Gas-phase catalytic synthesis of MTBE from MeOH and Bu t OH over various microporous H-zeolites

Indian Journal of Chemistry Vol. 48A, June 2009, pp. 788-792 Notes Gas-phase catalytic synthesis of MTBE from MeOH and Bu t OH over various microporous H-zeolites Q Zhang a, Q-H Xia a, *, X-H Lu a, X-T Ma b & K-X Su b a Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei University, Wuhan 430062, China Email: xia1965@yahoo.com b Jingchu University of Technology, Jingmen 448000, China Received 31 October 2008; revised and accepted 12 May 2009 Several H-zeolites such as HZSM-5, HY, Hβ, H-13X, H-K 10 and HA have been used to catalyze the gas-phase synthesis of methyl tert-butyl ether from methanol and tert-butanol in a continuous fixed-bed reactor. The ascending sequence of Brönsted acidity is: H-K 10 (0.86) < Hβ (2.04) < H-13X (2.83) < HY (3.93) < HZSM-5 (5.22), which is appreciably different from the order of Lewis acidity: H-K 10 (0.87) < HZSM-5 (1.15) < H-13X (1.75) < HY (4.94) < Hβ (6.10). The optimal reaction temperature varies in the order: Amberlyst-15 (80 110 C) Hβ (80 90 C) < HZSM-5 (120 C) < HY (140 150 C) < H-K 10 (150 C) << H-13X (220 C), and is dependent on several factors. The Hβ catalyst shows the best activity and on-stream stability (the selective conversion of >97.4 mol% Bu t OH to MTBE) at 80 90 C (comparable to that of commercial Amberlyst-15), along with increasing time-on-stream up to 110 h at 90 C. The highly selective conversion of Bu t OH to MTBE remains more or less constant on both the catalysts. Keywords: Catalysis, Acid catalysis, Gas phase synthesis, Zeolites, H-zeolites, Microporous zeolites IPC Code: Int. Cl. 8 B01J29/40; C10L1/85; C10L10/10 Due to the worldwide demand for high octane-number gasoline additives and pure iso-butene, the production of MTBE (methyl tert-butyl ether) has increased faster than most other commodity chemicals, with a total capacity of about 18.5 10 6 ton and an estimated increasing yearly demand of 15% 1,2. MBTE is industrially manufactured from methanol and iso-butene in the liquid phase using ion-exchanged Amberlyst-15 resin as the catalyst 3. Since the Amberlyst-15 resin catalyst suffers from severe drawbacks, including thermal and chemical instability, corrosive nature and polymerization of iso-butene 4,5, many alternative acid catalysts, such as zeolites 6-9, fluorinated SiO 2 /Al 2 O 3 10,11, sulphated ZrO 2 12, heteropoly acids 13,14, titanium silicates 15 have been developed to catalyze this reaction in the liquid phase. Several studies have been devoted, in recent years, to the gas-phase synthesis of MTBE from methanol and iso-butene 2, or from methanol and iso-butanol over an acidic catalyst, yielding the desired MTBE with water as a co-product 16. The liquid phase synthesis from methanol and iso-butene 17 has also been reported. Zeolites are reported to be less active than the Amberlyst-15, but more selective to MTBE 6,18. Ti-silicalite appears to be even more selective than HZSM-5 15. For the gas-phase synthesis of MTBE from methanol and iso-butene, Hβ zeolite was found to be as active as acidic Amberlyst-15, and noticeably superior to non- and dealuminated forms of HY, HZSM-5, zeolite omega, HMOR, SAPOs and pillared clays 2. A few studies have reported the liquidphase synthesis of MTBE from methanol (MeOH) and tert-butanol (Bu t OH) using solid acid catalysts, such as ion-exchanged resin 19, supported heteropoly acids 14,20, and ion-exchanged clays 14. Dodecatungstophosphoric acid supported on K 10 clay gave the best yield