Solid-Bases Using Methanol for C=C Bond Formation

石油学会誌 Sekiyu Gakkaishi, 36, (6), 421-435 (1993) 421

[Review Paper]

Catalytic Synthesis of α,β-Unsaturated Compounds over Solid-bases Using Methanol for C=C Bond Formation

Wataru UEDA

Dept. of Environmental Chemistry and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 227

(Received March 31, 1993)

A novel method for catalytically synthesizing α,β-unsaturated compounds was developed, where methanol was used as a key reagent for C=C bond formation. Reactants were saturated ketones, esters,

or nitriles, and their methyl or methylene groups at α-position are converted into vinyl group by addition of methanol. Magnesium oxide, activated by manganese ion or chromium ion, has been found to give the most effective catalytic performance. The most promising application of this synthetic method is demonstrated by the selective synthesis of acrylonitrile from acetonitrile and methanol over Mn-MgO catalyst. The selectivity for acrylonitrile was more than 95% and no deactivation of the catalyst was observed. The method was also applied to the conversion of acetone to methyl vinyl ketone and methyl propionate to methyl methacrylate. It was found that the surface property of MgO was modified by the presence of a transition metal ion; the addition of a metal ion with a larger ionic radius than Mg2+ increases the amount of surface base site, whereas the addition of a metal ion with an ionic radius smaller than that of Mg2+ induces the surface acid site without any appreciable changes in the amount of surface base site. Active catalysts were obtained in the latter case. Reaction mechanism was studied by isotopic tracer method. Reaction of deuterium-substituted acetonitrile and methanol revealed that the exchange reaction between hydroxyl hydrogen of methanol and methyl hydrogen of acetonitrile took place readily under the conditions of acrylonitrile synthesis, and that the isotopic distribution in acetonitrile after the reaction was very close to that of isotopic equilibrium. It was also found that the isotopic exchange reaction between methyl hydrogen of deuterated methanol and light methanol could occur under the same conditions. The key step in the reaction appears to be dehydrogenation of methanol to adsorbed formaldehyde species, which then reacts with the acetonitrile anion formed by proton abstraction with the surface base and, after cross-coupling and dehydration, yields acrylonitrile. It was concluded that surface acid property as well as surface base property played an important role in the course of the reaction.

1. Introduction oil crisis in the 1970's as a turning point, petrochemical industries have looked at various Methanol has become one of the major raw industrial processes based on olefins in order to get chemicals produced presently in chemical in- rid of their high dependencies on petroleum, and dustries, ranking third in volume behind ammonia have promoted studies on new processes based on and ethylene. The world production of methanol CO and methanol as carbon sources, the so-called is currently about 20 million tons per year. More C1 chemistry. This stimulated industrial and production of methanol is now expected since it research trends resulted in many new process will be utilized more in the future not only as a developments. Some of the methanol-based proc- chemical raw material but also as an energy carrier esses have been already industrialized, and some of for the various types of engines and for fuel cell1). the others are technologically ready for com- The development of the methods of transform- mercialization, having displaced the traditional ing methanol to industrial chemicals has had a synthetic routes based on olefins. history of over several decades2)-6). The develop- The object of this paper is to deal in detail with ment has progressed markedly since methanol was solid base-catalyzed synthetic reactions of α,β- manufactured via the hydrogenation of carbon unsaturated compounds with methanol7, which monoxide over solid catalysts at elevated tem- are some of the important and basic reactions peratures and pressures. Particularly, with the among the various methanol-based processes

石油学会誌 Sekiyu Gakkaishi, Vol. 36, No. 6, 1993 422 developed in the past several decades. using methanol as a reagent for C=C bond formation over solid-base catalysts promoted by 2. Catalytic Synthesis of α,β-Unsaturated Com- transition metal ions20). General reaction scheme pounds with Methanol over Solid-base is described as follows:

α,β-Unsaturated compounds are very important RCH2Z+CH3OH→M-MgO RCZ=CH2+H2+H2O chemicals in chemical industries, in particular for production of polymers. These compounds have traditionally been produced catalytically using olefins. For example, acrylonitrile is manufac- Z=C=OR', C=OOR', CN, phenyl tured by ammoxidation of propene over multi- component bismuth molybdate catalysts8). In R=Alkyl, H. (1) recent years, however, other synthetic methods based on the utilization of C1 chemicals such as The process is a base-catalyzed reaction where CO, methanol, and methane instead of olefins have methyl or methylene group activated by inductive been developed markedly because of the reasons electron withdrawal by such unsaturated sub- mentioned in the introduction. stituent as carbonyl, cyano or phenyl group is There are many promising methods claimed in converted into vinyl group by the addition of patents. An interesting example is the acryloni- methanol. trile synthesis by oxidative methylenation of 2.1. Synthesis of α,β-Unsaturated Nitriles acetonitrile with methane over metal oxide The most promising application of this method catalysts9). Since acetonitrile can be synthesized is demonstrated by the selective formation of from CO, H2, and NH3 catalytically10), the process acrylonitrile from acetonitrile and methanol21). may provide a promising route for acrylonitrile The conversion of acetonitrile proceeds catalytical- synthesis from C1 chemicals alone. Many similar ly at an elevated temperature (>300℃) and yields approaches have recently been undertaken exten- acrylonitrile(AN) selectively through dehydroge- sively11). Other examples are these classical nation, cross-condensation, and dehydration (Eq. processes where base- or acid-catalyzed condensa- (2)). tion reactions with formaldehyde are utilized12)-14). The well-known example is the condensation of CH3CN+CH3OH→CH2=CHCN+H2+H2O acetaldehyde, where acrolein can be normally (2) obtained because the initially formed aldol condensation product undergoes dehydration The catalysts are based on metal oxides which are simultaneously over the catalyst. In principle, well-known as solid-base materials, such as MgO, this reaction can be used for the synthesis of other CaO, La2O3, and so forth. These metal oxides unsaturated compounds. Alternatively, it is pos- themselves are, however, virtually inactive for this sible to carry out this type of reaction with in situ reaction, so that the catalytic properties of these generation of formaldehyde from methanol by basic oxides must be improved by the addition of oxidative dehydrogenation, and indeed some of its small amounts of transition metal cations. Table examples have recently been reported15)-19). As 1 shows examples for magnesium-based oxide another alternative, it is also possible to attain this catalysts. Obviously, a synergetic or bifunctional type of reaction directly with methanol by feature of the promoted M-MgO catalysts, depend- dehydrogenation. ing on the kind of element added, is significant for We have recently developed this type of reaction the catalytic activity and selectivity for the

Table 1 Synthesis of Acrylonitrile by the Reaction of Acetonitrile with Methanol over Various Metal Ion-containing MgO Catalystsa)

a) Reaction conditions are the same as those shown in Fig. 1. b) Commercial MgO.

