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Applied Catalysis A: General, 94 ( 1993 ) 91-106Elsevier Science Publishers B.V., Amsterdam A P C A T A2400 91 Catalytic conversion of methanol into light alkenes on mordenite-like zeolites A.J. Marchi and G.F. Froment Laboratorium voor Petrochemische Techniek, Rijksuniversiteit, Krijgslaan 281, B- 9000 Ghent (Belgium) (Received 24 September 1992) Abstract The paper deals with the effect of the number and strength of acid sites and with feed composition during methanol conversion into hydrocarbons on mordenite and dealuminated mordenite. The mordenite sample used in this work possesses principally aluminium-hydroxyl groups that absorb at 3650 and 3610 cm 1. They can interact strongly with product molecules. A rapid deactivation by coke was observed. The dealuminated mordenites exhibit principally aluminium hydroxyl groups that absorb at 3530 cm 1 Their interaction with the product molecules is less pronounced than that of the high frequency aluminium-hydroxyl groups. This and the reduction of the number of strong acid sites lead to a lower deactivation by coke and a higher selectivity to light alkenes. Decreasing the partial pressure of methanol and the addition of water to the feed lead to higher yields of light alkenes, principally of propene, and less coke deposition. Water competes with alkenes and aromatics for the Lewis and Bronsted acid sites. The coke deposition suppresses the conversion of alkenes into alkenes and aromatics so that the selectivity to propene, butene and C~+ alkenes is increased. Keywords: acidity; catalyst characterization (DRIFT); deactivation; light alkenes; methanol conversion; mordenite; zeolites. INTRODUCTION The catalytic conversion of methanol to lower alkenes is a process that attracts growing interest. Recently, Froment et al. [1] reviewed the activity, stability and selectivity of small, medium and large pore molecular sieves used as catalysts for this reaction. Most of the literature discusses the use of standard [2-10] or modified [11-13] HZSM-5 zeolite. Zeolite HZSM-5 has been widely studied because it deactivates rather slowly. This is attributed to the particular channel structure of this zeolite. The concentration and strength of acid sites is also an important factor in both the durable activity and the selecCorrespondence to. Dr. G.F. Froment, Laboratorium voor Petrochemische Techniek, Rijksuniversiteit, Krijgslaan 281, B-9000 Ghent, Belgium. Tel. ( + 32-91 )644516, fax. ( + 32-91 )644999, email L P T ~ A U T O C R T L . R U G . A C . B E . 0926-860X/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved. 92 A.J. Marchi and G.F. Froment/Appl. Catal. A 94 (1993) 91-106 tivity of the catalyst, however. Two ways of modifying the acidity of zeolites are dealumination and cation exchange. Dealuminated [14-19] and ion-exchanged zeolites [20] were shown to possess higher stability and selectivity than the parent zeolites. The work presented here is a continuation of the work on methanol conversion into lower alkenes by Dehertog and Froment [21] who used HZSM-5 catalysts and by Marchi and Froment [22 ] who used SAPO-34 and SAPO-11. Its aim is to investigate the effect of the feed composition and of the concentration and strength of acid sites on alkene formation, oligomerization, aromatization, alkylation and coke formation with mordenite-like zeolites. EXPERIMENTAL Three different mordenite samples from Zeocat (ZM-210, ZM-760 and ZM980) with framework silicon-to-aluminium atomic ratios of 10, 60 and 80 were used in this work. DRIFT (Diffuse Reflectance Infrared Fourier Transform) spectra of the fresh samples were recorded on a Bruker IF548 instrument equipped with a diffuse reflectance cell (Spectra-Tech). The samples were activated at 723 K under vacuum (10-6 mbar) overnight before recording the spectra. Potassium bromide was used as a reference. Textural properties were determined by N2 sorption at 77 K in a Micromeritics ASAP 2000 sorptometer. The catalytic conversion of methanol to hydrocarbons was carried out in a fixed-bed tubular (20 mm I.D.) titanium reactor. In all the experiments, the catalyst was diluted with inert material and the reactor was immersed in a salt bath to ensure a uniform temperature along