Specifics of the methanol and dimethyl ether conversion to hydrocarbons on modified Y and ZSM-5 zeolites

S.I.Abasov¹, F.A.Babaeva¹, B.B.Guliyev¹, N.N.Piriyev², M.I.Rustamov¹

¹ Institute of Petrochemical Processes named after Yu.G.Mammadaliyev under the Azerbaijan National Academy of Sciences 

Khojaly Avenue, 30, AZ1025 Baku, the Republic of Azerbaijan. E-mail: safa.abasov@rambler.ru

² Azerbaijan methanol company (AzMeCo)

20, Afiyaddin Jalilov Str. AZ1024 Baku, the Republic of Azerbaijan. E-mail: pnn@azmeco.com

The conversion of methanol to dimethyl ether and hydrocarbons on aluminum oxide, as well as the Y and ZSM-5 zeolites’ H-forms and forms modified with phosphoric acid has been studied. It has been suggested that the formation of the primary hydrocarbon - ethylene is structure-sensitive and depends on the distance between the Bronsted acid and Lewis basic centers responsible for the reaction.

Keywords: methanol, aluminum oxide, zeolite Y, zeolite ZSM-5, catalyst, conversion, DME, ethylene, olefins, hydrocarbons

The most efficient method of producing various hydrocarbons from natural gas reserves of which are significant is carries out by successive conversion of methane through the synthesis gas to methanol. Its final transformation into higher hydrocarbons on the relevant catalysts of acid type includes preliminary stage of dehydration to dimethyl ether (DME), and therefore the corresponding final products can also be obtained from this compound. [1]

It is known that the perspective of DME usage as an environmentally friendly energy carrier determines the increasing interest in the production of this ether [2]. In this regard, the new catalysts have been developed or mechanical mixtures of the synthesis gas to methanol conversion catalyst with the methanol to DME dehydration catalyst have been used [3-5]. This approach allows lowering of the requirements to the obtained synthesis gas and simplification of the technology of its production, and, consequently, reduction of the costs of the final products. Thus, the combination of the methanol production and its dehydration to DME processes is a flexible way to bring together the competitiveness of hydrocarbons obtained from natural gas and oil.

The successful solution of the problem of combining of the mentioned processes and the use of DME to produce the higher hydrocarbons requires the selection and synthesis of new highly selective catalysts for the conversion both of methanol to DME and methanol and DME to the corresponding hydrocarbons.

This paper deals with the results of the selection of catalysts and their subsequent conversion to hydrocarbons on aluminum oxide and zeolite catalysts.

The experimental part

We studied samples prepared by granulation of HY (Y) and NZSM-5 (ZSM-5) type zeolites (SiO₂/Al₂O₃ = 50) using aluminum hydroxide as a binder in the amount of 20% by weight (expressed as anhydrous Al₂О₃) from the main component. Formed extrudates were dried in the air at 80-120^оС, and then calcined at 550^оС (5h). Part of the granulated catalysts was additionally treated with an aqueous solution of phosphoric acid with stirring at 80-95^оС (3h), and then subjected to the above procedures of drying and calcination. The resulting samples contained phosphorus in the amount of 5% by weight expressed as Р₂О₅.

The catalyst samples were tested at a flow-through fixed-bed type installation with loading of 10 g of the catalyst at atmospheric pressure and a temperature of 280-450^оС and at a liquid methanol space velocity of 3,5-12 h^-1. The catalysts were pre-treated with the air for 1 hour at temperatures of 50-100^оС above the temperature of catalysis, followed by purging the system (0.5 hours) with a stream of nitrogen until a predetermined temperature.

Methanol conversion products were analyzed by gas chromatography (Auto System XL "Perkin Elmer" and Gazochrom).

The catalytic activity was characterized by conversion (X,%) of methanol and selectivity (S,%) of DME and hydrocarbons formation, which were calculated as follows:

where m0 and m is the number of reactant at the inlet and outlet of the reactor, respectively, in mol • g^-1_cat • h^-1; ni is the reactant conversion product (methanol or DME); Σni = Δm is the total amount of carbon-containing reactant conversion products; ratio 12/M is the atomic mass of carbon/molecular weight of reactant ratio (methanol -32, DME - 46).

Thereafter the selectivity of products was expressed as a percentage of the total number of carbon molecules involved in the reaction (% C).

Results and Discussion

According to the literature data [6] and our results (Fig. 1), aluminum oxide demonstrates consistently high activity in the conversion of methanol with selective formation of DME. Conversion of methanol under similar conditions on zeolite catalysts is more complex. As it can be seen from Figure 1, a, on the H-form of the ZSM-5 zeolite during the first 10 minutes of the experiment methanol is converted to hydrocarbons with high conversion and selectivity. However, after 30 minutes of catalysis the catalyst’s activity decreases rapidly (Fig. 1,b), and DME becomes the main product of reaction.