石油学会誌 Sekiyu Gakkaishi, Vol. 36, No. 6, 1993 423

oxide can catalyze hydrogenation of AN to propionitrile much more effectively. The addition of aluminum did not result in appreciable effect on both activity and selectivity. Metal ion-containing catalysts can be prepared by the following impregnation method: Com- mercial MgO (Soekawa Rika, 99.92%, surface area 11m2・g-1) is impregnated with an aqueous solution of the corresponding metal with stirring for 12h at ambient temperature. Water is then evaporated by heating, followed by drying in air at 110℃ for 24h. Heat treatment in a nitrogen The reaction was run at 623K with 1.3kPa of acetonitrile stream at about 600℃ is necessary in order to in N2 carrier, the methanol/acetonitrile molar ratio was decompose the impregnated metal nitrate and to 10, and the space velocity (CH3OH+CH3CN+N2) was

80ml・min-1・g-cat-1. desorb water and CO2. MgO having a high surface area (137m2・g-1) was prepared from Fig. 1 Activity and Selectivity Change of 3wt% Cr- commercial MgO by the treatment in a boiling MgO Catalyst for the Conversion of Acetonitrile water and calcination at 600℃ in a N2 stream. with Methanol to Acrylonitrile as a Function of Unless otherwise noted, thus prepared MgO was Time used. The effects of metal ion content in magnesium reaction22). The addition of manganese, chromi- oxide on the catalytic property is significant. The um, or iron has a pronounced effect on the course effect of addition of chromium ion in the range of of the reaction; overall catalytic activity based on metal ion loading (wt%) on MgO from zero to 15 is acetonitrile conversion is increased by a factor of shown in Fig 2. The formation of AN increases 100. The reaction is really selective and the by- with Cr content and then decreases after passing products are small amounts of propionitrile and through a maximum at 3wt% content. Excess methacrylonitrile which are formed by the con- addition causes a decrease in activity and induces secutive reactions of the main product, AN. the decomposition of methanol, and ultimately the Under the present optimal conditions, one-pass pure metal oxide used for dopant predominantly conversion of acetonitrile reaches more than 30%, catalyzes the decomposition of methanol, so that keeping the high selectivity (>90%) for AN over about 3wt% loading of metal ion is preferable for these catalysts. Stabilities of these catalysts are attaining the effective catalytic property for the extremely good (Fig. 1). A prolonged reaction, overall reaction. The selectivity for AN on the however, revealed that the activity decreased but basis of acetonitrile conversion was scarcely very slowly to an appreciable level. Selectivity affected by the content of metal ion. This activity was almost unchanged during the slow decay of change is not attributed to surface area changes activity. The slow coke formation on the surface, because all catalysts have nearly the same surface in particular on the surface basic site, seems areas (ca. 100m2・g-1). Therefore, the drastic responsible for the slow deactivation because the catalyst was slightly darkened. Almost the same activity as that of the fresh catalyst can be recovered by calcination of the used catalyst in an air stream at elevated temperatures, followed by activation of the catalyst. Looking at other promoted catalysts, less active catalysts were obtained by addition of nickel or copper because of the remarkable decay of activity in a short reaction time. Characteristically, magnesium oxide containing nickel was extremely inactive for the formation of AN but yielded propionitrile in relatively higher yields than the other catalysts. Some of the AN produced Reaction conditions are the same as those in Fig. 1. oligomerized over the nickel site to form higher molecular weight compounds. It seems that the Fig. 2 Addition Effect of Cr Ion on MgO Catalyst in added nickel ion on the surface of magnesium the Reaction of Acetonitrile with Methanol

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temperatures leads the increase in the selectivity for these by-products. More improvements on these catalyst systems are still necessary to attain high selectivities at high levels of conversion. Direct synthesis of propionitrile is also possible by the reaction of acetonitrile with methanol utilizing Ni element that has an ability for selective hydrogenation of the intermediate unsaturated compound (Eq. (3)) as described earlier22).

CH3CN+CH3OH→CH3CH2CN+H2O (3)

MgO containing both Ni (1wt%) and Mn (2wt%) Reaction conditions are the same as those in Fig. 1 except ions was found active and selective (Table 2). for the contact time. Propionitrile was synthesized selectively at high conversion of acetonitrile. Interestingly, the pro- Fig. 3 Effect of Contact Time on the Reaction of Acetonitrile with Methanol over 3wt% Cr-MgO pionitrile selectivity obtained over the Ni-Mn-MgO Catalyst catalyst was much higher than that over Ni-MgO catalyst. It is thought likely that acrylonitrile formed on the surface Mn ion site is hydrogenated increase in activity implies that the reaction needs predominantly with hydrogen on the surface Ni both functions, i.e., added metal ion and surface ion site. base site. The activity decrease by excess addition The above nitrile synthetic process can be is thought likely, due to the decrease in the surface applied for the methacrylonitrile (MAN) synthesis base site, which will be discussed later. from propionitrile (Eq. (4)). Figure 3 shows the effect of contact time on the conversion and selectivity in the AN synthesis over CH3CH2CN+CH3OH→CH3C=CH2CN+H2+H2O 3.1wt% Cr-MgO catalyst at 350℃. The AN synthesis from methanol and acetonitrile is an (4) endothermic reaction (15.4kcal/mol) and the Almost the same trend in catalytic performance is equilibrium constant at 350℃ is 7.1, so thermo- observed for MAN synthesis as that for AN dynamic restriction is not a problem. Thus the synthesis (Table 3). Hence the improved catalytic conversion can increase with increasing contact property of magnesium oxide is obtainable by the time. However, the selectivity depends strongly specific promotion effect of Mn, Cr, and Fe ions. on the conversion of acetonitrile. The formation By using these three catalysts, 95% selectivity for of saturated nitrile and the consecutive C-C bond MAN is achieved at about 30% conversion of formation to produce methacrylonitrile become propionitrile under the optimized conditions. more prominent at longer contact times, indi- By-products are isobutyronitrile and crotononi- cating that propionitrile is formed from AN either trile. by hydrogenation with the molecular hydrogen We have recently found that crotononitrile formed in the reaction or by the hydrogen transfer found in this reaction was formed by catalytic from methanol, and that methacrylonitrile is skeletal rearrangement of MAN23),24). produced by further reaction of the propionitrile formed with methanol. The same feature is RCH=C-CNR'→RC-CN=CHR' (5) observed by changing the reaction temperature; that is, higher conversions at higher reaction

Table 2 Synthesis of Propionitrile by the Reaction of Acetonitrile with Methanol over Ni Ion-containing MgO Catalystsa)

a) The reactions were run at 648K with 1.6kPa of acetonitrile in N2 carrier, the methanol/acetonitrile molar ratio was 10, and the space velocity was 63ml・min-1・g-cat-1.

