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Catal. Sustain. Energy 2017; 4: 59–61

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G.G. Volkova, V.A. Paukshtis of Dimethyl on Ag/

Cs1.5H1.5PW12O40: In-Situ FTIR Study of the Ag-Carbonyls https://doi.org/10.1515/cse-2017-0009 the catalyst. Moreover, iodide cocatalysts are carcinogens Received November 23, 2017; accepted December 6, 2017 and they should be avoided in “green” chemistry. These problems may be overcome by developing a heterogeneous Abstract: Halide-free carbonylation of dimethyl ether (DME) catalyst that can operate effectively without a halide to over heterogeneous catalysts offers a promoter. Dimethyl ether (DME) is more favorable for potentially green route to the production of important carbonylation than because it can be produced industrial chemicals. Previously we have shown that the from syn-gas more effectively [2]. addition of 1% of silver to Cs1.5H1.5PW12O40 led to an increase in activity by a factor of two. Formation of silver carbonyl CH -O-CH + CO ® CH -CO-O-CH clusters on the surface of the catalyst was demonstrated by 3 3 3 3 in situ FTIR spectroscopy. The v(CO) of silver carbonyl was Two types of acid catalysts are capable of activating observed at 2188 cm-1 and may be explained by the formation C-O bonds in methanol or in DME: zeolites and of cationic silver carbonyl Ag(CO)+ on the surface of the heteropolycompounds. Starting with Cu mordenite acidic cesium salt of 12-tungstophosphoric acid. In view of [3] BP Chemicals Limited continued their discoveries the increased efficiency, the novel bifunctional catalyst Ag/ in halide-free DME carbonylation with other acidic Cs H PW O is expected to hold significant promise for 1.5 1.5 12 40 zeolites – H-ZSM5, and H-FER [4]. Wegman was the practical use in new processes of basic organic chemistry. first to demonstrate that rhodium or iridium salts of tungstophosphoric heteropoly acid supported on silica Keywords: carbonylation, dimethyl ether, acidic can catalyze DME carbonylation to methyl acetate [5]. cesium salt of 12-tungstophosphoric acid, in situ FTIR We subsequently have shown that acidic cesium salts of spectroscopy, silver carbonyl 12-tungstophosphoric acid promoted with rhodium can lead to a 10-fold enhancement in the activity of halide-free DME carbonylation to methyl acetate [6]. Recently we have

1 Introduction observed that the addition of 1% of silver to Cs1.5H1.5PW12O40

led to an increase in activity by 100% [7], while Souma et is an industrial chemical which is manufactured al. have demonstrated the acceleration of carbonylation on a large scale. The main route to acetic acid is through of alcohols and by 5-25% in the presence of Ag/ the carbonylation of methanol which uses a homogeneous Nafion, Ag/ZSM-5, and Ag/SO4/ZrO2 [8]. rhodium or iridium catalyst and a halide promoter [1]. Here we report the characterization of silver carbonyl The main disadvantages of this process are: a) halides clusters on the surface of the acidic cesium salt of are highly corrosive and are poisons for many types of 12-tungstophosphoric acid using in situ FTIR spectroscopy. catalysts, b) it is difficult to separate the products from 2 Experimental

*Corresponding author: G.G. Volkova, Boreskov Institute of Catalysis, pr.Akademika Lavrentieva, 5, 630090, Novosibirsk, 2.1 Catalyst preparation Russia, E-mail: [email protected] E.A. Paukshtis, Boreskov Institute of Catalysis, pr.Akademika Lav- The samples of Cs H PW O catalysts 1) unpromoted rentieva, 5, 630090, Novosibirsk, Russia 1.5 1.5 12 40 and 2) promoted with silver were prepared by the

Open Access. © 2017 G.G. Volkova, V.A. Paukshtis, published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 License. 60 G.G. Volkova, V.A. Paukshtis dropwise addition of a 0.1 M aqueous solution of cesium CO, the v(CO) of silver carbonyl on Nafion was observed -1 -1 nitrate to the mixture of 0.1 M solutions of H3РW12O40 at 2173 cm [8]. The band at 2188 cm registered for the and silver nitrate taking into account the required ratio. 1%Ag/Cs1.5H1.5PW12O40 catalyst can therefore be assigned The resulting suspension after vigorous stirring for 24 to the formation of cationic silver carbonyl Ag(CO)+ on the hours was evaporated at 100oC to leave the solid. The surface of the acidic cesium salt of 12-tungstophosphoric

