DOCTORAL DISSERTATION Design, Preparation and Application of Zeolite Capsule Catalysts
Preface In this thesis, Pd sputtered zeolite capsule catalyst Co/SiO 2 -Z-Sp-Pd, zeolite capsule catalyst Cr/ZnO-SAPO46-PhyC with SAPO-46 zeolite shell, a tripartite zeolite capsule catalyst Ru/Al 2 O 3 -Pd/S and a dual-layer zeolite capsule catalyst Fe/SiO 2 -S-Z for synthesis of different target products by different tandem reactions were introduced. Tandem reaction process is a significant strategy to improve the efficiency of a chemical reaction, saving time and resources, while increasing
... , while increasing the reactant conversion and product yield. In these tandem processes, the reactants can be converted into target products through several sequential reactions that generally require different catalysts and several reactors. Methodologically, the separated catalysts for different sequential reactions are introduced together into one tandem reaction system. And a shared catalyst assembly style, physical mixing, is always adapted to combine the varied catalyst components. The obtained catalyst mixture is often called hybrid catalyst. One of the most used tandem reaction process proceeding on two or three types of different catalysts is illustrated as follow. Reactants enter this catalysis system to produce intermediates on the first catalyst A, and then part of the formed intermediates contact the second catalyst B to generate target products. Tandem reaction, however, can not proceed so smoothly on hybrid catalyst as designers expect, because some intermediates can escape from hybrid catalyst directly without further reaction on catalyst B. Moreover, the target product, generally, will not stop its further conversion, that is, it can be catalyzed again by either catalyst A or B to generate other undesired chemicals, consequently decreasing the selectivity of target products. Therefore, for tandem reaction, one challenge, but without being paid sufficient attention until now, is to improve the assembly style of different catalysts. However, as well known, different catalytic components in catalyst, especially for heterogeneous catalysis, can work more Preface Y. Jin II efficiently if with a well-designed assembly style. Precisely designing the catalyst structure, rather than simply loading different active metals on diverse supports, is very necessary. we demonstrate that changing the assembly style of general hybrid catalyst to core-shell-like zeolite capsule catalyst, that is, conbining these two or three types of different catalyst components in hybrid catalyst to a core-shell like zeolite catalyst, can significantly improve the oriented synthesis of target products and simultaneously cut off the over production of undesired chemicals. In chapter 1, The mm-sized capsule catalyst Co/SiO 2 -Z with the H-type zeolite (HZSM-5) as the shell, and the Co/SiO 2 -Z-Sp-Pd capsule catalyst with Pd loaded by sputtering method, as well as the Co/SiO 2 -Z-IW-Pd capsule catalyst, loading the Pd by incipient wetness impregnation, were prepared and used for isoparaffin direct synthesis by Fischer-Tropsch synthesis reaction from syngas with H 2 /CO ratio of 2/1 at 1.0MPa and 533 K. The analysis results of XRD, SEM, EDS and NH 3 -TPD showed that a compact HZSM-5 shell was formed on the Co/SiO 2 pellet, and the metallic Pd was well sputtered on the surface of the HZSM-5 shell for the Co/SiO 2 -Z-Sp-Pd catalyst. The isoparaffin and olefin selectivity increased in the FTS reactions on the HZSM-5 capsule catalyst than on the Co/SiO 2 catalyst. The selectivity of isoparaffins increased, and the selectivity of olefins decreased when using the Co/SiO 2 -Z-Sp-Pd catalyst, compared with the Co/SiO 2 -Z catalyst or the Co/SiO 2 -Z-IW-Pd catalyst, because metallic Pd introduced by sputtering on the zeolite shell could hydrogenate olefins efficiently. It was suitable to use the sputtering method for the preparation of the zeolite capsule catalyst loaded with metallic Pd, as conventional impregnation method could not reduce Pd cation due to the strong interaction from zeolite surface. The CO conversion, CH 4 and CO 2 selectivity on the catalyst of Co/SiO 2 -Z-Sp-Pd increased with temperature increasing from 513 K to 553 K. At elevated temperature, the isoparaffin selectivity also increased while the olefin selectivity was suppressed. Preface Y. Jin III In chapter 2, a simply manipulated approach, named "physically coating" (PhyC), was initially developed to construct a new core-shell-like silicoaluminophosphate (SAPO-46) encapsulated Cr/ZnO capsule catalyst. The method reported here is very accessible, repeatable, and scalable, overcoming much practical problems faced by the industrial preparation of zeolite capsule catalysts through traditional hydrothermal synthesis way. With the assistance of this method, the core catalyst Cr/ZnO is completely safe from the destruction caused by heat treatment and chemical corrosion. The one-step reaction process of syngas to dimethyl ether synthesis (STD reaction) is selected as the application to test the catalytic activity of the prepared capsule catalyst Cr/ZnO-SAPO46-PhyC. In comparison with the simple mixing structure of the traditional mixture catalyst, the specific core-shell structure of this new zeolite capsule catalyst has an obvious confinement effect on the controlled synthesis of DME from syngas. At the same time, it can effectively suppress the side reaction of the further dehydration of DME generating other by-products such as alkane/alkene. In chapter 3, a novel tripartite zeolite capsule catalyst that consists of a core (Ru/Al 2 O 3 )-microporous shell (Silicalite-1)-dopant (Pd) structure was presented. And then with the liquid-phase tandem reaction of glycerol conversion as a probe reaction, we demonstrate the previously unreported superiority of this tripartite zeolite capsule catalyst on the oriented synthesis of target products. The Pd doped microporous zeolite shell constructs a confined reaction space and provides molecular screening and refining ability to this tripartite zeolite capsule catalyst, which drives it to effectively realize the controlled synthesis of desired chemicals, simultaneously depressing undesired side-reactions, much better than conventional catalyst assembly style. The concept of catalyst encapsulated by a Pd doped micro porous zeolite shell and its application begin to bring new opportunities for studying the function of catalyst assembly style in varied tandem reaction systems, the correlations between catalyst assembly style and its Preface Y. Jin IV catalytic properties, as well as the nature of catalyst active sites and reaction mechanisms. In chapter 4, an iron-based H-ZSM-5 zeolite capsule catalyst (core catalyst of Fe/SiO 2 with 20 wt% Fe loading amount) was readily prepared by employing a dual-layer method under close-to-neutral conditions. The shell of the synthesized zeolite capsule catalyst has a dual-layer structure that enwrapped Fe/SiO 2 catalyst completely without any pinholes or cracks. For the direct synthesis of isoparaffin from syngas on this zeolite capsule catalyst, the formation of heavy hydrocarbon of C 12+ was suppressed completely, and the middle isoparaffins became the main products. The catalyst performances showed that its CO conversion and olefin selectivity decreased, and isoparaffin selectivity increased if compared with the mixture catalyst Fe/SiO 2 -M. This simple dual-layer method successfully solved the challenge that is synthesizing H-type zeolite shell on the silica-based catalyst under strong alkaline conditions.