with the highest selectivity in a batch reactor 14. However, while a few studies have been reported on the gas-phase synthesis of MTBE from methanol and tert-butanol over SO 2 4 /ZrO 2 /MCM-41 21, HPW/MCM-41, HAlMCM-41 22 and β zeolite ionexchanged with alkali metal ions 23, synthesis over H-zeolites in a continuous fixed-bed reactor has not been reported. For β- zeolite catalysts, the catalytic activity declined in the order of Hβ > Liβ > Naβ > Kβ > Rbβ > Csβ, in accordance with their surface acidity. This study compares the catalytic activity of various microporous acidic zeolite catalysts for the gas-phase synthesis of MTBE from methanol and tert-butanol. Under experimental conditions, Hβ zeolite shows the best activity and on-stream stability, comparable to that of commercial Amberlyst-15 resin catalyst. The deactivation tendency of these catalysts with time is also observed. Experimental A mixture of 120 g of 20 wt% tetraethyl ammonium hydroxide solution (Aldrich), 2.2267 g of NaAlO 2, 0.90 g of KCl and 0.15 g of NaOH was stirred to give a transparent solution and then Aerosil-200 silica (Aldrich, 32.59 g) was added. The

NOTES 789 resulting homogeneous sol was transferred into a Teflon-lined stainless-steel autoclave and heated to 170 C. After 40 h, the autoclave was quenched, the content filtered and the solid washed with deionized water. After drying at 96 C overnight, the solid was calcined at 540 C for 20 h. The resultant white solid Na-Alβ was ion-exchanged with 20 wt% of NH 4 NO 3 solution at 60 C three times to give NH 4 -Alβ. Each time, one gram of Na-Alβ was stirred with 25 ml of NH 4 NO 3 solution at 60 C for 3 h. Finally NH 4 -Alβ was decomposed into Hβ (Si/Al = 30.6) at 500 C for 3 h. HZSM-5 (Si/Al = 27.5), HY (6.7), HA (3.0), H-K 10 (2.7) and H-13X (2.3) were prepared from commercial powders of NaZSM-5, NaY, NaA, Na-K 10 (montmorillonite) and Na-13X zeolites (Aldrich) by using the same ion exchange procedure and re-calcination. The XRD patterns of powdered samples were recorded between 2θ = 3 and 50 with a Shimadzu XRD-6000 diffractometer using Ni-filtered Cu-Kα radiation operating at 40 kv and 30 ma. Autosorb-1 was used to measure the N 2 adsorption-desorption isotherms of the samples. Prior to the measurements, the samples were degassed at 300 C overnight. The BET specific surface area was calculated using the BET equation with relative pressure (p/po) at 0.05 0.25. The framework vibrational spectra of zeolites were recorded between 400 and 4000 cm 1 with a resolution of 4 cm 1 on a Shimadzu Prestige-21 FTIR spectrophotometer. The mixture of KBr and solid powder sample (200:1) was carefully ground and pressed into a round wafer under a pressure of 5 ton cm 2. IR spectra of adsorbed pyridine were recorded to determine the presence of Brönsted and Lewis acid sites over the catalysts. A self-supporting wafer (20 mg) of the catalyst was heated at 300 C for 3 h under a residual pressure of 10 6 mbar before adsorbing excess of pyridine at room temperature, followed by evacuation at 200 C for 30 min. Brönsted and Lewis acidities were quantified from the integrated areas of the bands at 1540 and at 1445 cm 1, respectively. The integrated areas of these two bands provide information only about the relative amount of pyridine, which interacts with Brönsted and Lewis sites; therefore the B, L, and B/L values should be regarded as relative indications. The average crystal sizes were calculated from SEM images, which were observed using a KYKY 1000B scanning electron microscope (SEM) operating at an accelerating voltage of 25 kv and tube current of 100 µa under vacuum at 10 6 mbar. ICP elemental analysis was performed with a Perkin Elmer Optima 3000DV spectrometer. Calibration standards with different concentrations were prepared by diluting corresponding standard metal solutions. For the gas phase synthesis of MTBE from MeOH and Bu t OH in a continuous fixed bed reactor, 0.20 g of the catalyst (40~60 mesh fraction) was predehydrated at 300 C for 2 h in a flow of helium before a mixture of MeOH and Bu t OH (molar ratio of 10:1) was pumped into the reactor by an ISCO syringe pump (0.25 in. o.d.) and heated at the given temperatures. During the reaction, a 13 ml min -1 helium flow was used as the carrier and diluting gas and the weight hourly space velocity (WHSV) was kept at 10 h 1. Each reaction reached steady state in 30 min and then the products were analysed on stream by a Shimadzu GC 2010 gas chromatograph equipped with a FID and an OV 1 capillary column. The conversion (mol %) of Bu t OH to MTBE, i.e., the selective conversion of Bu t OH to MTBE, was equal to the actual steady conversion of Bu t OH (mol %) into the products the selectivity of MTBE (%), regardless of excessive amounts of MeOH. The selectivity of MTBE below the optimal temperature was 100% for each reaction and thereafter decreased gradually with the rise of temperature. Results and discussion XRD patterns and IR framework vibration spectra of samples are illustrated in Figs 1 and 2, showing typical zeolite lattices for ZSM-5, Y, 13X, β, K 10 and A zeolites. The Si/Al ratios of zeolites undergoing ion exchange by NH 4 NO 3 solution and calcinations as determined by ICP was highest for Hβ (30.6) followed by HZSM-5 (27.5), HY (6.7), HA (3.0), H-K 10 (2.7) and H-13X (2.3). BET surface areas and pore volumes of various H-zeolites obtained by BET analysis were 406.5 m 2 /g and 0.22 cm 3 /g for HZSM-5, 821.3 m 2 /g and 0.43 cm 3 /g for HY, 862.4 m 2 /g and 0.42 cm 3 /g for H-13X, 506.2 m 2 /g and 0.27 cm 3 /g for Hβ, 243.3 m 2 /g and 0.26 cm 3 /g for H-K 10, and 155.7 m 2 /g and 0.14 cm 3 /g for HA, while the surface area of Amberlyst-15 was about 50 m 2 /g. Average crystal size observed by SEM were 0.5 1.2 µm for HZSM-5, 0.5 2.0 µm for HY, 0.6 2.1 µm for H-13X, 0.3 1.0 µm for Hβ, 0.8 2.5 µm for H-K 10, and 0.4 1.1 µm for HA.

790 INDIAN J CHEM, SEC A, JUNE 2009 Fig. 1 XRD patterns of various H-zeolites. Fig. 3 Pyridine-adsorption FTIR spectra of various H-zeolites. Table 1 Pyridine adsorption data on various H-zeolite samples a Catalysts B acid b L acid c B/L L/B HZSM-5 5.22 1.15 4.54 0.22 HY 3.93 4.94 0.80 1.26 H-13X 2.83 1.75 1.62 0.62 Hβ 2.04 6.10 0.33 2.99 H-K 10 0.86 0.87 0.99 1.01 HA 0 0.14 0 - Amberlyst-15 4.7 eq/g a Brönsted and Lewis acidities are quantified into integrated areas of the absorbances at 1540 and at 1445 cm 1, respectively. b at 1540 cm -1 c at 1445 cm -1 Fig. 2 IR framework vibration spectra of various H-zeolites. Since the synthesis of MTBE from MeOH and Bu t OH is typical of acid-catalyzed reactions, acidity of the catalysts is of importance. Figure 3 shows that the surface of HA has very few Lewis acid sites (0.14) as determined by the pyridine adsorption, possibly because the size of pyridine molecules is too large to enter the small pores (ca. 3 Å) of A zeolite. The surfaces of other zeolites contain relatively higher concentration of Brönsted and Lewis acidities, and the Brönsted acidity increases in the sequence: H-K10 <Hβ < H-13X < HY < HZSM-5, which is appreciably