Modification of ZSM-5 Zeolite with Trimethyl Phosphite. 2. Catalytic

In the present work, we have studied the conversion reactions of primary and secondary C1−C4 alcohols over trimethyl phosphite modified ZSM-5 zeolit...
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J. Phys. Chem. B 1998, 102, 5280-5286

Modification of ZSM-5 Zeolite with Trimethyl Phosphite. 2. Catalytic Properties in the Conversion of C1-C4 Alcohols Pekka Tynja1 la1 , Tuula T. Pakkanen,* and Saara Mustama1 ki Department of Chemistry, UniVersity of Joensuu, FIN-80101 Joensuu, Finland ReceiVed: January 6, 1998; In Final Form: March 10, 1998

In the present work, we have studied the conversion reactions of primary and secondary C1-C4 alcohols over trimethyl phosphite modified ZSM-5 zeolites possessing only weak acidity and the results were compared to those of unmodified HZSM-5 catalyst. FTIR experiments on the conversion of methanol clearly showed that the formation of carbon monoxide can take place via two different reaction mechanisms depending on the amount of water present in the reaction system. We believe that the conversion of methanol to dimethyl ether and then to CO takes place even on the weak acid sites, but that the conversion of dimethyl ether to hydrocarbons requires stronger acidity. Therefore, in the conversion of small alcohols (methanol and ethanol) over the trimethyl phosphite modified ZSM-5 carried out in a flow reactor, the formation of ethers was dominant, and no further conversion was observed due to the incapability of weak acid sites to catalyze the formation of hydrocarbons. In the case of higher alcohols the production of ethers decreased significantly probably due to the steric constraints caused by the channels, and therefore the dehydration of alcohols to corresponding alkenes became dominant.

1. Introduction The conversion of methanol to gasoline range hydrocarbons over zeolites with a pentasile structure was first reported by Mobil in 19761 and since then the zeolite catalyzed reactions of methanol, as well as those of higher alcohols, have been under a considerable study. It is well-known that light olefins are the initial products in the conversion of methanol to hydrocarbons over HZSM-5 zeolite to be then converted to higher alkenes, alkanes, and aromatics up to C10.2-4 To increase the formation selectivity of light olefins, reactions such as the oligomerization of alkenes, alkylation, aromatization, cracking, and disproportionation have to be prevented. The selective production of light alkenes can be attained by selecting optimum reaction conditions or by affecting the catalytic properties of the zeolite. Short contact time between the catalyst and methanol and low partial pressure of methanol, as well as increased reaction temperatures, have been found to enhance the formation of small olefins,5 while the high conversion of methanol shifts the product distribution toward aromatic and C5+ hydrocarbons.3 The olefin yield can also be increased by the modifications of the acidic or shape selective properties of the catalyst. It has been reported that high silicon to aluminum ratios (low concentration of strong acid sites) have resulted in increased selectivity toward light alkenes.6,7 Pentasil zeolites modified with gallium, chromium, vanadium, manganese, nickel, and iron have been found to be catalysts for the production of small olefins with both high selectivity and long catalyst lifetime.8-10 Postsynthesis modifications of ZSM-5 zeolites with phosphorus compounds have been observed to increase significantly the formation of C2-C4 olefins.11,12 Enhanced shape selective properties of the phosphorus modified ZSM-5 are believed to be due to the replacement of Bro¨nsted acid sites by new sites with weaker acid strength,13,14 as well as to the increased tortuosity and reduced dimensions of the zeolite channels.11

