Peng Feng
(Department of Chemical Engineering, South China University of Technology, GuangZhou 510641, China)

Received Jan. 24, 2000; Supported by the National Natural Science Foundation of China.(Grant No. 29903003)

Abstract A novel Mo/C catalyst has been found to have a high activity and selectivity for the vapor phase carbonylation of methanol to methyl acetate without addition of CH₃I promoter to the feed. A temperature around 573K is found to be optimum for ester production, the space-time yield of methyl acetate reaches 4.33mol/kgcat^.h.
Keywords methanol carbonylation, vapor phase carbonylation, Mo/C catalyst, supported catalyst, methyl acetate

The conversion of methanol into hydrocarbons has attracted significant research attention in C1 chemistry. Monsanto process for acetic acid via carbonylation of methanol is an example of the largest scale commercial application of this route^[1,2], Although iodide-promoted Rh-catalyzed carbonylation of methanol to acetic acid is one of the most successful examples of homogeneous catalysis employed industrially today, it is affected by the disadvantages associated with a very noble rhodium, a highly corrosive reaction medium due to the use of methyl iodide as promoter, and difficult product separation^[3]. In order to improve the current technology, the use of a heterogeneous catalyst seemed very interesting to us. Many research efforts have been indulged in the search for an appropriate catalyst to carry out vapor phase carbonylation under atmospheric pressure; for example, supported rhodium complexes^[4], and Ni/C catalyst.^[5,6] Ni/C catalyst was found to exhibit satisfactory activity and selectivity for the carbonylation of methanol. However, there has been very little success in finding heterogeneous catalysts that can operate effectively without a halide promoter. ^[7,8]According to the known carbonylation mechanism ^[1-4] ,the initial step is that methyl iodide (CH₃I) directly carbonylates with carbon monoxide to form CH₃COI which further interacts with methanol (MeOH) to form methyl acetate (AcOMe) and HI, and then MeOH reacts with HI to form CH₃I. Thus it seems likely that methanol does not carbonylate directly with CO during the catalytic cycle, in fact, this carbonylation reaction is indirect catalytic carbonylation. Without any halide in the feed as promoter, direct carbonylation of methanol is presented^[9,10].

1. EXPERIMENTAL
A commercially available granular activated carbon (coal based, in 20-40 mesh particle size, provided by Datong Activated Carbon Factory) was washed with deionized water. The carbon was then filtered, and kept in an oven at 393K for 12h.
Catalysts were prepared by incipient wetness impregnation of supports. Molybdeum was impregnated using ammonium heptamolybdate solution for 4-6h, followed by sulfidizing in (NH₄)₂S aqueous solution (S≥8% mass) for 4h, then drying in an air oven at 393K for 12h. The Mo content in the catalyst was 10% (mass). According to similar preparation procedure, a series of supported catalysts were prepared in the different impregnating solution (ammonium heptatungsten, chromium nitrate, and cobalt nitrate). Prior to the catalytic tests, the dried catalysts were treated in situ with H₂ (or N₂), at 673K for 2h.
Methanol carbonylation was carried out in a fixed bed reactor with a continuous flow system at atmospheric pressure. The reactor was made of glass with an inner diameter of 22mm. The mass of catalyst used was 2.0g. Carbon monoxide (99.9% Fushan) was saturated with methanol by bubbling the gas into a reservoir, which was kept at a suitable temperature to attain the desired methanol concentration in the reactant flow. A quantitative analysis of the products was carried out with a gas chromatograph (Shanghai 102) equipped with a flame ionization detector. The system allowed the separation of methane, ethene, dimethyl ether (DME), methanol, methyl acetate and acetic acid. Methanol conversion (X) and selectivity (S_i) for the reaction are defined as:
X=ΣX_i*N_i/(ΣX_i*N_i+X₀)×100%
Si=X_i*N_i/ΣX_i*N_i×100%
Where, X₀ = content of efflux of methanol (mol%);
X_i = content of efflux of product i (mol%);
N_i = number of methyl group in product i.

