Synthesis of the new framework phosphates and their catalytic activity in ethanol conversion into hydrocarbons

Catalysis Today 193 (2012) 37–41

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Synthesis of the new framework phosphates and their catalytic activity in ethanol conversion into hydrocarbons Margarita M. Ermilova a,∗ , Maxim V. Sukhanov a,b , Roman S. Borisov a , Natalia V. Orekhova a , Vladimir I. Pet’kov b , Svetlana A. Novikova c , Andrej B. Il’in c , Andrej B. Yaroslavtsev a,c a

A.V. Topchiev Institute of Petrochemical Synthesis RAS, Russia N.I. Lobachevsky State University of Nizhni Novgorod, Russia c N.S. Kurnakov Institute of General and Inorganic Chemistry RAS, Russia b

a r t i c l e

i n f o

Article history: Received 29 September 2011 Received in revised form 6 January 2012 Accepted 15 February 2012 Available online 20 March 2012 Keywords: Framework phosphate NASICON Catalyst Dehydration Dehydrogenation Ethanol Hydrocarbons

a b s t r a c t The series of new framework phosphate catalysts was synthesized and their activity in ethanol conversion to hydrocarbons was studied for the first time. The results were summarized for studies of NASICON (NZP) type phosphates formation A1±x Zr2−x Mx (PO4 )3 (where A = H3 O+ , Li+ , Na+ , Cu0.5 + , M = In, аt x = 0, 0.2) with the desired structure and properties controllable by variation of their chemical composition and synthesis parameters. The surface characteristics and catalytic properties of the resulting systems and the yields of the target products formed by ethanol transformations in inert atmosphere were analyzed on the basis of catalysts synthesis conditions. NaZr2 (PO4 )3 was the most active in hydrocarbons C3 + formation. On benzene yield the catalysts formed the row LiZr2 (PO4 )3 {II} ≈ Li1.2 Zr1.8 In0.2 (PO4 )3 {II} > LiZr2 (PO4 )3 {I} > NaZr2 (PO4 )3 {I}. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Synthesis of hydrocarbons from renewable sources, e.g., ethanol, prepared from biomass, is drawn attention of many researchers, which is highly topical in view of the limited world’s oil reserves and global environmental problems. Ethanol dehydration with diethyl ether (DEE) and/or ethylene formation occurs lightly on acid heterogeneous catalysts, particularly on alumina and zeolites. C3 + hydrocarbons, including up to 8–12% aromatic form with substantial yield only on zeolite catalysts with high concentration of Brønsted acid sites [1], but these catalysts activity decreases in time. The last years new catalysts family was developed on the base of complex phosphates of NASICON type, possessing the significant acidity and high thermal and phase stability [2]. These phosphates belong to framework materials, which zeolite-like structure is based on three-dimensional skeleton of corner-sharing PO4 tetrahedra and ZrO6 -octahedra AM2 (PO4 )3 , where A = H, Li, Na, Cu0.5 , H3 O, NH4 ; M = Zr, Fe, Cr, Ti, Hf, Sn, Ge and others [3] and include two kinds of cavities, which are populated by cations or

∗ Corresponding author. E-mail address: [email protected] (M.M. Ermilova). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2012.02.029

vacant. The fast ionic conductivity of these materials is due to migration out of framework cations in the interconnected channels. Thus, NASICON structures can be considered under certain conditions as nanoporous membranes. Although sodium containing NASICONs are extensively studied, their protonated forms are few reported despite their importance [4–6]. Wide variety of isomorphic ion substitutions at all crystallographic positions of their structure causes the multiplicity of local characteristics (polarity, Lewis acidity etc.) of framework sites and cavities. As result, the framework phosphates may show the basic, acidic and redox properties and are applicable as the catalysts of different organic reactions. NASICONS’s were drawn attention as active and stable catalysts of alcohol’s dehydration [7–12], dehydrogenation [13], paraffin isomerization and oxidation [14]. In our previous study [15] the conditions were found for effective dehydration of methanol and ethanol to respective ethers, which were shown to be the intermediates at C3 + and aromatic hydrocarbons formation. There was shown the possibility of preparation of composite membrane catalysts (CMC), in which good qualities of traditional heterogeneous catalysts are combined with the separation capabilities of membranes [16–18], on the base of alumosilicates [19] or framework phosphates of NASICON type [20,21]. The selective permeability of alumosilicate membrane for steam gave methanol