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Table 3 Synthesis of Methacrylonitrile by the Reaction of Propionitrile with Methanol over Various Metal Ion-containing MgO Catalystsa)

a) The reactions were run at 623K with 0.4kPa of propionitrile in N2 carrier, the methanol/propionitrile molar ratio was 10, and the space velocity was 81ml・min-1・g-cat-1.

Table 4 Skeletal Rearrangement of Methacrylonitrile over Various MgO and CaO Catalystsa)

a) Reaction conditions: reaction temp. 593℃, catalyst weight 50mg, pulse size 1μl, carrier gas(He)50ml・min-1.

This reaction is catalyzed by solid base, especially heat-treated basic magnesium carbonate, yielding cis and trans-crotononitrile with 80% selectivity at 70% conversion of MAN (Table 4). It was found that the reaction proceeds via CN migration mechanism as ascertained by the tracer study using 13C.

2.2. Synthesis of α,β-Unsaturated Esters Methyl methacrylate (MMA) can be synthesized by the reaction of methyl propionate and

CH3CH2COOCH3+CH3OH→ Reaction conditions are the same as those in Fig. 1 except CH3C=CH2COOCH3+H2+H2O (6) for the reaction temperature (673K). Ketones were a mixture of diethyl ketone and ethyl isopropenyl ketone.

Fig. 4 Addition Effect of Mn Ion on MgO Catalyst methanol7),25). Similarly, the magnesium oxide in the Reaction of Methyl Propionate with promoted by Mn ion is an effective catalyst. Methanol However, a slightly higher content of metal ion (from 5 to 17 wt%) than that of the catalyst employed for the AN synthesis gives a maximum Dependency on contact time for Mn-MgO yield of MMA as shown in Fig. 4. In addition, catalyst (16.7wt%) is given in Fig. 5. The general this reaction needs higher reaction temperatures dependency on contact time of the present syn- than the others. thetic method is also valid for methyl propionate Magnesium oxide became active at elevated conversion, where a higher selectivity for the temperatures to give MMA, but mainly diethyl desired product (methyl methacrylate) is attainned ketone (DEK), which was formed through the at shorter contact times. By using high space homo-coupling reaction of methyl propionate velocity to avoid the hydrogenation of MMA to over the surface base site, followed by decar- methyl isobutylate and further C-C bond forma- boxylation. The combining effect of metal ion tion, 65% selectivity for MMA was attained on the on the conversion is clear but it appears much more catalyst at 400℃. clearly in the selectivity change; selectivity to MMA It is also possible to produce methyl acrylate by increases with increasing content of manganese, the same process using methyl acetate and whereas the homo-coupling reaction of methyl methanol (Eq. (7))28). propionate is markedly suppressed.

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No polymerization of MVK was observed during the reaction and deactivation of the catalyst was negligible.

3. Characterization of Magnesium Oxide Cata- lysts Modified with Metal Ion

As shown in the general reaction scheme, the synthetic reactions need smooth sequential reactions of dehydrogenation, cross-coupling, and dehydration over the catalyst surface. To attain high selectivity, it is obviously necessary to

Reaction conditions are the same as those in Fig. 4 except suppress undesirable reactions, such as homo- the contact time. coupling, decomposition of methanol, polymerization and hydrogenation of the products. Fig. 5 Effect of Contact Time on the Reaction of Therefore, the roles of the surface base site and Acetonitrile with Methanol over 16.7wt% Mn- added metal ion in the present catalyst system are MgO Catalyst significant for the reaction. It is, however, unclear why the specific metal ions such as Mn, Cr, and Fe are effective and the others are not; and in CH3COOCH3+CH3OH→ addition, it is unclear what is the exact role of the CH2=CHCOOCH3+H2+H2O (7) surface base site in the reaction. Here, the bulk structure and the surface property of the modified However, high selectivity for methyl acrylate has catalysts were characterized by XRD, XPS, TPD of not yet been achieved because the homo-coupling CO2, and the reaction of 2-propanol. reaction of methyl acetate occurs to a higher extent 3.1. Solid-state Chemistry26) compared with the MMA synthesis. All XRD data obtained are summarized in Table 2.3. Synthesis of α,β-Unsaturated Ketones 5. All dried samples after impregnation of metal α,β-Unsaturated ketones are very important nitrates contain Mg(OH)2, so-called Brucite, and chemicals as starting materials for functionalized Mg3(OH)4(NO3)2. The catalytically active sam- polymer synthesis. However, highly efficient and ples (Fe-MgO, Mn-MgO, and Cr-MgO) and economic synthetic process has not yet been Al-MgO characteristically contain additional developed making it desirable to develope new phases that are identified as hydrotalcite-like processes. The present process (Eq. (8)) may con- phases. It is noteworthy that these phases tribute to this. Various kinds of α,β-unsaturated commonly appear when the additive metal ion is ketones can now be produced by this process using trivalent. Since the valency of manganese in the saturated ketones and methanol. The simplest starting material is 2, manganese ion seems to be example is the synthesis of methyl vinyl ketone partly oxidized into the tri- or tetravalent state (MVK) from acetone and methanol20). during impregnation. The valence change during the preparation was confirmed by XPS. CH3C=OCH3+CH3OH→CH2=CHC=OCH3+H2+H2O Mineralogical names of the hydrotalcite-like (8) phases found for Al, Fe, Cr, and Mn ion- containing materials are Hydrotalcite, Pytoaurite, Under optimum conditions, MVK selectivity of Barbertonite, and Desautelsite, respectively27). more than 60% is attained using Fe-MgO catalyst. When MgO was treated by the same impregnation

Table 5 Phases Formed During the Preparation of M-MgO Catalysts

a) These phases were observed only on highly loaded samples.

