Regulation of Framework Aluminum Siting and Acid Distribution in H-MCM-22 by Boron Incorporation and Its Effect on the Catalytic Performance in Methanol to Hydrocarbons

Abstract

As the process of methanol to hydrocarbons (MTH) is catalyzed by acid sites, the regulation of framework aluminum siting and acid distribution in a zeolite catalyst to enhance its performance in MTH is an important and challenging task. In this work, the regulation of framework aluminum siting in H-MCM-22 was achieved through boron incorporation; the relation between catalytic performance and acid distribution was investigated. The results illustrate that the distribution of framework aluminum and Bronsted acid sites among three types of pores in H-MCM-22 can be regulated through adjusting the content of boron incorporated during synthesis, due to the competitive occupancy of various framework T sites between boron and aluminum, whereas the textural properties and overall acid types and amounts are less influenced by boron incorporation. Incorporating a proper content of boron can concentrate the Bronsted acid sites in the sinusoidal channels rather than in the surface pockets and supercages. The acid sites located in the surface pockets and supercages are prone to carbonaceous deposition, whereas those acid sites in the sinusoidal channels are crucial for MTH in the steady reaction stage. Moreover, the acid sites in the sinusoidal channels are favorable to the olefin-based cycle that produces preferentially higher olefins. As a result, the incorporation of proper content of boron delivers the H-MCM-22 zeolite much greater stability and higher selectivity to higher olefins such as propene and butene in MTH than previously reported. These results help to clarify the relation between the catalytic performance of H-MCM-22 in MTH and its acid distribution and then bring forward an effective approach to develop better MTH catalysts by regulating the acid distribution.

Seed‐Assisted Synthesis of MWW‐Type Zeolite with Organic Structure‐Directing Agent‐Free Na‐Aluminosilicate Gel System

“…Indeed, it is difficult to assign the exact position of Al atoms in particular T-sites only by the presents ingle-pulse NMR results, although the different Al distribution can provideu sah int that the nature of the cation position( acid site) could be varied, at least in part, in seeds andt he product. There are no reports to clearly distinguish the accessible or inaccessible acid sites on T 6 and T 7 sites, althoughC hen et al [46] recently reporteda ni nteresting result.T hey regulated the Al distribution in H-form MCM-22 by keepingt he same Si/Al ratio and only changingt he boron content (Si/B ratio). Thep ossible location of acid sites in (T 1 + T 3 + T 4 + T 5 + T 8 )c an be roughly established according to previous works.…”

“…Ar elatively small number of additional peaks attributed to five-and six-coordinated extra-framework Al (d Al % 30 and 0ppm) was observed even after the calcination. [42][43][44][45][46] According to Kennedy et al, [42,43] the 27 Al DD-MASN MR peak attributed to four-coordinated framework Al can be resolved into three overlapped signals around d Al = 61, 56 and 50 ppm, assigned to the T 2 , T 1 + T 3 + T 4 + T 5 + T 8 ,a nd T 6 + T 7 sites, respectively (see Figure S8 in the Supporting Information for as chematic representation). Previous reports of varying zeolite framework topologies [8,9,13,17,18] adopting as eed-assisted, OSDA-free approach also showed products in the absence or ar elatively small amount of extra-framework Al.…”

“…As evidenced by 27 Al DD-MAS NMR measurements (Figure 3), the possible T-site (Al)d istribution (T 2 :T 1 + T 3 + T 4 + T 5 + T 8 :T 6 + T 7 ) for Na-form (24:53:22) and H-form (36:44:19) MWW-type zeolite products were different compared to that of the calcined H-form MCM-22 seeds (32:45:23). Kennedy et al [42,43] and Chen et al [46] reported that the T 4 site is mainly buried in the framework and becamei naccessiblet ot he probe molecules while the T 5 site is unstablea nd unfavorable for siting the Al atoms. In terms of (T 1 + T 3 + T 4 + T 5 + T 8 )s ites, apparently,s eeds (= 45) showed as imilar concentrationt ot he product (= 44).…”

Conversion of methanol to light olefins over H‐MCM‐22 dealuminated with different methods

“…Generally, the framework Al (56) species were ascribed to Al F species located in the sinusoidal channels, which contribute to Brőnsted acid sites in the sinusoidal channels. Meanwhile the framework Al (50) and Al (61) species were suggested to be Al F species located in the supercages and surface pockets, hence they contribute to Brőnsted acid sites in the supercages and surface pockets . As displayed in Table and Fig.…”

“…MCM‐22 zeolite has a unique lamellar structure with three independent pore systems, including two‐dimensional 10 membered‐ring (MR) sinusoidal channels (0.41 × 0.51 nm) within layer, large cylindrical supercages (0.71 × 0.71 × 1.82 nm) that are connected with 10 MR (0.4 × 0.55 nm) windows between layers and pockets (0.71 × 0.71 × 0.7 nm) on the external surface . Owing to its unique pore systems, MCM‐22 zeolite has been used in a number of catalytic processes, and recently it has been examined in the MTO reaction . Both experimental and theoretical work proposed that sinusoidal channels are favorable for the olefin‐based cycle with higher olefins formation, and the surface pockets and supercages are favorable for the formation of lower olefins and aromatics via an aromatic‐based cycle .…”