Dehydrogenation of Light Alkanes over Zeolites

Abstract

The conversion of light paraffins to olefins and the secondary reactions of the unsaturated compounds were investigated on H-ZSM5 and H-Y zeolites between 733 and 823 K. Steady stateand transient response-isotope tracing studies revealed that two mechanisms of dehydrogenation are operative. The main pathway is represented by monomolecular, protolytic dehydrogenation. This reaction contributes most to steady state olefin production. Additionally, at the initial stages of the reaction, extra framework aluminum moieties are speculated to participate in high dehydrogenation activity. This pathway is blocked at later stages of the reaction by product (hydrogen) inhibition. The intrinsic rates of protolytic dehydrogenation and olefin desorption range in the same order of magnitude. At high protolytic dehydrogenation rates, olefin desorption represents the rate determining step. Depending on the process conditions, olefins undergo secondary cracking, oligomerization, or isomerization. The latter proceeds via intramolecular rearrangement, possibly via a cyclopropylcarbenium ion at high temperatures and low conversions. At reaction conditions where bimolecular cracking prevails, isomerization is concluded to occur via secondary cracking of di-or oligomers.

Methane Transformation using Light Gasoline as Co-Reactant over Zn/H-ZSM11

“…Figure 3 shows that C2-C2=, C3 and C4 were present at initial stage of the reaction; the lowest C1 conversion levels indicated that they would be primary products from the C1 conversion. As C1 conversion increases, alkenes underwent secondary cracking, oligomerization and/or isomerization [19]. It could see that their yields pass through a maximum and then decrease, so they undergone a later transformation towards i-C4, C4= and finally to yield AH.…”

Effect of Sodium in Ferrierite on Selective Catalytic Reduction of NO by Acetylene

“…Whereas, Brönsted acids have been suggested to be essential for HC-SCR over many catalytic systems (e.g. ZSM-5 modified by Pd, Ga, In, Ce and Ag) [11][12][13][14][15][16][17][18]. The authors found that Brönsted acids contributed to the aimed reaction in different steps.…”

Rate Parameter Distributions for Isobutane Dehydrogenation and Isobutene Dimerization and Desorption over HZSM-5

“…These also fall most comfortably within the Site-2a, Brønsted acid range listed in Table 2. Other reported experimental dehydrogenation activation energies over HZSM-5 are 100 kJ mol −1 [14] and 136 ± 6 kJ mol −1 [1]. The latter was calculated using the same data sets and at the highest temperatures (>395 °C).…”

“…Many experimental and theoretical studies have been undertaken on isobutane adsorption and conversion reactions over zeolites. Experiments have provided a range of carbon-carbon bond cleavage and dehydrogenation mechanisms and rate parameters [1,4,[12][13][14][15]. Isobutene dimerization [16][17][18][19] and oligomerization [20] reactions over acidic catalysts in the liquid-phase have also been investigated.…”