Despite the great number of papers concerning the conversion of methanol to hydrocarbons, the reaction mechanism is still not fully understood. In the present study, we have approached the problem by using ZSM-5 catalysts with modified acidic properties in order to obtain more information about the catalytic role of weak and strong acid sites in the alcohol conversion reactions. 2. Experimental Section HZSM-5 zeolite was prepared according to the literature methods.15 The modifications which were carried out by both impregnation (P1ZSM-5) and CVD technique (P2ZSM-5), as well as the characterization of the catalysts, were described in the first part of this study.16 The reactions of methanol and ethanol over unmodified and phosphorus modified ZSM-5 were studied by in situ FTIR spectroscopy (Nicolet Magna 750 FTIR spectrometer). The sample chamber of a diffuse reflectance unit was equipped with nitrogen and vacuum line connections. Zeolite samples with a 50 wt % diamond powder matrix were dehydrated at 673 K for 2 h under nitrogen and vacuum conditions. Alcohols were introduced to the sample chamber under simultaneous vacuum and nitrogen flow at ambient temperature for 15 min. After the introduction of the reactant, the vacuum line was closed to prevent the escape of the alcohol or the reaction products. The temperature of the sample chamber was raised with the intervals of 25 K up to 673 K. After each increment, the system was allowed to stabilize for a few minutes before collecting the IR spectra (resolution 2 cm-1) at the reaction temperature. The conversions of methanol, ethanol, 1-propanol, 2-propanol, n-butanol, and 2-butanol were studied by using a continuous flow reactor. The catalysts were dehydrated in a nitrogen flow at 673 K overnight before the reactions. Reaction products were analyzed with a Hewlett-Packard 5890 Series II gas chromatography (on-line system) equipped with a flame ionization

S1089-5647(98)00672-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/12/1998

ZSM-5 Zeolite

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TABLE 1: Characterization of the Trimethyl Phosphite Modified ZSM-5 Zeolites modification crystallinity P acidity method Si/Al (%) (wt %) Al/P (µmol/g) HZSM-5 54.4 P1ZSM-5 CVD a P2ZSM-5 impregnated 57

99 a 87

2.2 2.7

0.52b 0.30

1060 930 935

a Not determined. b Determined from the aluminum content of unmodified HZSM-5.

detector. All reactions were carried out at 643 K, and the space velocity WHSV values for the reactants were calculated according to the equation WHSV ) (grams of alcohol)h-1(grams of zeolite)-1. 3. Results and Discussion The characterization of the materials used in the present work is presented in Table 1. The acidity of the phosphorus modified ZSM-5 zeolites was characterized by TPD of ammonia and FTIR (adsorption of pyridine) and 1H MAS NMR spectroscopies, and the results are described in the first part of this work.16 Both modification techniques resulted in materials with quite similar acidic properties. On the basis of the results we concluded the total loss of strong Bro¨nsted acid sites accompanied by the formation of new acidic centers with a weaker acid strength after the trimethyl phosphite modifications. However, the total number of acid sites stayed relative high in the modified zeolites. 3.1. Conversion of Methanol. In Situ FTIR Studies. The conversion of methanol to hydrocarbons has been reported to take place according to reactions 1-3.2 H2O

2 CH3OH y\ z CH3OCH3 +H O

(1)

CH3OCH3 f C2-C5 olefins

(2)

C2-C5 olefins f paraffins aromatics cycloparaffins C6+ olefins

(3)

Figure 1. In situ FTIR study on the conversion of methanol over unmodified zeolite HZSM-5.

SCHEME 1

2

The initial step in acid-catalyzed reactions of hydrocarbons is the hydrogen bonding of the reactant molecules with acidic hydroxyl groups of zeolite materials. The protonation of alcohols results in the formation of adsorbed oxonium ions (ROH2)+.17,18 After the introduction of methanol onto HZSM-5 at ambient temperature, the IR signals at 2950, 2920, and 2840 cm-1 as well as that at 1033 cm-1 assigned to adsorbed methanol, were detected (Figure 1). Lower wavenumbers compared with those of gas-phase methanol may be explained by the proton transfer between the acidic OH and methanol resulting in the formation of methyl oxonium ions (CH3OH2)+. At 175 °C, the formation of new bands at 2820 and 2890 cm-1, the elimination of water, and the disappearance of the bands above 1000 cm-1 were observed. The origin of the new bands was due to the formation of dimethyl ether via the elimination of water from the oxonium ions (Scheme 1, reaction 1). Above 250 °C the formation of hydrocarbons, possibly iso-butane or propene19 with an intensive IR band at 2967 cm-1, was detected. In the case of P2ZSM-5 (Figure 2) the conversion of oxonium ions to dimethyl ether took place at slightly higher temperature (200 °C) compared with that of the unmodified HZSM-5 (175 °C) due to the weaker acid strength of the catalyst. However,

