crystalline gallium silicates but these gallium silicates had not been subjected to a two step treatment according to the invention in experiment 5 carried out using a crystalline gallium silicate with a sio 2 ga 2 o 3 molar ratio of 160 the gallium silicate was subjected to the two step treatment no more than three times whereas the formula which expressed the relation between minimum number of times for the two step treatment to be carried out and sio 2 ga 2 o 3 molar ratio of the silicate stipulates that this number should be at least six on the results listed in table c the following may be remarked comparison of the results of experiments 1 3 shows that catalyst i per se shows acceptable performance which is enhanced by subjecting the catalyst three times experiment 2 or eight times experiment 3 to the two step treatment comparison of the results of experiments 4 9 shows that catalyst ii shows unacceptable performance which is insufficiently improved by subjecting the catalyst three times experiment 5 to the two step treatment subjecting catalyst ii to the two step treatment eight times experiment 6 or 16 times experiment 7 leads to acceptable performance subjecting catalyst ii to the two step treatment 20 times experiment 8 leads to optimum performance this performance does not improve significantly when the number is raised from 20 to 30 experiment 9 comparison of the results of experiments 6 and 10 shows that the number of two.