Indian Journ al of Chemical Technology Vol. 8, November2001 , pp. 482-486

Modifications of ZSl\1-5 zeolite catalyst for dimethyl ether conversion to olefins

Fran~oise Duprat* & Victor Cruz Morales ENSSPICAM, Av. Esc. Normandie-Niemen, F-13397 Marsei lle Cedex 20, France

Received 9 October 2000; revised 17 July 2001; accepted 28 July 2001

The influence of modifications of the ZSM-5 catalyst on the conversion of dimethyl ether to olefins is studied at low hydrocarbon conversion in order to explore the possibility of selective formation of C/-C6;. Three modifications designed in order to reduce catalyst acidi ty wi thout increasing steric hindrance are compared, namely. dealumination, parti al neutrali zation with ammonia and ion-exchange with lithium. The unmodified H-ZSM-5 produces 60 % of ethene and propene, 30 % of olefi ns C4 ; -C6; and 10 % of heavy olefins plus aromatics. In the same conversion range, the three modified catalysts promote the formation of C/ -C6; and even more the formation of C7;-C8;, and they lower the formation of light olefin s and aromatics. There is only a slight difference between modified catalysts : lithium exchanged ZSM-5 leads to higher ratio of C4 ;-C6; to heavy by-products. An enhanced diffusivi ty in modified catalysts probably pia) s a role in the increase in the average mol ecular weight of olefins produced.

Selective synthesis of olefins from methanol is of economic importance because of (i) the large availability of natural gas that can be converted to methanol with a high yield (> 99 %); (ii) the large demand of MTBE (methyl-tert-butylether) and higher ethers in the synthesis of which olefins C4--C6- is an alternative route. Methanol can be converted to high octane number gasoline over zeolite catalyst, H-ZSM-5: the synthetic fuel process MTG (methanol to gasoline) has been commercialized in 1985 in New Zealand 1. Olefins are intermediates in the production of alkanes and aromatics2·3. Zeolites are highly acidic, the most strongly acidic sites are more active towards the reactions of aromatization of olefins and high density of acidic sites enhances coke formation4.

The ways to increase the selectivity towards C2--C4-olefins consist mainly in controlling the reaction conditions as well as the acid properties and pore structure of the catalyst. The small pores zeolites such as erionite, zeolite T, SAP0-34 etc ... offer very high selectivity towards ethene and propene, but they are subjected to fast deactivation by coke formation 5. The intermediate pore zeolite, ZSM-5, is less selective but highly resistant to deactivation. High SiOi Al20 3 ratio of ZSM-52·6, modifications with phosphorus6·7 or by . h . h . 8 9 b . 10 h 10n-exc ange Wit magnesmm · , armm or ot er alkaline earth metals3 show increased selectivity to light olefins as well as an improved lifetime. However, ion-exchange with a bulky cation such as cesi um does not result in increased shape selectivitl.

*For correspondence: (duprat @spi-chim.u-3mrs.fr; Fax: 33-4 9 1 02 77 76)

This study aims at examining modifications of H-ZSM-5 enhancing the formation of intermedi ate olefins C/-C6=. Three ways of modifications in order to reduce acid strength without increasing steric hindrance have been compared. As the acidity is primarily connected with the Al/Si ratio, dealumination has been considered at first. Hydrothermal dealumination has been preferred to the chemical methods which remove efficiently aluminium atoms without decreasing significantly the acidity 11 ·12. In the second method, the most acidic sites of zeolite have been partially neutralized with ammonia. The higher the acidity of the site, the stronger will be the interaction between the site and NH3, and consequently higher will be the temperature needed to desorb NH3. Thus a careful control of the desorption temperature makes it possible to liberate only the less acidic sites. The catalyst obtained is stable at reaction temperatures below the desorption temperature. Alternatively, the acidity of zeolite has been reduced by proton-exchange with a small cation. Lithium was chosen because it imposes an extra steric hindrance in the pore structure as little as possible. For practical reasons, DME was used instead of methanol as the feed. Either feed will result in the same hydrocarbon distribution because the conversion between methanol and DME is reversible and fast.