the effect of the decreased acid strength on the conversion of methanol to dimethyl ether seemed not very remarkable. Above 375 °C and after a long contact time, a significant formation of methane (a band at 3018 cm-1 with a separation of the rotational lines 9 cm-1) as well as carbon monoxide was observed (reactions 3-5 in Scheme 1). No further changes in the spectra were observed even when the system was heated to 450 °C. On the basis of the above results, we believe that dimethyl ether is coordinated even on weak acid sites to be then converted to CH4, CO, and H2 according to the mechanism proposed by Haw,20 but the conversion of dimethyl ether to hydrocarbons requires the presence of strong acid sites. Another factor which may affect the formation of transition states, intermediates, or products in the methanol conversion reaction is the shape selective effect resulting from the reduced pore dimensions of the modified zeolites. However, on the basis of the pyridine adsorption experiments,16 the reduction of the channel dimensions of the modified materials was not drastic, and therefore, the inhibited conversion of small methanol molecules to

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Figure 3. In situ FTIR study on the conversion of methanol over phosphorus modified zeolite HZSM-5 (P1ZSM-5) with the presence of excess of water.

Figure 2. In situ FTIR study on the conversion of methanol over phosphorus modified zeolite HZSM-5 (P2ZSM-5).

hydrocarbons was related mainly to the reduced acid strength of the catalysts. Unlike in the case of P2ZSM-5, the formation of CO (observed at the wavenumber region of 2050-2250 cm-1) over P1ZSM-5 took place simultaneously with dimethyl ether (Figure 3). During the conversion of methanol over P1ZSM-5, the partial pressure of water in the sample chamber was high due to possible leaks of the reactor system. According to our observations, CO was formed via two different reaction routes. At the lower temperature the production of CO was possible only with the presence of the excess of water but the reaction mechanism is still unclear. At temperatures above 350 °C, the intensity of the CO bands was further increased due to the conversion of dimethyl ether according to the reactions 3-5 in Scheme 1. The effect of the excess of water on the production of CO was observed also in the conversion of methanol over HZSM-5 (Figure 4). At the temperatures above 225 °C, the amount of dimethyl ether began to decrease accompanied with the formation of carbon monoxide. Similarly, as in the case of P1ZSM5, the production of CO did not take place via the reaction mechanism presented in Scheme 1 since the concentration of methane at this stage was very low. It has been claimed that CO has no significance in the conversion of methanol to hydrocarbons.20,21 Our results are in agreement with this since the concentration of CO was not observed to decrease during the formation of hydrocarbons. Catalytic Studies. The results in the conversion of methanol over HZSM-5 and phosphorus modified ZSM-5 are presented in Table 2. Unmodified HZSM-5 produced a wide range of hydrocarbons ranging from methane to aromatics. Over phos-

Figure 4. In situ FTIR study on the conversion of methanol over unmodified zeolite HZSM-5 with the presence of excess of water.

phorus-modified ZSM-5 zeolites, the only product was dimethyl ether and no formation of hydrocarbons was observed. Even when the reaction temperature was increased from 370 to 400 °C and the WHSV value was decreased from 76 to 47, the only product formed in the reaction was dimethyl ether. The conversion of dimethyl ether to methane and CO over the phosphite-modified ZSM-5 requires very long contact time to take place, and therefore, the reaction was not observed in the flow reactor experiments. The initial step in the catalytic