石油学会誌 Sekiyu Gakkaishi, Vol. 36, No. 6, 1993 427 procedure with water containing no metal nitrate, only Mg(OH)2 phase was formed. Both brucite and hydrotalcite-like phases have a layer structure. In Mg(OH)2, the magnesium cation is octahedrally surrounded by hydroxyl groups. The resulting octahedra share edges to form infinite sheets. Some of the magnesium ion in this sheet can be isomorphously replaced by other metal cation. When the metal ion is of the same charged cation as magnesium, such as Ni2+ and Cue, no change occurs in the brucite lattice. When some of the magnesium ions are replaced by a higher valency cation, e.g., Al3+, Fe3+, Cr3+, and Mn3+, this infinite layer gains a positive charge which has to be neutralized by an appropriate number of interstitial anions. From the d-value of (00l) reflection observed for every hydrotalcite- like phase, it is assumed that hydroxyl anion or carbonate anion is situated between the positive sheets28),28). After the treatment at 873K in flowing N2, the above-mentioned phases all disappeared and then poorly crystallized magnesium oxide phase was Fig. 6 TPD Spectra of Adsorbed CO2 on Various formed. By heating up to this temperature, M-MgO Catalysts and Their Surface Areas dehydroxylation was completed and nitrate anion was removed. After the heat-treatment of the samples in which the metal ion loading was less On the other hand, the capacity of adsorbing than 3wt%, only magnesium oxide phase, without CO2 was found to depend strongly on the kind of any other minor phase was observed as ascertained metal ion added. A comparison of the amount of by the XRD analysis. However, an additional surface base site per unit surface area for each minor phase was observed for the highly loaded catalyst revealed that Cu2+ ion and Ni2+ ion show a samples. pronounced effect of increasing the amount of 3.2. Temperature-programmed Desorption surface base of MgO. The amount of surface base (TPD) of CO2 on these catalysts was 4-5 times greater than that The number of accessible surface base sites on on MgO and corresponded to about 50% of the total the modified magnesium oxides with various surface oxide ion. Such an increase was also metal ions was estimated by the amount of observed for the Cd2+ ion or Mn2+ ion-containing chemisorbed CO2 molecules30). The TPD spectra MgO catalyst but the increase was not so for chemisorbed CO2 were recorded after CO2 remarkable as that for the Cu or Ni ion-containing adsorption at ambient temperature on the catalysts one. The addition of Fe3+ ion or Cr3+ ion had no which were pretreated at 873K for 2h in a He effect on the surface basicity. In contrast to the stream (50ml・min-1). Thehcatingratewas20K・ above results, the amount of surface base per unit min-1. The TPD spectra are shown in Fig. 6 surface area was decreased by addition of Al3+ ion. together with the surface area of each catalyst and The amount of surface base was plotted against the amount of the surface base. The desorption of the ionic radius of added metal ion in Fig. 7. CO2 for every catalyst began at near 323K and MgO has a nondirectional simple ionic crystal substantially slowed down at about pretreatment with NaCl structure, and the coordination num- temperature (873K). Unfortunately, the TPD bers of both Mg2+ and O2- are 631). The values of spectra for the catalysts showed multiple desorp- ionic radii in Fig. 7 are those for the metal ions tion peaks with unidentifiable maxima in the with coordination number 6, considering that the whole temperature-programmed region. It is, metal ion introduced into the lattice of MgO settles therefore, difficult to see clear differences among down to the same circumstance as Mg2+ The the spectra. Nevertheless, the spectra suggest that valences of the metal ions are based on the starting no substantial change occurs in the distribution of materials in the preparation of the catalysts. As surface basicity on MgO by addition of various can be seen in Fig. 7, the amount of surface base metal ions. drastically increased with the addition of a metal

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Mn-MgO showed higher activities. However, their activity per unit surface area was virtually of the same level as that of MgO. The Cu-MgO catalyst showed the highest activity for the conversion of 2-propanol, as expected from the TPD results. In the case of the Ni-MgO catalyst, the conversion was almost the same as that of Mn-MgO. If the α,β-unsaturated Compound synthesis is controlled only by the surface base property, the order of catalytic activity of various metal ion- containing MgO should coincide with both CO2 adsorption capacity and the activity for 2-propanol Fig. 7 The Amount of Surface Base Site of Metal Ion- conversion. However, an apparent discrepancy containing MgO Catalysts res. the Ionic Radius arose, as can be seen in Table 1 and Table 6. The of the Corresponding Added Metal Ion addition of Fe3+, Cr3+, and Mn2+ to MgO, which gave good catalysts for the reaction, did not result ion with an ionic radius slightly larger than that of in the formation of any specific surface base as Mg2+,whereas the amount of base either decreased determined by TPD. Cu-MgO and Ni-MgO or remained unchanged when a metal ion with an showed quite high activity for 2-propanol ionic radius smaller than that of Mg2+was added to conversion and high CO2 adsorption capacity, MgO. The addition of metal ion with a far larger although their catalytic activity for the reaction ionic radius, such as Cd2+ is not so effective for was low. Therefore, some other catalytic prop- enhancement of the surface basicity. erties besides surface base should be taken into The relationship obtained between the amount account. This is supported furthermore by the of surface base of metal ion-containing MgO and fact that the catalytic activity to form α,β- the ionic radius of added metal ion is explained as unsaturated compounds is not necessarily pro- follows30): When a metal ion with a radius larger portional to the concentration of basic sites (Fig. than that of Mg2+ is incorporated into the MgO 8). lattice, a distortion occurs in the lattice that Table 6 also gives another important infor- surrounds the added metal ion. The distortion mation. One may note that some of the catalysts may result in the expansion of the Mg-O bond promote the dehydration of 2-propanol to form length and in the localization of an electron on the propene. The results are shown in the last oxygen atom. Consequently, the solid base column of the table. The catalysts, which were property may appear or increase. That the active for the formation of propene, were Al-MgO, addition of a metal ion with a far larger ionic radius shows such small effect may be ascribed to the fact that the metal ion is not able to incorporate into the lattice because of its large ionic radius. It was clarified by IR study32) that the surface base site on alkaline-earth oxide was ascribed to lattice oxide ion, but there were many ambiguous points concerning its base property. The above enhancement effect of added metal ion on surface basicity may prove an important structural factor for the solid base. 3.3. Dehydrogenation and Dehydration of 2- Propanol The reaction was carried out at 623K with 0.4kPa of In order to confirm the TPD results, 2-propanol propionitrile in nitrogen carrier, the methanol/ conversion was studied to test the dehydrogenation propionitrile molar ratio was 10, and the space velocity ability of the catalysts, as the dehydrogenation of 2- was 73ml・min/l・g-cat-1. The surface basicity was propanol is a well-known base catalyzed reac- determined by CO2 adsorption. tion33). The results are shown in Table 6. All Fig. 8 Effects of the Mn Ion Loading on the Catalytic catalysts mainly catalyzed the acetone formation. Activity, the Selectivity of Mn-MgO Catalyst in MgO and Al-MgO showed low dehydrogenation the Reaction of Propionitrile with Methanol, activities while Fe-MgO, Cr-MgO, and and the Surface Basicity

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Table 6 Dehydrogenation and Dehydration of 2-Propanol normally kept to some extent even after heat over M-MgO Catalystsa) treatment at high temperatures, so that a coordinatively unsaturated state may occur at some higher charged cation (Fig. 9). This situation presumably gives rise to the surface acid site. Although the above explanation has not been confirmed, it seems plausible that the formation of hydrotalcite-like phase strongly leads to active catalysts accompanied with the formation of a)Reactions were performed at 528K in a micropulse surface acid site. reactor constructed of quartz glass. 2-Propanol (0.5μl) was injected into a nitrogen carrier gas (20ml・min-1). Catalyst weight was 15mg. 4. Activation of Methanol and Mechanistic b) Commercial MgO. Aspects of the Reaction