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M.M. Ermilova et al. / Catalysis Today 193 (2012) 37–41

conversion into dimethyl ether in the membrane reactor 82.5% in comparison with 68.0% for tradition reactor [19]. In paper [20] NASICON membrane with steam permeability about 5 L/h m2 Bar was prepared from Na3 Ti2 Si2 PO12 by deposition to Moroccan clay. Fe0.33 Zr2 (PO4 )3 CMC was prepared in [21] by magnetron sputtering of NASICON on porous stainless steel sheet and tested in oxidative dehydrogenation of methanol. CMC was more active and selective for formaldehyde formation than the powder sample of Fe0.33 Zr2 (PO4 )3 in the same conditions. The selective removal of hydrogen through the similar membranes might give the promising control on the ethylene and C3 + hydrocarbons relation in the products of ethanol transformation. The aim of present work was the directed synthesis of acid framework phosphates AZr2 (PO4 )3 of NASICON type, which might be the precursors of CMC, and their approval as the catalysts of ethanol transformation into hydrocarbons C3 + .

1

2

10

20

30

40

50

60

2Θ

2. Experimental

Fig. 1. X-ray diffraction patterns of Li1.2 Zr1.8 In0.2 (PO4 )3 (1) and LiZr2 (PO4 )3 (2) powders.

2.1. Synthesis of the catalysts The samples of new NZP-phosphates of general formula AZr2 (PO4 )3 (A = H, Li, Na, Cu0.5 ) were prepared by solid phase, hydrothermal or sol–gel methods and characterized using XRD, IR-spectroscopy and microprobe analysis. Simple phosphates with NASICON structure – NaZr2 (PO4 )3 , Cu0.5 Zr2 (PO4 )3 and LiZr2 (PO4 )3 were obtained by traditional sol–gel method, denoted as {I}. Water solutions of ACl, ZrOCl2 and H3 PO4 were mixed in the stoichiometric amount. The precipitates were dried at 80 ◦ C and thermally treated at 250, 600 or 800 ◦ C with intermediate regrinding. LiZr2 (PO4 )3 and Zr-substituted Li1.2 Zr1.8 In0.2 (PO4 )3 were obtained by Pechini method [22] and denoted as {II}. ZrOCl2 , InCl3 , citric acid, NH4 H2 PO4 and LiCO3 were dissolved in ethylene glycol at 60 ◦ C with constant stirring. Then the mixture was dried at 150 ◦ C, 300 ◦ C and 650 ◦ C with intermediate regrinding. As a result the nanodispersed powders were obtained. The carbon content in the product does not exceed 2 wt%. (H3 O)Zr2 (PO4 )3 samples were obtained by hydrothermal method {III} or by ion exchange method {IV}. For (H3 O)Zr2 (PO4 )3 {III} synthesis 1 M water solutions of NH4 H2 (PO4 )3 and ZrOCl2 were mixed and heated in the autoclave at 250 ◦ C during 20 h. The obtained precipitate was annealed at 500 ◦ C during 72 h and boiled in purified water 36 h. The (H3 O)Zr2 (PO4 )3 {IV} sample was prepared from preliminary synthesized and annealed at 800 ◦ C Cu0.5 Zr2 (PO4 )3 by the treating by the solution of 0.5 M HCl with following washing by water and annealing at the temperature of 400 ◦ C.

