Methanol‐to‐Olefins Reaction over Large‐Pore Zeolites: Impact of Pore Structure on Catalytic Performance

Abstract

In the methanol-to-olefins (MTO) reaction, product flexibility is important for meeting the fluctuating chemicals demands. In this paper, the catalytic performance of recently reported zeolites having 12-ring pores, such as boroaluminosilicate CON-type zeolite ([Al,B]-CON), MCM-68, and YNU-5 zeolites, in the MTO reaction was compared to understand the relationship between zeolite framework and product selectivity. The catalytic performance of the synthesized zeolites was compared with that of Beta, mordenite, and ZSM-5. The highest selectivities for propylene and butylenes were achieved over MCM-68. The highest propylene-to-ethylene ratio in the products was obtained over [Al,B]-CON. ZSM-5 and [Al,B]-CON exhibited a superior ability to suppress coke formation compared to Beta, mordenite, MCM-68, and YNU-5.

Surfactant-Assisted Direct Crystallization of CON-Type Zeolites with Particle Size and Acid-Site Location Controlled

“…The observed size differences in the particle morphologies of the The XRD patterns of the H + -form samples are shown in Figure 1. For the samples without CTAB in the mother gel (Figure 1b−e), peak patterns almost identical to those of the CON-type zeolites 9,32,33,[36][37][38]40 were obtained, except for Si/ Al = 100 (Figure 1a). The formation of [Al, B]-CON-100 with a CON-type phase was not observed under the conditions applied in this study, suggesting that the synthetic window of the Si/Al molar ratio for the CON-type boroaluminosilicate zeolite ranges from ca.…”

“…The introduction of seed crystals into a synthetic gel is one of the most effective ways to promote crystallization kinetics that favor the formation of the desired zeolite crystal topology and size . As it is widely known that 12MR pore structures are topologically equivalent to those of CON phases, zeolite beta (*BEA) has been used as seeds in the synthesis of CON-type zeolite materials in several reports. ,,,− , Previously, Okubo and co-workers reported the considerable influence of the seed crystals CIT-1­(CON) and beta (*BEA) on the formation of CON-type zeolites and their crystal sizes, where 20 wt % of seed crystals relative to the silica source and tetraethylammonium hydroxide were used . This aroused our interest in the influence of the seed crystal on the formation of CON-type zeolites in the traditional synthetic method using 2 wt % of seeds and TMMAOH.…”

“…Zeolites that consist of three-dimensional channel systems with a 10MR and two 12MRs are fascinating candidates as catalysts for the selective production of propylene from methanol. Among these, CON-type zeolites have been viewed as potential catalysts or adsorbent materials based on their unique structure with high accessibility and diffusibility; the 10 and 12MR channels are connected and form a large void at the intersections, providing access to the crystal interior via two pores. , Since the discovery of the borosilicate CIT-1 as the first example of the pure CON phase, the design of various CON-type zeolites as well as their synthesis and resulting functions have attracted great attention. ,,− We have previously reported on the synthesis and characteristics of CON-type zeolites, together with their potential as catalyst materials for the selective production of propylene in the MTO reaction. − To improve the catalytic lifetime, control over the acidity and diffusibility are highly important. Along this line, it has been revealed that the use of high-silica CON-type boroaluminosilicate zeolites [Al, B]-CON and/or the isomorphous substitution of Al atoms by Ga atoms results in an improved catalytic lifetime with considerably high propylene selectivity, suggesting the effectiveness of heteroatom substitution onto the modulation of the acidity of zeolites.…”

Molecular Views on Mechanisms of Brønsted Acid-Catalyzed Reactions in Zeolites

“…The Davis group examined the selectivity of a large number of small-pore cage-type topologies and found that the olefin selectivity was quite well correlated with the cage-defining ring size, which is the critical diameter of the ellipse that can be inscribed in the cage (Figure ). The zeolites with the smallest cage-defining ring size had the highest selectivity to ethene versus propene, while very large cages produced longer olefins …”

Control of Competitive Structure-Directing Effect in CON-Type Zeolite Synthesis via Intermediate Addition Method

“…Owing to their high accessibility and diffusibility by large pores, as well as superior selectivity due to medium pores, a CON-type zeolite is expected to be a promising catalyst, especially for the methanol-to-olefins (MTO) reaction, which is considered a clean alternative path to produce light olefins. , An aluminoborosilicate CON-type zeolite, which can be prepared by both postsynthetic modification and direct synthesis to introduce acid sites generated by framework aluminum, has exhibited excellent catalytic performances. Unlike typical zeolites such as ZSM-5, SAPO-34, mordenite, and beta, − CON-type zeolites have demonstrated a long catalytic lifetime and high selectivity for key plastic feedstocks such as propylene and butane . However, two significant problems hinder the utilization of CON-type zeolite: (i) a long synthesis duration (7 days) and (ii) a highly expensive OSDA with a complex structure ( N , N , N -trimethyl-(−)- cis -myrtanylammonium (TMMAOH)).…”