Fe-MgO, Cr-MgO, and Mn-MgO. Since the 4.1. Brief Summary on Methanol Activation surface acid property can be estimated from the over Oxide Surface formation of propene from 2-propanol, the results In the solid-base catalyzed reaction with meth- indicate that these added metal ions can exist as anol, the C-H bond of the methyl or methylene acidic sites on the main basic surface of MgO. group of the reactants must be activated by electron Interestingly, the catalysts, except Al-MgO, active withdrawing substituents such as carbonyl, cyano, in the formation of propene are effective for the and phenyl groups. The intermediate carbon synthetic reaction with methanol. This coinci- anion, formed after the abstraction of acidic dence may suggest that the surface acid property as hydrogen by the surface basic site, reacts with the well as the basic property of the added metal ion is adsorbed methanol to give the product. necessary for the catalysts to actively promote the Therefore, one of the important factors in the reaction selectively. The reason for the poor mechanistic consideration is the activated state of activity of the Al-MgO catalyst in spite of its acid methanol on the oxide surface. property is the lack of surface base property (Fig. The activation feature is significantly different 7). The low activity of the Cu-MgO catalyst is between acid- and base-catalyzed reactions. For due to the lack of acidic property (Table 6). It can the methanol conversion to hydrocarbons, which thus be said that the catalysts which have both acid is acid-catalyzed reaction, several reaction mecha- and base properties are effective for the reaction, nisms have been so far proposed34),35). The most and those which show only either acid or base plausible mechanism is as follows: Methanol first property are ineffective. reacts with surface Bronsted acid site to be In addition, it is worth noting that a hydro- protonated, and then the protonated methanol talcite-like structure in the catalysts, which showed reacts with the surface lattice oxygen (weak base) surface acid property (Tables 5 and 6), has been near the strong Lewis acid site accompanied with observed. The dehydration of dried catalyst water formation. The resulting surface methoxy precursor is completed, and the nitrate ion is lost group may be strongly polarized in the form of by heating up to 873K, forming a poorly crystal- δ+δ- lized magnesium oxide phase. The morphology CH3-Obecause of the strong electron-withdrawing of the hydrotalcite-like layer structure can be ability of the Lewis acid site. Although the CH3+ ion or CH3 radical is the least stable of the corresponding alkyl species, these species have high activities on the surface at elevated temperatures to react with other molecules to form hydrocarbons. When the protonated methanol reacts with methanol (or methoxy anion), di- methyl ether will be than produced, which is well- known acid-catalyzed intermolecular dehydrogenation. If hydrogen iodide is present in the system instead of a solid acid, the reaction of protonated methanol with HI proceeds rapidly to form methyl iodide because water is easily displaced from the

Fig. 9 Relationship between the Structure of Catalyst protonated methanol by the weak basic halide ion. Precursor and the Catalytic Property Methyl iodide then oxidatively adds to the metal

石油学会誌 Sekiyu Gakkaishi, Vol-36, No. 6, 1993 430 complexes, forming alkyl metal intermediates which are accessible to various reactions, such as CO insertion. It is, therefore, noteworthy that C-O bond fission of methanol is not difficult in the presence of an acid. On the other hand, no C-O fission of methanol during the activation step occurs in the presence of a base. When methanol reacts with a solid base like MgO, O-H bond fission of methanol first takes place, forming methoxy anion and proton. The proton is adsorbed on the surface lattice oxygen (strong base) and the methoxy anion on the 1: CH3C=O-φ 3: CH3CN metal ion (weak acid) because methoxy anion is a 4: CH3CH2CN 2: CH3C=OCH3 5: CH3CH2COOCH3 stronger base than OH. The formation of 6: CH3-φ adsorbed methoxy anion on metal oxide is clearly Reaction conditions are the same as those shown in demonstrated by IR studies36),37). The methoxy Fig. 1. anion species can then change into a formaldehyde intermediate since the solid base donates an Fig. 10 Approximate Acidity (pKa) of α-Hydrogen of electron through the oxygen atom of methoxy the Reactants and the Rate of Reaction with anion to facilitate hydrogen elimination as a Methanol over 3wt% Cr-MgO and 3wt% Fe- MgO Catalysts hydride ion from the methyl group via C-H bond fission. Formaldehyde is, however, very reactive and adsorbed strongly, so that this intermediate the substrates; the reaction rate is higher for the does not normally desorb as a product such as substrate with a lower pKa value. Undesirable formaldehyde except in special cases, but reacts reactions of methanol such as its decomposition to further at elevated temperatures to produce H2 and CO were negligible in the reaction of each ultimate products such as H2 and CO through the substrate except toluene. This relationship sug- adsorbed formate species. The special cases are gests that the hydrogen abstraction step from copper-, zinc-, and silver-based catalysts, all of methyl or methylene group activated by the which adsorb methanol and formaldehyde weakly. inductive electron withdrawal effect of the un- Thus, these are well-known catalysts for form- saturated substituent on the surface basic site of the aldehyde and methyl formate production45)-47). oxide catalyst relates to the rate determining step. Although the above description discriminates By analogy with the aldol type reaction and with the function of base in the activation of methanol the proposed mechanism for the alkylation of from that of acid, cooperation of acid and base toluene to give styrene and/or ethylbenzene over a should be taken into account when co-reactants solid base catalyst38), it can be assumed that the exist, which is just the case of the present reaction present reaction proceeds via the formaldehyde and catalyst system. intermediate formed from methanol by dehydrogen- 4.2. Aspect for Reaction Mechanism ation, followed by the condensation of form- 4.2.1. Reaction Rate of Various Reagents with aldehyde with the intermediate anion that is formed Methanol on the surface basic site. However, since form- First, the reaction rates of six substrates (aceto- aldehyde was scarcely detected in the product phenone, acetone, acetonitrile, propionitrile, mixture, and it is reasonable to assume that methyl propionate, and toluene) with methanol methanol is not easily converted to formaldehyde using Cr-MgO and Fe-MgO catalysts were or to a formaldehyde-like intermediate adsorbed on determined at 625K and compared with pKa values the surface, it is necessary to take into account the of their α-hydrogen7). The results are given in methanol activation step in more detail in the Fig. 10. The reactions were conducted under the reaction mechanism consideration. We thus con- same conditions for each substrate as described in ducted deuterium tracer experiments to further the footnote of the figure, and the rates listed were investigate the reaction pathway. calculated for the formation of corresponding α,β- 4.2.2. Deuterium Tracer Experiments30) unsaturated compounds. Deuterium isotope effects were investigated for Acetophenone reacts with methanol with the the reaction of acetonitrile with methanol to form highest rate. Virtually no reaction of toluene was acrylonitrile as a representative reaction. The observed. There can be seen a correlation results are shown in Table 7. Under the reaction between the rates of reaction and the pKa values of conditions shown in the footnotes of the table, the