the catalyst bed. The dilution ratio, defined as the ratio of the weight of inert material to the weight of catalyst, WI/Wc, was 10. The experiments were carried out at 753 K. The space time, W / F o was varied between 10 and 32 g cat. × h/mol methanol. A detailed description of the equipment used can be found in a previous paper [23]. The products were analyzed in a Carle gas chromatograph provided with: (i) one RSL (polydimethylsiloxane) 160 capillary column (5/~m film, 0.32 m m I.D.) for analyzing C1-C4 compounds, CH3OH, (CH3)20, C~+ and aromatics, using a flame ion detector (FID); (ii) a 10% Sorbitol on a Chromosorb T column (length: 1 ft.) for resolving water, a 3.1% Carbowax 1540 on Porasil C (length: 10 ft.) and a 2.7% BIS 2,2-ethoxyethyl (EE) A on Chromosorb PAW (length: 17 ft. ) columns in series in order to resolve and identify all the compounds and isomers of the C3-C4 fraction, a Hayesep Q column (length: 7 ft. ) to resolve and identify ethane, ethene and carbon dioxide and a molecular sieve 13X (length: 9 ft.) to resolve and identify nitrogen (internal standard), methane and carbon monoxide, using a thermal conductivity detector (TCD) with helium as carrier gas; and (iii) a hydrogen transfer system (HTS) trans- A.J. Mar('hi and (LF. Froment/Appl. Catal. A 94 (1993) 91 106 93 ferring the hydrogen from the helium carrier gas into the nitrogen carrier gas stream, in order to detect it by means of a second TCD. The instantaneous carbon balance closure was better than 95%. R E S U L T S AND D I S C U S S I O N Influence of the silicon-to-aluminium ratio Experiments with mordenite samples of different Si/A1 ratios were carried out at 753 K and atmospheric pressure. The feed was a solution of methanol (30 wt.-% ) and water (70 wt.-% ). The initial total conversion of methanol was 100% in all cases for the whole range of W / F o used in this work (Fig. 1 ). The initial conversion of methanol to hydrocarbons increased as the silicon-to-aluminium atomic ratio decreased. This increase was accompanied by an important rise in the yield of methane (Tables 1 and 2). For the case of the sample with a framework silicon-to-aluminium atomic ratio of 10, the conversion of methanol dropped very fast from 100% to values below 20% (Fig. 1, curve a). In a similar way, the conversion to hydrocarbons fell from about 55% to almost zero, (Table 1 ). The initial yield of methane was higher t h a n 10%. The hydrocarbon fraction mainly consisted of C1-C4 alkanes and C,~-C4 alkenes (Table 1 ). The main alkene was ethene, whereas the main alkanes were methane, ethane and propane. Only small amounts of i-butane and i-butene and traces of aromatics and C5+ non-aromatic compounds were detected in the effluent. The ratios of light alkene yields were C J C 3 = 2.5-2.6 and C~/(C:~ + C4 ) -- 1.8-2.0 at 100% conversion. The ratios of the yields of total loo 80 6o 2 4O 2O "O.~ a ) 0 0 _ _ ~ q 015 i I___ 030 L 045 M (mol Me OH/gcat) Fig. 1. Methanol conversion on mordenite-like zeolites at 753 K and 1.04 bar total pressure versus the total amount of methanol fed per gram of catalyst. W / F o 10-32 g cat. h/mol methanol. Feed: methanol water 30:70 wt.-% mixture. (a) S i / A l = 10, (b) S i / A l = 6 0 ; (c) S i / A l = 8 0 . 94 TABLE1 A.J. Marchi and G.F. Froment/Appl. Catal. A 94 (1993) 91-I06 Conversion of methanol into hydrocarbons on mordenite Si/Al, 10; T, 753 K; P, 1.04 bar; W / F o, 18.3 g cat. × h/tool methanol; feed: water-methanol 70:30 wt.-% Ma tb XMeOH c XMH d 0.0064 0.12 100 55.5 12.3 9.7 7.3 3.9 10.0 1.5 0.7 0.9 0.0 Traces 0.0 0.0 0.0091 0.17 100 57.9 13.4 12.0 7.6 4.6 6.1 1.4 0.5 0.5 0.2 Traces 0.0 0.0 0.016 0,30 45.9 14.0 4.0 3.5 0.5 0.9 0.7 0.3 0.1 0.0 0.0 0.0 54.1 17.4 0.064 1.17 21.5 0.8 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 78.5 14.8 0.078 1.42 15.9 0.6 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 84.1 10.9 Yields e CH4 C2H4 C2H6 C3H6 C3Hs C4Hs n-C4Hlo i-C4H,0 CsH,o Aromatics CH30H C2H60 Hydrocarbon distribution l CH4 C~-C~ C2+ alkanes C5÷ alkenes 26.6 32.7 40.7 0.0 29.0 38.8 31.8 0.4 39.9 47.1 13.1 0.0 87.5 12.5 0.0 0.0 85.3 14.7 0.0 0.0 aM, mol methanol fed/g catalyst. bt, time on stream (h). CXMeOH, methanol conversion. ~XMH,tool hydrocarbons produced/100 mol methanol fed. eYield, g product/100 