ZSM-5 Zeolite

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TABLE 2: Conversion of Methanol and Ethanol to Hydrocarbons over HZSM-5 and Trimethyl Phosphite Modified ZSM-5a catalyst conversion of methanol (%) MeOH (wt %) dimethyl ether (wt %) WHSVb hydrocarbons (wt %) aliphatics aromatics linear branched olefins (wt %) C2 C3 C4 C2-C4 (total) C5-C6 (total) paraffins (wt %) C2 C3 C4 i-C4 C2-C4 (total) C5+ a

catalyst

HZSM-5

P1ZSM-5

P2ZSM-5

38.2 48.0 13.8 65.2

0.0 75.2 24.8 65.7

0.0 86.2 13.8 66.0

83.4 16.6 56.8 26.6

0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0

14.3 18.5 5.8 38.7 2.5

0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0

0.0 10.5 6.5 12.1 29.1 12.0

0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0

HZSM-5

P1ZSM-5

P2ZSM-5

63.6 30.8 5.6 68.2

16.8 58.6 24.6 66.0

11.4 67.8 20.8 70.7

98.2 1.8 90.1 8.1

100.0 0.0 100.0 0.0

100.0 0.0 100.0 0.0

79.2 3.1 3.9 86.2 1.0

100.0 0.0 0.0 100.0 0.0

100.0 0.0 0.0 100.0 0.0

0.7 1.7 1.1 3.4 6.9 4.3

0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0

conversion of ethanol (%) EtOH (wt %) diethyl ether (wt %) WHSVb hydrocarbons (wt %) aliphatics aromatics linear branched olefins (wt %) C2 C3 C4 C2-C4 (total) C5-C6 (total) paraffins (wt %) C2 C3 C4 i-C4 C2-C4 (total) C5+

Reaction conditions: T ) 370 °C, p ) 1 atm. b WHSV ) (grams of alcohol)h-1(grams of catalyst)-1.

Figure 5. In situ FTIR study on the conversion of ethanol over unmodified zeolite HZSM-5.

conversion of ethers to hydrocarbons is the coordination of the oxygen atom of the ether on the active site of the catalyst. It is likely that the adsorption of dimethyl ether followed by the conversion to CO over the phosphite-modified zeolites with only weak acid sites was possible, but the catalytic conversion of dimethyl ether to hydrocarbons did not take place. On the basis of the above results we believe that the conversion of methanol to hydrocarbons can be divided into two partial reactions (i) conversion of methanol to dimethyl ether and (ii) conversion of dimethyl ether to hydrocarbons requires different acid strength to take place. The elimination of water and H2 from adsorbed methanol to form dimethyl ether and CO, respectively, seems to take place quite easily even on weak acid sites. However,

Figure 6. In situ FTIR study on the conversion of ethanol over phosphorus modified zeolite HZSM-5.

the conversion of dimethyl ether to olefins and other hydrocarbons requires probably much stronger acidity. 3.2. Conversion of ethanol. In situ FTIR studies. In the FTIR spectrum of HZSM-5 introduced with ethanol, the bands at 2976, 2925, and 2900 cm-1 were observed (Figure 5). The conversion of ethanol to ethylene was detected at 175 °C by the appearance of sharp bands at 2988 and 950 cm-1 22. At temperatures above 200 °C, ethylene was further converted to hydrocarbons (IR band at 2970 cm-1). After the conversion of ethylene, the spectrum was almost identical with the corresponding spectrum of converted methanol shown in Figure 1. In the conversion of ethanol over phosphorus-modified ZSM-5 (Figure 6), the formation of ethylene required higher temperature (225 °C) than that in the case of unmodified catalyst