The specific surface areas of the samples were determined by BET method on adsorption of nitrogen in textural characteristics analysator ATX-06 Catacon. 2.3. Catalytic activity tests Ethanol transformations on the framework phosphate catalysts were investigated in conventional plug-flow system at atmospheric pressure and at the temperature from 220 ◦ C to 550 ◦ C with on-line gas chromatography and chromatomass-spectrometry. The identification of compounds, chemisorbed on the catalysts, was made by a method of programmed mass spectral thermodesorption (PMSTD). The catalyst samples of mass 0.300 г was mixed with quartz particles (average particle size 0.5 mm). Ethanol vapors were fed from thermostated bubbler to reactor in flow of argon (helium) carrier-gas with space velocity of 1.2 L/h. The reaction products were analyzed on an LKhM-5 chromatograph with a thermal conductivity detector, columns packed with activated charcoal and Porapak T, and computer processing of chromatograms. From the results of the analysis, the conversion of the alcohol x, selectivity S, and the yield of target products Y(%) and A–space time yield of products (mmol/g h) were calculated by the formulas: x=

ϕ0 − ϕ1 ϕ0

(1)

S=

ϕi ϕ0 − ϕ1

(2)

Y = xS100,

(3)

A= 2.2. Characterization of the catalysts The chemical composition and homogeneity of samples of the phosphates synthesized were monitored using a CamScan MV2300 (VEGA TS 5130MM) scanning electron microscope with YAG detectors of secondary and reflected electrons and a Link INCA ENERGY 200C energy-dispersive X-ray microanalyzer with a semiconductor Si(Li) detector. The PAP correction method was used to calculate the composition. The accuracy of determination of sample compositions was 2 at.%. The microprobe analysis of the phosphates has shown their homogeneity and correspondence to the theoretical composition. X-ray powder diffraction experiments were carried out by means of Rigaku D/MAX 2200 diffractometer with Ni-filtered Cu K␣ radiation, 2␪/␪ geometry and using a diffracted beam curved monochromator.

FxS . W

(4)

Here ϕ0 and ϕ1 are initial and final ethanol concentrations, ϕi is the part of ethanol consumed on reaction product i, F is feed rate of the alcohol vapor (mmol/h), and W is the mass of a catalyst (g). 3. Results and discussion The chemical composition and uniformity of the synthesized phosphates was confirmed according to SEM and the EDS X-ray microanalysis. The obtained compounds are homogeneous and belong to the NASICON structure type. Some X-ray diffraction patterns are shown in Fig. 1. The solid phase synthesis method [LiZr2 (PO4 )3 {I}] gave the materials of NASICON structure, crystallizing in monoclinic and orthorhombic syngony in relation 1:3. The samples contain no more

M.M. Ermilova et al. / Catalysis Today 193 (2012) 37–41

39

Table 1 Unit cell parameters and particle size of framework phosphate studied. Material

a (Å)

b (Å)

c (Å)

Particle size (nm)

NaZr2 (PO4 )3 I (solid phase method) LiZr2 (PO4 )3 I (solid phase method) LiZr2 (PO4 )3 II (Pecini method) Li1.2 Zr1.8 In0.2 (PO4 )3 II (Pecini method) (H3 O)Zr2 (PO4 )3 III (Hydrothermal method) (H3 O)Zr2 (PO4 )3 IV (Ion exchange)

8.811(1)

–

22.750(7)

>100

8.816(5) 8.863(14) 8.839(25)

8.935(3) 8.948(12) 8.933(7)

12.368(9) 12.423(20) 12.416(19)

>100 50–100 50–100

8.762(9)

–

23.711(1)

>100

13.401(5)

8.976(3)

8.844(3)

