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Ranges and levels of the experimental parameters.
| Factor | Symbol | − α | − 1 | 0 | + 1 | + α |
|---|---|---|---|---|---|---|
| Temperature/°C | A | 25 | 56 | 77 | 98 | 130 |
| Time/min | B | 30 | 111 | 165 | 219 | 300 |
| Catalyst loading/mg | C | 1.0 | 15.7 | 25.7 | 35.3 | 50.0 |
| Molar ratio of alcohol: acid | D | 1.0 | 3.7 | 5.5 | 7.3 | 10.0 |