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SCHEME 2

(175 °C). Ethylene was the final product and no further conversion of ethylene to hydrocarbons was observed. The formation of diethyl ether was not observed since diethyl ether is readily (even thermally) converted to ethylene. Catalytic Studies. The product distributions in the conversion of ethanol over HZSM-5 and phophorus modified ZSM-5 zeolites are presented in Table 2. The main product over unmodified catalyst was ethylene, although paraffins and higher olefins were formed as well. The amount of diethyl ether in the products was relative small. The reaction was very selective toward aliphatic and linear hydrocarbons, and the formation of aromatics, as well as paraffins, was much less significant than in the conversion of methanol. The only products over phosphorus-modified catalysts were diethyl ether and ethylene. The conversion of ethanol as well as higher alcohols may take place via two possible mechanism resulting in the formation of corresponding dialkyl ethers and alkenes. The proposed carbenium ion mechanisms are presented in Scheme 2. In the case of ethanol, the initial step should be the conversion of the bidentate ethyl oxonium ions to adsorbed ethyl carbenium ions via the elimination of water. The carbenium ions may then react with free ethanol molecules to form diethyl ether. Another possible reaction path for the adsorbed carbenium ions is the elimination of proton resulting in the production of ethylene. Since no conversion of diethyl ether or ethylene over phosphorus-modified ZSM-5 was observed, it is obvious that the weak acidic centers of the catalyst are unable to catalyze the formation of higher hydrocarbons. The catalytic behavior of phosphorus-modified ZSM-5 was also studied by using a reactant mixture with 50 mol % of methanol and 50 mol % of ethanol. The main product was ethyl methyl ether, although smaller amounts of dimethyl and diethyl ether, as well as ethylene, were formed. A similar product distribution is found in a homogeneous acid-catalyzed dehydration reaction of two different alcohols,23 which is known to take

place via a bimolecular nucleophilic substitution (SN2) mechanism. Therefore, it is likely that also the heterogeneous zeolite catalyzed reaction takes place in principle according to the same mechanism. 3.3. Conversion of C3 and C4 Alcohols. The conversions of 1-propanol and 2-propanol to hydrocarbons over unmodified HZSM-5 and trimethyl phosphite modified ZSM-5 zeolites are presented in Table 3. The conversion level of 2-propanol over HZSM-5 was notably higher than in the case of 1-propanol. This can be explained by the selectivity of the formation of carbenium ions which takes place according to a following order: primary alcohol < secondary alcohol < tertiary alcohol. However, there were no significant differences in the product distributions, although the production of propene was more efficient in the case of 2-propanol. Phosphorus modifications resulted in notable reduction in the conversion level of 1-propanol, and both of the modified catalysts were very selective toward the production of propene. The formation of paraffinic hydrocarbons were notably reduced due to the modifications, and the only identified products were propane and n-pentane. The conversion of 1-propanol to dipropyl ether was very minor, and the proportion of the ether in the reaction products varied from 0.6 to 2.6% ages. In the conversion of 2-propanol, the catalytic activity of the phosphorus modified zeolites was relatively high compared with that of an unmodified catalyst and the formation of propene was dominant especially over P2ZSM-5. The formation of diisopropyl ether was not observed in the reactions. The main side products in both reactions were cis- and trans-2-butene, 2-methyl-2-butene, and n-pentane. The product distributions in the conversion of 1-butanol and 2-butanol over HZSM-5 and phosphorus modified ZSM-5 are presented in Table 4. The catalytic activity of unmodified catalyst was slightly higher in the conversion of 2-butanol. The conversion level over P1ZSM-5 was quite low in the case of 1-butanol, whereas in the conversion of 2-butanol the activity