石油学会誌 Sekiyu Gakkaishi, Vol. 36, No. 6, 1993 431

variously deuterated methanol and acetonitrile Table 7 Kinetir Isotope Effect on the Rate of Acrylonitrile Formation in the Reaction of Acetonitrile with were formed. The distribution of four kinds of Methanol over 3wt% Mn-MgO Catalysta) deuterated acetonitriles was very close to that calculated for the isotopic equilibrium. The fraction of methanol-d3 increased in step with a decrease in the fraction of methanol-d4; the con- tents of methanol-d0, methanol-d1, and methanol- a) Reaction conditions are the sane as those shown in Fig. 1 d2 were almost unchanged. It is evident that the except the reaction temperature (603K) and the space velocity (315ml・min-1・g-cat-1). methanol-d3 formed is CD3OH, and the hydrogen of acetonitrrile is scrambled almost exclusively with the O-D group of the CD3OD (Eqs. (9), (10)) conversion of acetonitrile was less than 1% and the

reaction was nearly quantitative. The kinetic CD3OD→CD3O-+D+ (9) isotope effect on the rate was clearly observed only -CH2CN+D+→DCH2CN (10) when the methanol deuterated at the methyl group was allowed to react at 603K, giving k((H3OH)/ k(CD3OD)=2.2. No kinetic isotope effects were because the ratio of methanol-d3 to methanol-d4 caused by deuterium substitution at hydroxyl (0.36) is equal to the calculated ratio (0.36) of the hydrogen of methanol and acetonitrile. number of hydrogen atoms in acetonitrile to the The isotopic distribution in d-niethanol and d- number of deuterium atoms in the O-D group of acetonitrile were measured by mass spectrometry the deuterated methanol (after taking impurities alter the reactions over 3wt o Mn-MgO and MgO into account). These results clearly showed that catalysts. These experimental results ate the exchange reaction between hydroxyl hydrogen summarized in Figs. 11 and 12, respectively. of methanol and hydrogen of acetonitrile took When a mixture of methanol-d4 and acetonitrile-d0 place readily. Similarly, a mixture of methanol- (10:1) was passed over Mn-MgO catalyst at 603K, d0 and acetonitrile-d3, when passed over MgO

Reaction conditions are the same as those in Table 7.

Fig. 11 H-D Isotope Exchange between Methanol and Acetioniteile over 3wt% Mn-MgO Catalyst

Reaction conditions are the same as those in Table 7.

Fig. 12 H-D Isotope Exchange between Methanol and Acetonitrile over MgO Catalyst

石油学会誌 Sekiyu Gakkaishi, Vol 36, No. 6, 1993 432 catalyst at 603K, also gave variously deuterated reaction takes place between the methyl hydrogens products as shown in Fig. 12. The isotopic of absorbed methanols. The result, however, does distribution in acetonitrile, however, did not reach not give us any information whether the exchange equilibrium. reaction proceeds involving the surface or directly These two figures disclosed other important between the adsorbed molecules. information. When a mixture of methanol-d1 When only CH3OD was allowed to react, the (CH3OD) and acetonitrile-d3 was passed over exchange between methyl hydrogen and hydroxyl Mn-MgO catalyst under the same reaction hydrogen was observed, but its rate was not so conditions, almost identical isotopic distributions, rapid as for the above-observed exchange reaction within experimental error, for both methanol and between CD3OD and CH3OD. When a small acetonitrile were obtained before and after the amount of CD3CN (99.8% D, 1.6kPa) was added in reaction. The same result was observed with the the reactant feed, the rate of the exchange reaction MgO catalyst as shown in Fig. 12. These facts was slightly decreased because of the strong imply that the exchange reaction between methyl adsorption of acetonitrile, and no hydrogen atom hydrogen of methanol and hydrogen of aceto- was found in acetonitrile, although it was already nitrile is far more difficult than the exchange of revealed that the exchange reaction between the hydroxyl hydrogen of methanol with the hydrogen protonic hydrogen from methyl group of aceto- of acetonitrile. nitrile and the hydroxyl hydrogen of methanol had 4.2.3. Exchange Reaction between Methyl Hy- occurred quickly under the same conditions. The drogen of Methanol39),40) result suggests two pathways for the exchange; a We conducted exchange reactions between direct hydride transfer (adsorbed molecules to methyl hydrogen of adsorbed methanol over MgO molecules) and a hydride transfer via the surface catalyst at slightly lower reaction temperatures without scrambling with the surface proton. We than those for condensation. The results are further conducted the reaction of a mixture of illustrated in Fig. 13. When a mixture of CD3OD CD3OD, H2, and D2 over MgO catalyst. We ob- and CH3OD (1:1) was allowed to react over the served a reasonable rate of the H2-D2 exchange MgO catalyst at 603K, two partially deuterated reaction and an appreciable amount of CD3OH. methanols, CH2DOD and CHD2OD, were readily The result clearly indicates that there are both formed and a nearly isotopically equilibrated proton and hydride species on the MgO surface but mixture of deuterated methanols was formed at only proton is incorporated into methanol, that is, longer contact times. The deuterium content in a direct hydride transfer is the major route for the methyl group of methanol was found practically exchange reaction of the methyl hydrogen of unchanged after the reaction, so that it can be methanol. concluded that the 4 and d3-methanol do not The exchange reaction is, now, explained result from the uptake of the hydrogens present rationally on the basis of the mechanism proposed originally on the MgO catalyst surface by CD3OD. for the transfer hydrogenation of ketones with The result clearly reveals that the exchange alcohols. Once a small amount of adsorbed formaldehyde is formed by dehydrogenation of methanol, the hydrogen transfer

CD3O-(a)+HCHO(a)→DCDO(a)+CD2HO-(a)

(11)

may readily proceed directly through the adsorbed species and results in the formation of dx-methanol (Eq. (11)). Since the adsorbed formaldehyde species is always regenerated, formaldehyde can be regarded as a chain carrier. From this mechanism, it can be imaged that MgO surface is covered with many protons and adsorbed methanols, and hydride transfers quickly through the adsorbed Reaction conditions are as follows: partial pressure, methanol. CD3OD 8kPa, CH3OD 8kPa; N2 balance at atmospheric 4.2.4. Reaction Mechanism pressure; reaction temperature 603K. The exchange reaction between the hydroxyl

Fig. 13 Isotopic Exchange Reaction between Methyl hydrogen of methanol and the methyl hydrogen of Hydrogens of Methanol over MgO Catalyst acetonitrile (Fig. 11) takes place readily, and the