different from the order of Lewis acidity: H-K 10 < HZSM-5 < H-13X < HY < Hβ (Table 1). The B/L value varied in the order of HZSM-5 > H-13X > H-K 10 > HY > Hβ, while the sequence of L/B value was the very reverse: Hβ > HY > H-K 10 > H-13X > HZSM-5, indicating that the main acid sites were Brönsted acidity on HZSM-5, but Lewis acidity on Hβ. However, the catalytic activity of these H-zeolites in the gas-phase synthesis of MTBE from MeOH and Bu t OH showed a different variation from the sequence of both, their surface acidities and BET surface areas (Table 2). In this reaction, the Amberlyst-15 catalyst showed rather high activity

NOTES 791 with a selective conversion of about 97.3 mol% Bu t OH to MTBE at the temperatures between 80 and 110 C, which favored thermodynamics. HA showed relatively low activity due to the poor accessibility of small pores; the conversion of Bu t OH to MTBE was only 20.9 mol%, even at 280 C. The Hβ catalyst showed the best activity (the selective conversion of >97.4 mol% Bu t OH to MTBE) at 80 90 C, which is comparable to that of commercial Amberlyst 15 catalyst. The reaction on Hβ was sensitive to the slight change of temperature; once the temperature exceeded 100 C, the selectivity of by-products including hydrocarbons and oxygenates increased, leading to a linear reduction of the selectivity to MTBE with increasing the temperature. For each H-zeolite catalyst, there was an optimal temperature window at which the highest conversion was obtained (Table 2). The Table 2 Comparison of catalytic activities of various H-zeolite catalysts with Amberlyst-15 catalyst Catalysts Conversion (mol %) Selectivity (%) Optimal temp. ( C) HZSM-5 99.3 100 120 HY 96.6 99.6 140-150 H-13X 95.7 98.2 220 Hβ 97.4 100 80-90 H-K 10 93.7 100 150 Amberlyst-15 97.3 100 80-110 optimal reaction temperatures shifted gradually to higher values in the following order: Amberlyst-15 Hβ < HZSM-5 < HY < H-K 10 << H-13X, which may be dependent on several factors, such as the surface acidity, the micropore size, the framework composition, the surface area and the stability of zeolite, etc. Based on thermodynamics equilibrium, the higher temperature will largely promote the occurrence of side reactions over acid catalysts to produce other hydrocarbons or oxygenates. This may also be the reason for the bell-shape of the selective conversion curves fortert-butanol (in Fig. 4); the descending portions of the curves at higher temperatures correspond to the shift in the equilibrium towards lower yield of MTBE in most exothermic reactions. Studies on the on-stream activity of various catalysts were carried out at the respective optimal reaction temperatures. The catalytic activity of HZSM-5, H-13X and H-K 10 catalysts displayed almost a steep linear reduction of the conversion of tert-butanol to 13.7-mol% in 9 h for H-13X, to 15.7 mol% in 12 h for HZSM-5 and to 18.7-mol% in 20 h for H-K 10, while that of HY dropped stepwise to 20.8 mol% of the tert-butanol conversion for 70 h. This may be ascribed to the deactivation of H-zeolites due to the coking promoted by strong acidities and small pores, as revealed by the large weight losses between 200 C and 600 C shown in the TG-DTA analysis of the reacted zeolite catalysts (Fig. 5). The Hβ catalyst exhibited a Fig. 4 The catalytic activity of various H-zeolite and Amberlyst-15 catalysts for the gas-phase synthesis of MTBE from MeOH and Bu t OH. [1, HY; 2, H-K10; 3, HA; 4, HZSM-5; 5, Hβ; 6, H-13X; 7, Amberlyst-15]. Fig. 5 TG curves of the reacted H-zeolites.