ZSM-5 Zeolite

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TABLE 3: Conversion of 1-Propanol and 2-Propanol to Hydrocarbons over HZSM-5 and Trimethyl Phosphite Modified ZSM-5a catalyst conversion of 1-propanol (%) 1-propanol (wt %) dipropyl ether (wt %) WHSVb hydrocarbons (wt %) aliphatics aromatics linear branched olefins (wt %) C2 C3 C4 C2-C4 (total) C5-C6 (total) paraffins (wt %) C2 C3 C4 i-C4 C2-C4 (total) C5+ a

catalyst

HZSM-5

P1ZSM-5

P2ZSM-5

64.2 35.2 0.6 74.1

41.9 55.5 2.6 74.9

31.7 66.3 2.0 75.9

92.2 7.8 73.1 19.1

100.0 0.0 98.9 1.1

100.0 0.0 99.6 0.4

1.3 45.3 15.4 62.0 6.1

0.0 96.2 1.7 97.9 0.5

0.0 99.2 0.4 99.6 0.4

0.0 5.2 3.2 7.1 15.5 8.6

0.0 0.7 0.0 0.0 0.7 0.9

0.0 0.0 0.0 0.0 0.0 0.0

conversion of 2-propanol (%) 2-propanol (wt %) diisopropyl ether (wt %) WHSVb hydrocarbons (wt %) aliphatics aromatics linear branched olefins (wt %) C2 C3 C4 C2-C4 (total) C5-C6 (total) paraffins (wt %) C2 C3 C4 i-C4 C2-C4 (total) C5+

HZSM-5

P1ZSM-5

P2ZSM-5

83.0 17.0 0.0 70.0

70.3 29.7 0.0 69.5

79.4 20.6 0.0 75.9

94.3 5.7 74.7 19.6

100.0 0.0 98.2 1.8

100.0 0.0 99.4 0.6

1.1 53.7 9.8 64.6 9.0

0.0 92.2 4.3 96.5 2.2

0.0 97.6 1.0 98.6 0.8

0.0 3.9 2.6 5.6 12.1 8.5

0.0 0.0 0.0 0.0 0.0 1.3

0.0 0.0 0.0 0.0 0.0 0.5

Reaction conditions: T ) 370 °C, p ) 1 atm. b WHSV ) (grams of alcohol)h-1(grams of catalyst)-1.

TABLE 4: Conversion of 1-Butanol and 2-Butanol to Hydrocarbons over HZSM-5 and Trimethyl Phosphite Modified ZSM-5a catalyst conversion of 1-butanol (%) 1-butanol (wt %) dibutyl ether (wt %) WHSVb hydrocarbons (wt %) aliphatics aromatics linear branched olefins (wt %) C2 C3 C4 C2-C4 (total) C5-C6 (total) paraffins (wt %) C2 C3 C4 i-C4 C2-C4 (total) C5+ a

catalyst

HZSM-5

P1ZSM-5

P2ZSM-5

57.3 42.7 0.0 76.0

16.1 83.9 0.0 76.9

48.5 51.5 0.0 77.5

94.1 5.9 78.4 15.7

100.0 0.0 94.3 5.7

100.0 0.0 99.2 0.8

0.7 6.8 62.4 69.9 6.5

0.0 5.4 85.6 91.0 6.4

0.0 1.2 97.2 98.5 0.8

0.0 2.7 3.0 4.9 10.6 7.1

0.0 0.0 0.6 0.0 0.6 2.0

0.0 0.0 0.3 0.0 0.3 0.4

conversion of 2-butanol (%) 2-butanol (wt %) di-1-methylpropyl ether (wt %) WHSVb hydrocarbons (wt %) aliphatics aromatics linear branched olefins (wt %) C2 C3 C4 C2-C4 (total) C5-C6 (total) paraffins (wt %) C2 C3 C4 i-C4 C2-C4 (total) C5+

HZSM-5

P1ZSM-5

P2ZSM-5

69.8 30.2 0.0 76.5

74.1 25.9 0.0 76.9

55.9 44.1 0.0 73.2

96.1 3.9 83.9 12.2

100.0 0.0 98.5 1.5

100.0 0.0 96.8 3.2

0.5 5.2 72.0 77.7 5.7

0.0 1.7 95.5 97.2 2.0

0.0 3.3 91.8 95.2 3.3

0.0 1.8 2.1 3.5 7.4 5.5

0.0 0.0 0.0 0.0 0.0 0.8

0.0 0.0 0.2 0.0 0.2 1.3

Reaction conditions: T ) 370 °C, p ) 1 atm. b WHSV ) (grams of alcohol)h-1(grams of catalyst)-1.

