Table 5. Levels of the factors, experimental design and the results of the central composite design.

Run	Coded variable level		Real variable level		H2 yield (mol/mol-TRS)a
	X1	X2	X1	X2	Observed	Predicted
1	1	−1	17	5.5	1.62 ± 0.03	1.61
2	0	0	16	6.0	2.02 ± 0.05	2.00
3	−1.41	0	14.59	6.0	1.84 ± 0.01	1.86
4	−1	−1	15	5.5	1.79 ± 0.08	1.78
5	0	0	16	6.0	1.99 ± 0.02	2.00
6	0	0	16	6.0	1.98 ± 0.01	2.00
7	0	0	16	6.0	2.01 ± 0.02	2.00
8	0	−1.41	16	5.29	1.64 ± 0.03	1.66
9	0	1.41	16	6.71	1.71 ± 0.07	1.72
10	−1	1	15	6.5	1.85 ± 0.09	1.83
11	0	0	16	6.0	2.01 ± 0.02	2.00
12	1.41	0	17.41	6.0	1.61 ± 0.02	1.61
13	1	1	16	6.5	1.63 ± 0.04	1.63

^aValues were given by mean ± standard deviation (n = 3).