150–250

than 5% of Zr2 P2 O7 admixture. It is explained by low sintering temperature that was selected in order to obtain low particles size. The samples [(LiZr2 (PO4 )3 {II} and Li1.2 Zr1.8 In0.2 (PO4 )3 {II}] prepared by Pecini method had the NASICON structure and were crystallized in orthorhombic syngony (Fig. 1). The samples (H3 O)Zr2 (PO4 )3 {III} and {IV} crystallize in hexagonal syngony. Initial Cu0.5 Zr2 (PO4 )3 for (H3 O)Zr2 (PO4 )3 {IV} preparation was prepared by sol-gel method. Phosphate (H3 O)Zr2 (PO4 )3 {IV} contains up 20% of Cu0.5 Zr2 (PO4 )3 due to incomplete ion exchange during its preparation. The unit cell parameters of synthesized framework phosphates are shown in Table 1, as well as the size of particles, calculated from the X-ray data. The particle size depends strongly on the synthesis method. Thus, Pecini method application gives much smaller particles than that of solid phase method owing to forming of particles in rigid polymer matrix, which limits their size, and lower temperature of treatment. The specific surface area of the prepared samples depends both on the method of preparation (Table 2) and the temperature of samples synthesis. Pecini method gives the higher specific area. The minimal surface area was obtained at the synthesis temperature of 800 ◦ C. It can be mentioned that the increase in synthesis temperature from 600 ◦ C to 1000 ◦ C for NaZr2 (PO4 )3 results in the specific surface area decrease from 63 m2 /g to 8 m2 /g [23]. The synthesized framework phosphates are shown to be the active catalysts of ethanol transformations into hydrocarbons. At the temperatures below 300 ◦ C the basic products of ethanol conversion are hydrocarbons C2 and diethyl ether (DEE). The further temperature increasing up to 400 ◦ C causes the C3 –C6 hydrocarbons formation depending on the type of cations in phosphate structure. The Fig. 2 presents the temperature dependences of ethanol conversion on NaZr2 (PO4 )3 {I} and Li-containing catalysts, prepared by different methods. All samples gave practically the complete ethanol conversion. However, the temperature intervals of process depend strongly on the particle size (surface area) and on catalyst composition. Thus, in the case of NaZr2 (PO4 )3 {I} and LiZr2 (PO4 )3 {I}, obtained by solid phase method, the temperatures of process beginning were much higher than that for catalysts, prepared by Peceni method (curves 3 and 4, Fig. 2). This fact may be explained by above-mentioned higher specific area of samples, prepared by

X

1.0

0.8

0.6

0.4

0.2

4

2

1

3

T, 0C

0 200

250

300

350

400

450

500

550

Fig. 2. Ethanol conversion vs temperature for the catalysts Li1.2 Zr1.8 In0.2 (PO4 )3 (1); LiZr2 (PO4 )3 II (2); NaZr2 (PO4 )3 (3) and LiZr2 (PO4 )3 I (4).

Peceni method. Partial zirconium substitution in LiZr2 (PO4 )3 {II} by indium has decrease the temperature of the process beginning as well as the temperature of complete ethanol conversion reaching (ref. curves 1 and 2, Fig. 2). The distribution of reaction products is determined by the particle size and catalyst’s composition. The selectivity of NaZr2 (PO4 )3 {I} and LiZr2 (PO4 )3 {I} phosphates, prepared by solid phase method, are compared in Fig. 3. In case of NaZr2 (PO4 )3 the process starts from DEE formation at 250 ◦ C (curve 1, Fig. 3) and the following ethylene formation (curve 3, Fig. 3). At the temperatures higher than 350 ◦ C hydrocarbons C3 + are appeared. The yield of C3 + is shown in Table 2. On the LiZr2 (PO4 )3 {I} catalyst DEE formed only at the temperatures from 300 ◦ C up to 320 ◦ C (curve 2 Fig. 3). Then only ethylene is formed at the temperature range 330–450 ◦ C (curve 4 Fig. 3). Low C3 + hydrocarbons amounts are appeared after 450 ◦ C (curve 6 Fig. 3). Li-containing samples, prepared by Pecini method, have showed the high selectivity in relation of C3 + and benzene formation. As Fig. 4 shows, that the basic products of the process on LiZr2 (PO4 )3 {II} at the temperature range lower than 330 ◦ C were DEE and

S

1

2

1 4

0.8 Table 2 Temperature of synthesis and characteristics of materials studied. Material

NaZr2 (PO4 )3 I LiZr2 (PO4 )3 I LiZr2 (PO4 )3 II Li1.2 Zr1.8 In0.2 (PO4 )3 II (H3 O)Zr2 (PO4 )3 III (H3 O)Zr2 (PO4 )3 IV