石油学会誌 Sekiyu Gakkaishi, Vol. 36, No. 6, 1993 433 isotopic distribution in acetonitrile is close to formaldehyde, when passed over a solid base equilibrium, indicating that the dissociation step catalyst, gives acrylonitrile readily (573-623K). of the C-H bond of acetonitrile on the basic site to If the pKa value of the reactant is too high as in the form a proton and an anion is an equilibrium case of toluene, the rate-determining step should be reaction. Figure 11 also shows that the isotopes step (2). are scrambled completely in the methyl group of The correlation between the rate of reaction and acetonitrile and the hydroxyl group in methanol. the pKa value (Fig. 10) and the role of the added It is, therefore, evident that the dissociation of the metal ion are now discussed. A possible reaction O-H bond of methanol is not involved in the rate scheme is depicted in Fig. 14. The adsorption of determining step. This exchange reaction was substrates on added metal ion during the reaction not affected by the added metal ion in the MgO may be proved by the fact revealed in Table 8, catalyst, so that the two dissociation steps of the showing a comparison between the rate of the C-H bond of acetonitrile and of the O-H bond of reaction and that of the decomposition of methanol are promoted only by the surface base methanol during the reaction7). As has already site of MgO. been mentioned, methanol partly decomposed to No exchange reaction between the methyl CO and CH4, but the rate of decomposition is hydrogen of methanol and that of acetonitrile was seriously affected by the existence of substrates and observed but a clear kinetic isotope effect was by their kind. As can be seen in Table 8, methanol observed, thus strongly suggesting that the rate was easily decomposed by passing over the metal determining step is the dissociation of the C-H ion-containing catalyst, while MgO did not show bond of methanol. The exchange reaction of any activity for methanol decomposition. The methyl hydrogens between deuterated methanol fact indicates that methanol can adsorb on the and light methanol was explained by the presence added metal ion site, probably in the form of of a small amount of adsorbed formaldehyde species on the surface as a chain carrier. Therefore, this fact does not rule out but rather supports the fact that dissociation of C-H bond of methanol to be the rate determining step. On the basis of the above considerations, the following reaction steps are depicted as representative in the reaction of acetonitrile with methanol:

CH3OH(a)→CH3O-(a)+H+(a) Step(1)

CH3CN(a)→-CH2CN(a)+H+(a) Step(2)

CH3O-(a)→CH2O(a)+H-(a) Step(3)

CH2O(a)+-CH2CN(a)→

CH2=CHCN+-OH(a) Step(4)

H+(a)+H-(a)→H2 Step(5)

Steps (1) and (2) are in an equilibrium state. Step (3) is rate determining and forms an adsorbed hydride species. Step (4) is much faster than step Fig. 14 Reaction Scheme for the C=C Bond Formation (3) because a mixture of acetonitrile with and Methanol Decomposition

Table 8 Comparison between the Rate of C=C Bond Formation and Decomposition of Methanol

a) Reaction conditions are the same as those shown in Fig. 1. b) No decomposition of methanol took place on pure MgO catalyst.

石油学会誌 Sekiyu Gakkaishi, Vol. 36, No. 6, 1993 434

methoxy anion after hydrogen abstraction by the method is also applicable to the conversion of basic site, and is further dehydrogenated to give acetone to methyl vinyl ketone and of methyl CO. propionate to methyl methacrylate. The success When acetone is present in the feed gas, the rate of the present synthetic process can be attributed to of methanol decomposition drastically decreases. the added transition metal cations that modify the In the case of methanol-methyl propionate system, surface base property of MgO and add new methanol decomposition is fairly suppressed at the functions needed for the catalysis. As an same conditions as in acetone conversion. This is extension of this study, we have recently developed due to the fact that methoxy anion on added metal a new catalytic process for producing higher ion site cannot be further dehydrogenated because alcohols using methanol as the main building the coordinatively unsaturated state of added metal block40). Details will be reported elsewhere. ion, which is needed for dehydrogenation, is occupied by other substrate. References Methoxy anion formed by O-H dissociation on the surface base site may be adsorbed on the added 1) Tsuneshige, T., PETROTECH, 16, (3), 208 (1993). metal ion site because the metal ion is a stronger 2) Kein, W., "Catalysis in C1 Chemistry", D. Reidel Lewis acid than magnesium ion. Similarly the Publishing Co., Dordrecht (1983), p. 89. intermediate methylene anion formed by α- 3) Scheldon, R. A., "Chemicals from Synthesis Gas", D. Reidel Publishing Co., Dordrecht (1983), p. 127. hydrogen abstraction by the surface base site may 4) Wender, I., Catal. Rev.-Sci. Eng., 26, 303 (1984). also be adsorbed on the added metal ion site. 5) Calkins, W. H., Catal. Rev.-Sci. Eng., 26, 347 (1984). Methoxy anion is normally a much weaker base 6) Tominaga, H., Shokubai (Catalyst), 28, 184 (1986). than intermediate carbon anion; the intermediate 7) Ueda, W., Yokoyama, T., Kurokawa, H., Moro-oka, Y., methylene anions may be adsorbed more strongly Ikawa, T., Sekiyu Gakkaishi, 29, (1), 72 (1986). 8) Grasselli, R. K., Burrington, J. D., Brazdil, J. F., Faraday on the added metal ion site than methoxy anion. Discuss. Chem. Soc., 72, 203 (1982). The intermediate anion formed from the substrate 9) Khcheyan, Kh. E., Revenko, O. M., Shatalova, A. N., with a higher pKa value can be adsorbed more Gel'perina, F. G., Klebanova, F. D., Arunskaya, L. I., stably to the metal ion site because of the stronger Neftekhimiya, 17, 586 (1979). basic character. Therefore, the hydride-accepting 10) Monsanto, U.S. Pat. 4 179 462 (1979). 11) Firuzi, P. G., Petrol. Chem. U.S.S.R., 24, 80 (1984). ability of metal. ions from adsorbed methoxy 12) Vitcha, J. F., Sims, V. A., Ind. Eng. Chem., Prod. Res. Dev., anions may be affected by the adsorption of 5, 50 (1966). intermediate methylene anions, depending strong- 13) Standard Oil, Eur. Pat. 124 380 (1984). ly on the kind of substrate used. Since the 14) Eastman Kodak, U.S. Pat. 3 928 458 (1975). hydride-accepting ability of the metal ion is 15) British Petroleum, DE 1 905 763(1969). 16) ICI, Jpn. Kokai Tokkyo Koho 60-38 340 (1985). directly related to the rate determining step, the 17) BASF, GP 3 004 467 (1981). reaction rate must decrease with increasing pKa 18) Albanesi, G., Moggi, P., Appl. Catal., 6, 293 (1984). value of the substrate as observed in Fig. 10. 19) Ai, M., J. Catal., 112, 194 (1988). 20) Ueda, W., Yokoyama, T., Moro-oka, Y., Ikawa, T., J. 5. Summary Chem. Soc., Chem. Commun., 39 (1984). 21) Ueda, W., Yokoyama, T., Moro-oka, Y., Ikawa, T., Ind. Eng. Chem., Prod. Dev., 24, 340 (1985). A novel synthesizing process for α,β-unsaturated 22) Kurokawa, H., Kato, T., Ueda, W., Morikawa, Y., Moro- compounds has been developed by using methanol oka, Y., Ikawa, T., J. Catal., 126, 199 (1990). as a reagent for C=C bond formation. In this 23) Kurokawa, H., Nakamura, S., Ueda, W., Morikawa, Y., Moro-oka, Y., Ikawa, T., J. Chem. Soc., Chem. Commun., method, the C-H bond of methyl or methylene 658 (1989). group of the reactants must be activated by 24) Kurokawa, H., Nakamura, S., Ueda, W., Morikawa, Y., inductive electron withdrawal by unsaturated Ikawa, T., J. Catal., in press. substituents such as carbonyl, cyano, and phenyl 25) Ueda, W., Kurokawa, H., Moro-oka, Y., Ikawa, T., Chem. groups. This process is, thus, widely applicable Lett., 819 (1985). 26) Kurokawa, H., Ueda, W., Morikawa, Y., Moro-oka, Y., to synthesizing α,β-unsaturated compounds. Ikawa, T., MRS Int'l Mtg. on Adv. Mats. Vol.2, (1989), MgO, activated by manganese ion or chromium p.309. ion, has been found to give the most effective 27) Drits, V. A., Sokolova, T. N., Sokolova, G. V., Cherkashin, catalytic performance for the reaction. The most V. I., Clays Clay Miner., 35, 401(1987). promising application of this process is the 28) Miyata, S., Clays Clay Miner., 31, 305 (1983). synthesis of acrylonitrile from acetonitrile and 29) Reichle, W. T., Chemtech, 58 (1986). 30) Kurokawa, H., Kato, T., Yokoyama, T., Ueda, W., methanol. Selectivity of more than 95% is Morikawa, Y., Moro-oka, Y., Ikawa, T., J. Catal., 126, 208 attained at the optimum condition and no (1990). deactivation of the catalyst is observed. This 31) Dean, J. A., "Lange's Handbook of Chemistry", McGraw-

