dimethyl ether synthesis over novel silicotungstic acid incorporated nanostructur catalysts

INTERNATIONAL JOURNAL OF CHEMICAL

REACTOR ENGINEERING

Volume 8 2010 Article A45

Dimethyl Ether Synthesis over NovelSilicotungstic Acid Incorporated

Nanostructured Catalysts

Aysegul Ciftci∗ Dilek Varisli†

Timur Dogu‡

∗Middle East Technical University, [email protected]†Gazi University, [email protected]‡Middle East Technical University, [email protected]

ISSN 1542-6580Copyright c©2010 The Berkeley Electronic Press. All rights reserved.

Dimethyl Ether Synthesis over Novel SilicotungsticAcid Incorporated Nanostructured Catalysts∗

Aysegul Ciftci, Dilek Varisli, and Timur Dogu

Abstract

Dimethyl ether (DME), which is an excellent green diesel-fuel alternate withexcellent clean burning properties, is synthesized by dehydration of methanol overnovel solid acid catalysts, which are synthesized following a direct hydrothermalroute and using silicotungstic acid (STA) as the active compound. These meso-porous silicate structured catalysts have surface area values of 108-393 m2/g, de-pending upon their W/Si ratio. These catalysts showed very high methanol de-hydration activity and also very high DME selectivity values, approaching 100%.The STA-SiO2 mesoporous nanocomposite catalyst having a W/Si atomic ratio of0.33 showed the highest activity, with a DME selectivity over 99% and a methanolconversion over 60%, at 250◦C and at a space time of 0.27 s.g.cm−3. Effects ofW/Si atomic ratio, calcination temperature and the synthesis procedure on thecatalytic performance of these novel mesoporous catalytic materials were investi-gated.

KEYWORDS: DME, methanol, dehydration, silicotungstic acid, mesoporouscatalyst

∗METU Research Fund and TUBITAK grants are gratefully acknowledged. We thank Assist.Prof. Dr. Emrah Ozensoy from Bilkent University Chemistry Department for the EDX analyses,and Assoc. Prof. Dr. Naime A. Sezgi for her contributions.

1. INTRODUCTION

Fast increase of the rate of oil consumption and related environmental problems necessitated the development of sustainable alternative fuels. Being clean energy carriers, alcohols and ethers are considered as potential transportation fuel alternates. Dimethyl ether (DME) is an attractive transportation fuel substitute for compression ignition engines, in the sense that it has higher cetane number (55-60) than diesel fuel (40-55) and it does not produce black smoke. NOx emissions of DME derived diesel engines are also very low. DME is considered as a non-toxic and environmentally benign fuel, which can be produced from non-petroleum feedstocks (Dogu and Varisli, 2007; Olah et al., 2006).

DME can be synthesized by dehydration of methanol over solid acid catalysts. Methanol dehydration activity of different acidic catalysts, such as γ-Al2O3 (Yaripour et al, 2005; Raoof et al., 2008; Jun et al., 2002; Tokay, 2008), H-ZSM-5 (Fu et al., 2005), mesoporous aluminosilicates (Tokay, 2008; Varisli et al., 2009) and Nafion-silica nanocomposites (Ciftci et al., 2010) were tested in the literature. These catalysts were reported to show activity at different temperature ranges. γ-alumina and aluminosilicate type catalysts were reported to show good catalytic performance in a temperature range between 300-370oC. However, there are also some results reported at higher (up to 400oC) and lower temperatures. Our recent work with Nafion based nano-composite catalysts (Ciftci et al., 2010) showed good methanol dehydration activity at temperatures lower than 300oC. Green and sustainable aspects of heteropolyacid (HPA) catalysts are mentioned in the work of Misono (2000), by referring to some of their features, such as non-corrosiveness, no-waste production and pseudo-liquid phase behavior. The most important property of HPA’s is their high acidity, due to the high concentration of Brønsted acid sites in their structure (Okuhara et al., 1996). It was shown in our recent study that (Varisli et al., 2007); activity of silicotungstic acid (STA) was much higher than the activities of tungstophosphoric acid and molybdophosphoric acid, in ethanol dehydration reaction to produce ethylene. Their very low surface area values (1-5 m2/g) and high solubility in polar solvents limit the wide use of HPA’s as catalysts.