792 INDIAN J CHEM, SEC A, JUNE 2009 high on stream steady activity, comparable to that of the Amberlyst 15 at 90 C. With increasing time-onstream up to 110 h, the highly selective conversion of Bu t OH to MTBE remained more or less constant on both catalysts. This may be due to the relatively small diffusion hindrance and the high hydrophobicity of the two catalysts, since the β zeolite has a suitable pore size of about 6.7 Å and a high Si/Al ratio of 30.6. In the present study, Hβ (Si/Al=30.6), HZSM-5 (27.5), HY (6.7), HA (3.0), H-K 10 (2.7) and H-13X (2.3) were prepared from Naβ, NaZSM-5, NaY, NaA, Na-K 10 (montmorillonite) and Na-13X zeolites by using the same ion-exchange procedure and recalcinations. BET surface areas of various H-zeolites decreased in the order: H-13X (862.4 m 2 /g) > HY (821.3 m 2 /g) > Hβ (506.2 m 2 /g) > HZSM-5 (406.5 m 2 /g) > H-K 10 (243.3 m 2 /g) > HA (155.7 m 2 /g) >> Amberlyst-15 (~50 m 2 /g). The surface of HA has very few Lewis acidity sites (0.14) as determined by the pyridine adsorption. The B/L value varied in the order: HZSM-5 (4.54) > H-13X (1.62) > H-K 10 (0.99) > HY (0.80) > Hβ (0.33), while the sequence of L/B value was the very reverse: Hβ (2.99) > HY (1.26) > H-K 10 (1.01) > H-13X (0.62) > HZSM-5 (0.22), indicating that the main acid sites were Brönsted acidity on HZSM-5, but Lewis acidity on Hβ. However, the catalytic activity (i.e. the optimal temperature window) of these H-zeolites in the gasphase synthesis of MTBE from MeOH and Bu t OH varied in the order of Amberlyst-15 (80-110 C) Hβ (80-90 C) < HZSM-5 (120 C) < HY (140-150 C) < H-K 10 (150 C) << H-13X (220 C), and could not be correlated with the change in their surface acidities or their BET surface areas. Amongst the H-zeolites tested, Hβ showed the best activity and on-stream stability at the temperatures 80-90 C, comparable to that of commercial Amberlyst 15 catalyst. The deactivation of H-zeolites may be ascribed to the coking promoted by the strong acid sites and small pores, as revealed by TG-DTA analysis. Acknowledgement The authors acknowledge the financial support provided by the key project of Department of Education, Hubei Province (No. 2004D001), and by National Natural Science Foundation of China (No. 20673035). References 1 Peaff G, Chem Eng News, 101 (1994) 8. 2 Collignon F, Mariani M, Moreno S, Remy M & Poncelet G, J Catal, 166 (1997) 53. 3 Horvath T, Seiler M & Hunger M, Appl Catal A: Gen, 193 (2000) 227. 4 Hutchings G J, Nicolaides C P & Scurrell M S, Catal Today, 15 (1992) 23. 5 Le Van Mao R, Le T S, Faibairn M, Muntasar A, Xiao S & Denes S, Appl Catal A: Gen, 185 (1999) 221. 6 Chu P & Kuhl G H, Ind Eng Chem Res, 26 (1987) 365. 7 Kogelbauer A, Ocal M, Nikolopoulos A A, Goodwin Jr. J G & Macelin G, J Catal, 148 (1994) 157. 8 Nikolopoulos A A, Kogelbauer A, Goodwin Jr. J G & Macelin G, Appl Catal, 119 (1994) 69. 9 Ahemd S, El-Faer M Z, Abdillahi M M, Shirokoff J, Siddiqui M A B & Barri S A I, Appl Catal A: Gen, 161 (1997) 47. 10 Quiroga M E, Figoli N S & Sedran U A, React Kinet Catal Lett, 63 (1998) 75. 11 Nikolopoulos A A, Kogelbauer A, Goodwin Jr. J G & Macelin G, Catal Lett, 39 (1996) 173. 12 Quiroga M E, Figoli N S & Sedran U A, J Chem Eng, 67 (1997) 199. 13 Shibata S, Okuhara T & Misono M, J Mol Catal, 100 (1995) 49. 14 Yadav G D & Kirthivasan N, J Chem Soc Chem Commun (1995) 203. 15 Chang K-H, Kim G-J & Ahn W-S, Ind Eng Chem Res, 31 (1992) 125. 16 Nicolaides C P, Stotijn C J, van der Veen E R A & Visser M S, Appl Catal, 103 (1993) 223. 17 Collignon F, Loenders R, Martens J A, Jacobs P A & Poncelet G, J Catal, 182 (1999) 302. 18 Nikolopoulos A A, Oukaci R, Goodwin Jr. J G & Marcelin G, Catal Lett, 27 (1994) 149. 19 Matoug M-H & Goto S, Int J Chem Kinet, 25 (1993) 825. 20 Matoug M-H, Tagawa T & Goto S, J Chem Eng Japan, 26 (1993) 254. 21 Xia Q-H, Hidajat K & Kawi S, J Catal, 205 (2002) 318. 22 Xia Q-H, Hidajat K & Kawi S, J Catal, 209 (2002) 433. 23 Chen J-Y & Ko A-N, React Kinet Catal Lett, 71 (2000) 77.