Experimental Procedure

Catalyst preparation Unmodified catalyst (Union Carbide) consisted in

20 wt % H-ZSM-5 zeolite, with a Si/ AI atomic ratio

DUPRAT & MORALES: ZSM-5 ZEOLITE CATALYST FOR DIMETHYL ETHER CONVERSION TO OLEFINS 483

Table !-Product distribution in the reference experiments: hydrocarbon conversion and weight selectivity in olefins and

aromatics Feed 100 % DME, contact time, 3 gcAT h mor'

Temperature, °C 280 300 310 320

X He 0.05 0.14 0.21 0.30

s2 0.15 0.16 0.18 0.19 SJ 0.48 0.44 0.39 0.36

s4 0.22 0.22 0.21 0.19 s, 0.04 1 0.044 0.049 0.054 s6 0.041 0.047 0.053 0.058 s1 0.037 0.046 0.055 0.056 Ss 0.022 0.033 0.037 0.040

SAROM 0.023 0.028 0.034 0.046

of 60, on y-alumina. Different series of modified catalysts were prepared. As a general rule, a monotone influence of the modification was observed : higher the degree of neutralization, larger is the change in selectivity. Thus, only one sample of each type of catalyst is presented here. Dealuminated zeolite (DZSM-5) was obtained after 31 h of hydrothermal treatment at 600°C under a flow of pure steam. As shown by the temperature programmed desorption of ammonia (NH 3- TPD), the dealumina-tion removed most of the weak sites (namely 75 %) and not any strong sites. The total degree of neutralization was 60 %. Partially neutralized catalyst (NZSM-5) was prepared by ammonia adsorption at room temperature for 1.25 h, followed by desorption under a nitrogen flow for 20 min at 460 °C. The NH3-TPD profile showed 25 % of neutralization, due to the complete disappearance of the strong sites. Fully lithium ion-exchanged H-ZSM-5 (Li-ZSM-5) was prepared by suspending the catalyst in a solution containing the appropriate amount of lithium acetate for 15 h at room temperature. The suspension was then dried and calcinated at 400 °C in air to remove acetate ions. The degree of exchange was 100 %. From the NHr TPD profile, the lithium removed strong acidic sites and slightly increased the weak ones resulting in 20 % of neutralization .

Experimental set up and procedure The experiments were carried out in a stainless-

steel U-tube reactor (18 x 0.47 em), placed in a molten salt bath controlled by a temperature regulator. The catalyst was diluted with a-alumina in a ratio 1:10. Owing to the rapid dehydration reaction of methanol which takes place at equilibrium, the DME was used as the feed in order to reduce the heat of reaction, as about 50 % of the heat is evolved by the dehydration step. The combination of a salt bath , a small diameter tube, the use of DME as the reactant and the dilution

of the catalyst ensured the operation under essentially isothermal plug flow conditions. Gases were controlled by mass flow controllers. The reactor effluent was analyzed on-line in a gas chromatograph equipped with a gas injection valve, a flame ionization detector and a wide bore column GS-Q (30 m x 0.547 mm). Identification and calibration of the chromatogram were made using standard mixtures of olefins, paraffins and aromatics. From the products distribution, XHc, the conversion in total hydrocarbons and Si, the weight selectivity in each olefin ci= (the weight ratio of the amount of olefin to the total amount of hydrocarbons) were calculated.

Results

Reference experiments with unmodified catalyst Previously, experiments were carried out over the

unmodified H-ZSM-5 catalyst at different reaction temperature (280-400 °C), space time ( 1-8 gcAT h mol- 1) and DME/N2 molar ratio in the feed (20-100% ). It was observed that no light paraffins and a few percent of aromatics (mainly composed of xylenes) were formed . Low reaction temperature and high space velocity led to a low DME conversion and to the preferential formation of light olefins, whereas the opposite conditions favoured the formation of C5--Cs- . Since the heavy by-products seem to be less valuable than the light ones, it was decided to examine the effect of modifications of the catalyst at conversion lower than 0.3. The conditions of the reference experiments with unmodified catalyst are given in Table 1. The main product is propene (38-52 % of hydrocarbons) and butenes (20% ). The total amount of light olefins (C2-, C3-) represents about 60 %, in a proportion decreasing with the conversion . The target olefins (C4--C6-) are produced at a constant ratio of 30% in the conversion range. The total amount of "heavy products" (C7-, C8- plus aromatics) is about 10 %, in raising proportion as the conversion mcreases.