g methanol fed. /Hydrocarbon distribution, g hydrocarbon/100 g total hydrocarbons. light h y d r o c a r b o n s were CffC3 = 1.2-1.8 a n d C f f ( C 3 + C4) = 1.0-1.2. T h e s e ratios are in a g r e e m e n t w i t h t h o s e f o u n d for s m a l l - p o r e zeolites [22,24,25]. T h e D R I F T s p e c t r u m of t h e f r e s h s a m p l e s h o w e d a n a b s o r p t i o n b a n d w i t h a m a x i m u m a t 3650 c m - 1 a n d a s h o u l d e r at 3610 c m - ' (Fig. 2 ). Z M - 2 1 0 zeolite h a s a f r a m e w o r k Si/A1 r a t i o of 10. H o w e v e r , c o m p l e t e a n a l y s i s i n d i c a t e s t h a t t h e overall Si/A1 r a t i o is 5.5 ( T a b l e 3 ). T h i s m a y be e x p l a i n e d b y t h e p r e s e n c e o f e x t r a - f r a m e w o r k a l u m i n a . T h u s , t h e b a n d a t 3650 c m - ' m a y be a t t r i b u t e d to e x t r a - f r a m e w o r k m a t e r i a l [26 ], w h e r e a s t h e s h o u l d e r at 3610 c m - 1 m a y be due to f r a m e w o r k a l u m i n i u m - h y d r o x y l g r o u p s t r e t c h e s [22,27]. T h e s e hyd r o x y l g r o u p s p o s s e s s a high acid s t r e n g t h a n d c a n i n t e r a c t w i t h r e a g e n t a n d p r o d u c t m o l e c u l e s [22,27]. A s m a l l a b s o r p t i o n b a n d at a r o u n d 3735 c m - ' was A.J. Marchi atTd (;.F. Froment/Appl. Catal A 94 (1993) 91-106 T A B L E '2 Conversion of m e t h a n M into h y d r o c a r b o n s on d e a l u m i n a t e d m o r d e n i t e samples" 95 T, 753 K: P, 1.04 bar: W/I,'~, 21.5 g cal × h / m o l m e t h a n o l fed; feed: w a t e r m e t h a n o l 70::{0 wt. "~ Illixl ure Si/A1 ratio M [ 41) 0.015 ().;1;~ 100 :~5.t) 2.7 7.3 12.1 4.t) 5.1 1.2 5.4 0.7 2.4 (1.() 0.11 4.1 54.9 '.~1.5 80 0.032 0.70 11)0 47.1 9.5 !).5 l:L4 1.7 4.2 0.4 1.:~ 1.4 0.7 0.0 0.0 21.2 64.8 8.9 XMeOH ~'MH Yields CH~ C~H~ QH,; C:~H~ C4Hs n-C,~H ~,~ i-C~HI~ C~,+ Aromatics CH:~()H C2H~O CH4 C~ ('.4 Cs+ alkanes Aromatics C~, alkenes 0,042 0,90 74.5 :{5.4 8.2 6.2 10.0 0.6 5.5 0.2 0.2 1.4 0.2 2:L5 :{.1 24.9 44.2 '.L8 0.052 1.10 49.3 20.1:; 5.4 2.9 4.5 0.5 2.6 0.1 0.1 1.1 0.1 50.7 7.2 30.2 56.3 5.1 1.0 7.4 0.154 3.40 32.9 2.1) 0.9 0.1 0.1 0.0 0.0 0.0 0.0 0.1) 0.0 47.1 21.4 74.4 19.9 5.7 0.0 0.0 0.11) 2.17 100 30.7 0.8 2.4 21.5 1).9 9.4 //.3 2.8 2.2 2.;/ 0.1) 0.0 1.9 75.4 11.3 5.2 4.0 0.280 4.00 94.8 27.0 1.1 1.2 21.2 0.0 9.3 0.1 0.6 3.6 0.6 13.2 1.2 2.8 80.9 :L4 1.6 11.2 0.510 10.92 51.4 11.5 1).7 1).3 7.1 1).0 5.1 1).0 0.0 0.0 0.1 48.4 10.0 4.0 75.2 1.2 0.4 19.2 Hydrocarbon distributi,r7 5.9 1.4 1.5 :L6 0.6 4.5 "For an e x p l a n a t i o n o[" l e r m s see Table 1. also observed. This peak is normally assigned to silicon-hydroxyl groups [ 26 ] These groups do not interact with reagent and product molecules [22,26,27]. It was possible to feed a larger amount of methanol per gram of catalyst at 100% conversion when a dealuminated sample with a silicon-to-aluminium atomic ratio of 60 was used as catalyst, instead of the non-dealuminated sample (Fig. 1, curves a and b). The conversion of methanol dropped from 100% to values lower than 40%, but in this case it diminished more slowly than with the sample having a silicon-to-aluminium atomic ratio of 10. A similar behavior was observed for the conversion of methanol to hydrocarbons. The yield of methane was always less than 10%. The yield of light alkenes was higher than 20% at 100% conversion (Table 2). The ratios of light alkene yields were C2/ C:~ = 0.70-0.75 and C~/(C:~ + C~ ) = 0.40-0.50. The ratios of yields of total light hydrocarbons were C2/C:~ = 0.45-0.55 and C2/(C:~+ C4) = 0.30-0.40. These val- 96 5 7 3 5 c m -1 A.J. Marchi and G.F. Froment/Appl. Catal. A 94 (1993) 91-106 d if) I-z w o z rn rr 0 (,9 n~ I _ I I ~800 5600 3400 WAVENUMBER ( c m -t) Fig. 2. DRIFT spectra of mordenite and dealuminated mordenite samples used in this study. (a) Si/Al= 10; (b) Si/Al=60; (c) Si/Al=80. TABLE3 Textural properties of mordenite and dealuminated mordenite samples Sample Overall Si/A1 atomic ratio 5 60 80 Framework Si/A1 atomic ratio 10 60 80 Vpa (cm3/g) 0.19 0.28 0.30 Sgb (m2/g) 500 610 650 HM c DHM1 ~ DHM2 ~ aV., pore volume. bSg, specific surface area. OHM, H-mordenite zeolite. dDHM, dealuminated H-mordenite zeolite. ues are considerably lower than in