of the catalyst was very high. The activity of P2ZSM-5 was slightly lower compared with unmodified catalyst in both reactions. Phosphite modifications increased notably the selectivity of formation of butene isomers. The distribution of butene isomers was very similar in the conversions of both 1-butanol and 2-butanol over HZSM-5 and phosphorus-modified ZSM-5. The normalized amounts of formed 1-butene, as well as cis- and trans-2-butene, are presented in Figure 7. There were no significant differences in the production of 1-butene from 1-butanol and 2-butanol. This can be explained by the conversion of primary butyl carbenium ions to more stable secondary carbenium ions via a proton transfer in the case of 1-butanol according to the mechanism presented in Scheme 2. The production of 1-butene (30.2% and 24.1% for 1-butanol and

2-butanol, respectively, over HZSM-5) was notably higher compared with the amount of 1-butene in the equilibrium composition of butene isomers at 600 K24 (1-butene, 16%; trans2-butene, 52%; and cis-2-butene, 31%) probably due to the shape selective effect caused by the pore structure of zeolite ZSM-5. Again, the modifications reduced significantly the formation of paraffins resulting in the production of only very small amounts of butane and pentane. The formation of alkenes and dialkyl ethers from corresponding C1-C4 alcohols is presented in Figure 8. The production of ether decreased rapidly with increasing size of the alcohol and the formation of alkenes became dominant. The reduced formation of ethers may be explained by the restricted transition state or product selectivity of zeolite ZSM-5. As presented in Scheme 2, the conversion of alcohol to ether requires the attack

5286 J. Phys. Chem. B, Vol. 102, No. 27, 1998

Figure 7. Formation selectivity of butene isomers from 1-butanol and 2-butanol over unmodified and phosphite modified zeolite ZSM-5.

Tynja¨la¨ et al. intermediates was concluded and the reaction was possible with the presence of water eliminated only via the dehydration of methanol. In the case of the phosphite-modified ZSM-5, the catalytic activity of weak acid sites in the production of both dimethyl ether and CO was concluded. However, catalytic activity in the formation of hydrocarbons was not observed. Inhibited formation of hydrocarbons was supposed to be due mainly to the presence of only weak acid sites in the phosphorusmodified zeolites. The coordination of ethers on weak acid sites was probable, but the catalytic conversion of the ethers to hydrocarbons was supposed to be unlikely thus requiring the presence of strong acidity. In the case of higher alcohols the production of ethers decreased drastically and the formation of alkenes became dominant. The conversion of alcohols was concluded to take place via a carbenium ion mechanism in which the corresponding dialkyl ethers, as well as alkenes, may be formed. Reduced production of ethers with the increasing size of the reactant alcohol was explained by the shape selective effect of the pore structure of the modified ZSM-5 zeolites on the formation of dialkyl ether precursors. Acknowledgment. Financial support from the Technology Development Centre (TEKES) is warmly acknowledged. References and Notes

Figure 8. Production of dialkyl ethers and alkenes from corresponding C1-C4 alcohols over phosphorus modified zeolite ZSM-5.

of a free electron pair of an alcohol molecule to the positively charged carbon atom of an adsorbed carbenium ion. As the size of the alcohol increases, the reaction becomes more improbable due to the steric effects of the zeolite pores. The elimination of proton from adsorbed carbenium species is sterically more favorable than the formation of ether, and therefore, the formation of alkenes becomes more dominant in the cases of 1-propanol and higher alcohols. 4. Conclusions On the basis of earlier studies,16 the acidic properties of ZSM-5 zeolites modified with trimethyl phosphite by using both impregnation and CVD techniques were very similar and the presence of only weak acidity was observed. The effect of the phosphite modifications on the product distibution in the alcohol (primary and secondary C1-C4 alcohols) conversion reactions was also quite similar in the case of both modified materials, P1ZSM-5 and P2ZSM-5, although some slight differences in the catalytic activities (mostly in the conversion of butanols) were observed. In the conversion of small alcohols over the phosphorusmodified ZSM-5 zeolites, the main reaction products were the corresponding dialkyl ethers. In the methanol conversion reaction, the production of carbon monoxide via two different reaction routes was observed. In the first route taking place at ca. 200 °C the excess of water was found essential for the formation of CO. At higher temperatures, the conversion of dimethyl ether via methyl carbenium ion and formaldehyde