Modified catalysts The product distributions obtained with the three

modified catalysts are compared with the distribution obtained with the unmodified catalyst at the same hydrocarbon conversion. As the modified catalysts are less acidic, they are less active. The lower activity was balanced by an higher reaction temperature. The increments in reaction temperature were respectively 10-20 °C, 40-60 °C and 30-35 °C for dealuminated (DZSM-5), neutralized (NZSM-5) and lithium exchanged

484 INDIAN J. CHEM. TECHNOL. , NOVEMBER 2001

,.:r-: - -- C2• --a- C3--C4· -C5: -:s:-CS= · · + · ·C7•

"'

3 r ~ ... ________ -:i; ·.·. , .. ..l* .. CIF J 2,5 .. - .. Arom.1 1 2 r- · .. ·.::: :::: ::: ::::: :

"'oN ' ~--:s: :s: en 1,5 + --

1 t_ ~- - -------o--------

DUPRAT & MORALES: ZSM-5 ZEOLITE CATALYST FOR DIMETHYL ETHER CONVERSION TO OLEFINS 485

2,5

"' 2 "' (I) "! :J:

~ 1,5 " . >= l

U)

Products of reaction

Fig. 4-Comparison of the three catalyst modifications : average value in the conversion range of the ratio of the selectivity of modified catalyst to the one of H-ZSM-5 as a function of the number of carbon atom of products

0,7 --·---.. ······-1>31ight

0,6 Ill target

Dheavy

0,5 i!' > ;; u 0,4 .. ~ .. .. "' 0,3 e Cl> >

486 INDIAN J . CHEM. TECHNOL., NOVEMBER 2001

dealumination, partial neutralization with ammoma, and ion-exchange with lithium.

It should be noted that in the conditions of reaction (280-320 °C), the unmodified catalyst produces a lot of light olefins : the fraction c2·-c4- represents 84 % of hydrocarbons. As expected, modified catalysts enhanced the formation of c4·-c6·, however the extent of the improvement was only modest. Yield of aromatics decreased and the products distribution shifted to higher molecular weight. Unlike the other modified catalysts, Li-ZSM-5 favoured more C4--C6-

than heavy olefins. Thus, it is the most performing catalyst when heavy by-products are considered as

less valuable than light ones.

From a process point of view, low methanol conversion has some advantages: (i) the corresponding mild operating conditions preserve the catalyst lifetime; (ii) octane-boosting ethers are produced by reacting the olefin fraction with methanol. An ether plant is composed of a methanol-to-o lefins reactor, an isomerization reactor, for increas ing the ratio of iso-olefins and an etherification reactor. It would be interesting to recycle only the

unreacted materials from the etherification to methanol-to-olefins reactor.

References I Maiden C J, Stud Surf Sci Catal, 36 (1988) I. 2 Chang CD, Catal Rev Sci Eng, 26 (1984) 323. 3 Sano T, Kiyozumi Y & Shin S, Sekiyu Gakkaishi, 35 ( 1992) 429. 4 Guisnet M & Magnoux P, Zeolite Microporous Solids :

Synthesis, Structure and Reactivity (KJuwer academic Pub), 1992, 457.

5 Froment G F, Dehertog W J H & Marchi A J, Catalysis, 9 (1992) I.

6 Dehertog W J H & Froment G F, Appl Catal, 71 (1991 ) 153. 7 Oehlmann G, Jerschkewitz H G, Lischke G, Eckelt R, Parlitz B,

Schreier E, Zibrowius B & Loeffler E, Stud Surf Sci Catal. 65 (1991) I.

8 Vedrine, J, Auroux A, Dejaifve P, Ducarme V, Haser H & Zhou S, J Catal, 73 (1982) 147.

9 Levesque, P., Bianchi D, Le Van Mao R & Pajonk G M, Appl Catal, 57 ( 1990) 3 1.

10 Abdillahi , MM. , El-Nafaty U A & Al-Jara llah AM, Appl Catal A, 91 ( 1992) I.

II Komatowski J, Bauer W H, Pieper G, Rozwadowski M, Schmitz W & Cichowlas W, J Chem Soc Faraday Trans, 88 ( 1992) 1339.

12 Lago R M. Haag W 0 & Mikovsky R J, New Developments in Zeolite Science and Technology, Elsevier, Tokyo, ( 1986) 677.

13 Jansen von Rensburg L. Hunter R & Hutchings G J, Appl Caw/. 42 ( 1988) 29.

14 Sulikowsi B & Klinowski J, Appl Catal A, 89 (1992) 69. 15 Hashimoto K, Masuda T & Murakami N, in Zeolite Chemistry

and Catalysis (Elsevier, Amsterdam), 1991 , 477. 16 Nourredine T, Caracterisation par adsorption et diffusion de

zeolites echangees et etude des vitesses de diffUsion et d'echange d'hydrocarbures purs et en melange, These, Universite de Poitiers, 1990.