Temperature of synthesis (◦ C) 600 800 650 650 400 600

Specific surface area (m2 /g) 63 4 44 53 10 10

± ± ± ± ± ±

1 2 2 2 2 1

C3 + hydrocarbons yield (A) at 500 ◦ C (mmol/g h) 2.3 0.2 2.2 1.8 – 1.7

3

0.6 0.4

5

0.2

6

0 250

300

350

400

450

500

T, 0C 550

Fig. 3. The temperature dependences of catalysts selectivity (S) toward DEE–curves 1 and 2; ethylene–curves 3 and 4; hydrocarbons C3 + –curves 5 and 6 for the samples of NaZr2 (PO4 )3 {I} (curves 1,3 and 5) and LiZr2 (PO4 )3 {I} (curves 2,4 and 6).

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M.M. Ermilova et al. / Catalysis Today 193 (2012) 37–41

7

S 1.0

S

1

3

2

4

0.8

0.8

3 0.6

0.6

4

0.4

0.4

5

6

0.2

2 230

280

330

380

430

480

0

530

+

1.0 0.8 0.6

2

1

250

300

350

400

450

500

Fig. 6. The temperature dependences of catalysts selectivity (S) toward DEE–1, 2; ethylene–3 and 4; hydrocarbons C3 + –5 for the samples of H3 OZr2 (PO4 )3 I (1 and 3) and, H3 OZr2 (PO4 )3 II (2,4 and 5).

may be explained by the known dehydrogenation activity of Cu0.5 Zr2 (PO4 )3 {I} [13], remainder in (H3 O)Zr2 (PO4 )3 {IV} after ionic exchange. The more detail pattern of hydrocarbon products, formed at the temperatures 450–500 ◦ C, studied by chromatomassspectrometric analysis, showed that NaZr2 (PO4 )3 {I} and Licontaining NASICONs are active, in C3 –C6 paraffins and olefins formation. It is found also the alicyclic hydrocarbons C5 –C6 and aromatics. Fig. 7 shows benzene yield on the different framework phosphates studied in this work. Phosphates, prepared by Pecini method, are the most selective toward aromatics formation. This fact can be explained by the lower catalyst’s particles size, formed at catalyst preparation by this method (Table 1). On benzene yield the catalysts formed the row LiZr2 (PO4 )3 {II} ≈ Li1.2 Zr1.8 In0.2 (PO4 )3 {II} > LiZr2 (PO4 )3 {I} > NaZr2 (PO4 )3 {I}

It is interesting that only Li1.2 Zr1.8 In0.2 (PO4 )3 gave besides aromatics remarkable quantities of C4 –C6 dienic hydrocarbons, point out to peculiar process of dehydration–dehydrogenation on this catalyst. For details of this process it is need the chromatomass–spectometric study in wide temperature interval. The catalysts NaZr2 (PO4 )3 {I}, (H3 O)Zr2 (PO4 )3 {IV} and Li1.2 Zr1.8 In0.2 (PO4 )3 {II} were the most active in hydrocarbons C3 + formation (last column of Table 2). Most probably, the phosphates’ activity in process of ethanol conversion is determined by Zr+4 cations, which are present on the catalyst’s surface. They have the highest charge and polarized activity, and so they can be probably the sites for alcohol adsorption. However the nature of M+ cation in the matrix as well as their

