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Title: Alkylation of benzene and toluene with propane over bifunctional metal-acid catalysts
Author: Alotaibi, A. A.
ISNI:       0000 0004 7428 5889
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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Effective functionalization of light alkanes is one of the greatest challenges for catalysis science. Involving alkanes in reactions with other organic molecules, e.g., aromatic hydrocarbons, is important direction of light alkane utilization. The alkylation of benzene with ethane and propane to produce ethylbenzene (EtPh) and isopropylbenzene (iPrPh)) has attracted particular interest as these are the key intermediates in manufacturing styrene and phenol and produced commercially on a large scale by the well-established acid-catalysed alkylation of benzene by alkenes, ethene and propene. The replacement of ethene and propene by abundant and inexpensive alkanes would lead to more cost effective and environmentally benign production of these commodity chemicals. The first aim of this study is studying the direct alkylation of benzene with propane over bifunctional metal-acid catalysts in the gas phase using Pt and Pd as the metal components and silica-supported tungsten heteropoly acids (H3PW12O40 and H4SiW12O40, which are the strongest Keggin HPAs) as the acid components. It was found that with these catalysts, the alkylation yields isopropylbenzene (iPrPh) with high selectivity in a fixed-bed reactor at 250-350 oC and 1 bar pressure. Most efficiently the reaction occurs over Pt/H4SiW12O40/SiO2 catalyst, giving iPrPh with 90-93% selectivity at 6-8% conversion of benzene at 300 oC and an inlet C6H6/C3H8 molar ratio of 1:9. This significantly exceeds the efficiency of previously reported zeolite-based catalyst Pt/HZSM-5. There is important difference in performance between Pt/HPA/SiO2 and Pt/HZSM-5 catalysts. With the Pt/HZSM-5 catalyst, reaction selectivity is controlled by acid-catalysed transformations within zeolite microporous structure (product shape selectivity), leading to preferential formation of nPrPh together with cracking and transalkylation products such as MePh and EtPh rather that the desired iPrPh. In contrast, the mesoporous Pt/HPA/SiO2 catalyst gives selectively iPrPh, i.e., the alkylation product favoured from the carbenium ion mechanism. Next, the alkylation of benzene with propane was tested using Pd/HSiW and Pd/HZSM-5, similar to the corresponding platinum catalysts. The palladium catalysts were also active in this reaction under similar conditions. The mesoporous catalyst Pd/HSiW/SiO2 was much more selective to iPrPh (up to 88%) than the microporous Pd/HZSM-5 catalyst, which gave only 11-18% iPrPh selectivity. Overall, the Pd/HSiW and Pt/HSiW catalysts performed similarly in the alkylation of benzene by propane, with the Pt catalysts predictably more active than Pd ones per metal loading. This, however, is compensated for by the lower price (by a factor of ~1.5) of Pd as compared to Pt. Addition of gold to Pd-HSiW was tested in the alkylation of benzene and found to slightly enhance the activity of Pd catalysts. Finally, we looked at the direct alkylation of toluene with propane over the above bifunctional metal-acid catalysts in the gas phase in comparison with the corresponding catalysts based on zeolite HZSM-5. This reaction was found to occur most efficiently over silica-supported Pt/HPA/SiO2 catalysts giving p-cymene with up to 75% selectivity at 300 oC. Pt/HZSM-5 catalysts were much less selective (= 5.5% p-cymene selectivity), which may be explained by product shape selectivity control imposed by HZSM-5 microporous environment. The Pd-based bifunctional catalysts performed similarly to Pt catalysts, albeit with a lower activity per metal loading. Addition of gold to Pt and Pd in these catalysts was tested, but had little effect on their activity and p-cymene selectivity in toluene alkylation with propane.
Supervisor: Kozhevnikov, Ivan Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral