removed by extraction with Et2O (3×30 mL). All volatiles were removed in vacuo to yield 1,2-butylene carbonate 2a. The catalyst was dried in air overnight and reused. The conversion of the epoxide 1a and yield of the desired carbonate were determined either with isolated product or by 1 H NMR spectroscopy using mesitylene as internal standard.   Reaction conditions: 45 cm 3 stainless-steel autoclave, 1a (13.9 mmol, 1.0 equiv), 500 mg of the immobilized catalyst (1.0 mol% loading), solvent-free. Yield determined by 1 H NMR with mesitylene as internal standard. Reaction conditions: 45 cm 3 stainless-steel autoclave, 1a (13.9 mmol, 1.0 equiv), 500 mg of the immobilized catalyst (1.0 mol% loading), solvent-free. Yield determined by 1 H NMR with mesitylene as internal standard.  Reaction conditions: 45 cm 3 stainless-steel autoclave, 1a (13.9 mmol, 1.0 equiv), 500 mg of the immobilized catalyst (1.0 mol% or 2 mol% loading), solvent-free. Yield determined by 1 H NMR with mesitylene as internal standard.

SEM images and EDX mappings
The EDX mapping was performed within the boundary of the green frame (mapping area of 800×600 Pixel) to identify only elements within the central area of the corn.

Figure S3
SEM image of SiO2 support with scale (see also Figure 6, Ia in the manuscript).      Figure S11 EDX mapping with color coded intensity range for phosphorus of catalyst 5b@SiO2 with color scale.

Synthesis of cyclic carbonates 2
General procedure (GP) for the synthesis of various cyclic carbonates 2 using 5bb℗SiO2: A 45 cm 3 stainless-steel autoclave was charged with catalyst 5bb℗SiO2 (500 mg, 1.0 mol%) and epoxide 1 (13.9 mmol, 1.0 equiv). The autoclave was purged with CO2 and the reactor was heated to 45 °C for 6 h, while p(CO2, 45 °C) was kept constant at 1.0 MPa. The reactor was cooled with an ice bath below 20 °C and CO2 was released slowly. The crude mixture was diluted with EtOAc (30 mL) and filtered over SiO2. Subsequently, all volatiles were removed in vacuo to obtain carbonates 6.