Copper Phosphinate Complexes as Molecular Precursors for Ethanol Dehydrogenation Catalysts

Nowadays, the production of acetaldehyde heavily relies on the petroleum industry. Developing new catalysts for the ethanol dehydrogenation process that could sustainably substitute current acetaldehyde production methods is highly desired. Among the ethanol dehydrogenation catalysts, copper-based materials have been intensively studied. Unfortunately, the Cu-based catalysts suffer from sintering and coking, which lead to rapid deactivation with time-on-stream. Phosphorus doping has been demonstrated to diminish coking in methanol dehydrogenation, fluid catalytic cracking, and ethanol-to-olefin reactions. This work reports a pioneering application of the well-characterized copper phosphinate complexes as molecular precursors for copper-based ethanol dehydrogenation catalysts enriched with phosphate groups (Cu-phosphate/SiO2). Three new catalysts (CuP-1, CuP-2, and CuP-3), prepared by the deposition of complexes {Cu(SAAP)}n (1), [Cu6(BSAAP)6] (2), and [Cu3(NAAP)3] (3) on the surface of commercial SiO2, calcination at 500 °C, and reduction in the stream of the forming gas 5% H2/N2 at 400 °C, exhibited unusual properties. First, the catalysts showed a rapid increase in catalytic activity. After reaching the maximum conversion, the catalyst started to deactivate. The unusual behavior could be explained by the presence of the phosphate phase, which made Cu2+ reduction more difficult. The phosphorus content gradually decreased during time-on-stream, copper was reduced, and the activity increased. The deactivation of the catalyst could be related to the copper diffusion processes. The most active CuP-1 catalyst reaches a maximum of 73% ethanol conversion and over 98% acetaldehyde selectivity at 325 °C and WHSV = 2.37 h–1.


Ligands synthesis
(2-aminopropan-2-yl)phenylphosphinic acid (HAIPPA) HAIPPA was synthesized according to the literature method. 1 Commercially available benzyl carbamate, dichlorophenylphosphine, acetone, and acetic acid were used.The main product was hydrolyzed by hydrochloric acid, and the formed hydrochloride was transformed into free aminophosphonic acid by adding propylene oxide to its methanolic solution.Phenylphosphonic acid was observed as the main impurity.The boiling of the impure product with propan-2-ol, further filtration of the obtained slurry, and washing of the filtered precipitate with propan-2-ol allowed for the separation of the phenylphosphonic acid from the main product.

Synthesis of the complexescatalyst precursors {Cu(SAAP)}n (1)
Cu(NO3)2•2.5H2O (0.116 g, 0.500 mmol) was added to the vigorously stirred solution of NaHSAAP (0.169 g, 0.520 mmol) and NaOH (0.500 mmol) in 20 cm 3 of MeOH.The formed emerald-green solution was evaporated to dryness.The dry residue was suspended in tetrahydrofuran and filtered to separate the soluble complex from undissolved NaNO3.The solution was then dried, and the dried residue was dissolved in CH3CN (40 cm 3 ) and left to stand for crystallization.One week later, small, needle-like crystals were obtained.The crystals were filtered, washed with CH3CN, and dried in open air leading to 0.153 g of {Cu(SAAP)}n (yield of 83.9% based on P, M(C16H16CuNO3P) = 364.82g mol −1 ).Single crystals suitable for the single-crystal X-ray diffraction analysis were obtained by the vapor diffusion of acetone to the methanol solution of 1.

[Cu6(BSAAP)6] (2)
The solution of Cu(NO3)2•2.5H2O (0.116 g, 0.500 mmol) in 10 cm 3 of MeOH was added to the vigorously stirred solution of NaHBSAAP (0.202 g, 0.500 mmol) and NaOH (0.500 mmol) in 10 cm 3 of MeOH.The obtained emerald-green solution was left to stand for one day.Then, the solvent was evaporated to dryness.THF (10 cm 3 ) was added to the dried residue, and the formed solution was filtered to separate the soluble complex from undissolved NaNO3.The solution was evaporated to dryness, and the dried glass-like residue was dissolved in 20 cm 3 of CH3CN.The solution was left to stand for one week for crystallization.The formed crystalline precipitate was then filtered, washed with CH3CN, and dried in open air leading to 0.155 g of [Cu6(BSAAP)6] (yield of 69.9 % based on P, M(C96H90Br6Cu6N6O18P6) = 2662.31g mol −1 ).Single-crystals suitable for the single-crystal X-ray diffraction analysis were obtained by the complex crystallization from the CH3CN solution with a low concentration.