High surface area mesoporous materials are quite suitable for immobilization of active species in their framework. Development of silicate structured mesoporous solids (MCM-41, MCM-48, SBA-15 etc.) opened a new area in catalysis research (Taguchi and Schüth, 2005). Metals or metal oxides are generally incorporated into such mesoporous materials to increase their catalytic performance (Sener et al., 2006; Nalbant et al., 2008; Ozdogan et al., 2008). Said et al. (2007) and Vazquez et al. (2000) synthesized HPA/SiO2 catalysts by impregnation. A one-pot procedure was developed in our recent work (Varisli et al., 2009), for the synthesis of STA incorporated silicate structured mesoporous

1Ciftci et al.: DME Synthesis over Silicotungstic Acid-SiO2 Mesoporous Catalysts

Published by The Berkeley Electronic Press, 2010

catalysts. These catalysts and STA impregnated MCM-41 showed excellent catalytic performance in dehydration of ethanol (Varisli et al., 2008, 2009) to produce ethylene. In the present study, STA incorporated silicate structured catalysts were synthesized following a modified one-pot hydrothermal route and the catalysts containing different W/Si molar ratios in their structure were tested in the dehydration reaction of methanol to produce DME.

2. EXPERIMENTAL 2.1. Catalyst Synthesis and Characterization In this study, novel silicotungstic acid (STA) incorporated nano-composite catalysts were synthesized following a one-pot hydrothermal synthesis route. STA incorporated silicate structured mesoporous catalysts TRC-62(L), TRC-82(L) and TRC-92(L), containing different W/Si atomic ratios, were synthesized in our earlier study (Varisli et al., 2009, 2010). Following a modified procedure, new catalysts with a W/Si atomic ratio of 0.40, were synthesized in the present study. In the synthesis of this material, cetyltrimethylammonium bromide was used as the surfactant and TEOS (tetraethylorthosilicate) was used as the silica source. Predetermined amount of STA was dissolved in deionized water and added to the solution of TEOS and the surfactant. After mixing for 1 hour, the pH of the mixture was measured as about 1.0 and this solution was transferred into a Teflon-lined stainless steel autoclave, where it was kept at 120ºC for 96 hours. Hydrothermal synthesis was carried out under autogenous pressure in the fully sealed autoclave. The resulting material was washed successively with deionized water, until the pH of the washing liquid became constant. The solid product was dried at 40ºC and calcined in a tubular reactor in a continuous flow of dry air, at 350ºC (denoted as TRC-75(L)) or at 400ºC (denoted as TRC-75-400) for 8 hours. It was shown in our earlier study of STA incorporated mesoporous catalysts (TRC-62(L), TRC-82(L) and TRC-92(L)) that, calcination temperature had very important effects on the structure and on the catalytic performance of these materials. Significant loss of Brønsted acidity and consequently the activity was reported at calcination temperatures higher than 400oC. As an alternative technique, in the present work supercritical carbon dioxide extraction was used for surfactant removal from the material, which was synthesized by the one-pot hydrothermal route. Extraction was carried out at 100ºC, 350 bar and with a CO2 flow rate of 1mL/min, for 3 hours. Considering that the maximum reaction temperature of the catalytic performance tests was 350°C, the synthesized material was heated up to this temperature in the presence of dry air, after the supercritical CO2 extraction step. This material was denoted as TRC-75(L)-CO2.

