Probing the Accessible Sites for n-Butene Skeletal Isomerization over Aged and Selective H-Ferrierite with d3-Acetonitrile

Abstract

Aged H-ferrierite (H-FER) samples with different contents of de-posits were prepared and studied under differential catalytic conditions in a tapered element oscillating microbalance. Subsequently, these samples were examined using infrared spectroscopy to determine the nature of carbonaceous deposits and probing the type and number of accessible sites with d 3 -acetonitrile. From these results, for the first time, we have been able to calculate turnover frequencies (TOFs) for n-butene conversion and isobutene formation for both fresh and aged H-FER catalysts. It is observed that the deposition of carbonaceous species significantly lowers the number of accessible Brønsted sites. With short time on stream (TOS), cracking of the alkyl-aromatic deposits contributes to the overall isobutene production but simultaneously harms the selective catalytic action by inducing nonselective side reactions. With prolonged TOS these deposits become nonreactive and as a result the TOF drops, while isobutene is produced with high selectivity. It is demonstrated that at this stage no carbenium ions are detected, while Brønsted OH acid sites are still accessible for d 3 -acetonitrile on the extensively aged and highly selective H-FER. This indicates that the latter sites are responsible for the selective catalytic conversion of n-butene into isobutene. c 2002 Elsevier Science (USA)

Investigations into the origin of the remarkable catalytic performance of aged H-ferrierite for the skeletal isomerization of 1-butene to isobutene

“…This means that a nonnegligible amount of isobutene formed over fresh H-FER is produced by the monomolecular mechanism, although the nonselective bimolecular mechanism is prevailing at early TOS. As already suggested by many different groups [7,9,10,15,36], in addition, it appears that the catalytic action of aged H-FER is dominated by the monomolecular mechanism, in which 1-butene is selectively isomerized over the Brønsted acid sites near the 10-ring pore mouths. In other words, pore mouth catalysis, which was initially proposed to explain the highly selective nature of medium-pore molecular sieves with a one-dimensional 10-ring pore system, such as ZSM-22 and SAPO-11, for the hydroisomerization of long-chain n-alkanes to their branched isomers [37][38][39], could be the main origin of the remarkable isobutene selectivity observed for aged H-FER.…”

“…Because the pseudo-monomolecular mechanism does not require a large number of alkylaromatic tertiary carbenium ions to maintain high isobutene selectivity [13], the observed decrease in 1-butene conversion over H-FER caused by oxalic acid treatment makes the arguments against the existence of this mechanism stronger. In fact, there are several studies showing that the presence of coke deposits inside the H-FER pores is not crucial for achieving high isobutene selectivity [29,40,46,47], although they could contribute partly to the formation of by-products at early TOS. ferent crystal sizes, respectively.…”

“…Three lines of reasoning to support this idea are given below. First, a recent series of studies by van Donk et al [17,36,40,41] has shown that the 10-ring pore mouth inlets of aged H-FER are still accessible for 1-butene molecules, even in the case of deposition of high amounts (e.g., 6.8 wt%) of coke in the zeolite pores. Second, pore mouth shape selectivity allows us to explain the data of O'Young et al [42] wherein H-FER is more selective than ZSM-23, despite the fact that the free diameter (5.2 Å) at the FER channel intersection is slightly larger than that (4.8 Å) of 10-ring channels in ZSM- 23.…”

The Distribution and Strength of Brönsted Acid Sites on the Multi-Aluminum Model of FER Zeolite: A Theoretical Study

“…Ferrierite (FER) zeolite is a medium pore aluminosilicate material, including two-dimensional intersecting channels with 8-membered rings (4.8 Å × 3.5 Å) and 10-membered rings (5.4 Å × 4.2 Å) [6,7]. Due to its prominent pore structure and Brönsted acidity, it has been proved to be an efficient catalyst for butene isomerization [8,9]. Extensive experimental and theoretical studies have been performed to understand the underlying origin of the high reactivity of FER [8][9][10][11].…”

“…Due to its prominent pore structure and Brönsted acidity, it has been proved to be an efficient catalyst for butene isomerization [8,9]. Extensive experimental and theoretical studies have been performed to understand the underlying origin of the high reactivity of FER [8][9][10][11]. Although it is generally believed that the Brönsted acid site is the active site for the isomerization process and that the unique pore structure plays an important role in the reactivity, the properties of the catalyst, such as the acid strength, acid density, and the location of the acid sites and their effect on the reactivity are still unclear in the literature.…”