A slightly different picture is observed for the H-form of the Y-zeolite. It is seen (Fig. 1, a, b) that on this catalyst methanol is converted to DME (47%) and hydrocarbons (53%). Insufficiently high selectivity of the Y-zeolite catalyst with regard to the dehydration of methanol to DME or conversion into higher hydrocarbons, as well as instability of parameters of the catalytic activity of the ZSM-5 zeolite raises the problem of their modification. Considering the activity of the acid-type catalysts in the mentioned reactions, further researches of the methanol conversion were carried out on the samples treated with phosphoric acid.

Fig. 1. The influence of the duration of the experiment (a - 10 min, b - 30 min) on the conversion of methanol (1) on the initial catalysts, and selectivity of DME (2) and hydrocarbons (3) formation, T = 350^оС, space velocity is 3 h^-1

Fig. 2. The influence of the duration of the experiment (a - 10 min, b - 30 min) on the conversion of methanol (1) on the catalysts treated with phosphoric acid, and selectivity of DME (2) and hydrocarbons (3) formation, T = 350^оС, space velocity is 3 h^-1

From comparisons of the corresponding data in Fig. 1 and 2, it can be noted that the insertion of phosphoric acid into aluminum oxide virtually has no effect on the properties of this catalyst in the conversion of methanol. Treatment of zeolites with phosphoric acid promotes change of their catalytic properties. From Fig. 2 data it can be seen that the Y-zeolite catalyst modified with phosphoric acid demonstrates high selectivity in the dehydration of methanol to DME simultaneously maintaining a stable activity. Study of the influence of space velocity on the conversion of methanol to DME on this catalyst showed that it has retained its activity up to the space velocity of 10 h^-1. Therefore, after the insertion of Р₂О₅ the zeolite-based catalysts, along with high selectivity to DME, are characterized by high productivity, which makes them perspective for production of DME from synthesis gas in combination with synthesis gas to methanol conversion catalysts.

Modification of ZSM-5 with phosphoric acid also has a positive effect on the catalytic properties of this sample in the methanol conversion. This influence, as it can be seen from Fig. 2, comes down mainly to stabilization of high activity of the catalyst in the conversion of methanol to higher hydrocarbons.

Thus, the yield of the methanol conversion products depends on the structure type of zeolite. Comparison of methanol conversion on mesoporous aluminum oxide (diameter of mesopores is 2-4.5 nm) and wide-porous Y-zeolite (diameter of micropores is 1.32 nm) testifies that in both cases selective conversion to the dehydrating dimerization product is achieved in on Bronsted acid centers [7].

However on the ZSM-5 zeolite with more narrow pores (diameter of micropores is 0.55 nm) or on zeolites with a smaller diameter of the micropores (eg, SAPO-34, 0.38 nm) selective conversion of methanol to higher hydrocarbons is observed [8] despite the fact that the formation of the primary hydrocarbon intermediate - ethylene, as it is supposed in the [7] paper is preceded by the formation of DME. Based on the obtained results it can be suggested that, in contrast to the reaction of the dehydrating dimerization of methanol to DME the formation of the primary hydrocarbon - ethylene - depends on the structural characteristics of zeolites, ie, dimensional parameters of micropores, and therefore these reactions are structurally sensitive.

Let’s analyze the obtained results considering the known mechanism of the methanol conversion with participation of strong acids [1,9]. Conversion of methanol to DME can be described by the following scheme (1):

according to which the activation of the first molecule of alcohol takes place on Bronsted acid centers - ZOH. Further, according to [10], the interaction of tightly bound methanol (methoxonium groups) (I) with the second molecule of methanol from the gas phase takes place, which leads to the formation of an intermediate state with transfer of bonds. As the result of decomposition of this intermediate DME and Н₂О products are formed.

It is known that at the dehydration of aluminum oxide or zeolites the Bronsted acid centers contained in them transform into the centers representing the nucleophilic oxygen or the basic center (Lewis basic center), that is, in contrast to framework oxygen bound by proton in the ZOH, the oxygen in (Z1Z2)O is bounded with three-coordinated Z1 and Z2 aluminum atoms.

The resulting DME molecules are able to interact with similar centers, that is adsorb by the scheme (2), forming the intermediate (II).

In the case this process takes place on aluminum oxide or wide-porous Y-zeolite, desorption of the formed DME molecules from their pores does not meet with difficulties. However, in the ZSM-5 zeolite with more narrow pores, the micropores of which in addition to (Z1Z2)O contain the Bronsted acid centers ZOH as well, intermediate II can interact with strongly adsorbed methoxonium group I with formation of an intermediate activated complex (III) according to the scheme (3) .

Redistribution of bonds in this complex with the release of water molecules leads to the formation of a new complex (IV), which decomposes to release ethylene. In the scheme (3) it is noteworthy that formation of ethylene is accompanied by a transformation of Bronsted acid centers into Lewis basic centers:

This change in surface properties is of great importance for the implementation of catalytic processes and is the subject of further researches.