2 International Journal of Chemical Reactor Engineering Vol. 8 [2010], Article A45

http://www.bepress.com/ijcre/vol8/A45

Scanning Electron Microscopy (SEM) analyses of the synthesized catalysts were performed in Middle East Technical University (METU) Central Laboratory, by a Quanta 400F Field Emission SEM instrument, and Energy Dispersive Spectroscopy (EDS) analyses were done in METU Metallurgical and Materials Engineering Department by a JSM-6400 (JEOL) instrument equipped with NORAN System. Energy Dispersive X-Ray (EDX) elemental mapping analyses were performed in the Bilkent University Chemistry Department. SEM and EDX data were collected using a Zeiss EVO40 environmental SEM, that is equipped with a LaB6 electron gun, a vacuum SE detector, an elevated pressure SE detector, a backscattering electron detector (BSD), and a Bruker AXS XFlash 4010 detector. In order to get information about the relative intensities of Brønsted and Lewis acid sites of the synthesized catalysts, diffuse reflectance FT-IR spectroscopy (DRIFTS) analyses of pyridine adsorbed samples were performed by using a Perkin Elmer Spectrum One instrument. X-Ray Diffraction (XRD) analyses were made using the Rigaku D/MAX2200 diffractometer with a CuK radiation source, within a 2θ scanning range between 1-50°. Fourier transform infrared (FT-IR) spectroscopy analyses of the STA-silicate structured catalysts in the present work were performed by using a Bruker FTIR-IFS66/S instrument. Nitrogen adsorption-desorption analyses were carried out by a Quantachrome Autosorb-1-C/MS instrument, in the METU Central Laboratory. Multipoint BET surface area values, pore size distributions and pore volumes of the samples were analyzed by this characterization technique. XPS analysis of the used and the fresh catalysts was made with a SPECS instrument, also in the central laboratory of Middle East Technical University. Supercritical CO2 extraction was carried out with a SFX 3560 extractor. The synthesized STA incorporated nano-composite materials were also characterized by a Scanning-Transition Electron Microscopy (STEM) and Energy-Filtered Transmission Microscopy (EFTEM) analyses, by a Jeol 2100F instrument attached with Jeol EDX & HAADF detectors and a GIF Tridem STEM Pack filter. 2.2. Methanol dehydration reactions Methanol dehydration reactions were carried out in a flow system between 180-350ºC, with the TRC-62(L), TRC-75(L), TRC-82(L) and TRC-92(L) catalysts. Catalyst was placed into the stainless steel tubular reactor and heated up to the reaction temperature under continuous flow of helium. Methanol was first fed to an evaporator at a flow rate of 2.1mL/hr, by using a syringe pump. It was mixed with He gas in the evaporator to adjust the composition of the reactor feed stream. The volume fraction of methanol was adjusted as 0.48 in this stream, in the experiments carried out in the present study. Online analysis of the product stream was performed with a Varian CP 3800 gas chromatograph, equipped with a



Porapak T column. All the lines were heated to 150ºC, in order to avoid any condensation of reactants or products. Catalytic activity test experiments were carried out at different space times (at 0.14, 0.27 and 0.41 s.g.cm-3) by changing the amount of catalyst placed into the tubular reactor (0.1-0.3 g). 3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization Results The mesoporous catalytic materials synthesized in this work are composed of a porous silicate network, with STA being well dispersed in the structure as the active acidic component. Characterization results of TRC-62(L), TRC-82(L) and TRC-92(L) were reported elsewhere (Varisli et al., 2009, 2010). STEM and XRD analysis had indicated the presence of WOx nanorods dispersed within the mesoporous silicate matrix of the catalysts synthesized in our earlier study (Fig. 1). However, XRD analysis of the catalysts, which were synthesized in the present work in a sealed autoclave, did not show any sharp peaks corresponding to large WOx crystals (Fig. 2). This XRD spectrum indicated well dispersion of tungsten in the silicate lattice of TRC-75(L) catalyst. This XRD spectrum is different from the XRD spectra of former catalysts (TRC-62(L), TRC-82(L) and TRC-92(L)), for which XRD analysis had indicated sharp peaks corresponding to W20O58. This difference is thought to be due to the modification of the hydrothermal synthesis procedure used in the present study. In this work, synthesis of the new catalysts was achieved in a fully sealed autoclave, while in the previous synthesis procedure; hydrothermal synthesis had been achieved in an open vessel at atmospheric pressure.

Figure 1. TEM image of TRC-82(L).

Figure 2. XRD pattern of TRC-75(L).

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Physical properties of the catalysts used in this study in DME synthesis are given in Table 1. EDS analysis results indicated that STA was successfully incorporated into the structure of TRC-75(L). From the elemental mapping analysis of the sample (Fig. 3) it can be concluded that W and Si are very well dispersed in the nano-structured material. According to the results obtained from nitrogen physisorption analyses, pore volumes were in the range 0.47-0.37 cm3/g and average pore diameters were 7.8 nm, for the TRC-75 type catalytic materials (Table 1).

