Optimization of Process Variables in the Synthesis of Tributyl Citrate Using a Polyvinylpolypyrrolidone-Supported Brønsted Acidic Ionic Liquid Catalyst

Table 1

Coded values of the variables used in the experimental design.

Variables	Symbols	Variable levels
		−1	0	1
Mole ratio of n-butanol to citric acid		4.5	5.0	5.5
Reaction temperature (°C)		110	118	126
Amount of catalyst (wt%)		4	6	8
Reaction time (h)		4	5	6