Aromatic Disproportionation and Transalkylation over Modified Beta Zeolites

Abstract

Benzene, toluene and xylenes (BTX) are aromatic hydrocarbons that have a wide array of applications in the chemical and petrochemical industries. Xylenes, and especially the p-xylene isomer, are expected to see a growth rate of 7% by 2024 due to their use for production of purified terephthalic acid (PTA). As such, several processes based on zeolite catalysis have been developed to recover and maximize xylene production. Disproportionation and transalkylation are some of these processes, which utilize less valuable toluene that is in surplus and low value C9 aromatics to produce more valuable xylenes. In this thesis, toluene and 1,2,4-trimethylbenzene (TMB) disproportionation and their transalkylation has been studied over large pore beta zeolites. The effect of slight pressure fluctuations and varying feedstocks (toluene and/or 1,2,4-TMB) has been investigated over beta zeolites impregnated with 0.08% platinum metal. Also, the role of mesopore generation in both H-form and Pt-loaded beta zeolites has been evaluated in transalkylation activity. Lastly, the role of 3D-printing H-form and Pt-loaded beta zeolite monoliths with varying binder contents has also been examined in transalkylation. A fixed bed reactor was used to conduct experiments at 400 °C, WHSV of 5 h-1 with a feed of 50:50 wt. % toluene:1,2,4-TMB in transalkylation reactions. All activity was tested for 50 hours time-on-stream (TOS). In toluene disproportionation (TDP), highest toluene conversion and xylenes yields were attained in the order of 10 bar > 9 bar > 8 bar. Moreover, conversion and xylenes yields were maximized in transalkylation, followed by 1,2,4-TMB disproportionation and lastly toluene disproportionation. The introduction of mesopores in beta zeolites through the MWAC method (microwave-assisted chelation) was successful in rendering mesopores in the resultant beta zeolite, but was done at the expense of the parent zeolite’s intrinsic microporosity. This is evidenced through the loss of the resultant zeolite’s micropore area and volume upon mesopore formation. As such, lower conversion and xylenes yields were observed over mesoporous beta with more severe deactivation behaviour. Upon impregnation with platinum metal, mesoporous Pt-beta exhibited excellent catalytic performance wherein highest conversion and xylenes yield were attained at 63 wt. % and 42 wt. % respectively, similar to microporous Pt-beta, with minimal deactivation throughout 50 hours TOS. It appears that the hierarchical zeolite promoted homogenous dispersion of active component (platinum metal) on its surface, leading to enhanced catalytic stability with increased conversion and xylenes yield. The 3D-printed beta zeolite monoliths were successful at xylene production through transalkylation reactions. Among the metal-free 3D-printed catalysts, samples that were composed of highest zeolitic content (90 wt. %) and lowest binder content (10 wt. %) exhibited highest catalytic activity. Nevertheless, conversion and xylenes yield profiles were slightly lower than that of parent powder beta as the 3D-printed samples had less zeolitic content in their mixture and would thus be slightly less reactive. Upon platinum impregnation (0.08%), the 3D-printed Pt-beta monoliths exhibited higher conversion and xylenes yield than the metal-free 3D-printed monoliths and similar activity to powder parent Pt-beta. Highest conversion and xylenes yield were observed over sample F (80 wt. % Pt-beta:20 wt. % alumina binder) with values of 60 wt. % and 42 wt. % respectively. Although slightly less of all aromatic products were present in the product streams of the 3D-printed Pt-loaded samples, the reduction was not proportional to the reduction in the weight of the zeolite present in the monoliths. As such, they serve as cheaper economical alternatives to powder bifunctional zeolites as they contain less active zeolite component, replaced with inexpensive binder material, while giving similar catalytic results.

Date of Award	29 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Arthur Garforth (Main Supervisor) & Xiaolei Fan (Co Supervisor)