Chemical and Biological Engineering MS Thesis Defense by Elif Kocaman

KOÇ UNIVERSITY

GRADUATE SCHOOL OF SCIENCES & ENGINEERING

CHEMICAL AND BIOLOGICAL ENGINEERING

MS THESIS DEFENSE BY ELİF KOCAMAN

Title: Isobutene Oligomerization on Heteropolyacids Supported on Silica-Based High Surface Area Supports

Speaker: Elif Kocaman

Time: January 10, 2018 13:30-15:30

Place: ENG 127

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

Assoc. Prof. Alper UZUN (Advisor, Koc University)

Assoc. Prof. Seda KESKİN (Koc University)

Asst. Prof. Alican KIZILKAYA (Chemical Engineering Department, İzmir Institute of Technology)

Abstract:

Energy consumption of the world depends heavily on fossil fuels. Because the heavy crude oil is accessible at considerably lower prices than their lighter counterparts, the refineries are now motivated to process these heavy feedstocks. The units processing such feedstock produce a significant amount of light olefins as side products. A way of utilizing these low-valued side products is to convert them into more valuable liquid products. Among the alternative processes applied for this purpose, oligomerization offers a great degree of flexibility in terms of product composition. This process requires a solid acid catalyst, such as solid phosphoric acid, zeolites, cation exchange resins, and metal oxides. Because each of these catalysts has their own limitations, there is a strong need for a new type of catalyst. The requirement of a good alternative is having a high density of acid sites with tunable strength to allow controlling the product selectivity. With their unique physiochemical properties, Keggin type heteropolyacids (HPAs) offer a broad potential in this regard. Here, their potential for isobutene oligomerization is explored.  For this purpose, first a screening study was performed on tungstophosphoric acid, H3PW12O40 (TPA), tungstosilicic acid, H4SiW12O40 (TSA), and molybdophosphoric acid, H3PMo12O40 (MPA), impregnated on various silica-based high surface area supports, such as MCM-41, SBA-15, and SiO2, at various loadings. The catalytic performance of more than 28 catalysts were measured under identical conditions to determine the highly performing HPA-support combination. Data indicated that the selectivity towards distillate to gasoline ratio decreased with an increase in HPA loading on TSA/SBA-15 and TPA/MCM-41 catalysts. Because the TPA has the highest thermal stability and the strongest acid strength, the TPA/MCM-41 catalysts were selected for further study to elucidate the structure-performance relationships in more detail. For this purpose, TPA was loaded on MCM-41 at fourteen different loadings ranging from 1 to 90 wt%. Detailed characterization confirmed that the TPA clusters were successfully loaded on the support. The infrared (IR) spectroscopy and X-ray diffraction (XRD) results indicated the presence of interactions between the TPA clusters and MCM-41, especially at loadings below 50 wt%, where mostly monolayer TPA dispersion was present. These interactions led to the variations in acid site density and their corresponding strength as evidenced by the results of temperature programmed desorption of ammonia measurements. Catalytic performance measurements obtained at 393 K and 15 bar indicated that these TPA/MCM-41 catalysts provide more than 75% isobutene conversion at a weight hourly space velocity (WHSV) of 46 h-1, significantly higher than what the most of the solid acid catalysts provide under comparable conditions. Results further showed that the catalysts were more selective towards distillate range products especially at very low TPA loadings. The relative selectivity of trimers over dimers in the oligomerization product pool was four at a TPA loading of 1 wt% and decreased to 1.5 with increasing loading. Ruling out the presence of any strong correlations between the acid strength and catalytic performance, the data presented a strong dependence of the product selectivity on the availability and vicinity of the acid sites. These results present a broad potential of utilizing HPAs supported on high surface area supports as an alternative family of catalysts for high performance in olefin oligomerization. Ability to adjust the TPA loading in a wide range offers opportunities for tuning the product selectivity.