Table 1. Physical properties of STA incorporated silicate structured catalysts

Catalyst W/Si

atomic EDS

W/Si atomic Soln.

Multipoint BET Surface Area

(m2/g)

Pore volume (cm3/g)

Avg pore diameter

(nm) TRC-75(L) 0.33 0.40 252 0.37 7.8

TRC-75(L)-CO2 0.33 0.40 187 0.32 7.8 TRC-75-400 0.40 241 0.47 7.8

TRC-62(L) (*) 0.16 0.25 393 0.55 5.5 TRC-82(L) (*) 0.47 0.50 179 0.45 10.0 TRC-92(L) (*) 0.78 1.00 108 0.21 7.8

(*) Adapted from Varisli et al. (2009).

Figure 3. EDX mapping of TRC-75(L).

Nitrogen adsorption-desorption isotherms (Fig. 4) are Type IV, implying a

mesoporous structure. Physical properties of the supercritical CO2 treated catalyst (TRC-75(L)-CO2) were quite close to the physical properties of TRC-75(L).



Figure 4. Nitrogen adsorption-desorption isotherm of TRC-75(L).

STA incorporated catalysts synthesized in this work were calcined at 350ºC. At temperatures above 400°C, protons of STA in the nano-composite materials were reported to be lost (Varisli et al., 2009). Calcination of the synthesized material at 400ºC caused an increase in the pore volume (Table 1). Also, the pore size distribution of TRC-75-400 was broader than the pore size distributions of TRC-75(L) and TRC-75(L)-CO2 (Fig. 5).

Figure 5. Pore size distributions of TRC-75(L), TRC-75(L)-CO2 and TRC-75-400.

Morphologies of the samples were observed by scanning electron

microscopy (SEM) images. SEM image of TRC-75(L) is given in Fig. 6. SEM image of TRC-75-400 (Fig. 7) showed the formation of rod-like structures on the

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surface of the catalyst. Such a structure was also observed in our earlier work

(Varisli et al., 2009) for a STA/silica catalyst containing a W/Si ratio of 0.47. Results of FT-IR analysis of pure STA showed characteristic IR peaks at

780 cm-1 (W-O-W), 876 cm-1 (W-Ocorner-W), 921 cm-1 (Si-O) and 977 cm-1 (W=O) (Fig. 8). FT-IR spectra of TRC-75(L) and TRC-75(L)-CO2 also showed these bands (Fig. 8) at the same wave numbers. Although some decrease of intensities of these bands was observed, they were still quite sharp in the FT-IR spectra of the STA incorporated mesoporous materials. This decrease of intensity of the FT-IR bands is essentially due to the low STA content of the synthesized materials. Especially the band corresponding to W-Ocorner-W was quite weak in the synthesized materials. In the case of TRC-75-400, which was calcined at 400oC, the bands corresponding to the characteristic STA structure were not as sharp as the corresponding bands observed in the spectra of TRC-75(L) and TRC-75(L)-CO2. This result is an indication of some distortion of the STA structure during calcination at 400oC. Similar distortions were observed in our earlier studies (Varisli et al., 2009, 2010) for TRC-62, TRC-82 and TRC-92 type catalytic materials, which were calcined at temperatures higher than 350oC. FT-IR results obtained in this study indicated that the STA structure was not distorted within the lattice of the synthesized material which was calcined at 350oC and/or treated with supercritical CO2.

Figure 6. SEM image of TRC-75(L). Figure 7. SEM image of TRC-75-400.



Figure 8. FTIR spectra of TRC-75(L), TRC-75-400, TRC-75(L)-CO2 catalysts; and pure STA adapted from Varisli et al. (2009).

