Applied Catalysis A: General 213 (2001) 239–245
1-Hexene isomerization and n-hexane cracking over HMCM-22 Anup K. Talukdar a , Krishna G. Bhattacharyya a,∗ , Toshihide Baba b , Yoshio Ono b a
b
Department of Chemistry, Gauhati University, Guwahati 781014, Assam, India Department of Chemical Engineering, Tokyo Institute of Technology, Tokyo 152, Japan
Received 8 April 2000; received in revised form 28 September 2000; accepted 9 December 2000
Abstract Isomerization of 1-hexene and cracking of n-hexane over HMCM-22 were carried out in a continuous flow glass reactor. The results were compared with those obtained over HZSM-5 under similar conditions of conversion. The catalyst HMCM-22 caused much more double bond shift and less skeletal isomerization in 1-hexene isomerization than HZSM-5. Also, in n-hexane cracking, HMCM-22 gave a higher yield of olefins and a lower yield of aromatics than HZSM-5. The results can be explained on the basis of the lower acidic strength and the inferior hydrogen transfer capacity of HMCM-22 compared to those of HZSM-5. © 2001 Elsevier Science B.V. All rights reserved. Keywords: HMCM-22; 1-Hexene isomerization; n-Hexane cracking
1. Introduction Mobil [1] has patented the zeolite MCM-22 as a new member of the high silica materials. The framework topology of this material consists of two independent pore systems, both of which are accessed through rings composed of 10 tetrahedral atoms [2]. One of these pore systems has two-dimensional sinusoidal channels and the other consists of large supercages (7.1 Å × 7.1 Å × 18.2 Å). The pore topology of MCM-22 has been established by adsorption of various probe molecules [3,4] and by test reactions such as conversion of m-xylene [5,6], hydroisomerization of n-decane [6], alkylation of toluene and n-heptane cracking [2]. It has been shown from these studies that MCM22 has features of both 10-membered and 12-membered zeolite rings. The hydrogen-form of the zeolite, HMCM-22, has been postulated as having strong ∗ Corresponding author. Fax: +91-361-570133. E-mail address:
[email protected] (K.G. Bhattacharyya).
Bronsted and Lewis acid sites from FTIR and TPDA studies [7] after activation at 675 K. However, few systematic studies on the acidic properties of MCM-22 are available. The present work was aimed at investigating the acidic properties of MCM-22 by carrying out some test reactions of 1-hexene isomerization and n-hexane cracking over it as well as over ZSM-5. The 2- and 3-isomers of 1-hexene are the starting materials for production of a number of feedstocks and these are selectively synthesized by isomerization over moderately acidic zeolites. The skeletal isomers of 1-hexene have also become important as octane boosters for fuel oil. The present work had the additional objective of studying the effects of temperature and contact time on the product distribution pattern and thus evaluating the optimum conditions for the production of skeletal isomers of 1-hexene. 2. Experimental methods MCM-22 was synthesized from hexamethyleneimine, colloidal silica (20% SiO2 w/w), sodium
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aluminate (47.3% Al2 O3 , 34.8% Na2 O, 17.9% H2 O), sodium hydroxide and deionized water. A typical synthesis procedure for obtaining MCM-22 with SiO2 /Al2 O3 ratio of 30 (catalyst A) was as follows: 2.63 g of sodium hydroxide and 2.16 g of sodium aluminate were dissolved in 145.4 g of deionized water. To this solution, 14.8 g of hexamethyleneimine was added and the components were mixed well. The mixture was added drop by drop to 88.24 g of colloidal silica with constant stirring in an inert atmosphere of nitrogen. The resulting mixture was stirred for another 2 h to get a homogeneous gel. It was then transferred to a Teflon autoclave, sealed under nitrogen atmosphere and heated to a temperature of 423 K with constant stirring. The autoclave was allowed to cool after 7 days and the crystalline product was separated by centrifugation. This was washed several times with deionized water and was dried by keeping in an oven at a temperature of 393 K overnight. The organic template in the product was removed by calcination at 813 K. The template-free material was then converted to the ammonium-form by four-fold refluxing with 2 M NH4 Cl solution (353 K, 8 h). The separated product was allowed to dry first at room temperature and then at a temperature of 393 K by keeping it overnight in an oven. Another sample of MCM-22 with a SiO2 /Al2 O3 ratio of 70 (catalyst B) was also prepared, following an identical procedure. ICP–AES measurements showed the Si/Al ratio of catalyst A and catalyst B to be 15.3 and 29.2, respectively. The XRD patterns of the synthesized samples (both catalysts A and B) confirmed them to be zeolite MCM-22. The positions and the intensities of the diffraction peaks compare very well with those given in [1]. Two samples of ZSM-5 with Si/Al ratios of 21.5 (catalyst C) and 28.0 (catalyst D) were also synthesized following standard procedures and were converted to the ammonium forms by refluxing with NH4 Cl solution. Catalytic reactions were carried out in a fixed bed downflow glass reactor (i.d. 1 cm, length 38 cm). The ammonium forms of the zeolite were compacted and broken into particles of 16–32 mesh size to obtain the catalyst. Some (0.1 g) of the catalyst was introduced
into the reactor and was calcined at 773 K for 3 h in a flow of dry air to obtain the H-form of the zeolite. Prior to each reaction run at desired conditions, dry nitrogen gas was passed through the reactor for about 40 min. The reaction products were collected from a heated sampling zone with a previously heated syringe. An OV101 capillary column was used to analyze the products of the reactions. Porapak Q column was used to separate C1, C2 and C3 fractions in n-hexane cracking reaction.
3. Results and discussion 3.1. Isomerization of 1-hexene Isomerization of 1-hexene was carried out at 538 K and W/F (weight of the catalyst in gram per feed flow rate expressed in moles per hour; the feed
Fig. 1. Product distribution in 1-hexene isomerization over HMCM-22 (catalyst B, Si/Al 29.2) and HZSM-5 (catalyst D, Si/Al 28.0) at 538 K, W/F 0.14 g h mol−1 and 1-hexene partial pressure 40 kPa. M1–M4 and Z1–Z4, respectively represent total conversion, skeletal isomers, double bond isomers and cracking products for HMCM-22 and HZSM-5.
A.K. Talukdar et al. / Applied Catalysis A: General 213 (2001) 239–245
includes both the reactant and the carrier nitrogen gas) of 0.14 g h mol−1 . The reactant was at a partial pressure of 40 kPa. The reaction was run for a period of 3 h over HMCM-22 (catalyst B, Si/Al 29.2) and HZSM-5 (catalyst D, Si/Al 28.0). Fig. 1 shows the distribution of products in the isomerization reaction for the two catalysts. Some other C6 and beyond C6 products were also formed, but these were not identified. Both HMCM-22 and HZSM-5 show high stability with respect to the reaction, but the selectivities for the isomerization products exhibit a few remarkable differences. Selectivities towards skeletal isomers were ∼29 and ∼35%, respectively for HMCM-22 and HZSM-5. The selectivity towards cracking products was comparatively higher (∼4.5%) over HZSM-5
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than that over HMCM-22 (