the former case and agree with those found for large and medium pore zeolites. In addition, the initial yield of branched isomers and aromatics was much higher (Tables 1 and 2). Tetra-, penta- and hexa-methyl benzenes were detected in the effluent in considerable amounts. They were formed in the 12-membered ring channels. Further dealumination led to an important increase in the stability of catalyst and alkene yield, in agreement with results obtained by Bandiera et al. [ 17 ], whereas the initial conversion of methanol to hydrocarbons and the yield of methane were lower than in the former cases (Fig. I and Table 2 ). The yield of methane was always lower than 2%. The principal alkene was propene (Ta- A.J. Marchi and ( ;.F. Fr(~rnent/Appl. Catal. A 94 (I993) 91 106 97 ble 2). The ratios of light alkene yields were CJC:~=0.10-0.30 and C2/ (C3+C4) =0.05-0.25 at 100% conversion. The ratios of yields of total light hydrocarbons were C~/C:~= 0.07-0.25 and C2/(C:, + C4 ) = 0.05-0.15. These values are lower than in the former cases and correspond to those of large pore zeolites. The initial yields of alkanes and aromatics were also lower than in the case of the sample with a silicon-to-aluminium atomic ratio of 60 (Table 2). The DRIFT spectra (Fig. 2) showed some remarkable differences between the dealuminated samples and the non-dealuminated mordenite. The intensity of the absorption band at 3730 c m - I increased significantly. This could be attributed to the fact that on dealuminated H-mordenite defects are generated with the subsequent formation of silanol groups, which can not interact with reagent molecules. The high-frequency aluminium-hydroxyl group band shifted to higher wavenumbers and its intensity diminished as the silicon-to-aluminium ratio was raised. This band practically disappears for the sample with a silicon-to-aluminium ratio of 80. A new absorption band at 3530 c m - 1, that can be attributed to framework aluminium-hydroxyl group stretches [22,28] appeared. The intensity of this band increases as the Si/A1 ratio is raised. The band at 3610 c m - ~was not observed, probably due to lack of sensitivity since the dealumination process leads to an important decrease of the number of acid sites [17]. The dealuminated samples have a higher pore volume and specific surface area than the sample with a Si/A1 atomic ratio of 10 (Table 3), which is in agreement with the fact that the larger pores of ZM-210 could be partially blocked by extra-framework material. By pyridine desorption, Karge and Dondur [ 29 ] detected three types of acid sites in dealuminated mordenites: one Bronsted and two Lewis centers. Ammonia desorption showed four types of acid sites: two Bronsted and two Lewis centers. The second type of Bronsted site had a higher activation energy than the one detected by pyridine desorption. On the other hand, the pore structure of mordenite consists of parallel channels of 0.67 × 0.70 n m ( 12-membered ring pores) and 0.29 × 0.57 nm (8-membered ring pores), interconnected via small side pockets of 0.29 nm [30]. Lee et al. [16] suggested that in the alkylation of polynuclear aromatics, propene can diffuse through both the 12-membered ring and the 8-membered ring channels. Thus, the mordenite porous network may behave in a one-dimensional manner for large molecules, e.g. aromatics and branched isomers, and in a bi-dimensional manner for small molecules, such as light linear alkenes. On the basis of the results mentioned above, it may be concluded that the second type of Bronsted acid site might be located in the 8-membered ring channels of mordenite. The hydroxyl groups that absorb at 3530 cm-1 must be principally located in the large channels. In the non-dealuminated mordenite, they may be poisoned by cationic aluminium species from the extra-framework material. This can explain why they are not observed in the corresponding DRIFT spectrum. 