Propane aromatization

Contributions of three types of Ga sites in propane aromatization over GaiOs/Ga-MOR catalysts... 

The propane aromatization was conducted under the differential condition by using Ga203/Ga-MOR catalysts thus characterized. The contributions of L, HI, and H2 sites to the propane conversion and the aromatics formation were estimated by assuming that the observed reaction rates are the sum of the reaction rate on each site which is equal to the product of the turnover frequency (TFij) and the amount of active sites per weight of catalyst (Aj) ... 

Figure 4. Comparison of Propane Aromatization Performances of a Palladium Membrane Reactor (PMR) and a Conventional Reactor (CR) using a Ga-H-ZSM-5 Catalyst...

The pulse experiments demonstrated that active sites for propane dehydrogenation are formed upon exposure of the oxide form of gallium modified ZSM-5 to propane itself. A constant 1 1 ratio of hydrogen produced to propane consumed is attained after a number of pulses with little propene formation, which suggests that, after propane dehydrogenation to propane, aromatization proceeds through hydrogen transfer reactions. 

Kanazirev, V., Price, G.L, and Dooley, K.M. (1990) Enhancement in propane aromatization with Ga203/HZSM-5 catalysts. J. Chem. Soc. Chem. Commun., 712-713. 

Besides Ga, other metals such as Zn (11, 12) and Pt (13) have also been used in combination with ZSM-5 zeolite for C2-C4 aromatization. However, besides aromatization, Pt also catalyzes other undesired reactions, such as hydrogenolysis, hydrogenation and dealkylation that leads to excessive formation of methane and ethane, and limits the selectivity to aromatics. Therefore, Ga- and Zn-ZSM-5 catalysts are preferred over Pt-ZSM-5 except, perhaps, in the case of the more refractory ethane, in where a higher dehydrogenating function is needed to activate the reactant. The catalytic performance of Ga and Zn/ZSM-5 for propane aromatization is compared in Table 2.2. The results obtained on the purely acidic H-ZSM-5 are also included in the table. As observed, a higher conversion and yield of aromatics is obtained for the Ga/ZSM-5 catalyst. 

This suggests, for these two samples that the balance between the dehydrogenating function (GaxOy) and the acid function (H ) was almost unmodified. For sample 5%Ga A1, for which deactivation is faster, the selectivity for aromatics decreases with time on stream. This change appears to be mainly due to the decrease of the conversion rather than to a change in the balance between dehydrogenating and acid functions. It is Known that during propane aromatization CH4 is formed and that the main source of C-f comes from the first step of the reaction on the acid sites (2) (5). 

One of the major products in propane aromatization is hydrogen (eg 2C3H8 -> C5H6 + 5H2 ). It has been proposed (6) that changes In selectivity as a function of time on... 

Kusakabe, K. Yokoyama, S. Morooka, S. Hayashi, J. i. Nagata, H., Development of supported thin palladium membrane and application to enhancement of propane aromatization on Ga-silicate catalyst. Chemical Engineering Science 1996,51, (11), 3027. 

Influence of the proximity between Ga and H1 sites on propane aromatization... 

Activation and Deactivation of Ga/HZSM 5 Catalysts in Propane Aromatization at 773 K... 

Effect of silica binder on acidity, catalytic activity and deactivation due to coking in propane aromatization over H-gallosilicate (MFI)... 

Influence of the addition of silica, as a binder at a concentration of 10 or 50 wt%, to H-gallosilicate (MFI) zeolite on its inter- and intracrystalline acidity, initial activity, product selectivity and distribution of aromatics formed in the propane amortization (at 550°C) and also on its deactivation due to coking in the aromatization process has been thoroughly investigated. Silica binder caused an appreciable decrease in the zeolitic acidity (both external and intracrystalline acid sites) and also in the propane conversion/aromatization activity. Because of it, the deactivation due to coking of the zeolite in the propane aromatization is, however, decreased. The deactivation rate constant for the initial fast deactivation is decreased but that for the later slow deactivation is increased because of the binder. The aromatics selectivity for aromatics and para shape selectivity of the zeolite, particularly at lower conversions, are increased but the propylene selectivity and dehydrogenation/cracking activity ratio are decreased due to the presence of binder in the zeolite catalyst. 