[Cu3(NAAP)3] (3)
The solution of Cu(NO3)2•2.5H2O (0.144 g, 0.621 mmol) in 10 cm 3 of MeOH was added to the vigorously stirred solution of NaHNAAP (0.233 g, 0.621 mmol) and NaOH (0.621 mmol) in 10 cm 3 of MeOH.The obtained emerald-green solution was left to stand for one day.Then the solvent was evaporated to dryness.THF (10 cm 3 ) was added to the dried residue, and the formed solution was filtered to separate the soluble complex from undissolved NaNO3.The solution was evaporated to dryness, and the dried glass-like residue was dissolved in 50 cm 3 of CH3CN and then the solution volume was reduced to 5-10 cm 3 till the crystallization began.After one week, crystals were filtered, washed with a small amount of acetonitrile, and dried in open air leading to 0.211 g of [Cu3(NAAP)3] (yield of 81.9% based on P, M(C60H54Cu3N3O9P3) = 1244.65g mol −1  The molecular formulas and molecular weights corresponding to the data obtained from the single-crystal X-ray analysis were used in this table.These formulas could differ from the ones obtained by elemental analysis of dried samples. .

Figure S11
. PXRD patterns of complexes 1-3 calcined at 1000 °C with the heating rate of 5 °C min -1 under an air atmosphere.After temperature was reached, heating was stopped.Bar plots display PDF cards corresponding to the Cu2P2O7 (PDF: 00-044-0182) 6,7 and Cu3(PO4)2 (PDF: 00-080-0992). 9 . 1 H and 31 P { 1 H} (inset) NMR spectra of triethyl phosphate (TEP) formed during the catalytic process on the CuP-3-TEP catalyst with approximately tenfold loading of Cu.Spectra were recorded in C6D6.The reaction products, formed during 24h time-on-stream (TOS), were collected in an ice-cooled trap.Volatile compounds were isolated using a rotary evaporator, and the residues were dissolved in 0.5 cm 3 of C6D6.

Figure S1 .
Figure S1.ESI-MS spectra of 1 in positive mode (fragmentor 50 V).Insets represent calculated (red) and experimental (black) isotopic patterns of the most intense peaks.

Figure S2 .
Figure S2.ESI-MS spectra of 1 in positive and negative modes (fragmentor 100 V).Insets represent calculated (red) and experimental (black) isotopic patterns of the most intense peaks.

Figure S3 .
Figure S3.ESI-MS spectra of 2 in positive mode (fragmentor 50 V).Insets represent calculated (red) and experimental (black) isotopic patterns of the most intense and molecular peaks.

Figure S4 .
Figure S4.ESI-MS spectra of 2 in positive and negative modes (fragmentor 100 V).Insets represent calculated (red) and experimental (black) isotopic patterns of the most intense and molecular peaks.

Figure S5 .
Figure S5.ESI-MS spectra of 3 in positive mode (fragmentor 100 V, lower concentration).Inset represents calculated (red) and experimental (black) isotopic patterns of the most intense molecular peak.

Figure S6 .
Figure S6.ESI-MS spectra of 3 in positive and negative modes(fragmentor 100 V, higher concentration).Insets represent calculated (red) and experimental (black) isotopic patterns of the most intense and molecular peaks.

Figure S13 .
Figure S13.N2 adsorption and desorption isotherm of Aerosil 300 used as support for the preparation of all catalysts.

Figure S14 .
Figure S14.N2 adsorption and desorption isotherms of the prepared catalysts and benchmark samples.

Figure S15 .
Figure S15.STEM-EDS analysis of CuP-Y after catalysis, red rectangle determines measured EDS area.

Figure S16 .
Figure S16.STEM-EDS analysis of CuP-P after catalysis, red rectangle determines measured EDS area.

Table S1 .
). Selected crystallographic data and structure refinement parameters for the complexes 1-3.

Table S4 .
Used masses of Cu phosphinate complexes for catalysts preparation

Table S5 .
XPS surface measurement of Cu, P, and C content in Cu-phosphate/SiO2 samples and comparison with the benchmark catalysts.
b Calculated from H2 consumption.