98 A.J. Marchi and G.F. Froment/Appl. Catal. A 94 (I993) 91-106 The presence of extra-framework material in the larger pores, which reduces the effective pore size of these, and Bronsted acid sites in the 8-membered ring channels of non-dealuminated mordenite, provides an explanation for the results obtained in the conversion of methanol into hydrocarbons when a mordenite sample with a framework silicon-to-aluminium ratio of 10 was used. The 12-membered ring channels must be blocked or partially blocked by the extraframework material, likely amorphous alumina. Thus, the effective pore size of ZM-210 zeolite is lower than can be expected. This explains the similarity in catalytic behavior of this sample with that of small-pore zeolites. The small amounts of i-C4 and aromatics detected in the effluent can be formed on the acid sites present on the catalyst external surface and/or inside the partially blocked 12-membered ring channels. Due to the reduction of the effective pore size of these pores by extra-framework alumina, the diffusion of aromatics and branched hydrocarbons becomes difficult, and their yield is small. The rapid deactivation of this catalyst may be explained by coke formed on the external surface and inside of the partially blocked 12-membered ring channels of the non-dealuminated mordenite, which can block the entrance of the one-dimensional channels. Due to the small effective pore size and the onedimensional pore structure, blockage of the pores by coke deposition leads to a dramatic decrease of the concentration of the Br~nsted acid centers available for interaction with the reagent molecules and, therefore, of the catalytic activity of the zeolite ZM-210 [31-33]. The high initial yield of methane is indicative of a rapid loss of activity of this mordenite sample by coke deposition (Table 1 ) [34]. It is generally accepted that a "methoxy" group can be formed on Bronsted acid sites. This "methoxy" species could then be converted to methane by hydrogen transfer reactions [35]. This seems to be favored in the case of ZM-210 because of the rapid deactivation by coke deposition. The methyloxonium species has also been suggested as an intermediate for the initial C-C bond formation [36,37]. Thus, C2-C4 alkene formation through this mechanism might be plausible in the case of ZM-210. The high yields of ethane and propane could be due to rapid conversion of alkenes into alkanes on the strong acid sites present in ZM-210. The high ratio of ethene to propene may be due to steric inhibition of the methanol alkylation reaction, diffusion constraints and higher reactivity of propene in hydrogen transfer reactions and oligomer formation, which is in agreement with the product distribution observed in this work (Table 1 ). The dealumination would remove the extra-framework alumina [17], leading to an enlargement of the effective pore size and to the uncovering of the Bronsted acid sites located in the 12-membered ring channels. Thus, methanol can diffuse through them and be converted to linear and branched alkenes and alkanes and aromatics. The almost complete removal of the extra-framework alumina and the reduction of the number of strong acid sites permits a mordenite to be obtained with a high stability and selectivity for conversion of A.J. Mar('hi and ~;.F. Fr.mertt/Appl. Catal. A 94 (1993) 91 106 99 methanol into light alkenes. The most predominant Bronsted-type sites in dealuminated mordenite are those due to silicon-hydroxyl and aluminiumhydroxyl groups that can absorb at 3730 and 3530 cm-1, respectively. The silanol groups can not interact with reagent and product molecules [ 22,26,27 ]. The Bronsted-type sites that absorb at 3530 cm i interact with oxygenates and hydrocarbons, but the interaction seems to be weaker than in the case of the hydroxyls that absorb at 3610 cm ~. In addition, the number of aluminium-hydroxyl sites is so low, as revealed by DRIFT, that they are very isolated from each other [14,15,18]. These two features explain the higher stability of this catalyst. The smaller interaction between hydrocarbon molecules and hydroxyl groups and the lower probability that two molecules of aromatics a n d / or alkenes adsorbed on strong acid sites can interact lead to a lower production of coke and, therefore, less deactivation. The mesopores created during dealumination might also play an important role on mordenite stability. The reduction of the number of strongest acid sites and the small concentration of low frequency aluminium-hydroxyl groups also explain the lower conversion of alkenes into alkanes and aromatics. The absence of