Exploring the void structure and activity of RUB-39 based expanded materials using the hydroconversion of decane

Abstract

The layered silicate RUB-39 can be transformed by topotactic condensation into RUB-41 (RRO), a zeolite with 8- and 10- ring pores. If the layered RUB-39 is first silylated with dichlorodimethylsilane (DCDMS) or hexamethyldisiloxane (HMDS), an interlayer expanded structure is created after calcination. The DCDMS expanded material contains 10- and 12-ring pores instead of 8- and 10-ring pores. Detailed physicochemical characterization showed that the Al content is not significantly changed during the expansion. In the hydroconversion of decane, the expanded materials have a significantly increased activity, as demonstrated by the lower temperatures at which isomerization and cracking occur. Detailed comparison of the product selectivities obtained with RUB-41 or with its expanded analogs shows that the void structure of the expanded materials is significantly less constrained, as reflected in the distribution of methylnonane isomers, of the ethyloctane vs. methylnonane isomers, and in the ratio of monobranched vs. dibranched isomers.

Introduction

Zeolites are widely used as heterogeneous catalysts for the production of petrochemicals and fine chemicals. Because of the permanent quest for more sustainable processes and because of the ever more demanding requirements to the catalysts for these processes, there is a continued search for new zeolite materials with improved catalytic properties [1]. In an innovative approach to zeolite synthesis, layered silicates are prepared in a first step; upon calcination, these may transform by topotactic condensation into three-dimensional silicate frameworks. In some cases, the topotactic condensation of new layered materials results in well-known structures. For example, PREFER transforms upon calcination into ferrierite [2].

In other cases, the calcination of the layered precursor materials results in previously unknown structures. For instance, EU-19 [3], Nu-6 [4], PLS-1 [5], or RUB-18 [6] transform upon calcination into EU-20b, Nu-6(2), CDS-1, and RUB-24, with CAS, NSI, CDO, and RWR topologies, respectively. However, in all these materials, the pores are narrow and hardly accessible to reactants. In order to create more attractive materials for catalysis, the layered precursor can be treated after synthesis [7], [8]. Delamination can be achieved via swelling of the lamellar precursor with a surfactant. For instance, full delamination of the precursor material MCM-22(P) resulted in the formation of ITQ-2 [9]. Alternatively, MCM-22(P) can be pillared by the use of a swelling agent and a treatment with tetraethylorthosilicate, resulting in a material named MCM-36 [10]. ITQ-2 and MCM-36 showed an improved access to the catalytic sites in the hydroconversion of n-decane compared with MCM-22 [11]. Recently, zeolite UTL was converted into a lamellar material with zeolite-like layers. This new material can be further modified in a similar way to other layered materials [12].

A new and versatile method of converting 2D lamellar precursors has been reported by the groups of Wu, Tatsumi, and coworkers [7], [13]. They prepared Interlayer Expanded Zeolites (IEZ) by inserting monomeric Si compounds into the interlayer spaces via a one-step dialkoxysilylation followed by the removal of the organic moieties. IEZ materials were prepared starting from layered silicates PLS-1, MWW(P), PREFER, and MCM-47. Diethoxydimethylsilane (DEDMS) or dichlorodimethylsilane (DCDMS) was employed as silylating agents. The two methyl groups serve to avoid intermolecular condensation; if triethoxymethylsilane or tetraethoxysilane (TEOS) are used, condensation of the silanes can occur and amorphous silica is deposited on the zeolite crystals.

A recent addition to the group of layered silicate precursors is RUB-39. Upon calcination, this material yields the zeolite RUB-41 with RRO topology [14], [15]. RUB-41 contains a 2-dimensional pore system with intersecting 8- and 10-ring pores. From structure analysis, their dimensions are determined as 0.58 × 0.41 nm (8MR) and 0.59 × 0.41 nm (10MR). In previous work, we demonstrated the selective uptake of trans-2-butene and cis-2-butene from an isomeric butene mixture on the all silica RUB-41 [16]. This selectivity was ascribed to the distorted 10-ring present in RUB-41. In later work, Al was incorporated into this structure. (H)Al-RUB-41 was employed in the amination reaction of methanol with ammonia. (H)Al-RUB-41 showed a high selectivity (87%) toward monomethylamine and dimethylamine, a behavior which is typically expected for a small pore zeolite. This was attributed to the unique pore architecture of the material with its distorted 10 channels, which for this reaction seem spatially more restrictive than the 10-MR in, e.g., MFI or MEL zeolites [17].

Here, we report on materials obtained by interlayer silylation of the layered RUB-39 precursor material. After exposure of RUB-39 to dichlorodimethylsilane (DCDMS) or hexamethyldisiloxane (HMDS), followed by calcination, new expanded structures are formed. In this work, the catalytic effect of the interlayer expansion is investigated using the hydroconversion of decane as a test reaction. The composition of the branched isomerization products formed from decane is used to evaluate whether effectively the interlayer expansion results in a modified intracrystalline zeolite pore architecture.