Effect of water on the yield of DME conversion product on ZSM-5; Т=450^оС; the space velocity is 1 h^-1

Thus, the redistribution of bonds represented by the scheme (3) suggests a probable stage of formation of the primary C-C bond, which is the limiting one in the conversion of methanol to hydrocarbons. Such a statement is based on the fact that the dehydration of ethanol to ethylene, unlike methanol, easily runs on aluminum oxide and Y-zeolite [11].

Behavior of the process occurring on the ZSM-5 by the scheme (3) is in good agreement with the known concepts regarding the participation of three molecules of methanol in the formation of ethylene [12,13] and the intermediate similar in its structure to methoxyethane [7.14]. Therefore, the distances between the Bronsted acid centers (ZOH) and Lewis basic centers (Z₁Z₂O) restricted by the dimensional parameters of the zeolite micropores promote formation of activated complexes III in them. Moreover, on the basis of the received results it can be concluded that with decrease of the micropore diameter from 1,32 (Y) to 0.55 nm (ZSM-5), the probability of formation of such complex increases to the limit value (absence of DME among the products), and therefore, determines the structural sensitivity of primary ethylene formation in the conversion of methanol and DME.

In this connection it should also be noted that in the case of SAPO-34 and SAPO-17 zeolites having more narrow pores (pore diameters within 0,32-0,38 nm) the assumption of the formation of ethylene via ethanol (using two molecules of methanol) [15] also can be described by the scheme (3). However, in this case, due to the small distances between Bronsted acid centers and Lewis basic centers relatively to the ZSM-5, the place of methanol (methoxonium) in the intermediate I and III is taken by water molecule (hydroxonium).

Conversion of methanol to the higher hydrocarbons on the H-ZSM-5 zeolite involves the successive formation of DME and primary ethylene. However, as it can be seen from Fig. 3, all other conditions being equal, the different distribution of hydrocarbon products of the methanol and DME conversion is observed. If in the case of methanol the main products are olefins with low molecular weight, then at the conversion of DME the aliphatic hydrocarbons with isomeric structure and higher molecular weight are mainly formed.

Fig. 3 Distribution (selectivity of formation) of the methanol and DME conversion products on the ZSM-5 catalyst (Т=350^оС, the space velocity is 3 h^-1): 1 - C_V-C_Y – alkanes, 2 - С₂₌ - С₄₌ - olefins, 3 - С₄₊ - aliphatic hydrocarbons, 4 - aromatic hydrocarbons, 5 - DME.

Amount of water released in the conversion of methanol to hydrocarbons is twice as much as in the similar conversion of DME. In this connection it is interesting to consider the effect of the water released in the methanol conversion on the yield of DME conversion products.

As an example, the table shows the data of distribution of the DME conversion products (without water and with water dilution), comparison of which shows that the additional amount of water (corresponding to the water released at 100% conversion of methanol, DME: H₂O = 1:1) effects on the yield of the conversion products. It should be noted that under the influence of the supplied H₂O vapor the yield of propylene increases, the yield of ethylene slightly changes, and almost a twofold reduction of С₅₊ hydrocarbons is observed as well.

Thus, addition of water vapor does not affect the yield of the primary hydrocarbon - ethylene, and only affect its subsequent oligomerization, reducing the yield of hydrocarbons having the higher molecular weight. However, it should be noted that the water vapor inhibiting oligomerization of the produced olefins with low molecular weight, has no effect on the alkylation of primary ethylene. Therefore, variation of the DME (methanol): H₂O ratio allows purposeful regulation of the yield of hydrocarbon products of their conversion.

Another conclusion made from the analysis of the received results is that while the formation of the primary hydrocarbon - ethylene - from methanol or DME can be attributed to the structure-sensitive reactions depending on the distances between the Bronsted acid centers (ZOH) and Lewis basic centers (Z₁Z₂O) restricted by dimensional parameters of the zeolite micropores within the range of 0.32 - 0.55 nm, then the reactions of other hydrocarbons formation are mainly structure-insensitive, and their output can be adjusted by choosing both the catalysts and conditions of the methanol and DME conversion.

The results of researches showed that the modification of wide-porous zeolites of Y-type with phosphoric acid improves their productivity in the methanol to DME dehydration reaction (at the conversion of methanol and yield of DME close to equilibrium). Analysis of the received results and the literature data allows suggestion that the formation of the primary hydrocarbon - ethylene is a structure-sensitive reaction, which passes with the participation of the Bronsted acid and Lewis basic centers of the catalyst, and is defined by the distance between these centers. With decrease of diameter of the zeolite micropores from 0.55 (ZSM-5) to 0.38 nm (SAPO-34) in the formation of primary ethylene one of the methanol molecules is replaced by the water molecule.

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