extra-framework alumina and the reduction of the number of strong acid sites permits catalysts to be obtained with high durable activity and selectivity for light alkene production, as in the case of dealuminated mordenite with a silicon-to-aluminium atomic ratio of 80. Influence of water and methanol partial pressure and coke deposition Experiments with mixtures of methanol-water and methanol-nitrogen of various concentrations were carried out at 753 K and 1.04 bar on the sample with a silicon-to-aluminium atomic ratio of 80. It was observed that the catalyst stability was improved as the concentration of methanol in the feed was lowered by the addition of water (Fig. 3). In all cases, after a certain length of time, the conversion dropped from 100% to a final value, which depended upon the concentration of water. The higher this concentration, the lower the conversion at the final state. The principal reaction that takes place in this final state is the equilibrium reaction methanoldimethyl ether, with water as a product. As a consequence, increasing the concentration of water in the feed shifts the equilibrium to lower conversions of methanol. A similar behavior was observed for methanol conversion on SAPO34 molecular sieves [22]. As the concentration of methanol was lowered from 100 to 30% by adding water, which means a decrease of the partial pressure of methanol from 1.0 to 0.2 bar, the maximum yield of alkenes increased from 23 to 35 g of light alkenes per 100 g of methanol fed. The initial yield of alkanes decreased from about 30 to 14 g of light alkanes per 100 g of methanol fed and the initial yield of aro- 100 A.J. Marchi and G.F. Froment/Appl. Catal. A 94 (1993) 91-106 O f0 8 O ~ 6o x 4O ~ 2 0 Ooo - c o!2 04 I o'6 o's ,o M (mol M e O H / g c a t ) Fig. 3. Methanol conversion on dealuminated mordenite (Si/A1 = 80) at 753 K and 1.04 bar total pressure versus the total amount of methanol fed per gram of catalyst. W / F ° 10-32 g cat. h / m o l methanol. Feed: (a) methanol 100 %; (b) methanol-water 60 : 40 wt.- %; (c) methanol-water 30 : 70 wt.-%. { /7o \ oo 02 04 06 o8 M ( m o [ M e O H / g cat) ~o Fig. 4. Light alkene yields on dealuminated mordenite ( S i / A I : 8 0 ) at 753 K and 1.04 bar total pressure versus the total amount of methanol fed per gram of catalyst. W / F ° : 10-32 g cat. h / m o l methanol. Feed: methanol-water 30:70 wt.-% mixture. (a) Propene; (b) total butenes; (c) ethene. matics from about 10 to 5 g of aromatics per 100 g of methanol fed, indicating that the conversion of alkenes to alkanes and aromatics is lower. In all cases, the yield of propene and butenes increased with the amount of methanol fed per gram of catalyst, until a maximum was reached (Fig. 4). This increase was less pronounced when the concentration of methanol in the feed was lowered. The maximum yield of propene and butene becomes more important as the water concentration in the feed is raised (Fig. 5). After the maximum, the yield of C3-C4 alkenes diminished faster according to the amount of methanol fed per gram of catalyst as the concentration of methanol in the A.J. Marchi and G.F. Froment/Appl Catal. A 94 (1993) 91-106 101 24 0 =~ 20 g 0 -x 16 g ....I r~ 8 0 . ± __ i i 20 40 60 80 Water content (wt.-°/,,) Fig. 5. M a x i m u m yield of light alkenes as a function of water content in the teed at 753 K and 1.04 bar total pressure. (a) Propene, (b) total butenes. 9 ~z O o 0 g3 o © 02 04 06 M (real M e O H / g 08 cot) Fig. 6. Light alkane yields on dealuminated mordenite ( S i / A I = 8 0 ) at 753 K and 1.04 bar total pressure versus the total a m o u n t of methanol fed per gram of catalyst. W / F ° : 10 32 cat. h / m o l methanol. Feed: m e t h a n o l - w a t e r 30 : 70 wt.-% mixture. (a) i-Butane; (b) propane; (c) n-butane. feed was raised. In all cases, the ethene yield decreased monotonically with the amount of methanol fed per gram of catalyst. Propene and butenes are the most important alkenes. Considering the maximum yields, the ratio C J C 4 increased from 1.6 to 2.3 as the concentration of water in the feed was raised from 0 to 70 wt.