(1) Meisel, S. C.; McCullough, J. P.; Lechtale, C. H.; Weisz, P. B. Chemtech. 1976, 6, 86. (2) Chang, C. D.; Silvestri, A. J. J. Catal. 1977, 47, 249. (3) Chu, C. T-W.; Chang, C. D. J. Catal. 1984, 86, 297. (4) Anderson, M. W.; Klinowski, J. J. Am. Chem. Soc. 1990, 112, 10. (5) Dehertog, W. J. H.; Froment, G. F. Appl. Catal. 1991, 71, 153. (6) Chang, C. D.; Chu, C. T.; Socha, R. F. J. Catal. 1984, 86, 289. (7) Wu, M. M.; Kaeding, W. W. J. Catal. 1984, 88, 478. (8) Szostak, R.; Nair, V.; Thomas, T. L. J. Chem. Soc., Faraday Trans. I 1987, 83, 487. (9) Inui, T.; Yamase, O.; Fukuda, K.; Itoh, A.; Tarumoto, J.; Morinaga, M.; Hagiwara, T.; Takegami, T. Progress of the Eighth International Congress on Catalysis, West Berlin; Verlag Chemie: Weinheim, 1984, Vol. III, p 569. (10) Wichterlova´, B.; Beran, S.; Bedna´rova´, S.; Nedomova´, K.; Dudikova´, L.; Jiru, P. Stud. Surf. Sci. Catal. 1988, 37, 199. (11) Kaeding, W. W.; Butter, S. A. J. Catal. 1980, 61, 155. (12) Ve´drine, J. C.; Auroux, A.; Dejaifve, P.; Ducarme, V.; Hoser, H.; Zhou, S. J. Catal. 1982, 73, 147. (13) Jentys, A., Rumplmayr, G. and Lercher, J. A. Appl. Catal. 1989, 53, 299. (14) Romannikov, V. N.; Tissler, A. J.; Thome, R. React. Kinet. Catal. Lett. 1993, 51, 125. (15) Plank, C. J.; Rosinski, E. J.; Schwartz A. B. U.S.Patent 3,926,782, 1975. (16) Tynja¨la¨, P.; Pakkanen, T. T. Microporous and Mesoporous Materials; 1998, in press. (17) Haase, F.; Sauer, J. J. Phys. Chem. 1994, 98, 3083. (18) Nusterer, E.; Blo¨chl, P. E.; Schwatz, K. Angew. Chem., Int. Ed. Engl. 1996, 35, 175. (19) Kolbe, S. Acta Chem. Scand. 1988, A42, 185. (20) Munson, E. J.; Kheir, A. A.; Lazo, N. D.; Haw, J. F. J. Phys. Chem. 1992, 96, 7740. (21) Munson, E. J.; Lazo, N. D.; Moellenhoff M. E.; Haw, J. F. J. Am. Chem. Soc. 1991, 113, 2783. (22) Moser, W. R.; Chiang, C. C.; Thompson, R. W. J. Catal. 1989, 115, 532. (23) Morrison, R. T.; Boyd, R. N. Organic Chemistry, 4th ed.; Allyn and Bacon, Inc.: Boston, 1983, p 534. (24) Alberty, R. A.; Gehrig, C. A. J. Phys. Chem. Ref. Data 1985, 14, 803.