DRIFTS analysis of the pyridine adsorbed samples gave IR absorption bands at 1539, 1488 and 1445 cm-1 (Fig. 9). The first band at 1539 cm-1 corresponds to pyridinium ion and shows the presence of Brönsted acid sites on the catalyst surface. The third band is associated with the adsorbed molecules on the Lewis acid sites (Damyanova et al., 1999). The second band corresponds to another form of adsorbed pyridine molecules. For TRC-75(L), all of these bands were observed. However it is clear that Brønsted acid sites were stronger than Lewis acid sites. As stated by Herrera et al. (2008) and also as reported in our earlier publications (Ciftci et al., 2010; Varisli et al., 2009), presence of Brønsted acid sites is the major indication of the activity of the catalysts in alcohol dehydration reactions. As originally proposed by Bandiera and Naccache (1991) and later supported in the theoretical analysis of Blaszkowski and Van Santen (1996), DME formation on zeolite type catalytic materials was expected to take place by the reaction of the [CH3.OH2]+ and [CH3O]- surface species, which were adsorbed on the Brønsted acid and on the adjacent Lewis basic sites. Besides the Brønsted acidity, surface area is also expected to contribute to the activity of such solid acid catalysts in dehydration of alcohols. As shown in Figure 9, the Brønsted acidity of the material prepared by the supercritical CO2 extraction process (TRC-75(L)-CO2) was even stronger than the Brønsted acidity of TRC-75(L).

The results of the characterization studies indicated that, by the synthesis of the nanocomposite catalysts following a one-pot hydrothermal route, catalytic properties of STA were enhanced by increasing its surface area to around 200

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m2/g. Acidic characteristics of STA were preserved. Also, these catalysts did not lose their acidic character even after successive washing steps.

3.2. Methanol Dehydration Results

Methanol dehydration reactions were carried out in vapour phase. The observed products were dimethyl ether and formaldehyde, which were produced according to the following dehydration and dehydrogenation reactions:

OHOCHCHOHCH 23332 +⎯→← (1)

223 HOCHOHCH +⎯→← (2)

Typical methanol conversion data obtained with the catalysts containing different W/Si ratios are shown in Fig. 10. Results reported in this figure correspond to the average of at least three data points obtained during the first hour of operation at a given temperature. Temperature dependence of DME selectivity values obtained with these catalysts are given in Fig. 11. The main superiority of the new STA incorporated mesoporous catalysts synthesized in this work over the conventional alumina catalysts is their very high DME selectivity (approaching to 100%) and quite high activity at relatively low temperatures. These catalysts showed methanol dehydration activity at temperatures as low as 180oC. The catalyst containing a W/Si ratio of 0.16 gave the lowest activity in methanol dehydration in a temperature range of 180-350°C. The lowest activity of TRC-62(L) is due to its less STA content. In fact, the Brønsted acidity of this material was reported to be less than the Brønsted acidities of TRC-82(L) and TRC-92(L) (Varisli et al., 2009). The highest DME yield obtained with the STA

Figure 9. DRIFT results of pyridine adsorbed samples

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incorporated nano-structured catalysts was about 0.60, at a space time of 0.27 s.g.cm-3 and at 250ºC. TRC-82(L), TRC-92(L) and TRC-75(L) exhibited similar trends in dehydration of methanol in the temperature range of 180-350ºC. As far as DME selectivity was concerned, TRC-75(L) showed the best performance, approaching to 100%. Relative intensity of the band (at 1539 cm-1) corresponding to Brønsted acidity of TRC-75(L) (Fig. 9) was even higher than the relative intensities of the bands corresponding to Brønsted acidities of TRC-82(L) and TRC-92(L), which were reported in our earlier publications (Varisli et al., 2009, 2010). These results supported the very high DME selectivity values obtained with this catalyst.

Surface area of the STA incorporated mesoporous catalysts decreased with an increase in the STA content (Table 1) (Varisli et al., 2009). Decrease of surface area and increase of Brønsted acidity have opposing effects on the activity of STA incorporated catalytic materials synthesized in this work. Although it contains a less W/Si ratio (0.33), TRC-75(L) is as active as TRC-82(L) and TRC-92(L), which have W/Si ratios of 0.47 and 0.78, respectively. Better catalytic performance of TRC-75(L) can be attributed to the well dispersion of the active phase (STA) within the catalyst. Best operation temperature is between 200-250°C for this catalyst. Decrease of conversion at higher temperatures indicated deactivation by coke formation. Some experiments were repeated for periods longer than 6 hours at high temperatures. XPS analyses of the fresh and the used