-% (Fig. 5). The alkane yield decreased monotonically to zero as the amount of methanol fed per gram of catalyst increased (Fig. 6). This decrease was less pronounced when the concentration of methanol in the feed was lowered. Propane and i-butane were the principal alkanes. Ethane was not detected in the effluent of the reactor. Very small amounts of C5+ alkanes could be detected. The same behavior as for C3-C 4 alkanes was observed in the case of aromatics. C5+ alkenes followed a behavior similar to that observed in the case of C.~C4 alkenes (Fig. 7). In previous work, it has been observed that with a feed of nitrogen-methanol 102 6 A.J. Marchi and G.F. Froment/Appl. Catal. A 94 (1993) 91-106 I 0 4 0 o) o o 2 o .c: @ @ 02 04 0.6 08 M(molMeOH/g 10 c0t) Fig. 7. Aromatic and C.~+ alkene yields on dealuminated mordenite (Si/A1 = 80) at 753 K and 1.04 bar total pressure versus the total amount of methanol fed per gram of catalyst. W / F ° : 10-32 g cat. h/tool methanol. Feed: methanol-water 30:70 wt.-% mixture. (a) Total aromatics; (b) Cs+ alkenes. mixtures, the evolution of the conversion of methanol with the total amount of methanol fed per gram of catalyst was close to that observed with a feed of 100% methanol [22,38]. Thus, the higher total amount of methanol fed per gram of catalyst when water was used as diluent can not be attributed to the partial pressure of methanol, but to the presence of water in the feed. These results suggest that water plays a very important role in the suppression of the steps that lead to coke formation in the catalytic conversion of methanol on SAPO-34 [22]. For dealuminated mordenite, the influence of water on the total amount of methanol fed per gram of catalyst was less pronounced than in the case of SAPO-34, probably because of a different acidity. However, it was observed that after 0.015 mol of methanol per gram of catalyst was fed, i.e., almost at the beginning of the reaction, and a mixture of methanol-nitrogen with a partial pressure of methanol of 0.2 bar was used as feed, the yield of C2-C4 alkenes was 18.3%, while the yield of Cs+ non-aromatics was 6.0%. When a mixture of methanol-water with the same partial pressure of methanol was fed, the yield of C2-C4 alkenes was 21.9% and the yield of C~+ non-aromatics was less than 3.0%, for the same amount of methanol fed per gram of catalyst. According to the results obtained in the present work, the hydrocarbon distribution changes dramatically with the methanol partial pressure. The yield of coke precursors decreased as the concentration of methanol in the feed was lowered, thus leading to a slower deactivation by coke formation. The change in the hydrocarbon distribution may be attributed to both the partial pressure of methanol and, to some extent, to the interaction between water molecules and Bronsted acid sites. The decrease of the partial pressure of methanol leads to the suppression of the alkene conversion steps. On the other hand, it is known that water can interact with both Lewis and Bronsted acid centers [39,40]. Jentys et al. [39] have shown by means of IR and mass spectroscopy A.J. Marchi and (;.F. Fronwr~t/Appl. Catal. ,4 94 (1993)91 106 103 that water can be adsorbed on Bronsted-type sites, yielding a hydroxonium ion. Thus, the Bronsted acid sites on which water is adsorbed are not available for interaction with the reagent and the product molecules of the methanol conversion reaction. This is in agreement with the results of Vedrine et al. [41], who demonstrated that water weakens the acid sites. Therefore, the interaction between alkene molecules and these acid sites diminishes and so does the conversion of light alkenes into oligomers, alkanes and aromatics, thereby increasing the initial selectivity to C~-C4. In addition, water can transform Lewis into Bronsted acid centers. The reduction of the concentration of Lewis acid centers by water leads to a lower deactivation by coke, since coke formation seems to be faster on this type of sites [40]. The propene yield was more affected than the butene yield by the reduction of methanol concentration. This indicates that the first interacts more strongly with the active acid sites and must be the principal compound to be converted into oligomers, alkanes and aromatics. On the other hand, according to the results obtained in this work, in agreement with results reported by Naccache et al. for dealuminated mordenites [18], deactivation seems to take place in two stages. In the early stage of the reaction, the concentration of Bronsted acid sites seems to be gradually reduced by coke precursors that are strongly adsorbed. Thus, the probability of interaction with alkenes diminishes and so does their conversion into oligomers, alkanes and aromatics. This explains the increase of propene, butene and C.