2. RESULTS AND DISCUSSION
2.1 Methanol carbonylation over different supported catalysts
Table1 shows the effect of different catalysts on the activity and selectivity of methanol carbonylation. Although three kinds of metals (Cr. Mo. W) belong to same group elements (VIB), only sulfurized Mo/C catalyst is active. Both sulfurized Cr/C and W/C catalysts produce dimethyl ether as the main product; no methyl acetate is formed. Sulfurized Co catalyst (I) is active for methanol carbonylation, and its selectivity to methyl acetate is similar to the observed for sulfurized Mo catalyst (I). However, methanol conversion is significantly higher in the latter.
After these sulfurized catalysts were reduced, the phenomenon of peculiar interest was observed. Cobalt catalyst (II) is inactive for methanol carbonylation (methanol conversion is only 1.4%, no methyl acetate is found), and its performance is similar to the observed for Co catalyst (III). This is due to the reduction of sulfurized Co catalyst into metal cobalt catalyst, which leads to catalyst deactivation. On the other hand, this reduced Mo catalyst (II) reveals surprisingly higher catalytic activity for methanol conversion than sulfurized Mo catalyst (I). However, direct reduced-unsulfurized Mo catalyst(III) is significantly lower catalytic activity and selectivity than the reduced-sulfurized Mo catalyst (II). It seemed plausible to us that a novel reduced Mo/C catalyst is very active to catalyze the carbonylation reaction in the gas phase without any promoter. This reduced sulfide has not received much attention for methanol carbonylation; the characterization of this catalyst structure is being carried on.

Table 1. Methanol carbonylation by the various catalysts

Reaction conditions: T=573K, P=100KPa, GHSV=3250 L/kgcat ^. h.
MeOH/CO=1/11 (molar ratio), Time on stream = 4h
(I)_Sulfurized catalyst was treated in N₂ (673K, 2h)
(II)_Sulfurized catalyst was reduced in H₂ (673K, 2h)
(III)_Unsulfurized catalyst was reduced in H₂ (673K, 2h)
Compared to the known liquid phase methanol carbonylation processes, some advantages of this direct methanol carbonylation over the novel Mo/C catalyst are as follows: (1) halide promoters are not required, corrosive attack to the reactors and pipes of iodide-acid solution may be avoided; (2) noble metal rhodium are not required, this catalyst is cheap; (3) the reaction is carried out under atmospheric pressure.

2.2 Effect of reaction temperature
Table 2 shows the effect of reaction temperature on the activity and selectivity. Data for temperature examinations are collected with the novel Mo/C catalyst on stream for at least 4h. The data are considered as steady state date. The selectivity to methyl acetate decrease with the decrease of the temperature, the conversion of methanol increase with the increase of the temperature. A maximum of carbonylation activity is reached at 573K, TOF of this catalyst reaches 4.16 h^-1. The space time yield of methyl acetate is 4.33 mol/(kgcat ^. h), this datum is surprisingly higher than that of methanol carbonylation reported in the literatures^[7,8]. This result suggests that the novel Mo/C catalyst may be promising for methyl acetate production from the carbonylation of methanol at atmospheric pressure, without requiring methyl iodide as promoter.
From table 2, we can see that methane is formed as the main byproduct, DME is formed at a lower extent. Methanol carbonylation is found to be a very selective reaction to methyl acetate at lower temperature (T<553K). Higher temperatures lead a sharp decrease of the carbonylation selectivity and to an increase of methane selectivity.

Table 2 Effect of reaction temperatures on the carbonylation of methanol over the novel Mo/C catalyst(II)

Reaction conditions as in table 1.
Y=X ^. S(AcOMe),
TOF(turnover frequency)=AcOMe mol/(mol Mo ^. h).
In this work we have demonstrated that initial step is the formation of a methyl-metal complex which is believed to be rapidly transformed into an acetyl-metal complex. In accordance with this scheme, molybdenum is very active site where methyl fragments are formed through methanol decomposition, higher temperatures enhance hydrogenation of methyl group (methanation) to produce methane. The detailed mechanism of direct carbonylation reaction is unclear, further work to clarify intermedium is in progress.

3.CONCLUSION
A series of supported sulfides catalysts were prepared. A novel reduced-sulfurized Mo/C is found to have a significantly high activity and selectivity in the carbonylation of methanol at atmospheric pressure without using any halide as promoter. A temperature around 533K is optimum for higher methyl acetate production, the selectivity to methyl acetate is 78.1%(mol), the conversion of methanol exceeds 90%. Higher temperature decreases the carbonylation selectivity sharply and increases methane selectivity.

REFERENCES
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