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Figure 10. Conversion of methanol obtained with hydrothermally synthesized catalysts containing different W/Si ratios at 0.27 s.g.cm-3

Figure 11. DME selectivity obtained with hydrothermally synthesized catalysts containing different W/Si ratios at 0.27 s.g.cm-3



catalysts in these experiments clearly indicated the formation of coke on the catalyst surface. C/Si atomic ratio was about 1.5 for the used catalysts in these experiments. Some activity decrease was observed at long test periods, especially at reaction temperatures higher than of 250°C. However, the catalyst was easily reactivated to its original activity by its re-calcination at 350°C in the presence of dry air. The tolerance of TRC-75(L) to coke deposition is apparently higher than TRC-82(L) and TRC-92(L) (Fig. 10).

As it is expected, DME yield values increased with an increase in space time (80% at a space time of 0.41 s.g.cm-3), at about 250°C (Fig. 12). At this temperature, DME selectivity was 100%.

DME yield values obtained with the catalysts containing a W/Si ratio of 0.4 (in the solution) but calcined at different conditions or treated with supercritical CO2 are shown in Fig. 13. The catalyst prepared by removing the surfactant from the structure using supercritical CO2 (TRC-75(L)-CO2) appears to be as active as TRC-75(L). DRIFTS analysis of pyridine adsorbed materials had indicated that the strength of the Brønsted acid sites of TRC-75(L)-CO2 was somewhat higher than the Brønsted acidity of TRC-75(L). However, the surface area of this material was lower than TRC-75(L) (Table 1). Apparently, disadvantage of lower surface area was compensated by higher Brønsted acidity (Fig. 9), so that the catalytic performance of TRC-75(L)-CO2 was quite close to the performance of TRC-75(L). Calcination at 400ºC caused some decrease of the activity, indicating some loss of protons of the catalyst. In fact, the FT-IR analysis results reported in

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Figure 12. DME yields at different space times (0.14, 0.27, 0.41 s.g.cm-3) with TRC-75(L)

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TRC-75(L)TRC-75-400TRC-75(L)-CO2

Figure 13. DME yields obtained with TRC-75(L), TRC-75-400 and TRC-75(L)-CO2



Figure 8 had also indicated some distortions of the STA structure when the synthesized material was calcined at 400oC. DME selectivities of TRC-75(L) and TRC-75(L)-CO2 were close, approaching to 100 %. Formaldehyde forms at lower temperatures. Formaldehyde selectivity of TRC-62(L) was the highest, approaching to 56 % at 180ºC. Trace amount of ethylene formation was also observed with these catalysts at high temperatures.

4. CONCLUSIONS It was shown that, dimethyl ether, which is considered as a green transportation fuel alternate, can be produced at very high yields, using the novel STA incorporated mesoporous silicate structured catalysts synthesized in this work. Especially, the catalysts containing a W/Si ratio of 0.4 in the synthesis solution (TRC-75(L)) gave very high DME selectivity values approaching to 100 %. Best operating temperature of methanol dehydration was between 200-250°C, with the new STA incorporated mesoporous catalysts. Low temperature activity and very high DME selectivity are important superiorities of these new catalytic materials. Formaldehyde formation was observed as the main by-product at lower temperatures. Results indicated that the procedure used during the hydrothermal synthesis and the calcination temperature of the synthesized materials are quite important, as far as their catalytic performances were concerned. Supercritical CO2 extraction caused no defect in the catalyst structure or activity loss. Increase of calcination temperature from 350°C to 400ºC caused some decrease in DME yield, which was considered to be due to the loss of some Brønsted acidity and partial deformation of the STA structure. The results obtained in this study showed the possibility of synthesizing this non-petroleum fuel alternate (DME) at very high yields starting from methanol and using the nanocomposite STA-silicate structured mesoporous catalysts synthesized here.



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dimethyl ether synthesis over novel silicotungstic acid incorporated nanostructur catalysts

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diesel fuel

high dme selectivity

silicotungstic acid

aluminosilicate type

benign fuel

wsi ratio

highest activity

synthesis procedure