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Hill, New York (1973). (1968). 32) Iizuka, T., Hattori, H., Ohono, Y., Sohma, J., Tanabe, K., 37) Kondo, J., Sakata, Y., Maruya, K., Tamaru, K., Onishi, T., J. Catal., 22, 130 (1971). Appl. Surf. Sci., 28, 475 (1987). 33) Ueda, W., Yokoyama, T., Moro-oka, Y., Ikawa, T., Chem. 38) Itoh, H., Hattori, T., Suzuki, K., Murakami, Y., J. Catal., Lett., 1059 (1985). 79, 21 (1983). 34) Chang, C. D., Catal. Rev-Sci. Eng., 25, 1 (1983). 39) Ueda, W., Kuwabara, T., Kurokawa, H., Morikawa, Y., 35) Ono, Y., Mori, T., J. Chem. Soc., Faraday Trans. I, 77, 2209 Chem. Lett., 265 (1990). (1981). 40) Ueda, W., Ohshida, T., Kuwabara, T., Morikawa, Y., 36) Kagel, R. O., Greenler, R. G., J. Chem. Phys., 49, 1638 Catal. Lett., 12, 97 (1992).

要旨

メタノールを増炭剤とする固体塩基による接触的 α,β-不飽和化合物合成

上田渉

東京工業大学総合理工学研究科化学環境工学専攻, 227横浜市緑区長津田町4259

メタノールをC=C結合形成の反応基材とした接触的 α,β- た。すなわち, Mgイオンよりイオン半径の小さい金属イオン不飽和化合物合成プロセスを開発した。反応物は種々の飽和ケは表面塩基性を変えないかあるいは減少させるのに対し, Mg トン, エステル, ニトリルで, これらのα位のメチル, あるイオンより大きいイオン半径を持つ金属イオンは増加の方向にいはメチレン基がメタノールとの反応によりビニル基に変換さ作用する。ただし, 大きすぎるとその効果は小さくなる。このれる (次式)。現象はイソプロパノールの接触脱水素反応に対する活性の面からも確認された。加えて, この反応でのプロペン生成から鉄, RCH2Z+CH3OH→M-MgO RCZ=CH2+H2+H2O クロム, マンガンのイオンを添加した触媒は表面酸性質を有することが分かり, 酸性質が触媒活性に関係していることが示唆された。表面酸性質の発現は, 触媒調製段階で形成されるハイ Z=C=OR',C=OOR',CN,phenyl ドロタルサイト様構造に起因すると結論した。 R=Alkyl, H. (1) 本反応の反応機構を速度論的同位体効果, 水素-重水素同位この反応には固体塩基性金属酸化物触媒が必要で, なかでも酸体交換反応等により検討し, 考察した。メタノールとアセトニ化マグネシウムがよく, これに適切な金属イオンを添加して活トリルの反応で, メタノールのメチル基の水素を重水素で置換性化する。有効な添加金属イオンは鉄, クロム, マンガンの各した場合のみ速度論的同位体効果が現れた。メタノールの水酸イオンであった。これら金属イオン添加酸化マグネシウム触媒基の水素とアセトニトリルのメチル水素は触媒の塩基の作用にを用いることにより, たとえばメタノールとアセトニトリルとより容易に交換する。一方, メタノールのメチル基の水素とアの反応で95%以上の選択率でアクリロニトリルを合成するこセトニトリルのメチル水素とは全く交換しない。ただし, メタとができた。触媒性能は長時間安定であった。この触媒を種々ノール間でのメチル水素の交換は, 吸着物間の水素移行によりの類縁反応に応用し, その性能を評価した。容易に進行する。これらの結果より, 本反応の律速段階は触媒金属イオン添加酸化マグネシウムの物性を検討した。試料のの添加金属イオン上でのメタノールのC-H結合の解離にある表面塩基性をCO2の吸着能で調べたところ, 添加金属イオンと結論した。この段階はもう一つの反応物がルイス酸性の添加によってそれは大きく変化することが見い出され, この変化は金属イオン上にメタノールと共吸着するため, 反応物によって添加金属イオンのイオン半径によって整理できることが分かっ大きく影響を受ける。

Keywords

Methanol, Solid base catalyst, α,β-Unsaturated compound, Condensation, Surface property,

Reaction mechanism

石油学会誌 Sekiyu Gakkaishi, Vol. 36, No. 6, 1993