~,+ alkene yields and the decrease of alkane and aromatic yields with the amount of methanol fed per gram of catalyst in the early stage of the reaction. The ethene yield is not affected in the same way as the other alkenes, in agreement with the observation that ethane was not detected in the effluent. Light and C:,, alkenes are converted into alkanes, oligomers and aromatics. Propene seems to be principally converted into propane and butenes into butanes. These results are in agreement with mechanisms proposed in the literature [2,10,42,431. In the later stages of the reaction, the growth of the coke becomes more important, leading to the blockage of the pores, which reduces the number of acid centers rapidly, so that the yield of light alkenes decreases. At the early stage of the reaction, poly-substituted aromatics can be formed and diffuse easily through the 12-membered ring channels. As the reaction progresses, the effective pore size becomes smaller and thereby the diffusion of these aromatic molecules becomes more difficult, favoring coke growth. CONCLt!SIONS Three types of acid sites due to aluminium-hydroxyl groups have been detected in the mordenite samples used in this work. One of them absorbed at 3650 cm - 1 was attributed to extra-framework material. The other two, at 3610 104 A.J. Marchi and G.F. Froment/Appl. Catal. A 94 (1993) 91-106 and 3540 c m - 1, correspond to framework aluminum-hydroxyl groups of the zeolite. In the non-dealuminated mordenite, the low-frequency aluminiumhydroxyl groups seems to be poisoned by the extra-framework alumina. As the silicon-to-aluminium ratio is raised by dealumination, the concentration of aluminium-hydroxyl groups that absorb at 3650 and 3610 c m - 1decreases. The removal of extra-framework alumina opens the 12-membered ring channels and makes the aluminium-hydroxyl groups that absorb at 3540 cm-1 available. The decrease of the high-frequency aluminium-hydroxyl groups leads to a catalyst with high durable activity and selectivity for light alkenes. This is the case for mordenite samples with silicon-to-aluminium atomic ratios of 80 or higher. In these samples, the 12-membered ring channels are completely free and large molecules like poly-substituted benzenes can be formed and diffuse through the porous structure of the zeolite. The interaction of hydrocarbons with the catalyst surface is less pronounced than in the case of samples with higher silicon-to-aluminium ratios, due to the low number and the type of acid sites that are present in these dealuminated mordenites. The durable activity of the dealuminated mordenites and the selectivity to light alkenes can also be improved by decreasing the partial pressure of the methanol and adding water to the feed. The decrease of the partial pressure of methanol leads to a higher yield of light alkenes and a lower yield of alkanes, oligomers and aromatics. Water competes with light alkenes for the Br~nsted and Lewis acid sites. The adsorption of water on these acid centers reduces their strength and concentration and, thereby, the probability of their interaction with hydrocarbons. As a consequence, the initial conversion of light alkenes, principally propene, into oligomers, aromatics and coke decreases. Both the lower yield of coke precursors and the weaker interaction between coke precursors and acid sites reduces the deactivation by coke. At the early stages of the reaction, the gradual reduction of the number of acid sites decreases the probability of their interaction with light alkene molecules. Thereby, the conversion of light alkenes into alkanes and aromatics decreases and the selectivity to light alkenes increases. ACKNOWLEDGMENTS The authors are grateful to the Belgian "Ministerie voor de Programmatie van het Wetenschapsbeleid" for the IUAP Center of Excellence Grant. 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