Propane aromatization reaction (at 550°C) was carried out at atmospheric pressure in a continuous flow quartz reactor (id 13 mm), using a propane-nitrogen mixture (33.3 mol-% propane) as a feed with a space velocity of 3100 cm g h". The catalytic activity and selectivity were measured as a function of time-on-stream (up to about 6.7 + 0.2 h). The reaction products were analyzed by an on-line GC with FID, using Poropak-Q (3 mm x 3 m) and Benton-34 (5%) and dinonylphthalate (5%) on Chromosorb-W (3 mm x 5 m) columns. The activity and selectivity data at different space velocities in the absence of catalyst deactivation (i.e. initial activity/selectivity) at 550°C were obtained by the square pulse technique by passing the reaction mixture at different space velocities over fresh catalyst for a short period (2-5 min) under steady state and then replacing the reactant mixture by pure Nj during the product analysis by the GC. 

Data on acidity of H-GaMFI catalyst with silica binder at different concentrations and its initial activity and coke deposition in the propane aromatization (GHSV = 3100 cm g h ). 

In order to study the influence of binder on the deactivation of the zeolite due to coking in the propane aromatization, the time-on-stream activity/selectivity of the zeolite catalyst with or without silica binder has been determined for a period of 6.7 + 0.2 h. The results are shown in Figs. 1 and 2. Data on the amoimt of coke formed on the catalysts during their time-onstream are included in Table 1. It was shown earlier that the deactivation of H-GaMFI zeolite in the propane aromatization is mainly due to coking [11]. 

Fig. 2 Influence of time-on-stream on the selectivity for aromatios and undesired products (nnethane and ethane) and the p-X/m-X product ratio in the propane aromatization over H-GaMFI with and without silica binder (SO wt%).

Deactivation rate constant (k[Pg.428]

Data on the initial activity (total conversion of propane and its conversion-to-aromatics) in the propane aromatization over the zeolite catalyst containing silica binder at different concentrations (0-50 wt%) are included in Table 1. The initial activity for both the total conversion of propane and its conversion-to-aromatics is decreased markedly with increasing the concentration of silica binder in the zeolite catalyst. The influence of the silica binder on the initial activity is consistent with its influence on the acidity of the zeolite catalyst. The observed dependence of the catalytic activity on the acidity is also consistent with that observed earlier [11-13]. 

Comparison of H-GaMFI zeolite catalysts with and without silica binder (50 wt%) for their product selectivity, dehydrogenation / cracking (D/C) and aromatization / cracking (A/C) activity ratios, and aromatization / (methane + ethane) mass ratio and p-X / m-X ratio in the propane aromatization at isoconversion of propane (x). 

Both the intracrystalline and intercrystalline (or external) acid sites of the zeolite are decreased by the silica binder. The changes in the intracrystalline acidity of the zeolite are reflected in its propane aromatization activity the activity is reduced significantly by the silica binder. The aromatics selectivity and the dehydrogenation / cracking and aromatization / cracking activity ratios and aromatization/(methane + ethane) mass ratio are also affected appreciably by the silica binder. The shape selectivity of the zeolite is increased markedly by the silica binder. Also because of the binder, the deactivation rate constant for initial fast deactivation is decreased but for the later slow deactivation is increased. 

Modification of H-ZSM-5 zeolites through solid-state reaction with ZnO was described by Yang et al. [32]. On the basis of XPS results they reported that, upon heat-treatment of a ZnO/H-ZSM-5 mixture, Zn ions migrated from the outer surface into the channels of the zeolite. This finding was supported by TPDA, IR (decrease of acidic Brpnsted sites upon solid-state reaction between ZnO and H-ZSM-5) and temperature-programmed reduction (TPR). The latter showed increased uptake and reducibility after thermal treatment of ZnO/H-ZSM-5 compared to ZnO. Zeolites Zn,H-ZSM-5 exhibited, after reduction in H2, pronounced selectivity in propane aromatization. 

HZSM5 zeolites catalyze the transformation of propane aromatic products. This aromatization requires various steps of propene, oligomerization of propene, cyclization of oligomers and hydrogen transfer from naphtenes to olefinic compounds with formation of aromatics. The cracking of Cg-Cg oligomers leads to C2-C5 olefins which, like propene, participate in the formation of aromatics. 

Zn increased the activity of HZSMB for propane conversion and the selectivity for aromatic products. This effect of zinc was not due to a change in the acidity of HZSMB. Indeed there was practically no modification in the amnonia thermal desorption spectrum (Figure 1). The nature of Zn species and their role in propane aromatization are discussed. 

Zn species introduced in the form of zinc chloride in ZSMS zeolites increase their activity and their selectivity for propane aromatization. On ZnZSMS catalysts propane aromatization can be considered as a bifunctional process, zinc species catalyzing propane activation (step 1) and naphtene dehydrogenation (step S) and the acid sites of HZSMS catalyzing steps 2-4. The increase in selectivity for aromatics is particularly significant because on zinc species step S occurs without... 

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