8 7

Вenzene yield, Y, %

C2 H4 . Hydrocarbons C3 were obtained at the higher temperatures, achieving the selectivity about 25 at.%. The peculiarity of indium doped catalyst Li1.2 Zr1.8 In0.2 (PO4 )3 {II} was the formation of acetaldehyde already at the temperature of 230 ◦ C simultaneously with appearance of DEE. This fact may be assigned to improvement of LiZr2 (PO4 )3 II catalytic properties by zirconium partial substitution by indium. In this case, the In doping cause the additional incorporation of lithium ions in M2 position where their mobility is significantly higher [24]. The Fig. 5 presents the temperature dependence of ethanol conversion on (H3 O)Zr2 (PO4 )3 {III} and (H3 O)Zr2 (PO4 )3 {IV}. It is evident that catalyst prepared by hydrothermal method is more active than that one prepared by ionic change (curves 1 and 2, Fig. 5). Since the specific surface area for both (H3 O)Zr2 (PO4 )3 samples did not differ within the error (Table 2), this result may be explained by incomplete replacement of copper in the sample (H3 O)Zr2 (PO4 )3 {IV}. In this case the mobility of H3 O+ decreases strongly [25]. It is logical to assume that the process of ethanol transformation on framework phosphates takes place through the mechanism of acid catalysis. Therefore, the use of materials with protoncontaining groups could lead to results, better than that of Li- and Na-containing catalysts. However all these catalysts gave the similar pattern of temperature dependence of ethanol conversion (refer curves 2–4 of Fig. 2 and curves of Fig. 5). The products distribution on the (H3 O)Zr2 (PO4 )3 {III} catalyst had some peculiarities. It gave the practically 100% yield of ethylene at the temperatures as high as 400 ◦ C (Fig. 6 curve 3), i.e. this catalyst, prepared by hydrothermal method, shows only dehydration activity. On the (H3 O)Zr2 (PO4 )3 {IV} sample up to 22% of hydrocarbons C3 + were formed besides ethylene at temperatures as high as 340 ◦ C (Fig. 6 curves 4 and 5). This difference between (H3 O)Zr2 (PO4 )3 {III} and (H3 O)Zr2 (PO4 )3 {IV} catalysts

T, 0C

0

580

Fig. 4. The temperature dependences of catalysts selectivity (S) toward DEE–1, 2; ethylene–curves 3 and 4; hydrocarbons C3 –C6 (curves 5 and 6) and acetaldehyde (curve 7) for the catalysts LiZr2 (PO4 )3 {II} (curves 1,3 and 5) and Li1.2 Zr1.8 In0.2 (PO4 )3 {II} (curves 2,4,6 and 7).

0.4

5

0.2

T, C

0

X

1

1.0

6 5 4 3 2 1

0.2

T, 0C

0 200

250

300

350

400

450

500

550

Fig. 5. Ethanol conversion vs temperature for the catalysts H3 OZr2 (PO4 )3 {III} (curve 1) and H3 OZr2 (PO4 )3 {IV} (curve 2).

0

1

2

3

4

Fig. 7. Benzene yields (Y) at the temperature 500 ◦ C for the catalysts NaZr2 (PO4 )3 (1), Li1.2 Zr1.8 In0.2 (PO4 )3 (2), LiZr2 (PO4 )3 I (3) and LiZr2 (PO4 )3 II (4).

M.M. Ermilova et al. / Catalysis Today 193 (2012) 37–41

mobility also essentially determine the rate and the direction of catalytic processes. 4. Conclusions The series of framework phosphates of NASICON type was synthesized by different methods and their activity in ethanol conversion to hydrocarbons was studied for the first time. It was shown that these materials are promising catalyst of hydrocarbons production from ethanol. At the temperatures as high as 400 ◦ C the selectivity to hydrocarbons up to 95% was achieved, the product distribution being depended on the type of NASICON-cation and catalyst’s synthesis method. Acknowledgments The authors acknowledge the support of this work by RFBR (Project 09-03-00579) and by Presidium of RAS (Project no 3). References [1] V.R. Choudhary, V. Nayak, Zeolites 5 (1985) 325. [2] M.V. Sukhanov, M.M. Ermilova, N.V. Orekhova, G.F. Tereshchenko, V.I. Petkov, I.A. Shchelokov, Bull. Lobachevski Nizegorodski Univ. (2007) 89–94. [3] V.I. Pet’kov, G.I. Dorokhova, A.I. Orlova, Kristallografiya 46 (2001) 76–81. [4] M.A. Subramanian, B.D. Roberts, A. Clearfield, Mater. Res. Bull. 19 (1984) 1471–1478. [5] P.R. Rudolf, M.A. Subramanian, A. Clearfield, J.D. Jorgensen, Solid State Ionics 17 (1985) 337–342.

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