Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Hugo de Lasa

Abstract

Light olefins are key intermediates in the petrochemical industry. Nowadays, finding economic alternative routes for light olefin synthesis is among the main goals for many petrochemical companies. The conversion of natural gas to light olefins is one of the promising routes in which DME is an intermediate that can be synthesized directly from syngas. This route requires having DME as a feedstock to produce light olefins (DTO) using an HZSM-5 catalyst. In this catalytic process, the DME converts first to light olefins that may further transformed, forming heavier C4+ hydrocarbons. Thus, selecting a proper catalyst and suitable operating conditions that may lead to minimize olefins further conversion is a key for the implementation of the DTO process. In order to accomplish this, kinetic modeling and catalyst design is very critical. To our knowledge, there is no study cited in the technical literature addressing these topics jointly and this is the case, despite their great significance for DTO reactor scale-up.

Given the originality and the economic incentive of the above subject, the aim of the present research is to investigate HZSM-5 as a potential selective and robust catalyst for light olefin production from DME. The detailed objectives of this PhD dissertation include catalyst preparation, characterization, testing under reaction conditions and kinetic modeling. The catalyst characterization addresses the influence of the SiO2/Al2O3 ratio (30, 80, and 280) on HZSM-5 physicochemical properties. The reactivity runs, on the hand, are intended to achieve the maximum catalyst performance and light olefin selectivity by varying the SiO2/Al2O3 ratio in the HZSM-5 catalyst. The kinetic study involves a reaction scheme and the development of a model suitable to describe the reaction network. The proposed kinetic model includes establishing the rates for the individual steps, assembling the individual chemical species balances, and using the experimental data to determine numerically the phenomenologically based kinetic parameters.

Regarding the catalyst characterization in the present PhD dissertation, N2 isotherm data show a dual hysteresis loop for HZSM-5 with a SiO2/Al2O3 = 280 ratio compared to a single hysteresis loop produced by HZSM-5 with a lower SiO2/Al2O3 ratio. The NLDFT cylindrical model confirmed the HZSM-5 characteristic 5.5 Ǻ pore size. On the basis of the XRD and N2-isotherm results, the SiO2/Al2O3 ratio appears not to have an effect on HZSM-5 structural properties. NH3-TPD and Pyridine-FTIR confirmed that in HZSM-5, weak acidity encompasses Lewis and hydrogen-bonded sites. The strong acidity mainly involves the Brönsted acid sites with a ratio of weak to strong acid sites and a total acidity being reduced with the SiO2/Al2O3 ratio. Furthermore, NH3 desorption kinetics allows the calculation of activation energies and rate constants for both strong and weak acid sites. This allows one to show that desorption rate constants vary with the changes in weak and strong acid sites as observed with NH3-TPD and Pyridine-FTIR.

Concerning catalyst reactivity and more specifically DME conversions and catalyst deactivation, it is proven in the present study, that both augment with the acidity of the HZSM-5 (SiO2/Al2O3 in the 30-80 range) and the reaction temperature. The HZSM-5 catalyst with a SiO2/Al2O3 = 280 ratio, however, provides a stable operation with higher light olefin selectivity (ethylene, propene and butene). As a result, this catalyst was chosen in the present PhD dissertation, for the development of the DTO reaction network. In this respect, for this catalyst, light olefins were found to be the major products at low conversion (2%) with minor heavy olefins and with no paraffins or aromatics produced. With the raising of the DME conversion up to 45%, C5+ olefins, paraffins, and aromatics selectivities were more abundant, increasing consistently at the expense of the light olefins. In addition, at DME conversions higher than 45%, the change of the product selectivities became less pronounced. At the maximum DME conversion of 74%, the attained selectivites for light olefins, heavy olefins, paraffins, and aromatics were 48%, 32%, 9%, and 11%, respectively.

The reaction elementary steps were modeled using carbenium ion chemistry. In the DTO reaction network, surface bound methoxy species play a vital role as methylating agents for both olefins and aromatics. Furthermore, the resulting kinetic model for DTO was established using rigorous statistical analysis. This was done estimating DTO hydrocarbon species reaction rate parameters using non-linear regression and experimental data. It was observed that the pre-exponential factors for methylating olefins and aromatics decreased consistently with the increase of the carbon number of the species being methylated. The activation energy for methylating light olefins was found to be slightly higher (169 kJ/mol) when compared to that for the heavy olefins (156 kJ/mol). On the other hand, the aromatic methylation reaction, displayed a higher activation energy as the number of methyl groups in the aromatic ring increased (104 -191 kJ/mol).


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