Cs1.5H1.5PW12O40 composition of the acidic cesium salt was acid. It is known that solid heteropolyacids and their salts chosen as the most active among other CsxH3-xPW12O40 can exhibit pseudoliquid phase behavior, so the rate of so catalysts [6]. called “bulk-type” reaction may be more than 100 times higher than the rate of “surface-type” reaction [11]. We can assume that the close value of v(CO) at 2186 to 2188 cm-1 + 2.2 In situ FTIR spectroscopy for Ag(CO) in H2SO4 and on the surface of Cs1.5H1.5PW12O40 characterization is connected with the pseudoliquid phase behavior of the acidic cesium salt of 12-tungstophosphoric acid. FTIR spectroscopy in a thermostatted in situ IR cell was applied to determine the formation of carbonyls on the surface of the Ag/Cs1.5H1.5PW12O40 and Cs1.5H1.5PW12O40 catalysts by monitoring the adsorption of CO [9]. The samples were pressed into self supporting wafers 2 containing 30 mg/cm of material and 50% of BaF2. Samples were reduced by in the IR cell at 200oC for 60 min. and then treated with CO (250 torr) at 200oC for 60 min. All the spectra were recorded at 200oC in a CO atmosphere in the range of 400-6000 cm-1 with 4 cm-1 resolution, using a Shimadzu FTIR-8300 spectrometer.

3 Results and discussion

The FTIR spectra of adsorbed CO on unpromoted acidic Fig. 1. FTIR spectra of CO adsorption on Cs H PW O (1) and on Ag/ cesium salts of H PW O (Fig. 1, curve 1) revealed a very 1.5 1.5 12 40 3 12 40 Cs H PW O (2) after 60 min reduction in H at 200oC and admis- -1 1.5 1.5 12 40 2 week band at 2170cm . This band can be attributed to CO sion of 250 torr CO at 200oC. adsorption on the surface of the Cs1.5H1.5PW12O40 catalyst. Ponec et al. assigned the band at 2167 cm-1 to CO adsorbed Previously, for rhodium promoted Cs H PW O catalysts, on 1% Na/WO /Al O catalysts. For pure WO /Al O sample x 3-x 12 40 3 2 3 3 2 3 we have shown that 1) activation of the C-O bond in the the CO stretching band was found at 2193 cm-1[10]. Authors DME and formation of a metal-alkyl bond of this paper explained the difference in IR bands (2193 occurs in the presence of strong Bronsted acid sites and and 2167 cm-1) by various sites of CO adsorption. The band 2) these acid sites act in conjunction with Rh carbonyl at 2193 cm-1 they assigned to CO adsorption on W+6 in WO / 3 complexes, which are responsible for CO insertion and Al O catalyst and the band at 2167 cm-1 to CO adsorbed 2 3 acetate formation [12]. The same mechanism could in the neighbourhood of Na+ ions or on them in 1% Na/ occur in the case of the silver promoted acidic cesium WO /Al O catalysts. In our case we can suggest that 3 2 3 salt of 12-tungstophosphoric acid in halide-free DME CO adsorption occurs on the Cs+ ions in Cs H PW O 1.5 1.5 12 40 carbonylation. catalyst. The low intensity of the band 2170 cm-1 indicates Thus, this study gave us understanding of the reason that only trace amount of CO can be adsorbed on the for the 100% increase in activity of Ag/Cs H PW O surface of the Cs H PW O sample. 1.5 1.5 12 40 1.5 1.5 12 40 catalyst as compared with Ag/Nafion, Ag/ZSM-5 and Ag/ Adsorption of on 1%Ag/ SO /ZrO catalysts (5-25%). It may be associated with the Cs H PW O prereduced at 200oC results in the 4 2 1.5 1.5 12 40 difference of v(CO) in the two catalysts: Ag/Cs H PW O appearance of one band at 2188 cm-1 with high intensity 1.5 1.5 12 40 catalyst having a v(CO) of 2188 cm-1 and Ag/Nafion having (Fig.1, curve 2). Souma et al. have observed the v(CO) in a v(CO) of 2173 cm-1, and it is quite possible that cationic the IR spectra at 2186 cm-1 for Ag(CO)+ in H SO liquid 2 4 silver carbonyl clusters obtained in strong acid media are media. When the solid catalyst Ag/Nafion was exposed to the most active in carbonylation reactions. Carbonylation of Dimethyl Ether on Ag/Cs1.5H1.5PW12O40 61

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